Research on pi-conjugated organic materials in the recent past has produced enormous developments in the field of organic electronics and it is mainly due to their applications in electronic devices such as organic field effect transistors (OFETs), organic light emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). The primary goal of this research work is to design and synthesize high performing charge transport organic semiconductors. One of the criteria for better performance of the organic thin film transistor (OTFT) is to have high uniform thin film morphology of the organic semiconductor layer on the substrate. The first project in this dissertation has been directed towards improving the thin film morphology of the functionalized pentacenes through liquid crystalline behaviour. The results have suggested the possibility of thermotropic liquid crystalline phases in 6,13-bis(diisopropylhexylsilylethynyl) pentacene which has no pi-stacking in its solid state and the presence of silyl group at the peri-position is crucial for the stability of the functionalized pentacenes. In the second project, i have investigated the effect of alkyl groups with varying chain length on the anthradithiophene chromophore on the performance of the charge transporting devices. Organic blend cell based on solution processable 2,8-diethyl-5,12-bis(triethylsilylethynyl) anthradithiophene has showed 1% power conversion efficiency and the performance is mainly attributed to the large crystalline phase segregation of the functionalized anthradithiophene from the amorphous soluble fullerene derivative matrix. OTFT study on alkyl substituted functionalized anthradithiophenes suggested the need of delegate balance between thin film morphology and the crystal packing. Third project has been directed towards synthesizing halogen substituted functionalized anthradithiophenes and their influence in the performance of OFETs. OTFT made of 2,8-difluoro-5,12-bis(triethylsilylethynyl) anthradithiophene produced devices with thin film hole mobilities greater than 1 cm2/Vs. The result suggested that the device is not contact limited rather this high performance OTFTs are due to the contact induced crystallinity of the organic semiconductor.

The field of organic electronics has become an increasingly important field of research in recent years. Organic based semiconductors have the potential for creating inexpensive, solution processed devices on flexible substrates. Some of the applications of organic semiconductors include organic field effect transistors, organic light emitting diodes and organic photovoltaics. Functionalized pentacenes have been proven to be viable donor materials for use in organic photovoltaic devices. The goal of this research is to synthesize and test the viability of novel electron deficient pentacenes and pentacene based materials as acceptors to be used as drop-in replacements for PCBM in bulk-heterojunction organic solar cells. Our goal was to tune and improve the efficiencies of these solar cells in a two pronged approach. First we tuned the open circuit voltage of these devices by determining the optimal energy levels of these acceptors by varying the number of electron withdrawing substituents on the acene core. We also tuned the short circuit current by chemically altering the solid state packing and optimizing device processing conditions. A preliminary structure-property relationship of these small molecule acceptors and photovoltaic device efficiency was established as a result.

Stable organic semiconductors are critical to produce inexpensive, efficient and flexible thin film organic solar cells. A current chemical focus is the synthesis of stable, electron-accepting materials to be utilized as an acceptor layer in photovoltaics.1 The Anthony group has shown that the functionalization of pentacene with suitable electron withdrawing groups provides a catalog of suitable acceptors for this purpose.2 These pentacenes can be further modified to pack in a unique 1-dimensional "sandwich herringbone" crystal packing, leading to improved device current.3 To improve the stability of acene acceptors, we have taken two hetero-atom themed approaches. First, we have studied the acenequinone as an electron-accepting chromophore.4 Further, we replaced the terminal aromatic rings with heterocycles, such as furan or thiophene. In order to enhance the crystal engineering versatility of the chromophore, we utilize c-fused heterocycles (rather than the more commonly used b-fused cycles seen in e.g. anthradithiophenes). The c-fused acenequinones can be tetra-functionalized with silylethynyl groups to influence crystal packing and increase solubility.5 The silylethyne groups are known to increase the photostability and lower the energy gap (Eg) of pentacenes.5 The functionalization of the silylethyne groups also aids in lowering the lowest unoccupied orbital (LUMO) of acene structures.5

