Dibenzothiophene derivatives as host materials for high efficiency in deep blue phosphorescent organic light emitting diodes

Article abstract of DOI:10.1039/c1jm12421h

Various dibenzothiophene derivatives were synthesized as high triplet energy host materials for deep blue phosphorescent organic light-emitting diodes (PHOLEDs) and their device performances were investigated. Dibenzothiophene host materials with a hole transport type carbazole and electron transport type phosphine oxide moieties showed high quantum efficiency and low driving voltage due to balanced charge injection and transport. A high quantum efficiency of 20.2% and only 10% reduction of quantum efficiency at 1000 cd m^-2 were demonstrated from the deep blue PHOLEDs with the dibenzothiophene based high triplet energy host materials. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1jm12421h

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PAPER
Dibenzothiophene derivatives as host materials for high efficiency in deep blue
phosphorescent organic light emitting diodes†
*
Sook Hee Jeong and Jun Yeob Lee
Received 30th May 2011, Accepted 12th July 2011
DOI: 10.1039/c1jm12421h
Various dibenzothiophene derivatives were synthesized as high triplet energy host materials for deep
blue phosphorescent organic light-emitting diodes (PHOLEDs) and their device performances were
investigated. Dibenzothiophene host materials with a hole transport type carbazole and electron
transport type phosphine oxide moieties showed high quantum efficiency and low driving voltage due
to balanced charge injection and transport. A high quantum efficiency of 20.2% and only 10%
reduction of quantum efficiency at 1000 cd mꢀ2 were demonstrated from the deep blue PHOLEDs with
the dibenzothiophene based high triplet energy host materials.
Other than the core structures mentioned above, dibenzo-
Introduction
thiophene (DBT) was also reported to be the high triplet energy
core structure.16–19 A DBT compound substituted with diphe-
nylphosphine oxide was applied as an electron transport type
exciton blocking layer in blue PHOLEDs and showed good
performances.16,17 This compound was also used as the host
material in combination with the hole transport type host
material in the double emitting layer structure.18 The DBT
compound was effective in the double layer emitting structure,
but the efficiency of the single emitting layer device with the DBT
type host was poor due to the strong electron transport proper-
ties. On the other hand, the DBT unit can be used as the core
structure of high triplet energy materials.
The development of deep blue phosphorescent organic light-
emitting diodes (PHOLEDs) is important because they can
greatly improve the power consumption of full colour organic
light-emitting diode panels. Although a high quantum efficiency
was achieved in deep blue PHOLEDs, it is essential to enhance
the quantum efficiency of deep blue PHOLEDs.1 In particular,
the quantum efficiency at high luminance should be increased for
practical applications.
Most of the studies to improve the quantum efficiency of deep
blue PHOLEDs focused on the synthesis of high triplet energy
host materials for efficient energy transfer to deep blue phos-
phorescent dopants.1–15 Several core structures, such as 9-phe-
In this study, a series of DBT derivatives with different
substituents to manage the charge transport properties were
synthesized and high efficiency deep blue PHOLEDs were
fabricated using high triplet energy host materials. Substitution
of the DBT core with one carbazole and one diphenylphosphine
oxide was the most effective in improving the quantum efficiency
and efficiency roll-off of deep blue PHOLEDs. High quantum
efficiency (>20%) and little efficiency roll-off (<10% of maximum
quantum efficiency at 1000 cd mꢀ2) were achieved in the deep
blue PHOLEDs using the DBT derivatives. This efficiency is one
of the best reported up to now.
nylcarbazole,1–3
N,N-dicarbazolyl-3,5-benzene
(mCP),4–8
tetraphenylsilane,9,10 triazine11,12 and sterically hindered
biphenyl,13 were developed as high triplet energy cores and
modified with a range of functional groups. Hole transport type
cores, such as 9-phenylcarbazole, were functionalized with elec-
tron transport type units, such as diphenylphosphine oxide,
whereas electron transport type cores were substituted with hole
transport type substituents, such as carbazole, to obtain the
highest occupied molecular orbital (HOMO)/lowest unoccupied
molecular orbital (LUMO) levels for bipolar charge transport
properties and efficient charge injection. Up to now, the most
effective host material for deep blue PHOLEDs was phenyl-
carbazole derivative modified with diphenylphosphine oxide and
a high quantum efficiency of 19.2% was reported.1
Experimental section
Synthesis
The synthetic route of the DBT based host materials is shown in
Scheme 1.
