Thermally-stable 2,3-diphenylated benzotiophene containing host materials for red phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2014.06.002

This study reports a series of benzo[b]thiophene (BT) derivatives with good electronic properties as host materials for red phosphorescent organic light-emitting diodes (PHOLEDs). We applied thermally-stable materials for red PHOLEDs which contain BT and carbazole moieties which also provide a good charge transport ability, as well as a good charge confinement effect (T₁ > 2.25 eV). To control the appropriate charge transporting ability, two different structures having one or two carbazole units connected to the single BT moiety were prepared. Using this approach, we found that the material with a single carbazole and a single BT unit showed the best current efficiency and external quantum efficiency (up to 23.6 cd/A and 12.8%, respectively). The results of the experiment also suggest that BT could be used as an electron-transporting unit when utilized with carbazole moiety, although it has previously been utilized as a p-type material.

Full text of DOI:10.1016/j.dyepig.2014.06.002

Accepted Manuscript
Thermally-stable 2,3-diphenylated benzotiophene containing host materials for red
phosphorescent organic light-emitting diodes
Joon Ho Jeon, Nam-Jin Lee, Ji-Hoon Lee, Min Chul Suh
PII:
S0143-7208(14)00226-5
10.1016/j.dyepig.2014.06.002
DYPI 4410
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 2 April 2014
Revised Date: 22 May 2014
Accepted Date: 2 June 2014
Please cite this article as: Jeon JH, Lee N-J, Lee J-H, Suh MC, Thermally-stable 2,3-diphenylated
benzotiophene containing host materials for red phosphorescent organic light-emitting diodes, Dyes and
Pigments (2014), doi: 10.1016/j.dyepig.2014.06.002.
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(Revised)
Thermally-stable 2,3-diphenylated benzotiophene containing
host materials for red phosphorescent organic light-emitting
diodes
Joon Ho Jeona,†, Nam-Jin Leeb,†, Ji-Hoon Leeb,*, and Min Chul Suha,*
aDepartment of Information Display, Kyung Hee University,
Dongdaemoon-Gu, Seoul 130-701, Republic of Korea
bDepartment of Polymer Science and Engineering & Department of IT Convergence,
50 Daehak-ro, Chungju-Si, Chungbuk 380-702, Republic of Korea
Abstract
This study reports a series of benzo[b]thiophene (BT) derivatives with good electronic
properties as host materials for red phosphorescent organic light-emitting diodes (PHOLEDs).
We applied thermally-stable materials for red PHOLEDs which contain BT and carbazole
moieties which also provide a good charge transport ability, as well as a good charge
confinement effect (T1 > 2.25 eV). To control the appropriate charge transporting ability, two
different structures having one or two carbazole units connected to the single BT moiety were
prepared. Using this approach, we found that the material with a single carbazole and a single
BT unit showed the best current efficiency and external quantum efficiency (up to 23.6 cd/A
and 12.8%, respectively). The results of the experiment also suggest that BT could be used as
an electron-transporting unit when utilized with carbazole moiety, although it has previously
been utilized as a p-type material.
Keywords: Host materials; Red phosphorescent OLED; Benzothiophene; Carbazole; Lower lying LUMO level.
†
Both authors have equally contributed to this work.
*Corresponding authors. Tel : +82-43-841-5427/ +82-2-961-0694; Fax: +82/43-841-5420/+82-2-968-6924
E-mail: jihoonli@ut.ac.kr (Prof. J.-H. Lee), mcsuh@khu.ac.kr (Prof. M.C. Suh)
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1. Introduction
Phosphorescent organic light-emitting diodes (PHOLEDs) have attracted much attention
because of their high quantum efficiency as compared to conventional fluorescent OLEDs
through utilizing both singlet and triplet excitons for emission [1-5]. In fact, red
phosphorescent materials have already been applied in the main-display mode of commercial
mobile phones since 2007. Green PHOLEDs have been utilized in the same application as
well. More specifically, there has been progress in research on the external quantum
efficiency (EQE) of PHOLEDs. Kim and co-authors reported that their extremely high
efficiency (>30% of EQE) could be achieved if horizontally-oriented emitters are used [6].
