Utilising high work function metal oxides as hole extracting layers for organic photovoltaic cells

A substantial amount of research has already been undertaken towards creating commercially viable organic photovoltaics (OPVs). This is due to the potential use of OPV cells as an inexpensive source of renewable energy. There are many factors to consider in OPV cell design, including photo-active materials, cell architecture and electrode selection. However, additional interlayers for use between the photo-active materials and the electrodes were identified to be as important and need to be developed to optimise cell performance. The work presented here focuses on the influence of various metal oxide hole extracting layers in different OPV systems. Metal oxides such as molybdenum oxide (MoOx) have shown great promise in polymer cells as a hole extracting layer, and here we investigate their use in small molecule cells. An optimised MoOx layer thickness of 5 nm provides a ~ 60 % increase in overall power conversion efficiency (ηp) for chloroaluminium phthalocyanine (ClAlPc) / fullerene (C60) cells in comparison to those fabricated on bare ITO. A similar improvement of ηp is reported when using the MoOx layer in a boron subphthalocyanine chloride (SubPc) / C60 system. For both high ionisation potential donor materials, the cells containing MoOx achieve a significantly higher open circuit voltage (Voc). Conversely, cells utilising the lower ionisation potential donor materials such as copper phthalocyanine (CuPc) and pentacene produce similar Voc values when deposited on both ITO and MoOx. Hence, the ηp is marginally reduced with the MoOx layer. To attain a deeper understanding, the factors behind these performance differences were explored by UV-vis absorption spectroscopy, ultra-violet photoemission spectroscopy (UPS), X-ray diffraction (XRD) and atomic force microscopy (AFM). Thermally evaporated vanadium oxide (V2Ox) was used as an alternative hole extracting layer to MoOx, achieving analogous performance to MoOx when used in SubPc / C60 and CuPc / C60 cells. The electronic properties of the V2Ox layer are investigated using UPS, and it is demonstrated to have substoichiometric n-type character in contrast to the p-type behaviour previously reported. Additionally, the in-situ fabrication and characterisation of organic layers using UPS indicate Fermi level pinning of the organic to the metal oxide. A solution processed vanadium oxide (V2Ox(sol)) layer was developed and characterised as an alternative method of layer fabrication. The atmospheric processing conditions are found to have a dramatic effect on cell performance, and are studied using x-ray photoelectron spectroscopy (XPS). Layers spin-coated under a nitrogen atmosphere exhibit a larger composition of V4+ states. Kelvin probe and UPS experiments indicate the V2Ox(sol) is also a high work function, n-type layer, with the V2Ox(sol) hole extracting layer producing similar cell performance to the thermally evaporated metal oxide layers. Cells deposited on the V2Ox(sol) layer demonstrate good operational stability characteristics, outperforming a commonly used solution processable hole extracting layer

Nanostructured copper electrodes for organic photovoltaics

This thesis describes a body of original research focused on the development of a viable alternative to the indium-tin oxide (ITO) glass window electrode used in organic photovoltaic (OPV) devices, based on the use of ultra-thin Cu films. The first results chapter describes a low cost, robust Cu | Al bilayer window electrode that simultaneously functions as the low work function electron-extracting electrode and as a sink for oxygen/water molecules in OPVs. When the electrode is exposed to air, an ultra-thin oxide layer forms at its surface without any increase in surface roughness, and the sheet resistance of the electrode actually decreases. However, this electrode has the disadvantage of a lower far-field transparency than ITO glass. The second results chapter describes how the transparency of ultra-thin Cu films can be increased to a level comparable to that of ITO glass across most of the spectrum over which OPVs harvest light using an overlayer of tungsten sub-oxide (WO3-x) which is spontaneously doped with Cu, increasing both its refractive index and electrical conductivity. Unfortunately these electrodes are not air stable. The third results chapter describes how the developments described in the previous two chapters might be integrated to realise an electrode that is both air-stable and highly transparent. The final results chapter describes a very different approach to coupling light into an OPV based on a Cu electrode with a dense array of sub-optical wavelength apertures. These electrodes absorb light strongly, concentrating it as surface plasmon excitations. It is shown that this trapped light can be absorbed by the light harvesting organic semiconductor in organic photovoltaics so that electrodes with very low far-field transparency can perform as well as more transparent electrodes

Electrodes for top-illuminated organic photovoltaic devices.

