Energy Level Engineering of Donor Polymers via Inductive and Resonance Effects for Polymer Solar Cells: Effects of Cyano and Alkoxy Substituents

Article abstract of DOI:10.1021/acs.chemmater.5b03256

Fine tuning the energy levels of donor polymers is a critically important step toward achieving high power conversion efficiencies in polymer solar cells (PSCs). We systematically controlled the energy levels of donor polymers by introducing cyano (CN) and alkoxy (OR) groups into the 4,4′-didodecyl-2,2′-bithiophene (BT) unit in a step-by-step fashion, thereby varying the inductive and resonance effects. The three monomer units (BT, BTC, and BTCox) were polymerized with benzo[1,2-b:4,5-b′]dithiophene (BDT) as a counter unit to afford three polymers (PBDT-BT, PBDT-BTC, and PBDT-BTCox). The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels decreased significantly upon the introduction of CN groups, and these levels increased slightly upon attachment of the OR groups, in good agreement with the measured open-circuit voltages of the three polymer devices. The strong inductive and resonance effects present in PBDT-BTCox narrowed the polymer band gap to 1.74 eV to afford a power conversion efficiency of 5.06%, the highest value achieved among the three polymers.

Full text of DOI:10.1021/acs.chemmater.5b03256

Subscriber access provided by EPFL | Scientific Information and Libraries
Article
Energy Level Engineering of Donor Polymers via Inductive and Resonance
Effects for Polymer Solar Cells: Effects of Cyano and Alkoxy Substituents
Heung Gyu Kim, Min Kim, J. Arul Clement, Jaewon Lee,
Jisoo Shin, Hyeongjin Hwang, Dong Hun Sin, and Kilwon Cho
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03256 • Publication Date (Web): 25 Sep 2015
Downloaded from http://pubs.acs.org on September 26, 2015
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth
Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 38
Chemistry of Materials
1
2
3
4
5
6
7
Energy Level Engineering of Donor Polymers via Inductive and
Resonance Effects for Polymer Solar Cells: Effects of Cyano and
Alkoxy Substituents
8
9
Heung Gyu Kim,† Min Kim,† J. Arul Clement,† Jaewon Lee, Jisoo Shin, Hyeongjin Hwang,
Dong Hun Sin, and Kilwon Cho*
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Department of Chemical Engineering
Pohang University of Science and Technology
Pohang 790ꢀ784, Korea
Email: kwcho@postech.ac.kr
† The authors contributed equally to this work.
Abstract
Fine tuning the energy levels of donor polymers is a critically important step toward
achieving high power conversion efficiencies in polymer solar cells (PSCs). We
systematically controlled the energy levels of donor polymers by introducing cyano (CN) and
alkoxy (OR) groups into the 4,4'ꢀdidodecylꢀ2,2'ꢀbithiophene (BT) unit in a stepꢀbyꢀstep
fashion, thereby varying the inductive and resonance effects. The three monomer units (BT,
BTC, and BTCox) were polymerized with benzo[1,2ꢀb:4,5ꢀb’]dithiophene (BDT) as a
counter unit to afford three polymers (PBDTꢀBT, PBDTꢀBTC, and PBDTꢀBTCox). The
highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels
decreased significantly upon the introduction of CN groups, and these levels increased
slightly upon attachment of the OR groups, in good agreement with the measured openꢀ
circuit voltages of the three polymer devices. The strong inductive and resonance effects
present in PBDTꢀBTCox narrowed the polymer band gap to 1.74 eV to afford a power
conversion efficiency of 5.06%, the highest value achieved among the three polymers.
1
ACS Paragon Plus Environment

Chemistry of Materials
Page 2 of 38
1
2
3
4
5
Introduction
6
7
8
9
Polymer solar cells (PSCs) have attracted wide interest as possible candidates for
realizing lightweight flexible energy sources using lowꢀcost largeꢀarea solution processes.1–3
Until recently, bulk heterojunction (BHJ)ꢀbased PSCs prepared from blends of conjugated
donor polymers and an acceptor, [6,6]ꢀphenylꢀC61ꢀbutyric acid methyl ester (PC61BM) or
[6,6]ꢀphenylꢀC71ꢀbutyric acid methyl ester (PC71BM), have yielded the best enhancements in
the power conversion efficiencies (PCEs).4,5 Donor polymers with a narrow band gap and a
deep highest occupied molecular orbital (HOMO) energy level are needed to enhance light
absorption and achieve a high shortꢀcircuit current density (Jsc) and high open circuit voltage
(Voc) because Voc is strongly related to the difference between the HOMO energy level of the
donor material and the lowest unoccupied molecular orbital (LUMO) energy level of the
acceptor. Therefore, fineꢀtuning the band gap and energy level of the donor polymer is
critically important for achieving a high PCE.6 To this end, three synthetic strategies have
been developed to engineer the band gaps and control the energy levels of the donor
polymers: (1) alternating donorꢀacceptor (D–A) structures, (2) aromaticity and bond length
alternation in a polymer backbone (quinoid polymer), and (3) implementation of the
inductive and resonance effects.7 Among these strategies, the alternating D–A concept, in
which electronꢀrich (donor) and electronꢀdeficient (acceptor) units are combined, has thus far
been most widely applied and examined in donor polymers. This concept enables the facile
tuning of the band gap, absorption spectrum, and energy levels of a donor polymer by
varying the donor and acceptor units. Tremendous research efforts have been devoted to this
strategy and exceptional advances have been achieved, with high PCEs of up to 10%.8–14
Quinoid polymers present another strategy for controlling the energy levels and band gaps.
For instance, poly (thiophene) exhibits a relatively large band gap of 2.0 eV, whereas poly
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(benzo[c]thiophene), which includes a benzene ring bound to a thiophene ring, provides an
2
ACS Paragon Plus Environment

Page 3 of 38
Chemistry of Materials
1
2
3
4
5
6
7
8
9
exceedingly small band gap of 1.0 eV due to the resonance energy of the benzene ring.15,16
Recently, the PTB7ꢀTh quinoid polymer has been tested in PSCs, providing a high PCE
(>10%) with a narrow band gap (1.59 eV).17,18
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Finally, inductive and resonance effects may be tuned by incorporating electronꢀdonating
or electronꢀwithdrawing substituents directly into the aromatic units of the main conjugated
polymer backbone. The inductive and resonance effects are effective in adjusting the energy
levels and band gaps in the donor polymer. In general, electronꢀdonating groups increase the
HOMO energy level, whereas electronꢀwithdrawing groups reduce the LUMO energy level.19
For instance, the cyano (CN) group is a typical electronꢀwithdrawing group and can lower
both the HOMO and the LUMO energy levels with respect to the corresponding levels of the
analogues prepared without the CN groups.20–23 Alternatively, the HOMO energy level may
be increased and the band gap narrowed upon the introduction of an alkoxy (OR) electronꢀ
donating group to the backbone of the conjugated polymer.24 Therefore, the energy level and
band gap of a donor polymer may be finely tuned by modulating the inductive and resonance
effects. Synthetic strategy of mainly utilizing inductive and resonance effects has not yet been
extensively studied for donor polymers in PSCs despite the recognized effectiveness of
inductive and resonance strategies for energy level control.
This work primarily focused on the systematic study of inductive and resonance effects in
an effort to engineer the energy levels and band gaps of donor polymers. To this end, three
monomer units were designed and synthesized by successively introducing CN and OR
groups onto the 4,4'ꢀdidodecylꢀ2,2'ꢀbithiophene (BT) unit. The BT moiety was used as a
reference structure. First, we introduced CN groups as electronꢀwithdrawing groups into the
BT unit to synthesize the 4,4'ꢀdidodecylꢀ3,3'ꢀdicyanoꢀ2,2'ꢀbithiophene (BTC) unit. Second, an
OR electronꢀdonating group was introduced into the BTC unit to synthesize 4,4’ꢀ
didodecyloxyꢀ3,3'ꢀdicyanoꢀ2,2'ꢀbithiophene (BTCox). These monomer units were
3
ACS Paragon Plus Environment

Chemistry of Materials
Page 4 of 38
1
2
3
4
5
6
7
8
9
polymerized with benzo[1,2ꢀb:4,5ꢀb’]dithiophene (BDT) as a counter unit to afford three
polymers (PBDTꢀBT, PBDTꢀBTC, and PBDTꢀBTCox). The PBDTꢀBT reference polymer
consisted only of donor units without any acceptor moieties, to exclude D–A alternating
effects. We then investigated the effects of the CN and OR groups on the energy level, band
gap, and device performance in detail. The HOMO energy level was dramatically lowered
from –5.37 eV to –5.58 eV upon introduction of a CN group, and the band gap narrowed
significantly from 2.1 eV to 1.74 eV upon introduction of both CN and OR groups due to
inductive and resonance effects. Moreover the crystallinity of the polymers changed
considerably upon introduction of the substituents. PBDTꢀBTCox yielded the highest
crystallinity because the CN and OR groups formed a strong resonance structure, and the SꢀO
nonꢀcovalent interactions in the polymer backbone enhanced the planarity of the polymer
backbone. The best PCE, 5.06%, was achieved in the PBDTꢀBTCoxꢀbased singleꢀjunction
device under AM 1.5G illumination. We demonstrated that the inductive and resonance
effects offered a powerful synthetic strategy for fineꢀtuning the energy levels and band gaps
of the conjugated polymers.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Results and Discussion
Synthesis of Monomers and Polymers
The general synthetic route to the monomers is described in Scheme 1. The BDT
monomer was synthesized according to the procedure reported in the literature.25 4ꢀDodecylꢀ
3ꢀcyanothiophene (5) and 4ꢀdodecyloxyꢀ3ꢀcyanothiophene (8) were synthesized according to
methods described previously and were coupled to afford the BTC monomer (6) and BTCox
monomer (10).21,26 As described in Scheme 2, a Stille coupling polymerization between the
BDT monomer and three monomers (BT, BTC, and BTCox) gave three new polymers
(PBDTꢀBT, PBDTꢀBTC, and PBDTꢀBTCox) after standard purification. The synthesized
4
ACS Paragon Plus Environment