Hence, numerous attempts have been undertaken to combine this extremely high efficiency
with high device stability. To make such devices, many ideas are still needed on how to
extract the internally trapped emission on a waveguide or a substrate mode. However, the
development of thermally or electrochemically stable host materials to improve lifetime and
internal quantum efficiency still needs to realize highly stable PHOLEDs, because PHOLEDs
generally have a much shorter lifetime compared to fluorescent ones. Indeed, PHOLEDs
normally need a more or less complicated evaporation system, such as mixed host approach,
to obtain a much longer lifetime, although a very high efficiency could sometimes be
achieved from a single host system. [7,8] Thus, many research groups focus on the
elaboration of ideal host materials with bipolar characteristics, as a minute change in mixing
the ratio of hole transport type and electron transport type host materials could change the
resultant device characteristics, as well as negatively impact the reliability of active matrix
OLED products. To synthesize the bipolar host material, attempts have been made to
combine the hole transport type functional groups, with the electron transport type functional
groups, As a result of these efforts, improvements of the PHOLED device efficiency by using
certain bipolar host materials have been reported [9-13].
This study reports two different types of new host materials for red PHOLEDs having high
triplet (>2.25 eV) with thermally-stable benzo[b]thiophene (BT) moieties to improve their
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electron transport ability. As a result of using BT moieties at host material, PHOLEDs
devices that have novel host materials showed an improved EQE (up to 12.8%).
2. Results and Discussion
2.1. Materials
Two bipolar host materials to realize a highly efficient thermally-stable red PHOLEDs
were prepared. Due to its frequent occurrence in nature and useful properties in material
science, BT moiety was selected as a basic functionality [14-17]. In particular, the triplet
energy level of BT (~2.99 eV) is similar to that of 9-H or 9-phenyl carbazole (~3.01 eV) [18-
21]. This is a very useful merit for making new host materials and tuning their triplet energy
levels. On the other hand, these BT derivatives including multiply arylated
benzo[b]thiophenes and further π-extended benzo[1,2-b;4,5-b’]dithiophenes have recently
aroused considerable interest in the field of organic electronics[22-24]. However, to the best
of our knowledge, no OLEDs using host materials based on BT moiety have been
investigated to date.
Scheme. 1 shows the schematic diagram of the synthetic route to prepare such compounds.
Herein the 2,3-diphenylated BT derivative was used as an electron-transporting functionality.
Indeed, there have scarcely been any attempts to make new bipolar host materials that do not
contain any electron-transporting moieties, such as pyridine, triazine, benzimidazole,
phosphine oxide, and so on. Meanwhile, we changed the numbers of carbazole moiety
connected to the central benzene core unit which act as hole transporting functional units.
Fig. 1 shows the geometries of the core parts of the two resultant host materials obtained
from the simulation at a DNP/GGA(PBE) level of theory using Dmol3 module (Material
Studio 7.0, Accelrys). BT moieties were rather twisted to the core benzene unit (dihedral
angles ca. 38.75~42.05o) which could expectedly arouse a broken conjugation so that these
materials can be used as red host materials (T1 > 2.25 eV). In addition, the molecular orbital
distribution of Synthesis of 9,9'-(5-(triphenylen-2-yl)-1,3-phenylene)bis(9H-carbazole) (BT-
mCP), and Synthesis of 9-(4-(2,3-diphenylbenzo[b]thiophen-5-yl)phenyl)-9H-carbazole (BT-
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PC) are also shown in Fig. 1. As expected from the molecular structures, the electrons in
lowest unoccupied molecular orbital (HOMO) were localized on the N-phenylcarbazole (PC)
or 1,3-phenylene)bis(9H-carbazole) (mCP) units, while the electrons in lowest unoccupied
molecular orbital (LUMO) were dispersed over the 2,3-diphenylbenzo[b]thiophene core units.
In other words, the hole carriers injected from the hole transport layer (HTL) could move
through PC or mCP units, while the electrons injected from the electron transporting layer
(ETL) could be transported through BT moieties.
Fig. 2(a) shows the UV-Vis absorption and photoluminescence spectra of new host
material. The band gap (Eg) of BT-mCP was slightly larger than that of BT-PC (BT-mCP
3.48eV, BT-PC: 3.39eV). Both compounds showed vibronic fine structures in their emission
spectra, especially at low temperature (77 K). The triplet energies (T1) of new host materials
determined by the first phosphorescence peak from the shorter wavelength region were 2.26,
2.26 eV for BT-mCP and BT-PC, respectively. The energy level of HOMO was collected
from cyclic voltammetry while Eg was determined from the band edge of UV-visible
absorption spectrum (see Table 1). The LUMO level of material was determined from the
HOMO level and Eg. Fig. 2(b) and 2(c) shows the thermal stability behavior of the materials
synthesized in this study. Novel host materials with benzothiophene moieties have high
thermal stability. BT-mCP and BT-PC showed the decomposition temperature (Td) of 430,
393°C as shown in Fig. 2(b). Meanwhile, they showed glass temperature (Tg) of 150, 110°C,
respectively as shown in Fig. 2(c). The representative results are also summarized in Table 1.
