Thiourea with sulfur-donor as an effective additive for enhanced performance of lead-free double perovskite photovoltaic cells

Lead-free inorganic Cs₂AgBiBr₆ double perovskites have emerged as promising materials in perovskite solar cells (PSCs) to tackle the inferior stability and toxicity issues of organic–inorganic hybrid PSCs. However, the power conversion efficiencies (PCEs) of Cs₂AgBiBr₆ solar cells are remarkably restricted by the intrinsic and extrinsic defects of Cs₂AgBiBr₆ films. More specifically, the fast crystallization process in the formation of Cs₂AgBiBr₆ films strongly prevents the homogeneous growth of perovskite crystals, leading to inferior Cs₂AgBiBr₆ film quality. This work introduces a facile strategy to retard the crystallization of Cs₂AgBiBr₆ perovskites by introducing Lewis base additives into the precursor solution. The incorporation of a strongly coordinated thiourea additive with a sulfur donor leads to the generation of a Lewis acid base adduct, which retards the crystallization process for Cs₂AgBiBr₆ crystals, improves the quality of the Cs₂AgBiBr₆ film, decreases the defect density and inhibits charge carrier recombination. After optimization, the cell delivers a superior PCE of 3.07%, surpassing that reported for most Cs₂AgBiBr₆-based solar cells in the literature, and exhibits outstanding stability with a PCE retention rate of 95% after 20 days of storage in air. This work provides an effective strategy for further improvements in the performance of inorganic lead-free Cs₂AgBiBr₆-based photovoltaic cells.