Light-induced trap emptying revealed by intensity-dependent quantum efficiency of organic solar cells

Abstract

Revisiting the intensity-dependent quantum efficiency (IDQE) technique in the context of non-fullerene acceptors, we find that at forward-bias conditions, the response exhibits what seems to be anomalous behavior that is not consistent with light excitation induced trap filling. Analysis based on the Shockley-Read-Hall model leads to the conclusion that the contacts cause the traps to be completely full in the dark. The role of the light excitation is to half-empty the traps, and thus, the "anomalous"behavior is created. By fitting the IDQE at several bias levels, we find that the trapping is consistent with multiphonon capture by a state close to the middle of the gap. As trap-assisted recombination is a significant loss mechanism, it is essential to fully monitor it for indoor applications as well as to cross the single junction 20% power conversion efficiency limit.