Interstitial Iodine Induced Deep-Trap-Pinning Suppresses Self-Healing at the TiO₂/Perovskite Interface

Defects significantly influence charge transport in CH₃NH₃PbI₃ (MAPbI₃) perovskite solar cells, particularly at interfaces. Using quantum dynamics simulation, we reveal a distinct interstitial iodine (I_i) defect behavior at different positions in the TiO₂/MAPbI₃ system. In the perovskite bulk-like region, I_i exhibits high mobility and dissociates detrimental iodine trimers, facilitating small-to-large polaron transition and promoting shallow trap formation. In contrast, the interfacial I_i defect enhances local structural rigidity due to its strong interaction with undercoordinated Ti atoms and MA molecular dipoles, which unexpectedly pins the deep trap state and suppresses its inherent self-healing capability. This leads to polaron localization and accelerates nonradiative recombination by 2 orders of magnitude. The results reveal the mechanism of deep-trap-pinning due to an interstitial I_i defect at perovskite interfaces, which offers theoretical guidance for minimizing charge losses in highly efficient perovskite solar cells.