九州大学学術情報リポジトリ
Kyushu University Institutional Repository
Blue light-emitting organic electroluminescent
devices
Adachi, Chihaya
Department of Materials Science and Technology, Graduate School of Engineering Sciences,
Kyushu University
Tsutsui, Tetsuo
Department of Materials Science and Technology, Graduate School of Engineering Sciences,
Kyushu University
Saito, Shogo
Department of Materials Science and Technology, Graduate School of Engineering Sciences,
Kyushu University
http://hdl.handle.net/2324/19440
出版情報：Applied Physics Letters. 56 (9), pp.799-801, 1990-02-26. American Institute of
Physics
バージョン：
権利関係：Copyright 1990 American Institute of Physics. This article may be downloaded for
personal use only. Any other use requires prior permission of the author and the American
Institute of Physics.
Blue light-emitting organic electroluminescent devices
Chihaya Adachi, Tetsuo Tsutsui, and Shogo Saito
Department o/Materials Science
and
Technology, Graduate School a/Engineering Sciences,
Kyushu University, Kasuga-shi, Fukuoka 816, Japan
(Received
11
September 1989; accepted for publication
18
December 1989)
Organic electroluminescent
(EL)
devices with multilayered thin-film structures which emitted
bright blue light were constructed.
Two
empirical guides for
the
selection
of
blue-emitting
materials were established.
The
keys to obtain the
EL
cells with high
EL
efficiency were
excellent film-forming capability
of
an
emitter layer
and
the appropriate combinations
of
emitter
and
carrier
transport
materials for avoiding the formation
of
exciplexes.
In
one
of
our
organic electroluminescent devices, blue emission with a luminance
of
700
cd/m
2
was achieved
at
a
current
density
of
100
mA/cm
2
and
a
dc
drive voltage
of
10
V.
Organic materials have been expected
to
be applicable
for practical electroluminescent
(EL)
devices because
of
their
high fluorescence efficiency
and
semiconducting prop-
erties.
One
of
the
most fascinating advantages
of
organic
materials is the possibility
of
a wide selection
of
emission
colors, particularly in
the
blue region, in
EL
displays
through
the molecular design
of
organic materials. Recent-
ly,
organic
EL
devices
with
bright green
and
yellow emission
which
possessed multilayered thin-film structures consisting
of
an
emitter layer
and
carrier
transport
layers have been
reported.
1
•
2
However,
the
fabrication
of
bright
EL
devices
with blue emission has
not
been sllccessful.
From
the view-
point
of
the working principle
of
organic
EL
cells, no diffi-
culty specific
to
blue emission is anticipated.
In
fact, we have
already reported
that
blue light emission was possible in or-
ganic
EL
devices with an anthracene film as
an
emitter lay-
er,; although the blue emission was not bright enough for
practical display devices. We expect
that
the
development
of
new emitter materials surely holds the prospect
of
bright
blue emission.
In
this letter, we examined
15
organic materials for blue
emission
and
obtained
two
empirical guides for their selec-
tion.
The
keys to obtaining
EL
cells with high
EL
efficiency
were excellent
film-forming capability
of
an
emitter layer
and
the appropriate combinations
of
emitter
and
carrier
transport
materials for avoiding the formation
of
exciplexes.
When
we used properly selected emitter
and
carrier trans-
port
materials, high luminance, over 500
cd/m
2
in the blue
region, was achieved.
Three types
ofEL
cell structures were used in
our
study:
type
A-indium-tin-oxide
substrate
(ITO)4/hole
transport
layer/emitter
layer/MgAg,
type
B-ITO/emitter
layerl
electron
transport
layer/MgAg,5
and
type
C-ITO/hole
transport
layer/emitter
layer/electron
transport
layer/
MgAg.'
The
thickness
of
the
organic layers was 500 A unless
otherwise specified.
Organic layers were deposited on a pre-
cleaned
ITO
glass substrate by vacuum deposition,
and
a
cathode
MgAg
layer was deposited
on
the organic layer by
codeposition.
The
deposition rate for organic layers was
about
2-4
A/s.
