Les polyoxométallates (POM) sont des composés anioniques constitués par l’assemblage de polyèdres d’oxydes métalliques {MOy}, (avec M, MoVI ou WVI) reliés entre eux par des atomes d'oxygène. Les POM forment ainsi une classe remarquable de clusters d’oxydes métalliques inorganiques nanométriques, avec une grande variété de charges et de structures. Il est possible de former des systèmes hybrides incluant la partie inorganique du POM et une partie organique greffée, permettant d’apporter de nouvelles fonctionnalités aux POM, tel que l’auto-assemblage. Nous avons consacré ces travaux de thèse à la caractérisation de systèmes classiques, hybrides et auto-assemblés de POM par spectrométrie de masse couplée à la spectrométrie à la mobilité ionique (IMS-MS). Une première approche expérimentale par spectrométrie de mobilité ionique en tube de dérive (DTIMS) nous a permis de déterminer les sections efficaces de collisions (CCS) de POM étalons dans l’hélium et dans l’azote. Les CCS des étalons POM nous ont ensuite permis d’étalonner une cellule IMS de type Travelling Wave (TWIMS). L’analyse par IMS-MS de POM hybrides organiques-inorganiques seuls ou en présence de PdCl2 a mis en évidence la présence de systèmes auto-assemblés triangulaires [POM3·cation3], carrés [POM4·cation4] ou pentagonaux [POM5·cation5] avec différents états de charges. Des valeurs de CCS de ces auto-assemblages ont également pu être estimées à partir de l’étalonnage de la cellule TWIMS. Par une approche théorique, nous avons modélisé plusieurs structures de POM standards avec et sans contre-ion tetrabutylammonium (TBA+) par la théorie de la fonctionnelle de la densité (DFT). Les structures optimisées ont été utilisées afin de déterminer des CCS théoriques grâce au logiciel MOBCAL, auquel nous avons incorporé les atomes de molybdène et de tungstène pour lesquels nous avons optimisé de nouveaux paramètres de potentiel de Lennard Jones. La correspondance des CCS expérimentales et théoriques des structures de POM standards offre de nouvelles possibilités pour une attribution structurale pour les POM hybrides auto-assemblés par coordination en présence de cations métalliques. / Polyoxometalates (POM) are anionic compounds formed by the assembly of metal oxide polyhedra {MOy}, (with M, MoVI or WVI) linked together by oxygen atoms. POM thus form a remarkable class of nanometric inorganic metal oxide clusters, with a wide variety of charges and structures. It is possible to form hybrid systems including the inorganic part of the POM and a grafted organic part, allowing new functionalities to be added to the POM, such as selfassembly. We have dedicated this thesis work to the characterization of standards, hybrid and self-assembled POM systems by mass spectrometry coupled to ion mobility spectrometry (IMS-MS). A first experimental approach using drift tube ion mobility spectrometry (DTIMS) allowed us to determine the collision cross sections (CCS) of standard POM in helium and nitrogen. The CCS of the POM standards then allowed us to calibrate an IMS cell of a Travelling Wave ion mobility instrument (TWIMS). The analysis by IMS-MS of organic-inorganic hybrid POMs alone or in the presence of transition metal cations revealed the presence of self-assembled triangular [POM3·cation3], square [POM4·cation4] or pentagonal [POM5·cation5] systems with different charge states. CCS values of these self-assemblies was estimated from the calibration of the TWIMS cell. Using a theoretical approach, we modelled several standard POM structures with and without tetrabutylammonium counterion (TBA+) using density functional theory (DFT). The optimized structures were used to determine theoretical CCS using the trajectory method of the MOBCAL software, in which we incorporated molybdenum and tungsten atoms for which we optimized new Lennard Jones potential parameters. The correspondence of experimental and theoretical CCS of standard POM structures offers new possibilities for structural attribution of self-assembled hybrid POM by coordination in the presence of metal cations.

