In situ self-elimination of defects via controlled perovskite crystallization dynamics for high-performance solar cells.

Understanding and controlling the crystallization process is critical to achieving high-quality perovskite films and efficient solar cells. Herein, we identified that one key issue in the formation of formamidinium lead iodide (FAPbI 3 ) defects is the participation of the intermediates in the crystallization process. A comprehensive picture of structural and carrier evolution during crystallization was demonstrated using a combination of in situ grazing-incidence wide-angle X-ray scattering, UV-Vis, and photoluminescence spectroscopies. Three crystallization stages were registered for the commonly observed conversion pathway of precursors - δ-FAPbI 3 intermediate - α-FAPbI 3 , during which the spontaneous generation of defects was found. We found that a hydrogen sulfate-based ionic liquid additive enabled the phase conversion pathway of precursors - solvated intermediates - α-FAPbI 3 , during which the spontaneous generation of δ-FAPbI 3 can be effectively circumvented. Both growth kinetics at the initial crystallization stage and the following recrystallization of α-FAPbI 3 were significantly prolonged because of the additional exchange process between solvents and FA + , which resulted in the self-elimination of defects during crystallization. Therefore, the improved crystallization dynamics led to larger grain sizes and fewer defects within thin films. The improved perovskite crystallization dynamics finally transformed into high-performance solar cells under one-sun illumination (AM 1.5 G) with efficiency as high as 25.14% at 300 K. These cells delivered a PCE of 26.12% at 240 K, which might open up a new era of application of this emerging photovoltaic technology to low-temperature environments such as near-space and polar regions. This article is protected by copyright. All rights reserved.