The
emitting
area
in
the
cells was 0.2 X 0.2
cm
2
•
For
a hole
transport
material, aromatic diamine,6
which was known
to
transport
holes selectively, was used.
For
an
electron
transport
material, we used
an
oxadiazole
derivative
(PBD).
In
a previous
letter/'
we showed
that
the
PED
operated as
an
excellent electron
conductor
in organic
layered structures.
In
Fig. 1
the
molecular structures
of
two
carrier
transport
materials
and
organic fluorescent materi-
als? with intense blue fluorescence
are
listed.
An
the
emitter
materials were purified by a
train
sublimation
method
8
be-
fore use.
The
quality
of
the emitter films was observed
under
a microscope with
lOOOX
magnification.
First,
11
organic emitter materials were examined using
type
A cells. Table I summarizes
the
emitter materials, the
qualities
of
emitter films deposited
on
an
amorphous
dia-
mine film,
and
photoluminescence
(PL)
and
EL
emission
wavelengths.
The
first requirement for achieving high per-
formance in blue
EL
cells is good quality
of
the
emitter
films.
Six materials
(El
to
E6),
which are known as typical or-
ganic scintillators, did not give dense
thin
films.
The
surfaces
of
the
evaporated films appeared
to
be rugged;
that
is to say,
the
growth
of
about 1 f.tm scale large crystals was observed.
The
capability
offorming
smooth
layers was closely related
to
the molecular
structure
of
the
emitter
materials.
In
gen-
eral, organic compounds with blue fluorescence have simple
molecular structures with
high symmetry
and
with few sub-
stituents. Thus, such molecules
possess a tendency
to
give
large crystals, even when they
are
forced
to
form
thin
films.
Poor
quality
of
these emitter films resulted in inferior
EL
cell
characteristics, even
though
we
could
fabricate several
EL
cells
and
observed
EL
emission from these cells.
In
contrast,
five
materials
(E7-E11)
gave
smooth
pinhole-free
thin
films
on
the amorphous dense layers
of
a diamine. Evidently,
the
emission efficiencies
of
the
EL
cells with
poor
quality
of
the
emitter films (E2, E3, E4, E5,
and
E6)
were quite low,
and
also deviations from a linear relation in luminance-current
(L-J)
characteristics were observed.
On
the
other
hand,
the
EL
cells with smooth, pinhole-free films showed high
EL
efficiency as shown in the
EL
cells
with
the
emitters (E7, E8,
ElO,
and
Ell).
One notices
that
the
symmetry
ofthese
com-
pounds is
rather
low
and/or
bulky substituents are attached
in these compounds. Thus, we established a rough criterion
for the selection
of
emitter
compounds.
A second requirement concerns the formation
of
exci-
pIe xes between an emitter
and
a hole
transport
material
(diamine).
The
emitter
materials
(E6-E9)
were found
to
form exciplexes with the diamine
in
solid states,
and
resulted
in
a long-wavelength
PL
emission due to the exciplexes. A
typical example
of
exciplex formation can be demonstrated
799
Appl. Phys. Lett.
56
(9), 26 February 1990
0003-6951/90/090799-03$02.00
@ 1990 American Institute
of
Physics 799
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~
a
I I
a a
FI
G.
I.
Molecular
structur~s
of
two
carrier
transport
and
15
emitter
materi-
als.
by
the
combination
of
the
E8 and diaminc (Table O.
The
evaporated film
of
a single E8 layer showed its
PL
at
around
435 nm. However, when
the
E8
and
diamine were mixed in
solid states, its
PL
peak shifted
to
530 nm.
In
addition,
the
EL
emission spectrum in this
EL
cell coincided well with the
exciplex emission. Thus,
it
is
suggested
that
an
exciplex was
formed
at
the
interface
cf
emitter
and
diamine layers
and
also served as
carrier
recombination
and
emission sites.
For-
mation
of
the
exciplcx
at
the
interface between
thc
two or-
ganic layers must be avoided for obtaining blue emission.
