Lead-free double perovskites: how divalent cations tune the electronic structure for photovoltaic applications

Recently, the efficiency of perovskite-based solar cells has reached 25.7%. Due to lead toxicity, lead-free double perovskites have received great attention as potential candidates for photovoltaic applications. Keeping in mind the significant challenges (indirect and wide bandgaps) related to most of the present double perovskites, we used ab initio calculations and incorporated divalent and tetravalent dopant cations at the mixed B-site of Cs2AgSbCl6 to tune the electronic structure for optoelectronic applications. The doped perovskites are structurally and thermodynamically stable. The tunability of the electronic structure is shown for Sn2+- and Ge2+-doped configurations due to the strong hybridization between the dopant orbitals (s(2)p(0)) and the host orbitals at the band edges. The predicted bandgaps of the doped perovskites are of a direct character and in the ideal range (1.70-1.90 eV) for the top cell in tandem solar cells. The inclusion of Zn2+, Cd2+, Sn4+, and Ge4+ with the electronic configurations of d(10)s(0) and s(0)p(0) resulted in a negligible modification of the electronic structure, and hence did not show an indirect-direct bandgap transition. Therefore, the contribution from the dopant orbitals at the band edges dominates the electronic structure tunability of the doped perovskites. Furthermore, an enhanced absorption efficiency is observed for the Sn2+- and Ge2+-doped perovskites. This work provides a computational guide for exploring low-cost and non-toxic dopants for the electronic structure-engineering of double perovskites as photo-absorbers in future solar cell applications.