Improved efficiency in OLEDs with a thin Alq3 interlayer

doi:10.1016/j.jlumin.2006.01.253

Journal of Luminescence

Volumes 122–123, January–April 2007, Pages 660-662

https://doi.org/10.1016/j.jlumin.2006.01.253 Get rights and content

Abstract

We demonstrate an improved efficiency in OLEDs with a thin Alq₃ interlayer, which is inserted into the hole-transport layer for adjusting the hole-injection and transport, and improving the hole-electron balance. The thin Alq₃ interlayer can effectively influence the electrical performance and electroluminescence (EL) efficiency of the devices. The devices with an optimum Alq₃ interlayer exhibit a maximum EL efficiency of around 3.3 cd/A, which is improved by a factor of two over the conventional devices (1.6 cd/A) without the interlayer.

Introduction

Tris-8-hydroxyquinoline aluminum (Alq₃) is one of the most widely used materials as the emission layer (EML) and electron transport layer (ETL) in organic light-emitting devices (OLEDs) due to its good luminescent properties, high electron mobility, and excellent thermal stability [1], [2]. However, in typical vacuum-deposited OLEDs with diamine derivatives (e.g. TPD or NPB) as the hole-transport layer (HTL) and Alq₃ as the EML/ETL, the majority carriers are holes due to the relatively efficient hole-injection from the HTL–anode interface and the sufficiently high hole-mobility in the HTL [2], [3], [4]. It is generally recognized that the balanced carriers is desirable for high electroluminescence (EL) efficiency.

It has been reported that the hole-injection from anode can be adjusted by inserting a buffer layer between the indium tin oxide (ITO) anode and HTL. Copper phthalocyanine (CuPc) [5], silicon oxynitride [6], and LiF [7], have been deposited onto ITO surface as an effective buffer layer for efficiency and stability enhancement. Liew et al. have found that OLEDs with a thin Alq₃ buffer layer upon ITO anode possesses higher EL efficiency and longer stability due to the reduction of hole-injection from ITO to HTL [8]. Qiu et al. [9] have demonstrated OLEDs with improved hole-electron balance by using CuPc/NPB multiple-quantum-well structure, which could decrease the hole-mobility effectively.

In this article, we demonstrate an efficiency improvement in OLEDs by inserting a thin Alq₃ interlayer into HTL so as to adjust the hole-injection and transport, and to improve the hole-electron balance. The devices with a thin Alq₃ interlayer exhibit significantly improved EL efficiency compared to the conventional devices. We propose that the improvement may result from enhanced hole-electron balance or double-layer light emission.

Section snippets

Experiment

The device structure used in this study is ITO/TPD (x nm)/Alq₃ (5 nm)/TPD (40-x nm)/Alq₃ (55 nm)/Al, where x is the distance between the 5 nm Alq₃ interlayer and ITO anode, varied from 0 to 30 nm. The bilayer devices with structure of ITO/TPD (40 nm)/Alq₃ (60 nm)/Al have been fabricated as the conventional devices for comparison. All devices were prepared in a high vacuum chamber at a base pressure of 6×10⁻⁴ Pa. The TPD, Alq₃ (from Tokyo Kasei Kogyo Co. Ltd. and used as received without purification),

Results and discussion

Fig. 1 shows the typical J–V (a) and EL efficiency-J (b) characteristics of the devices with 5 nm Alq₃ interlayer, in which the distance between the Alq₃ interlayer and ITO anode is varied from 0 to 30 nm. Compared to the conventional device, the devices with the Alq₃ interlayer apparently exhibit lower current density but higher EL efficiency. As the distance between the Alq₃ interlayer and ITO anode decreases, the hole injection from ITO to TPD or/and transport in TPD decrease due to the

Conclusion

We have demonstrated an efficiency improvement in OLEDs by introducing a thin Alq₃ interlayer into HTL layer to adjust the hole-injection and transport, and to improve the hole-electron balance. The thin Alq₃ interlayer effectively influenced the electrical performance and balanced the hole-electron carriers. OLEDs with a thin Alq₃ interlayer, inserted at an optimum location in HTL, exhibited a maximum EL efficiency of around 3.3 cd/A, which was about twice as that of conventional devices (1.6

Acknowledgement

This work is partially supported by the Department of Science and Technology of Guangzhou Municipality under Grant no. 2004J1-C0011, and the Doctoral Foundation from Ministry of Education of China under Grant no. 20040558008.

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A study on the spectroscopic, energy band, and optoelectronic properties of α,ω-dihexylsexithiophene/tris(8-hydroxyquinolinate) gallium blends; DH6T/Gaq3 composite system
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Optical and morphological modifications in post-thermally treated tris(8-hydroxyquinoline) gallium films deposited on quartz substrates
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It is well known that tris (8-hydroxyquinoline) aluminium (Alq3) metal can be a viable candidate for the application in organic light emitting diodes (OLED)s [1–5].
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Organic chelate metal complexes Mq3 (M = metal, q = 8-hydroxyquinolinate) have received considerable attention since the first Alq3 organic electroluminescent device was reported by Tang and Van Slyke in 1987 [1]. Thereafter, Alq3 has been successfully incorporated in the multilayer organic light emitting diodes (OLEDs) [2,3] and pioneered significant achievement in the field of organic flat panel displays. Besides, its use as buffer electron transport layer (ELT) [4] and dopant material into the donor and/or acceptor layers in the organic solar cells (OSCs) [5] has been demonstrated.
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Improved efficiency in OLEDs with a thin Alq3 interlayer

Abstract

Introduction

Section snippets

Experiment

Results and discussion

Conclusion

Acknowledgement

Appl. Phys. Lett.

Appl. Phys. Lett.

Proc. SPIE.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Improved efficiency in OLEDs with a thin Alq₃ interlayer