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Endothermic energy transfer: A mechanism for
generating very efficient high-energy
phosphorescent emission in organic materials
Adachi, Chihaya
Center for Photonics and Optoelectronic Materials (POEM), Department of Electrical Engineering
and the Princeton Materials Institute, Princeton University
Kwong, Raymond C.
Universal Display Corporation
Djurovich, Peter
Baldo, Marc.A.
Center for Photonics and Optoelectronic Materials (POEM), Department of Electrical Engineering
and the Princeton Materials Institute, Princeton University
他
http://hdl.handle.net/2324/19449
出版情報：Applied Physics Letters. 79 (13), pp.2082-2084, 2001-09-24. American Institute of
Physics
バージョン：
権利関係：Copyright 2001 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.
Endothermic energy transfer: A mechanism for generating very efficient
high-energy phosphorescent emission in organic materials
Chihaya Adachi, Raymond C. Kwong,
a)
Peter Djurovich,
a)
Vadim Adamovich,
b)
Marc A. Baldo, Mark E. Thompson,
b)
and Stephen R. Forrest
c)
Center for Photonics and Optoelectronic Materials (POEM), Department of Electrical Engineering,
Princeton University, Princeton, New Jersey 08544
共Received 21 June 2001; accepted for publication 16 July 2001兲
Intermolecular energy transfer processes typically involve an exothermic transfer of energy from a
donor site to a molecule with a substantially lower-energy excited state 共trap兲. Here, we demonstrate
that an endothermic energy transfer from a molecular organic host 共donor兲 to an organometallic
phosphor 共trap兲 can lead to highly efficient blue electroluminescence. This demonstration of
endothermic transfer employs iridium共III兲bis共4,6-di-fluorophenyl兲-pyridinato-N,C
2
⬘
兲picolinate as
the phosphor. Due to the comparable energy of the phosphor triplet state relative to that of the 4,4
⬘
-
N,N
⬘
-dicarbazole-biphenyl conductive host molecule into which it is doped, the rapid exothermic
transfer of energy from phosphor to host, and subsequent slow endothermic transfer from host back
to phosphor, is clearly observed. Using this unique triplet energy transfer process, we force emission
from the higher-energy, blue triplet state of the phosphor 共peak wavelength of 470 nm兲, obtaining
a very high maximum organic light-emitting device external quantum efficiency of 共5.7⫾0.3兲% and
a luminous power efficiency of 共6.3⫾0.3兲lm/W. © 2001 American Institute of Physics.
关DOI: 10.1063/1.1400076兴
Energy transfer from a conductive host to a luminescent
dopant can result in high external quantum efficiencies in
organic thin-film light-emitting devices. For example, we
have recently demonstrated high-efficiency green and red or-
ganic electrophosphorescent devices which harvested both
singlet and triplet excitons, leading to internal quantum effi-
ciencies (
int
) approaching 100%.
1–5
In these cases, high
efficiencies were obtained by energy transfer from both the
host singlet and triplet states to the phosphor triplet, or via
direct trapping of charge on the phosphor, thereby harvesting
up to 100% of the excited states. These transfers entail a
resonant, exothermic process. As the triplet energy of the
phosphor increases, it becomes less likely to find an appro-
priate host with a suitably high-energy triplet state. The very
large excitonic energies required of the host also suggest that
this material layer may not have appropriate energy-level
alignments with other materials used in an OLED structure,
hence, resulting in a further reduction in efficiency. To elimi-
nate this competition between the conductive and energy
transfer properties of the host, a route to efficient blue elec-
trophosphorescence may involve the endothermic energy
transfer from a near-resonant excited state of the host to the
higher triplet energy of the phosphor.
6,7
Provided that the
energy required in the transfer is not significantly greater
than the thermal energy, this process can be very efficient.
Here, we demonstrate blue electrophosphorescence us-
ing energy transfer from a conductive organic host to the
iridium complex: iridium共III兲bis关4,6-di-fluorophenyl兲-
pyridinato-N,C
2
⬘
兴picolinate 共FIrpic兲.
