Study of a thiophene-based polymer for optoelectronic applications S. Cheylan a, * , A. Fraleoni-Morgera b , J. Puigdollers c , C. Voz c , L. Setti b , R. Alcubilla c , G. Badenes a , P. Costa-Bizzarri b , M. Lanzi b a ICFO, Institut de Cie `ncies Foto `niques, Edificio NEXUS II, c. Jordi Girona 29, 08034 Barcelona, Spain b Department of Industrial and Materials Chemistry, University of Bologna, V. Risorgimento 4, 40136 Bologna, Italy c Departamento de Ingenierı ´a Electro ´ nica, Universidad Polite ´cnica de Catalun ˜a, UPC, Campus Nord Edifici C4, c/ Jordi Girona 1-3, 08034 Barcelona, Spain Received 18 May 2005; received in revised form 14 September 2005; accepted 14 September 2005 Available online 21 October 2005 Abstract A thiophene-based conjugated polymer bearing a cyano group ( – CN) as a side chain substituent was successfully synthesized. The polymer evidences an excellent filmability from various organic solvents as well as an enhanced photoluminescence. The polymer has been characterized optically (Fourier Transformed Infrared spectroscopy, absorption and photoluminescence) in solution and in film, while X-ray diffraction measurements (XRD) of thin films were performed to investigate its bulk morphological features. From the absorption edge of the spectrum of a thin polymer film, the optical band gap of the polymer is estimated to be 2.0 eV, which corresponds to orange emission. Furthermore, a single layer light emitting diode (LED) was fabricated. The device produced bright stable electroluminescence at room temperature. All of the results indicate that this polymer is a promising emissive material for application in polymeric LEDs. D 2005 Elsevier B.V. All rights reserved. Keywords: Electroluminescence; Conjugated polymer; Optical properties; Thiophenes 1. Introduction Since the discovery of conductivity in polymers and certain organic molecules, remarkable progress has been made in synthesizing organic materials, in understanding their proper- ties and in developing them for use in electronic and optical devices [1,2]. Currently, polymer and organic light-emitting diodes (LEDs) [3], photovoltaic cells [4] and field effect transistors [5] are being pushed towards commercialization. So far, a wide range of conjugated polymer systems has been developed, such as poly(1,4-phenylenevinylene) (PPV) [6,7], poly( p -phenylene) (PPP) [8], polyfluorene (PF) [9] and polythiophene (PT) [10], and their derivatives. Among these, polythiophenes are being regarded as one of the most promising materials for practical applications because of their good thermal and chemical stability, as well as for the tunability of their electronic and optical properties [11–14]. Indeed, by attaching different functional groups as a side chain substituent and controlling their regioregularity, light emission ranging from blue to near-infrared has been demonstrated [11,12]. Among the possible substituents attachable to the main polythiophenic chain, the cyano group (–CN) is very interesting, since it has been found that its presence in a conjugated polymer may be of help in enhancing the PL [15]. Also, it has been widely demonstrated that the presence of a cyano group may be very helpful for the tuning of the electronic properties of conjugated polymers, as in the case for example in CN-PPVs, or in CN-substituted polyfluorenes [16–18]. In this view, we present here a novel –CH 2 CH 2 OPhCN substituted thiophenic copolymer, poly{[3-hexylthiophene]-co- 3-[2-( p -cyano-phenoxy)ethyl]thiophene} (in brief PTOPhCN), in an effort to develop efficient thiophene-based light-emitting polymers. The copolymer may be easily and cheaply prepared by oxidative polymerization and evidenced an excellent filmability from various organic solvents, exhibiting notable photoluminescence in solution and in solid, spin coated films. Finally, we present preliminary results on the electrical properties of this copolymer, which shows promise for its use in optoelectronic devices. Indeed, a single layer LED based on the polymer was fabricated, and showed bright and stable orange electroluminescence at high bias voltages. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.09.177 * Corresponding author. Tel.: +34 93 413 7942; fax: +34 93 413 7943. E-mail address: Stephanie.cheylan@icfo.es (S. Cheylan). Thin Solid Films 497 (2006) 16 – 19 www.elsevier.com/locate/tsf