Design, synthesis, optical studies, and application of all-inorganic layered double perovskites as stabilizers in ambient air processed perovskite solar cells

•Layered double perovskites (LDPs) were synthesized for perovskite solar cells (PSCs).•Cs4CuSb2Cl12 had 1.1 eV bandgap and truncated hexagonal trapezohedron morphology.•Adding Cs4CuSb2Cl12 (CCSbCl) to MAPbI3 boosted the performance of PSCs.•The highest power conversion efficiency of 6.81% was measured using 1% CCSbCl.•PSC with 1% CCSbCl in MAPbI3 displayed the lowest charge transfer resistance.