Characterization by Ab Initio Calculations of an Intermediate Band Material Based on Chalcopyrite Semiconductors Substituted by Several Transition Metals

Recent work has proposed that enhanced efficiency can be achieved in photovoltaic cells through implementation of the intermediate band (IB) concept in which a narrow band inserted within the band gap of a semiconductor is used to make the combined absorption of two sub-band gap energy photons lead to the generation of current at the higher voltage corresponding to the full band gap. Trying to tell which materials could have the IB properties necessary for this, quantum calculations within density functional theory at the generalized gradient approximation level have been carried out in this work for chalcopyrite-type copper gallium sulphide structures where gallium is partially substituted by transition metal atoms (Ti, V, Cr, Mn), as these materials are presumed to be candidates for developing the IB electronic structure able to realize this concept. The computed electronic structures characterized by density-of-state curves and band dispersion diagrams, show transition metal-induced spin-polarized characteristics and additional bands appearing in the band gap of the parent sulphide compound. In the results reported here for these compounds, the multiplicity, degree of filling, and energy position of the electronic levels depends on the number of electrons and the electronegativity of the transition element, and they are analyzed in terms of the crystal field splitting produced by the crystalline structure in the metal $3d$ orbital manifold. For the Ti- and Cr-derived structures (the more interesting ones from the point of view of the intermediate band photovoltaic concept), the stability of these materials is also assessed by computing the energetics of their decomposition in appropriate, known stable compounds. Although this decomposition is found to be favorable, the corresponding energy difference is relatively small, and it is envisaged that they can be made effective experimentally. With these results, the suitability of these materials for use as IB compounds in photovoltaic cells is discussed, and the Ti-substituted one is proposed as the best candidate of this type.