Real-time optical waveguide measurements of dye adsorption into nanocrystalline TiO₂ films with relevance to dye-sensitized solar cells

Optical waveguide spectroscopy has been used to study the real-time adsorption of ruthenium 535-bisTBA (N-719) dye in mesoporous nanocrystalline titanium dioxide films of the type used in dye-sensitized solar cells (DSCs). Porous titania films were prepared on top of gold substrates, and prism coupling was used to create a guided wave in the nanocrystalline film. The conditions under which a guided mode can be excited are dependent on both the refractive index and the extinction coefficient of the mesoporous layer, where the mesoporous layer refers to both the nanocrystals of TiO2 and the composition of the pores. It was therefore possible to track changes in dye concentration in the pores in real time. The total concentration of dye in the film appeared to continue increasing even after 22 h, in contrast to the amount of dye in the pores that was able to absorb light at 632.8 nm, which saturated after 5 h. The total concentration of dye molecules was 2.47 10-4 mol cm-3 as a function of the total pore volume at equilibrium; assuming a regular array of spherical particles with a porosity of 0.5, this translates to 10-6 moles m-2. The value of surface coverage obtained from OWS is similar to that calculated by dye desorption studies and is close to the value previously reported for the N3 dye. (Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gratzel, M. J. Am. Chem. Soc. 2001, 123, 1613). A preliminary model that simulates the increase in dye as measured by optical modes has also been developed and shows that the data does not fit a single diffusion coefficient.