Terthiophene-Based D-A Polymer with an Asymmetric Arrangement of Alkyl Chains That Enables Efficient Polymer Solar Cells

Article abstract of DOI:10.1021/jacs.5b08556

We report a series of difluorobenzothiadizole (ffBT) and oligothiophene-based polymers with the oligothiophene unit being quaterthiophene (T4), terthiophene (T3), and bithiophene (T2). We demonstrate that a polymer based on ffBT and T3 with an asymmetric arrangement of alkyl chains enables the fabrication of 10.7% efficiency thick-film polymer solar cells (PSCs) without using any processing additives. By decreasing the number of thiophene rings per repeating unit and thus increasing the effective density of the ffBT unit in the polymer backbone, the HOMO and LUMO levels of the T3 polymers are significantly deeper than those of the T4 polymers, and the absorption onset of the T3 polymers is also slightly red-shifted. For the three T3 polymers obtained, the positions and size of the alkyl chains play a critical role in achieving the best PSC performances. The T3 polymer with a commonly known arrangement of alkyl chains (alkyl chains sitting on the first and third thiophenes in a mirror symmetric manner) yields poor morphology and PSC efficiencies. Surprisingly, a T3 polymer with an asymmetric arrangement of alkyl chains (which is later described as having an "asymmetric bi-repeating unit") enables the best-performing PSCs. Morphological studies show that the optimized ffBT-T3 polymer forms a polymer:fullerene morphology that differs significantly from that obtained with T4-based polymers. The morphological changes include a reduced domain size and a reduced extent of polymer crystallinity. The change from T4 to T3 comonomer units and the novel arrangement of alkyl chains in our study provide an important tool to tune the energy levels and morphological properties of donor polymers, which has an overall beneficial effect and leads to enhanced PSC performance.

Full text of DOI:10.1021/jacs.5b08556

Article
pubs.acs.org/JACS
Terthiophene-Based D−A Polymer with an Asymmetric Arrangement
of Alkyl Chains That Enables Efficient Polymer Solar Cells
†
†
,§,#
†,#
†
†
‡,§
†
,‡
†
,†,⊥
†
Huawei Hu,
Kui Jiang, Guofang Yang, Jing Liu, Zhengke Li, Haoran Lin, Yuhang Liu,
∥
∥
Jingbo Zhao, Jie Zhang, Fei Huang, Yongquan Qu, Wei Ma,* and He Yan*
†Department of Chemistry and Energy Institute, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,
Hong Kong
‡
§
State Key Laboratory for Mechanical Behavior of Materials, and Center for Applied Chemical Research, Frontier Institute of Science
and Technology, Xi’an Jiaotong University, Xi’an 710049, China
∥
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South
China University of Technology, Guangzhou 510640, People’s Republic of China
⊥
HKUST-Shenzhen Research Institute, No. 9 Yuexing first RD, Hi-tech Park, Nanshan, Shenzhen 518057, China
S Supporting Information
*
ABSTRACT: We report a series of difluorobenzothiadizole (ffBT) and
oligothiophene-based polymers with the oligothiophene unit being
quaterthiophene (T4), terthiophene (T3), and bithiophene (T2). We
demonstrate that a polymer based on ffBT and T3 with an asymmetric
arrangement of alkyl chains enables the fabrication of 10.7% efficiency thick-
film polymer solar cells (PSCs) without using any processing additives. By
decreasing the number of thiophene rings per repeating unit and thus
increasing the effective density of the ffBT unit in the polymer backbone, the
HOMO and LUMO levels of the T3 polymers are significantly deeper than
those of the T4 polymers, and the absorption onset of the T3 polymers is
also slightly red-shifted. For the three T3 polymers obtained, the positions
and size of the alkyl chains play a critical role in achieving the best PSC
performances. The T3 polymer with a commonly known arrangement of
alkyl chains (alkyl chains sitting on the first and third thiophenes in a mirror
symmetric manner) yields poor morphology and PSC efficiencies. Surprisingly, a T3 polymer with an asymmetric arrangement of
alkyl chains (which is later described as having an ”asymmetric bi-repeating unit”) enables the best-performing PSCs.
Morphological studies show that the optimized ffBT-T3 polymer forms a polymer:fullerene morphology that differs significantly
from that obtained with T4-based polymers. The morphological changes include a reduced domain size and a reduced extent of
polymer crystallinity. The change from T4 to T3 comonomer units and the novel arrangement of alkyl chains in our study
provide an important tool to tune the energy levels and morphological properties of donor polymers, which has an overall
beneficial effect and leads to enhanced PSC performance.
INTRODUCTION
polymers that enable the fabrication of high-efficiency PSCs.
Wei You and co-workers reported that fluorination of the
benzothiadizole unit effectively lowered both the highest
occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) levels of the
■
Polymer solar cells (PSCs) based on conjugated polymers and
fullerene derivatives have been extensively investigated due to
their unique advantages, including mechanical flexibility, light
weight, large area and low cost fabrications.
decade, great efforts have been spent to maximize the
1
−4
Over the past
27,28
polymers,
and LUMO levels could be finely tuned by the density of
and that the extent of the shift in the HOMO
5
−15
performance of PSCs;
tuning of the energy levels and optical band gaps of the donor
an important approach has been
29
fluorination on the backbone of the polymer. A similar
electronic effect upon fluorination was also observed for the
PTB7 polymer family with the HOMO and LUMO levels of
polymers using a donor−acceptor (D−A) copolymer strat-
1
6−21
egy.
