Design of bicarbazole type host materials for long-term stability in blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2017.01.021

Two bicarbazole type host materials, 9-(dibenzo [b,d]thiophen-4-yl)-9?-phenyl-9H,9′H-3,3?-bicarbazole (DBTBCz) and 9,9?-bis(dibenzo [b,d]thiophen-4-yl)-9H,9′H-3,3?-bicarbazole (DDBTBCz), were developed as lifetime enhancing host materials for blue phosphorescent organic light-emitting diodes (PhOLEDs). The DBTBCz and DDBTBCz host materials were prepared by substituting one or two dibenzothiophene units to a 3,3?-bicarbazole backbone structure for the purpose of improving thermal stability and rigidity of the host materials for stable operational lifetime. Device characterization of the host materials revealed that the dibenzothiophene modification via 4- position is better than that via 2- position for improved lifetime of blue PhOLEDs.

Full text of DOI:10.1016/j.orgel.2017.01.021

Organic Electronics 43 (2017) 130e135
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Design of bicarbazole type host materials for long-term stability in
blue phosphorescent organic light-emitting diodes
Hyeong Min Kim, Jeong Min Choi, 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
Two bicarbazole type host materials, 9-(dibenzo [b,d]thiophen-4-yl)-9ʹ-phenyl-9H,90H-3,3ʹ-bicarbazole
(DBTBCz) and 9,9ʹ-bis(dibenzo [b,d]thiophen-4-yl)-9H,90H-3,3ʹ-bicarbazole (DDBTBCz), were developed
as lifetime enhancing host materials for blue phosphorescent organic light-emitting diodes (PhOLEDs).
The DBTBCz and DDBTBCz host materials were prepared by substituting one or two dibenzothiophene
units to a 3,3ʹ-bicarbazole backbone structure for the purpose of improving thermal stability and rigidity
of the host materials for stable operational lifetime. Device characterization of the host materials
revealed that the dibenzothiophene modification via 4- position is better than that via 2- position for
improved lifetime of blue PhOLEDs.
Article history:
Received 14 November 2016
Received in revised form
15 January 2017
Accepted 15 January 2017
Available online 17 January 2017
Keywords:
Dibenzothiophene
Lifetime
© 2017 Elsevier B.V. All rights reserved.
Blue phosphorescent device
Host
1. Introduction
terms of QE of the blue PhOLEDs [11]. There are many reasons for
the poor lifetime of blue PhOLEDs and lack of stable high triplet
Lifetime is one of the most important device performances of
organic light-emitting diodes (OLEDs) in addition to quantum ef-
ficiency because short lifetime device cannot be applied in practical
applications [1e5]. In particular, the lifetime of the blue OLEDs is
relatively short compared with that of red and green OLEDs, which
encouraged researchers to develop stable blue OLEDs [6]. Through
extensive studies and effort, the lifetime of blue fluorescent OLEDs
was significantly extended although it is still inferior to that of red
and green fluorescent OLEDs [7]. However, the blue fluorescent
OLEDs suffer from low quantum efficiency (QE) limited by spin
statistics defining singlet and triplet exciton formation, which
incited the development of high efficiency blue phosphorescent
OLEDs (PhOEDs) with 100% internal quantum efficiency (QE)
theoretically [8].
The theoretical maximum QE of the blue PhOLEDs has already
been reached through molecular engineering of hosts, phospho-
rescent emitters and charge transport materials. Device engineer-
ing to confine all excitons and carriers inside an emitting layer also
aided the full utilization of singlet and triplet excitons for blue
phosphorescent emission [9,10]. However, the lifetime of blue
PhOLEDs is still very short in spite of the great advances made in
energy materials is one of critical problems [12e14]. Although
many high triplet energy host and charge transport materials were
reported, most materials could not work stably due to instability of
the high triplet energy organic materials [15,17,18,22e24]. Only
several host and charge transport materials were developed as the
stable organic materials to obtain improved lifetime. For instance, a
CN modified host material, 9-(300-(carbazol-9-yl)-[1,10,30,100-ter-
phenyl]-3-yl)-carbazole-3-carbonitrile, was reported as a lifetime
boosting host material by strengthened chemical bond [25], and
thiophene-carbazole derivatives were also lifetime-extending host
materials [20,26]. Therefore, the development of the host materials
to fulfil the stability requirement of the blue PhOLEDs is essential to
prolong the lifetime of the blue PhOLEDs.
