In Situ Grain Boundary Functionalization for Stable and Efficient Inorganic CsPbI2Br Perovskite Solar Cells

The phase instability and large energy loss are two obstacles to achieve stable and efficient inorganic-CsPbI3-xBrx perovskite solar cells. In this work, stable cubic perovskite (alpha)-phase CsPbI2Br is successfully achieved by Pb(Ac)(2) functioning at the grain boundary under low temperature. Ac- strongly coordinates with CsPbI2Br to stabilize the alpha-phase and also make the grain size smaller and film uniform by fast nucleation. PbO is formed in situ at the grain boundary by decomposing Pb(Ac)(2) at high-temperature annealing. The semiconducting PbO effectively passivates the surface states, reduces the interface recombination, and promotes the charge transport in CsPbI2Br perovskite solar cells. A 12% efficiency and good stability are obtained for in situ PbO-passivated CsPbI2Br solar cells, while Pb(Ac)(2)-passivated device exhibits 8.7% performance and the highest stability, much better than the control device with 8.5% performance and inferior stability. This article highlights the extrinsic ionic grain boundary functionalization to achieve stable and efficient inorganic CsPbI3-xBrx materials and the devices.