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Abstract

Despite recent advances leading to unprecedented performance in organic photovoltaic devices, the underlying processes of charge generation in these semiconductors are still unclear. Furthermore, the operational stability of organic solar cells - a key requirement for successful application outside the laboratory - is often neglected. This thesis addresses these urgent and complex challenges by investigating the photophysics and degradation mechanisms of two high-efficiency material systems by employing ultrafast transient spectroscopy. The first part is devoted to the understanding of charge generation in PffBT4T-2OD:PC70BM which acts as a model system for a new class of organic photovoltaic materials. It is unambiguously shown that the separation of electron-hole pairs is field-dependent, with significant implications for the research of novel combinations of materials with low energy offsets. Based on these results, the second part of this thesis focuses on the environmental stability of the aforementioned system which is shown to be exceptionally sensitive to the influence of oxygen. The observed results can be comprehended by oxygen-induced p-doping of the active layer, resulting in rapid deterioration of the device properties. Finally, the photophysics and degradation of solar cells based on the small molecule donor DRCN5T, representative of a new trend in solar cell design, are addressed. These devices display remarkable stability which is accredited to an ultrafast energy transfer from the unstable to the stable components. This insight can potentially influence design rules for future research on organic solar cells. Therefore, this work contributes substantially to the understanding of the photophysics at short timescales and the stability of organic solar cells with high relevance for the field.