Star-shaped oligotriarylamines with planarized triphenylamine core: Solution-processable, high-Tg hole-injecting and hole-transporting materials for organic light-emitting devices

Article abstract of DOI:10.1021/cm1018585

Two novel star-shaped oligotriarylamines with planar triphenylamine core and peripheral triarylamine groups, namely FATPA-T and FATPA-Cz, were synthesized by Suzuki cross-coupling reaction. The molecular design imparts the materials with the following fea

Full text of DOI:10.1021/cm1018585

Chem. Mater. 2011, 23, 771–777 771
DOI:10.1021/cm1018585
Star-Shaped Oligotriarylamines with Planarized Triphenylamine Core:
Solution-Processable, High-Tg Hole-Injecting and Hole-Transporting
Materials for Organic Light-Emitting Devices†
Zuoquan Jiang,‡ Tengling Ye,§ Chuluo Yang,*,‡ Dezhi Yang,§ Minrong Zhu,‡
Cheng Zhong,‡ Jingui Qin,‡ and Dongge Ma*,§
‡Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials,
Wuhan University, Wuhan 430072, People’s Republic of China, and State Key Laboratory of Polymer
§
Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, People’s Republic of China
Received July 3, 2010. Revised Manuscript Received November 7, 2010
Two novel star-shaped oligotriarylamines with planar triphenylamine core and peripheral triaryl-
amine groups, namely FATPA-T and FATPA-Cz, were synthesized by Suzuki cross-coupling reaction.
The molecular design imparts the materials with the following features: (i) excellent thermal stabilities
with quite high glass transition temperatures (237 °C for FATPA-T and 272 °C for FATPA-Cz);
(ii) good solution-processability; (iii) good hole mobility, efficient hole injection, and electron-blocking
functions. Furthermore, their optoelectronic properties can be modulated by the peripheral triaryl-
amine groups. For example, FATPA-T with triphenylamine peripheries shows the significantly red-
shifted absorption and emission, as well as the small band gap as compared to FATPA-Cz with
carbazole peripheries. Double-layer Alq3-emitting OLEDs using FATPA-T or FATPA-Cz as hole-
transport layer by spin-coating method were fabricated, and the FATPA-Cz-based devices show
greatly improved performance as compared to standard NPB-based device by vacuum-evaporation
of NPB. The optimized three-layer Alq3-emitting OLEDs by using FATPA-Cz and NPB as double
hole-transport layers exhibit the maximum current efficiency of 6.83 cd/A, which is the highest
for the Alq3-based green emission under the similar device structures. The advantages of solution-
processablity and very high Tg make the star-shaped oligotriarylamines ideal substitutes for conventional
arylamines as hole-inject and hole-transport materials.
1. Introduction
N,N0-bis(3-methylphenyl)-1,10-biphenyl-4, 40-diamine
(TPD), are 96 and 65 °C, respectively.3 The expansion of
the hole-transporting layer (HTL) in a thermally stressed
OLED can cause the device degradation. This effect can
be reduced using HTL materials with high Tg.4 To improve
the amorphous nature, various oligotriarylamines with linear,
star-shaped, branched or dendritic structures have been
developed.5 Shirota et al. have reported several families
of high Tg star-shaped oligotriarylamines with different
cores (benzene, triphenylbenzene or triphenylamine); further-
more, in these systems the extended conjugation can result in
a low oxidation potential, in consequence a low barrier for
hole injection from anode.6 The most efficient conjugation
Triarylamine derivatives have been widely used as hole-
transporting materials in thin layer optoelectronic devices
such as organic light-emitting diodes (OLEDs), organic
photovoltaic cells (OPV), and organic field-effect transistors
(OFETs).1 However, the monomeric molecules do not exhibit
a high hole-transport efficiency due to their easy crystallinity
and/or low thermal stability.2 In many OLEDs, the hole-
transporting materials are the weakest link, with regard to
thermal stability. For example, the glass-transition tem-
perature (Tg) for aluminum tris(8-hydroxyquinoline) (Alq3),
a common electron-transporting material, is 175 °C, whereas
those for the standard hole-transporting materials, such
as 1, 4-bis(1-naphthylphenyl)amino)biphenyl (NPB) and
(3) Naito, K.; Miura, A. J. Phys. Chem. 1993, 97, 6240.