Page 5 of 38
Chemistry of Materials
1
2
3
4
5
6
7
8
9
polymers were soluble in chlorinated solvents, such as chlorobenzene (CB), oꢀ
dichlorobenzene (oꢀDCB), and chloroform. The numberꢀaverage molecular weights (Mn) and
polydispersity indices (PDIs) of the polymers were evaluated by gel permeation
chromatography (GPC) at 40 °C using CB as the eluent. The Mn values of PBDTꢀBT, PBDTꢀ
BTC, and PBDTꢀBTCox were 14.8, 11.8, and 37.6 kDa, with PDIs of 1.93, 3.37, and 1.77,
respectively. The polymers exhibited good thermal stability with a decomposition
temperature (Td) exceeding 390 °C, determined using thermogravimetric analysis (TGA), as
shown in Figure S1 and Table 1. The thermotropic properties of the polymers were
characterized by differential scanning calorimetry (DSC). DSC measurements did not reveal
any endothermic or exothermic behavior between 30 °C and 250 °C. Although there is a
slight deviance of PDIs and Mn of the synthesized polymers, these differences might not
largely change intrinsic polymer properties such as absorption, energy level, crystallinity and
DFT calculation according to previous reports.27–29
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Optical Properties
The UVꢀvisible absorption spectra of the polymers were obtained in chloroform solutions
and in thin films, as shown in Figure 1a. The absorption spectra gradually redꢀshifted with the
introduction of CN (PBDTꢀBTC) and OR groups (PBDTꢀBTCox). The maximum absorption
wavelengths (λmax) of PBDTꢀBT, PBDTꢀBTC, and PBDTꢀBTCox, measured in solution,
were at 468, 448, and 629 nm, respectively, whereas the corresponding values measured in
the thin films were 518, 529, and 635 nm. The absorption spectra of the polymers were redꢀ
shifted in the thin films due to the increased πꢀπ interchain interactions upon aggregation. The
optical band gaps (Egopt) of PBDTꢀBT, PBDTꢀBTC, and PBDTꢀBTCox were calculated from
the absorption edges in the thin films and were found to be 2.10, 1.96, and 1.74 eV,
respectively. The PBDTꢀBT reference polymer exhibited a large band gap of 2.10 eV, as
5
ACS Paragon Plus Environment

Chemistry of Materials
Page 6 of 38
1
2
3
4
5
6
7
8
9
expected. The PBDTꢀBTC polymer prepared with CN groups yielded a band gap that was
slightly reduced, to 1.96 eV, because the CN groups slightly stabilized the quinoid structure
of the BT unit via the inductive and resonance effects of the CN groups. The band gap of the
PBDTꢀBTCox polymer, prepared with CN and OR groups, decreased significantly to 1.74 eV
because the quinoid structure throughout the polymer backbone was strongly stabilized by the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
opt
inductive and resonance effects of the CN and OR groups. The measured λmax and Eg
values are listed in Table 1. Moreover the effects of the optical absorption were investigated
by measuring the absorption coefficients (α) of the polymers. PBDTꢀBTCox displayed the
highest maximum among the polymers, an α of 635 nm, which was twice the value of α for
PBDTꢀBTC (Figure S2).
Resonance Structures
The influence of the substituents on the electronic structure of the polymer may be
described in terms of two effects: the inductive and resonance effects. The inductive effect
involves the transmission of electrostatic charge through the σꢀbonds, mediated by the
electronegativity of the atoms, whereas the resonance effect involves the transmission of
electron density through the πꢀsystem of the molecules via the electronꢀdonating or ꢀ
withdrawing properties of the substituents, as described using the relevant resonance
structures. Figure 2 shows the resonance structures of the respective monomer moieties. In
the reference structure comprising the BT unit with alkyl groups, the carbon atom was
slightly more electronegative than the hydrogen atom in the alkyl chain. The small dipole
moment of this structure pulled electron density away from hydrogen and toward the carbon
in the alkyl chain. The slight buildup of electron density on the carbon in the alkyl group
induced a small degree of electron donation via the inductive effect (+I). The CN groups on
the BTC unit withdrew electron density through inductive (–I) and resonance (–R) effects.
6
ACS Paragon Plus Environment

Page 7 of 38
Chemistry of Materials
1
2
3
4
5
6
7
8
9
The quinoid structure of the BTC unit was more stabilized than the BT unit as a result of the
inductive and resonance effects of the CN groups. The oxygen atom on the BTCox unit
bearing OR and CN groups was more electronegative than the carbon atom, and the electron
density was withdrawn as a result of the inductive effect (–I). Simultaneously, the lone pair
electrons of the oxygen atom could be donated back into the thiophene ring through
resonance effect (+R). In this case, the resonance effect (+R) of the OR groups was more
powerful than the inductive effect (–I). The quinoid structure of the BTC unit was strongly
stabilized by both the resonance effect (+R) of the OR groups and the resonance effect (–R)
of the CN groups.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Electrochemical Properties
The electrochemical properties of the synthesized polymers were investigated using
cyclic voltammetry (CV), as shown in Figure S3. The HOMO/LUMO energy levels of
PBDTꢀBT, PBDTꢀBTC, and PBDTꢀBTCox were calculated to be –5.37/–3.25, –5.58/–3.53,
and –5.50/–3.51 eV, with ferrocene as a reference (Table 1 and Figure 1b). PBDTꢀBTC and
PBDTꢀBTCox exhibited deep HOMO energy levels (<–5.5 eV) upon the addition of the CN
groups, thereby ensuring good air stability and a high Voc in PSC applications. PBDTꢀBTC
with PBDTꢀBT were compared, revealing that the HOMO energy level of PBDTꢀBTC
dramatically decreased, from –5.37 to –5.58 eV, and the LUMO energy level also decreased,
from –3.25 to –3.53 eV, as a result of the inductive (–I) and resonance effects (–R) of the CN
cv
groups. The band gap (Eg ) decreased slightly, from 2.12 to 2.05 eV, due to the inductive (–
I) and resonance effects (–R) of the CN groups. The PBDTꢀBTCox polymer bearing OR and
CN groups yielded a HOMO energy level that slightly increased, from –5.58 to –5.50 eV, due
to the resonance effect (+R) of the OR groups. In particular, PBDTꢀBTCox displayed both
7
ACS Paragon Plus Environment

Chemistry of Materials
Page 8 of 38
1
2
3
4
5
6
7
inductive and resonance effects in the thiophene units, thereby dramatically reducing the
cv
band gap (Eg ) to 1.99 eV due to the strong stabilization of the quinoid structure.
8
9
Molecular simulations of the polymers were performed using density functional theory
(DFT) calculation at the B3LYP/6ꢀ31G* level to investigate the effects of the CN and OR
groups. The alkyl groups in the structures were replaced with methyl groups that do not
significantly affect the equilibrium geometries or electronic properties of the molecules. The
calculated HOMO and LUMO levels were found to agree well with the energy levels
measured using CV. Although DFT calculations usually provide higher values than CV
measurements, both methods revealed similar trends in the energy levels. The
HOMO/LUMO energy levels of PBDTꢀBT were found to be –4.74/–2.22 eV. A comparison
of PBDTꢀBTC with PBDTꢀBT revealed that the LUMO energy level decreased dramatically
from –2.22 to –2.92 eV, and the HOMO energy level also decreased from –4.74 to –5.35 eV
as a result of the inductive (–I) and resonance effects (–R) of CN groups. A comparison of
PBDTꢀBTCox with PBDTꢀBTC revealed that the LUMO energy level decreased from –2.92
to –3.10 eV whereas the HOMO energy level increased from –5.35 to –5.22 eV due to the
strong resonance effect (+R) of the OR groups. The resulting theoretical (DFT) and
experimental (CV) HOMO and LUMO levels agreed well, as illustrated in Figure 1b.
We further examined the structural properties of the polymers. Two dihedral angles, the
first between the BDT unit and the thiophene unit (θ1) and the second between the two
thiophene units (θ2) in the polymers, were calculated, as shown in Figure 3. The θ1 and θ2
values obtained from PBDTꢀBT were calculated to be 19.54° and 13.42°. The dihedral
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
angles of PBDTꢀBTC (θ1: 30.35°, θ2: 19.72°) became more twisted upon introduction of the
CN groups into the BT moiety due to the increased steric hindrance relative to the hindrance
introduced by hydrogen atoms. Interestingly, the dihedral angles (θ1: 6.71, θ2: 0.29) of
8
ACS Paragon Plus Environment

