Improving Performance Of Dye-Sensitized Solar Cells By Controlling Morphologies Of TiO₂Nanocrystalline Materials

Dye-sensitized solar cells （DSCs） have received much attention for two decades due totheir advantages of low cost, easy fabrication, high light-to-electricity conversion efficiency,and environment-friendly features. Further increasing conversion efficiencies up to15%isthe key challenging in DSCs. TiO₂nanocrystalline porous films that function as adsorbing dyemolecules and transporting the photogenerated electrons injected from the dye molecules toconducting substrates are the key components in DSCs, which play an important role indetermining photocurrent densities and conversion efficiencies of the DSCs. Aiming at furtherincreasing the conversion efficiency of DSCs, this work focus on improving the electrontransport property of TiO₂porous films by controlling the morphology of TiO₂nanoparticles,and increasing the light capturing efficiency of TiO₂porous films by applying the scatteringlayer composed of TiO₂mesocrystal.In the first part of the current work, the morphology control of TiO₂nanoparticles wasrealized by adding （diethylenetriamine） DETA into the hydrothermal system where thetitanate nanotubes would transform into TiO₂nanocrystalline materials. The mixed TiO₂powders composed of rhombus particles with smaller size and rod-shaped particles withlarger size are obtained by controlling the amount of DETA added. The mixed TiO₂powdersand the powders composed of smaller TiO₂nanoparticles prepared from the system withoutDETA were applied to fabricate the TiO₂porous film photoanodes for DSCs. It is shown that,compared with the films composed of smaller TiO₂nanoparticles, the films made from themixed TiO₂powders adsorb comparable amount of dye but show better electron transportproperty. Consequently, the DSCs based on the photoanodes made from the mixed powdersexhibit larger photocurrent densities and higher conversion efficiencies than the DSCs basedof the photoanodes composed of smaller TiO₂nanoparticles.In the second part of this work, ellipsoid-shaped anatase TiO₂mesocrystals weresuccessfully synthesized by the solvothermal method, in which tetrabutyl titanate and aceticacid were used as the reactants. The mesocrystals are assemblies of crystallographicallyoriented TiO₂nanocrystals. The effect of tetrabutyl titanate concentration on the size andspecific surface area of the mesocrystals has been investigated, and the mesocrystals with thespecific surface area of as large as108m²/g are obtained. The adsorbed dye amountmeasurement and the electrochemical impedance spectrum analysis show that, the filmscomposed of the TiO₂mesocrystals exhibit good electron transport properties and excellentlight scattering ability, which are suitable to be used as the scattering layer in double-layer photoanodes for DSCs. Based on the comparison on the performances of the DSCs based onthe one-layer and double-layer photoanodes, respectively, it is revealed that the utilization ofthe film composed of the TiO₂mesocrystals as the top scattering layer improves the lightcapturing efficiency and the electron collecting efficiency of the photoanodes, and thuseffectively increase the photocurrent density and conversion efficiency of the DSCs.