High triplet energy electron transport type exciton blocking materials for stable blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2016.02.025

High triplet energy electron transport materials with dibenzothiophene and dibenzofuran cores modified with a diphenyltriazine unit were investigated as electron transport type exciton blocking materials for stable blue phosphorescent organic light-emitting diodes. The two exciton blocking materials showed high triplet energy above 2.80 eV and enhanced quantum efficiency of the blue phosphorescent devices by more than 40% while maintaining stability of the pristine blue devices without the high triplet energy exciton blocking layer.

Full text of DOI:10.1016/j.orgel.2016.02.025

Organic Electronics 32 (2016) 109e114
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
High triplet energy electron transport type exciton blocking materials
for stable blue phosphorescent organic light-emitting diodes
Yu Jin Kang, Jun Yeob Lee*
School of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
High triplet energy electron transport materials with dibenzothiophene and dibenzofuran cores modified
with a diphenyltriazine unit were investigated as electron transport type exciton blocking materials for
stable blue phosphorescent organic light-emitting diodes. The two exciton blocking materials showed
high triplet energy above 2.80 eV and enhanced quantum efficiency of the blue phosphorescent devices
by more than 40% while maintaining stability of the pristine blue devices without the high triplet energy
exciton blocking layer.
Received 29 August 2015
Received in revised form
12 February 2016
Accepted 15 February 2016
Available online xxx
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Exciton blocking layer
Dibenzothiophene
Dibenzofuran
Diphenyltriazine
1. Introduction
confining excitons and injecting holes [6,9e12]. However, most
high triplet energy hole transport type exciton blocking materials
Blue phosphorescent organic light-emitting diodes (PHOLEDs)
require triplet exciton blocking functional layers specially designed
to have high triplet energy above 2.70 eV to put down triplet
exciton quenching and leakage [1e3]. The triplet exciton blocking
functional layers are essential in the blue PHOLEDs and are inserted
at the interface between phosphorescent emitting layers and
charge transport layers. Both hole transport type and electron
transport type exciton blocking layers are inserted because triplet
excitons can diffuse into both hole and electron transport layers
[4e12]. Therefore, the development of the triplet exciton blocking
layers was a challenging issue to promote the light-emitting per-
formances of the blue PHOLEDs.
had negative effect on the operational lifetime of the blue PHOLEDs
because of instability of the exciton blocking materials. The
degradation of the blue PHOLEDs was even more serious when
electron transport type exciton blocking materials were adopted
[13e16]. Therefore, most stable blue PHOLEDs could not use any
electron transport type exciton blocking materials in spite of low
EQE by triplet exciton quenching caused by low triplet energy of
common electron transport materials [17]. The problem of the low
EQE of the stable blue PHOLEDs can be resolved by design and
synthesis of stable electron transport type exciton blocking
materials.
In this work, two electron transport type exciton blocking ma-
Intense research and development of the exciton blocking ma-
terials upgraded external quantum efficiency (EQE) of the blue
PHOLEDs by suppressing leakage and quenching of triplet excitons
by charge transport materials. Several groups demonstrated above
20% EQE in the blue PHOLEDs by development of hole and electron
transport type exciton blocking functional layers [6e8]. Carbazole
or aromatic amine functionalized organic materials worked effi-
ciently as a hole transport type exciton blocking materials by
terials,
2,8-bis(4,6-diphenyl-1,3,5-triazin-2-yl)dibenzo[b,d]thio-
phene (DBTTrz) and 2,8-bis(4,6-diphenyl-1,3,5-triazin-2-yl)
dibenzo[b,d]furan (DBFTrz) were introduced as the stable high
triplet energy exciton blocking functional materials of blue PHO-
LEDs. DBTTrz and DBFTrz had only stable aromatic moieties in the
molecular structure and enhanced the EQE of the blue PHOLEDs
with the lifetime of the blue PHOLEDs unchanged. Therefore, the
DBTTrz and DBFTrz exciton blocking materials can be used to in-
crease the EQE of stable blue PHOLEDs.
* Corresponding author.
E-mail address: leej17@skku.edu (J.Y. Lee).
http://dx.doi.org/10.1016/j.orgel.2016.02.025
1566-1199/© 2016 Elsevier B.V. All rights reserved.

