Device Physics of Organic Solar Cells: Drift-Diffusion Simulation in Comparison with Experimental Data of Solar Cells Based on Small Molecules

This thesis deals with the device physics of organic solar cells. Organic photovoltaics (OPV) is a field of applied research which has been growing rapidly in the last decade leading to a current record value of power-conversion efficiency of 10 percent. One major reason for this boom is a potentially low-cost production of solar modules on flexible (polymer) substrate. Furthermore, new application are expected by flexible or semitransparent organic solar cells. That is why several OPV startup companies were launched in the last decade. Organic solar cells consist of hydrocarbon compounds, deposited as ultrathin layers (some tens of nm) on a substrate. Absorption of light leads to molecular excited states (excitons) which are strongly bound due to the weak interactions and low dielectric constant in a molecular solid. The excitons have to be split into positive and negative charges, which are subsequently collected at different electrodes. An effective dissociation of excitons is provided by a heterojunction of two molecules with different frontier orbital energies, such that the electron is transfered to the (electron) acceptor and the positive charge (hole) remains on the donor molecule. This junction can be realized by two distinct layers forming a planar heterojunction or by an intermixed film of donor and acceptor, resulting in a bulk heterojunction. Electrodes are attached to the absorber to collect the charges by providing an ohmic contact in the optimum case. This work focuses on the electrical processes in organic solar cells developing and employing a one-dimensional drift-diffusion model. The electrical model developed here is combined with an optical model and covers the diffusion of excitons, their separation, and the subsequent transport of charges. In contrast to inorganics, charge-carrier mobilities are low in the investigated materials and charge transport is strongly affected by energy barriers at the electrodes. The current-voltage characteristics (J-V curve) of a solar cell reflect the electrical processes in the device. Therefore, the J-V curve is selected as means of comparison between systematic series of simulation and experimental data. This mainly qualitative approach allows for an identification of dominating processes and provides microscopic explanations. One crucial issue, as already mentioned, is the contact between absorber layer and electrode. Energy barriers lead to a reduction of the power-conversion efficiency due to a decrease in the open-circuit voltage or the fill factor by S-shaped J-V curve (S-kink), which are often observed for organic solar cells. It is shown by a systematic study that the introduction of deliberate barriers for charge-carrier extraction and injection can cause such S-kinks. It is explained by simulated electrical-field profiles why also injection barriers lead to a reduction of the probability for charge-carrier extraction. A pile-up of charge carriers at an extraction barrier is confirmed by measurements of transient photocurrents. In flat heterojunction solar cells an additional reason for S-kinks is found in an imbalance of electron and hole mobilities. Due to the variety of reasons for S-kinks, methods and criteria for a distinction are proposed. These include J-V measurements at different temperatures and of samples with varied layer thicknesses. Most of the studies of this this work are based on experimental data of solar cells comprisiing the donor dye zinc phthalocyanine and the acceptor fullerene C60. It is observed that the open-circuit voltage of these devices depends on the mixing ratio of ZnPc:C60. A comparison of experimental and simulation data indicates that the reason is a changed donor-acceptor energy gap caused by a shift of the ionization potential of ZnPc. A spatial gradient in the mixing ratio of a bulk heterojunction is also investigated as a donor(acceptor)-rich mixture at the hole(electron)-collecting contact is supposed to assist charge extraction. This effect is not observed, but a reduction of charge-carrier losses at the “wrong” electrode which is seen at an increase in the open-circuit voltage. The most important intrinsic loss mechanism of a solar cell is bulk recombination which is treated at the example of ZnPc:C60 devices in the last part of this work. An examination of the dependence of the open-circuit voltage on illumination intensity shows that the dominating