Intermediate Composition Engineering for Even Halide Distribution in Efficient Perovskite/Silicon Tandem Solar Cells

Wide-bandgap (WBG) perovskites hold tremendous potential for realizing efficient tandem solar cells. However, the rapid crystallization rate of the Br-rich composition leads to nonuniform halide distribution and bulk defects, posing significant challenges in reaching the theoretical efficiency limit and long-term stability. Here, we provide an intermediate-composition engineering (ICE) strategy by substituting lead bromide (PbBr 2 ) with lead chloride (PbCl 2 ) and methylammonium bromide (MABr). This approach forms a metastable 2D intermediate phase A 2 PbI x Br 3– x Cl (A includes FA, MA, Cs, 0 < x < 3), which inhibits the preferential nucleation of the Br-rich phase and facilitates uniform halide crystallization in WBG perovskite films, thereby improving film quality. The resulting perovskite solar cells (1.67 eV) deliver a power conversion efficiency (PCE) of 22.5% with an impressive open-circuit voltage ( V OC ) of 1.280 V, and the champion tandem cells yield a certified efficiency of 30.5% (1.21 cm 2 ). This work provides a new route for addressing the phase inhomogeneity and improving the film quality of mixed halide perovskites.