Suppressing charge recombination in a methylammonium-free wide-bandgap perovskite film for high-performance and stable perovskite solar cells†

Wide-bandgap (WBG) formamidinium–cesium (FA–Cs) hybrid lead iodide–bromide mixed perovskites (∼1.7 eV) have gained great attention with the potential of enabling highly efficient tandem photovoltaics when integrated with crystalline silicon and other low-bandgap solar cells. However, their power conversion efficiencies (PCEs) are still insufficient compared to their methylammonium (MA) counterparts, mainly owing to the high open-circuit voltage (V_OC) deficits (>0.43 V). Here, by incorporating rubidium iodide (RbI) in the FA_0.8Cs_0.2Pb(I_0.75Br_0.25)₃ perovskite precursor, the film crystallinity and bulk defects are significantly optimized. In addition, we propose an all-around interface engineering strategy sequentially constructing a surface heterojunction and using trioctylphosphine oxide (TOPO), which can significantly passivate grain boundaries and undercoordinated defects, as well as optimize the energy band. As a result, the target MA-free WBG n–i–p solar cells at 1.685 eV have achieved a record efficiency of 23.35% and a high V_OC of 1.30 V (with a record voltage deficit of 0.385 V). Most importantly, the unencapsulated solar cells also display impressive air storage stability, operating stability and thermal stability. Moreover, a PCE of 19.54% on a 1 cm² WBG solar cell and a PCE of 21.31% on a 0.04 cm² p–i–n inverted WBG solar cell are also demonstrated.