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Optical Design & Engineering

White organic light emitting diodes show improved performance

Novel fluorescent and phosphorescent materials and optimized device configuration enable enhanced pure-white-light generation.
20 October 2008, SPIE Newsroom. DOI: 10.1117/2.1200810.1309

Organic light-emitting diodes (OLEDs) operate by passing an electric current through a fluorescent or phosphorescent organic layer, resulting in photon emission at a certain wavelength specific to the emitter. OLEDs continue to draw attention in scientific and industrial contexts as promising candidates for the construction of large-area full-color flat-panel displays due to their ease of fabrication and convenience for many applications.

White OLEDs (WOLEDs) have attracted significant interest because of their demonstrated applicability in full-color displays with color filters, as backlights for liquid-crystal displays, and for paperlike illumination sources. To enter the lighting market, however, several critical problems must be solved: they should be more power efficient, reliable, and cost-effective. One possible solution uses fluorescent or phosphorescent emitting materials that are characterized by high power efficiency and stability, while optimization of the device configuration is also seen as a useful approach to produce commercially competitive lighting. WOLEDs are usually constructed by fabricating a device that emits and mixes light of either three primary colors (red, green, and blue) or only blue and yellow (or orange) in a suitable ratio. Therefore, development of high-efficiency monochromatic emitters and an optimal configuration would improve device performance.

Figure 1. Normalized absorption spectra of N; N-di(naphthalen-2-yl)-N; N-diphenyl-benzene (NPB) and tris(8-hydroxyquinolinato) aluminum (Alq) and the photoluminescence (PL) spectrum of 1,2,3,4,5,6-hexakis(9,9-diethyl-9H-fluoren-2-yl) benzene (HKEthFLYPh) in arbitrary units (a.u.). The dash-dotted ellipses link the curves to their respective vertical axes. The inset shows the molecular structure of the HKEthFLYPh starburst material.

We have synthesized high-efficiency fluorescent and phosphorescent materials and fabricated high-performance OLEDs.1–3 For instance, production of high-brightness yellow OLEDs was achieved using fluorescent silole as the high-efficiency emitter.1 Employing a novel iridium-complex material,2,3 we successfully constructed4 phosphorescent WOLEDs with a maximum luminance of 15,460cd/m and a power efficiency of 8.1lm/W. In addition, we investigated the use of certain novel fluorene materials for blue emission5. Also, a derivative of 6,6 ′ -(9H-fluoren-9,9-diyl)bis(2,3-bis(9,9-dihexyl-9H-fluoren-2-yl)quinoxaline) (BFLBBFLYQ) was synthesized and characterized. Exciplex (excited-state complex) emission and chromism (a reversible color change induced by an external stimulus) were observed after UV irradiation in a blend of BFLBBFLYQ and N,N ′ -biphenylN,N ′ -bis-(3-methylphenyl)-1,1 ′ -biphenyl-4,4 ′ -diamine (TPD) film and in polar solvent, respectively.6,7

Furthermore, low-energy emission bands from 530 to 600nm originate from the photoluminescence (PL) spectra of BFLBBFLYQ-evaporated films deposited with fluorenone defects. The latter were introduced through thermal oxidization and photo-oxidization.8 In most cases, highly-efficient WOLEDs can be produced by a careful control of both host material and dopant through co-evaporation, particularly for red, blue, and yellow light-emitting materials. Meticulous dopant-concentration control is normally imperative to produce pure white light. This is time consuming, complicates fabrication, and reduces reproducibility. It also invariably wastes organic functional materials. Therefore, we concentrate on optimizing device configuration and developing novel materials for energy transfer, aimed at creating high-efficiency devices using non-doped layers.9–13

Förster resonance energy transfer occurs in fluorescent materials if a singlet state in the host matrix is equivalent to the corresponding energy level of the dopant. The PL spectrum of 1,2,3,4,5,6-hexakis(9,9-diethyl-9H-fluoren-2-yl) benzene (HKEthFLYPh) starburst material exhibits good overlap with the absorption of N; N-di(naphthalen-2-yl)-N; N-diphenyl-benzene (NPB), and part overlap with the absorption spectrum of tris(8-hydroxyquinolinato) aluminum (Alq) (see Figure 1). Therefore, efficient Förster energy transfers from HKEthFLYPh to NPB and Alq do occur.11

We also constructed WOLEDs by mixing blue and yellow-green light based on novel HKEthFLYPh material, which showed a current efficiency from 1.8 to 1.4cd/A from 5.5 to 15V with Commission Internationale de L'Eclairage (CIE) chromaticity (x; y) coordinates (0.29, 0.36) at 8V.12 Pure white emission with CIE coordinates (0.33, 0.33) was obtained by inserting an ultrathin layer of rubrene as yellow emitter.13

We have developed high-performance rigid and flexible OLEDs based on novel fluorescent/phosphorescent materials and optimized their configuration to promote growth into the commercial market.14 We are now focusing on developing organic optoelectronic devices (including solar cells and OLEDs) with higher efficiencies and longer lifetimes.

This work was partially supported by the National Science Foundation of China (NSFC) through grants 60425101 and 20674049, the Foundation for Innovative Research Groups of the NSFC through grant 60721001, and the Program for New Century Excellence Talents in Universities through grant NCET-06-0812.

Junsheng Yu, Shuangling Lou, Yadong Jiang
State Key Laboratory of Electronic Thin Films and Integrated Devices
School of Optoelectronic Information
University of Electronic Science and Technology of China
Chengdu, China

Junsheng Yu obtained his PhD from Tokyo University of Agriculture and Technology in 2001 and is currently a professor. His research interests focus on optoelectronic materials and devices.

Shuangling Lou received her BS (2003) and MS (2006) from the University of Electronic Science and Technology of China (UESTC). She is a PhD student with research interests in organic optoelectronic materials and devices.

Yadong Jiang obtained his BS (1986), MS (1989), and PhD (2001) from UESTC, where he now is a professor and dean of the School of Optoelectronic Information. His major research interests include optoelectronic materials and devices, sensitive materials, and sensors.

Qing Zhang
Department of Polymer Science
School of Chemistry and Chemical Technology
Shanghai Jiaotong University
Shanghai, China

Qing Zhang received his PhD from the University of South Carolina in 1997. He is a professor with research interests in organic functional-material design and synthesis.