Donor-acceptor polymers incorporating alkylated dithienylbenzothiadiazole for bulk heterojunction solar cells: Pronounced effect of positioning alkyl Chains

Article abstract of DOI:10.1021/ma902241b

4,7-Di(thiophen-2-yl)benzothiadiazole (DTBT) has been used to construct a number of donor-acceptor low band gap polymers for bulk heterojunction (BHJ) photovoltaics with high efficiency numbers. Its strong tendency to π-stack often leads to polymers with low molecular weight and poor solubility, which could potentially be alleviated by anchoring solubilizing chains onto the DTBT unit. A systematic study of the effect of positioning alkyl chains on DTBT on properties of polymers was implemented by investigating a small library of structurally related polymers with identical conjugated backbone. This series of donor-acceptor polymers employed a common donor unit, benzo[2,1-b:3,4-b′] dithiophene (BDT), and modified DTBT as the acceptor unit. Three variations of modified DTBT units were prepared with alkyl side chains at (a) the 5-and 6-positions of 2,1,3-benzothiadiazole (DTsolBT), (b) 3-positions of the flanking thienyl groups (3DTBT), and (c) 4-positions (4DTBT), in addition to the unmodified DTBT. Contrary to results from previous studies, optical and electrochemical studies disclosed almost identical band gap and energy levels between PBDT-4DTBT and PBDT-DTBT. These results indicated that anchoring solubilizing alkyl chains on the 4-positions of DTBT only introduced a minimum steric hindrance within BDT-DTBT, maintaining the extended conjugation of the fundamental structural unit (BDT-DTBT). More importantly, the additional high molecular weight and excellent solubility of PBDT-4DTBT led to a more uniform mixture with PCBM, with better control on the film morphology. AU these features of PBDT-4DTBT led to a significantly improved efficiency of related BHJ solar cells (up to 2.2% has been observed), triple the efficiency obtained from BHJ devices fabricated from the "conventional" PBDT-DTBT (0.72%). Our discovery reinforced the importance of high molecular weight and good solubility of donor polymers for BHJ solar cells, in addition to a low band gap and a low HOMO energy level, in order to further enhance the device efficiencies.

Full text of DOI:10.1021/ma902241b

Macromolecules 2010, 43, 811–820 811
DOI: 10.1021/ma902241b
Donor-Acceptor Polymers Incorporating Alkylated
Dithienylbenzothiadiazole for Bulk Heterojunction Solar Cells:
Pronounced Effect of Positioning Alkyl Chains
Huaxing Zhou,† Liqiang Yang,‡ Shengqiang Xiao,†,§ Shubin Liu,^ and Wei You*,†
†Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
27599-3290, ‡Curriculum in Applied Sciences and Engineering, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3287, §State Key Lab of Advanced Technology for Materials Synthesis
and Processing, Wuhan University of Technology, Wuhan, P. R. China 430070, and ^Research Computing Center,
University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 25599-3420
Received October 8, 2009; Revised Manuscript Received November 24, 2009
ABSTRACT: 4,7-Di(thiophen-2-yl)benzothiadiazole (DTBT) has been used to construct a number of
donor-acceptor low band gap polymers for bulk heterojunction (BHJ) photovoltaics with high efficiency
numbers. Its strong tendency to π-stack often leads to polymers with low molecular weight and poor solubility,
which could potentially be alleviated by anchoring solubilizing chains onto the DTBT unit. A systematic study of
the effect of positioning alkyl chains on DTBT on properties of polymers was implemented by investigating a
small library of structurally related polymers with identical conjugated backbone. This series of donor-
acceptor polymers employed a common donor unit, benzo[2,1-b:3,4-b0]dithiophene (BDT), and modified DTBT
as the acceptor unit. Three variations of modified DTBT units were prepared with alkyl side chains at (a) the 5-
and 6-positions of 2,1,3-benzothiadiazole (DTsolBT), (b) 3-positions of the flanking thienyl groups (3DTBT),
and (c) 4-positions (4DTBT), in addition to the unmodified DTBT. Contrary to results from previous studies,
optical and electrochemical studies disclosed almost identical band gap and energy levels between
PBDT-4DTBT and PBDT-DTBT. These results indicated that anchoring solubilizing alkyl chains on the
4-positions of DTBT only introduced a minimum steric hindrance within BDT-DTBT, maintaining the
extended conjugation of the fundamental structural unit (BDT-DTBT). More importantly, the additional
high molecular weight and excellent solubility of PBDT-4DTBT led to a more uniform mixture with PCBM,
with better control on the film morphology. All these features of PBDT-4DTBT led to a significantly improved
efficiency of related BHJ solar cells (up to 2.2% has been observed), triple the efficiency obtained from BHJ
devices fabricated from the “conventional” PBDT-DTBT (0.72%). Our discovery reinforced the importance of
high molecular weight and good solubility of donor polymers for BHJ solar cells, in addition to a low band gap
and a low HOMO energy level, in order to further enhance the device efficiencies.
