Synthesis of novel triphenylamine-based conjugated polyelectrolytes and their application as hole-transport layers in polymeric light-emitting diodes

Article abstract of DOI:10.1039/b603704f

Alternating triphenylamine-based copolymers poly[N-(4- sulfonatobutyloxyphenyl)-4,4′-diphenylamine-alt-1,4-phenylene] sodium salt (PTPOBS) and poly[N-(4-sulfonatophenyl)-4,4′-diphenylamine-alt-N-(p- trifluoromethyl)phenyl-4,4′-diphenylamine] sodium salt (PTFTS) were synthesized via palladium-catalyzed Suzuki coupling reaction. These polymers are soluble only in polar solvents, such as dimethyl sulfoxide (DMSO) and a mixed solvent of methanol and N,N-dimethyl formamide (DMF), rather than in non-polar solvents such as toluene and xylene. The electrochemical and photophysical properties of the resulted copolymers were investigated. The HOMO levels of the polymers (-5.08 eV for PTPOBS and -5.24 eV for PTFTS) were close to the work function of PEDOT. The relatively high-lying LUMO levels (-2.21 eV for PTPOBS and -2.24 eV for PTFTS) revealed that they had good electron-blocking capabilities. Devices with a PTPOBS or PTFTS layer inserted between ITO or PEDOT and of red and green-emitting polymers showed lower turn-on voltages and enhanced efficiency compared with the reference devices composed of bare ITO or ITO/PEDOT as anode. These polymers can be used as an independent hole-transport/electron-blocking layer or in combination with a PEDOT layer for fabrication of multilayer devices without intermixing with the subsequent EL layer by solution processing in full-color display applications. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b603704f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis of novel triphenylamine-based conjugated polyelectrolytes and
their application as hole-transport layers in polymeric light-emitting diodes
Wei Shi, Suqin Fan, Fei Huang, Wei Yang, Ransheng Liu and Yong Cao*
Received 13th March 2006, Accepted 11th April 2006
First published as an Advance Article on the web 5th May 2006
DOI: 10.1039/b603704f
Alternating triphenylamine-based copolymers poly[N-(4-sulfonatobutyloxyphenyl)-
4,49-diphenylamine-alt-1,4-phenylene] sodium salt (PTPOBS) and poly[N-(4-sulfonatophenyl)-
4,49-diphenylamine-alt-N-(p-trifluoromethyl)phenyl-4,49-diphenylamine] sodium salt (PTFTS)
were synthesized via palladium-catalyzed Suzuki coupling reaction. These polymers are soluble
only in polar solvents, such as dimethyl sulfoxide (DMSO) and a mixed solvent of methanol and
N,N-dimethyl formamide (DMF), rather than in non-polar solvents such as toluene and xylene.
The electrochemical and photophysical properties of the resulted copolymers were investigated.
The HOMO levels of the polymers (25.08 eV for PTPOBS and 25.24 eV for PTFTS) were close
to the work function of PEDOT. The relatively high-lying LUMO levels (22.21 eV for PTPOBS
and 22.24 eV for PTFTS) revealed that they had good electron-blocking capabilities. Devices
with a PTPOBS or PTFTS layer inserted between ITO or PEDOT and of red and green-emitting
polymers showed lower turn-on voltages and enhanced efficiency compared with the reference
devices composed of bare ITO or ITO/PEDOT as anode. These polymers can be used as an
independent hole-transport/electron-blocking layer or in combination with a PEDOT layer for
fabrication of multilayer devices without intermixing with the subsequent EL layer by solution
processing in full-color display applications.
single-layer device configuration is that OLEDs with multiple
Introduction
layer structures allow the recombination zone to be confined
Organic light-emitting diodes (OLEDs) are generally fabri-
cated from ether small molecules,1–3 dendrimers4 or con-
(electron-blocking) and electron-transport (hole-blocking)
in a very narrow emission layer between the hole-transport
jugated polymers.5 Generally, OLEDs are thin-film multilayer
layers. A challenge to the fabrication of multilayer devices
structures composed of a hole-transporting, an emitting,
via solution processing is that the application of subsequent
and an electron-transporting layer sandwiched between two
layers may result in the erosion of the layers that have been
electrodes. Charge carriers (holes and electrons) are injected
applied in the previous steps. One of the solutions to the
and transported from the anode and cathode, and are
fabrication of erosion-resistant HTLs is to make such
recombined in the emitting layer to emit light.6 It is highly
materials insoluble by thermal or photochemical crosslinking
desirable to develop a suitable hole-transporting material
prior to the processing of the emitting layer. Varieties of
(HTM) that possesses good mechanical, thermal and electro-
photo-crosslinkable11 and thermally crosslinkable12 HTMs
chemical properties. The HTM facilitates the hole injection
have been synthesized recently. An alternative solution is to
and transport from the anode and also in many cases serve as
use HTMs which are processable in solvents in which emitting
an electron-blocking layer which blocks the electron move-
polymers are not soluble. A typical example is the widely-used
ment to the anode and confine excitons within the emissive
anode buffer PEDOT-PSS,13 which is
a microemulsion
layer.1,7 Triarylamine (TAA) derivatives, small molecules8 or
polymers with a TAA unit in the polymer backbone9a–c or in
pendant9d–f groups, are one of the most widely-used hole-
transporting materials because they are easily oxidized to form
stable radical cations.10
in water. Recently, Gong et al.14 reported ethanol-soluble,
sulfonated poly(N-vinylcarbazole) (PVK) and PVK-SO3Li,15
which can be used as the hole-transport layer in a multilayer
white-light emission PLED.