The work in this thesis focuses on the development of electrodes for top-illuminated organic photovoltaics (OPVs) and studies how their complex interactions with other layers affect the device. The development of a novel substrate electrode based on an Al | Cu bilayer capped with an ultra-thin Al layer is initially shown. This electrode offers the rare combination of high reflectivity, a very low work function of ~3.2 eV, and high stability towards oxidation. Photoelectron spectroscopy studies shows that an Al capping layer of ~1 nm in thickness is sufficient to block oxidation of the underlying Cu, which is remarkable given that the self-limiting oxide thickness for bulk Al is ≥2 nm. This promising substrate electrode is used to elucidate a new design rule for top-illuminated bulk-heterojunction OPVs. It is shown that for OPVs utilising high performance donor-type organic semiconductors in conjunction with a low work function electron extracting electrode, a barrier to hole-extraction spontaneously forms at the donor | electron-extracting electrode interface, blocking unwanted hole-extraction and negating the need for a hole-blocking layer, which simplifies the device architecture. This electrode design rule is underpinned by studies of the interfacial energetics with five widely used solution processed organic semiconductors as well as device based investigations. A novel organo-molybdenum oxide bronze is also developed which combines the function of wide band-gap interlayer for efficient hole-extraction with the role of a metal electrode seed layer, enabling the fabrication of highly transparent, low-sheet-resistance silver window electrodes for top-illuminated OPVs. Additionally, preliminary results relating to the fabrication of a model nanostructured reflective electrode are shown. This is designed to investigate the extent to which absorption of light can be enhanced in a top-illuminated OPVs by texturing the reflective substrate electrode

Synthesis of Cyanine Polyelectrolytes and Anions for Organic Electronic Devices

Many practical applications among organic electronic devices have been demonstrated over the last decades. They are considered as promising alternatives to inorganic semiconductor technologies due to the potential of cost-effective fabrication, excellent performance and versatile functionalities such as flexibility, portability and transparency. In this work, we mainly focused on the investigation of a special class of organic semiconductors, cyanine dyes. Starting with the synthesis of cyanine polyelectrolytes (Cy-Poly) and novel anions, new functionalities such as orthogonal solubility, crosslinking capability and photosensitizing ability were introduced and the corresponding materials as active components in organic electronic devices were explored. The utilization of cyanines allows for the mixed ionic/electronic conduction in solid organic semiconducting thin films. Due to the presence of mobile anions, an ionic junction is created as a result of the ionic motion towards respective metal electrodes. This leads to electrochemical oxidation and reduction within the organic thin film and the establishment of a built-in electric field across the intrinsic region with potential energy shifts. The electronic conductivity is considerably enhanced in the doped zones, which facilitates charge injection from the electrodes. The remarkable potential drop in the intrinsic region enables light emission of the electroluminescent materials as well as a photovoltaic response upon white light illumination. A special interest was given to the stabilization of ionic junctions in cyanine dye organic electronic devices by chemically fixing the ionic carriers in the desired position. Unexpected exciton quenching behavior was observed in the devices containing the immobilizable phenyl azide anions, which was attributed to the triplet sensitization effect of trimethine cyanines on the photodecomposition of 4-azido benzoate ions

