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

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 (VI ), a typical deep defect in CH3 NH3 PbI3 (MAPbI3 ), is elucidated. The structural and electronic behaviors of VI 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(I0.96 Br0.04 )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.