These results strongly suggest that the Jsc of PbS/CdS q-dot solar cells can be increased by varying the thickness of the CdS layer on a same thick PbS layer. Hence, it can be suggested that the enhance in Jsc of PbS/CdS solar cell after coating of CdS on PbS is not due to light harvesting effect. The different role played by the CdS layer in PbS/CdS design could be that it can function as a passivation layer in minimizing the surface defects in PbS q-dots. For the verification of surface passivation properties of CdS on PbS surface, we performed photovoltage decay measurements and electrochemical impedance spectroscopy measurements of PbS/CdS photoanode with different thicknesses of PbS and CdS.