CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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Therefore the study of interfacial electron transfer dynamics between the dye and
semiconductor could give information not only on the dynamics but it could also throw light
on the relationship between the dynamical variables and the efficiency of the solar cell. The
studies on the dye sensitized TiO 2 revealed that the injection times for most of the dyes take
place in ~ 100fs [3.3-3.4]. This is due to the fact that the TiO 2 has a very large density of
states owing to a very large electron effective mass, leading to a large electron coupling
between the semiconductor and the dyes. However, the back electron transfer times show
considerable variation between different dyes. The time scales for the back electron transfer
process ranges from hundreds of fs to milliseconds [3.5-3.13]. Electron transfer parameters
are very important factors for designing an efficient solar cell. One of the dyes that have been
widely studied for sensitization is alizarin, due to the fact that this dye has absorption from
400-500 nm and is known to form a strong complex with TiO 2 . This leads to an interesting
phenomenon, i.e. formation of a charge transfer complex on the surface of TiO 2 [3.3, 3.4,
3.14]. This type of CT complex leads to a direct injection of the electron from the HOMO of
the dye to the conduction band of the electron. But this issue to still not understand well,
because recent theoretical results show that even in a very strong binding dye like alizarin
molecule, electron transfer takes place from the LUMO of the dye to the TiO 2 conduction
band rather than via a CT mechanism. The electron transfer in this case takes place
adiabatically within 6 fs, which is expected of a strong binding dye [3.4]. Huber et al [3.4] in
the study of electron injection from alizarin to TiO 2 reasoned that in a strong coupling dye
like alizarin, the ultrafast electron injection can be explained on the basis of adiabatic
process. The adiabaticity in the electron transfer process implies that the electron transfer
takes place in the same Born-Oppenheimer surface and therefore is less dependent on the