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Confinement of charge carriers and molecular
excitons within 5-nm-thick emitter layer in
organic electroluminescent devices with a
double heterostructure
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/2324/19441
出版情報：Applied Physics Letters. 57 (6), pp.531-533, 1990-08-06. 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.
Confinement of charge carriers and
molecu~ar
excitons within 5
a
nm
a
thick
emitter layer
in
organic electroluminescent devices with a double
heterostructure
Chihaya Adachi, Tetsuo Tsutsui, and Shogo Saito
Department
of
Materials Science
and
Technology, Graduate School
of
Engineering Sciences,
Kyushu University, Kasuga-shi.. Fukuoka 816, Japan
(Received 29
January
1990; accepted for publication 30
May
1990)
Organic electroluminescent devices with a double-heterostructure indium-tin-oxide
substrate/
hole transport
layer/emitter
layer/electron
transport
layer/MgAg
have been fabricated
by vacuum vapor
deposition.
The
organic carrier transport
and
emitter layers were composed
of
amorphous films.
In
the
double-heterostructure devices,
the
luminance continued
to
lie in high level, even when the emitter thickness was 50
A.
The
confinement
of
charge
carriers
and
molecular excitons within a narrow emitter layer was achieved.
Recent progress in high performance organic electrolu-
minescent
(EL)
devices owes to
the
use
of
multilayer cell
structures which are composed
of
an emitter layer
(EML)
and carrier transport layers.
l
-
7
Tang
and
VanSlyke
l
re-
ported
that
the combination
of
the
EML
which possessed
electron transporting tendency with a hole transport layer
(HTL)
was essential for high luminance and high stability
of
EL
cells.
1
We proposed a three-layer cell structure in
which
an
EML
was sandwiched between hole
and
electron
transport layers based on
the
idea that
an
electron trans-
port
layer
(ETL)
as well as
HTL
should
playa
major role
in the increase
of
EL
efficiency, when a variety
of
emitter
materials was employed.
Due
to
a lack
of
adequate
BTL
materials, however,
our
preliminary test indicated
that
the
introduction
of
ETL
did
not
contribute to
the
enhance-
ment
of
EL
efficiency
at
that
time.
4
In
the
wide search
of
ETL
materials, we found
an
outstanding electron transport
material, an oxadiazole derivative. Judging from its molec-
ular structure,
it was
not
assumed to be a perfect electron
conductor,
but
it really worked as an excellent
ETL
in
EL
devices.
Thus
using this material we, for the first time,
succeeded in fabricating
the
EL
device having hole con-
ductor as an
EM
L
6
Now, we have both
HTL
and
ETL
materials
and
are
ready to construct ideal three-layer structure cells.
It
is
expected
that
the confinement
of
holes
and
electrons within
a thin
EML
is attained in
the
three layer cell, which we call
a double heterostructure
(DH)
hereafter.
In
this letter, we report the fabrication
of
high perfor-
mance
DH
cells
and
compare
those cells with conventional
two-layer (single heterostructure) cells.
The
effect
of
the
thickness
of
the
EML
on
EL
efficiencies demonstrated the
realization
of
confinement
of
holes
and
electrons within the
very narrow
EML.
This also indicates the effective confine-
ment
of
molecular excitons produced by the recombination
of
holes
and
electrons within the
EML.
Two types
of
EL
cell structures, single heterostruc-
tures (SHs)
and
DHs, were used in
our
experiment as
shown in Fig.
1.
The
organic materials used in
the
exper-
iment are also shown in
the
figure. We used an aromatic
diamine
(TAD)
as the hole transport material
and
an
ox-
adiazole
derivative
(PED)
as the electron transport mate-
rial.
For
emitter
material,
triphenylamine
derivative
(NSD)
was used.
In
a previous paper,
6
we showed
that
the
SH
device with
NSD
and
PBO
layers gave high
EL
effi-
ciency in the case when the thickness
of
EML
and
ETL
was fixed
at
500
A.
In
this study,
the
thickness
ofHTL
and
ETL
was fixed
at
500
A,
and
the thickness
of
EML
was
varied from
500 to 50
A.
Organic layers were deposited
on
a precleancd indium-
tin-oxide
(ITO)
glass substrate
and
a cathode MgAg layer
was deposited on
ETL
by codeposition in a vacuum
of
10
- 7
Torr
at
room temperature.
The
thickness
of
the
ITO
layer was 1000 A
and
sheet resistance was about 20
U/D.
The
deposition rate for organic layers was about 2
A/s.
