Synthesis Of Nitrogen-doped Graphene And Its Application As A Cathode Material For Dye-sensitized Solar Cells

Solar energy is an inexhaustible energy resource. In recent years, tremendous investigations have been conducted for exploring photovoltaic materials. Dye sensitized solar cell(DSC) has attracted more and more attentions because of low cost, easy-fabrication, and high photoelectric conversion efficiency. Recently, many carbon electrode materials in place of traditional platinum cathodes have been developed. When combining with iodine-free electrolyte, carbon based electrodes in DSSCs give rise to higher power conversion efficiency and lower fabrication cost. Graphene is featured as good conductivity, high catalytic activity.These advantages make it to be a promising material for preparing Pt-free counter electrode.However, graphene has no band gap, which impedes the effective modulation on its conductivity. Also the surfaces of graphene are smooth and inactive. As a result, the adhesion of graphene onto other materials is more or less difficult.In this thesis, nitrogen-doping onto graphene framework is employed for improving its catalytic activity. This attempt is realized by using both hydrothermal preparation and pyrolysis method. In the first pathway, pristine graphene oxide is reacted with urea in an inhomogenous solution. This method prevents the gelation of graphene and leads to nitrogen content of 7.6 at%. According to TEM measurements, the resultant nitrogen doped graphene can be exfoliated easily into nanosheets, which facilitates the fabrication of nitrogen-doped graphene cathode. As for the second method, melamine is employed as nitrogen source during the pyrolysis process. A higher nitrogen content of 10.43 at % is achieved.XPS analysis on nitrogen doped graphene indicates the pyridine-N groups, which is electrochemically active, is thermally unstable at the early stage of the hydrothermal process.Heat stability can be improved by extending the hydrothermal reaction time. We found the nitrogen doped graphene with the hydrothermal reaction time of 12 h gives rise to a better electrochemical activity. And the electrochemical activity is highly relative to the atomic ratio of nitrogen to oxygen. After annealing at 350 o C, the nitrogen doped graphene cathode yields a higher photoelectric conversion efficiency of 8.2%, which is beyond the photovoltaic performance of the DSC with reduced graphene oxide cathode. This result can be explained as the significant increase of the exchange current density owing to the embedding of nitrogen atoms into graphene framework.