Efficient hole-transporter for phosphorescent organic light emitting diodes with a simple molecular structure

Article abstract of DOI:10.1016/j.orgel.2015.08.011

N,N-diphenyl-4-(quinolin-8-yl)aniline (SQTPA), which composes a triphenylamine group and a quinoline group, has been synthesized and employed as a hole-transporter in phosphorescent OLEDs. It has been proved that SQTPA has efficient hole-transport propert

Full text of DOI:10.1016/j.orgel.2015.08.011

Organic Electronics 26 (2015) 481–486
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Efficient hole-transporter for phosphorescent organic light emitting
diodes with a simple molecular structure
⇑
Xiaoxia Yang, Xiaoyang Du, Silu Tao , Yun Huang, Xulin Ding, Rui Xue
School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 June 2015
Received in revised form 30 July 2015
Accepted 13 August 2015
N,N-diphenyl-4-(quinolin-8-yl)aniline (SQTPA), which composes a triphenylamine group and a quinoline
group, has been synthesized and employed as a hole-transporter in phosphorescent OLEDs. It has been
proved that SQTPA has efficient hole-transport property with a hole-mobility of 3.60 ꢀ 10ꢁ5 cm2/V s at
the electric field of 800 (V/cm)1/2, which is higher than that of NPB (1.93 ꢀ 10ꢁ5 cm2/V s). Blue, orange
and green phosphorescent OLEDs have been fabricated based on FIrpic, Ir(2-phq)3, Ir(ppy)3 with typical
structures by using SQTPA as the hole-transporter. The SQTPA-based devices show maximum external
quantum efficiencies and power efficiencies of 17.5%, 32.5 lm/W for blue, 12.3%, 20.5 lm/W for orange
and 20.3%, 64.5 lm/W for green. The performances of SQTPA-based devices are much better than that
of NPB-based phosphorescent OLEDs with similar structures. Thought of its very simple molecular struc-
ture and easy synthetic route, SQTPA should be an efficient hole-transporter for phosphorescent OLEDs.
Ó 2015 Elsevier B.V. All rights reserved.
Keywords:
OLEDs
Hole-transporter
Phosphorescent
1. Introduction
based derivatives have been widely used in OLEDs for their rela-
tively high hole-transporting property and appropriate highest
Phosphorescent OLEDs have been shown to harvest 100% of the
excitions generated by electrical injection, corresponding to a four
times increment in efficiency compared to that obtained in singlet-
harvesting fluorescent OLEDs [1–3]. Thus, OLEDs have attracted a
great deal of attention because of their huge applications in full-
color flat-panel displays and solid-state lightings [4,5]. Although
considerable progresses have been made in OLEDs, there are still
two key focuses. One concern is to improve the performance espe-
cially the efficiency and the lifetime of the OLEDs. The other issue is
to achieve a vast reduction in the cost of the OLEDs, which includes
the cost on the fabrication process and the organic materials. To
occupied molecular orbital level. The triphenylamine-based hole
transporters, such as TAPC, are significant to increase the efficiency
of phosphorescent OLEDs [17–20]. Bipolar materials combining
with a hole transporting unit and an electron transporting unit
have been widely investigated in OLEDs in these years [21–25]. It
has been reported that some bipolar materials have exhibited with
a high hole transporting property, which should be used as poten-
tial good hole-transporters [23–25].
Moreover, the molecular structures and the synthetic routes of
the charge transporters should be simple as much as possible,
which can reduce the cost of OLEDs massively due to a great quan-
tity of use in the fabrication process. Thus, it is important to design
and synthesize efficient charge transporters with very simple
molecular structure, which can bring high performance and reduce
the cost in phosphorescent OLEDs.