Department of Polymer Science and Engineering, Dankook University,
126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi, 448-701, Republic of
Korea. E-mail: leej17@dankook.ac.kr; Fax: +82 31 8005 3585; Tel: +82
31 8005 3585
Synthesis of 9-(8-bromodibenzo[b,d]thiophene-2-yl)-9H-carba-
zole (1). 2,8-Dibromo-dibenzothiophene (3.00 g, 8.77 mmol),
9H-carbazole (1.76 g, 10.52 mmol), potassium carbonate (5.09 g,
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm12421h
14604 | J. Mater. Chem., 2011, 21, 14604–14609
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1
DBT2: yield 65%. Tg 115 ꢁC. H NMR (200 MHz, CDCl3):
d 8.59–8.53 (d, 1H), 8.30 (s, 1H), 8.18–7.97 (m, 4H), 7.75–7.66
(m, 6H), 7.54–7.31 (m, 12H). 13C NMR (200 MHz, CDCl3):
d 144.6, 141.4, 138.8, 136.7, 135.5, 135.4, 133.3, 132.5, 132.4,
132.2, 130.3, 130.2, 129.7, 129.0, 128.9, 128.7, 127.1, 126.4, 126.3,
124.5, 123.7, 123.5, 123.4, 121.1, 120.7, 120.5, 109.9. MS (FAB)
m/z [(M + H)+] calcd for C36H24NOPS 550.13, found 550. Anal.
Calcd for C36H24NOPS: C, 78.67; H, 4.40; N, 2.55; S, 5.83.
Found: C, 78.78; H, 4.45; N, 2.49; S, 5.59%.
Scheme 1 Synthetic scheme of host materials.
2,8-Bis(diphenylphosphoryl)dibenzo[b,d]thiophene (DBT3). 2,8-
Dibromo-dibenzothiophene (2.50 g, 7.31 mmol) in THF (30 mL)
was placed into a 100 mL two-necked flask. The reaction flask
was cooled to ꢀ78 ꢁC and n-BuLi (2.5 M in hexane, 7.30 mL) was
added dropwise. The whole solution was stirred at this temper-
ature for 3 h, followed by the addition of a solution of chlor-
odiphenylphosphine (4.03 g, 18.27 mmol) under an argon
atmosphere. The resulting mixture was warmed gradually to
ambient temperature and quenched with methanol (10 mL). The
mixture was extracted with dichloromethane. The combined
organic layers were dried over magnesium sulfate, filtered and
evaporated under reduced pressure. The white powdery product
was obtained (1.82 g, yield 46%). The powder was dissolved in
dichloromethane (20 mL) and hydrogen peroxide (4 mL), and
stirred overnight at room temperature. The organic layer was
separated and washed with dichloromethane and water. The
extract was evaporated to dryness to produce a white solid.
36.84 mmol), copper iodide (0.42 g, 2.19 mmol) and 18-crown-6
(0.23 g, 0.88 mmol) were dissolved in anhydrous o-dichloro-
benzene under nitrogen atmosphere. The reaction mixture was
stirred for 12 h at 100 ꢁC. The mixture was diluted with
dichloromethane and washed three times with water (100 mL).
The organic layer was dried over anhydrous MgSO4 and evap-
orated in vacuo to give the crude product, which was purified by
column chromatography using n-hexane as an eluent. The final
white powdery product was obtained in 43% yield.
Synthesis of 2,8-di(9H-carbazol-9-yl)dibenzo[b,d]thiophene
(DBT1). 2,8-Dibromo-dibenzothiophene (2.00 g, 5.85 mmol),
9H-carbazole (2.93 g, 17.54 mmol), potassium carbonate (3.39 g,
24.56 mmol), copper iodide (0.28 g, 1.46 mmol) and 18-crown-6
(0.15 g, 0.58 mmol) were dissolved in anhydrous o-dichloro-
benzene under a nitrogen atmosphere. The reaction mixture was
stirred for 12 h at 100 ꢁC. The mixture was diluted with
dichloromethane and washed three times with water (100 mL).
