Development of a new conjugated polymer containing dialkoxynaphthalene for efficient polymer solar cells and organic thin film transistors

Article abstract of DOI:10.1002/pola.24526

A new semiconducting polymer, poly((5,5-E-α-((2-thienyl)methylene)-2- thiopheneacetonitrile)-alt-2,6-[(1,5-didecyloxy)naphthalene])) (PBTADN), an alternating copolymer of 2,3-bis-(thiophene-2-yl)-acrylronitrile and didecyloxy naphthalene, is synthesized and used as an active material for organic thin film transistors (OTFTs) and organic solar cells. The incorporation of 2,3-bis-(thiophene-2-yl)-acrylronitrile as an electron deficient group and didecyloxy naphthalene as an electron rich group resulted in a relatively low bandgap, high charge carrier mobility, and finally good photovoltaic performances of PBTADN solar cells. Because of the excellent miscibility of PBTADN and PC₇₁BM, as confirmed by Grazing Incident X-ray Scattering (GIXS) measurements and Transmission Electron Microscopy (TEM), homogeneous film morphology was achieved. The maximum power conversion efficiency of the PBTADN:PC₇₁BM solar cell reached 2.9% with a V_oc of 0.88 V, a short circuit current density (J_sc) of 5.6 mA/cm², and a fill factor of 59.1%. The solution processed thin film transistor with PBTADN revealed a highest saturation mobility of 0.025 cm²/Vs with an on/off ratio of 10⁴. The molecular weight dependence of the morphology, charge carrier mobility, and finally the photovoltaic performances were also studied and it was found that high molecular weight PBTADN has better self assembly characteristics, showing enhanced performance.

Full text of DOI:10.1002/pola.24526

Development of a New Conjugated Polymer Containing
Dialkoxynaphthalene for Efficient Polymer Solar Cells and
Organic Thin Film Transistors
1
1
1
1
2
JUN HO KWON, JI-YOUNG AN, HANMAE JANG, SOLJI CHOI, DAE SUNG CHUNG,
3
2
1
2
3
MIN-JUNG LEE, HYO-JUNG CHA, JIN-HEE PARK, CHAN-EON PARK, YUN-HI KIM
1
Kyeong-Nam Science High School, Jinsung 660-851, Korea
2
Department of Chemical Engineering, Polymer Research Institute, Pohang University of Science
and Technology, Pohang 790-784, Korea
3
Department of Chemistry and RINS, Gyeongsang National University, Chinju 660-701, Korea
Received 12 July 2010; accepted 20 November 2010
DOI: 10.1002/pola.24526
Published online 3 January 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A new semiconducting polymer, poly((5,5-E-a-((2-
thienyl)methylene)-2-thiopheneacetonitrile)-alt-2,6-[(1,5-didecy-
loxy)naphthalene])) (PBTADN), an alternating copolymer of 2,3-
bis-(thiophene-2-yl)-acrylronitrile and didecyloxy naphthalene,
is synthesized and used as an active material for organic thin
film transistors (OTFTs) and organic solar cells. The incorpora-
tion of 2,3-bis-(thiophene-2-yl)-acrylronitrile as an electron defi-
cient group and didecyloxy naphthalene as an electron rich
group resulted in a relatively low bandgap, high charge carrier
mobility, and finally good photovoltaic performances of
PBTADN solar cells. Because of the excellent miscibility of
PBTADN and PC71BM, as confirmed by Grazing Incident X-ray
Scattering (GIXS) measurements and Transmission Electron
Microscopy (TEM), homogeneous film morphology was
achieved. The maximum power conversion efficiency of the
PBTADN:PC71BM solar cell reached 2.9% with a Voc of 0.88 V, a
2
short circuit current density (Jsc) of 5.6 mA/cm , and a fill factor
of 59.1%. The solution processed thin film transistor with
2
PBTADN revealed a highest saturation mobility of 0.025 cm /Vs
4
with an on/off ratio of 10 . The molecular weight dependence
of the morphology, charge carrier mobility, and finally the pho-
tovoltaic performances were also studied and it was found that
high molecular weight PBTADN has better self assembly char-
acteristics, showing enhanced performance. VC 2011 Wiley Peri-
odicals, Inc. J Polym Sci Part A: Polym Chem 49: 1119–1128,
2011
KEYWORDS: dialkoxynaphthalene; organic thin film transistor;
polymer solar cell
INTRODUCTION Organic semiconducting materials show
(HOMO) and the lowest unoccupied molecular orbital
(LUMO) of PC BM limits the magnitude of the short circuit
current and open circuit voltage.
physical properties similar to those of typical inorganic semi-
6
1
1
–11
25
conductors.
