Dibenzothiophene-based planar conjugated polymers for high efficiency polymer solar cells

Article abstract of DOI:10.1021/ma301622g

Three D-A alternating copolymers P1-3 with 3,7-linked 2,8-bis(alkoxy) dibenzothiophene as the donor unit and benzothiadiazole (P1 and P2) or 3,4-bis(octyloxy)benzothiadiazole (P3) as the acceptor unit have been designed and synthesized. P1-3 show two broa

Full text of DOI:10.1021/ma301622g

Article
pubs.acs.org/Macromolecules
Dibenzothiophene-Based Planar Conjugated Polymers for High
Efficiency Polymer Solar Cells
Enquan Jin, Chun Du, Ming Wang, Weiwei Li, Cuihong Li,* Hedi Wei, and Zhishan Bo*
Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing
100875, China
S
* Supporting Information
ABSTRACT: Three D−A alternating copolymers P1−3 with 3,7-linked
2,8-bis(alkoxy)dibenzothiophene as the donor unit and benzothiadiazole
(P1 and P2) or 3,4-bis(octyloxy)benzothiadiazole (P3) as the acceptor
unit have been designed and synthesized. P1−3 show two broad
absorption peaks in the visible region, and the internal charge transfer
(ICT) absorptions at about 530 nm in solutions and 560 nm in films of
P3 are much stronger than that of P1 and P2. All the polymers show
narrow band gaps below 2.0 eV and the low-lying HOMO energy levels
of approximately −5.30 eV. The hole mobilities of polymer films spin-cast
from 1,2-dichlorobenzene (DCB) solutions are 3.0 × 10−4, 2.7 × 10−4,
and 2.3 × 10−3 cm2 V−1 s−1 for P1, P2, and P3, respectively. Under
simulated solar illumination of AM 1.5G (100 mW/cm2), a PCE of 4.48%
with a Voc of 0.83 V, a Jsc of 9.30 mA/cm2, and an FF of 0.58 has been
achieved for PSCs with the P3:PC71BM blend (1:3, by weight) as the
active layer in simply processed devices, whereas after the optimization, PCEs of only 1.02% and 1.71% have been obtained for
P1- and P2-based devices, respectively. This is the first report on dibenzothiophene-based conjugated polymers used for high
efficiency polymer solar cells, demonstrating that photovoltaic performance can be improved by fine-tuning the conjugated
polymer structure.
conjugated polymer donors is usually several orders lower than
the electron mobility of PCBM. In order to increase the hole
mobility of polymer donors, polymer chains are required to
have a planar structure and closely pack in films.6
INTRODUCTION
■
In recent years, polymer solar cells (PSCs) have attracted
considerable attention due to their advantages of lightweight,
flexibility, and large-scale manufacturing with low-cost roll-to-
roll process.1 Polymer solar cells have experienced a rapid
development since the invention of the bulk-heterojunction
(BHJ) device structure, which has been proven to be the most
efficient device structure until now.2 For polymer solar cells
power conversion efficiency (PCE) higher than 7% has been
achieved by several groups via the optimization of the polymer
and device structure.3 The BHJ structure is a phase-separated
bicontinuous network formed by blending electron-donating
polymers and electron-accepting fullerene derivatives, such as
(6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) or (6,6)-
phenyl-C71-butyric acid methyl ester (PC71BM). To achieve
highly efficient BHJ polymer cells, polymer donors should have
a strong and broad absorption to effectively absorb sunlight, an
appropriate highest occupied molecular orbital level (HOMO)
and an appropriate lowest unoccupied molecular orbital level
(LUMO) to maximize the short-circuit current (Jsc) and open-
circuit voltage (Voc),2f,4 and most importantly the polymer
donor and PCBM blends (active layer) should have a balanced
hole and electron mobility to promote the continuous
generation of electrons without recombination reactions or
saturation of charges.2f,4a The electron mobility of PCBM is
around 2 × 10−3 cm2 V−1 s−1,5 whereas the hole mobility of
Main chain donor−acceptor (D−A) alternating design is a
very efficient way to tune the absorption and energy level of
conjugated polymers through the intramolecular charge transfer
from donor to acceptor unit.3g,7 On the one hand,
benzothiadiazole is a commonly used acceptor unit in
construction of D−A alternating conjugated polymers, and
high power conversion efficiency has been achieved for
benzothiadiazole containing D−A alternating copolymers.8
On the other hand, 2,7-linked fluorene (two benzene ring
bridged by carbon atom),7e,8a,9 2,7-linked carbazole (two
benzene rings bridged by nitrogen atom),2e,10 and 2,7-linked
silafluorene (two benzene rings bridged by silicon atom)6c,11
have been used as donor units for the construction of the D−A
copolymers. However, until now there is no report on using
3,7-linked dibenzothiophene (two benzene rings bridged by
sulfur atom) as donor unit in synthesizing D−A alternating
copolymers. It is worthy noting that Wang et al. have found
that marked chalcogen−chalcogen interactions exist in
Received: August 1, 2012
Revised: September 5, 2012
Published: September 18, 2012
© 2012 American Chemical Society
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analyzer) with a standard three-electrode electrochemical cell in a 0.1
M tetrabutylammonium tetrafluoroborate solution in CH3CN at room
temperature under an atmosphere of nitrogen with a scanning rate of
0.1 V/s. A Pt plate working electrode, a Pt wire counter electrode, and
an Ag/AgNO3 (0.01 M in CH3CN) reference electrode were used.
The experiments were calibrated with the standard ferrocene/
ferrocenium (Fc) redox system and assumption that the energy level
of Fc is 4.8 eV below vacuum.
Fabrication and Characterization of Organic Field-Effect
Transistors (OFETs). Top-contact devices were fabricated based on
Si/SiO2 substrates (the back low resistance Si as gate, SiO2 (500 nm)
with a capacitance of 7.5 nF cm−2 as gate insulator). The substrate of
Si/SiO2 was sequentially cleaned with pure water, hot concentrated
sulfuric acid−hydrogen peroxide solution (concentrated sulfuric
acid:hydrogen peroxide, 2:1 by volume), pure water, pure ethanol,
and pure acetone successively and then treated with a monolayer of
trichloro(octadecyl)silane (OTS) by the normal vapor deposition
method described elsewhere. Polymer or polymer/PC71BM films were
cast or spin-coated on the OTS-modified Si/SiO2 substrate from DCB
with the polymer concentration of 10 mg/mL. Electrodes of Au (25
nm) were vacuum-deposited on films with width/length = 50 (channel
width = 2.5 mm, channel length = 50 μm). J−V characteristics were
obtained using an Agilent B2902A source meter with a Micro-
manipulator 6150 probe station in a clean and shielded box at room
temperature in air.
Polymer Solar Cell Fabrication and Characterization.
Polymer solar cells (PSCs) were fabricated with the device
configuration of ITO/PEDOT:PSS/Polymer:PC71BM (1:3)/LiF/Al.
The conductivity of ITO was 20 Ω/□. PEDOT:PSS is Baytron Al
4083 from H.C. Starck and was filtered with a 0.45 mm PVDF film
before use. A thin layer of PEDOT:PSS was spin-coated on top of
cleaned ITO substrate at 3000 rpm/s for 60 s and dried subsequently
at 130 °C for 15 min on a hot plate before transferred into a glovebox.
The thickness of the PEDOT:PSS layer was about 40 nm. The blend
of polymers and PC71BM was dissolved in ODCB, heated at 90 °C for
overnight to ensure the sufficient dissolution, and then spin-coated
onto PEDOT:PSS layer. The top electrode was thermally evaporated,
with a 0.6 nm LiF layer, followed by 100 nm of aluminum at a pressure
of 10−4 Pa through a shadow mask. Five cells were fabricated on one
substrate with an effective area of 0.04 cm2. The measurement of
devices was conducted in air without encapsulation. Current−voltage
characteristics were recorded using an Agilent B2902A source meter
under an AM1.5G AAA class solar simulator (model XES-301S, SAN-
EI) with an intensity of 100 mW cm−2 as the white light source, and
the intensity was calibrated with a standard single-crystal Si
photovoltaic cell. The temperature while measuring the J−V curves
was ∼25 °C.
