Synthesis, characterization and performance of P3HT-azide-PCBM microgel

Article abstract of DOI:10.1166/jnn.2016.12049

Spherical Poly(3-hexylthiophene)-azide-[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT-azide-PCBM) microgel was synthesized by introducing an azide cross-linkable group into the conjugated polymer. Its UV-vis spectrum shifts blue as a result of the systematic disruption of planarity along the P3HT backbone and a reduction in long range conjugation. And the corresponding fluorescence spectrum also indicates that a strong intra-molecular photo-induced electron transfer is originated from the covalent linkage of PCBM moiety to the P3HT backbone via the hexyl bridge upon photo-excitation. Combined Gel Permeation Chromatography (GPC), dynamic and static light scattering (DLS and SLS) measurements prove that the resultant P3HT-azide-PCBM copolymer is a uniform spherical microgel with a hydrodynamic radius of 130 nm at room temperature. The spherical P3HT-azide-PCBM microgel can be used as electron donor or compatibilizer in bulk-heterojunction (BHJ) devices, but its introduction reduces the photovoltaic performance compared with the blend of P3HT/PCBM, because the hole mobility in the crosslinked polythiophene backbones is largely decreased.

Full text of DOI:10.1166/jnn.2016.12049

Article
Journal of
Nanoscience and Nanotechnology
Vol. 16, 5639–5645, 2016
Copyright © 2016 American Scientific Publishers
All rights reserved
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Printed in the United States of America
Synthesis, Characterization and Performance of
P3HT-Azide-PCBM Microgel
Guangmin Wei1ꢀ2, Xinxin Li2, Di Jia2, and He Cheng1ꢀ3ꢀ∗
1Dongguan Institute of Neutron Science (DINS), Dongguan 523808, China
2State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, CAS, Beijing 100190, China
3China Spallation Neutron Source (CSNS), Institute of High Energy Physics (IHEP), CAS, Dongguan 523803, China
Spherical Poly(3-hexylthiophene)-azide-[6,6]-phenyl-C61-butyric acid methyl ester (P3HT-azide-
PCBM) microgel was synthesized by introducing an azide cross-linkable group into the conjugated
polymer. Its UV-vis spectrum shifts blue as a result of the systematic disruption of planarity along
the P3HT backbone and a reduction in long range conjugation. And the corresponding fluorescence
spectrum also indicates that a strong intra-molecular photo-induced electron transfer is originated
from the covalent linkage of PCBM moiety to the P3HT backbone via the hexyl bridge upon photo-
excitation. Combined Gel Permeation Chromatography (GPC), dynamic and static light scattering
(DLS and SLS) measurements prove that the resultant P3HT-azide-PCBM copolymer is a uni-
form spherical microgel with a hydrodynamic radius of 130 nm at room temperature. The spherical
P3HT-azide-PCBM microgel can be used as electron donor or compatibilizer in bulk-heterojunction
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(BHJ) devices, but its introduction reduces the photovoltaic performance compared with the blend
IP: 185.71.3.155 On: Fri, 06 May 2016 07:14:10
of P3HT/PCBM, because the hole mobility in the crosslinked polythiophene backbones is largely
Copyright: American Scientific Publishers
decreased.
Keywords: P3HT-Azide-PCBM, Microgel, Bulk Heterojunction Solar Cells, Stability,
Nano-Morphology.
1. INTRODUCTION
can be easily produced by spin casting of a blend solu-
tion on substrates, the control of donor–acceptor blend
morphology is extremely difficult since the morphology
is obtained by kinetically trapping of a non-equilibrium
state during the process of crystallization coupled with
spinodal decomposition. Most BHJ polymer solar cells are
not thermally stable because subsequent exposure to heat
will drive further development of the morphology toward
a state of crystallization and macrophase separation in the
micrometer scale.9ꢀ10
One major approach to improve the thermal stabil-
ity of conjugated polymer-fullerene BHJ photovoltaic
devices is the use of chemical cross-linkable units to
prevent phase segregation in active layer. In 2005,
Gaudiana et al. synthesized cross-linkable fullerene deriva-
tives containing glycidyl functionality. They succeeded
in suppressing the large aggregation of the fullerene
derivative under thermal annealing after acid-catalyzed
cross-linking.11 In 2009, Hashimoto et al. prepared a
cross-linkable regioregular poly(3-(5-hexenyl)thiophene)
Polymer based organic photovoltaics have attracted
lots of attentions as a potential candidate of cost-
effectiveness light-weighted flexible solar cells.1ꢀ2 A sig-
nificant breakthrough has been achieved with the advent of
bulk-heterojunction (BHJ) devices based on ꢁ-conjugated
polymers as electron donors and fullerene derivatives as
electron acceptors.3ꢀ4 Although a number of polymer-
fullerene combinations have been tested,5ꢀ6 BHJ solar
cells based on poly(3-hexylthiophene) (P3HT) and the
fullerene derivative [6,6]-phenyl-C61-butyric acid methyl
ester (PCBM) still represent the state of the art in
organic photovoltaics with reproducible efficiencies as
high as 5%.7
A key parameter for efficient BHJ solar cells is
the three-dimensional nano-morphology of the active
layer that provides a large interfacial area for exciton
dissociation.8 Though the active layer of BHJ solar cells
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 6
1533-4880/2016/16/5639/007
doi:10.1166/jnn.2016.12049
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Synthesis, Characterization and Performance of P3HT-Azide-PCBM Microgel
Wei et al.