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D. E.; Thompson, M. E. Adv. Mater. 1998, 10, 1108.
† Accepted as part of the “Special Issue on π-Functional Materials”.
*Corresponding author. E-mail: clyang@whu.edu.cn (C.Y.); mdg1014@
ciac.jl.cn (D.M.).
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2010 American Chemical Society
Published on Web 11/23/2010
pubs.acs.org/cm

772 Chem. Mater., Vol. 23, No. 3, 2011
Jiang et al.
was observed in the 4,40,400-tris(diphenylamino)triphenyl-
amine (TDATA) series in which the nitrogen atoms are
connected through a phenylene group via paralinkages.6b
The layer-by-layer evaporation has been proven to be
a successful technique in the fabrication of OLEDs; how-
ever, it is also restricted by high cost, time-consuming and
low process yield. Convenient solution processing such as
spin coating or inkjet printing has been desired as an easy
preparation of devices with low cost and large area.7 For
example, PVK (polyvinylcarbazole), has been widely used
as solution-processable hole-transporting material in
OLEDs.8 However, as compared to vacuum-evaporated
materials, the solution-processable, small-molecule-based
hole-transporting materials have little reported to date.
Hellwinkel et al. initially reported a series of bridged
triarylamine compounds.9 Recently, the bridged triaryl-
amine derivatives attracted attentions in organic opto-
electronics.10 We reported a new fully diarylmethene-bridged
triphenylamine (FATPA), which has an almost planar tri-
phenylamine (TPA) skeleton and exhibits excellent thermal
and morphological stability.11 In this paper, we report
the synthesis and the structures of two novel star-shape
arylamines with the planarized FATPA as molecular
core and triphenylamine or carbazole as peripheral group.
The new star-shaped arylamines are applicable in the OLEDs
as morphologically stable hole-transport materials having a
good hole mobility, efficient hole injection, and electron-
blocking functions. Moreover, the novel materials are
solution-processable. All the merits make them very attrac-
tive hole-transporting materials in OLEDs.
measuring their weight loss while heating at a rate of 10 °C min-1
from 25 to 600 °C. UV-vis absorption spectra were recorded on
Shimadzu UV-2550 spectrophotometer. PL spectra were recorded
on Hitachi F-4500 fluorescence spectrophotometer. Cyclic voltam-
metric measurements were carried out on a computer-controlled
EG&G potentiostat/galvanostat model 283 using tetrabutyl-
ammonium hexafluorophosphate (0.1 M) in freshly distilled CH2Cl2
solution as supporting electrolyte and conventional three-electrode
cell with a Pt button working electrode of 2 mm diameter, a platinum-
wire counter electrode, and a Ag/AgCl reference electrode with
ferrocenium-ferrocene (Fcþ/Fc) as the internal standard.
Synthesis of FATPA-Br. To a solution of FATPA (1.0 g,
1.22 mmol) in 20 mL of chloroform was added NBS (0.68 g,
3.82 mmol) at 0 °C. The mixture was stirred for 12 h at room
temperature, and then filtered. The filtrate was washed with water
three times, and the organic layers were dried with anhydrous
sodium sulfate. After removal of organic solvent, the resulting solid
was recrystallized from ethanol to give a white solid (Yield:
95%). 1H NMR (300 MHz, CDCl3, δ): 6.91 (s, 6H), 6.86 (d, J=
8.1 Hz, 12 H), 6.55 (d, J=8.1 Hz, 12 H), 2.30 (s, 18 H); 13C NMR
(75 MHz, CDCl3, δ): 141.81, 136.13, 134.26, 130.77, 130.70, 130.01,
128.70, 116.10, 55.29, 21.12. Anal. Calcd. for C63H48Br3N (%):
C, 71.47; H, 4.57; N, 1.32. Found: C, 71.23; H, 4.40; N, 1.35;
MALDI-TOF-MS: m/z 1059.2 (Mþ).