In addition, the selection of proper alkyl chains is
11,30,31
the polymers significantly reduced upon fluorination.
We
essential because they have great influence on the molecular
weights, intermolecular interactions, charge transport, and
polymer:fullerene morphology, which all play significant roles
recently reported a D−A copolymer based on ffBT and
quaterthiophene (T4), named PffBT4T-2OD, that in several
22−26
in the performance of PSCs.
Difluorobenzothiadizole (ffBT)-based D−A copolymers have
emerged as one of the most successful families of donor
Received: August 13, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b08556
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
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Figure 1. (a) Chemical structures of the repeating units for T2-, T3-, and T4-based polymers. (b−g) Illustration of possible arrangements of alkyl
chains on the oligothiophene units.
Scheme 1. Synthetic Routes to the Polymers Presented in This Study
cases allowed the fabrication of high-efficiency (>10%) PSCs
when the polymer was combined with different fullerenes. The
RESULTS AND DISCUSSION
■
5
Materials Synthesis and Optoelectronic Properties.
PffBT4T-2OD polymer features second-position branched alkyl
chains (2OD) sitting between the thiophenes in the T4 unit.
Such a T4-2OD structure motif introduces a highly temper-
ature-dependent aggregation property (for the PffBT4T-2OD
polymer) that can be used to achieve a near-optimum
polymer:fullerene morphology and highly efficient PSCs. It is
important to note that most high-efficiency PSCs were achieved
on the basis of a solution containing an additive named 1,8-
Density functional theory (DFT) calculations were carried out
on several polymer backbones (Figure 1a) that have an
increasing density of the ffBT units. The calculated HOMO and
LUMO levels are −4.82 and −2.98, −4.88 and −3.02, and
−4.96 and −3.11 eV, for the T4, T3, and T2 polymers,
respectively. The results of these DFT calculations indeed
support that deeper HOMO and LUMO levels can be obtained
as the density of the ffBT units increases.
5
,8,11,32
Motivated by the positive results of the DFT calculation,
several ffBT polymers based on T4, T3, and T2 units were
synthesized. To obtain polymers that are soluble and easily
processable for the fabrication of solar cells, it is important to
attach branched alkyl chains on the beta positions of the
diiodooctane,
which is a toxic and persistent (due to its
high boiling point) chemical that is undesirable in industry
processes. The PSC community is actively looking for an
approach to achieve high-efficiency PSCs without using any
additives.
33
thiophene rings. The use of second-position branched alkyl
chains between the thiophene rings is critical, because they are
the key structural feature that enables the highly temperature-
In this work, we report a D−A polymer based on ffBT and
terthiophene with an asymmetric arrangement of alkyl chains
that enables the fabrication of highly efficient PSCs up to 10.7%
without any additives. The main design rationale for the new
polymers is to increase the beneficial electronic effects of ffBT
units by reducing the number of thiophene rings per repeating
unit and thus increasing the effective density of ffBT units in
the polymer backbone. This should enable further decrease in
the HOMO and LUMO levels of the ffBT-oligothiophene
polymers.
5
dependent aggregation property of the polymers. Several
possible arrangements of alkyl chains are illustrated in Figure
1
b−g. For the positioning of the alkyl chains, the first principle
is to avoid putting two alkyl chains on the “head-to-head”
positions of two adjacent thiophenes.
34,35
For example, when
positions 1 and 2 in the T2 polymer (Figure 1b) are attached
with branched alkyl chains, the two thiophene units will be
strongly twisted due to the steric hindrance effect caused by the
B
DOI: 10.1021/jacs.5b08556
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Article
two head-to-head alkyl chains (shown in Figure 1b). Therefore,
the T2 polymer has only one possible alkyl chain arrangement
which correspond to the T4, T3, and T2 polymers, respectively.
Note that PffBT-T4-2HD and PffBT-T3(1,2)-1 were selected
to compare the differences in the properties between the T4
and T3 polymers, because PffBT-T4-2HD and PffBT-T3(1,2)-
1 have the same 2HD alkyl chains and allow a fair comparison
between the solubility properties of the T4 and T3 polymers.
The optical bandgaps and energy levels of the investigated
polymers (summarized in Table 1) were estimated on the basis
(Figure 1c). The common approach for the T4 polymer is to
attach alkyl chains on the beta positions of the first and fourth
thiophenes and allow the alkyl chains to point inside toward the
second and third thiophene rings in a C2 symmetric manner
(Figure 1g). For the T3 polymers, there are three possible
arrangements of alkyl chains on the terthiophene unit as shown
in Figure 1d−f. Among these three possible arrangements, a
reasonable and commonly used arrangement is to attach two
alkyl chains on the first and third thiophene units in a mirror
symmetric manner with reference to the T3 unit (Figure 1d), as
Table 1. Molecular Weight and Electrochemical Properties
of Polymers
36,37
M
n
M
w
E
HOMO
(eV)
E
g,opt
(eV)
E
LUMO
c
reported in the literature.
Another possibility is to attach
a
b
polymer
(kDa)
(kDa)
(eV)
the alkyl chains on the first and second thiophene units in a
head-to-tail manner (Figure 1e) to form an asymmetric T3 unit.
