Passivating Detrimental DX Centers in CH₃ NH₃ PbI₃ for Reducing Nonradiative Recombination and Elongating Carrier Lifetime.

Abstract

After a period of rapid, unprecedented development, the growth in the efficiency of perovskite solar cells has recently slowed. Further improvement of cell efficiency will rely on the in-depth understanding and delicate control of defect passivation. Here, the formation mechanism of iodine vacancies (V_I ), a typical deep defect in CH₃ NH₃ PbI₃ (MAPbI₃ ), is elucidated. The structural and electronic behaviors of V_I are like those of a DX center, a kind of detrimental defect formed by large atomic displacement. Aided by the passivation mechanism of DX centers in tetrahedral semiconductors, it is found that the introduction of Br strengthens chemical bonds and prevents large atomic displacements during defect charging. It therefore reduces the defect states and diminishes electron-phonon coupling. Using time-domain density functional theory (DFT) combined with nonadiabatic molecular dynamics, it is found that the carrier lifetime can be enhanced from 3.2 ns in defective MAPbI₃ to 19 ns in CH₃ NH₃ Pb(I_0.96 Br_0.04 )₃ . This work advances our understanding of how a small amount of Br doping improves the carrier dynamics and cell performance of MAPbI₃ . It may also provide a route to enhance the carrier lifetimes and efficiencies of perovskite solar cells by defect passivation.