Ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells on transparent conductive oxide (TCO) back contacts combine advantages of ultrathin cells for reducing material consumption of rare indium and gallium and TCO-transparency benefited applications in tandems, bifacial configurations etc. However, their efficiencies are still limited and the back barrier potential is a primary reason from an electrical perspective. In this work, we explore the effects of Nadoping by post deposition treatment (PDT) on the performance of ultrathin CIGSe solar cells on ITO (Sn:In2O3)-coated Na-free glass substrates. Na doping enhances not only the open circuit-voltage (Voc) by increasing the doping level, but also the fill factor (FF) by switching the Schottky contact to an Ohmic contact at the CIGSe/ITO interface, which we propose is due to the increased recombination at the back interface. The optimum performance is achieved at a NaF dose of 2 mg with a top efficiency of 12.9%, which exhibits an enhancement by nearly 48% relative compared to the references without Na doping. To our best knowledge, this is the highest efficiency achieved for ultrathin cells (<500 nm absorber thickness) on TCO without additional antireflection or back reflecting layer. Therefore, the results show that sodium control offers a solid basis for the development of ultrathin CIGSe cells on TCO in above-mentioned promising applications. © 2021 Elsevier B.V.

Semi-transparent ultrathin Cu(In,Ga)Se 2 (CIGSe) solar cells offer many promising applications but their efficiencies are still limited. In this work, SiO 2 point contact structures were prepared using nanosphere lithography on Sn:In 2 O 3 (ITO) substrate and ultrathin CIGSe solar cells were fabricated on top. It was discovered that the SiO 2 point contact structure at the CIGSe/ITO interface behaves significantly different from at the CIGSe/Mo interface and does not improve the rear interface reflectivity or short-circuit current density. However, the SiO 2 point contact structure brings pronounced electrical benefits, arising from the joint effect of reduced back recombination and induced field effect by internal charges. Semi-transparent ultrathin CIGSe solar cells with an absorber thickness of 380 nm exhibited a significant increase in open circuit voltage of 26 mV and fill factor of 9.4% (absolute). Consequently, the efficiency is improved by 23% (from absolute 6.8%–8.4%). © 2019

We provide a computational method for quickly determining the correct distribution of optically active nanoparticles for a desired response. This is achieved by simulating the optical response of single nanoparticles and performing a statistical averaging over different sizes. We find good agreement between experiment and theory for the transmission, reflectance and absorption of both an ordered and disordered array. By repeating the simulation for different particle distributions, we show that the method is capable of accurately predicting the correct nanoparticle distribution for a desired optical response. We provide a referential graph for predicting the optical response of different Ag nanoparticle distributions on a glass substrate, which can be extended to other substrate and particle materials, and particle shapes and sizes. © 2018 IOP Publishing Ltd.

In this work, light trapping effects based on dielectric nanoparticles (NPs) are numerically evaluated in ultrathin CuIn1-xGaxSe2 (CIGSe) solar cells for different locations of NPs, cell architectures and illumination directions, with relevant implications and optimized NP parameters being specified. The severe absorption in Mo is the main constraining factor for the effective implementation of light trapping NPs in ultrathin CIGSe cells. For a significant light absorption enhancement, it is favoured to integrate dielectric NPs at the rear interface of CIGSe/back contact and employ transparent conductive oxide (TCO) back contacts rather than the conventional Mo. It is demonstrated, that under front illumination, the low-index (n = 1.5) hemispherical NPs at the CIGSe/TCO interface cause significant light trapping effects. The NP-patterned ultrathin cells achieve a maximum short circuit current density (Jsc) of 36.4 mA/cm2 at an upper limit of 500 nm CIGSe thickness, which is as high as 94% Jsc of their thick flat counterparts with a CIGSe thickness of 2000 nm. In contrast, under back illumination, the patterned ultrathin cells realize comparable absorption to the corresponding thick counterpart at a CIGSe thickness of only 300 nm and the maximum Jsc (35.2 mA/cm2) saturates at a CIGSe thickness of 425 nm. Further, Jsc is less attenuated by the parasitic absorption in Al:ZnO (AZO) under back illumination than under front illumination in module configuration, where a much thicker AZO is required. This suggests that patterned ultrathin CIGSe solar cells under back illumination will be a promising cell architecture simultaneously for high efficiencies and less material usage in industrial module production. © 2018 Elsevier Ltd

Light concentration opens up the path to enhanced material efficiency of solar cells via increased conversion efficiency and decreased material requirement. For true material saving, a fabrication method allowing local growth of high quality absorber material is essential. We present two scalable fs-laser based approaches for bottom-up growth of Cu(In,Ga)Se2 micro islands utilizing either site-controlled assembly of In(,Ga) droplets on laser-patterned substrates during physical vapor deposition, or laser-induced forward transfer of (Cu,In,Ga) layers for local precursor arrangement. The Cu(In,Ga)Se2 absorbers formed after selenization can deliver working solar devices showing efficiency enhancement under light concentration. © 2018 SPIE.

