Conjugated polymers based on dicarboxylic imide-substituted isothianaphthene and their applications in solar cells

Article abstract of DOI:10.1002/pola.25021

Four new polymers containing a benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic imide (DIITN) unit including the homopolymer and three donor-acceptor copolymers were designed, synthesized, and characterized. For these copolymers, DIITN unit with low bandgap was selected as an electron acceptor, whereas 5,5′-(2,7-bisthiophen-2-yl)-9-(2-decyltetradecyl)-9H-carbazole), 5,5′-(3,3′-di-n-octylsilylene-2,2′-bithiophene), and 5,5′-(2,7-bisthiophen-2-yl-9,9-bisoctyl-9H-fluoren-7-yl) were chosen as the electron donor units to tune the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels of the copolymers for better light harvesting. These polymers exhibit extended absorption in the visible and near-infrared range and are soluble in common organic solvents. The relative low lying HOMO of these polymers promises good air stability and high open-circuit voltage (V_oc) for photovoltaic application. Bulk heterojunction solar cells were fabricated by blending the copolymers with [6,6]-phenyl-C61-butyric acid methyl ester or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The best power conversion efficiency of 1.6% was achieved under simulated sunlight AM 1.5G (100 mW/cm²) from solar cells containing 20 wt % of the fluorene copolymer poly[5,5′-(2,7-bisthiophen-2-yl-9,9-bisoctyl-9H- fluoren-7-yl)-alt-2,9-(benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic imide)] and 80 wt % of PC71BM with a high open-circuit voltage (V_oc) of 0.84 V.

Full text of DOI:10.1002/pola.25021

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Conjugated Polymers Based on Dicarboxylic Imide-Substituted
Isothianaphthene and Their Applications in Solar Cells
Hairong Li,1,2y Shuangyong Sun,1y Teddy Salim,1 Swarnalatha Bomma,1
Andrew C. Grimsdale,1,2 Yeng Ming Lam1,2
1School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue,
Singapore 639798, Singapore
2Energy Research Institute, Nanyang Technological University, Research Technoplaza, Singapore 637553
Correspondence to: A. C. Grimsdale (E-mail: acgrimsdale@ntu.edu.sg) or Y. M. Lam (E-mail: ymlam@ntu.edu.sg)
Received 21 July 2011; accepted 6 September 2011; published online 14 October 2011
DOI: 10.1002/pola.25021
ABSTRACT: Four new polymers containing a benzo[c]thiophene-
N-dodecyl-4,5-dicarboxylic imide (DIITN) unit including the
homopolymer and three donor–acceptor copolymers were
designed, synthesized, and characterized. For these copoly-
mers, DIITN unit with low bandgap was selected as an electron
acceptor, whereas 5,50-(2,7-bisthiophen-2-yl)-9-(2-decyltetra-
for photovoltaic application. Bulk heterojunction solar cells
were fabricated by blending the copolymers with [6,6]-phenyl-
C61-butyric acid methyl ester or [6,6]-phenyl-C71-butyric acid
methyl ester (PC71BM). The best power conversion efficiency
of 1.6% was achieved under simulated sunlight AM 1.5G (100
mW/cm2) from solar cells containing 20 wt % of the fluorene
copolymer poly[5,50-(2,7-bisthiophen-2-yl-9,9-bisoctyl-9H-fluo-
ren-7-yl)-alt-2,9-(benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic
imide)] and 80 wt % of PC71BM with a high open-circuit volt-
decyl)-9H-carbazole),
5,50-(3,30-di-n-octylsilylene-2,20-bithio-
phene), and 5,50-(2,7-bisthiophen-2-yl-9,9-bisoctyl-9H-fluoren-7-
yl) were chosen as the electron donor units to tune the highest
occupied molecular orbital/lowest unoccupied molecular or-
bital (HOMO/LUMO) levels of the copolymers for better light
harvesting. These polymers exhibit extended absorption in the
visible and near-infrared range and are soluble in common or-
ganic solvents. The relative low lying HOMO of these polymers
C
V
age (Voc) of 0.84 V. 2011 Wiley Periodicals, Inc. J Polym Sci
Part A: Polym Chem 50: 250–260, 2012
KEYWORDS: conjugated polymers; dicarboxylic imide; fuller-
promises good air stability and high open-circuit voltage (Voc
)
enes; isothianaphthene; low bandbap; solar cells; thin films
INTRODUCTION The poly(3-hexylthiophene) (P3HT):[6,6]-
phenyl-C61-butyric acid methyl ester (PC61BM) blend has
been reported as one of the most promising systems for
polymer bulk heterojunction (BHJ) solar cells, with demon-
strated power conversion efficiencies (PCEs) for the best
devices of 5–6%.1–4 However, this standard system with
P3HT as donor and PCBM as acceptor is limited by its low
open-circuit voltage Voc (ꢀ0.6 V)5 and unoptimized light harvest-
ing ability (low short-circuit current Jsc) due to the relatively
large bandgap of P3HT.6,7 It has been reported that the Voc of BHJ
organic solar cells can be estimated from the energy difference
between highest occupied molecular orbital (HOMO) of donor
and lowest unoccupied molecular orbital (LUMO) of acceptor.8
Furthermore, the bandgap of polymer donors can be tuned either
by moving the LUMO away from vacuum or moving HOMO level
closer to vacuum to enhance the absorption. However, the LUMO
difference between polymer and fullerene must be at least 0.3 eV
to allow efficient charge dissociation.8 Therefore, an ideal poly-
mer should have a suitable LUMO level that has sufficient driving
force for electron transfer from donor to acceptor and a deep
lying HOMO level to obtain a high solar cell Voc while maintaining
low bandgap. In recent years, many novel low-bandgap polymers
synthesized using this approach for solar cell applications with
efficiencies of more than 7% are reported.9–12
In theory, the intrinsic bandgap of conjugated polymers can
be tuned by manipulating the bond length alternation, aro-
matic units, conjugation length, substituents, and intermolec-
ular interactions.13,14 One feasible approach to synthesize
low-bandgap polymers is to design alternating donor–
acceptor (D-A) push–pull structure, where the intramolecular
charge transfer (ICT) from the electron-rich unit to electron-
poor unit can tune the HOMO and LUMO energetic levels of
the copolymers. To fulfill the requirement for highly efficient
solar cells as discussed earlier, electron donor and acceptor
units of the conjugated copolymers must be well chosen.
Recently, several D-A copolymers systems have been success-
fully developed to achieve remarkable PCEs, consisting of
alkyl-substituted fluorene and carbazole units as donors,
†Hairong Li and Shuangyong Sun contributed equally to this work.
C
V
2011 Wiley Periodicals, Inc.
250
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 250–260

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 1 Chemical structures of polymers (a) PDIITN, (b) P(CT-DIITN), (c) P(TS-DIITN), and (d) P(FT-DIITN).
claiming to have good chemical tenability, high mobility, and
environmental stability.15–19
Column chromatography was carried out with Merck silica gel
(230–400 mesh), whereas thin layer chromatography was per-
formed on Merck silica gel 60 Al-backed plates (20 cm ꢁ 20
cm). The NMR spectra were collected on a Bruker DPX 400
spectrometer using chloroform-d as a solvent and tetramethyl-
silane as an internal standard. Matrix-assisted laser desorption/
ionization time-of-flight (MALDI-TOF) mass spectra were
obtained on a Shimadzu AXIMA Performance. Elemental analy-
sis was obtained via a Thermo Scientific Flash 2000 Series
CHNS Analyzer. Thermogravimetric analysis (TGA) was carried
out using a TA Instrument Q500 Thermogravimetric Analyzer
at a heating rate of 10 C/min up to 900 C and at a nitrogen
flow rate of 75 cm3/min. Differential scanning calorimetry
(DSC) was run on a TA Instrument Q10. UV–vis absorption
spectra were recorded on a Shimadzu UV–Vis 2501 Spectrome-
ter. Cyclic voltammetry (CV) was performed on a CHI 660 elec-
trochemical workstation with a three-electrode cell in a solution
of Bu4NBF4 (0.1 M) of anhydrous dichloromethane (DCM) at a
scanning rate of 100 mV/s. A platinum wire was used as the
working electrode and an Ag/AgNO3 (0.1 M) electrode was
used as the reference electrode.
