Effects of Graphene in Graphene/TiO2 Composite Films Applied to Solar Cell Photoelectrode

A simple and effective method has been developed to prepare a composite porous film that incorporates graphene sheets and anatase TiO2 nanoparticles. After sensitization, the films have been investigated as dye-sensitized solar cell photoelectrodes. The cell performances showed that the incorporation of an optimized graphene content of 1.2 wt % increases the power conversion efficiency by 12% due to the enhancement of the short-circuit current density (J(sc)). The photoelectrodes have been characterized by various techniques, and the cell functioning has been studied by impedance spectroscopy over a large applied potential range. The electronic structure, charge carrier lifetime (tau(n)), transport/collection time (tau(tr)), and electron transport parameters of the layers have been determined. We conclude that photoelectrodes with and without graphene show no limitation due to the transport of the I-/I-3(-) redox shuttle. The rate of the charge transfer (recombination) parasitic reaction is unchanged with the presence of graphene. The electron transport in the photoelectrode is significantly faster for the composite film due to a quantified 60% increase in the layer conductivity. However, we have also shown that the charge-carrier collection efficiency is very high even without graphene, and that this parameter is not key to explain the cell-performance enhancement. Graphene also increases the film specific internal surface area. The composite films have a higher dye loading. They exhibit a better solar light absorption and a J(sc) enlargement.