Synthesis and photovoltaic properties of 4,9-dithien-2′-yl-2,1,3-naphthothiadiazole-based D-A copolymers

Article abstract of DOI:10.1016/j.polymer.2015.10.021

A series of donor-acceptor (D-A) conjugated copolymers (PBDTNT, PCDTNT, PFDTNT, and PNDTNT) based on a new acceptor unit of 4,9-dithien-2′-yl-2,1,3-naphthothiadiazole (DTNT) and different donor unit of benzodithiophene, carbazole, fluorene, or naphthalene, were synthesized and characterized as donor in polymer solar cells (PSCs). The effect of different donor units on the physicochemical properties of the polymers was investigated systematically. The copolymers showed relatively low-lying HOMO energy levels of around -5.45 eV, which is favorable for achieving high open-circuit voltage (V_oc). The photovoltaic properties of these copolymers were studied by fabricating the PSCs with the polymer as donor and PC₇₀BM as acceptor. PSCs based on PBDTNT:PC₇₀BM (1:4, w/w) exhibited a power conversion efficiency (PCE) of 2.73%, a high V_oc of 0.90 V, a short-circuit current density (J_sc) of 9.45 mA cm^-2, and a fill factor of 32.1% via optimizing the device fabrication conditions, including the polymer/PC₇₀BM blend ratio, solvent additive and treatment with methanol. With the same fabrication method, PSCs based on PCDTNT, PFDTNT, and PNDTNT showed PCEs of 2.26, 1.57 and 1.45%, and high V_oc values of 0.90, 0.92 and 0.92 V, respectively.

Full text of DOI:10.1016/j.polymer.2015.10.021

Polymer 79 (2015) 119e127
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and photovoltaic properties of 4,9-dithien-20-yl-2,1,3-
naphthothiadiazole-based D-A copolymers
,
b
,
,
, **
Ping Shen a *, Haijun Bin a, Lixia Chen a b, Zhi-Guo Zhang a, Yongfang Li a
a CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China
b College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan
411105, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of donor-acceptor (D-A) conjugated copolymers (PBDTNT, PCDTNT, PFDTNT, and PNDTNT)
based on a new acceptor unit of 4,9-dithien-20-yl-2,1,3-naphthothiadiazole (DTNT) and different donor
unit of benzodithiophene, carbazole, fluorene, or naphthalene, were synthesized and characterized as
donor in polymer solar cells (PSCs). The effect of different donor units on the physicochemical properties
of the polymers was investigated systematically. The copolymers showed relatively low-lying HOMO
energy levels of around ꢀ5.45 eV, which is favorable for achieving high open-circuit voltage (Voc). The
photovoltaic properties of these copolymers were studied by fabricating the PSCs with the polymer as
donor and PC70BM as acceptor. PSCs based on PBDTNT:PC70BM (1:4, w/w) exhibited a power conversion
efficiency (PCE) of 2.73%, a high Voc of 0.90 V, a short-circuit current density (Jsc) of 9.45 mA cmꢀ2, and a
fill factor of 32.1% via optimizing the device fabrication conditions, including the polymer/PC70BM blend
ratio, solvent additive and treatment with methanol. With the same fabrication method, PSCs based on
PCDTNT, PFDTNT, and PNDTNT showed PCEs of 2.26, 1.57 and 1.45%, and high Voc values of 0.90, 0.92 and
0.92 V, respectively.
Received 21 May 2015
Received in revised form
18 August 2015
Accepted 7 October 2015
Available online 22 October 2015
Keywords:
D-A conjugated copolymers
4,9-dithien-20-yl-2,1,3-naphthothiadiazole
Polymer solar cells
Crown Copyright © 2015 Published by Elsevier Ltd. All rights reserved.
1. Introduction
crystallinity and solubility for high hole mobility and for optimal
morphology, and well-matched HOMO/LUMO energy levels be-
Polymer solar cells (PSCs) have great potential as the future
energy source because of the advantages of low cost, solution
processability, lightweight, being mechanically flexible, and suit-
able for large-area fabrication [1]. The bulk heterojunction (BHJ)
active layer based on the blend of conjugated polymers as donor
and fullerene derivatives (such as [6,6]-phenyl-C61-butyric acid
methyl ester (PC60BM) and [6,6]-phenyl-C71-butyric acid methyl
ester (PC70BM)) as acceptor is the most successful device structure
for PSCs so far [2]. Many new conjugated polymers have been
synthesized for high performance PSCs [3e5]. The main design
rules for donor polymers are low bandgap for large solar photon
harvest and thus high short-circuit current density (Jsc), low-lying
HOMO energy level for high open-circuit voltage (Voc), balanced
tween polymers and fullerene derivatives for efficient charge sep-
aration [1]. Now it is well recognized that the donor-acceptor (D-A)
copolymer strategy is effective in constructing efficient low
bandgap polymers because not only the absorption bands but also
HOMO/LUMO energy levels and charge carrier mobilities of the
target polymers can be feasibly tuned by intramolecular charge
transfer (ICT) from donor to acceptor moieties. Most of the high
efficiency conjugated polymer donors reported so far are the D-A
copolymers, with several of them showing over 9% power conver-
sion efficiencies (PCEs) in the PSCs [5].
In recent years, a large number of donor units for DꢀA co-
polymers have been reported, which include fluorene, carbazole,
cyclopentadithiophene, dithienosilole, indacenodithiophene, ben-
zodithiophene etc. [1c,d]. However, efficient acceptor units are
relatively rare [6]. Thus, the exploration of new acceptor units for D-
A copolymers is urgent and it is a compelling subject in chemistry.
Both 2,1,3-benzothiadiazole (BT) and 4,7-dithien-2-yl-2,1,3-
benzothiadiazole (DTBT) are the most successful and popular
acceptor units for highly efficient low bandgap polymers, and PSCs
* Corresponding author. College of Chemistry and Key Laboratory of Environ-
mentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan
University, Xiangtan 411105, People's Republic of China.
** Corresponding author.
E-mail addresses: shenping802002@163.com (P. Shen), liyf@iccas.ac.cn (Y. Li).
http://dx.doi.org/10.1016/j.polymer.2015.10.021
0032-3861/Crown Copyright © 2015 Published by Elsevier Ltd. All rights reserved.

120
P. Shen et al. / Polymer 79 (2015) 119e127
based on these polymers exhibited prominent PCEs [5].
2. Experimental section
2,1,3-Naphthothiadiazole (NT) can be seen as an analogue of
2,1,3-benzothiadiazole (BT) with the extended conjugation through
a fused ring structure (Fig. 1). Compared to the BT unit, the organic
small molecules and polymers based on NT are expected to possess
a decreased bandgap due to the extended conjugation length in the
molecular backbone whereas without the complicated modifica-
tions and whilst maintaining the structural analogues of BT-based
polymers. Consequently, the NT-based molecules showed inter-
esting optoelectronic properties and widely applied in organic
light-emitting diodes [7]. However, there are few reports on the NT-
based small molecules or polymers [4a,8] for photovoltaic mate-
rials. To expanding the knowledge on the NT-based conjugated
polymers, we designed a new acceptor unit of 4,9-dithien-20-yl-
2,1,3-naphthothiadiazole (DTNT) by linking two alkylated thienyl
units [9] in the 4,9-position of 2,1,3-naphthothiadiazole, for the
synthesis of new D-A copolymers (Fig. 1). Herein, we present the
synthesis of a series of new D-A copolymers (as shown in Fig. 1)
based on DTNT acceptor unit copolymerized with different donor
moieties: benzodithiophene (PBDTNT), carbazole (PCDTNT), fluo-
rene (PFDTNT), and naphthalene (PNDTNT). Design motif of this
series of acceptors from BT to NT to DTNT is also outlined in Fig. 1.
The thermal, photophysical and electrochemical properties of these
polymers have been characterized and the effect of different donor
units was carefully investigated. Indeed, our results indicate that
the optoelectronic properties of the target D-A copolymers can be
fine-tuned by varying the donor units with different electron-
donating ability. The PSCs based on these copolymers as donor
and PC70BM as acceptor were fabricated. Preliminary tests of the
photovoltaic performance based on PBDTNT showed a PCE of 2.73%
with a high Voc of 0.90 V, indicating DTNT could be a promising
acceptor unit for building high Voc D-A copolymer photovoltaic
materials.
2.1. Measurements and characterization
1H NMR and 13C NMR spectra were measured on a Bruker
DMXꢀ400 spectrometer with chloroform as the solvent and tri-
methylsilane as the internal reference. UVevisible (UVevis) ab-
sorption spectra were measured on a Hitachi U-3010 UVevis
spectrophotometer. Mass spectra (MS) were recorded on a Shi-
madzu spectrometer and MALDI-TOF-MS spectra were determined
on a Bruker BIFLEX III mass spectrometer. Thermogravimetric
analysis (TGA) was conducted on a PerkineElmer TGAꢀ7 ther-
mogravimetric analyzer at a heating rate of 20 ꢁC minꢀ1 and under
a nitrogen flow rate of 100 mL minꢀ1. Molecular weights of the
polymers were measured by gel permeation chromatography (GPC)
method on Waters 515e2410 with polystyrenes as standard and
tetrahydrofuran (THF) as an eluent. The electrochemical cyclic
voltammetry (CV) was performed on a Zahner IM6e Electro-
chemical Workstation, with a Pt disk coated with the polymer film,
Pt wire and Ag/AgCl electrode as the working electrode, counter
electrode and reference electrode respectively, in a 0.1 mol Lꢀ1
tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile
solution. The morphology of blend films was measured using an
atomic force microscope (AFM, Digital Instrument Multimode
Nanoscope IIIA) using the tapping mode.
2.2. PSC device fabrication and characterization
The PSC devices were fabricated with a structure of ITO/
PEDOT:PSS/polymer:PC70BM/Ca/Al. The patterned ITO glass was
precleaned in an ultrasonic bath of acetone and isopropanol and
treated in an ultraviolet-ozone chamber (Jelight Co.) for 30 min. A
thin layer of PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly
(a)
S
S
R
R
N
N
N
N
S
N
N
Extended conjugation
Improved solubiltiy
S
S
Benzothiadiazole (BT)
Naphthothiadiazole (NT)
Alkylthienyl substituted NT (DTNT)
(b)
C4H9
C2H5
S
S
C6H13
C6H13
C8H17
C6H13
N
N
C8H17
S
C6H13
N
N
S
N
S
S
n
S
S
S
n
S
PCDTNT
PBDTNT
C2H5
C4H9
OC8H17
S
S
C12H25
C12H25C6H13
C6H13
N
N
N
N
C6H13
C6H13
S
S
S
S
n
n
OC8H17
PNDTNT
PFDTNT
Fig. 1. (a) Design motif of this series of acceptors from BT to NT to DTNT; (b) Molecular structures of the DTNT-based D-A copolymers PBDTNT, PCDTNT, PFNTNT, and PNDTNT.

P. Shen et al. / Polymer 79 (2015) 119e127
121
(styrenesulfonate)) was spin-cast on precleaned ITO-coated glass
from a PEDOT:PSS aqueous solution (Baytron P VP AI 4083 from H.
C. Starck) at 3000 rpm and dried subsequently at 150 ꢁC for 30 min
in air, then the device was transferred to a glove box, where a blend
solution of the polymer and PC70BM with the polymer concentra-
tion of 10 mg mLꢀ1 was spin-coated onto the PEDOT:PSS layer.
Finally, a Ca/Al metal top electrode was deposited in vacuum onto
the active layer at a pressure of ca. 5 ꢂ 10ꢀ5 Pa. The active area of
the device was ca. 4 mm2. The current densityevoltage (JeV)
characteristics were measured on a computer-controlled Keithley
236 SourceeMeasure Unit. A xenon lamp (150 W) coupled with AM
1.5 solar spectrum filter was used as the light source, and the optical
power at the sample was 100 mW cmꢀ2. External quantum effi-
ciency (EQE) spectrum was measured by a Stanford Research Sys-
tems model SR830 DSP lock-in amplifier coupled with WDG3
monochromator and a 150 W xenon lamp.
7.25 (2H, s), 2.76 (4H, t, J ¼ 7.8), 1.31e1.77 (16H, m), 0.90 (6H, t,
J ¼ 8.0). 13C NMR (100 MHz, CDCl3,
d/ppm): 151.67, 143.44, 136.05,
132.86, 131.82, 127.10, 126.81, 123.85, 122.60, 31.76, 30.64, 30.47,
29.16, 22.69, 14.16. MALDI-TOF: m/z 518.1.
2.3.4. Synthesis of 4,9-di(5-bromo-4-hexylthiophen-2-yl)-2,1,3-
naphthothiadiazole (M1)
NBS (1.30 g, 7.23 mmol) in DMF (15 mL) was added dropwise to a
solution of compound 3 (1.5 g, 2.89 mmol) in DMF (50 mL), which
was cooled in an ice bath. After complete addition, the reaction
mixture was warmed to room temperature and stirred for over-
night. The reaction mixture was poured into water and then
extracted with chloroform. The organic layers were combined,
washed with sodium chloride solution and water, dried with so-
dium magnesium. Further purification was performed through
silica gel chromatography using hexane/dichloromethane (6:1 by
volume) to generate 5 (1.41 g, 72%) as a deep red solid. 1H NMR
2.3. Synthesis of monomers and polymers
(400 MHz, CDCl3, d/ppm): 8.10e8.13 (2H, m), 7.31e7.33 (2H, m),
7.09 (2H, s), 2.70 (4H, t, J ¼ 7.6), 1.35e1.71 (16H, m), 0.90 (6H, t,
Naphthalene-2,3-diamine, 3-hexylthiophene, NBS, 2,7-dibromo
carbazole, 2,7-dibromofluorene, 2,6-dibromonaphthalene-1,5-diol,
and Pd(PPh3)4 were purchased from J&K Chemical or Alfa Aesar.
Toluene was distilled from sodium benzophenone under nitrogen
before use. All other reagents and solvents used in this work were
commercially purchased and used without further purification. All
chromatographic separations were carried out on silica gel
(200e300 mesh). Compounds tributyl(4-hexylthiophen-2-yl)stan-
nane [10], M2 [11], M3 [12], M4 [13], and M5 [14] were synthesized
according to the procedure reported in the literatures. The detailed
synthetic processes of other compounds are as follows.
J ¼ 7.0). 13C NMR (100 MHz, CDCl3,
d/ppm): 151.32, 142.35, 135.86,
132.74, 131.56, 127.20, 126.81, 123.04, 111.89, 31.67, 30.00, 29.73,
29.04, 22.66, 14.13. MALDI-TOF: m/z 676.0.
2.3.5. Synthesis of the copolymer PBDTNT
In a 25 mL flask, M1 (169 mg, 0.25 mmol) and M2 (226 mg,
0.25 mmol) were dissolved in 8 mL toluene, and the solution was
flushed with argon for 15 min, then 30 mg Pd(PPh3)4 was added
into the solution. The mixture was again flushed with argon for
20 min. Then the reaction solution was heated to reflux for 12 h and
cooled to room temperature and added dropwise to 200 mL
methanol. The precipitate was collected and further purified by
Soxhlet extraction with methanol, hexane, and chloroform in
sequence. The chloroform fraction was concentrated and added
dropwise into methanol. Finally, the precipitates were collected
and dried under vacuum overnight to get PBDTNT as a black solid
2.3.1. Synthesis of 1,4-dibromonaphthalene-2,3-diamine (1)
A mixture of 2.40 mL of bromine (9.60 g, 60.0 mmol) and 30 mL
of glacial acetic acid was added dropwise into a solution of naph-
thalene-2,3-diamine (2.00 g, 12.6 mmol) in 100 mL of glacial acetic
acid, with vigorous stirring at room temperature. After 2 h, the
precipitate was filtered off and washed subsequently with aqueous
K2CO3 solution and water. A brown powder was obtained after
(224.4 mg, 82.1% yield). 1H NMR (400 MHz, CDCl3,
d/ppm):
8.46e8.45 (br, 2H), 7.82 (s, 2H), 7.49e7.47 (br, 2H), 7.39e7.30 (br,
4H), 6.92 (s, 2H), 4.56 (br, 1H), 3.00e2.87 (br, 8H), 1.79e1.68 (br,
2H), 1.54e1.26 (br, 32H), 0.97e0.89 (br, 18H).
drying (3.33 g, 10.1 mmol, 85%). 1H NMR (400 MHz, CDCl3,
d/ppm):
8.00e8.02 (2H, m), 7.38e7.40 (2H, m), 2.79 (4H, s). MS (EI): m/z 316.
2.3.6. Synthesis of the copolymer PFDTNT
2.3.2. Synthesis of 4,9-dibromo-2,1,3-naphthothiadiazole (2)
M1 (169 mg, 0.25 mmol), M4 (188.5 mg, 0.25 mmol), Pd(PPh3)4
(30 mg), and phase-transfer catalyst Aliquat 336 (two drops) were
dissolved in toluene (10 mL). A 2 M aqueous solution of K2CO3
(6 mL) was added. The mixture was stirred vigorously for 48 h
under argon and cooled to room temperature. Then the polymer
was precipitated by slowly adding the mixture into 200 mL
methanol. The precipitate was collected and further purified by
Soxhlet extraction with methanol, hexane, and chloroform in
sequence. The chloroform fraction was concentrated and added
dropwise into methanol. Finally, the precipitates were collected
and dried under vacuum overnight to get PFDTNT as a purple solid
A solution of 1 (3.67 g, 11.6 mmol) in 150 mL chloroform was
added dropwise into a mixture of thionyl chloride (5.9 mL,
81.3 mmol), 40 mL chloroform and 14 mL pyridine, with vigorous
stirring in a ice-water bath. After the dropwise addition, the solu-
tion was stirred for 2 h at room temperature, and refluxed over-
night. Evaporation of the solvent and purification by column
chromatography on silica gel with dichloromethane/hexane (1:4 by
volume) as eluent were performed, and pure product 2 (1.71 g, 43%)
was finally obtained. 1H NMR (400 MHz, CDCl3,
d/ppm): 8.43e8.46
(2H, m), 7.61e7.63 (2H, m). 13C NMR (100 MHz, CDCl3,
d/ppm):
150.44, 133.19, 128.67, 127.83, 112.61. MS (EI): m/z 344.
(149.6 mg, 58.8%). 1H NMR (400 MHz, CDCl3,
d/ppm): 8.56e8.54 (br,
1H), 7.81 (s, 2H), 7.70e7.69 (br, 1H), 7.60e7.54 (br, 5H), 7.43 (s, 2H),
7.05 (s, 1H), 2.89e2.84 (br, 4H), 2.08e2.02 (br, 4H), 1.78e1.75 (br,
4H), 1.54e1.11 (br, 36H), 0.90e0.82 (br, 12H).
2.3.3. Synthesis of 4,9-di(4-hexylthiophen-2-yl)-2,1,3-
naphthothiadiazole (3)
A solution of compound 2 (3.0 g, 8.2 mmol) and tributyl(4-
hexylthiophen-2-yl)stannane (11.2 g, 24.6 mmol) in freshly
distilled toluene (35 mL) was degassed. Under an argon atmosphere
the Pd(PPh3)4 (115 mg, 0.1 mmol) was added. And then the mixture
heated reflux for 24 h, the toluene was removed by vacuum
distillation. The crude product was purified through the silica gel
column with hexane/dichloromethane (4:1 by volume) to give pure
compound 3 as a brick-red solid (2.88 g, 68%). 1H NMR (400 MHz,
2.3.7. Synthesis of the copolymer PCDTNT
PCDTNT was synthesized by following the same procedure for
PFDTNT. M1 (169 mg, 0.25 mmol) and M3 (164.3 mg, 0.25 mmol)
were used as starting materials. Finally, PCDTNT was obtained as a
purple solid (131.0 mg, 56.9% yield). (400 MHz, CDCl3, d/ppm): 8.57
(br, 1H), 8.18 (br, 2H), 7.82e7.51 (br, 7H), 7.27 (br, 1H), 7.07 (s, 1H),
4.56 (br, 1H), 4.04 (br, 1H), 2.94e2.90 (br, 4H), 1.80 (br, 4H),
1.54e1.15 (br, 40H), 0.88e0.78 (br, 12H).
CDCl3, d/ppm): 8.31e8.33 (2H, m), 7.41e7.43 (2H, m), 7.28 (2H, s),

122
P. Shen et al. / Polymer 79 (2015) 119e127
2.3.8. Synthesis of the copolymer PNDTNT
extracting with methanol, hexane, and chloroform, respectively.
The chloroform solution was concentrated and the product repre-
cipitated in methanol to obtain the resulting copolymers. Table 1
summarizes the polymerization results and thermal properties of
the copolymers.
PNDTNT was synthesized by following the same procedure for
PFDTNT. M1 (169 mg, 0.25 mmol) and M5 (159.1 mg, 0.25 mmol)
were used as starting materials. Finally, PNDTNT was obtained as a
purple solid (120.8 mg, 53.7% yield). (400 MHz, CDCl3, d/ppm):
8.52e8.50 (br, 1H), 8.22e8.04 (br, 2H), 7.61e7.59 (br, 2H),
7.580e7.54 (br, 2H), 7.49e7.43 (br, 2H), 7.05 (s, 1H), 4.18e3.70 (br,
4H), 2.78e2.65 (br, 4H), 1.88e1.00 (br, 40H), 0.90e0.72 (br, 12H).
The copolymers can be readily dissolved in common organic
solvents, such as chloroform, tetrahydrofuran (THF), chloroben-
zene, and 1,2-dichlorobenzene (DCB) at room temperature. The
molecular weights and polydispersity indices (PDIs) of the co-
polymers were determined by gel permeation chromatography
(GPC) analysis with a polystyrene standard calibration and THF as
an eluent. The weight-average molecular weight (Mw) of these
polymers ranged from 26.7 kDa to 142.6 kDa with PDI ranging from
2.31 to 4.18, respectively (Table 1). Compared to the reported ana-
logues [8] of PCDTNT, the obviously higher molecular weight in-
dicates that the introduction of alkyl into two thiophene units of
DTNT benefits to improve the solubility of the resulting polymer.
The high molecular weight is desirable because it will help to
3. Results and discussion
3.1. Synthesis and characterization
The chemical structures and synthesis routes of monomers and
the four D-A copolymers are outlined in Fig. 1 and Scheme 1,
respectively. Naphthalene-2,3-diamine was brominated with
bromine in acetic acid to give 4,9-dibromo-2,1,3-naphthothiadia
zole (compound 1) in a good yield (85%). Treating compound 1
with excess thionyl chloride in a mixed solution of pyridine and
chloroform generated dibromo-NT compound 2 by intramolecular
cyclization. Then, the NT-based compound reacted with alkylth-
iophene to afford compound 3 (DTNT) via a Stille coupling reaction
which was converted into the corresponding dibromide monomer
M1 by treatment with NBS in DMF. The other four monomers M2
[11], M3 [12], M4 [13], and M5 [14] were prepared according to the
literature methods. PCDTNT, PFDTNT, and PNDTNT were synthe-
Table 1
Polymerization results and thermal properties of the four copolymers.
Copolymer
Yield (%)
Mn (kDa)a
Mw (kDa)a
PDI
Td (ꢁC)b
PBDTNT
PCDTNT
PFDTNT
PNDTNT
82.1
56.9
58.8
53.7
8.1
34.1
25.6
18.9
26.7
142.6
75.2
3.30
4.18
2.94
2.31
370
375
356
365
43.8
sized by
a Suzuki coupling reaction in moderate yields
a
Determined by GPC in THF based on polystyrene standards.
The onset decomposition temperature, determined by TGA in nitrogen.
(53.7e58.8%), whereas PBDTNT was prepared by a Stille coupling
reaction in a high yield of 82.1%. Crude copolymers were purified by
b
S
S
H2N
NH2
H2N
NH2
Br
N
N
SnBu3
SOCl2, Pyridine
CHCl3
Br2, HOAc
C6H13
Br
Br
S
Br
Toluene
Pd(PPh3)4
C4H9
C2H5
1
2
S
N
N
C6H13
Br
N
N
C6H13
C6H13
C6H13
S
NBS
DMF
S
Br
S
S
S
Sn
Sn
S
S
S
3
M1
M2
C2H5
C4H9
C8H17
C8H17
OC8H17
C12H25 C12H25
O
N
O
O
O
O
B
O
O
B
O
B
B
B
O
B
O
O
O
OC8H17
M3
M4
M5
PCDTNT
M3
Pd(PPh3)4, aq336
Toulene, K2CO3 ( 2 M)
Pd(PPh3)4
Toulene
M4
M5
+ M 1
M1
+ M 2
PBDTNT
PFDTNT
PNDTNT
Scheme 1. Syntheses of the monomers and D-A copolymers based on 4,9-dithien-20-yl-2,1,3-naphthothiadiazole.

P. Shen et al. / Polymer 79 (2015) 119e127
123
improve the film-forming ability and photovoltaic performance.
based polymers are quite similar to typical D-A copolymer [8].
There is a normal ICT between the different donor units and
acceptor DTNT unit in these copolymers. In addition, it found that
there is a non-planar conformation with strong twist angles be-
tween the two thienyl and NT units, supporting our presumption
aforementioned about the reduced intensity of the ICT band.
Compared to the solution absorption, the absorption of the
copolymer films (Fig. 3b) becomes broaden and shows a red shift,
indicating good intermolecular interactions in the solid state. The
lmax of ICT for PBDTNT, PCDTNT, PFDTNT, and PNDTNT films were
red-shifted by 44, 14, 12, and 14 nm, respectively (Table 2),
compared with those of the polymer solutions. Among the four
polymers, the film absorption peak of PBDTNT exhibited the largest
red-shift relative to the absorption peak in solution (Fig. 3b). This
result suggests the presence of strong intermolecular interactions
in the PBDTNT film compared with the other three polymers,
despite the larger steric hindrance in the bulky 2-ethylhexyl
substituted thiophene side chain. Empirically, the donor strength
is expected to increase in the order benzodithiophene
> carbazole > fluorene > naphthalene [1c,d]. A visible red-shift in
the lmax is observed as the donor comonomer is varied from
naphthalene to fluorene to carbazole to benzodithiophene, indic-
ative of increasing ICT strength with the increase of donor strength
[16]. The absorption edges (ledge) of the film absorption bands for
PBDTNT, PCDTNT, PFDTNT, and PNDTNT are at 730, 725, 687 and
655 nm (Table 2), respectively. The optical bandgaps (Eogpt), esti-
mated from the absorption edge of the thin film absorption spectra,
are between 1.69 and 1.89 eV. As expected, the four polymers based
on NT possess a decreased bandgap relative to the BT-based ana-
logues [17], due to the extended the conjugation length in the
molecular backbone [8]. Moreover, PBDTNT with the strongest ICT
interaction has the narrowest optical bandgap and broadest ab-
sorption, implying that PBDTNT-based PSCs could achieve higher
short-circuit current density (Jsc) compared to other three
polymers.
3.2. Thermal properties
Thermal stability of polymers is a very important criterion for
their use in electronic devices. Fig. 2 shows the TGA curves of the
copolymers at the heating rate of 20 ꢁC minꢀ1 under a nitrogen
atmosphere. The onset decomposition temperatures (Td) of the
copolymers are in the range of 356e375 ꢁC (Table 1), indicating that
these DTNT-based copolymers possess good thermal stability.
Obviously, the thermal stability of the copolymers is adequate for
the fabrication processes of PSCs and other optoelectronic devices.
3.3. Optical properties
Electronic absorption spectra of polymers in the chloroform
solutions and drop-casted films were measured. The detailed ab-
sorption data including the absorption maximum wavelength
(lmax) in both solution and films, the absorption edge (onset
wavelength of the absorption peak, ledge) of the polymer films, and
optical bandgaps (Eogpt) are summarized in Table 2.
As shown in Fig. 3a, all of the absorption spectra recorded from
dilute chloroform solutions feature dual-band of a D-A system: one
located at 300e450 nm is assigned to localized
p-p* transitions,
and another broader band from 450 to 700 nm in the long wave-
length region, is corresponding to intramolecular charge transfer
(ICT) between the different donor units (benzodithiophene,
carbazole, fluorene, and naphthalene) and acceptor DTNT unit.
Obviously, PBDTNT possesses stronger absorption than that of
PCDTBT, PFDTNT, and PNDTNT in the long wavelength region. This
result demonstrates that PBDTNT has a better ability of light-
harvesting than that of other three polymers, indicating the
former would give a higher Jsc than the latter. Moreover, it should
be noted that the copolymer PBDTNT shows one additional high-
energy band located at approximately 350 nm, which could be
assigned to the conjugated thiophene side chains. For all the
polymers, one can see that the absorption intensity of the ICT bands
3.4. Redox properties and HOMO/LUMO energy levels
is obviously weaker than that of p-p* absorption bands. This could
To investigate the redox behavior of the copolymers and
determine the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) energy levels, the
electrochemical properties of the polymers were investigated by
cyclic voltammetry (CV) [18]. A platinum (Pt) electrode covered
with a polymer film by dip-coating was used as the working elec-
trode, a platinum wire was used as the counter electrode, and Ag/
AgCl served as the reference electrode. The CV measurements were
carried out in a Bu4NPF6/acetonitrile electrolyte (0.1 M) at room
be ascribed to the relatively weak electron-withdrawing nature of
the NT unit relative to other acceptor units (e.g. BT [15]) or/and a
strong steric interaction between NT and adjacent thienyl units in
the polymer [6b,8] slightly impeding the effective ICT between the
donor and acceptor units. According to the previous density func-
tional theory calculation on the analogues of PCDTNT, these DTNT-
temperature under nitrogen atmosphere at
a scan rate of
100 mV sꢀ1. All the electrochemical data are summarized in Table 2.
The HOMO/LUMO energy levels of the polymers was deduced from
the oxidation/reduction onsets, assuming that the energy level of
ferrocene (Fc) was 4.8 eV below the vacuum level (Fc/Fcþ measured
as 0.40 V vs. Ag/AgCl, Fig. 4).
The observed oxidation CVs in Fig. 4 show quasi-reversible
oxidation waves for all the DTNT-based D-A copolymers. The
onset of oxidation (Eoonxset) for PBDTNT, PCDTNT, PFDTNT, and
PNDTNT occurred at 1.07, 1.06, 1.09 and 1.11 V (Fig. 4), which cor-
responds to HOMO energy level [19] of ꢀ5.47, ꢀ5.46, ꢀ5.49,
and ꢀ5.51 eV, respectively (Table 2). In generally, the oxidation
potentials gradually become less positive with increasing strength
of the donor comonomer. PNDTNT with the weakest donor unit of
naphthalene possessed the lowest lying HOMO energy level. The
low-lying HOMO energy level in these DTNT-based copolymers
suggests that they are oxidatively stable hole transporting mate-
rials [20]. In addition, low-lying HOMO energy levels are desired in
Fig. 2. TGA plots of copolymers with
atmosphere.
a
heating rate of 20 ꢁC minꢀ1 under N2

124
P. Shen et al. / Polymer 79 (2015) 119e127
Table 2
Optical and electrochemical properties of the four copolymers.
Copolymer
Solution ls,max (nm)a
Film lf,max (nm)b
ledge (nm)c
Eogpt (eV)d
HOMO (eV)e
LUMO (eV)e
Eegc (eV)f
PBDTNT
PCDTNT
PFDTNT
PNDTNT
346, 562
360, 544
350, 544
340, 536
418, 606
364, 558
356, 556
342, 550
730
725
687
655
1.69
1.71
1.80
1.89
ꢀ5.47
ꢀ5.46
ꢀ5.49
ꢀ5.51
ꢀ3.78g
ꢀ3.54
ꢀ3.56
ꢀ3.55
e
1.92
1.93
1.96
a
Measured in dilute chloroform solution.
b
c
d
e
f
Measured on quartz plate by polymers cast from chloroform solution.
Absorption edge of the thin films.
Estimated from the onset wavelength of the absorption spectra: Eogpt ¼ 1240/ledge
.
onset
Calculated according to the equation: HOMO/LUMO ¼ ꢀe (E
þ 4.40) (eV).
ox/red
Bandgap obtained from ELUMO ꢀ EHOMO
.
g
LUMO ¼ (Eogpt þ HOMO) (eV).
BHJ PSCs as an effective approach to maximize the open-circuit
voltage (Voc), which is known to be related to the difference in
the donor polymer HOMO and the acceptor (fullerene derivatives,
e.g. PC70BM) LUMO energy levels.
On the other hand, the copolymers PCDTNT, PFDTNT, and
PNDTNT showed quasi-reversible or irreversible reduction waves
with the onset reduction potentials (Eoonxset) of ꢀ0.86 to ꢀ0.84 V (vs.
Ag/AgCl) (Fig. 4). The estimated LUMO energy levels [19] of the
three copolymers were in the range of ꢀ3.54 to ꢀ3.56 eV (Table 2).
The three DTNT-based D-A copolymers essentially have an identical
LUMO energy level of ꢀ3.55 eV, which is not surprising in
considering the constant DTNT acceptor moiety among all the
copolymers. The present DTNT-based copolymers have a slightly
higher-lying LUMO energy level compared to previously reported
NT-based analogues (LUMO ¼ ꢀ3.63 eV) [8] and BT-based ana-
logues (LUMO ¼ ꢀ3.6 eV) [15]. In generally, it has been found that a
LUMO energy level offset between polymer and fullerene derivative
(e.g. PC70BM) should be at least 0.3 V for the efficient exciton
dissociation into charge carriers [20] and that higher offsets
enhance the driving force for charge generation and improve
photocurrent in bulk heterojunction photovoltaic devices [20c].
Consequently, the elevated LUMO energy levels of the three poly-
mers are favorable for charge generation and improve photovoltaic
performance. Unfortunately, it was difficult to directly obtain the
LUMO energy level for the copolymer PBDTNT using this method,
because no obvious reduction wave can be observed from the CV
(Fig. 4). Thus the LUMO energy level of this polymer is estimated
from the optical bandgap (Eogpt) and the HOMO energy level based
on the equation of LUMO ¼ (Eogpt þ HOMO) eV [21]. The electro-
chemical bandgap (Eegc) is determined to be 1.92 eV for PCDTNT,
1.93 eV for PFDTNT, and 1.96 eV for PNDTNT (Table 2), which are
0.07e0.21 eV larger than the optically determined ones (Eogpt
)
(Table 2). This difference can be explained by the exciton binding
energy of conjugated polymers which is thought to be in the range
of 0.2e1.0 eV [22].
The results of optical and electrochemical properties prove that
the absorption bands as well as the molecular energy levels of
DTNT-based polymers can be tuned by copolymerizing with
Fig. 4. Cyclic voltammograms of the copolymer films on platinum electrode in
acetonitrile solution containing 0.1 mol Lꢀ1 Bu4NPF6 at a scan rate of 100 mV sꢀ1. The
cyclic voltammogram of ferrocene was also put in the figure for the potential
calibration.
Fig. 3. UVevis absorption spectra of copolymers in chloroform solution (a) and in the
thin films (b).

P. Shen et al. / Polymer 79 (2015) 119e127
125
different donor units. In many aspects, these newly synthesized
polymers closely resemble the ideal low bandgap polymers sug-
gested by the previous reports in design rules of polymers for PSCs
applications [20a,b].
and Jsc (Table 3). The values of Voc and Jsc up to 0.82 V and
7.27 mA cmꢀ2, respectively, is obtained with CN as an additive,
whereas the maximum Voc and Jsc could only reach 0.76 V and
6.84 mA cmꢀ2 without additive while keeping the same film
thickness (~100 nm). In our recent work, we found that post-
solvent treatment usually can further improve the performance of
photovoltaic cells [19]. Here we employ a polar solvent methanol to
further optimize the photovoltaic performance of PSCs. The detail
process of methanol treatment can be found in the previous report
[19a]. As shown in Table 3, one can see that after the methanol
treatment the photovoltaic performance of the copolymer has been
enhanced. When the mixing ratio of PBDTNT:PC70BM is 1:3, the
PCE is further improved up to 2.20% (C3) with an increased Jsc from
6.01 mA cmꢀ2 (C1) to 7.06 mA cmꢀ2 (C3), and a Voc from 0.82 V to
0.94 V. Similarly for C5 and C7 with the mixing ratio of 1:4, the PCE
is improved from 1.78% (C5) up to 2.28% (C7) with a significantly
increased Voc of 0.92 V and a slight increased Jsc of 7.26 mA cmꢀ2
(Table 3). In view of the addition of CN as the additive and the
solvent treatment can effectively improve the photovoltaic per-
formance, we further optimize the performance using two tech-
niques together. Higher PCEs of 2.49% and 2.73% are achieved for
solar cells C4 and C8, respectively, with 3 vol % CN as the additive
and the methanol treatment (Fig. 5 and Table 3). For C4 the main
reason is the increases both Voc (0.96 V) and Jsc (8.06 mA cmꢀ2),
whereas C8 is mainly ascribed to the remarkable improvement of Jsc
(9.45 mA cmꢀ2) even with a slight drop of Voc (0.90 V). Based on the
above observations, the photovoltaic properties of the other three
polymers were further improved by using the same approach.
Finally, the devices based on the other three polymers present the
enhanced PCEs of 2.26% (C10), 1.57% (C12), and 1.45% (C14) for
PCDTNT, PFDTNT, and PNDTNT, respectively. Obviously, the
improved PCE is mainly ascribed to the significant enhancement of
both Voc and Jsc. It is worth mentioning that the photovoltaic per-
formance of PCDTNT is superior to the previous reported analogues
[8], mainly due to the obviously higher Voc, and Jsc of the former.
This relates to higher molecular weight of PCDTNT as compared
with the analogues. The result indicates that the introduction of
alkyl into two thiophene units of DTNT benefits to improve the
solubility, film-forming ability and photovoltaic performance of the
resulting polymer. The improved efficiency may originate from
enhanced charge transport, or improved efficiency of carrier
collection, which can all result from the creation of an optimal
morphology under the conditions of CN as the processing additive
and treatment with methanol. It should be noted that the cells
based on these DTNT-based polymers exhibit quite high Voc values
above 0.90 V (C3, C4, C5, C7, C8, C10, C12, and C14), which can be
readily understood for the low-lying HOMO energy level (Table 2).
Moreover, the Voc is as high as 0.96 V in the case of C4. There are few
reports about such a high Voc for a single-junction polymer solar
cell to the best of our knowledge [25].
3.5. Photovoltaic properties
To investigate the photovoltaic properties of this series of DTNT-
based copolymers, bulk-heterojunction solar cells with a configu-
ration of ITO/PEDOT:PSS/polymer:PC70BM/Ca/Al were fabricated.
The device performance can be significantly improved by opti-
mizing the morphology in active layer with utilization of process-
ing additives [23] and solvent polar solvent treatment [19]. The
polymer:PC70BM
active
layers
spin-coated
from
1,2-
dichlorobenzene (DCB) or DCB with 3 vol % 1-chloronaphthalene
(CN). Fig. 5 shows the current density-voltage characteristic (J-V) of
the PSCs based on the blends of polymer:PC70BM under illumina-
tion of AM 1.5G, 100 mW cmꢀ2. The corresponding open-circuit
voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and
PCE values of the devices are summarized in Table 3.
At first, the blend ratio of the polymer donor and PC70BM
acceptor was optimized. When the mixing ratio of PBDTNT and
PC70BM increases from 1:3 to 1:4, the PCE of the cell shows a
various degree of improvement under the same fabrication con-
dition. For example, the solar cell C6 shows higher PCE (2.02%) than
that of C2 (1.88%) when the cells prepared from DCB with 3 vol% CN
as additive. The same conclusion can be found in the case of C3 vs.
C7 and C4 vs. C8. The main reason is the increase of Jsc although Voc
decreased to some extent (Table 3). It is observed that an increase in
the PC70BM content resulted in a slight decrease in the Voc. This has
previously been observed for other donor polymers [24], including
ours [19]. The reduction in the Voc was proposed to correlate with
the decrease in the energy of the charge transfer (CT) states with
increasing PC70BM concentrations [24c]. However, for the other
three polymers, all of them have the optimal blend ratios of 1:3.
With the optimal blend ratio, the highest PCE is 1.46%, 1.23% and
1.14% for PCDTNT (C9), PFDTNT (C11), and PNDTNT (C13), respec-
tively (Table 3).
Next, we find that photovoltaic performance can be improved
when 3 vol % CN was used as a process additive. Take PBDTNT as an
example, the PCE of the cell C5 with a mixing donor/acceptor ratio
of 1:4 is 1.78%, using DCB as pristine solvent. PCE can be increased
up to 2.02% for C6 when 3 vol % CN is used as the additive. The
improved PCE can essentially be attributed to both the higher Voc
2
0
-2
-4
-6
The external quantum efficiencies (EQE) of the solar cells were
measured under the illumination of monochromatic light. Fig. 6
depicts the EQE curves of the best performance of poly-
mer:PC70BM blends at the optimal weight ratios with 3 vol % CN as
the additive and the methanol treatment. PSCs based on these
copolymers exhibit EQE responses ranging from 300 to 700 nm
with maximum EQE values of 66% at 445 nm, 62% at 425 nm, 47% at
430 nm and 39% at 465 nm for PBDTNT, PCDTNT, PFDTNT, and
PNDTNT, respectively. We attribute these EQE responses in the
visible region to the corresponding absorbance of the polymer/
PC70BM blends, resulting from both the intrinsic absorptions of the
polymer and the presence of PC70BM, which also absorbs signifi-
cantly at 300e400 nm. These cells display relatively higher EQE
responses at wavelengths of 300e550 nm than those longer than
550 nm because of both the obviously strong absorbance of the
C8
-8
-10
-12
C10
C12
C14
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
Fig. 5. JeV characteristics of PSCs based on the four copolymers under the best
fabricated conditions.

126
P. Shen et al. / Polymer 79 (2015) 119e127
Table 3
Photovoltaic properties of the PSCs based on these copolymer:PC70BM with different fabricated conditions, under the illumination of AM1.5, 100 mW cmꢀ2
.
Cell
Active Layer
CN
MeOH
Voc (V)
Jsc (mA cmꢀ2
)
FF (%)
PCE (%)
C1
C2
C3
C4
C5
C6
C7
C8
PBDTNT:PC70BM ¼ 1:3
PBDTNT:PC70BM ¼ 1:3
PBDTNT:PC70BM ¼ 1:3
PBDTNT:PC70BM ¼ 1:3
PBDTNT:PC70BM ¼ 1:4
PBDTNT:PC70BM ¼ 1:4
PBDTNT:PC70BM ¼ 1:4
PBDTNT:PC70BM ¼ 1:4
PCDTNT:PC70BM ¼ 1:3
PCDTNT:PC70BM ¼ 1:3
PFDTNT:PC70BM ¼ 1:3
PFDTNT:PC70BM ¼ 1:3
PNDTNT:PC70BM ¼ 1:3
PNDTNT:PC70BM ¼ 1:3
ꢀ
ꢀ
ꢀ
þ
þ
ꢀ
ꢀ
þ
þ
ꢀ
þ
ꢀ
þ
ꢀ
þ
0.82
0.86
0.94
0.96
0.76
0.82
0.92
0.90
0.80
0.90
0.82
0.92
0.80
0.92
6.01
6.54
7.06
8.06
6.84
7.27
7.26
9.45
5.59
7.65
4.75
5.68
4.62
5.38
35.2
33.4
33.1
32.2
34.3
34.0
34.1
32.1
32.6
32.8
31.7
30.1
30.9
29.3
1.73
1.88
2.20
2.49
1.78
2.02
2.28
2.73
1.46
2.26
1.23
1.57
1.14
1.45
3%
ꢀ
3%
ꢀ
3%
ꢀ
3%
ꢀ
C9
C10
C11
C12
C13
C14
3%
ꢀ
3%
ꢀ
3%
compared with the other three copolymers, in present we chose
this polymer as a representative to explore the effect of the
morphology of active layers on the photovoltaic performance. We
observed the surface morphologies of the blend films
(PBDTNT:PC70BM ¼ 1:4) using atomic force microscopy (AFM).
Fig. 7 gives the AFM phase images of the active layer surfaces. As
shown in Fig. 7a, without the additive the surface of the active
layer of polymer:PC70BM (C5) is very smooth, with a root-mean-
square (RMS) of 0.50 nm, which means that there is no obvious
phase separation between the polymer donor and fullerene
acceptor in the active layer. It is unfavorable to the exciton
dissociation and charge transport and results in poor photovoltaic
performance (C5). After adding the additive of 3 vol % CN, the
surface of the active layers (C6) becomes rougher with an
increased RMS of 3.64 nm, and the domain size of phase separa-
tion is raised in some extent (Fig. 7b). Moreover, after methanol
treatment the surface of the blend film (C8) shows more rougher
with a further increased RMS of 5.85 nm and demonstrates a more
uniform distribution of PBDTNT and PC70BM (see Fig. 7c). The
relatively larger RMS indicates the improved aggregation of the
polymer donor and fullerene acceptor in the active layer. As some
of the polar solvent molecules penetrated the active layer, PC70BM
particles redistributed and aggregated to reduce the contact areas
with the polar solvents [19a,26]. As a result, solvent-dependent
PC70BM migration within the polymer chains enabled the forma-
tion of nanoscale phase separation of the blend films (Fig. 7c). This
relatively course surface and the long-range interpenetrating
network are beneficial to the exciton dissociation and charge
transport, thus leading to an increase in Jsc and Voc as well as the
device efficiency (Table 3). Obviously, the appropriate domain
sizes and phase separation can produce the best photovoltaic
performance.
Fig. 6. EQE curves of solar cells based on the four copolymers under the best fabricated
conditions.
polymer and the presence of PC70BM in the short wavelength range
(Fig. 3b). Although the EQE response covers a not very broad
spectrum, the EQE value is relatively high EQE value over 65%
around 445 nm, which elucidates the obviously higher Jsc of
PBDTNT-based cells relative to the other three copolymers. Overall,
the EQE results are consistent with the Jsc values of the corre-
sponding PSCs.
3.6. Morphology study
Considering the better photovoltaic properties of PBDTNT as
Fig. 7. AFM phase images of (a) PBDTNT:PC70BM (1:4), (b) PBDTNT:PC70BM (1:4, 3 vol% CN), (c) PBDTNT:PC70BM (1:4, 3 vol% CN, treated with methanol).

P. Shen et al. / Polymer 79 (2015) 119e127
2938e2944.
127
4. Conclusions
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A series of DTNT-based D-A copolymers PBDTNT, PCDTNT,
PFDTNT, and PNDTNT, were designed and synthesized in combi-
nation with various donor units such as benzodithiophene, carba-
zole, fluorene, and naphthalene. The optical and electrochemical
properties showed that the absorbance and bandgaps can well
tuned by changing the electron-donating ability of the donor units.
The copolymers showed the relatively low-lying HOMO energy
levels of around ꢀ5.45 eV. PSCs with the polymers as donor and
PC70BM as acceptor were fabricated and photovoltaic performances
were optimized with blend ratio, processing additive and methanol
treatment. Photovoltaic devices based on these DTNT-based co-
polymers showed high Voc over 0.90 V thanking to their lower-lying
HOMO energy levels. Among the four copolymers, PBDTNT
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exhibited the best PCE of 2.73% with Voc
¼
0.90 V,
Jsc ¼ 9.45 mA cmꢀ2 and FF ¼ 32.1% after device optimization. The
high Jsc and FF values of PBDTNT devices were attributed to their
rather broad absorption and optimized phase separation of the
active layer. This work may provide certain insight for the explo-
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This work was supported by the Natural Science Foundation of
China (Grant no. 91433117), Natural Science Foundation of Hunan
Province, China (Grant no. 2015JJ3019) and the Key Research
Foundation of Education Bureau of Hunan Province, China (Grant
no. 14A147).
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