Fluorination Effects on Indacenodithienothiophene Acceptor Packing and Electronic Structure, End-Group Redistribution, and Solar Cell Photovoltaic Response

Article abstract of DOI:10.1021/jacs.8b13653

Indacenodithienothiophene (IDTT)-based postfullerene electron acceptors, such as ITIC (2,2′-[[6,6,12,12-tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene-2,8-diyl]-bis[methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile]), have become synonymous with high power conversion efficiencies (PCEs) in bulk heterojunction (BHJ) polymer solar cells (PSCs). Here we systematically investigate the influence of end-group fluorination density and positioning on the physicochemical properties, single-crystal packing, end-group redistribution propensity, and BHJ photovoltaic performance of a series of ITIC variants, ITIC-nF (n = 0, 2, 3, 4, and 6). Increasing n from 0 → 6 contracts the optical bandgap, but only marginally lowers the LUMO for n > 4. This yields enhanced photovoltaic short-circuit current density and good open-circuit voltage, so that ITIC-6F achieves the highest PCE of the series, approaching 12% in blends with the PBDB-TF donor polymer. Single-crystal diffraction reveals that the ITIC-nF molecules cofacially interleave with ITIC-6F having the shortest π-π distance of 3.28 ?. This feature together with ZINDO-level computed intermolecular electronic coupling integrals as high as 57 meV, and B3LYP/DZP-level reorganization energies as low as 147 meV, rival or surpass the corresponding values for fullerenes, ITIC-0F, and ITIC-4F, and track a positive correlation between the ITIC-nF space-charge limited electron mobility and n. Finally, a heretofore unrecognized solution-phase redistribution process between the 2-(3-oxo-indan-1-ylidene)-malononitrile-derived end-groups (EGs) of IDTT-based NFAs, i.e., EG¹-IDTT-EG¹ + EG²-IDTT-EG² ? 2 EG¹-IDTT-EG², with implications for the entire ITIC PSC field, is identified and mechanistically characterized, and the effects on PSC performance are assessed.

Full text of DOI:10.1021/jacs.8b13653

Subscriber access provided by TULANE UNIVERSITY
Article
Fluorination Effects on Indacenodithienothiophene
Acceptor Packing and Electronic Structure, End-Group
Redistribution, and Solar Cell Photovoltaic Response
Thomas J Aldrich, Micaela Matta, Weigang Zhu, Steven M. Swick, Charlotte L. Stern,
George C. Schatz, Antonio Facchetti, Ferdinand S. Melkonyan, and Tobin J. Marks
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 23 Jan 2019
Downloaded from http://pubs.acs.org on January 23, 2019
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 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 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.
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 15
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Fluorination Effects on Indacenodithienothiophene Acceptor
Packing and Electronic Structure, End-Group Redistribution,
and Solar Cell Photovoltaic Response
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
Thomas J. Aldrich,† Micaela Matta,*,† Weigang Zhu,† Steven M. Swick,† Charlotte L. Stern,† George C.
Schatz,*,† Antonio Facchetti,*,†,§ Ferdinand S. Melkonyan,*,† Tobin J. Marks*,†,‡
† Department of Chemistry and the Materials Research Center and ‡ Department of Materials Science and Engineering,
Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
§ Flexterra Corporation, 8025 Lamon Avenue, Skokie, Illinois 60077, United States
ABSTRACT: Indacenodithienothiophene (IDTT)-based postfullerene electron acceptors, such as ITIC (2,2′-[[6,6,12,12-tetrakis(4-
hexylphenyl)-6,12-dihydrodithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene-2,8-diyl]-bis[methylidyne(3-oxo-1H-indene-
2,1(3H)-diylidene)]]bis[propanedinitrile]), have become synonymous with high power conversion efficiencies (PCEs) in bulk
heterojunction (BHJ) polymer solar cells (PSCs). Here we systematically investigate the influence of end-group fluorination
density and positioning on the physicochemical properties, single-crystal packing, end-group redistribution propensity, and BHJ
photovoltaic performance of a series of ITIC variants, ITIC-nF (n = 0, 2, 3, 4, and 6). Increasing n from 0 → 6 contracts the optical
bandgap, but only marginally lowers the LUMO for n > 4. This yields enhanced photovoltaic short-circuit current density and good
open-circuit voltage, so that ITIC-6F achieves the highest PCE of the series, approaching 12% in blends with the PBDB-TF donor
polymer. Single-crystal diffraction reveals that the ITIC-nF molecules cofacially interleave with ITIC-6F having the shortest π–π
distance, 3.28Ǻ. This feature together with ZINDO-level computed intermolecular electronic coupling integrals as high as 57 meV,
and B3LYP/DZP-level reorganization energies as low as 147 meV, rival or surpass the corresponding values for fullerenes, ITIC-
0F, and ITIC-4F, and track a positive correlation between the ITIC-nF space-charge limited electron mobility and n. Finally, a
heretofore unrecognized solution-phase redistribution process between the 2-(3-oxo-indan-1-ylidene)-malononitrile-derived end-
groups (EGs) of IDTT-based NFAs, i.e., EG1-IDTT-EG1 + EG2-IDTT-EG2
2 EG1-IDTT-EG2, with implications for the entire
ITIC PSC field, is identified and mechanistically characterized, and the effects on PSC performance are assessed.
INTRODUCTION
π-Conjugated organic semiconductors are essential
components in polymer solar cells (PSCs) containing
interpenetrating bulk heterojunction (BHJ) blend photoactive
layers of solution-processable p-type (donor) polymers and n-
type (acceptor) molecules.1–5 While the dominant acceptor
materials for much of the past decade have been based on
fullerenes and rylene diimides,3,6–10 the recent advent of
indacenodithienothiophene
(IDTT)-based
nonfullerene
acceptors (NFAs) has enabled monumental increases in PSC
power conversion efficiency (PCE),11–15 now demonstrated to
exceed 17% in tandem PSCs.16
These remarkable
performance increases are partially attributed to advantageous
IDTT-based NFA properties, which include: strong oscillator
strengths, low optical bandgaps, high electron mobilities, low
internal reorganization energies, rapid charge transfer with
donor materials, and synthetic tunability.17–23
Since the first report of the IDTT-based NFA ITIC-0F
(Figure 1a),17 diverse structural variants have shown promise
in high-efficiency PSCs.11,12,14,24 Among the most noteworthy
to date is tetrafluorinated ITIC-4F (Figure 1a), which when
paired with certain wide-bandgap donor polymers, has broad
applicability in high-efficiency binary25–30 and ternary single-
junction31 as well as tandem PSCs.32 IDTT NFA end-group
fluorination downshifts the frontier molecular orbitals
Figure 1. (a) Chemical structures of the symmetrical and
unsymmetrical ITIC-nF NFAs (n = 0, 2, 3, 4, 6) and the donor
polymer PBDB-TF. (b) Thin film UV-vis spectra. (c) CV-
derived frontier MO energetics.
(FMOs), thereby lowering the open-circuit voltage (VOC
)
values in BHJ PSCs relative to those with ITIC-0F. However,
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 15
Scheme 1. Synthetic Routes to the Trifluorinated End-Group and the ITIC-nF Electron Acceptors
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
fluorination also contracts the NFA optical bandgap, and the
subsequent enhancements in short-circuit current density (JSC)
enabled by the broader BHJ optical cross-section more than
symmetrical NFAs (EG-IDTT-EG + EG’-IDTT-EG’), end-
group redistribution results in the formation of all possible
symmetrical and unsymmetrical NFA combinations, i.e., EG-
IDTT-EG, EG-IDTT-EG’, and EG’-IDTT-EG’. We also
present a materials processing methodology which minimizes
redistribution. Importantly, note that ITIC-0F, ITIC-2F, and
ITIC-4F readily participate in end-group redistribution,
whereas ITIC-3F and ITIC-6F are less reactive. 13C isotopic
compensate for the aforementioned reductions in VOC
,
affording overall higher PCEs.26,29,33,34
Nevertheless,
minimizing the tradeoff between JSC and VOC has been
identified as a critical strategy for optimizing NFA-based PSC
performance.35 However, despite the potential of fluorinated
IDTT acceptors in high-performance PSCs, systematic studies
of fluorination density and positioning effects on acceptor
properties and photovoltaic performance have not been
labeling studies yield results consistent with
a [2+2]
cycloaddition/cycloreversion mechanism for this process.
Finally, the ITIC-nF acceptors are investigated here in
BHJ PSCs with the wide-bandgap donor polymer poly{[4,8-
bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-
b']dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-
ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c']dithiophene-
1,3-diyl]]} (PBDB-TF, Figure 1a),18,41,42 previously reported in
high-efficiency binary PSCs with ITIC-0F and ITIC-
4F.25,28,30,31,42 PBDB-TF:ITIC-nF PSC chemical structure-
morphology-performance relationships are investigated using
UV-vis spectroscopy, DSC, single-crystal diffraction, cyclic
voltammetry, AFM, grazing incidence wide-angle X-ray
scattering (GIWAXS), as well as photovoltaic and space-
charge limited current (SCLC) mobility measurements. The
results underscore the beneficial effects of IDTT NFA
thoroughly carried out, especially from
experiment - theory standpoint.
a
combined
In this study, we compare and contrast the chemical
properties, electronic structures, crystal packings, chemical
reactivities, and BHJ photovoltaic performance characteristics
of an IDTT-based NFA series, ITIC-nF (Figure 1a), where the
end-group fluorination density and positioning are
systematically varied. X-ray data on other IDTT-related
acceptors anticipate close intermolecular π–π end-group
interactions,18,36–38 and therefore we hypothesized that
unsymmetrical EG-IDTT-EG’ structures with one fluorinated
and one non-fluorinated end-group (ITIC-2F and ITIC-3F,
Figure 1a) might enhance molecular ordering and electron
transport in informative ways relative to symmetrical EG-
IDTT-EG NFAs.39,40 It will be seen that comparison of the
ITIC-0F, ITIC-4F, and ITIC-6F low-temperature crystal
structures offers insight into their solid state intermolecular
interactions as does the strength and directionality of ZINDO-
level computed intermolecular electronic coupling integrals.
