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A Vinylene-Linked Benzo[1,2-b:4,5-b’]dithiophene-2,1,3-Benzothiadiazole
Low-Bandgap Polymer
Alessandro Abbotto,1 Mirko Seri,2 Milind S. Dangate,1 Filippo De Angelis,3
Norberto Manfredi,1 Edoardo Mosconi,3 Margherita Bolognesi,4 Riccardo Ruffo,1
Matteo M. Salamone,1 Michele Muccini1
1
Department of Materials Science, Milano-Bicocca Solar Energy Research Center—MIB-Solar,
University of Milano-Bicocca, Via Cozzi 53, Milano I-20125, Italy
2
Consiglio Nazionale delle Ricerche, Istituto per lo Studio dei Materiali Nanostrutturati (CNR-ISMN),
Via P. Gobetti101, Bologna I-40129, Italy
3
Consiglio Nazionale delle Ricerche, Istituto CNR di Scienze e Tecnologie Molecolari (CNR-Italy),
Via Elce di Sotto 8, Perugia I-06213, Italy
4
Laboratory MIST E-R, Via P. Gobetti101, Bologna I-40129, Italy
Correspondence to: A. Abbotto (E-mail: [email protected]) or R. Ruffo (E-mail: [email protected])
Received 14 January 2012; accepted 8 March 2012; published online 30 March 2012
DOI: 10.1002/pola.26046
ABSTRACT: A new heteroarylene-vinylene donor–acceptor
polymer P(BDT-V-BTD) with reduced bandgap has been synthesized and its photophysical, electronic and photovoltaic
properties investigated both experimentally and theoretically.
The structure of the polymer comprises an unprecedented
combination of a strong donor (4,8-dialkoxy-benzo[1,2-b:4,5b’]dithiophene, BDT), a strong acceptor (2,1,3-benzothiadiazole, BTD) and a vinylene spacer. The new polymer was
obtained by a metal-catalyzed cross-coupling Stille reaction
and fully characterized by NMR, UV–vis absorption, GPC,
TGA, DSC and electrochemistry. Detailed ab initio computations with solvation effects have been performed for the
monomer and model oligomers. The electrochemical investigation has ascertained the ambipolar character of the polymer and energetic values of HOMO, LUMO and bandgap
matching materials-design rules for optimized organic photovoltaic devices. The HOMO and LUMO energies are consis-
tently lower than those of previous heteroarylene-vinylene
polymer while the introduction of the vinylene spacer
afforded lower bandgaps compared to the analogous system
P(BDT-BTD) with no spacer between the aromatic rings.
These superior properties should allow for enhanced photovoltages and photocurrents in photovoltaic devices in combination with PCBM. Preliminary photovoltaic investigation
afforded relatively modest power conversion efficiencies of
0.74% (AM 1.5G, 100 mW/cm2), albeit higher than that of previous heteroarylene-vinylene polymers and comparable to
C 2012 Wiley Periodicals, Inc. J Polym
that of P(BDT-BTD). V
Sci Part A: Polym Chem 50: 2829–2840, 2012
INTRODUCTION Low-bandgap p-conjugated semiconducting
polymers are attracting an increasing interest in a number of
materials science fields, including electrochromics, organic
transistors and organic photovoltaics (OPV).1–4 In bulk heterojunction (BHJ) OPV devices a conjugated polymer, acting as
the donor, transfers an electron from the sunlight promoted
excited state to a fullerene derivative,5 typically PC61BM
([6,6]-phenyl-C61-butyric acid methyl ester) or PC71BM
([6,6]-phenyl-C71-butyric acid methyl ester), acting as the
acceptor.6,7 The separated electron-hole pair is then transported and collected at the electrodes. In order to achieve
high power conversion efficiencies (PCEs) an efficient sun-
light harvesting is needed to yield high short-circuit current
densities (Jsc). This aspect necessarily requires the use of
low bandgap conjugated polymers as the donors, able to efficiently absorb up to the visible lower energy portion of the
solar spectrum. Recent design rules have established that
the best donor polymers should have a bandgap energy in
the range of 1.2–1.7 eV.8 Further material-design rules
require an energy of the lowest unoccupied molecular orbital
(LUMO) of the polymer higher than that of the LUMOPCBM,
with a minimum offset of 0.3 eV, which corresponds to a
value of 4.0 eV (or higher) assuming 4.3 eV for
LUMOPCBM, and, accordingly, an energy of the Highest
KEYWORDS: bulk heterojunction solar cells; conducting poly-
mers; conjugated polymers; donor–acceptor polymers; electrochemistry; heteroaromatics; heteroatom-containing polymers;
low bandgap polymers; polyaromatics; quantum chemistry
Additional Supporting Information may be found in the online version of this article.
C 2012 Wiley Periodicals, Inc.
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FIGURE 2 Structure of polymer P(BDT-BTD).
FIGURE 1 Structure of donor BDT and acceptor BTD constituting units of the investigated D-A polymers.
Occupied Molecular Orbital (HOMO) of the polymer ranging
from 5.2 to 5.7 eV.
Spectral engineering in semiconducting polymers is an important tool in order to achieve large OPV efficiencies.9 A
very common strategy to low-bandgap polymers is based on
the donor–acceptor approach, where p electron-rich donor
(D) and p electron-poor acceptor (A) fragments are alternated along the p-conjugated backbone.10 This derives from
the fact that D groups raise the HOMO energy and concomitantly A groups lower the LUMO energy, with the global
effect of decreasing the polymer bandgap. In the last years
researchers have used several D and A groups, including
sophisticated systems.11 In particular, our attention was
attracted by two highly efficient fragments, 4,8-dialkoxybenzo[1,2-b:4,5-b’]dithiophene (BDT) as a donor group and
2,1,3-benzothiadiazole (BTD) as an acceptor group (Fig. 1).
