Synthetic Metals. 28 (1989) C335-C340 C335 PHOTOEXCITATION SPECTROSCOPY OF POLY(3-METHYLTHIOPHENE): ELECTRONIC BANDS AND VIBRATIONAL MODES J. POPLAWSKI and E. EHRENFREUND Department of Physics and Solid State Institute, Technion-Israel Institute of Technology, Haifa 32000, (Israel) S. GLENIS and A.J. FRANK Solar Energy Research Institute, 1617 Cole Boulevard, Golden, Colorado 80401 (U.S.A.) ABSTRACT Photoexcitation spectroscopy, including photoluminescence, resonant Raman scattering and photoinduced absorption in the energy range 0.1eV to 2.5eV, is reported for the conjugated polymer poly(3-methylthiophene). Photoinduced electronic bands and infrared active vibrations characteristic of bipolarons were observed. Amplitude mode analysis of the Raman and photoinduced infrared modes indicates for this non-degenerate ground state system a one-dimensional gap of ~2.2eV and an extrinsic component of ~0.22eV. Broad cw photoluminescence band with phonon side bands are observed below the band gap and is assigned to weakly bound excitons. INTRODUCTION Poly(3-methylthiophene) [PMT] is a derivative of polythiophene (PT) where a hydro- gen atom at the three carbon position on the thiophene ring is replaced by a CH 3 group (see Fig. 1). Both PMT and PT belong to the class of conjugated polymers with a non- degenerate ground state symmetry [1]. As a result, polarons and bipolarons should be the important non-linear excitations [2,3,4]. Studies of doped non-degenerate ground state conducting polymers by means of light induced electron spin resonance (LESR) and op- tical absorption spectrophotometry, showed indeed that polarons and bipolarons are the predominant charge excitations [5-9]. Photoinduced absorption (PA) and LESR studies of PT on a wide energy range (0.1 to 2.5eV) have shown that the long lived optically induced charged defects are probably bipolarons [10]. Recent photoinduced absorption and doping induced absorption studies of PMT in the infrared (<_ leV) have revealed similar spectra to that of PT. Therefore, it was concluded that the predominant charged defects in PMT are also bipolarons [5]. Bipolarons have also been suggested as the predominant charge storage mechanism in the related polymer polyselenophene [111. Photoexcited electron-hole pairs in PT give rise to strong photoluminescence (PL) due to long lived deeply trapped photocarriers [12,13,14]. From cw photoluminescence excitation 0379-6779/89/$3.50 © Elsevier Sequoia/Printed in The Netherlands