Lead free A₂NaInI₆ (A = Cs, Rb, K) double perovskites for optoelectronic and thermoelectric applications

Alkali metal substitution in double perovskites is an appropriate approach to deliberate tuning of band edges which play a vital role in bandgap engineering of emerging semiconducting materials. Here we tuned the optoelectronic behavior by carefully engineering the band edges in A(2)NaInI(6) (A = Cs, Rb, K) systems and by tuning the alkali atoms at A site using density functional theory based WIEN2K code. First of all, the unit cell is relaxed to find the optimized lattice parameters. The substitution of smaller-sized cation at the A site leads to the reduction in lattice parameter which shifts the absorption edge towards a shorter wavelength and reduces the bandgap energy. The value of bandgap energy is noticed at 1.60 eV for Cs2NaInI6 which increases to 1.65 and 1.70 eV for Rb2NaInI6 and K2NaInI6 respectively. The computation of the elastic constant's dependent modulus of elasticity and the Pugh and Poisson ratio confirm their brittle nature. Their viability for solar-thermoelectric generators is carried out by computing the temperature-dependent transport parameters using the BoltzTrap code showing the potential of these systems for solar thermoelectric generators.