High-power-efficiency blue electrophosphorescence enabled by the synergistic combination of phosphine-oxide-based host and electron-transporting materials

Article abstract of DOI:10.1021/cm4037555

A new host material of (4-{1-[4-(diphenylphosphoryl)phenyl]cyclohexyl} phenyl)bis(4-methylphenyl)amine (POPCPA) is designed and synthesized by integrating electron-donating 4,4′-dimethyldiphenylamine unit and electron-accepting diphenylphosphine oxide group into the cyclohexane skeleton. The design strategy endows the host material with a high triplet energy of 2.93 eV, a shallow HOMO level of -5.24 eV, and a bipolar charge transporting feature. In addition, a new electron-transporting (ET) material of 1,3,5-tri[3- (diphenylphosphoryl)phenyl]benzene (TP3PO), which possesses a high triplet of 2.78 eV, a deep HOMO level of -6.40 eV, and a good ET ability, is constructed by the suitable combination of three diphenylphosphine oxide groups and the triphenylbenzene skeleton. These features render these phosphine-oxide-based functional materials ideal for blue phosphorescent organic light-emitting diodes (PhOLEDs). By employing these functional materials, a blue device exhibits low driving voltages of 2.6, 3.6, and 5.4 V at the luminance of 1, 100, and 1000 cd m^-2, respectively, and the highest power efficiency (up to 45.3 lm W^-1) to date for iridium(III) bis(4′,6′- difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6)-based blue PhOLEDs, which is significantly higher than those of the FIr6-based PhOLEDs with n-doped electron-transporting layer or p-i-n structure. These results suggest that the high-power-efficiency blue PhOLEDs can be achieved by elaborate designing of host and electron-transporting materials systematically to suit the blue emitter and emission zone structure.

Full text of DOI:10.1021/cm4037555

Article
pubs.acs.org/cm
High-Power-Efficiency Blue Electrophosphorescence Enabled by the
Synergistic Combination of Phosphine-Oxide-Based Host and
Electron-Transporting Materials
Shaolong Gong,†,§,⊥,‡ Yi-Lu Chang,§,‡ Kailong Wu,† Robin White,§ Zheng-Hong Lu,*,§ Datong Song,⊥
and Chuluo Yang*,†
†Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China
§Department of Materials Science and Engineering, University of Toronto, 184 College St., Toronto, Ontario M5S 3E4, Canada
⊥Davenport Chemical Research Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto,
Ontario M5S 3H6, Canada
S
* Supporting Information
ABSTRACT: A new host material of (4-{1-[4-(diphenylphosphoryl)phenyl]-
cyclohexyl}phenyl)bis(4-methylphenyl)amine (POPCPA) is designed and synthe-
sized by integrating electron-donating 4,4′-dimethyldiphenylamine unit and
electron-accepting diphenylphosphine oxide group into the cyclohexane skeleton.
The design strategy endows the host material with a high triplet energy of 2.93 eV,
a shallow HOMO level of −5.24 eV, and a bipolar charge transporting feature. In
addition, a new electron-transporting (ET) material of 1,3,5-tri[3-
(diphenylphosphoryl)phenyl]benzene (TP3PO), which possesses a high triplet
of 2.78 eV, a deep HOMO level of −6.40 eV, and a good ET ability, is constructed
by the suitable combination of three diphenylphosphine oxide groups and the
triphenylbenzene skeleton. These features render these phosphine-oxide-based
functional materials ideal for blue phosphorescent organic light-emitting diodes
(PhOLEDs). By employing these functional materials, a blue device exhibits low
driving voltages of 2.6, 3.6, and 5.4 V at the luminance of 1, 100, and 1000 cd m−2,
respectively, and the highest power efficiency (up to 45.3 lm W−1) to date for iridium(III) bis(4′,6′-difluorophenylpyridinato)-
tetrakis(1-pyrazolyl)borate (FIr6)-based blue PhOLEDs, which is significantly higher than those of the FIr6-based PhOLEDs
with n-doped electron-transporting layer or p-i-n structure. These results suggest that the high-power-efficiency blue PhOLEDs
can be achieved by elaborate designing of host and electron-transporting materials systematically to suit the blue emitter and
emission zone structure.
