A high triplet energy, high thermal stability oxadiazole derivative as the electron transporter for highly efficient red, green and blue phosphorescent OLEDs

Article abstract of DOI:10.1039/c4tc02348j

A high glass transition temperature (T_g = 220 °C), high triplet energy gap (E_T = 2.76 eV) and high electron mobility material bis(m-terphenyl)oxadiazole was readily synthesized. It can serve as a universal electron transporter for bl

Full text of DOI:10.1039/c4tc02348j

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Journal Name
RSCPublishing
ARTICLE
A high triplet energy, high thermal stability
oxadiazole derivative as the electron transporter
for highly efficient red, green and blue
phosphorescent OLEDs
Cite this: DOI: 10.1039/x0xx00000x
Received , xx
Accepted , xx
DOI: 10.1039/x0xx00000x
Cheng-Hung Shih, P. Rajamalli, Cheng-An Wu, Ming-Jai Chiu, Li-Kang Chu and
Chien-Hong Cheng*
www.rsc.org/
A high glass transition temperature (Tg = 220 °C), high triplet energy gap (ET= 2.88 eV) and high electron
mobility material bis(m-terphenyl)oxadiazole was readily synthesized. It can serve as a universal electron
transporter for blue, green and red phosphorescent OLEDs with excellent efficiencies. The material shows
high current density compared to the other electron transport materials and exhibits reduced driving voltage
for all color PhOLEDs irrespective of the energy level of the host materials, due to efficient electron
injection from 2,5-di([1,1':3',1''-terphenyl]-5'-yl)-1,3,4-oxadiazole (TPOTP) to host material. For the green
PhOLED, a maximum external quantum efficiency (EQE) over 25%, current efficiency of 97.6 cd/A and
power efficiency of 100.6 lm/W were achieved. The red and blue devices using TPOTP as the electron
transporter also show EQE higher than 23% with very low roll-off in efficiencies in practical brightness level.
1 Introduction
a device is very important.7 In addition, ETM should possess a
high triplet energy gap to prevent dopant or host triplet excitons
Organic lightꢀemitting diodes (OLEDs) have attracted great
attention because of their applications in both flat and flexible
displays and lighting.1 Phosphorescent OLEDs are particularly
fascinating due to the fact that their internal/external quantum
efficiencies can be 4 times higher than the traditional
fluorescence device.2,3 To achieve excellent efficiencies,
OLEDs are generally constructed with multilayers including
anode, hole injection, holeꢀtransporting, emissive (host and
dopant), hole blocking, electronꢀtransporting, electron injection
and cathode layers.4 For a RGB fullꢀcolour OLED display, a
large number of materials should be used, if the red, green and
red devices all use different materials. It would greatly simplify
the manufacturing process and reduce the cost, if some of the
materials can be shared with different devices to reduce the
number of materials.
In the last few decades, many types of semiconducting hole
transporting materials (HTM) and electron transporting
materials (ETM) have been reported.5 Among these two types
of materials, Hole transporting semiconducting materials have
become practical due to their high chargeꢀcarrier mobility and
excellent operational durability.6 Therefore, the development of
ETM with high electron mobility are necessary to improve the
OLED performance because the balance of hole and electron in
in the emissive layer to diffuse to the ET layer.8 The ETM also
should have an optimal lowest unoccupied molecular orbital
(LUMO) energy level for the smooth electron injection from
cathode to ETM and electron transport from ETM to the host
layer and have the highest occupied molecular orbital (HOMO)
level low enough to block the hole to get into the electron
transporting layer.
There are many electron transport materials developed to
reduce the driving voltage and improve efficiency of
PhOLEDs.9 Although the electron mobilities of these electron
transport materials are improved, the LUMO levels are deep
which are not suitable for electron injection into the common
carbazoleꢀbased host materials with a shallow LUMO levels of
2.2 ꢀ 2.4 eV. Moreover, a high glass transition temperature for
ETM is required to maintain the device stability. In general
high Tg needs a rigid structure and high molecular weight. It is
a great challenge to design a ETM having high Tg, and high
energy gap as well, because these two properties are
contradictory.
We are interested in synthesizing efficient thermally stable
electron transport materials. Recently, we reported two
oxadiazoleꢀbased ETMs for blue devices. Although these
devices are efficient, their Tg and the electron mobility are
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relatively low.10 To improve these two properties, we designed state is likely due to the strong
and synthesized a new electron transporting material, 2,5ꢀ molecules.
