A multicomponent rhenium-based triplet emitter for organic electroluminescence

Article abstract of DOI:10.1016/j.cplett.2006.12.044

A novel Re complex functionalized by a hole-transport and an electron-transport groups was utilized to fabricate organic light-emitting devices (OLEDs). A current efficiency up to 8.2 cd A^-1 corresponding to a power efficiency of 4.6 ml W^-1 and a peak brightness as high as 5500 cd m^-2 were obtained. These results represent the best values reported for OLEDs based on rhenium complexes. Short lifetime, enhanced carrier injection capability of Re complex, and efficient charge-trapping followed by exciton confinement in the emissive layer should be responsible for the outstanding electrophosphorescent performances.

Full text of DOI:10.1016/j.cplett.2006.12.044

Chemical Physics Letters 435 (2007) 54–58
www.elsevier.com/locate/cplett
A multicomponent rhenium-based triplet emitter for
organic electroluminescence
c
a,
a
c
c,
a
C.B. Liu a,b, J. Li , B. Li *, Z.R. Hong , F.F. Zhao , S.Y. Liu *, W.L. Li
a
Key Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,
Changchun 130033, PR China
b
Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
State Key Laboratory of Integrated Optoelectronics, Jilin University, Changchun 130023, PR China
c
Received 10 October 2006; in final form 5 December 2006
Available online 16 December 2006
Abstract
A novel Re complex functionalized by a hole-transport and an electron-transport groups was utilized to fabricate organic light-emit-
ting devices (OLEDs). A current efficiency up to 8.2 cd Aꢀ1 corresponding to a power efficiency of 4.6 ml Wꢀ1 and a peak brightness as
high as 5500 cd mꢀ2 were obtained. These results represent the best values reported for OLEDs based on rhenium complexes. Short life-
time, enhanced carrier injection capability of Re complex, and efficient charge-trapping followed by exciton confinement in the emissive
layer should be responsible for the outstanding electrophosphorescent performances.
ꢀ 2006 Elsevier B.V. All rights reserved.
1. Introduction
opens the way to high EL efficiency up to 100%, three
times higher than the fluorescent dyes, by utilizing both
singlet and triplet excitons [4]. So far, great success has
been achieved in application of iridium complexes as
emitters in OLEDs [5,6]. However, due to the long exci-
ton decay time of phosphorescent complexes, nonradia-
tive relaxation of triplet excitons, i.e. triplet–triplet
annihilation dramatically decrease the EL efficiency with
increasing current density. It is favorable to use phospho-
rescent materials with short lifetime for high EL efficiency
at high currents. Re(I) complexes possess the merits such
as intense phosphorescence emission, high stability and
short phosphorescent lifetime less than 1 ls [7], thus pro-
viding the opportunity to use them for highly efficient
OLEDs. Li et al. presented efficient devices based on
(2,9-dimethyl-1,10-phenanthroline)Re- (CO)3Cl (dmphen-
Re) doped into 4,40-N,N0-dicarbazole-biphenyl (CBP)
with a high current efficiency up to 7.15 cd Aꢀ1 [8]. Wang
et al. achieved yellow electrophosphorescence devices with
power efficiency of 1.6 lm Wꢀ1 by utilizing energy transfer
from BePP2 (PP = 2-2(hydroxyphenyl)-pyridine) to
(Butbpy) Re(CO)3Cl (Butbpy = 4,40-bi(tert-butyl) 2,20-
bipyridine) [9]. In addition, white eletrophosphorescence
Currently, research on organic light-emitting devices
(OLEDs) has focused on the improvement in electrolu-
minscent (EL) performances such as efficiency and stabil-
ity by exploring novel materials and optimizing device
architectures [1,2]. Reasonable device structure offers bal-
anced charge injection and transport, which ensure the
exciton formation occurring in the emissive layer. An effi-
cient way to harvest exciton energy for light emission is
guest–host system, in which exciton can form either on
the host sites and then activate the guest, or directly on
the guest molecules by charge trapping. Efficient exciton
formation and confinement in the emissive layer can
make for efficient electroluminescence [3]. Application of
phosphorescent complexes incorporating heavy metal ions
*
Corresponding authors. Address: Key Laboratory of Excited State
Processes, Changchun Institute of Optics, Fine Mechanics and Physics,
Chinese Academy of Sciences, Changchun 130033, PR China. Fax: +86
431 6176935.
E-mail addresses: lib020@ciomp.ac.cn (B. Li), syliu@mali.jlu.edu.cn
(S.Y. Liu).
0009-2614/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.12.044