This type of head-to-tail arrangement of alkyl chains is not
commonly seen in D−A-type donor polymers, but was used to
construct small molecule donors that yielded organic solar cells
with high fill factor (FF) and PCEs. Surprisingly, we found
that this unusual arrangement of alkyl chains (Figure 1e)
enabled a completely different and more favorable polymer:-
fullerene morphology and thus dramatically enhanced the
performance of the PSCs as compared to the reported
approach of symmetric arrangement of alkyl chains on the
T3 unit as shown in Figure 1d. (Note that the polymer in
Figure 1f was not successfully synthesized because the
thiophene monomer substituted with two long-branched alkyl
chains is extremely difficult to synthesize and purify.)
PffBT-T4-2HD
PffBT-T3(1,3)
PffBT-T3(1,2)-1
PffBT-T3(1,2)-2
PffBT-T2
13.9
37.7
72.2
66.1
58.4
25.8
56.0
−5.21
−5.35
−5.31
−5.31
−5.38
1.65
1.60
1.63
1.63
1.61
−3.56
−3.75
−3.68
−3.68
−3.77
129.0
109.6
98.0
38
a
b
Measured by cyclic voltammetry. Estimated on the basis of film
absorption onset. Calculated by using HOMO and Eg,opt.
c
The synthesis of the new polymers involved in this study is
illustrated in Scheme 1. The T4 polymer and the T3 polymer
with alkyl chains on the first and third thiophenes (PffBT-
T3(1,3)) can be synthesized via a commonly used synthetic
route from the dibromide of 5,6-difluoro-4,7-bis(thiophen-2-
yl)-2,1,3-benzothiadiazole (T-ffBT-T) and the distannyl
reagent of thiophene or bithiophene. However, the synthesis
of the T2 polymer or the other T3 polymers must be
performed via a different route involving the distannyl reagent
of T-ffBT-T. Note that each repeating unit of the T4 and
T3(1,3) polymers contains only one ffBT unit, whereas the
repeating units of the T2 and T3(1,2) polymers contain two
ffBT units that are not chemically equivalent due to the
different positions of the alkyl chains relative to those of the
ffBT units. In some sense, the T3(1,2) polymers can be
described as having an “asymmetric bi-repeating unit”.
Figure 2. Normalized UV−vis absorption spectra of polymer films.
of the film UV−vis absorption spectra (Figure 2) and cyclic
voltammetric measurements (Figure S3). As shown in Figure 2,
all of the polymers exhibited an absorption peak at ∼640 nm,
while PffBT-T4-2HD and PffBT-T2 showed another absorp-
tion peak at about 693 and 713 nm, which originate from the
strong polymer aggregation, as demonstrated in a previous
Regarding the size of the alkyl chains, the common choices of
second branched alkyl chains include 2HD (2-hexyldecyl,
C6C10), 2OD (2-octyldodecyl, C8C12), and 2DT (2-
decyltetradecyl, C10C14). The general guideline of the choice
of alkyl chain is to minimize the size of the branched alkyl
chains, because unnecessarily long alkyl chains may cause many
negative effects, including a reduced absorption coefficient, a
5
report. On the other hand, when the oligothiophene unit
changes from T4 to T3 then to T2, the effective density of ffBT
unit is increased, and the electronic effect of the ffBT unit is
thus enhanced, which led to lower HOMO and LUMO levels
and also red-shifted absorption. This is the reason that PffBT-
T2 exhibits red-shifted absorption compared to PffBT-T4-
2HD. Similar observations on the different properties of
polymers based on odd and even oligothiophene units have
5
,39,40
lower domain purity, and lower efficiency for PSCs.
At the
same time, it is necessary to use an alkyl chain that is sufficiently
large to provide sufficient solubility for the polymer. For the T3
polymers, the choices of alkyl chains are 2HD and 2HN (2-
hexylnonyl, C6C9). For the T4 polymers, the choices of alkyl
chains are 2HD and 2OD. However, the T2 polymers with
41
been reported for isoindigo polymers. The T3 polymers
indeed exhibit deeper HOMO and LUMO levels than the T4
polymers, which is consistent with the shift in HOMO levels
predicted by the DFT calculations and with the change in the
open circuit voltages (VOC) of the corresponding PSCs (which
will be shown later). A deeper HOMO level and smaller
bandgaps should help to achieve higher VOC and larger short-
2HD, 2OD, and even 2DT alkyl chains exhibit extremely poor
solubility. Therefore, an especially long alkyl chain of 2TH (2-
tetradecylhexadecyl, C14C16) must be incorporated in the T2
polymer structure to obtain a polymer that is possible to
process. In this work, we first examine the differences in the
properties of PffBT-T4-2HD, PffBT-T3(1,2)-1, and PffBT-T2,
circuit current (J ) for the PSCs. It is important to note that
SC
the LUMO level of the T2 polymer is −3.77 eV, which might
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Figure 3. Illustration of the orientations of alkyl chains on the PffBT-T4-2HD (a) and PffBT-T3(1,2)-1 (b) polymers.
a
Table 2. Photovoltaic Properties of PSCs Based on Polymers/PC BM
7
1
μ (cm V s−1
2
−1
)
VOC (V)
JSC (mA cm−2
)
FF (%)
PCE (%)
polymer
h
1.3 × 10−
6.7 × 10
1.2 × 10−
3
0.73 ± 0.01
0.77 ± 0.01
0.82 ± 0.01
0.82 ± 0.01
0.88 ± 0.01
0.77 ± 0.01
14.8 ± 0.2
10.6 ± 0.3
18.5 ± 0.2
18.7 ± 0.2
10.1 ± 0.3
18.2 ± 0.2
64.4 ± 0.6
45.3 ± 0.9
64.1 ± 0.7
68.3 ± 0.5
48.6 ± 0.5
73.2 ± 0.7
7.0 ± 0.2
3.7 ± 0.2
9.7 ± 0.3
10.5 ± 0.2
4.3 ± 0.2
10.2 ± 0.3
PffBT-T4-2HD
PffBT-T3(1,3)
PffBT-T3(1,2)-1
PffBT-T3(1,2)-2
PffBT-T2
−
4
3
3
4
2
−
2.4 × 10
8.0 × 10−
1.3 × 10−
PffBT-T4-2OD
a
The values for VOC, J , and FF are the averages of about 15 devices.
