9In this paper, we introduce the sequential deposition method for enhancing the 10 performance of nanocomposite counter electrodes (CEs) in quantum dot sensitized solar 11 cells (QDSCs). Here, various CEs are made by the deposition of CuS, PbS, and 12 CuS/PbS nanocomposite layers on an FTO (fluorine doped tin oxide) coated glass 13 substrate by simple successive ionic layer adsorption and reaction (SILAR) method. The 14 result indicates that efficiency of the cells with CuS/PbS CEs enhances by 2.65 % 15 compare with the bare CuS (0.99 %) or PbS (1.47 %) CEs. Also, no considerable 16 improvement is observed for PbS/CuS CEs (1.09 %), which indicates that the order of 17 PbS and CuS deposition is critical for efficiency enhancement. Here, efficiency of the 18 cells with CuS/Pbs CEs was demonstrated to be systematically enhanced by one by one 19 deposition of CuS and PbS layers (sequential deposition). Also, for the CdS/CdSe QD 20 sensitized cells and sequentially deposited CuS/PbS CEs, the cell performance of 3.58 % 21is obtained which is enhanced by more than three and two times compared with the 22 efficiency of the cells with CuS (0.99%) and PbS (1.47%) CEs, respectively. The fill 23 factor is obtained as 0.53 which is the maximum value for CuS/PbS CEs, to our 24 knowledge. Impedance spectroscopy (IS) is performed on the QDSCs in order to 25 2 understand the origin of the performance increment. Results indicate that the electron 26 transfer resistance is reduced in the case of sequentially deposited CEs, while the series 27 resistance is almost the same. This general study represents that sequential deposition 28 method could offer new opportunities for the simple fabrication of solution-processed 29CEs in order to make efficient QDSCs. 30

In this paper, we introduce graphene/CuS/PbS nanocomposite as an effective counter electrode (CE) for quantum dot sensitized solar cells (QDSCs). The result indicated that the catalytic performance of CuS/PbS CEs was enhanced by pre deposition of graphene (G) sheets on the fluorine doped tin oxide (FTO) coated glass substrates. Here, various CEs are made by the simple successive ionic layer adsorption and reaction (SILAR) method. The result indicates that the fill factor of the cells with FTO/G/CuS/PbS CEs enhances in compare with the FTO/CuS/PbS CEs. Here, for the CdS/CdSe QD sensitized cells and G/CuS/PbS CEs, the cells' efficiency and fill factor is obtained 3.21 % and 0.51 respectively which is enhanced compared to the CuS/PbS CEs with 2.54 % efficiency and 0.36 fill factor. The present results indicate that the compact structure of the CuS/PbS CEs, replaces with a porous structure after graphene pre deposition which is expected to enhance the electrolyte/CE interface and consequently the fill factor. According to the obtained results, in spite of the poor catalytic activity of the graphene in the polysulfide electrolyte, it simply modifies the morphology of the CuS/PbS CEs for enhancing their performance. The results also demonstrated that G pre deposition can be systematically applied to enhance the performance of CEs in QDSCs.