Crystallization Modulation and Comprehensive Defect Passivation by Carbonyl Functionalized Spacer Cation Towards High-Performance Inverted Perovskite Solar Cells.

The inverted cesium/formamidinium (CsFA)-based methylammonium-free perovskite solar cells possess great potential in simultaneously realizing high power conversion efficiency (PCE) and excellent stability. However, the uncontrollable crystallization process and poor film quality hinder further enhancement of photovoltaic performance and operational stability. Herein, we propose a synergistic modulation strategy of perovskite crystallization and the defects at grain boundaries (GBs) and interface by using a novel carbonyl functionalized spacer cation. L-Alanine benzyl ester hydrochloride (L-ABEHCl) containing carbonyl functionalized ammonium cation is incorporated into perovskite precursor solution, increasing the nucleation rate and reducing the crystal growth rate because of its strong interaction with precursor components, leading to increased grain size and crystallinity. No 2D perovskite is formed for L-ABEHCl as additive whereas 2D perovskite is formed upon L-ABEHCl post-treatment. It is revealed that FA and Cs in precursor solution suppress the formation of 2D perovskite. As a result, the L-ABEHCl passivates the defects at GBs in the form of organic salts and passivates interface defects in the form of 2D perovskite. Due to minimized carrier nonradiative recombination losses, the inverted devices using synergistic modulation strategy achieve a maximum PCE of 25.77 % (certified stabilized PCE of 25.59 %), which is one of the highest PCEs ever reported for the devices based on vacuum flash evaporation method. The unencapsulated target device maintains 90.85 % of its initial PCE after 2300 h of continuous maximum power point tracking, among the most excellent stabilities accomplished by inverted devices.