Construction of high Tg bipolar host materials with balanced electron-hole mobility based on 1,2,4-thiadiazole for phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1021/cm403388s

A novel electron-transporting moiety, 1,2,4-thiadiazole, was first introduced to construct bipolar host molecules for phosphorescent organic light-emitting diodes (PhOLEDs). By incorporating 1,2,4-thiadiazole with typical hole-transporting carbazole moieties, a series of thiadiazole/carbazole hybrids, o-CzTHZ, m-CzTHZ, and p-CzTHZ, were synthesized. All the hybrids exhibit very high glass transition temperatures (T_g ≥ 167 °C) and show good thermal and morphological stability in films. Moreover, these host materials possess good bipolar charge transporting properties; electron and hole mobilities of these bipolar thiadiazole/carbazole hybrids can be tuned by simply adjusting the linkage modes between thiadiazole and carbazole moieties. The maximum external quantum efficiencies (η_{EQE, max}) in the green PhOLEDs with o-CzTHZ, m-CzTHZ, and p-CzTHZ as the hosts reached 26.1%, 24.0%, and 22.9%, respectively, and their EQE were still over 20% even at the high luminance of 10,000 cd/m². This study demonstrates that 1,2,4-thiadiazole should be an excellent electron-transporting unit for bipolar phosphorescent hosts.

Full text of DOI:10.1021/cm403388s

Article
pubs.acs.org/cm
Construction of High Tg Bipolar Host Materials with Balanced
Electron−Hole Mobility Based on 1,2,4-Thiadiazole for
Phosphorescent Organic Light-Emitting Diodes
Jiangjiang Jin,† Wenzhi Zhang,† Bo Wang,† Guanyuan Mu,† Peng Xu,† Lei Wang,*,† Hong Huang,†
Jiangshan Chen,*,‡ and Dongge Ma‡
†Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and
Technology, Wuhan, 430074 Hubei, P. R. China
‡State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022 Jilin, P. R. China
S
* Supporting Information
ABSTRACT: A novel electron-transporting moiety, 1,2,4-thiadiazole, was first
introduced to construct bipolar host molecules for phosphorescent organic light-
emitting diodes (PhOLEDs). By incorporating 1,2,4-thiadiazole with typical
hole-transporting carbazole moieties, a series of thiadiazole/carbazole hybrids, o-
CzTHZ, m-CzTHZ, and p-CzTHZ, were synthesized. All the hybrids exhibit
very high glass transition temperatures (Tg ≥ 167 °C) and show good thermal
and morphological stability in films. Moreover, these host materials possess good
bipolar charge transporting properties; electron and hole mobilities of these
bipolar thiadiazole/carbazole hybrids can be tuned by simply adjusting the
linkage modes between thiadiazole and carbazole moieties. The maximum
external quantum efficiencies (ηEQE, max) in the green PhOLEDs with o-CzTHZ,
m-CzTHZ, and p-CzTHZ as the hosts reached 26.1%, 24.0%, and 22.9%,
respectively, and their EQE were still over 20% even at the high luminance of
10,000 cd/m2. This study demonstrates that 1,2,4-thiadiazole should be an excellent electron-transporting unit for bipolar
phosphorescent hosts.
as the emitter, in which a maximum external quantum efficiency
(ηEQE, max) of 20% and maximum power efficiency (ηp, max) of
59.3 lm/W were achieved. Additionally, they replaced the
carbazole with triphenylamine to form another compound of
m-TPA-o-OXD,11 and the ηEQE, max and ηp, max were improved
to 21% and 86.4 lm/W, respectively. Takizawa et al.12
presented two green PhOLEDs based on the bipolar host
materials, TPBI-Cz and TPBI-Da, showing maximum current
efficiencies (ηc, max) of 48 cd/A and 60 cd/A, respectively. Chou
et al.13 synthesized a bipolar material (BCPO) based on a
carbazole/phosphine oxide hybrid, and the green PhOLED
with BCPO as the host showed ηEQE, max of 21.6%, ηc, max of 83.4
cd/A and ηp, max of 87.5 lm/W. In our previous work, a green
device based on a bipolar host of mPhBINCP was reported,8
which exhibited a ηc, max of 77.6 cd/A and a ηEQE, max of 21%.
