Donor conjugated polymers-based on alkyl chain substituted oligobenzo[c]thiophene derivatives with well-balanced energy levels for bulk heterojunction solar cells

Article abstract of DOI:10.1039/c3ra41518j

We designed and synthesized two donor conjugated polymers (poly(2Z,2′Z)-2,2′-(2,5-bis(octyloxy)-1,4-phenylene) bis(3-(5-(3-(3-hexylthiophen-2-yl)benzo[c] thiophen-1-yl)thiophen-2-yl) acrylonitrile (CN-PHTBTPB) and poly(2Z,2′Z)-2,2′-(2,5-bis(octyloxy)- 1,4-phenylene)bis(3-(5-(3-(3-octylthiophen-2-yl)benzo[c]thiophen-1-yl) thiophen-2-yl)acrylonitrile (CN-POTBTPB)), comprised of hexyl and octyl chains substituted at the 3-position of 1,3-di(thiophene-2-yl)benzo[c]thiophene moieties as a donor unit coupled with a 1,4-bis-(cyano-2-thienylvinylene) phenylene based precursor by means of Knoevenagel polycondensation followed by a chemical oxidative polymerization method. Both the synthesized polymers (CN-PHTBTPB and CN-POTBTPB) exhibited reduced optical band gaps of about 1.86 eV. The resulting polymers were experimentally found to possess deep HOMO (about -5.50 to 5.78 eV) and high-lying LUMO (about -3.78 to 3.82 eV) energy levels. Theoretical calculations revealed that the variation in the HOMO and LUMO energy levels are consistent with cyclic voltammetry measurements. Using these polymers, the bulk heterojunction photovoltaic devices with the structure of ITO/PEDOT:PSS/Synthesized donor polymer:PCBM/LiF/Al was fabricated in open air. The device based on the CN-PHTBTPB and CN-POTBTPB polymers delivered respectively power conversion efficiencies of 1.65% and 1.98% under the illumination of AM 1.5G, 100 mW cm^-2, which indicates that this kind of conjugated polymer is a highly promising material for organic solar cell application. The clear correlation between the blend film morphology and the power conversion efficiency of the devices was observed via atomic force microscopic studies. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra41518j

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Donor conjugated polymers-based on alkyl chain
substituted oligobenzo[c]thiophene derivatives with
well-balanced energy levels for bulk heterojunction
solar cells3
Cite this: DOI: 10.1039/c3ra41518j
Michael Ruby Raj and Sambandam Anandan*
We designed and synthesized two donor conjugated polymers (poly(2Z,29Z)-2,29-(2,5-bis(octyloxy)-1,4-
phenylene)bis(3-(5-(3-(3-hexylthiophen-2-yl)benzo[c]
thiophen-1-yl)thiophen-2-yl)acrylonitrile
(CN-
PHTBTPB) and poly(2Z,29Z)-2,29-(2,5-bis(octyloxy)-1,4-phenylene)bis(3-(5-(3-(3-octylthiophen-2-yl)ben-
zo[c]thiophen-1-yl)thiophen-2-yl)acrylonitrile (CN-POTBTPB)), comprised of hexyl and octyl chains
substituted at the 3-position of 1,3-di(thiophene-2-yl)benzo[c]thiophene moieties as a donor unit coupled
with a 1,4-bis-(cyano-2-thienylvinylene) phenylene based precursor by means of Knoevenagel poly-
condensation followed by a chemical oxidative polymerization method. Both the synthesized polymers
(CN-PHTBTPB and CN-POTBTPB) exhibited reduced optical band gaps of about 1.86 eV. The resulting
polymers were experimentally found to possess deep HOMO (about 25.50 to 5.78 eV) and high-lying
LUMO (about 23.78 to 3.82 eV) energy levels. Theoretical calculations revealed that the variation in the
HOMO and LUMO energy levels are consistent with cyclic voltammetry measurements. Using these
polymers, the bulk heterojunction photovoltaic devices with the structure of ITO/PEDOT:PSS/Synthesized
donor polymer:PCBM/LiF/Al was fabricated in open air. The device based on the CN-PHTBTPB and CN-
POTBTPB polymers delivered respectively power conversion efficiencies of 1.65% and 1.98% under the
Received 28th March 2013,
Accepted 4th June 2013
22
illumination of AM 1.5G, 100 mW cm , which indicates that this kind of conjugated polymer is a highly
promising material for organic solar cell application. The clear correlation between the blend film
morphology and the power conversion efficiency of the devices was observed via atomic force microscopic
studies.
DOI: 10.1039/c3ra41518j
www.rsc.org/advances
3
–5
efficiencies approaching 5–8%.
The limitations of the rr-
Introduction
P3HT/PCBM system in further improving the efficiency of
polymer solar cells is due to the (i) the relatively large band gap
of rr-P3HT (y1.9 eV) limiting the absorption of solar light with
wavelengths below 650 nm, while a majority of the peak
photon intensity of the solar spectrum is located at about 700
Organic photovoltaic solar cells based on p-conjugated
polymers have been emerging as an attractive and cost-
effective route to the wide use of solar energy for low-cost
1
power generation. Bulk heterojunction (BHJ) solar cells,
consisting of a light-absorbing narrow band gap conjugated
donor polymer blended with acceptor fullerene derivatives in
an interpenetrated network with nanoscale-sized domains,
have emerged as the highest performance organic solar cells to
6
nm and (ii) a relatively small energy difference between the
LUMO of PCBM and the HOMO of rr-P3HT results in a lower
open-circuit voltage (Voc = 0.6–0.65 V), since Voc is linearly
correlated to the energy level offset between the HOMO of the
2
date. Conceptionally, regioregular poly(3-hexylthiophene) (rr-
7
donor polymer and the LUMO of the electron acceptor.
P3HT) is the most represented electron donor polymer
blended with [6,6]-phenyl- C61-butyric acid methyl ester
Therefore, maximizing the overall power conversion efficiency
requires a delicate balancing of narrow band gap conjugated
polymers in order to make the absorption spectra of the
conjugated polymer match well with the maximum photon
flux of the solar spectrum. Furthermore, a low-lying HOMO
energy level of the conjugated polymer represents one of the
most efficient strategies to develop high performance solar
(PCBM) as the photoactive layer in the organic BHJ solar cells,
because it has delivered state-of-the-art power conversion
Nanomaterials & Solar Energy Conversion Lab, Department of Chemistry, National
Institute of Technology, Tiruchirappalli-620 015, India. E-mail: sanand@nitt.edu;
sanand99@yahoo.com
1
8,9
3
Electronic supplementary information (ESI) available: H NMR spectrum of
cells.
monomers (10 and 11) and polymers, TGA curves, XRD data and DFT data (XYZ
coordinates) of the polymers. See DOI: 10.1039/c3ra41518j
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Currently, there are three main design considerations that
could improve the narrow energy band gap, extend the light
absorption in the near-infrared region and give a higher
absorption coefficient of the conjugated donor polymer and
improved charge mobility, which are rationally based on these
approaches: (i) chemical rigidification of aromatic rings in the
RSC Advances
Recently, we designed a strategy based on a D–A conjugated
donor polymer (poly(2-(4-{1-cyano-2-[5-(3-thiophen-2-yl-ben-
zo[c] thiophen-1-yl)-thiophen-2-yl]-vinyl}-2,5-bis-hexyloxy-phe-
nyl)-but-2-enenitrile, CN-PTBTBPB, with a wide optical band
gap (E ) y 1.95 eV) and with the electron-rich nature of the
g
1,3-di(thiophene-2-yl)benzo[c]thiophene moieties incorpo-
rated into the 1,4-bis-(cyano-2-thienylvinylene) phenylene
precursors which behaved as a promising electron donor
polymer. The fabricated bulk heterojunction solar cell using
the above polymer as a potential donor delivered a power
1
0,11
conjugated polymer systems,
(ii) increasing the quinoidal
1
2
character of the aromatic basic units (iii) creation of an
photoinduced intramolecular charge transfer (ICT) through
the association of alternating conjugated electron rich and
electron poor moieties in the conjugated molecular back-
2
7a
conversion efficiency of 1.33% in combination with PCBM.