SC
a
Table 3. Summary of GIWAXS Data
(
100) d spacing (Å)
(100) coherence length
(010) d spacing (Å)
[±0.01]
(010) coherence length
percentage of face-on area
materials
[±0.01]
(Å) [±3.0]
(Å) [±0.7]
(%) [±2]
PffBT-T4-2HD:PC BM
18.18
21.61
19.81
19.27
24.09
22.22
170.4
159.1
80.1
3.50
3.47
3.67
3.65
3.48
3.55
78.4
48.4
31.9
39.7
75.8
78.5
0
68
71
PffBT-T3(1,3):PC BM
71
PffBT-T3(1,2)-1:PC BM
100
100
100
100
71
PffBT-T3(1,2)-2:PC BM
85.1
71
PffBT-T2:PC BM
129.1
235.4
71
PffBT-T4-2OD:PC BM
71
a
The values in brackets represent typical errors of the measurements.
be too deep to provide a sufficient LUMO offset with PCBM
LUMO, −4.0 eV), because it is commonly believed that a
LUMO offset of 0.3 eV is needed to ensure highly efficient
addition, the temperature-dependent aggregation properties of
the T3 and T4 polymers are also compared. It can be clearly
seen in Figure S1 that the PffBT-T4-2HD polymer exhibits a
strong absorption peak at 693 nm at room temperature, which
indicates strong polymer aggregation in solution at room
temperature. The PffBT-T3(1,2)-1 does not exhibit such an
aggregation peak, indicating much weaker aggregation of the
PffBT-T3(1,2)-1 polymer in solution.
(
42,43
exciton dissociations for polymer:fullerene cells.
Polymer Solubility and Crystallinity. The following
discussions mainly focus on the comparison of the T3 and
T4 polymers, both of which exhibit reasonably good PSC
performance. The properties and performance of the T2
polymer will be explained separately in a later paragraph.
The greater solubility of the PffBT-T3(1,2)-1 as compared to
the PffBT-T4-2HD polymer is consistent with its lower
crystallinity and weaker aggregation. It has been commonly
observed in polymer semiconductors that polymers with greater
crystallinity often exhibit lower solubility because stronger π−π
stacking between polymer chains makes it more difficult to
(
Regarding the T2 polymer, it exhibits an extremely poor
solubility as compared to the T3 and T4 polymers, because it
has only one branched alkyl chain for each ffBT repeating unit
and the T3 and T4 polymers have two branched alkyl chains for
each repeating unit. The extremely poor solubility of the T2
polymer partially contributes to the poor PSC performance of
the PffBT-T2-based devices.) Comparing the T3 and T4
polymers with the same 2HD alkyl chains (PffBT-T4-2HD and
PffBT-T3(1,2)-1), the PffBT-T3(1,2)-1 polymer exhibits a
44,45
dissolve the polymers.
From the perspective of chemical
structure, it is also reasonable for the PffBT-T4-2HD polymer
to have greater crystallinity and stronger lamellar packing
because the quaterthiophene comonomer (with two 2HD alkyl
chains) has a C2 symmetry, which allows the PffBT-T4-2HD
polymer to form a regioregular polymer structure. As shown in
Figure 3a, the 2HD alkyl chains have a regular and parallel
arrangement, which can help the interdigitation of the alkyl
chains along the lamellar packing direction. The role of C2
symmetric monomers in the formation of regioregular polymer
significantly enhanced solubility. The M of PffBT-T4−2HD is
n
only 14 kDa; yet it could not be dissolved in hot toluene but is
only soluble in hot chlorobenzene. In contrast, the PffBT-
T3(1,2)-1 polymer with the same 2HD alkyl chains can be
readily dissolved in toluene even though the polymer molecular
weight is 5 times higher. The PffBT-T4-2HD polymer is found
to be significantly more crystalline by grazing incidence wide-
angle X-ray scattering (GIWAXS). As shown later, the (100)
coherence length (CL) of PffBT-T4-2HD is about 17 nm,
which is double that for the PffBT-T3(1,2)-1 polymer. In
5
,45−47
structures is well described in the literature.
In contrast,
the T3 unit is not C2 symmetric, and the 2HD alkyl chains on
the PffBT-T3(1,2)-1 are not all parallel, which contributes to
the lower extent of crystallinity of the PffBT-T3(1,2)-1
D
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2
Figure 4. (a) Current−voltage plots under illumination with AM 1.5G solar simulated light (100 mW/cm ) and (b) EQE spectra of the BHJ solar
cells with PC BM.
71
polymer. As a result, the extent of alkyl chain interdigitation is
significantly stronger for the PffBT-T4-2HD polymer than for
the PffBT-T3(1,2)-1 polymer, as evidenced by the small
lamellar stacking distance of the PffBT-T4-2HD polymer film
undesirable DIO additive in the active layer processing. These
results clearly indicate the beneficial effects of reducing the
number of thiophenes and increasing the effective density of
the ffBT units.
(
data summarized in Table 3).
Photovoltaic Performance. The performance of the PSC
Morphological Characterization and Correlation to
Device Performances. Comparison of T3- and T4-Based
Polymers. To further understand the performance differences
between polymers in this study, GIWAXS and resonant soft X-
devices was investigated using an inverted device structure
(
current density−voltage curves of the optimized devices
based on these five materials are presented in Figure 4a. For
the T4 polymer with 2HD alkyl chains (PffBT-T4-2HD), a
ITO/ZnO/polymer:PC BM/V O /Al). The characteristic
7
1
2
5
48,49
ray scattering (RSoXS)
were used to reveal the relationship
between device performance and the polymer:fullerene
morphology. First, the morphology and the poor performance
of PffBT-T4-2HD were investigated. For the PffBT-T4-2HD
polymer, although it exhibits better interdigitation and higher
crystallinity than the PffBT-T3(1,2) polymers, the low face-on
percentage of the PffBT-T4-2HD:PC BM blend should reduce
PCE of 7.0% (7.2% max) with a V of 0.73 V, a JSC of 14.8 mA
OC
−2
cm , and a FF of 64.4% was obtained. The PffBT-T4-2HD
polymer has very poor solubility, which makes it difficult to
process and to obtain good device performance. Previous
studies have shown that the optimal choice of alkyl chain for
the T4 polymer is 2OD and that it is important to obtain
reasonably high molecular weights to achieve high-efficiency
devices. The performance of the T4 polymer with 2OD alkyl
chains has been reported and listed in Table 2 (the average
efficiency of PffBT-T4-2OD:PC BM devices is 10.2%). In
comparison, the T3 polymer exhibits significantly enhanced
solubility. As a result, even the T3 polymer with the 2HD alkyl
chain exhibits excellent solubility for device processing. For
PffBT-T3(1,2)-1, a VOC as high as 0.82 V was observed, and
combined with its high JSC of 18.5 mA cm and FF of 64.1%, a
high PCE of 9.7% (10.0% max) was obtained. As the solubility
of PffBT-T3(1,2)-1 is still more than sufficient, the size of the
alkyl chains on PffBT-T3(1,2)-1 could be further reduced to
minimize the negative effects of alkyl chains. For this reason, a
T3 polymer with a combination of 2HD and 2-hexylnonyl
71
the charge transport ability of the polymer in the vertical
direction, which hurts the mobility of the polymer despite its
high crystallinity. As a result, the hole mobility of the polymer is
1.3 × 10− cm V s , which is even lower than the PffBT-
T3(1,2) polymers. Moreover, the PffBT-T4-2HD polymer has
very poor solubility, which makes it difficult to process and to
obtain good device performance. Our R-SoXS data also show
that the average domain purity of the PffBT-T4-2HD:PC BM
3
2
−1 −1
7
1
7
1
blend is ∼78% of that in the PffBT-T4-2OD:PC BM film. The
71
low domain purity also contributes to the lower FF of PffBT-
−2
T4-2HD:PC BM-based cells. Next, the morphology of the T3
71
polymers was studied and compared to that of the T4 polymers
(PffBT-T4-2HD or PffBT-T4-2OD). The change of the
oligothiophene unit from T4 to T3 has both positive and
negative effects on the polymer:fullerene morphology. The
negative effect is that the two PffBT-T3(1,2) polymers are
significantly less crystalline than the T4-based polymers, as
explained in an earlier paragraph. The positive effect is that the
median domain sizes (estimated by RSoXS, Figure 5) of the
polymer:fullerene blends based on the T3 polymers are
reduced from 40 nm for PffBT-T4-2OD to about 31 nm for
PffBT-T3(1,2)-1 and 23 nm for PffBT-T3(1,2)-2. The reduced
domain size should be beneficial for the performance of PSCs
because the 40 nm domain size of PffBT-T4-2OD is slightly
larger than the generally accepted optimal domain size (∼20
(
2HN) alkyl was synthesized (structure shown in Scheme 1).
Devices based on PffBT-T3(1,2)-2 showed a PCE of 10.5%
(
6
−2
10.7% max) with VOC = 0.82 V, JSC = 18.7 mA cm , and FF =
8.3%. Both the JSC and the FF are better for the cells based on
PffBT-T3(1,2)-2 than for the cells based on PffBT-T3(1,2)-1.
The VOC of the T4-polymer-based cells is 0.77 V, and that of
the cells based on T3 polymers is about 0.82 V; this difference
can be attributed to the deeper HOMO level of the T3
polymers. In addition, the onset of the absorption and external
quantum efficiency (EQE) spectra (Figure 4b) of the T3
polymers is red-shifted by ∼10 nm as compared to that of the
T4 polymers, which contributes to the enhanced J of the
PffBT-T3(1,2)-2 cells. Importantly, the high performance of
the PffBT-T3(1,2)-2-based cells was achieved without using the
50−52
nm) for PSCs.
On the other hand, the reduced extent of
polymer crystallinity led to slightly lower FF (68.3%) for the
PSCs based on T3 polymers than for their T4 counterparts
(73.2%). Still, the FF of 68.3% for PffBT-T3(1,2)-2 is an
impressive value, as the thickness of the active layer is relatively
SC
E
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Journal of the American Chemical Society
Article
of PSCs not only depends on the HOMO level of the polymer,
but also other factors such as radiative and nonradiative
54
recombination loss. As the PffBT-T3(1,3):PC BM blend
7
1
exhibits an excessively large domain size, the recombination loss
in the PffBT-T3(1,3):PC BM blend could be significantly
71
larger than that of the PffBT-T3(1,2)-1 (or -2):PC BM blend.
