Long-Lived and Highly Efficient TADF-PhOLED with “(A)n–D–(A)n” Structured Terpyridine Electron-Transporting Material

Article abstract of DOI:10.1002/adfm.201800429

The electron-transporting material (ETM) is one of the key factors to determine the efficiency and stability of organic light-emitting diodes (OLEDs). A novel ETM with a “(Acceptor)_n–Donor–(Acceptor)_n” (“(A)_n–D–(A)_n”) structure, 2,7-di([2,2′:6′,2″-terpyridin]-4′-yl)-9,9′-spirobifluorene (27-TPSF), is synthesized by combining electron-withdrawing terpyridine (TPY) moieties and rigid twisted spirobifluorene, in which the TPY moieties facilitate electron transport and injection while the spirobifluorene moiety ensures high triplet energy (T₁ = 2.5 eV) as well as enhances glass transition temperature (T_g = 195 °C) for better stability. By using tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)₃) as the emitter, the 27-TPSF-based device exhibits a maximum external quantum efficiency (η_{ext, max}) of 24.5%, and a half-life (T₅₀) of 121, 6804, and 382 636 h at an initial luminance of 10 000, 1000, and 100 cd m^?2, respectively, which are much better than the commercialized ETM of 9,10-bis(6-phenylpyridin-3-yl)anthracene (DPPyA). Furthermore, a higher efficiency, a η_{ext, max} of 28.2% and a maximum power efficiency (η_PE, _max) of 129.3 lm W^?1, can be achieved by adopting bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate) (Ir(ppy)₂tmd) as the emitter and 27-TPSF as the ETM. These results indicate that the derivative of TPY to form “(A)_n–D–(A)_n” structure is a promising way to design an ETM with good comprehensive properties for OLEDs.

Full text of DOI:10.1002/adfm.201800429

FULL PAPER
Electron-Transporting Materials
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Long-Lived and Highly Efficient TADF-PhOLED
with “(A)n–D–(A)n” Structured Terpyridine Electron-
Transporting Material
Mengying Bian, Dongdong Zhang, Yuanxun Wang, Yao-Hsien Chung, Yang Liu,
Hungkit Ting, Lian Duan, Zhijian Chen, Zuqiang Bian, Zhiwei Liu,* and Lixin Xiao*
(ηint) of 100%.[2] To further meet the
The electron-transporting material (ETM) is one of the key factors to
determine the efficiency and stability of organic light-emitting diodes
(OLEDs). A novel ETM with a “(Acceptor)n–Donor–(Acceptor)n” (“(A)n–D–
(A)n”) structure, 2,7-di([2,2′:6′,2″-terpyridin]-4′-yl)-9,9′-spirobifluorene (27-
TPSF), is synthesized by combining electron-withdrawing terpyridine (TPY)
moieties and rigid twisted spirobifluorene, in which the TPY moieties facilitate
electron transport and injection while the spirobifluorene moiety ensures high
triplet energy (T1 = 2.5 eV) as well as enhances glass transition temperature
(Tg = 195 °C) for better stability. By using tris[2-(p-tolyl)pyridine]iridium(III)
(Ir(mppy)3) as the emitter, the 27-TPSF-based device exhibits a maximum
external quantum efficiency (ηext, max) of 24.5%, and a half-life (T50) of 121,
6804, and 382 636 h at an initial luminance of 10 000, 1000, and 100 cd m−2,
respectively, which are much better than the commercialized ETM of
9,10-bis(6-phenylpyridin-3-yl)anthracene (DPPyA). Furthermore, a higher
efficiency, a ηext, max of 28.2% and a maximum power efficiency (ηPE, max) of
129.3 lm W−1, can be achieved by adopting bis(2-phenylpyridine)iridium(III)
(2,2,6,6-tetramethylheptane-3,5-diketonate) (Ir(ppy)2tmd) as the emitter and
27-TPSF as the ETM. These results indicate that the derivative of TPY to form
“(A)n–D–(A)n” structure is a promising way to design an ETM with good
comprehensive properties for OLEDs.
requirements of commercialization for an
ideal phosphorescent OLED (PhOLED),
low efficiency roll-off and long operational
lifetime are requested simultaneously.
