PVK를 이용한 고분자 EL 소자의 전기-광학적 특성 연구

권승호, 권오준, 백상윤, 유재수

중앙대학교 화학공학과

Study on the Electro-Optical Characteristics of Polymer EL Device using PVK

Seung Ho Kwon, Oh Jun Kwon, Sang Yoon Paik, Jae Soo Yoo

Dept. of Chemical Engeering, Chung-Ang University

Introduction

Small molecule-based devices are now at commercial level, while the performance of polymer-based devices are enhancing and new processing techniques are tried for full-color EL displays[1]. One of the advantages of polymers over small molecules is film-forming property. Polymer films can be formed with easy by spin-coating or dip-coating using polymer solution. The production cost of polymer-based devices can be considerably lower than that of small molecule-based devices[2]. Compared with intrinsically conductive polymers, dye-doped polymers, which are polymers containing functional groups as side groups or in the main chain, are much more flexible in material design. By selecting a proper dopant molecule and the concentration, it is possible to optimize carrier transport properties as well as emission color. Thus dye-doped polymers are ideally suited for the fabrication of single layer EL devices[3].

In order to develop portable polymer EL device with the luminance over 1000cd/m2 operating at turn-on voltage below 5V, it is essential to down the turn-on voltage. Reducing the turn-on voltages may be possible by optimizing film thickness as well as anode and cathode. Also, introducing buffer layer such as PEDOT/PSS can be effective, because the charge injection efficiency for the PEDOT/PSS is much stronger than ITO[4].

Organic electroluminance(EL) film device having the single layer structure of indium-tin-oxide(ITO) / poly(n-vinylcarbazole, PVK) / cathode was fabricated to examine the turn-on voltage. Here, tris-8-hydroxyquinoline aluminum(Alq3) and nile red were also co-doped to PVK as electron transporting material and luminescent dye, respectively. Lithium-aluminum alloy was used as cathode. The parameters to have influence on the reduction of turn-on voltage were identified and the methodology for low-power consumption will be discussed from the material points as well as EL structure in this presentation.

Experimental

ITO-glass with sheet resistance of 8.31□/□ was cleaned with trichloroethylene(TCE), acetone, methanol. Polymer films were formed by spin-coating using chloroform solution containing PVK(0.2g), Alq3(20wt%), and dye(0.5%wt) on the ITO-glass with the size of 30mm×30mm. After baking, Al-Li alloy as cathode was deposited by vacuum deposition method. The thickness of film and Al-Li layer was measured by Ellipsometer. And then, the electro-optical properties of device were investigated.

Results and discussion

The device structure is a single layer structure having the doped PVK layer sandwitched between electrodes. The cell configuration and the molecular structures of the materials used in this work are shown in Fig. 1. Electro-optical properties of each film classified by thickness were investigated. At first, electro-optical characteristics of doped PVK layer containing only 0.5wt% of nile red as red emitting center were investigated. From this device, turn-on voltage of each film was about 10∼11V, as shown in Fig. 2. Luminance of the film with 64nm thick was 12.6cd/m2 at 18V.

Fig. 1 Configuration of the EL device and molecular

structure of material used.

Fig. 3 shows current vs. voltage characteristics of ITO/PVK+Alq3+Nile red/Li-Al device. The turn-on voltage was about 11∼12V. Maxinum luminance was 259cd/m2 at 26V with 84.3nm of film thickness, as shown in Fig. 4. Luminous efficiency were calculated by following equation;

[eqn: eta ~=~ { pi L} over {10JV/A }]

Here, L and A are luminance(cd/m2) and cell area(cm2), respectively, and

also J and V are current(mA) and voltage(V). Maximun efficiency was 0.021

lm/W at 21V.

Generally, luminous efficiency may improve by reducing both voltage and

current.

(a)                                     (b)

Fig. 2 Current-Voltage Characteristics  Fig. 3 Current-Voltage characteristics

of ITO/PVK+Nile Red/Li-Al device  of ITO/PVK+Alq3+Nile Red/Li-Al device

(a)                                     (b)

Fig. 4 (a)Luminance-voltage characteristics and (b)Efficiency-voltage                 characteristics of EL devices according to film thickness

EL spectra of each device were measured at 20V. The peak seen at 586nm may originated from nile red and 506nm from Alq3. Emission peak of device(A) was stronger than that of device(B) by adding Alq3, as shown in Fig. 5-(a). Voltage increased, as shown in Fig. 5-(b). But current decreased relatively. Generally, turn-on voltage may increased according to the concentration of dopant, that is, the more amount of dopant increase, the more turn-on voltage also increased.

Our work was not optimized yet. Our objective is to identify the materials and structure for reducing the turn-on voltage and improve the OLED performance. The optimization of the proper thickness of film as well as to control doping concentration of Alq3 and dye should be continued. Also, oxygen-plasma treatment of ITO glass and the adoption of buffer layer will be presented in this presentation.

(a)                                    (b)

Fig. 5 Comparison of (a)EL spectra and (b) current-voltage characteristics          with ITO/PVK+Nile Red device(A) and ITO/PVK+Alq3+Nile Red/Li-Al        device(B)

Summary

It has been reported that polymer-based devices with single or double layer structure operating below 10V, which have luminance over 1000cd/m2 with long life time, have been fabricated. There are some advantages in polymer based devices compared with small molecule-based devices such as the intensity of film, simple process, and low cost. Our objective is to down turn-on voltage below 5V. Practically, EL device operating below 5V would be reported. Also, the basic understanding on the effects of material and EL structure on the turn-on voltage may lead to the practical OLED development.

References

[1] J. Kido, Proceedings of the Fifth International Display, 98IDW, December,     1998.

[2] J. Kido, Trends in Polymer Science, 10, 350∼355, 1994.

[3] J. Kido, H. Shionoya, and K. Nagai, Appl. Phys. Lett. 67(16) October,         1995.

[4] Y. Yang, J. Bharathan, SID 98 Digest, 1998.

Acknowledgement

This work was supported by Korea Science and Engineering Foundation(KOSEF) [# 1999-1-307-003-3]