Synthesis and properties of phenothiazylene vinylene-based polymers: New organic semiconductors for field-effect transistors and solar cells

Article abstract of DOI:10.1002/pola.23814

A series of new phenothiazylene vinylene-based semiconducting polymers, poly[3,7-(4′-dodecyloxyphenyl)phenothiazylene vinylene] (P1), poly[3,7-(4′-dodecyloxyphenyl)phenothiazylene vinylene-alt-1 ,4-phenylene vinylene] (P2), and poly[3,7-(4′-dodecyloxyphenyl)phenothiazylene vinylene-alt-2,5thienylene vinylene] (P3), have been synthesized via a HornerEmmons reaction. FTIR and ¹H NMR spectroscopies confirmed that the configurations of the vinylene groups in the polymers were all-trans (E). The weight-averaged molecular weights (Mw) of P1, P2, and P3 were found to be 27,000, 22,000, and 29,000, with polydispersity indices of 1.91, 2.05, and 2.25, respectively. The thermograms for P1, P2, and P3 each contained only a broad glass transition, at 129, 167, and 155 °C, respectively, without the observation of melting features. UVvisible absorption spectra of the polymers showed two strong absorption bands in the ranges 315-370 nm and 450-500 nm, which arose from absorptions of the phenothiazine segments and the conjugated main chains. Solution-processed fieldeffect transistors fabricated from these polymers showed ptype organic thin-film transistor characteristics. The field-effect mobilities of P1, P2, and P3 were measured to be 1.0 × 10^-4, 3.6 × 10^-5, and 1.0 × 10^-3 cm² V^-1 s^-1, respectively, and the on/off ratios were In the order of 10² for P1 and P2, and 10³ for P3. Atomic force microscopy and X-ray diffraction analysis of thin films of the polymers show that they have amorphous structures. A photovoltaic device in which a P3/PC⁷¹BM (1/5) blend film was used as the active layer exhibited an open-circuit voltage (V_oc) of 0.42 V, a short circuit current (J_sc) of 5.17 mA cm^-2, a fill factor of 0.35, and a power conversion efficiency of 0.76% under AM 1.5 G (100 mW cm^-2) illumination.

Full text of DOI:10.1002/pola.23814

Synthesis and Properties of Phenothiazylene Vinylene-Based Polymers:
New Organic Semiconductors for Field-Effect Transistors and Solar Cells
SEON-KYOUNG SON,1 YOON-SUK CHOI,1 WOO-HYUNG LEE,1 YONGTAEK HONG,2 JAE-RYOUNG KIM,3 WON-SUK SHIN,3
SANG-JIN MOON,3 DO-HOON HWANG,4 IN-NAM KANG1
1Department of Chemistry, Catholic University of Korea, Bucheon, Gyeonggi-do 420-743, Korea
2Department of Electrical Engineering and Computer Science, Seoul National University, 599 Gwanak-ro, Gwanak-gu,
Seoul 151-744, Korea
3Energy Materials Research Division, Korea Research Institute of Chemical Technology, P. O. Box 107, Yuseong,
Daejeon 305-600, Korea
4Department of Applied Chemistry, Kumoh National Institute of Technology, Kumi 730-701, Korea
Received 28 September 2009; accepted 3 November 2009
DOI: 10.1002/pola.23814
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A series of new phenothiazylene vinylene-based
semiconducting polymers, poly[3,7-(40-dodecyloxyphenyl)phe-
nothiazylene vinylene] (P1), poly[3,7-(40-dodecyloxyphenyl)phe-
nothiazylene vinylene-alt-1,4-phenylene vinylene] (P2), and
poly[3,7-(40-dodecyloxyphenyl)phenothiazylene vinylene-alt-2,5-
thienylene vinylene] (P3), have been synthesized via a Horner-
Emmons reaction. FTIR and 1H NMR spectroscopies confirmed
that the configurations of the vinylene groups in the polymers
were all-trans (E). The weight-averaged molecular weights
(Mw) of P1, P2, and P3 were found to be 27,000, 22,000, and
29,000, with polydispersity indices of 1.91, 2.05, and 2.25,
respectively. The thermograms for P1, P2, and P3 each con-
tained only a broad glass transition, at 129, 167, and 155 ꢀC,
respectively, without the observation of melting features. UV–
visible absorption spectra of the polymers showed two strong
absorption bands in the ranges 315–370 nm and 450–500 nm,
which arose from absorptions of the phenothiazine segments
and the conjugated main chains. Solution-processed field-
effect transistors fabricated from these polymers showed p-
type organic thin-film transistor characteristics. The field-effect
mobilities of P1, P2, and P3 were measured to be 1.0 ꢁ 10ꢂ4
,
3.6 ꢁ 10ꢂ5, and 1.0 ꢁ 10ꢂ3 cm2 Vꢂ1
s
ꢂ1, respectively, and the
on/off ratios were in the order of 102 for P1 and P2, and 103 for
P3. Atomic force microscopy and X-ray diffraction analysis of
thin films of the polymers show that they have amorphous
structures. A photovoltaic device in which a P3/PC71BM (1/5)
blend film was used as the active layer exhibited an open-cir-
cuit voltage (VOC) of 0.42 V, a short circuit current (JSC) of 5.17
mA cmꢂ2, a fill factor of 0.35, and a power conversion effi-
ciency of 0.76% under AM 1.5 G (100 mW cmꢂ2) illumination.
C
V
2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
48: 635–646, 2010
KEYWORDS: charge carrier mobility; charge transport; conju-
gated polymers; heteroatom-containing polymers; organic so-
lar cell; organic thin film transistor; phenothiazine; PPV;
semiconducting polymer; spin coating
INTRODUCTION In recent years, conjugated polymers have
been developed for applications such as polymer light-emit-
ting diodes,1–4 organic thin-film transistors (OTFTs),5–9 and
photovoltaic solar cells.10–13 OTFTs based on solution-pro-
cessable organic semiconductors have been increasingly
investigated for use in low-cost memory devices, large-area
display devices, and flexible electronic devices.14–17 Molecu-
lar design and synthesis of new conjugated semiconducting
polymers with high charge-carrier mobilities may allow real-
ization of high-performance solution-processable OTFTs.