To achieve such demands, a valid strategy
utilizing thermally activated delayed fluo-
rescent (TADF) material as the host for
PhOLED (TADF-PhOLED) has been pro-
posed, boosting device performances with
high efficiency, low driving voltage, and
reduced efficiency roll-off by alleviating the
triplet-involved annihilations.[1c,3] Despite
the state-of-the-art devices’ performances
continue to be improved, the device life-
times of TADF-PhOLEDs are still far from
satisfactory.[4] Because the development
of stable host and emitter materials has
been the focus in the past decade, the lack
of ideal electron-transporting materials
(ETMs) has become the principle element
that limited the device lifetimes.[5]
A desired ETM requires high electron
transport (ET) mobility, good exciton
confinement as well as feasible charge
injection from the anode.[4b,6] Techni-
cally, a high triplet energy (T1) is necessary for good exciton
confinement, which can be achieved by limited molecule con-
jugation structures. Numerous works have been devoted to
developing high T1 ETMs, utilizing strong electron-deficient
units like pyridine, pyrimidine,[7] triazine,[8] benzoimidazole,[9]
such as diphenyl-bis[4-(pyridin-3-yl)phenyl]silane (DPPS),[10]
1. Introduction
Organic light-emitting diodes (OLEDs) have been commercial-
ized as flat panel displays and have great potential in developing
energy-saving solid-state-lighting.[1] By using phosphorescent
emitters, it is possible to obtain an internal quantum efficiency
M. Bian, Dr. Y.-H. Chung, Dr. H. Ting, Prof. Z. Chen, Prof. L. Xiao
State Key Laboratory for Mesoscopic Physics and Department of Physics
Peking University
Y. Wang
Valiant Corporation
Shandong 264006, P. R. China
Beijing 100871, P. R. China
E-mail: xiao66@pku.edu.cn
Y. Liu, Prof. Z. Bian, Prof. Z. Liu
Beijing National Laboratory for Molecular Sciences
State Key Laboratory of Rare Earth Materials Chemistry and Applications
College of Chemistry and Molecular Engineering
Peking University
Dr. D. Zhang, Prof. L. Duan
Key Lab of Organic Optoelectronics and Molecular Engineering of
Ministry of Education
Department of Chemistry
Beijing 100871, P. R. China
Tsinghua University
E-mail: zwliu@pku.edu.cn
Beijing 100084, P. R. China
Prof. L. Xiao
College of Physics and Electronics Engineering
Hengyang Normal University
Hengyang 421008, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201800429.
DOI: 10.1002/adfm.201800429
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1,2,4,5-tetra(3-pyrid-3-yl-phenyl)benzene (TemPPB),[11] and
1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPB).[12] How-
ever, the limited conjugation structure of those high T1 ETMs
goes against charge transport and their stabilities are not satis-
fied. Very recently, Duan et al. proposed a pyridine-based ETM
9,10-bis(6-phenylpyridin-3-yl)anthracene (DPPyA) utilizing
anthracene core for TADF-PhOLED, realizing the lowest opera-
tion voltage as well as the highest power efficiency under prac-
tical luminance above 5000 cd m−2, showing the latent capacity
of new ETMs for TADF-PhOLED.[4b] However, the low T1 of
DPPyA requires the finely tuned recombination zone to alle-
viate exciton quenching, which increases the device fabrication
difficulty. It is therefore highly desired to develop new ETMs
with high T1 for TADF-PhOLED to confine triplet excitons and
particularly to further improve device stabilities. Terpyridine
(TPY) intrinsically possesses stronger electron-accepting char-
acter than pyridine. We once reported a deep-blue emitter of
2,7-di(2,2′:6′,2″-terpyridin-4-yl)-9,9-dioctyl-9H-fluorene (DTPF)
by using TPY as the acceptor to improve its ET property.[13]
Recently, Kido et al. investigated a series of TPY isomers,[14]
achieving 23.5% of a maximum external quantum efficiency
(ηext, max) in green PhOLED. To further improve the thermal
stability of DTPF, a rigid twisted spirobifluorene could be intro-
duced to replace fluorene.[15] Moreover, with sp3 carbon and
non-coplanar structure, spirobifluorene could prevent the inter-
molecular interaction to achieve high T1.[16]
Here, a novel ETM with a “(Acceptor)n–Donor–(Acceptor)n”
(“(A)n–D–(A)n”) structure, 2,7-di([2,2′:6′,2″-terpyridin]-4′-yl)-
9,9′-spirobifluorene (27-TPSF), is synthesized by combining
electron-withdrawing TPY moieties with rigid spirobifluorene.
