Hydrophobic 2D Perovskite‐Modified Layer with Polyfunctional Groups for Enhanced Performance and High Moisture Stability of Perovskite Solar Cells

The charges stuck in trap sites hinder charge transport and lead to V oc below the radiative limit, which seriously restrict the performance and stability of organic–inorganic halide perovskite solar cells (PSCs). Chemical passivation is an effective method to reduce defects and suppress nonradiative recombination. Herein, a new passivation molecule l ‐cysteine methyl ester hydrochloride (CME) with thiol and ester groups is designed to modify the interface between the perovskite layer and hole transport layer (HTL). It reveals that thiol possesses outstanding moisture resistance and ester suppresses nonradiative recombination by coordinating with undercoordinated Pb 2+ . Furthermore, the 2D modified layer at the grain boundaries and surface passivates surface defects and promotes hole extraction. As a result, the CME device achieves the highest PCE of 20.33% with an enhanced open‐circuit voltage ( V oc ) of 1.11 V. Due to the barrier of highly hydrophobic 2D perovskites, the modified devices show excellent stability while exposed to humidity and high‐temperature environment. A facile and effective strategy to design organic molecular structures with polyfunctional groups to passivate trap‐assisted nonradiative recombination at the surface and grain boundaries is provided.