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 361 / Diese 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. Abgesehen von den Exzitonen gilt als weitere Besonderheit einer organischen Solarzelle, dass sie aus amorphen, intrinsischen und sehr schlecht leitfähigen Absorberschichten besteht. Elektrische Effekte sind an der Strom-Spannungskennlinie (I-U ) sichtbar, die in dieser Arbeit als Hauptvergleichspunkt zwischen experimentellen Solarzellendaten und den Simulationsergebnissen dient. Durch einen weitgehend qualitativen Vergleich können dominierende Prozesse bestimmt und mikroskopische Erklärungen gefunden werden. Ein wichtiger Punkt ist der schon erwähnte Kontakt zwischen Absorberschicht und Elektrode. Dort auftretende Energiebarrieren führen zu einem Einbruch im Solarzellenwirkungsgrad, der sich durch eine Verringerung der Leerlaufspanung und/oder S-förmigen Kennlinien (S-Knick) bemerkbar macht. Anhand einer systematischen Studie der Grenzfläche Lochleiter/Donator wird gezeigt, dass Energiebarrieren sowohl für die Ladungsträgerextraktion als auch für die -injektion zu S-Knicken führen können. Insbesondere die Tatsache, dass Injektionsbarrieren sich auch negativ auf den Photostrom auswirken, wird anhand von simulierten Ladungsträger- und elektrischen Feldprofilen erklärt. Das Aufstauen von Ladungsträgern an Extraktionsbarrieren wird durch Messungen transienter Photoströme bestätigt. Da S-Knicke in organischen Solarzellen im Allgemeinen häufig beobachtet werden, werden weitere Methoden vorgeschlagen, die die Identifikation der Ursachen ermöglichen. Dazu zählen I-U Messungen in Abhängigkeit von Temperatur und Schichtdicken. Als eine weitere Ursache von S-Knicken werden unausgeglichene Ladungsträgerbeweglichkeiten in einer Solarzelle mit flachem Übergang identifiziert und von den Barrierefällen unterschieden. Weiterer Forschungsgegenstand dieser Arbeit sind Mischschichtsolarzellen aus dem Donator-Farbstoff Zink-Phthalozyanin ZnPc und dem Akzeptor Fulleren C60. Dort wird beobachtet, dass die Leerlaufspannung vom Mischverhältnis abhängt. Ein Vergleich von Experiment und Simulation zeigt, dass sich das Ionisationspotenzial von ZnPc und dadurch die effektive Energielücke des Mischsystems ändern. Zusätzlich zu homogenen Mischschichten werden Solarzellen untersucht, die einen Gradienten im Mischungsverhältnis aufweisen. Die Vermutung liegt nahe, dass ein hoher Donatorgehalt am Löcherkontakt und ein hoher Akzeptorgehalt nahe des Elektronenkontakts die Ladungsträgerextraktion begünstigen. Dieser Effekt ist in dem hier untersuchten System allerdings vergleichsweise irrelevant gegenüber der Tatsache, dass der Gradient das Abfließen bzw. die Rekombination von Ladungsträgern am “falschen” Kontakt reduziert und somit die Leerlaufspannung erhöht. Der wichtigste intrinsische Verlustmechanismus einer Solarzelle ist die Rekombination von Ladungsträgern. Diese wird im letzten Teil der Arbeit anhand der ZnPc:C60 Solarzelle behandelt. Messungen der Leerlaufspannung in Abhängigkeit von der Beleuchtungsintensität zeigen, dass sich der dominierende Rekombinationsprozess mit zunehmender Intensität von Störstellenrekombination zu direkter Rekombination von freien Ladungsträgern verschiebt. Eine gezielte Variation des Absorptionsprofils in der Absorberschicht zeigt, dass die Ladungsträgerextraktionswahrscheinlickeit vom Ort der Ladungsträgergeneration abhängt. Dieser Effekt wird hervorgerufen durch unausgeglichene Elektronen- und Löcherbeweglichkeiten und äußert sich im Füllfaktor. Weitere Simulationsergebnisse bezüglich des Einflusses von Ladungsträgerbeweglichkeiten und verschiedener Rekombinationsmechanismen auf die I-U Kennlinie und die experimentelle Identifikation eines Photoshunts, der den Photostrom in Rückwärtsrichtung unter Beleuchtung dominiert, runden die Arbeit ab.: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 361