By using tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)3)[17] as
the green phosphorescent emitter to dope into a TADF host,
the optimized device with 27-TPSF as ETM exhibits a turn on
voltage (Von) of 2.4 V, a ηext, max of 24.5%, and a half-life (T50)
of 121 h at initial luminance of 10 000 cd m−2, corresponding
to 6804 h at 1000 cd m−2 and 382 636 h at 100 cd m−2, which
are much better than the device with a commercialized ETM
of DPPyA. Additionally, by adopting an emitter with prefer-
able horiziontal orientations, even higher ηext, max of 28.2% and
a maximum power efficiency (ηPE, max) of 129.3 lm W−1 were
realized.[18]
catalyzed by Pd catalysts, which is simple and easy to scale up.
27-TPSF has been characterized by H NMR, electrospray ioni-
zation mass spectra (ESI-MS), and elemental analyses, and sub-
limated before using.
1
2.2. Photophysical Properties
Figure 1a shows the room-temperature UV/vis absorption and
fluorescence spectra of 27-TPSF in chloroform (CHCl3) (10−5 m)
and film state. In solution, there are two absorption peaks at
297 and 308 nm, as shown in Table 1, which are attributed to
the π–π* transition of the fluorene units in spirobifluorene.[19]
Another two absorption peaks of 254 and 280 nm are ascribed
to the TPY.[14] The strong absorption band above 310 nm sug-
gests that there is electronic coupling between the spirobi-
fluorene and the two TPY components.[20] The maxima is at
341 nm which occurs obviously bathochromic shift relative to
a series of spirobifluorene derivatives substituted by different
electron accepting units, crediting to the more extended con-
jugation between the 2,7-substituted spirobifluorene and the
TPY.[7,15a] The maximum molar absorption coefficient reaches
414 000 m−1 cm−1 at the wavelength of 280 nm. The absorption
peaks in film state are located at 300 and 344 nm, and tailing to
387 nm, corresponding to an optical band gap (Eop) of 3.2 eV.
Due to the twisted structure of spirobifluorene, 27-TPSF is
non-coplanar structure which restrains from the formation
of π–π stacking. Therefore, comparing the absorption band of
27-TPSF in solution with film state, there is no obvious shift
of absorption peak. 27-TPSF exhibited deep blue emission
in CHCl3 with photoluminescence peak located at 395 nm,
and the bathochromic shift of the emission peak obtained in
neat film is 13 nm, presumably due to the weak intermolec-
ular interaction (as shown in Figure S1, Supporting Informa-
tion). The phosphorescence spectrum was obtained in frozen
2-methyl-tetrahydrofuran (10−5 m) at 77K (Figure 1a). The first
phosphorescence peak of 27-TPSF was located at 504 nm, thus
the transition energy of T1–S0 was estimated to be ≈2.5 eV,
which is higher than that of the emitter of Ir(mppy)3) (T1
=
2.4 eV), and much higher than that of commercialized ETM of
DPPyA (1.8 eV).[4b]
2.3. Theoretical Calculations and Electrochemical Properties
2. Results and Discussion
2.1. Synthesis and Characterization
Theoretical calculations were carried out by using Gaussian
09 software with 6-311+G(d,p)/6-31G(d) basis set. As shown
in Figure 2a, its highest occupied molecular orbital (HOMO)
is mainly located at the electron-rich spirobifluorene moiety.
27-TPSF was synthesized as shown in Scheme 1. TPY was
linked to spirobifluorene through Suzuki coupling reaction
Scheme 1. Synthesis of the 27-TPSF.
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Figure 1. a) Absorption, fluorescence, and phosphorescence spectra of 27-TPSF. b) The CV curves for 27-TPSF at a scan rate of 100 mV s−1 (inset:
multiple CV of 27-TPSF in dilute DMF with 20 cycles).