The rapidly increasing power conversion efficiencies (PCEs) of organic solar cells (OSCs) above 10% were made possible by concerted international research activities in the last few years, aiming to understand the processes that lead to the generation of free charge carriers following photon absorption. Despite these efforts, many details are still unknown, especially how these processes can be improved already at the drawing board of molecular design. To unveil this information, dicyanovinyl end-capped oligothiophene derivatives (DCVnTs) are used as a model system in this thesis, allowing to investigate the impact of small structural changes on the molecular properties and the final solar cells. On thin films of a methylated DCV4T derivative, the influence of the measurement temperature on the charge carrier generation process is investigated. The observed temperature activation in photoinduced absorption (PIA) measurements is attributed to an increased charge carrier mobility, increasing the distance between the charges at the donor/acceptor (D/A) interface and, thus, facilitating their final dissociation. The correlation between the activation energy and the mobility is confirmed using a DCV6T derivative with lower mobility , exhibiting a higher activation energy for charge carrier generation. Another parameter to influence the charge carrier generation process is the molecular structure. Here, alkyl side chains with varying length are introduced and their influence on the intramolecular energy levels as well as the absorption and emission properties in pristine and blend films with the acceptor C60 are examined. The observed differences in intermolecular order (higher order for shorter side chains) and phase separation in blend layers (larger phase separation for shorter side chains) are confirmed in PIA measurements upon comparing the temperature dependence of the triplet exciton lifetimes. A proposed correlation between the side chain length and the coupling between D and A, which is crucial for efficient charge transfer, is not confirmed. The presented flat heterojunction solar cells underline this conclusion, giving similar photocurrent densities for all compounds. Differences in PCE are related to shifts of the energy levels and the morphology of the blend layer in bulk heterojunction devices. Furthermore, the impact of the electric field on the charge carrier generation yield is investigated in a proof-of-principle study, introducing PIA measurements in transmission geometry realized using semitransparent solar cells. The recombination analysis of the photogenerated charge carriers reveals two recombination components. Trapped charge carriers or bound charge pairs at the D/A interface are proposed as an explanation for this result. The miscibility of D and A, which can be influenced by heating the substrate during layer deposition, is of crucial importance to obtain high PCEs. In this work, the unusual negative influence of the substrate temperature on DCV4T:C60 blend layers in solar cells is investigated. By using optical measurements and structure determination tools, a rearrangement of the DCV4T crystallites is found to be responsible for the reduced absorption and, therefore, photocurrent at higher substrate temperature. The proposed blend morphology at a substrate temperature of 90° C is characterized by a nearly complete demixing of the D and A phases. This investigation is of particular relevance, because it shows the microscopic origins of a behavior that is contrary to the increase of the PCE upon substrate heating usually reported in literature. Finally, the optimization steps to achieve a record PCE of 7.7% using a DCV5T derivative as donor material are presented, including the optimization of the substrate temperature, the active layer thickness, and the