Foram desenvolvidos sistemas de medidas visando a caracterização de células solares de sílico monocristalino. Para isso, foram determinadas as características I x V no escuro para diferentes níveis de iluminação. Curvas de resposta espectral e capacitância em função da tensão inversa aplicada foram também obtidas. Foi feita uma avaliação do comportamento dessas células em função da temperatura e realizadas medidas de profundidade de junção utilizando-se três métodos distintos. Os principais parâmetros, que determinam o desempenho dessas células, foram obtidos boa concordância com a teoria e com os resultados apresentados na literatura. / Systems of measurements were developed for the characterization of single crystal silicon solar cells. For that, the curves I x V were measured in the dark and for different intensity of illumination. Curves of spectral response and of capacitance as a function of the reciprocal of the voltage were also measured. The behavior of the cells as a function of temperature was analysed and also measurements of junction depth were made by three different methods. Values for the parameters that characterize the cells were obtained, showing a good agreement with theoretical values and also with already reported values.

No description available.

<div>In this paper various applications of axial tensile load, bending load, and rolling loading has been applied to a Copper Indium Gallium Diselenide (CIGS) Solar Cell to lean how it would affect the solar cell parameters of: Open circuit voltage (Voc), Short circuit current, (Isc), Maximum power (Pmax), and Efficiency (EFF), and Fill Factor (FF). These Relationships were found for with three different experiments. The first experiment the applies axial tensile stress is to a CIGS solar cell ranging from 0 to 200 psi with various strain rates: 0.0001, 0.001, 0.01, and 0.1 in/sec as well as various relaxation time: 1min, 5min, and 10 min while the performance of solar cell is measured. The results of this gave several trends couple pertaining the Voc . The first is that open circuit voltage increases slightly with increasing stress. The second is the rate of increase (the slope) increases with longer relaxation times. The second set of trend pertains to the Isc. The first is that short circuit current generally is larger with larger stress. The second is there seems to be a general increase in the Isc up to a given threshold of stress. After that threshold the Isc seems to decrease. The threshold stress varies depending on strain rate and relaxation time. The second set of experiments consisted of holding a CIGS solar cell in a fixed curved position while it was in operational use. The radii of the curved cells were: 0.41, 0.20, 0.16, 0.13, 0.11, 0.094, and 0.082 m. The radii were performed for both concave and convex cell curvature. The trends for this show a slight decrease in all cell parameters with decreasing radii, the exception being Voc which is not effecting, the convex curvature causing a slightly faster decrease than the concave. This set of experiments were also processed to find the trends of the single diode model parameters of series resistance (Rs), shunt resistance (Rsh), dark current (I0), and saturation current (IL), which agreed with the experimental results. The second experiment consisted of rolling a CIGS solar cell in tensile (cells towards dowel.) and compression (cells away from dowel) around a dowel to create internal damage. The diameter of the dowels decreased. The dowel diameters were: 2. 1.75, 1.25, 1, 0.75, 0.5, and 0.25 inches. This experiment showed similar trends as the bending one but also had a critical diameter of 1.75 in where beyond that damage much greater. Finally a parametric study was done in COMSOL Multiphysics® to examine how changes in the CIGS material properties of electron mobility (EM), electron life time, (EL), hole mobility 15 (HM), and Hole life time (HL) effect the cell parameters. The trends are of an exponential manner that converges to a given value as the material properties increase. When EL, EM, HL are very small, on the order of 10-4 times smaller than their accepted values, a transient like responses occurs.<br></div>

O trabalho realizado nesta tese esteve apoiado em dois objetivos principais. O primeiro centrado na otimização das etapas e processos de fabricação de células solares de silício de alto rendimento envolvendo redução de custos. O segundo objetivo foi direcionado na implementação de células solares eficientes e não dependentes do armadilhamento de impurezas através da difusão de alumínio. Para levar a cabo estes objetivos de forma planejada, o trabalho dividiu-se em otimizações teóricas e implementações experimentais. As otimizações teóricas foram realizadas utilizando dois programas: um programa desenvolvido (simulacell.pas) e implementado no próprio LME (versão 2), e o outro adquirido comercialmente, PC1D. De acordo com os resultados obtidos em estruturas completas n+p e n++n+p foi possível concluir que tanto as estruturas formadas através de emissores homogêneos como as obtidas utilizando emissores duplamente difundidos permitem alcançar eficiências elevadas, 25,5% a 26,0%, respectivamente, em um amplo intervalo de espessuras e concentrações superficiais de dopantes. No que tange aos desenvolvimentos experimentais, este trabalho se inicia com o desenvolvimento de um processo simplificado de baixo custo, em células solares de silício Cz de baixa resistividade com estrutura n+pp+, tipo \"mesa\". Este processo simplificado também está baseado na difusão de fósforo e alumínio (P/Al), utilizando gases industriais e reagentes químicos de grau \"para análise\", como uma transposição do processo de fabricação anteriormente desenvolvido no LME-EPUSP em substratos de silício FZ utilizando tecnologia planar. A célula solar mais representativa do processo implementado, A-16-1, permitiu atingir eficiências no entorno de 17%. As implementações experimentais visaram inicialmente o desenvolvimento de um procedimento visando à qualificação de materiais de partida (silício), utilizando a técnica de decaimento fotocondutivo (PCD) através de dois procedimentos de passivação de superfícies; oxidações térmicas e difusões suaves de fósforo. Posteriormente, utilizando o sistema PCD, novas otimizações dos emissores de tipo n+ homogêneos e regiões de tipo p foram realizadas, seguidos por oxidações térmicas passivadoras hidrogenadas, preservando-se o tempo de vida do volume em valores elevados (aproximadamente 1ms, após a realização de todas as etapas térmicas). Estes resultados qualificam o silício e os materiais de consumo utilizados, assim como, o novo processo de fabricação desenvolvido. Esta técnica também permitiu qualificar os emissores com perfil Gaussianos processados, atingindo valores da ordem de 45fA/cm2 para densidades de recombinação em estruturas n+pn+. Desenvolveram-se também estruturas n+p em materiais Cz de baixa resistividade 2-3W.cm de dois diferentes fabricantes, e silício FZ com 0,5W.cm. Pôde ser comprovada a qualidade das etapas que compõem o processo completo otimizado tendo-se obtido tensões de circuito aberto-implícitas de 652,4mV (Si-Cz fabricante 1) e 662,6mV (Si-Cz fabricante 2), e 670,8mV (FZ). De acordo com simulações realizadas utilizando parâmetros habituais de dispositivos do próprio LME, estas tensões, quando associadas a um conjunto óptico frontal típico das células solares de alto rendimento do LME (texturização química aleatória e filme de SiO2), permitirão atingir valores entre 19% - 20%. Entretanto, utilizando texturização e camada dupla torna-se plausível atingir o marco de 21% de rendimento, ultrapassando assim a barreira dos 17% (recorde nacional), e comprovando a potencialidade da infra-estrutura deste laboratório para o desenvolvimento de células solares não dependentes do efeito do armadilhamento de impurezas através da difusão de alumínio. / The work developed at this thesis has been based on two main objectives. First, it was focused on the optimization of the steps and processes for the fabrication of high efficiency solar cells, reducing production costs. The latter objective was directed to develop solar cells that were efficient and non-dependent on impurities gettering performed through the aluminum diffusion. In order to attend the planned objectives the work was divided into the theoretical objectives and experimental developments. The theoretical optimizations were performed using two different program codes: one was developed at LME (simulacell.pas), being upgraded afterwards (version 2); and the other was acquired commercially, the PC1D. According to the obtained results in complete structures n+p and n++n+p, it was possible to conclude that the homogeneous and double diffused emitter structures can provide high efficiencies, from 25,5% to 26,0%, respectively, for a wide range of thicknesses and surface doping levels. Concerning the experimental developments, this work starts with a low cost simplified process, using Cz silicon solar cells with low base resistivity and the structure n+pp+, \"mesa\" type. This simplified process was also based on the phosphorus/ aluminum diffusion (P/Al), using industrial gases and for analysis grade chemical reagents, as a fabrication process transposition of the process previously developed at LME-EPUSP using silicon substrates with planar technology. The most representative solar cells of the implemented process, A-16-1, provided about a 17% efficiency. The experimental implementations aimed the development of procedure for starting material (silicon) qualification, by using the photoconductive decay technique (PCD) with two surface passivation procedures: thermal oxidation and light phosphorus diffusion. Later, using PCD system, new optimizations of n+ homogeneous emitters and p-type region were performed, followed by passivating thermal oxidations with hydrogenation, maintaining the volume lifetime at high values (approximately 1ms, after each thermal step). These results qualified the used silicon and the consumer materials, as well the new fabrication process developed. This technique has also allowed qualifying the processed Gaussian profile emitters, providing values about 45fA/cm2 for the recombination current density in n+pp+ structures. N+p structures were also developed using Cz silicon with low resistivity 2- 3W.cm of two different manufacturers and FZ with 0.5W.cm. It could be proved the quality of the steps of a complete optimized process resulting implicit open circuit voltages of 652.4mV (Cz silicon - manufacturer type 1), 662.6mV (Cz silicon - manufacturer type 2), and 670.8mV (FZ silicon). According to the theoretical simulations performed using the usual parameters of devices processed at LME (random chemical texturization and SiO2 film), efficiencies between 19%-20% can be reached. However, using a random texturization and a double layer anti-reflection system, a 21% efficiency becomes possible, surpassing the 17% barrier (national record), and proving the potentiality of this laboratory facility for the development of solar cells non-dependent on impurity gettering through the aluminum diffusion.