In this paper, N,N-diphenyl-4-(quinolin-8-yl)aniline (SQTPA),
which composes a triphenylamine group and a quinoline group,
has been synthesized and characterized. It has been proved that
SQTPA has efficient hole-transport property with a hole-mobility
of 3.60 ꢀ 10ꢁ5 cm2/V s at the electric field of 800 (V/cm)1/2, mean-
while the hole-mobility of NPB is 1.93 ꢀ 10ꢁ5 cm2/V s. Blue, orange
and green phosphorescent OLEDs have been fabricated based on
FIrpic, Ir(2-phq)3, Ir(ppy)3 with typical device structures by using
SQTPA as the hole-transporter. The devices show maximum exter-
nal quantum efficiencies and power efficiencies of 17.5%, 32.5 lm/
achieve highly-efficient OLEDs, maintaining
a good balance
between electron and hole currents in OLEDs is an important factor
[6]. Various strategies have been developed to meet the require-
ment of charge balance, which includes the employment of highly
efficient charge transporters and the introduction of bipolar or
multi-functional molecules [7–12]. Efficient charge transporters
including hole transporters and electron transporters are the most
important materials to achieve high performance OLEDs. Pyridine-
based electron transporters, such as TmPyPB, etc., developed by
Kido group [13–16], have been widely used to largely improve
the performance of the phosphorescent OLEDs. Triphenylamine-
⇑
Corresponding author.
E-mail address: silutao@uestc.edu.cn (S. Tao).
http://dx.doi.org/10.1016/j.orgel.2015.08.011
1566-1199/Ó 2015 Elsevier B.V. All rights reserved.

482
X. Yang et al. / Organic Electronics 26 (2015) 481–486
W for blue, 12.3%, 20.5 lm/W for orange and 20.3%, 64.5 lm/W for
green, respectively. The results of SQTPA-based devices are much
better (35% higher for blue, 37% higher for orange and 46% higher
for green) than that of NPB-based phosphorescent OLEDs with sim-
ilar device structures. Thought of its very simple molecular struc-
ture and easy synthetic route, SQTPA should be an efficient hole-
transporter.
3. Results and discussions
The synthetic route of SQTPA is shown in Fig. 1. SQTPA can be
obtained by a one-step Suzuki coupling reaction with a high yield
of 65%. The molecule structure of SQTPA was confirmed with 1H
NMR spectrometry, EA and MS. The thermal stability was deter-
mined by thermo gravimetric analysis (TGA), showing a thermal
decomposition temperature (Td, corresponding to 5% weight loss)
of 248 °C. By differential scanning calorimetry (DSC), no distinct
glass transition was observed. The highest occupied molecular
orbital (HOMO) energy level of SQTPA was measured to be 5.5 eV
by ultraviolet photoelectron spectroscopy (UPS), and the lowest
unoccupied molecular orbital (LUMO) was 2.5 eV.
2. Experiment
2.1. Chemical and instrument
All powders and solvents were used in this reaction as received
from commercial sources without further purification. The final
product was demonstrated with 1H NMR spectra, elemental analy-
sis (EA) and Mass Spectrometry (MS). The glass transition temper-
ature (Tg) was measured with differential scanning calorimetry
(DSC) by using Perkin-Elmerpyris DSC6 operated at a heating rate
of 10 °C minꢁ1. The decomposition temperature (Td) was mea-
sured by thermo gravimetric analysis (TGA) which was performed
on a TA SDT Q600 instrument at a heating rate of 10 °C minꢁ1
under nitrogen flowed. Optical absorption and emission spectra
were recorded with a Hitachi U-3010 UV–vis spectrophotometer
and a Hitachi F-4500 fluorescence spectrophotometer, respec-
tively. Highest occupied molecular orbital (HOMO) energy level
was confirmed via ultraviolet photoelectron spectroscopy (UPS).
Lowest unoccupied molecular orbital (LUMO) energy level was cal-
culated by the difference valves between the HOMO energy level
and energy gap (Eg) obtained from the optical absorption edge.
Fig.
2
shows the room-temperature UV–vis absorption,
dilute dichloromethane
photoluminescence (PL) spectra in
a
(CH2Cl2) solution and phosphorescence spectra in dilute 2-methyl-
tetrahydrofuran (2-MeTHF) at 77 K of SQTPA, respectively. The
room-temperature (RT) short-wavelength absorptions peaked at
301 nm in dilute CH2Cl2 solution can be ascribed to the p–
p⁄ tran-
sitions from triphenylamine unit [26–28]. The singlet state (ES) and
triplet state (ET) energy of SQTPA were calculated to be 2.54 eV and
2.28 eV, respectively.
