Elsevier

Displays

Volume 29, Issue 4, October 2008, Pages 323-326
Displays

Multilayer cathode for organic light-emitting devices

https://doi.org/10.1016/j.displa.2007.09.015Get rights and content

Abstract

A thin layer of samarium (Sm) was introduced in the cathode fabrication of the organic light-emitting devices (OLEDs). An efficient cathode Sm/LiF/Al used to improve the performance of OLEDs was reported. Standard N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl 4,4′-diamine (NPB)/tris-(8-hydroxyquinoline) aluminum (AlQ) devices with Sm/LiF/Al cathode showed dramatically enhanced electroluminescent (EL) brightness and efficiency. The optimized device with the 0.5 nm layer of Sm with the cathode structure of Sm/LiF/Al showed the improved power efficiency of 40% than the control device of the conventional LiF/Al cathode at 100 mA/cm2. The drive voltage of the device with the Sm/LiF/Al cathode was decreased about 2 V at 500 mA/cm2 compared with the conventional LiF/Al cathode device. The enhanced properties of the device with such a multilayer cathode are considered to the improved balance of electron/hole injection in the emitting layer.

Introduction

Organic light-emitting diodes (OLEDs) based on organic small molecule and polymers have been extensively studied for potential applications, especially in the field of flat-panel displays [1], [2], [3], [4], [5], [6], [7]. In general, OLEDs are fabricated with multilayer structures consisting of active luminescent layers, hole transporting layers (HTLs) and electron transporting layers (ETLs) to achieve a balanced charge-carrier injection and enhanced luminescence efficiency [1], [2], [3]. Due to the work function differences between the cathode metal and ETL, an effective cathode structure for efficient electron injection is critical to performances of OLEDs. There is a potential energy barrier that limits electron injection at the cathode-organic layer interface. Therefore, metals having a low work function such as Li, Ca, and Mg are used to reduce the energy barrier height [2]. However, such metals are very reactive and susceptible to oxidation in air without an appropriate passivation. Therefore, more environmentally stable electrodes such as Al are needed. However, the high work function of Al results in lower luminous efficiency and higher operating voltage in OLEDs with an Al cathode. Recently, a thin insulating layer such as LiF, CsF, MgF2, etc. deposited between an organic layer and Al cathode has been shown to greatly enhance the electron injection and the EL efficiency [4], [5], [6], [7]. Since such a scheme usually involves particular chemical interaction of metal/EIL/ETL, its effectiveness is sensitive to the metal used and very often limits the choice of cathode metals to Al only. Although a lot of researches have carried out to understand the mechanism of the thin insulating layer for the enhanced electron injection, it is still not clearly understood. Recently, Cannon Inc. and other groups reported that cesium carbonate (Cs2CO3), either vacuum deposited as an individual layer over the organic ETL or codeposited with ETL, effectively facilitates electron injection from a wide range of metal electrodes [8], [9]. While device performances using such cathode structures are encouraging, mechanisms of this injection scheme are not understood. People are still searching for the high efficient cathode for OLEDs.

In this paper, we report the application of a thin samarium (Sm) layer overlapped by the LiF and Al layer as efficient cathode in OLEDs. Compared to LiF/Al cathode, Sm/LiF/Al cathode improves significantly the electroluminescent (EL) brightness and efficiency, the power efficiency is enhanced up to 40% with respect to LiF/Al cathode. The drive voltage was also reduced from 12.5 V for device with LiF/Al cathode to 10.2 V for device with the Sm (0.5 nm)/LiF/Al cathode at the current density of 500 mA/cm2.

Section snippets

Experimental

In this study, three kinds of devices (ITO/NPB/AlQ/Sm/LiF/Al) with different Sm thickness have been fabricated and their device properties were compared with the control device (ITO/NPB/AlQ/LiF/Al). Chemical structures of molecules used in the experiment and the configuration of the devices are shown in Fig. 1. N,N′-Bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl 4,4′-diamine (NPB)/tris-(8-hydroxyquinoline) aluminum (AlQ) were selected as the HTL and ETL, respectively. A cathode Sm/LiF/Al was used

Results and discussion

It was found experimentally that the thicknesses of Sm affect significantly on the EL performance of devices. The devices with different thickness of Sm layer have been made. Current density–voltage (JV) characteristics of the devices with the cathode of LiF/Al or LiF/Al together with Sm layer with different thickness are presented in Fig. 2a. As can be seen from this figure, the current densities of the devices with Sm/LiF/Al cathode are higher than that of the control device. The device with

Conclusion

An efficient cathode with the structure of Sm/LiF/Al was introduced in the cathode fabrication of the OLEDs. The device with 0.5 nm layer of Sm with the cathode of Sm/LiF/Al showed the improved power efficiency of 40% than the control device of the conventional Al/LiF cathode at 100 mA/cm2. The device with 0.5 nm Sm layer has the highest brightness of 12356 cd/m2 at 16 V, yet the control device has the brightness of about 6870 cd/m2 at the same voltage. The drive voltage of the device with the

Acknowledgements

This research was supported by National Nature Science Foundation of China under contract No. 90201004 and by Beijing Science Foundation under contract No. H0304300204.

References (18)

  • Hee-Young Kang et al.

    Mater. Sci. Eng.

    (2004)
  • Y.Q. Li et al.

    Chem. Phys. Lett.

    (2003)
  • C.W. Tang et al.

    Appl. Phys. Lett.

    (1987)
  • M.A. Baldo et al.

    Nature

    (1998)
  • Y. Cao et al.

    Nature

    (1998)
  • L.S. Hung et al.

    Appl. Phys. Lett.

    (1997)
  • H.W. Choi et al.

    Appl. Phys. Lett.

    (2005)
  • S.T. Zhang et al.

    Appl. Phys. Lett.

    (2004)
  • Hasegawa et al.

    SID Int. Symp. Digest Tech. Papers

    (2004)
There are more references available in the full text version of this article.

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