Poly(isothianaphthene) (PITN) derivatives have attracted
much attention in recent years because of their very low
bandgaps (down to ꢀ1 eV) and good solubility in organic
solvents.20 For instance, poly(benzo[c]thiophene-N-2-ethyl-
hexyl-4,5-dicarboxylic imide), P(EHI-PITN), exhibits low
bandgap of ꢀ1.29 eV with HOMO level of 4.88 eV and LUMO
level of 3.59 eV.11,21 Since their first introduction, several
PITN derivatives have been synthesized and used in BHJ
solar cells. However, to date, the best reported efficiency of
solar cells containing PITN derivatives is only 0.9%.22
ꢂ
ꢂ
Herein, we report the synthesis of a new homopolymer
poly (benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic imide
(PDIITN), which has a basic chemical structure similar to that
of EHI-PITN, and of a series of D-A alternating copolymers
poly [5,50-(2,7-bisthiophen-2-yl)-9-(2-decyltetradecyl)-9H-carba-
zole)-alt-2,9-(benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic im-
ide)], (P(CT-DIITN)), poly[5,50-(3,30-di-n-octylsilylene-2,20-bithio-
phene)-alt-2,9-(benzo[c]thiophene-N-dodecyl-4,5-dicarbox-
ylic imide)], (P(TS-DIITN)), and poly[5,50-(2,7-bisthiophen-2-
yl-9,9-bisoctyl-9H-fluoren-7-yl)-alt-2,9-(benzo[c]thiophene-N-
dodecyl-4,5-dicarboxylic imide)], (P(FT-DIITN)), which contain
a DIITN unit as an acceptor (Fig. 1). We also report on the results
of testing blends of these copolymers with fullerene acceptors in
BHJs solar cells.
Solar Cells Fabrication and Characterization
Polymer:PCBM were blended with different weight ratios
(1:1, 1:2, and 1:4) with polymer concentration of 10 mg/mL.
Indium tin oxide (ITO)/glass substrates were purchased
from Kintec Company (7 X/sq). The ITO/glass substrates
were first successively cleaned by detergent, deionized water,
acetone, and isopropanol for 15 min each in ultrasonic water
bath. Before spin coating of PEDOT:PSS, the substrates were
plasma cleaned for 2 min. A layer of 30 nm of PEDOT:PSS
EXPERIMENTAL
Materials and Equipments
ꢂ
Chemicals and reagents were purchased from either Aldrich or
Alfa-Aesar and were used without further purification unless
otherwise stated. N-Bromosuccinimide (NBS) was recrystallized
from distilled water to get white crystals (previously yellow).
film was deposited on ITO and then baked at 140 C for 10
min in a glovebox. The polymer:PCBM blend solutions were
then spin coated onto the PEDOT:PSS layer to yield films
thickness of ꢀ70 nm. Finally, 100 nm of aluminum was
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 250–260
251

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
deposited in a vacuum (10ꢃ6 Torr) through a shadow mask
on the active layer to form the cathode. The active area for
each device was 0.07 cm2. Current density–voltage (J–V)
characteristics of the devices were measured using Agilent
4155C Semiconductor Parameter Analyzer under simulated
AM 1.5G illumination (100 mW/cm2). The external quantum
efficiency (EQE) test was performed by Merlin radiometer
(Newport) with calibrated Si photodiode (Hamamatsu) as
reference. AFM images were recorded using the tapping
mode on the Digital Instruments Nanoscope IIIa.
20H, CH2), 0.86 (t, J ¼ 6.8 Hz, 3H, CH3). 13C NMR d: 167.9,
143.5, 133.0, 126.3, 38.8, 32.5, 30.9, 30.2, 30.1, 30.0, 29.8,
29.2, 27.5, 23.4, 20.9, 14.7. MS (MALDI-TOF) m/z: 501.05
(Mþ). Calculated for C22H31Br2NO2: C, 52.71; H, 6.23; N,
2.79%. Found C, 52.44; H, 6.40; N, 2.95%.
Benzo[c]thiophene-N-dodecyl-2,7-dihydro-4,
5-dicarboxylic Imide (5)
To a suspension of sodium sulfide nonahydrate (5.3 g, 22.0
mmol) in absolute ethanol (200 mL) was added dropwise a
solution of compound 4 (10 g, 20.0 mmol) in chloroform (80
mL). After addition the suspension was brought to reflux for 1
h, cooled down, and evaporated. The residue was dissolved in
DCM, and the solution was filtered. The filtrate was condensed
and the residue was adsorbed onto silica; elution with EA:hex-
ane (1:4) gave 5 as a white powder (4.5 g, 60%).
Synthesis
The synthetic route is summarized in Scheme 1. 4,5-Di-
methyl-cis-4-enetetrahydrophthalic anhydride (1)23 and 2,7-
dibromocarbazole (9)24 were synthesized according to the
literature procedures.
1H NMR d: 7.67 (s, 2H, ArH), 4.32 (s, 4H, SCH2), 3.65 (t, J ¼
7.2 Hz, 3H, NCH2), 1.65 (m, 2H, NCH2CH2), 1.23–1.37 (m
20H, CH2), 0.86 (t, J ¼ 6.8 Hz, 3H, CH3). 13C NMR d: 168.6,
147.7, 132.2, 120.2, 38.8, 38.2, 32.5, 30.3, 30.2, 30.1, 30.0,
29.8, 29.2, 27.5, 23.3, 14.8. MS (MALDI-TOF) m/z: 373.15
(Mþ). Calculated for C22H31NO2S: C, 70.74; H, 8.36; N, 3.75;
S, 8.58%. Found C, 70.79; H, 8.41; N, 3.98; S, 8.33%.
N-Dodecyl-4,5-dimethyl-cis-4-tetrahydrophthalimide (2)
Compound 1 (15 g, 83.2 mmol), 1-dodecylamine (17.0 g, 91.7
mmol), and nickel acetate tetrahydrate (1.5 g, 8.3 mmol) was
dissolved in the solvent mixture of dimethylformamide (50 mL)
and pyridine (15 mL). The solution was heated up to reflux for
1 day, cooled down and poured into water, and extracted with
DCM. The extract was washed with water, dried over MgSO4,
and filtered. The filtrate was evaporated and the residue was
purified by chromatography on silica eluting with hexane:DCM
(1:1) to yield 2 as a colorless liquid (27.5 g, 96%).
1H NMR (CDCl3) d: 3.42 (d, J ¼ 7.2 Hz, 2H, NCH2), 2.98 (m,
1H, OACCH), 2.21–2.44 (m, 4H, CHCH2), 1.64 (s, 6H, CCH3),
1.45 (m, 2H, NCH2CH2), 1.18–1.32 (m, 20H, CH2), 0.86 (t, J ¼
6.8 Hz, 3H, CH2CH3). 13C NMR (CDCl3) d: 181.0, 127.5, 40.4,
32.5, 31.4, 30.2, 30.1, 30.0, 29.9, 28.2, 27.1, 23.3, 19.8, 14.7. MS
(MALDI-TOF) m/z: 347.22 (Mþ). Calculated for C22H37NO2: C,
76.03; H, 10.73; N, 4.03%. Found: C, 76.20; H, 10.87; N, 4.27%.
Benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic Imide (7)
A solution of m-chloroperbenzoic acid (m-CPBA) (2.8 g, 16.1
mmol) in chloroform (50 mL) was added dropwise to a solu-
tion of compound 5 (6.0 g, 16.1 mmol) in chloroform (150
mL) in an ice-water bath. After addition the reaction was
maintained at 0 ꢂC for 2 h, during which the intermediate
benzo[c]thiophene-1-oxide-N-dodecyl-2,7-dihydro-4,5-dicarbox-
ylic imide (6) was formed. The solution was brought to reflux
for another 4 h for dehydration, cooled down, condensed, and
the residue was adsorbed onto silica; elution with EA:hexane
(1:4) produced 7 as white crystals (5.35 g, 90%).
N-Dodecyl-4,5-dimethylphthalic Imide (3)
1H NMR d: 8.12 (s, 2H, ArH), 8.00 (s, 2H, ThH), 3.70 (t, J ¼
7.2 Hz, 3H, NCH2), 1.68 (m, 2H, NCH2CH2), 1.23–1.35 (m
20H, CH2), 0.86 (t, J ¼ 6.8 Hz, 3H, CH3). 13C NMR d: 168.6,
139.0, 126.7, 123.5, 120.2, 39.0, 32.5, 30.4, 30.3, 30.2, 30.1,
30.0, 29.9, 29.1, 27.6, 23.3, 14.8. MS (MALDI-TOF) m/z:
371.10 (Mþ). Calculated for C22H29NO2S: C, 71.12; H, 7.87;
N, 3.77; S, 8.63%. Found C, 71.01; H, 7.90; N, 3.85; S, 8.60%.
Compound
3 was prepared according to a literature
method21 as white crystals in 77% yield.
1H NMR d: 7.58 (s, 2H, ArH), 3.62 (t, J ¼ 7.2 Hz, 2H, NCH2),
1.63 (m, 2H, NCH2CH2), 1.23–1.30 (m, 20H, CH2), 0.87 (t, J ¼
6.8 Hz, CH2CH3). 13C NMR d: 169.5, 144.0, 130.8, 124.8, 38.6,
32.6, 30.3, 30.2, 30.1, 30.0, 29.9, 29.3, 27.5, 23.3, 21.2, 14.8.
MS (MALDI-TOF) m/z: 343.20 (Mþ). Calculated for C22H33NO2:
C, 76.92; H, 9.68; N, 4.08%. Found C, 76.69; H, 9.85; N, 4.24%.
2,9-Dibromo-benzo[c]thiophene-N-dodecyl-4,
5-dicarboxylic Imide (8)
N-Dodecyl-4,5-bis(bromomethyl)phthalimide (4)
To a solution of compound 7 (2.2 g, 6.0 mmol) in chloroform
(50 mL) cooled in an ice-water bath was added dropwise bro-
mine (1.93 g, 12.0 mmol) in chloroform (50 mL). The solution
was warmed up to room temperature and stirred overnight,
condensed, and the residue was adsorbed onto silica; elution
with DCM: hexane (1:1) gave 8 as yellow flakes (2.85 g, 90%).
To a refluxing solution of compound 3 (15.4 g, 44.8 mmol)
in anhydrous benzene (200 mL), was added a mixture of
NBS (18.4 g, 103.3 mmol) and benzoyl peroxide (20 mg) in
three portions every 40 min under 100 W tungsten bulb
irradiation. One hour after the last addition of NBS, the reac-
tion mixture was cooled down, evaporated, the residue was
extracted with DCM, and the extract was washed with water.
The organic layer was condensed and loaded onto silica for
chromatography eluting with ethyl acetate (EA):hexane (1:6)
to obtain 4 as white crystals (12.1 g, 55%).
1H NMR d: 7.99 (s, 2H, ArH), 3.72 (t, J ¼ 7.2 Hz, 3H, NCH2),
1.70 (m, 2H, NCH2CH2), 1.24–1.35 (m 20H, CH2), 0.88 (t, J ¼
6.8 Hz, 3H, CH3). 13C NMR d: 167.7, 138.6, 128.0, 119.1,
110.6, 39.3, 32.6, 30.3, 30.2, 30.1, 30.0, 29.8, 29.0, 27.6, 23.3,
14.8. MS (MALDI-TOF) m/z: 529.00 (Mþ). Calculated for
1H NMR d: 7.83 (s, 2H, ArH), 4.69 (s, 4H, CH2Br), 3.66 (t, J
¼ 7.2 Hz, 3H, NCH2), 1.64 (m, 2H, NCH2CH2), 1.23–1.41 (m
C22H27Br2NO2S: C, 49.92; H, 5.14; N, 2.65; S, 6.06%. Found
C, 49.87; H, 5.13; N, 2.77; S, 6.01%.
252
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 250–260

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
SCHEME 1 Synthetic routes to polymer PDIITN, P(CT-DIITN), P(TS-DIITN), and P(FT-DIITN).
2-Dodecyl-tetradecyl-bromide (10)
room temperature and stirred for 2 days. The solvent was
evaporated, the residue was extracted with hexane, and the
extract was filtered. The filtrate was adsorbed to silica; elu-
tion with hexane produced 10 as a colorless liquid (22.1 g,
95%).
2-Dodecyl-tetradecanol (20 g, 56.4 mmol) and triphenylphos-
phine (14.8 g, 56.4 mmol) were dissolved in 200 mL of DCM
in an ice-water bath. NBS (12 g, 67.7 mmol) was added in
portion over 2 h. The solution was then warmed up to
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 250–260
253

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
1H NMR d: 3.44 (d, J ¼ 4.4 Hz, BrCH2), 1.87 (m, 1H,
BrCH2CH), 1.20–1.39 (br, 40 H, CH2), 0.89 (t, J ¼ 7.2 Hz, 6H,
CH3). 13C NMR d: 40.2, 33.2, 32.66, 32.65, 32.3, 30.5, 30.4,
30.3, 30.1, 30.0, 27.2, 23.4, 14.8. MS (MALDI-TOF) m/z:
416.21 (Mþ).
poured into water, and extracted with diethyl ether. The
extract was washed thoroughly with water, and the organic
layer was collected and evaporated to obtain the product 13
as a colorless liquid, which was used without further
purification.
3,3-Di-n-octylsilylene-2,20-bithiophene (14)
Compound 14 was synthesized by the literature procedure25
as a colorless oil in 60% yield.
2,7-Dibromo-9-(2-decyltetradecyl)-9H-carbazole (11)
To a solution of 2,7-dibromo-9H-carbazole 9 (4 g, 12.3
mmol) in dimethyl sulfoxide (DMSO) (40 mL) was added
powered potassium hydroxide (0.83 g, 14.7 mmol) in one
portion.. After stirring for 1 h, compound 10 (6.1 g, 14.7
mmol) in DMSO (30 mL) was added dropwise. The solution
was stirred overnight, poured into water, extracted with
DCM, and the extract was evaporated. The crude product
was further purified by chromatography on silica eluting
with hexane to obtain 11 as a colorless oil (7.15 g, 88%).