To investigate the hole-transporting property of SQTPA, hole-
only device was fabricated with the structure of ITO/MoO3
(10 nm)/SQTPA (80 nm)/MoO3 (10 nm)/Al. MoO3 serves as hole
injecting layer (HIL) and electron blocking layer (EBL). For compar-
ison,
NPB
(4,40-bis(N-(1-naphthyl)-N-phenylamino)biphenyl)
based hole-only device was also fabricated with the structure of
ITO/MoO3 (10 nm)/NPB (80 nm)/MoO3 (10 nm)/Al. In the devices,
most of carriers can be restrained in the organic layer due to the
large energy gap between the LUMO of MoO3 (2.3 eV) and the work
function of Al cathode (4.3 eV). Holes can be injected from the
anode to the organic layer, thus the hole-mobility of SQTPA and
NPB can be obtained. Fig. 3 (insert) describes the current density
versus voltage (J–V) curves of the devices. Remarkable higher
hole-current density in the SQTPA-based device is obvious to
demonstrate that SQTPA is capable of good hole-transporting
2.2. Materials synthesis
4-(8-Quinolyl)–triphenylamine (SQTPA). 4-Bromotriphenylamine
(2 mmol), 8-quinoline boronic acid (2.5 mmol), Pd(PPh3)4
(0.5 mmol), aqueous Na2CO3 (2.0 M, 20 ml), ethanol (15 ml), and
toluene (20 ml) were mixed in a flask. The mixture was degassed
and the reaction was refluxed at 100 °C for 24 h under nitrogen.
After being cooled, the solvent was evaporated under vacuum
and the product was extracted with dichloromethane (CH2Cl2).
The CH2Cl2 solution was washed with water and dried with MgSO4.
Evaporation of the solvent, followed by column chromatography
on silica gel (petroleum ether/CH2Cl2) yielded white powder. Yield:
65%. 1H nuclear magnetic resonance: (CDCl3, 300 MHz) d: 8.97–
8.96 (m, 1H), 8.21–8.18 (d, 1H), 7.80–7.74 (m, 2H), 7.64 (d, 2H),
7.59 (d, 1H), 7.43–7.40 (m, 1H), 7.30–7.26 (m, 4H), 7.21–7.17 (m,
6H), 7.03 (d, 2H); Mass Spectrometry (ESI+): m/z 372.2 (M+); ele-
mental analysis: Anal. Calcd. For C27H20N2 C: 87.07%, H: 5.41%, N:
7.52%. Found: C: 87.10%, H: 5.40%, N 7.32%.
N
B
N
N
HO OH
N
Pd(PPh3)4,Toluene,Ethanol
2M Na2CO3, N2
Br
Fig. 1. The synthetic route of SQTPA.
2.3. Device fabrication and measurement
Indium tin oxide (ITO)-coated glass substrates with a sheet
resistance of 15
X per square were used as the starting substrates.
Before device fabrication, the ITO glass substrates were pretreat-
ment carefully by washing with isopropyl alcohol and deionized
water, dried in an oven at 120 °C over 2 h and then treated with
ultraviolet-ozone for 30 min before loading in the vacuum deposi-
tion chamber. All organic layers were deposited on the ITO glass
substrates with
a a base pressure
rate of 1–2 Å sꢁ1 under
5 ꢀ 10ꢁ4 Pa. The electron-injecting layer LiF and cathode Al were
deposited with a rate of 0.1 Å sꢁ1 and 10 Å sꢁ1, respectively. EL
spectra, CIE coordinates, and current density–voltage–luminance
(J–V–L) characteristics were evaluated with a Spectrascan PR655
photometer and
a computer-controlled Keithley model 2400
source meter under ambient atmosphere.
Fig. 2. Absorption and photoluminescence properties of SQTPA.

X. Yang et al. / Organic Electronics 26 (2015) 481–486
483
!
rffiffiffi
V
L
l
¼
l
0 exp
b
ð2Þ
where er is relative permittivity (ꢂ3.00), e0 is vacuum permittivity
(ꢂ8.85 ꢀ 10ꢁ14 F cmꢁ1),
l is the carrier mobility determined by
SCLC measurements, L is the cathode–anode distance (100 nm for
the device), l0 is zero-field mobility and b is a coefficient that is
proportional to the Poole–Frenkel factor [30]. The value of l0 and
b are calculated to be 2.88 ꢀ 10ꢁ6 cm2/V s, 2.82 ꢀ 10ꢁ3 (V cmꢁ1 ꢁ1/2
)
for SQTPA. By using the obtained values, the field-dependent holes
mobility can be obtained by Eq. (2). The characters of hole-mobility
against electric field (E1/2) were shown in Fig. 3. The hole mobility of
SQTPA and NPB are 3.60 ꢀ 10ꢁ5 cm2/V s and 1.93 ꢀ 10ꢁ5 cm2/V s at
high electric field of 800 (V/cm)1/2, respectively. These results imply
that the hole-transporting property of SQTPA is better than that of
the widely used hole-transporter NPB at the same electric field.
Thought of the good hole-transporting property and the triplet
energy (2.28 eV) of SQTPA, phosphorescent orange OLED using
SQTPA as a hole transporter was fabricated with a structure of
ITO/MoO3 (1 nm)/SQTPA (40 nm)/CBP: 4%Ir(2-phq)3 (30 nm)/TPBi
(40 nm)/LiF (1 nm)/Al. ITO and Al are anode and cathode, MoO3
and LiF function as hole injecting layer (HIL) and electron injecting
layer (EIL), respectively. 4,40-Bis(N-carbazolyl)-1,10-biphenyl (CBP)
is the host of EML; 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)
phenyl (TPBi) is electron transporting layer (ETL) which can con-
fine the triplet excitions in EML; In the device, tris(2-
phenylquinoline) iridium(III) (Ir(2-phq)3) acts as phosphorescent
orange dopant. The doping concentration of Ir(2-phq)3 was exactly
maintained at 4%. For comparison, a control device was also pre-
pared using the widely used hole-transporter NPB (4,40-bis(N-(1-
naphthyl)-N-phenylamino)biphenyl) with the same structure.
Fig. 4 shows the EL spectra of the orange OLEDs based on SQTPA
and NPB. At 1000 cd/m2, the devices both show orange emission
with an emission peak at 588 nm. The CIE coordinates are (0.56,
0.43) for the SQTPA-based device, (0.57, 0.43) for the NPB-based
device. There is no redundant emission from the host or transport
layers, suggesting that the device structure with rational design
can confine excitions within the EML effectively and effective
energy transfer from host to dopant occurred.
Fig. 3. Mobility character at different electric field and current density versus
voltage (inset) of hole-only devices.
Fig. 4. Normalized EL spectra at 1000 cd/m2 of orange OLEDs with SQTPA and NPB.
property, which should be attributed to the much higher electron-
donating effect of the triphenylamine moiety.
Fig.5(a) reveals the luminous efficiencies characteristics of both
devices. The SQTPA-based device owns maximum efficiencies of
12.3% for external quantum efficiency (EQE), 27.1 cd/A for current
efficiency (CE) and 20.5 lm/W for power efficiency (PE). Mean-
while, the orange device containing NPB shows the maximum effi-
ciencies of 10%, 22.9 cd/A, 15 lm/W, respectively. The results show
that the power efficiency of SQTPA-base device is 37% higher than
To further certificate the hole-mobility, SCLC method is used for
the hole-only device [29]. The formula can be described as follows:
9
J ¼ ere0
8
V2
L3
l
ð1Þ
and
Fig. 5. The current density, luminance versus voltage characteristic and the luminance efficiency properties of orange OLEDs with SQTPA and NPB, respectively.

484
X. Yang et al. / Organic Electronics 26 (2015) 481–486
Table 1
Performances summary of the devices in this work.
Device
Von (V)
EQEa (%), CEb (cd/A), PEc (lm/W)
10 cd mꢁ2
100 cd mꢁ2
1000 cd mꢁ2
Max
CIEd
O1
O2
G1
G2
B1
B2
3.4
4.0
3.0
3.2
3.6
3.8
7.0, 25.8, 20.3
9.3, 21.6, 14.5
20.3, 70.0, 61.0
13.8, 49.3, 43.5
17.5, 41.8, 32.9
14.0, 32.2, 24.0
11.8, 26.8, 19.1
9.9, 22.7, 13.9
14.6, 52.8, 38.7
12.4, 45.3, 36.2
14.4, 34.2, 22.6
14.4, 33.9, 19.5
12.1, 26.2, 14.6
8.9, 20.6, 10.0
10.8, 37.8, 17.6
11.5, 41.1, 18.1
12.3, 29.4, 14.6
11.6, 27.7, 12.1
12.3, 27.1, 20.5
10.0, 22.9, 15.0
20.3, 71.9, 64.5
13.8, 49.3, 44.3
17.5, 41.5, 32.5
15.0, 35.3, 24.1
(0.57, 0.43)
(0.56, 0.43)
(0.32, 0.62)
(0.32, 0.62)
(0.18, 0.40)
(0.18, 0.40)
Blue, orange and green OLEDs with SQTPA were named as B1, O1 and G1, respectively.
Blue, orange and green OLEDs with NPB were named as B2, O2 and G2, respectively.
a
External quantum efficiency.
Current efficiency.
b
c
Power efficiency.
d
Measured at 1000 cd mꢁ2
.
Fig. 8. Normalized EL spectra at 1000 cd/m2 of blue OLEDs with SQTPA and NPB.
Fig. 6. Normalized EL spectra at 1000 cd/m2 of green OLEDs with SQTPA and NPB.
clearly reveals that the current density of the SQTPA-based device
is much higher than that of the NPB-based device. The device with
SQTPA exhibits a lower turn-on voltage (at a brightness of
1 cd mꢁ2) of 3.4 V than that of the device with NPB (4.0 V). Thought
of the similar HOMO level of NPB and SQTPA, the lower turn-on
voltage should be attributed to the higher hole-transporting prop-
erty of SQTPA than NPB [31–33]. Table 1 summarizes key perfor-
mance parameters of the devices.
Moreover, green phosphorescent OLEDs with a configure of ITO/
MoO3 (1 nm)/SQTPA or NPB (40 nm)/CBP: 8%Ir(ppy)3 (30 nm)/TPBi
(40 nm)/LiF (1 nm)/Al were fabricated. The emission layer consists
of CBP doped with 8 wt% of the Ir(ppy)3. Fig. 6 gives the EL spectra
that of NPB-based device. Maintaining a good balance between
electron and hole currents in OLEDs is an important factor to
achieve highly-efficient OLEDs, which means that the mobility of
the hole transporter and the electron transporter should be compa-
rable. Although the hole injection barrier at the ITO/SQTPA and
ITO/NPB interfaces are almost the same due to the similar HOMO
levels of SQTPA and NPB. The higher hole mobility of SQTPA should
enhance the hole injection and the probability of excitions recom-
bination in EML. Thus, a better balance between electrons and
holes in the EML was obtained, which induced a higher efficiency.
Fig.5(b) describes J–V–L characteristics of two orange devices. It
Fig. 7. The luminance efficiency properties curve and the current density, luminance versus voltage characteristic of green OLEDs with SQTPA and NPB, respectively.

X. Yang et al. / Organic Electronics 26 (2015) 481–486
485
Fig. 9. The luminance efficiency properties curve and the current density, luminance versus voltage characteristic of blue OLEDs with SQTPA and NPB, respectively.
of the green OLEDs. At 1000 cd/m2, the two devices both exhibit a
pure green emission from Ir(ppy)3 (peaked at 516 nm) and CIE
coordinates of (0.32, 0.62). Fig. 7(a) displays luminous efficiencies
characteristics of the green devices. The green device with SQTPA
shows EQE, CE and PE of 20.3%, 71.9 cd/A and 64.5 lm/W, which
are much higher than that of NPB (13.8%, 49.3 cd/A and 44.3 lm/
W). In this device, the excitions recombination zone should be
located near the interface of the emission layer (EML) and
electron-transporting layer (ETL) due to the high hole transporting
property of SQTPA. The triplet energy of TPBi is high enough to
confine the formed triplet excitions at the interface. Thus, although
the ET of SQTPA (2.28 eV) is lower than that of Ir(ppy)3 (2.42 eV),
the EQE of SQTPA-based device is obtained as high as 20.3%. Some
other groups have found similar results in their green phosphores-
cent devices and blue phosphorescent devices by using a low tri-
plet energy hole transporter [34–35]. Fig. 7(b) gives the J–V–L
characteristic of the green devices. The turn-on voltage of the
SQTPA-based device (3.0 V) is lower than that of NPB-based device
(3.2 V).
