Half-filled intermediate bands in doped inorganic perovskites for solar cells.

Efficiency of solar cells can be improved by introducing intermediate bands. However, width and occupation requirements on the intermediate bands bring challenges for materials design. In this study, we systematically investigate the electronic structure of doped CsPbX (X = Cl, Br, or I). A screening in transition metals suggests that Cr and Mo doped perovskites have dispersive and half-filled intermediate bands, which is desirable for building solar cells. However, intermediate bands originated from degenerated d orbitals are easily split when the octahedral symmetry of the local chemical environment around the dopant is broken. To avoid this problem, we further perform a screening of non-transition metals aiming to generate sp orbital-based intermediate bands. It turns out that In and Ga can generate a half-filled intermediate band, which is not sensitive to the local symmetry. The estimated efficiency of In and Ga doped CsPbCl at a doping concentration of 8.3% is 57.97% and 54.21%, notably higher than the Shockley-Queisser limit (40.7%). Results presented here demonstrate the potential of intermediate band engineering in photovoltaic applications.