Ladder-type oligo- p -phenylene-containing copolymers with high open-circuit voltages and ambient photovoltaic activity

Article abstract of DOI:10.1021/ja909111p

Four ladder-type oligo-p-phenylene containing donor-acceptor copolymers were designed, synthesized, and characterized. The ladder-type oligo-p-phenylene was used as an electron donor unit in these copolymers to provide a deeper highest occupied molecular orbital (HOMO) level for obtaining polymer solar cells with a higher open-circuit voltage, while 4,7-dithien-2-yl-2,1,3- benzothiadiazole or 5,8-dithien-2-yl-2,3-diphenylquinoxaline was chosen as an electron acceptor unit to tune the electronic band gaps of the polymers for a better light harvesting ability. These copolymers exhibit field-effect mobilities as high as 0.011 cm²/(V s). Compared to fluorene containing copolymers with the same acceptor unit, these ladder-type oligo-p-phenylene containing copolymers have enhanced and bathochromically shifted absorption bands and much better solubility in organic solvents. Photovoltaic applications of these polymers as light-harvesting and hole-conducting materials are investigated in conjunction with [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). Without extensive optimization work, a power conversion efficiency (PCE) of 3.7% and a high open-circuit voltage of 1.06 V are obtained under simulated solar light AM 1.5 G (100 mW/cm²) from a solar cell with an active layer containing 20 wt % ladder-type tetra-p-phenylene containing copolymer (P3FTBT6) and 80 wt % PC₆₁BM. Moreover, a high PCE of 4.5% was also achieved from a solar cell with an active layer containing 20 wt % P3FTBT6 and 80 wt % PC ₇₁BM.

Full text of DOI:10.1021/ja909111p

Published on Web 01/26/2010
Ladder-Type Oligo-p-phenylene-Containing Copolymers with
High Open-Circuit Voltages and Ambient Photovoltaic Activity
Qingdong Zheng, Byung Jun Jung, Jia Sun, and Howard E. Katz*
Department of Materials Science and Engineering and Department of Chemistry, The Johns
Hopkins UniVersity, 102 Maryland Hall, 3400 North Charles Street, Baltimore, Maryland 21218
Received October 29, 2009; E-mail: hekatz@jhu.edu
Abstract: Four ladder-type oligo-p-phenylene containing donor-acceptor copolymers were designed,
synthesized, and characterized. The ladder-type oligo-p-phenylene was used as an electron donor unit in
these copolymers to provide a deeper highest occupied molecular orbital (HOMO) level for obtaining polymer
solar cells with a higher open-circuit voltage, while 4,7-dithien-2-yl-2,1,3-benzothiadiazole or 5,8-dithien-
2-yl-2,3-diphenylquinoxaline was chosen as an electron acceptor unit to tune the electronic band gaps of
the polymers for a better light harvesting ability. These copolymers exhibit field-effect mobilities as high as
0.011 cm2/(V s). Compared to fluorene containing copolymers with the same acceptor unit, these ladder-
type oligo-p-phenylene containing copolymers have enhanced and bathochromically shifted absorption bands
and much better solubility in organic solvents. Photovoltaic applications of these polymers as light-harvesting
and hole-conducting materials are investigated in conjunction with [6,6]-phenyl-C61-butyric acid methyl
ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Without extensive optimization
work, a power conversion efficiency (PCE) of 3.7% and a high open-circuit voltage of 1.06 V are obtained
under simulated solar light AM 1.5 G (100 mW/cm2) from a solar cell with an active layer containing 20 wt
% ladder-type tetra-p-phenylene containing copolymer (P3FTBT6) and 80 wt % PC61BM. Moreover, a high
PCE of 4.5% was also achieved from a solar cell with an active layer containing 20 wt % P3FTBT6 and 80
wt % PC71BM.
1. Introduction
the current density and voltage at the maximum power output;
and FF is the fill factor. In-depth investigation of the solar cell
devices based on the P3HT:PCBM system reveals that its
efficiency is limited by low open-circuit voltage Voc (∼0.6 V)
and the relatively large band gap of P3HT which limits its
harvesting ability (low Jsc). Therefore, in recent years many low
band gap polymers have been developed to increase the light
absorbing ability of solar cell devices,11-21 because the light
In the past decade, there has been increasing interest in
polymer solar cells due to the growing demand for “green” and
sustainable energy. Compared to today’s inorganic solar cells,
conjugated polymer-based solar cells are expected to be cheaper
because they can be fabricated at a much lower cost with the
aid of large area solution casting or roll-to-roll manufacturing
of flexible modules.1-3 Since the introduction of the bulk
heterojunction (BHJ) concept,4 regioregular poly(3-hexyl-
thiophene) (P3HT) has been chosen as the standard electron
donor material in polymer BHJ solar cells. The highest power
conversion efficiency reported for the polymer blend of P3HT
and [6,6]-phenyl C61 butyric acid methyl ester (PC61BM, or
PCBM) is ∼5%.5–10 It is well-known that the power conversion
efficiency and fill factor of a solar cell device are calculated
according to the following equations:
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η ) (FF × Jsc × Voc)/Pin
(1)
(2)
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FF ) (Jmax × Vmax)/(Jsc × Voc)
where Pin is the input power; Jsc and Voc are the short-circuit
current and open-circuit voltage, respectively; Jmax and Vmax are
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Ladder-Type Oligo-p-phenylene-Containing Copolymers
A R T I C L E S
absorbing ability is directly related to the value of short-circuit
current Jsc. However, the open-circuit voltages for most of these
low band gap polymer-based solar cells are in the range
0.35-0.80 V.11–13,15,18,21 It has been reported that there is a
correlation between Voc and the difference between the highest
occupied molecular orbital (HOMO) of the donor and the lowest
unoccupied molecular orbital (LUMO) of the acceptor used in
bulk heterojunction (BHJ) solar cells. Therefore, reduction of
the band gap of a donor polymer will also reduce the open-
circuit voltage, because of a decreased energy difference
between the HOMO of a polymer and the LUMO of an acceptor
(e.g., PC61BM).
To achieve a polymer solar cell device with a high light
harvesting ability (low band gap) as well as a high open-circuit
voltage, one feasible approach is to design alternating donor-
acceptor copolymers, where the electron donor unit may provide
a deeper HOMO level and the electron acceptor unit is used to
tune the electronic band gap of the polymers. Recently, this
kind of donor-acceptor polymer has been successfully used
for high performance solar cells by choosing fluorene or
carbazole as the electron donor and benzothiadiazole, quinoxa-
line, or thienopyrazine as the electron acceptor.22-32
compared to fluorene derivatives and, thus, result in an enhanced
solar light harvesting. At the same time, for every repeat unit
in the ladder-type oligo-p-phenylene containing polymers, there
are at least four alkyl chains on the polymer backbone.
