Development of Dye-sensitized Solar Cells Fabricated with PVDF-HFP-type Polymeric Solid Electrolytes and Titanium Dioxide Nanotubes

Abstract

Dye-sensitized solar cells (DSCs) are photoelectrochemical cells consisting of mesoporous TiO₂ electrodes sensitized with organic dyes such as ruthenium dyes, Pt counter-electrodes, and I⁻/I₃⁻ redox electrolytes. Increasing the durability and power conversion efficiency of DSCs are critical goals that will have to be met before DSCs can be put on the market on a large scale. For increasing durability, we have developed new gel-type polymeric solid electrolytes (PSEs) based on poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) to reduce leakage of the electrolyte solution, which is one of the main factors of poor DSC durability. The use of PSEs, however, is almost always accompanied by a decrease in the short-circuit current density (J_sc). We then studied the electrochemical properties of two different kinds of DSCs to determine why the conversion efficiency is lower in PSE-based DSCs than in liquid electrolyte-based DSCs. The diffusion coefficient of I₃⁻ and the cell-gap (the distance between the surface of the transparent conducting oxide substrate for the TiO₂ electrode and that of the Pt counter-electrode) of DSCs were eventually understood to be key factors affecting the J_sc. This indicates that the design of the DSC structure is quite important for achieving higher conversion efficiency in a PSE-based DSC. Further, with the aim of increasing the power conversion efficiency, we have developed ultrahigh-aspect-ratio TiO₂ nanotubes (TNTs), made by anodic oxidation of Ti metals in an extremely dilute perchloric acid solution, to establish good carrier pathways. Unlike TiO₂ nanoparticles (NPs), TNTs of suitable dimensions serve as efficient light scatterers while also providing large surface areas for charge separation. We have succeeded in enhancing the power conversion efficiency of a DSC using a TiO₂ electrode with a new bilayer structure, which consists of a light-scattering TNT layer formed upon a light-absorbing NP layer. We also found a promising application for TNTs formed on Ti substrates, i.e. their use in the fabrication of flexible, back-side illuminated DSCs. In this review we present our recent research on DSCs fabricated with PVDF-HFP-type PSEs and ultrahigh-aspect-ratio TNTs.