High-performance OLEDs based on 4,5-diaza-9,9′-spirobifluorene ligated rhenium(I) complex with enhanced steric hindrance

Article abstract of DOI:10.1016/j.orgel.2012.09.020

Highly efficient and emitter concentration insensitive phosphorescent electroluminescent devices based on a novel rhenium(I) [Re(I)] complex, i.e., (4,5-diaza-9,9′-spirobifluorene)Re(CO)₃Br (Re-DSBF), were established. Non-doped device based on

Full text of DOI:10.1016/j.orgel.2012.09.020

Organic Electronics 13 (2012) 3138–3144
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
High-performance OLEDs based on 4,5-diaza-9,90-spirobifluorene
ligated rhenium(I) complex with enhanced steric hindrance
Xiao Li a, , Hai-Jun Chi a, Gong-Hao Lu a, Guo-Yong Xiao a, Yan Dong a, Dong-Yu Zhang b,
⇑
⇑
,
Zhi-Qiang Zhang a, Zhi-Zhi Hu a
a School of Chemical Engineering, University of Science and Technology Liaoning (USTL), Anshan 114051, People’s Republic of China
b Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly efficient and emitter concentration insensitive phosphorescent electroluminescent
devices based on a novel rhenium(I) [Re(I)] complex, i.e., (4,5-diaza-9,90-spirobifluo-
rene)Re(CO)3Br (Re-DSBF), were established. Non-doped device based on Re-DSBF as emit-
ter exhibited outstanding performances with the peak luminance of 8531 cd mꢀ2 and
maximum current efficiency of 16.8 cd Aꢀ1, which were the highest reported to date for
non-doped phosphorescent electroluminescent devices based on Re(I) emitters. Such
excellent performances are closely related to the steric hindrance, large Stokes shift, and
short luminescent lifetime of Re-DSBF complex. The luminescent mechanisms of those
devices were also investigated.
Received 17 May 2012
Received in revised form 18 September
2012
Accepted 23 September 2012
Available online 6 October 2012
Keywords:
4,5-Diaza-9,90-spirobifluorene
Rhenium complex
Phosphorescence
OLEDs
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
efficient devices based on [2-(40-diphenylaminophenyl)-
1,10-phenanthroline]Re(CO)3Cl with a maximum current
Organic light-emitting diodes (OLEDs) possess latent
application prospects in eco-friendly flat-panel displays
and energy-saving solid-state lightings owing to rapid pro-
gress in material design and device fabrication [1–3]. Of
the light-emitting organic materials available, phosphores-
cent materials have attracted extensive attention in recent
years owing to their promising advantages of achieving a
maximum internal quantum efficiency of nearly 100% [4–
6]. In view of the distinguished qualities of rhenium(I)
[Re(I)] complexes which possess such as photophysical,
photochemical, excited state redox properties and with
the aim of further exploring novel phosphorescent materi-
als, many researchers pay considerable attention to this
field [7–10]. For example, Mu et al. [11] recently reported
efficiency of 12.2 cd Aꢀ1 and
a peak brightness of
7308 cd mꢀ2 by a solution process, which represented the
best values reported for electrophosphorescent devices
based on solution processable Re(I) complexes, but these
values were still much lower than those of Re(I)-doped de-
vices fabricated by vacuum deposition methods [12,13].
Very recently, Mauro et al. [14] reported phosphorescent
OLEDs with outstanding external quantum efficiency of ca.
10% using binuclear Re(I) complexes as dopants, which
was the highest reported for phosphorescent OLEDs with
Re(I)-based phosphors up to now.
Due to the poor carrier mobility and luminescence self-
quenching of the intrinsic phosphorescent dopants, high-
performance phosphorescent OLEDs are usually fabricated
by doping the emitters into a charge-transporting host
matrix in a rather low and narrow concentration range.
However, the complicated low-doped process which usu-
ally requires a precise control of dopant ratio limits the
practical applications in mass production. Therefore, it is
of significant importance and interest for OLEDs applica-
⇑
Corresponding authors. Tel.: +86 412 5929627; fax: +86 412
5929637.