Introduction
low band gap polymers have been demonstrated with decent
efficiencies (3-6%) when they were blended with PCBM or
In recent years there has been tremendous progress in the field of
bulk heterojunction (BHJ) polymer solar cells since its inception in
1995.1 The overall energy conversion efficiency has been steadily
improving from an initial 1% via a blend of poly[2-methoxy-
5-(30,70-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV)
with [6,6]-phenyl C60-butyric acid methyl ester (PCBM)2 to an
impressive ∼5% achieved by poly(3-hexylthiophene) (P3HT):
PCBM via extensive morphology optimization.3-5 In order to
further improve the efficiency of BHJ polymer solar cells, the
research community is enthusiastically searching for new poly-
mers. From the materials perspective, these donor polymers
should not only have a low band gap (to increase the short circuit
current, Jsc) but also bear a low energy level of the highest oc-
cupied molecular orbital (HOMO) (to improve the open circuit
voltage, Voc).6 Typically, a low band gap polymer is designed via a
“donor-acceptor” (D-A) approach, which is to incorporate
electron-rich and electron-deficient moieties in the polymer back-
bone. The low band gap is mainly caused by the intramolecular
charge transfer between donor and acceptor units.7 Indeed, several
[6,6]-phenyl C70-butyric acid methyl ester (PC70BM) in typical
BHJ devices.8-28 For example, PCDTBT, a low band gap polymer
synthesized from a copolymerization of alkylated carbazole and
4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT), recently demon-
strated a record high efficiency of 6.1% when it was blended in a
BHJ composite with PC70BM.17
One common feature of these successful low band gap polymers is
the predominant employment of 2,1,3-benzothiadiazole (BT)12,14,24
or di-2-thienyl-2,1,3-benzothiadiazole (DTBT)8-10,15-17,23 as the
acceptor units. Compared with the BT unit, DTBT has a few
advantages. First, the two flanking thienyl units relieve the otherwise
possibly severe steric hindrance between the acceptor BT unit and
donor aromatic units (especially when benzene-based aromatics are
used). Thus, the synthesized donor-acceptor polymers would adopt
a more planar structure, thereby reducing the band gap by enhancing
the D-A conjugation. In addition, a more planar conjugated
backbone would facilitate the chain-chain interactions among
polymers, improving the charge carrier (usually hole) mobility.
Second, while the electron accepting BT unit maintains the low band
gap, the two electron rich, flanking thienyl units would help improve
the hole mobility, since thiophene-based polymers (such as P3HT)
have shown very high hole mobility.29 Because of these advantages,
*To whom all correspondence should be addressed. E-mail: wyou@
email.unc.edu.
r 2009 American Chemical Society
Published on Web 12/29/2009
pubs.acs.org/Macromolecules

812 Macromolecules, Vol. 43, No. 2, 2010
Zhou et al.
the widespread usage of the DTBT unit has resulted in a number of
polymers with high BHJ solar cell efficiencies.8-10,15-17,20,23,30,31
Unfortunately, the strong stacking ability of polymers incor-
porating the DTBT unit also introduces several concomitant
technical difficulties. For example, the additional thiophene rings
could result in polymers that are poorly soluble.30 This low
solubility can lead to low molecular weight of the polymer and
unnecessary difficulty in processing the polymer. For BHJ solar
cells, high molecular weight polymers are desirable, which have
been shown to help enhance the efficiency of related BHJ devices,
presumably by improved inter-polymer interaction to enhance
the current.32-34 Even if one could manage to obtain high
molecular weight polymers, the excessive stacking may result in
aggregation of polymers upon spin-casting of the film. This
aggregation leads to polymer-only domains on a micrometer
scale, in contrast to the desired morphology: nanometer
sized, separated phases of donor polymers and fullerenes.35
In order to fully utilize the aforementioned merits of DTBT
units, it is important to search for new strategies to modify
the structure of DTBT so as to improve the solubility and
molecular weight of polymers incorporating DTBT units. In
addition, such modification of DTBT units should also have
minimum impact on the band gap and energy levels one would
obtain from “conventional” polymers incorporating unmodified
DTBT units.
There have been attempts to incorporate alkyl or alkoxy chains
on the DTBT unit in order to enhance both the solubility and
molecular weight of resulting polymers. In one study, Shi et al.
copolymerized alkylated fluorene with DTBT modified with
alkoxy chains on the 3-position of these thienyl units, resulting
in a polymer with much higher molecular weight (number-
average molecular weight, Mn, 68 kg/mol) and better solubility36
than those of original polymers without any soluble chains on the
DTBT unit.30 However, these electron-donating alkoxy groups
on the DTBT unit caused the HOMO level of the resulting
polymer to increase, which led to a lower Voc. More recently,
Wang et al. attempted to add additional alkylated thiophene
units to extend the DTBT unit.37 The resulting D-A polymer
with alkylated fluorene as the donor unit showed a smaller band
gap, higher molecular weight, and better solubility compared
with the original copolymer that incorporated alkylated fluorene
and DTBT (PF-DBT).30,38 Unfortunately, the overall efficiency
of this new polymer (PFO-M3) was smaller (2.63%) than that of
the original PF-DBT (up to 3.5%),9 in spite of a lower band gap
and a similar HOMO energy level of PFO-M3. In another study,
Song et al. prepared polymers incorporating a DTBT unit
modified with either alkyl chains on the 4-position of these
thienyl units or alkoxy chains on the 3-position.39 Compared
with polymers incorporating DTBT without any decorated
chains, these modified ones were more soluble. Again, the overall
BHJ device efficiencies of polymers with such modified DTBT
were lower than that of the corresponding DTBT-based polymer
without alkoxy/alkyl chains, mainly due to the lower Jsc in the
former case. Computational simulation revealed that a severe
steric hindrance was introduced by these alkyl/alkoxy chains,
leading to a twisted conjugated backbone in both polymers with
such modified DTBT units.40 Therefore, the hole mobilities of
the polymers incorporating such modified DTBT units were
noticeably lower than that of polymers with unmodified DTBT
unit, which accounted for a smaller Jsc in the former case.40 In an
earlier study, Jayakannan et al. prepared the homopolymers
of alkylated DTBT by varying alkyl chains on either 3- or
4-positions of thienyl groups.41 Though relatively high molecular
weight polymers were obtained, the steric hindrance introduced
by these alkyl chains in these polymers led to much larger band
gaps than that of the homopolymer of unmodified DTBT.42 All
these studies acknowledged the advantage of employing DTBT
Figure 1. Chemical structures of PBDT-DTBT, PBDT-4DTBT,
PBDT-3DTBT and PBDT-DTsolBT.
units in constructing D-A low band gap polymers for BHJ solar
cells. More importantly, these previous studies underscored the
importance of the functionalization of DTBT units to reach high
molecular weight and good solubility of resulting polymers,
which should lead to PV devices with higher efficiencies. Un-
fortunately, no improvements on BHJ devices efficiency were
observed from these reported polymers that incorporated modi-
fied DTBT units.