Synthesis of 2,8-Dihydroxydibenzothiophene-S,S-dioxide
(3). To a solution of 2,8-dibromodibenzothiophene (1) (20.0 g,
58.47 mmol) in THF (150 mL) cooled to −78 °C was added n-
butyllithium (58.47 mL, 2.5 M in hexane, 146.18 mmol) by syringe.
The mixture was stirred at −78 °C for 2 h, triisopropyl borate (43.00
mL, 185.17 mmol) was added rapidly, and the mixture was stirred
overnight and allowed to warm to room temperature gradually. Dilute
hydrochloric acid was added, and the mixture was extracted with ether.
The organic layer was separated, washed with brine, and dried over
anhydrous MgSO4. The solvent was removed under reduced pressure,
and the residue (crude benzothiophene-S,S-dioxide-2,8-diboronic acid,
2) was dissolved in acetic acid (150 mL). The solution was cooled to 0
°C with an ice bath, hydrogen peroxide (30%, 80 mL) was added
dropwise, and the resulted mixture was stirred at 0 °C overnight.
Removal of the solvent under reduced pressure afforded the crude 2,8-
dihydroxydibenzothiophene-S,S-dioxide, 3 (13.35 g, 92%), as a
colorless solid, which was used for the next step without further
purification.
chalcogen−annulated perylene solid films, which may facilitate
charge transport.12 In this work, we design and synthesize three
D−A alternating copolymers with 3,7-linked 2,8-bis(alkoxy)-
dibenzothiophene as the donor unit and benzothiadiazole or
3,4-bis(octyloxy)benzothiadiazole as the acceptor unit. We
attached two alkoxy substituents on the 2,8-positions of
dibenzothiophene to ensure that the two alkoxy chains do
not hinder the π−π stacking of polymer backbones in solid
films and generates soluble polymers.. For the first time, we
demonstrate that 2,8-bis(alkoxy)-substituted dibenzothiophene
can be a useful donor unit for constructing main chain D−A
alternating narrow band gap conjugated polymer for high
efficiency polymer solar cells. In addition, we have found that
conjugated polymers (P3) with 2,8-bis(alkoxy)-
dibenzothiophene as the donor unit and 5,6-bis(octyloxy)-
substituted benzothiadiazole as the acceptor unit show a strong
internal charge transfer (ICT) absorption in the long
wavelength region, whereas copolymers (P1 and P2) with
2,8-bis(alkoxy)dibenzothiophene as the donor unit and
benzothiadiazole as the acceptor unit show a relative weak
ICT absorption. As expected, the strong absorbed P3-based
polymer solar cells show higher PCE than the weak absorbed
P1- and P2-based polymer solar cell devices. Under simulated
solar illumination of AM 1.5G (100 mW/cm2), a PCE of 4.48%
has been achieved for PSCs with P3:PC71BM blend as the
active layer in simply processed devices. The devices illustrate a
considerably high Voc of 0.83 V, a Jsc of 9.30 mA/cm2, and a
comparable fill factor (FF) of 0.58. The hole moblity of P3
reached 2.3 × 10−3 cm2 V−1 s−1. The above result demonstrates
that the absorption and photovoltaic performance can be
improved by fine-tuning the conjugated polymer structure.
EXPERIMENTAL PART
■
Materials and Instruments. Unless otherwise noted, all
chemicals were purchased from Aldrich or Acros and used without
further purification. The catalyst precursor Pd(PPh3)4 was prepared
according to the literature and stored in a Schlenk tube under a
nitrogen atmosphere.13 2,8-Dibromodibenzothiophene was synthe-
sized according to the literature procedure.14 Tetrahydrofuran (THF)
and diethyl ether (Et2O) were distilled from sodium with
benzophenone as an indicator under a nitrogen atmosphere. Hexane
and dichloromethane (DCM) were distilled from CaH2. Chloroform
was distilled before use. All reactions were performed under an
atmosphere of nitrogen and monitored by thin layer chromatography
(TLC) on silica gel 60 F254 (Merck, 0.2 mm). Column
1
chromatography was carried out on silica gel (200−300 mesh). H
and 13C NMR spectra were recorded on a Bruker AV 400
spectrometer. Fluorescence spectra were recorded on a FluoroMax-4
spectrofluorometer. UV−vis absorption spectra were obtained on a
PerkinElmer UV−vis spectrometer model Lambda 750. Elemental
analyses were performed on a Flash EA 1112 analyzer. Thermal
gravimetric analysis (TGA) and differential scanning calorimetry
(DSC) measurements were performed on TA2100 and PerkinElmer
Diamond DSC instrument, respectively, under a nitrogen atmosphere
at a heating rate of 10 °C/min to record TGA and DSC curves. The
gel permeation chromatography (GPC) measurements were per-
formed at 150 °C on a PL-220 (Polymer Laboratories) chromatograph
connected to a differential refractometer with 1,2,4-trichlorobenzene as
an eluent. Atomic force microscopy (AFM) measurements were
performed under ambient conditions using a Digital Instrument
Multimode Nanoscope IIIA operating in the tapping mode. The
thickness of the blend films was determined by a Dektak 6 M surface
profilometer. The powder X-ray diffraction (XRD) patterns were
collected using a PANalytical X’Pert PRO MPD diffractometer with
Cu Kα radiation. The electrochemical behavior of the polymers was
investigated using cyclic voltammetry (CHI 630A electrochemical
General Procedure for the Synthesis of 2,8-Bis(alkoxy)-
dibenzothiophene-S,S-dioxide (4). A mixture of compound 3,
alkyl bromide, potassium carbonate, and butanone was heated to reflux
and stirred under a nitrogen atmosphere for 2 days. The solvent was
removed under reduced pressure, the residue was partitioned between
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methylene dichloride and water, the organic layer was separated, and
the aqueous layer was extracted with CH2Cl2. The combined organic
layers were dried over anhydrous MgSO4 and evaporated to dryness.
The residue was purified by flash column chromatography on silica gel
eluted with petroleum ether (60−90 °C)/dichloromethane (1:2, v/v)
to give 3,6-bis(alkoxy)dibenzothiophene-S,S-dioxide (4) as colorless
solids.
Synthesis of 2,8-Bis(dodecyloxy)dibenzothiophene-S,S-dioxide
(4a). 3,6-Dioxylbenzothiophene-S,S-dioxide (3) (6.00 g, 24.17
mmol), dodecyl bromide (15.06 g, 60.43 mmol), potassium carbonate
(10 g, 72.5 mmol), and butanone (250 mL) were used. 4a was
obtained in a yield of 89% (12.58 g). 1H NMR (400 MHz, CDCl3) δ:
7.64−7.61 (d, 2H), 7.10−7.09 (d, 2H), 6.90−6.88 (dd, 2H), 4.00−
3.97 (t, 4H), 1.79−1.72 (m, 4H), 1.44−1.37 (m, 4H), 1.27−1.20 (m,
32H), 0.82−0.79 (t, 6H). 13C NMR (100 MHz, CDCl3) δ: 163.65,
133.70, 130.48, 123.49, 115.77, 107.46, 68.80, 31.91, 29.65, 29.62,
29.58, 29.54, 29.34, 29.03, 25.94, 22.68, 14.11. Anal. Calcd for
C36H56O4S: C, 73.93; H, 9.65. Found: C, 73.75; H, 9.38.
MHz, CDCl3) δ: 158.97, 130.82, 129.11, 125.43, 113.57, 103.43,
71.11, 38.45, 29.50, 28.14, 22.91, 21.97, 13.07, 10.32. Anal. Calcd for
C28H38Br2O4S: C 53.34, H 6.08. Found: C 52.97, H 6.22.
