Synthesis of low bandgap polymers based on thienoquinodimethane units and their applications in bulk heterojunction solar cells

Article abstract of DOI:10.1039/c2jm33637e

A non-fused ring building block of an electron-rich quinoid structure, 2,5-thienoquinodimethane, has been synthesized and used in the synthesis of novel donor (D)-acceptor (A) type low bandgap polymers for the first time. Namely, 2,5-thienoquinodimethane with 4-(tert-butyl)phenyl or 4-(octyloxy)phenyl side chain as a solubilizing group was copolymerized with an electron-deficient diketopyrrolopyrrole subunit (PQD1 and PQD2, respectively). These polymer films exhibited broad and intense absorption bands in the region of 400-1000 nm. Photovoltaic devices with active layers consisting of PQD1 or PQD2 with [6,6]-phenyl-C₇₁-butyric acid methyl ester ([70]PCBM) revealed a broad photoresponse range covering from 400 to 1000 nm, whereas the power conversion efficiencies (η) were found to be moderate (1.44% for PQD1 and 0.96% for PQD2) under the illumination of AM 1.5G, 100 mW cm^-2. The superior η value of the PQD1:[70]PCBM-based device relative to the PQD2:[70]PCBM-based device can be attributed to the more favorable phase separation nanostructure in the active layer as well as the higher crystallinity of PQD1 than PQD2. These results provide valuable, basic guidelines for rational designs of quinoidal heterole-based low bandgap polymers for high performance organic solar cells. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm33637e

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PAPER
Synthesis of low bandgap polymers based on thienoquinodimethane units and
their applications in bulk heterojunction solar cells†
Tomokazu Umeyama,ab Yusuke Watanabe,a Masaaki Oodoi,a Douvogianni Evgenia,a Tetsuya Shishidoa
ac
*
and Hiroshi Imahori
Received 6th June 2012, Accepted 2nd August 2012
DOI: 10.1039/c2jm33637e
A non-fused ring building block of an electron-rich quinoid structure, 2,5-thienoquinodimethane, has
been synthesized and used in the synthesis of novel donor (D)–acceptor (A) type low bandgap polymers
for the first time. Namely, 2,5-thienoquinodimethane with 4-(tert-butyl)phenyl or 4-(octyloxy)phenyl side
chain as a solubilizing group was copolymerized with an electron-deficient diketopyrrolopyrrole subunit
(PQD1 and PQD2, respectively). These polymer films exhibited broad and intense absorption bands in
the region of 400–1000 nm. Photovoltaic devices with active layers consisting of PQD1 or PQD2 with
[6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) revealed a broad photoresponse range covering
from 400 to 1000 nm, whereas the power conversion efficiencies (h) were found to be moderate (1.44% for
PQD1 and 0.96% for PQD2) under the illumination of AM 1.5G, 100 mW cmꢀ2. The superior h value of
the PQD1:[70]PCBM-based device relative to the PQD2:[70]PCBM-based device can be attributed to the
more favorable phase separation nanostructure in the active layer as well as the higher crystallinity of
PQD1 than PQD2. These results provide valuable, basic guidelines for rational designs of quinoidal
heterole-based low bandgap polymers for high performance organic solar cells.
requirements for the low bandgap polymers in [60]PCBM-
1. Introduction
based OPVs are appropriate HOMO (ꢁꢀ5.4 eV) and LUMO
(ꢁꢀ3.9 eV) energy levels for broad light-harvesting, efficient
charge separation, and high open circuit voltage.1,3 The balanced
crystallinity and solubility of the polymers are also of significance
for high hole mobility and optimal film morphology.1a
Polymer-based organic photovoltaics (OPVs) have stimulated
broad interest due to their potential advantages such as facile
fabrication, low cost, lightweight, and flexibility.1 On the basis of
the concept of bulk heterojunction (BHJ) structure, where elec-
tron donor and acceptor materials form bicontinuous phase
separation structures with tens of nanometer-sized domains,
OPVs made by blending poly(3-hexylthiophene) (P3HT) as a
donor and [6,6]-phenyl-C61-butyric acid methyl ester ([60]
PCBM) as an acceptor have been intensively investigated,
leading to power conversion efficiencies (h) of up to 4–5%.2
However, the h values of P3HT-based OPVs were mainly limited
by their narrow absorption spectra ranging from 300–650 nm.
Therefore, low bandgap polymers are highly desirable to serve as
donors of the active layers in OPVs aiming at matching their
absorptions well with the solar radiation spectrum.1 The key
Synthesis of low bandgap polymers can date back to 1984,
when poly(isothianaphthene) (PITN) with a bandgap as low as
1.0 eV was synthesized by electrochemical polymerization and
chemical oxidation methods.4 The main chain of PITN favors
quinoid resonance forms to preserve the benzene aromaticity at
the expense of the thiophene aromaticity, which endows a planar
and rigid polymer backbone with an extensively delocalized p-
conjugation system. However, the poor solubility of PITN in
common organic solvents resulted in low processability and thus
hindered the device applications. Following the synthesis of
PITN, poly(arylene-methine)s (PAMs) combining hetero-
quinodimethane and heteroaromatic units in the main chain
were prepared through the acid-catalyzed polycondensation
reactions between heteroaromatic compounds and aldehydes
and subsequent oxidations.5–10 Although this approach yielded
poly((oligo)thienylene-methine)s,5,6 poly(ITN-methine)s,7,8 and
poly(pyrrolylene-methine)s9 with a low bandgap of ꢁ1.2 eV,
these polymers suffered from irregularly linked heteroaromatic
units and contamination of residual oxidants. In addition, the
number-average molecular weights were found to be typically
less than 5000. Thus, the material applications of PAMs in
aDepartment of Molecular Engineering, Graduate School of Engineering,
Kyoto University, Nishikyo-ku, Kyoto, 615-8510, Japan
bPRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho,
Kawaguchi, Saitama 332-0012, Japan
cInstitute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto
University, Nishikyo-ku, Kyoto 615-8510, Japan. E-mail: imahori@scl.
kyoto-u.ac.jp; Fax: +81-75-383-2571; Tel: +81-75-383-2566
† Electronic supplementary information (ESI) available: Details of
monomer synthesis, NMR spectra and GPC traces of the monomers,
schematic illustrations of the OPV devices, cyclic voltammograms, and
XRD patterns. See DOI: 10.1039/c2jm33637e
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optical and electrical devices such as OPVs and organic field-
effect transistors have never been reported so far.
of the residual oxidant and irregularly linked thiophenes, as they
can be removed by careful monomer purifications. The charac-
terizations and thermal, optical, and photovoltaic properties of
PQDs are described in this report. Furthermore, the relationship
between OPV device performances and nanostructured phase
separations in the active layer is also discussed.