Page 9 of 38
Chemistry of Materials
1
2
3
4
5
6
7
8
9
PBDTꢀBTCox were incredibly reduced due to the strong stabilization of the quinoid structure
of the polymer backbone via the inductive and resonance effects of both the CN and OR
groups and the effects of SꢀO nonꢀcovalent interactions (interꢀlocking).30–32 These results
indicate that introducing substituents could considerably change the planarity of the polymer
backbone in addition to changing the energy levels.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
X-ray Diffraction Properties
Grazing incidence Xꢀray diffraction (GIXD) measurements were collected to investigate
the crystalline properties of the polymers.33–36 The GIXD patterns obtained from the three
neat polymer films revealed distinct structural differences, as shown in Figure 4. The outꢀofꢀ
plane (OOP) GIXD profile of the neat PBDTꢀBT film displayed pronounced lamellar
stacking (h00) peaks at 0.31 Å–1 and 0.62 Å–1 arising from the alkyl chain packing, with a dꢀ
spacing of 20 Å. The inꢀplane (IP) profile of the neat PBDTꢀBT film showed a weak (100)
peak at 0.31 Å–1 and a strong πꢀπ stacking (010) peak at 1.54 Å–1, which corresponded to
(010) πꢀπ stacking with a dꢀspacing of 4.08 Å. These results suggested that the neat PBDTꢀ
BT film was formed predominantly with an edgeꢀon molecular orientation with respect to the
substrate. The OOP profiles collected from the neat PBDTꢀBTC film showed weak lamellar
stacking (h00) peaks at 0.29 Å–1 and a pronounced πꢀπ stacking (010) peak at 1.54 Å–1. The
IP profile revealed a stronger (100) peak in the OOP profile, indicating that a faceꢀon
molecular orientation dominated the neat PBDTꢀBTC film. By comparison, the GIXD
profiles of the neat PBDTꢀBTCox film showed prominent (h00) and (010) peaks in both the
OOP and IP profiles, suggesting that the PBDTꢀBTCox polymer assumed a bimodal
orientation distribution containing both edgeꢀon and faceꢀon orientations. The GIXD patterns
obtained from the neat PBDTꢀBTC films revealed lowꢀorder and lowꢀintensity lamellar
stacking (h00) peaks compared to the corresponding peaks collected from the PBDTꢀBT
9
ACS Paragon Plus Environment

Chemistry of Materials
Page 10 of 38
1
2
3
4
5
6
7
8
9
polymer. These results confirmed that the steric hindrance of the CN groups in the BT unit
disrupted molecular packing. On the other hand, the GIXD pattern of the neat PBDTꢀBTCox
polymer exhibited prominent higherꢀorder (h00) diffraction peaks that were attributed to the
enhanced planarity of the polymers due to stabilization of the quinoid structure of the
polymer backbone and the effects of the SꢀO interaction, as fully supported by the DFT
molecular simulations and circularly integrated XRD profiles (Figure S5). Interestingly, the
molecular planarity and packing compactness of the polymers also reflected the πꢀπ stacking
distance between polymer chains, revealing that the PBDTꢀBTCox molecules (3.80 Å) were
shorter than the PBDTꢀBT molecules (4.08 Å). The presence of the fullerene (in a blend film
comprising PC71BM) did not fully disrupt the crystal structures of the polymer, and the
diffraction peak of PC71BM was clearly observed. The crystal structure of the PBDTꢀBTCox
polymer was beneficial for charge transport in the photovoltaic devices because it assumed a
faceꢀon orientation and a reduced πꢀπ stacking distance between the polymer backbones.37–39
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Solar Cell Performance
Polymer solar cells were fabricated using the three polymers as a donor material and
PC71BM as an acceptor material. The device architecture was ITO/MoO3/polymer:PC71BM/
LiF/Al, and the device area was 0.055 cm2. The MoO3 layer was used as a hole transport
layer with a high work function that was energetically aligned to the deep HOMO energy
level (< –5.5 eV).40,41 The thickness and blend weight ratios of the polymer/PC71BM film
were optimized, as listed in Table 2. The 1:2 weight blend ratio of the polymer/PC71BM films
processed from oꢀDCB exhibited the best performance for all three polymers. The addition of
the solvent additive, 1,8ꢀdiiodooctane (DIO), enhanced the photovoltaic performances only in
the case of PBDTꢀBTCox and not in the cases of the PBDTꢀBT and PBDTꢀBTC polymers, as
10
ACS Paragon Plus Environment

Page 11 of 38
Chemistry of Materials
1
2
3
4
5
6
7
explained by the atomic force microscopy (AFM) morphological characterization images
(Figure 6).
8
9
The current density versus voltage curves (J-V) collected under AM 1.5 conditions with
an intensity of 100 mW cm–2 are presented in Figure 5a, and the average device parameters
are listed in Table 2. Among the three polymer devices, the PBDTꢀBTCox device exhibited
the highest power conversion efficiency, with Voc = 0.86 V, Jsc = 10.1 mA cmꢀ2, FF = 58.4%,
and PCE = 4.80% (PCEmax = 5.06%). The Voc values of PBDTꢀBT (0.82 V), PBDTꢀBTC
(0.90 V), and PBDTꢀBTCox (0.86 V) exactly corresponded to the differences between the
measured HOMO energy levels of the three polymers (–5.37 eV, –5.58 eV, and –5.50 eV,
respectively) and the LUMO energy levels of PC71BM. The external quantum efficiencies
(EQEs) of these devices are shown in Figure 5b. The EQE spectra of the PBDTꢀBTC and
PBDTꢀBTCox devices were redꢀshifted (29 and 86 nm, respectively) compared to those of
the PBDTꢀBT device, in good agreement with the UVꢀVis absorption results (Figure S7).
Despite the redꢀshifted absorption maximum of the PBDTꢀBTC device, the maximum of
EQE did not exceed 10% and resulted in a poor Jsc. This decrease relates to the largely phaseꢀ
separated morphology on a length scale of hundreds of nanometers, as illustrated in the AFM
images. On the other hand, the PBDTꢀBTCox device exhibited the highest EQE maximum
(~55%) and Jsc (10.1 mA cm–2), with an absorption maximum that was redꢀshifted from 300
nm to 721 nm and agreed well with the UVꢀVis spectrum.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Insight into the fill factors of the polymer/PC71BM devices was sought by measuring the
charge transport properties using the spaceꢀchargeꢀlimited current (SCLC) method.42–45 The
results were fit to the model J = (9 /8)εꢀ(V2 / L3 ), where ε is the static dielectric constant of
the medium and ꢁ is the carrier mobility, in holeꢀdominant devices with the structure
ITO/MoO3/polymer: PC71BM/MoO3/Al.46,47 The fits to the J-V curve, based on the SCLC
11
ACS Paragon Plus Environment

Chemistry of Materials
Page 12 of 38
1
2
3
4
5
6
7
8
9
model, are shown in Figure S4. The hole mobilities were calculated to be 3.34 × 10–6 cm2 V–
1s–1 for PBDTꢀBT/PC71BM, 1.23 × 10–8 cm2 V–1s–1 for PBDTꢀBTC/PC71BM, and 5.64 × 10–5
cm2 V–1s–1 for PBDTꢀBTCox/PC71BM (weight ratio of the donor versus acceptor is 1:2),
respectively. The hole mobilities were closely related to the molecular structures and
crystallinities. The closely packed molecular structure of the PBDTꢀBTCox polymer
particularly contributed to the high hole mobility and fill factor (59%) of the PBDTꢀ
BTCox/PC71BM device compared to the corresponding values measured from the PBDTꢀBT
(46%) and PBDTꢀBTC (41%) devices. The differences in hole mobilities of the three
polymers agreed with the crystallinities of the polymers, as measured using GIXD.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
AFM images were collected to assess the morphological structures of the
polymer/PC71BM blend films. The PBDTꢀBT/PC71BM blend film assumed a phaseꢀseparated
morphology on the ~100 nm length scale (Figure 6a). This morphology was not beneficial for
the BHJ structure due to the limited exciton diffusion length, on the ~10 nm length scale.48,49
The AFM image obtained from the PBDTꢀBTC/PC71BM blend film also revealed significant
phase separation, with PC71BM aggregates about ~200 nm in length (Figure 6b). This
morphology clearly explained the low Jsc value measured from the PBDTꢀBTC device.
Unfortunately, the morphologies of the PBDTꢀBT and PBDTꢀBTC blend films changed
slightly when processed with DIO (Figure 6d, 6e). The addition of DIO did not improve the
photovoltaic performances of the PBDTꢀBT and PBDTꢀBTC devices. On the contrary, the
PBDTꢀBTCox/PC71BM blend film revealed a nanoscale intermixed morphology in the wellꢀ
developed BHJ structure (Figure 6c). When processed with DIO, the PBDTꢀBTCox blend
morphology also revealed nanoscale phase separation that improved the efficiency of charge
separation (Figure 6f). The morphological differences among the polymer:PC71BM blend
films could be explained by comparing the measured surface energies of the polymers and
PC71BM.50 We confirmed that the observed severe phase separation in the PBDTꢀ
12
ACS Paragon Plus Environment