The stacked polyphase bridges (SPB) converter topology is investigated in the presentthesis as a fault-tolerant choice for permanent-magnet synchronous reluctance (PMSyn-Rel) motor control. Integrated motor drive systems are studied as they offer great benefitsfor propulsion applications. Moreover, the importance of a modular topology, like theSPB, for an electric powertrain is discussed. The latter consists of a number of seriesconnected, 3-phase 2-level inverter submodules that supply separate sets of windings ina multi-star motor. The specifications of building a four-board SPB setup are examined,while the challenges of an active voltage balancing controller are analyzed. The designprocess is explained step-by-step and the final printed circuit boards (PCBs) are presented.Furthermore, the significance of low electromagnetic interference design for a converterthat requires high speed communication is highlighted. Finally, the prototype is testedthoroughly and the expected fault-tolerant capabilities are validated on a PMSynRel motor. / I detta examensarbete unders¨ok SPB-omriktartopologin (stacked polyphase bridges converter)i termer av ett feltolerant elektriskt drivsystem f¨or en permamentmagnetassisteradsynkron reluktansmaskin (PMSynRel). SPB-omriktaren best°ar av ett antal seriekoppladetrefasomriktare av tv°aniv°atyp som, var och en, f¨orser effekt till en trefaslindningtillh¨orande en modul¨ar elmaskin av multifastyp. Specifikation, design och konstruktion aven SPB-omriktare med fyra seriekopplade moduler studeras. Designproceduren presenterasi en steg-f¨or-steg-process och de tillverkade kretskorten presenteras utf¨orligt. Kommunikationenmellan de olika kretskorten unders¨oks med s¨arskild tonvikt p°a l°ag elektromagnetiskinterferens vilket ¨ar n¨odv¨andigt om h¨og kommunikationshastighet skall kunnauppn°as. Den f¨ardigst¨allda prototypen har utv¨arderats experimentellt och kapaciteten f¨orfeltolerans har demonstrerats vid drift av en PMSynRel-maskin utrustad med en multifaslindning.

Sedan outsourcing blev vanligare på den svenska marknaden har det även blivit vanligt att i förvaltning upphandla sin drift i konkurrens. I ett mycket tidigt skede uppmärksammandes en problematik med samarbetet mellan beställare och driftentreprenör som innebar att fakturahanteringen mellan parterna var omfattande och tidkrävande. För att bland annat minska administrationen upprättades en självriskmodell som många beställare idag tillämpar i kontrakten, vilken innebär att driftentreprenören ersätter felavhjälpande underhållsåtgärdskostnader upp till ett visst gränsbelopp.  Syftet med fallstudien är att undersöka modellens uppbyggnad och tillämpning samtatt analysera vad beställare och entreprenör anser om självriskmodellen. Studien bygger främst på primär insamlad data genom ett antal intervjuer med främst en beställare, Locum samt en sekundär datainsamling i form av en enkätundersökning besvarad av verksamma entreprenörer inom branschen. För att bibehålla det offentliga fastighetsbeståndet och för att se till brukarens bästa måste en förståelse finnas mellan beställare och entreprenör. Den lösningen som idag finns för att upprätthålla kvalité och service i en funktionsupphandling är att använda självriskmodellen. För beställaren fungerar modellen som ett incitament att funktionskraven uppnås och för att minska fakturahanteringen. Hur modellen uppfattas på marknaden med positiva och negativa aspekter skiljer sig markant beroende på vilka personliga erfarenheter en part har av den. / Since outsourcing became more common in the Swedish market, it has also become common for the management to procure its operation competitively. In a very early stage there was a problem recognized, the invoice between client and contractor was extensive and time consuming. Partly to reduce administration a deductible model was established which many clients today applies to the contracts. This means that the operating contractor himself may replace corrective maintenance action costs up to a certain amount. The purpose of this case study is to examine the model's structure and implementation, as well as to analyze clients and contractors consider of the deductible model. The study is based on primary data collected through a number of interviews with primarily client and a secondary data collection in the form of a questionnaire answered by contractors within the industry. In order to maintain the public housing stock and to ensure the patient's best interests there must be an understanding between the client and the contractor. The solution to maintain quality and service in a procurement function the client uses today the deductible model. The model serves as an incentive for the client to make sure the functional requirements are achieved and to reduce invoice processing. How the model is perceived in the market with positive and negative aspects differ markedly depending on the different experiences of it

