New dithienophosphole-based donor-acceptor alternating copolymers: Synthesis and structure property relationships in OFET

Article abstract of DOI:10.1016/j.dyepig.2015.09.026

Two donor-acceptor conjugated copolymers comprised of a novel dithienophosphole (DTP) as the acceptor unit and bithiophene (BT) or decylthiophenebenzodithiophene (DTBDT) as the donor unit were synthesized and characterized to elucidate the relationship between donor-acceptor (D-A) architecture and organic field-effect transistors (OFETs) performance. These characteristics of D-A copolymers were affected by the structural suitability of donor and acceptor units. Poly(DTP-BT) had an ordered structure that facilitated charge carrier transfer. The crystallinity of poly(DTP-BT) increased as annealing temperature (T_ann) increased. In contrast, poly(DTP-DTBDT) was amorphous regardless of T_ann due to the long alkyl chains of the DTBDT units. OFET devices made with poly(DTP-BT) and annealed at 200°C exhibit a highly crystalline morphology and a relatively high field-effect mobility (4.9 × 10^-3 cm²/(V·s)).

Full text of DOI:10.1016/j.dyepig.2015.09.026

Accepted Manuscript
New dithienophosphole-based donor-acceptor alternating copolymers: synthesis and
structure property relationships in OFET
Min Jae Sung, Yebyeol Kim, Sang Bong Lee, Gi Back Lee, Tae Kyu An, Hyojung
Cha, Prof. Se Hyun Kim, Prof. Chan Eon Park, Prof. Yun-Hi Kim
PII:
S0143-7208(15)00373-3
10.1016/j.dyepig.2015.09.026
DYPI 4937
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 18 August 2015
Revised Date: 11 September 2015
Accepted Date: 15 September 2015
Please cite this article as: Sung MJ, Kim Y, Lee SB, Lee GB, An TK, Cha H, Kim SH, Park CE, Kim Y-H,
New dithienophosphole-based donor-acceptor alternating copolymers: synthesis and structure property
relationships in OFET, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.09.026.
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GRAPHICAL ABSTRACT

ACCEPTED MANUSCRIPT
Submitted to Dyes and Pigments, August, 2015
New dithienophosphole-based donor-acceptor alternating copolymers:
synthesis and structure property relationships in OFET
a
b
c,
a
d
Min Jae Sung, Yebyeol Kim , Sang Bong Lee, Gi Back Lee , Tae Kyu An , Hyojung
b
e
b
c
Cha , Se Hyun Kim *, Chan Eon Park *, and Yun-Hi Kim *
a
Department of Polymer Science and Engineering, Gyeongsang National University, Jin-ju,
660-701, Republic of Korea
b
POSTECH Organic Electronics Laboratory, Polymer Research Institute, Department of
Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784,
Republic of Korea
c
Department of Chemistry and RIGET, Gyeongsang National University, Jin-ju, 660-701,
Republic of Korea
d
Department of Polymer Science & Engineering, Korea National University of Transportation,
Chungju, Republic of Korea
e
School of Chemical Engineering, Medical and Polymer Materials, Yeungnam University,
Gyeongsan, 712-749, Republic of Korea

ACCEPTED MANUSCRIPT
*
Corresponding Author
a
1
. Prof. Yun-Hi Kim *
Department of Chemistry and ERI, Gyeongsang National University, Jin-ju, 660-701,
Republic of Korea
E-mail: ykim@gnu.ac.kr
2. Prof. Chan Eon Park
POSTECH Organic Electronics Laboratory, Polymer Research Institute, Department of
Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784,
Republic of Korea
E-mail: cep@postech.ac.kr
3. Prof. Se Hyun Kim
Medical and Polymer Materials, Yeungnam University, Gyeongsan, 712-749, Republic of
Korea
E-mail: shkim97@yu.ac.kr
Min Jae Sung and Yebyeol Kim contributed equally to this work.

