Intrinsic electrochemical doping in blue light emitting polymer devices utilizing a water soluble anionic conjugated polymer

We report on light emitting device related properties of the water soluble blue light emitting anionic poly{1,4-phellylene- [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 handling 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 luminance in forward bias direction as well as in reverse bias direction. (c) 2007 Elsevier B.V. All rights reserved.