Synthesis and characterization of a novel symmetrical sulfone-substituted polyphenylene vinylene (SO2EH-PPV) for applications in light emitting devices

Article abstract of DOI:10.1021/jp3080964

A novel symmetrical alkylsulfonyl-substituted poly(phenylenevinylene) derivative, poly [2,5-bis-(2′-ethylhexylsulfonyl)-1,4-phenylene)vinylene] (SO₂EH-PPV), was synthesized via palladium-catalyzed Stille coupling, and its electronic and optical properties were investigated. The novel PPV derivative was characterized by NMR, UV-visible absorption, photoluminescence, gel permeation chromatography, infrared spectroscopy, and cyclic voltammetry (CV). The polymer with M_w of 27 800 and a polydispersity index of 2.6 is readily soluble in common organic solvents, such as THF, chloroform, and toluene. The fluorescence quantum yield of the polymer, determined against rhodamine 6G in dilute aqueous solutions, was 0.95. The HOMO and LUMO levels of SO₂EH-PPV were calculated to be -6.0 and -3.61 eV, respectively. The results obtained by CV suggest that SO₂EH-PPV is a strong electron acceptor polymer. Single layer stable polymer light-emitting diode devices with the configuration of (ITO/PEDOT:PSS/SO₂EH-PPV polymer/Al) were fabricated exhibiting a green light emission.

Full text of DOI:10.1021/jp3080964

Article
pubs.acs.org/JPCB
Synthesis and Characterization of a Novel Symmetrical Sulfone-
Substituted Polyphenylene Vinylene (SO2EH-PPV) for Applications in
Light Emitting Devices
Emir Hubijar,† Alexios Papadimitratos,∥ Doyun Lee,† Anvar Zakhidov,‡,§ and John P. Ferraris*,†,§
‡
§
†Department of Chemistry, Department of Physics, and The Alan G. MacDiarmid Nanotech Institute, The University of Texas at
Dallas, 800 West Campbell Rd, Richardson, Texas 75080-3021, United States
∥Solarno Inc., 153 Hollywood Drive, Coppell, Texas 75019, United States
S
* Supporting Information
ABSTRACT: A novel symmetrical alkylsulfonyl-substituted poly(phenylenevinylene)
derivative, poly [2,5-bis-(2′-ethylhexylsulfonyl)-1,4-phenylene)vinylene] (SO2EH-PPV),
was synthesized via palladium-catalyzed Stille coupling, and its electronic and optical
properties were investigated. The novel PPV derivative was characterized by NMR, UV−
visible absorption, photoluminescence, gel permeation chromatography, infrared
spectroscopy, and cyclic voltammetry (CV). The polymer with Mw of 27 800 and a
polydispersity index of 2.6 is readily soluble in common organic solvents, such as THF,
chloroform, and toluene. The fluorescence quantum yield of the polymer, determined
against rhodamine 6G in dilute aqueous solutions, was 0.95. The HOMO and LUMO
levels of SO2EH-PPV were calculated to be −6.0 and −3.61 eV, respectively. The results obtained by CV suggest that SO2EH-
PPV is a strong electron acceptor polymer. Single layer stable polymer light-emitting diode devices with the configuration of
(ITO/PEDOT:PSS/SO2EH-PPV polymer/Al) were fabricated exhibiting a green light emission.
