Cs2TlBiI6: a new lead-free halide double perovskite with direct band gap.

The recent development of halide double perovskites A2B'B"X6 with favorable band gaps have provided a new search direction for stable Pb-free perovskite solar cells. Here, we propose a new lead free double perovskite Cs2TlBiI6 as a potential candidate for perovskite solar cell absorber. We probe the structural, electronic and optical properties of this material through density functional theory calculations. Our calculations on this material show that Cs2TlBiI6 adopt cubic double perovskite structure with space group Fm-3m. Using PBE exchange-correlation functional we obtain direct band gap of about 1.37 eV at the centre of Brillouin zone. The direct band gap and strong optical absorption of this material in the visible energy range of solar spectrum implies that Cs2TlBiI6 can be a potential candidate for perovskite based solar cell. Our fundamental calculations on this material would open up further possibility of experimental studies on TlBi based perovskite.