Synthesis of FATPA-T. A mixture of FATPA-Br (0.60 g, 0.57
mmol), N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-
N-phenylbenzenamine (0.84 g, 2.26 mmol), Pd(PPh3)4 (40 mg,
2% mmol) and sodium carbonate (1.81 g, 17.1 mmol) in 40 mL
of toluene and 9 mL of distilled water was stirred at 90 °C for 36 h
under argon. The mixture was extracted with toluene, and the
organic layers were washed with brine, dried over anhydrous sodium
sulfate. The crude product was purified by column chromatog-
raphy on silica gel using 3:1 (v/v) petroleum/chloroform as the eluent
to afford a yellow solid (Yield: 81%). 1H NMR (300 MHz,
CDCl3, δ): 7.18-7.15 (m, 18 H), 7.05-6.91 (m, 30 H), 6.78 (d,
2. Experimental Section
J=7.2 Hz, 12 H), 6.65 (d, J=7.2 Hz, 12 H), 2.21 (s, 18 H); 13
C
1
General Information. H NMR and 13C NMR spectra were
NMR (75 MHz, CDCl3, δ): 165.17, 165.13, 149.00, 147.91, 144.45,
136.60, 135.70, 135.08, 131.47, 130.56, 129.52, 128.19, 127.03, 125.64,
125.25, 124.13, 56.93, 22.32. Anal. Calcd. for C117H90N4 (%):
C, 90.54; H, 5.85; N, 3.61. Found: C, 91.01; H, 6.16; N, 3.36.
MALDI-TOF-MS: m/z 1551.5 (Mþ).
measured on Varian Unity 300 MHz spectrometer using CDCl3
as solvent. Elemental analyses of carbon, hydrogen, and nitro-
gen were performed on a Vario EL-III microanalyzer. EI-MS
spectra were recorded with a VJ-ZAB-3F-Mass spectrometer.
MALDI-TOF mass spectrometric measurement was performed
on Bruker Biflex III MALDI TOF instrument. Differential
scanning calorimetry (DSC) was performed on a NETZSCH
DSC 200 PC unit at a heating rate of 10 °Cmin-1 from 30 to 600 °C
under argon. Thermogravimetric analysis (TGA) was undertaken
with a NETZSCH STA 449C instrument. The thermal stability
of the samples under a nitrogen atmosphere was determined by
Synthesis of FATPA-Cz. A mixture of FATPA-Br (0.60 g,
0.57 mmol), 9-(4-tert-butylphenyl)-3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9H-carbazole (0.28 g, 2.26 mmol), Pd(PPh3)4
(40 mg, 2% mmol) and sodium carbonate (1.81 g, 17.1 mmol) in
40 mL of toluene and 9 mL of distilled water was stirred at 90 °C
for 36 h under argon. The mixture was extracted with toluene,
and the organic layers were washed with brine, dried over anhydrous
sodium sulfate. The crude product was purified by column chro-
matography on silica gel using 3:1 (v/v) petroleum/chloroform
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Chem., Int. Ed. 2007, 46, 1149. (b) Ge, Z.; Hayakawa, T.; Ando, S.;
Ueda, M.; Akiike, T.; Miyamoto, H.; Kajita, T.; Kakimoto, M.
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as the eluent to afford a yellow solid (Yield: 73%). H NMR
(300 MHz, CDCl3, δ): 7.99 (d, J=8.1 Hz, 3H), 7.52 (d, J=8.1 Hz,
9H), 7.37-7.26 (m, 24H), 6.83-6.76 (m, 27H), 2.25 (s, 18H),
1.34 (s, 27H); 13C NMR (75 MHz, CDCl3, δ): 150.40, 143.28,
141.37, 140.09, 135.39, 135.19, 134.85, 134.18, 132.94, 130.24, 129.24,
128.15, 126.97, 126.67, 126.61, 126.54, 126.45, 125.85, 124.99, 123.47,
123.26, 120.17, 119.68, 117.95, 109.92, 109.87, 55.69, 30.88, 29.67,
20.94. Anal. Calcd. for C129H108N4 (%): C, 90.38; H, 6.35; N,
3.27. Found: C, 90.71; H, 6.63; N, 2.94. MALDI-TOF-MS: m/z
1712.9 (Mþ).
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Chen, L.; Zhu, L.; Qin, J.; Cao, Y. J. Mater. Chem. 2007, 17, 3451.
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Device Fabrication and Measurements. The ITO-coated glass
substrates were cleaned with special detergent and deionized water.
After cleaning, the substrates were baked at 120 °C for 20 min
followed by O2 plasma treatment. FATPA-T or FATPA-Cz were
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Org. Lett. 2009, 11, 1503.

