Employing Pentacene To Balance the Charge Transport in Inverted Organic Solar Cells

The charge-transport balance is regarded as a prerequisite for high-performance organic photovoltaics (OPVs) because it is able to suppress the space-charge accumulation and reduce the probability of charge-carrier recombination before they are extracted. However, electrons usually transport faster than holes in most fullerene-based bulk heterojunction active layers, which leads to the unbalanced charge transport and thereby degrades the device performance. Here, we report an easy-processed approach to overcome this unbalance. Upon doping high-hole-mobility pentance (Pc) into the active layer, we found that the hole transport improves significantly, and the original nanomorphology of the active layer undergoes subtle changes. Time-resolved photoluminescence spectrum reveals that the lifetime of exciton within the blend films decreases in the presence of Pc, indicating that the Pc dopant may participate in the charge-transfer process and facilitate the exciton dissociation. As a result, a light-doping level of 0.2% significantly improves the power conversion efficiency by 38%, from 3.46 to 4.79% in P3HT/PC71BM-based solar cells, which demonstrates a feasible approach to increase the efficiency of OPVs.