Prof. Dr. Roland Schmechel

Nanostructural Engineering
University of Duisburg-Essen

Contact

Hub
  • A Stochastic Large-Signal Model for Printed High-Frequency Rectifiers Used for Efficient Generation of Higher Harmonics
    Neumann, K. and Kuhnel, L. and Langer, F. and Rennings, A. and Benson, N. and Schmechel, R. and Erni, D.
    IEEE Transactions on Microwave Theory and Techniques 68 (2020)
    This article investigates the stochastic Schottky barrier variations of printed distributed Schottky diodes consisting of a self-assembled arrangement of crystalline silicon microcones onto a metal layer. The microcone formation emerges from an inkjet printed Si nanoparticle film after laser sintering, yielding a Schottky diode when a corresponding top metallization is applied. The I - V characteristic and the voltage-dependent impedance of such diode is measured. By using the simulation software Advanced Design System (ADS), we develop a new scalable circuit model consisting of many different elementary diodes, which can explain the measured behavior. The elementary microcone diodes differ electrically in their barrier height, which is modeled as a stochastic process with a Gaussian distribution. A comparison between this model and a single diode model based on the thermionic field emission theory is conducted. We show that the distributed model outperforms the single-diode model in every regard and allows a prediction of the power levels of the harmonic frequency generation. Through more in-depth research, we find that a distributed barrier height leads to a smoother I - V curve, which in turn can lead to higher second and third harmonic power levels. By adjusting the barrier height distribution, the desired harmonics can be increased. © 1963-2012 IEEE.
    view abstract10.1109/TMTT.2020.2990561
  • Influence of the cathode microstructure on the stability of inverted planar perovskite solar cells
    Sirotinskaya, S. and Schmechel, R. and Benson, N.
    RSC Advances 10 (2020)
    One of the main challenges for perovskite solar cells (PSC) is their environmental stability, as oxygen and water induced aging may result in mobile decomposition compounds, which can enhance the recombination rate and react with charge carrier extraction layers or the contact metallization. In this contribution the importance of the microstructure of the contact metallization on the environmental cell stability is investigated. For this purpose, the storage stability of inverted planar methylammonium lead iodide (MAPI)-based perovskite solar cells without encapsulation is tested, using the metals aluminum (Al), silver (Ag), gold (Au) and nickel (Ni) as representative cathode materials. For this study, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis of the different electrodes as well as the perovskite is correlated with PSC device current-voltage (J-V) and impedance measurements. Our findings substantiate that the metal microstructure has a significant influence on the PSC aging properties. While a strong perovskite decomposition and iodide diffusion to the contacts were detected for devices using Al, Ag or Au cathodes with a polycrystalline microstructure, these effects were strongly reduced when Ni metallization was employed, where a nanocrystalline microstructure was exhibited under the chosen process conditions. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/d0ra00195c
  • Ultra-fast measurement circuit for transient space charge limited current in organic semiconductor thin films
    Rojek, K. and Schmechel, R. and Benson, N.
    Measurement Science and Technology 31 (2020)
    The charge carrier mobility is a crucial parameter determining the device performance for numerous different semiconductor applications. Consequently, an accurate measurement of this quantity is crucial. For this purpose, the transient space charge limited current (SCLC) method is commonly applied and is preferable over, for example, Hall or field effect measurements, as the analyzed current direction is in line with typical device architectures. For the transient SCLC method, a voltage step is applied and the transit time of injected charge carriers is determined using displacement currents. Consequently, the difficulty of this method is the use of an adequate RC time constant for the sample charging, as it needs to be much shorter than the transit time. This parameter generally limits the application of transient SCLC strongly, in terms of obtainable charge carrier mobility or minimum required film thickness. Here, we demonstrate a measurement circuit with a low RC time constant, which works in a wide current range (1 ϵA-0.5 A) and thus allows for significant flexibility in terms of minimum film thickness or detectable charge carrier mobility. The circuit is fast enough to measure, for example, charge carrier mobilities of up to 10-4 cm2 Vs-1 for a 76 nm thick 4,4',4"-Tris[phenyl(m-tolyl)amino]triphenylamine (MTDATA) layer, without using limited bridge circuitry. For this purpose, a capacitor coupled fast transistor switch generates a voltage step to avoid voltage oscillations and a fast operational amplifier is used for amplification of the voltage over a variable measurement resistor. We demonstrate the circuit working principle by measuring benchmarked MTDATA diodes and discuss its range of application. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6501/ab3b2d
  • Analysis of stochastic Schottky barrier variations within printed high frequency rectifiers for harmonics generation
    Neumann, K. and Kuehnel, L. and Langer, F. and Rennings, A. and Benson, N. and Schmechel, R. and Erni, D.
    IMWS-AMP 2019 - 2019 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (2019)
    In this paper we investigate the stochastic Schottky barrier variations of printed distributed Schottky diodes consisting of a self-assembled arrangement of crystalline silicon microcones onto a metal layer. The microcone formation emerges from an inkjet printed Si nanoparticle film after laser sintering yielding a Schottky diode when a corresponding top metallization is applied. The elementary microcone diodes differ electrically in their barrier height, which is modelled as a gaussian distribution. The circuit simulation software Advanced Design System (ADS) is used to analyze the rectification abilities of the overall structure. The results show that a distributed barrier height leads to a smoother IV-characteristic, which can also be interpreted as an aggregated diode with a smaller turn on voltage. A Fourier analysis of the rectified time-domain signal shows an amplification of the frequency components up to the third harmonic in comparison to a non distributed single diode. © 2019 IEEE.
    view abstract10.1109/IMWS-AMP.2019.8880082
  • Experimental evidence for the separation of thermally excited bipolar charge carries within a p-n junction: A new approach to thermoelectric materials and generators
    MacUlewicz, F. and Wagner, T. and Arzi, K. and Hartmann, N. and Weimann, N. and Schmechel, R.
    Journal of Applied Physics 125 (2019)
    Common thermoelectric generators are based on the Seebeck effect, which describes the thermal diffusion of majority charge carriers within a temperature gradient in a solid. It is a unipolar transport phenomenon that gets suppressed if bipolar charge carriers occur. Here, we demonstrate by experiments that thermally excited bipolar charge carriers can be separated by the built-in field without external bias within a p-n junction. Such a phenomenon has been predicted theoretically before but was never proven experimentally. In the experiment, a nominal intrinsic silicon wafer (doping concentration less than 1013cm-3) was inserted in a p-i-n structure. It could be shown that electric power can be extracted from the space charge region (i-region), while the conventional thermoelectric contribution from the p-and n-regions is suppressed by short-circuiting. While the measured and simulated overall Seebeck effect of intrinsic silicon predicts a zero crossing of output power with increasing hot-side temperature due to a transition from p-type to n-type transport, the measured and simulated output power of the p-i-n structure increases monotonically with increasing hot-side temperature, indicating clearly the different nature of both mechanisms. © 2019 Author(s).
    view abstract10.1063/1.5081998
  • MIS-TSC: A combination of the thermally stimulated current method and a metal-insulator-semiconductor device for unipolar trap spectroscopy
    Rojek, K. and Schmechel, R. and Benson, N.
    Applied Physics Letters 114 (2019)
    To determine the density of states distribution of traps within a semiconductor, the thermally stimulated current (TSC) method is often applied. However, the bipolar nature of the typical device structure does not allow for strict unipolar operation, and therefore the method does not allow for the separate evaluation of electron and hole traps. The recombination between electrons and holes makes the interpretation of the data difficult, which becomes an essential drawback of this method. To address these issues, we propose the use of a metal insulator semiconductor (MIS) device structure for TSC measurements, which can be operated strictly unipolar by the sign of the applied voltage during the charging process. Thus, the problem of recombination and bipolar contribution to the measurement signal is avoided. As an additional benefit, the MIS device structure typically results in very low leakage currents, and thus a low noise level for the measurement. This permits precise measurements even below 1 pA, and consequently increases the resolution of the method. This aspect is especially important for fractional TSC, as the measurement time is long and the current low when compared to the envelope measurement. Here, we demonstrate the basic principle of this TSC approach, which we name MIS-TSC, using the well-studied organic semiconductor P3HT as a benchmark. © 2019 Author(s).
    view abstract10.1063/1.5090947
  • Spatially resolved investigation of the defect states in methylammonium lead iodide perovskite bicrystals
    Sirotinskaya, S. and Fettkenhauer, C. and Okada, D. and Yamamoto, Y. and Lupascu, D.C. and Schmechel, R. and Benson, N.
    Journal of Materials Chemistry C 7 (2019)
    Organic-inorganic halide perovskites are one of the most promising novel materials for photovoltaic applications. One of the most common and well-researched compounds in this material class is methylammonium lead iodide (MAPI). However, the formation of different kinds of defects in the polycrystalline MAPI thin films and their influence on the electrical performance of solar cells is still a matter of debate. In this work, we use single MAPI crystals grown by inverse temperature crystallization as a model system for the systematic experimental evaluation of defects as suggested by current theoretical work on MAPI thin films. The macroscopic nature of these crystallites enables an experimental approach that allows the separate evaluation of the grain boundary and crystallite bulk. For this purpose, we have employed a combination of μ-PL and μ-XPS measurements to probe the defect types suggested by the literature to determine the most likely defect types in the MAPI crystallites/thin film areas. © The Royal Society of Chemistry 2019.
    view abstract10.1039/c8tc06622a
  • Thermoelectrics versus thermophotovoltaics: Two approaches to convert heat fluxes into electricity
    Okanimba Tedah, I.A. and Maculewicz, F. and Wolf, D.E. and Schmechel, R.