2.2. Device Characteristics
Bipolar characteristics of host materials with proper triplet energy levels are very important
for the enhancement of efficiency, because they help to broaden the recombination zone
inside the emission layer of PHOLEDs. In addition, the precisely controlled charge balance at
the emission layer is also very important to improve the device stability, as well as its
efficiency. To investigate the relative charge carrier transport ability of emission layer (EML),
we prepared the hole only devices (HODs), as well as electron only devices (EODs) [25-27]
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of 4,4-N,N’-Dicarbazole-1,1’-biphenyl (CBP) and newly synthesized host materials by using
molybdenum oxide (MoO3) and lithium quinolate (LiQ) as charge carrier injection layers
from indium tin oxide (ITO) and aluminum (Al) as follows.
HOD A: ITO / MoO3 (0.75 nm) / CBP (100 nm) / MoO3 (10 nm) / Al (100 nm)
HOD B: ITO / MoO3 (0.75 nm) / BT-mCP (100 nm) / MoO3 (10 nm) / Al (100 nm)
HOD C: ITO / MoO3 (0.75 nm) / BT-PC (100 nm) / MoO3 (10 nm) / Al (100 nm)
EOD A: Al (50 nm) / LiQ (1.5 nm) / CBP (100 nm) / LiQ (1.5 nm) / Al (100 nm)
EOD B: Al (50 nm) / LiQ (1.5 nm) / BT-mCP (100 nm) / LiQ (1.5 nm) / Al (100 nm)
EOD C: Al (50 nm) / LiQ (1.5 nm) / BT-PC (100 nm) / LiQ (1.5 nm) / Al (100 nm)
Fig. 3(a) shows energy band diagram of HODs and EODs of host materials utilized in this
study. 0.75 nm of MoO3 were deposited on the front and rear of host materials of HOD
devices. MoO3 helps the effective ohmic injection of hole carriers into the HOMO and block
the flow of electrons from cathode to the LUMO [25]. At EODs, LiQ/Al were used to inject
the electron into the LUMO of new host materials, because it has also been widely known as
an effective cathode system towards tris(8-hydroxyquinolinato)aluminum [28]. The current
density – voltage (J - V) characteristics obtained from the aforementioned HODs and EODs
are shown in Fig. 4(a). Our results suggest that while the hole current density was
significantly reduced in the order of HOD A (CBP) > HOD B (BT-mCP) > HOD C (BT-PC),
the electron current density was reduced in the order of EOD C (BT-PC) > EOD A (CBP) ≈
EOD B (BT-mCP). To estimate the charge balance when the materials were used as host
materials in emission layer, we plotted the relative charge density of those devices by
dividing the hole current density of HODs by electron current density of EODs [e.g. J(hole) /
J(electron) where J — current density, mA/cm2] as shown in Fig. 4(b) [1]. From this plot, we
could expect that BT-PC could give the most desirable device performance as the relative
charge density value was very close to unity, although the differences of HOMOs and
LUMOs between host and dopant materials should be additionally considered. Consequently,
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from this result it could be expected that BT-PC would show the most efficient device
behavior.
CBP was selected as a reference material to verify the effect of new synthetic host
materials, because one of the major charge functional units of new host materials is carbazole
moiety which is similar to those of CBP. Fig. 5 shows the perspective image of the OLED
devices fabricated in this study. The chemical structures of the used materials are provided in
Fig. 5. We used ITO as an anode, N,N’-bis(naphthalen-1-yl)-N,N’-bis(phenyl)benzidine
(NPB) as a hole transport layer, 4,4’,4’’-tris(carbazol-9-yl)-triphenylamine (HATCN) as a
charge generation layer, CBP and new materials synthesized in this study as host materials
for EML and iridium(III) bis(2-Phenylquinoline) acetylacetonate (Ir(phq)2acac) as a dopant
material for EML, 4,7- diphenyl-1,10-phenanthroline (TpPyPB) as an electron transport layer,
as well as a hole blocking layer, lithium fluoride (LiF) as an electron injection layer, Al as a
cathode. The exact device configuration used in this work was as follows.