Recently, a facile method to afford low bandgap polymers
by combining electron-rich (donor) and electron-deficient
(acceptor) moieties in their repeating units has been extensively
explored.1 Such internal donor (D)–acceptor (A) structures can
also enhance charge carrier mobility due to the reduced inter-
chain p–p stacking distance.11 In this regard, a number of elec-
tron-rich units including cyclopenta[2,1-b:3,4-b0]dithiophene,12
dithieno[3,2-b:20,30-d]silole,13 and benzo[1,2-b:4,5-b0]dithio-
phene14 as well as electron-deficient units including ester- or
ketone-substituted thieno[3,4-b]thiophene,15 diketopyrrolo[3,4-c]
pyrrole-1,4-dione (DPP),16 2,1,3-benzothiadiazole,17 and thieno
[3,4-c]pyrrole-4,6-dione18 have been developed. It should be
pointed out that all these units are fused aromatic rings, which
exhibit more rigid and planar structures than monocyclic
aromatic rings, thereby leading to a decreased chain-folding,
enhanced intermolecular stacking, and increased effective p
conjugation length. So far, a combination of electron-rich benzo
[1,2-b:4,5-b0]dithiophene and electron-deficient thieno[3,4-b]
thiophene has led to one of the highest h values of 7–8%.15 In
addition to the D–A intramolecular interaction, conjugated
polymers containing thieno[3,4-b]thiophene units in the main
chain tend to possess low bandgaps because their main chains
favor quinoid forms to preserve the fused thiophene aromaticity
at the expense of the aromaticity of thiophene in the main chain,
which is in analogy with ITN-based polymers.4
2. Experimental section
2.1. Instruments
The microwave-assisted procedures were carried out with a CEM
Discover BenchMate microwave. 1H NMR spectra were
obtained with a JEOL JNM-EX400 NMR spectrometer. FT-IR
spectra were recorded using a Thermo Fisher Scientific Nicolet
6700 FT-IR (attenuated total reflectance (ATR)). High resolu-
tion mass spectra (HRMS) were obtained with a JEOL JMS-
T100CS. Gel permeation chromatography (GPC) measurements
were carried out using a SHIMADZU Prominence equipped
with JAIGEL-3HAF and JAIGEL-2HAF columns (Japan
Analytical Industry) for estimations of polymer molecular
weights and monomer purities, respectively, using chloroform as
an eluent after calibration with standard polystyrenes. UV-VIS-
NIR absorption spectra were measured using a PerkinElmer
Lambda 900 UV/VIS/NIR spectrometer. Steady-state fluores-
cence spectra were recorded using a SPEX Fluoromax-3 spec-
trofluorometer (HORIBA) in the visible region and
a
SHIMADZU NIR-PL System in the near-infrared region. A
time-correlated single photon counting (TCSPC) method was
used for the fluorescence lifetime measurements in the nano-
second and sub-nanosecond time-scale, and the time resolution
was ꢁ80 ps (fwhm) for an excitation wavelength of 483 nm.20
Atomic force microscopy (AFM) analyses were carried out using
an Asylum Technology MFP-3D-SA in the AC mode. Ther-
mogravimetric analysis (TGA) measurements were conducted
using a SHIMADZU DTG-60H under flowing nitrogen at a scan
rate of 10 ꢂC minꢀ1. Differential scanning calorimetry (DSC)
analysis was made using a SHIMADZU DSC-60 under flowing
nitrogen at a scan rate of 10 ꢂC minꢀ1. Cyclic voltammetry (CV)
measurements were performed using an ALS 630A electro-
chemical analyzer in benzonitrile containing 0.1 M tetrabuty-
lammonium hexafluorophosphate as a supporting electrolyte.
An ITO (Geomatec) working electrode, Ag/AgNO3 (0.01 M in
acetonitrile) reference electrode, and Pt wire counter electrode
were employed. Photoemission yield spectroscopy in air (PYSA)
was conducted using a AC-3 (Riken Keiki). X-Ray diffraction
(XRD) analyses were conducted using a Rigaku A2 diffrac-
tometer using Cu Ka radiation. The samples were cast on indium
tin oxide (ITO)-coated SiO2 substrates. Photocurrent–voltage
characteristics were measured by a PECK2400-N with a PEC-
L11 solar simulator (Peccell Technologies) under standard two-
electrode conditions (100 mW cmꢀ2, AM1.5). Photocurrent
action spectra were recorded with a PEC-S20DC (Peccell
Technologies).
On the other hand, Jenekhe and Chen reported the synthesis of
quinoid thiophene oligomers (i.e., monomer to trimer) as model
compounds of PAMs.19 These compounds exhibited high
planarity and efficient p-electron delocalization with absorption
maxima of 480–676 nm. Moreover, the bandgap and ionization
potential decreased with the increasing length of the quinoid
oligomer.19 Therefore, these electron-rich quinoidal thiophene
units can be regarded as one of the highly promising building
blocks for D–A conjugated polymers with tunable bandgap and
HOMO energy levels. Although the incorporation of quinoidal
structures is a versatile approach for the creation of novel low
bandgap polymers as well as the utilization of the fused-ring
structures, metal-catalyzed polycondensation using difunction-
alized quinoid compounds has yet to be reported so far.
Herein we report on the first synthesis of D–A type conjugated
polymers (PQD1, 2) containing quinoidal thiophene units as
electron-rich moieties in the polymer for OPVs. 2,5-Thienoqui-
nodimethane19 with 5-trimethylstannyl-2-thienyl groups at both
methylene carbons (compound 3) was prepared via 1 and 2 and
then used for copolymerization by Stille coupling reaction
(Scheme 1). DPP unit with two 2-ethylhexyl chains at N-atoms
(compound 4)16 was chosen as the A unit, as it has strong elec-
tron-withdrawing properties, rigid coplanar structure, and
sufficient solubility. The resulting polymers, PQDs, are expected
to possess relatively narrow bandgaps due to the highly conju-
gated quinoidal thiophene units as well as D–A interactions in
the main chain. Phenyl groups with bulky tert-butyl or long,
electron-donating octyloxy groups were introduced at methine
carbons next to the quinoidal thiophene to improve the solubility
of the polymers in organic solvents. In contrast to the acid-
catalyzed polymerization,5–10 this metal-catalyzed conventional
polymerization method is potentially free from contaminations
2.2. Materials
All solvents and chemicals were of reagent grade quality,
purchased commercially and used without further purification
unless otherwise noted. Synthesis details for 4-(tert-butyl)phenyl
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Scheme 1 Synthetic routes to comonomer 3 and polymer PQD.
2-thienyl ketone and 4-(octyloxy)phenyl 2-thienyl ketone are
described in the ESI.†
(400 MHz, CD2Cl2; Me4Si): 7.5–6.8 (aromatic, 14H), 6.7–6.4
(quinoid, 2H), 3.99 (OCH2, 4H), 1.79 (OCH2CH2, 4H), 1.5–1.3
(CH2, 20H), 0.89 (CH3, 6H). IR (ATR): 3072, 3036, 2927, 2854,
1604, 1572, 1505, 1465, 1387, 1299, 1282, 1246, 1174, 1139, 1107,
1051, 975, 792, 695 cmꢀ1. HRMS (m/z): calcd for C42H50O2S3,
682.2967; found 682.2952.