While ITIC-nF acceptors pack cofacially, ITIC-6F interleaves
with the shortest π–π stacking distances and greatest computed
electronic coupling between neighboring molecules.
Additionally, heavy fluorination is shown to reduce the
computed internal reorganization energies to as low as 147
meV, lower than those in fullerenes.
fluorination
hexafluorinated ITIC-6F achieving the highest PCEs,
approaching 12%, due to enhanced JSCs and comparable VOC
on
PSC
performance
metrics,
with
s
relative to ITIC-4F-based PSCs. Lastly, the effects of end-
group redistribution on the performance and BHJ morphology
of binary blend PSCs containing unsymmetrical ITIC-nF
acceptors as well as ternary blend PSCs containing two
different symmetrical ITIC-nF acceptors are scrutinized.
RESULTS AND DISCUSSION
We begin with the synthesis of the NFA series, ITIC-nF
(n = 0, 2, 3, 4, 6), and systematically investigate the effects of
end-group fluorination on their optical absorption,
electrochemical, and thermal properties as well as on their
electronic structures, including B3LYP-computed internal
We also report a previously unrecognized NFA end-group
redistribution process occurring in the blend solutions prior to
BHJ film deposition. Namely, in solutions containing one
unsymmetrical NFA (EG-IDTT-EG’) or a mixture of two
reorganization energies.
Single-crystal X-ray diffraction
ACS Paragon Plus Environment

Page 3 of 15
Journal of the American Chemical Society
elucidates the ITIC-nF geometry, solid state packing, and shown in Figures 1c and S2. All of the ITIC-nF exhibit
1
2
3
4
5
6
7
8
ZINDO-calculated intermolecular electronic coupling. ITIC-
nF solution-phase redistribution is assessed in photovoltaic
blend solutions by NMR spectroscopy and the reaction
mechanism is investigated by 13C isotopic labeling
experiments. The photovoltaic properties of the ITIC-nF
series are characterized in BHJ PSCs with the donor polymer,
PBDB-TF. The effects of redistribution on ITIC-nF PSC
performance are evaluated by comparing with BHJ blends
prepared following protocols that suppress it. The PSC BHJ
blend morphologies are characterized by AFM and GIWAXS,
and their charge transport characteristics are determined by
SCLC mobility measurements. Taken together, these results
demonstrate the photovoltaic performance benefits achieved
via structurally advantageous NFA fluorination.
reasonable energy level alignment relative to the PBDB-TF
donor polymer FMOs.11 The ITIC-nF LUMO energies
monotonically decrease with increasing n, consistent with
electron-
Table 1. ITIC-nF Acceptor Optical Absorption Properties
and Calculated Internal Reorganization Energies
Acceptor Solutiona
Filmb
opt
λmax
ε × 10–5
λmax
Eg
λint
9
(nm)
(M–1cm–1) (nm)
(eV)
1.59
1.56
1.55
1.54
1.51
(meV)c
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
ITIC-0F
ITIC-2F
ITIC-4F
ITIC-3F
ITIC-6F
668
676
682
685
695
1.62
1.74
1.95
1.70
2.00
708
722
728
730
745
155d
158
158
152
147
ITIC-nF Acceptor Synthesis.
The synthesis and
characterization of the ITIC-nF acceptors are summarized in
Scheme 1 and in the Supporting Information (Schemes S1 and
S2). First, the trifluorinated end-group 2-(4,5,6-trifluoro-3-
oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (4) is
synthesized by the condensation of malononitrile with dione 3.
Because two resulting isomers are possible, the structure of 4
is confirmed by ITIC-6F single-crystal X-ray diffraction data
(vide infra), which is itself synthesized via the Knoevenagel
condensation reaction between 4 and 6,6,12,12-tetrakis(4-
hexylphenyl)-6,12-dihydrodithieno[2,3-d:2',3'-d']-s-
a
b
Solution UV-vis spectra in PhCl (0.010 mg mL–1).
UV-vis
c
spectra of PhCl-cast films.
Calculated internal reorganization
energies at the B3LYP/DZP level of theory. d See ref. 18.
withdrawing fluorine effects.34,44 Note however that the
difference in LUMO energy between ITIC-4F and ITIC-6F
(~10 meV) is dramatically smaller than that between ITIC-0F
and ITIC-2F (70 meV), which suggests that fluorination of the
end-group fragment at the 4-position has less electronic
influence than does fluorination at the 5- or 6-positions (see
indaceno[1,2-b:5,6-b']dithiophene-2,8-dicarboxaldehyde
Scheme 1).
(5,
A two-step synthetic sequence was initially attempted for
unsymmetrical ITIC-2F. First, reaction of 5 with 1 equiv of
fluorine-free 2-(3-oxo-indan-1-ylidene)-malononitrile (6)
also Figures S5–S9).34
The ITIC-nF acceptor thermal
properties were next investigated by DSC (Figure S3). None
of the present acceptors exhibit a thermal transition below 200
°C. Rather, ITIC-2F, ITIC-3F, and ITIC-6F exhibit cold
crystallization transitions in the first DSC heating cycles at
202 °C, 213 °C, and 243 °C, respectively (Table S3).42 In
contrast, no thermal transitions are observed in the respective
second DSC heating/cooling cycles of these molecules.
affords the mono-condensed product 8.
However, the
subsequent reaction of 8 with 1.5 equiv of 2-(5,6-difluoro-3-
oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (7) yields
three major products identified to be ITIC-0F, ITIC-2F, and
ITIC-4F, indicating that both condensation and retro-
condensation of end-groups with IDTT are facile under these
reaction conditions.31,43 Nevertheless, ITIC-2F is obtained,
albeit in 15% yield due to purification challenges. We believe
that the reversibility of end-group condensation with IDTT
will ultimately aid in the concise synthesis of chemically
diverse NFA libraries. As a proof of concept, the one-pot
reaction of 5, 6, and 7 easily generates three different NFAs:
ITIC-0F, ITIC-2F, and ITIC-4F (Scheme S2). Furthermore,
this principle is utilized in the synthesis of unsymmetrical
ITIC-3F from the reaction of ITIC-6F with 1.3 equiv of 6
(Scheme 1).
Interestingly,
the
B3LYP/DZP-level
internal
reorganization energies, λint, of ITIC-2F and ITIC-4F are
slightly greater than that of ITIC-0F, while those of ITIC-3F
and ITIC-6F are lower (Table 1). Note that the λint of ITIC-6F
(147 meV) is also lower than those for fullerenes, PC61BM and
PC71BM.45 Reorganization energy is a measure of the
energetic costs attributed to structural relaxation following
ionization (electron addition), and the lower values for ITIC-
3F and ITIC-6F may favor greater intermolecular electron
carrier mobility.18,46
Solid State Packing and Intermolecular Electronic
Coupling. Solid state ordering plays a critical role in defining
both the strength and directionality of electronic coupling
interactions in organic semiconductors.47–50 Despite the
complex and often predominantly amorphous nature of BHJ
photoactive layers, the analysis of molecular single-crystal
packing provides complementary insight into structure/self-
assembly relationships that can assist in both the rational
design of new materials and in the understanding of their
charge transport properties in BHJ blends.18,51 Currently
however, there is a surprising paucity of single-crystal
structural data for IDTT-derived acceptors, including ITIC-4F,
considering their broad functionality and fundamental
importance. Consequently, single-crystals of ITIC-0F, ITIC-
2F, ITIC-4F, ITIC-6F, and a cocrystal of ITIC-0F with ITIC-
4F were grown via slow vapor diffusion, and diffraction data
were then acquired (see SI for complete detail).52
ITIC-nF Acceptor Characterization. ITIC-nF optical
absorption properties are summarized in Table 1.
In
chlorobenzene (PhCl) solutions, ITIC-nF fluorination
generally red-shifts the absorption profile and λmax (Figure S1),
however ITIC-3F is slightly red-shifted versus ITIC-4F, in
agreement with the computationally predicted trend (Table
S7). The solution extinction coefficients also exhibit an
increasing trend with ITIC-nF fluorination, consistent with
previous reports.26,33 The ITIC-nF thin film optical absorption
spectra are shown in Figure 1b. Again, increased fluorination
generally red-shifts the λmax and absorption onset, resulting in
a lower optical bandgap (Egopt), however ITIC-3F (1.54 eV)
opt
exhibits a slightly lower Eg than ITIC-4F (1.55 eV) and
ITIC-6F exhibits the lowest (1.51 eV).
The ITIC-nF frontier molecular orbital (FMO) energies
estimated from the CV oxidation and reduction potentials are
ACS Paragon Plus Environment

Journal of the American Chemical Society
The crystal structures of ITIC-0F, ITIC-4F, and ITIC-6F
are shown in Figure 2. While all three molecules crystallize in
triclinic unit cells, striking differences in intermolecular
packing and conjugated IDTT-core planarity are evident.
Notably, all three NFAs exhibit intramolecular S···O=C
interactions with distances closer than the sum of the S and O
van der
Page 4 of 15
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
Figure 2. Single-crystal structures of (a) ITIC-0F, (b) ITIC-4F, and (c) ITIC-6F. (Left) π-face-on perspective of the single molecules
highlighting the intramolecular S···O distance with the last digit uncertainty in parentheses. (Right) Crystal packing diagrams illustrating
the relationships between an arbitrary central molecule (colored) and four near neighbors. Calculated electronic coupling values, |J|, are
relative to central molecule. Alkyl chains are truncated to methyl groups and hydrogen atoms are omitted for ease of viewing.