E-mail addresses: aslixiao@yahoo.com.cn (X. Li), dyzhang2012@ya-
hoo.com.cn (D.-Y. Zhang).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.09.020

X. Li et al. / Organic Electronics 13 (2012) 3138–3144
3139
tions to explore new phosphorescent materials that can
serve as efficient high-doped or non-doped emitters.
It is well-known that 4,5-diaza-9,90-spirobifluorene
were added into the flask under nitrogen atmosphere, then
the mixture solvents of THF (0.5 mL) and 1,2-dichloroeth-
ane (0.1 mL) were added. Subsequently, the system was
heated and stirred for about 20 min to active the magne-
sium. Then 2-bromobiphenyl (1.9 mL, 11 mmol) in 9.5 mL
THF was added dropwise into this mixture. The reaction
was stopped until the magnesium disappeared completely.
Under refluxing, the flask containing above resulting 2-
biphenylmagnesium bromide was added into 4,5-diazaflu-
oren-9-one (0.91 g, 5 mmol) in 25 mL THF. The mixture
was refluxed for about 12 h, and then quenched with water
after cooling to ambient temperature and extracted with
CHCl3. The combined organic solution was dried with
MgSO4 and concentrated by rotary evaporation. The result-
ing crude solid was washed with n-hexane and dried to
give 5-biphenyl-2-yl-5H-cyclopenta [2,1-b;3,4-b0]dipyri-
din-5-ol as a brown solid (1.232 g, yield: 73%). The result-
ing intermediate of 5-biphenyl-2-yl-5H-cyclopenta [2,1-
b;3,4-b0]dipyridin-5-ol, 100 mL acetic acid and 1.5 mL sul-
furic acid were refluxed for about 24 h under nitrogen
atmosphere. The reaction was quenched with cold water
after cooling to RT and neutralized with saturated NaOH
(aq) to basic, then extracted with CHCl3 and dried with
MgSO4. The combined organic solution was concentrated
by rotary evaporation, washed with n-hexane and dried
to give DSBF as light brown solid. Yield: 51%. MS (APCI):
m/z 319.2 [M + H+]. 1H NMR (500 MHz, CDCl3, TMS):d 8.7
(s, 2 H), 7.87 (d, J = 7.63 Hz, 2 H), 7.41 (dd, J = 7.445 Hz, 2
H), 7.14–7.12 (m, 6 H), 6.73 (d, J = 7.585 Hz, 2 H); IR (KBr,
cmꢀ1):3026, 1402, 1167, 752.
(DSBF), which has two almost mutually perpendicular
p-
system linked by a common tetrahedral atom and rigid
backbone, exhibits high electron affinity [15]. Wong et al.
[16] reported a DSBF derivative as efficient electron injec-
tion/transport material and blue emitter in OLEDs. Then,
Wu et al. [17] reported highly efficient single-layered so-
lid-state light-emitting electrochemical cells based on irid-
ium (III) complexes containing DSBF ligand which could
greatly enhance the steric hindrance of iridium (III) com-
plexes and reduce self-quenching of luminescence. Lately,
Du et al. [18] reported a DSBF functionalized europium
(III) complex with efficient photo- and electroluminescent
properties.
Hinted by aforementioned those, highly efficient and
emitter concentration insensitive phosphorescent OLEDs
based on a novel Re(I) emitter, i.e., (4,5-diaza-9,90-spirobi-
fluorene)Re(CO)3Br (Re-DSBF), were achieved. More
importantly, the non-doped device based on Re-DSBF
emitter showed prominent performances with the peak
luminance of 8531 cd mꢀ2 and maximum current effi-
ciency of 16.8 cd Aꢀ1
.
