Hydrothermally synthesized titania nanotubes as a promising electron transport medium in dye sensitized solar cells exhibiting a record efficiency of 7.6% for 1-D based devices

Abstract

Hydrothermally synthesized TiO₂ nanotubes (TNTs) with a diameter of approximately 10 nm and a length of 250 nm are successfully employed in dye-sensitized solar cells (DSSCs) based on N719 dye and iodide/triiodide electrolyte and exhibiting an efficiency of 7.6% at 1 sun illumination. Randomly oriented TiO₂ nanotubes are deposited on FTO glass by the electrophoretic deposition method, and the thickness of the TNT layer and hence the solar cell performance have been shown to depend on the deposition time and the nanotube concentration in the electrolyte solution. The highest efficiency is obtained for the solar cell fabricated with the 6-minute electrophoretically deposited TiO₂ film having a film thickness of ∼6 μm. These pristine TNT photoelectrodes exhibit a short-circuit current density (J_sc), an open-circuit voltage (V_oc), a fill factor (FF) and an efficiency (η) of 2.4 mA cm⁻², 899 mV, 78% and 1.7%, respectively. TiCl₄ treatment of pristine TNT photoelectrodes enhances the J_sc, V_oc, FF and η to 13.2 mA cm⁻², 819 mV, 70.4% and 7.6%, respectively. The TiCl₄ treatment is found to be vital for the enhancement of the solar cell performance of hydrothermally synthesised TiO₂ nanotube based devices. Enhancement of electron lifetime is noted after treatment of bare TiO₂ nanotubes with 0.5 M TiCl₄ solution. The electron transport resistance, electron lifetime and charge recombination properties of bare and TiCl₄ treated TiO₂ nanotubes are investigated by electrochemical impedance measurements. The increased performance of the DSSC fabricated with TiCl₄ treated TNTs is due to several factors favouring the enhancement of efficiency. These include the dye loading amount, the decrease in electron transport resistance, the increase in density of states and enhancement of light scattering.