Waals radii (3.25 Å), potentially suggesting a conformational
lock.53,54 While the central IDTT units are generally planar,
torsions between the IDTT cores and the end-groups are
observed. Both ITIC-0F and ITIC-6F exhibit relatively small
torsions of 4.1(5)° and 8.0(8)°, respectively, while that of
ITIC-4F is larger, 16.1(7)°. Furthermore, dramatic differences
in intermolecular packing are evident. Specifically, ITIC-0F
and ITIC-6F pack face-to-face in “brick-like” planes separated
by their alkyl substituents, while ITIC-4F packs in both a face-
to-face and an edge-to-face fashion (Figure 2). Note that
ITIC-2F as well as the ITIC-0F:ITIC-4F cocrystal adopt a
packing structure similar to that of ITIC-4F, however due to a
high degree of disorder about the crystallographic inversion
center and the resulting absence of any preferential
difluorinated/non-fluorinated end-group ordering, additional
analysis of these crystals was not performed (See SI and
Figures S13 and S14).55 In ITIC-0F and ITIC-6F, substantial
π–π interactions between the electron deficient end-groups
with interplanar spacings of ~3.4 Å for ITIC-0F, and 3.95 Å
and 3.28 Å for ITIC-6F are observed. Note that the close 3.28
Å ITIC-6F π–π distance is one of the shortest yet observed for
photovoltaically efficient IDTT NFAs,18,36–38,56,57 and is similar
to those observed in crystalline fullerene acceptors, such as
PC61BM and PC71BM.58,59 Additionally, remarkably close
3.16 Å CN···π distances are observed between neighboring
ITIC-6F molecules (Figure 2c). These interactions may reflect
favorable lone-pair···π electrostatic effects, which can be
observed with highly electron-deficient π-systems including
perfluoroarenes.60–62 In contrast, the closest intermolecular
distances for ITIC-4F are 3.21 Å and 3.35 Å, resulting from
S···π (lone-pair···π) and π–π interactions, respectively (Figure
2b).63,64
We next assess the impact of single-crystal packing on
intermolecular electronic coupling. First, an arbitrary ‘central’
molecule within the crystal lattice was selected as a reference.
Next, the transfer integrals between the reference and its
nearest neighbors were computed using a ZINDO Hamiltonian
(see SI for details and Tables S4–S6).18,65 The crystal structure
of ITIC-0F features four neighbors having non-negligible
transfer integrals. The largest |J| coupling value of 11.4 meV
results from the closer intermolecular π–π contacts (Figure
2a).
Tetrafluorinated ITIC-4F also exhibits four near-
neighbors, but the largest coupling arises from edge-to-face
interactions with |J| = 17.1 meV (Figure 2b). Interestingly, a
previous analysis of another ITIC-0F polymorph exhibiting
edge-to-face packing found a similar result, with a maximum
|J| value (16 meV) also arising from edge-to-face
interactions.18,56
In marked contrast, ITIC-6F exhibits
dramatically larger electronic couplings to its nearest
neighbors with |J| values of 45.2 and 56.8 meV (Figure 2c).
Note that the trend in computed couplings parallels that found
in the experimental electron mobilities (vide infra), and that
the electronic coupling in ITIC-6F is the largest yet reported
for an IDTT-based NFA and even exceeds those reported for
ACS Paragon Plus Environment

Page 5 of 15
Journal of the American Chemical Society
crystalline PC61BM.66 Interestingly, stronger coupling is not
The extent of end-group redistribution in these mixtures
was assessed by solution 19F{1H} NMR spectroscopy because
1H NMR is uninformative (Figure S18). Surprisingly, signals
attributed to both ITIC-2F and ITIC-4F are present in the
19F{1H} NMR spectrum of the conventionally-prepared
PBDB-TF:ITIC-2F active layer solution (ITIC-2F SCR
spectrum, Figure 3b). The peak ratio of ITIC-2F:ITIC-4F is
10:6, implying a 10:3 molar ratio due to ITIC-4F symmetry.
The formation of ITIC-4F in this blend indicates the existence
of a solution-phase redistribution process between ITIC-nF
1
2
3
4
5
6
7
8
found between the closely spaced π–π pairs, but rather for the
pairs sharing the close CN···π interactions, in accord with the
importance of nitrile interactions to both the intermolecular
coupling and electron mobility of some n-type organic
semiconductors.67
Lastly, the electronic coupling
directionality was assessed by summing all |J| contributions
along each crystallographic axis (see SI, Figure S4). While
coupling in ITIC-0F has the lowest magnitude, it is the most
isotropic and may enable more robust charge transport in
tortuous BHJ blends.68 In contrast, ITIC-4F and ITIC-6F
exhibit large |J| anisotropy along their a and b crystallographic
axes, respectively.
9
end-groups, i.e., 2 ITIC-2F
ITIC-4F + ITIC-0F. Note that
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
the reaction stoichiometry requires the formation of ITIC-0F
and ITIC-4F in equal quantities. Although ITIC-0F cannot be
directly quantified by 1H NMR spectroscopy due to peak
overlap (Figure S18), its presence in the final active layer
mixture is supported by HRMS analysis, where molecular ions
consistent with those expected for ITIC-0F, ITIC-2F, and
ITIC-4F are observed (Figure 3c). Thus, redistribution in the
PBDB-TF:ITIC-2F active layer blend solution generates a
final acceptor composition of ITIC-2F:ITIC-0F:ITIC-4F in a
10:3:3 molar ratio. Consequently, this blend is determined to
ITIC-nF Solution-Phase Redistribution and Chemical
Instability. Due to the reversible end-group reactivity
be 60% scrambled.
Analogously, scrambling in the
conventionally-prepared PBDB-TF:ITIC-3F active layer blend
solution is found to generate ITIC-0F and ITIC-6F (Figure
S17). However, scrambling in the ITIC-3F blend solution is
less extensive, only 5% (Table S11). This suggests slower
redistribution reaction kinetics for the trifluorinated end-group
relative to both the di- and non-fluorinated ones. Scrambling
of ITIC-2F was also investigated in PhCl solutions containing
neither PBDB-TF nor DIO, and while it is found to be
somewhat slower, it nevertheless occurs (Figure S20).
The reversibility of the end-group redistribution reaction
was additionally evaluated in active layer blend solutions
containing two different symmetrical ITIC-nF acceptors in a
1:1 molar ratio. These blend solutions were prepared
following the same conventional procedures as noted above.
Based on the combined NMR spectroscopic and HRMS data
on these blends, we conclude that the end-group redistribution
reaction is reversible. Specifically, after 16 h at 50 °C, the
PBDB-TF:ITIC-0F:ITIC-4F blend solution is found to
additionally contain the unsymmetrical acceptor ITIC-2F
(ITIC-0F:ITIC-4F SCR spectrum, Figure 3b). Likewise the
PBDB-TF:ITIC-0F:ITIC-6F blend solution is found to
additionally contain ITIC-3F (Figure S17). The extent of end-
group scrambling in the ITIC-0F:ITIC-4F and ITIC-0F:ITIC-
6F blend solutions is determined by 19F{1H} NMR
spectroscopy to be 34% and 4%, respectively (Table S11).
Again, these results indicate a reduced propensity of the
trifluorinated end-group to participate in the redistribution
process.
Figure 3. Analysis of ITIC-nF end-group redistribution in
photovoltaic active layer solutions prepared according to
conventional scrambled (SCR) conditions.
(a) Chemical
structures of ITIC-2F and ITIC-4F. (b) 19F{1H} NMR spectra of
the purified ITIC-2F and ITIC-4F NFAs compared to those of
ITIC-2F SCR and ITIC-0F:ITIC-4F SCR active layer solutions.
(c) HRMS analysis of the ITIC-2F SCR active layer solution.
observed during the syntheses of ITIC-2F and ITIC-3F, the
chemical stability of ITIC-nF molecules within photovoltaic
blend solutions was additionally evaluated. Specifically,
active layer blend solutions of PBDB-TF and ITIC-2F as well
as of PBDB-TF and ITIC-3F were first prepared following
conventional procedures, i.e., by dissolving PBDB-TF and
NFA (1:1 mass ratio) in anhydrous PhCl containing 0.5 vol%
of the commonly-utilized PSC processing additive 1,8-
diiodooctane (DIO) at 50 °C for 16 h inside a glovebox (see SI
for complete details).25,28,42 Note that the unsymmetrical ITIC-
nF acceptors were deliberately selected because end-group
redistribution in these molecules will form chemically distinct
products. As will be seen below, these conventional active
layer preparation conditions result in ITIC-nF end-group
redistribution and consequently blends prepared by these
methods will be referred to as “scrambled” and will be
denoted by SCR.
These results indicate that under standard active layer
solution preparation conditions, the end-groups of IDTT-based
NFAs undergo reversible redistribution.
In solutions
containing NFAs with chemically different end-groups, as is
the case with unsymmetrical ITIC-nF molecules or with two
different symmetrical ITIC-nF molecules, redistribution
results in the generation of new and chemically distinct
acceptors which were not initially present in the solution. Due
to the unexpected nature of this chemical instability and the
absence of any obvious catalyst, those factors giving rise to
this reactivity were further investigated.
ITIC-nF
End-Group
Redistribution
Reaction
Mechanism. The mechanism of the end-group redistribution
ACS Paragon Plus Environment

Journal of the American Chemical Society
occurring under conventional “scrambled” active layer
Page 6 of 15
1
2
3
4
5
6
7
8
solution preparation conditions was next examined. While
transition-metal-catalyzed C=C/C=C cross metathesis
reactions have been extensively characterized,69,70 relatively
less is known about uncatalyzed olefin exchange reactions,
such as the present redistribution process, including their
mechanisms.
thermally
Previously, photocatalyzed71–73 as well as
induced associative [2+2]
cycloaddition/cycloreversion (CA/CR) mechanisms via
cyclobutane intermediates have been proposed for C=C/C=C
9
redistributions.74–76
Similarly, C=N/C=C redistribution
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
between imines and Knoevenagel compounds has recently
been shown to proceed through a four-membered ring
intermediate via a CA/CR mechanism.77 However, a H2O-
catalyzed
dissociative
retro-condensation/condensation
(RC/C) mechanism via aldehyde intermediates has also been
proposed for C=C/C=C redistribution in other reports.78 Thus,
we next outline a series of 13C isotopic labeling experiments
Figure 4. Mechanistic investigation of the ITIC-nF end-group redistribution reaction. (a) Reaction between unlabeled ITIC-0F (1.0
equiv) and 13CHO-IDTT(13C)-2F (1.0 equiv) in PhCl:DIO (99.5:0.5 v:v) with PBDB-TF at 50 °C for 24 h. Expected products of the RC/C
and CA/CR mechanisms are shown. Selected regions of the (b) 19F{1H}, (c) 13C, and (d) 1H NMR spectra of the product mixture.
performed on model ITIC-nF molecules that enable the
CA/CR and RC/C redistribution mechanisms to be rigorously
distinguished. Note that ITIC-nF scrambling occurs under
light-free conditions, precluding a photocatalyzed pathway
(Figure S16).