In this paper, we report the synthesis of two sulfonate
substituted polytriphenylamine polyelectrolytes, which are
soluble only in polar solvents. The redox potentials of the
Polymer light-emitting diodes (PLEDs) have recently
attracted researchers’ intense interest due to their possible
applications in large-area flat panel displays based on solution
printing techniques. The advantage of OLEDs with multilayer
device configuration, typically fabricated by thermal deposi-
tion, over polymer light-emitting devices (PLEDs) with typical
two polymers can be adjusted by the attachment of electron-
2
donating (butyloxy) or electron-withdrawing (–CF3, –SO3
)
groups to the main chain. Excellent hole-transporting
and electron-blocking properties in combination with good
solubility in polar solvents make them promising
candidates for hole transport layers in the fabrication of
high-efficiency multilayer polymer light-emitting devices and
displays.
Institute of Polymer Optoelectronic Materials and Devices, Key
Laboratory of Special Functional Materials and Advanced
Manufacturing Technology, South China University of Technology,
Guangzhou 510640, China. E-mail: poycao@scut.edu.cn
This journal is ß The Royal Society of Chemistry 2006
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ethanol was added. After this mixture was refluxed for 1 h, 1,4-
butanesulfonate (0.82 g, 6.0 mmol) was added via syringe. The
mixture was stirred at reflux in a nitrogen flow for 24 h. The
product precipitated after being cooled to room temperature.
The filtrate was recrystallized from absolute ethanol to give a
Experimental
Materials
All manipulations involving air-sensitive reagents were per-
formed under an atmosphere of dry nitrogen. All reagents,
unless otherwise specified, were obtained from Aldrich,
Acros, and TCI Chemical Co. and were used as received.
All the solvents used were further purified before use.
1
white powder (1.6 g, 58%). H NMR (300 MHz, DMSO-d6) d
(ppm): 7.44–7.38 (d, 4H, aromatic H), 7.03–7.01 (d, 2H,
aromatic H), 6.95–6.92 (d, 2H, aromatic H), 6.89–6.86 (d, 4H,
aromatic H), 3.94–3.92 (t, 2H, –OCH2–), 2.46–2.45 (t, 2H,
–CH2SO3Na), 1.77–1.70 (m, 4H, –CH2–CH2–), 13C NMR
(75 MHz, DMSO-d6) d (ppm): 156.75, 147.13, 139.27, 132.75,
128.34, 124.63, 116.38, 114.24, 68.14, 51.64, 28.61, 22.48.
Elem. Anal. Calcd for C22H20Br2NO4SNa?1.0H2O: C, 44.37;
H, 3.70; N, 2.35; S, 5.38. Found: C, 44.40; H, 3.70; N, 2.06;
S, 6.09%.
N,N-Di-p-bromophenyl-(p-trifluoromethyl)aniline,16
N,N-
bis(4-bromophenyl)aniline17 and poly(9,9-dioctylfluorene-co-
4,7-dithien-2-yl-2,1,3-benzothiadiazole (PFO-DBT15)18 were
prepared according to the published procedures. Poly(9,9-
dioctylfluorene-co-2,1,3-benzothiadiazole) (PFO-BT15) was
synthesized using a procedure similar to that described in
the literature.12b
4-(N,N-Bis(4-bromophenyl)amino)benzene sulfonic sodium
salt (4). N,N-Bis(4-bromophenyl)aniline (2.27 g, 5.63 mmol)
was dissolved in 35 ml of dichloromethane. Added dropwise to
this solution was chlorosulfonic acid (0.5 ml, 7.319 mmol) at
room temperature. The reaction mixture was stirred at a room
temperature for 1 h and then was poured into crushed ice. 50%
NaOH (aq) was added to this mixture until pH > 9. The
precipitate was filtered off and recrystallized from distilled
water, then vacuum dried at 80 uC for 24 h to a give silvery
4-Methoxy-N,N-bis(4-bromophenyl)aniline (1). Toluene
(50 ml), 4-methoxyaniline (6.16 g, 50.0 mmol), 1-bromo-4-
iodobenzene (31.8 g, 110.0 mmol), phenanthroline (0.337 g,
1.87 mmol), cuprous chloride (CuCl) (0.196 g, 1.87 mmol) and
potassium hydroxide (KOH) (flakes, 86%, 26.8 g, 391 mmol)
were added sequentially to a 250 ml three-necked flask
equipped with condenser, magnetic stirrer, nitrogen inlet and
outlet under nitrogen. The reaction mixture was heated to
reflux in 30 min and was stirred at reflux for 48 h. Then the
mixture was cooled to 75 uC, 150 ml of toluene and 100 ml of
distilled water were used in extraction. The toluene phase was
separated, dried with anhydrous magnesium sulfate (MgSO4),
filtered and vacuum distilled. The crude product was purified
by silica column chromatography using petroleum ether–ethyl
acetate (19 : 1 v/v) as eluent to give 1 (8 g, 37%) as a colorless
viscous liquid. 1H NMR (300 MHz, CDCl3) d (ppm): 7.34–
7.31 (d, 4H, aromatic H), 7.07–7.04 (d, 2H, aromatic H), 6.93–
6.90 (d, 4H, aromatic H), 6.89–6.86 (d, 2H, aromatic H), 3.83
(s, 3H, –OCH3), 13C NMR (75 MHz, CDCl3) d (ppm): 156.75,
146.83, 139.74, 132.17, 127.41, 124.48, 115.04, 114.56, 55.50
(–OCH3).
