Detrimental\nDefect Cooperativity at TiO<sub>2</sub>/CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Interface: Decreased\nStability, Enhanced Ion Diffusion, and Reduced Charge Lifetime and\nTransport

Interfaces are essential for solar cell performance since\nthey\ngovern charge separation and transport. Using quantum dynamics simulation,\nwe demonstrate that at interfaces, common defects that are benign\non their own, iodine vacancy in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> (V<sub>I</sub>) and oxygen vacancy in TiO<sub>2</sub> (V<sub>O</sub>), are responsible synergistically for poor stability and\ncharge losses. V<sub>O</sub> promotes V<sub>I</sub> diffusion and\naccelerates iodine migration. A midgap trap state appears, inhibiting\ncharge transport and accelerating charge recombination by an order\nof magnitude. Strong structural distortions strengthen electron-vibrational\ninteractions and activate high-frequency TiO<sub>2</sub> phonons.\nBecause of the widely reported high defect-tolerance of lead-halide\nperovskites, the synergistic detrimental influence of perovskite defects\nwith defects in other materials is often overlooked. The interfacial\ndefect pairing could be a major reason for poor stability and charge\nlosses in perovskite solar cells. The results suggest that either\na high-quality perovskite or high-quality charge extraction layer\nmay be sufficient to achieve high performance.