Prof. Dr. Thomas Kirchartz

associated member

IEK5 Photovoltaik
Forschungszentrum Jülich GmbH

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  • A Bias-Free, Stand-Alone, and Scalable Photovoltaic–Electrochemical Device for Solar Hydrogen Production
    Lee, M. and Turan, B. and Becker, J.-P. and Welter, K. and Klingebiel, B. and Neumann, E. and Sohn, Y.J. and Merdzhanova, T. and Kirchartz, T. and Finger, F. and Rau, U. and Haas, S.
    Advanced Sustainable Systems 4 (2020)
    Although photovoltaic–electrochemical (PV–EC) water splitting is likely to be an important and powerful tool to provide environmentally friendly hydrogen, most developments in this field have been conducted on a laboratory scale so far. In order for the technology to make a sizeable impact on the energy transition, scaled up devices must be developed. Here a scalable (64 cm2 aperture area) artificial PV–EC device composed of triple-junction thin-film silicon solar cells in conjunction with an electrodeposited bifunctional nickel iron molybdenum water-splitting catalyst is shown. The device shows a solar to hydrogen efficiency of up to 4.67% (5.33% active area, H2 production rate of 1.26 μmol H2/s) without bias assistance and wire connection and works for 30 min. The gas separation is enabled by incorporating a membrane in a 3D printed device frame. In addition, a wired small area device is also fabricated in order to show the potential of the concept. The device is operated for 127 h and initially 7.7% solar to hydrogen efficiency with a PV active area of 0.5 cm2 is achieved. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adsu.202000070
  • Analyzing Interface Recombination in Lead-Halide Perovskite Solar Cells with Organic and Inorganic Hole-Transport Layers
    Haddad, J. and Krogmeier, B. and Klingebiel, B. and Krückemeier, L. and Melhem, S. and Liu, Z. and Hüpkes, J. and Mathur, S. and Kirchartz, T.
    Advanced Materials Interfaces 7 (2020)
    The interfaces between absorber and transport layers are shown to be critical for perovskite device performance. However, quantitative characterization of interface recombination has so far proven to be highly challenging in working perovskite solar cells. Here, methylammonium lead halide (CH3NH3PbI3) perovskite solar cells are studied based on a range of different hole-transport layers, namely, an inorganic hole-transport layer CuOx, an organic hole-transport layer poly(triarylamine) (PTAA), and a bilayer of CuOx/PTAA. The cells are completed by a [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/bathocuproine/Ag electron contact. Energy levels are characterized using photoelectron spectroscopy and recombination dynamics by combining steady-state photoluminescence and transient photoluminescence with numerical simulations. While the PTAA-based devices hardly show any interface recombination losses and open-circuit voltages >1.2 V, substantial losses are observed for the samples with a direct CuOx/perovskite interface. These losses are assigned to a combination of energetic misalignment at the CuOx/perovskite interface coupled with increased interface recombination velocities at the perovskite/PCBM interface. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/admi.202000366
  • Bifunctional CoFeVOx Catalyst for Solar Water Splitting by using Multijunction and Heterojunction Silicon Solar Cells
    Lee, M. and Ding, X. and Banerjee, S. and Krause, F. and Smirnov, V. and Astakhov, O. and Merdzhanova, T. and Klingebiel, B. and Kirchartz, T. and Finger, F. and Rau, U. and Haas, S.
    Advanced Materials Technologies 5 (2020)
    Photovoltaic driven electrochemical (PV-EC) water splitting technology is considered as one of the solutions for an environmental-friendly hydrogen supply. In a PV-EC system, efficient catalysts are required to increase the rate of both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, the development of a CoFeVOx bifunctional catalyst produced by a simple electrodeposition method is presented. It is found that after the water splitting reaction, vanadium is almost completely depleted in the mixture of elements for OER, while its concentration at the HER catalyst is similar or even higher after the reaction. For the OER catalyst, the depletion of vanadium might lead to the formation of pores, which could be correlated with the activity enhancement. The developed catalyst is integrated into PV-EC devices, coupled with different types of silicon PV. An average solar to hydrogen efficiency of 13.3% (9.6 cm2 PV aperture area) is achieved with a shingled module consisting of three laterally series-connected silicon heterojunction solar cells. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/admt.202000592
  • Device Performance of Emerging Photovoltaic Materials (Version 1)
    Almora, O. and Baran, D. and Bazan, G.C. and Berger, C. and Cabrera, C.I. and Catchpole, K.R. and Erten-Ela, S. and Guo, F. and Hauch, J. and Ho-Baillie, A.W.Y. and Jacobsson, T.J. and Janssen, R.A.J. and Kirchartz, T. and Kopidakis, N. and Li, Y. and Loi, M.A. and Lunt, R.R. and Mathew, X. and McGehee, M.D. and Min, J. and Mitzi, D.B. and Nazeeruddin, M.K. and Nelson, J. and Nogueira, A.F. and Paetzold, U.W. and Park, N.-G. and Rand, B.P. and Rau, U. and Snaith, H.J. and Unger, E. and Vaillant-Roca, L. and Yip, H.-L. and Brabec, C.J.
    Advanced Energy Materials (2020)
    Emerging photovoltaics (PVs) focus on a variety of applications complementing large scale electricity generation. Organic, dye-sensitized, and some perovskite solar cells are considered in building integration, greenhouses, wearable, and indoor applications, thereby motivating research on flexible, transparent, semitransparent, and multi-junction PVs. Nevertheless, it can be very time consuming to find or develop an up-to-date overview of the state-of-the-art performance for these systems and applications. Two important resources for recording research cells efficiencies are the National Renewable Energy Laboratory chart and the efficiency tables compiled biannually by Martin Green and colleagues. Both publications provide an effective coverage over the established technologies, bridging research and industry. An alternative approach is proposed here summarizing the best reports in the diverse research subjects for emerging PVs. Best performance parameters are provided as a function of the photovoltaic bandgap energy for each technology and application, and are put into perspective using, e.g., the Shockley–Queisser limit. In all cases, the reported data correspond to published and/or properly described certified results, with enough details provided for prospective data reproduction. Additionally, the stability test energy yield is included as an analysis parameter among state-of-the-art emerging PVs. © 2020 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH
    view abstract10.1002/aenm.202002774
  • Efficient Area Matched Converter Aided Solar Charging of Lithium Ion Batteries Using High Voltage Perovskite Solar Cells
    Kin, L.-C. and Liu, Z. and Astakhov, O. and Agbo, S.N. and Tempel, H. and Yu, S. and Kungl, H. and Eichel, R.-A. and Rau, U. and Kirchartz, T. and Merdzhanova, T.
    ACS Applied Energy Materials 3 (2020)
    Efficient solar charging of a battery has been demonstrated in the past by sizing batteries many times that of a solar cell to reduce the effective current density experienced by the battery. Although efficient, such a strategy of coupling a battery up to 10 times larger with a solar cell will make solar-battery integration more challenging and limit the size, and thus maximum power output, of an integrated device. Area matched LFP-LTO (lithium iron phosphate, lithium titanate) battery solar charging using high voltage lead halide perovskite solar cells with a boost converter gave a maximum overall efficiency of 9.9% and a high 14.9% solar to battery charging efficiency. Two differently sized systems were compared using the same converter, and an exergy analysis was performed, showing limitations of converter usage in solar-powered internet of things (IoT) devices and size dependent battery losses. © 2019 American Chemical Society.
    view abstract10.1021/acsaem.9b01672
  • Femto- To Microsecond Dynamics of Excited Electrons in a Quadruple Cation Perovskite
    Jung, E. and Budzinauskas, K. and Öz, S. and Ünlü, F. and Kuhn, H. and Wagner, J. and Grabowski, D. and Klingebiel, B. and Cherasse, M. and Dong, J. and Aversa, P. and Vivo, P. and Kirchartz, T. and Miyasaka, T. and Van Loosdrecht, P.H.M. and Perfetti, L. and Mathur, S.
    ACS Energy Letters 5 (2020)
    Quadruple cation mixed halide perovskite, GA0.015Cs0.046MA0.152FA0.787Pb(I0.815Br0.185)3, single crystals were grown for the first time using an inverse temperature crystallization process. Solar cell devices in n-i-p stack configuration using thin films of the same materials showed power conversion efficiency above 20%. Complementary time-resolved spectroscopy confirmed that polycrystalline thin films and single crystals identically composed exhibit similar carrier dynamics in the picosecond range. Cooling of excited carriers and bandgap renormalization occur on the same time scale of 200-300 fs. The radiative recombination coefficient (1.2 × 10-9 cm3/s) is comparable to values reported for a GaAs semiconductor. At low excitation density, a long carrier lifetime of 3.2 μs was recorded possibly due to the passivation of recombination centers. This study clarifies discrepancies about the lifetime of hot carriers, the impact of radiative recombination, and the role of recombination centers on solar cell performance. The quadruple cation perovskites displayed short time dynamics with slow recombination of charge carriers. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acsenergylett.9b02684
  • How To Quantify the Efficiency Potential of Neat Perovskite Films: Perovskite Semiconductors with an Implied Efficiency Exceeding 28%
    Stolterfoht, M. and Grischek, M. and Caprioglio, P. and Wolff, C.M. and Gutierrez-Partida, E. and Peña-Camargo, F. and Rothhardt, D. and Zhang, S. and Raoufi, M. and Wolansky, J. and Abdi-Jalebi, M. and Stranks, S.D. and Albrecht, S. and Kirchartz, T. and Neher, D.
    Advanced Materials 32 (2020)
    Perovskite photovoltaic (PV) cells have demonstrated power conversion efficiencies (PCE) that are close to those of monocrystalline silicon cells; however, in contrast to silicon PV, perovskites are not limited by Auger recombination under 1-sun illumination. Nevertheless, compared to GaAs and monocrystalline silicon PV, perovskite cells have significantly lower fill factors due to a combination of resistive and non-radiative recombination losses. This necessitates a deeper understanding of the underlying loss mechanisms and in particular the ideality factor of the cell. By measuring the intensity dependence of the external open-circuit voltage and the internal quasi-Fermi level splitting (QFLS), the transport resistance-free efficiency of the complete cell as well as the efficiency potential of any neat perovskite film with or without attached transport layers are quantified. Moreover, intensity-dependent QFLS measurements on different perovskite compositions allows for disentangling of the impact of the interfaces and the perovskite surface on the non-radiative fill factor and open-circuit voltage loss. It is found that potassium-passivated triple cation perovskite films stand out by their exceptionally high implied PCEs > 28%, which could be achieved with ideal transport layers. Finally, strategies are presented to reduce both the ideality factor and transport losses to push the efficiency to the thermodynamic limit. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adma.202000080
  • Minimum doping densities for p–n junctions
    Kirchartz, T. and Cahen, D.
    Nature Energy (2020)
    view abstract10.1038/s41560-020-00708-2
  • Optimization of Transparent Passivating Contact for Crystalline Silicon Solar Cells
    Kohler, M. and Finger, F. and Rau, U. and Ding, K. and Pomaska, M. and Zamchiy, A. and Lambertz, A. and Duan, W. and Lentz, F. and Li, S. and Smirnov, V. and Kirchartz, T.
    IEEE Journal of Photovoltaics 10 (2020)
    A highly transparent front contact layer system for crystalline silicon (c-Si) solar cells is investigated and optimized. This contact system consists of a wet-chemically grown silicon tunnel oxide, a hydrogenated microcrystalline silicon carbide [SiO2/μc-SiC:H(n)] prepared by hot-wire chemical vapor deposition (HWCVD), and a sputter-deposited indium doped tin oxide. Because of the exclusive use of very high bandgap materials, this system is more transparent for the solar light than state of the art amorphous (a-Si:H) or polycrystalline silicon contacts. By investigating the electrical conductivity of the μc-SiC:H(n) and the influence of the hot-wire filament temperature on the contact properties, we find that the electrical conductivity of μc-SiC:H(n) can be increased by 12 orders of magnitude to a maximum of 0.9 S/cm due to an increased doping density and crystallite size. This optimization of the electrical conductivity leads to a strong decrease in contact resistivity. Applying this SiO2/μc-SiC:H(n) transparent passivating front side contact to crystalline solar cells with an a-Si:H/c-Si heterojunction back contact we achieve a maximum power conversion efficiency of 21.6% and a short-circuit current density of 39.6 mA/cm2. All devices show superior quantum efficiency in the short wavelength region compared to the reference cells with a-Si:H/c-Si heterojunction front contacts. Furthermore, these transparent passivating contacts operate without any post processing treatments, e.g., forming gas annealing or high-temperature recrystallization. © 2011-2012 IEEE.
    view abstract10.1109/JPHOTOV.2019.2947131
  • Overcoming the Limitations of Transient Photovoltage Measurements for Studying Recombination in Organic Solar Cells
    Azzouzi, M. and Calado, P. and Telford, A.M. and Eisner, F. and Hou, X. and Kirchartz, T. and Barnes, P.R.F. and Nelson, J.
    Solar RRL 4 (2020)
    Transient photovoltage (TPV) measurements are frequently used to study recombination processes in thin-film solar cells by probing the decay of a small optically induced voltage perturbation to infer the charge carrier dynamics of devices at open circuit. However, the validity of this method to probe organic semiconductors has recently come into doubt due to large discrepancies in the reported carrier lifetime values for the same systems and the reporting of unrealistic reaction order values. Herein, the validity of TPV to extract reliable charge carrier lifetimes in thin-film solar cells is explored through the use of time-dependent drift-diffusion simulations and measurements. It is found that in low-mobility materials, TPV serves primarily as a probe of charge carrier redistribution in the bulk rather than bulk recombination dynamics and that the extracted time constant is highly mobility dependent. To address this shortcoming, transient photocharge, a new technique to measure the charge carrier density during photovoltage decay, is introduced and applied to study the recombination dynamics in a series of (fullerene and nonfullerene) organic solar cell systems. It is shown that using this technique the charge carrier recombination lifetime in the active layer is more accurately determined. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/solr.201900581
  • Photoluminescence-Based Characterization of Halide Perovskites for Photovoltaics
    Kirchartz, T. and Márquez, J.A. and Stolterfoht, M. and Unold, T.
    Advanced Energy Materials 10 (2020)
    Photoluminescence spectroscopy is a widely applied characterization technique for semiconductor materials in general and halide perovskite solar cell materials in particular. It can give direct information on the recombination kinetics and processes as well as the internal electrochemical potential of free charge carriers in single semiconductor layers, layer stacks with transport layers, and complete solar cells. The correct evaluation and interpretation of photoluminescence requires the consideration of proper excitation conditions, calibration and application of the appropriate approximations to the rather complex theory, which includes radiative recombination, non-radiative recombination, interface recombination, charge transfer, and photon recycling. In this article, an overview is given of the theory and application to specific halide perovskite compositions, illustrating the variables that should be considered when applying photoluminescence analysis in these materials. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201904134
  • Pitfalls and prospects of optical spectroscopy to characterize perovskite-transport layer interfaces
    Hutter, E.M. and Kirchartz, T. and Ehrler, B. and Cahen, D. and Von Hauff, E.
    Applied Physics Letters 116 (2020)
    Perovskite photovoltaics has witnessed an unprecedented increase in power conversion efficiency over the last decade. The choice of transport layers, through which photo-generated electrons and holes are transported to electrodes, is a crucial factor for further improving both the device performance and stability. In this perspective, we critically examine the application of optical spectroscopy to characterize the quality of the transport layer-perovskite interface. We highlight the power of complementary studies that use both continuous wave and time-resolved photoluminescence to understand non-radiative losses and additional transient spectroscopies for characterizing the potential for loss-less carrier extraction at the solar cell interfaces. Based on this discussion, we make recommendations on how to extrapolate results from optical measurements to assess the quality of a transport layer and its impact on solar cell efficiency. © 2020 Author(s).
    view abstract10.1063/1.5143121
  • Relationship between fill factor and light intensity in solar cells based on organic disordered semiconductors: The role of tail states
    Xiao, B. and Calado, P. and MacKenzie, R.C.I. and Kirchartz, T. and Yan, J. and Nelson, J.
    Physical Review Applied 14 (2020)
    The origin of the relationship between fill factor (FF) and light intensity (I) in organic disordered-semiconductor-based solar cells is studied. An analytical model describing the balance between transport and recombination of charge carriers, parameterized with a factor, Γm, is introduced to understand the FF-I relation, where higher values of Γm correlate to larger FFs. Comparing the effects of direct and tail-state-mediated recombination on the FF-I plot, we find that, for low-mobility systems, direct recombination with constant transport mobility can deliver only a negative dependence of Γm,dir on light intensity. By contrast, tail-state-mediated recombination with trapping and detrapping processes can produce a positive Γm,t versus sun dependency. The analytical model is validated by numerical drift-diffusion simulations. To further validate our model, two material systems that show opposite FF-I behavior are studied: Poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-[4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl]} (PTB7-Th):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) devices show a negative FF-I relation, while PTB7-Th:(5Z,5′Z)-5,5′-{[7,7′-(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl)]bis(methanylylidene)}bis(3-ethyl-2-thioxothiazolidin-4-one) (O-IDTBR) devices show a positive correlation. Optoelectronic measurements show that the O-IDTBR device presents a higher ideality factor, stronger trapping and detrapping behavior, and a higher density of trap states, relative to the PC71BM device, supporting the theoretical model. This work provides a comprehensive understanding of the correlation between FF and light intensity for disordered-semiconductor-based solar cells. © 2020 American Physical Society.
    view abstract10.1103/PhysRevApplied.14.024034
  • Understanding the interplay of stability and efficiency in A-site engineered lead halide perovskites
    Ünlü, F. and Jung, E. and Haddad, J. and Kulkarni, A. and Öz, S. and Choi, H. and Fischer, T. and Chakraborty, S. and Kirchartz, T. and Mathur, S.
    APL Materials 8 (2020)
    Organic-inorganic hybrid lead halide perovskites have gained significant attention as light-harvesting materials in thin-film photovoltaics due to their exceptional optoelectronic properties and simple fabrication process. The power conversion efficiency of perovskite solar cells (PSCs) has surged beyond 25% in a short time span. Their transition to commercial market is a "work in progress"due to limited long-term operational stability and the persisting environmental concern due to the presence of lead. Comprehensive investigations on the interplay of material composition and interfacial effects on the device performance of PSCs based on methylammonium lead iodide have shown the crucial role of an A-site cation in incipient deterioration of the material through external stimuli (moisture, light, oxygen, or heat). Consequently, a partial or complete replacement of A-site cations by up to four isoelectronic substituents has resulted in many new perovskite compositions. The correlations between the chemical composition and the optoelectronic properties are, however, not always easy to determine. A-site cation management is governed by stability and charge neutrality of the lattice, and the choices include Cs+-cations and organic cations such as CH3NH3+ or CH(NH2)2+ and combinations thereof. Since the size of the cations is an important structural parameter, an adequate compositional engineering of the A-site could effectively optimize the stability by reducing non-radiative defect sites and enhancing carrier lifetimes. This Perspective reflects on the experimental strategies for A-site cation management and their direct impact on the stability and device performance. It also highlights the opportunities and challenges for further research and industrial commercialization of PSCs. © 2020 Author(s).
    view abstract10.1063/5.0011851
  • Understanding the Light-Intensity Dependence of the Short-Circuit Current of Organic Solar Cells
    Hartnagel, P. and Kirchartz, T.
