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transmission measurements are more sensitive to changes in reflectivity than the
PDS technique, it is unlikely that these contributed to Voss et al’s data given the
thickness of their samples. While our 5a curves show an increase for hv <
0. 9eV, Voss et al report a second peak in 5T/T at 0.43 eV. They suggest that
these two peaks are due to transitions from bipolaron gap states photo-induced
by absorption in the tt - n* peak. More detailed data will be needed to improve
upon this conjecture.
Dark and pumped spectra for PDA-4BCMU are shown in Figure 5. The
photo-induced changes in absorption are clearly smaller than for MEHPPV and
data from thicker samples will be needed to more carefully defined the exact
spectral shape of the effect. As in the case of MEHPPV, the photo-induced
effects take hours to fully develop and decay, and it is clear from Figure 5 that
there are at least two spectral regions where marked enhancements of a are
seen.
The dependence of the induced absorption on pump beam intensity is
shown for MEHPPV in Figure 6 and PDA-4BCMU in Figure 7. In both cases
the values of 5a seen at ~l/2 hr are plotted, and, within the error of
measurement, they both scale as the square root of pump light intensity. An
observation such as this suggests the applicability of bimolecular recombination
kinetics:
d(Sa)/dt = G - B (5a)2 (1)
where G is the creation rate of defects, proportional to the pump light intensity,
and B is the volume recombination rate for the defects which are associated
with the induced absorption, 5a. The steady state solution of Equation 1 is 5a =
(G/B)0-5. The general solution is a hyperbolic tangent function of time which
is plotted in Figure 1. Clearly, the fit to the observed time dependence of 5a is
poor. Since the pump beam is strongly absorbed, we must account for that fact
that the G value depends on position in the film; in the strong absorption limit
this correction produces the somewhat slower time dependence shown in Figure
1, which does not adequately reproduce the extremely slow growth or decay
(not shown) of 5a.
These slow kinetics raise the issue of whether even long periods in the
dark leave remnants of photo-induced absorption. The a curve for a sample
that has been in the dark for two days (Figure 2) does show a small peak at
-1.35 eV, very much like the photo-induced peaks.
Dark and photo-induced spectra are shown for P3HT in Figure 8. After
sitting 2 days in the dark the absorption behavior has not gone back to the
virgin spectra in this case, and the final spectral shape is quite different from
either the virgin or the pumped curves. This suggests that irreversible chemical
or structural changes may be caused by extended pump light exposure.
Preliminary observations of P3HT films immersed in Flourinert that has not
been de-oxygenated show more severe degradation. These data only represent
scattered observations; a proper experimental approach to this issue has not been
attempted. Along these lines we have also observed that there are also small
remnant absortpions in MEHPPV which do not have the spectral characteristics
of the data shown in Figure 4. Again, not purging 02 from the Flourinert
seems to exacerbate this problem.
SUMMARY AND CONCLUSIONS
We have established that large changes in the sub-gap absorption of all the
measured polymers occur when they are exposed to light above the 7t -n*
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Seager, Carleton H.; Sinclair, Michael B.; Mc Branch, D.; Heeger, A. J. & Baker, G. L. Measurements of photo-induced changes in conjugated polymers, article, January 1, 1991; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1089307/m1/6/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.