Band-gap energies and strain effects in CuIn1-xGaxS2 based solar cells

The optical properties of CuIn1-xGaxS2 (CIGS) polycrystalline films and solar cells were characterized by room and low (20 K) temperature photoreflectance (PR) and electroreflectance (ER) spectroscopy for two different compositions of the CIGS absorber ([Ga]/([In]+[Ga]) = 0.04 and 0.12). The E-a and E-b band energies of the three-split energy gap of ternary and quaternary absorbers were determined at room (300 K) and low (20 K) temperatures. In the ER spectra at 300 K, the E-a and E-b bands were split into four sub-bands originated by differently strained regions of the active absorber layer. The strain evolution in the layer was analysed with respect to the band separation. Modulated reflectance was proven to be a versatile tool for the investigation of material properties. The results of the present study are discussed together with the results of SEM and XRD studies.