High performance in organic electronic devices requires the
development of methodologies that allow precise control of
the polymer electronic structure to achieve a desired band
gap, and highest occupied molecular orbital (HOMO)–lowest
unoccupied molecular orbital (LUMO) energy levels. Consider-
able effort has been devoted to understand the relationship
between energy levels in a polymer and OTFT performance.18
The control over morphology and crystallinity of polymer thin
films is also important, and chemical and physical methods
have been used to this end, for example, side-chain-induced
self-assembly and thermal treatment of polymer thin films. To
realize such objectives, much work has focused on introducing
chemical modifications to the conjugated moieties or incorpo-
rating new conjugated building blocks into the polymer
chains.19–23 Among the conjugated polymers studied to date,
polythiophenes,24,25 fluorene-based copolymers,26,27 and
poly(p-phenylene vinylene) derivatives8,28–33 have yielded
high performances in OTFT applications. Phenothiazine-based
Correspondence to: I.-N. Kang (E-mail: inamkang@catholic.ac.kr)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 635–646 (2010) 2009 Wiley Periodicals, Inc.
PHENOTHIAZYLENE VINYLENE-BASED POLYMERS, SON ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
conjugated polymers have also been investigated for use in var-
ious electronic devices as they have good electron-donating
and transporting behavior because of the presence of electron-
rich sulfur and nitrogen atoms.34–37 In our early work, we syn-
thesized conjugated homopolymers and copolymers of pheno-
thiazine and fluorene units to give hole and charge mobilities of
10ꢂ4 cm2 Vꢂ1 sꢂ1 with an on/off ratio of 103.38 However, the
mobility of the polymer remained low, possibly due to the non-
optimized chemical structure. To modulate the electronic prop-
erties of the phenothiazine-based polymers, we introduced a vi-
nylene linkage into the p-conjugated systems, which provided a
new building block for organic semiconductors.
mer. Atomic force microscopy (AFM) measurements were
obtained in tapping mode using a Multimode IIIa Digital
Instrument AFM. X-ray diffraction (XRD) analyses of the
polymer films were performed on an XPERT-PRO MRD. Elec-
trical characteristics of the TFTs were measured under ambi-
ent conditions using both Keithley 2400 and 236 source/
measure units. All measurements were obtained with chan-
nel lengths (L) of 100 lm and channel widths (W) of 1000
lm. Field-effect mobilities were extracted in the saturation
regime from the slope of the source-drain current.
Fabrication of the Organic Thin-Film Transistor Devices
Thin-film transistors were fabricated on silicon wafers using
a top contact geometry (channel length L ¼ 100 lm, width
W ¼ 1000 lm) under ambient conditions without taking
special precautions to exclude air, moisture, or light. A heav-
ily n-doped silicon wafer with a 300 nm thermal silicon diox-
ide (SiO2) layer was used as the substrate/gate electrode,
with the top SiO2 layer serving as the gate dielectric. The
SiO2 surface of the wafer substrate was first cleaned with pi-
ranha solution (H2O2/H2SO4) at 130 ꢀC for 20 min, and the
cleaned wafer was immersed in a solution of 1.0 mM octyl-
trichlorosilane (OTS-8) in toluene at room temperature for 2
h under nitrogen. The semiconductor layer was spin coated
at 2000 rpm from a 0.5 wt % chloroform solution to a thick-
ness of 60 nm. Subsequently, a series of gold source/drain
electrode pairs were deposited by vacuum evaporation
through a shadow mask. Silicon oxide at the backside of the
silicon wafer of the TFT device was removed with HF to pro-
vide a conductive gate contact.
Here, we report the synthesis and characterization of a new
class of phenothiazylene vinylene-based semiconducting
polymers: poly[3,7-(40-dodecyloxyphenyl)phenothiazylene vi-
nylene] (P1), poly[3,7-(40-dodecyloxyphenyl)phenothiazylene
vinylene-alt-1,4-phenylene vinylene] (P2), and poly[3,7-(40-
dodecyloxyphenyl)phenothiazylene
vinylene-alt-2,5-thieny-
lene vinylene] (P3), which function as p-type semiconductors
in OTFTs (Scheme 2). To increase the solubility of the poly-
mer, an easily accessible dialkoxy-substituted phenylene ring
was incorporated in the phenothiazine units. The effect of
intramolecular ordering on organic devices was studied by
introducing structurally rigid phenylene units and electron-
rich thienylene units into the polymer backbones to control
intrachain and interchain p–p interactions. The influence of
structural variation on field-effect mobility in OTFTs was
investigated. Furthermore, bulk heterojunction-type devices
with PC71BM acceptors were fabricated to investigate the
photovoltaic properties of the polymers. Devices using P3
and PC71BM (30H-cyclopropa[8,25][5,6]fullerene-C70-D5h(6)-
30-butanoic acid, 30-phenyl-, methyl ester)-blend films as the
active layer exhibited 0.76% power conversion efficiency
(PCE).
Fabrication of the Organic Photovoltaic Device
Composite solutions with the P3 polymer and PC71BM were
prepared using 1,2-dichlorobenzene (DCB). The polymer
photovoltaic devices were fabricated with a typical sandwich
structure of ITO/PEDOT:PSS/active layer/LiF/Al. The indium
tin oxide (ITO)-coated glass substrates were cleaned by a
routine cleaning procedure, including sonication in detergent
followed by sonication in distilled water, acetone, and 2-pro-
panol. A 45-nm thick layer of PEDOT:PSS (Baytron P) was
spin coated onto a cleaned ITO substrate after exposing the
ITO surface to ozone for 10 min. The PEDOT:PSS layer was
baked on a hot plate at 120 ꢀC for 10 min. The active layer
was spin coated from the predissolved composite solution af-
ter filtering through 0.45 lm PP syringe filters. The device
structure was completed by depositing 0.6 nm LiF and then
130 nm Al cathode as the top electrode onto the polymer
active layer under a 3 ꢁ 10ꢂ6 torr vacuum in a thermal
evaporator.