comparable sizes.1,18 Due to these limitations, different
acceptor-types of polymers, including cyano-substituted
INTRODUCTION
■
Conjugated polymers are a novel class of semiconductors that
have been intensively studied as electroluminescent materials
alkoxy-substituted PPVs (poly[(2-(2′-ethylhexyloxy)-5-me-
for application in polymer light-emitting diodes (PLEDs).1−6
thoxy)-1,4-phenylene-(1-cyanovinylene)]) (MEH-CN-PPV)
have been extensively investigated and showed great improve-
They offer the advantages of low cost and easy processing,
which make them potential replacements for expensive
ments in electroluminescence (EL) efficiency due to their high
electron affinity.16,19−25 The cyano (electron-withdrawing)
substituent on the vinylene linkage in the backbone of the
polymer serves to lower both the LUMO and HOMO energy
levels.26,27 Lowering the LUMO energy level enhances the
electron injection which then allows for an effective
recombination of electrons and holes resulting in a higher
device performance. Sulfone moieties are also widely known for
their electron withdrawing properties (comparable to cyano)
and can be directly attached to the polymer’s backbone to
afford electron acceptor materials.28−31
inorganic counterparts.7,8 Among the organic conjugated
polymers, poly(p-phenylenevinylenes) (PPVs) have received
considerable attention for application in PLEDs after the first
report by Burroughes et al. in 1990.9 PPV polymers have good
chemical and mechanical properties, high hole mobilities, and
require relatively low driving voltages in optoelectronic devices
making them a suitable material in PLEDs.10−12 The
performance of the PLED devices using conjugated polymers
can be limited due to large mobility mismatch between holes
and electrons, leading to a recombination zone very close to
one of the electrodes where traps and defects are likely to be
prevalent.1,13,14 Since hole mobility is often larger than electron
mobility in conjugated polymers, the recombination zone is
likely to occur close to the cathode.15 Apart from the polymer’s
charge transport properties, the operation of a PLED device
relies greatly upon injection of charges from the corresponding
electrodes. Hence, charge injection efficiency depends upon the
suitable selection of electrode materials as well.1 Therefore, the
size of the energy barrier between the work function of the
electrode and the energy level of the HOMO (for the anode)
or LUMO (for the cathode) level of the polymer must be
minimal for efficient charge injection.16,17 Because balanced
charge injection is required, the energy barriers should be of
The attractive combination of excellent optical and electronic
properties of the conjugated PPVs has encouraged our
synthesis of a novel symmetrical alkylsulfonyl-substituted
poly(phenylenevinylene) derivative, poly[2,5-bis-(2′-ethylhex-
ylsulfonyl)-1,4 phenylene)vinylene] (SO2EH-PPV) utilizing the
Stille-coupling reaction. In this paper, we report detailed
synthesis and characterization of this novel polymer and
Special Issue: Paul F. Barbara Memorial Issue
Received: August 14, 2012
Revised: November 19, 2012
Published: November 20, 2012
© 2012 American Chemical Society
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Scheme 1. Synthetic Routes to Precursor Monomers and Polymer
Reagents and conditions: (i) 20 mol % CuI, neocuproine, 4 equiv. K3PO4,DMF, 2-ethylhexanethiol, 115 °C, 48 h; (ii) Br2, I2, CH2Cl2, 3 d at room
temp; (iii) H2O2, AcOH, 3 h reflux; (iv) trans-1,2-bis (tri-n-butylstannyl) ethylene, toluene, Pd(PPh3)4, 120 °C, 24 h.
preliminary results using this electroluminescent material in
light emitting diodes.
resulting films were dried in vacuum at 50 °C for 2 h. A
solution of tetrabutylammonium hexafluorophosphate
(TBAPF6) in dry acetonitrile (0.1 M) was used as the
electrolyte, and the reference electrode was Ag/Ag+ (0.01 M
AgNO3 in TBAPF6/dry acetronitrile). The counter electrode
was a platinum wire. CV measurements of the SO2EH-PPV
homopolymer were scanned at constant scan rate of 50 mV/s.
TMAFM (tappping mode atomic force microscopy) studies
were carried out on a VEECO-dimension 5000 scanning probe
microscope with a hybrid xyz head equipped with Nano-Scope
Software. AFM images were obtained using silicon cantilevers
with nominal spring constant of 42 N/m and nominal
resonance frequency of 300 kHz (OTESPa). Nanoscope 7.30
software was used for surface imaging and image analysis. All
AFM measurements were conducted under ambient conditions.
Cantilever oscillation amplitude was equal to ca. 380 mV, and
all images were acquired at 1 Hz scan frequency. Sample scan
area was 1 μm. The AFM images were obtained on the actual
PLED devices. Samples were prepared from chloroform (10
mg/mL) via spin-casting. Light emitting devices were fabricated
using glass/indium tin oxide substrates. The glass substrates
were high quality display glass (Corning 1737) with very low
surface roughness. The sheet resistance of the indium tin oxide
(ITO) layer was about 10 Ω/□, with a film thickness of 120
nm and an optical transparency of 85%. The hole injection
layers, PEDOT:PSS (CLEVIOS PVP CH 8000), were spun
cast at 5000 rpm, and the layer thickness was 70 nm on average.
