Structural, electronic, optical and thermoelectric properties of FrSnI3-xClx (X=0, 1, 2, 3) perovskites using the TB-mBJ approach

Mixed halide inorganic perovskites represent a significant advance in energy conversion materials, with research on these materials paving the way for more sustainable and accessible energy solutions. In this work we investigated the effects of chlorine atom substitution on the structural, electronic, optical and thermoelectric properties of mixed halide perovskites FrSnI_3-xCl_x (x = 0, 1, 2, 3). The FP-LAPW approach within the Wien2k package has been employed, using the modified Becke-Johnson potential (TB-mBJ) for the exchange correlation functionals. For FrSnI₃ and FrSnCl₃ the calculated lattice parameters are in good agreement with theoretical results.The formation energies calculated for FrSnI_3-xCl_x (x = 0, 1, 2, 3) confirm the thermodynamic stability of all these materials. Analysis of the electronic properties, including partial density of states (PDOS), total density of states (TDOS) and band structures, indicates that these materials exhibit p-type semiconductor behaviour with direct band gaps ranging from 1.169 to 1.953 eV In terms of optical properties, the FrSnI_3-xCl_x compounds exhibit high optical absorption (α(ω) > 10⁴ cm⁻¹) in the visible region. The broad absorption range extends from visible to ultraviolet energy. The low reflectivity values observed (below 30 %) and their minimal energy loss suggest potential applications in optoelectronic devices. The thermoelectric properties of these materials were also studied over a temperature range of 100 to 950 K. They exhibit high Seebeck coefficients, high electrical conductivity, and low thermal conductivity. Notably, at room temperature, FrSnI₂Cl and FrSnICl₂ show the highest figures of merit, reaching 1.31 and 0.61, respectively, demonstrating their high efficiency for thermoelectric devices