Highly Sensitive Absorption Measurements in Organic Thin Films: Charge Transfer Complexes and Light-Induced Changes
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Highly Sensitive Absorption Measurements in Organic Thin Films: Charge Transfer Complexes and Light-Induced Changes ANDREW SKUMANICH IBM Almaden Research Center, K67/802, 650 Harry Rd. San Jose, CA 95120.
ABSTRACT Using the sensitive absorption technique of Photothermal Deflection Spectroscopy (PI1S) it is possible to detect subtle spectral features in the absorption spectra of polymeric thin films that cannot be detected by other means. This paper describes observations of weak charge-transfer complex (C*I•) formation and also light-induced changes in various organic films. The filns studied were those commonly used as charge transport materials in organic photoconductors, comprised of a small transport molecule and a polymer binder. The optically measured CTC bands can be used to establish single electron energy levels of the constituents. Reproducible photo-induced changes are manifest both in the strong UV transitions as well as in the "sub-gap" absorption and appear to be correlated with the photo-chemical stability of the constituents. The light-fatigue has both a reversible as well as irreversible component. 1. Introduction An important signature of material properties is the absorption spectrum. For optical materials it becomes critical to determine the spectral properties to a high degree of sensitivity. Spectral shifts may adversely affect. the behavior and performance of a given media. Of particular interest is the photo-stability since the materials may be repeatedly exposed to illumination. This is the case for the organic photoreceptors used in electrophotography, as well as non-linear organic compounds, and photorefractives. Various issues include, the short and long term stability to exposure, as well as the photobleaching and photo-conversion processes that are occurring. Often these may be effects that are hard to detect. In the case of two component photorcceptors, there may also be spectral shifts from the interaction of the substituents by CTC formation, which again, may affect the material properties. For weak absorption in thin fdms or for subtle spectral features, standard measurement techniques are inadequate. Ideally suited for this regime is photothermal deflection spectroscopy (l'DS) which can give several order of magnitude greater sensitivity than transmission techniques.2 This paper reports on CTC formation and light-fatigue in materials commonly used as charge transport materials in organic photoconductors and photorefractives. 2. Samples The thin films are comprised of a small transport molecule in a polymer binder. The small molecule (electron donating) set included: (I) dliethylaminobenzaldchyde diphEnythydrazone (DEl,1); (2) a related t 1 set based on DlFI with the phenyl count varying from I to 4, (labeled as DEll () thru DEll (4) or alternatively as MPII, DEll, MTPHI, TI'H respectively); (3) phenyl-diethylamino-styryl-pyrazolinc (DEASP), and methoxy-1)EASP (Mx-DEASP); (4) various carbazole derivatives; and (5) l).II mixed with an acceptor molecule nitrobcnzophcnonc (NPB). For
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