Bromine-substituted triphenylamine derivatives with improved hole-mobility for highly efficient green phosphorescent OLEDs with a low operating voltage

Article abstract of DOI:10.1039/c6tc03020c

Charge transport materials play a crucial role in organic light-emitting diodes (OLEDs) due to their effect on reducing the operating voltage and enhancing the power efficiency. To develop hole transport materials with improved hole-mobility, two novel br

Full text of DOI:10.1039/c6tc03020c

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BromineꢀSubstituted Triphenylamine Derivatives with Improved
HoleꢀMobility for Highly Efficient Green Phosphorescent OLEDs with
Low Operating Voltage
Xiaoyang Du,a Juewen Zhao,a Wei Liu,b Kai Wang,b Shaolin Yuan,a Caijun Zheng,*,a
Hui Lin,a Silu Tao,*,a XiaoꢀHong Zhangb
a School of Optoelectronic Information, University of Electronic Science and Technology of China
(UESTC), Chengdu 610054, PR China
b Functional Nano and Soft Materials Laboratory (FUNSOM) and Collaborative Innovation
Center of Suzhou Nano Science and Technology Soochow University, Suzhou, Jiangsu 215123, PR.
China
Abstract
Charge transport materials play a crucial role in organic lightꢀemitting diodes (OLEDs)
due to their effect on reducing operating voltage and enhancing power efficiency. To develop
hole transport materials with improved holeꢀmobility, two novel bromineꢀsubstituted
triphenylamine
derivatives:
(4ꢀBromoꢀphenyl)ꢀbisꢀ(4ꢀquinolinꢀ8ꢀylꢀphenyl)ꢀamine)
BrꢀDQTPA and (9,9ꢀBisꢀ(4ꢀtriphenylꢀamine)ꢀ2,7ꢀdibromoꢀ9Hꢀfluorene) BrꢀDTF were
designed and synthesized. The hole mobilities of BrꢀDQTPA and BrꢀDTF show one order of
magnitude
enhancement
compared
with
nonꢀbrominated
and
(Phenylꢀbisꢀ(4ꢀquinolinꢀ8ꢀylꢀphenyl)ꢀamine)
DQTPA
(9,9ꢀBisꢀ(4ꢀtriphenylꢀamine)ꢀ9Hꢀfluorene) DTF at the same electric field. Attributing to the
improved hole mobilities, traditional green phosphorescent OLEDs that using BrꢀDQTPA or
*Corresponding author: Eꢀmail: zhengcaijun@uestc.edu.cn (C. J. Zheng), silutao@uestc.edu.cn (S. L.
Tao).

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BrꢀDTF as hole transport material both show ultralow operating voltage at 1cd/m2 (below
2.4V) and very high luminous efficiencies (over 21% and 90 lm/W) without light outcoupling
improvement. Those results are much better than that of DQTPAꢀand DTFꢀ, even NPBꢀbased
green devices, indicating bromineꢀsubstitution is a promising and convenient way to achieve
novel hole transport materials with improved holeꢀmobility.
Key words: bromineꢀsubstitution, holeꢀmobility, operating voltage, OLEDs
Introduction
In the last three decades, organic semiconductor materials act as the most active
components in organic thin film devices and attract great attention both in scientific and
industry fields.[1ꢀ4] To organic semiconductors, carrier mobility is one of the most important
factors because it not only determines the electric conductivity of semiconductor materials,
but also affects the charge transfer in the devices.[5ꢀ8] Especially for organic light emitting
diodes (OLEDs), the high carrier mobility could even reduce the operating voltage and then
greatly enhance the power efficiency, which is significant for energy conservation.[9ꢀ12] Thus,
in OLEDs, improving the carrier mobility of organic charge transport materials is always
essential. Throughout the development of organic charge transport materials, traditional
strategies, like enlarging the conjugation,[13,14] introducing heterocyclic groups[15,16] et al,
have already widely used in the past years. It is required to develop new strategy to further
enhance the carrier mobility of organic charge transport materials.
In the past studies, introducing the halogen atoms into the organic semiconductor
molecules has been reported on the different enhancements of their carrier mobilities[17ꢀ19]
Fluorine atom, with the strong electronegativity, can enhance the electronꢀdeficiency of