1
white solid (1.51 g, 53%). H NMR (300 MHz, DMSO-d6) d
(ppm): 7.51–7.49 (d, 2H, aromatic H), 7.44–7.40 (d, 4H,
aromatic H), 6.93–6.91 (d, 2H, aromatic H), 6.92–6.89 (d, 4H,
aromatic H), 13C NMR (75 MHz, DMSO-d6) d (ppm): 146.93,
146.70, 144.44, 133.01, 127.83, 126.17, 123.82, 115.51. Elem.
Anal. Calcd for C18H12Br2NO3SNa?1.4H2O: C, 40.74; H, 2.79;
N, 2.64; S, 6.04. Found: C, 40.98; H, 2.80; N, 2.46; S, 6.43%.
N,N-Bis[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phe-
nyl]-(p-trifluoromethyl)aniline (5). In a 250 ml three-necked
flask in an argon flow, N,N-di-p-bromophenyl-(p-trifluoro-
methyl)aniline (9.0 g, 19.05 mmol) was dissolved in 120 ml of
anhydrous tetrahydrofuran (THF). The solution was cooled to
278 uC, and then n-butyllithium solution (n-BuLi) (1.6 M in
hexane, 25 ml, 40 mmol) was slowly added. The reaction
mixture was stirred at this temperature for 2 h. Then
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (30 ml,
128.6 mmol) was quickly added via syringe. The resulting
mixture was stirred at 278 uC for 2 h, and then was slowly
warmed to room temperature and stirring continued for 48 h.
Distilled water (50 ml) was added to the reaction mixture
which was extracted using dichloromethane (3 6 100 mL).
The organic phase was separated and dried over MgSO4
overnight. The solvent was distilled, and the crude product
was purified by recrystallization from hexane–ethyl acetate
(19 : 1 v/v) to give a white solid (5.7 g, 53%). 1H NMR
(300 MHz, CDCl3) d (ppm): 7.76–7.73 (d, 4H, aromatic H),
7.48–7.45 (d, 2H, aromatic H), 7.15–7.12 (d, 2H, aromatic
ring), 7.12–7.09 (d, 4H, aromatic H), 1.36 (s, 24H, –CH3). 13C
NMR (75 MHz, CDCl3) d (ppm): 150.23, 149.31, 136.14,
126.39, 126.34, 126.07, 124.19, 123.92, 123.00, 83.78, 24.87.
4-Hydroxy-N,N-bis(4-bromophenyl)aniline (2). A solution of
boron tribromide (BBr3) (1.75 ml, 18.5 mmol) in 18.5 ml of dry
dichloromethane (CH2Cl2) was added dropwise to a solution
of 1 (5.3 g, 12.2 mmol) in 90 ml of dry CH2Cl2 at room
temperature. After being stirred at room temperature over-
night, the mixture was slowly poured into 250 ml of distilled
water and stirred for another hour. The organic phase was
separated and the water phase was extracted by CH2Cl2 until
colorless. The organic phase was combined and dried over
MgSO4. Column chromatography using silica gel and petro-
leum ether–CH2Cl2 (1 : 3 v/v) as eluent to afford 2 (4.5 g, 88%)
as pale green solids. 1H NMR (300 MHz, CDCl3) d (ppm):
7.34–7.31 (d, 4H, aromatic H), 7.01–6.98 (d, 2H, aromatic H),
6.92–6.89 (d, 4H, aromatic H), 6.82–6.79 (d, 2H, aromatic H),
5.25 (s, 1H, –OH), 13C NMR (75 MHz, CDCl3) d (ppm):
152.69, 146.80, 139.89, 132.19 127.61, 124.30, 116.57, 114.61.
4-N,N-Bis(4-bromophenyl)amino-1-(sulfonatobutyloxy)ben-
zene sodium salt (3). Added into a 100 ml three-necked round-
bottomed flask were 2 (2.0 g, 4.8 mmol) and sodium methoxide
(0.38 g, 7 mmol) under nitrogen and then 45 ml of absolute
Poly[N-(4-sulfonatobutyloxyphenyl)-4,49-diphenylamine-alt-
1,4-phenylene] sodium salt (PTPOBS). Added into a 250 ml
2388 | J. Mater. Chem., 2006, 16, 2387–2394
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three-necked flask was 4-N,N-bis(4-bromophenyl)amino-1-
(sulfonatobutyloxy)benzene sodium salt (3) (0.866 g,
1.5 mmol), 1,4-phenyldiboronic acid (0.249 g, 1.5 mmol),
Na2CO3 (1.5 g, 14.2 mmol) and Pd(OAc)2 (palladium acetate)
(0.010 g, 3 mol%) under nitrogen. The content was purged with
nitrogen for 30 min, then 70 ml of distilled water and 30 ml of
DMF were injected by syringe after 1 hour deaeration. The
reaction mixture was heated to 85–90 uC and stirred for 48 h.
