Scalable fabrication of wide-bandgap perovskites using green solvents for tandem solar cells

Commercializing perovskite-based tandems necessitates environmentally friendly solvents for scalable fabrication of efficient wide-bandgap (WBG) (1.65–1.80 eV) perovskites. However, the green solvents developed for formamidinium lead iodide-based ~1.50-eV-bandgap perovskites are unsuitable for WBG perovskites due to the low solubility of caesium and bromide salts, leading to reliance on toxic N,N-dimethylformamide solvent. Here we present a green solvent system comprising dimethyl sulfoxide and acetonitrile to effectively dissolve the named salts, with the addition of ethyl alcohol to prevent precursor degradation and to extend the solution processing window. Using this green solvent mixture, we achieve blade-coated WBG perovskite solar cells with power conversion efficiencies of 19.6% (1.78 eV) and 21.5% (1.68 eV). We then demonstrate 20.25-cm² all-perovskite tandem solar modules with a power conversion efficiency of 23.8%. Furthermore, we achieved WBG perovskites deposited in ambient air and narrow-bandgap perovskites fabricated using the same green solvents, which promotes the viability of environmentally friendly fabrication.