Sustainable p-type copper selenide solar material with ultra-large absorption coefficient† †Electronic supplementary information (ESI) available: X-ray crystallographic data, in CIF format, for the single crystal structure refinements of Cu4TiSe4 at 300 K. Tables S1–S3 and Fig. S1–S9. see DOI: 10.1039/c8sc00873f

We report the synthesis of CTSe, a p-type titanium copper selenide semiconductor. Its band gap (1.15 eV) and its ultra-large absorption coefficient (105 cm–1) in the entire visible range make it a promising Earth-abundant solar absorber material.
Earth-abundant solar absorber materials with large optical absorption coefficients in the visible enable the fabrication of low-cost high-efficiency single and multi-junction thin-film solar cells. Here, we report a new p-type semiconductor, Cu4TiSe4 (CTSe), featuring indirect (1.15 eV) and direct (1.34 eV) band gaps in the optimal range for solar absorber materials. CTSe crystallizes in a new noncentrosymmetric cubic structure (space group F4[combining macron]3c) in which CuSe4 tetrahedra share edges and corners to form octahedral anionic clusters, [Cu4Se4]4–, which in turn share corners to build the three-dimensional framework, with Ti4+ ions located at tetrahedral interstices within the channels. The unique crystal structure and the Ti 3d orbital character of the conduction band of CTSe give rise to near-optimal band gap values and ultra-large absorption coefficients (larger than 105 cm–1) throughout the visible range, which are promising for scalable low-cost high-efficiency CTSe-based thin-film solar cells.