Comparative study of charge characteristics in PCPDTBT:fullerenes solar cells

Highlights: • Spectral signal of PCPDTBT⁺ species was obtained by spectroelectrochemistry. • Fullerene downwards shifts the HOMO level of PCPDTBT⁺ near the D-A interface. • Fullerene promotes hole transport from near the D-A interface to polymer bulk phase. • Charge charateristics in PCPDTBT:fullerene solar cells were studied. PCPDTBT is a representative donor material for polymer solar cells, and the performance of PCPDTBT-based devices depends on the type of acceptors. To study the effect of fullerene structure on the performance of PCPDTBT-based device, the correlations among morphology, optical and electrical properties of photovoltaic devices were analyzed. The results showed that PCPDTBT:PC₇₁BM had pronounced crystal order and phase separation comparing to that of PCPDTBT:PC₆₁BM, which could be helpful for charge transport. Furthermore, we find the HOMO energy level of PCPDTBT⁺ near the donor–acceptor interface was down-shifted due to interaction between donor and acceptor. The bended energy level at the interface could facilitate the hole transport from near the donor–acceptor interface to the bulk-phase of polymer, and thus reducing the charge recombination at the interface. Impedance test demonstrated that PCPDTBT:PC₇₁BM device had smaller leakage current and better carrier transport, so that it showed higher power conversion efficiency than PCPDTBT:PC₆₁BM device.