Current Position: Research Scientist, Bridgestone
Undergraduate Institution: University of Massachusetts, Amherst

Ph.D. Thesis Research:

I will be researching the use of electroluminescent materials based upon dye-doped block copolymers as the active element in efficient organic displays. Electroluminescent polymer materials offer the promise of thin flat panel displays, which would be lightweight, mechanically flexible, and potentially inexpensive.  Emissive display technology has the potential to be ten times as efficient as current liquid-crystal displays (LCDs), and could revolutionize the world of portable electronic systems.  Moreover, such materials can be deposited on flexible or even woven substrates, greatly expanding the range of potential applications compared with either LCDs or conventional cathode-ray tubes. An immediate application area for displays based on elecroluminescent polymers would be portable electronic devices, such as cell phones and laptops.  For such applications, the ruggedness of a mechanically flexible display (with a plastic screen instead of glass), coupled with light weight and efficiency (for long battery life) will be crucial.  In addition, entirely new applications can be envisioned, based on simple few-pixel displays that could be prepared at costs low enough to make them disposable.

Current organic light-emitting devices (OLEDs) based on small molecules are now being introduced in selected consumer products. Polymers have advantages in processing that would make them suitable for the production of low cost OLEDs. However, current polymer-based systems do not meet all of the material requirements, such as high efficiency, high brightness, and long lifetime, mandating further materials development if the promise of electroluminescent displays is to be realized. Among the better materials developed to date are polymer-small molecule blends [1], where the emitting layer contains: 1) a hole-transport polymer such as poly(N-vinylcarbazole), PVK, as a majority component, 2) an electron-transporting small molecule, typically containing oxadiazole groups, and 3) a small quantity of a dye, which determines the emission color. However, the electron transport molecule must be added in large quantities and tends to recrystallize, degrading device performance. Also, such materials are difficult to pattern using recently developed inkjet and dye diffusion methods because of the mobility of the electron transport molecule. Recent material developments have included random copolymers of N-vinylcarbazole (NVK) with an oxadiazole containing monomer [2]. However, devices fabricated from these random copolymers show carrier injection efficiencies and mobilities lower than for blends, leading to reduced currents and brightnesses at operating voltages. Also, excited state complexes within these random copolymers substantially influence the ease of energy transfer to the dye [3].

The ideal material for such a display would provide separate transport pathways for electrons and holes, 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. These material requirements could be met through the synthesis of block copolymers of hole and electron transporting monomers, instead of the random copolymers that have previously been made with the same monomers. Synthesis work has already begun on such materials using a sequential “normal”/living free-radical polymerization. A difunctional initiator can first initiate polymerization of one of the monomers, producing polymer and leaving the other initiator as an end group. The product from this first step is then used as a macroinitiator in the polymerization of the second monomer. 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 all the elements of a full-color display.

[1] C.C. Wu, J.C. Sturm, R.A. Register, J. Tian, E.P. Dana, and M.E. Thompson, “Efficient Organic Electroluminescent Devices Using Single-Layer Doped Polymer Thin Films with Bipolar Carrier Transport Abilities”, IEEE Trans. Elec. Dev., 44, 1269 (1997).

[2] X. Jiang, R.A. Register, K.A. Killeen, M.E. Thompson, F. Pschenitzka, and J.C. Sturm, “Statistical Copolymers with Side-Chain Hole and Electron Transport Groups for Electroluminscent Device Applications”, Chem. Mater., 12, 2542 (2000).

[3] X. Jiang, R.A. Register, K.A. Killeen, M.E. Thompson, F. Pschenitzka, T.R. Hebner, and J.C. Sturm, “Effect of Carbazole-Oxadiazole Excited-State Complexes on the Efficiency of Dye-Doped Light-Emitting Diodes”, J. Appl. Phys., 91, 6717 (2002).