Herein, we describe the synthesis of bicarbazole backbone
structure based host materials possessing 4- position modified
dibenzothiophene as substituent of the backbone structure. The
bicarbazole backbone structure was either substituted with one
dibenzothiophene unit or two dibenzothiophene units to yield 9-
(dibenzo
[b,d]thiophen-4-yl)-9ʹ-phenyl-9H,90H-3,3ʹ-bicarbazole
(DBTBCz) and 9,9ʹ-bis(dibenzo [b,d]thiophen-4-yl)-9H,90H-3,3ʹ-
bicarbazole (DDBTBCz) as the host materials. The two host mate-
rials with the dibenzotiophene unit linked via 4- position to the
bicarbazole backbone structure outperformed the host material
with the same dibenzothiophene unit linked via 2- position to the
same backbone structure as the host of blue emitting tris [1-(2,4-
* Corresponding author.
E-mail address: leej17@skku.edu (J.Y. Lee).
http://dx.doi.org/10.1016/j.orgel.2017.01.021
1566-1199/© 2017 Elsevier B.V. All rights reserved.

H.M. Kim et al. / Organic Electronics 43 (2017) 130e135
131
diisopropyldibenzo [b,d]furan-3-yl)-2-phenylimidazole] (Ir (dbi)3)
Lifetime devices had the following stack structure.
triplet emitter.
ITO/DNTPD (60 nm)/BPBPA (30 nm)/host:Ir (dbi)3 (25 nm,
5e15% doping)/LG201 (35 nm)/LiF (1 nm)/Al (200 nm).
2. Experimental
ITO represents indium tin oxide, DNTPD is N,Nʹ-diphenyl-N,Nʹ-
bis-[4-(phenyl-m-tolyl-amino)-phenyl]-biphenyl-4,4ʹ-diamine, and
BPBPA is N,N,NʹNʹ-tetra [(1,1ʹ-biphenyl)-4-yl]-(1,1ʹ-biphenyl)-4,4ʹ-
diamine. Lifetime data of the blue PHOLEDs were gathered at an
interval of 0.07 h at a constant current mode.
2.1. General information
All chemicals and reagents were used without further purifi-
cation. Copper iodide and ( )-trans-1,2-diaminocyclohexane pur-
chased from Sigma Aldrich Co. were used without purification. 3-
Bromo-9H-carbazole, (9-phenyl-9H-carbazol-3-yl)boronic acid,
and tetrakis (tripheylpholsphine) palladium (0) from P&H Tech Co.
were used as received. 4-Iododibenzo [b,d]thiophene and 9H,90H-
3,3ʹ-bicarbazole were purchased from INCO Co.. Potassium phos-
phate tribasic, potassium carbonate, dimethylformamide, tetrahy-
drofuran, dichloromethane and magnesium sulfate were products
of Duksan Sci. Co.. 9-Phenyl-9H,90H-3,3ʹ-bicarbazole was synthe-
sized according to the literature [21].
3. Results and discussion
Several bicarbazole backbone derived host materials have been
known as the host materials of blue PhOLEDs [19,20] and the
merits of the bicarbazole backbone structure are rigidity and
adjusted highest occupied molecular orbital (HOMO) level for
good hole injection. The adjusted HOMO level for hole injection
from an aromatic amine type hole transport material or carbazole
type hole transport material may prevent charge accumulation at
the interface between hole transport layer and emitting layer. The
rigidity of the backbone structure may allow the host material to
have thermal stability and amorphous character for suppressed
crystallization. Moreover, the two carbazole units in the backbone
structure are linked by stable sp2 CeC bond which can be stable
under long-term electrical operation. These benefits of the
bicarbazole backbone based host materials can be strengthened
by modifying the bicarbazole moiety with a dibenzothiophene
moiety as reported in our previous work [20]. The bicarbazole
type host material modified with 2- position activated dibenzo-
thiophene (BCzDBT) was better than other bicarbazole host ma-
terials substituted with dibenzofuran or biphenyl. Although it was
found that the 2- position activated dibenzothiophene modifica-
tion worked effectively, the lifetime was rather short. In order to
resolve the shortcoming of the 2- position modification of
dibenzothiophene, 4- position activated dibenzothiophene was
adopted to construct the bicarbazole type host materials. Main
reason of designing the hosts using 4- position substituted
dibenzothiophene was to stabilize the molecule by added rigidity
as reported in other works [26] and also to improve hole transport
properties to properly manage the carrier recombination zone in
the emitting layer.