transport layers. / Der rasante Anstieg des Wirkungsgrads von organischen Solarzellen über die Marke von 10% war nur durch länderübergreifende Forschungsaktivitäten während der letzten Jahre möglich. Trotz der gemeinsamen Anstrengungen, die Prozesse, die zwischen der Absorption der Photonen und der Ladungsträgererzeugung liegen, genauer zu verstehen, sind einige Fragen jedoch immer noch ungelöst, z.B. wie diese Prozesse schon auf dem Reißbrett durch die gezielte Änderung bestimmter Molekülstrukturen optimiert werden können. Um dieses Ziel zu erreichen, werden in dieser Arbeit Dicyanovinyl-substituierte Oligothiophene (DCVnTs) verwendet. Diese Materialien bieten die Möglichkeit, kleine strukturelle Änderungen vorzunehmen, deren Einfluss auf die molekularen und auf die Solarzelleneigenschaften untersucht werden soll. Der Einfluss der Messtemperatur auf den Prozess der Ladungsträgertrennung wird hier an einer methylierten DCV4T-Verbindung in einer dünnen Schicht untersucht. Die bei photoinduzierter Absorptionsspektroskopie (PIA) beobachtete Aktivierung dieses Prozesses mit zunehmender Temperatur wird auf eine erhöhte Ladungsträgerbeweglichkeit zurückgeführt. Der dadurch erhöhte effektive Abstand der Ladungen an der Grenzfläche zwischen Donator (D) und Akzeptor (A) erleichtert die endgültige Trennung der Ladungsträger. Durch den Vergleich mit einer DCV6T-Verbindung wird der Zusammenhang zwischen der Aktivierungsenergie und der Beweglichkeit bekräftigt. Die kleinere Beweglichkeit äußert sich dabei in einer größeren Aktivierungsenergie. Darüber hinaus kann der Ladungsträgergenerationsprozess auch von der Molekülstruktur abhängen. In dieser Arbeit wird untersucht, wie sich die Länge von Alkylseitenketten auf die Energieniveaus der Moleküle, aber auch auf die Absorptions- und Lumineszenzeigenschaften der Materialien in reinen und in Mischschichten mit dem Akzeptor C60 äußert. Die ermittelten Unterschiede bezüglich der Molekülordnung (geordneter für kürzere Seitenketten) und der Phasengrößen in Mischschichten (größere Phasen bei kürzerer Kettenlänge) werden in der Untersuchung der Temperaturabhängigkeit der Lebensdauer von Triplettexzitonen mittels PIA-Messungen bestätigt. Für Solarzellen ist von Bedeutung, ob sich die Seitenkettenlänge auf die Wechselwirkung zwischen D und A auswirkt. Der vermutete Zusammenhang wird hier nicht bestätigt. Ein ähnlicher Photostrom für alle untersuchten Verbindungen in Solarzellen mit planaren Heteroübergängen unterstreicht diese Schlussfolgerung. Unterschiede im Wirkungsgrad werden auf Änderungen der Energieniveaus und die Morphologie in Mischschichtsolarzellen zurückgeführt. Des Weiteren wird in einer Machbarkeitsstudie der Einfluss des elektrischen Felds auf die Generationsausbeute freier Ladungsträger untersucht. Dafür werden halbtransparente Solarzellen verwendet, die es ermöglichen, PIA-Messungen in Transmissionsgeometrie durchzuführen. Als mögliche Erklärung für das Auftreten zweier Rekombinationskomponenten in der Analyse des Rekombinationsverhaltens der durch Licht erzeugten Ladungsträger werden eingefangene Ladungsträger und gebundene Ladungsträgerpaare an der D/A-Grenzfläche genannt. Das Mischverhalten von D und A kann durch ein Heizen des Substrates während des Verdampfungsprozesses eingestellt werden, was von entscheidender Bedeutung für eine weitere Steigerung des Wirkungsgrades ist. Für DCV4T:C60-Mischschichtsolarzellen wird jedoch eine Verschlechterung des Wirkungsgrads zu höheren Substrattemperaturen beobachtet. Durch optische Messungen und Methoden zur Schichtstrukturbestimmung wird dieser Effekt auf eine Umordnung der DCV4T-Kristallite für hohe Substrattemperaturen und die damit verbundene Verringerung der Absorption und damit auch des Photostroms zurückgeführt. Bei einer Substrattemperatur von 90° C sind die D- und A-Komponenten fast vollständig entmischt. Dieses Beispiel ist von besonderer Bedeutung, weil hier die Ursachen für ein Verhalten aufgezeigt werden, das entgegen den Beispielen aus der Literatur eine Abnahme des Wirkungsgrads beim Aufdampfen der aktiven Schicht auf ein geheiztes Substrat zeigt. Schließlich werden die Optimierungsschritte dargelegt, mit denen Solarzellen mit einer DCV5T-Verbindung als Donatormaterial auf einen Rekordwirkungsgrad von 7,7% gebracht werden. Dabei wird die Substrattemperatur, die Dicke der aktiven Schicht und die Transportschichten angepasst.

