Measurements of Photo-Induced Changes in Conjugated Polymers
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Measurements of Photo-Induced Changes in
Conjugated Polymers
Carleton H. Seager and Michael B. Sinclair Sandia National Laboratories, Albuquerque, N. M. D. Mc Branch and A. J. Heeger University of California at Santa Barbara, Santa Barbara, G. L. Baker Bell Communications Research, Red Bank, N. J.
Ca.
ABSTRACT We have used the highly sensitive technique of Photothermal Deflection Spectroscopy (PDS) to measure changes in the infra-red absorption spectra of MEHPPV, P3HT and Polydiacetylene-4BCMU induced by pumping these polymers with light above the ir - it* transition energy. In contrast to previous chopped light transmission measurements of these effects, the PDS technique can directly measure the buildup or decay of the absorption coefficient, m, on the time scale of seconds to days. In the case of MEHPPV we observe that above-gap light causes the appearance of a broad infra-red peak in a, which continues to grow-in hours after the pump light is first applied. For this polymer the general shape of the absorption spectra in the unpumped state mimics the photo-induced changes, suggesting that remnant photo-induced states determine the maximum transparency observed under normal experimental conditions. For P3HT and to a lesser extent, MEHPPV, we also observe irreversible photo-induced absorption components which we tentatively identify with photo-induced oxidation of the polymer matrix.
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 r - n* 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/cm 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: 1. broad, Urbach-like tails on the 7r - iO* 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 7r - 7r* 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 excita
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