However, the lowest unoccupied molecular orbital (LUMO)
is distributed over the whole molecule, leading to high elec-
tron mobility. The optimized structure of 2,2′:6′,2″-terpyridine
exhibits a nearly planar configuration due to the intramolecular
H-bonds interaction (as shown in Figure S1, Supporting Infor-
mation).[21] The HOMO and LUMO energy levels for 27-TPSF
were calculated at −5.9 and −2.1 eV, respectively. In order to
investigate the charge injection ability, the electron density of
27-TPSF from the total self-consistent field density were per-
formed by using the electrostatic potential (ESP) method by
Singh and Kollman.[22] As illustrated in Figure 2b, the highly
electronegative regions around nitrogen atoms suggest poten-
tial interaction between nitrogen and lithium, and the coordi-
nation between nitrogen atom in the heteroaromatic ring with
metals efficiently enhances electron injection (EI), as suggested
by Adachi et al.[23] The spin density distribution (SDD) is the
difference between α and β spin contributions to the total elec-
tron density. The SDD of the T1 state of 27-TPSF (Figure 2c)
shows that the T1 state is thoroughly localized on the spirobiflu-
orene and the core pyridine, surrounded by the side pyridines
of the molecules.[24] The quenching of the excitons occurred
through the Dexter transfer mechanism can be further hin-
dered by the peripheral pyridines.[6a]
Nevertheless, the HOMO energy level of 27-TPSF in film
measured by Riken AC-2 was −5.9 eV. There is a little differ-
ence between the results of photoelectron spectroscopy and
CV measurements, because they were measured in solid state
and solution, respectively. Electrochemical stability of charge
transporting material is prerequisite for ensuring that OLEDs
remain stable during operation. Multiple CV measurement in
dimethyl formamide (DMF) was performed that reversibility
and unchanged reduction potential were obtained in 20 cycles
(inset of Figure 1b), indicating the excellent stability of the
anion unities of 27-TPSF, which may be attributed to the
extended conjugation that reduces the charge density.[27] To
further confirm the above speculation, the optimized geom-
etries and the electronic properties of the anion (i.e., the neu-
tral 27-TPSF molecule in the presence of an extra electron)
were calculated. It is clear that the LUMO of the anion extend
over one fluorene group of spirobifluorene and bis-TPY, as
shown in Figure 2d, suggesting the molecular conformational
stability with the charge injection.[4b]
2.4. Thermal Properties
Cyclic voltammetry (CV) was adopted to obtain the rela-
tive energy levels of 27-TPSF in solution, as shown in Figure
1b. On the basis of the onset potentials for reduction and
oxidation, the HOMO and the LUMO energy level were esti-
mated with regard to ferrocene (4.8 eV below vacuum).[25] The
HOMO level of 27-TPSF is figured to be −6.2 eV by using
an empirical formula (EHOMO = – (Eox – EFc/Fc+) eV + (−4.8)
eV)[26] and the LUMO level is estimated to be −2.7 eV by using
(ELUMO = – (Ere – EFc/Fc+) eV + (−4.8) eV). The narrower Eop
is obtained by introducing TPY units instead of phenyl unit
(pyridine, pyrimidine) to the C2 position of spirofluorene,
mainly by pulling the LUMO energy levels deeper.[7,15a]
The thermal behavior of 27-TPSF was evaluated by means of
differential scanning calorimetry (DSC) and thermo gravi-
metric analysis (TGA) under nitrogen atmosphere. 27-TPSF
shows great thermal stability with the decomposition tempera-
ture (Td) of 468 °C (as shown in Figure S2, Supporting Infor-
mation) and Tg of 195 °C (as shown in Figure S3, Supporting
Information), much higher than DPPyA (Tg = 135 °C).[18] The
higher Tg is the consequence of the existence of spirobifluorene
which possesses a rigid and twisted structure, benefitting for
maintaining the morphological stability of amorphous film
under thermal evaporating at high temperature and contrib-
uting to the enhancement of the device lifetime.
Table 1. Physical properties of 27-TPSF.
Material
27TPSF
HOMOc)/ HOMOd) [eV]
LUMOc)/ LUMOd) (eV)
ET [eV]
λabsa) [nm]
λabsb) [nm]
λPLa) [nm]
λPLb) [nm]
Tg [°C]
254, 280, 297, 308, 344
300, 344
395
410
195
2.5
−5.9/−6.2
−2.7/−2.7
a)Measured in CHCl3; b)Measured in neat film; c)Measured using photoelectron spectrometer (Riken AC-2), LUMOc) = HOMOc) + Eg; d)Measured using CV.
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Figure 2. a) The HOMO and the LUMO distributions of 27-TPSF. b) The isosurface of 27-TPSF ESP. c) The distribution of the triplet density. d) LUMO
of the 27-TPSF anion.
