SnO2/ZnO composite dye-sensitized solar cells with graphene-based counter electrodes

We have pioneered in developing dye-sensitized solar cells with interconnected nanoparticles of semiconductors other than TiO2 and we found that despite impressive properties of semiconducting materials such as ZnO and SnO2 which should theoretically give better performances than those based on TiO2, they give quite inferior performances and we explained this unprecedented fact by considering faster recombination of injected electrons in the latter semiconductor nanoparticles than those in TiO2 nanoparticles. To circumvent this problem, we introduced a novel technology in which we covered the surfaces of interconnected nanoparticles of SnO2 by an ultra-thin layer of a wide band-gap semiconductor or an insulator where the thickness of the layer is similar to 1 nm or less such that electron injection from the excited dye molecules to the conduction band of the semiconductor nanoparticles is possible but injected electrons are incapable of penetrating the insulating barrier layer. In this way, we were able to suppress recombination significantly and such composite film based dye-sensitized solar cells gave efficiencies comparable to those based on TiO2 nanoparticles. We then realized that platinum used in dye-sensitized solar cell counter electrodes accounts for nearly 40% of the cost of these solar cells and hence limits their practical application. We then worked on alternative materials and found that carbon based counter electrodes can be tailored to give efficiencies closer to those of Pt counter electrode based solar cells. In extending this study we now worked on graphene as counter electrode material for composite SnO2/ZnO dye-sensitized solar cells. We report in this manuscript the optimizations of this low-cost counter electrode based dye-sensitized solar cell to give performance closer to those of expensive platinum based counterparts.