Synthesis of 3,7-Dibromo-2,8-bis(octyloxy)dibenzothiophene-
S,S-dioxide (5c). Bromine (20.29 g, 126.94 mmol) and 2,8-
bis(octyloxy)dibenzothiophene-S,S-dioxide (15.00 g, 31.73 mmol)
were used. 5c was obtained as a colorless crystal in a yield of 85%
(17.01 g). 1H NMR (400 MHz, CDCl3) δ: 7.77 (s, 2H), 6.92 (s, 2H),
4.13−4.10 (t, 4H), 1.96−1.89 (m, 4), 1.61−1.54 (m, 4H), 1.44−1.33
(m, 16H), 0.93−0.89 (t, 6H). 13C NMR (100 MHz, CDCl3) δ: 159.82,
131.77, 129.96, 126.37, 114.43, 104.38, 69.89, 31.80, 29.29, 29.21,
28.93, 26.03, 22.68, 14.13. Anal. Calcd for C28H38Br2O4S: C 53.34, H
6.08. Found: C 53.42, H 5.78.
General Procedure for the Synthesis of 3,7-Dibromo-2,8-
bis(alkoxy)dibenzothiophene (6). To an ice bath cooled
suspension of LiAlH4 in dry diethyl ether (100 mL) was added a
solution of 2,7-dibromo-3,6-bis(alkoxy)dibenzothiophene-S,S-dioxide
(5) in dry diethyl ether (200 mL). The mixture was stirred under a
nitrogen atmosphere for 20 min at 0 °C. Ethyl acetate (50 mL) was
added to quench the reaction, and the solvent was moved under
reduced pressure. The residue was partitioned between chloroform
150 (mL) and dilute hydrochloric acid aqueous solution (100 mL, 0.01
M), the organic layer was separated, and the aqueous layer was
extracted with chloroform (100 mL × 3). The combined organic layers
were washed with water (200 mL), dried over anhydrous MgSO4, and
evaporated to dryness. The residue was purified by flash column
chromatography on silica gel eluting with petroleum ether (60−90 °C)
to afford the desired product 6.
Synthesis of 2,8-Bis(2-ethylhexyloxy)dibenzothiophene-S,S-diox-
ide (4b). 3,6-Dioxylbenzothiophene-S,S-dioxide (3) (6.00 g, 24.17
mmol), 2-ethylhexyl bromide (11.67 g, 60.43 mmol), potassium
carbonate (10.00 g, 72.46 mmol), and butanone (150 mL) were used.
1
4b was obtained in a yield of 76% (8.68 g). H NMR (400 MHz,
CDCl3) δ: 7.61−7.59 (d, 2H), 7.11−7.10 (d, 2H), 6.89−6.86 (dd,
4H), 3.87−3.86 (d, 4H), 1.45−1.41 (m, 2H), 1.40−1.20 (m, 18H),
0.89−0.85(t, 6H), 0.84−0.80 (t, 6H). 13C NMR (100 MHz, CDCl3) δ:
163.85, 133.67, 130.38, 123.36, 115.86, 107.41, 71.18, 65.26, 41.96,
39.30, 30.43, 30.12, 29.11, 29.05, 23.80, 23.34, 23.08, 23.00, 14.06,
11.11, 11.09. Anal. Calcd for C28H40O4S: C, 71.15; H, 9.53. Found: C,
71.77; H, 9.92.
Synthesis of 2,8-Bis(octyloxy)dibenzothiophene-S,S-dioxide (4c).
3,6-Dioxylbenzothiophene-S,S-dioxide (3) (8.40 g, 33.8 mmol), n-
octyl bromide (16.34 g, 84.6 mmol), potassium carbonate (14.01 g,
101.4 mmol), and butanone (200 mL) were used. 4c was obtained in a
yield of 92% (15.98 g). 1H NMR (400 MHz, CDCl3) δ: 7.61−7.59 (d,
2H), 7.08−7.07 (d, 2H), 6.88−6.86 (dd, 2H), 3.99−3.95 (t, 4H),
1.78−1.71 (m, 4H), 1.44−1.37 (m, 4H), 1.33−1.18 (m, 16H), 0.84−
0.80 (t, 6H). 13C NMR (100 MHz, CDCl3) δ: 163.65, 133.69, 130.43,
123.44, 115.81, 107.41, 68.78, 31.79, 29.30, 29.21, 29.03, 25.95, 22.65,
14.10. Anal. Calcd for C28H40O4S: C 71.15, H 9.53. Found: C 71.41, H
9.67.
Synthesis of 3,7-Dibromo-2,8-bis(dodecyloxy)dibenzothiophene
(6a). 5a (1.8 g, 2.42 mmol) and LiAlH4 (0.368 g, 9.69 mmol) were
used, and 6a was obtained as a colorless solid (1.60 g, 93%). 1H NMR
(400 MHz, CDCl3) δ: 7.85 (s, 2H), 7.35 (d, 2H), 4.08−4.04 (t, 4H),
1.88−1.81 (m, 4H), 1.52−1.46 (m, 4H), 1.33−1.19 (m, 32H), 0.82−
0.79 (t, 6H). 13C NMR (100 MHz, CDCl3) δ: 152.35, 134.02, 131.72,
125.77, 111.77, 103.81, 68.76, 30.91, 28.67, 28.64, 28.60, 28.58, 28.37,
28.34, 28.18, 25.07, 21.67, 13.09. Anal. Calcd for C36H54Br2O2S: C
60.84, H 7.66. Found: C 61.02, H 7.95.
Synthesis of 3,7-Dibromo-2,8-bis(2-ethylhexyloxy)-
dibenzothiophene (6b). 5b (4.00 g, 6.34 mmol) and LiAlH4 (0.96
g, 25.38 mmol) were used, and 6b was obtained as a colorless solid
1
(3.42 g, 90%). H NMR (400 MHz, CDCl3) δ: 7.95 (s, 2H), 7.46 (s,
2H), 4.07−4.05 (d, 4H), 1.89−1.85 (m, 2H), 1.67−1.56 (m, 18H),
1.54−1.51 (t, 6H), 1.49−1.45 (t, 6H). 13C NMR (100 MHz, CDCl3
δ):153.55, 135.06, 132.62, 126.79, 112.86, 104.53, 71.91, 39.57, 30.63,
29.19, 24.05, 23.06, 14.11, 11.29. Anal. Calcd for C28H38Br2O2S: C
56.19, H 6.40. Found: C 56.42, H 6.69.
General Procedure for the Synthesis of 3,7-Dibromo-2,8-
bis(alkoxy)dibenzothiophene-S,S-dioxide (5). To an ice bath
cooled solution of 2,8-bis(alkoxy)dibenzothiophene-S,S-dioxide and a
small amount of iodine in chloroform (200 mL) was added dropwise a
solution of bromine in chloroform (60 mL) over 0.5 h. The mixture
was stirred at room temperature for 6 h. An aqueous solution of
Na2SO3 (10%, 100 mL) was added to quench the excess Br2. The
organic layer was separated, and the aqueous one was extracted with
CHCl3. The combined organic layers were washed with water, dried
over with anhydrous MgSO4, and evaporated to dryness. The residue
was recrystallized from a solvent mixture of dichloromethane and
petroleum ether (2:1, v/v) to afford 5.
Synthesis of 3,7-Dibromo-2,8-bis(octyloxy)dibenzothiophene
(6c). 5c (10.00 g, 15.86 mmol) and LiAlH4 (2.41 g, 63.44 mmol)
1
were used, and 6c was obtained as a colorless solid (9.02 g, 95%). H
NMR (400 MHz, CDCl3) δ: 7.87 (s, 1H), 7.38 (s, 1H), 4.10−4.06 (t,
4H), 1.88−1.81 (m, 4), 1.53−1.46 (m, 4H), 1.36−1.18 (m, 16H),
0.84−0.81 (t, 6H). 13C NMR (100 MHz, CDCl3) δ: 153.38, 135.05,
132.76, 126.81, 112.80, 104.87, 104.85, 69.79, 31.82, 29.69, 29.33,
29.24, 29.18, 26.08, 22.67, 14.10. Anal. Calcd for C28H38Br2O2S: C
56.19, H 6.40. Found: C 56.33, H 6.68.