Polymer solar cells have been developed as a
promising cost effective alternative to inorganic-based solar
cells, offering to realize low cost, light weight, and flexible
For an effective polymer solar cell, the two most decisive
parameters are the open-circuit voltage (V ) and the short-
oc
1
2,13
cells.
Bulk-heterojunction-type solar cells use a phase
circuit current (J ). J is primarily determined by the light
sc
sc
separated blend of organic electron donor and acceptor
components, where the conjugated polymer is often used as
the donor and the fullerene derivative is used as the
absorption ability of the material, the charge-separation effi-
ciency, and the high and balanced carrier mobilities. Voc is
limited by the difference in the HOMO of the donor and
1
4–22
acceptor.
LUMO of the acceptor, where a small V represents a smaller
oc
2
6
driving force for the PV process.
Highly efficient polymer solar cells based on poly(3-hexyl-
thiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl
ester (PC BM) have been reported with power conversion
Fullerene derivatives are commonly used as electron accept-
ors for polymer solar cells. Therefore, changing the band gap
and energy levels of the donor material by accordingly
designing the molecular structure is a convenient approach
6
1
2
3,24
efficiencies of 4–5%.
However, further improvement of
the efficiency of P3HT-based polymer solar cell (PSC)s is
constrained due to their absorption not matching well with
the solar spectrum. Furthermore, the relatively small energy
level offset between the highest occupied molecular orbital
to achieve a high Voc and J . Low-bandgap donor–acceptor
sc
alternating conjugated polymers have received considerable
interest as active materials for donor materials of organic
Correspondence to: C.-E. Park (E-mail: cep@postech.ac.kr) or Y.-H. Kim; (E-mail: ykim@gsnu.ac.kr)
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 1119–1128 (2011) V
2011 Wiley Periodicals, Inc.
C
DEVELOPMENT OF A NEW CONJUGATED POLYMER CONTAINING DIALKOXYNAPHTHALENE, KWON ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthetic route for the polymer.
photovoltaics. Typical donor–acceptor conjugated polymers
are composed of electron deficient aromatic heterocycles and
electron rich derivatives.
naphthalene unit offers high mobility of charges due to
enhanced stacking properties through interdigitaion. It can
also act as an electron rich unit with a high Voc compared
with reported polythiophene, although the alkoxy group
slightly decreases the Voc. Most of all, the (1,5-didecyloxy)-
naphthalene unit can be easily prepared. Thus, the newly
designed D–A alternating copolymer is expected to provide
promising solar cell performance due to improved charge
transport, high self-organization capability, and appropriate
2
7–33
Recently, we reported an alternating copolymer, composed of
alkoxy naphthalene and bithiophene, that showed high
6
charge carrier mobility and high oxidation stability. Naph-
thalene is one of the smallest linear planar p-systems and
has a relatively low HOMO level. The introduction of sym-
metrical long dialkoxy groups could increase the stacking
6
,34
Voc. In addition, the newly designed D–A alternating copoly-
properties through easy interdigitation.
mer incorporating planar p–p system allows long range
intermolecular p–p stacking and enhances the stacking prop-
erties through easy interdigition, which can lead to high
charge carrier mobility.
Here, we designed a new semiconducting polymer, poly((5,5-
E-a-((2-thienyl)methylene)-2-thiopheneacetonitrile)-alt-2,6-
[
(1,5-didecyloxy)naphthalene])) (PBTADN), an alternating
copolymer of 2,3-bis-(thiophene-2-yl)-acrylronitrile and di-
decyloxy naphthalene. The introduced 2,3-bis-(thiophene-2-
yl)-acrylronitrile unit has a vinyl linkage between adjacent
thiophene units. An advantage of incorporating this group is
that it serves to extend the conjugation length, which could
lead to a decreased bandgap. Furthermore, the nitril group
in the vinyl linkage can effectively attract electrons through
the vinyl linkage. Thus, 2,3-bis-(thiophene-2-yl)-acrylronitrile
is an electron deficient unit. In addition, the (1,5-didecyloxy)-
RESULTS AND DISCUSSION
The synthetic route for the polymer is shown in Scheme 1.
The borate compound was prepared by bromination, alkyla-
tion, and litation. (E)-2,3-bis-(5-bromothiophen-2-yl)-acrylo-
nitrile was obtained by bromination and aldol condensation.
Polymerization was carried out by a Suzuki coupling reac-
tion. After stirring for 48 h, the end-capping reaction was
TABLE 1 Molecular Weight and Optical and Electrochemical Properties of Polymer
b
c
b
d
e
f
k
absmax
k
emmax
k
emmax
E
gopt
HOMO
(eV)
LUMO
(eV)
a
w
a
Polymer
M
PDI
(film, nm)
(solution, nm)
(film, nm)
(eV)
PBTADN-1
PBTADN-2
a
14,100
44,800
1.37
1.46
561 (523)
571 (531)
595
606
e
680
685
1.89
1.86
5.4
5.4
3.51
3.54
Determined by GPC, relative to polystyrene standards.
