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Efficient blue-to-violet organic light-emitting diodes

Chengfeng Qiu, Haiying Chen, Man Wong*, Hoi S. KwokDepartment of Electrical and Electronic Engineering, Center for Display Research,

The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

Received 22 April 2002; received in revised form 27 January 2003; accepted 8 May 2003

Abstract

Organic light-emitting diodes emitting in the range of 400 nm (violet) to 460 nm (blue) are reported. The basic device structure consists of

indiumtin oxide/N,N0-diphenyl-N,N0-bis-(3-methylphenyl)-1,10-biphenyl-4,40-diamine (TPD)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthro-

line (BCP)/lithium fluoride (LiF)/aluminum. Offset of the energy levels at the TPD/BCP interface favors blocking of holes on the TPD side of

the interface. Voltage-induced color change is observed and explained in terms of a switching from emission dominated by interfacial

exciplex-induced recombination at low applied bias to one dominated by bulk exciton-induced recombination at high applied bias. With the

addition of copper(II) phthalocyanine (CuPc) as an anode buffer layer and tris-8-(hydroxyquinoline) aluminum (Alq 3) as a cathode buffer

layer, external quantum efficiencies as high as 0.5% at blue emission and 0.4% at violet emission have been obtained.

# 2003 Elsevier B.V. All rights reserved.

Keywords: Organic light-emitting diode; Violet light; Voltage-tunable emission; Exciplex

1. Introduction

Organic light-emitting diodes (OLEDs) [1] are challen-ging liquid-crystals as an alternative flat-panel display tech-

nology because of their relative merits of being self-emitting

with a wider viewing angle, having a faster switching speed

and being easier to manufacture because of their all solid-

state nature.

Using N,N0-diphenyl-N,N0-bis-(3-methylphenyl)-1,10-

biphenyl-4,40-diamine (TPD) as a hole transport layer and

4-dicyanomethylene-6-(p-dimethylaminostyryl)-2-methyl-

4H-pyran (DCM)-doped tris-8-(hydroxyquinoline) alumi-

num (Alq3) as an emission layer, Mori et al. [2] obtained

voltage-tunable emission from red to green. Using two

electron transport layers, Hamaguchi et al. [3] and Liu

et al. [4] fabricated green-to-blue and red-to-orange vol-

tage-tunable OLEDs. Kalinowski et al. fabricated an orange-

to-greenish yellow voltage-tunable device by inserting a

layer of red-emitting perylene-bis(2-phenyl) imide between

an Alq3 electron transport layer and a magnesium cathode

[5] and a red-to-blue device using a perylene-bis(2-phenyl)

imide-doped TPD [6].

Despite the variety of voltage-tunable small-molecule

OLEDs, their emissions are typically in the longer wave-

length regime of the visible spectrum. In this paper, the

fabrication and characterization of an efficient blue-to-violet

emitting OLED using TPD as a hole transport layer and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as a

hole-blocking [7,8] electron transport layer are reported.

When the effective energy barriers against carrier injection

are reduced by incorporating copper(II) phthalocyanine

(CuPc) as an anode buffer layer [9] and Alq3 as a cathode

buffer layer, efficiencies as high as 0.5% at blue emission

and 0.4% at violet emission have been obtained. A max-

imum luminance of greater than 2000 cd/m2 has been

measured for the violet emission.

2. Materials and device fabrication

Glass coated with 70 nm indiumtin oxide (ITO) was

used as the starting substrate. The sequence of pre-cleaning

prior to loading into the evaporation chamber consisted of

ultrasonic detergent soak for 30 min, de-ionized (DI) water

spray for 10 min, ultrasonic DI water soak for 30 min, oven

bake-dry for 12 h and ultraviolet ozone illumination for

9 min [10].

The constituent organic layers for the OLEDs were

deposited on the ITO glass substrates by thermal vacuum

evaporation of commercial grade TPD, BCP, Alq3 and CuPc

(Fig. 1) powder sources loaded in resistively heated eva-

Synthetic Metals 140 (2004) 101104

* Corresponding author. Tel.: 852-2358-7050; fax: 852-2358-1485.

E-mail address: eemwong@ee.ust.hk (M. Wong).

0379-6779/$ see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0379-6779(03)00359-X

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poration cells. The base pressure in the evaporator was

8 mTorr. The deposition rates of the organic thin films

were 0.20.4 nm/s. While the ITO formed the anodes of the

OLEDs, 0.1 nm lithium fluoride (LiF) topped with 150 nm

aluminum (Al) composite layers were used as the cathodes.

The deposition rates of LiF and Al were 0.020.05 and 1

1.5 nm/s, respectively. Film thickness was determined in situ

using a crystal monitor.

Two types of 4 mm diameter OLEDs were fabricated

using a set of shadow masks. These are type D devices

without the electrode buffer layers: ITO (75 nm)/TPD

(60 nm)/BCP (30 nm)/LiF (1 nm)/Al (150 nm), and typeB devices with the electrode buffer layers: ITO

(75 nm)/CuPc (20 nm)/TPD (40 nm)/BCP (30 nm)/Alq3(30 nm)/LiF (1 nm)/Al (150 nm). The devices were char-

acterized in room ambient and temperature without encap-

sulation. Electroluminescence (EL) intensity was measured

using a Kollmorgen Instruments PR650 SpectraScan spec-

trophotometer and currentvoltage (IV) characteristics

were measured using a Hewlett-Packard HP4145B semi-

conductor parameter analyzer.

The 100 nm thick organic layers for measuring the photo-

luminescence (PL) and absorption spectra were deposited,

respectively, on sapphire and on quartz substrates by thermal

vacuum evaporation. Photoluminescence and absorption

spectra of Alq3, BCP, TPD and a 4:1 co-evaporated mixture

of BCP:TCP were measured. The excitation light for PL was

obtained from a HeCd laser at a wavelength of 337 nm.

3. Results and discussion

Typical luminance (L)current density (J)V character-

istics of the two types of devices are shown in Fig. 2. Since

both hole and electron injection efficiencies are improved

with the incorporation of the electrode buffer layers, the

power efficiency of type B device is clearly higher because

of both higher luminance current efficiency (defined as the

ratio of L to J) and lower threshold voltage for L and J.

Typical voltage-dependent EL spectra of type D and B

devices are shown in Fig. 3a and b, respectively. For a type D

device biased at 10 V, a spectral peak can be observed at a

wavelength (l) of466 nm (blue). As the spectral intensity

decreases with decreasing l, a minor plateau can be seen at

l 400 nm (violet). With increasing bias, the major peak

shifts to shorter l from 466 nm and the intensity at 400 nm

increases. At a bias of 16 V, the peak shifts to 420 nm,

equivalent to a deep blue emission. A respective externalquantum efficiency (x) and maximum power efficiency (Zp)

of0.4% and 0.3 lm/W have been obtained for blue. The

corresponding numbers for violet were x 0:2% and

Zp 0:08 lm/W for violet. A maximum luminance of

360 cd/m2 has been measured.

For a type B device biased at 10 V, similar spectral peak

and plateau can be observed at l 466 and 400 nm, respec-

tively. With increasing bias, the major peak shifts to a shorter

l. At a bias of 16 V, the peak shifts to 400 nm, though the

intensity at 420 nm is still strong. A respective external

quantum efficiency (x) and maximum power efficiency (Zp)

of0.5% and 0.5 lm/W have been obtained for blue. The

corresponding numbers for violet were x 0:4% and

Zp 0:2 lm/W for violet. A maximum luminance of

2010 cd/m2 has been measured.

The energy-level diagram of a type B device is shown in

the inset in Fig. 4a [7,11]. Because of the lower highest-

occupied molecular orbital (HOMO) of the electron-trans-

porting BCP compared to that of the hole-transporting TPD,

holes drifting through the TPD are blocked on the TPD side

of the TPD/BCP interface. Similarly, because of the higher

lowest-unoccupied molecular orbital (LUMO) of TPD,

electrons are blocked on the BCP side of the TPD/BCP

interface. The energy barrier against electron injection is

Fig. 1. Molecule structures of the constituent organic materials used in

OLED construction. Fig. 2. LJV characteristics of type D and B devices.

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smaller than that against hole injection at the TPD/BCP

interface.

The photoluminescence spectra of Alq3, BCP, TPD and a

4:1 co-evaporated mixture of BCP:TCP are summarized in

Fig. 4a. PL peaks are observed for Alq3 at 520 nm, for BCP

in the ultraviolet range and for TPD at 420 and 400 nm.

Therefore, it is possible to identify TPD as being responsible

for the EL emission at 420 and 400 nm. The PL spectrum of

the BCP:TPD mixture peaks at 452 nm, a wavelength sig-

nificantly longer than the peak locations of pure BCP or

TPD. This peak, the closest that can be attributed to the EL

emission at 466 nm, is attributed to exciplex recombina-

tion [12] associated with the LUMO and HOMO of BCP and

TPD, respectively. Compared to the absorption spectra of

pure TPD and BCP shown in Fig. 4b, no new absorption

peaks were observed on the BCP:TPD mixture. This is

further evidence that the PL emission attributed to exciplex

is not induced by any aggregate type effects. Lastly, the most

likely cause of the relative red-shift of the EL peak from the

PL peak is microcavity effect [13].

It is presently proposed that the emission at 466 nm is

associated with exciplex emission at the TPD/BCP interface

(inset of Fig. 4a). At low applied bias, both the injection of

carriers across the interface and their drift in the bulk of the

transport layers are limited. Exciplex emission dominates,

involving the excited state of BCP and the ground state of

TPD. As the applied bias is increased, more electrons than

holes are injected across the interface because of the smaller

Fig. 3. Normalized EL spectra of (a) type D and (b) type B OLEDs.

Voltage-induced spectral shifts are clearly visible. Shown in insets are the

relative EL spectra before normalization and the corresponding device

structures.

Fig. 4. (a) PL spectra of TPD, BCP, Alq3 and a 4:1 mixture of BCP:TPD

on sapphire. The energy-level diagram of a type B device is shown in

the inset. (b) Absorption spectra of TPD, BCP, and a 4:1 mixture of

BCP:TPD on quartz.

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energy barrier against electron injection. Exciton formation

in the bulk of TPD and their subsequent recombination

become dominant. This explains shifting of the emission

peaks to 420 and 400 nm at higher applied bias. Such

voltage-induced switching of exciplex (yellow) to exciton

(green) dominated emission has been observed in OLEDs

involving a different set of constituent materials [14].

4. Conclusion

Efficient blue-to-violet voltage-tunable organic light-emit-

ting diodes, with quantum efficiencies of 0.4 and 0.2%,

respectively, for blue and violet, have been fabricated and

characterized. With the incorporation of electrode buffer

layers, the efficiency can be further improved to, respectively,

0.5% for blue and 0.4% for violet. The voltage-controlled

emission has been explained in terms of a switching from

interfacial emission dominated by exciplex recombination to

bulk emission dominated by exciton recombination.

Acknowledgements

This research was supported by a grant from the Research

Grants Council of the Hong Kong Special Administrative

Region.

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