Intrinsic electrochemical doping in blue light emitting polymer devices utilizing a water soluble anionic conjugated polymer S. Sax a,b , G. Mauthner a,b , T. Piok a,b , S. Pradhan c , U. Scherf c , E.J.W. List a,b, * a Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology, A-8010 Graz, Austria b JOANNEUM RESEARCH Institute of Nanostructured Materials and Photonics, A-8160 Weiz, Austria c Bergische Universita ¨ t Wuppertal, Makromolekulare Chemie, Fachbereich Chemie Gaußstraße 20, D-42119 Wuppertal, Germany Received 4 December 2006; received in revised form 31 May 2007; accepted 3 July 2007 Available online 20 July 2007 Abstract We report on light emitting device related properties of the water soluble blue light emitting anionic poly{1,4-pheny- lene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} copolymer (PBS-PFP) with alkyl sulfonic acid sodium side chains. Compared to organic solvents water has the advantage of being environmentally friendly, which simplifies the han- dling during device fabrication by novel preparation techniques such as ink-jet printing. Light emitting devices fabricated from this polymer in thin film configuration show stable blue emission with an emission maximum at approximately 420 nm. Investigations concerning the current–density/voltage and the luminance/voltage device characteristics as well as the current–density over operation time at constant biases allow the identification of intrinsic electrochemical doping effects caused by ionic groups attached on the side chains of the polymer. Due to this fact, firstly the electroluminescence onset voltage for devices using high work-function electrodes for electron injection such as aluminum, which has the advantage of being air stable, can be distinctly reduced, and secondly devices fabricated from this polymer showed lumi- nance in forward bias direction as well as in reverse bias direction. Ó 2007 Elsevier B.V. All rights reserved. PACS: 78.20.Bh; 85.60.Jb; 78.60.Fi; 78.66.Bz Keywords: Light emitting electrochemical cell; Conjugated polymer; Water soluble polymer; Blue emitter; Polyelectrolyte 1. Introduction During the last years organic semiconductors – especially in the field of organic light emitting devices – have found their way into commercial applications. Organic light emitting devices devel- oped for flat panel displays or solid state lighting 1566-1199/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2007.07.001 * Corresponding author. Address: Christian Doppler Labora- tory Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology, A-8010 Graz, Austria. Tel.: +43 316 873 8468; fax: +43 316 873 8478. E-mail address: e.list@tugraz.at (E.J.W. List). Available online at www.sciencedirect.com Organic Electronics 8 (2007) 791–795 www.elsevier.com/locate/orgel