1
3
bone. A major advantage in combining the donor–acceptor
conjugated polymers allows the narrowing of the energy band
gap to the desired magnitude and extending the absorption
band to the near-infrared region, in addition to the introduc-
tion of electronic push-pull driving forces which facilitates
electron delocalization by the creation of quinoid character (D
Furthermore, in order to reduce the optical band gap of the
conjugated donor polymer and to improve the co-planarity,
additional branched alkyl side chains with different lengths
were introduced into the polymer backbone of CN-PTBTBPB
since the introduction of alkyl side chains helps polymer
organization and intermolecular p–p stacking interaction in
+
2
14
28
=
A A D = A ) over the conjugated backbone.
Following these design rationales, great pieces of design
strategy have been used by researchers to attain a reduced
band gap (E = 1.5–2.0 eV) and narrow band gap (E = ,1.5
conjugated polymers consisting of alternating electro-
the solid state. Hence, two structurally related D–A con-
jugated donor polymers (Scheme 1) consisting of alkyl side
chains (i.e., hexyl and octyl groups) substituted at the third
position of the 1,3-di(thiophene-2-yl)benzo[c]thiophene donor
moiety incorporated at the 1,4-bis-(cyano-2-thienylvinylene)
phenylene precursors were used to investigate the influence of
alkyl side chains on their photovoltaic properties.
g
g
1
5,16
eV)
nic push-pull motifs towards extending their light absorption
into the near-infrared (NIR) region and show that the
1
7–21
fabricated BHJ solar cells may have high efficiencies.
Following the above concept, the electron-rich nature of alkyl,
alkoxy side chain substituted thiophene ring based hetero-
cycles incorporated into the 1,4-bis-(1-cyano-2-thienylvinylene)
phenylene precursors (CN-PPV-based derivatives) have been
developed to fabricate BHJ solar cells with efficiencies
Results and discussion
Material synthesis and structural characterization
Scheme 1 summarizes the structures and the synthetic
methods for the synthesis of the benzo[c]thiophene analogs
of carbaldehyde compounds, monomers and polymers. The
key monomers 10 and 11 were prepared according to the
procedure described in the experimental section. As outlined
in Scheme 1, the polymers were synthesized by a chemical
2
2–25
exceeding from 0.3 to 3.3% at AM 1.5G illumination.
Although the results from the BHJ devices indicate that these
conjugated polymers are promising photovoltaic materials, the
performances of these materials are much lower than the
theoretical values as expected from their optical band gap and
frontier molecular orbitals energy values. This extensive
deviation reveals that the photovoltaic performances of
conjugated polymers are not exclusively determined by their
optical band gap and molecular orbital energy levels. Besides
the charge carrier mobility and solubility of the donor
polymers in the processing solvent prior to device fabrication
are also important for improving the photovoltaic perfor-
mance of the polymers. Nevertheless, the properties of these
conjugated polymers are highly sensitive for designing the
molecular structure of the polymers, including the backbone
and alkyl chain architectures, which can be used to fine-tune
the optical band gap of the conjugated polymers. From this
point of view, the introduction of a branched alkyl chain
substituent on the oligobenzo[c]thiophene unit in the main
chain of the conjugated polymer structures are of importance
in increasing the charge carrier mobility, solubility, broad-
ening the absorption spectrum via the strong p–p stacking
3
oxidative polymerization with anhydrous FeCl according to
the conditions described in the experimental section. The
yielded crude polymers were washed extensively with metha-
nol, followed by Soxhlet extraction with methanol and acetone
successively to remove byproducts and oligomers. Finally, the
polymers were extracted with chloroform, recollected by
precipitation in methanol, and dried under vacuum. The
isolated polymers were soluble in almost all common organic
solvents, such as chloroform, dichloromethane, THF, chlor-
obenzene and o-dichlorobenzene. The molecular structures of
the key monomers and polymers were confirmed by FT–IR,
1
mass spectroscopy, H-NMR spectroscopy and elemental
analysis.
Characterization of the CN-PHTBTPB and CN-POTBTPB
polymers
The synthesized polymers have relatively high molecular
2
1
21
weights (CN-PHTBTPB: M
= 42 kg mol , M = 46 kg mol
w
n
2
1
21
2
6
and CN-POTBTPB: M = 39 kg mol , M = 44 kg mol ) and
n
w
ability and also the appropriate frontier molecular orbitals
HOMO and LUMO energy levels), which serve as potential
low polydispersity indexes (PDI) (CN-PHTBTPB:1.08 and CN-
POTBTPB: 1.11), which were determined by Gel Permeation
Chromatography (GPC) in THF relative to a narrow low
(
electron donors in polymer BHJ solar cells.
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Scheme 1 Synthesis of various compounds, monomers and polymers.
molecular weight polystyrene standard and the data is
summarized in Table 1. The molecular structures of the
S1 and Fig. S2, ESI
assigned peaks are found to be consistent with the proposed
structure.
3
in the supporting information), the
1
synthesized polymers were confirmed by H-NMR and ele-
1
mental analysis. The H-NMR spectra of the polymers show
Thermal properties
the chemical shift of the branched alkyl side chains at 0.78–
2
.67 ppm and the methyleneoxy protons (–CH
2
O–) appear in
Thermogravimetric analysis was employed to evaluate the
thermal stability of the polymers since it is one of the
imperative parameters for photovoltaic device fabrication. The
thermal properties of the CN-PHTBTPB and CN-POTBTPB
polymers were investigated by means of thermal gravimetric
analysis in a nitrogen atmosphere using a 20 uC min
temperature ramp from 50 to 800 uC. The thermograms
revealed that both the polymers exhibit a two-stage weight loss
(See Fig. S5a and S5b, ESI in the supporting information). The
3
first stage weight loss occurs at 140 uC, which is assigned to
aromatic ring degradation, and the second stage weight loss
takes place at 330 uC, after which the polymer is then stable up
to 455 uC, this can be attributed to decomposition of the
conjugated polymer back bone. Thus, both the polymers
exhibit good thermal stability and also less than 46% of
material remains available at above 500 uC. Consequently, this
assures us that these polymers are good potential candidates
for the fabrication of optoelectronic devices.
the range of 3.94–4.14 ppm. Furthermore, the aromatic phenyl
ring protons appear at 6.95–7.06 ppm and the vinyl protons
adjacent to the phenyl rings appear at 7.35–7.47 ppm. The
protons of the 1.3-di(thiophene-2-yl) ring of benzo[c]thiophene
moiety adjacent to the oligobenzo[c]thiophene rings and the
aromatic phenyl protons on the oligobenzo[c]thiophene
system are found in the range of 7.05–8.00 ppm. Finally, the
aliphatic protons resonate at 0.84, 1.25, 1.56 and 2.64 ppm
2
1
1
respectively. Thus from the H NMR spectral studies (See Fig.
Table 1 Polymerization results for the CN-PHTBTPB and CN-POTBTPB polymers
2
1 a
21 a
b
Polymer
M
n
(kg mol
)
M
w
(kg mol
)
PDI
Yield
CN-PHTBTPB
CN-POTBTPB
428
398
464
442
1.08
1.11
50%
55%
a
Determined from GPC (eluent: THF; polystyrene standards).