The incorporation of hole-transporting and electron-trans-
porting moieties into the multifunctional host materials has
been proved to be an efficient method to broaden the
recombination zone, improve the device efficiency, and simplify
the device structure. Therefore, the development of bipolar
INTRODUCTION
■
Phosphorescent organic light-emitting diodes (PhOLEDs) have
continued to attract great interest because they can approach
100% internal quantum efficiency (IQE) by utilizing both
singlet and triplet excitons.1 To achieve highly efficient
electrophosphorescence by reducing competitive factors, such
as concentration quenching and triplet−triplet annihilation,
phosphorescent emitters of heavy-metal complexes are usually
doped into a suitable host material.2 It is desirable that the host
materials ought to have a large enough triplet energy gap for
efficient energy transfer to the guest, good carrier transport
properties for a balanced recombination of holes and electrons
in the emitting layer, and energy-level matching with
neighboring layers for effective charge injection.3 In recent
years, the design of bipolar host materials configured with
electron-donating groups as donors (D) and electron-with-
drawing groups as acceptors (A) seems to be the most
appealing strategy because they can provide more balance in
electron and hole fluxes and simplify device structures.4
Electron-deficient moieties such as 1,3,4-oxadiazole,5 benzoi-
midazole,6 pyridine,7 phenanthroimidazole,8 1,3,5-triazine,9 and
phosphine oxide10 have all been employed as efficient electron-
transporting acceptors. Tao et al.5b reported a carbazole/
oxadiazole hybrid o-CzOXD for green PhOLEDs with Ir(ppy)3
Received: October 24, 2013
Revised: March 14, 2014
Published: March 14, 2014
© 2014 American Chemical Society
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Scheme 1. Synthetic Routes to o-CzTHZ, m-CzTHZ, and p-CzTHZ
carbon, hydrogen, and nitrogen were measured on an Elementar
(Vario Micro cube) analyzer. UV−vis absorption and PL spectra were
recorded on a Shimadzu UV−vis-NIR spectrophotometer (UV-3600)
and Edinburgh instruments (FLSP920 spectrometers), respectively.
Cyclic voltammetry measurements were carried out on computer-
controlled EG&G Potentiostat/Galvanostat model 283 at room
temperature with a conventional three electrode cell consisting of a
Pt button working electrode of 2 mm in diameter, a platinum wire
counter electrode, and a Ag/AgNO3 (0.1 M) reference electrode.
Nitrogen-purged dichloromethane containing 0.1 M tetrabutylammo-
niumhexafluorophosphate (Bu4NPF6) was used as the supporting
electrolyte. The redox potential was determined from the intersection
of two tangents drawn at the rising and background current of the
cyclic voltammogram. Differential scanning calorimetry (DSC) was
recorded on a PE Instruments DSC 2920 unit with a heating rate of 10
°C/min from 25 to 250 °C under nitrogen. The glass transition
temperature (Tg) was determined from the second heating scan.
Thermogravimetric analysis (TGA) was undertaken using a
PerkinElmer Instruments (Pyris1 TGA) at a heating rate of 10 °C/
min from 30 to 700 °C under a nitrogen environment. The thermal
stability of the samples were determined by the recorded temperature
at 5% weight loss. In the time-of-flight (TOF) transient-photocurrent
technique, those samples were illuminated at 355 nm through the ITO
electrode from an UV-FQ q-switched DPSS laser (pulse: 4 ns). The
transient currents were measured using a digital phosphor oscilloscope
(DPO 3034; bandwidth: 300 MHz).
host materials containing new moieties is still interesting for
practical applications. In addition, the thermal stability of the
bipolar hosts ought to be further improved, which is
advantageous to lengthen the lifetime of PhOLEDs for practical
applications.
As we know, the carbazole group has been widely used in
host materials because of its high triplet energy and excellent
hole-transporting ability.14 For example, carbazole-based CBP
has been used as the host for green and red phosphorescent
emitters.15 However, the device performance with CBP was still
unsatisfactory due to its low electron-transporting ability.16
Another disadvantage is the low glass transition temperatures
(Tg) (about 62 °C) leading to easy crystallization, which is
notorious for the poor lifetimes of PhOLEDs.17 Thus, the
coupled electron-transporting groups should play very
important roles to overcome the above issues in the
carbazole-based bipolar hosts. Here, 1,2,4-thiadiazole as a
novel electron-transporting group was first used to construct
PhOLED bipolar host materials, and their optical and electronic
performances have been studied systematically. We synthesized
three carbazole/thiadiazole bipolar hosts, o-CzTHZ, m-
CzTHZ, and p-CzTHZ, through adjusting the linkage mode
between the carbazole and 1,2,4-thiadiazole moieties. Though
these materials have relatively low molecular masses, their Tg
values are as high as around 170 °C, which are much higher
than those of the reported bipolar hosts. More interestingly,
nearly equal hole and electron mobilities were realized in o-
CzTHZ and m-CzTHZ, which will be expected to be beneficial
for broadening recombination zones. Combining with the hole
trapping-recombination, the ηEQE, max in the green PhOLEDs
with o-CzTHZ and m-CzTHZ as the bipolar hosts reached
26.1% and 24.0%, respectively, which could correspond to
nearly 100% IQE. This study indicates that 1,2,4-thiadiazole is
not only a novel electron-transporting moiety but also an
excellent unit for building bipolar host materials with high
thermal stability.
