Three carbazole-based host materials: facile synthesis, photophysical properties and performances in PhOLED

Article abstract of DOI:10.1016/j.tet.2016.10.044

Three host materials based on the carbazole were designed and facilely synthesized, i. e., 9-(6-(9-carbazolyl)hexyl)carbazole (hCP), 9-(4-(2-thiophene-based)phenyl) carbazole (sCP) and 9-(4-(9-carbazolyl)phenyl)carbazole(pCP). The relationships between the structures and photophysical, electrochemical and electrophosphorescent performances were investigated. Their emission peaks are located in UV region and the optical band gap are 3.51, 3.57, and 3.89 eV, respectively. Hence, they can be used as hosts in phosphorescent organic light-emitting devices (PhOLEDs). The PHOLEDs were fabricated by doping tris(2-phenylpyridine)iridium (Ir(ppy)₃) in host as light-emitting materials. The maximum current efficiency of the PhOLEDs employed sCP as host was 44.88 cd/A.

Full text of DOI:10.1016/j.tet.2016.10.044

Accepted Manuscript
Three carbazole-based host materials: facile synthesis, photophysical properties and
performances in PhOLED
Tingting Yang, Huixia Xu, Bo Zhao, Peng Tao, Peng Sun, Hua Wang, Bingshe Xu,
Wai-Yeung Wong
PII:
S0040-4020(16)31075-4
10.1016/j.tet.2016.10.044
TET 28185
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 3 August 2016
Revised Date: 14 October 2016
Accepted Date: 17 October 2016
Please cite this article as: Yang T, Xu H, Zhao B, Tao P, Sun P, Wang H, Xu B, Wong W-Y, Three
carbazole-based host materials: facile synthesis, photophysical properties and performances in
PhOLED, Tetrahedron (2016), doi: 10.1016/j.tet.2016.10.044.
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ACCEPTED MANUSCRIPT
Three host materials (hCP, sCP and pCP) by changing the introduction of different
connection groups were designed and synthesized. The sCP exhibited bipolar. The
green PhOLEDs using Ir(ppy)3 as phosphorescent dopants were fabricated.

ACCEPTED MANUSCRIPT
Three Carbazole-based Host Materials: Facile Synthesis, Photophysical
Properties and Performances in PhOLED
Tingting Yang a, b, Huixia Xu a, b, Bo Zhao a, b, Peng Tao a, b, Peng Sun a, b, Hua Wang a, b, Bingshe
Xu a, b*, Wai-Yeung Wong a, c
aKey Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of
Education, Taiyuan University of Technology, Taiyuan 030024, China
bShanxi Research Center of Advanced Materials Science and Technology, Taiyuan 030024, China
cInstitute of Molecular Functional Materials, Department of Chemistry and Institute of Advanced
Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, P. R. China.
Abstract: Three host materials based on the carbazole were designed and facilely
synthesized,
i.
e.
9-(6-(9-carbazolyl)hexyl)carbazole
carbazole (sCP)
(hCP),
and
9-(4-(2-thiophene-based)phenyl)
9-(4-(9-carbazolyl)phenyl)carbazole(pCP). The relationships between the structures
and photophysical, electrochemical and electrophosphorescent performances were
investigated. Their emission peaks are located in UV region and the optical band gap
are 3.51, 3.57 and 3.89 eV, respectively. Hence, they can be used as hosts in
phosphorescent organic light-emitting devices (PhOLEDs). The PHOLEDs were
fabricated by doping tris(2-phenylpyridine)iridium (Ir(ppy)3) in host as light-emitting
materials. The maximum current efficiency of the PhOLEDs employed sCP as host
was 44.88 cd/A.
