Mixed perovskites have achieved substantial successes in boosting solar cell efficiency, but the complicated perovskite crystal formation pathway remains mysterious. Here, we report for the first time the detailed crystallization process of mixed perovskites (FA0.83MA0.17Pb(I0.83Br0.17)3) during spin-coating by in situ GIWAXS measurements, and three phase formation stages have been identified: I. precursor solution; II. hexagonal δ-phase (2H); III. complex phases including hexagonal polytypes (4H, 6H), MAI-PbI2-DMSO intermediate phases and perovskite α-phase. The correlated device performance and ex situ characterizations suggest the existence of “annealing window” covering the duration of stage II. The spin-coated film should be annealed within the annealing window to avoid the formation of hexagonal polytypes (Stage III) during the perovskite crystallization process, thus achieving a good device performance. Remarkably, the crystallization pathway can be manipulated by incorporating Cs+ ions in mixed perovskites. Combined with density functional theory (DFT) calculations, the perovskite system with sufficient Cs+ will bypass the formation of secondary phases in stage III by promoting the formation of α-phase both kinetically and thermodynamically, thereby significantly extending the annealing window. This study provides underlying reasons of the time sensitivity of fabricating mixed-perovskite devices and insightful guidelines for manipulating the perovskite crystallization pathways towards higher performance.
Manuscript by Qin et al. was published at Advanced Materials, 2019, 1901284, https://doi.org/10.1002/adma.201901284.