O trabalho realizado nesta tese esteve apoiado em dois objetivos principais. O primeiro centrado na otimização das etapas e processos de fabricação de células solares de silício de alto rendimento envolvendo redução de custos. O segundo objetivo foi direcionado na implementação de células solares eficientes e não dependentes do armadilhamento de impurezas através da difusão de alumínio. Para levar a cabo estes objetivos de forma planejada, o trabalho dividiu-se em otimizações teóricas e implementações experimentais. As otimizações teóricas foram realizadas utilizando dois programas: um programa desenvolvido (simulacell.pas) e implementado no próprio LME (versão 2), e o outro adquirido comercialmente, PC1D. De acordo com os resultados obtidos em estruturas completas n+p e n++n+p foi possível concluir que tanto as estruturas formadas através de emissores homogêneos como as obtidas utilizando emissores duplamente difundidos permitem alcançar eficiências elevadas, 25,5% a 26,0%, respectivamente, em um amplo intervalo de espessuras e concentrações superficiais de dopantes. No que tange aos desenvolvimentos experimentais, este trabalho se inicia com o desenvolvimento de um processo simplificado de baixo custo, em células solares de silício Cz de baixa resistividade com estrutura n+pp+, tipo \"mesa\". Este processo simplificado também está baseado na difusão de fósforo e alumínio (P/Al), utilizando gases industriais e reagentes químicos de grau \"para análise\", como uma transposição do processo de fabricação anteriormente desenvolvido no LME-EPUSP em substratos de silício FZ utilizando tecnologia planar. A célula solar mais representativa do processo implementado, A-16-1, permitiu atingir eficiências no entorno de 17%. As implementações experimentais visaram inicialmente o desenvolvimento de um procedimento visando à qualificação de materiais de partida (silício), utilizando a técnica de decaimento fotocondutivo (PCD) através de dois procedimentos de passivação de superfícies; oxidações térmicas e difusões suaves de fósforo. Posteriormente, utilizando o sistema PCD, novas otimizações dos emissores de tipo n+ homogêneos e regiões de tipo p foram realizadas, seguidos por oxidações térmicas passivadoras hidrogenadas, preservando-se o tempo de vida do volume em valores elevados (aproximadamente 1ms, após a realização de todas as etapas térmicas). Estes resultados qualificam o silício e os materiais de consumo utilizados, assim como, o novo processo de fabricação desenvolvido. Esta técnica também permitiu qualificar os emissores com perfil Gaussianos processados, atingindo valores da ordem de 45fA/cm2 para densidades de recombinação em estruturas n+pn+. Desenvolveram-se também estruturas n+p em materiais Cz de baixa resistividade 2-3W.cm de dois diferentes fabricantes, e silício FZ com 0,5W.cm. Pôde ser comprovada a qualidade das etapas que compõem o processo completo otimizado tendo-se obtido tensões de circuito aberto-implícitas de 652,4mV (Si-Cz fabricante 1) e 662,6mV (Si-Cz fabricante 2), e 670,8mV (FZ). De acordo com simulações realizadas utilizando parâmetros habituais de dispositivos do próprio LME, estas tensões, quando associadas a um conjunto óptico frontal típico das células solares de alto rendimento do LME (texturização química aleatória e filme de SiO2), permitirão atingir valores entre 19% - 20%. Entretanto, utilizando texturização e camada dupla torna-se plausível atingir o marco de 21% de rendimento, ultrapassando assim a barreira dos 17% (recorde nacional), e comprovando a potencialidade da infra-estrutura deste laboratório para o desenvolvimento de células solares não dependentes do efeito do armadilhamento de impurezas através da difusão de alumínio. / The work developed at this thesis has been based on two main objectives. First, it was focused on the optimization of the steps and processes for the fabrication of high efficiency solar cells, reducing production costs. The latter objective was directed to develop solar cells that were efficient and non-dependent on impurities gettering performed through the aluminum diffusion. In order to attend the planned objectives the work was divided into the theoretical objectives and experimental developments. The theoretical optimizations were performed using two different program codes: one was developed at LME (simulacell.pas), being upgraded afterwards (version 2); and the other was acquired commercially, the PC1D. According to the obtained results in complete structures n+p and n++n+p, it was possible to conclude that the homogeneous and double diffused emitter structures can provide high efficiencies, from 25,5% to 26,0%, respectively, for a wide range of thicknesses