CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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study. Based on the injection dynamics study we have arrived at the model for the injection
into the semiconductor considering the discreteness in the CB. For the sake of comparison
we compared the present results with that of bulk TiO 2 /alizarin, this revealed a
monoexponential pulse width limited injection. Furthermore, the BET dynamics revealed that
it is considerably slower on the B-TiO 2 compared to that of bulk TiO 2 system. This was
explained within the framework of Marcus inversion, where an increase free energy due to
upward shift of the conduction band levels and also its discrete nature which in turn slow
down the carrier relaxation time and eventually increase the lifetime of the electron at higher
energy level, and finally led to an decrease in the BET. These observations are significant as
previously it was believed that in a strong coupling dye the ET takes place via adiabatic
route, but the present set of results seem to indicate that ET event is nonadiabatic based on
the multiexponential injection. Based on this observation we can also predict that in a
relatively weaker binding dye the injection time will be still slower and the BET rate will be
more slow, which has tremendous implication for the solar cell application because the
organic dye have gone out of favor among the scientists working on the solar cell because
they have a very fast BET rate as compared to Ru-polypyridyl complexes. This study proves
that this problem can be solved using ultrasmall TiO 2 . Alternately one can design solar cells
with ultrasmall TiO 2 and Ru based sensitizer to further improve the efficiency from the
present best efficiency of 10%.
3.5. References
1. O’Regan, B.; Graetzel, M. Nature 1991, 353, 737.