For PbS electrode, a rapid photovoltage decay is noted com- pared to CdS electrode. When a CdS layer is coated on PbS layer, a notable slower voltage decay was observed and the voltage decay further slowed down with the increase of the thickness of the CdS layer. The lifetime values obtained from Fig. 3 using Eq. (1) are shown in the inset in Fig. 3 and the estimated lifetimes of PbS, CdS, 1PbS/1CdS, 1PbS/2CdS and 1PbS/3CdS were 0.02, 0.8, 0.08, 0.26, and 0.64 s at 0.25 V, respectively. It can be clearly observed that the lifetime of the quantum dot based on PbS is extremely low and the lifetime of PbS/CdS cells increase with the increases of number of CdS cycles.