    Advanced Theory and Simulations 3 (2020)
    In organic solar cells, bimolecular recombination is a key factor limiting the device performance and creating the need for characterization. Light-intensity-dependent short-circuit current density measurements are a frequently used tool to qualitatively analyze bimolecular recombination in a device. When applying a 0D model, bimolecular recombination is expected to reduce the otherwise linear correlation of the short-circuit current density Jsc and the light intensity Φ to a sublinear trend. It is shown by numerical simulations that the slope of the Jsc–Φ curve is affected by the recombination mechanism (direct or via traps), the spatial distribution of charge carriers and—in thick solar cells—by space charge effects. Only the combination of these effects allows proper explanation of the different cases, some of which cannot be explained in a simple 0D device model. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/adts.202000116
  • What is a deep defect? Combining Shockley-Read-Hall statistics with multiphonon recombination theory
    Das, B. and Aguilera, I. and Rau, U. and Kirchartz, T.
    Physical Review Materials 4 (2020)
    Slow nonradiative recombination is a key factor in achieving high open-circuit voltages or high luminescence yields in any optoelectronic material. Whether a defect is contributing substantially to nonradiative recombination is often estimated by defect statistics based on the model by Shockley, Read, and Hall. However, defect statistics are agnostic to the origin of the capture coefficients and therefore conclude that essentially every defect between the two quasi-Fermi levels is equally likely to be a recombination-active defect. Here, we combine Shockley-Read-Hall statistics with microscopic models for defect-assisted recombination to study how the microscopic properties of a material affect how recombination active a defect is depending on its energy level. We then use material parameters representative of typical photovoltaic absorber materials (CH3NH3PbI3, Si, and GaAs) to illustrate the relevance, but also the limitations of our model. © 2020 American Physical Society.
    view abstract10.1103/PhysRevMaterials.4.024602
  • Charge Carrier Collection and Contact Selectivity in Solar Cells
    Rau, U. and Kirchartz, T.
    Advanced Materials Interfaces 6 (2019)
    The electronic properties of the contacts to a photovoltaic absorber material are important for the final efficiency of any type of solar cell. For highly efficient solar cells based on high quality absorber materials like single-crystalline silicon, polycrystalline Cu(In,Ga)Se2, CdTe, or metal-halide perovskites, contact formation is even the decisive processing step determining the final efficiency. The present paper combines recently developed quantitative concepts for the description of contacts to solar cells in terms of their selectivity toward a more general description that is valid for all types of solar cells and all types of contacts. It is shown that the built-in voltage is an important parameter to influence the selectivity of contacts to photovoltaic absorber materials. It is also shown that the contact selectivity is mathematically related to the collection efficiency which can be measured by luminescence based techniques. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/admi.201900252
  • Charge-Carrier Recombination in Halide Perovskites
    Dequilettes, D.W. and Frohna, K. and Emin, D. and Kirchartz, T. and Bulovic, V. and Ginger, D.S. and Stranks, S.D.
    Chemical Reviews 119 (2019)
    The success of halide perovskites in a host of optoelectronic applications is often attributed to their long photoexcited carrier lifetimes, which has led to charge-carrier recombination processes being described as unique compared to other semiconductors. Here, we integrate recent literature findings to provide a critical assessment of the factors we believe are most likely controlling recombination in the most widely studied halide perovskite systems. We focus on four mechanisms that have been proposed to affect measured charge carrier recombination lifetimes, namely: (1) recombination via trap states, (2) polaron formation, (3) the indirect nature of the bandgap (e.g., Rashba effect), and (4) photon recycling. We scrutinize the evidence for each case and the implications of each process on carrier recombination dynamics. Although they have attracted considerable speculation, we conclude that multiple trapping or hopping in shallow trap states, and the possible indirect nature of the bandgap (e.g., Rashba effect), seem to be less likely given the combined evidence, at least in high-quality samples most relevant to solar cells and light-emitting diodes. On the other hand, photon recycling appears to play a clear role in increasing apparent lifetime for samples with high photoluminescence quantum yields. We conclude that polaron dynamics are intriguing and deserving of further study. We highlight potential interdependencies of these processes and suggest future experiments to better decouple their relative contributions. A more complete understanding of the recombination processes could allow us to rationally tailor the properties of these fascinating semiconductors and will aid the discovery of other materials exhibiting similarly exceptional optoelectronic properties. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acs.chemrev.9b00169
  • Electron-Beam-Induced Current Measurements of Thin-Film Solar Cells
    Abou-Ras, D. and Kirchartz, T.
    ACS Applied Energy Materials 2 (2019)
    The present tutorial review provides a practical guide to the analysis of semiconductor devices using electron-beam-induced currents (EBICs). The authors focus on cross-sectional EBIC measurements that provide an experimental assay of the efficiency of charge carrier collection in a semiconductor diode. The tutorial covers the fundamental physics of the technique, specimen preparation, data acquisition, and numerical simulation and analysis of the experimental data. A key focus is put on application cases from the field of thin-film photovoltaics as well as specific pitfalls that may occur, such as effects occurring under high-level injection and at grain boundaries of polycrystalline materials. © 2019 American Chemical Society.
    view abstract10.1021/acsaem.9b01172
  • Extracting Recombination Parameters from Impedance Measurements on Organic Solar Cells
    Zonno, I. and Zayani, H. and Grzeslo, M. and Krogmeier, B. and Kirchartz, T.
    Physical Review Applied 11 (2019)
    Nongeminate recombination in organic solar cells is one of the main causes of reduced device performances. The quantification, and the subsequent reduction, of nongeminate recombination losses requires the development of methods, which allow an accurate and reliable determination of the charge-carrier density, mobility, and lifetime. Here, we present a method, based on the charge-density dependence of the recombination current, to determine the recombination parameters from impedance measurements of organic solar cells. We show that the use of the difference between the capacitance under illumination and the capacitance in the dark for the calculation of the charge density present in the active layer, allows the identification of the dominant recombination mechanism and the subsequent determination of either the charge-carrier lifetime or the bimolecular recombination coefficient. © 2019 American Physical Society.
    view abstract10.1103/PhysRevApplied.11.054024
  • Factors Controlling Open-Circuit Voltage Losses in Organic Solar Cells
    Azzouzi, M. and Kirchartz, T. and Nelson, J.
    Trends in Chemistry 1 (2019)
    The performance of solar cells based on molecular electronic materials is limited by relatively low open-circuit voltage (V oc )relative to the absorption threshold. These voltage losses must be reduced to achieve competitive power-conversion efficiencies. Voltage losses are assigned to the molecular heterojunction required to dissociate photogenerated excitons and to relatively fast electron–hole recombination. Recent studies using luminescence have helped quantify these losses and understand their molecular origin. Recently, higher voltages and lower losses have been achieved using new molecular acceptors in place of traditional fullerenes, suggesting that optimizing chemical structure could enable improved device performance. This mini-review combines a device-physics perspective with a body of experimental observations to explore the practical and theoretical limits to V oc . © 2019
    view abstract10.1016/j.trechm.2019.01.010
  • Fermi-level pinning in methylammonium lead iodide perovskites
    Gallet, T. and Grabowski, D. and Kirchartz, T. and Redinger, A.
    Nanoscale 11 (2019)
    Hybrid organic inorganic perovskites are ideal candidates for absorber layers in next generation thin film photovoltaics. The polycrystalline nature of these layers imposes substantial complications for the design of high efficiency devices since the optoelectronic properties can vary on the nanometre scale. Here we show via scanning tunnelling microscopy and spectroscopy that different grains and grain facets exhibit variations in the local density of states. Modeling of the tunneling spectroscopy curves allows us to quantify the density and fluctuations of surface states and estimate the variations in workfunction on the nanometre scale. The simulations corroborate that the high number of surface states leads to Fermi-level pinning of the methylammonium lead iodide surfaces. We do not observe a variation of the local density of states at the grain boundaries compared to the grain interior. These results are in contrast to other reported SPM measurements in literature. Our results show that most of the fluctuations of the electrical properties in these polycrystalline materials arise due to grain to grain variations and not due to distinct electronic properties of the grain boundaries. The measured workfunction changes at the different grains result in local variations of the band alignment with the carrier selective top contact and the varying number of surface states influence the recombination activity in the devices. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9nr02643f
  • Guide for the perplexed to the Shockley–Queisser model for solar cells
    Guillemoles, J.-F. and Kirchartz, T. and Cahen, D. and Rau, U.
    Nature Photonics 13 (2019)
    The Shockley–Queisser model is a landmark in photovoltaic device analysis by defining an ideal situation as reference for actual solar cells. However, the model and its implications are easily misunderstood. Thus, we present a guide to help understand and to avoid misinterpreting it. © 2019, Springer Nature Limited.
    view abstract10.1038/s41566-019-0479-2
  • High open-circuit voltages in lead-halide perovskite solar cells: Experiment, theory and open questions
    Kirchartz, T.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 377 (2019)
    One of the most significant features of lead-halide perovskites is their ability to have comparably slow recombination despite the fact that these materials are mostly processed from solution at room temperature. The slow recombination allows achieving high open-circuit voltages when the lead-halide perovskite layers are used in solar cells. This perspective discusses the state of the art of our understanding and of experimental data with regard to recombination and open-circuit voltages in lead-halide perovskites. A special focus is put onto open questions that the community has to tackle to design future photovoltaic and optoelectronic devices based on lead-halide perovskites and other semiconductors with similar properties. © 2019 The Author(s) Published by the Royal Society. All rights reserved.
    view abstract10.1098/rsta.2018.0286
  • Highly Compact TiO 2 Films by Spray Pyrolysis and Application in Perovskite Solar Cells
    Möllmann, A. and Gedamu, D. and Vivo, P. and Frohnhoven, R. and Stadler, D. and Fischer, T. and Ka, I. and Steinhorst, M. and Nechache, R. and Rosei, F. and Cloutier, S.G. and Kirchartz, T. and Mathur, S.
    Advanced Engineering Materials 21 (2019)
    Transparent and pinhole free hole-blocking layers such as TiO 2 grown at low temperatures and by scalable processes are necessary to reduce production costs and thus enabling commercialization of perovskite solar cells. Here, the authors compare the transport properties of TiO 2 compact layers grown by spray pyrolysis from commonly used titanium diisopropoxide bisacetylacetonate ([Ti(OPr i ) 2 (acac) 2 ]) precursor to films grown by spray pyrolysis of TiCl 4 . Spray pyrolysis provides insights into the interdependence of precursor chemistry and electron transport properties of TiO 2 films and their influence on the performance of the perovskite solar cells. X-ray diffraction and X-ray photoelectron spectroscopy data confirm the chemical and structural composition of the obtained films. Thin film deposition at lower temperature (150 °C) are conducted using TiCl 4 to evaluate the influence of crystal growth and topography by scanning electron microscopy and atomic force microscopy as well as thickness (profilometry) and transmittance (UV/Vis spectroscopy) on the power conversion efficiency of perovskite solar cells. TiO 2 compact layers grown from TiCl 4 enhance the power conversion efficiency by acting as superior electron transfer medium and by reducing hysteresis behavior, when compared to films grown using titanium diisopropoxide bisacetylacetonate. UV/Vis spectroscopy and external quantum efficiency studies reveal the correlation of transmittance on the power conversion efficiency. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adem.201801196
  • How to Report Record Open-Circuit Voltages in Lead-Halide Perovskite Solar Cells
    Krückemeier, L. and Rau, U. and Stolterfoht, M. and Kirchartz, T.
    Advanced Energy Materials (2019)
    Open-circuit voltages of lead-halide perovskite solar cells are improving rapidly and are approaching the thermodynamic limit. Since many different perovskite compositions with different bandgap energies are actively being investigated, it is not straightforward to compare the open-circuit voltages between these devices as long as a consistent method of referencing is missing. For the purpose of comparing open-circuit voltages and identifying outstanding values, it is imperative to use a unique, generally accepted way of calculating the thermodynamic limit, which is currently not the case. Here a meta-analysis of methods to determine the bandgap and a radiative limit for open-circuit voltage is presented. The differences between the methods are analyzed and an easily applicable approach based on the solar cell quantum efficiency as a general reference is proposed. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201902573
  • Impact of Marginal Exciton-Charge-Transfer State Offset on Charge Generation and Recombination in Polymer:Fullerene Solar Cells
    Vezie, M.S. and Azzouzi, M. and Telford, A.M. and Hopper, T.R. and Sieval, A.B. and Hummelen, J.C. and Fallon, K. and Bronstein, H. and Kirchartz, T. and Bakulin, A.A. and Clarke, T.M. and Nelson, J.
    ACS Energy Letters 4 (2019)
    The energetic offset between the initial photoexcited state and charge-transfer (CT) state in organic heterojunction solar cells influences both charge generation and open-circuit voltage (Voc). Here, we use time-resolved spectroscopy and voltage loss measurements to analyze the effect of the exciton-CT state offset on charge transfer, separation, and recombination processes in blends of a low-band-gap polymer (INDT-S) with fullerene derivatives of different electron affinity (PCBM and KL). For the lower exciton-CT state offset blend (INDT-S:PCBM), both photocurrent generation and nonradiative voltage losses are lower. The INDT-S:PCBM blend shows different excited-state dynamics depending on whether the donor or acceptor is photoexcited. Surprisingly, the charge recombination dynamics in INDT-S:PCBM are distinctly faster than those in INDT-S:KL upon excitation of the donor. We reconcile these observations using a kinetic model and by considering hybridization between the lowest excitonic and CT states. The modeling results show that this hybridization can significantly reduce Voc losses while still allowing reasonable charge generation efficiency. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acsenergylett.9b01368
  • Open-Circuit Voltages Exceeding 1.26 v in Planar Methylammonium Lead Iodide Perovskite Solar Cells
    Liu, Z. and Krückemeier, L. and Krogmeier, B. and Klingebiel, B. and Márquez, J.A. and Levcenko, S. and Öz, S. and Mathur, S. and Rau, U. and Unold, T. and Kirchartz, T.
    ACS Energy Letters 4 (2019)
    We demonstrate open-circuit voltages exceeding 1.26 V for inverted planar CH 3 NH 3 PbI 3 solar cells fabricated using a combination of lead acetate and PbCl 2 precursors leading to smooth films and large grain sizes. Surface recombination is suppressed by careful optimization of the PTAA hole transport and PCBM electron transport layers. Suppression of bulk and surface recombination is verified by absolute photoluminescence measurements with external quantum efficiencies of ∼5% in complete cells. In addition, we find exceptionally long photoluminescence lifetimes in full cells and in layer stacks involving one or two contact layers. Numerical simulations reveal that these long photoluminescence lifetimes are only possible with extremely low recombination velocities at the interfaces between absorber and contact materials. © 2018 American Chemical Society.
    view abstract10.1021/acsenergylett.8b01906
  • Photon Management in Perovskite Solar Cells
    Kirchartz, T.
    Journal of Physical Chemistry Letters 10 (2019)
    view abstract10.1021/acs.jpclett.9b02053
  • Role of Electron-Phonon Coupling in the Thermal Evolution of Bulk Rashba-Like Spin-Split Lead Halide Perovskites Exhibiting Dual-Band Photoluminescence
    Steele, J.A. and Puech, P. and Monserrat, B. and Wu, B. and Yang, R.X. and Kirchartz, T. and Yuan, H. and Fleury, G. and Giovanni, D. and Fron, E. and Keshavarz, M. and Debroye, E. and Zhou, G. and Sum, T.C. and Walsh, A. and Hofkens, J. and Roeffaers, M.B.J.
    ACS Energy Letters 4 (2019)
    The optoelectronic properties of lead halide perovskites strongly depend on their underlying crystal symmetries and dynamics, sometimes exhibiting a dual photoluminescence (PL) emission via Rashba-like effects. Here we exploit spin- and temperature-dependent PL to study single-crystal APbBr3 (A = Cs and methylammonium; CH3NH3) and evaluate the peak energy, intensity, and line width evolutions of their dual emission. Both perovskites exhibit temperature trends governed by two temperature regimes - above and below approximately 100 K - which impose different carrier scattering and radiative recombination dynamics. With increasing temperature, high-energy optical phonons activate near 100 K to drive energy splitting of the dual bands and induce line width broadening via electron-phonon coupling, with a stronger coupling constant inferred for carriers recombining by the spin-split indirect bands, compared to the direct ones. We find that the unusual thermal evolutions of all-inorganic and hybrid bulk lead bromide perovskites are comparable, suggesting A-site independence and the dominance of dynamic effects, and are best understood within a framework that accounts for Rashba-like effects. Copyright © 2019 American Chemical Society.
    view abstract10.1021/acsenergylett.9b01427
  • Simulating nanocrystal-based solar cells: A lead sulfide case study
    Lin, W.M.M. and Yazdani, N. and Yarema, O. and Volk, S. and Yarema, M. and Kirchartz, T. and Wood, V.
    Journal of Chemical Physics 151 (2019)
    Nanocrystal-based solar cells are promising candidates for next generation photovoltaic applications; however, the most recent improvements to the device chemistry and architecture have been mostly trial-and-error based advancements. Due to complex interdependencies among parameters, determining factors that limit overall solar cell efficiency are not trivial. Furthermore, many of the underlying chemical and physical parameters of nanocrystal-based solar cells have only recently been understood and quantified. Here, we show that this new understanding of interfaces, transport, and origin of trap states in nanocrystal-based semiconductors can be integrated into simulation tools, based on 1D drift-diffusion models. Using input parameters measured in independent experiments, we find excellent agreement between experimentally measured and simulated PbS nanocrystal solar cell behavior without having to fit any parameters. We then use this simulation to understand the impact of interfaces, charge carrier mobility, and trap-assisted recombination on nanocrystal performance. We find that careful engineering of the interface between the nanocrystals and the current collector is crucial for an optimal open-circuit voltage. We also show that in the regime of trap-state densities found in PbS nanocrystal solar cells (∼1017 cm-3), device performance exhibits strong dependence on the trap state density, explaining the sensitivity of power conversion efficiency to small changes in nanocrystal synthesis and nanocrystal thin-film deposition that has been reported in the literature. Based on these findings, we propose a systematic approach to nanocrystal solar cell optimization. Our method for incorporating parameters into simulations presented and validated here can be adopted to speed up the understanding and development of all types of nanocrystal-based solar cells. © 2019 Author(s).
    view abstract10.1063/1.5129159
  • Tail state limited photocurrent collection of thick photoactive layers in organic solar cells
    Wu, J. and Luke, J. and Lee, H.K.H. and Shakya Tuladhar, P. and Cha, H. and Jang, S.-Y. and Tsoi, W.C. and Heeney, M. and Kang, H. and Lee, K. and Kirchartz, T. and Kim, J.-S. and Durrant, J.R.
    Nature Communications 10 (2019)
    We analyse organic solar cells with four different photoactive blends exhibiting differing dependencies of short-circuit current upon photoactive layer thickness. These blends and devices are analysed by transient optoelectronic techniques of carrier kinetics and densities, air photoemission spectroscopy of material energetics, Kelvin probe measurements of work function, Mott-Schottky analyses of apparent doping density and by device modelling. We conclude that, for the device series studied, the photocurrent loss with thick active layers is primarily associated with the accumulation of photo-generated charge carriers in intra-bandgap tail states. This charge accumulation screens the device internal electrical field, preventing efficient charge collection. Purification of one studied donor polymer is observed to reduce tail state distribution and density and increase the maximal photoactive thickness for efficient operation. Our work suggests that selecting organic photoactive layers with a narrow distribution of tail states is a key requirement for the fabrication of efficient, high photocurrent, thick organic solar cells. © 2019, The Author(s).
    view abstract10.1038/s41467-019-12951-7
  • The impact of energy alignment and interfacial recombination on the internal and external open-circuit voltage of perovskite solar cells
    Stolterfoht, M. and Caprioglio, P. and Wolff, C.M. and Márquez, J.A. and Nordmann, J. and Zhang, S. and Rothhardt, D. and Hörmann, U. and Amir, Y. and Redinger, A. and Kegelmann, L. and Zu, F. and Albrecht, S. and Koch, N. and Kirchartz, T. and Saliba, M. and Unold, T. and Neher, D.