Page 13 of 38
Chemistry of Materials
1
2
3
4
5
6
7
8
9
BT:PC71BM and PBDTꢀBTC:PC71BM blend films corresponded to the large differences
among the surface energies of the polymer and fullerene (PBDTꢀBT: 21.78 mN m–1, PBDTꢀ
BTC: 16.51 mN m–1, PC71BM: 23.66 mN m–1). PBDTꢀBTCox displayed a surface energy
(22.51 mN m–1) similar to that measured in PC71BM, consistent with the observed nanoꢀscale
separated morphology (Figure S6).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Conclusions
We systematically studied the energy levels and band gaps of the donor polymers in
which the inductive and resonance effects had been tuned through synthetic engineering.
Three monomer units were designed and synthesized by introducing the CN and OR groups
onto the BT unit in a stepꢀbyꢀstep manner. The synthesized monomers were polymerized with
the BDT unit as a counter monomer to afford three polymers (PBDTꢀBT, PBDTꢀBTC, and
PBDTꢀBTCox). We investigated the effects of the CN and OR groups on the energy level,
band gap, and device performance. The HOMO energy level decreased dramatically, from –
5.37 to –5.58 eV, upon the introduction of CN groups, and the band gap (Egopt) narrowed
significantly, from 2.1 to 1.74 eV, upon introduction of both CN and OR groups via the strong
inductive and resonance effects. The resultant photovoltaic performances were characterized
by high openꢀcircuit voltages (Voc) ranging from 0.82 V to 0.98 V. In the polymer series, the
polymer solar cell based on a blend of PBDTꢀBTCox and PC71BM gave the best photovoltaic
performance, with a Voc of 0.86 V and a PCE of 5.06%. We demonstrated that synthetically
tuning the inductive and resonance effects provided a powerful strategy for fineꢀtuning the
energy levels and band gaps of the donor polymers.
Experimental Section
Materials
13
ACS Paragon Plus Environment

Chemistry of Materials
Page 14 of 38
1
2
3
4
5
6
7
8
9
All reagents were purchased from Aldrich, TCI and Acros, and used without further
purification. All anhydrous organic solvents for synthesis and device fabrication such as
tetrahydrofuran (THF), chloroform, toluene, N, Nꢀdimethylformamide (DMF), chlorobenzene
(CB), oꢀdichlorobenzene (oꢀDCB) were purchased form Aldrich. The compounds, 5,5'ꢀ
dibromoꢀ4,4'ꢀdidodecylꢀ2,2'ꢀbithiophene, 3ꢀcyanoꢀ4ꢀdodecylthiophene, and BDT monomer
were synthesized according to literature procedures.21,24,25
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Characterization
1H and 13C NMR spectra were recorded on a Bruker AVANCE 400 operating at 400 MHz
and 100 MHz respectively in chloroformꢀd solutions with tetramethylsilane (TMS) as the
internal standard. Numberꢀaverage (Mn) and weightꢀaverage (Mw) molecular weights were
measured by gel permeation chromatography (GPC) using a Shimadzu LC solution with CB
as the eluent and a calibration curve of polystyrene standards at 40 °C. UVꢀvis absorption
spectra were obtained with a Varian CARY 5000 UV/Visible Spectrophotometer. Differential
scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed by using
a DSC 2910 (TA Instruments) and a TGA 2050 (TA Instruments) under a nitrogen
atmosphere at a heating and cooling rate of 10 °C minꢀ1. Cyclic voltammograms were
recorded using a PowerLab/AD Instruments model system. A 0.1 M tetrabutylammonium
hexafluorophosphate (Bu4NPF6) solution in acetonitrile was used as the electrolyte solution.
The three electrode system consisted of an Ag/AgCl reference electrode, a glassy carbon
working electrode, and a platinum counter electrode. Polymer thin films were prepared by
drop casting polymer solutions in chloroform onto the working electrode. The potential of the
Ag/AgCl reference electrode was internally calibrated by using the ferrocene/ferrocenium
redox couple (Fc/Fc+). The energy levels were estimated with the equations: EHOMO = −(4.80
+ Eonset, ox ), and ELUMO = −(4.80 + Eonset, red). The film morphologies were characterized with
14
ACS Paragon Plus Environment

Page 15 of 38
Chemistry of Materials
1
2
3
4
5
6
7
8
9
atomic force microscopy (AFM, Digital Instruments Multimode). The thicknesses of the
polymer films were measured by using an ellipsometer (Mꢀ2000V, J. A. Woollam Co., Inc.).
2D grazing incidence Xꢀray diffraction grazing incidence Xꢀray diffraction (GIXD) was
performed by using the synchrotron source at the Pohang Accelerator Laboratory (PAL) in
Korea. 2D GIXD patterns were recorded with a 2D CCD detector (Rayonix SX165) and the
Xꢀray irradiation time was 1 ~ 10 s depending on the saturation level of the detector.
Computational details
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Density functional theory (DFT) calculations were performed using the Gaussian 09
package with the nonlocal hybrid Becke threeꢀparameter LeeꢀYangꢀParr (B3LYP) function
and the 6ꢀ31G* basis set after optimizing the geometry of oligomers.
Monomer synthesis
2-Bromo-3-dodecylthiophene 1. 3ꢀdodecylthiophene (5 g, 19.8 mmol) was added to a dry
chloroform (30 mL) and acetic acid (30 mL) solution. NBS (3.53 g, 19.8 mmol) was added
slowly and then stirred for 2 h at room temperature. After addition of water (50 mL), organic
phase was separated and extracted with chloroform (2 × 20 mL). The combined extracts were
dried over anhydrous MgSO4, filtered and distillation of solvent under reduced pressure. The
oily residue was purified by column chromatography using hexane as the eluent to afford 5.4
g of compound 1 (82% yield) as a colorless oily liquid. 1H NMR (400 MHz, CDCl3, ppm) δ:
7.19 (d, 1H, J = 5.6 Hz), 6.81 (d, 1H, J = 5.6 Hz), 2.58 (t, 2H, J = 7.8 Hz), 1.58 (m, 2H, J =
7.5 Hz), 1.36ꢀ1.30 (m, 18H), 0.91 (t, 3H, J = 7.0 Hz). 13C NMR (100 MHz, CDCl3, ppm) δ:
141.9, 128.2, 125.1, 108.8, 31.9, 29.8, 29.7, 29.6, 29.5, 29.4, 29.3, 29.2, 22.7, 14.1.
5,5'-Dibromo-4,4'-didodecyl-2,2'-bithiophene
2.
The
mixture
of
a
Bis(benzonitrile)palladium (II) chloride (0.09 g, 0.24 mmol), 20 mL of DMSO, and 2ꢀbromoꢀ
3ꢀdodecylthiophene (5 g, 15.09 mmol) was stirred. To the resulting mixture, silver (I)
fluoride (3.82 g, 2 equiv) was added and heated at 60 °C for 12 h. Then, the mixture was
15
ACS Paragon Plus Environment

Chemistry of Materials
Page 16 of 38
1
2
3
4
5
6
7
8
9
cooled to room temperature and passed through a Celite pad, which was successively washed
with chloroform. The filtrate was washed with water and the combined organic layer was
dried over anhydrous MgSO4 and concentrated under reduced pressure. The crude residue
was purified by column chromatography using hexane as the eluent. Recrystallization of
crude product using dichloromethane and methanol gave 3.9 g of compound 2 (78% yield) as
a yellow solid. 1H NMR (400 MHz, CDCl3, ppm) δ: 6.79 (s, 2H), 2.52 (t, 4H, J = 7.8 Hz),
1.56 (m, 4H, J = 7.3 Hz), 1.34ꢀ1.28 (m, 36H), 0.89 (t, 6H, J = 7.0 Hz). 13C NMR (100 MHz,
CDCl3, ppm) δ: 142.9, 136.1, 124.4, 107.8, 31.9, 29.7, 29.6, 29.5, 29.4, 29.3, 29.2, 22.7, 14.1.
2,3,5-Tribromo-4-dodecylthiophene 3. Bromine (5.12 mL, 100 mol) was added dropwise
over a period of 1.5 h to a solution of 3ꢀdodecylthiophene (8 g, 31.68 mmol) in anhydrous
chloroform (50 mL) in the presence of a catalytic amount of Fe powder. The reaction mixture
was stirred for 2 h. After careful addition of water (50 mL), the mixture was slowly
neutralized by an aqueous solution of 2M NaOH (40 mL). The aqueous phase was extracted
with dichloromethane (2 × 50 mL). The organic layer was collected and washed with a
saturated aqueous solution of Na2S2O3, dried over MgSO4, and concentrated under reduced
pressure. The crude residue was purified by column chromatography using hexane as the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
eluent to afford 13.8 g of compound 3 (89 % yield) as an orange oil. H NMR (400 MHz,
CDCl3, ppm) δ: 2.68 (t, 2H, J = 7.8 Hz), 1.58 (m, 2H, J = 7.5 Hz), 1.36ꢀ1.30 (m, 18H), 0.91 (t,
3H, J = 7.0 Hz). 13C NMR (100 MHz, CDCl3, ppm) δ: 141.7, 118.2, 109.1, 108.8, 31.9, 29.8,
29.7, 29.6, 29.5, 29.4, 29.3, 29.2, 22.7, 14.1.
3-Bromo-4-dodecylthiophene 4. nꢀBuLi (2.5M in hexane, 21.3 mL, 53.25 mmol) was
slowly added over a period of 2 h to a solution of 2,3,5ꢀtribromoꢀ4ꢀdodecylthiophene (13 g,
26.57 mmol) in anhydrous Et2O (100 mL) cooled to –5 °C. After 20 min of additional stirring
at –5 °C, the reaction mixture was carefully hydrolyzed at –5 °C with vigorous stirring by
addition of an aqueous solution of 4M HCl (20 mL). The mixture was diluted with Et2O (50
16
ACS Paragon Plus Environment

