Manipulation of Cation Combination and Configuration of Halide Double Perovskites for Solar Cell Absorbers

Pb-free halide double perovskites, A₂B⁺B³⁺X₆ (A = Cs, B⁺/B³⁺ = metal cation, X = halogen anion) have been proposed to replace hybrid halide perovskites (e.g., CH₃NH₃PbI₃) as stable, Pb-free materials for high efficiency solar cell absorbers. Here, using first-principles density functional theory calculations and symmetry analysis we show that the overall electronic properties of A₂B⁺B³⁺X₆ depend strongly on the atomic orbitals and site occupation of the B⁺ and B³⁺ cations. For the ordered A₂B⁺B³⁺X₆ compounds, in order to have a direct band gap with allowed optical transitions, both of the B⁺ and B³⁺ cations should possess the lone-pair s state. Thus, only the family of compounds stratifies this criterion. On the other hand, we reveal that the electronic structures of A₂B⁺B³⁺X₆ can be greatly tuned by controlling the site occupation ordering parameter of the B⁺ and B³⁺ cations. Compared to their ordered counterparts, the band gaps of the disordered A₂B⁺B³⁺X₆ alloys can be reduced significantly and the band gap character can be switched from indirect to direct, which greatly expanded the possible candidates for solar cell applications. Our results thus set a new direction and guidelines for the design of Pb-free halide double perovskites for solar cells.