Carbon nitride transparent counter electrode prepared by magnetron sputtering for a dye-sensitized solar cell

Carbon nitride (CN_x) films supported on fluorine-doped tin oxide (FTO) glass are prepared by radio frequency magnetron sputtering, in which the film thicknesses are 90–100 nm, and the element components in the CN_x films are in the range of x = 0.15–0.25. The as-prepared CN_x is for the first time used as counter electrode for dye-sensitized solar cells (DSSCs), and show a preparation-temperature dependent electrochemical performance. X-ray photoelectron spectroscopy (XPS) demonstrates that there is a higher proportion of sp² CC and sp³ CN hybridized bonds in CN_x-500 (the sample treated at 500 °C) than in CN_x-RT (the sample without a heat treatment). It is proposed that the sp² CC and sp³ C–N hybridized bonds in the CN_x films are helpful for improving the electrocatalytic activities in DSSCs. Meanwhile, Raman spectra also prove that CN_x-500 has a relatively high graphitization level that means an increasing electrical conductivity. This further explains why the sample after the heat treatment has a higher electrochemical performance in DSSCs. In addition, the as-prepared CN_x counter electrodes have a good light transmittance in the visible light region. The results are meaningful for developing low-cost metal-free transparent counter electrodes for DSSCs.