Silicon-based carbazole and oxadiazole hybrid as a bipolar host material for phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2016.08.021

A silicon-based bipolar compound, 2-(4-((4-(9H-carbazol-9-yl)phenyl)dimethylsilyl)phenyl)-5-phenyl-1,3,4-oxadiazole (COHS), was designed and prepared as a host material for phosphorescent organic light-emitting diodes (OLEDs). The conjugated analogue of COHS, 2-(4′-(9H-carbazol-9-yl)biphenyl-4-yl)-5-phenyl-1,3,4-oxadiazole (COH), was also prepared to investigate their structure–property relationships. Thermal-, photophysical- and electrochemical properties as well as their single-crystal X-ray structures were studied for COHS and COH. The central silicon atom in COHS successfully disconnected the electronic communication between the carbazole and oxadiazole groups, resulting in relatively high triplet energy of ca. 2.71?eV, which were capable of hosting green phosphorescent emitters. DFT calculations were conducted to investigate the electronic structures of COHS and COH, and the results showed good correlation to experimental results. Finally, COHS and COH were used as a bipolar host material for a green phosphorescence organic light-emitting diode (PHOLED) devices with Ir(ppy)₃ (tris[2-phenylpyridinato-C²,N]iridium(III)) as a dopant. The resulting device with COHS (device I) showed higher performance than the device with COH (device II), exhibiting high efficiencies and low-efficiency roll-off. Device I achieved maximum external quantum efficiencies (EQE) of 15.8%, whereas device II exhibited a relatively lower EQE of 13.0%.

Full text of DOI:10.1016/j.orgel.2016.08.021

Organic Electronics 38 (2016) 222e229
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Silicon-based carbazole and oxadiazole hybrid as a bipolar host
material for phosphorescent organic light-emitting diodes
Ah-Rang Lee, Jiwon Lee, Jiewon Lee, Won-Sik Han*
Department of Chemistry, Seoul Women's University, Seoul 01797, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A silicon-based bipolar compound, 2-(4-((4-(9H-carbazol-9-yl)phenyl)dimethylsilyl)phenyl)-5-phenyl-
1,3,4-oxadiazole (COHS), was designed and prepared as a host material for phosphorescent organic light-
emitting diodes (OLEDs). The conjugated analogue of COHS, 2-(40-(9H-carbazol-9-yl)biphenyl-4-yl)-5-
phenyl-1,3,4-oxadiazole (COH), was also prepared to investigate their structureeproperty relation-
ships. Thermal-, photophysical- and electrochemical properties as well as their single-crystal X-ray
structures were studied for COHS and COH. The central silicon atom in COHS successfully disconnected
the electronic communication between the carbazole and oxadiazole groups, resulting in relatively high
triplet energy of ca. 2.71 eV, which were capable of hosting green phosphorescent emitters. DFT calcu-
lations were conducted to investigate the electronic structures of COHS and COH, and the results showed
good correlation to experimental results. Finally, COHS and COH were used as a bipolar host material for
Received 8 June 2016
Received in revised form
4 August 2016
Accepted 21 August 2016
Keywords:
Bipolar host
OLEDs
High triplet energy
Structure-property relationship
a
green phosphorescence organic light-emitting diode (PHOLED) devices with Ir(ppy)3 (tris[2-
phenylpyridinato-C2,N]iridium(III)) as a dopant. The resulting device with COHS (device I) showed
higher performance than the device with COH (device II), exhibiting high efficiencies and low-efficiency
roll-off. Device I achieved maximum external quantum efficiencies (EQE) of 15.8%, whereas device II
exhibited a relatively lower EQE of 13.0%.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
To achieve this, several strategies have been suggested for the
development of host materials with limited
example, 1) both silicon [5e8] or sp3ehybridized carbon [9] atoms
were introduced into a host material to block the -conjugation,
p-conjugation. For
Phosphorescent organic light-emitting diodes (PHOLEDs) have
been deemed potential candidates for flat-panel displays and solid-
state lighting sources because the heavy-metal phosphors can
theoretically achieve 100% internal quantum efficiency by har-
vesting both electronically generated singlet and triplet excitons
[1]. To avoid efficiency roll-off induced by concentration quenching
and tripletetriplet annihilation, the heavy-metal phosphors are
normally homogeneously dispersed into a host matrix [2,3].
Therefore, developing host materials with great performance is of
equal importance to the dopant material for achieving high per-
formance in a PHOLED device.
In general, an appropriate host material is required to satisfy
several conditions. First, the triplet energy level (ET) of the host
material should be higher than that of the dopant material to
facilitate total energy transfer from the host to the phosphorescent
emitter and confine the triplet exciton within the emitting layer [4].
p
and 2) changing the interconnection between aromatic functional
groups to the meta position, rather than ortho or para positions, to
increase the distortion of the backbone structure [10e12]. Second,
the desired host materials should have a high glass transition
temperature (Tg) that can enhance the thermal and morphological
stability, extending the operational lifetime of the device. Another
requirement is that it should has proper carrier transport proper-
ties. If charge hopping occurs in the emission layer, the charge
recombination zone would decrease and the exciton will be
generated on the surface between the hole-transporting or
electron-transporting layer, resulting in a decrease in device effi-
ciency [13]. In addition, charge and exciton accumulation at in-
terfaces may cause efficiency roll-off and device degradation owing
to tripletetriplet annihilation [14]. Therefore, it is crucial that the
host material possesses efficient and balanced charge transport
properties. For this reason, electron acceptors such as benzimid-
azole [15e17], phosphine oxide [18e21], triazine [22e25],
* Corresponding author.