SO2EH-PPV solutions in chloroform were prepared at a
concentration of 10 mg/mL. The polymeric solutions were
spun cast at 2000 rpm onto the glass/ITO/PEDOT:PSS
substrates with average film thicknesses of 100 nm according to
profilometry measurements (Ambios Technology, XP-1). An
interface modification thin layer of cesium carbonate (Cs2CO3)
was spun cast before cathode fabrication to improve electron
injection. The cesium carbonate was dissolved in 2-
ethoxyethanol at 0.2 wt %. The patterned aluminum (Al)
cathode was deposited using thermal evaporation through a
shadow mask. All device characterizations were done using a
Keithly 236 source measurement unit and a Photo Research
EXPERIMENTAL SECTION
■
Materials. All chemicals, unless otherwise stated, were
obtained from Sigma Aldrich Chemical Co. and were used as
received. All solvents were distilled over appropriate drying
agent(s) prior to use and were purged with nitrogen. Water
content (<20 ppm) was determined with a Karl Fischer titrator
(Denver Instruments, model 270).
Characterization. UV−visible absorption spectra were
collected using a Shimadzu UV-1601PC UV−visible spectro-
photometer controlled by UV-1601PC software. Emission
spectra were collected using a Jobin-Yvon fluorimeter
controlled by Datamax software version 1.03. FT-IR spectra
were collected using a Nicolet/Avatar 360 FT-IR controlled by
EZ-OMNIC software. The fluorescence quantum yield in
chloroform solution was determined relative to a Rhodamine
6G standard in dilute aqueous solution. Corrections for
refractive indices and differences in optical densities were
applied. Molecular weights were obtained via gel permeation
chromatography (GPC) on two series-connected ViscoGel I-
Series (I-MBHMW-3078) columns with a Viscotek triple array
detector 302 (TDA 302: refractrometer, light scattering, and
viscometer) using THF eluent at 1 mL/min flow rate.
Polystyrene standards were used for calibration. Data were
analyzed using Viscotek OmniSEC software, version 3.0. High
resolution mass spectra (HRMS) were recorded on a mass
1
spectrometer operating in positive ESI(+) mode. HNMR and
13CNMR spectra were obtained at room temperature using a
JEOL FX-270 MHz spectrometer using TMS as an internal
standard. TGA thermograms were obtained using a Perkin-
Elmer PYRIS 1 thermogravimetric analyzer heated from 60 to
800 °C at 10 °C/min. Cyclic voltammograms (CV) were
obtained using a standard three-electrode cell connected to an
EG&G Princeton Applied Research potentiostat/galvanostat
273A with PowerSuite software. All CV measurements were
done inside a nitrogen-filled glovebox. Chloroform solutions of
the PPVs were drop cast onto platinum electrodes and the
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PR-650 Spectrocolorimeter that provided the current density−
voltage (J−V) and luminance−voltage (L−V).
2H, Car−H), 3.39−3.34 (dd, 4H, SO2−CH2), 1.97−2.096
(septet, 2H, CH), 1.65−1.21 (m, 16H, other CH2), 0.92−0.86
(t, 12H, CH3). 13CNMR (CDCl3, 270 MHz): δ 144.7 (s, Car−
SO2), 138.1 (s, Car-H), 120.5 (Car−Br), 57.4 (s, SO2−CH2),
34.4 (s, CH), 32.4, 28.2, 25.9, 22.7 (s, CH2), 14.1 (s, CH3),
10.2 (s, other CH3). HRMS (ESI+) calculated for
C22H36Br2O4S2: 609.0314 (M+Na+). Found: 609.0325 (M
+Na+).
Monomer Synthesis. Monomer 1 (1,4-dibromo-2,5-bis(2-
ethylhexylsulfonyl)benzene), monomer 2 (1,4-bis(2-ethylhex-
ylsulfonyl)-2,5-diethynylbenzene), and all intermediates were
synthesized according to the reported literature with
modifications. The synthetic pathways to all monomers are
displayed in Scheme 1.