After being cooled to room temperature, the mixture was
poured into 1 L of acetone. The precipitate was collected and
redissolved into a minimum amount of DMSO, filtered off and
dialyzed using a membrane with an 8000 molecular weight
cutoff for 3 days. Water was vacuum distilled and the final
product was obtained after drying in vacuum at 80 uC for 24 h
(0.25 g, 40%). 1H NMR (300 MHz, DMSO-d6) d (ppm): 7.70–
7.64 (m, 8H, aromatic H), 7.08–6.98 (m, 8H, aromatic H), 3.97
(m, 2H, –OCH2–), 2.50 (m, 2H, –CH2SO3Na), 1.77–1.70 (m,
4H, –CH2–CH2–), 13C NMR (75 MHz, DMSO-d6) d (ppm):
156.50, 147.40, 139.84, 138.55, 133.52, 128.37, 127.94, 127.11,
123.09, 116.29, 68.14, 51.69, 28.66, 22.50. Elem. Anal. Calcd
for C28H24NO4SNa?0.8H2O: C, 66.22; H, 5.05; N, 2.76; S,
6.31. Found: C, 65.00; H, 6.08; N, 3.45; S, 6.10%.
performed with a multi-angle photometer (DAWN-DSP,
Wyatt Technology Co.) at 633 nm and at 25 uC with DMSO
eluent at a flow rate of 1.0 ml min21. Elemental analyses were
performed on a Vario EL Elemental Analysis Instrument
(Elementar Co.). UV-visible absorption spectra were recorded
on a HP 8453 UV-Vis spectrophotometer. Cyclic voltammetry
was carried out on a CHI660A electrochemical workstation
with platinum electrodes at a scan rate of 50 mV s21 against a
saturated calomel reference electrode with a nitrogen-saturated
solution of 0.1 M tetrabutylammonium hexafluorophosphate
(Bu4NPF6) in acetonitrile (CH3CN). Thermogravimetric
analyses (TGA) were conducted on a NETZSCH TG 209
at a heating rate of 10 uC min21 and a nitrogen flow rate of
20 mL min21
.
LED fabrication and characterization
PLEDs with the triphenylamine-based polyelectrolytes as HTL
were investigated. The device structures were ITO/(PEDOT)/
HTL/emissive polymer/Ba/Al. Patterned indium tin oxide
(ITO) coated glass substrates were cleaned with acetone,
detergent, distilled water and isopropanol in an ultrasonic
bath, and were treated with oxygen plasma before use. If
necessary, an 80 nm layer of PEDOT was spin-coated on the
cleaned ITO from an aq. emulsion of PEDOT : PSS (Baytron
P, Bayer A.G.) [poly(3,4-ethylenedioxythiophene) (PEDOT)
doped with poly(styrenesulfonic acid) (PSS)] followed by
drying in a vacuum oven at 80 uC for 8 h. About 1 wt% of
hole-transport materials PTPOBS and PTFTS were dissolved
in the mixed solvent of methanol and DMF (CH3OH : DMF =
4 : 1 v/v), filtered through a 0.45 mm filter and spin-coated on
top of ITO or PEDOT inside a dry box. The film thicknesses
of the polyelectrolytes were around 40 nm, as measured
with a Tencor Alfa step 500 surface profiler. Emissive
Poly[N-(4-sulfonatophenyl)-4,49-diphenylamine-alt-N-(p-tri-
fluoromethyl)phenyl-4,49-diphenylamine] sodium salt (PTFTS).
Added into a 250 ml three-necked flask were 4-(N,N-bis(4-
bromophenyl)amino)benzene sulfonic sodium salt (4) (1.01 g,
2.0 mmol), N,N-bis[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)phenyl]-(p-trifluoromethyl)aniline (5) (1.132 g, 2.0 mmol),
Na2CO3 (2.0 g, 18.9 mmol), 95 ml of distilled water, 40 ml of
DMF and 25 ml of THF. Into the mixture was added
Pd(OAc)2 (0.014 g, 3 mol%) after being purged with nitrogen
for 30 min. The reaction was carried out at 85–90 uC and
vigorously stirred for 24 h. The second portion (0.014 g) of
Pd(OAc)2 was added and stirring continued at 85–90 uC for
another 24 h. After being cooled to room temperature, the
mixture was poured into 1 L of acetone. The precipitate was
collected and redissolved into a minimum amount of DMSO,
filtered off and dialyzed using a membrane with an 8000
molecular weight cutoff for 3 days. Water was vacuum distilled
and the final product was obtained after drying in vacuo at
80 uC for 24 h (1.25 g, 40%). 1H NMR (300 MHz, DMSO-d6) d
(ppm): 7.65–7.56 (m, 12H, aromatic H), 7.22–7.02 (m 12H,
aromatic H), 13C NMR (75 MHz, DMSO-d6) d (ppm): 151.01,
147.36, 146.62, 145.40, 143.71, 136.20, 134.40, 128.15, 128.00,
127.59, 127.09, 126.18, 124.48, 123.53, 123.26, 121.84, 121.20.
Elem. Anal. Calcd for C37H24F3N2O3SNa?1.3H2O: C, 65.35;
H, 3.92; N, 4.12; S, 4.71. Found: C, 64.19; H, 4.82; N, 4.12;
S, 5.05%.
material
poly(9,9-dioctylfluorene-co-4,7-dithien-2-yl-2,1,3-
benzothiadiazole) (PFO-DBT15, DOF : DBT = 85 : 15)
and poly(9,9-dioctylfluorene-co-2,1,3-benzothiadiazole) (PFO-
BT15, DOF : BT = 85:15) were dissolved in toluene, filtered
through a 0.45 mm filter and spin-coated onto the anode to
form a film of about 80 nm. Ba and Al layers were vacuum-
evaporated on the top of an EL polymer in a vacuum of 1 6
1024 Pa. To reveal the hole-transport effect caused by these
two polyelectrolytes, devices without HTL having structures
ITO/emissive polymer (80 nm)/Ba/Al, and ITO/PEDOT : PSS
(80 nm)/emissive polymer (80 nm)/Ba/Al were made as
references. The current density–voltage–luminance (J–V–L)
characteristics were measured by a Keithley 236 source-
measurement unit and a calibrated silicon photodiode which
was calibrated for luminance by a PR-705 SpectraScan
Spectrophotometer (Photo Research). The external EL quan-
tum efficiency (QE) was collected by measuring the total light
output in all directions in an integrating sphere (IS-080,
Labsphere).