General Procedure for the Synthesis of 3,7-Bis(thiophene-2-
yl)-2,8-bis(alkoxy)dibenzothiophene (7). A mixture of 3,7-
dibromo-2,8-bis(alkoxy)dibenzothiophene (6), 4,4,5,5-tetramethyl-2-
(thiophen-2-yl)-1,3,2-dioxaborolane, K2CO3, toluene, water, and
tetrabutylammonium bromide (TBAB) was carefully degassed before
and after Pd(PPh3)4 was added. The reaction mixture was stirred at
120 °C under a nitrogen atmosphere for 3 days. The organic layer was
separated, the aqueous one was extracted with CH2Cl2, and the
combined organic layers were dried over anhydrous MgSO4 and
evaporated to dryness. The residue was chromatographically purified
on silica gel column eluting with CH2Cl2/petroleum ether (60−90
°C) (1:10, v/v).
Synthesis of 3,7-Dibromo-2,8-bis(dodecyloxy)dibenzothiophene-
S,S-dioxide (5a). 2,8-Bis(octyloxy)dibenzothiophene-S,S-dioxide (2.00
g, 3.42 mmol) and bromine (2.19 g, 13.68 mmol) were used. 5a was
1
obtained as a colorless crystal in a yield of 81% (2.06 g). H NMR
(400 MHz, CDCl3) δ: 7.85−7.84 (d, 2H), 7.01−6.99 (d, 2H), 4.15−
4.12 (t, 4H), 1.95−1.88 (m, 4H), 1.60−1.52 (m, 4H), 1.41−1.27 (m,
32H), 0.90−0.87 (t, 6H). 13C NMR (100 MHz, CDCl3) δ: 159.94,
131.89, 130.33, 126.63, 114.56, 104.49, 70.01, 31.94, 29.69, 29.66,
29.60, 29.55, 29.36, 29.33, 28.97, 26.03, 22.70, 14.11. Anal. Calcd for
C36H54Br2O4S: C 58.22, H 7.33. Found: C 58.43, H 7.58.
Synthesis of 3,7-Dibromo-2,8-bis(2-ethylhexyloxy)-
dibenzothiophene-S,S-dioxide (5b). 2,8-Bis(2-ethylhexyloxy)-
dibenzothiophene-S,S-dioxide (6.00 g, 12.69 mmol) and bromine
(8.12 g, 50.77 mmol) were used. 5b was obtained as a colorless crystal
1
in a yield of 78% (6.24 g). H NMR (400 MHz, CDCl3) δ: 7.63 (s,
3,7-Bis(thiophene-2-yl)-2,8-bis(dodecyloxy)dibenzothiophene
(7a). 3,7-Dibromo-2,8-bis(dodecyloxy)dibenzothiophene (6a) (5.00 g,
8.35 mmol), 4,4,5,5-tetramethyl-2-(thiophen-2-yl)-1,3,2-dioxaborolane
2H), 6.84 (s, 2H), 3.94−3.93 (d, 2H), 1.79−1.74 (m,2H), 1.56−1.45
(m, 18H), 1.43−1.40 (t, 6H), 1.35−1.31 (t, 6H). 13C NMR (100
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(7.02 g, 33.42 mmol), K2CO3 (11.53 g, 83.55 mmol), toluene (150
mL), water (50 mL), TBAB (1.0 g, 3.1 mmol), and Pd(PPh3)4 (193.2
mg, 0.17 mmol) were used. 7a was obtained as a yellow solid (5.80 g,
washed with brine, dried over anhydrous MgSO4, and evaporated to
dryness under reduced pressure. The residue was recrystallized from
acetone to afford 9 as a colorless solid (7.63 g, 72%). H NMR (400
MHz, DMSO) δ: 8.06 (s, 2H), 8.00 (s, 2H), 7.87 (m, 4H), 4.22−4.19
(t, 4H), 1.88−1.81 (m, 4H), 1.52−1.45 (m, 4H), 1.37−1.28 (m, 16H),
0.88−0.87 (t, 6H). 13C NMR (100 MHz, DMSO) δ: 160.67, 137.82,
131.75, 129.44, 104.59, 68.30, 31.18, 28.72, 28.66, 28.63, 25.58, 22.05,
13.89. Anal. Calcd for C28H42B2O6S: C 63.65, H 8.01. Found: C 63.95,
H 8.92.
1
1
91%). H NMR (400 MHz, CDCl3) δ: 7.98 (s, 2H), 7.59−7.53 (d,
2H), 7.46 (s, 2H), 7.31−7.29 (d, 2H), 7.06−7.04 (t, 2H), 4.17−1.14
(t, 4H), 1.93−1.90 (m, 4H), 1.54−1.48 (m, 4H), 1.46−1.20 (m, 32H),
0.83−0.79 (t, 6H). 13C NMR (100 MHz, CDCl3) δ: 152.48, 138.38,
134.13, 131.82, 125.73, 124.91, 124.68, 123.01, 120.97, 103.35, 68.47,
30.92, 28.68, 28.65, 28.61, 28.57, 28.43, 28.34, 25.33, 21.67, 13.08.
Anal. Calcd for C44H60O2S3: C 73.69, H 8.43. Found: C 73.94, H 8.72.
3, 7-Bis(thiophene-2-yl)-2, 8-bis(2-ethylhexyloxy)-
dibenzothiophene (7b). 3,7-Dibromo-2,8-bis(2-ethylhexyloxy)-
dibenzothiophene (6b) (3.50 g, 5.85 mmol), 4,4,5,5-tetramethyl-2-
(thiophen-2-yl)-1,3,2-dioxaborolane (4.91 g, 23.40 mmol), K2CO3
(8.00 g, 57.97 mmol), toluene (50 mL), water (15 mL), TBAB
(1.00 g, 3.10 mmol), and Pd(PPh3)4 (135 mg, 0.117 mmol) were used.
7b was obtained as a yellow solid (3.15 g, 89%). 1H NMR (400 MHz,
acetone-d6) δ: 8.24 (s, 2H), 8.04 (s, 2H), 7.72−7.71 (d, 2H), 7.54−
7.52 (d, 2H), 7.15−7.14 (t, 2H), 4.18−4.17 (d, 4H), 1.91−1.88 (m,
2H), 1.71−1.27 (m, 16H), 1.00−0.96 (t, 6H), 0.92−0.89 (t, 6H). 13C
NMR (100 MHz, CDCl3) δ: 154.61, 139.93, 136.38, 133.44, 127.55,
127.03, 126.70, 124.74, 122.61, 105.84, 72.18, 40.52, 31.54, 24.88,
23.71, 14.36, 11.54. Anal. Calcd for C36H44O2S3: C 71.48, H 7.33.
Found: C 71.17, H 7.63.
General Procedure for the Synthesis of 3,7-Linked 2,8-
Bis(alkoxy)dibenzothiophene-Containing Conjugated Poly-
mers (P1, P2, and P3). A mixture of dibromide monomer 8, 4,7-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzo[c][1,2,5]-
thiadiazole (10), THF, toluene, H2O, and NaHCO3 was carefully
degassed before and after Pd(PPh3)4 was added. The mixture was
stirred and refluxed under nitrogen for 3 days. Phenylboronic acid and
Pd(PPh3)4 were added and refluxed for 4 h; after that bromobenzene
was added, and the mixture was refluxed overnight to complete the
end-capping reaction. After being cooled to room temperature, water
and chlorobenzene were added, and the organic layer was separated
and washed three times with water. And then, the solution was heated
to 100 °C to dissolve the polymers and filtered. After the removal of
most solvent, the residue was poured into a large amount of acetone,
and the resulting precipitates were collected by filtration and washed
with acetone. The crude product was redissolved in a minimum
amount of chlorobenzene and precipitated into a large amount of
acetone. The formed precipitates were collected by filtration and dried
under high vacuum.
