Julia Kraus
Physik molekularer Donator-Akzeptor Solarzellen
Betreuer: Prof. Dr. Wolfgang Brütting [Experimentalphysik IV]
Datum der mündlichen Prüfung: 09.07.2013
318 Seiten, deutsch
Organic photovoltaic cells (OPVCs) use organic semiconductors, i.e. conjugated polymers or molecular materials, as active layer for the direct conversion of sunlight into electricity. In their simplest form, OPVCs comprise two electrodes, enclosing a light-harvesting active layer with a total thickness of only a few hundred nanometers. Moderate material costs, fast and cheap production at low temperature as well as the possibility to produce large area flexible modules make organic photovoltaics a promising technology for thin-film solar cells including the compatibility with role-to-role processing for high volume output. Thanks to intensive research, power conversion efficiencies exceeding ten percent can already be achieved with laboratory scale cells. However, the commercial breakthrough of organic photovoltaics as cost-efficient energy source still depends on further progress concerning challenges - amongst others, improving efficiencies in modules, enhancing long-term stability, increasing device area and solving encapsulation issues. While the progress during the first years of research on OPVCs was largely achieved by trial and error experiments and material screening, further efficiency improvements require a detailed understanding of the fundamental processes occurring in organic solar cells. This work contributes to this approach by establishing a correlation between structural and electronic properties, microscopic transport phenomena and macroscopic parameters determining solar cell performance for different prototypes of organic molecular materials. In this context, organic/organic and electrode/organic interfaces play a major role - as they crucially influence exciton dissociation, charge carrier recombination and charge extraction. Within this work, relations between growth parameters and the resulting structures and morphologies between molecular materials with different shapes are identified - either deposited subsequently forming a planar interface or simultaneously in molecular blends. Besides morphological aspects, the electronic structure of these interfaces plays an equally important role in OPVCs. While the electrode/organic contacts are responsible for charge carrier extraction, efficient exciton dissociation and charge carrier generation is decisively influenced by the donor/acceptor interface. Thus, all processes and device parameters are strongly dependent on adjusted energy level alignment and optimized offsets at these interfaces. A variety of studies on polymeric solar cells identified the energetics at the donor/acceptor interface as a limiting factor for the efficiency of polymer devices. This work provides an experimentally validated understanding for these limitations in OPVCs based on molecular materials which allows quantifying recombination losses and working out the peculiarities of crystalline organic thin-film solar cells as compared to their polymeric counterparts. Altogether, the present thesis provides a comprehensive picture of true electronic structures at the relevant interfaces, the dependence of film growth conditions on structure and morphology, and finally identifies correlations to device function. Concerning the material choice, pioneering work on the new donor material diindenoperylene (DIP) in combination with the fullerene C60 is carried out - finally leading to its successful application in organic solar cells.