Efficient bandgap widening in co-evaporated MAPbI₃ perovskite

Co-evaporated perovskite solar cells (PSCs) have demonstrated outstanding properties, such as great scalability, intrinsic stability, high-power conversion efficiency (PCE), and fabrication adaptability even on rough surfaces. At present, MAPbI₃ is the most used co-evaporated perovskite due to the complexity of forming multi-component compositions by thermal evaporation. Even though PSCs with high PCEs have been obtained, the MAPbI₃ bandgap (∼1.60 eV) is not ideal for multijunction devices. In this work, we propose a facile method to increase the bandgap of co-evaporated MAPbI₃ (∼1.60 eV) through a MABr-based treatment. The best MABr-treated perovskite composition films show a bandgap of 1.66 eV (MAPb(Br_0.18I_0.82)₃) and exhibit good spectral stability under continuous 1-sun illumination at the ambient conditions of 28 °C and 70% relative humidity. This hybrid method works efficiently for thick co-evaporated MAPbI₃ films (∼750 nm), which is unusual for hybrid processes. The n-i-p PSCs built from the MAPb(Br_0.18I_0.82)₃ films exhibit a blue-shifted external quantum efficiency and a V_oc increase of ∼30 mV as compared to the pure MAPbI₃ PSCs, in agreement with the bandgap widening observed in the films. This hybrid method to crete wide bandgap perovskites can be universally applied to MAPbI₃ deposited on both flat and textured surfaces and shows great promise for its integration in monolithic tandems.