Isolated polymer yield upon polymerization.
b
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Table 2 Optical properties, electrochemical properties, and electronic energy levels of the CN-PHTBTPB and CN-POTBTPB polymers
21
21
a
opt
b
onset
ox
c
d
onset
red
e
f
ec
g
g
Polymer
l
max(nm)/(emax
M
cm
)
ledge(nm)
lem(nm)
E
g
(eV)
E
(V) /HOMO(eV)
E
(V) /LUMO(eV)
E (eV)
CN-PHTBTPB
CN-POTBTPB
405 (36 009)
30 (6685)
410 (66 196)
32 (5378)
663
666
529
528
1.87
1.86
+0.79/25.53
+0.82/25.56
20.935/23.80
20.960/23.78
1.73
1.78
5
5
a
b
c
d
Onset point from absorption spectrum. Optical band gap. Onset point from oxidation potential. HOMO level derived from onset point
e
f
of oxidation potential. Onset point from reduction potential. LUMO level derived from onset point of reduction potential.
g
Electrochemically determined energy band gap.
Photophysical measurement
Electrochemical analysis
The normalized UV-vis absorption spectra and molar extinc-
Cyclic voltammograms were recorded using thin films of CN-
PHTBTPB and CN-POTBTPB polymers prepared via a drop-
casted technique as described in the experimental section. The
CV curves of both polymers showed three quasi-reversible
reduction waves and three quasi-reversible oxidation waves at
a moderately high reduction and oxidation potential respec-
tively, as shown in Fig. 3. On the anodic sweep, both polymers
showed an oxidation peak with an onset potential of 0.79 V for
CN-PHTBTPB and 0.82 V for CN-POTBTPB respectively. In
contrast, the cathodic sweep showed an onset reduction
tion coefficient spectra of polymers CN-PHTBTPB and CN-
2
5
POTBTPB were acquired in chloroform solution (1 6 10 M)
and their absorption data is given in Table 2 (Fig. 1 (a and b).
Both the polymers exhibit two distinct absorption bands: one
broad absorption band in the far UV region (330–430 nm),
which is probably ascribed to the p–p* electronic transition
derived from the close analogues of the alkyl chain substituted
1,3-di(thiophene-2-yl)benzo[c]thiophene conjugated system,
and the other longer wavelength absorption band is in the
visible region (450–530 nm) with a relatively strong absorption
peak which can be assigned to the intramolecular charge
transfer (ICT) of the conjugated polymer framework. Thus,
both the polymers exhibit a reduced band gap of 1.86 eV based
on their respective onset absorption spectrum. The absorption
spectra of both CN-PHTBTPB and CN-POTBTPB in chloroform
solution were essentially identical to each other, with the latter
having a small (ca. 5 nm) red-shift of the absorption
maximum, as well as on onset.
Thus, the absorption spectrum of the synthesized con-
jugated polymers is linearly relative to the effective conjuga-
tion length induced steric hindrance due to the introduction
of branched alkyl side chains at the third position. This
approach significantly reduces the optical band gap (E ) of the
g
resulting polymers (CN-PHTBTPB and CN-POTBTPB) by 0.1–
0
.11 eV, in comparison with the donor polymer without alkyl
2
6a
side chain substitution.
Consequently, these polymers are
suitable for improving the light-harvesting properties.
The florescence spectra of both the polymers were also
2
5
acquired in chloroform (1 6 10
M) with an excitation
wavelength at 400 nm (Fig. 2(a and b)) and the summarized
data is collated in Table 3. Upon excitation of the polymer
solution at 400 nm, both polymers exhibit single emission
band. Additionally, the excitation spectra of both polymers
have a vibronic structure, with peak maxima in the wavelength
range of 420–435 nm and one shoulder peak at 385 nm, while
fluorescence peaks appear at 530 nm. Time-resolved fluores-
cence recorded at the emission maxima (lmax = 525 nm) shows
that the residual (charge transfer) emission of both polymers
in chloroform has two-exponential decay functions (Fig. 2(c
and d)).
Fig. 1 (a) Normalized absorption spectra of CN-PHTBTPB and CN-POTBTPB
2
5
polymers in CHCl
3
of concentration 1 6 10 M. (b) Molar extinction coefficient
spectra of CN-PHTBTPB and CN-POTBTPB polymers.
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Fig. 2 The fluorescence emission spectra of polymers: (a) CN-PHTBTPB emission
max
spectrum (l emi = 529 nm when excited at 400 nm) - excitation spectrum
max
(l
exi = 425 nm detected with an emission wavelength of 530 nm); (b) CN-
max
POTBTPB emission spectrum (l emi = 528 nm when excited at 400 nm) -
max
excitation spectrum (l exi = 430 nm detected with an emission wavelength of
525 nm). Time-resolved emission decay kinetics of (c) CN-PHTBTPB; and (d) CN-
POTBTPB polymer with 525 ¡ 2 nm pulsed excitation.
potential of 20.94 V for CN-PHTBTPB and 20.96 V for CN-
POTBTPB. This was based on the standard redox potential of
the ferrocene/ferrocenium ion (Fc/Fc ), which is assumed to
have an absolute energy level of 24.8 eV relative to vacuum
level and the value of NHE was used as 24.74 eV for
conjugated polymers throughout the study. We evaluated the
energy parameters, the HOMO and LUMO levels, as well as the
electrochemical band gap (Eg,ec) using the following equations:
Fig. 3 Cyclic voltammetric curves of (a) CN-PHTBTPB and (b) CN-POTBTPB
4 6
polymer thin films in acetonitrile (ACN) solution containing 0.1 M n-Bu NPF as
+
the supporting electrolyte. The measurement was recorded from thin films of
the polymers drop-casted from chloroform solutions on a glassy carbon disk as
+
the working electrode (3.0 mm in diameter) against AgCl/Ag as a reference.
21
Each measurement was calibrated with ferrocene at scan rate of 100 mV s
.
Table 2. The HOMO energy levels were thus estimated from the
onset
onset oxidation potentials to be 25.53 eV for CN-PHTBTPB and
HOMO (eV) = 2e (Eox + 4.74) (eV)
25.56 eV for CN-POTBTPB respectively. Indeed, the incorporation
of the electron pushing ability of longer alkyl chains at the
3-position of the 1,3-di(thiophene-2-yl)benzo[c]thiophene donor
moiety in the main chain of conjugated polymer afforded
significant changes in the HOMO energy level of the resulting
polymers. It is worth mentioning that the alteration in the HOMO
energy levels of these polymers is essential to achieve the expected
onset
LUMO (eV) = 2e (Ered + 4.74) (eV)
ec
g
onset
onset
E
= Eox 2 Ered (eV)
onset
onset
where Eox and Ered are the oxidation and reduction potentials
relative to the Ag/Ag electrode. The estimated redox potentials
higher open circuit voltage (V ) values and also making them
oc
+
electron-donor polymers in BHJ solar cells. The estimated LUMO
and the HOMO and LUMO energy levels are summarized in
energy levels from the onset reduction potentials were 23.80 eV
2
Table 3 Photovoltaic properties of the BHJ solar cells based on CN-PHTBTPB:PCBM (D1) and CN-POTBTPB:PCBM (D2) solar device (AM 1.5G, 100 mW cm ) at open
2
air condition
2
2
Device
Active layer thickness (nm)
Jsc (mA cm
)
Voc (V)
FF
PCE (%)
a
b
D1
D2
100
100
4.325
4.887
0.72
0.74
0.53
0.55
1.65 (1.45 ¡ 0.21)
1.98 (1.91 ¡ 0.05)
a
b
The maximum PCE at AM 1.5G condition. The average value of PCE ¡ standard deviation of four devices.