The rapidly increasing power conversion efficiencies (PCEs) of organic solar cells (OSCs) above 10% were made possible by concerted international research activities in the last few years, aiming to understand the processes that lead to the generation of free charge carriers following photon absorption. Despite these efforts, many details are still unknown, especially how these processes can be improved already at the drawing board of molecular design. To unveil this information, dicyanovinyl end-capped oligothiophene derivatives (DCVnTs) are used as a model system in this thesis, allowing to investigate the impact of small structural changes on the molecular properties and the final solar cells. On thin films of a methylated DCV4T derivative, the influence of the measurement temperature on the charge carrier generation process is investigated. The observed temperature activation in photoinduced absorption (PIA) measurements is attributed to an increased charge carrier mobility, increasing the distance between the charges at the donor/acceptor (D/A) interface and, thus, facilitating their final dissociation. The correlation between the activation energy and the mobility is confirmed using a DCV6T derivative with lower mobility , exhibiting a higher activation energy for charge carrier generation. Another parameter to influence the charge carrier generation process is the molecular structure. Here, alkyl side chains with varying length are introduced and their influence on the intramolecular energy levels as well as the absorption and emission properties in pristine and blend films with the acceptor C60 are examined. The observed differences in intermolecular order (higher order for shorter side chains) and phase separation in blend layers (larger phase separation for shorter side chains) are confirmed in PIA measurements upon comparing the temperature dependence of the triplet exciton lifetimes. A proposed correlation between the side chain length and the coupling between D and A, which is crucial for efficient charge transfer, is not confirmed. The presented flat heterojunction solar cells underline this conclusion, giving similar photocurrent densities for all compounds. Differences in PCE are related to shifts of the energy levels and the morphology of the blend layer in bulk heterojunction devices. Furthermore, the impact of the electric field on the charge carrier generation yield is investigated in a proof-of-principle study, introducing PIA measurements in transmission geometry realized using semitransparent solar cells. The recombination analysis of the photogenerated charge carriers reveals two recombination components. Trapped charge carriers or bound charge pairs at the D/A interface are proposed as an explanation for this result. The miscibility of D and A, which can be influenced by heating the substrate during layer deposition, is of crucial importance to obtain high PCEs. In this work, the unusual negative influence of the substrate temperature on DCV4T:C60 blend layers in solar cells is investigated. By using optical measurements and structure determination tools, a rearrangement of the DCV4T crystallites is found to be responsible for the reduced absorption and, therefore, photocurrent at higher substrate temperature. The proposed blend morphology at a substrate temperature of 90° C is characterized by a nearly complete demixing of the D and A phases. This investigation is of particular relevance, because it shows the microscopic origins of a behavior that is contrary to the increase of the PCE upon substrate heating usually reported in literature. Finally, the optimization steps to achieve a record PCE of 7.7% using a DCV5T derivative as donor material are presented, including the