Because no intermediates indicative of either the CA/CR
or RC/C mechanisms [i.e., cyclobutanes or aldehydes,
respectively] are observed in the 1H NMR spectra of the
photovoltaic blends prepared following conventional
procedures (Figure S18), the C=C/C=C end-group
redistribution mechanism of the ITIC-nF molecules is probed
here by isotopic labeling (Scheme S5). First, the redistribution
reaction between ITIC-0F and the 13C-labeled difluorinated
mono-condensed 13CHO-IDTT(13C)-2F was investigated under
the conventionally-prepared active layer blend solution
conditions (Figure 4a). Note that the 13CHO-IDTT(13C)-2F
aldehyde group (13CHO) reactivity is predicted to depend
dramatically on the operative reaction mechanism.
Specifically, in a CA/CR process, the 13CHO moieties (v,
Figure 4a) should be unreactive.79 In marked contrast, in a
RC/C process, the 13CHO moieties (ii, Figure 4a) are expected
to be reactive, and an equivalent of 12CHO moieties will be
formed for every 13CHO which reacts with a nucleophilic end-
group (i or iii, Figure 4a). Thus, the CA/CR and RC/C
mechanisms should be distinguishable by observing whether
or not the 13C-enrichment of the aldehyde group is preserved
during the reaction.
After reaction of ITIC-0F with 13CHO-IDTT(13C)-2F for
24 h at 50 °C, the 19F{1H} NMR spectrum reveals ~20% end-
group redistribution conversion with the major fluorinated
product identified as ITIC-2F (Figures 4b and S21).80 The 13
C
NMR analysis also indicates ~20% conversion with the major
13C-labeled product identified as 13CHO-IDTT(13C)-0F
(Figures 4c and S22). Note that the 13C signal from the
ACS Paragon Plus Environment

Page 7 of 15
Journal of the American Chemical Society
aforementioned generated ITIC-2F is largely absent indicating
that it is unlabeled (Figure S22). Next, the 1H NMR low field
1
2
3
4
5
6
7
8
9
region was examined to assess the isotopic distribution of the
aldehyde-containing products. An overlapped set of doublets
assignable to 13C-bound aldehydic H atoms on both 13CHO-
IDTT(13C)-0F and 13CHO-IDTT(13C)-2F is observed, however
virtually no singlet aldehyde signal from 12CHO groups is
present (Figure 4d and S23). This indicates that the 13CHO-
IDTT(13C)-2F aldehyde is essentially unreactive under the
conventional photovoltaic active layer solution conditions.
Analogous results are obtained when either unlabeled ITIC-4F
or ITIC-6F is reacted with 13CHO-IDTT(13C)-0F (Section 11
in the SI). Collectively, these findings indicate that the end-
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
group redistribution process does not proceed via
a
dissociative H2O-catalyzed RC/C mechanism. Moreover, no
evidence of the RC/C mechanism can be observed even under
forcing conditions, e.g. with 1 vol% H2O in the PhCl solvent
(Figure S23).81 Instead, these results are consistent with an
associative CA/CR end-group redistribution mechanism
occurring through a four-membered cyclobutane intermediate
(v, Figure 4a). As this mechanism does not produce ‘free’
nucleophilic end-groups (i and iii, Figure 4a) through a retro-
condensation process, we also conclude that end-group
redistribution in the active layer solutions proceeds by a
distinctly different mechanism than during ITIC-nF synthesis
(vide supra).
ITIC-nF Binary Blend Solar Cell Performance. Next,
PSCs with the inverted cell architecture, ITO/ZnO/active
layer/MoO3/Ag, containing a binary BHJ active layer blend of
PBDB-TF and ITIC-nF were fabricated and evaluated to
assess the photovoltaic (PV) response of the ITIC-nF
acceptors.
First, an “unscrambled” material processing
methodology was developed to reduce ITIC-nF end-group
redistribution observed in the active layer solutions prepared
following conventional procedures (vide supra). Specifically,
PBDB-TF was first pre-dissolved separately in a mixture of
PhCl:DIO (99.5:0.5 v:v)25,28 at 50 °C for 16 h, cooled to 25 °C,
and then added to the ITIC-nF acceptor to achieve a 1:1 mass
ratio in
Figure 5. Comparison of the PBDB-TF:ITIC-nF binary and ternary PSC photovoltaic performances. (a) J–V response of the binary PSCs
and (d) corresponding EQE spectra. (b) Comparison of the J–V response for the scrambled (SCR) and unscrambled binary PSCs and (e)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 15
corresponding EQE spectra. (c) Comparison of the J–V response for the scrambled (SCR) and unscrambled ternary PSCs and (f)
1
corresponding EQE spectra.
2
3
Table 2. Photovoltaic Metrics of the PBDB-TF:ITIC-nF-Based PSCs
4
5
VOC
JSC
FF
PCE
(%)c
µh × 104
µe × 104
Acceptor(s) Conditiona Scramb. %b
(V)c
(mA cm–2)c (%)c
(cm2V–1s–1)
(cm2V–1s–1)
6
7
8
9
ITIC-0F
ITIC-2F
ITIC-4F
ITIC-3F
ITIC-6F
-
-
1.00 (1.00)
0.91 (0.92)
0.91 (0.91)
0.83 (0.83)
0.90 (0.89)
0.89 (0.89)
0.81 (0.82)
14.8 (15.1)
16.7 (17.3)
16.3 (16.9)
17.6 (18.3)
17.8 (19.4)
17.9 (18.4)
21.2 (21.6)
56.5 (57.6)
8.33 (8.72)
2.0 ± 0.6
1.3 ± 1.0
0.9 ± 0.2
2.0 ± 1.1
0.5 ± 0.1
0.7 ± 0.2
1.2 ± 0.3
2.7 ± 1.0
2.0 ± 0.3
5 ± 2
-
3
65.6 (65.7)
64.8 (65.5)
67.7 (68.4)
66.0 (66.5)
64.1 (64.5)
66.7 (67.4)
10.03 (10.38)
9.67 (10.07)
9.91 (10.39)
10.55 (11.44)
10.21 (10.54)
11.41 (11.89)
SCR
60
-
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
-
5 ± 2
-
<1
5
4.4 ± 1.7
5 ± 3
SCR
-
-
3.6 ± 1.9
-
2
0.91 (0.91)
0.91 (0.91)
0.89 (0.89)
0.89 (0.89)
16.1 (17.7)
17.0 (18.6)
16.3 (17.4)
15.9 (17.0)
65.2 (65.8)
65.5 (65.1)
63.2 (63.2)
62.6 (63.1)
9.61 (10.64)
10.19 (11.01)
9.20 (9.77)
8.84 (9.51)
1.9 ± 1.1
1.3 ± 0.2
0.9 ± 0.4
1.5 ± 0.8
b
6 ± 4
ITIC-0F:
ITIC-4F
SCR
-
34
<1
4
3.6 ± 1.9
5 ± 2
ITIC-0F:
ITIC-6F
SCR
8 ± 4
a
Active layer solutions prepared following either unscrambled (-) or conventional scrambled (SCR) conditions.
Scrambling extent
assessed by 19F{1H} NMR analysis of blend solution. Photovoltaic metrics reported as the mean average of ≥10 separate devices, and
c
data for champion cell of each type are in parentheses. Complete statistical data are available in Table S10 and Figure S15.
the final solution (See SI for complete details). The PBDB-
TF:ITIC-nF solutions were stirred for only 10 min. at 25 °C
and then spin-coated onto ZnO-coated ITO substrates. This
processing methodology can easily be adapted for different
solvents and solvent additives. It also reduces the end-group
redistribution extent to ≤ 3% and enables compositional
control over BHJ blends containing unsymmetrical ITIC-nF
acceptors or blends containing multiple symmetrical ITIC-nF
acceptors (Table 2). Finally, MoO3 and Ag were vapor
deposited through a shadow mask to complete the PSC device
fabrication. In contrast to previous reports where non-
fluorinated ITIC-derivatives achieved enhanced photovoltaic
performance by BHJ thermal annealing,18,42 ITIC-3F- and
ITIC-6F-based PSCs annealed at their respective cold
crystallization temperatures exhibit dramatically reduced PV
metrics (Table S10), and consequently only PSCs with as-cast
active layers are considered here.
Current density–voltage (J–V) characteristics of the
champion PSCs for each binary blend under air mass global
(AM 1.5G) simulated solar illumination are shown in Figure
5a and complete PV metrics are summarized in Table 2. The
VOC values for these “unscrambled” PSCs exhibit a monotonic,
but not unexpected, decrease with increasing ITIC-nF
fluorination (Figure 6a). However, note that the differences in
VOC between PBDB-TF:ITIC-2F and PBDB-TF:ITIC-3F PSCs
as well as between PBDB-TF:ITIC-4F and PBDB-TF:ITIC-6F
PSCs are relatively small (~0.01 V), and parallel the trend in
ITIC-nF LUMO energies (Figure 6a). The JSC values increase
ACS Paragon Plus Environment

Page 9 of 15
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Figure 6. Analysis of unscrambled binary and ternary PBDB-TF:ITIC-nF PSC performance metrics. (a) Correlation of ITIC-nF LUMO
energy with PSC VOC values. (b) Correlation of ITIC-nF optical bandgap with PSC JSC values. VOC values for the unscrambled binary and
ternary PBDB-TF:ITIC-nF PSCs versus mole fraction (x) of (c) difluorinated and (d) trifluorinated end-groups within the acceptor phase.
