Non-doped organic light-emitting diodes based on phenanthroimidazole-triphenylamine derivatives with a low efficiency roll-off of 9% at a high luminance of 10?000 cd m-2

Article abstract of DOI:10.1039/d0tc03379k

Simultaneously achieving high exciton utilization efficiency (ηS) and a low efficiency roll-off at high brightness is challenging for fluorescent organic light-emitting diodes (OLEDs). In this work, we tried to realize this goal by constructing "hot exciton"materials by fine-tuning the lowest triplet excited state (T1) features. Two phenanthroimidazole-triphenylamine based donor-acceptor (D-A) derivatives TPPI-AQ and TPPI-BZPCN have been designed and synthesized. The photophysical studies and theoretical calculations reveal that TPPI-BZPCN has an obvious "hot exciton"feature and an aggregation-induced emission (AIE) characteristic. A big gap between T1 and T2 exists in TPPI-BZPCN which provides the possibility for a reverse intersystem system crossing (RISC) process from the upper energy level of T2 to S1. As a result, the non-doped electroluminescent device based on TPPI-BZPCN achieves a maximum external quantum efficiency (EQE) of 3.33%. The electroluminescence (EL) spectrum exhibits an emission peak at 592 nm, and the brightness can be up to 55?657 cd m-2. Also, the device is able to retain an EQE of 3.03% at the high luminance of 10?000 cd m-2, with a very low efficiency roll-off of 9%, which is the lowest efficiency roll-off currently reported in the orange-red emitters based on the "hot exciton"mechanism. Moreover, the ηS of the non-doped device reaches 48%, which exceeds the limitation of 25% in conventional fluorescence OLEDs.

Full text of DOI:10.1039/d0tc03379k

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Nosang V. Myung, Yong-Ho Choa et al.
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ARTICLE
Non-doped
organic
light-emitting
diodes
based
on
phenanthroimidazole-triphenylamine derivatives with a low
efficiency roll-off of 9% at a high luminance of 10000 cd m−2
Received 00th January 20xx,
Accepted 00th January 20xx
Chunya Du, Futong Liu, Hui Liu, Xin He, Dongyan Jiang, Zijun Feng, Lei Gao, and Ping Lu*
DOI: 10.1039/x0xx00000x
Simultaneously achieving high exciton utilization efficiency (ηS) and low efficiency roll-off at high brightness is challenging
for fluorescent organic light-emitting diodes (OLEDs). In this work, we tried to realize this goal by constructing “hot exciton”
materials via fine-tuning the lowest triplet excited state (T1) characters. Two phenanthroimidazole-triphenylamine based
donor-acceptor (D-A) derivatives TPPI-AQ and TPPI-BZPCN have been designed and synthesized. The photophysical studies
and theoretical calculations reveal that TPPI-BZPCN has an obvious “hot exciton” feature and aggregation-induced emission
(AIE) characteristic. A big gap between T1 and T2 exists in TPPI-BZPCN which provides the possibility for reverse intersystem
system crossing (RISC) process from upper energy level of T2 to S1. As a result, the non-doped electroluminescent device
based on TPPI-BZPCN achieves a maximum external quantum efficiency (EQE) of 3.33%. The electroluminescence (EL)
spectrum exhibits an emission peak at 592 nm, and the brightness can be up to 55657 cd m-2. Also, the device is able to
remain EQE of 3.03% at the high luminance of 10000 cd m-2, with a very low efficiency roll-off of 9%, which is the lowest
efficiency roll-off currently reported in the orange-red emitters based on “hot exciton” mechanism. Moreover, the ηS of the
non-doped device reaches 48%, which exceeds the limitation of 25% in conventional fluorescence OLEDs.
