Size-tunable mesoporous spherical TiO 2 (MS TiO 2 ) with a surface area of $110 m 2 g À1 have been prepared through combination of ''dilute mixing''-driven hydrolysis of titanium(iv) tetraethoxide and solvothermal treatment. The hierarchically structured MS TiO 2 are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and nitrogen sorption analysis. Using three different MS TiO 2 (587, 757, and 1554 nm in diameter) as a scattering overlayer on a transparent nanocrystalline TiO 2 film, bi-layered dye-sensitized solar cells (DSCs) have been fabricated. Since the MS TiO 2 particles are comprised of $10 nm nanocrystallites that cluster together to form large secondary spheres, they can function as light scatterers without sacrificing the surface area for dye-uptake. As a result, the present MS TiO 2 -based cells perform a noticeable improvement in the overall efficiency: maximum 9.37% versus 6.80% for the reference cell made of a TiO 2 nanocrystalline film. This extraordinary result is attributed to the dual effects of enhanced dye loading and light scattering.

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Phenyltrimethoxysilane (PTMS) was anchored onto the sensitized TiO 2 nanoparticles. This insulating molecular layer effectively inhibited the charge recombination at the interface of TiO 2 /electrolyte in the dye-sensitized solar cells (DSCs) without sacrificing the dye-loading capacity of the nanocrystalline TiO 2 . DSCs using PTMSmodified TiO 2 exhibited a short-circuit current (J SC ) of 15.9 mA/cm 2 , an open-circuit voltage (V OC ) of 789 mV, and a fill factor (FF) of 68.2%, yielding an overall conversion efficiency () of 8.55% under 100 mW/cm 2 illumination. The resulting cell efficiency was improved by ~10% as compared with the reference cell.