<p>The miniaturization and thinning of wearable, soft robotics and medical devices are soon to require higher performance modeling as the physical flexibility causes direct impacts on the electrical characteristics of the circuit – changing its behavior. As a representative flexible electronic component, the organic field effect transistor (OFET) has attracted much attention in its manufacturing as well as applications. However, as the strain and stress effects are integrated into multiphysics modelers with deeper interactions, the computational complexity and accuracy of OFET modeling is resurfacing as a limiting bottleneck.</p><p>The dissertation was organized into three interrelated studies. In the first study, the Mass-Spring-Damper (MSD) model for an inverted staggered thin film transistor (TFT) was proposed to investigate the TFT’s internal stress/strain fields, and the strain effects on the overall characteristics of the TFT. A comparison study with the finite element analysis (FEA) model shows that the MSD model can reduce memory usage and raises the computational convergence speed for rendering the same results as the FEA. The second study developed the generalized solid-state model by incorporating the density of trap states in the band structure of organic semiconductors (OSCs). The introduction of trap states allows the generalized solid-state model to describe the electrical characteristics of both inorganic TFTs and organic field-effect transistors (OFETs). It is revealed through experimental verification that the generalized solid-state model can accurately characterize the bending induced electrical properties of an OFET in the linear and saturation regimes. The third study aims to model the transient and steady-state dynamics of an arbitrary organic semiconductor device under mechanical strain. In this study, the fractional drift-diffusion (Fr-DD) model and its computational scheme with high accuracy and high convergence rate were proposed. Based on simulation and experimental validation, the transconductance and output characteristics of a bendable OFET were found to be well determined by the Fr-DD model not only in the linear and saturation regimes, but also in the subthreshold regime.</p>

The topic of this thesis is a diffusion process on a potential landscape which is given by a smooth Hamiltonian function in the regime of small noise. The work provides a new proof of the Eyring-Kramers formula for the Poincaré inequality of the associated generator of the diffusion. The Poincaré inequality characterizes the spectral gap of the generator and establishes the exponential rate of convergence towards equilibrium in the L²-distance. This result was first obtained by Bovier et. al. in 2004 relying on potential theory. The presented approach in the thesis generalizes to obtain also asymptotic sharp estimates of the constant in the logarithmic Sobolev inequality. The optimal constant in the logarithmic Sobolev inequality characterizes the convergence rate to equilibrium with respect to the relative entropy, which is a stronger distance as the L²-distance and slightly weaker than the L¹-distance. The optimal constant has here no direct spectral representation. The proof makes use of the scale separation present in the dynamics. The Eyring-Kramers formula follows as a simple corollary from the two main results of the work: The first one shows that the associated Gibbs measure restricted to a basin of attraction has a good Poincaré and logarithmic Sobolev constants providing the fast convergence of the diffusion to metastable states. The second main ingredient is a mean-difference estimate. Here a weighted transportation distance is used. It contains the main contribution to the Poincaré and logarithmic Sobolev constant, resulting from exponential long waiting times of jumps between metastable states of the diffusion.

