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

Abstract

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 (s²p⁰) 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 d¹⁰s⁰ and s⁰p⁰ 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.