Tailored Cysteine‐Derived Molecular Structures toward Efficient and Stable Inorganic Perovskite Solar Cells

Abstract Surface–defect‐triggered non‐radiative charge recombination and poor stability have become the main roadblock to continued improvement in inorganic perovskite solar cells (PSCs). Herein, the main culprits are identified on the inorganic perovskite surface by first‐principles calculations, and to purposefully design a brand‐new passivator, Boc‐S‐4‐methoxy‐benzyl‐l‐cysteine (BMBC), whose multiple Lewis‐based functional groups (NH, S and CO) to suppress halide vacancies and coordinate with undercoordinated Pb²⁺ through typical Lewis baseacid reactions. The tailored electron‐donating methoxyl group (CH₃O–) can cause an increased electron density on the benzene ring, which strengthens the interaction with undercoordinated Pb²⁺ via electrostatic interactions. This BMBC passivation can reduce the surface trap density, enlarge grains, prolong the charge lifetime, and cause a more suitable energy‐level alignment. In addition, the hydrophobic tert‐butyl in butoxycarbonyl (Boc‐) group ensures that BMBC is uniformly covered and prevents harmful aggregation through steric repulsion at the perovskite/hole–transporting layer (HTL) interface, thus providing a hydrophobic umbrella to resist moisture invasion. Consequently, the combination of the above increases the efficiency of CsPbI_3−xBr_x PSC from 18.6% to 21.8%, the highest efficiency for this type of inorganic metal halide PSCs so far, as far as it is known. Moreover, the device exhibits higher environmental and thermal stability.