1H NMR d: 7.85 (d, J ¼ 8.4 Hz, 2H, 4,5-2H on carbazole),
7.48 (d, J ¼ 1.6 Hz, 2H, 1,8-2H on carbazole), 7.33 (dd, J ¼
8.4 Hz, 2H, 3,6-2H on carbazole), 3.99 (d, J ¼ 7.6 Hz, 2H,
NCH2), 2.06 (br, 1H, NCH2CH), 1.22–1.40 (br, 40 H, CH2),
0.90 (t, J ¼ 7.2 Hz, 6H, CH3). 13C NMR d: 142.5, 123.1, 122.0,
121.9, 120.3, 112.9, 48.5, 38.2, 32.66, 32.64, 32.3, 30.6, 30.3,
30.2, 30.09, 30.05, 27.10, 23.4, 14.8. MS (MALDI-TOF) m/z:
661.24 (Mþ). Calculated for C36H55Br2N: C, 65.35; H, 8.38; N,
2.12%. Found C, 65.31; H, 8.39; N, 2.33%.
1H NMR d: 7.19 (d, J ¼ 4.8 Hz, 2H, ThH), 7.05 (d, J ¼ 4.8 Hz,
2H, ThH), 1.38 (m, 4H, SiCH2), 1.21–1.35 (m, 20H, CH2),
0.85–0.93 (m, 10H, CH2CH3). 13C NMR d: 149.8, 142.3,
130.3, 125.6, 33.8, 32.5, 29.87, 29.85, 24.8, 23.3, 14.8, 12.5.
MS (MALDI-TOF) m/z: 417.35 (Mþ).
5,50-Bis(tributylstannyl)-3,30-di-n-octylsilylene-2,
20-bithiophene (15)
Compound 15 was synthesized by the same method as com-
pound 13 and was used without further purification because
of its instability on silica.
2,7-Bis(5-(tributylstannyl)thiophen-2-yl)-9,
90-bisoctyl-9H-fluorene (16)
Compound 16 was synthesized by the same method as com-
pound 13 and was used without further purification.
Poly(benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic imide
A solution of 7 (0.8 g, 2.15 mmol) in anhydrous chloroform
(70 mL) was added to a 100-mL one-neck flask charged
with anhydrous FeCl3 (1.4 g, 8.6 mmol) and triethylamine
(0.87 g, 8.6 mmol) under a nitrogen atmosphere; the solu-
tion was immediately turned dark red. The solution was
stirred for 3 days, poured into methanol (400 mL), and the
resulting solid was collected by filtration. The solid was
washed with water and methanol, dedoped with ammonium
hydroxide solution (28% aqueous) and EDTA, followed by
Soxhlet extraction with methanol and acetone to obtain
PDIITN as a black solid (0.47 g, 60%).
9-(2-Decyltetradecyl)-2,7-di(thiophen-2-yl)-9H-
carbazole (12)
Compound 11 (2 g, 3.0 mmol) and thiophene-2-boronic acid
(0.97, 7.5 mmol) were added to a 100-mL round-bottom
flask containing a solvent mixture of THF (40 mL) and aque-
ous potassium carbonate solution (2 M, 30 mL). The reaction
solution was brought to reflux for 1 day under nitrogen,
cooled down and poured into water, extracted with DCM,
and the extract was filtered. The filtrate was condensed and
adsorbed onto silica; chromatography with hexane as eluent
gave 12 as a white fluffy solid (1.7 g, 85%).
1H NMR d: 8.03 (d, J ¼ 8.4 Hz, 2H, 4,5-2H on carbazole),
7.57 (s, 2H, 1,8-2H on carbazole), 7.50 (d, J ¼ 8.4 Hz, 2H,
3,6-2H on carbazole), 7.40 (d, J ¼ 4.8 Hz, 2H, ThH), 7.30 (d,
J ¼ 4.8 Hz, 2H, ThH), 7.12 (m, 2H, ThH), 4.20 (d, J ¼ 7.2 Hz,
2H, NCH2), 2.17 (br, 1H, NCH2CH), 1.21–1.43 (br, 40 H, CH2),
0.88 (t, J ¼ 7.2 Hz, 6H, CH3). 13C NMR d: 146.3, 142.6, 132.7,
128.7, 125.1, 1236.6, 122.8, 121.2, 118.4, 106.6, 48.1, 38.5,
32.6, 32.2, 30.6, 30.3, 30.0, 29.7, 27.32, 23.3, 14.8. MS
(MALDI-TOF) m/z: 667.20 (Mþ). Calculated for C44H61NS2: C,
79.10; H, 9.20; N, 2.10; S, 9.60%. Found C, 79.07; H, 9.26 N,
2.37; S, 9.31%.
1H NMR d: 8.41 (br, 2H, ArH), 3.73 (br, 2H, NCH2), 1.65 (br,
2H, NCH2CH2), 1.25 (br, m, 20H, CH2), 0.87 (t, J ¼ 6.8 Hz,
3H, CH3). Calculated for C22H27NO2S: C, 71.51; H, 7.36; N,
3.79; S, 8.68%. Found C, 70.45; H, 7.03 N, 3.70; S, 7.96%.
Poly[5,50-(2,7-bisthiophen-2-yl)-9-(2-decyltetradecyl)-9H-
carbazole)-alt-2,9-(benzo[c]thiophene-N-dodecyl-4,5-
dicarboxylic imide)]
A solution of 8 (0.3 g, 0.57 mmol) and 13 (0.78, 0.63 mmol)
in THF (30 mL) was heated under reflux for 2 days under
N2, then cooled down and poured into methanol, and fil-
tered. The resulting solid was purified by Soxhlet extraction
using methanol, hexane, and acetone, and the residue was
dissolved in DCM, which was then evaporated under reduced
pressure to obtain P(CT-DIITN) as a black solid (0.32 g,
55%).
2,7-Bis(5-(tributylstannyl)thiophen-2-yl)-9-
(2-decyltetradecyl)-9H-carbazole (13)
To a solution of compound 12 (1.5 g, 2.2 mmol) in anhy-
drous THF (30 mL) at ꢃ78 ꢂC was added dropwise a solu-
tion of n-BuLi in hexane (1.6 M, 3 mL, 4.8 mmol). After addi-
tion the solution was warmed up to ꢂroom temperature for 5
min and cooled down again to ꢃ78 C, followed by addition
of tributyltin chloride (1.56 g, 4.8 mmol). The solution was
warmed up to room temperature and stirred overnight, then
1H NMR d: 8.26 (br, 2H, ArH), 8.07 (br, 2H, CbzH), 7.31–7.52
(br, m, CbzH, ThH), 4.24 (br, 2H, CbzNCH2), 3.74 (br, 2H,
NCH2), 2.17 (br, 1H, NCH2CH), 1.72 (br, 2H, NCH2CH), 1.20–
1.50 (br, m, 60H, CH2), 0.86 (m, 9H, CH3). Calculated for
254
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 250–260

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 1 Molecular Weights and Thermal Properties of
Polymers PDIITN, P(CT-DIITN), P(TS-DIITN), and P(FT-DIITN)
C66H86N2O2S3: C, 76.55; H, 8.37; N, 2.71; S, 9.29%. Found C,
75.13; H, 7.99; N, 2.75; S, 8.78%.
Poly[5,50-(3,30-di-n-octylsilylene-2,20-bithiophene)-alt-2,9-
(benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic imide)]
P(TS-DIITN) was synthesized by copolymerization of com-
pound 8 and 15 by the same method as for polymer P(CT-
DIITN) as a black in 60% yield.
