Optical absorption and dispersion characterizations of CuGaSe2 thin films prepared by flash evaporation technique

Thin films of CuGaSe2 have been prepared by flash evaporation technique. The optical properties of the prepared films were investigated using spectrophotometric measurements of the transmittance and reflectance at normal incidence of light in the wavelength range from 400 to 2500 nm. The optical constants as refractive index, n, and absorption index, k, were calculated and found to be independent of film thickness in the range of the film thickness 132–423 nm. The analysis of the photon energy against the absorption coefficient showed three direct optical transitions (one of them is allowed while the others are forbidden). This direct transition was ascribed to the crystal field and spin orbital splitting of the upper most valence band. The crystal field and spin orbital splitting of CuGaSe2 were found to be − 0.15 eV and 0.45 eV, respectively. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple–DiDomenico (WD) model. The single oscillator energy (Eo), the dispersion energy (Ed), the high frequency dielectric constant (ε∞), the lattice dielectric constant (εL) and the ratio of free charge carrier concentration to the effective mass (N / m*) were estimated. The capacitance–voltage measurements of CuGaSe2/p-Si heterojunction showed that the diode is abrupt junction diode. The carrier concentration and the built-in voltage were estimated. The current–voltage characteristics of the device under illumination were investigated and photovoltaic properties of the device were evaluated.

Research Highlights
► Thin films of CuGaSe2 were successfully prepared by flash evaporation technique.
► Three direct optical transitions were obtained and ascribed to the crystal field and spin orbital splitting of the upper most valence band.
► CuGaSe2/p-Si is abrupt junction diode and the photovoltaic parameters were evaluated using J-V characteristics under illumination.