recombination mechanism shifts from trap-assisted to direct recombination for higher intensities. A variation of the absorption profile within the blend layer shows that the probability of charge-carrier extraction depends on the locus of charge-carrier generation. This results in a fill factor dependent on the absorption profile. The reason is an imbalance in charge-carrier mobilities which can be influenced by the mixing ratio. The work is completed by a simulation study of the influence of charge-carrier mobilities and different recombination processes on the J-V curve and an identification of a photoshunt dominating the experimental linear photocurrent-voltage characteristics in reverse bias.:Abstract - Kurzfassung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v 1 Introduction 1.1 Energy supply and climate change . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Development of (organic) photovoltaics . . . . . . . . . . . . . . . . . . 3 1.3 Structure and scope of this thesis . . . . . . . . . . . . . . . . . . . . . . 6 I Basics 2 Photovoltaic Energy Conversion 2.1 Fundamentals of solar thermal energy conversion . . . . . . . . . . .11 2.1.1 The solar spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.1.2 Black-body irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 2.1.3 Maximum power-conversion efficiency . . . . . . . . . . . . . . . . . 15 2.2 Basics of semiconductor physics . . . . . . . . . . . . . . . . . . . . . . 16 2.2.1 Band structure, electrons and holes . . . . . . . . . . . . . . . . . . 16 2.2.2 Quasi-Fermi levels and electrochemical potentials . . . . . . . . . .22 2.3 Transformation of thermal radiation into chemical energy . . . . . 28 2.4 From chemical energy to electrical energy . . . . . . . . . . . .. . . . . 29 2.5 Possible solar-cell realizations . . . . . . . . . . . . . . . . . . . . . . . . 33 2.5.1 The p-n junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.5.2 Heterojunction and dye solar cells . . . . . . . . . . . . . . . . . . . . 36 2.5.3 The p-i-n concept with wide-gap transport layers . . . . . . . . . 37 2.6 Maximum efficiency – Shockley-Queisser limit . . . . . . . . . . . . . .38 2.7 Novel concepts and classification of solar cells . . . . . . . . . . . . . 41 3 Organic Solar Cells 3.1 Energetics of organic molecules . . . . . . . . . . . . . . . . . . . . . . . 43 3.1.1 From atoms to molecules . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.1.2 From single molecules to a molecular solid . . . . . . . . . . . . . . 50 3.2 Energy and charge transport in organic semiconductors . . . . . . 52 3.2.1 Exciton transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.2.2 Charge transport - Gaussian disorder model . . . . . . . . . . . . .53 3.3 Working principle of donor-acceptor heterojunction solar cells . .57 3.3.1 Particle losses, quantum efficiency, and photocurrent . . . . . . .57 3.3.2 Energy losses, potential energy, and photovoltage . . . . . . . . 62 3.3.3 Maximum power-conversion efficiency . . . . . . . . . . . . . . . . . 66 3.3.4 Understanding the J-V curve in the MIM picture . . . . . . . . . . .68 3.3.5 Introduction to analytical models describing the photocurrent 70 3.4 Metal-organic interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 3.4.1 Conventional metal-semiconductor interfaces: Barriers and Schottky contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 3.4.2 Metal-organic interfaces: Disorder and ICT . . . . . . . . . . . . . . 79 3.5 Experimental realization of small-molecule solar cells . . . . . . . . 80 3.5.1 Stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 3.5.2 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83 3.5.3 Fabrication details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 3.6 Basic characterization methods . . . . . . . . . . . . . . . . . . . . . . . 92 3.6.1 Current-voltage characteristics . . . . . . . . . . . . . . . . . . . . . . 92 3.6.2 Spectrally resolved measurements . . . . . . . . . . . . . . . . . . . 93 3.6.3 Transient measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 95 4 Modeling 4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 4.2 The drift-diffusion model in general . . . . . . . . . . . . . . . . . . . . 99 4.2.1 Derivation and conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 99 4.2.2 The Einstein Relation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .103 4.2.3 Poisson’s equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 4.2.4 Differential equation system . . . . . . . . . . . . . . . . . . . . . . . .105 4.3 Implementation of the algorithm . . . . . . . . . . . . . . . . . . . . . . 