Note that the PBDB-TF:ITIC-2F VOC data in (c) is obscured by that of PBDB-TF:ITIC-0F:ITIC-4F.
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
monotonically with the inverse of ITIC-nF optical bandgap
ITIC-nF Ternary and Redistributed Quaternary
Blend Solar Cell Performance. Additionally, the influence
of end-group redistribution on the solar cell performances of
the unsymmetrical ITIC-nF acceptors was evaluated. First,
active layer solutions of PBDB-TF:ITIC-2F, and PBDB-
TF:ITIC-3F were prepared following conventional procedures,
identical to the “unscrambled” PSCs with the one alteration
being that both PBDB-TF and ITIC-nF were co-dissolved for
16 h at 50 °C before active layer deposition. As demonstrated
in Figure 3, ITIC-nF acceptors participate in an end-group
redistribution process under these conditions. Thus, these are
actually quaternary photovoltaic BHJ blends containing the
donor polymer as well as three chemically distinct ITIC-nF
acceptors. The PV metrics for these “scrambled” ‘binary’
blend PSCs are denoted by SCR in Table 2. The champion J–
V responses and corresponding EQE spectra for the SCR
blends are compared directly to their “unscrambled”
counterparts in Figures 5b,e. Interestingly, the PCEs of the
ITIC-2F SCR and ITIC-3F SCR PSCs are only marginally
lower than those of the corresponding unscrambled blend
PSCs with similar VOC and JSC values, but slightly decreased
FF values (Table 2).
Next, considering the current intense interest in BHJ
optimization via ternary component strategies as well as the
demonstrated high performance of ternary PSCs containing
two IDTT-derived NFAs,31,82–85 PSCs containing ITIC-0F and
either ITIC-4F or ITIC-6F were further investigated. While
perhaps not optimal for photovoltaic performance, the two
NFAs were studied at a 1:1 molar ratio to enable comparison
with the binary PBDB-TF:ITIC-2F and PBDB-TF:ITIC-3F
PSCs, where a 1:1 fluorinated:non-fluorinated end-group ratio
opt
(Figure 6b). Specifically, the highest Eg NFA, ITIC-0F,
opt
delivers a JSC of 15.1 mA cm–2 and the lowest Eg ITIC-6F-
based PSCs deliver an impressive JSC of 21.6 mA cm–2. These
trends in JSC are in close agreement with the external quantum
efficiency (EQE) spectra (Figure 5d) and the respective
integrated current densities (Table S10). The fill factor (FF)
values for the present ITIC-nF PSCs are all similar ~66-68%,
except for those of the ITIC-0F-based PSCs, which are lower,
~57%. Consequently, due to the advantages of high JSC and
FF as well as acceptable VOC, the ITIC-6F PSCs achieve nearly
12% PCE, which is greater than the PCEs obtained with any of
the other NFAs studied here. PSCs containing ITIC-3F
achieve the second highest performance, delivering up to
11.44% PCE. The ITIC-2F and ITIC-4F PSCs achieve similar
performances of 10.38% and 10.39%, respectively -- far
higher than those obtained with ITIC-0F (8.72%), and in
accord with previous reports.26,29
Overall, the present ITIC-nF acceptors containing the
trifluorinated end-group (ITIC-3F and ITIC-6F) achieve
superior PV performance metrics versus the analogous
acceptors containing the difluorinated end-group (ITIC-2F and
ITIC-4F). Notably, the trifluorinated end-group imparts a
lower ITIC-nF optical bandgap, which appears to be beneficial
for increased PSC JSC values (Figure 6b) due to more-efficient
low-energy photon collection between ~770–850 nm (Figure
5d).
However, the additional fluorine atom on the
trifluorinated end-group only marginally reduces ITIC-nF
LUMO energy and PSC VOC value versus those for the
respective difluorinated end-group-derived ITIC-nF acceptors
(Figure 6a). These two factors, enhanced JSC and minimally
reduced VOC, enable higher photovoltaic PCEs for the PBDB-
TF:ITIC-6F and PBDB-TF:ITIC-3F PSCs versus those for the
PBDB-TF:ITIC-4F and PBDB-TF:ITIC-2F PSCs.35
is also present.
Active layer solutions were prepared
following both “scrambled” and “unscrambled” conditions.
Again, note that ternary active layers prepared following
“scrambled” conditions are actually quaternary blends. The
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 15
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
Figure 7. 2D GIWAXS patterns of the scrambled and unscrambled PBDB-TF:ITIC-nF binary and ternary BHJ blends and of the pristine
PBDB-TF film.
champion J–V responses and corresponding EQE spectra for
all ‘ternary’ PSCs are shown in Figures 5c,f and all PV data
are tabulated in Table 2. Interestingly, the VOC values of the
scrambled and unscrambled blends are similar. The effects of
end-group scrambling on the ‘ternary’ PSC PCEs are
divergent. Specifically, the ITIC-0F:ITIC-4F SCR devices
deliver higher PCEs (up to 11.01%) versus their unscrambled
counterparts (10.64% PCE) due to marginally enhanced JSC
values. However, ITIC-0F:ITIC-6F SCR PSCs (9.77% PCE)
exhibit slightly reduced performance versus the respective
unscrambled PSCs (9.51% PCE) due to modestly lower JSC
values.
BHJ Film Morphology and Charge Transport. The
influence of ITIC-nF fluorination and end-group redistribution
on the BHJ blend film morphologies and charge transport
were next investigated. The BHJ blends were subjected to the
same optimal conditions as for the PSC fabrication described
above. The pristine material and BHJ blend morphologies and
surface topologies were characterized by GIWAXS and AFM,
respectively.107,108 The pristine ITIC-0F, ITIC-2F, ITIC-3F,
and ITIC-4F films exhibit a weak out-of-plane π–π scattering
reflection with d010 ≈ 3.4-3.5 Å (Figure S24).82 Interestingly,
the ITIC-6F pristine film exhibits virtually no scattering
reflections, consistent with minimal crystallinity and in accord
with the DSC results discussed above. Similar to previous
reports, the pristine PBDB-TF donor polymer films exhibit
strong preferential π-face-on orientation with a π–π distance of
d010 = 3.67 Å and in-plane lamellar stacking reflections with
d100 = 21.2 Å (Figure 7).18,41
The 2D GIWAXS patterns of the BHJ films are shown in
Figure 7 and data are summarized in Tables S12 and S13. The
binary and ternary BHJ photovoltaic blend films all exhibit an
out-of-plane π–π stacking reflection with d010 ≈ 3.6 Å and an
in-plane lamellar stacking reflection with d100 ≈ 21 Å, both
attributable to the PBDB-TF donor. Due to overlap between
the π–π scattering reflections of PBDB-TF and those of the
ITIC-nF, as well as lack of additional BHJ processing
treatments which tend to crystallize IDTT NFAs, such as
thermal annealing,18,42 no crystalline stacking reflections
attributable to the ITIC-nF species are observed in the BHJ
blend GIWAXS patterns (Figures 7 and S25). Additionally,
no significant trends in PBDB-TF crystallographic parameters
correlate with the observed PSC performance metrics.
Furthermore, the AFM topographical and phase images of the
PBDB-TF:ITIC-nF blend film surfaces (Figures S27 and S28)
are all similar, with roughnesses of ~2–4 nm, with no
significant influence of ITIC-nF fluorination or end-group
scrambling. The collective GIWAXS and AFM results
indicate that all of the present binary, ternary, and quaternary
BHJ blends exhibit essentially indistinguishable morphologies,
not influenced by end-group redistribution or the choice of
ITIC-nF acceptor(s), in accord with recent observations on
ternary PBDB-TF:IDTT-based NFA BHJ blends.31,82
Interestingly, no significant NFA scrambling effects on
the PV metrics of the present binary and ternary PSCs are
observed (Table 2). We attribute the PV similarities to two
factors: 1) The present ITIC-nF acceptors exhibit a high
degree of miscibility within the BHJ blends and consequently
form an “electronic alloy”,86–89 as evidenced by the linear VOC
dependence on the mole fraction of fluorinated end-groups
within the acceptor phase (Figures 6c,d).90,91 The tuning of
PSC VOC through acceptor composition variation was recently
reported in ternary blend PSCs containing two other IDTT-
based NFAs.82,85
Furthermore, as noted above, such
miscibility is also evident in the ITIC-2F single-crystal
diffraction data (Figure S13) as well as in the ITIC-0F + ITIC-
4F cocrystal (Figure S14), where preferred fluorinated/non-
fluorinated end-group ordering is absent. Thus, scrambling
likely does not affect PSC VOC values to a large degree
because the generated acceptors are likely to also be miscible
with the majority of the acceptor phase and the mole fraction
of fluorinated end-groups in the acceptor material phase
remains constant. 2) Redistribution does not significantly
impact the BHJ morphology or charge transport (vide infra) --
properties which heavily influence JSC and FF metrics.5,92
Note that scrambling between other less-miscible NFAs
having different FMO energies may more significantly impact
PSC performance and BHJ morphology than in the present
systems.93 Consequently, we believe that many of the reported
‘ternary’ blend PSCs fabricated following conventional active
layer solution procedures and containing two or more IDTT-
derived NFAs, each with different end-groups, warrant further
scrutiny to assess both the extent and impact of scrambling on
PSC performance.30,31,82,94–106
The charge transport properties in the direction
orthogonal to the substrate of the pristine materials and BHJ
ACS Paragon Plus Environment

Page 11 of 15
Journal of the American Chemical Society
blends were investigated using a SCLC model.109 Hole-only
cylcloaddition/cycloreversion mechanism.