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compounds and endow them with improved electron mobilities. Like perfluorinated
oligo(pꢀphenylene)s (PFꢀ6P) and perfluoroaryl (FAr)ꢀsubstituted tetracene (FPPT), both of
them exhibit 100 times higher electron mobilities than that of their nonꢀfluorinated
counterpart.[20,21] Moreover, with the good stability of fluorine, fluorineꢀsubstituted organic
compounds with improved electronꢀmobility have already been employed in organic
photovoltaic cells and OLEDs.[22ꢀ24] Correspondingly, with a heavy atomic weight, iodine
atom can induce a significant decrease in the local holeꢀvibration coupling, thus resulting in
an improvement of hole mobilities for iodineꢀsubstituted compounds.[27,29] Such as
1,4ꢀdiiodobenzene (DIB) and 2,6ꢀdiiododithieno[3,2ꢀb:2’,3’ꢀd]thiophene (DTTꢀ2I), both of
them are marked for their extremely large hole mobilities about 10ꢀ2~10 cm2 Vꢀ1sꢀ1.[25ꢀ30]
However, due to the poor electrochemical and thermal stabilities of iodine, iodineꢀsubstituted
compounds can’t be utilized in practical OLEDs application. Thus, although halogenation is
an efficient and convenient way to enhance carrier mobility, it is still highly required to
develop halogenꢀsubstituted compounds with improved hole mobility but without the
sacrifice of electrochemical and thermal stabilities.
Compared with iodine atom, bromine atom can possess similar electronegativity, also
heavy atomic weight, but relatively better electrochemical and thermal stabilities. So
bromineꢀsubstituted compounds should have the potential to achieve both improved hole
mobilities and good stability. Given on this point, we designed and synthesized two novel
hole transport materials (HTMs), ((4ꢀbromoꢀphenyl)ꢀbisꢀ(4ꢀquinolinꢀ8ꢀylꢀphenyl)ꢀamine)
BrꢀDQTPA and (9,9ꢀbisꢀ(4ꢀtriphenylꢀamine)ꢀ2,7ꢀdibromoꢀ9Hꢀfluorene) BrꢀDTF, by
introducing bromine atom into the common holeꢀtransporting triphenylamine

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derivatives.[31ꢀ34] Both two compounds successfully show good thermal stabilities in
thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements.
And the hole mobilities of BrꢀDQTPA and BrꢀDTF are respectively measured as 1.1×10ꢀ5 cm2
Vꢀ1sꢀ1 and 3.0×10ꢀ5 cm2 Vꢀ1sꢀ1 at electric field of 600 (V cmꢀ1)1/2 by spaceꢀchargeꢀlimited
current (SCLC) model, which are much higher than that of DQTPA (1.6×10ꢀ6 cm2 Vꢀ1sꢀ1) and
DTF (3.1×10ꢀ6 cm2 Vꢀ1sꢀ1) at the same electric field. To further evaluate the potential
application of Brꢀsubstituted compounds in the OLEDs, traditional green phosphorescent
devices using (bis(2ꢀphenylpyridine)(acetylacetonato)iridium(III)) Ir(ppy)2acac as the dopant
were constructed with new HTMs. Both the devices based on BrꢀDQTPA or BrꢀDTF show
high luminous efficiencies (over 21% and 90 lm/W) and ultralow operating voltage at 1cd/m2
(below 2.4V) without light outcoupling improvement. These results are not only much higher
than that of DQTPAꢀ and DTFꢀbased devices (15.9%, 56.0 lm/W and 15%, 63.3 lm/W), but
also better than that of traditional NPBꢀbased device (17.5% and 78.3 lm/W), proving
Brꢀsubstitution is a fruitful approach to develop novel HTMs with improved the hole mobility
but without sacrificing the stability.
Experimental
Chemical and Instrument
All powders and solvents were used as received from commercial suppliers without
1
further purification. The final products were confirmed with HNMR spectra, elemental
analysis (EA) and mass spectrometry (MS). Glass transition temperature (Tg) was measured
o
by using a PerkinꢀElmerpyris DSC6 system operated at a heating rate of 10 C minꢀ1.
Decomposition temperature (Td, corresponding to 5% weight loss) was measured by a TA