106 4.3.1 Basics of the algorithm and discretization . . . . . . . . . . . . . . 107 4.3.2 Calculation of the electric field . . . . . . . . . . . . . . . . . . . . . . 108 4.3.3 Calculation of rates of change . . . . . . . . . . . . . . . . . . . . . . 109 4.3.4 Calculation of the time step . . . . . . . . . . . . . . . . . . . . . . . . 111 4.3.5 Detection of steady state and transient currents . . . . . . . . . 111 4.4 Implemented models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 4.4.1 Charge carrier mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 4.4.2 Recombination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 4.4.3 Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 4.4.4 Gaussian density of states . . . . . . . . . . . . . . . . . . . . . . . . 120 4.5 Contacts as boundary conditions . . . . . . . . . . . . . . . . . . . . . 121 4.6 Organic-organic interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 124 4.6.1 Charge transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 4.6.2 Generation and recombination . . . . . . . . . . . . . . . . . . . . . . 127 4.7 The simulation tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 4.8 Verification with analytical solutions . . . . . . . . . . . . . . . . . . . 129 4.8.1 Single-carrier devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 4.8.2 The p-n junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 4.9 Experimental determination of material properties . . . . . . . . . 136 4.10 Summary and main input parameters . . . . . . . . . . . . . . . . . 140 II Results and Discussion 5 Simulation Study on Single-Layer Bulk-Heterojunction Solar Cells 5.1 Investigated device structure and definitions . . . . . . . . . . . . . 144 5.2 On the optimum mobility, contact properties, and the open-circuit voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 5.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .146 5.2.2 Investigated mobility and recombination models . . . . . . . . . .147 5.2.3 Recombination only in the BHJ (selective contacts) . . . . . . . . 149 5.2.4 Recombination (also) at electrodes (non-selective contacts) . .155 5.2.5 Injection barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .158 5.2.6 Effect of energy-level bending on the open-circuit voltage . . . 161 5.3 Photocurrent and characteristic points in simulated J-V curves . .163 5.3.1 Negligible bulk recombination . . . . . . . . . . . . . . . . . . . . . . . .164 5.3.2 Bulk-recombination-limited photocurrent . . . . . . . . . . . . . . . 167 5.4 The effect of disorder on the open-circuit voltage . . . . . . . . . . .169 5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .172 6 Influence of Injection and Extraction Barriers on Open-Circuit Voltage and J-V Curve Shape studied at a Variation of Hole Transport Layer and Donor Materials 6.1 Methodological approach . . . . . . . . . . . . . . . . . . . . . . . . . . . .174 6.2 Current-voltage data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 6.2.1 Fingerprints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 6.2.2 Current-voltage characteristics under illumination . . . . . . . . . 181 6.3 Detailed microscopic explanations . . . . . . . . . . . . . . . . . . . . . .181 6.3.1 Injection barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .184 6.3.2 Extraction barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .187 6.3.3 Comparison between flat and bulk heterojunction . . . . . . . . . 188 6.4 Current-voltage curves in a logarithmic plot . . . . . . . . . . . . . . .188 6.5 Detailed analysis of the material combination MeO-TPD and BPAPF as donor and hole transport layer . . . . . . . . . . . . . . . . . . . . . . . . . . 190 6.5.1 The interfaces BPAPF/MeO-TPD and MeO-TPD/BPAPF measured by photoelectron spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 190 6.5.2 Dependence of the J-V curve shape on layer thicknesses . . . . 195 6.5.3 Dependence of the S-kink on temperature . . . . . . . . . . . . . . 198 6.5.4 Transient measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 200 6.6 Summary and final remarks . . . . . . . . . . . . . . . . . . . . . . . . . . 207 7 Imbalanced Mobilities causing S-shaped J-V Curves in Planar Heterojunction Solar Cells 7.1 Imbalanced mobilities in simulation . . . . . . . . . . . . . . . . . . . . . 209 7.2 Experimental verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 7.2.1 Current-voltage characteristics . . . . . . . . . . . . . . . . . . . . . . 