8
The introduction of
the electron withdrawing fluorine complex results in an in-
crease of the triplet exciton energy and, hence, a blueshift of
the phosphorescence compared with that of Ir共ppy)
3
.We
obtained a maximum external quantum electroluminescent
共EL兲 efficiency (
ext
)of共5.7⫾0.3兲% and a luminous power
efficiency (
p
)of共6.3⫾0.3兲lm/W, representing a significant
improvement of the efficiencies compared with the blue fluo-
rescent emitters reported to date.
9–11
Figure 1共a兲 shows photoluminescent 共PL兲 spectra of
three different iridium-based phosphors, bis共2-phenyl-
pyridinato-N,C
2
⬘
兲iridium共acetylacetonate兲
关
ppy
2
Ir共acac)],
bis关4, 6-di-fluorophenyl兲-pyridinato-N, C
2
⬘
兴iridium共acety-
lacetonate兲关FIr共acac兲兴, and FIrpic, demonstrating a spectral
shift with ligand modification. The presence of the heavy
metal iridium results in strong spin-orbit coupling and metal
ligand charge transfer, allowing for rapid intersystem cross-
ing of excitons into the radiative triplet manifold of the
ligand.
8
All three complexes give high photoluminescent ef-
ficiencies of ⌽
pl
⫽0.5–0.6 in fluid solution. With introduc-
tion of fluorine atoms into the 4,6-positions in
2-phenylpyridine, the triplet excited state experiences a blue-
shift of ⬃40 nm in the PL peak FIr共acac兲 as compared with
the green emitting ppy
2
Ir共acac). Furthermore, replacement
of the acetylacetonate ligand of FIr共acac兲 with picolinate
共i.e., FIrpic兲 resulted in an additional ⬃20 nm blueshift.
Organic light-emitting devices were grown on a glass
substrate precoated with a ⬃130-nm-thick indium–tin–oxide
共ITO兲 layer with a sheet resistance of ⬃20 ⍀/䊐. Prior to
organic layer deposition, the substrate was degreased with
solvents and cleaned for 5 min by exposure to an UV–ozone
ambient, after which it was immediately loaded into the
evaporation system. With a base pressure of ⬃4⫻ 10
⫺ 8
Torr,
the organic and metal cathode layers were grown succes-
sively without breaking vacuum using an in vacuo mask ex-
a兲
Universal Display Corporation, 375 Phillips Blvd., Ewing, NJ 08618.
b兲
Department of Chemistry, University of Southern California, Los Angeles,
Los Angeles, CA 90089.
c兲
Electronic mail: forrest@princeton.edu
APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 13 24 SEPTEMBER 2001
20820003-6951/2001/79(13)/2082/3/$18.00 © 2001 American Institute of Physics
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change mechanism. First, a 10-nm-thick copper phthalocya-
nine 共CuPc兲 hole injection layer followed by a 30-nm-thick
4,4
⬘
-bis关N-共1-naphthyl兲-N-phenyl-amino兴biphenyl 共
␣
-NPD兲
hole transport layer 共HTL兲 were deposited. Next, a 30-nm-
thick light-emitting layer 共EML兲 consisting of 6% FIrpic
doped into a 4,4
⬘
-N,N
⬘
-dicarbazole-biphenyl 共CBP兲 host
was prepared via thermal codeposition. Finally, a 30-nm-
thick layer of 4-biphenyloxolato aluminum共III兲bis共2-methyl-
8-quinolinato兲4-phenylphenolate 共BAlq兲 was used to trans-
port and inject electrons into the EML. A shadow mask with
rectangular 2 mm⫻2 mm openings was used to define the
cathode consisting of a 1-nm-thick LiF layer, followed by a
100-nm-thick Al layer. After deposition, the device was en-
capsulated using an UV-epoxy resin under a nitrogen atmo-
sphere with ⬍1 ppm oxygen and water. Given that the peak
CBP triplet wavelength
9
is ⫽484 nm 关共2.56⫾0.10兲 eV兴,
compared to ⫽475 nm 关共2.62⫾0.10兲 eV兴 for FIrpic 共see
spectra in Fig. 3兲, endothermic transfer may be interrupted
by nonradiative defect states of intermediate energy. Intro-
duction of oxygen or water may be the source of such de-
fects. Indeed, we have found that breaking vacuum at any
point in the fabrication process and exposure to air or puri-
fied oxygen 共⬍1 ppm oxygen and water兲 results in a decrease
in efficiency of at least a factor of 2 below the values re-
ported here. A similar ambient sensitivity is not observed for
green and red electrophosphorescence OLEDs employing
conventional exothermic energy transfer mechanisms.