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SDT Q600 instrument at a heating rate of 10 oC minꢀ1 under nitrogen flow. Optical absorption
and emission spectra were recorded with a Hitachi Uꢀ3010 UVꢀVS spectrophotometer and a
Hitachi Fꢀ4600 fluorescence spectrophotometer, respectively. The highest occupied molecular
orbital (HOMO) energy level was measured with ultraviolet photoelectron spectroscopy
(UPS). And the lowest unoccupied molecular orbital (LUMO) energy level was determined
from the difference between the HOMO energy level and the energy gap (Eg) obtained from
the optical absorption edge. Film morphologies were investigated by Atomic Force
Microscope (AFM, BrukerꢀDimension Icon).
Materials synthesis
Synthesis of 4,4'-Dibromotriphenylamine: Triphenylamine (5 mmol, 1.23 g),
N,Nꢀdimethylformamide (DMF) (12.5 ml) were mixed and fully dissolved in a flask.
Nꢀbromosuccinimide (NBS) (10 mmol, 1.78 g) was dissolved in DMF (25 ml) and the
mixture was added drop wisely in above mentioned triphenylamine solution at 0 oC. Then the
mixture was kept in dark and stirred overnight at room temperature. The solution was diluted
with water (30 mL) and extracted by dichloromethane. A white powder of crude product was
obtained and purified by recrystallization with dichloromethane and petroleum ether. Then the
pure product as a white solid was achieved with yield of 89%.
Synthesis of Tris(4-bromophenyl)amine: Triphenylamine (5 mmol, 1.23 g),
N,Nꢀdimethylformamide (DMF) (20 ml) were mixed and fully dissolved in a flask.
Nꢀbromosuccinimide (NBS) (15 mmol, 2.67 g) was dissolved in DMF (25 ml) and the
mixture was added drop wisely in above mentioned triphenylamine solution at 0 oC. Then the
mixture was kept in dark and stirred overnight at room temperature. The solution was diluted

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with water (50 mL) and extracted by dichloromethane. A white powder of crude product was
obtained and purified by recrystallization with dichloromethane and petroleum ether. Then the
pure product as a white solid was achieved with yield of 83%.
Synthesis of Phenyl-bis-(4-quinolin-8-yl-phenyl)-amine (DQTPA): Tris(4ꢀbromophenyl)amine
(2 mmol, 0.81 g), 8ꢀquinolinboronic acid (4.5 mmol, 0.78 g), Pd(PPh3)4 (0.1 g), aqueous
Na2CO3 (2.0 M, 20 ml), ethanol (15 ml) and toluene (20 ml) were mixed in a flask. The
o
mixture was degassed three times and then heated and refluxed at 100 C for 24h under a
nitrogen atmosphere. After the reaction mixture cooled to room temperature, the solution was
evaporated and extracted with dichloromethane. The extracted solution was then washed with
water and dried with MgSO4. Evaporate the solution and gave light yellow solid as crude
product. The product was first purified by column chromatography on silica gel (petroleum
ether: dichloromethane, 3:1) and then future purified by recrystallization to give a pure
product as a white solid with the yield of 64%. 1H NMR: (CDCl3, 300 MHz, δ): 7.04ꢀ7.15 (m,
1H), 7.25ꢀ7.34 (m, 8H), 7.39ꢀ7.45 (m, 2H), 7.50ꢀ7.63 (t, 2H), 7.65ꢀ7.70 (d, 4H), 7.75ꢀ7.82 (t,
4H), 8.18ꢀ8.23 (m, 2H), 8.96ꢀ9.00 (m, 2H). MS (EI) m/z: [M]+:499.2. EA: anal. calcd. for
C36H25N3, C: 86.55%, H: 5.04%, N: 8.41%; found: C: 86.85%, H: 5.08%, N: 8.26%.
Synthesis
of
(4-Bromo-phenyl)-bis-(4-quinolin-8-yl-phenyl)-amine
(Br-DQTPA):
4,4'ꢀDibromotriphenylamine (2 mmol, 0.96 g), 8ꢀquinolinboronic acid (4 mmol, 0.69 g),
Pd(PPh3)4 (0.1 g), aqueous Na2CO3 (2.0 M, 20 ml), ethanol (15 ml) and toluene (20 ml) were
mixed in a flask. The mixture was degassed three times and then heated and refluxed at 100
oC for 24h under a nitrogen atmosphere. After the reaction mixture cooled to room
temperature, the solution was evaporated and extracted with dichloromethane. The extracted

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solution was then washed with water and dried with MgSO4. Evaporate the solution and gave
yellow solid as crude product. The product was first purified by column chromatography on
silica gel (petroleum ether: dichloromethane, 3:1) and then future purified by recrystallization
to give a pure product as a white solid with the yield of 60%. 1H NMR: (CDCl3, 300 MHz, δ):
7.15ꢀ7.23 (d, 2H), 7.27ꢀ7.34 (d, 4H), 7.35ꢀ7.48 (m, 4H), 7.55ꢀ7.64 (t, 2H), 7.65ꢀ7.73 (d, 4H),
7.74ꢀ7.86 (m, 4H), 8.15ꢀ8.23 (m, 2H), 8.95ꢀ9.05 (m, 2H). MS (EI) m/z: [M]+: 577.1. EA: anal.
calcd. for C36H24N3Br, C: 74.74%, H: 4.18%, N: 7.26%; found: C: 73.91%, H: 4.12%, N:
6.98%.
Synthesis of 9,9-Bis-(4-triphenyl-amine)-9H-fluorene (DTF): Fluorenꢀ9ꢀone (3 mmol, 0.54 g),
triphenylꢀamine (50 mmol, 12.25 g) and methanesulfonic acid (1 ml) were mixed in a flask.
o
The mixture was degassed three times and then heated at 140 C for 10h under a nitrogen
atmosphere. After the reaction mixture cooled to room temperature, the mixture was dissolved
and extracted with dichloromethane. The extracted solution was then washed with water and
dried with MgSO4. Evaporate the solution and gave blue solid as crude product. The product
was first purified by column chromatography on silica gel (petroleum ether: dichloromethane,
4:1) and then future purified by recrystallization to give a pure product as a white solid with
1
the yield of 50%. H NMR: (CDCl3, 300 MHz, δ): 6.85ꢀ6.94 (d, 4H), 6.95ꢀ7.02 (t, 4H),
7.03ꢀ7.09 (m, 12H), 7.15ꢀ7.24 (t, 8H), 7.26ꢀ7.32 (d, 2H), 7.33ꢀ7.41 (t, 2H), 7.43ꢀ7.48 (d, 2H),
7.71ꢀ7.79 (d, 2H). MS (EI) m/z: [M]+: 652.3. EA: anal. calcd. for C49H36N2: C: 90.15%, H:
5.56%, N: 4.29%; found: C: 90.03%, H: 5.48%, N: 4.15%.
Synthesis
of
9,9-Bis-(4-triphenyl-amine)
-2,7-dibromo-9H-fluorene
(Br-DTF):
2,7ꢀDibromoꢀfluorenꢀ9ꢀone (3 mmol, 1.01 g), triphenylꢀamine (50 mmol, 12.25 g) and

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methanesulfonic acid (1 ml) were mixed in a flask. The mixture was degassed three times and
then heated at 140 oC for 10h under a nitrogen atmosphere. After the reaction mixture cooled
to room temperature, the mixture was dissolved and extracted with dichloromethane. The
extracted solution was then washed with water and dried with MgSO4. Evaporate the solution
and gave blue solid as crude product. The product was first purified by column
chromatography on silica gel (petroleum ether: dichloromethane, 4:1) and then future purified
by recrystallization to give a pure product as a yellow solid with the yield of 74%. 1H NMR:
(CDCl3, 300 MHz, δ): 6.86ꢀ6.94 (d, 6H), 6.95ꢀ7.04 (m, 6H), 7.05ꢀ7.12 (d, 6H), 7.18ꢀ7.26 (t,
10H), 7.45ꢀ7.54 (m, 4H), 7.56ꢀ7.62 (d, 2H). MS (EI) m/z: [M]+: 810.1. EA: anal. calcd. for
C49H34Br2N2: C: 72.60%, H: 4.23%, N: 3.46%; found: C: 72.29%, H: 4.07%, N: 3.33%.
Device fabrication and Measurement
Indium tin oxide (ITO) coated glasses with a sheet resistance of 15 Ohm per square was
used as substrates. Before device fabrication, the ITO glass substrates were carefully
pretreated by washing with isopropyl alcohol and deionized water, dried in an oven at 120 oC
over 1 hour and then treated with ultravioletꢀozone for 25 mins before loaded in a vacuum
deposition chamber with a base vacuum better than 10ꢀ6 Torr. Organic layers were
successively deposited on the ITO glass substrates with
a rate of 1ꢀ2 Å/s.
Hole/electronꢀinjecting layer of MoO3/LiF and cathode of Al were respectively deposited with
the rates of 0.1 and 10 Å/s.
The electroluminescence (EL) spectra, CIE coordinates, and luminance characteristics
were all measured with a Spectrascan PR655 photometer under ambient atmosphere. The

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current densityꢀvoltage (J-V) characteristic was measured by a computerꢀcontrolled Keithley
2400 source meter.
Results and discussion
Scheme 1. Synthetic route and molecule structure of DQTPA, BrꢀDQTPA, DTF and BrꢀDTF
Scheme 1 depicts the synthetic routes and molecule structures of DQTPA, BrꢀDQTPA, DTF
1
and BrꢀDTF. The molecule structures of the final products were confirmed with HNMR
spectrometry, EA and MS. Thermal properties of the four compounds were studied with the
TGA and DSC measurements as shown in Figure 1. BrꢀDQTPA shows a Td of 339 oC and a Tg
of 111 oC, whereas DQTPA shows a thermal decomposition temperature of 356 oC and a glass
o
transition temperature of 108 C. Likewise, BrꢀDTF and DTF respectively show the Tds of
o
o
380 and 362 C, and Tgs of 101 and 98 C. Compared with their original states, both

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BrꢀDQTPA and BrꢀDTF show similar Tg and Td, suggesting that introducing bromine atom in
the molecule has little influence on the thermal stability of compounds.
Figure 1. TGA and DSC (inset) measurements of (a) DQTPA, (b) BrꢀDQTPA, (c) DTF and (d)
BrꢀDTF
Figure 2. (a): UVꢀVis absorption and PL spectra of DQTPA and BrꢀDQTPA, in dilute
dichloromethane solution at room temperature, and the phosphorescence spectra of DQTPA and
BrꢀDQTPA in 2ꢀmethyltetrahydrofuran at 77K; (b): UVꢀVis absorption and PL spectra of DTF and
BrꢀDTF in dilute dichloromethane solution at room temperature, and the phosphorescence spectra
of DTF and BrꢀDTF in 2ꢀmethyltetrahydrofuran at 77K.
Figure 2 reveals the UVꢀVis absorption and PL spectra of DQTPA, BrꢀDQTPA, DTF and
BrꢀDTF in dilute dichloromethane solution at room temperature, and the phosphorescence

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spectra of DQTPA, BrꢀDQTPA, DTF and BrꢀDTF in 2ꢀmethyltetrahydrofuran (2ꢀMeTHF) at
77K. The low temperature phosphorescence spectra of BrꢀDQTPA and BrꢀDTF show similar
waveform with DQTPA and DTF, respectively. Triplet energies (ETs) of DQTPA and
BrꢀDQTPA are determined from the first emission peak of their phosphorescence spectra to be
2.28 and 2.30 eV. In the same manner, the ETs of DTF and BrꢀDTF are determined to be 2.86
and 2.73 eV. Furthermore, DQTPA and BrꢀDQTPA respectively show the HOMO energy
levels of ꢀ5.34 and ꢀ5.48 eV as determined by UPS and the LUMO energy levels of ꢀ2.30 and
ꢀ2.39 eV obtained by the values of their HOMO and optical gaps. Similarly, DTF and BrꢀDTF
present the HOMO energy levels of ꢀ5.48 and ꢀ5.51 eV and the LUMO energy levels of ꢀ2.04
and ꢀ2.15 eV, respectively. The molecular orbital distributions of DQTPA, BrꢀDQTPA, DTF
and BrꢀDTF are characterized by density functional theory calculations at the
B3LPY/6ꢀ31G(d) level as shown in Figure 3. The LUMOs of DQTPA and BrꢀDQTPA are
primarily localized at quinolone unit, whereas the HOMOs are located at triphenylamine and
bromineꢀsubstituted triphenylamine group. For DTF and BrꢀDTF, both HOMOs are dominant
on triphenylamine group and the LUMOs respectively distribute on fluorine and
bromineꢀsubstituted fluorine unit. These results imply that bromination could slightly affect
the frontier orbital energy levels of molecules. Key physical data of those compounds are also
listed in Table 1.

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Figure 3. Calculated spatial distributions of the LUMO/HOMO of DQTPA, BrꢀDQTPA, DTF and
BrꢀDTF.
Table 1. Key physical data of DQTPA, BrꢀDQTPA, DTF and BrꢀDTF
Materials
DQTPA
BrꢀDQTPA
DTF
λabs (nm) [a]
291/358
290/353
310
λPL (nm) [b]
496
Tg/Td (oC) [c]
108/356
111/340
98/362
ET (eV)[d]
2.28
HOMO/LUMO (eV)[e]
ꢀ5.34/ꢀ2.30
ꢁ (cm2/Vs)[f]
1.6×10ꢀ6
1.1×10ꢀ5
3.1×10ꢀ6
3.0×10ꢀ5
468
2.30
ꢀ5.48/ꢀ2.39
428
2.86
ꢀ5.48/ꢀ2.04
BrꢀDTF
295
408
101/380
2.73
ꢀ5.51/ꢀ2.15
[a], [b] is the room temperature absorption spectra and phosphorescence spectra from dilute dichloromethane solution,
respectively; [c] is glass transition temperature and thermal decomposition temperature obtained from DSC and TGA
[d]
[e]
measurements;
is the triplet energy level;
is the highest occupied molecular orbital and lowest unoccupied
molecular orbital energy levels determined by the ultraviolet photoelectron spectroscopy (UPS) and the absorption
edge. [f] is the hole mobility at 600 (V cmꢀ1)1/2
To further investigate the electrical properties of the Brꢀsubstituted compounds,
holeꢀonly devices were fabricated with the structure of ITO/MoO3 (10 nm)/HTM (80
nm)/MoO3 (10 nm)/Al, where HTM is DQTPA or BrꢀDQTPA or DTF or BrꢀDTF. In the
device, holes can be easily injected from the anode to the organic layer, whereas electrons are
restrained due to the large barrier between the conduction band of MoO3 (ꢀ2.3 eV) and the
work function of Al cathode (ꢀ4.3 eV). Hole mobilities of the four compounds are
investigated by SCLC method, [35] which is described as:

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9
V 2
J = εrε0ꢀ
(1)
3
8
L
and
V
L
(2)
ꢀ
=
ꢀ
0 exp(
β
)
8.85×10−14 F cmꢀ1), ꢀ is the
carrier mobility determined by SCLC measurements, L is the cathodeꢀanode distance (100 nm
ε
ε
Where is relative permittivity (≈3.00), is vacuum permittivity (≈
0
r
ꢀ
for the device), is zeroꢀfield mobility and is a coefficient that is proportional to the
β
0
ꢀ0
PooleꢀFrenkel factor, [36] the value of
and
were calculated by linear fitting. By using
β
the obtained values, the holeꢀmobility can be obtained by eq (2). Figure 4 shows the
holeꢀmobility versus electric field (E1/2) characteristics of DQTPA, BrꢀDQTPA, DTF and
BrꢀDTF. As listed in Table 1, the holeꢀmobilities of BrꢀDQTPA and BrꢀDTF are
respectively 1.1×10ꢀ5 cm2 Vꢀ1sꢀ1 and 3.0×10ꢀ5 cm2 Vꢀ1sꢀ1 at an electric field of 600 (V
cmꢀ1)1/2, which are much higher than that of DQTPA (1.6×10ꢀ6 cm2 Vꢀ1sꢀ1) and DTF
(3.1×10ꢀ6 cm2 Vꢀ1sꢀ1) at the same electric field. These results strongly suggest that the
bromination of DQTPA and DTF can efficiently improve the hole mobilitis. The big
difference in mobility should be primarily associated with the heavy bromine atoms. Similar
with the iodine, bromine substitution can also induce a significant decrease in the local
holeꢀvibration coupling. Thus, the electronic coupling for holes is much larger than the
holeꢀvibration coupling for the Brꢀsubstituted compounds, which leads to an improvement in
hole mobility.[27,29]

