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Volume 29, Issue 3, July 2008, Pages 268-272
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Turn-on voltage reduction of organic light-emitting diode using a nickel-doped indium tin oxide anode prepared by single target sputtering

https://doi.org/10.1016/j.displa.2007.08.009Get rights and content

Abstract

Organic light-emitting diodes (OLEDs) with a nickel (Ni)-doped indium tin oxide (ITO) anode were fabricated. The Ni-doped ITO anode was prepared using sputter deposition of Ni–ITO single targets consisting of 1, 3 and 5 wt% of nickel. Turn-on voltage of OLED devices with the Ni-doped ITO anode was reduced by 2.5, 4 and 3.8 V for 1, 3 and 5 wt% targets, respectively. Half-luminance lifetime was improved by 2.5 times with a Ni(3 wt%)-ITO single target. The successful development in preparing Ni-doped ITO films by Ni–ITO single target sputtering allows this approach to be adopted for OLED manufacturing.

Introduction

Indium tin oxide (ITO) is a well known transparent degenerate n-type semiconductor that is commonly used as an anode material for organic light-emitting diodes (OLEDs). For this application, ITO requires high surface work function and smooth surfaces in addition to high transparency and conductivity. Adjusting surface chemical states of an ITO anode by gaseous plasma bombardment [1], [2], [3], immersion in solutions [4], [5], forming self assembled monolayers [6], [7], [8], introducing a top metallic oxide [9], [10], [11] or metal elements into ITO matrix [12], [13] has been proved to be able to raise ITO surface work function, while chemical mechanical polishing [14], [15] or plasma bombardment [16], [17] can lead to an improved surface morphology. Raising ITO surface work function enhances hole injection from ITO into organics due to the reduced potential barrier at the ITO/organics interfaces. This should generally result in the improvement in the optoelectrical characteristics of OLED devices. Smoothing ITO surface topography on the other hand minimizes the occurrence of local high current flow and hence diminishes joule heat caused lifetime degradation.

Until now, gaseous plasma surface treatment of ITO has been the major approach to raise ITO surface work function in the industrial manufacturing of OLEDs, whereas surface smoothing is generally conducted by the process control of ITO deposition or post-surface polishing. However, our recent studies which add high work function nickel (Ni) atoms into ITO matrix have shown their high potential in improving overall OLED performances [18]. This is because Ni is a high work function material (Φ  5.0 eV) and can form NiOx phases when introduced into ITO matrix. The existence of NiOx phase can raise ITO work function to a level of 5.4 eV which is much higher than that of the gaseous plasma treated ITO (Φ  4.6 eV). Hence, a large reduction in the operation voltage of OLED devices with a Ni–ITO/organics interface is usually observed. Our previous work has demonstrated that the turn-on voltage can be reduced by 2.3 V for a OLED device with an Ni(1.8 wt%)-doped ITO anode. Especially noticed is that the incorporation of Ni in the ITO matrix also yields a smoother ITO surface, leading to the enhanced device lifetime [19], [20].

Yet, the preparation of the Ni-doped ITO anode was conducted by a Ni–ITO co-sputter method that is difficult to be implemented into an in-line mass production system. In this article, we report on the formation of Ni-doped ITO film using a Ni–ITO single target with various Ni concentrations. The effects of the Ni-doped ITO anode prepared with this single target approach on the characteristics of OLED devices are investigated, and the results are compared with those fabricated with the Ni–ITO co-sputter method. The use of Ni–ITO single target allows the synthesis of Ni-doped ITO film be directly adopted in the current production line.

Section snippets

Device fabrication

OLED devices with a structure of Al/tris(8-hydroxyquinoline) aluminum (Alq3)/N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′biphenyl-4,4′-diamine(NPB)/ nickel-doped ITO/PET film were employed as an test vehicle. Fabrication of the device began with the deposition of a 100 nm undoped ITO film on a PET substrate using d.c. sputtering at substrate temperature of 100 °C. A 50 nm Ni-doped ITO film was then immediately deposited in the same sputter chamber with a Ni–ITO single target using an r.f. power of 30 

Results and discussions

To investigate the effects of Ni-doped ITO films on the performance of OLED devices, the stacked Ni-doped ITO/ITO films were characterized with a four-point probe and a Hitachi 3310 UV spectrometer for sheet resistance and optical transmittance, respectively. The in-depth Ni atomic concentration was analyzed using an Auger Electron Spectrometer (AES; VG MicroLab 310-F), and the surface morphology was measured by an atomic force microscope. The average Ni concentration calculated from AES depth

Conclusions

This article demonstrated that OLED devices with a Ni-doped ITO anode fabricated using a Ni–ITO single target performed better characteristics than that without Ni-doped one. A threshold voltage drop up to 1.52 V and a turn-on voltage reduction by 4 V have been achieved for the device with an anode sputtered with a Ni(3 wt%)-ITO single target. The half-luminance lifetime was enhanced by 2.5 times, attributed to the improved characteristics and surface roughness. The characteristics of OLED devices

Acknowledgements

The authors would like to thank the National Science Council of Taiwan for financially supporting this work under Contract No. NSC-95-2221-E-218-047. Thanks also go to Center for Advanced Optoelectronics of National Cheng Kung University for partially funding this project and providing analytical instruments.

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