The
thickness
of
organic layers was determined from fre-
quency shifts on a
quartz
oscillating thickness monitor
I-+---+-+--<I---+--l--
MgAg
.
ETl
-EMl
L-.---.~_
.>
- fTO
~I:i
FIG.
L Structures
of
the
EL
devices
and
the
molecular structures
of
the ma-
terials used
for
the
devices.
531
Appi. Phys. Lett. 57 (6), 6 August 1990
0003-6951/90/320531-03$02.00
Ce,
1990 American Institute of Physics
531
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
1000
86
0
I:J
IlII
100
IIIi
E
II
-c
10·
u
-"""
FIG.
2
Luminance
a1
the
III
current
density
of
100
mAl
u
::
O:DH
I'i:I
cm
2
as a function
of
emitter
c
'E
0.1
IlISH
thickness
in
DH
lind SH
:l
IIIi
c~lls;
Ca)
ITO/TAD/NSDI
...l
0.01
PBD/MgAg,
and
(b)
ITOI
NSD/PBD/MgAg.
0
200
40.0
Thickness
/ A
which was placed very
dose
to
an
organic film on
the
substrate in
the
vacuum
deposition chamber.
The
emitting
area in
EL
cells
was
0.2XO.2
cm
2
•
AU
of
the
deposited
organic films were found
to
be
amorphous
based
on
the
observation
under
a polarizing microscope.
Figure
2 shows
the
thickness dependence
of
luminance
at
a fixed
current
density
of
100
mA/cm
2
for
the
DH
and
SH
cells.
When
the
emitter
thickness was 500
A.,
the
lumi-
nance
of
1000
cd/m2
was observed in
both
of
the
SH
and
DH
cens.
The
SH
cell
structure
gave a high
EL
efficiency,
and
there
was
no
effect
of
the
insertion
of
HTL
(DR).
However, when
the
thickness
of
the
emitter
layer was re-
duced to less
than
300
A,
a new feature appeared.
The
SH
cell showed a
drastic
decrease in luminance with
the
de-
crease
of
the
emitter
thickness.
In
contrast,
constant
high
luminance was retained in
the
wide range
of
the
emitter
thicknesses in
the
DH
cell.
The
luminance
continued
to
lie
in high level, even when
the
emitter
thickness was reduced
to
50
A.
The
uniform
emission indicates
the
uniformity
of
the
ultrathin
emitter
layer.
Figure
3 shows
the
EL
spectrum
of
the
DH
cell with
50
A
emitter
thickness
at
the
current
density
of
10
mAl
cm
2
•
Also,
the
photoluminescence
(PL)
spectra
of
NSD,
TAD,
and
PED
layers are included.
The
EL
spectrum
corresponded exactly
to
the
PL
spectrum
of
the
NSD
film.
No
emission from
the
TAD
or
the
PBD
layer was ob-
served.
This
indicates
that
the
site
of
carrier recombination
located only within
the
EML.
In
other
words,
the
confine-
ment
of
charge carriers within
the
EML
is attained; elec-
trons
injected
from
the
PRO layer into
the
NSD
layer
are
blocked
at
the
TAD/NSD
boundary,
and
holes injected
from
the
TAD
layer
into
the
NSD
layer are blocked
at
the
NSD/PBD
boundary.
300
532
I\~
I
~\
I \
,
500.
700
Wave!ength
/nm
J
900
FIG.
).
EL
spectrum
of
thc
DR
cell
and
PL
spectra
of
TAD,
NSD,
and
PBD
layers;
(a)
EL
spectrum
in
a
[ITOI
TAD(500
A)/NSD(50
A)I
PBD
(SOO
A)IMgAgj
cell,
(b)
PI_
spectrum
in a
TAD
film,
(e)
PL
sp~ctrllm
in a
NSD
film,
and
(d)
I'L
spec-
trum
in a
PBD
film.
Appi. Phys. Lett., Vol. 57,
No.6.
6 August 1990
The
comparison
of
EL
and
PL
spectra also gives
the
evidence
of
the
confinement
of
molecular excitcns within
the
EML.
The
carrier
transport
materials,
TAD
and
PHD,
which possess emission peaks
at
around
400
nm,
are as-
sumed
to
have exciton energies larger
than
that
of
the
NSD
layer. Thus,
the
excitons
created
within
the
EML
have
no
ability
to
migrate
into
TAD
or
PEO
layers. These consid-
erations lead
to
the
conclusion
that
the
confinement
of
both
charge carriers
and
molecular
excitons within an
EML
was achieved even in
the
50-A.-thick
EML.