(P3HNT). P3HNT underwent cross-linking at the vinyl
groups of the side chains upon a thermal treatment and
the formation of large aggregations of PCBM was pre-
vented in P3HNT:PCBM films even after prolonged ther-
mal annealing.12 In 2012, Kim et al. introduced azide
cross-linkable units to P3HT and produced a solvent-
resistant hole-transporting material for OTFTs and an
in-situ compatibilizer in OPVs.13 Unfortunately, the chem-
ical cross-linking will lead to significantly reduced device
performance in spite of the stabilized P3HT/PCBM
morphology.11ꢀ12 Therefore, the development of materi-
als with both stable morphology and high performance
remains a challenge, though the cross-linking concept is
simple and powerful.
2.2. Materials
3-hexylthiophene was purchased from Puyang Huicheng
Electronic Material Co., Ltd. 3-bromothiophene, N -bromo-
succinimide (NBS), n-butyllithium (n-BuLi, 1.6
solution in hexanes), 1,6-dibromohexane, 1,3-dibromo-
5,5-dimethylhydantoin, Lithium diisopropylamide (LDA,
2.0 M solution in THF), anhydrous ZnCl2, Ni(dppp)Cl2
and 18-crown-6 were purchased from J&K Scientific Ltd.
NaN3 was purchased from Xiya Reagent. PCBM was pur-
chased from Solarmer Energy, Inc. All of these chemicals
were used as received. THF and n-hexane were dried over
Na/benzophenone. DMF was dried by anhydrous MgSO4.
M
2.3. Synthesis of Monomers
2.3.1. Synthesis of 2-Bromo-3-Hexylthiophene (2)
3-hexylthiophene (1) (5.00 g, 29.7 mmol) was dissolved
in dry THF (50 mL) under an argon atmosphere. NBS
(5.81 g, 32.7 mmol) was separated into 5 parts, and each
In the previous studies, in-situ chemical cross-linking in
BHJ films right after the formation of the optimal mor-
phology is the typical strategy, because it is supposed to
keep the pre-formed suitable structure while improving
morphological stability.11ꢀ14ꢀ15 However, the decrease of
the contrast during chemical reaction and possible reac-
tion induced phase separation result in an elusive mor-
phology in the active layer. Therefore, compared with the
insoluble and inmeltable photo-crosslinked active layer,
it is more desirable to prepare the conducting polymer-
fullerene copolymer to adjust the morphology directly.
In this paper, the synthesis, characterization, optical and
ꢀ
part was added every 5 min at 0 C. After adding the
last part of NBS, the solution was stirred at 0 ꢀC for
10 min. Then the solution was stirred at room temperature
for 2 hrs. The solvent was removed in vacuum and 100 mL
of hexane was added (to precipitate all the succinimide).
The mixture was filtered and the solvent was removed in
vacuum. The title compound was purified by the column
chromatography (silica/hexane).17ꢀ18
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photovoltaic properties of a cross-linkable P3HT-azide-
PCBM copolymer microgel are reported. In order to gain a
deeper insight into the structure of the P3HT-azide-PCBM,
dynamic and static light scattering are used. Although
there are many choices for the reactive function, we adopt
an azide group at the end of the side chain as the cross-
linkable site because this kind of cross-linkable unit is easy
to prepare and highly reactive.
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2.3.2. Synthesis of 3-(6-Bromohexyl)Thiophene (4)
Copyright: American Scientific Publishers
3-bromothiophene (3) (10.20 g, 62.6 mmol) was dissolved
in dry hexanes (100 mL) under an argon atmosphere, and
cooled to −78 ꢀC. n-BuLi (40 mL, 62.6 mmol) was added
dropwise to the stirred solution, after which the solu-
ꢀ
tion was stirred at −78 C for 10 min. Then, dry THF
was added dropwise via a syringe (ca. 10–15 mL) until
white 3-lithiothiophene salt precipitated. The solution was
ꢀ
then stirred at −78 C for 1 hr, and allowed to warm
2. EXPERIMENTAL DETAILS
2.1. Characterization
to −10 ꢀC. Dry THF (10 mL) and 1,6-dibromohexane
(40 mL, 260.0 mmol) were added to ꢀthe solution, after
which the solution was stirred at −10 C for 10 min and
then warmed to room temperature. After stirred at room
temperature for 2 hrs, the solution was diluted with dry
hexane (100 mL). The separated organic layer was washed
with water, dried over anhydrous MgSO4, and concen-
trated in vacuum. Excess remained 1,6-dibromohexane in
crude product was removed by vacuum distillation, and
then the product was purified by column chromatography
using hexane as the eluent.13ꢀ14
1HNMR spectra were recorded at 298 K with a Bruker
Avance 400 NMR spectrometer. Number average (Mn) and
weight average (Mw) molecular weights were determined
by GPC with polystyrene as standard and CHCl3 as eluent.
