Ultrahigh-Efficiency Green PHOLEDs with a Voltage under 3 V and a Power Efficiency of Nearly 110 lm W?1 at Luminance of 10 000 cd m?2

Article abstract of DOI:10.1002/adma.201702847

Maintaining high power efficiency (PE) under high brightness is still a pressing problem for the practical application of organic light-emitting diodes (OLEDs). Here, ultrahigh-efficiency green phosphorescent OLEDs (PHOLEDs) with a record-low voltage at luminance above 5000 cd m^?2 are fabricated, by developing a novel anthracene/pyridine derivative as the electron-transporting material (ETM) combined with a material displaying thermally activated delayed fluorescence as the host. The pyridine units of the ETM not only facilitate charge injection, but also enhance the electron-transporting mobility, profiting from the closely packed molecules caused by the intermolecular H-bonding. The optimized green PHOLEDs show record-low driving voltages of 2.76 and 2.92 V, with EQEs/PEs of 28.0%/102 lm W^?1 and 27.9%/97 lm W^?1 at 5000 and 10 000 cd m^?2, respectively. Furthermore, device optimization exhibits an unprecedented high PE of 109 lm W^?1 at 10 000 cd m^?2 with voltage under 3 V. Those values are the state-of-the-art among all reported green OLEDs so far, paving their way toward practical applications.

Full text of DOI:10.1002/adma.201702847

CommuniCation
Organic Light-Emitting Diodes
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Ultrahigh-Efficiency Green PHOLEDs with a Voltage
under 3 V and a Power Efficiency of Nearly 110 lm W−1
at Luminance of 10 000 cd m−2
Dongdong Zhang, Juan Qiao, Deqiang Zhang, and Lian Duan*
One efficient strategy to lower the
Maintaining high power efficiency (PE) under high brightness is still a
pressing problem for the practical application of organic light-emitting
diodes (OLEDs). Here, ultrahigh-efficiency green phosphorescent OLEDs
(PHOLEDs) with a record-low voltage at luminance above 5000 cd m−2 are
fabricated, by developing a novel anthracene/pyridine derivative as the
electron-transporting material (ETM) combined with a material displaying
thermally activated delayed fluorescence as the host. The pyridine units of
the ETM not only facilitate charge injection, but also enhance the electron-
transporting mobility, profiting from the closely packed molecules caused by
the intermolecular H-bonding. The optimized green PHOLEDs show record-
low driving voltages of 2.76 and 2.92 V, with EQEs/PEs of 28.0%/102 lm W−1
and 27.9%/97 lm W−1 at 5000 and 10 000 cd m−2, respectively. Furthermore,
device optimization exhibits an unprecedented high PE of 109 lm W−1 at
10 000 cd m−2 with voltage under 3 V. Those values are the state-of-the-art
among all reported green OLEDs so far, paving their way toward practical
applications.
voltage of PHOLEDs is to utilize materials
with thermally activated delayed fluores-
cence (TADF) as the hosts.[9] Benefiting
from the low singlet–triplet splittings
(ΔESTs) of the TADF hosts, which lead
to small energy gaps and then moderate
frontier energy levels, feasible charge
injection from the adjacent layers into the
hosts can be anticipated in the prerequi-
site of the high triplet energies to confine
excitons on the dopant.[10] Our group first
adopted TADF materials as the host for
conventional green phosphors, tris(2-phe-
nylpyridine)iridium(III) (Ir(ppy)3), real-
izing a turn-on voltage as low as 2.19 V.[9]
Also, Kido and co-workers reported a host
material with ΔEST of 0.14 eV and the turn-
on voltage of the Ir(ppy)3-based device is
only 2.11 V.[11] Additionally, a bonus can
be anticipated is that the efficiency roll-off
of PHOLEDs based on TADF host can be
The prospects of organic light-emitting diodes (OLEDs) as
glare-free light sources and display panels has been flourishing
recently.[1–3] Given the predicted per meter square manufac-
turing cost of large-area illumination as well as the limited aper-
ture ratio of the high-resolution displays, the brightness levels
of OLEDs are required to be above 5000 cd m−2.[4,5] How to
maintain a high device efficiency under high brightness, espe-
cially the power efficiency (PE), from the point view of energy
saving, thereof becomes a pressing problem. Both high external
quantum efficiency (EQE) and low operation voltage should be
achieved simultaneously to realize high PEs. So far, the max-
imum EQEs (EQEmaxs) of phosphorescent OLEDs (PHOLEDs)
have been pushed up to above 30% with suppressed efficiency
roll-off.[6–8] The key issue for the high PE, thereafter, becomes
how to reduce the driving voltage.
greatly suppressed, given that, besides from the directly short-
range Dexter energy transfer (DET), the host triplets can be
up-converted to its singlet ones and then transferred to the
dopant by virtue of the long-range Förster interaction (FET),
significantly decreasing the exciton density.[12]
Besides from the host materials, charge-transporting mate-
rials, especially electron-transporting materials (ETMs), are
also crucial to reduce device voltages. By adopting a novel com-
bination of electron-transport layer (ETL) and electron-injec-
tion layer, Kido and co-workers developed an Ir(ppy)3-based
OLED operating at 2.22 V under 100 cd m−2.[13] Ideal ETMs
for PHOLEDs should possess large electron-transporting
mobilities (µes), good exciton confinement as well as good
electron-injection ability simultaneously. What is well estab-
lished is that a good exciton confinement requires ETMs pos-
sessing high triplet energy (T1), which, however, usually means
a weak molecule conjugation and a low µe consequently.[14]
Alternately, high µes can be obtained from low T1-ETMs with
large molecule conjugation nondonor–acceptor structures.
Our group developed a series of anthracene derivations with
high µes, exhibiting high performance in fluorescent OLEDs
and PHOLEDs.[15,16] Though the T1 energy of anthracene unit
is low, good confinement of excitons can be realized by steri-
cally encapsulating the anthracene unit with bulk side group
units or tuning the recombination zone away from the emit-
ting layer/ETM interfaces.[16]
Dr. D. D. Zhang, Dr. J. Qiao, Dr. D. Q. Zhang, Prof. L. Duan
Key Lab of Organic Optoelectronics and Molecular Engineering
of Ministry of Education
Department of Chemistry
Tsinghua University
Beijing 100084, P. R. China
E-mail: duanl@tsinghua.edu.cn
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201702847.
DOI: 10.1002/adma.201702847
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Based on the above concept, ultrahigh-efficiency green
PHOLEDs with a record-low voltage at luminance above
5000 cd m−2 were developed, by utilizing a novel anthracene/
pyridyl derivative as the ETM combined with a TADF host. The
pyridyl units of the ETM not only facilitate the charge injection,
but also enhance electron transporting ability profiting from the
closely packed molecules caused by the intermolecular H-bonding.
The optimized green PHOLED showed record-low driving vol-
tages of 2.76 and 2.92 V, with EQEs/PEs of 28.0%/102 lm W−1
and 27.9%/97 lm W−1 at 5000 and 10 000 cd m−2, respectively.
Further device optimization exhibits unprecedented high PE of
109 lm W−1 at 10 000 cd m−2 with the voltage under 3 V.
the phenyl planes are only 23.05° and 27.54°, respectively. The
twisted structure of DPPyA benefits to realize good thermal
stability. The thermal properties of DPPyA were determined
by differential scanning calorimetry and thermogravimetric
analysis as shown in Figure S1 in the Supporting Information,
revealing that the thermal-decomposition temperatures (Td, cor-
responding to 5% weight loss) and the glass-transition tempera-
ture (Tg) are 397 and 135 °C, respectively.
The unique packing structure of the DPPyA crystal is also
shown in Figure 1c. Owing to the pyridyl units in the ETMs,
intra- and intermolecular H-bonding interactions were con-

firmed. The obtained CH N bond distances are 2.718–2.929 Å
The synthesis process of 9,10-bis(6-phenylpyridin-3-yl)anthra-
cene (DPPyA) is illustrated in Figure 1a. DPPyA was synthesized
from anthraquinone and 5-bromo-2-phenylpyridine, which
was synthesized from pyridin-3-ylboronic acid and 5-bromo-
2-iodopyridine.[17] Owing to the large conjugation structure,
the anthracene plane was chosen as the core unit to enhance
the charge-transporting ability. The pyridyl units were directly
attached on the 9,10-position of the anthracene unit. It has been
established that the coordination effect of the nitrogen atom in
pyridyl unit with lithium cations induces an effective electron
injection and thus a low driving voltage.[14a,18] The pyridyl unit,
thereof, was adopted in DPPyA to enhance the charge-injec-
tion ability. Furthermore, the electron-deficient characteristic
of pyridyl unit can deep the lowest unoccupied energy level
(LUMO) of the ETM, which also facilitates the charge injec-
tion. Thus, it can be anticipated that DPPyA has the potential to
achieve high µe and good charge injection simultaneously.
The molecular configurations of the DPPyA were further
determined by X-ray crystallography, as shown in Figure 1b. A
twisted single-crystal structure was observed, with the dihedral
angles between the anthracene and the pyridyl planes are 74.66°
and 74.07° arising from the large steric effect of the anthracene
unit. Meanwhile, the dihedral angles between the pyridyl and
and 2.557–2.598 Å for inter- and intramolecular, respectively,
which can be classified as H-bonding and have already been
reported in films based on pyridine rings.[19] A large number of
contributions of multifurcated H-bonding can be an adequate
trigger for the effective construction of the supramolecular
structure. Owing to the strong intermolecular interactions, con-
densed molecule packing can be realized. The hopping model
between molecules has been established as the main pathway
for the charge transport in the organic films, which requires
efficient molecule contact to achieve a large intermolecular
orbital overlap.[20] Thereof, the tightly packed molecules facili-
tate the charge transport between molecules.
To better understanding the relationship between the charge-
transporting properties and the molecule structure, the highest
occupied molecule orbital (HOMO) and LUMO distributions
of DPPyA were investigated using Gaussian 09 software with
B3LYP/6-31G* basis sets. As can be seen from Figure 2a, both
the HOMO and LUMO are mainly located on the anthracene
core in DPPyA with little of them extending to the pyridyl units,
indicating that the anthracene units are mainly responsible for
the charge transport, as we anticipated. Since the HOMO and
the LUMO are located on the same unit, the charge-transfer
integrals for electrons and holes are thereafter similar, which
will lead to balanced electron- and hole-transport properties
according to the Marcus theory.[21] The spin density distribution
of the triplet state of DPPyA was also calculated as depicted in
Figure 2a. The T1 state is mainly delocalized on the anthracene
core and the pyridine units. Owing to the large conjugation of
the anthracene unit, low T1 of 1.71 eV was calculated based on
the time-dependent DFT basis, corresponding to the previous
calculated ones with the anthracene unit.[16]
As an ideal ETM, the charge-injection ability is also cru-
cial. As illustrated in Figure 2a, the electron density of DPPyA
from total SCF density was calculated, revealing that the nega-
tive charges are accumulated around the nitrogen atoms of the
pyridyl units, which renders the feasible interaction between
nitrogen and lithium. Adachi and co-workers suggested that the
coordination effect of the nitrogen atom in the heteroaromatic
ring with metals efficiently enhances electron injection.[22] Kido
et al. also observed this phenomenon from a series of pyridyl
derivations utilized as the ETMs.[14a,18] Therefore, it can be
anticipated that the pyridyl units in DPPyA can also induce an
effective electron injection used as an ETM.
The photo-physical features of DPPyA were measured in
dichloromethane and are illustrated in Figure 2b. The absorption
spectrum of DPPyA exhibits peaks at 378 and 400 nm, which
can be assigned to the π–π* transitions of the compounds.[15]
Figure 1. a) The synthesis process of DPPyA. b) The single crystal struc-
ture of DPPyA. c) The molecule packing structure of DPPyA.
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Figure 2. a) The HOMO and LUMO distributions of DPPyA. b) The distribution of the triplet density as well as the electron density of DPPyA. c) The
absorption and the emission spectra of DPPyA in CH2Cl2 solution. d) The CV curves of DPPyA. e) The charge mobility versus electrical field of DPPyA film.
The emission spectrum shows a peak at 435 nm, corresponding
to singlet energies of 2.85 eV. To measure the triplet energy of
DPPyA, the emission of Firpic: DPPyA mixed film at 77 k was
measured as shown in Figure S2 in the Supporting Information,
where Firpic is bis[2-(4,6-difluorophenyl)pyridinato-C2,N](pico-
linato)iridium(III), utilized as the triplet sensitizer to increase
the triplet density of DPPyA. The measured phosphorescence
emission is peaked at 703 nm, corresponding to triplet energy of
1.77 eV, agreeing well with the calculated ones.
The electrochemical properties of DPPyA were studied by
cyclic voltammetry (CV) using a platinum (Pt) disk electrode as
the working electrode while Ag/AgCl was used as the reference
electrode. Oxidation CV was performed in dichloromethane
containing with 0.1 m n-Bu4NPCl6 as the supporting electrolyte
at a scan rate of 250 mV s−1 while reduction CV was performed
in N,N-dimethylformamide (DMF) with 0.1 m n-Bu4NClO4
as the supporting electrolyte at a scan rate of 250 mV s−1. As
shown in Figure 2c, the CV of DPPyA reveals both reversible
oxidation and reduction potential, indicating that both radical
cations and anions of DPPyA are stable. To reveal the effect of
charge injection on the molecular conformational stability, the
optimized geometries and the electronic properties of both the
anion and cation (i.e., the neutral DPPyA molecule in the pres-
ence of an extra electron and an extra hole, respectively) were
calculated Gaussian 09 0software with B3LYP/6-31G* basis
sets. It is clear that both the HOMO of the cation and LUMO of
the anion extend over the whole molecule, as shown in Figure S3
in the Supporting Information. Such delocalization of the extra
charges facilitates to reduce the charge density, conducive to the
stability of the cation and anion unities of DPPyA.[23] The HOMO
level of DPPyA is −5.9 V, which was determined from the anodic
oxidation potential. Similarly, the LUMO level of DPPyA was
calculated to be as deep as −3.0 eV, benefiting promotion of the
electron injection, as we anticipated. The LUMO energy level of
DPPyA is deeper than other anthracene derivatives reported pre-
viously by our group, resulting from the electron-deficient prop-
erty of the pridyl units directly attached on the anthracene unit.[15]
The carrier mobility of DPPyA thin film was characterized
by the time-of-flight (TOF) transient photocurrent technique.
The configuration of the device is ITO/DPPyA (1.4 µm)/Ag
(200 nm). The carrier mobility (µ) was calculated from the
values of the transient time (Tt), the sample thickness (D), and
the applied voltage (V) using the following equation:
µ = D/(TtE) = D2/(VTt )
(1)
Figure 2d shows the field-dependent hole and electron
mobilities of DPPyA, both of which follow the Poole–Frenkel
µ(E) = µ(0) exp(0)β E
(PF) relationship
, where β is the
PF slope, and µ(0) is the zero-field mobility.[23] It can be
observed that DPPyA displays balanced mobility for holes
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and electrons, agreeing well with our previous work, in which
several anthracene derivations exhibited ambipolar trans-
port properties being reported. The µ0(h) and µ0(e) values are
5.1 × 10−4 and 4.6 × 10−4 cm2 V−1 s−1 while the βh and βe values
are 1.48 × 10−3 and 1.56 × 10−3 cm−1/2 V1/2, respectively. As
reported, the zero-field mobilities only depend on the reorgani-
zation energy and the charge-transfer integral.[23] The almost
identical µ0(h) and µ0(e) values indicate similar reorganization
energy and charge-transfer integrals for electrons and holes,
proving that holes and electrons are transported in similar envi-
ronments, that is mainly through the anthracene units. Notably,
at an electric field of 5.5 × 105 V cm−1, the electron mobility
of DPPyA reaches 1.5 × 10−3 cm2 V−1 s−1, which is about three
orders of magnitude higher than that of Alq3.[24]
High electron-transporting mobility with good electron-
injection ability renders DPPyA to be adequate ETMs used
in the OLEDs. To evaluate the performance of DPPyA as the
ETM, green PHOLEDs were fabricated, with the device struc-
ture of ITO/HATCN (5 nm)/NPB (40 nm)/TCTA (5 nm):15%
Ir(mppy)3/DIC-TRZ:15% Ir(mppy)3 (15 nm)/DPPyA (50 nm)/
LiF(0.5 nm)/Al(150 nm), where HATCN, NPB, TCTA, DIC-
TRZ, and Ir(mppy)3 are dipyrazino[2,3-f:2′,3′-h]quinoxaline-
2,3,6,7,10,11-hexacarbonitrile, N,N′-bis(1-naphthalenyl)-N,N′-
[2,3-a]carbazole-11-yl)-1,3,5-triazine, and tris[2-(p-tolyl)pyridine]
iridium(III), respectively. Here, a well-known TADF material,
DIC-TRZ, was chosen as the host.[10] It has been demon-
strated that DIC-TRZ as the host for a green dopant, Ir(ppy)3,
can greatly boost device efficiency with a suppressed efficiency
roll-off. Due to the small ΔEST of the host, the triplet of DIC-
TRZ can be up-converted into its singlet ones, followed by
being transferred to the dopant through the long-range FET
mechanisms.[10] Combined with the directly short-range DET,
such an energy transfer process can greatly reduce the triplet
exciton density in the emitting layers, benefiting suppression
of the efficiency roll-off. Also, originating from its small ΔEST,
though the triplet energy of DIC-TRZ is as high as 2.76 eV, its
singlet energy is only 2.82 eV and energy gap of only 2.9 eV.
