Cu(In,Ga)Se-2 (CIGS) is one of the most promising light-absorbing materials. The high-efficiency CIGS solar cells normally have included a CdS buffer layer deposited by a chemical bath deposition (CBD) technique. To measure the thickness of the CBD-CdS, the quartz crystal microbalance (QCM) system was introduced in this study. Using this system, the effect of the CdS thickness (reaction temperature versus reaction time), the impact of the reaction rate with constant thickness (reaction temperature versus thickness), and the characteristics of the CdS annealing effect were investigated. When the reaction time was fixed, the efficiency was affected by the thickness of CdS. Nevertheless, when the thickness of CdS was fixed in the thickness of the best efficiency, the reaction temperature did not affect it. By annealing the CdS film, the cell performance was improved. In the case of the sample with a low reaction temperature (60 degrees C), the efficiency improvement was better than with different reaction temperatures because impurity of CdS was reduced more after the heat treatment. On the other hand, according to the XPS spectra, CdS with high temperatures had less impurity, so the annealing process had less impact on the efficiency. (C) 2015 Elsevier B.V. All rights reserved.