Detrimental Defect Cooperativity at TiO₂/CH₃NH₃PbI₃ Interface: Decreased Stability, Enhanced Ion Diffusion, and Reduced Charge Lifetime and Transport

Interfaces are essential for solar cell performance since they govern charge separation and transport. Using quantum dynamics simulation, we demonstrate that at interfaces, common defects that are benign on their own, iodine vacancy in CH3NH3PbI3 (VI) and oxygen vacancy in TiO2 (VO), are responsible synergistically for poor stability and charge losses. VO promotes VI diffusion and accelerates iodine migration. A midgap trap state appears, inhibiting charge transport and accelerating charge recombination by an order of magnitude. Strong structural distortions strengthen electron-vibrational interactions and activate high-frequency TiO2 phonons. Because of the widely reported high defect-tolerance of lead-halide perovskites, the synergistic detrimental influence of perovskite defects with defects in other materials is often overlooked. The interfacial defect pairing could be a major reason for poor stability and charge losses in perovskite solar cells. The results suggest that either a high-quality perovskite or high-quality charge extraction layer may be sufficient to achieve high performance.