216 7.2.2 Transient photocurrents . . . . . . . . . . . . . . . . . . . . . . . . . . 219 7.3 Field-dependent exciton dissociation as an additional source of S-kinks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .221 7.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 8 Open-Circuit Voltage and J-V Curve Shape of ZnPc:C60 Solar Cells with Varied Mixing Ratio and Hole Transport Layer 8.1 Experimental approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . .223 8.2 The open-circuit voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . .225 8.3 The role of the hole transport layer and of doping . . . . . . . . . .228 8.4 Explaining the open-circuit voltage as a function of mixing ratio 230 8.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 9 Effect of Concentration Gradients in ZnPc:C60 Bulk Heterojunction Solar Cells 9.1 Investigated devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 9.2 Current-voltage results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238 9.2.1 Fill factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 9.2.2 Short-circuit current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 9.2.3 Open-circuit voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 9.3 Voltage dependent external quantum efficiency data . . . . . . . . 245 9.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .247 10 Role of the Generation Profile and Recombination in ZnPc:C60 Solar Cells 10.1 Idea and solar-cell design . . . . . . . . . . . . . . . . . . . . . . . . . . 249 10.1.1 Absorption data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 10.1.2 Simulated generation profiles . . . . . . . . . . . . . . . . . . . . . . 253 10.2 Correlation of fill factor with generation profile and imbalance in mobilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 10.2.1 Current-voltage data . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 10.2.2 Monochromatic J-V curves . . . . . . . . . . . . . . . . . . . . . . . . 258 10.2.3 Voltage dependent external quantum efficiency . . . . . . . . . 259 10.3 Recombination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 10.3.1 Exponential region of dark J-V curves . . . . . . . . . . . . . . . . 261 10.3.2 J-V data dependent on illumination intensity . . . . . . . . . . . 265 10.3.3 Lifetime of charge carriers . . . . . . . . . . . . . . . . . . . . . . . . 271 10.4 Comparison with simulations . . . . . . . . . . . . . . . . . . . . . . . . 273 10.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 11 Linear Saturation Behavior 11.1 Definition of the photoshunt . . . . . . . . . . . . . . . . . . . . . . . . 279 11.2 Quasi-linear photocurrent in simulation . . . . . . . . . . . . . . . . 280 11.3 Experimental approach and results . . . . . . . . . . . . . . . . . . . 281 11.3.1 Identification of the main source of the photoshunt . . . . . . 283 11.3.2 Investigation of the thickness dependence of the saturation 285 11.3.3 Photoshunt in flat heterojunction ZnPc/C60 solar cells . . . . 289 11.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 III Summary and Outlook 12 Main Results 12.1 Interpretation of current-voltage curves . . . . . . . . . . . . . . . . 295 12.2 Stack design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300 12.3 Main conclusions on the applicability of the developed drift-diffusion simulation to organic solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . 302 13 Further Analyses and Possible Extensions of the Simulation 13.1 Frequency response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 13.2 Reverse tunneling currents and tandem cells . . . . . . . . . . . . . 307 13.2.1 Reverse current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 13.2.2 J-V curves of tandem cells . . . . . . . . . . . . . . . . . . . . . . . . 309 13.3 Further points to examine . . . . . . . . . . . . . . . . . . . . . . . . . . 