Synchronous Phase Transformation for Efficient Wide‐Bandgap Perovskite Photovoltaics

Abstract Mixed‐halogen wide‐bandgap (WBG) perovskite materials are employed in tandem solar cells (TSCs) due to their continuous tunability of bandgap. However, inhomogeneous halogen phases are often observed in bromine‐rich perovskite films, which restricts the performance of WBG perovskite solar cells (PSCs) and TSCs. Here, homogeneous halogen‐phase perovskite is proposed to form film by a synchronous halogen‐phase transformation strategy. 1,3‐Dimethyl‐2‐imidazolidinone (DMI) is introduced into the perovskite precursor solution, due to its stronger binding energy with lead halide (PbX 2 ). The homogeneous DMI‐PbX 2 adducted intermediate phase is stable in precursor solution and at spin‐coating stage. And it then synchronously transforms into a homogeneous halide‐phase perovskite film at the annealing stage. Benefited from efficient carrier extraction and suppressed carrier recombination, the resulting 1.76 eV‐bandgap PSC achieves a record power conversion efficiency (PCE) of 21.42% (certified 21.18%) among devices with a bandgap wider than 1.74 eV. Based on the high transmittance of semitransparent‐WBG PSC, a 4‐terminal all‐perovskite TSC achieves a PCE of 29.66%.