and surface doping levels. Concerning the experimental developments, this work starts with a low cost simplified process, using Cz silicon solar cells with low base resistivity and the structure n+pp+, \"mesa\" type. This simplified process was also based on the phosphorus/ aluminum diffusion (P/Al), using industrial gases and for analysis grade chemical reagents, as a fabrication process transposition of the process previously developed at LME-EPUSP using silicon substrates with planar technology. The most representative solar cells of the implemented process, A-16-1, provided about a 17% efficiency. The experimental implementations aimed the development of procedure for starting material (silicon) qualification, by using the photoconductive decay technique (PCD) with two surface passivation procedures: thermal oxidation and light phosphorus diffusion. Later, using PCD system, new optimizations of n+ homogeneous emitters and p-type region were performed, followed by passivating thermal oxidations with hydrogenation, maintaining the volume lifetime at high values (approximately 1ms, after each thermal step). These results qualified the used silicon and the consumer materials, as well the new fabrication process developed. This technique has also allowed qualifying the processed Gaussian profile emitters, providing values about 45fA/cm2 for the recombination current density in n+pp+ structures. N+p structures were also developed using Cz silicon with low resistivity 2- 3W.cm of two different manufacturers and FZ with 0.5W.cm. It could be proved the quality of the steps of a complete optimized process resulting implicit open circuit voltages of 652.4mV (Cz silicon - manufacturer type 1), 662.6mV (Cz silicon - manufacturer type 2), and 670.8mV (FZ silicon). According to the theoretical simulations performed using the usual parameters of devices processed at LME (random chemical texturization and SiO2 film), efficiencies between 19%-20% can be reached. However, using a random texturization and a double layer anti-reflection system, a 21% efficiency becomes possible, surpassing the 17% barrier (national record), and proving the potentiality of this laboratory facility for the development of solar cells non-dependent on impurity gettering through the aluminum diffusion.

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

Organic solar cells (OSCs) based on non-fullerene acceptors (NFAs) have attracted a great deal of attention in recent years due to their rapidly increasing efficiency and enormous potential. In this work, the optical and electronic properties of systems containing the very promising non-fullerene acceptor PYT have been thoroughly studied with the use of the density functional theory (DFT) and the time-dependent density functional theory (TDDFT). By changing the electron linker from thiophene to furan and selenophene, respectively, the PYT was divided into three variants, each of which was studied independently. In addition, these three systems were combined with the donor PBDB-T to generate two distinct interface conformations. The properties studied in this work include the optimized geometries, HOMO-LUMO levels, UV-Vis spectra, frontier molecular orbitals (FMOs), natural transition orbitals (NTOs), density of states (DOS), dipole moments, open-circuit voltages, exciton binding energies, and local exciton (LE) and charge transfer (CT) energies. The calculations were performed in chlorobenzene solution utilizing the polarizable continuum model (PCM). It was discovered that PBDB-T/PY-Se exhibited remarkable flatness employing the π-π stacking conformation which corresponds well with the excellent D/A compatibility observed experimentally. All interfaces displayed appropriate positioning of the HOMO-LUMO levels, with the acceptor dominating the LUMO and the donor dominating the HOMO, with HOMO-LUMO gaps ranging between 1.34 and 1.38 eV. The differences in the interchanging of the electron linker were not that significant, and neither was the change in interface conformation in terms of the HOMO-LUMO levels. This may indicate that the system can be effective even without the presence of a π-π stacking conformation. The first excited states for all interface systems were shown to be pure CT transitions, and on average, 80% of the states exhibit CT character. The remaining contributions consisted of transitions within the pure materials, with a larger contribution within the acceptor. The theoretical results of this study indicate that systems containing the novel polymer acceptor PYT and its variants PY-O and PY-Se exhibit very intriguing properties, and further development of OSCs containing these polymers