We investigate the influence of the built-in voltage on the performance of organic bulk heterojuction solar cells that are based on a p-i-n structure. Electrical doping in the hole and the electron transport layer allows to tune their work function and hence to adjust the built-in voltage: Changing the doping concentration from 0.5 to 32 wt% induces a shift of the work function towards the transport levels and increases the built-in voltage. To determine the built-in voltage, we use electroabsorption spectroscopy which is based on an evaluation of the spectra caused by a change in absorption due to an electric field (Stark effect). For a model system with a bulk heterojunction of BF-DPB and C60, we show that higher doping concentrations in both the electron and the hole transport layer increase the built-in voltage, leading to an enhanced short circuit current and solar cell performance.

Diese Arbeit beschäftigt sich mit der Untersuchung der Langzeitstabilität organischer Solarzellen. Die Solarzellen als Gegenstand dieser Untersuchungen sind dabei aus Materialien aufgebaut, die mittels thermischer Gasphasenabscheidung im Vakuum hergestellt werden. Das unterscheidet diese von vielen in der Literatur vorgestellten Alterungsstudien, die Polymersolarzellen behandeln. Als Standardsystem werden einfache pii-Bauelemente ausgewählt, die in ZnPc und C60 zwei gut untersuchte Materialien in der aktiven Donor-Akzeptor-Schicht nutzen. Die Ergebnisse dieser Arbeit sind dabei in drei Kapiteln zusammengefasst. In Kapitel 5 wird untersucht, wie sich verschiedene Faktoren auf die Lebensdauer der Solarzellen auswirken. Für verkapselte Solarzellen mit MeO-TPD in der Lochtransportschicht wird die thermische Beschleunigung der Degradation mit einem Arrhenius’schen Verhalten beschrieben und eine Aktivierungsenergie EA=712 meV gefunden. Aus dieser Beschreibung wird für verkapselte Solarzelle bei 100 mW cm-2 und 45°C eine Lebensdauer von 62.000 h extrapoliert, die experimentell nicht verifiziert werden kann. Auch der Einfluss der Beleuchtungsintensität auf die Degradationsgeschwindigkeit wird untersucht und kann systematisch erklärt werden: Die Beschleunigung, die sich aus einer Erhöhung der Intensität weißen Lichtes ergibt, kann beschrieben werden, indem man die Anzahl extrahierter Ladungsträger berechnet. Bei Alterungen unter verschiedene Intensitäten ist diese Zahl identisch, wenn man die Messung bei gleichem Grad der Degradation betrachtet. Diese Modell kann auch auf monochromatische Beleuchtung ausgedehnt werden und es zeigt sich bei einem Vergleich über alle untersuchten Wellenlängen, dass der Anstieg der fallenden Kurven umso steiler wird, je höher die kürzeste Wellenlänge des jeweiligen Spektrums ist. Der zweite Teil dieses Kapitels ist der Degradation unverkapselter Solarzellen mit BF-DPB als Lochtransportmaterial gewidmet. Durch Variation von Temperatur und relativer Luftfeuchte konnten beide Einflussfaktoren in einem kombinierten Modell, dem Peck-Modell, zusammengefasst werden. Dieses wurde bisher nicht zur Beschreibung des Degradationsverhaltens von Solarzellen verwendet. Eine Vorhersage der Lebensdauer bei beliebigen Werten für beide Parameter ist somit möglich. Deutlich sticht in diesem Experiment hervor, dass die Anwesenheit von Wasser die Degradation klar dominiert. Darauf folgen Messungen, bei denen die Wasserpermeationsrate (WVTR) der Verkapselung variiert wird. Dabei stellt sich heraus, dass diese besser als 10-3 g m-2 d-1 sein muss, um die Stabilität zu verbessern. Durch eine Trennung der WVTR der äußeren Barriere und jener der Aluminiumelektrode ist es möglich, den Wert WVTR(Al) zu ermitteln. Dieser beträgt 8 x10-4 g m-2 d-1. Zusätzlich kann die Wassermenge, die benötigt wird, um die untersuch-ten Solarzelle auf 50% des Startwertes zu degradieren, zu 10 mg m2 bestimmt werden. Kapitel 6 zeigt eine umfassende Charakterisierung von an Luft degradierten Solarzellen. Mit den chemischen Analysemethoden TOF-SIMS und LDI-TOF-MS können mehrere Reaktionen der verwendeten Materialien mit Luft identifiziert werden. Dabei sticht die Oxidation der BPhen-Aluminium-Grenzschicht, die zur Bildung von Al2O3 führt, hervor. Weitere Reaktionsprodukte, vor allem in Verbindung mit Fluor, welches aus der Zersetzung von C60F36 stammt, werden gezeigt. Die Oxidation der Organik-Aluminium-Grenzschicht kann mit Hilfe von Elektrolumineszenzmessungen untersucht werden. Bei diesen zeigt sich, dass die Abnahme