A pyridine-containing anthracene derivative with high electron and hole mobilities for highly efficient and stable fluorescent organic light-emitting diodes

Article abstract of DOI:10.1002/adfm.201002691

A pyridine-containing anthracene derivative, 9,10-bis(3-(pyridin-3-yl) phenyl)anthracene (DPyPA), which comprehensively outperforms the widely used electron-transport material (ETM), tris(8-quinolinolato) aluminum (Alq ₃), is synthesized. DPyPA

Full text of DOI:10.1002/adfm.201002691

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A Pyridine-Containing Anthracene Derivative with High
Electron and Hole Mobilities for Highly Efficient and Stable
Fluorescent Organic Light-Emitting Diodes
Yongduo Sun, Lian Duan, Deqiang Zhang, Juan Qiao, Guifang Dong, Liduo Wang,
a n d Yong Qiu*
aluminum (Alq3), although it possesses
low electron mobility, and the instability
of cationic Alq3 is considered to be respon-
sible for OLED degradation.[4] Thus, devel-
oping high-performance ETMs with high
electron mobility and good stability is
crucially important for improving OLED
performance.[3] To improve the electron
mobility of organic semiconductors, aza-
aromatics such as phenanthroline, qui-
noline, pyridine, and triazole have been
incorporated into π-conjugated systems.[5]
However, OLEDs that use aza-aromatics
as ETMs usually show short lifetimes.[5]
In this paper, we report the synthesis and
investigation of a new anthracene-based
compound: 9,10-bis(3-(pyridin-3-yl)phenyl)
anthracene (DPyPA), which incorporates
A pyridine-containing anthracene derivative, 9,10-bis(3-(pyridin-3-yl)phenyl)
anthracene (DPyPA), which comprehensively outperforms the widely used
electron-transport material (ETM), tris(8-quinolinolato) aluminum (Alq3),
is synthesized. DPyPA exhibits ambipolar transport properties, with both
electron and hole mobilities of around 10−3 cm−2 V−1 s−1 ; about two orders of
magnitude higher than that of Alq3. The nitrogen atom in the pyridine ring of
DPyPA coordinates to lithium cations, which leads to efficient electron injec-
tion when LiF/Al is used as the cathode. Electrochemical measurements dem-
onstrate that both the cations and anions of DPyPA are stable, which may
improve the stability of devices based on DPyPA. Red-emitting, green-emitting,
and blue-emitting fluorescent organic light emitting diodes with DPyPA as
the ETM display lower turn-on voltages, higher efficiencies, and stronger
luminance than the devices with Alq3 as the ETM. The power efficiencies of
the devices based on DPyPA are greater by 80–140% relative to those of the
Alq3-based devices. The improved performance of these devices is attributed
to the increased carrier balance. In addition, the device employing DPyPA as
the ETM possesses excellent stability: the half-life of the DPyPA-based device
is 67 000 h—seven times longer than that of the Alq3-based device—for an
initial luminance of 5000 cd m−2 .
3-(pyridin-3-yl)phenyl—an
electron-transport
outstanding
chromophore[5b]—as
the substituent. DPyPA was found to
possess high electron mobility (around
10−3 cm2 V−1 s−1 ) and easy injection of
electrons from the cathode as well as
appropriate highest occupied molecular
orbital (HOMO) and lowest unoccupied
1. Introduction
molecular orbital (LUMO) energy levels. Furthermore, cyclic
voltammetry (CV) measurements reveal that both the cations
and anions of DPyPA are stable, which may lead to improved
device stability. Red-emitting, green-emitting, and blue-emitting
devices that employ DPyPA as the ETM exhibit lower turn-on
voltages, higher efficiencies, and larger luminances than the
devices with Alq3 as the ETM. In addition, the OLED employing
DPyPA as the ETM also possesses better stability than the Alq3-
based device, which indicates that it is possible to create mate-
rials containing aza-aromatics as ETMs that can form OLEDs
with both high performance and good stability.