Charge Carrier Trap Spectroscopy on Organic Hole Transport Materials

Electronic circuits comprising organic semiconductor thin-films are part of promising technologies for a renewable power generation and an energy-efficient information technology. Whereas TV and mobile phone applications of organic light emitting diodes (OLEDs) got ready for the market awhile ago, organic photovoltaics still lack in power conversion efficiencies, especially in relation to their current fabrication costs. A major reason for the low efficiencies are losses due to the large number of charge carrier traps in organic semiconductors as compared to silicon. It is the aim of this thesis to identify and quantify charge carrier traps in vacuum-deposited organic semiconductor thin-films and comprehend the reasons for the trap formation. For that, the techniques impedance spectroscopy (IS), thermally stimulated currents (TSC), and photoelectron spectroscopy are utilized. In order to assess the absolute energy of charge carrier traps, the charge carrier transport levels are computed for various hole transport materials such as MeO-TPD, pentacene, and ZnPc. Unlike inorganics, organic semiconductors possess in first-order approximation Gaussian distributed densities of states and temperaturedependent transport levels. The latter shift by up to 300 meV towards the energy gap-mid when changing from room temperature to 10 K as it is done for TSC examinations. The frequency-dependent capacitance response of charge carrier traps in organic Schottky diodes of pentacene and ZnPc are studied via impedance spectroscopy. In undoped systems, deep traps with depths of approx. 0.6 eV and densities in the order of 1016...1017 cm−3 are prevailing. For pentacene, the deep trap density is reduced when the material undergoes an additional purification step. Utilizing p-doping, the Fermi level is tuned in a way that deep traps are saturated. Vice versa, the freeze-out of p-doped ZnPc provides further insight into the influence of trap-filling, impurity saturation and reserve on the Fermi level position in organic semiconductors. Furthermore, charge carrier traps are investigated via thermally stimulated currents. It is shown that the trap depths are obtained correctly only if the dispersive transport of the released charge carriers until their extraction is considered. For the first time, the polarity of charge carrier traps in MeO-TPD, ZnPc, and m-MTDATA is identified from TSC’s differences in release time when spacer layers are introduced in the TSC samples. Simultaneously, tiny hole mobilities in the order of 10−13 cm2 Vs−1 are detected for low-temperature thin-films of the hole transporter material Spiro-TTB. It is shown for Spiro-TTB co-evaporated with the acceptor molecule F6-TCNNQ and a p-doped ZnPc:C60 absorber blend that the doping process creates shallow trap levels. Finally, various organic hole transport materials are examined upon their stability in water and oxygen atmosphere during sample fabrication and storage of the organic electronics. In case of pentacene, ZnPc, MeO-TPD, and m-MTDATA, hole traps are already present in unexposed thin-films, which increase in trap density upon oxygen exposure. A global trap level caused by oxygen impurities is found at energies of 4.7...4.8 eV that is detrimental to hole transport in organic semiconductors.Elektronische Bauelemente aus Dünnschichten