    Journal of Physics D: Applied Physics 52 (2019)
    Direct conversion of heat fluxes into electricity is usually done by thermoelectric generators (TEGs). For hot-side temperatures above 1000 K, thermal radiation carries a high energy density, comparable with the energy density extracted from TEGs and therefore a direct conversion of thermal radiation into electricity, named thermophotovoltaics (TPV), would also be an option. This paper compares both methods with respect to efficiency and extractable power density. The physical limits are estimated under simplified but realistic boundary conditions. For TPV the radiative detailed balance limit under black body radiation, which was calculated for different hot-side temperatures from 310 K to 3000 K for an optimized bandgap of the applied material was used. But, since very narrow bandgaps leading to strong non-radiative recombination mechanisms, the bandgap was limited to Eg ≥ 0.5 eV. The effect of suppressing sub-bandgap radiation as well as an enhanced radiation density in the nearfield (near-field TPV) were also included. The TEG efficiency and power density was calculated under thermal matching conditions with a heat transfer coefficient of Kcont = 250 W m-2 K-1 and an average device ZT̄ = 1. The results are compared with experimental data for TPV and TEGs from literature. It is shown, that up to 600 K hot-side temperature TEGs are superior to TPV, due to a significant higher power density. Above 1000 K TPV profits from higher efficiency by a similar power density. But above 2000 K TPV suffers from cell heating. The range 600 K to 1000 K is currently captured by high temperature thermoelectrics, but near-field TPV (NF-TPV) has good chances to compete with TEGs in this temperature range in the future. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/ab1833
  • Efficient p-n junction-based thermoelectric generator that can operate at extreme temperature conditions
    Chavez, R. and Angst, S. and Hall, J. and Maculewicz, F. and Stoetzel, J. and Wiggers, H. and Thanh Hung, L. and Van Nong, N. and Pryds, N. and Span, G. and Wolf, D.E. and Schmechel, R. and Schierning, G.
    Journal of Physics D: Applied Physics 51 (2018)
    In many industrial processes, a large proportion of energy is lost in the form of heat. Thermoelectric generators can convert this waste heat into electricity by means of the Seebeck effect. However, the use of thermoelectric generators in practical applications on an industrial scale is limited in part because electrical, thermal, and mechanical bonding contacts between the semiconductor materials and the metal electrodes in current designs are not capable of withstanding thermal-mechanical stress and alloying of the metal-semiconductor interface when exposed to the high temperatures occurring in many real-world applications. Here we demonstrate a concept for thermoelectric generators that can address this issue by replacing the metallization and electrode bonding on the hot side of the device by a p-n junction between the two semiconductor materials, making the device robust against temperature induced failure. In our proof-of-principle demonstration, a p-n junction device made from nanocrystalline silicon is at least comparable in its efficiency and power output to conventional devices of the same material and fabrication process, but with the advantage of sustaining high hot side temperatures and oxidative atmosphere. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aa9b6a
  • Anisotropic layered Bi2 Te3 -In2 Te3 composites: Control of interface density for tuning of thermoelectric properties
    Liu, D. and Li, X. and De Castro Borlido, P.M. and Botti, S. and Schmechel, R. and Rettenmayr, M.
    Scientific Reports 7 (2017)
    Layered (Bi1-xInx) 2 Te3 -In2 Te3 (x = 0.075) composites of pronounced anisotropy in structure and thermoelectric properties were produced by zone melting and subsequent coherent precipitation of In2 Te3 from a (Bi1-xInx) 2 Te3 (x > 0.075) matrix. Employing solid state phase transformation, the Bi2 Te3 /In2 Te3 interface density was tuned by modifying the driving force for In2 Te3 precipitation. The structure-property relationship in this strongly anisotropic material is characterized thoroughly and systematically for the first time. Unexpectedly, with increasing Bi2 Te3 /In2 Te3 interface density, an increase in electrical conductivity and a decrease in the absolute Seebeck coefficient were found. This is likely to be due to electron accumulation layers at the Bi2 Te3 /In2 Te3 interfaces and the interplay of bipolar transport in Bi2 Te3. Significantly improved thermoelectric properties of Bi2 Te3 -In2 Te3 composites as compared to the single phase (Bi1-xInx) 2 Te3 solid solution are obtained. © The Author(s) 2017.
    view abstract10.1038/srep43611
  • Film forming properties of silicon nanoparticles on SixNy coated substrates during excimer laser annealing
    Caninenberg, M. and Kiesler, D. and Benson, N. and Schmechel, R.
    Optics and Laser Technology 90 (2017)
    In this article we investigate the film forming properties of excimer laser annealed silicon nanoparticles on non-silicon substrates. In contrast to their film forming properties on oxide free silicon substrates, the nanoparticle thin film tends to dewet and form a porous µ-structure on the silicon nitrite covered glass model substrates considered for our investigation. This is quantified using a SEM study in conjunction with image processing software, in order to evaluate the µ-structure size and inter µ-structure distance in dependence of the laser energy density. To generalize our results, the film forming process is described using a COMSOL Multiphysics ® fluid dynamics model, which solves the Navier Stokes equation for incompressible Newtonian fluids. To account for the porous nanoparticle thin film structure in the simulation, an effective medium approach is used by applying a conservative level set one phase method to our mesh. This effort allows us to predict the Si melt film formation ranging from a porous Si µ-structure to a compact 100% density Si thin film in dependence of the substrate / thin film interaction, as well as the laser energy used for the nanoparticle processing. © 2016 Elsevier Ltd
    view abstract10.1016/j.optlastec.2016.11.010
  • Microstructure and thermoelectric properties of Si-WSi2 nanocomposites
    Stoetzel, J. and Schneider, T. and Mueller, M.M. and Kleebe, H.-J. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Acta Materialia 125 (2017)
    Nanocomposites of n-doped Si/WSi2 were prepared and morphologically and thermoelectrically investigated. The composites were densified by spark-plasma-sintering of doped Si nanoparticles with WSi2 nanoinclusions. The nanoparticles were synthesized in a gas-phase process. The microstructure of the bulk nanocomposite shows an inhomogeneous distribution of the WSi2 nanoinclusions in form of WSi2-rich and -depleted regions. This inhomogeneity is not present in the starting material and is assigned to a self-organizing process during sintering. The inhomogeneities are in the micrometer range and may act as scattering centers for long-wavelength phonons. The WSi2 nanoinclusions grow during sintering from originally 3–7 nm up to 30–143 nm depending on the total W content and might act as scattering centers for the medium wavelength range of phonons. Further, the growth of Si grains is suppressed by the WSi2 inclusions, which leads to an enhanced grain boundary density. Adding 1 at% W reduces lattice thermal conductivity by almost 35% within the temperature range from 300 K to 1250 K compared to pure, nanocrystalline silicon (doped). By addition of 6 at% W a reduction of 54% in lattice thermal conductivity is achieved. Although little amounts of W slightly reduce the power factor an enhancement of the thermoelectric figure of merit of 50% at 1250 K compared to a tungsten-free reference was realized. © 2016
    view abstract10.1016/j.actamat.2016.11.069
  • Rodlike Tetracene Derivatives
    Roth, M. and Ahles, M. and Gawrisch, C. and Schwalm, T. and Schmechel, R. and Melzer, C. and von Seggern, H. and Rehahn, M.
    Chemistry - A European Journal 23 (2017)
    Efficient and versatile synthetic access to rodlike tetracene derivatives was developed by means of Diels–Alder cycloaddition, halogenation, halogen–metal exchange, and transition metal mediated coupling reactions. Herein, the synthesis and structural, electrical, and charge-transport properties of three of the resulting materials, namely, 2-(tetracen-2-yl)tetracene, 1,4-bis(2-tetracenyl)benzene, and 2,5-bis(2-tetracenyl)thiophene, are presented. Good crystallization behavior on SiO2 substrates, narrowing of the bandgap by 0.2 eV, and a decrease of the ionization potential of more than 0.5 eV compared to tetracene were observed. Charge-carrier field-effect mobilities on the order of 10−1 cm2 V−1 s−1, on/off ratios of 105, and threshold voltages Vth<15 V were found in thin-film organic field-effect transistors prepared by standard high-vacuum deposition techniques. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201702382
  • Soluble metal oxo alkoxide inks with advanced rheological properties for inkjet-printed thin-film transistors
    Meyer, S. and Pham, D.V. and Merkulov, S. and Weber, D. and Merkulov, A. and Benson, N. and Schmechel, R.