800
Appl. Phys. Lett., Vol. 56,
No.9,
26 February 1990
TABLE
I. Quality
of
emitter
films,
PL
peaks
of
emitter
films,
and
EL
peaks
in
type
A cells.
Quality 1'1.
peak
EL peak
Emitter
of
film
(run)"
(nln)b
El
r:'lir
420
425
1'.2
fair 472
475
E3
fair
432
430
E4
fair 452
460
E5
t:1ir
370
E6
fair 478 590"
E7 good
460
520"
E8
good
435
530"
E9
good 440 560"
EIO
good
460
460
Ell
good
467
465
a
Peak
wavelength
of
a
PL
spectrum.
b
Peak
wavelength
of
an
EL
spectrum.
"Luminance
at a
curren!
density
of
100
rnA/em'.
dEL
intensity was
too
weak
to
get
the
EL
peak.
"Formation
of
exeiplex between
an
emitter
and
a diamine.
Luminance
(cd/m')"
0.Q2
0.09
6
1
0.004
0.09
35
12
0.08
70
120
In
the
survey
of
the
emitter
materials with intense blue
emission, we found
that
phenyl-substituted cyclopentadiene
derivatives
(E1O
and
E 11) overcame
the
two difficulties de-
scribed above.
They
possess intense flucrescence
at
~460
nm
in solid states
and
also
form
dense
and
pinhole-free films.
Thc
EL
cells with these emitters showed good
EL
perfor-
mance.
The
EL
cell
with
the
emitter
Ell
behaved like a
rectifier.
In
a forward bias
of
the
ITO
electrode, a large injec-
tion
current
was observed in comparison with a small cur-
rent
of
reverse bias.
Luminance
was linearly proportional
to
thc
current
in a wide
current
range,
and
the
luminance
of
120
cd/m2
was achieved
at
a
current
density
of
100
mA/cm
2
and
a dc voltage
of
13
V.
The
emission peak was at - 465
nm
and
coincided well with
the
PL
spectrum
of
the
E
11
solid film,
indicating
no
formation
of
an exeiplcx.
Success in high
EL
efficiency was also obtained with
type B cells. Table
II
summarizes
the
EL
characteristics
of
the
fluorescent materials suitable for
the
type B cell struc-
TABLE
H.
Quality
of
emitter
films,
PI.
peaks
of
emitter
films,
and
EL
peaks in type B
and
type
C cells.
Type
Bcells
Quality
PL
peak
EL peak
Emitter
of
film
(nm)"
(nm)h
EI2
good
478
470
EI3
good
450
460
EI4
good
469
480
E15
good
459
460
Type
Cedi
Quality
PL
peak
EL
peak
Emitter
of
film
(nm)a
(nm)"
E3
good
432 430
"Peak wavelength
of
a
PL
spectrum.
"Peak wavelength
of
an
EL
spectrum.
''Luminance
at
a
current
density
of
100
rnA/em'.
Luminance
(cd/m')'
300
220
180
400
Luminance
(cd/em')"
700
Adachi, Tsutsui, and Saito
800
Downloaded 06 Apr 2011 to 133.5.128.1. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
::.j
:.:i
III
III
'<,
"
>
>
..
'j
. ;
c
•
i
.r.;
....!
§.
FIG.
2.
Photoluminescent
and
electroluminescent spectra. (solid jine)
EL
spectrum
in a
ITO/diamine/E3/PBD/MgAg
cell,
(broken
line)
PL
spec-
trum
of
the emitter layer E3.
ture.
The
empirical guides,
mentioned
before,
are
also appli-
cable in this cell structure.
These
EL
cells also exhibited
rectification,
where
the
forward
bias
corresponded
to
the
positive
on
the
ITO electrode.
In
the
case
of
the
cell
with
E15
as
an
emitter, a
luminance
of
400
cd/m
2
at
a
current
density
of
100
rnA/cm
2
was
obtained
with
a
dc
voltage
of9
V.
Finally, we discuss
our
best
blue-emitting device.
The
cell
structure
was
type
C
with
the
emitter
£3,
350 A
in
thick-
ness.