der aktiven Fläche in vollem Umfang Ursache für die Reduktion der Kurzschlussstromdichte ist. Als Eintrittskanäle für Sauerstoff und Wasser werden neben pinholes auch die Seitenkanten der Solarzelle identifiziert. Hinweise für die flächige Diffusion von Wasser werden zusätzlich erbracht. Erster Ansatz zur Verbesserung der Langzeitstabilität ist der Austausch von BPhen durch ein dotiertes Elektronen-transportmaterial. Eine Variation von fünf Materialien zeigt, dass ein Zusammenhang zwischen Rauigkeit dieses Materials und der Lebensdauer besteht: So werden die besten Stabilität für Materialien wie C60 und Bis-HFl-NTCDI gezeigt, die mit einer geringen Rauigkeit aufwachsen. Die Lebensdauer beträgt am Beispiel von Bis-HFl-NTCDI bei [T=65°C; rH=2,2%] T50=762 h und ist damit etwa viermal so groß wie bei Verwendung von BPhen. Weitere Optimierungsversuche, zum Beispiel durch Variation der Elektrode, des p-Dotanden, oder der Invertierung der Struktur zeigen zwar zusätzliche Degradationspfade auf, führen aber zu keiner Verbesserung der Stabilität. Auf Basis zuvor durchgeführter Überlegungen zu Redoxreaktionen (organischer) Materialien mit Wasser und Sauerstoff wird zum Abschluss der Arbeit ein möglicher Aufbau für luftstabile organische Solarzellen vorgeschlagen.:1 Einleitung 2 Grundlagen 2.1 Organische Halbleiter 2.2 Grundlagen der Photovoltaik 2.3 Quasi-Fermi-Niveaus & Würfel-Modell 2.4 Organische Solarzellen 3 Grundlagen zu Langzeitmessungen 3.1 Anforderungen an organische Solarzellen 3.2 Materialien 4 Experimentelle Grundlagen 4.1 Herstellung der Solarzellen und Ca-Tests 4.2 Verkapselung 4.3 Charakterisierungsmethoden 4.4 Alterungsmessungen 4.5 Verwendeter Probenaufbau 5 Variation der Alterungsbedingungen 5.1 Alterung in Inertatmosphäre 5.2 Alterung in Anwesenheit von Luft 5.3 Korrelation von Laborbewitterung und Außenmessung 5.4 Zusammenfassung 6 Charakterisierung gealterter Solarzellen 6.1 TOF-SIMS 6.2 UI-Kennlinien, EQE und Absorption 6.3 Elektrolumineszenzmessungen 6.4 Elektrische Simulation 6.5 LDI-TOF-MS 6.6 Zusammenfassung 7 Optimierung der Lebensdauer 7.1 Variation der Elektronentransportschicht in pin-Solarzellen 7.1.5.2 Rauigkeit 7.1.6 Zusammenfassung 7.2 Invertierte Struktur: pin vs. nip 7.3 Variation des Deckkontaktes 7.4 Variationen in der Lochtransportschicht 7.5 Zusammenfassung: Optimierung der Lebensdauer 8 Zusammenfassung und Ausblick 8.1 Zusammenfassung 8.2 Ausblick

This study focuses on the architectural modification of pin-type small-molecule organic solar cells, in particular on the absorption layer and its influence on the key solar cell parameters, such as short circuit current density, fill factor and open circuit voltage. Three different approaches have been applied to improve the match between the solar spectrum and the spectral sensitivity of organic solar cells. In the first part, deposition parameters such as substrate temperature, gradient strength and (graded) absorption layer thickness are evaluated and compared to organic solar cells with homogeneously deposited absorption layers. Moreover, the gradient-like distribution of the absorption layer is characterized optically and morphological effects have been extensively studied. In order to isolate the origin of the efficiency improvement due to the graded architecture, voltage-dependent spectral response measurements have been performed and gave new insights. The second part concentrates on the efficient in-coupling of converted UV light, which is usually lost because of the cut off properties of organic light in-coupling layers. Via Förster resonance energy transfer, the absorbed UV light is re-emitted as red light and contributes significantly to higher short circuit current densities. The correlation between doping concentration, simple stack architecture modifications and the performance improvement is duly presented. In the third and last part, the impact of tri-component bulk heterojunction absorption layers is investigated, as these have potential to broaden the sensitivity spectrum of organic solar cells without chemical modification of designated absorber molecules. Along with the possibility to easily increase the photocurrent, an interesting behavior of the open circuit voltage has been observed. Knowledge about the impact of slight modifications within the solar stack architecture is important in order to be able to improve the device efficiency for the production of cheap and clean energy.