Figure 1共b兲 shows the EL spectrum with a maximum at
the peak wavelength of
max
⫽475 nm and additional sub-
peaks at
sub
⫽495 and 540 nm 共arrows兲, which generally
agrees with the PL spectral shape. The Commission Interna-
tionale de L’Eclairage 共CIE兲 coordinates of 共x⫽0.16,
y⫽0.29兲 for a FIrpic OLED is shown in the inset of Fig. 1共b兲
along with the coordinates of green
关
Ir共ppy)
3
] and red
关
Btp
2
Ir共acac)] electrophosphorescence devices.
Figure 2 shows
ext
and
p
as functions of current den-
sity. A maximum
ext
⫽共5.7⫾0.3兲% and a luminous power
efficiency (
p
)of共6.3⫾0.3兲lm/W are achieved at J⫽5 and
0.1 mA/cm
2
, respectively. While the device shows a gradual
decrease in
ext
with increasing current which has previously
been attributed to triplet–triplet annihilation,
12
a maximum
luminance of 6400 cd/m
2
with
ext
⫽3.0% was obtained even
at a high current of J⫽ 100 mA/cm
2
. These values compare
FIG. 1. 共Color兲共a兲 Molecular structures of the iridium complexes:
ppy
2
Ir共acac), FIr共acac兲, and FIrpic, with their photoluminescence spectra in
a dilute 共10
⫺ 5
M兲 chloroform solution. 共b兲 Electroluminescence spectra of
the following OLED structure: ITO/CuPc 共10 nm兲/
␣
-NPD共30 nm兲/CBP host
doped with 6% FIrpic 共30 nm兲/BAlq 共30 nm兲/LiF 共1nm兲/Al 共100 nm兲.The
EL spectrum has a maximum at the peak wavelength of
max
⫽475 nm and
additional subpeaks at
sub
⫽495 and 540 nm 共arrows兲, which agrees with
the PL spectral shape. 共Inset兲 CIE coordinates of FIrpic 共x⫽0.16, y⫽0.29兲,
Ir共ppy)
3
共x⫽0.28, y⫽0.62兲, and btp
2
Ir共acac) 共x⫽0.67, y⫽0.33兲, and a color
photograph of an array of four FIrpic OLEDs.
FIG. 2. External electroluminescent quantum 共
ext
: filled squares兲 and
power 共
p
: open circles兲 efficiencies of the following OLED structure:
ITO/CuPc 共10 nm兲/
␣
-NPD 共30 nm兲/CBP host doped with 6% FIrpic 共30
nm兲/BAlq 共30 nm兲/LiF 共1nm兲/Al 共100 nm兲. 共Inset兲 Energy-level diagram of
triplet levels of a CBP host and a FIrpic guest. Due to the energy lineup of
CBP and FIrpic triplet levels, both extothermic and endothermic transfer is
possible. Here,
g
and
h
are the radiative decay rates of triplets on the
guest 共phosphor兲 and host molecules, and the rates of exothermic 共forward兲
(
F
) and endothermic 共reverse兲 (
R
) energy transfers between CBP and
FIrpic are also indicated.
2083Appl. Phys. Lett., Vol. 79, No. 13, 24 September 2001 Adachi
et al.