2.5. Film Morphology
Device B: ITO/HAT-CN (2 nm)/NPB (40 nm)/TCTA (5 nm)/
DIC-TRZ: Ir(mppy)3 (7 wt%, 20 nm)/DPPyA (45 nm)/LiF/Al.
In Device A, HAT-CN (2 nm) was used as the hole-injecting
material, a 40 nm of NPB and 5 nm of TCTA as hole trans-
porting material, a 20 nm layer of emitting material (EM)
consisting of Ir(mppy)3 (7 wt%) as the phosphorescent
emitter, codeposited with the TADF host material DIC-TRZ,
and a 45 nm of 27-TPSF as ETM. For comparison, DPPyA
was adopted as the ETM to fabricate Device B. As shown in
Figure 5, the Von, the J and the ηPE, max of Device A is compa-
rable with those of Device B, revealing that the EI and ET prop-
erties of 27-TPSF (µe = 5.70 × 10−7cm2 V−1 s−1 at zero-electric
field, as shown in Figure S4, Supporting Information) are as
good as DPPyA. Moreover, 27-TPSF-based device achieved a max-
imum current efficiency (ηCE, max) of 84.7 cd A−1 and ηext, max of
24.5%, higher than those of DPPyA-based device with a ηCE, max
of 76.3 cd A−1 and ηext, max of 22.1% as shown in Table 2. The
obvious advantage of 27-TPSF can be attributed to the adequate
high T1 of 27-TPSF, much higher than DPPyA. For PhOLEDs,
good exciton confinement in the emitting layer is one of the
significant premises, otherwise, the excitons quench through
energy transfer from the high T1 of host or EM to the lower
energy ones of ETM. This energy transfer occurs through the
Dexter transfer mechanism which is a short-range process that
can be hindered by increasing the mutual molecule distances,
and which can be accomplished by introducing large side groups
to the molecule.[6a] Whereas, the SDD of the triplet state of
DPPyA was reported that the T1 state is almost delocalized on the
In order to observe the morphology of films of 27-TPSF and
its reference DPPyA, atomic force microscopy (AFM) was per-
formed in tapping mode. The surface roughness of 50 nm thick
films of DPPyA and 27-TPSF deposited on indium-tin-oxide
(ITO) exhibited a root-mean-square roughness (Rrms) of 8.75 and
1.39 nm, respectively (as shown in Figure 3). Obviously, the
film of DPPyA exhibited a much rougher surface which may be
caused by the strong intermolecular H-bonds interaction that
makes molecules packed densely. Completely planar moiety
generally has a strong propensity to form π-stack and easy to
crystalize. Combination of twisted rigid moiety spirobifluorene
and planar TPY moiety helps to achieve amorphous smooth
stable film which is the basis for preparing stable OLEDs.[28]
2.6. Device Performance
To investigate the potential of 27-TPSF as ETM, as demon-
strated in Figure 4, a series of devices was designed and fabri-
cated. The device structures are listed as follows:
Device A: ITO/dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-
hexacarbonitrile (HAT-CN) (2 nm)/4,4′-bis[N-(1-naphthyl)-N-
phenyl-amino]biphenyl (NPB) (40 nm)/tris(4-(9H-carbazol-9-yl)
phenyl)amine (TCTA) (5 nm)/2,4-diphenyl-6-bis(12-phenylin-
dolo)[2,3-a]carbazole-11-yl)-1,3,5-triazine (DIC-TRZ): Ir(mppy)3
(7 wt.%, 20 nm)/27-TPSF (45 nm)/LiF/Al.
Figure 3. 3D topography images and Rrms values of 50 nm thick films of a) DPPyA (Rrms = 8.75 nm), and b) 27-TPSF (Rrms = 1.39 nm).
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Figure 4. Device structures, chemical structures, and energy diagram of materials.
whole molecule. Therefore, the quenching of excitons cannot be
avoided, influencing the acquisition of high performance devices.
27-TPSF has adequate high T1 and its peripheral pyridines can
further hinder the quenching of the triplet excitons. To verify
the triplet exciton confinement ability of the ETM, the transient
photoluminescence was measured and all the transient decay
curves were observed at 519 nm (Figure S5, Supporting Informa-
tion). It was observed that the exciton lifetimes of the guest in
the films are in the order of DIC-TRZ: Ir(mppy)3 (7 wt%, 30 nm)
(1.2 µs) ≈ DIC-TRZ: Ir(mppy)3 (7 wt%, 30 nm)/27-TPSF (45 nm)
(1.2 µs) > DIC-TRZ: Ir(mppy)3 (7 wt%, 30 nm)/DPPyA (45 nm)
(0.8 µs), demonstrating the relatively good exciton confinement
of 27-TPSF compared with that of DPPyA.[6a] In addition, it is
worthy to note that 27-TPSF-based device also showed excellent
efficiency stability, with small roll-off on the ηCE and ηext with
increasing brightness up to 10 000 cd m−2. The device showed
a high ηCE of 84.7 cd A−1 (or 77.0 cd A−1) and a ηext of 24.2% (or
22.5%) under 1000 cd m−2 (or 10 000 cd m−2).
4948 h at 1000 cd m−2 (246 658 h at 100 cd m−2) for Device A
and Device B, respectively.
The better operational stability of 27-TPSF-based device can
be attributed to a combination of several reasons. First, the
pretty low T1 state of DPPyA can induce exciton quenching and
further lead to the degradation of the device. 27-TPSF possesses
sufficiently high T1 to avoid exciton quenching, averting effi-
ciency roll-off. Second, the flatness of the organic thin layer is
important for good contact between the organic layer and the
cathode or anode, which is a key element for realizing the long-
term stability of the electroluminescent diode.[30] Analyzing
through AFM, the evaporated 27-TPSF film is amorphous and
much smoother than DPPyA. We can reasonably speculate that
the contact of the 27-TPSF film with the cathode is better, and
the rough surface morphology of DPPyA can cause the traps to
accumulate at or near electron-transporting layer/cathode inter-
face. To fill those traps, immobile charge density will increase,
further causing a significant rise in voltage, which suggests that
the device will degrade rapidly, as shown in Figure 5f.[31] Third,
another parameter that influences device lifetime is the thermal
stability of the materials. If the device operating heat or other
heat treatments exceed the specific Tg of the materials, interdif-
fusion between the organic layers as well as crystallization of
the amorphous materials may occur.[32] The absolutely higher
Tg of 27-TPSF than that of DPPyA ensures the attainment of
the amorphous stable film to achieve longer operational life-
time of the device.
At an initial luminance of 10 000 cd m−2, a lifetime T50 of
121 h was realized with 27-TPSF, which is higher than 78 h
achieved with DPPyA. For display applications, the device life-
time of green emitters is usually measured at an initial lumi-
nance of 1000 cd m−2. By adopting a degradation acceleration
factor that relates luminance to lifetime,[29] viz
10 000 cd m–2 n
T50 (1000 cd m–2 ) = T50 (10 000 cd m–2 ) ×
(1)
1000 cd m–2




As we know, the high horizontal dipole ratio combined with
high photoluminescence efficiency of Ir(ppy)2tmd is expected
to give a high ηext in OLED.[1b] Therefore, in order to acquire
With n = 1.75, the speculated lifetimes are 6804 h at an ini-
tial luminance of 1000 cd m−2 (382 636 h at 100 cd m−2) and
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Figure 5. a) Current density–voltage (J–V), b) power efficiency–current density (ηPE–J), c) current efficiency–current density (ηCE–J), and d) external
quantum efficiency–current density (ηext–J) curves of Device A and Device B. e) The lifetimes of the devices measured at an initial luminance of
1000 cd m−2. f) The operating voltage–time of the devices measured at an initial luminance of 10 000 cd m−2
.
high efficiency, we adopted Ir(ppy)2tmd as the emitter and fab-
ricated the following OLEDs:
Device C: ITO/HAT-CN (2 nm)/1,1-bis[4-[N,N-di(p-tolyl)-
amino]phenyl]-cyclohexane (TAPC) (60 nm)/TCTA (5 nm)/DIC-
TRZ: Ir(ppy)2tmd (7 wt%, 20 nm)/27-TPSF (45 nm)/LiF/Al.
Device D: ITO/HAT-CN (2 nm)/TAPC (60 nm)/TCTA
(5 nm)/DIC-TRZ: Ir(ppy)2tmd (7 wt%, 20 nm)/DPPyA (45 nm)/
LiF/Al.
The performance of Device C and Device D is pre-
sented in Figure 6 and Table 2. A ηCE, max of 107.0 cd A−1
and ηext, max of 28.2% were achieved in Device C, with ηCE of
103.9 and 96.9 cd A−1, ηext of 27.4% and 25.6% at brightness
of 1000 cd m−2 and 10 000 cd m−2 remaining, respectively,
showing low efficiency roll-off, which also behaves better than
the DPPyA-based device (Device D).