Polymers
Mw (kg/mol) Mn (kg/mol) PDI Tg (ꢂC) Td (ꢂC)
PDIITN
20.9
9.5
17.7
15.7
15.9
2.2 211
2.6 160
1.8 134
2.1 153
304
295
291
292
P(CT-DIITN) 46.0
P(TS-DIITN) 28.8
P(FT-DIITN) 33.5
1H NMR d: 8.47 (br, 2H, PhH), 7.43 (br, m, 2H, ThH), 3.74
(br, 2H, NCH2), 1.68–1.70 (br, NCH2CH2), 1.13–1.48 (br, m,
CH2), 0.85–0.97 (br, m, CH2CH3). Calculated for
PDIITN, P(CT-DIITN), P(TS-DIITN), and P(FT-DIITN) have
glass transition temperature (Tg) of 211, 160, 134, and 153
ꢂC, respectively. The reduction in Tg of copolymers P(CT-
DIITN), P(TS-DIITN), and P(TS-DIITN) compared to that of
homopolymer PDIITN results from the increased flexibility
of the polymer main chains due to the incorporation of bulky
dialkyl-substituted aromatic donor units. The decomposition
temperatures (Td) (5% weight loss for the polymers) deter-
C46H63NO2S3Si: C, 70.27; H, 8.08; N, 1.78; S, 12.23%. Found
C, 69.46; H, 8.31; N, 2.21; S, 12.72%.
Poly[5,50-(2,7-bisthiophen-2-yl-9,9-bisoctyl-9H-fluoren-7-
yl)-alt-2,9-(benzo[c]thiophene-N-dodecyl-4,5-dicarboxylic
imide)]
P(FT-DIITN) was synthesized by copolymerization of com-
pound 8 and 16 by the same method as for P(CT-DIITN) as
a black solid in 50% yield.
ꢂ
mined by TGA are around 300 C, which are consistent with
those of previously reported PITN derivatives.26,27
1H NMR d: 8.45 (br, 2H, PhH), 7.49–7.70 (br, m, PhH, ThH),
7.32 (br, ThH), 3.76 (br, 2H, NCH2), 2.10 (br, 4H, C(CH2)2),
1.71 (br, 2H, NCH2CH2), 1.10–1.35 (br, m, CH2), 0.80–0.87 (br,
m, CH2CH3). Calculated for C59H71NO2S3: C, 76.83; H, 7.76; N,
1.52; S, 10.43%. Found C, 76.30; H, 7.65; N, 2.12; S, 9.87%.
Optical Properties
Figure 2 shows the UV–vis–NIR absorption spectra of the
four polymers in chloroform (20 lg/mL) normalized at 544
nm, and their optical properties are summarized in Table 2.
The homopolymer PDIITN has a maximum absorption peak
at 418 nm and a shoulder at 540 nm, which are attributed
to the p–p* absorption of the conjugated polymer back-
bone.21 The onset of absorption (around 750 nm) corre-
sponds to an optical bandgap of 1.65 eV. The incorporation
of aromatic electron donors in the copolymers results in ICT
between donor and acceptor, causing electronic delocaliza-
tion along the polymer backbones. This improves the effec-
tive conjugation length and leads to an extension of absorp-
tion.28 The absorption spectra of copolymers P(CT-DIITN)
and P(TS-DIITN) are both red-shifted and their maxima are
red-shifted 6 and 22 nm, respectively. The larger peak shift
and the rising second broadened peak at 628 nm of P(TS-
DIITN) is ascribed to the much stronger ICT between the
silylene-bithiophene electron-donating unit and the benzo-
thiophene-dicarboxylic imide acceptor unit in the polymer
main chain than that between bisthiophene-carbazole donor
and benzothiophene-dicarboxylic imide acceptor in P(CT-
DIITN).29,30 However, for P(FT-DIITN) copolymer, its
absorption peak and shoulder are blue-shifted to 414 and
517 nm, respectively, while the absorption edge shows a 90-
nm blue shift compared to the homopolymer. This is because
the two alkyl chains on the fluorene units induce twisting of
the polymer chain by steric hindrance, thus reducing the
intramolecular interaction between the electron donor and
acceptor and cause poorer p–p stacking of the polymer back-
bones.31 Copolymers P(FT-DIITN) and P(CT-DIITN) have
analogous chemical structures, and their absorption spec-
trums are quite similar. Both of them have a much enhanced
absorption peak as the p–p* transitions of bisthiophene-car-
bazole and bisthiophene-fluorene units are around 400
nm.31,32 However, the nitrogen bridge in the carbazole unit
has only one alkyl substituent permitting a more planar
RESULTS AND DISCUSSION
Material Synthesis
The synthesis routes to these new polymers are shown in
Scheme 1. First, monomers 2,9-dibromo-benzo[c]thiophene-
N-dodecyl-4,5-dicarboxylic imide (8), 2,7-bis(5-(tributyl-
stannyl)thiophen-2-yl)-9-(2-decyltetradecyl)-9H-carbazole (13),
5,50-bis(tributylstannyl)-3,30-di-n-octylsilylene-2,20-bithiophene
(15), and 2,7-bis(5-(tributylstannyl)thiophen-2-yl)-9,90-bis-
octyl-9H-fluorene (16) were prepared. Monomer (8) was
obtained after a long step of reactions starting from com-
pound (1) (4,5-dimethyl-cis-4-enetetrahydrophthalic anhy-
dride). Monomer 13 was made from 2,7-dibromocarbazole
(9) and 2-dodecyl-tetradecyl-bromide (10). Monomer 15
was obtained by stannylation of 3,3-di-n-octylsilylene-2,20-
bithiophene (14). Copolymers P(CT-DIITN), P(TS-DIITN),
and P(FT-DIITN) were synthesized by a palladium-catalyzed
Stille coupling reaction in THF solution between monomer
(8) and, respectively, monomers 13, 15, and 16. To study
the effect of electron-donating ability of donor moieties on
the intrinsic properties of D-A copolymers, the homopolymer
PDIITN was also prepared from monomer (8) for compari-
son. The final structures of these polymers were confirmed
by NMR spectroscopy.
Polymer Characterization
Thermal Properties
The molecular weights of the polymers synthesized were
characterized by gel permeation chromatography. These
polymers have weight-averaged molecular weight (Mw) rang-
ing from 20 to 34 kg/mol with polydispersity indices (PDIs)
of 1.8–2.6 (Table 1). Their thermal properties are also sum-
marized in Table 1. As characterized by DSC, the polymers
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 250–260
255

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
exclude doping as a probable explanation). This suggests
that blending of PDIITN with PCBM may not result in work-
ing solar cell device because of the lack of electron-donating
component in the system. Indeed, as investigated in this
study, the PDIITN:PC61BM blend system did not show any
photovoltaic behavior. As such, PDIITN will be excluded in
the discussion of the polymeric blends with fullerene. The
bandgap of PDIITN was estimated from the onset value of
its absorption spectrum in Figure 2 to be 1.65 eV. Both the
D-A copolymers P(CT-DIITN) and P(TS-DIITN) show lower
bandgaps, which is a result of the intrachain charge transfer
between donor and acceptor in the polymer backbones.35,36
However, P(FT-DIITN) exhibits a larger bandgap of 1.91 eV
with HOMO/LUMO levels of ꢃ5.35/ꢃ3.44 eV, which are con-
sistent to its narrow absorption range showed earlier, as the
twisted nature of polymer backbone disrupts the effective
conjugation. The estimated LUMO levels of P(CT-DIITN),
P(TS-DIITN), and P(FT-DIITN) are ꢃ3.70, ꢃ3.91, and ꢃ3.44
eV, respectively. The relatively low LUMO energy levels of
these copolymers result from the strong electron-accepting
ability of the benzothiophene-dicarboxylic imide unit. Com-
pared to P(CT-DIITN), which exhibits HOMO/LUMO values
of ꢃ5.16/ꢃ3.70 eV, the carbazole-containing P(TS-DIITN)
has a HOMO/LUMO of ꢃ5.26/ꢃ3.91 eV. The lower lying
HOMO level of P(TS-DIITN) is contrary to the observation
that a stronger donor in the D-A copolymers normally
results in a higher HOMO energy level37 and is attributed to
the disturbance of the ICT and bandgap tuning due to twist-
ing of the silylene-bithiophene unit. It should be noted that
the values of electrochemical bandgaps of P(CT-DIITN),
P(TS-DIITN), and P(FT-DIITN) are quite close to those of
their respective optical bandgaps (<0.07 eV) calculated from
the absorption profiles in Figure 2. Such small differences in
the bandgap values have commonly been observed in amor-
phous polymers with rigid polymer backbones.38
FIGURE 2 Absorption spectra of polymers PDIITN, P(CT-DIITN),
P(TS-DIITN), and P(FT-DIITN) in CHCl3.
conformation, and the polymer backbone of P(CT-DIITN)
exhibits a more extended configuration, thus resulting in a
red-shifted absorption relative to P(FT-DIITN).33
Electrochemical Properties
To apply these conjugated polymers in solar cells, it is im-
portant to know their HOMO and LUMO levels. CV was per-
formed in a three-electrode configuration in a solution of
Bu4NBF4 (0.1 M) of anhydrous DCM at a scan rate of 100
mV/s with platinum electrodes as working and counter elec-
trode and an Ag/AgNO3 (0.1 M) electrode as reference elec-
trode. As the redox potential of ferrocene Fc/Fcþ, which has
an absolute energy level of ꢃ4.8 eV relative to the vacuum
level, is located at 0.4 eV in 0.1 M DCM solution of
Bu4NBF4,34 the values of HOMO and LUMO as well as the
bandgap of the polymers can be calculated according to the
following equations:HOMO ¼ ꢃ4.4 eV ꢃ EoxLUMO ¼ ꢃ4.4
eV þ Ered,where Eox and Ered are the onset oxidation poten-
tial and the onset reduction potential relative to Ag/Agþ,
respectively. The calculated electrochemical orbital energy
values of these polymers are summarized in Table 2. The
homopolymer PDIITN has a LUMO energy level of ꢃ3.57 eV,
but its oxidation potential was not observed in the CV mea-
surement, which is possibly due to the strong n-type charac-
ter of the polymer (polymer was extensively dedoped so we
Blend Characterization
Optical Characterization
Figure 3 shows the UV–vis absorption spectra of P(CT-
DIITN), P(TS-DIITN), and P(FT-DIITN) copolymers blend
films with PC61BM at 1:1 weight ratio. These films are spin
coated from dichlorobenzene (DCB), and the absorption
spectra are normalized with respect to the absorption max-
ima of PC61BM at 330 nm. With the incorporation of
PC61BM, significant absorption enhancements are observed
TABLE 2 HOMO/LUMO Levels and Absorption Maxima of Polymers PDIITN, P(CT-DIITN),
P(TS-DIITN), and P(FT-DIITN) in CHCl3
a
b
c
Polymers
HOMO (eV) LUMO (eV) Eg (eV) Eg (eV) kmax (nm)
kons (nm)
PDIITN
N/A
ꢃ3.57
ꢃ3.70
ꢃ3.91
ꢃ3.44
–
1.65
1.53
1.33
1.88
418, 540
750
P(CT-DIITN) ꢃ5.12
P(TS-DIITN) ꢃ5.26
P(FT-DIITN) ꢃ5.35
1.42
1.46
1.91
424 (4.96), 560 (2.63) 810
440 (3.52), 628 (4.40) 935
414 (4.51), 517 (2.06) 660
a
Electrochemical bandgap.
b
Optical bandgap.
c
Values in the parenthesis are the molar absorptivity in the unit of ꢁ104 Mꢃ1 cmꢃ1
.
256
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 250–260

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
contrast increase and interconnected networks can be easily
observed. However, at highest PC61BM loading (80%), large
numbers of PC61BM aggregates appear in P(CT-
DIITN):PC61BM film because of the limited miscibility. By
contrast, P(TS-DIITN):PC61BM films are less affected by the
high content of PC61BM, and the surface of the film is still
even. For P(FT-DIITN):PC61BM AFM images, at all blend
ratios, films always show a bicontinuous nanoscale phase
separation, which is generally considered advantageous in
blends for BHJ solar cells.
Photovoltaic Cell
The device performance of solar cells is strongly related to
the nanomorphology of polymer-fullerene blends, which can
be affected by a number of parameters, such as chemical na-
ture of polymer, polymer–fullerene ratio, solution concentra-
tion, and solvent. Polymers P(CT-DIITN), P(TS-DIITN), and
P(FT-DIITN) have different molecular structures, molecular
weight, and polydispersity, and they possess different inter-
action strengths with PCBM, which further influences the
miscibility between polymer and PCBM, and hence, the final
film morphology. Therefore, the optimized blend ratios for
the different polymers may be different even under the same
preparation conditions.
FIGURE 3 Absorption spectra of P(CT-DIITN):PC61BM, P(TS-
DIITN):PC61BM, and P(FT-DIITN):PC61BM composite films with 1:1
weight ratio spin coated from DCB.
around 370 nm range, and at the same time absorption co-
efficients at longer wavelength are reduced. This effect can
be explained by a decrease in the amount of donor polymer
and the interference effect between polymers and PC61BM.
From the figure, we found that the absorption of these poly-
mers is weaker than that of PC61BM; thus, the solar cells
based on the polymer blends may have low Jsc. Compared to
their absorption profiles in solution, the absorption spectra
of P(CT-DIITN):PC61BM, P(TS-DIITN):PC61BM, and P(FT-
DIITN):PC61BM composite films are notably red-shifted. It
should be noted that for P(CT-DIITN) blend film, the absorp-
tion shoulder exhibits a 45-nm red shift. However, for P(FT-
DIITN):PC61BM film, the absorption shoulder is absent.
While for P(TS-DIITN), the first absorption peak of the film
vanished and the second absorption peak is strongly red-
shifted by 142 nm. These large red-shifts from solution to
solid are attributed to the twisting of the polymer backbones
in solution and improved conjugation in the films.
The DIITN-based copolymers were blended with PC61BM to
make BHJ solar cells with structure configuration of ITO/
PEDOT:PSS/polymer:PC61BM/Al. Active layer films of ꢀ70
nm were spin coated from DCB solution. The photovoltaic
parameters, such as short-circuit current (Jsc), open-circuit
voltage (Voc), and fill factor (FF), of P(CT-DIITN):PC61BM,
P(TS-DIITN):PC61BM, and P(FT-DIITN):PC61BM devices
with different blend ratios are summarized in Table 3. It is
widely accepted that the Voc of BHJ organic solar cells can be
estimated by the energy differences between the HOMO of
donor and LUMO of acceptor;8 therefore, the maximum pos-
sible Voc of P(CT-DIITN):PC61BM, P(TS-DIITN):PC61BM, and
P(FT-DIITN):PC61BM is 0.92, 1.06, and 1.15 V, respectively
(taking LUMO of PC61BM as ꢃ4.2 eV). The measured Voc of
solar cells based on P(CT-DIITN) and P(FT-DIITN) are both
high (>0.7 V) and are about 0.3 V below the maximum val-
ues, consistent with loss of energy due to an exciton binding
energy of 0.3 eV as has been seen for other conjugated poly-
mers, whereas that of the P(TS-DIITN)-based device is
lower than expected. This discrepancy from expectation may
arise from a number of factors such as carrier recombina-
tion, resistance due to film thickness, and the degree of
phase separation, which can affect the actual Voc.39 It is
revealed that for P(CT-DIITN):PC61BM and P(FT-
DIITN):PC61BM devices, the Voc remains almost constant at
low PCBM content (1:1 and 1:2) and reduces at high PCBM
loading (1:4). This is followed by reduction of the shunt
resistances as calculated from J–V curves for the blends with
high PCBM loading. A possible explanation is that the large
amount of PCBM in the blend may create more direct paths
between the cathode and anode, thus reducing the Voc.40 By
contrast for P(TS-DIIN):PCBM device, the highest Voc is
obtained at 1:2 weight ratio. The Jsc trends of these polymer
based solar cells are also quite different. For P(CT-DIITN)-
Morphology Properties
The morphology of active layer films is crucial for device
performance, and it can be affected by the blend ratio.
Here, different ratios of P(CT-DIITN):PC61BM, P(TS-
DIITN):PC61BM, and P(FT-DIITN):PC61BM films were pre-
pared from DCB solution for morphological investigation. Fig-
ure 4 presents the AFM height images of P(CT-DIITN):PC61BM,
P(TS-DIITN):PC61BM, and P(FT-DIITN):PC61BM films at
weight ratios of 1:1, 1:2, and 1:4. In general, these images
show relatively smooth surfaces of the polymer blends and
good morphologies with interpenetrating networks for
charge carrier transport. Compared to the other two polymer
blend films, P(CT-DIITN):PC61BM films are more compact, as
the more twisted nature of the silicon-bridged P(TS-DIITN)
and carbon-bridged P(FT-DIITN) polymers backbones hin-
ders polymer chain packing. For P(CT-DIITN) and P(TS-
DIITN) blend films, at polymer:PC61BM weight ratio of 1:1,
polymers and PC61BM show high miscibility and the images
look homogeneous. With higher loading of PC61BM, films
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 250–260
257

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 4 AFM height images of P(CT-DIITN):PC61BM device at the ratio of (a) 1:1, (b) 1:2, and (c) 1:4; P(TS-DIITN):PC61BM device
at the ratio of (d) 1:1, (e) 1:2, and (f) 1:4; P(FT-DIITN):PC61BM device at the ratio of (g) 1:1, (h) 1:2, and (i) 1:4.
based devices, Jsc decreases with the increase in PCBM
amount, whereas for P(TS-DIITN)-based devices, Jsc is opti-
mized at a blend ratio of 1:2, and Jsc increases with PCBM
loading in P(FT-DIITN)-based device. However, FF tends to
increase with PCBM concentration for all polymer blends. It
is apparent that the behaviors of Jsc and FF are strongly
related to the PCBM content. Larger amounts of PCBM can
reduce the absorption of the active layer because of its
poorer extinction coefficient of absorption in the visible
range, but this may enhance the hole mobility of polymer
donors, which results in a more balanced charge transport.
As such, there is a compromise between charge generation
(blend absorption) and charge transport. Generally, higher
PCBM amount is necessary for amorphous polymer:PCBM
systems.41–43
Figure 5 shows the current density–voltage (J–V) characteris-
tic of the best cells based on the three copolymers under
simulated solar light (1 sun). P(CT-DIITN)-, P(TS-DIITN)-,
and P(FT-DIITN)-based devices obtain their best perform-
ance when polymer:PCBM ratio is 1:1, 1:2, and 1:4, respec-
tively. P(FT-DIITN):PC61BM solar cell demonstrates the high-
est efficiency of 0.9%, with Jsc of 3.06 mA/cm2, Voc of 0.84 V,
and FF of 0.35. The higher Voc is consistent to the low lying
HOMO of P(FT-DIITN) in the CV measurement. In compari-
son, the optimized P(CT-DIITN)-based device shows Jsc of
258
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 250–260

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 3 PCE Summaries of Solar Cells-Based P(CT-
DIITN):PC61BM, P(TS-DIITN):PC61BM, and P(TS-DIITN):PC61BM
at Different Weight Ratios
range. This is attributed by the asymmetry nature of PC71BM
molecules with a more extended absorption in the visible
range.44,45 The current density–voltage (J–V) characteristics of
P(FT-DIITN):PC61BM and P(FT-DIITN):PC71BM solar cells are
shown in Figure 6(b). Optimized devices based on P(FT-
DIITN):PC71BM exhibit higher PCE of 1.6%, with Jsc of 5.62
mA/cm2, Voc of 0.84 V, and FF of 0.34. As expected, the
increased performance is attributed to the improved Jsc, which
agrees well with the EQE data. The integrated Jsc as calculated
from the EQE also demonstrates an increase from 3.16 to 5.31
mA/cm2 after the replacement of PC61BM with PC71BM. Fur-
ther effort in thermal annealing did not help increase the PCE.
Besides the low FF, the low absorptivity of the polymer could
be another factor for low PCE. P(FT-DIITN) has molar absorp-
tivity of 4.51 ꢁ 104 Mꢃ1 cmꢃ1 at 414 nm and 2.06 ꢁ 104 Mꢃ1
cmꢃ1 at 517 nm, the low absorption at longer wavelength was
probably due to cross-conjugation effect, where the conjuga-
tion along the polymer backbones is shunted to the DIITN
units and/or the electrons are localized within DIITN units,
Polymer:PCBM JSC
VOC
g
Polymers
ratio
(mA/cm2) (V)
FF
(%)
P(CT-DIITN) 1:1
1.63
1.25
0.64
1.13
1.15
0.99
1.24
2.48
3.06
0.70 0.46 0.51
0.74 0.47 0.43
0.67 0.53 0.23
0.62 0.42 0.29
0.67 0.44 0.33
0.64 0.51 0.32
0.88 0.26 0.29
0.87 0.32 0.70
0.84 0.35 0.90
1:2
1:4
P(TS-DIITN) 1:1
1:2
1:4
P(FT-DIITN)
1:1
1:2
1:4
1.63 mA/cm2, Voc of 0.70 V, FF of 0.46, and PCE of 0.51,
whereas P(TS-DIITN):PC61BM device has Jsc of 1.15 mA/
cm2, Voc of 0.67, FF of 0.44, and PCE of 0.33%. The signifi-
cant difference in PCE of the different copolymer systems
indicates that solar cell performance may be tuned by incor-
porating appropriate electron donors in the DIITN-based D-A
conjugated copolymers.
To further improve the device performance of P(FT-DIITN)-
based solar cells, we substituted PC61BM with PC71BM as an
acceptor, as the larger fullerene shows stronger absorption in
the visible region of the spectrum. Figure 6(a) shows the EQE
responses of photovoltaic cells based on P(FT-DIITN):PC61BM
and P(FT-DIITN):PC71BM with blend ratios of 1:4. The devices
have similar EQE profiles, which have wide signal responses
in the visible range, with two peaks at 400 and 505 nm. The
P(FT-DIITN) blend with PC71BM gives much higher EQE
response (EQE max ¼ 38%) than that with PC61BM (EQE max
¼ 28%) and is more photoresponsive in the 350 to 700 nm
FIGURE 6 (Upper)
External
quantum
efficiency
(EQE)
responses of devices based on P(FT-DIITN):PC61BM and P(FT-
DIITN):PC71BM at the weight ratio of 1:4 in DCB. (Bottom) Cur-
rent density–voltage (J–V) curves of solar cell devices based on
P(FT-DIITN):PC61BM and P(FT-DIITN):PC71BM at the weight
ratio of 1:4 in DCB.
FIGURE 5 Current density–voltage (J–V) curves of polymer
solar cells: P(CT-DIITN):PC61BM (1:1 wt %), P(TS-DIITN):PC61BM
(1:2 wt %), and P(FT-DIITN):PC61BM (1:4 wt %).
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 250–260
259

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
15 Scherf, U.; List, E. J. W. Adv Mater 2002, 14, 477–487.
leading to weaker quinoid structure along the polymer
backbone.46
16 Kitazawa, D.; Watanabe, N.; Yamamoto, S.; Tsukamoto, J.
Appl Phys Lett 2009, 95, 053701–053703.
17 Gedefaw, D.; Zhou, Y.; Hellstrom, S.; Lindgren, L.; Ander-
sson, L. M.; Zhang, F.; Mammo, W.; Inganas, O.; Andersson,
M. R. J Mater Chem 2009, 19, 5359–5363.
CONCLUSIONS
In summary, we have successfully synthesized a series of novel
conjugated copolymers containing benzothiophene-dicarboxylic
imide unit as an acceptor. Different electron-donating moieties,
including bisthiophene-carbazole unit, silylene-bithiophene
unit, and bisthiophene-fluorene unit, are coupled in the poly-
mer main chains. These copolymers show an extension of
absorption spectrum up to 900 nm and have electrochemical
band gaps in the range of 1.42–1.91. The different HOMO and
LUMO levels of these polymers demonstrate that the bandgap
of polymers can be fine-tuned by carefully selecting the donor
and acceptor units. The relatively low HOMO energy levels of
these polymer render them air stable and are suitable for high
Voc solar cells. As a demonstration, these copolymers were
blended with PCBM to make solar cells. It is found that the poly-
mer solar cell based on P(FT-DIITN) shows the highest per-
formance (1.6%), with a high Voc of 0.84 V. Further studies to
understand the origins of low FF and absorptivity are impera-
tive to improve the device performance.
18 Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon,
J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat Photon
2009, 3, 297–302.
19 Lindgren, L. J.; Zhang, F.; Andersson, M.; Barrau, S.; Hell-
stro´ m, S.; Mammo, W.; Perzon, E.; Ingana´s, O.; Andersson, M.
R. Chem Mater 2009, 21, 3491–3502.
20 Lee, B.-L.; Yamamoto, T. Macromolecules 1999, 32, 1375–1382.
21 Meng, H.; Wudl, F. Macromolecules 2001, 34, 1810–1816.
22 Kim, J. Y.; Qin, Y.; Stevens, D. M.; Kalihari, V.; Hillmyer, M.
A.; Frisbie, C. D. J Phys Chem C 2009, 113, 21928–21936.
23 Hahn, W. E.; Bartnik, R.; Epsztajn, J.; Zielinski, T. Acta Pol
Pharm 1987, 44, 292–304.
24 Dierschke, F.; Grimsdale, A. C.; Mullen, K. Synthesis 2003,
¨
2003, 2470–2472.
25 Usta, H.; Lu, G.; Facchetti, A.; Marks, T. J. J Am Chem Soc
2006, 128, 9034–9035.
26 Hung, T.-T.; Chen, S.-A. Polymer 1999, 40, 3881–3884.
27 van Asselt, R.; Vanderzande, D.; Gelan, J.; Froehling, P. E.;
Aagaard, O. Synth Met 2000, 110, 25–30.
H. Li thanks Nanyang Technological University and the Singa-
pore Ministry of Education for funding through the Academic
Research Fund Tier1 (Start-up grant 14/06 and research grant
RG19/07). This work is supported by the Science and Engineer-
ing Research Council, Agency for Science, Technology and
Research (A*STAR), Singapore.
28 Wu, P.-T.; Kim, F. S.; Champion, R. D.; Jenekhe, S. A. Mac-
romolecules 2008, 41, 7021–7028.
29 Ohshita, J. Macromol Chem Phys 2009, 210, 1360–1370.
30 Zhan, X.; Risko, C.; Amy, F.; Chan, C.; Zhao, W.; Barlow, S.;
Kahn, A.; Bre´das, J.-L.; Marder, S. R. J Am Chem Soc 2005,
127, 9021–9029.
31 Rodrigues, R. F. a.; Charas, A.; Morgado, J.; Mac¡anita, A. n.
REFERENCES AND NOTES
Macromolecules 2009, 43, 765–771.
32 Wu, P.-T.; Kim, F. S.; Champion, R. D.; Jenekhe, S. A. Mac-
romolecules, 2008, 41, 7021–7028.
1 Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P.
G.; Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D.
C.; Nelson, J. Nat Mater 2008, 7, 158–164.
33 Qi, T.; Liu, Y.; Qiu, W.; Zhang, H.; Gao, X.; Liu, Y.; Lu, K.;
Du, C.; Yu, G.; Zhu, D. J Mater Chem 2008, 18, 1131–1138.
2 Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery,
34 Zhou, H.; Zhao, X.; Huang, T.; Lu, R.; Zhang, H.; Qi, X.; Xue,
K.; Yang, Y. Nat Mater 2005, 4, 864–868.
P.; Liu, X.; Zhang, X. Org Biomol Chem 2011, 9, 1600–1607.
3 Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Adv Funct
35 Zhang, Q. T.; Tour, J. M. J Am Chem Soc 1998, 120, 5355–5362.
36 Akoudad, S.; Roncali, J. Chem Commun 1998, 2081–2082.
Mater 2005, 15, 1617–1622.
4 Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. Adv Funct
Mater 2003, 13, 85–88.
37 Li, Y.; Xue, L.; Li, H.; Li, Z.; Xu, B.; Wen, S.; Tian, W. Macro-
molecules 2009, 42, 4491–4499.
5 Zhao, G.; He, Y.; Li, Y. Adv Mater 2010, 22, 4355–4358.
38 Zheng, Q.; Jung, B. J.; Sun, J.; Katz, H. E. J Am Chem Soc
6 Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl Phys
2010, 132, 5394–5404.
Lett 2006, 88, 093511–093513.
39 Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. J
7 Al-Ibrahim, M.; Roth, H. K.; Zhokhavets, U.; Gobsch, G.; Sens-
Am Chem Soc 2009, 131, 9281–9286.
fuss, S. Sol Energy Mater Sol Cell 2005, 85, 13–20.
40 Xue, L.; Li, Y.; Dong, F.; Tian, W. Nanotechnology, 2010, 21,
155201–155201.
8 Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf,
C.; Heeger, A. J.; Brabec, C. J. Adv Mater 2006, 18, 789–794.
¨
41 Andersson, L. M.; Inganas, O. Appl Phys Lett 2006, 88,
082103–082103.
9 Chen, C.-P.; Chan, S.-H.; Chao, T.-C.; Ting, C.; Ko, B.-T. J Am
Chem Soc 2008, 130, 12828–12833.
42 Pacios, R.; Nelson, J.; Bradley, D. D. C.; Brabec, C. J Appl
Phys Lett 2003, 83, 4764–4766.
10 Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.;
Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. J Am Chem Soc
2008, 130, 3619–3623.
43 Melzer, C.; Koop, E. J.; Mihailetchi, V. D.; Blom, P. W. M.
Adv Funct Mater 2004, 14, 865–870.
11 Muhlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller,
¨
D.; Gaudiana, R.; Brabec, C. Adv Mater 2006, 18, 2884–2889.
44 Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.;
Heeger, A. J.; Bazan, G. C. Nat Mater 2007, 6, 497–500.
12 Mondal, R.; Miyaki, N.; Becerril, H. A.; Norton, J. E.; Parmer,
J.; Mayer, A. C.; Tang, M. L.; Bre’das, J.-L.; McGehee, M. D.;
Bao, Z. Chem Mater 2009, 21, 3618–3628.
45 Yao, Y.; Shi, C.; Li, G.; Shrotriya, V.; Pei, Q.; Yang, Y. Appl
Phys Lett 2006, 89, 153507–153503.
13 Roncali, J. Chem Rev 1997, 97, 173–206.
46 Fraind, A. M.; Tovar, J. D. J Phys Chem B 2010, 114,
3104–3116.
14 Winder, C.; Sariciftci, N. S. J Mater Chem 2004, 14, 1077–1086.
260
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 250–260