Is a heterojunction essential for high-efficiency organic solar cells?

Innovation in high-performance semiconducting polymers have led to a 10-fold increase in the efficiency of polymer based solar cells. However, the record efficiency reported is often less than one-third of the thermodynamic limit anticipated for these newly developed polymers. Remarkably, almost half of this efficiency gap could be attributed to the voltage loss at the donor-acceptor heterojunction (HJ). In this work, it is demonstrated that poor charge collection caused by defect induced electric field screening is a significant bottleneck for the high efficiency HJ-free device design. Our results imply that low J_SC in HJ-free devices has been misattributed to exciton bottleneck, rather than defect screening. The results suggest synthesis of new polymers with high purity with the reassessment of the prospects of HJ-free polymer-only device design.