2.2. Synthesis
2.2.1. 9-(dibenzo[b,d]thiophen-4-yl)-9ʹ-phenyl-9H,90H-3,3ʹ-
bicarbazole (DBTBCz)
9H,90H-3,3ʹ-bicarbazole (1.50 g, 3.67 mmol), 4-iododibenzo [b,d]
thiophene (1.48 g, 4.77 mmol), copper iodide (0.21 g, 1.10 mmol),
potassium phosphate (2.34 g, 11.02 mmol) and ( )-trans-1,2-
diaminocyclohexane (0.13 ml, 1.10 mmol) in 1,4-dioxane (50 ml)
were refluxed under nitrogen for 18 h and cooled to room tem-
perature. After filtering the solution, the filtrate was dried and
extracted with dichloromethane and distilled water. The organic
layer was dehydrated with magnesium sulfate and then dried un-
der vacuum condition. After purification, a white solid was
collected by column chromatography (dichloromethane: hexane
(1: 1)).
Yield 76%, 1.00 g. 1H NMR (500 MHz, DMSO-d6):
d 8.75 (s, 1H),
8.70 (s, 1H), 8.63 (d, 1H, J ¼ 9.0Hz), 8.48 (d, 2H, J ¼ 7.5Hz), 8.44 (d,
2H, J ¼ 8.0Hz), 8.34 (d, 2H, J ¼ 7.5Hz), 7.94 (d, 2H, J ¼ 7.5Hz),
7.93e7.80 (m, 4H), 7.72e7.66 (m, 4H), 7.58e7.32 (m, 9H), 7.20 (d,
1H, J ¼ 8.5Hz), 7.13 (d,1H, J ¼ 8.0Hz). 13C NMR (125 MHz, DMSO-d6):
d
140.6, 140.2, 139.3, 138.8, 138.2, 137.6, 137.0, 136.9, 135.1, 133.6,
133.3, 131.4, 130.2, 127.7, 127.6, 126.8, 126.6, 126.4, 126.3, 125.6,
125.5, 125.1, 123.6, 123.5, 123.2, 123.1, 123.0, 122,6, 122.3, 121.0,
120.8, 120.4, 120.1, 118.8, 118.6, 110.5, 110.2, 110.0, 109.7. LC/MS (m/
z): found, 590.7 ([M þ H]þ); Calcd. for C42H26N2S, 590.7.
Synthesis procedure of DDBTBCz followed the synthetic method
of other bicarbazole type compounds as depicted in Scheme 1. The
only difference was that 9-phenyl-9H,90H-3,3ʹ-bicarbazole was
replaced with 9H,90H-3,3ʹ-bicarbazole. DBTBCz was produced from
9H,90H-3,3ʹ-bicarbazole intermediate with only one active carba-
zole for substitution.
2.2.2. 9,90-Bis(dibenzo[b,d]thiophen-4-yl)-9H,90H-3,3ʹ-bicarbazole
(DDBTBCz)
The synthesized compounds were electrochemically and pho-
tophysically characterized after sublimation to measure the HOMO,
lowest unoccupied molecular orbital (LUMO), singlet energy, and
triplet energy. Electrochemical characterization of DBTBCz and
DDBTBCz by cyclic voltammetry (CV) estimated the HOMO and
LUMO from oxidation and reduction onset of the voltage scan in
Fig. 1. The HOMO of the two host materials was ꢀ5.85 eV, and the
LUMO values of DBTBCz and DDBTBCz were ꢀ2.65 eV and ꢀ2.60 eV,
respectively, by calibrating the oxidation and reduction onset
voltages using Ferrocene as the standard material. In order to un-
derstand the electrochemical characterization data, the HOMO and
LUMO calculation results were obtained after optimizing the ge-
ometry of the host material using Gaussian 09 program providing
B3LYP 6-31G* basis set. The HOMO calculation results of DBTBCz
and DDBTBCz in Fig. 2 were of little difference and the HOMO was
evenly delocalized over the bicarbazole backbone structure. The
LUMO calculation results were also similar in the two host mate-
rials because the LUMO was localized on the dibenzothiophene
unit. These similar orbital calculation results in the two compounds
9,90-Bis(dibenzo
[b,d]thiophen-4-yl)-9H,90H-3,3ʹ-bicarbazole
was synthesized in concordance with the synthetic procedure of
DBTBCz except for use of 9H,90H-3,3ʹ-bicarbazole instead of 9-
phenyl-9H,90H-3,3ʹ-bicarbazole.
Yield 48%, 1.50 g. 1H NMR (500 MHz, DMSO-d6):
d 8.74 (s, 1H),
8.60 (d, 1H, J ¼ 7.5Hz) 8.49 (d, 1H, J ¼ 8.0Hz), 8.42 (d, 1H, J ¼ 7.5Hz),
7.90 (d, 1H, J ¼ 8.0Hz), 7.80e7.75 (m, 3H), 7.56e7.49 (m, 2H), 7.39 (t,
1H, J ¼ 15.0Hz), 7.33 (t,1H, J ¼ 14.5Hz), 7.16 (d,1H, J ¼ 8.5Hz), 7.11 (d,
1H, J ¼ 8.0Hz),. 13C NMR (125 MHz, DMSO-d6):
d 140.6, 138.8, 138.2,
137.6, 137.0, 135.1, 133.6, 131.4, 127.7, 126.7, 126.4, 126.3, 125.5, 125.1,
123.6, 123.1, 122.5, 122.3, 121.0, 120.3, 118.9, 110.5, 110.1. LC/MS (m/
z): found, 696.9 ([M þ H]þ); Calcd. for C48H28N2S2, 696.9.
2.3. Device fabrication and measurements
Device fabrication process, device structure and device perfor-
mance measurement of the blue PHOLEDs with the DBTBCz and
DDBTBCz hosts were the same as those described in other work.20

132
H.M. Kim et al. / Organic Electronics 43 (2017) 130e135
Scheme 1. Synthetic scheme of DBTBCz and DDBTBCz.
can explain the similar HOMO and LUMO level of DBTBCz and
DDBTBCz.
Photophysical characterization results of DBTBCz and DDBTBCz
are presented in Fig. 3. Singlet energy and triplet energy were
calculated from the photophysical measurement results by fluo-
rescence spectrometer. The singlet energy from the solution pho-
toluminescence (PL) peak position was 3.09 eV in the two
compounds and the triplet energies from the first phosphorescent
emission peak of low temperature PL were 2.92 eV and 2.94 eV in
the DBTBCz and DDBTBCz, respectively. The introduction of the
dibenzothiophene moiety through 4- position had little influence
on the singlet and triplet energy of the host materials judging from
the similar singlet and triplet energy of DBTBCz and DDBTBCz.
Comparing the PL emission spectra of the two host materials with
ultravioletevisible (UVevis) absorption spectrum of Ir (dbi)3 [16], it
can be projected that they may be good energy transferring host
material of Ir (dbi)3. High singlet energy (>3.0 eV) and triplet en-
ergy (>2.9 eV) of DBTBCz and DDBTBCz may activate both Forster
and Dexter energy transfer processes between the host materials
and Ir (dbi)3, and can lead to full harvesting of singlet and triplet
excitons in the phosphorescent emission process of Ir (dbi)3.
UVevis absorption spectra were also quite similar in the DBTBCz
and DDBTBCz.
