Sequential deposition as a route for efficient counter electrodes in quantum dot sensitized solar cells

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