According to the energy gap law, a narrow bandgap
between the lowest singlet excited state (S1) and the ground
state (S0) will result in a severe nonradiative internal conversion
Introduction
Organic light-emitting diodes (OLEDs) have attracted great
attentions and considered as the next generation flat-panel
displays and white lighting technologies owing to their unique
features such as flexibility, lightweight, superior color gamut, no
turn-on delay and wide viewing angle.1-6 Tremendous efforts
have been made to accelerate the development and
commercialization of OLEDs. How to maximize the exciton
utilization efficiency (ηS) is the crucial issue to improve the
performance of OLEDs.7,8 For this purpose, phosphoresce-
ence,9-13 thermally activated delayed fluorescence (TADF),1,14-16
triplet-triplet annihilation (TTA),17-19 “hot exciton”20 and
radicals21-23 represent the main approaches to make use of
triplet excitons. Among them, TADF and “hot exciton” materials,
which are metal-free, relatively easy to synthesize and able to
obtain 100% internal quantum efficiency theoretically, have
been rapidly developed in recent years. Materials with
relatively wide bandgaps such as blue and green emitters based
on these two mechanisms are well-developed and some
materials have been reported to show excellent performance in
OLED applications.24-28 However, materials with narrower
bandgaps, including orange and red emitters with good OLED
performance still remain challenging.
(IC), which inevitably leads to lower photoluminescent quantum
yield (PLQY).29-31 Besides, the character of lowest triplet excited
state (T1) has a crucial influence on the property of the resulting
materials. A donor-acceptor design strategy is commonly used
to achieve emitter with narrow bandgap. In general, the T1
value of the whole molecule is determined by the lowest T1 of
respective building blocks. In fact, in the design of TADF
materials, the constructing unit with low T1 energy of localized
state (3LE) should be avoided because the 3LE tends to weaken
or even quench the delayed fluorescence due to the spin
forbidden transition and big energy difference when 3LE is
converted to S1 of charge transfer state (1CT).32 In this case,
some traditional electron-withdrawing units with low T1 values
are not suitable for construction of TADF materials but may
show potential applications in the design of “hot exciton”
materials since the reverse intersystem system crossing (RISC)
process occurs in the upper level and T1 value shows almost no
effect on the harvesting of triplet excitons. Also, as compared
with TADF materials which suffer from the low PLQYs due to the
separated highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energy levels and
rapid efficiency roll-offs at high brightness (>1000 cd m-2)14,33-36
on account of the accumulation of triplet excitons, materials
based on “hot exciton” possess relatively high PLQYs, short
fluorescent lifetimes, and usually show ideal efficiency roll-offs
in OLEDs.20,37-40 The “hot exciton” material with narrower
bandgap needs further investigation.41-43
a. State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Qianjin Street No. 2699, Changchun, 130012, P. R. China. E-mail: lup@jlu.edu.cn.
†Electronic
Supplementary
Information
(ESI)
available:
See
DOI:10.1039/x0xx00000x
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In this work, we tried to obtain “hot exciton” materials by
Journal Name
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DOI: 10.1039/D0TC03379K
fine-tuning the T1 characters of electron acceptors.
Benzothiadiazole (BZP) is a five-membered heterocyclic ring
that contains two sp2 non-equivalent hybridized nitrogen atoms
and one sp equivalent hybridized sulfur atom. BZP is a classical
electron-withdrawing group with a T1 lower than 2.0 eV.
Moreover, BZP possesses a high PLQY and a large energy gap
between T1 and T2 (0.9 eV), which will facilitate the realization
of the “hot exciton” feature. Here, the cyano group, which
possesses a higher T1 than BZP, is adopted to connect with BZP
to form a stronger electron acceptor, cyano-substituted BZP
(BZPCN), aiming at extending the emission wavelength of the
resulting molecule. In our prior studies, phenanthroimidazole
(PI) is found to have decent PLQY and excellent thermal stability
owing to the rigid planar π conjugation.19 The intrinsically
bipolar electronic structure of PI is beneficial to achieve
balanced charge transport and high performance in OLEDs.44,45
According to previous works, connecting acceptor at the C2
position and donor at the N1 position of PI core will be benefit
for achieving high-performance PI-based bipolar emitters.46
Herein, we further introduce triphenylamine (TPA) at the N1
position of PI unit. Thus, TPA and PI are combined to form a
whole stronger new donor (TPPI). The propeller-shaped
structure of TPA can sterically inhibit the π-π molecular stacking
and avoid aggregation induced quenching (ACQ) in the solid
state47 as well as partially increase the solubility of molecules
for lowering the difficulty of purification. TPPI-BZPCN is thus
designed with the expectation to be a “hot exciton” material
with low T1 value. Besides, another acceptor anthraquinone
(AQ), which has better electron-withdrawing ability deriving
from the existence of two carbonyl groups but possesses a
higher T1 of 2.5 eV, is also selected as constructing unit to
replace BZPCN group to get TPPI-AQ for comparison purpose.