Previously, we have demonstrated that by incorporating a
donor;bithiophene fused with a benzene moiety (benzo[2,1-
b:3,4-b0]dithiophene, BDT);and an acceptor;benzothiadiazole
(BT);a low band gap polymer (PBDT-BT) was obtained
together with a low HOMO energy level.43 However, an effi-
ciencyof only 0.6% was obtained for PBDT-BT, mainly due to a
small Jsc of 2.06 mA/cm2. This low current was ascribed to a low
hole mobility (4.21 ꢀ 10-6 cm2/(V s)) and a low molecular
3
weight (Mn: 10.1 kg/mol), both of which would be alleviated by
introducing the modified DTBT units. Thus, a library of poly-
mers incorporating BDT as the donor and modified DTBT as the
acceptor unit was envisioned and synthesized (Figure 1). Alkyl
chains, rather than alkoxy chains, were used to mitigate the
possible elevation of the HOMO energy levels of the resulting
polymers. Three variations of modified DTBT units were pre-
pared: alkyl side chains at (a) the 5- and 6-positions of 2,1,3-
benzothiadiazole (DTsolBT), (b) 3-positions of the flanking
thienyl groups (3DTBT), and (c) 4-positions (4DTBT). For
comparison, the polymer with unmodified DTBT (PBDT-
DTBT) was also synthesized.
As expected, much higher molecular weights (∼30 kg/mol) and
better solubility in processing solvents were observed for all three
polymers with alkylated DTBT units than those of PBDT-
DTBT (9 kg/mol). Interestingly, contrary to results from previous
studies, optical and electrochemical studies disclose almost iden-
tical band gap and energy levels between PBDT-4DTBT and
PBDT-DTBT. These results indicate that anchoring solubilizing
alkyl chains on the 4-positions of DTBT only introduces a
minimum steric hindrance within BDT-DTBT, thereby main-
taining the extended conjugation of the fundamental structural
unit (BDT-DTBT). Furthermore, the polymer containing the
“properly” modified DTBT unit, PBDT-4DTBT, shows an
improved hole mobility of 9.2 ꢀ 10-6 cm2/(V s) than that of
3
PBDT-DTBT (3.9 ꢀ 10-6 cm2/(V s)) or PBDT-BT (4.21 ꢀ
3
10-6 cm2/(V s)). This noticeably higher mobility of PBDT-
3
4DTBT, together with its low band gap and relatively low
HOMO energy level inherited from PBDT-DTBT, leads to a
significantly improved efficiency of related BHJ solar cells (up to
2.2% has been observed), triple the efficiency obtained from BHJ
devices fabricated from either PBDT-DTBT (0.72%) or
PBDT-BT (0.6%).43 A thorough investigation of this library
of structurally related polymers unveils a complete picture of the
influence of alkyl chains on different positions of DTBT units on

Article
Macromolecules, Vol. 43, No. 2, 2010 813
Scheme 1. Synthesis of Monomer A1 (Brominated DTsolBT)
Scheme 2. Synthetic Route for A2 and A3 (Brominated 3DTBT and 4DTBT)
the optical, electrical, and photovoltaic properties of resulting
polymers.
selectively occurred at the more acidic 5-position, followed by
stannylation to yield (7). The other isomer (8), with tributyltin
anchored at the 2-position, was obtained via 2-bromo-3-
octylthiophene as the intermediate, since the bromination via
NBS was selective at the more nucleophilic 2-position. Then
halogen exchange with n-BuLi led to the lithiated 2-position,
which was subsequently quenched by (n-Bu)3SnCl to yield the
2-stannylated (8). The unmodified DTBT monomer was
synthesized following a literature report.46
Polymer Synthesis. Four polymers were therefore pre-
pared by a palladium-catalyzed Stille cross-coupling poly-
condensation of benzo[2,1-b:3,4-b0]dithiophene (BDT)
distannane (D0) with brominated DTBT and its derivatives
(Scheme 3). The polymerization of PBDT-DTBT was
stopped after 18 h when precipitation was observed. The
crude polymer was then precipitated from methanol and
extracted via a Soxhlet apparatus with acetone, hexane, and
finally chloroform. Only the chloroform-soluble portion was
collected in order to obtain a high molecular weight, solution
processable polymer. A noticeable amount of residue re-
mained in the extraction thimble after the extraction with
chloroform. This presumably high molecular weight
portion was discarded since it would not be solution proces-
sable, which consequently led to a relatively low yield (51%)
Results and Discussion
Monomer Synthesis. In order to obtain a high molecular
weight polymer with good solubility, long, branched alkyl
chains were attached at the central BT unit during the
synthesis of DTsolBT (M1 in Scheme 1). 4-Ethyloct-1-yne
was prepared from commercially available 2-ethylhexyl bro-
mide with overall yield of >60%. A Sonogashira coupling
was employed to attach the 4-ethyloct-1-yne to 1,2-dibromo-
4,5-dinitrobenzene (2) obtained by nitration of 1,2-dibromo-
benzene (1), leading to the alkylated dinitrobenzene (3).
Both triple bonds and nitro groups were reduced simulta-
neously via Pd/H2, yielding alkylated diaminobenzene (5).
Treating (5) with thionyl chloride under basic conditions
afforded compound (6), the alkylated benzothiadiazole
(solBT), which then underwent several conventional halo-
genations and coupling reaction to monomer A1, the bro-
minated DTsolBT.
Synthesis of A2 and A3 is depicted in Scheme 2. They were
synthesized via slightly modified literature procedures.41,44,45
It is worth noting that the deprotonation of 3-octylthiophene

814 Macromolecules, Vol. 43, No. 2, 2010
Scheme 3. Polymerization by Stille Coupling Reaction
Zhou et al.
Table 1. Polymerization Results for Polymers
of the polymerization. Not surprisingly, the measured
molecular weight of PBDT-DTBT is low: the number-
average (Mn) is only 9 kg/mol, and the weight-average
(Mw) is 12 kg/mol, mainly due to the poor solubility of
PBDT-DTBT.
b
yield
[%]
Mna [kg/
mol]
Mwa [kg/
mol]
Td
PDIa [ꢀC]
PBDT-DTBT
PBDT-4DTBT
PBDT-3DTBT
PBDT-DTsolBT
51
75
88
87
9
27
37
30
12
54
84
92
1.31 270
1.80 420
3.07 436
2.44 440
On the other hand, adding solubilizing chains to the DTBT
unit significantly increases the molecular weight of the
resulting polymers and improves their solubility in com-
monly employed solvents such as THF and chloroform.
Both of these features, high molecular weight and good
solubility, also explain the high yields of these polymeriza-
tions even after crude polymers were extensively purified
(Table 1). The structures of these purified polymers were
confirmed by 1H NMR and elemental analysis (Supporting
Information). Neither glass transition nor melting transition
was observed for all three polymers with alkylated DTBT,
indicating the amorphous nature of these polymers. A
slightly observable glass transition at ∼125 ꢀC and a barely
visible melting transition at ∼190 ꢀC were obtained for
PBDT-DTBT, partly due to its low molecular weight and
more rigid backbone. Please note that microwave-assisted
polymerization47-49 was employed in synthesizing PBDT-
4DTBT, with satisfactorily high molecular weight (comp-
arable to that of PBDT-3DTBT or PBDT-DTsolBT)
obtained in much shorter time (half an hour compared with
days in conventional heating).
Optical and Electrochemical Properties. The positioning of
attached solubilizing alkyl chains on the DTBT unit see-
mingly has little impact on the molecular weight and solubi-
lity of related polymers: high molecular weight and good
solubility have been unanimously obtained for polymers
incorporating alkylated DTBT units. However, dramatic
effects were observed on the optical and electrochemical
properties of polymers incorporating alkylated DTBT units,
depending upon where these alkyl chains are attached on the
DTBT units. It is surprising to discover that anchoring these
alkyl chains on the 4-positions of these thienyl groups on
the DTBT unit have negligible impact on the absorption
and band gap of PBDT-4DTBT compared with those
of PBDT-DTBT in solution (Figure 2a). In thin films,
the unsubstituted PBDT-DTBT has a strong tendency to
a Determined by GPC in tetrahydrofuran (THF) using polystyrene
standards. b The temperature of degradation corresponding to a 5%
weight loss determined by TGA at a heating rate of 10 ꢀC/min.
π-stack: a pronounced absorption increase was observed
from about 550 nm, extending up to almost 800 nm. The
alkylated PBDT-4DTBT shows similar behavior of strong
stacking, though the absorption edge is slightly blue-shifted
compared with that of PBDT-DTBT (735 nm vs 761 nm)
(Table 2), indicative of a slight steric hindrance from these
extra alkyl chains in the solid state. Nevertheless, similarly
small band gaps (<1.7 eV) have been observed for both
polymers. These results suggest that anchoring alkyl chains
on the 4-positions of the thienyl groups (i.e., 4DTBT)
introduces minimum steric hindrance to the original con-
jugated backbone of PBDT-DTBT, maintaining the elec-
tron delocalization from D-A structures.
When these alkyl chains are located at either the 3-posi-
tions of the thienyl groups (3DTBT) or the 5- and 6-positions
of the 2,1,3-benzothiadiazole (DTsolBT), severe steric hin-
drance is introduced between the flanking thienyl groups and
the central BT unit. The conjugated backbone is thereby
twisted at the D-A linkage, significantly affecting the
effective conjugation between the donor and the acceptor
(BT). Large band gaps were observed for these two polymers
(2.21 eV for PBDT-3DTBT and 2.48 eV for PBDT-
DTsolBT). The even larger band gap of PBDT-DTsolBT
implies much stronger steric hindrance (thereby twisting of
the conjugated backbone) is introduced when alkyl chains
are located at the central BT unit. The twisting of the
backbone due to the DTsolBT unit also explains the fact
that nearly negligible stacking was observed for PBDT-
DTsolBT in thin film, while an appreciable red shift in thin
film was still observed for PBDT-3DTBT. Finally, both

Article
Macromolecules, Vol. 43, No. 2, 2010 815
PBDT-DTBT and PBDT-4DTBT are much more absorp-
tive in the solid state than either PBDT-3DTBT or
PBDT-DTsolBT: similar absorption coefficients of about
1.5 ꢀ 10-5 cm-1 for the former two polymers at their maxi-
mum absorption wavelength, significantly greater than those
of the latter two (up to 0.9 ꢀ 10-5 cm-1) at their maxima.
Probing this library of polymers with identical conjugated
backbone via cyclic voltammetry provides direct evidence on
how the difference in positioning these alkyl chains affects
the energy levels of these related polymers. The lowest
unoccupied molecular orbital (LUMO) is essentially domi-
nated by the common acceptor unit (BT), which explains
why all four polymers show similar reduction potential and
LUMO energy levels (Figure 3 and Table 2). However, the
HOMO energy levels disclose dramatic differences. The
HOMO energy levels of PBDT-4DTBT and PBDT-DTBT
differ only slightly (- 5.27 eV vs - 5.21 eV), further indicating
that anchoring alkyl chains at the 4-positions of thienyl
groups has almost negligible influence on the extended
conjugation of the D-A polymer. The introduction of two
electron rich thienyl groups to the original PBDT-BT
increases the electron density of the conjugated backbone,
leading to an elevated HOMO energy level of PBDT-DTBT
(- 5.21 eV) compared with that of PBDT-BT (- 5.34 eV).43
Shifting these alkyl chains to the 3-positions of thienyl
groups (PBDT-3DTBT) leads to an appreciable conjuga-
tion twisting at the linkage of thienyl groups with the central
BT unit. The apparently reduced electron delocalization
renders a lowered HOMO energy level of -5.38 eV. In the
case of PBDT-DTsolBT, a very low HOMO level of -5.69
eV was observed, strikingly similar to that of the homopo-
lymer of BDT (HMPBDT, -5.70 eV).43 This apparent
coincidence implies that the severe steric hindrance from
DTsolBT may disrupt the conjugation between the “donor”
and the “acceptor” in PBDT-DTsolBT. Thus, the HOMO is
essentially localized at the BDT unit (and partially on
thienyls), which explains similar HOMO levels of
PBDT-DTsolBT and HMPBDT.
Computational Study. Computational study of this series
of polymers provides insightful information to account for
the observed difference of optical and electrochemical prop-
erties. To simplify the calculation, only one repeating unit of
each polymer was subject to the calculation, with alkyl
chains replaced by CH3 groups. The optimized geometry,
HOMO and LUMO energy levels, and their electron density
distributions were calculated at the B3LYP/6-311þG* level
of theory50,51 using density functional theory and Gaussian
03 package (Figure 4).52
The dihedral angles between two thienyl groups with
central BT unit as well as the donor BDT unit quantita-
tively measure the steric hindrance introduced by these
alkyl chains. All three dihedral angles for PBDT-DTBT
are small (Table 3), indicative of complete conjugation of all
Figure 2. UV-vis spectra of all the polymers: (a) PBDT-DTBT and
PBDT-4DTBT polymers in chloroform solution (solid line) and in
solid film (dash line) and (b) PBDT-DTsolBT and PBDT-3DTBT
polymers in chloroform solution (solid line) and in solid film (dash line).
Figure 3. Cyclic voltammograms of the oxidation and reduction beha-
vior of thin films of PBDT-DTBT, PBDT-4DTBT, PBDT-3DTBT,
and PBDT-DTsolBT.
Table 2. Optical and Electrochemical Data of All Polymers
UV-vis absorption
CHCl3 solution
PL
film
CHCl3 solution
cyclic voltammetry
DFT
calcd
a
a
ox
onest
red
E
onest
λmax
[nm]
λonset
[nm]
Eg
[eV]
λmax
[nm]
Eg
[eV]
λmax
[nm]
E
(V)/HOMO [eV]
polymer
λonset [nm]
(V)/LUMO [eV] HOMO [eV]
PBDT-DTBT
PBDT-4DTBT
PBDT-3DTBT
PBDT-DTsolBT
550
567
475
425
738
726
560
500
1.68
1.70
2.21
2.48
666
641
520
435
761
735
628
528
1.63
1.69
1.97
2.35
628
619
654
580
0.47/-5.27
0.41/-5.21
0.58/-5.38
0.89/-5.69
-1.64/-3.16
-1.82/-2.98
-1.67/-3.13
-1.79/-3.01
-5.22
-5.19
-5.33
-5.43
a Calculated from the intersection of the tangent on the low energetic edge of the absorption spectrum with the baseline.

816 Macromolecules, Vol. 43, No. 2, 2010
Zhou et al.
Figure 4. Calculated HOMO (left) and LUMO (right) orbitals of polymers.
Table 3. Calculated Dihedral Angles of Polymersa
dihedral angle 1 dihedral angle 2 dihedral angle 3
participating aromatic units. The electron density is well
delocalized along the conjugated backbone, as displayed by
the isosurface of the HOMO energy level of PBDT-DTBT
(Figure 4). There is only a slight increase of the dihedral
angle between the 4-substituted thienyl group and BDT
unit in PBDT-4DTBT, confirming the minimum steric
hindrance introduced by the 4DTBT. The electron density
is also delocalized in the HOMO of PBDT-4DTBT, though
slightly biased toward the BDT unit compared with that
of PBDT-DTBT. These very similar;however slightly
different;HOMO isosurfaces explain that only a slight
difference (0.06 eV) was observed for the HOMO energy
levels of PBDT-DTBT and PBDT-4DTBT. Furthermore,
the electron density of LUMO energy levels of both PBDT-
DTBT and PBDT-4DTBT are essentially localized on the
DTBT unit, supporting observed similar LUMO energy
levels. All these similarities contribute to the similar UV-vis
absorptions and band gaps of PBDT-DTBT and PBDT-
4DTBT.
polymer
(deg)
(deg)
(deg)
PBDT-DTBT
PBDT-4DTBT
PBDT-3DTBT
PBDT-DTsolBT
4.1
5.2
50.7
58.0
10.9
14.3
36.2
55.2
14.1
30.2
17.7
19.9
a Calculations were carried out for one repeating unit of each poly-
mer, in gas phase, at temperature 0 K and in vacuum.
between thienyl groups and central BT unit, as shown by a
dramatic numerical increase in dihedral angles 1 and 2. For
example, over 50ꢀ angles have been calculated for the
dihedral angels between thienyl groups and BT in PBDT-
DTsolBT. This severe steric hindrance essentially breaks the
conjugation at the linkages between thienyl groups and BT,
thereby rendering the HOMO of PBDT-DTsolBT localized
at the donor BDT unit. Compared with PBDT-DTsolBT,
smaller torsional angles between thienyl groups and BT unit
are calculated in the case of PBDT-3DTBT. Therefore, the
electron density is slightly extended to the BT unit in the
HOMO of PBDT-3DTBT. Comparing the isophases of
all three polymers incorporating modified DTBT unit clearly
explains why the measured HOMO energy level of
Moving these alkyl chains away from the vicinity of BDT
unit in the case of PBDT-3DTBT and PBDT-DTsolBT
relieves the steric hindrance between substituted DTBT and
BDT unit, recovering small numbers on dihedral angle 3
(Table 3). However, greater steric hindrance is formed

Article
Macromolecules, Vol. 43, No. 2, 2010 817
PBDT-3DTBT is between that of PBDT-4DTBT and that
of PBDT-DTsolBT.
polymer-based BHJ PV cells. As discussed earlier, anchoring
solubilizing chains at 4-positions of these thienyl groups
(PBDT-4DTBT) has a minimum impact on the band gap
and energy levels, compared with those of PBDT-DTBT.
More importantly, the newly acquired solubility/processa-
bility of PBDT-4DTBT renders a much improved intermix-
ing with PCBM without severe aggregation of polymers.
Therefore, the BHJ solar cell of PBDT-4DTBT:PCBM
displays a Voc of 0.75 V (0.2 V higher than that of PBDT-
DTBT-based solar cell). The low band gap (1.69 eV) leads to
a much higher Jsc (5.92 mA/cm2), leading to an overall
efficiency of 1.83%. By applying 3% 1,8-diiodoocane as
an additive into the processing solvent (chloroform) to
modify the film morphology,53,54 a higher efficiency of
2.17% is achieved, mainly due to the noticeably increased
FF (Table 4). Adding additives appears to promote more
ordering in the polymer domains, as indicated by a ∼50 nm
red shift in the absorption maximum of blends processed
with additive (Figure 6).55 This noticeable red shift in the
absorption maximum might account for the improved Jsc of
the devices processed with additives.
Shifting alkyl chains to the inner core of DTBT introduces
significant steric hindrance between the aromatic units on
the conjugated backbone, leading to much increased band
gaps. The severe steric hindrance would also weaken the
interaction between polymer chains, leading to low hole
mobilities. Furthermore, the electron density of HOMO
energy levels of PBDT-3DTBT and PBDT-DTsolBT are
essentially localized at the BDT unit (Figure 4). The lack of
delocalization would reduce the possibility of excitons
moving to donor/acceptor interface and increase the gemi-
nate recombination of the recently dissociated excitons.
Therefore, low efficiencies were observed for both PBDT-
3DTBT (0.21%) and PBDT-DTsolBT (0.01%). The very
small efficiency of PBDT-DTsolBT is largely due to severely
disrupted conjugation between thienyl groups and the BT
unit (Table 3 and Figure 4).
Photovoltaic Properties. Bulk heterojunction photovoltaic
devices were constructed to investigate the influence on
photovoltaic properties introduced by the subtle change in
positioning alkyl chains. For fair comparison, care was taken
during the processing to maintain similarly structured de-
vices: (a) because this series of polymers have the identical
conjugated backbone with only variations on the size and
position of side chains, all polymers were blended with
PCBM at 1:1 weight ratio in chloroform at 5 mg/mL; (b)
identical spin rate (1100 rpm) and time (1 min) were em-
ployed to achieve similar film thicknesses. A typical fabri-
cated solar cell has a configuration of glass/ITO/PEDOT:
PSS(40 nm)/polymer:PCBM blend (∼100 nm)/Ca(30 nm)/
Al(70 nm) (Experimental Section). The current-voltage
characteristics of solar cells based on these four polymers
blended with PCBM are shown in Figure 5. Representative
performance parameters of solar cells are listed in Table 4.
Because of its low solubility, PBDT-DTBT was dissolved
into chloroform at high temperature (∼60 ꢀC). Polymers
noticeably aggregated when the solution cooled. Therefore,
the solution of PBDT-DTBT and PCBM was sonicated
during cooling, which precipitated the polymer, resulting in a
processable dispersion.21 Films of PBDT-DTBT:PCBM are
the thinnest (65 nm) among all the polymer:PCBM films,
which is attributed to the low viscosity of the dispersion. A
relatively low Voc of 0.55 V was observed, likely due to the
elevated HOMO energy level of PBDT-DTBT in the ag-
gregated state. Excessive aggregation would also likely lead
to form polymers-only domains so large that excitons cannot
reach a donor/acceptor interface before they decay to the
ground state.53 Thus, the measured Jsc is only 3.53 mA/cm2
despite the low band gap (1.63 eV) of PBDT-DTBT.
Together with a FF of 0.37, an overall conversion efficiency
of 0.72% is obtained for PBDT-DTBT. Not surprisingly,
attaching these alkyl chains greatly improved the solubility
of resulting polymers; however, the anchoring positions
significantly impact the photovoltaic properties of related
The BHJ devices of PBDT-DTBT and PBDT-4DTBT
were further tested for their incident photon to current
efficiency (IPCE). For comparison, the IPCE data are shown
Figure 5. Characteristic J-V curves of the optimized devices of poly-
Figure 6. Comparison of absorption of PBDT-4DTBT:PCBM (1:1)
thin films with and without additive.
mer-based BHJ solar cells under 1 Sun condition (100 mW/cm2).
Table 4. PV Performances of Polymers
polymer
polymer:PCBM
thickness (nm)
Voc (V)
Jsc (mA/cm2)
FF (%)
η (%)
PBDT-4DTBT (3% diiodooctane)
PBDT-4DTBT
1:1
1:1
1:1
1:1
1:1
95
100
85
80
65
0.67
0.75
0.89
0.43
0.55
6.38
5.92
0.94
0.12
3.53
50.79
41.27
24.74
26.35
36.8
2.17
1.83
0.21
0.01
0.72
PBDT-3DTBT
PBDT-DTsolBT
PBDT-DTBT

818 Macromolecules, Vol. 43, No. 2, 2010
Table 5. Mobility of Polymers under SCLC Conditions
Zhou et al.
polymer only
thickness (nm)
55
mobility cm2/(V s)
polymer:PCBM
thickness (nm)
mobility cm2/(V s)
3
3
PBDT-4DTBT
4.36 ꢀ 10-6
2.91 ꢀ 10-6
1:1 þ 3% diiodooctane
75
75
65
1.60 ꢀ 10-5
9.20 ꢀ 10-6
3.94 ꢀ 10-6
1:1
1:1
PBDT-DTBT
55
improving the hole mobility. In addition, using additives
appears to improve the film morphology with more ordering
in the polymer domains, which explains an even higher hole
mobility (1.60 ꢀ 10-6 cm2/(V s)). These observations signify
the importance of high molecular weight and good solubility
of polymers in improving device efficiencies.
Conclusions
It has been demonstrated that introducing alkyl chains onto
various positions of the dithienylbenzothiadiazole (DTBT) can
significantly increase the molecular weight and solubility of
related polymers. However, the anchoring positions of these
alkyl chains have a strong influence on the optical and electro-
chemical properties of these structurally similar polymers with
identical conjugated backbones. Contrary to previous reports,
our study indicates that attaching alkyl chains on the 4-positions
of these thienyl groups (i.e., 4DTBT) only introduces minimum
steric hindrance into the related D-A polymer (PBDT-
4DTBT). Therefore, PBDT-4DTBT maintains almost identical
band gap and energy levels compared with PBDT-DTBT
(unmodified DTBT). More importantly, the additional high
molecular weight and excellent solubility of PBDT-4DTBT lead
to a more uniform mixture withPCBM, with better control onthe
film morphology. All these features of PBDT-4DTBT contri-
bute to a much enhanced efficiency (up to 2.2%) of PBDT-
4DTBT, significantly higher than that of PBDT-DTBT-based
devices (0.7%). Our discovery reinforces the importance of high
molecular weight and good solubility of donor polymers for BHJ
solar cells, in addition to a low band gap and a low HOMO
energy level, in order to further enhance the device efficiencies.
Finally, we believe the strategy of “properly” modifying the
acceptor unit can be applied to other D-A polymers as well.
For example, 4DTBT can be employed in conjugation with other
fused bithiophene-based polycyclic aromatics to construct D-A
polymers with low band gap and low HOMO energy levels. If a
higher mobility together with better controlled morphology can
be achieved by these new polymers, an even higher efficiency can
be expected.
Figure 7. IPCE and absorption of PBDT-DTBT and PBDT-4DTBT
(absorption is normalized by film thickness).
together with the absorption of blended thin films (Figure 7).
These two films absorb light rather equally when the absorp-
tion is normalized by film thickness, though the PBDT-
DTBT based film has slightly more absorption in the near-IR
region. The IPCE curves follow individual film absorptions,
with maxima at 430 and 600 nm. However, the maximum
IPCE value of PBDT-4DTBT-based device is almost twice
as much as that of PBDT-DTBT-based device (29% vs 16%
at 600 nm). Since the film thickness of PBDT-4DTBT-based
device is only about 50% thicker than that of PBDT-
DTBT (100 nm vs 65 nm), these indicate that charges have
much greater chance to reach the electrodes in the case of
PBDT-4DTBT-based device, possibly due to a higher hole
mobility in PBDT-4DTBT devices.
In order to further understand the dramatically different
PV performance of these two polymers, PBDT-DTBT and
PBDT-4DTBT, hole mobility values were estimated via
space-charge limit current (SCLC) by fabricating hole-only
devices.56,57 For pure polymers, the measured mobilities are
similar for both polymers (Table 5), with PBDT-4DTBT
having slightly higher hole mobility. The hole mobility
difference is more noticeable in the polymer/PCBM blend.
The PBDT-4DTBT/PCBM blend demonstrates a much
higher mobility, double that of the PBDT-DTBT/PCBM
blend (Table 5). As indicated earlier, PBDT-DTBT has a
strong tendency to stack, which is desirable in improving
mobility within a polymer-only domain. However, these low
molecular weight polymers, i.e., short chains, limit the
interaction between different domains, leading to an overall
suppressed mobility.29 On the other hand, the properly
positioned alkyl chains offer the excellent solubility and high
molecular weight of PBDT-4DTBT. The high molecular
weight of PBDT-4DTBT (i.e., long polymer chains) would
connect locally ordered individual domains, thereby improv-
ing the hole mobility in the polymer-only devices.58 Further-
more, the good solubility of PBDT-4DTBT ensures a good
miscibility with PCBM in the processing solvent. The clus-
tering of PCBM during solvent evaporation could help
further aggregation of PBDT-4DTBT while maintaining
the connection between polymer only domains, further
Experimental Section
Reagents and Instrumentation. All reagents and chemicals
were purchased from commercial sources (Aldrich, Acros,
Strem, Fluka) and used without further purification unless
stated otherwise. Reagent grade solvents were dried when
necessary and purified by distillation. Microwave-assisted
polymerizations were conducted in a CEM Discover Benchmate
microwave reactor. Gel permeation chromatography (GPC)
measurements were performed on a Waters 2695 separations
module apparatus with a differential refractive index detector
with tetrahydrofuran (THF) as eluent. The obtained molecular
weight is relative to the polystyrene standard. Thermogravi-
metric analysis (TGA) measurements were carried out with a
PerkinElmer thermogravimetric analyzer (Pyris 1 TGA) at a
heating rate of 10 ꢀC min-1 under a nitrogen atmosphere. The
temperature of degradation (Td) is correlated to a 5% weight
loss. Differential scanning calorimetry (DSC) measurements
were carried out with a model 2920 MDSC from TA Instru-
ments under a nitrogen atmosphere. 1H nuclear magnetic
resonance (NMR) measurements were recorded either with a
Bruker Avance 300 MHz AMX or a Bruker 400 MHz DRX

Article
Macromolecules, Vol. 43, No. 2, 2010 819
spectrometer. 13C nuclear magnetic resonance (NMR) measure-
ments were carried out with a Bruker 400 MHz DRX spectro-
meter. Chemical shifts were expressed in parts per million
(ppm), and splitting patterns are designated as s (singlet),
d (doublet), and m (multiplet). Coupling constants J are re-
ported in hertz (Hz).
cathode at a pressure of ∼1 ꢀ 10-6 mbar. There are eight devices
per substrate, with an active area of 12 mm2 per device. Device
characterization was carried out under AM1.5G irradiation
with the intensity of 100 mW/cm2 (Oriel 91160, 300 W) cali-
brated by a NREL certified standard silicon cell. Current versus
potential (I-V) curves were recorded with a Keithley 2400
digital source meter. EQE were detected under monochromatic
illumination (Oriel Cornerstone 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 fabrication 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 (40 nm)/copolymer-PCBM/Pd (50 nm)
were fabricated. The experimental dark current densities J of
polymer:PCBM blends were measured when applied with vol-
tage 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 resis-
tance, 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 Information), hole mobilities of copolymers can be
deduced from
Electrochemistry. Cyclic voltammetry measurements were
carried out using a Bioanalytical Systems (BAS) Epsilon poten-
tiostat equipped with a standard three-electrode configuration.
Typically, a three-electrode cell equipped with a glass carbon
working electrode, a Ag/AgNO3 (0.01 M in anhydrous acet-
onitrile) reference electrode, and a Pt wire counter electrode was
employed. The measurements were done in anhydrous acetoni-
trile with tetrabutyl ammonium 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 chloro-
form solution and dried under house nitrogen stream prior to
measurements. The electrochemical onsets were determined at
the position where the current starts to differ from the base-
line. The potential of Ag/AgNO3 reference electrode was
internally calibrated by using the ferrocene/ferrocenium redox
couple (Fc/Fcþ), which has a known reduction potential
of -4.8 eV.59,60 The highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels of copolymers were calculated from the onset
oxidation potentials (Eoonxest) and onset reductive potentials
(Eroendest), respectively, according to eqs 1 and 2. The electroche-
mically determined band gaps were deduced from the difference
between onset potentials from oxidation and reduction of
copolymers as depicted in eq 3.
9
V2
h L3
J ¼ εrε0μ
8
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.
ox
onset
HOMO ¼ -ðE
LUMO ¼ -ðE
þ 4:8Þ ðeVÞ
ð1Þ
ð2Þ
ox
onset
þ 4:8Þ ðeVÞ
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 Office of Naval Research (Grant
N000140911016).
EC
gap
ox
onset
red
E
¼ ðE
-E
onset
Þ
ð3Þ
Spectroscopy. UV-vis absorption spectra were obtained by a
Shimadzu UV-2401PC spectrophotometer. Fluorescence spec-
tra were recorded on a Shimadzu RF-5301PC spectrofluoro-
photometer. For the measurements of thin films, polymers were
spun-coated onto precleaned glass slides from 10 mg/mL poly-
mer solutions in chloroform. The thicknesses of films were
recorded by a profilometer (Alpha-Step 200, Tencor In-
struments).
Supporting Information Available: Synthesis of all mole-
cules; H spectra, fluorescence spectra, and DSC curves of all
1
0.5
polymers; 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.
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
ITO pattern had a resistivity of 15 Ω/0. Prior to use, the
substrates were ultrasonicated for 20 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 30 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 w/w, 10 mg/mL for poly-
mers) was dissolved in chloroform with heating at 60 ꢀC
for 6 h. All the solutions (except in the case of PBDT-DTBT,
which has a poor solubility) were filtered through a 0.45 μm
poly(tetrafluoroethylene) (PTFE) filter and spun-cast at 1100
rpm for 60 s onto PEDOT:PSS layer. The substrates were then
dried at room temperature in the glovebox under a nitrogen
atmosphere for 12 h. The thicknesses of films were recorded by a
profilometer (Alpha-Step 200, Tencor Instruments). The de-
vices were finished for measurement after thermal deposition of
a 30 nm film of calcium and a 70 nm aluminum film as the
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