FTIR spectra were obtained with Bruker EQUINOX55
FTIR spectrometer. Fluorescence spectra were performed
on a Cary Eclipse spectrometer. The viscosity of o-DCB
was measured by Ubbelohde viscometers (capillary diam-
eter is 0.2 0.01 mm). Light scattering experiments were
carried out on a modified LS spectrometer equipped with a
multi-ꢂ digital time correlator (ALV5000).16 The electro-
chemical cyclic voltammetry experiments were conducted
on a CHI650D Electrochemical Workstation with glassy
carbon disk, Pt wire, and an Ag/Ag+ electrode as the work-
ing electrode, counter electrode, and reference electrode,
respectively in a 0.1 mol/L tetrabutylammonium hexafluo-
rophosphate (Bu4NPF6ꢃ acetonitrile solution.
2.3.3. Synthesis of 2-Bromo-3-(6-Bromohexyl)
Thiophene (5)
3-(6-bromohexyl)thiophene (4) (4.33 g, 17.5 mmol) was
dissolved in dry THF (60 mL). The solution was cooled
in an ice bath, and 1,3-dibromo-5,5-dimethylhydantoin
(2.63 g, 9.2 mmol) was added and the solution was
stirred at room temperature for 2 hrs. After concentrating
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Wei et al.
Synthesis, Characterization and Performance of P3HT-Azide-PCBM Microgel
the solution in vacuum, dry hexane was added, and the
solution was filtered and concentrated in vacuum again.
Further purification was carried out via silica gel column
chromatography, using hexane as the eluent.14
of P3HT/PCBM (1:1, weight ratio), P3HT-azide-
PCBM/PCBM (1:0.8, weight ratio) and P3HT/P3HT-
azide-PCBM/PCBM (1:0.2:1.12, weight ratio) in o-DCB
(10 mg mL−1, based on the polymer weight concentration)
ꢀ
were prepared and stirred at 110 C for more than 24 hrs.
After spin-coating a 30 nm layer of poly(3,4-ethylene
2.4. Synthesis of Polymers
2.4.1. Synthesis of P3HT-Br (6)
dioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)
onto a pre-cleaned ITO coated glass substrates, the solu-
tion was spin-coated. The devices were completed by
evaporating Ca/Al metal electrodes with area of 0.04 cm2
defined by masks. The current–voltage curves are mea-
sured under 100 mW cm−2 standard AM 1.5 G spectrum
using a XES-70S1 (SAN-EI ELECTRIC CO., LTD.) solar
simulator (AAA grade, 70 mm×70 mm photo-beam size).
2 × 2 cm Monocrystalline silicon reference cell (SRC-
1000-TC-QZ) was purchased from VLSI Standards Inc.
The mismatch between solar simulator and the standard
solar spectrum has been considered. The average PCE
were obtained from more than 30 pieces of devices.19
LDA (15 mL, 30.0 mmol) was added dropwise to the mix-
ture of 2-bromo-3-(6-bromohexyl)thiophene (5) (1.96 g,
6.0 mmol) and 2-bromo-3-hexylthiophene (2) (6.68 g,
27.0 mmol) in dry THF (80 mL) at −78 ꢀC. After
1 hr reaction at −78 ꢀC, anhydrous ZnCl2 (5.11 g,
37.5 mmol)/dry THF (50 mL) solution was added drop-
ꢀ
wise via a syringe. The solution was stirred at −78 C
for 30 min and then warmed slowly to room temperature.
Polymerization initiated by adding Ni(dppp)Cl2 (81.3 mg,
0.15 mmol) to the solution and the reaction was carried
out at room temperature for overnight. The polymerization
was ended by 5 mL 1.0 M aqueous HCl. The polymer
was precipitated with methanol (450 mL) containing 7 M
NH3 (6 mL) to neutralize it, and the resulting precipitate
was then filtered. Oligomers and impurities in the product
were removed by soxhlet extractions with methanol and
hexane, followed by chloroform extraction. The resulting
solid was dried under vacuum to yield the product (6).13
3. RESULTS AND DISCUSSION
3.1. Synthesis of the Microgel
Scheme 1 depicts the synthetic approach to obtain P3HT-
azide-PCBM. Cycloaddition of the azide unit to PCBM
completes the synthesis.20ꢀ21 The azido route for the attach-
ment of PCBM, rather than atom transfer radical addi-
tion (ATRA) is used as it is simple and versatile.22ꢀ23
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2.4.2. Synthesis of P3HT-Azide (7)
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1HNMR integration of the proton at the 4-position of the
P3HT-Br (6) (500 mg) was dissolved in THF (125 mL).