ACCEPTED MANUSCRIPT
ABSTRACT
Two donor-acceptor conjugated copolymers comprised of a novel dithienophosphole (DTP)
as the acceptor unit and bithiophene (BT) or decylthiophenebenzodithiophene (DTBDT) as
the donor unit were synthesized and characterized to elucidate the relationship between
donor-acceptor (D-A) architecture and organic field-effect transistors (OFETs) performance.
These characteristics of D-A copolymers were affected by the structural suitability of donor
and acceptor units. Poly(DTP-BT) had an ordered structure that facilitated charge carrier
transfer. The crystallinity of poly(DTP-BT) increased as annealing temperature (Tann
)
increased. In contrast, poly(DTP-DTBDT) was amorphous regardless of Tann due to the long
alkyl chains of the DTBDT units. OFET devices made with poly(DTP-BT) and annealed at
200°C exhibit a highly crystalline morphology and a relatively high field-effect mobility
-3
2
(
4.9×10 cm /(Vꢀs)).
KEYWORDS: Organic field effect transistor (OFET); dithienophosphole; donor-acceptor
D-A) architecture; X-ray diffraction
(

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1. Introduction
Polymer semiconductors have made major advances due to new material development
s and device optimization. Because polymer semiconductors have advantages such as easy
chemical modification, flexibility, light weight, and economically processable in applications
such as gravure or inkjet printing [1-4], they have been widely researched for organic field-
effect transistors (OFETs) flexible displays, memory, or sensors [5-11]. In these fields of
organic electronics, OFETs have been actively researched to improve their charge transport
property. Particularly, chemical modification is a key factor to improve charge transport
property of OFETs by controlling chain curvature, torsional angle, and electronic structure of
the polymer chains [12].
Recent studies have actively investigated chemical modification with donor-acceptor (D-A)
copolymer architectures that have fast intramolecular charge transfer (ICT) and possibility to
fine-tune chemical structure [13-16]. Although many research groups have devoted signif
icant attention to D-A type OFETs, a large amount of research is still being conducte
d to improve the OFET performance and reliability of D-A type polymers. However,
only a few of the acceptor units have presented good performance as the active layer of
OFET, e.g., diketopyrrolopyrrole, isoindigo, naphthalene diimide, benzothiadiazole, and
benzobisthiadiazole [9, 17-24].
As part of the effort to material research, we used dithieno[3,2-b:2’,3’-d]phosphole (DTP)
moiety which exhibits strong electron-withdrawing property as the acceptor unit in a D-A
type polymer. The DTP moiety forms an unusual hyperconjugation ring and a rigid pyramidal
geometry that is induced by a σ*-π* interactions between the π-conjugated scaffold and
exocyclic substituents. This unusual structure decreases the energy level of the lowest
unoccupied molecular orbital (LUMO) of DTP blocks, thereby making them strong electron
acceptors [25-27]. As a result, D-A polymers in which DTP blocks have shown high

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performance in both OPVs and OLEDs. In addition, the optoelectrical properties of these
copolymer systems can be tuned by chemical modification [25, 28]. Unfortunately, although
DTP-based D-A polymers previously researched for OLEDs and OPVs, none of the previous
research attempted to apply them as acceptor units in OFETs, or to analyze charge carrier
transport at the semiconductor/dielectric interface. Therefore, it is necessary to figure out the
charge transport property of OFETs developed from DTP-based D-A type polymer.
Additionally, in a D-A polymer semiconductor, electrical and geometrical suitability
between donor and acceptor, as well as its individual characteristic are very important. The
presence of a donor group that is bulkier than the acceptor group imposes steric hindrance on
inter-chain packing between adjacent polymer chains. Therefore, to elucidate the structural
suitability for the charge transport in OFETs, a study of DTP-based polymer with various
donor building blocks is required [8, 13, 29-34].
The present study explored the effect of structural suitability on film morphology and t
he OFET performance of DTP-based D-A copolymers by combining the DTP unit wit
h bithiophene (BT) or 4,8-bis(5-decylthiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (DTB
DT) donor building blocks with different size of conjugated core and alkyl side chains (
FIGURE 1). Incorporation of adequate electron-donating properties and geometrical sha
pe of BT units in DTP-based copolymers can induce the formation of well-ordered cr
ystal structures that are conductive to fast charge carrier transport [35]. Conversely, bul
ky and long alkyl side chain of DTBDT hinder inter-chain packing and do not allow
the formation of specific crystal structures. The effect of donor moiety (BT or DTBD
T) on performance of OFETs based on DTP polymers was investigated.
2. Experimental Section
2.1. Material Synthesis and Characterization

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1
13
Measurements. H NMR and C NMR were recorded using a Bruker Advance
spectrometer. Thermal analysis was performed on a TA TGA 2100 thermogravimetric
analyzer in N
2
atmosphere at a rate of 10°C/min. Differential scanning calorimeter (DSC)
on a TA instrument 2100 DSC. UV–vis absorption spectra were
was conducted under N
2
measured using a UV-1650PC spectrophotometer. Electrochemical analysis was performed in
an electrolyte solution of 0.1 M tetrabutylammonium perchlorate in acetonitrile at room
temperature (RT) under N . Molecular weights and polydispersities of the copolymers were
2
determined using gel permeation chromatography with polystyrene standard calibration.
Materials. Solvents and reagents were purchased from Aldrich, Alfa Aesar, or TCI.
Catalysts used in coupling reactions were purchased from Umicore. Other materials were of
common commercial grade and were used as received. All solvents were further purified
before use.
Synthesis of 2,6-dibromo-3,5-didodecyl-4-phenylphospholo[3,2-b:4,5-b']dithiophene 4-
oxide (DDTP) (1). The synthesis procedure followed literature methods [28]. The product
was purified by column chromatography using hexane:ethyl acetate (10:1 [v/v]) following
1
recrystallization in ethanol. Yield: 25%. H NMR (300 MHz, CDCl
3
) [ppm] δ = 7.74 (m,
2H), 7.56 (m, 1H), 7.45 (m, 2H), 2.57 (m, 2H), 2.37 (m, 2H), 1.43-1.10 (br, 40H), 0.92-0.87
1
3
(
m, 6H). C NMR (500 MHz, CDCl
3
)[ppm] δ = 144.26, 144.08, 143.20, 143.09, 137.98,
1
2
3
37.12, 133.00, 131.22, 131.13, 129.52, 129.42, 128.71, 111.92, 111.78, 32.32, 30.02, 29.82,
-1
9.74, 29.69, 29.57, 29.04, 23.08, 14,49. MS (EI) m/z = 782 (M+). FT-IR (cm , neat): 3087,
053, 3021, 2917, 2852, 1673, 1603, 1469, 1437, 1272, 1197, 1092, 787, 749, 713, 693.
Synthesis of 2,2'-bithiophene (2). Magnesium (16.4 g, 0.675 mol) was added to a flask and
the apparatus was flame-dried under vacuum. 2-Bromothiophene (110.0 g, 0.675 mol) and
dry diethyl ether (800 mL) was added to the flask over the course of 1 h. After addition, the
reaction mixture was heated and stirred at 40°C for 2 h. The solution was then allowed to

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cool to RT. A second flame-dried flask containing 2-bromothiophene (100.0 g, 0.613 mol),
[
1,3-bis(diphenylphosphino)propane]dichloronickel(II) (3.32 g, 6.13 mmol) and dry diethyl
ether (800 mL). Grignard reagent solution was added dropwise at 0°C. The reaction mixture
was stirred at RT for 12 h. The reaction was quenched with 2M HCl. The organic layer was
extracted with diethyl ether and the combined organic layer was dried over anhydrous
magnesium sulfate. The solvent was removed under reduced pressure and the resulting
residue was purified by silica gel chromatography using hexane as the eluent. Yield: 75.8 g
1
(
74%). H NMR (300 MHz, CDCl
3
)[ppm] δ = 7.19 (d, 2H), 7.16 (d, 2H), 7.00 (d, 2H).
Synthesis of 5,5'-bis(trimethylstannyl)-2,2'-bithiophene (3). A solution of 2,2-bithiophene
(1.0 g, 0.006 mol) in dry tetrahydrofuran (100 mL), n-butyllithium in hexane (2.5 M) (5.3
mL, 0.013 mol) was added to dropwise at -78°C. The reaction mixture was warmed to RT
and stirred for 2 h. Trimethyltin chloride (2.6 g, 0.013 mol) was added to one portion
at -78°C. The reaction mixture was warmed to RT and stirred for 2 h. The organic layer was
extracted with diethyl ether and the combined organic layer was dried over anhydrous
magnesium sulfate. The solvent was removed under reduced pressure and the product was
further purified by recrystallization from hexane to give a light yellow crystal. Yield: 1.8 g
1
(
61%). H NMR (300 MHz, CDCl ) [ppm] δ = 7.27 (d, 2H), 7.09 (d, 2H), 0.37 (s, 18H).
3
Synthesis of 4,8-bis(5-decylthiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (DTBDT) (4). A
solution of 2-decylthiophene (7.3 g, 0.032 mol) in dry tetrahydrofuran (150 mL), n-
butyllithium in hexane (2.5 M) (13.0 mL, 0.032 mol) was added to dropwise at 0°C. The
reaction mixture was then warmed to 50°C and stirred for 2 h, then benzo[1,2-b:4,5-
b']dithiophene-4,8-dione (3.0 g, 0.013 mol) was added to one portion and stirred for 1.5 h at
5
0°C. The solutions were cooled to RT, then SnCl
10%) was added and stirred for 2 h. The reaction mixture was poured into ice water and the
organic layer was extracted with diethyl ether; the combined organic layer was dried over
2 2
•2H O (9.2 g, 0.041 mol) in 10 mL HCl
(

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anhydrous magnesium sulfate. The solvent was removed under reduced pressure and the
resulting residue was purified by silica gel chromatography using hexane:ethyl acetate (10:1
[
v/v]) as the eluent. The product was further purified by recrystallization from methanol to
1
give pale yellow solid. Yield: 4.5 g (48%). H NMR (300 MHz, CDCl
3
) [ppm] δ = 7.72 (d,
2
H), 7.43 (d, 2H), 7.30 (d, 2H), 7.15 (d, 2H), 2.88 (d, 4H), 1.80 (m, 4H), 1.53-1.18 (br, 28H),
.92-0.82 (m, 6H).
Synthesis of
0
(4,8-bis(5-decylthiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-
diyl)bis(trimethylstannane) (5). A solution of 4,8-bis(5-decylthiophen-2-yl)benzo[1,2-b:4,5-
b']dithiophene (4) (4.5 g, 0.007 mol) in dry tetrahydrofuran (200 mL), n-butyllithium in
hexane (2.5 M) (6.2 mL, 0.015 mol) was added to dropwise at 0°C. The reaction mixture was
warmed to 50°C and stirred for 2 h. The mixture was cooled to RT, then trimethyltin chloride
(3.4 g, 0.017 mol) was added and stirred for 6 h. The reaction mixture was poured into ice
water and the organic layer was extracted with diethyl ether. The combined organic layer was
dried over anhydrous magnesium sulfate. The solvent was removed under reduced pressure
and the product was further purified by recrystallization from methanol to give yellow
1
crystal. Yield: 5.0 g (73%). H NMR (300 MHz, CDCl
3
) [ppm] δ = 7.69 (t, 2H), 7.49 (d, 2H),
7
.18 (d, 2H), 3.01 (d, 4H), 1.81 (m, 4H), 1.56-1.34 (br, 28H), 0.99-0.92 (m, 6H), 0.56-0.33 (t,
8H).
Polymerization of poly(DTP-BT). The polymer was synthesized (FIGURE 1a) by Stille
coupling under N . 2,6-Dibromo-3,5-didodecyl-4-phenylphospholo[3,2-b:4,5-b']dithiophene
-oxide (1) (0.500 g 0.638 mmol) and bis(trimethylstannyl)-2,2'-bithiophene (3) (0.314 g
.638 mmol) were dissolved in chlorobenzene (7.5 mL). Pd (dba) (0.011 g, 2 mol%) and
1
2
4
0
2
3
3
P(o-tol) (0.015 g, 8 mol%) were added to the mixture, then it was heated to 110°C and
stirred for 48 h. 2-Bromothiophene (0.2 mL) was added with a small amount of catalysts for
end capping, and the reaction mixture was further refluxed for 6 h. Tributyl-thiophen-2-yl-

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stannane (0.2 mL) was then added. After another 6 h, the reaction mixture was poured into
methanol (200 mL) and passed through a glass filter. The soxhlet was carried out with
methanol, hexane, and chloroform. The chloroform solution containing polymer was
1
reprecipitated in methanol and filtered out. Yield: 0.35 g (70%). H NMR (500 MHz, CDCl
3
)
[
ppm] δ = 7.77-7.71 (br, 2H), 7.49-7.38 (br, 4H), 7.04-6.95 (br, 3H), 2.71 (br, 2H), 2.51 (br,
H), 1.22-1.03 (br, 40H), 0.80 (br, 6H).
Polymerization of poly(DTP-DTBDT). The polymer was prepared in the same way
FIGURE 1b) as poly(DTP-BT), but using 2,6-dibromo-3,5-didodecyl-4-
2
(
phenylphospholo[3,2-b:4,5-b’]dithiophene 4-oxide (0.367 g, 0.468 mmol) (1) and (4,8-bis(5-
decylthiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl)bis(trimethylstannane) (5) (0.450
1
g 0.468 mmol). Yield: 0.31 g (53%). H NMR (300 MHz, CDCl
3
) [ppm] δ = 7.83 (br, 2H),
7.64-7.45 (br, 5H), 7.30 (br, 2H), 6.91 (br, 2H), 2.96-2.58 (br, 8H), 1.80 (br, 4H), 1.48-1.09
-1
(
br, 68H), 0.91 (br, 12H). FT-IR (cm , neat): 3065, 2918, 2851, 1464, 1434, 1203, 1181,
1
2
109, 802, 746, 715, 692.
.2. Morphological Characterization
X-ray diffraction (XRD) experiments were performed at the energy of 11.57 keV at the 5A
beamline at the Pohang Accelerator Laboratory (PAL), Pohang, Korea. The measurement
angle was gradually increased from 1° to 15° in 0.05° steps in an out-of-plane direction.
Atomic force microscopy (AFM) experiments were conducted using an XE-100 Advanced
Scanning Probe Microscope operating in tapping mode with a silicon cantilever. The thin-
film samples used in the XRD and AFM studies were fabricated by spin-coating at 6000 rpm
with the 0.2% chloroform solution on an octadecyltrichlorosilane (ODTS)-modified silicon
wafer, followed by drying under vacuum at RT. After film deposition, the samples were
annealed at 100°C, 150°C, or 200°C to observe the effect of annealing temperature (Tann
)

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2.3. Fabrication and Characterization of Field-Effect Transistors
Heavily As-doped silicon wafers were used as the gate electrode of bottom-gated, top-
contact OFETs. A thermally-grown 3000 Å-thick oxidized layer was used as the gate
dielectric. The SiO /Si substrates were cleaned with piranha solution [H (40
mL)/concentrated H SO (60 mL)], and chemically modified with ODTS. As a
2
2 2
O
2
4
semiconductor layer, poly(DTP-BT) or poly(DTP-DTBDT) films were constructed by spin
coating at 6000 rpm for 30 s with a 0.2 wt% solution in chloroform, yielding films with
average thicknesses of 50 nm, as confirmed by ellipsometery (J.A.WOLLAM Co., Inc.).
Gold source and drain electrodes of thickness 1000 Å were thermally evaporated onto the
semiconductor layer. The dimensions of the source and drain electrodes were determined by
the ratio of width (W)/length (L).The un-encapsulated OFETs were characterized under
ambient conditions in a dark room using Keithley 236 source/measure units. A voltage-
-8
2
capacitance scan confirmed that the gate dielectric capacitance (Cdiel = 10 nF/cm ) remained
almost constant. The transfer character was obtained in the saturation region (drain voltage =
–
80 V). The threshold voltage (Vth) and field-effect mobility (µFET) were extracted from a
1
/2
IDS -V (I : drain current, V : gate voltage) plot using the equation: I = µFET Cdiel
G
DS
G
DS
2
(
G
W/2L)(V -Vth) .
3
. Results and Discussion
.1. Synthesis and Characterization
The Synthesis schemes of the monomers and polymers used in this study are depicted in
3
FIGURE 1. The structures of the synthesized polymers were confirmed by H-NMR and IR
spectroscopy (FIGURE S1). Both polymers were good solubility in common organic solvent
such as chloroform and chlorobenzene. The good solubility was likely due to the long alkyl

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substituents of DTP. The number average molecular weight of poly(DTP-BT) and poly(DTP-
DTBDT) were 11 kg/mol with a polydispersity index (PDI) = 4.02, and 19.1 kg/mol with PDI
=
1.63, respectively.
In thermogravimetric analyses (TGA) of poly(DTP-BT) and poly(DTP-DTBDT) (FIGURE
S2), the materials decomposed (5% weight loss) at 445.86°C and 390.18°C, respectively;
thus both compounds have good thermal stability. The lower decomposition temperature of
poly(DTP-DTBDT) can be attributed to the long alkyl chains of the DTBDT units and the
low density of the amorphous film.
3.2. Electrochemical and Photophysical Properties
Cyclic voltammetry (CV) was used to estimate the highest occupied molecular orbital
(
HOMO) and LUMO energy levels of synthesized polymers (FIGURE S3a). The poly
mers showed quasi-reversible redox waves. Poly(DTP-BT) had onset reduction potentia
onset
onset
l (Ere
DT) had Ere
URE S3b) were estimated as LUMO = Ere
) = −0.81 V and onset oxidation potential (Eox ) = 0.81 V; poly(DTP-DTB
onset
onset
= −0.73 V and Eox
= 1.18 V. The LUMO and HOMO levels (FIG
onset
onset
+ 4.44 eV; HOMO = Eox
+ 4.44 e
V [36R]. LUMO levels of the polymers were similar because they have the same DT
P acceptor moiety. The HOMO levels and band gaps differ due to differences in don
or strength [37, 38].
UV-absorption spectra (FIGURE 2) were recorded from poly(DTP-BT) and poly(DTP-
DTBDT) films annealed at various temperatures. In the two copolymers, the push-pull
interactions caused by the D-A copolymer architecture reduce the energy of the optical band
gap, thereby increasing the degree of intramolecular charge transfer (ICT) [13]. The strong
D-A interactions and aligned molecular packing of the poly(DTP-BT) films induced the red-
shift in the absorption maximum of the poly(DTP-BT) relative to that of poly(DTP-DTBDT).

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On transition from solution to thin-film, both polymers showed a significant red-shift, which
indicates enhanced π-π stacking [15]. The optical band gaps in the thin-film state were
verified from the onset of film absorption (TABLE 1). To induce organization of the polymer
chains, the films were annealed at various temperatures. Overall absorption intensity
increased with Tann in For poly(DTP-BT) films, but was not affected by Tann in poly(DTP-
DTBDT) films.
3.3 Morphological Characteristics
The images of top surface for poly(DTP-BT) and poly(DTP-DTBDT) films were
provided using the AFM observation. It is clearly shown that poly(DTP-BT) film has crystal
structure, while poly(DTP-DTBDT) film has amorphous morphology. Additionally, thermal
annealing also differently affected the surface morphologies of poly(DTP-BT) and poly(
DTP-DTBDT) films (FIGURE 3). As Tann increased, the poly(DTP-BT) formed partiall
q
y-aggregated film morphology; as a result, R increased from 0.86 nm in the as-cast fi
lm to 1.34 nm in the film annealed at 200 ˚C film (FIGURE 3a-d). Deep holes or tr
enches were observed in films annealed at 200 ˚C; these may have been charge trap
sites [6, 39, 40]. In contrast, none of the poly(DTP-DTBDT) films showed aggregations;
all had very smooth surfaces with R
C.
q
~ 0.3 nm even after thermal annealing at 200 ˚
XRD analyses verified the dependence of crystalline structure depending on Tann for
the poly(DTP-BT) and poly(DTP-DTBDT) films (FIGURE 4). In the XRD spectra of as-
cast poly(DTP-BT) film, the distinct (100) diffraction peak at 2θ = 2.92° (d-spacing = 2.10
nm) indicate the presence of ordered lamellar sheets in direction of surface normal. On the
other hand, the (100) peak of poly(DTP-DTBDT) was not discernible. The poly(DTP-BT
)
showed enhanced crystal morphology while poly(DTP-DTBDT) exhibited featureless

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amorphous with increasing Tann. In the poly(DTP-BT) film, the enhanced crystallinity by t
hermal annealing was confirmed by an increase in the intensity of the (100) peak (FI
GURE 4a) and the appearance of (200) and (300) diffraction peaks. The decreased
interlayer spacing (d-spacing) also indicate the closely packed lamellae. The (100) diffractio
n peak shifted from 2θ = 2.92° in the as-cast film to 2θ = 2.72° (d-spacing = 2.26
nm) in films annealed at 200°C. The decrease in inter-chain distance with increasing
Tann in poly(DTP-BT) film is attributed to more closely packed chains, which facilitat
e charge carrier transport after annealing [41-43]. These phenomenon which crystal
morphology was enhanced by thermal annealing indicate that the kinetically-limited crystal
lization of poly(DTP-BT) films at RT is activated by thermal treatment. Whereas, in po
ly(DTP-DTBDT) film, any diffraction peaks was not verified even after thermal annealing. T
he absence of diffraction peak may be caused by interruption of chain self-organization
by the relatively long alkyl group of DTBDT.
The AFM and XRD data indicate that poly(DTP-BT) is crystalline and poly(DTP-
DTBDT) is amorphous. The crystalline morphology of poly(DTP-BT) may result from its
strong electron-donating properties and the geometric size of the BT moiety, which is
appropriate for the formation of ordered structures. Conversely, the bulky size of the
DTBDT moiety prevents alignment between adjacent chains.
3.4. OFET Characteristics
Over 6 individual devices were measured and OFET characterized according to Tann
.
Transfer characteristics of OFETs and their OFET parameters fabricated with poly(DTP-BT)
and poly (DTP-DTBDT) semiconductors were affected by Tann (Figure 5a,b and Table 2).
µ
FET of poly(DTP-BT) increased gradually as Tann increased, whereas µFET of poly(D
TP-DTBDT) was less affected by Tann. These different trends may be due to changes

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in the crystal morphologies of semiconductor films during thermal annealing. XRD
spectra showed that the poly(DTP-BT) film underwent significant increase in crystallin
ity with increasing Tann (FIGURE 4a), whereas the poly(DTP-DTBDT) film remained
amorphous regardless of Tann (FIGURE 4b). In this case, the thermal energy applied d
uring annealing removed only residual solvent and did not induce structural rearrange
ment. In most OFETs based on polymer semiconductors, thermal annealing is used to
increase molecular ordering along the π-π stacking direction, thereby enhancing charge
transport in the semiconductor layer [44]. This effect was observed here, where µFET of
crystalline poly(DTP-BT) OFETs increased with increasing Tann, whereas those of the
amorphous poly(DTP-DTBDT) OFETs remained the same.
However, the Vth of poly(DTP-BT) OFETs gradually shifted in the negative V
G
direc
tion as Tann increased (from -25 V in as-cast film to -29 V in films that had been a
nnealed at 200°C). This negative shift in Vth during thermal annealing is attributed to
enlargement of the subthreshold region, which is defined as the range from turn-on to
Vth. Subthreshold region is intimately related to the amount of the trap to should be filled
in order to form a conducting channel of OFETs; i.e., as subthreshold regions expand V
th
becomes more negative. In annealed poly(DTP-BT) devices, the negatively shifted Vth
than that of as-cast devices, implies increased the number of traps in the band gap than as-
cast devices (assuming that the turn-on voltages do not change). This effect may be d
ue to the presence of grain boundaries (GBs). As Tann increased, the roughness of the
poly(DTP-BT) film increased (FIGUREs 3a-d). The increase in surface roughness indi
cates increases in heterogeneity of morphology and the proportion of GB regions in t
he annealed film; these changes would increase the number of deep traps (far from th
e band edge), the number of shallow traps (near it), or both [6, 45-47]. Therefore, Dee
p holes or trenches in the annealed-poly(DTP-BT) may act as deep trap and expanded the

ACCEPTED MANUSCRIPT
subthreshold or off- region [48, 49].
Consequently, thermal annealing of poly (DTP-BT) increases its crystallinity and thereby
enables fast charge transport, and also increases the proportion of GB regions and thereby
causes a negative shift in Vth
.
4. Conclusion
A strongly electron-deficient DTP moiety with a rigid structure was used as an acceptor
building block in D-A alternating copolymers. The DTP moiety was combined with two
different donor units to yield two electronically and morphologically different materials
(
crystalline poly(DTP-BT) and amorphous poly(DTP-DTBDT)). Although as-cast films of
both materials exhibited similar µFET values, poly(DTP-BT) films that had been annealed at
00°C exhibited a µFET that was 15 times higher than that of amorphous poly(DTP-DTBDT).
2
This difference was due the enhanced crystallinity of the poly(DTP-BT) film. Thermal
annealing of poly (DTP-BT) also increased the GB region, and this increase may have
resulted in an increased number of trap sites, and consequently in an expansion of the
subthreshold region and a more negative Vth than that of poly(DTP-DTBDT).
Acknowledgement
This research was supported by a grant (2011-0031639) from the Center for Advanced Soft
Electronics under the Global Frontier Research Program of the Ministry of Education,
Science and Technology, Korea and NRF grant funded by the Korea government (MSIP)
(2010-0029084 & 2012M3A7B4049647). XRD experiments were performed at the 5A
beamline of the Pohang Accelerator Laboratory in Korea.

ACCEPTED MANUSCRIPT
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FIGURES AND TABLES
Table 1. Optical properties of the DTP-based polymers.
Table 2. Performance of FETs.
Figure 1. Synthesis routes of monomers and copolymers: (a) Poly(DTP-BT) and (b) poly(DTP-
DTBDT).
Figure 2. UV-visible absorption spectra of (a) poly(DTP-BT), and (b) poly(DTP-DTBDT) in
dilute CHCl and thin films according to annealing temperature.
3
Figure 3. (top) AFM images and (bottom) cross section images of (a-d) poly(DTP-BT) and (e-f)
poly(DTP-DTBDT).
Figure 4. Out-of-plane XRD spectra of poly(DTP-BT) (a), and poly(DTP-DTBDT) (b) according
to annealing temperature.
Figure 5. Transfer characteristics at a fixed V of -80 V for OFETs of poly(DTP-BT) (a) and
DS
poly(DTP-DTBDT) (b). Output characteristics of (c) poly(DTP-BT) annealed at 200 °C (d) and
poly(DTP-DTBDT) annealed at 150 °C.

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Table 1. Optical Properties of the DTP-based polymers.
λmaxsol
nm]
λmaxfilm
[nm]
λedgefilm
[nm]
Egoptical
[eV]
Polymer
[
Poly(DTP-BT)
519
16
580
545
693
675
1.79
1.84
poly(DTP-
DTBDT)
5

ACCEPTED MANUSCRIPT
Table 2. Performance of FETs.
Annealing
temp. [°C]
µ
µ
FET,AVG
FET,MAX
2
on/off
Vth [V]
2
[cm /Vs]
5.7ꢀ10-4
1.0ꢀ10-3
4.0ꢀ10-3
5.5ꢀ10-3
2.2ꢀ10-4
[cm /Vs]
5.7ꢀ10-4
8.8ꢀ10-4
3.3ꢀ10-3
5.1ꢀ10-3
1.1ꢀ10-4
As-cast
6ꢀ103
7ꢀ103
1ꢀ104
2ꢀ104
5ꢀ103
-25
-27
-27
-29
-11
-4.0
-6.0
-3.4
1
1
2
00
50
00
poly(DTP-BT)
As-cast
1
1
2
00
50
00
3.4ꢀ10-4
2.5ꢀ10-4
8ꢀ104
poly(DTP-
DTBDT)
3.2ꢀ10-4
3.0ꢀ10-4
3ꢀ104
3.3ꢀ10-4
2.8ꢀ10-4
4ꢀ104
*
The V and on/off ratio of each annealing temperatures was calculated from transfer curves
th
having a maximum µFET.

ACCEPTED MANUSCRIPT
Figure 1. Synthesis routes of monomers and copolymers: (a) Poly(DTP-BT) and (b) poly(DTP-DTBDT).
S
n-BuLi / Trimethyltin chloride
Tetrahydrofuran
S
S
Sn
Sn
S
2
3
C
10
H
21
10 21
C H
S
S
S
n-BuLi / Trimethyltin chloride
Tetrahydrofuran
S
Sn
Sn
S
S
S
S
C H
10
21
10 21
C H
4
5
O
O
Pd
2
(dba)
3
/ P(o-tol)
3
C
12
H
25
12 25
C H
C
12
H
25
C
12
H
25
P
P
S
Sn
+
Sn
S
Chlorobenzene
S
S
S
Br
S
S
S
S
S
Br
n
poly(DTP-BT)
1
10 21
C H
C H
10 21
O
S
P
C
12
H
25
S
12 25
C H
Pd
2 3 3
(dba) / P(o-tol)
S
S
O
S
C
12
H
25
C
12
H
25
Sn
Sn
S
S
P
+
Chlorobenzene
S
S
S
n
Br
Br
S
S
S
S
C
10
H
21
C
10
H
21
1
poly(DTP-DTBDT)

ACCEPTED MANUSCRIPT
Figure 2. UV-visible absorption spectra of (a) poly(DTP-BT), and (b) poly(DTP-DTBDT) in dilute CHCl
3
and thin films according to annealing temperature.

ACCEPTED MANUSCRIPT
Figure 3. (top) AFM images and (bottom) cross section images of (a-d) poly(DTP-BT) and (e-f)
poly(DTP-DTBDT).

ACCEPTED MANUSCRIPT
Figure 4. Out-of-plane XRD spectra of poly(DTP-BT) (a), and poly(DTP-DTBDT) (b) according to
annealing temperature.
(
a)
200
(
100)
1
7
50
0
(
200)
pristine
(
300)
2
00
(
b)
150
0
7
pristine
2
4
6
8
10
2θ [degree]