Abstract: The effect of low thickness of CdTe on optical, front, and back recombination losses and hence on the efficiency of a CdS/CdTe solar cell is studied theoretically in this work. It is found that the optical losses are about 23%–24% and depend weakly on CdTe thickness. The recombination losses are about 28% at dCdTe = 0.45 m and decrease to 23% at dCdTe = 1.1 m because the recombination losses have significant effects at thinner layers. The recorded efficiency is in the 9.5%–10.5% range corresponding to the thickness of CdTe of 0.45–1.1 mand it is considered in good agreement with experimental results. The electron diffusion length is in the range of 1.6–15.8 m and corresponds to 10−9–10−7 s of the electron lifetime is sufficient to make the current density reaches its maximum value (16.3 mA/cm2) at dCdTe = 1.1 mwith efficiency of 10.5%.When the CdTe thickness is assumed to be 5 m, which is often used in the fabrication of CdTe-based solar cells, the calculated current density is about 20 mA/cm2 and the corresponding efficiency is 13%. The present results lead to the shrinking of the gap between the theoretical and practical results and contribute to improving the efficiency of CdS/CdTe cells in the future.