Role of charged defects on organic solar cell performance: Prospect of heterojunction-free device design

Heterojunction-free organic photovoltaic (HJ-free OPV) devices promise to double the efficiency of bulk heterojunction cells, mainly by improving the open circuit voltage. Unfortunately, photocurrent of HJ-free OPV cell is poor, which is generally attributed to incomplete exciton dissociation. In this paper, we show that it is the defect induced electric field screening, not exciton dissociation, which lowers charge collection and degrades the photocurrent. The hypothesis is confirmed by careful device simulation and impedance spectroscopy. The defect-limited theory of photocurrent consistently interprets the voltage dependent quantum efficiency and intensity dependent power conversion efficiency widely observed for HJ-free OPVs. This work suggests bright prospect of HJ-free OPV design, provided the material defects are minimized.