Synthesis and Photophysical Studies of Thiadiazole[3,4-c]pyridine Copolymer Based Organic Field-Effect Transistors

Article abstract of DOI:10.1007/s10895-016-1792-5

A novel thiadiazolo[3,4-c]pyridine]?based donor-acceptor (D-A) copolymer, poly[4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-[4,7-bis(4-(2-ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine] (PTBDTPT), containing triisopropylsilylethynyl(TIPS)benzo[1,2-b:4,5-b′]dithiophene as a donor is synthesized by Stille polymerization reaction. All the important photo physical prerequisites for organic field-effect transistor (OFET) application such as strong and broad optical absorption, thermal stability, and compatible HOMO-LUMO levels can be accomplished and combined on one macromolecule. Optical band gap of the polymer was found to be 1.61?eV as calculated from its film onset absorption edge. The hole mobility of bottom gate OFET using the synthesized polymer as an active channel is found to be 1.92 X 10^?2?cm?V^?1?s^?1 with the On/Off ratio of 25. The photophysical study suggests that PTBDTPT is promising candidate for future large area organic electronic applications.

Full text of DOI:10.1007/s10895-016-1792-5

J Fluoresc
DOI 10.1007/s10895-016-1792-5
ORIGINAL ARTICLE
Synthesis and Photophysical Studies of Thiadiazole[3,4-c]
pyridine Copolymer Based Organic Field-Effect Transistors
Chinna Bathula1,2 & Sang Kyu Lee3 & Pranav Kalode4 & Sachin Badgujar3
Ningaraddi S. Belavagi5 & Imtiyaz Ahmed M. Khazi5 & Youngjong Kang 1,2
&
Received: 5 November 2015 /Accepted: 27 March 2016
#
Springer Science+Business Media New York 2016
. .
Keywords Thiadiazolo[3,4-c]pyridine] Benzodithiophene
Abstract A novel thiadiazolo[3,4-c]pyridine] based
donor-acceptor (D-A) copolymer, poly[4,8-bis
(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene-
2,6-diyl-alt-[4,7-bis(4-(2-ethylhexyl)thiophen-2-yl)-[1,2,
5]thiadiazolo[3,4-c]pyridine] (PTBDTPT), containing
triisopropylsilylethynyl(TIPS)benzo[1,2-b:4,5-b′]
dithiophene as a donor is synthesized by Stille polymeriza-
tion reaction. All the important photo physical prerequisites
for organic field-effect transistor (OFET) application such as
strong and broad optical absorption, thermal stability, and
compatible HOMO-LUMO levels can be accomplished and
combined on one macromolecule. Optical band gap of the
polymer was found to be 1.61 eV as calculated from its film
onset absorption edge. The hole mobility of bottom gate
OFET using the synthesized polymer as an active channel is
found to be 1.92 X 10−2 cm V−1 s−1 with the On/Off ratio of
25. The photophysical study suggests that PTBDTPT is
promising candidate for future large area organic electronic
applications.
.
.
Photophysical studies Stille reaction Organic field-effect
transistors
Introduction
Organic electronics offers attractive balance between cost and
performance due to versatility in its molecular structure.
Remarkable progress has been made on the solution-
processed organic semiconductors to replace amorphous sili-
con as the active layer in bulk heterojunction(BHJ) organic
photovoltaics (OPVs), organic field-effect transistors (OFETs)
due to their potential applications in flexible and low-cost E-
paper, smart card, radio frequency identification, and displays
[1–3]. Conjugated polymer backbones containing alternating
electron rich donor and electron-poor acceptor units have
emerged as a popular approach in the design of low band
gap materials [4]. By careful consideration of the repeating
donor and acceptor units, control over the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) energy levels of these polymers is possible
[5]. This facilitates the design of a variety of chromophores
with optimal light absorption properties for the injection and
extraction of charges in OFET devices [6–8]. Some of the
highest performing polymers for organic electronic applica-
tions utilize this concept [6].
* Chinna Bathula
cdbathula9@gmail.com
1
Department of Chemistry, Research Institute for Natural Sciences,
Hanyang University, Seoul 133-791, Republic of Korea
2
Institute of Nanoscience and Technology, Hanyang University,
The mobility in polymeric FETs especially at ambient con-
ditions is generally low due to the poor packing of those ma-
terials. In order to solve this problem, one of the most effective
approaches is to strengthen the intermolecular interactions be-
tween the neighbouring molecules by enhancing their molec-
ular orbital overlapping. Although the large number of high
performing materials have been explored for OFETs,
diketopyrrolopyrrole remains as one of the most versatile
Seoul 133-791, Republic of Korea
3
Energy Materials Research Center, Korea Research Institute of
Chemical Technology (KRICT), 141 Gajeong-ro, Yuseong-gu,
Daejeon 305-600, Republic of Korea
4
Department of Physics and Energy Systems Research, Ajou
University, Yeongtong-gu, Suwon 443-749, Republic of Korea
5
CPEPA, Department of Chemistry, Karnatak University,
Dharwad 580 003, India

J Fluoresc
and widely used structural motifs [9, 10]. It was found that the
fused aromatic rings such as thieno[3,2- b]-thiophene [11]
provide a clear tendency to form a large orbital overlapping
area, which is useful for charge carrier transport. Additionally,
development of the alternating donor-acceptor (D – A) copol-
ymers may be another versatile way to enhance mobility [12],
because these polymers can exhibit closer intermolecular
π − π stacking in view of the attractive forces between the
donor and acceptor units. Conjugated polymers containing 2,
1,3-benzothiadiazole (BTz) based D–A type of have been ex-
plored in OFET [5]. Among the reported blocks, benzo[1,2-
b:4,5-b]dithiophene (BDT) [13–16], naphthodithiophene
(NDT) [17, 18], and 4,7-dithien-2-yl-2,1,3-benzothiadiazole
(DTBT) [5] have been proven to be excellent donor and ac-
ceptor units. Dithienothiadiazole [3,4-c]pyridine (DTPT)
offers a pyridyl N-atom for possible Lewis acid binding,
which is more basic and accessible than the BTz coun-
terpart [19], and makes it a stronger acceptor than BTz. The
thiadiazolopyridine unit is a strong electron acceptor [20]
which can lead to charge-transfer characteristics when
coupled with complementary donor fragments. Copolymers
incorporating the DTPT unit have been reported as effective
narrow band gap materials and are shown to exhibit enhanced
optoelectronic properties [21, 22].
In this research we report, a novel donor–acceptor
types of conjugated polymer PTBDTPT based on
dithienothiadiazole[3,4-c]pyridine acceptor and
triisopropylsilylethynyl(TIPS)benzo[1,2-b:4,5-b′]
dithiophene as donor for application in OFET. All the
important photo physical prerequisites for organic field-
effect transistor (OFET) application such as strong and broad
optical absorption, thermal stability, and compatible HOMO-
LUMO levels are determined in one macromolecule. Due to
the presence triisopropylsilylethynyl in the donor and ethyl
hexyl alkyl chain on the dithienothiadiazole[3,4-c]pyridine
acceptor, the polymer PTBDTPT exhibited good solubility
in common processing solvents for fabrication of OFET
such as chloroform, chlorobenzene and O-dichlorobenzene.
The hole mobility of bottom gate OFET using the poly-
mer PTBDTPT as an active channel found to be 1.92 X
10−2 cm V−1 s−1 with the On/Off ratio of 25. This work
successfully demonstrates that dithienothiadiazole [3,4-c-
]pyridine and triisopropylsilylethynyl(TIPS)benzo[1,2-b:4,5-
b′] dithiophene are promising units to build D-A copolymer
for future large area organic electronics.
(DMF), tetrakis(triphenylphosphine)palladium, tributyl(4-(2-
ethylhexyl)thiophen-2-yl)stannane, N-bromosuccinimide,
bis(triphenylphosphine)palladium(II) dichloride and toluene
(99.8 %, anhydrous) were purchased from Aldrich. 3, 4-
diaminopyridine was Purchased from TCI. All chemicals
were used without further purification. The monomers 2,6-
Bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)-benzo[1,2-
b:4,5-b′]dithiophene [12] and 4,7-Bis(5-bromo-4-(2-
ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo [3,4-c]pyridine
[19] were prepared by previously described methods.
Material Characterization
1
The synthesized compounds were characterized with H
NMR and 13C NMR spectra obtained using a Bruker DPX-
300 NMR spectrometer. UV-visible analysis was performed
using a Lambda 20 (Perkin Elmer) diode array spectropho-
tometer. The number and average molecular weights of the
polymers were determined by gel permeation chromatography
(GPC; Viscotek) equipped with a TDA 302 detector and a PL-
gel (Varian) column, using chloroform as the eluent and poly-
styrene as the standard. Cyclic voltammetry experiment is
carried out using Weis-500 work station. Thermogravimetric
analysis (TGA) was performed under a nitrogen atmosphere at
a heating rate of 10 °C min−1 with a Dupont 9900 analyzer.
Synthesis of Monomer and Polymer
Synthesis of 2,5-dibromo-3,4-diaminopyridine (2).
In a three-neck 100 mL round-bottom flask, was added 3,4-
diaminopyridine (1) (5.0 g, 45.2 mmol) with 9.3 mL of 48 %
hydrobromic acid. The resulting mixture was heated to reflux
while stirring and bromine (7.6 mL, 148.5 mmol) was added
dropwise. The reaction mixture was refluxed for 5 h and
cooled to ambient temperature. The mixture was filtered and
washed with a saturated solution of Na2S2O3 (100 mL),
Na2CO3 (100 mL) and finally water (100 mL). To complete
the neutralization process, the crude product was refluxed
30 min in a 10 % solution of Na2CO3. The mixture was then
filtered and washed with water. The brown-dark product was
first purified by column chromatography ethyl acetate/hexane
(3:2) and by recrystallization from methanol to give com-
Experimental
Materials
1
pound 2 as an orange powder (3.1 g, 25.5 %). HNMR
(Acetone-d6, 300 MHz, δ/ppm): 7.65 (s, 1H), 5.54 (br, 2H),
4.66 (br, 2H). 13C NMR (100 MHz, Acetone-d6, δ/ppm):
140.66, 140.45, 129.81, 128.30, 106.24. EI-MS m/z: [M +
H]+ Cacld for C5H5Br2N3: 264.88, Found: 264.70.
Triisopropylsilyl acetylene, N,N,N,N-tetramethyl-ethane-1,2-
diamine (TMEDA), trimethyltinchloride, dimethylformamide

J Fluoresc
7-bromo-4-chloro [1,2,5]thiadiazolo[3,4-c]pyridine (3).
(s, 1H), 7.05 (s, 1H), 2.64 (t, 4H),1.66 (m, 2H), 1.21–1.42 (m,
16H), 0.95 (m, 12H). 13C NMR (CDCl3, 100 MHz, δ/ppm):
155.06, 148.18, 146.46, 144.12, 143.34, 141.28, 140.76,
136.15, 133.70, 129.93, 126.68, 120.30, 120.63, 40.46,
34.71, 32.60, 29.03, 25.77, 23.23, 14.33, 11.02. EI-MS m/z:
[M + H]+ Cacld for C29H39N3S3: 525.84, Found: 525.23.
A 250 mL flame-dried flask fitted with a condenser was
charged with 2.00 g (7.5 mmol) of 2,5-dibromo-3,4-
diaminopyridine (2) and 41.00 ml (560 mmol) of thionyl chlo-
ride. The mixture was refluxed for 5 h then cooled to room
temperature. The excess of thionyl chloride was evaporated
and the crude product slowly neutralized with 50 mL of satu-
rated NaHCO3. Once the reaction quenching was completed,
100 mL of chloroform was added and the mixture transferred
to a separatory funnel. The chloroform layer was separated
and the aqueous layer washed two times with chloroform.
The organic layers were combined, dried with MgSO4 and
the solvent removed under reduce pressure. The crude product
was recrystallized from methanol to afford compound 3 pale
4,7-Bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)-[1,2,
5]thiadiazolo[3,4-c]pyridine (5)
In a dry two-neck 50 mL round bottom flask, 4,7-Bis(4-(2-
ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]Pyridine (4)
(0.50 g, 0.95 mmol) was dissolved in dry THF (20 mL) at room
temperature. Then the reaction mixture was stirred in an ice bath
for 15 min, NBS (0.440 g, 2.35 mmol) was added in small
portions in dark. After stirring for about 10 h, the reaction mix-
ture was washed with water (100 mL), and the organic layer was
dried over anhydrous Na2SO4. The solvent was removed to give
the product as a red solid. The solid was purified by column
chromatography on silica gel using petroleum ether/
dichloromethane (10:1) as eluent to get the crude product
Further purification was carried out by recrystallization with
ethanol to obtain the pure compound 5 as red crystals
(0.430 g, 67 %).1H NMR (CDCl3, 300 MHz, δ/ppm): 8.69 (s,
1H), 8.32 (s, 1H), 7.74 (s, 1H), 2.58 (t, 4H), 1.71 (m, 2H), 1.20–
1.40 (m, 16H), 0.91 (m, 12H). 13C NMR (CDCl3, 100 MHz, δ/
ppm): 154.66, 147.80, 145.52, 143.58,142.68, 140.86, 140.19,
135.82, 133.24, 129.15, 119.95,117.22, 112.62, 40.10, 34.16,
32.60, 28.91, 25.84, 23.23, 14.32, 11.02. EI-MS m/z: [M +
H]+ Cacld for C29H37Br2N3S3: 683.64, Found: 683.40.
1
yellow needles as the title product (1.15 g. 62 %). H NMR
(300 MHz, CDCl3, δ/ppm): 8.54 (s, 1H), 13C NMR
(100 MHz, CDCl3, δ/ppm): 156.30, 148.68, 145.40, 144.84,
111.11, EI-MS m/z: [M + H]+ Cacld for C5HBrClN3S:
248.87, Found: 248.62.
4,7-Bis(4-(2-ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,
4-c]Pyridine (4)
In a dry two-neck 100 mL round bottom flask, 7-bromo-4-
chloro [1,2,5]thiadiazolo[3,4-c]pyridine (3) (0.50 g,
2.00 mmol), tributyl(4-(2-ethylhexyl)thiophen-2-yl)stannane
(1.64 g, 4.60 mmol) and Pd(PPh3)2Cl2 (0.10 g) were added
under the protection of N2. Dry THF (12 mL) and DMF
(12 mL) were injected through a syringe. The reaction mixture
was heated to reflux for 12 h, then the mixture was allowed to
cool down to room temperature and poured into water
(50 mL). The organic phase was next separated and the aque-
ous phase extracted by chloroform (3 X 50 mL). After removal
of the solvent, the solid was purified by column chromatogra-
phy on silica gel using petroleum ether/dichloromethane (10:1)
as eluent to get the crude product. Further purification was
carried out by recrystallization with ethanol to obtain the pure
compound 4 as red crystals (0.63 g, 70 %), 1H NMR (CDCl3,
300 MHz, δ/ppm): 8.81 (s, 1H), 8.50 (s, 1H), 7.93 (s, 1H), 7.18
Synthesis of poly[4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,
5-b′]dithiophene-2,6-diyl-alt-[4,7-bis(4-(2-ethylhexyl)thiophen-
2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine] (PTBDTPT).
.

J Fluoresc
Scheme 1 Synthetic route for DTPT monomer
2,6-Bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)-
benzo[1,2-b:4,5-b′]dithiophene (6) (262 mg, 0.3 mmol), 4,7-
Bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)-[1,2,
5]thiadiazolo[3,4-c]pyridine (5) (205 mg, 0.3 mmol) and
Pd(PPh3)4 (14 mg, 0.05 eq.) were added to a 50 mL round-
bottom flask. Then, anhydrous DMF (1 mL) and anhydrous
toluene (4 mL) were added. The polymerization was carried
out at 105 °C under argon protection. Then, anhydrous DMF
(1 mL) and anhydrous toluene (4 mL) were added. The poly-
merization was carried out at 105 °C under argon protection.
After 24 h, the reaction mixture was cooled to about 50 °C and
added slowly to a vigorously stirred methanol (50 mL). The
polymer fibers were collected by filtration. The polymer was
dissolved in chlorobenzene and precipitated again into meth-
anol. The precipitate was then subjected to Soxhlet extraction
with methanol, acetone, hexanes, and chloroform. The final
polymer is obtained by evaporation of chloroform and precip-
itating in methanol. The polymer is filtered and dried in vacuo
at 40 °C for 12 h. Dark blue shining material is obtained in the
yield of 73 %. 1H NMR (CDCl3, 300 MHz, δ/ppm): 8.64 (br,
1H), 8.26 (br, 1H), 7.58 (br, 2H), 7.62 (br, 1H), 2.47 (br, 4H),
1.68 (br, 2H), 1.20–1.40 (br, 58H), 0.93(br, 12H). Anal.calcd:
C, 68.31; H, 7.95; N, 3.81, S, 14.54; found C, 68.20; H, 7.86;
N, 3.72, S, 14.48.
Results and Discussion
Synthesis and Characterization of the Monomer
and Polymer
The acceptor monomer 4,7-Bis(5-bromo-4-(2-
ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine
was synthesized as depicted in (Scheme 1). First step involves
bromination of 3,4- diaminopyridine with 48 % hydrobromic
acid resulting in the formation of 2,5-dibromo-3,4-
diaminopyridine (2). Thus obtained intermediate is subjected
for ring closure reaction using thionyl chloride to obtain 7-
bromo-4-chloro [1,2,5]thiadiazolo[3,4-c]pyridine (3). The
next step consisted of palladium catalyzed Stille reaction of
compound (3) and tributyl(4-(2-ethylhexyl)thiophen-2-
yl)stannane leading to an intermediate 4,7-Bis(4-(2-
ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]Pyridine
(4), followed by bromination using NBS to accomplish the
synthesis of monomer 4,7-Bis(5-bromo-4-(2-
ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine
(5). The thiadiazolo [3,4-c]pyridine] based co-polymer
PTBDTPT is synthesized by polycondensation of 2,6-
bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)-benzo[1,2-
b:4,5-b′]dithiophene (6) and 4,7-Bis(5-bromo-4-(2-
Scheme 2 Synthetic route for
PTBDTPT copolymer

J Fluoresc
ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine
(5) through the palladium catalyzed Stille reaction (Scheme
2). The crude polymers were extracted with chloroform and
precipitated in methanol followed by washing with methanol
and acetone, successively, via Soxhlet apparatus to remove
by-products and oligomers. The final polymers were obtained
by evaporation of chloroform and precipitating in methanol.
The number average molecular weights (Mn) were determined
by gel permeation chromatography (GPC) against polystyrene
standards in a chloroform eluent (Table 1). Mn of the polymer
is found to be 12.9 kg mol−1 with a polydispersity index of
1.93 (Table 1).
Fig. 1 TGA plots for the polymer, obtained with a heating rate of
10 °C min−1 under an inert atmosphere
Thermal Properties
Electrochemical Properties
The thermal properties of copolymers were evaluated by
thermogravimetric analysis (TGA) under a nitrogen at-
mosphere at a heating rate of 10 °C min−1 (Fig. 1). The
polymer exhibited good thermal stability, showing a
weight loss of less than 5 % at temperature up to
350 °C indicating sufficient thermal stability for OFET
applications. The physical properties of the polymer are
summarized in (Table 1).
We studied the electrochemical properties of the polymer by
cyclic voltammetry in order to estimate their HOMO and
LUMO energy levels. The onset oxidation and reduction po-
tentials obtained are correspond to the HOMO and LUMO
energy levels, respectively. The electrochemical properties of
polymer as thin film on a platinum electrode in 0.1 M
−1
Bu4NPF6 acetonitrile solution at a scan rate of 50 mV s
are determined. As seen in Fig. 3, the onset oxidation
potential of PTBDTPT is found to be at 0.50 eV versus
Ag/AgCl respectively. Using ferrocence as a reference
value with −4.8 eV below the vacuum level [21], the
HOMO energy levels of the polymers approximating
using Eq. (1),
Optical Properties
The optical properties of the polymer are obtained by UV-Vis
with chloroform solution and thin film are shown in Fig. 2.
The absorption bands of PTBDTPT for solution were ob-
served at 350 nm, 430 nm and 620 nm while those of film
were seen at 360 nm, 450 nm and 650 nm. The higher energy
absorbances were attributed to localized π-π* transitions
while the lower energy bands were associated with an intra-
molecular charge transfer (ICT) between the donor and accep-
tor similar to those characterized for by Jespersen et al. [23,
24]. The optical properties of the polymer film are slightly red-
shifted by 30 nm most likely due to high co-planarity and/or
enhanced intermolecular electronic interactions in the solid
state such that lead to stability of a lower energy excited state.
The optical band gap obtained by extrapolating the absorption
edge of the film is 1.61 eV. These results support that the
polymer showed low band gap. The optical properties are
summarized in Table 2.
EHOMO ¼ −ðEOX −EFc þ 4:8ÞðeVÞ
ð1Þ
wherein EFc is the potential of the internal standard,
the ferrocene/ferrocenium ion (Fc/Fc+) couple. The
value of EFc, which is determined under the same
experimental conditions is approximately 0.25 eV vs
Ag/Ag + .
Table 1 Physical properties of the polymer
Polymer
Mna [kg/mol] Mw a [kg/mol] PDI Yield [%] Td b [oC]
25.3 1.93 73 350
PTBDTPT 12.9
a The molecular weights were determined by using gel permeation chro-
matography (GPC) against polystyrene standards in chloroform
eluent,
Fig. 2 UV-Vis absorption spectra of the polymer in chloroform solution
and thin film
b Temperature resulting in 5 % weight loss based on initial weight

J Fluoresc
Table 2 Optical and electrochemical properties of the polymer
λmaxabs,Sol a λmaxabs,Film a HOMO b LUMO c Egopt d
Polymer
[nm]
[nm]
[eV]
[eV]
[eV]
PTBDTPT 620
650
−5.15
−3.54
1. 61
a The UV-Vis absorption spectra of the polymers were measured in chlo-
roform solution and thin film,
b HOMO levels of the polymer were determined from onset voltage of the
first oxidation potential with reference to ferrocene at −4.8 eV,
c LUMO levels of the polymer were estimated from the optical band gaps
and the HOMO energy levels,
d Optical band gap was calculated from the UV-Vis absorption onset in film
Fig. 4 Transfer characteristics of OFET fabricated with Polymer
PTBDTPT at VD = −100 V. Inset shows a schematic of FET structure
used in this study
The HOMO energy levels of the polymer is found to be
−5.15 eV. From this value, the LUMO energy levels of the
polymer is obtained using Eq. (2)
Organic Filed Effect Transistors (OFET)
The effects of the donor segment on the electrical transport
properties of the resulting materials were examined by fabri-
cating OFET. Figure 4 illustrates typical transfer and output
characteristics of OFET using PTBDTPT as an active chan-
nel. Inset shows a schematic of OFET device used in this
study. Top contact bottom gate FETs were fabricated on sili-
con (Si) wafer covered with 300 nm silicon oxide (SiO2).
Samples were first cleaned with Piranha solution for 20 min
at 80 °C followed by carefully rinsing with distilled water,
ethanol and finally blown dry with a stream of nitrogen gas.
Then SiO2/Si samples were treated with UVozone for 2 min to
make its surface hydrophilic. The PTBDTPT polymer layer
in chlorobenzene solution was spin-coated on UVozone treat-
ed SiO2/Si substrate with concentration of 5 mg mL−1 at 3000
RPM for 30s. Samples were further dried at 80 °C for 10 min
on hot plate to remove residual solvents. 50 nm thick Au
electrodes were deposited as source and drain through metal
shadow mask (channel length (L) = 50 μm and width
(W) = 100 μm) in thermal evaporator. All the electrical char-
acterizations were performed in dark under ambient air using
semiconductor parameter analyzer Agilent 4155C. The hole
mobility of polymer in the saturation region [18] was calcu-
lated using following equation,
opt
ELUMO ¼ EHOMO þ Eg ðeVÞ
ð2Þ
The LUMO values are determined to be −3.54 eV
such that the energy gap between the HOMO and
LUMO to be 1.61 eV. The electrochemical properties
are summarized in Table 2. These data are determined
by UV-visible and cyclic voltametry (CV) experiments
show that the presence of the strong acceptor in the
polymer resulted in a lower energy HOMO as well as
LUMO while maintaining a lower overall band gap.
Cyclic voltammetry is used in order to estimate
HOMO and LUMO energy levels and UV visible test
is performed to determine optical band gap (Egopt) of
the polymer material. By careful consideration of the
repeating donor and acceptor units, control over the
highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels of
these polymers is possible. This facilitates the optimal
light absorption properties for the injection and extraction of
charges in OFET devices.
2
Ids ¼ ðWCi=2LÞμðVG−VTÞ
ð3Þ
where Ids is the drain-source current in the saturated region, W
and L are the channel width and length, respectively, μ is the
field-effect mobility, Ci is the capacitance per unit area of the
Table 3 Device characteristics of solution processed OFET
Polymer
μ
max [cm2V−1 s−1
1.92 X 10−2
]
aIon/Ioff
PTBDTPT
25
a Ion/Ioff refers to the corresponding on-to-off ratio and the threshold volt-
Fig. 3 Cyclic Voltammogram of the thin film of the polymer
age at the maximum hole mobility

J Fluoresc
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9. Bathula C, Sachin B, Belavagi NS, Lee SK, Kang Y, Khazi IM
(2016) Synthesis, characterization and optoelectronic properties of
benzodithiophene Based copolymers for Application in Solar cells.
J Fluoresc 26:371–376
10. Kim J-H, Lee M, Yang H, Hwang D-H (2014) High molecular
weight triisopropylsilylethynyl (TIPS)-benzodithiophene and
Diketopyrrolopyrrole-Based copolymer for high performance or-
ganic photovoltaic cells. J Mater Chem A 2:6348–6352
11. Yuning L, Samarendra S, Prashant S (2010) A high mobility p-type
DPP-Thieno[3,2-b]thiophene copolymer for Organic Thin-Film
Transistors. Adv. Mate 22:4862–4866
insulation layer (SiO2, 300 nm), and VG and VT are the gate
and threshold voltages, respectively.
The bottom gate field effect mobility of polymer PTBDTPT
in saturation region was found to be was found to be 1.92 X
10−2 cm2V−1S−1 with ON/OFF ratio more than 25 and Drain
voltage VD = −100 V. The OFET devices made using the poly-
mer PTBDTPTexhibited typical p-type characteristics. Results
are summarized in Table 3. These results clearly demonstrate
that both the backbone and side chains of the polymers have a
great influence on the corresponding OFET performance. The
enhanced electrical properties can be also attributed to the co-
planarity of PTBDTPT polymer that results into layer by layer
self-assembly during spin coating process. This ultimately re-
sults in improving on current which evidently improves mobil-
ity and On/Off ratio. Further studies are currently underway to
gain better understanding of charge transport mechanism.
12. Bathula C, Song CE, Sachin B, Hong S-J, Kang I-N, Moon S-J, Lee
J, Cho S, Shim H-K, Lee V (2012) New TIPS-substituted benzo[1,
2-b:4,5-b′]dithiophene-based copolymers for application in poly-
mer solar cells. J Mater Chem 22:22224–22232
13. Liang Y, Xu Z, Xia J, Tsai S-T, Wu Y, Li G, Ray C, Yu L (2010) For
the Bright Future-Bulk heterojunction polymer Solar cells with
Power Conversion efficiency of 7.4 %. Adv Mater 22:E135–E138
14. Zhou HX, Yang LQ, Stuart AC, Price SC, Liu SB, You W (2011)
Development of Fluorinated Benzothiadiazole as a Structural unit
for a polymer Solar Cell of 7 % efficiency. Angew Chem Int Ed 50:
2995–2998
15. Price SC, Stuart AC, Yang L, Zhou H, You W (2011) Fluorine
substituted Conjugated polymer of medium band gap yields 7 %
efficiency in polymer − fullerene solar cells. J Am Chem Soc 133:
4625–4631
16. Bathula C, Kim M, Song CE, Shin WS, Hwang D-H, Lee
J-C, Kang I-N, Lee SK, Park T (2015) Concentration-dependent
pyrene-driven self-assembly in benzo[1,2-b:4,5-b′]dithiophene(BDT)
Thienothiophene(TT) − Pyrene copolymers. Macromolecules 48:
3509–3515
Conclusions
In summary, the present work successfully demonstrated the syn-
thesis and characterization of novel donor–acceptor type of conju-
gated polymer (PTBDTPT) based on the dithienothiadiazole[3,4-
c]pyridine and triisopropylsilylethynyl(TIPS)benzo[1,2-b:4,5-b
′]dithiophene units, for application in OFET. The hole mobility
of bottom gate OFET using the polymer as an active channel
found to be to be 1.92 X 10−2 cm2V−1S−1 with the On/Off ratio
just over 25, at room temperature in ambient conditions.
Photophysical studies suggest the polymer to be promising candi-
date for organic electronics. Additional modifications to the poly-
mer structure and device are currently under study to further im-
prove the OFET performance.
Acknowledgments We gratefully acknowledge the support by
Samsung Research Funding Center of Samsung Electronics under
Project Number SRFC-MA1401-05.
17. Sanjaykumar SR, Badgujar S, Song CE, Shin WS, Moon S-J, Kang
I-N, Lee J, Cho S, Lee SK, Lee J-C (2012) Synthesis and charac-
terization of a novel naphthodithiophene-based copolymer for use
in polymer solar cells. Macromolecules 45:6938–6945
18. Bathula C, Song CE, Sachin B, Hong S-J, Park S-J, Shin WS, Lee J-
C, Cho S, Ahn T, Cho S, Moon S-J, Lee SK (2013) Naphtho[1,2-b:
5,6-b′]dithiophene-based copolymers for applications to polymer
solar cells. Polym Chem 4:2132–2139
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