Article
Chem. Mater., Vol. 23, No. 3, 2011 773
spin-coated from the solution in chlorobenzene onto the pre-
cleaned ITO substrates. After baking at 120 °C for 30 min, the
emitting layer of Alq3 was deposited by thermal evaporation at a
base pressure <1 ꢀ 10-4 Pa. At last, a 1 nm thick layer of LiF
and a 100 nm thick film Al were thermally deposited as a cathode
under vacuum. If needed, NPB was also deposited by thermal
evaporation in the same condition. The thickness of the films
deposited by thermal evaporation was monitored by the crystal
thickness meter and the thickness of spin-coated films was mea-
sured by the terrace detector. The active area of the device are
16 mm2, determined by the cross breadth between the cathode
and the ITO. The EL spectra were measured by the JY SPEX
CCD3000 spectra. Steady current-luminance-voltage was mea-
sured by Keithley 2400 with silicon photodiode. All tests were
performed under air atmosphere and at room temperature.
The samples for the TOF measurement were prepared by
vacuum deposition using the structure: glass/Ag (30 nm)/HTL
(1.1 μm for FATPA-Cz and 1.3 μm for FATPA-T/Al (150 nm).
Optical excitation of the carrier packet was achieved by illumi-
nation through the Ag electrode using an intense short duration
(5-6 ns) light pulse from a frequency-tripled (355 nm) Nd:YAG
laser. The transient current was measured across the load resistor
using Agilent 54825A Infinium Digital Oscilloscope.
in toluene solution and film state, and the photophysical
data are summarized in Table 1. Both compounds show
almost no absorption beyond 400 nm. The high optical
transparency to visible light allows the passage of light
emitted from the device to ensure a high light collecting
efficiency. The energy gaps are evaluated from the onset
of optical absorption of thin films to be 2.88 and 3.02 eV
for FATPA-T and FATPA-Cz, respectively. FATPA-T
shows the significantly red-shifted absorption and emission,
as well as the small band gap as compared to FATPA-Cz,
which could be attributed to its larger conjugation than
that of FATPA-Cz (Table 1).
The oxidative electrochemical behavior of the compounds
was examined by cyclic voltammetry (CV; see Figure 2).
FATPA-T and FATPA-Cz show four and three well-defined
stepwise-oxidation waves, respectively. Furthermore, the
two compounds all exhibit the first reversible oxidation
process around 0.29 V (vs Fcþ/Fc). Considering that the
identical FATPA core of the two molecules, we assigned
the first wave to the oxidation of the central triphenyl-
amino core. Further multistep oxidations occur at the
terminal triarylamine units.14 Their ionization potential
(IP) values estimated from E1/2 (half-wave potential) of
the oxidation are -5.03 and -5.05 eV with regard to the
energy level of ferrocene (4.8 eV below vacuum), which
are significantly higher than that of central core FATPA
(-5.22 eV),11 hence there must be some delocalization of
the charge onto one or more arms of extended structures.
The IPs of the two new compounds are close to that of
ITO (-4.70 eV), which implies that they could serve as
efficient hole-injection materials. The electron affinity (EA)
values are deduced from IP values and energy gaps to
be -2.15 eV for FATPA-T and -2.04 eV for FATPA-Cz,
which suggests that they could also act as efficient electron-
blocking materials. The estimate of EA is a gross approx-
imation that does not take the exciton binding energy into
account.
We initially fabricated the electroluminescent devices with
a simple configuration (Structure I): ITO/HTL (50 nm)/Alq3
(40 nm)/LiF (1 nm)/Al (100 nm). In these devices, FATPA-T
or FATPA-Cz was processed by spin-coating method to
serve as hole-transport layer (HTL), and Alq3 was used as
both emissive and electron-transport layer (EML and
ETL). A reference device using NPB as HTL by vacuum
vapor deposition under the identical configuration was
also fabricated for comparison. All the devices show the
exclusive green emission originating from Alq3, indicat-
ing no exciplex formation at the interface of HTL/EML.
Figure 3 shows the current-voltage-luminance (J-V-L)
characteristics and current efficiency vs. current density
curves of the three devices. The current densities and
luminances of the devices with FATPA-T or NPB as HTL
are very similar, while those of the device with FATPA-Cz
as HTL are markedly low under the same driving voltage.