As a result, experimental investigations demonstrate that TPPI-
BZPCN owns obvious “hot exciton” state feature and
aggregation-induced emission (AIE) characteristic. Theoretical
calculations indicate a low T1 in TPPI-BZPCN which facilitate to
enlarge the energy gap between the T1 and T2. The “hot exciton”
character is not observed in TPPI-AQ due to the higher T1 energy
and resulting much more IC processes among T1 and Tn.
Scheme. 1 Synthesis routes for TPPI-BZPCN and TPPI-AQ.
(a) Suzuki coupling: Pd (PPh3)4, K2CO3 (2 M aq), toluene, 90 °C, 36 h under N2
atmosphere; (b) CsF, DMSO, 140 °C, N2 protection, 6 h; (c) SnCl2·2H2O, CH3CH2OH, N2
protection, 70 °C, 100 min; (d) CH3COONH4, acetic acid, 120 °C, N2 protection, 2 h; (e)
bis(pinacolato)diboron, Pd (dppf) Cl2, KOAc, 1,4-dioxane, 90 °C, N2 protection, 48 h.
refluxed in acetic acid. Then, TPPIB was accessible through
reaction between bis(pinacolato)diboron and TPPIBr. Finally,
TPPI-BZPCN was successfully synthesized by using BZPCNBr and
TPPIB as reactants via palladium-catalyzed Suzuki coupling.
Similarly, TPPI-AQ was obtained in the same way by employing
TPPIB and 2-bromoanthracene-9,10-dione as starting materials.
The molecular structures of TPPI-BZPCN and TPPI-AQ were
characterized by mass spectrometry spectroscopy (MS), nuclear
magnetic resonance (NMR) and elemental analysis, which are
corresponded well with their predicted results.
Thermal properties
The thermal properties of TPPI-BZPCN and TPPI-AQ were
investigated by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC). For TPPI-BZPCN and
TPPI-AQ, the thermal degradation temperatures (Td) are
The non-doped device based on TPPI-BZPCN shows a high
ηS of 48% and a high brightness of 55657 cd m-2. Moreover, the
non-doped device achieves a maximum external quantum
efficiency (EQE) of 3.33% at 1000 cd m−2, and remains EQE of
3.03% at the high luminance of 10000 cd m−2, with a very low
efficiency roll-off of 9%. These results suggesting the non-doped
device keeps relatively high efficiency as well as a low efficiency
roll-off at high brightness.
Results and Discussion
Synthesis and Characterization
The molecular structures and synthesis routes of TPPI-BZPCN
and TPPI-AQ are shown in Scheme 1. TPPIBr was first prepared
by the one-pot reaction,48 in which TPA-NH2, 4-
bromobenzaldehyde and phenanthrene-9,10-dione were
Fig. 1 (a) TGA and (b) DSC curves of TPPI-BZPCN and TPPI-AQ.
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estimated to be 508 and 473 °C (Fig 1a). Such high values of Td
indicate that TPPI-BZPCN and TPPI-AQ are suitable for
fabricating OLEDs through evaporation methods and tolerant
for joule heat generated during the preparation process.
Additionally, only two slight endothermic signals were observed
for TPPI-BZPCN and TPPI-AQ during the second heating scan (Fig
1b), which should be ascribed to glass transition temperatures
(Tg) at 146 and 147 °C of TPPI-BZPCN and TPPI-AQ, respectively.
However, no apparent crystallization temperatures (Tc) or
melting points (Tm) were found during the increasing process of
two components, indicating that TPPI-BZPCN and TPPI-AQ both
show excellent morphological stability, which is in favour of
OLED applications.
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DOI: 10.1039/D0TC03379K
Fig.
2
(a) Molecular structures of TPPI-BZPCN and TPPI-AQ and their optimized
geometries of ground state S0 and excited state S1. (b) HOMO and LUMO of TPPI-BZPCN
and TPPI-AQ at the geometry of S0 state.
coexistence of LE and charge-transfer (CT) components.
Meanwhile, the T1 state of TPPI-BZPCN is LE state with a low
3
Theoretical Calculations
energy value of 1.74 eV, which fits our original vision well.