C.B. Liu et al. / Chemical Physics Letters 435 (2007) 54–58
55
from the mixing of yellow emission from (4,40-dimethyl-
2. Experimental details
2,20-bipyridine)Re(CO)3Cl (Dmbpy-Re) and blue emission
from N,N0-di-1-naphthyl-N,N0-diphenylbenzidine (NPB)
was also reported [10]. All of the above achievements
indicate that rhenium complexes have emerged as promis-
ing candidates for OLED applications. Additionally, it
has been shown that the introduction of charge transport-
ing groups into emissive molecules can also improve the
device performance [11,12]. Gong and Gordon groups
reported the application of funtionalized rhenium(I) com-
plexes as emitters in EL devices, showing much space for
further enhancement on EL performances [13,14]. Never-
theless, to date, efficient rhenium(I) complexes used for
OLEDs are few and further improvement in performance
is desired. Therefore, it is crucial to design novel func-
tionalized Re(I) complexes suitable for OLEDs
applications.
Fig. 1 outlines the synthetic protocol for the Re-com-
plex. First, 2,40-triphenylamino)-imidazo[4,5-f]1,10-phe-
nanthroline (TPIP) and 2-(4-(4-bromo-butyloxy)phenyl)-
5-phenyl-1,3,4-oxadiazole (BOXD) were synthesized sepa-
rately according to literature procedures [15,16]. And then,
TPIP reacts with BOXD to afford OXD-TPIP in the
presence of sodium hydride. Finally, Re-complex (Re-
OXD-TPIP) was synthesized by direct complexation of
OXD-TPIP with Re(CO)5Br according to the conventional
literature method [17]. The synthetic details are in the sup-
plement. The identity of ligand OXD-TPIP was verified by
single crystal X-ray diffraction analysis. A perspective
drawing of the structure of OXD-TPIP with atomic num-
bering is shown in Fig. 2. The purity and composition of
1
Re-OXD-TPIP were confirmed by HNMR and positive-
In this letter, we wish to report the synthesis, photolu-
minescent (PL) and EL properties of a novel multicompo-
ion FAB-MS.
To investigate the EL properties of Re-OXD-TIPP, the
devices with various doping concentrations were fabricated
using Re-OXD-TPIP as an emitting dopant and CBP as
the host. Vacuum vapor deposition was used to fabricate
the emissive layers since Re-OXD-TPIP is stable enough
to be sublimed. The EL devices are composed of indium
tin oxide (ITO)/4,40,40 0-tris[3-methylphenyl(phenyl) amino]
triphenylamine (m-MTDATA) (30 nm)/N,N0-di-1-naph-
thyl-N,N0-diphenylbenzidine (NPB) (20 nm)/CBP: x wt.%
Re-OXD-TPIP (30 nm)/4,7-diphenyl-1,10-phenanthroline
nent Re(I) complex prepared by introducing
a
triphenylamine and an oxadiazole moieties in the frame-
work of 1,10-phenanthroline molecule. Thus, the hole
transporting, electron transporting, and luminescent func-
tional groups are incorporated into a single molecule. The
devices based on the developed Re(I) complex emit strong
yellowish–green light with a peak emission at ꢁ554 nm.
The maximum luminance exceeding 5500 cd mꢀ2 at
17 V, maximum current efficiency up to 8.2 cd Aꢀ1 corre-
sponding to the power efficiency of 4.3 lm Wꢀ1 at 6 V
were obtained. To our knowledge, these results represent
the best values reported for devices with Re-complexes
as emitters so far.
(Bphen)
(20 nm)/tris(8-hydroxyquinoline)
aluminum
(Alq3) (20 nm)/LiF (0.8 nm)/Al (200 nm). m-MTDATA
was used as hole-injection material and NPB was employed
as the hole-transporter. Bphen, Alq3,and LiF/Al were used
Fig. 1. The synthetic route to Re-OXD-TPIP.

