In recent decades, perovskite solar cells (PSCs) have attracted significant attention due to their remarkable rise in power conversion efficiencies (PCEs) reaching 27%.[1] Building on this success, mixed-halide lead PSCs merit extensive research owing to their tunable bandgaps, which broaden their potential application.[2] To surpass the PCE limit of single-junction cells dictated from the Shockley-Queisser theory, tandem solar cells have emerged as a highly promising strategy.[3] In these configurations, wide-bandgap (WBG) PSCs (the bandgap energy, Eg = 1.7–1.9 eV) play a crucial role due to their ability to harvest higher-energy photons.[4] The state-of-the-art WBG PSCs recently achieve the open-circuit voltage (V_OC) of 1.30–1.38 V for Eg = 1.84–1.85 eV,[5] while their V_OC deficit is still larger than those of the conventional iodine-rich PSCs (V_OC = 1.19 V for Eg = 1.51 eV).[1]

Controlling the crystallization process of WBG PSCs remains challenging in the presence of high bromine (Br) content inherent to WBG perovskites, because the ionic radius and solubility of Br are smaller than those of iodine (I).[6] Consequently, WBG perovskites often undergo a rapid and uncontrollable crystallization process, where Br-rich phases tend to crystallize prematurely at the perovskite/air interface, while I-rich phases preferentially accumulate at the bottom.[7] This vertical phase separation results in a bandgap mismatch, creating superfluous barriers that hinder efficient carrier transport.[8] Furthermore, when employing the conventional antisolvent method for WBG perovskite films, wrinkled surface morphologies can occasionally occur under certain processing conditions such as direct chlorobenzene treatment.[9] This poor surface quality impairs the contact between the perovskite layer and the hole/electron transport layers (HTL/ETL), leading to interfacial resistance, current leakage, and degradation.[10] Moreover, the rapid mixing of the antisolvent and the precursor solution induces a spontaneous transition to supersaturation during spin-coating, generating excessive nuclei that compete with subsequent grain growth.[11] This results in small grains with a high density of grain boundaries. These boundaries act as the accumulation sites of vacancies and impurities, accelerating nonradiative recombination[12] and providing pathways for ion migration.[13] Moreover, the rapid evaporation of organic solvents during post-annealing often leaves uncoordinated Pb²⁺ defects on the surface.[7] These defects serve as nonradiative recombination centers as well, significantly reducing the VOC.[14] Accordingly, mitigating the V_OC loss by a precise control of crystallization process remains a critical challenge for WBG PSCs.

Herein, we report a synergistic strategy of crystallization regulation that combines a gas-quenching process and Pb(SCN)₂ additive engineering to produce high-quality, high Br-containing (40%) WBG perovskite films. Compared to the conventional antisolvent method, gas-quenching provides a gentler transition to supersaturation, suppresses excessive nucleation, and relieves residual strain. These effects lead to smooth films with large grains without wrinkles caused by the residual stress. Furthermore, Pb(SCN)₂ modifies the coordination environment of the precursor solution and promotes the formation of stable intermediate species, which retards grain growth during thermal annealing as revealed by in-situ UV–vis absorption and photoluminescence spectroscopies. Consequently, film quality improves significantly with the enhanced crystallographic orientation and suppressed halide segregation, yielding WBG PSCs with an increased V_OC from 1.087 to 1.252 V and a power conversion efficiency from 9.24 to 12.82%.