Insights into the Dynamic Electron–Hole Separation Process Induced by a Trapped Electron in Lead Halide Perovskites in the Presence of Solutions

Metal halide perovskite solar cells show great promise, in terms of their high-power conversion efficiency. However, the dynamic electron-hole separation process remains elusive. Using ab initio molecular dynamics, we discover that the presence of photogenerated electron trapped at a Pb²⁺ ion can induce significant electron-hole separations on the CH₃NH₃PbI₃ perovskite in the presence of HI solution. In this dynamic process, the separated electron is transferred to the Pb⁺ ion to form a Pb⁰ atom, while the separated hole is trapped in an I dimer. The reason behind this induced electron-hole separation is clearly revealed. Furthermore, the charge carrier transfer mechanism is elucidated, which not only explains the carrier migration but also the degradation of the perovskite in a humid environment. Comparing the atomic motions in CH₃NH₃PbI₃ and CH₃NH₃PbCl₃ quantitatively demonstrates that CH₃NH₃PbI₃ is more active but less stable than CH₃NH₃PbCl₃. The proposed mechanism for the electron-hole separation mechanism and perovskite degradation in humid conditions provides insights into the design of a highly efficient perovskite with good stability.