Organic light-emitting diodes (OLEDs) have attracted a great
deal of attention due to their potential applications in full-color
flat-panel displays and illumination.[1] The charge balance in
OLEDs is considered to be essential for achieving high per-
formance. Many high-mobility hole-transport materials (HTMs)
have been designed and synthesized.[2] However, electron-
transport materials (ETMs) with high mobility are relatively
rare.[3] The most widely used ETM is still tris(8-quinolinolato)
Y. Sun, Dr. L. Duan, D. Zhang, Dr. J. Qiao, Prof. G. Dong, Prof. L. Wang,
Prof. Y. Qiu
Key Lab of Organic Optoelectronics and Molecular Engineering
of Ministry of Education
2. Results and Discussion
2.1. Synthesis and Characterization
Department of Chemistry
Tsinghua University
Beijing 100084, PR China
E-mail: qiuy@mail.tsinghua.edu.cn
Scheme 1 illustrates the synthetic approach to DPyPA.
Anthraquinone reacted with (3-bromophenyl) lithium,
which was synthesized from 3-bromo-iodobenzene and
DOI: 10.1002/adfm.201002691
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Br
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
Solution
Thin film
I
1.0
0
-85 C
n-BuLi
1.
2.
0.8
Br
Abs
PL
Br
0.6
Li
O
O
0.4
0.2
3. KI/NaH PO , H O,
2
2
2
Br
CH CO H, reflux
3
2
0.0
N
300
400
Wavelength (nm)
500
B(OH)
2
N
Figure 1. Absorption and PL-emission spectra of CH2Cl2 solution and
thermally evaporated film of DPyPA.
N
9,10-bis(3-(pyridin-3-yl)phenyl)anthracene
Scheme 1. Synthetic routes of DPyPA
using the previously reported HOMO of ferrocene (4.8 eV)
as a reference.[7b] The LUMO was calculated fom the HOMO
value and the energy gap (Eg) from the edge of the absorp-
tion spectrum to be about 2.8 eV.[7c] For Alq3 under identical
experimental conditions, however, both the oxidation potential
and reduction potential are irreversible. And the HOMO and
LUMO of Alq3 are 5.7 eV and 3.0 eV, respectively.
n-butyllithium, to form 9,10-bis(3-bromophenyl)anthracene.
Then, 9,10-bis(3-bromophenyl) anthracene reacted with
pyridin-3-ylboronic acid via a palladium-catalyzed Suzuki
coupling reaction to form the desired product, 9,10-bis(3-
(pyridin-3-yl)phenyl)anthracene (DPyPA).[6] The structure
of DPyPA was confirmed by mass spectrometry, elemental
2.4. Carrier-Transport Properties
1
analysis, H NMR, and 13C NMR. All materials used in the
devices were purified by repeated temperature-gradient
vacuum sublimation.
The carrier mobilities of the DPyPA thin film were character-
ized by the time-of-flight (TOF) transient-photocurrent tech-
nique.[8] The configuration of the device is ITO/Ag (60 nm)/
organic layer (1.4 μm)/Ag (200 nm). The carrier mobility
(μ) was calculated from the values of the transit time (Tt),
2.2. Thermal and Photophysical Properties
The thermal properties of DPyPA were determined by differen-
tial scanning calorimetry (DSC) and thermogravimetric analysis
(TGA). DPyPA has a glass-transition temperature (Tg) of 97 °C,
a melting temperature (Tm) of 268 °C, and a decomposition
temperature (Td, corresponding to 5% weight loss) of 442 °C.
The absorption spectra and photoluminescence (PL) spectra
of DPyPA in CH2Cl2 (2 × 10−4 M) and the vacuum-evaporated
DPyPA film on the quartz substrate are shown in Figure 1. The
absorption spectra of DPyPA in the solution and solid state
exhibit peaks at 375 nm and 381 nm, respectively, which can be
attributed to π–π∗ transitions. The solution-PL emission peak
occurs at 432 nm, while the thin-film PL emission is around
443 nm.