Dans le traitement de langue et des système de dialogue, il est courant de pré-entraîner des modèles de langue sur corpus humain avant de les affiner par le biais d'un simulateur et de résolution de tâches. Malheuresement, ce type d'entrainement tend aussi à induire un phénomène connu sous le nom de dérive du langage. Concrétement, les propriétés syntaxiques et sémantiques de la langue intiallement apprise se détériorent: les agents se concentrent uniquement sur la résolution de la tâche, et non plus sur la préservation de la langue. En s'inspirant des travaux en sciences cognitives, et notamment l'apprentigssage itératif Kirby and Griffiths (2014), nous proposons ici une approche générique pour contrer cette dérive du langage. Nous avons appelé cette méthode Seeded iterated learning (SIL), ou apprentissage itératif capitalisé. Ce travail a été publié sous le titre (Lu et al., 2020b) et est présenté au chapitre 2. Afin d'émuler la transmission de la langue entre chaque génération d'agents, un agent étudiant est d'abord pré-entrainé avant d'être affiné de manière itérative, et ceci, en imitant des données échantillonnées à partir d'un agent enseignant nouvellement formé. À chaque génération, l'enseignant est créé en copiant l'agent étudiant, avant d'être de nouveau affiné en maximisant le taux de réussite de la tâche sous-jacente. Dans un second temps, nous présentons Supervised Seeded iterated learning (SSIL) dans le chapitre 3, où apprentissage itératif capitalisé avec supervision, qui a été publié sous le titre (Lu et al., 2020b). SSIL s'appuie sur SIL en le combinant avec une autre méthode populaire appelée Supervised SelfPlay (S2P) (Gupta et al., 2019), où apprentissage supervisé par auto-jeu. SSIL est capable d'atténuer les problèmes de S2P et de SIL, i.e. la dérive du langage dans les dernier stades de l'entrainement tout en préservant une plus grande diversité linguistique. Tout d'abord, nous évaluons nos méthodes dans sous la forme d'une preuve de concept à traver le Jeu de Lewis avec du langage synthetique. Dans un second temps, nous l'étendons à un jeu de traduction se utilisant du langage naturel. Dans les deux cas, nous soulignons l'efficacité de nos méthodes par rapport aux autres méthodes de la litterature. Dans le chapitre 1, nous discutons des concepts de base nécessaires à la compréhension des articles présentés dans les chapitres 2 et 3. Nous décrivons le problème spécifique du dialogue orienté tâche, y compris les approches actuelles et les défis auxquels ils sont confrontés : en particulier, la dérive linguistique. Nous donnons également un aperçu du cadre d'apprentissage itéré. Certaines sections du chapitre 1 sont empruntées aux articles pour des raisons de cohérence et de facilité de compréhension. Le chapitre 2 comprend les travaux publiés sous le nom de (Lu et al., 2020b) et le chapitre 3 comprend les travaux publiés sous le nom de (Lu et al., 2020a), avant de conclure au chapitre 4. / In task-oriented dialogue, pretraining on human corpus followed by finetuning in a simulator using selfplay suffers from a phenomenon called language drift. The syntactic and semantic properties of the learned language deteriorates as the agents only focuses on solving the task. Inspired by the iterative learning framework in cognitive science Kirby and Griffiths (2014), we propose a generic approach to counter language drift called Seeded iterated learning (SIL). This work was published as (Lu et al., 2020b) and is presented in Chapter 2. In an attempt to emulate transmission of language between generations, a pretrained student agent is iteratively refined by imitating data sampled from a newly trained teacher agent. At each generation, the teacher is created by copying the student agent, before being finetuned to maximize task completion.We further introduce Supervised Seeded iterated learning (SSIL) in Chapter 3, work which was published as (Lu et al., 2020a). SSIL builds upon SIL by combining it with the other popular method called Supervised SelfPlay (S2P) (Gupta et al., 2019). SSIL is able to mitigate the problems of both S2P and SIL namely late-stage training collapse and low language diversity. We evaluate our methods in a toy setting of Lewis Game, and then scale it up to the translation game with natural language. In both settings, we highlight the efficacy of our methods compared to the baselines. In Chapter 1, we talk about the core concepts required for understanding the papers presented in Chapters 2 and 3. We describe the specific problem of task-oriented dialogue including current approaches and the challenges they face: particularly, the challenge of language drift. We also give an overview of the iterated learning framework. Some sections in Chapter 1 are borrowed from the papers for coherence and ease of understanding. Chapter 2 comprises of the work published as (Lu et al., 2020b) and Chapter 3 comprises of the work published as (Lu et al., 2020a). Chapter 4 gives a conclusion on the work.

