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Electronic Imaging & Signal Processing

New structures for highly-efficient and robust blue organic light-emitting devices

A new structure for OLEDs can provide high electroluminescent efficiency, a long operational lifetime of 10,000 hours, and strong saturated color for the vitally important but elusive blue emitter.

1 February 2006, SPIE Newsroom. DOI: 10.1117/2.1200601.0012

Organic light-emitting diodes (OLED) have considerable advantages over today's ubiquitous liquid crystal displays (LCDs) in terms of the higher efficiency, lower power consumption, and faster response times. With the ability to be self-emissive and both extremely thin and light, they are particularly suitable for flexible display applications. They are thus one of the most promising new display technologies today.

Three kinds of pixelation scheme have been demonstrated for fabricatingfull-color OLED displays: discreet lateral RGB emitters;1 blue emitters with a color-conversion layer;2 and white emitters with color filters.3 All of these require a robust blue emitter with high electroluminescent (EL) efficiency and long operational lifetime. In additional, the color of a blue OLED also needs to be saturated, preferably with a y-value of less than 0.15 on the Commission Internationale d'Eclairage coordinate (CIEy) scale, to significantly reduce the power consumption of a full-color display (see Figure 1).

 
Figure 1. The importance of the blue CIEy color coordinate for power consumption in the composition of white balance in a full-color OLED.
 

An OLED consists of a series of organic thin films sandwiched between two conductive electrodes. The choice of organic materials and the layer structure determine the device performance: such as emissive color, operating lifetime, and EL efficiency. It is well-known that the guest-host-doped emitter system can significantly improve the device performance and modify the color.4 Furthermore, the modification of the anode/cathode contact or hole/electron transport layer has been shown to improve charge balance and recombination, as the injected hole is usually more mobile than the injected electron under the same electric field in conventional OLED.5,6 However, to date, blue-doped emitter systems with all the desirable attributes of high EL efficiency, long operational lifetime, and saturated blue color are still rare.7,8

We have successfully demonstrated an anthracene-based blue host material, 2-methyl-9,10-di(2-naphthyl)anthracene (MADN), which can form a stable thin-film morphology upon thermal evaporation.9 When fabricated as an OLED, the device achieved an EL efficiency of 1.4cd/A with a CIEx,y color coordinate of [0.15, 0.10]. Subsequently it was discovered that—by using a novel composite hole transport layer (c-HTL) composed of copper phthalocyanine (CuPc)/N,N'-bis(1-naphthyl)-N,N'-diphenyl,1,1'-biphenyl-4,4'-diamine (NPB) at a specific ratio—the EL efficiency was boosted by more than a factor of two to 3.0cd/A.10 The increased EL efficiency is attributed to an improved balance between hole and electron currents arriving at the recombination zone due to the c-HTL layer. This can efficiently reduce hole mobility, a notion that has been supported by the hole-only devices we reported previously.11

We found the EL efficiency of blue OLEDs can be further improved by judicious selection of a matching guest material for efficient Foerster-energy transfer. This is highly dependent on the spectral overlap between the emission of the host and the absorption of the guest. For instance, the di(styryl)amine based light-blue dopant—p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph)—possesses a high fluorescence quantum yield at a λmax of 458nm, and a well-matched spectral overlay between the guest DSA-Ph and the host MADN. Thus, this blue-doped emitter12 achieved a high EL efficiency of 9.7cd/A with a greenish-blue color of CIEx,y [0.16, 0.32], and a long operational lifetime of 46,000h at a initial brightness of 100cd/m2. However, the color saturation is far from adequate for full-color OLED displays.

To achieve saturated-blue OLED, a deeper-blue dopant with a more hypsochromic (blue) shift of the emission from the DSA-Ph is needed. From the material-design and synthetic point of view, the most straightforward approach is to shorten the conjugation length (chromophore) of the di(styryl)amine-based material to the mono(styryl)amine-based core. By modifying the substituents attached to the nitrogen as well as to the styrene-part of the molecule, it is possible to obtain emissions in the deeper-blue visible region between 430–450nm. When doped in MADN, the device13 with the deep-blue dopant (BD-1) achieved an EL efficiency of 2.2cd/A with a saturated blue of CIEx,y [0.15, 0.11] and a long lifetime of 10,000h at initial brightness 100cd/m2.

Carrier recombination and the balance of holes and electrons are two key factors in producing high-EL-efficiency OLEDs. Incorporating the c-HTL layer in our sky-blue and deep-blue emitter systems with DSA-Ph and BD-1 in host MADN, gave devices with very high EL efficiency of 17cd/A and 5.4cd/A and blue color coordinate of CIEx,y [0.14, 0.28] and [0.14, 0.13], respectively.11,13 These devices with c-HTL showed twice the EL efficiency with comparable device operational lifetimes to those without c-HTL (see Figure 2).

 
Figure 2. EL performances and the improved factor of sky- and deep-blue doped OLEDs with c-HTL.
 

To the best of our knowledge, the EL performance of our sky-blue and deep blue OLEDs using c-HTL represent some of the best reported to date, making them suitable for use in full color OLED displays.


Authors
Chin H. Chen
Display Institute, National Chiao Tung University
Hsinchu, Taiwan
Microelectronics and Information Systems Research Ctr., National Chiao Tung University
Hsinchu, Taiwan
 
Meng-Ting Lee and Chi-Hung Liao
Department of Applied Chemistry, National Chiao Tung University
Hsinchu, Taiwan

References:
1. S. Miyaguch, S. Ishizuks, T. Wakimoto, J. Funaki, Y. Fukuda, H. Kubota, K. Yoshida, T. Watanabe, H. Ochi, T. Sakamoto, M. Tsuchida, I. Ohshita, T. Thoma, OLED full-color passive matrix display, Proc. 9th Int. Workshop on Inorganic and Organic electroluminescence, pp. 197, 1998.
2. C. Hosokawa, E. Eida, M. Matuura, F. Fukuoka, H. Nakamura, T. Kusumoto, Organic multicolor el display with fine pixels, Proc. Society of Information Display, pp. 1073-1076, 1997.
3. J. Kido, K. Hongawa, K. Okutama, K. Nagai, White light-emitting organic electroluminescent devices using poly(N-vinylcarbazole) emitter layer doped with three fluorescent dyes, Appl. Phys. Lett., Vol: 64, no. 7, pp. 815-817, 1994.
4. C. W. Tang, S. A. VanSlyke, C. H. Chen, Electroluminescence of doped organic thin film, J. Appl. Phys., Vol: 65, no. 9, pp. 3610-3616, 1989.
5. L. S. Hung, C. W. Tang, M. G. Mason, Enhanced electron injection in organic electroluminescence devices using an Al/LiF electrode, Appl. Phys. Lett., Vol: 70, no. 2, pp. 152-154, 1997.
6. S. Naka, H. Onnagawa, T. Tsutsui, High electron mobility in bathophenanthroline, Appl. Phys. Lett., Vol: 76, no. 2, pp. 197-199, 2000.