CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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drastically reduced leading to an increase in the efficiency of the solar cell.
Additionally, the injection dynamics reveal that although dynamics followed at 550
nm and 900 nm are similar in terms of their lifetimes indicating similar origin to their
injection events, the contribution to the individual injection components is considerably
different. It can be clearly seen from the kinetics at two wavelengths that the fastest injection
components contribution increases when monitored at 900 nm compared to 550 nm. This
could be explained on the basis of the fact that while 550 nm wavelength corresponds to the
cation absorption which is solely dependent on the concentration of the cation produced,
where the molar extinction co-efficient of cation does not change. As a result kinetics at 500
nm gives the true picture of ET dynamics. However, at 900 nm optical density depends on
the concentration of electrons in the conductions band and also cross-section (extinction coefficient)
of the injected electron. The cross-section of injected electron depends on the
density of states at energetic position of the injected in the CB. So when the injected electron
lies in the higher energy level where the levels are more closely packed and has a higher
absorption coefficient as compared to the region near the minima of the conduction band.
The density of states near the conduction band minima is given by
( EdE )
*
2m
3/2
0 2 3
2
EdE
3.2
In the above equation, m is the effective mass of the electron and the rest of the symbols
have their usual meaning [3.15]. The above expression clearly shows that the density of
states nears the conduction band minima increase as we move higher in energy.