Temperature dependence of the optical energy band gap in CuIn3Se5 and CuGa3Se5

The temperature dependence of the optical band gap EG in bulk crystals of CuIn3Se5 and CuGa3Se5 has been analyzed by separately considering the contributions due to electron–phonon interaction and thermal expansion. For the former contribution, we use an expression related to the mean frequency of phonons, defined by a temperature θ, that participate in the shift of the fundamental energy gap with temperature. For the latter, a term that explicitly takes into account the temperature variation of the thermal expansion coefficient and the pressure dependence of the band gap is employed.