Functional layers of electrolyte and spectrum conversion for Dye-sensitized TiO2 solar cells

Abstract

Two functional layers have been developed for enhanced stability of Dye-sensitized TiO2 solar cells (DSSCs) : double layer gel-type electrolyte and spectrum conversion layer with 1,8-naphthalimide derivatives. In addition, a facile hierarchical microporous TiO2 film fabrication method has been introduced for easy infiltration of quasi-solid state electrolyte. The key element for long-term stability is the choice of sensitizing dye, electrolyte and UV cut-off filter, especially for outdoor condition. Ruthenium complex dye (N719) has been widely used due to high photo-to-current conversion efficiency and proved stability. However, there has been contradiction between photo-to-current conversion efficiency and long-term stability in the choice of electrolyte and UV cut-off filter. To enhance the photo-to-current conversion efficiency of long-term stable UV-cured polymer gel electrolyte, additional nano-gel electrolyte layer was introduced between the above-mentioned electrolyte and counter electrode. Resultant double layer gel-type electrolyte showed a lower charge transfer resistance due to the increase of exchange-reaction-based charge diffusion effect despite the negative influence from increase of electrolyte thickness. A crucial factor using quasi-solid state electrolyte is decreasing contact resistance between TiO2 nanoparticulate film and electrolyte. To accomplish easy infiltration of electrolyte into the TiO2 nanoparticulate film, micro-crater structure was formed using acetylene-black as an evaporated material at high temperature. This morphology control improved light scattering effect without using additional light scattering layer. Titanium dioxide nanoparticles provide the support for binding of sensitizing dye, however, in case of UV light irradiation, they act as a photocatalyst inducing the degradation of sensitizing dye and decrease of redox couple. To convert damaging UV light into useful longer wavelength light in advance, additional spectrum conversion layer with UV absorbing 1,8-naphthalimide derivatives was introduced on the photo-anode. This functional layer absorbed effectively the UV light in advance and emitted a useful longer wavelength light by Stokes fluorescence. In this research, three new methods were developed using Dye-sensitized TiO2 solar cells with Ruthenium complex dye to avoid unexpected influence from environment. However, these new techniques can be applied extensively to the next generation solar cells regardless of the kind of device to improve the long-term stability while maintaining high photo-to-current conversion efficiency.