A dual-retarded reaction processed mixed-cation perovskite layer for high-efficiency solar cells

Mixed-cation perovskite solar cells (PSCs) have become of enormous interest because of their excellent efficiency, which is now crossing 23.7%. Their broader absorption, relatively high stability with low fabrication cost compared to conventional single phase ABX ₃ perovskites (where A: organic cation; B: divalent metal ion; and X: halide anion) are key properties of mixed-halide mixed-cation perovskites. However, the controlling reaction rate and formation of extremely dense, textured, smooth, and large grains of perovskite layer is a crucial task in order to achieve highly efficient PSCs. Herein, a new simple dual-retarded reaction processing (DRP) method is developed to synthesize a high-quality mixed-cation (FAPbI ₃ ) _0.85 (MAPbBr ₃ ) _0.15 (where MAPbBr ₃ stands for methylammonium lead bromide and FAPbI ₃ stands for formamidinium lead iodide) perovskite thin film via intermediate phase and incorporation of nitrogen-doped reduced graphene oxide (N-rGO). The reaction rate is retarded via two steps: first the formation of intermediate phase and second the interaction of the nitrogen groups on N-rGO with hydrogen atoms from formamidinium cations. This DRP process allows for the fabrication of PSCs with maximum conversion efficiency higher than 20.3%.