Photoluminescence and Odmr Studies of Pristine and Photodegraded Poly (3-Hexylthiophene) Films and Solutions
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PHOTOLUMINESCENCE AND ODMR STUDIES OF PRISTINE AND PHOTODEGRADED POLY(3-HEXYLTHIOPHENE) FILMS AND SOLUTIONS L. S. SWANSON, * J. SHINAR, * L. R. LICHTY, * AND K. YOSHINO** *Ames Laboratory - USDOE and Physics Department, Iowa State University, Ames, IA 50011-3020 **Department of Electrical Engineering, Faculty of Engineering, Osaka University, Osaka 565, Japan. ABSTRACT The photoluminescence (FL) and optically detected magnetic resonance (ODMR) spectra of poly(3-hexylthiophene) in solutions of CCI 4 , toluene, and CH2 C12 , and films cast from these solutions, are described and discussed. In solution, the PL yield is 7%, and increases with increasing temperature. In oxygen-free, protected films cast from these solutions, the room-temperature yield is -0.7%, the spectra is red shifted by -0.5eV, and strong peaks at 1.52, 1.69, and 1.86eV appear. In films and CH2 Cl 2 solutions, the ODMR is resolvable into narrow and broad components, and its intensity A./L is -5x10-i at room temperature. A strong photoinduced oxidation process quenches the PL upon exposure to 02 but not water, at a rate possibly limited by 02 diffusion. INTRODUCTION The photoexcitations in polythiophene (PT) and its 3-alkyl derivatives, a prototypical weakly nondegenerate ground state n-conjugated polymer, have been the subject of numerous studies 11-13]. Thus, the nonradiative decay channels of photogenerated e-h pairs into polarons within a few picoseconds, and of polarons into bipolarons within a few milliseconds, are well established [1,2,4,9,13]. Similar processes are observed during oxidative p-doping [3,8]. Yet the nature of the radiative decay processes is still not well understood. The observations of anisotropic photoluminescence (PL) in stretch oriented films 16] and an enhanced PL yield of -7% in dilute solutions vs. films [111 provide strong evidence that the radiative channel is intrachain, and enhanced by conformational defects [10,15-18] that reduce the conjugation length. Yet while previous studies of Kaneto and coworkers [5-7,14] provided evidence that the PL is due to exciton decay, recent optically detected magnetic resonance (ODMR) measurements support the polaron-to-band channel [19-21]. This work, while not offering a conclusive verdict on this issue, provides additional results on poly(3-hexylthiophene) (P3HT) relevant to this problem. In particular, the temperature dependence of the PL and ODMR spectra in solutions and films is described, as well as the photoinduced oxidation process, which quenches the PL and the ODMR. EXPERIMENTAL PROCEDURE Free standing films of P3HT were prepared as described elsewhere [14,15). These films were stored in the dark, but not in vacuo. Thus, to clean the films from physisorbed oxygen, the free standing films were dissolved in CCl4 , toluene, or CH2 C12 , and the solution was then degassed by several cooling-pumping cycles. Clean films were then deposited by casting onto quartz substrates, in vacuo or a protective inert atmosphere, and sealed in quartz tubes. Degassed solutions were also seale
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