    ACS Applied Materials and Interfaces 9 (2017)
    Semiconductor inks containing an indium-based oxo alkoxide precursor material were optimized regarding rheology requirements for a commercial 10 pL inkjet printhead. The rheological stability is evaluated by measuring the dynamic viscosity of the formulations for 12 h with a constant shear rate stress under ambient conditions. It is believed that the observed superior stability of the inks is the result of effectively suppressing the hydrolysis and condensation reaction between the metal oxo alkoxide precursor complex and atmospheric water. This can be attributed to a strong precursor coordination and the resulting reduction in ligand exchange dynamics of the solvent tetrahydrofurfuryl alcohol which is used as the main solvent in the formulations. It is also shown that with a proper selection of cosolvents, having high polar Hansen solubility parameter values, the inks drop formation properties and wettability can be finetuned by maintaining the inks rheological stability. Good drop jetting performance without satellite formation and high drop velocities of 8.25 m/s were found with the support of dimensionless numbers and printability windows. By printing single 10 pL ink dots onto short channel indium-tin-oxide electrodes, In2O3 calcination at 350 °C and a solution-processed back-channel protection, high average saturation mobility of approximately 10 cm2/(V s) are demonstrated in a bottom-contact coplanar thinfilm transistor device structure. © 2017 American Chemical Society.
    view abstract10.1021/acsami.6b12586
  • Anisotropic n-Type Bi2Te3-In2Te3 Thermoelectric Material Produced by Seeding Zone Melting and Solid State Transformation
    Liu, D. and Stötzel, J. and Seyring, M. and Drüe, M. and Li, X. and Schmechel, R. and Rettenmayr, M.
    Crystal Growth and Design 16 (2016)
    Seeding zone melting is applied to produce bulk Bi1.625In0.375Te3 with 7.5 atom % In in solid solution. The concentration distribution is markedly homogeneous and exhibits pronounced anisotropic electrical and thermal conductivity. Subsequent precipitation from the solid solution leads to the formation of a highly anisotropic composite thermoelectric material consisting of aligned microscaled Bi2Te3 and extended micro- to nanoscaled In2Te3 plates. By the precipitation, an increase of zT by a factor of 6 compared with the parent supersaturated solid solution crystal is achieved. This is attributed to the combination of a decrease of In concentration from 7.5 to 3 atom % in the Bi2Te3 layer and an increasing interface density due to the precipitation of In2Te3. The Bi2Te3/In2Te3 interface is determined as coherent, and the crystallographic orientation between the two phases is determined as «2¯11»In2Te3//«11¯00»Bi2Te3, {111}In2Te3//{0001}Bi2Te3. © 2015 American Chemical Society.
    view abstract10.1021/acs.cgd.5b01015
  • Modelling of electron beam induced nanowire attraction
    Bitzer, L.A. and Speich, C. and Schäfer, D. and Erni, D. and Prost, W. and Tegude, F.J. and Benson, N. and Schmechel, R.
    Journal of Applied Physics 119 (2016)
    Scanning electron microscope (SEM) induced nanowire (NW) attraction or bundling is a well known effect, which is mainly ascribed to structural or material dependent properties. However, there have also been recent reports of electron beam induced nanowire bending by SEM imaging, which is not fully explained by the current models, especially when considering the electro-dynamic interaction between NWs. In this article, we contribute to the understanding of this phenomenon, by introducing an electro-dynamic model based on capacitor and Lorentz force interaction, where the active NW bending is stimulated by an electromagnetic force between individual wires. The model includes geometrical, electrical, and mechanical NW parameters, as well as the influence of the electron beam source parameters and is validated using in-situ observations of electron beam induced GaAs nanowire (NW) bending by SEM imaging. © 2016 AIP Publishing LLC.
    view abstract10.1063/1.4945674
  • Silicon-based nanocomposites for thermoelectric application
    Schierning, G. and Stoetzel, J. and Chavez, R. and Kessler, V. and Hall, J. and Schmechel, R. and Schneider, T. and Petermann, N. and Wiggers, H. and Angst, S. and Wolf, D.E. and Stoib, B. and Greppmair, A. and Stutzmann, M. and Brandt, M.S.
    Physica Status Solidi (A) Applications and Materials Science 213 (2016)
    Here we present the realization of efficient and sustainable silicon-based thermoelectric materials from nanoparticles. We employ a gas phase synthesis for the nanoparticles which is capable of producing doped silicon (Si) nanoparticles, doped alloy nanoparticles of silicon and germanium (Ge), SixGe1-x, and doped composites of Si nanoparticles with embedded metal silicide precipitation phases. Hence, the so-called "nanoparticle in alloy" approach, theoretically proposed in the literature, forms a guideline for the material development. For bulk samples, a current-activated pressure-assisted densification process of the nanoparticles was optimized in order to obtain the desired microstructure. For thin films, a laser annealing process was developed. Thermoelectric transport properties were characterized on nanocrystalline bulk samples and laser-sintered-thin films. Devices were produced from nanocrystalline bulk silicon in the form of p-n junction thermoelectric generators, and their electrical output data were measured up to hot side temperatures of 750°C. In order to get a deeper insight into thermoelectric properties and structure forming processes, a 3D-Onsager network model was developed. This model was extended further to study the p-n junction thermoelectric generator and understand the fundamental working principle of this novel device architecture. Gas phase synthesis of composite nanoparticles; nanocrystalline bulk with optimized composite microstructure; laser-annealed thin film. The authors fabricated thermoelectric nanomaterials from doped silicon and silicon and germanium alloy nanoparticles, as well as composites of Si nanoparticles with embedded metal silicide nanoparticles. Processing was performed applying a current-activated pressure-assisted densification process for bulk samples and a laser annealing process for thin film samples. Devices were produced in the form of pn junction thermoelectric generators. A 3D-Onsager network model was used to understand the fundamental working principle of this novel device architecture. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201532602
  • Super-resolution for scanning light stimulation systems
    Bitzer, L.A. and Neumann, K. and Benson, N. and Schmechel, R.
    Review of Scientific Instruments 87 (2016)
    Super-resolution (SR) is a technique used in digital image processing to overcome the resolution limitation of imaging systems. In this process, a single high resolution image is reconstructed from multiple low resolution images. SR is commonly used for CCD and CMOS (Complementary Metal-Oxide-Semiconductor) sensor images, as well as for medical applications, e.g., magnetic resonance imaging. Here, we demonstrate that super-resolution can be applied with scanning light stimulation (LS) systems, which are common to obtain space-resolved electro-optical parameters of a sample. For our purposes, the Projection Onto Convex Sets (POCS) was chosen and modified to suit the needs of LS systems. To demonstrate the SR adaption, an Optical Beam Induced Current (OBIC) LS system was used. The POCS algorithm was optimized by means of OBIC short circuit current measurements on a multicrystalline solar cell, resulting in a mean square error reduction of up to 61% and improved image quality. © 2016 Author(s).
    view abstract10.1063/1.4961748
  • Two-Step Annealing Leading to Refined Bi2Te3-In2Te3 Lamellar Structures for Tuning of Thermoelectric Properties
    Liu, D. and Li, X. and Schmechel, R. and Rettenmayr, M.
    Journal of Electronic Materials 45 (2016)
    A two-step annealing process was applied to control the morphology of Bi2Te3-In2Te3 composite materials via precipitation of In2Te3 from supersaturated (Bi,In)2Te3. Finer lamellae were obtained via two-step as compared with single-step isothermal annealing. The microstructure was optimized by exploiting thermodynamic and kinetic effects during nucleation and growth of In2Te3. The relationship between the morphologies and thermoelectric properties was analyzed. With preannealing at a lower temperature, refined morphologies lead to an enhanced power factor and zT in the temperature range from room temperature to ∼100°C. The enhancement is mainly caused by an increased Seebeck coefficient, most probably due to energy-dependent scattering processes. However, the thermal conductivity is dominated by bipolar thermal transport that compensates the low lattice thermal conductivity completely. © 2015, The Minerals, Metals & Materials Society.
    view abstract10.1007/s11664-015-4151-4
  • Concepts for medium-high to high temperature thermoelectric heat-to-electricity conversion: a review of selected materials and basic considerations of module design
    Schierning, G. and Chavez, R. and Schmechel, R. and Balke, B. and Rogl, G. and Rogl, P.
    Translational Materials Research 2 (2015)
    Within the last decade, novel materials concepts and nanotechnology have resulted in a great increase of the conversion efficiency of thermoelectric materials. Despite this, a mass market for thermoelectric heat-to-electricity conversion is yet to be opened up. One reason for this is that the transfer of the lab records into fabrication techniques which enable thermoelectric generator modules is very challenging. By closing the gap between record lab values and modules, broad industrial applications may become feasible. In this review, we compare three classes of materials, all designed for medium-high to high temperature applications in the field of waste heat recovery: skutterudites, half-Heusler compounds, and silicon-based materials. Common to all three classes of thermoelectric materials is that they are built from elements which are neither scarce (e.g. tellurium) nor toxic (e.g. lead) and therefore may be the foundation of a sustainable technology. Further, these materials can provide both, n-type and p-type materials with similar performance and thermomechanical properties, such that the fabrication of thermoelectric generator modules has already been successfully demonstrated. The fabrication processes of the presented materials are scalable or have already been scaled up. The availability of thermoelectric materials is only one important aspect for the development of thermoelectric generator modules and heat conversion systems based on this technology. The design and configuration of the thermoelectric generator modules is similarly important. Hence, basic considerations of module configuration and different fundamental layouts of the thermoelectric heat-to-electricity conversion system are discussed within an additional chapter of this review.
    view abstract10.1088/2053-1613/2/2/025001
  • Sample temperature profile during the excimer laser annealing of silicon nanoparticles
    Caninenberg, M. and Verheyen, E. and Kiesler, D. and Stoib, B. and Brandt, M.S. and Benson, N. and Schmechel, R.
