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Eco-friendly Bismuth Based Double Perovskites X$_2$NaBiCl$_6$ (X=Cs, Rb, K) for Optoelectronic and Thermoelectric Applications: A First-Principles Study

Owing to the energy shortages and various severe adverse effects of traditional fossil fuel power generation mechanisms, photovoltaic and thermoelectric materials are considered as the potential candidates for building non-traditional, efficient, and eco-friendly power generation portfolios. Lead-based perovskites have emerged as highly efficient, abundantly available, and low-cost materials for such applications but there are two major challenges i.e. chemical instability and the danger of toxic lead leaching that can badly affect both the environment and human health. Therefore, in search of lead-free perovskites, the replacement of lead with eco-friendly elements like bismuth and sodium may be a good strategy. Bismuth has very similar electronic properties as that of lead so it gives very efficient perovskites. Therefore, double perovskites containg Bi element X$_2$NaBiCl$_6$ (X=Cs, Rb, K) are explored here in terms of the structural, opto-electronic , and thermoelectric properties using the first-principles approach. The materials are attractive for optoelectronic applications such as ultraviolet sensors and detectors due to the prominent absorption peaks in ultraviolet region (10eV-30eV), low reflection (5-7%), and high optical conductivity. In addition to the alluring optoelectronic features, the compounds under study have figure of merit being close to unity so these are also promising for thermoelectric applications.