The organic layer was dried over anhydrous MgSO4 and evap-
orated in vacuo to give the crude product, which was purified by
column chromatography using n-hexane as the eluent to afford
a white powdery product.
1
DBT3: yield 46%. Tg 100 ꢁC. H NMR (200 MHz, CDCl3):
d 8.50 (s, 1H), 8.44 (s, 1H), 7.99–7.94 (m, 2H), 7.81–7.42 (m,
22H). 13C NMR (200 MHz, CDCl3): d 143.2, 134.6, 134.4, 132.5,
131.8, 131.8, 131.7, 131.5, 129.9, 129.8, 129.2, 128.4, 128.3, 125.7,
125.6, 122.8, 122.7. MS (FAB) m/z [(M + H)+] calcd for
C36H26O2P2S 585.11, found 585. Anal. Calcd for C36H26O2P2S:
C, 73.96; H, 4.48; S, 5.48. Found: C, 73.61; H, 4.47; S, 5.36%.
1
DBT1: yield 43%. Tg 128 ꢁC. H NMR (200 MHz, CDCl3):
d 8.31 (s, 2H), 8.17–8.13 (d, 5H), 7.45–7.69 (m, 2H), 7.55–7.24
(m, 13H). 13C NMR (200 MHz, CDCl3): d 141.2, 139.2, 136.6,
134.9, 126.6, 126.1, 124.4, 123.4, 120.6, 120.4, 120.1, 109.6. MS
(FAB) m/z [(M + H)+] calcd for C36H22N2S 515.15, found 515.
Anal. Calcd for C36H22N2S: C, 84.02; H, 4.31; N, 5.44; S, 6.23.
Found: C, 83.39; H, 4.35; N, 5.24; S, 5.80%.
Device preparation and measurements
DBT1, DBT2 and DBT3 were tested as the host materials for blue
PHOLEDs. The device structure of the blue PHOLEDs was
indium tin oxide (ITO, 150 nm)/N,N0-diphenyl-N,N0-bis-[4-
(phenyl-m-tolyl-amino)-phenyl]-biphenyl-4,40-diamine (DNTPD,
9-(8-(Diphenylphosphoryl)dibenzo[b,d]thiophene-2-yl)-9H-carba-
zole (DBT2). 9-(8-Bromodibenzo[b,d]thiophene-2-yl)-9H-carba-
zole (1) (1.60 g, 3.74 mmol) in THF (30 mL) was placed into
a 100 mL two-necked flask. The reaction flask was cooled to
ꢀ78 ꢁC and n-BuLi (2.5 M in hexane, 1.79 mL) was added
dropwise. The whole solution was stirred at this temperature for
60 nm)/N,N0-di(1-naphthyl)-N,N0-diphenylbenzidine(NPB,
5
nm)/N,N0-dicarbazolyl-3,5-benzene (mCP, 10 nm)/DBT1 or
DBT2 or DBT3: bis((3,5-difluoro-4-cyanophenyl)pyridine)
iridium picolinate (FCNIrpic) (30 nm)/diphenylphosphine oxide-
4-(triphenylsilyl)phenyl (TSPO1, 20 nm)/LiF(1 nm)/Al(200 nm).
The doping concentration of the FCNIrpic was 15% in all devices.
A hole only device with a device structure of ITO/DNTPD
(60 nm)/NPB (5 nm)/mCP (10 nm)/DBT1 or DBT2 or DBT3
(30 nm)/DNTPD (5 nm)/Al and an electron only device with
a device structure of ITO/TSPO1 (5 nm)/DBT1 or DBT2 or DBT3
(30 nm)/TSPO1 (25 nm)/LiF/Al were also fabricated as single
charge devices. All organic materials were evaporated thermally at
a vacuum pressure of 8.0 ꢂ 10ꢀ7 Torr at a deposition rate of 0.1 nm
sꢀ1 except for the FCNIrpic. All devices were encapsulated with
a CaO getter and a glass lid after Al deposition. The device
performance of the blue PHOLEDs was measured using a Keith-
ley 2400 source measurement unit and a CS1000 spectroradi-
ometer. Ultraviolet-visible (UV-vis) and photoluminescence (PL)
3
h, followed by the addition of a solution of chlor-
odiphenylphophine (0.99 g, 4.48 mmol) in an argon atmosphere.