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Fig. 4 Energy band diagram for CN-PHTBTPB and CN-POTBTPB donor
polymers with a PCBM acceptor.
Fig. 5 B3LYP/6–31g (d) optimized ground state geometry of CN-PHTBTPB.
for CN-PHTBTPB and 23.78 eV for CN-POTBTPB respectively. The
electrochemical band gap is found to be ca. 1.73 eV for CN-
PHTBTPB and 1.78 eV for CN-POTBTPB respectively. A complete
picture of the energy band diagram of both polymers (Fig. 4) was
constructed based on the electrochemical results and shows the
deeper HOMO energy level (ca. 25.53 eV for CN-PHTBTPB and ca.
molecular orbitals (HOMO, HOMO-1, HOMO-2 and HOMO-3,
LUMO, LUMO + 1, LUMO + 2 and LUMO + 3) and the
corresponding energy level diagram computed at B3LYP/6-
31g(d) level. As clearly shown in Fig. 7 and 8, the HOMO,
HOMO-2 and LUMO, and LUMO + 1 of both the polymers are
evenly localized on the entire p-conjugated polymer system,
whereas the HOMO-1, HOMO-3 and LUMO + 2 are mainly
localized on the branched alkyl chain substituted 1,3-
25.56 eV for CN-POTBTPB) for the polymers, which is well below
the cut-off for the ambient air stability to oxidation (i.e. ca. 25.2
29
eV). The energy level differences (LUMO offset) was greater than
yca. 0.5 eV between the LUMO levels of both polymers and
LUMO of PCBM (24.3 eV). This assures that the energy level
differences of both the polymers could provide a sufficient driving
force for the direct electron transfer from the photoexcited
polymer to PCBM. Besides, the LUMO energy levels that CN-
PHTBTPB (ca. 23.80 eV) and CN-POTBTPB (ca. 23.78 eV)
possessed were higher than that of the parent CN-PTBTBPB
di(thiophene-2-yl)benzo[c]thiophene
conjugated
system.
Nevertheless, the LUMO + 3 levels of both the polymers are
localized on the alkyl side chain substituted 1,3-di(thiophene-
2- yl)benzo[c]thiophene moiety, as well as the electron-
withdrawing character of the cyano-phenylenevinylene unit.
On the other hand, the HOMO-1, HOMO-3 and LUMO + 2 are
predominantly localized on either ends of both the polymer
systems which indicates that the HOMO A LUMO + 2, HOMO-
27a
polymer (ca. 23.87 eV) without alkyl side chain substitution,
1
A LUMO and HOMO-3 A LUMO + 2 transitions, as
which indicates that the alkyl chains with different substituent
lengths on the 1,3-di(thiophene-2-yl)benzo[c]thiophene analogs
also have a significant effect on the LUMO energy levels (i.e.,
shifting the reduction potentials) of the resulting polymers.
Consequently, the HOMO and LUMO energy levels of the resulting
donor polymers can be fine-tuned further by modifying the
molecular structure of the conjugated polymers as the platform for
highly efficient polymer solar cells.
expected, arise from intramolecular charge-transfer (ICT) and
the other absorptions are mainly of a p A p* nature of polymer
Theoretical calculations
In order to support and clarify the experimental results
further, the structural and electronic properties of CN-
PHTBTPB and CN-POTBTPB at the molecular level, DFT
calculations were carried out with the Gaussian 09 programme
suite polarized at the B3LYP/6-31G(d) level. The optimized
geometries of CN-PHTBTPB and CN-POTBTPB that are given
in Fig. 5 and 6 indicate that both the CN-PHTBTPB and CN-
POTBTPB polymers look like asymmetric molecules. The
computed results revealed that the all the C–C bonds in both
the polymer backbones are partial double bonds, which is
most important for comprehensive p-delocalization to occur
on the polymer system. Fig. 7 and 8 show the frontier
Fig. 6 B3LYP/6–31g (d) optimized ground state geometry of CN-POTBTPB.
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BHJ device structures are represented as follows: [ITO/
PEDOT:PSS(40 nm)/CN-PHTBTPB:PCBM(100 nm)(1 : 1,w/w)/
LiF(5 nm)/Al(110 nm)(D1)] and [ITO/PEDOT:PSS(40 nm)/CN-
POTBTPB:PCBM(100 nm)(1 : 1,w/w)/LiF(5 nm)/Al(110 nm)
(
D2)]. In order to examine the device performance, the semi-
logarithmic current density (J)-voltage (V) characteristics
Fig. 9) of the fabricated devices were measured in the dark
(
and under AM 1.5G illuminations from a calibrated solar
simulator and also J–V curves of the devices (D1 and D2) are
shown in Fig. S6, ESI . The overall results of the maximum and
3
average power conversion efficiency of the solar devices are
summarized in Table 3. From Table 3, the device D1 delivered
an open circuit voltage (Voc) of about 0.72 V, a short circuit
2
2
current (Jsc) of about 4.33 mA cm and a fill factor (FF) of
about 0.53, leading to a power conversion efficiency of about
1
1
.65%. In contrast, the device D2 exhibited an efficiency of
2
2
.98% with a short circuit current (J ) of about 4.89 mA cm
,
sc
Fig. 7 Computed frontier molecular orbitals and the corresponding energy
levels diagram of CN-PHTBTPB.
an open circuit voltage (V ) of about 0.74 V, and a fill factor of
about 0.55.
oc
The typical photoaction spectra (IPCE) of devices (D1 and
D2) are depicted in Fig. 10. The figures show an onset at about
660 nm (1.87 eV) for CN-PHTBTPB:PCBM (D1) and at about
segments. The estimated band gap for both the polymers from
the low energetic edge of the absorption spectrum appearing
opt
at 663 nm is E
g
= 1.87 eV for CN-PHTBTPB and at 666 nm is
opt
E
g
= 1.86 eV for CN-POTBTPB, respectively. The calculated
band gaps from the HOMO and LUMO energy levels of both
DFT
the polymers by DFT are found to be E
PHTBTPB and E ,
g
g
,
= 1.72 eV for CN-
DFT
= 2.36 eV for CN-POTBTPB, respectively.
Consequently, the calculated band gap is in line with the
experimentally determined reduced optical band gap.
Photovoltaic properties
In order to evaluate the photovoltaic properties, typical BHJ
devices were made by sandwiching a mixture of as synthesized
donor polymers (CN-PHTBTPB and CN-POTBTPB) with a
soluble fullerene-based component (PCBM) between the
PEDOT:PSS coated ITO and Al electrodes, respectively. The
Fig. 9 J–V characteristics of (a) ITO/PEDOT:PSS (40 nm)/CN-PHTBTPB:PCBM
(100 nm) (1 : 1 w/w)/LiF(5 nm)/Al(110 nm) (D1), (b) ITO/PEDOT:PSS/CN-
POTBTPB:PCBM (100 nm) (1 : 1 w/w)/LiF(5 nm)/Al (110 nm) (D2).
Fig. 8 Computed frontier molecular orbitals and the corresponding energy
levels diagram of CN-POTBTPB.
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Fig. 10 Photoaction spectra (IPCE) of the BHJ devices (D1 and D2) based on the
CN-PHTBTPB and CN-POTBTPB polymers.
