Enhanced photovoltaic performance of dye-sensitized solar cells based on TiO2 nanosheets/graphene composite films

Dye-sensitized solar cells (DSSCs) based on TiO2 nanosheets (TiO2-NSs)/graphene nanocomposite films were fabricated, and the effects of graphene on the microstructures and photoelectric conversion performance of the as-fabricated DSSC were investigated. The graphene loading clearly influences the textural properties (including specific surface areas, porosity and pore volume) and the optical absorption properties. Moreover, the charge transfer and transport versus the charge trapping and recombination is also affected by the graphene loading. As a consequence, the photoelectric conversion efficiency of the TiO2-NSs/graphene nanocomposite film electrodes can be improved to a great extent upon graphene loading, dependent on the loading amount of graphene. A moderate amount of graphene (<0.75 wt%) obviously enhanced the DSSC efficiency. Graphene not only reduced the electrolyte-electrode interfacial resistance and the charge recombination rate, but also enhanced the transport of electrons from the films to the fluorine doped tin oxide (FTO) substrates. Furthermore, the incorporated graphene improved the light harvesting and thus increased the number of photoinduced electrons. Also, the modified porous structures of the composite photoanode facilitated diffusion of the electrolyte in the cell, which in turn helped to regenerate the dye, which is important for the photovoltaic response of the solar cells. However, excessive graphene loading (>0.75 wt%) largely lowered the DSSC performance. Higher graphene loading not only impaired the crystallinity of the TiO2-NSs, but also shielded the light adsorption of the dyes and reduced the number of photogenerated electrons.