Comprehensive analysis of optoelectronic and photovoltaic properties of Cs2ScAgX6 (X = Br, I) double perovskites for tandem cells using DFT and SCAPS-1D methods

•An in-depth DFT study was conducted on Cs2ScAgX6 (X = Br, I) double halide perovskites, analyzing structural, optical, and electronic properties..•Band structures and density of states indicate key photovoltaic applications.•The gap energy of Cs2ScAgI6 favors its use as an absorber in tandem solar cells.•SCAPS-1D simulation of a Cs2ScAgI6/CIGS tandem cell achieved 28.88% efficiency. Double perovskites, which have substantial benefits over conventional lead-based perovskites in terms of chemical stability and non-toxicity, have become a viable substitute in recent years. This study provides a comprehensive evaluation of Cs2ScAgX6 (X = Br, I) as a potential photovoltaic (PV) material, based on a density functional (DFT) calculation using the generalised gradient approach (GGA) with the modified Becke-Johnson (mBJ) potential, which show that this material is characterised by a band gap of 1.8 eV for X = Br and 1.55 eV for X = I, as well as excellent optical properties, including a high absorption coefficient greater than 105cm-1. The results obtained highlight the suitability of Cs2ScAgX6 as a photovoltaic material. More specifically, it can play the role of absorber for the top cell in tandem solar cells. This study focuses on the use of Cs2ScAgI6 as an absorber for top cell, in combination with CIGS as an absorber for bottom cell, to construct a two-junction tandem solar cell. Current matching, essential for the operation of the tandem cell, is achieved by using a filtered spectrum to illuminate the lower cell, pre-calibrated based on experimental values from the literature. This condition is achieved with a thickness of 490 nm of Cs2ScAgI6, offering exceptional performance: 1.795 V for the open circuit voltage (Voc), 18.45 mA/cm2 for the short-circuit current density (Jsc), a fill factor (FF) of 87.2 %, and a power conversion efficiency (PCE) of 28.88 %.