Poly[3,7-(2,8-bis(dodecyloxy)dibenzothiophene-alt-5′,5″-di-
(thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (P1). 8a (114 mg, 0.13
mmol), 10 (51 mg, 0.13 mmol), THF (10 mL), toluene (10 mL),
H2O (2 mL), NaHCO3 (0.2 g, 2.4 mmol), and Pd(PPh3)4 (3 mg, 2.6
μmol) were used for Suzuki polycondensation. Phenylboronic acid (20
mg, 0.16 mmol), Pd(PPh3)4 (3.03 mg, 2.6 μmol), and bromobenzene
(20 μL, 1.91 mmol) were used for the end-capping reaction. P1 was
obtained as a dark red solid in a yield of 47% (52 mg). Since P1 has
poor solubility in CHCl3 or chlorobenzene (CB) at room temperature,
13C NMR is difficult to obtain. 1H NMR (400 MHz, C6D6) δ: 8.42 (br,
General Procedure for the Synthesis of 3,7-Bis(5-bromo-
thiophene-2-yl)-2,8-bis(alkoxy)dibenzothiophene (8). A mix-
ture of 3,7-bis(thiophene-2-yl)-2,8-bis(alkoxy)dibenzothiophene (7),
N-bromosuccimide (NBS), chloroform, and acetic acid was stirred at
room temperature in dark for 24 h. The reaction mixture was
partitioned between aqueous NaHCO3 solution (0.5 M) and
chloroform, the organic layer was separated, and the aqueous layer
was extracted with chloroform for several times. The combined
organic layers were washed with aqueous NaHCO3 solution for several
times, dried over anhydrous MgSO4, and evaporated to dryness. The
residue was chromatographically purified on silica gel column eluting
with CH2Cl2/petroleum ether (1:10, v/v) to afford compound 8.
3,7-Bis(5-bromothiophene-2-yl)-2,8-bis(dodecyloxy)-
dibenzothiophene (8a). 3,7-Bis(thiophene-2-yl)-2,8-bis(dodecyloxy)-
dibenzothiophene (7a) (5.00 g, 6.56 mmol), NBS (4.82 g, 16.94
mmol), chloroform (150 mL), and acetic acid (30 mL) were used. 8a
2H), 8.30 (br, 2H), 7.80 (br, 2H), 7.75 (br, 2H), 7.74 (br, 2H), 4.08
(br, 4H), 2.04 (br, 4H), 1.70 (br, 4H), 1.38 (br, 32H), 1.03−1.00 (br,
6H). Anal. Calcd for C50H62N2O2S4: C 70.54, H 7.34, N 3.29. Found:
C 71.27, H 7.53, N 2.82. GPC (PS standards): Mw = 6.4 kg/mol, Mn =
4.7 kg/mol, PDI = 1.4.
1
was obtained as a yellow solid (5.36 g, 85%). H NMR (400 MHz,
CDCl3) δ: 7.91 (s, 2H), 7.42 (s, 2H), 7.26−7.25 (d, 2H), 7.00−6.99
(d, 2H), 4.17−4.14 (t, 4H), 1.96−1.89 (m, 4H), 1.56−1.50 (m, 4H),
1.48−1.20 (m, 32H), 0.83−0.81 (t, 6H). 13C NMR (100 MHz,
CDCl3) δ: 152.93, 140.41, 135.14, 132.75, 129.26, 124.85, 123.08,
120.63, 113.33, 103.93, 69.51, 31.95, 29.73, 29.69, 29.67, 29.62, 29.49,
29.39, 29.24, 26.36, 22.71, 14,14. Anal. Calcd for C44H58Br2O2S3: C
60.40, H 6.68. Found: C 60.64, H 6.93.
3,7-Bis(5-bromothiophene-2-yl)-2,8-bis(2-ethylhexyloxy)-
dibenzothiophene (8b). 3,7-Bis(thiophene-2-yl)-2,8-bis(2-
ethylhexyloxy)dibenzothiophene (7b) (517 mg, 0.85 mmol), NBS
(312 mg, 1.75 mmol), chloroform (50 mL), and acetic acid (5 mL)
were used. 8c was obtained as a yellow solid (554 mg, 85%). 1H NMR
(400 MHz, CDCl3) δ: 7.89 (s, 2H), 7.43 (s, 2H), 7.23−7.22 (d, 2H),
6.99−6.98 (d, 2H), 4.06−4.04 (d, 4H), 1.91−1.85 (m, 2H), 1.61−1.45
(m, 16H), 1.36−1.34 (t, 6H), 1.33−1.31 (t, 6H). 13C NMR (100
MHz, CDCl3) δ: 153.42, 139 Anal. Calcd for C36H42Br2O2S3: C 56.69,
H 5.55. Found: C 56.93, H 5.87.
Synthesis of 2,8-Bis(octyloxy)dibenzothiophene-3,7-diboronic
Acid (9). To a solution of 6c (12.0 g, 20.1 mmol) in dry diethyl
ether (150 mL) was added dropwise n-BuLi (20.05 mL, 2.5 M in
hexane, 50.13 mmol) at −78 °C under a nitrogen atmosphere. The
mixture was stirred at −78 °C for 30 min, allowed to warm to room
temperature (rt) gradually, and then stirred at rt for a further 50 min.
The solution was then cooled down to −78 °C again, triisopropyl
borate (13.97 mL, 60.16 mmol) was added rapidly, and the resulted
mixture was stirred overnight and allowed to warm to rt gradually. The
mixture was partitioned between aqueous dilute hydrochloric acid and
diethyl ether, the organic layer was separated, and the aqueous layer
was extracted with diethyl ether (200 mL × 3). The organic layer was
Poly[3,7-(2,8-bis(2-ethylhexyloxy)dibenzothiophene-alt-5′,5″-di-
(thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (P2). 8b (200 mg, 0.26
mmol), 10 (102 mg, 0.26 mmol), THF (10 mL), toluene (10 mL),
H2O (2 mL), NaHCO3 (0.3 g, 3.57 mmol), and Pd(PPh3)4 (6 mg, 5.2
μmol) were used for Suzuki polycondensation. Phenylboronic acid (20
mg, 0.16 mmol), Pd(PPh3)4 (6 mg, 5.2 μmol), and bromobenzene (20
μL, 1.91 mmol) were used for the end-capping reaction. P2 was
obtained as a dark red solid in a yield of 63% (122 mg). 1H NMR (400
MHz, C6D6) δ: 8.51 (br, 2H), 8.17 (br, 2H), 8.08 (br, 2H), 7.92 (br,
2H), 7.60 (br, 2H), 4.53 (br, 4H), 2.25−1.82 (br, 2H), 1.66−1.55 (br,
16H), 1.31−1.29 (br, 6H), 1.23−1.13 (br, 6H). 13C NMR (100 MHz,
C6D6) δ: 154.14, 153.02, 139.81, 134.84, 134.15, 126.17, 125.96,
104.38, 71.87, 40.08, 31.07, 29.59, 29.39, 24.51, 24.35, 23.43, 23.29,
14.33, 11.54, 11.38, 1.24. Anal. Calcd for C42H46N2O2S4: C 68.25, H
6.27, N 3.79. Found: C 68.83, H 6.45, N 3.27 GPC (PS standards):
Mw = 16.3 kg/mol, Mn = 14.6 kg/mol, PDI = 1.2.
