Crystallization suppression of mixed-halide intermediates for perovskite/Cu(In,Ga)Se2 tandem solar cells with improved efficiency

Perovskite/Cu(In,Ga)Se 2 (CIGS) tandem solar cells hold particular promise for flexible and lightweight applications, yet their efficiency still lags behind other perovskite-based tandem architectures. To achieve optimal spectral matching with the CIGS bottom cell, the perovskite top cell requires a wide bandgap, obtainable with a mixed-halide composition with high bromide content. However, perovskite films made with high bromide content suffer from inhomogeneous halide distribution, especially for scalable film fabrication method. Here we use 2-pyrrolidinone as the coordinating solvent to suppress the crystallization of the halide intermediates, maintaining a homogeneous halide distribution during ambient blade-coating of the perovskite film. We demonstrate a flexible monolithic two-terminal perovskite/CIGS tandem cell with a power conversion efficiency of 27.3%, with minimal efficiency loss over 500 h of operation.