Device A: ITO / NPB (60 nm) / HATCN (5 nm) / NPB (60 nm) / CBP : Ir(phq)2(acac) (8%,
30nm) / TpPyPB (25 nm) / LiF (1.5 nm) / Al (100 nm)
Device B: ITO / NPB (60 nm) / HATCN (5 nm) / NPB (60 nm) / BT-mCP : Ir(phq)2(acac)
(8%, 30nm) / TpPyPB (25 nm) / LiF (1.5 nm) / Al (100 nm)
Device C: ITO / NPB (60 nm) / HATCN (5 nm) / NPB (60 nm) / BT-PC : Ir(phq)2(acac)
(5%, 30nm) / TpPyPB (25 nm) / LiF (1.5 nm) / Al (100 nm)
All devices were prepared using thick HTL by using NPB (60 nm) / HATCN (5 nm) / NPB
(60 nm) to reduce a leakage current level to obtain reasonable efficiency values. The energy
diagram of the materials utilized for those devices is also depicted in Fig. 3(b).
Fig. 6(a) shows the current density – voltage (J – V) and luminance – voltage (L – V)
characteristics of the fabricated red devices; the representative results are summarized in
Table 2. At a given constant voltage of 5.0 V, current density values of 8.26, 8.29 and 9.49
mA/cm2 were observed in the fabricated Devices A, B, and C, respectively. The reason why
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BT-PC showed the greatest current density value could be due to the reasonable exciton
formation inside the EML layer. In other words, the recombination zone may be formed close
to the HTL / EML interface, because there is a huge difference in HOMO energy levels in-
between BT-PC and Ir(phq)2acac. Nevertheless, the electron current could unproblematically
be reached in this interface, as the electron barrier in-between BT-PC and TpPyPB is
relatively small (~0.4 eV). This is somewhat different in the BT-mCP case. While the
relatively shallow HOMO level of BT-mCP may result in an easier hole injection, the
electron injection into BT-mCP is very difficult. Besides, a fairly low electron conductivity of
BT-mCP may inhibit reasonable recombination of the hole and electron carriers. While CBP
also exhibits large tapping behavior in-between HTL / EML interface because of the big
difference of HOMO energy levels in-between CBP and dopant, they also showed moderate
electron injection barrier (~ 0.5 eV). Our expectations from the calculation of relative current
density and experiment results are somewhat different. Specifically, we expected that the
device prepared with BT-mCP would show a better efficiency behavior than with CBP.
However, the results suggest that the two show similar efficiency values. This means that the
real energetic situation in-between host and dopant materials in EML could lead to the
emergence of very complicated device properties. Nevertheless, as expected (see Fig. 4(b)),
BT-PC have emerged as good candidates for excellent host materials for red PHOLEDs.
Fig. 7 shows normalized EL spectra at the brightness of 1000 cd/m2. All devices radiate the
peak wavelength of Ir(phq)2acac (603nm, 604nm, 600nm for Devices A, B, and C,
respectively.
At a given constant luminance of 1000 cd/m2, the current and power efficiencies were 13.6
cd/A and 8.7 lm/W for the Device A, 13.7 cd/A and 8.70 lm/W for the Device B, 19.1 cd/A
and 13.2 lm/W for the Device C, respectively. These efficiency data correspond to 9.72, 10.3
and 12.8 % external quantum efficiencies of Devices A, B and C, respectively. The
maximum current and power efficiencies were 13.6 cd/A and 13.4 lm/W for the Device A,
19.5 cd/A and 20.4 lm/W for the Device B, 23.6 cd/A and 24.7 lm/W for the Device C,
respectively. All of detail device characteristics are summarized in Table 2. Our expectations
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from the calculation of relative current density and experiment results have somewhat
difference. In other words, we expected that the device prepared with BT-mCP could show
better efficiency behavior than that with CBP, while they showed similar efficiency values.
This means that the real energetic situation in between host and dopant materials in EML
could cause very complicated device properties. Nevertheless, BT-PC has emerged as good
candidates for excellent host materials for red PHOLEDs as we expected from Fig. 6 (b).
Fig. 7 shows normalized EL spectra at a brightness of 1000 cd/m2. All devices radiate the
peak wavelength of Ir(phq)2acac (603nm, 604nm, 600nm for the Devices A, B and C,
respectively.)
3. Conclusion
In this study, we verified the effect of novel bipolar host materials for red PHOLEDs.
Although materials with benzothiophene moieties were expected to have high hole
transporting ability, they manifested reasonable electron carrier characteristics. Our red
PHOLED devices showed fairly good current efficiency and EQE values, as their relative
charge density is close to unity. Finally, 46% of the efficiency improvement had been
achieved by using newly synthesized BT-PC as red host
4. Experimental
4.1. Instruments
1H-NMR and 13C-NMR spectra were recorded on a Bruker Avance 400 MHz NMR
spectrometer and chemical shifts were referenced to chloroform (7.26 ppm). UV-Vis spectra
were recorded with a HEWLETT PACKARD 8453 UV-Vis spectrophotometer.
Photoluminescence Spectrometer was used PERKIN ELMER LS 50B model. The molecular
mass value of the compound was analyzed by Synapt G2 HDMS quadrupole time-of-flight
(TOF) mass spectrometer equipped with an electrospray ion source (Waters, Milford, MA,
USA). The instrument was calibrated using NaF solution. The sample was dissolved in 100 %
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MeOH and introduced by direct infusion at a flow rate of 20 ꢀl/min into the ion source
operating in positive mode. All spectra were acquired at a range of 50 to 1500 m/z. Leucine
enkephalin was used as the lock mass for the exact mass measurement correction. Thermal
properties were measured by differential scanning calorimetry (DSC) under nitrogen
atmosphere using a (TA instruments) and thermogravimetric analysis (TGA) using TGA-
1000 (SINCO). Cyclic voltammetry were measured by a Autolab/PGSTAT 2 model at room
temperature in a solution of tetra-n-butylammonium hexafluorophospate(n-Bu4NPF6)(0.1 N)
in acetonitrile under nitrogen gas protection at a scan rate of 100 or 50 mV/s. The working
electrode was Pt disk type and reference electrode was Ag/0.1M AgNO3.
4.2. Synthesis of new host materials
4.2.1. Synthesis of 9-(4-bromophenyl)-9H-carbazole (1)
9H-carbazole (5.0 g, 29.9 mmol), 1-bromo-4-iodobenzene (8.88 g, 31.4 mmol), copper(I)
iodide (0.86 g, 4.5 mmol), potassium phosphate (27.5 g, 125.6 mmol) and trans-1,2-
diaminocyclohexane (1.1 ml, 9.0 mmol) in 200 ml of anhydrous toluene was stirred at 120 °C
for 48 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed
three times with distilled water and extracted with chloroform. The organic layer was
separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure.
The crude product was purified by silica gel column chromatography using n-hexane-
1
tetrahydrofuran (5:1) eluent to afford (1) a white powder, yield 65%. H-NMR (CDCl3, 400
MHz) 8.16-8.14 (d, J = 7.6 Hz, 2H), 7.75-7.73 (d, J = 8.4 Hz, 2H), 7.48-7.45 (d, J = 8.4 Hz,
2H), 7.43-7.37 (m, 4H), 7.34-7.29 (t, J = 8 Hz, 2H) ppm.
4.2.2. Synthesis of 9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole
(2)
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n-Butyllithium (26 mL, 41.59 mmol) was added to a solution of 9-(4-bromophenyl)-9H-
carbazole (6.7 g, 20.79 mmol) in anhydrous tetrahydrofuran (70 mL) at -78 ꢀ under
nitrogen atmosphere. The mixture was stirred for 30 minutes at the same temperature. And
then, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (21.2 mL, 103.97 mmol) was
added dropwise to the reaction mixture and stirred for 12 h at room temperature. The reaction
mixture was poured into water and extracted with chloroform. The organic layer was
separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure.
The crude product was purified by silica gel column chromatography using toluene eluent to
1
afford (2) a white powder, yield 54%. H-NMR (CDCl3, 400 MHz) 8.17-7.15 (d, J = 8 Hz,
2H), 8.10-8.08 (d, J = 8 Hz, 2H), 7.64-7.62 (d, J = 8.4 Hz, 2H), 7.49-7.47 (d, J = 8 Hz, 2H),
7.45-7.41 (t, J = 6.8 Hz, 2H), 7.33-7.29 (t, J = 6.8 Hz, 2H), 1.43 (s, 12H) ppm.