π
ꢀ
π
stacking of the TPOTP
di([1,1':3',1''ꢀterphenyl]ꢀ5'ꢀyl)ꢀ1,3,4ꢀoxadiazole (TPOTP). The
The singlet energy gap ES of TPOTP as determined from
oxadiazole unit was selected due to the advantages: i) high the intersection of its absorption and emission spectra is 3.59
electron affinity; ii) wide energy gap; and iii) readily eV. The HOMO and LUMO energy levels were estimated to be
maintaining the planarity of the molecule. The metaꢀterphenyl 5.85 and 2.26 eV. The LUMO is calculated from the reduction
unit was connected to enhance the thermal stability without potential (Fig. S1) and HOMO is determined from LUMO + ES
.
much decrease of the energy gap. TPOTP possesses important The triplet energy gap (ET) was estimated to be 2.76 eV, from
properties such as high triplet energy, optimal HOMO and the high energy peak of phosphorescent spectrum in the solid
LUMO levels, high electron mobility and high Tg. This film at 77 K.
compound is utilized as
a
universal ETM for the
phosphorescent blue, green and red OLEDs and all have
achieved excellent device performance.
FL in 10-5M DCM
FL in thin film
Ph in 2-MeTHF
Ph in thin film
Abs. in 10-5M DCM
Abs. in thin film
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2 Results and discussion
2.1 Synthesis and characterization
As shown in Scheme 1, TPOTP could be synthesized from
Suzuki coupling reaction of 2,5ꢀbis(3,5ꢀdibromophenyl)ꢀ1,3,4ꢀ
oxadiazole (OBr4) with phenyl boronic acid in good yield. One
of the two starting materials, 5ꢀ(3,5ꢀdibromophenyl)ꢀ1Hꢀ
tetrazole (35Brtaz), was prepared from 3,5ꢀdibromobenzonitrile
and sodium azide in the presence of zinc chloride according to
a literature procedure.11 The details for the preparation of these
compounds are given in the experimental section. This product
was further purified by gradient temperature sublimation before
300
400
500
600
700
Wavelength (nm)
Fig.
1 The absorption (Abs.) and fluorescence (Fl.) spectra of TPOTP in
1
device fabrication and was characterized by its H, 13C NMR,
dichloromethane (10-5 M) solution and thin film measured at room temperature;
the phosphorescence (Ph) spectra measured in 2-MeTHF (10-5 M) and thin film at
77 K.
mass and elemental analysis data.
2.3 Thermal properties and DFT calculation
The thermal properties of TPOTP were determined by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurement and the results are shown in
Fig. 2 and Fig. S2. Even though TPOTP appears to be a simple
molecule, it shows an extremely high glass transition
temperature
conventionally
(
Tg)
of 220 °C
much higher than the
used ETL
TPOTP
Scheme 1 Synthesis of TPOTP.
2.2 Photophysical properties
Tg
The UVꢀvis absorption and photoluminescence spectra at room
temperature and at 77 K of TPOTP are shown in Fig. 1,
whereas the peak maxima of these spectra are summarized in
Table 1. The absorption peak appears at 260 nm in
dichloromethane (10ꢀ5 M) solution and 267 nm in the thinꢀfilm,
Tm
100
150
200
250
which can be attributed to πꢀπ* transitions of the molecule (see
Temperature, oC
Fig. 3 for the HOMO and LUMO). The fluorescence peak
showed at 350 nm in dichloromethane (10ꢀ5 M) solution and
368 nm in the thin film. The observed red shift in the thin film
Fig. 2 Differential scanning calorimetry (DSC) trace of TPOTP with a scan rate of
10 C/min.
°
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Table 1 Physical and thermal properties of TPOTP
λabs
λabs
λem
λem
λem
λem
ETM
Tg (°C)e Tm (°C)e Td (°C)f LUMO (eV)g HOMO (eV)h Eg (eV)i ET(eV)j
(nm)a (nm)b (nm)a (nm)b (nm)c (nm)d
TPOTP
260
267
350
368
461
489
220
278
425
2.26
5.85
3.59
2.76
aMeasured in DCM (1×10ꢀ5 M) at room temperature. bMeasured in thin film at room temperature. cPhosphorescence measured in 2ꢀMeTHF (1×10ꢀ5 M) at 77
K. dPhosphorescence measured in thin film at 77 K. eObtained from DSC measurements. fObtained from TGA measurement. gMeasured from the reduction
potential in 10ꢀ3 M THF solution by cyclic voltammetry. hMeasured from LUMO+Eg. iEstimated from the optical absorption threshold. jEstimated from the
onset of phosphorescence spectrum.
