First principles calculation of isolated intermediate bands formation in a transition metal-doped chalcopyrite-type semiconductor

Density Functional Theory (DFT) calculations at the GGA level have been carried out for Ti-substituted chalcopyrite-type CuGaS2 as it might constitute an intermediate band material of the kind that has been proposed to lead to enhanced efficiency photovoltaic cells. According to these calculations an intermediate band appears when Ti substitutes Ga at a 25% level in this structure, resulting in a magnetic half-metallic compound. This intermediate band slightly overlaps the conduction band and, when a higher accuracy calculation approach like the introduction of a Hubbard-type empirical correction is used (GGA + U method), it splits leaving a filled narrow band, well isolated inside the band gap. Considering the nanocrystalline form in which these chalcopyrite-type compounds are used in solar cells, an assessment of the effects of a small crystal size in this system have been carried out with a slab model. In this calculation a decreased bandgap width is observed, which can be as a result of surface termination effects. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.