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

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 ${Cs}_{2}ScAg{X}_6$ (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 ${10}^5{cm}^{-1}$. The results obtained highlight the suitability of ${Cs}_{2}ScAg{X}_6$ 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 ${Cs}_{2}ScAg{I}_6$ 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 ${Cs}_{2}ScAg{I}_6$, offering exceptional performance: 1.795 V for the open circuit voltage ($V_{oc}$), 18.45 mA/${cm}^2$ for the short-circuit current density ($J_{sc}$), a fill factor (FF) of 87.2 %, and a power conversion efficiency (PCE) of 28.88 %.