The resulting mixture was warmed gradually to ambient
temperature and quenched with methanol (10 mL). The mixture
was extracted with dichloromethane. The combined organic
layers were dried over magnesium sulfate, filtered, and evapo-
rated under reduced pressure. The white powdery product
(1.30 g) was obtained (yield 65%). It was dissolved in dichloro-
methane (20 mL) and hydrogen peroxide (5 mL), which was
stirred overnight at room temperature. The organic layer was
separated and washed with dichloromethane and water. The
extract was evaporated to dryness to afford a white solid.
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measurements of the host materials were carried out using 1.0 ꢂ
10ꢀ4 M tetrahydrofuran solution. Low temperature PL measure-
ments were carried out using a solution of host materials dissolved
in tetrahydrofuran under liquid nitrogen. A UV-vis spectrometer
(Shimadzu, UV-2501PC) and a fluorescence spectrometer (Hita-
chi, F-7000) were used to obtain the UV-vis absorption and PL
emission data. Cyclic voltammetry was performed using the host
films on an ITO substrate in an acetonitrile solution with tetra-
butylammonium perchlorate at 0.1 M concentration. Ag was used
as the reference electrode and Pt was used as the counter electrode.
Ferrocene was used as the standard material for the CV
measurement. A potentiostat (Ivium IV) was used to scan the
voltage and measure the change in current. The glass transition
temperature (Tg) of the host materials was measured using
a differential scanning calorimeter (Mettler DSC 822) at a heating
rate of 10 ꢁC minꢀ1 in a nitrogen atmosphere.
LUMO distribution of all host materials synthesized in this
study. The substituent mostly affected the HOMO distribution
of the DBT derivatives and had little effect on LUMO distri-
bution. The DBT core has an electron withdrawing sulfur unit,
which makes the aromatic ring electron deficient, inducing
LUMO localization in the DBT core. On the other hand, the
HOMO is generally localized in the electron donating unit and is
heavily affected by the electron donating substituent. The
HOMO of DBT1 with two carbazole moieties was localized
mostly over the electron donating carbazole group and the
HOMO of DBT2 was also localized in the carbazole unit.
Carbazole is an electron donating unit, inducing localization of
the HOMO on the carbazole unit. On the other hand, the
HOMO of the DBT3 was distributed mostly over the dibenzo-
thiophene core due to the strong electron withdrawing properties
of the diphenylphosphine oxide. Therefore, it is expected that the
HOMO of DBT3 is shifted downward due to the electron with-
drawing character of the dibenzothiophene core. Fig.
1
Results and discussion
summarizes the simulated HOMO and LUMO levels. The
HOMO of the DBT3 showed a large value compared to that of
DBT1 and DBT2, whereas the LUMO in DBT3 was not changed
greatly. The simulated HOMO of DBT2 was similar to that of
DBT1 because the HOMO was localized on the same carbazole
unit.
The DBT core structure has a high triplet energy of 3.01 eV,
which is high enough for energy transfer to a deep blue emitting
phosphorescent dopant, such as FCNIrpic with a triplet energy
of 2.74 eV.19 The DBT core has electron transport character due
to the electron withdrawing sulfur group, which makes the
aromatic ring electron deficient. Therefore, DBT was typically
used as the high triplet energy core for the electron transport type
exciton blocking layer.16,17 Although DBT was utilized as the
core of the exciton blocking layers, the DBT core can also be
used as the backbone structure of a high triplet energy host
material by controlling the charge transport properties.