organischer Halbleiter sind Teil möglicher Schlüsseltechnologien zur regenerativen Energiegewinnung und energieeffizienten Informationstechnik. Während Fernseh- und Mobilfunkanwendungen organischer Leuchtdioden (OLEDs) bereits vor einiger Zeit Marktreife erlangt haben, ist die organische Photovoltaik (OPV) noch durch zu hohe Fertigungskosten in Relation zu unzureichenden Effizienzen unrentabel. Ein wesentlicher Grund für die niedrigen Wirkungsgrade sind Verluste durch die im Vergleich zu Silizium hohe Zahl an Ladungsträgerfallen in organischen Halbleitern. Ziel dieser Arbeit ist es, mittels Impedanz-Spektroskopie (IS), thermisch stimulierten Strömen (TSC) und Photoelektronenspektroskopie methodenübergreifend Ladungsträgerfallen in vakuumverdampften organischen Dünnschichten zu identifizieren, zu quantifizieren und ihre Ursachen zu ergründen. Um die Energie von Ladungsträgerfallen absolut beziffern zu können, wird zunächst für verschiedene Lochtransportmaterialien wie z.B. MeO-TPD, Pentazen und ZnPc die Transportenergie aus den in erster Ordnung gaußförmigen Zustandsdichten berechnet. Im Gegensatz zu anorganischen Halbleitern ist die Transportenergie in organischen Halbleitern temperaturabhängig. Sie verschiebt sich beim Übergang von Raumtemperatur zu 10 K, wie für TSC Untersuchungen bedeutsam, um bis zu 300 meV in Richtung der Bandlückenmitte. Mittels Impedanz-Spektroskopie wird die frequenzabhängige Kapazitätsantwort von Ladungsträgerfallen in organischen Schottky-Dioden aus Pentazen und ZnPc untersucht. In undotierten Systemen dominieren Defekte mit Tiefen um 0.6 eV, deren Dichte in der Größenordnung von 1016...1017 cm−3 liegt, sich aber im Fall von Pentazen durch einen zusätzlichen Materialaufreinigungsschritt halbieren lässt. Über p-Dotierung wird das Fermi-Level so eingestellt, dass tiefe Fallen abgesättigt werden können. Umgekehrt liefert das Ausfrieren von p-dotiertem ZnPc weitere Belege für den Einfluss von Fallenzuständen, Störstellen-Erschöpfung und Reserve auf das Fermi-Level in dotierten organischen Halbleitern. Im Weiteren werden Ladungsträgerfallen über thermisch stimulierte Ströme untersucht. Es wird gezeigt, dass die Fallentiefen nur dann konsistent bestimmt werden, wenn der dispersive Transport von freigesetzten Ladungsträgern zur Extraktionsstelle berücksichtigt wird. Durch Einführung von ’Abstandshalterschichten’ werden erstmalig über TSC die Polaritäten von Ladungsträgerfallen in MeO-TPD, ZnPc und m-MTDATA per Laufzeitunterschied bestimmt. Gleichzeitig werden geringste Löcherbeweglichkeiten in der Größenordnung von 10−13 cm2 Vs−1 für stark gekühlte Dünnschichten des Lochtransporters Spiro-TTB gemessen. Wie für Spiro-TTB koverdampft mit dem Akzeptormolekül F6-TCNNQ und p-dotierte Mischschichten der Absorbermaterialien ZnPc und C60 gezeigt, erzeugt Dotierung relativ flache Störstellen. Abschließend werden verschiedene organische Lochtransporter-Materialien auf ihre Stabilität in Wasser- und Sauerstoffatmosphären während der Prozessierung und der Lagerung fertiger elektronischer Bauelemente untersucht. Für Pentazen, ZnPc, MeO-TPD und m-MTDATA werden Löcherfallen in intrinsischen Dünnschichten nachgewiesen. Bei Kontakt mit Sauerstoff nimmt deren Defektdichte zu. Es findet sich ein universales Fallenniveau bei rund 4.7...4.8 eV, verursacht durch Sauerstoffverunreinigungen, welches den Lochtransport in organischen Halbleitern limitiert

