Charge balance materials for homojunction and heterojunction OLED applications

Abstract

In a homojunction device, a single organic layer assumes the multiple roles of hole, electron transportation, and emitter. Its ease in processing is highly desirable from the manufacturing point of view. In this paper, we shall describe the synthesis of a range of bipolar small molecules and conductive vinyl polymers for application in homojunction and heterojunction organic light emitting diodes (OLEDs). The bipolar materials, in general, consist of three basic building blocks: an arylamine, a 1,3,4-oxadiazole, and a polycyclic aromatic moiety. The achievement of charge balance can be validated either by direct measurement of electron/hole mobility or indirectly via optimization of device properties. A series of conductive vinyl copolymers containing hole transporting N-(4-methoxyphenyl)-N-(4-vinylphenyl) naphthalen-1-amine (4MeONPA) and electron transporting 2-phenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole (OXA) at different compositions was applied for heterojunction and homojunction OLEDs. For heterojunction devices employed the copolymers as the hole transporting layer and Alq₃ as the electron transporting and emitting layer, a maximum luminance and current efficiency of over 23000 cd/m² and 4.2 cd/A (PL of Alq₃), respectively, were achieved at the charge balance composition. Homojunction devices for the copolymers were demonstrated by the addition of rubrene as a dopant. The single layer devices at the optimal copolymer composition has Ca 1500 cd/m² and 0.74 cd/A.

Keywords

homojunction devices / charge balance / bipolar small molecules / conductive vinyl copolymers

References

[1] Tang C W, Van Slyke S A. Organic electroluminescent diodes. Applied Physics Letters, 1987, 51(12): 913-915

[2] Yang Y. Polymer electoluminescent devices. MRS Bulletin, 1997, 22(6): 31-34

[3] Adachi C, Tsutsui T, Saito S. Blue light-emitting organic electroluminescent devices. Applied Physics Letters, 1990, 56(9): 799-801

[4] Hou L D, Duan L, Qiao J, Li W, Zhang D Q, Qiu Y. Efficient single layer solution-processed blue-emitting electrophosphorescent devices based on a small-molecule host. Applied Physics Letters, 2008, 92(26): 263301

[5] Higginson K A, Zhang X M, Papadimitrakopoulos F. Thermal and morphological effects on the hydrolytic stability of aluminum tris(8-hydroxyquinoline) (Alq₃). Chemistry of Materials, 1998, 10(4): 1017-1020

[6] Hung L S, Lee S T. Electrode modification and interface engineering in organic light- emitting diodes. Materials Science and Engineering: B, 2001, 85(2-3): 104-108

[7] Lee T H, Tong K L, So S K, Leung L M. Synthesis and electroluminescence of thiophene-based bipolar small molecules with different arylamine moieties. Synthetic Metals, 2005, 155(1): 116-124

[8] Xiang N J, Lee T H, Gong M L, Tong K L, So S K, Leung L M. Synthesis of 2-phenylquinoline-based ambipolar molecules containing multiple 1,3,4-oxadiazole spacer groups. Synthetic Metals, 2006, 156(2-4): 270-275

[9] Lee T H. “Synthesis and characterization of electroluminescent bipolar small molecules and polymers”. Dissertation for the Doctoral Degree. Hong Kong: Hong Kong Baptist University, 2007

[10] Lee T H, Tong K L, So S K, Leung L M. High-mobility hole-transporting polymers for electroluminescence applications. Japanese Journal of Applied Physics, 2005, 44(1): 543-545

[11] Yuan J B, Leung L M, Gong M L. Synthesis of efficient blue and red light emitting phenanthroline derivatives containing both hole and electron transporting properties. Tetrahedron Letters, 2004, 45(33): 6361-6363

[12] Goodbrand H B. US Patent, 5648539, 1997

[13] Behl M. Hattemer E, Brehmer M, Zentel R. Tailored semiconducting polymers: living radical polymerization and NLO-functionalization of triphenylamines. Macromolecular Chemistry Physics, 2002, 203(3): 503-510

[14] So S K, Choi W K, Cheng C H, Leung L M, Kwong C F. Surface preparation and characterization of indium tin oxide substrates for organic electroluminescent devices. Applied Physics A: Materials Science and Processing, 1999, 68(4): 447-450

[15] Lee T H, Lai K M, Leung L M. Hole-limiting conductive vinyl copolymers for AlQ₃-based OLED applications. Polymer, 2009, 50(19), 4602-4611

Acknowledgements

This work was supported by a Hong Kong SAR government research grant (No. HKBU2105/06E). We would also like to thanks the TOF mobility measurement conducted by Prof. S.K. So and Mr. K.L Tong at the Physics Department of HKBU.

RIGHTS & PERMISSIONS

2014 Higher Education Press and Springer-Verlag Berlin Heidelberg