Size-Controlled Anatase Titania Single Crystals with Octahedron-like Morphology for Dye-Sensitized Solar Cells

Abstract

A simple hydrothermal method with titanium tetra-isopropoxide (TTIP) as a precursor and triethanolamine (TEOA) as a chelating agent enabled growth in the presence of a base (diethylamine, DEA) of anatase titania nanocrystals (HD1-HD5) of controlled size. DEA played a key role to expedite this growth, for which a biphasic crystal growth mechanism is proposed. The produced single crystals of titania show octahedron-like morphology with sizes in a broad range of 30-400 nm; a typical, extra large, octahedral single crystal (HD5) of length 410 nm and width 260 nm was obtained after repeating a sequential hydrothermal treatment using 1103 and then 1104 as a seed crystal. The nanocrystals of size similar to 30 nm (HD1) and similar to 300 nm (HD5) served as active layer and scattering layer, respectively, to fabricate N719-sensitized solar cells. These HD devices showed greater V-oc than devices of conventional nanoparticle (NP) type; the overall device performance of HD attained an efficiency of 10.2% power conversion at a total film thickness of 28 mu m, which is superior to that of a NP-based reference device (eta = 9.6%) optimized at a total film thickness of 18-20 mu m. According to results obtained from transient photoelectric and charge extraction measurements, this superior performance of HD devices relative to their NP counterparts is due to the more rapid electron transport and greater TiO2 potential.