Chemical characterization
The 1H and 13C NMR spectra were collected on a Bruker
DRX 300 spectrometer operating at 300 MHz (for 1H) and
75 MHz (for 13C), in deuterated chloroform or DMSO
solution with tetramethylsilane as reference. Number-average
(Mn) and weight-average (Mw) molecular weights were deter-
mined by multi-angle laser light scattering combined with
size exclusion chromatography (MALLS-SEC), which was
Results and discussion
Synthesis and characterization
The synthesis route is shown in Scheme 1. Compound 1 was
synthesized by the improved Ullmann reaction using the
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Scheme 1 Synthesis route of the monomers and polymers and chemical structure of PLED emissive layers, PFO-DBT15 and PFO-BT15.
catalyst system composed of phenanthroline, CuCl and KOH,
which was used to synthesize triphenylamine derivatives
according to Goodbrand and Hu.19 By the treatment of 1
with BBr3 in dry dichloromethane, 2 was obtained as pale
green solid in 88% yield after column chromatography
using silica gel and petroleum ether–CH2Cl2 (1 : 3 v/v) as
eluent. Compound 2 was alkoxylated by 1,4-butanesulfonate
in the presence of sodium methoxide in refluxing absolute
ethanol to give monomer 3. N,N-Bis(4-bromophenyl)aniline
reacted with chlorosulfonic acid in CH2Cl2 at a room
temperature, and was then neutralized by aqueous NaOH
to give monomer 4. Several attempts to induce sulfonic
groups by using conc. H2SO4 (98%) were unsuccessful. Due
to the existence of the sulfonate group in the side chain,
monomers 3 and 4 are not soluble in most common organic
non-polar solvents, such as chloroform, toluene and xylene,
but are soluble in polar solvents, such as water, DMF
and DMSO. N,N-Di-p-bromophenyl-(p-trifluoromethyl)ani-
line was first lithiated by n-BuLi in dry THF and
then reacted with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane to give the boronic acid ester substituted
product 5 in 53% yield.
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The polymers were synthesized by Pd(OAc)2-catalyzed
Suzuki cross-coupling polymerization as described by Child
and Reynolds20 and Kim et al.21 for the preparation of
poly(p-phenylene) analogues. A mixture of an 0.2 M aqueous
solution of Na2CO3 and DMF (70 : 30 v/v) was utilized as
the polymerization medium. The reactions were conducted at
85–90 uC for 48 h. A difference in the synthesis of PTPOBS
and PTFTS is that, since N,N-bis[4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl]-(p-trifluoromethyl)aniline (5) is not
soluble in the reaction mixture, we added some THF to
dissolve monomer 5 and make its hydrolysis faster. Another
portion of Pd(OAc)2 was added to complete the reaction in the
synthesis of PTFTS, similar to what Balanda et al. described
for the synthesis of cationic poly(p-phenylene).22 The resulting
polymers were precipitated in acetone after being cooled to
room temperature. The precipitates were redissolved in DMSO
and filtered to remove the Pd particles. The filtrate was
dialyzed using a membrane with a cut-off of 8000 molecular
weight for 3 days. In this process we changed fresh distilled
water several times to remove DMSO, ionic impurities and low
molecular weight oligomers. The final products were obtained
by the evaporation of the water. The yield was about 40%
after both polymers were dried in a vacuum oven at 80 uC. The
obtained polymers PTPOBS and PTFTS were readily dis-
solved in DMSO and in a mixture of methanol–DMF, but they
were insoluble in non-polar organic solvents, such as chloro-
form, toluene, xylene and chlorobenzene. As they can with-
stand the erosion of commonly-used non-polar organic
solvents, these two polymers can efficiently avoid interface
mixing when they are used as HTL during the fabrication of
multilayer PLEDs. Multi-angle laser light scattering combined
with size exclusion chromatography (MALLS-SEC) analysis23
showed the average molecular weights of Mn = 36 580 (¡3%)
and Mw = 61 400 (¡4%) for PTPOBS, Mn = 11 600 (¡8%)
and Mw = 26 ,820 (¡4%) for PTFTS.
Fig. 1 Thermogravimetric analysis of PTPOBS and PTFTS in a
nitrogen atmosphere.
blue-shifted. The main absorption peaks of PTPOBS and
PTFTS were 376 and 357 nm, respectively. The optical band
gaps of the copolymers estimated from the onset of the UV-vis
absorption band (432 nm for PTPOBS, 415 nm for PTFTS)
are 2.87 and 2.99 eV, respectively. The two polymers emit blue
light when excited by UV light. The PL spectra reached
maxima at 450 nm for PTPOBS and at 425 nm for PTFTS.
There are two possible explanations for the blue-shift of
the absorption and PL spectra of PTFTS compared with
PTPOBS. First, this shows that the conjugation length of
the PTPOBS backbone is longer than that of PTFTS. For
PTPOBS the triphenylamine groups are connected to the
phenylene group alternately in the polymer backbone, which
may result in a longer p-conjugation than PTFTS without
phenylene moieties in the backbone. Then, two electron-
withdrawing groups (–CF3 and –SO32) are directly connected
to the polymer main chain, resulting in greater reduction of
the electron density of the PTFTS backbone than that of
PTPOBS.
The chemical structures of all the monomers and polymers
1
The electrochemical behavior of the polymers was investi-
gated by cyclic voltammetry (CV). The CV was performed in a
solution of Bu4NPF6 (0.1 M) in acetonitrile at a scan rate of
50 mV s21 at room temperature under argon. A platinum
electrode was used as the working electrode, a Pt wire was used
as the counter electrode, and a saturated calomel electrode was
used as the reference electrode. As shown in Fig. 3, for
were verified by H, 13C NMR and elemental analysis.
The thermal properties of these two polymers were
investigated using thermogravimetric analysis (TGA). We note
that there is always some water remaining in both polymers,24
even if the polymers were dried for a long time in a vacuum
at 100 uC. The amounts of water in the polymers were
estimated at about 3% for both polymers by TGA. Fig. 1
shows the thermograms of these two polymers. The tempera-
ture ranges from 25 to 800 uC in a nitrogen atmosphere. There
is a small amount of water loss at a lower temperature for
both polymers.24 The onset of degradation temperatures for
PTPOBS and PTFTS are about 237 and 233 uC, which is
associated with desulfonation.21
Optical and electrochemical properties
Transparent uniform films of the polymers were prepared on
quartz substrates by spin-casting from a mixed solvent DMF–
MeOH (1 : 4 v/v) solution of PTPOBS and PTFTS. Fig. 2
shows UV-visible absorption and PL spectra of the two
polymers in the solid form under 325 nm excitation from a
HeCd laser diode. Compared with PTPOBS, both the absorp-
tion and PL emission of PTFTS copolymer are significantly
Fig. 2 UV-vis absorption and PL emission spectra of PTPOBS and
PTFTS copolymers in solid-state films.
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Fig. 4 Energy-level diagram of PLED components; the band-edge
energy levels were determined by CV or taken from the literature.
Fig. 3 Cyclic voltammograms of the polymer films coated on
platinum electrodes in 0.1 mol L21 Bu4NPF6, CH3CN solution.
PTPOBS the oxidative process started at around 0.68 V and
gave a sharp oxidative peak at 1.10 V. The oxidation is
reversible, and the corresponding deoxidation peak appears at
0.70 V. The CV behavior of PTPOBS, including the oxidation
onset, oxidation and deoxidation peaks and CV curve profile,
is very similar to that of poly[N-(4-butylphenyl)amino(1,19 :
49,10-terphenyl-49,40-ylene)] (PBPITP),9b which has a similar
chemical structure to PTPOBS. Similar to PTPOBS, one
oxidation peak was observed during the anodic scan of the
PTFTS copolymer. The oxidation started at around 0.83 V
with a peak around 1.40 V, highly reversible and was
deoxidized at 0.70 V. The energy levels of the highest occupied
molecular orbitals (HOMOs) of PTPOBS and PTFTS were
estimated from the onset of the oxidation wave, calculated
Fig. 5 EL spectra of red and green emitting PLEDs with PTPOBS or
PTFTS as HTL, structure ITO/(PEDOT : PSS)/(PTPOBS or PTFTS)/
PFO-DBT15 or PFO-BT15/Ba/Al.
according to an empirical formula, EHOMO = 2e(Eox
+
(PFO-BT15) emitting polymers (see Scheme 1) were used to
fabricate multilayer devices with PTPOBS and PTFTS as
HTL on the top of bare ITO or PEDOT : PSS. Fig. 4 shows
the energy-level diagram of different layers in the PLEDs
composed of these materials.
4.4) (eV),25 to be 25.08 and 25.23 eV, respectively. In the
cathodic scan, no reduction peak (down to 22.5 V vs. SCE)
was observed for both PTPOBS and PTFTS copolymers. The
level of the lowest unoccupied molecular orbital (LUMO) can
be estimated by subtracting the optical band gap energy from
the HOMO, as determined by electrochemistry. The LUMO
levels of PTPOBS and PTFTS were respectively estimated at
22.21 and 22.24 eV. The relatively high-lying LUMO levels
reveal that it is difficult to inject electrons to these two
polymers. In other words, they may serve as an electron-
blocking layer when they are inserted between an emission
layer and an anode.
Fig. 5 shows EL spectra of PFO-DBT15 and PFO-BT15 in
different device configurations. The similar EL emission for
both EL emitting polymers, regardless of HTL material used,
indicates that the recombination zone is always confined in the
emissive layer when PTPOBS and PTFTS are inserted between
the emissive layer and anode.
Fig. 6a compares the external quantum efficiency of red-
emitting devices based on PFO-DBT15 with PTPOBS or
PTFTS inserted between ITO or PEDOT and the emissive
layer. Both PTPOBS and PTFTS enhance the device efficiency
significantly compared with the devices only composed of ITO
or PEDOT without HTL (PTPOBS or PTFTS). It is important
to note that the efficiency of a device with PTPOBS and
PTFTS as HTL on the top of ITO substrate without PEDOT
layer is much higher than that of a device with the very well
The UV-visible absorption, electrochemical and photo-
luminescence properties of these polymers in solid films are
summarized in Table 1.
Electroluminescence properties
To verify the hole-injection and transport properties of these
new hole-transport polymers, red (PFO-DBT15) and green
Table 1 UV-Vis absorption, electrochemical, photoluminescence properties of the polymers (in solid films)
Polymers
labsmax/nm
Optical band gapa/eV
Eox/V
EHOMO/eV
ELUMOb/eV
lPLmax/nm
PTPOBS
PTFTS
a
376
357
2.87
2.99
0.68
0.83
25.08
25.23
22.21
22.24
450
425
b
Estimated from the onset wavelength of the optical absorption in the solid state film. Calculated from the HOMO level and
optical band gap.
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Fig. 6 Device characteristics of the red-emitting PLEDs with
structure ITO/(PEDOT)/(PTPOBS or PTFTS)/PFO-DBT15/Ba/Al.
(a) External quantum efficiency–bias (V) characteristics. (b)
Brightness–bias (V) characteristics.
investigated PEDOT layer, although their HOMO levels are
close. This may be because the relatively high-lying LUMO
levels of PTPOBS and PTFTS served as an electron-blocking
layer. The LUMO energy gap between PFO-DBT15 and
PTPOBS, PTFTS was about 1.3 eV; this energy barrier
blocks the movement of electrons to the anode and confine
electrons in the emissive layer. Consequently, the hole–electron
recombination took place in a more balanced way in the
emissive layer resulting in high efficiency (Fig. 6, 7). Since
PEDOT has as LUMO of around 3.4 eV (Fig. 4), electrons
can easily move from the emissive layer across the PEDOT
layer reaching the anode.
Fig. 7 Device characteristics of the green-emitting PLEDs with
structure ITO/(PEDOT)/(PTFTS)/PFO-BT15/Ba/Al. (a) External
quantum efficiency–current density characteristics. (b) Brightness–
current density characteristics.
layer as HTL, giving a saturated red emission with luminance
of 2000 cd m22 at 7.6 V and CIE coordinate of (0.68, 0.31)
with an external QE over 1%. In Table 2 we compare the
device performance with different HTLs at a current density of
around 33 mA cm22
.
Fig. 6b shows luminance vs. bias for the same devices as
in Fig. 6a. The turn-on voltage (turn-on voltage is defined
as the voltage required for a device to reach a luminance of
y1 cd m22) is 6.2 V for the device with bare ITO as anode.
When a HTL was insert between ITO and the EML, the turn-
on voltage was reduced significantly to 4.4 V for PTPOBS and
3.2 V for PTFTS. We note that the turn-on voltage with
PTFTS is also lower than that of the device with PEDOT
(3.6 V) as anode buffer layer. The lower turn-on voltage is due
to the reduced barrier height in the anode–EM polymer
interface as a result of the insertion of PTPOBS or PTFTS
layers. As can be seen from the energy-level diagram of PLED
components shown in Fig. 4, the energy barrier for holes
directly injected from ITO to the PFO-DBT15 emissive layer is
about 0.8 eV. This relatively large energy level mismatch
makes hole injection difficult. By the insertion of a hole-
transport layer, the hole injection barrier height was reduced
and hence hole injection was enhanced. The best device
performance was realized for a device with a PEDOT/PTFTS
Similar results were obtained for the green-emitting poly-
mer, PFO-BT15, with PTFTS as HTL. Fig. 7a shows that a
significant improvement in device efficiency can be observed
for the device with a PTFTS layer on top of ITO or PEDOT,
as in the case of red-emitting polymers. The device with
Table 2 Device performance of the red-emitting (PFO-DBT15)
devices at a current density of about 33 mA cm22
Device performancea
Device structure Bias/V I/mA cm22 Luminance/cd m22 QEext (%)
PTPOBS
PTFTS
PEDOT/PTPOBS 7.9
PEDOT/PTFTS 5.2
Bare ITO
PEDOT : PSS
8.3
6.9
36.5
33.7
33.1
31.3
34.8
37.3
201
176
165
160
38
1.5
1.4
1.3
1.4
0.3
0.9
8.9
5.4
124
a
Device active area 0.15 cm2.
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Table 3 Device performance of the green-emitting (PFO-BT15)
devices at a current density of about 33 mA cm22
References
1 C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett., 1987, 51, 913.
2 M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
3 B. W. D’Andrade, M. A. Baldo, C. Adachi, J. Brooks,
M. E. Thompson and S. R. Forrest, Appl. Phys. Lett., 2001, 79,
1045.
4 J. P. J. Markham, S.-C. Lo, S. W. Magennis, P. L. Bum and
I. S. W. Samuel, Appl. Phys. Lett., 2002, 80, 2645.
5 J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks,
K. Mackay, R. H. Friend, P. L. Burn and A. B. Holmes, Nature,
1990, 347, 539.
Device performancea
Device structure Bias/V I/mA cm22 Luminance/cd m22 QEext (%)
PTFTS 9.4
PEDOT/PTFTS 7.5
35.3
32.2
34.1
38.4
1357
843
31
2.3
1.5
0.054
1.1
Bare ITO
PEDOT : PSS
9.2
5.9
738
a
Device active area 0.15 cm2.
6 M. Strukelj, F. Papadimitrakopoulos, T. M. Miller and
L. J. Rothberg, Science, 1995, 267, 1969.
PTFTS alone on top of the ITO substrate exhibits the most
impressive performance with a turn-on voltage of 3.7 V
and luminance of 5150 cd m22 at a current density of
154 mA cm22(Fig. 7b), the maximum EL quantum efficiency
reaching 2.4% at 7.2 V. Table 3 summarizes the device
performance of green-emitting polymers with different HTL
7 C. Adachi, T. Tsutsui and S. Saito, Appl. Phys. Lett., 1990, 57, 531.
8 (a) C. W. Tang, S. A. VanSlyke and C. H. Chen, J. Appl. Phys.,
1989, 65, 3610; (b) C. Adachi, T. Tsutsui and S. Saito, Appl. Phys.