might further aid in the development of high-performance OSCs. / Organiska solceller (OSC) baserade på icke-fullerenacceptorer (NFA) har väckt stor uppmärksamhet de senaste åren på grund av dess snabbt ökande effektivitet och enorma potential. I detta arbete har de optiska och elektroniska egenskaperna hos system innehållande den mycket lovande icke-fullerenacceptorn PYT studerats grundligt med användning av täthetsfunktionalteorin (DFT) och den tidsberoende täthetsfunktionalteorin (TDDFT). Genom att ändra elektronförbindelsen från tiofen till furan respektive selenofen så delades PYT upp i tre varianter som var och en studerades oberoende av varandra. Dessutom kombinerades dessa tre system med donatorn PBDB-T för att generera två distinkta gränssnittskonformationer. Egenskaperna som studeras i detta arbete inkluderar optimerade geometrier, HOMO-LUMO-nivåer, UV-vis spektra, gränsmolekylära orbitaler (FMO), naturliga övergångsorbitaler (NTO), tillståndstäthet (DOS), dipolmoment, tomgångsspänning, excitonbindningsenergi samt lokal exciton (LE) och laddningsöverförings (CT) energier. Beräkningarna utfördes i klorbensenlösning med användning av den polariserbara kontinuummodellen (PCM). I resultatet uppvisade PBDB-T/PY-Se en anmärkningsvärd planhet med användning av π-π staplingskonformationen som överensstämmer väl med den utmärkta D/A-kompatibiliteten som observerats experimentellt. Alla gränssnitt visade lämplig positionering av HOMO-LUMO-nivåerna, med acceptorn som dominerade LUMO och donatorn som dominerade HOMO, med HOMO-LUMO-gap mellan 1.34 och 1.38 eV. Skillnaderna i utbytet av elektronförbindelsen visade sig inte vara signifikanta och inte heller skillnaden i gränssnittskonformation när det gäller HOMO-LUMO-nivåerna. Detta kan indikera att systemet kan vara effektivt även utan förekomst av π-π staplingskonformation. De första exciterade tillstånden för alla gränssnittssystem visade sig vara rena CT-övergångar och i genomsnitt uppvisade 80% av tillstånden CT-karaktär. Resterande andel bestod av övergångar inom de rena materialen med en större andel inom acceptorn. De teoretiska resultaten av denna studie indikerar att system innehållande den nya polymeracceptorn PYT och dess varianter PY-O och PY-Se uppvisar mycket spännande egenskaper samt att vidareutveckling av OSC:er som innehåller dessa polymerer ytterligare kan hjälpa till i utvecklingen av högpresterande OSC:er.

Three mathematical models, two of primary alkaline battery cathode discharge, and one of primary alkaline battery discharge, are developed, presented, solved and investigated in this thesis. The primary aim of this work is to improve our understanding of the complex, interrelated and nonlinear processes that occur within primary alkaline batteries during discharge. We use perturbation techniques and Laplace transforms to analyse and simplify an existing model of primary alkaline battery cathode under galvanostatic discharge. The process highlights key phenomena, and removes those phenomena that have very little effect on discharge from the model. We find that electrolyte variation within Electrolytic Manganese Dioxide (EMD) particles is negligible, but proton diffusion within EMD crystals is important. The simplification process results in a significant reduction in the number of model equations, and greatly decreases the computational overhead of the numerical simulation software. In addition, the model results based on this simplified framework compare well with available experimental data. The second model of the primary alkaline battery cathode discharge simulates step potential electrochemical spectroscopy discharges, and is used to improve our understanding of the multi-reaction nature of the reduction of EMD. We find that a single-reaction framework is able to simulate multi-reaction behaviour through the use of a nonlinear ion-ion interaction term. The third model simulates the full primary alkaline battery system, and accounts for the precipitation of zinc oxide within the separator (and other regions), and subsequent internal short circuit through this phase. It was found that an internal short circuit is created at the beginning of discharge, and this self-discharge may be exacerbated by discharging the cell intermittently. We find that using a thicker separator paper is a very effective way of minimising self-discharge behaviour. The equations describing the three models are solved numerically in MATLABR, using three pieces of numerical simulation software. They provide a flexible and powerful set of primary alkaline battery discharge prediction tools, that leverage the simplified model framework, allowing them to be easily run on a desktop PC.