Synthesis. All the reagents and materials were received from
Aldrich and used without further purification unless otherwise noted.
1,2,4-Thiadiazole was purchased from Wuhan Sunshine optoelec-
tronics Technology Co. Ltd. Synthetic routes of the three compounds
are outlined in Scheme 1.
2,5-Bis(2-(9H-carbazol-9-yl)phenyl)-1,2,4-thiadiazole (o-CzTHZ).
A mixture of 3-bromo-5-chloro-1,2,4-thiadiazole (0.50 g, 2.5 mmol),
(2-(9H-carbazol-9-yl)phenyl)boronic acid (1.61 g, 5.6 mmol), [Pd
(PPh3)4] (196 mg, 0.17 mmol), K2CO3 (2.0 M aqueous solution, 5.0
mL, 10.0 mmol), toluene (50 mL), and ethanol (25.0 mL) was stirred
at 100 °C for 24 h. After cooling to room temperature, dichloro-
methane (200.0 mL) was added to the reaction mixture. The organic
phase was separated and washed with brine before being dried over
anhydrous MgSO4. The solvent was evaporated, and the solid residues
were purified by column chromatography on silica gel with petroleum
ether:dichloromethane (2:1) to afford the product as a white powder
EXPERIMENTAL SECTION
■
1
(1.28 g, 89%). H NMR (400 MHz, DMSO-d6) δ [ppm]: 8.24−8.22
General Procedures. 1H NMR and 13C NMR spectra were
conducted on a Bruker-AF301 AT 400 MHz spectrometer. Mass
spectra were performed on an Agilent (1100 LC/MSD Trap) using
APCI ionization. The experimental conditions and equipment used
have been described in our previous works.8,14 Elemental analyses of
(d, J = 8.0 Hz, 2H), 8.19−8.15 (t, J = 6.4 Hz, 3H), 7.86−7.82 (t, J =
6.0 Hz, 1H), 7.77−7.69 (m, 3H), 7.49−7.45 (t, J = 6.4 Hz, 1H), 7.35−
7.21 (m, 9H), 7.10−7.08 (d, J = 8.0 Hz, 1H), 6.92−6.90 (d, J = 8.0 Hz,
2H), 6.64−6.62 (d, J = 8.0 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ
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Figure 1. a) Absorption and emission spectra of o-CzTHZ, m-CzTHZ, and p-CzTHZ in toluene solution at room temperature. b) Phosphorescence
spectra of o-CzTHZ, m-CzTHZ, and p-CzTHZ in 2-methyltetrahydrofuran at 77 K.
[ppm]: 180.97, 168.69, 142.17, 140.70, 136.08, 135.99, 132.05, 132.14,
131.70, 131.17, 130.47, 129.51, 129.41, 129.18, 128.72, 126.35, 125.78,
124.50, 123.35, 120.78, 120.42, 119.89, 110.00, 109.72; APCI MS (m/
RESULTS AND DISCUSSION
■
Synthesis. The hybrids of o-CzTHZ, m-CzTHZ, and p-
CzTHZ were synthesized by a Suzuki coupling reaction of 3-
bromo-5-chloro-1,2,4-thiadiazole with carbazole intermediates,
to give the desired compounds with excellent isolated yields.
Carbazole intermediates were prepared according to a
previously reported procedure.19 The detailed procedures and
characterization of these new compounds are described in the
Experimental Section. Repeated temperature-gradient vacuum
sublimation is required for further purification of these
materials used in PhOLEDs.
Photophysical and Electrochemical Properties. Figure
1 shows the electronic absorption and fluorescence spectra of
the compounds o-CzTHZ, m-CzTHZ, and p-CzTHZ. The
absorption at around 290 nm for all of the compounds could be
assigned to the carbazole-centered n-π transition. The longer-
wavelength absorption at around 330−340 nm could be
attributed to the π−π* transition from the electron-donating
carbazole moiety to the electron-accepting thiadiazole moiety.