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favorably with
ext
⫽2.4% for fluorescent devices with a
similar blue color emission spectrum. Since the triplet energy
level of a CBP host 共2.56⫾0.10兲 eV is slightly less than that
of FIrpic at 共2.62⫾0.10兲 eV 共inset of Fig. 2兲, exothermic
energy transfer from FIrpic to CBP is inferred. The pro-
nounced roll off at small J is indicative of the sensitivity of
backward energy transfer to the presence of energy dissipa-
tive pathways, reducing the efficiency via nonradiative triplet
recombination when the density of triplets is too low to satu-
rate these parasitic mechanisms.
Figure 3 shows a streak image of the transient decay of a
6% FIrpic:CBP film at T⫽100 K with two time-resolved
emission spectra. In addition to the prompt phosphorescence
of FIrpic, we observe an extremely long decay component
lasting for
⬃10 ms, which follows the CBP triplet lifetime.
Since the PL spectrum of the slow component coincides with
that of FIrpic PL, this supports the conclusion that exother-
mic energy transfer from FIrpic to CBP occurs. The triplet
state then migrates through the CBP host molecules, and fi-
nally, is endothermally transferred back to FIrpic, resulting in
the delayed phosphorescence observed. Due to the signifi-
cant difference of lifetimes of the excited states,
h
Ⰶ
g
, 共
h
and
g
are the radiative decay rates of the triplets on the host
and guest molecules, respectively兲, the triplet exciton decay
originates from FIrpic, as desired. The blue emission cen-
tered at
max
⫽400 nm in the prompt emission spectrum is
due to fluorescence of CBP, with a transient lifetime Ⰶ100
ns, which is significantly shorter than the decay of FIrpic.
Figure 4 shows the temperature dependence of the tran-
sient decay and the relative PL efficiency (
PL
) of FIrpic
doped into CBP. After a slight enhancement of
PL
as the
temperature is increased from 50 to 200 K, it once again
decreases at yet higher temperatures. The transient decay
characteristics are also temperature dependent. In particular,
a significant decrease in the nonexponential decay time was
observed at T⫽50 and 100 K. The increase of
PL
from
T⫽300 to 200 K is due to the suppression of nonradiative
decay of FIrpic. The decrease below T⬃200 K, however, is a
signature of retardation of the endothermic process of energy
transfer from CBP to FIrpic, leading to loss of the radiative
triplet excitons. Since we observe no delayed component at
T⫽300 K, energy transfer from CBP to FIrpic is very effi-
cient with thermal assistance. In contrast, the PL intensity of
Ir共ppy兲
3
:CBP shows no temperature dependence along with
no evidence for such a slow component at low temperature,
suggesting the absence of backward energy transfer in that
system.
In summary, we demonstrated efficient blue electrophos-
phorescence using FIrpic as the phosphor molecule. The
transient phosphorescence decay suggests the presence of en-
dothermic energy transfer between the phosphor and the con-
ductive CBP host.
This work was funded by the Universal Display Corpo-
ration, the Defense Advanced Research Projects Agency, and
the Air Force Office of Scientific Research.
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FIG. 3. Streak image of a 6% FIrpic:CBP film 共100 nm thick兲 onaSi
substrate under nitrogen pulse excitation 共⬃500 ps兲 at T⫽100 K. Two dis-
tinct decay processes, prompt and delayed phosphorescence, are demon-
strated along with their photoluminescent spectra: dashed line⫽delayed and
solid line⫽prompt. Also shown is the CBP phosphorescence spectrum ob-
tained at 10 K.
FIG. 4. Transient photoluminescence decay characteristics of a 6% FIrpic:
CBP film 共100 nm thick兲 on a Si substrate under nitrogen pulse excitation
共⬃500 ps兲 at T⫽50, 100, 200, and 300 K. 共Inset兲 Temperature dependences
of the relative photoluminescence efficiency (
PL
) of the film.
2084 Appl. Phys. Lett., Vol. 79, No. 13, 24 September 2001 Adachi
et al.
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