PPV Polymer Synthesis. Polymerization was carried out
via Stille coupling between monomer 3 and trans-1,2-bis(tri-n-
butylstannyl)ethylene following the reported literature with
modifications. The synthetic pathway is displayed in Scheme 1.
Poly[2,5-bis(2-ethylhexylsulfonyl)-1,4-phenylene-vi-
nylene]. To a 3-neck 50 mL flask were added 0.11 g (0.17
mmol) of trans-1,2-bis(tri-n-butylstannyl)ethylene and 0.1 g
(0.17 mmol) of 1,4-dibromo-2,5-bis(2-ethylhexylsulfonyl)-
benzene. Then, 6 mL of dry toluene were added under the
protection of nitrogen. The solution was flushed with nitrogen
for 10 min, and then 0.004 g (1% molar) of Pd (PPh3)4
dissolved in 2 mL of toluene was added to the reaction mixture.
After another flushing with nitrogen for 20 min, the reaction
was heated at reflux (120 °C) for 48 h. After cooling to room
temperature, the polymerization mixture was poured into 300
mL of methanol. The precipitated polymer was collected by
filtration using a Millipore Durapore 0.45 μm membrane filter.
The collected polymer was then subjected to Soxhlet extraction
with methanol, hexane, and chloroform. The polymer was then
dried under vacuum overnight at 50 °C. FTIR (KBr pellet:
1,4-Bis-(2-ethylhexylthio)benzene (1). 1,4-Diiodoben-
zene (5 g, 15.2 mmol), copper iodide (0.58 g, 21 mol %),
neocuproine (0.63 g, 20 mol %), and potassium phosphate
tribasic (13.0 g, 4.1 equiv) were placed in a 250 mL RB-flask
while maintaining a nitrogen flow. DMF (100 mL, anhydrous)
was injected, and the reaction assumed a clear, deep red color.
2-Ethylhexanethiol (5.8 mL, 2.2 equiv) was injected into the
reaction, and the color turned orange. The reaction mixture was
heated to 110 °C where-upon the reaction turned to a white
turbid color and was left to stir overnight under nitrogen. TLC
on silica analysis (hexanes) after 48 h of reaction time showed
disubstituted product without the presence of the monosub-
stituted side product. The reaction mixture was left to cool to
room temperature and ethyl acetate (30 mL) was added and
then filtered. The organic layer was collected, washed with
deionized water, 7.5% sodium bicarbonate solution, and brine
(2 × 100 mL), and dried over sodium sulfate. The solvent was
removed under reduced pressure affording a deep purple
colored oil. The oil was purified through chromatography on a
silica gel packed column, using hexanes as the eluent to afford
1
cm−1): 2859, 2929 (aliphatic C−H), 1151, 1351 (−SO2). H
1
4.45 g (80%) of clear colorless oil. H NMR (CDCl3, 270
NMR (CDCl3, 270 MHz): δ 8.31 (s, 2H, Car−H), 3.6−3.3 (dd,
4H, SO2−CH2), 2.1−1.8 (septet, 2H, CH) 1.65−1.21 (m, 16H,
other CH2), 0.92−0.86 (t, 12H, CH3). GPC (THF): Mn = 10
600, Mw = 27 800, polydispersity index = 2.6. UV−vis (solution
10−5 M in CHCl3): λabs.max/nm = 442, (as film): 444. PL
(solution 10−5 M in CHCl3): λem.max/nm = 505, 534 (as film):
548. TGATd: 250 °C (onset, 5% weight loss). Cyclic
voltammetry (HOMO/LUMO = −6.0 eV/−3.61eV).
MHz): δ 7.24−7.23 (s, 4H, Car−H), 2.87−2.85 (d, 4H, S−
CH2), 1.60−1.26 (m, 18H, other CH2), 0.92−0.86 (t, 12H,
CH3). 13CNMR (CDCl3, 270 MHz): δ 135.0 (s, Car−S), 129.7
(s, Car−C), 39.1 (s, S−CH2), 38.5 (s, CH), 32.4, 25.7, 23.1 (s,
CH2), 14.2 (s, CH3), 10.8 (s, other CH3).
