Polycyclic aromatics with flanking thiophenes: Tuning energy level and band gap of conjugated polymers for bulk heterojunction photovoltaics

Article abstract of DOI:10.1021/ma902164q

In the pursuit of donor-acceptor low band gap polymers for photovoltaic applications, finding an optimal donor monomer which maximizes the photovoltaic efficiency is a complex synthetic optimization problem. We synthesized three different bithiophenes flanking a center aromatic ring (pyrrole, benzene, or pyridine) as the donor monomers (DTPr, DTBn, and DTPn) with decreasing electron-donating ability. An array of six electrochemically and optically unique polymers were prepared by copolymerizing these three monomers with 2,1,3-benzothiadiazole and with thiophene. The optical, electronic, and photovoltaic properties of these polymers were investigated. Among this series, we found that the HOMO energy level of the polymer is dominated by the most electron-rich ring in the polymer backbone. The optical band gap, conversely, involves the entire polymeric system. Among the three donor monomers investigated, we identified that DTBn-based polymers exhibited the most potential in photovoltaic applications due to their moderately low band gaps and low HOMO energy levels.

Full text of DOI:10.1021/ma902164q

Macromolecules 2010, 43, 797–804 797
DOI: 10.1021/ma902164q
Polycyclic Aromatics with Flanking Thiophenes: Tuning Energy Level and
Band Gap of Conjugated Polymers for Bulk Heterojunction Photovoltaics
Samuel C. Price, Andrew C. Stuart, and Wei You*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
27599-3290
Received September 30, 2009; Revised Manuscript Received November 30, 2009
ABSTRACT: In the pursuit of donor-acceptor low band gap polymers for photovoltaic applications,
finding an optimal donor monomer which maximizes the photovoltaic efficiency is a complex synthetic
optimization problem. We synthesized three different bithiophenes flanking a center aromatic ring (pyrrole,
benzene, or pyridine) as the donor monomers (DTPr, DTBn, and DTPn) with decreasing electron-donating
ability. An array of six electrochemically and optically unique polymers were prepared by copolymerizing
these three monomers with 2,1,3-benzothiadiazole and with thiophene. The optical, electronic, and photo-
voltaic properties of these polymers were investigated. Among this series, we found that the HOMO energy
level of the polymer is dominated by the most electron-rich ring in the polymer backbone. The optical band
gap, conversely, involves the entire polymeric system. Among the three donor monomers investigated, we
identified that DTBn-based polymers exhibited the most potential in photovoltaic applications due to their
moderately low band gaps and low HOMO energy levels.
Introduction
template, since fused thiophenes have produced several high
mobility semicrystalline polymers, and the flanking thiophenes
As a cost-effective alternative to current mainstream silicon solar
cells, polymeric solar cells, in particular, bulk heterojunction (BHJ)
solar cells based on fullerene and its derivatives, have attracted
extensive research attention and witnessed significant progress in
recent years.1-4 The record high efficiency has continually ad-
vanced to its current value of more than 6%5,6 due to synergetic
advancements in device optimization, the understanding of organic
semiconductor physics, and new material development.
would reduce steric hindrance and create smaller dihedral angles
with adjacent monomers. Then pyrrole, benzene, and pyridine
were inserted into the bithiophene template to create three donor
monomers of decreasing electron-donating ability. These three
monomers were then copolymerized with 2,1,3-benzothiadiazole
and with thiophene in order to study the optical, electronic, and
photovoltaic properties of these polymers and to identify the
most promising monomer for future study.
In PC61BM-based BHJ solar cells, the theoretical maximum Voc
of a device is determined by highest occupied molecular orbital
(HOMO) energy level of the conjugated polymer, and the theore-
tical maximum Jsc of a device is largely dependent on the band gap
of this conjugated polymer.1 The most successful low band gap
polymer design strategy to date for photovoltaic devices is the
donor-acceptor alternating copolymer (or internal charge trans-
fer copolymer) strategy, which has produced several polymers with
photovoltaic power conversion efficiencies larger than 4.0%.5-14
The HOMO level of these donor-acceptor copolymers is deter-
mined almost exclusively by the donor monomer.15,16 In order
to decrease the HOMO level (and raise the Voc of the result-
ing photovoltaic cell), donors with “weaker” electron-donating
ability should be applied to this strategy. However, while “weaker”
donors will improve the Voc by lowering the HOMO, they will also
widen the band gap since the interaction between the electron-rich
donor and the electron-poor acceptor is what lowers the band gap
of these copolymers. Additionally, polymers with HOMO levels
below -5.9 eV exhibit higher rates of geminate charge recombina-
tion.17 Thus, finding an optimal donor monomer which maximizes
the photovoltaic efficiency is a complex synthetic optimization
problem.
