Regulating the Crystallization Process of IntermediatePhase for Wide-Bandgap Perovskite Reduces Open-Circuit Voltage Deficit

Wide-bandgap (WBG) perovskite solar cells with a bandgap exceeding 1.65 eV effectively complement the spectral response of monocrystalline silicon, making them promising candidates for high-performance perovskite–silicon tandem cells. However, achieving WBG perovskites with a bandgap of 1.65 eV typically requires a Br-rich composition, which often results in severe light-induced I–Br phase segregation. In this work, a Cs-rich composition, FA_0.5Cs_0.5Pb(I_0.93Br_0.07)₃ perovskite with a bandgap of 1.65 eV, was studied and an arylformamidium additive-assisted approach was adopted to regulate the crystallization process of the intermediate phase. The arylformamidium molecules selectively adsorb onto the Cs₂PbI₂Cl₂ intermediate grains, inhibiting the interplay between DMSO and Cs₂PbI₂Cl₂, significantly increasing the grain size of intermediate-phase Cs₂PbI₂Cl₂ and improving its crystallinity. The highly uniform Cs₂PbI₂Cl₂ intermediate grains enhance the nucleation consistency and growth uniformity of the perovskite, ultimately forming a homogeneous perovskite film with large grain sizes and preferential orientation, while reducing the defect state densities. As a result, the 2-thienformamidium iodide (TFAI)-modified WBG perovskite solar cells achieved a power conversion efficiency (PCE) of 21.94%, alongside an impressive open-circuit voltage of 1.27 V. Moreover, the devices retained over 90% of their initial PCE after >1000 h of maximum power point (MPP) tracking degradation testing, and negligible phase segregation was detected after 450 h of AM 1.5G illumination.