<p>  </p> <p>The study of two-phase flow in different orientations can allow for greater understanding of the fundamentals of two-phase flow dynamics. While a large amount of work has been performed for vertical flows and recent work has been done for horizontal flows, limited research has been done studying inclined upward two-phase flows between those two orientations. Studying two-phase flows at various inclinations is important for developing physical models and simulations of two-phase flow systems and understanding the changes between what is observed for symmetric vertical flows and asymmetric horizontal flows. The present work seeks to systematically characterize the effects of inclination on adiabatic concurrent air-water two-phase flows in straight pipes. An experimental database is established for local and global two-phase flow parameters in a novel inclinable 25.4 mm inner diameter test facility using four-sensor conductivity probes, high speed video capabilities, a ring-type impedance meter, a pressure transducer, and a gamma densitometer. Rotatable measurement ports are employed to allow for local conductivity probe measurements across the flow profile to capture asymmetric parameter distributions during experiments without stopping the flow. Some of the major effects of inclination are investigated, including the effects on flow regime transition, bubble distribution, frictional pressure loss, and relative motion between the two phases. Flow visualization and machine-learning methods are employed to identify the transitions between flow regimes for inclined orientations, and these transitions are compared against existing theoretical flow regime transition criteria proposed in literature. The theoretical transitions in literature agree well with both methods for vertical flow, but additional work is necessary for angles between 0 degrees and 60 degrees. The effect of inclination on two-phase frictional pressure drop is explored, and a novel adaption of the Lockhart-Martinelli pressure drop correlation is proposed, which is able to predict the pressure drop for the conditions investigated with an absolute percent difference of 2.6%. To explore the relationships between orientation, void fraction, and relative motion, one-dimensional drift flux analyses are performed for the data at each angle investigated. It is observed that the relative velocity between phases decreases as the angle is reduced, with a relative velocity near zero at some intermediate angles and a negative relative velocity for near-horizontal orientations.  Existing modeling capabilities that have been developed for vertical and horizontal flows are evaluated based on the local two-phase parameters collected at multiple orientations. The performance of the one-dimensional interfacial area transport equation for vertical and horizontal flows is tested against experimental data and a novel model for horizontal and inclined-upward bubbly flows is proposed. Finally, an evaluation of existing momentum transfer relations is performed for the two-fluid model using three-dimensional computational fluid dynamics tools for horizontal and inclined. The prediction of the void fraction distribution and gas velocity profiles are compared against experimental data, and improvements to the lift force model are identified based on changes in the relative velocity between phases. </p>

A short (11 page) plain language summary is available under the filename "Research Summary_Peer Support Institutionalization - Troubling Everyday Work.pdf" / This study explores how the everyday work of peer supporters working within institutionalized settings are shaped by institutional forces (“ruling relations”), through a series of four (peer support) focus groups and interviews with five peer support workers in Ontario. I explore peer supporters’ approaches to writing, reading, and verbally sharing information about their peers (“documentation work”), and reveal how their experiences and “felt troubles” relating to documentation are shaped by ideas of (clinical) confidentiality constructed in the Personal Health Information Protection Act (2004). I also explore how both lived experience and peer support are devalued through the ways organizations and clinicians determine and describe the value of healthcare roles (“de/valuing work”), and reveal how peer supporters’ experiences of being (de)valued are shaped by discourses of “professional/ism” which equate being a professional to having a post-secondary education and working through clinical frameworks. I describe the work that peer supporters, clinicians, and organizations (can) engage in to ground peer support workers within peer values and approaches (“values work”) through accessing peer community and fostering environments of peer culture. I draw on these suggestions and the findings of the study to provide recommendations for peer support workers, organizations and clinical workers, the peer support sector as a whole, and research/ers. / Thesis / Master of Social Work (MSW)