Progress of Surface Science Studies on ABX(3)-Based Metal Halide Perovskite Solar Cells

ABX(3) type metal halide perovskite solar cells (PSCs) have shown efficiencies over 25%, rocketing toward their theoretical limit. To gain the full potential of PSCs relies on the understanding of the device working mechanisms and recombination, the material quality, and the match of energy levels in the device stacks. In this review, the importance of designing PSCs from the viewpoint of surface/interface science studies is presented. For this purpose, recent case studies are discussed to demonstrate how probing of local heterogeneities (e.g., grains, grain boundaries, atomic structure, etc.) in perovskites by surface science techniques can help correlate material properties and PSC device performance. At the solar cell device level with active areas larger than millimeter scale, the ensemble average measurement techniques can characterize the overall average properties of perovskite films as well as their adjacent layers and provide clues to understand better the solar cell parameters. How generation and healing of electronic defects in perovskite films limit the device efficiency, reproducibility, and stability, and induce the time-dependent transient behavior in the current-voltage curves are also the central focus of this review. On the basis of these studies, strategies to further improve efficiency and stability, as well as reducing hysteresis are presented

Strategies for Optimizing Organic Solar Cells: Correlation between Morphology and Performance in DCV6T - C60 Heterojunctions

This work investigates organic solar cells made of small molecules. Using the material system α,ω-bis(dicyanovinylene)-sexithiophene (DCV6T) - C60 as model, the correlation between the photovoltaic active layer morphology and performance of the solar cell is studied. The chosen method for controlling the layer morphology is applying different substrate temperatures (Tsub ) during the deposition of the layer. In neat DCV6T layers, substrate heating induces higher crystallinity as is shown by X-ray diffraction and atomic force microscopy (AFM). The absorption spectrum displays a more distinct fine structure, a redshift of the absorption peaks by up to 11 nm and a significant increase of the low energy absorption band at Tsub = 120°C compared to Tsub = 30°C. Contrary to general expectations, the hole mobility as measured in field effect transistors and with the method of charge extraction by linearly increasing voltage (CELIV) does not increase in samples with higher crystallinity. In mixed layers, investigations by AFM and UV-Vis spectroscopy reveal a stronger phase separation induced by substrate heating, leading to larger domains of DCV6T. This is indicated by an increased grain size and roughness of the topography, the increase of the DCV6T luminescence signal, and the more distinct fine structure of the DCV6T related absorption. Based on the results of the morphology analysis, the effect of different substrate temperatures on the performance of solar cells with flat and mixed DCV6T - C60 heterojunctions is investigated. In flat heterojunction solar cells, a slight increase of the photocurrent by about 10% is observed upon substrate heating, attributed to the increase of DCV6T absorption. In mixed DCV6T : C60 heterojunction solar cells, much more pronounced enhancements are achieved. By varying the substrate temperature from -7°C to 120°C, it is shown that the stronger phase separation upon substrate heating facilitates the charge transport, leading to a significant increase of the internal quantum efficiency (IQE), photocurrent, and fill factor. Consequently, the power conversion efficiency (PCE) increases from 0.5% at Tsub = -7°C to about 3.0 % at Tsub ≥ 77°C. Subsequent optimization of the DCV6T : C60 mixing ratio and the stack