Electrochemical and photophysical measurements of DBTBCz
and DDBTBCz substantiated that the two host materials are elec-
trochemically and photophysically similar. However, the two hosts
were dissimilar in terms of thermal properties represented by glass
transition temperature (Tg) and thermal decomposition tempera-
ture (Td). The Tg were extracted from second heating scan data of
differential scanning calorimeter (DSC) under nitrogen in Fig. 4 and
the Td was estimated from heating scan data of thermogravimetric
analyser (TGA) under nitrogen in Fig. 5. The 4- position
Fig. 1. CV curves of DBTBCz and DDBTBCz.
Fig. 2. HOMO and LUMO distribution and energy levels of DBTBCz and DDBTBCz.
Fig. 3. UVeVis, solution PL and low temperature PL spectra of DBTBCz and DDBTBCz.

H.M. Kim et al. / Organic Electronics 43 (2017) 130e135
133
Fig. 6. Energy level diagram of the blue PHOLEDs.
Fig. 4. DSC curves of DBTBCz and DDBTBCz.
Fig. 7. Current densityevoltage and luminance-voltage curves of DBTBCz and
DDBTBCz devices doped with Ir (dbi)3.
Fig. 5. TGA curves of DBTBCz and DDBTBCz.
modification of dibenzothiophene increased the Tg up to 148 ꢁC and
an additional dibenzothiophene modification further increased the
Tg up to 189 ꢁC due to restricted molecular motion by large mo-
lecular size and rigidity by additional 4- position modification. In
previous work, the Tg of the bicarbazole host material with two
dibenzothiophene units was 172 ꢁC, but it was increased by 17 ꢁC
just by changing the interconnect position of dibenzothiophene
owing to steric hindrance by 4- position connection [20]. This also
affected the Td at 5 wt% loss of the DBTBCz and DDBTBCz, resulting
in high Td of 478 ꢁC and 564 ꢁC in the DBTBCz and DDBTBCz,
respectively. The thermal analysis results of DBTBCz and DDBTBCz
suggest that the 4- position interconnection of dibenzothiophene
differentiates the DBTBCz and DDBTBCz host materials from pre-
vious host materials in terms of thermal stability [20]. All material
characterization results are summarized in Table 1.
The material characterization data described above proposed
that the DBTBCz and DDBTBCz host materials are appropriate as the
hosts of blue phosphorescent Ir (dbi)3 emitter. Therefore, the two
hosts were evaluated in the device structure optimized for blue
PhOLEDs. Fig. 6 shows the device structure along with energy levels
of each layer comprising the blue PhOLEDs. The blue PhOLEDs were
grown by doping Ir (dbi)3 at an optimized doping concentration of
15% in the DBTBCz and DDBTBCz hosts. Electrical measurement
data of the DBTBCz and DDBTBCz devices obtained by voltage
Fig. 8. Quantum efficiencyeluminance plots of the DBTBCz and DDBTBCz devices
doped with Ir (dbi)3.
sweep at an interval of 0.5 V are summarized in Fig. 7. The current
density obtained from the voltage scan was of little difference in the
two devices because of the similar HOMO and LUMO levels from CV
measurements. Luminance obtained from the voltage scan was also
plotted according to voltage in Fig. 7 in addition to the current
density. The luminance were quite similar in the DBTBCz and
DDBTBCz devices and only slight difference of the luminance was
observed. As the current density at the same voltage was similar in
the two devices, the similar luminance forecasts similar efficiency
in the two devices.
Table 1
Photophysical, electrochemical and thermal properties of the materials.
Singlet energy (eV)
Triplet energy (eV)
HOMO (eV)
LUMO (eV)
Tg (ꢁC)
Td (ꢁC)
DBTBCz
DDBTBCz
3.09
3.09
2.92
2.94
ꢀ5.85
ꢀ5.85
ꢀ2.65
ꢀ2.60
148
189
478
564

134
H.M. Kim et al. / Organic Electronics 43 (2017) 130e135
Fig. 11. Lifetime plots of the DBTBCz and DDBTBCz devices doped with Ir (dbi)3 at an
initial luminance of 1000 cd/m2.
Emission spectra of the DBTBCz and DDBTBCz devices in Fig. 10
indicate that only Ir (dbi)3 emission was activated in the blue device
by energy transfer process from the hosts to Ir (dbi)3. As the device
structure was designed to block charge or exciton leakage from the
device, pure Ir (dbi)3 emission peak appeared in the device emis-
sion spectra. All device performances are listed in Table 2.
