Elsevier

Microelectronics Journal

Volume 37, Issue 9, September 2006, Pages 916-918
Microelectronics Journal

Bright green organic light-emitting devices having a composite electron transport layer

https://doi.org/10.1016/j.mejo.2006.01.013Get rights and content

Abstract

A bright green organic light-emitting device employing a co-deposited Al–Alq3 layer has been fabricated. The device structure is glass/indium tin oxide (ITO)/ N, N′-diphenyl-N, N′- (3-methylphenyl)-1, 1′-biphenyl-4, 4′-diamine (TPD)/tris(8-quinolinolato) aluminum (Alq3)/ Al–Alq3/Al. In this device, Al–Alq3 is used as electron transport layer (ETL). The device shows an operation voltage of 6.1 V at 20 mA/cm2. At optimal condition, the brightness of a device at 20 mA/cm2 is 2195 cd/m2 achieved a luminance efficiency of 5.64lm/W. The result proves that the composite Al–Alq3 layer is suitable for the ETL of organic light-emitting devices (OLEDs).

Introduction

Since Tang and Vanslyke developed multi-layer organic light-emitting devices (OLEDs) [1], OLEDs are of considerable importance for their potential as a generation of flat panel displays because of their high brightness, high luminance efficiency, wide color range, easy fabrication process, low operation voltage and possibility for flexible displays [2]. Therefore, tremendous efforts have been made toward improving the device performance. It was known that the performance of OLEDs depends heavily on the efficiency of carrier injection and their recombination, which generate molecular excitions, as well as the balance of the holes and electrons [3], [4]. In order to achieve maximal efficiency, more carrier injection into the emission layer is a must in the device. Generally, the injected hole is usually more mobile than the injected electron under the same electric field. If we could increase the carriers heavily, especially the injected electrons, it is sure that the device performance, including the operation voltage, the brightness, the luminance efficiency etc, could be obviously improved. Finding a way of increasing the number of electrons or reducing the number of holes reaching the emission zone is considered as one of most direct and economic solutions to improve device efficiency.

Up until now, there are several approaches to increase the number of injected electrons including modification of the cathode contact or electron transport layer which have been known to improve carrier balance and recombination [5], [6]. One of the most simple and often used methods is to insert a buffer layer, such as the inorganic Li compounds (Li2O or LiF), between an emitting layer (EL) and the cathode which can improve the device efficiency and stability [5], [7]. Junki Kido et al who used a buffer layer of Alq3 doped Li got the maximal brightness of 30,000 cd/m2, while 3400cd/m2 for the device without the buffer layer [8].

Consequently, we consider whether we can also improve the device performance by using the Alq3 doped Al as the ETL. Another reason for this idea is that the process will be much simpler if we use such a layer.

In this letter, we will demonstrate that this OLED structure can significantly improve the device performance by increasing the number of injected electrons. The OLED shown in this letter has also delivered a luminance efficiency of 5.64 lm/W, a brightness of 2195 cd/m2, and the operation voltage of 6.1 V at 20 mA/cm2.

In our experiment, two green devices have been fabricated. Device I is a standard green device, and Device II is a green device with a composite electron transport layer. The devices structures are shown in Fig. 1. For the Device I, LiF/Al is the bi-layer cathode, tris(8-quinolinolato) aluminum (Alq3) is the ETL and the green EL.For the Device II,Al–Alq3,which is co-deposited, is the whole ETL, and the tris(8-quinolinolato) aluminum (Alq3)is the green emitter layer. After the routine cleaning procedure of ultrasonicating the indium-tin oxide (ITO) coated glass in organic solvents, deionized water, and dried in N2 atmosphere, then treated by O2 plasma. The N, N'-diphenyl-N, N'-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) (30 nm) layer, which is hole transport layer(HTL),was spin coated. The multi-layer structure of Alq3 (10 nm) /LiF(1 nm)/Al(220 nm) and the Al–Alq3(∼1:1) (atom/molecule) (5 nm) layer were deposited on the substrate by resistive heating. Devices were then fabricated, encapsulated, and ready for characterized.

Another Device III [Glass/ITO/TPD (30 nm)/Alq3 (15 nm)/Al (220 nm)] without any buffer layer was fabricated, for the purpose of comparing with the Device II with such a buffer layer. Its structure is shown in Fig. 2. Alq3 is both the ETL and the EL.

The current–voltage-brightness characteristics of the devices were measured after all devices were hermetically sealed at room temperature.

Fig. 3 shows the curve of JV characteristics of the green devices. Lower operation voltage was observed in the Device II with the Al–Alq3 ETL than that of the standard green Device I and the Device III. For instance, the operation voltage at 20 mA/cm2 is 6.1 V for the Device II with the Al–Alq3 ETL, 6.3 V for the standard green Device I, and 8.0 V for the Device III. It indicates that the barrier height for electron injection has been reduced and then the electrons can be more easily injected from the cathode to the EL of the device with the Al–Alq3 ETL structure.

Fig. 4 shows the curve of brightness–voltage (BV) characteristics of the green devices. Considerably higher brightness was observed in the Device II with the Al–Alq3 ETL compared to that of the standard green Device I and the Device III which we believe is due to the increased injected electrons because of the Al–Alq3 ETL that results in more balanced charge carriers injected and more carrier recombination at the emitting zone. Therefore, the luminance efficiency of the device is also improved by 28%. The brightness at 20 mA/cm2 is 2195 cd/m2 for the Device II with the Al–Alq3 ETL, 1775 cd/m2 for the standard green Device I and 1000 cd/m2 for the Device III, respectively. The luminance efficiency for the Devices I, II and III at 20 mA/cm2 is 4.42, 5.64, and 1.96 lm/W, respectively.

These results clearly demonstrate that the co-deposited Al–Alq3 at the cathode interface is effective in increasing the number of injected electrons, lowering the operation voltage, keeping the balance the hole–electron injection, resulting in higher brightness and device efficiencies. We believe that the reason for this result is the following explanation. The co-deposited Al–Alq3 layer has two contributions here. One contribution is the existence of such a buffer layer: Al–Alq3 reduces the barrier height for electron injection and improves the electron conductivity of the device, so the number of injected and transported electrons to the emitting zone are increased, resulting the balance with the injected holes from the anode is kept, then the efficiency of their recombination is increased. Another contribution is: Al–Alq3 as a dielectric buffer layer reduces quenching in the OLEDs efficiently, because the brightness of the device (using the Alq3 as the EL) will be quenched severely if the distance between Alq3 film and cathode is shortened to a certain extent [9], [10].

In conclusion, we have succeeded in obtaining the bright green device through the use of a composite electron transport layer. The incorporation of Al–Alq3 (for ETL) not only results in such an evident increase in device efficiency and brightness, but also emits color that is essentially identical to that of the device without the Al–Alq3 ETL. Another important advantage is that fabrication of this device is easier than any other operations using other materials as the ETL, so the cost will be much lower.

Section snippets

Acknowledgement

This work was supported in part by NSF (No.60276026).

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