The assignment is manifested by the fact that the intensity of
the π−π* transition for o-CzTHZ and m-CzTHZ is remarkably
reduced in comparison to the dominant π−π* transition for p-
CzTHZ. This may be ascribed to the good π-conjugation
between the carbazole and thiadiazole moieties when the
carbazole is linked at the para position of the 3,5-diphenyl-
1,2,4-thiadiazole for p-CzTHZ.
All of the compounds emit deep-blue light with the emission
peaks in the range of 410−430 nm in the solution state. The
triplet energies (ET), determined by the highest-energy vibronic
sub-band of the phosphorescence spectra at 77 K, follow the
trend: o-CzTHZ (2.62 eV) > m-CzTHZ (2.58 eV) > p-CzTHZ
(2.48 eV), implying that they may act as appropriate host
materials for green and red phosphorescent emitters. The ET of
o-CzTHZ is higher than those of m-CzTHZ and p-CzTHZ,
which is most likely due to the strong steric effects in o-
CzTHZ.20
Electrochemical Properties. Cyclic voltammetry (CV)
was performed to study the electrochemical properties of
bipolar host materials o-CzTHZ, m-CzTHZ, and p-CzTHZ.
Their cyclic voltammograms are shown in Figure 2, and the
respective electrochemical data are summarized in Table 1. All
compounds exhibited irreversible oxidation peaks at around 1.0
V, which is most likely resulting from the electrochemical
property of the carbazole moiety.21 The irreversible oxidations
of the three bipolar molecules were due to the dimerization or
electropolymerization of the carbazole units at the C3 and/or
C6 positions.6a On the other hand, both o-CzTHZ, m-CzTHZ,
and p-CzTHZ exhibited a well-defined quasireversible reduc-
z): 569.5; Elemental analysis calcd (%) for C38H24N4S: C 80.26, H
4.25, N 9.85; found: C 80.26, H 4.24, N 9.83.
2,5-Bis(3-(H-carbazol-9-yl)phenyl)-1,2,4-thiadiazole (m-CzTHZ).
Compound m-CzTHZ was prepared according to the same procedure
as compound o-CzTHZ but using (3-(9H-carbazol-9-yl)phenyl)-
boronic acid instead of (2-(9H-carbazol-9-yl)phenyl)boronic acid.
1
Yield: 84%; H NMR (400 MHz, CDCl3) δ [ppm]: 8.60 (s, 1H),
8.48−8.46 (d, J = 8.0 Hz, 1H), 8.27 (s, 1H), 8.16−8.13 (d, J = 8.0 Hz,
4H), 8.11−8.09 (m, 1H), 7.75−7.74 (d, J = 8.0 Hz, 2H), 7.72−7.66
(m, 2H), 7.46−7.38 (m, 8H), 7.32−7.26 (m, 4H); 13C NMR (100
MHz, CDCl3) δ [ppm]: 187.43, 173.11, 140.91, 140.69, 139.01,
138.34, 134.61, 132.45, 130.99, 130.55, 130.31, 129.13, 127.38, 127.20,
126.43, 126.22, 126.05, 125.99, 123.65, 123.51, 120.48, 120.40, 120.33,
120.06, 109.78, 109.54; APCI MS (m/z): 569.2 [M+1]+; Elemental
analysis calcd (%) for C38H24N4S: C 80.26, H 4.25, N 9.85; found: C
80.27, H 4.26, N 9.85.
2,5-Bis(4-(H-carbazol-9-yl)phenyl)-1,2,4-thiadiazole (p-CzTHZ).
Compound p-CzTHZ was prepared according to the same procedure
as compound o-CzTHZ but using (4-(9H-carbazol-9-yl)phenyl)-
boronic acid instead of (2-(9H-carbazol-9-yl)phenyl)boronic acid.
Yield: 80%. 1H NMR (400 MHz, CDCl3) δ [ppm]: 8.67−8.65 (d, J
= 8.0 Hz, 2H), 8.33−8.31 (d, J = 8.0 Hz, 2H), 8.17−8.15 (d, J = 8.0
Hz, 4H), 7.80−7.74 (m, 4H), 7.53−7.51 (d, J = 8.0 Hz, 4H), 7.47−
7.41 (m, 4H), 7.35−7.29 (m, 4H); 13C NMR (100 MHz, CDCl3) δ
[ppm]: 187.38, 173.23, 141.25, 140.63, 140.36, 139.75, 131.69, 130.01,
129.26, 129.16, 127.47, 127.09, 126.26, 126.11, 123.89, 123.70, 120.65,
120.52, 120.40, 120.29, 109.86, 109.75; APCI MS (m/z): 569.7;
Elemental analysis calcd (%) C38H24N4S: C 80.26, H 4.25, N 9.85;
found: C 80.25, H 4.26, N 9.84.