Solubilizing alkyl chains can be easily introduced into this
unique molecular backbone, which may provide a better solution
processability of the target polymers. As a result, indenofluorene
or ladder-type oligo-p-phenylene has been chosen as a building
block for materials with various applications.33-38 However,
to the best of our knowledge, there is no work on the ladder-
type oligo-p-phenylene containing copolymers for photovoltaic
(PV) applications. In this work, we choose ladder-type oligo-
p-phenylenes as electron donor building blocks and 4,7-dithien-
2-yl-2,1,3-benzothiadiazole or 5,8-dithien-2-yl-2,3-diphenylqui-
noxaline as an electron acceptor building block to obtain
copolymers with deeper HOMO energy levels, broader spectral
absorption ranges, and improved phase separation properties
with PCBMs. Among polymers for PV applications, the hexyl
group has a good balance between crystallinity and miscibility
in the bid to achieve optimal morphology. On the other hand,
polymers with decyl groups have better solubility compared to
those with hexyl groups,39 and the decyl group was reported to
be a good side chain for fluorene-containing polymers to achieve
high power conversion efficiency.28 Therefore, in this work, both
hexyl and decyl are chosen as side chains for targeted soluble
copolymers. We report on the synthesis, characterization,
photophysical properties, field effect transistor behaviors, and
photovoltaic properties of these ladder-type oligo-p-phenylene
containing copolymers.
The structure of ladder-type oligo-p-phenylenes consists of
several “linearly overlapping” fluorenes, and it would serve as
a good electronic donor owing to its extended π-conjugation.
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derivatives may lead to a broader, more intense absorption band
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2.1. Synthesis and Characterization. The syntheses of the
four new copolymers are shown in Scheme 1. They were
prepared by a palladium-catalyzed Suzuki coupling reaction
between 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (3)
or 5,8-bis-(5-bromothiophen-2-yl)-2,3-diphenylquinoxaline (4),
and three diboronated ladder-type oligo-p-phenylenes (1a-b,
2). For each copolymer, an end-capping reaction was performed
using bromobenzene and phenyl boronic acid to increase the
stability of the polymer. The synthesis of monomers 1a-b and
2 is outlined in Scheme 2. As shown in Scheme 2, the
preparation of comonomers 1a-b started from compounds
5a-b, which were selectively brominated at the 2- and
8-positions, with copper(II) bromide on an aluminum oxide
matrix in carbon tetrachloride affording 2,8-dibromo-6,6′,12,12′-
tetraalkyl-6,12-dihydroindeno-[1,2b]fluorene (6a-b). Then com-
pounds 6a-b were reacted with 4,4,4′,4′,5,5,5′,5′-octamethyl-
2,2′-bi(1,3,2-dioxaborolane) to yield compounds 1a-b with the
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Zheng et al.
Scheme 1. Synthesis of Ladder-Type Oligo-p-phenylene-Containing Copolymers
aid of Pd(dppf)Cl2. In the first step of the synthesis of
comonomer 2, compound 8 was prepared by a 2-fold Suzuki
coupling reaction between 2,7-bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9,9-dihexylfluorene (7) and methyl 2-bro-
mobenzoate, using (PPh3)4Pd(0) as catalyst in a mixture of
toluene and an aqueous solution of K2CO3 (2 M). Nucleophilic
addition to the carboxylic ester (8) by n-hexyllithium gave the
corresponding tertiary alcohol, and then the resulting tertiary
alcohol was converted to compound 9 by a ring-closure reaction
in the presence of boron trifluoride etherate, via an intramo-
lecular Friedel-Crafts alkylation. In the third step, compound
9 was selectively brominated at the 2- and 10-positions by
copper(II) bromide on an aluminum oxide matrix in carbon
tetrachloride. Finally compound 10 was reacted with 4,4,4′,4′,
5,5,5′,5′-octamethyl-2,2′-bi(1,3,2-dioxaborolane) using catalytic
amounts of Pd(dppf)Cl2 and potassium acetate in DMF to
provide compound 2. The synthesis of the acceptor comonomers,
4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (3) and 5,8-
bis(5-bromothiophen-2-yl)-2,3-diphenylquinoxaline (4), were
prepared according to a literature procedure.40 For comparison
purposes, we also prepared a fluorene-containing copolymer
(PFTBT6) as shown in Scheme 1. The structures of the
polymers were determined with NMR spectroscopy. Gel
permeation chromatography (GPC) results showed that these
polymers have weight-averaged molecular weights of 21-64
kg/mol with polydispersity indexes (PDI) of 1.8-2.7 (Table 1).
These polymers have very good solubility in many organic
solvents such as THF, toluene, chloroform, chlorobenzene, etc.
due to the four or six alkyl chains flanked on both sides of the
oligo-phenylenes. For example, the solubility of the indenof-
luorene-containing copolymer in chlorobenzene is at least
several times higher than its analogous fluorene-containing
copolymer with the same side chains, which will make the
solution processing of solar cell devices easier.
2.2. Optical Absorption and Photoluminescence. The optical
absorption and emission spectra of the four polymer solutions
(in chlorobenzene) are shown in Figure 1a. For comparison,
the absorption spectrum of PFTBT6 is also included in Figure
1a. The pure polymer films and the films blended with PC61BM
(or PC71BM) are shown in Figure 1b-c. Similar to the fluorene-
containing polymer PFTBT6, all of the four copolymers
exhibitedtwoabsorptionpeaks,atypicalfeaturefordonor-acceptor
copolymers. As shown in Figure 1a, the absorption peaks around
400 nm originate from the π-π* transition of indenofluorene
or ladder-type tetra-p-phenylene units, while the absorption
(40) (a) Hou, Q.; Xu, Y.; Yang, W.; Yuan, M.; Peng, J.; Cao, Y. J. Mater.
Chem. 2002, 12, 2887–2892. (b) Tsami, A.; Bunnagel, T. W.; Farrell,
T.; Scharber, M.; Choulis, S. A.; Brabec, C. J.; Scherf, U. J. Mater.
Chem. 2007, 17, 1353–1355.