materials such as BCP (80 °C) and TAZ (70 °C).12 Also, it has emissive layer particularly for blue phosphorescent emitter, the
a decomposition temperature (Td) of 425 °C higher than that of transient PL decay of the TPOTP:Firpic (8%) doped thin film
OXDꢀ7 (331 °C).10 The high Tg of TPOTP suggests high was measured under nitrogen atmosphere at room temperature.
morphologic stability of the amorphous phase in the film, an As shown in Fig S3, the film exhibits nearly monoꢀexponential
important property for the device. To gain insight into the decay with a lifetime of 1.5 ꢁs.13 The result suggests that the
electronic states of TPOTP, DFT calculation was performed. triplet energy of the TPOTP is large enough to prevent leakage
The results show that the HOMO is mostly distributed over the of the blue phosphorescent triplet excitons to ETL.13
oxadiazole group and two central phenylene groups and to a
less extent over the two peripheral phenyl groups (Fig. 3).
Similar to the HOMO, the LUMO is also localized mainly on
the oxadiazole moiety and the two central phenylene groups
with negligible contribution from the four peripheral phenyls.
Further, the HOMO, LUMO and triplet energy levels were
estimated by the timeꢀdependent density functional theory
(TDDFT). The calculated HOMO and LUMO energy levels are
6.04 and 1.66 eV, respectively and the triplet energy gap is 2.75
eV. These values are in reasonable agreement with the
experimental data in Table 1.
1000
100
10
OXD-7
TPBi
BCP+Alq3
BAlq
1
TPOTP
tOXD-mTP
TmPyPB
0.1
0.01
0
5
10
15
20
V
Fig. 4 Current density vs voltage characteristics of the electron-only devices.
Fig. 3 Calculated spatial distributions of the HOMO and LUMO of TPOTP
To evaluate the performance of TPOTP as ETL, device G
was fabricated using Ir(ppy)3 as the dopant with the following
device structure: ITO/NPB (20 nm)/ TCTA (10 nm)/BCPO:
Ir(ppy)3 (8% 30 nm)/TPOTP (40 nm)/LiF (1 nm)/Al (100 nm),
To compare the electron transporting properties of this
compound with other wellꢀknown transporting materials,
several electron only devices using different EMTs were
fabricated. These devices consist of the following structure:
ITO/BCP (15 nm)/ ETM (40 nm)/LiF (1 nm)/Al (100 nm)
where BCP = 2,9ꢀdimethylꢀ4,7ꢀdiphenylꢀ1,10ꢀphenanthroline.
The current density vs voltage curves of these electronꢀonly
devices are shown in Fig. 4. TPOTP gave the lowest driving
voltage and the highest current density at any applied voltage
among the ET materials tested except TmPyPB. It is known
that TmPyPB has very high electron mobility among the ETMs.
However, TPOTP appears to have higher current density at low
and high applied voltages than TmPyPB in the electronꢀonly
devices (Fig. 4). The results indicate that TPOTP is one of the
highest electron mobility materials. The planar conformation,
where
NPB
=
N4
,
N4'ꢀdi(naphthalenꢀ1ꢀyl)ꢀN4
,
N4'
ꢀ
ꢀ
diphenylbiphenylꢀ4,4'ꢀdiamine and TCTA = 4,4′,4′′ꢀtris(
N
carbazolyl)triphenylamine are holeꢀtransporting materials and
BCPO
=
9,9'ꢀ(4,4'ꢀ(phenylphosphoryl)bis(4,1ꢀ
phenylene))bis(9
H
ꢀcarbazole) is the host. The energy levels and
the molecular structures of each layer in the device are shown
in Fig. 5, while the performances of the device are summarized
in Table 2. Device G shows a low turnꢀon voltage of 2.5 V, a
remarkably external quantum efficiency, current efficiency, and
power efficiency of 25.2%, 97.6 cd/A and 101 lm/W,
respectively. In addition, the device gave
a maximum
luminance of 115000 cd mꢀ2 with CIE of (x = 0.30, y = 0.64).