Quantum Calculation. Theoretical calculations on the geo-
metrical and electronic properties were peformed on the Amsterdam
Density Functional (ADF) 2009.01 program package. B3LYP (Becke
three parameters hybrid functional with Lee-Yang-Perdew correlation
functionals) and the 6-31G(d) atomic basis set were used to determine
and optimize the structures.18 Molecular orbitals were visualized using
ADFview.
OLED Fabrication and Measurement. The EL devices were
fabricated by vacuum thermal evaporation technology according to the
methods modified from our previous approach.8,14 Devices with an
area of 3 mm × 3 mm were fabricated by vacuum deposition at 5 ×
10−6 Torr. An indium tin oxide (ITO) with a sheet resistance of 20 Ω/
square was used as the substrate. The ITO glass substrates were
cleaned with acetone, ethanol, and deionized water and then dried in
an oven at 120 °C. Before device fabrication, the clear ITO was treated
with oxygen plasma for 5 min. The deposition rate of organic
compounds was 0.9−1.1 Å s−1. MnO3 was deposited on the ITO
surface and then followed by the other organic layers. Finally, a
cathode composed of LiF and Al metal was deposited sequentially
onto the substrate. The L−V−J of the devices was tested using a
Keithley 2400 Source meter and PR655 under ambient conditions.
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in the range of 370−420 °C, and their corresponding Tg are
167, 170, and 168 °C, respectively. The Tg values of these new
compounds are much higher than those of the most reported
bipolar hosts, such as the analogous oxadiazole/cabazole
hybrids (o-CzOXD, p-CzOXD and m-CzOXD).5b This clearly
indicates that the 1,2,4-thiadiazole moiety should play a
dominant role in the thermal properties of the bipolar host
materials. The high Td and Tg highlight the suitability of these
materials to render high quality amorphous thin films upon
vacuum thermal evaporation, which is the prerequisite for the
preparation of devices with high efficiency and high stability.
To get an insight into the thermal stabilities of the vacuum
evaporated films of o-CzTHZ, m-CzTHZ, and p-CzTHZ, the
effects of annealing temperatures on the morphologies were
studied by atomic force microscopy (AFM). As shown in
Figure 5, the pristine solid films of o-CzTHZ, m-CzTHZ, and
p-CzTHZ exhibit a fairly smooth surface morphology with the
root-mean-square (RMS) roughness of 0.429, 0.469, and 0.415
nm, respectively. After thermal annealing at 150 °C for 30 min
under a nitrogen atmosphere, the RMS roughness of the
corresponding films are still as low as 0.638, 0.449, and 0.641
nm, respectively. These results indicate that all three hosts have
excellent thermal and morphological stability, which would be
advantageous to the stability of PhOLEDs.
Bipolar Transporting Characteristics. In order to study
the bipolar transporting properties of o-CzTHZ, m-CzTHZ,
and p-CzTHZ, the time-of-flight (TOF) technique was used to
determine their charge carrier mobilities. Figure 6 shows typical
TOF transients of hole/electron for o-CzTHZ, m-CzTHZ, and
p-CzTHZ under an applied field. Figure 7 shows the hole and
electron mobilities of o-CzTHZ, m-CzTHZ, and p-CzTHZ
plotted as a function of the square root of the applied electric
field.
The almost balanced hole and electron mobilities were found
for o-CzTHZ. The hole mobilities of o-CzTHZ are in the range
of 3.72 × 10−4 to 4.47 × 10−4 cm2/(V s) with fields varying
from 3.6 × 105 to 5.6 × 105 V/cm, and the electron mobilities
are in the range of 3.41 × 10−4 to 3.91 × 10−4 cm2/(V s) with
fields varying from 3.3 × 105 to 4.7 × 105 V/cm. The values of
hole and electron mobilities for m-CzTHZ are also very close,
in which its hole mobilities lie in the range of 7.61 × 10−5 to
8.31 × 10−5 cm2/(V s) during 3.6−4.6 × 105 V/cm, and its
electron mobilities lie in the range of 6.78 × 10−5 to 8.06 ×
10−5 cm2/(V s) during 4.2−5.0 × 105 V/cm. These results
indicate that the o-CzTHZ and m-CzTHZ show bipolar
charge-transport properties with excellent balanced electron
and hole mobilities. The hole mobilities of p-CzTHZ lie in the
range of 3.52 × 10−4 to 4.56 × 10−4 cm2/(V s) when the fields
vary from 2.5 × 105 to 3.8 × 105 V/cm, while the electron
mobilities lie in the range of 2.46 × 10−5 to 3.28 × 10−5 cm2/(V
s) with fields varying from 4.8 × 105 to 6.4 × 105 V/cm, which
means that the hole mobilities are an order of magnitude higher
than the electron mobilities. This study demonstrates that the
Figure 2. Cyclic voltammograms of o-CzTHZ, m-CzTHZ, and p-
CzTHZ in CH2Cl2 solution.