Page 17 of 38
Chemistry of Materials
1
2
3
4
5
6
7
8
9
mL) and slowly warmed to room temperature. The aqueous phase was separated and
extracted with Et2O (2 × 50 mL). The organic phase was separated and washed with water (2
× 150 mL), dried over MgSO4, and evaporated. The oily residue was purified by column
chromatography on silica gel using hexane as the eluent to afford 8.1 g of compound 4 (92 %
yield) as a slightly yellow oil. 1H NMR (400 MHz, CDCl3, ppm) δ: 7.24 (d, 1H, J = 3.5 Hz),
6.98 (d, 1H, J = 3.4 Hz), 2.60 (t, 2H, J = 7.5 Hz), 1.58 (m, 2H, J = 7.5 Hz), 1.36ꢀ1.31 (m, 18
H), 0.91 (t, 3H, J = 7.02 Hz). 13C NMR (100 MHz, CDCl3, ppm) δ: 128.3, 122.6, 120.5, 112.7,
31.9, 29.9, 29.7, 29.4, 29.3, 29.2, 22.7, 14.1.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4-Dodecyl-3-cyanothiophene 5. A mixture of 3ꢀBromoꢀ4ꢀdodecylthiophene (8 g, 24.1
mmol) and CuCN (2.5 g, 27.9 mmol) in anhydrous DMF (50 mL) was refluxed for 38 h. The
reaction mixture was cooled to 60 °C before a solution of FeCl3 (7.8 g, 48.3 mmol) in an
aqueous solution of 8M HCl (40 mL) was added. The mixture was then cooled to room
temperature and extracted with dichloromethane. The organic layer was washed with water,
dried over MgSO4, and evaporated under reduced pressure. The oily residue was purified by
column chromatography on silica gel using dichloromethane and hexane (1:4) to afford 6.4 g
1
of compound 5 (95 % yield) as an orange oil. H NMR (400 MHz, CDCl3, ppm) δ: 7.86 (d,
1H, J = 3.3 Hz), 7.01 (d, 1H, J = 3.2 Hz), 2.75 (t, 2H, J = 7.7 Hz), 1.63 (m, 2H), 1.41–1.31 (m,
13
18 H), 0.90 (t, 3H, J = 7.01 Hz). C NMR (100 MHz, CDCl3, ppm) δ: 145.5, 134.4, 122.8,
116.1, 112.9, 31.8, 29.7, 29.4, 29.2, 29.1, 29.0, 22.6, 14.1
5,5'-Dibromo-4,4'-didodecyl-3,3'-dicyano-2,2'-bithiophene 6. To a dry chloroform
solution (25 mL) containing 4ꢀdodecylꢀ3ꢀcyanothiophene (1 g, 3.61 mmol), catalytic amount
of Fe powder was added. Bromine (0.63 g, 3.97 mmol) was added slowly and then stirred for
12 h at room temperature. Organic phase was separated and extracted with chloroform (2 ×
20 mL). The combined extracts were dried over anhydrous MgSO4, filtered and distillation of
solvent under reduced pressure. The crude product was used next step without further
17
ACS Paragon Plus Environment

Chemistry of Materials
Page 18 of 38
1
2
3
4
5
6
7
8
9
purification. The mixture of a Bis(benzonitrile)palladium (II) chloride (0.03 g, 0.08 mmol),
20 mL of DMSO, and 5ꢀbromoꢀ4ꢀdodecylthiopheneꢀ3ꢀcarbonitrile (1 g, 2.80 mmol) was
stirred. To the resulting mixture, silver (I) fluoride (0.70 g, 2 equiv) was added and heated at
60 °C for 12 h. Then, the mixture was cooled to room temperature and passed through a
Celite pad, which was successively washed with chloroform. The filtrate was washed with
water and the aqueous layer was extracted with chloroform (3 × 50 mL). The combined
organic layer was dried over MgSO4 and concentrated under reduced pressure to give a crude
solid. The crude solid was purified by column chromatography on silica gel using
dichloromethane and hexane (1:2) as the eluent. Recrystallization of the product using
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
dichloromethane and methanol gave 0.4 g of compound 6 (40% yield) as a yellow solid. H
NMR (400 MHz, CDCl3, ppm) δ: 2.36 (t, 4H, J = 7.4 Hz), 1.68ꢀ1.63 (m, 4H), 1.32ꢀ1.27 (m,
36H), 0.90 (t, 6H, J = 6.8 Hz). 13C NMR (100 MHz, CDCl3, ppm) δ: 144.8, 140.3, 113.6,
112.8, 110.8, 31.9, 29.6, 29.4, 29.3, 29.2, 29.1, 22.7, 14.1. Elemental analysis: calc. for
C34H50Br2N2S2: C, 57.46; H, 7.09; N, 3.94; S 9.02. Found: C, 57.54; H, 7.03; N, 3.86; S,
9.08%.
4-Dodecyloxy-3-cyano-2,5-dihydrothiophene 7. To
a
solution of 3ꢀcyanoꢀ4ꢀ
oxotetrahydrothiophene (2 g, 15.73 mmol) in 15 mL of DMF, Cs2CO3 (6.14 g, 1.2 equiv) and
dodecyl methanesulfonate (5 g, 1.2 equiv) were added. The solution was irradiated in CEM
microwave oven (T = 80 °C) for 10 min. After the completion of the reaction, the mixture
was poured into water, and extracted with dichloromethane (3 × 50 mL). The combined
organic layer was washed several times with water and dried with MgSO4. After evaporation
of the solvent, the residue was purified through column chromatography using hexane and
ethyl acetate (10:1) as the eluent to afford 3.4 g of compound 7 (73% yield) as a pale yellow
1
oily liquid. H NMR (400 MHz, CDCl3, ppm) δ: 4.30 (t, 2H, J = 6.4 Hz), 3.76 (s, 4H), 1.74ꢀ
18
ACS Paragon Plus Environment

Page 19 of 38
Chemistry of Materials
1
2
3
4
5
6
1.67 (m, 2H), 1.29ꢀ1.23 (m, 18H), 0.86 (t, 3H, J = 6.8 Hz). 13C NMR (100 MHz, CDCl3, ppm)
δ: 168.9, 115.7, 80.6, 72.3, 36.0, 34.1, 31.9, 29.6, 29.5, 29.4, 29.3, 29.2, 25.6, 22.7, 14.1.
7
8
9
4-Dodecyloxy-3-cyanothiophene 8.
A
solution of 4ꢀdodecyloxyꢀ3ꢀcyanoꢀ2,5ꢀ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
dihydrothiophene (3 g 10.15 mmol) in dichloromethane (50 mL) was stirred at 50 °C then a
solution of DDQ (2.77 g, 1.2 equiv) in THF (20 mL) was slowly added. After addition, the
reaction mixture was stirred for 12 h. After addition of water (100 mL), the mixture was
extracted with dichloromethane (3 × 50 mL). The organic phase was dried over MgSO4 and
evaporated under reduced pressure. The resulting oil was purified by column chromatography
on silica gel using hexane and ethyl acetate (10:1) as the eluent to afford 2.5 g of compound 8
(84% yield) as a colorless solid. 1H NMR (400 MHz, CDCl3, ppm) δ: 7.77 (s, 1H, ArH), 6.27
(s, 1H, ArH), 4.0 (t, 2H, J = 6.6 Hz), 1.86ꢀ1.79 (m, 2H), 1.48ꢀ1.43 (m, 2H), 1.28 (m, 16H),
0.89 (t, 3H, J = 6.8 Hz). 13C NMR (100 MHz, CDCl3, ppm) δ: 157.6, 134.1, 113.4, 104.1, 97.9,
71.3, 31.9, 29.6, 29.5, 29.4, 29.3, 28.9, 25.9, 22.7, 14.1.
5-Bromo-4-dodecyloxy-3-cyanothiophene 9. NBS (1.82 g, 1.2 equiv) was slowly added
to a solution of 4ꢀdodecyloxyꢀ3ꢀcyanothiophene (2.5 g, 8.51 mmol) in chloroform (30 mL).
The reaction mixture was heated at 50 °C for 10 h in dark. After addition of water (30 ml),
the mixture was extracted with dichloromethane (3 × 50 mL). The organic phase was dried
over MgSO4 and evaporated under reduced pressure. The resulting oil was purified by
column chromatography on silica gel using hexane and ethyl acetate (10:1) as the eluent to
afford 2.9 g of compound 9 (91% yield) as a pale yellow liquid. 1H NMR (400 MHz, CDCl3,
ppm) δ: 7.79 (s, 1H, ArH), 4.24 (t, 2H, J = 6.6 Hz), 1.84ꢀ1.77 (m, 2H), 1.53ꢀ1.48 (m, 2H),
1.33ꢀ 1.28 (m, 16H), 0.896 (t, 3H, J = 6.8 Hz). 13C NMR (100 MHz, CDCl3, ppm) δ: 154.5,
133.8, 112.9, 106.6, 98.2, 75.0, 31.9, 29.9, 29.7, 29.6, 29.5, 29.4, 29.3, 25.7, 22.7, 14.1.
5,5’-Dibromo-4,4’-didodecyloxy-3,3'-dicyano-2,2'-bithiophene 10. The mixture of a
Bis(benzonitrile)palladium (II) chloride (0.06 g, 0.16 mmol), 30 mL of DMSO, and 5ꢀbromoꢀ
19
ACS Paragon Plus Environment