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A R T I C L E S
Scheme 2. Synthesis of Comonomers 1a-b and 2
Table 1. Molecular Weight and Optical Absorption of Polymers in Solution and Solid Statea
absorption
emission
Mn
(kg/mol)
Mw
(kg/mol)
polymers
PDI
solution
film
solution
film
Egopt (eV)b
PIFTBT10
PIFDTQ10
PIFTBT6
P3FTBT6
26.3
7.8
22.5
14.2
63.5
21.0
45.9
24.9
2.42
2.65
2.03
1.75
402,542
400,525
402,542
415,538
399,543
395,530
399,541
408,531
613
572
613
613
618
671
618
618
1.97
2.00
1.97
1.96
a Polymers are dissolved in chlorobenzene for absorption and emission spectra of solution samples. b Estimated from the onset of the absorption
spectra of thin films.
peaks around 540 nm come from the π-π* transition of the
low band gap acceptor unit. Copolymers PIFTBT10 and
PIFTBT6 have the same conjugated polymer backbone, but
with different lengths of the alkyl chains. In Figure 1a, we can
see that their absorption spectra are identical indicating that the
alkyl chains will not change the π-conjugated system of
polymers. Compared to PFTBT6, both peaks of PIFTBT6
bathochromically shift around 16-19 nm due to the extended
π-conjugated system by replacing the fluorene unit with the
indenofluorene unit. At the same time, an increased absorption
extinction coefficient was found for PIFTBT6 compared to that
for PFTBT6. Furthermore, the copolymer P3FTBT6 shows a
more red-shifted linear absorption compared to PFTBT6
especially for the short wavelength peak, where there is a 32
nm shift from 383 to 415 nm. Besides the bathochromically
shifted absorption band, the absorption extinction coefficient
for P3FTBT6 at the short wavelength peak is nearly double
compared to PFTBT6, which also suggests an improved
sunlight absorption ability. The difference between PIFTBT10
and PIFDTQ10 is the acceptor unit. PIFTBT10 shows a red-
shifted linear absorption compared to PIFDTQ10 because the
4,7-dithien-2-yl-2,1,3-benzothiadiazole unit in PIFTBT10 is a
stronger acceptor compared to the 5,8-dithien-2-yl-2,3-diphen-
ylquinoxaline unit in PIFDTQ10. The three copolymers with
the 4,7-dithien-2-yl-2,1,3-benzothiadiazole repeat unit show
almost the same emission spectrum either in solution phase or
in the solid film state (Figure S1 in the Supporting Information).
PIFDTQ10 exhibits an ∼41 nm hypsochromically shifted
emission compared to PIFTBT10 in a chlorobenzene solution.
However, in the solid film state PIFDTQ10 shows a 53 nm
bathochromically shifted emission compared to PIFTBT10. This
can be attributed to the two phenyl groups in the 5,8-dithien-
2-yl-2,3-diphenylquinoxaline unit stretching out from the plane
of the polymer backbone in the solution state, whereas in the
solid state they may be coplanar with the quinoxaline plane.
For some polymers such as P3HT, there is a large red shift of
the absorption peak in going from solution phase to solid phase.
However, the other three polymers with the 4,7-dithien-2-yl-
2,1,3-benzothiadiazole repeat unit show little difference for the
absorption and emission spectra in solutions and solid state films.
This may be due to the more rigid polymer backbones in these
three polymers which prevent their conformal change in the
solid state. For polymers PIFTBT10 and PIFTBT6, the optical
band gap was found to be 1.97 eV as determined from the onset
of the absorption spectra for the solid state films. The optical
band gaps of PIFDTQ10 and P3FTBT6 were determined to
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A R T I C L E S
Zheng et al.
Figure 2. Cyclic voltammetry (CV) of the ladder-type oligo-p-phenylene
containing copolymers.
(Bu4NBF4) in acetonitrile. Under these conditions, the onset
oxidation potential (E1/2 ox) of ferrocene was -0.02 V versus
Ag/Ag+. The HOMO energy level of polymers was determined
from the oxidation onset of the second scan from CV data. It is
assumed that the redox potential of Fc/Fc+ has an absolute
energy level of -4.80 eV to vacuum.41 The energy of HOMO
and LUMO levels were calculated according to the following
equations:
EHOMO ) -(φox + 4.82) (eV)
ELUMO ) -(φred + 4.82) (eV)
(3)
(4)
where φox and φred are the onset oxidation potential and the onset
reduction potential vs Ag/Ag+ respectively. Cyclic voltammo-
grams of these four polymers (PIFTBT10, PIFDTQ10, PIFT-
BT6, P3FTBT6) are shown in Figure 2. Both reversible
reduction and oxidation behaviors were observed for all four
polymers, indicating the good structural stability of these
polymers in the charged state. For PIFTBT10, the onset
potentials for oxidation are located around +650 mV versus
Ag/AgNO3, which corresponds to a highest occupied molecular
orbital of -5.47 eV. The LUMO energy level of PIFTBT10 is
calculated to be ∼ -3.44 eV based on the onset potential for
reduction at around -1.37 eV. The electrochemical bandgap
of PIFTBT10 is determined to be 2.03 eV, which is quite close
to the band gap (1.97 eV) predicted by solid state optical
absorption of the polymer films. The small difference (<0.1 eV)
between electrochemical band gap and optical band gap for
PIFTBT10 is similar to some other amorphous polymers,30 and
it could be due to the more rigid polymer backbone for
PIFTBT10 compared to other crystalline polymers (such as
P3HT) that usually have larger differences between their
electrochemical band gaps and optical band gaps. Using similar
methods, the HOMO and LUMO levels and electrochemical
band gaps for the other three polymers are calculated and listed
in Table 2. The HOMO and LUMO energy levels of the four
polymers are in good agreement with the optimal levels for
obtaining good performance in photovoltaic cells using PC61BM
(LUMO: -3.7 eV) or PC71BM (LUMO: -3.75 eV) as an
Figure 1. Optical absorption spectra of the targeted polymers in solutions
(a), pure polymer films (b), and polymer:fullerene blends.
be 2.00 and 1.96 eV, respectively. Not surprisingly, the optical
absorption of the P3FTBT6/PC61BM blend has an increased
absorption extinction coefficient at ∼400 nm compared to
PIFTBT10 or PIFTBT6 blends with PC61BM at the same blend
ratio (polymer:PC61BM ) 1:3 by weight).
2.3. Electrochemical Study. For polymers used for solar cells,
it is important to know the positions of their HOMO and LUMO
levels. Cyclic voltammograms (CV) were performed in a three-
electrode cell using platinum electrodes at a scan rate of 50
mV s-1 and a Ag/Ag+ (0.1 M of AgNO3 in acetonitrile)
reference electrode in an anhydrous and nitrogen-saturated
solution of 0.1 M tetrabutylammonium tetrafluoroborate
(41) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Ba¨ssler, H.;
Porsch, M.; Daub, J. AdV. Mater. 1995, 7, 551–554.
9
5398 J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010

Ladder-Type Oligo-p-phenylene-Containing Copolymers
A R T I C L E S
Table 2. Eletrochemical Properties of Polymersa
and P3FTBT6 are found to be (4.2 ( 0.2) × 10-3, (9.5 ( 0.7)
× 10-4, (1.1 ( 0.1) × 10-2, and (9.7 ( 0.3) × 10-3 cm2/(V s),
in that order. All the devices show clear on/off behavior which
will minimize leakage currents. The structure difference in
PIFTBT10 and PIFTBT6 is the length of side chains. We found
a decrease of hole mobility with the longer alkyl chains. It is
expected that more bulky side chains may increase the steric
hindrance for the intermolecular packing, thus resulting in a
decrease in mobility in going from PIFTBT6 to PIFTBT10.
