A simple D-π-A hybrid mode for highly efficient non-doped true blue OLEDs with CIEy < 0.05 and EQE up to 6%

Article abstract of DOI:10.1039/c8tc03777a

Although great advances have been achieved in organic electroluminescence fields, highly efficient true blue organic electroluminescence with small CIE coordinates and satisfactory color purity are still limited. Here, we practise a simple and effective m

Full text of DOI:10.1039/c8tc03777a

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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A simple D–π–A hybrid mode for highly efficient non-doped true
blue OLEDs with CIEy < 0.05 and EQE up to 6%
Received 00th January 20xx,
Accepted 00th January 20xx
Jinjin Shi,a Qi Ding,a Lei Xu,a Xianhao Lv,a Zhongwei Liu,a Qikun Sun,a Yuyu Pan,b Shanfeng Xue*a
and Wenjun Yang*a
DOI: 10.1039/x0xx00000x
Abstract text goes here. While great advances have been achieved in organic electroluminescence fields, highly efficient
true blue organic electroluminescence with small CIE coordinates and satisfactory color purity are still limited. Here, we
practise a simple and effective molecular avenue for true blue organic electroluminescence, and the strategy is to use the
biphenyl-bridging hybrids of 1,2,4-triazoles (acceptors) and various donors (D) as the emitters. The as-prepared D–π–A
hybrid (TPATZ) containing 2,3,4-triphenyl-1,2,4-triazole and triphenylamine emits bright blue light in both solution and
solid states, and the non-doped device with TPATZ as emitter exhibits true blue electroluminescence (EL) with the
emission peak at 430 nm and the Commission Internationale de L’Eclairage coordinate of (0.155, 0.047). Moreover, the
satisfactory EL color purity with the full width at half maximum (FWHM) of only 50 nm and the maximum external
quantum efficiency of 5.92% are obtained, which is among the best true blue OLEDs reported to date. This impressive EL
properties are experimentally and theoretically analyzed and emphasize that this simple D–π–A hybrid mode is an
www.rsc.org/
effective
molecular
strategy
for
highly
efficient
true
blue
EL
emitters.
external quantum efficiency (EQE) of 19.2% and CIE coordinate
of (0.148, 0.098)[11b] and by Chou et al. based on a 2-(styryl)-
triphenylene derivative with the maximum EQE of 10.2% and
CIE coordinate of (0.14, 0.14).[12b] Recently, a new method of
the combinational molecular design is promising for highly
efficient deep-blue emitters by Bian.[12c] However, highly
efficient non-doped true blue OLEDs with small CIE
coordinates (such as x ≤ 0.16, y ≤ 0.06), short peak emission
wavelength (such as λ ≤ 430 nm), and narrow full width at half
maximum (FWHM) are still limited and challenging. Such true
blue EL is not only the important member of the three primary
colors but also serves as the excitation light source and the
host matrix to generate other emissions.[16-20] However, the
true blue emission signifies the narrower band gap and smaller
molecular conjugation size of organic EL emitters, which
generally causes inferior charge transporting and also restrains
molecular design space.[20b] As well known, integrating
electron donor (D) and acceptor (A) units into a single
molecule is a common way for obtaining highly efficient EL
emitters, but the selection of D and A units and the linking
spacer (π-conjugation bridge) and position can play a crucial
role in determining the EL color and properties. For example,
some strongly twisted D–π–A motifs with short spacers (such
as phenyl) might form TADF emitters, but the strong
intramolecular charge transfer (ICT) effect has red-shifted EL to
greatly deviate from true blue emissions,[21-24] whereas the
incorporation of long and/or planar conjugation spacers
usually extends π-conjugation to move the EL out of the true
blue region.[25] Therefore, not any D and A combinations can
certainly optimize the highly efficient true blue EL emitters,
Introduction
Organic luminophores have persistently attracted research
attention due to their promising applications in light-emitting
diodes (LEDs), fluorescent sensors and switches, and nonlinear
optics.[1-6] Considering that the highly efficient phosphorescent
materials are mostly expensive Ir and Pt complexes and the
satisfactory deep-blue phosphorescent LEDs are not obtained,
organic fluorescent emitters are recently refocused because of
their unique advantages of low cost, structure diversity,
fantastic flexibility, appreciable stability, and good process-
ability.[7-10] Up to now, a large number of green, red, and light
blue organic emitters with excellent electroluminescence (EL)
performances have been developed by designing new principle
and concept materials with thermally activated delayed
fluorescence (TADF),[11] triplet–triplet annihilation (TTA),[12]
hybridized local and charge transfer (HLCT) excited state,[13]
triplet–polaron interaction (TPI) induced upconversion,[14] and
aggregation-induced emission natures.[15] The most impressive
blue OLEDs are the doped devices reported by Adachi et al.
based on carbazole−cyaphenine hybrid with the maximum
a.Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province
(QUST) School of Polymer Science & Engineering Qingdao University of Science &
Technology 53-Zhengzhou Road, Qingdao 266042, P. R. China. *E-mail:
sfxue@qust.edu.cn, ywjph2004@qust.edu.cn
b.School of Petrochemical Engineering Shenyang University of Technology 30
Guanghua Street, Liaoyang, 111003, P. R. China.
Electronic Supplementary Information (ESI) available: NMR and other
characterization. See DOI: 10.1039/x0xx00000x
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Journal Name
and these design conflicts have made the highly efficient true 980 spectrometer with an EPL-375 optical laser. The natural
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DOI: 10.1039/C8TC03777A
transition orbitals (NTOs) of S0 → S1 were calculated by using
blue EL still scarce and challenging.
the TD-M062X/6-31G(d, p) method based on the optimized
lowest singlet excited state configuration. Single-crystal X-ray
diffraction data were collected using a Rigaku RAXIS-PRID
diffractometer with graphite monochromator Mo Kα radiation.
The structure was solved with direct methods using the
SHELXTL programs and refined with ful-matrix least squares on
F2. Anisotropic thermal parameters were refined for all the
nonhydrogen atoms. All the hydrogen atoms of ligands were
generated geometrically. CCDC 1587403 containing the
crystallographic data can be obtained free from the Cambridge
Encouragingly, preliminary progresses have been achieved
recently. Tong et al. link the donors at the 6,9-positions of
phenanthroimidazole to form a V-type D–π–A hybrid and
fabricate a non-doped OLED with the maximum EQE of 7.20%
and CIE coordinate of (0.150, 0.063),[13e] and Li et al. synthesize
the branched triazole-based D–π–A hybrids and obtain non-
doped OLEDs with the maximum EQE of 6.3–6.8% and the CIE
coordinates of (0.158, 0.038–0.043).[14c] Very recently, we have
demonstrated that the simple 3,4,5-triphenyl-1,2,4-triazole–9-
phenylcarbazole hybrid (PCZTZ) can fabricate the non-doped
OLED emitting near-ultraviolet EL (408 nm) with the maximum
EQE of 6.6% and CIE coordinate of (0.17, 0.07).[26] Moreover,
PCZTZ is found to exhibit high and balanced hole and electron
mobilities up to 10−4 cm2 V−1 s−1. This signifies that triaryl-
triazoles are weak electron-withdrawing and good electron-
transporting units, and the distorted structure is favor of solid-
state fluorescence. In the current work, we further design and
facilely synthesize a new 1,2,4-triazole-based D–π–A hybrid
(TPATZ) with triphenylamine (TPA) as the donor. It is found
that the non-doped OLEDs still display true blue emission with
the CIE coordinate of (0.155, 0.047) and a narrow FWHM of 50
nm although TPA is a strong donor unit and can red shift the
emission. Moreover, the maximum EQE is up to 5.92% and
among the best true blue OLEDs reported to date. These
results further signify that constructing quasi-linear 1,2,4-
triazole-based D–π–A hybrids with biphenyl as the conjugation
bridge is an universal and effective avenue for highly efficient
true blue EL emitters.
Crystallographic
Data
Centre
via
www.ccdc.cam.
ac.uk/data_request/cif.
Synthesis of N'-benzoyl-4-bromobenzohydrazide (M1)
4-Bromobenzoic chloride (3.63 g, 16.5 mmol) was dissolved in
dichloromethane (25 mL) and added dropwise into a solution
of benzoyl hydrazine (2.25 g, 16.5 mmol) and triethylamine (3
mL) in dichloromethane (25 mL). The resulting mixture was
stirred for 4 h at room temperature and then washed with
water. The organic phase was evaporated to remove the
solvent, and the residue was dried under reduced pressure
and purified by recrystallization using absolute ethyl alcohol to
1
afford a white powder. (4.8 g, yield 91 %). H NMR (500 MHz,
CDCl3) δ 10.62 (s, 2H), 7.85 (d, J = 7.2 Hz, 2H), 7.75 (d, J = 7.4 Hz,
2H), 7.64 (d, J = 7.0 Hz, 4H), 7.40−7.52 (t, J = 7.2 Hz, 1H).
Synthesis of 3-(4-bromophenyl)-4, 5-diphenyl-1,2,4-triazole (M2)
Aniline (5.59 g, 60.0 mmol) and o-dichlorobenzene (60 mL)
were added into a 500 mL of two necked round-bottom flask
equipped with a reflux condenser. PCl3 (2.06 g, 15.0 mmol)
was slowly added dropwise to the flask through syringe under
nitrogen atmosphere. Subsequently, M1 (3.19 g, 10.0 mmol) in
o-dichlorobenzene (60 mL) was added by dropping funnel, The
resulting mixture was stirred under N2 at 150°C for 12 h and
then cooled to room temperature. Diethyl ether (200 mL) was
poured into mixture, and an orange precipitate was formed,
filtered, and washed with Et2O. The solid was dried under
reduced pressure and further purified by a silica gel column
chromatography using ethyl acetate/dichloromethane mixture
as the eluent. A white powder was obtain (2.7 g, yield 72 %).
1H NMR (500 MHz, CDCl3) δ 7.53–7.39 (m, 7H), 7.36 (t, J = 7.4
Hz, 1H), 7.33–7.27 (m, 4H), 7.15 (d, J = 7.2 Hz, 2H). 13C NMR
(126 MHz, CDCl3) δ 154.97, 153.82, 134.92, 131.72, 131.67,
130.28, 130.12, 130.08, 129.79, 129.70, 129.21, 128.73, 128.40,
127.67, 126.65, 125.82, 124.22. MALDI-TOF MS (mass m/z):
375.04 [M+]. Anal. calcd. for C20H14BrN3: C, 63.84; H, 3.75; Br,
21.24; N, 11.17. Found: C, 63.79; H, 3.72; N, 11.16.
Experiment section
General information
Materials
All raw materials were commercially purchased from Aldrich
Chemical Co. or Energy Chemical Co., China. Tetrahydrofuran
(THF) was distilled over metallic sodium over calcium hydride
before use. The other organic solvents and reagents were all
commercially available analytical-grade products and used as
received without further purification.
Measurements
1H and 13C NMR spectra were recorded on a Bruker AC500
spectrometer at 500 and 125 MHz, respectively, when the
compounds were dissolved in deuterated hloroform (CDCl3).
The chemical shift for each signal was reported in ppm units
with tetramethylsilane (TMS) as an internal reference, where δ
(TMS) = 0. The MALDI-TOF-MS mass spectra were measured
using an AXIMA-CFRTM plus instrument. Elemental analysis
was characterized by a Flash EA 1112 instrument. UV-vis
absorption and fluorescence spectra of solution and films were
measured using a Hitachi U-4100 spectrophotometer and a
Hitachi F-4600 spectrophotometer, respectively. Fluorescence
quantum yield, low temperature fluorescence and phosphor-
escence spectra were recorded using an FLS980 spectrometer.
The lifetimes of solutions were measured on an Edinburgh FLS-
Synthesis of 4'-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)-N, N-diphenyl-
[1,1'-biphenyl]-4-amine (TPATZ)
A mixture of (4-(diphenylamino)phenyl)boronic acid (1.84 g,
6.4 mmol), M2 (2 g, 5.3 mmol), potassium carbonate (1.1 g, 8.0
mmol), THF (100 mL), deionized water (20 mL), Pd(PPh3)4 (0.18
g, 0.2 mmol) was refluxed at 70°C for 24 h under nitrogen.
After cooling to the room temperature, deionized water (100
mL) was added and extracted with dichloromethane for three
times. After the organic phase was dried over anhydrous
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magnesium sulphate, the solvent was evaporated from the hybridized LE and CT (HLCT) state and were commonly favor of
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DOI: 10.1039/C8TC03777A
filtrate. The residue was purified through silica gel column fluorescence emission.
chromatography using ethyl acetate/dichloromethane as the
eluent to give a white solid (2.47 g, 86%). H NMR (500 MHz,
(a)
65 nm
1
n-hexane
n-butyl ether
isopropyl ether
ethyl ether
ethyl acetate
teterahydrofuran
dichloromethane
acetone
CDCl3) δ 7.53–7.39 (m, 11H), 7.36 (t, J = 7.3 Hz, 1H), 7.29 (dd, J
= 14.9, 7.9 Hz, 5H), 7.25 (s, 1H), 7.19 (d, J = 7.3 Hz, 2H), 7.11
(dd, J = 7.9, 5.1 Hz, 6H), 7.04 (t, J = 7.3 Hz, 2H). 13C NMR (125
MHz, CDCl3) δ 154.77, 154.60, 147.71, 147.48, 141.62, 135.31,
133.49, 129.98, 129.60, 129.29, 129.08, 128.80, 128.38, 127.87,
127.62, 126.94, 126.36, 125.13, 124.56, 123.55, 123.13.
MALDI-TOF MS (mass m/z): 540.145 [M+]. Anal. calcd. for
C38H28N4: C, 84.42; H, 5.22; N, 10.36. Found: C, 84.44; H, 5.26;
N, 10.32.
acetonitrile
Results and Discussion
Synthesis and Characterization
400
450
500
550
600
650
Wavelength (nm)
Starting from commercially available low-cost 4-bromobenzoyl
chloride and benzohydrazide, N'-benzoyl-4-bromobenzohydra-
zide (M1) was formed and then cyclized with aniline to afford
M2 in an overall yield of 64%. The target compound TPATZ was
readily synthesized by the Suzuki coupling reaction of M2 with
the inexpensive (4-(diphenylamino)phenyl)boronic acid in a
high yield of 86% (Scheme 1). NMR, mass spectroscopy, and
elemental analysis were employed to confirm the compound’s
structure and composition (Fig. S1-S3).
Scheme 1 Molecular structure and synthetic route of TPATZ.
Fig. 1 (a) Solvatochromic PL spectra of TPATZ with the increased solvent polarity;
(b) Linear fitting of Lippert–Mataga model (the semisolid squares represent the
Stokes shifts in different solvents, and the lines are fitted for solvatochromic
model).
TPATZ was soluble in common organic solvents and
exhibited a distinct solvatochromic effect. Upon changing the
solvents from the non-polar hexane, low-polar dialkylethers, to
middle-polar THF and ethyl acetate, to high-polar acetone and
acetonitrile, the photo-luminescence spectra were gradually
red-shifted by a total spectral shift of 65 nm (from 393 to 458
nm) (Fig. 1a). The ground-state dipole moment (μe) was
estimated to be 5.975 D through the DFT (density function
theory) calculation at the level of B3LYP/6-31G-(d,p). To
understand the intramolecular charge transfer (CT) nature of
TPATZ under the excited states, we employed the Lippert–
Mataga relationship to estimate the excited state dipole
moment (μe).[27] The plot of the Stokes shift (νa − νf) versus the
solvent polarity function f revealed a two-section linear
relation with the increase of solvent polarities (Fig. 1b). The
calculated dipole moments were 12.78 and 18.47 D in the low-
and high-polarity regions, respectively, which suggested the
coexistence of a weak CT (locally excited, LE) state and a strong
CT state. Such excited state characters have been named the
To theoretically examine excited state natures of TPATZ
molecule, the quantum chemical calculations were carried out
(Fig. 2). The optimized molecular geometry showed that both
the conjugation backbone and the peripheral phenyl units
were seriously distorted to some extent. The frontier
molecular orbital (FMO) calculation indicated that the HOMO
electrons were mainly located on donor unit and biphenyl
bridge, whereas the LUMO electrons dominantly concentrated
on the 5-phenyltriazole-biphenyl moiety. That is, the middle
biphenyl unit acted as an integral part of both orbitals and
contributed much to the HLCT state nature. The calculated
natural transition orbital (NTO) revealed that the hole was
primarily distributed on the donor and biphenyl moieties with
a little residual on the triazole ring. In contrast, the particle
was mostly localized on the biphenyl bridge. The good overlap
between hole and particle indicated the existence of the HLCT
state, which was qualitatively consistent with above
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Fig. 3 UV and PL spectra of TPATZ in both THF solution and evaporated film.
HOMO/LUMO analysis and solvatochromic effect. Organic
luminophores with distinct HLCT state nature and high
oscillator strength (f = 1.2774 for TPATZ) might be promising
light-emitting materials.
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104
103
102
101
100
DOI: 10.1039/C8TC03777A
n-hexane
isopropyl ether
diethyl ether
tetrahydrofuran
acetonitrile
0
20
40
60
Time (ns)
Fig. 4 Lifetime measurement of TPATZ in n-hexane, isopropyl ether, diethyl
ether, teterahydrofuran and acetonitrile.
To further elucidate the excited-state properties, the
transient PL spectra of TPATZ in different solvents were
measured at room temperature. As shown in Fig. 4, the
molecule exhibited the mono-exponential decay in different
solvents of n-hexane (1.08 ns), n-butyl ether (1.49 ns), THF
(1.97 ns), and acetonitrile (2.77 ns). These slightly increased
life-times within nanosecond scales with increase of solvent
polarity signified the HLCT state character of the molecule.
Meanwhile, we also measured the fluorescence and
phosphorescence spectra of TPATZ in THF solution at 77 K (Fig.
5). The fluorescence emission (no delay) was in true blue
region with the dual emission peak of 412 and 424 nm, and the
delay emission spectrum showed the peak wavelength at 512
nm. Based on the low temperature fluorescence and
phosphorescence spectra, the energy levels of S1 and T1 were
calculated to be 3.01 and 2.42 eV, respectively. Thus, the
energy gap between S1 and T1 (ΔEST) was estimated to be 0.59
eV, which is not favorable for the effective reverse intersystem
crossing (RISC) from low-lying T1 to S1 states. On the other
hand, the large band gap between S1 and T1 was also revealed
by quantum chemical calculation (0.75 eV). Detailed
photophysical parameters are listed in Table 1.
Fig. 2 NTO for S1/ S0 transition in TPATZ. Herein, f represents for the oscillator
strength, and the percentage weights of hole–particle are given for the S1/S0
emission.
Photophysical, thermal, and electrochemical properties
Generally, the linear D‒π‒A molecules with distinct
solvatochromic effect should show the gradually or sharply
decreased fluorescence efficiency with the increase of solvent
polarity. However, TPATZ solutions in n-hexane, diisopropyl
ether, diethyl ether, THF, and acetonitrile could qualitatively
show an increasing fluorescence efficiency of 60%, 77%, 86%,
92%, and 86%, respectively, measured by a fluorescence
integrating sphere, which was possibly related to its strong LE
state nature. The vacuum-deposited film of TPATZ showed the
absorption and emission peaks at 358 nm and 444 nm,
respectively, which were slightly red-shifted relative to those
in THF solution (350 and 437 nm) (Fig. 3). Importantly, the film
still maintained a commendable fluorescence efficiency of
49%. The optical band gap was estimated to be 2.92 eV based
on the onset of absorption.
THF
UV
412 nm
3.01 eV
512 nm
2.42 eV
no delay
PL
delay 1 ms
Film
UV
77 K in THF
PL
400
450
500
550
600
650
700
300
400
500
600
Wavelength (nm)
Wavelength (nm)
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Fig. 5 Fluorescence and phosphorescence of TPATZ in THF at 77 K.
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DOI: 10.1039/C8TC03777A
Table 1. Photophysical, thermal, and electrochemical data of TPATZ.
a)
b)
c)
d)
λabs
λem
Tg
Td
HOMOe)
[eV]
LUMOe)
[eV]
Egf)
[eV]
Compound
TPATZ
[nm]
[nm]
[°C]
[°C]
350/358
430/454
103
422
-5.24
-2.25
2.99
a)UV-Vis absorption spectra measured in THF solution/film state at room temperature; b)Fluorescence spectra were measured in THF solution/film state at room temperature; c)Tg =
glass transition temperature; d)Td = decomposition; e)Measured by cyclic voltammetry; f)Optical gap calculated from the absorption onset in THF.
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were performed in a nitrogen atmosphere to
evaluate the thermal properties of TPATZ solid. TGA measure-
ment found no weight loss before 400 °C (Fig. S4). Owing to its
twist structure resulted from the carbazole group, its
crystallization was suppressed and DSC analysis indicated the
appearance of a relatively high glass transition temperature at
103 °C (Fig. S5).[28] Thus, TPATZ had the excellent thermal
stability and good morphological stability, which is essential
for solid state applications such as light-emitting devices. Cyclic
X-ray crystal structure analysis and crystal function
voltammetry measurements afforded the onset oxidation and
reduction potentials of 0.66 and −2.46 V, respectively (Fig. S6),
which corresponded to the HOMO and LUMO energy levels of
−5.24 and −2.25 eV, respectively. The calculated
electrochemical band gap was 2.99 eV and was well consistent
with the optical band gap 2.92 eV (vide supra).
Fig. 6 (a) The single molecule structure of TPATZ determined by single crystal; (b)
Existed short range interactions of compound TPATZ with the neighboring
molecule.
The single crystal of TPATZ was obtained by sublimation and
analyzed by X-ray crystallography. The corresponding crystallo-
graphic data were listed in Table S1. The results revealed that
TPATZ adopted a twisted molecular conformation, and the
torsion angles between triazol and 5-phenyl, 4-phenyl, and 3-
phenyl were 32.68o, 74.24o, 32.84o, respectively (Fig. 6). The
biphenyl bridge was also twisted by 34.04, and the triphenyl-
amine donor was propeller-like. These twisted molecules are
packed together and formed the triclinic system containing 16
molecules with a Fdd2 space group and the centrosymmetric
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alignment without strong π–π interactions. The twisted confor- the EBU standard of 0.06. The EL spectra hardly changed with
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DOI: 10.1039/C8TC03777A
mation and packing structure were stabilized by both intra- the increase of applied voltage from 4 to 10 V (Fig. S7). The
molecular and intermolecular C–H···π interactions (Fig. 6b). desirable CIE coordinates were ascribed to the short EL
The optimized geometry and crystal analysis signified that emission wavelength and narrow full width at half maximum
TPATZ molecules should also tend to aggregate in a twisted (FWHM). The FWHM is measured to be 50 nm and is one of
conformation in the amorphous state, which could explain the the narrowest true blue EL emissions reports to date. More
commendable film fluorescence efficiency.
Electroluminescence properties
encouragingly, the device showed the excellent EL perform-
ance. The turn-on voltage and maximum luminance were 3.1 V
and 4970 cd m-2, respectively (Fig. 7b). The calculated
maximum current efficiency, maximum power efficiency, and
maximum external quantum efficiency (EQE) were 2.41 cd A-1,
2.20 lm W-1, and 5.92%, respectively, and the efficiency roll-off
of the device is also small (Fig. 7c, d). The corresponding data
are listed in Table 2. This commendable EL performance was
among the best ones reported previously for the saturated
true blue emission with with CIEy < 0.06, and such typical
reports have been summarized in Table S2. This comparison,
combined with our previous results, underlined that
constructing quasi-linear 1,2,4-triazole-based D–π–A hybrids
with biphenyl conjugation bridge should be an universal and
effective avenue for highly efficient true blue EL emitters.
To evaluate the potential of TPATZ as a light-emitting material
for blue OLEDs, a non-doped device was fabricated with the
structure of ITO/HATCN (5 nm)/TAPC (50 nm)/(TPATZ) (20 nm)
/TPBi (55 nm)/LiF (1 nm)/Al (100 nm), where ITO and LiF/Al
were used as the anode and cathode, respectively, and HATCN
(dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11
nitrile, TAPC (1,1-Bis[4-N,N′-di(p-
-hexa-carbo-
tolyl)amino]phenyl)cyclohexane, and TPBi (1,3,5-tris(1-phenyl-
1H-benzimidazol-2-yl)benzene served as the hole injection
layer, hole transporting layer, and the electron transporting
and hole blocking layer, respectively. As shown in Fig. 7a, the
device showed a true blue EL emission with the peak
wavelength of 430 nm and the saturated and much deeper CIE
coordinates of (0.155, 0.047). The y value is much smaller than
Table 2. Electroluminescent properties of non-doped device based on TPATZ.
f)
g)
EQEe)
[%]
λEL
FWHMEL
[nm]
CIEh)
[x, y]
Radiative
exciton
ratio
a)
VTurn-on
[V]
CE b)
PE c)
Ld)
Device
TPATZ
[cd A-1]
[lm W-1]
[cd m-2]
[nm]
3.1
2.41
2.20
4970
5.92%
60%-40%
430
50
(0.155, 0.047)
a)Turn on voltage at a luminance of 1 cd m-2
;
b)Maximal current efficiency; c)Maximal power efficiency; d)Maximum luminance; e)Maximal external quantum efficiency; f)Maximal EL
peak value; g)Full width half maximum of EL spectrum; h)Observer: 2o; obtained at 5 V. Device structure of ITO/HATCN (5 nm)/TAPC (50)/TPATZ (20 nm)/TPBi (55 nm)/LiF (1 nm)/Al
(100nm).
6 | J. Name., 2012, 00, 1-3
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Fig. 7 (a) EL spectrum of the device I, the inset shows the CIE coordinates of the EL (at 5 V); (b) The current density –voltage–luminance characteristics of the device;
(c) The current efficiency–luminance–power efficiency characteristics of the device; (d) The external quantum efficiency–luminance curve, the inset shows the device
structure (ITO/HATCN (5 nm)/TAPC (50 nm)/TPATZ (20 nm)/TPBi (55 nm)/LiF (1 nm)/Al (100 nm)).
The maximum EQE was up to 5.92% and among the highest was not favor of TADF process. TTA was another way for
value of the nonoped OLEDs with CIEy < 0.06 (the EBU blue harvesting the triplet and the induced delayed fluorescence
standard). Generally, the EQE for fluorescent OLEDs could be was proportional to the square of the triplet population
expressed as EQE = Xs × ΦPL × ηr × ηout, where Xs was the ratio density. However, a linear correlation between the luminance
of singlets to the total excitions, ΦPL was the fluorescence and current density was observed (Fig. S9), together with no
quantum efficiency of the amorphous emitter, ηr was the delayed fluorescence in the transient PL decay measured at 77
fraction of excition formation of the injected charge carriers, K, revealing the lack of the TTA process. D–A-type molecules
and ηout was the light out-coupling efficiency. Based on the with singlet formation ratio higher than 25% could also occur
experimental EQE (5.92%) and film fluorescence efficiency at the higher-level excited states, e.g., the RISC from T2 to the
(49%), we used the maximal ηr (100%) and the common ηout singlet manifold. However, this type required the large energy
(20%–30%) to calculate the Xs. The obtained Xs was in the gap between T2 and T1 (such as > 1 eV) to lead to inefficient
range of 60%–40%, which greatly exceeded the theoretical inter-conversion from T2 to T1 and thus T2 was converted to
limitation of 25% from the spin statistics for fluorescent OLED singlet. Time-dependent DFT calculation indicated that the
(singlet/triplet ratio ≈ 1/3). This signified that some triplet energy gap between T2 and T1 of TPATZ was small (0.44 eV),
excitons may have been converted to singlet states (reverse which did not benefit for this process.
intersystem crossing, RISC) to generate light emission in the
While TTA, TADF, and higher level RISC can not explain why
device. At present, RISC could occur through the thermally- the singlet formation ratio is higher than 25% in the TPATZ
activated delayed fluorescence (TADF), the triplet–triplet device, we have alternatively considered that the extra singlets
annihilation (TTA), and the higher-level triplet to singlet were attributed to the triplet–polaron interaction (TPI) induced
excited states for the D–A molecules with HLCT state nature, up-conversion from triplet to singlet. It has been theoretically
etc. Among them, TADF was a highly efficient tactics for and experimentally investigated that the singlet formation
harvesting triplets but required the small ΔEST (commonly, ≤ cross section could be larger than that of triplets in some
0.3 eV). To our case, the ΔEST was up to 0.59 eV polymers and oligomers, and triplet excitons could be
(experimentally, Fig. 5) and 0.75 eV (theoretically, Fig. S8), and converted to singlet gound state or singlet excitons through
the fluorescence lifetimes of both film and solution were much TPI process.[14] Moreover, a recent publication has verified the
shorter than the known TADF materials (vide supra), which existence of TPI process in the device with a similar branched
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D–A molecule as the emitting layer, based on the magneto- Shandong Province of China (No. 2018GGX102022), the Key
View Article Online
DOI: 10.1039/C8TC03777A
current (MC) of the working device increased with both the Project of Higher Educational Science and Technology Program
applied voltage and the magnetic field. It was also conjectured of Shandong Province of China (No. J18KZ001), and the Open
that some deep-blue OLEDs with much higher singlet ratios Project of the State Key Laboratory of Supramolecular
than 25% and without observable delayed fluorescence should Structure and Materials of Jilin University (SKLSSM-201828).
exist the TPI-induced conversion from triplet to singlet. We We appreciate that the State Key Laboratory of Luminescent
thought that the MC effect of the EL devices using linear D–A Materials and Devices of South China University of Technology
molecules as the emitters (such as the present TPATZ and the provides convenience for fabricating OLED devices.
previous PCZTZ) should be more remarkable, although the
measurement was not conducted because of the apparatus
limitation. The existence of strong couplings between the
Notes and references
donor and acceptor moieties of neighbor molecules was a little
like “intermolecular heterojunction” in organic photovoltaic
devices. It was considered to be possible that the triplet
excitons in the OLEDs were dissociated at the “intermolecular
heterojunction,” then the separated charge recombinated with
the opposite charge at the neighbor molecules, which could
offer the possibility of converting triplet excitons to singlet
excitons through TPI.
1
2
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triazole derivatives as the acceptors to link electron-donating
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for realizing new true blue luminophores. The incorporation of
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 51573081 and 51673105), the
Natural Science Foundation of Shandong Province of China (No.
ZR2016JL016), the Key Resarch and Development Program of
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A new true blue fluorescent emitter (TPATZ) with simple structure is designed and facilely
synthesized. The non-doped device exhibits true blue electroluminescence with CIE coordinates of
(0.155, 0.047) and the narrow FWHM of 50 nm, and the y value is fairly small and beyond the
European Broadcasting Union standard of 0.06. The device shows an impressive maximum
external quantum efficiency up to 5.92%.