optimization of the substrate temperature, the active layer thickness, and the transport layers.:Abstract - Kurzfassung Publications Contents 1 Introduction 2 Elementary Processes in Organic Semiconductors 2.1 Introduction 2.2 Optical Excitations in Organic Materials 2.2.1 Introduction 2.2.2 Radiative Processes: Absorption and Emission 2.2.3 Non-radiative Relaxation Processes 2.2.4 Triplet Excitons and Intersystem Crossing 2.3 Polarization Effects and Disorder 2.4 Transport Processes in Disordered Organic Materials 2.4.1 Charge Transport 2.4.1.1 The Bässler Model 2.4.1.2 Marcus Theory for Electron Transfer 2.4.1.3 Small Polaron Model 2.4.1.4 Functional Dependencies of the Charge Carrier Mobility 2.4.2 Diffusive Motion 2.4.3 Exciton Transfer Mechanisms 2.4.4 Characteristics of Exciton Diffusion 2.5 Charge Photogeneration in Pristine Materials 3 Organic Photovoltaics 3.1 General Introduction to Solar Cell Physics 3.2 Introduction to the Donor/Acceptor Heterojunction Concept 3.3 The Open-Circuit Voltage in Organic Solar Cells 3.4 Doping of Organic Semiconductors 3.5 Introduction to the p-i-n Concept 3.6 Charge Transfer Excitons in Donor/Acceptor Heterojunction Systems 3.6.1 Introduction 3.6.2 Verification of Charge Transfer Excitons in Donor/Acceptor Systems 3.7 The Process Cascade for Free Charge Carrier Generation in Donor/Acceptor Heterojunction Systems 3.7.1 The Initial Charge Transfer Step 3.7.2 The Binding Energy of the Charge Transfer Exciton 3.7.3 \"Hot\" Charge Transfer Exciton Dissociation 3.7.4 \"Cold\" Charge Transfer Exciton Dissociation 3.7.5 Supposed Influence Factors on Charge Transfer Exciton Dissociation 3.7.6 Recombination Pathways for Charge Transfer Excitons 3.7.7 Free Charge Carrier Formation and Recombination 4 Experimental Methods 4.1 Sample Preparation 4.2 Material Characterization Methods 4.2.1 Optical Characterization 4.2.2 Cyclic Voltammetry 4.2.3 Ultraviolet Photoelectron Spectroscopy 4.2.4 Atomic Force Microscopy 4.2.5 Grazing Incidence X-Ray Diffraction 4.2.6 Organic Field-Effect Transistor 4.3 Photoinduced Absorption Spectroscopy 4.3.1 Introduction 4.3.2 Derivation of the PIA Signal 4.3.3 Recombination Dynamics 4.3.4 Intensity Dependence of the PIA Signal 4.4 Solar Cell Characterization 4.4.1 External Quantum Efficiency 4.4.2 Spectral Mismatch Correction 4.4.3 Current-Voltage Characteristics 4.4.4 Optical Device Simulations 4.4.5 Optical Device Transmission Measurements 5 The Oligothiophene Material System 5.1 Introduction 5.2 Thermal Stability 5.3 Energy Levels 5.4 Optical Properties of the Pristine Materials 5.5 The Donor/Acceptor Couple: DCVnT and C60 5.6 Solar Cell Devices 5.7 Summary 6 Temperature Dependence of Charge Carrier Generation 6.1 Introduction 6.2 Principal Introduction to the PIA Measurements 6.2.1 Interpretation of the Spectra 6.2.2 Interpretation of the Frequency Scans 6.3 Temperature Dependence of the Spectra 6.4 Discussion of the Temperature Dependent Processes in the Blend Layer 6.5 Temperature Activated Free Charge Carrier Generation 6.5.1 Evaluation of the Activation Energy for the DCV4T-Me:C60 Blend 6.5.2 Comparison to a Sexithiophene Derivative (DCV6T-Me) 6.6 Summary 7 Side Chain Investigation on Quaterthiophene Derivatives 7.1 Energy Levels 7.2 Optical Properties 7.2.1 Solution and Pristine Films 7.2.2 Mixed Films with C60 7.3 Influence of the Side Chain Length on the Intermolecular Coupling 7.3.1 PIA Spectra of Pristine and Blend Layers at 10K 7.3.2 Recombination Analysis for Pristine and Blend Films at 10K 7.4 The Influence of the Side Chain Length on the Offset Charge Carrier Generation Rate at Low Temperature 7.5 In the High-Temperature Limit: Implications for Solar Cell Devices 7.5.1 PIA Spectra in Pristine and Blend Films at 200K 7.5.2 Recombination