Organic light-emitting diodes using 3- or 5-hydroxyflavone-metal complexes

Article abstract of DOI:10.1063/1.120330

3- or 5-hydroxyflavone-metal complexes were synthesized and applied to organic light-emitting diodes (OLEDs) as an emitting material and an electron transport material. When bis(5-hydroxyflavonato)beryllium [Be(5Fla)₂] was used as an emitting material, the OLED showed a luminance of 1900 cd/m² with yellow. On the other hand, when used as an electron transport material in an organic three-layer cell, a high luminance of more than 10 000 cd/m² was obtained. Furthermore, the turn-on voltage of the OLED could be lowered to 2.4 V and its lifetime was more than 1300 h. Be(5Fla)₂ was found to have a good electron transporting capability and good stability.

Full text of DOI:10.1063/1.120330

Organic light-emitting diodes using 3- or 5-hydroxyflavone–metal
complexes
Yuji Hamada, Takeshi Sano, Hiroyuki Fujii, Yoshitaka Nishio, Hisakazu Takahashi et al.
Citation: Appl. Phys. Lett. 71, 3338 (1997); doi: 10.1063/1.120330
View online: http://dx.doi.org/10.1063/1.120330
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v71/i23
Published by the American Institute of Physics.
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Organic light-emitting diodes using 3- or 5-hydroxyflavone–metal
complexes
Yuji Hamada,a) Takeshi Sano, Hiroyuki Fujii, Yoshitaka Nishio, Hisakazu Takahashi,
and Kenichi Shibata
New Materials Research Center, Sanyo Electric Company, 1-18-13, Hashiridani, Hirakata,
Osaka 573, Japan
͑
Received 23 June 1997; accepted for publication 6 October 1997͒
- or 5-hydroxyflavone-metal complexes were synthesized and applied to organic light-emitting
3
diodes ͑OLEDs͒ as an emitting material and an electron transport material. When
bis͑5-hydroxyflavonato͒beryllium ͓Be͑5Fla͒ ͔ was used as an emitting material, the OLED showed
2
2
a luminance of 1900 cd/m with yellow. On the other hand, when used as an electron transport
material in an organic three-layer cell, a high luminance of more than 10 000 cd/m was obtained.
2
Furthermore, the turn-on voltage of the OLED could be lowered to 2.4 V and its lifetime was more
than 1300 h. Be͑5Fla͒ was found to have a good electron transporting capability and good stability.
2
©
1997 American Institute of Physics. ͓S0003-6951͑97͒00349-5͔
Organic light-emitting diodes ͑OLEDs͒ have been ex-
pected to find application as a new type of display since
Tang and co-workers reported on high-performance
dinated with metal ions on oxygen and nitrogen in
Alq , BeBq . However, complexes which have an oxygen,
3
2
oxygen-coordinate bond have seldom been reported as mate-
rials for OLEDs. A complex with an oxygen, oxygen coor-
dinate bond ͑o, o-complex͒ is expected to be useful for
OLEDs because the o, o-complex, such as the morin–metal
complex, is known to have fluorescence in analytical chem-
istry. In this letter we report on new o, o-complexes, which
were constructed with 3- or 5-hydroxyflavone and metal
ions. These complexes showed high performance in OLEDs
as an electron transport material or an emitting material.
The four complexes of tris͑5-hydroxyflavonato͒ alumi-
1
OLEDs. Chelate–metal complexes are often used as a car-
rier transport material, an emitting material, and a host ma-
terial in the OLEDs because they have a good carrier trans-
porting capability and strong fluorescence. They also exhibit
good stability in films ͑no crystal deposition with time͒, and
offer heat resistance because of their high melting points.