4.2.3. Synthesis of 2,3-dibromobenzo[b]thiophene (3)
Benzo[b]thiophene (10 g, 74.5 mmol) was dissolved in 250 ml of chloroform. The resulting
solution was cooled down to 0°C and bromine (7.87 ml, 152.8 mmol) was added. The
mixture was stirred for 12 h. The reaction mixture was poured into water and extracted with
chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and
evaporated under reduced pressure to afford (3) as a white solid, yield 90%. 1H-NMR (CDCl3,
400 MHz) 7.76-7.74 (d, J = 8.8 Hz, 1H), 7.73-7.71 (d, J = 8 Hz, 1H), 7.45-7.41 (t, J = 7.6 Hz,
1H), 7.41-7.36 (t, J = 8 Hz, 1H) ppm.
4.2.4. Synthesis of 2,3-diphenylbenzo[b]thiophene (4)
2,3-dibromobenzo[b]thiophene (6.3 g, 21.6 mmol), phenylboronic acid (5.5 g, 45.3 mmol),
tetrakis(triphenylphosphine)palladium(0) (0.6 g, 0.5 mmol), and 1.0 M Na2CO3 (86 ml) in
220 ml of toluene was stirred at 130 °C for 48 h under nitrogen atmosphere. After the
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reaction had finished, the mixture was washed three times with distilled water and extracted
with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate,
and evaporated under reduced pressure. The crude product was purified by silica gel column
chromatography using n-hexane-toluene (5:1) eluent to afford (4) a white powder, yield 80%.
1H-NMR (CDCl3, 400 MHz) 7.90-7.88 (dd, J = 6.4 Hz, 1H), 7.62-7.59 (dd, J = 7.6 Hz, 1H),
7.41-7.32 (m, 10H), 7.25-7.24 (d, J = 2 Hz, 2H) ppm.
4.2.5. Synthesis of 5-bromo-2,3-diphenylbenzo[b]thiophene (5)
2,3-diphenylbenzo[b]thiophene (6.1 g, 21.3 mmol) was dissolved in 70 ml of chloroform.
The resulting solution was cooled down to 0 °C and bromine (1.1 ml, 21.9 mmol) was added.
The mixture was stirred for 12 h. The reaction mixture was poured into water and extracted
with chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate,
and evaporated under reduced pressure. The crude product purified by recrystallization in
1
acetone to afford (5) as a white solid, yield 70%. H-NMR (CDCl3, 400 MHz) 8.01 (s, 1H),
7.43 (s, 2H), 7.41-7.39 (d, J = 7.2 Hz, 3H), 7.31-7.29 (m, 5H), 7.25-7.24 (d, J = 3.6 Hz, 2H)
ppm.
4.2.6. Synthesis of 9,9'-(5-bromo-1,3-phenylene)bis(9H-carbazole) (6)
1,3,5-Tribromobenzene (5 g, 15.9 mmol), 9H-carbazole (5.31 g, 31.8 mmol), copper(I)
iodide (0.91 g, 4.7 mmol), potassium phosphate (41.71 g, 190.6 mmol) and trans-1,2-
diaminocyclohexane (1.2 ml, 9.5 mmol) in 160 ml of anhydrous toluene was stirred at 120 °C
for 48 h under nitrogen atmosphere. After the reaction had finished, the mixture was washed
three times with distilled water and extracted with chloroform. The organic layer was
separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure.
The crude product was purified by silica gel column chromatography using n-hexane-
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1
tetrahydrofuran (5:1) eluent to afford (6) a white powder, yield 27%. H-NMR (CDCl3, 400
MHz) 8.16-8.14 (d, J = 7.6 Hz, 4H), 7.87 (s, 2H), 7.8 (s, 1H), 7.57-7.54 (d, J = 8.4 Hz, 4H),
7.49-7.45 (t, J = 7.2 Hz, 4H), 7.36-7.32 (t, J = 7.6 Hz, 4H) ppm.
4.2.7. Synthesis of 2-(2,3-diphenylbenzo[b]thiophen-5-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (7)
5-Bromo-2,3-diphenylbenzo[b]thiophene (10 g, 27.4 mmol), bis(pinacolato) diborate (8.3 g,
32.8 mmol), [1,10-bis(diphenylphosphino)ferrocene]dichloropalladium(II) (0.4 g, 0.5 mmol),
and potassium acetate (13.6 g, 136.9 mmol) were dissolved in anhydrous 1,4-dioxane (120
ml) and refluxed under nitrogen for 24 h. After the reaction had finished, the mixture was
washed three times with distilled water and extracted with chloroform. The organic layer was
separated, dried over anhydrous magnesium sulfate, and evaporated under reduced pressure.