2.3. Monomer synthesis
2.3.1. 6,7-Bis(4-(tert-butyl)phenyl)-6,7-bis(2-thienyl)-2,5-thie-
noquinodimethane (2a). Compound 2a was synthesized according
to the reported procedure.19 Anhydrous hexane (7 mL) was added
to a 50 mL two-necked, round-bottomed flask under argon
protection. Tetramethylethylenediamine (TMEDA, 0.66 mL),
thiophene (0.14 mL, 1.8 mmol), and n-butyllithium (1.65 M in
hexane, 2.66 mL, 4.4 mmol) were added to the stirred solution at
room temperature. The reaction mixture was refluxed for 1 h and
then cooled to ꢀ40 ꢂC. The solution of 4-(tert-butyl)phenyl thienyl
ketone (1.08 g, 4.4 mmol) in distilled diethyl ether (7 mL) was
added dropwise to the reaction mixture. Then, the mixture was
warmed up to room temperature and stirred overnight. After the
reaction was quenched with 1 M NH4Cl aq. (20 mL), the organic
layer was extracted with dichloromethane and ethyl acetate. The
combined organic layer was dried over MgSO4 and the solvent was
evaporated. After drying under vacuum, the obtained crude
product 1a was dissolved in toluene (30 mL) and a solution of
Na2S2O4 (4.6 g, 26.4 mmol) and 57% HI (4.4 mL) in distilled water
(30 mL) was added. The two-phase system was vigorously stirred
at room temperature for 24 h. An orange colored reaction mixture
was neutralized with NaHCO3 and extracted several times with
chloroform. The combined organic layer was dried over Na2SO4.
After evaporation of the solvent, the product was chromato-
graphed on a silica gel (dichloromethane : hexane ¼ 1 : 3).
Recrystallization of the product with ethanol gave 2a as orange
solids. Yield 0.34 g (35%, two step). 1H NMR dH (400 MHz,
CD2Cl2; Me4Si): 7.7–7.0 (aromatic, 14H), 6.7–6.4 (quinoid, 2H),
1.14 (CH3, 18H). IR (ATR): 3065, 2960, 2929, 2900, 2864, 1691,
2.3.3. 6,7-Bis(4-(tert-butyl)phenyl)-6,7-bis(5-trimethylstannyl-2-
thienyl)-2,5-thienoquinodimethane (3a). The solution of 2a (0.27 g,
0.50 mmol) and TMEDA (0.16 mL) in distilled tetrahydrofuran (5
mL) was added to a 30 mL two-necked, round-bottomed flask
under argon protection. The reaction mixture was cooled to ꢀ78
ꢂC, and n-butyllithium (1.65 M in hexane, 0.64 mL, 1.1 mmol) was
added to the stirred solution. The reaction mixture was refluxed for
1 h and then cooled again to ꢀ78 ꢂC and trimethyl tin chloride (1.0
M in THF, 1.2 mL) was added dropwise to the reaction mixture.
After 3 h, the reaction mixture was warmed up to room tempera-
ture and stirred overnight. The reaction was quenched with 1 M
NH4Cl aq. (10 mL). The organic layer was extracted with chloro-
form, washed with brine and dried over Mg2SO4. After evapora-
tion of the solvent, reprecipitation with chloroform/methanol gave
1
3a as orange solids. Yield 0.32 g (74%). H NMR dH (400 MHz,
CDCl3; Me4Si): 7.5–7.0 (aromatic, 12H), 6.7–6.4 (quinoid, 2H),
1.36 (CH3, 18H), 0.36 (Sn–CH3, 18H). IR (ATR): 3048, 2962, 2904,
2864, 1555, 1503, 1457, 1383, 1207, 1155, 1057, 1025, 942,
771 cmꢀ1. HRMS (m/z): calcd for C40H50S3Sn2, 864.1113; found
864.1088. 1H NMR spectrum and GPC trace are shown in Fig. S1
and S2, respectively (see ESI†).
2.3.4. 6,7-Bis(4-(octyloxy)phenyl)-6,7-bis(5-trimethylstannyl-
2-thienyl)-2,5-thienoquinodimethane (3b). Compound 3b was
synthesized following the same procedure for 3a. Compound 3b
as slurry orange liquid was obtained. Yield 0.48 g (96%). 1H
NMR dH (400 MHz, CDCl3; Me4Si): 7.5–6.8 (aromatic, 12H),
6.7–6.4 (quinoid, 2H), 3.99 (OCH2, 4H), 1.79 (OCH2CH2, 4H),
1.5–1.3 (CH2, 20H), 0.89 (CH3, 6H), 0.36 (Sn–CH3, 18H). IR
(ATR): 3045, 2925, 2855, 1605, 1506, 1469, 1285, 1246, 1174,
1152, 1028, 943, 798, 534 cmꢀ1. HRMS (m/z): calcd for
C48H66O2S3Sn2, 1008.2257; found 1008.2243. 1H NMR spec-
trum and GPC trace are shown in Fig. S1 and S2,† respectively.
1656, 1558, 1510, 1459, 1216, 1143, 1063, 1024, 940, 977 cmꢀ1
.
HRMS (m/z): calcd for C34H34S3, 538.1823; found 538.1799.
2.3.2. 6,7-Bis(4-(octyloxy)phenyl)-6,7-bis(2-thienyl)-2,5-thie-
noquinodimethane (2b). Compound 2b was synthesized following
the same procedure for 2a. The product was chromatographed
on a silica gel (chloroform : hexane ¼ 1 : 4) to afford 2b as
1
slurry orange liquid. Yield 0.49 g (40%, two step). H NMR dH
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2.4. Polymer synthesis
3. Results and discussion
2.4.1. PQD1. A test tube was charged with 3a (43 mg,
0.050 mmol), 4 (34 mg, 0.050 mmol), Pd2dba3 (dba ¼ diben-
zylideneacetone) (2.3 mg, 0.0025 mmol), tri(o-tolyl)phosphine
(8.8 mg, 0.029 mmol) and anhydrous chlorobenzene (1 mL),
and bubbled with argon for 10 min. The reaction mixture was
stirred at 170 ꢂC for 18 h with a microwave reactor. The cooled
mixture was subsequently poured into methanol and the black
precipitate was collected on a membrane filter. The product was
washed with methanol and hexane, and purified by reprecipi-
tation with chloroform/methanol to give PQD1 as a dark blue
solid. Yield 31 mg (58%). 1H NMR: dH (400 MHz; CDCl3) 7.6–
7.0 (aromatic, 16H), 6.7–6.4 (quinoid, 2H), 4.02 (NCH2, 4H),
1.90 (NCH2CH, 2H), 1.5–1.2 (CH2, C–CH3, 34H), 0.8–0.9
(CH2CH3, 12H). IR (ATR): 3064, 2957, 2921, 2860, 1656, 1540,
1521, 1466, 1425, 1342, 1319, 1225, 1147, 1100, 1072, 1015,
3.1. Monomers and polymers synthesis and thermal properties
The synthetic routes to the comonomer compounds 3a, 3b and
the polymers PQD1, PQD2 are shown in Scheme 1. The crude
precursors 1a with 4-(tert-butyl)phenyl and 1b with 4-(octyloxy)
phenyl groups were obtained by the reaction between 2,5-dili-
thiothiophene and the corresponding phenyl thienyl ketone.