The current densityꢀvoltage and luminanceꢀvoltage curves
are shown in Fig. 6a, while the EQE vs luminance curve is
displayed in Fig. 6b. At a brightness level of 100 and 1000 cd
mꢀ2, the external quantum efficiencies still retain as high as
that would enhance the intermolecular
πꢀπ interaction, likely
accounts for the high current density of the TPOTPꢀbased
device. Further, to see the exciton confinement within the
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Fig. 5 The structures of the materials used and HOMO/LUMO levels in the EL devices
Table 2 EL performances of the devices with TPOTP as an ETM
PEmax (lm/W,
V)c
CIE (x,y),
at 8 V
Devicea
Vd (V)b
2.5
Lmax (cd/m2, V)c
115000, 16.5
35600, 17.0
EQEmax (%, V)c
25.2, 3.5
CEmax(cd/A, V)c
97.6, 3.5
λmax, (nm)d at 8V
G
B
R
101, 3.0
41.8, 3.5
20.2, 4.5
514
472
620
(0.30,0.64)
3.2
23.1, 4.0
49.1, 4.0
(0.15,0.33)
(0.67,0.33)
3.0
38000, 17.5
23.6, 5.5
29.9, 5.5
aDevice configurations: device G _ ITO/NPB (20 nm)/TCTA (10 nm)/BCPO: Ir(ppy)3 (8 wt%) (30 nm)/ TPOTP (40 nm)/LiF (1 nm)/Al (100 nm); device B ꢀ
ITO/NPB (30 nm)/mCP (20 nm)/BCPO: Firpic (8 wt%) (30 nm)/ TPOTP (40 nm)/LiF (1 nm)/Al (100 nm); device R ꢀ ITO/NPB (10 nm)/TCTA (20
nm)/CzPPQ: (piq)2Ir(acac) (4 wt%) (30 nm)/ TPOTP (30 nm)/LiF (1 nm)/Al (100 nm); bVd, The operating voltage at a brightness of 1 cd/m2; cLmax, Maximum
d
luminance; EQEmax, Maximum external quantum efficiency; CEmax, Maximum current efficiency; PEmax, Maximum power efficiency; and λmax, The
wavelength where the EL spectrum having the maximum intensity.
25.0% and 23.3%. To compare the device performance with transporting material and also as an exciton blocker to prevent
conventional electron transporting materials, device G1 was diffusion of exciton to the NPB layer.
fabricated using BCP and Alq3 as an ETM and results are
Device B (Table 2) reveals a low turnꢀon voltage of 3.2 V
summarized in Table S1. Importantly, device G show higher and high maximum external quantum efficiency, current
EQE, power and current efficiency compared to those of the efficiency and power efficiency of 23.1%, 49.1 cd/A, and 41.8
Ir(ppy)3ꢀbased green device G1 using BCP and Alq3.8b Our lm/W, respectively (Fig. 6). Moreover, a maximum luminance
newly developed ETM not only possesses thermal stability, but of 35600 cd/m2 with CIE values of (0.15, 0.33) was reached.
also reduces the device complexity with improved efficiency. Interestingly, this device shows much improved performance
The EQE and current efficiency vs. luminance of this device is (EQEꢀ23.1% compared to the previously reported blue FIrpic
displayed in Figure 4b. Unlike previous reports,14 this device device (EQEꢀ11.4%) using OXDꢀ7 as an ETM,10 Device R
shows low efficiency rollꢀoff at higher current density. The exhibits a low turnꢀon voltage of 3.0 V, maximum external
EQE of device G at luminance 1000 cd mꢀ2 retains 92.5% of quantum efficiency, current efficiency and power efficiency of
their maximum values, indicating
recombination even at a high current density (Table S2).