Measured in thin film onto fused quartz plates.
Measured in chloroform solution.
Calculated using the empirical equation: Ip(HOMO) ¼ ꢀ(Eonset,ox þ 4.3)
(eV).
b
c
f
Calculated from the HOMO level and optical band gap.
d
The optical band gap, Egopt, is taken as the absorption onset of the
UV-vis spectrum of the polymer film.
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1
FIGURE 1 H NMR and IR spectroscopies of polymer.
afforded by the addition of bromobenzene following benzene
boronic acid. The polymer was purified by precipitation, col-
umn chromatography, Soxhlet, and precipitation. The poly-
mer was fractionated by a column chromatography solvent.
PBTADN-2 was obtained by column chromatography using
chloroform as an eluent after PBTADN-1 was obtained by
DEVELOPMENT OF A NEW CONJUGATED POLYMER CONTAINING DIALKOXYNAPHTHALENE, KWON ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 TGA (a) and DSC (b) thermograms of polymer.
column chromatography using toluene as an eluent. After
PBTADN-2 and PBTADN-1 were, respectively, purified by
Soxhlet using methanol for elimination of catalyst, they were
further purified by Soxhlet using toluene. PBTADN-2 was
soluble in chloroform, chlorobenzene, dichlorobenzene, and
hot toluene whereas PBTADN-1 was soluble in toluene, chlo-
roform, chlorobenzene, and dichlorobenzene at room tem-
perature. A gel permeation chromatography (GPC) analysis
in chloroform solution relative to polystyrene standards
revealed that the molecular weight depended on the fractio-
nated solvent, as shown in Table 1. PBTADN-1 has a weight
cyano group produces a medium intensity sharp band in the
ꢀ
1
triple bond region at 2210 cm
in the IR spectrum. The
CAH stretching of aromatic and vinyl protons appeared at
ꢀ
1
3
060 and 3010 cm
and CAH stretching of aliphatic
ꢀ
1
appeared at 2950 cm . The thermogravimetric analysis
(
TGA) thermogram revealed that the onset decomposition
ꢁ
temperature of the polymer under nitrogen flow was 340 C,
indicating good thermal stability [Fig. 2(a)]. Differential scan-
ning calorimetry (DSC) showed a reversible transition at 200
ꢁ
C, which suggests the possibility of melting of the long alkyl
side chain [Fig. 2(b)].
w
average molecular weight (M ) of 14,100 with a polydisper-
sity index (PDI) of 1.37. PBTADN-2 has a weight average mo-
The absorption and photoluminescence (PL) spectra of the
polymer in the film state are presented in Figure 3.
PBTADN-1 in chloroform solution has an absorption maxi-
mum at 496 nm, whereas PBTADN-1 of the film has a well
defined vibronic structure with an absorption maximum at
523 nm and a red-shifted new shoulder at 561 nm, indica-
tive of strong intermolecular interactions in a solid state. For
the PBTADN-2 of the film, the absorption maximum was
lecular weight (M ) of 44,800 with PDI of 1.46. The struc-
w
1
ture of the polymer was confirmed by H NMR and IR
spectroscopies (Fig. 1). In the H NMR, aromatic and vinyl
1
protons appeared at 7–8 ppm and oxy-methylene protons
and other aliphatic protons appeared at 4 ppm and 2–0.8
ppm, respectively. The integration ratio of aromatic, oxy-
methylene, and other aliphatic protons was 9:4:38. The
FIGURE 3 The absorption (a) and photoluminescence (b) spectra of PBTADN-1 and PBTADN-2 measured at both solution and thin
film. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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ARTICLE
FIGURE 4 J–V curves of a PBTADN-1: PC71BM (1:4) (a) and PBTADN-2: PC71BM (1:4) (b) solar cells (red line: dark current). [Color
figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
observed at 531 nm with an intense shoulder peak at 571
nm. This result indicates that the relatively high molecular
weight of PBTADN-2 resulted in expanded conjugation length
and increased intermolecular interaction in the film state
due to a reduced end group, compared with PBTADN-1. The
optical band gap of PBTADN-1 and PBTADN-2 was calculated
from the absorption edge and found to be 1.89 eV and 1.86
eV, which are 0.10–0.15 eV smaller than that of the reported
ity despite that it is a donor–acceptor polymer. The LUMO
was calculated to be 3.55 eV from the optical band gap and
HOMO.
