New Development Of Graphene And Its Application In TiO₂ Photoanode

Dye sensitized solar cells(DSSCs)have attracted lots of interests because of their relatively low cost,high efficiency and easy in fabrication.Although?13%conversion efficiency was obtained,further improvements are still necessary for competition with other kind of solar cells.The major bottleneck in achieving a high power conversion efficiency is the transport of photogenerated electrons across the TiO2 nanoparticle network,which competes with the charge recombination.To suppress the recombination and enhance the transport,graphene,the stable two-dimensional carbon nanomaterial with exceptional charge transport properties(blillions of times larger than that in TiO2),was introduced into DSSCs photoanode to efficiently collect photo-generated electrons and achieve high conversion efficiency.As such,in this project high quality and uniform graphene-TiO2 nanocomposite were produced and used as the photoanode of DSSCs under mild condition to achieve a high conversion efficiency.To achieve reduction under mild condition,graphene(RGO)are reduced from graphene oxide with vitamin C as a reducing agent in an aqueous solution through simple stirring and centrifugation to from graphene-TiO2 nanocomposite.Then,RGO-TiO2 films with varying amount of RGO were prepared as photoanode in DSSC.With the same film thickness,DSSC with 0.75 wt%RGO obtained 30.2%increase in conversion efficiency as compared to its no RGO counterpart.With detailed investigation on the interfacial electrical properties,RGO-TiO2 composites have longer electron life-time and longer electron diffusion length.This increase is attributed to the effect of graphene leading to the improved photogenerated electron transfer ability and reduced charge recombination.The effect of film thickness is investigated subsequently in order to obtain appropriate thickness of the RGO-TiO2 composite film for application in dye-sensitized solar cells.The result shows that the DSSCs with RGO-TiO2 composite film's thickness of 16 ?m get the highest conversion efficiency.Moreover,the size effect of graphene sheets on the cell performance was investigated.The experimental results indicated that cells loaded with the smaller graphene sheets yielded larger enhancement.The smaller graphene sheets improved the dye adsorption,leading to higher conversion efficiency.However,the reduction of GO to produce graphene cannot remove all structural defects introduced by the oxidation process and results in lots of basal-plane defects which seriously reduce its conductivity,thus limit the improvement in conversion efficiency.Exfoliated graphene sheets(EGS)are obtained using a simple liquid phase sonication and then mixed with TiO2 nanoparticles in dye sensitized solar cell photoanode to achieve higher electrical conductivity and faster electron transfer due to much fewer defects as compare to conventional reduced graphene oxide(RGO):ID/IG of 0.256 in EGS as compared with that of 1.128 in RGO.The EGS-TiO2 photoanode yields a conversion efficiency of 8.24%,over 19%increase comparing with that of RGO-TiO2 photoanode,and 43%increase over that of TiO2 alone under the strict identical preparation conditions.The increase is believed to come from the increased conductivity of EGS owing to fewer basal plane defects as there is no chemical reduction process in comparison with the case of RGO,and to the much better connectivity to the case of TiO2-only photoanode.