A study on the optical and microstructural characteristics of quaternary Cu(In,Ga)Se2 polycrystalline thin films

The optical and microstructural properties of polycrystalline CuIn _1-yGa_ySe₂ (CIGS) thin film deposited by coevaporation are reported within the boundaries of an orthogonal experimental design investigating the effects of Cu flux, Ga/(Ga+In) composition. Se rate, substrate temperature, T_s and substrate type. The optical bandgaps for near-stoichiometric CuIn_1-yGa_ySe₂ are smaller and exhibit bowing behavior which follows the relationship E_g=1.011+0.664y+0.249y(y -1). In comparison, Cu-poor films exhibit a linear variation with zero bowing given by E_g=1.0032+0.71369y. The increase in E_g with decreasing Cu may result in part from lattice shrinkage as measured by X-ray diffraction (XRD). Optical absorption below the band edge appears to be dependent upon both Cu and Ga content. Absorption coefficients of α⩾10³ cm^-1 within this region are indicative of Cu-rich films. Absorption ⩽10³ cm^-1 may be dictated more by surface morphology and possible phase separation in films containing ⩾50% Ga. The magnitude of α varies from ≅2×10⁴ near the band edge tip to 10⁵ cm^-1 at 1 ev above the edge for near-stoichiometric films, with the absorption in Cu-poor films being slightly less