    Energy and Environmental Science 12 (2019)
    Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (VOC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence yield of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in pin- and nip-type cells including high efficiency devices (21.4%). Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the VOC by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the VOC of the device. Importantly, the VOC equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells, while in poor performing devices, the VOC is substantially lower than the QFLS. Using ultraviolet photoelectron spectroscopy and differential charging capacitance experiments we show that this is due to an energy level mis-alignment at the p-interface. The findings are corroborated by rigorous device simulations which outline important considerations to maximize the VOC. This work highlights that the challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in proper energy level alignment and in suppression of defect recombination at the interfaces. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9ee02020a
  • Vacancy doping and charge transport in B i2 S3 nanoparticle films for photovoltaic applications
    Mock, J. and Klingebiel, B. and Schillings, D. and Nuys, M. and Flohre, J. and Wang, S. and Kirchartz, T. and Carius, R.
    Physical Review Materials 3 (2019)
    Native point defect doping via thermal treatment is an easy and promising method to tune the electrical transport properties of semiconductors made for renewable-energy conversion. In this study, we investigate the vacancy doping of the lowly toxic semiconductor Bi2S3 using electrical conductivity as well as thermoelectric power measurements. We enhance the electrical conductivity of bismuth sulfide nanoparticle layers by more than four orders of magnitude by a stepwise thermal treatment in a moderate temperature range (300-480 K). Via thermoelectric power measurements we attribute this enhancement to an increase in charge-carrier mobility by two orders of magnitude and to an increase in charge-carrier density by more than two orders of magnitude. We find that the energetic position of the electron-doping sulfur vacancies of bismuth sulfide nanoparticles is significantly shallower than previously reported for bulk material. Subsequently, we implement Bi2S3 nanoparticles doped with sulfur vacancies by thermal annealing in photovoltaic devices using P3HT as an electron donor molecule. We find that annealing up to 383 K yields the best compromise between improving charge-carrier transport and increasing defect densities. ©2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.105406
  • Balancing electrical and optical losses for efficient 4-terminal Si-perovskite solar cells with solution processed percolation electrodes
    Ramírez Quiroz, C.O. and Shen, Y. and Salvador, M. and Forberich, K. and Schrenker, N. and Spyropoulos, G.D. and Heumüller, T. and Wilkinson, B. and Kirchartz, T. and Spiecker, E. and Verlinden, P.J. and Zhang, X. and Green, M.A. and Ho-Baillie, A. and Brabec, C.J.
    Journal of Materials Chemistry A 6 (2018)
    The unprecedented rise in efficiency of perovskite-based photovoltaics has sparked interest in semi-transparent devices, particularly for tandem structures. Despite promising reports regarding efficiency and reduced parasitic absorption, many devices still rely on processes from the gas phase, compromising both applicability and cost factors. Here, we report all-solution perovskite solar cells with improved infrared transparency ideally suited as top-cells for efficient multi-junction device configurations. We demonstrate the functionality of copper(i) thiocyanate as antireflective layer and as selective contact between the transparent conductive oxide and the perovskite. This concept allows us to fabricate an opaque device with steady state efficiency as high as 20.1%. By employing silver nanowires with robust environmental stability as the bottom electrode, we demonstrate different regimes of device performance that can be described through a classical percolation model, leading to semi-transparent solar cells with efficiencies of up to 17.1%. In conjunction with the implementation of an infrared-tuned transparent conductive oxide contact deposited on UV-fused silica, we show a full device average transmittance surpassing 84% between 800 and 1100 nm (as opposed to 77% with PEDOT:PSS as the selective contact). Finally, we mechanically stacked optimized perovskite devices on top of high performing PERL and IBC silicon architectures. The measured imputed output efficiency of the 4-terminal perovskite-silicon solar cell was 26.7% and 25.2% for the PERL-perovskite and IBC-perovskite, respectively. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c7ta10945h
  • Charge Transport in Spiro-OMeTAD Investigated through Space-Charge-Limited Current Measurements
    Röhr, J.A. and Shi, X. and Haque, S.A. and Kirchartz, T. and Nelson, J.
    Physical Review Applied 9 (2018)
    Extracting charge-carrier mobilities for organic semiconductors from space-charge-limited conduction measurements is complicated in practice by nonideal factors such as trapping in defects and injection barriers. Here, we show that by allowing the bandlike charge-carrier mobility, trap characteristics, injection barrier heights, and the shunt resistance to vary in a multiple-trapping drift-diffusion model, a numerical fit can be obtained to the entire current density-voltage curve from experimental space-charge-limited current measurements on both symmetric and asymmetric 2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) single-carrier devices. This approach yields a bandlike mobility that is more than an order of magnitude higher than the effective mobility obtained using analytical approximations, such as the Mott-Gurney law and the moving-electrode equation. It is also shown that where these analytical approximations require a temperature-dependent effective mobility to achieve fits, the numerical model can yield a temperature-, electric-field-, and charge-carrier-density-independent mobility. Finally, we present an analytical model describing trap-limited current flow through a semiconductor in a symmetric single-carrier device. We compare the obtained charge-carrier mobility and trap characteristics from this analytical model to the results from the numerical model, showing excellent agreement. This work shows the importance of accounting for traps and injection barriers explicitly when analyzing current density-voltage curves from space-charge-limited current measurements. © 2018 American Physical Society.
    view abstract10.1103/PhysRevApplied.9.044017
  • Exploring the validity and limitations of the Mott-Gurney law for charge-carrier mobility determination of semiconducting thin-films
    Röhr, J.A. and Moia, D. and Haque, S.A. and Kirchartz, T. and Nelson, J.
    Journal of Physics Condensed Matter 30 (2018)
    Using drift-diffusion simulations, we investigate the voltage dependence of the dark current in single carrier devices typically used to determine charge-carrier mobilities. For both low and high voltages, the current increases linearly with the applied voltage. Whereas the linear current at low voltages is mainly due to space charge in the middle of the device, the linear current at high voltage is caused by charge-carrier saturation due to a high degree of injection. As a consequence, the current density at these voltages does not follow the classical square law derived by Mott and Gurney, and we show that for trap-free devices, only for intermediate voltages, a space-charge-limited drift current can be observed with a slope that approaches a value of two. We show that, depending on the thickness of the semiconductor layer and the size of the injection barriers, the two linear current-voltage regimes can dominate the whole voltage range, and the intermediate Mott-Gurney regime can shrink or disappear. In this case, which will especially occur for thicknesses and injection barriers typical of single-carrier devices used to probe organic semiconductors, a meaningful analysis using the Mott-Gurney law will become unachievable, because a square-law fit can no longer be achieved, resulting in the mobility being substantially underestimated. General criteria for when to expect deviations from the Mott-Gurney law when used for analysis of intrinsic semiconductors are discussed. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/aaabad
  • Figures of Merit Guiding Research on Organic Solar Cells
    Kirchartz, T. and Kaienburg, P. and Baran, D.
    Journal of Physical Chemistry C 122 (2018)
    While substantial progress in the efficiency of polymer-based solar cells was possible by optimizing the energy levels of the polymer and more recently also the acceptor molecule, further progress beyond 10% efficiency requires a number of criteria to be fulfilled simultaneously, namely, low energy-level offsets at the donor-acceptor heterojunction, low open-circuit voltage losses due to nonradiative recombination, and efficient charge transport and collection. In this feature article we discuss these criteria considering thermodynamic limits, their correlation to photocurrent and photovoltage, and effects on the fill factor. Each criterion is quantified by a figure of merit (FOM) that directly relates to device performance. To ensure a wide applicability, we focus on FOMs that are easily accessible from common experiments. We demonstrate the relevance of these FOMs by looking at the historic and recent achievements of organic solar cells. We hope that the presented FOMs are or will become a valuable tool to evaluate, monitor, and guide further development of new organic absorber materials for solar cells. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcc.8b01598
  • How Contact Layers Control Shunting Losses from Pinholes in Thin-Film Solar Cells
    Kaienburg, P. and Hartnagel, P. and Pieters, B.E. and Yu, J. and Grabowski, D. and Liu, Z. and Haddad, J. and Rau, U. and Kirchartz, T.
    Journal of Physical Chemistry C 122 (2018)
    An absorber layer that does not fully cover the substrate is a common issue for thin-film solar cells such as perovskites. However, models that describe the impact of pinholes on solar cell performance are scarce. Here, we demonstrate that certain combinations of contact layers suppress the negative impact of pinholes better than others. The absence of the absorber at a pinhole gives way to a direct electrical contact between the two semiconducting electron and hole transport layers. The key to understand how pinholes impact the solar cell performance is the resulting nonlinear diodelike behavior of the current across the interface between these two layers (commonly referred to as a shunt current). Based on experimentally obtained data that mimic the current-voltage characteristics across these interfaces, we develop a simple model to predict pinhole-induced solar cell performance deterioration. We investigate typical contact layer combinations such as TiO 2 /spiro-OMeTAD, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)/phenyl-C 61 -butyric acid methyl ester, and TiO 2 /poly(3-hexylthiophene). Our results directly apply to perovskite and other emerging inorganic thin-film solar cells, and the methodology is transferable to CIGS and CdTe. We find substantial differences between five commonly applied contact layer combinations and conclude that it is not sufficient to optimize the contact layers of any real-world thin-film solar cell only with regard to the applied absorber. Instead, in the context of laboratory and industrial fabrication, the tolerance against pinholes (i.e., the mitigation of shunt losses via existing pinholes) needs to be considered as an additional, important objective. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcc.8b09400
  • Impact of Small Phonon Energies on the Charge-Carrier Lifetimes in Metal-Halide Perovskites
    Kirchartz, T. and Markvart, T. and Rau, U. and Egger, D.A.
    Journal of Physical Chemistry Letters 9 (2018)
    Metal-halide perovskite (MHP) solar cells exhibit long nonradiative lifetimes as a crucial feature enabling high efficiencies. Long nonradiative lifetimes occur if the transfer of electronic into vibrational energy is slow due to, e.g., a low trap density, weak electron-phonon coupling, or the requirement to release many phonons in the electronic transition. Here, we combine known material properties of MHPs with basic models for electron-phonon coupling and multiphonon-transition rates in polar semiconductors. We find that the low phonon energies of MAPbI3 lead to a strong dependence of recombination rates on trap position, which we deduce from the underlying physical effects determining nonradiative transitions. This is important for nonradiative recombination in MHPs, as it implies that they are rather insensitive to defects that are not at midgap energy, which can lead to long lifetimes. Therefore, the low phonon energies of MHPs are likely an important factor for their optoelectronic performance. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpclett.7b03414
  • Nonradiative Energy Losses in Bulk-Heterojunction Organic Photovoltaics
    Azzouzi, M. and Yan, J. and Kirchartz, T. and Liu, K. and Wang, J. and Wu, H. and Nelson, J.
    Physical Review X 8 (2018)
    The performance of solar cells based on molecular electronic materials is limited by relatively high nonradiative voltage losses. The primary pathway for nonradiative recombination in organic donor-acceptor heterojunction devices is believed to be the decay of a charge-transfer (CT) excited state to the ground state via energy transfer to vibrational modes. Recently, nonradiative voltage losses have been related to properties of the charge-transfer state such as the Franck-Condon factor describing the overlap of the CT and ground-state vibrational states and, therefore, to the energy of the CT state. However, experimental data do not always follow the trends suggested by the simple model. Here, we extend this recombination model to include other factors that influence the nonradiative decay-rate constant, and therefore the open-circuit voltage, but have not yet been explored in detail. We use the extended model to understand the observed behavior of series of small molecules:fullerene blend devices, where open-circuit voltage appears insensitive to nonradiative loss. The trend could be explained only in terms of a microstructure-dependent CT-state oscillator strength, showing that parameters other than CT-state energy can control nonradiative recombination. We present design rules for improving open-circuit voltage via the control of material parameters and propose a realistic limit to the power-conversion efficiency of organic solar cells. © 2018 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevX.8.031055
  • Optical design of spectrally selective interlayers for perovskite/silicon heterojunction tandem solar cells
    Bittkau, K. and Kirchartz, T. and Rau, U.
    Optics Express 26 (2018)
    Monolithic perovskite/c-Si tandem solar cells have the potential to exceed the Shockley-Queisser limit for single junction solar cells. However, reflection losses at internal interfaces play a crucial role for the overall efficiency of the tandem devices. Significant reflection losses are caused by the charge selective contacts which have a significantly lower refractive index compared to the absorber materials. Here, we present an approach to overcome a significant part of these reflection losses by introducing a multilayer stack between the top and bottom cell which shows spectrally selective transmission/reflection behavior. The layer stack is designed and optimized by optical simulations using transfer matrix method and a genetic algorithm. The incident sun light is split into a direct part and an isotropic diffuse part. The tandem solar cell with interlayer shows an absolute improvement of short-circuit current density of 0.82 mA/cm2. © 2018 Optical Society of America.
    view abstract10.1364/OE.26.00A750
  • Optical Gaps of Organic Solar Cells as a Reference for Comparing Voltage Losses
    Wang, Y. and Qian, D. and Cui, Y. and Zhang, H. and Hou, J. and Vandewal, K. and Kirchartz, T. and Gao, F.
    Advanced Energy Materials 8 (2018)
    The voltage loss, determined by the difference between the optical gap (Eg) and the open-circuit voltage (VOC), is one of the most important parameters determining the performance of organic solar cells (OSCs). However, the variety of different methods used to determine Eg makes it hard to fairly compare voltages losses among different material systems. In this paper, the authors discuss and compare various Eg determination methods and show how they affect the detailed calculation of voltage losses, as well as predictions of the maximum achievable power conversion efficiency. The aim of this paper is to make it possible for the OSC community to compare voltage losses in a consistent and reasonable way. It is found that the voltage losses for strongly absorbed photons in state-of-the-art OSCs are not much less than 0.6 V, which still must be decreased to further enhance efficiency. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201801352
  • Performance Evaluation of Semitransparent Perovskite Solar Cells for Application in Four-Terminal Tandem Cells
    Kirchartz, T. and Korgitzsch, S. and Hüpkes, J. and Quiroz, C.O.R. and Brabec, C.J.
    ACS Energy Letters 3 (2018)
    The efficiency of perovskite-based tandem solar cells and the respective efficiency gain over the single-junction operation of the bottom cell strongly depend on the performance of the component cells. Thus, a fair comparison of reported top cells is difficult. We therefore compute the tandem cell efficiency for the combination of several semitransparent perovskite top solar cells and crystalline silicon or chalcopyrite bottom cells from the literature. We focus on four-terminal configurations but also estimate and discuss the differences between four- and two-terminal configurations. For each top cell, we thereby determine the tandem cell performance as a function of the bottom cell efficiency, which results in a linear relationship. From these data, we extract two parameters to quantify the suitability of the top cell: (i) the slope of the tandem vs. bottom cell efficiency, which is the effective transparency of the top cell, and (ii) the tandem cell efficiency for a targeted bottom cell. These two figures of merit were calculated for a representative set of bottom cells and may serve for comparison of semitransparent perovskite top cells in the future. Copyright © 2018 American Chemical Society.
    view abstract10.1021/acsenergylett.8b00598
  • Research Update: Recombination and open-circuit voltage in lead-halide perovskites
    Kirchartz, T. and Krückemeier, L. and Unger, E.L.
    APL Materials 6 (2018)
    The high open-circuit voltage and the slow recombination in lead-halide perovskite solar cells has been one of the main contributors to their success as photovoltaic materials. Here, we review the knowledge on recombination in perovskite-based solar cells, compare the situation with silicon solar cells, and introduce the parameters used to describe recombination and open-circuit voltage losses in solar cells. We first discuss the effect of lifetimes and surface recombination velocities on photovoltaic performance before we study the microscopic origin of charge-carrier lifetimes. The lifetimes depend on defect positions and densities and on the kinetic prefactors that control the phonon-assisted interaction between the extended states in the conduction and valence band and the localized defect states. We finally argue that the key to understand the long lifetimes and high open-circuit voltages is a combination of a low density of deep defects and a slow dissipation of energy via multiphonon processes due to the low phonon energies in the lead-halide perovskites. © 2018 Author(s).
    view abstract10.1063/1.5052164
  • Robust nonfullerene solar cells approaching unity external quantum efficiency enabled by suppression of geminate recombination
    Baran, D. and Gasparini, N. and Wadsworth, A. and Tan, C.H. and Wehbe, N. and Song, X. and Hamid, Z. and Zhang, W. and Neophytou, M. and Kirchartz, T. and Brabec, C.J. and Durrant, J.R. and McCulloch, I.
    Nature Communications 9 (2018)
    Nonfullerene solar cells have increased their efficiencies up to 13%, yet quantum efficiencies are still limited to 80%. Here we report efficient nonfullerene solar cells with quantum efficiencies approaching unity. This is achieved with overlapping absorption bands of donor and acceptor that increases the photon absorption strength in the range from about 570 to 700 nm, thus, almost all incident photons are absorbed in the active layer. The charges generated are found to dissociate with negligible geminate recombination losses resulting in a short-circuit current density of 20 mA cm-2 along with open-circuit voltages >1 V, which is remarkable for a 1.6 eV bandgap system. Most importantly, the unique nano-morphology of the donor:acceptor blend results in a substantially improved stability under illumination. Understanding the efficient charge separation in nonfullerene acceptors can pave the way to robust and recombination-free organic solar cells. © 2018 The Author(s).
    view abstract10.1038/s41467-018-04502-3
  • Spin-coated planar Sb2S3 hybrid solar cells approaching 5% efficiency
    Kaienburg, P. and Klingebiel, B. and Kirchartz, T.
    Beilstein Journal of Nanotechnology 9 (2018)
    Antimony sulfide solar cells have demonstrated an efficiency exceeding 7% when assembled in an extremely thin absorber configuration deposited via chemical bath deposition. More recently, less complex, planar geometries were obtained from simple spincoating approaches, but the device efficiency still lags behind. We compare two processing routes based on different precursors reported in the literature. By studying the film morphology, sub-bandgap absorption and solar cell performance, improved annealing procedures are found and the crystallization temperature is shown to be critical. In order to determine the optimized processing conditions, the role of the polymeric hole transport material is discussed. The efficiency of our best solar cells exceeds previous reports for each processing route, and our champion device displays one of the highest efficiencies reported for planar antimony sulfide solar cells. © 2018 Kaienburg et al.
    view abstract10.3762/bjnano.9.200
  • Statistics of the Auger Recombination of Electrons and Holes via Defect Levels in the Band Gap - Application to Lead-Halide Perovskites
    Staub, F. and Rau, U. and Kirchartz, T.
