Nonadiabatic molecular dynamics study on effect of Ge/Sn alloy on hot carrier relaxation of CsPbBr<sub>3</sub> perovskite

Abstract

Perovskite solar cells have been a prominent focus in the field of photovoltaics in recent decades, owing to their exceptional performance: easy synthesis, and cost-effectiveness. The all-inorganic cesium-based perovskite CsPbBr<sub>3</sub>, known for its remarkable thermal stability, has become a star material in the field of optoelectronics due to its outstanding luminescent properties. Despite the high efficiency of lead-based perovskite solar cells, the toxicity associated with lead and the poor long-term stability of these devices remain significant barriers to their large-scale commercialization. As is well known, non-radiative electron-hole recombination significantly shortens the carrier lifetime, acting as a primary pathway for excited state charge to loss energy. This phenomenon directly affects the photovoltaic conversion efficiency and charge transfer performance of perovskite materials. Therefore, maximizing the reduction of non-radiative recombination energy loss in perovskite solar cells has become a crucial research focus. In this study, a systematic exploration is conducted by using a non-adiabatic molecular dynamics approach combined with time-dependent density functional theory to investigate the excited-state carrier dynamics of CsPbBr<sub>3</sub> and its alloyed structures, CsPb<sub>0.75</sub>Ge<sub>0.25</sub>Br<sub>3</sub> and CsPb<sub>0.5</sub>Ge<sub>0.25</sub>Sn<sub>0.25</sub>Br<sub>3</sub>. The study comprehensively analyzes the non-radiative electron-hole recombination scenarios and the mechanisms for reducing charge energy loss based on crystal structure, electronic properties, and excited-state properties. The research findings reveal that alloying with Sn/Ge can reduce the bandgap, increase non-adiabatic coupling, and shorten the decoherence time. The interplay of reduced quantum decoherence, smaller bandgap, and larger non-adiabatic coupling effectively decelerates the electron-hole recombination process. Consequently, the carrier lifetime of the CsPb<sub>0.75</sub>Ge<sub>0.25</sub>Br<sub>3</sub> system extends by 1.6 times. Moreover, under the joint influence of Sn/Ge, the carrier lifetime of the CsPb<sub>0.5</sub>Ge<sub>0.25</sub>Sn<sub>0.25</sub>Br<sub>3</sub> system extends by 4.2 times compared with those of the original system. The overall sequence follows CsPb<sub>0.5</sub>Ge<sub>0.25</sub>Sn<sub>0.25</sub>Br<sub>3</sub> > CsPb<sub>0.75</sub>Ge<sub>0.25</sub>Br<sub>3</sub> > CsPbBr<sub>3</sub>. This study underscores the significant influence of binary alloying of B-site metal cations (in the perovskite structure <i>ABX</i><sub>3</sub>, where B-site refers to the metal cation) on the non-radiative electron-hole recombination of CsPbBr<sub>3</sub>.This research presents an effective alloying scheme that substantially prolongs the carrier lifetime of perovskites, offering a rational approach to optimizing solar cell performance. It lays the groundwork for the future design of perovskite solar cell materials.