The impact of tail states on the performance of organic bulk heterojunction solar cells

Abstract

This thesis is concerned with quantification of the effects of tail states on charge transport and non-geminate recombination loss in organic bulk heterojunction solar cells. After the description of the context of this work, the theoretical background and methodology employed are introduced. In this thesis, a variety of donor/acceptor blend systems are investigated. The first results chapter reports a study of the changes in charge carrier densities and charge carrier lifetimes following PCBM oxidation and burn-in degradation systems compared to fresh devices using charge extraction and transient photovoltage analysis. The resulting changes in tail state density and recombination dynamics of the photoactive layer are quantified, and shown to explain changes in device performance, specifically reductions Voc and FF. The second results chapter analyses the changes in device performance of different donor/acceptor blend systems under high and low light operation. The associated light intensity dependent changes in optoelectronic properties are investigated, in particular, the origins of the reduced Voc and increased FF observed at low light levels. Differences in performance are related to the extent to which tail states can particularly affect device performance under low light level operation. In the third results chapter, the impact of active layer thickness on the efficiency of organic bulk heterojunction systems is explored by combining experiment and 1D drift-diffusion modelling. In particular, the factors limiting the photocurrent generation efficiency of thick devices is discussed through the analysis of carrier collection and recombination dynamics using a combination of charge extraction (CE) and transient photovoltage (TPV) on a series of thickness dependent devices. For thick devices, the depletion width is determined to be less than the device thickness, resulting in losses at short circuit and lower on photocurrent collection. Different situations that can cause small depletion layer widths are explored and discussed. This study suggests that for the donor/acceptor blends studied are not, the space charge layer width is not limited by unintentional doped, rather the origin of this small space charge layer is because of the accumulation, under irradiation, of charge carriers in sub-bandgap tail states. The final results chapter presents an analysis of the effects of carrier collection and recombination dynamics on FF of different systems. The charge carrier density across the J-V iv curve and its recombination loss are quantified. Qualitative correlations between the values of FF and mobility-life product, Langevin and non-Langevin behaviour are explored.