placed on the typical phosphor of iridium(III) bis(4′,6′-
difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate
(FIr6).10−12 Since then, several research groups have
endeavored to develop high-power-efficiency FIr6-based
PhOLEDs through optimizing device structures.13,14 Xue et
al. demonstrated a maximum PE of 36 lm W−1 by incorporating
a double emitting layer into p-i-n type cell architecture;13b Chu
et al. reported a highly efficient FIr6-based PhOLED with a
peak PE of 39.2 lm W−1 by utilizing the mixed host system and
n-doped electron-transporting layer (ETL).14b
Despite efficiency improvement, these reported FIr6-based
PhOLEDs require complicated device structures that inevitably
result in high driving voltages, control difficulties, and high cost,
and thus restrict further enhancement of PE and the
commercialization of PhOLEDs for lighting. In terms of
INTRODUCTION
■
Organic light-emitting diodes (OLEDs) are considered the
next-generation technology for flexible flat-panel displays and
low-cost general lighting sources. Phosphorescent OLEDs
(PhOLEDs) are particularly attractive because the introduction
of phosphors can boost the internal quantum efficiency up to
100% through the utilization of both singlet and triplet
excitons.1−3 Although external quantum efficiencies (EQEs)
over 25% have been realized for multicolor PhOLEDs without
any light out-coupling enhancement,4−7 there is still a long way
for the commercialization of PhOLEDs for solid lighting
sources because of their high power consumption and poor
device stability. To reduce the power consumption, two key
issues need to be addressed in the enhancement of power
efficiency (PE) of PhOLEDs: improving the EQE, and reducing
the device driving voltage. To date, green and red PhOLEDs
with high power efficiencies have been achieved,4a,5b,8 but high-
power-efficiency blue PhOLEDs remain to be further
developed.9 Among the blue PhOLEDs, the focus is still
Received: November 12, 2013
Revised: January 20, 2014
© XXXX American Chemical Society
A
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materials, rational design of suitable functional materials for the
FIr6-based PhOLEDs is a viable alternative to improve the PE
without the introduction of complicated device structures.15
Because 1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane
(TAPC) with high triplet energy of 2.87 eV16 and good hole-
transporting ability has been widely used as hole-transporting
layers (HTLs) in the blue PhOLEDs,13b,14,15 the remaining key
point has become how to develop efficient host and ET
materials for high-power-efficiency blue PhOLEDs. The
common requirement for host and ET materials is a higher
triplet energy (ET) than that of FIr6 (2.72 eV)10 so as to
maintain effective energy transfer from host to guest, that is,
confining excitons inside the emissive layer (EML). Besides this
criterion, the host materials must possess bipolar charge
transporting properties to balance charge transport in the
EML, and well-matched highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
levels with the adjacent HTL and ETL to facilitate carrier
injection into the EML.3b,17 The ideal ET material should
possess efficient ET ability to maintain the charge balance in
the devices and low HOMO level to block holes.18 In this
context, it becomes a significant challenge in the search of the
host and ET materials that simultaneously meets all the
respective requirements for high-power-efficiency blue PhO-
LEDs.
Recently, phosphine oxide derivatives of aromatic molecules
have emerged as a new class of efficient host and ET materials
for blue PhOLEDs, which showed promise in reducing the
driving voltage.7a,19,20 Inspired by this, we design and synthesize
a new host material of POPCPA by integrating electron-
donating 4,4′-dimethyldiphenylamine unit and electron-accept-
ing diphenylphosphine oxide group into the cyclohexane
skeleton. As expected, the proposed host material possesses a
high ET of 2.93 eV, a shallow HOMO level of −5.24 eV, and a
bipolar charge transporting feature. In addition, we also
construct a new ET material of TP3PO by incorporating
three diphenylphosphine oxide groups into the triphenylben-
zene skeleton. The design strategy endows the ET material with
a high ET (2.78 eV), a deep HOMO level (−6.40 eV), and
good ET ability. As a result, the synergistic combination of
these two materials leads to a FIr6-based blue PhOLED with a
peak EQE of 20.8% and low driving voltages of 2.6, 3.6, and 5.4
V at the luminance of 1 (turn-on), 100, and 1000 cd m−2,
respectively. Without any light out-coupling enhancement, the
combination of high EQE and low driving voltage provides an
outstanding maximum PE of 45.3 lm W−1. To the best of our
knowledge, this is the highest power efficiency for FIr6-based
blue PhOLEDs and is even significantly higher than those of
FIr6-based PhOLEDs with n-doped ETL or p-i-n structure.
spectrometer as the optical detector. The films (50 nm thick)
of codeposited host molecules and blue phosphor were formed
on precleaned quartz substrates by thermal vacuum deposition
(a base pressure of 10−7 Torr) and the doping concentration
was 10 wt %. The absolute PLQYs were calculated by taking
appropriate ratios of the excitation and emission peak areas of
spectra recorded with (1) the sphere empty, (2) the excitation
focused directly onto the sample, and (3) the excitation focused
onto the wall of the sphere, indirectly exciting the sample. The
accuracy of the system was verified against literature value for
the green phosphor fac-tris(2-phenylpyridinato)iridium(III) (2
wt %, Ir(ppy)3) in N,N′-dicarbazolyl-4,4′-biphenyl (CBP)
film.21a Differential scanning calorimetry (DSC) was performed
on a NETZSCH DSC 200 PC unit at a heating rate of 10 °C
min−1 from 20 to 200 °C under argon. The glass transition
temperature (Tg) was determined from the second heating
scan. Thermogravimetric analysis (TGA) was undertaken with
a NETZSCH STA 449C instrument. The thermal stability of
the samples under a nitrogen atmosphere was determined by
measuring their weight loss while heating at a rate of 10 °C
min−1 from 25 to 800 °C.
Device Fabrication and Measurement. The hole-
injection material MoO3 and electron-injection material LiF
were purchased from Sigma-Aldrich and used as received. The
hole-transporting material TAPC, host material N,N′-dicarba-
zolyl-3,5-benzene (mCP), electron transport materials 1,3,5-
tri(m-pyrid-3-yl-phenyl)benzene (Tm3PyPB) and 1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene (TPBI) were purchased from
Luminescence Technology Corporation and purified by train
sublimation prior to deposition. Devices were fabricated in a
Kurt J. Lesker LUMINOS cluster tool with a base pressure of
10−8 Torr without breaking vacuum. The ITO anode is
commercially patterned and coated on glass substrates with a
thickness of 120 nm and sheet resistance of 15 Ω per square.
Prior to loading, the substrate was degreased with standard
solvents, blow-dried using a N2 gun, and treated in a UV−
ozone chamber. All doping concentrations used in this work are
by weight percentage. The active area for all devices was 2 mm2.
Before removing the devices from the vacuum for character-
ization they were encapsulated by a 500 nm thick layer of SiO
deposited by thermal evaporation. Luminance−voltage meas-
urements were carried out using a Minolta LS-110 Luminance
Meter. Current−voltage characteristics were measured using an
HP4140B pA meter. The electroluminescence spectra were
measured using an Ocean Optics USB4000 spectrometer
calibrated with a standard halogen lamp. The radiant flux for
calculating EQEs was measured using an integrating sphere
equipped with an Ocean Optics USB4000 spectrometer with
NIST traceable calibration using a halogen lamp.22 UPS
measurements were performed using a PHI 5500 Multi-
Technique system, with attached organic deposition chamber
with a base pressure of 10−10 Torr. Additional details regarding
device fabrication, characterization, and UPS measurements
have been described elsewhere.23
EXPERIMENTAL SECTION
■
1
General Information. H NMR and 13C NMR spectra
were measured on a MECUYR-VX300 spectrometer. Elemental
analyses of carbon, hydrogen, and nitrogen were performed on
a Vario EL III microanalyzer. Mass spectra were measured on a
Thermo Trace DSQ II GC/MS. UV−vis absorption spectra
were recorded on a Shimadzu UV-2500 recording spectropho-
tometer. Photoluminescence (PL) and phosphorescence
spectra were recorded on a Hitachi F-4500 fluorescence
spectrophotometer. The PL quantum yields (PLQYs) of films
were determined using a previously reported integrating sphere
method21 with a 365 nm LED (Thor-laboratories M365L2) as
the excitation source and an Ocean Optics Maya 2000
Materials. {4-[1-(4-Aminophenyl)cyclohexyl]phenyl}amine
(1) and 1,3,5-tri(3-bromophenyl)benzene (4) were prepared
according to previously reported procedures, respectively.24,25
The solvents were dried using standard procedures. All other
reagents were used as received from commercial sources unless
otherwise stated.
Synthesis of 1,1′-Cyclohexane-1,1-diylbis(4-iodobenzene)
(2). An aqueous solution (15 mL) of NaNO2 (2.97 g, 43 mmol)
was added dropwise to a stirring mixture of 1 (5.33 g, 20
B
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Scheme 1. Synthetic Routes and Chemical Structures of POPCPA and TP3PO
mmol) in 25% H2SO4 (47 g, 120 mmol) at 0 °C for 1 h. After
stirring at 0 °C for another 2 h, the formed suspension was
dropped slowly into a stirring aqueous solution (100 mL) of KI
(8.63 g, 52 mmol) at 50 °C and the reaction mixture was stirred
vigorously at 50 °C for 6 h. After cooling to room temperature,
the mixture was extracted with CH2Cl2, washed with an
aqueous Na2S2O3 solution and brine, and dried over anhydrous
Na2SO4. After removal of the solvent, the residue was purified
by column chromatography on silica gel using petroleum as the
eluent to give a white powder. Yield: 54%. 1H NMR (300 MHz,
CDCl3, 25 °C, TMS): δ 7.58 (d, J = 8.7 Hz, 4H), 6.99 (d, J =
8.7 Hz, 4H), 2.21−2.17 (m, 4H), 1.57−1.50 (m, 6H). 13C
NMR (75 MHz, CDCl3, 25 °C): δ 147.84, 137.49, 129.40,
91.43, 46.02, 36.85, 26.32, 22.89. MS (EI): m/z 488.0 (M+).
Synthesis of {4-[1-(4-Iodophenyl)cyclohexyl]phenyl}bis(4-
methylphenyl)amine (3). A mixture of 2 (8.2 g, 16.8 mmol),
4,4′-dimethyldiphenylamine (2.36 g, 12 mmol), CuI (114 mg,
0.6 mmol), tBuOK (2.02 g, 18 mmol), and ( )-trans-1,2-
diamino-cyclohexane (0.144 mL, 1.2 mmol) in 1,4-dioxane (30
mL) was refluxed under an argon atmosphere for 18 h. After
cooling to room temperature, the reaction mixture was poured
into brine, extracted with CH2Cl2, washed with brine, and dried
over anhydrous Na2SO4. After removal of the solvent, the
residue was purified by column chromatography on silica gel
using dichloromethane/petroleum (1:5 by vol) as the eluent to
2.22−2.18 (m, 4H), 1.56−1.50 (m, 6H). 13C NMR (75 MHz,
CDCl3, 25 °C): δ 145.54, 145.31, 137.15, 132.15, 129.73,
129.44, 127.42, 124.38, 122.24, 110.82, 110.71, 90.67, 45.49,
36.88, 26.24, 22.81, 20.79. MS (EI): m/z 557.5 (M+).
Synthesis of (4-{1-[4-(Diphenylphosphoryl)phenyl]-
cyclohexyl}phenyl)bis(4-methylphenyl)amine (POPCPA). A
mixture of NiCl2·6H2O (84 mg, 0.35 mmol), zinc (460 mg,
7.0 mmol), 2,2′-bipyridine (112 mg, 0.72 mmol), 3 (2.0 g, 3.59
mmol), and diphenylphosphoine oxide (798 mg, 3.95 mmol) in
N,N-dimethylacetamide (20 mL) was stirred under argon at
150 °C for 48 h. After cooling to room temperature, the
mixture was diluted with water, extracted with CH2Cl2, and
washed with brine. Then the organic phases were dried with
anhydrous MgSO4. After removing the solvent, the residue was
purified by column chromatography on silica gel using
methanol/CH2Cl2 (1:20 by vol) as the eluent to give a light
yellow power. Yield: 42%. 1H NMR (300 MHz, CDCl3, 25 °C,
TMS): δ 7.72−7.64 (m, 4H), 7.57−7.50 (m, 4H), 7.48−7.42
(m, 4H), 7.40−7.36 (m, 2H), 7.06−7.02 (m, 6H), 6.96 (d, J =
8.4 Hz, 4H), 6.90 (d, J = 8.7 Hz, 2H), 2.29 (s, 6H), 2.23−2.22
(m, 4H), 1.53−1.50 (m, 6H). 13C NMR (75 MHz, CDCl3, 25
°C): δ 145.75, 145.39, 132.29, 132.20, 132.10, 131.90, 131.87,
129.87, 128.57, 128.45, 127.70, 127.53, 127.40, 124.56, 122.28,
46.02, 37.02, 26.35, 22.90, 20.89. MS (EI): m/z 631.7 (M+).
Anal. Calcd for C44H42NOP (%): C, 83.65; H, 6.70; N, 2.22.
Found: C, 83.52; H, 6.56; N, 2.27.
1
give a white powder. Yield: 63%. H NMR (300 MHz, CDCl3,
25 °C, TMS): δ 7.58 (d, J = 8.4 Hz, 2H), 7.06−7.02 (m, 8H),
6.95 (d, J = 8.7 Hz, 4H), 6.91−6.88 (m, 2H), 2.29 (s, 6H),
Synthesis of 1,3,5-Tri[3-(diphenylphosphoryl)phenyl]-
benzene (TP3PO). Prepared as a white solid according to the
C
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same procedure as POPCPA but using 1,3,5-tri(3-
1
bromophenyl)benzene (4). Yield: 38%. H NMR (300 MHz,
CDCl3, 25 °C, TMS): δ 8.08 (d, J = 12 Hz, 3H), 7.80 (d, J = 6
Hz, 3H), 7.73−7.67 (m, 15H), 7.55−7.48 (m, 24H). 13C NMR
(75 MHz, CDCl3, 25 °C): δ 141.21, 140.85, 140.71, 133.68,
132.73, 131.84, 131.34, 130.89, 130.67, 128.41, 128.27, 125.48.
MS (EI): m/z 906.1 (M+). Anal. Calcd for C60H45O3P3 (%): C,
79.46; H, 5.00. Found: C, 79.27; H, 4.90.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthetic routes
and chemical structures of the host material of (4-{1-[4-
(diphenylphosphoryl)phenyl]cyclohexyl}phenyl)bis(4-
methylphenyl)amine (POPCPA) and the electron-transporting
material of 1,3,5-tri[3-(diphenylphosphoryl)phenyl]benzene
(TP3PO) are depicted in Scheme 1. The starting material 1
was subjected to a diazo reaction to produce 2, which was
coupled with 4,4′-dimethyldiphenylamine to yield the key
intermediate 3.26 The desired products of POPCPA and
TP3PO were prepared through cross-coupling reactions of
diphenylphosphine oxide with 3 and 4, respectively.27
Figure 2. Normalized UV−vis absorption and fluorescence spectra of
POPCPA and TP3PO in hexane at 5 × 10−6 M at room temperature,
and phosphorescence spectra of POPCPA and TP3PO in 2-
methyltetrahydrofuran at 77 K.
the photoluminescence quantum yield (PLQY) of the film with
10 wt % FIr6 doped in POPCPA. For comparison, we also
measured the PLQY of the film with 10 wt % FIr6 doped in
mCP. Both doped films showed relatively high PLQYs (86% for
POPCPA:10 wt % FIr6 and 88% for mCP:10 wt % FIr6), which
are indicative of potentially good host material of POPCPA for
blue phosphor FIr6.
HOMO and LUMO Levels. The HOMO levels of the new
compounds were probed by ultraviolet photoemission spectro-
scopic (UPS) measurements under a vacuum (about 10−10
Torr). Figure 3 panels a and b show the UPS valence band
spectra (used for determining HOMO offset) and the
secondary electron spectra (used for determining work
function) of POPCPA and TP3PO, respectively. Our measure-
ments show that the HOMO level of POPCPA is at −5.24 eV,
which is slightly higher than that of hole-transporting material
TAPC (−5.5 eV),28 that is, no hole-injection barrier from
TAPC to POPCPA in the device. The HOMO level of TP3PO
is at −6.40 eV, suggesting that TP3PO can act as an efficient
ET/hole-blocking (HB) layer in the PhOLEDs. The LUMO
level of TP3PO deduced from the Eg (3.95 eV) and HOMO of
TP3PO is −2.45 eV, which is suitable for electron injection
from the cathode.
POPCPA and TP3PO were characterized with H and 13C
1
NMR spectroscopy, mass spectrometry, and elemental analysis
(see the Experimental Section). POPCPA and TP3PO
decompose at 406 and 525 °C (decomposition temperature
(Td), corresponding to 5% weight loss, determined by
thermogravimetric analysis (TGA)), respectively, and display
glass transition temperatures (Tg, determined by the differential
scanning calorimetry (DSC)) of 90 and 123 °C, respectively
(Figure 1). Such high Td and Tg can benefit the stability of the
devices at elevated temperatures.
Electroluminescent Devices. To evaluate the charge-
transporting character of POPCPA, a hole-only device having
the structure of ITO/MoO3 (1 nm)/TAPC (60 nm)/POPCPA
(70 nm)/MoO3 (1 nm)/Al and an electron-only device having
the configuration of Al/LiF (1 nm)/POPCPA (70 nm)/
TP3PO (40 nm)/LiF (1 nm)/Al were fabricated. The current
density versus voltage curves show that the hole-current density
is comparable to the electron-current density in POPCPA
(Figure 4), indicating the bipolar transporting nature of
POPCPA.
To assess the properties of POPCPA and TP3PO in
electroluminescence (EL), we fabricated the FIr6-based blue
device (device A) by employing these two compounds as the
host and ETL materials, respectively, with the configuration of
ITO/MoO3 (1 nm)/TAPC (60 nm)/POPCPA:10 wt % FIr6
(15 nm)/TP3PO (40 nm)/LiF (1 nm)/Al. In the device,
MoO3 and LiF were used as hole- and electron-injecting layers,
respectively; TAPC and TP3PO served as the hole- and
electron-transporting layers, respectively; FIr6 doped into
POPCPA was used as the emitting layer. For comparison, we
also fabricated the control device (device B) employing the
Figure 1. TGA traces of POPCPA and TP3PO recorded at a heating
rate of 10 °C min−1. Inset: DSC traces of POPCPA and TP3PO
recorded at a heating rate of 10 °C min−1.
Photophysical Properties. Absorption, fluorescence, and
phosphorescence spectra of POPCPA and TP3PO are shown
in Figure 2, and the key parameters are summrized in Table 1.
Their bandgaps (Eg) estimated from their absorption edges in
the film state (Supporting Information Figure S1) are 3.40
(POPCPA) and 3.95 eV (TP3PO). The ET of the compounds
are determined to be 2.93 (POPCPA) and 2.78 eV (TP3PO)
by the highest-energy vibronic sub-band of their phosphor-
escence spectra in 2-methyltetrahydrofuran at 77 K, which are
substantially higher than that of the blue phosphor FIr6 (2.72
eV), indicating that they can confine excitons on the blue
phosphor inside the EML. To further evaluate the suitability of
POPCPA as host material for blue phosphor FIr6, we measured
D
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Table 1. Physical Data of POPCPA and TP3PO
a
b
c
c
d
e
f
g
compound
Tg [°C]
Td [°C]
λabs [nm]
λem max
368
306, 349
,
[nm]
Eg [eV]
ET [eV]
HOMO [eV]
LUMO [eV]
POPCPA
TP3PO
90
406
303
259
3.40
3.95
2.93
2.78
−5.24
−6.40
−1.84
−2.45
123
525
a
b
c
d
Obtained from DSC measurements. Obtained from TGA measurements. Measured in hexane. Determined from the onset of absorption in film
state. Measured in 2-methyltetrahydrofuran at 77 K. Obtained from UPS measurements. Calculated from HOMO and Eg.
e
f
g
Figure 3. (a) Valence photoemission spectra of POPCPA (bottom)
and TP3PO (top). (b) Secondary electron spectra of POPCPA
(bottom) and TP3PO (top).
Figure 5. (a) Current density−voltage−luminance characteristics for
Figure 4. Current density versus voltage characteristics of the hole-
only and electron-only devices. Hole-only device: ITO/MoO3 (1
nm)/TAPC (60 nm)/POPCPA (70 nm)/MoO3 (1 nm)/Al. Electron-
only device: Al/LiF (1 nm)/POPCPA (70 nm)/TP3PO (40 nm)/LiF
(1 nm)/Al.
devices A and B. (b) Power efficiency and external quantum efficiency
and versus luminance curves for devices A and B.
(Supporting Information Figure S2), and a maximum EQE
(EQEmax) of 20.8%, which is significantly higher than that of
the control device B (CEmax of 26.9 cd A−1 and EQEmax of
14.6%). Considering the fact that the PLQYs for the doped
films are almost the same (86% for POPCPA:10 wt % FIr6 and
88% for mCP:10 wt % FIr6), the superior device performance
of POPCPA-based device A over mCP-based device B could be
attributed to the bipolar character of POPCPA, resulting in the
balanced hole and electron transport in the EML. Without any
out-coupling enhancement, the combination of low driving
voltage and high EQE provides an outstanding maximum
power efficiency (PEmax) of 45.3 lm W−1 for device A. To the
best of our knowledge, this is the highest power efficiency for
FIr6-based blue PhOLEDs and is much higher even than those
of FIr6-based PhOLEDs with n-doped ETL (PEmax of 39.2 lm
W−1)14b or p-i-n structure (PEmax of 36 lm W−1).13b In addition,
device A shows better color purity with CIE coordinates of
(0.15, 0.26) than the control device B with CIE coordinates of
(0.19, 0.28), which could be attributed to the shift of
recombination zone in device B owing to the unbalanced
charge transporting ability of mCP.
