Unleashing the Potential of All-Perovskite Tandem Solar Cell Structures: A Hybrid DFT and Numerical Simulation Insights

The widespread use of lead-based perovskites in photovoltaic (PV) applications is hindered by their toxicity, instability, and environmental concerns, creating a critical need for sustainable alternatives. In this study, we explore the structural and optoelectronic properties of the lead-free, all-inorganic Sr$_{\mathrm{3}}$NX$_{\mathrm{3}}$ (X = I, Br, Cl, F) perovskite family using density functional theory (DFT) and many-body perturbation theory (GW-BSE). These materials exhibit direct bandgaps tunable from 1.04 eV to 2.65 eV and high absorption coefficients ( 10$^{5}$ cm $^{-1}$), demonstrating their potential as efficient absorber layers in photovoltaic devices. Numerical simulations using SCAPS-1D are performed to evaluate device performance in both uni-junction and tandem solar cell (TSC) architectures. Among the uni-junction configurations, the Sr$_{\mathrm{3}}$NBr$_{\mathrm{3}}$-based device achieved the highest power conversion efficiency (PCE) of 23.06%. A monolithic TSC composed of Sr$_{\mathrm{3}}$NBr$_{\mathrm{3}}$ (top) and Sr$_{\mathrm{3}}$NI$_{\mathrm{3}}$ (bottom) absorbers attained a PCE of 30.22%, with an open circuit voltage ($V_{oc}$) of 1.75 V and short circuit current density ($J_{sc}$) of 20.8 mA/cm$^{2}$. This work demonstrates the practical viability of lead-free Sr-based perovskites and offers a promising pathway toward high-efficiency, stable, and environmentally friendly solar energy technologies.