311 Appendix A Lists A.1 List of symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 A.2 List of abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318 A.3 List of constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 B Simulation data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 C Experimental data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 Acknowledgments - Danksagung 361Diese Dissertation beschäftigt sich mit der Physik organischer Solarzellen. Die organische Photovoltaik ist ein Forschungsgebiet, dem in den letzten zehn Jahren enorme Aufmerksamkeit zu Teil wurde. Der Grund liegt darin, dass diese neuartigen Solarzellen, deren aktueller Rekordwirkungsgrad bei 10 Prozent liegt, ein Potential für eine kostengünstige Produktion auf flexiblem (Polymer)substrat aufweisen und aufgrund ihrer Vielfältigkeit neue Anwendungsbereiche für die Photovoltaik erschließen. Organische Solarzellen bestehen aus ultradünnen (einige 10 nm) Schichten aus Kohlenwasserstoffverbindungen. Damit der photovoltaische Effekt genutzt werden kann, müssen die durch Licht angeregten Molekülzustände zu freien Ladungsträgern führen, wobei positive und negative Ladung an unterschiedlichen Kontakten extrahiert werden. Für eine effektive Trennung dieser stark gebundenden lokalisierten angeregten Zustände (Exzitonen) ist eine Grenzfläche zwischen Molekülen mit unterschiedlichen Energieniveaus der Grenzorbitale erforderlich, sodass ein Elektron auf einem Akzeptor- und eine positive Ladung auf einem Donatormolekül entstehen. Diese Grenzschicht kann als planarer Heteroübergang durch zwei getrennte Schichten oder als Volumen-Heteroübergang in einer Mischschicht realisiert werden. Die Absorberschichten werden durch Elektroden kontaktiert, wobei es für effiziente Solarzellen erforderlich ist, dass diese einen ohmschen Kontakt ausbilden, da ansonsten Verluste zu erwarten sind. Diese Arbeit behandelt im Besonderen die elektrischen Prozesse einer organischen Solarzelle. Dafür wird ein eindimensionales Drift-Diffusionsmodell entwickelt, das den Transport von Exzitonen, deren Trennung an einer Grenzfläche und die Ladungsträgerdynamik beschreibt. 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Es wird informiert über eine qualitativ-inhaltsanalytische Einzelfallstudie, in welcher Gesprächsinhalte ausgewählter Transkriptionen der ambulanten Kurztherapie einer weiblichen Angstpatientin systematisch kodiert und therapieverlaufsbezogen ausgewertet wurden. Als Ergebnisse werden die Kategorien "Beziehung der Patientin zu ihrem Ehemann" und "Beziehung der Patientin zum Therapeuten" vorgestellt. Die Behandlung ermöglichte es der Patientin, ihre Unzufriedenheit mit der ehelichen Beziehung zu artikulieren und gleichzeitig an den Therapeuten libidinöse Wünsche zu richten

Simultaneous lattice engineering and defect control via cadmium incorporation for high‐performance inorganic perovskite solar cells

Doping of all‐inorganic lead halide perovskites to enhance their photovoltaic performance and stability has been reported to be effective. Up to now most studies have focused on the doping of elements in to the perovskite lattice. However, most of them cannot be doped into the perovskite lattice and the roles of these dopants are still controversial. Herein,the authors introduce CdI2 as an additive into CsPbI3−xBr x and use it as active layer to fabricate high‐performance inorganic perovskite solar cells (PSCs). Cd with a smaller radius than Pb can partially substitute Pb in the perovskite lattice by up to 2 mol%. Meanwhile, the remaining Cd stays on the surface and grain boundaries (GB) of the perovskite film in the form of Cs2CdI4−xBr−x, which is found to reduce non‐radiative recombination. These effects result in prolonged charge carrier lifetime, suppressed defect formation, decreased GBs, and an upward shift of energybands in the Cd‐containing film. A champion efficiency of 20.8% is achieved for Cd‐incorporated PSCs, together with improved device ambient stability. This work highlights the importance of simultaneous lattice engineering, defectcontrol and atomic‐level characterization in achieving high‐performance inorganic PSCs with well‐defined structure‐property relationships

When photoluminescence, electroluminescence, and open-circuit voltage diverge : light soaking and halide segregation in perovskite solar cells

Perovskite solar cells suffer from various instabilities on all time scales. Some of them are driven by light, in particular when employing compounds with mixed halides. Such light soaking effects have been observed in performance changes of solar-cell devices. They have also been spectroscopically investigated in detail on films, where the formation of a low-gap iodine rich phase, seen in a red shift of the PL has been made responsible for a reduced open-circuit voltage. However, studies synchronously examining device performance and its relation to spectroscopy data, are scarce. Here, we perform an in-operandum study, where we investigate changes of open-circuit voltage (Voc) and photocurrent during light soaking and complement it with photo- (PL) and electroluminescence (EL) data on devices, which allow analysis of the Voc-limiting processes using optical and optoelectronic