Efficient red organic light-emitting devices based on a europium complex

Article abstract of DOI:10.1063/1.1626022

An efficient red organic light-emitting devices based on a europium complex were fabricated. The devices show the maximum luminance up to 800 cd/m², an external quantum efficiency of 4.3%, current efficiency of 4.7 cd/A, and power efficiency of 1.6 lm/W. Current-voltage characteristics were also measured by using a Keithley source measurement unit. A pure red light with a peak of 612 nm and a half bandwidth of 3 nm, which is the characteristics emission of trivalent europium ion, is observed.

Full text of DOI:10.1063/1.1626022

Efficient red organic light-emitting devices based on a europium complex
Junfeng Fang and Dongge Ma
Citation: Applied Physics Letters 83, 4041 (2003); doi: 10.1063/1.1626022
View online: http://dx.doi.org/10.1063/1.1626022
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/83/19?ver=pdfcov
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APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 19
10 NOVEMBER 2003
Efficient red organic light-emitting devices based on a europium complex
Junfeng Fang and Dongge Maa)
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
͑Received 21 July 2003; accepted 23 September 2003͒
An efficient organic light-emitting device using
Eu(Tmphen)(TTA)3 (TTAϭthenoyltrifluoroacetone,
a
trivalent europium ͑Eu͒ complex
Tmphenϭ3,4,7,8-tetramethyl-1,10-
phenanthroline) as the dopant emitter was fabricated. The devices were a multilayer structure
of indium tin oxide/N,N-diphenyl-N,N-bis͑3-methylphenyl͒-1,1-biphenyl-4,4-diamine ͑40 nm͒/
Eu complex:4,4-N,N-dicarbazole-biphenyl ͑1%, 30 nm͒/2,9-dimethyl,4,7-diphenyl-1,10-
phenanthroline ͑20 nm͒/AlQ ͑30 nm͒/LiF ͑1 nm͒/Al ͑100 nm͒. A pure red light with a peak of
612 nm and a half bandwidth of 3 nm, which is the characteristic emission of trivalent europium
ion, was observed. The devices show the maximum luminance up to 800 cd/m2, an external
quantum efficiency of 4.3%, current efficiency of 4.7 cd/A, and power efficiency of 1.6 lm/W.
At the brightness of 100 cd/m2, the quantum efficiency reaches 2.2% ͑2.3 cd/A͒. © 2003 American
Institute of Physics. ͓DOI: 10.1063/1.1626022͔
Organic light-emitting devices ͑OLEDs͒ are one of the
most promising next generation low cost full color flat panel
displays alterative to liquid crystal-based ones. At present,
organic materials including fluorescent organic molecules
and polymers, and phosphorescent organic molecules with
heavy metals have been widely used as active mediums in
OLEDs.1–3 These organic materials show good electrolumi-
nescent performance in OLEDs, particularly, the phosphores-
cent molecules have been demonstrated the prospect of ob-
taining devices with the internal quantum efficiencies of
100% through radiative recombination of both singlet and
triplet excitons.4 However, the broad nature of the lumines-
cent spectra of these organic molecules leads to the poor
luminescent purity, which are dull and, thus, not suited for
actual display applications. For OLEDs, the development of
high-performance red emission is still much in demand com-
pared with both green and blue emission, which are already
available. Therefore, how to obtain red OLEDs with high
efficiency as well as good purity of color is necessary.
It is well known that rare-earth complexes emit sharp
spectral band due to inner f orbitals of the central rare-earth
metal ions and are expected to show high luminescence ef-
ficiency since both singlet and triplet excitons are involved in
the luminescence process.5 Of rare-earth complexes studied,
europium complexes, which exhibit strong photolumines-
cence corresponding to the 5D0 –7F2 transition of Eu3ϩ ions,
appear most attractive in view of red emission ability. At
present, OLEDs based on europium complexes have
achieved certain progress in electroluminescent ͑EL͒
performance,6–9 but the EL efficiency, particularly in the
case of high brightness such as 100 cd/m2, is not satisfied
compared to the fluorescence organic molecules and electro-
phosphorescent organic molecules. The design of europium
complexes as active medium in OLEDs to further improve
EL efficiency is therefore crucial for practical applications.
In this letter, we developed a europium complex and
studied its EL properties. The europium complex is
Eu(Tmphen)(TTA)3 (TTAϭthenoyltrifluoroacetone,
Tmphenϭ3,4,7,8-tetramethyl-1,10-phenanthroline). The
Eu(Tmphen)(TTA)3 is synthesized by a conventional
method.10 Europium chloride was prepared by suspend an-
hydrous europium oxide ͑99.99% pure͒ in hydrochloride
acid and digesting on a steam bath until completely dis-
solved. The ligands 3,4,7,8-tetramethyl-1,10-phenanthroline
and thenoyltrifluoroacetone were dissolved in 95% hot etha-
nol, then sodium hydroxide was added. The mixture was
stirred while europium chloride solution was added drop-
wise. The mixture was cooled after 2 h, and washed with
water and ethanol, in turn, and finally recrystallized. Figure 1
shows the chemical structure of Eu(Tmphen)(TTA)3 .
The EL properties of Eu(Tmphen)(TTA)3 were studied
by using multilayer structure of indium-tin-oxide ͑ITO͒/TPD
͑40 nm͒/CBP: Eu(Tmphen)(TTA)3 ͑30 nm͒/BCP
(20 nm)/Alq3 ͑30 nm͒/LiF ͑1 nm͒/Al ͑100 nm͒ ͑Fig. 1͒,
where TPD is N,N-diphenyl-N,N-bis͑3-methylphenyl͒-1,1-
biphenyl-4,4-diamine as the hole transport layer, CBP is 4,4-
N,N-dicarbazole-biphenyl as the host, BCP is 2,9-dimethyl-
4,7-diphenyl-1,10-phenanthroline as the electron transport/
hole block layer, and Alq3 is tris͑8-hydroxyquinoline͒
aluminum as the electron transport layer. LiF/Al is as the
cathode. All the organics were evaporated with rate in the
range 0.1–0.3 nm/s under high vacuum (р3ϫ10Ϫ4 Pa).
The metallic cathode was evaporated at higher rate ͑0.8–1
nm/s͒ without opening the vacuum. Current–brightness–
voltage characteristics were measured by using a Keithley
source measurement unit ͑Keithley 2400 and Keithley 2000͒
with a calibrated silicon photodiode. The EL spectra were
measured by JY SPEX CCD3000 spectrometer. All the mea-
surements were carried out in ambient atmosphere at room
temperature. The active area of device is 9 mm2.
Because
the
doping
concentration
of
Eu(Tmphen)(TTA)3 in CBP significantly affects the lumi-
nescence efficiency, we first optimized the doping concentra-
tion. A 1% Eu(Tmphen)(TTA)3 doped CBP device exhibits
a͒
Electronic mail: mdg1014@ciac.jl.cn
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4042
Appl. Phys. Lett., Vol. 83, No. 19, 10 November 2003
J. Fang and D. Ma
FIG. 3. EL efficiency as a function of current density of the 1%
Eu(Tmphen)(TTA)3 doped CBP device.
increasing current, typical of luminescence originating from
recombination of long-lived triplet excitons, which has been
explained as triplet–triplet annihilation on CBP molecules
following back transfer from TTA due to the near resonance
of TTA and CBP triplet states.6
FIG. 1. Chemical structure of Eu(Tmphen)(TTA)3 and structure of the elec-
troluminescence device.
Figure
4
shows the EL spectra of the 1%
Eu(Tmphen)(TTA)3 doped CBP device at different voltages.
It can be seen that a pure red Eu3ϩ ion emission based on
the best efficiency and luminance. Figure 2 shows the
7
5D0→ F2 transition with a peak of 612 nm and a full width
current–brightness–voltage characteristics of
a
1%
at half maximum of 3 nm was observed at a wide voltage
range. In the case of a doped organic system, carrier trapping
Eu(Tmphen)(TTA)3 doped CBP device. The turn-on voltage
was 7 V. The maximum brightness of 800 cd/m2 was ob-
tained at drive voltage of 24.5 V. The EL quantum
efficiency–current characteristic of the same device was also
measured, as shown in Fig. 3. A maximum current efficiency
of 4.7 cd/A, corresponding to an EL quantum efficiency of
4.3%, was obtained at current density of 0.1 mA/cm2. The
maximum power efficiency reached 1.6 lm/W. At the bright-
ness of 100 cd/m2, the quantum efficiency yet reaches 2.2%
͑2.3 cd/A͒ at current density of 4.3 mA/cm2. Note, as seen in
Fig. 3, that the EL quantum efficiency also decreases with
¨
and Forster energy transfer are thought of as two main lumi-
nescent mechanisms.11 The two mechanisms may simulta-
neously exist in the luminescence process, or one of them is
dominant, which depends on the used dopant and organic
host. For the case of europium complex doped in CBP, gen-
erally considered that the energy transfer between CBP and
europium complexes is incomplete.6 The fact that no emis-
sion from CBP in an Eu(Tmphen)(TTA)3 doped CBP device
was observed demonstrates that the carrier trapping is the
main luminescence process. In this case, the injected elec-
trons and holes through CBP are first trapped by
Eu(Tmphen)(TTA)3 , and then form excitons on
¨
Eu(Tmphen)(TTA)3 . However, the Forster energy transfer
FIG. 2. Current density–brightness–voltage characteristics of the 1%
FIG. 4. Electroluminescence spectra of the 1% Eu(Tmphen)(TTA)3 doped
CBP device at different voltage.
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Eu(Tmphen)(TTA)3 doped CBP device.
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Appl. Phys. Lett., Vol. 83, No. 19, 10 November 2003
J. Fang and D. Ma
4043
from excited CBP singlet energies to Eu(Tmphen)(TTA)3
singlet states, which is partial, cannot also be completely
excluded in the EL process.
Eu(Tmphen)(TTA)3 exhibits high EL efficiency, even at
higher brightness. We attribute the high EL efficiency to the
excellent planar structure of Tmphen ligand due to the intro-
duction of four methyl groups. Our results demonstrate that
it is possible to achieve high emission efficiency in the lan-
thanide complexes by optimized design of molecular struc-
tures as well as reasonable device structures.
In conclusion, we developed an europium complex
Eu(Tmphen)(TTA)3 , and studied its EL characteristics us-
ing structure ITO/TPD/Eu(Tmphen)(TTA)3 :CBP/BCP/
Alq3 /LiF/Al. A pure red light with a peak of 612 nm and a
half bandwidth of 3 nm at a wide voltage range was ob-
served. The device shows a luminance up to 800 cd/m2, an
external quantum efficiency of 4.3%, current efficiency of
4.7 cd/A. At the brightness of 100 cd/m2, the quantum effi-
ciency yet remains 2.2%, which is a better result reported so
far using europium complexes as active medium in OLEDs.
Our results indicate that the lanthanide complexes showing
their advantages as phosphorescent materials to achieve high
EL performance in OLEDs are worth anticipating.
One of the authors ͑Dr. D.M.͒ is grateful to Hundreds
Talents program of Chinese Academy of Sciences, Outstand-
ing Youth Foundation of Jilin Research Council and Ministry
of Science and Technology of China ͑973 Program No.
2002CB613403͒ for financial support.
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