Thanks to the low T1 energy level, there is a significant
energy gap (0.98 eV) between T2 (2.72 eV) and T1 (1.74 eV), as
well as a negligible energy gap (0.03 eV) between T2 (2.72 eV)
and S1 (2.69 eV). Thus, it provides the possibility to open up the
RISC process from T2 to S1 and the IC from T2 to T1 may be
effectively inhibited. These characters might endow TPPI-
BZPCN with high PLQY and high ηs. As compared, the situation
in TPPI-AQ is quite different from TPPI-BZPCN. As shown in Fig.
3b, the S2 and T4 states of TPPI-AQ manifest the coexistence of
LE and CT components. However, the energy gap of T4 (2.86 eV,
Table S2) and T3 (2.75eV) is 0.11 eV, and a relatively small
energy gap (0.06 eV) is observed between T1 (2.56 eV) and T2
(2.62 eV), which is conducive to the sequential IC process from
T4 to T1. Eventually, the RISC process in the upper level states is
greatly reduced, and plenty of triplet excitons would be lost
through the non-radiative transition from T1 to S0. On the
whole, compared with TPPI-AQ, TPPI-BZPCN can be expected to
achieve both high PLQY and high ηs simultaneously, and further
to maximize EQE in OLEDs. Therefore, a low-lying energy of T1
as well as a large energy gap between T1 and T2 are essential for
constructing “hot exciton” materials with high exciton
utilization.
The S0 and S1 geometries for TPPI-BZPCN and TPPI-AQ were
optimized by density functional theory (DFT) and time-
dependent density functional theory (TDDFT) methods at the
basis set level of B3LYP/6-31G (d, p) to comprehend the relation
between the molecular structure and the emission behavior of
two materials. Their optimized geometry and structure
parameters are shown in Fig. 2a. For ground state, the twist
angles θ1 between TPPI and phenyl bridge are 29° and 25° for
TPPI-BZPCN and TPPI-AQ, and torsion angles θ2 between
bridging phenyl ring and two acceptors are both 35° for TPPI-
BZPCN and TPPI-AQ. The two adjacent hydrogen atoms on the
benzene bridge and the acceptor (BZPCN unit or AQ unit) have
more robust space steric hindrance, resulting in a larger torsion
angle θ2 comparing to θ1. For the excited state of the two
molecules, twist angles θ1 and θ2 change very little. As a result,
the appropriate twist angles contribute to the increase of
conjugated length and the generation of long wavelength
emission. Also, the HOMO and LUMO orbits are mostly located
on the donor and acceptor groups, respectively, and partially
overlapped on the phenyl bridge (Fig. 2b), which will be
beneficial to enhance the locally excited (LE) proportion and
further improve the PLQY.
Then, we calculated natural transition orbital (NTO) to
analyze electron transition characters for the emission of TPPI-
BZPCN and TPPI-AQ. As shown in Fig. 3a, the particle for the S1
state of TPPI-BZPCN is mainly dispersed on the acceptor BZPCN,
but the hole is distributed over the long axis of TPPI-BZPCN
which contains BZPCN and PI group, demonstrating the
Photophysical Properties
Also, the ultraviolet-visible (UV-vis) and photoluminescence
(PL) spectra of TPPI-BZPCN and TPPI-AQ were measured in
various solvents for further observation the ground state and
the excited state properties. As shown in Fig. S8, the absorption
Fig. 3 (a) The natural transition orbitals for S1, T1, and T2 and model for exciton relaxation process of TPPI-BZPCN. (b) The natural transition orbitals for S1, S2, T3 and T4, and model for
exciton relaxation process of TPPI-AQ.
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spectra of TPPI-BZPCN and TPPI-AQ are kept almost unchanged
with the increase of solvent polarity, indicating hardly variations
for dipole distance of the ground state in different solvents.