Chemistry of Materials
Page 20 of 38
1
2
3
4
5
6
7
8
9
4ꢀdodecyloxyꢀ3ꢀcyanothiophene (2.9 g, 7.79 mmol) was stirred. To the resulting mixture,
silver (I) fluoride (2.17 g, 2.2 equiv) was added and heated at 60 °C for 7 h. Then, the
mixture was cooled to room temperature and passed through a Celite pad, which was
successively washed with chloroform. The filtrate was washed with water and extracted with
chloroform (3 × 50 mL). The combined organic layer was dried over MgSO4 and
concentrated under reduced pressure to give a crude solid. The crude solid was purified by
column chromatography on silica gel using dichloromethane and hexane (1:2) as the eluent.
Recrystallization of the product using dichloromethane and methanol gave 1.6 g of
compound 10 (55% yield) as a yellow solid. 1H NMR (400 MHz, CDCl3, ppm) δ: 4.28 (t, 4H,
J = 6.6 Hz), 1.86ꢀ1.79 (m, 4H), 1.55ꢀ1.47 (m, 4H), 1.28 (m, 32H), 0.90 (t, 6H, J = 6.8 Hz).
13C NMR (100 MHz, CDCl3, ppm) δ: 155.4, 138.2, 112.3, 105.2, 100.3, 75.5, 31.9, 29.9, 29.7,
29.6, 29.5, 29.4, 29.3, 25.6, 22.7, 14.1. Elemental analysis: calc. for C34H50Br2N2O2S2: C,
54.98; H, 6.79; N, 3.77; S 8.63. Found: C, 54.88; H, 6.69; N, 3.78; S, 8.78%.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Polymer synthesis
PBDT-BT. Compound 2 (0.26 g, 0.39 mmol), BDT monomer (0.30 g, 0.39 mmol), and
Pd(PPh3)4 (9 mg) were dissolved in a mixture of toluene (13 mL) and DMF (1 mL). The
reaction flask was purged with nitrogen for 1 h. The solution was refluxed for 3 days at
120 °C, and then cooled to room temperature. The polymer solution was poured into
methanol (200 mL). The resulting solid was filtered and Soxhlet extracted with methanol,
acetone, hexane, and chloroform until the wash solution of each extraction was colorless. The
chloroform fraction was concentrated and poured into methanol, filtered, and dried under
1
vacuum to afford 0.26 g (72% yield) of red solid. H NMR (400 MHz, CDCl3), δ (ppm):
7.67ꢀ7.07 (m, 4H), 4.31ꢀ4.21 (br, 4H), 2.96ꢀ2.52 (br, 4H), 1.95ꢀ0.84 (br, 76H). GPC (CB,
40 °C): Mn = 14.85 kDa, Mw = 28.63 kDa, PDI = 1.93. Elemental analysis: calc. for
C58H88O2S4: C, 73.67; H, 9.38; S 13.56. Found: C, 73.26; H, 9.24; S, 13.83%.
20
ACS Paragon Plus Environment

Page 21 of 38
Chemistry of Materials
1
2
3
4
5
PBDT-BTC. Compound 6 (0.184 g, 0.26 mmol) and BDT monomer (0.20 g, 0.26 mmol)
1
6
7
were used as the monomers. Yield: 0.17 g (66%). H NMR (400 MHz, CDCl3), δ (ppm):
8
9
7.72ꢀ6.94 (m, 2H), 4.32ꢀ4.16 (br, 4H), 3.14ꢀ2.97 (br, 4H), 1.94ꢀ0.75 (br, 76H). GPC (CB,
40 °C): Mn = 11.82 kDa, Mw = 39.83 kDa, PDI = 3.37. Elemental analysis: calc. for
C60H86N2O2S4: C, 72.38; H, 8.71; N, 2.81; S 12.88. Found: C, 72.13; H, 8.48; N, 2.61; S,
13.02%.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
PBDT-BTCox. Compound 10 (0.25 g, 0.337 mmol) and BDT monomer (0.26 g, 0.337
1
mmol) were used as the monomers. Yield: 0.31 g (89%). H NMR (400 MHz, CDCl3), δ
(ppm): 8.70ꢀ6.71 (m, 2H), 4.68ꢀ3.79 (br, 8H), 1.97ꢀ0.74 (br, 76H). GPC (CB, 40 °C): Mn =
37.59 kDa, Mw = 66.73 kDa, PDI = 1.77. Elemental analysis: calc. for C60H86N2O4S4: C,
70.13; H, 8.44; N, 2.73; S 12.48. Found: C, 70.17; H, 8.24; N, 2.62; S, 12.59%.
Fabrication and characterization of the polymer solar cells
OPV fabrication. An ITO patterned glass (15 ꢂ sqꢀ1) was cleaned by sonication with warm
acetone and isopropyl alcohol followed by UVꢀozone treatment. After ITO cleansing, 9 nm
MoO3 film was formed by thermal evaporation of MoO3 powder. PC71BM (99.5%, Solenne,
Inc.) were used as acceptor materials in this study. The polymer:PC71BM blend solutions
were prepared in 1,2ꢀdichlorobenzene solution at 70 °C for 1 day. 1,8ꢀdiiodooctane (DIO)
was added into the solution (1.5 v%) and stirred for 20 minutes before spinꢀcoating. The
DIOꢀadded blend solutions were then spinꢀcast onto the MoO3 coated ITO glasses and dried
for 1 h in N2 environment, giving total thickness of 90~110 nm depending on optimum
thicknesses at respective conditions. The blend films were dried under high vacuum (<10ꢀ6
Torr) for 12 hours to completely remove the residual solvent. On the tops of these films, 6 Å
21
ACS Paragon Plus Environment

Chemistry of Materials
Page 22 of 38
1
2
3
4
5
6
7
of LiF and 120 nm of Al were formed by thermal evaporation as the cathode. The working
areas for devices were 0.055 cm2.
8
9
Electrical Characterization. The currentꢀvoltage (J-V) characteristics were measured using
a Keithley 4200 power source under AM 1.5G illumination at an intensity of 100 mW cmꢀ2
(Oriel 1 kW solar simulator) with PVM 132 reference cell certified by NREL. All electrical
measurements and fabrication processes were performed under inert N2 environment. The
external quantum efficiency (EQE) was measured using a photomodulation spectroscopy
setup (Merlin, Oriel) with monochromatic light from a xenon lamp. The power density of the
monochromatic light was calibrated using a Si photodiode certified by the National Institute
for Standards and Technology (NIST).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Supporting information
Detail characterization of polymers (TGA, CV, absorption coefficient, XRD), SCLC, contact
angle, and normalized EQE curves. This information is available free of charge via the
Internet at http://pubs.acs.org/.
Acknowledgements
H. G. Kim, M. Kim, and J. Arul Clement contributed equally to this work. This work was
supported by a grant (Code No. 2011–0031628) from the Center for Advanced Soft
Electronics under the Global Frontier Research Program of the Ministry of Science, ICT and
Future Planning, Korea. The authors thank the Pohang Accelerator Laboratory for providing
the synchrotron radiation sources at 3C and 9A beam lines used in this study.
22
ACS Paragon Plus Environment