In ternary
1
2
3
4
5
6
7
8
ITO/MoO3/organic/MoO3/Ag diodes and electron-only
ITO/ZnO/organic/LiF/Al diodes were measured to extract the
hole (μh) and electron (μe) mobilities of the organic films,
respectively. Data are summarized in Tables 2 and S14.
Overall the μe values of the pristine ITIC-nF films increase
with greater n, consistent with previous observations of
fluorination effects in IDTT-related electron acceptors (Figure
S29).26,33,34 In the present work, the pristine ITIC-0F, ITIC-4F,
and ITIC-6F μes trend upward from 3.7 to 5.1 to 5.7 × 10–4
cm2V–1s–1, which mirrors the trend in single-crystal
intermolecular electronic couplings (Figure 2). However, the
photovoltaic blends containing two different symmetrical
ITIC-nF acceptors, this redistribution reaction results in the in-
situ generation of the respective unsymmetrical acceptor.
Therefore, the extent and impact of end-group redistribution
on PSC performance in ternary blends of two or more different
IDTT-related acceptors should be scrutinized in future studies.
The present results highlight the importance of understanding
and controlling the solution-phase stability of ITIC-derived
postfullerene acceptors, and identify the hexafluorinated ITIC-
6F as a promising material for future high-efficiency PSCs.
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
ASSOCIATED CONTENT
Supporting Information
Material synthesis and characterization, UV-vis and NMR spectra,
CV, DSC, PSC device fabrication, AFM, GIWAXS, SCLC
measurements, and DFT computations are available in the
Supporting Information. This material is available free of charge
via the Internet at http://pubs.acs.org.
present
pristine
ITIC-nF
films
are
largely
amorphous/nanocrystalline as noted in the GIWAXS data
discussion (vide supra). Consequently, this correlation may
indicate that the enhanced intermolecular coupling seen in the
increasingly fluorinated acceptor crystals is also present in the
less crystalline spin-coated films (Figure S24).
The μes of the present pristine ITIC-nF films are similar in
magnitude to the μes for the corresponding BHJ blends,
indicating that efficient electron transport pathways are largely
unaffected by the presence of the donor polymer, similar to
other previous reports on IDTT-related materials.21,26
However, the μe trend observed in the present ITIC-nF pristine
films is not preserved in the BHJ blends. The BHJ blend μh
and μe values are in the range of 0.5–2 × 10–4 cm2V–1s–1 and 2–
8 × 10–4 cm2V–1s–1, respectively (Table 2). Overall, no
significant differences in either μh or μe are detected between
AUTHOR INFORMATION
Corresponding Authors
*E-mail micaela.matta@northwestern.edu
*E-mail g-schatz@northwestern.edu
*E-mail a-facchetti@northwestern.edu
*E-mail f-melkonyan@northwestern.edu
*E-mail t-marks@northwestern.edu
ORCID
any of the blends.
This similarity, despite dramatic
differences in PSC performance metrics, indicates that charge
transport within the present blends is likely not a limiting
factor in PSC performance. Instead, the collective data
strongly suggest that structurally advantageous fluorination of
ITIC-nF acceptors optimizes the optical cross-sections and
enhances JSC values (Figure 6b), while minimally lowering the
LUMO energy levels and VOC values (Figure 6a), and plays a
significant role in determining the photovoltaic responses of
the present NFAs.35
Thomas J. Aldrich: 0000-0001-7169-4052
Micaela Matta: 0000-0002-9852-3154
Weigang Zhu: 0000-0002-5888-4481
George C. Schatz: 0000-0001-5837-4740
Antonio Facchetti: 0000-0002-8175-7958
Ferdinand S. Melkonyan: 0000-0001-8228-9247
Tobin J. Marks: 0000-0001-8771-0141
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
CONCLUSIONS
A series of IDTT-based postfullerene electron acceptors, ITIC-
nF, was synthesized and the effects of fluorination extent and
distribution on the chemical properties, crystal structures,
Notes
The authors declare no competing financial interest.
redistribution
propensities,
charge
mobilities,
BHJ
morphologies, and PSC photovoltaic metrics were
systematically investigated. Increasing ITIC-nF end-group
fluorination narrows the optical bandgap and deepens the
LUMO energy. Single-crystal structure analysis reveals that
ITIC-6F packs cofacially with π–π distances as short as 3.28
Å, calculated intermolecular electronic couplings as high as 57
meV, and internal reorganization energies as low as 147 meV.
These data rival or surpass the corresponding values for
fullerenes, ITIC-0F, and ITIC-4F. Relative to the state-of-the-
art acceptor ITIC-4F, hexafluorinated ITIC-6F exhibits a
lower optical bandgap but only a minimally deepened LUMO
energy, enabling greater PSC performance metrics with PCE
approaching 12% in BHJ blends with the donor polymer
PBDB-TF.
ACKNOWLEDGMENT
This research was supported in part by the Center for Light
Energy Activated Redox Processes (LEAP), an Energy Frontier
Research Center funded by the U.S. Department of Energy, Office
of Science, Office of Basic Energy Sciences under Award DE-
SC0001059, by the U.S. Department of Energy, Office of
Science, and Office of Basic Energy Sciences under Award
Number DE-FG02-08ER46536, AFOSR grant FA9550-18-1-
0320, and the Northwestern University Materials Research
Science and Engineering Center under NSF grant DMR-1720139.
F.S.M. was supported by award 70NANB14H012 from the U.S.
Department of Commerce, National Institute of Standards and
Technology as part of the Center for Hierarchical Materials
Design (CHiMaD). This work made use of the EPIC, Keck-II,
and/or SPID facilities of Northwestern’s NUANCE Center, which
received support from the Soft and Hybrid Nanotechnology
Experimental (SHyNE) Resource (NSF ECCS-1542205). We
thank the Integrated Molecular Structure Education and Research
Center (IMSERC) for characterization facilities supported by
Northwestern University, National Science Foundation (NSF)
Lastly, we report on a heretofore unrecognized solution-
phase redistribution process occurring between the end-groups
of IDTT-based postfullerene acceptors, i.e., EG1-IDTT-EG1 +
EG2-IDTT-EG2
2 EG1-IDTT-EG2. Investigation of the
redistribution reaction with isotopically labeled ITIC-nF
model substrates yields results consistent with a [2+2]
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 15
under NSF CHE-1048773, Soft and Hybrid Nanotechnology
1
2
3
4
5
6
7
8
Experimental (SHyNE) Resource (NSF NNCI-1542205), the State
of Illinois, and International Institute for Nanotechnology (IIN).
Use of the Advanced Photon Source, an Office of Science User
Facility operated for the U.S. Department of Energy Office of
Science by Argonne National Laboratory, was supported by the
U.S. DOE under Contract DE-AC02-06CH11357. We thank N.
Clark (Northwestern University) for assistance with GIWAXS
data collection. T.J.A. and S.M.S. thank the NSF for predoctoral
fellowships.
9
REFERENCES
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) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer
Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal
Donor-Acceptor Heterojunctions. Science 1995, 270, 1789–1791.
(2) Inganäs, O. Organic Photovoltaics over Three Decades. Adv.
Mater. 2018, 1800388.
(3) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L.
Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev.
2015, 115, 12666–12731.
(19) Yao, H.; Cui, Y.; Yu, R.; Gao, B.; Zhang, H.; Hou, J. Design,
Synthesis, and Photovoltaic Characterization of Small Molecular
Acceptor with an Ultra-Narrow Band Gap. Angew. Chem. 2017, 129,
3091–3095.
a
(20) Eastham, N. D.; Logsdon, J. L.; Manley, E. F.; Aldrich, T. J.;
Leonardi, M. J.; Wang, G.; Powers-Riggs, N. E.; Young, R. M.; Chen, L.
X.; Wasielewski, M. R.; Melkonyan, F. S.; Chang, R. P. H.; Marks, T. J.
Hole-Transfer Dependence on Blend Morphology and Energy Level
Alignment in Polymer:ITIC Photovoltaic Materials. Adv. Mater. 2018, 30,
1704263.
(21) Yi, X.; Gautam, B.; Constantinou, I.; Cheng, Y.; Peng, Z.; Klump,
E.; Ba, X.; Ho, C. H. Y.; Dong, C.; Marder, S. R.; Reynolds, J. R.; Tsang,
S.-W.; So, F. Impact of Nonfullerene Molecular Architecture on Charge
Generation, Transport, and Morphology in PTB7-Th-Based Organic Solar
Cells. Adv. Funct. Mater. 2018, 28, 1802702.
(22) Ziffer, M. E.; Jo, S. B.; Zhong, H.; Ye, L.; Liu, H.; Lin, F.; Zhang,
J.; Li, X.; Ade, H. W.; Jen, A. K.-Y.; Ginger, D. S. Long-Lived, Non-
Geminate, Radiative Recombination of Photogenerated Charges in a
Polymer/Small-Molecule Acceptor Photovoltaic Blend. J. Am. Chem. Soc.
2018, 140, 9996–10008.
(23) Fei, Z.; Eisner, F. D.; Jiao, X.; Azzouzi, M.; Rohr, J. A.; Han, Y.;
Shahid, M.; Chesman, A. S. R.; Easton, C. D.; McNeill, C. R.;
Anthopoulos, T. D.; Nelson, J.; Heeney, M. An Alkylated
Indacenodithieno[3,2-b]thiophene-Based Nonfullerene Acceptor with
High Crystallinity Exhibiting Single Junction Solar Cell Efficiencies
Greater than 13% with Low Voltage Losses. Adv. Mater. 2018, 30,
1705209.
(24) Yao, Z.; Liao, X.; Gao, K.; Lin, F.; Xu, X.; Shi, X.; Zuo, L.; Liu,
F.; Chen, Y.; Jen, A. K.-Y. Dithienopicenocarbazole-Based Acceptors for
Efficient Organic Solar Cells with Optoelectronic Responses Over 1000
nm and an Extremely Low Energy Loss. J. Am. Chem. Soc. 2018, 140,
2054–2057.
(25) Zheng, Z.; Hu, Q.; Zhang, S.; Zhang, D.; Wang, J.; Xie, S.; Wang,
R.; Qin, Y.; Li, W.; Hong, L.; Liang, N.; Liu, F.; Zhang, Y.; Wei, Z.;
Tang, Z.; Russell, T. P.; Hou, J.; Zhou, H. A Highly Efficient Non-
Fullerene Organic Solar Cell with a Fill Factor over 0.80 Enabled by a
Fine-Tuned Hole-Transporting Layer. Adv. Mater. 2018, 1801801.
(26) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J.
Molecular Optimization Enables over 13% Efficiency in Organic Solar
Cells. J. Am. Chem. Soc. 2017, 139, 7148–7151.
(27) Li, S.; Ye, L.; Zhao, W.; Yan, H.; Yang, B.; Liu, D.; Li, W.; Ade,
H.; Hou, J. A Wide Band Gap Polymer with a Deep Highest Occupied
Molecular Orbital Level Enables 14.2% Efficiency in Polymer Solar
Cells. J. Am. Chem. Soc. 2018, 140, 7159–7167.
(28) Li, W.; Ye, L.; Li, S.; Yao, H.; Ade, H.; Hou, J. A High-
Efficiency Organic Solar Cell Enabled by the Strong Intramolecular
Electron Push-Pull Effect of the Nonfullerene Acceptor. Adv. Mater.
2018, 1707170.
(4) Zhang, J.; Tan, H. S.; Guo, X.; Facchetti, A.; Yan, H. Material
insights and challenges for non-fullerene organic solar cells based on
small molecular acceptors. Nat. Energy 2018, 3, 720–731.
(5) Mazzio, K. A.; Luscombe, K. The future of organic photovoltaics.
Chem. Soc. Rev. 2015, 44, 78-90.
(6) Heeger, A. J. 25th Anniversary Article: Bulk Heterojunction Solar
Cells: Understanding the Mechanism of Operation. Adv. Mater. 2014, 26,
10–28.
(7) Holliday, S.; Li, Y.; Luscombe, C. K. Recent advances in high
performance donor-acceptor polymers for organic photovoltaics. Prog.
Polym Sci. 2017, 70, 34–51.
(8) Hartnett, P. E.; Timalsina, A.; Matte, H. S. S. R.; Zhou, N.; Guo,
X.; Zhao, W.; Facchetti, A.; Chang, R. P. H.; Hersam, M. C.;
Wasielewski, M. R.; Marks, T. J. Slip-Stacked Perylenediimides as an
Alternative Strategy for High Efficiency Nonfullerene Acceptors in
Organic Photovoltaics. J. Am. Chem. Soc. 2014, 136, 16345–16356.
(9) Zhong, Y.; Trinh, M. T.; Chen, R.; Purdum, G. E.; Khlyabich, P.
P.; Sezen, M.; Oh, S.; Zhu, H.; Fowler, B.; Zhang, B.; Wang, W.; Nam,
C.-Y.; Sfeir, M. Y.; Black, C. T.; Steigerwald, M. L.; Loo, Y.-L.; Ng, F.;
Zhu, X.-Y.; Nuckolls, C. Molecular helices as electron acceptors in high-
performance bulk heterojunction solar cells. Nat. Commun. 2015, 6, 8242.
(10) Sisto, T. J.; Zhong, Y.; Zhang, B.; Trinh, M. T.; Miyata, K.;
Zhong, X.; Zhu, X.-Y.; Steigerwald, M. L.; Ng, F.; Nuckolls, C. Long,
Atomically Precise Donor–Acceptor Cove-Edge Nanoribbons as Electron
Acceptors. J. Am. Chem. Soc. 2017, 139, 5648–5651.
(11) Yan, C.; Barlow, S.; Wang, Z.; Yan, H.; Jen, A. K.-Y.; Marder, S.
R.; Zhan, X. Non-fullerene acceptors for organic solar cells. Nat. Rev.
Mater. 2018, 3, 18003.
(12) Hou, J.; Inganäs, O.; Friend, R. H.; Gao, F. Organic solar cells
based on non-fullerene acceptors. Nat. Mater. 2018, 17, 119–128.
(13) Zhang, G.; Zhao, J.; Chow, P. C. Y.; Jiang, K.; Zhang, J.; Zhu, Z.;
Zhang, J.; Huang, F.; Yan, H. Nonfullerene Acceptor Molecules for Bulk
Heterojunction Organic Solar Cells. Chem. Rev. 2018, 118, 3447–3507.
(14) Wadsworth, A.; Moser, M.; Marks, A.; Little, M. S.; Gasparini,
N.; Brabec, C. J.; Baran, D.; McCulloch, I. Critical review of the
molecular design progress in non-fullerene electron acceptors towards
commercially viable organic solar cells. Chem. Soc. Rev. 2018,
DOI:10.1039/C7CS00892A.
(15) Che, X.; Li, Y.; Qu, Y.; Forrest, S. R. High fabrication yield
organic tandem photovoltaics combining vacuum- and solution-processed
subcells with 15% efficiency. Nat. Energy 2018, 3, 422–427.
(16) Meng, L.; Zhang, Y.; Wan, X.; Li, C.; Zhang, X.; Wang, Y.; Ke,
X.; Xiao, Z.; Ding, L.; Xia, R.; Yip, H.-L.; Cao, Y.; Chen, Y. Organic and
solution-processed tandem solar cells with 17.3% efficiency. Science
2018, 361, 1094–1098.
(17) Lin, Y.; Wang, J.; Zhang, Z.-G.; Bai, H.; Li, Y.; Zhu, D.; Zhan,
X. An Electron Acceptor Challenging Fullerenes for Efficient Polymer
Solar Cells. Adv. Mater. 2015, 27, 1170-1174.
(18) Swick, S. M.; Zhu, W.; Matta, M.; Aldrich, T. J.; Harbuzaru, A.;
Navarette, J. T. L.; Ortiz, R. P.; Kohlstedt, K. L.; Schatz, G. C.; Facchetti,
A.; Melkonyan F. S.; Marks, T. J. Closely packed, low reorganization
energy π-xtended postfullerene acceptors for efficient polymer solar cells.
Proc. Natl. Acad. Sci. U.S.A. 2018, 115, E8341–E8348.
(29) Fan, B.; Du, X.; Liu, F.; Zhong, W.; Ying, L.; Xie, R.; Tang, X.;
An, K.; Xin, J.; Li, N.; Ma, W.; Brabec, C. J.; Huang, F.; Cao, Y. Fine-
tuning of the chemical structure of photoactive materials for highly
efficient organic photovoltaics. Nat. Energy 2018, 3, 1051–1058.
(30) Yu, R.; Yao, H.; Hong, L.; Qin, Y.; Zhu, J.; Cui, Y.; Li, S.; Hou,
J. Design and application of volatilizable solid additives in non-fullerene
organic solar cells. Nat. Commun. 2018, 9, 4645.
(31) Gao, B.; Yao, H.; Hou, J.; Yu, R.; Hong, L.; Xu, Y.; Hou, J.
Multi-component non-fullerene acceptors with tunable bandgap structures
for efficient organic solar cells. J. Mater. Chem. A. 2018, 6, 23644–23649.
(32) Chen, W.; Zhang, J.; Xu, G.; Xue, R.; Li, Y.; Zhou, Y.; Hou, J.;
Li, Y. A Semitransparent Inorganic Perovskite Film for Overcoming
ACS Paragon Plus Environment

Page 13 of 15
Journal of the American Chemical Society
Ultraviolet Light Instability of Organic Solar Cells and Achieving 14.03%
Efficiency. Adv. Mater. 2018, 1800855.
(33) Zhao, F.; Dai, S.; Wu, Y.; Zhang, Q.; Wang, J.; Jiang, Li, Ling,
Q.; Wei, Z.; Ma, W.; You, W.; Wang, C.; Zhan, X. Single-Junction
Binary-Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency.
Adv. Mater. 2017, 29, 1700144.
(34) Dai, S.; Zhao, F.; Zhang, Q.; Lau, T.-K.; Li, T.; Liu, K.; Ling, Q.;
Wang, C.; Lu, X.; You, W.; Zhan, X. Fused Nonacylic Electron Acceptors
for Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2017, 139, 1366–
1343.
(35) Gasparini, N.; Wadsworth, A.; Moser, M.; Baran, D.; McCulloch,
I.; Brabec, C. J. The Physics of Small Molecule Acceptors for Efficient
and Stable Bulk Heterojunction Solar Cells. Adv. Energy Mater. 2018, 8,
1703298.
(36) Shi, X.; Zuo, L.; Jo, S. B.; Gao, K.; Lin, F.; Liu, F.; Jen, A. K.-Y.
Design of a Highly Crystalline Low-Band Gap Fused-Ring Electron
Acceptor for High-Efficiency Solar Cells with Low Energy Loss. Chem.
Mater. 2017, 29, 8369–8376.
(37) Liang, Z.; Li, M.; Zhang, X.; Wang, Q.; Jiang, Y.; Tian, H.; Geng,
Y. Near-infrared absorbing non-fullerene acceptors with selenophene as π
bridges for efficient organic solar cells. J. Mater. Chem. A 2018, 6, 8059–
8067.
(38) Shi, X.; Liao, X.; Gao, K.; Zuo, L.; Chen, J.; Zhao, J.; Liu, F.;
Chen, Y.; Jen, A. K.-Y. An Electron Acceptor with Broad Visible-NIR
Absorption and Unique Solid State Packing for As-Cast High
Performance Binary Organic Solar Cells. Adv. Funct. Mater. 2018,
1802324.
(39) Williams, J. H. The Molecular Electric Quadrupole Moment and
Solid-State Architecture. Acc. Chem. Res. 1993, 26, 593–598.