The device based on FATPA-T and FATPA-Cz exhibit a
maximum current efficiency (ηc max) of 3.39 and 4.41 cd/A
3. Results and Discussion
The synthesis of the star-shaped arylamines, namely
FATPA-T and FATPA-Cz, is outlined in Scheme 1. The
intermediate 3Br-FATPA was prepared by the bromination
of FATPA with NBS in a high yield of 95%. FATPA-T and
FATPA-Cz were synthesized by the Suzuki cross-coupling
reactions of 3Br-FATPA with 4-triphenylamine pinacol
borate or 3-(9-(4-t-butyl-phenyl)-9H-carbazole) pinacol
borate in good yields of 70-80%. Their chemical structures
1
were fully characterized by H NMR, 13C NMR, mass
spectrometry, and elemental analysis (see the Experimental
Section).
The excellent thermal stability of the compounds is indi-
cated by their high decomposition temperatures (Td, cor-
responding to 5% weight loss) of 511 °C for FATPA-T
and 506 °C for FATPA-Cz in the thermogravimetric
analysis (TGA). Distinct glass transition temperatures
(Tg) were observed at 237 °C for FATPA-T and 272 °C
for FATPA-Cz from the differential scanning calorimetry
(DSC), which are much higher than the parent core FATPA
(178 °C). To our knowledge, these values are among the
highest Tg for hole-transporting materials to date.12 The
bulky star-shaped molecular configuration and the aryl
peripheries on the rigid FATPA core could cause entan-
glement in the amorphous state and hinder recrystalliza-
tion, which is highly desirable to improve the efficiency
and lifetime of OLEDs, especially for high-temperature
applications of devices.13
Figure 1 shows the absorption spectra of the com-
pounds in toluene and the PL spectra of the compounds
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774 Chem. Mater., Vol. 23, No. 3, 2011
Jiang et al.
Scheme 1. Synthesis of FATPA-T and FATPA-Cz
device were achieved by using wet technique to fabricate
the hole-transporting layer, which means a more simple
process and low cost in comparison with the vacuum vapor
deposition for NPB.
To gain insight into the device performances with dif-
ferent HTL, we measured the hole mobilities of the star-
shaped arylamines by the time-of-flight (TOF) transient
photocurrent technique at room temperature. Both com-
pounds exhibit nondispersive hole-transport characteristics
as indicated by the TOF transients. The field-dependence of
mobilities determined from the TOF transients (Figure 4)
follows the nearly universal Poole-Frenkel relationship.15
The hole mobilities of 3.1ꢀ 10-4 cm2 V-1 s-1 for FATPA-
Cz and 6.5ꢀ10-4 cm2 V-1 s-1 for FATPA-T at an electric
Figure 1. Normalized absorption in toluene and emission spectra in toluene
solution and film.
(Table 2), respectively, whicharecomparableorsignificantly
higher as compared to the NPB-based device (3.14 cd/A).
It is worthy to mention that the high efficiencies of the
€
€
(15) (a) Bassler, H. Int. J. Mol. Phys. 1994, 8, 847. (b) Bassler, H. Philos.
Mag. B 1992, 65, 795. (c) Gill, W. D. J. Appl. Phys. 1972, 43, 5033.

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Chem. Mater., Vol. 23, No. 3, 2011 775
Table 1. Thermal, Electrochemical, and Photophysical Data of the Compounds
Td (°C)
Tg (°C)
IP (eV)
EA (eV)
Abs λmax (nm) soln/film
PL λmax (nm) soln/film
FATPA-T
FATPA-Cz
511
506
237
272
-5.03 a
-5.05 a
-2.15 b
-2.04 b
378 c/383 d
344 c/357 d
414 c/429 d
404 c/419 d
a Estimated from the half-wave potential. b Deduced from the IP and the optical energy gap. c Measured in toluene. d Measured in film.
injection barrier from HTL to EML of Alq3 (0.2 eV) and a
higher hole mobility than FATPA-T, the two factors are
counteracted to result in the similar current densities and
comparable efficiencies for the FATPA-T and NPB-based
devices. (ii) Comparing the FATPA-T and FATPA-Cz,
both have the same hole-injection barriers, but the hole
mobility of FATPA-Cz is smaller than that of FATPA-T,
and thus the usage of FATPA-Cz with moderate hole mobil-
ity would decrease the hole current, consequently result in
more balanced recombination of holes and electrons in
the emitting layer. This point could be supported by the
lower current density of the FATPA-Cz based device at
the given voltage relative to those of FATPA-T or NPB-
based devices as discussed above.
Figure 2. Cyclic voltammogrames of the compounds in CH2Cl2 for
oxidation.
Because the hole mobility of FATPA-Cz is still signif-
icantly higher than electron mobility of Alq3,17 the efficien-
cies of FATPA-Cz based device were further improved by
optimizing the thickness of HTL (90, 115, 135, 150 nm).
The best performance is attained in a device with 115 nm
thick HTL, and its maximum current efficiency reaches
5.81 cd/A (Figure 6). This indicates that the carrier balance
factor is optimal in such a device. For thinner FATPA-Cz
layer (50, 90 nm), the holes could be in excess, on the
contrary, for thicker FATPA-Cz layer (135, 150 nm), the
electrons could be overflow.
Another possible origin of the high device efficiency for
FATPA-Cz could be attributed to its efficient electron-
blocking function. To confirm the assumption, device struc-
ture II: ITO/HTL(115 nm)/Alq3 (5, 15, 25, or 35 nm)/
DCJTB (1 nm)/Alq3 (35, 25, 15, or 5 nm)/LiF (1 nm)/Al
(100nm),wasdesignedtomeasuretheexcitonsdistribution.18
In structure II, a thin layer (l nm) of 4-(dicyanomethylene)-
2-(t-butyl)-6-methyl-4H-pyran (DCJTB) was inserted into
the EML at different distances from the interface between
the EML and HTL. When excitons were distributed over
and decayed in the layer of DCJTB, the corresponding
emission spectrum represented a composite of the Alq3
and DCJTB emissions as shown in Figure 7. The dominant
red emission at 5 nm distance away from the HTL layer
indicates that the recombination zones of holes and elec-
trons are near the interface of HTL/EML. The small hole
mobility relative to electron mobility in Alq3 layer cause
the recombination zone to approach the HTL, whereas
the efficient electron blocking function of FATPA-Cz make
the recombination occur near the interface of HTL/EML.
In terms of the IP values of HTLs, we finally optimized
the device structures by using FATPA-Cz and NPB as double
hole-transport layers (Structure III): ITO/FATPA-Cz
Figure 3. (a) Current density-voltage-luminance characteristics.
(b) Current efficiency versus current density curves for device structure I.
field of ca. 1ꢀ105 V cm-1 are lower than that of the typical
HTL material NPB (1 ꢀ 10-3 cm2 V-1 s-1).16
In view of the hole-injection and hole-transport char-
acters of the HTLs, the device performance could be
elucidated as follows: (i) Comparing the FATPA-T and
NPB, FATPA-T has a substantially lower hole injection
barrier from ITO to HTL (∼0.35 eV) than NPB (∼0.9 eV)
(Figure 5), whereas NPB has a significantly lower hole
(17) Vanslyke, S. A.; Chen, C. H.; Tang, C. W. Appl. Phys. Lett. 1996,
69, 2160.
(18) Wu, Z.; Wang, L.; Lei, G.; Qiu, Y. J. Appl. Phys. 2005, 97, 103105.
(16) Lin, L. B.; Young, R. H.;Mason, M. G.; Jenekhe, S. A.; Borsenberger,
P. M. Appl. Phys. Lett. 1998, 72, 864.

776 Chem. Mater., Vol. 23, No. 3, 2011
Jiang et al.