The donor BDT group has been recently used in a significant
number of highly efficient polymers for BHJ devices,12
including some of the present record polymers with overall
conversion efficiencies approaching 8%, PTB7 and PBDTTT.13
The acceptor BTD ring has lately received great attention
thanks to its electron-withdrawing strength and ability to
effectively lower the LUMO energy.14 Again, the use of BTD
permitted to build a new polymer with top-ranked PCE
(6.1%) and a remarkable internal quantum efficiency
approaching unity.15
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where D and A are heteroaromatic fragments separated by a
vinylene (V) spacer, have been described. We believe that
this event, mostly due to the more stringent synthetic issues
and to the lack of convenient commercial precursors, represents an important limitation in the field of semiconducting
polymers for OPV in view of the significant advantages
that the vinylene spacer might offer such as promoted coplanarity of adjacent aromatic units, extended p-conjugation,
enhanced intermolecular p-stacking, and, accordingly,
reduced bandgaps.17 Swager and coworkers have pioneered
the use of V-linked D-A polymers with donor alkoxybenzene
rings18 as alternatives to conventional poly(p-phenylenevinylene) (PPV) systems.19 Naso and coworkers have investigated several fluorinated PPVs.20 Reynolds and coworkers
have recently reported a vinylene-linked BTD-based polymer
with a low bandgap energy (1.7 eV) but still modest OPV
efficiencies (0.3%).21 Our group has recently described two
V-linked D-A polymers where A was a pyridine ring with different substitution patterns and D either an electron-rich
3,4-ethylenedioxithiophene (EDOT),22 or a pyrrole moiety.23
The energetic characterization of the EDOT polymers P(2,5Py-V-EDOT) and P(2,6-Py-V-EDOT) revealed HOMO (5.1/
5.0 eV), LUMO (3.4 eV), and narrow bandgap (1.6/1.7
eV) energies fitting materials-design rules for optimized OPV.
However, as for Reynolds’s case, photovoltaic devices in combination with PC71BM afforded a relatively modest PCE of
0.5% (AM 1.5G, 100 mW/cm2), which was mostly attributed to the low molecular-weight of the polymers accessible
via the chemical route.
Furthermore, the use of the donor BDT and/or acceptor BTD
led to polymers with optimal HOMO, LUMO, and bandgap
energies, closely matching those of the ideal conjugated polymer in OPV devices, showing that these polycyclic heteroaromatic rings are very effective constituting units for the
design of performing p-conjugated polymers for BHJ cells.
Nevertheless, very few reports have described the simultaneous use of both fragments in the same polymeric backbone.16 Surprisingly, in none of these cases vinylene spacers
were used but an aryl–aryl bonding was always present,
with the BDT and BTD groups either directly bonded to each
other or alternated with thienyl spacers. The most simple
combination, the polymer P(BDT-BTD), was investigated by
Hou et al., Yang and coworkers (Fig. 2).16(a) An electrochemical bandgap of 1.9 eV and a PCE of 0.90 in combination with
PC61BM was reported.
We present here the first example of a vinylene-linked BDTBTD low-bandgap polymer P(BDT-V-BTD) (Fig. 3). The polymer has been synthesized via metal catalyzed poly-coupling
reaction and fully characterized in its optical, electrochemical, charge carrier mobility, and photovoltaic properties.
Despite the very large variety of D-A polymers, relatively few
examples of vinylene-linked conjugated polymers for OPV,
FIGURE 3 Structure of the polymer P(BDT-V-BTD) investigated
in this work.
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SCHEME 1 Synthesis of the polymer P(BDT-V-BTD).
DFT/TDDFT computations have been performed for model
oligomers to analyze their optical and electronic properties.
EXPERIMENTAL
Chemical and Spectroscopic Characterization
1
H NMR spectra were recorded on a Bruker AMX-500 instrument operating at 500.13 MHz. All reagents were obtained
from commercial suppliers at the highest purity grade and
used without further purification. Anhydrous toluene was
purchased from Sigma Aldrich and used as received in an
argon-filled glove-box. Absorption spectra were recorded on
a V-570 Jasco spectrophotomer. Transition temperatures
were determined by differential scanning calorimetry (DSC)
using a Mettler Toledo DSC 821 instrument with a heating
and cooling rate of 15 C/min under nitrogen. Thermogravimetric analyses (TGA) were performed with a Mettler Toledo
TGA/DSC STARe system at a heating rate of 10 C/min under
nitrogen. Gel permeation chromatography (GPC) analyses
were recorded on a Waters 1515 separation module using
polystyrene as a standard and THF as an eluant.
Synthesis of P(BDT-V-BTD)
In a glove-box filled with argon ([O2] < 1 ppm) a mixture of
4,7-bis((E)-2-bromovinyl)benzo[c][1,2,5]thiadiazole (2)21 (265
mg, 0.77 mmol) and 2,6-bis(trimethyltin)-4,8-dioctyloxybenzo[1,2-b:4,5-b’]dithiophene (1)12(f) (Scheme 1) (590 mg,
0.77 mmol) in toluene (40 mL) was put in a schlenk flask and
stirred for 40 min. Then Pd(PPh3)4 (88 mg, 0.077 mmol) was
added to the flask and the mixture stirred for another 40 min.
The flask was tightly closed, removed from the glove-box, and
stirred for 18 h at 110 C under an argon atmosphere. After
cooling down to room temperature MeOH (80 mL) was added
to the reaction mixture. The formed precipitate was collected
by filtration into a Soxhlet thimble and extracted with n-hexane and CHCl3. The solid, obtained by removing the solvent
from the CHCl3 fraction under reduced pressure at T below
40 C, was dried under vacuum for 1 day to give the polymer
(200 mg, 0.32 mmol, 42%) as a violet solid. 1H NMR
(C2D2Cl4): d 8.5 – 6.5 (8H, m, aromatic and vinylene protons),
4.3 (4H, broad, OCH2), 1.9 (4H, broad, OCH2CH2), 1.8–1.2
(20H, m, remaining CH2), 0.9 (6H, broad, CH3).
Electrochemical Characterization
The electrochemical characterization was performed by differential pulsed voltammetry (DPV) and cyclic voltammetry
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(CV) in a two compartment three electrode cell assembled in
an Argon filled glove box ([O2] <1 ppm) using an EG&G
PARSTAT 2263 potentiostat/galvanostat. A gold disc, a Pt
flag, and a Ag/AgCl wire were used as working, counter, and
pseudoreference electrode, respectively. The electrolyte was
a 0.1 M solution of tetrabutylammonium perchlorate (Fluka,
electrochemical grade, =99.0%) in anhydrous dichloromethane (Sigma Aldrich >99.8%). The working electrode
disc was well polished with an 0.1 lm alumina suspension,
sonicated for 15 min in deionized water and washed with 2propanol before use. The pseudoreference electrode was calibrated either by adding ferrocene (0.5 mM) to test solution
(reductive DPV) or externally by a 0.5 mM solution of ferrocene in the electrolyte (in absence of the polymer). Both
calibrations provided the same result; the corresponding
cathodic peak potentials differed for less than 5 mV.
Computational Investigation
All the calculations have been performed with the Gaussian
03 program package.24 3-21G* and 6-31G* basis sets were
used for geometry optimizations. A 6-31G* basis set were
used for single point energy calculations. For all calculations
we used a polarizable continuum model (PCM) to describe
solvation effects.25(a) For the optical properties, in previous
papers on similar highly conjugated systems,26,27 we found a
considerable red-shift of the lowest TDDFT excitation energy
calculated by the B3LYP functional,25(b) compared to the experimental absorption maxima. This shift was as large as
0.6–0.8 eV and was related to the inaccurate description of
charge-transfer excited states in highly delocalized systems.26–28 Moreover, further red-shifts were calculated by
introducing solvation effects. To overcome this limitation we
used here the B3LYP functional for geometry optimizations
and the MPW1K functional,29 containing 42% of Hartree–
Fock exchange, for TDDFT excitation energies.27 The
increased amount of Hartree–Fock exchanges ensures a correction of the self interaction error typical of conventional
functionals and also improves the long-range exchange
behavior. The MPW1K excitation energies of the investigated
polymers are in excellent agreement with the experimental
values. As far as the electrochemical properties are concerned, we proceeded to evaluate the energy of HOMO and
LUMO of the investigated polymers applying the vertical
approximation due to the Koopmans theorem, that is, by taking the negative of the HOMO and LUMO single particle
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eigenvalues at the B3LYP/6-31G* level in solution. Also in
this case we are able to nicely reproduce the measured
energy values of HOMO and LUMO, with a deviation from
the experimental data in the range of 0.1–0.2 eV.
Fabrication and Characterization of Photovoltaic
Cells and OFETs
Photovoltaic devices were fabricated with the standard
architecture ITO/PEDOT:PSS/P(BDT-V-BTD):PCBM/LiF/Al. Indium-Tin Oxide (ITO) covered glass substrates (sheet resistance 10 X/square were cleaned sequentially by ultrasonic
treatment in deionized water, acetone and isopropyl alcohol,
then dried and placed in a Oxygen/UV plasma chamber
cleaner for 10 mins. A poly(3,4-ethylenedioxythiophene) /
poly(styrenesulfonate) (PEDOT:PSS) (Baytron P) layer about
80 nm thick was spin-coated at 2000 rpm (60 s) onto the
substrates, followed by annealing at 150 C for 30 mins.
PC61BM and PC71BM were bought from American Dye
Source. Two blended solutions of P(BDT-V-BTD) and PC61BM
in dry CHCl3 were prepared, with polymer concentration of
7 mg/mL and PC61BM concentrations of 7 and 10.5 mg/mL,
leading to solutions with polymer: fullerene weight ratios of
1:1 and 1:1.5, respectively. Another solution was prepared
with P(BDT-V-BTD) (7 mg/mL) and PC71BM (10.5 mg/mL)
in dry CHCl3 leading to a solution with P(BDT-VBTD):PC71BM weight ratio of 1:1.5. Blend solutions were
kept in an ultrasonic bath for 2 hours, then spin-coated in
air at 1500 rpm for 60 s onto glass/ITO/PEDOT:PSS substrates (active layer thickness 50 nm). LiF and Al cathodes
(0.6 nm and 70 nm thick) were deposited sequentially
through a mask on top of the active layer in a vacuum
chamber at a pressure of 106 Torr. The active area of the
device was 6 mm2. Current density-voltage curves were
recorded with a Keithley 236 source-meter unit by illuminating a single device through a mask with simulated AM 1.5G
irradiation (100 mW/cm2) from an Abet Technologies Sun
2000 Solar Simulator. All devices were tested in oxygen and
water free environment inside a glove box filled with nitrogen. The thicknesses of the films were measured by a profilometer. Atomic force microscopy (AFM) images were taken
with a Solver Pro (NTMDT) scanning probe microscope in
tapping mode. Absorption of thin films was measured using
a JASCO spectrometer.
External Quantum Efficiency (EQE) was measured with a
home built system on encapsulated devices: monochromatic
light was obtained with a Xenon arc lamp from Lot-Oriel
(300 Watt power) coupled with a Spectra-Pro monochromator. The photocurrent produced by the device passed
through a calibrated resistance (51 Ohms) and the Voltage
signal was collected after the resistance with a Merlin LockIn Digital Amplifier. Signal was pulsed by means of an optical
chopper (around 300 Hz frequency). Internal Quantum Efficiency (IQE) was calculated starting from the EQE and
absorption spectra, taking into account the following
assumptions: (i) 7% of incident light is reflected at the air/
glass interface, (ii) the Al contact is considered a perfect mirror, therefore the fraction of light absorbed by the active
layer (A%) is calculated from the doubled absorption spec-
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trum (Abs) of the active layer, as A% ¼ 1–10(2*Abs). Thus
IQE ¼ EQE/A%.
Bottom-gate/top-contact OFETs were fabricated on hexamethyldisilazane (HMDS)-treated, p-doped Silicon wafers
covered with a SiO2 dielectric layer. Trimethylsilation of the
SiO2 surface was done by exposure to HMDS vapors in airfree, room temperature reaction. P(BDT-V-BTD):PC61BM 1:1.5
(w/w) or P(BDT-V-BTD) only films were prepared in the
same condition used for the OPVs active layers preparation.
To complete the OFET devices, 50 nm of Au was thermally
evaporated over the semiconducting layer through a shadow
mask in a vacuum chamber at a pressure of 106 Torr to
yield the source and drain electrodes (channel length ¼
1000 lm and channel width ¼ 70 lm).
RESULTS AND DISCUSSION
Synthesis, Chemical Characterization, and
Optical Properties
The polymer P(BDT-V-BTD) was obtained according to
Scheme 1 by a Stille poly-cross-coupling reaction catalyzed
by Pd(PPh3)4 starting from the bis-trimethyltin derivative 1
and the bis-2-bromovinyl BTD derivative 2.
Polymerization proceeded in good yields producing materials
with moderate number-average molecular weights. The polymer was precipitated in methanol, collected by filtration,
and purified by Soxhlet extraction. Low-molecular-weight
oligomers were removed by extraction with n-hexane. The
soluble higher-molecular-weight fraction was extracted
with chloroform with a significant portion of solid residue,
not soluble in common organic solvents, remaining in the
Soxhlet thimble.
The new polymer was characterized by 1H NMR and UV–vis
spectroscopy, GPC, DSC, and TGA. Figure 4 shows the 1H
NMR spectrum of P(BDT-V-BTD). Table 1 summarizes the
main structural, optical, and thermal properties of the polymer along with those of our previously reported vinylene
EDOT-pyridine polymers P(2,6-Py-V-EDOT) and P(2,6-Py-VEDOT),22 and of the reference polymer P(BDT-BTD) containing the same donor and acceptor heteroaromatic units but
without the vinylene spacer.16(a)
Figure 5 shows the UV–vis spectra in solution and as thin
films of P(BDT-V-BTD). A second polymer, where the two
linear octyl chains of the BDT unit were replaced by two 2ethylhexyl branched chains, was prepared starting from
the corresponding bis-trimethyltin precursor.16(b) Although
photophysical properties were similar to that of the octyl
derivative, solubility and film processability unexpectedly
resulted poorer and no significant photovoltaic response
in OPV cells in combination with PC61BM or PC71BM was
measured. For these reasons the 2-ethylhexyl substituted
polymer was no further considered in this work.
The GPC data revealed molecular-weights which are somewhat smaller than P(BDT-BTD) but higher than previous
vinylene polymers P(2,6-Py-V-EDOT) and P(2,5-Py-V-EDOT).
A considerable bathochromic effect due to the introduction
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FIGURE 4 1H NMR spectrum of P(BDT-V-BTD) in 1,1,2,2-tetrachloroethane-d2 (6.0 ppm from TMS) (number of protons refer to the
repeat unit).
radation temperature above 300 C and higher than that of
the reference polymers (Table 1). The thermal stability is
therefore satisfactory for use in OPVs and other optoelectronic devices. The DSC scan revealed no obvious thermal
transitions in the temperature range from 25 to 400 C.
of the new donor and acceptor fragments has been observed,
which in turn yielded smaller optical bandgap energies
(from 1.9–2.0 to 1.7 eV), in agreement with that observed
for P(BDT-BTD).
When measured as thin film, the absorption spectrum of
P(BDT-V-BTD) is broadened in the low-energy portion, likely
due to aggregated networks from p-p interactions in the
solid state arising also from the vinylene-induced increased
planarity. This results in lower absorption onsets and optical
bandgaps (1.5 eV), again as similarly found for P(BDT-BTD).
A thermogravimetric (TGA) and differential scanning calorimetry (DSC) analysis was carried out in order to determine
the thermal stability and transitions of the new polymer. The
polymer exhibited a reasonable thermal stability with a deg-
Electrochemical Characterization
The polymer powder was dissolved (0.5 mM) in an electrolyte solution of 0.1 M tetrabutylammonium perchlorate
(TBAClO4) in CH2Cl2. Cyclic voltammetries (CVs) performed
in the whole potential range between 0.7 and 1.8 V
showed several redox processes (Fig. 6). Oligomer oxidation
and reduction processes lied at potential above 0.2 and
below 1.5 V, respectively, and the corresponding peak positions and currents are stable upon cycling. Before both
TABLE 1 Structural and Optical Properties of P(BDT-V-BTD) in CHCl3 Solution and as a Thin Film and Comparison with Reference
Polymers
CHCl3solution
kmax
(nm)
konset
(nm)
Egapopt
(eV)f
Film
kmax
(nm)
konset
(nm)
Egapopt
(eV)f
Polymer
Mw
(kg mol1)a
P(BDT-V-BTD)
12.3
7.2
1.7
560
730
1.7
561
850
1.5
–
330
5.2
5.0
1.0
353
650
1.9
364
760
1.6
110
240
2.3
1.0
415
615
2.0
424
770
1.6
–
265
1.7
591i
730i
1.7i
595
850
1.5
–
270
P(2,6-Py-V-EDOT)g
P(2,5-Py-V-EDOT)g
P(BDT-BTD)h
Mn
(kg mol1)b
2.3
31
18
PDIc
a
e
b
f
Weight-average molecular weight obtained by GPC.
Number-average molar mass obtained by GPC.
c
Polydispersity index (PDI ¼ Mw/Mn).
d
Optical bandgap (calculated on the low energetic edge of the absorption spectrum).
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Tg
( C)d
Td
( C)e
Glass transition temperature determined by DSC.
Decomposition temperature determined by TGA (5% weight loss).
g
From ref. 22.
h
From ref. 16(a).
i
Data in THF.
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FIGURE 5 Absorption spectra of P(BDT-V-BTD) measured in
CHCl3 solution (solid line) and as thin film (dashed line).
FIGURE 7 CVs of P(BDT-V-BTD) in 0.1 M TBAPF6 CH2Cl2 solution at 20 mV/s performed in different potential ranges.
processes, sharp charge-trapping-like peaks were observed
the current and potential of which shifted cycle by cycle. The
reversible redox process around 1.0 V was observed only
after the first scan. The process increased upon cycling and
is likely due to the redox system of coupled chains which
grow at high oxidative potentials (>0.5 V).
To better understand the electrochemical behavior we separately explored the two potential regions wherein oligomers
oxidation and reduction take place (Fig. 7). Upon doing this
the charge trapping peaks were no longer visible and the
current relative to the intermediate potential redox process
accordingly did not increase. HOMO-LUMO energy values
were determined by Differential Pulsed Voltammetry (Fig. 8).
Since both peaks showed irreversible features compared to
the peak of the internal standard (ferrocene), in this specific
case energy values were estimated by the onsets of the rising currents. The onsets were calculated by taking the intercepts between peak tangent and current baseline. The
FIGURE 6 Full range CV of P(BDT-V-BTD) in 0.1 M TBAPF6
CH2Cl2 solution at 20 mV/s.
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results are collected in Table 2 along with those of the reference polymers.
As expected, the stronger acceptor character of the BTD moiety decreased both the HOMO and LUMO energies compared
to previous vinylene-linked polymers, with the polymer
being harder to oxidize and easier to reduce than the pyridine derivatives. In contrast, the different energetic values
with respect to P(BDT-BTD) are more difficult to explain,
being the acceptor and donor components identical. Indeed,
we found that if the data for P(BDT-BTD)16(a) are recalibrated (the pseudo-reference Ag electrode recalibrated vs.
ferrocene) the oxidation/reduction onsets lie in a potential
region very similar to the vinylene-linked polymer. Thus, the
recalculated HOMO/LUMO levels of the P(BDT-BTD) were
actually in excellent agreement with those of P(BDT-V-BTD).
DFT/TDDFT Calculations
To gain insight into the structural, electronic, and optical
properties of the investigated systems, we performed DFT/
FIGURE 8 DPVs of P(BDT-V-BTD) in 0.1 M TBAPF6 CH2Cl2 solution at 10 mV/s.
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TABLE 2 Electrochemical Properties of P(BDT-V-BTD) in 0.1 M TBAPF6 CH2Cl2 Solution and Comparison with Reference Polymersa
Eonsetox (V)
Polymer
Eonsetred (V)
HOMO (eV)
LUMO (eV)
EgapEC (eV)b
0.30
5.5
1.50
3.7
1.8
P(2,6-Py-V-EDOT)c
0.23
5.0
1.80
3.4
1.6
c
0.11
5.1
1.78
3.4
1.7
P(BDT-V-BTD)
P(2,5-Py-V-EDOT)
5.1(5.6)e
P(BDT-BTD)d
a
þ
3.2 (3.7)e
1.9
All potentials are reported vs. Fc/Fc and HOMO and LUMO energies
are derived from the electrochemical data based on the assumption
that the Fc/Fcþ redox couple is 5.2 eV relative to vacuum.
b
Electrochemical bandgap, obtained from the difference between the
reduction and the oxidation potential onset (or LUMO and HOMO
energies).
c
TDDFT calculations, including solvation effects, on the monomers BDT-V-BTD and BDT-BTD and on their selected oligomers (BDT-V-BTD)n and (BDT-BTD)n, with the aim to simulate
the properties of the polymeric species. To reduce the computational overhead we replaced the n-octyl and n-dodecyl
chains of the monomers BDT-V-BTD and BDT-BTD with a
methyl (ACH3) substituent, thus looking at the P(Me-BDT-VBTD) and P(Me-BDT-BTD) models. For the characterization
of the optical and electrochemical properties of this type of
polymers we followed the same approach developed in our
previous work.22 In particular, the theoretical study of a
polymeric systems can be performed via two different
approaches: (i) by considering the infinite polymer as a periodic system, and (ii) by analyzing extended systems with
increasing but finite dimension. Here, we chose the second
approach, where the polymer is studied with a growing up
approach starting from the principal monomeric unit. Such
‘‘cluster’’ approach allows us to exploit all the computational
machinery developed for isolated systems,30 including the
calculation of excited states using TDDFT and solvation
effects in a simple yet effective way, by means of PCM.25(a)
Geometry optimization in vacuum of the monomers was performed by the B3LYP functional using both 3-21G* and 631G* basis sets, finding minimal differences between the two
data sets. Thus, for the larger system, we used the 3-21G*
basis set, allowing the description of the extended oligomeric
systems, whereby a large number of atoms, and thus a large
associated computational overhead, needs to be considered.
The optimized geometries of representative systems are
shown in Figure 9. The optimized structures are planar in all
From ref. 21.
From ref. 16(a).
e
HOMO and LUMO recalculated from ref. 16(a) by considering that the
Fc/Fcþ redox couple is 5.2 eV relative to vacuum.
d
FIGURE 9 Optimized geometries of (Me-BDT-V-BTD)n and (Me-BDT-BTD)n monomers (n ¼ 1) and oligomers (n ¼ 3, 5).
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TABLE 3 Calculated HOMO,a LUMO,a and Excitationb Energies of (Me-BDT-V-BTD)n and (Me-BDT-BTD)n
(Me-BDT-BTD)n
(Me-BDT-V-BTD)n
N
Exc. En (eV)
HOMO (eV)
LUMO (eV)
Egap (eV)
Exc. En (eV)
HOMO (eV)
LUMO (eV)
Egap (eV)
1
2.56 (2.63)
5.21 (5.13)
2.69 (2.68)
2.52 (2.45)
2.91 (2.88)
5.39 (5.27)
2.63 (2.62)
2.76 (2.65)
2
2.18 (2.21)
4.98 (4.90)
2.87 (2.85)
2.11 (2.05)
2.36 (2.35)
5.21 (5.09)
2.90 (2.86)
2.31 (2.23)
3
2.07 (2.07)
4.91 (4.83)
2.94 (2.92)
1.97 (1.91)
2.21 (2.18)
5.13 (5.02)
3.00 (2.97)
2.13 (2.05)
4
2.01 (2.00)
4.79 (4.79)
2.95 (2.95)
1.84 (1.84)
2.14 (2.10)
5.10 (4.98)
3.05 (3.02)
2.05 (1.96)
5
1.99 (1.97)
4.86 (4.78)
2.99 (2.96)
1.87 (1.82)
2.11 (2.06)
5.08 (4.96)
3.07 (3.04)
2.01 (1.92)
Fit
1.80 (1.83)
4.73 (4.68)
3.06 (3.03)
1.67 (1.65)
1.88 (1.84)
5.00 (4.89)
3.18 (3.14)
1.82 (1.75)
a
TDDFT excitation energies calculated on the B3LYP/3-21G* optimized
geometries with the MPW1K functional.
b
cases, apart from the methoxy groups which lie outside the
plane.
ative measured electrochemical potentials and absolute values referred to the vacuum. In this work we thus evaluated
the oxidation and reduction potentials by taking the negative
of the HOMO and LUMO single particle eigenvalues. We then
adopted the selected hybrid level of theory to extended our
investigations to the whole oligomer series, from the monomer (n ¼ 1) to the pentamer (n ¼ 5) (Table 3). Using a linear fit31 and plotting the HOMO, LUMO and excitation energies of the oligomers against 1/n we can extrapolate the
values of the infinite polymer as the intercept with ordinate
axes (Fig. 10 and Table 3).
We performed TDDFT excited state calculations (Table 3) on
the optimized structures of the monomers and oligomers up
to five monomeric units (n ¼ 5). In our previous work we
found that B3LYP provides accurate ground state geometries,27 whereas the MPW1K functional,29 which has an
increased percentage of Hartree–Fock exchange, provides excitation energies in much closer agreement with experimental values.27
Both approaches have their merits, with Koopmans theorem
offering a simple but approximate computational procedure,
requiring only a calculation on the neutral species. The calculation of Gibbs free energies, on the other hand, is accurate but computationally very demanding, requiring calculation of geometries and vibrational frequencies in vacuo and
geometries in solution, for the neutral, cation, and anion species. In our previous work we have tested both approaches
for the P(2,6-Py-V-EDOT) and P(2,5-Py-V-EDOT) monomers
and trimers,22 the latter being the largest systems for which
calculation of Gibbs free energies was possible within a reasonable computer overhead.
As a matter of fact, we found both approaches to be
adequate to describe the investigated systems, especially
considering the somewhat uncertain conversion between rel-
HOMO and LUMO energies in THF (in vacuo in parentheses) calculated
at the B3LYP/6-31G* level on the B3LYP/3-21G* optimized geometries.
The calculated extrapolated values of the absorption energies
in solution (Table 3, fit n ¼ 1–5) reproduced the experimental absorption onsets, located at about 1.7 eV for both systems. The computed B3LYP/6-31G* HOMO and LUMO levels
of (Me-BDT-V-BTD)n and (Me-BDT-BTD)n are listed in Table 3
for the oligomers series (n ¼ 1–5) (see Fig. 11 for isodensity
contour plots).
We notice that the calculated single particle HOMO and
LUMO values are in excellent agreement with the experimental estimates (Table 3). In particular for P(Me-BDT-V-BTD)
the calculated HOMO and LUMO energies are 4.7 and 3.1
eV, respectively, reproducing the experimental optical gap
(1.7 eV) and the electrochemical measurements. For the
P(Me-BDT-BTD) polymer, the calculated HOMO and LUMO
FIGURE 10 Calculated trends of lowest excitation energies (left), and HOMO and LUMO values (right) for P(Me-BDT-V-BTD)
(squares) and P(Me-BDT-BTD) (diamonds).
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devices are almost independent on the blend composition
and on the acceptor (PC61BM or PC71BM). The mean Voc
value of 0.68 V well matches with the Voc reported for the
solar cell based on the analogous single bonded P(BDT-BTD)
donor material (see Table 4), in agreement with the similar
molecular structure and the HOMO energy levels of the two
polymers (Tables 2 and 3). On the other hand, within all
P(BDT-V-BTD) based solar cells, the measured FF and JSC did
not exceed 45% and 2.69 mA cm2, respectively. This could
be due to unfavorable thin-film electrical and/or morphological characteristics hampering the PCEs of the devices, despite the optimization of the polymer optoelectronic
properties.
FIGURE 11 Isodensity plots of HOMOs and LUMOs for P(MeBDT-V-BTD) and P(Me-BDT-BTD) monomers and trimers.
energies are 5.0 and 3.2 eV, respectively, showing a calculated HOMO-LUMO difference of 1.8 eV in excellent agreement with the optical and electrochemical data shown in
Tables 1 and 2, respectively.
Photovoltaic Properties
The use of P(BDT-V-BTD) in combination with fullerene
derivatives (PC61BM and PC71BM) as donor and acceptor
materials in BHJ OPV devices was investigated. The standard
OPV configuration glass/ITO/PEDOT:PSS/donor:acceptor/
LiF/Al was used. Devices fabrication details are reported in
the ‘‘Experimental’’ section. Figure 12 shows the characteristic current density-voltage (J-V) plots of the most representative solar cells with different donor:acceptor weight ratios
and different acceptors (PC61BM or PC71BM), measured
under standard AM 1.5G illumination.
The best performing device yielded a PCE of 0.74% with
current density (JSC), open circuit voltage (Voc), and fill factor
(FF) of 2.69 mA cm2, 0.68 V, and 41%, respectively (Table
4). The Voc values measured for the P(BDT-V-BTD) based
Many efforts to improve the device performances were carried out by following a variety of strategies. In general, the
variation of the donor:acceptor blend ratios is well known to
affect significantly the photovoltaic parameters. By increasing
the polymer amount in the blend films, going from 1:1 to
1.2:1 P(BDT-V-BTD):PC61BM weight ratio, corresponding to
blend films with an excess of polymer, we observed a slight
decrease of the overall OPV performance (PCE going from
0.58 to 0.52, mainly due to decreased Jsc and FF) (Table 4).
On the opposite, by increasing the PC61BM content, from 1:1
to 1:1.5 D:A weight ratio, a raise of the JSC (from 2.34 to
2.69 mA cm2) and FF (from 35 to 41%) were obtained,
leading to a PCE of 0.74%. This result could be likely
ascribed to enhanced charge separation and transport processes in the active blend due to a more favorable self-organization of the interpenetrating donor and acceptor phases. A
further increase of the PC61BM content led to reduced JSC,
and consequently PCEs (Table 4), confirming the best
donor:acceptor blend weight ratio of 1:1.5.
It is well known that thermal and solvent annealing of the
active layer or of complete BHJ OPV devices could lead to
improved blend film nano-morphology, electrical properties
and device performances.32 We found that thin film thermal
annealing in the range 60–140 C, and for different times,
resulted in film degradation, adversely affecting photovoltaic
parameters. Moreover, different solvents (e.g., toluene, chlorobenzene, and ortho-dichlorobenzene) were used to spin-
TABLE 4 Photovoltaic Parameters of Optimized P(BDT-V-BTD):
PCBM-Based Devices, Compared to Reference Polymer P(BDTBTD) Solar Cell
FIGURE 12 J-V curves, under illumination, of optimized P(BDTV-BTD): PCBM solar cells.
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Materials
D:A
Ratio
(wt/wt)
Jsc
(mA/cm2)
Voc
(V)
FF
(%)
PCE
(%)
P(BDT-V-BTD): PC61BM
1.2:1
2.20
0.70
34
0.52
P(BDT-V-BTD): PC61BM
1:1
2.34
0.70
35
0.58
P(BDT-V-BTD): PC61BM
1:1.5
2.69
0.68
41
0.74
P(BDT-V-BTD): PC71BM
1:1.5
2.05
0.71
45
0.66
P(BDT-V-BTD): PC61BM
1:1.8
2.24
0.68
38
0.58
P(BDT-BTD): PC61BMa
1:1
2.97
0.68
44
0.90
a
Values taken from ref. 16(a).
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cast the blends, but poor quality films were obtained as a
consequence of the limited polymer solubility. The most
macroscopically homogeneous and electrically uniform films
were here obtained from chloroform solutions, resulting in
the best OPV performances.
In order to increase the device photocurrent generation,
strategies to improve the blend film light collection ability
were carried out. The photocurrent generation, together with
other parameters such as hole mobility, optimum annealing
temperature and ideal morphology, is also determined by the
amount of photons absorbed by the active layer, which in
turns depends on the onset wavelength of the polymer
(Eopt
gap ) and on the blend film absorbance.
A high polymer absorption molar coefficient (e) is obtained
in solution (emax ¼ 3.5 105 mol1 cm1 considering Mw ¼
12.3 kg mol1), which is in the same order of magnitude of
other well performing OPV polymers.33 Therefore, in addition to the intrinsically high polymer molar absorption, the
active layer film thickness was increased with the aim to
enhance the overall blend film absorbance. Indeed, in P(BDTV-BTD):PC61BM based devices, increasing the film thickness
up to 50 nm led to an improvement in photocurrent generation and overall efficiency. However, a further increase in the
active layer thickness, although leading to enhanced film absorbance, did not afford improved JSC and PCE. Optimized
50-nm thick active layer films showed absorbance values
lower than 0.3 over the visible spectral window (see Supporting Information Fig. 1). Therefore, it is likely that such
low film absorption, limited by thickness (50 nm), besides
other morphological and electrical device features, is an important factor limiting the photocurrent response of P(BDTV-BTD) based devices. On the other hand, since the active
layers in optimized devices still have >50% transparency in
the visible region, their potential use as photoactive materials in semitransparent cells for tandem or stackable devices
could be considered in future works on P(BDT-V-BTD) based
devices.34
An additional strategy to improve the photocurrent response
of P(BDT-V-BTD) based solar cells is based on the replacement of PC61BM with PC71BM as acceptor material in the
active layer, in order to exploit the superior light harvesting
properties of the latter in the visible region.35 Although an
enhancement in the absorption spectrum of the P(BDT-VBTD):PC71BM blend film was recorded in comparison with
the analogous PC61BM based film (Supporting Information
Fig. 1), no significant improvements of PCE values were
observed (Table 4). This result might be attributed to the
possible PC71BM aggregation tendency and its poor miscibility with the polymer, resulting in lower thin-film quality
with limited nanoscale morphology, phase separation and,
thereby, inferior electrical properties compared to PC61BM
based devices.36
Hole mobility (lh) of P(BDT-V-BTD) based films was measured in order to investigate the polymer charge carrier
transport properties which, together with other parameters
such as optical absorption, morphology and energetic levels
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alignment, affect the performances of a donor material in
BHJ OPV devices. We measured lh of the polymer in pristine
and blended films (with PC61BM) in organic field effect transistor (OFET) devices.37 The transfer plots of a top-contact
OFET prepared with optimized P(BDT-V-BTD) film spun on
Si/SiO2/HDMS showed, as expected, a p-type behavior with
lh ¼ 6.3 105 cm2 V1 s1, in agreement with other low
band-gap polymers showing similar OFET hole mobilities
and comparable or better photovoltaic performances compared to P(BDT-V-BTD).22,38 Moreover, OFET devices prepared with optimized P(BDT-V-BTD):PC61BM film exhibited a
lh value of 4.6 105 cm2 V1 s1, very similar to that
measured for the polymer alone, indicating that the presence
of PC61BM does not significantly affect the morphology and
the continuity of the polymer phase over the film surface.
Polymer carrier mobility is strictly correlated with molecular
weight and nano-morphology. In particular, recent works on
P3HT clearly confirmed the relationship between polymer
charge carrier mobility, OPV performance, and molecular
weight.39 In fact it was demonstrated that P3HT OFET mobility is controlled by the probability of carriers to cross the
low-conductive disordered regions lying within the highly conductive crystalline domains. This probability is higher for high
molecular weight P3HT samples where molecular connections
bridge the crystalline domains, while it is lower in low molecular weight P3HT films where the intercrystalline molecular
connections are missing. Therefore, it is possible that the relatively low molecular weight of P(BDT-V-BTD), which affects
the polymer electrical and morphological characteristics, could
represent one of the critical factor that limits the photovoltaic
performance of the corresponding BHJ solar cells.
To get an insight into the micro- and nano-structural organization of the polymer in the active layer, we investigated by
tapping-mode AFM the morphological features of the P(BDTV-BTD):PC61BM and P(BDT-V-BTD):PC71BM blend films
affording the best OPV performances (see Supporting
Information Fig. 2). The measurements revealed surface
roughness (Root Mean Square, rms) of 1.3 and 1.5 nm
for the P(BDT-V-BTD):PC61BM and the P(BDT-V-BTD):PC71BM
films, respectively, suggesting a smooth and homogeneous
surface of the blends. The two thin films have similar morphologies but different domain size, which depends on the
nature of the acceptor in the blend. P(BDT-V-BTD):PC61BM
film exhibits smaller features with respect to the P(BDT-VBTD):PC71BM film, likely due to the lower aggregation tendency of PC61BM compared to PC71BM.36 This might also justify the better performance of the PC61BM based devices.
To analyze the spectral response of the best performing solar
cell, External Quantum Efficiency (EQE) of P(BDT-V-BTD):
PC61BM 1:1.5 (wt/wt) based device was measured. The EQE
spectrum (Fig. 13) shows the highest generated-electrons
over incident-photons ratio, comprised within 0.15 and 0.17,
in the spectral range between 450 and 650 nm, which corresponds to the lower-energy absorption band of the polymer.
This indicates that photocurrent mainly arises from the
absorption of P(BDT-V-BTD) in the photoactive layer. The
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FIGURE 13 EQE (white spots) and IQE (black squares) spectra
of optimized P(BDT-V-BTD): PC61BM, 1:1.5 (w/w) devices. The
absorption spectrum of the active layer (black line) is included
for comparison.
short-circuit current density of the device inferred from convoluting the EQE over the entire wavelength range (350–750
nm) matches well, within the experimental error, with the
JSC value obtained in the J-V curve measurement (2.53 and
2.69 mA cm2, respectively). An estimation of the Internal
Quantum Efficiency (IQE) of the (BDT-V-BTD):PC61BM 1:1.5
(w/w) device is also given in Figure 13. While IQE values inferior to unity over the entire spectral range reveal limited
photocurrent generation capability of the device,40 the fraction of light absorbed by the active layer (A%, calculated
from Absorbance, see ‘‘Experimental’’ section) results as high
as 0.8 in the range 450–650 nm, which is consistent with
the similarity of the IQE and EQE curves. Therefore, the
modest photocurrent generation capability in the optimized
P(BDT-V-BTD):PC61BM devices has rather to be attributed to
limited polymer charge mobility than to poor active layer
light collection efficiency.
CONCLUSIONS
A new heteroarylene-vinylene donor–acceptor low bandgap
polymer is presented. The design has been based on the unprecedented combination of strong donor (BDT) and
acceptor (BTD) alternating units together with a vinylene
spacer in order to exploit increased planarization along the
p backbone and efficient p delocalization with respect to the
most common approach in the literature presenting direct
aryl–aryl connection. The Stille metal-catalyzed cross-coupling reaction afforded polymeric species with larger molecular weights compared to previously reported heteroarylenevinylene donor–acceptor polymers,22,23 though still nonoptimal for best photovoltaic devices.
The electrochemical investigation has highlighted the ambipolar character of the new polymer, with both p- and n-doping processes being observable. In agreement with the introduction of a strong acceptor fragment in the polymeric
backbone, HOMO and LUMO energies were significantly
down shifted by 0.3–0.5 V with respect to their counterparts
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containing a pyridine ring as acceptor.22 This result is important in view of the expected higher photovoltage of OPV
cells, being Voc proportional to the difference LUMOPCBM –
HOMOpolymer.8 On the other hand, the presence of the vinylene spacer allowed for lower bandgap energies, in excellent
agreement with the computational prediction, and improved
thermal stability compared to the corresponding species
P(BDT-BTD) with direct bonding between the two heteroaromatic units. In conclusion, the experimental and computational studies have ascertained structural, photophysical, and
electronic properties matching optimal materials design rules
for efficient donor polymers in OPV devices. Indeed, investigation of the photovoltaic properties in devices containing
PCBM as the acceptor has shown higher photovoltages and
photocurrents compared to those of the previous heteroarylene-vinylene polymers. The still nonoptimal thin-film nanomorphologies and hole mobility properties, likely due to the
relatively low molecular weight, could account for the modest PCE lower than 1%, similar to those obtained for P(BDTBTD). Efforts are under way to efficiently convert the
encouraging properties of these heteroarylene-vinylene donor–acceptor polymers into more efficient OPV devices.
ACKNOWLEDGMENTS
The authors thank Fondazione Cariplo (Grant 2007-5085) for financial support. They are grateful to Alberto Bianchi for kind assistance in GPC measurements. Financial support from EU
through projects FP7-ICT-248052 (PHOTO-FET) and FP7-ICT287594 (SUNFLOWER), from Consorzio MISTE-R through project
FESR-Tecnopolo AMBIMAT, and from CNR through project EFOR
is acknowledged. F.D.A. and E.M. thank Fondazione Istituto Italiano di Tecnologia, Project SEED 2009 ‘‘HELYOS’’ for financial support. They also thank G. Ruani for useful discussions.
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