Grain boundary cracks patching and defect dual passivation with ammonium formate for high-efficiency triple-cation perovskite solar cells

Triple-cation perovskite solar cells exhibit better long-term stability as compared to FAPbI 3 devices but also have more ions and vacancies defects in film. Herein, ammonium formate (NH 4 HCO 2 ) is introduced and forms a stable NH 4 HCO 2 -PbI 2 adduct onto the surface of perovskite to patch grain boundary cracks and passivate interfacial defects. The density functional theory calculation results indicate that there is a strong interface interaction between perovskite and NH 4 HCO 2 , and the defects are well anchored by forming Pb··COOH bond and I··NH 4 bond. The density of states proves that surface trap states by the I vacancy is also effectively eliminated, which is consistent with the experimental results. As a result, the optimized devices achieve a power conversion efficiency of 24.62% and exhibit remarkable long-term stability in air. This work provides a simple defect multiple passivation strategy to prepare perovskite solar cells with high efficiency and stability. Triple-cation perovskite solar cells are more stable than formamidinium lead iodide but possess more defects. Here, grain boundary cracks and passivate interfacial defects are patched using ammonium formate which forms a stable adduct on the perovskite surface.