Thermodynamics of the formation of Ti- and Cr-doped CuGaS2 intermediate-band photovoltaic materials

Following previous electronic structure calculations that indicated that the substitution of part of the Ga atoms in CuGaS(2) chalcopyrite by Ti or Cr produces an intermediate band material that could make new photovoltaic cells of enhanced efficiency possible, new DFT calculations have now been carried out to assess the thermodynamic viability of these substituted structures. Calculations of the total energy, disorder entropy, and vibration (phonon) contributions are combined to obtain the free energy of formation of those materials relative to a combination of the most closely related known stable compounds into which they could decompose. It is found that, except for low substituent concentrations and high temperatures, the substituted structures are less stable than the combination of simpler compounds (the formation of which would thus be more favorable), but to a smaller extent than that previously found for Mn-doped GaAs, which can be prepared in the laboratory. The solubility is computed to be lower for Ti than for Cr; the latter also has a tendency to cluster.