widely used host of mCP for blue electrophosphorescence with
the same configuration.10 The current density−voltage−
luminance (J−V−L) characteristics and efficiency versus
luminance curves for the devices are shown in Figure 5, and
the key EL data are summarized in Table 2. The POPCPA-
based device A showed extremely low driving voltages of 2.6,
3.6, and 5.4 V at the luminance of 1 (turn-on), 100, and 1000
cd m−2, respectively, compared to voltages of 3.0, 4.2, and 6.2 V
at the same luminance for the mCP-based control device B.
This can be attributed to the shallower HOMO level of
POPCPA (−5.24 eV) compared with that of mCP (−5.9 eV),10
thereby leading to no hole injection barrier from TAPC
(HOMO level of −5.5 eV) to POPCPA with respect to a large
injection barrier of 0.4 eV from TAPC to mCP. Furthermore,
the low driving voltages of device A are even comparable to
those of FIr6-based PhOLED with p-i-n configuration, which
required 3.0, 3.4, and 4.3 V at the luminance of 0.1, 100, and
1000 cd m−2, respectively.13b Remarkably, device A shows a
maximum current efficiency (CEmax) of 37.5 cd A−1
E
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a
Table 2. Electroluminescence Characteristics of the Devices
b
c
d
d
d
e
device
A
host
ETL
V [V]
CEmax [cd A−1
]
PEmax [lm W−1
]
EQEmax [%]
CIE (x, y)
POPCPA
TP3PO
2.6, 3.6, 5.4
3.0, 4.2, 6.2
2.8, 4.0, 7.0
2.8, 3.8, 6.2
37.5
45.3
20.8
0.15, 0.26
0.19, 0.28
0.17, 0.25
0.17, 0.26
36.7 0.8
26.9
43.5 1.8
28.2
20.4 0.4
14.6
B
mCP
TP3PO
TPBI
26.2 0.8
12.1
26.6 1.7
12.9
14.2 0.4
6.6
C
D
POPCPA
POPCPA
11.8 0.4
28.4
12.5 0.5
31.9
6.4 0.2
15.7
Tm3PyPB
28.2 0.4
31.6 0.4
15.6 0.2
a
Abbreviations: ETL, electron-transporting layer; V, voltage; CE, current efficiency; PE, power efficiency; EQE, external quantum efficiency; CIE [x,
b
y], Commission International de I’Eclairage coordinates. Device structure: ITO/MoO3 (1 nm)/TAPC (60 nm)/Host:10 wt % FIr6 (15 nm)/ETL
c
d
e
(40 nm)/LiF (1 nm)/Al (100 nm). In the order of onset, 100, and 1000 cd m−2. The statistics (italic) based on 10 cells of each type. Measured at
3.6 V.
Generally, energy transfer from host to guest and direct
charge trapping on dopants are two main emission mechanisms
in OLEDs. A distinction between them can be made from the
dependence of driving voltage on guest concentration.29 For
the energy transfer, current density−voltage (J−V) character-
istics are not sensitive to the variation of the doping
concentration, whereas for direct charge trapping, the dopants
act as the shallow trapping centers, leading to a dependence of
J−V characteristics on the doping concentration. To figure out
the emission mechanism of device A, we fabricated four devices
by changing the doping concentration of FIr6 from 5, to 10, to
15, and to 20 wt % in POPCPA with the structure of ITO/
MoO3 (1 nm)/TAPC (60 nm)/POPCPA: x wt % FIr6 (15
nm)/TP3PO (40 nm)/LiF (1 nm)/Al (100 nm). As shown in
Figure 6, J−V characteristics are almost independent of FIr6
However, the excellent device performance of device A
cannot completely be explained by the superiority of host
material. We thought that the ETL material also plays an
important role in improving device efficiencies. To figure out
the contribution of ETL material to the device performance, we
fabricated a set of devices based on the host of POPCPA and
the widely used ETL materials of TPBI30 (device C) and
Tm3PyPB18 (device D), respectively. Figure 7 shows J−V−L
characteristics and efficiency versus luminance curves for the
devices C and D. Apparently, TP3PO-based device A
outperforms device C (PEmax of 12.9 lm W−1 and EQEmax of
6.6%) and D (PEmax of 31.9 lm W−1 and EQEmax of 15.7%),
which definitely indicates the superiority of TP3PO over TPBI
Figure 6. Current density versus voltage characteristics as a function of
doping concentration in devices with the structure of ITO/MoO3 (1
nm)/TAPC (60 nm)/POPCPA: x wt % FIr6 (15 nm)/TP3PO (40
nm)/LiF (1 nm)/Al (100 nm), x = 5, 10, 15, and 20.
