Elsevier

Displays

Volume 28, Issues 4–5, December 2007, Pages 186-190
Displays

Highly efficient styrylamine-doped blue and white organic electroluminescent devices

https://doi.org/10.1016/j.displa.2007.07.006Get rights and content

Abstract

We have developed highly efficient blue and white organic electroluminescent devices based on a blue fluorescent styrylamine dopant EBDP. The blue and white organic light emitting diodes (OLEDs) with the structures: Indium–tin oxide (ITO)/copper phthalocyanine(CuPc)/N,N′-bis-(1-naphenyl)-N,N′-biphenyl-1,1′-bipheny1-4-4′-diamine (NPB)/2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN):EBDP/tris(8-hydroxyquinoline)aluminum(Alq3)/LiF/Al and ITO/CuPc/NPB/TBADN:EBDP: 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)/Alq3/LiF/Al were studied by using EBDP as blue dopant. For the blue device, the maximum luminance and maximum efficiency were 26961 cd/m2 and 8.29 cd/A, respectively, the luminance at a current density 20 mA/cm2 was 1597 cd/m2. For the white device, the maximum luminance of 32,291 cd/m2, maximum efficiency 8.31 cd/A and the luminance of 1413 cd/m2 at a current density 20 mA/cm2 were obtained. The slow decrease of efficiency with the increase of current density indicates weak exciton–exciton annihilation, which is resulted from the large steric hindrance due to the non-planar structure of the fluorescence dye EBDP.

Introduction

Organic light emitting diodes (OLEDs) have attracted wide attention [1], [2] in view of practical applications for displays and lighting [3], [4], since the demonstration of efficient electroluminescence (EL) from a bilayer device by Tang et al. [5]. It is well known that the EL efficiency, color, and operational stability of organic EL devices can be significantly improved with the use of a doped emitter [6], [7]. White light emission has been conventionally realized by combining red or yellow dye with a blue emitter. Since human eyes are relatively less sensitive to blue light, it is therefore important to increase the blue portion in the white OLEDs. To date, there are two major blue-doped emitter systems that have been applied in organic EL devices. One was reported by Hosokawa et al. [8], in 1995, which utilized a class of distyrylarylene derivatives as blue host material and styrylamine as dopant. The device efficiency recorded then was about 3.4 cd/A producing bright emission in the blue–green region. The other major blue-doped emitter was developed by Shi and Tang [9] at Kodak, in 2002, which utilized the diphenylanthracene derivatives 9,10-di(2-naphthyl)anthracene (ADN) as blue host and 2,5,8,11-tetra(t-butyl)-perylene (TBPe) as dopant to generate a somewhat deeper blue emission. The blue EL device was reported to produce an EL efficiency of around 3.5 cd/A with a half-life of 4000 h at an initial light output of 700 cd/m2. Based on efficient blue emitter, doping with an appropriate combination of red or orange fluorescent dyes can produce white light. Jiang et al. [10] obtained a highly efficient white device by doping blue dye perylene and a red dye in 9,10-bis(3′,5′-diaryl) phenyl anthracene, which has the maximum luminance of 14850 cd/m2 with CIE(0.32, 0.38).

In this paper, we prepared high efficient blue and white devices using a new blue dopant EBDP. This new blue dye not only improved luminance and efficiency, but also showed weak current annihilation. It is excellent in the passive matrix OLED displays.

Section snippets

Experiment

The molecular structures of the main organic materials used in the study are shown in Fig. 1. The special feature of EBDP is its non-planar molecular structure, that is, amine at both ends of the molecule are forced to twist by steric hindrance. The molecular structure was designed in order to avoid concentration quenching at high doping concentration, leading to the improvement of energy transfer from host to dopant molecule. By using EBDP as blue dopant, efficient blue and white organic light

Blue devices

In order to find an optimized EBDP doping concentration, the detailed EL performances of the device (BEBDP) measured at 20 mA/cm2 are shown in Table 1. In these devices, the EBDP volume percentages are varied from 1% to 5% to investigate the effect of the EBDP doping ratio on the characteristics of the devices. We have found that the optimal EBDP concentration in TBADN is about 3%, providing an EL efficiency of 7.99 cd/A and 3.07 lm/W at 8.07 V with a CIEx,y (x = 0.15, y = 0.30). The doped devices show

Conclusions

Efficient blue and white devices were made with dopant EBDP. For the blue device, the maximum luminance was 26961 cd/m2, and its maximum efficiency was 8.29 cd/A, luminance at a current density 20 mA/cm2 was 1597 cd/m2. The luminance and efficiency are 1.8× that of the device doped with TBPe. Based on the high efficient blue luminescence, a white device was made by further doping of DCJTB. For the white device, the maximum luminance 32291 cd/m2, maximum efficiency of 8.31 cd/A, and a luminance of 1413

Acknowledgment

This work was supported by the National Natural Science Foundation of China [No. 90202034, No. 60477014, and No. 60577041] and 973 Project of China [No. 2002CB613400].

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