The photophysical properties of the host materials were
analyzed by UV-vis and PL. Fig. 2 shows UV-vis and PL spectra
of the host materials. DBT1 showed the absorption of the
carbazole and dibenzothiophene units, and DBT2 also exhibited
the absorption peaks of the carbazole unit and dibenzothiophene
core. The absorption peak at 335 nm corresponds to the p–p*
transition of the carbazole moiety, whereas the absorption peaks
at short wavelengths were assigned to the absorption of the DBT
core. DBT3 exhibited only the absorption of the p–p* transition
of the dibenzothiophene core. The absorption spectra of DBT1
and DBT2 were red-shifted compared with that of DBT3 because
of the carbazole centered absorption. The optical band gap could
DBT was modified with diphenylphosphine oxide and carba-
zole to tailor the HOMO/LUMO levels and charge transport
properties. Diphenylphospine oxide was used as the substituent
to improve the electron transport properties,20 whereas carbazole
was introduced to enhance the hole transport properties. Three
compounds based on DBT, DBT1, DBT2 and DBT3, were
synthesized. DBT1 had two carbazole groups at the 2 and 8
positions of DBT, whereas DBT3 possessed two diphenylphos-
phine oxide groups at the 2 and 8 positions. DBT2 had an
asymmetrical molecular structure with one carbazole and one
diphenyl phosphine oxide. Scheme 1 shows the synthetic scheme
of the host materials. All compounds were synthesized from
a 2,8-dibromodibenzothiophene intermediate. The diphenyl-
phosphine oxide unit was substituted to the backbone structure
by the lithiation and phosphorylation of the brominated inter-
mediate followed by oxidation with hydrogen peroxide. The
carbazole unit was reacted with the brominated intermediate
using CuI as a catalyst, yielding the carbazole modified host
materials. In the synthesis of asymmetric compound, the carba-
zole unit was first linked to the core structure followed by
diphenylphosphine oxide formation. All compounds were puri-
fied by recrystallization and column chromatography to obtain
high purity products for device evaluation.
Density functional theory (DFT) calculations of the host
materials were carried out using a suite of the Gaussian 09
program to study the molecular orbital distribution. The
nonlocal density functional of Becke’s 3-parameters employing
Lee–Yang–Parr functional (B3LYP) with 6-31G* basis sets was
used for the calculation. Fig. 1 shows the simulated HOMO and
Fig. 1 Molecular simulation results of DBT1, DBT2 and DBT3. HOMO
and LUMO orbital distribution and simulated energy levels are shown.
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Fig. 2 UV-vis and PL spectra of DBT1, DBT2 and DBT3.
be calculated from the absorption edge of the UV-vis spectrum
and the band gap of DBT1 and DBT2 was 3.52 eV. The band gap
of DBT3 was wider than that of DBT1 and DBT2 (3.70 eV). The
PL spectrum also showed a similar trend to the UV-vis absorp-
tion spectrum. The PL emission was red-shifted in DBT1 and
DBT2 due to the reduced optical band gap of the host materials.
The maximum emission peak of DBT1 and DBT2 was 386 nm,
whereas that of DBT3 was 350 nm. The emission spectra of the
three host materials were well overlapped with the absorption of
the FCNIrpic dopant, indicating that energy transfer from the
host materials to the dopant is efficient.
Fig. 3 Hole only (a) and electron only (b) device data of DBT1, DBT2
and DBT3.
derivatives. The hole current density was high in the DBT1 and
DBT2 devices, whereas it was significantly lower in the DBT3
device. The low hole current density of the DBT3 device is due to
the electron deficient unit in DBT3. DBT3 has an electron defi-
cient dibenzothiophene core and diphenylphosphine oxide
substituent. All chemical groups in the DBT3 have electron
withdrawing character, making DBT3 electron deficient. There-
fore, the hole transport properties of DBT3 are poor. In addition,
the HOMO level of 6.69 eV hinders hole injection from the hole
transport layer to the DBT3 host due to the high energy barrier
for hole injection of 0.59 eV (hole transport layer HOMO
6.1 eV). The high energy barrier for hole injection and the poor
hole transport properties of DBT3 greatly reduced the hole
current density. The high hole current density of DBT1 and
DBT2 is due to the hole transport carbazole group. The carba-
zole unit is a good hole transport unit and is effective in
improving the hole transport properties of the organic material.
The HOMO was dispersed over the hole transport carbazole unit
in DBT1 and DBT2, resulting in a high hole current density in
DBT1 and DBT2. In addition to the hole transport properties,
there was no energy barrier for hole injection in DBT1 and
DBT2. The good hole transport properties and little energy
barrier for hole injection were responsible for the high hole
current density of DBT1 and DBT2.
