Degradation effect of polymer hole transport layer on organicelectroluminescence device performance

Jeong-Woo Choia,*, Joo Sung Kima, Se Yong Oha, Hee-Woo Rheea,Won Hong Leea, Sang Baek Leeb

aDepartment of Chemical Engineering, Sogang University, C.P.O. Box 1142, Seoul 100-611, South KoreabDepartment of Chemical Engineering, Cheju National University, Cheju 690-756, South Korea

Abstract

The effect of heat treatment to the polymer hole transport layer (HTL) on the performance of the organic light emitting devices (OLEDs)

was investigated. Poly-N- (p-diphenylamine) phenyl methacrylamide [PDPMA] was used as a polymer HTL. Polymer HTL was coated onto

ITO substrate by spin coating method. The effect of polymer HTL degradation on the turn-on voltage of the prepared OLEDs was

investigated by I±V measurement at the various conditions of temperature and exposure time. OLEDs were fabricated with the structure

of ITO/Polymer HTL/Emitting layer (Alq3)/Electrode (Al), and polymer HTL was treated by heat. The different surface morphologies of

polymer HTL was observed by atomic force microscopy (AFM) at the variation of exposure time and temperature. The degree of the polymer

HTL degradation was increased as the exposure time and temperature were increased, and it was observed that the increase of turn-on voltage

was primarily caused by the polymer HTL degradation. q 2000 Elsevier Science S.A. All rights reserved.

Keywords: Organic electroluminescence device; Atomic force microscopy; Thermal degradation

1. Introduction

In the electroluminescence devices industry, the remark-

able technological improvement has been made with the

initial discovery of conjugated polymer electrolumines-

cence [1,2]. It has been widely known that the polymer

light emitting diode (LED) displays have several advan-

tages, such as easiness of fabrication and commercially

useful lifetime [3], compared with the inorganic-based

LED. Polymer LED shows no image ¯icker, image latency,

and loss of contrast or color change, compared with the

monomer-based LED. For the commercialization of organic

electroluminescence devices, two major criteria should be

considered, i.e. quantum ef®ciency and lifetime. In the

viewpoint of the importance of the hole mobility and the

density in the emitting layer (EML), the number of injected

holes from the anode should be balanced to enhance the EL

quantum ef®ciency [4]. It has been considered that humidity

and Joule heat would be the crucial factors mainly affecting

the degradation process of the devices [5±7]. Although the

encapsulation method has been used as one of the best poli-

cies to protect the EL device from degradation sources and

to extend the lifetime of devices, the Joule heat caused by

large current ¯ow irrespective of light emission could not be

eliminated. Other methods to reduce the current ¯ow with-

out the reduction of the light emitting performance of the

device have also been reported, such as the fabrication of the

polymer/organic hetero-structure devices with various

organic layers for hole blocking [8]. Even though the

Joule heat can be prevented by fabricating this kind of struc-

ture, the ef®ciency of electroluminescence device can be

reduced if polymer layer degradation occurs. It has also

been reported that the poor electroluminescence ef®ciency

can be resulted from the device degradation caused by the

crystallization of the HTLs [9,10] or the delimitation at the

cathode/Alq3 interface [11,12]. However, the time course

behavior or the temperature dependency of the polymer

HTL degradation caused by the Joule heat has not been

reported yet. In this study, the degradation processes of

polymer HTL were investigated by the observation of

morphology change using atomic force microscopy

(AFM) at the different conditions of temperature and expo-

sure time. Maximum temperature in the experiment was

958C due to the consideration of local Joule heat [13]. The

effects of the surface degradation of polymer HTL on the I±

V characteristics and turn-on voltage of the fabricated elec-

troluminescence device were investigated.

Thin Solid Films 363 (2000) 271±274

0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.

PII: S0040-6090(99)01001-9

www.elsevier.com/locate/tsf

* Corresponding author. Tel.: 1 82-2-705-8480; fax: 1 82-2-711-0439.

E-mail address: jwchoi@ccs.sogang.ac.kr (J.-W. Choi)

2. Experimental

Tris(8-quinolinolato)aluminum (Alq3), used as light-

emitting material, was purchased from Sigma (St. Louis,

USA). Poly-N-(p-diphenylamine) phenyl methacrylamide

[PDPMA] was synthesized and used as a polymer HTL

[14]. The OLED consist of ITO/HTL (polymer)/EML

(Alq3)/Electrode (Al). The ITO-coated glass plates were

kindly obtained from Samsung Display Device Co.

(Korea). The ITO-coated glass (transparent anode, sheet

resistance ,20 V/cm2) was treated by chromic-sulfuric

acid solution followed by rinsing with de-ionized water,

methanol, and acetone, and ®nally dried under nitrogen

gas. ITO-coated glass was treated with O2 plasma treatment.

The polymer HTL was prepared onto the pretreated ITO

glass by spin coating method (PWM32, Headway Research

Co., USA). The thickness of polymer HTL was found as

about 800 AÊ based on the measurement by the ellipsometry

analysis. Each polymer HTL prepared onto ITO glass was

thermally treated at 55 and 958C, respectively. The exposure

times to temperature variation were 3, 5, 7 and 14 days,

respectively. The surface morphologies of the thermally

treated polymer HTL according to the variation of exposure

time and temperature were obtained by AFM (Autoprobe

CP, Park Scienti®c Instruments, USA). The Alq3 (EML) and

aluminum electrode were then sequentially deposited onto

polymer HTL by vacuum evaporation (1026 Torr). The

thickness of Alq3 emitting layer and of Al to be deposited

were set to 800 and 1000 AÊ , respectively (CRTM-5000,

SINKU-KIKO Co., Japan). The I±V characteristics of the

fabricated device were measured using the source measur-

ing unit (SMU 236, Keithley, USA).

3. Results and discussion

Since the ITO surface was used as a template for the

J.-W. Choi et al. / Thin Solid Films 363 (2000) 271±274272

Fig. 1. The PDPMA morphology after 3 days (at 258C).

Fig. 2. The PDPMA morphology after 14 days (at 258C).

Fig. 3. The PDPMA morphology after 7 days (at 558C).

Fig. 4. The PDPMA morphology after 14 days (at 558C).

polymer HTL deposition and the surface morphology of the

polymer HTL can be affected by the template morphology,

the ITO surface morphology could be a very important

factor to affect the effective interface contact and effective

hole transport. O2 plasma treatment leads to improvement of

uniformity of the ITO surface, and thereby increases the

effective interfacial area. The change of the polymer HTL

morphology would be affected mainly by the thermal treat-

ment, not by the change of the template morphology.

Because, the severe changes of the template (ITO) morphol-

ogy was not be observed with respect to the various condi-

tions of time and temperature.

The surface morphologies of PDPMA ®lms fabricated by

spin coating method at the various temperature and exposure

time were shown in Figs. 1±6. It was observed that the thin

layer of PDPMA showed a fairly good surface morphology

and its surface morphology was not changed with the expo-

sure time at room temperature (258C) (Figs. 1 and 2). In the

case of the PDPMA ®lms treated at 558C, the quality of

surface morphology became worse than those treated at

room temperature in Figs. 3 and 4. There was not a remark-

able change in the surface morphology until 5 days, but the

number of voids (dark spots) was increased and the size

became broaden from 7 days. From Figs. 5 and 6. it was

observed that the trend in morphology degradation at 958Cwas similar to that at 558C except that the remarkable degra-

dation occurred from 5 days. In order to elucidate the rela-

tionship between the degradation of PDPMA morphology

and device performance, the I±V characteristics of the

devices fabricated at different conditions were also investi-

gated. In Figs. 3±8, bounces were observed in the I±V curves.

These irregularities should be related with the thermal treat-

ment conditions such as the variation of time and tempera-

ture. Since the degradation of the polymer HTL would result

J.-W. Choi et al. / Thin Solid Films 363 (2000) 271±274 273

Fig. 5. The PDPMA morphology after 5 days (at 958C). Fig. 6. The PDPMA morphology after 14 days (at 958C).

Fig. 7. The I±V Characteristic of ITO/PDPMA(PHTL)/Alq3(EML)/Al with time (at 558C): (X), ®rst; (W), 7 days; (P), 14 days; (L) 21 days).

in the decrease of the effective interfacial area for hole trans-

port which causes the decrease of the effective recombination

in emitting layer, the degradation of polymer HTL by thermal

treatment lead to the irregular I±V characteristics that caused

the increase of turn-on voltage as shown in Table 1. When the

operation time of OLEDs becomes longer, degradation of

PHTL morphology is caused by the local joule heat about

1008C. The external form of a thin ®lm can be in¯uenced by

thermal treatment. Void is a common morphological change

of a thin ®lm, which results in serious yield and reliability

problems in microelectronic devices. From the point of view

of device application, a morphological change such as the

formation of many tiny voids in a ®lm is serious problem in

turn±on voltage and lifetime. To clarify the thermal degrada-

tion below the glass transition temperature the void forma-

tion was more investigated.

Acknowledgements

This work has been ®nancially supported by university

basic research project of Ministry of information and

communication (96057-BT-II1).

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J.-W. Choi et al. / Thin Solid Films 363 (2000) 271±274274

Fig. 8. The I±V Characteristic of ITO/PDPMA(PHTL)/Alq3(EML)/Al with time (at 958C): (X), ®rst; (W), 7 days; (P), 14 days; (L) 21 days).

Table 1

The change of turn on voltage with exposure times and temperatures

3 days 5 days 7 days 14 days

258C 7.00 V 7.00 V 7.25 V 7.00 V

558C 7.25 V 7.25 V 7.75 V 8.40 V

958C 7.25 V 7.50 V 8.00 V 9.25 V