Bulk heterojunction polymer solar cells were fabricated
using PBTADN-1 and PBTADN-2 as an electron donor mate-
rial and (6,6)-phenyl-C -butyric acid methyl ester (PC BM)
7
1
71
as an electron acceptor. The device structures are ITO/
PEDOT-PSS/polymer:PC71BM (1:4 weight ratio)/LiF/Al after
careful optimization processes. We have optimized the
PBTADN:PCBM devices using both PC BM and PC BM and
1
5
value of poly(alkylthiophene) homopolymer. The photolu-
minescence emission spectra of the polymer show a red
emission with kmax at 595 nm in a dilute chloroform solution
and a broad, featureless very weak emission band at 685 nm
in a thin film. It is suggested that the narrow band gap and
the weak emission originated from ICT (intramolecular
charge transfer) interaction of the polymer with D–A. The
HOMO and LUMO energy levels of the polymer thin film are
characterized by cyclic voltammetry (CV). The onset oxida-
tion potential was 1.0 V, corresponding to a HOMO of 5.4 eV,
6
1
71
changing the processing solvents. We have found that
PBTADN provided maximum photovoltaic performances
when mixed with PC BM (1:4 ratio) in dichlorobenzene so-
7
1
lution. Figure 4 shows the J–V curve of a PBTADN-1 and
PBTADN-2 solar cell under an AM 1.5 condition at 100 mW/
2
cm . The maximum PCE of the PBTADN-2:PC BM solar cell
7
1
reached 2.9% with a Voc of 0.88 V, a short circuit current
2
density (J ) of 5.6 mA/cm , and a fill factor of 59.1%. In the
sc
3
5
which is 0.3 eV higher than that of P3HT. The polymer was
found to have a deep HOMO level with a narrow band-gap,
with potential for a high Voc as well as high oxidation stabil-
case of PBTADN-1, the maximum PCE of the solar cell was
1.4% with a V of 0.8 V, a short circuit current density (Jsc)
oc
2
of 4.1 mA/cm , and a fill factor of 43%. From the results, we
FIGURE 5 Optical absorption spectra of PBTADN-1: PC71BM (1:4) (a) and PBTADN-2: PC71BM (1:4) (b) blend films on glass/ITO/
PEDOT substrates. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
DEVELOPMENT OF A NEW CONJUGATED POLYMER CONTAINING DIALKOXYNAPHTHALENE, KWON ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 7 TEM images of PBTADN-1:PC71BM (1:4) and
PBTADN-2:PC71BM (1:4).
Although both PBTADN-1:PC BM and PBTADN-2:PC BM
7
1
71
blends have homogeneous morphology with domain size
below 10 nm, lower molecular weight and amorphous-like
PBTADN-1:PC BM has much finer morphology with domain
7
1
size even below 5 nm. It is well known that there are two
morphological parameters that should be achieved altogether
for efficient bulk heterojunction cells. The first is large do-
nor–acceptor interfacial area, which can guarantee sufficient
exciton dissociation into holes/electrons. The second is an
interpenetrating network structure, which can serve as an
effective percolation pathway for each charge carrier to
reach collecting electrodes. Therefore, nanoscale (10–20 nm)
phase separation with a continuous network is believed to
be the ideal morphology for high performance of bulk heter-
ojunction cells. In the PBTADN-1:PC BM (1:4) blend film,
FIGURE 6 GIXS patterns of PBTADN-1, PBTADN-1:PC71BM
(
1:4), PBTADN-2, and PBTADN-2:PC71BM (1:4).
found that the polymer has a relatively high Voc whereas the
Jsc value is quite low relative to those of widely used poly-
thiophenes. Furthermore, we also found that the associated
photovoltaic parameters, including V , the short circuit cur-
oc
7
1
rent density (J ), and the fill factor depend on the molecular
sc
although large donor–acceptor interfacial area was achieved
by excellent miscibility, it appears that a percolating pathway
for each charge carrier was not established, while those mor-
phologies were partially achieved in the PBTADN-2:PC BM
weight of the polymer. Figure 5 shows the optical absorption
spectrum of
a PBTADN-2:PC BM (1:4) and PBTADN-
71
1:PC71BM (1:4) blend film using dichlorobenzene on glass/
7
1
ITO/PEDOT. The absorption bands of both blend films were
distributed similarly within 400–600 nm. It is reported that
Jsc is strongly dependent on the charge transport, absorption
blend film. These morphological characteristics can explain
the low photovoltaic performance of PBTADN-1 (lower
short-circuit current density and thus low PCE) despite its
high Voc and the good photovoltaic performance of PBTADN-
2. Figure 8 represents the IPCE (incident photon to current
efficiency) of the PBTADN-1:PC BM (1:4) and PBTADN-
3
6
coefficient, and morphology of the active layer.