The present work is the fourth (and final) contribution to an inter-laboratory collaboration that was planned at the 3rd International Summit on Organic Photovoltaic Stability (ISOS-3). The collaboration involved six laboratories capable of producing seven distinct sets of OPV devices that were degraded under well-defined conditions in accordance with the ISOS-3 protocols. The degradation experiments lasted up to 1830 hours and involved more than 300 cells on more than 100 devices. The devices were analyzed and characterized at different points of their lifetimes by a large number of non-destructive and destructive techniques in order to identify specific degradation mechanisms responsible for the deterioration of the photovoltaic response. Work presented herein involves time-of-flight secondary ion mass spectrometry (TOF-SIMS) in order to study chemical degradation in-plane as well as in-depth in the organic solar cells. Various degradation mechanisms were investigated and correlated with cell performance. For example, photo-oxidation of the active material was quantitatively studied as a function of cell performance. The large variety of cell architectures used (some with and some without encapsulation) enabled valuable comparisons and important conclusions to be drawn on degradation behaviour. This comprehensive investigation of OPV stability has significantly advanced the understanding of degradation behaviour in OPV devices, which is an important step towards large scale application of organic solar cells. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

Organic solar cells offer attractive properties for novel applications and continuous advances in material and concept development have led to significant improvements in device performance. To exploit their full potential (roll-to-roll production of flexible and top-illuminated devices, using e.g. opaque metal foil or textile as substrate), highly transparent, conductive, mechanically flexible, and cost-efficient top electrodes are of great importance. The current standard material indium tin oxide (ITO) is rigid, expensive and requires a high energy / high temperature deposition process, limiting ITO (and other transparent conductive oxides) to bottom electrode applications. This work presents fundamental investigations to understand and control the properties of transparent conductors and documents four different approaches to prepare transparent electrodes on top of efficient small molecule organic solar cells, with the aim to replace ITO. Fullerene C60 layers are investigated as completely carbon-based electrodes. For an optimized doping concentration, sheet resistance and transmittance are improved and efficient solar cells are realized. Since the lateral charge transport is still limited, a combination with a microstructured conductor is suggested. Pulsed laser deposition allows for the first time a damage-free preparation of gallium doped zinc oxide (ZnO:Ga) layers on top of organic devices by careful optimization of the deposition atmosphere. ZnO:Ga electrodes with a transmittance of Tvis = 82.7 % and sheet resistance Rs = 83 Ohm/sq are obtained. The formation of local shunts due to ZnO:Ga droplets is identified and then prevented by a shadow mask between the target and the sample, enabling solar cells with similar efficiency (2.9 %) compared to a reference device using a state-of-the-art metal top contact. Another very promising alternative are intrinsically flexible, ultra-thin silver layers. By introducing an oxide interlayer, the adverse interpenetration of silver and organic materials is prevented and the charge extraction from the solar cells is improved. With a second oxide layer on top, the silver electrode is significantly stabilized, leading to an increased solar cell lifetime of 4500 h (factor of 107). Scanning electron micrographs of Ag thin films reveal a poor wetting on organic and oxide substrates, which strongly limits the electrode performance. However, it is significantly improved by a 1 nm thin seed layer. An optimized Au/Ag film reaches Tvis = 78.1 % and Rs = 19 Ohm/sq, superior to ITO. Finally, silver electrodes blended with calcium show a unique microstructure which enables unusually high transmittance (84.3 % at 27.3 Ohm/sq) even above the expectations from bulk material properties and thin film optics. Such values have not been reached for transparent electrodes on top of organic material so far. Solar cells with a Ca:Ag top electrode achieve an efficiency of 7.2 %, which exceeds the 6.9 % of bottom-illuminated reference cells with conventional ITO electrodes and defines a new world record for top-illuminated organic solar cells. With these electrodes, semi-transparent and large-area devices, as well as devices on opaque and flexible substrates are successfully prepared. In summary, it is shown that ZnO:Ga and thin metal electrodes can replace ITO and fill the lack of high performance top electrodes. Moreover, the introduced concepts are not restricted to specific solar cell architectures or organic compounds but are widely applicable for a variety of organic devices.

In the last 25 years, organic or "plastic" solar cells have gained commercial interest as a light-weight, flexible, colorful, and potentially low-cost technology for direct solar energy conversion into electrical power. Currently, organic solar cells with a maximum power conversion effciency (PCE) of 12% can compete with classical silicon technology under certain conditions. In particular, a variety of strongly absorbing organic molecules is available, enabling custom-built organic solar cells for versatile applications. In order to improve the PCE, the charge carrier mobility in organic thin films must be improved. The transport characterization of the relevant materials is usually done in neat layers for simplicity. However, the active layer of highly efficient organic solar cells comprises a bulk heterojunction (BHJ) of a donor and an acceptor component necessary for effective charge carrier generation from photo-generated excitons. In the literature, the transport properties of such blend layers are hardly studied. In this work, the transport properties of typical BHJ layers are investigated using space-charge limited currents (SCLC), conductivity, impedance spectroscopy (IS), and thermally stimulated currents (TSC) in order to model the transport with numerical drift-diffusion simulations. Firstly, the influence of an exponential density of trap states on the thickness dependence of SCLCs in devices with Ohmic injection contacts is investigated by simulations. Then, the results are applied to SCLC and conductivity measurements of electron- and hole-only devices of ZnPc:C60 at different mixing ratios. Particularly, the field and charge carrier density dependence of the mobility is evaluated, suggesting that the hole transport is dominated by exponential tail states acting as trapping sites. For comparison, transport in DCV5T-Me33:C60, which shows better PCEs in solar cells, is shown not to be dominated by traps. Furthermore, a temperature-dependent IS analysis of weakly p-doped ZnPc:C60 (1:1) blend reveals the energy-resolved distribution of occupied states, containing a Gaussian trap state as well as exponential tail states. The obtained results can be considered a basis for the characterization of trap states in organic solar cells. Moreover, the precise knowledge of the transport-relevant trap states is shown to facilitate modeling of complete devices, constituting a basis for predictive simulations of optimized device structures.:1 Introduction 2 Organic Semiconductors and Solar Cells 2.1 Structural, Optical, and Energetic Properties 2.2 Charge Carrier Transport 2.2.1 Classical Transport Models 2.2.2 Hopping and Tunneling Transport 2.2.3 Limitations of Transport Characterization 2.3 Doping 2.4 Single Carrier Devices 2.4.1 Theory of Space-Charge Limited Currents 2.4.2 Electrical Potential Mapping by Thickness Variation 2.4.3 Influence of the Contacts 2.5 Organic Solar Cells 2.5.1 Principles 2.5.2 The p-i-n Concept 2.5.3 Recombination 2.5.4 Electrical Characterization 3 Numerical Drift-Diffusion Simulations 3.1 Modeling Organic Semiconductors 3.2 System of Differential Equations 3.3 Simulation Algorithm and Modules 4 Exploiting Contact Diffusion Currents for Trap Characterization in Organic Semiconductors 4.1 Motivation 4.2 Drift-Diffusion Model 4.3 Results and Discussion 4.4 Conclusion 5 Transport Characterization of Donor-Acceptor Blend Layers 5.1 Motivation 5.2 Device Fabrication 5.3 Hole Transport in ZnPc:C60 Blends with Balanced Mixing Ratios 5.3.1 Current-Voltage Measurements 5.3.2 Drift-Diffusion Model 5.3.3 Modeling Results 5.3.4 Discussion 5.4 Hole Transport in Fullerene-Rich ZnPc:C60 Blends 5.4.1 Results and Discussion 5.5 Electron Transport in ZnPc:C60 (1:1) 5.5.1 Results and Discussion 5.6 Transport in Blend Layers with the High Efficiency Donor DCV2-5T-Me33 5.6.1 Hole Transport in DCV2-5T-Me33:C60 5.6.2 Electron Transport in DCV2-5T-Me33:C60 5.7 Conclusions for Transport in Blend Layers 6 Doping-Enabled Density of States Determination in Donor-Acceptor Blend Layers 6.1 Motivation 6.2 Theory 6.3 Methods 6.4 Results 6.4.1 Impedance Spectroscopy 6.4.2 Fermi level, Mott-Schottky Analysis, and Band Diagram 6.4.3 DOOS Determination 6.4.4 Thermally Stimulated Currents 6.4.5 Solar Cell Characteristics 6.5 Discussion 6.6 Conclusions on the DOS of ZnPc:C60 (1:1) 7 Conclusion and Outlook Materials, Symbols, Abbreviations Bibliography / Organische oder "Plastik"-Solarzellen haben in den letzten 25 Jahren eine rasante Entwicklung durchlaufen. Kommerziell sind sie vor allem wegen ihres geringen Gewichts, Biegsamkeit, Farbigkeit und potentiell geringen Herstellungskosten interessant, was zukünftig auf spezielle Anwendungen zugeschnittene Solarzellen ermöglichen wird. Die Leistungseffzienz von 12% ist dabei unter günstigen Bedingungen bereits mit klassischer Siliziumtechnologie konkurrenzfähig. Um die Effzienz weiter zu steigern und damit die Wirtschaftlichkeit zu erhöhen, muss vor allem die Ladungsträgerbeweglichkeit verbessert werden. In organischen Solarzellen werden typischerweise Donator-Akzeptor-Mischschichten verwendet, die für die effziente Generation freier Ladungsträger aus photo-induzierten Exzitonen verantwortlich sind. Obwohl solche Mischschichten typisch für organische Solarzellen sind, werden Transportuntersuchungen der relevanten Materialien der Einfachheit halber meist in ungemischten Schichten durchgeführt. In der vorliegenden Arbeit wird der Ladungstransport in Donator-Akzeptor-Mischschichten mithilfe raumladungsbegrenzter Ströme (space-charge limited currents, SCLCs), Leitfähigkeit, Impedanzspektroskopie (IS) und thermisch-generierter Ströme (thermally stimulated currents, TSC) untersucht und mit numerischen Drift-Diffusions-Simulationen modelliert. Zunächst wird mittels Simulation der Einfluss exponentiell verteilter Fallenzustände auf das schichtdickenabhängige SCLC-Verhalten unipolarer Bauelemente mit Ohmschen Kontakten untersucht. Die Erkenntnisse werden dann auf Elektronen- und Lochtransport in ZnPc:C60-Mischschichten mit verschiedenen Mischverhältnissen angewendet. Dabei wird die Beweglichkeit als Funktion von elektrischem Feld und Ladungsträgerdichte dargestellt, um SCLC- und Leitfähigkeitsmessungen zu erklären, was mit einer exponentiellen Fallenverteilung gelingt. Zum Vergleich werden dieselben Untersuchungen in DCV2-5T-Me33:C60, dem effizientesten der bekannten Solarzellenmaterialien dieser Art, wiederholt, ohne Anzeichen für fallendominierten Transport. Des weiteren