concentration at the doping concentration from 5 to 10 wt %,
which implies that the primary emission mechanism of FIr6 at
low doping concentration (5 and 10 wt %) is the energy
transfer from POPCPA to FIr6. However, in the case of the
doping concentration variation from 5, to 15, and to 20 wt %,
the current density increases with the increasing concentration
of FIr6 at a constant driving voltage. This indicates that the
primary emission mechanism of FIr6 at high doping
concentration (15 and 20 wt %) is the direct charge trapping
on FIr6. Therefore, for device A with 10 wt % FIr6 doped in
POPCPA, the primary emission mechanism is the energy
transfer from POPCPA to FIr6.
Figure 7. (a) Current density−voltage−luminance characteristics for
devices C and D. (b) Power efficiency and external quantum efficiency
and versus luminance curves for devices C and D.
F
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Chemistry of Materials
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and Tm3PyPB. To evaluate ET abilities of the three
compounds, electron-only devices with a structure of Al/LiF
(1 nm)/TP3PO, TPBI or Tm3PyPB (100 nm)/LiF (1 nm)/Al
were constructed (Figure 8). TP3PO and TPBI showed the
by elaborate designing of host and ET materials systematically
to suit the blue emitter and emission zone structure. This
reported high PE should help advance the application of
PhOLEDs for displays and lighting.
ASSOCIATED CONTENT
■
S
* Supporting Information
Absorption spectra of POPCPA and TP3PO in film state,
current efficiency versus luminance curves for devices A and B,
and normalized EL spectra and CIE coordinates of devices A
and B at a driving voltage of 3.6 V. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: clyang@whu.edu.cn. (C. Y.)
*E-mail: zhenghong.lu@utoronto.ca. (Z.-H. L.)
Author Contributions
Figure 8. Current density versus voltage characteristics of the electron-
only devices based on different ETL materials. Electron-only devices:
Al/LiF (1 nm)/TP3PO, TPBI, or Tm3PyPB (100 nm)/LiF (1 nm)/
Al.
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
similar electron current density under the same voltage, which
is slightly lower than that of Tm3PyPB. This suggests that the
ET ability of TP3PO is comparable to that of TPBI, and slightly
lower than that of Tm3PyPB. On the other hand, the ET of
TP3PO (2.78 eV) is sufficiently higher than that of FIr6 (2.72
eV), indicative of the efficient exciton-confining ability of
TP3PO, whereas the ET of TPBI (2.67 eV)30 or Tm3PyPB
(2.75 eV)18 is lower than or close to that of FIr6 (2.72 eV),
respectively, thereby possibly leading to triplet exciton
quenching on TPBI or Tm3PyPB. Therefore, the excellent
device performance of TP3PO-based device A surpassing those
of devices C and D can be rationalized by a combination of
good ET ability and high triplet energy of TP3PO.
Providing the contributions of both host and ETL materials
in blue devices, the extremely high power efficiency of blue
device A can be attributed to the synergistic combination of the
superiorities from both the host of POPCPA and the electron-
transporting material of TP3PO, that is, the matching HOMO
level of POPCPA to facilitate the hole injection to the EML, the
bipolar nature of POPCPA and good ET ability of TP3PO to
render charge balance and a broad distribution of recombina-
tion region in the EML, and high triplet energies of POPCPA
and TP3PO to ensure efficient energy transfer from the host to
guest and effective exciton confinement inside the EML.
ACKNOWLEDGMENTS
■
C.Y. thanks the National Basic Research Program of China
(973 Program 2013CB834805), the Research Fund for the
Doctoral Program of Higher Education of China
(20120141110029), and the National Science Fund for
Distinguished Young Scholars of China (51125013). Z.-H. Lu
thanks the Natural Sciences and Engineering Research Council
of Canada for financial support.
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