Elsevier

Thin Solid Films

Volume 498, Issues 1–2, 1 March 2006, Pages 249-253
Thin Solid Films

Improved efficiency of organic light-emitting diodes using CoPc buffer layer

https://doi.org/10.1016/j.tsf.2005.07.120Get rights and content

Abstract

In this paper, the remarkable improvements in turn-on voltage and luminance have been demonstrated in an organic light-emitting diode (OLED) using cobalt phthalocyanine (CoPc) layer as a hole injection layer (HIL) in a device structure of ITO/CoPc/NPB (60 nm)/Alq3 (75 nm)/LiF (1 nm)/Al (200 nm). The J–V, L–V and ηp–V characteristics were measured at room temperature with a thickness variation of CoPc layer. The improvement of the luminance intensity by a factor of more than two has been obtained and the turn-on voltage at 1 cd/m2 decreased about 1 V with the CoPc layer. We propose that such an improvement is mainly due to the relative energy level of the highest-occupied molecular orbital (HOMO) band between that of the NPB hole transport layer (HTL) and the Fermi level of ITO anode, leading to enhance the hole injection from the ITO to organic layer.

Introduction

Organic light-emitting diodes (OLEDs) are currently considered as promising candidates for large-area, full-color and flat-panel displays due to their prominent advantages such as ease in fabrication and convenience in application [1]. For OLEDs, the injection efficiency is a critical parameter and depends in an important way on the work function of the electrode. The potential barrier between ITO and emissive severely limits the efficiency of hole injection. The treatment of the ITO surface usually has a strong influence on the performance of the OLED device [2], [3], [4], [5], [6], such as the O2 plasma treatment increases the work function of ITO and removes its surface contaminants, thus enhancing the hole injection [7]. Yet the work function of the O2 plasma-treated ITO is not high enough, and the energy barrier exists for the hole injection from ITO to the organic layer. Another problem with the ITO anode is the diffusion of indium into the organic layer during device operations, which was found to be correlated with the decay of device performance [8].

In recent years, an important enhancement of device stability was achieved by depositing a copper phthalocyanine (CuPc) layer on the ITO anode [9], [10] because of the good match of its HOMO (5.1 eV below vacuum [11]) to the work function of ITO. Moreover, CuPc has very weak absorption of light with wavelengths between 400 and 500 nm, making it suitable for use in blue and green OLEDs [12]. Scott et al. [13] reported that the oxygen released from ITO to HTL would result in the formation of luminescence quenching centers, and increase the operational voltage of the OLEDs. While CuPc was inserted between the ITO anode and HTL, it could reduce the ITO-induced degradation on the HTL materials. Moreover, Nuesch et al. [14] found that a thin CuPc layer was deposited on top of the ITO, the oxygen in ITO would diffuse into CuPc layer and an interfacial space charge layer was formed. It resulted in the effective ITO work function pinned at the HOMO orbital energy of CuPc and the injection efficiency of charge carriers did not depend significantly on the surface treatment of the ITO electrode [14].

The two most widely investigated materials of this family are H2Pc and CuPc compounds. This is mainly due to their early synthesis, dating back to 1907 and 1927, respectively, as well as to their extensive use in the dye industry. It is why CoPc and other M-Pc in comparison have received less attention, which is the motivation of present work. The chemical structure of MPc allows tuning of its oxidation potential (or ionization potential) by altering the central atom in Pc macrocycles. In this paper, we fabricated an OLED by inserting the CoPc thin film as an HIL on a glass substrate and evaluated the OLED characteristics. We also systematically investigated the cyclic voltammetry (CV) and UV–VIS absorption characteristics of CoPc and then the HOMO and LUMO levels were determined.

Section snippets

Experimental details

The ITO-coated glass substrates (Merck Display Technologies) used have a film thickness of 0.2 μm and sheet resistance of 10 Ω/sq. They were initially degreased by scrubbing them in the detergent (Merck Extran) solution. Then they were immersed sequentially in heated ultrasonic bath of DI water, isopropyl alcohol and ethanol for 15 min each followed by being rinsed in DI water. Finally, the substrates were blown dry with nitrogen gas and then treated by O2 plasma for 2 min prior to use.

Luminance–current density–voltage and EL characteristics of OLEDs

The J–V of the devices with respect to the CoPc thickness under forward bias are shown in Fig. 1. As can be clearly seen, the J–V performance of the devices is strongly dependent on the presence and the thickness of the CoPc buffer layer. Moreover, with the inserting of the CoPc buffer film, the threshold voltages decrease markedly. Fig. 2 shows the L–V for the devices in Fig. 1. Similarly, for the same emission intensity, the devices with CoPc buffer layer have the lower voltage. The turn-on

Conclusion

In this paper, we have demonstrated that inserting the CoPc buffer layer between ITO and HTL can significantly improve the turn-on voltage, luminance and current density of OLEDs due to the enhancement of the hole injection. The determined energy level alignments measured using CV and UV–VIS measurements have provided direct evidence to reveal the origins of the improved device performance. The red shift observed in the EL spectra was increased with the thickness of the CoPc layer, which may be

Acknowledgment

This work was supported by Ministry of Economic Affairs of the Republic of China (No. 91-EC-17-A-07-S1-0018) and Micro-Nanotechnology Research Center of the NCKU.

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