30
20
10
0
-10
-20
-30
2.3. Electrochemical Properties
2.0
1.5
1.0
0.5
0.0 -0.5 -1.0 -1.5 -2.0
Potential (V)
The electrochemical properties of DPyPA were studied by CV.
As shown in Figure 2, one quasireversible oxidation poten-
tial occurs at 1.50 V and one reversible reduction potential
at –1.68 V, which indicates that both the radical cations and
anions are stable entities.[7a] The HOMO of DPyPA is 5.9 eV,
which was determined from the anodic oxidation potential by
Figure 2. CV of DPyPA. Working electrode: Pt disc; reference elec-
trode: Ag/AgCl. Oxidation CV was performed in dichloromethane con-
taining 0.1 M n-Bu4NPCl6 as the supporting electrolyte at a scan rate of
300 mV s−1 . Reduction CV was performed in N,N-dimethyl formamide
(DMF) with 0.1 M n-Bu4NClO4 as the supporting electrolyte at a scan rate
of 200 mV s−1
.
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Figure 3. Electron and hole mobilities of DPyPA film plotted as a function
of the square root of the electric field.
the sample thickness (D) and the applied voltage (V) by
using the following equation: μ = D2/(VTt). As shown in
Figure 3, DPyPA displays high electron mobility (around 1 ×
10−3 cm2 V−1 s−1 ) at an electric field of 4 × 105 V cm−1 , which
is comparable to the highest reported electron mobility,[5b]
and is about two orders of magnitude higher than that of
Alq3.[9] DPyPA also exhibits high hole mobility that is close to
the electron mobility.
To verify the ambipolar nature of DPyPA, we fabricated a
single-layer device with a structure of ITO/MoO3 (2 nm)/
DPyPA (100 nm)/Mg: Ag (150 nm)/Ag (50 nm). MoO3 was
used as the hole-injection layer. This device emitted a deep
blue light that comes from DPyPA itself. The device pro-
duced a maximum current efficiency of 0.9 cd A−1 , a max-
imum brightness of 3301 cd m−2 , and a turn-on voltage of
4.8 V. The results demonstrate that DPyPA is capable of func-
tioning as both electron-transport and hole-transport mate-
rial. The electroluminescence (EL) emission spectrum and
typical current density–luminance–voltage (J–L–V) character-
istics of the single-layer device are shown in the Supporting
Information (SI).
Figure 4. The molecular-orbital surfaces of HOMO and LUMO of DPyPA
calculated at the DFT//B3LYP/6-31G level.
2.6. Electroluminescent Properties
To evaluate the possible applications of DPyPA in OLEDs, red,
green, and blue OLEDs were fabricated with DPyPA as the ETM.
Devices with Alq3, the most widely used ETM, were also fabri-
cated for comparison. The structures of the devices were as listed
below, where NPB is 4,4’-N,N’-bis[N-(1-naphthyl)-N-phenyl-
amino]biphenyl, DNTPD is N,N’-di(4-(N,N’-diphenyl-amino)
phenyl)-N,N’-diphenylbenzidine, DTDP is dibenzo([f,f’]-
4,4’,7,7’-tetraphenyl)diindeno(1,2,3-cd:1’,2’,3’-lm)perylene,
F4TCNQ is 2,2’-(perfluorocyclohexa-2,5-diene-1,4-diylidene)
dimalononitrile, ADN is 9,10-di(naphthalen-2-yl)anthracene,
PADNis9,10-di(naphthalen-2-yl)-2-phenylanthracene,DPAVBiis
4,4’-(1E,1’E)-2,2’-(biphenyl-4,4’-diyl)bis(ethene-2,1-diyl)bis(N,N-
dip-tolylaniline), rubrene is 5,6,11,12-tetraphenyltetracene, HAT
is dipyrazino[2,3-f:2’,3’-h]quinoxaline-2,3,6,7,10,11-hexacarboni-
trile and DTBTTD is 2,6-di-tert-butyl-N9,N9,N10,N10-tetrap-
tolylanthracene-9,10-diamine.
