Synchronous Phase Transformation for Efficient Wide-Bandgap Perovskite Photovoltaics.

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). The homogeneous DMI-PbX 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%.