Table 2. Characteristics of the Electroluminescent Devices
device
HTL
Vona (V)
Lmaxb (cd m-2
)
ηc.maxc (cd A-1
)
ηp.maxd (lm W1-
)
ηext.maxe (%)
I
I
I
I
I
I
I
III
III
III
III
FATPA-T (50 nm)
FATPA-Cz (50 nm)
NPB (50 nm)
2.7
2.7
2.5
2.9
3.1
3.3
3.5
4.9
4.9
4.9
5.1
11068
19541
17719
15683
11470
7194
3.39
4.41
3.14
5.21
5.81
5.43
5.43
6.83
6.82
6.19
6.03
2.35
2.63
2.32
3.25
2.84
2.44
2.12
2.77
2.77
2.42
1.54
1.01
1.05
1.38
1.64
1.80
1.61
1.70
2.07
2.01
1.84
1.85
FATPA-Cz (90 nm)
FATPA-Cz (115 nm)
FATPA-Cz (135 nm)
FATPA-Cz (150 nm)
FATPA-Cz/NPB (10 nm)
FATPA-Cz/NPB (20 nm)
FATPA-Cz/NPB (30 nm)
FATPA-Cz/NPB (40 nm)
6581
16405
16976
14082
11966
a Turn on voltage. b Maximum luminance. c Maximum current efficiency. d Maximum power efficiency. e Maximum external quantum efficiency.
Figure 4. Hole mobilities versus E1/2 for films of FATPA-T and FATPA-Cz.
Figure 7. EL spectra for device structure IV.
Figure 5. Energy levels of the materials used in the devices.
Figure 8. Current efficiency versus current density curves for device
structure III.
layer FATPA-Cz, subsequently to the second hole-transport
layer NPB, and then to the emitting layer Alq3. All the devices
exhibit high current efficiencies with ηc max in the range of
6.03-6.83 cd/A (Figure 8). The high device efficiencies
could be attributed to: (i) The energy barriers for hole
injection are lowered by the use of double hole-transport
layer structure. The FATPA-Cz here is referred as a hole-
injection buffer layer. (ii) When only FATPA-Cz was
used as HTL, the recombination zone was located near
the interface of HTL/EML since the hole injection barrier
Figure 6. Current efficiency versus current density curves for FATPA-Cz
from HTL to EML was large. When NPB with faster hole
based devices with different HTL thickness.
mobility and lower IP was inserted between FATPA-Cz
(115 nm)/NPB (10, 20, 30, or 40 nm)/Alq (40 nm)/LiF
(1 nm)/Al (100 nm). Hole carriers are injected in a stepwise
process from the anode ITO to the first hole-transport
and EML, the hole injection to EML became facile and
the recombination zone was extended in the EML and the
exciton quenching was efficiently suppressed, in consequence

Article
Chem. Mater., Vol. 23, No. 3, 2011 777
the device efficiency was further improved.19 To the best
of our knowledge, the maximum current efficiency of
6.83 cd/A obtained at the 10 nm NPB thickness is the highest
for the Alq3-based green emission under the similar device
structures until now.20 Noticeably, the excellent device
performance is achieved by using spin-coating method for
the hole-transporting layer.
layer Alq3 emitting OLEDs using FATPA-T or FATPA-Cz
as hole-transport layer by spin-coating method were
fabricated, and the FATPA-Cz-based device show sig-
nificantly improved performance as compared to standard
NPB-based device by vacuum-evaporation of NPB. Three-
layer Alq3 emitting OLEDs by using FATPA-Cz and
NPB as double hole-transport layers exhibit high current
efficiencies with ηc max in the range of 6.03-6.83 cd/A,
which are the highest for the Alq3-based green emission
under the similar device structures. The advantages of
solution-processablity and very high Tg make the star-shaped
oligotriarylamines ideal substitutes for conventional aryl-
amines as hole-injection and hole-transport materials.
4. Conclusion
In summary, we reported two novel star-shaped oligo-
triarylamines based on a rigid planar triphenylamine core
for the first time. The molecular design imparts the materials
with the excellent thermal and morphological stabilities,
good solution-processability, and low oxidation potential,
which are promising properties for organic optoelectronic
devices. Furthermore, their optoelectronic properties can
be tuned by the peripheral triarylamine groups. Double-
Acknowledgment. We thank the National Natural Science
Foundation of China (Project Nos. 90922020, 50773057), the
National Basic Research Program of China (973 Program-
2009CB623602, 2009CB930603), the Open Research Fund
of State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, for financial support.
(19) Tao, Y.; Wang, Q.; Yang, C.; Wang, Q.; Zhang, Z.; Zou, T.; Qin, J.;
Ma, D. Angew. Chem., Int. Ed. 2008, 47, 8104.
(20) Li, J.; Ma, C.; Tang, J.; Lee, C. S.; Lee, S. T. Chem. Mater. 2005,
17, 615.