Analysis: Triplet Excitons and Free Charge Carriers 7.6 Solar Cells 7.6.1 Flat Heterojunction Devices 7.6.2 Bulk Heterojunction Devices 7.7 Summary 8 Electric-Field Dependent PIA Measurements on Complete Solar Cell Devices 8.1 Introduction 8.2 Semitransparent Organic Solar Cells 8.3 Photoinduced Absorption Measurements 8.4 Summary and Outlook 9 The Effect of Substrate Heating During Layer Deposition on the Performance of DCV4T:C60 BHJ Solar Cells 9.1 Introduction 9.2 The Importance of Morphology Control for BHJ Solar Cells 9.3 The Impact of Substrate Heating on DCV4T:C60 BHJ Solar Cells 9.4 Absorption and Photoluminescence 9.5 Topographical Investigations (AFM) 9.6 X-ray Investigations 9.6.1 1D GIXRD Measurements 9.6.2 2D GIXRD Measurements 9.7 Proposed Morphological Picture and Confirmation Measurements 9.7.1 Morphology Sketch of the DCV4T:C60 Blend Layer 9.7.2 Confirmation Measurements 9.8 The Equivalence of Temperature and Time 9.9 Summary 10 Record Solar Cells Using DCV5T-Me33 as Donor Material 10.1 Introduction 10.2 The Influence of the Substrate Temperature 10.3 Determination of the Optical Constants 10.4 Stack Optimization 10.5 Summary and Outlook 11 Conclusions and Outlook 11.1 Summary of the Photophysical Investigations 11.2 Summary of Device Investigations 11.3 Future Challenges Appendix A Detailed Description of the Experimental Setup for PIA Spectroscopy Appendix B Determination of the Triplet Level by Differential PL Measurements Appendix C Additional Tables and Figures Appendix D Reproducibility of the Solar Cell Results (Statistics) Appendix E Lists Bibliography Acknowledgments / Der rasante Anstieg des Wirkungsgrads von organischen Solarzellen über die Marke von 10% war nur durch länderübergreifende Forschungsaktivitäten während der letzten Jahre möglich. Trotz der gemeinsamen Anstrengungen, die Prozesse, die zwischen der Absorption der Photonen und der Ladungsträgererzeugung liegen, genauer zu verstehen, sind einige Fragen jedoch immer noch ungelöst, z.B. wie diese Prozesse schon auf dem Reißbrett durch die gezielte Änderung bestimmter Molekülstrukturen optimiert werden können. Um dieses Ziel zu erreichen, werden in dieser Arbeit Dicyanovinyl-substituierte Oligothiophene (DCVnTs) verwendet. Diese Materialien bieten die Möglichkeit, kleine strukturelle Änderungen vorzunehmen, deren Einfluss auf die molekularen und auf die Solarzelleneigenschaften untersucht werden soll. Der Einfluss der Messtemperatur auf den Prozess der Ladungsträgertrennung wird hier an einer methylierten DCV4T-Verbindung in einer dünnen Schicht untersucht. Die bei photoinduzierter Absorptionsspektroskopie (PIA) beobachtete Aktivierung dieses Prozesses mit zunehmender Temperatur wird auf eine erhöhte Ladungsträgerbeweglichkeit zurückgeführt. Der dadurch erhöhte effektive Abstand der Ladungen an der Grenzfläche zwischen Donator (D) und Akzeptor (A) erleichtert die endgültige Trennung der Ladungsträger. Durch den Vergleich mit einer DCV6T-Verbindung wird der Zusammenhang zwischen der Aktivierungsenergie und der Beweglichkeit bekräftigt. Die kleinere Beweglichkeit äußert sich dabei in einer größeren Aktivierungsenergie. Darüber hinaus kann der Ladungsträgergenerationsprozess auch von der Molekülstruktur abhängen. In dieser Arbeit wird untersucht, wie sich die Länge von Alkylseitenketten auf die Energieniveaus der Moleküle, aber auch auf die Absorptions- und Lumineszenzeigenschaften der Materialien in reinen und in Mischschichten mit dem Akzeptor C60 äußert. Die ermittelten Unterschiede bezüglich der Molekülordnung (geordneter für kürzere Seitenketten) und der Phasengrößen in Mischschichten (größere Phasen bei kürzerer Kettenlänge) werden in der Untersuchung der Temperaturabhängigkeit der Lebensdauer von Triplettexzitonen