110
Y.J. Kang, J.Y. Lee / Organic Electronics 32 (2016) 109e114
2. Experimental
2.2.3. 2,8-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)dibenzo
[b,d]furan
2,7-Diiodobenzene (4.0 g, 12.0 mmol), Bis(pinacolato)diboron
2.1. General information
(6.0 g, 26.0 mmol) and potassium acetate (3.0 g, 36.0 mmol) were
dissolved in 250 ml of 1,4-dioxane. [1,10-Bis(diphenylphosphino)
ferrocene]palladium(II) dichloride dichloromethane adduct (0.3 g,
0.4 mmol) are added in the mixture and it was refluxed for over-
night. After cooling the solution to room temperature, it was
quenched with distilled water and the solution was extracted using
methylene chloride. The crude product was purified by column
chromatography on silica gel using n-hexane/MC and the product
was obtained as a yellowish white powder after sublimation (2.5 g,
yield 65%). MS (APCI) miz 420.2 [(M)þ].
2-Chloro-4,6-diphenyl-1,3,5-triazine (Sun fine global Co.), tet-
rakis(triphenylphosphine)palladium(0) (P&H Tech), potassium
carbonate, n-hexane, magnesium sulfate anhydrous (Duksan Sci.
Co.). These chemical were used without further purification.
Tetrahydrofuran was distilled over sodium and calcium hydride.
Chemical analysis of the synthesized materials referred to the
method reported in other literature [8].
2.2. Synthesis
2.2.1. 2,8-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)dibenzo
[b,d]thiophene
2.2.4. 2,8-Bis(4,6-diphenyl-1,3,5-triazin-2-yl)dibenzo[b,d]furan
(DBFTrz)
Under nitrogen, 2,8-dibromodibenzo[b,d]thiophene (1.0 g,
3.0 mmol), Bis(pinacolato)diboron (1.85 g, 8.0 mmol) and potas-
sium acetate (0.9 g, 10.0 mmol) were dissolved in 100 ml of 1,4-
2,8-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)dibenzo
[b,d]furan (0.7 g, 1.6 mmol) and 2-chloro-4,6-diphenyl-1,3,5-
triazine (1.0 g, 3.8 mmol) were dissolved in 10 ml of anhydride
tetrahydrofuran. After stirring for 30 min at room temperature,
tetrakis(triphenylphosphine)palladium (12.0 mg, 0.1 mmol) was
added in mixture and followed by addition of 2 M aqueous po-
tassium carbonate (2.8 g in 10 ml distilled water). Work-up pro-
cedure of DBFTrz was the same as that of DBTTrz and the product
was obtained as a white powder after sublimation (1.1 g, yield 73%).
1H NMR (400 MHZ, CDCl3): 9.57e9.56 (d, 1H, J ¼ 0.8 Hz),
9.04e9.01 (d, 1H, J ¼ 5.2 Hz), 8.87e8.85 (d, 4H, J ¼ 5.0 Hz), 7.81 (s,
1H), 7.79 (s, 1H), 7.72e7.63 (m, 12H), 7.54e7.51 (m, 6H) MS (APCI)
miz 630.7 [(M)þ].
dioxane.
[1,10-Bis(diphenylphosphino)ferrocene]palladium(II)
dichloride dichloromethane adduct (7.1 mg, 0.1 mmol) are added in
the mixture. It was refluxed for overnight and quenched with
distilled water followed by extracting with ethyl acetate. The
mixture was purified by column chromatography on silica gel using
ethyl acetate and hexane. The yellowish white powder was ob-
tained to 1.2 g. MS (APCI) miz 436.2 [(M)þ].
2.2.2. 2,8-Bis(4,6-diphenyl-1,3,5-triazin-2-yl)dibenzo[b,d]
thiophene (DBTTrz)
2,8-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)dibenzo
[b,d]thiophene (0.6 g, 1.3 mmol), and 2-chloro-4,6-diphenyl-1,3,5-
triazine (0.8 g, 3.1 mmol) were dissolved in 10 ml of anhydride
tetrahydrofuran. After stirring for 30 min at room temperature,
tetrakis(triphenylphosphine)palladium(0) (0.01 g, 0.1 mmol) was
added and 2 M aqueous potassium carbonate (2.8 g in 10 ml
distilled water) was put to the reaction mixture instantly. The re-
action mixture was refluxed overnight and a white powder are
formed. The crude product was separated by filtration and purified
by washing using tetrahydrofuran and hexane. A white product was
finally obtained (1.2 g, 90% yield).