tion process at −2.04, −2.18, and −2.13 V (vs Ag/AgNO3),
respectively, which is dependent on the linkage modes of the
carbazole and 1,2,4-thiadiazole. The highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels of these compounds were calculated
from the onset potentials of oxidation and reduction. The
HOMO/LUMO levels were estimated to be −5.64/−2.64,
−5.51/−2.50, and −5.50/−2.55 eV, respectively, by comparing
with ferrocene (Fc) (HOMO = −(Eox + 4.68) eV) and
ferrocene (Fc) (LUMO = −(Ered + 4.68) eV). This implies that
holes and electrons can transfer into the bipolar hosts with
small obstruction from TCTA (−5.3/−2.36 eV) and TmPyPB
(−6.68/−2.73 eV),22 respectively, and then be well-confined in
the emissive layers.
Theoretical Calculations. To gain insight into the
electronic structures of these compounds, a density functional
theory (DFT) calculation was performed. As shown in Figure 3,
the calculated HOMO/LUMO values are in the range of 5.38−
5.55/2.55−2.64 eV, which also correlates well with the
experimental data. The electron distributions in the HOMO
level for these new compounds are located on the phenyl-
carbazole linked to the C3 of 1,2,4-thiadiazole, and in the
LUMO the electron distributions are mostly located on 1,2,4-
thiadiazole and phenyl linked to C5 of 1,2,4-thiadiazole, which
could be ascribed to the more strong electronegativity of the
C3 of 1,2,4-thiadiazole. All of these materials have almost
complete separation of the HOMO and LUMO. The
separation between the HOMO and LUMO is beneficial for
efficient charge-carrier transport and the prevention of reverse
energy transfer.23
Thermal Properties. The thermal properties of o-CzTHZ,
m-CzTHZ, and p-CzTHZ were determined by thermogravi-
metric analysis (TGA) and differential scanning calorimetry
(DSC), as shown in Figure 4. The decomposition temperatures
(Td, corresponding to 5% weight loss) and the Tg are also listed
in Table 1. The Td of o-CzTHZ, m-CzTHZ, and p-CzTHZ are
Table 1. Photophysical, Electrochemical, and Thermal Properties of the o-CzTHZ, m-CzTHZ, and p-CzTHZ
a
a
b
b
c
d
e
f
Abs (nm) PL (nm) Eox (V) Ered (V) HOMO/LUMO (eV) HOMO/LUMOcal (eV) Eg (eV) ET (eV) Td/Tg (°C)
o-CzTHZ
m-CzTHZ
p-CzTHZ
330/291
338/281
342/290
421
428
413
0.96
0.83
0.82
−2.04
−2.18
−2.13
−5.64/−2.64
−5.51/−2.50
−5.50/−2.55
−5.55/−2.84
−5.38/−2.58
−5.43/−2.67
3.00
3.01
2.95
2.62
2.58
2.48
370/167
394/170
420/168
a
b
c
d
Measured in CH2Cl2. The onset potential of oxidation and reduction as referenced to ferrocene (Fc). Values from DFT calculation. The band
e
f
gap energies were estimated from CV. Measured in 2-MeTHF at 77 K. Measured from DSC and TGA.
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Figure 3. Spatial distributions of the HOMO and LUMO levels for o-CzTHZ, m-CzTHZ, and p-CzTHZ.
bipolar charge transporting properties of the thiadiazole/
carbazole hybrids can be simply adjusted by the linkage
mode. The relatively high carrier mobility of o-CzTHZ will be
benificial for the fabrication of a high efficiency device.
Electrophosphorescent OLED Characterization. To
evaluate the performance of the thiadiazole/carbazole asym-
metric hybrids as hosts, PhOLEDs were fabricated by using a
green emitter of Ir(ppy)3 as the guest. The device configuration
is ITO/MoO3 (10 nm)/TAPC (50 nm)/TCTA (5 nm)/Host:
x Ir(ppy)3 (20 nm)/TmPyPB (45 nm)/LiF (1 nm)/Al (100
nm) (o-CzTHZ for device A, m-CzTHZ for device B, and p-
CzTHZ for device C). The energy level diagram is shown in
Figure 8. TAPC (4,4′-cyclohexylidenebis[N,N-bis(4-
methylphenyl)benzenamine]) was used as the hole-trans-
porting material, TmPyPB (1,3,5-tri[(3-pyridyl)phen-3-yl]-
benzene) was used as both the electron-transporting and
hole-blocking layer, and MoO3 and LiF served as the hole and
electron-injecting layers, respectively. In the devices, TCTA
(tris(4-carbazoyl-9-ylphenyl)amine) was chosen as the exciton
blocker to prevent the diffusion of excitons to the hole-
Figure 4. TGA thermograms of o-CzTHZ, m-CzTHZ, and p-CzTHZ
recorded at a heating rate of 10 °C min−1. Inset: DSC thermograms of
o-CzTHZ, m-CzTHZ, and p-CzTHZ recorded at a heating rate of 10
°C min−1.
Figure 5. Atomic force microscopic pictures of o-CzTHZ, m-CzTHZ, and p-CzTHZ. (a), (b), (c) without thermal treatment and (d), (e), (f) after
thermal treatment at 150 °C for 30 min.
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Figure 6. Log of the photocurrent vs the log of time in a time-of-flight measurement. The electron mobility (μ) was calculated from the values of the
transit time (Tt), the sample thickness, and the applied voltage (V) according to the equation μ = D2/(VTt).
Figure 8. Energy level diagram of the devices based on the 1,2,4-
thiadiazole derivatives.
The 3 wt %-doped device (A1−C1) shows good efficiencies, and
the EQE of the three hosts are all over 20%, but they are still
lower than the 6 wt %-doped devices (A2−C2). This
Figure 7. . Electron and hole mobilities versus E1/2 for o-CzTHZ, m-
CzTHZ, and p-CzTHZ.
demonstrates that the triplet energy transfer from the host to
the dopant is not complete. The 9 wt %-doped device (A3−C3)
shows the lowest efficiencies, which suggests considerable self-
transporting layer. Meanwhile, TCTA can also reduce the hole
injection barrier from TAPC to the host. The dopant
concentration was optimized through concentration-dependent
experiments with the dopant amount ranging from 3 to 9 wt %.
quenching of the dopant upon excitation when excess Ir(ppy)3
has been codeposited. Above all, we find that all the devices
with 6 wt % dopant exhibit the best efficiencies (see Figure S3
and Table S1).
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Figure 9. Green electrophosphorescence properties of devices A2, B2, and C2 with o-CzTHZ, m-CzTHZ, and p-CzTHZ as the host: a) J−V−L
curves; b) current efficiency and power efficiency versus brightness curves; c) EQE versus brightness curves; d) EL spectra.
a
Table 2. Electroluminescence Properties of the Devices A2−C2
b
b
b
c
d
device
host
Von [V]
[ηc] [cd/A]
[ηp] [lm/W]
[ηEQE
]
[%]
[ηEQE/] [%]
CIE [x, y]
A2
B2
C2
o-CzTHZ
m-CzTHZ
p-CzTHZ
3.1
3.4
3.1
92.3( 1.2%)
86.4( 1.3%)
80.2( 0.2%)
78.8( 0.4%)
61.8( 1.4%)
26.1( 1.5%)
24.0( 0.8%)
22.9( 0.9%)
20.3( 0.5%)
20.6 ( 0.5%)
(0.30, 0.62)
(0.31, 0.62)
(0.30, 0.62)
56.7( 0.3%)
21.2 ( 0.5%)
a
b
c
Von: turn-on voltage at 1 cd/m2; ηc: current efficiency; ηp: power efficiency. Order of measured value: maximum. Measured at 10000 cd/m2.
d
Measured at 100 cd/m2.
As shown in Figure 9 and Table 2, the optimized devices A2−
when the current density increased. It should be noted that the
hole mobility of TAPC26 is higher than the electron mobility of
TmPyPb;27 therefore, the more stronger hole trap of o-CzTHZ
may be beneficial to the hole/electron balance in the emissive
layer.
C2 display turn-on voltages from 3.1−3.4 V, and the efficiencies
of them are in the order: A2 > B2 > C2. Device A2 based on o-
CzTHZ achieves a ηc, max of 92.3 cd/A and a ηp, max of 78.8 lm/
W, particularly, its ηEQE, max reaches 26.1%. The maximum
efficiencies for devices B2 and C2 are 86.4 cd/A (61.8 lm/W
and 24.0%) and 80.2 cd/A (56.7 lm/W and 22.9%),
respectively. Even at a high brightness of 10000 cd/m2, the
EQE of devices A2, B2, and C2 are still as high as 20.3%, 20.6%,
and 21.2%, respectively, indicating the devices show low
efficiency roll-off.
Peak wavelengths of the EL spectra of green devices A2−C2
exhibit an emission similar to the phosphorescent emitter
Ir(ppy)3. This suggests that the emission peaks are indeed
derived from the dopant, and the triplet energy transfer from
the hosts to the dopant are complete. The current density−
voltage-luminance (J-V-L) characteristics, efficiency-luminance
characteristics, and EL spectra of the devices are shown in
Figure 9, and the key parameters are summarized in Table 2. In
order to reflect the reproducibility of the higher efficiencies
based on 6 wt %-doped Ir(ppy)3, all three hosts were tested for
three times under the same conditions, which can be seen from
Figure 9, Figure S4, and Table S2, and the efficiencies all exhibit
only small deviations.
The reason why the o-CzTHZ based device possesses the
best efficiencies can be elucidated from the following aspects.
First, the higher triplet energies of o-CzTHZ could efficiently
suppress adverse energy back-transfer from the guest Ir(ppy)3
(2.40 eV)24 to the host, consequently resulting in good
performances of device A2. Second, the host o-CzTHZ exhibits
higher charge mobilities than that of m-CzTHZ and p-CzTHZ,
as shown in Figure 7. Third, the hole trapping-recombination
mechanism for o-CzTHZ is another factor that affects the
efficiency of device A2. To confirm the hypothesis, the hole-
only device of ITO/MoO3 (5 nm)/TCTA (10 nm)/Hosts:
Ir(ppy)3 (0%, 6%) (40 nm)/TCTA (10 nm)/MoO3 (5 nm)/Al
was fabricated. As depicted in Figure S5, it can be clearly seen
that the current density of a doped hole-only device based on o-
CzTHZ reduces more obviously than that of m-CzTHZ and p-
CzTHZ, which is induced by the deepest hole traps of o-
CzTHZ. At the low current density, hole-trapping could
provide an additional emission process and will improve the
efficiency.25 Regrettably, the more trapped holes will lead to
triplet-charge annihilation and induce the efficiency roll-off
CONCLUSIONS
■
In conclusion, the 1,2,4-thiadiazole moiety was employed as the
electron-transporting moiety for bipolar host materials, and a
series of simple thiadiazole/carbazole hybrids were developed.
Their bipolar charge transporting properties can be tuned by
changing the linkage mode and the balanced electron, and hole
mobilities were achieved when carbazole was attached to 1,2,4-
thiadiazole via ortho and para positions. Besides, the
introduction of the 1,2,4-thiadiazole moiety can significantly
improve the thermal stability of the bipolar host materials,
which is beneficial to the device stability. The devices with o-
CzTHZ, m-CzTHZ, and p-CzTHZ as the hosts achieve
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ASSOCIATED CONTENT
* Supporting Information
■
S
The HOMO and LUMO of TAPC and TCTA were measured
cyclic voltammetry (CV) in CH2Cl2, and their cyclic
voltammograms are shown in Figures S1 and S2. PE-CE-
Brightness, EQE-Brightness plots, and table of efficiencies. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: wanglei@mail.hust.edu.cn.
*E-mail: jschen@ciac.ac.cn.
■
(19) You, J.; Li, G.; Wang, Z. Org. Biomol. Chem. 2012, 10, 9481.
(20) Fan, C.; Zhao, F.; Gan, P.; Yang, S.; Liu, T.; Zhong, C.; Ma, D.;
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(21) Mondal, E.; Hung, W.-Y.; Dai, H.-C.; Wong, K.-T. Adv. Funct.
Mater. 2013, 23, 3096.
Notes
(22) Bin, J. K.; Cho, N. S.; Hong, J. I. Adv. Mater. 2012, 24, 2911.
(23) Ge, Z.; Hayakawa, T.; Ando, S.; Ueda, M.; Akiike, T.;
Miyamoto, H.; Kajita, T.; Kakimoto, M.-a. Adv. Funct. Mater. 2008, 18,
584.
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Pudzich, R.; Salbeck, J. Appl. Phys. Lett. 2004, 85, 3911.
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Yan, P. Chem. Mater. 2013, 25, 4966. (b) Zhao, F.; Sun, N.; Zhang, H.;
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research work was supported by the NSFC/China
(21161160442, 51203056), the National Basic Research
Program of China (973 Program 2013CB922104), and the
Analytical and Testing Centre at Huazhong University of
Science and Technology. The authors thank Dr. M. P. Aldred
for the English polishing.
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(27) Su, S.-J.; Chiba, T.; Takeda, T.; Kido, J. Adv. Mater. 2008, 20,
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