E-mail address: chemisthan@gmail.com (W.-S. Han).
http://dx.doi.org/10.1016/j.orgel.2016.08.021
1566-1199/© 2016 Elsevier B.V. All rights reserved.

A.-R. Lee et al. / Organic Electronics 38 (2016) 222e229
223
oxadiazole [26e28], and electron donors such as carbazole [29,30],
triarylamine [31], and dibenzothiophene [32,33] were introduced
to modify the bipolar carrier abilities of host materials; however,
because of possible charge delocalization between the hole and
electron-transport moieties, this type of material usually exhibits a
relatively small energy band gap and low ET. Therefore, it is
imperative that the bipolar host materials possess weak donor-
acceptor interactions to retain a high ET. Thus, the first strategy
mentioned above which incorporating silicon atom can be applied
in designing a bipolar host material.
Carbazoles, possessing the advantage of high triplet energies up
to 3 eV [34], have been mostly used as building blocks for the
design of host materials. One commonly used host for phospho-
rescent emitters is 4,40-bis(9-carbazolyl)-biphenyl (CBP); however,
owing to the presence of a biphenyl group in the molecule, this
material has a triplet energy of 2.56 eV [35], relatively lower than
dopant materials for green or blue OLED applications.
couple. 1,4-Dibromobenzene, carbazole, benzhydrazide, 4-benzoyl
chloride, sodium bicarbonate, copper sulfate, potassium carbon-
ate, phosphorus oxychloride, bis(pinacolato)diboron, potassium
acetate, 1,4-dibromo-2,5-dimethylbenzene, [1,10-bis(diphenyl-
phosphino)ferrocene]dichloropalladium(II)
complex
with
dichloromethane, tetrakis(triphenylphosphine)palladium(0), n-
BuLi (2.5 M solution in n-hexane), dimethyldichlorosilane, and
sodium carbonate were purchased from Aldrich or TCI and used
without further purification. The starting materials, 2-(4-
bromophenyl)-5-phenyl-1,3,4-oxadiazole (DPO-Br) [37], 9-(4-
bromophenyl)carbazole (PCeBr) [5], 9-(4-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-20-yl)phenyl)-9H-carbazole (PC-BE) [38] were
prepared according to the previously reported procedures.
2.2. Synthesis
2.2.1. 2-(4-((4-(9H-carbazol-9-yl)phenyl)dimethylsilyl)phenyl)-5-
phenyl-1,3,4-oxadiazole (COHS)
Oxadiazole derivatives are among the most widely investigated
electron-transporting materials for OLEDs because of their rela-
tively high electron affinities. For example, the oxadiazole moiety,
9-(4-bromophenyl)carbazole (PCeBr) (2.13 g, 6.6 mmol) in
diethyl ether (40 mL) was stirred at 0 ꢀC under dry argon atmo-
sphere and treated with a solution of n-BuLi (2.9 mL, 2.5 M in n-
hexane, 7.3 mmol). The resulting mixture was kept at 0 ꢀC for 1 h,
and then dichlorodimethylsilane (6.6 mL, 54.9 mmol) was added
slowly. The reaction temperature was warm to room temperature
and the reaction mixture was stirred for overnight. After the reac-
tion, LiCl salts were filtered by canuula and reaction solvent and
excess dichlorodimethlysilane in filtrate were removed under
reduced pressure and stored in Ar atmosphere. In another flask, 2-
(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole (DPO-Br) (2.22 g,
7.4 mmol) in dry THF (40 mL) was prepared and a 2.5 M solution of
n-BuLi in n-hexane (2.9 mL, 7.3 mmol) was added dropwise
at ꢁ78 ꢀC. After stirring for 30 min, the previously prepared 9-[4-
(chlorodimethylsilyl)phenyl]-9H-carbazole (PCS) in THF (10 mL)
was added slowly at ꢁ78 ꢀC. After addition, the reaction tempera-
ture was warm to room temperature and the reaction mixture was
stirred for overnight. The reaction mixture was quenched by
addition of distilled water, extracted with dichloromethane and
washed with water. The combined organic layers were dried over
MgSO4 and filtered. The filtrate was evaporated under reduced
pressure and the residue was purified by silica-gel column chro-
matography using ethyl acetate/n-hexane (v/v ¼ 1:4) as an eluent.
The resulting white solid was dissolved in dichloromethane and
evaporated slowly to give single crystals. Yield: 36%; 1H NMR
2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxdiazole
(PBD),
showed an electron affinity of 2.16 eV and was first used as an
electron-transporting material in a bilayer OLED [36]; however,
vacuum-evaporated amorphous PBD thin films have low stability
(Tg ¼ 60 ꢀC) and crystallize during device operation. Therefore,
enhancement of thermal and morphological stability of the oxa-
diazole moieties is still required.
In this study, we have prepared 2-(4-((4-(9H-carbazol-9-yl)
phenyl)dimethylsilyl)phenyl)-5-phenyl-1,3,4-oxadiazole (COHS), a
bipolar host material comprising carbazole and oxadiazole units. A
silicon atom was introduced between the two units to disconnect
their electronic communication and enhance their thermal prop-
erties. To investigate the role of the silicon atom, its analogous
compound, 2-(40-(9H-carbazol-9-yl)biphenyl-4-yl)-5-phenyl-1,3,4-
oxadiazole (COH), was also prepared. Single-crystal X-ray molec-
ular structures for both compounds were determined and the
thermal-, photophysical- and electrochemical properties of these
compounds were systematically investigated. Thereafter, green
PHOLEDs using COHS and COH as bipolar host materials with
Ir(ppy)3 as a dopant material were fabricated and their efficiencies
were reported.
2. Experimental
(300 MHz, CDCl3,
7.6e7.53 (m, 5H), 7.48e7.38 (m, 4H), 7.31e7.26 (m, 2H), 1.58 (s, 3H),
0.70 (s, 3H); 13C NMR (75 MHz, CDCl3,
): 142.9, 140.6, 138.8, 136.6,
d
): 8.18e8.13 (m, 6H), 7.76 (t, J ¼ 15.9 Hz, 4H),
2.1. General information
d
All the experimental procedures were carried out under a dry
nitrogen or argon atmosphere using standard Schlenk techniques.
Tetrahydrofuran (THF) was distilled freshly over sodium benzo-
phenone. The 1H and 13C NMR spectra were recorded on a Bruker
Fourier 300 MHz spectrometer, which was operated at 300.1 and
75.4 MHz, respectively. 1H and 13C NMR chemical shifts were
measured in CDCl3 and referenced to relative peaks of CHCl3
(7.26 ppm for 1H NMR) and CDCl3 (77.16 ppm for 13C NMR). The
elemental analyses were performed using a Carlo Erba Instrument
CHNSeO EA 1108 analyzer. The HR-MS analysis was performed by
high sensitive LC/MS/MSn (n ¼ 10) spectrometer (Thermo Fisher
Scientific, LCQ Fleet Hyperbolic Ion Trap MS/MSn Spectrometer).
Cyclic voltammetry (CV) was performed in an electrolytic so-
135.7, 134.9, 131.8, 129.1, 126.9, 126.3, 126.1, 125.9, 124.5, 123.9,
123.4, 120.3, 120.0,109.8, e2.46. HRMS: calculated for C34H27N3OSi:
521.1923, Found: 521.1931. Elemental analysis: calculated for
C34H27N3OSi: C, 78.28; H, 5.22; N, 8.05, Found: C, 78.29; H, 5.23; N,
8.03.
2.2.2. 2-(40-(9H-carbazol-9-yl)biphenyl-4-yl)-5-phenyl-1,3,4-
oxadiazole (COH)
A mixture of 9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-20-yl)
phenyl)-9H-carbazole (PC-BE) (1.49 g, 4.0 mmol), 2-(4-
bromophenyl)-5-phenyl-1,3,4-oxadiazole (DPO-Br) (1.33 g,
4.4 mmol), tetrakis(triphenylphosphine)palladium (0.46 g,
0.4 mmol), and Na2CO3 (1.71 g, 16.0 mmol) in tetrahydrofuran
(THF)/H2O (v:v ¼ 21 mL:7 mL) was stirred at 70 ꢀC for overnight
under argon atmosphere. After cooling to room temperature, the
mixture was poured into distilled water and extracted with
dichloromethane. The combined organic layers were dried over
MgSO4 and evaporated under reduced pressure. The crude product
was purified by recrystallization in dichloromethane/n-hexane to
lution prepared using 0.1
M tetrabutyl ammonium hexa-
fluorophosphate (NBu4PF6) at room temperature under an
atmosphere of argon. For this purpose, CHI600E was used. Glassy
carbon, platinum wire, and Ag/AgNO3 (0.1 M) were used as work-
ing, counter, and reference electrodes, respectively. All the poten-
tials were calibrated to the ferrocene/ferrocenium (Fc/Fcþ) redox

224
A.-R. Lee et al. / Organic Electronics 38 (2016) 222e229
give a white solid. Yield: 76%; 1H NMR (300.1 MHz, CDCl3,
d
): 8.29
compounds COHS and COH were optimized using the B3LYP den-
sity functional theory (DFT) and 6/31G (d,p) basis set. Then, time-
dependent DFT (TDDFT) calculations were performed using the
same functional theory and basis set, which were previously used
to estimate the energies and oscillator strengths of derivatives. The
contours of electron density were plotted using Chem3D ver.10
[43].
(d, J ¼ 8.5 Hz, 2H), 8.21e8.16 (m, 4H), 7.89 (t, J ¼ 8.0 Hz, 5H), 7.71 (d,
J ¼ 8.5 Hz, 2H), 7.59e7.55 (m, 2H), 7.51e7.41 (m, 4H), 7.34e7.29 (m,
2H); 13C NMR (75.4 MHz, CDCl3,
d): 143.5, 140.7, 138.8, 137.8, 131.8,
129.1, 128.6, 127.7, 237.6, 127.5, 137.0, 126.0, 123.9, 123.5, 123.0,
120.4, 120.1, 109.78. HRMS: calculated for C32H21N3O: 463.1685,
Found: 463.1689. Elemental analysis: calculated for C32H21N3O: C,
82.92; H, 4.57; N, 9.07, Found: C, 82.90; H, 4.58; N, 9.08.
2.8. Device fabrication and characterization
2.3. Crystal structure determination
The OLED devices were fabricated on glass substrates, which
were pre-coated with a 150 nm indium tin oxide (ITO) layer having
sheet resistance of 10 U/square. The ITO glass was pre-cleaned by a
Single crystals of COHS and COH were mounted on a diffrac-
tometer. To perform preliminary examination and data collection,
we used a Bruker SMART CCD detector system single-crystal X-ray
diffractometer that was equipped with a sealed-tube X-ray source
conventional solvent cleaning method. Using oxygen plasma
treatment, the ITO surface was cleaned again immediately before
depositing the hole injection layer. At a background pressure of
~3 ꢂ 10ꢁ7 Torr, thermal evaporation was performed to sequentially
deposit organic and Al layers. The OLED devices were packaged in a
glove box filled with N2 of ultra-high purity; the concentrations of
oxygen and water were maintained at approximately 0.5 ppm in
this box. A glass lid was attached to the substrate through an epoxy
seal, which was applied around the perimeter of this box. The
current-voltage characteristics of OLEDs were measured using a
source measurement unit (Keithley 2635B). The electrolumines-
cence spectra, luminance, and CIE coordinates were measured us-
ing a spectroradiometer (Konica Minolta CS-2000). Assuming
Lambertian emission, we calculated the external quantum effi-
ciency (EQE) from the luminance, current density, and electrolu-
minescence spectrum.
(50 kV ꢂ 30 mA); monochromatic Mo Ka radiation ( ¼ 0.71073 Å)
l
corresponding to graphite was obtained from the X-ray source. At
this preliminary stage, the unit cell constants were determined
from a set of 45 narrow-frame (0.3ꢀ in
u) scans. A double-pass
method of scanning was used to exclude any noise. The collected
frames were integrated using an orientation matrix, which was
developed using narrow frame scans. The SMART software package
was used for data collection, while SAINT software package was
used for frame integration [39]. Final cell constants were deter-
mined by conducting global refinement of the xyz centroids of the
reflections, which were harvested from the entire data set. The
structural solution and refinement were carried out using SHELXTL
PLUS software package [40].
2.4. Absorption and emission spectra
3. Results and discussion
The absorption and photoluminescence spectra were recorded
on an SHIMADZU UV-2450 UV/VIS/NIR scanning spectrophotom-
eter and a VARIAN Cary Eclipse fluorescence spectrophotometer,
respectively. The emission quantum yields are estimated using 95%
ethanol solution of anthracene (Ff ¼ 0.27) as a standard [41].
3.1. Synthesis
Scheme 1 illustrates the synthetic procedures used for prepa-
ration of COHS and COH. Initially, the starting compounds, 2-(4-
bromophenyl)-5-phenyl-1,3,4-oxadiazole (DPO-Br) [37], 9-(4-
2.5. Phosphorescence emission spectra
bromophenyl)carbazole
(PCeBr)
[5],
and
9-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-20-yl)phenyl)-9H-carbazole (PC-
BE) [38] were prepared according to previously reported proced-
ures. The starting compound, 9-(4-bromophenyl)carbazole
(PCeBr), was prepared by an Ullmann condensation between 9H-
carbazole and 1,4-dibromobenzene. 2-(4-bromophenyl)-5-phenyl-
1,3,4-oxadiazole (DPO-Br) was prepared starting from the reaction
of benzhydrazide with 4-bromobenzoyl chloride, followed by
treatment with POCl3 to give the starting compound 2-(4-
bromophenyl)-5-phenyl-1,3,4-oxadiazole (DPO-Br) in modest
yield. COHS was synthesized by performing a direct substitution
reaction between (4-(9H-carbazol-9-yl)phenyl)lithium and a large
excess of dichlorosilane to give the mono-substituted product, 9-
(4-(chlorodimethylsilyl)phenyl)-9H-carbazole (PCS), which was
handled under an Ar atmosphere owing to its instability in air. In-
situ reaction with the initial mono-substituted product, PCS, and
(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)lithium gave the final
product, COHS, as a white solid in 36% yield. To prepare its analogue
compound, COH, PC-Br was converted to PC-BE by reaction with
4,4,40,40,5,5,50,50-octamethyl-2,20-bi(1,3,2-dioxaborolane) catalyzed
with Pd(dppf)Cl2. Suzuki-Miyaura coupling reaction catalyzed by
Pd(PPh3)4 with PC-BE and DPO-Br in the presence of base gave COH
as a white solid in 76% yield. Both compounds were purified by
silica-gel column chromatography. The structures and chemical
At 77 K, the third harmonic (355 nm) pulse of a Q-switched
Nd:YAG laser (Continuum, Surelite II; pulse width of 4.5 ns) was
used to excite the molecules. The phosphorescence spectra were
recorded by an ICCD detector (Andor, iStar) equipped to a mono-
chromator (DongWoo Optron, Monora 500i). The temporal profiles
were measured using a monochromator equipped with a photo-
multiplier (Zolix Instruments Co., CR 131) and a digital oscilloscope
(Tektronix, TDS-784D). Oxygen in sample solutions was removed
through the argon-purging method.
2.6. Thermal properties
TGA study was performed using a TGA/DSC 1 (Mettler-Toledo
Inc.) thermogravimetric analyzer. TGA was conducted several
times, and the data were confirmed. The sample was loaded into a
weight-tared alumina pan, and heated at a rate of 5 ꢀC/min from 25
to 180 ꢀC under flowing nitrogen (50 mL/min). Thermal properties
were measured using differential scanning calorimetry (Pyris Dia-
mond DSC; Perkin Elmer). A heating rate of 10 ꢀC/min was used for
first melting the compound followed by a rapid cooling rate of
40 ꢀC/min to room temperature.
2.7. DFT calculations
purities of the final products were adequately verified by 1H and 13
C
NMR spectroscopy, mass spectrometry and elemental analysis (see
Experimental Section). Single crystals for X-ray analysis of both
The derivatives were theoretically calculated using the
Gaussian09 package [42]. The ground-state geometries of
COHS and COH were grown from slow evaporation of a

A.-R. Lee et al. / Organic Electronics 38 (2016) 222e229
225
Me
n-BuLi
N
N
N
Br
+
Me2SiCl2
N
Si Cl
Me
o
diethyl ether, 0 C
PC-Br
PCS
Me
Me
Si
Br
O
N
O
n-BuLi
Si Cl
Me
N
+
THF, -78 o
C
N
N
N
Me
PCS
DPO-Br
COHS
Br
O
B
O
O
N
Pd(PPh3)4, Na2CO3
THF/H2O, reflux
O
+
N
N
N
N
PC-BE
DPO-Br
Scheme 1. Synthetic procedures and chemical structures of COHS and COH.
COH
dichloromethane/n-hexane solution. Before fabrication of the OLED
device, both compounds were further purified by train sublimation.
with CeSieC bond angles of 109.5(4)ꢀ (averaged value). The planes
of the carbazole and oxadiazole rings were twisted significantly
relative to each other with a dihedral angle of 85.0(1)ꢀ. This twist
results from the tetrahedral environment around the central silane
core which imposes a non-coplanar structure in the molecule. COH
comprises central biphenyl rings with periphery carbazole and
oxadiazole rings. The oxadiazole ring is attached to another phenyl
ring. The central biphenyl rings were notably twisted relative to
each other (16.5(1)ꢀ), which can be attributed to spatial torsion
3.2. Crystal structure
The crystal structures of COHS and COH were elucidated by
single-crystal X-ray analysis. Fig. 1a shows their crystal structures
with thermal ellipsoids drawn at the 30% probability level and
Fig. 1b shows packing structures. Table S1 summarizes the X-ray
structural parameters, and Tables S2ꢁS5 summarize bond lengths
and angles. COHS comprises a silicon atom bonded to four pe-
ripheral molecular groups, resulting in a tetrapod array. The silicon
atom has a tetrahedral geometry defined by the four peripheral
ipso-carbon atoms having a bond distance of 1.873(1) Å (averaged
value). The four carbons were arranged in a tetrahedral structure
disrupting the
p-conjugation between the carbazole and oxadia-
zole units. The crystal packing structures of COHS and COH are
shown in Fig. 1b. As can be seen in their crystal packing structures,
they have different intermolecular interactions. First, three packed
molecules have intermolecular interactions in the COHS packing
structure, whereas only two packed molecules have intermolecular
Fig. 1. (a) Single crystal structures of COHS (left) and COH (right) at 30% probability for the thermal ellipsoids. Hydrogen atoms were omitted for clarity. (b) Crystal packing structure
of COHS (left) and COH (right) in a unit cell.