Tris͑8-hydroxyquinolinato͒aluminum (Alq ) and bis͑8-
3
hydroxyquinolinato͒zinc ͑Znq ) are well-known materials
2
1,2
and exhibit good properties for OLEDs. We showed the
molecular design of a chelate–metal complex in a previous
num Al͑5Fla͒ , bis͑5-hydroxyflavonato͒zinc Zn͑5Fla͒ ,
͓
͔
͓
͔
3
2
3
article. Since the film of a chelate–metal complex is ordi-
bis͑5-hydroxyflavonato͒beryllium
Be͑5Fla͒ ,
and
͓
͔
2
narily deposited by vacuum vapor deposition, the chelate–
metal complex must be easily sublimed. It was found that an
inner complex salt offers good sublimation in a vacuum and
its film was deposited successfully. The complexes men-
tioned above, such as Alq and Znq , have a structure with
bis͑3-hydroxyflavonato͒zinc ͓Zn͑3Fla͒ ͔, were newly syn-
thesized. The molecular structure of these complexes are
shown in Fig. 1. For example Al͑5Fla͒3 was prepared
2
3
2
an inner complex salt. We have developed various chelate–
metal complexes. Bis͑10-hydroxybenzo͓h͔quin-
4
olinato͒beryllium (BeBq2) showed a higher performance
than Alq when used as an electron transport material. A cell
3
using BeBq showed a long lifetime of 3500 h at an initial
2
2
luminance of 500 cd/m and more than 15 000 h at 100
cd/m . The chelate–metal complexes show various emit-
ting colors. For example, azomethine–zinc complexes
2
5,6
showed
blue
or
violet,3
zinc ͑ZnTPP͒
and
5,10,15,20-
and bis͑4-
tetraphenylporphrinato
hydroxyacridinato͒zinc (ZnAc ) showed a red emission.
7
2
Bis͑2-͑2-hydroxyphenyl͒benzothiazolato͒zinc ͓Zn͑BTZ͒2͔
showed a whitish emission, having a spectrum with a wide
8
half band of more than 150 nm. White emission is important
if OLEDs are applied to the backlights or full-color displays
with color filters.
These complexes almost always have an oxygen,
nitrogen-coordinate bond in relation to bonding between
ligands and metal ions. For example, the ligands were coor-
FIG. 1. The molecular structures of 3- or 5-hydroxyflavone-metal com-
plexes.
a͒Electronic mail: n43hamada@hr155.a1.sanyo.co.jp
3338
Appl. Phys. Lett. 71 (23), 8 December 1997
0003-6951/97/71(23)/3338/3/$10.00
© 1997 American Institute of Physics
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di͑1-napthyl͒-1,1Ј-biphenyl-4,4Ј-diamine ͑Ref. 10͒ ͑host ma-
terial͒ and rubrene ͑dopant, 5 wt % against NPB͒. The
rubrene doped system was used in this cell because this
system showed high luminance and high stability.5
4
,4Ј,4Љ-tris͑3-methylphenylphenylamino͒triphenylamine was
11
used in the hole transport layer. The molecular structures
mentioned above are shown in Fig. 2.
The photoluminescence ͑PL͒ spectra of the organic films
͑1000 Å͒ were measured with a fluorescence spectrophotom-
eter. The ionization potential of these films was measured by
atmosphere ultraviolet photoelectron analysis using a Riken
Keiki AC-1 under ambient atmosphere.
These complexes were examined first as an emitting ma-
terial. Their films were easily deposited and were uniform.
The PL peak wavelengths are shown in Table I. These PL
intensities are very weak compared with Alq . Three 5-
3
hydroxyflavone-metal complexes had almost the same PL
peaks of around 560 nm. The PL of the 3-hydroxyflavone-
metal complex, Zn͑3Fla͒ , could not be measured because of
2
its weak intensity. The electroluminescence ͑EL͒ results are
shown in Table I. The OLEDs which used these complexes
showed yellow–orange emissions. The luminance of these
complexes was not relatively high. Although the Be͑5Fla͒2
2
cell showed the highest luminance of all at 1900 cd/m , the
Al͑5Fla͒3 cell and Zn͑5Fla͒2 cell showed luminance of
2
around 1000 cd/m and the Zn͑3Fla͒ cell showed luminance
2
2
of only 186 cd/m . The reason for this low luminance is that
these complexes had a weak PL intensity. The turn-on volt-
age, which was defined as the voltage when the luminance
was 1 cd/m , was also measured. The turn-on voltage of the
FIG. 2. The molecular structures of the hole transport materials and host
material.
2
Be͑5Fla͒ cell was 2.4 V, which was the lowest voltage of
2
through the reaction of 5-hydroxyflavone and aluminum iso-
propoxide in methanol with reflux, and purified by train
sublimation. These complexes are an inner complex salt and
all. The Be͑5Fla͒ cell had the highest luminance and the
2
9
lowest turn-on voltage although Be͑5Fla͒ had a weak PL. It
2
seems that Be͑5Fla͒ has a good electron transporting capa-
are easily sublimed in a vacuum.
2
bility.