Poly[3,7-(2,8-bis(octyloxy)dibenzothiophene-alt-5′,5″-di-
(thiophen-2-yl)-5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole] (P3). A
mixture of 9 (471 mg, 0.89 mmol) and 4,7-bis(5-bromothiophen-2-
yl)-5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole (11) (637 mg, 0.89
mmol), THF (30 mL), H2O (6 mL), and NaHCO3 (1.50 g, 17.83
mmol) was carefully degassed before and after Pd(PPh3)4 (20.6 mg,
17.83 μmol) was added. The mixture was stirred and refluxed under
nitrogen for 3 days. Phenylboronic acid (50 mg, 0.41 mmol) and
Pd(PPh3)4 (20.6 mg, 17.83 μmol) were added and refluxed for 4 h;
after that bromobenzene (500 μL, 47.76 mmol) was added, and the
mixture was refluxed overnight to complete the end-capping reaction.
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Scheme 1. Synthesis of Monomers and Copolymers
After being cooled to room temperature, water and chlorobenzene
were added, and the organic layer was separated and washed three
times with water. And then, the solution was heated to 100 °C to
dissolve the polymers and filtered. After the removal of most solvent,
the residue was poured into a large amount of acetone, and the
resulting precipitates were collected by filtration and washed with
acetone. The crude product was redissolved in a minimum amount of
chlorobenzene and precipitated into a large amount of acetone. The
formed precipitates were collected by filtration and dried in high
vacuum. P3 was obtained as a dark red solid in a yield of 86% (762
mg). 1H NMR (400 MHz, C6D6) δ: 8.96 (br, 2H), 8.45 (br, 2H), 8.09
(br, 2H), 7.94 (br, 2H), 4.46 (br, 8H), 2.29−2.20 (br, 8H), 1.76−1.69
(br, 8H), 1.54−1.41 (br, 32H), 1.04−1.02 (br, 12H). 13C NMR (100
MHz, C6D6) δ: 154.10, 152.09, 151.33, 141.58, 135.66, 135.03, 134.80,
126.16, 124.40, 121.87, 117.93, 104.64, 69.54, 32.20, 32.16, 31.00,
30.07, 29.84, 29.79, 29.67, 26.65, 26.53, 23.02, 14.33, 1.24, 1.19. Anal.
Calcd for C58H78N2O4S4: C 69.98, H 7.90, N 2.81. Found: C 69.10, H
7.97, N 2.08. GPC (PS standards): Mw = 86.6 kg/mol, Mn = 27.7 kg/
mol, PDI = 3.1.
RESULTS AND DISCUSSION
■
Material Synthesis and Characterization. The syntheses
of the monomers and polymers are outlined in Scheme 1. 2,8-
Dibromodibenzothiophene was reacted with n-BuLi at −78 °C
followed by quenching the anions with triisopropyl borate to
afford the crude dibenzothiophene-2,8-diboronic acid (2),
which was used for the next step without further purification.
The oxidation of 2 with H2O2 in acetic acid afforded 2,8-
dihydroxydibenzothiophene-S-dioxide (3) in a total yield of
92%. Williamson etherification with alkyl bromides and K2CO3
in butanone afforded 2,8-bis(alkoxy)dibenzothiophene-S,S-
dioxides (4a−c) in yields of 76−92%. Bromination of 4a−c
with bromine in chloroform at room temperature furnished 3,7-
dibromo-2,8-bis(alkoxy)dibenzothiophene-S,S-dioxides (5a−c)
in yields of 78−85%. The reduction of 5a−c with LiAlH4 in
diethyl ether at 0 °C afforded 3,7-dibromo-2,8-bis(alkoxy)-
dibenzothiophene (6a−c) in yields of 90−95%. Suzuki cross-
coupling of 6a−c and 4,4,5,5-tetramethyl-2-(thiophen-2-yl)-
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Table 1. Physical, Electronic, and Optical Properties of P1, P2, and P3
a
a
b
c
polymer
Mn (kg/mol)
Mw (kg/mol)
PDI
Tg (°C)
λmax [nm] solution
λmax [nm] film
Eg,opt (eV)
HOMO (eV)
LUMO (eV)
P1
P2
P3
4.7
14.6
27.7
6.4
16.3
86.6
1.4
1.2
3.1
383
391
324
401, 530
400, 529
401, 532
408, 566
414, 567
402, 560
1.80
1.79
1.98
−5.23
−5.29
−5.34
−3.43
−3.50
−3.37
a
Mn and PDI of the polymers were determined by GPC using polystyrene standards at 150 °C with 1,2,4-trichlorobenzene as an eluent.
b
c
Decomposition temperature determined by TGA in N2 based on 5% weight loss. Calculated from the absorption band edge of the copolymer film,
Eg,opt = 1240/λedge
.
Figure 1. UV−vis absorption spectra of P1−3 in CB solution (a) and in film (b) and P1−3/PC71BM blending film (1:3 by weight) (c).
1,3,2-dioxaborolane was carried out in a biphasic mixture of
aqueous K2CO3 and toluene with Pd(PPh3)4 as the catalyst
precursor and tetrabutylammonium bromide as the phase
transfer catalyst to afford 3,7-bis(thiophene-2-yl)-2,8-bis-
(alkoxy)dibenzothiophene (7a−c) in yields of 89−91%.
Bromination of 7a−c with NBS in a solvent mixture of
chloroform and acetic acid at room temperature afforded the
desired dibromide monomers (8a−c) in yields 85−89%. 2,8-
Bis(alkoxy)dibenzothiophene containing conjugated polymers
P1 and P2 were synthesized as dark red solids in yields of 47
and 63%, respectively, by Suzuki polymerization of dibromide
monomers 8a and 8b with diboronic ester monomer 4,7-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzo[c]-
[1,2,5]thiadiazole (10) in a biphasic mixture of toluene/
aqueous NaHCO3 with freshly prepared Pd(PPh3)4 as the
catalyst precursor. After the polymerization, phenylboronic acid
and bromobenzene were added successively in an interval of 4
h to cap the end groups. Low yields for the polymerization are
probably due to the formation of dark red precipitates during
the polymerization, which cannot be fully dissolved even in
chlorobenzene. It is worth noting that the obtained P1−2 are
soluble in chlorobenzene (CB), 1,2-dichlorobenzene (DCB),
and 1,2,4-trichlorobenzene at elevated temperature. The
molecular weight and molecular weight distribution of P1−2
were determined by gel permeation chromatography (GPC) at
150 °C using 1,2,4-trichlorobenzene as an eluent calibrated
with polystyrene standards and are summarized in Table 1. It
should be noted that polymers P1−2 only carry two flexible
alkyl chains on the dibenzothiophene at each repeating unit. To
increase the molecular weight and solubility, conjugated
polymer P3 carrying four alkyl chains at each repeating unit
was designed and synthesized. Miyaura reaction of 4,7-
dibromo-5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole with bis-
(pinacolato)diboron is failed to afford the desired 4,7-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-5,6-bis-
(octyloxy)benzo[c][1,2,5]thiadiazole. Therefore an alternative
synthetic route was adopted to synthesize polymer P3 as shown
in Scheme 1. 3,7-Dibromo-2,8-bis(octyloxy)dibenzothiophene
(6d) was converted to the corresponding 2,8-bis(octyloxy)-
dibenzothiophene-3,7-diboronic acid (9) by treatment with n-
Buli at −78 °C and followed by quenching the formed anions
with triisopropyl borate. P3 was synthesized as a dark red solid
in a yield of 86% by Suzuki polycondensation of diboronic acid
9 and 4,7-bis(5-bromothiophen-2-yl)-5,6-bis(octyloxy)benzo-
[c][1,2,5]thiadiazole (11). As expected, polymer P3 had higher
molecular weight, and it was soluble in organic solvents such as
chloroform, CB, and DCB. As shown in Table 1, P3 showed a
number-average molecular weight (Mn) of 27.7 kg/mol, a
weight-average molecular weight (Mw) of 86.6 kg/mol, and a
polydispersity index (PDI) of 3.1. Thermogravimetric analysis
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Figure 2. (a) J−V curves for the BHJ solar cells derived from the blend of polymer:PC71BM (1:3, by weight) and (b) EQE curves for the BHJ solar
cells derived from the blend of polymer:PC71BM (1:3, by weight).19
(TGA) indicated that P1−3 have good thermal stability with
the decomposition temperature up to 300 °C under a nitrogen
atmosphere. Differential scanning calorimetry (DSC) revealed
that there is no obvious glass transition for P1−3. Hole
mobility of narrow band gap conjugated polymers is closely
related to their packing in the solid state. Therefore, X-ray
diffractions (XRD) of powdery samples of P1−3 were
measured. As shown in Figure S1 (see Supporting Informa-
tion), all polymers exhibit two diffraction peaks: the first peak at
small-angle region reflecting the distance between polymer
backbones separated by alkyl side chains and the peak at wide-
angle region reflecting the π−π distance between the polymer
backbones. The distances between polymer backbones
separated by alkyl chains are 22.09 Å for P1, 18.82 Å for P2,
and 18.58 Å for P3. The π−π distances between the polymer
backbones are 4.29 Å for P1, 4.62 Å for P2, and 4.20 Å for P3.