TPPI-BZPCN and TPPI-AQ have the same absorption peak at 363
nm, which arises from the π−π* transition of the PI moiety. For
TPPI-BZPCN, the absorption band around 420 nm can be
attributed to the intramolecular CT and the 307 nm absorption
peak comes from the π−π* transition of BZP moiety. As for TPPI-
AQ, the 311 nm absorption band is ascribed to the π−π*
transition of AQ, and the 410 nm absorption peak can be
attributed to the intramolecular CT. Besides, the absorption
onsets of TPPI-BZPCN and TPPI-AQ in THF correspond to 497
and 487 nm, and the optical gaps are estimated to be 2.49 and
2.10 eV, respectively. The emission spectra of TPPI-BZPCN and
TPPI-AQ are presented in Fig. 4. With the increase of solvent
polarity, the emission spectra of the two molecules are
gradually broadened as well as become structureless and red-
shifted, testifying the natural CT ingredient in their excited
states.
In order to get a better insight into the property of excited
state, the Lippert-Mataga relation was applied to plot the linear
fitting analysis of the Stokes shifts (νa-νf) and the orientation
polarizability (f). The slope of the line is corresponding to the
dipole moment (μe) of excited state.49 As shown in Fig. S11,
either TPPI-BZPCN and TPPI-AQ present a constant slope
suggesting that the feature of S1 hardly change; thus, both of
them possess a quasi-equivalent hybridized excited state with
evaluated dipole moments to be 18.9 D (r = 0.97) and 20.6 D (r
= 0.94), respectively. These dipole moment values between the
23 D (typical CT compound DMABN) and 8 D (typical LE
compound) are in consistent with the “hot exciton” character.50
Moreover, in the measurement of transient PL decay, they both
exhibit single-exponential lifetimes in nonpolar hexane solvent,
which are 3.80 ns for TPPI-BZPCN and 1.75 ns for TPPI-AQ,
respectively (Fig. S12). No long lifetime was observed in their
films under degassed condition (Fig. S13). These results suggest
these compounds are not emitters with delayed fluorescence.
The investigations of the AIE activity of TPPI-BZPCN and
TPPI-AQ were also implemented. As shown in Fig.5a and 5b, in
100% THF solution, TPPI-BZPCN exhibits an orange emission
peaking at 563 nm. Once the fw exceeds 0%, the emission
intensity decreases rapidly. The emission intensity remains
weakened until fw increased to 60%, accompanied by slightly
red-shifted emission peak. This can be interpreted by the effect
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DOI: 10.1039/D0TC03379K
Fig. 4 Emission spectra of TPPI-BZPCN (a) and TPPI-AQ (b) in different solvents.
of intermolecular CT. When fw is over 60%, the emission
intensity is increased sharply with a gradual blue-shift in
emission peaks owing to the AIE effect. These results attest that
TPPI-BZPCN possesses AIE feature.8,40,51 As shown in Fig. 5c and
5d, the PL intensity of TPPI- AQ remains low when the fw is less
than 80% in the THF/water mixture, but it increased rapidly
when the fw is up to 90%. The sudden increase of emission
intensity of TPPPI-AQ deriving from the strong aggregation
limits the non-radiative transition caused by vibration and
rotation, which is a commonly accepted mechanism of AIE
phenomenon.52-55
The PLQY of TPPI-BZPCN in the nonpolar solvent n-hexane
is 50%. However, no matter in nonpolar solvent or high polar
solvent, the PLQY of TPPI-AQ is quite low, generally about only
0.5% (Table S1). This may arise from the enhanced nonradiative
transition rate. The TPPI-BZPCN neat film shows an emission peak at
573 nm (Fig. S10), and the corresponding PLQY is 35%. However, the
Table 1 Key thermal and photophysical properties of TPPI-BZPCN and TPPI-AQ.
a
b
e
Compound
Td
Tg
HOMO/LUMO c
(eV)
PLQY d
(%)
Eg
(°C)
508
473
(°C)
146
147
(eV)
2.49
2.10
TPPI-BZPCN
TPPI-AQ
-5.63/-3.37
-5.64/-3.68
35.0
0.82
a Td is thermal-decomposition temperature at a weight percentage of 95%. b Tg is glass
transition temperatures. c HOMO and LUMO levels were calculated using the oxidation
Fig. 5 PL spectra of TPPI-BZPCN (a) and TPPI-AQ (c) (10×10−5 M) in THF/water mixtures
with different water fractions (fw). Plot of I1 (emission intensity)/I0 (emission intensity in
THF solution) of TPPI-BZPCN (b) and TPPI-AQ (d) versus water content of the solvent
mixture.
d
onset potentials and the reduction onset potentials, respectively. The absolute PL
quantum yield of neat film estimated by integrating sphere. e Optical gap calculated from
the absorption onset in dilute THF (10-5 M).
