Effect of Bromine Substitution on the Ion Migration and Optical Absorption in MAPbI3 Perovskite Solar Cells: The First-Principles Study

In the past few years, the remarkable energy conversion efficiency of lead-halide-based perovskite solar cells (PSCs) has drawn extraordinary attention. However, some exposed problems in PSCs such as the low chemical stability and so forth are tough to eliminate. A fundamental understanding of ionic transport at the nanoscale is essential for developing high-performance PSCs based on the anomalous hysteresis current-voltage (I-V) curves and the poor stability. Our work is to understand the ionic transport mechanism by introducing suitable halogen substitution with insignificant impact on light absorption to hinder ion diffusion and thereby to seek a method to improve the stability. Herein, we used first-principles density functional theory (DFT) to calculate the band gaps and the optical absorption coefficients, and the interstitial and the vacancy defect diffusion barriers of halide in the orthogonal phase MAPbX(3) (MA = CH3NH3, X = I, Br, I0.5Br0.5) perovskite, respectively. The research results show that a half bromine substitution not only prevents ion migration in perovskite, but also maintains a favorable light absorption capacity. It may be helpful to maintain the PSC's property of light absorption with a similar atomic substitution. Furthermore, smaller atomic substitution for the halogen atoms may be essential for increasing the diffusion barrier.