    ACS Omega 3 (2018)
    Recent evidence for bimolecular nonradiative recombination in lead-halide perovskites poses the question for a mechanistic origin of such a recombination term. A possible mechanism is Auger recombination involving two free charge carriers and a trapped charge-carrier. To study the influence of trap-assisted Auger recombination on bimolecular recombination in lead-halide perovskites, we combine estimates of the transition rates with a detailed balance compatible approach of calculating the occupation statistics of defect levels using a similar approach as for the well-known Shockley-Read-Hall recombination statistics. We find that the kinetics resulting from trap-assisted Auger recombination encompasses three different regimes: low injection, high injection, and saturation. Although the saturation regime with a recombination rate proportional to the square of free carrier concentration might explain the nonradiative bimolecular recombination in general, we show that the necessary trap density is higher than reported. Thus, we conclude that Auger recombination via traps is most likely not the explanation for the observed nonradiative bimolecular recombination in CH3NH3PbI3 and related materials. © 2018 American Chemical Society.
    view abstract10.1021/acsomega.8b00962
  • The role of fullerenes in the environmental stability of polymer:fullerene solar cells
    Lee, H.K.H. and Telford, A.M. and Röhr, J.A. and Wyatt, M.F. and Rice, B. and Wu, J. and De Castro Maciel, A. and Tuladhar, S.M. and Speller, E. and McGettrick, J. and Searle, J.R. and Pont, S. and Watson, T. and Kirchartz, T. and Durrant, J.R. and Tsoi, W.C. and Nelson, J. and Li, Z.
    Energy and Environmental Science 11 (2018)
    Environmental stability is a common challenge for the commercialisation of low cost, encapsulation-free organic opto-electronic devices. Understanding the role of materials degradation is the key to address this challenge, but most such studies have been limited to conjugated polymers. Here we quantitatively study the role of the common fullerene derivative PCBM in limiting the stability of benchmark organic solar cells, showing that a minor fraction (<1%) of photo-oxidised PCBM, induced by short exposure to either solar or ambient laboratory lighting conditions in air, consistent with typical processing and operating conditions, is sufficient to compromise device performance severely. We identify the effects of photo-oxidation of PCBM on its chemical structure, and connect this to specific changes in its electronic structure, which significantly alter the electron transport and recombination kinetics. The effect of photo-oxidation on device current-voltage characteristics, electron mobility and density of states could all be explained with the same model of photoinduced defects acting as trap states. Our results demonstrate that the photochemical instability of PCBM and chemically similar fullerenes remains a barrier for the commercialisation of organic opto-electronic devices. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c7ee02983g
  • Understanding Thermal Admittance Spectroscopy in Low-Mobility Semiconductors
    Wang, S. and Kaienburg, P. and Klingebiel, B. and Schillings, D. and Kirchartz, T.
    Journal of Physical Chemistry C 122 (2018)
    Thermal admittance spectroscopy (TAS) is frequently used to analyze the properties of trap states in semiconductor devices. We perform detailed simulations in combination with experiments to understand the effect of low carrier mobility on the analysis of trap states by TAS. We show that the apparent characteristic peak in the differential capacitance spectra is strongly dominated by the dielectric relaxation (DR) peak caused by low carrier mobilities for the case of shallow traps and low trap densities. The model for the DR dominated case is successfully applied to interpret the experimental results from poly(3-hexylthiophene-2,5-diyl) (P3HT) based diodes. In contrast, for deep states with high density of states, we are able to properly estimate the energetic position, but the low carrier mobility affects the correct determination of the attempt-to-escape frequency as well as the capture cross section. Our results reveal that low carrier mobilities cause inherent obstacles in accurately determining the trap properties and thereby affect the analysis of the origin and nature of the trap states by admittance spectroscopy. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcc.8b01921
  • What Makes a Good Solar Cell?
    Kirchartz, T. and Rau, U.
    Advanced Energy Materials 8 (2018)
    Recent years have seen a substantial efficiency improvement for a variety of solar cell technologies as well as the rise of a new class of photovoltaic absorber materials, the metal-halide perovskites. Conversion efficiencies that are coming closer and closer to the thermodynamic limits require a physical description of the corresponding solar cells that is compatible with those limits. This progress report summarizes recent work on the interdependence of basic material properties of semiconductor materials with their efficiency potential as photovoltaic absorbers. The connection of the classical Shockley–Queisser approach, with the band gap energy as the only parameter, to a more general radiative limit and to situations where nonradiative recombination dominates is discussed. The authors delineate a consistent loss analysis that enables a quantitative comparison between different solar cell technologies. In a next step, bulk material properties that influence the photovoltaic performance of a semiconductor like absorption coefficient, densities of states of the free carriers, or phonon energies are considered. It is shown that variations of these properties have a big influence on the optimized design of a solar cell but not necessarily on the achievable efficiency. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201703385
  • What Remains Unexplained about the Properties of Halide Perovskites?
    Egger, D.A. and Bera, A. and Cahen, D. and Hodes, G. and Kirchartz, T. and Kronik, L. and Lovrincic, R. and Rappe, A.M. and Reichman, D.R. and Yaffe, O.
    Advanced Materials 30 (2018)
    The notion that halide perovskite crystals (ABX3, where X is a halide) exhibit unique structural and optoelectronic behavior deserves serious scrutiny. After decades of steady and half a decade of intense research, the question which attributes of these materials are unusual, is discussed, with an emphasis on the identification of the most important remaining issues. The goal is to stimulate discussion rather than to merely present a community consensus. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adma.201800691
  • Decreasing Radiative Recombination Coefficients via an Indirect Band Gap in Lead Halide Perovskites
    Kirchartz, T. and Rau, U.
    Journal of Physical Chemistry Letters 8 (2017)
    High absorption coefficients imply also high radiative recombination coefficients due to detailed balance. It is therefore worthwhile to investigate whether a combination of two transitions with different absorption coefficients (such as a direct and an indirect band gap) could be used to reduce radiative recombination while at the same time retaining the high absorption coefficient. We show here that a longer radiative lifetime helping charge collection can indeed be achieved, while an increase in open-circuit voltage by adding an indirect band gap below the direct one is impossible. We also show that the absorption coefficient in CH3NH3PbI3 could indeed consist of a direct and an indirect contribution; however, the indirect one seems to dominate luminescence and therefore radiative recombination. Thus, the condition that the direct gap is mainly responsible for absorption and emission would not be valid for CH3NH3PbI3. Therefore, we would not expect any benefit of an indirect gap in the radiative limit. However, there may be benefits for charge collection but not open-circuit voltage if nonradiative recombination is dominant. © 2017 American Chemical Society.
    view abstract10.1021/acs.jpclett.7b00236
  • Efficiency Potential of Photovoltaic Materials and Devices Unveiled by Detailed-Balance Analysis
    Rau, U. and Blank, B. and Müller, T.C.M. and Kirchartz, T.
    Physical Review Applied 7 (2017)
    A consistent mathematical approach is presented that connects the Shockley-Queisser (SQ) theory to the analysis of real-world devices. We demonstrate that the external photovoltaic quantum efficiency QePV of a solar cell results from a distribution of SQ-type band-gap energies and how this distribution is derived from experimental data. This leads us to the definition of a photovoltaic band-gap energy EgPV as a reference value for the analysis of the device performance. For a variety of solar-cell devices, we show that the combination of QePV and electroluminescence measurements allows for a detailed loss analysis that is fully compatible with the principle of detailed balance. © 2017 American Physical Society.
    view abstract10.1103/PhysRevApplied.7.044016
  • Manipulating the Net Radiative Recombination Rate in Lead Halide Perovskite Films by Modification of Light Outcoupling
    Staub, F. and Kirchartz, T. and Bittkau, K. and Rau, U.
    Journal of Physical Chemistry Letters 8 (2017)
    Photon recycling is a fundamental physical process that becomes especially important for photovoltaic devices that operate close to the radiative limit. This implies that the externally measured radiative decay rate deviates from the internal radiative recombination rate of the material. In the present Letter, the probability of photon recycling in organic lead halide perovskite films is manipulated by modifying the underlying layer stacks. We observe recombination kinetics by time-resolved photoluminescence that is controlled by the optical design of the chosen layer structure. Quantitative simulations of decay rates and emission spectra show excellent agreement with experimental results if we assume that the internal bimolecular recombination coefficient is ∼66% radiative. © 2017 American Chemical Society.
    view abstract10.1021/acs.jpclett.7b02224
  • On the correct interpretation of the low voltage regime in intrinsic single-carrier devices
    Röhr, J.A. and Kirchartz, T. and Nelson, J.
    Journal of Physics Condensed Matter 29 (2017)
    We discuss the approach of determining the charge-carrier density of a single-carrier device by combining Ohm's law and the Mott-Gurney law. We show that this approach is seldom valid, due to the fact that whenever Ohm's law is applicable the Mott-Gurney law is usually not, and vice versa. We do this using a numerical drift-diffusion solver to calculate the current density-voltage curves and the charge-carrier density, with increasing doping concentration. As this doping concentration is increased to very large values, using Ohm's law becomes a sensible way of measuring the product of mobility and doping density in the sample. However, in the high-doping limit, the current is no longer governed by space-charge and it will no longer be possible to determine the charge-carrier mobility using the Mott-Gurney law. This leaves the value for the mobility as an unknown in the mobility-doping density product in Ohm's law. We also show that, when the charge-carrier mobility for an intrinsic semiconductor is known in advance, the carrier density is underestimated up to many orders of magnitude if Ohm's law is used. We finally seek to establish a window of conditions where the two methods can be combined to yield reasonable results. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/aa66cc
  • Reducing the efficiency-stability-cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells
    Baran, D. and Ashraf, R.S. and Hanifi, D.A. and Abdelsamie, M. and Gasparini, N. and Röhr, J.A. and Holliday, S. and Wadsworth, A. and Lockett, S. and Neophytou, M. and Emmott, C.J.M. and Nelson, J. and Brabec, C.J. and Amassian, A. and Salleo, A. and Kirchartz, T. and Durrant, J.R. and McCulloch, I.
    Nature Materials 16 (2017)
    Technological deployment of organic photovoltaic modules requires improvements in device light-conversion efficiency and stability while keeping material costs low. Here we demonstrate highly efficient and stable solar cells using a ternary approach, wherein two non-fullerene acceptors are combined with both a scalable and affordable donor polymer, poly(3-hexylthiophene) (P3HT), and a high-efficiency, low-bandgap polymer in a single-layer bulk-heterojunction device. The addition of a strongly absorbing small molecule acceptor into a P3HT-based non-fullerene blend increases the device efficiency up to 7.7 ± 0.1% without any solvent additives. The improvement is assigned to changes in microstructure that reduce charge recombination and increase the photovoltage, and to improved light harvesting across the visible region. The stability of P3HT-based devices in ambient conditions is also significantly improved relative to polymer:fullerene devices. Combined with a low-bandgap donor polymer (PBDTTT-EFT, also known as PCE10), the two mixed acceptors also lead to solar cells with 11.0 ± 0.4% efficiency and a high open-circuit voltage of 1.03 ± 0.01 V. © 2016 Nature Publishing Group
    view abstract10.1038/nmat4797
  • Selection metric for photovoltaic materials screening based on detailed-balance analysis
    Blank, B. and Kirchartz, T. and Lany, S. and Rau, U.
    Physical Review Applied 8 (2017)
    The success of recently discovered absorber materials for photovoltaic applications has been generating increasing interest in systematic materials screening over the last years. However, the key for a successful materials screening is a suitable selection metric that goes beyond the Shockley-Queisser theory that determines the thermodynamic efficiency limit of an absorber material solely by its band-gap energy. In this work, we develop a selection metric to quantify the potential photovoltaic efficiency of a material. Our approach is compatible with detailed balance and applicable in computational and experimental materials screening. We use the complex refractive index to calculate radiative and nonradiative efficiency limits and the respective optimal thickness in the high mobility limit. We compare our model to the widely applied selection metric by Yu and Zunger [Phys. Rev. Lett. 108, 068701 (2012)PRLTAO0031-900710.1103/PhysRevLett.108.068701] with respect to their dependence on thickness, internal luminescence quantum efficiency, and refractive index. Finally, the model is applied to complex refractive indices calculated via electronic structure theory. © 2017 American Physical Society.
    view abstract10.1103/PhysRevApplied.8.024032
  • Simultaneous topographical, electrical and optical microscopy of optoelectronic devices at the nanoscale
    Kumar, N. and Zoladek-Lemanczyk, A. and Guilbert, A.A.Y. and Su, W. and Tuladhar, S.M. and Kirchartz, T. and Schroeder, B.C. and McCulloch, I. and Nelson, J. and Roy, D. and Castro, F.A.
    Nanoscale 9 (2017)
    Novel optoelectronic devices rely on complex nanomaterial systems where the nanoscale morphology and local chemical composition are critical to performance. However, the lack of analytical techniques that can directly probe these structure-property relationships at the nanoscale presents a major obstacle to device development. In this work, we present a novel method for non-destructive, simultaneous mapping of the morphology, chemical composition and photoelectrical properties with <20 nm spatial resolution by combining plasmonic optical signal enhancement with electrical-mode scanning probe microscopy. We demonstrate that this combined approach offers subsurface sensitivity that can be exploited to provide molecular information with a nanoscale resolution in all three spatial dimensions. By applying the technique to an organic solar cell device, we show that the inferred surface and subsurface composition distribution correlates strongly with the local photocurrent generation and explains macroscopic device performance. For instance, the direct measurement of fullerene phase purity can distinguish between high purity aggregates that lead to poor performance and lower purity aggregates (fullerene intercalated with polymer) that result in strong photocurrent generation and collection. We show that the reliable determination of the structure-property relationship at the nanoscale can remove ambiguity from macroscopic device data and support the identification of the best routes for device optimisation. The multi-parameter measurement approach demonstrated herein is expected to play a significant role in guiding the rational design of nanomaterial-based optoelectronic devices, by opening a new realm of possibilities for advanced investigation via the combination of nanoscale optical spectroscopy with a whole range of scanning probe microscopy modes. © The Royal Society of Chemistry 2017.
    view abstract10.1039/c6nr09057e
  • Transient Optoelectronic Analysis of the Impact of Material Energetics and Recombination Kinetics on the Open-Circuit Voltage of Hybrid Perovskite Solar Cells
    Wheeler, S. and Bryant, D. and Troughton, J. and Kirchartz, T. and Watson, T. and Nelson, J. and Durrant, J.R.
    Journal of Physical Chemistry C 121 (2017)
    Transient optoelectronic measurements were used to evaluate the factors determining the open-circuit voltage of a series of planar photovoltaic devices based on hybrid perovskite layers with varying iodine/bromine ratios. Employing differential charging and transient photovoltage measurements, we used a simple device model based on the charge-carrier-density dependence of nongeminate recombination to re-create correctly not only the measured device open-circuit voltage (VOC) as a function of light intensity but also its dependence on bromine substitution. The 173 (±7) mV increase in device voltage observed with 20% bromine substitution is shown to result from a 227 (±8) mV increase in effective electronic band gap, which was offset in part by a 56 (±5) mV voltage loss due to faster carrier recombination. The faster recombination following 20% bromine substitution can be avoided by indene-C60 bisadduct (ICBA) substitution into the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) electron-collection layer, resulting in a further 73 (±7) mV increase in device VOC. These results are consistent with surface recombination losses at the perovskite/fullerene interface being the primary limitation on the VOC output of bromine-substituted devices. This study thus presents, and experimentally validates, a simple model for the device physics underlying voltage generation in such perovskite-based solar cells and demonstrates that this approach can provide key insights into factors limiting this voltage output as a function of material energetics. © 2017 American Chemical Society.
    view abstract10.1021/acs.jpcc.7b02411
  • Understanding Mott-Schottky Measurements under Illumination in Organic Bulk Heterojunction Solar Cells
    Zonno, I. and Martinez-Otero, A. and Hebig, J.-C. and Kirchartz, T.
    Physical Review Applied 7 (2017)
    The Mott-Schottky analysis in the dark is a frequently used method to determine the doping concentration of semiconductors from capacitance-voltage measurements, even for such complex systems as polymer:fullerene blends used for organic solar cells. While the analysis of capacitance-voltage measurements in the dark is relatively well established, the analysis of data taken under illumination is currently not fully understood. Here, we present experiments and simulations to show which physical mechanisms affect the Mott-Schottky analysis under illumination. We show that the mobility of the blend has a major influence on the shape of the capacitance-voltage curve and can be obtained from data taken under reverse bias. In addition, we show that the apparent shift of the built-in voltage observed previously can be explained by a shift of the onset of space-charge-limited collection with illumination intensity. © 2017 American Physical Society.
    view abstract10.1103/PhysRevApplied.7.034018
  • Absorption and Photocurrent Spectroscopy with High Dynamic Range
    Christian, T. and Müller, M. and Kirchartz, T.
    Advanced Characterization Techniques for Thin Film Solar Cells: Second Edition 1-2 (2016)
    view abstract10.1002/9783527699025.ch8
  • Advanced Characterization Techniques for Thin Film Solar Cells: Second Edition
    Abou-Ras, D. and Kirchartz, T. and Rau, U.
    Advanced Characterization Techniques for Thin Film Solar Cells: Second Edition 1-2 (2016)
    The book focuses on advanced characterization methods for thin-film solar cells that have proven their relevance both for academic and corporate photovoltaic research and development. After an introduction to thin-film photovoltaics, highly experienced experts report on device and materials characterization methods such as electroluminescence analysis, capacitance spectroscopy, and various microscopy methods. In the final part of the book simulation techniques are presented which are used for ab-initio calculations of relevant semiconductors and for device simulations in 1D, 2D and 3D. Building on a proven concept, this new edition also covers thermography, transient optoelectronic methods, and absorption and photocurrent spectroscopy. © 2016 Wiley-VCH Verlag GmbH & Co.KGaA. All rights reserved.
    view abstract10.1002/9783527699025
  • Beyond Bulk Lifetimes: Insights into Lead Halide Perovskite Films from Time-Resolved Photoluminescence
    Staub, F. and Hempel, H. and Hebig, J.-C. and Mock, J. and Paetzold, U.W. and Rau, U. and Unold, T. and Kirchartz, T.
    Physical Review Applied 6 (2016)
    Careful interpretation of time-resolved photoluminescence (TRPL) measurements can substantially improve our understanding of the complex nature of charge-carrier processes in metal-halide perovskites, including, for instance, charge separation, trapping, and surface and bulk recombination. In this work, we demonstrate that TRPL measurements combined with powerful analytical models and additional supporting experiments can reveal insights into the charge-carrier dynamics that go beyond the determination of minority-charge-carrier lifetimes. While taking into account doping and photon recycling in the absorber layer, we investigate surface and bulk recombination (trap-assisted, radiative, and Auger) by means of the shape of photoluminescence transients. The observed long effective lifetime indicates high material purity and good passivation of perovskite surfaces with exceptionally low surface recombination velocities on the order of about 10 cm/s. Finally, we show how to predict the potential open-circuit voltage for a device with ideal contacts based on the transient and steady-state photoluminescence data from a perovskite absorber film and including the effect of photon recycling. © 2016 American Physical Society.
    view abstract10.1103/PhysRevApplied.6.044017
  • Compositional and electrical properties of line and planar defects in Cu(In,Ga)Se2 thin films for solar cells - a review
    Abou-Ras, D. and Schmidt, S.S. and Schäfer, N. and Kavalakkatt, J. and Rissom, T. and Unold, T. and Mainz, R. and Weber, A. and Kirchartz, T. and Simsek Sanli, E. and van Aken, P.A. and Ramasse, Q.M. and Kleebe, H.-J. and Azulay, D. and Balberg, I. and Millo, O. and Cojocaru-Mirédin, O. and Barragan-Yani, D. and Albe, K. and Haarstrich, J. and Ronning, C.