Results and Discussion
Monomer and Polymer Synthesis. Two structural units,
dithieno[3,2-b:20,30-d ]pyrrole (DTPr)18,19 and benzo[1,2-
b:4,5-b0]dithiophene (DTBn),20 were synthesized according
to literature procedures. Long branched alkyl chains were
attached in the center pyrrole or benzene unit to ensure the
solubility of resulting polymers. Both units were readily
converted into distannylated monomers to copolymerize
with dibrominated comonomers via Stille coupling polym-
erization, offering corresponding polymers (Scheme 1). The
third structural unit, dithieno[3,2-b:20,30-e]pyridine (DTPn),
has also been reported in the literature.21 However, in our
efforts to synthesize dithienopyridines with long solubilizing
alkyl chains, we encountered a great deal of difficulty with
the established synthetic route.21 The literature procedure
employed a bromination (Scheme 1A) in the final steps of
the synthesis, which offered the target molecules with very
low yield when long alkyl chains were involved (6% when
R = n-dodecyl).22 With our particular substrates which
possessed more bulky alkyl chains at the 8-position of the
DTPn, the reported bromination procedure resulted in either
inseparable mixtures or exceptionally low yields.
To systematically discover this optimal donor monomer, a
design strategy based upon fusing aromatic rings of different
oxidation potentials into bithiophene was envisioned. First,
bithiophenes flanking a center aromatic ring was chosen as a
The distannyl intermediate of the literature bromination
procedure could also serve as a polymerizable monomer for
Stille coupling polymerization; however, metalation of 1
with n-BuLi according to the literature procedure resulted
in a mixture of monostannyl and distannyl compounds
*To whom all correspondence should be addressed. E-mail: wyou@
email.unc.edu.
r 2009 American Chemical Society
Published on Web 12/31/2009
pubs.acs.org/Macromolecules

798 Macromolecules, Vol. 43, No. 2, 2010
Price et al.
Figure 1. The library of structurally related polymers. “DT” stands for dithiophene. “Pr”, “Bn”, and “Pn” stand for fused pyrrole, benzene, and
pyridine, respectively. “T” stands for thiophene and “BT” for benzothiadiazole.
Scheme 1. (A) Literature Procedure To Synthesize the Dibrominated DTPn; (B) Deuterium Labeling Experiment; and (C) Modified Synthetic
Procedure of Dibrominated DTPn

Article
Macromolecules, Vol. 43, No. 2, 2010 799
which could not be separated. In an attempt to optimize the
stannylation procedure, 1 was lithiated with n-BuLi and then
quenched with deuterated methanol (Scheme 1B). This
deuterium labeling experiment reveals that even after 5 h,
the lithiation was not complete, and some starting material
remained according to NMR (Figure 2). Attempts to opti-
mize the metalation procedure using tert-BuLi resulted in
complete deprotonation with no traces of starting material;
however, the deprotonation was not selective, resulting in a
57:43 mixture of R:β deprotonation. Therefore, a new syn-
thetic route to prepare polymerizable 8-alkyldithienopyri-
dines was required.
Because ofthese deficiencies withthe literaturepreparation,
a modified synthetic scheme (Scheme 1C) that introduced a
bromine in the R-position of the thiophene before condensa-
tion to form the pyridine ring was envisioned. The bromine
was introduced in the first step by bromination of the com-
mercially available thiophene-3-carboxylic acid.23 The acid
(2) was then subjected to a Curtius rearrangement to produce
the Boc-protected amine (3) in good yield. The Boc protecting
group stabilized the easily oxidized amine and was readily
deprotected in situ in the subsequent acid-catalyzed condensa-
tion step. The aldehyde (4) was chosen to ensure the good
solubility of the resulting polymers. The TFA-catalyzed con-
densation then afforded the final monomer (5) as a solid,
which would aid in preparing high molecular weight polymers
from typical condensation polymerizations where rigorous
control of stoichiometry is required. This monomer (5) was
then copolymerized with 2,5-bis(trimethyltin)thiophene or
2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol ester) to
offer the desired polymers, PDTPn-T and PDTPn-BT.
Figure 2. NMR results of lithiation study with deuterium labeling: (A)
starting material, (B) product of n-BuLi lithiation and quench with
CD3OD, and (C) product of tert-BuLi lithiation and quench with CD3OD.
Scheme 2. Synthesis of Polymers via Palladium-Catalyzed Coupling Reactionsa
a Shown on the right are the representative solutions of each polymer.

800 Macromolecules, Vol. 43, No. 2, 2010
Price et al.
The crude copolymers were washed extensively with
methanol, followed by the Soxhlet extraction with methanol,
and ethyl acetate successively to remove byproducts and
oligomers. Finally, the polymers were Soxhlet extracted with
hexanes or chloroform and recollected by precipitation into
into methanol and dried under vacuum. The molecular
observed. Since the newly substituted benzene or pyridine
ring is no longer the most electron-rich ring in the system, the
HOMO is relatively independent of which ring is substituted
for the pyrrole ring. For example, nearly the same HOMO
energy levels (- 5.55 eV vs -5.56 eV) are measured for
PDTBn-BT and PDTPn-BT. This is because the HOMO
behavior is dominated by the two flanking thiophene rings.
The same trend is apparent in the case of PDTBn-T and
PDTPn-T, as the HOMO behavior is dominated by the
copolymerized thiophene ring and causes the HOMO posi-
tion to be roughly equal for both polymers despite their
structural differences.
Despite the HOMO energy level depending greatly on the
most electron-rich ring in the aromatic system, the optical
band gap involves the entire polymeric system. Even though
the HOMO energy level of the DTPn polymer series is little
changed from the DTBn series, the band gaps of the DTPn
series are significantly wider than these of the DTBn series.
For example, the band gap of PDTPn-BT (2.1 eV) is noticeably
larger than that of PDTBn-BT (1.8 eV) due to the electron-
deficient pyridine, which diminishes the donor-acceptor inter-
action in the copolymer that gives the low band gap.
On the basis of electrochemical and optical considerations
alone, DTBn is the most promising candidate for use in
photovoltaic devices. The low measured HOMO energy
levels predict high values of the Voc, and the measured optical
band gap remains low. Conversely, these results exhibit the
DTPn and DTPr units less favorably as possible successful
candidates for use in photovoltaic devices. The band gaps of
polymers made from the DTPn donor unit are significantly
wider than the others in this study, while the HOMO levels
are almost identical to the HOMO levels of the DTBn series.
While polymers based upon DTPr should have large theore-
tical maximum currents due to their lower band gaps, the
HOMO levels of this series of polymers remain too high to
yield devices with high efficiencies.
Photovoltaic Properties. These structurally similar, yet
optically and electrochemically very different, polymers were
tested in bulk heterojunction photovoltaic devices. Standard
BHJ device configuration was used (ITO/PEDOT/polymer:
PCBM/Ca/Al). A calibrated AM 1.5G light source (100
mW/cm2, 1 sun) was employed to simulate the irradiation
from the sun. The blending ratio of polymer vs PCBM and
the thickness of the active layer were varied to achieve the
best device performance under semioptimized conditions
(Table 3). Representative J-V curves of these polymers
under 1 sun condition are displayed in Figure 5. In addition,
the UV-vis and IPCE spectra of the PDTBn-BT/PCBM
blend are displayed together (Figure 6) in order to reveal
their correlation.
1
structures of both polymers were confirmed by H NMR
spectroscopy. The yields and molecular weights of polymers
are listed in Table 1. The low yield of PDTPr-T can be
attributed to its low solubility, as large quantities of solid
remained in the Soxhlet extraction thimble after chloroform
extraction. The opposite problem explains the low polymer
yield for PDTBn-BT, in that a large quantity of material was
collected in the hexane fraction of the polymerization.
However, this hexane fraction consisted of low molecular
weight polymer, and only the high molecular weight chloro-
form fraction was used, decreasing the polymerization yield
significantly. The molecular weights were determined by gel
permeation chromatography (GPC) in THF or 1,2,4-trichlor-
obenzene by referring to polystyrene standards (Table 1). The
molecular weights of DTPn-based polymers (PDTPn-T and
PDTPn-BT) are noticeably low, which can be attributed to
their lower solubility during the polymerization reaction.
Optical and Electrochemical Properties. This collection of
structurally similar yet energetically diverse polymers allows
for the interplay between the electron-donating ability of the
donor monomer and the optical and electronic properties to
be studied in detail. The optical and electrochemical proper-
ties of these polymers are shown in Figures 3 and 4, and
extracted data are summarized in Table 2.
Interestingly, the HOMO energy level of the entire con-
jugated polymer is dominated primarily by the most elec-
tron-rich aromatic unit in the polymer, regardless of what
other rings it is fused to. For example, nearly identical
HOMO levels of -4.89 and -4.94 eV were observed in
PDTPr-T and PDTPr-BT, respectively, because the most
easily oxidized ring in these two polymers is the pyrrole ring.
Therefore, the monomer DTPr is polymerized with (BT or T)
has little effect on the HOMO energy level, and this phenom-
ena is common with other polymer systems.24 However,
when the pyrrole ring is substituted for a benzene or pyridine
ring, a drastic reduction in the HOMO energy level is
Table 1. Polymerization Results for Polymers
yield [%]
Mn [kg/mol]
Mw [kg/mol]
PDI
PDTPr-T
PDTPr-BT
PDTBn-T
PDTBn-BT
PDTPn-T
PDTPn-BT
13
63
93
8
90
76
12.2a
8.0b
50.3a
17.6b
109.6b
150.7b
10.7a
2.6a
4.1
2.20
2.02
2.15
2.47
1.68
54.1b
69.9b
4.3a
1.5a
The PDTBn series immediately exhibits superior photo-
voltaic properties when compared to the rest of the polymers
in this study. The theoretical predicted Voc of 0.95 and 0.75 V
for PDTBn-BT and PDTBn-T,25-27 respectively, are close to
a Determined by GPC in 1,2,4-trichlorobenzene using polystyrene
standard at 150 °C. b Determined by GPC in THF using polystyrene
standards.
Table 2. Optical and Electrochemical Data of All Polymers
UV-vis absorption
cyclic voltammetry
ox
red
CHCl3 solution
film
E
(V)
E
(V)
onest
onest
polymer
λmax [nm]
λonset [nm]
Ega [eV]
λmax [nm]
λonset [nm]
Ega [eV]
HOMO [eV]
LUMO [eV]
PDTPr-T
PDTPr-BT
PDTBn-T
PDTBn-BT
PDTPn-T
PDTPn-BT
575
697
537
648
465
508
659
837
566
688
535
580
1.9
1.5
2.2
1.8
2.3
2.1
609
727
543
650
465
540
663
875
584
694
534
579
1.9
1.4
2.1
1.8
2.3
2.1
0.09/-4.89
0.14/-4.94
0.55/-5.35
0.75/-5.55
0.63/-5.43
0.76/-5.56
-2.24/-2.56
-1.73/-3.07
-2.18/-2.62
-1.55/-3.25
-1.83/-2.97
-1.47/-3.33
a Calculated from the intersection of the tangent on the low energetic edge of the absorption spectrum with the baseline.

Article
Macromolecules, Vol. 43, No. 2, 2010 801
the experimentally determined values of 0.77 and 0.88 V. Not
surprisingly, the lower HOMO of the polymers (e.g.,
PDTBn-BT) yields relatively higher Voc values. The moderately
low band gaps in the DTBn series are sufficient to produce
satisfactory short-circuit currents. Additionally, the symmetri-
cal nature of the monomer combined with the alkyl
chain branching being located on a carbon that is not adjacent
to the polymer backbone should yield polymers films with
higher crystallinity. Thus, despite a wider band gap, the high
molecular weight and possible increased crystallinity of the
blended film cause the short-circuit current of the DTBn series
to be very close to that of the DTPr series of polymers.
As predicted, the low Voc of the polymers in the DTPr
series remains the limiting factor in their solar cell perfor-
mance, with both cells recording Voc values lower than 0.5 V.
PDTPr-BT’s especially low band gap does lead to a 32%
increase in the Jsc compared with that of PDTBn-BT; how-
ever, a significantly higher current must be obtained in
order to offset its low Voc (0.46 V) and fill factor (0.39).
While these parameters can be improved by increases in
molecular weight and further optimizing the polymer
structure (modifying the alkyl chain length, varying the
acceptor other than BT, etc.), the D-A polymers based on
DTPr are intrinsically limited by their high-lying HOMO
energy levels. Therefore, DTPr is not a particularly promising
candidate for constructing new polymers as high efficiency
photovoltaic materials. However, applications which require
especially low band gaps (for example, infrared detectors) are
still a possibility for this electron-rich donor monomer.
BHJ device.32 However, further optimization of the polymer
structure was not warranted given the poor energetic and
optical properties of this series of polymers.
The relatively low efficiencies of these polymer-based BHJ
devices are mainly due to their low current, though low band
gaps have been achieved in some of these polymers (e.g.,
PDTPr-BT and PDTBn-BT). In addition to a low band gap,
a high hole mobility, comparable to the electron mobility of
PCBM, is also required to achieve a high current. The hole
mobility of these polymers in the BHJ blends was calculated
via the space-charge limited current (SCLC) model by con-
structing hole-only devices with 40 nm of palladium (see
Experimental Section for more details).33 For comparison,
“polymer only” devices were also fabricated to probe the
mobility of polymers in the absence of PCBM.34 The mea-
sured hole mobilities of these polymers in the absence of
PCBM are generally much lower than the electron mobility
of PCBM (∼10-4 cm2/(V s)) (Table 4), which explains the
low efficiencies of these devices when compared with
P3HT and other high-performance polymers. Additionally,
the film morphology of these devices is not optimal. For
example, AFM images of PDTBn-T based devices (see
Supporting Information) reveal micrometer-sized phase
The DTPn series does not behave as expected in terms of
Voc. The experimentally determined values are significantly
lower than the Voc expected given the low HOMO levels of
the polymers in this series. The poor performance of PDTPn-
BT is likely due to its low molecular weight. 1.2 kg/mol
represents a degree of polymerization of 2.25, and the effects
of low molecular weight on all aspects of solar cell perfor-
mance are well documented.28-31 Additionally, the large
branched alkyl chain adjacent to the polymer backbone in
the DTPn (and DTPr) series could lead to amorphous films,
which would reduce the efficiency of charge separation in the
Figure 4. Cyclic voltammograms of all polymers.
Figure 3. (A) Solution absorption of all polymers in chloroform. (B) Film absorptions.
Table 3. PV Performances of Polymers
polymer
polymer:PCBM
thickness (nm)
Voc (V)
Jsc (mA/cm2)
FF
η (%)
IPCE (%)
Rs (Ω)
PDTPr-T
PDTPr-BT
PDTBn-T
PDTBn-BT
PDTPn-T
PDTPn-BT
1:1
1:3
1:2
1:1
1:3
1:1
50
100
190
155
50
0.29
0.46
0.88
0.77
0.41
0.31
1.89
4.0
2.17
3.02
1.93
0.75
0.32
0.39
0.54
0.41
0.34
0.37
0.18
0.72
1.02
0.94
0.27
0.09
0.58
33.1
8.62
16.1
10.1
5.16
117
78
147
80
48.7
80
45

802 Macromolecules, Vol. 43, No. 2, 2010
Price et al.
Two key structure/property trends have also emerged. The
behavior of the HOMO energy level is dominated by the most
electron-rich ring in the polymer backbone. Conversely, the
optical band gap is a function of the electronic properties of the
entire conjugated aromatic backbone. Electron-deficient pyridine
diminishes the donor-acceptor interaction in the copolymer that
gives the low band gap. This explains the 0.3 eV change in the
band gap between DTBn-BT and DTPn-BT and the 0.3 eV
change in band gap between DTBn-T and DTPr-T.
Lastly, the photovoltaic performance of DTBn polymer series
is largely inhibited by the low hole mobility of the two polymer
candidates. Future study will focus on improving the hole
mobility and crystallinity of these polymers in order to improve
their photovoltaic performance.
Experimental Section
General Methods. 1H nuclear magnetic resonance (NMR)
spectra were obtained at 400 or 300 MHz as solutions in CDCl3.
13C NMR spectra were obtained at 100 MHz as solutions in
CDCl3. Chemical shifts are reported in parts per million (ppm,
δ) and referenced from tetramethylsilane. Coupling constants
are reported in hertz (Hz). Spectral splitting patterns are desig-
nated as s, singlet; d, doublet; t, triplet; m, multiplet; and br,
broad. Melting points are uncorrected.
UV-vis absorption spectra were obtained by a Shimadzu
UV-2401PC spectrophotometer. Fluorescence spectra were re-
corded on a Shimadzu RF-5301PC spectrofluorophotometer.
For the measurements of thin films, the polymer was spin-
coated at 600 rpm onto precleaned glass slides from 10 mg/mL
polymer solution in o-dichlorobenzene and dried slowly in a
Petri dish for 3 h.
Figure 5. Characteristic I-V curves of the optimized devices of all
polymer-based BHJ solar cells under 1 sun condition.
Table 4. Mobility
thickness
(nm)
mobility polymer: thickness mobility
cm2/V s
polymer
PCBM
(nm)
cm2/V s
3
3
PDTPr-T
PDTPr-BT
PDTBn-T
PDTBn-BT
PDTPn-T
PDTPn-BT
130
120
80
105
60
1.11 ꢀ 10-5 1:1
6.41 ꢀ 10-5 1:3
2.57 ꢀ 10-5 1:2
2.75 ꢀ 10-5 1:1
5.98 ꢀ 10-6 1:3
5.35 ꢀ 10-6 1:1
75
75
65
240
45
45
8.17 ꢀ 10-6
1.15 ꢀ 10-5
2.14 ꢀ 10-6
3.88 ꢀ 10-7
3.55 ꢀ 10-6
2.25 ꢀ 10-6
100
Gel permeation chromatography (GPC) measurements were
performed on two different machines, depending upon the solubi-
lity of the polymers in THF. For THF-soluble polymers, a Waters
2695 separations module apparatus with a differential refractive
index detector (at UNC Chapel Hill) was used, employing tetra-
hydrofuran (THF) as the eluent. For THF nonsoluble polymers, a
Polymer Laboratories PL-GPC 220 instrument (at University of
Chicago) was used, using 1,2,4-trichlorobenzene as the eluent
(stabilized with 125 ppm BHT) at 150 °C. The obtained molecular
weight is relative to polystyrene standards.
Cyclic voltammetry measurements were carried out using a
Bioanalytical Systems (BAS) Epsilon potentiostat equipped
with a standard three-electrode configuration. Typically, a
three-electrode cell equipped with a glassy carbon working
electrode, a Ag/AgNO3 (0.01 M in anhydrous acetonitrile)
reference electrode, and a Pt wire counter electrode was em-
ployed. The measurements were done in anhydrous acetonitrile
with tetrabutylammonium hexafluorophosphate (0.1 M) as the
supporting electrolyte under an argon atmosphere at a scan rate
of 100 mV/s. Polymer films were drop-cast onto the glassy
carbon working electrode from a 2.5 mg/mL chloroform solu-
tion and dried under house nitrogen stream prior to measure-
ments. The potential of Ag/AgNO3 reference electrode was
internally calibrated by using the ferrocene/ferrocenium redox
couple (Fc/Fcþ). The electrochemical onsets were determined at
the position where the current starts to differ from the baseline.
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels were cal-
culated from the onset oxidation potential (Eox) and onset
reductive potential (Ered), respectively, according to eqs 1 and 2.
Figure 6. IPCE spectrum of BHJ photovoltaic device of ITO/PEDOT:
PSS (45 nm)/PDTBn-BT:PCBM (1:1, w/w)/Ca (30 nm)/Al (100 nm)
and the optical absorptions for the corresponding film of the blend.
segregation (even though PDTBn-T based devices exhibited
the highest efficiency). Thus, these devices offer a prelimin-
ary look at the potential of these polymers, and employing
additional optimization techniques beyond simple device
thickness and ratio of polymer:PCBM could allow for an
increase in device performance.
Conclusion
We investigated a collection of six electrochemically and
optically unique polymers based upon three donor monomers.
All three donor monomers are structurally related, featuring the
same flanking thiophenes, but incorporating different center
aromatic units. DTBn-based polymers exhibited the most poten-
tial in photovoltaic applications due to its moderately low band
gap and low HOMO energy level. The other polymers in this
study either demonstrated high-lying HOMO energy levels
(DTPr-based polymers) or unacceptably wide band gaps
(DTPn-based polymers).
HOMO ¼ -ðEox þ 4:8Þ ðeVÞ
LUMO ¼ -ðEred þ 4:8Þ ðeVÞ
ð1Þ
ð2Þ
Polymer Solar Cell Fabrication and Testing. Glass substrates
coated with patterned indium-doped tin oxide (ITO) were
purchased from Thin Film Devices, Inc. The 150 nm sputtered

Article
Macromolecules, Vol. 43, No. 2, 2010 803
ITO pattern had a resistivity of 15 Ω/0. Prior to use, the
substrates were ultrasonicated for 10 min in acetone followed
by deionized water and then 2-propanol. The substrates were
dried under a stream of nitrogen and subjected to the treatment
of UV-ozone over 20 min. A filtered dispersion of PEDOT:PSS
in water (Baytron PH500) was then spun-cast onto clean ITO
substrates at 4000 rpm for 60 s and then baked at 140 °C for
10 min to give a thin film with a thickness of 40 nm. A blend of
polymer and PCBM (1:1, 1:2, or 1:3, w/w, depending upon the
polymer; see Table 3) at 10 mg/mL (for polymer) was dissolved
in dichlorobenzene (for PDTPr-BT) or chlorobenzene (for other
polymers) with heating at 60 °C for 6 h, filtered through a 0.45
μm poly(tetrafluoroethylene) (PTFE) filter, and spun-cast bet-
ween 800 and 1200 rpm for 60 s onto a PEDOT:PSS layer. The
substrates were then dried at room temperature under N2 for
12 h. The devices were finished for measurement after thermal
deposition of a 30 nm film of calcium and a 100 nm aluminum
film as the cathode at a pressure of ∼1 ꢀ 10-6 mbar. There are
eight devices per substrate, with an active area of 12 mm2 per
device. The thicknesses of films were recorded by a profilometer
(Alpha-Step 200, Tencor Instruments). Device characterization
was carried out under AM 1.5G irradiation with the intensity of
100 mW/cm2 (Oriel 91160, 300 W) calibrated by a NREL
certified standard silicon cell. Current vs potential (I-V) curves
were recorded with a Keithley 2400 digital source meter. EQE
were detected under monochromatic illumination (Oriel Cor-
nerstone 260 1/4 m monochromator equipped with Oriel
70613NS QTH lamp), and the calibration of the incident light
was performed with a monocrystalline silicon diode. All fabri-
cation steps after adding the PEDOT:PSS layer onto ITO
substrate, and characterizations were performed in gloveboxes
under a nitrogen atmosphere. For mobility measurements, the
hole-only devices in a configuration of ITO/PEDOT:PSS
(45 nm)/polymer-PCBM/Pd (40 nm) were fabricated. The ex-
perimental dark current densities J of polymer:PCBM blends
were measured when applied with voltage from 0 to 6 V. The
applied voltage V was corrected from the built-in voltage Vbi,
which was taken as a compensation voltage Vbi = Voc þ 0.05 V
and the voltage drop Vrs across the indium tin oxide/poly(3,4-
ethylene-dioxythiophene):poly(styrenesulfonic acid) (ITO/
PEDOT:PSS) series resistance and contact resistance, which is
found to be around 35 Ω from a reference device without the
polymer layer. From the plots of J0.5 vs V (Supporting Infor-
mation), hole mobilities of copolymers can be deduced from
tert-Butyl 5-Bromothiophen-3-ylcarbamate (3). 2-Bromothio-
phene-4-carboxylic acid (11.584 g, 55.95 mmol) was combined
with anhydrous toluene (225 mL) in a dry flask under argon.
Triethylamine (11.8 mL, 83.93 mmol) was added to the slurry,
which became homogeneous after addition. Diphenylphosphoryl
azide (12.1 mL, 55.95 mmol) was then added at room temperature,
and the reaction mixture was stirred for 2.5 h. The mixture was
then heated to 80 °C and stirred for 1 h. Anhydrous tert-butanol
(16.0 mL, 167.9 mmol) was then added, and the reaction mixture
was stirred for 16 h at 80 °C. The mixture was then concentrated
and purified by column chromatography using a 3:2 toluene:
hexanes solution as the eluent. The fractions were concentrated,
affording a beige powder of sufficient purity for the following steps.
Yield: 11.807 g (76%). Analytical purity was obtained by recrys-
tallization from cyclohexanes (refluxed, then cooled to 4 °C),
yielding colorless prisms which were stable in atmosphere for over
a week. Colorless crystalline solid; mp 88-91 °C. 1H NMR
(CDCl3, 300 MHz, δ): 7.04 (br s, 1H), 6.94 (br s, 1H), 6.61 (br s,
1H), 1.50 (s, 9H). 13C NMR (CDCl3, 100 MHz, δ): 152.5, 135.8,
123.7, 111.7, 108.7, 80.9, 28.2. ESI-TOF MS: [M þ Na]þ
=
299.9667 (calcd [M þ Na]þ = 299.9670).
2,6-Dibromo-8-(1-hexylnonyl)dithieno[3,2-b:20,30-e]pyridine
(5). 3 (2.039 g, 7.33 mmol) and 2-hexyldecanal (881 mg, 3.67
mmol) were dissolved in methylene chloride (40 mL). Trifluor-
acetic acid (1.8 mL) was slowly added over 3 min, and the
resulting mixture was heated to reflux. After 18 h, the reaction
mixture was partitioned between CH2Cl2 and ice cold 10%
NaOH solution and separated, and the organic layer was dried
over Na2SO4. The mixture was then purified by column chro-
matography eluting with a 7:1 mixture of hexanes:ethyl acetate.
The resulting yellow oil was then concentrated under vacuum
(0.5 mmHg) for 24 h, resulting in a pale brown solid. Yield: 615
1
mg (30%). Crystalline brown solid; mp 60-61 °C. H NMR
(CDCl3, 300 MHz, δ): 7.57 (s, 2H), 2.99 (m, 1H), 2.01 (m, 2H),
1.84 (m, 2H), 1.16 (m, 20H), 0.86 (m, 6H). 13C NMR (CDCl3,
100 MHz, δ): 154.98, 141.66, 127.66, 120.93, 47.02, 33.51,
33.47, 31.72, 31.49, 29.48, 29.19, 29.10, 27.79, 22.55, 22.48,
13.99, 13.91. ESI-TOF MS [M þ H]þ = 558.0506 (calcd [M þ
H]þ = 558.0499).
2-Hexyldecanal (4). Dimethyl sulfoxide (10.2 mL, 143.6
mmol) was dissolved in anhydrous methylene chloride (350
mL) and chilled to -78 °C under argon. Oxalyl chloride (6.48
mL, 75.6 mmol) was then slowly added dropwise, while main-
taining the temperature at -78 °C. The mixture was stirred for 30
min, and then 2-hexyldecan-1-ol (20.9 mL, 71.99 mmol) was added
dropwise at -78 °C. The mixture was stirred for 35 min carefully
maintaining -78 °C. TEA (30 mL, 215 mmol) was added, and a
thick white precipitate formed. The mixture was stirred for 10 min
at -78 °C and then allowed to warm to room temperature. The
mixture was poured into 1 M HCl and extracted with methylene
chloride. The organic layer was then washed repeatedly with
distilled water and dried over MgSO4. The mixture was then
filtered, concentrated, and filtered through a short plug of silica
gel. The silica gel was washed with hexanes, and the filtrate was
concentrated and distilled under reduce pressure. The desired
aldehyde was obtained at a distillate temperature of 105 °C at
0.6 mmHg. Yield: 12.087 g (70%). Colorless oil. 1H NMR (CDCl3,
300 MHz, δ): 9.55 (d, J = 3.3 Hz, 1H), 2.21 (m, 1H), 1.60 (m, 2H),
1.45 (m, 2H), 1.27 (m, 20H), 0.88 (t, J = 6.6 Hz, 6H). 13C NMR
(CDCl3, 100 MHz, δ): 205.45, 51.94, 31.80, 31.59, 29.66, 29.35,
29.32, 29.18, 28.88, 27.03, 26.99, 22.59, 22.52, 14.00, 13.95.
9
V2
J ¼ εrε0μ
8
h L3
where ε0 is the permittivity of free space, εr is the dielectric
constant of the polymer which is assumed to be around 3 for the
conjugated polymers, μh is the hole mobility, V is the voltage
drop across the device, and L is the film thickness of active layer.
Reagents. All solvents are ACS grade unless otherwise noted.
Anhydrous THF was obtained by distillation from sodium/
benzophenone prior to use. Anhydrous methylene chloride was
dried over magnesium sulfate and filtered directly into the
reaction flask prior to use. Anhydrous toluene was used as
received. Anhydrous tert-butanol was obtained by treatment
with sodium metal and then distillation. 2-Bromothiophene-4-
carboxylic acid,23 2,6-di(trimethyltin)-N-(1-octylnonyl)dithi-
eno[3,2-b:20,30-d]pyrrole,18 2,5-bis(trimethyltin)thiophene,35
4,7-dibromo-2,1,3-benzothiadiazole,36 and 2,1,3-benzothiadia-
zole-4,7-bis(boronic acid pinacol ester)37 were prepared accord-
ing to established literature procedures. 2,6-Bis(trimethyltin)-
4,8-(3-hexylundecyl)benzo[1,2-b:4,5-b0]dithiophene6,20 and
N-(1-octylnonyl)-2,6-dibromodithieno[3,2-b;20,30-d]pyrrole38,39
were prepared using procedures analogous to established literature
procedures, and 1H and 13C NMR spectra for each monomer are
shown in the Supporting Information. All other chemicals were
purchased from commercial sources (Acros, Alfa Aesar, Aldrich,
Fisher Scientific, Oakwood Chemical) and used without further
purification.
Deuterium Lithiation Experiments. The following lithiation
conditions are reproduced from ref 21. 1 (151 mg, 0.363 mmol)

804 Macromolecules, Vol. 43, No. 2, 2010
Price et al.
was dissolved in 1.5 mL of anhydrous THF in a dry flask. The
solution was cooled to -78 °C, and a 2.5 M solution of n-BuLi in
hexanes (0.35 mL, 0.87 mmol) was added dropwise. The solu-
tion was stirred at -78 °C for 1 h and then at 0 °C for 5 h. The
solution was then cooled back down to -78 °C, and methanol-d4
(0.5 mL) was added in one portion. The solution was stirred at
-78 °C for 5 min and then warmed to room temperature. The
reaction mixture was then filtered through a silica plug (250 mg),
and the plug was washed with 1 mL of CH2Cl2. The resulting
filtrate was then concentrated, and the residue was analyzed by
NMR. The procedure was repeated using 2.05 equiv of t-BuLi
at -78 °C for 1 h and then quenching with methanol-d4.
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Representative Stille Coupling Polymerization Procedure. 2,6-
Bis(trimethyltin)-4,8-(3-hexylundecyl)benzo[1,2-b:4,5-b]dithio-
phene (498 mg, 0.502 mmol), 4,7-dibromo-2,1,3-benzothiadia-
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purged with argon for 20 min. Then tris(dibenzylidene-
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acetate, hexanes, and chloroform. The chloroform extracts were
then concentrated and precipitated into methanol. The resulting
solids were filtered and washed with methanol, and residual
solvent was removed under vacuum at 0.5 mmHg affording
polymer PDTBn-BT as a blue-black powder. Yield: 32 mg (8%).
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Acknowledgment. This work was supported by the Univer-
sity of North Carolina at Chapel Hill, the National Science
Foundation STC Program at UNC Chapel Hill (CHE-
9876674), a DuPont Science and Engineering Grant, a DuPont
Young Professor Award, and the Office of Naval Research
(Grant N000140911016). S.C.P. gratefully acknowledges Ap-
plied Materials for a graduate fellowship. We acknowledge Mr.
Huaxing Zhou for CV measurements. We thank Prof. Richard
Jordan and Mr. Zhongliang Shen of the University of Chicago
for GPC measurements.
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Supporting Information Available: 1H and 13C spectra of
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0.5
molecules; J
vs V plots of mobility measurement of all
polymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Heeney, M.; McCulloch, I.; Nelson, J.; Bradley, D. D. C.; Durrant,
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