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A.-R. Lee et al. / Organic Electronics 38 (2016) 222e229
interactions in the COH packing structure. Second, they have
different types of interactions. COHS has three different intermo-
of COHS shows distinguished absorption peaks at 287, 293, 304, 319
and 340 nm. Among them, absorption peaks at 287, 304, 319 nm
originated from DPO, whereas the peaks at 293 and 340 nm orig-
inated from PC. Thus, the central silicon atom successfully disturbs
electronic communication between the PC and DPO units in the
ground state. In contrast, COH shows a different absorption spec-
trum which exhibits a very broad and strong absorption from 302
to 380 nm. This absorption behavior can be rationalized by the
generation of charge transfer character between the two periphery
groups, even in the ground state. Excited state behaviors were
investigated by photoluminescence spectra. Photoluminescence
spectra was measured in a toluene solution with excitation at
290 nm for all compounds. As shown in Fig. 2b, COHS and COH
showed significantly different emission properties in toluene so-
lution. COHS showed an emission spectrum with two distinct
vibronic structures at 349 and 364 nm, which resemble the emis-
sion spectrum of N-phenylcarbazole. Thus, emission of COHS can be
lecular
leetoeoxadiazole, and phenyletoephenyl interactions, and one
CeH$$$ interaction. Among the three different intermolecular
interactions, the phenyletoephenyl interaction between the
pe
p
interactions, phenyletoeoxadiazole, oxadiazo-
p
pep
inner and outer phenyl rings of the DPO groups has the shortest
distance of 3.86 Å while the distances of intermolecular phenyl-to-
oxadiazole and oxadiazole-to-oxadiazole interactions were 3.96
and 4.62 Å, respectively. The crystal packing structure of COHS also
contains CeH/p interactions between the hydrogen atom of
methyl group and the outer phenyl ring of the DPO group, with
distance of 2.69 Å. In contrast, COH has only one type of intermo-
lecular interaction, a phenyl-to-oxadiazole interaction with a dis-
tance of 3.60 Å. Even though COH has a shorter intermolecular
pep
interaction, the highly ordered intermolecular interactions in
pep
COHS are expected facilitate charge-hopping, resulting in an
improvement in device efficiency (vide infra).
unambiguously assigned to the lowest
p-p* transition (locally
excited state, LE) of the N-phenylcarbazole chromophore in the
molecule. In contrast, the emission spectrum of COH exhibits broad
and structureless emission with an emission wavelength red-
shifted to 400 nm relative to COHS and the parent molecules, N-
phenylcarbazole and diphenyloxadiazole. These results indicate
existence of the charge transfer state of COH in the excited state.
Because both compounds have a bipolar structure, solvent
dependent experiments were conducted. As shown in Fig. S4, both
compounds show different emission behavior depending on sol-
vent polarity. COHS showed local emission from the PC unit in n-
hexane. With increasing solvent polarity, COHS showed dual
emission properties from local and charge transfer excited states in
dichloromethane. Increasing the solvent polarity through the use of
acetonitrile, emissions from charge transfer excited state were
further red-shifted owing to the stabilization of the charge transfer
excited state. COH showed emission from a local excited state in n-
hexane. For COH, emission wavelengths were more significantly
red-shifted with increasing solvent polarity from n-hexane to
acetonitrile. The LipperteMataga plot, the plot of the Stokes shift of
the emission versus the solvent polarity, was analyzed to calculate
their dipole moments [46]. The difference between the maximum
absorption and emission wavelengths, expressed in wavenumbers
3.3. Thermal properties
High thermal stability with a high glass transition temperature
is desirable in an efficient OLED device fabrication process to help
prevent morphological changes and suppresses aggregate forma-
tion upon heating. In this regard, the thermal stabilities of COHS
and COH were investigated by thermogravimetric analysis and
differential scanning calorimetry (DSC). Both COHS and COH
exhibit high thermal decomposition temperatures (Td) at 441 ꢀC
and 416 ꢀC, respectively, as shown in Fig. S2. This result indicates a
substantially higher thermal stability for COHS compared with
COH. Their morphological stability was further investigated by DSC
measured under a nitrogen atmosphere with a heating rate of
10 ꢀC minꢁ1. The glass transition temperature (Tg) for COHS was
observed at 72 ꢀC, whereas Tg for COH was not observed, even in
the second and third DSC scans, as shown in Fig. S3.
3.4. Photophysical properties
To investigate the photophysical properties of COHS and COH,
UV/Vis absorption and photoluminescence spectra in toluene were
measured at room temperature as shown in Fig. 2a and b, respec-
tively. For comparison, independent units of N-phenylcarbazole
(PC) and diphenyloxadiazole (DPO) were also measured under the
same conditions. Photophysical properties for all compounds are
summarized in Table 1. As shown in Fig. 2a, characteristic absorp-
tion peaks around 293 and 340 nm can be assigned to the 1La ) 1A
(Dy), is given by the following eqn (1):
!
ꢀ
ꢁ
ꢂ
ꢃ
2
2
hca30
ε ꢁ 1
n ꢁ 1
va ꢁ vf
¼
ꢂ
¼
ꢁ
ꢂ
me
ꢁ
mg
2ε þ 1 2n ꢁ 1
(1)
!
2
hca30
ꢂ
D
f ꢂ ðDmÞ2
and 1Lb
)
1A transitions, respectively, of the PC chromophore
[44,45]. In contrast, DPO exhibits strong absorption at 287 nm and
absorption shoulders at 297 and 311 nm. The absorption spectrum
(a)
(b)
COHS
COH
N-Phenylcarbazole
Diphenyloxadiazole
COHS
COH
N-Phenylcarbazole
Diphenyloxadiazole
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
300
350
400
450
500
550
300
350
400
450
Wavelength (nm)
Wavelength (nm)
Fig. 2. (a) Normalized UVeVis absorption spectra and (b) normalized emission spectra of COHS, COH, N-phenylcarbazole, and diphenyloxadiazole in toluene solution at room
temperature.