Comparing PIFTBT10 with PIFDTQ10, it is found that the
mobility of the former is almost 5 times larger than the latter,
which may be attributed to the fact that two bulky phenyls in
the 5,8-dithien-2-yl-2,3-diphenylquinoxaline repeat unit of
PIFDTQ10 do not lead to a better π-π packing compared to
PIFTBT10 with the 4,7-dithien-2-yl-2,1,3-benzothiadiazole
repeat unit. For the three polymers with the 4,7-dithien-2-yl-
2,1,3-benzothiadiazole repeat unit, PIFTBT6 and P3FTBT6
show over double the mobility compared to PIFTBT10. This
could be due to the π-π packing difference of the polymer
backbone as well as the morphological difference induced by
the length of the side chains. We performed atomic force
microscopy (AFM) surface measurements of these films, and
the topographic images are shown in Figure S2. In general, the
AFM images of the four polymers show uniform and flat films
with roughnesses in the range 0.6-1.0 nm. We did not observe
crystalline domains for these polymer films, indicating that all
four polymers are amorphous. The films for polymers with
longer side chains (PIFTBT10 and PIFDTQ10) show an
increased roughness compared to the films for those with short
alkyl chains (PIFTBT6 and P3FTBT6).
Eox
onset (V)
Ered
onset (V)
HOMO
(eV)
LUMO
(eV)
Eg
(eV)
polymers
PIFTBT10
PIFDTQ10
PIFTBT6
P3FTBT6
0.65
0.63
0.67
0.63
-1.38
-1.46
-1.36
-1.36
-5.47
-5.45
-5.49
-5.45
-3.44
-3.36
-3.46
-3.45
2.03
2.09
2.03
2.00
a Measured in a 0.1 M solution of Bu4NPF6 in CH3CN with a Pt
electrode and a Ag/AgNO3 reference electrode. In these conditions, the
onset oxidation potential (E1/2 ox) of ferrocene was -0.02 V versus Ag/
Ag+. It is assumed that the redox potential of Fc/Fc+ has an absolute
energy level of -4.80 eV to vacuum.
acceptor. The deeper lying HOMO levels of these polymers will
provide a higher open circuit voltage (Voc) according to the
theoretical prediction.42
Figure 3 depicts the electron-state-density distribution of the
HOMO and LUMO of geometry optimized structures (DFT
B3LYP/6-31G) of analogous monomers (a) PIFTBT6 (PIFT-
BT10), (b) P3FTBT6 using the Gaussium 03 program. The
results indicate that the electron density of LUMO is mainly
localized on the acceptor unit, while the electron density of
HOMO is distributed over the entire conjugated molecule (both
the acceptor unit and donor unit). Calculated results show that
PIFTBT6 has a HOMO energy level of -5.08 eV, which is
somewhat low-lying compared to P3FTBT6 (-5.03 eV), which
has a half-fluorene extended π-conjugation system. These
calculated changes for LUMO, HOMO levels and band gaps
in going from PIFTBT6 (PIFTBT10) to P3FTBT6 are also in
agreement with the experiment results.
2.4. Field Effect Transistors. Among the requirements for
polymer solar cells are an efficient photoinduced charge transfer
from the donor molecules (polymers) to acceptors (such as
PCBMs) and sufficient transport properties of the polymer blend.
It has been demonstrated that PCBMs are good electron
transporting materials with sufficient mobility for high perfor-
mance solar cells.43 To ensure the hole transporting properties
of the polymers, we fabricated field effect transistors (FETs).
Top-contact bottom-gate transistors were fabricated under
ambient conditions by spin-casting chlorobenzene solutions of
the synthesized polymers on the heavily doped Si(100) substrates
treated with hexamethyldisilazane (HMDS) or without any
surface treatment. Figure 4a-d show the typical current-voltage
characteristics of the polymeric FET devices with a channel
width and length of approximately 6.5 mm and 270 µm,
respectively, where Id, Vd, Vg represent the source-drain current,
source-drain voltage, and gate voltage, respectively. The
saturation region mobilities were calculated from the transfer
characteristics of the FETs using the slope derived from the
square root of the absolute value of the current as a function of
gate voltage between -50 and -30 V. The threshold voltages
of the polymeric FETs were derived from the onsets of the
transfer curves. Device performance data of the four different
polymers on two different surface treated substrates are listed
in Table 3. From the table, the FETs devices with HMDS
treatment gave 4-23-fold higher mobilities compared to those
without surface treatment using the same polymer as the
semiconducting material. Among the FET devices with HMDS
treatment, the motilities for PIFTBT10, PIFDTQ10, PIFTBT6,
2.5. Solar Cells. As shown in the inset of Figure 5a, the BHJ
solar cells were fabricated with a device structure of ITO/
PEDOT:PSS/polymer:fullerene/Cs2CO3/Al. After spin-coating
a ∼40 nm poly(ethylenedioxythiophene polystyrenesulfonate)
(PEDOT:PSS) layer on the anode (ITO), the active layers with
a thickness of 100-180 nm were spin-coated from a mixed
solvent (chlorobenzene:o-dichlorobenzene ) 4:1) solution of
polymers and fullerenes. Then the cathode, a bilayer of a thin
(1.0 nm) Cs2CO3 layer covered with 100 nm Al, was thermally
evaporated.
Figure 5a-b show the current density-voltage (J-V) char-
acteristics of photovoltaic devices based on the four polymer
blends under simulated solar light (AM 1.5 G, 100 mW/cm2,
RT, ambient). Representative characteristics of the solar cells
are summarized in Table 4. The cell based on BisDMO-
PFDTBT/PC61BM serves as a standard reference for the solar
cell performance of the four polymers developed in this work.44
BisDMO-PFDTBT is a fluorene containing copolymer exactly
the same as PFTBT6 except the alkyl chains change from
n-hexyl to 3,7-dimethyloctyl, which affords better solubility.44
We did not use PFTBT6 due to its extremely low solubility in
organic solvents. The data in Table 4 show all these polymers
showed better performance than the analogous fluorene contain-
ing copolymers (BisDMO-PFDTBT) except for PIFTBT6.
Compared to PIFTBT6, PIFTBT10 which contains longer side
chains has a higher power conversion efficiency of 2.44% with
respect to 0.97% for PIFTBT6. This can be due to the solubility
differences found for these two polymers which may lead to
their phase separation conditions, although the hole mobility
of PIFTBT6 is more than double that of PIFTBT10, indicating
that the hole mobility of a polymer is not the only parameter
(42) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.;
Heeger, A. J.; Brabec, C. L. AdV. Mater. 2006, 18, 789–794.
(43) Mihailetchi, V. D.; van Duren, J. K. J.; Blom, P. W. M.; Hummelen,
J. C.; Janssen, R. A. J.; Kroon, J. M.; Rispens, M. T.; Verhees,
W. J. H.; Wienk, M. M. AdV. Funct. Mater. 2003, 13, 43–46.
(44) Chen, M.-H.; Hou, J.; Hong, Z.; Yang, G.; Sista, S.; Chen, L.-M.;
Yang, Y. AdV. Mater. 2009, 21, 4238–4242.
