Intense green light from a silyl-substituted poly(p-phenylenevinylene)- based light-emitting diode with air-stable cathode

Article abstract of DOI:10.1039/a904179f

A silicon-containing poly(p-phenylenevinylene) derivative, poly[2,5- bis(butyldimethylsilyl)-1,4-phenylenevinylene] (BS-PPV), was synthesized via the Gilch reaction. The polymer is fully solution processable with high thermal stability. The UV-Vis absorpt

Full text of DOI:10.1039/a904179f

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Intense green light from a silyl-substituted
poly(p-phenylenevinylene)-based light-emitting diode with air-stable
cathode
Zhi-Kuan Chen,a Lian-Hui Wang,b En-Tang Kangb and Wei Huanga
a Institute of Materials Research and Engineering (IMRE), National University of Singapore, 10
Kent Ridge Crescent, Singapore 119260, Republic of Singapore. E-mail: wei-huang=imre.org.sg
b Department of Chemical and Environmental Engineering, National University of Singapore, 10
Kent Ridge Crescent, Singapore 119260, Republic of Singapore
Received 25th May 1999, Accepted 8th July 1999
A silicon-containing poly(p-phenylenevinylene) derivative, poly[2,5-bis(butyldimethylsilyl)-1,4-
phenylenevinylene] (BS-PPV), was synthesized via the Gilch reaction. The polymer is fully solution processable
with high thermal stability. The UV-Vis absorption and Ñuorescent emission spectra demonstrate that
BS-PPV is a promising green emissive material for light-emitting device application. Cyclic voltammetric
measurements indicate that it can be reversibly n-doped and irreversibly p-doped with the onset oxidation and
reduction potentials of 1.16 and [ 1.81 V, respectively. The HOMO and LUMO energy levels of BS-PPV
were estimated to be 5.56 and 2.59 eV, respectively. Single layer devices with the conÐguration
ITO/BS-PPV/Al were fabricated, which showed a turn-on voltage of 6 V and intense green light was observed
at around 7.5 V. The performance is better than that of devices fabricated with other silicon-containing
PPV-based polymers.
readily processable material with high PL and EL quantum
Introduction
efficiencies. Single layer devices using BS-PPV as the emissive
layer and air stable Al as the cathode with the conÐguration
of ITO/BS-PPV/Al were fabricated. The characteristics of the
single layer devices will be discussed.
Enormous e†orts have been made to develop conjugated elec-
troluminescent polymers for application in light-emitting
diodes (LEDs) in the past decade.1h12 Polymer LEDs
(PLEDs) have demonstrated some attractive advantages such
as low cost processing through simple spin-coating or casting
techniques, color tuning by way of chemical modiÐcation of
the active polymers and ease of realization of larger area Ñat
panel displays over traditional inorganic LEDs. It is recog-
nized that good processability, high photoluminescent
quantum efficiency and high thermal and optical stability are
basic prerequisites for electroluminescent polymers to be com-
mercialized for LED displays. Poly(p-phenylene vinylene)
(PPV) and its derivatives are typical examples and are the
most extensively investigated materials for PLEDs. Among
PPV derivatives, increasing attention is being paid to silyl-
substituted PPVs since the Ðrst silicon-containing PPV was
reported by Wudl and co-workers in 1994.13,14 Silyl-
substituted PPVs are attractive owing to their extremely high
photoluminescent (PL) quantum efficiencies compared with
PPV itself or alkoxy-substituted PPVs. Very high PL effi-
ciency (60% or above for thin Ðlm samples) and EL efficiency
based on silyl-substituted PPVs have been achieved.15,16 The
high EL performance is attributed to a good balance for hole
and electron injections into the emissive material and the low
barrier energies between the polymer Ðlm and the elec-
trodes.15 In addition, silicon-containing PPVs have demon-
strated very good solubility and uniform Ðlm morphology
without any crystalline features in the Ðlm state.15
Experimental
Measurements
NMR spectra were collected on a Bruker ACF 300 spectro-
meter with chloroform-d as solvent and tetramethylsilane as
internal standard. FT-IR spectra were recorded on a Bio-Rad
FTS 165 spectrometer by dispersing samples in KBr disks.
UV-Vis and Ñuorescence spectra were obtained on a Shi-
madzu UV-NIR 3100 spectrophotometer and on a Perkin-
Elmer LS 50B luminescence spectrometer, respectively.