The HOMO levels of the host materials were determined by
cyclic voltammetry (CV) (Fig. S1†). The HOMO levels of DBT1,
DBT2 and DBT3 were 6.10, 6.09 and 6.69 eV, respectively. The
measured HOMO levels showed a similar trend to the simulated
HOMO levels in Fig. 1. The oxidation site of DBT1 and DBT2 is
the carbazole unit, whereas that of DBT3 is the dibenzothio-
phene core. Therefore, similar HOMO levels were observed in
DBT1 and DBT2. Table 1 lists the HOMO levels. The LUMO
levels could be calculated from the HOMO and optical band gap,
and they were 2.58, 2.57 and 2.99 eV for DBT1, DBT2 and
DBT3, respectively.
The triplet energy of the host materials was measured from
a low temperature PL experiment as the peak of PL emission.
The triplet energies of the DBT1, DBT2 and DBT3 were 2.94,
2.92 and 2.96 eV, respectively. All host materials showed a high
triplet energy (>2.90 eV) and could effectively transfer energy to
the deep blue emitting FCNIrpic dopant material with a triplet
energy of 2.74 eV.
Hole only and electron only devices of DBT-based host
materials were fabricated to correlate the molecular structure
with the charge transport properties of host materials. Fig. 3
shows the hole only and electron only device data of the DBT
The electron current density was quite different from the hole
current density. DBT2 and DBT3 showed much higher current
density than DBT1. Electron deficient dibenzothiophene and
diphenylphosphine oxide in DBT2 and DBT3 enhanced the
electron transport properties, leading to a high electron current
density. On the other hand, the carbazole group in DBT1
reduced the electron deficiency of the dibenzothiophene core and
had a negative effect on the electron current density. A
comparison of the hole and electron current densities revealed
Table 1 Photophysical and thermal properties of host materials
UV-visa/
nm
PLa/ HOMO/ LUMO/ Band
ET/ Tg/
ꢁC
nm
eV
eV
gap/eV eV
DBT1 325, 338 386 6.10
DBT2 326, 338 387 6.09
DBT3 293, 315, 327 350 6.69
2.58
2.57
2.99
3.52
3.52
3.70
2.94 128
2.92 115
2.96 100
a
Measured in tetrahydrofuran solvent.
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Fig. 5 Quantum efficiency–luminance curves of the deep blue PHO-
LEDs with the DBT1, DBT2 and DBT3 host materials.
Fig. 4 Current density–voltage–luminance curves of DBT1, DBT2 and
DBT3 devices.
quantum efficiency of the DBT3 device. In particular, the effi-
ciency decrease at high current density was significant due to
unbalanced charge injection at high luminance. The maximum
quantum efficiency of the DBT2 device was 20.2% and the
quantum efficiency at 1000 cd mꢀ2 was 17.7%. There was only
DBT1 to have strong hole transport properties and DBT3 to
have strong electron transport properties. DBT2 was similar to
DBT1 for hole transport and to DBT3 for electron transport.
Therefore, it is expected that DBT2 would be better than DBT1
and DBT3 in terms of the holes and electrons balance.
about 10% decrease of the quantum efficiency at 1000 cd mꢀ2
.
As the DBT type host materials had a high triplet energy for
energy transfer to the deep blue phosphorescent dopant, they
were evaluated as the host materials for deep blue PHOLEDs.
Fig. 4 shows the current density–voltage–luminance curves of the
deep blue PHOLEDs with DBT derivatives as the host materials.
The deep blue emitting FCNIrpic dopant was doped at a doping
concentration of 15% and the device performance was compared.
The FCNIrpic has a high triplet energy of 2.74 eV and HOMO/
LUMO levels of 5.72/2.98 eV.21 The DBT2 device showed higher
current density than the DBT1 and DBT3 devices. The current
density generally depends on the hole and electron densities in
the device. Both the hole and electron current densities were high
in the DBT2 hole only and electron only devices, resulting in
a high current density in the DBT2 device. The higher current
density of the DBT1 device was attributed to the high hole
current density. The hole current density was much higher than
the electron current density and the total current density was
dominated by the hole current density. Therefore, the current
density of the DBT1 device was high and slightly lower than that
of the DBT2 device. The current density of the DBT3 device was
low because of the low hole current density despite the high
electron current density. The luminance followed the same trend
as the current density, even though the luminance of the DBT1
device was greatly degraded.