655 nm (1.89 eV) for CN-POTBTPB:PCBM (D2), which is
relatively identical to their respective absorption spectra of the
polymers. The calculated IPCE values of D1 and D2 in the wide
wavelength range from 750 to 300 nm, indicates that both
polymers contribute in the conversion of solar energy into
electrical energy. The IPCE (%) of the D1 device exhibits a
maximum of 34% at 465 nm, which is noticeably lower than
that of device D2 (56% at 473 nm).
Fig. 11 AFM topographic image of (a) CN-PHTBTPB:PCBM (1 : 1 w/w) blend
film in the chlorobenzene and annealed at 120 uC (RMS value (Rq) = 3.259 nm).
(b) The phase contrast image of the blend film.
Both polymers have characteristic moderate open circuit
voltages (around 0.70–0.74 V) due to the electron-pushing
properties of the alkyl chain substituted at the 3-position of
the 1,3-di(thiophene-2-yl)benzo[c]thiophene segment by the
benzene ring system through the cyanovinylene linkage which
plays a key role in fine-tuning the energy levels of the
polymers. The exhibited open circuit voltage of the resulting
polymers is 0.2 ¡ 0.01 V lower than the theoretical voltage
value (0.93 V for CN-PHTBTPB and 0.96 V for the CN-POTBTPB
polymer). The reason for such a difference is due to the
dissociation of the geminate-polaron pair and subsequent
losses in the relaxation of the electron and hole into their
charge transfer state. From the IPCE spectrum of the polymer
blend film (Fig. 10), it can be further seen that a relatively
limited fill factor is observed in the case of the D1 device,
which could be ascribed to the limited exciton dissociation
rate compared to that of the D2 device.
PHTBTPB:PCBM and 1.47 nm for CN-POTBTPB:PCBM blend
film).
As seen clearly from Fig. 11a and 12a, the surface
morphology of the D1 blend film exhibits phase separation
into relatively large islands with domain sizes in the range of
30–110 nm and root-mean-square (rms) roughness (Rq = 3.259
nm for the CN-PHTBTPB:PCBM blend film), whereas the D2
blend film shows small islands with root-mean-square (rms)
roughness (Rq = 2.389 nm for the CN-POTBTPB:PCBM blend
film) which is well supported by their corresponding phase
contrast image (Fig. 11b and Fig. 12b). This large degree of
aggregation domains (domain sizes) is much larger than the
3
0
31
exciton diffusion length of ca. 5–20 nm, which leads to
charge recombination and low charge carrier generation at the
polymer/PCBM heterointerfaces. The XRD analysis results of
the polymer films prepared via a drop-cast technique are
3
amorphous in nature (See Fig. S7, ESI ) and pronounced
intermolecular p–p stacking peaks are observed at 2h = 21.20u
for the CN-PHTBTPB polymer film and at 2h = 21.22u for the
CN-POTBTPB polymer film.
AFM studies
The study of the topographic film morphology and phase
images using AFM on the fabricated devices have provided
deep insight into the significant distinction in the perfor-
mance of the active layers of CN-PHTBTPB:PCBM (D1;
Fig. 11a) and CN-POTBTPB:PCBM (D2; Fig. 12a) devices.
However, both active layers surfaces are smoother, as
32
Experimental section
Materials synthesis
evidenced by their low mean roughness (R
a
) values less than
All reagents and chemicals were purchased from Alfa Aesar
and Sigma-Aldrich unless stated otherwise. All solvents were
2
nm (i.e., 1.96 nm mean roughness for the CN-
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1,4-bis(cyanomethyl)-2,5-bis(octyloxy)benzene (2)
Finely crushed sodium cyanide (0.72 g, 14.82 mmol, 4 eqiv.)
and 1,4-bis(chloromethyl)-2,5-bis(octyloxy)benzene (1) (1.6 g,
3.71 mmol, 1 equiv.) were dissolved in 25 ml of anhydrous
DMF and the mixture was heated to 110 uC and maintained at
the same temperature for 2 h. After being cooled to room
temperature, the mixture was poured into 50 g crushed ice,
upon which a brown solid precipitated. The solid was collected
via filtration and washed with hexane (2 6 10 ml). The crude
brown solid product was again purified by recrystallization
1
from hexane. Yield: 1.4 g (off-white solid), 84%. H-NMR (300
MHz, CDCl , ppm): d 6.89 (s, 2H), 3.98 (m, 4H), 3.95 (s, 4H),
3
1.79 (m, 4H), 1.45–1.30 (m, 20H), 0.90 (t, 6H). Anal. Calcd. for
C H N O : C, 75.68; H, 9.77; N, 6.79. Found: C, 74.96; H,
2
6
40 2 2
9.80; N, 6.70.
1-(3-hexylthiophen-2-yl)-3-(thiophen-2-yl)benzo[c]thiophene
(6)
The titled compound 6 was obtained by a reaction of
thiolactone (3) (3.0 g, 13.90 mmol) with a pale yellow solution
of 3-hexyl-2-thienylmagnesium bromide (4) (prepared via the
slow addition of 20.83 mmol of 3-hexyl-2-bromothiophene
with 16.65 mmol of Mg turnings in THF under nitrogen
atmosphere at RT, resulting in the appearance of a greenish
yellow colour) and Lawesson’s reagent (7.0 mmol) according to
3
4
reported experimental procedure. The solvent was removed
under vacuum to give the crude product, which was
chromatographed on neutral alumina using hexane as eluent
Fig. 12 AFM topographic image of (a) CN-POTBTPB:PCBM (1 : 1 w/w) blend
film in chlorobenzene and annealed at 120 uC (RMS value (Rq) = 2.389 nm). (b)
The phase contrast image of blend film.
to afford the greenish yellow thin liquid (2.82 g, 53% yield).
1
3
H-NMR (400 MHz, CDCl , ppm): d 7.95 (d, 1H, J = 8.8 Hz), 7.49
(
d, 1H, J = 2.7 Hz), 7.38 (d, 2H, J = 5.2 Hz), 7.35 (d, 1H, J = 2.8
Hz), 7.16 (t, 1H, J = 3.6 Hz), 7.07 (t, 1H, J = 5.2 Hz), 6.97 (d, 2H, J
5.2 Hz), 2.75 (t, 2H, J = 7.8 Hz), 1.60 (q, 2H, J = 7.4 Hz), 1.22
freshly distilled prior to use: Tetrahedrofuran was distilled
using Na/benzophenone; dichloromethane was distilled over
=
(
anhydrous phosphorus pentaoxide (P
passed through an alumina column to remove the ethanol
before being distilled over K CO . The tert-butanol (t-BuOH)
was freshly distilled over CaH . All column chromatographic
2
O
5
) and chloroform was
+
m, 6H), 0.84 (t, 3H, J = 7.2 Hz); ESI (MS): m/z: 405.61 [M + Na] .
Anal. Calcd. for C22 : C, 69.06; H, 5.80; S, 25.14, Found: C,
22 3
H S
2
3
6
8.78; H, 5.78; S, 24.85.
2
1
-(3-octylthiophen-2-yl)-3-(thiophen-2-yl)benzo[c]thiophene (7)
separations were carried out on Merck silica gel (230–400
mesh) and neutral alumina (Aluminium Oxide Active,
neutral Activity I–II). Aqueous solutions of poly-(3,4-ethylene-
dioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) and
The titled compound (7) was obtained by a reaction of
thiolactone 3 (3.55 g, 16.43 mmol) with a pale yellow solution
of 3-octyl-2-thienylmagnesium bromide (5) (prepared via the
slow addition of 24.65 mmol of 3-octyl-2-bromothiophene with
[
6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were pur-
chased from Sigma-Aldrich and used as such. The following
compounds were synthesized according to previously reported
procedures:
(
(
(
19.72 mmol of Mg turnings in THF under nitrogen atmo-
sphere at RT, resulting in the appearance of a reddish orange
colour) and Lawesson’s reagent (8.22 mmol) according to the
1,4-bis(chloromethyl)-2,5-bis(octyloxy)benzene
2
7b
33
3
4
1);
1,4-bis(cyanomethyl)-2,5-bis(octyloxy) benzene (2); 1-
experimental procedure described above. The solvent was
removed under vacuum to give crude product, which was
chromatographed on neutral alumina using hexane as eluent
3
4
3-hexylthiophen-2-yl)-3-(thiophen-2-yl)benzo[c]thiophene (6); 1-
3
4
3-octylthiophen-2-yl)-3-(thiophen-2-yl)benzo[c]thiophene (7);
-(1-(3-hexylthiophen-2-yl)benzo[c]thiophen-3-yl)thiophene-2-car-
5
to afford the deep greenish yellow thin liquid (3.5 g, yield:
3
4a
1
baldehyde (8);
thiophene-2-carbaldehyde (9).
thetic routes for the synthesis of various compounds, mono-
mers and polymers are outlined in Scheme 1. The detailed
synthetic processes of the other compounds, monomers and
polymers are as follows.
5-(1-(3-octylthio-phen-2-yl)benzo[c]thiophen-3-yl)
5
2%). H-NMR (400 MHz, CDCl
3
, ppm): d 7.72 (d, 1H, J = 8.8
3
4a
The structure and the syn-
Hz), 7.58 (d, 1H, J = 9.8 Hz), 7.38 (d, 2H, J = 5.1 Hz), 7.35 (d, 1H,
J = 2.6 Hz), 7.28 (t, 1H, J = 4.6 Hz), 7.20 (t, 1H, J = 5.4 Hz), 7.04
(d, 2H, J = 5.4 Hz), 2.76 (t, 2H, J = 7.9 Hz), 1.59 (q, 2H, J = 7.2
Hz), 1.35 (m, 10H), 0.86 (t, 3H, J = 7.2 Hz). ESI (MS): m/z: 409.10
+
[
M 2 H] . Anal. Calcd. for C24
H
26
S
3
: C, 70.19; H, 6.38; S, 23.42;
Found: C, 69.87; H, 6.32; S, 23.11.
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5
in a mixture of tert-butanol (t-BuOH)/tetrahyrofuran (1 : 1)
with 9.8 equiv. of potassium tert-butoxide (t-BuOK) based on
the following procedure.
carbaldehyde (8)
The titled compound (8) was synthesized from the formylation
3
4a
of compound (6) based on a previously reported procedure.
A mixture of POCl (1.5 ml, 15.70 mmol) in DCM (50 ml) was
Monomer (10)
3
A mixture of compound (2) (0.265 g, 0.64 mmol) and the
formylated compound (8) (0.66 g, 1.60 mmol) was dissolved in
a 25 : 25 ml mixture of freshly distilled t-BuOH/THF at room
temperature in a 100 ml flame-dried round bottom flask. A
catalytic amount of potassium tert-butoxide (0.71 g, 6.3 mmol)
was added into the mixture under nitrogen atmosphere and
stirred for 25–30 h at reflux temperature (70–75 uC). After being
cooled to room temperature 50 ml of water was added into the
reaction mixture and stirred for 5–10 min and then the
mixture was extracted two times with (2 6 25 ml) chloroform.
slowly added to DMF (1.2 ml, 15.50 mmol) at 0 uC for 30 min.
The mixture was stirred until a pale yellow thick mass formed.
Subsequently, the pale yellow mass was slowly added into (6)
(
2.0 g, 5.23 mmol) dissolved in 50 ml of DCM at 0 uC. The
resulting reaction mixture was stirred at room temperature for
0–12 h. After completion of the reaction, the solvent was
1
completely removed under vacuum to give a thick reddish
brown liquid. Aqueous NaOH (1 N) was added into the thick
liquid and heated on a steam bath for 1 h and then cooled to
room temperature. The crude product was filtered and air
dried. Then it was purified by column chromatography on
neutral alumina using 50% chloroform/hexane as eluent to
2 4
The chloroform layer was dried over anhydrous Na SO and
the solvent was evaporated under vacuum to afford the
product as a sticky reddish yellow semisolid. The yielded
crude product was chromatographed on silica gel 230–400
mesh using hexane as eluent to afford the pure reddish yellow
afford the product as an orange to reddish solid (1.37 g, yield:
1
8
1
7
3
2
0%). H-NMR (400 MHz, CDCl
3
, ppm): d 9.86 (s, 1H), 7.98 (d,
H, J = 8.8 Hz), 7.59 (d, 1H, J = 9.8 Hz), 7.52 (d, 1H, J = 7.9 Hz),
.40 (d, 1H, J = 5.3 Hz), 7.35 (d, 1H, J = 2.8 Hz), 7.20 (t, 1H, J =
.6 Hz), 7.12 (t, 1H, J = 5.8 Hz), 6.98 (d, 1H, J = 5.3 Hz), 2.64 (t,
H, J = 8.0 Hz), 1.60 (q, 2H, J = 7.2 Hz), 1.21 (m, 6H), 0.88 (t, 3H,
J = 7.0 Hz). HR-MS (ESI): m/z: 410.08 [M] . Anal. Calcd. for
2
1
semisolid (0.35 g, yield: 45%). FT-IR (KBr, cm ): 3074
Aromatic LCH), 2925, 2847 (aliphatic C–H), 2870 (aromatic
(
1
C–H), 2272 (CLN). H NMR (300 MHz, CDCl , ppm): d 8.01 (d,
3
+
1H, J = 8.4 Hz), 7.83(dd, 2H, J = 3.6 Hz & J = 5.4 Hz), 7.68 (s,
8
3
1
H), 7.50 (t, 2H, J = 2.1 Hz), 7.06 (t, 4H, J = 5.4 Hz), 6.96 (d, 2H),
.72 (d, 4H, J = 2.1 Hz), 3.21(t, 4H, J = 7.8 Hz), 1.84 (m, 8H),
23 3
C H22OS : C, 67.28; H, 5.40; S, 23.43. Found: C, 66.98; H, 5.36;
S, 23.15.
.42–1.27 (m, 32H), 0.88 (m, 12H). HR-MS (ESI) (m/z): 1196.21
+
5-(1-(3-octylthiophen-2-yl)benzo[c]thiophen-3-yl)thiophen-e-2-
[M 2 H] . Anal. Calcd. for C H N O S : C, 72.20; H, 6.73; N,
7
2
80 2 2 6
carbaldehyde (9)
2.34; O, 2.67; S, 16.06, Found: C, 72. 50; H, 9.52; N, 2.06; S,
5.79.
1
The titled compound (9) was synthesized from the formylation
of compound (7) based on the described experimental
Monomer (11)
3
4a
procedure.
DCM (50 ml) was slowly added to DMF (2.0 ml, 25.83 mmol) at
uC for 30 min. The mixture was stirred until a pale yellow
A mixture of POCl3 (2.55 ml, 26.70 mmol) in
A mixture of compound (2) (0.18 g, 0.44 mmol) and the
formylated compound (9) (0.48 g, 1.1 mmol) was dissolved in a
0
25 : 25 ml mixture of freshly distilled t-BuOH/THF at room
thick mass formed. Subsequently, the pale yellow mass was
slowly added into (7) (3.4 g, 8.28 mmol) dissolved in 50 ml of
DCM at 0 uC. The resulting reaction mixture was stirred at
room temperature for 10–12 h. After completion of the
reaction, the solvent was completely removed under vacuum
to give a thick reddish brown liquid. Aqueous NaOH (1 N) was
added into the thick liquid and heated on a steam bath for 1 h
and then cooled to room temperature. The crude product was
filtered and air dried. Then it was purified by column
chromatography on neutral alumina using 50% chloroform/
temperature in a 100 ml flame-dried round bottom flask. A
catalytic amount of potassium tert-butoxide (0.71 g, 6.3 mmol)
was added into the mixture under nitrogen atmosphere and
stirred for 25–30 h at reflux temperature (70–75 uC). After being
cooled to room temperature, 50 ml of water was added into the
reaction mixture and stirred for 5–10 min and then the
mixture was extracted two times with (2 6 25 ml) chloroform.
