T.W. Hagler

Nature of the primary photoexcitations in poly(arylene-vinylenes)

Abstract We present an overview of the optical properties and related experimental results obtained from poly( para -phenylene vinylene), PPV, and its soluble derivatives, and we critically examine the interpretation of these data in the context of both the band model and the exciton model of the electronic structure of these semiconducting and luminescent polymers. Results obtained from highly oriented and structurally ordered PPVs demonstrate that the observed disparities between the physical properties of PPV and the polydiacetylenes (PDAs) are not due to disorder; these differences are intrinsic and arise from the fundamental difference in the nature of the low energy photoexcited states: excitons in the PDAs and free carriers in the PPVs.

Measuring internal electric fields in organic light-emitting diodes using electroabsorption spectroscopy

A widely applicable electroabsorption technique to measure internal electric fields in organic light-emitting diodes is presented. The technique exploits the change in the a.c. electroabsorption response in the presence of a d.c. electric field. The electroabsorption signal is modulated at the fundamental frequency of the a.c. test signal, in addition to the usual modulation at the second harmonic frequency, when a d.c. bias is present. In metal/organic film/metal devices employing different metal contacts there is a built-in electric field in the organic film caused by the difference in work function between the two contacts. The electroabsorption response at the fundamental frequency of the applied a.c. bias is measured as a function of an external d.c. bias. The electroabsorption signal is nulled when the applied d.c. bias cancels the built-in electric field established by the different metals. We apply this technique to measure changes in metal–polymer Schottky barrier heights as a function of the contact metal. In metal/multiple organic films/metal structures the electroabsorption signals from the constituent organic films are identified spectroscopically and measured at both the fundamental and second harmonic frequency of the a.c. test signal. The amplitudes of the electroabsorption responses are then used to determine the a.c. and d.c. electric fields present in the organic layers. We apply this technique to determine the d.c. electric field distribution within a multi-layer organic light-emitting diode. These results highlight the general applicability of electroabsorption methods to probe internal electric fields in organic light-emitting diodes.

Optical Studies of Polyphenylenevinylene and Derivatives: Raman and Luminescence Spectroscopy

Abstract We have done Raman and luminescence measurements on a variety of PPV derivatives and oriented polymer blends. Changes in the observed Raman spectra for the polymers studied are consistent with molecular level changes in the polymer structure. Low frequency modes, which may be torsional modes of the polymer, have been observed for the first time. By comparison to site-selective luminescence spectra, it is found that at least two different phonons contribute to the vibrationally resolved luminescence spectra.

Luminescence Excitation Spectroscopy in Poly(p-Phenylene-Vinylene) Derivatives: Implications for a Semiconductor Band VS. Exciton Model in Conjugated Polymers

Abstract High-resolution luminescence excitation spectroscopy (site-selective flourescence) results are presented for two soluble derivatives of poly(p phenylene-vinylene): poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV), and poly(2,5-bis(cholestanoxy)-p-phenylene vinylene) (BCHA-PPV). Measurements have been performed on both thin films of these polymers, and dilute blends in polyethylene oriented by tensile drawing. By examining the effects of oxygen exposure in cast films, and the energy- dependence of the luminescence anisotropy in the oriented blends, we provide evidence that the electronic states in the absorption tail arise from localized states in an amorphous semiconductor.

Nature of the primary photo-excitations in poly(arylene-vinylenes)

Summary form only given. We present an overview of the optical properties and related experimental results obtained from poly(paraphenylene vinylene), PPV, and its soluble derivatives, and we critically examine the interpretation of these data in the context of both the band model and the exciton model of the electronic structure of these semiconducting and luminescent polymers. Results obtained from highly oriented and structurally ordered PPVs demonstrate that the observed disparities between the physical properties of PPV and the polydiacetylenes (PDAs) are not due to disorder; these differences are intrinsic and arise from the fundamental difference in the nature of the low energy photo-excited states: excitons in the PDAs and free carriers in the PPVs.