2,5-Dibromo-1,4-bis-(2-ethylhexylthio)benzene (2). A
catalytic amount of iodine (0.066 g) was added to a solution of
3 g of 1,4-bis-(2′-ethylhexylthio)benzene (1) (8.18 mmol) in
CH2Cl2 (30 mL) maintained at 0 °C. With constant stirring in
the dark, bromine (1.048 mL 20.5 mmol) was added to the
solution which was then stirred at room temperature for 3 days.
The reaction was terminated by adding 10% aqueous solution
of sodium sulfite Na2SO3 and then the mixture was extracted
with chlroform. The organic phase was collected and washed
three times with water and dried over sodium sulfate. The
solvent was removed under reduced pressure to afford a yellow
oil. The oil was purified through a silica gel packed column,
using hexanes as the eluent to yield 2.9 g (68% yield) of a clear
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Polymer. Synthetic
steps toward monomers and a SO2EH-PPV polymer are
outlined in Scheme 1. In the first step, S-alkylation was
completed using a modified synthetic pathway based on the
Ullman-type copper(I)-catalyzed cross-coupling reaction of aryl
halides with alkanethiols.32−34 Once S-alkylation was com-
pleted, the final monomer was obtained via direct bromination
of the1,4-bis-(2′-ethylhexylthio)benzene at the para sites,
followed by oxidation of sulfur to sulfone (Scheme 1).30,35−37
The synthesis of the SO2EH-PPV was carried out by palladium-
catalyzed Stille coupling reaction utilizing the monomer 3 and
commercially available trans-1,2-bis(tri-n-butylstannyl)-
ethylene.26,38 The polymer obtained with a Mw of 27 800 and
a polydispersity index (PDI) of 2.6 was readily soluble in
common organic solvents, such as THF, chloroform and
toluene. The good solubility can be attributed to the two
sulfone-alkyl side chains attached to phenyl ring. All of the
polymerization data are summarized in Table 1. The polymer
forms smooth and uniform films on glass substrates when cast
from chloroform solutions, which can be an advantage for their
potential applications as active layers in electronic devices. The
chemical structures of all of the monomers and the polymer
1
colorless oil. H NMR (CDCl3, 270 MHz): δ 7.36 (s, 2H, Car-
H), 2.87−2.85 (d, 4H, S-CH2), 1.69−1.28 (m, 18H, other
CH2), 0.92−0.86 (t, 12H, CH3). 13CNMR (CDCl3, 270 MHz):
δ 137.0 (s, Car−S), 131.8 (s, Car−H), 123.4 (Car−Br), 39.1 (s,
S−CH2), 38.5 (s, CH), 33.1, 28.9, 25.4, 23.8 (s, CH2), 14.2 (s,
CH3), 10.8 (s, other CH3).
1,4-Dibromo-2,5-bis(2-ethylhexylsulfonyl)benzene
(3). To a solution of 2 g (3.81 mmol) of 2,5-dibromo-1,4-bis-
(2′-ethylhexylthio)benzene in 14 mL of warm glacial acetic acid
was added 8 mL of 35% hydrogen peroxide in 7.2 mL of glacial
acetic acid. When the initial reaction had subsided, another 8
mL of the 35% hydrogen peroxide was added, and the solution
was refluxed for 3 h at 120 °C. The crystalline white solid was
obtained when the mixture was poured into water. Mp. 104−
106 °C. FTIR (KBr pellet: cm−1): 2852, 2924 (aliphatic C−H),
1
were verified with NMR and FTIR analyses. In the H NMR
1
1149, 1350 (−SO2). H NMR (CDCl3, 270 MHz): δ 8.49 (s,
spectrum in Figure S1, the signals at around δ 7.5−8.6 ppm are
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Table 1. Polymerization Data for the Conjugated Polymer
a
a
a
b
polymer
Mn
10 600
Mw
27 800
PDI
2.61
yield (%)
64
Td (°C)
SO2EH-PPV
250
a
Determined using GPC (eluent: THF; polystyrene standards).