Therefore, we can see that these molecular weight depend-
ent photovoltaic performances of PBTADN are not due to the
difference in the light absorbing ability, but rather differen-
ces of morphology, structure, and the resulting charge carrier
mobility.
7
1
2:PC BM (1:4) blend solar cell. The IPCE data indicate that
7
1
PBTADN-1 or PBTADN-2 acts as a photoexcited electron
The molecular organization and the blend film morphology
were investigated by GIXS and TEM (Figs. 6 and 7). Although
the pristine PBTADN-2 shows a high crystalline nature with
distinct Bragg refraction peaks of GIXS patterns, PBTADN-
2
:PC BM shows lower crystalline peaks. These results indi-
71
cate that the PBTADN-2:PC BM (1:4) blend has excellent
7
1
miscibility. In the same manner, PBTADN-1 also shows
decreased crystalline peaks in the blend film compared with
the pristine film, revealing excellent miscibility. However, it
is important to note that the overall crystallinity of PBTADN-
2
is much higher than that of PBTADN-1 in terms of both
pristine and blend films. This can be explained by the fact
that PBTADN-1 with relatively low molecular weight has
many end groups, leading to increased solubility with
PC BM and inhibition of the ordered structure. TEM images
FIGURE 8 IPCE (incident photon to current efficiency) of the
PBTADN-1:PC71BM (1:4) blend solar cell and PBTADN-
2:PC71BM (1:4) blend solar cell.
7
1
also show similar results (Fig. 7).
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ARTICLE
TABLE 2 Electrical Parameters of PBTADN-Based OFETs and Solar Cells
a
M
J
sc
PCE
(%)
2
2
Polymer
(cm /Vs)
I
on/Ioff
V
t
(V)
(mA/cm )
V
oc
FF
3
PBTADN-1
PBTADN-2
a
0.002
0.025
>10
ꢀ15
ꢀ10
4.1
5.6
0.8
43
1.4
2.9
4
>10
0.88
59.1
2
Top contact OTFT was fabricated with a OTS treated-SiO of 300 nm dielectric and 45 nm of polymer as
organic semiconductor. Channel lengths (L) of 100 lm and channel widths (W) of 2000 lm are used.
donor although it is a minor contributor (20%) to photocur-
EXPERIMENTAL
rent in the device.
Materials
The effects of thermal treatment of the polymers were
also investigated. By application of an annealing process,
both PBTADN-1 and PBTADN-2 showed a tendency to re-
All reagents and solvents were purchased from Aldrich
Chemical Co. and Fluka. Only analytical grade quality chemi-
cals were used. PEDOT was purchased from Bayer. Spectro-
crystallize, leading to an undesired morphology for
bulkheterojunction.
a
scopic grade CHCl (Aldrich) was used for all absorption and
3
emission experiments. All other compounds were used as
received.
Field Effect Transistors
The charge transport properties of the polymer were investi-
gated by fabricating an OFET with a bottom-gate top-contact
Measurement and Characterization
H NMR spectra data are expressed in ppm relative to the
1
internal standard and were obtained on a DRX 500 MHz
NMR spectrometer. FT-IR spectra were obtained with a
Bomem Michelson series FT-IR spectrometer, and the UV-vis
absorption spectra were obtained in chloroform on a Shi-
madzu UV 3100 spectrometer. The molecular weight and
polydispersity of the polymer were determined by a GPC
analysis with polystyrene standard calibration (Waters high
pressure GPC assembly model M590 pump l-styragel col-
umns of 105, 104, 103, 500, and 100 Å, refractive index
2
device geometry. Onto the heavily n-doped Si/SiO substrate,
a spin-coated polymer film was prepared using chloroform
as a solvent. Field effect mobility was measured in the satu-
ration regime using the relationship lsat ¼ (2IDSL)/[WC(Vg–
2
V ) ], where I is the saturation drain current, C is the ca-
th
DS
pacitance of the oxide dielectric, V is the gate bias, and V
g
th
is the threshold voltage. The mobility and on/off ratios of
PBTADN-1 and PBTADN-2 are summarized in Table 2. As
shown in the XRD spectrum of PBTADN-2 (Fig. 9), diffraction
detectors, solvent CHCl ). An elemental analysis was per-
3
ꢁ
ꢁ
peaks are observed at 2h ¼ 4.53 (100), 9.10 (200), indicat-
ing a high degree of crystallinity in the film. The XRD spec-
trum of the PBTADN-1 film also showed weak diffractions
formed using a Leco Co. CHNS-932. TGA measurements were
performed on a Perkin–Elmer series 7 analysis system under
ꢁ
N at a heating rate 10 C/min. The photoluminescence spec-
2
ꢁ
ꢁ
with peaks at 2h ¼ 4.06 (100) and 8.38 (200). The diffrac-
tra were recorded on a Perkin–Elmer LS-50 fluorometer uti-
lizing a lock-in power meter (Newport 818-SL). Cyclic
ꢁ
ꢁ
tion peak at 4.53 of PBTADN-2 and that at 4.06 of
PBTADN-1 correspond to an interlayer lamellar d (100)-
spacing of 19.5 Å and 21.7 Å, respectively. The measured
interlayer distance suggests that the alkoxy chains of the
polymer interdigitate. It also indicates that high molecular
weight PBTADN-2 with smaller end groups has a more inter-
digitated structure, leading to closer d (100)-spacing com-
pared with PBTADN-1. Because of its high crystallinity of
PBTADN-2 with high molecular weight yielded superior an
electrical performance to that of low-crystalline PBTADN-1
with low molecular weight, as expected from XRD results. As
shown in Figure 10(a,b), PBTADN-2 revealed typical transfer
2
curves, a highest saturation mobility of 0.025 cm /Vs, and an
4
on/off ratio of 10 , from the OTS-pretreated device.