Such short distances indicate that the polymer backbones are of
a relatively planar conformation and can be closely stacking in
the solid state.15
Optical and Electrochemical Properties. The UV−vis
absorption spectra of polymers P1−3 in dilute CB solutions
and as thin films at 25 °C are shown in Figure 1, and the data
are also summarized in Table 1. In dilute CB solutions, P1 and
P2 displayed a broad absorption with two peaks located at
about 401 and 530 nm, respectively. As shown in Table S1 (see
Supporting Information), molar extinction coefficient values of
P1 and P2 in solutions at 530 nm are 6.02 × 104 and 5.04 × 104
M−1 cm−1, respectively. The molar absorption coefficient of the
high-energy peak is larger than the low-energy one. Different
from P1 and P2, P3 in dilute CB solution exhibited a weak
absorption peak at 401 nm and an intense absorption peak at
532 nm. The molar extinction coefficient of P3 in solution at
530 nm is 9.94 × 104 M−1 cm−1, and the data are also
summarized in Table S1. It is worthy noting that the molar
absorption coefficient of the high-energy peak is smaller than
the low-energy one. On going from solution to film, the
absorption spectra of all polymers became broader and red-
shifted as shown in Figure 1b. Especially, the absorption of P1
and P2 in films is much broader than that of P3. The
absorption onsets of P1, P2, and P3 in films are 690, 694, and
627 nm, respectively. The optical band gaps (Eg,opt) of P1, P2,
and P3 in films were therefore calculated to be 1.80, 1.79, and
1.98 eV, respectively. The band gap of P3 is larger than that of
P1 and P2 mainly because of the electronic effect.
Unsubstituted benzothiadiazole is a strong acceptor unit; the
attaching two electron-donating alkoxy chains on the
benzothiadiazole ring will weaken its electron withdrawing
ability, and as a result the low-energy absorption peak of P3 in
solution should be blue-shifted in comparison with that of P1
and P2, namely, in solution the optical band gaps of P1 and P2
should be narrower than that of P3. However, the S−O
electronic interaction of P3 will lead to a planarization of the
polymer backbone,16 and as a result, the absorption spectrum
of P3 will red-shift and the optical band gap of P3 in solution
will decrease. Considering these two effects, the optical band
gaps of P1, P2, and P3 in solutions happen to be almost the
same as shown Figure 1a. On going from solution to film, the
polymer backbone of P1 and P2 could become more planar
due to the π−π stacking, resulting in a drastic decreasing of
their optical band gap. For P3, due to the S−O interaction, the
polymer backbone is probably already of a more planar
structure in solution. On going from solution to film, the
packing of polymer chains cannot cause a significant decreasing
of the optical band gap. Therefore, in films the band gap of P3
should be higher than that of P1 and P2. The HOMO energy
level was determined by cyclic voltammogram according to the
equation EHOMO = −e(Eox + 4.71) (eV) and the LUMO energy
17
level was calculated by the equation ELUMO = EHOMO + Eg,opt
.
HOMO and LUMO energy levels were therefore determined
to be −5.23 and −3.43 eV for P1, −5.29 and −3.50 eV for P2,
and −5.34 and −3.37 eV for P3. The data are summarized in
Table 1. Considering the LUMO level of PCBM is −4.2 eV, the
offsets between donor polymers P1−3 and the acceptor PCBM
are in the range of 0.7−0.9 eV,2f,4a,18 which should provide an
enough driving force for efficient exciton dissociation. As
shown in Figure 1c, the P1−3/PC71BM (1:3 by weight) blend
films demonstrate a broad absorption in the range of 300−700
nm.
Photovoltaic Properties. Devices used for the evaluation the
photovoltaic performances of polymers were fabricated with a
device configuration of ITO/PEDOT:PSS/Polymer:PC71BM/
LiF/Al. The thickness of PEDOT:PSS layer, LiF layer, and Al
electrode is about 40, 0.6, and 100 nm, respectively. The
photovoltaic properties of P1−3 were screened by blending
P1−3 and PC71BM in DCB in different weight ratios, different
concentration, and different spin-coating speeds with or
without 1,8-diiodooctane (DIO) as a processing additive.
After the optimization, solar cells fabricated with the blends of
polymer and PC71BM in a weight ratio of 1:3 and with DCB as
the processing solvent gave the best performance. The typical
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Table 2. Photovoltaic Parameters of PSCs Based on P1−3 as Donor and PC71BM as Acceptor with a Weight Ratio of 1:3 under
the Illumination of AM1.5G, 100 mW/cm2
b
active layer
solvent
thickness (nm)
Voc (V)
Jsc (mA/cm2)
FF
PCE (%) (max)
PCE (%) (av)
μhole (cm2 V−1 s−1
)
P1:PC71BM
P2:PC71BM
P3:PC71BM
P3:PC71BM
DCB
DCB
DCB
92
95
84
80
0.79
0.72
0.79
0.83
2.60
6.80
9.07
9.30
0.51
0.35
0.59
0.58
1.02
1.71
4.19
4.48
0.99
1.69
1.3 × 10−4
2.7 × 10−4
2.3 × 10−3
a
DCB
4.32
a
b
0.5% DIO additive (v/v). Measured by using the organic field-effect transistor (OFET) method.
Figure 3. Output curves at different gate voltages (VG) for OFETs using spin-coated P1 (a), P2 (b), and P3 (c) on OTS-treated Si/SiO2 substrate.
current density−voltage (J−V) curves of photovoltaic cells with
the blends of P1−3/PC71BM (1:3, by weight) as the active
layer under 1 sun of simulated AM 1.5G solar radiation (100
mW/cm2) are shown in Figure 2. The device results of each
polymer-based solar cells are summarized in Table 2. The film
spin-coated from P1:PC71BM (1:3, by weight) in DCB solution
show extremely rough surface with visible particles, which
might be due to the poor solubility of P1 in DCB solution.
Therefore, the device fabricated from P1:PC71BM (1:3, by
weight) in DCB solution provided unsatisfactory result. PCE of
1.02% with a Voc of 0.79 V, a Jsc of 2.60 mA/cm2, and an FF of
0.51 was achieved for P1-based sloar cells. For P2, the
incorporation of branched side chains endowed the polymer
with better solubility in solutions than P1, the photovoltaic
performance was enhanced a little with PCE of 1.71% with a
Voc of 0.72 V, a Jsc of 6.80 mA/cm2, and an FF of 0.35 by the
device fabricated from P2:PC71BM (1:3, by weight) in DCB
solution. It is worth mentioning that P3-based devices exhibited
higher PCE of 4.19% with a Voc of 0.79 V, a Jsc of 9.07 mA/cm2,
and an FF of 0.59. Significantly, with the addition of DIO as the
additive into the processing solvent, the PCE of P3 based on
solar cells could be further increased to 4.48% with a Voc of 0.83
V, a Jsc of 9.30 mA/cm2, and an FF of 0.58. The best
photovoltaic performance for P3:PC71BM (1:3, by weight) was
achieved with DCB as the processing solvent, the concentration
of the blend 30 mg/mL, and DIO (0.5 vol%) as the additive.
