Acceptor aggregation induced hole mobility degradation in polymer solar cells

Abstract

Charge carrier transport plays a critical role in determining the stability of organic solar cells (OSCs). It is widely acknowledged that changes in the morphology of donor and acceptor structural domains influence the degradation of hole and electron mobility. However, there has been limited research conducted on the temporal evolution and correlation between electron and hole mobility in highly mixed bulk heterojunction (BHJ) films. This research gap hampers our understanding of the mechanism behind charge carrier decay and impedes further improvements in device stability for OSCs. Here, we observed that the molecular conformation of electron acceptors significantly influences the evolution of hole mobility under different conditions, even when the same donor material and weight fraction are used. Notably, we find that the decay trends of electron mobility exhibit a cooperative behavior with those of hole mobility in both systems. Additionally, we observed that the variation in hole transport properties becomes less sensitive in donor-rich cases (with a weight ratio of 99 : 1), further confirming that the distinct decay trends of hole mobilities in PM6-based films are caused by the presence of acceptor molecules. Through morphological characterization, we have concluded that changes in acceptor aggregation during the aging process impact the distribution of polymer donors, thereby influencing the degradation of hole carriers. This research not only provides direct evidence of an acceptor-induced mechanism for hole carrier decay but also offers perspectives on improving the device stability of OSCs.