Towards highly efficient blue-phosphorescent organic light-emitting diodes with low operating voltage and excellent efficiency stability

Article abstract of DOI:10.1002/chem.201001981

Let there be light! An effective strategy for the design of highly efficient phosphine oxide light-emitting-diode hosts with an ortho-linked structure has been demonstrated by synthesizing 4-diphenylphosphoryl dibenzofuran. The ortho linkage of the phosph

Full text of DOI:10.1002/chem.201001981

COMMUNICATION
DOI: 10.1002/chem.201001981
Towards Highly Efficient Blue-Phosphorescent Organic Light-Emitting
Diodes with Low Operating Voltage and Excellent Efficiency Stability
Chunmiao Han,[a] Guohua Xie,[b] Hui Xu,*[a] Zhensong Zhang,[b] Donghui Yu,[a]
Yi Zhao,*[b] Pengfei Yan,[a] Zhaopeng Deng,[a] Qiang Li,[a] and Shiyong Liu[b]
Since the pioneering report by Forrest et al., phosphores-
cent organic light-emitting diodes (PHOLEDs) have been
of much interest because of their ideal characteristics for
highly energy-efficient, flat-panel displays and promising
candidates for the next generation of solid-state lighting,
such as the potential 100% internal quantum efficiency.[1]
To reduce the triplet–triplet (T–T) annihilation and concen-
tration quenching, most of the phosphorescent dyes of
heavy-metal complexes are doped in host materials to im-
prove the device performance.[2] Nevertheless, stable and ef-
ficient blue PHOLEDs remain a significant challenge.[3] In
the host–guest doping of blue electrophosphorescent devi-
ces, the efficient positive energy transfer to the guest re-
quires a very high first triplet energy level (T1) of the host
(approaching 3.0 eV).[4] Usually, a host with a high triplet
energy level exhibits a high driving voltage due to poor car-
rier-transporting properties originating from the intrinsic
wide band gap. For practical use, the low operating voltage
is another significant indicator, which requires fitting of the
frontier molecular orbitals (FMOs) of the host and the
neighbouring carrier-transporting layers. It indicates that the
low first singlet energy level (S1) of the host can decrease
the driving voltage of OLEDs.[5] Therefore, development of
host materials with both high T1 and relatively low S1 be-
comes one of the critical points for high-performance blue
PHOLEDs.
Most of the hosts with high T1 are designed with the in-
corporation of meso or insulating linkages, such as N,N-di-
carbazoly-3,5-benzene (mCP)[4] and tetraaryl silane deriva-
tives.[6] However, the operating voltages of the devices
remain high (ꢀ10 V at practical luminance). The high
LUMO of mCP (2.4 eV) and the electrical inertia of silicon
are the main factors for the high driving voltages.[7a] Recent-
ly, hosts based on aryl phosphine oxides (APO) have attract-
ed intensive interest.[7] The phosphine oxide moieties are be-
lieved to efficiently polarise the molecules without extend-
ing the conjugated length of the active chromophore.[7b]
Such characteristics make APOs obtain good carrier-injec-
tion/transporting abilities and, at the same time, retain high
T1 levels. Nevertheless, only a few blue PHOLEDs based on
APO hosts exhibit low driving voltages (<5 V) and low effi-
ciency roll-offs (<30%) at
a practical luminance of
1000 cdmꢁ2.[5,7a,d,e] Nearly all of the APO hosts reported
have diphenylphosphine oxide (DPPO) moieties substituting
the chromophores along the long axis of the molecules, such
as 2,7-substitution of fluorene,[7b,f–h] 3,6-substitution of carba-
zole[7a] and 2,8-substitution of dibenzofuran.[7c] Such symmet-
rical structures are unfavourable to retain T1 and efficiently
polarising the chromophores.
Although only a few hosts are designed with the ortho
linkage,[8,7f] this modification is advantageous in the con-
struction of high-performance blue-electrophosphorescent
APO hosts because 1) the ortho-linked phosphine oxide fa-
cilitates the retention of T1; 2) through ortho substitution,
the DPPO moieties are on the short axis of the chromo-
phores; this facilitates the polarisation of the chromophores
and 3) the unsymmetrical structure of the ortho linkage
would suppress intermolecular aggregation and further limit
the efficiency roll-off at high luminance.
