Symmetrical Conjugated Molecular Additive for Defect Passivation and Charge Transfer Bridge in Perovskite Solar Cells

Defect passivation of functional additives is an effective approach to suppress the nonradiative recombination centers in perovskite solar cells. Here, a symmetrical conjugated molecular additive (4,4′-bipyridyl, BPY[4,4]) with bilateral nitrogen atoms is employed for effective defect passivation by bonding with uncoordinated Pb2+ cations within perovskite. Meanwhile, π-conjugated BPY[4,4] affords large bridging interaction for better charge carrier transfer pathway between perovskite grains than control, as witnessed by increased electron and hole mobility. Consequently, the device with optimal BPY[4,4] device displays a power conversion efficiency of 20.82% higher than control device of 18.38% and exhibits better long-term thermal stability. It is worth noting that BPY[4,4] has symmetrical and conjugated molecular structures, which benefit from π–π stacking and face-on orientation structure, so it has more advantages than asymmetric and nonconjugated molecular additives, such as charge transfer, defect passivation, and device performance. Our work demonstrates that symmetrical conjugated molecule additives can construct bridged charge transfer channels between perovskite grains as well as effective defect passivation.