High-Efficiency Quasi-2D Perovskite Solar Cells Incorporating 2,2′-Biimidazolium Cation

Quasi-2D perovskites are attractive because of their improved stability compared with 3D perovskites counterparts; however, they suffer from poor performance due to the insulating organic cation spacers. To resolve this issue, a strategy of replacing the insulating spacer with conducting spacer is proposed which successfully converts the spacer from a charge-transporting “barrier” to charge-transporting “bridge.” Specifically, an alkyl linker-free, fully conjugated aromatic 2,2′-biimidazolium (BIDZ) cation is introduced as a spacer to compose quasi-2D perovskites. Density functional theory (DFT) simulation results show that the lowest unoccupied molecular orbital (LUMO) level localizes on BIDZ and the highest occupied molecular orbital (HOMO) level is on the perovskite. However, both HOMO and LUMO levels localize on perovskite slabs for the well-known phenethylammonium (PEA)-based 2D perovskites. The strong electronic coupling between BIDZ and 3D perovskite slabs improves carrier mobilities even for a low-weak-crystallinity and random-orientated quasi-2D perovskite film. As a result, a remarkable power conversion efficiency up to 11.4% (n = 5) is achieved, which is much higher than that of PEA-based random-orientated quasi-2D perovskites with the same processing condition (6.5%). The strategy paves the way to highly efficient and stable quasi-2D perovskites solar cells through designing new organic spacer cations.