Light concentration has proven beneficial for solar cells, most notably for highly efficient but expensive absorber materials using high concentrations and large scale optics. Here, we investigate the light concentration for cost-efficient thin-film solar cells that show nano- or microtextured absorbers. Our absorber material of choice is CuIn; GaSe2 (CIGSe), which has a proven stabilized record efficiency of 22.6% and which-despite being a polycrystalline thin-film material-is very tolerant to environmental influences. Taking a nanoscale approach, we concentrate light in the CIGSe absorber layer by integrating photonic nanostructures made from dielectric materials. The dielectric nanostructures give rise to resonant modes and field localization in their vicinity. Thus, when inserted inside or adjacent to the absorber layer, absorption and efficiency enhancement are observed. In contrast to this internal absorption enhancement, external enhancement is exploited in the microscaled approach: mm-sized lenses can be used to concentrate light onto CIGSe solar cells with lateral dimensions reduced down to the micrometer range. These micro solar cells come with the benefit of improved heat dissipation compared with the large scale concentrators and promise compact high-efficiency devices. Both approaches of light concentration allow for reduction in material consumption by restricting the absorber dimension either vertically (ultrathin absorbers for dielectric nanostructures) or horizontally (microabsorbers for concentrating lenses) and have significant potential for efficiency enhancement. ©2017 Society of Photo-Optical Instrumentation Engineers (SPIE).

This work elaborates on the high scattering which dielectric nanorods exhibit and how it can be exploited to control light propagation across material interfaces. A detailed overview of how dielectric nanorods interact with light through a combination of dipolar scattering and leaky modes is performed via outward power flux calculations. We establish and account for design parameters that best result in light magnification owing to resonant behavior of nanorods. Impact of material parameters on scattering and their dispersion have been calculated to establish that low loss dielectric oxides like ZnO when nanostructured show excellent antenna like resonances which can be used to control light coupling and propagation. Interfacial scattering calculations demonstrate the high forward directivity of nanorods for various dielectric interfaces. A systematic analysis for different configurations of single and periodic nanorods on air dielectric interface emphasizes the light coupling tendencies exhibited by nanorods to and from a dielectric. Spatial characteristics of the localized field enhancement of the nanorod array on an air dielectric interface show focusing attributes of the nanorod array. We give a detailed account to tailor and selectively increase light propagation across an interface with good spectral and spatial control. © 2017 The Author(s).

We propose an application of a stack of glass slides as a broadband neutral density (ND) filter with high (&#x003E;100 W/cm<formula><tex>$^2$</tex></formula>) damage threshold. The influence of multiple reflections on the filter transmittance is analyzed with transfer matrix method. The numerical study proves that the filter spectrum is determined by material dispersion and degree of wavelength averaging occurring due to light incoherence and limited measurement resolution. Varying the angle of light incidence can be used to extend the possible optical density range of the filters. The numerical results are verified by spectral measurements with setups containing coherent and incoherent light sources. The applicability of the filters is proven in a concentrated sun simulator. OAPA

Ultra-thin perovskite absorber layers have attracted increasing interest since they are suitable for application in semi-transparent perovskite and tandem solar cells. In this study, size and density controlled plasmonic silver nanoparticles are successfully incorporated into ultra-thin perovskite solar cells through a low temperature spray chemical vapor deposition method. Incorporation of Ag nanoparticles leads to a significant enhancement of 22.2% for the average short-circuit current density. This resulted in a relative improvement of 22.5% for the average power conversion efficiency. Characterization by surface photovoltage and photoluminescence provides evidence that the implemented silver nanoparticles can enhance the charge separation and the trapping of electrons into the TiO2 layer at the CH3NH3PbI3/TiO2 interface. The application of these silver nanoparticles therefore has promise to enhance the ultra-thin perovskite solar cells. © The Royal Society of Chemistry.

Background: Nanoparticles exhibit unique light scattering properties and are applied in many research fields. Methods: In this work, we perform angular resolved scattering measurements to study the scattering behaviour of random and periodic silver (Ag), and periodic polystyrene (PS) nanoparticles. Results: The random Ag nanoparticles, with a wide particle size distribution, are able to broadbandly scatter light into large angles. In contrast, both types of periodic nanoparticles are characterized by a strong scattering zone where scattering angles are increasing as the wavelength increases. Conclusions: Angular resolved scattering measurements enable experimentally revealing the particular scattering properties of different nanostructures. © 2017, The Author(s).

Indium islands on molybdenum coated glass can be grown in ordered arrays by surface structuring using a femtosecond laser. The effect of varying the molybdenum coated glass substrate temperature and the indium deposition rate on island areal density, volume and geometry is investigated and evaluated in a physical vapor deposition (PVD) process. The joined impact of growth conditions and spacing of the femtosecond laser structured spots on the arrangement and morphology of indium islands is demonstrated. The results yield a deeper understanding of the island growth and its precise adjustment to industrial requirements, which is indispensable for a technological application of such structures at a high throughput, for instance as precursors for the preparation of Cu(In,Ga)Se2 micro concentrator solar cells. © 2016 Elsevier B.V.

We demonstrate the fabrication and characterization of nanostructured transparent conductive oxides and via numerical simulation observe the parametric optical transmission trend. These structures can be used for customized light coupling to and from optoelectronic devices. © 2017 OSA.

