Preparation of brookite TiO2 nanoparticles with small sizes and the improved photovoltaic performance of brookite-based dye-sensitized solar cells

Abstract

Brookite TiO₂ nanoparticles with small sizes (hereafter denoted as BTP particles) were synthesized through the hydrothermal treatment of TiCl₄ solution with Pb(NO₃)₂ as an additive. The obtained BTP particles have a large specific surface area (∼122.2 m² g⁻¹) and relatively uniform particle sizes (∼10 nm) with the coexistence of a small quantity of nanorods with a length of ∼100 nm. When used as a photoanode material for dye-sensitized solar cells (DSSCs), the BTP particles show a much higher dye-loading content than the brookite TiO₂ quasi nanocubes (denoted as BTN particles) with a mean size of ∼50 nm and a specific surface area of ∼34.2 m² g⁻¹ that were prepared through a similar hydrothermal process but without the addition of Pb(NO₃)₂. The fabricated BTP film-based solar cell with an optimized film thickness gives a conversion efficiency up to 6.36% with a 74% improvement when compared to the BTN film-based one (3.65%) under AM 1.5G one sun irradiation, while the corresponding bilayer brookite-based solar cell by using brookite TiO₂ submicrometer particles as an overlayer of the BTP film displays a significantly enhanced efficiency of 7.64%. Both of them exceed the current record (5.97%) for the conversion efficiency of pure brookite-based DSSCs reported in the literature. The present results not only demonstrate a really simple synthesis of brookite TiO₂ nanoparticles with both high phase purity and a large surface area, but also offer an efficient approach to improve the photovoltaic performance of brookite-based solar cells by offsetting brookite's inherent shortages such as lower dye-loading and poor conductivity as compared to anatase.