7
1
This could an important reason why the VOC of PffBT-
T3(1,3):PC BM cell is lower than that estimated by its
71
HOMO level. PffBT-T3(1,3) exhibits greater crystallinity than
the other two T3 polymers, as shown by its large (010) and
(
100) CL. However, the hole mobility of PffBT-T3(1,3) is the
−4
2
−1 −1
worst (6.7 × 10 cm V s ), because it does not have a
preferential face-on orientation like those of the other two T3
polymers. In contrast, both PffBT-T3(1,2)-1 and PffBT-
T3(1,2)-2 films exhibit a (010) peak that is preferentially
located in an out-of-plane direction. These negative morpho-
logical features can largely explain the poor PSC performance
of cells based on PffBT-T3(1,3).
Figure 5. R-SoXS data of polymer:PC BM blend films.
71
Our comparison of the structures, morphology, and
performance of the other two T3 polymers PffBT-T3(1,2)-1
and PffBT-T3(1,2)-2 also provide interesting results. While
PffBT-T3(1,2)-1 has two 2HD alkyl chains, PffBT-T3(1,2)-2
has one 2HD alkyl chain and one slightly shorter 2HN alkyl
chain on each terthiophene repeating unit. The design rationale
of PffBT-T3(1,2)-2 is to slightly reduce the alkyl chains and the
solubility of PffBT-T3(1,2)-1 and thus to increase the
crystallinity and hole mobility of the polymer. Indeed, the
GIWAXS data show that the (010) CL of PffBT-T3(1,2)-2
increases to 3.97 nm, whereas that of PffBT-T3(1,2)-1 is 3.19
nm. The (010) d-spacing of PffBT-T3(1,2)-2 is also slightly
reduced from 3.67 to 3.65 Å. The lamellar stacking of PffBT-
T3(1,2)-2 is also stronger than that of PffBT-T3(1,2)-1; the
(100) CL increases from 8.01 to 8.51 nm, and the (100) d-
spacing decreases from 19.81 to 19.27 nm. The SCLC hole
thick (∼250 nm). It is well-known that achieving high FFs for
thick-film PSCs is quite challenging. The lower FF of PffBT-
T3(1,2)-2-based cells is consistent with its reduced space
charge limited current (SCLC) hole mobility in comparison
with that of PffBT-T4-2OD. As shown in Table 2, the SCLC
hole mobilities of PffBT4T-2OD and PffBT3T(1,2)-2 are about
.2 × 10− and 2.4 × 10 cm
2
−3
2
V
−1
s , respectively.
−1
1
Nevertheless, the overall PSC performance of the devices
based on PffBT-T3(1,2)-2 is improved because the benefits of
an increased VOC, widened absorption ranges, and a reduced
domain size outweigh the loss in FF due to the weaker polymer
2
8,45
crystallinity.
As shown in Figure 4b, the EQE spectrum of
PffBT3T(1,2)-2-based devices is red-shifted by ∼10 nm as
compared to that of PffBT-T4-2HD, which can be attributed to
the red-shifted absorption onset of PffBT3T(1,2)-2 (761 nm
versus 752 nm for PffBT-T4-2HD). The results show that a
change of the comonomer structure from T4 to T3 provides an
important tool to optimize the energy levels and the
morphology of the polymer:fullerene blends, which results in
an overall beneficial effect that led to enhanced PSC efficiencies
−3
2
−1
mobility of PffBT-T3(1,2)-2-based cells is 2.4 × 10 cm V
−1
s , which is double that of the cells based on PffBT-T3(1,2)-1
(1.2 × 10− cm V s ). Although the greater crystallinity of
PffBT-T3(1,2)-2 than PffBT-T3(1,2)-1 is the expected result of
the reduced alkyl chain size, it is surprising to observe that the
domain size of polymer:fullerene blends is significantly smaller
for PffBT-T3(1,2)-2 (23.5 nm) than for PffBT-T3(1,2)-1 (31
nm). The reduced domain size of PffBT-T3(1,2)-2 is possibly
due to the fact that the polymer contains a mixture of 2HD and
2HN alkyl chains, which appears to have a positive effect on
reducing the domain size of polymer:fullerene blends. In
addition, the topography of the polymer:fullerene films is
investigated by atomic force microscopy (Figure S2); it reveals
that the blend films exhibit a smooth surface with reasonable
domain sizes except for PffBT-T3(1,3):PC71BM, which is in
agreement with the RSoXS data.
3
2
−1 −1
48
even without using the DIO additive. Previous study has
shown that the use of DIO is critical for PTB7:PC BM-based
7
1
materials to achieve small domain sizes of ∼20 nm that is
optimal for PSC operation. In our study, however, the blend of
PffBT-T3(1,2)-2:PC BM exhibits an optimal domain size
7
1
without using the DIO additive in the solution.
Comparison between the T3 Polymers. It is important that
the three T3 polymers can achieve significantly different PSC
performance despite the relatively small differences in their
chemical structures. In a previous study, a series of T3-based
53
polymers were investigated, but the alkyl chains of the
reported T3 polymers were arranged in a symmetric manner on
the T3 unit. In our study, the T3 polymer (PffBT-T3(1,3))
with a known and symmetric arrangement of alkyl chains
exhibits the worst performance. In such an arrangement, the
two 2HD alkyl chains are pointed toward each other in a head-
to-head manner with only one thiophene spacer. This could
resulte in a significant steric hindrance effect and thus limit the
Comparison between T2 polymer with T3/T4 polymers.