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PLQY of TPPI-AQ neat film is only 0.8% with an emission peak
located at 610 nm. Then we calculated the radiative rate (kr) and
nonradiative rate (knr) of neat film through the following
equations,
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kr = Φ
(1)
(2)
τ
1 ― Φ
knr
=
τ
where Φ stands for the PLQY, and τ denotes the transient
fluorescent lifetime. Taking TPPI-BZPCN as an example, in neat
film, the value of Φ is 35% (Table 1) and the τ is 10.17 ns (Fig.
S13). The kr value of TPPI-BZPCN is calculated to be 3.4 × 107 s−1,
and the knr is 6.4 × 107 s−1. In the same way, the kr and knr for
TPPI-AQ are 4.0 × 105 and 4.8 × 107 s−1, respectively. The data
comparison between the two compounds is shown in the Fig.
S14. For TPPI-BZPCN, the kr and knr are similar on the same order
of magnitude. In contrast, the knr of TPPI-AQ is two orders of
magnitude larger than kr, demonstrating that the exciton of S1
returns to S0 mainly by the nonradiative process. As a result, the
PLQY of TPPI-AQ is quite low no matter in solution or the neat
film, and it also accounts for the different AIE properties.
Detailed information is summarized in Table S1.
Fig. 6 Cyclic voltammograms of TPPI-BZPCN and TPPI-AQ.
2.8 V. The maximum EQE is obtained at high luminescence of
1000 cd m-2 of 3.33%, with only 9% roll-off even at high
luminance of 10000 cd m-2 (EQE = 3.03%). The efficiency roll-off
is negligible when comparing to most orange-red TADF
emitters, which suffer from serious efficiency roll-offs at high
luminance (>1000 cd m-2). Although the non-doped device of
TPPI-AQ exhibits red-shifted emission peaking at 632 nm in
contrast to TPPI- BZPCN. Nevertheless, the EQE of the non-
doped device of TPPI-AQ is only 0.48%, and the maximum
luminance is 1396 cd m-2. In addition, the device based on TPPI-
AQ showed a very high Von of 4.1 V. The PLQY of TPPI-AQ neat
film is only 0.82%, while the PLQY of TPPI-BZPCN neat film is
35%, nearly 42 times higher than TPPI-AQ, suggesting the
luminance of device based on TPPI-AQ needs higher voltage to
reach 1 cd m-2. Thus, the TPPI-AQ-based device showed very
high Von. The non-doped device based on TPPI-BZPCN is much
superior to TPPI-AQ in performance. This may be attributed to
the utilization of more triplet excitons for TPPI-BZPCN.
Electrochemical properties
Cyclic voltammetry (CV) measurements were performed to
estimate the HOMO and the LUMO levels of TPPI-BZPCN and
TPPI-AQ (Fig. 6). Nearly equal HOMO levels were found for TPPI-
BZPCN (-5.63 eV) and TPPI-AQ (-5.64 eV), which are originated
from their same donor group of TPPI. Differently, TPPI-AQ
exhibits a LUMO energy level of -3.68 eV, which is much lower
than that of TPPI-BZPCN (-3.37 eV). The shallower LUMO level
of TPPI-BZPCN is attributed to the weaker electron-withdrawing
ability of BZPCN relative to AQ. Moreover, the higher LUMO
energy level of TPPI-BZPCN compared with TPPI-AQ is closer to
the LUMO energy level of the classical electron transporting
layer material 3,3’-(5’-(3-(pyridin-3-yl)phenyl)-[1,1’:3’,1’’-
terphenyl]3,3’’-diyl)dipyridine (TmPyPb, -2.7 eV), which is
beneficial for electron transportation to the emitting layer and
improve the device efficiency.