Temperature at 5% weight loss by a heating rate of 10 °C/min
b
under nitrogen.
assigned to the aromatic and conjugated vinylene protons
which are shifted downfield due to the electron-withdrawing
nature of sulfone units. Moreover, the methine proton signal
can be seen at 2.6 ppm as a broadened peak as opposed to a
septet previously observed in monomers 3 and is shifted
1
downfield. H NMR spectrum also shows the characteristic
peak of the methylene protons adjacent to the sulfone at 3.1−
3.3 ppm. Finally, a cluster of broadened peaks between 0.2 and
1.9 ppm correspond to the rest of the proton signals from the
side alkyl chains. The chemical structure of the SO2EH-PPV
polymer is also reflected in the FTIR spectra, which are shown
in Figure S2. The vinylic carbon−carbon double bond stretch
was observed at 1015 and 1032 cm−1. The absorption bands at
1151 and 1351 cm−1 correspond to sulfonyl groups, and the
signal observed from 3000 to 2800 cm−1 is attributed to C−H
stretch from the aliphatic chains.
Figure 2. Normalized UV−visible absorption and photoluminescence
of SO2EH-PPV polymer in solution (10−5 M in CHCl3).
Thermal Properties. The thermal properties of the
resulting polymer were investigated by thermogravimetric
analysis (TGA) at a heating rate of 10 °C/min under nitrogen
flow (Figure 1). The polymer exhibited good thermal stability
Figure 3. Normalized solid-state UV−visible absorption and photo-
luminescence of SO2EH-PPV polymer (film cast from 10 mg/mL in
CHCl3 solutions).
Table 2. Optical Properties of the Polymer SO2EH-PPV
λabs.max (nm, UV−vis)
λem.max (nm, PL)
a
b
a
b
c
polymer
solution
film
solution
film
Φ
SO2EH-PPV
442
444
505, 534
548
0.95
a
b
1 × 10−5 M in CHCl3. Spin-coated from a 10 mg/mL solution.
c
Quantum yield of the polymer solution using Rhodamine 6G (Φ =
0.95) as a standard.
Figure 1. Thermogravimetric analysis of the SO2EH-PPV polymer
under N2.
gap for the polymer SO2EH-PPV, estimated from the
absorption spectra’s onset in a solid state, is 2.39 eV (Table
3). The fluorescence quantum yield of the polymer, determined
against rhodamine 6G in dilute aqueous solutions, is 0.95.
Morphology. The morphology of the polymer SO2EH-PPV
in PLED devices was investigated using TMAFM microscopy.
Films were deposited by spin-casting from chloroform solutions
with a concentration of 10 mg/mL followed by annealing at
120 °C for 5 min. As can be seen from the AFM topography
images in Figure 4, the film from the resulting SO2EH-PPV/
chloroform solution has a smooth surface. The rms roughness
values were obtained by completing roughness analysis of the
surface which revealed that the average root-mean- square
roughness (R rms) over 2 μm × 2 μm is below 3 nm.
with a 5% weight loss onset occurring at 250 °C with significant
weight losses at 380 °C (30% loss) and 410 °C (50% loss). The
thermal stability of SO2EH-PPV is adequate for use in LED or
other electronic applications.
Optical Properties. The optical properties of the novel
polymer were investigated by measuring UV−visible absorption
and photoluminescence (PL) spectra in both dilute chloroform
solutions and as thin films. UV−visible absorption and PL
spectra in solution and as thin films are shown in Figures 2 and
3, respectively, and are summarized in Table 2. The absorption
and emission maxima in dilute solutions appear at 442 and 505
nm (with a shoulder signal at 534 nm), respectively. Figure 3
shows that spun cast film of the polymer has a red-shifted
emission maxima compared to its corresponding solution
spectra. The polymer in the solid state emits an intense green
light upon excitation. The absorption and emission maxima of
thin films are 444 and 548 nm, respectively. The optical band
Electrochemical Properties. Cyclic voltammetry (CV)
was performed to investigate electrochemical behavior and to
estimate electronic energy levels (HOMO and LUMO levels)
of the SO2EH-PPV polymer. Figure 5 shows the CV of the
homopolymer using TBAPF6 as supporting electrolyte in
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Table 3. Electrochemical Properties of the Polymer SO2EH-PPV
b
c
polymer
λabs.max (nm, film)
λonset (nm)
Eopt.gap
Eonset.ox(V)/HOMO (eV)
Eonset.red(V)/LUMO (eV)
a
SO2EH-PPV
444
518
2.39
1.3/−6.0
−1.10/−3.61
a
b
c
Estimated from the onset wavelength of optical absorption. HOMO was calculated from the LUMO level and optical band gap. LUMO was
calculated from the onset reduction potential.
Figure 4. TMAFM height image of the SO2EH-PPV based PLED devices spin cast from the chloroform solutions (10 mg/mL); (b) 3D height
TMAFM image of the SO2EH-PPV based PLED devices deposited by spin-casting from chloroform solution (10 mg/mL).Scan size: 2 μm × 2 μm.
reoxidation). Figure 5 shows the onset reduction potential of
SO2EH-PPV of −1.10 V vs Ag/Ag+, which corresponds to the
lowest unoccupied molecular orbital (LUMO) energy level of
−3.61 eV according to eq 2. Ionization potential value of −6.0
eV (HOMO level) was calculated by utilizing the onset
oxidation potential (1.3 V vs Ag/Ag+) into the eq 1. Compared
with the n-doping/dedoping process, the cyclic voltammogram
of the SO2EH-PPV polymer showed an irreversible oxidation
peak. The optical band gap for the resulting polymer, calculated
from the onset of absorption, was 2.39 eV, which is in a good
agreement with the electrochemical band gap of 2.4 estimated
from the reduction (−1.1 eV) and oxidation (1.3 eV) onset as
seen in Figure 5. Cyclic voltammetry suggests that SO2EH-PPV
is a strong electron-acceptor and could be an attractive material
for other applications in electronic devices.
Figure 5. Cyclic voltammogram of SO2EH-PPV.
Electroluminescent Properties. PLED devices based on
the SO2EH-PPV polymer were fabricated with the following
device configurations: glass/ITO/PEDOT:PSS/SO2EH-PPV/
Al and glass/ITO/PEDOT:PSS/SO2EH-PPV/CsCO3/Al. In-
dium tin oxide (ITO) with a sheet resistance of 10 Ω /□ was
used as an anode. An aluminum (Al) cathode was deposited
onto the device using thermal evaporation. The Fermi levels of
ITO and Al were approximately −4.7 and −4.2 eV,
respectively.1,15 The hole transporting layer composed of
poly(3,4-ethylene dioxythiophene):polystyrene sulfonate, PE-
DOT:PSS with a work function of 5.1 eV,39,40 was spun cast as
a thin layer of approximately 70 nm onto the ITO/glass
substrate. The active/emissive layer was prepared via spin
coating of the polymer solution in chloroform (10 mg/mL) on
top of the ITO/PEDOT:PSS covered ITO anode, and was
annealed at 120 °C for 5 min. The thickness of the emissive
layer was approximately 100 nm for both configurations. The
current density−voltage (J−V) and luminescence-voltage (L−
V) characteristics of glass/ITO/PEDOT:PSS/SO2EH-PPV/Al
acetonitrile solution with platinum working electrodes, a
platinum wire counter electrode and an Ag/AgNO3 reference
electrode under the N2 atmosphere. The ionization potential
(HOMO level) and electron affinity (LUMO level) of the
SO2EH-PPV polymer were calculated from the onset potentials
of oxidation and reduction relative to the vacuum. Hence, the
HOMO and LUMO levels as well as the energy band gap
(Eg,ec) could be computed according to the following equations:
ox
HOMO = −e(E
+ 4.71) (eV)
(1)
(2)
(3)
onset
red
onset
LUMO = −e(E
+ 4.71) (eV)
ox
onset
ox
onset
Eg,ec = E
− E
(eV)
ox
red
where E
and E
are the measured potentials relative to
onset
onset
Ag/Ag+. Detailed electrochemical properties as well as the
energy level parameters of the PPV homopolymer are listed in
Table 3. The cyclic voltammogram of SO2EH-PPV displays a
distinct reversible n-doping/dedoping process (reduction/
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and glass/ITO/PEDOT:PSS/SO2EH-PPV/CsCO3/Al devices
are shown in Figure 6. The color of EL spectra is typically
Figure 8. Schematic diagram of the PLED device configurations and
illustration of SO2EH-PPV’s HOMO and LUMO levels.