PBTADN-1 exhibited an order of magnitude lower hole mo-
2
3
bility of 0.002 cm /Vs, and an on/off ratio of 10 . The
enhanced charge carrier mobility in the higher molecular
weight polymer is likely due to a reduction in defects arising
from chain ends and enhanced intermolecular interaction.
Therefore, we can confirm that the molecular weight de-
pendent photovoltaic performances of PBTADN are due to
the differences of morphology and charge carrier mobility as
predicted.
FIGURE 9 Out-of-plane XRD spectra of PBTADN thin films
(Inset: Log plot to compare peak positions between two poly-
mers). [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
DEVELOPMENT OF A NEW CONJUGATED POLYMER CONTAINING DIALKOXYNAPHTHALENE, KWON ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 10 Transfer characteristics of thin film transistors based on (a) PBTADN-1 and (b) PBTADN-2. Output characteristics of
those based on (c) PBTADN-1 and (d) PBTADN-2. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
voltammetry was carried out in a two-compartment cell with
a model with platinum electrodes at a scan rate of 10 mV/s
against an Ag/Ag (0.1 M AgNO3 in acetonitrile) reference
Fabrication of the OFET Devices
Top contact OFETs were fabricated on a common gate of
highly n-doped silicon with a 300 nm thick thermally grown
þ
electrode in an anhydrous and nitrogen-saturated solution of
2
SiO dielectric layer. To produce hydrophobic dielectric surfa-
0
.1 M Bu NBF in acetonitrile.
ces, octyltrichlorosilane (OTS) layers were coated onto the
SiO2 dielectric layer. Films of the organic semiconductor
were spin coated under various conditions, with a nominal
thickness of 45 nm. Gold source and drain electrodes were
evaporated with a shadow mask on top of each semiconduc-
tor (100 nm). Channel lengths (L) of 100 lm and channel
widths (W) of 2000 lm were used for all measurements.
The electrical characteristics of the FETs were measured in
air using Keithley 2400 and 236 source/measure units. The
field-effect mobilities were extracted in the saturation regime
from the slopes of the source-drain currents.
4
4
Solar Cell Device Fabrication and Measurement
The acceptor compound PC BM was co-dissolved in 1,2-
7
1
dichlorobenzene (o-DCB). ITO-coated glass (Sinhan SNP
Korea) substrates were cleaned stepwise in water, acetone,
and isopropyl alcohol under ultrasonication for 10 min and
treated with oxygen plasma before use. PEDOT:PSS (Baytron
P VP A1 4083) was then spin-coated (5000 rpm, 40 nm
thick) on top of ITO-coated glass. After being soft baked at
ꢁ
150 C for 1 min, a polymer/PC BM composites active layer
7
1
(
9
80 nm thick) was then spin-cast from the blend solutions at
00 rpm for 40 s on the ITO/PEDOT:PSS substrate. LiF (1
nm) and Al (100 nm) layers were deposited in sequence
Synthesis
(1) Synthesis of 2,6-Dibromo-1,5-dihydroxy Naphthalene
1,5-Dihydroxy naphthalene (20 g, 0.12 mol) and a catalytic
amount of iodine were added to acetic acid solvent (550 mL).
ꢀ
6
under a vacuum of 2 ꢂ 10 torr, and were subsequently
ꢁ
ꢁ
annealed at 150 C for 18 min. Measurement of the devices
After the mixture was heated to 80 C, bromine (40 g, 0.25
was performed under simulated AM 1.5G irradiation (100
mol) was slowly added. After the mixture was stirred for 30
min, the flask was cooled to room temperature. The crude
product was filtered and washed with petroleum ether.
2
mW/cm ). The effective area of the film was measured to be
2
0
.09 cm .