Plasmonic and photonic nanoparticles have proven beneficial for solar cells in the aspect of light management. For improved exploitation of nanoparticles in solar cells, it is necessary to reveal the absorption enhancement mechanism from the nanoparticles. In this study, we investigated the nanoparticle-enhanced solar cells in near-field regime with optic and opto-electric scanning near-field optical microscopy (SNOM). The near-field distribution of regularly arranged silver and polystyrene nanoparticles produced by nanosphere lithography on Cu(In,Ga)Se2 (CIGSe) solar cells is characterized using a custom-built SNOM, which gives insight into the optical mechanism of light trapping from nanoparticles to solar cells. On the other hand, the photocurrent of CIGSe solar cells with and without nanoparticles is studied with an opto-electric SNOM by recording the photocurrent during surface scanning, further revealing the opto-electrical influences of the nanoparticles. In addition, finite element method simulations have been performed and agree with the results from SNOM. We found the dielectric polystyrene spheres are able to enhance the absorption and benefit the generation of charge carriers in the solar cells. Copyright © 2017 SPIE.

A procedure to fabricate CuInSe2 (CISe) micro-absorbers and solar cells for concentrator applications is presented. The micro-absorbers are developed from indium precursor islands, which are deposited on a molybdenum coated glass substrate (back contact), followed by deposition of copper on top and subsequent selenization as well as selective etching of copper selenides. In order to compare the properties of the locally grown absorbers to those of conventional large area CISe films, we systematically examine the compositional and morphological homogeneity of the micro absorbers and carry out photoluminescence measurements. Preliminary devices for micro-concentrator solar cell applications are fabricated by optimizing the copper to indium ratio and the size of the indium precursor islands. The resulting micro solar cells provide a characteristic I–V curve under standard illumination conditions (1 sun). © 2017 Elsevier Ltd

The local efficiency of lamellar shaped Cu(In,Ga)Se2 solar cells has been investigated using scanning near-field optical microscopy (SNOM). Topographic and photocurrent measurements have been performed simultaneously with a 100 nm tip aperture. The lamellar shaped solar cell with monolithic interconnects (P scribe) has been investigated on a nanometre scale for the first time at different regions using SNOM. It was found that, the cell region between P1 and P2 significantly contributes to the solar cells overall photocurrent generation. The photocurrent produced depends locally on the sample topography and it is concluded that it is mainly due to roughness changes of the ZnO:Al/i-ZnO top electrode. Regions lying under large grains of ZnO produce significantly less current than regions under small granules. The observed photocurrent features were allocated primarily to the ZnO:Al/i-ZnO top electrode. They were found to be independent of the wavelength of the light used (532 nm and 633 nm). © 2017 The Authors Journal of Microscopy © 2017 Royal Microscopical Society

CuIn1– xGaxSe2 (CIGSe) solar cells have achieved record efficiency values as high as 22.6% for small areas, with module efficiency values of 16.5%. However, for economic viability these values must be achieved with reduced material consumption (especially indium), which requires reducing the CIGSe absorber thickness from 2000–3000 nm to below 500 nm. Soft-imprinted SiOx nanoparticles (NPs) beneath a conformal CIGSe layer enable this thickness reduction. Optically, they enhance the absorption of light through Fabry–Pérot and waveguided resonances within the CIGSe layer, preventing current loss. For CIGSe solar cells on ITO with an absorber thickness of only 390 nm and a nanophotonic contact the current density (Jsc) increases from 25.7 to 32.1 mA cm−2. At the same time, the nanopatterned contact reduces the back barrier, leading to an increased open-circuit voltage (518 to 558 mV) and fill factor (50.7% to 55.2%). Combined, these effects increase the efficiency value from 6.8% to 10.0% for this initial demonstration. With the addition of an antireflection coating, the champion NP-enhanced cell achieves a Jsc of 34.0 mA cm−2, corresponding to 93% of the Jsc achieved by the thick world-record cell. This result shows that optoelectronic nanopatterning provides a path to high efficiency cells with reduced materials consumption. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

One of the promising approaches to improve the efficiency of conventional single-crystalline silicon (c-Si) solar cells is their integration in a tandem arrangement. In this perspective, inorganic-organic perovskites are an ideal blend of materials to combine with c-Si owing to their complementary light absorption characteristics. Even though interesting and promising combinations of perovskite/c-Si-based solar cells have been presented, their overall efficiency has been limited by the photocurrent reduction occurring in both perovskite and silicon due mostly to reflection and parasitic losses. Here, we envision and model a new design strategy for an efficient light-to-current conversion through the use of a nanopillar array based perovskite/c-Si tandem solar cell. The optical-electrical performance of the proposed architecture is analyzed by a 3D finite-element numerical model. In particular, we have searched for the best optical enhancement conditions through the tuning of the cell geometrical parameters, demonstrating the importance of optical resonances. Afterward, we have evaluated the electrical response of the optimized structures in a four-terminal (4-T) configuration by studying the current-voltage characteristics and power conversion efficiency. In particular, the introduced solar cell yields a conversion efficiency of 27%, with contributions of 18.5% and 8.51% from perovskite and c-Si, respectively. We have compared our proposed nanopatterned design with its planar counterpart characterized by the same quantity of active material, obtaining a relative efficiency enhancement of 21%. Importantly, the conversion efficiency of our proposed design surpasses the efficiency of single-junction perovskite and c-Si solar cells, and, similarly, it represents a new achievement for 4-T perovskite/c-Si tandem solar cells. © 2017 American Chemical Society.

Single-pulse femtosecond laser-induced forward transfer (LIFT, 30 fs, 790 nm) is used to deposit micron-sized dots of copper and/or indium onto a molybdenum layer on glass. Such systems can serve as precursors for the bottom–up manufacturing of micro-concentrator solar cells based on copper–indium–gallium–diselenide. The influence of the thickness of the copper, indium, and combined copper–indium donor layers on the quality of the transferred dots was qualified by scanning electron microscopy, energy-dispersive X-ray analysis, and optical microscopy. The potential for manufacturing of a spatial arrangement adapted to the geometry of micro-lens arrays needed for micro-concentrator solar cells is demonstrated. © 2017, Springer-Verlag GmbH Germany.

Light management has gained wide interest for various types of solar cells. This paper reviews the application of nanostructures for light management to chalcopyrite (CIGSe) type solar cells. Firstly, the relevance of light management for CIGSe solar cells will be introduced and concepts of nanostructures for absorption enhancement discussed. The development of ultra-thin CIGSe solar cells and examples for nanoparticle fabrication techniques together with their chances and challenges for application to CIGSe will be presented. Particular attention will be paid to nanostructures that have been applied to CIGSe solar cells, revealing many theoretical and some experimental results. Metallic and dielectric nanostructures as well as intrinsic nanotextures will be covered. For the future, combined considerations of optical and electrical properties will gain in importance. © 2017 IOP Publishing Ltd.

Ag nanoparticles have attracted interest for plasmonic absorption enhancement of solar cells. For this purpose, well-defined particle sizes and densities as well as very low deposition temperatures are required. Thus, we report here a new spray chemical vapour deposition method for producing Ag NP films with independent size and density control at substrate temperatures even below 100 °C, which is much lower than for many other techniques. This method can be used on different substrates to deposit Ag NP films. It is a reproducible, low-cost process which uses trimethylphosphine (hexafluoroacetylacetonato) silver as a precursor in alcoholic solution. By systematic variation of deposition parameters and classic experiments, mechanisms of particle growth and of deposition processes as well as the low decomposition temperature of the precursor could be explained. Using the 3D finite element method, absorption spectra of selected samples were simulated, which fitted well with the measured results. Hence, further applications of such Ag NP films for generating plasmonic near field can be predicted by the simulation. © 2017, Springer Science+Business Media Dordrecht.

Concentrator photovoltaics (CPV) is a cost-effective method for generating electricity in regions that have a large fraction of direct solar radiation. With the help of lenses, sunlight is concentrated onto miniature, highly efficient multi-junction solar cells with a photovoltaic performance above 40%. To ensure illumination with direct radiation, CPV modules must be installed on trackers to follow the sun's path. However, the costs of huge concentration optics and the photovoltaic technology used, narrow the market possibilities for CPV technology. Efforts to reduce these costs are being undertaken by the promotion of Cu(Inx,Ga1-x)Se2 solar cells to take over the high cost multi-junction solar cells and implementing more compact devices by minimization of solar cell area. Micrometer-sized absorbers have the potential of low cost, high efficiencies and good thermal dissipation under concentrated illumination. Heat dissipation at low (&lt;10×) to medium (10 × to 100×) flux density distributions is the key point of high concentration studies for macro- and micro-sized solar cells (from 1 μm2 to 1 mm2). To study this thermal process and to optimize it, critical parameters must be taken in account: absorber area, substrate area and thickness, structure design, heat transfer mechanism, concentration factor and illumination profile. A close study on them will be carried out to determine the best structure to enhance and reach the highest possible thermal management pointing to an efficiency improvement. © 2017 IOP Publishing Ltd.

Light concentration has proven beneficial for solar cells, most notably for highly efficient but expensive absorber materials using high concentrations and large scale optics. Here we investigate light concentration for cost efficient thinfilm solar cells which show nano-or microtextured absorbers. Our absorber material of choice is Cu(In,Ga)Se2 (CIGSe) which has a proven stabilized record efficiency of 22.6% and which-despite being a polycrystalline thin-film material-is very tolerant to environmental influences. Taking a nanoscale approach, we concentrate light in the CIGSe absorber layer by integrating photonic nanostructures made from dielectric materials. The dielectric nanostructures give rise to resonant modes and field localization in their vicinity. Thus when inserted inside or adjacent to the absorber layer, absorption and efficiency enhancement are observed. In contrast to this internal absorption enhancement, external enhancement is exploited in the microscale approach: mm-sized lenses can be used to concentrate light onto CIGSe solar cells with lateral dimensions reduced down to the micrometer range. These micro solar cells come with the benefit of improved heat dissipation compared to the large scale concentrators and promise compact high efficiency devices. Both approaches of light concentration allow for reduction in material consumption by restricting the absorber dimension either vertically (ultra-thin absorbers for dielectric nanostructures) or horizontally (micro absorbers for concentrating lenses) and have significant potential for efficiency enhancement. © 2016 SPIE.

2-D closely packed SiO2 nanosphere arrays serving as the photonic structure for light absorption enhancement on top of ultra-thin Cu(In1-xGax)Se2 solar cells are investigated both theoretically and experimentally. It is theoretically demonstrated that whispering gallery modes and high order Mie resonances contribute to the light absorption enhancement for the large spheres and an anti-reflection effect is prominent for small ones. The ultra-thin CIGSe solar cells achieve the optimum absorption enhancement for the small sphere array with a diameter of 110 nm, contrary to the larger spheres used in Si solar cells. The reason is attributed to the strong parasitic absorption in the AZO/ZnO/CdS front layers. They absorb mainly in the short wavelength range where the Mie resonances occur. Additionally, it is shown that the 110-nm-diameter sphere array exhibits a better angular tolerance than a conventional planar anti-reflection layer, which shows the potential as a promising anti-reflection structure. © 2016 Elsevier B.V. All rights reserved.

We evaluate the potential of inserting metallic, metal-dielectric core-shell, and fully dielectric nanoparticles in ultrathin chalcopyrite solar cells to enhance absorption which experiences a significant drop for absorber thicknesses below 500 nm. For different integration positions at the front or at the rear of the solar cell structure theoretical expectations and potential benefits originating from light scattering, near-field enhancement and coupling into waveguide modes by the nanoparticles are presented. These benefits are always balanced against experimental challenges arising for particular geometries due to the very specific fabrication processes of chalcopyrite solar cells. In particular high absorber deposition temperatures as well as contact layers that are relatively thick compared to other devices need to be considered. Based on this, we will need to go beyond some geometries that have proven beneficial for other types of solar cells and identify the most promising configurations for chalcopyrite-based devices. © 2016 Materials Research Society.

A bottom-up approach is presented for the production of arrays of indium islands on a molybdenum layer on glass, which can serve as micro-sized precursors for indium compounds such as copper-indium-gallium-diselenide used in photovoltaics. Femtosecond laser ablation of glass and a subsequent deposition of a molybdenum film or direct laser processing of the molybdenum film both allow the preferential nucleation and growth of indium islands at the predefined locations in a following indium-based physical vapor deposition (PVD) process. A proper choice of laser and deposition parameters ensures the controlled growth of indium islands exclusively at the laser ablated spots. Based on a statistical analysis, these results are compared to the non-structured molybdenum surface, leading to randomly grown indium islands after PVD. © 2016 AIP Publishing LLC.

Ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells pose challenges of incomplete absorption and back contact recombination. In this work, we applied the simple collodial nanosphere lithography and fabricated 2D SiO2 nanomeshes (NMs), which simultaneously benefit ultrathin CIGSe solar cells electrically and optically. Electrically, the NMs are capable of passivating the back contact recombination and increasing the minimum bandgap of absorbers. Optically, the parasitic absorption in Mo as a main optical loss is reduced. Consequently, the SiO2 NMs give rise to an increase of 3.5 mA/cm2 in short circuit current density (Jsc) and of 57 mV in open circuit voltage increase (Voc), leading to an absolute efficiency enhancement as high as 2.6% (relatively 30%) for CIGSe solar cells with an absorber thickness of only 370 nm and a steep back Ga/[Ga + In] grading. © 2016 American Chemical Society.

We present a method for calculating the plasmonic and photonic enhancement of the absorption in solar cells. The method involves coupling between a transfer matrix method to describe light propagation in the layered stack and Mie theory for calculating the absorption and angular scattered field distribution from the nanoparticles. We also compare the method to rigorous simulations. © 2015 IEEE.

To realize the high efficiency potential of perovskite/chalcopyrite tandem solar cells in modules, hydrogenated In2O3 (IO:H) as electrode is investigated. IO:H with an electron mobility of 100 cm2 V-1 s-1 is demonstrated. Compared to the conventional Sn doped In2O3 (ITO), IO:H exhibits a decreased electron concentration and leads to almost no sub-bandgap absorption up to the wavelength of 1200 nm. Without a trade-off between transparency and lateral resistance in the IO:H electrode, the tandem cell keeps increasing in efficiency as the IO:H thickness increases and efficiencies above 22% are calculated. In contrast, the cells with ITO as electrode perform much worse due to the severe parasitic absorption in ITO. This indicates that IO:H has the potential to lead to high efficiencies, which is otherwise constrained by the parasitic absorption in conventional transparent conductive oxide electrode for tandem solar cells in modules. © 2015 AIP Publishing LLC.

To investigate the process temperature on the growth of ultra-thin (≤500 nm) Cu(In,Ga)Se (CIGSe) absorbers and the corresponding performance of solar cells, the process temperature was set to 610 °C and 440 °C, respectively. It was found that the low process temperature (440 °C) could reduce the inter-diffusion of Ga-In and thus result in a higher back [Ga]/([Ga]+[In]) ([Ga]/[III]) grading than at the temperature of 610 °C. The higher back [Ga]/[III] grading at 440 °C was evidenced to both electrically and optically contribute to the efficiency enhancement of the solar cells in contrast to the lower back [Ga]/[III] grading at 610 °C. It was also implied that the high back [Ga]/[III] grading was beneficial to the collection of carriers generated from the back-reflected light. © 2014 Elsevier B.V.

Integration of plasmonic Ag nanoparticles as a back reflector in ultra-thin Cu(In,Ga)Se2 (CIGSe) solar cells is investigated. X-ray photoelectron spectroscopy results show that Ag nanoparticles underneath a Sn:In2O3 back contact could not be thermally passivated even at a low substrate temperature of 440 °C during CIGSe deposition. It is shown that a 50 nm thick Al2O3 film prepared by atomic layer deposition is able to block the diffusion of Ag, clearing the thermal obstacle in utilizing Ag nanoparticles as a back reflector in ultra-thin CIGSe solar cells. Via 3-D finite element optical simulation, it is proved that the Ag nanoparticles show the potential to contribute the effective absorption in CIGSe solar cells. © 2015 Elsevier B.V. All rights reserved.

We experimentally demonstrate photocurrent enhancement in ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells with absorber layers of 460 nm by nanoscale dielectric light scattering patterns printed by substrate conformal imprint lithography. We show that patterning the front side of the device with TiO2 nanoparticle arrays results in a small photocurrent enhancement in almost the entire 400-1200 nm spectral range due to enhanced light coupling into the cell. Three-dimensional finite-difference time-domain simulations are in good agreement with external quantum efficiency measurements. Patterning the Mo/CIGSe back interface using SiO2 nanoparticles leads to strongly enhanced light trapping, increasing the efficiency from 11.1% for a flat to 12.3% for a patterned cell. Simulations show that optimizing the array geometry could further improve light trapping. Including nanoparticles at the Mo/CIGSe interface leads to substantially reduced parasitic absorption in the Mo back contact. Parasitic absorption in the back contact can be further reduced by fabricating CIGSe cells on top of a SiO2-patterned In2O3:Sn (ITO) back contact. Simulations show that these semitransparent cells have similar spectrally averaged reflection and absorption in the CIGSe active layer as a Mo-based patterned cell, demonstrating that the absorption losses in the Mo can be partially turned into transmission through the semitransparent geometry. © 2015 American Chemical Society.

A scanning near-field optical microscope (SNOM) is a powerful tool to investigate optical effects that are smaller than Abbe's limit. Its greatest strength is the simultaneous measurement of high-resolution topography and optical nearfield data that can be correlated to each other. However, the resolution of an aperture SNOM is always limited by the probe. It is a technical challenge to fabricate small illumination tips with a well-defined aperture and high transmission. The aperture size and the coating homogeneity will define the optical resolution and the optical image whereas the tip size and shape influence the topographic accuracy. Although the technique has been developing for many years, the correlation between simulated near-field data and measurement is still not convincing. To overcome this challenge, the mapping of near-field plasmonic interactions of silver nanoparticles is investigated. Different nanocluster samples with diverse distributions of silver particles are characterized via SNOM in illumination and collection mode. This will lead to topographical and optical images that can be used as an input for SNOM simulations with the aim of estimating optical artifacts. Including tip, particles, and substrate, our finite-elementmethod (FEM) simulations are based on the realistic geometry. Correlating the high-precision SNOM measurement and the detailed simulation of a full image scan will enable us to draw conclusions regarding near-field enhancements caused by interacting particles. © 2015 SPIE.

Thin film solar cells already benefit from significant material and energy savings. By using photon management, the conversion efficiency and the power density can be enhanced further, including a reduction of material costs. In this work, micrometer-sized Cu(In,Ga)Se2 (CIGS) thin film solar cells were investigated under concentrated white light illumination (1-50×). The cell design is based on industrially standardized, lamellar shaped solar cells with monolithic interconnects (P-scribe). In order to characterize the shunt and serial resistance profiles and their impact on the device performance the cell width was reduced stepwise from 1900 to 200 μm and the P1-scribe thickness was varied between 45 and 320 μm. The results are compared to macroscopic solar cells in standard geometry and dot-shaped microcells with ring contacts. Under concentrated white light, the maximal conversion efficiency could be increased by more than 3.8% absolute for the lamellar microcells and more than 4.8% absolute in case of dot-shaped microcells compared to their initial values at 1 sun illumination. The power density could be raised by a factor of 51 and 70, respectively. But apparently, the optimum concentration level and the improvement in performance strongly depend on the chosen cell geometry, the used contact method and the electrical material properties. It turns out, that the widely used industrial thin film solar cell design pattern cannot simply be adapted to prepare micro-concentrator CIGS solar modules, without significant optimization. Based on the experimental and simulated results, modifications for the cell design are proposed. Copyright © 2015 John Wiley & Sons, Ltd.

Both metallic nanoparticles exhibiting plasmonic effects and dielectric nanoparticles coupling the light into resonant modes have shown successful applications to photovoltaics. On a larger scale, microconcentrator optics promise to enhance solar cell efficiency and to reduce material consumption. Here, we want to create a link between the concentrators on the nano- and on the microscale. From metallic nanospheres, we turn to dielectric ones and then look at increasing radii to approach the microscale. The lenses are investigated with respect to their interaction with light using three-dimensional simulations with the finite-element method. Resulting maps of local electric field distributions reveal the focusing behavior of the dielectric spheres. For larger lens sizes, ray tracing calculations, which give ray distributions in agreement with electric field intensities, can be applied. Calculations of back focal lengths in geometrical optics coincide with ray tracing results and allow insight into how the focal length can be tuned as a function of particle size, substrate refractive index, and the shape of the microlens. Despite the similarities we find for the nano- and the microlenses, integration into solar cells needs to be carefully adjusted, depending on the goals of material saving, concentration level, focal distance, and lens size. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE).

Efficient light management in optoelectronic devices requires nanosystems where high optical qualities coincide with suitable device integration. The requirement of chemical and electrical passivation for integrating nanostrutures in e.g.Thin film solar cells points towards the use of insulating and stable dielectric material, which however has to provide high scattering and near-fields as well. We investigate metal@dielectric core-shell nanoparticles and dielectric nanorods. Whereas core-shell nanoparticles can be simulated using Mie theory, nanorods of finite length are studied with the finite element method. We reveal that a metallic core within a thin dielectric shell can help to enhance scattering and near-field cross sections compared to a bare dielectric nanoparticle of the same radius. A dielectric nanorod has the benefit over a dielectric nanosphere in that it can generate much higher scattering cross sections and also give rise to a high near-field enhancement along its whole length. Electrical benefits of e.g. Ag@oxide nanoparticles in thin-film solar cells and ZnO nanorods in hybrid devices lie in reduction of recombination centers or close contact of the nanorod material with the surrounding organics, respectively. The optical benefit of dielectric shell material and elongated dielectric nanostructures is highlighted in this paper. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.

Phase transitions have been investigated for silver deposition onto In<inf>2</inf>S<inf>3</inf> precursor layers by spray chemical vapor deposition from a trimethylphosphine (hexafluoroacetylacetonato) silver (Ag(hfacac)(PMe<inf>3</inf>)) solution. The formation of Ag nanoparticles (Ag NPs) on top of the semiconductor layer set on concomitant with the formation of AgIn<inf>5</inf>S<inf>8</inf>. The increase of the diameter of Ag NPs was accompanied by the evolution of orthorhombic AgInS<inf>2</inf>. The formation of Ag<inf>2</inf>S at the interface between Ag NPs and the semiconductor layer was observed. Surface photovoltage spectroscopy indicated charge separation and electronic transitions in the ranges of corresponding band gaps. The phase transition approach is aimed to be applied for the formation of plasmonic nanostructures on top of extremely thin semiconducting layers. © 2015 Elsevier B.V.

We calculate the complex refractive index of inhomogeneous thin films using the transfer matrix method and reflection/transmission measurements. To this end we have developed a model for both the 3D distribution of inhomogeneities inside thin films and for light propagation through the inhomogeneities. The model involves splitting the light into contributions from the homogeneous section of the film (modelled coherently) and the inhomogeneous sections (modelled incoherently). Measurements of the film implied an isotropic inhomogeneity distribution, which was replicated in the simulation. The model for light propagation inside a film was implemented into a transfer matrix program allowing for the evaluation of the reflection and transmission of the thin film on a substrate. Using this result and experimental data for the reflection and transmission, the complex refractive index, n+ik, of an inhomogeneous CuInSe2 film was calculated. The resulting n and k were in much closer agreement to the n and k for a homogeneous CuInSe2 film than those for the standard transfer matrix approach applied to the data of the inhomogeneous sample. The n value at short wavelengths deviates from the homogeneous value suggesting a breakdown of the scalar scattering theory for short wavelengths. © 2014 IOP Publishing Ltd.

We use scanning near-field optical microscopy (SNOM) to characterize different plasmonic-nanoparticle situations with high spatial and spectral resolution in this comparative study. The near-field enhancement is measured with an aperture probe (Al coated glass fiber) and two CCD spectrometers for simultaneous detection of reflection and transmission. The images of transmission and reflection show a correlation to the topography. We present a new way to access the relative absorption and discuss the results with consideration of artifact influences. Near-field enhancements are deeper understood by imaging isolated particles. This near field will be compared to measurements of random-particle distributions. Therefore, we will show normalized reflection and transmission images of random structures that lay the foundation for an absolute interpretation of near-field images. The normalization considers both the far-field UV/VIS results and a reference image of the substrate. The near-field reflection of nanoparticle arrays shows an enhancement of 25 %. In view of specific applications, particle distributions implemented in two ways: as far-field scatters and as near field enhancing objects. © 2014 SPIE.

Metallic nanoparticles exhibiting plasmonic effects as well as dielectric nanoparticles coupling the light into resonant modes have both shown successful application to photovoltaics. On the larger scale, microconcentrator optics promise to enhance solar cell efficiency and reduce material consumption. Here we want to make the link between concentrators on the nano- And on the microscale. From metallic nanospheres we turn to dielectric ones and then look at increasing radii to approach concentrator optics on the mircoscale. The nano- And microlenses are investigated with respect to their interaction with light using 3D simulations with the finite element method. Resulting maps of local electric field distributions reveal the focusing behavior of the dielectric spheres. For larger lens sizes, ray tracing calculations can be applied which give ray distributions in agreement with areas of high electric field intensities. Calculations of back focal lengths using ray tracing coincide with results from geometrical optics simulations. They give us insight into how the focal length can be tuned as a function of particle size, but also depending on the substrate refractive index and the shape of the microlens. Turning from spherical to segment-shaped lenses allows us to approach the realistic case of microconcentrator optics and to draw conclusions about focus tuning and system design. Despite the similarities of focusing behavior we find for the nano- And the microlenses, the integration into solar cells needs to be carefully adjusted, depending on the ambition of material saving, concentration level, focal distance and lens size, all being closely related. © 2014 SPIE.

We investigate the influence of substrate and its temperature on the optical constants of CuIn1-xGaxSe2 (CIGSe) thin films using the transfer-matrix method. The optical constants of a CIGSe layer on top of a transparent conducting oxide (TCO) layer were calculated considering the realistic optical constants of the TCO layer after CIGSe deposition. It was found that TCO substrates could influence the optical constants of CIGSe layers and that the ITO (Sn doped In2O3) substrate had a greater impact than IMO (Mo doped In2O3) for the CIGSe (x = 0.4) film when compared to a reference on bare glass substrate. Additionally, the varied substrate temperatures did not impact the optical constants of CGSe (x = 1). For CIGSe (x = 0.4), the refractive index n stayed relatively independent although at low temperature the grain size was reduced and the Ga/(Ga+In) profile was altered compared to that at high temperature (610 °C). In contrast, the extinction coefficient k at low temperature showed higher absorption at longer wavelengths because of a lower minimum bandgap (E g,min) originating from reduced inter-diffusion of Ga-Se at a low substrate temperature. © 2014 IOP Publishing Ltd.

We theoretically compare the scattering and near field of nanoparticles from different types of materials, each characterized by specific optical properties that determine the interaction with light: metals with their free charge carriers giving rise to plasmon resonances, dielectrics showing zero absorption in wide wavelength ranges, and semiconductors combining the two beforehand mentioned properties plus a band gap. Our simulations are based on Mie theory and on full 3D calculations of Maxwell's equations with the finite element method. Scattering and absorption cross sections, their division into the different order electric and magnetic modes, electromagnetic near field distributions around the nanoparticles at various wavelengths as well as angular distributions of the scattered light were investigated. The combined information from these calculations will give guidelines for choosing adequate nanoparticles when aiming at certain scattering properties. With a special focus on the integration into thin film solar cells, we will evaluate our results. © 2014 Schmid et al.

Plasmonic absorption enhancement by metal nanoparticles strongly relies on the local electric field distributions generated by the nanoparticles. Therefore, here we study random assemblies of metal nanoparticles as they are widely considered for solar cell application with scanning near-field optical microscopy. A collective scattering behavior is observed despite a resolution on the particle size. We find variations in scattering intensity on a length scale several times larger than in the topography. FDTD (finite-difference time domain) simulations show the impact of irregularities and size variations on the scattering behavior. An understanding of the plasmonic scattering behavior at the nanometer scale will support the successful application of nanoparticles for absorption enhancement in thin-film solar cells. © 2013 IOP Publishing Ltd.

Ultra-thin solar cells on transparent back contacts constitute the basis for highly efficient tandem solar devices which can surpass the single cell efficiency limit. The material reduction related to ultra-thin high efficiency devices additionally lowers the price. Despite the fact that they are ultra-thin the absorbers still have to remain optically thick and therefore require adequate light management. A promising approach for enhanced absorption is plasmonic scattering from metal nanoparticles. In this paper we discuss the experimental incorporation of Ag nanoparticles in ultra-thin wide-gap chalcopyrite solar cells on transparent back contacts. A 6.9% efficient 500 nm Cu(In,Ga)S2 solar cell on In2O3:Mo (at this point without nanoparticles) is the starting point. For the predicted optimum design of including particles at the rear side the stability of the nanostructures integrated in the back contact is investigated in detail. As a first step towards proof-of-concept, absorption enhancement from the nanoparticles included in the complete solar cell is experimentally shown in optical properties. © 2012 Elsevier B.V.

To investigate the effect of surface roughness on the calculation of optical constants, e.g., the complex refractive index n + i k or (n, k) of CuIn1-xGaxSe2 (CIGSe) thin films, we took CuInSe2 (CISe) and CuGaSe2 (CGSe) as examples and applied the "Modified Transfer-Matrix (MTM)" method to calculate optical constants with considering the effect of scattering due to surface roughness. Compared to the Transfer-Matrix (TM) method without considering surface roughness, it was revealed that the MTM method could improve the accuracy of calculation. The calculated refractive index values from the MTM method increase by 6.89% for CISe and 2.59% for CGSe in contrast to those from the TM method. In addition, bromine solution was confirmed via Scanning Electron Microscopy and Atomic Force Microscopy to be able to reduce the surface roughness. Calculated results from smoothened samples showed that the accuracy of calculated optical constants was further improved. Finally, optical constants calculated by the MTM method were compared to those from smoothened samples, validating that the MTM method could eliminate the influence of surface roughness on the calculation of optical constants more effectively for CGSe with low surface roughness than for CISe with high surface roughness. © 2013 AIP Publishing LLC.

Plasmonic scattering from metal nanostructures presents a promising concept for improving the conversion efficiency of solar cells. The determination of optimal nanostructures and their position within the solar cell is crucial to boost the efficiency. Therefore we established a one-dimensional optical model combining plasmonic scattering and thin-film optics to simulate optical properties of thin-film solar cells including metal nanoparticles. Scattering models based on dipole oscillations and Mie theory are presented and their integration in thin-film semi-coherent optical descriptions is explained. A plasmonic layer is introduced in the thin-film structure to simulate scattering properties as well as parasitic absorption in the metal nanoparticles. A proof of modeling concept is given for the case of metal-island grown silver nanoparticles on glass and ZnO:Al/glass substrates. Using simulations a promising application of the nanoparticle integration is shown for the case of CuGaSe2 solar cells. © 2011 IOP Publishing Ltd.