Fig. 9. Current density-voltage curves of hole only devices of DBTBCz and DDBTBCz.
One important objective of designing the DBTBCz and DDBTBCz
host materials was to develop stably operating host for extended
lifetime. Therefore, the long-term operation behaviour of the blue
PhOLEDs was investigated by driving the DBTBCz and DDBTBCz
devices at a fixed luminance of 1000 cd/m2. Fig. 11 plots the
luminance change of the DBTBCz and DDBTBCz blue PhOLEDs. It is
clear from the luminance-operation time data that DBTBCz per-
formed much better than DDBTBCz as the lifetime extending host
material. In particular, the DBTBCz was even better than the host
with 2- position modified dibenzothiophene in the bicarbazole
backbone structure. At the same driving condition, the lifetime up
to 70% of initial luminance was 55 h in our work, but it was 25 h in
the blue PhOLEDs with the 2- position modified dibenzothiophene
based host material [20]. This proves that 4- position modifying
approach is better than 2- position modifying approach in the
development of dibenzothiophene modified bicarbazole host ma-
terials. The improved lifetime of the DBTBCz device can be under-
stood by two main factors related with material parameters.
Rigidity of the molecular structure by 4- position modification
seems to be one key reason for the improved lifetime of the DBTBCz
and DDBTBCz devices compared to the BCzDBT device with a 2-
position modified dibenzothiophene. The other key factor is good
hole transport character of the host materials because facile hole
transport would widen the exciton formation zone and stabilize
the device by reduced triplet-triplet and triplet-polaron annihila-
tion [2].
Fig. 10. EL spectra of the DBTBCz and DDBTBCz devices doped with Ir (dbi)3.
The combined electrical and optical measurements provided
quantum efficiency (QE) of the DBTBCz and DDBTBCz devices in
Fig. 8. As projected from the current density and luminance data,
there was similarity of the QE in the DBTBCz and DDBTBCz devices
by providing maximum QE of 22.6% and 22.5%, respectively. High
QE was achieved in the two devices by good hole transport prop-
erties of the two hosts overcoming the strong hole trapping effect
by Ir (dbi)3 as predicted from the energy levels of the host and
dopant materials. The bicarbazole backbone structure is respon-
sible for the good hole transport character of the DBTBCz and
DDBTBCz hosts. DBTBCz and DDBTBCz behaved quite similarly as
the host of Ir (dbi)3 judging from the current density, luminance
and QE data. However, the efficiency roll-off was rather significant
in the two devices because of strong hole trapping effect by Ir (dbi)3
which narrows the emission zone of the Ir (dbi)3 devices [27]. It was
relatively insignificant in the DBTBCz device because of better hole
transport properties of DBTBCz than that of DDBTBCz as presented
in Fig. 9.
4. Conclusions
The synthesis and device evaluation of high triplet energy hosts
having dibenzothiophene unit linked to a bicarbazole backbone
Table 2
Summarized device performances of the DBTBCz and DDBTBCz devices.
CIE (x,y)
EL lmax (nm)
Quantum efficiency (%)
Current efficiency (cd/A)
[1000 cd/m2]
[Max]
[1000 cd/m2]
[Max]
DBTBCz
DDBTBCz
DBTBCz (lifetime)
DDBTBCz (lifetime)
(0.21, 0.43)
(0.21, 0.43)
(0.20, 0.41)
(0.20, 0.43)
475
475
475
475
14.13
14.93
4.59
22.61
22.46
4.91
34.41
36.50
11.14
12.53
53.55
53.77
12.05
17.42
5.05
6.83

H.M. Kim et al. / Organic Electronics 43 (2017) 130e135
135
(2002) 162.
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linking the dibenzothiophene via 4- position is better than that via
2- position in terms of long-term stability of the devices. The
DBTBCz host with the bicarbazole backbone structure and one
dibenzothiophene unit exhibited both high efficiency and
improved lifetime Although absolute lifetime of the blue PhOLEDs
is still short, but the design approach used in this work can open a
way of improving the stability of the blue PhOLEDs.
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