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Figure 4. Holeꢀmobility versus electric field characteristics of DQTPA, BrꢀDQTPA, DTF and
BrꢀDTF
As BrꢀDTF shows highest hole mobility among these compounds, DTF and BrꢀDTF were
chose to investigate the influence of bromineꢀsubstitution on film morphology. Figure S1
shows the AFM morphologies of DTF and BrꢀDTF deposited on quartz substrate, the
rootꢀmeanꢀsquare surface roughnesses of DTF and BrꢀDTF films are 0.38 and 0.37 nm,
respectively. Suggesting the bromine atom nearly has no influence on morphology structure
and film forming ability.
Considering the good hole transport abilities of Brꢀsubstituted compounds, we further
evaluated their capability of the HTM in OLEDs. A series of traditional phosphorescent
OLEDs using Ir(ppy)2acac as the green dopant were fabricated with the configuration of
ITO/MoO3 (1 nm)/ DQTPA or BrꢀDQTPA or DTF or BrꢀDTF (40 nm)/CBP: 8%Ir(ppy)2acac
(30 nm)/B3PYMPM (50 nm)/LiF (1 nm)/Al (100 nm). In the devices, MoO3 acts as
holeꢀinjecting layer; DQTPA, BrꢀDQTPA, DTF and BrꢀDTF are the holeꢀtransporting layers
(HTLs), 4,4’ꢀbis(Nꢀcarbazolyl)biphenyl (CBP) is used as the host material;
4,6ꢀbis(3,5ꢀdi(pyridinꢀ3ꢀyl)phenyl)ꢀ2ꢀmethylpyrimidine (B3PYMPM) works as an

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electronꢀtransporting layer as well as a holeꢀblocking layer for its good electronꢀtransporting
ability and low HOMO energy level; ITO and LiF/Al are the anode and cathode, respectively.
The device structures and energy levels of the functional materials are shown in Figure 5. As
shown, both BrꢀDQTPA and BrꢀDTF have high enough LUMO energy level to resist the
electron transfer from the emitting layer to HTL.
Figure 5. Device structures and energy levels of the used materials
Figure 6. EL spectra at 1000 cd/m2 of DQTPAꢀ, BrꢀDQTPAꢀ, DTFꢀ and BrꢀDTFꢀ based devices
Figure 6 is the EL spectra of the above mentioned phosphorescent devices measured at
1000 cd/m2. All of those devices show similar EL spectra and a same emission peak of 528

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nm, which suggests that the generated excitons are well confined in emission layer and the
green emission fully originates from Ir(ppy)2acac. Figure 7 shows the current densityꢀvoltage
curves of those green OLEDs. It is easy to observe that BrꢀDQTPAꢀ and BrꢀDTFꢀbased
devices show higher current density than that of DQTPAꢀ and DTFꢀbased devices, which is in
accordance with hole mobility varying regularity of those materials. Moreover, the
BrꢀDQTPAꢀ and BrꢀDTFꢀbased devices also exhibit much lower turnꢀon voltages (Von) than
DQTPAꢀ and DTFꢀbased devices. Especially, BrꢀDTFꢀbased device owns a very low Von of
2.3 V.
Figure 7. Current densityꢀvoltage curve of DQTPAꢀ, BrꢀDQTPA, DTF and BrꢀDTF based devices;

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Figure 8. Luminous efficiencies at different brightness of the DQTPAꢀ, BrꢀDQTPA, DTFꢀ and
BrꢀDTFꢀ based green phosphorescent OLEDs
Figure 8 presents the power efficiency and EQE versus luminance curves of four green
phosphorescent OLEDs. BrꢀDQTPAꢀ and BrꢀDTFꢀbased devices show extremely high EQEs
and power efficiencies (over 21% and 90 lm/W) without light outcoupling enhancement,
proving both two Brꢀsubstituted compounds can well act as the HTM in the OLEDs. For
comparison, DQTPAꢀ and DTFꢀbased green phosphorescent devices show the lower
efficiencies of 15%, 63.3 lm/W and 15.9%, 56.0 lm/W for EQE and power efficiency,
respectively, which should be ascribed to the poorer hole mobilities of DQTPA and DTF. To
further evaluate the hole transport ability of Brꢀsubstituted compounds, green phosphorescent
OLEDs without hole injection layer MoO3 have been fabricated as follows: ITO/ DQTPA or
BrꢀDQTPA or DTF or BrꢀDTF (40 nm)/ CBP: 8% Ir(ppy)2acac (30 nm)/ B3PYMPM ( x nm)/
LiF (1 nm)/Al (100 nm), x ranged from 30 nm to 60 nm. Here, the thickness of B3PYMPM is
adjusted for charge injected balance. Figure S2 show the EQEs versus luminance
characteristics of these devices, it’s noteworthy that although the Von of these devices show
little increase than the devices with MoO3, the luminous efficiencies of the devices reveal

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great enhancement than that device before. Especially the device using BrꢀDTF as HTL bursts
a maximum efficiency of 27% (90.4 cd/A, 109.2 lm/W). All the device data are listed in Table
S1. Furthermore, to compare the performance of BrꢀDQTPA and BrꢀDTF with the commonly
used hole transport materials, such as (4,4’ꢀcyclohexylidenebis[N,Nꢀbis(4ꢀmethylphenyl)
aniline]) TAPC and (4,4’,4’’ꢀtris(Nꢀcarbazolyl)triphenylamine) TCTA, control devices using
TAPC and TCTA as HTLs were constructed. The EQEs curves of the control devices are
revealed in Figure S3, TAPC and TCTA based control devices respectively show maximum
efficiency of 20.1% (74.3 cd/A, 80.4 lm/W) and 20.7% (75.5 cd/A, 90.9 lm/W), which are
lower than Brꢀsubstituted compoundsꢀbased devices, indicating BrꢀDQTPA and BrꢀDTF are
excellent HTMs.
Table 2. Summary of the green phosphorescent devices in this work
Voltage
@1 cd/m2
CE (cd/A) / PE (lm/W) / EQE (%)
CIE
@1000 cd/m2
HTL of Device
Maximum
100 cd/m2
1000 cd/m2
DQTPA
BrꢀDQTPA
DTF
3.0
2.4
2.7
2.3
59.6/56.0/15.9 59.0/45.7/15.4 46.3/26.5/12.6
73.3/92.1/21.4 73.2/68.0/19.5 62.8/40.0/16.7
60.5/63.3/15.9 57.0/49.7/14.9 38.9/23.7/10.0
78.7/98.9/21.0 77.2/79.8/20.5 68.4/50.0/18.2
(0.35, 0.62)
(0.35, 0.62)
(0.35, 0.62)
(0.35, 0.62)
BrꢀDTF
Conclusion
In summary, we designed and synthesized two novel Brꢀsubstituted HTMs, BrꢀDQTPA
and BrꢀDTF in this work. Both the two compounds show good thermal stability in DSC and
TGA measurements. The holeꢀmobilities of BrꢀDQTPA and BrꢀDTF are calculated to be

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1.1×10ꢀ5 cm2 Vꢀ1sꢀ1 and 3.0×10ꢀ5 cm2 Vꢀ1sꢀ1 at electric field of 600 (V cmꢀ1)1/2 respectively,
which show an order of magnitude enhancement than that of DQTPA (1.6×10ꢀ6 cm2 Vꢀ1sꢀ1)
and DTF (3.1×10ꢀ6 cm2 Vꢀ1sꢀ1) at the same electric field. Moreover, traditional green
phosphorescent OLEDs that using Ir(ppy)2acac as the dopant were fabricated to investigate
the potential of Brꢀsubstituted compounds as the HTMs. Both the devices with BrꢀDQTPA
and BrꢀDTF as HTMs show high EQEs over 21% and high power efficiencies close to 100
lm/W. The results are not only much higher than that of DQTPAꢀ and DTFꢀbased devices
(15.9%, 56.0 lm/W and 15%, 63.3 lm/W), but also higher than that of NPBꢀbased traditional
green device (17.5% and 78.3 lm/W). This work demonstrates the good application of
bromineꢀsubstituted compounds in OLEDs and gives an approach to develop novel HTMs
with improved hole mobility.
Acknowledgement
The work was supported by the National Natural Science Foundation of China (NSFC
Grant Nos. 51373029, 61421002, and 51533005), “Preeminent Youth Fund of Si Chuan
Province (No. 2015JQ0006),” “the Fundamental Research Funds for the Central Universities
(ZYGX2015J048).”
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High performance green phosphorescent OLEDs using novel bromine-substituted compounds as
hole transport layers.