Origin of electric field-dependent charge generation in organic photovoltaics with planar and bulk heterojunctions

Abstract

In organic photovoltaics (OPVs), electric field-dependent charge generation often degrades the fill factor and power conversion efficiency, but its origin remains unclear. This work evaluates the electric field dependence for various non-fullerenes, fullerenes, and electron-donating polymers in OPVs with bulk heterojunction (BHJ) and planar heterojunction (PHJ) structures. We found that the electric field dependence in the charge generation process is more obvious in PHJ than in BHJ structures, even when the same electron donor and acceptor material are used. The insufficient energy difference between singlet excited and charge transfer states primarily causes the electric field-dependent charge generation. In addition, the relative molecular orientation between the donor and acceptor molecules at the interfaces could also be an essential factor determining the charge generation. The results suggest that the end-on orientation of the acceptor molecules to the donor domain is advantageous for efficient field-independent charge generation.