Red device A1: ITO/4.2% F4TCNQ-doped DNTPD (150 nm)/
NPB (20 nm)/0.4% rubrene-doped ADN and DPDT (4: 6)
(30 nm)/DPyPA (20 nm)/LiF (0.5 nm)/Al (150 nm).
Red device A2: ITO/4.2% F4TCNQ-doped DNTPD (150 nm)/
NPB (20 nm)/0.4% rubrene-doped ADN and DPDT (4: 6)
(30 nm)/Alq3 (20 nm)/LiF (0.5 nm)/Al (150 nm).
Green device A1: ITO/HAT (5 nm)/NPB (20 nm)/9%
DTBTTD-doped PADN (30 nm)/DPyPA (20 nm)/LiF (0.5 nm)/
Al (150 nm).
Green device A2: ITO/HAT (5 nm)/NPB (20 nm)/9%
DTBTTD-doped PADN (30 nm)/Alq3 (20 nm)/LiF (0.5 nm)/Al
(150 nm).
2.5. Quantum Chemical Calculations
Theoretical calculations on the electronic states of DPyPA
were carried out at the DFT//B3LYP/6-31G level in the Gaus-
sian 03 program[10] to acquire a better understanding of the
ambipolar properties. As shown in Figure 4, both HOMO and
LUMO are mainly located on the anthracence core in DPyPA,
which results in similar charge-transfer integrals for elec-
trons and holes. According to Marcus theory, similar charge-
transfer integrals for electrons and holes will lead to balanced
electron- and hole-transport properties,[11] so the ambipolar
transport properties of DPyPA may result from the overlap of
frontier orbitals on the anthracene core. This agrees well with
our previous work, in which several naphtho[2,3-c][1,2,5]thia-
diazole derivatives exhibited ambipolar transport properties
because of the similar charge-transfer integrals for electrons
and holes.[12]
Blue device C1: ITO/NPB (50 nm)/4% DPAVBi-doped ADN
(30 nm)/DPyPA (30 nm)/LiF (0.5 nm)/Al (150 nm).
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the OLEDs. At an initial luminance of
5000 cd m−2 , devices B1 (DPyPA-based
device) and B2 (Alq3-based device) were
operated at a constant current for approxi-
mately 2800 h. It can be seen from Figure 6
that the OLED with DPyPA exhibited
better stability. The extrapolated lifetimes
of devices B1 and B2, at an initial lumi-
nance of 5000 cd m−2 , are about 8900 and
67100 h, respectively. Here, we used a
stretched exponential decay (SED) as pre-
viously reported: L/L0 = exp[–(t/τ)β], where
L0 is the initial luminance, L is the lumi-
nance, β and τ are fitting parameters, and
t is the test time.[15]
Figure 5. Energy-level diagram of the multilayer devices: HOMO and LUMO levels of DPyPA
and Alq3 were measured by CV, while those of NPB, ADN, PADN, DPDT, ITO, and LiF/Al are
previously reported values.[13]
Blue device C2: ITO/NPB (50 nm)/4% DPAVBi-doped ADN
(30 nm)/Alq3 (30 nm)/LiF (0.5 nm)/Al (150 nm).
Although OLEDs with aza-aromatics as the ETM usually
exhibit poor stability, DPyPA seems to be an expection.[5] As
shown in Figure 2, DPyPA exhibits one quasireversible oxida-
tion potential and one reversible reduction potential. This indi-
cates that both the radical cations and radical anions of DPyPA
are stable.[7,16] The stable radical cations and radical anions may
be the main reason for the improved operational stability of the
OLEDs made with DPyPA.
The structures of the devices and the energy levels of the
materials are shown in Figure 5. The LUMO level of DPyPA is
near to that of Alq3, while the HOMO level of DPyPA is 0.2 eV
lower than that of Alq3, which may act as a hole barrier. All the
devices were fabricated by high-vacuum thermal evaporation.
The current density–voltage (J–V) characteristics and the
luminance–voltage (L–V) characteristics are shown in Figure 6,
and the device performances are summarized in Table 1. It can
be seen that devices employing DPyPA as the ETM possessed
lower turn-on voltages (defined as the voltage required to give a
luminance of 1 cd m−2 ) and higher efficiencies and luminances
than those of the devices with Alq3 as the ETM. At 5000 cd m−2 ,
the driving voltages of the devices A1, A2, B1, B2, C1, and C2
are 1.7, 2.5, 2.5, 3.3, 3.4, and 5.9 V, respectively. The driving
voltage at 5000 cd m−2 decreased by 1.6, 2.9, and 4.5 V for
devices A, B, and C, respectively, by using DPyPA instead of
Alq3. The power efficiencies at 5000 cd m−2 of the devices are
3.51, 1.91, 18.32, 8.91, 11.26, and 7.63 lm W−1 for devices A1,
A2, B1, B2 C1, and C2, respectively. The power efficiencies
increased by about 84%, 106%, and 143%, for devices A, B, and
C, respectively, compared with those of the devices with Alq3 as
the ETM.
3. Conclusion
We have designed and synthesized a high-performance ETM,
DPyPA. Highly efficient red-emitting, green-emitting, and
blue-emitting fluorescent OLEDs were achieved by employing
DPyPA as the ETM. The increased efficiency of the OLEDs
with the newly developed ETM is due to the improved elec-
tron transport and injection. The excellent operational sta-
bility may be attributed to the stable radical cations and
radical anions of DPyPA. The concept, presented in this
paper, of designing molecules with stable cations and anions
should be extended to the design of other organic charge-
transport materials.
The high efficiencies of the devices based on DPyPA are
attributed to the increased carrier balance, which is due to
the improved electron transport and injection. The electron
mobility of DPyPA is about two orders of magnitude higher
than that of Alq3, and near to that of NPB.25 The LUMO level
of DPyPA is near to that of Alq3, so together with the pos-
sible effect of the coordination of the pyridine nitrogen atom
to the lithium cations,[14] efficient electron injection from the
cathode into DPyPA is guaranteed. The low driving voltages
of the devices with DPyPA also result from the enhanced elec-
tron transport and injection. The experimental details in the SI
also demonstrate that it is easier for coordination to take place
between the LiF/Al bilayer cathode and DPyPA than between
LiF/Al and Alq3.
4. Experimental Section
General Experiments: 1H-NMR and 13C-NMR spectra were recorded
on an ECA600 NMR spectrometer with tetramethylsilane as the internal
standard and CDCl3 as solvent. Electron-impact (EI) mass spectra
were recorded on a GCT-MS Micromass UK instrument. Elemental
analysis was carried out on an Elementary Vario EL CHN elemental
analyzer. Absorption and PL spectra were recorded with a UV–vis
spectrophotometer (Agilent 8453) and a fluorospectrophotometer (Jobin
Yvon, FluoroMax-3), respectively. TGA was recorded with a Netzsch
simultaneous thermal analyzer (STA) system (STA 409PC) under a
dry-nitrogen flow at a heating rate of 10 °C min−1 . Glass-transition
temperatures were recorded by DSC at a heating rate of 10 °C min−1
with a thermal analysis instrument (DSC 2910 modulated calorimeter).
CV was performed on a Princeton Applied Research potentiostat/
galvanostat model 283 voltammetric analyzer with a platinum plate as
the working electrode, a silver wire as the pseudo-reference electrode,
and a platinum wire as the counter electrode. Ferrocene was selected as
the internal standard. The solutions were bubbled with argon for 10 min
before measurements.