EXPERIMENTAL
Instrumentation
1H and 13C NMR spectra were recorded on a Bruker AVANCE
400 spectrometer, with tetramethyl silane as an internal ref-
erence. Fast atom bombardment (FAB) mass spectra were
recorded on a JEOL, JMS-AX505WA, HP5890 Series II. Melt-
ing points were measured on a Yamato melting point appara-
tus (model-MP-21). The optical absorption spectra were
measured on a Shimadzu UV-3100 UV–VIS-NIR spectrometer.
FTIR spectra were recorded on a Perkin-Elmer Spectrum
2000 system. The number- and weight-averaged molecular
weights of polymers were determined by gel permeation
chromatography (GPC) on a Viscotek T60A instrument, using
tetrahydrofuran (THF) as eluent and polystyrene as a stand-
ard. Differential scanning calorimetry (DSC) and thermal
gravimetric analysis were obtained using a TA Q100 instru-
ment operated under a nitrogen atmosphere. Cyclic voltam-
metry (CV) was performed on an AUTOLAB/PG-STAT12
model system with a three-electrode cell in an acetonitrile
Synthesis of Monomers and Polymers
1-Bromo-4-dodecyloxybenzene (1)
Compound 1 was prepared according to the literature
method.39 Excess 1-bromododecane was removed by distilla-
tion, and the rꢀesidual oil was crystallized in methanol. Yield
94.7%, mp 34 C.
solution of Bu4NBF4 (0.10 M) at a scan rate of 50 mV sꢂ1
.
Polymer film coatings on Pt wire electrodes were formed by
dipping electrodes into a solution of the appropriate poly-
1H NMR (300 MHz, CDCl3, d): 7.34 (d, 2H), 6.77 (d, 2H), 3.90
(t, 2H), 1.76 (m, 2H), 1.40 (m, 18H), 0.88 (t, 3H). 13C NMR
636
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ARTICLE
(CDCl3, 80 MHz, d): 158.24, 132.16, 116.28, 112.52, 68.25,
31.91, 29.55, 22.68, 14.11. FABþ-Mass m/e 342.
cosolvent (n-hexane:ethyl acetate ¼ 5:1 [v/v]) as the eluent.
The product yield was 48% (2.8 g).
1H NMR (300 MHz, CDCl3, d): 9.72 (s, 2H), 7.44 (s, 2H), 7.32
(m, 4H), 7.14 (d, 2H), 6.22 (d, 2H), 4.05 (t, 2H), 1.85 (m,
2H), 1.38 (m, 18H), 0.88 (t, 3H). 13C NMR (CDCl3, 80 MHz,
d): 189.63, 159.63, 147.93, 131.90, 131.2, 129.87, 127.42,
119.80, 116.98, 116.02, 68.49, 31.91, 29.39, 26.07, 22.68,
14.12. FABþ-Mass m/e 515. Anal. Calcd. for C32H37NO3S: C,
74.53; H, 7.23; N, 2.72; O, 9.31; S, 6.22. Found: C, 74.20; H,
7.10; N, 2.75; O, 9.78; S, 6.17.
10-(40-Dodecyloxyphenyl)phenothiazine (2)
Phenothiazine (10.0 g, 50 mmol), 1-bromo-4-dodecyloxyben-
zene (20.5 g, 60 mmol), Pd2(dba)3[tris(dibenzylidineace-
tone)dipalladium(0)] (2.29 g, 2.4 mmol), sodium tert-butoxide
(7.4 g, 72 mmol), and tricyclohexylphosphine (0.98 g, 3.6
mmol) were dissolved in 200 mL of toluene and kept under
nitrogen at 130 ꢀC for 5 h. When the reaction was over, the
reaction mixture was cooled to room temperature, quenched
by addition of 200 mL of water, and then extracted three
times using dichloromethane and water. The organic layer
was separated and dried with anhydrous magnesium sulfate,
and then the solvent was removed using a rotary evaporator.
The crude product was purified by column chromatography
using a cosolvent (toluene:hexane ¼ 1:2 [v/v]) as the eluent.
The product yield was 86% (19.7 g).
3,7-Bis(hydroxymethyl)-10-(40-dodecyloxyphenyl)-
phenothiazine (5)
Compound 4 (5.0 g, 9.7 mmol) and 80 mL of anhydrous THF
were placed in a 250-mL two-necked flask and stirred at
ꢂ30 ꢀC under nitrogen. LiAlH4 (2.3 mL, 5.3 mmol) was
added slowly to the solution over 30 min, and then the mix-
ture was stirred for 1 h. The mixture was poured into water,
extracted with ethyl acetate, and dried with anhydrous
MgSO4. The solvent was removed via rotary evaporation, and
the crude product was purified by column chromatography
using a cosolvent (n-hexane:ethyl acetate ¼ 1:1 [v/v]) as elu-
ent. The product yield was 72% (3.67 g).
1H NMR (300 MHz, CDCl3, d): 7.27 (d, 2H), 7.08 (d, 2H), 6.97
(d, 2H), 6.81 (s, 2H), 6.19 (s, 2H), 4.02 (t, 2H), 1.85 (m, 2H),
1.39 (m, 18H), 0.88 (t, 3H). 13C NMR (CDCl3, 80 MHz, d):
158.84, 126.78, 116.34, 115.62, 68.31, 31.92, 29.42, 26.10,
22.70, 14.13. FABþ-Mass m/e 459. Anal. Calcd. for
C30H39NOS: C, 78.04; H, 8.51; N, 3.03; O, 3.47; S, 6.94. Found:
C, 78.40; H, 8.50; N, 3.02; O, 3.45; S, 6.98.
1H NMR (300 MHz, CDCl3, d): 7.06 (m, 4H), 6.92 (d, 2H),
6.75 (m, 2H), 6.11 (m, 2H), 4.42 (s, 4H), 4.02 (t, 2H), 2.05 (s,
2H), 1.82(q, 2H), 1.40 (m, 18H), 0.88 (t, 3H). 13C NMR (80
MHz, CDCl3, d): 158.78, 143.74, 134.91, 133.09, 132.06,
125.77, 125.49, 119.51, 116.46, 115.53, 68.38, 64.07, 31.98,
22.75, 29.74, 26.16, 22.75, 14.20. FABþ-Mass m/e: 519. Anal.
Calcd. for C32H41NO3S: C, 73.95; H, 7.95; N, 2.69; O, 9.24; S,
6.17. Found: C, 74.10; H, 7.97; N, 2.65; O, 9.18; S, 6.10.
3,7-Dibromo-10-(40-dodecyloxyphenyl)phenothiazine (3)
Compound 2 (16.0 g, 34.8 mmol) was dissolved in 100 mL
of chloroform and 150 mL of acetic acid, and then N-bromo-
succinimide (NBS; 13.6 g, 76.7 ꢀmmol) was added and the so-
lution was stirred for 2 h at 0 C. Dilute aqueous sodium hy-
droxide was then added and the reaction mixture was
stirred for further 30 min. The reaction mixture was
extracted three times using chloroform and brine; the or-
ganic layer was separated and dried with anhydrous magne-
sium sulfate, and then the solvent was removed using a ro-
tary evaporator. The crude product was precipitated in
methanol. The product yield was 93% (20.4 g).
3,7-Bis(methylphosphonate)-10-(40-dodecyloxyphenyl)-
phenothiazine (6)
Into a solution of Compound 5 (1.7 g, 3.3 mmol) in 20 mL
of toluene, 0.52 mL of SOCl2 (7.2 mmol) was added (by sy-
ꢀ
ringe) over 30 min at 0 C, and then the mixture was stirred
for 5 h. The mixture was then poured into water, extracted
with CHCl3, and dried with anhydrous MgSO4. The solvent
was removed via rotary evaporation. The crude product (1.9
g) was used in the next step without further purification
because of its low environmental stability. The crude product
was dissolved with excess triethyl phosphite (10 mL) and
then refluxed for 10 h. Excess triethyl phosphite was
removed by vacuum distillation and the residual oil was
purified by column chromatography using ethyl acetate as
eluent. The product yield was 82% (2.04 g).
1H NMR (300 MHz, CDCl3, d): 7.23 (d, 2H), 7.06 (m, 4H),
6.89 (d, 2H), 6.01 (d, 2H), 4.01 (t, 2H), 1.83 (m, 2H), 1.39
(m, 18H), 0.88 (t, 3H). 13C NMR (CDCl3, 80 MHz, d): 158.84,
126.78, 116.34, 115.62, 68.31, 31.92, 29.42, 26.10, 22.70,
14.13. FABþ-Mass m/e 617. Anal. Calcd. for C30H35Br2NOS: C,
58.35; H, 5.71; Br, 25.88; N, 2.27; O, 2.59; S, 5.19. Found: C,
58.60; H, 5.60; Br, 25.50; N, 2.20; O, 2.95; S, 5.15.
3,7-Diformyl-10-(40-dodecyloxyphenyl)phenothiazine (4)
Into a solution of Compound 3 (7.0 g, 11.3 mmol) in THF
(100 mL), 15.6 mL (25.0 mmol) of n-BuLi (1.6 M in hexane)
was added (by syringe) at ꢂ78 ꢀC. The mixture was stirred
1H NMR (300 MHz, CDCl3, d): 7.16 (m, 4H), 6.87 (t, 2H),
6.72 (m, 2H), 6.08 (d, 2H), 4.04 (m, 10H), 2.91 (d, 4H), 1.83
(q, 2H), 1.48 (m, 2H), 1.28 (m, 28H) 0.88 (t, 3H). 13C NMR
(80 MHz, CDCl3, d): 158.86, 143.40, 132.87, 132.05, 128.11,
128.03, 127.55, 127.47, 125.26, 125.14, 119.52, 116.34,
115.56, 62.29, 62.12, 62.04, 33.31, 31.90, 31.47, 29.58,
29.40, 29.34, 29.27, 26.08, 22.67, 16.45, 16.37, 14.11. FABþ-
Mass m/e: 759. Anal. Calcd. for C40H59NO7P2S: C, 63.22; H,
7.83; N, 1.84; O, 14.74; P, 8.15; S, 4.22. Found: C, 64.02; H,
7.90; N, 1.82; O, 13.96; P, 8.10; S, 4.20.
ꢀ
at ꢂ78 C for 3 h. 1-Formylpiperidine (3.14 mL, 28.2 mmol)
was added to the solution, and the resulting mixture was
ꢀ
stirred at ꢂ78 C for 1 h and then warmed to room temper-
ature and stirred for further 12 h. The mixture was poured
into water, extracted with ether, and dried with anhydrous
MgSO4. The solvent was removed via rotary evaporation, and
the residue was purified by column chromatography using a
PHENOTHIAZYLENE VINYLENE-BASED POLYMERS, SON ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthetic routes to the monomers.
1,4-Bis(methyl phosphonate)benzene (7)
by methanol. The polymer, P1, was a greenish brown solid
(0.33 g, 80%).
This compound was synthesized by the reported method.40
1H NMR (CDCl3, 300 MHz, d): 6.82 (s, 2H), 4.07 (m, 8H),
3.30 (d, 4H), 1.29 (m, 12H). 13C NMR (CDCl3, 80 MHz, d):
131.7, 127.3, 62.2, 28.9, 27.0, 16.2. FABþ-Mass m/e 385.
1H NMR (300 MHz, CDCl3, d): 7.23 (m, Ar-H), 7.07 (m, Ar-H),
6.84 (m, Ar-H), 6.66 (m, vinyl-H), 6.06 (m, Ar-H), 4.02 (t,
OCH2), 1.83–1.27 (m, CH2), 0.88 (t, CH3). Anal. Calcd. for
C32H37NOS: C, 79.46; H, 7.71; N, 2.90; O, 3.31; S, 6.63. Found:
C, 79.30; H, 7.82; N, 2.84; O, 3.34; S, 6.70.
2,5-Bis(methylphosphonate)thiophene (8)
This compound was prepared according to the reported
method.40
Poly[3,7-(40-dodecyloxyphenyl) phenothiazylene vinylene-
alt-1,4-phenylene vinylene] (P2)
1H NMR (CDCl3, 300 MHz, d): 7.25 (s, 4H), 4.00 (m, 8H),
3.13 (d, 4H), 1.23 (m, 12H). 13C NMR (CDCl3, 80 MHz, d):
130.3, 62.1, 33.3, 32.4, 16.3. FABþ-Mass m/e 379.
Poly[3,7-(40-dodecyloxyphenyl)phenothiazylene vinylene]
(P1)
Compound 4 (0.40 g, 0.77 mmol) and Compound 7 (0.29 g,
0.78 mmol) were dissolved in anhydrous THF (30 mL) under
nitrogen. A 1.94 mL aliquot of potassium tert-butoxꢀide (1.0
M in anhydrous THF) was slowly added for 1 h at 0 C, after
which the mixture was stirred for 2 h at 0 ꢀC. The reaction
was quenched by the addition of a small amount of acetic
acid. The neutralized mixture was poured into methanol, and
the polymer was collected by filtration. The polymer was
purified by reprecipitation from CHCl3 to methanol, and then
Soxhlet extraction by methanol. The polymer, P2, was a pale
red solid (0.23 g, 52%).
Compound 4 (0.20 g, 0.39 mmol) and Compound 6 (0.30 g,
0.39 mmol) were dissolved in 25.0 mL of anhydrous THF
under nitrogen. A 0.88 mL (0.86 mmol) aliquot of potassium
tert-butoxide (1.0 M in anhydrous THF) was slowly added
ꢀ
over 30 h at 0 C. The mixture was stirred for 2 h and then
warmed to RT and maintained at that temperature over 24
h. The reaction was quenched by the addition of a small
amount of acetic acid, and the resulting mixture was poured
into methanol. The precipitated polymer was collected by fil-
tration. The polymer was purified through Soxhlet extraction
1H NMR (300 MHz, CDCl3, d): 7.41 (m, Ar-H), 7.15–7.04 (m,
Ar-H), 6.88 (m, vinyl-H), 6.10 (m, Ar-H), 4.02 (t, OCH2),
1.83–1.17 (m, CH2), 0.88 (m, CH3). Anal. Calcd. for
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ARTICLE
SCHEME 2 Chemical structures and synthetic routes to the polymers P1, P2, and P3.
C40H43NOS: C, 82.01; H, 7.40; N, 2.39; O, 2.73; S, 5.47. Found:
C, 81.74; H, 7.63; N, 2.33; O, 2.88; S, 5.42.
Poly[3,7-(40-dodecyloxyphenyl) phenothiazylene vinylene-
alt-2,5-thienylene vinylene] (P3)
Compound 4 (0.40 g, 0.77 mmol) and Compound 8 (0.27 g,
0.78 mmol) were dissolved in anhydrous THF (20 mL) under
nitrogen. A 1.70 mL aliquot of potassium tert-butoxꢀide (1.0
M in anhydrous THF) was slowly added for 2 h at 0 C, after
which the mixture was stirred for 1 h at 0 ꢀC. The reaction
was quenched by the addition of a small amount of acetic
acid. The neutralized mixture was poured into methanol, and
the polymer was collected by filtration. The polymer was
purified by reprecipitation from CHCl3 to methanol, and then
Soxhlet extraction by methanol. The polymer, P3, was a red
solid (0.42 g, 88%).
1H NMR (300 MHz, CDCl3, d): 7.22 (m, Ar-H), 7.06 (m, Ar-H),
6.92 (m, Ar-H), 6.83 (vinyl-H), 6.62 (m, Ar-H), 6.05 (m, Ar-
H), 4.02 (t, OCH2), 1.82–1.25 (m, CH2), 0.88 (t, CH3). Anal.
FIGURE 1 FTIR spectra of monomers 4, P1, P2, and P3
polymers.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
sizedꢀfrom the dilithiation of Compound 3 using n-BuLi at
ꢂ78 C, followed by a formylation reaction with dried N-for-
mylpiperidine. The low yield obtained for this reaction was
attributed to production of the monoformylated compound.
The dialdehyde groups of Compound 4 were transformed
into dihydroxymethyl groups (Compound 5) by reduction
with LiAlH4 in anhydrous THF. The hydroxymethyl groups of
Compound 5 were transformed into chloromethyl groups
using thionyl chloride in toluene with catalytic quantities of
pyridine. The chloromethylated compound was moisture sen-
sitive, so it was used in the subsequent reaction without fur-
ther purification to give the diphosphonate compound 6
with 80% yield. As shown in Scheme 2, Compounds 7 and 8
were synthesized according to methods described in the lit-
erature.40 The Horner-Emmons coupling reaction is a useful
method for synthesizing well-defined polymers because it
provides strict regioselectivity and all-trans configura-
tions.41,42 Similar to the conditions used in the Horner-
Emmons reaction, polymerization was performed in anhy-
drous THF with potassium tert-butoxide as the basic con-
densing agent.43 FTIR spectra of the dialdehyde monomer 4
and the polymers P1, P2, and P3 are shown in Figure 1. In
the case of P1, P2, and P3, strong C¼¼O vibrations from dia-
ldehydes at 1683 cmꢂ1 had disappeared, and CAH out-of-
plane vibrations of the trans-vinylene were observed at 960
cmꢂ1. Therefore, the newly formed vinylene double bonds
had all-trans (E) configurations. The molecular weights and
polydispersity indices of the polymers were determined by
GPC using THF as the eluent and polystyrene as the stand-
ard. The weight-averaged molecular weights (Mw) of P1, P2,
and P3 were found to be 27,000, 22,000, and 29,000, with
polydispersity indices of 1.91, 2.05, and 2.25, respectively.
P1 showed good solubility in common organic solvents such
as chloroform, chlorobenzene, and toluene, but P2 and P3
polymers exhibited relatively poor solubilities when com-
pared with P1 because of the incorporation of the rigid
phenylene and thienylene units, respectively, into the poly-
mer backbones. The polymer yields were 52–88% after puri-
fication. The thermal properties of the polymers were inves-
tigated by DSC [Fig. 2(a)] and TGA [Fig. 2(b)]. The
thermograms for P1, P2, and P3 each conꢀ tained only a
broad glass transition, at 129, 167, and 155 C, respectively,
without observation of melting features. None of the poly-
mers showed melting- or crystalline-transition behaviors,
suggesting that they may have amorphous structures, or that
the transition temperature was higher than temperatures
studied. The excellent thermal stabilities of the P1, P2, and
P3 polymers were evident from their TGA profiles, with
FIGURE 2 (a) DSC and (b) TGA of the phenothiazylene vinyl-
ene-based polymers at a heating rate 10 ꢀC minꢂ1 under an
inert atmosphere.
Calcd. for C38H41NOS2: C, 77.11; H, 6.98; N, 2.37; O, 2.70; S,
10.84. Found: C, 76.87; H, 7.28; N, 2.33; O, 2.66; S, 10.86.
RESULTS AND DISCUSSION
Synthesis and Characterization of Polymer
The synthetic routes to the monomers are outlined in
Scheme 1. The synthetic routes to the polymers P1, P2, and
P3 are given in Scheme 2. The monomers required for
Horner-Emmons condensation polymerization were easily
prepared by conventional organic synthesis reactions. As
shown in Scheme 1, phenothiazine on the nitrogen atom was
substituted with Compound 1 using a palladium-catalyzed
coupling reaction to provide Compound 2. In the preparation
of the polymers, an aryl group provided sufficient resonance
stabilization to increase the polymer’s ability to transport
hole carriers.37 A linear alkoxy chain was introduced in place
of a branched chain to promote intermolecular interactions
and molecular self-organization. Compound 2 was converted
to the dibromo compound 3 in good yield with NBS in the
presence of a polar protic solvent. Compound 4 was synthe-
TABLE 1 Physical Properties of the P1, P2, and P3 Polymers
Polymer
Mw (g molꢂ1
)
Mw/Mn
Tg (ꢀC)
Td (ꢀC)a
P1
P2
P3
a
27,000
22,000
29,000
1.91
2.05
2.25
129
167
155
399
422
423
Temperature for 5% weight loss.
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TABLE 2 UV–Visible Maximum Absorption Wavelength (kmax), Band Gap Energy (Eg), and
Ionization Potential (EHOMO) of the P1, P2, and P3 Polymers
Polymers
Solutiona (kmax,nm)
Film (kmax,nm)
Eg (eV)b
EHOMO (eV)
ELUMO (eV)c
P1
P2
P3
a
316, 454
347, 456
370, 494
317, 460
347, 465
373, 500
2.33
2.30
2.12
5.10
5.12
5.09
2.77
2.82
2.97
Measurements performed in CHCl3.
b
c
Estimated from the onset of the absorption in thin films (Eg ¼ 1240/konset eV).
Calculated using optical band gap energy.
decomposition temperatures above 400 ꢀC. The weight
losses for P1, P2, and P3 were 5.2, 3.6, and 3.2% at 400 C,
respectively. These results for the synthesized polymers are
summarized in Table 1.
Bu4NBF4 supporting electrolyte solution with a scan rate of
50 mV sꢂ1. All polymers showed partial reversibility in the
monoelectronic oxidation process. The HOMO levels of P1,
P2, and P3 were estimated to be 5.10, 5.12, and 5.09 eV,
respectively. These results showed that the introduction of
different units (phenylene and thienylene units) in the main
chains had little influence on the HOMO level. The reduction
curves of these polymers were not obtained, which sug-
gested that these polymers were intrinsic p-type semicon-
ductors. Given that the optical band gaps of the P1, P2, and
ꢀ
Optical and Electrochemical Properties of the Polymers
UV–visible absorption spectra (both in chloroform solution
and as thin solid films) of all polymers are presented in Ta-
ble 2 and Figure 3. All polymers showed two strong absorp-
tion peaks in the ranges 315–370 nm and 450–500 nm,
respectively. The relative absorbances of P2 and P3 at short
wavelengths were reduced when compared with the homo-
polymer (P1), which indicated that the absorption peak at
short wavelengths may have resulted from the phenothiazine
moieties.37 The UV–visible absorption peaks at longer wave-
lengths for P1, P2, and P3 appeared at 454, 456, and 494
nm in solution and 460, 465, and 500 nm in thin solid films,
respectively, and may have originated from the p–p* transi-
tion. As expected, the absorption maxima in P3 solutions
and films (with thienylene vinylene units) were red-shifted
by 40 nm when compared with those of P1 and P2 poly-
mers. This effect was attributed to an elongation of the effec-
tive conjugation length by the incorporation of thienylene vi-
nylene units in the polymer backbone, which enhanced
intra/intermolecular orbital overlap, resulting in a red-shift
in absorption. It is interesting to note that an absorption
red-shift of only 2 nm was obtained for P2 (with phenylene
vinylene units). These results suggest that incorporation of
the thiophene moiety in the polymer backbone was more
effective in elongating the conjugation length than incorpora-
tion of the phenylene ring. The absorption spectra of the
polymer thin films were similar to the spectra obtained in
dilute solutions, except for the slight red-shift observed in
the thin films. These results imply similar amorphous mor-
phological properties in both solution and thin solid films.
The absorption maxima shifts observed for P1, P2, and P3
in the film state relative to the solution state (Dk) were
found to be 6, 9, and 6 nm, respectively. These results sug-
gest the absence of polymer aggregation or crystallization
during thin film formation. The electrochemical properties of
the polymers were characterized by CV at room temperature.
The oxidative stabilities of the polymer thin films were esti-
mated from CV measurements of dip-coated thin films on
ITO-coated glass, which formed the working electrode. A
platinum rod was used as a counter electrode, and Ag/Agþ
as a reference electrode in acetonitrile with a 0.1 M n-
FIGURE 3 UV–visible spectra of the phenothiazylene vinylene-
based polymers (a) in chloroform solution and (b) in thin films.
PHENOTHIAZYLENE VINYLENE-BASED POLYMERS, SON ET AL.
641

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 4 Output characteristics of the P3 device at different gate biases (a), and transfer curves at constant VD ¼ ꢂ60 V for devi-
ces using P1 (b), P2 (c), and P3 (d).
P3 thin films, as determined from the absorption onset,
were 2.33, 2.30, and 2.12 eV, respectively, we calculated the
LUMO levels of the P1, P2, and P3 thin films to be 2.77,
2.82, and 2.97 eV, respectively. The electrochemical poten-
tials are summarized in Table 2. In addition, the HOMO lev-
els of all polymers matched the work function of metallic
gold (5.1 eV), and efficient hole-charge injection between the
electrode and semiconductor is anticipated.44
common gate consisting of highly n-doped silicon with a
300-nm thick thermally grown SiO2 dielectric layer. The sub-
strates were modified with OTS-8. Organic semiconductor
films with a thickness of 50 nm were spin coated at 2000
rpm from a 0.5 wt % chloroform solution, followed by
TFT Characteristics of the Polymer Thin Films
OTFTs based on P1, P2, and P3 were fabricated by means of
solution processing. Top-contact OTFTs were fabricated on a
TABLE 3 Annealing Behaviors (at 150 8C/5 min under N2
Atmosphere) of the P1, P2, and P3 Polymer Thin-Film
Transistors Spin Coated on OTS-Modified SiO2/n-Doped
Si Substrates
Before Annealing
After Annealing
l (cm2 Vꢂ1 sꢂ1
Ion/off
Polymers
l (cm2 Vꢂ1 sꢂ1
)
Ion/off
)
P1
P2
P3
1.0 ꢁ 10ꢂ4
3.6 ꢁ 10ꢂ5
1.0 ꢁ 10ꢂ3
ꢃ102
ꢃ102
ꢃ103
1.5 ꢁ 10ꢂ4
1.5 ꢁ 10ꢂ5
1.2 ꢁ 10ꢂ3
ꢃ102
ꢃ102
ꢃ103
FIGURE 5 X-Ray diffraction patterns of P1, P2, and P3 in thin
film state.
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ARTICLE
FIGURE 7 J–V characteristics of photovoltaic cells prepared
from polymers:PC71BM (1:5) blend active materials.
2
Saturated regime ðVD > VGÞ : ID ¼ ðW=2LÞlCiðVG ꢂ VTÞ ;
where ID is the drain current in the saturated regime, W and
L are the channel width and length, respectively, l is the
field-effect mobility, Ci is the capacitance per unit area of the
gate dielectric layer, and VG and VT are the gate and threshold
voltages, respectively. All OTFTs containing P1, P2, and P3
were found to exhibit typical p-channel characteristics. Figure
4 shows the drain current (IDS) versus drain-source voltage
(VDS) characteristics of the P3 transistor and the transfer
characteristics of the P1-, P2-, and P3-based field effect tran-
sistors. The output curve showed good saturation behavior
with clear saturation currents. The saturation mobilities of
P1, P2, and P3 were found to be 1.0 ꢁ 10ꢂ4, 3.6 ꢁ 10ꢂ5, and
1.0 ꢁ 10ꢂ3 cm2 Vꢂ1
s
ꢂ1, respectively, and the on/off ratio
was around 102 for P1, P2, and 103 for P3. The threshold
voltages (VT) of the devices were determined from the linear
fit of the square root of IDS in the saturated regime versus VG,
and VG was determined by extrapolating the measured data
to IDS ¼ 0. The calculated VT of the P1, P2, and P3 devices
were 13, 15, 20 V, respectively. OTFTs based on the P3 poly-
mer afforded the highest values of mobility and on/off ratio
among the synthesized polymers. OTFT properties of P1, P2
and, P3 polymers are summarized in Table 3. To investigate
FIGURE 6 Atomic force microscopy height images for (a) P1,
(b) P2, and (c) P3 thin films spin-coated onto OTS-treated SiO2
substrates. Image size: 10 lm ꢁ 10 lm.
TABLE 4 Photovoltaic Properties of the Polymer:PC71BM
Devices
Polymer:
PCBM
Film
drying at 100 ꢀC (for 5 min) to remove the solvent. On top
of the thin film surface, gold was deposited through a
shadow mask to produce the source and drain electrodes.
The channel length L and width W was 100 lm and 1000
lm, respectively. The OTFT characteristics were determined
under ambient conditions, and no precautions were taken to
insulate the materials and devices from exposure to air,
moisture, or light. The field-effect mobility was calculated
according to the following equation:45
Thickness
(nm)
JSC
VOC
PCE
(%)
Ratio (wt)
(mA cmꢂ2
)
FF
(V)
P1 (1:5)
P2 (1:5)
P3 (1:5)
P3 (1:3)
P3 (1:1)
71
70
73
70
71
1.89
1.57
5.17
4.36
2.16
0.30
0.29
0.35
0.34
0.32
0.59
0.61
0.42
0.49
0.52
0.33
0.28
0.76
0.73
0.36
PHENOTHIAZYLENE VINYLENE-BASED POLYMERS, SON ET AL.
643

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
the annealing behaviors of the polymer thin films, we fulfilled
the preꢀbaking of all the polymer thin films (P1, P2, and P3)
at 100 C for 5 min under N2 atmosphere, and then annealed
ꢀ
at 150 C for 5 min under N2 gas condition, whose tempera-
ture is above Tg of the polymers. No significant mobility
changes were obseꢀrved after annealing the thin films of P1,
P2, and P3 at 150 C/5 min under N2 atmosphere. XRD anal-
ysis and AFM were performed to determine the structural
characteristics of the polymers and their surface morpholo-
gies in the thin film state, respectively. No reflection peaks
were observed in the thin film state XRD patterns of P1, P2,
and P3 (Fig. 5). These findings indicate that the films of P1,
P2, and P3 consist of amorphous collections of randomly ori-
ented polymer chains. The AFM height images of the poly-
mers, obtained on OTS-8-modified wafer substrates, are
shown in Figure 6. In all cases, the images indicated a smooth
flat surface with a root-mean-square (RMS) surface rough-
ness of 1.5–2.4 nm. These results suggest that all the polymer
thin films have amorphous structures.
Organic Photovoltaic Characteristics of the Polymer
Thin Film
A bulk heterojunction solar cells were fabricated with a
sandwich structure of ITO/PEDOT:PSS (50 nm)/poly-
mer:PC71BM/LiF (0.6 nm)/Al (130 nm). After deposition of
PEDOT:PSS layer on the top of the anode (ITO), the active
materials was deposited via spin coating from 1,2-DCB solu-
tion of polymer and PC71BM. In the bulk heterojunction solar
cells, the polymers were used as an electron donor and
PC71BM was used as an electron acceptor. The J–V character-
istics of the polymer solar cells prepared from the polymers
(P1, P2, and P3):PC71BM (1:5) blend materials are shown in
Figure 7. And the open circuit voltage (VOC), short-circuit
current density (JSC), fill factor (FF), and PCE of these de-
vices are summarized in Table 4. For these devices based on
the P1–P3:PC71BM (1:5) blend materials, an increase in per-
formance was observed from P1 to P3 polymer. The PCE of
P1–P3:PC71BM (1:5) devices were found to be 0.33, 0.28,
and 0.76, respectively. And the open-circuit voltage (VOC),
which is related to the difference between the HOMO level
of the donor polymer and the LUMO level of the acceptor
(PCBM),46,47 of the P1–P3:PC71BM (1:5) devices were found
to be 0.59, 0.61, and 0.42, respectively. These values are
slightly corresponding with the results of CV analysis (com-
pare with Table 2). The greatest changes of these devices
were seen in short-circuit current density (JSC), which gener-
ally depends on the light absorption and carrier mobility of
donor polymers. When compared with the thin film using
P3, the fraction of light absorption and the TFT mobility
shows the highest values among the synthesized polymers,
and therefore, the PCE of P3:PC71BM (1:5) device exhibits
the highest value among the solar cell devices, which well
corresponds with a previous work.48 To investigate the sur-
face morphology of the blend films, AFM images of P1–
P3:PC71BM (1:5) films were measured (Fig. 8). From the
images, a clear phase separation was seen in the P3:PC71BM
(1:5) film, whereas the P1:PC71BM (1:5) and P2:PC71BM
(1:5) films showed more smooth and homogeneous surface.
FIGURE 8 AFM phase images of thin films cast from (a)
P1:PC71BM (1:5), (b) P2:PC71BM (1:5), and (c) P3:PC71BM (1:5).
Scan size is 500 nm ꢁ 500 nm. [Color figure can be viewed in
the online issue, which is available at www.interscience.
wiley.com.]
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ARTICLE
ferences in organic photovoltaic (OPV) performance between
the donor polymers. These results suggest that the differen-
ces of PCE on P1–P3:PC71BM (1:5) devices may be results of
a decrease in the interfacial area and ability of hole transpor-
tation of the donor polymers. Especially, the PCE and JSC val-
ues were found to increase as the fraction of PC71BM in the
blend films increased for the P3:PC71BM blend devices. PCE
of the P3:PC71BM (1:1), (1:3), and (1:5) devices were found
to be 0.36, 0.73, and 0.76, respectively. We also examined
the surface differences by using the AFM (Fig. 9). In the case
of the P3:PC71BM (1:1) blend films, the significant phase
separation was not observed, whereas for the P3:PC71BM
(1:3) and P3:PC71BM (1:5) blend films, the clear phase sepa-
ration was observed. Generally, photocurrent is increased as
the fraction of phase boundary in blend films, and therefore,
the lower PCE of the P3:PC71BM (1:1) device may be due to
a decrease of the donor–acceptor phase boundary.49
CONCLUSIONS
In summary, we have synthesized new solution-processable
phenothiazylene vinylene-based polymers by the Horner-
Emmons reaction. The polymers possessed good solubilities
in common organic solvents and high thermal stabilities. The
synthesized polymers exhibited behaviors characteristic of
an amorphous structure, as demonstrated by XRD analysis
and AFM imaging. UV–visible absorption spectra of the poly-
mers showed two strong absorption bands in the ranges of
315–370 nm and 450–500 nm, which resulted from absorp-
tion by the phenothiazine segments (maxima at 315–370
nm) and the conjugated main chains (maxima at 450–500
nm). OTFTs based on the P3 polymer provided superior FET
performance, affording a hole mobility of 1.0 ꢁ 10ꢂ3 cm2
Vꢂ1 sꢂ1 with an on/off ratio of 103. The photovoltaic device
using a P3/PC71BM (1/5) blend film as the active layer
exhibited an open-circuit voltage (VOC) of 0.42 V, a short-cir-
cuit current (JSC) of 5.17 mA cmꢂ2, an FF of 0.35, and a PCE
of 0.76% under AM 1.5 G (100 mW cmꢂ2) illumination.
These results suggest that the phenothiazylene vinylene-
based polymers represent a useful class of solution-process-
able semiconductors for organic electronic devices.
This work was partly supported by the IT R&D program of
MKE/KEIT (2009-F-017-01, Development of Fundamental
Technologies for Flexible Combined-Function Organic Elec-
tronic Device) and by the Catholic University of Korea, Research
Fund, 2008. The authors thank the cooperative R&D Program
funded by the Korea Research Council Industrial Science and
Technology, Republic of Korea, for support of this research.
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FIGURE 9 AFM phase images of (a) P3:PC71BM (1:1), (b)
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