Recent developments of graphene-TiO2 composite nanomaterials as efficient photoelectrodes in dye-sensitized solar cells: A review

The energy demand is ever increasing on aspect of primary and secondary energy sources to fulfill the energy requirement in this century. High energy supply is required especially in the developing countries to sustain the lifestyle due to the growth in the world's population and techno-economic city. In fact, the sources of fossil fuel-based energy are limited and the use of fossil fuels can lead to the environmental pollution problem. Accordingly, the combustion of fossil fuels sources will give rise to the global warming issue, i.e., the greenhouse effect of carbon dioxide resulting in erratic weather patterns, such as floods, draughts, etc. In the last two decades, solar photovoltaic cells have been used as the alternatives to generate renewable, sustainable and green energy. High dye absorption rates, efficient charge separation of exciton, and free charge carrier formation as well as large area production at low costs contribute to the emerging photovoltaic technologies. Generally, the materials involved in photoanode section (acting as the platform for the electron collection) are the main key to produce high-efficiency photovoltaic cells. Currently, multijunction based solar panels showed the highest performance of 40% efficiency as compared to that of the conventional Si based solar panels with 22-27% efficiency. Apart of that, DSSCs sensitized by the silyl-anchor and carboxy-anchor dyes have attained the highest efficiency of 14%. In recent years, the metal doping or binary oxide photo-catalyst systems, particularly graphene-TiO2 nanocomposites, have attracted much attention due to their extraordinary characteristics, i.e., (i) photocatalyst activity enhancement, and (ii) accelerated electron mobility to reduce the charge recombination. The high efficiency of the graphene-TiO2 nanocomposites in DSSCs' reaction basically requires a suitable architecture that minimizes electron loss during excitation state and maximizes photon absorption. In order to further improve the immigration of photo-induced charge carriers during excitation state, considerable efforts have to be exerted to further improve the charge mobility and maximize the dye loading. It has also been reported that the properties of graphene-TiO2 nanocomposites primarily depend on the nature of the preparation method, and the role of optimum dopants content incorporated into the graphene photoanode to improve the dye molecules loading and minimize the charge recombination. Therefore, we review recent achievements for different nanoarchitecture of TiO2, as well as graphene-TiO2 composite nanostructured based photoanode in DSSCs performance with future prospects and challenges. Indeed, graphene-TiO2 composite photoanode has proven to be of great interest for use in DSSCs.