mittels PIA-Messungen bestätigt. Für Solarzellen ist von Bedeutung, ob sich die Seitenkettenlänge auf die Wechselwirkung zwischen D und A auswirkt. Der vermutete Zusammenhang wird hier nicht bestätigt. Ein ähnlicher Photostrom für alle untersuchten Verbindungen in Solarzellen mit planaren Heteroübergängen unterstreicht diese Schlussfolgerung. Unterschiede im Wirkungsgrad werden auf Änderungen der Energieniveaus und die Morphologie in Mischschichtsolarzellen zurückgeführt. Des Weiteren wird in einer Machbarkeitsstudie der Einfluss des elektrischen Felds auf die Generationsausbeute freier Ladungsträger untersucht. Dafür werden halbtransparente Solarzellen verwendet, die es ermöglichen, PIA-Messungen in Transmissionsgeometrie durchzuführen. Als mögliche Erklärung für das Auftreten zweier Rekombinationskomponenten in der Analyse des Rekombinationsverhaltens der durch Licht erzeugten Ladungsträger werden eingefangene Ladungsträger und gebundene Ladungsträgerpaare an der D/A-Grenzfläche genannt. Das Mischverhalten von D und A kann durch ein Heizen des Substrates während des Verdampfungsprozesses eingestellt werden, was von entscheidender Bedeutung für eine weitere Steigerung des Wirkungsgrades ist. Für DCV4T:C60-Mischschichtsolarzellen wird jedoch eine Verschlechterung des Wirkungsgrads zu höheren Substrattemperaturen beobachtet. Durch optische Messungen und Methoden zur Schichtstrukturbestimmung wird dieser Effekt auf eine Umordnung der DCV4T-Kristallite für hohe Substrattemperaturen und die damit verbundene Verringerung der Absorption und damit auch des Photostroms zurückgeführt. Bei einer Substrattemperatur von 90° C sind die D- und A-Komponenten fast vollständig entmischt. Dieses Beispiel ist von besonderer Bedeutung, weil hier die Ursachen für ein Verhalten aufgezeigt werden, das entgegen den Beispielen aus der Literatur eine Abnahme des Wirkungsgrads beim Aufdampfen der aktiven Schicht auf ein geheiztes Substrat zeigt. Schließlich werden die Optimierungsschritte dargelegt, mit denen Solarzellen mit einer DCV5T-Verbindung als Donatormaterial auf einen Rekordwirkungsgrad von 7,7% gebracht werden. Dabei wird die Substrattemperatur, die Dicke der aktiven Schicht und die Transportschichten angepasst.:Abstract - Kurzfassung Publications Contents 1 Introduction 2 Elementary Processes in Organic Semiconductors 2.1 Introduction 2.2 Optical Excitations in Organic Materials 2.2.1 Introduction 2.2.2 Radiative Processes: Absorption and Emission 2.2.3 Non-radiative Relaxation Processes 2.2.4 Triplet Excitons and Intersystem Crossing 2.3 Polarization Effects and Disorder 2.4 Transport Processes in Disordered Organic Materials 2.4.1 Charge Transport 2.4.1.1 The Bässler Model 2.4.1.2 Marcus Theory for Electron Transfer 2.4.1.3 Small Polaron Model 2.4.1.4 Functional Dependencies of the Charge Carrier Mobility 2.4.2 Diffusive Motion 2.4.3 Exciton Transfer Mechanisms 2.4.4 Characteristics of Exciton Diffusion 2.5 Charge Photogeneration in Pristine Materials 3 Organic Photovoltaics 3.1 General Introduction to Solar Cell Physics 3.2 Introduction to the Donor/Acceptor Heterojunction Concept 3.3 The Open-Circuit Voltage in Organic Solar Cells 3.4 Doping of Organic Semiconductors 3.5 Introduction to the p-i-n Concept 3.6 Charge Transfer Excitons in Donor/Acceptor Heterojunction Systems 3.6.1 Introduction 3.6.2 Verification of Charge Transfer Excitons in Donor/Acceptor Systems 3.7 The Process Cascade for Free Charge Carrier Generation in Donor/Acceptor Heterojunction Systems 3.7.1 The Initial Charge Transfer Step 3.7.2 The Binding Energy of the Charge Transfer Exciton 3.7.3 \"Hot\" Charge Transfer Exciton Dissociation 3.7.4 \"Cold\" Charge Transfer Exciton Dissociation 3.7.5 Supposed Influence Factors on Charge Transfer