The incorporation of GaCI3 into the usual CdCl2 post-deposition treatment solution of CdTe-based solar cells has been investigated. Both CdS and CdTe layers used in this work were prepared by electroplating technique and they are n-type in electrical conduction as observed from photoelectrochemical cell measurement technique. Before applying the chemical treatments to the device structures, the effect of GaCI3 incorporation into the usual CdCl2 treatment was first studied on the structural and optical properties of CdTe thin films. The results of the optical properties show that the bandgap of CdTe thin films treated with a mixture of GaCl3 + CdCl2 is closer to that of bulk CdTe layers than the ones treated only with CdCl2 solution. The structural properties also showed that CdTe thin films treated with GaCl3 + CdCl2 is more crystalline than CdTe thin films treated only with CdCl2 solution. The addition of GaCl3 into the CdCl2 solution have been seen to drastically enhance the solar-to-electric conversion efficiency of CdS/CdTe based solar cells. One of the effects of Ga incorporation into the usual CdCl2 treatment was seen in the series resistance reduction which ultimately leads to enhancement in the observed short-circuit current density, fill factor and overall solar cell efficiency. For the glass/FTO/n-CdS/n-CdTe device structures, the cell efficiencies were observed in the range 1.9-2.1% after being treated with CdCl2 solution only. When treated with CdCl2 + GaCl3, the efficiency increased to 6.1-6.4%. Subsequent study on multi-junction graded bandgap solar cells using the GaCl3 + CdCl2 chemical solution for the surface treatment of glass/FTO/n-ZnS/n-CdS/n-CdTe device structures results in solar cell efficiency >10%. (C) 2016 Elsevier B.V. All rights reserved.