Structural Design And Interfacial Modification Of TiO₂ Nanocrystals For Dye-sensitized Solar Cells

Dye-sensitized solar cells(DSSCs) with the advantages of low-cost, simple manufacture procedure and high theoretical energy conversion efficiencies have been recognized as a promising alternative to conventional silicon-based solar cells. A semiconductor porous film with TiO2 nanoparticles possesses a large specific surface,and plays an important role in dye absorption and electron transfer of DSSCs. The photoelectric conversion efficiency is still limited by some problems in the photoanode film: the porous film with tiny nanoparticles shows a relatively high transmittance to the solar light and utilizes some of the incident light, leading to a low light harvesting efficiency; the film have abundant defects and grain boundaries,which impedes electron transfer seriously; the porous nature of the TiO2 layer could leave a portion of FTO conducting surface uncovered, which allows the redox electrolyte solution to penetrate the porous structure to reach the uncovered FTO surface, and further leads to electron leakage. To overcome these problem, we focus on the controllable structural design, preparation and interfacial modification of titanium dioxide nanocrystals.(1) Hierarchical scattering TiO2 particles were synthesized by a simple hydrothermal method. The mixture solution of low-cost inorganic salt of Ti(SO4)2 and(NH4)2SO4 was hydrothermal treated 120 °C for 15 h, and then hierarchical TiO2 particles were formed after a sintering at 600 °C for 2 h. The as-prepared spheres(150-200 nm) assembled by closely connected nanocrystals had a relatively high specific surface area of 43.2 m2 g-1. By using TiO2 nanospheres as scattering layer on top of semi-transparent TiO2 layer, the power conversion efficiency of DSSCs improved from 7.24% to 9.07%. This great improvements compared to commercial scattering particles(8.33%) are mainly attributed to the higher surface area for increasing the loading of dye molecule.(2) Well-connected anatase TiO2 nanocrystals were synthesized by one-stepthermal decomposition of the double salt ammonium titanyl sulfate at 700 °C for 2 h.The as-prepared TiO2 nanocrystals with a crystalline size of 15~30 nm had a relatively high specific surface area of 72.82 m2 g-1 for adsorbing more dye. The TiO2 nanocrystals inside the aggregates were densely packed with every grains having well contacts to neighbor grains. The connected structure between the crystallites decreases the negative effects of electron grain boundary crossing and reduce recombination within the aggregate. The DSSC based on the TiO2 nanoparticles as the photoanode exhibited an excellent short-circuit current density(15.94 mA cm-2) and a highly efficient power conversion efficiency(8.16%), which is higher than that of commercial aero-process P25 nanoparticles.(3) Concave-convex nanostructured TiO2 blocking layer film with a thickness of100 nm was prepared on FTO by a liquid deposition process from(NH4)2TiF6. The as-prepared film composed of 10~30 nm TiO2 particles and with a concave-convex nanostructure had a high surface roughness. DSSCs with TiO2 film on top of FTO improved the conversion efficiency from 7.93 to 9.16%, mainly because of the prevention of FTO/I3- charge recombination and an improved contact between FTO and porous TiO2 film.