Thermogravimetric analysis (TGA) was conducted on a
DuPont Thermal Analyst 2100 system with a TGA 2950 ther-
mogravimetric analyzer at a heating rate of 20 ¡C min~1 and
an air Ñow rate of 75 ml min~1. Elemental microanalyses were
performed on a Perkin-Elmer 2400 elemental analyzer for C,
H, N and S determination. Cyclic voltammetry (CV) was per-
formed on an EG&G Parc Model 273A potentiostat/
galvanostat system with a three-electrode cell in a solution of
Bu NBF (0.10 M) in acetonitrile at a scan rate of 10 mV s~1.
4
4
Polymer Ðlms were coated on a square Pt electrode (0.50 cm2)
by dipping the electrode into the corresponding solutions and
then drying in air. A Pt wire was used as the counter electrode
In our previous work, a silicon-containing PPV-based
copolymer with an alternating structure showed that the PL
quantum efficiency could be as high as 65%.17 In this paper
we report a facile way for the synthesis of a new PPV deriv-
ative, poly[2,5-bis(butyldimethylsilyl)-1,4-phenylenevinylene]
(BS-PPV), through introducing two silyl groups as the substit-
uents on the phenylene ring, aiming at developing a more
and a Ag/AgNO (0.10 M in acetonitrile) electrode was used
3
as the reference electrode. Prior to each series of measure-
ments the cell was deoxygenated with argon. Gel permeation
chromatographic (GPC) analysis was conducted on a Perkin-
Elmer Model 200 HPLC system equipped with Phenogel
MXL and MXM columns using polystyrene as standard and
THF as eluent.
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Single layer LED devices were fabricated using an indium
tin oxide (ITO)-coated glass substrate with a sheet resistance
of about 250 ) K~1. The substrate was cleaned by sonicating
in hexane and propan-2-ol successively prior to use. A
uniform polymer Ðlm was obtained by spin coating at room
temperature from 10 mg ml~1 of xylene solution at 2000 rev
min~1 with thickness of about 1000 Ó. The Ðlm was dried at
room temperature in a vacuum oven for 24 h before the
cathode electrode was deposited. An aluminum electrode with
thickness of about 1000 Ó was evaporated (JEOL-400 vacuum
evaporator) through a mask at a pressure of 3 ] 10~7 Torr
and a rate of 1 Ó s~1 (TM-200R thickness monitor, Maxtek).
The active area of the LED was approximately 7 mm2. The
electrical contacts were Ðxed using a conductive epoxy 14G
adhesive. All of the sample processing, handling and optic-
electronic measurements were performed under ambient con-
ditions. The determination of currentÈvoltage (IÈV )
characteristics was carried out using a Keithley 238 High
Current Source Measure unit.
Poly[2,5-bis(butyldimethylsilyl)-1,4-phenylenevinylene] (BS-
PPV). A solution of 0.984 g (2.0 mmol) of 2,5-bis(butyldi-
methylsilyl)-1,4-bisbromomethylbenzene in 30 ml of
anhydrous THF was charged into a 100 ml Ñask. To this solu-
tion were added dropwise 12 ml of a 1.0 M solution of pot-
assium tert-butoxide (12.0 mmol) in anhydrous THF at room
temperature with stirring. The mixture was continuously
stirred for 24 h and then poured into 300 ml of methanol. The
resulting yellow precipitate was washed with de-ionized water
and dried under vacuum. The crude polymer was dissolved in
chloroform and reprecipitated in methanol twice. After
Soxhlet extraction with methanol and acetone for 12 h suc-
cessively, the polymer was Ðnally dried under vacuum. A 0.290
g amount (44% yield) of yellow solid was obtained. FT-IR
(KBr), m /cm~1 3044, 2961, 2923, 2855, 1467, 1250, 1194,
max
1164, 1108, 1078, 1064, 959, 836, 810, 769, 720, 641, 464. 1H
NMR (CDCl , ppm), d 7.96È7.80 (br, 1H), 7.78È7.62 (br, 1H),
3
7.58È7.36 (br, 2H), 1.47È1.20 (br, 8H), 1.04È0.72 (br, 10H),
0.56È0.24 (br, 12H). Anal. Calcd. for (C
H, 10.36; Found: C, 72.59; H, 10.24%.
H
Si ) : C, 72.65;
20 34 2 n
Materials
Results and discussion
Tetrahydrofuran (THF) was distilled over sodiumÈ
benzophenone.