    Physica Status Solidi - Rapid Research Letters 10 (2016)
    The present review gives an overview of the various reports on properties of line and planar defects in Cu(In,Ga)(S,Se)2 thin films for high-efficiency solar cells. We report results from various analysis techniques applied to characterize these defects at different length scales, which allow for drawing a consistent picture on structural and electronic defect properties. A key finding is atomic reconstruction detected at line and planar defects, which may be one mechanism to reduce excess charge densities and to relax deep-defect states from midgap to shallow energy levels. On the other hand, nonradiative Shockley-Read-Hall recombination is still enhanced with respect to defect-free grain interiors, which is correlated with substantial reduction of luminescence intensities. Comparison of the microscopic electrical properties of planar defects in Cu(In,Ga)(S,Se)2 thin films with two-dimensional device simulations suggest that these defects are one origin of the reduced open-circuit voltage of the photovoltaic devices. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) This review gives an overview on the current understanding of line and planar defects in Cu(In,Ga)Se2 thin films and their impacts on the corresponding solar-cell devices. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssr.201510440
  • Detailed balance analysis of photovoltaic materials and devices
    Rau, U. and Blank, B. and Kirchartz, T.
    Conference Record of the IEEE Photovoltaic Specialists Conference 2016-November (2016)
    The recent years have seen amazing progress in various photovoltaic technologies, like new world records for Cu(In, Ga)Se2, Si, and GaAs solar cells or the unprecedented rise of organo-metal halide materials. Furthermore, new nano-electronic and nano-photonic concepts claim to challenge traditional limits like the Shockley-Queisser-limit for the maximum power conversion efficiency or the Yablonovitch-limit for the maximum light path enhancement. In the light of the recent developments, it appears necessary to revisit fundamental theories in order to extend the description of limiting cases towards a consistent top-down approach based on detailed balance. Such an approach allows one to quantify departures from the ideal situation as well us to judge whether or not proposed concepts can really deliver what they promise. The present contribution proposes a method that allows us to measure the prospects of materials and devices with direct relation to the thermodynamic limits. The method applies to the direct experimental analysis of materials as well as to the quantification of theoretical material prospects from first principle calculations. In this way, we gain by simple experiments figures of merit for both the electrical and the optical quality of materials and devices. © 2016 IEEE.
    view abstract10.1109/PVSC.2016.7750332
  • Discriminating between surface and bulk recombination in organic solar cells by studying the thickness dependence of the open-circuit voltage
    Zonno, I. and Krogmeier, B. and Katte, V. and Lübke, D. and Martinez-Otero, A. and Kirchartz, T.
    Applied Physics Letters 109 (2016)
    In case of thin-film solar cells, it is often rather difficult to determine what the dominant recombination mechanism is. In particular, it is difficult to distinguish recombination at the interface between the absorber layer and the electrodes (typically called surface recombination) from recombination in the bulk of the absorber - or in case of organic solar cells at the internal donor-acceptor interfaces. Here, we suggest a method to distinguish surface and bulk recombination in thin-film solar cells based on the thickness dependence of the saturation current density, which we derive from the open-circuit voltage and the photocurrent at short circuit or reverse bias. By means of numerical simulations, we show that surface and bulk recombination currents scale differently with thickness assuming the material properties to be unchanged. We test our predictions on a range of organic solar cell data from our laboratory and from literature and show that in the field of organic photovoltaics the whole range of cases, from mostly surface limited to purely bulk limited, is observed. © 2016 Author(s).
    view abstract10.1063/1.4966613
  • Electroluminescence Analysis of Solar Cells and Solar Modules
    Kirchartz, T. and Huhn, V. and Gerber, A. and Pieters, B.E. and Rau, U.
    Advanced Characterization Techniques for Thin Film Solar Cells: Second Edition 1-2 (2016)
    view abstract10.1002/9783527699025.ch3
  • Extracting Information about the Electronic Quality of Organic Solar-Cell Absorbers from Fill Factor and Thickness
    Kaienburg, P. and Rau, U. and Kirchartz, T.
    Physical Review Applied 6 (2016)
    Understanding the fill factor in organic solar cells remains challenging due to its complex dependence on a multitude of parameters. By means of drift-diffusion simulations, we thoroughly analyze the fill factor of such low-mobility systems and demonstrate its dependence on a collection coefficient defined in this work. We systematically discuss the effect of different recombination mechanisms, space-charge regions, and contact properties. Based on these findings, we are able to interpret the thickness dependence of the fill factor for different experimental studies from the literature. The presented model provides a facile method to extract the photoactive layer's electronic quality which is of particular importance for the fill factor. We illustrate that over the past 15 years, the electronic quality has not been continuously improved, although organic solar-cell efficiencies increased steadily over the same period of time. Only recent reports show the synthesis of polymers for semiconducting films of high electronic quality that are able to produce new efficiency records. © 2016 American Physical Society.
    view abstract10.1103/PhysRevApplied.6.024001
  • Fundamental Electrical Characterization of Thin-Film Solar Cells
    Kirchartz, T. and Ding, K. and Rau, U.
    Advanced Characterization Techniques for Thin Film Solar Cells: Second Edition 1-2 (2016)
    view abstract10.1002/9783527699025.ch2
  • Impact of Photon Recycling on the Open-Circuit Voltage of Metal Halide Perovskite Solar Cells
    Kirchartz, T. and Staub, F. and Rau, U.
    ACS Energy Letters 1 (2016)
    Photon recycling has been recently shown to be measurable in perovskite solar cells. Here we discuss the impact of this effect on the open-circuit voltage of perovskite solar cells and show how the voltage boost due to photon recycling depends on electronic properties, such as Shockley-Read-Hall lifetimes, and on the optical properties of the device, such as the amount of parasitic absorption and the efficiency of light outcoupling.
    view abstract10.1021/acsenergylett.6b00223
  • Introduction to Thin-Film Photovoltaics
    Kirchartz, T. and Abou-Ras, D. and Rau, U.
    Advanced Characterization Techniques for Thin Film Solar Cells: Second Edition 1-2 (2016)
    view abstract10.1002/9783527699025.ch1
  • Low Open-Circuit Voltage Loss in Solution-Processed Small-Molecule Organic Solar Cells
    Tuladhar, S. M. and Azzouzi, M. and Delval, F. and Yao, J. and Guilbert, A. A. Y. and Kirchartz, T. and Montcada, N. F. and Dominguez, R. and Langa, F. and Palomares, E. and Nelson, J.
    ACS Energy Letters 1 (2016)
    We analyze the voltage losses at open circuit in solution processed, small-molecule:fullerene blend solar cells, using electroluminescence and external quantum efficiency measurements and the reciprocity relationship between light absorption and emission. For solar cells made from oligo-thienylenevinylene-based donors and phenyl-C-71, butyric acid methyl ester (PC71BM), we find that the voltage loss due to the finite breadth of the absorption edge is remarkably small, less than 0.01 eV in the best cases, while the voltage loss due to nonradiative recombination reaches 0.29 eV, one of the smallest values reported for an organic solar cell. As a result, the open-circuit voltage reaches around 1.0 V for an optical gap of 1.6 eV, greatly exceeding the voltage of a high-performance polymer-based system with similar optical gap. We assign the remarkably small absorption broadening loss to a low degree of energetic disorder in the small-molecule system that allows efficient charge separation typical conjugated polymer blends.
    view abstract10.1021/acsenergylett.6b00162
  • One-Dimensional Electro-Optical Simulations of Thin-Film Solar Cells
    Pieters, B.E. and Decock, K. and Burgelman, M. and Stangl, R. and Kirchartz, T.
    Advanced Characterization Techniques for Thin Film Solar Cells: Second Edition 2-2 (2016)
    view abstract10.1002/9783527699025.ch23
  • Optoelectronic Properties of (CH3NH3)(3)Sb2I9 Thin Films for Photovoltaic Applications
    Hebig, J. C. and Kuhn, I. and Flohre, J. and Kirchartz, T.
    ACS Energy Letters 1 (2016)
    We present solution-based fabrication and characterization of the lead-free perovskite-related methylammonium antimony iodide (CH3NH3)(3)Sb2I9 compound. By photothermal deflection spectroscopy (PDS), we determined a peak absorption coefficient a approximate to 10(5) cm(-1) and an optical band gap of 2.14 eV for amorphous films of (CH3NH3)(3)Sb2I9. Compared to the related Bi compound, the Sb-perovskite shows no exciton peak in its absorption spectrum. The photoluminescence emission (PL) is observed at 1.58 eV, and the Urbach tail energy of this amorphous compound is E-u = 62 meV, indicating a substantial amount of energetic disorder. We fabricate a planar heterojunction solar cell with a (CH3NH3)(3)Sb2I9 absorber layer that yields a power conversion efficiency of eta approximate to 0.5%, already featuring a decent fill factor (FF) of 55% and open-circuit voltage of 890 mV but low photocurrent densities. The result of this basic study on (CH3NH3)Sb2I9 shows that this compound is a possible starting point for further research into Sb-based lead-free perovskite solar cells.
    view abstract10.1021/acsenergylett.6b00170
  • Physical aspects of ferroelectric semiconductors for photovoltaic solar energy conversion
    Lopez-Varo, P. and Bertoluzzi, L. and Bisquert, J. and Alexe, M. and Coll, M. and Huang, J. and Jimenez-Tejada, J.A. and Kirchartz, T. and Nechache, R. and Rosei, F. and Yuan, Y.
    Physics Reports 653 (2016)
    Solar energy conversion using semiconductors to fabricate photovoltaic devices relies on efficient light absorption, charge separation of electron–hole pair carriers or excitons, and fast transport and charge extraction to counter recombination processes. Ferroelectric materials are able to host a permanent electrical polarization which provides control over electrical field distribution in bulk and interfacial regions. In this review, we provide a critical overview of the physical principles and mechanisms of solar energy conversion using ferroelectric semiconductors and contact layers, as well as the main achievements reported so far. In a ferroelectric semiconductor film with ideal contacts, the polarization charge would be totally screened by the metal layers and no charge collection field would exist. However, real materials show a depolarization field, smooth termination of polarization, and interfacial energy barriers that do provide the control of interface and bulk electric field by switchable spontaneous polarization. We explore different phenomena as the polarization-modulated Schottky-like barriers at metal/ferroelectric interfaces, depolarization fields, vacancy migration, and the switchable rectifying behavior of ferroelectric thin films. Using a basic physical model of a solar cell, our analysis provides a general picture of the influence of ferroelectric effects on the actual power conversion efficiency of the solar cell device, and we are able to assess whether these effects or their combinations are beneficial or counterproductive. We describe in detail the bulk photovoltaic effect and the contact layers that modify the built-in field and the charge injection and separation in bulk heterojunction organic cells as well as in photocatalytic and water splitting devices. We also review the dominant families of ferroelectric materials that have been most extensively investigated and have provided the best photovoltaic performance. © 2016
    view abstract10.1016/j.physrep.2016.07.006
  • Reciprocity between Charge Injection and Extraction and Its Influence on the Interpretation of Electroluminescence Spectra in Organic Solar Cells
    Kirchartz, T. and Nelson, J. and Rau, U.
    Physical Review Applied 5 (2016)
    Reciprocity relations based on the principle of detailed balance have been frequently used to analyze luminescence intensity and the spectrum of organic solar cells. These reciprocity relations were derived for cases where a linear extrapolation of equilibrium conditions to the nonequilibrium situations present during measurements is possible and therefore requires semiconductors with linear recombination mechanisms. Here, we discuss the impact of nonlinear recombination typically found in organic solar cells on the analysis of luminescence spectra and estimate criteria under which reciprocity relations can still be used to analyze the data. We find that depending on the exact application, only for low mobilities μ<10-4 cm2/V s or very asymmetric mobilities do substantial disagreements between simulation and analytical equations occur. © 2016 American Physical Society.
    view abstract10.1103/PhysRevApplied.5.054003
  • Reduced voltage losses yield 10% efficient fullerene free organic solar cells with >1 V open circuit voltages
    Baran, D. and Kirchartz, T. and Wheeler, S. and Dimitrov, S. and Abdelsamie, M. and Gorman, J. and Ashraf, R.S. and Holliday, S. and Wadsworth, A. and Gasparini, N. and Kaienburg, P. and Yan, H. and Amassian, A. and Brabec, C.J. and Durrant, J.R. and McCulloch, I.
    Energy and Environmental Science 9 (2016)
    Optimization of the energy levels at the donor-acceptor interface of organic solar cells has driven their efficiencies to above 10%. However, further improvements towards efficiencies comparable with inorganic solar cells remain challenging because of high recombination losses, which empirically limit the open-circuit voltage (Voc) to typically less than 1 V. Here we show that this empirical limit can be overcome using non-fullerene acceptors blended with the low band gap polymer PffBT4T-2DT leading to efficiencies approaching 10% (9.95%). We achieve Voc up to 1.12 V, which corresponds to a loss of only Eg/q - Voc = 0.5 ± 0.01 V between the optical bandgap Eg of the polymer and Voc. This high Voc is shown to be associated with the achievement of remarkably low non-geminate and non-radiative recombination losses in these devices. Suppression of non-radiative recombination implies high external electroluminescence quantum efficiencies which are orders of magnitude higher than those of equivalent devices employing fullerene acceptors. Using the balance between reduced recombination losses and good photocurrent generation efficiencies achieved experimentally as a baseline for simulations of the efficiency potential of organic solar cells, we estimate that efficiencies of up to 20% are achievable if band gaps and fill factors are further optimized. © The Royal Society of Chemistry 2016.
    view abstract10.1039/c6ee02598f
  • Transient Optoelectronic Characterization of Thin-Film Solar Cells
    Deibel, C. and Kirchartz, T.
    Advanced Characterization Techniques for Thin Film Solar Cells: Second Edition 1-2 (2016)
    view abstract10.1002/9783527699025.ch6
  • Zero-dimensional (CH3NH3)3Bi2I9 perovskite for optoelectronic applications
    Öz, S. and Hebig, J.-C. and Jung, E. and Singh, T. and Lepcha, A. and Olthof, S. and Jan, F. and Gao, Y. and German, R. and van Loosdrecht, P.H.M. and Meerholz, K. and Kirchartz, T. and Mathur, S.
    Solar Energy Materials and Solar Cells 158 (2016)
    We present the preparation and characterization of solution processed lead free films of zero-dimensional methylammonium iodo bismuthate (CH3NH3)3Bi2I9 perovskite. Structural characterization reveals the formation of hexagonal micro-crystals with preferred growth orientation along the c-axis. The material exhibits a wide band gap of 2.9eV and upon optical excitation the photoluminescence emission is observed at 1.65eV (751nm). Photoelectron spectroscopy confirms the stoichiometry of the (CH3NH3)3Bi2I9 perovskite and yields an ionization energy of 6.24eV, while Raman spectra confirm the vibrational modes of the bioctahedral inorganic anion framework (Bi2I9)3- in the low wavenumber regime (< 200cm-1) regime. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT) are employed to evaluate the experimental results. We apply this novel bismuth-based hybrid perovskite in proof of principle simple heterojunction solar cell devices, which yield power conversion efficiencies of ~0.1%. Further enhancement is expected once the film morphology and device stack are optimized. © 2016 Elsevier B.V.
    view abstract10.1016/j.solmat.2016.01.035
  • Classification of solar cells according to mechanisms of charge separation and charge collection
    Kirchartz, T. and Bisquert, J. and Mora-Sero, I. and Garcia-Belmonte, G.
    Physical Chemistry Chemical Physics 17 (2015)
    In the last decade, photovoltaics (PV) has experienced an important transformation. Traditional solar cells formed by compact semiconductor layers have been joined by new kinds of cells that are constituted by a complex mixture of organic, inorganic and solid or liquid electrolyte materials, and rely on charge separation at the nanoscale. Recently, metal organic halide perovskites have appeared in the photovoltaic landscape showing large conversion efficiencies, and they may share characteristics of the two former types. In this paper we provide a general description of the photovoltaic mechanisms of the single absorber solar cell types, combining all-inorganic, hybrid and organic cells into a single framework. The operation of the solar cell relies on a number of internal processes that exploit internal charge separation and overall charge collection minimizing recombination. There are two main effects to achieve the required efficiency, first to exploit kinetics at interfaces, favouring the required forward process, and second to take advantage of internal electrical fields caused by a built-in voltage and by the distribution of photogenerated charges. These principles represented by selective contacts, interfaces and the main energy diagram, form a solid base for the discussion of the operation of future types of solar cells. Additional effects based on ferroelectric polarization and ionic drift provide interesting prospects for investigating new PV effects mainly in the perovskite materials. © the Owner Societies 2015.
    view abstract10.1039/c4cp05174b
  • Electric properties and carrier multiplication in breakdown sites in multi-crystalline silicon solar cells
    Schneemann, M. and Kirchartz, T. and Carius, R. and Rau, U.
    Journal of Applied Physics 117 (2015)
    This paper studies the effective electrical size and carrier multiplication of breakdown sites in multi-crystalline silicon solar cells. The local series resistance limits the current of each breakdown site and is thereby linearizing the current-voltage characteristic. This fact allows the estimation of the effective electrical diameters to be as low as 100nm. Using a laser beam induced current (LBIC) measurement with a high spatial resolution, we find carrier multiplication factors on the order of 30 (Zener-type breakdown) and 100 (avalanche breakdown) as new lower limits. Hence, we prove that also the so-called Zener-type breakdown is followed by avalanche multiplication. We explain that previous measurements of the carrier multiplication using thermography yield results higher than unity, only if the spatial defect density is high enough, and the illumination intensity is lower than what was used for the LBIC method. The individual series resistances of the breakdown sites limit the current through these breakdown sites. Therefore, the measured multiplication factors depend on the applied voltage as well as on the injected photocurrent. Both dependencies are successfully simulated using a series-resistance-limited diode model. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4921286
  • Influence of Surface Recombination on Charge-Carrier Kinetics in Organic Bulk Heterojunction Solar Cells with Nickel Oxide Interlayers
    Wheeler, S. and Deledalle, F. and Tokmoldin, N. and Kirchartz, T. and Nelson, J. and Durrant, J.R.
    Physical Review Applied 4 (2015)
    The choice of electrode for organic photovoltaics is known to be of importance to both device stability and performance, especially regarding the open-circuit voltage (VOC). Here we show that the work function of a nickel oxide anode, varied using an O2 plasma treatment, has a considerable influence on the open-circuit voltage VOC of an organic solar cell. We probe recombination in the devices using transient photovoltage and charge extraction to determine the lifetime as a function of charge-carrier concentration and compare the experimental results with numerical drift-diffusion simulations. This combination of experiment and simulations allows us to conclude that the variations in VOC are due to a change in surface recombination, localized at the NiO anode, although only a small change in carrier lifetime is observed. © 2015 American Physical Society.
    view abstract10.1103/PhysRevApplied.4.024020
  • Organic photovoltaic greenhouses: A unique application for semi-transparent PV?
    Emmott, C.J.M. and Röhr, J.A. and Campoy-Quiles, M. and Kirchartz, T. and Urbina, A. and Ekins-Daukes, N.J. and Nelson, J.
