Ellipsoidal TiO2 mesocrystals as bi-functional photoanode materials for dye-sensitized solar cells

Ellipsoidal TiO2 mesocrystals (MCs) with varying particle sizes depending on the amount of titanium precursor are synthesized by solvothermal method using acetic acid and tetrabutyl titanate. The large one (MC-1) is predominant 700 nm in length and 400 nm in width, while the small one (MC-2) is 250 nm in length and 100 nm in width. SEM and TEM images reveal that the mesocrystals exhibit single-crystalline-like structures, which are constructed by the orderly aggregating of tiny primary TiO2 nanocrystals. The specific surface areas of MC-1 and MC-2 are 119.5 and 132.4 m2/g respectively. The mesocrystals are potential photoanode materials in DSSCs, which can not only enhance the light scattering and dye adsorption, but also enhance the electron transport. The highest efficiency of 7.16% is achieved for the bi-layer NC-MC-2 DSSCs when the MC-2 is applied as a scattering layer on top of the nanocrystalline underlayer. The improvement of the efficiency is mainly attributed to the high specific surface area as well as the efficient light scattering, both of which result in the increase of light harvesting. Meanwhile, the electron transport of the photoanodes is also investigated. It is found that the electron transport of the single-crystalline-like mesocrystals is superior than nanoparticles, corresponding to the excellent energy conversion efficiency.