This work represents a theoretical analysis of a new type of thin film solar cells with structure glass/ITO/CdS/PbS/AL. The transmission spectrum was calculated based on the multi-reflections effect from all cell layers as well as absorption effect in both ITO and CdS layers. The calculations of spectral internal quantum efficiency were carried out based on the front and back surface recombination of PbS layer. The recombination losses in space-charge region were studied. The effect of thickness of the absorber layer on the calculations of short-circuit current density was studied under illumination condition of AM1.5 solar irradiation. The data of open circuit voltage, maximum voltage, maximum current density, fill factor, output power density and efficiency of CdS/PbS solar cell were estimated from the J–V characteristics curve under illumination effect. This study ignored the effect of metallic back contact and thus the incomplete absorption losses (transmission losses) take place at thin absorber layer. The obtained results showed that the 2 lm thickness of the absorber layer is not sufficient to absorb all the transmitted photons from the window layer (CdS) and the selection of theses thin thicknesses of PbS in order to correspond with experimental implementations of these devices. Both the optical and recombination losses lead to decrease the current density by 82% (JSC = 7.28 mA/cm2) at thickness 0.5 lm of PbS and these losses decreased to 67% (JSC = 16 mA/cm2) at thickness 2 lm. The output power density and the cell efficiency increased with increasing the thickness of effective layer. The maximum cell efficiency of 4.13% was obtained at thickness of 2 lm which is considered greater than those obtained experimentally.