Understanding Interfacial Recombination Processes in Narrow-Band-Gap Organic Solar Cells

Recombination losses in organic photovoltaics (OPVs) remain a performance-limiting factor, including bulk trap-assisted recombination and interfacial recombination at the electrode:active layer interface. In this work, we demonstrate the role of the front electrode:active layer interface in a narrow-band-gap system, PCE10:COTIC-4F, a promising candidate for semitransparent organic photovoltaics. We systematically address charge generation, recombination, and extraction, with a focus on interfacial recombination via surface traps by a comparison of four device structures with electrodes based on ZnO, ZnO/PFN-Br, PEDOT:PSS, and a self-doped conjugated polyelectrolyte (CPE-K). The amount of interfacial recombination is affected significantly by the electrode choice, while similar levels of bulk recombination are maintained. For the studied blend, we identify ZnO as a suitable choice, pairing low surface recombination rates with beneficial charge carrier generation, favorable energy level alignment, and efficient extraction. In contrast, PEDOT:PSS-based devices suffer from interfacial recombination, which can be suppressed when CPE-K is used instead.