Improved photovoltaic performance of quantum dot-sensitized solar cells based on highly electrocatalytic Ca-doped CuS counter electrodes

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Herein we report the fabrication of highly efficient Ca-doped CuS counter electrodes (CEs) in order to modify the photovoltaic characteristics of quantum dot-sensitized solar cells (QDSSCs). CuS CEs with different degrees of Ca doping are deposited on fluorine-doped tin oxide (FTO) coated glass surfaces via the widely adopted chemical bath deposition (CBD) method and used directly as CEs for TiO2/CdS/CdSe/ZnS photoelectrode based QDSSCs. The results indicate that the QDSSCs incorporating 20% Ca-doped CuS CEs exhibit an excellent conversion efficiency (η) of 4.92%, short circuit current density (Jsc) of 15.47 mA cm−2, open circuit photovoltage (Voc) of 0.611 V, and fill factor (FF) of 0.521 under illumination of one sun (AM 1.5, 100 mW cm−2). These values are significantly higher than those obtained for QDSSCs incorporating bare CuS CEs (η = 3.51%, Jsc = 12.03 mA cm−2, Voc = 0.596 V, FF = 0.490) under similar conditions. The improved surface morphology of Ca-doped CuS materials offers more active sites for the catalytic reactions and an improved pathway for fast charge transport and a lower electron recombination rate for the electrolyte redox couple. The film thickness, elemental composition, crystallographic phase, surface topography, and chemical bond configurations of Ca-doped CuS CEs were characterized by scanning electron microscopy, inductively coupled plasma – atomic emission spectrometry, energy dispersive X-ray analysis, X-ray diffraction, atomic force microscopy, and X-ray photoelectron spectroscopy analysis. More importantly, impedance spectroscopy analysis and Tafel polarization curves indicate that Ca-doping on CuS CEs reduces charge transfer resistance and leads to high and consistent catalytic activity for the reduction of polysulfide electrolyte. This study demonstrates that an appropriate amount of Ca-doping of CuS CEs is an effective method of enhancing the performance of QDSSCs.