1126
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
1
Yield: 57%. H NMR (500 MHz, CDCl ) [ppm] d 7.75 (d, 2H),
7
were added in a solution of 200 mL of methanol and 1.3 g
3
ꢀ
1
ꢀ2
.64 (d, 2H), 6.0 (s, 2H) FT-IR (KBr, cm ): 2950 (aliphatic
CAH str), 3050 (aromatic CAH str), 1180 (aromatic CABr
str), C NMR (500 MHz, CDCl ) [ppm] d 152.8, 131.0, 130.1,
19.3, 113.5 Anal. Calcd. for C H Br O2:C, 37.98; H, 1.90;
(2.5 ꢂ 10 mol) of sodium methoxide. After the reaction mix-
ture was stirred for 24 h at room temperature, the sediment
was filtered. The crude product was purified by column chro-
matography using methylene dichloride as an eluent.
1
13
3
1
10 6 2
Found: C, 37.75; H, 1.92.
Yield: (4.7 g) 52%, H NMR (CDCl , ppm) d 7.27 (d,1H), 7.26
3
ꢀ
1
(
s,1H), 7.10–7.07 (t,2H), 7.02 (d,1H) FT-IR (KBr, cm ): 3024
(aromatic CAH str), 2214 (nitrile CBN str) C NMR (500
(
2
2) Synthesis of 2,6-Dibromo-1,5-didecyloxy Naphthalene
1
3
ꢀ
2
,6-Dibromo-1,5-dihydroxynaphthalene (15 g, 4.5 ꢂ 10
MHz, CDCl ) [ppm] d 139.4, 138.8, 133.3, 131.3, 131.1,
mol), 1-bromodecane (22 g, 0.1 mol), KOH, and KI (0.75 g, 4.5
3
ꢀ
3
130.7, 127.3, 118.6, 113.3, 102.6 Anal. Calcd. For
ꢂ 10 mol) were mixed in ethanol (200 mL) and refluxed for
C H Br NS : C, 35.40; H, 1.35; Found: C, 35.38; H, 1.37.
4
8 h. After reflux, the flask was cooled to room temperature.
11
5
2
2
The mixture was washed by a 10% KOH aqueous solution (5.5
g, 0.1 mol) to remove residual HBr, and washed again using
water to remove salt. The mixture was stirred with MgSO for
4
drying, and then filtered. The crude product was purified by
column chromatography by hexane as an eluent.
(
6) Polymerization
The polymer was prepared from a palladium catalyzed Suzuki
coupling reaction. All handling of catalysts and polymerization
was done in a nitrogen atmosphere. Pd(PPh ) (0.01 g, 8.0 ꢂ
3
4
0
ꢀ
3
0
1
0
mol%) was added to a mixture of 2,6-(1 ,2 -ethylborate)-
1
ꢀ4
Yield: 25%, H NMR (500 MHz, CDCl ) [ppm] d 7.76 (d, 2H),
1,5-didecylxyloxynaphthalene (0.15 g, 3.0 ꢂ 10 mol), (E)-
3
7
.62 (d, 2H), 4.08 (t, 4H), 1.96 (m, 4H), 1.59 (m, 4H), 1.37 (m,
2,3-bis-(5-bromothiophen-2-yl)-acrylonitrile (0.173 g, 2.5 ꢂ
ꢀ
1
ꢀ4
24H), 0.91 (m, 6H). FT-IR (KBr, cm ): 2950 (aliphatic CAH str),
10 mol), 10 mL THF and 4 mL 2 M K CO . The reaction mix-
2
3
13
ꢁ
3
050 (aromatic CAH str), 1180(aromatic CABr str), C NMR
500 MHz, CDCl ) [ppm] d 152.8, 131.0, 130.1, 119.3, 113.5,
4.6, 31.9, 30.2, 29.8, 29.6, 29.5, 29.3, 26.0, 22.7, 14.1 Anal. Calcd.
For C H Br O : C, 60.38; H, 7.79 Found C, 60.35; H, 7.78.
ture was heated at 80 C under a nitrogen atmosphere for 48
ꢀ
3
ꢀ5
(
7
3
h. Bromobenzene (5.0 ꢂ 10 g, 3.2 ꢂ 10 mol) was added
ꢀ3 ꢀ5
and then phenyl boronic acid (5.0 ꢂ 10 g, 4.1 ꢂ 10 mol)
was added with small amounts of catalysts for end-capping.
After 2 h, the reaction mixture was poured into methanol (50
mL) and filtered with a glass filter. The residue was dissolved
in CHCl3 and washed with water. The precipitation step was
repeated twice with chloroform/methanol.