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J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010 5399

A R T I C L E S
Zheng et al.
Figure 3. DFT-calculated LUMO and HOMO of the geometry optimized structures of analogous monomers of (a) PIFTBT6, PIFTBT10 and (b) P3FTBT6.
1/2
Figure 4. Id-Vd curves of FET devices as a function of Vg (left), Id-sat, and Id-sat vs Vg (right) based on spin-cast films for the polymers (a) PIFTBT10,
(b) PIFDTQ10, (c) PIFTBT6, and (d) P3FTBT6.
for achieving high performance solar cells. At the same time,
the results suggest that the length of the solubilizing group in
the indenofluorene unit may play an important role in the
solubility and miscibility with fullerene which could lead to a
difference in the nanoscale morphology of polymer-fullerene
blends. It is well recognized that the performance of solar cells
is related to the nanoscale morphology of polymer-fullerene
blends influenced by parameters including the solvent, the
processing temperature, the solution concentration, the relative
ratio in composition between polymer and fullerene, the thermal
annealing process, the chemical structure of polymer, etc. Since
we have not yet fully optimized the conditions for the device
fabrication of all the polymers, the data shown in Table 4 do
not represent the best performance for all four polymers. The
comparison here is merely based on one specific device
fabrication condition. The different solar cell performances
found for PIFTBT6 and PIFTBT10 show that the performance
of this type of copolymer can be further improved by introducing
some possibly better side chains such as branched alkyl chains.
For PIFDTQ10, we chose both PC61BM and PC71BM as
acceptors in the active layer. PIFDTQ10 shows a high short-
circuit current (Jsc) of 7.57 mA/cm2, an open-circuit voltage (Voc)
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5400 J. AM. CHEM. SOC. VOL. 132, NO. 15, 2010

Ladder-Type Oligo-p-phenylene-Containing Copolymers
A R T I C L E S
Table 3. Mobilities, On-Off Current Ratios, and Threshold
Table 4. Summary of Device Parameters of the ITO/PEDOT:PSS/
Voltages of Polymers Based OFETs Measured in Aira
Polymer:Fullerene/Cs2CO3/Al Devicesa
substrate
treatment
blend ratio
with PC61BM
Voc
(V)
Jsc
(mA cm-2
fill
factors
polymers
µ (cm2 V-1 s-1
)
Vth (V)
on/off
polymers
efficiencies
)
PIFTBT10
PIFTBT10
PIFDTQ10
PIFDTQ10
PIFTBT6
PIFTBT6
P3FTBT6
P3FTBT6
none
HMDS
none
HMDS
none
HMDS
none
(1.8 ( 0.4) × 10-4
(4.2 ( 0.2) × 10-3
(2.3 ( 0.2) × 10-4
(9.5 ( 0.7) × 10-4
(1.1 ( 0.1) × 10-3
(1.1 ( 0.1) × 10-2
(6.1 ( 0.5) × 10-4
(9.7 ( 0.3) × 10-3
7-25
6-12
3-13
3-8
3-10
5-8
103
103
103
103
103
103
103
103
PIFDTQ10
PIFDTQ10
PIFTBT10
PIFTBT6
P3FTBT6
P3FTBT6
1:3.0
1:3.5b
1:3.0
1:3.0
1:3.0
1:4.0
1:4.0b
1:3.0
1.01
1.00
1.00
0.98
1.06
1.06
1.04
0.94
5.53
7.57
6.10
3.25
6.73
7.80
10.3
4.45
0.42
0.40
0.40
0.31
0.39
0.44
0.42
0.42
2.32
3.04
2.44
0.97
2.81
3.67
4.50
1.75
1-13
4-7
P3FTBT6
BisDMO-PFDTBT
HMDS
a Threshold voltage (Vth). HMDS: hexamethyldisilazane.
a Short-circuit current density (Jsc), open-circuit voltage (Voc). b Blend
with PC71BM.
higher power conversion efficiency of 2.81% in comparison to
0.97% for PIFTBT6:PC61BM with the same device configu-
ration. Surprisingly, the P3FTBT6:PC61BM blends show a
higher Voc of 1.06 V compared to 0.98 V for PIFTBT6:PC61BM,
although the HOMO level of P3FTBT6 is in fact slightly higher
than that for PIFTBT6. We also fabricated P3FTBT6:PC61BM
solar cell devices with a higher fullerene blend ratio (1:4), and
the results are shown in Table 4. Although no extensive
optimization work has been carried out, an outstanding short-
circuit current (Jsc) of 7.80 mA/cm2 and a high open-circuit
voltage (Voc) of 1.06 V are reached for the P3FTBT6 blends
with PC61BM (1:4). The best results were a power conversion
efficiency of 3.67% and a fill factor of 0.44. Furthermore, when
PC71BM was chosen as an electron acceptor material (P3FTBT6:
PC71BM ) 1:4) for the solar cells, a short-circuit current (Jsc)
of 10.3 mA/cm2 and a high open-circuit voltage (Voc) of 1.04
V are achieved yielding an outstanding power conversion
efficiency of 4.5%. As shown in Figure 5, devices fabricated
from both blend ratios of P3FTBT6 to PC61BM showed a high
open circuit voltage of 1.06 V. This observation indicated that
Voc is correlated not only to the difference between the HOMO
of the donor and the LUMO of the acceptor but also to the
molecular structural attributes of a polymer such as rigidity and
planarity. The open circuit voltage of 1.06 V is higher than
analogous donor-acceptor copolymers using fluorene (0.94 V)
as the donor as shown in Table 4. It is also higher than other
donor-acceptor copolymers using carbazoles (0.88 V) as the
donor unit30,31 and much higher than the P3HT/PC61BM system
(0.60 V) with similar short circuit currents.7 We should point
out that our devices are fabricated and tested in an ambient
environment without any encapsulation. We realize that the fill
factors of our devices are not high (0.44), and the photocurrents
in reverse bias show a mild field dependence due to the field-
dependent exciton dissociation rate, and thus reduced fill factors.
These low fill factors can also be attributed to the unbalanced
transport of charge carriers and the relatively thick active
polymer-fullerene layers (>100 nm) and high trap densities in
the devices under ambient environment. An optical spacer or a
hole blocker such as titanium oxide (TiOx) between the active
polymer-fullerene layer can be used to increase the fill factors
of solar cell devices.31 Therefore with all these considerations,
there should be opportunities to further optimize the solar cell
performance by fabricating and testing devices in the glovebox,
excluding oxygen and water to reduce traps, by engineering the
active layer-electrode interface, and by carefully tracing sources
of parasitic external resistances throughout our device structures.
Figure 5. (a) Current density-voltage (J-V) characteristics of PV devices
based on PIFTBT10 (or PIFTBT6, P3FTBT6):PC61BM polymer blends
under simulated solar light (AM 1.5 G, 100 mW/cm2, RT, ambient). (b)
Current density-voltage (J-V) characteristics of PV devices based on
PIFDTQ10(P3FTBT6):PC71BM(PC61BM) polymer blends under simulated
solar light (AM 1.5 G, 100 mW/cm2, RT, ambient). Inset is the schematic
device structure for the polymer/fullerene bulk heterojunction solar cells.
of 1.00 V, a fill factor of 0.40, and a PCE of 3.04% when it
was blended with PC71BM at a ratio of (1:3.5 by weight). In
comparison, when it is blended with PC61BM, a moderate Jsc
of 5.53 mA/cm2, a Voc of 1.01 V, and a fill factor of 0.42 are
achieved with the resulting power conversion efficiency of
2.32%. The increased efficiency for PIFDTQ10:PC71BM can
be attributed to the increasing optical absorption of the polymer
blends, which leads to the increased short-circuit current.45
P3FTBT6 and PIFTBT6 have the same side chains, but
P3FTBT6 has an increased π-conjugation length (one-half the
length of a fluorene). P3FTBT6:PC61BM blends show a much
(45) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen,
J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed. 2003,
42, 3371–3375.
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A R T I C L E S
Zheng et al.
Figure 6. External quantum efficiencies of photovoltaic cells calculated
from the photocurrents under short-circuit conditions based on P3FTBT6:
PC61BM (blue) and P3FTBT6:PC71BM (red).
The external quantum efficiencies (EQE) as a function of
wavelength of the photovoltaic cells based on P3FTBT6:
PC61BM (blue) and P3FTBT6:PC71BM (red) are shown in
Figure 6. As shown in the figure, the P3FTBT6:PC61BM blend
shows an EQE value of 0.82 at ∼410 nm. This EQE peak is
very close to the absorption peak (408 nm) of the copolymer,
which indicates that the photovoltaic conversion arises from the
absorption of the copolymer. For the device based on P3FTBT6:
PC71BM, increased EQE values are observed in the range
450-700 nm, which is due to the increased absorption by
PC71BM compared to PC61BM. The broader profile and higher
values of EQE found for the P3FTBT6:PC71BM are consistent
with the higher Jsc measured in solar cells compared to
P3FTBT6:PC61BM.
The morphological requirement for the active layer in high
performance PSCs is nanoscale phase separation, which enables
a large interface area for exciton dissociation and, in the mean
time, a continuous percolating path for hole and electron
transport to the corresponding electrodes. In this work, AFM
was used to characterize the morphology of the polymer:PCBMs
blends. These AFM topography and phase images are shown
in Figures 7 and 8. As shown in Figure 7a and b, the AFM
measurement demonstrates the nanoscale (>10 nm) phase
separation for PIFTBT10:PC61BM and PIFDTQ10:PC61BM
blends, which result in ∼100 nm sized clusters for the blended
films. Measurements on polymers:PC61BM with different blend
ratios reveal that the dark areas are attributed to PC61BM
domains. Phase separation with the formation of PC61BM-rich
domains facilitates an improved charge transport and carrier
collection efficiency, which results in a reduction of recombina-
tion losses and an increase in short-circuit current density. From
Figures 7 and 8, one may find that the length of side chains has
some impact on the roughness as well as the phase separation
size. In general, longer alkyl chains (decyl) lead to an increased
roughness as well as enlarged domain sizes. For PIFTBT10
and PIFDTQ10 with the 10-carbon alkyl chains, the roughness
of the blend films with PC61BM is in the range 3.5-4.0 nm,
whereas, for PIFTBT6 and P3FTBT6 with 6-carbon alkyl
chains, the roughness is in the range 1.3-1.5 nm. In Figure 7a,
PIFTBT10 is homogeneously distributed in the matrix with a
domain size of ∼20 nm in comparison to a domain size of 10
nm for PIFTBT6 in Figure 7. As shown in Figure 8a and b,
two different blend ratios of P3FTBT6 to PC61BM led to
Figure 7. AFM topography images of PIFTBT10:PC61BM (a), PIFDTQ10:
PC61BM (b), and PIFTBT6:PC61BM (c) films spin-cast from a mixture of
chlorobenzene and o-dichlorobenzene (left) and surface phase images of
the films (right).
different domain sizes of the films as well as the shapes of the
interpenetrated network. These morphology differences are also
reflected by the photovoltaic behavior of the P3FTBT6:PC61BM
films. In Figure 8b, for the P3FTBT6/PC61BM blended film,
the bright patterns of P3FTBT6 domains show an interpenetrat-
ing network which enables a large interface area for exciton
dissociation as well as a continuous percolated path for hole
and electron transport to the corresponding electrodes. This may
be responsible for the improved performance of the device from
2.81% (1:3) to 3.67% (1:4). Figure 8c shows the AFM
topography and phase images of P3FTBT6:PC71BM. When the
electron acceptor material changed from PC61BM to PC71BM,
an increased roughness of ∼2.5 nm is observed compared to
1.5 nm for P3FTBT6:PC61BM, which could be due to the
increased molecular size of PC71BM. However, pronounced
nanoscale phase separation (>10 nm) is still seen for the
P3FTBT6:PC71BM blend.
3. Conclusions
In conclusion, we have successfully synthesized a series of
donor-acceptor ladder-type oligo-p-phenylene-containing co-
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Ladder-Type Oligo-p-phenylene-Containing Copolymers
A R T I C L E S
copolymers are promising candidates for achieving BHJ solar
cells with high conversion efficiencies and high open circuit
voltages.
4. Experimental Section
Reagents were purchased from Aldrich Inc. and Lancaster
Synthesis Ltd. and used without further purification unless otherwise
stated. 6,6′,12,12′-Tetradecyl-6,12-dihydroindeno[1,2b]fluorene, 4,7-
bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole,
5,8-bis(5-bro-
mothiophen-2-yl)-2,3-diphenylquinoxaline, and 2,8-dibromo-
6,6′,12,12′-tetradecyl-6,12-dihydroindeno-[1,2b]fluorene were prepared
according to the literature procedures.33,35,40 Column chromatog-
raphy was conducted with silica gel 60 (400 mesh). 1H NMR spectra
were recorded at either 300 or 400 MHz. Absorption and fluores-
cence spectra were acquired using a spectrophotometer (Cary 50
UV/vis) and a Jobin-Yvon Fluolog FL-3.11 spectrofluorometer,
respectively.
2,8-Dibromo-6,6′,12,12′-tetradecyl-6,12-dihydroindeno-[1,2b]flu-
orene (6b). To a solution of 6,6′,12,12′-tetradecyl-6,12-dihy-
droindeno[1,2b]fluorene (5b) (4.0 g; 4.9 mmol) in CCl4 (100 mL)
was added copper(II) bromide (5.6 g) on aluminum oxide (11.2
g). After the reaction mixture had been refluxed for 24 h, it was
filtered, and the organic filtrate was washed with water and dried
over magnesium sulfate. Removing the solvent afforded the crude
product, which was purified by column chromatography on silica
gel, eluting with petroleum ether. The title compound 6b (4.5 g,
1
90%) was collected as a yellow crystalline solid. H NMR (300
MHz, CDCl3, δ): 7.58 (d, J ) 8.4 Hz, 2H), 7.54 (s, 2H), 7.47-7.44
(m, 4H), 1.98 (t, J ) 8.1 Hz, 8H), 1.55-1.03 (m, 56H), 0.89-0.83
(m, 12 H), 0.60 (bs, 8H); HRMS calcd for C60H92Br2, 970.5566;
found, 970.5552.
2,2′-(6,6,12,12-Tetradecyl-6,12-dihydroindeno[1,2b]fluorene-
2,8-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (1b). 2,8-
Dibromo-6,6′,12,12′-tetradecyl-6,12-dihydroindeno-[1,2b]fluo-
rene 6b (4.5 g, 4.6 mmol), bis(pinacolato)diboron (4.6 g, 18.1
mmol), PdCl2(dppf) (250 mg, 0.3 mmol), and KOAc (3.0 g, 30
mmol) in degassed DMF (50 mL) were stirred at 65 °C overnight.
The reaction was quenched by adding water, and the resulting
mixture was washed with petroleum ether (100 mL × 3). The
organic layers were washed with brine, dried over Na2SO4, and
concentrated in vacuo to a white solid. The solid was purified by
silica gel chromatography by 20% methylene chloride in hexane
Figure 8. AFM topography images of P3FTBT6:PC61BM films with two
different blend ratios and P3FTBT6:PC71BM films spin-cast from a mixture
of chlorobenzene and o-dichlorobenzene (left) and surface phase images
of the films (right).
1
to give the desired compound as a white solid (3.75 g, 78%). H
NMR(400 MHz, CHCl3, δ): 7.77 (d, J ) 8.4 Hz, 2H), 7.74 (d, J )
5.6 Hz, 2H), 7.63 (s, 2H), 2.10-1.90 (m, 8H), 1.40 (s, 24 H),
1.20-1.00 (m, 56H), 0.81 (t, J ) 7.2 Hz, 12H), 0.56 (bs, 8H).
HRMS calcd for C72H116B2O4, 1066.9060; found, 1066.9078.
2,2′-(6,6,12,12-Tetrahexyl-6,12-dihydroindeno[1,2b]fluorene-
2,8-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (1a). Com-
pound 1a was prepared as a white crystalline solid according to
the same procedure as that for compound 1b. Yield: 58%. 1H NMR
(400 MHz, CHCl3, δ): 7.79 (d, J ) 8.4 Hz, 2H), 7.75 (d, J ) 5.4
Hz, 4H), 7.63 (s, 2H), 2.07-1.99 (m, 8H), 1.40 (s, 24 H), 1.08-1.00
(m, 24H), 0.71(t, J ) 6.6 Hz, 12H), 0.61 (bs, 8H). HRMS calcd
for C56H84B2O4, 842.6556; found, 842.6559.
polymers, PIFDTQ10, PIFTBT10, PIFTBT6, and P3FTBT6.
Incorporation of indenofluorene or ladder-type tetra-p-phenylene,
an electron donating building block to the polymer backbone,
leads to an enhanced and bathochromically shifted absorption
band compared to the fluorene containing analogous copolymers.
At the same time, the HOMO energy levels of these polymers
were kept low, which results in a more than 60% increase in
operating voltage compared to P3HT and many low band gap
polymers. Without extended optimization, a high power conver-
sion efficiency of 3.67% and a high Voc of 1.06 V were achieved
from a P3FTBT6:PC61BM (1:4) blend under the ambient
environment, which is superior to that of the analogous fluorene
containing copolymer (BisDMO-PFDTBT) cell (1.75%) under
the same experimental conditions. A high power conversion
efficiency of 4.50% and a high open-circuit voltage of 1.04 V
were also achieved from a polymer solar cell device with an
active layer containing 20 wt % P3FTBT6 and 80 wt %
PC71BM. The absorption range of this type of donor-acceptor
copolymer can be further bathochromically tuned to better match
the solar spectrum by using a still stronger acceptor. These
results show that ladder-type oligo-p-phenylene containing
Synthesis of Compound 2. Compound 2 was prepared according
to the same procedure as that for compound 1b from compound
10.35 Yield: 48%. H NMR (400 MHz, CHCl3, δ): 7.78 (d, J )
1
8.4 Hz, 2H), 7.74 (d, J ) 4.8 Hz, 4H), 7.65 (s, 2H),7.63 (s, 2H),
2.10-2.03 (m, 12H), 1.41 (s, 24 H), 1.03 (bs, 40H), 0.72-0.66
(m, 30H). HRMS calcd for C75H112B2O4, 1098.8747; found,
1098.8769.
Synthesis of PIFDTQ10. 2,2′-(6,6,12,12-Tetradecyl-6,12-
dihydroindeno[1,2b]fluorene-2,8-diyl)bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane) (0.56 g, 0.53 mmol), 5,8-bis(5-bromo-2-thienyl)-
2,3-diphenylquinoxaline (0.317 g, 0.53 mmol), and 20 mL of K2CO3
solution (2 M in H2O) were added into a three-neck flask containing
50 mL of toluene. The flask was connected to a reflux condenser
9
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A R T I C L E S
Zheng et al.
and filled with nitrogen. After bubbling the toluene solution for 30
min, 50 mg of Pd(PPh3)4 (ca. 5 mol %) were added. The degassed
reaction mixture was stirred under nitrogen at 95 °C for 2 days.
Workup of the reaction mixture and isolation of the polymer
occurred by extraction with chloroform; washing of the combined
organic extracts with 1 M HCl, a sat. aqueous NaHCO3 solution,
and water; and drying over MgSO4. Then this organic extract was
filtered through a short florisil column. After partial evaporation
of the solvent, the concentrated solution was precipitated into
methanol, and the solid was collected by filtration and extracted in
a Soxhlet setup with methanol for 24 h and then with hexane for
another 24 h. The insoluble remainders were redissolved in
chloroform and precipitated into methanol. After filtration and
drying in vacuo at 50 °C overnight, 0.30 g of the polymers was
obtained (45% yield). 1H NMR (400 MHz, CDCl3, δ): 8.24 (s, 2H),
7.96-7.47 (m, 22H), 2.11 (bs, 8H), 1.20-1.00 (m, 56H), 0.84-0.77
(m, 20H). GPC: Mn ) 7.8 kg/mol, Mw ) 21 kg/mol, PDI ) 2.65.
Synthesis of PIFTBT10. PIFTBT10 was prepared according
to the same procedure as that for PIFDTQ10. 71% yield, 1H NMR
(300 MHz, CDCl3, δ): 8.18 (s, 2H), 7.97 (s, 2H), 7.79-7.53 (m,
10H), 2.10 (bs, 8H), 1.21-1.00 (m, 56H), 0.83-0.73 (m, 20H).
GPC Mn ) 26.3 kg/mol, Mw ) 63.5 kg/mol, PDI ) 2.42.
spin-casting (1500 rpm) a blend of P3FTBT6:PC61BM or P3FTBT6:
PC71BM in a 1:4 w/w ratio on the ITO/PEDOT:PSS substrates.
Then the devices were kept at room temperature for 24 h. Then,
the cathode, a bilayer of a thin (1.0 nm) Cs2CO3 layer covered with
100 nm Al, was thermally evaporated. The thermal evaporation of
Cs2CO3 and Al was done under a shadow mask. The active area of
the devices was fixed at 0.07 cm2. As shown in the inset of Figure
5, an island-type cathode design was chosen to exclude any excess
photocurrent generated from the parasitic solar cell structure, where
the conductive PEDOT layer can act as an effective anode.46 Four
polymers and PC61BM (or PC71BM) were dissolved together in a
mixture of chorobenzene and o-dichlorobenzene (4:1, v/v) to give
an overall 20 mg/mL solution. PIFDTQ10 and PC71BM were
dissolved together in a mixture of chorobenzene and o-dichlo-
robenzene (4:1, v/v) to give an overall 28 mg/mL solution.
Device Characterization. Device characterization in ambient
environment was performed under AM 1.5G irradiation (100 mW/
cm2) on an Oriel Xenon solar simulator. The solar cell devices were
illuminated through their ITO sides. The current density-voltage
curves were measured in air by an Agilent 4155C semiconductor
parameter analyzer with the ICS lite software. Since there are some
deviations in the spectral output of the solar simulator with respect
to the standard AM1.5 spectrum, the Jsc values for the solar cell
devices were corrected by introducing a mismatch factor as shown
in eq 5:
Synthesis of PIFTBT6. PIFTBT6 was prepared according to
1
the same procedure as that for PIFDTQ10. 31% yield, H NMR
(300 MHz, CDCl3, δ): 8.18 (s, 2H), 7.98 (s, 2H), 7.80-7.54 (m,
10H), 2.11 (bs, 8H), 1.17 (bs, 24H), 0.77-0.68 (m, 20H). GPC Mn
) 22.5 kg/mol, Mw ) 45.9 kg/mol, PDI ) 2.03.
Jsc ) Jsc(measured) × m
(5)
Synthesis of P3FTBT6. P3FTBT6 was prepared according to
1
where m is the spectral mismatch factor calibrated using an NREL-
calibrated PV-measurement mono-Si solar cell. For all short circuit
currents and power conversion efficiencies we used a mismatch
factor of 0.87. External quantum efficiencies (EQEs) ware calculated
from the photocurrents under short-circuit conditions. The solar cell
device was illuminated through its ITO side with a 100 W Xe lamp
(PhotoMax) coupled to a 1/4 m monochromator (Oriel Cornerstone).
Incident irradiances were measured using an optometer (Graesby
Optronics S370 with a United Detector Technology silicon detector),
and photocurrents were measured using an electrometer (Keithley
617).
Atomic Force Microscopy. AFM was performed in tapping
mode using a Digital Instruments microscope (Molecular Imaging
PicoPlus). Films for AFM were prepared on PEDOT:PSS-coated
ITO substrates prepared in identical fashion to those prepared for
device fabrication. The average roughnesses of films were calculated
from AFM topography images of scanned areas using the Gwyddion
analysis tool (downloadable from http://gwyddion.net).
the same procedure as that for PIFDTQ10; 43% yield; H NMR
(300 MHz, CDCl3, δ): 8.19 (s, 2H), 7.98 (s, 2H), 7.80-7.53 (m,
12H), 2.11 (bs, 12H), 1.10 (bs, 36H), 0.75-0.70 (m, 30H). GPC
Mn ) 14.2 kg/mol, Mw ) 24.9 kg/mol, PDI ) 1.75.
Computational Methods. All of the DFT calculations were
performed with the Gaussian 03 program package (Gaussian, Inc.).
The structures shown in Figure 3 were fully optimized without any
symmetry restrictions at the B3LYP level. A split-valence plus
polarization basis set, 6-31G(d), was used. The DFT/B3LYP/6-
31G(d)-optimized structures for the model compounds were used
for electronic-structure analysis. HOMO and LUMO isosurfaces
shown in Figure 3 were plotted using the Gabedit software (http://
gabedit.sourceforge.net).
Field-Effect Transistors. Top contact FETs of copolymers were
fabricated on hexamethyldisilazane (HMDS)-treated or untreated
SiO2/Si substrates. The semiconducting films were deposited by
spin-casting pure polymers (∼7 mg/mL in chlorobenzene) at 1500
rpm on the substrates. Gold top contact source and drain electrodes
of ∼50 nm thickness were vapor deposited through a shadow mask.
The channel widths and lengths were ∼6.5 mm and 270 µm,
respectively. Devices were measured in air using an Agilent 4155C
semiconductor parameter analyzer with the ICS lite software.
Solar Cell Fabrication. The indium tin oxide (ITO)-covered
glass substrates were cleaned by ultrasonification sequentially in
detergent, water, acetone, and isopropyl alcohol for 30 min each
and then dried in an oven at 90 °C overnight. A thin layer (∼40
nm) of poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid)
PEDOT/PSS (Baytron P Al 4083) was spin-coated onto the ITO
surface, which was pretreated by oxygen plasma for 5 min. The
substrates were baked at 120 °C for 2 h. An active layer (150-180
nm) was fabricated in the ambient by spin-casting (800 rpm) a blend
of polymer:PC61BM/polymer:PC71BM (purchased from Nano C)
in a 1:3 (or 1:3.5) w/w ratio on the ITO/PEDOT:PSS substrates
without further special treatment. For optimization of P3FTBT6,
an active layer of (100-110 nm) was fabricated in the ambient by
Acknowledgment. We thank Prof. Peter Searson for access to
AFM instrumentation and Prof. Gerald J. Meyer and Mr. John
Rowley (Department of Chemistry, Johns Hopkins University) for
their help in EQE measurements. We are grateful to the AFOSR
(Contract Number FA9550-06-1-0076), DOE Office of Basic
Energy Sciences (Contract Number DE-FG01-07ER-46465), and
JHU MRSEC for funding.
Supporting Information Available: Emission spectra and
AFM topography images. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA909111P
(46) Kim, M.-S.; Kang, M.-G.; Guo, L. J.; Kim, J. Appl. Phys. Lett. 2008,
92, 133301/1–133301/3.
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