Comparative photovoltaic and impedance spectroscopic study on carbon counter electrode based CdS quantum dot sensitized solar cell using polysulfide and iodide/triiodide as redox liquid electrolytes

•CdS quantum dots (QDs) prepared using SILAR method were used as sensitizer.•Photovoltaic and impedance spectroscopic measurements were performed on QDSSCs.•The device with polysulfide electrolyte showed a significant improvement in its performance.•The photovoltaic parameters of polysulfide based device were observed higher.•Polysulfide based device showed very large improvement in the values of Rs and Rsh. In this investigation we report comparative study on the quantum dot sensitized solar sells (QDSSCs) using polysulfide and iodide/triiodide as redox liquid electrolytes aimed for the improvement in performance of CdS sensitized solar cell. The CdS films were deposited upon bare FTO and nanocrystalline mesoporous TiO2 coated FTO substrates by following a successive ionic layer adsorption and reaction (SILAR) method. The films were characterized by using atomic force microscopy (AFM) measurements and UV–Visible (UV–vis) spectroscopy. Analysing the AFM images height of the nanocrystals was found to be less than 2.84 nm. The estimated size of the CdS quantum dots (QDs) obtained using the UV–vis data and hyperbolic band model (HBM) was 4.94 nm. Photovoltaic performance and impedance spectroscopic measurements were performed on these FTO/TiO2/CdS/Electrolyte/Carbon type QDSSCs under an AM1.5 illumination (100 mW.cm−2) and dark, respectively. The overall power conversion efficiency, short circuit current and the fill factor of polysulfide based device were observed higher than the iodide/triiodide based cell by 280%, 123%, and 95%, respectively. Improvement in series resistance (Rs) and shunt resistance (Rsh) values of polysulfide based device were observed by 830%, and 348%, respectively. The results achieved using low cost ploysulfide electrolyte and carbon as counter electrode are observed higher than the results of the recent attempts made by other research groups for enhancing performance of the CdS QDSSCs using platinum as counter electrode. The better performance of FTO/TiO2/CdS/Polysulfide/Carbon type QDSSC could be attributed to the higher hole-recovery rate of polysulfide leading to a lower probability for charge recombination and the higher electrocatalytic activity at the CE/electrolyte interface resulting the fast electron injection and transfer from CdS QDs into the photoanode substrate.