The energy gap of DIC-TRZ is relatively low compared with the
conventional host with the same triplet energy but larger ΔEST,
conducive to the charge injection from the adjacent layers.
For PHOLEDs, a good exciton confinement in the EML is
one of the key prerequisites for high device performance. The
triplet energy of DPPyA is only 1.77 eV, much lower than that
of Ir(mppy)3 (2.4 eV). Therefore, DPPyA as the ETM may
quench the dopant excitons. One method to avoid this problem
is to tune the recombination zone away from the EML/ETL
interface. As illustrated from Figure 3a, no energy barrier is
observed between the HOMO energy level of NPB and that
diphenyl-[1,1′-biphenyl]-4,4′-diamine,
N,N,N-tris(4-(9-carba-
zolyl) phenyl) amine, 2,4-diphenyl-6-bis(12-phenylindolo)
Figure 3. a) The energy diagram of the PHOLEDs. b) The current density–voltage–brightness curves of the PHOLEDs. c) The EQE–brightness curves
of the PHOLEDs. d) The PE–brightness curves of the PHOLEDs.
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of Ir(mppy)3, suggesting that the holes are directly trapped by
the dopant. From previous carrier-mobility studies, it is known
that trapping of charges on the Ir(mppy)3 strongly reduces the
device current density, indicating that the hole transport in the
device is slowed down.[25] On the contrary, the electrons will
be directly injected into the DIC-TRZ host, due to the deeper
LUMO of the host than the dopant. Given the high µ(e) of DIC-
TRZ, which was reported to be in the range of 10−3 cm2 V−1 s−1,
the electrons in DIC-TRZ can be quickly transported to the
TCTA:DIC-TRZ interface and piled up at the interface owing to
the large energy barrier between TCTA and DIC-TRZ.[1c] Taking
into consideration of the high µ(e)s of the DPPyA and the host,
it can be well assumed that the dominating type of charge car-
rier in the EML is electrons. The accumulated electrons on
the DIC-TRZ change the internal electric-field distributions,
shifting the HOMO of DIC-TRZ to a shallower level.[26,27] Holes
thereafter become accessible from the Ir(mppy)3 to the DIC-
TRZ and then recombine with the electrons gathered at the
interface very quickly and efficiently. Then the formed triplet
excitons of DIC-TRZ can be either directly transferred to the
dopant through the short-range DET or up-converted into its
singlet ones, from where the energy being transferred to the
dopant through the long-range FET mechanisms, giving out
bright emission. Since the recombination is away from the
EML/ETM interface, the quenching problem resulted from the
low triplet energy of DPPyA is thereafter avoided.
Remarkably, an extremely low driving voltage was achieved
for the device with DPPyA as the ETM with the voltages of 2.35,
2.56, 2.76, and 2.92 V at 100, 1000, 5000, and 10 000 cd m−2,
respectively, without any techniques for enhancement of light
outcoupling. The voltage at 100 cd m−2 is even lower than the
hv of the Ir(mppy)3, which should be 2.4 V. The reason can be
attributed to thermally assisted electroluminance, which has
been theoretically and experimentally demonstrated.[13] From a
theoretical perspective, Leo and co-workers have predicted the
thermodynamic limit of the operating voltage using an equation
based on black-body radiation and the thermodynamic limit, if
the operating voltage in a green OLED, for example, is evaluated
to be 1.95 V at 100 cd m−2.[28] The voltage at 100 cd m−2 obtained
here is only 0.4 eV higher than the theoretical limit value. Also,
the voltage of the device at 10 000 cd m−2 is even lower than
3 V. To our knowledge, the voltage of the device with the lumi-
nance above 5000 cd m−2 is the lowest reported so far, as can
be seen from Table 1. The extremely low voltage originates
from the high µ(e) and the good injection ability of DPPyA,
combined with the utilization of a material with TADF as the
host. For comparison, devices with 4,7-diphenyl-1,10-phenan-
throline (Bphen) and 9,10-bis(3-(pyridin-3-yl)phenyl)anthracene
(DPyPA), as well as a well-known commercial ETM, LG201,
as the ETMs were also fabricated.[15,29] The reported µ(e)s of
Bphen and DPyPA both are in the range of 10−4 cm2 V−1 s−1,
lower than the one of DPPyA.[15,30] As illustrated in Figure 3b,
the voltages of those devices at 5000 cd m−2 are 3.53, 3.09, and
4.47 eV for Bphen-, DPyPA-, and LG201-based devices. The
higher voltages of these devices can be attributed to their lower
µ(e)s compared with that of DPPyA.