2. Experimental details
2.1. Materials and instruments
Commercially available reagents were used without
further purification unless otherwise stated. Re(CO)5Br
was available from Alfa Aesar. Solvents were dried by stan-
dard procedures prior to use. NMR spectra were recorded
on a Bruker AC 500 spectrometer. Chemical shifts were re-
ported in ppm down field from tetramethysilane (TMS)
with the solvent resonance as the internal standard. Ele-
mental analysis was performed on Vario EL III CHNS instru-
ment. FTIR spectra were recorded with samples as KBr
pellets using WQF 200 FTIR spectrophotometer. Thermo-
gravimetric analysis (TGA) was undertaken under nitrogen
atmosphere at a heating rate of 10 °C minꢀ1 on a Perkin-El-
mer Diamond TG–DTA 6300 thermal analyzer. UV–vis
absorption spectrum was obtained on a Perkin-Elmer
Lambda 900 spectrophotometer. Photoluminescent (PL)
spectra were measured on a Perkin-Elmer LS 55 fluores-
cence spectrophotometer. PL quantum yield of the Re(I)
complex was measured according to the reference’s meth-
od [7]. The luminescent lifetimes of the Re(I) complex were
detected by a system equipped with a TDS 3052 digital
phosphor oscilloscope pulsed Nd:YAG laser with a Third-
Harmonic-Generator 355 nm output. Cyclic voltammetry
experiments were conducted using a CHI832B electro-
chemical analyzer with a scan rate of 200 mV sꢀ1. All mea-
surements were carried out at room temperature (RT).
2.2.2. Synthesis of Re-DSBF
DSBF (0.33 g, 1.1 mmol) and Re(CO)5Br (0.41 g, 1 mmol)
were refluxed in 100 mL of toluene for 3 h under nitrogen
atmosphere. After the mixture was cooled to RT, the sol-
vent was removed and the resulting yellow solid was puri-
fied by silica–gel column chromatography with acetic
ether and petroleum ether as mobile phase. Yield: 76%.
1H NMR (500 MHz, CD2Cl2, TMS):d 8.68–8.66 (m, 2 H),
7.87 (d, J = 7.695 Hz,1 H), 7.82 (d, J = 7.635 Hz, 1 H), 7.43
(dd, J = 7.565 Hz, 1 H), 7.37 (dd, J = 7.558 Hz, 1 H), 7.32–
7.28 (m,
J = 7.55 Hz, 1 H), 6.89 (d, J = 7.625 Hz, 1 H), 6.68 (d,
J = 7.635 Hz,
H); 13C NMR (125 MHz, CD2Cl2,
4 H), 7.18 (dd, J = 7.60 Hz, 1 H), 7.08 (d,
1
TMS):161.705, 150.669, 143.003, 142.441, 141.590,
141.494, 134.063, 129.561, 129.324, 128.780, 128.381,
127.214, 124.045, 123.986, 120.964, 120.782; IR(KBr,
cmꢀ1):2025, 1927, 1886, 1283, 1230; Elemental analysis
for C26H17BrN2O3Re. Calcd: C, 46.50; H, 2.55; N, 4.17;
Found: C 46.52, H 2.53, N 4.19.
2.3. Device fabrication and EL measurements
OLEDs were fabricated through vacuum deposition of
the materials at about 1 ꢁ 10ꢀ6 Torr onto ITO-coated glass
substrates with a sheet resistance of 25
X
sqꢀ1. The ITO-
2.2. Material synthesis
coated substrates were routinely cleaned by ultrasonic
treatment in solvents and then cleaned by exposure to a
UV-ozone ambient. All organic layers were deposited in
succession without breaking vacuum. The devices were pre-
pared with the conventional structures of ITO/m-MTDATA
2.2.1. Synthesis of DSBF
DSBF was synthesized according to the reference meth-
od [16]. Magnesium (0.24 g, 10 mmol) and a little iodine

3140
X. Li et al. / Organic Electronics 13 (2012) 3138–3144
(10 nm)/NPB (20 nm)/CBP: x% Re-DSBF (30 nm)/Bphen
(10 nm)/Alq3 (20 nm)/LiF (1 nm)/Al (120 nm), in which m-
MTDATA {4,40,400-tris[3-methylphenyl(phenyl)amino]tri-
phenyl-amine}, NPB {4,40-bis[N-(1-naphthyl)-N-phenyl-
amino]biphenyl}, CBP (4,40-N,N0- dicarbazolebiphenyl),
Bphen (4,7-diphenyl-1,10-phenanthroline), and Alq3
[tris(8-hydroxyquinoline)aluminum] were used as hole
injection layer, hole transporting layer, host, exciton block-
ing layer and electron transporting layer, respectively, and
LiF/Al as the composite cathode. Deposition rates and thick-
nesses of the layers were monitored in situ using oscillating
quartz monitors. Thermal deposition rates for organic
materials, LiF, and Al were ꢂ1, ꢂ1, and ꢂ10 Å sꢀ1, respec-
tively. EL spectra were measured by a PR655 spectra scan
spectrometer. The luminance–current–voltage (L–I–V)
characteristics were recorded simultaneously with the
measurement of EL spectra by combining the spectrometer
with a Keithley 2400 source meter. All measurements were
carried out at RT under ambient conditions.
pseudooctahedral fac-ReðCOÞþ3 [19]. All evidences indicated
that the resulting Re(I) complex conformed to the pro-
posed structure.
The thermal stability of the complex was carried out by
TGA under a nitrogen stream. Re-DSBF was thermally sta-
ble up to ca. 330 °C corresponding to 5% weight loss, as
determined in Fig. 1, which was beneficial to the long-term
stability of OLEDs fabricated from the material.
3.2. Photophysical properties
Fig. 2 showed the UV absorption and PL spectra of Re-
DSBF in dilute dichloromethane solution and solid PL spec-
trum of 25% Re-DSBF doped in CBP film. Absorption spec-
trum of Re-DSBF was very similar to those of analogous
Re(I) tricarbonyl complexes [20], and the assignments
had been made accordingly. By comparison to the absorp-
tion of the free ligand DSBF, the absorption bands of Re-
DSBF located at ca. 270 and 305 nm could be assigned to
spin-allowed 1p pꢃ DSBF-centered electronic transitions.
–
3. Results and discussion
The moderately intense, poorly distinguished absorption
bands extending into the visible region from ca. 350 to
500 nm were tentatively assigned to an admixture of me-
3.1. Synthesis and characterization
tal-to-ligand charge transfer states, d
p
(Re) ? pꢃ (DSBF)
The synthetic routes of Re-DSBF complex were showed
in Scheme 1. The synthesis of spiroconfigured ligand DSBF
involved three steps including oxidation, nucleophilic
addition and electrophilic substitution reactions using
1,10-phenanthroline as starting material [16]. Among the
synthesis processes, it should be specially pointed out that
the preparation for Grignard reagent based on 2-bromobi-
phenyl required strict condition control because of large
steric hindrance of 2-bromobiphenyl. Then, Re-DSBF was
smoothly obtained by direct complexation of DSBF with
Re(CO)5Br in refluxing toluene for good isolated yield.
The molecular structure was fully characterized by 1H
NMR, 13C NMR, and element analysis, respectively. The
molecular structure of Re-DSBF was further confirmed by
IR spectra. Three C„O stretching vibration bands were ob-
served in IR spectrum, characteristic of monomeric
(1MLCT and 3MLCT). The PL spectrum of Re-DSBF in dichlo-
romethane showed a green emission with a maximum
emission centered at 530 nm, which had been assigned
as MLCT-based luminescence, typical of this type of com-
plex [21]. From solid PL spectrum (25% Re-DSBF doped in
CBP), strong emission at 545 nm including weak emission
at about 420 nm which should be attributed to the emis-
sion of CBP can be detected, indicating that efficient energy
transfer occurs between CBP and Re-DSBF.
The PL quantum yield of Re-DSBF in chloroform solu-
tion was measured to be 0.15 using quinine sulfate
(u = 0.546) as a calibration standard, which was much
higher than those of previously reported Re(I) complexes
based on 4,5-diazafluorene derivatives [22]. The spirocon-
figured DSBF ligand effectively enhanced the steric hin-
drance of Re(I) complex and reduced the self-quenching
Scheme 1. Synthetic routes of Re-DSBF complex.

