Electroluminescent Polymers

In a collaboration with Professor James Sturm (Electrical Engineering) and Professor Mark Thompson (Chemistry, USC), we developed a range of electroluminescent (EL) polymer materials for flat panel displays. Such emissive display technology has the potential to be ten times as efficient as liquid-crystal displays, and could revolutionize the world of portable electronic systems. Several consumer electronic devices, such as the Samsung Galaxy S5 smartphone, are already using small-molecule EL displays (usually referred to as OLEDs, organic light-emitting diodes), but polymer-based EL displays potentially offer both simpler processing and more robust mechanical properties, particularly useful for the flexible displays of the future.  Some of the notable achievements from our work have been: a) a high-efficiency EL device employing air-stable electrodes, b) new EL polymers from which devices can be prepared by a room-temperature self-assembly route, c) the ability to tune the emission color by changing the component ratio in a polymer blend system, allowing red, green, and blue devices to be prepared from the same materials, and d) an integrated red-green-blue display element, which comprises the building block of any full-color display. A magnified photograph of the RGB element is shown below.

Integrated red-green-blue device prepared from dye-doped polymers
Integrated red-green-blue device prepared from dye-doped polymers. Actual size of each pixel is approximately 1.5 mm on a side. Photograph taken under normal room illumination; device brightnesses are approximately 100 cd/m2 (Wu et al., Appl. Phys. Lett., 69, 3117 (1996)).

For these laboratory advances to form the basis for commercial EL flat-panel displays, the operating lifetimes of these devices must be extended. The devices shown above are based on hole-transporting poly(N-vinylcarbazole) blended with electron-transporting small-molecule oxadiazoles, then lightly doped with a dye to tune the emission color. As the oxadiazoles are crystallizable species, slow recrystallization can lead to device failure. To avoid this problem, we synthesized copolymers containing both electron- and hole-transporting units. One successful example is shown below, where both the chemical structure and the device characteristics are illustrated.

Random copolymers of electron- and hole-transport monomers for EL devices
Random copolymers of electron- and hole-transport monomers for EL devices. Chemical structure of the mers of the copolymer: N-vinylcarbazole (NVK) at left, oxadiazole-bearing monomer at right.

Random copolymers of electron- and hole-transport monomers for EL devices
Random copolymers of electron- and hole-transport monomers for EL devices. Characteristics of a device prepared from a copolymer containing 83 mol% N-vinylcarbazole, lightly doped with nile red (Jiang et al., Chem. Mater., 12, 2542 (2000)).

Though these random copolymers can produce devices with good quantum efficiency, the hole-blocking nature of the oxadiazole units (and electron-blocking nature of the carbazole units) leads to poor mobilities for both types of carriers, and consequently poor power efficiency.  To circumvent this limitation, we sought to synthesize block copolymers of the same hole- and electron-transport monomers, so that both types of carriers would have independent transport pathways through the material, with a large internal interfacial area for carrier recombination.  Nitroxide-mediated radical polymerization (NMRP) was employed; unfortunately, NVK propagates very slowly under NMRP, making this synthetic route impractical with these monomers.

Supported by the New Jersey Commission on Science and Technology through the New Jersey Center for Organic Optoelectronics

Group Members Involved, and Their Project Titles:

John Fang PDRA – “Living Radical Polymerization of Carbazole and Oxadiazole Monomers”
Xuezhong Jiang PDRA – “Carbazole-Oxadiazole Copolymers: Synthesis and Device Fabrication”
Jeffrey Quinn PhD *08 – “Nitroxide-Mediated Radical Polymerization and Functional Group-Containing Block Copolymers”
Chung-Chih Wu PhD EE *97 – “Light-Emitting Devices Based on Doped Polymer Thin Films”

Relevant Group Publications:

J.D. Quinn and R.A. Register, “Nitroxide-Mediated Radical Polymerization of N-ethyl-2-vinylcarbazole”, Polym. Adv. Technol., 19, 556-559 (2008) [Special Issue for Founding Editor Menachem Lewin’s 90th Birthday].

X. Jiang, A.K.-Y. Jen, G.D. Phelan, D. Huang, T.M. Londergan, L.R. Dalton, and R.A. Register, “Efficient Emission from a Europium Complex Containing Dendron-Substituted Diketone Ligands”, Thin Solid Films, 416, 212-217 (2002).

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, 6717-6724 (2002).

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 Single-Layer Electroluminescent Device Applications”, Chem. Mater., 12, 2542-2549 (2000).

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, 1269-1281 (1997).

C.-C. Wu, J.C. Sturm, R.A. Register, and M.E. Thompson, “Integrated Three-Color Organic Light-Emitting Devices”, Appl. Phys. Lett., 69, 3117-3119 (1996).

J. Tian, C.-C. Wu, T.A. Ronneberg, M.E. Thompson, J.C. Sturm, and R.A. Register, “Photophysical Properties, Self-Assembled Thin Films and Light-Emitting Diodes of Poly(p-pyridylvinylene)s and Poly(p-pyridinium vinylene)s”, Chem. Mater., 7, 2190-2198 (1995).

J. Tian, C.-c. Wu, M.E. Thompson, J.C. Sturm, R.A. Register, T.M. Swager, and M.J. Marsella, “Electroluminescent Properties of Self-Assembled Polymer Thin Films”, Adv. Mater., 7, 395-398 (1995).

C.C. Wu, J.K.M. Chun, P.E. Burrows, J.C. Sturm, M.E. Thompson, S.R. Forrest, and R.A. Register, “Poly(p-phenylene vinylene)/Tris(8-hydroxy)quinoline Aluminum Heterostructure Light Emitting Diode”, Appl. Phys. Lett., 66, 653-655 (1995).