Crystalline ordered states of CuIn1−xGaxSe2 (x = 0, 0.3, and 1.0) thin-films on different substrates investigated by Raman scattering spectroscopy

Abstract

Structural properties of Cu(In,Ga)Se2 absorber layers have been examined for photovoltaic applications. Thin-films with three different chemical compositions, CuInSe2 (CIS), Cu(In,Ga)Se2 (CIGS) and CuGaSe2 (CGS), were grown by co-evaporation on two kinds of substrates: Mo-coated soda-lime glass and bare soda-lime glass. Intriguing morphology and grain-growth behaviors were found in the surface of the films. X-ray diffraction of the films exhibited phase formation of the stoichiometric chalcopyrite phase of the materials while signs of secondary phases like Cu2Se and Cu–Se2 were also observed. The optical transmittance of the films was measured to obtain their optical bandgaps, which were well matched with the bulk values of CIS, CIGS, and CGS, which are 1.1, 1.4, and 1.7 eV, respectively. Using Raman scattering spectroscopy, the A1 mode was observed to shift from 177 cm−1 for CIS to 189 cm−1 for CGS as the Ga content increased. The films on Mo substrates are likely to have secondary phases, which is not the case for soda-lime glass. An indication of the formation of the CuAu structure is obtained from the CIS thin-films.