Electric Field Induced Ionization of the Exciton in Poly(Phenylene Vinylene)
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Electric Field Induced Ionization of the Exciton in Poly(phenylene vinylene) Jian Wang, Daniel Moses, Alan J. Heeger, N. Kirova1 and S. Brazovski1 Institute for Polymers and Organic Solids, University of California, Santa Barbara, CA 93103, U.S.A. 1 LPTMS, Bat.100, Universite Paris-Sud, 91405, Orsay-Cedex, France ABSTRACT The exciton binding energy (Eb) and the band gap energy (Eg) of poly(phenylene vinylene), PPV, have been determined by photoconductivity excitation profile spectroscopy as a function of light polarization, applied electric field, and temperature. The spectral signature of the exciton is a narrow peak (100 meV full width at half maximum) that emerges just below the band edge upon increasing the external field, the temperature or the defect density. The exciton peak is observed only for light polarized parallel to the chain axis. The exciton binding energy is obtained from the energy of the exciton peak with respect to the band edge and, independently from analysis of the field dependence of the exciton dissociation. It is Eb ≈ 60 meV. INTRODUCTION A central issue in the field of conjugated polymers is the strength of the electron-electron (elel) interaction relative to the bandwidth [1]: Is the attraction of a geminate electron-hole pair so strong that the photoexcitations are localized and strongly correlated Frenkel excitons? Or rather, are the charge carriers sufficiently well screened that a band picture supplemented by the electron-phonon interaction (polaron formation) and the el-el interaction (weakly bound excitons) is justified? Determination of the exciton binding energy (Eb) is critically important to answering these questions and thereby to understanding the electronic structure of semiconducting polymers. Traditionally, photoexcitation of carriers in conjugated polymers has been assumed to be a secondary process, resulting from dissociation of primary excitons [1-5]. Theoretical models of such a process predict a strong dependence of the carrier quantum yield on external field and temperature. However, the experimental results are in variance with these predictions; the carrier photogeneration mechanism is nearly independent of temperature [6-9] and external field in low to moderate fields [8]. Moreover, ultrafast (t < 100 fs) carrier photogeneration has been observed with relatively high quantum efficiency (~10%) implying direct photogeneration of charge carriers [10,11]. Nevertheless, the most basic aspects of the electronic structure of semiconducting polymers remain controversial; e.g. published values for the exciton binding energy in poly(phenylene vinylene), PPV, range from 0.1 - 0.9 eV [1,12-16]. We have addressed these questions through excitation profile spectroscopy of the steady state photocurrent (Iphoto) in oriented PPV at various external fields (F) and temperatures, and in samples with different defect concentrations. The spectral signature of the exciton is observed in the spectrum as a narrow peak that emerges just below the band edge upon increasing the external elect
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