Surface Reconstruction for Efficient Cs2AgBiBr6 Solar Cells

Lead-free double perovskite Cs2AgBiBr6 is considered to serve as a substitute for toxic lead-based perovskite components and is a promising candidate for environmentally friendly perovskite solar cells (PSCs). However, the high deep-level trap density in Cs2AgBiBr6 films hinders the device performance, which is caused by the different solubilities of components in the precursor solution and the solution-process fabrication method. In this work, we reported a surface reconstruction (SR) strategy, in which a dissolution process is initiated by dimethyl sulfoxide (DMSO) and followed by the introduction of methylammonium bromide (MABr) to generate a mixed organic–inorganic intermediate phase and induce an orderly recrystallization. The dissolution–recrystallization process effectively improves the overall crystalline quality and surface composition stoichiometry ratio, passivates the bromine vacancies enriched in the film, and forms a dense as well as ordered film surface, resulting in reduced trap density, extended carrier lifetime, better interfacial contact, and faster carrier extraction. The differences in underlying mechanisms and effects between the conventional interfacial passivation (IP) strategy based on the passivator/isopropanol (IPA) system and the SR strategy proposed in this work were also compared by multiple characterizations. All these advantages enable the Cs2AgBiBr6-based PSCs fabricated by the SR strategy to achieve a significantly higher power conversion efficiency (PCE) of 2.05% and an ultrahigh fill factor (FF) of 83.80% as well as open-circuit voltage (VOC) of 1.201 V, with good stability and broad applicability in the hole transport layer (HTL)-free PSCs. Therefore, this work points out the fundamentally different mechanisms of two post-treatment strategies on the overall regulation of Cs2AgBiBr6 films and provides a clear clue to further enhance the quality of lead-free double perovskite films and optimize the photovoltaic performance of PSCs.