    Energy and Environmental Science 8 (2015)
    Organic photovoltaics are an emerging solar power technology which embody properties such as transparency, flexibility, and rapid, roll to roll manufacture, opening the potential for unique niche applications. We report a detailed techno-economic analysis of one such application, namely the photovoltaic greenhouse, and discuss whether the unique properties of the technology can provide advantages over conventional photovoltaics. The potential for spectral selectivity through the choice of OPV materials is evaluated for the case of a photovoltaic greenhouse. The action spectrum of typical greenhouse crops is used to determine the impact on crop growth of blocking different spectral ranges from the crops. Transfer matrix optical modelling is used to assess the efficiency and spectrally resolved transparency of a variety of commercially available semi-conducting polymer materials, in addition to a non-commercial low-band-gap material with absorption outside that required for crop growth. Economic analysis suggests there could be a huge potential for OPV greenhouses if aggressive cost targets can be met. Technical analysis shows that semi-transparent OPV devices may struggle to perform better than opaque crystalline silicon with partial coverage, however, OPV devices using the low-band-gap material PMDPP3T, as well as a high efficiency mid-band-gap polymer PCDTBT, can demonstrate improved performance in comparison to opaque, flexible thin-film modules such as CIGS. These results stress the importance of developing new, highly transparent electrode and interlayer materials, along with high efficiency active layers, if the full potential of this application is going to be realised. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c4ee03132f
  • Quantifying losses in open-circuit voltage in solution-processable solar cells
    Yao, J. and Kirchartz, T. and Vezie, M.S. and Faist, M.A. and Gong, W. and He, Z. and Wu, H. and Troughton, J. and Watson, T. and Bryant, D. and Nelson, J.
    Physical Review Applied 4 (2015)
    The maximum open-circuit voltage of a solar cell can be evaluated in terms of its ability to emit light. We herein verify the reciprocity relation between the electroluminescence spectrum and subband-gap quantum efficiency spectrum for several photovoltaic technologies at different stages of commercial development, including inorganic, organic, and a type of methyl-ammonium lead- halide CH3NH3PbI3-xClx perovskite solar cells. Based on the detailed balance theory and reciprocity relations between light emission and light absorption, voltage losses at open circuit are quantified and assigned to specific mechanisms, namely, absorption edge broadening and nonradiative recombination. The voltage loss due to nonradiative recombination is low for inorganic solar cells (0.04-0.21 V), while for organic solar cell devices it is larger but surprisingly uniform, with values of 0.34-0.44 V for a range of material combinations. We show that, in CH3NH3PbI3-xClx perovskite solar cells that exhibit hysteresis, the loss to nonradiative recombination varies substantially with voltage scan conditions. We then show that for different solar cell technologies there is a roughly linear relation between the power conversion efficiency and the voltage loss due to nonradiative recombination. © 2015 American Physical Society.
    view abstract10.1103/PhysRevApplied.4.014020
  • Role of Polymer Fractionation in Energetic Losses and Charge Carrier Lifetimes of Polymer: Fullerene Solar Cells
    Baran, D. and Vezie, M.S. and Gasparini, N. and Deledalle, F. and Yao, J. and Schroeder, B.C. and Bronstein, H. and Ameri, T. and Kirchartz, T. and McCulloch, I. and Nelson, J. and Brabec, C.J.
    Journal of Physical Chemistry C 119 (2015)
    Nonradiative recombination reduces the open-circuit voltage relative to its theoretical limit and leads to reduced luminescence emission at a given excitation. Therefore, it is possible to correlate changes in luminescence emission with changes in open-circuit voltage and in the charge carrier lifetime. Here we use luminescence studies combined with transient photovoltage and differential charging analyses to study the effect of polymer fractionation in indacenoedithiophene-co-benzothiadiazole (IDTBT):fullerene solar cells. In this system, polymer fractionation increases electroluminescence emission at the same injection current and reduces nonradiative recombination. High-molecular-weight and fractionated IDTBT polymers exhibit higher carrier lifetime-mobility product compared to that of their nonfractionated analogues, resulting in improved solar cell performance. (Graph Presented). © 2015 American Chemical Society.
    view abstract10.1021/acs.jpcc.5b05709
  • Understanding the effect of unintentional doping on transport optimization and analysis in efficient organic bulk-heterojunction solar cells
    Deledalle, F. and Kirchartz, T. and Vezie, M.S. and Campoy-Quiles, M. and Tuladhar, P.S. and Nelson, J. and Durrant, J.R.
    Physical Review X 5 (2015)
    In this paper, we provide experimental evidence of the effects of unintentional p-type doping on the performance and the apparent recombination dynamics of bulk-heterojunction solar cells. By supporting these experimental observations with drift-diffusion simulations on two batches of the same efficient polymer-fullerene solar cells with substantially different doping levels and at different thicknesses, we investigate the way the presence of doping affects the interpretation of optoelectronic measurements of recombination and charge transport in organic solar cells. We also present experimental evidence on how unintentional doping can lead to excessively high apparent reaction orders. Our work suggests first that the knowledge of the level of dopants is essential in the studies of recombination dynamics and carrier transport and that unintentional doping levels need to be reduced below approximately 7 × 1015 cm-3 for full optimization around the second interference maximum of highly efficient polymer-fullerene solar cells.
    view abstract10.1103/PhysRevX.5.011032
  • Device Modelling of Organic Bulk Heterojunction Solar Cells
    Kirchartz, T. and Nelson, J.
    Multiscale Modelling of Organic and Hybrid Photovoltaics 352 (2014)
    We review the methods used to simulate the optoelectronic response of organic solar cells and focus on the application of one-dimensional drift-diffusion simulations. We discuss how the important physical processes are treated and review some of the experiments necessary to determine the input parameters for device simulations. To illustrate the usefulness of drift-diffusion simulations, we discuss several case studies, addressing the influence of charged defects on transport in bipolar and unipolar devices, the influence of defects on recombination, device performance and ideality factors. To illustrate frequency domain simulations, we show how to determine the validity range of Mott-Schottky plots for thin devices. Finally, we discuss an example where optical simulations are used to calculate the parasitic absorption in contact layers.
    view abstract10.1007/128_2013_473
  • Effect of localized states on the reciprocity between quantum efficiency and electroluminescence in Cu(In,Ga)Se2 and Si thin-film solar cells
    Müller, T.C.M. and Pieters, B.E. and Kirchartz, T. and Carius, R. and Rau, U.
    Solar Energy Materials and Solar Cells 129 (2014)
    We present the electroluminescence and quantum efficiency of three different types of thin-film solar cells based on absorbers made from Cu(In,Ga)Se2 as well as from μc-Si:H and a-Si:H. Simulations of our experimental results show that the main contribution to the electroluminescence spectrum originates from band-to-tail transitions in the case of Cu(In,Ga)Se2, from tail-to-tail transitions in μc-Si:H, and from tail-to-tail as well as from band-to-midgap-defect transitions in a-Si:H. By comparing the electroluminescence with the quantum efficiency, we analyze the effect of localized states on the optoelectronic reciprocity relation in the three material systems. The relatively steep band-tail density of states in Cu(In,Ga)Se2 is compatible with the reciprocity relation at room temperature while the shallower band-tail density of states in μc-Si:H and the deep mid-gap defect states in a-Si:H lead to substantial deviations from the reciprocity. © 2014 Elsevier B.V.
    view abstract10.1016/j.solmat.2014.04.018
  • Electron collection as a limit to polymer:PCBM solar cell efficiency: Effect of blend microstructure on carrier mobility and device performance in PTB7:PCBM
    Foster, S. and Deledalle, F. and Mitani, A. and Kimura, T. and Kim, K.-B. and Okachi, T. and Kirchartz, T. and Oguma, J. and Miyake, K. and Durrant, J.R. and Doi, S. and Nelson, J.
    Advanced Energy Materials 4 (2014)
    The poor photovoltaic performance of state-of-the-art blends of poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fl uoro-2-[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl] (PTB7) and [6,6]-phenyl-C61-butyric acid (PCBM) at large active layer thicknesses is studied using space-charge-limited current mobility and photovoltaic device measurements. The poor performance is found to result from relatively low electron mobility. This is attributed to the low tendency of PTB7 to aggregate, which reduces the ability of the fullerene to form a connected network. Increasing the PCBM content 60-80 wt% increases electron mobility and accordingly improves performance for thicker devices, resulting in a fill factor (FF) close to 0.6 at 300 nm. The result confirms that by improving only the connectivity of the fullerene phase, efficient electron and hole collection is possible for 300 nm-thick PTB7:PCBM devices. Furthermore, it is shown that solvent additive 1,8-diiodooctane (DIO), used in the highest efficiency PTB7:PCBM devices, does not improve the thickness dependence and, accordingly, does not lead to an increase in either hole or electron mobility or in the carrier lifetime. A key challenge for researchers is therefore to develop new methods to ensure connectivity in the fullerene phase in blends without relying on either a large excess of fullerene or strong aggregation of the polymer. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/aenm.201400311
  • On the thermodynamics of light trapping in solar cells
    Rau, U. and Kirchartz, T.
    Nature Materials 13 (2014)
    view abstract10.1038/nmat3837
  • Parameter free calculation of the subgap density of states in poly(3-hexylthiophene)
    Frost, J.M. and Kirkpatrick, J. and Kirchartz, T. and Nelson, J.
    Faraday Discussions 174 (2014)
    We investigate the influence of intra-chain and inter-chain interactions on the sub-gap density of states in a conjugated polymer using a combination of atomistic molecular dynamics simulation of polymer film structure and tight-binding calculation of electronic energy levels. For disordered assemblies of poly-3-hexylthiophene we find that the tail of the density of hole states is approximately exponential with a characteristic energy of 37 meV, which is similar to experimental values. This tail of states arises mainly from variations in the electronic coupling between neighbouring monomers, and is only slightly influenced by interchain coupling. Thus, knowledge of the disorder in torsion between neighbouring monomers is sufficient to estimate the density of states for the polymer. However, the intrachain torsional disorder is determined largely by the packing of the chains rather than the torsional potential alone. We propose the combination of methods as a tool to design higher mobility conjugated polymers. © The Royal Society of Chemistry 2014.
    view abstract10.1039/c4fd00153b
  • Spectroscopic evaluation of mixing and crystallinity of fullerenes in bulk heterojunctions
    Guilbert, A.A.Y. and Schmidt, M. and Bruno, A. and Yao, J. and King, S. and Tuladhar, S.M. and Kirchartz, T. and Alonso, M.I. and Goñi, A.R. and Stingelin, N. and Haque, S.A. and Campoy-Quiles, M. and Nelson, J.
    Advanced Functional Materials 24 (2014)
    The microstructure of blend fi lms of conjugated polymer and fullerene, especially the degree of mixing and crystallization, impacts the performance of organic photovoltaic devices considerably. Mixing and crystallization affect device performance in different ways. These phenomena are not easy to screen using traditional methods such as imaging. In this paper, the amorphous regiorandom poly(3-hexylthiophene) is blended with the potentially crystalline fullerene [6,6]-phenyl-C61-butyric acid methyl ester PCBM and the amorphous bis-adduct. First, the degree of mixing of polymer: fullerene blends is evaluated using UV-Vis absorption, steady-state and ultra-fast photoluminescence spectroscopy. The blue-shift of the polymer emission and absorption onset are used in combination with the saturation of the polymer emission decay time upon fullerene addition in order to infer the onset of aggregation of the blends. Second, the crystallinity of the fullerene is probed using variable angle spectroscopic ellipsometry (VASE), electroluminescence and photoluminescence spectroscopy. It is shown that the red-shift of charge transfer emission in the case of PCBM based blends cannot be explained solely by a variation of optical dielectric constant as probed by VASE. A combination of optical spectroscopy techniques, therefore, allows to probe the degree of mixing and can also distinguish between aggregation and crystallization of fullerenes. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201401626
  • Thermodynamics of light management in photovoltaic devices
    Rau, U. and Paetzold, U.W. and Kirchartz, T.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    This paper elaborates a comprehensive theory of the thermodynamics of light management in solar cells explicitly considering imperfect light trapping, parasitic absorption and nonradiative recombination losses. A quantitative description of the entropic losses that reduce the open-circuit voltage and the energy conversion efficiency from the radiative limit towards realistic situations is presented. The theory embraces the fundamental limits for idealized solar cell devices given by the Yablonovitch limit and the Shockley-Queisser limit. We discriminate between reversible and irreversible entropic loss processes for four fundamental light management concepts: (i) conventional light trapping as an integral part of the device, (ii) geometric concentration of incident light, (iii) angular restriction of incoming and outgoing light, and (iv) light concentration by luminescent solar collectors. Based on this discrimination, a comprehensive discussion of the interplay between the loss processes and light management is presented. As part of this analysis, a new figure of merit for efficient light trapping in solar cells is introduced as well as an example of a deterministic light trapping concept which induces almost optimal light trapping. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.035211
  • Understanding the apparent charge density dependence of mobility and lifetime in organic bulk heterojunction solar cells
    Deledalle, F. and Shakya Tuladhar, P. and Nelson, J. and Durrant, J.R. and Kirchartz, T.
    Journal of Physical Chemistry C 118 (2014)
    Energetic disorder in organic semiconductors leads to strong dependence of recombination kinetics and mobility on charge density. However, observed mobilities and reaction orders are normally interpreted assuming uniform charge carrier distributions. In this paper, we explore the effect of the spatial distribution of charge on the determination of mobility and recombination rate as a function of average charge density. Since the spatial gradient changes when the thickness of a device is varied, we study thickness series of two different polymer:fullerene systems and measure the charge density dependence of mobility and lifetime. Using simulations, we can show that the high apparent reaction orders frequently observed in the literature result from the spatial gradient of charge density at open circuit. However, the mobilities, measured at short circuit, are less affected by the gradients and therefore may show substantially different apparent charge density dependence than the recombination constants, especially for small device thicknesses. © 2014 American Chemical Society.
    view abstract10.1021/jp502948y
  • Analysis of the series resistance in pin-type thin-film silicon solar cells
    Müller, T.C.M. and Pieters, B.E. and Rau, U. and Kirchartz, T.
    Journal of Applied Physics 113 (2013)
    The series resistance of microcrystalline hydrogenated silicon thin-film pin-type solar cells is investigated using illumination dependent current/voltage characteristics. We present a simple analytical model describing the total series resistance of low-mobility pin-type solar cells. The model thus provides insight into the influence of the material properties of the intrinsic layer on the series resistance. Our model allows us to separate the voltage dependent internal resistance of the intrinsic layer from the residual, external resistance. We verified our model over a wide range of parameters relevant to thin-film silicon devices by comparison to numerical simulations. Finally, we demonstrate that our model can consistently describe the series resistance of experimental a μc-Si:H pin-type solar cell. Furthermore, the fitting of the model with experimental data yields the external series resistance and information of the carrier mobilities and effective density of states in the bands of the intrinsic layer in the device. © 2013 American Institute of Physics.
    view abstract10.1063/1.4798393
  • Influence of crystallinity and energetics on charge separation in polymer-inorganic nanocomposite films for solar cells
    Bansal, N. and Reynolds, L.X. and MacLachlan, A. and Lutz, T. and Ashraf, R.S. and Zhang, W. and Nielsen, C.B. and McCulloch, I. and Rebois, D.G. and Kirchartz, T. and Hill, M.S. and Molloy, K.C. and Nelson, J. and Haque, S.A.
    Scientific Reports 3 (2013)
    The dissociation of photogenerated excitons and the subsequent spatial separation of the charges are of crucial importance to the design of efficient donor-acceptor heterojunction solar cells. While huge progress has been made in understanding charge generation at all-organic junctions, the process in hybrid organic:inorganic systems has barely been addressed. Here, we explore the influence of energetic driving force and local crystallinity on the efficiency of charge pair generation at hybrid organic:inorganic semiconductor heterojunctions. We use X-ray diffraction, photoluminescence quenching, transient absorption spectroscopy, photovoltaic device and electroluminescence measurements to demonstrate that the dissociation of photogenerated polaron pairs at hybrid heterojunctions is assisted by the presence of crystalline electron acceptor domains. We propose that such domains encourage delocalization of the geminate pair state. The present findings suggest that the requirement for a large driving energy for charge separation is relaxed when a more crystalline electron acceptor is used.
    view abstract10.1038/srep01531
  • Influence of diffusion on space-charge-limited current measurements in organic semiconductors
    Kirchartz, T.
    Beilstein Journal of Nanotechnology 4 (2013)
    Numerical simulations of current-voltage curves in electron-only devices are used to discuss the influence of charged defects on the information derived from fitting space-charge-limited current models to the data. Charged, acceptor-like defects lead to barriers impeding the flow of electrons in electron-only devices and therefore lead to a reduced current that is similar to the situation where the device has a built-in voltage. This reduced current will lead to an underestimation of the mobilities and an overestimation of characteristic tail slopes if analytical equations are used to analyze the data. Correcting for the barrier created by the charged defects can, however, be a successful way to still be able to obtain reasonably accurate mobility values. © 2013 Kirchartz; licensee Beilstein-Institut.
    view abstract10.3762/bjnano.4.18
  • Influence of doping on charge carrier collection in normal and inverted geometry polymer:fullerene solar cells
    Dibb, G.F.A. and Muth, M.-A. and Kirchartz, T. and Engmann, S. and Hoppe, H. and Gobsch, G. and Thelakkat, M. and Blouin, N. and Tierney, S. and Carrasco-Orozco, M. and Durrant, J.R. and Nelson, J.
    Scientific Reports 3 (2013)
    While organic semiconductors used in polymer:fullerene photovoltaics are generally not intentionally doped, significant levels of unintentional doping have previously been reported in the literature. Here, we explain the differences in photocurrent collection between standard (transparent anode) and inverted (transparent cathode) low band-gap polymer:fullerene solar cells in terms of unintentional p-type doping. Using capacitance/voltage measurements, we find that the devices exhibit doping levels of order 1016 acm -3, resulting in space-charge regions ∼100nm thick at short circuit. As a result, low field regions form in devices thicker than 100nm. Because more of the light is absorbed in the low field region in standard than in inverted architectures, the losses due to inefficient charge collection are greater in standard architectures. Using optical modelling, we show that the observed trends in photocurrent with device architecture and thickness can be explained if only charge carriers photogenerated in the depletion region contribute to the photocurrent.
    view abstract10.1038/srep03335
  • Investigation of a conjugated polyelectrolyte interlayer for inverted polymer:fullerene solar cells
    Xia, R. and Leem, D.-S. and Kirchartz, T. and Spencer, S. and Murphy, C. and He, Z. and Wu, H. and Su, S. and Cao, Y. and Kim, J.S. and Demello, J.C. and Bradley, D.D.C. and Nelson, J.
    Advanced Energy Materials 3 (2013)
    Inverted bulk heterojunction solar cells are fabricated using a conjugated polyelectrolyte (PFN) as a cathode interlayer. Enhanced photovoltaic performance is achieved by adjusting the PFN thickness. Measurements of the optical transmittance, cathode work function (via UPS) and surface atomic composition (via XPS) provide insights into this optimization. Drift-diffusion simulations point to a reduction in recombination of holes at the cathode as the main cause for improving Voc. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/aenm.201200967
  • Isostructural, deeper highest occupied molecular orbital analogues of poly(3-hexylthiophene) for high-open circuit voltage organic solar cells
    Bronstein, H. and Hurhangee, M. and Fregoso, E.C. and Beatrup, D. and Soon, Y.W. and Huang, Z. and Hadipour, A. and Tuladhar, P.S. and Rossbauer, S. and Sohn, E.-H. and Shoaee, S. and Dimitrov, S.D. and Frost, J.M. and Ashraf, R.S. and Kirchartz, T. and Watkins, S.E. and Song, K. and Anthopoulos, T. and Nelson, J. and Rand, B.P. and Durrant, J.R. and McCulloch, I.