The crude product was purified by silica gel column chromatography using n-hexane-toluene
1
(1:1) eluent to afford (7) a white powder, yield 70%. H-NMR (CDCl3, 400 MHz) 7.36 (s,
1H), 7.75-7.73 (d, J = 8 Hz, 1H), 7.59-7.57 (d, J = 8 Hz, 1H), 7.43-7.36 (m, 3H), 7.34-7.31
(m, 4H), 7.25-7.23 (m, 3H), 1.38 (s, 12H) ppm.
4.2.8. Synthesis of 9-(4-(2,3-diphenylbenzo[b]thiophen-5-yl)phenyl)-9H-carbazole (BT-PC)
2-(2,3-Diphenylbenzo[b]thiophen-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.0 g, 2.74
mmol),
9-(4-bromophenyl)-9H-carbazole
(0.86
g,
2.67
mmol),
tetrakis(triphenylphosphine)palladium(0) (0.14 g, 0.12 mmol), and 1.0 M Na2CO3 (9.7 ml) in
50 ml of toluene was stirred at 130 °C for 48 h under nitrogen atmosphere. After the reaction
had finished, the mixture was washed three times with distilled water and extracted with
chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and
evaporated under reduced pressure. The crude product was purified by silica gel column
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chromatography using n-hexane-tetrahydrofuran (3:1). The final product was obtained as a
white powder after purification by vacuum sublimation at a synthetic yield of 63%. 1H-NMR
(CDCl3, 400 MHz) 8.20 (s, 1H), 8.18-8.16 (d, J = 7.6 Hz, 2H), 7.92-7.90 (d, J = 6.4 Hz, 2H),
7.73-7.71 (d, J = 8 Hz, 1H), 7.69-7.67 (d, J = 8.8 Hz, 3H), 7.52-7.50 (d, J = 8 Hz, 2H), 7.46-
13
7.27 (m, 14H) ppm; C-NMR (CDCl3, 400 MHz) 138.6, 138.1, 138.0, 137.8, 137.4, 134.6,
133.2, 131.9, 130.9, 128.2, 127.4, 126.5, 126.4, 126.2, 125.6, 125.3, 125.2, 123.8, 121.8,
121.5, 121.2, 118.2, 118.1, 117.8, 107.6 ppm. HRMS (ESI, m/z): [M+H]+ calcd for C38H26NS,
528.1786; found, 528.1777. Anal. calcd for C38H25NS: C 86.49, H 4.78; found: C 86.43, H
4.79 %.
4.2.9. Synthesis of 9,9'-(5-(triphenylen-2-yl)-1,3-phenylene)bis(9H-carbazole) (BT-mCP)
9,9'-(5-Bromo-1,3-phenylene)bis(9H-carbazole)
(1.1
g,
2.26
mmol),
2-(2,3-
diphenylbenzo[b]thiophen-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.96 g, 2.33 mmol),
tetrakis(triphenylphosphine)palladium(0) (0.08 g, 0.07 mmol), and 1.0 M Na2CO3 (4.8 ml) in
30 ml of toluene was stirred at 130 °C for 48 h under nitrogen atmosphere. After the reaction
had finished, the mixture was washed three times with distilled water and extracted with
chloroform. The organic layer was separated, dried over anhydrous magnesium sulfate, and
evaporated under reduce pressure. The crude product was purified by silica gel column
chromatography using n-hexane-tetrahydrofuran (5:1). The final product was obtained as a
white powder after purification by vacuum sublimation at a synthetic yield of 58%. 1H-NMR
(DMSO-d6, 400 MHz) 8.71 (s, 1H), 7.30-8.28 (d, J = 8 Hz, 4H), 8.21 (s, 1H), 7.98-7.95 (d, J
= 8.8 Hz, 1H), 7.88 (s, 1H), 7.70-7.68 (d, J = 8.4 Hz, 4H), 7.57-7.54 (d, J = 8.8 Hz, 1H), 7.52-
7.46 (m, 6H), 7.44-7.42 (d, J = 6.8 Hz, 1H), 7.35-7.31 (m, 11H) ppm; 13C-NMR (CDCl3, 400
MHz) 144.6, 141, 140.6, 139.9, 139.8, 135.7, 135.2, 134, 133.1, 130.4, 129.6, 128.9, 128.5,
128, 127.7, 126.3, 124.5, 124, 123.9, 123.7, 120.7, 120.6, 120.5, 109.8 ppm. HRMS (ESI,
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m/z): [M+H]+ calcd for C50H33NS, 693.2364; found, 693.2342. Anal. calcd for C50H32NS:
C 86.67, H 4.66; found: C 86.60, H 4.67 %.
