Thermal annealing effect on internal electrical polarization in organic solar cells

Abstract

This article reports experimental studies on internal electrical polarization effects by using optical absorption and photoexcitation assisted capacitance–voltage (C–V) measurements based on morphological development in the standard P3HT:PCBM solar cell. We observe that morphological development can increase absorption intensity upon thermal annealing. The increase of absorption intensity essentially reflects an enhancement of absorption coefficient of local donor and acceptor structures. We attribute the enhancement of absorption coefficient to the well-known morphological developments: increased polymer crystallinity and molecular aggregations caused by thermal annealing. Furthermore, the enhancement in absorption coefficient indicates stronger electrical polarizations in the P3HT:PCBM film. The C–V studies find that increasing local electrical polarizations can lead to an enhancement on the generation of charge carriers at donor:acceptor interfaces and the transport of generated charge carriers to respective electrode interfaces in the P3HT:PCBM device. Our experimental findings suggest that local electrical polarizations play an important role in the development of photovoltaic processes in organic solar cells.