The chloroform layer was dried over anhydrous Na SO and
2
4
the solvent was evaporated under vacuum to afford the
product as a sticky reddish yellow semisolid. The yielded
crude product was chromatographed on silica gel 230–400
mesh using hexane as eluent to afford the pure reddish yellow
hexane as eluent to afford the product as an orange to reddish
1
solid (0.5 g, yield: 85%). H-NMR (400 MHz, CDCl , ppm): d
3
9
(
.84 (s, 1H), 7.78 (d, 1H, J = 8.8 Hz), 7.60 (d, 1H, J = 10 Hz), 7.54
d, 1H, J = 8.2 Hz), 7.36 (d, 1H, J = 5.2 Hz), 7.35 (d, 1H, J = 2.6
Hz), 7.24 (t, 1H, J = 4.6 Hz), 7.17(t, 1H, J = 5.4 Hz), 7.01 (d, 1H, J
5.3 Hz), 2.74 (t, 2H, J = 7.9 Hz), 1.58 (q, 2H, J = 7.2 Hz), 1.34
m, 10H), 0.84 (t, 3H, J = 7.0 Hz). HR-MS (ESI): m/z: 439.11 [M +
2
1
semisolid (1.2 g, yield: 56%). FT-IR (KBr, cm ): 3083
(
Aromatic LCH), 2924, 2854 (aliphatic C–H), 2860 (aromatic
1
C–H), 2269 (CLN). H NMR (300 MHz, CDCl , ppm): d 7.50 (d,
3
=
(
1H, J = 3.6 Hz), 7.44 (d, 1H, J = 5.1 Hz), 7.35 (dd, 2H, J = 3.6 Hz
+
& J = 5.1 Hz), 7.25 (s, 8H), 7.15 (t, 2H, J = 3.6 Hz), 7.07 (t, 4H, J =
H] . Anal. Calcd. for C H OS : C, 68.45; H, 5.97; S, 21.93.
2
5
26
3
5.1 Hz), 6.97 (d, 2H, J = 4.8 Hz), 3.94 (d, 4H, J = 2.1 Hz), 2.67 (t,
Found: C, 68.12; H, 5.90; S, 21.68.
4
H, J = 7.8 Hz), 1.62 (m, 8H), 1.25 (m, 40H), 0.85 (m, 12H). HR-
+
Synthesis of monomers
MS (ESI) (m/z): 1252.42[M
2
H] . Anal. Calcd. for
C
76
H
88
N
2
O
2
S
6
: C, 72.80; H, 7.07; N, 2.23; S, 15.34. Found: C,
The synthesis of the monomers was accomplished by
Knoevenagel condensation of the 1,4-bis(cyanomethyl)-2,5-
bis(octyloxy)benzene (2) with aldehyde compounds (8 and 9)
72.63; H, 9.91; N, 1.97; S, 15.05.
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General polymerization procedure
Photophysical measurements
The polymers CN-PHTBTPB and CN-POTBTPB were synthe-
The UV-vis absorption spectra and emission spectra were
recorded on a double beam ultraviolet-visible spectrophot-
ometer (Model: T90+, PG Instruments Ltd, UK) and Shimadzu
spectrofluorometer (RF-5301 PC), respectively. The optical
band gap was calculated from the onset of the absorption
spectra. Fluorescence lifetime measurements were carried out
in a picosecond time correlated single photon counting
sized by chemical oxidative polymerization using anhydrous
3
5
FeCl based on the Sugimoto et al. procedure. The dissolved
3
monomer (10; 0.70 g, 0.58 mmol and 11; 1.0 g, 0.80 mmol) in
chloroform were added into the suspended anhydrous FeCl
1.1 equivalents) in 20 ml chloroform under nitrogen atmo-
3
(
sphere. The resulting mixture was stirred at room temperature
for 48 h and diluted with more chloroform (50 ml). The dark
mixture was treated with a dilute solution of 100% hydrazine
(
(
TCSPC) spectrometer with
TSUNAMI, Spectra physics, USA) as the excitation source of
a tunable Ti-sapphire laser
the 400–420 nm excitation wavelength. The fluorescence decay
curves were analyzed using the software provided by IBH (DAS-
2 2 2
hydrate (NH ?NH ?H O). Upon solvent evaporation, the pro-
ducts were afforded as given below:
6).
CN-PHTBTPB polymer
Electrochemical analysis
2
1
3
3
Dark violet powder, Yield: 50%. UV-vis (CHCl ): lmax (e 6 M
2
1
Electrochemical measurements were recorded on a computer
controlled AUTOLAB-potentiostat-galvanostat in a conven-
tional three-electrode configuration using polymer thin film
drop-casting from chloroform on a glassy carbon disk as the
working electrode (3.0 mm in diameter), platinum-wire as the
cm ): 530 (6685) and 405 nm (36 009). Fluorescence in CHCl
excited at 400 nm): 530 nm. FT-IR (KBr, cm ): 3078 (aromatic
2
1
(
LCH), 2928, 2850 (aliphatic C–H), 2872 (aromatic C–H), 2269
1
(
CLN). H NMR (300 MHz, CDCl , ppm): d 7.56 (d, 1H, J = 8.4
3
Hz), 7.37 (dd, 2H, J = 3.6 Hz & J = 5.4 Hz), 7.17 (s, 8H), 7.11 (t,
+
counter electrode, and Ag/Ag (0.1 M) as the reference
2
3
1
H, J = 2.1 Hz), 7.08 (t, 4H, J = 5.4 Hz), 6.97 (d, 2H, J = 4.8 Hz),
electrode in argon-saturated acetonitrile (CH CN, HPLC grade)
3
.92 (d, 4H, J = 2.1 Hz), 2.65 (t, 4H, J = 7.8 Hz), 1.60 (m, 8H),
2
1
solution containing 0.1 M tetrabutylammonium hexafluoro-
phosphate (Bu NPF ) as the supporting electrolyte. The
4 6
.23 (m, 32H), 0.84 (m, 12H). GPC analysis: M
n
= 42 kg mol
,
2
1
Mw = 46 kg mol , PDI = 1.08. Anal. Calcd. for C H N O S :
72
80 2 2 6
potential was internally calibrated using ferrocene/ferroce-
C, 72.20; H, 6.73; N, 2.34; S, 16.06; Found: C, 73.10; H, 9.49; N,
.06; S, 15.80.
+
nium (Fc/Fc ) as the redox couple which was estimated to have
2
an oxidation potential of 24.8 eV vs. vacuum. The HOMO and
CN-POTBTPB polymer
LUMO energy levels of the copolymer were estimated based on
a value of 24.8 eV for Fc/Fc with respect to the zero vacuum
level.
