Specific capacitance of CoS encapsulated g-C3N4 core shell nanocomposite as extremely efficient counter electrode in quantum dots solar cells

In this context, the electrical conductivity and surface activity of cobalt sulphide (CoS) as counter electrode CE is improved by incorporating with graphitic carbon nitride (g-C3N4) to enhance the power conversion efficiency (PCE) of quantum dot sensitized solar cells (DSSCs). Core-shell nanocomposite CoS @g-C3N4 composed of CoS core encapsulated by graphitic carbon nitride shell in different weight ratios of CoS to constant ratio of g-C3N4 was synthesised. The prepared composites were assigned as 1:1 (CoS-1), 5:1 (CoS-5), 7:1(CoS-7), 9:1(CoS-9) and 11:1(CoS-11). The different prepared CoS@g-C3N4 nanocomposites were applied as counter electrodes (CEs) in QDSSCs based on TiO2 nanorod arrays with 84.37-nm length and 5.24-nm width as working electrode, cadmium sulphide (CdS) as quantum dots and polysulfide electrolyte. The results show that CoS-7 has superior efficiency with clear improvement in cell performance up to 10.15%, short current density J(sc) of 19.5 mA/cm(2) , open circuit voltage V-oc of 0.68 mV and fill factor (FF) of 0.78. The improved performance is attributed to the formation of synergistic heterojunction between g-C3N4 and CoS which facilitates a fast electron transfer at the interface between them; also, more photoelectrons could be produced due to the formation of carbon vacancies in g-C3N4 which made g-C3N4 to hold more excitable electrons and inhibit the recombination of photogenerated carriers. Matching the valence and conduction bands of CoS to those of g-C3N4 also catalyses collecting and mobility rate of the electrons and holes and decreases the recombination rate effectively. Cyclic voltammetry, impedance spectroscopy, the equivalent circuit, analysis of capacitance-frequency spectra, charge transfer spectra and charge recombination were also studied.