X. Li et al. / Organic Electronics 13 (2012) 3138–3144
3141
determine the intrinsic triplet-state lifetime (
s0) of Re-
DSBF, the lifetimes of Re-DSBF in degassed chloroform at
different concentrations from 1.0 ꢁ 10ꢀ5 to 1.0 ꢁ 10ꢀ3
-
mol Lꢀ1 were measured. The Stern–Volmer kinetics equa-
tion s0
/
s
= 1 + kqꢄC could be used to depict the
relationship of the measured lifetimes
s
and Re-DSBF con-
centration C, where kq was the quenching rate constant.
s0 and kq were calculated to be 0.5
l
s and 2.53 ꢁ 108 -
molꢀ1 sꢀ1, respectively. The intrinsic lifetime s0 exhibited
a slight change at different concentrations and kq was be-
low the diffusion-controlled limit of ꢂ1010 molꢀ1 sꢀ1 [23].
This phenomenon further demonstrated that there existed
weak concentration quenching in Re-DSBF solution.
3.3. Electrochemical properties
Fig. 1. TGA trace of Re-DSBF in N2 atmosphere at a heating rate of
10 °C minꢀ1
.
The electrochemical behavior of Re-DSBF was investi-
gated by CV, which was shown in Fig. 3. The anodic waves
are associated with a ReI-based oxidation process (ReI/ReII),
and the cathodic waves are associated with a ligand
(L)-based reduction process ([ReIBr(CO)3(L)]/ [ReIBr(CO)3
(Lꢀ)]ꢀ) [24,25]. Re-DSBF shows irreversible anodic waves
at E1/2 = +1.53 V with an onset oxidation potential of
+1.06 V. The energy level of the highest occupied molecular
orbital (HOMO) is calculated from the onset oxidation
Ox
onset
(E
) with the formula EHOMO = ꢀ4.8 ꢀ EOx (ꢀ4.8 V for
onset
saturated calomel electrode with respect to the zero
vacuum level) to be ꢀ5.86 eV for Re-DSBF. The energy
band gap of Re-DSBF obtained from the optical absorption
spectrum is 2.68 eV, so the lowest unoccupied molecular
orbital (LUMO) is determined to be 3.18 eV.
3.4. Density functional theory calculations on Re(I) complex
Fig. 2. UV absorption and PL spectra of Re-DSBF in dilute dichlorometh-
ane solution and solid PL spectrum of 25% Re-DSBF doped in CBP film.
The ground state geometries and electronic structures
of Re-DSBF were calculated according to density functional
theory (DFT) calculations using the GAUSSIAN-03 software
package (Gaussian, Inc.) at the B3LYP/LANL2DZ level since
such investigations had been proven to be very helpful in
understanding the photophysical properties of Re(I) com-
plexes [26]. Therefore, it would be instructive to examine
the HOMOs and the LUMOs of this complex.
effect, so PL measurement showed highly retained quan-
tum yield.
As shown in Fig. 2, there was little overlap between the
absorption and emission spectra of the Re(I) complex. The
Stokes shift for Re-DSBF which was obtained between the
maximum of the lowest energy absorption band and the
maximum of the emission band was as large as about
180 nm. The large Stokes shift may be caused by significant
structural differences between the ground state and ex-
cited state upon photo excitation. More importantly, such
a large Stokes shift was especially valuable for light-emit-
ting materials used in high doped or non-doped OLEDs
since the absence of self-absorption would definitely facil-
itate efficient light emission.
It has been well recognized that phosphorescence life-
time has significant influence in triplet–triplet annihilation.
The longer phosphorescence lifetime of a material increases
the possibility of triplet–triplet annihilation it potentially
has. A very short lifetime (0.12 ls) of Re-DSBF in solid state
powder was obtained owing to molecule–molecule interac-
tions. Therefore, the short phosphorescence lifetime of Re-
DSBF could efficiently suppress the phosphorescence self-
quenching to exhibit high efficiency. In order to further
Fig. 3. Cyclic voltammograms of Re-DSBF measured in CH2Cl2 (vs. SCE) at
a scan rate of 200 mV sꢀ1
.

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X. Li et al. / Organic Electronics 13 (2012) 3138–3144
Fig. 4. Contour plots of HOMOs and LUMOs for Re-DSBF as determined by DFT calculations.
Table 1
EL performance parameters of different Re(I)-doped electrophosphorescent devices.
Device
kmaxa/nm
CIE (x,y) at 6 V
Lmaxb/cd mꢀ2
gLc/cd Aꢀ1
gLd/cd Aꢀ1
I
II
III
IV
548
548
550
557
(0.363,0.591)
(0.365,0.587)
(0.370,0.572)
(0.395,0.576)
4840
7195
8990
8531
5.4
7.4
12.1
16.8
4.0
6.5
8.9
8.5
a
b
c
Maximum emission wavelength.
Maximum luminance.
Maximum current efficiency.
d
Current efficiency at 100 mA cmꢀ2
.
As could be seen from Fig. 4, the ground state geome-
tries of Re-DSBF displayed a distorted octahedral configu-
ration of ligand around the metal center. The calculated
geometries of Re-DSBF showed that the biphenyl unit
was significantly twisted against to the 4,5-diazafluorene
core, resulting in a non-planar structure in Re-DSBF mole-
cule. The geometrical characteristic could effectively sup-
press molecular packing. The HOMOs of Re-DSBF were
mainly composed of the
p orbitals of the carbonyl group,
the bromine, and the d orbitals of Re cation which were
in antibonding coordination with the axial bromine group
and bonding with the three carbonyl groups. While the LU-
MOs of Re-DSBF were essentially pꢃ orbital localized on al-
most the whole 4,5-diazafluorene moiety with slim
contributions from Re cation. These results also indicated
that introduction of biphenyl unit in Re-DSBF hardly af-
fected the HOMOs and LUMOs levels of Re-DSBF, and so
its green emission was similar to that of our previously re-
ported Re(I) complexes based on 4,5-diazafluorene deriva-
tives [22]. Our calculated results were well in agreement
with previous reports [26].
Fig. 5. EL spectra of devices I–IV at the voltage of 6 V.
voltage of 6 V are shown in Fig. 5. It can be found that EL
spectra of devices I–IV consist of very weak emission bands
at about 400–430 nm. The weak emissions in doped de-
vices should originate from CBP, compared with solid PL
spectrum of 25% Re-DSBF doped in CBP. The inconspicuous
emissions from CBP support a model of charge trapping
[27]. However, the weak emission at about 420 nm in
non-doped device should be attributed to NPB emission.
The maximum EL emissions of devices I, II and III are al-
most same, which appear at ca. 548 nm. The maximum
emission of non-doped device IV appears at 557 nm, which
is slightly red-shifted for about 10 nm probably owing to
the stronger intermolecular interaction. Compared with
PL spectrum in dichloromethane solution, a red shift of
about 20 nm is observed in the EL spectra, which is fre-
quently observed among light-emitting materials mainly
because of the effect of the electrical field on the excited
states.
3.5. Electroluminescent properties
To investigate EL properties of Re-DSBF as emitter, four
devices with a typical multi-layer configuration of ITO/m-
MTDATA/NPB/CBP: x% Re-DSBF/Bphen/Alq3/LiF/Al were
fabricated. Devices I, II and III used Re-DSBF doped in
CBP with different doping levels (8% in device I, 15% in II
and 25% in III), while device IV utilized Re-DSBF as a
non-doped emitter. Key EL parameters of devices I–IV were
summarized in Table 1.
All devices exhibit yellowish-green emissions, which
are insensitive to the applied voltages at the wide range
of 3–16 V. The EL spectra of devices I–IV at the same

X. Li et al. / Organic Electronics 13 (2012) 3138–3144
3143
Fig. 6. L–I–V characteristic of non-doped device IV.
Fig. 8. Current density–voltage curve of electron-only single-carrier
device with the structure of ITO/BCP/Re-DSBF/BCP/LiF/Al.
with the structure of ITO/BCP (10 nm)/Re-DSBF (30 nm)/
BCP (10 nm)/LiF (1 nm)/Al) was fabricated. As shown in
Fig. 8, current density is greatly increasing with voltage,
demonstrating that Re-DSBF has superior electron-
transporting ability.
As shown in inset of Fig. 7, there is 0.5 eV HOMO hole
barrier at NPB/CBP interface, and the holes transferred
from NPB will accumulate at the NPB/CBP interface. Those
accumulated holes are in favor of electrons trapped on
dopant. The device efficiency increases with the voltage,
suggesting that carrier trapping plays a role in this work
[30]. These results demonstrated that Re-DSBF is a very
promising candidate for practical applications in OLEDs.
Fig. 7. Current efficiency–current density curves of devices I–IV. Inset:
the proposed energy levels diagram.
4. Conclusions
The L–I–V characteristics of device IV are plotted in
Fig. 6. The non-doped device exhibits a peak luminance
of 8531 cd mꢀ2 at 8.5 V, which is better than those of the
most Re(I)-based OLEDs [12,13]. The turn-on voltage of
the device is lower than 4 V. The device efficiency varies
with the doping concentration of Re-DSBF, as shown in
Fig. 7. Device IV shows the highest current efficiency of
16.8 cd Aꢀ1 at the current density of 28 mA cmꢀ2, which
is the highest ever reported for non-doped phosphorescent
OLEDs based on Re(I)-complex emitters and not far from
those of the Re(I)-doped OLEDs [12–14]. The higher effi-
ciency at high doping concentrations probably results from
largely suppressed self-quenching of Re-DSBF. It is particu-
larly interesting that the EL efficiency firstly increases at
relatively low current density until it reaches a maximum
value. This phenomenon could be elucidated as follows: at
low current density, excess hole injection generates be-
In conclusion, highly efficient phosphorescent OLEDs
had been achieved by using a new emitter Re(I) complex
with enhanced steric hindrance. In particularly, non-doped
device based on this Re(I) complex exhibited the peak
luminance of 8531 cd mꢀ2 and the maximum efficiency
of 16.8 cd Aꢀ1, which were among the highest reported to
date for non-doped OLEDs based on Re(I) complexes. Such
excellent performances were probably attributed to the
steric hindrance, large Stokes shift, and short luminescent
lifetime of Re-DSBF complex. Those encouraging results
imply that the phosphorescent transition metal complexes
containing ligands with good electron-transporting prop-
erty and superior steric hindrance are excellent candidates
for highly efficient OLEDs in commercial production.
Acknowledgements
cause of the much higher hole mobility of NPB (lh = 5.1 -
ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1) than the electron mobility of Alq3
Authors gratefully acknowledge the supports from the
National Natural Science Foundation of China (Grant No.
61204021), the Project from Department of education of
Liaoning Province (Grant No. L2012094), Open Fund of
Key Laboratory of Functional Materials, Colleges and Uni-
versities in Liaoning Province (Grant No. USTLKL 2012-
02) and Research Project for Young Teachers of USTL
(Grant No. 2010Y08).
(l
e = 1.4 ꢁ 10ꢀ6 cm2 Vꢀ1 sꢀ1
) [28,29], and consequently
charge carrier balance is not well established. With
increasing current density, the good electron-transporting
ability of Re-DSBF could improve the charge balance, then
resulting in gradually increasing efficiency.
In order to further evaluate electron-transporting
behavior of Re-DSBF, electron-only single-carrier device

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X. Li et al. / Organic Electronics 13 (2012) 3138–3144
[16] K.-T. Wong, R.-T. Chen, F.-C. Fang, C.-C. Wu, Y.-T. Lin, Org. Lett. 7
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