First principles insight into band gap tuning in bismuth based double perovskites X2NaBiCl6 (X = Cs, Rb, K) for enhanced optoelectronic and thermoelectric properties

Owing to the energy shortages and various severe adverse effects of traditional fossil fuel power generation mechanisms, photovoltaic and thermoelectric materials are considered as 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 the search for lead-free perovskites, the replacement of lead with eco-friendly elements like bismuth (Bi3+) and sodium (Na+) may be a good strategy. Bismuth has very similar electronic properties as that of lead so it gives very efficient perovskites. Therefore, double perovskites containing Bi element X2NaBiCl6 (X = Cs, Rb, K) are explored here in terms of the structural, opto-electronic, and thermoelectric properties using the first-principles approach. The band gap decreased by the replacement of Cs with Rb and K and hence the materials became more attractive for optoelectronic applications such as ultraviolet sensors and detectors due to the prominent absorption peaks in the ultraviolet region (10 eV-30 eV), low reflection (5–7 %), and high optical conductivity (∼1015 sec-1). In addition to the alluring optoelectronic features, the compounds under study have the figure of merits close to unity hence also promising for thermoelectric applications.