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INTRODUCTION
Long chain conjugated polymers have come under recent scrutiny because
of their interesting electrical and optical properties [1,2]. In particular, their
potential use in waveguide structures and optical switching elements [3] has
focussed attention on their optical properties at wavelengths longer than the 7r -
7r* transition [4,5], which is typically in the 2.0 - 2.5 eV range. In order for
polymer waveguides to be practical, the attenuation must be a few db/cnt or
less at the wavelength of interest. Several sources of residual absorption in the
subgap region can determine the ultimate transparency of such materials. These
include: I. broad, Urbach-like tails on the ir - ir* edge due to disorder, strain,
internal electric fields, or other causes. 2. Overtones of localized vibrational
modes. 3. Residual absorptions induced by exciting the polymer with light
above the % - z* excitation energy. A number of recent observations of these
induced absorptions have been made which indicate that their spectral character
is consistent with that expected for trapped bipolarons [6]. Most of these
observations have been made in a chopped mode in the 5 to 10 Hz range. In
the present study we have used Photothermal Deflection Spectroscopy (PDS) to
study the long time behavior of these induced absorptions as they grow in
during excitation and as they decay when the pump light is removed. We find
that the extremely slow kinetic behavior of these excitations may, in fact, cause
them to become the dominant factor in the near-IR transparency.
While sub-gap absorption studies may be useful in understanding the
mechanisms of charge excitation and trapping in these materials, they may also
be a useful probe of their chemical stability. Small changes in local order or
chemical composition due to various degradation mechanisms like photo-
oxidation are likely to result in readily observable absorption changes in this
spectral region as opposed to areas where ir - ir* transitions dominate the
spectral characteristics. Extended measurements of some of the present samples
have indeed revealed shifts in the subgap absorption which have distinctly
different spectral dependences from the induced absorption peaks, suggesting
small chemical changes may be occuring.
EXPERIMENTAL
The samples studied here were produced by spin casting the polymers onto
pre-scribed fused silica substrates from precursor/solvent solutions. After
drying, these substrates were then broken into ~ 0.5 x 1.0 cm pieces in an inert
gas environment and loaded into 1 cm2 optical cuvettes containing Flourinert
(FC-75). This commercial flourocarbon solvent was de-oxygenated prior to
loading using a dry-Helium gas bubbling technique. After loading, the sample-
containing cuvette was hermetically sealed and transferred to the optical
spectrometer. Results on several polymers will be discussed here:
Polydiacetylene-4BCMU (PDA-4BCMU), poly(2-methoxy,5-(2’ ethyl-
hexoxy)-PPV (MEHPPV), and po!y(3-hexylthiophene) (P3HT).
General descriptions of the PDS technique and the specific apparatus
employed here are given in the companion paper presented here [7] and
elsewhere [8,9]. Except for a distinct OH overtone absorption in the quartz
substrates which varies in magnitude from manufacturer to manufacturer and
from sample to sample, the influence of the substrate is minimal in the present
measurements. In addition to verifying this assertion by making companion
measurements on substrates where the polymer had been stripped off, we note
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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/m1/4/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.