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.