Polymer light-emitting diodes (PLEDs) have attracted much scientific and technological research interest since their first discovery in 1990 [1]. Utilization of electroluminescent polymeric materials shows several advantages over organic small molecules for use in PLEDs: better processability, and high flexibility [2]. Furthermore, inexpensive wet-processes such as spin-coating and ink-jet can be applied for the fabrication of PLED devices, which is essential in order to apply PLEDs to display and lighting technologies.

Unfortunately, it is generally recognized that polymeric devices show lower performance (efficiency, life time) compared with devices fabricated with a vacuum process based on low-molecular weight materials. Low efficiency in polymeric devices is partially due to difficulty in fabricating the devices with a layered-structure. To overcome the drawbacks, a breakthrough is necessary from the point of the molecular design and the control the morphology in the active layer. We have showed the advantage of block copolymers consisting of hole and electron transporting blocks as the host materials in phosphorescent devices [3-5]. Block copolymers were prepared via a nitroxide mediated living radical polymerization.

Block copolymers assemble into microor nano-phase separated structures with various domain shapes such as lamella, cylinder, or sphere. Exploiting nanostructures of block copolymers with appropriate designs can improve performance of applications due to allocation of functionality to each domain [6]. The other groups also reported several block copolymers for PLED applications [7,8].

More recently Tan et al. synthesized the different type of block copolymers for EL applications via the Suzuki coupling polymerization followed by the C-N coupling polymerization [9,10], which consisted of light emitting and electron transporting polyfluorene (PF) unit and hole transporting polytriarylamine (PTAA) unit. It was revealed that the introduction of PTAA increased emission efficiencies compared with PF homopolymer. This is due to the facile hole injection from the anode and/or the efficient electron block by PTAA moieties, which are located in the vicinity of the PEDOT/PSS coated on the anode through the hydrogen bonding of trioxyethylene group with PSS [10].

Here we proposed the novel methodology for the increase the efficiency in PLED. That is the novel molecular design of full functional polymers, which are block copolymers consisting of hole transporting unit and electron transporting unit with emitting moiety at the junction point. In the previous devices utilizing block copolymers as host polymers, emitting materials are low molecular weight phosphorescent dyes dispersed in host materials. In this case, emitting parts are randomly distributed in the active layer. Emission process is resulted from the recombination of holes and electrons, followed by the energy transfer from recombination centers to emitting moieties and/or the charge trap predominately occurred at the emitting sites. Therefore if the emitting parts are located in the vicinity of the interface between hole and electron transporting domains, more efficient energy transfer and carrier trap are anticipated. In order to attain the situation, a new molecular design is proposed. Target polymers are block copolymers consisting of hole transporting unit and electron transporting unit with emitting moiety at the junction point. If an ideal phase separation occurs, the emitting moiety exists at the interface between both domains.

In this study, the synthetic strategy we established for PF-b-PTAA (Suzuki coupling followed by C-N coupling polymerization) is modified to prepare a block copolymer consisting of PF and PTAA functionalized with green emitting phenoxazine moiety at the junction point of two blocks. Phenoxazine derivatives are known as a emitting dye [11,12], and have been utilized as the component of EL copolymers [13,14]. EL characteristics were preliminarily investigated for the comparison with random copolymer.