    Optics and Laser Technology 74 (2015)
    Based on the heat diffusion equation we describe the temperature profile of a silicon nanoparticle thin film on silicon during excimer laser annealing using COMSOL Multiphysics. For this purpose system specific material parameters are determined such as the silicon nanoparticle melting point at 1683 K, the surface reflectivity at 248 nm of 20% and the nanoparticle thermal conductivity between 0.3 and 1.2 W/m K. To validate our model, the simulation results are compared to experimental data obtained by Raman spectroscopy, SEM microscopy and electrochemical capacitance-voltage measurements (ECV). The experimental data are in good agreement with our theoretical findings and support the validity of the model. © 2015, Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.optlastec.2015.05.020
  • Scanning light stimulation system with active focus correction at μm resolution for PV applications
    Bitzer, L.A. and Elagin, M. and Semtsiv, M.P. and Masselink, W.T. and Benson, N. and Schmechel, R.
    IEEE Journal of Photovoltaics 5 (2015)
    Recently, we introduced a scanning light stimulation system with an automated focus correction. The method is however resolution limited by a CMOS sensor used to focus the light beam. Here, to achieve a higher resolution, we enhanced the light beam focusing process by combining it with an edge detection technique. By this modification, the system resolution is improved down to a 1/e2 -diameter of 4.3 μm as demonstrated using an example measurement on a multicrystalline solar cell. Furthermore, it is shown that the obtained resolution is mainly limited by height variations of the positioning system, and methods to compensate these limitations are discussed using example measurements on a quantum well solar cell. © 2011-2012 IEEE.
    view abstract10.1109/JPHOTOV.2015.2392933
  • Silicon-based nanocomposites for thermoelectric high temperature waste heat recovery
    Schierning, G. and Wiggers, H. and Schmechel, R.
    ECS Transactions 69 (2015)
    Thermoelectric generators can recover waste heat from high temperature heat sources. Given a scalable and affordable technology, this may be part of a future energy mix. For this, suitable thermoelectric converter materials need to be indentified and their efficiency improved. Besides, thermoelectric generators may also be further developed for high temperature applications. We here present results on silicon-based nanocomposites and thermoelectric generators which can meet these criteria. © The Electrochemical Society.
    view abstract10.1149/06909.0003ecst
  • Thermoelectric properties of pulsed current sintered nanocrystalline Al-doped ZnO by chemical vapour synthesis
    Gautam, D. and Engenhorst, M. and Schilling, C. and Schierning, G. and Schmechel, R. and Winterer, M.
    Journal of Materials Chemistry A 3 (2015)
    ZnO is a promising n-type oxide thermoelectric material, which is stable in air at elevated temperatures. In the present study, we report the bottom-up approach to create Al-doped ZnO nanocomposites from nanopowders, which are prepared by chemical vapour synthesis. With our synthesis route, we are able to create highly doped Al-containing ZnO nanocomposites that exhibit bulk-like electrical conductivity. Moreover, the impact of the microstructure of the nanocomposites on their thermal conductivity is enormous, with a value of 1.0 W m-1 K-1 for 1% Al-ZnO at room temperature, which is one of the lowest values reported, to date, on ZnO nanocomposites. The optimization of the Al-doping and microstructure with respect to the transport properties of bulk Al-ZnO nanocomposites leads to a zT value of about 0.24 at 950 K, underlining the potential of our technique. This journal is © The Royal Society of Chemistry 2015.
    view abstract10.1039/c4ta04355c
  • Thermoelectric transport properties of boron-doped nanocrystalline diamond foils
    Engenhorst, M. and Fecher, J. and Notthoff, C. and Schierning, G. and Schmechel, R. and Rosiwal, S.M.
    Carbon 81 (2015)
    Natural diamond is known for its outstanding thermal conductivity and electrical insulation. However, synthetic production allows for doping and tailoring microstructural and transport properties. Despite some motivation in the literature and the ongoing search for abundant and non-toxic thermoelectric materials, the first experimental study on a set of eight substrate-free boron-doped nanocrystalline diamond foils is presented herein. All transport coefficients were determined in the same direction within the same foils over a broad temperature range up to 900 °C. It is found that nanostructuring reduces the thermal conductivity by two orders of magnitude, but the mobility decreases significantly to around 1 cm2 V-1 s-1, too. Although degenerate transport can be concluded from the temperature dependence of the Seebeck coefficient, charge carriers notably scatter at grain boundaries where sp2-carbon modifications and amorphous boron-rich phases form during synthesis. A detailed analysis of doping efficiency yields an acceptor fraction of only 8-18 at%, meaning that during synthesis excess boron thermodynamically prefers electrically inactive sites. Decent power factors above 10-4Wm-1 K-2 at 900 °C are found despite the low mobility, and a Jonker-type analysis grants a deeper insight into this issue. Together with the high thermal conductivity, the thermoelectric figure of merit zT does not exceed 0.01 at 900 °C. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2014.10.002
  • Thermoelectrics from silicon nanoparticles: the influence of native oxide
    Petermann, N. and Stötzel, J. and Stein, N. and Kessler, V. and Wiggers, H. and Theissmann, R. and Schierning, G. and Schmechel, R.
    European Physical Journal B 88 (2015)
    Thermoelectric materials were synthesized by current-assisted sintering of doped silicon nanoparticles produced in a microwave-plasma reactor. Due to their affinity to oxygen, the nanoparticles start to oxidize when handled in air and even a thin surface layer of native silicon oxide leads to a significant increase in the oxide volume ratio. This results in a considerable incorporation of oxygen into the sintered pellets, thus affecting the thermoelectric performance. To investigate the necessity of inert handling of the raw materials, the thermoelectric transport properties of sintered nanocrystalline silicon samples were characterized with respect to their oxygen content. An innovative method allowing a quantitative silicon oxide analysis by means of electron microscopy was applied: the contrast between areas of high and low electrical conductivity was attributed to the silicon matrix and silicon oxide precipitates, respectively. Thermoelectric characterization revealed that both, electron mobility and thermal conductivity decrease with increasing silicon oxide content. A maximum figure of merit with zT = 0.45 at 950 °C was achieved for samples with a silicon oxide mass fraction of 9.5 and 21.4% while the sample with more than 25% of oxygen clearly indicates a negative impact of the oxygen on the electron mobility. © 2015, EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
    view abstract10.1140/epjb/e2015-50594-7
  • A facile solution-doping method to improve a low-temperature zinc oxide precursor: Towards low-cost electronics on plastic foil
    Weber, D. and Botnaraş, S. and Pham, D.V. and Merkulov, A. and Steiger, J. and Schmechel, R. and De Cola, L.
    Advanced Functional Materials 24 (2014)
    Optimization of thin-film transistors performance is usually accompanied by an increase of the process temperature. This work presents a method to raise the field effect mobility by a factor of 3 without a change of the process parameters. The modification involves a solution doping process where an ammine zinc complex is formed in the presence of metal ions of the 13th group, namely gallium and indium. Morphological studies, including scanning electron microscopy and atomic force microscopy, reveal the difference among the resulting films. Moreover, X-ray diffraction results show that the doping affects the preferred orientation of the zinc oxide crystals in the resulting film. The electrical properties vary distinctly and are best for a solution doped with both gallium and indium. With a double-layer system the performance of this new precursor exceeds field effect mobility values of 1 cm2 V-1 s-1 after a maximum process temperature of 160 °C. The performance of ZnO-based field-effect transistors is improved by a simple solution-doping procedure using ions of the 13th group. The method has a strong influence on the film morphology and orientation of the crystallites. This leads to field effect mobility values comparable to amorphous silicon. The low conversion temperature allows the fabrication on flexible substrates. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201303461
  • Fabrication of high-temperature-stable thermoelectric generator modules based on nanocrystalline silicon
    Kessler, V. and Dehnen, M. and Chavez, R. and Engenhorst, M. and Stoetzel, J. and Petermann, N. and Hesse, K. and Huelser, T. and Spree, M. and Stiewe, C. and Ziolkowski, P. and Schierning, G. and Schmechel, R.
    Journal of Electronic Materials 43 (2014)
    High-temperature-stable thermoelectric generator modules (TGMs) based on nanocrystalline silicon have been fabricated, characterized by the Harman technique, and measured in a generator test facility at the German Aerospace Center. Starting with highly doped p- and n-type silicon nanoparticles from a scalable gas-phase process, nanocrystalline bulk silicon was obtained using a current-activated sintering technique. Electrochemical plating methods were employed to metalize the nanocrystalline silicon. The specific electrical contact resistance ρ c of the semiconductor-metal interface was characterized by a transfer length method. Values as low as ρ c < 1 × 10-6 Ω cm2 were measured. The device figure of merit of a TGM with 64 legs was approximately ZT = 0.13 at 600°C as measured by the Harman technique. Using a generator test facility, the maximum electrical power output of a TGM with 100 legs was measured to be roughly 1 W at hot-side temperature of 600°C and cold-side temperature of 300°C. © 2014 TMS.
    view abstract10.1007/s11664-014-3093-6
  • High temperature thermoelectric device concept using large area PN junctions
    Chavez, R. and Angst, S. and Hall, J. and Stoetzel, J. and Kessler, V. and Bitzer, L. and Maculewicz, F. and Benson, N. and Wiggers, H. and Wolf, D. and Schierning, G. and Schmechel, R.
