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

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-/I-3(-) 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 I-3(-) 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.