30
46
2 2
(3) Synthesis of 2,6-Bis(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olan-2-yl)-1,5-didecyloxynaphthalene
2
,6-Dibromo-1,5-didecyloxy naphthalene (5.0 g, 8.3 ꢂ 10ꢀ3
mol) was dissolved in THF solvent, and then the mixture was
1
ꢁ
ꢀ2
Yield: 45%, H NMR (300 MHz, CDCl ) [ppm] d 7–8 (m, 9H),
4.0 (d, 4H), 0.5–2 (m, 38 H), FT-IR (KBr, cm ): 3100 (aro-
matic CAH str), 2950 (aliphatic CAH str), 2214 (nitrile CBN
str), C NMR (500 MHz, CDCl ) [ppm] d 152.0, 143.7, 139.6,
then stirred at ꢀ78 C. n-BuLi (7 mL, 1.8 ꢂ 10 mol) was
3
ꢀ
1
ꢁ
dropped, and stirred at ꢀ78 C for 1 h. In the mixture, 2-iso-
propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (14 g, 7.4 ꢂ
1
3
ꢀ
2
10
mol) was added and the mixture was stirred at room
3
1
1
3
38.3, 133.4, 132.5, 131.2, 129.7, 129.0, 128.2, 127.7, 126.4,
25.7, 123.8, 122.7, 119.3, 117.0, 102.1, 74.7, 32.0, 30.5,
0.2, 29.7, 29.4, 26.3, 26.1, 22.7, 14.2 Anal. Calcd. For
temperature for overnight. After reaction, the mixture was ter-
minated with water. The obtained crude product was then
purified by column chromatography using hexane as eluent.
1
(
C H NO S ) : C, 75.30; H, 7.88; Found: C, 75.13; H, 7.94.
41 51 2 2 n
Yield: 35%, H NMR (500 MHz, CDCl ) [ppm] d 7.94 (m,
3
2
1
H), 7.76 (m, 2H), 4.08 (m, 4H), 1.96 (m, 4H), 1.59 (m, 4H),
.41 (s, 24H), 1.37 (m, 24H), 0.91 (m, 6H) FT-IR (KBr,
CONCLUSIONS
A novel polymeric semiconductor (PBTADN) with a donor–
acceptor alternating structure was synthesized and charac-
terized. Optical characterizations showed that PBTADN has a
substantially good absorbance and deep HOMO level, and is
thus suitable for solar cell applications. From GIXS and TEM
experiments, it was demonstrated that high molecular weight
PBTADN-2 has better self assembly characteristics, thus
achieving both donor–acceptor interfacial area and percolat-
ing pathways for both charge carriers compared with low
molecular weight PBTADN-1. On the basis of these photovol-
taic performance measures, we believe that this new syn-
thetic strategy provides a good approach for further develop-
ment of highly efficient polymer solar cells.
ꢀ
1
cm ): 3100 (aromatic CAH str), 2900 (aliphatic CAH str).
(
2
4) Synthesis of 5-Bromo-2-thiopene Acetonitrile
ꢀ
2
-Thiopene acetonitrile (10 g, 8.1 ꢂ 10 mol) and 15 g (8.4
ꢀ2
ꢂ 10 mol) of N-bromosuccinimide were added to 200 mL
of DMF. After stirring for 5 h, the reaction was quenched
with water. The crude product was obtained from extraction
followed by evaporation. The crude product was purified by
column chromatography using petroleum ether as an eluent.
1
Yield: 72% H NMR (500 MHz CDCl
6
3
) [ppm] d 6.7 (m, 1H),
ꢀ
1
.7 (m, 1H), 3.8 (s, 2H) FT-IR (KBr, cm ): 3059 (aromatic
1
3
CAH str), 2251 (nitrile CN str) C NMR (500 MHz, CDCl3)
ppm] d 132.2, 129.9, 127.5, 116.2, 112.15, 18.6 Anal. Calcd.
For C H BrNS: C, 35.64; H, 2.01; Found: C, 35.79; H, 2.04.
[
This work was supported by KOFAC (R&E 2009), Mid-Career
Researcher Program through NRF grant funded by the MEST
(2010-0027732). Min-Jung Lee thanks to the Ministry of
Knowledge Economy (MKE) and Korea Institute for Advance-
ment in Technology (KIAT) through the Workforce Develop-
ment Program in Strategic Technology.
6
4
(5) Synthesis of (E)-2,3-Bis-(5-bromothiophen-2-yl)-
acrylonitrile
5
4
ꢀ
2
-Bromo-2-thiophene acetonitrile (5.0 g, 2.5 ꢂ 10 mol) and
.7 g (2.5 ꢂ 10ꢀ mol) of 5-bromothiophene-2-cabaldehyde
2
DEVELOPMENT OF A NEW CONJUGATED POLYMER CONTAINING DIALKOXYNAPHTHALENE, KWON ET AL.
1127

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
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