werden erstmals schwach p-dotierte ZnPc:C60-Mischschichten mit temperaturabhängiger IS untersucht, um direkt die Dichte besetzter Lochfallenzustände zu bestimmen. Dabei werden wiederum exponentielle Fallenzustände sowie eine Gaußförmige Falle beobachtet. Insgesamt tragen die über Fallenzustände in Mischschichten gewonnenen Erkenntnisse zum Verständnis von Transportprozessen bei und bilden damit eine Grundlage für die systematische Identifizierung von Fallenzuständen in Solarzellen. Außerdem wird gezeigt, dass die genaue Beschreibung der transportrelevanten Fallenzustände die Modellierung von Bauelementen ermöglicht, auf deren Grundlage zukünftig optimierte Probenstrukturen vorhergesagt werden können.:1 Introduction 2 Organic Semiconductors and Solar Cells 2.1 Structural, Optical, and Energetic Properties 2.2 Charge Carrier Transport 2.2.1 Classical Transport Models 2.2.2 Hopping and Tunneling Transport 2.2.3 Limitations of Transport Characterization 2.3 Doping 2.4 Single Carrier Devices 2.4.1 Theory of Space-Charge Limited Currents 2.4.2 Electrical Potential Mapping by Thickness Variation 2.4.3 Influence of the Contacts 2.5 Organic Solar Cells 2.5.1 Principles 2.5.2 The p-i-n Concept 2.5.3 Recombination 2.5.4 Electrical Characterization 3 Numerical Drift-Diffusion Simulations 3.1 Modeling Organic Semiconductors 3.2 System of Differential Equations 3.3 Simulation Algorithm and Modules 4 Exploiting Contact Diffusion Currents for Trap Characterization in Organic Semiconductors 4.1 Motivation 4.2 Drift-Diffusion Model 4.3 Results and Discussion 4.4 Conclusion 5 Transport Characterization of Donor-Acceptor Blend Layers 5.1 Motivation 5.2 Device Fabrication 5.3 Hole Transport in ZnPc:C60 Blends with Balanced Mixing Ratios 5.3.1 Current-Voltage Measurements 5.3.2 Drift-Diffusion Model 5.3.3 Modeling Results 5.3.4 Discussion 5.4 Hole Transport in Fullerene-Rich ZnPc:C60 Blends 5.4.1 Results and Discussion 5.5 Electron Transport in ZnPc:C60 (1:1) 5.5.1 Results and Discussion 5.6 Transport in Blend Layers with the High Efficiency Donor DCV2-5T-Me33 5.6.1 Hole Transport in DCV2-5T-Me33:C60 5.6.2 Electron Transport in DCV2-5T-Me33:C60 5.7 Conclusions for Transport in Blend Layers 6 Doping-Enabled Density of States Determination in Donor-Acceptor Blend Layers 6.1 Motivation 6.2 Theory 6.3 Methods 6.4 Results 6.4.1 Impedance Spectroscopy 6.4.2 Fermi level, Mott-Schottky Analysis, and Band Diagram 6.4.3 DOOS Determination 6.4.4 Thermally Stimulated Currents 6.4.5 Solar Cell Characteristics 6.5 Discussion 6.6 Conclusions on the DOS of ZnPc:C60 (1:1) 7 Conclusion and Outlook Materials, Symbols, Abbreviations Bibliography

Organic photovoltaic (OPV) is a promising technology for renewable energy at low cost. Over the last five years, the power conversion efficiency (PCE) has doubled to 12%, which still is clearly lower than commercially available inorganic solar cells with a PCE around 20%. One approach to improve the PCE is to complement the conversion of light into electrical energy with the infrared (IR) part of the solar spectrum. However, the fundamental difference between organic and inorganic semiconductors is the exciton binding energy. Compared to inorganic semiconductors, in organic materials the exciton binding energy is more than 20 times higher, leading to strongly bound electron hole pairs, which are primarily generated upon photo-excitation. To dissociate these charges, in OPV, a donor-acceptor system is used. However, the energetics of this donor-acceptor system lowers the obtained open circuit voltage, representing one major loss of OPV as compared to inorganic solar cells. In the first part of this work, three benzannulated 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (aza-BODIPY) infrared absorbing donor dyes with methyl, methoxy, or without side-group attached are investigated. The solar cells with the highest PCE, i.e. the devices using the donor molecule without a side-group, exhibit a difference between the optical and the effective gap of 0.17 eV. It reflects the "driving force" for the electron to transfer from the donor to the acceptor, and is lowest for the best performing device, indicating that in the devices used efficient charge dissociation does not require large electron transfer energy loss. A - for OPV - relatively high open circuit voltage of 0.81 V is measured and when compared to the optical gap of the donor, a voltage loss of 0.74 V is obtained, reflecting the high voltage losses in OPV. In inorganic devices these voltage loss is around 0.4 V. One approach to lower this difference in the voltage loss is to lower the exciton binding energy of neat organic materials, leading to a larger exciton size. A saturation of the exciton size at about 1.2 nm is calculated by time dependent density functional method (TD-DFT) for one dimensional conjugated organic molecules with a size larger than 4nm. For the largest size of the exciton, provided by the poly(benzimidazobenzophenanthroline) (BBL), a Coulomb interaction of 0.4 eV between the electron and hole wave function is calculated, leading to an estimated exciton binding energy of about 0.2 eV, serving as a lower limit for the organic molecules investigated. The exciton binding energy can further be lowered by increasing the dielectric constant or by introducing a charge transfer (CT) state between two adjacent molecules. It is shown for the ladder polymer BBL that the dielectric function, including ionic and electronic contributions, can be calculated by a new method within the DFT and TD-DFT framework. In agreement with ellipsometry measurements, a highly anisotropic dielectric constant is obtained, which is 8.3 along the backbone of the polymer and around 3 perpendicular to the polymer. The high dielectric constant along the backbone originates from the