Electrochemical impedance spectra of CdSe quantume dots sensitized nanocrystalline TiO2 solar cells

Quantum dots sensitized nanocrystalline TiO2 solar cells (QDSSCs) are promising third-generation photovoltaic devices. In comparison with conventional dye-sensitized solar cells (DSSCs), the efficiency of QDSSCs is still very low (about 3%). In this paper, the electrochemical impedance spectroscopy technology has been adopted to investigate the quasi-Fermi level and the carrier dynamics of the colloidal CdSe QDs sensitized TiO2 eletrode with S2-/S-x(-) redox electrolytes and the series resistance of the QDSSCs. In comparison with the conventional DSSCs with Gamma(3)/Gamma as redox electrolytes, the energy difference between the conduction band edge and the quasi-Fermi levels of the TiO2 films (or the Fermi levels of the redox electrolytes) in QDSSCs has been decreased by about 0.3 V, resulting in the decrease of V-proportional to by this value. The increases of the electrolyte diffussion resistance and the charge transfer resistance between Pt counter electrodes and S2-/S-x(-) redox electrolytes were attributed to the decrease of the fill factors. However, the electron lifetime and electron diffussion length for QDSSCs are longer than those for DSSCs due to the retardation of the electron recombination by the adsorbed cysteine at the surfaces of the TiO2 films. It is indicated that electron recombination at the TiO2/electrolyte interface is not the main reason for the lower J(sc) of the colloidal QDs sensitized QDSSCs. Improving light harvesting efficiency and photoelectron injection efficiency should be considered in the future for such kind of QDSSCs.