The NaN3 (150 mg, 2.28 mmol) and 18-crown-6 (600 mg,
2.28 mmol) was dissolved in DMF (40 mL) and sonica-
tion was applied to obtain homogeneous solution. DMF
solution was added to THF solution and then refluxed for
overnight under an argon atmosphere. The polymer was
precipitated with methanol and the resulting precipitate
was then filtered. Residual NaN3 was removed by soxh-
let extractions with methanol for overnight. The resulting
solid was dried under vacuum to yield the product (7).13
Copyright: American Scientific Publishers
thienyl ring shows that the HT regioregularity of P3HT-Br
is ∼98% with respect to the ꢅ-methylene protons (2H
at 2.78 ppm). The molar ratio of 3-(azidohexyl)thiophene
monomer in P3HT-azide copolymer, x, can be estimated
from the integral area ratio of the peaks for –CH2N3 (ꢆ =
0ꢇ92 ppm) and –CH3 (ꢆ = 3ꢇ28 ppm) groups. According
to the following equation,
A
3ꢈ1 −xꢃ
2x
3ꢇ145
0ꢇ252
−CH3ꢀꢆ=0ꢇ92 ppm
=
=
(1)
A
−CH2N3ꢀꢆ=3ꢇ28 ppm
2.4.3. Synthesis of P3HT-Azide-PCBM (8)
P3HT-azide (500 mg) and PCBM (250 mg) were dissolved
in o-DCB (15 mL), and the solution was stirred at 150 C
for overnight. The solution was cooled to room temper-
ature and filtered with 0.45 ꢄm Millex Millipore PTFE
membrane. The polymer was precipitated with methanol
and the resulting precipitate was then filtered. Residual
PCBM was removed by soxhlet extractions with methanol
for overnight. The resulting solid was dried under vacuum
to yield the product (8).
x can be calculated as 0.11. GPC analysis indicates that the
number average (Mnꢃ and weight average (Mwꢃ molecu-
lar weights, and polydispersity index (PDI) of P3HT-azide
copolymer are 5800 g/mol, 7400 g/mol and 1.26, respec-
tively. Based on the molar ratio of 3-(azidohexyl)thiophene
monomer in P3HT-azide copolymer (xꢃ and the number
average molecular weights (Mnꢃ, a degree of polymeriza-
tion (DP) for P3HT-azide copolymer can be estimated.
The result shows that each P3HT-azide copolymer chain
contains three 3-(azidohexyl)thiophene monomers and 31
3-hexylthiophene monomers on average.
In Scheme 1, the bromine groups of P3HT-Br are
replaced with azide units (N3ꢃ by NaN3 and 18-crown-6 as
a phase transfer catalyst in THF/DMF mixture. Then the
azide units of P3HT-azide have an addition reaction with
PCBM in o-DCB solution. The formation of the azide and
ꢀ
2.5. Device Fabrication and Characterization
Indium-tin oxide (ITO) coated glass substrates were
subjected to ultrasonication in different solvent sys-
tems including acetone, 2% soap in water, deionized
water, and then isopropanol. Three different solutions
J. Nanosci. Nanotechnol. 16, 5639–5645, 2016
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Synthesis, Characterization and Performance of P3HT-Azide-PCBM Microgel
Wei et al.
Scheme 1. Synthetic route for P3HT-azide-PCBM.
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its subsequent functionalization with PCBM can be moni-
measured in Figure 2. The absorption peak at 460 nm
is the extensive ꢁ–ꢁ∗ transition due to the conjugated
P3HT unit,23 and the absorption peak at 330 nm is from
the PCBM moiety.22ꢀ25 When P3HT and PCBM are phys-
ically mixed, the corresponding UV-vis spectrum of the
blend has two distinct absorption peaks at 463 nm and
330 nm, respectively. However, after the introduction and
cycloaddition of azide unit to PCBM, the absorption peak
of P3HT-azide-PCBM has an obvious blue shift as a result
of the systematic disruption of planarity along the back-
bone and a reduction in long range conjugation. This can
be further proved by fluorescence measurement.
Copyright: American Scientific Publishers
tored by IR spectroscopy.15ꢀ24 As shown in Figure 1, after
azidation, a strong characteristic band for the azide group
at 2090 cm−1 (Fig. 1(b)) is observed in FTIR spectrum.
After cycloaddition to PCBM, the peak at 2090 cm−1 com-
pletely disappears (Fig. 1(c)). The appearance and disap-
pearance of the sharp peak of the azide unit at 2090 cm−1
,
conform that azidation and cycloaddition reactions are
successful.