Subsequent reactions with HI/Na2S2O4 converted the precursors
to the corresponding conjugated quinoid compounds 2a and
2b.19 The quinoid compounds are likely to contain three diaste-
reomers, i.e., E,E, E,Z, and Z,Z isomers. However, the difficulty
in the separation forced us to use 2a and 2b as mixtures of dia-
stereomers for the following step of distannylation, yielding the
quinoidal thiophene comonomers 3a and 3b as E,E, E,Z, and
Z,Z mixtures. The compounds 3a and 3b were copolymerized
with DPP comonomer 4 bearing two 2-ethylhexyl chains at N-
atoms through Stille coupling reactions to afford the target
polymers PQD1 and PQD2 with 4-(tert-butyl)phenyl and 4-
(octyloxy)phenyl groups as solubilizing substituents, respec-
tively. The polymers were obtained as a dark blue solid and
purified by repeated reprecipitation with chloroform/methanol.
The polymers PQD1 and PQD2 showed excellent solubility in
common organic solvents such as toluene, chloroform, and
chlorobenzene, which permits the polymers to be readily pro-
cessed by a solution technique for the film formation. The
structural irregularity, i.e., coexistence of E,E, E,Z, and Z,Z
isomers in the main chain, as well as solubilizing side chains may
contribute to the high solubility. The number-average molecular
weight (Mn) and polydispersity index (PDI) were estimated to be
4700 and 1.5 for PQD1 and 8200 and 2.5 for PQD2, respectively,
by GPC measurements with polystyrene standards in chloro-
form. Different reaction conditions such as conventional heating
using oil baths, catalysts (Pd(PPh3)4, PdCl2(PPh3)3, and
(C6H5CN)2PdCl2), and solvents (o-dichlorobenzene, toluene,
and DMF) resulted in lower molecular weights. The reason for
the relatively low molecular weight of PQD1 and PQD2 is
currently unclear, but an insufficient purity of the quinoid and/or
DPP monomers may be responsible for the low Mn value.22 It is
noteworthy that both solid and solution samples of PQD1 and
PQD2 were stable in air for more than several months in ambient
atmosphere.
794 cmꢀ1
.
2.4.2. PQD2. Polymer PQD2 was synthesized following the
same procedure for PQD1. The product was washed with
methanol and hexane, and purified by reprecipitation with
chloroform/methanol. Yield 19 mg (32%). 1H NMR dH (400
MHz, CDCl3; Me4Si): 7.6–6.8 (aromatic, 16H), 6.6–6.3 (quinoid,
2H), 4.1–3.9 (OCH2, NCH2, 8H), 1.9 (NCH2CH, 2H), 1.83
(OCH2CH2, 4H), 1.5–1.2 (CH2, 36H), 0.8–0.9 (CH3, 18H). IR
(ATR): 3075, 2925, 2855, 1662, 1604, 1553, 1506, 1455, 1424,
1303, 1245, 1174, 1074, 1023, 793, 733 cmꢀ1
.
2.5. Fabrication and characterization of polymer solar cells
The OPV devices were fabricated as follows.21 Indium tin oxide
(ITO) on a glass substrate with a sheet resistance of 5 U sqꢀ1
(Geomatec) was used. The substrates were sonicated consecu-
tively with acetone and ethanol for 15 min. After blow-drying
and UV-ozone treatment, the substrates were spin coated at
4000 rpm with poly(ethylene dioxythiophene) doped with
polystyrene sulfonic acid (PEDOT:PSS, Clevios P VP AI 4083)
and dried with a hot plate at 200 ꢂC for 10 min. Under a
nitrogen atmosphere, an active layer of polymer:[6,6]-phenyl-
C71-butyric acid methyl ester ([70]PCBM) (American Dye
Source, Inc.) film was formed by spin coating a chlorobenzene
solution of polymer:[70]PCBM (1 : 2 (w/w), [polymer] ¼ 15 mg
mLꢀ1, [[70]PCBM] ¼ 30 mg mLꢀ1) at 4000 rpm for 1 min onto
the ITO/PEDOT:PSS. The thicknesses of the PQD1:[70]PCBM
and PQD2:[70]PCBM films were estimated to be 70 and 68 nm,
respectively, by AFM measurements. Then, a TiOx layer was
prepared by spin coating an ethanol solution of titanium(IV)
isopropoxide (Wako Pure Chemical Industries, Ltd.) at 2000
rpm, and then leaving for 30 min under ambient atmosphere.
The titanium(IV) isopropoxide was hydrolyzed to yield the TiOx
layer. Finally, an Al (The Nilaco Corporation) layer was
deposited by thermal evaporation under vacuum (5 ꢃ 10ꢀ5 Pa)
to yield the layered device structure (denoted as ITO/
PEDOT:PSS/polymer:[70]PCBM/TiOx/Al). Schematic illustra-
tions of the top and side views of the device are depicted in
Fig. S3 (see ESI†). Photocurrent–voltage characteristics were
measured under ambient atmosphere and simulated solar light,
air mass (AM) 1.5 (100 mW cmꢀ2).
The thermal stabilities of PQD1 and PQD2 were investigated
by thermogravimetric analysis (TGA). Both polymers were
thermally stable withꢂdecomposition temperatures (5% weight
loss) of 387 and 399 C for PQD1 and PQD2, respectively, in
nitrogen atmosphere. The high-thermal stabilities of the
obtained polymers are beneficial for their application in the
active layers of OPVs. On the other hand, differential scanning
calorimetry (DSC) did not display any transition temperatures
ꢂ
such as glass transition or melting up to 350 C, implying that
these polymers are thermally robust.
3.2. Optical and electrochemical properties
The optical properties of the copolymers were investigated by
visible-near infrared (vis-NIR) absorption spectra in diluted
chlorobenzene solutions and film states spin-coated on glass
substrates (Fig. 1). The optical data of the polymers are listed in
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ESI†). Although both polymers showed oxidation peaks, the
onsets of the oxidation signals were not evident, making it
difficult to estimate the HOMO energy levels from the voltam-
mograms.23 Conversely, the HOMO energy levels of PQD1 and
PQD2 were determined to be ꢀ5.2 and ꢀ4.9 eV by using
photoemission yield spectroscopy in air (PYSA, Fig. 2).24 The
electron-donating feature of octyloxy groups raised the HOMO
level of PQD2 considerably compared to that of PQD1. The
LUMO levels were estimated to be ꢀ4.0 and ꢀ3.7 eV for PQD1
and PQD2, respectively, by adding the thin-film optical bandgap
to the HOMO level.24,25 The energy levels of PQD1, PQD2, [70]
PCBM, buffer layers, and electrodes used in the OPV devices
(vide infra) are illustrated in Fig. 3.21 As it has been well recog-
nized that the binding energy of the excitons is ca. 0.3–0.5 eV, the
LUMO energy level of the donor must be positioned above that
of the acceptor (i.e., [70]PCBM) by 0.3–0.5 eV to ensure an
efficient photoinduced electron transfer (ET) from the donor
excited state to the acceptor.1 Thus, the driving force for electron
transfer from the PQD1 excited state to [70]PCBM is slightly
insufficient, whereas that from the PQD2 excited state to [70]
PCBM is sufficient for efficient ET.
Fig. 1 Vis-NIR absorption spectra of chlorobenzene solutions of PQD1
(solid line, 19 mg mLꢀ1) and PQD2 (solid line with circle) and films of
PQD1 (dashed line) and PQD2 (solid line with square). The spectra of
PQD2 solution and film samples were normalized to that of PQD1
solution for comparison.