Electroluminescence Properties
In order to study their electroluminescence (EL) performances,
two non-doped devices were fabricated with the configuration
of ITO/HATCN (6 nm)/TAPC (30 nm)/TCTA (10 nm)/EML (20
nm)/TmPyPb (40 nm)/LiF (1 nm)/Al (100 nm), in which indium
tin oxide (ITO) was anode, 1,4,5,8,9,11-hex-aazatriphenylene-
hexacarbonitrile
(HATCN)
and
di-(4-(N,N-
ditolylamino)phenyl)cyclohexane (TAPC) were employed as
hole injection and transportation layer, respectively. Tris(4-
carbazoyl-9-ylphenyl)amine (TCTA) was the exciton blocking
layer, LiF and TmPyPb corresponded to electron injection and
transportation layer, Al was applied as the cathode. TPPI-BZPCN
and TPPI-AQ were used as emitting layers (EML). The non-
doped device based on TPPI-BZPCN displays an EL spectrum
peaking at 592 nm with a maximum luminance of 55657 cd m-2
(Fig. 7b). It exhibits an extremely low turn-on voltage (Von) of
Fig. 7 (a) The energy level diagram and the used materials in the devices. (b) Normalized
EL spectra and external quantum efficiency versus luminance curves of non-doped
devices based on TPPI-BZPCN and TPPI-AQ. (c) Current density-voltage-luminance (J-V-
L) curves.
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Table 2 Key performance parameters of the EL performances of OLEDs based on TPPI-BZPCN and TPPI-AQ.
DOI: 10.1039/D0TC03379K
a
b
c
d
f
Device
Von
Lmax
CEmax
PEmax
EQE (%) e
λmax
CIE g
(cd m-2)
55657
(cd A-1)
7.6
(lm W-1)
4.28
max/1000/10000 cd m-2
3.33/3.33/3.03
(nm)
592
(x, y)
(0.53,0.46)
(V)
2.8
TPPI-BZPCN
TPPI-AQ
4.1
1396
0.51
0.36
0.48/0.08/ --
632
(0.47,0.52)
a Turn-on voltage at 1 cd m-2. b Maximum luminance. c Maximum current efficiency. d Maximum power efficiency. e Maximum external quantum efficiency/1000/10000 cd m-2. f EL
emission peak of EL spectrum at 100 cd m-2. g CIE coordinate of at 100 cd m-2.
The exciton utilization efficiency of the devices can be Science and Technology Department (20180201084GX) and the
evaluated according to the EQE equation of OLED as below.
Fundamental Research Funds for the Central Universities.
(
)
EQE = γ × ηS × ΦPL × ηout
(3)
Notes and references
γ
where is the recombination efficiency of injected electron and
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≈
Φ
PL
designed. ηout is light out-coupling efficiency ( 0.20), and
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device performances of non-doped EL devices are listed in Table
2 for TPPI-BZPCN and TPPI-AQ.
Conclusions
In conclusion, two new orange-red fluorescent emitters TPPI-
BZPCN and TPPI-AQ have been successfully developed by
incorporating PI and TPA to form a new donor and BZPCN/ AQ
separately as acceptor. Benefiting from contributions of “hot
exciton” in the upper level and AIE characteristic, the ηS value
of non-doped device based on TPPI-BZPCN reaches 48%.
Besides, the maximum EQE is 3.33%, which is acquired at the
luminance of 1000 cd m-2. Even at a high luminescence of 10000
cd m-2, the EQE can remain 3.03%, with only 9% reduction,
which is quite a small value of efficiency roll-off. Additionally,
the luminance can be up to a maximum of 55657 cd m-2. As far
as we know, this is one of the best non-doped devices among
orange-red materials in the mechanism of “hot excitons”. Our
experimental results may provide a new way for further
realizing high ηs and low efficiency roll-off at high luminescence
in terms of orange-red emitters, which based on “hot excitons”
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research is supported by the National Key R&D Program of
China (Grant No. 2016YFB0401001), National Natural Science
Foundation of China (91833304, 21774047), the Jilin Provincial
6 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry C
Page 8 of 8
The “hot exciton” material TPPI-BZPCN is successfully achieved by fine-tuning the T1 characters of
electron acceptor. The non-doped device based on TPPI-BZPCN exhibits a maximum EQE of 3.33% at the
View Article Online
−2
−2
DOI: 10.1039/D0TC03379K
luminance of 1000 cd m , and remains 3.03% at a high luminescence of 10000 cd m , with an only 9%
reduction. Additionally, the luminance can reach a maximum of 55657 cd m−2.