2.3. Device fabrication and measurements
The device structure of the blue PHOLEDs, indium tin oxide/
DNTPD (60 nm)/BPBPA (30 nm)/mCBP: Ir(dbi)3 (25 nm, 10% doping)/
LG201 (35 nm)/LiF (1 nm)/Al (200 nm). was the same as that
described in the literature [18]. The only difference was that DBTTrz
or DBFTrz was inserted between 3,3-di(9H-carbazol-9-yl)biphenyl
(mCBP):
tris[1-(2,4-diisopropyldibenzo[b,d]furan-3-yl)-2-
phenylimidazole] iridium(III) (Ir(dbi)3) emitting layer and 9,10-
di(naphthalene-2-yl)anthracen-2-yl-(4,1-phenylene)(1-phenyl-1H-
benzo[d]imidazole (NAPIm) electron transport layer. DNTPD and
BPBPA were N,N0-diphenyl-N,N0-bis-[4-(phenyl-m-tolyl-amino)-
phenyl]-biphenyl-4,40-diamine, N,N,N0N0-tetra[(1,10-biphenyl)-4-yl]-
(1,10-biphenyl)-4,40-diamine, respectively. The thickness of the
exciton blocking layer was 5 nm. Lifetime data of the blue PHOLEDs
1H NMR (400 MHZ, CDCl3): 9.83 (s, 1H), 8.98e8.96 (d, 1H,
J ¼ 5.0 Hz), 8.91e8.87 (d, 4H, J ¼ 4.8 Hz), 8.10e8.08 (t, 2H,
J ¼ 2.93 Hz), 7.71e7.63 (m, 12H), 7.54e7.51 (m, 6H) MS (APCI) miz
646.7 [(M)þ]. Elemental Analysis (calculated for C42H26N6S): C,
78.00; H, 4.05; N, 12.99. Found: C, 78.46; H, 4.25; N, 12.99.
Scheme 1. Synthetic scheme of DBTTrz and DBFTrz.

Y.J. Kang, J.Y. Lee / Organic Electronics 32 (2016) 109e114
111
Fig. 1. HOMO and LUMO distribution of DBTTrz and DBFTrz.
Fig. 4. Current density-voltage-luminance curves of DBTTrz and DBFTrz devices.
Fig. 2. UVevis, Solution PL and low temperature PL spectra of DBTTrz and DBFTrz.
Fig. 5. Current density-voltage plots of single charge devices of DBTTrz and DBFTrz.
3. Results and discussion
Fig. 3. Reduction curves of DBTTrz and DBFTrz during cyclic voltammetry scan.
Two important parameters to be considered in the design of the
stable exciton blocking materials for high EQE and stable lifetime
are triplet energy and bond dissociation energy of chemical moi-
eties included in the molecular structure. From the chemical
were collected at 1000 cd/m2 at a fixed current. Luminance change
and voltage rise were traced according to driving time of the devices.

112
Y.J. Kang, J.Y. Lee / Organic Electronics 32 (2016) 109e114
and long lifetime can be satisfied by proper management of
chemical structure of the exciton blocking materials.
Main backbone structures of two exciton blocking materials
designed in this work were dibenzothiophene and dibenzofuran for
high triplet energy and chemical stability. Those moieties were
already proven as chemical units for long lifetime in the design of
host materials due to rigidity and planar molecular structure. It was
reported that the dibenzothiophene and dibenzofuran moieties are
superior to biphenyl for long lifetime in the blue PHOLEDs, which
encouraged us to use those moieties in the design of exciton
blocking materials. Moreover, those moieties may increase thermal
stability of the exciton blocking materials due to rigidity. However,
the two core moieties have weak electron carrying ability as elec-
tron transport type exciton blocking materials, which was
compensated by modification of the core structures with a diphe-
nyltriazine moiety which has good electron transporting character,
chemical stability and high triplet energy. Therefore, the DBTTrz
and DBFTrz exciton blocking materials which have high triplet
energy and stable chemical building blocks may fulfil the require-
ment of the electron transport type exciton blocking materials.
