Towards Intermediate Phase‐Assisted Perovskite Crystallization via Aqueous Solution for High‐Performance Solar Cells

Abstract Halide perovskite absorbers are promising materials for next‐generation perovskite solar cells (PSCs), which rely on a solution made of several chemicals dissolved in toxic solvents. Such precursor solution processing is not compatible with a green chemical route. Aqueous precursor solutions are cost‐effective and eco‐friendly fabrication protocols; however, their device performances are inferior to those in regular. Here, an effective means of making high‐performance PSCs is reported, enabled by proposing a doping strategy that incorporates RbCl into the Pb(NO ₃ ) ₂ aqueous precursor. The incorporation of RbCl into the Pb(NO ₃ ) ₂ film can form two intermediate phases, namely Pb(NO ₃ ) ₂ ·RbCl and (PbI ₂ ) ₂ RbCl. The formation of these intermediate phases spontaneously facilitates the transformation of the resultant PbI ₂ from a stacking layered structure into discrete [PbI ₆ ] ⁴⁻ octahedra. This promotes the coordination of A‐site cations with the [PbI ₆ ] ⁴⁻ octahedra, thereby accelerating perovskite crystallization and reducing the residual Pb(NO ₃ ) ₂ or PbI ₂ . Hence, RbCl‐doped PSCs exhibit excellent long‐term stability, retaining 96% of their initial power conversion efficiency (PCE) after 2016 h under ISOS‐D‐1I protocol, and achieving a PCE of 22.75% with a fill factor (FF) of 82.62%. To the best of knowledge, this represents the highest reported FF and one of the highest PCE for PSCs fabricated from aqueous solutions.