We have investigated the optical properties of CuIn1-xGaxSe2 (CIGS) thin film solar cells using their electroreflectance (ER) at room temperature. The ER spectra exhibited one broad and two narrow signal regions. Using the photoluminescence (PL) and photocurrent (PC) spectra, the peaks in the low-energy region (1.02-1.35 eV) can be assigned to the CIGS thin film. The PC results implied that the peaks in the high-energy region (2.10-2.52 eV) can be assigned to the CdS band-gap energy. Using the applied bias voltage, the broad signals in the 1.35-2.09 eV region can be assigned to the Franz-Keldysh oscillation (FKO) due to the internal electric field. The ER spectra exhibited a distorted CdS signal for the CIGS thin film solar cell with low shunt resistance and efficiency. (C) 2014 Elsevier B. V. All rights reserved.