+
2
1
3
Dark violet powder, Yield: 55%. UV-vis (CHCl ): l
(e 6 M
3
max
2
1
cm ): 532 (5378) and 410 nm (66 196). Fluorescence in CHCl
2
1
(excited at 400 nm): 529 nm. FT-IR (KBr, cm ): 3081(aromatic
Film morphology
LCH), 2929, 2852 (aliphatic C–H), 2860 (aromatic C–H), 2269
Topographic and phase images of the blend polymer (active
material) were measured using an atomic force microscope
1
3
(CLN). H NMR (300 MHz, CDCl , ppm): d 7.50 (d, 1H, J = 3.6
Hz), 7.44 (d, 1H, J = 5.1 Hz), 7.35 (dd, 2H, J = 3.6 Hz & J = 5.1
Hz), 7.25 (s, 8H), 7.15 (t, 2H, J = 3.6 Hz), 7.07 (t, 4H, J = 5.1 Hz),
(
AFM, AFM, XE-100, Park Systems), operated in a non-contact
mode with commercially available Si cantilevers (crystal silicon
tip), with a resonant frequency (F ) of 300–350 KHz) and a
typical spring constant (K) of 20–100 N m . The scanner was a
00 micrometer scanner, and the scan rate was 0.50–1 Hz in
6.97 (d, 2H, J = 4.8 Hz), 3.94 (d, 4H, J = 2.1 Hz), 2.67 (t, 4H, J =
o
7
.8 Hz), 1.62 (m, 8H), 1.25 (m, 40H), 0.85 (m, 12H). GPC
21
2
1
21
n w
analysis: M = 39 kg mol , M = 44 kg mol ; PDI = 1.11.
1
Anal. Calcd. for C H N O S : C, 72.80; H, 7.07; N, 2.23; S,
7
6
88 2 2 6
air. The off-line image processing and analysis (i.e., roughness)
were performed using DI off-line software (XEI Corp). Several
locations of the film were examined to ensure uniformity and
reproducibility.
15.34. Found: C, 72.63; H, 9.90; N, 1.96; S, 15.05.
Characterization methods
1
H-NMR spectra were realized on a 300/400 MHz BRUKER
spectrometer in deuterated chloroform solution at 298 K,
unless specified otherwise. Chemical shifts (d values) were
recorded in units of ppm relative to tetramethylsilane (TMS) as
an internal standard. The average molecular weights and
polydispersity index of the polymers were determined by a Gel
Permeation Chromatography (GPC) method in THF as eluent
by referring to polystyrene standards. Elemental analyses were
Density functional theory
All calculations were performed using the Gaussian 09
3
6
programme package. The polymers were optimized at the
hybrid exchange–correlation B3LYP functional in conjugation
with the 6–31 g(d) basis set. It has already been shown that
B3LYP/6–31 g(d) gives decent ground state structures of
3
7
conjugated polymers.
performed on
Thermogravimetric analysis (TGA) was conducted on
Perkin Elmer Pyris-1 thermogravimetric analyzer with a
a
Vario EL III elemental analyzer.
Polymer solar cell fabrication and characterization
a
For the solar cell device fabrication, indium tin oxide (ITO)-
2
1
22
heating rate of 20 uC min under a nitrogen atmosphere.
coated conducting glass substrates (15–20 V cm ) were used
The electrochemical cyclic voltammetry was conducted on a
as the anode and aluminium as the cathode. First, ITO
substrates were patterned in isolated configuration, followed
by ozonization for 90 min and then cleaned with detergent,
ultrasonicated in deionized water, acetone, and isopropanol
computer
controlled
AUTOLAB-potentiostat/galvanostat,
Netherlands.
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for 20 min in each solvent and finally dried in an oven at 100
uC for 1 h. The thickness and roughness of ITO were 110 and
calculated for each wavelength using a calibrated monocrystal-
line silicon diode as reference.
10 nm, respectively, measured using a Tencor Alpha Step 500
thickness profilometer. hole transporting layer of
A
PEDOT:PSS was spin-coated on the ITO substrate at 1000
rpm for 1 min and was annealed at 100 uC for 30 min on a hot
plate under vacuum. A blend mixture consisting of donor
polymers (CN-PHTBTPB and CN-POTBTPB):PCBM was made
in anhydrous chlorobenzene (1 : 1 weight ratio at a concentra-
Conclusion
In conclusion, conjugated donor polymers comprising alkyl
side chains with different lengths substituted at the 3-position
of the 1,3-di(thiophene-2-yl)benzo[c]thiophene donor moiety
coupled with 1,4-bis-(cyano-2-thienylvinylene) phenylene pre-
cursors were synthesized by Knoevenagel condensation reac-
tion followed by chemical oxidative polymerization. The
presence of the branched alkyl side chains with different
lengths at the 3-position of 1,3-di(thiophene-2-yl)benzo[c]thio-
phene unit resulted in deeper HOMO and higher-lying LUMO
energy levels and a reduced band gap which is propitious to
increase the moderate VOC values and light-harvesting proper-
ties. The maximum PCE of the device based on the CN-
POTBTPB polymer reached about 1.98%, whereas the device
based on the CN-PHTBTPB polymer exhibited a PCE of 1.65%.
Nevertheless, the fabricated device based on the CN-PHTBTPB
polymer blend film renders a relatively limited fill factor and
efficiency, which is due to the phase separation into relatively
large island domains found via AFM study and thus, a clear
correlation was revealed between the blend film morphology
and the power conversion efficiency of the devices. Further,
the AFM images of both the blend films also correlated well
with the IPCE spectra of the respective active layer.
2
1
tion of 10 mg mL ). In order to attain homogeneity, the
solution was stirred for 5 h at 60 uC and then filtered using a
PTFE filter (0.2 mm pore size). Then the above mixture was
spin-coated on top of the PEDOT:PSS layer at 800 rpm for 1
min and the resulting films were dried at 100 uC under high
vacuum for 30 min. The thickness of the PEDOT:PSS (40 nm)
and donor polymer:PCBM (100 nm) layers were measured
using the above mentioned profilometer after the surface of
the films had been cut with a diamond cutter to expose the
substrate. Then, the lithium fluoride (LiF) and aluminium
layer of thickness 5 nm and 110 nm was deposited on the
polymer films by thermal evaporation at a pressure of less
2
6
than 5 6 10 mbar to form the cathode. We fabricated a
polymer solar cell device with the typical structures as follows:
ITO/PEDOT:PSS(40 nm)/CN-PHTBTPB:PCBM (100 nm)
(1 : 1,w/w)/LiF(5 nm)/Al(110 nm)(D1) and ITO/PEDOT-PSS(40
nm)/CN-POTBTPB:PCBM (100 nm) (1 : 1,w/w)/LiF(5 nm)/
Al(110 nm)(D2). Such a fabricated device consisted of 4
2
isolated devices per substrate with an active area of 0.25 cm
per device (Fig. 13) and the thickness of the photoactive layer
was 100 nm. After encapsulation by UV-curing glue, the
electrical characterization (such as the short circuit photo-
current density (Jsc), open circuit voltage (Voc) and fill factor
Acknowledgements
(FF)) of the encapsulated devices was performed in air using a
Authors thank DST, New Delhi (Ref. No.SR/S1/PC-49/2009) and
DST-FIST, New Delhi (SR/FT/CSI-190/2008 dated 16th Mar
Keithley 2400 semiconductor characterization system and an
Oriel white light solar simulator operating at an intensity of
2
2
2008) for the sanction of research fund towards the develop-
100 mW cm under AM 1.5 G (air mass 1.5 global) condition.
ment of new facilities. The authors thank Prof. S. Sundar
Kumar Iyer for given permission to fabricate solar cell devices
in Organic Electronics laboratory, IITK, India.
The incident photon to current conversion efficiency (IPCE)
as a function of wavelength was measured with an Oriel 300 W
Xe Arc lamp in combination with an Oriel Cornerstone 260J
monochromator. The number of incident photons was
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