To further demonstrate the application of SQTPA in phosphores-
cent OLEDs, blue devices have been fabricated with the configura-
tion of ITO/MoO3 (1 nm)/SQTPA or NPB (40 nm)/mCP (5 nm)/
SPPO1: 10% FIrpic (30 nm)/SPPO1 (30 nm)/LiF (1 nm)/Al. In the
device, bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iri
dium(III) (FIrpic) is utilized as the blue dopant. Due to the triplet
energy is lower than that of FIrpic (SQTPA:2.28 eV; FIrpic:2.68 eV),
1,3-bis(carbazol-9-yl)benzene (mCP) is used as an excition block-
ing layer (EBL) for its high-lying triplet energy of 2.9 eV, which
can confine the triplet excitions in EML effectively; 9,9-spirobifluo
ren-2-yl-diphenyl-phosphine oxide (SPPO1) is utilized as the host
and the electron transporter in the device due to its high triplet
energy (2.9 eV) and good electron-transporting property. The dop-
ing concentration of FIrpic in the device is optimized to be 10%.
Fig. 8 shows the EL spectra of the blue devices. At 1000 cd/m2,
the devices both give blue emission with two peaks centered at
472 and 497 nm, and the same CIE coordinates of (0.18, 0.40).
Fig. 9 shows the current efficiency–luminance–power efficiency
plots and J–V–L curves of blue devices. The SQTPA-based device
demonstrates the performances with a maximum external quan-
tum efficiency (EQE) of 17.5%, a maximum current efficiency (CE)
of 41.5 cd/A, and a maximum power efficiency (PE) of 32.5 lm/W
(at 8.3 cd/m2). Whereas that of NPB-based device gives corre-
sponding efficiencies of 15%, 35.3 cd/A and 24.1 lm/W. The effi-
ciency of blue device based on SQTPA has dramatically increased
(by 36%) compared to that of the blue device based on NPB, which
should be partially attributed to the characteristics of hole injec-
tion and transportation of HTL material. In addition, the good
device structure can confine the excitions in EML effectively, which
is beneficial for the high performance. The turn-on voltage of
SQTPA based device is 3.6 V, which is also lower than that of device
with NPB (3.8 V).
Owing to the simple molecular structure, easy synthetic route
and the excellent performances of the phosphorescent devices
using the new hole transporter, it is believed that SQTPA should
be an efficient hole-transporter for phosphorescent OLEDs.
4. Conclusion
A simple molecule SQTPA, which composes a triphenylamine
group and a quinoline group, has been synthesized and employed
as an excellent hole-transporter in phosphorescent OLEDs. The
hole-transporting property was examined in simple monochro-
matic phosphorescent OLEDs. FIrpic, Ir(2-phq)3, Ir(ppy)3 were
respectively used in blue, orange and green phosphorescent OLEDs.
Maximum external quantum efficiencies, current efficiencies and
power efficiencies of 17.5%, 41.5 cd/A and 32.5 lm/W for blue,
12.3%, 27.1 cd/A and 20.5 lm/W for orange, 20.3%, 71.9 cd/A and
64.5 lm/W for green were obtained. The performances of SQTPA-
based devices are much better than that of NPB-based phosphores-
cent OLEDs with similar structures. Thought of its very simple
molecular structure and easy synthetic route, SQTPA should be
an efficient hole-transporter for phosphorescent OLEDs.
Acknowledgments
The work was supported by the National Natural Science Foun-
dation of China (NSFC Grant 51373029, 61421002, 51533005),
‘‘Preeminent Youth Fund of Si Chuan Province (2015JQ0006)”,
‘‘Program for New Century Excellent Talents in the University of
the Chinese Ministry of Education (NCET-11-0067)” and ‘‘the
Opened Fund of the State Key Laboratory on Integrated Optoelec-
tronics, China (IOSKL2013KF07)”.
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