strong delocalization of the electrons along the π-system. The ionic contribution increases the mean value of the dielectric constant from 3.6 to 4.2. In order to further increase the dielectric constant, different polar side-chains are attached to the ladder polymer BBL and their dielectric constant is calculated. A strong increase of the dielectric constant to about 17 is obtained by attaching a zwitterionic side-chain to the BBL monomer. In order to lower the exciton binding energy by a CT state, a charge transfer from a donor to an acceptor molecule must be introduced. The Coulomb binding energy of intermolecular CT states are calculated. It is shown that an intermolecular CT state of two π-stacked BBL oligomers does not exhibit a lower Coulomb binding energy as compared to the intramolecular binding energy. However, by a spatial separation of the donor and the acceptor molecule, in-line of the polymer backbone, the Coulomb binding energy is reduced from 0.40 eV to 0.24 eV. Combining such CT states with the high dielectric constant obtained by zwitterionic side-chains would lead to an exciton binding energy close to the thermal energy, resulting in spontaneous free carrier generation on neat materials. This could potentially reduce the voltage losses and increase the PCE in OPV devices significantly. / Die organische Photovoltaik stellt eine kostengünstige, erneuerbare und daher zukunftsgerichtete Energieversorgung dar. Die Umwandlungseffizienz organischer Solarzellen von Sonnenenergie in elektrische Energie konnte über die letzten fünf Jahre auf 12% verdoppelt werden. Kommerziell erhältliche anorganische Solarzellen weisen im Vergleich dazu eine Effizienz von ca. 20% auf. Eine Möglichkeit, die Effizienz organischer Solarzellen zu erhöhen, ist die Umwandlung von Licht in Elektrizität nicht nur im sichtbaren Bereich, sondern zusätzlich auch im infraroten Bereich des Sonnenspektrums. Der größte Unterschied zwischen den organischen und anorganischen Solarzellen liegt allerdings in der Exzitonbindungsenergie, welche in organischen Materialien ca. 20 Mal größer ist. Um das Exziton in freie Ladungsträger zu trennen, wird in organischen Solarzellen deshalb ein Donator-Akzeptor-Übergang benutzt, welcher unter anderem auch für den Spannungs- und damit für den Effizienzverlust von organischen Solarzellen verantwortlich ist. Im ersten Teil der Dissertation werden verschiedene funktionalisierte Donator-Moleküle, die infrarotes Licht absorbieren, untersucht. Die Donator-Moleküle ohne zusätzliche Funktionalisierungsgruppe weisen dabei die höchste Umwandlungseffizienz auf. In den besten Zellen kann ein Unterschied zwischen der optischen und effektiven \"Bandlücke\" von 0,17 eV gemessen werden. Dieser Unterschied stellt die treibende Kraft für den Übergang des Elektrons vom Donator zum Akzeptor dar. Da jedoch dieser Unterschied in der besten Solarzelle am geringsten ist, scheint die Dissoziation der Ladungsträger in den untersuchten Donator-Akzeptor-Systemem nicht vom ihm abzuhängen. Die gemessene relative hohe Leerlaufspannung von 0,81 V ist 0,74 V kleiner als die effektive Bandlücke und zeigt die hohen Spannungsverluste organischer Solarzellen. Die Spannungsverluste anorganischer Solarzellen liegen im Bereich von 0,4 V. Ein Ansatz, um die Spannungsverluste zu verkleinern, liegt in der Reduzierung der Exzitonbindungsenergie, woraus ein größeres Exziton erfolgt. Mit der zeitabhängigen Dichtefunktionaltheorie wird an einer Reihe organischer Moleküle gezeigt, dass die Exzitongröße bei einer Moleküllänge (oder Konjugationslänge) größer als 4nm bei 1,2nm sättigt. Für das größte Exziton, welches im Leiterpolymer Poly(benzimidazobenzophenanthroline) (BBL) vorhanden ist, wird eine Coulomb-Bindungsenergie von 0,4 eV berechnet und eine Exzitonbindungsenergie von 0,2 eV abgeschätzt. Die Exzitonbindungsenergie kann entweder durch Erhöhung der Dielektrizitätskonstante oder durch Erzeugung eines Ladungstransfer-Zustandes weiter verringert werden. Es wird gezeigt, dass mit einer neu entwickelten Methode auf Basis der Dichtefunktionaltheorie die ionischen und elektronischen Beiträge zur dielektrischen Funktion von BBL berechnet werden können. Die berechneten anisotropen Werte stimmen gut mit Werten aus Ellipsometriemessungen überein. Entlang der Polymerkette erhalten wir eine hohe Dielektrizitätskonstante von 8,3 und senkrecht dazu von ca. 3. Die hohe Dielektrizitätskonstante entlang der Polymerkette kann auf die starke Delokalisation der π-Elektronen zurückgeführt werden. Der Mittelwert der Dielektrizitätskonstante wird durch die ionischen Beiträge von 3,6 auf 4,2 erhöht. Um die Dielektrizitätskonstante weiter zu erhöhen, werden verschiedene polare Seitenketten am BBL-Polymer angebracht und die Dielektrizitätskonstante berechnet. Es wird gezeigt, dass die Anbringung einer zwitterionischen Seitenkette am BBL-Monomer die Dielektrizitätskonstante auf 17 erhöht. Um die Exzitonbindungsenergie durch einen Ladungstransfer-Zustand zu verringern, werden ein Donator- und ein Akzeptor-Molekül benötigt. Die Coulomb-Bindungsenergien der intermolekularen Ladungstransfer-Zustände werden berechnet. Es wird gezeigt, dass intermolekulare Ladungstransfer-Zustände zwischen zwei π-gestapelten BBL-Oligomeren keine Verringerung der Coulomb-Bindungsenergie bewirken. Bei einer räumlichen Trennung des Donator- und Akzeptor-Moleküls entlang der Polymerkette kann die Coulomb-Bindungsenergie von 0,40 eV auf 0,24 eV gesenkt werden. Eine Kombination aus diesem Ladungstransfer und der Erhöhung der Dielektirizitätskonstante durch zwitterionische Seitenketten kann zu einer niedrigen Exzitonbindungsenergie, nahe der thermischen Energie, und damit zu freien Ladungsträgern führen. Der damit verringerte Spannungsverlust kann die Umwandlungseffizienz organischer Solarzellen signifikant erhöhen.

