John Zheng Fang
Current Position: Project Leader, Cytec Surface Specialties
Ph.D. Institution: University of Akron
Princeton Postdoctoral Research:
I am currently interested in patterned electroluminescent displays based on polymer LEDs. Polymers have several intrinsic advantages as organic display components: 1) rapid and easy processing into thin films by spin-coating, 2) tunability of composition through copolymerization, and 3) easy doping with a variety of emitter molecules (dyes) to control the emission color, and 4) straightforward and rapid patterning into high-pixel-density displays. The ideal material for such a display will provide separate transport pathways for electrons and holes (thus permitting high mobility of both carriers simultaneously), but where the pathways are sufficiently small in scale that a large interfacial area between the hole- and electron-transporting regions exists for carrier recombination. In addition, the material should be compatible with a variety of small-molecule dye dopants. We propose that these material requirements could be met through the synthesis of block copolymers of hole- and electron-transporting monomers, such as N-vinylcarbazole (for holes) and a styrene-substituted oxadiazole (for electrons). While random copolymers of these monomers has been successfully synthesized in this group, the random sequence of the units does not provide separate carrier transport pathways for electrons and holes, leading to devices with only fair performance. However, these random copolymers did demonstrate the importance of excited-state complexes within the polymer in facilitating energy transfer to the dye.
In this project, we propose to synthesize block copolymers by sequential “normal”/living free radical polymerization, using difunctional initiators capable of initiating both types of polymerization. This route will provide for precise and independent control of the molecular weights of both blocks, allowing sensitive tuning of the connectivity of the carrier transport pathways. When the incompatibility between blocks is sufficiently large, the blocks will self-assemble into nanoscale domain structures, producing contiguous but independent transport paths for holes and electrons sharing a high interfacial area for recombination. These devices can be doped with minute concentrations of emitters (dyes) of various colors to produce the elements of a full-color display. The nanodomain morphology in device-thickness films of these block copolymers will be characterized via electron or scanning-probe microscopy.