Cadmium telluride is a promising material for large scale photovoltaic applications. In this paper we study CdS/CdTe heterojunction solar cells with electroreflectance spectroscopy. Both CdS and CdTe layers in solar cells were grown sequentially without intermediate processing by the close-space sublimation method. Electroreflectance measurements were performed in the temperature range of T = 100-300 K. Two solar cells were investigated with conversion efficiencies of 4.1% and 9.6%. The main focus in this work was to study the temperature dependent behavior of the broadening parameter and the bandgap energy of CdTe thin film in solar cells. Room temperature bandgap values of CdTe were E-g = 1.499 eV and E-g = 1.481 eV for higher and lower efficiency solar cells, respectively. Measured bandgap energies are lower than for single crystal CdTe. The formation of CdTe1-xSx solid solution layer on the surface of CdTe is proposed as a possible cause of lower bandgap energies. (C) 2013 Elsevier B.V. All rights reserved.