The world consumes several tens of terawatts (TW) of electricity. If solar energy should have a notable share in the energy generation of the future, the fabrication of solar modules has to be changed from nowadays batch-to-batch processes that operate in the gigawatt regime to a reliable production that allows TW`s. Large area roll-to-roll (R2R) printing enables solar cell manufacturing to proceed to TW production.
Organic photovoltaics (OPV) are one of the very promising technologies for printing production. The 10 % hurdle has been overcome after huge progress in increasing the efficiency of OPV over the past years. Now, mainly large scale production and stability issues are in the research focus. Developing the interface layers, situated between active layer and electrodes, of organic solar cells (OSCs) is one of the central parts to solve these challenges. The interface layers are used to maximize
efficiency, dene the polarity and increase the lifetime of the devices. Apart from their functionality, the interface layers should fulfill the requirements for a large scale printing production. Metal oxides are a very promising option to provide functionality at the required processing conditions. On the n-type side, this thesis concentrates on zinc oxide (ZnO) and doped zinc oxide as electron transport layer (ETL). The low conductivity (typically 10E-6 S/cm) of intrinsic ZnO restricts the layer thickness of such an ETL to a few tens
of nanometers. A conductivity in the order of 10E-3 S/cm was derived from electrical simulations as sufficient to increase the interface layer thickness to over 1 µm which would provide all the desired freedom for this parameter. This conductivity can be achieved via doping with Aluminum (Al). In this thesis, Al-doped ZnO (AZO) is introduced as interface layer from precursor solution with comparable performance to ZnO in thin films. AZO maintains the performance also in thick films, while ZnO devices suffer from electrical losses.
Further work was performed to improve the deposition parameters, especially the annealing temperature. The annealing temperature could be improved to a technologically relevant regime of below 150 °C via the engineering of the AZO precursor solution. For the precursor approach, fully functional interface layers with up to 680 nm are experimentally realized to verify the relevance of the simulated results.
Thick interface layers may improve the lifetime of the devices and this ETL fulfills the requirements to start lifetime tests.
The surface of the ETL plays a deciding role in the contact formation to the active layer. The surface of an AZO ETL was systematically manipulated using self-assembled monolayers (SAMs) to study its impact on device performance.
The performance of the resulting devices varied between 13 % and 115 % of the unmodied reference device depending on the used SAM. This demonstrates the crucial role of the surface of metal oxide interlayers.
Furthermore, the developed ETL`s were tested with a solution processed electrode. Nowadays preferred transparent electrode indium-tin-oxide (ITO) is expensive and brittle. Both make it unattractive for future R2R production on flexible substrates. Solution processed silver nanowires are a candidate to replace ITO. Silver nanowires are solution processable at low temperatures and flexible.
The AgNW`s form a network with a quite rough morphology that needs to be filled and smoothed by an interface layer. The developed ETL`s (ZnO and AZO) are found to be fitting solutions and fully functional devices with 2.7 % efficiency and over 60 % fill factor (FF) are demonstrated. Especially the high FF expresses the excellent electrical functionality of the low temperature, solution processed transparent electrode. This work was an essential step towards fully solution processed and semitransparent devices that have been realized afterwards.
Normal architecture solar cells usually employ rather unstable, low work function metals as cathode that limits device lifetime. One approach to make more stable devices is to use metal oxides with high workfunction metals as cathode instead.
The previously investigated precursors showed no functionality when applied on top of active layers in the normal architecture. Therefore an AZO nanoparticle (NP) dispersion is employed that shows comparable electrical parameters to the previously used precursors at only 80 °C annealing temperature. The nanoparticle dispersion enables the use of an AZO ETL even on top of previously deposited organic active layers without affecting the morphology or harming the sensitive materials. AZO NP are a replacement with comparable performance to unstable Ca as cathode and can be employed together with stable Ag for normal architecture solar cells.
On the hole transport side, the commonly employed organic poly(3,4-ethylendioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was found to be a major source of degradation through its hygroscopic and acidic nature. Transition metal oxides are investigated in this thesis as promising candidates with better intrinsic stability. At first, MoOX deposited from a nanoparticle suspension is introduced as HTL in organic solar cells. The performance is found to match PEDOT:
PSS with improved parallel resistance, but an oxygen plasma treatment is
necessary for functional films.
The experience gained with MoOX was applied to develop an improved approachwith WOX. WOX was chosen because of the possibility to get very small and mono disperse particles that can be stabilized without organic ligands in an alcoholic solvent system. This enables the deposition smooth films that only need a very low temperature (80 °C) treatment without oxygen plasma for functional films and devices. This WOX dispersion can even be used in the inverted architecture where the layer is deposited on top of the organic active layer.
Hereby "metal oxide interface only" inverted solar cell with a commercially available active material with high efficiency of around 6 % are made possible. This layer stack contains only electrode materials that are considered as intrinsically stable and that fulfill the previously discussed large area production requirements to a large extent. Promising lifetime studies with devices containing metal oxides
that were developed in this thesis, can be started now.Weltweit werden mehrere Dutzend Terawatt Elektrizität konsumiert. Wenn Solarenergie einen bedeutenden Teil zur Energieerzeugung der Zukunft beitragen soll, muss die Fabrikation von Solarmodulen von der heutigen Einzelfertigung
im Gigawatt-Bereich auf Prozesse umgestellt werden, die Terawatt erlauben.
Großflächiger, Rolle-zu-Rolle (R2R) Druck ermöglicht Solarzellenfertigung im Terawatt-Bereich.
Organische Photovoltaik ist eine der sehr vielversprechenden Technologien für die Produktion in Druckverfahren. Die 10 % Hürde wurde nach großen Fortschritten in der Effizienzsteigerung in den letzen Jahren genommen. Jetzt stehen
hauptsächlich die Lösung von großtechnischen Produktions- und Haltbarkeitsproblemen im Fokus der Wissenschaft. Die Entwicklung der Zwischenschichten organischer Solarzellen, die sich zwischen aktiver Schicht und Elektrode befinden, ist von zentraler Bedeutung um diese Herausforderungen zu lösen. Die Zwischenschichten werden dazu genutzt die Effizienz zu maximieren, die Polarität der Bauteile zu definieren und um die Lebensdauer zu erhöhen. Abgesehen von ihrer Funktionalität, sollen die Zwischenschichten auch die Anforderung für die
großflächige Produktion im Druckverfahren erfüllen. Metalloxide sind sehr vielversprechende Kandidaten um die Funktionalitäten bei den geforderten Prozessierungsbedingungen zu gewährleisten