For the T2-based polymer, its PSC performance is significantly
worse than that of the best T3 and T4 polymers. First, the
solubility of the T2-based polymers is extremely poor, because
there is only one alkyl chain for each ffBT unit. As a result,
when 2HD or 2OD alkyl chains were used on the T2 polymer
backbone, the solubility of the obtained polymers is too poor to
be soluble in hot chlorobenzene. Even when a much longer
2DT alkyl chain is used on the T2 polymer backbone, the
polymer obtained is only slightly soluble in boiling
molecular weight of the polymer. The M of PffBT-T3(1,3) is
n
37.7 kDa, which is significantly lower than that of the other two
T3-based polymers. It is also found that the polymer:fullerene
domain size (estimated by RSoXS) for PffBT-T3(1,3) is 165
nm, which is excessively large for optimal PSC operation. It is
noted that the VOC of PffBT-T3(1,3) is slightly lower than that
of PffBT-T3(1,2)-1. It has been generally accepted that the VOC
chlorobenzene. The M of the T2 polymer with 2DT alkyl
n
chains is only 5 kDa, because the molecular weight is limited by
the poor solubility of the polymer. As a result, an excessive long
F
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Journal of the American Chemical Society
Article
alkyl chain (2TH) is used on the T2 polymer backbone to
obtain a polymer that has reasonably high molecular weight and
good solubility. In a previous report, excessively long alkyl
polymer can be processed at 110 °C, but the higher molecular
weight polymer requires the solution and substrate be
preheated at 130−140 °C (the substrate also needs to attached
onto a preheated metal chuck and spun together on the spin-
coater; the preheated metal chuck serves as a “heat reservoir” to
slow the cooling of the substrate during the spin-coating
process) to obtain a workable active layer film. The poor
solubility of the high molecular weight PffBT-T4-2HD polymer
led to lower PSC efficiencies of 6.2% (6.4% max).
5
chains have been shown to have many negative effects such as
impure domains and poor lamellar stacking. As the 2DT alkyl
chain was already proved to be excessively long and to have
caused several negative effects in our report, the current 2TH
alkyl chains have six additional carbons as compared to 2DT
and are likely to cause more serious negative effects. In
addition, the LUMO level of the T2-based polymer is −3.77
eV, which may be too deep to offer a sufficient LUMO offset to
ensure efficient exciton dissociation.
Influence of Alkyl Chains. To summarize the effect of
alkyl chains for the T4 polymer, the 2HD chain is too short and
caused poor solubility, while the 2DT chain is too long and
5
Effect of the Molecular Weight and Related Dis-
cussion. It is well-known that polymer molecular weight is an
important material parameter that could influence polymer
aggregation and the morphology of the polymer:fullerene
introduced other negative morphology effects. The optimal
alkyl chain should be the one that is the shortest possible while
ensuring sufficient solubility for processing. Apparently, the
2OD chain is a better choice of alkyl chain than 2HD for the
T4 polymer. For the PffBT-T3(1,2) polymers, however, the
solubility of the polymer family is greatly enhanced due to the
less regular polymer structure and thus lower polymer
crystallinity. As a result, the T3 polymers with 2HD or slightly
shorter chains are still highly soluble for device processing.
Therefore, the optimal alkyl chain for the T3 polymers is not
2OD; instead, it should be 2HD or even shorter chains. To
further support this point, we synthesized the T3 polymers with
2OD alkyl chains or a mixture of 2OD and 2HD alkyl chains
(shown in Scheme 1). These two polymers exhibit better
solubility even in dichloromethane. It has been demonstrated in
our work and it is general rule of the PSC field that donor
polymers with too good solubility do not work efficiently in
PSCs. Not surprisingly, the PCE decreases dramatically with
longer alkyl chains (Table S1).
5
5−58
blend.
Therefore, it is important to study the effect of
molecular weights on the performance of the PffBT-T3(1,2)-2
polymer. While the polymer batch with the best 10.7%
efficiency has a M of 66.1 kDa, a lower molecular weight
n
(
M = 47.8 kDa) polymer batch was intentionally synthesized
n
for comparison (Table S1). First, the lower molecular weight
polymer batch exhibits a better solubility as it can be extracted
using chloroform. In contrast, the higher molecular weight
polymer is not soluble in hot chloroform and can only be
extracted using hot toluene or chlorobenzene. The temper-
ature-dependent aggregation properties of these two polymer
batches are then compared in Figure S1. Overall, the shape and
the trend of red-shift for the UV−vis absorption spectra of the
two polymer batches are rather similar. The absorption spectra
of the higher molecular weight polymer are only slightly
different with a small shoulder at about 730 nm (Figure S1).
PSC devices were fabricated based on the lower molecular
weight PffBT-T3(1,2)-2 polymer, and a respectably high
efficiency of 10.2% was obtained. These results show that the
PffBT-T3(1,2)-2 polymer can yield high-performance (>10%)
PSCs when the polymer batches have lower polymer molecular
weight and very different solubility properties. This is an
important advantage for the scale-up of polymer batches as
there is less stringent requirement to control the molecular and
solubility properties of the polymer.
Comparing the optimal polymer in the T3 family (PffBT-
T3(1,2)-2) with the best one in the T4 family (PffBT-T4-
2
OD), when using the same PC BM as the acceptor, the
71
performance of PffBT-T4-2OD is about 10.5%, while the PCE
of PffBT-T3(1,2)-2 is 10.7%. However, it is important to point
out that the 10.5% efficiency of PffBT-T4-2OD was achieved
using DIO, which is a problematic high-boiling and halogenated
additive that is not desirable in industry processes. In contrast,
the 10.7% PCE of PffBT-T3(1,2)-2 can be achieved without
using any additives at all. This offers a tremendous advantage
for the easy processing and optimization of PSCs. Because of its
higher VOC and wider absorption range than the T4 polymer,
the PffBT-T3(1,2)-2 polymer should have more room to
further increase its FF and JSC and thus PCE upon extensive
device optimizations.
It is also noted that some polymers in Table 1 have
significantly lower molecular weights than PffBT-T3(1,2)-2.
Therefore, it is important to clarify whether the lower
molecular weight is the reason that caused their inferior PSC
performance. For the PffBT-T4-2HD polymer (M = 13.9
n
kDa), our main conclusion was that it exhibits much poorer
solubility than the PffBT-T3 polymers. If the low molecular
weight version of PffBT-T4-2HD already exhibits dramatically
lower solubility than the PffBT-T3 polymer, the solubility of
the higher molecular weight version should be even worse. To
prove this point, we attempted to synthesize the PffBT-T4-
CONCLUSIONS
■
In summary, a series of ffBT and oligothiophene-based D−A
copolymers are synthesized and systematically studied. It is
shown that the T3 polymer with an asymmetric arrangement of
alkyl chains enables highly efficient thick-film PSCs with PCE
up to 10.7%. In addition, this high efficiency of T3 polymer-
based PSCs was achieved without using any processing
additives, which greatly simplifies the processing of PSCs and
makes the polymer more suitable for industry applications. By
reducing the number of thiophene units per repeating unit, the
HOMO and LUMO levels of the polymers are reduced and the
absorption onsets of the polymer films are also slightly red-
shifted. These positive changes contribute to higher VOC and
JSC values for the T3 polymer than for the T4 polymers. By
comparing the three T3 polymers that differ in the positions
and size of their alkyl chains, it is surprisingly found that the T3
2HD polymer with higher molecular weights. Because of the
poor solubility of the PffBT-T4-2HD polymer, it tends to
solidify in the polymerization reaction glassware once the
molecular weight reaches a certain level. It is thus generally
challenging to synthesize polymers with higher molecular
weights. By further synthesis optimizations, a PffBT-T4-2HD
polymer batch was obtained with a high M of 65.6 kDa. Not
n
surprisingly, the solubility of this polymer batch is much poorer
and its processing is extremely difficult as compared to the low
molecular weight batch. For the processing of the polymer:-
fullerene solution, the lower molecular weight PffBT-T4-2HD
G
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Journal of the American Chemical Society
Article
polymer with an unusual head-to-tail arrangement of alkyl
chains on the first and second thiophenes exhibits a more
favorable morphology and dramatically enhanced performance
than the T3 polymer with mirror-symmetric alkyl chains on the
first and third thiophenes. Our study also shows that the change
from a T4 to a T3 comonomer unit introduces significant
differences in the energy levels, solubility, crystallinity,
polymer:fullerene morphology, and PSC performances between
the T3 and T4 polymers. The polymer design rationales (using
a T3 unit with an asymmetric arrangement of alkyl chains)
demonstrated in our work provide an effective approach to tune
the energy levels and morphology of donor polymers that can
be adopted to further increase the efficiency of PSCs.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
(19) Huang, Y.; Guo, X.; Liu, F.; Huo, L.; Chen, Y.; Russell, T. P.;
■
Han, C. C.; Li, Y.; Hou, J. Adv. Mater. 2012, 24, 3383.
Corresponding Authors
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*msewma@mail.xjtu.edu.cn
hyan@ust.hk
(21) McCulloch, I.; Ashraf, R. S.; Biniek, L.; Bronstein, H.; Combe,
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Author Contributions
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H.H. and K.J. contributed equally to this work.
Notes
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(24) Zhou, H.; Yang, L.; You, W. Macromolecules 2012, 45, 607.
(25) Cabanetos, C.; El Labban, A.; Bartelt, J. A.; Douglas, J. D.;
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Mateker, W. R.; Frechet, J. M.; McGehee, M. D.; Beaujuge, P. M. J.
Am. Chem. Soc. 2013, 135, 4656.
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The work was partially supported by the National Basic
Research Program of China (973 Program; 2013CB834705 and
2
(
014CB643501), the Hong Kong Research Grants Council
T23-407/13-N, N_HKUST623/13, and 606012), the Na-
tional Science Foundation of China (nos. 21374090,
1361165301, 21504006, and 21534003), and HKUST
president’s office through SSTSP scheme (project ref number:
(
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29) Li, W.; Albrecht, S.; Yang, L.; Roland, S.; Tumbleston, J. R.;
5
(
59
EP201). X-ray data were acquired at beamlines 7.3.3 and
(
6
0
1
1.0.1.2 at the Advanced Light Source, which is supported by
McAfee, T.; Yan, L.; Kelly, M. A.; Ade, H.; Neher, D.; You, W. J. Am.
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the Director, Office of Science, Office of Basic Energy Sciences,
of the U.S. Department of Energy under contract no. DE-
AC02-05CH11231.
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DOI: 10.1021/jacs.5b08556
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX