Dithienobenzochalcogenodiazole-based electron donor-acceptor polymers for organic electronics

Article abstract of DOI:10.1016/j.dyepig.2016.01.035

Appositely functionalized dithienobenzo-thiadiazole and dithienobenzo-oxadiazole-monomers were prepared and used in the synthesis of conjugated electron donor-acceptor (D-A) polymers. Detailed systematic investigations were carried out to study the effects of chalcogen atoms and three donor units on the optical and electrochemical properties as well as photovoltaic and field-effect transistor performance of the D-A polymers. All polymers displayed good thermal properties. Polymers containing benzooxadiazole moiety showed deeper LUMO levels as compared to their benzothiadiazole-containing analogues, whereas those derived from weak donor unit exhibited deeper HOMO levels than those with stronger donors. Photovoltaic power conversion efficiency of over 2% and hole field-effect mobility of 2.6 × 10^-2 cm²V^-1s^-1 and on/off ratio of over 10⁵ were obtained. These results demonstrate that dithienobenzo-chalcogenodiazole structures are potentially useful electron acceptor building blocks for the construction of D-A polymers for organic electronics applications.

Full text of DOI:10.1016/j.dyepig.2016.01.035

Dyes and Pigments 129 (2016) 90e99
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Dithienobenzochalcogenodiazole-based electron donoreacceptor
polymers for organic electronics
,
, **
Amsalu Efrem a, Yanlian Lei b, Bo Wu b, Mingfeng Wang a *, Siu Choon Ng d
,
,
***
Beng S. Ong c
a School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459, Singapore
b Department of Physics, Research Centre of Excellence for Organic Electronics and Institute of Advanced Materials, Hong Kong Baptist University,
Kowloon Tong, Kowloon, Hong Kong
c Research Centre of Excellence for Organic Electronics, Institute of Creativity and Department of Chemistry, Hong Kong Baptist University,
Kowloon Tong, Kowloon, Hong Kong
d Singapore Institute of Technology, 10 Dover Drive, 138683, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Appositely functionalized dithienobenzo-thiadiazole and dithienobenzo-oxadiazole-monomers were
prepared and used in the synthesis of conjugated electron donor-acceptor (D-A) polymers. Detailed
systematic investigations were carried out to study the effects of chalcogen atoms and three donor units
on the optical and electrochemical properties as well as photovoltaic and field-effect transistor perfor-
mance of the D-A polymers. All polymers displayed good thermal properties. Polymers containing
benzooxadiazole moiety showed deeper LUMO levels as compared to their benzothiadiazole-containing
analogues, whereas those derived from weak donor unit exhibited deeper HOMO levels than those with
stronger donors. Photovoltaic power conversion efficiency of over 2% and hole field-effect mobility of
2.6 ꢀ 10ꢁ2 cm2Vꢁ1sꢁ1 and on/off ratio of over 105 were obtained. These results demonstrate that
dithienobenzo-chalcogenodiazole structures are potentially useful electron acceptor building blocks for
the construction of D-A polymers for organic electronics applications.
Received 13 December 2015
Received in revised form
29 January 2016
Accepted 31 January 2016
Available online 18 February 2016
Keywords:
Dithienobenzochalcogenodiazole
Electron donor-acceptor polymers
Field-effect transistors
Field-effect mobility
Organic photovoltaics
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
device performance. From these perspectives, conjugated electron
donor and acceptor (D-A) polymers have been a plausible structural
Research on organic semiconductors has advanced rapidly over
the past decade owing to their potential as electrically active ma-
terials in producing low-cost, light weight, large area, and flexible
electronic devices such as organic light-emitting diodes (OLEDs),
organic photovoltaic (OPVs), and organic field-effect transistors
(OFETs) [1e3]. Structurally, organic semiconductors can be readily
modified to tune molecular properties such as crystallinity, energy
levels, optical band gap, etc. to achieve desirable electrical perfor-
mance characteristics [4e7]. Thus, understanding the relationships
between molecular structure, material properties, and device per-
formance would enable rational materials design for improved
design strategy to achieve the required materials properties
through controlled intramolecular charge transfer from the donor
to acceptor moiety within the polymer framework [8].
Chalcogenodiazole chromophore has received great interest as
an electron acceptor moiety for the construction of D-A polymers
for organic electronics applications. Many known D-A polymers
utilize 2,1,3-benzothiadiazole (BTz) structure [9e12], which has
high electron affinity and relatively low HOMO level as compared to
other acceptor moieties [13e16]. D-A polymers with the BTz moiety
modified via substitution of sulfur with other heteroatoms have led
to interesting D-A systems, offering enhanced OPVs and OFET
performance characteristics [17e21]. Using BTz-based D-A poly-
mers as donors, Liu et al. has recently reported PCE as high as over
10% in OPV devices [22].
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: mfwang@ntu.edu.sg (M. Wang), SiuChoon.Ng@SingaporeTech.
edu.sg (S.C. Ng), bong@hkbu.edu.hk (B.S. Ong).
Another design modification of the BTz moiety in D-A polymers
is through fusion of its benzo function with other aromatics to
maximize
p
-orbital overlap by restricting intramolecular rotation
stacking and thus to
such as to induce optimal face-to-face
p-p
http://dx.doi.org/10.1016/j.dyepig.2016.01.035
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

A. Efrem et al. / Dyes and Pigments 129 (2016) 90e99
91
facilitate charge transport via intermolecular hopping [23e26].
Thus, some of recent works on fusion of BTz moiety with thiophene,
indole, and thieno[3,2-b]pyrrole have shown significant improve-
ments in physical, electrochemical and device properties of the
resulting polymers [27e34]. D-A polymers derived from another
acceptor structure, benzooxadiazole (BOz) have also been shown to
exhibit enhanced electronic device performance [35e42]. To the
best of our knowledge, few D-A small molecules and polymers
based on thiophene-fused-benzooxadiazole have been reported for
organic electronics applications [43,44].
The measurements of photovoltaic performances were carried out
under illumination of AM1.5G simulated solar light at 940 W mꢁ2
.
Thus the PCE is calculated as Voc*Jsc*FF/0.94. All of the polymers
with concentration of 10 mg/mL were mixed with PC70BM in wt.
ratio of 1:2, and dissolved in the chlorobenzene (CB) solvent with 3%
of 1,8-diiodooctane (DIO) as an additive for spin-coating at 500 and
1000 rpm, where the photovoltaic properties of active layer pre-
pared at 500 rpm is described in the supporting information.
2.4. OFET fabrication and characterization
In this work, we report the synthesis and optoelectronical char-
acterization of five D-A copolymers based on two acceptor units,
dithieno[30,20:3,4; 200,300:5,6]benzo[1,2-c] [1,2,5] thiodiazole (DT-
BTD) and dithieno[30,20:3,4; 200,300:5,6] benzo [1,2-c] [1,2,5] oxadia-
zole (DT-BOD), alternating with three donor moieties, thienothio-
phene (TT), bithiophene (BT), and alkylidenefluorene [45]. We also
studied the effects of chalcogene atoms (S and O) on their physico-
chemical properties and OPV and OFET device performances.
OFET performance characteristics of all polymers were studied
using a bottom gate, top contact configuration fabricated on heavily
n-doped silicon wafer <100 > with a 300-nm thermally grown SiO2
as substrate/gate electrode. The SiO2/Si wafer substrate was first
modified with OTS-8 (or OTS-18) [8]. Then, a solution of D-A
polymer dissolved in chlorobenzene (3 mg/mL) was spin coated on
SiO2 gate dielectric at a spin speed of 800 rpm for 120 s in glove box.
Subsequently, the resulting polymer film was placed into a vacuum
oven to remove the solvent. A series of gold source/drain electrode
pairs were deposited on top of the polymer film by vacuum thermal
evaporation through a shadow mask to create a set of OFET devices.
All of the devices were evaluated in ambient condition using a dual-
channel Keithley 2636B SouceMeter. The FET mobility was extrac-
ted using the following equation in the saturation regime from the
2. Experimental section
2.1. Materials
Unless stated otherwise, all starting materials were obtained
from SigmaeAldrich and used without further purification. The
anhydrous solvents like tetrahydrofuran (THF), diethyl ether, and
toluene for the reaction were collected from PureSolv MD solvent
purification system of model PS-400-5-MD. Compound 5, 6, 7a, and
7b were prepared according reported procedures [46]. All the re-
actions were carried out under nitrogen atmosphere.
gate sweep: Ids
¼
m
Ci(Vg ꢁ Vth
)
2 (W/2L).
is the field-effect mobility, Ci
Where Ids is the drain current,
m
(10 nF cmꢁ2) is the capacitance per unit area of the gate dielectric
layer, and Vg and Vth are gate voltage and threshold voltage. Vth was
derived from the relationship between the square root of Ids at the
saturated regime and Vg by extrapolating the measured data to
2.2. Characterization
Ids ¼ 0. W (1500
mm) and L (160 or 80 mm) are channel width and
length, respectively.
1H NMR spectra were recorded on a Bruker AV 300 spectrometer
with TMS as the internal reference. Chemical shifts of 1H NMR were
reported in ppm. Splitting patterns were designated as s (singlet),
d (doublet), t (triplet), m (multiplet), and br (broad). Molecular
weights of the polymers were obtained on a Gel permeation
chromatography (GPC) using CHCl3 at 80 ꢂC as eluent after cali-
bration with standard polystyrene. Elemental analyses were per-
formed on a Vario EL elemental analysis instrument (Elementar).
UVevis absorption spectra were recorded on a Cary 4000 spec-
trophotometer. Thermogravimetric analysis (TGA) was carried out
on a Perꢁk1in Elmer Diamond TGA/DTA, at a heating rate of
10 ꢂC min from 25 to 600 ꢂC under a nitrogen atmosphere. Cyclic
voltammetry (CV) measurements were conducted on a three-
electrode electrochemistry workstation (Model CHI006D) in
0.1 M anhydrous acetonitrile solution of tetrabutylammonium
hexafluorophosphate (Bu4NPF6) at 25 ꢂC under nitrogen atmo-
sphere. A glassy carbon disk coated with a thin layer of polymer
film was used as working electrode, a silver wire as pseudo-
reference electrode and a platinum wire as counter electrode.
2.5. Synthesis of monomers
2.5.1. Synthesis of 5,8-dibromodithieno [30,20:3,4; 200,300:5,6]benzo
[1,2-c] [1,2,5] thiadiazole (8a)
To a 250-mL round-bottom flask, equipped with a stir bar and a
condenser was added DT-BTD (0.623 g, 2.5 mmol, 1 equiv), chlo-
roform (200 mL), and bromine (0.282 mL, 0.8819 g, 5.519 mmol,
2.2 equiv.). The reaction mixture was warmed to 80e85 ꢂC, and
stirred for 12 h. The reaction mixture was cooled to room tem-
perature, and the resulting solid was filtered. The filtrate cake was
collected, dried, and recrystallized from o-dichlorobenzene, to give
8a as yellow crystalline solid (0.76 g, 75% yield). 1H NMR (300 MHz,
hot-chlorobenzene-d):
d (ppm): 7.95 (s, 2H).
2.5.2. Synthesis of 5,8-dibromodithieno[30,20:3,4; 200,300:5,6]benzo
[1,2-c]oxadiazole (8b)
The reaction was carried using the same procedure as 8a. The
workup has been modified as follows: after 12 h of reaction it was
allowed to cool to room temperature, while cooling the mixture
was diluted with aqueous Na2S2O3 solution. Subsequently, the
mixture was separated and washed with saturated aqueous
NaHCO3 solution and brine. Then dried over MgSO4, filtered and
concentrated under vacuum, and the solid product was recrystal-
lized from o-dichlorobenzene to give 8b as a yellow crystalline solid
Tapping-mode AFM height images (5 ꢀ 5
m
m2) of pristine polymer
films on OTS-modified Si/SiO2 substrates for all polymers annealed
at 120 ꢂC were taken by using scanning probe microscope (SPM)
digital instrument dimension V.
2.3. OPV fabrication and characterization
(69% yield).1H NMR (300 MHz, CDCl3)
d (ppm): 7.92 (s, 2H).
The photovoltaic properties of all polymers were investigated
using bulk heterojunction organic photovoltaics (BHJ-OPVs) with
configuration of ITO/ZnO/active layer/MoO3/Ag, where the active
layer was composed of D-A polymers as electron donor and PCBM as
an electron acceptor. For all polymers, ZnO was fabricated by spin
coating, while MoO3 and Ag werefabricated by thermal evaporation.
2.5.3. Synthesis of 5,8-bis(4-(2-octyldodecyl) thiophen-2-yl)
dithieno [30,20:3,4; 200,300:5,6] benzo[1,2-c] [1,2,5]thiadiazole (9a)
Compound 8 (0.2 g, 0.492 mmol) and Compound 4 (0.675 g,
1.28 mmol) were added to 40 mL toluene in a 100 mL round-bottom
flask and purged with N2 for 30 min. Pd(PPh3)4 (29 mg,

92
A. Efrem et al. / Dyes and Pigments 129 (2016) 90e99
0.025 mmol) was added to the mixture and refluxed for 48 h. After
the mixture was cooled to room temperature, the solvent was
evaporated under reduced pressure, and then dissolved in 50 mL
diethyl ether and extensively washed with water and brine. The
organic phase was separated and dried over anhydrous MgSO4.
After removal of the solvent by vacuum evaporation, the residue
was purified by column chromatography on silica gel (CH2Cl2/
hexane ¼ 1:10) to give Compound 9 as a yellow oil (0.34 g, 71%).
2.6.2. Poly[5-(3-(2-methyldodecyl)-[2,20:50,200-terthiophen]-5-yl)-
8-(4-(2-octyldodecyl)thiophen-2-yl)-dithieno[30,20:3,4;200,300: 5,6]
benzo[1,2-c] [1,2,5]thiadiazole] (P2)
Yield: 47%. Anal. Calcd for C66H92N2S7: C, 69.66; H, 8.15; N, 2.46;
S, 19.73. Found: C, 67.55; H, 8.375; N, 2.421; S, 19.77.
2.6.3. Poly[5-(4-(2-methyldodecyl)-5-(thieno[3,2-b]thiophen-2-yl)
thiophen-2-yl)-8-(4-(2-octyldodecyl)thiophen-2-yl)dithieno
[30,20:3,4;200,300:5,6]benzo[1,2-c] [1,2,5]oxadiazole] (P3)
1H NMR (300 MHz, CDCl3)
d (ppm): 8.0 (d, 2H), 7.15 (s, 2H), 6.90
(s, 2H), 2.57 (d, 4H), 1.65 (br, 2H), 1.38e1.13 (m, 64H), 0.92e0.81 (m,
12H).
Yield: 41%. Anal. Calcd for C64H90N2OS6: C, 70.15; H, 8.28; N,
2.56; O, 1.46; S, 17.56. Found: C, 65.96; H, 7.854; N, 2.813; S, 18.33.
2.7. Synthesis of D-A polymers by Suzuki polycondensation
2.5.4. Synthesis of 5,8-bis(4-(2-octyldodecyl) thiophen-2-yl)
dithieno[30,20:3,4; 200,300:5,6] benzo[1,2c]- [1,2,5] oxadiazole (9b)
Compound 9b was synthesized in the same procedure with
Compound 9a. Yield: 84%.
d (ppm): 7.90 (d, 2H), 7.15 (d, 2H),
6.92 (d, 2H), 2.55 (d, 4H), 1.64 (br, 2H), 1.38e1.13 (m, 64H), 0.92e0.8
To
a 25- mL round-bottom flask dibromide monomer
(0.1 mmol),2,20-(9-(heptadecan-9-ylidene)-9H-fluorene-2,7-diyl)
bis (4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.1 mmol), and dry
toluene (4 mL), and 2 M K2CO3 aqueous solution (1 mL) were added.
After degassed by N2 for 15 min, Pd(PPh3)4 (~3 mol%) was added
under nitrogen stream and degassed via nitrogen bubbling for
about 30 min. Then, the reaction mixture was heated to 110 ꢂC and
stirred for 3 days. After being cooled to room temperature, the
reaction mixture was poured into 100 mL of methanol and filtered.
The resulting crude product was then subjected to Soxhlet extrac-
tion with methanol, acetone, hexane, and chloroform in sequence.
The chloroform fraction was evaporated to dryness and reprecipi-
tated from methanol to give PF-O as a dark red and PF-S were
collected from hexane fraction and repricipitated in methanol to
give an orange-red solid.
1H NMR (300 MHz, CDCl3)
(m, 12H).
2.5.5. Synthesis of 5,8-bis(5-bromo-4-(2-octyl dodecyl)thiophen-2-
yl)dithieno [30,20: 3,4; 200,300: 5,6]- benzo [1,2-c] [1,2,5]thiadiazole
(10a)
N-Bromosuccinimide (0.137 g, 0.768 mmol) was added portion-
wise to a solution of Compound 9 (0.34 g, 0.349 mmol) in 80 mL of
THF at room temperature. After the mixture was stirred for 12 h,
NaHCO3 solution was added, and the mixture was extracted with
CH2Cl2. The organic layer was washed with water and brine and
then dried over anhydrous MgSO4. The solvent was removed by
vacuum evaporation, and the residue was purified by column
chromatography on silica gel (CH2Cl2/hexane ¼ 1:10) to give
Compound 10a as a yellow oil (0.277 g, 70%).
2.7.1. Poly[5-(5-(9-(heptadecan-9-ylidene)-9H-fluoren-2-yl)-4-(2-
octyldodecyl)thiophen-2-yl)-8-(4-(2-octyldodecyl)thiophen-2-yl)
dithieno[30,20:3,4;200,300:5,6]benzo[1,2-c] [1,2,5]thiadiazole] (PF-S)
Yield: 69%. Anal. Calcd for C88H128N2S5: C, 76.91; H, 9.39; N,
2.04; S, 11.67. Found: C, 68.37; H, 9.246; N, 1.871; S, 9.666.
1H NMR (300 MHz, CDCl3)
d (ppm): 7.92 (s, 2H), 6.99 (s, 2H), 2.52
(d, 4H), 1.70 (br, 2H), 1.38e1.12 (m, 64H), 0.93e0.78 (m, 12H).
2.7.2. Poly[5-(5-(9-(heptadecan-9-ylidene)-9H-fluoren-2-yl)-4-(2-
octyldodecyl)thiophen-2-yl)-8-(4-(2-octyldodecyl)thiophen-2-yl)
dithieno[30,20:3,4;200,300:5,6]benzo[1,2-c] [1,2,5]oxadiazole] (PF-O)
Yield: 82%. Anal. Calcd for C88H128N2OS4: C, 77.82; H, 9.50; N,
2.06; O, 1.18; S, 9.44. Found: C, 76.48; H, 10.05; N, 2.172; S, 9.302.
2.5.6. Synthesis of 5,8-bis(5-bromo-4-(2-octyl dodecyl)thiophen-2-
yl)dithieno [30,20: 3,4; 200,300: 5,6]- benzo[1,2-c] [1,2,5]oxadiazole
(10b)
Compound 10b was synthesized in the same procedure with
Compound 10a. Yield: 92%.
1H NMR (300 MHz, CDCl3)
d (ppm): 7.80 (s, 2H), 6.97 (s, 2H), 2.52
3. Results and discussion
(d, 4H), 1.69 (br, 2H), 1.41e1.02 (m, 64H), 0.94e0.76 (m, 12H).
3.1. Synthesis of monomers and D-A polymers
2.6. Synthesis of D-A polymers by Stille polycondensation
The syntheses of monomers, 10a and 10b, are depicted in
Scheme 1. The intermediate 7a was synthesized in 79% yield from
the diketone 6 [46] via derivatization to its corresponding diamine
followed by SOCl2 treatment. The intermediate 7b was obtained
from the same diketone 6 by refluxing it with hydroxyl amine-
hydrochloride in a pressure vessel at 140 ꢂC for 3 days to get a
yellow solid. Bromination of 7a and 7b with Br2 provided the cor-
responding dibromides, 8a (75% yield) and 8b (69% yield), respec-
tively. While 8a and 8b had limited solubility in common organic
solvents except dichlorobenzene, hot chlorobenzene, and toluene,
their respective bis[4-(2-octyldodecyl)-2-thienyl)-functionalized
products, 9a and 9b exhibited greatly improved solubility charac-
teristics. Subsequent bromination provided the respective dibro-
mides, 10a and 10b. The chemical identities of all the intermediates
1 to 10 in Scheme 1 were confirmed using NMR spectroscopy. The
synthesis of 4-(2-octyldodecyl)-2-(trimethylstannyl)thiophene for
the conversion of 8 to 9 is depicted in Scheme S1 (Supporting
information). Its synthesis could not be accomplished via the
usual Kumada cross-coupling reaction, mostly likely due to steric
To a 25 mL round-bottom flask dibromide monomer (0.1 mmol),
bis(trimethylstannyl)-substituted monomers (0.1 mmol), and dry
toluene (4 ml) were added. After degassed by N2 for 15 min,
Pd(PPh3)4 (3 mol%) was added under nitrogen stream and degassed
via nitrogen bubbling for about 30 min. Then, the reaction mixture
was heated to 110 ꢂC and stirred for 3 days. After being cooled to
room temperature, the reaction mixture was poured into 100 mL of
methanol and filtered. The resulted crude product was then sub-
jected to Soxhlet extraction with methanol, acetone, hexane, and
chloroform in sequence. The chloroform fraction was evaporated to
dryness and reprecipitated from methanol to give Pl, P2 and P3 as a
black powder.
2.6.1. Poly[5-(4-(2-methyldodecyl)-5-(thieno[3,2-b]thiophen-2-yl)
thiophen-2-yl)-8-(4-(2-octyldodecyl)thiophen-2-yl)dithieno
[30,20:3,4;200,300:5,6]benzo[1,2-c] [1,2,5]thiadiazole] (P1)
Yield: 68%. Anal. Calcd for C64H90N2S7: C, 69.13; H, 8.16; N, 2.52;
S, 20.19. Found: C, 65.36; H, 7.529; N, 3.077; S, 22.05.

A. Efrem et al. / Dyes and Pigments 129 (2016) 90e99
93
Scheme 1. Synthesis of acceptor monomers (i) n-BuLi, ꢁ78 ꢂC, CuBr/LiBr then oxalyl chloride, THF, 78%. (ii) FeCl3, CH2Cl2, r.t, 82%. (iii) 7a) HONH2$HCl, 10% Pd/C, 78 ꢂC then
N2H4$H2O, ~60 ꢂC, EtOH, 34%. then SOCl2, Et3N, DCM, reflux, 79%. (iii) 7b) HONH2$HCl, 85 ꢂC for 24 h then, 140 ꢂC for 60hr, EtOH, 30%. (iv) Br2, CHCl3, reflux, 75% for 8a and 69% for
8b. (v) Pd(PPh3)4, toluene, 110 ꢂC. 71% for 9a and 84% for 9b (vi) NBS, THF, r.t. 70% for 10a and 92% for 10b.
interference from the long branched alkyl chain. Instead, it was
obtained from the Grignard reaction (i-prMgCl$LiCl complex) of 3-
bromothiophene with 2-octydodecyl aldehyde, followed by dehy-
droxylation with AlH3 generated in situ from a mixture of
LiAlH4eAlCl3 and stannylation with Sn(Me)3Cl.
nitrogen. Fig. 1 shows TGA where the onset decomposition tem-
perature (Td) with 5% weight loss is in a range of 316e416 ꢂC,
indicating that all polymers have excellent thermal stability for use
in optoelectronic devices such as OPVs and OFETs.
The D-A polymers, P1, P2, and P3 were synthesized via Stille
polycondensation of intermediate 10 and 2,5-bis(trimethyl
stannyl)-thieno[3,2-b]thiophene or 5,50-bis(trimethyl-stannyl)-
2,20-bithiophene, while both PF-S and PF-O were obtained from
Suzuki polycondensation between intermediate 10 and
dioxaborolan-alkylidenefluorene in the presence of Pd(PPh3)4
catalyst in refluxing toluene (110 ꢂC) for 72 h. All D-A polymer
products were isolated via precipitation from methanol and puri-
fied by sequential Soxhlet extractions with methanol, acetone,
hexane, and chloroform. All of the D-A polymers have displayed
relatively good solubility in chlorinated solvents such as chloroform
and chlorinated benzene.
D-A polymers, P1 and P2, which were utilized in our OFETstudies,
were obtained by our previous synthesis procedure as depicted in
Scheme 2 [34]. First, 2-bromo-3-(2-octyldodecyl) thiophene were
coupled with the donor units 2,5-bis(trimethyl stannyl)-thieno[3,2-
b]thiophene or 5,50-bis(trimethylstannyl)-2,20-bithiophene fol-
lowed by stannylation and Stille polycondensation with 8a, though
the molecular weights and overall reaction yields of the polymers
were lower as compared to the one obtained by using Scheme 3.
The molecular weights and polydispersity index (PDI) of the D-A
polymers were determined by gel permeation chromatography
(GPC) at 80 ꢂC with a polystyrene standard calibration and CHCl3 as
the eluent. As shown in Table 1, most D-A polymers have the
weight-average molecular weights (Mw) as high as 20 kDa and
number-average molecular weight (Mn) ranging from 5.4 to
12.6 kDa. The polydispersity index (PDI) for all the polymers is
relatively narrow, except P1 which shows a relatively broad PDI of
2.54 which might be due to its strong tendency to aggregate in
solution as evidenced by its solution UVevis absorption spectra
(Fig. S2 of Supporting Information).
3.3. Optical properties
The absorption spectra of the D-A polymers in dilute chloroform
solutions and spin-coated films on quartz substrates are shown in
Fig. 2, while their respective optical data, including the maximum
absorption peak wavelengths (lmax), absorption edge wavelengths
(
(
lonset), shoulder maximum absorption peak wavelengths
lshoulder), and optical band gap (Eogpt; calculated from lonset), are
summarized in Table 2. The significant red-shift and broad ab-
sorption bands of P1-3 compared to those of PF-S and PF-O could
be attributed to the strong intramolecular charge transfer (ICT)
interaction between the electron-rich moieties and electron-
deficient segments in the former polymers. The absorption max-
ima for P1, P2, and P3 are 522, 520, 500 nm respectively and all of
them displayed two shoulder peaks at ca. 560 and 600 nm, sug-
gesting polymer aggregation even in solution (Fig. S2 of supporting
information) [30]. The solution absorption maxima of PF-S and PF-
O were at shorter wavelength region owing to the weaker electron
acceptor strength of alkylidenefluorene moiety. In thin films, as
expected, all polymers (P1, P2, P3, PF-S, and PF-O) showed red-
shifts for up to ca. 15e53 nm, indicating higher molecular orders
from intermolecular
p-p stacking interactions. Specifically, the
oxadiazole-based polymers (P3 and PF-O) exhibited larger red-
shifts than their corresponding thiadiazole-based counterparts
(P1 and PF-S), likely due to decreased steric interference to mo-
lecular interaction from the oxygen-containing oxadiazole-based
moiety as compared to larger surfur-containing thiadiazole-based
moiety [47]. Furthermore, increased intensity of 0-0 vibrational
peaks (~600 nm) relative to those of the 0e1 vibrational peaks was
observed in the film spectra of P1, P2, and P3, suggesting relatively
planar polymer backbone structures and formation of J-aggregates
[47,48]. Finally, the absorption edges (lonset) of film spectra of P1,
P2, and P3 were in the range of 650e660 nm, which correspond to
an optical band gap (Eogpt) of 1.89 eV while lonset for PF-S and PF-O
is about 580 nm, corresponding to Eogpt of 2.1 eV.
3.2. Thermal properties
Thermal stability of all D-A polymers was investigated by ther-
mogravimetric analysis (TGA) at a heating rate of 10 ꢂC minꢁ1 under

94
A. Efrem et al. / Dyes and Pigments 129 (2016) 90e99
Scheme 2. Previous synthetic route to D-A polymers, P1 and P2.
Scheme 3. Improved synthetic route to D-A polymers, P1, P2, P3, PF-S and PF-O.

A. Efrem et al. / Dyes and Pigments 129 (2016) 90e99
95
Table 1
Molecular weights and thermal properties of D-A polymers.
D-A Polymer
Yield (%)
aMn (ꢀ 103 g/mol)
aMw (ꢀ 103 g/mol)
aPDI
bTd (ꢂC)
P1
P2
P3
PF-S
PF-O
68
47
41
70
82
8.0
9.1
7.6
12.6
5.4
20.5
17.3
14.8
17.9
8.6
2.54
1.89
1.93
1.42
1.59
401
416
368
316
343
a
Determined by GPC in CHCl3 at 80 ꢂC using polystyrene standards.
Decomposition temperature, determined by TGA in nitrogen, based on 5% weight loss.
b
3.4. Electrochemical properties
100
90
The cyclic voltammetry (CV) experiment were carried out on D-
A polymer films to examine their electronic energy levels (Fig. S1,
Supporting Information). The highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) energy
levels were calculated from the onset oxidation potentials (E
80
onset
)
ox
onset
70
P1
P2
and the onset reduction potentials (E
) [49], respectively and
red
summarized in Table 3. Within experimental errors, the band gaps
calculated from the measured CV (Eeglc) data were essentially similar
to those estimated from optical absorption edges (Table 2). It is
obvious that the replacement of sulfur atom of thiadiazole moiety
with oxygen atom to form oxadiazole moiety had a noticeable ef-
fect on the energy levels of the resulting D-A polymers. Thus, with
the same donor moiety, the oxadiazoleebased polymer was
observed to display a slightly lower LUMO energy level than its
corresponding thiadiazole-based polymer, attributable to the
stronger electro-negativity of oxygen.
60
P3
PF-S
PF-O
50
40
100
200
300
400
500
600
o
( C)
Temprature
Fig. 1. TGA plots of the five D-A polymers with a heating rate of 10 ꢂC minꢁ1 under
nitrogen.
in soln.
in film
1.0
0.8
0.6
0.4
0.2
0.0
P1
P2
P3
PF-S
PF-O
P1
1.0
P2
P3
0.8
0.6
0.4
0.2
0.0
PF-S
PF-O
400
500
600
700
800
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 2. UVevis absorption spectra of D-A polymers in chloroform solution and thin films for D-A polymers, P1, P2, P3, PF-S, and PF-O.
Table 2
Optical data of the D-A polymers.
D-A Polymer
Solution
Film
almax[nm]
blshold[nm]
clonset[nm]
almax[nm]
blshold[nm]
clonset[nm]
dEogpt(eV)
P1
P2
P3
PF-S
PF-O
522
520
500
480
465
597
600
601
e
660
651
655
570
547
552
552
553
495
485
597
592
595
e
658
654
655
578
581
1.88
1.89
1.89
2.15
2.13
e
e
a
Absorption maxima.
b
Absorption maxima at shoulder peak.
The onset edge absorption.
c
d
Estimated from the onset edge of absorption in films (lonset): dEogpt(eV) ¼ 1240/lonset [nm].

96
A. Efrem et al. / Dyes and Pigments 129 (2016) 90e99
Table 3
Electrochemical properties of D-A polymers.
aHOMOelc (eV)
bLUMOopt (eV)
LUMOelc (eV)
Eg
(eV)
onset
onset
elc
D-A polymer
E
(V)
E
(V)
ox
red
P1
P2
P3
PF-S
PF-O
1.02
0.93
1.00
1.42
1.34
ꢁ0.77
ꢁ0.75
ꢁ0.74
ꢁ0.68
ꢁ0.66
ꢁ5.42
ꢁ5.33
ꢁ5.40
ꢁ5.82
ꢁ5.74
ꢁ3.54
ꢁ3.44
ꢁ3.51
ꢁ3.67
ꢁ3.61
ꢁ3.63
ꢁ3.65
ꢁ3.66
ꢁ3.72
ꢁ3.74
1.79
1.68
1.74
2.10
2.00
a
HOMOele ¼ ꢁe(EOX þ 4.4) (eV).
b
LUMOopt ¼ Eogpt þ EHOMO
.
3.5. Photovoltaic properties
3.6. Field effect transistor properties
The photovoltaic properties of the D-A polymers were investi-
gated in bulk heterojunction organic photovoltaic (BHJ-OPV) de-
vices using solar simuator (San_El Electric, XEC-301S) and Agilent
U2722A Modular Source Measure Unit. The device structure had
the configuration of ITO/ZnO/active layer/MoO3/Ag with the active
layer being composed of D-A polymer as the electron donor and
PCBM as an electron acceptor. The observed photovoltaic proper-
ties, i.e., open circuit voltage (Voc), short circuit current (Jsc), fill
factor (FF), and power conversion efficiency (PCE) are summarized
in Table 4. The PCEs ranging from 0.21% to 2.2% were recorded for
these devices. It was noted that the devices with active layer spin
cast at spin speed of 1000 rpm displayed higher PCEs than those at
spinning speed of 500 rpm (Supporting Information: Fig. S3 and
Table S1), which could be attributed to the better film quality of
the higher spinning speed. As can be seen from the current density-
voltage plots of Fig. 3, the devices using PF-S and PF-O as donor
polymers exhibited much higher Voc (0.93 and 0.98 V, respectively)
than those with P1, P2, and P3 (6.8 V) as donor polymers. These are
consistent with their HOMO levels, since Voc is proportional to the
energy difference between HOMO level of donor and LUMO level of
the acceptor (PCBM) [49].
The field-effect transistor properties of D-A polymers were
evaluated using a bottom-gate, top-contact transistor configura-
tion. A heavily n-doped silicon wafer <100 > with a 300-nm ther-
mally grown silicon dioxide (SiO2) was used as the substrate/gate
electrode. The surface of SiO2/Si substrate was modified with either
octyltrichlorosilane (OTS-8) or octadecyltrichlorosilane (OTS-18) to
enhance transistor performance. The D-A polymer was dissolved in
chlorobenzene (3 mg/mL) and filtered through a syringe fitted with
a 10-micron filter. The filtered solution was spin coated on top of
OTS-8- or OTS-18-modified SiO2 surface, vacuum dried at 60 ꢂC for
several hours. The devices were optionally annealed at 120 ꢂC for
30 min when required. A series of gold source/drain electrode pairs
were deposited by vacuum thermal evaporation through a shadow
mask, thus creating a set of OFET devices with various channel
length (L) and width (W) dimensions for electrical evaluation. All
the polymers had shown p-type semiconductor behavior, and their
OFET characteristics are summarized in Tables 5 and 6 for devices
treated with OTS-8 and OTS-18, respectively. Typical Transfer and
output characteristics as well as hysteresis behavior of OFET devices
with P2 as channel semiconductor on OTS-18-modified substrate
are provided in Fig. 5. The transfer and output characteristics and
hysteresis behaviors of other D-A polymers are given in Fig. S4/S5
(Supporting Information). The extracted field-effect mobility, cur-
rent on/off ratio and threshold voltage of OFET devices on OTS-8-
and OTS-18-modifed substrates are summarized in Tables 5 and
6, respectively. As can be noted, the results showed that P1, P2,
and P3 gave essentially the same field-effect properties, with P2
performed slightly better and P3 slightly poorer. On the other hand,
both PF-S and PF-O gave significantly poorer OFET performance,
which may be attributed to the donor moiety, alkylidenefluorene,
being not a strong donor. In addition, we observed that those de-
vices built on ODTS-18-modified SiO2 substrates afforded much
better performance than those on OTS-8-modified substrates. Both
mobility and on/ratio are much higher in devices on OTS-18-
modified substrates, which are in agreement with the higher effi-
ciency of octadodecyl chain in facilitating molecular ordering of the
D-A polymers. In addition, noticeable improvements in mobility
and on/off ratio were noted after the devices were thermally
annealed at 120 ꢂC as thermal treatment was expected to further
promote molecular ordering of D-A polymers. The highest mobility
of 2.6 ꢀ 10ꢁ2 cm2Vꢁ1sꢁ1 and on/ratio of 3 ꢀ 105 were obtained from
devices using P2 as channel semiconductor.
The device with donor polymer, P2, afforded the best PCE of 2.2%
and Jsc (4.7 mA/cm2) comparable to that of P1, which might be due
to its higher fill factor (66%). It has been suggested that series
resistance (Rs) and shunt resistance (Rsh) are two of the factors that
determine FF [50], thus, a device having lower Rs and higher Rsh
would yield higher FF and improved PCE. P2 showed smallest Rs of
8.2
four polymers, while PF-O, which gave the lowest PCE of 0.21%, had
the highest Rs of 1.9 and Rsh of 2.3 k
cm2. Therefore, the observed
U U
cm2 and a relatively higher Rsh of 1.9 k cm2 than the other
U
PCE values for all five polymers were quite consistent with their FF,
Rs and Rsh values. The highest PCE for P2 may be attributed to its
lower band-gap and broader absorptions which enable more effi-
cient solar energy harvesting.
The incident photon to current efficiency, IPCE, of the devices for
P1, P2 and P3 (Fig. 4) shows reasonably wide external quantum
efficiency (EQE) graphs in the visible region (300e700 nm) with
large EQE values in the 300e730 nm region and lower values
beyond 730 nm. The P1 devices exhibited the highest EQE (up to
47.5%) at 380 nm, in good agreement with the result for the highest
photocurrent value, Jsc of 4.71 mA/cm2 whereas, P2 and P3 showed
EQE values of 45.5% and 37.4%, respectively.
Table 4
Photovoltaic performance of D-A polymers with PC70BM under illumination of AM 1.5G, 940 W/m2 (Active layer: D-A. Polymer/PC70BM (1/2 by weight) with 3% of DIO; 10e12
devices were evaluated for each polymer).
D-A Polymer
V
oc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
Rs (kU
cm2)
Rsh (kU
cm2)
P1
P2
P3
PF-S
PF-O
0.68
0.68
0.68
0.93
0.98
4.7
4.7
3.9
1.7
0.68
56
66
55
52
30
1.9
2.2
1.6
0.88
0.21
0.010
1.1
1.9
0.98
2.1
2.3
8.2 ꢀ 10ꢁ3
0.019
0.80
1.9

A. Efrem et al. / Dyes and Pigments 129 (2016) 90e99
97
1
0
0.0
-0.5
-1.0
-1.5
PF-S
PF-O
P1
P2
P3
-1
-2
-3
-4
-5
-6
-2.0
0.0
0.5
Voltage (V)
1.0
-0.2
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Fig. 3. Current density-voltage of photovoltaic devices with active layer of D-A polymer/PC70BM (1/2 by weight) and 3% of DIO additive.
A good correlation between the oxidative potentials of the
polymers and the threshold voltages of the corresponding devices
was noted. For example, the HOMO levels of P1, P2, and P3 are
higher than those of PF-S and PF-O, thus their threshold voltages
(Vth) were more positive (ꢁ6 V for P1, ꢁ4 V for P2 and ꢁ10 V for P3)
as compared with those of PF-S and PF-O, which were more
negative Vth (ꢁ14.8 V for PF-S and ꢁ15 V for PF-O). This was likely
related to the hole-injection barrier from the gold contact to the
polymer semiconductor as PF-S and PF-O polymers have deeper
HOMO levels (ꢁ5.82 to ꢁ5.74 eV) which are not energetically
compatible with gold which has a work function of ꢁ5.1 eV.
Interestingly P1, P2, and P3 polymers showed very small hysteresis
in their transfer characteristics, indicating less trapping in polymer
films [51].
The AFM images of pristine films of P1-3 and PF-S, PF-O spin-
cast on OTS modified Si/SiO2 substrates from CB solution at
800 rpm and annealed at 120 ꢂC are shown in Fig. S6 (Supporting
Information). P2 formed ordered biocontinuous networks of cy-
lindrical structures, which may contribute to its superior perfor-
mance among all of the polymers in OFET devices. In contrast to P2,
both P1 and P3 formed relatively smooth films without obvious
phase separation. Both PF-S and PF-O formed noncontinuous
spherical aggregates, consistent with their poor performances
50
40
30
20
10
0
P1
P2
P3
300
400
500
600
700
Wavelength (nm)
Fig. 4. IPCE spectra of the OPV devices using P1, P2 and P3 as donor polymers and
PC70BM as acceptor.
Table 5
Performance characteristics of OFET devices with D-A polymer semiconductor on OTS-8-treatd substrate fabricated and measured under ambient conditions.
D-A polymer
Average Mobilitya (cm2Vꢁ1sꢁ1
As fabricated, 60 ꢂC
)
Vth (V)
On/off ratio
Annealed at 120 ꢂC
Annealed at 120 ꢂC
P1
P2
P3
PF-S
PF-O
0.7 ꢀ 10ꢁ2 (0.8 ꢀ 10ꢁ2
)
)
1.1 ꢀ 10ꢁ2 (1.3 ꢀ 10ꢁ2
)
)
ꢁ6.0
¡4.0
2 ꢀ 104
6 ꢀ 104
3 ꢀ 105
2 ꢀ 104
5 ꢀ 104
1.1 ꢀ 10ꢁ2 (1.2 ꢀ 10ꢁ2
1.6 ꢀ 10ꢁ2 (1.7 ꢀ 10ꢁ2
0.5 ꢀ 10ꢁ2 (0.6 ꢀ 10ꢁ2
1.6 ꢀ 104
)
1.5 ꢀ 10ꢁ2 (1.9 ꢀ 10ꢁ2
2.6 ꢀ 10ꢁ4
)
ꢁ10.1
ꢁ14.8
ꢁ15.0
4 ꢀ 104
1.1 ꢀ 10ꢁ3
a
Maximum mobility shown in parentheses.
Table 6
Performance characteristics of OFET devices with D-A polymer semiconductor on OTS-18-treatd substrate fabricated and measured under ambient conditions.
D-A polymer
Average Mobilitya (cm2Vꢁ1sꢁ1
As fabricated, 60 ꢂC
)
Vth (V)
On/off ratio
Annealed at 120 ꢂC
Annealed at 120 ꢂC
P1
P2
P3
1.5 ꢀ 10ꢁ2 (1.7 ꢀ 10ꢁ2
)
)
2.3 ꢀ 10ꢁ2 (2.4 ꢀ 10ꢁ2
)
)
ꢁ12.0
¡12.0
ꢁ7.1
105
2.1 ꢀ 10ꢁ2 (2.4 ꢀ 10ꢁ2
2.4 ꢀ 10ꢁ2 (2.6 ꢀ 10ꢁ2
3 ꢀ 105
0.6 ꢀ 10ꢁ2 (0.7 ꢀ 10ꢁ2
)
1.9 ꢀ 10ꢁ2 (2.0 ꢀ 10ꢁ2
)
3 ꢀ 105
a
Maximum mobility shown in parentheses.

98
A. Efrem et al. / Dyes and Pigments 129 (2016) 90e99
0.0
10-6
10-6
10-7
-4.0x10-7
-8.0x10-7
-1.2x10-6
-1.6x10-6
-2.0x10-6
10-7
10-8
10-8
10-9
10-9
-10 V
-20 V
-30 V
-40 V
-50 V
-60 V
10-10
10-11
10-10
10-11
forward
backward
(c)
(b)
(a)
-60 -50 -40 -30 -20 -10
Vg (V)
0
-60 -50 -40 -30 -20 -10
Vg (V)
0
-60 -50 -40 -30 -20 -10 0
Vds (V)
Fig. 5. Typical transfer (a), output characteristics curves (b), and forward/backward gate voltage sweeps (hysteresis) (c) of OFET devices with P2 as channel semiconductor on OTS-
18-modified SiO2/Si substrate (W/L ¼ 1500 m/160 m devices).
m
m
[2] Guo X, Baumgarten M, Müllen K. Designing
p-conjugated polymers for
(Table 5) in OFET devices. Their root-mean square (RMS) surface
roughness values are P1 (1.86 nm), P2 (4.57 nm), P3 (3.50 nm), PF-S
(3.93 nm), and PF-O (2.52 nm). In addition, one can see that the
exchange of S/O atoms did not result in any significant change in
film morphology, which is consistent with the results reported by
Huang et al. and Shi et al. in similar polymers [30,47].
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4. Conclusion
D-A polymers comprising dithienobenzochalcogenodiazole
acceptor units alternating with thienothiophene, bithiophene, and
alkylidenefluorene donor units were synthesized for evaluation as
semiconductor polymers for OPV and OFET devices. The D-A
polymers based on thienothiophene and bithiophene structures
have exhibited lower optical bandgaps as compared to their alky-
lidenefluorene analogues. From solution to thin-film state, all
polymers exhibited red shifts in their absorption spectra accom-
panied with the appearance of shoulder peaks, indicating forma-
tion of more planar polymer backbone orderings or some
J-aggregates in their solid states. UVevis and CV studies revealed
that oxadiazole-containing polymers displayed deeper LUMO
levels and more red-shifted thin-film absorption spectra when
compared to their respective thiadiazole-containing counterparts.
Among all the polymers, P2 provided the highest PCE in OPVs and
highest mobility and on/off ration in OFETs. Also worthy of note is
the finding that many of these polymer (i.e., P1, P2 and P3)
exhibited little hysteresis effects in OFET devices. We believe that
the electronic device performance can be further improved through
structural and synthesis optimization.
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Acknowledgments
M.W. is grateful for the start-up funding support of Nanyang
Assistant Professorship by Nanyang Technological University. We
gratefully acknowledge A*STAR, Singapore (Grant No.102 170 0135)
and Hong Kong Baptist University Strategic Development fund
(SDF13-0531-A02) for financial support. We thank Kai Wang and
Marc Courte for help in AFM.
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at http://
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