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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: [email protected] (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.
102 C. Qiu et al. / Synthetic Metals 140 (2004) 101104
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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.
C. Qiu et al. / Synthetic Metals 140 (2004) 101104 103
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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.
References
[1] C.W. Tang, S.A. Van Slyke, Organic electroluminescent diode, Appl.Phys. Lett. 51 (1987) 913915.
[2] T. Mori, K. Miyachi, T. Mizutani, A Study of the electrolumines-
cence process of an organic electroluminescence diode with an Alq3
emission layer using a dye-doping method, J. Phys. D: Appl. Phys.
28 (7) (1995) 14611467.
[3] M. Hamaguchi, A. Fujii, Y. Ohmori, K. Yoshino, Voltage- and
polarity-tunable multicolor organic electroluminescent devices, Jpn.
J. Appl. Phys. 35 (1996) L1462L1464.
[4] Z. Liu, C. Tang, W. Zhao, Z. Zhang, X. Jiang, L. Wang, S. Xu,
Voltage-tunable-color triple-layer organic light emitting diodes, SPIE
3175 (1998) 142145.
[5] J. Kalinowski, P. Di Marco, M. Cocchi, V. Fattori, N. Camaiono, J.
Duff, Voltage-tunable-color multiplayer organic light emitting diode,
Appl. Phys. Lett. 68 (17) (1996) 23172319.
[6] J. Kalinowski, P. Di Marco, V. Fattori, L. Giulietti, M. Cocchi,
Voltage-induced evolution of emission spectra in organic light-
emitting diodes, J. Appl. Phys. 83 (8) (1998) 42424248.
[7] Y. Kijima, N. Asai, S. Tamura, A blue organic light emitting diode,
Jpn. J. Appl. Phys. 38 (1999) 52745277.
[8] M. Lkai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Highly efficient
phosphorescence from organic light-emitting devices with an
exciton-block layer, Appl. Phys. Lett. 79 (2) (2001) 156158.
[9] S.A. Van Slyke, C.H. Chen, C.W. Tang, Organic electroluminescent
devices with improved stability, Appl. Phys. Lett. 69 (15) (1996)
21602162.
[10] C. Qiu, H. Chen, M. Wong, H.S. Kwok, Dependence of the luminous
and power efficiencies of organic light-emitting diode on the
thickness of the constituent organic layers, IEEE Trans. Electron.
Devices 48 (9) (2001) 21312137.
[11] K. Okumoto, Y. Shirota, Development of high-performance blue
violet-emitting organic electroluminescent devices, Appl. Phys. Lett.
79 (9) (2001) 12311233.
[12] D.D. Gebler, Y.Z. Wang, J.W. Blatchford, S.W. Jessen, D.K. Fu, T.M.
Swager, A.G. MacDiarmid, A.J. Epstein, Exciplex emission in
bilayer polymer light-emitting devices, Appl. Phys. Lett. 70 (13)
(1997) 16441646.
[13] V. Bulovic, V.B. Khalfin, G. Gu, P.E. Burrows, D.Z. Garbuzov, S.R.
Forrest, Weak microcavity effects in organic light-emitting diodes,
Phys. Rev. B 58 (7) (1998) 37303740.
[14] K. Itano, H. Ogawa, Y. Shirota, Exciplex formation at the organic
solid-state interface: yellow emission in organic light-emitting diodesusing green-fluorescent tris(8-quinolinolato)aluminum and hole-
transporting molecular materials with low ionization potentials,
Appl. Phys. Lett. 72 (6) (2001) 636638.
104 C. Qiu et al. / Synthetic Metals 140 (2004) 101104