The balanced charge injection enhanced the quantum efficiency
and suppressed the efficiency roll-off at high luminance. Table 2
lists the performance of all devices. The quantum efficiency of the
DBT2 device is one of the best efficiency values reported for the
deep blue PHOLEDs.21–24 The best quantum efficiency reported
in the literature was 25.3%.21 Although the quantum efficiency of
the DBT2 device was lower than that of the best efficiency value,
a high quantum efficiency (>20%) was obtained using the DBT2
host.
Fig. 6 presents the electroluminescence (EL) spectra of the blue
PHOLEDs with the DBT based host materials. All devices
showed a main emission peak at 460 nm and a shoulder peak at
479 nm. The main emission peak was not changed depending on
the host materials, but the shoulder peak was intensified in the
DBT3 device. DBT3 is an electron transport type host material
and shifts the emission zone from the electron transport layer
side to the hole transport layer. The recombination zone shifted
to the hole transport layer side induces excimer formation at the
interface between the mCP hole transport layer and the DBT3
host, leading to an intensified shoulder peak at the long wave-
length region. The optical effect by the change in recombination
zone also increased the intensity of the shoulder peak. The colour
coordinates of the DBT1 and DBT2 devices at 1000 cd mꢀ2 were
(0.14, 0.21), whereas those of the DBT3 device were (0.14, 0.25).
Fig. 5 shows the quantum efficiency–luminance curves of the
three devices. The DBT2 device showed higher quantum effi-
ciency than the DBT1 and DBT3 devices. As shown in the hole
only and electron only devices, DBT2 exhibited both high hole
and electron density. This improves the hole and electron balance
in the emitting layer, increasing the quantum efficiency of the
DBT2 device. On the other hand, the hole current density in the
DBT1 device was much higher than the electron density,
degrading the holes and electrons balance in the emitting layer.
In the case of the DBT3 device, the charge balance was better
than that of the DBT1 device, resulting in higher quantum effi-
ciency than the DBT1 device. Nevertheless, the low hole current
density of the DBT3 device could not improve further the
Table 2 Device performances of the blue PHOLEDs with DBT1, DBT2
and DBT3 hosts
Maximum
quantum
efficiency
(%)
Maximum Maximum
current power
efficiency/ efficiency/
Quantum
efficiencya Color
(%)
indexb
cd Aꢀ1
lm Wꢀ1
DBT1 13.2
DBT2 20.2
DBT3 19.0
11.5
17.9
16.0
(0.14, 0.21) 18.2
(0.14, 0.21) 28.9
(0.14, 0.25) 29.9
9.7
28.9
17.8
a
b
Quantum efficiency was measured at 1000 cd mꢀ2
.
Color index was
measured at 1000 cd mꢀ2
.
14608 | J. Mater. Chem., 2011, 21, 14604–14609
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with carbazole and diphenylphosphine oxide groups. Holes and
electrons were balanced in DBT2 with a carbazole and a diphe-
nylphosphine oxide, and a high quantum efficiency of 20.2% was
achieved. In addition, the efficiency roll-off of the deep blue
PHOLEDs was also greatly suppressed. The dibenzothiophene
core was proven to be effective as the core structure of high
triplet energy host materials.
Notes and references
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according to doping concentration.
The device performance of the DBT2 devices was optimized
further by changing the doping concentration of the FCNIrpic
dopant. Fig. 7 shows the quantum efficiency of the DBT2 devices
according to the doping concentration. The maximum quantum
efficiency of the DBT2 device was similar over the doping
concentration range from 3% to 15%. The doping concentration
had little effect on the quantum efficiency, even though the
quantum efficiency at high luminance was rather high at high
doping concentrations. The bipolar charge transport properties
of DBT2 induced little dependency of the quantum efficiency on
the doping concentration. The colour coordinates of the DBT2
devices were blue-shifted at low doping concentrations and were
(0.14, 0.20) at 3% and 5%.
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Dibenzothiophene-based host materials with different functional
groups were synthesized by modifying the dibenzothiophene
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