Table 2. List of device performances.
a)
b)
b)
b)
Von
CIEc) (x, y)
ηCE, max/1000/10 000 ηPE, max/1000/10 000 ηext, max/1000/10 000
[cd A−1 [lm W−1
[%]
[V]
2.4
2.4
]
]
A
B
C
D
84.7/84.7/77.0
76.3/72.0/59.2
94.7/82.0/52.6
93.2/66.4/34.3
129.3/65.3/38.1
128.9/59.1/34.1
24.5/24.2/22.5 (0.30, 0.62)
22.1/21.0/17.2 (0.30, 0.63)
28.2/27.4/25.6 (0.31, 0.65)
24.9/24.8/23.0 (0.31, 0.65)
3. Conclusions
2.3 107.0/103.9/96.9
2.3 94.4/94.1/86.9
In summary, we have synthesized a novel ETM with a “(A)n–D–
(A)n” structure by introducing rigid twisted spirobifluorene to
electron-withdrawing TPY moieties, which ensure high T1
a)At 1 cd m−2 b)Maximum, at 1000 cd m−2, at 10 000 cd m−2 c)At 100 cd m−2
; ; .
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Figure 6. a) Current density–voltage (J–V), b) power efficiency–current density (ηPE–J), c) current efficiency–current density (ηCE–J), and d) external
quantum efficiency–current density (ηext–J) curves of Device C and Device D.
carried out on a Perkin-Elmer Lambda 35 spectrometer. Emission spectra
were recorded using Hitachi F7000 fluorescence spectrophotometer. In
a nitrogen-purged dichloromethane (CH2Cl2) or DMF solution at room
temperature, CV was measured using a CHI600C voltammetric analyzer
of 2.5 eV and Tg of 195 °C. Moreover, multiple cyclic voltam-
metry measurement indicates the excellent stability of the anion
unities of 27-TPSF. Its ET/EI properties and operational sta-
bility were investigated via multilayered TADF-PhOLED using
with tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 m) as the
Ir(mppy)3 as the emitter doped into the TADF host of DIC-TRZ.
The device that incorporates 27-TPSF exhibited a Von of 2.4 V,
and a ηext, max of 24.5% which is better than the commercial-
ized ETM of the DPPyA-based reference device, indicating that
27-TPSF provides superior ET/EI properties. Under constant
current operation at an initial luminescence of 10 000 cd m−2,
the T50 of 121 h is realized with 27-TPSF, corresponding
to 6804 h at 1000 cd m−2 and 382 636 h at 100 cd m−2 which
is longer than the DPPyA-based device. Furthermore, higher
efficiency can be achieved by adopting higher horizontal dipole
ratio Ir(ppy)2tmd as the emitter with 27-TPSF as the ETM, with
a ηext, max of 28.2% and a ηPE, max of 129.3 lm W−1. The state-
of-the-art device performances greatly manifest the potential of
such ETM design stragtegy and pave the way for TADF-PhOLED
toward practical application. These results indicate that the inte-
gration of TPY and spirobifluorene to form “(A)n–D–(A)n” struc-
ture is a promising way to design an ideal ETM.
supporting electrolyte and ferrocene as an external standard. A platinum
working electrode, a platinum wire auxiliary electrode, and a Ag/AgCl wire
pseudo reference electrode make up the conventional three electrodes. By
using atmospheric ultraviolet photoelectron spectroscopy (Rikken Keiki
AC-2), HOMO level in film state was estimated. The current density J,
voltage V, and luminance L characteristics of OLEDs were acquired using
absolute ηext measurement system (C9920-12, Hamamatsu Photonics)
with the photonic multichannel analyzer (PMA-12, Hamamatsu
Photonics) as well as a computer controlled Keithley 2400 Sourcemeter
unit with a calibrated silicon diode.
Materials: HAT-CN, TAPC, NPB, TCTA, DIC-TRZ, Ir(ppy)2tmd,
Ir(mppy)3, and DPPyA were commercially available and used without
further purification.
