Dye Regeneration by Spiro-MeOTAD in Solid State Dye-Sensitized Solar Cells Studied by Photoinduced Absorption Spectroscopy and Spectroelectrochemistry

Photoinduced absorption spectroscopy is presented as a tool for the systematic study of dye regeneration and pore filling in solid state dye-sensitized solar cells. Oxidn. potentials and extinction coeffs. for oxidized species of the perylene dye, ID28, on TiO2 and of the hole conductor, 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD), were detd. by spectroelectrochem. The onset of oxidn. of a solid film of spiro-MeOTAD was found to be 0.15 V vs. ferrocene/ferrocenium and extinction coeffs. of spiro-MeOTAD+ were found to be 33,000 M-1/cm-1 at 507 nm and 8500 M-1/cm-1 at 690 nm. Electrons in TiO2 films were shown to alter the ground-state absorption spectra of ID28 attached to TiO2. Photoinduced absorption measurements indicated a good contact between ID28 and spiro-MeOTAD for different spiro-MeOTAD concns. for both 2- and 6-μm thick TiO2 films. We discuss the possibility of estg. the quality of pore filling from the positions of absorption peaks. Results suggested that with a spiro-MeOTAD concn. of 300 mg/mL in chlorobenzene, a uniform distribution of spiro-MeOTAD in the pores of the 6-μm thick TiO2 film could be achieved.