2,5-Dibromo-p-xylene,
potassium
tert-
The synthetic route for BS-PPV is facile and the detailed con-
ditions are outlined in Scheme 1. The monomer was prepared
by radical bromination of 2,5-butyldimethylsilyl-p-xylene,
which was synthesized through coupling of the Grignard
butoxide (1.0 M solution in THF), N-bromosuccinimide
(NBS) and magnesium turnings, were purchased from Aldrich
and butyldimethylsilyl chloride from Fluka. All chemicals
were used without further puriÐcation.
reagent
2,5-dibromomagnesium-p-xylene
and
butyl-
dimethylsilyl chloride in THF. The monomer was polymerized
through the Gilch route in the presence of potassium tert-
butoxide in THF. The polymer thus obtained is a yellow solid
after precipitation of its chloroform solution twice in meth-
anol. The polymer is fully soluble in common organic solvents
such as chloroform, THF, toluene, and xylene. Its molecular
weight was measured by means of GPC using THF as eluent
and polystyrene as standard to be M \ 73 400 and M \
187 400 with a polydispersity index of 2.55. The degree of
polymerization of the polymer was calculated as 222. A high
molecular weight and long conjugated chain of the backbone
are good for the preparation of smooth and uniform Ðlms for
polymer LED device fabrication and can lower the speed of
crystallization of the thin Ðlms in LED devices.
The thermal stability of the polymer in air was evaluated by
TGA and di†erential thermal analysis (DTA). The TGA
results showed that the polymer has high thermal stability.
The thermogram is depicted in Fig. 1. The onset of degrada-
tion of the polymer is at about 312 ¡C. It can be seen that
there are two weight loss steps for the polymer. The Ðrst step
is from 312 to 532 ¡C with a maximum weight loss rate at
2,5-Bis(butyldimethylsilyl)-p-xylene. The Grignard reagent
2,5-bisbromomagnesium-p-xylene was prepared by reÑuxing
the mixture of 2,5-dibromo-p-xylene (5.28 g, 20 mmol) and
magnesium turnings (1.06 g, 44 mmol) in 20 ml of anhydrous
THF for 4 h. To this solution, cooled in an ice-bath, was
added a solution of butyldimethylsilyl chloride (6.63 g, 44
mmol) in 20 ml of THF. The mixture was reÑuxed for 24 h
and then cooled in an ice-bath. After it had been quenched
with saturated ammonium chloride aqueous solution, THF
was evaporated and the residue was extracted twice with 30
ml of hexane. The combined organic layer was washed with
water, brine and then dried over anhydrous magnesium
sulfate. After the solvent had been evaporated under reduced
pressure, the residue was puriÐed by silica gel chromatog-
raphy with hexane as eluent. A 3.54 g amount of colorless
liquid was obtained with a yield of 53%. MS, m/z 334. 1H
n
w
NMR (CDCl , ppm), d 7.19 (2H, s), 2.42 (6H, s), 1.42È1.27
3
[8H, m, È(CH ) È], 0.93È0.79 (10H, m, ÈSiCH È and ÈCH ),
2 2
2
3
Si : C,
2
0.30 [12H, s, ÈSi(CH ) ]. Anal. Calcd. for C
3 2
H
20 38
71.77; H, 11.44; Found: C, 71.93; H, 11.50%.
2,5-Bis(butyldimethylsilyl)-1,4-bisbromomethylbenzene.
A
5.01 g (15.0 mmol) amount of 2,5-bisbutyldimethylsilyl-p-
xylene, 5.40 g (30.0 mmol) of NBS, a catalytic amount of
benzyl peroxide (BPO) and 150 ml of benzene were charged
into a Ñask. The reaction mixture was stirred at ambient tem-
perature and irradiated with a tungsten lamp for 2 h. The
solution was then washed with water three times followed by
brine. The organic phase was dried over anhydrous magne-
sium sulfate. After Ðltration and evaporation of the solvent,
the crude product was puriÐed by silica gel chromatography
to give 3.01 g of colorless liquid product (41% yield). MS, m/z
492. FT-IR (KBr), m /cm~1 3075, 2949, 2929, 2848, 1469,
max
1413, 1376, 1343, 1249, 1208, 1191, 1168, 1126, 910, 877, 865,
842, 826, 810, 789, 753, 719, 701, 681, 659, 538, 482, 458, 439.
1H NMR (CDCl , ppm), d 7.49 (2H, s), 4.58 (4H, s, ÈCH Br),
3
2
1.44È1.27 [8H, m, È(CH ) È], 0.92È0.85 (10H, m, ÈSiCH È and
2 2
ÈCH ), 0.39 [12H, s, ÈSi(CH ) ]. 13C NMR (CDCl , ppm), d
3 2
142.0, 140.0, 137.6, 34.1, 26.4, 26.0, 16.0, 13.7, 1.6. Anal. Calcd.
2
3
3
Scheme 1 The synthetic route for BS-PPV. Reagents and conditions:
(i) MgÈTHFÈC H (CH ) SiCl; (ii) NBSÈbenzeneÈBPOÈhm; (iii) tert-
for C
H
Br Si : C, 48.77; H, 7.37; Br, 32.45; Found: C,
20 36
2 2
4
butoxideÈTHF.
9
3 2
48.53; H, 7.42; Br, 32.29%.
3790
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(Bu NBF ) in acetonitrile, which was obtained in a three-
4
4
electrode cell. The polymer Ðlms exhibited
a partially
reversible n-doping process when a cathodic scan was per-
formed. The cathodic peak occurred at [ 1.95 V with a corre-
sponding anodic peak at [ 1.78 V (vs. SCE), which represents
a charging and discharging process. Along with the potential
scan, the color of the Ðlms changed from green to purple at
Ðrst, then to brown after oxidation. The onset for the n-
doping process is [ 1.81 V. It is noted that BS-PPV also
shows a p-doping process. The peak potential is 1.37 V,
accompanied by a color change from green to blueÈgreen and
to yellowÈgreen after reduction. The onset of p-doping took
place at 1.16 V. The negative sweep of the polymer Ðlm could
be repeated for at least Ðve cycles without any signiÐcant
change in the CV curves, whereas for the positive sweep the
corresponding current intensity gradually decreased with the
repeated cycles.
From the onset potentials of the oxidation and reduction
processes, it can be estimated that the bandgap of BS-PPV is
2.97 eV, which is higher than the value obtained from the
optical absorption spectrum. The di†erence may be caused by
the interface barrier between the polymer Ðlm and the elec-
trode surface. The optical value corresponds to the energy gap
between the valence band and the conduction band, while the
electrochemical value may be a combination of the results of
the optical bandgap coupled with the interface barrier for
charge injection, which makes it larger. However, the electro-
chemical system is much closer to the practical PLEDs device
conÐguration. Hence the bandgap obtained should be more
meaningful as a reference for PLED device design.
Fig. 1 TGA/DTA of BS-PPV in air.
484 ¡C. The second step is in the range 532È680 ¡C and the
maximum weight loss rate occurs at 624 ¡C. At 400 ¡C, the
weight loss in air is just about 5%. Above 680 ¡C, no further
weight loss takes place and there is about 20% of residue
remaining, which may be charcoal and silicon oxide produced
during the degradation. From the percentage weight loss, it
can be deduced that the Ðrst weight loss is related to the side-
chain degradation and the second weight loss corresponds to
the decomposition of the polymer backbone.
Fig. 2 shows the UV-Vis absorption and photoluminescence
spectra of BS-PPV. The UV-Vis absorption spectra of the Ðlm
and solution samples are similar, with the same wavelength of
maximum absorption at 434 nm. The spectrum of the Ðlm
sample is slightly broader than that of the solution, which is
associated with more pÈp* transition energy levels along the
polymer backbone in the Ðlm state. The bandgap can be esti-
mated to be 2.45 eV from the absorption edge of the Ðlm
sample. The photoluminescence spectra of solution and Ðlm
samples reveal some di†erences. The main peak for the solu-
tion sample is at 494 nm with a shoulder at 526 nm, whereas
the maximum emission for the Ðlm samples takes place at 512
nm with a well resolved side peak at 543 nm, which means the
emission area lies in the green region. The side peaks of the
solution and Ðlm samples are associated with vibronic coup-
ling of excitons. Using quinine sulfate (ca. 1 ] 10~5 M) in 0.1
M H SO as the standard, the quantum yield of BS-PPV in
chloroform (ca. 1 ] 10~6 M) was measured.18 The absolute
quantum yield is as high as 72%, which is much higher than
that of MEH-PPV (14%) measured under the same conditions
in our laboratory.
The electrochemical properties of BS-PPV were investi-
gated by CV. The electrochemical redox behavior of the
polymer Ðlm is an important parameter for its consideration
as a light-emitting device. In addition, the energy levels of the
lowest unoccupied molecular orbital (LUMO) and the highest
occupied molecular orbital (HOMO) of the polymer can be
determined from the oxidation and reduction potentials of the
polymer Ðlms. Fig. 3 presents the cyclic voltammogram of
BS-PPV in 0.10 M tetrabutylammonium tetraÑuoroborate
According to the empirical relationship obtained by Ðtting
of the experimental data through the valence e†ective Hamil-
tonian (VEH) technique, the solid state ionization potential
(I ) and electron affinity (E ) has a correlation with the E@
p
a
ox
and E@ of the onset potentials for oxidation and reduction.19
red
These onset potentials are indicated in Fig. 3. The correlation
can be expressed as: I \ E@ ] 4.4 eV and E \ E@ ] 4.4
p
ox
a
red
eV. The electrochemical measurement determined that E@ and
ox
E@ are 1.16 and [ 1.81 V, respectively, and I and E of the
polymer can be roughly estimated as 5.56 and 2.59 eV, respec-
tively.