Page 23 of 38
Chemistry of Materials
1
2
3
4
5
References
(1) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerꢀBased Organic Solar
Cells. Chem. Rev. 2007, 107, 1324.
6
7
8
9
(2) Thompson, B. C.; Fréchet, J. M. J. Polymer–Fullerene Composite Solar Cells. Angew.
Chem., Int. Ed. 2008, 47, 58.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(3) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater.
2001, 11, 15.
(4) Park, J. H.; Kim, J. S.; Lee, J. H.; Lee, W. H.; Cho, K. Effect of Annealing Solvent
Solubility on the Performance of Poly(3ꢀhexylthiophene)/Methanofullerene Solar Cells. J.
Phys. Chem. C 2009, 113, 17579.
(5) Kim, J.ꢀH.; Park, J. H.; Lee, J. H.; Kim, J. S.; Sim, M.; Shim, C.; Cho, K. Bulk
Heterojunction Solar Cells Based on Preformed Polythiophene Nanowires via Solubilityꢀ
Induced Crystallization. J. Mater. Chem. 2010, 20, 7398.
(6) Zhou, H.; Yang, L.; You, W. Rational Design of High Performance Conjugated Polymers
for Organic Solar Cells. Macromolecules 2012, 45, 607.
(7) Cheng, Y.ꢀJ.; Yang, S.ꢀH.; Hsu, C.ꢀS. Synthesis of Conjugated Polymers for Organic
Solar Cell Applications. Chem. Rev. 2009, 109, 5868.
(8) Coffin, R. C.; Peet, J.; Rogers, J.; Bazan, G. C. Streamlined MicrowaveꢀAssisted
Preparation of NarrowꢀBandgap Conjugated Polymers for High Performance Bulk
Heterojunction Solar Cells. Nat. Chem. 2009, 1, 657.
(9) Zhang, Y.; Hau, S. K.; Yip, H.ꢀL.; Sun, Y.; Acton, O.; Jen, A. K.ꢀY. Efficient Polymer
Solar Cells Based on the Copolymers of Benzodithiophene and Thienopyrroledione. Chem.
Mater. 2010, 22, 2696.
(10) Zhou, N.; Guo, X.; Ortiz, R. P.; Li, S.; Zhang, S.; Chang, R. P. H.; Facchetti, A.; Marks,
T. J. Bithiophene Imide and Benzodithiophene Copolymers for Efficient Inverted Polymer
Solar Cells. Adv. Mater. 2012, 24, 2242.
(11) Lee, J.; Jo, S. B.; Kim, M.; Kim, H. G.; Shin, J.; Kim, H.; Cho, K. Donor–Acceptor
Alternating Copolymer Nanowires for Highly Efficient Organic Solar Cells. Adv. Mater.
2014, 26, 6706.
(12) Nguyen, T. L.; Choi, H.; Ko, S.ꢀJ.; Uddin, M. A.; Walker, B.; Yum, S.; Jeong, J.ꢀE.; Yun,
M. H.; Shin, T. J.; Hwang, S.; Kim, J. Y.; Woo, H. Y. SemiꢀCrystalline Photovoltaic
Polymers with Efficiency Exceeding 9% in a ~300 nm Thick Conventional SingleꢀCell
Device. Energy Environ. Sci. 2014, 7, 3040.
(13) Ho, C.ꢀC.; Chen, C.ꢀA.; Chang, C.ꢀY.; Darling, S. B.; Su, W.ꢀF. IsoindigoꢀBased
Copolymers for Polymer Solar Cells with Efficiency over 7%. J. Mater. Chem. A 2014, 2,
8026.
23
ACS Paragon Plus Environment

Chemistry of Materials
Page 24 of 38
1
2
3
4
5
6
7
(14) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H.
Aggregation and Morphology Control Enables Multiple Cases of HighꢀEfficiency Polymer
Solar Cells. Nat. Commun. 2014, 5, 5293.
8
9
(15) Roncali, J. Molecular Engineering of the Band Gap πꢀConjugated Systems: Facing
Technological Applications. Macromol. Rapid Commun. 2007, 28, 1761.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(16) Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.; Yang, Y.; Yu, L. P.;
Wu, Y.; Li, G. Polymer Solar Cells with Enhanced OpenꢀCircuit Voltage and Efficiency. Nat.
Photonics 2009, 3, 649.
(17) He, Z.; Xiao, B.; Liu, F.; Wu, H.; Yang, Y.; Xiao, S.; Wang, C.; Russell, T. P.; Cao, Y.
SingleꢀJunction Polymer Solar Cells with High Efficiency and Photovoltage. Nat. Photonics
2015, 9, 174.
(18) Chen, J.ꢀD.; Cui, C.; Li, Y.ꢀQ.; Zhou, L.; Ou, Q.ꢀD.; Li, C.; Li, Y.; Tang, J.ꢀX. Singleꢀ
Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Adv. Mater.
2015, 27, 1035.
(19) Zhang, Q. T.; Tour, J. M. Alternating Donor/Acceptor Repeat Units in Polythiophenes.
Intramolecular Charge Transfer for Reducing Band Gaps in Fully Substituted Conjugated
Polymers. J. Am. Chem. Soc. 1998, 120, 5355.
(20) Thompson, B. C.; Kim, Y.ꢀG.; McCarley, T. D.; Reynolds, J. R. Soluble Narrow Band
Gap and Blue PropylenedioxythiopheneꢀCyanovinylene Polymers as Multifunctional
Materials for Photovoltaic and Electrochromic Applications. J. Am. Chem. Soc. 2006, 128,
12714.
(21) Hergué, N.; Mallet, C.; Savitha, G.; Allain, M.; Frére, P.; Roncali, J. Facile Synthesis of
3ꢀAlkoxyꢀ4ꢀcyanothiophenes as New Building Blocks for DonorꢀAcceptor Conjugated
Systems. Org. Lett. 2011, 13, 1762.
(22) Rudenko, A. E.; Khlyabich, P. P.; Thompson, B. C. Random Poly(3ꢀhexylthiopheneꢀcoꢀ
3ꢀcyanothiophene) Copolymers via Direct Arylation Polymerization (DArP) for Organic
Solar Cells with High OpenꢀCircuit Voltage. ACS Macro Lett. 2014, 3, 387.
(23) Casey, A.; Han, Y.; Fei, Z.; White, A. J. P.; Anthopoulos, T. D.; Heeney, M. Cyano
substituted benzothiadiazole: a novel acceptor inducing nꢀtype behaviour in conjugated
polymers. J. Mater. Chem. C 2015, 3, 265.
(24) Huo, L.; Zhang, S.; Guo, X.; Xu, F.; Li, Y.; Hou, J. Replacing Alkoxy Groups with
Alkylthienyl Groups: A Feasible Approach to Improve the Properties of Photovoltaic
Polymers. Angew. Chem., Int. Ed. 2011, 50, 9697.
(25) Hou, J.; Park, M.ꢀH.; Zhang, S.; Yao, Y.; Chen, L.ꢀM.; Li, J.ꢀH.; Yang, Y. Bandgap and
Molecular Energy Level Control of Conjugated Polymer Photovoltaic Materials Based on
Benzo[1,2ꢀb:4,5ꢀb′]dithiophene. Macromolecules 2008, 41, 6012.
24
ACS Paragon Plus Environment

