Surface Purification and Compensation of I Interstitial in Quasi-2D CsPbI₃

Photovoltaic devices based on Quasi-2D CsPbI₃ draw a lot of research interest, however, they currently exhibit poorer efficiencies than the bulk ones, possibly because of an enlarged surface area that may lead to more defects and extra coupling between surface and defect states. To verify this phenomenon, we apply first-principles static and molecular dynamics simulations and find a stable and detrimental I interstitial in Quasi-2D CsPbI₃ under room temperature. Moreover, to effectively regulate the I interstitial, a purification or compensation on the I interstitial by a volatile H atom as surfactants is investigated for different surface attached sites. However, for less volatile surfactants of Li and Na atoms, only the compensation effects occur. Based on these results, we propose potential criteria of volatility to select surfactants for a two-stage surfactant strategy, which purifies the defect at the as-grown stage followed by compensating the defect states and stabilizing the surface during the post annealing.