2.7. Stability of OLEDs
For OLEDs with aza-aromatics as the ETM, the lifetime
is of great concern. We therefore studied the stability of
Computational Methodology: The geometrical and electronic
properties of the DPyPA were calculated with the Gaussian 03 program
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1200
package. The geometry was optimized by means of B3LYP with a 6-31G
basis set.
Preparation of Materials: All reactants and solvents, unless otherwise
stated, were purchased from commercial sources and used as
received.
Device A1
1000
10000
1000
100
Device A2
800
9,10-bis(3-bromophenyl)anthracene: n-butyllithium (2.9 M, 9.60 mL,
27.4 mmol) was slowly added to a solution of 3-bromo-iodobenzene
(8.00 g, 28.4 mmol) in dry tetrahydrofuran (120 mL) at –85 °C under
argon before anthraquinone (2.51 g 12.0 mmol) was added all at
once. The resulting mixture was stirred for 2 h, during which time the
temperature rose to 20 °C. Cold water (100 mL) was added and the
organic phase was isolated. The aqueous phase was extracted with ethyl
acetate (2 × 50 mL). The combined organic fractions were dried over
magnesium sulfate and the volatiles were removed in vacuo to deliver
a foamy residue. To this residue were added potassium iodide (8.00 g,
48.2 mmol), sodium hypophosphite (7.62 g, 72.1 mmol), and acetic acid
(100 mL), and the mixture was heated under reflux for 2 h. After cooling,
the yellow precipitate was collected, washed with plenty of water, and
dried. The 9,10-bis(3-bromophenyl)anthracene (5.57 g) was obtained
(the yield ratio: 95.1%).
DPyPA: 9,10-bis(3-bromophenyl)anthracene (5.36 g, 11.0 mmol),
pyridine-3-boronic acid (3.10 g, 24 mmol), palladium chloride
(PdCl2, 0.266 g, 1.5 mmol), triphenyl phosphite (0.786 g, 3 mmol),
potassium carbonate (12.4 g, 90 mmol), toluene (166 mL), ethyl
alcohol (120 mL), and distilled water (153 mL) were added to a
reaction flask under a nitrogen atmosphere. The reaction mixture
was heated under reflux for five h before the organic fraction was
isolated. The volatiles were removed in vacuo to leave a grey residue
that was purified by column chromatography (silica gel) with
petroleum ether/ethyl acetate (1:1) as eluent. A white solid (3.68 g,
76.0%) was obtained.
600
400
200
0
10
0
2
4
6
8
10
Voltage (V)
400
300
200
100
0
Device B1
Device B2
10000
1000
100
10
0
1
2
3
4
5
6
7
8
9
10 11 12
Spectroscopic characterization of DPyPA: 1H NMR (600 MHz, 25 °C,
CDCl3, δ): 8.97 (s, 2H), 8.61 (d, J = 7.2 Hz, 2H), 7.97 (d, J = 12.4 Hz,
2H), 7.81 (s, 2H), 7.78–7.74 (m, 8H), 7.56 (t, J = 10 Hz, 2H), 7.39–7.37
(m, 6H); 13C-HNR (600 MHz, 25 °C, CDCl3, δ): 148.81, 148.54, 134.54,
131.20, 129.97, 129.38, 126.96, 126.36, 125.44, 123.70; MS (EI): MW
484.59, m/z 484; elemental anal. calcd for (%): C, 89.23; H, 4.99; N,
5.78; found: C, 89.21; H, 5.01; N, 5.72; mp 276.7 °C.