Exciton Dissociation 3.7.6 Recombination Pathways for Charge Transfer Excitons 3.7.7 Free Charge Carrier Formation and Recombination 4 Experimental Methods 4.1 Sample Preparation 4.2 Material Characterization Methods 4.2.1 Optical Characterization 4.2.2 Cyclic Voltammetry 4.2.3 Ultraviolet Photoelectron Spectroscopy 4.2.4 Atomic Force Microscopy 4.2.5 Grazing Incidence X-Ray Diffraction 4.2.6 Organic Field-Effect Transistor 4.3 Photoinduced Absorption Spectroscopy 4.3.1 Introduction 4.3.2 Derivation of the PIA Signal 4.3.3 Recombination Dynamics 4.3.4 Intensity Dependence of the PIA Signal 4.4 Solar Cell Characterization 4.4.1 External Quantum Efficiency 4.4.2 Spectral Mismatch Correction 4.4.3 Current-Voltage Characteristics 4.4.4 Optical Device Simulations 4.4.5 Optical Device Transmission Measurements 5 The Oligothiophene Material System 5.1 Introduction 5.2 Thermal Stability 5.3 Energy Levels 5.4 Optical Properties of the Pristine Materials 5.5 The Donor/Acceptor Couple: DCVnT and C60 5.6 Solar Cell Devices 5.7 Summary 6 Temperature Dependence of Charge Carrier Generation 6.1 Introduction 6.2 Principal Introduction to the PIA Measurements 6.2.1 Interpretation of the Spectra 6.2.2 Interpretation of the Frequency Scans 6.3 Temperature Dependence of the Spectra 6.4 Discussion of the Temperature Dependent Processes in the Blend Layer 6.5 Temperature Activated Free Charge Carrier Generation 6.5.1 Evaluation of the Activation Energy for the DCV4T-Me:C60 Blend 6.5.2 Comparison to a Sexithiophene Derivative (DCV6T-Me) 6.6 Summary 7 Side Chain Investigation on Quaterthiophene Derivatives 7.1 Energy Levels 7.2 Optical Properties 7.2.1 Solution and Pristine Films 7.2.2 Mixed Films with C60 7.3 Influence of the Side Chain Length on the Intermolecular Coupling 7.3.1 PIA Spectra of Pristine and Blend Layers at 10K 7.3.2 Recombination Analysis for Pristine and Blend Films at 10K 7.4 The Influence of the Side Chain Length on the Offset Charge Carrier Generation Rate at Low Temperature 7.5 In the High-Temperature Limit: Implications for Solar Cell Devices 7.5.1 PIA Spectra in Pristine and Blend Films at 200K 7.5.2 Recombination Analysis: Triplet Excitons and Free Charge Carriers 7.6 Solar Cells 7.6.1 Flat Heterojunction Devices 7.6.2 Bulk Heterojunction Devices 7.7 Summary 8 Electric-Field Dependent PIA Measurements on Complete Solar Cell Devices 8.1 Introduction 8.2 Semitransparent Organic Solar Cells 8.3 Photoinduced Absorption Measurements 8.4 Summary and Outlook 9 The Effect of Substrate Heating During Layer Deposition on the Performance of DCV4T:C60 BHJ Solar Cells 9.1 Introduction 9.2 The Importance of Morphology Control for BHJ Solar Cells 9.3 The Impact of Substrate Heating on DCV4T:C60 BHJ Solar Cells 9.4 Absorption and Photoluminescence 9.5 Topographical Investigations (AFM) 9.6 X-ray Investigations 9.6.1 1D GIXRD Measurements 9.6.2 2D GIXRD Measurements 9.7 Proposed Morphological Picture and Confirmation Measurements 9.7.1 Morphology Sketch of the DCV4T:C60 Blend Layer 9.7.2 Confirmation Measurements 9.8 The Equivalence of Temperature and Time 9.9 Summary 10 Record Solar Cells Using DCV5T-Me33 as Donor Material 10.1 Introduction 10.2 The Influence of the Substrate Temperature 10.3 Determination of the Optical Constants 10.4 Stack Optimization 10.5 Summary and Outlook 11 Conclusions and Outlook 11.1 Summary of the Photophysical Investigations 11.2 Summary of Device Investigations 11.3 Future Challenges Appendix A Detailed Description of the Experimental Setup for PIA Spectroscopy Appendix B Determination of the Triplet Level by Differential PL Measurements Appendix C Additional Tables and Figures Appendix D Reproducibility of the Solar Cell Results (Statistics) Appendix E Lists Bibliography Acknowledgments