Alkali Cation Doping for Improving the Structural Stability of 2D Perovskite in 3D/2D PSCs.

3D/2D hybrid perovskite systems have been intensively investigated to improve the stability of perovskite solar cells (PSCs). Whereas, undesired crystallization of 2D perovskite during the film formation process could undermine the structural stability of 2D perovskite materials, which cause serious hysteresis of PSCs after aging. This issue is however rarely studied. Stability study for 3D/2D hybrid systems to date are all under one-direction scan, and the lack of detailed information on the hysteresis after aging compromises the credibility of the stability results. In this work, by correlating the hysteresis of the hybrid PSCs with the 2D crystal structure, we find that the prompt 2D perovskite formation process easily induces numerous crystal imperfections and structural defects. These defects are susceptible to humidity attack and decompose the 2D perovskite to insulating long chain cations and 3D perovskite, which hinder charge transfer or generate charge accumulation. Therefore, large hysteresis is exhibited after aging the 3D/2D hybrid PSCs in ambient, even though reverse-scan power conversion efficiency (PCE) is found to be well-preserved. To address this issue, alkali cations K+ and Rb+ are introduced into the 2D perovskite to exquisitely modulate the crystal formation, which gives rise to higher crystallinity of 2D perovskite and better film morphology with less defects. We achieved PCE beyond 21% due to the preferable charge transfer process and reduced non-radiative recombination losses. The structural features also bring about impressive moisture stability, which results in the corresponding PSCs retaining 93% of its initial PCE and negligible hysteresis after aging in ambient atmosphere for 1200 hours.