    Chemistry of Materials 25 (2013)
    We present the synthesis and characterization of two novel thiazole-containing conjugated polymers (PTTTz and PTTz) that are isostructural to poly(3-hexylthiophene) (P3HT). The novel materials demonstrate optical and morphological properties almost identical to those of P3HT but with HOMO and LUMO levels that are up to 0.45 eV deeper. An intramolecular planarizing nitrogen-sulfur nonbonding interaction is observed, and its magnitude and origin are discussed. Both materials demonstrate significantly greater open circuit voltages than P3HT in bulk heterojunction solar cells. PTTTz is shown to be an extremely versatile donor polymer that can be used with a wide variety of fullerene acceptors with device efficiencies of up to 4.5%. It is anticipated that this material could be used as a high-open circuit voltage alternative to P3HT in organic solar cells. © 2013 American Chemical Society.
    view abstract10.1021/cm4022563
  • Local junction voltages and radiative ideality factors of a-Si:H solar modules determined by electroluminescence imaging
    Tran, T.M.H. and Pieters, B.E. and Schneemann, M. and Müller, T.C.M. and Gerber, A. and Kirchartz, T. and Rau, U.
    Materials Research Society Symposium Proceedings 1536 (2013)
    In this contribution, we show that the dominant electroluminescent emission of hydrogenated amorphous silicon (a-Si:H) thin-film solar cells follows a diode law, whose radiative ideality factor nr is larger than one. This is in contrast to crystalline silicon and Cu(In, Ga)Se2 solar cells for which nr equals one. As a consequence, the existing quantitative analysis for the extraction of the local junction voltage V j(r) from luminescence images fails for a-Si:H solar cells. We expand the existing analysis method, and include the radiative ideality factor n r into the model. With this modification, we are able to determine the local junction voltage Vj(r) for a-Si:H solar cells and modules. We investigated the local junction voltage Vj(r) and the radiative ideality factor nr for both initial and stabilized a-Si:H solar modules. Furthermore, we show that the apparent radiative ideality factor is affected by the spectral sensitivity of the used camera system. © 2013 Materials Research Society.
    view abstract10.1557/opl.2013.817
  • Measurement and modeling of reverse biased electroluminescence in multi-crystalline silicon solar cells
    Schneemann, M. and Kirchartz, T. and Carius, R. and Rau, U.
    Journal of Applied Physics 114 (2013)
    Calibrated microscopic measurements of electroluminescent emission spectra of reverse biased multi-crystalline silicon solar cells in a wide range of photon energies E (0.8 eV ≤ E ≤ 4 eV) are reported. The observed spectra originating directly from point-like sources exhibit a broad maximum around 0.8 eV followed by a high photon energy tail. A model for intraband emission accurately fits microscopically measured spectra obtained from single point sources. Furthermore, we do not find significant features from interband recombination. From the fits to the intraband transition model, we extract an effective charge carrier temperature of around 4000 K for all investigated spots. The analysis also yields the different depths of the sources, which are shown to be consistent with the dimension of the space charge region. From the areas around the point sources, we observe indirect emission of internally reflected light. Due to the multiple paths through the wafer, this indirect emission exhibits a maximum at a photon energy slightly lower than the band gap energy Eg. We demonstrate that global, non-microscopic measurements are strongly influenced by this indirect radiation and therefore prone to misinterpretation. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4824099
  • On the differences between dark and light ideality factor in polymer:Fullerene solar cells
    Kirchartz, T. and Deledalle, F. and Tuladhar, P.S. and Durrant, J.R. and Nelson, J.
    Journal of Physical Chemistry Letters 4 (2013)
    Ideality factors are derived from either the slope of the dark current/voltage curve or the light intensity dependence of the open-circuit voltage in solar cells and are often a valuable method to characterize the type of recombination. In the case of polymer:fullerene solar cells, the ideality factors derived by the two methods usually differ substantially. Here we investigate the reasons for the discrepancies by determining both ideality factors differentially as a function of voltage and by comparing them with simulations. We find that both the dark and light ideality factors are sensitive to bulk recombination mechanisms at the internal donor:acceptor interface, as is often assumed in the literature. While the interpretation of the dark ideality factor is difficult due to resistive effects, determining the light ideality factor differentially indicates that the open-circuit voltage of many polymer:fullerene solar cells is limited by surface recombination, which leads to light ideality factors decreasing below one at high voltage. © 2013 American Chemical Society.
    view abstract10.1021/jz4012146
  • Photocurrent Enhancement from Diketopyrrolopyrrole Polymer Solar Cells through Alkyl-Chain Branching Point Manipulation
    Meager, I. and Ashraf, R. S. and Mollinger, S. and Schroeder, B. C. and Bronstein, H. and Beatrup, D. and Vezie, M. S. and Kirchartz, T. and Salleo, A. and Nelson, J. and McCulloch, I.
    Journal of the American Chemical Society 135 (2013)
    Systematically moving the alkyl-chain branching position away from the polymer backbone afforded two new thieno[3,2-b]thiophene diketopyrrolopyrrole (DPPTT-T) polymers. When used as donor materials in polyrner:fullerene solar cells, efficiencies exceeding 7% were achieved without the use of processing additives. The effect of the position of the alkyl-chain branching point on the thin-film morphology was investigated using X-ray scattering techniques and the effects on the photovoltaic and charge-transport properties were also studied. For both solar cell and transistor devices, moving the branching point further from the backbone was beneficial. This is the first time that this effect has been shown to improve solar cell performance. Strong evidence is presented for changes in microstructure across the series, which is most likely the cause for the photocurrent enhancement.
    view abstract10.1021/ja406934j
  • Quantitative evaluation method for electroluminescence images of a-Si: H thin-film solar modules
    Tran, T.M.H. and Pieters, B.E. and Schneemann, M. and Müller, T.C.M. and Gerber, A. and Kirchartz, T. and Rau, U.
    Physica Status Solidi - Rapid Research Letters 7 (2013)
    This work presents a method for extracting the absolute local junction voltage of a-Si:H thin-film solar cells and modules from electroluminescence (EL) images. It is shown that the electroluminescent emission of a-Si:H devices follows a diode law with a radiative ideality factor nr larger than one. We introduce an evaluation method that allows us to determine the absolute local junction voltage in cases of nr &gt; 1, while existing approaches rely on the assumption of nr = 1. Furthermore, we find that the experimentally determined values of nr vary from sample to sample. It is also explained why the derived radiative ideality factor is influenced by the spectral sensitivity of the camera system used in the experiment. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssr.201308039
  • Relating recombination, density of states, and device performance in an effi cient polymer:fullerene organic solar cell blend
    Hawks, S.A. and Deledalle, F. and Yao, J. and Rebois, D.G. and Li, G. and Nelson, J. and Yang, Y. and Kirchartz, T. and Durrant, J.R.
    Advanced Energy Materials 3 (2013)
    We explore the interrelation between density of states, recombination kinetics, and device performance in effi cient poly[4,8-bis-(2-ethylhexyloxy)- benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxy-1-one) thieno[3,4-b]thiophene-2,6-diyl]:[6,6]-phenyl-C 71-butyric acid methyl ester (PBDTTT-C:PC 71 BM) bulk-heterojunction organic solar cells. We modulate the active-layer density of states by varying the polymer:fullerene composition over a small range around the ratio that leads to the maximum solar cell effi ciency (50-67 wt% PC 71 BM). Using transient and steady-state techniques, we fi nd that nongeminate recombination limits the device effi-ciency and, moreover, that increasing the PC 71 BM content simultaneously increases the carrier lifetime and drift mobility in contrast to the behavior expected for Langevin recombination. Changes in electronic properties with fullerene content are accompanied by a signifi cant change in the magnitude or energetic separation of the density of localized states. Our comprehensive approach to understanding device performance represents signifi cant progress in understanding what limits these high-effi ciency polymer:fullerene systems.© 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.
    view abstract10.1002/aenm.201300194
  • Transient phenomena in Cu(In,Ga)Se2 solar modules investigated by electroluminescence imaging
    Tran, T.M.H. and Pieters, B.E. and Ulbrich, C. and Gerber, A. and Kirchartz, T. and Rau, U.
    Thin Solid Films 535 (2013)
    Industrially fabricated thin-film modules are investigated by electroluminescence (EL) photography. We observe metastable transients in a series of successive images recorded during application of forward bias immediately after the Cu(In,Ga)Se2 modules were kept in the dark for several hours. Our experiments are conducted in the dark either under constant current (monitoring the module voltage) or vice versa. For both situations, the EL intensities increase with time, whereas we observe a decrease of the overall module voltage (at fixed current) or an increase of the current (at fixed voltage). We ascribe our observations to the simultaneous decrease of the bulk series resistance Rs and the reduction of recombination currents (an increase in the junction resistance Rj) during the forward bias soaking. A quantitative analysis of our data shows that the reduction of R s we observe is much stronger than the increase of Rj. We also show that the bulk series resistance can strongly adulterate the determination of the sheet resistance of the front ZnO from EL images. © 2012 Elsevier B.V.
    view abstract10.1016/j.tsf.2012.10.039
  • Understanding the effect of donor layer thickness and a MoO3 hole transport layer on the open-circuit voltage in squaraine/C60 bilayer solar cells
    Ryan, J.W. and Kirchartz, T. and Viterisi, A. and Nelson, J. and Palomares, E.
    Journal of Physical Chemistry C 117 (2013)
    Small molecule organic solar cells are becoming increasingly efficient through improved molecular design. However, there is still much to be understood regarding device operation. Here we study bilayer solar cells employing a 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) donor and fullerene acceptor to probe the effect of donor layer thickness and a MoO 3 electron transport layer on device performance. The thickness of SQ is seen to drastically affect the open-circuit voltage (VOC) and fill factor (FF), while the short circuit current is not altered significantly. The fact that the VOC of the bilayers with thin (6 nm) donor layers shows a strong dependence on the material and workfunction of the anode cannot be explained with a model for a perfect bilayer. Recombination of electrons from C60 at the anode contact has to be possible to understand the strong effect of the anode workfunction. Using numerical simulations and a simple two-diode model we show that the most likely interpretation of the observed effects is that for thin SQ layers, the roughness of the interface is high enough to allow electrons in the C60 to tunnel through the SQ to recombine directly at the anode. Thicker SQ layers will block most of these recombination pathways, which explains the drastic dependence of VOC on thickness. Bulk-heterojunction devices were also fabricated to illustrate the effect of anode material on the VOC. © 2013 American Chemical Society.
    view abstract10.1021/jp406472t
  • Understanding the reduced efficiencies of organic solar cells employing fullerene multiadducts as acceptors
    Faist, M.A. and Shoaee, S. and Tuladhar, S. and Dibb, G.F.A. and Foster, S. and Gong, W. and Kirchartz, T. and Bradley, D.D.C. and Durrant, J.R. and Nelson, J.
    Advanced Energy Materials 3 (2013)
    The use of fullerenes with two or more adducts as acceptors has been recently shown to enhance the performance of bulk-heterojunction solar cells using poly(3-hexylthiophene) (P3HT) as the donor. The enhancement is caused by a substantial increase in the open-circuit voltage due to a rise in the fullerene lowest unoccupied molecular orbital (LUMO) level when going from monoadducts to multiadducts. While the increase in the open-circuit voltage is obtained with many different polymers, most polymers other than P3HT show a substantially reduced photocurrent when blended with fullerene multiadducts like bis-PCBM (bis adduct of Phenyl-C61-butyric acid methyl ester) or the indene C 60 bis-adduct ICBA. Here we investigate the reasons for this decrease in photocurrent. We find that it can be attributed partly to a loss in charge generation efficiency that may be related to the LUMO-LUMO and HOMO-HOMO (highest occupied molecular orbital) offsets at the donor-acceptor heterojunction, and partly to reduced charge carrier collection efficiencies. We show that the P3HT exhibits efficient collection due to high hole and electron mobilities with mono- and multiadduct fullerenes. In contrast the less crystalline polymer Poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5- thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl (PCDTBT) shows inefficient charge carrier collection, assigned to low hole mobility in the polymer and low electron mobility when blended with multiadduct fullerenes. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/aenm.201200673
  • Charge extraction and photocurrent in organic bulk heterojunction solar cells
    Petersen, A. and Kirchartz, T. and Wagner, T.A.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    The pseudo point-symmetry of the photocurrent-voltage characteristics J ph(V) (defined as the difference between light and dark currents) has been utilized to determine important properties of organic bulk-heterojunction (BHJ) solar cells, e.g., contact recombination velocities, the dominant charge recombination mechanism, or the presence of a field dependent exciton dissociation process. In order to improve the theoretical understanding of the photocurrent generation in BHJ solar cells, we apply a numerical drift-diffusion model to investigate the effect of injection barriers, selective contacts, different recombination mechanisms, and series resistances on J ph(V). We show the consistency of the model with experimental data from literature and reduce different experimental observations to a single, fundamental mechanism in solar cells with intrinsic absorber layers: position dependent equilibrium concentrations and lifetimes of the charge carriers. Based on this result, we discuss the special points of the photocurrent-voltage characteristic such as the point of symmetry and the compensation voltage. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.045208
  • Comparison of device models for organic solar cells: Band-to-band vs. tail states recombination
    Soldera, M. and Taretto, K. and Kirchartz, T.
    Physica Status Solidi (A) Applications and Materials Science 209 (2012)
    The efficiency-limiting recombination mechanism in bulk-heterojunction (BHJ) solar cells is a current topic of investigation and debate in organic photovoltaics. In this work, we simulate state-of-the-art BHJ solar cells using two different models. The first model takes into account band-to-band recombination and field dependent carrier generation. The second model assumes a Shockley-Read-Hall (SRH) recombination mechanism via tail states and field independent carrier generation. Additionally, we include in both cases optical modelling and, thus, position-dependent exciton generation and non-ideal exciton collection. We explore both recombination mechanisms by fitting light and dark current-voltage (JV) characteristics of BHJ cells of five materials: P3HT, MDMO-PPV, MEH-PPV, PCDTBT and PF10TBT, all blended with fullerene derivatives. We show that although main device parameters such as short circuit current, open circuit voltage, fill factor and ideality factor are accurately reproduced by both Langevin and tail recombination, only tail recombination reproduces also the ideality factor of dark characteristics accurately. Nevertheless, the model with SRH recombination via tail states needs the inclusion of external circuitry to account for the heavy shunt present in all the blends, except P3HT:PCBM, when illuminated. Finally, we propose a means to find analytical expressions for the short circuit current by assuming a linear relation between the recombination rate and the concentration of free minority carriers. The model reproduces experimental data of P3HT cells at various thickness values using realistic parameters for this material. Dark JV measurement (circles) of a PCDTBT:PC 70BM solar cell (Park et al., Nature Photon. 3, 297 (2009) [1]), the fit with the model including recombination via tail states (solid line) and the fit with the model reported by (Koster et al., Phys. Rev. B 72, 085205 (2005) [2]) that includes bimolecular band-to-band recombination and charge transfer state (CTS) dissociation. The inset shows the JV curves under white light. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201127264
  • Competition between the charge transfer state and the singlet states of donor or acceptor limiting the efficiency in polymer: Fullerene solar cells
    Faist, M.A. and Kirchartz, T. and Gong, W. and Ashraf, R.S. and McCulloch, I. and De Mello, J.C. and Ekins-Daukes, N.J. and Bradley, D.D.C. and Nelson, J.
    Journal of the American Chemical Society 134 (2012)
    We study the appearance and energy of the charge transfer (CT) state using measurements of electroluminescence (EL) and photoluminescence (PL) in blend films of high-performance polymers with fullerene acceptors. EL spectroscopy provides a direct probe of the energy of the interfacial states without the need to rely on the LUMO and HOMO energies as estimated in pristine materials. For each polymer, we use different fullerenes with varying LUMO levels as electron acceptors, in order to vary the energy of the CT state relative to the blend with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). As the energy of the CT state emission approaches the absorption onset of the blend component with the smaller optical bandgap, E opt,min min{E opt,donor; E opt,acceptor}, we observe a transition in the EL spectrum from CT emission to singlet emission from the component with the smaller bandgap. The appearance of component singlet emission coincides with reduced photocurrent and fill factor. We conclude that the open circuit voltage V OC is limited by the smaller bandgap of the two blend components. From the losses of the studied materials, we derive an empirical limit for the open circuit voltage: V OC ≲ E opt,min/e - (0.66 ± 0.08)eV. © 2011 American Chemical Society.
    view abstract10.1021/ja210029w
  • Field-dependent exciton dissociation in organic heterojunction solar cells
    Petersen, A. and Ojala, A. and Kirchartz, T. and Wagner, T.A. and Würthner, F. and Rau, U.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    In organic heterojunction solar cells, the generation of free charge carriers takes place in a multistep process which involves charge transfer (CT) states, that is, bound electron-hole pairs at the interface between donor and acceptor molecules. Past efforts to model the CT-state dissociation during solar cell operation were not able to consistently reproduce the experimentally observed field and temperature dependence. This discrepancy between model and experiment was partly due to the field-dependent free charge carrier collection process, which plays an important role in the widely used bulk heterojunction cell configuration and superimposes a possible field-dependent charge carrier generation process. In order to distinguish between generation and collection of free charge carriers, we propose the planar heterojunction cell configuration as a model system to study the field-dependent charge carrier generation process in organic heterojunction solar cells. We apply this model system to check current CT-state dissociation models against experimental data. Although the models can quantitatively account for the photocurrent's dependence on the applied voltage and the device thickness, they fail to account for the virtually negligible temperature dependence of the field-dependent charge-generation process. This discrepancy is traced back to a common feature of the models: an Arrhenius-like temperature dependence, distinctive of all processes involving a thermally activated jump over an energy barrier. As a solution to the problem, we introduce an exciton dissociation model based on a field-dependent tunnel process and demonstrate its consistency with the experimental observations. Our results indicate that the current microscopic picture of the charge-generation process in organic heterojunction solar cells being limited by the CT-state dissociation process needs to be reconsidered. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.245208
  • Influence of energetic disorder on electroluminescence emission in polymer: Fullerene solar cells
    Gong, W. and Faist, M.A. and Ekins-Daukes, N.J. and Xu, Z. and Bradley, D.D.C. and Nelson, J. and Kirchartz, T.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    Electroluminescence (EL) spectroscopy and imaging can be useful techniques to analyze various loss mechanisms in solar cells, but the interpretation of the results is not trivial in solar cells made from disordered materials such as organic semiconductors. In this case the interpretation of EL measurements may be affected by the presence of a tail of localized states. Here, we study several polymer:fullerene systems and show that, despite the presence of tail states, the shape of the EL spectrum is insensitive to the applied voltage. This indicates that the emission originates mainly from mobile charges in higher lying states recombining at the polymer:fullerene interface and that most charges in deeper tail states do not contribute to the EL spectrum. The consequence of our finding is that simple models of EL emission in ideal semiconductors can be applied to polymer:fullerene solar cells and can therefore be used to evaluate the potential of different material systems in terms of recombination losses and to study resistive losses using luminescence imaging. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.024201
  • Meaning of reaction orders in polymer:fullerene solar cells
    Kirchartz, T. and Nelson, J.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    Nongeminate recombination in polymer:fullerene solar cells is frequently characterized using transient optoelectronic measurements that allow the determination of recombination rates, charge carrier lifetimes, and average charge carrier concentrations as a function of voltage. These data are often interpreted in terms of an empirical reaction order defining how recombination depends on measured charge density. In polymer:fullerene solar cells, the empirical reaction orders are often considerably larger than 2, which had previously been explained in terms of the nonlinear relationship between mobile and trapped charge carriers in the presence of an exponential tail of localized states. Here, we show that experimentally determined reaction orders depend not only on the shape of the density of states but also on the spatial distribution of carriers. In particular, in solar cells with small depletion regions due to small active layer thicknesses or due to large unintentional background doping of the polymers, the reaction order can assume values that are much larger than the value expected from the shape of the density of states alone. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.165201
  • Merocyanine/C60 planar heterojunction solar cells: Effect of dye orientation on exciton dissociation and solar cell performance
    Ojala, A. and Petersen, A. and Fuchs, A. and Lovrincic, R. and Pölking, C. and Trollmann, J. and Hwang, J. and Lennartz, C. and Reichelt, H. and Höffken, H.W. and Pucci, A. and Erk, P. and Kirchartz, T. and Würthner, F.