56
C.B. Liu et al. / Chemical Physics Letters 435 (2007) 54–58
Fig. 2. A perspective drawing of OXD-TPIP with atomic numbering scheme. Thermal ellipsoids at the 10% probability level.
as the hole blocker, the electron transporter and the cath-
ode, respectively. Re-OXD-TPIP was doped into the host
material CBP with mass ratios of 2–10 wt.%, and acted
as light-emitting layer. The pressure of the chamber is
below 3 · 10ꢀ4 Pa. UV–vis absorption spectra were
recorded using Shimadzu UV-3000 spectrophotometer.
PL spectra were measured with a RF-5301Pc spectroflu-
orophotometer. EL spectra and Commission Internatio-
nale De L’Eclairage (CIE) coordination of these devices
were measured by a PR650 spectrascan spectrometer. The
luminance–current density–voltage characteristics were
recorded simultaneously with the measurement of EL spec-
tra by combining the spectrometer with a Keithley model
2400 programmable voltage–current source. All measure-
ments were carried out in air at room temperature.
3. Results and discussion
Fig. 3 shows the current efficiency versus current density
characteristics of the EL devices based on Re-OXD-TPIP
with various doping concentrations. Benefiting from the
short triplet lifetime of Re-OXD-TPIP in solid state
(0.16 ls), a very slow decrease of the current efficiency with
an increase of current density was observed for each device.
The efficiencies at the benchmark luminance of 100 cd mꢀ2
are almost equal to the maximum efficiency, as summarized
in Table 1. Thanks to the improvement of the carrier’s injec-
tion ability by introducing a triphenylamine and an oxadi-
zaole moieties in Re-OXD-TPIP molecules, the turn-on
voltage of all four devices is lower than 5 V. As in the case
of other phosphorescent OLEDs, the device performance
Fig. 3. Dependence of current efficiency on current density at different concentrations of Re-OXD-TPIP in CBP at 6 V. Inset: Current density–
brightness–voltage characteristics of a device based on 7 wt.% Re-OXD-TPIP in CBP.

C.B. Liu et al. / Chemical Physics Letters 435 (2007) 54–58
57
Table 1
EL device performances of Re-OXD-TPIP with different concentrations
a
e
f
Concentration (wt.%)
gmax
gb
gc
gd
gmax
Bmax
2
5
7
10
a
1.4
7.5
8.2
5.5
0.7
2.9
2.6
1.7
0.6
1.7
1.3
0.7
1.1
6.7
7.5
5.3
0.6
3.4
4.3
2.6
2906
5682
5510
2920
Maximum current efficiency (cd Aꢀ1).
b
c
d
e
f
Current efficiency at 100 mAcmꢀ2 (cd Aꢀ1).
Current efficiency at 400 mAcmꢀ2 (cd Aꢀ1).
Current efficiency at 100 cd mꢀ2 (cd Aꢀ1).
Maximum power efficiency (lm Wꢀ1).
Maximum brightness (cd mꢀ2).
shows a strong dependence on the doping concentration.
Comparison of performance of these four devices suggests
that the device with 7 wt.% Re-OXD-TPIP in CBP offers
the highest device efficiency. The inset of Fig. 3 shows the
current density–brightness–voltage characteristics of the
optimized device. A maximum brightness of 5505 cd mꢀ2
at a current density of J = 572 mA cmꢀ2 with voltage of
17 V and a maximum current efficiency of 8.2 cd Aꢀ1 at a
current of 0.23 mA cmꢀ2 with the luminance of 18 cd mꢀ2
were achieved. A luminance of 1000 cd mꢀ2 corresponding
to current efficiency of 3.6 cd Aꢀ1 was observed. Even at a
high luminance of 2500 cd mꢀ2 and high current density
of 100 mA cmꢀ2, the current efficiency of this device
remained as high as 2.6 cd Aꢀ1. These values demonstrated
the device stability under continuous operation is improved
compared to the previous reports with rhenium(I) com-
plexes as emitters in OLEDs [7–10]. We believe that the rel-
atively short lifetime of Re-OXD-TPIP is an important
factor to improve the EL performances. Another main fac-
tor to enhance the device performances is efficient exciton
formation on the Re-OXD-TPIP molecules by direct
charge-trapping and confinement within the emissive layer.
This can be inferred from the following analysis of the PL
spectra, EL spectra of Re-OXD-TPIP doped films in
CBP, and the energy levels of the guest and the host.
Fig. 4. PL and EL (17 V) spectra of Re-OXD-TPIP with various
concentrations in CBP. Inset: Absorption spectrum for Re-OXD-TPIP
in chloroform solution (a), PL spectra of CBP flim (b) and of Re-OXD-
TPIP (c).
tration of 2 wt.%, as displayed in Fig. 4b. The significant
differences between the EL and PL spectra of Re-OXD-
TPIP doped films in CBP indicate that direct charge-trap-
ping dominates the EL process. Fig. 5 shows the proposed
energy level diagram of the EL device. The highest occu-
pied molecular orbital (HOMO) energy level of Re-OXD-
TPIP was determined to be 5.8 eV, by cyclic voltammetry.
The lowest unoccupied molecular orbital (LUMO) energy
level was calculated to be 3.1 eV according to the absorp-
tion band edge of Re-OXD-TPIP. Other data are based
on references [18–21]. The energy levels of Re-complex lie
between the band gap of CBP, which meets the require-
ment for efficient carrier trapping [22]. So charge carriers
in the emissive layer can be directly trapped by the Re-com-
plex and await the arrival of opposite carrier for recombi-
nation to form excitons. The existence of a triphenylamine
and an oxadiazole moieties in complex can facilitate carrier
The inset of Fig. 4 plotted the absorption spectrum of
Re-OXD-TPIP in chloroform solution, PL spectra of
CBP and Re-OXD-TPIP. Two strong absorption bands
around 295 and 345 nm can be assigned to spin-allowed
p ꢀ p transitions on the ligand OXD-TPIP. The weak
*
broad absorption band that centers at 437 nm are attrib-
uted to the spin-allowed metal(dp) to ligand (p ) charge-
*
transfer (1MLCT) transition. There exists a partial overlap
between the fluorescence spectrum of CBP and the absorp-
tion spectrum of Re-OXD-TPIP. Consequently, as shown
by the PL spectra of Re-OXD-TPIP doped films in CBP
(Fig. 4a), the emission from the host (CBP) can not be
completely quenched even with more than 10 wt.% of Re-
OXD-TPIP. This implies Fo¨rster energy transfer from the
1
singlet-excited state in the host (CBP) to the MLCT state
of the guest (Re-OXD-TPIP) occurs only at a moderate
level. In a sharp contrast to the PL spectra, no CBP emis-
sion from the OLEDs was observed even at dopant concen-
Fig. 5. The energy level diagram of electrophosphorescence device.