    Journal of Electronic Materials 43 (2014)
    A new high temperature thermoelectric device concept using large area nanostructured silicon p-type and n-type (PN) junctions is presented. In contrast to conventional thermoelectric generators, where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we experimentally demonstrate a device concept in which a large area PN junction made from highly doped densified silicon nanoparticles is subject to a temperature gradient parallel to the PN interface. In the proposed device concept, the electrical contacts are made at the cold side eliminating the hot side substrate and difficulties that go along with high temperature electrical contacts. This concept allows temperature gradients greater than 300 K to be experimentally applied with hot side temperatures larger than 800 K. Electronic properties of the PN junctions and power output characterizations are presented. A fundamental working principle is discussed using a particle network model with temperature and electric fields as variables, and which considers electrical conductivity and thermal conductivity according to Fourier's law, as well as Peltier and Seebeck effects. © 2014 TMS.
    view abstract10.1007/s11664-014-3073-x
  • Note: Automated optical focusing on encapsulated devices for scanning light stimulation systems
    Bitzer, L.A. and Benson, N. and Schmechel, R.
    Review of Scientific Instruments 85 (2014)
    Recently, a scanning light stimulation system with an automated, adaptive focus correction during the measurement was introduced. Here, its application on encapsulated devices is discussed. This includes the changes an encapsulating optical medium introduces to the focusing process as well as to the subsequent light stimulation measurement. Further, the focusing method is modified to compensate for the influence of refraction and to maintain a minimum beam diameter on the sample surface. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4892379
  • A new adaptive light beam focusing principle for scanning light stimulation systems
    Bitzer, L.A. and Meseth, M. and Benson, N. and Schmechel, R.
    Review of Scientific Instruments 84 (2013)
    In this article a novel principle to achieve optimal focusing conditions or rather the smallest possible beam diameter for scanning light stimulation systems is presented. It is based on the following methodology: First, a reference point on a camera sensor is introduced where optimal focusing conditions are adjusted and the distance between the light focusing optic and the reference point is determined using a laser displacement sensor. In a second step, this displacement sensor is used to map the topography of the sample under investigation. Finally, the actual measurement is conducted, using optimal focusing conditions in each measurement point at the sample surface, that are determined by the height difference between camera sensor and the sample topography. This principle is independent of the measurement values, the optical or electrical properties of the sample, the used light source, or the selected wavelength. Furthermore, the samples can be tilted, rough, bent, or of different surface materials. In the following the principle is implemented using an optical beam induced current system, but basically it can be applied to any other scanning light stimulation system. Measurements to demonstrate its operation are shown, using a polycrystalline silicon solar cell. © 2013 American Institute of Physics.
    view abstract10.1063/1.4791795
  • A new thermoelectric concept using large area PN junctions
    Chavez, R. and Becker, A. and Kessler, V. and Engenhorst, M. and Petermann, N. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Materials Research Society Symposium Proceedings 1543 (2013)
    A new thermoelectric concept using large area silicon PN junctions is experimentally demonstrated. In contrast to conventional thermoelectric generators where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we demonstrate a large area PN junction made from densified silicon nanoparticles that combines thermally induced charge generation and separation in a space charge region with the conventional Seebeck effect by applying a temperature gradient parallel to the PN junction. In the proposed concept, the electrical contacts are made at the cold side eliminating the need for contacts at the hot side allowing temperature gradients greater than 100K to be applied. The investigated PN junction devices are produced by stacking n-type and p-type nanopowder prior to a densification process. The nanoparticulate nature of the densified PN junction lowers thermal conductivity and increases the intraband traps density which we propose is beneficial for transport across the PN junction thus enhancing the thermoelectric properties. A fundamental working principle of the proposed concept is suggested, along with characterization of power output and output voltages per temperature difference that are close to those one would expect from a conventional thermoelectric generator. © 2013 Materials Research Society.
    view abstract10.1557/opl.2013.954
  • A novel adaptive focusing principle for scanning light stimulation systems down to 2μm resolution
    Bitzer, L.A. and Benson, N. and Schmechel, R.
    Conference Record of the IEEE Photovoltaic Specialists Conference (2013)
    A new principle to achieve optimal focusing conditions or rather the smallest possible beam diameter for scanning light stimulation systems is presented. It is based on the following three steps: First, a reference point is introduced on a CMOS sensor to adjust the beam diameter. The distance between the light focusing optic and the reference point is then determined using a laser displacement sensor. In a second step, this displacement sensor is used to obtain the topography of the sample under investigation. Finally, the actual measurement is conducted, using optimal focusing conditions in each measurement point on the sample surface. They are determined by the height difference between the CMOS sensor and the sample topography. This principle is independent of optical or electrical sample properties, the used light source or the selected wavelength. Furthermore, the samples can be tilted, rough, bent or of different surface materials. The described focusing principle can be applied to any scanning light stimulation system. Here, it is implemented using an optical beam induced current (OBIC) setup with a laser light source. © 2013 IEEE.
    view abstract10.1109/PVSC.2013.6744233
  • A thermoelectric generator concept using a p-n junction: Experimental proof of principle
    Becker, A. and Chavez, R. and Petermann, N. and Schierning, G. and Schmechel, R.
    Journal of Electronic Materials 42 (2013)
    Conventional thermoelectric generators (TEGs) use single p- and n-doped legs for thermoelectric energy harvesting. We explore a concept using thermoelectric p-n junctions made from densified silicon nanoparticles. The nanoparticle powder was synthesized in a microwave plasma reactor using silane, diborane and phosphine as precursors. To achieve a bulk sample with a p-n junction, a layer of boron-doped nanoparticle powder was stacked on a layer of phosphorus-doped powder and compacted by current-activated pressure- assisted densification. To use the p-n structure as a TEG, a temperature gradient was applied along the p-n junction. It is expected that this temperature gradient leads to electron-hole pair generation and separation in the junction and diffusion of the charge carriers. A reference method was used to characterize the open-circuit voltage of the p-n junction TEG. © 2013 TMS.
    view abstract10.1007/s11664-012-2399-5
  • Corrigendum to " Mechanical layer compaction for dispersion processed nanoparticulate zinc oxide thin film transistors" [Microelectron. Eng. 96 (2012) 36-39]
    Bubel, S. and Schmechel, R.
    Microelectronic Engineering 103 (2013)
    view abstract10.1016/j.mee.2012.10.002
  • Electrical contact resistance of electroless nickel to nanocrystalline silicon and the fabrication of a thermoelectric generator
    Kessler, V. and Dehnen, M. and Chavez, R. and Engenhorst, M. and Stoetzel, J. and Petermann, N. and Hesse, K. and Huelser, T. and Spree, M. and Schierning, G. and Schmechel, R.
    Materials Research Society Symposium Proceedings 1553 (2013)
    We present the fabrication of a high-temperature stable thermoelectric generator based on nanocrystalline silicon. Highly doped silicon nanoparticles were sintered by a current activated sintering technique to get nanocrystalline bulk silicon. The metalization of silicon was realized by (electro-)chemical plating and the specific electrical contact resistance ρc of the semiconductor-metal interface was measured by a transfer length method. Values as low as \rho -C < 1 \cdot 10^{ - 6} \,\Omega cm-2 were measured. The metalized nanocrystalline silicon legs were sintered to metalized ceramic substrates using a silver-based sinter paste. The device figure of merit of the thermoelectric generator was determined by a Harman measurement with a maximum ZT of approximately 0.13 at 600 °C. Copyright © Materials Research Society 2013.
    view abstract10.1557/opl.2013.863
  • Erratum: Thermoelectric properties of nanocrystalline silicon from a scaled-up synthesis plant (Advanced Engineering Materials (2013) 15 (379) DOI: 10.1002/adem.201200233)
    Kessler, V. and Gautam, D. and Hülser, T. and Spree, M. and Theissmann, R. and Winterer, M. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Advanced Engineering Materials 15 (2013)
    view abstract10.1002/adem.201300182
  • Excimer laser doping using highly doped silicon nanoparticles
    Meseth, M. and Kunert, B.C. and Bitzer, L. and Kunze, F. and Meyer, S. and Kiefer, F. and Dehnen, M. and Orthner, H. and Petermann, N. and Kummer, M. and Wiggers, H. and Harder, N.-P. and Benson, N. and Schmechel, R.
    Physica Status Solidi (A) Applications and Materials Science 210 (2013)
    Laser doping of crystalline Si (c-Si) using highly doped Si nanoparticles (NPs) as the dopant source is investigated. For this purpose Si NPs are deposited onto c-Si substrates from dispersion using a spin coater and subsequently laser annealed by scanning over the sample with a 248 nm line profile excimer laser. Scanning electron microscope (SEM) investigations demonstrate that the laser intensity as well as the oxide concentration in the NP thin film strongly influence the film forming properties of the annealed NPs. Substrate doping is substantiated using electrochemical capacitance voltage (ECV) measurements on realized pn-junctions. In dependence of the laser fluencies ranging from 0.81 to 2.54 J cm-2, the effective doping depth is determined to be in the range of 50 to 250 nm. The rectifying behaviour of the pn- or np-junctions is verified by current voltage measurements. A homogeneous in-plane doping distribution realized by the laser doping process is demonstrated on the μm scale by light beam induced current measurements. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201329012
  • Impact of rapid thermal annealing on thermoelectric properties of bulk nanostructured zinc oxide
    Engenhorst, M. and Gautam, D. and Schilling, C. and Winterer, M. and Schierning, G. and Schmechel, R.
