A Roadmap to 30.5% Efficiency in All‐Perovskite Tandem Solar Cells Using 3D Finite Element Analysis of CuSCN@GeSe Core‐Shell and CeO ₂ Electron Transport Layer Optimization

All‐perovskite tandem solar cells hold great promise for next‐generation photovoltaics, yet their efficiency is often limited by current mismatch between subcells and inefficient charge transport at interfaces. In this work, we address these challenges through three‐dimensional finite element simulations of a three‐terminal (3T) tandem device. A wide‐bandgap FACsPb(IBr) 3 absorber was used in the top cell and a narrow‐bandgap MASnI 3 absorber in the bottom cell. To enhance light harvesting and hole transport in the bottom subcell, we incorporated a CuSCN@GeSe core–shell nanostructure. In addition, TiO 2 in the top subcell was replaced by CeO 2 as the electron transport layer to improve electron extraction and suppress recombination. Under optimized and well‐passivated interfacial conditions, the proposed design indicates substantial performance enhancement, with the simulated short‐circuit current density ( J sc ) increasing from 11.22 to 15.33 mA/cm 2 , the open‐circuit voltage ( V oc ) from 1.90 to 2.37 V, the fill factor from 0.76 to 0.84, and the power conversion efficiency approaching 30.5%.This study demonstrates the synergistic benefits of nanostructure engineering and charge‐transport layer optimization, offering a practical strategy to achieve highly efficient and stable all‐perovskite tandem photovoltaics.