reciprocity relations. We find that changes in the Voc for stable single halide compositions are quantitatively correlated with changes in the PL intensity, showing that the Voc follows changes in the quasi-Fermi level splitting. In contrast, changes in Voc for the mixed halide composition are not correlated to the emergence of the low-gap phase, confirming that this phase is not the sole culprit for a low and instable Voc. Instead, non-radiative voltage losses influenced by mobile ions are dominant in devices containing compositions with high Br content. Interestingly, the low-gap phase contributes less to photocurrent, as seen by a wavelength-dependent PL quenching at short circuit. This observation might be explained by the formation of emissive but partially insulated iodine-rich regions in the film. Such an effect is also possible for single halide systems, when the perovskite composition is not stable, seen in an increase of PL at short circuit during light soaking. This indicates that ion migration in general causes photovoltaically inactive regions, without enhancing non-radiative recombination. EL measurements confirm that Rau’s reciprocity relation between external EL quantum efficiency and Voc cannot readily be applied to absorbers with such different phases.Perovskite solar cells suffer from various instabilities on all time scales. Some of them are driven by light, in particular when employing compounds with mixed halides. Such light soaking effects have been observed in performance changes of solar-cell devices. They have also been spectroscopically investigated in detail on films, where the formation of a low-gap iodine rich phase, seen in a red shift of the PL has been made responsible for a reduced open-circuit voltage. However, studies synchronously examining device performance and its relation to spectroscopy data, are scarce. Here, we perform an in-operandum study, where we investigate changes of open-circuit voltage (Voc) and photocurrent during light soaking and complement it with photo- (PL) and electroluminescence (EL) data on devices, which allow analysis of the Voc-limiting processes using optical and optoelectronic reciprocity relations. We find that changes in the Voc for stable single halide compositions are quantitatively correlated with changes in the PL intensity, showing that the Voc follows changes in the quasi-Fermi level splitting. In contrast, changes in Voc for the mixed halide composition are not correlated to the emergence of the low-gap phase, confirming that this phase is not the sole culprit for a low and instable Voc. Instead, non-radiative voltage losses influenced by mobile ions are dominant in devices containing compositions with high Br content. Interestingly, the low-gap phase contributes less to photocurrent, as seen by a wavelength-dependent PL quenching at short circuit. This observation might be explained by the formation of emissive but partially insulated iodine-rich regions in the film. Such an effect is also possible for single halide systems, when the perovskite composition is not stable, seen in an increase of PL at short circuit during light soaking. This indicates that ion migration in general causes photovoltaically inactive regions, without enhancing non-radiative recombination. EL measurements confirm that Rau’s reciprocity relation between external EL quantum efficiency and Voc cannot readily be applied to absorbers with such different phases

Double layer composite electrode strategy for efficient perovskite solar cells with excellent reverse-bias stability

Perovskite solar cells (PSCs) have become the representatives of next generation of photovoltaics; nevertheless, their stability is insufficient for large scale deployment, particularly the reverse bias stability. Here, we propose a transparent conducting oxide (TCO) and low-cost metal composite electrode to improve the stability of PSCs without sacrificing the efficiency. The TCO can block ion migrations and chemical reactions between the metal and perovskite, while the metal greatly enhances the conductivity of the composite electrode. As a result, composite electrode-PSCs achieved a power conversion efficiency (PCE) of 23.7% (certified 23.2%) and exhibited excellent stability, maintaining 95% of the initial PCE when applying a reverse bias of 4.0 V for 60 s and over 92% of the initial PCE after 1000 h continuous light soaking. This composite electrode strategy can be extended to different combinations of TCOs and metals. It opens a new avenue for improving the stability of PSCs