Improved Defect Tolerance and Charge Carrier Lifetime in Tin-Lead Mixed Perovskites: Ab Initio Quantum Dynamics

Simulations by nonadiabatic (NA) molecular dynamics demonstrate that mixing tin with lead in CH3NH3PbI3 can passivate the midgap state created by an interstitial iodine (Ii) via the imposed compressive strain and upshifted valence band maximum, reduce NA coupling by decreasing electron-hole wave functions overlap, and shortens puredephasing time by introducing high-frequency phonon modes. Thus, the charge carrier lifetime extends to 3.6 ns due to the significantly reduced nonradiative electron-hole recombination, which is an order of magnitude longer than the Ii-containing CH3NH3PbI3, over 2.5 times longer than the pristine CH3NH3PbI3 (1.4 ns), and even 1.7 times longer than the tin-lead mixed perovskite without the I-i defects (2.1 ns). Tin-lead alloying simultaneously increases the I-i defect formation energy to 0.402 eV from 0.179 eV in CH3NH3PbI3, which effectively enhances defect tolerance by reducing the defect concentration. The study reveals the factors controlling the enhanced performance of tin- lead mixed perovskite solar cells.