Key words: Carbazole, PhOLED, Bipolar host material, Thiophene
1. Introduction
The PHOLEDs have been paid much attention as the candidate for full-color
flat-panel displays and light sources due to the unique characteristics of harvesting
both singlet and triplet excitons and the theoretical internal quantum efficiency can
reach 100%.1-8 However, the serious concentration quenching and triplet-triplet (T-T)
1
*Corresponding author: Xu Bingshe, E-mail: xubs@tyut.edu.cn

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annihilation limit their applications.8-12 The phosphorescent light-emitting materials
must be doped in host as light-emitting layer in PhOLEDs. The characteristics of host
materials directly determine the performances of PhOLEDs and generally require
wide band gap, high carrier mobility and triplet state (T1) level.13, 14
Molecules based carbazole have been widely used as host materials for
PhOLEDs due to their high T1 and excellent hole-transporting properties.15 For
example, 4,4'-bis(9H-carbazol-9-yl)biphenyl (CBP) is a prominent host for green and
red triplet emitters.16 However, CBP is prone to crystallization due to its rather-low
glass-transition temperature (62 oC),15b especially under low dopant concentrations.16b
Thus, it is highly urgent to develop the host material that has a T1 higher than that of
the phosphorescent guest emitter, a relatively wider Eg with an appropriate the highest
occupied molecular orbital (HOMO) level, and good carrier-transporting properties.
Consequently, highly efficient, low-driving-voltage PhOLEDs could be realized.
In this work, to overcome these issues, we designed and synthesized a series of
phosphorescent host materials with higher triplet energy level and higher Tg than CBP
by the simple method. The phosphorescent devices based on them (xCP) have been
prepared. To increase the solubility of host, the compound of hCP was synthesized by
using alkyl chain to connect two carbazole. To obtain higher T1, the compound of
pCP was synthesized by breaking the conjugation of hCP. To improve the
charger-transport ability, the bipolar host materials sCP was synthesized by
introducing the thiophene moiety as electron-withdrawing and carbazole as
electron-donating groups. The synthetic scheme of the xCP is represented in Scheme
1.
2. Experimental section
2.1 General information
All of the chemicals and reagents are used in their original form without further
purification. Solvents for chemical synthesis are purified according to the standard
procedures. All chemical reactions are performed in a nitrogen atmosphere.
13
1H NMR and C NMR data are recorded using a Switzerland Bruker DR×600
NMR spectrometer relative to tetramethylsilane (TMS) as internal standard.
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Elemental analyses were performed using a Vario EL elemental analyzer. The single
crystals grow from the mixed solutions of CH2Cl2 and hexane. The final structure was
confirmed by X-ray diffraction analysis (Bruker-Nonius SMART APEX II CCD),
with graphite-monochromatic Mο Kα (λ = 0.71073 Å). Crystallographic data for the
structural analyses have been deposited with the Cambridge Crystallographic Data
Center (CCDC). CCDC reference numbers for hCP, pCP and sCP are 1500354,
1500355 and 1500356, respectively. UV-vis absorption spectra were recorded on a
Hitachi U-3900 spectrophotometer and the photoluminescence (PL) spectra were
recorded using a Fluoromax-4 spectrophotometer in diluted CH2Cl2 solution at room
temperature. Thermogravimetric analyses (TGA) was performed using a Netzsch TG
209 at a heating rate of 10 oC/min in nitrogen atmosphere. The melting point (Tm) was
measured by the SGW®X-4 micro melting point measuring instrument. Differential
scanning calorimetry (DSC) measurements were performed at both heating rate of 10
o
oC/min and cooling rate of 20 C/min under nitrogen atmosphere under nitrogen
atmosphere, using DSC Q100 V9.4 Build287 apparatus. Cyclic voltammetry was
performed using Autolab/PG STAT302 in a one-compartment electrolysis cell
consisting of a platinum wire as working electrode, a platinum electrode as counter,
and a calomel electrode as reference. Tetrabutylammonium perchlorate was used as a
supporting electrolyte (0.1 M), and cyclic voltammetric runs were monitored at a scan
rate of 50 mV/s.
All calculations were performed using the Gaussian 03 package by employing
experimental parameters determined by X-ray singlecrystal diffraction as an input file.
The geometry optimization by density functional theory (DFT) was carried out using
the B3LYP functional with 6-31G(d) basis sets.17-19
2.2 Device Fabrication and Characterization
To investigate the electroluminescent properties of the xCP, doped
electroluminescent devices with the configuration of ITO/ MoO3 (30 nm)/ TCTA (30
nm)/ Ir(ppy)3 (7 wt%): xCP (20 nm)/ TPBi (45 nm)/ LiF (1 nm)/ Al (300 nm) were
fabricated by high-vacuum (3×10−4 Pa) thermal evaporation onto a glass substrate
pre-coated with ITO. MoO3 and Tris(carbazol-9-yl)-triphenylamine (TCTA) were
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used as hole injecting/transporting materials, and Ir(ppy)3 acts as emitting layer.
