High-efficiency layered sulfur-doped reduced graphene oxide and carbon nanotube composite counter electrode for quantum dot sensitized solar cells

A high-efficiency layered sulfur-doped reduced graphene oxide and carbon nanotube composite counter electrode for quantum dot sensitized solar cells is prepared via ultrasonic dispersion followed with simultaneous reduction and doping by hydrothermal using Na2S as a reductant and sulfur source. Scanning and Transmission electron microscopy show the layered composite with carbon nanotube-bridges between reduced graphene sheets. An increase in sp(2)C/sp(3)C ratio of composite is revealed for the first time by the analysis of X-ray photoelectron spectra. Electrochemical tests show that the charge transfer resistance is 1.76 Omega. The power conversion efficiency of 5.70% is obtained for the quantum dot sensitized solar cell based the composite counter electrode, and is 18.26%, 76.47%, and 126.19% higher than those of unsulfured composite, reduced graphene, and carbon nanotube counter electrodes, respectively. This is mainly due to the improvement of morphological and property of sulfur-doped reduced graphene and carbon nanotube composite, which presents a high specific surface area, high conductivity, providing abundant catalytic active sites and fast electron transport channels for S-n(2-) reduction.