Optoelectronic and thermoelectric properties of novel stable lead-free cubic double perovskites A2NaIO6 (A = Ca, Sr) for renewable energy applications

Abstract

Over the past decade, perovskites have received considerable attention because of their record power conversion efficiency (25.7%) in solar cells. These materials have also received recent research interest in thermoelectrics, most likely due to their high carrier mobility, large power factor, and ultralow thermal conductivity. Therefore, in the present work, we have examined the optoelectronic and thermoelectric properties of A₂NaIO₆ (A = Ca, Sr) double perovskites using first-principles calculations. Stability has been confirmed using reliable and accurate descriptors of formation energy and phonon calculations. The optimized lattice constant and volume show an increasing tendency with changing A site cation (Ca → Sr). The computed band structures depict the semiconducting nature with direct band gap values of 2.64 eV (Ca₂NaIO₆) and 2.48 eV (Sr₂NaIO₆). The absorption was found to start in the visible range where the reflectivity was less than 10%. Moreover, the high Seebeck coefficient, large electrical conductivity, and fairly low thermal conductivity result in ZT values of 0.724 for Ca₂NaIO₆ and 0.686 for Sr₂NaIO₆ at 1000 K. With their optimum band gap, excellent light absorption capacity, and high ZT values, A₂NaIO₆ emerge as promising candidates for optoelectronic and thermoelectric applications.