Here, we would like also
to
mention
another
advantage
of
the
use
of
the
DB,
that
is,
the
insertion
of
ETL or
HTL
between
EML
and
electrodes because
it
serves
to
prevent
the
quenching
of
excitons
at
electrode surfaces.
8
_
9
Accord-
ing
to
our
separate experiment,
the
PL
intensity from
the
EML
in
a
ITO/TAD(200
A)/NSD(25
A)/PBD(200
A)/MgAg
cell was six times larger
than
that
in an
ITO!
TAD(200
A)/NSD(25
A)/MgAg
cell. Evidently,
the
in-
serted
PED
layer prevented
the
quenching
of
the
molecu-
lar
excitons
produced
by
photoexcitation.
This
result
supports
the
assumption
that
the
insertion
of
the
carrier
transport
layers contribute
to
prevent
the
excitons from
quenching at
the
electrode surfaces. This effect is particu-
larly
important,
when
EML
thickness becomes
narrower
than
the
average migration length
of
excitons.
Based
on
the
above considerations, we
can
point
out
two
major reasons for
the
lower
EL
efficiency in
the
SH
cells with
EML
at
the
thickness less
than
300
A.
First, a
large
portion
of
electrons injected from
the
PBO
layer pass
through
the
EML
without
the
encounter
with holes. Sec-
ond, a significant portion
of
the
molecular excitons pro-
duced
within
EML
reach
the
ITO
electrode
during
a mi-
gration process
and
are
quenched.
It
should
be stressed
that
the
success in
the
construc-
tion
of
the
DH
with
extremely
thin
EML
stimulates
our
EL
study
towards
exciting possibility in organic thin-film
devices.
The
increase
of
the
density
of
charge
carriers
and
molecular excitons within extremely
thin
EML
can
be at-
tained by means of
the
DR.
Even
the
use
of
a single mo-
lecular layer for
EML
may
be possible.
The
increase
of
excited states within a very
narrow
region opens
the
pros-
pect for
the
attainment
of
population
inversion
of
excited
states.
The
second prospect is concerned
with
the
fabrication
of
molecular size devices.
We
deal with
the
EML
thickness
of
about
50
A,
which corresponds
to
the
stack
of
about
ten
emitter
molecules. Therefore,
our
EL
devices
now
work
at
the
molecular size
at
least along
the
direction
of
applied
field.
We
have obtained a way
to
observe electronic
and
optical
phenomena
on
a few
molecules
sandwiched
between flat
and
smooth
organic
carner
injection layen;,
This approach, we believe, gives one
of
the
promising en-
trances
towards
the
investigation
of
molecular size elec-
tronic devices.
We
would like
to
acknowledge
Mr.
M.
Hashimoto
and
Mr.
M.
Ohta
in
RICOH
Co. Ltd.,
Japan
for preparing
the
TAD
and
the
NSD.
!e. w.
Tang
and
S. A. VanSlyke, Appl.
1'11Ys.
Lett.
51, 913
(1987).
Adachi, Tsutsui, and Saito
532
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2c.
Adachi,
S.
Tokito, T. Tsutsui, and
S.
Saito, Jpn.
1.
Appl. Phys. 27,
L269 (1988).
Ie.
Adachi,
S.
Tokito, T. Tsutsui,
and
S.
Saito,
lpn.
l.
App!. Phys. 27,
LiB
(1988).
4e.
Adachi, M. Morikawa,
S.
Takltn, T. Tsutsui, and
S.
Saito, Proceed-
ings
of
the 4th International Workshop
on
Eleclroluminescence, Octoher
11-14, 1988,
Totton,
Japan, Springer Proceedings in Physics (Springer,
Berlin, 1989), Vol. 38,
p.
358.
533
Appl. Phys. Lett., Vo:' 57,
No.6,
6 August 1990
5e.
W. Tang,
S.
A. VanSlyke,
and
C.
H. Chen,
J.
AppJ. Phys. 65, 3610
(1989).
6e.
Adachi, T. Tsutsui, and
S.
Saito, AppL Phys. Lett. 55, 1489 (1<)89).
7
e.
Adachi, T. Tsutsui, and
S.
Saito, App!. Phys. LeU. 56, 799 (1990).
'K.
C.
Kao and
W.
Hwang, Electrical Transport in Solids (Pergamon,
New
York, 1981), p. 418.
"H.
Kurczewski
and
H. Bassler, J. Lumines. 15, 26! (1977).
Adachi, Tsutsui, and Saito 533
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