However, for P1 and P2, the use of additive (DIO or 1-
chloronaphthalene) did not show any positive effect on their
photovoltaic performances. Obviously, higher Jsc for P3-based
solar cells accounts for the better photovoltaic performance
than P1 and P2. Jsc can be improved mainly from the increased
light absorption, the good hole mobility of polymer for charge
transfer, and optimal film nanoscale morphology for charge
separation and transport. Since the molecular weight of donor
polymers can significantly influence the performance of
polymer solar cells, usually the high molecular weight sample
gives better photovoltaic performance than the low molecular
weight sample. To elucidate the influence of molecular weight
on polymer solar cell device performance, we use low molecular
weight sample of P2 to fabricate devices. The low molecular
weight P3 has an Mn of 6060 g/mol, an Mw of 8300 g/mol, and
a PDI of 1.4. Using the same device fabrication conditions like
high molecular weight P3, the low molecular weight P3-based
devices give a worse performance with a PCE of 3.1%, which is
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Figure 4. AFM images (5 × 5 μm2) of the blends of polymer:PC71BM (1:3, by weight) spin-cast from DCB solutions: (a) P1, (b) P2, (c) P3, and
(d) P3 (containing 0.5% DIO in volume).
still markedly higher than that of P2. This result demonstrates
that both the chemical structure and the molecular weight can
influence on the performance of the polymers. Obviously,
higher Jsc for P3 was attributed, at least partially, to the intenser
absorption in the visible region, the higher hole mobility, and
the better nanoscale film morphology of its active layer. The
above results demonstrate that the photovoltaic performance
can be improved by fine-tuning the conjugated polymer
corresponding absorption spectra (Figure 2b) can be observed
for each polymer-based solar cell, which indicates PC71BM
compensates for the absorption valley of the polymers and the
contribution from PC71BM to the photocurrent is very
pronounced. The EQE value of the devices for P3-based
PSCs reached above 50% from nearly the entire visible range of
350−600 nm. Especially, the device for P3-based PSCs with the
addition of DIO (0.5 vol %) exhibited the maximum EQE of
65% at 418 nm. Significantly, all current intensity (Jsc) values
calculated from the integration of the EQEs of the devices agree
well with the Jsc values obtained from the J−V measurements.
Transport Properties. The transport properties of the
polymers were investigated by fabricating organic thin film
field effect transistors (OFETs). Typical p-channel field-effect
transistor behavior was obtained. The hole mobility (μ) was
estimated in the saturated regime from the derivative plots of
the square root of source-drain current (ISD) versus gate voltage
(VG) through equation ISD = (W/2L)Ciμ(VG − VT)2 where W
is the channel width, L is the channel length, Ci is the
structure. Additionlly, the theoretical Voc was estimated
donor
according to the formula Voc = (|E
HOMO| − |EPCBM
LUMO|)/e − 0.3 V2f to be 0.73, 0.79, and 0.84 V for P1, P2,
and P3-based polymer solar cells, respectively, which are
roughly consistent with measured values as shown in Table 2.
To evaluate the accuracy of measurements, the external
quantum efficiency (EQE) curves of the PSC devices fabricated
from the blend of the corresponding polymer and PC71BM in
DCB solution were measured under illumination of mono-
chromatic light. As shown in Figure 3, a significant and broader
photo-to-current response from 300 to 700 nm than the
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capacitance per unit area of the gate dielectric layer (SiO2, 500
nm, Ci = 7.5 nF/cm2), and VT is the threshold voltage. The
output and transfer characteristic curves of the spin-coated film
of P1, P2, and P3 on OTS-treated Si/SiO2 substrates are
shown in Figure 3. The hole mobilities of P1 and P2 film spin-
cast from DCB solution by annealing at 110 °C for 3 min are
1.3 × 10−4 and 2.7 × 10−4 cm2 V−1 s−1, respectively.
Significantly, the hole mobility of the film spin-cast from the
blend of P3 showed better charge transfer ability with the hole
mobility of 2.3 × 10−3 cm2 V−1 s−1 without further device
optimization. The higher hole mobility is attributed to the
stronger intermolecular interactions between the planar
polymer chains that can bring the polymer chain into close
proximity as evidenced by the XRD results. The side chains on
the benzothiadiazole ring will not generate steric hindrance,
since the electrostatic attraction between ether oxygens and
thienyl sulfur atoms of P3 will enhance planarization.16 Closer
distance between the polymer chains can facilitate charge
hopping in the polymer.20 Obviously, the hole mobility of the
film spin-cast from the blend of P3 exhibited 1 order of
magnitude higher than P1 and P2. Such high mobility is
beneficial for charge transfer in the resulting PSCs. The
mobility data are summarized in Table 2. The transport
properties indicate P3 is a promising material for solar cell
applications.
Film Morphologies. The morphology of the blend film is
crucial for the photovoltaic performance, which largely affects
the charge separation and transport. Therefore, the morphol-
ogies of polymer/PC71BM blend films spin-coated from DCB
solutions were investigated by atomic force microscopy (AFM)
using tapping mode. AFM height images of the blend films of
P1−3:PC71BM (1:3, by weight) are shown in Figure 4. The
P1:PC71BM (1:3, by weight) and P2:PC71BM (1:3, by weight)
blend films spin-coated from DCB solutions showed rough
surfaces with larger domain size of several hundered nanome-
ters and the root-mean-square (rms) of 3.56 nm for P1 and
4.77 nm for P2. An interpenetrating bicontinuous network
between polymer and PCBM with an ideal domain size of 10−
20 nm is desirable for high-performance PSCs.21 Such large
phase separation for P1- and P2-based blend films increases the
distance to the nearest interface, leading to a lower photo-
current. The blend films of P3:PC71BM (1:3, by weight) from
DCB show very smooth surfaces which are strikingly different
from that of P1 and P2. No large domain was observed,
indicating no obvious phase separation occurs.22 The root-
mean-square (rms) roughness for the P3:PC71BM (1:3, by
weight) blend films are 0.326 nm, which is lower than P1 or
P2:PC71BM (1:3, by weight) blend films. With the addition of
DIO (0.5 vol %), AFM images still exhibited uniform and finer
nanostructures and rms has been increased a little to 0.384 nm
as shown in Figure 3d. Neither larger nor smaller phase
separation is favorable for efficient charge separation and
transportation. Only appropriate phase separation and domain
size can produce the best photovoltaic performance.
and 530 nm, respectively, and P3 exhibits much stronger ICT
absorption than P1 and P2. The photovoltaic performance of
P3-based polymer solar cell devices is much better than that of
P1- and P2-based devices. Polymer solar cells with the P3/
PC71BM (1:3, by weight) blend as active layer show the best
photovoltaic performance with a PCE of 4.48%. Our results
demonstrate that the photovoltaic performance can be
improved by fine-tuning the structure of conjugated polymers.
ASSOCIATED CONTENT
* Supporting Information
Molar extinction coefficient data, XRD patterns, DSC and TGA
curves, cyclic voltammograms, PL spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: licuihong@bnu.edu.cn (C.L.); zsbo@bnu.edu.cn
(Z.B.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We express thanks for the financial support by the 973
Programs (2011CB935702 and 2009CB623603), the NSF of
China (20834006, 51003006, and 21161160443), and the
Fundamental Research Funds for the Central Universities.
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CONCLUSIONS
■
In summary, narrow band gap conjugated polymers with 3,7-
linked 2,8-bis(alkoxy)dibenzothiophene as the donor unit and
benzothiadiazole (P1 and P2) or 3,4-bis(octyloxy)-
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