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

Defects significantly influence charge transport in CH 3 NH 3 PbI 3 (MAPbI 3 ) 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 TiO 2 /MAPbI 3 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.