    Advanced Functional Materials 22 (2012)
    In this study the charge dissociation at the donor/acceptor heterointerface of thermally evaporated planar heterojunction merocyanine/C60 organic solar cells is investigated. Deposition of the donor material on a heated substrate as well as post-annealing of the complete devices at temperatures above the glass transition temperature of the donor material results in a twofold increase of the fill factor. An analytical model employing an electric-field-dependent exciton dissociation mechanism reveals that geminate recombination is limiting the performance of as-deposited cells. Fourier-transform infrared ellipsometry shows that, at temperatures above the glass transition temperature of the donor material, the orientation of the dye molecules in the donor films undergoes changes upon annealing. Based on this finding, the influence of the dye molecules' orientations on the charge-transfer state energies is calculated by quantum mechanical/molecular mechanics methods. The results of these detailed studies provide new insight into the exciton dissociation process in organic photovoltaic devices, and thus valuable guidelines for designing new donor materials. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201101697
  • Modelling of photo- and electroluminescence of hydrogenated microcrystalline silicon solar cells
    Müller, T.C.M. and Pieters, B.E. and Kirchartz, T. and Carius, R. and Rau, U.
    Physica Status Solidi (C) Current Topics in Solid State Physics 9 (2012)
    Photoluminescence (PL) and electroluminescence (EL) have received much attention as characterization techniques for photovoltaic devices. The methods are applied to study e.g. optical band-gap, defect states, or quasi-Fermi level splitting. Spatially resolved EL imaging is used to derive local junction voltage differences making it a fast inline characterization method for solar modules. However, the interpretation of EL and PL experiments on hydrogenated microcrystalline silicon (μc-Si:H) solar cells is more complex hampering the direct determination of local voltage differences. In this work we integrated an existing model for PL of μc-Si:H silicon with a commercial device simulator for thin-film silicon devices. This way we extended the existing model from a spatially zero dimensional model to a one dimensional model which can also model EL. Furthermore the connection with an electrical device simulator enables the consistent modeling of EL, PL, and the electrical properties of the device. We compared experimental and simulation results for EL, PL and dark-, and illuminated- current/voltage characteristics over a wide temperature range (80 - 300 K). The simulations and experiments are in good agreement in the temperature range from 170 K up to room temperature. In experiments we observed several effects which cannot be explained in the previous zero dimensional model. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssc.201200428
  • Sensitivity of the mott-schottky analysis in organic solar cells
    Kirchartz, T. and Gong, W. and Hawks, S.A. and Agostinelli, T. and MacKenzie, R.C.I. and Yang, Y. and Nelson, J.
    Journal of Physical Chemistry C 116 (2012)
    The application of Mott-Schottky analysis to capacitance-voltage measurements of polymer:fullerene solar cells is a frequently used method to determine doping densities and built-in voltages, which have important implications for understanding the device physics of these cells. Here we compare drift-diffusion simulations with experiments to explore the influence and the detection limit of doping in situations where device thickness and doping density are too low for the depletion approximation to be valid. The results of our simulations suggest that the typically measured values on the order of 5 × 10 16 cm -3 for doping density in thin films of 100 nm or lower may not be reliably determined from capacitance measurements and could originate from a completely intrinsic active layer. In addition, we explain how the violation of the depletion approximation leads to a strong underestimation of the actual built-in voltage by the built-in voltage V MS determined by Mott-Schottky analysis. © 2012 American Chemical Society.
    view abstract10.1021/jp300397f
  • Understanding the thickness-dependent performance of organic bulk heterojunction solar cells: The influence of mobility, lifetime, and space charge
    Kirchartz, T. and Agostinelli, T. and Campoy-Quiles, M. and Gong, W. and Nelson, J.
    Journal of Physical Chemistry Letters 3 (2012)
    We investigate the reasons for the dependence of photovoltaic performance on the absorber thickness of organic solar cells using experiments and drift-diffusion simulations. The main trend in photocurrent and fill factor versus thickness is determined by mobility and lifetime of the charge carriers. In addition, space charge becomes more and more important the thicker the device is because it creates field free regions with low collection efficiency. The two main sources of space-charge effects are doping and asymmetric mobilities. We show that for our experimental results on Si-PCPDTBT:PC71BM (poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30-d]silole)-2,6-diyl-alt-(4, 7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,50-diyl]:[6,6]-phenyl C71-butyric acid methyl ester) solar cells, the influence of doping is most likely the dominant influence on the space charge and has an important effect on the thickness dependence of performance. © 2012 American Chemical Society.
    view abstract10.1021/jz301639y
  • Analysis of the relationship between linearity of corrected photocurrent and the order of recombination in organic solar cells
    Dibb, G.F.A. and Kirchartz, T. and Credgington, D. and Durrant, J.R. and Nelson, J.
    Journal of Physical Chemistry Letters 2 (2011)
    We address the claim that the dependence of the "corrected photocurrent" (de fined as the difference between the light and dark currents) upon light intensity can be used to determine the charge recombination mechanismin an organic solar cell. We analyze a poly(3-hexylthiophene):[6,6]- phenyl C61-butyric acid methyl ester (P3HT:PCBM) device using corrected photocurrent and transient photovoltage experiments and show that whereas the corrected photocurrent is linear in light intensity the charge recombination rate scales superlinearly with charge carrier density.We explain this apparent discrepancy by measuring the charge carrier densities at different applied voltages and light intensities.We show that it is only safe to infer a linear recombination mechanism from a linear dependence of corrected photocurrent on light intensity under the following special conditions: (i) the photogenerated charge carrier density is much larger than the dark carrier density and (ii) the photogenerated carrier density is proportional to the photogeneration rate. (Graph Presented). © 2011 American Chemical Society.
    view abstract10.1021/jz201104d
  • Approaching the lambertian limit in randomly textured thin-film solar cells
    Fahr, S. and Kirchartz, T. and Rockstuhl, C. and Lederer, F.
    Optics Express 19 (2011)
    The Lambertian limit for solar cells is a benchmark for evaluating their efficiency. It has been shown that the performance of either extremely thick or extremely thin solar cells can be driven close to this limit by using an appropriate photon management. Here we show that this is likewise possible for realistic, practically relevant thin-film solar cells based on amorphous silicon. Most importantly, we achieve this goal by relying on random textures already incorporated into state-of-the-art superstrates; with the only subtlety that their topology has to be downscaled to typical feature sizes of about 100 nm. © 2011 Optical Society of America.
    view abstract10.1364/OE.19.00A865
  • Approaching the lambertian limit in randomly textured thin-film solar cells
    Lederer, F. and Fahr, S. and Rockstuhl, C. and Kirchartz, T.
    Materials Research Society Symposium Proceedings 1391 (2011)
    The Lambertian limit represents a benchmark for the enhancement of the effective path length in solar cells, which is important as soon as the absorption length exceeds the absorber thickness. In previous publications it has been shown that either extremely thick or extremely thin solar cells can be driven close to this limit by exploiting up to date photon management. In this contribution we show that the Lambertian limit can also be achieved with thin-film solar cells based on amorphous silicon for practically relevant absorber thicknesses. Departing from superstrates, which are currently incorporated into state-of-the-art thin-film solar cells, we show that their topology has simply to be downscaled to typical feature sizes of about 100 nm in order to achieve this goal. By systematically studying the impact of the modulation height and the lateral feature sizes of the incorporated textures and of the absorber thickness we are able to deduce intuitive guidelines how to approach the Lambertian limit in randomly textured thin-film solar cells. © 2012 Materials Research Society.
    view abstract10.1557/opl.2012.694
  • Characterization and simulation of a-Si:H/μc-Si:H tandem solar cells
    Ding, K. and Kirchartz, T. and Pieters, B.E. and Ulbrich, C. and Ermes, A.M. and Schicho, S. and Lambertz, A. and Carius, R. and Rau, U.
    Solar Energy Materials and Solar Cells 95 (2011)
    We simulated device characteristics of a-Si:H single junction, μc-Si:H single junction and a-Si:H/μc-Si:H tandem solar cells with the numerical device simulator Advanced Semiconductor Analysis (ASA). For this purpose we measured and adjusted electrical and optical input parameters by comparing measured and simulated external quantum efficiency, current-voltage characteristic and reflectivity spectra. Consistent reproducibility of experimental data by numerical simulation was achieved for all types of cells investigated in this work. We also show good correspondence between the experimental and simulated characteristics for a-Si:H/μc-Si:H tandem solar cells with various absorber thicknesses on both Asahi U-type SnO2:F and sputtered/etched (Jülich) ZnO:Al substrates. Based on this good correlation between experiment and theory, we provide insight into device properties that are not directly measurable like the spatially resolved absorptance and the voltage-dependent carrier collection. These data reveal that the difference between tandem solar cells grown on Asahi U-type and Jülich ZnO substrates primarily arises from their optical properties. In addition, we find out that the doped layers do not contribute to the photocurrent except for the front p-layer. We also calculated the initial efficiencies of a-Si:H/μc-Si:H tandem solar cells with different combinations of a-Si:H and μc-Si:H absorber layer thicknesses. The maximum efficiency is found at 260 nm/1500 nm for tandem solar cells on Asahi U-type substrates and at 360 nm/850 nm for tandem solar cells on Jülich ZnO substrates. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.solmat.2011.07.023
  • Electroluminescence Analysis of Solar Cells and Solar Modules
    Kirchartz, T. and Helbig, A. and Pieters, B.E. and Rau, U.
    Advanced Characterization Techniques for Thin Film Solar Cells (2011)
    view abstract10.1002/9783527636280.ch3
  • Influence of the indium tin oxide/organic interface on open-circuit voltage, recombination, and cell degradation in organic small-molecule solar cells
    Schäfer, S. and Petersen, A. and Wagner, T.A. and Kniprath, R. and Lingenfelser, D. and Zen, A. and Kirchartz, T. and Zimmermann, B. and Würfel, U. and Feng, X. and Mayer, T.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    In this paper we investigate the performance and stability of small-molecule organic solar cells with respect to the indium tin oxide (ITO)/organic interface. Different zinc-phthalocyanine (ZnPc)/fullerene (C 60) cell architectures with and without ITO O2-plasma treatment are compared and tested with respect to their degradation behavior under illumination in inert atmosphere. Photoelectron spectroscopy (UPS and XPS) shows that the O2-plasma treatment increases the ITO work function from 4.3 eV up to 5.6 eV. We find that both the increased ITO work function as well as the introduction of an electron blocking layer between ITO and the mixed donor/acceptor layer increases the open-circuit voltage Voc by more than 200 mV. For both cases our continuum approach device simulation quantitatively relates the increase of Voc to a reduced contact recombination and thus a reduced dark current. For cells built on ozone treated ITO we find a fast cell degradation caused by the UV part of the AM 1.5 spectrum. We identify the degradation, which manifests itself in a decrease of Voc of up to 25%, as a partial reversion of the plasma induced ITO work function increase. Additionally, we demonstrate that the degradation can be reduced by structural changes in the cell architecture, leading to improved cell stability. We present a comprehensive study of the recombination at the ITO/organic interface and its influence on the open-circuit voltage and the cell stability. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.165311
  • Introduction to Thin-Film Photovoltaics
    Kirchartz, T. and Rau, U.
    Advanced Characterization Techniques for Thin Film Solar Cells (2011)
    view abstract10.1002/9783527636280.ch1
  • Modeling nongeminate recombination in P3HT:PCBM solar cells
    MacKenzie, R.C.I. and Kirchartz, T. and Dibb, G.F.A. and Nelson, J.
    Journal of Physical Chemistry C 115 (2011)
    By introducing tail states into a continuum drift diffusion model, we are able to self-consistently reproduce the experimental voltage-dependent carrier concentration, the current-voltage curve, and the nongeminate recombination prefactor as measured by transient photovoltage (TPV) measurements of poly-3-hexylthiophene:phenyl-C61-butyric acid methyl ester (P3HT:PCBM). A mobility edge is introduced into the density of states (DoS) for both electrons and holes thus separating each population into mobile and trapped carriers. The effective mobility for each charge type is calculated from the ratio of free to trapped carriers within the DoS and is thus carrier density dependent. By introducing this effective mobility into Langevin's formula for charge recombination, we are able to reproduce the experimentally observed quasi-third-order recombination rates. Furthermore, we evaluate three possible DoS shapes, a pure Gaussian, a Gaussian with an exponential superimposed, and a pure exponential. It is found that an exponential DoS is essential to reproduce the experimental data. © 2011 American Chemical Society.
    view abstract10.1021/jp200234m
  • Recombination via tail states in polythiophene:fullerene solar cells
    Kirchartz, T. and Pieters, B.E. and Kirkpatrick, J. and Rau, U. and Nelson, J.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    State-of-the-art models used for drift-diffusion simulations of organic bulk heterojunction solar cells based on band transport are not capable of reproducing the voltage dependence of dark current density and carrier concentration of such devices, as determined by current-voltage and charge-extraction measurements. Here, we show how to correctly reproduce this experimental data by including an exponential tail of localized states into the density of states for both electrons and holes, and allowing recombination to occur between free charge carriers and charge carriers trapped in these states. When this recombination via tail states is included, the dependence of charge-carrier concentration on voltage is distinctly different from the case of band-to-band recombination and the dependence of recombination current on carrier concentration to a power higher than 2 can be explained. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.115209
  • Enhanced light trapping in thin amorphous silicon solar cells by directionally selective optical filters
    Ulbrich, C. and Peters, M. and Tayyib, M. and Blaesi, B. and Kirchartz, T. and Gerber, A. and Rau, U.
    Proceedings of SPIE - The International Society for Optical Engineering 7725 (2010)
    Optical absorption losses limit the efficiency of thin-film solar cells. We demonstrate how to increase the absorption in hydrogenated amorphous silicon solar cells by using a directionally selective optical multilayer filter covering the front glass. The filter transmits perpendicularly incident photons in the wavelength range 350 nm - 770 nm. In the regime of low absorptance, i.e. large optical absorption lengths, however, it blocks those photons impinging under oblique angles. Thus, the incoming radiation is transmitted with almost no loss while the emitted radiation is mostly blocked due to its wider angle distribution. We determine the enhancement in the optical path length from reflectivity measurements. In the weakly absorbing high wavelength range (650 nm - 770 nm) we observe a peak optical path length enhancement of κ ∼3.5. The effective path length enhancement κ ∼ calculated from the external quantum efficiency of the solar cell with filter, however, peaks at a lower value of only κ ∼1.5 in the same wavelength range. Parasitic absorption in the layers adjacent to the photovoltaic absorber limit the increase in the effective light path enhancement. Nonetheless we determine an increase of 0.2 mAcm-2 in the total short circuit current density. © 2010 Copyright SPIE - The International Society for Optical Engineering.
    view abstract10.1117/12.853799
  • Enhanced light trapping in thin-film solar cells by a directionally selective filter
    Ulbrich, C. and Peters, M. and Bläsi, B. and Kirchartz, T. and Gerber, A. and Rau, U.
    Optics Express 18 (2010)
    A directionally selective multilayer filter is applied to a hydrogenated amorphous silicon solar cell to improve the light trapping. The filter prevents non-absorbed long-wavelength photons from leaving the cell under oblique angles leading to an enhancement of the total optical path length for weakly absorbed light within the device by a factor of kr = 3.5. Parasitic absorption in the contact layers limits the effective path length improvement for the photovoltaic quantum efficiency to a factor of kEQE = 1.5. The total short-circuit current density increases by ΔJsc = 0.2 mAcm2 due to the directional selectivity of the Bragg-like filter. © 2010 Optical Society of America.
    view abstract10.1364/OE.18.00A133
  • Modeling of photoluminescence spectra and quasi-Fermi level splitting in μc-Si:H solar cells
    Pieters, B.E. and Kirchartz, T. and Merdzhanova, T. and Carius, R.
    Solar Energy Materials and Solar Cells 94 (2010)
    We developed a model describing the photoluminescence spectra from hydrogenated microcrystalline silicon (μc-Si:H). From the model we derived analytical relations between the separation of the quasi-Fermi levels and PL-peak energy and intensity. These relations may be useful when photoluminescence or electroluminescence based methods are applied for characterization of μc-Si:H solar cells and modules. We compared the model with experimental PL spectra from a μc-Si:H solar cell. Our model can consistently explain the relation between the measured PL-peak intensity, energy and the measured open circuit voltage of the μc-Si:H solar cell. © 2010 Elsevier B.V.
    view abstract10.1016/j.solmat.2010.06.005
  • Quantitative electroluminescence analysis of resistive losses in Cu(In, Ga)Se2 thin-film modules
    Helbig, A. and Kirchartz, T. and Schaeffler, R. and Werner, J.H. and Rau, U.
    Solar Energy Materials and Solar Cells 94 (2010)
    Cu(In,Ga)Se2 mini-modules are investigated by electroluminescence imaging under different bias voltage conditions. The images typically show 10-20 localized shunts on the investigated test modules of an area of 20×17 cm2 with 42 cells. The consequences of these shunts on the performance of the individual cells and of the entire module are quantitatively analyzed by evaluating the electroluminescence images. Our analysis considers the electroluminescence intensity at each surface position of the module to depend on the actual voltage drop across the junction at this specific location. Hence, the analysis of the electroluminescence intensity permits the reconstruction of the current/voltage characteristics of all individual cells of the module. Additionally, we determine the sheet resistance of the ZnO window layer and the Mo back contact from the spatial dependence of the electroluminescence intensity across one solar cell in the module. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.solmat.2010.01.028
  • Reverse biased electroluminescence spectroscopy of crystalline silicon solar cells with high spatial resolution
    Schneemann, M. and Helbig, A. and Kirchartz, T. and Carius, R. and Rau, U.
    Physica Status Solidi (A) Applications and Materials Science 207 (2010)
    Electroluminescence spectra of different types of crystalline silicon solar cells obtained under reverse bias are analysed on a macroscopic as well as on a microscopic scale. The calibrated spectra of all samples exhibit a dominant peak at the band-gap energy E g ≈ 1.1 eV. Although the fraction of visible light may vary over one order of magnitude, the shapes of the calibrated spectra are qualitatively similar. Single emission sites have been investigated with respect to the externally applied voltage V and their onset voltage V O by a microscope attached to a spectrometer. While the fraction of visible light within the spectra of individual sites increases with higher absolute onset voltage V O of the respective site, the actual applied voltage V has no influence on the shape of the spectra. Furthermore, we find that the emission intensity of all investigated single sites increases linearly with the applied voltage V. A nonlinear increase of local emission intensities as reported in the literature results from consecutive appearance of distinct breakdown sites in close distance to each other. Such series of breakdown events are not resolved without using microscopic measurements and therefore mistaken as the illumination intensity of a single site. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201026309
  • organic and hybrid photovoltaics

  • organic photovoltaics

  • photovoltaics

  • polymer solar cells

  • solar cells

  • thin films

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