For
the
emitter
material,
1,1
,4,4-tetraphenyl-l ,3-buta-
diene, E3,
which
did
not
give a pinhole-free
thin
film in a
type A cell, was used. A
luminance
of
700
cd/m2
was
achieved
at
a
current
of
100
mA/cm
2
by
applying a
de
vol-
tage
of
10
V.
At
this
emission intensity, a
luminance
effi-
ciency
ofO.221m/W
was estimated.
The
EL
efficiency
of
this
cell was
about
100 X
larger
than
that
of
the
type
A cell with
the
emitter
E3.
The
peak
of
the
EL
spectrum
was located
at
430
nm
and
coincided
wen
with
the
PL
spectrum
of
the
emit-
ter
film,
although
the
full
width
at
half
maximum
(FWHM)
of
the
EL
spectrum
was
broader
than
that
of
the
PL
spec-
801 Appl. Phys.
Lett,
Vo!. 56,
No.9,
26
February 19S0
trum
(Fig.
2).
One
of
the
reasons for high
EL
efficiency
in
this
three-layer cell is
the
formation
of
a stable, dense,
and
homogeneous
emitter
layer.
The
deposition
of
an
electron
transport
layer
on
the
emitter
layer
contributed
to
keeping
the
morphology
of
the
emitter
layer in as-deposited form,
drastically
retarding
the
crystallization
of
the
emitter
layer .
Also,
the
effect
of
confinement
of
carriers
and
excitoos with-
in
the
emitting layer
by
the
double
heterostructure
should
be
considered.
We
cannot
specify, however,
which
effect is
dominant
in this cell
at
this
stage.
The
stability
of
the
type
C cell
with
the
E3
emitter
was
tested
under
a
constant
current
density
of
10
mA/cm
2
•
The
initial
luminance
of
90
cd/m2
showed
a relatively fast de-
crease
(50%
in
2
h).
At
a
temperature
of
77
K,
a
constant
luminance
was retained
over
5 h.
One
of
the
most
likely
origins
of
the
degradation
of
the
cells is
the
crystallization
of
the
organic layers
due
to
produced
heat.
In
summary,
we have
obtained
the
organic
EL
devices
with
bright
blue emission. Excellent
performance
of
the
EL
devices
can
be
attributed
t6
the
fabrication
of
pinhole-free
films
and
the
exclusion
of
exciplex
formation
at
the
inter-
faces between
the
two
organic layers.
Our
empirical guide-
lines
shown
in
this
letter
promise
further
development
of
blue-emitter materials in
multilayered
EL
devices.
IC.
W.
Tang and
S.
A. VanSlyke, ApI'L Phys. Lett. 51, 913
(1987).
'c. Adachi,
S<
Tokito, T. Tsutsui,
and
S.
Saito, Jpn.
J.
Appl. Phys. 27, L713
(
\988).
'CO
Adachi,
S.
Tokito, T. Tsutsui,
and
S.
Saito, Jpn.
J.
ApI'\. Phys. 27, L269
(1988) .
<'ITO was purchased from
HOY
A Co.
Ltd.,
Tokyo, Japan.
The
thickness
of
the
ITO
is
lOOO
A
"nd
sheet resistance is - 20
H/D.
"c.
Adachi, T. TsutSlli,
and
S.
Saito, Appl. Phys. Lett. 55, 1489
(\989).
"M. Abkowitz and D. M. Pai, Philos. Mag. B
53,193
(l9X6).
7The
compound
(E7)
was kindly
donated
from Mitsubishi-kasci Co. Ltd.,
Fukuoka,
Japan.
The
compounds
(E12~E15)
were kindly
donated
from
RICOH
Co., Ltd., Shizuoka, Japan.
Other
compounds
were
purchased
from Aldrich Chemical Company, Inc
..
Milwaukee, Wisconsin,
532.33.
'HJ
Wagner, R. O. Loutfy,
and
C. Hsiao,
1.
Mater. Sci. 17, 2781
(1982).
Adachi, Tsutsui, and Saito 801
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