The preparation of DBTTrz and DBFTrz was made by simple
Suzuki coupling reaction of boronic ester modified dibenzofuran
and dibenzothiophene with chlorinated diphenyltriazine. Pd cata-
lyst assisted the coupling reaction and production yields of DBTTrz
and DBFTrz were 90% and 73%, respectively. The DBTTrz and DBFTrz
compounds were ascertained by nuclear magnetic resonance, mass
and elemental analysis. Scheme 1 shows synthetic route of DBTTrz
and DBFTrz.
Fig. 6. Quantum efficiency-luminance curves of the blue PHOLED devices.
The appropriateness of the two compounds as exciton blocking
materials was investigated by calculating the molecular orbital
with Gaussian 09 Software. A B3LYP 6-31G* basis set based calcu-
lation result in Fig. 1 represents the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) of DBTTrz and DBFTrz. The HOMO and LUMO of DBTTrz and
DBFTrz were similarly distributed with the HOMO localization on
the dibenzothiophene and dibenzofuran core, and the LUMO
extension from the core to the triazine moiety. The strong electron
deficiency of the triazine moiety determined the HOMO and LUMO
distribution. As the LUMO electronic orbital was uniformly spread
over the core and two triazine moieties, the DBTTrz and DBFTrz
compounds are expected to transport electrons effectively through
large overlap integral for electron hopping.
Fig. 7. Energy level diagram of the blue PHOLED devices.
Reference
100
DBFTrz
DBTTrz
80
60
40
20
0
Two important photophysical parameters, HOMO-LUMO gap
and triplet energy, were extracted from ultraviolet-visible (UVevis)
absorption and low temperature photoluminescence (PL) spectra in
Fig. 2. Tetrahydrofuran solution of the two materials was used for
the photophysical analysis. Main UVevis absorption peaks of
DBTTrz and DBFTrz corresponding to
difference, but the UVevis absorption edge reflecting n-
p
ꢀ
p
* transition were of little
p
* transi-
0
20
40
60
80
tion was different. The UVevis absorption edge was extended to
long wavelength in the DBTTrz by the loosely bound non-bonding
electron of dibenzothiophene. HOMO-LUMO gaps of DBTTrz and
DBFTrz from the UVevis absorption edge were 3.41 eV and 3.57 eV,
respectively. Phosphorescent emission peak from low temperature
PL measurement provided a high triplet energy of 2.80 eV and
2.94 eV in DBTTrz and DBFTrz, respectively. The assembly of the
high triplet energy core and diphenyltriazine gave high triplet en-
ergy in the DBTTrz and DBFTrz.
Estimation of the HOMO and LUMO levels is essential to study
the applicability of DBTTrz and DBFTrz as exciton blocking layers.
Therefore, cyclic voltammetry (CV) analysis of DBTTrz and DBFTrz
was carried out to estimate ionization potential (IP) and electron
affinity (EA). However, IP could not be calculated because oxidation
behaviour was not stable possibly due to poor electron donating
character of DBTTrz and DBFTrz. Only the reduction behaviour was
Time (h)
Fig. 8. Lifetime curves of the blue PHOLED devices. Lifetime was measured at an initial
luminance of 1000 cd mꢀ2
.
stability point of view, pure aromatic moieties are favourable due to
high dissociation energy of the chemical bond in the aromatic
moieties. Small dihedral angle between aromatic moieties is also
favoured because extended conjugation structure stabilizes the
chemical bond between aromatics and can extend the lifetime of
the devices. From the EQE point of view, only high triplet energy
moieties need to be used to construct the exciton blocking mate-
rials and conjugation between aromatic moieties should be mini-
mized. Therefore, the two main requirements of high triplet energy

Y.J. Kang, J.Y. Lee / Organic Electronics 32 (2016) 109e114
113
Fig. 9. Dihedral angle between diphenyltriazine and core moieties.
maximum EQE from 8.3% to 13.0% was demonstrated by inserting
the DBTTrz and DBFTrz exciton blocking layers. As can be easily
projected from the energy level diagram in Fig. 7, triplet exciton
blocking and hole blocking effect of DBTTrz and DBFTrz were
dominant factors for the enhanced EQE.
The stable molecular structure of DBTTrz and DBFTrz may be
helpful to stabilize the blue PHOLEDs for long term driving.
Therefore, the effect of the exciton blocking materials on the life-
time of the blue PHOLEDs was examined by collecting luminance
data according to driving time at a constant current driving mode.