Additionally, EQEmax as high as 28.2% was realized as
illustrated in Figure 3c, suggesting that the low T1 energy
of the ETM does not quench the excitons of the EML due to
the wisely tuned recombination zone, as discussed above.
Particularly, the efficiency roll-off of the device is extremely
small, with EQE remaining 28.0%/27.9% even at practical
luminance of 5000 cd m−2/10 000 cd m−2. Derived from the
low driving voltage and high EQE, maximum power effi-
ciencies as high as 107 lm W−1 were achieved (Figure 3d),
remaining 102 and 97 lm W−1 at 5000 and 10 000 cd m−2,
respectively. Comparably, the EQEs/PEs at 5000 cd m−2
were only 21.2%/61.2 lm W−1, 20.2%/66.5, lm W−1, and
15.0%/34.3 lm W−1 for Bphen, DPyPA, and LG201, respectively.
The inferiority of PE originates from the relatively higher oper-
ation voltages. Further, the devices’ lifetimes were measured
at an initial luminance of 1000 cd m−2. As can be seen from
Figure S4 in the Supporting Information, after more than 40 h,
there is no luminance decay for the device based on DPPyA
while significant decay for the device with Bphen as the EML
layer, indicating long-term stability of DPPyA. Those perfor-
mances here testify the great potential of DPPyA as the ETM.
Considering that the dominating type of charge carrier
in the EMLs of the above devices is electrons, a mixed host
composed
of
N4,N4,N4′,N4′-tetra([1,1′-biphenyl]-4-yl)-[1,1′-
biphenyl]-4,4′-diamine (BPBPA): DIC-TRZ was adopted to
achieve more balanced charges in the EML.[31] The device
Table 1. Summary of the green PHOLEDs.
Ref.
Voltage
[V]
EQE
[%]
PE
[lm W−1
]
5000 cd m−2
2.76
10 000 cd m−2
5000 cd m−2
10 000 cd m−2
27.9
5000 cd m−2
10 000 cd m−2
This work
This work
Ref. [6a]
2.92
2.95
6.3
28.0
31.7
–
102
115
–
97
109
–
2.78
31.5
≈4.8a)
–
≈27a
Ref. [6b]
4.2
–
27.8
–
77
≈70a)
≈78a)
110.2
≈95a)
Ref. [13]
2.95
≈3.4a)
≈4.3a)
≈3.8a)
≈4a)
≈58a)
≈90a)
≈80a)
Ref. [18a]
Ref. [32a]b)
Ref. [32b]
–
–
≈4.8a)
≈4.4a)
≈34a)
30
37.2
≈32a)
a)Obtained Read from the figures of the device performances; b)Based on an electrode structure of high-index TiO2 layers and low-index hole-injection layers sandwiching
graphene electrodes.
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structure is ITO/HATCN (5 nm)/BPBPA (50 nm)/DIC-TRZ
(20 nm):BPBPA (30%):15% Ir(mppy)3/DPPyA (50 nm)/LiF
(0.5 nm)/Al (150 nm). The triplet energy of BPBPA is about
2.5 eV, higher than that of Ir(mppy)3, benefiting the achieve-
ment of good exciton confinement. The concentration of
BPBPA was finely optimized to be 30%. The energy diagram of
the PHOLEDs was shown in Figure 4a.
As can be seen from Figure 4b, the voltages of device at
5000 and 10 000 cd m−2 are also as low as 2.78 and 2.95 V,
respectively. The slightly increased voltage compared with the
one using NPB as the HTM can be attributed to the relatively
lower hole-transporting mobility of BPBPA. Remarkably,
EQEmax/PEmax as high as 31.8%/118 lm W−1 was achieved, as
is shown in Figure 4c, with EQEs/PEs of 31.7%/115 lm W−1
and 31.5%/109 lm W−1 remaining at 5000 and 10 000 cd m−2,
respectively. Benefiting from the high EQE and low voltage,
the PE values are the highest among all the reported green
PHOLEDs under practical brightness above 5000 cd m−2 as
illustrated in Table 1, greatly manifesting the potential of such
a design strategy. It is evident from the improved device perfor-
mance that no exciton quenching happens even if the mixed
host was adopted. The reason can be attributed to, on the
one hand, although the mixed host may broaden recombina-
tion zone, the recombination is still away from the EML/ETL
interface owing to the high electron-transporting mobilities,
while the relatively lower hole-transporting mobility of BPBPA.
It can be demonstrated by the fact that when the BPBPA con-
centration is higher than 30%, the device efficiencies are grad-
ually reduced, attributed to the shifted recombination zone
toward the EML/ETL interface, which leads to more exciton
quenching. On the other hand, we compared the exciton-
quenching ability of DPPyA with that of DPyPA as shown in
Figure S5 in the Supporting Information by comparing the
photoluminance (PL) transient lifetime of the Ir(mppy)3 in
binary films of EML/ETLs, showing that the linear structure
of the side group of DPPyA benefits the achievement of better
exciton confining ability than the distorted side-group struc-
ture of DPyPA. This implies that subtle modulation of the side
group can significantly change the properties of the anthra-
cene derivatives, and may shed a new light on more efficient
electron materials.
a
700
b
600
500
400
300
200
100
0
10000
1000
100
10
In conclusion, a novel anthracene/pyridine derivation with
high µe and good electron injection ability was designed as
the ETM, greatly boosting the PE of PHOLEDs, together with
a TADF material as the host. The optimized green PHOLEDs
exhibited extremely record-low driving voltages of 2.76 and
2.92 V at practical brightness of 5000 and 10 000 cd m−2, respec-
tively. Further device optimization even leads to an unprece-
dented record-high PE nearly 110 lm W−1 at 10 000 cd m−2 with
voltage of under 3 V. This work shows the great importance of
such ETM design and device structure optimization strategy,
paving their way toward further practical applications.
1
4.0
1.0
1.5
2.0
2.5
3.0
3.5
Voltage (V)
c
50
40
30
20
10
0
120
100
80
60
40
20
0
Experimental Section
TOF Mobility Measurement: DPPyA was purified by sublimation
before use in subsequent analyses and device fabrication. The TOF
device was prepared by vacuum deposition. The third harmonic of a
Nd:YAG laser (355 nm, 15 ns) was used as the excitation light source.
By switching the polarity of the applied dc bias, the transit times Tt,
the time that the carriers pass through the organic film to reach the
counter electrode, was measured by a digital storage oscilloscope.
With the applied bias V and the thickness of the organic layer D, which
was much larger than the optical absorption depth of the excitation,
the carrier mobilities could be calculated according to the formula:
µ = D/(TtE) = D2/(VTt).
N
N
0
5000 10000 15000 20000 25000 30000
Brightness (cd/m2)
Fabrication of OLEDs: ITO substrates with a sheet resistance of
−1

15 Ω
were cleaned and treated with oxygen plasma before use. The
OLEDs were fabricated through vacuum deposition of the materials at
10−6 Torr onto the ITO glass. The encapsulated devices were characterized
using a Keithley 4200 semiconductor characterization system in ambient
Figure 4. a) The energy diagram of the PHOLEDs. b) The current density–
voltage–brightness curves of the PHOLEDs. c) The EQE–brightness–PE
curves of the PHOLEDs.
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Photo Research PR705 spectrophotometer. Electroluminescence
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found, 485.2. Anal. calcd for C36H24N2: C 89.23, H 4.99, N 5.78; found:
C 89.15, H 5.03, N 5.82.
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Supporting Information
Supporting Information is available from the Wiley Online Library or
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The authors would like to thank the National Key Basic Research
and Development Program of China (Grant Nos. 2016YFB041003,
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Keywords
electron transporting materials, green phosphorescent OLEDs, high
power efficiency, low voltage
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Adv. Mater. 2017, 1702847
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2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim