Regulating the Crystallization Process of Intermediate Phase 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, FA0.5Cs0.5Pb(I0.93Br0.07)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.