Table 1. The chlorobenzene solutions of PQD1 and PQD2
showed broad absorption bands in visible and NIR regions with
maximum absorption peaks at 734 and 686 nm, respectively, due
to the p–p* transitions. These broad absorption features are
similar to other polymers based on DPP and electron-rich fused-
ring units.16 Despite the lower molecular weight of PQD1 than
that of PQD2, the absorption band of PDQ1 in chlorobenzene is
red-shifted compared with PQD2. This may result from the
lower solubility of PQD1 than that of PQD2 due to the short side
chains at phenyl groups, i.e., tert-butyl group, which would
induce a certain degree of packing even in the diluted chloro-
benzene solution. Consistently, a cast film of PQD1 displayed an
absorption band with a peak at 733 nm that is comparable to that
in chlorobenzene, whereas the PQD2 film exhibited significant
bathochromic shifts relative to the solution samples. This red
shift can be attributed to the more aggregated configuration
formed in the solid state. The edges of the film absorption bands
for PQD1 and PQD2 were 1030 and 1000 nm, respectively, both
of which correspond to optical bandgaps of ca. 1.2 eV. Mean-
while, measurements of steady-state fluorescence spectra of the
polymers revealed no detectable emissions in visible and NIR
regions. Besides, the fluorescence lifetime measurements in the
same region did not display any decay components. These results
indicate the occurrence of the ultrafast nonradiative vibrational
relaxation of the polymer excited state to the ground state as well
as the possibilities for quenching by defect sites and residual
impurities in the polymer samples, which are beyond the time
resolution of the measurement system (<80 ps).
3.3. Photovoltaic properties
To examine the photovoltaic properties of the polymers, OPV
devices with a sandwich configuration of ITO/PEDOT:PSS/
polymer:[70]PCBM/TiOx/Al, where the active layer is placed
between the hole transport (PEDOT:PSS) and hole blocking
(TiOx) layers, were fabricated (Fig. S3†). The active layers of
Fig. 2 Photoemission yield spectroscopy in air of PQD1 (circle) and
PQD2 (square).
Cyclic voltammetry measurements of PQD1 and PQD2 films
on ITO electrodes wꢀere also performed in acetonitrile containing
0.1 M n-Bu4N+PF6 as a supporting electrolyte (Fig. S4, see
Table 1 Optical and electrochemical data of PQD1 and PQD2
Abs. lmax/nm Abs. ledge/nm
opt a
Eg
eV
/
HOMOb/ LUMOc/
Polymer Solution Film Film
eV
eV
PQD1
PQD2
734
686
733 1030
732 1000
1.2
1.2
ꢀ5.2
ꢀ4.9
ꢀ4.0
ꢀ3.7
a
Determined from the onset wavelength in the absorption spectra of film
samples. b Determined by PYSA. c Determined from HOMO levels and
optical bandgaps.
Fig. 3 Energy level diagrams of PQD1, PQD2, [70]PCBM, buffer
layers, and electrodes used in OPV devices.
24398 | J. Mater. Chem., 2012, 22, 24394–24402
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PQD1:[70]PCBM and PQD2:[70]PCBM were spin-coated from
the chlorobenzene solutions (1 : 2 (w/w)). [70]PCBM was chosen
as the electron acceptor instead of [60]PCBM, because [70]
PCBM possesses electronic properties similar to [60]PCBM, but
a considerably higher absorption coefficient in the visible region
of 400–600 nm, which complements the absorption of low
bandgap polymers.26 Fig. 4 depicts the current–voltage charac-
teristics of the OPV devices under simulated AM 1.5G solar
irradiation at 100 mW cmꢀ2. The OPV device based on the
PQD1:[70]PCBM layer demonstrated a h value of 1.44% with an
open circuit voltage (VOC) of 0.53 V, short circuit current density
(JSC) of 6.58 mA cmꢀ2, and fill factor (FF) of 0.42, which are
superior to those of the PQD2:[70]PCBM-based device (h ¼
0.96%, JSC ¼ 5.58 mA cmꢀ2, VOC ¼ 0.45 V, FF ¼ 0.41) (Table 2).
The difference between HOMO of the donor and LUMO of the
acceptor is known to influence the VOC value.5 The high HOMO
level of PQD2 relative to PQD1 (Fig. 3) due to the introduction
of the electron-donating octyloxy groups led to the lower VOC
value of the PQD2 device (0.45 V) than the PQD1 device
(0.53 V). It should be noted here that the VOC value of the PQD1
device is still inferior to those of the devices consisting of other
DPP-based conjugated polymers,16 reflecting the strong electron-
donating feature of 2,5-thienoquinodimethane units.19 Mean-
while, the JSC and FF values of both the PQD1:[70]PCBM- and
PQD2:[70]PCBM-based devices were also lower than those of
the reported OPV devices.12–18 Considering that the film thick-
nesses and absorption intensities of the PQD1:[70]PCBM and
PQD2:[70]PCBM films are comparable to those of the active
layers in the reported OPV devices,27 the poor JSC values may
result from inefficient charge separation and charge transport. A
plausible reason for the poor charge separation efficiency is the
ultrafast nonradiative vibrational relaxation of the polymer
excited state to the ground state, which are experimentally vali-
dated by fluorescence lifetime measurements (vide supra). In
addition, the low FF values may be due to poor charge collection
efficiency at the interface between the polymer:[70]PCBM and
TiOx/Al.28
The photocurrent action spectra of both the PQD1:[70]
PCBM- and PQD2:[70]PCBM-based devices displayed a wide
range of photocurrent generation in visible and NIR regions up
to 1000 nm (Fig. 5), reflecting the broad absorption band of the
polymers (Fig. 6). It should be emphasized here that the incident
photon-to-current efficiencies (IPCE) of the PQD1:[70]PCBM-
based device in the 400–600 nm region are high compared to
those in the longer wavelength region (600–1000 nm), which does
not match the feature of the absorption spectrum. In contrast,
the IPCE profile for the PQD2:[70]PCBM-based device is largely
similar to the corresponding absorption spectrum. These results
indicate that the absorption of PQD1 makes less contribution to
the photocurrent generation than that of [70]PCBM owing to
the insufficient driving force for the photoinduced ET from the
PQD1 excited state to [70]PCBM. This is consistent with the
HOMO and LUMO energy levels of the polymers estimated with
PYSA and optical bandgaps (Fig. 3). The higher IPCE values of
the PQD1:[70]PCBM-based device in the region of the [70]
PCBM absorption rendered a JSC value higher than that of the
PQD2:[70]PCBM-based device. The results of photocurrent
action spectra provide guidelines to improve the JSC values, i.e.,
for the high efficiency of ET from the excited state of the polymer
to [70]PCBM, electron-donating side groups such as alkox-
yphenyls should be attached to the polymer main chains.
3.4. Film structures
To shed light on the relationship between the film nanostructure
and the device performances, the surface morphologies of the
active layers were evaluated by atomic force microscopy (AFM)
(Fig. 7). Although both composite films showed grain patterns
with tens of nanometer sizes at surfaces, the PQD2:[70]PCBM
Fig. 5 Photocurrent action spectra of OPV devices with PQD1:[70]
PCBM and PQD2:[70]PCBM.