The cells were fabricated on indium-tin-oxide ͑ITO, an-
ode͒ substrates which were cleaned by an ultraviolet ozone
cleaner immediately before use. Organic layers and a cath-
ode layer (Mg In ) were deposited by conventional vapor
Be͑5Fla͒ was examined as an electron transport mate-
2
rial. Simultaneously, conventional electron transport materi-
als such as Alq , BeBq , and Zn͑BTZ͒ , were also examined
3
2
2
0
.9 0.1
Ϫ6
for comparison with Be͑5Fla͒ . The cell structure in this ex-
vacuum deposition in 5ϫ10 Torr. The emitting area was
ϫ2 mm . The cell structures used in this experiment are
2
2
periment was the organic three-layer structure mentioned
above. The results are shown in Table II. All cells showed a
yellow emission which originated in rubrene because each
EL peak was almost identical to the PL peak of rubrene ͑560
nm͒. The luminance-voltage characteristics of these cells are
shown in Fig. 3. The Be͑5Fla͒2, BeBq2, and Zn͑BTZ͒2 cells
showed better luminance-voltage properties than the Alq3
cell. The BeBq2 cell showed the highest maximum lumi-
2
shown as follows: A two organic layer type of ͓ITO/hole
transport layer ͑500 Å͒/emitting layer ͑500 Å͒/MgIn͔ was
employed when the chelate–metal complexes were exam-
ined as an emitting material. The hole transport layer was
composed of N,NЈ-diphenyl-N,NЈ-di͑3-methylphenyl͒-1,1Ј-
biphenyl-4,4Ј-diamine. A three organic layer type of ͓ITO/
hole transport layer ͑500 Å͒/emitting layer ͑200 Å͒/electron
transport layer ͑400 Å͒/MgIn͔ was employed when the com-
plexes were examined as an electron transport material.
The emitting layer was composed of N,NЈ-diphenyl-N,NЈ-
2
nance of all at 44 000 cd/m . However, the turn-on voltage
of the Be͑5Fla͒ cell was 2.4 V, which was the lowest of all,
2
although the Be͑5Fla͒ cell had a low maximum luminance
2
TABLE I. PL and EL results of 3-, or 5-hydroxyflavone-metal complexes used as an emitting material.
Emitting
materials
Chromaticity
coordinates
Turn-on
voltage ͑V͒
Maximum luminance
2
PL ͑nm͒
EL ͑nm͒
(cd/m )
Al͑5Fla͒3
Zn͑5Fla͒2
Be͑5Fla͒2
Zn͑3Fla͒2
563
568
554
598
591
559
607
xϭ0.467, yϭ0.511
xϭ0.472, yϭ0.516
xϭ0.460, yϭ0.524
xϭ0.474, yϭ0.476
4.0
4.2
2.4
6.2
1276 ͑10 V͒
878 ͑9 V͒
1900 ͑7 V͒
186 ͑12 V͒
weak
Appl. Phys. Lett., Vol. 71, No. 23, 8 December 1997
Hamada et al.
3339
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TABLE II. EL results of Be͑5Fla͒ used as an electron transport material
2
and comparison with other materials.
Electron
transport
materials EL ͑nm͒
Maximum
luminance
͑cd/m )
Chromaticity
coordinates
Turn-on
voltage ͑V͒
2
Be͑5Fla͒2
BeBq2
Alq3
559
553
556
559
xϭ0.484, yϭ0.513
xϭ0.467, yϭ0.518
xϭ0.445, yϭ0.552
xϭ0.490, yϭ0.498
2.4
2.8
3.6
3.5
10 670 ͑10 V͒
44 000 ͑10 V͒
29 200 ͑14 V͒
10 090 ͑12 V͒
Zn͑BTZ͒2
2
of 10 670 cd/m as compared with the BeBq cell. Inciden-
2
tally, the turn-on voltage of the BeBq cell was 2.7 V, and
2
FIG. 4. The life test of the Be͑5Fla͒ cell.
that of the Alq cell was 3.6 V. The current of the Be͑5Fla͒2
2
3
cell was stable and no noisy current was observed in the low
is lower than that of BeBq ͑368 °C͒. When the cells showed
2
voltage region ͑1–3 V͒. It seems that Be͑5Fla͒ plays a role
2
the maximum luminance, the heat was generated in the cells
because the large current flowed. Therefore, the Be͑5Fla͒2
cell was broken in low luminance compared with the BeBq2
cell.
as the good electron transport layer in the low voltage region.
The reason for the low turn-on voltage of the Be͑5Fla͒2
cell is that the electron injection barrier between Be͑5Fla͒2
and MgIn ͑0.5 eV͒ is smaller than that of Alq /MgIn ͑0.8
3
The life test of the Be͑5Fla͒ cell was examined at room
2
eV͒ and BeBq /MgIn͑0.8 eV͒. ͓Ionization potential:
2
temperature when the cell structure of ͓ITO/MTDATA/TPD
Be͑5Fla͒ ͑5.7 eV͒, BeBq ͑5.5 eV͒, Alq ͑5.6 eV͒, electron
2
2
3
ϩrubrene 5 wt %/Be͑5Fla͒ /MgIn] was employed. The lu-
2
affinity: Be͑5Fla͒ ͑3.2 eV͒, BeBq ͑2.9 eV͒, Alq ͑2.9 eV͒,
2
2
3
minous efficiency of this cell was 2.3 lm/W at 547
work function: MgIn ͑3.7 eV͒.͔ Electron affinity was calcu-
lated to subtract the optical band gap from the ionization
potential. Furthermore, it seems that the electron transporting
of Be͑5Fla͒ and BeBq is better than that of Alq because
2
cd/m .The life test was carried out at an initial luminance of
2
2
2
10 cd/m under a constant current density of 6 mA/cm .
The result of the life test is shown in Fig. 4. The life was
defined as the half decay time of initial luminance. The lu-
2
2
3
their luminance-voltage curves are located in a lower voltage
minance of the Be͑5Fla͒ cell maintained 74% of the initial
2
region compared with the Alq cell in Fig. 3.
3
luminance after 1300 h passed. Be͑5Fla͒ was chemically
2
The reason for the low maximum luminance of the
stable and no crystal deposition of Be͑5Fla͒ was observed
2
Be͑5Fla͒ cell is that the melting point of Be͑5Fla͒ ͑272 °C͒
2
2
on the film.
Be͑5Fla͒ was found to be an excellent electron transport
2
material, equal to BeBq . This is the first time that Be͑5Fla͒2
2
has been found to be useful as an oxygen, oxygen-coordinate
complex.
1
C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 51, 913 ͑1987͒.
Y. Hamada, T. Sano, M. Fujita, T. Fujii, Y. Nishio, and K. Shibata, Jpn. J.
2
Appl. Phys., Part 2 32, L514 ͑1993͒.
Y. Hamada, T. Sano, M. Fujita, T. Fujii, Y. Nishio, and K. Shibata, Jpn. J.
3
Appl. Phys., Part 2 32, L511 ͑1993͒.
Y. Hamada, T. Sano, M. Fujita, T. Fujii, Y. Nishio, and K. Shibata, Chem.
4
Lett., 905 ͑1993͒.
5
Y. Hamada, T. Sano, K. Shibata, and K. Kuroki, Jpn. J. Appl. Phys., Part
2
34, L824 ͑1995͒.
6
T. Sano, Y. Hamada, and K. Shibata, Extended Abstracts of Inorganic and
Organic Electroluminescence, Berlin, Germany, 1996, Vol. EL96, p. 249.
Y. Hamada, IEEE Trans. Electron Devices 44, 1208 ͑1997͒.
Y. Hamada, T. Sano, H. Fujii, Y. Nishio, H. Takahashi, and K. Shibata,
Jpn. J. Appl. Phys., Part 2 35, L1339 ͑1996͒.
7
8
9
0
J. Wagner, R. O. Loutfy, and C. K. Hsiao, J. Mater. Sci. 17, 2781 ͑1982͒.
S. A. VanSlyke, C. H. Chen, and C. W. Tang, Appl. Phys. Lett. 69, 2160
1
1
͑
1996͒.
1
FIG. 3. The luminance-voltage characteristics of organic three-layer cells.
Y. Shirota, Y. Kuwabara, H. Inaba, T. Wakimoto, H. Nakada, Y.
͑
᭿͒ BeBq , ͑ᮀ͒ Be͑5Fla͒ , ͑ϫ͒ Zn͑BTZ͒ , ͑᭡͒ Alq .
Yonemoto, S. Kawami, and K. Imai, Appl. Phys. Lett. 65, 807 ͑1994͒.
2
2
2
3
3340
Appl. Phys. Lett., Vol. 71, No. 23, 8 December 1997
Hamada et al.
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