design of the solar cell lead to devices with PCE of 4.9±0.2 %. Using optical simulations, the IQE of these devices is studied in more detail to identify major remaining loss mechanisms. The evaluation of the absorption pattern in the wavelength range from 300 to 750 nm shows that only 77 % of the absorbed photons contribute to the exciton generation in photovoltaic active layers, while the rest is lost in passive layers. Furthermore, the IQE of the photovoltaic active layers, consisting of an intrinsic C60 layer and a mixed DCV6T : C60 layer, exhibits a lower exciton diffusion efficiency for C60 excitons compared to DCV6T excitons, attributed to exciton migration into the adjacent electron transport layer.:1 Introduction 2 Physical Properties of Organic Semiconductors 2.1 Organic Solids 2.2 Molecules with Conjugated π-Electron Systems 2.2.1 Energy Splitting in Molecular Orbital Theory 2.2.2 Extended π-Conjugated Systems 2.3 Optical Excitations in Organic Molecules 2.4 From Molecules to Solids 2.4.1 Self-Polarization in Organic Solids 2.4.2 Excitations in Organic Solids 2.4.3 Charge Carriers and Transport 3 Organic Photovoltaics 3.1 Solar Cell Physics 3.1.1 Conversion of Radiation into Chemical Energy 3.1.2 Conversion of Chemical Energy into Electrical Energy 3.1.3 Conventional pn-Junction as Photodiode 3.1.4 Simple Equivalent Circuit 3.2 Organic Solar Cells 3.2.1 Donor-Acceptor Heterojunction 3.2.2 Recombination Processes 3.2.3 Transport Layers – The p-i-n Concept 4 Experimental 4.1 Materials 4.1.1 C60 4.1.2 Transport Materials 4.2 Sample Preparation 4.3 Experimental Methods 4.3.1 X-Ray Diffraction 4.3.2 Optical Characterization 4.3.3 Topography Characterization 4.3.4 Mobility Measurements 4.3.5 Electrical Characterization of Solar Cells 4.3.6 Optical Simulation 4.3.7 Ultraviolet Photoelectron Spectroscopy 4.4 Standard Reporting Conditions and Mismatch 5 The Material System DCV6T - C60 5.1 Oligothiophenes as Donors in Heterojunctions with C60 5.2 Basic Material Properties of DCV6T 5.2.1 Optical Properties 5.2.2 Electronic Properties 5.3 Effect of Substrate Heating on Layer Morphology 5.3.1 Neat DCV6T Layers 5.3.2 Mixed DCV6T : C60 Layers 5.4 Effect of Substrate Heating on Mobility 6 DCV6T - C60 Solar Cells 6.1 Effect of Substrate Heating in DCV6T - C60 Solar Cells 6.1.1 Flat Heterojunction Solar Cells 6.1.2 Mixed Heterojunction Solar Cells 6.2 Influence of the Mixing Ratio 6.3 Optimizing the Layer Stack 6.3.1 Influence of the Transport Layer Thickness 6.3.2 Influence of the Mixed Layer Thickness 6.3.3 Discussion of Quantum Efficiency and Loss Mechanisms 6.4 Thermal Annealing 7 Conclusions and Outlook 7.1 Conclusions 7.2 Outlook Appendix Bibliography AcknowledgementsDiese Arbeit befasst sich mit organischen Solarzellen aus kleinen Molekülen. Anhand des Materialsystems α,ω-bis(Dicyanovinylen)-Sexithiophen (DCV6T) - C60 wird der Zusammenhang zwischen Morphologie der photovoltaisch aktiven Schicht und dem Leistungverhalten der Solarzellen untersucht. Zur Beeinflussung der Morphologie werden verschiedene Substrattemperaturen (Tsub ) während des Schichtwachstums der aktiven Schicht eingestellt. Beim Heizen des Substrates weisen DCV6T Einzelschichten eine erhöhte Kristallinität auf, die mittels Röntgenbeugung und Rasterkraftmikroskopie (AFM) erkennbar ist. Zudem bewirkt die Erhöhung der Substrattemperatur von 30°C auf 120°C eine ausgeprägtere Feinstrukturierung des Absorptionsspektrums, eine Rotverschiebung um bis zu 11 nm und eine Verstärkung der niederenergetischen Absorptionsbande. Entgegen den Erwartungen wird weder in Feldeffekttransistoren noch mit der Methode der Ladungsextraktion bei linear steigenden Spannungspulsen (CELIV) eine Verbesserung der Löcherbeweglichkeit in Zusammenhang mit der erhöhten Kristallinität gemessen. Mischschichten mit C60 weisen bei erhöhten Substrattemperaturen eine stärkere