Improve the crystallinity and morphology of perovskite films by suppressing the formation of intermediate phase of CH3NH3PbCl3

Crystallinity and morphology of perovskite films can greatly influence the performance of perovskite solar cells (Pero-SCs) in terms of charge carrier generation and transportation. Incorporation of Cl (from either PbCl2 or CH3NH3Cl) in crystal growth of CH3NH3PbI3 is known as an effective method to acquire larger crystal grains and longer carrier diffusion length. It has been reported that an intermediate phase of CH3NH3PbCl3 was usually observed during crystallization, which should be critical to the quality of the resulting perovskite, however, how the intermediate phase influence the crystal growth was lack of comprehensive understanding. In this study, through varying the mixing ratio of the precursors PbI2:2CH3NH3I and PbCl2:3CH3NH3I, we were able to systematically tune the content and the converting time of CH3NH3PbCl3. We found that suppressing the formation of CH3NH3PbCl3 within the as-cast films can effectively improve the quality (crystallinity and morphology) of the resulting perovskite films. The improvement led to significant enhancement of the performance of the corresponding p-i-n planar Pero-SCs. Under optimized conditions, the best PCE was increased from 12.8% to 16.2%, which should be attributed to the alleviated charge recombination due to the improved quality of perovskite films by suppressing the formation of CH3NH3PbCl3.