58
C.B. Liu et al. / Chemical Physics Letters 435 (2007) 54–58
injection, and consequently, leads to efficient exciton for-
mation on the Re-OXD-TPIP molecules and efficient elec-
trophosphorescence. Furthermore, an additional dim blue
emission originating from NPB was observed at concentra-
tion of 2 wt.% and 5 wt.% except the strong Re-OXD-TPIP
emission and vanished when the concentration is higher
than 7 wt.%. This is the result of charge trapping. Since
hole injection from the NPB HOMO into the CBP HOMO
is energetically unfavourable, so when the dopant concen-
tration is low, accumulated holes in NBP layer can recom-
bine with the electrons injected from the emissive layer,
resulting in NPB emission in addition to exciton formation
at Re-OXD-TPIP. With increasing doping concentrations,
more and more electrons can be intercepted and trapped by
Re-OXD-TPIP and the contribution from NPB decreased.
At Re-OXD-TPIP concentrations higher than 5 wt.%, no
electrons were injected into NPB layer, so the NPB emis-
sion disappeared. Another important feature in our devices
is that white EL emission (Commission International de
L’Eclairage chromaticity coordinates at X = 0.33, Y =
0.33) with the maximum current efficiency of 1.4 cd Aꢀ1
is observed from the device with 2 wt.% Re-OXD-TPIP
in CBP at 17 V, suggesting that the combination of
Re-OXD-TPIP and a blue emitter may result in pure white
OLEDs. The work on improving the efficiency of white
OLEDs based on Re-OXD-TPIP and other blue emitters
is currently in progress.
Acknowledgement
The authors gratefully thank the financial supports of
One Hundred Talents Project from Chinese Academy of
Sciences and the NSFC (Grant No. 20571071).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.cplett.2006.
12.044.
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4. Conclusions
In summary, we designed and synthesized a novel Re(I)
complex for OLEDs application. The EL device based on
Re-OXD-TPIP showed typical MLCT emission centering
3
at 554 nm, a peak brightness of 5510 cd mꢀ2, and a high
current efficiency of 8.2 cd Aꢀ1 corresponding to a power
efficiency of 4.3 lm Wꢀ1. The present results indicate that
molecular engineering is an effective method to enhance
the EL performance of organic metal complexes. The emis-
sive colors from Re-OXD-TPIP and NPB complement
each other to result in white EL emission with X = 0.33,
Y = 0.33 in a low concentration of 2 wt.%, suggesting that
the combination of Re-OXD-TPIP and other blue emitters
may give rise to efficient white OLEDs.
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