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′In_1-xBi_xI₆ (B′ = Li, Na, K, x = 0.25 and 0.75), we discover the Cs₂B′In_0.75Bi_0.25I₆ (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′In_1-xBi_xI₆ 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.