JOURNAL OF LUMINESCENCE ELSEVIER Journal of Luminescence 76&77 (1998) 416-419 Excitation dynamics in a-sexithiophene single crystals and UHV-grown films M. HopmeieP*, R.N. Marksb, R. Michelb, M. Muccinib, M. Murgiab, R. Zambonib, C. Talianib, G. Horowitzc, F. Garnierc, M. Oestreicha, R.F. Mahrt” ’ FB Ph.wik and Zenirum fir Material~,issenschafien der Philipps-Unirersitiit Marburg. Rmthof5. 35032 Marburg. Germa~~>~ ’ CNR. Inst. di Spettroscopia Molecolare. Via P. Gohetti 101. Bologna. Ital> ’ Lahoratoire des Materiaux Moleculaires. Centre National de la Recherche Scientifrque, 2 rue Henri Dunant. 94320 ThtaiA. France Abstract We have investigated the disorder-determined optical properties of cc-sexithiophene by comparing measurements of an evaporated film and a single crystal. We observed for the first time sharp lines in absorption and photoluminescence from both samples representing the intrinsic electronic properties of the material. The time-integrated emission spectra of both samples are dominated by radiative trap emission, which is more pronounced in the film due to the higher degree of disorder. Photoluminescence excitation and site-selective photoluminescence measurements show the different origin and character of the traps in the samples. We attribute the single crystal traps to the so-called X-traps while the nature of the traps in the film is still undetermined. Time-resolved photoluminescence measurements of the film show a fast transfer process of the sharp bulk emitters to the unstructured trap emission within the first 50 ps after excitation. A comparable process is observable in the single crystal, showing a slower dynamics due to lower trap density. i: 1998 Elsevier Science B.V. All rights reserved. Keywords: Oligothiophene; Structural disorder; Optical properties; Time-resolved photoluminescence 1. Introduction The optical and electronic properties of con- jugated organic materials are actually a matter of intense research. Their possible application in devi- ces (light-emitting diodes, field-effect transistors) causes high interest in the fundamental processes determining the optical behaviour of this materials [l, 21. In this area, poly-thiophene and especially oligo-thiophenes play an important role for inves- *Corresponding author. Fax: (49) 642 1 287036; e-mail: Mar- kus.Hopmeier@physik.uni-marburg.de. tigating basic mechanisms of optical properties due to their well-defined structural properties in the solid state and their various and excellent prepara- tion possibilities [3-73. Furthermore, a direct pos- sibility to use these materials in device structures has been shown [8], In this work, we focus on the influence of structural disorder on optical proper- ties in a-sexithiophene (6 T). We investigated the optical behaviour of a 6 T single crystal and a dis- ordered 6 T evaporated film by a variety of optical measurement techniques. Our results show a strong influence due to structural disorder, masking the intrinsic optical properties. 0022.2313/98/$19.00 ‘C 1998 Elsevier Science B.V. All rights reserved PII SOO22-23 13(97)00229-9