Grain Boundary Cracks Patching and Defect Dual Passivation with Ammonium Formate for High-Efficiency Triple-Cation Perovskite Solar Cells

Abstract Triple-cation perovskite solar cells (PSCs) exhibit better long-term stability as compared to FAPbI₃ devices but also have more defects such as undercoordinated lead ions (Pb²⁺), halide vacancies, and organic cation vacancies in film. Herein, ammonium formate (NH₄HCO₂) is introduced and forms a stable NH₄HCO₂-PbI₂ adduct onto the surface of perovskite (FA0.945MA0.025Cs0.03Pb(I0.975Br0.025)₃) 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 surface and NH₄HCO₂, and the defects are well anchored by forming Pb··COOH bond and I··NH₄ bond. The density of states (DOS) proves that surface trap states (around the Fermi level) created by the I vacancy is effectively eliminated, which is consistent with the experimental results of suppressing non-radiative recombination at the interface. As expected, the optimized PSCs achieve a champion power conversion efficiency (PCE) of 24.62%, which is much higher than the value of control devices with a PCE of 23.45%. Moreover, the unencapsulated devices exhibit remarkable long-term stability in air with 40% RH at 25°C. This work provides a simple defect multiple passivation strategy to build PSCs with high efficiency and stability.