Resonance Raman study of interfacial electron transfer in dye sensitized solar cells

Resonance Raman spectroscopy was used in correlation with absorption and luminescence spectroscopy to study the ultrafast (∼100 fs) electron injection from dye molecules into colloidal suspensions of TiO2 nanoparticles. Cis-Ru(4,4'-dicarboxy-2,2' -bipyridine)2(NCS)2, commonly referred to as "N3," is the sensitizer molecule of choice in these studies due to its broad absorption band and highly efficient electron injection in the dye sensitized solar cells. The nature of the electronic states of N3 dye and the solvent effects on the efficiency of electron injection were the focus of this research.; Resonance Raman spectra revealed only slight perturbations of the ground state structure of the N3 dye upon adhering to the TiO2 colloidal nanoparticles in ethanol. There were also generally minor differences between the ground state structures of the adhered N3 in ethanol and acetonitrile, with the only interesting difference being a shift in the SCN stretch, which is a functional group of N3 not thought to be associated with binding, but rather with the electron injection. Raman cross sections, which provide information about the excited state dynamics of the dye, were similar for the free and adsorbed dye in ethanol, but were generally higher for the adsorbed dye in acetonitrile. It will be shown that these results, in correlation with luminescence and absorption spectra indicate that the solvent does affect the interfacial electron injection in the N3/TiO2 system.; No quenching of the N3 luminescence occurred in ethanol upon adhering to the surface of the TiO2 colloidal particles in this solvent. There was, however, considerable quenching of the N3 emission when the N3 was adhered to TiO2 in the presence of acetonitrile compared to that measured when ethanol was used as a solvent. This is consistent with electron injection in the acetonitrile, but none in the ethanol. Absorption spectra show only a very minor red shift of N3 upon adhesion to TiO2 in ethanol, but a dramatic apparent red shift in acetonitrile. The red shift in acetonitrile is consistent with strong electronic coupling between the dye and the semiconductor in the acetonitrile. This stronger coupling in acetonitrile helps to explain the better emission quenching on the colloidal nanoparticles in this solvent.; Finally, wavelength dependent depolarization ratios were found to be dispersed and varied dramatically from the value of 1/3 that is expected for resonance enhancement with only one excited state. This is consistent with literature reports that there are multiple states within N3's broad absorption band. It is also consistent with the fact that there is weak resonance Raman enhancement in N3's blue band, which could also result from multiple states.