Page 25 of 38
Chemistry of Materials
1
2
3
4
5
6
7
(26) Blanchard, P.; Verlhac, P.; Michaux, L.; Frére, P.; Roncali, J. Fine Tuning of the
Electronic Properties of Linear πꢀConjugated Oligomers by Covalent Bridging. Chem. Eur. J.
2006, 12, 1244.
8
9
(27) Lee, H. K. H.; Li, Z.; Constantinou, I.; So, F.; Tsang, S. W.; So, S. K. BatchꢀtoꢀBatch
Variation of Polymeric Photovoltaic Materials: its Origin and Impacts on Charge Carrier
Transport and Device Performances. Adv. Energy Mater. 2014, 4, 1400768.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(28) Liu, C.; Wang, K.; Hu, X.; Yang, Y.; Hsu, C.ꢀH.; Zhang, W.; Xiao, S.; Gong, X.; Cao, Y.
Molecular Weight Effect on the Efficiency of Polymer Solar Cells. ACS Appl. Mater.
Interfaces 2013, 5, 12163.
(29) Bartelt, J. A.; Douglas, J. D.; Mateker, W. R.; Labban, A. E.; Tassone, C. J.; Toney, M.
F.; Fréchet, J. M. J.; Beaujuge, P. M.; McGehee, M. D. Controlling SolutionꢀPhase Polymer
Aggregation with Molecular Weight and Solvent Additives to Optimize PolymerꢀFullerene
Bulk Heterojunction Solar Cells. Adv. Energy Mater. 2014, 4, 1301733.
(30) Hwang, H.; Khim, D.; Yun, J.ꢀM.; Jung, E.; Jang, S.ꢀY.; Jang, Y. H.; Noh, Y.ꢀY.; Kim,
D.ꢀY. Quinoidal Molecules as a New Class of Ambipolar Semiconductor Originating from
Amphoteric Redox Behavior. Adv. Funct. Mater. 2015, 25, 1146.
(31) Kim, H. G.; Kang, B.; Ko, H.; Lee, J.; Shin, J.; Cho, K. Synthetic Tailoring of SolidꢀState
Order in DiketopyrrolopyrroleꢀBased Copolymers via Intramolecular Noncovalent
Interactions. Chem. Mater. 2015, 27, 829.
(32) Jackson, N. E.; Savoie, B. M.; Kohlstedt, K. L.; de la Cruz, M. O.; Schatz, G. C.; Chen,
L. X.; Ratner, M. A. Controlling Conformations of Conjugated Polymers and Small
Molecules: The Role of Nonbonding Interactions. J. Am. Chem. Soc. 2013, 135, 10475.
(33) Baker, J. L.; Jimison, L. H.; Mannsfeld, S.; Volkman, S.; Yin, S.; Subramanian, V.;
Salleo, A.; Alivisatos, A. P.; Toney, M. F. Quantification of Thin Film Crystallographic
Orientation Using Xꢀray Diffraction with an Area Detector. Langmuir 2010, 26, 9146.
(34) Rivnay, J.; Mannsfeld, S. C. B.; Miller, C. E.; Salleo, A.; Toney, M. F. Quantitative
Determination of Organic Semiconductor Microstructure from the Molecular to Device Scale.
Chem. Rev. 2012, 112, 5488.
(35) Olivier, Y.; Niedzialek, D.; Lemaur, V.; Pisula, W.; Mullen, K.; Koldemir, U.; Reynolds,
J. R.; Lazzaroni, R.; Cornil, J.; Beljonne, D. HighꢀMobility Hole and Electron Transport
Conjugated Polymers: How Structure Defines Function. Adv. Mater. 2014, 26, 2119.
(36) Chen, M. S.; Niskala, J. R.; Unruh, D. A.; Chu, C. K.; Lee, O. P.; Fréchet, J. M. J.
Control of PolymerꢀPacking Orientation in Thin Films through Synthetic Tailoring of
Backbone Coplanarity. Chem. Mater. 2013, 25, 4088.
(37) Kim, K.ꢀH.; Park, S.; Yu, H.; Kang, H.; Song, I.; Oh, J. H.; Kim, B. J. Determining
Optimal Crystallinity of DiketopyrrolopyrroleꢀBased Terpolymers for Highly Efficient
Polymer Solar Cells and Transistors. Chem. Mater. 2014, 26, 6963.
(38) Rand, B. P.; Cheyns, D.; Vasseur, K.; Giebink, N. C.; Mothy, S.; Yi, Y.; Coropceanu, V.;
25
ACS Paragon Plus Environment

Chemistry of Materials
Page 26 of 38
1
2
3
4
5
6
7
Beljonne, D.; Cornil, J.; Bredas, J.ꢀL.; Genoe, J. The Impact of Molecular Orientation on the
Photovoltaic Properties of a Phthalocyanine/Fullerene Heterojunction. Adv. Funct. Mater.
2012, 22, 2987.
8
9
(39) Tumbleston, J. R.; Collins, B. A.; Yang, L.; Stuart, A. C.; Gann, E.; Ma, W.; You, W.;
Ade, H. The Influence of Molecular Orientation on Organic Bulk Heterojunction Solar Cells.
Nat. Photon. 2014, 8, 385.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(40) Jasieniak, J. J.; Seifter, J.; Jo, J.; Mates, T.; Heeger, A. J. A SolutionꢀProcessed MoOx
Anode Interlayer for Use within Organic Photovoltaic Devices. Adv. Funct. Mater. 2012, 22,
2594.
(41) Kroger, M.; Hamwi, S.; Meyer, J.; Riedl, T.; Kowalsky, W.; Kahn, A. Role of the Deepꢀ
Lying Electronic States of MoO3 in the Enhancement of HoleꢀInjection in Organic Thin Films.
Appl. Phys. Lett. 2009, 95, 123301.
(42) Bartesaghi, D.; Perez, I. C.; Kniepert, J.; Roland, S.; Turbiez, M.; Neher, D.; Koster, L. J.
A. Competition Between Recombination and Extraction of Free Charges Determines the Fill
Factor of Organic Solar Cells. Nat. Comm. 2015, 6, 7083.
(43) Proctor, C. M.; Love, J. A.; Nguyen, T.ꢀQ. Mobility Guidelines for High Fill Factor
SolutionꢀProcessed Small Molecule Solar Cells. Adv. Mater. 2014, 26, 5957.
(44) Kim, M.; Park, J. H.; Kim, J. H.; Sung, J. H.; Jo, S. B.; Jo, M.ꢀH.; Cho, K. Lateral
Organic Solar Cells with SelfꢀAssembled Semiconductor Nanowires. Adv. Energy Mater.
2015, 5, 1401317.
(45) Li, W.; Albrecht, S.; Yang, L.; Roland, S.; Tumbleston, J. R.; McAfee, T.; Yan, L.; Kelly,
M. A.; Ade, H.; Neher, D.; You, W. MobilityꢀControlled Performance of Thick Solar Cells
Based on Fluorinated Copolymers. J. Am. Chem. Soc. 2014, 136, 15566.
(46) Lampert, M. A. et al, Current Injection in Solids, Academic Press, New York, 1970.
(47) Mihailetchi, V. D.; Koster, L. J. A.; Blom, P. W. M.; Melzer, C.; Boer, B.; Duren, J. K. J.;
Janssen, R. A. J. Compositional Dependence of the Performance of Poly(pꢀphenylene
vinylene): Methanofullerene BulkꢀHeterojunction Solar Cells. Adv. Funct. Mater. 2005, 15,
795.
(48) Sim, M.; Shin, J.; Shim, C.; Kim, M.; Jo, S. B.; Kim, J. H.; Cho, K. Dependence of
Exciton Diffusion Length on Crystalline Order in Conjugated Polymers. J. Phys. Chem. C
2014, 118, 760.
(49) Markov, D. E.; Amsterdam, E.; Blom, P. W. M.; Sieval, A. B.; Hummelen, J. C. Accurate
Measurement of the Exciton Diffusion Length in a Conjugated Polymer Using a
Heterostructure with a SideꢀChain CrossꢀLinked Fullerene Layer. J. Phys. Chem. A 2005, 109,
5266.
(50) Kim, J. S.; Lee, Y.; Lee, J. H.; Park, J. H.; Kim, J. K.; Cho, K. HighꢀEfficiency Organic
Solar Cells Based on EndꢀFunctionalꢀGroupꢀModified Poly(3ꢀhexylthiophene). Adv. Mater.
2010, 22, 1355.
26
ACS Paragon Plus Environment

Page 27 of 38
Chemistry of Materials
1
2
3
4
5
Figure Captions
6
7
8
9
Scheme 1. Synthetic route of monomers. Reagents and conditions: (a) NBS,
chloroform/acetic acid, 2 h, RT, 82%; (b) AgF, Bis(benzonitrile)palladium (II) chloride,
DMSO, 60 °C, 12 h, 78%; (c) Br2, Fe, chloroform, RT, 2 h, 89% (d) nꢀBuLi, Et2O, –5 °C, 2 h,
HCl, 92%; (e) CuCN, DMF, 120 °C, 38 h, 95% (f) 1) NBS, chloroform, 50 °C, 10 h; 2) AgF,
Bis(benzonitrile)palladium (II) chloride, DMSO, 60 °C, 7 h, 40%; (g) dodecyl
methanesulfonate, Cs2CO3, DMF, microwave, 80 °C, 10 min, 73%; (h) DDQ,
THF/dichloromethane, 50 °C, 12 h, 84%; (i) NBS, chloroform, 50 °C, 10 h, 91%; (j) AgF,
Bis(benzonitrile)palladium (II) chloride, DMSO, 60 °C, 7 h, 55%.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 2. Synthetic route of new conjugated polymers.
Figure 1. (a) Normalized UVꢀvis absorption spectra of polymers in solution and thin films.
(b) Energy level diagram of polymers.
Figure 2. Resonance structures of (a) PBDTꢀBT, (b) PBDTꢀBTC and (c) PBDTꢀBTCox.
Figure 3. Calculated optimized geometries and molecular orbitals for trimers of (a) PBDTꢀ
BT, (b) PBDTꢀBTC and (c) PBDTꢀBTCox (B3LYP/6ꢀ31G*).
Figure 4. (a) 2D GIXD patterns of the polymer thin films (top) neat polymers and (bottom)
polymers/PC71BM blend films. (b) The corresponding GIXD diffractogram profiles along the
inꢀplane and outꢀofꢀplane directions.
Figure 5. (a) J-V curves of the BHJ solar cells based on PBDTꢀBT, PBDTꢀBTC and PBDTꢀ
BTCox under an illumination of AM 1.5G, 100 mA cmꢀ2. (b) EQE curves of BHJ solar cells.
Figure 6. AFM topography of PBDTꢀBT/PC71BM, PBDTꢀBTC/PC71BM and PBDTꢀ
BTCox/PC71BM without DIO (a, b, c) and with DIO (d, e, f). Scale bar = 500 nm.
27
ACS Paragon Plus Environment

Chemistry of Materials
Page 28 of 38
1
2
3
4
5
Table 1. Intrinsic properties of polymers
6
7
8
Table 2. Photovoltaic properties of polymers
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
28
ACS Paragon Plus Environment

Page 29 of 38
Chemistry of Materials
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 1
29
ACS Paragon Plus Environment