Fig.
4 Current–voltage characteristics of OPV devices based on
PQD1:[70]PCBM and PQD2:[70]PCBM under illumination of AM 1.5,
100 mW cmꢀ2 white light.
Table 2 Photovoltaic properties
Active layer
JSC/mA cmꢀ2
VOC/V
FF
h/%
Fig. 6 Vis-NIR absorption spectra of PQD1:[70]PCBM and PQD2:[70]
PCBM thin films spin-coated from the chlorobenzene solutions onto
ITO/PEDOT:PSS substrates.
PQD1:[70]PCBM
PQD2:[70]PCBM
6.58
5.58
0.53
0.45
0.42
0.41
1.44
0.96
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Fig. 8 Out-of-plane XRD patterns of PQD1 and PQD2 films.
crystallinity of the PQD1 and PQD2 polymer backbone were
absent in both out-of-plane and in-plane profiles (Fig. 8 and
Fig. S5†). This is in sharp contrast to the highly crystalline P3HT
that displays a peak arising from p–p stacking with a spacing
ꢀ
of ꢁ3.8 A in the in-plane XRD patterns, suggesting the edge-on
configuration of the P3HT film.2 These results indicate that the
p–p stacking crystallinity of the PQD1 and PQD2 backbone is
insufficient to induce the corresponding XRD peak, and the low
crystallinity may lead to low charge transport efficiencies and JSC
values in comparison with other recently reported devices.2,12–19
Conclusions
In conclusion, we have designed and synthesized donor–acceptor
type low bandgap polymers containing quinoidal thiophene as a
novel non-fused donor unit and DPP as an acceptor unit for the
first time (PQD1 and PQD2 with 4-(tert-butyl)phenyl and 4-
(octyloxy)phenyl side chains, respectively). The polymers
exhibited broad and intense absorption bands in visible and NIR
regions. The devices with the polymer:[70]PCBM active layer
showed a wide range of photocurrent generation in the region of
400–1000 nm, whereas the h values were found to be moderate
(1.44% for PQD1 and 0.96% fro PQD2). The superior h value of
the PQD1-based device relative to the PQD2-based device can be
attributed to the more favorable nanostructure of the phase
separation between PQD1 and [70]PCBM as well as the higher
crystallinity of PQD1. Although these h values are lower than
those on the state-of-the-art OPV devices, we believe that the
OPV device performance with this novel type of copolymer can
be further improved by modulating the side-chain structures29
and molecular weights30 of the polymers to induce more efficient
polymer–polymer interactions. Optimization of the film struc-
tures by thermal annealing or the use of solvent additives31 may
also lead to improvements in device performances. These results
obtained here demonstrate that heteroquinodimethane struc-
tures are promising, simple, and versatile comonomer building
blocks for photoactive materials in polymer bulk heterojunction
OPV applications.
Fig. 7 Tapping-mode atomic force micrographs of (a) PQD1:[70]PCBM
and (b) PQD2:[70]PCBM thin films spin-coated on the ITO/PEDOT:PSS
substrate. The color scale represents the height topography, with bright
and dark representing the highest and lowest features, respectively. The
RMS surface roughnesses are (a) 0.45 and (b) 0.76 nm, respectively.
thin film exhibited a larger root-mean-square (RMS) surface
roughness of 0.76 nm than that of the PQD1:[70]PCBM film
(RMS ¼ 0.45 nm). This result indicates the occurrence of an
unfavorably large nano-scale phase separation in the PQD2:[70]
PCBM layer, lowering the charge separation efficiency at the
polymer–[70]PCBM interface.1 In addition, poor electrical
communication at the interface between the PQD2:[70]PCBM
layer and electrodes (i.e., TiOx/Al and ITO/PEDOT:PSS) as a
result of the rough surface structure may lead to inferior device
performance. X-ray diffraction (XRD) measurements of the
polymer films drop-cast from the chlorobenzene solutions were
also conducted to assess the bulk morphology of the polymers.
Fig. 8 and S5 (see ESI†) provide the out-of-plane and in-plane
XRD patterns of the thin films of PQD1 and PQD2. The PQD1
film exhibited a peak at 2q ¼ 4.6ꢂ (Fig. 8), which can be assigned
to the polymer lamellar sheet with an inter-polymer distance of
ꢀ
19 A. A similar result was obtained for the in-plane XRD profile
(Fig. S3†), indicating that there is no preferential orientation of
the lamellar sheet in PQD1 relative to the substrate. On the other
hand, the PQD2 film showed no obvious signals in both profiles,
suggesting that the polymer is almost amorphous. The more
irregular alignment of PQD2 may inhibit the efficient charge
transport, resulting in inferior device performances despite the
sufficient driving force of electron transfer from the polymer to
[70]PCBM. It is noteworthy that signals of p–p stacking
Acknowledgements
H. I. thanks NEDO for financial support. The authors thank Ms
Kati Stranius, Prof. N. V. Tkachenko and Prof. H. Lemmetyinen
for the TCSPC measurements, and Ms M. L. C. de Jesus, Prof. T.
Sagawa (Kyoto University), Dr Y. Nakajima, Mr Y. Iwai and
Mr S. Fujiwara (Riken Keiki, Co. Ltd.) for the PYSA
measurements.
24400 | J. Mater. Chem., 2012, 22, 24394–24402
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W. Zhang, Z. Zhang, D. Qian, Y. Huang, J. Hou and Y. Li, Adv.
Mater., 2012, 24, 1476.
References
16 (a) M. M. Wienk, M. Turbiez, J. Gilot and R. A. J. Janssen, Adv.
Mater., 2008, 20, 2556; (b) A. P. Zoombelt, S. G. J. Mathijssen,
M. G. R. Turbiez, M. M. Wienk and R. A. J. Janssen, J. Mater.
Chem., 2010, 20, 2240; (c) A. T. Yiu, P. M. Beaujuge, O. P. Lee,
1 (a) H. Hoppe and N. S. Sariciftci, J. Mater. Res., 2004, 19, 1924; (b)
R. Gaudiana and C. Reabec, Nat. Photonics, 2008, 2, 287; (c)
G. Dennler, M. C. Scharber and C. J. Brabec, Adv. Mater., 2009,
21, 1323; (d) C. Deibel and V. Dyakonov, Rep. Prog. Phys., 2010,
73, 096401.
2 (a) K. M. Coakley and M. D. McGehee, Chem. Mater., 2004, 16,
4533; (b) W. Ma, C. Yang, X. Gong, K. Lee and A. J. Heeger, Adv.
Funct. Mater., 2005, 15, 1617; (c) G. Li, V. Shrotriya, J. Huang,
Y. Yao, T. Moriarty, K. Emery and Y. Yang, Nat. Mater., 2005, 4,
864.
ꢁ
C. H. Woo, M. F. Toney and J. M. J. Frechet, J. Am. Chem. Soc.,
2012, 134, 2180.
17 (a) R. C. Coffin, J. Peet, J. Rogers and G. C. Bazan, Nat. Chem., 2009,
1, 657; (b) S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates,
J. S. Moon, D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat.
Photonics, 2009, 3, 297; (c) H. Zhou, L. Yang, A. C. Stuart,
S. C. Price, S. Liu and W. You, Angew. Chem., Int. Ed., 2011, 50,
2995.