1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) was employed as
electron-transporting and hole-blocking material. LiF was the electron injection layer,
and Al was evaporated as cathode. Luminance-voltage-current density (L-V-J)
characteristics of devices were recorded on Keithley 2400 Source Meter and L-2188
spot Brightness Meter. The active area of ITO was (3×3) mm2. The
electroluminescent spectra were recorded on PR-655 spectrophotometer. The
voltage-current density (V-J) characteristics of PhOLEDs were recorded using
Keithley 2400 Source Meter and ST-900M Spot Brightness Meter.
2.3 Synthesis of complexes
2.3.1 Synthesis of hCP
Carbazole (5.02 g, 30 mmol) and 1,6-dibromohexane (3.66 g, 15 mmol) were
dissolved in 80 mL toluene. Tertbutyl ammonium bromide (TBAB) (0.5 g, 1.5 mmol)
dissolved in 5 mL deionized water and the 2 M K2CO3 solution were added
subsequently. The mixture was heated to reflux for 24 h and then washed with
deionized water and dried over Na2CO3. The precipitate was purified by column
chromatography (silica gel, dichloromethane: petroleum ether = 1:20) to obtain the
white crystal product (4.99 g, yield: 80%). 1H NMR (600 MHz, CDCl3, TMS): δ 8.09
(d, J = 7.8 Hz, 4H), 7.43 (ddd, J1 = 7.2 Hz, J2 = 6.0 Hz, J3 = 1.2 Hz, 4H), 7.33 (d, J =
8.4 Hz, 4H), 7.22 (ddd, J1 = 7.2 Hz, J2 = 6.0 Hz, J3 = 1.2 Hz, 4H), 4.24 (t, J = 7.2 Hz,
13
4H), 1.85-1.81 (m, 4H), 1.41-1.37 (m, 4H). C NMR (150 MHz, CDCl3): δ 143.28,
128.50, 125.74, 123.26, 121.67, 111.48, 45.71, 31.70, 29.96. Anal. Calcd for
C30H28N2 (%): C, 86.54; H, 6.73; N, 6.73; found: C, 87.23; H, 6.78; N, 6.67.
2.3.2 Synthesis of pCP
A mixture of CuI (0.57 g, 3 mmol), 18-Crown-6 (0.264 g, 1 mmol), K2CO3 (16.6
g, 120 mmol), 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU) (2 mL),
dibromobenzene (7.1 g, 30 mmol) and carbazole (10 g, 60 mmol) were heated at
170 °C for 11 h under nitrogen. After cooling to room temperature, the mixture was
quenched with 1 N HCl, the precipitate was filtered and was washed with NH3·H2O
and water. The grey solid was recrystallized twice from chloroformto afford as
colorless crystals (19.3 g, yield: 79%). 1H NMR (600 MHz, CDCl3, TMS): δ 8.19 (d,
J = 7.8 Hz, 4 H), 7.82 (s, 4 H), 7.57 (d, J = 8.4 Hz, 4 H), 7.48(ddd, J1 = 7.2 Hz, J2 =
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6.0 Hz, J3 = 1.2 Hz, 4 H), 7.34 (ddd, J1 = 7.2 Hz, J2 = 6.0 Hz, J3 = 1.2 Hz, 4 H). 13C
NMR (150 MHz, CDCl3): δ 146.36, 143.72, 139.76, 136.50, 131.16, 130.38, 130.19,
128.92, 128.26, 126.53, 123.26, 122.93, 112.72. Anal. Calcd for C30H20N2 (%): C,
88.24; H, 4.90; N, 6.86; found: C, 88.28; H, 4.98; N, 6.74.
2.3.3 Synthesis of sCP
9-(4-bromophenyl)carbazole: A mixture of CuI (0.190 g, 1 mmol), 18-Crown-6
(0.159
g,
0.6
mmol),
K2CO3
(8.291
g,
60
(20
mmol),
mL),
1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone
(DMPU)
dibromobenzene (4.718 g, 20 mmol) and carbazole (1.672 g, 10 mmol) were heated at
180 °C for 16 h under nitrogen. After cooling to room temperature, the mixture was
quenched with 1 N HCl, the precipitate was filtered and washed with NH3·H2O and
water. The grey solid was purified with column chromatography using hexane as
eluant (1.5 g, yield: 46.6%). 1H NMR (600 MHz, CDCl3, TMS): δ 8.25 (d, J = 7.8 Hz,
2 H), 7.86 (dt, J1 = 3 Hz, J2 = 1.8 Hz, J3 = 8.4 Hz, 2 H), 7.61 (d, J1 = 3 Hz, J2 = 1.8 Hz,
J3 = 8.4 Hz, 2 H), 7.43-7.45 (m, 2 H), 7.39 (d, J = 8.4 Hz, 2 H), 7.29-7.31 (m, 2 H).
A
mixture of 9-(4-bromophenyl)carbazole (3.22 g, 10 mmol) and
2-thiopheneboronic acid (1.28 g, 10 mmol) was added to anhydrous toluene (50 mL).
After 2 M aqueous K2CO3 was added to the stirred mixture, the reaction mixture was
degassed with nitrogen for 1 h. Then Pd(PPh3)4 (0.03 g, 0.05 mmol) was added to the
mixture. The solution was further refluxed under a nitrogen atmosphere for 24 h.
After the reaction solution was cooled to room temperature, the whole mixture was
poured into water, and the organic layer was separated and washed with water. The
organic extracts were dried over MgSO4. The crude product was further purified by
column chromatography on silica gel using hexane as the eluent to yield compound as
a white solid (2.27 g, yield: 70%). 1H NMR (600 MHz, CDCl3, TMS): δ 8.15 (ddd, J1
= 7.8 Hz, J2 = 7.2 Hz, J3 = 0.6 Hz, 2 H), 7.85-7.83 (m, 2 H), 7.60-7.58 (m, 2 H), 7.46
(ddd, J1 = 8.40 Hz, J2 = 1.2 Hz, J3 = 0.6 Hz, 2 H), 7.44-7.41 (m, 3 H), 7.35 (dd, J1 =
5.40 Hz, J2 = 1.2 Hz, 1 H), 7.30 (ddd, J1 = 7.8 Hz, J2 = 6.6 Hz, J3 = 1.2 Hz, 2 H), 7.15
(dd, J1 = 5.40 Hz, J2 = 3.6 Hz, 1 H). 13C NMR (150 MHz, CDCl3): δ 143.47, 140.84,
136.88, 133.61, 128.29, 127.49, 127.30, 126.05, 125.38, 123.65, 123.52, 120.39,
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120.10, 109.85. Anal. Calcd for C22H15NS (%): C, 81.23; H, 4.61; N, 4.31; S, 9.85;
found: C, 81.75; H, 4.69; N, 4.17; S, 9.32.
Scheme. 1 Synthetic scheme of hCP, pCP and sCP
3. Results and discussion
3.1 Single-crystal structures
The X-ray single-crystal structure diffraction studies were carried out to reveal
their exact structures. The structure data are shown in Table 1. Their single crystals
were grown from CH2Cl2/hexane solution. Their crystal structures are all the
orthogonal crystal system (Fig. 1). The space group of hCP is P2(1)2(1)2(1) in which
the bond length of C(13)-N(1) and C(18)-N(2) are 1.453(3) and 1.457(3) Å are
slightly smaller than the standard C-N key bond length (1.47-1.50 Å).19 Thus bond
energy is bigger and the material is more stable. In addition, at both ends of carbazole
groups for the planar configuration, the torsion angle between alkyl chain and
carbazole groups are -178.7(2) and 176.0(2) ̊, respectively.
The pCP is Pbca space group in which the bond length of C(13)-N(1) and
C(16)-N(2) are 1.430(3) and 1.430(3) Å and smaller than that of hCP.20 The torsion
angle between carbazole and benzene are -124.4(2) and 124.6(2)
̊
respectively. The
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torsion angle of C(19)-N(2)-C(16) is 124.6(2)
torsion angle of C(11)-N(1)-C(8) is 126.40(19)
̊
̊
and pCP are both smaller than that of hCP. Aomg them the torsion angle of
C(x)-N(1)-C(y) of hCP is the smallest, the flexibility is the best. The rigid strong
stereo space configuration is beneficial to increase the molecular steric hindrance,
weaken the conjugate effect between adjacent carbazole groups and widen exciton
diffusion area which can help to improve the T1 level.
Table 1 Crystallographic data for hCP, sCP and pCP
hCP
sCP
pCP
Empirical formula
Crystal size/mm3
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C(x)-N-C(y) (deg)
a (Å)
C30H28N2
0.36×0.21×0.20
416.54
C22H15NS
0.23 × 0.19 × 0.13
325.41
C30H20N2
0.2 × 0.15 × 0.12
408.48
296(2)
299.84
299.59
0.71073
Orthorhombic
P2(1)2(1)2(1)
-178.7(2)
5.844(2)
12.001(5)
32.444(13)
90
0.71073
Orthorhombic
Pbca
0.71073
Orthorhombic
Pbca
124.93(19)
16.9032(6)
7.7378(3)
25.4653(9)
90
108.4(2)
8.210(2)
16.562(4)
32.036(8)
90
b (Å)
c (Å)
a (deg)
b (deg)
90
90
90
g (deg)
90
90
90
Volume/ Å3
Z
2275.5(16)
4
3330.7(2)
8
4355.7(18)
8
Reflections collected
16228
31414
40995
Goodness-of-fit on F2 1.009
R indexes (all data) R1 = 0.0963,
1.048
0.989
R1 = 0.0934,
R1 = 0.1905,
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wR2 = 0.1118
wR2 = 0.2699
wR2 = 0.2120
hCP
sCP
pCP
Fig. 1 Single-crystal structures of hCP, pCP and sCP
3.2 Thermal properties
The good thermal stability of compounds is indicated by their high decomposition
temperatures Td (corresponding to 5% weight loss). The Td of hCP, pCP and sCP are
321, 301 and 161 oC, respectively, as can be seen in Fig. 2a. In addition, both hCP and
pCP with two carbazole groups have higher Td owing to the better rigidity of
carbazole. In Fig. 2b, the glass-transition temperatures (Tg) of hCP and pCP are 93 oC
and 129 oC, respectively, which are much higher than that of analogous
carbazole-based host such as CBP (62 oC) and 1,3-di(9H-carbazol-9-yl)benzene (mCP)
o
(60 C).14b And the higher Tg is expected to improve the stability of the device. The
flexibility of hCP molecular is better than sCP and pCP and easy to improve the
decomposition temperature. And the Tm of hCP, pCP and sCP are 127.9-128.4,
335.4-336.1 and 188.1-188.9 oC, respectively.
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Fig. 2 TG (a) and DSC (b) curves of hCP, sCP and pCP
3.3 Photophysical properties
Fig. 3 displayed the UV-vis and photoluminescent (PL) spectra of xCP in dilute
CH2Cl2 solution (5×10-6 mol/L) and the PL emission spectra of sCP in different
solution. The absorption peaks around 236-264 nm corresponds to the π-π* electron
transition of benzene ring. And the absorption peak around 293 nm can be assigned to
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the carbazole-centered matrix π-π* transitions, whereas the absorptions in the range of
320-346 nm could be attributed to the n-π* electron transfer of c arbazole.21-23 The
absorption maximum (λmax) was observed at 237 nm (molar extinction coefficient ɛ=
1.14 × 105 M-1 cm-1) for hCP, 241 nm (ɛ = 1.25 × 105 M-1 cm-1) for pCP and 235 nm
(ɛ = 1.58 × 105 M-1 cm-1) for sCP.
Fig. 3 (a) The UV-Vis absorption and PL spectra of hCP, sCP and pCP in CH2Cl2 solution and (b)
The PL spectra of sCP in different solutions
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The optical energy gaps (Eg) of the compounds were calculated according to
their absorption edges and are listed in Table 2. Eg of hCP, sCP and pCP are 3.51, 3.57
and 3.89 eV, which are higher than that of CBP.24 The compounds hCP and sCP show
lower Eg than pCP due to the increased conjugation.
The PL emission spectra of hCP, sCP and pCP were measured in CH2Cl2 solution
which show near-ultraviolet emissions (Fig. 3a). The emission spectra of hCP have
the maximum peaks at 351 and 368 nm, and the emission wavelength of sCP is
located at 382 nm and pCP appears at around 355 and 363 nm respectively. Compared
with that of CBP, the maximum emission peaks have the obvious blue shifts.24, 25 The
emission peaks of pCP has samller red shift than that of sCP due to the larger
distortion. The conjugate degree of hCP is the minimum so as to the blue shift than
that of pCP and sCP.
The PL emission spectra of hCP and pCP have almost no change with the change
of solution polarity. The PL emission spectra of sCP in different solution were
measured, as can been seen in Fig. 3(b). The emission spectra have obvious red
shifted 43 nm and the fine structure disappeared with the increase of solvent polarity.
This result indicated that there are intramolecular charge transfer (ICT) state.
Fig. 4 The PL spectra of hCP, sCP and pCP in 2-methyltetrahydrofuran matrix at 77 K
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The properties of triplet excited states for the compounds were studied by
low-temperature (77 K) PL spectra in 2-methyltetrahydrofuran (2-MeTHF), as shown
in Fig. 4. The T1 of hCP, sCP and pCP were calculated as 3.01, 3.02 and 2.84 eV,
respectively. They are suitable for the phosphorescent host materials and the T1 of
Ir(ppy)3 is 2.4 eV. 13
In order to further investigate the luminescent properties of these three host
materials, their ground state structures were optimized and the frontier molecular
orbitals were shown in Fig. 5. As for hCP, the HOMO are largely located at one of
carbazole groups and the lowest unoccupied molecular orbital (LUMO) are absolutely
located at another carbazole groups. The HOMO and LUMO are well separated,
without overlapping, leading to the bule-shift spectra. The HOMO of sCP distributed
on the molecular skeleton and the LUMO distributed in thiophene group and benzene
ring. The HOMO and LUMO of the sCP are well overlapping and the ambipolar
property of it leads easily to transfer electron.
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HOMO
LUMO
hCP
pCP
sCP
Fig. 5 Optimized molecular structure and molecular orbital amplitude plots for the HOMO
and LUMO of hCP, pCP and sCP
Compared with hCP and pCP, the fluorescence emission of sCP has a large red
shift, and the HOMO level is decreased which make a wider band gap formed and
reduce the hole injection barrier. The HOMO of pCP distributed in the molecular
skeleton and the LUMO distributed in the carbazole groups at both ends of the
molecule.
3.4 Electrochemical properties
Cyclic voltammetry (CV) were performed to investigate the electrochemical
properties of the compounds (Fig. 6). And the values of EHOMO and ELUMO were
obtained according to the equation ELUMO = Eg - EHOMO and are also listed in Table 2.
The HOMO/LUMO levels of hCP, sCP and pCP are -5.98/-2.47, -6.04/-2.57 and
-5.95/-2.06 eV.
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Fig. 6 The cyclic voltammetry curves of hCP , sCP and pCP
Table 2 Photophysical, electrochemical and thermal properties of xCP
a
c
Host
λab
λPL
T1
LUMOb
(eV)
Eg
Tg
Td
(eV)
(eV)
(oC)
(oC)
（nm）
（nm）
hCP
sCP
pCP
264, 294, 346
257, 302, 325
257, 302, 325
412
410
436
3.01
3.02
2.84
-2.47
-2.57
-2.06
3.51
3.57
93
321
299
161
3.89
129
a
b
c
PL spectra at 77 K in 2-methyltetrahydrofuran solution. LUMO = HOMO - Eg. Eg obtained
from the onset of UV-vis absorption spectra.
3.5 Electroluminescent properties
To prove the bipolar property of sCP, the hole-only and electron-only devices
were fabricated with the structures of ITO/ MoO3 (3 nm)/ NPB (30 nm)/ sCP (40 nm)/
NPB (10 nm)/ Al (hole-only) and ITO/ Bphen (10 nm)/ sCP (40 nm)/ Bphen (10 nm)/
LiF (1 nm)/ Al (electron-only), respectively. Where MoO3 and LiF served as hole-
and
electron-injecting
layers,
N,N'-Bis-[(1-naphthalenyl)-N,N'-bis-phenyl]-(1,1'-biphenyl)-4,4'-diamine (NPB) was
used as the hole-transport layer and 4,7-diphenyl-1,10-phenanthroline (BPhen) as
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electron-transport/hole-block layer (Fig. 7). The current density (J) of electron-only
based sCP was much higher than that of hole-only device at the same voltage, which
indicated that the electron-transport ability is superior to that of hole-transport.
Fig. 7 The J−V characteristic curves of single-carrier based-sCP devices
Based on the triplet energies and other agreeable parameters of these three host
materials, PhOLEDs were fabricated with an doping concentration of 7 wt% Ir(ppy)3
as the emitter. The structures of devices A-C are ITO/ MoO3 (30 nm)/ TCTA (30 nm)/
Ir(ppy)3 (7 wt%): host (20 nm)/ TPBi (45 nm)/ LiF (1 nm)/ Al (300 nm), which host
are hCP (device A), sCP (device B), pCP (device C), respectively. The EL spectra of
devices A-C exhibit the same peak at 512 nm with Commission Internationale de
L'Eclairage (CIE) of around (0.30, 0.62), which are identical to the PL spectra of
Ir(ppy)3 in Fig. 8. This result demonstrated that the effective energy transfers were
achieved from host to guest. The spectra have no obvious with the change of voltage,
indicating that the devices exhibited the good color stability.
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Fig. 8 EL spectra of devices A-C at voltage of 8 V
The current density-voltage-luminance (J-V-L) characteristics for the devices
A-C are in Fig. 9 and the corresponding data are listed in Table 2. The maximum
luminance (Lmax) of the device B is 35 610 cd/m2 and has a lower turn-on voltage (Von:
the voltage at the luminance of 1 cd/m2) of 3.0 V. The based-pCP device (device C)
has inferior performances with the Lmax of 9 497 cd/m2 and Von of 5.5 V.
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Fig. 9 The J-V and L-V curves of devices A-C
Fig. 10 shows the current efficiency (ηL) - current density (J) for the devices A-C.
The device B exhibited the excellent electroluminescent properties with the maximum
ηL of 44.88 cd/A, the maximum power efficiency (ηp) of 37.75 lm/W and the external
quantum efficiency (EQE) of 13.08%, as shown in Table 3. On the other hand, the
device C displayed the ηL.max of 35 cd/A and maximum EQE of 10.07%. The balanced
carrier transport improved the performances in device B owing to the bipolar property
of sCP. What’s more, the higher T1 of sCP can effectively prevent back energy from
guest to host.
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Fig. 10 The current efficiency- current density of devices A-C
Table 3 Devices performances of PhOLEDs
a
b
c
d
Device
Host
Von
Lmax
ηL.max
η pmax
CIE
EQE
(%)
(V)
4.5
3
(cd/m2)
2876
(cd/A)
21.3
44.88
35
(lm/W)
16.08
37.75
13.42
(x, y)e
A
B
C
hCP
sCP
pCP
(0.30,0.62)
(0.31,0.60)
6.0
35610
9497
13.08
10.07
5.5
(0.30,0.62)
a
b
c
d
Voltages at 1 cdꢀm-2. Maximum luminance. Maximum current efficiency. Maximum power
efficiency. e CIE measured at 8 V.
4. Conclusions
In summary, we have synthesized three host materials by changing the
introduction of different connection groups. The emission spectra of hCP have the
maximum peaks at 351 and 368 nm, and the emission wavelength of sCP is located at
382 nm and pCP appears at around 355 and 363 nm respectively. The T1 of hCP, sCP
and pCP are 3.01, 3.02 and 2.84 eV, respectively. The PhOLEDs used sCP as host
exhibited higher maximum ηL of 44.88 cd/A with the ηp 37.75 lm/W and the
maximum EQE of 13.08%. Thus, these hosts have the potential applications in the
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blue PhOLEDs.
Acknowledgements
This work was financially supported by the International Science and Technology
Cooperation Program of China (2012DFR50460), the National Natural Scientific
Foundation of China (61274056, 61307030 and 61307029), Shanxi Provincial Key
Innovative Research Team in Science and Technology (201513002-10), the Program
for Changjiang Scholar and Innovation Research Team in University (IRT0972), the
Natural Science Foundation of Shanxi Province (2015021070 and 201601D011031)
and the Key Scientific and Technological Project of Shanxi Province (20120321019).
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