In Situ Grain Boundary Functionalization for Stable and Efficient Inorganic CsPbI 2 Br Perovskite Solar Cells

The phase instability and large energy loss are two obstacles to achieve stable and efficient inorganic‐CsPbI 3− x Br x perovskite solar cells. In this work, stable cubic perovskite (α)‐phase CsPbI 2 Br is successfully achieved by Pb(Ac) 2 functioning at the grain boundary under low temperature. Ac − strongly coordinates with CsPbI 2 Br to stabilize the α‐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 CsPbI 2 Br perovskite solar cells. A 12% efficiency and good stability are obtained for in situ PbO‐passivated CsPbI 2 Br 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 CsPbI 3− x Br x materials and the devices.