Size-dependent light-scattering effects of nanoporous TiO₂ spheres in dye-sensitized solar cells

Submicron-sized monodispersed TiO₂ spheres (SPs) with high porosity were synthesized by a controlled hydrolysis of titanium tetraisopropoxide (TTIP) and subsequent hydrothermal treatment at 230 °C. By adjusting the ratio of TTIP to water (the r-factor) in the hydrolysis reaction, the diameters of SPs were selectively controlled to 260, 350, 450, 560, 800, and 980 nm. The prepared SPs in the pure anatase phase were highly porous structures with crystallite sizes of ∼15 nm and surface areas of 101-121 m²g^-1. The synthesized nanoporous SPs in different sizes were then applied as the light-scattering layer (LSL) of dye-sensitized solar cells (DSCs) for efficient utilization of solar spectrum, and the size-dependent light-scattering effects of those SPs were systematically investigated. The 450 nm sized SP (SP450) provided the highest light-scattering efficiency among those in the 260-800 nm range. Relatively higher efficiency is caused by the characteristic light-scattering effect based on its unique diameter and also by the photonic reflection effect originating from its size-uniformity and long-range ordering. As a result the photovoltaic conversion efficiency (η) of DSC was improved from 6.92 to 9.04% with introducing the nanoporous SP450 as LSL.