A.-R. Lee et al. / Organic Electronics 38 (2016) 222e229
227
Table 1
Photophysical, electrochemical, and thermal properties of COHS and COH.
At room temperature
At 77 K
em (nm)b
HOMO (eV)c
LUMO (eV)d
Tg (ꢀC)
Td (ꢀC)
l
abs (nm)a
l
em (nm)a
QY (%)
l
COHS
COH
287, 293, 304, 319, 340
293, 327, 340
349
399
23.4
52.6
2.71
2.47
ꢁ5.71
ꢁ5.74
ꢁ2.17
ꢁ2.52
72
441
416
N.D.e
a
Measured in toluene.
b
c
Measured in 2-MeTHF. The HOMO and LUMO levels were determined using the following equations.
EHOMO(eV) ¼ ꢁeðEoonxset þ 4:8Þ:
ELUMO(eV) ¼ eðEHOMO þ EgoptÞ:
Not detected.
d
e
where
m
eemg
(
Dm) is the difference between the dipole moments of
cyclic voltammetry (CV) was performed in a 0.1 M tetra(n-butyl)
ammonium hexafluorophosphate (n-Bu4NPF6) dichloromethane
solution using a three electrode cell system: a glassy carbon elec-
trode was used as the working electrode; platinum wire and Ag/
AgNO3 were used as counter and reference electrodes, respectively.
As shown in Fig. 4, irreversible oxidation waves appear at Epa ¼ 0.91
and 0.94 V (versus Fc/Fcþ) for COHS and COH, respectively, which
originate from the carbazole center. In the reversed scan, for both
compounds, two reduction waves at Epa ¼ 0.79 and 0.51 V for COHS
and Epa ¼ 0.80 and 0.51 V for COH are observed, respectively. These
additional reduction waves at 0.51 V are due to the instability of
their radical cations [47]. Moreover, both compounds showed
irreversible reduction waves at Epc ¼ ꢁ1.45 and ꢁ1.44 V (versus Fc/
Fcþ) for COHS and COH, respectively. These peaks can be ascribed to
a reduction of the oxadiazole group. Using oxidation potentials
determined by CVs and optical HOMO-LUMO gaps by UV/Vis
spectra, the energy levels of the highest occupied molecular orbital
(HOMO) in COHS and COH were estimated as ꢁ5.71 and ꢁ5.74 eV,
respectively, and the lowest unoccupied molecular orbital (LUMO)
were estimated as ꢁ2.17 and ꢁ2.52 eV, respectively (See Table 1).
Thus, presence of silicon atom in COHS effectively blocks the
electronic communication between PC and DPO groups which re-
sults relatively wider band gap that COH.
the excited and ground states, c is the speed of light, h is Planck's
constant and a0 is the radius of the Onsager cavity around the
fluorophore. The solvent dielectric constant (ε) and refractive index
(n) are included in the term
Df, known as the orientation polariz-
ability. The Onsager radius of 10.25 and 10.76 Å for COHS and COH,
respectively, were taken from the single-crystal X-ray structures. As
shown in Fig. S5, the Stokes shift for charge transfer emissions in
various solvents were observed to change linearly with
Df, from
which increases in the dipole moments (Dm) for COHS and COH
were estimated to be 19.6 and 16.6 Debye, respectively. Assuming
that a dipole moment of 4.8 Debye corresponds to a positive charge
and an electron held at 1 Å apart, the observed changes of the
dipole moments indicate that one electron (unit charge) travels a
distance of ~4 Å, which corresponds to the distance between PC and
DPO.
The triplet energy level of host materials is important for use of
these materials in phosphorescent OLEDs; back energy transfer
from the triplet state of dopant to the triplet state of host will
decrease emission efficiency. To investigate the triplet energy level
of COHS and COH, emission spectra were measured in 2-MeTHF at
77 K. As shown in Fig. 3, COHS showed a higher triplet energy level
(2.71 eV) compared with COH (2.47 eV), estimated by taking the
highest-energy peak of phosphorescence as the T1 / S0 transition
energy which corresponds to the vibronic 0e0 transition between
these two electronic states. The high triplet energy in COHS was
3.6. DFT calculations
caused by p-conjugation-blocking and reduced intermolecular in-
To understand transition states, we conducted quantum
chemical calculations (DFT/B3LYP/6-31G(d,p)) using Gaussian09
package [42]. Thereafter, we analyzed their transition states using
the optimized geometries. Spatial distributions for the frontier
orbitals of COHS and COH are depicted in Fig. 5. In the case of
COHS, the HOMO and the LUMO are completely separated by the
central silicon atom. The HOMO was mainly localized on the PC
moiety, whereas the LUMO was localized on the DPO moiety. As
the HOMO and LUMO orbital distributions are almost completely
teractions due to the presence of a silicon atom. This indicates the
effective confinement of the triplet excitons on the dopant and the
prevention of back energy transfer from the dopant to the host
material.
3.5. Electrochemical properties and theoretical calculations
To investigate the electrochemical properties of COHS and COH,
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
COHS
COH
T1 = 2.71 eV
T1 = 2.47 eV
350 400 450 500 550 600 650 700
Wavelength (nm)
Fig. 4. Cyclic voltammograms of COHS and COH in dichloromethane solution con-
Fig. 3. Emission spectra of COHS and COH in 2-MeTHF at 77 K.
taining 0.1 M n-Bu4NPF6 as electrolyte, at a scan rate of 0.1 V sꢁ1
.

228
A.-R. Lee et al. / Organic Electronics 38 (2016) 222e229
on COHS and COH and designated as devices I and II, respectively.
The configuration of the devices includes ITO/HAT-CN (10 nm)/
TAPC (90 nm)/COHS or COH:Ir(ppy)3 (25 nm, 8%)/EF29 (60 nm)/Liq
(1 nm)/Al (150 nm), where HAT-CN is dipyrazino[2,3-f:20,30-h]qui-
noxaline-2,3,6,7,10,11-hexacarbonitrile,
TAPC
is
4,40-cyclo-
hexylidenebis[N,N-bis(4-methylphenyl)benzenamine], Ir(ppy)3 is
tris[2-phenylpyridinato-C [2],N]iridium(III) and EF29 is a triazine
based silicone material [48]. Chemical structures and energy level
diagrams of the materials used are shown in Fig. S6. TAPC serves as
the hole-transporting layer and EF29 serves as the electron-
transporting layer. Fig. 6 shows device performances and Table 2
summarizes the two device characteristics. Fig. 6a shows the
current-voltage-luminance (IeVeL) characteristics. In the IeVeL
curves, device I showed slightly better performance than device II.
This may be owing to the existence of
adjacent, repeating molecules in the crystal packing, as noted in the
pep interactions between
Fig. 5. Spatial distributions and energy levels of the frontier orbitals for COHS and
COH.
crystal packing structure. Current efficiency versus current density
Fig. 6. (a) Current density-voltage-luminance characteristics for device I and II. (b) External quantum efficiency and power efficiency versus current density curves for device I and
II.
Table 2
Device performances of the device I, II, and III.
Device
Turn on (V)
Power efficiency (lm/W)
Current efficiency (Cd/A)
EQE (%)
Max
CIE
Max
400 nit
Max
400 nit
400 nit
Device I
Device II
<3.2
<3.0
54.55
50.19
31.25
24.04
55.84
46.48
55.71
45.92
15.83
13.02
15.79
12.86
(0.30, 0.63)
(0.31, 0.62)
spatially separated, we propose that HOMO‒LUMO excitation
restricts the electron density distribution to one side of COHS.
The emission spectrum of COHS perfectly supports this assump-
tion. In contrast, in the case of COH, the HOMO was delocalized
over the PC and DPO units, as shown in Fig. 5. Furthermore,
distribution of the LUMO level was delocalized over the entire
curves for device I and II are shown in Fig. S7. Overall, device I,
using COHS as host material, showed improved performance/peak
external electroluminescent (EL) quantum and power efficiencies
of hext ¼ 15.83% and hp ¼ 54.55 lm/W compared with device II using
COH as host material, with hext ¼ 13.02% and hp ¼ 50.19 lm/W. EL
spectra of device I and II showed typical emission spectrum of
Ir(ppy)3, which indicates triplet excitons are confined in the dopant
material, as shown in Fig. S8. Note that both devices exhibit a low
efficiency rolleoff, as shown in Fig. 6b. The low efficiency rolleoff at
high luminance for both devices might be attributed to the use of
the bipolar host materials, which may have resulted in balanced
charge fluxes and a broad distribution of the recombination region
within the emitting layer.
molecule, indicating that
a charge transfer state would be
generated in the excited state of COH. Again, this prediction
correlates very well with the emission spectrum of COH. Sup-
ported by DFT calculations, the assignments of the emission
spectra of COHS and COH were confirmed. Moreover, to calculate
the triplet energy, TD-DFT calculations were conducted. The
calculated triplet energy levels of COHS and COH were observed
to be 2.80 and 2.63 eV, respectively, which agree well with the
experimental values.
4. Conclusions
3.7. Device performances
A tetrahedral silicon motif was introduced to maintain bipolar
character and enhance the triplet energy level. To determine the
role of silicon in the molecular array, single-crystal X-ray structural
studies were conducted. The typical tetrahedral geometry owing to
To examine the structureeproperty relationship on device effi-
ciency, a series of green emitting PHOLEDs were fabricated based

A.-R. Lee et al. / Organic Electronics 38 (2016) 222e229
229
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of the two functional units in a molecule. The thermal properties
improved greatly with the introduction of the silicon atom, with a
maximum value of Td ¼ 441 ꢀC and Tg ¼ 72 ꢀC. Furthermore, the
incorporation of silicon atom in COHS effectively isolated the two
functional units from each other, resulting in a slightly wider
HOMOeLUMO band gap compared with COH in both the singlet
and triplet energies. In the green PHOLED devices using COHS as a
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