    Materials Research Society Symposium Proceedings 1543 (2013)
    In search for non-toxic thermoelectric materials that are stable in air at elevated temperatures, zinc oxide has been shown to be one of only few efficient n-type oxidic materials. Our bottom-up approach starts with very small (< 10 nm) Al-doped ZnO nanoparticles prepared from organometallic precursors by chemical vapor synthesis using nominal doping concentrations of 2 at% and 8 at%. In order to obtain bulk nanostructured solids, the powders were compacted in a current-activated pressure-assisted densification process. Rapid thermal annealing was studied systematically as a means of further dopant activation. The thermoelectric properties are evaluated with regard to charge carrier concentration and mobility. A Jonker-type analysis reveals the potential of our approach to achieve high power factors. In the present study, power factors larger than 4× 10-4 Wm-1K-2 were measured at temperatures higher than 600 °C. © 2013 Materials Research Society.
    view abstract10.1557/opl.2013.932
  • Laser-doping of crystalline silicon substrates using doped silicon nanoparticles
    Meseth, M. and Lamine, K. and Dehnen, M. and Kayser, S. and Brock, W. and Behrenberg, D. and Orthner, H. and Elsukova, A. and Hartmann, N. and Wiggers, H. and Hülser, T. and Nienhaus, H. and Benson, N. and Schmechel, R.
    Thin Solid Films 548 (2013)
    Crystalline Si substrates are doped by laser annealing of solution processed Si. For this experiment, dispersions of highly B-doped Si nanoparticles are deposited onto intrinsic Si and laser processed using an 807.5 nm continuous wave laser. During laser processing the particles as well as a surface-near substrate layer are melted to subsequently crystallize in the same orientation as the substrate. The doping profile is investigated by secondary ion mass spectroscopy revealing a constant B concentration of 2 × 10 18 cm- 3 throughout the entire analyzed depth of 5 μm. Four-point probe measurements demonstrate that the effective conductivity of the doped sample is increased by almost two orders of magnitude. The absolute doping depth is estimated to be in between 8 μm and 100 μm. Further, a pn-diode is created by laser doping an n-type c-Si substrate using the Si NPs. © 2013 Published by Elsevier B.V.
    view abstract10.1016/j.tsf.2013.09.056
  • Note: High resolution alternating current/direct current Harman technique
    Chavez, R. and Becker, A. and Bartel, M. and Kessler, V. and Schierning, G. and Schmechel, R.
    Review of Scientific Instruments 84 (2013)
    This note describes the construction and engineering of a high precision Harman set-up for metrology of the thermoelectric figure of merit (ZT) of modules and materials based on steady state AC and DC measurements. The Harman technique presented in this article has a resolution of milli-ZT and it does not employ lock-in amplifiers or AC bridges; rather, the technique is developed to avoid typical complications experienced in AC Harman systems. By one-time reference measurements the best operation point for the system is chosen, minimizing the effects of capacitive loads due to AC signals. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4825118
  • Reduced Coulomb interaction in organic solar cells by the introduction of high-k SrTiO3 nanoparticles
    Benson, N. and Engel, M. and Schaefer, D. and Erni, D. and Kern, J. and Deibel, C. and Herzig, E.M. and Muller-Buschbaum, P. and Schmechel, R.
    Conference Record of the IEEE Photovoltaic Specialists Conference (2013)
    A concept is introduced which allows for reduced Coulomb interaction in organic solar cells and as such for enhanced power conversion efficiencies. The concept is based on the introduction of electrically insulating, nanostructured high-k materials into the organic matrix, which do not contribute to the charge carrier transport, however, enhance the effective permittivity of the organic active layer. Using an analytical model, it is demonstrated that even at a distance of 20 nm to the organic/inorganic interface of the nanostructure, the Coulomb interaction in the organic semiconductor can be reduced by more than 15 %. The concept is substantiated experimentally by realizing P3HT:PCBM solar cells with integrated SrTiO3 nanoparticles. It could be demonstrated that in comparison to a reference cells without nanoparticles, the power conversion efficiency is improved by ∼17 %. This effect is interpreted to be the result of an organic active layer effective permittivity enhancement, which is supported by the result of transient absorption as well as grazing incidence wide angle x-ray scattering measurements. © 2013 IEEE.
    view abstract10.1109/PVSC.2013.6745113
  • Reduced Coulomb interaction in organic solar cells by the introduction of inorganic high-k nanostructured materials
    Engel, M. and Schaefer, D. and Erni, D. and Benson, N. and Schmechel, R.
    Physica Status Solidi (A) Applications and Materials Science 210 (2013)
    In this paper a concept is introduced, which allows for reduced Coulomb interaction in organic solar cells and as such for enhanced power conversion efficiencies. The concept is based on the introduction of electrically insulating, nanostructured high-k materials into the organic matrix, which do not contribute to the charge transport; however, enhance the effective permittivity of the organic active layer and thereby reduce the Coulomb interaction. Using an analytical model, it is demonstrated that even at a distance of 20 nm to the organic/inorganic interface of the nanostructure, the Coulomb interaction in the organic semiconductor can be reduced by more than 15%. The concept is implemented using P3HT:PCBM solar cells with integrated high-k nanoparticles (strontium titanate). It could be demonstrated that in comparison to a reference cell without integrated nanoparticles, the power conversion efficiencies is improved by ∼17%. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201228771
  • Thermoelectric properties of nanocrystalline silicon from a scaled-up synthesis plant
    Kessler, V. and Gautam, D. and Hülser, T. and Spree, M. and Theissmann, R. and Winterer, M. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Advanced Engineering Materials 15 (2013)
    Silicon based thermoelectrics are promising candidates for high temperature energy scavenging applications. We present the properties of thermoelectrics made from highly boron doped silicon nanoparticles. The particles were produced by a continuous gas phase process in a scaled-up synthesis plant enabling production rates in the kg h-1 regime. The silicon nanoparticles were compacted by direct current assisted sintering to yield nanocrystalline bulk silicon with average crystallite size between 40 and 80 nm and relative densities above 97% of the density of single crystalline silicon. The influence of the sintering temperature on the thermoelectric properties is investigated. It was found that high sintering temperatures are beneficial for an enhancement of the power factor, while the thermal conductivity was only moderately affected. The optimization of the compaction procedure with respect to the transport properties leads to zT values of the p-type nanosilicon of 0.32 at 700 °C, demonstrating the potential of our method. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201200233
  • A sintered nanoparticle p-n junction observed by a Seebeck microscan
    Becker, A. and Schierning, G. and Theissmann, R. and Meseth, M. and Benson, N. and Schmechel, R. and Schwesig, D. and Petermann, N. and Wiggers, H. and Ziolkowski, P.
    Journal of Applied Physics 111 (2012)
    A nanoparticular p-n junction was realized by a field-assisted sintering process, using p-type and n-type doped silicon nanoparticles. A spatially resolved Seebeck microscan showed a broad transition from the positively doped to the negatively doped range. Overshoots on both sides are characteristic for the transition. Despite the tip size being much larger than the mean particle size, information about the dopant distribution between the particles is deduced from modeling the measured data under different assumptions, including the limited spatial resolution of the tip. The best match between measured and modeled data is achieved by the idea of doping compensation, due to the sintering process. Due to a short time at high temperature during the field-assisted sintering process, solid state diffusion is too slow to be solely responsible for the observed compensation of donors and acceptors over a wide range. Therefore, these measurements support a densification mechanism based on (partial) melting and recrystallization. © 2012 American Institute of Physics.
    view abstract10.1063/1.3693609
  • Influence of annealing atmospheres and synthetic air treatment on solution processed zinc oxide thin film transistors
    Busch, C. and Schierning, G. and Theissmann, R. and Schmechel, R.
    Journal of Applied Physics 112 (2012)
    Thin film transistors (TFTs) based on active layers of zinc oxide prepared from a solution process were fabricated under different annealing conditions. The influence of the annealing gas as well as the influence of a subsequent exposure to synthetic air to the device properties is considered. Annealing under N 2 or H 2 atmosphere leads to a strong negative threshold voltage shift. With respect to known defect states in ZnO, two different donor states are suggested to be responsible for the negative threshold voltage. A subsequent synthetic air treatment causes in general a positive threshold voltage shift. However, transistors annealed under H 2 degrade very fast under synthetic air in contrast to transistors annealed under N 2. In order to obtain more information about the density of states (DOS) distribution, a transistor model for thin film transistors in the hopping transport regime (Vissenberg model) was utilized. For positive threshold voltages, the DOS distribution is independent from the gas treatment and the threshold voltage within the experimental accuracy. This indicates a shift of the Fermi-level within an exponentially decaying DOS. The change in the charge carrier density is either due to shallow donors or due to a charge transfer with acceptors at the surface. In contrast, for negative threshold voltages, the DOS distribution parameter rises, indicating a flatter DOS distribution. We suggest that the difference is due to the change from accumulation mode to the depletion mode of the device. © 2012 American Institute of Physics.
    view abstract10.1063/1.4742976
  • Laser-sintered thin films of doped SiGe nanoparticles
    Stoib, B. and Langmann, T. and Matich, S. and Antesberger, T. and Stein, N. and Angst, S. and Petermann, N. and Schmechel, R. and Schierning, G. and Wolf, D.E. and Wiggers, H. and Stutzmann, M. and Brandt, M.S.