With power generation and consumption placed further away from each other, with for example increased offshore power production, the need for HVDC transmission systems increases. As voltage levels in the HVDC transmission system are raised, the losses can be decreased, enabling efficient power transfer over longer distances. Such an increase in voltage levels comes with questions regarding insulation performance due to increased electrical field stress in high voltage apparatus, particularly in and around cable terminations. Thus, physical measurements of electrical fields or voltage potentials in air are of interest to improve the understanding of the electrical fields around cable terminations as well as to verify and develop simulation models for use at ultra high voltage. In this thesis, different measurement systems and sensors for electrical field measurements are investigated, and their benefits and drawbacks are compared to a known previously implemented reference measurement system with known strengths and limitations. Two new conceptual measurement systems with a sensor concept and positioning system are developed and proposed for a set of given conditions, such as measurement around cable terminations in air. The first proposed system is based on a shutter field mill sensor placed on variable electric potential. The second system is based on the reference system's sensor design modified to remove the need for a large positioning system. The feasibility of the two measurement systems is investigated further utilizing a COMSOL model and a mechanical prototype. The simulation model is used for electrical field simulations around cable terminations in a 2D-axisymmetric geometry as well as a 3D geometry to verify the first measurement system. The mechanical prototype is utilized to test and verify the possibility of implementing the second system's positioning system. The two final proposed measurement systems can be further developed and used as a foundation for a future implemented measurement system. / Ökad elproduktion i form av exempelvis vindkraftsverk placerade till havs leder till elproduktion och konsumtion placerad med större avstånd från varandra, vilket skapar ett behov av HVDC transmissionssystemm. Genom att höja spänningsnivåerna i transmissionssystemet kan förluster minskar, vilket möjligör effektiv kraftöverföring över längre avstånd. En sådan ökning av spänningsnivåerna i transmissionssystemet kommer med obesvarade frågor angående isoleringsprestanda i högspänningsutrustningen på grund av en höjd elektrisk stress, med särskilt intresse gällande påverkan på kabelavslut. Därför är fysiska mätningar av elektriska fält eller potentialer i luft av intresse för att förbättra förståelsen av det elektriska fältet kring kabelavslut, samt att verifiera och utveckla simuleringsmodeller för användning vid ultrahög spänningspotentialer. I detta examensarbete har olika mätsystem och sensorer för elektriska fältmätningar utredits och deras fördelar och nackdelar jämförts med ett känt tidigare implementerat rereferensmätsystem med kända styrkor och begränsningar. Två nya konceptuella mätsystem med sensorkoncept med tillhörande positioneringssystem utvecklas och föreslås för en uppsättning givna förhållanden, som t.ex mätning kring kabelavslutningar i luft. Det första föreslagna systemet är baserat på en fältkvarnssensor placerad på variabel elektrisk potential. De andra systemet är baserat på referenssystemets sensordesign modifierad för att ta bort behovet av ett stort positioneringssystem. Genomförbarheten av de två mätsystem undersöks vidare med användning av en COMSOL-modell och en mekanisk prototyp. Simuleringsmodellen används för elektriska fält simuleringar kring ett kabelavslut i en 2D-axelsymmetrisk geometri samt i en 3D-geometri för att verifiera det första mätsystemet. Mekaniska prototypen används för att testa och verifiera möjligheten att implementera andra systemets positioneringssystem. De två föreslagna mätsystemen kan vidareutvecklas och användas som en grund för ett framtida implementerat mätsystem.