Direct band-gap iodide double perovskite solar cell materials by doping strategy: First-principles predictions

As alternatives to lead-based halide perovskite materials, the lead-free halide double-perovskite materials have many advantages but often show wide indirect band gaps and exceed the optimum band-gap range of 0.9–1.6 eV. Based on first-principles calculations, we scan a serial of halide double-perovskites Cs2B′B''X6 (B′ = Li, Na, K; B'' = In, Bi; X = Cl, Br, I) and find that most of Cs2B′B''X6 double-perovskite materials have direct band gaps but larger than 1.6 eV, while the direct band gaps of Cs2LiInI6 and Cs2NaInI6 are smaller than 0.9 eV. We modulate the electronic structure of Cs2B′InI6 (B′ = Li, Na, K) by an ionic doping strategy through substituting partial In ions with Bi ions and discover more halide double-perovskite solar cell materials with suitable band gap. By exploring the stability, electronic structures and optical properties of the doped double-perovskite Cs2B′In1-xBixI6 (B′ = Li, Na, K, x = 0.25 and 0.75), we discover the Cs2B′In0.75Bi0.25I6 (B′ = Li, Na and K) DP materials show direct band gaps within the optimal band-gap range of 0.90 − 1.60 eV for promising solar-cell materials. Meanwhile, the doped Cs2B′In1-xBixI6 DP materials show good carrier mobility as high as 103 cm2S−1V−1. Moreover, the predicted new materials exhibit excellent light absorption coefficients of 106 cm−1 in the visible light range. These new lead-free inorganic DP materials may offer great promise as candidates for highly efficient solar absorber materials for photovoltaic applications.