The influence of copper on the optical band gap of heterometallic iodido antimonates and bismuthates.

Halogenido metalates of heavy main group elements are versatile semiconductor materials with broad applications. Especially the iodido metalates generally show small optical band gaps, making them suitable for photovoltaics. However, the most promising results have been generated using toxic lead-based materials, raising environmental concerns, while the related and far less toxic bismuth compounds show band gaps too large for direct use in photovoltaics. The introduction of heterometals such as copper can significantly lower the band gap of iodido bismuthates and antimonates, but no clear trend could yet be established in this regard. In this work a short overview over all known copper iodido bismuthates and antimonates is given and this small family of compounds is expanded with nine charged as well as neutral complexes [EkMlIm(P(R)3)n]q- (E = Sb, Bi; M = Cu, Ag; R = Ph, o-tol). The compounds' crystal structures, stability and optical properties are investigated and compared to the findings of quantum chemical investigations. The main excitation is shown to be a copper to antimony or copper to bismuth charge transfer while the relative energetic position of the organic ligand orbitals influences the magnitude of the band gap. This reveals that the nature of the ligands and the coordination environment at the copper atom is crucial for designing new copper iodido antimonates and bismuthates with specific band gaps.