red
p
a
2
4
The I and E values, optical bandgap, energy barriers of
p
a
holes injected from ITO (*E ) and electrons from Al (*E )
h
e
into the polymer and the imbalance of the energy barrier (*E
e
[ *E ) are listed in Table 1. For the sake of comparison, the
h
data related to PPV and MEH-PPV are also included. The
work functions of ITO and Al are 4.6 and 4.2 eV, respec-
tively.20,21 The energy barriers for hole and electron injection
from ITO and Al into BS-PPV Ðlm are 0.96 and 1.61 eV,
respectively. It can be seen that there is a 0.65 eV barrier dif-
ference (*E [ *E ) for the injection of the two kinds of elec-
e
h
Fig. 3 Cyclic voltammograms of BS-PPV Ðlms coated on platinum
Fig. 2 UV-Vis absorption and PL spectra of BS-PPV in solution
and in the solid state. (a) UV, solution; (b) UV, Ðlm; (c) PL, solution;
(d) PL, Ðlm.
plate electrodes in acetonitrile containing 0.10 M Bu NBF . Platinum
4
4
wire as the counter electrode. The potentials are referenced against
Ag/0.10 M AgNO in acetonitrile. Scan rate: 10 mV s~1.
3
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Table 1 Comparison of optical bandgap, ionization potential (I ),
Conclusions
p
electron affinity (E ), the energy barrier and barrier imbalance of
a
charge injection of BS-PPV, PPV and MEH-PPV
Fully soluble poly(p-phenylenevinylene) containing two silyl
groups as pendants was synthesized through a polymerization
reaction. TGA indicated that the polymer is stable below
300 ¡C in air. UV-Vis spectra of the polymer solution and Ðlm
samples exhibited the same maximum absorption. The optical
bandgap was measured to be 2.45 eV from the absorption
edge of the Ðlm samples. PL spectra indicated that the
polymer emits intense green light. A quantum efficiency of
72% of the polymer solution was observed. CV indicated that
the polymer is electrochemically active, and can be both
n-doped and p-doped. The n-doping process is partially
reversible. However, the p-doping cycle is irreversible, which
could be attributed to the instability of the doped state. The
onset potentials of the oxidation and reduction processes were
determined and the HOMO and LUMO energy levels were
estimated to be 5.56 and 2.59 eV, respectively. The HOMO is
much lower than those of PPV and MEH-PPV which can be
ascribed to the introduction of silyl side-chains into the
polymer. Single layer LEDs which deployed BS-PPV as the
emissive layer sandwiched between ITO and Al were fabri-
cated. The turn-on voltage of the device is about 6 V and
visible green light can be observed at above 7.5 V. Internal
quantum efficiencies of 0.1È0.2% were obtained based on the
single layer devices.
E (optical)/ I /
E /
*E / *E / *E È*E /
g
p
a
h
e
e
eV
h
Polymer
eV
eVa eVa
eVb
eVb
BS-PPV
PPVc
2.45
2.5
5.56 2.59 0.96
5.0
4.9
1.61 0.65
2.5
2.8
0.4
0.3
1.7
1.4
1.3
1.1
MEH-PPVc 2.1
a The ionization potential (I ) and electron affinity (E ) values of
p
a
BS-PPV were measured using an electrochemical method. b *E and
h
*E are the energy barrier of holes and electrons injected into the
e
polymer from ITO and Al, respectively. c The data for PPV,
MEH-PPV and the work functions of Al and ITO were taken from
the literature.20h22
trical charges. However, this di†erence is much lower than
those of PPV20 and MEH-PPV,22 which are 1.3 and 1.1 eV,
respectively as shown in Table 1. The relatively smaller barrier
di†erence for injection of electrons and holes may provide a
chance to fabricate devices using an air stable Al cathode with
more balanced carriers and high electroluminescence quantum
efficiency. From Table 1, we can see that the di†erence in the
E values of the three polymers is in a small range, whereas I
a
p
of BS-PPV is much lower than those of PPV and MEH-PPV,
which leads to a small barrier di†erence between *E and
e
*E . This could be attributed to the silyl groups introduced
h
into the polymer as the side-chains which increase the oxida-
References
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can be expected that the performance would be better if the
devices were fabricated and measured under an inert atmo-
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Fig. 4 Current density vs. bias voltage characteristics of an ITO/BS-
PPV/Al device.
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Phys. Chem. Chem. Phys., 1999, 1, 3789È3792