To get a deeper insight into the possible changes
in molecular packing of polymers due to azide unit
introduction and cycloaddition, UV-vis spectrum is
1.6
P3HT-Br
P3HT-azide
P3HT-azide-PCBM
P3HT
2.5
1.4
1.2
1.0
0.8
0.6
a
P3HT/PCBM blend
P3HT-azide-PCBM
a
2.0
1.5
1.0
0.5
0.0
2090 cm–1
b
b
c
c ~ 0.08 mg/g
c
1200
1800
2400
3000
3600
300
400
500
600
700
800
Wavenumber (cm–1
)
Wavelength (nm)
Figure 1. FTIR spectrum of P3HT-Br (a), P3HT-azide (b), and P3HT-
azide-PCBM (c). The dash line is used to guide the eye.
Figure 2. UV-vis spectra of P3HT (a), P3HT/PCBM blend (b) and
P3HT-azide-PCBM (c) in o-DCB at room temperature.
5642
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Wei et al.
Synthesis, Characterization and Performance of P3HT-Azide-PCBM Microgel
It is well known that the fluorescence is useful for prob-
1.2
1.0
0.8
0.6
η = 0.1847 + 0.8390e–(t–21.19)/59.49
ing the intramolecular charge transfer dynamics between
donor and acceptor.26 Figure 3 illustrates the fluorescence
spectrum of P3HT-azide-PCBM in o-DCB in compari-
son with those of pure P3HT and P3HT/PCBM blend.
To make a quantitative comparison, a P3HT/PCBM (1:0.5,
weight ratio) blend solution is prepared to ensure that the
molar ratio of PCBM in the blend is similar (11%) to
that in P3HT-azide-PCBM. Because of the intermolecular
photo-induced electron transfer, the blend has only ∼8.2%
quenching of the fluorescence of pure P3HT (Figs. 3(a)
and (b)). While P3HT-azide-PCBM has ∼58.8% quench-
ing according to the integral of the fluorescence band
(see Figs. 3(a) and (c)). These results clearly prove that a
strong intramolecular photoinduced electron transfer orig-
inating from the covalent linkage of PCBM moiety to
the P3HT backbone via the hexyl bridge in P3HT-azide-
PCBM. The intra-molecular hybrid structure of P3HT-
azide-PCBM enables closer contact of the PCBM moiety
with the thienyl ring via the hexyl bridge, and facili-
tates their ꢁ–ꢁ interactions. However, in P3HT/PCBM
blend counterpart, the ꢁ–ꢁ interactions between P3HT
and PCBM can only take place intermolecularly, resulting
in a weaker quenching.
10
20
30
40
50
60
70
80
t (ºC)
Figure 4. The temperature dependence viscosity of the o-DCB.
decay time, ꢂ, as temperature increases, and the hydro-
dynamic radius Rh has a little decrease correspondingly.
Figure 5(b) shows the hydrodynamic radius of ꢀP3HT-
azide-PCBM in o-DCB is about 130 nm at 20 C and
decreases slightly with temperature. It implies that it is not
a copolymer chain, but a microgel of P3HT-azide-PCBM
in o-DCB solution. From GPC measurement, the number
average molecular weight (Mnꢃ of P3HT-azide copolymer
is about 5.8 kg/mol. If each PCBM reacts with only one
azide unit in cycloaddition, the number average molecular
weight of P3HT-azide-PCBM should be about 8.5 kg/mol,
when each P3HT-azide copolymer chain contains three
3-(azidohexyl)thiophene monomers on average. However,
the hydrodynamic radius of P3HT-azide-PCBM in o-DCB
is about 130 nm at 20 ꢀC, which is too large for a polymer
3.2. Characterization of the Solution Structure
To investigate the solution structure of P3HT–azide-PCBM
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chain with 8.5 kg/mol molecular weight.
The basic shape of P3HT-azide-PCBM in o-DCB can
be determined from the ratio between its radius of gyra-
copolymer, Light scattering experiments are carried out.
IP: 185.71.3.155 On: Fri, 06 May 2016 07:14:10
In dynamic light scattering (DLS),Ctohpeyrhiygdhrto: dAymnaemricican Scientific Publishers
radius Rh can be determined from the Stokes-Einstein rela-
tion Rh = kBT /6ꢁꢉD, where ꢉ is the solvent viscosity.25
Because the viscosity will change with temperature,27 the
temperature dependence of viscosity are measured first.
As shown in Figure 4, the relation between viscosity and
temperature of o-DCB can be represented by the equation:
tion and hydrodynamic radius, Rg/Rh. This is because they
are defined from different ways: Rg is related to the square
average of the segmental distribution ꢁri2j ꢂ and reflects the
density distribution in real space, while Rh is related to
the inverse segmental distribution ꢁꢃrij ꢃ−1Rgꢂ and is equiv-
alent to the radius of a hard sphere with the same trans-
lational diffusion coefficient.28 Theoretically, for a coiled
chain in a good solvent, Rg/Rh = 1ꢇ5, and for a uniform
none draining sphere Rg/Rh = 0ꢇ78.29 Figure 6 is the tem-
perature evolution of Rh, Rg, Rg/Rh of P3HT-azide-PCBM
in o-DCB solution. The results indicate that the Rg/Rh
of the P3HT-azide-PCBM particles keeps a constant of
∼0.80 at different temperatures. Therefore, it is reasonable
to conclude that the shape of P3HT-azide-PCBM micro-
gel in o-DCB solution is close to a uniform sphere where
PCBM acts as crosslinker.