Voltage (V)
3000
2500
2000
1500
1000
500
Device C1
Device C2
10000
1000
100
Fabrication of OLEDs: ITO substrates with sheet resistance of 15 Ω/ꢀ
(square resistance) were sufficiently cleaned and treated with oxygen
plasma before use. The EL devices were fabricated through vacuum
deposition of the materials at 10−6 torr onto ITO glass. All the
´
organic layers were deposited at a rate of 2.0 Å s−1 . The cathode was
completed through thermal deposition of LiF at a deposition rate of
10
´
0.1 Å s−1 , and then capped with Al metal by thermal evaporation at
0
´
a rate of 5.0 Å s−1 . The encapsulated device was characterized with a
Keithley 4200 semiconductor characterization system in ambient
conditions. The EL spectra were collected with a Photo Research PR705
spectrophotometer.
0
2
4
6
8
10
12
14
Voltage (V)
TOF Mobility Measurement: DPyPA was purified by sublimation before
use in analyses and device fabrication. The TOF device was prepared
by vacuum deposition using the following structure: ITO/Ag (60 nm)/
DPyPA (1.4 µm)/Ag (200 nm). A third harmonic of the Nd:YAG laser
(355 nm, 15 ns) is used as the excitation light source, which passes
through the semitransparent electrode (Ag) and photogenerates a thin
sheet of excess carriers. By switching the polarity of the applied dc bias,
the transit time (Tt, the time that the carriers pass through the organic
film to reach the counter electrode) is measured by a digital storage
oscilloscope. With the applied bias V and the thickness, D, of the
organic layer which is much greater than the optical absorption depth of
the excitation, the carrier mobilities could be calculated according to the
formula μ = D/(TtE) = D2/(VTt).
Figure 6. a–c) The current density–voltage (J–V) curves and the
luminance–voltage (L–V) curves of devices A1 and A2 (red OLEDs), B1
and B2 (green OLEDs), and C1 and C2 (blue OLEDs); d) luminance-
deterioration curves of devices B1 and B2 (green OLEDs).
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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T a b le 1 . Electroluminescent data of the devices.
Red OLEDs
Green OLEDs
Blue OLEDs
device parametera)
Von [V]
device A1 with DPyPA
device A2 with Alq3
device B1 with DPyPA
device B2 with Alq3
device C1 with DPyPA device C2 with Alq3
1.7
4.9
2.5
6.5
2.5
3.7
3.3
6.8
3.4
7.0
5.9
11.5
V/Vb)
ηc [cd A−1 b)
]
5.98
3.89
21.69
18.32
5.50
17.19
8.91
11.26
5.05
7.63
p [lm W−1 b)
ext [%]b)
em [nm]
]
3.51
1.91
2.08
η
η
3.71
2.62
4.35
5.66
3.68
608
608
520
520
472
472
λ
CIE(x, y)
(0.650, 0.349)
32112
6.22
(0.648, 0.351)
18921
4.11
(0.368, 0.612)
35667
22.52
19.60
5.71
(0.375, 0.608)
35018
17.52
10.69
4.43
(0.178, 0.337)
25950
12.31
6.24
(0.175, 0.304)
21627
8.23
Lmax [cd m−2
]
η
η
η
c, max [cd A−1
]
p, max [lm W−1
]
5.69
3.11
2.46
4.07
2.69
6.20
3.97
ext, max [%]
a)Abbreviations: Von: turn-on voltage; V: Voltage; Lmax: maximum luminance; L: luminance; λem: emission wavelength; CIE(x,y): Commission International de I’Eclairage
coordinates; ηp, max: maximum power efficiency; ηc, max: maximum current efficiency; ηext, max: maximum external quantum efficiency; ηp: power efficiency; ηc: current effi-
ciency; ηext: external quantum efficiency; b)Data taken at the luminance 5000 cd m−2
.
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Acknowledgements
We thank the National Natural Science Foundation of China (No.
50990060) and the National Key Basic Research and Development
Program of China under Grant No. 2009CB623604 for support. Note:
This article has been amended on May 24, 2011 to correct an error in
Figure 6c of the version first published online.
Received: December 22, 2010
Published online: April 12, 2011
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