    Applied Physics Letters 100 (2012)
    We present a study of the morphology and the thermoelectric properties of short-pulse laser-sintered (LS) nanoparticle (NP) thin films, consisting of SiGe alloy NPs or composites of Si and Ge NPs. Laser-sintering of spin-coated NP films in vacuum results in a macroporous percolating network with a typical thickness of 300 nm. The Seebeck coefficient for LS samples is the same as for bulk samples prepared by current-assisted sintering and is typical for degenerate doping. The electrical conductivity of LS films is influenced by two-dimensional percolation effects and rises with increasing temperature, approximately following a power-law. © 2012 American Institute of Physics.
    view abstract10.1063/1.4726041
  • Mechanical layer compaction for dispersion processed nanoparticulate zinc oxide thin film transistors
    Bubel, S. and Schmechel, R.
    Microelectronic Engineering 96 (2012)
    Using rolling as a roll-to-roll compatible compaction process for solution processable electronics, we demonstrate improved layer morphology and field effect transistor performance of nanoparticulate zinc oxide (ZnO) thin films. Semiconducting ZnO layers have been processed from a polyvinylpyrrolidone (PVP) stabilized nanoparticulate dispersion at low temperatures. Maximum saturation mobilities of 7×10-3cm2/V s, improvements in mobility of more than one order of magnitude and a reduction in threshold voltage by more than 30% are shown. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.mee.2012.02.044
  • Metal oxide thin-film transistors from nano particles and solutions
    Busch, C. and Bubel, S. and Theissmann, R. and Schmechel, R.
    NanoScience and Technology 79 (2012)
    This article compares several non-vacuum-based low-temperature deposition techniques of semiconducting oxides for thin-film transistor applications. After an introduction into basic thin-film transistor theory it summarizes in short the development in the field of semi conducting oxides. Three different deposition techniques are considered in more detail: (1) a direct deposition of semi conducting oxide nanoparticles from a carrier gas stream, on the example of SnOx and In 2O 3, (2) a wet-deposition of nanodispersions of ZnO, and (3) a deposition of liquid precursors with subsequent transformation into the semiconducting oxide, on the example of ZnO. The advantages and disadvantages of the several methods are discussed critically also with respect to results from the literature. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-28546-2_15
  • N-type perylene to fill voids in solution processed nanoparticulate zinc oxide thin films
    Bubel, S. and Ringk, A. and Strohriegl, P. and Schmechel, R.
    Physica E: Low-Dimensional Systems and Nanostructures 44 (2012)
    Using nanoparticle dispersions for printing of semiconductors would be the easiest way to evolve from classic printing technologies towards printed electronics. However, nanoparticular thin films are unfavorable in transistor applications due to two reasons: (i) The charge transport in the thin film or at its interfaces to the gate dielectric is disturbed by the voids between the nanoparticles. (ii) These layers are highly sensitive to surface adsorbates due to their high surface to volume ratio. Atmospheric surface adsorbates, e.g. on metal oxides are known to influence the electrical properties of the thin films. In order to overcome the disadvantages of the nanoparticulate thin film, this work targets both issues with a combined approach. By choosing a qualified surface adsorbate, the perturbing surface of the nanoparticles will be passivated. By using the surface adsorbate as a linker to an electron conducting organic molecule, the n-type organic will be eligible for filling the voids between the particles. We present the synthesis of a new pyrrolidone functionalized n-type perylene diimide and its application in hetero-layer nanoparticulate zinc oxide (ZnO) field-effect transistors. © 2012 Elsevier B.V.
    view abstract10.1016/j.physe.2012.06.027
  • Nanoparticles from the Gas Phase: Formation, Structure, Properties
    Lorke, A. and Winterer, M. and Schmechel, R. and Schulz, C.
    NanoScience and Technology 79 (2012)
    view abstract10.1007/978-3-642-28546-2
  • Reduced exciton binding energy in organic semiconductors: Tailoring the Coulomb interaction
    Engel, M. and Kunze, F. and Lupascu, D.C. and Benson, N. and Schmechel, R.
    Physica Status Solidi - Rapid Research Letters 6 (2012)
    For organic photovoltaics (OPV) the maximum in obtainable power conversion efficiency is limited by a low semiconductor permittivity and the resulting enhanced Coulomb interaction (CI). This, however, is an aspect rarely addressed in the OPV development. Here, a concept is introduced which allows a reduced CI in organic semiconductors. This is the result of a device structure, which upon exciton formation forces part of the electric field between complementary charges through a high-k material, resulting in partial field screening and as such a reduced CI. The feasibility of this concept is substantiated by an investigation on the exciton separation efficiency in pentacene devices. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssr.201105488
  • The effect of Peltier heat during current activated densification
    Becker, A. and Angst, S. and Schmitz, A. and Engenhorst, M. and Stoetzel, J. and Gautam, D. and Wiggers, H. and Wolf, D.E. and Schierning, G. and Schmechel, R.
    Applied Physics Letters 101 (2012)
    It is shown that current-activated pressure-assisted densification (CAPAD) is sensitive to the Peltier effect. Under CAPAD, the Peltier effect leads to a significant redistribution of heat within the sample during the densification. The densification of highly p-doped silicon nanoparticles during CAPAD and the properties of the obtained samples are investigated experimentally and by computer simulation. Both, simulation and experiments, indicate clearly a higher temperature on the cathode side and a decreasing temperature from the center to the outer shell. Furthermore, computer simulations provide additional insights into the temperature profile which explain the anisotropic properties of the measured sample. © 2012 American Institute of Physics.
    view abstract10.1063/1.4731272
  • The realization of a pn-diode using only silicon nanoparticles
    Meseth, M. and Ziolkowski, P. and Schierning, G. and Theissmann, R. and Petermann, N. and Wiggers, H. and Benson, N. and Schmechel, R.
    Scripta Materialia 67 (2012)
    Si nanoparticles (Si-NPs) are a non-toxic and low-cost material resource that can be processed from dispersion for electrical thin film or from powder for bulk application using various sintering techniques. So far research on electronic applications using Si-NPs is limited. Few reports exist on thermoelectric research, or hybrid photovoltaic applications. In the following we demonstrate the realization of the first Si pn-diode using only Si-NPs in combination with field-assisted sintering. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2012.04.039
  • Thin-film transistors with a channel composed of semiconducting metal oxide nanoparticles deposited from the gas phase
    Busch, C. and Schierning, G. and Theissmann, R. and Nedic, A. and Kruis, F.E. and Schmechel, R.
    Journal of Nanoparticle Research 14 (2012)
    The fabrication of semiconducting functional layers using low-temperature processes is of high interest for flexible printable electronics applications. Here, the one-step deposition of semiconducting nanoparticles from the gas phase for an active layer within a thin-film transistor is described. Layers of semiconducting nanoparticles with a particle size between 10 and 25 nm were prepared by the use of a simple aerosol deposition system, excluding potentially unwanted technological procedures like substrate heating or the use of solvents. The nanoparticles were deposited directly onto standard thin-film transistor test devices, using thermally grown silicon oxide as gate dielectric. Proof-of-principle experiments were done deploying two different wide-band gap semiconducting oxides, tin oxide, SnO x, and indium oxide, In 2O 3. The tin oxide spots prepared from the gas phase were too conducting to be used as channel material in thin-film transistors, most probably due to a high concentration of oxygen defects. Using indium oxide nanoparticles, thin-film transistor devices with significant field effect were obtained. Even though the electron mobility of the investigated devices was only in the range of 10 -6 cm 2V -1s -1, the operability of this method for the fabrication of transistors was demonstrated. With respect to the possibilities to control the particle size and layer morphology in situ during deposition, improvements are expected. © 2012 Springer Science+Business Media B.V.
    view abstract10.1007/s11051-012-0888-3
  • Artificially nanostructured n-type SiGe bulk thermoelectrics through plasma enhanced growth of alloy nanoparticles from the gas phase
    Stein, N. and Petermann, N. and Theissmann, R. and Schierning, G. and Schmechel, R. and Wiggers, H.
    Journal of Materials Research 26 (2011)
    SiGe alloys belong to the class of classic high temperature thermoelectric materials. By the means of nanostructuring, the performance of this well-known material can be further enhanced. Additional grain boundaries and point defects added to the alloy structure result in a strong decrease in thermal conductivity because of reduced lattice contribution to the overall thermal conductivity. Hence, the figure of merit can be increased. To obtain a nanostructured bulk material, a nanosized raw material is essential. In this work, a new approach toward nanostructured SiGe alloys is presented where alloyed nanoparticles are synthesized from a homogeneous mixture of the respective precursors in a microwave plasma reactor. As-prepared nanoparticles are compacted to a dense bulk material by a field assisted sintering technique. A figure of merit of zT = 0.5 ± 0.09 at 450 °C and a peak zT of 0.8 ± 0.15 at 1000 °C could be achieved for a nanostructured, 0.8% phosphorus-doped Si 80Ge20 alloy without any further optimization. Copyright © Materials Research Society 2011.
    view abstract10.1557/jmr.2011.117
  • Electronic properties of polyvinylpyrrolidone at the zinc oxide nanoparticle surface : PVP in ZnO dispersions and nanoparticulate ZnO thin films for thin film transistors
    Bubel, S. and Mechau, N. and Schmechel, R.