€
3 M. S. Scharber, D. Muhlbacher, M. Koppe, P. Denk, C. Waldauf,
A. J. Heeger and C. J. Brabec, Adv. Mater., 2006, 18, 789.
4 (a) F. Wudl, M. Kobayashi and A. J. Heeger, J. Org. Chem., 1984, 49,
3382; (b) M. Kobayashi, N. Colaneri, M. Boysel, F. Wudl and
A. J. Heeger, J. Chem. Phys., 1985, 82, 5717.
5 (a) S. A. Jenekhe, Nature, 1986, 322, 345; (b) A. O. Patil and F. Wudl,
Macromolecules, 1988, 21, 540; (c) W.-C. Chen and S. A. Jenekhe,
Macromolecules, 1995, 28, 454; (d) W.-C. Chen and S. A. Jenekhe,
Macromolecules, 1995, 28, 465; (e) W.-C. Chen, C.-L. Liu,
C.-T. Yen, F.-C. Tsai, C. J. Tonzola, N. Olson and S. A. Jenekhe,
Macromolecules, 2004, 37, 5959.
ꢁ
18 (a) Y. Zou, A. Najari, P. Berrouard, S. Beaupre, R. A. Badrou,
Y. Tao and M. Leclerc, J. Am. Chem. Soc., 2010, 132, 5330; (b)
C. Piliego, T. W. Holcombe, J. D. Douglas, C. H. Woo,
ꢁ
P. M. Beaujuge and J. M. J. Frechet, J. Am. Chem. Soc., 2010, 132,
7595; (c) T.-Y. Chu, J. Lu, S. Beaupr, Y. Zhang, J.-R. Pouliot,
S. Wakim, J. Zhou, M. Leclerc, Z. Li, J. Ding and Y. Tao, J. Am.
Chem. Soc., 2011, 133, 4250; (d) Z. Li, S.-W. Tsang, X. Du,
L. Scoles, G. Robertson, Y. Zhang, F. Toll, Y. Tao, J. Lu and
J. Ding, Adv. Funct. Mater., 2011, 17, 3331.
6 (a) P. Stagnaro, G. Costa and A. Gandini, Macromolecules, 2001, 34,
26; (b) P. Stagnaro, G. Costa, F. Pioli and A. Gandini, Polymer, 2003,
44, 1359; (c) M. B. Zaman and D. F. Perepichka, Chem. Commun.,
2005, 4187; (d) P. Stagnaro, F. Pioli, M. Panizza and A. Gandini,
Macromolecules, 2009, 42, 2455.
7 (a) H. Goto, K. Akagi and H. Shirakawa, Synth. Met., 1997, 84, 385;
(b) R. Kiebooms, H. Goto and K. Akagi, Synth. Met., 2001, 119, 117;
(c) R. H. L. Kiebooms, H. Goto and K. Akagi, Macromolecules, 2001,
34, 7989.
19 W.-C. Chen and S. A. Jenekhe, Macromol. Chem. Phys., 1998, 199,
655.
20 (a) E. Maligaspe, N. V. Tkachenko, N. K. Subbaiyan, R. Chitta,
M. E. Zandler, H. Lemmetyinen and F. D’Souza, J. Phys. Chem. A,
2009, 113, 8478; (b) N. V. Tkachenko, L. Rantala, A. Y. Tauber,
J. Helaja, P. H. Hynninen and H. Lemmetyinen, J. Am. Chem.
Soc., 1999, 121, 9378; (c) V. Vehmanen, N. V. Tkachenko,
H. Imahori, S. Fukuzumi and H. Lemmetyinen, Spectrochim. Acta,
Part A, 2001, 57, 2229; (d) M. Isosomppi, N. V. Tkachenko,
A. Efimov and H. Lemmetyinen, J. Phys. Chem. A, 2005, 109,
4881.
21 (a) A. Hayakawa, O. Yoshikawa, T. Fujieda, K. Uehara and
S. Yoshikawa, Appl. Phys. Lett., 2007, 90, 163517; (b) T. Umeyama,
T. Takamatsu, N. Tezuka, Y. Matano, Y. Araki, O. Yoshikawa,
T. Sagawa, S. Yoshikawa and H. Imahori, J. Phys. Chem. C, 2009,
113, 10819; (c) T. Umeyama, M. Oodoi, O. Yoshikawa, T. Sagawa,
S. Yoshikawa, D. Evgenia, N. Tezuka, Y. Matano, K. Stranius,
N. V. Tkachencko, H. Lemmetyinen and H. Imahori, J. Mater.
Chem., 2011, 21, 12454; (d) T. Umeyama, E. Douvogianni and
H. Imahori, Chem. Lett., 2012, 41, 354; (e) T. Umeyama,
K. Hirose, K. Noda, K. Matsushige, T. Shishido, H. Hayashi,
Y. Matano, N. Ono and H. Imahori, J. Phys. Chem. C, 2012, 116,
1256; (f) T. Umeyama, K. Hirose, K. Noda, K. Matsushige,
8 (a) A. O. Patil and F. Wudl, Macromolecules, 1988, 21, 540; (b)
€
H. Neugebauer, C. Kvarnstrom, C. Brabec, N. S. Sariciftci,
R. Kiebooms, F. Wudl and S. Luzzati, J. Chem. Phys., 1999, 110,
12108; (c) R. Kiebooms and F. Wudl, Synth. Met., 1999, 101, 40.
€
9 (a) H. Braunling, R. Becker and G. Blochl, Synth. Met., 1991, 42,
1539; (b) J. M. Toussaint and J. L. Bredas, J. Chem. Phys., 1991,
€
ꢁ
94, 8122; (c) H. Aota, T. Reikan, A. Matsumoto and M. Kamachi,
Chem. Lett., 1997, 26, 527; (d) H. Goto and K. Akagi, J. Polym.
Sci., Part A: Polym. Chem., 2005, 43, 616; (e) H. Aota, T. Ishikawa,
Y. Amiuchi, H. Yano, T. Kunimoto and A. Matsumoto, Chem.
Lett., 2010, 39, 1288.
10 (a) Q. Zhang, Y. Li and M. Yang, J. Mater. Sci., 2004, 39, 6089; (b)
Q. Zhang, Y. Li and M. Yang, Synth. Met., 2004, 146, 69; (c)
M. Yang, Q. Zhang, P. Wu, H. Ye and X. Liu, Polymer, 2005, 46,
6626; (d) Q. Zhang, J. Feng, K. Liu, D. Zhu, M. Yang, H. Ye and
X. Liu, Synth. Met., 2006, 156, 804.
€€
T. Shishido, H. Saarenpaa, N. V. Tkachenko, H. Lemmetyinen,
N. Ono and H. Imahori, J. Phys. Chem. C, DOI: 10.1021/
jp305001p; (g) S. Kitaura, K. Kurotobi, M. Sato, T. Umeyama,
Y. Takano and H. Imahori, Chem. Commun., 2012, 48, 8550.
€
11 (a) L. Burgi, M. Turbiez, R. Pfeiffer, F. Bienewald, H. Kirner and
C. Winnewisser, Adv. Mater., 2008, 20, 2217; (b) Y. Li, S. P. Singh
and P. Sonar, Adv. Mater., 2010, 22, 4862; (c) H. N. Tsao,
D. M. Cho, I. Park, M. R. Hansen, A. Mavrinskiy, D. Y. Yoon,
22 GPC traces of 3a and 3b showed a sharp peak with a slight tailing
(Fig. S2†), suggesting the existence of small amounts of impurities.
€
23 J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, H. Bassler,
M. Porsch and J. Daub, Adv. Mater., 1995, 7, 551.
€
R. Graf, W. Pisula, H. Spiess and K. Mullen, J. Am. Chem. Soc.,
2011, 133, 2605.
€
12 (a) D. Muhlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller,
R. Gaudiana and C. Brabec, Adv. Mater., 2006, 18, 2884; (b)
24 (a) H. Bronstein, Z. Chen, R. S. Ashraf, W. Zhang, J. Du,
J. R. Durrant, P. S. Tuladhar, K. Song, S. E. Watkins, Y. Geerts,
M. M. Wienk, R. A. J. Janssen, T. Anthopoulos, H. Sirringhaus,
M. Heeney and I. McCulloch, J. Am. Chem. Soc., 2011, 133, 3272;
(b) R. J. Kumar, J. M. MacDonald, T. B. Singh, L. J. Waddington
and A. B. Holmes, J. Am. Chem. Soc., 2011, 133, 8564; (c)
I. Osaka, M. Shimawaki, H. Mori, I. Doi, E. Miyazaki,
T. Koganezawa and K. Takimiya, J. Am. Chem. Soc., 2012, 134, 3498.
25 Because the reduction onsets of PQD1 and PQD2 films were relatively
evident in the cyclic voltammograms (Fig. S2†), the LUMO energy
levels could be calculated to be ꢀ3.9 and ꢀ3.7 eV, respectively,
both of which are comparable to the values obtained by PYSA
measurements and optical bandgaps (Table 1).
26 M. M. Wienk, J. M. Kroon, W. J. H. Verhees, J. Knol,
J. C. Hummelen, P. A. van Hal and R. A. J. Janssen, Angew.
Chem., Int. Ed., 2003, 42, 3371.
27 (a) K.-H. Ong, S.-L. Lim, H.-S. Tan, H.-K. Wong, J. Li, Z. Ma,
L. C. H. Moh, S.-H. Lim, J. C. de Mello and Z.-K. Chen, Adv.
Mater., 2011, 23, 1409; (b) Y. Sun, C. Cui, H. Wang and Y. Li,
Adv. Energy Mater., 2011, 1, 1058; (c) L. Yang, H. Zhou,
S. C. Price and W. You, J. Am. Chem. Soc., 2012, 134, 5432.
€
Z. Zhu, D. Waller, R. Gaudiana, M. Morana, D. Muhlbacher,
M. C. Scharber and C. Brabec, Macromolecules, 2007, 40, 1981.
13 (a) L. Liao, L. Dai, A. Smith, M. Durstock, J. Lu, J. Ding and Y. Tao,
Macromolecules, 2007, 40, 9406; (b) J. Hou, H. Chen, S. Zhang, G. Li
and Y. Yang, J. Am. Chem. Soc., 2008, 130, 16144; (c) C. V. Hoven,
X.-D. Dang, R. C. Coffin, J. Peet, T.-Q. Nguyen and G. C. Bazan,
Adv. Mater., 2010, 22, E63.
14 (a) H. Pan, Y. Li, Y. Wu, P. Liu, B. Ong, S. Zhy and G. Xu, Chem.
Mater., 2006, 18, 3237; (b) L. Huo, J. Hou, S. Zhang, H.-Y. Chen
and Y. Yang, Angew. Chem., Int. Ed., 2010, 49, 1500; (c) M. Wang,
X. Hu, P. Liu, W. Li, X. Gong, F. Huang and Y. Cao, J. Am.
Chem. Soc., 2011, 133, 9368.
15 (a) Y. Y. Liang, Y. Wu, D. Q. Feng, S.-T. Tsai, G. Li, H. J. Son and
L. P. Yu, J. Am. Chem. Soc., 2009, 131, 56; (b) Y. Y. Liang,
D. Q. Feng, Y. Wu, S.-T. Tsai, G. Li, C. Ray and L. P. Yu, J. Am.
Chem. Soc., 2009, 131, 7792; (c) Y. Y. Liang, Z. Xu, J. B. Xia,
S. T. Tsai, Y. Wu, G. Li, C. Ray and L. P. Yu, Adv. Mater., 2010,
22, E135; (d) Z. He, C. Zhong, X. Huang, W.-Y. Wong, H. Wu,
L. Chen, S. Su and Y. Cao, Adv. Mater., 2011, 23, 4636; (e) Z. Tan,
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 24394–24402 | 24401

View Online
28 (a) C. J. Brabec, S. E. Shaheen, C. Winder, N. S. Sariciftci and
P. Denk, Appl. Phys. Lett., 2002, 80, 1288; (b) D. Gupta,
S. Mukhopadhyay and K. S. Narayan, Sol. Energy Mater. Sol.
Cells, 2010, 94, 1309.
29 (a) J. M. Szarko, J. Guo, Y. Liang, B. Lee, B. S. Rolczynski,
J. Strzalka, T. Xu, S. Loser, T. J. Marks, L. Yu and L. X. Chen,
Adv. Mater., 2010, 22, 5468; (b) S. Subramaniyan, H. Xin,
F. S. Kim, S. Shoaee, J. R. Durrant and S. A. Jenekhe, Adv. Energy
Mater., 2011, 1, 854; (c) S. Ko, E. T. Hoke, L. Pandey, S. Hong,
R. Mondal, C. Risko, Y. Yi, R. Noriega, M. D. McGehee,
30 (a) P. Schilinsky, U. Asawapirom, U. Scherf, M. Biele and
C. J. Brabec, Chem. Mater., 2005, 17, 2175; (b) A. M. Ballantyne,
L. Chen, J. Dane, T. Hammant, F. M. Braun, M. Heeney,
W. Duffy, I. McCulloch, D. D. C. Bradley and J. Nelson, Adv.
Funct. Mater., 2008, 18, 2373.
31 (a) F. Padinger, R. S. Rittberger and N. S. Sariciftci, Adv. Funct.
Mater., 2003, 13, 85; (b) J. Peet, C. Soci, R. C. Coffin,
T. Q. Nguyen, A. Mikhailovsky, D. Moses and G. C. Bazan, Appl.
Phys. Lett., 2006, 89, 252105; (c) J. Peet, J. Y. Kim, N. E. Coates,
W. L. Ma, D. Moses, A. J. Heeger and G. C. Bazan, Nat. Mater.,
2007, 6, 497; (d) F.-C. Chen, H.-C. Tseng and C.-J. Ko, Appl. Phys.
Lett., 2008, 92, 103316.
ꢁ
J.-L. Bredas, A. Salleo and Z. Bao, J. Am. Chem. Soc., 2012, 134,
5222.
24402 | J. Mater. Chem., 2012, 22, 24394–24402
This journal is ª The Royal Society of Chemistry 2012