New complex intermediate band materials for highly efficient photovoltaic cells

Several complex compounds that result from inserting atoms of certain transition metals in known semiconductors are proposed as intermediate band materials that, used as light absorbers, would allow to increase substantially photovoltaic efficiency. In particular, these are sulphides containing indium in octahedral coordination as the spinel CdIn2S4 or scandium (Sc2S3, with a defect NaCl structure) with the trivalent element partially substituted by Ti or V, and the silicon clathrate of type II (having a fcc Bravais lattice with 34 Si atoms per primitive cell) in which V substitutes partially Si or Ag is occluded in the intracrystalline cavities. DFT calculations show that in several of these structures the added metal gives rise to a partially occupied, relatively narrow band located between the conduction and valence bands. Some of these structures have been synthesized experimentally, and we show UV-Vis-NIR spectra agreeing with the DFT predictions