a
balanced carrier 23.6%, 29.9 cd/A, and 20.2 lm/W, respectively with maximum
luminance 38000 cd mꢀ2. Devices B and R also show negligible
In addition to the green phosphorescent OLED, TPOTP can rollꢀoff similar to that of device G, at the practical brightness
also be applied as a ETL/EBL for blue and red phosphorescent level of 100 and 1000 cd mꢀ2; the EQEs retain as high as 21.9%
devices. The commonly used Firpic and Ir(piq)2acac were and 20.2% for device B and 23% and 23.2% for device R. The
selected as the blue and red dopant, respectively. The structures EQEs of devices B and R at luminance 1000 cd mꢀ2 retain 92.2
of blue and red devices are ITO/NPB (30 nm)/mCP (20 and 98.3%, respectively (Table S2), of their maximum values,
nm)/BCPO: FIrpic (8%) (30 nm)/TPOTP (40 nm)/LiF (1 indicating a balanced carrier recombination even at a high
nm)/Al (100 nm) and ITO/NPB (10 nm)/TCTA (20 nm)/ current density. Although TPOTP possess shallow LUMO
(CzPPQ): dopant (4% 30 nm)/TPOTP (30 nm)/LiF (1 nm)/Al level, all devices show low turnꢀon voltages. This suggests that
(100 nm), respectively, where, CzPPQ
=
9ꢀ(4ꢀ(4ꢀ the electron injection from ETM to the host materials is very
phenylquinolinꢀ2ꢀyl)phenyl)ꢀ9Hꢀcarbazole is the host.15 Here, smooth and efficient. Since all these blue, green and red devices
1,3ꢀbis(9ꢀ
H
ꢀcarbazolꢀ9ꢀyl)benzene (
m
CP) is used as holeꢀ revealed high external quantum, current and power efficiencies.
It is fascinating to mention that by using TPOTP as the
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common EBL/ETL, the performance of these primaryꢀcolor spectra were measured on a Hitachi Fꢀ4500 spectrophotometer.
devices can be improved. To the best of our knowledge, this is The electrochemical properties were measured by 600s A CH
the first report using a common ETL material for blue, green, Instruments. The HRMS were measured on a MATꢀ95XL
and red phosphorescent devices with high performance.
HRMS. Elemental analysis was measured on an Elementar
vario EL III. The glass transition temperature was determined
by DSC under nitrogen atmosphere using
a DSCꢀQ10
350
instrument. The decomposition temperature was determined by
TGA using TG/DTA Seiko SSCꢀ5200 instrument. The
molecular geometry optimizations and electronic properties
were computed by carrying out the Gaussian 03 program with
density functional theory (DFT) and timeꢀdependent DFT
(TDDFT) calculations, in which the Becke’s threeꢀparameter
functional combined with Lee, Yang, and Parr’s correlation
functional (B3LYP) hybrid exchangeꢀcorrelation functional
with the 6ꢀ31G* basic set were used. The molecular orbitals
were visualized on the Gaussview 4.1 software.
a)
100000
10000
1000
100
300
250
200
150
100
50
B
G
R
10
1
5
10
15
4.2 Synthesis of 5-(3,5-dibromophenyl)-1H-tetrazole (35Brtaz)11a
V
To a 250 mL roundꢀbottom flask, 3,5ꢀdibromo benzonitrile
(15.65 g, 60.0 mmol), sodium azide (7.8 g, 120 mmol), zinc
chloride (16.35 g, 120 mmol), and 100 mL of DMF was added.
The reaction mixture was stirred at 120 °C for 24 h. After
completion, the reaction was cooled to room temperature and
then poured into 1N HCl solution (300 mL). The precipitated
solid was filtered and washed with water. After dried under
vacuum, white solid 35Brtaz was obtained in 70% yield.
Expected product was confirmed by 1H and 13C NMR
30
25
20
15
10
5
b)
B
G
R
1.0
0.8
0.6
0.4
0.2
0.0
1
spectroscopy. H NMR (400 MHz, DMSOꢀd6, δ): 8.12 (s, 2H),
7.92 (s, 1H); 13C NMR (100 MHz, DMSOꢀd6, δ): 155.5, 135.0,
129.6, 128.4, 123.2.
400
500
600
700
Wavelength, nm
0
10
100
1000
10000
100000
4.3 Synthesis of 2,5-bis(3,5-dibromophenyl)-1,3,4-oxadiazole
(OBr4)11b
2
Luminance, cd/m
Fig. 6 a) Current density and luminance vs driving voltage, b) External quantum
efficiency vs luminance and inset: luminance spectra of B, G and R.
A mixture of 5ꢀ(3,5ꢀdibromophenyl)ꢀ1
3,5ꢀdibromobenzoyl chloride (22.0mmol) in
H
ꢀtetrazole (20.0 mmol),
ꢀxylene (50 mL)
o
was refluxed for 8 h under nitrogen atmosphere. After
completion, the reaction mixture was cooled to room
temperature and the solvent was removed under reduced
pressure and recrystallized from n-hexane to give white solid
OBr4 in 65% yield. The desired product was confirmed by H
NMR spectroscopy and mass spectrometry. 1H NMR (400
3 Conclusion
In summary, a new highly thermally stable (Tg = 220 °C) and
high triplet energy gap (ET = 2.76 eV) with shallow LUMO
level oxadiazole based ET material, TPOTP, was synthesized.
1
The material was used as
a
universal electron
MHz, CDCl3, δ): 8.20 (d,
J = 2 Hz, 4H), 7.85 (t, J = 2 Hz, 2H);
transporter/exciton blocker for blue, green and red
phosphorescent devices. The material effectively transports
electrons and blocks the holes and excitons and improves the
charge balance in the EML of the devices, resulting in high
device efficiencies for all of the devices, particularly the green
one with an EQE of 25.2%, current efficiency of 97.6 cd/A and
power efficiency of 100.6 lm/W. Moreover, all devices show
very low efficiency rollꢀoff in the practical ranges.
HRMS (m/z): [M+] calcd. for C14H6Br4N2O, 533.7214; found,
533.7216.
4.4
Synthesis
of
2,5-di([1,1':3',1''-terphenyl]-5'-yl)-1,3,4-
oxadiazole (TPOTP)
Pd(PPh3)4 (346 mg, 0.30 mmol) was added under nitrogen
atmosphere to the solution of OBr4 (2,5ꢀbis(3,5ꢀ
dibromophenyl)ꢀ1,3,4ꢀoxadiazole) (5.34 g, 10.0 mmol),
phenylboronic acid (5.5 g, 45.0 mmol) and K2CO3 (9.3 g, 67.7
mmol) in toluene (90 mL), ethanol (40 mL) and water (40 mL)
solvent mixture. Then, the reaction mixture was heated at 120
4 Experimental section
4.1 General information
°
C for 24 h under nitrogen atmosphere. After completion, the
UVꢀvis absorption spectra were recorded using a Hitachi Uꢀ
3300 spectrophotometer. PL spectra and phosphorescent
reaction mixture was cooled to room temperature and poured
into water to precipitate the product and filtered. The crude
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product was dissolved in dichloromethane and washed with
water. The organic layer was concentrated under reduced
pressure and the residue was purified by a silica gel column
chromatography using dichloromethane/methanol mixture
(20:1, v/v) as the eluent to give a white solid TPOTP in 70%
yield. Product was confirmed by 1H and 13C NMR spectroscopy
and mass spectrometry. 1H NMR (400 MHz, CDCl3, δ): 8.35 (s,
4H), 7.97 (s, 2H), 7.72 (d, 8H), 7.50 (t, J = 7.4 Hz, 8H), 7.42 (t,
J = 7.2 Hz, 4H); 13C NMR (100 MHz, CDCl3, δ): 164.77,
142.87, 139.91, 129.41, 129.00, 128.08, 127.35, 124.85,
124.45; HRMS (m/z): [M+] calcd. for C38H26N2O, 526.2045;
found, 526.2042; Anal. calcd. for C38H26N2O: C 86.67, H 4.98,
N 5.32, O 3.04; Found: C 86.50, H 5.01, N 5.34, O 3.15.
47, 8865; j) C.ꢀH. Fan, P. Sun, T.ꢀH. Su and C.ꢀH. Cheng, Adv.
Mater. 2011, 23, 2981.
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DOI: 10.1039/C4TC02348J
TOC graphics:
A high triplet energy, high thermal stability oxadiazole derivative
as the electron transporter for highly efficient red, green and blue
phosphorescent OLEDs
Cheng-Hung Shih, P. Rajamalli, Cheng-An Wu, Ming-Jai Chiu, Li-Kang Chu and Chien-Hong
Cheng*
A high glass transition, triplet energy gap and electron mobility material, a bis(m-
terphenyl)oxadiazole derivative TPOTP, is readily synthesized. It can serve as an efficient
universal electron transporter and exciton blocker for blue, green and red phosphorescent OLEDs
with maximum external quantum efficiencies all exceeding 23%.