The aim of this work was to investigate silver nanowire as well as carbon nanotube networks as transparent conducting electrodes for small molecule organic solar cells. In the framework of the nanowire investigations, a low-temperature method at less than 80 °C is developed to obtain highly conductive networks directly after the deposition and without post-processing. In detail, specific non-conductive organic materials act as a matrix where the nanowires are embedded in such that a mutual attraction based on capillary forces and hydrophobic interaction is created. This process is mediated by the ethanol contained in the nanowire dispersion and works only for sublayer materials which exhibit hydrophobic and hydrophilic groups at the same time. In contrast to high-temperature processed reference electrodes (210 °C for 90 min) without matrix, a slightly lower sheet resistance of 10.8 Ohm/sq at a transparency of 80.4 % (including substrate) is obtained by using polyvinylpyrrolidone as the sublayer material. In comparison to annealed silver nanowire networks, the novel approach yields a performance enhancement in corresponding organic solar cells which can compete with ITO-based devices. Furthermore, a novel approach for scalable, highly conductive, and transparent silver nanowire top-electrodes for organic optoelectronic devices is introduced. By utilizing a perfluorinated methacrylate as stabilizer, silver nanowires with high aspect ratio can be transferred into inert solvents which do not dissolve most organic compounds making this modified dispersion compatible with small molecule and polymer-based organic optoelectronic devices. The inert silver nanowire dispersion yields highly performing top-electrodes with a sheet resistance of 10.0 Ohm/sq at 80.0 % transparency (including substrate) directly after low-temperature deposition at 30 °C and without further post-processing. In comparison to similarly prepared reference devices comprising a thin-metal film as transparent top-electrode, reasonable power conversion efficiencies are demonstrated by spray-coating this dispersion directly on simple, air-exposed small molecule-based organic solar cells. Moreover, a deeper understanding of the percolation behavior of silver nanowire networks has been achieved. Herein, direct measurements of the basic network parameters, including the wire-to-wire junction resistance and the resistance of a single nanowire of pristine and annealed networks have been carried out for the first time. By putting the values into a simulation routine, a good accordance between measurement and simulation is achieved. Thus, an examination of the electrical limit of the nanowire system used in this work can be realized by extrapolating the junction resistance down to zero. The annealed silver nanowires are fairly close to the limit with a theoretical enhancement range of only 20 % (common absolute sheet resistance of approximately 10 Ohm/sq) such that a significant performance improvement is only expected by an enlargement of the nanowire length or by the implementation of new network geometries. In addition, carbon nanotube networks are investigated as alternative network-type, transparent bottom-electrode for organic small molecule solar cells. For that purpose, cleaning and structuring as well as planarization procedures are developed and optimized which maintain the optoelectronic performance of the carbon nanotube electrodes. Furthermore, a hybrid electrode consisting of silver nanowires covered with carbon nanotubes is fabricated yielding organic solar cells with only 0.47 % power conversion efficiency. In contrast, optimized electrodes comprising only carbon nanotubes show significantly higher efficiency. In comparison to identically prepared ITO devices, comparable or lower power conversion efficiencies of 3.96 % (in p-i-n stack), 4.83 % (in cascade cell) as well as 4.81 % (in p-n-i-p architecture) are demonstrated. For an inverted n-i-p stack design, the highest power conversion efficiency of 5.42 % is achieved.

Fe und Cu wurden als Schlüsselspezies für die Betrachtung von Oberflächenkontamination in der Prozessierung von c-Si-Solarzellen identifiziert. Studien mit gezielt aufgebrachten Metallkonzentrationen vor verschiedenen Passivierungs- und Diffusionsprozessschritten ergaben relativ hohe kritische Werte für Cu, außer bei thermischer Oxidation. Niedrige Werte wurden für beide Elemente vor Hochtemperaturschritten im n-Typ-Hocheffizienzprozess beobachtet, wobei sich die B-Diffusion als etwas weniger empfindlich darstellte. Temporäre Gettereffekte für Fe (in p-Typ-Si) und Cu (in n-Typ-Si) wurden beobachtet. Es zeigte sich, dass As-Cut-Wafer, unabhängig von der Sägeart (SiC-slurry oder Diamantdraht) sehr hohe Metallverunreinigungen (im Bereich 1*1011 – 5*1014 cm-2) in den Prozess einbringen. Das alkalische Ätzen verringert diese Menge kaum, was hohe Anforderungen an die anschließende Reinigung ergibt. Die Optimierung von HF/O3-Reinigungslösung für diese Flächen ergab beste Reinigungsergebnisse bei niedrigen HF-Konzentrationen, abhängig vom alkalischen Ätzschritt und anschließendem HCl/HF-Dip.