Synthesis and Characterization:Synthesis of 2,7-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9,9′-spirobifluorene(1):
2,7-dibromo-9,9′-
spirobifluorene (30.0 mmol), bis(pinacolato)diboron (90.0 mmol) and
KOAc (120.0 mmol) were mixed in a flask containing nitrogen-saturated
1,4-dioxane (300 mL) before Pd(PPh3)4 (0.3 mmol) was added. The
resulting mixture was heated to 95 °C and reacted for 5.0 h keeping
the temperature constant. Then 200 mL cold water was added and the
solvent was eluted to give a gray solid mixture after cooling to room
temperature. The residue was purified by column chromatography using
petroleum ether as the eluent, and the isolate was recrystallized from
THF:petroleum ether (1:5) to give white pure solid. Yield: 53% (9.0 g).
Synthesis of 27-TPSF: Compund (1) (2.20 mmol), 4′-bromo-2,2′:6′,2″-
terpyridine (4.80 mmol), K2CO3 (8.80 mmol), and 18-Crown-6 (0.08 mmol)
were mixed in a flask containing nitrogen-saturated toluene and water
before Pd(PPh3)4 (0.13 mmol) was added. Then the solution was heated
to 95 °C and reacted for 36 h keeping the temperature constant. After
cooling to room temperature, the resulting mixture was filtered and
washed with water to neutral. The separated organic phase was dried with
4. Experimental Section
General: The 1H NMR spectra were measured on Varian 300 MHz,
Bruker 400 MHz NMR spectrometer. ESI-MS in positive or negative ion
mode was recorded on Bruker Apex IV Fourier transform ion cyclotron
resonance mass spectrometer. Elemental analyses were performed on a
VARIO elemental analyzer from Elementar Analysensysteme GmbH. Under
nitrogen atmosphere at a heating rate of 10 °C min−1, DSC was undertaken
on a Perkin-Elmer Diamond DSC Pyris instrument while TGA was taken on
a SEIKO EXSTAR 6000 TG/DTA 6200 unit. UV–vis absorption spectra were
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anhydrous Na2SO4 and evaporated to give the red crude product. The
crude product was purified by neutral alumina column chromatography
with dichloroethane (C2H4Cl2) /petroleum ether (1/3, v/v) as the eluent,
and the isolate was recrystallized from C2H4Cl2:petroleum ether (3:1) to
give white pure solid (2). Yield: 43% (738 mg). mp: 388 °C; 1H NMR
(500 MHz, CDCl3): δ 8.68 (d, J = 4.1 Hz, 1H), 8.60 (d, J = 7.9 Hz, 1H), 8.53
(s, 1H), 8.05 (d, J = 8.0 Hz, 1H), 7.97 (dd, J = 8.0, 1.4 Hz, 1H), 7.90 (d, J =
7.6 Hz, 1H), 7.85 (td, J = 7.8, 1.6 Hz, 1H), 7.40 (t, J = 7.2 Hz, 1H), 7.33
(dd, J = 6.9, 5.3 Hz, 1H), 7.22 (d, J = 1.1 Hz, 1H), 7.15 (t, J = 7.5 Hz, 1H),
6.82 (d, J = 7.6 Hz, 1H); 13C NMR (CDCl3): δC = 156.22, 155.78, 150.37,
148.98, 148.02, 142.02, 138.72, 136.77, 128.07, 127.96, 127.62, 124.13,
123.68, 122.81, 121.38, 120.73, 120.41, 118.91, 77.25, 77.00, 76.74, 66.27;
13C DEPT NMR (THF-d8) δ 149.04 (+ ve), 148.94 (+ ve), 136.42 (+ ve),
127.91 (+ ve), 127.45 (+ ve), 123.91 (+ ve), 123.63 (+ ve), 122.27 (+ ve),
121.11 (+ ve), 120.73 (+ ve), 120.64 (+ ve), 120.23 (+ ve), 118.41 (+ ve),
118.30 (+ ve), 66.97 (+ ve), 66.79 (+ ve), 66.62 (+ ve), 24.84 (+ ve), 24.69
(+ ve), 24.53 (+ ve); HRMS (ESI, m/z): [M + H]+, calcd for C55H34N6, m/z
= 778.28450, found m/z = 778.29046; Anal. Calc. for C55H34N6: C, 84.81;
H, 4.40; N, 10.79. Found: C, 84.87; H, 4.49; N, 10.80.
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Keywords
electron-transporting, long lifetime, organic light-emitting diodes,
phosphorescence, terpyridine, thermally activated delayed fluorescence
Received: January 17, 2018
Revised: March 14, 2018
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