[a] C. Han, Dr. H. Xu, D. Yu, Prof. P. Yan, Z. Deng, Dr. Q. Li
Key Laboratory of Functional Inorganic Material Chemistry
Ministry of Education, Heilongjiang University
74 Xuefu Road, Harbin 150080 (P.R. China)
Fax : (+86)451-86608042
E-mail: hxu@hlju.edu.cn
[b] G. Xie, Z. Zhang, Prof. Y. Zhao, Prof. S. Liu
State Key Laboratory on Integrated Optoelectronics
College of Electronics Science and Engineering
Jilin University, 2699 Qianjin Street, Changchun 130012 (P.R. China)
Fax : (+86)431-5095592
Herein, we report a novel dibenzofuran phosphine oxide
host, 4-diphenylphosphoryl dibenzofuran (o-DBFPPO),
which is formed through ortho linkage of dibenzofuran and
a DPPO moiety. Low driving voltages (3.1 V at 100 cdmꢁ2)
E-mail: zhao_yi@jlu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001981.
Chem. Eur. J. 2011, 17, 445 – 449
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
445

and low efficiency roll-offs (4% external quantum efficiency
(EQE) for 100–1000 cdmꢁ2) of the corresponding blue
PHOLEDs were demonstrated. o-DBFPPO was convenient-
ly prepared through a three-step procedure of regioselective
lithiation, coupling to the phosphine and oxidation, with a
good total yield over 70% (Scheme 1). The structural char-
age. The improved thermal properties of o-DBFPPO en-
hance the stability of the film morphology and suppress the
potential phase separation during operation.
The more efficient polarisation of dibenzofuran by the
ortho linkage of DPPO was proved by DFT calculations of
o-DBFPPO and its para-linked isomer, p-DBFPPO. The
structure optimisation was performed by using the Gaussi-
an 03 package at the B3LYP 6-31G* level.[10] The calculated
HOMOs of o-DBFPPO and p-DBFPPO are ꢁ6.15 and
ꢁ6.20 eV, respectively, whereas the calculated LUMOs of o-
DBFPPO and p-DBFPPO are ꢁ1.20 and ꢁ1.15 eV, respec-
tively (Figure 2). The HOMO–LUMO energy gap (Eg) of o-
Scheme 1. Synthesis of o-DBFPPO. TMEDA=N,N,N’,N’-tetramethyle-
thylenediamine.
acterisation was established on the basis of mass spectrome-
try, NMR spectroscopy and elemental analysis (see the Sup-
porting Information). The molecular structure was further
confirmed by X-ray crystallography (Figure 1).[9] Because of
the hindrance of the ortho linked DPPO, no p–p stacking
between neighbouring o-DBFPPO molecules is observed
(Figure S1 in the Supporting Information), in contrast to the
para-linked dibenzofuran phosphine oxide derivatives.[7c]
Thus, the ortho linkage has a remarkable effect on reducing
intermolecular interactions.
Figure 2. DFT calculation results of o-DBFPPO and p-DBFPPO.
DBFPPO is 4.95 eV, which is 0.1 eV smaller than that of p-
DBFPPO. With the lower LUMO and higher HOMO, o-
DBFPPO is expected to be more predominant in carrier in-
jection and transporting than p-DBFPPO. Although all of
the FMO electron clouds are mainly located on dibenzofur-
an, the LUMO and HOMO electron cloud distributions of
the two compounds are very different. For p-DBFPPO, para
substitution of electron-withdrawing DPPO does not affect
the uniform distribution of the FMO electron cloud on di-
benzofuran. However, for o-DBFPPO, the phenyl linking
makes a major contribution to the LUMO, but only a minor
contribution to the HOMO (Figure 2). It means that diben-
zofuran is bipolarised to some degree by the ortho linkage
of DPPO. This facilitates the balance of carrier injection
and transporting in the emitting layer (EML). The LUMO
and HOMO energy levels of o-DBFPPO were further deter-
mined by cyclic voltammetry (CV) to have values of ꢁ2.86
and ꢁ5.96 eV, respectively.
Figure 1. ORTEP diagram of o-DBFPPO with thermal ellipsoids of 30%
probability.
The thermal properties of o-DBFPPO were investigated
by thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC). The decomposition temperature
(Td) of DBFPPO is 2968C, which makes the fabrication of
the device feasible through vacuum evaporation. The DSC
result shows that o-DBFPPO has a melting point (Tm) of
1888C. The temperature of the glass transition (Tg) of
DBFPPO is about 898C, which is much higher than that of
mCP (608C). The higher Tg may be attributed to the more
compact structure of o-DBFPPO because of the ortho link-
The absorption and fluorescence spectra of o-DBFPPO in
films and dilute solutions are shown in Figure 3. The absorp-
tions at around 290 nm are attributed to p!p* transitions
of dibenzofuran. This band is enhanced in films due to the
p–p interaction between neighbouring dibenzofurans. The
first singlet excited energy level (S1) of o-DBFPPO is
3.89 eV estimated from the absorption edge (319 nm). The
photoluminescence (PL) emission of o-DBFPPO at room
temperature is in the range of 300–415 nm with a maximum
intensity at 326 nm, which is thoroughly overlapped with the
446
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Chem. Eur. J. 2011, 17, 445 – 449

Phosphine oxide-Doped Light-Emitting Diodes
COMMUNICATION
weaker than that of BPhen. Although among the I-type de-
vices I-B showed the highest efficiency of 28 LmWꢁ1
(30 cdAꢁ1, 13.3%) at 90 cdmꢁ2 (Figure 4c and Figure S4 in
the Supporting Information), device I-A exhibited the most
stable efficiency between 100–1000 cdmꢁ2, the percentages
of efficiency roll-off are only 23% for power efficiency
(PE), 4% for current efficiency (CE) and 4% for EQE. To
the best of our knowledge, this is among the highest levels
reported for single-host/FIrpic-based PHOLEDs and com-
parable to mixed-host-based devices.[5,7g,h]
Because FIrpic and BPhen have similar T1 values (T1 =
2.78 eV), the electron-transporting tri[3-(3-pyridyl)mesityl]-
borane (3TPYMB), which has a much higher T1 value
(2.95 eV), was inserted between the EML and BPhen layers
as exiton-blocking layer to prevent the diffusion of T1 exci-
ton from EML to ETL. This led to the construction of four
II-type devices (device II-A, II-B, II-C and II-D), the config-
urations of which were ITO/MoOx (2 nm)/m-MTDATA:-
Figure 3. Absorption and emission spectra of o-DBFPPO in a film and in
CH2Cl2 (1ꢁ10ꢁ5 m) and phosphorescence spectrum of o-DBFPPO in
CH2Cl2 (1ꢁ10ꢁ5 m) at 77 K: : absorption in the film, : absorption in
&
*
~
*
CH2Cl2, &: emission in the film, : emission in CH2Cl2 and : phosphor-
escence in CH2Cl2.
MoOx (3:1, 10 nm)/m-MTDATA (30 nm)/[Ir
DBFPPO:FIrpic (10:1, 10 nm)/3TPYMB (y nm)/BPhen
((40ꢁy) nm)/LiF(1 nm)/Al (for II-A, y=0; for II-B, y=5;
ACHTUNGRTEN(NUNG ppz)3] (10 nm)/
metal-to-ligand charge-transfer (MLCT) absorption peak of
ACHTUNGTRENNUNG
the blue-phosphorescent-emitting bis
N
for II-C, y=10; for II-D, y=15). The combined thickness of
3TPYMB and BPhen was 40 nm. Therefore, from II-A to II-
D, the thickness of the 3TPYMB layer increased from 0 to
15 nm, whereas the thickness of the BPhen layer decreased
from 40 to 25 nm. Devices II-A and II-B showed similar cur-
rent–voltage characteristics. The driving voltages of II-C and
II-D gradually increased compared with II-A and II-B; this
may be attributed to the strong hole-blocking ability of
3TPYMB (HOMO=ꢁ6.77 eV) (Figure 4b). Because of the
efficient T1 exciton blocking, the highest CE and EQE
values of II-C (139 cdmꢁ2) were improved to 35.5 cdAꢁ1and
15.5%, respectively. However, with the lower operating
voltage, device II-B showed the highest PE of 36 LmWꢁ1 at
83 cd mꢁ2 (Figure 4d and Figure S5 in the Supporting Infor-
mation). Simultaneously, device II-B had the most stable ef-
ficiencies among the II-type devices with very low efficiency
roll-offs of 25% for PE, 9% for CE and 9% for EQE (100–
1000 cdmꢁ2).
The electroluminescent (EL) spectra of the I- and II-type
devices at 1000 cdmꢁ2 are shown in Figure 4a and b. The
main peaks appear at 468 nm. The shoulder peaks at 500 nm
are much weaker. The Commission Internationale de
L’Eclairage (CIE) coordinates are (0.16, 0.32) and (0.15,
0.29) for the I- and II-type devices, respectively. These coor-
dinates are remarkably smaller than for other PHOLEDs
based on phosphine oxide hosts and FIrpic.[7b–h] The vibra-
tional peak at 500 nm is attributed to the recombination
zones close to the interfaces between carrier-transporting
layers and EMLs.[7h] The ortho linkage endows o-DBFPPO
with both hole- and electron-injection/transporting abilities,
which induce the recombination zones to shift to the centre
of EML and further reduce the vibrational peaks. The more
efficient polarisation of dibenzofuran in o-DBFPPO is also
the reason for the extremely stable EL efficiencies of the I-
and II-type devices. For most phosphine oxide hosts, stron-
ger electron-injection/transporting ability leads limitations
pyridinato-N,C2)picolinatoiridium(III) (FIrpic) constituting
AHCTUNGTRENNUNG
the basis of the diode. The phosphorescence spectrum of o-
DBFPPO was also measured at 77 K. T1 of o-DBFPPO esti-
mated from the u0,0 transition identified as the highest-
energy band (394 nm) was as high as 3.15 eV, which was
0.4 eV higher than that of FIrpic (2.75 eV). Therefore, an ef-
ficient exothermic energy transfer from o-DBFPPO to
FIrpic can be expected. Simultaneously, the energy gap be-
tween S1 and T1 (DEST) of o-DBFPPO is only 0.74 eV. The
small DEST reduces the driving voltage of PHOLEDs.[5]
To investigate the performance of o-DBFPPO as the host
material, three I-type devices (device I-A, I-B and I-C) were
fabricated through vacuum evaporation with the same con-
figuration of ITO/MoOx (2 nm)/m-MTDATA: MoOx (3:1,
10 nm)/m-MTDATA (30 nm)/[IrACHTNUGRTENUNG(ppz)3] (10 nm)/o-DBFPPO:
FIrpic (10:1, x nm)/BPhen ((50ꢁx) nm)/LiF (1 nm)/Al (for
I-A, x=10; for I-B, x=20; for I-C, x=30), in which MoOx
and LiF served as hole- and electron-injecting layers, m-
MTDATA is 4,4’,4’’-tri(N-3-methylphenyl-N-phenylamino)-
triphenylamine, which served as a hole-transporting layer
(HTL), BPhen is 4,7-diphenyl-1,10-phenanthroline, which
served as an electron-transporting layer (ETL), and [Ir-
ACHTUNGTRENNUNG(ppz)3] is tris(phenylpyrazole)iridium, which was used as
both hole-transporting and electron-blocking materials, re-
spectively. The combined thickness of EML and ETL was
50 nm. Therefore, from I-A to I-C, the thickness of EML in-
creased from 10 to 30 nm, whereas the thickness of the ETL
decreased from 40 to 20 nm. Device I-A showed the lowest
turn-on voltage of 2.6 V at 3 cdmꢁ2, and the driving voltages
at 100 and 1000 cdmꢁ2 were as low as 3.1 and 4.0 V, respec-
tively (Figure 4a). These are the lowest operating voltages
reported so far among the single-host/FIrpic-based PHO-
LEDs.[5,7] Along with increasing thickness of the EML, the
driving voltages also gradually increased, which implies that
the electron-injection/transporting ability of o-DBFPPO is
Chem. Eur. J. 2011, 17, 445 – 449
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
447

H. Xu, Y. Zhao et al.
ꢁ2
~
*
Figure 4. a) Voltage–luminance characteristics of I-type devices (&: I-A; : I-B and : I-C), and their EL spectra at 1000 cdm (inset). b) Voltage–lumi-
ꢁ2
~
!
&
*
nance characteristics of II-type devices ( : II-A, : II-B, : II-C, : II-D), and their EL spectra at 1000 cdm (inset). c) Luminance–efficiency curves of
~
~
!
&
*
*
I-type devices (&: I-A, : I-B and : I-C). d) Luminance–efficiency curves of II-type devices ( : II-A, : II-B, : II-C and : II-D).
of the recombination zone to a narrow zone close to the
HTL side. The balanced carrier-injection/transporting ability
of o-DBFPPO expanded the recombination zone to the
bulk of EML, instead of the interfaces. Therefore, the exci-
ton concentration quenching and triplet–triplet annihilation
were reduced in the I- and II-type devices at high lumi-
nance.
colour purity and low operating voltage make o-DBFPPO
one of the most effective hosts for blue PHOLEDs.
In summary, an effective strategy for the design of highly
efficient phosphine oxide hosts with an ortho-linked struc-
ture has been demonstrated by a simple dibenzofuran phos-
phine oxide host. The ortho linkage of the DPPO moiety
has the dual effect of retaining the high T1 and efficiently
Table 1 lists the EL perfor-
Table 1. EL performance of representative FIrpic-based devices.
mance of several representative
FIrpic-based PHOLEDs. Com-
pared with o-DBFPPO the re-
lated para-linked 2,8-bis(diphe-
nylphosphine oxide)dibenzofur-
Host[a]
Driving voltage [V]
Max. efficiency[b]
Efficiency roll- off [%][b]
CIE (x, y)
2,8-DBFDPPO[7c]
PCF[7d]
3.3,[c] 5.6[d]
20.1, 20.2, –
26.2, 30.8, 14.8
47, 50, 22
65, 39, –[d]
25, 13, 13[e]
35, –, 10[f]
70, –, 40[e]
27, –, 5[e]
–, –
0.13, 0.35
–, –
0.195, 0.438
–, –
–, –
–, –
–, –
0.15, 0.29
2.6,[c] 4.3[e]
BCBP[5c]
3.0,[c] 4.8,[f] 7.0[e]
3.0,[c] 6.1[e]
SPPO1[7h]
SPPO11[7f]
BCPO[7i]
28, –, 11.5
an
(2,8-DBFDPPO)
shows
3.5,[c] 5.5[e]
24.7, 35.6, 17.5
40.6, 45.1, 23.5
55, 52, –
26.7, 30.6, 15.7
36, 35.5, 15.5
lower maximum efficiencies
2.8[c]
–, –, 18[e]
(20.1 LmW<-M>1
and
CzTP[5a]
3.0,[c] 3.4,[f] 4.1[e]
3.0,[c] 5.0,[f] ꢀ7.0[e]
2.6,[c] 3.1,[f] 4.0[e]
47, 27, –[e]
40, 22, 24[f]
23, 4, 4[e]
CzSi[6b]
20.2 cdAꢁ1), worse efficiency
roll-off (65% for PE and 39%
for CE at 800 cdmꢁ2) and
higher operating voltages (3.3 V
o-DBFPPO
[a] PCF=2,7-bis(diphenylphosphine oxide)-9-(9-phenylcarbazol-3-yl)-9-phenylfluorene, BCBP=2,20-bis(4-car-
bazolylphenyl)-1,10-biphenyl, SPPO1=2-(diphenylphosphoryl)spirofluorene, SPPO11=4-(diphenylphosphor-
yl)spirofluorene,
[1,1’;3’1’’]terphenyl-5’-yl-9H-carbazole,
[b] Appearing in the following order: PE [lmWꢁ1], CE [cdAꢁ1] and EQE [%]. [c] Onset voltage. [d] At
art efficiency stability, high 800 cdmꢁ2. [e] At 1000 cdmꢁ2. [f] At 100 cdmꢁ2
BCPO=bis-4-(N-carbazolyl)phenyl)phenylphosphine
oxide,
CzTP=9-phenyl-3,6-bis-
onset
and
5.6 V
at
CzSi=9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole.
800 cdmꢁ2).[7c] The state-of-the-
.
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Phosphine oxide-Doped Light-Emitting Diodes
COMMUNICATION
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Acknowledgements
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This work was financially supported by the National Key Basic Research
and Development Program of China under grant no. 2010CB327701,
NSFC under Grant 50903028 and 60977024, Natural Science Foundation
of Heilongjiang province under Grant QC08C10, the Supporting Pro-
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Keywords: density functional calculations
fluorescence · light-emitting diodes · phosphorus
·
doping
·
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Received: July 13, 2010
Published online: December 3, 2010
Chem. Eur. J. 2011, 17, 445 – 449
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