Lett., 1989, 55, 1489; (c) S. A. VanSlyke, C. H. Chen and
C. W. Tang, Appl. Phys. Lett., 1996, 69, 2160.
9 (a) M. Nomura, Y. Shibasaki, M. Ueda, K. Tugita, M. Ichikawa
and Y. Taniguchi, Macromolecules, 2004, 37, 1204; (b) W.-L. Yu,
J. Pei, W. Huang and A. J. Heeger, Chem. Commun., 2000, 681; (c)
B.-J. Jung, J.-I. Lee, H. Y. Chu, L.-M. Do and H.-K. Shim,
Macromolecules, 2002, 35, 2282; (d) E. S. Kolb, R. A. Gaudiana
and P. G. Mehta, Macromolecules, 1996, 29, 2359; (e) E. Bellmann,
S. E. Shaheen, R. H. Grubbs, S. R. Marder, B. Kippelen and
N. Peyghambarian, Chem. Mater., 1999, 11, 399; (f) S. Liu,
X. Jiang, H. Ma, M. S. Liu and A. K.-Y. Jen, Macromolecules,
2000, 33, 3514.
10 M. Thelakkat, Macromol. Mater. Eng., 2002, 287, 442.
11 (a) E. Bacher, M. Bayerl, P. Rudati, N. Reckefuss, C. D. Muller,
K. Meerholz and O. Nuyken, Macromolecules, 2005, 38, 1640; (b)
A. Bacher, C. H. Erdelen, W. Paulus, H. Ringsdorf, H.-W. Schmidt
and P. Schuhmacher, Macromolecules, 1999, 32, 4551.
12 (a) G. Klarner, J.-I. Lee, V. Y. Lee, E. Chan, J.-P. Chen, A. Nelson,
D. Markiewicz, R. Siemens, J. C. Scott and R. D. Miller, Chem.
Mater., 1999, 11, 1800; (b) H. Yan, P. Lee, N. R. Armstrong,
A. Graham, G. A. Evmenenko, P. Dutta and T. J. Marks, J. Am.
Chem. Soc., 2005, 127, 3172.
13 (a) Y. Cao, G. Yu, C. Zhang, R. Menon and A. J. Heeger, Synth.
Met., 1997, 87, 171; (b) T. M. Brown, J. S. Kim, R. H. Friend,
F. Cacialli, R. Daik and W. J. Feast, Appl. Phys. Lett., 1999, 75,
1679; (c) B. L. Groenendaal, F. Jonas, D. Freitag, H. Pidlartzik
and J. R. Reynolds, Adv. Mater., 2000, 12, 481.
14 X. Gong, S. Wang, D. Moses, G. C. Bazan and A. J. Heeger, Adv.
Mater., 2005, 17, 2053.
15 S. Wang, Z. Zeng, S. Yang, L.-T. Weng, P. C. L. Wong and K. Ho,
Macromolecules, 2000, 33, 3232.
16 C. R. Towns and R. O’Dell, US Pat., 2002, 6858324.
17 S. W. Kim, S. C. Shim, D. Y. Kim and C. Y. Kim, Synth. Met.,
2001, 122, 363.
18 Q. Hou, Y. S. Xu, W. Yang, M. Yuan, J. B. Peng and Y. Cao,
J. Mater. Chem., 2002, 12, 1703.
19 H. B. Goodbrand and N.-X. Hu, J. Org. Chem., 1999, 64, 670.
20 A. D. Child and J. R. Reynolds, Macromolecules, 1994, 27, 1975.
21 S. Kim, J. Jackiw, E. Robinson, K. S. Schanze, J. R. Reynolds,
J. Baur, M. F. Rubner and D. Boils, Macromolecules, 1998, 31,
964.
layers at a current density of around 33 mA cm22
.
The results indicate that PTPOBS and PTFTS are excellent
HTL materials, superior to PEDOT in PLED applications.
These polymers can be used alone or in combination with a
PEDOT layer for the fabrication of multilayer devices by
using solution processing techniques in full-color display
applications.
Conclusions
We have developed two conjugated polyelectrolytes based
on triphenylamine, PTPOBS and PTFTS, through Suzuki
coupling reaction. The low oxidation potentials and high-lying
LUMO levels make these two polymers excellent hole-injecting
and electron-blocking materials. These polymers are soluble
only in polar solvents, rather than in non-polar solvents, in
which most light-emitting polymers are processed. We have
proved that, after inserting these two polymers between ITO or
PEDOT and red light-emitting (PFO-DBT15) or green-
emitting (PFO-BT15) layers, the devices showed lower turn-
on voltage, higher brightness and enhanced external quantum
efficiencies compared with a single-layer device with bare ITO
as anode or a device with PEDOT as HTL. Excellent hole-
transporting and electron-blocking properties together with
good solubility in polar solvents make them promising
candidates for hole transport layers in the fabrication of
high-efficiency multilayer polymer light-emitting devices and
displays by solution processing techniques.
22 P. B. Balanda, M. B. Ramey and J. R. Reynolds, Macromolecules,
1999, 32, 3970.
23 F. Huang, X. H. Wang, D. L. Wang, W. Yang and Y. Cao,
Polymer, 2005, 46, 12010.
24 F. Huang, H. B. Wu, D. L. Wang, W. Yang and Y. Cao, Chem.
Mater., 2004, 16, 708.
25 D. M. Leeuw, M. M. J. Simenon, A. R. Brown and
R. E. F. Einerhand, Synth. Met., 1997, 87, 53.
Acknowledgements
The authors greatly appreciate the financial support from the
National Natural Science Foundation of China (Project No.
50433030) and the Ministry of Science and Technology,
project no. 2002CB613402.
2394 | J. Mater. Chem., 2006, 16, 2387–2394
This journal is ß The Royal Society of Chemistry 2006