Phasentrennung auf, die zu größeren DCV6T Domänen innerhalb der Schicht führt. Dieser Effekt wird zum Einen durch größere Körnung und Rauigkeit der Topographie, zum Anderen durch die Erhöhung des Lumineszenzsignals von DCV6T sowie der Ausprägung der Feinstruktur im Absorptionsspektrum nachgewiesen. Ausgehend von den Ergebnissen der Morphologieuntersuchung werden die Auswirkungen von verschiedenen Substrattemperaturen auf das Leistungsverhalten von DCV6T - C60 Solarzellen mit planarem und Volumen-Heteroübergang analysiert. Solarzellen mit planarem Heteroübergang weisen eine geringe Verbesserung des Photostromes von etwa 10 % beim Heizen des Substrates auf. Diese wird durch die Erhöhung der DCV6T Absorption verursacht. In Volumen-Heteroübergängen führt die stärkere Phasentrennung bei steigender Substrattemperatur im untersuchten Temperaturbereich von -7°C bis 120°C zu einer Verbesserung des Ladungsträgertransports. Dadurch verbessern sich die interne Quanteneffizienz (IQE), der Photostrom und der Füllfaktor. Der Wirkungsgrad der Solarzellen erhöht sich von 0.5 % bei Tsub = -7°C auf 3.0 % bei Tsub ≥ 77°C. Eine weitere Optimierung des DCV6T : C60 Mischverhältnisses und des Schichtaufbaus ermöglicht Solarzellen mit Wirkungsgraden von 4.9±0.2 %. Mittels optischer Simulationen wird die IQE dieser Solarzellen näher untersucht, um verbleibende Verlustmechanismen zu identifizieren. Es ergibt sich, dass innerhalb des Wellenlängenbereichs von 300 bis 750 nm nur 77 % der absorbierten Photonen tatsächlich in den photovoltaisch aktiven Schichten absorbiert werden, während der Rest in nicht aktiven Schichten verloren geht. Des Weiteren kann nachgewiesen werden, dass C60 Exzitonen aus der aktiven Schicht, bestehend as einer intrinsischen C60 Schicht und einer DCV6T : C60 Mischschicht, durch Diffusion in die angrenzende Elektronentransportschicht verloren gehen.:1 Introduction 2 Physical Properties of Organic Semiconductors 2.1 Organic Solids 2.2 Molecules with Conjugated π-Electron Systems 2.2.1 Energy Splitting in Molecular Orbital Theory 2.2.2 Extended π-Conjugated Systems 2.3 Optical Excitations in Organic Molecules 2.4 From Molecules to Solids 2.4.1 Self-Polarization in Organic Solids 2.4.2 Excitations in Organic Solids 2.4.3 Charge Carriers and Transport 3 Organic Photovoltaics 3.1 Solar Cell Physics 3.1.1 Conversion of Radiation into Chemical Energy 3.1.2 Conversion of Chemical Energy into Electrical Energy 3.1.3 Conventional pn-Junction as Photodiode 3.1.4 Simple Equivalent Circuit 3.2 Organic Solar Cells 3.2.1 Donor-Acceptor Heterojunction 3.2.2 Recombination Processes 3.2.3 Transport Layers – The p-i-n Concept 4 Experimental 4.1 Materials 4.1.1 C60 4.1.2 Transport Materials 4.2 Sample Preparation 4.3 Experimental Methods 4.3.1 X-Ray Diffraction 4.3.2 Optical Characterization 4.3.3 Topography Characterization 4.3.4 Mobility Measurements 4.3.5 Electrical Characterization of Solar Cells 4.3.6 Optical Simulation 4.3.7 Ultraviolet Photoelectron Spectroscopy 4.4 Standard Reporting Conditions and Mismatch 5 The Material System DCV6T - C60 5.1 Oligothiophenes as Donors in Heterojunctions with C60 5.2 Basic Material Properties of DCV6T 5.2.1 Optical Properties 5.2.2 Electronic Properties 5.3 Effect of Substrate Heating on Layer Morphology 5.3.1 Neat DCV6T Layers 5.3.2 Mixed DCV6T : C60 Layers 5.4 Effect of Substrate Heating on Mobility 6 DCV6T - C60 Solar Cells 6.1 Effect of Substrate Heating in DCV6T - C60 Solar Cells 6.1.1 Flat Heterojunction Solar Cells 6.1.2 Mixed Heterojunction Solar Cells 6.2 Influence of the Mixing Ratio 6.3 Optimizing the Layer Stack 6.3.1 Influence of the Transport Layer Thickness 6.3.2 Influence of the Mixed Layer Thickness 6.3.3 Discussion of Quantum Efficiency and Loss Mechanisms 6.4 Thermal Annealing 7 Conclusions and Outlook 7.1 Conclusions 7.2 Outlook Appendix Bibliography Acknowledgement