Luminance decay curves in Fig. 8 suggests that the DBTTrz and
DBFTrz exciton blocking materials had little negative effect on the
lifetime of the blue PHOLEDs. Although initial drop of luminance
was slightly significant in the DBTTrz and DBFTrz devices, the
luminance decay after initial decrease was similar. Typically, it is
difficult to introduce the high triplet energy exciton blocking layer
because they quickly degrade the blue PHOLEDs due to intrinsic
chemical instability of the high triplet energy compound. However,
the DBTTrz and DBFTrz are composed of stable chemical moieties
and are widely conjugated by small dihedral angle (0ꢁ) between the
dibenzothiophene or dibenzofuran core and diphenyltriazine
moiety (Fig. 9), which allowed them to maintain the lifetime of the
reference device without the exciton blocking layer. Additionally,
high glass transition temperature of DBTTrz (195 ꢁC) and DBFTrz
(140 ꢁC) as shown in differential scanning calorimeter (DSC) ther-
mograms in Fig. 10 thermally stabilized the DBTTrz and DBFTrz
exciton blocking materials and made a contribution to the stable
operation of the DBTTrz and DBFTrz devices. Therefore, the DBTTrz
and DBFTrz enhanced the EQE of blue PHOLEDs without any
degradation of long-term stability.
Fig. 10. DSC thermograms of DBTTrz and DBFTrz.
clear from reverse bias scan of CV. Reduction curves of DBTTrz and
DBFTrz in Fig. 3 suggested reduction onsets of ꢀ1.61 V and ꢀ1.73 V,
which were changed into ꢀ3.19 eV and ꢀ3.07 eV as EA values of
DBTTrz and DBFTrz using ferrocene as a standard material. There-
fore, the HOMO/LUMOs of DBTTrz and DBFTrz were proposed
as ꢀ6.60/ꢀ3.19 eV and ꢀ6.64/ꢀ3.07 eV, respectively, using the
measured LUMO values and HOMO-LUMO gap from absorption
edge of the UVevis measurement. The HOMO and LUMO of DBTTrz
and DBFTrz were deep enough for hole blocking and electron in-
jection as exciton blocking materials.
As DBTTrz and DBFTrz were confirmed to have triplet energy for
triplet exciton blocking, HOMO level for hole blocking and LUMO
level for electron injection, they were inserted as an exciton
blocking layer of blue PHOLEDs. Voltage scan of the blue PHOLEDs
produced the current density-voltage and luminance-voltage
curves in Fig. 4. A reference device without the DBTTrz or DBFTrz
exciton blocking layer was also evaluated to judge the effectiveness
of the exciton blocking materials. The current density and lumi-
nance of the DBFTrz device were higher than those of the DBTTrz
device, indicating that electron carrying ability of DBFTrz is better
than that of DBTTrz, which was verified by electron current density
of electron only devices in Fig. 5. The high electron current density
of the DBFTrz electron only device supports the good electron
transport character of DBFTrz, which is due to high electronega-
tivity of oxygen of DBFTrz. The relatively electron withdrawing
character of oxygen to sulfur enhances electron accepting character
of the DBFTrz, improving electron transport performance. However,
the current density of the DBFTrz device was not increased
compared to that of the reference device without the exciton
blocking layer.
4. Conclusions
Design, synthesis, and device evaluation of DBTTrz and DBFTrz
exciton blocking materials were investigated and the two com-
pounds were effective to improve the EQE of blue PHOLEDs without
any degradation of device lifetime. Small dihedral angle between
aromatic moieties of DBTTrz and DBFTrz was proposed as the key
factor for the stable operation of the blue PHOLEDs in the presence
of the high triplet energy exciton blocking materials. Therefore, the
dibenzothiophene or dibenzofuran derived triazine type com-
pounds can be suitable for developing stable blue PHOLEDs and
various modifications of the DBTTrz and DBFTrz may produce
better exciton blocking materials than DBTTrz and DBFTrz in terms
of EQE and efficiency.
Acknowledgements
EQE plot obtained from the current density and luminance data
of the blue PHOLEDs is shown in Fig. 6. Dramatic increase of
This paper was supported by Samsung Research Fund, Sung-
kyunkwan University, 2015

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Y.J. Kang, J.Y. Lee / Organic Electronics 32 (2016) 109e114
[9] M.-F. Wu, S.-J. Yeh, C.-T. Chen, H. Murayama, T. Tsuboi, W.-S. Li, I. Chao, S.-
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