4.3. Fabrication of PHOLEDs
4.3.1 Materials : NPB / HATCN / NPB as a hole injection layer, Ir(phq)2acac as a red
dopant, TpPyPB as a hole blocking layer, and ETL, LiF, LiQ as an electron injection layer
and Al as a cathode were purchased from commercial suppliers and were used without
purification.
4.3.2 Device Fabrication: To fabricate OLED devices, clean glass substrates pre-coated
with a 150-nm-thick ITO layer with a sheet resistance of ~12 Ω/sq were used. Line patterns
of ITO were formed on glass by photolithography process. The ITO glass was cleaned by
sonification in an isopropylalcohol and acetone, rinsed in deionized water, and finally
irradiated in a UV-ozone chamber. All organic materials were deposited by the vacuum
evaporation technique under a pressure of ~1x10-7 Torr. The deposition rate of organic layers
was about 0.5 Å/s. Deposition rates of LiF and Al were 0.1 Å/s and 3 Å/s, respectively.
4.3.3 Measurements: The current density-voltage (J-V) and luminance-voltage (L-V) data
of OLEDs were measured by Keithley 2635A and Minolta CS-100A, respectively. The
OLED area was 4 mm2 for all the samples studied in this work. Electroluminescence (EL)
spectra and CIE coordinate were obtained using a Minolta CS-2000A spectroradiometer.
Acknowledgments
Prof. M. C. Suh was supported by the Industrial Strategic Technology Development
Program (Grant No. 10042590). Prof. J.-H. Lee was supported by a grant from Basic Science
Research Program through the National Research Foundation of Korea (NRF) funded by the
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Ministry of Education, Science and Technology (Grant No. NRF-2010-0012463). We would
like to thank Dr. Young Hye Kim (KBSI) for the ESI MS analysis.
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FIGURE CAPTIONS
Scheme 1. Synthesis scheme of novel red host materials
Fig. 1. Geometries of two host materials having benzothiophene.
Fig. 2. (a) UV-visible absorption and photoluminescence spectra (at 298 and 77K),
(b) DSC and TGA analyses of red host materials
Fig. 3. Energy band diagrams of used materials in this work.
Fig. 4. (a) Hole current characteristics of HOD A~C and electron current characteristics of
EOD A ~C. (b) Relative charge density in host materials. (J(hole) / J(electron))
Fig. 5. Schematic diagram of device architecture and molecular structure used in this study.
Fig. 6. (a) J – V – L characteristics of fabricated red devices. (b) Luminance vs current
efficiency and power efficiency characteristics of fabricated red PHOLEDs.
Fig. 7. The normalized EL spectra at a brightness of 1000 cd/m2 of fabricated red devices.
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Table. 1. Summary of physical properties of new host materials.
λpl. [nm]
T1
T1
HOMO
[eV]
LUMO
[eV]
Eg
Tg
Tm
Td
λabs [nm]
Compound
(eV)
(Exp)
(eV)
(Cal)
[eV]
[ꢀ]
[ꢀ]
[ꢀ]
(CV)
Solution
393
Film
404
245, 293,
326, 340
BT-mCP
BT-PC
2.26
2.26
2.25
2.27
- 5.66
- 5.96
- 2.18
- 2.57
3.48
3.39
150
110
255
258
430
393
244, 294,
330
398
416
Table 2. Summary of device performance of all fabricated PHOLED devices.
Turn-on
voltage
[V][a]
Driving
voltage
[V][b]
Current
Efficiency
[cd/A][b][c]
Power
Device
Efficiency
EQE [%][b]
CIE (x, y)[b]
[cd/A][b][c]
13.6
(13.6)
8.70
(13.4)
Device A
Device B
Device C
2.58
2.53
2.54
4.90
4.90
4.49
9.72
10.3
12.8
(0.620, 0.379)
(0.624, 0.374)
(0.626, 0.381)
13.7
(19.5)
8.70
(20.4)
19.1
(23.6)
13.2
(24.7)
[a] Measured at 1 cd m−2
[b] Measured at 1000 cd m−2
[c] The values in parentheses mark the maximum efficiency
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