It is known that o-DCB is a good solvent for both P3HT
and PCBM, if P3HT-azide-PCBM formed in cycloaddition
is a copolymer chain, the chain conformation should be a
random coil with Rg/Rh ∼1.5. However, the hydrodynamic
radius of P3HT-azide-PCBM in o-DCB is about 130 nm
which is too large for a random coil, and its Rg/Rh ∼ 0ꢇ8.
Therefore, we consider that the structure of P3HT-azide-
PCBM formed in cycloaddition is a spherical microgel,
which consists of solvent-penetrated crosslinked three-
dimensional polymer networks with dimensions between
ꢉ = 0ꢇ1847 +0ꢇ8390e−ꢈt−21ꢇ19ꢃ/59ꢇ49
(2)
Figure 5(a) is the temperature evolution of intensity–
intensity time correlation function of P3HT-azide-PCBM
in o-DCB. There is a slight decrease of the characteristic
P3HT
P3HT-azide-PCBM
P3HT/PCBM blend
a
300
200
100
0
b
Excitation: 450nm
c ~ 0.2mg/g
c
500
600
700
800
900
Wavelength (nm)
Figure 3. Fluorescence spectra of P3HT (a), P3HT-azide-PCBM (b),
and P3HT/PCBM blend (c) in o-DCB at room temperature. The excita-
tion wavelength was 450 nm.
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Synthesis, Characterization and Performance of P3HT-Azide-PCBM Microgel
Wei et al.
(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
20ºC
30ºC
40ºC
50ºC
60ºC
70ºC
80ºC
90ºC
100ºC
20ºC
30ºC
40ºC
50ºC
60ºC
70ºC
80ºC
90ºC
100ºC
1.0
0.8
0.6
0.4
0.2
0.0
decrease
10–3 10–2 10–1 100 101 102 103 104
101
102
103
Rh (nm)
τ (ms)
Figure 5. The intensity–intensity time correlation functions (a) and the corresponding hydrodynamic radius (b) of P3HT-azide-PCBM in o-DCB at
different temperatures.
50 nm to a few micrometers,30 as illustrated in Figure 7.
According to the literature, PCBM is an electron-acceptor
and contains pentagonal and hexagonal rings.15 Thus, the
reaction of azide with PCBM may proceed through mono,
tri, even higher addition.31 The multi-addition leads to
cross-linking microgel, where PCBM is just like a chemi-
cal cross-linker.
self assemble. Through this morphological evolution, the
featureless and homogeneous blend gradually reaches a
bicontinuous interpenetrating D–A network with optimal
D–A domain size.7 As a result, maximum interfacial area
for efficient charge generation and a higher degree of
nanoscale P3HT crystallinity can be achieved for better
charge transport.32 Unfortunately, this optimal morphol-
ogy is a kinetically trapped metastable state that read-
ily moves toward a more thermodynamically stable state
if thermal annealing at elevated temperature is applied
constantly.10ꢀ33ꢀ34 The P3HT domain continues to undergo
further crystallization, while the spherical PCBM, with
its high molecular mobility, tends to diffuse out of the
3.3. Photovoltaic Properties
The photovoltaic properties of the devices fabri-
cated under optimal conditions are listed in Table I.
For the BHJ OPV devices based on P3HT-azide-
PCBM:PCBM and P3HT:P3HT-azide-PCBM:PCBM, the
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polymer matrix and aggregates into larger clusters or sin-
donor/acceptor (D/A) weight ratio of 1:0.8 and 1:0.2:1.12
IP: 185.71.3.155 On: Fri, 06 May 2016 07:14:10
gle crystals.35 Considering that a photovoltaic device must
are used in the active layer. As a reference, the
donor/acceptor (D/A) weight ratio for the device of
P3HT:PCBM should be 1:1. Compared to the reference,
the Voc is slightly reduced when P3HT-azide-PCBM is
used in the OPV devices. For the device of P3HT-azide-
PCBM:PCBM, P3HT-azide-PCBM is used as the donor,
and the Jsc and FF reduce greatly. While, for the device
of P3HT:P3HT-azide-PCBM:PCBM, P3HT-azide-PCBM
is used as the compatibilizer, and the Jsc and FF reduce
slightly. The decrease of Jsc and FF may be attributed to
the decrease in mobility with the presence of crosslinked
P3HT-azide-PCBM microgel.
Copyright: American Scientific Publishers
be exposed to long-term sunlight irradiation, the accumu-
lated heat may raise the operational temperature above the
glass transition temperature (Tg) of polymers, thus destroy-
ing the optimal morphology, and deteriorating the device
performance.36 In order to enhance the thermal stability of
the active layer, preserving the morphology by chemical
cross linking of donor and acceptor is an ideal method,12ꢀ37
but the introduction of cross-linking bridges in conju-
gated polymers often disturbs their molecular packing,
significantly reducing their performance.11 According to
the previous studies, in the poly(3-alkylthiophene)s films,
the ꢁ–ꢁ stacking of the polymers is largely affected
by the packing of the side chains.38ꢀ39 The introduc-
tion of large functionalities to the side chains can result
in the decrease of crystallinity, thereby decreasing the
hole mobility in the polythiophene chains.40 Furthermore,
Thermal annealing of P3HT:PCBM composite provides
external energy, which drives the P3HT to reorganize and
200
180
160
140
120
100
140
120
100
80
1.2
0.9
0.6
0.3
Rh (nm)
Rg (nm)
Rg / Rh
60
15
30
45
60
75
90
105
T (ºC)
Figure 6. Rh, Rg, Rg/Rh of P3HT-azide-PCBM in o-DCB at different
Figure 7. Schematic representation of the two different routes in
temperatures.
cycloaddition.
5644
J. Nanosci. Nanotechnol. 16, 5639–5645, 2016

Wei et al.
Synthesis, Characterization and Performance of P3HT-Azide-PCBM Microgel
Table I. Photovoltaic performance of the device based on P3HT-
azide-PCBM:PCBM, P3HT:P3HT-azide-PCBM:PCBM and P3HT:PCBM
under the illumination of AM 1.5G, 100 mW/cm2.
9. E. Klimov, W. Li, X. Yang, G. G. Hoffmann, and J. Loos,
Macromolecules 39, 4493 (2006).
10. H. Zhong, X. Yang, B. deWith, and J. Loos, Macromolecules 39, 218
(2005).
11. M. Drees, H. Hoppe, C. Winder, H. Neugebauer, N. S. Sariciftci,
W. Schwinger, F. Schaffler, C. Topf, M. C. Scharber, Z. Zhu, and
R. Gaudiana, J. Mater. Chem. 15, 5158 (2005).
12. S. Miyanishi, K. Tajima, and K. Hashimoto, Macromolecules
42, 1610 (2009).
Voc
(V)
Jsc
(mA/cm2ꢃ
FF
(%)
PCE
(%)
Samples
P3HT-azide-PCBM:PCBM
P3HT:P3HT-azide-PCBM:PCBM 0.626
P3HT:PCBM 0.635
0.627
1.41
5.26
5.73
30.7 0ꢇ272
64.6 2ꢇ11
69.7 2ꢇ53
13. H. J. Kim, A. R. Han, C.-H. Cho, H. Kang, H.-H. Cho, M. Y. Lee,
J. M. J. Fréchet, J. H. Oh, and B. J. Kim, Chem. Mater. 24, 215
(2011).
14. B. J. Kim, Y. Miyamoto, B. Ma, and J. M. J. Fréchet, Adv. Funct.
Mater. 19, 2273 (2009).
P3HT-azide-PCBM is a chemical cross-linked spherical
microgel, and its chain mobility is largely reduced in the
active layer.
15. B. Gholamkhass and S. Holdcroft, Chem. Mater. 22, 5371 (2010).
16. D. Jia, J.-F. Yang, H. Cheng, and C. C. Han, Acta Polym. Sin. 5, 564
(2015).
17. R. S. Loewe, S. M. Khersonsky, and R. D. McCullough, Adv. Mater.
11, 250 (1999).
18. Z. Gu, Y. Tan, K. Tsuchiya, T. Shimomura, and K. Ogino, Polymers
3, 558 (2011).
19. Y. Huang, X. Guo, F. Liu, L. Huo, Y. Chen, T. P. Russell, C. C. Han,
Y. Li, and J. Hou, Adv. Mater. 24, 3383 (2012).
20. X. Yang and J. Loos, Macromolecules 40, 1353 (2007).
21. J. Roncali, Chem. Rev. 92, 711 (1992).
4. CONCLUSIONS
In summary, the synthesis, characterization, optical and
photovoltaic properties of a cross-linkable P3HT-azide-
PCBM microgel are reported. P3HT-azide-PCBM is syn-
thesized by introducing an azide cross-linkable group into
the conjugated polymer. Compared with P3HT/PCBM
blend, the absorption band of P3HT-azide-PCBM shifts
blue greatly and its fluorescence intensity is deeply
quenched. According to DLS measurement, in o-DCB
solution, the P3HT-azide-PCBM copolymer is not a poly-
mer chain, but a chemical cross-linked spherical micro-
gel. The spherical shape of P3HT-azide-PCBM has large
influence on its photovoltaic performance because the hole
mobility in the polythiophene chains is reduced.
22. S. Xiao, S. Wang, H. Fang, Y. Li, Z. Shi, C. Du, and D. Zhu,
Macromol. Rapid Comm. 22, 1313 (2001).
23. B. Gholamkhass, T. J. Peckham, and S. Holdcroft, Polym. Chem.
1, 708 (2010).
24. M. H. van der Veen, B. de Boer, U. Stalmach, K. I. van de Wetering,
and G. Hadziioannou, Macromolecules 37, 3673 (2004).
25. G. Wei, D. Yao, Z. Li, Y. Huang, H. Cheng, and C. C. Han,
Delivered by Ingenta to: McMaster University
Macromolecules 46, 1212 (2013).
IP: 185.71.3.155 On: Fri, 06 May 2016 07:14:10
Copyright: American S2c6i.enMt.ifiÁc. PHuerbralnisz,hBe.rsIllescas, N. Martín, C. Luo, and D. M. Guldi,
Notes
J. Org. Chem. 65, 5728 (2000).
The authors declare no competing financial interest.
27. R. T. Sanderson, Chemistry 41, 368 (1949).
28. Y. Huang, H. Cheng, and C. C. Han, Macromolecules 44, 5020
(2011).
29. C. Wu, A. Niu, Y. Zhao, C. Li, and Y. Yang, Macromol. Rapid
Comm. 22, 704 (2001).
30. H. Cheng and G. Z. Zhang, Hydrogel Micro and Nanoparticles,
edited by L. A. Lyon and M. J. Serpe, Wiely-VCH, Germany (2012),
Vol. 1, p. 1.
31. C. J. Hawker, Macromolecules 27, 4836 (1994).
32. H. Hoppe and N. S. Sariciftci, J. Mater. Chem. 16, 45 (2006).
33. X. Yang, J. K. J. van Duren, R. A. J. Janssen, M. A. J. Michels, and
J. Loos, Macromolecules 37, 2151 (2004).
34. A. Swinnen, I. Haeldermans, M. vande Ven, J. D’Haen,
G. Vanhoyland, S. Aresu, M. D’Olieslaeger, and J. Manca, Adv.
Funct. Mater. 16, 760 (2006).
35. X. Yang, J. K. J. van Duren, M. T. Rispens, J. C. Hummelen, R. A. J.
Janssen, M. A. J. Michels, and J. Loos, Adv. Mater. 16, 802 (2004).
36. M. Jørgensen, K. Norrman, and F. C. Krebs, Sol. Energ. Mat. Sol. C
92, 686 (2008).
Acknowledgments: This work is supported by the
National Natural Scientific Foundation of China (Grant
No. 21174152), the Innovation Project of the Institute
of High Energy Physics CAS (2015IHEPYJRC901), and
Capital (Beijing) Science and Technology Resources Plat-
form (Z131110000613065).
References and Notes
1. K. M. Coakley and M. D. McGehee, Chem. Mater. 16, 4533 (2004).
2. M. S. Ramasamy, S. S. Mahapatra, and J. W. Cho, J. Nanosci.
Nanotechnol. 14, 7553 (2014).
3. G. Yu, J. Gao, J. Hummelen, F. Wudl, and A. Heeger, Science
270, 1789 (1995).
4. S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger,
T. Fromherz, and J. C. Hummelen, Appl. Phys. Lett. 78, 841 (2001).
5. J. K. Lee, W. L. Ma, C. J. Brabec, J. Yuen, J. S. Moon, J. Y. Kim,
K. Lee, G. C. Bazan, and A. J. Heeger, J. Am. Chem. Soc. 130, 3619
(2008).
37. Y.-J. Cheng, C.-H. Hsieh, P.-J. Li, and C.-S. Hsu, Adv. Funct. Mater.
21, 1723 (2011).
38. Y. Zhang, K. Tajima, K. Hirota, and K. Hashimoto, J. Am. Chem.
Soc. 130, 7812 (2008).
39. B. C. Thompson, B. J. Kim, D. F. Kavulak, K. Sivula, C. Mauldin,
and J. M. J. Fréchet, Macromolecules 40, 7425 (2007).
40. K. Tajima, Y. Suzuki, and K. Hashimoto, J. Phys. Chem. C 112, 8507
(2008).
6. J.-F. Qu, J. Liu, and Z.-Y. Xie, Chinese J. Polym. Sci. 31, 815 (2013).
7. W. Ma, C. Yang, X. Gong, K. Lee, and A. J. Heeger, Adv. Funct.
Mater. 15, 1617 (2005).
8. X. Yang, J. Loos, S. C. Veenstra, W. J. H. Verhees, M. M. Wienk,
J. M. Kroon, M. A. J. Michels, and R. A. J. Janssen, Nano Lett.
5, 579 (2005).
Received: 12 December 2014. Accepted: 28 January 2015.
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