    Journal of Materials Science 46 (2011)
    We investigated the electrical effects of polyvinylpyrrolidone (PVP), used as a dispersion agent in zinc oxide (ZnO) nanodispersions. We found PVP reduces the high surface conductivity and atmospheric sensitivity. Compared with polymer free ZnO thin films, the nanoparticulate layers with PVP exhibit a smaller density of thermally active charge carriers, a reduced density of trap states, and a Fermi level shift toward the valence band, yielding improved performance, vanishing hysteresis characteristics and reduced atmospheric sensitivity in thin film transistors (TFT). In addition, we discuss the attachment of PVP to the ZnO surface. © 2011 Springer Science+Business Media, LLC.
    view abstract10.1007/s10853-011-5757-4
  • Erratum: Artificially nanostructured n-type SiGe bulk thermoelectrics through plasma enhanced growth of alloy nanoparticles from the gas phase (Journal of Materials Research (2011) 26: 15 (1872) DOI: 10.1557/jmr.2011.117)
    Stein, N. and Petermann, N. and Theissmann, R. and Schierning, G. and Schmechel, R. and Wiggers, H.
    Journal of Materials Research 26 (2011)
    view abstract10.1557/jmr.2011.311
  • From nanoparticles to nanocrystalline bulk: Percolation effects in field assisted sintering of silicon nanoparticles
    Schwesig, D. and Schierning, G. and Theissmann, R. and Stein, N. and Petermann, N. and Wiggers, H. and Schmechel, R. and Wolf, D.E.
    Nanotechnology 22 (2011)
    Nanocrystalline bulk materials are desirable for many applications as they combine mechanical strength and specific electronic transport properties. Our bottom-up approach starts with tailored nanoparticles. Compaction and thermal treatment are crucial, but usually the final stage sintering is accompanied by rapid grain growth which spoils nanocrystallinity. For electrically conducting nanoparticles, field activated sintering techniques overcome this problem. Small grain sizes have been maintained in spite of consolidation. Nevertheless, the underlying principles, which are of high practical importance, have not been fully elucidated yet. In this combined experimental and theoretical work, we show how the developing microstructure during sintering correlates with the percolation paths of the current through the powder using highly doped silicon nanoparticles as a model system. It is possible to achieve a nanocrystalline bulk material and a homogeneous microstructure. For this, not only the generation of current paths due to compaction, but also the disintegration due to Joule heating is required. The observed density fluctuations on the micrometer scale are attributed to the heat profile of the simulated powder networks. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/22/13/135601
  • High performance low temperature solution-processed zinc oxide thin film transistor
    Theissmann, R. and Bubel, S. and Sanlialp, M. and Busch, C. and Schierning, G. and Schmechel, R.
    Thin Solid Films 519 (2011)
    Amorphous zinc oxide thin films have been processed out of an aqueous solution applying a one step synthesis procedure. For this, zinc oxide containing crystalline water (ZnO· × H2O) is dissolved in aqueous ammonia (NH3), making use of the higher solubility of ZnO· × H2O compared with the commonly used zinc oxide. Characteristically, as-produced layers have a thickness of below 10 nm. The films have been probed in standard thin film transistor devices, using silicon dioxide as dielectric layer. Keeping the maximum process temperature at 125 °C, a device mobility of 0.25 cm2V- 1s- 1 at an on/off ratio of 106 was demonstrated. At an annealing temperature of 300 °C, the performance could be optimized up to a mobility of 0.8 cm2V- 1s- 1. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2011.02.073
  • Influence of the annealing atmosphere on solution based zinc oxide thin film transistors
    Busch, C. and Theissmann, R. and Bubel, S. and Schierning, G. and Schmechel, R.
    Materials Research Society Symposium Proceedings 1359 (2011)
    Zinc oxide layers with a thickness of less than 10 nanometers have been synthesized from an aqueous solution for the application as active layer in thin film transistors. They have been conditioned by applying different oxidizing and reducing atmospheres during an annealing process at a temperature of 125°C. It is shown that the charge carrier mobility and threshold voltage is strongly influenced by the annealing atmosphere. Samples annealed in 10% forming gas (H 2 in N 2 - reducing atmosphere) show the highest field-effect-mobility of 0.6 cm 2V -1s -1, but no saturation of the drain current, due to a high free carrier concentration. Samples treated under oxygen (strongest oxidizing atmosphere) show significantly lower mobilities. Subsequently, the samples have been exposed to synthetic air, with varying exposure times. Samples which have been annealed under hydrogen atmospheres show a pronounced decay of the drain current if exposed to synthetic air, whereas all samples conditioned under hydrogen-free atmospheres are significantly more stable under synthetic air. This enhanced sensitivity against oxygen after hydrogen treatment is attributed to residual hydrogen content in the sample that supports the formation of OH-groups which act as electron acceptors. © 2011 Materials Research Society.
    view abstract10.1557/opl.2011.754
  • Role of oxygen on microstructure and thermoelectric properties of silicon nanocomposites
    Schierning, G. and Theissmann, R. and Stein, N. and Petermann, N. and Becker, A. and Engenhorst, M. and Kessler, V. and Geller, M. and Beckel, A. and Wiggers, H. and Schmechel, R.
    Journal of Applied Physics 110 (2011)
    Phosphorus-doped silicon nanopowder from a gas phase process was compacted by DC-current sintering in order to obtain thermoelectrically active, nanocrystalline bulk silicon. A density between 95 and 96 compared to the density of single crystalline silicon was achieved, while preserving the nanocrystalline character with an average crystallite size of best 25 nm. As a native surface oxidation of the nanopowder usually occurs during nanopowder handling, a focus of this work is on the role of oxygen on microstructure and transport properties of the nanocomposite. A characterization with transmission electron microscopy (TEM) showed that the original core/shell structure of the nanoparticles was not found within the sintered nanocomposites. Two different types of oxide precipitates could be identified by energy filtered imaging technique. For a detailed analysis, 3-dimensional tomography with reconstruction was done using a needle-shaped sample prepared by focused ion beam (FIB). The 3-dimensional distribution of silicon dioxide precipitates confirmed that the initial core/shell structure breaks down and precipitates are formed. It is further found that residual pores are exclusively located within oxide precipitates. Thermoelectric characterization was done on silicon nanocomposites sintered between 960 C and 1060 C with varying oxygen content between room temperature and 950 C. The higher sintering temperature led to a better electrical activation of the phosphorus dopant. The oxidic precipitates support densification and seem to be able to reduce the thermal conductivity therefore enhancing thermoelectric properties. A peak figure of merit, zT, of 0.5 at 950 C was measured for a sample sintered at 1060 C with a mean crystallite size of 46 nm. © 2011 American Institute of Physics.
    view abstract10.1063/1.3658021
  • Field effects on SnOx and SnO2 nanoparticles synthesized in the gas phase
    Roy Chowdhury, D. and Ivaturi, A. and Nedic, A. and Einar Kruis, F. and Schmechel, R.
    Physica E: Low-Dimensional Systems and Nanostructures 42 (2010)
    The present study reports for the first time the influence of stoichiometry of SnO2 nanoparticles synthesized in the gas phase at atmospheric pressure towards the field effect behaviour. The field effect was measured by using the nanoparticles as active material in a transistor channel. The transistors fabricated from the stoichiometric SnO2 nanoparticles (∼20 nm) obtained by post-deposition low-temperature (300 °C) oxidation of the SnO nanoparticles clearly demonstrate n-type behaviour in contrast to the high electrical conductance exhibited by the non-stoichiometric SnOx nanoparticles obtained by high temperature (650 °C) in-flight oxidation. X-ray Photoelectron Spectroscopy (XPS) studies confirm the stoichiometry of the in-flight as well as the post-oxidized nanoparticles.
    view abstract10.1016/j.physe.2010.06.005
  • Trap states and space charge limited current in dispersion processed zinc oxide thin films
    Bubel, S. and Mechau, N. and Hahn, H. and Schmechel, R.
    Journal of Applied Physics 108 (2010)
    The electric transport properties of nanoparticulate zinc oxide (ZnO) thin films are investigated in nitrogen and ambient atmosphere with respect to the effects of polymer adsorbates, in order to study the origin of hysteresis behavior of ZnO thin film transistors. A strong dependence on the polymer adsorbate of the conductivity in nitrogen atmosphere is observed. Utilizing the space charge limited current theory, the trap depth and concentration in the films have been estimated. According to this analysis, the low conductivity of polymer free thin films in ambient atmosphere is caused by an increase in deep traps, compensating free charge carriers and not by a reduction in donorlike defect states. Furthermore, polymeric additives seem to induce similar trap states, which make the transport properties less sensitive against atmospheric influences. However, the strongly compensated semiconductor created in this way, causes a slow trap and release behavior resulting in a strong hysteresis in the transistor characteristics and long-term instabilities. It is shown, that ignoring these time-dependent characteristics, straight forward derived transistor parameters like the field effect mobility can be easily overestimated. © 2010 American Institute of Physics.
    view abstract10.1063/1.3524184
  • electronics

  • functional layers

  • nanocomposites

  • photovoltaics

  • printable electronics

  • sintering

  • thermoelectrics

  • thin films

  • transport

« back