Journal of Luminescence 122–123 (2007) 610–613 Blue light-emitting diodes based on novel polyfluorene copolymers Lars J. Lindgren a,c,Ã , Fengling Zhang b,c , Shimelis Admassie b,c , Xiangjun Wang b,c , Mats R. Andersson a,c , Olle Ingana¨s b,c a Department of Chemical and Biological Engineering, Polymer Technology, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden b Department of Physics, Linko¨ping University, SE-581 83 Linko¨ping, Sweden c Center of Organic Electronics, Linko¨ping University, Linko¨ping, Sweden Available online 20 March 2006 Abstract This study presents the synthesis and characterisation of a series of fluorene-based conjugated copolymers, together with the preparation and characterisation of the corresponding light-emitting devices. The polymers consist of alkoxyphenyl-substituted fluorene units together with different amounts of a hole-transporting triphenylamine-substituted fluorene unit: 0%, 10%, 25% and 50%. All polymers (P0, P1, P2, and P3) show high photoluminescence efficiency (Z PL ) and light emission (both PL and EL) in the blue spectral region. Electrochemical studies show improved hole injection as the ratio of the triphenylamine-substituted segment is increased. The electroluminescence quantum efficiencies (EQEs) of the devices increase six times going from P0 to P1. Compared with P1, polymers P2 and P3 show lower efficiencies in devices. These findings indicate the presence of an optimal polymer composition, where balance between the charge-carrier mobilities has been reached. r 2006 Elsevier B.V. All rights reserved. Keywords: Polyfluorenes; Copolymers; PLEDs; Blue-light emission; Charge injection 1. Introduction Since the discovery of the electroluminescent properties of poly(p-phenylene vinylene) (PPV) [1], conjugated poly- mers for light-emitting applications have been intensively researched. Among the polyphenylene-based materials, polyfluorenes (PFs) have emerged as promising blue- light-emitting materials for polymer light-emitting diodes (PLEDs) [2]. Poly(9,9-dialkylfluorene)s can be prepared in good yield and with high purity. However, they have two major deficiencies as candidate materials for PLEDs. The first negative aspect is that poly(9,9-dialkylfluorene)s exhibit a red-shifted emission after thermal treatment or during device operation. It has been shown that bulky substitution in the 9-position of polyfluorenes leads to an improved colour stability [3]. The second problem to overcome is the imbalance of charge-carrier mobilities in polyfluorene-based light-emit- ting devices. This imbalance is caused by large injection barriers between electrodes and polymer together with a difference in charge-carrier mobilities in the polymer. Recently, Ego et al. [4] presented the synthesis and properties of a polyfluorene with hole-transporting triphe- nylamine units introduced in the 9-position. This new homopolymer showed a pure blue emission and improved hole injection compared to standard poly(dialkylfluorene)s. Here we present the synthesis and device preparation of a series of fluorene-based copolymers. The polymers (P0–P3 in Fig. 1) consist of a hole-transporting monomer and a bulky alkoxy-phenyl-substituted monomer in differ- ent compositions. Optical and electrochemical properties of the polymers together with device performances can be correlated to the relative composition of monomers in each polymer. In this way an optimal composition with balanced charge-carrier mobilities can be achieved. ARTICLE IN PRESS www.elsevier.com/locate/jlumin 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.238 Ã Corresponding author. Department of Chemical and Biological Engineering, Polymer Technology, Chalmers University of Technology, SE-412 96 Go¨ teborg, Sweden. Tel.: +4631772 34 11; fax: +46 31 772 34 18. E-mail address: lila@chalmers.se (L.J. Lindgren).