Solar cells convert sunlight into electrical energy using the photo effect. With a mar-ket share of 60%, multicrystalline silicon (mc-Si) is the most frequently used absorber material. Standard mc-Si ingots are directionally solidified in a fused silica (SiO2) crucible, which exhibits a silicon nitride (Si3N4) inner coating. After the entire raw material has been melted, the nucleation takes place on the Si3N4 inner coating at the bottom of the crucible. This results in an inhomogeneous initial grain structure and an increased fraction of dislocation clusters in the upper part of the ingot, which decrease the quality of standard mc-Si. Therefore, the global goal is the development of a cost-effective technology that reduces the formation of clusters and enhances the quality of mc-Si ingots. One way of achieving that goal is to produce the so-called \"high performance multi crystalline silicon\" (HPM-Si). During the directional solidification silicon raw material remains unmelted at the bottom of the SiO2 crucible, whereby crystallization does start on the silicon feedstock a few millimeters above the crucible bottom. Compared to standard mc-Si, a finer grained structure with many small grains is formed, which are separated by so-called random grain boundaries. Since the movement of dislocations across this grain boundary type has rarely been observed, the risk of formation of dislocation clusters, which have a negative impact on the efficiency of solar cells, is greatly reduced for HPM-Si. However, the disadvantage of the HPM-Si compared to the mc-Si is the yield loss resulting from the unmelted raw material at the crucible bottom. Hence, the aim of the present work is to produce mc-Si with a fine-grained structure in combination with a high fraction of random grain boundaries without the disad-vantage of yield loss. In order to investigate the grain structure in dependence of the nucleation conditions G1 ingots having a mass of 14.5 kg and dimensions of 220x220x130 mm³ were directional solidified in a furnace. The analysis of the grain structure with respect to the grain size, grain orientation and the random grain boundary length fraction and the comparison with the HPM-Si reference crystal took place on horizontal wafers with a thickness of 3mm. One possibility to influence the grain structure of mc-Si could be the variation of the cooling conditions before the start of crystallization at the crucible bottom. In a first series of experiments, a gas-flowed cooling plate, positioned below the crucible, was used. An increased gas flow increases the axial heat flow downwards and the cooling rate below the crucible bottom in the same direction. The detected cooling rate, measured by a thermocouple in the silicon melt 5 mm above the crucible bottom, varied in a range between 0.06-1.5 K/min. An increased cooling rate increases the supercooling, with a maximum of 2K. The analysis of the grain structure shows that a reduction in the cooling rate in combination with the lowest supercooling minimizes the average grain size and increases the fraction of random grain boundaries. However, an HPM-Si like grain structure (grain size and fraction of random grain boundaries comparable to HPM-Si) could not completely produced. Furthermore, due to the extended process time, the wafer yield is reduced, whereby the reduction of the cooling rate is not a preferable method for the industrial process. In a second experimental series, which took place under constant cooling rates, the influence of an additional nucleation layer on the initial grain structure was investigated. For this purpose, the additional nucleation layer was applied on the already existing Si3N4 inner coating on the crucible bottom. In order to adjust a HPM-Si like grain structure, the contact angle of the silicon melt on the additional nucleation layer should be lower than on the Si3N4 inner coating. The theoretical basis for this hypothesis is the relationship between the contact angle and the nucleation energy, which states that a reduced contact angle lowers the nucleation energy and can ultimately lead to more nuclei. Furthermore, in order to avoid melting, the additional nucleation layer must have a higher melting point than silicon. Suitable materials for the application as a foreign seed sample are SiC, SiO2 and Al2O3, which are used in the form of particles with different sizes. The production of the additional nucleation layer was carried out by a spraying as well as by an embedding procedure. These layers exhibit different thermal conductivity as well as surface roughness. Embedded nucleation layers generate higher roughness values than sprayed nucleation layers. The analysis of the grain structure identified the surface roughness as the main influencing factor on the initial grain size. While an increased surface roughness (Rq>100μm) results in a fine-grained structure (average grain size: <2mm²) comparable to HPM-Si, the average grain size increases (>2 mm²) with a reduced surface roughness (Rq<100μm). However, the analysis of the grain boundary relationship shows that the fraction of random grain boundaries does not correlate with the average grain size. Only a ma-terial dependency was detected. All SiO2 nucleation layers generate an increased fraction of random grain boundaries, comparable to the HPM-Si material. In contrast, the fraction of random grain boundaries was reduced for all SiC nucleation layers. This result is probably established with the different thermal conductivities of the used materials. The increased thermal conductivity of the sample with the SiC nucleation layers increases the cooling rate, promoting dendritic growth. In contrast the lower thermal conductivity of the SiO2 nucleation layers reduces the cooling rate and dendritic growth is suppressed. Since dendrites exhibit a Σ3 grain boundary relationship in the center, the fraction of this grain boundary type increases for SiC nucleation layers and the fraction of random grain boundaries decreases. In this thesis, various possibilities for influencing the grain structure have been pre-sented. A SiO2 nucleation layer with a roughness value Rq> 200μm represents an industrially relevant solution for the production of mc-Si with comparable properties to the HPM-Si without the disadvantages of yield loss. Hence, it was possible to in-crease the yield with comparable material quality, whereby the production costs could be reduced. Some first crucible manufacturers have already transferred the use of the SiO2 nucleation layers on top of the already existing Si3N4 inner coating at the crucible bottom to production.