diagrams in Figure 8 show two different types of devices
where the thin Cs2CO3 layer decreases the effective work
function of the cathode to 2.2 eV by forming Al−O−Cs
complex43,44 and eliminates the electron injection barrier
between the Al and LUMO level of SO2EH-PPV. As evidenced
by the obtained results, the more evenly matched injection of
carriers from both electrodes yields a more balance charge ratio
and hence better efficiency in PLED devices. In this particular
polymer, the sulfone substituent (electron withdrawing) which
is directly attached to the PPV benzene ring has a large
influence on lowering both LUMO and HOMO levels of the
polymer and consequently contributes to the poor efficiency of
the constructed LED devices. In order to mitigate these
limitations and take advantage of the excellent intrinsic
properties of SO2EH-PPV, we are currently optimizing/
designing new LED device configurations to better match the
HOMO and LUMO levels of the polymer with the anode and
cathode. This would potentially improve the injection of holes
and electrons, and overall efficiency of the device.
Figure 6. Current density−voltage (J−V) and luminescence-voltage
(L−V) curves of devices: (a) glass:ITO/PEDOT:PSS/polymer
(SO2EH-PPV)/Al and (b) glass:ITO/PEDOT:PSS/polymer
(SO2EH-PPV)/Cs2CO3/Al.
defined by their CIE (Commission Internationale de
L’Eclairage) chromaticity coordinates (x, y).41,42 The CIE
coordinates for the EL spectra of SO2EH-PPV are 0.36 and
0.55. Both device configurations based on SO2EH-PPV
polymer showed uniform and stable green emission. The EL
spectrum of the polymer is almost identical to the PL (see
Figure 7) indicating that the recombination zone in EL and the
CONCLUSION
■
In this paper, we have successfully synthesized a novel
symmetrical sulfone-substituted polyphenylene vinylene
(PPV) polymer (SO2EH-PPV) via Stille-coupling reaction.
The polymer with Mw of 27 800 and a polydispersity index
(PDI) of 2.6 is readily soluble in common organic solvents,
such as THF, chloroform, and toluene. In addition, it exhibits a
good thermal stability with 5% weight loss onset occurring at
250 °C. The optical band gap for the resulting polymer,
calculated from the absorption spectra’s onset in a solid state,
was 2.39 eV. The fluorescence quantum yield of the polymer,
determined against a rhodamine 6G standard in dilute aqueous
solution, was 0.95. Stable PLED devices based on SO2EH-PPV
polymer were fabricated but exhibited low efficiencies due to
the deep HOMO level of the SO2EH-PPV and its mismatch
with the work function of the ITO anode. The cyclic
voltammogram of SO2EH-PPV displays a distinct reversible
n-doping/n-dedoping process, with the onset reduction
potential at −1.1 V vs Ag/Ag+. Using this potential, the
LUMO level of the polymer was calculated to be −3.61 eV.
Due to the small current associated with the p-doping in the
oxidation process, the HOMO level was calculated (−6.0 eV)
from the polymer’s optical energy gap and its experimentally
determined LUMO level. Cyclic voltammetry suggests that
SO2EH-PPV is a strong electron acceptor conjugated polymer
that can be an attractive material for applications in
photovoltaic devices as an electron acceptor.
Figure 7. PL and EL spectra of devices glass:ITO/PEDOT:PSS/
polymer (SO2EH-PPV)/Al.
emission mechanism in PL originated from the same excited
state. The poor efficiency of the fabricated devices is expected
because of a large mismatch of a HOMO level of SO2EH-PPV
and the ITO anode work function (see Figure 8). Therefore, a
large hole injection barrier is formed at the interface of the
anode and the active layer affecting the efficiency of the PLED
devices. The best performance was obtained with the ITO/
PEDOT:PSS/SO2EH-PPV/Al devices, whereas the fabricated
devices with the additional electron injection layer of Cs2CO3
had lower luminance. Figure 6 shows that the devices with
Cs2CO3 are operating at higher current density, indicating
improved electron injection from the cathode to the active/
emissive layer (SO2EH-PPV). However, the performance of
PLED devices suffered by utilizing an electron injection layer
(Cs2CO3) due to unbalanced ratio of positive and negative
charges during operation of the PLEDs. The schematic
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(26) Zou, Y.; Hou, J.; Yang, C.; Li, Y. Macromolecules 2006, 39,
8889−8891.
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR, IR, and quantum yield data are included. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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Financial support from the Robert A. Welch Foundation
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