(40) Okamoto, T.; Nakahara, K.; Saeki, A.; Seki, S.; Oh, J. H.;
Akkerman, H. B.; Bao, Z.; Matsuo, Y. Aryl−Perfluoroaryl Substituted
Tetracene: Induction of Face-to-Face π−π Stacking and Enhancement of
Charge Carrier Properties. Chem. Mater. 2011, 23, 1646−1649.
(41) Zhang, M.; Guo, X.; Ma, W.; Ade, H.; Hou, J. A Large-Bandgap
Conjugated Polymer for Versatile Photovoltaic Applications with High
Performance. Adv. Mater. 2015, 27, 4655–4660.
(42) Aldrich, T. J.; Swick, S. M.; Melkonyan, F. S.; Marks, T. J.
Enhancing Indacenodithiophene Acceptor Crystallinity via Substituent
Manipulation Increases Organic Solar Cell Efficiency. Chem. Mater.
2017, 29, 10294–10298.
(43) Tietze, L. F.; Beifuss, U. The Knoevenagel Reaction. In
Comprehensive Organic Synthesis; Trost, B. M., Eds.; Pergamon Press:
New York, 1991; Vol. 2, pp 341–394.
(44) Zhang, Q.; Kelly, M. A.; Bauer, N.; You, W. The Curious Case of
Fluorination of Conjugated Polymers for Solar Cells. Acc. Chem. Res.
2017, 50, 2401-2409.
(45) Tummala, N. R.; Zheng, Z.; Aziz, S. G.; Coropceanu, V.; Brédas,
J.-L. J. Phys. Chem. Lett. 2015, 6, 3657–3662.
(46) Friederich, P.; Meded, V.; Poschlad, A.; Neumann, T.; Rodin, V.;
Stehr, V.; Symalla, F.; Danilov, D.; Lüdemann, G.; Fink, R. F.; Kondov,
I.; von Wrochem, F.; Wenzel, W. Molecular Origin of the Charge Carrier
Mobility in Small Molecule Organic Semiconductors. Adv. Funct. Mater.
2016, 26, 5757–5763.
(47) Sutton, C.; Risko, C.; Brédas, J.-L. Noncovalent Intermolecular
Interactions in Organic Electronic Materials: Implications for the
Molecular Packing vs Electronic Properties of Acenes. Chem. Mater.
2016, 28, 3–16.
(48) Burnett, E. K.; Ly, J.; Niazi, M. R.; McCuskey, S. R.; Amassian,
A.; Smilgies, D.-M.; Mannsfeld, S. C. B.; Briseno, A. L. Bistetracene
Thin Film Polymorphic Control to Unravel the Effect of Molecular
Packing on Charge Transport. Adv. Mater. Interfaces 2018, 5, 1701607.
(49) Olivier, Y.; Niedzialek, D.; Lemaur, V.; Pisula, W.; Müllen, K.;
Koldemir, U.; Reynolds, J. R.; Lazzaroni, R.; Cornil, J.; Beljonne, D. 25th
Anniversary Article: High-Mobility Hole and Electron Transport
Conjugated Polymers: How Structure Defines Function. Adv. Mater.
2014, 26, 2119–2136.
(50) Tsutsui, Y.; Schweicher, G.; Chattopadhyay, B.; Sakurai, T.;
Arlin, J.-B.; Ruzié, C.; Aliev, A.; Ciesielski, A.; Colella, S.; Kennedy, A.
R.; Lemaur, V.; Olivier, Y.; Hadji, R.; Sanguinet, L.; Castet, F.; Osella, S.;
Dudenko, D.; Beljonne, D.; Cornil, J.; Samorì, P.; Seki, S.; Geerts, Y. H.
Unraveling Unprecedented Charge Carrier Mobility through Structure
Property Relationship of Four Isomers of Didodecyl[1]benzothio[3,2-
b][1]benzothiophene. Adv. Mater. 2016, 28, 7106–7114.
1885952; ITIC-2F: 1885955; ITIC-4F: 1885953; ITIC-0F:ITIC-4F:
1885956; ITIC-6F: 1885954.
1
2
3
4
5
6
7
8
(53) Jackson, N. E.; Savoie, B. M.; Kohlstedt, K. L.; Olvera de la
Cruz, M.; 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, 10475–10483.
(54) Huang, H.; Yang, L.; Facchetti, A.; Marks, T. J. Organic and
Polymeric Semiconductors Enhanced by Noncovalent Conformational
Locks. Chem. Rev. 2017, 117, 10291–10318.
(55) Sherman, J. B.; Moncino, K.; Baruah, T.; Wu, G.; Parkin, S. R.;
Purushothaman, B.; Zope, R.; Anthony, J.; Chabinyc, M. L. Crystalline
Alloys of Organic Donors and Acceptors Based on TIPS-Pentacene. J.
Phys. Chem. C 2015, 119, 20823–20832.
(56) Han, G.; Guo, Y.; Song, X.; Wang, Y.; Yi, Y. Terminal π–π
stacking determines three-dimensional molecular packing and isotropic
charge transport in an A–π–A electron acceptor for non-fullerene organic
solar cells. J. Mater. Chem. C 2017, 5, 4852–4857.
(57) Yan, D.; Liu, W.; Yao, J.; Zhan, C. Fused-Ring Nonfullerene
Acceptor Forming Interpenetrating J-Architecture for Fullerene-Free
Polymer Solar Cells. Adv. Energy Mater. 2018, 1800204.
(58) Rispens, M. T.; Meetsma, A.; Rittberger, R.; Brabec, C. J.;
Sariciftci, N. S.; Hummelen, J. C. Influence of the solvent on the crystal
structure of PCBM and the efficiency of MDMO-PPV:PCBM ‘plastic’
solar cells. Chem. Commun. 2003, 0, 2116–2118.
(59) Umeyama, T.; Miyata, T.; Jakowetz, A. C.; Shibata, S.; Kurotobi,
K.; Higashino, T.; Koganezawa, T.; Tsujimoto, M.; Gélinas, S.; Matsuda,
W.; Seki, S.; Friend, R. H.; Imahori, H. Regioisomer effects of
[70]fullerene mono-adduct acceptors in bulk heterojunction polymer solar
cells. Chem. Sci. 2017, 8, 181–188.
(60) Giese, M.; Albrecht, M.; Rissanen, K. Anion–π Interactions with
Fluoroarenes. Chem. Rev. 2015, 115, 8867–8895.
(61) Schottel, B. L.; Chifotides, H. T.; Dunbar, K. R. Anion-π
interactions. Chem. Soc. Rev. 2008, 37, 68–83.
(62) Mallet, C.; Moussallem, C.; Faurie, A.; Allain, M.; Gohier, F.;
Skene, W. G.; Frère, P. Rational Topological Design of Fluorescence
Enahcement upon Aggregation of Distyrylfuran Derivatives. Chem. Eur.
J. 2015, 21, 7944–7953.
(63) Beno, B. R.; Yeung, K.-S.; Bartberger, M. D.; Pennington, L. D.;
Meanwell, N. A. A Survey of the Role of Noncovalent Sulfur Interactions
in Drug Design. J. Med. Chem. 2015, 58, 4383–4438.
(64) Morgado, C. A.; McNamara, J. P.; Hillier, I. H.; Burton, N. A.;
Vincent, M. A. Density Functional and Semiempirical Molecular Orbital
Methods Including Dispersion Corrections for the Accurate Description of
Noncovalent Interactions Involving Sulfur-Containing Molecules. J.
Chem. Theory Comput. 2007, 3, 1656–1664.
(65) Kirkpatrick, J. An Approximate Method for Calculating Transfer
Integrals Based on the ZINDO Hamiltonian. Int. J. Quantum Chem. 2006,
108, 51–56.
(66) Gajdos, F.; Oberhofer, H.; Dupuis, M.; Blumberger, J. Correction
to “Inapplicability of Electron-Hopping Models for the Organic
Semiconductor Phenyl-C61-butyric Acid Methyl Ester (PCBM)”. J. Phys.
Chem. Lett. 2014, 5, 2765–2766.
(67) Gruntz, G.; Lee, H.; Hirsch, L.; Castet, F.; Toupance, T.; Briseno,
A. L.; Nicolas, Y. Nitrile Substitution Effect on Triphenodioxazine-Based
Materials for Liquid-Processed Air-Stable n-Type Organic Field Effect
Transistors. Adv. Electron. Mater. 2015, 1, 1500072.
(68) Heiber, M. C.; Kister, K.; Baumann, A.; Dyakonov, V.; Deibel,
C.; Nguyen, T.-Q. Impact of Tortuosity on Charge-Carrier Transport in
Organic Bulk Heterojunction Blends. Phys. Rev. Appl. 2017, 8, 05043.
(69) Trnka, T. M.; Grubbs, R. H. The Development of L2X2Ru=CHR
Olefin Metathesis Catalysts: An Organometallic Success Story. Acc.
Chem. Res. 2001, 34, 18–29.
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
(70) Connon, S. J.; Blechert, S. Recent Developments in Olefin Cross-
Metathesis. Angew. Chem. Int. Ed. 2003, 42, 1900–1923.
(71) Wei, P.; Zhang, J.-X.; Zhao, Z.; Chen, Y.; He, X.; Chen, M.;
Gong, J.; Sung, H. H.-Y.; Williams, I. D.; Lam, J. W. Y.; Tang, B. Z.
Multiple yet Controllable Photoswitching in a Single AIEgen System. J.
Am. Chem. Soc. 2018, 140, 1966–1975.
(72) Chung J. W.; You, Y.; Huh, H. S.; An, B.-K.; Yoon, S.-J.; Kim, S.
H.; Lee, S. W.; Park, S. Y. Shear- and UV-Induced Fluorescence
Switching in Stilbenic π-Dimer Crystals Powered by Reversible [2 + 2]
Cycloaddition. J. Am. Chem. Soc. 2009, 131, 8163–8172.
(73) Novak, K.; Enkelmann, V.; Wegner, G.; Wagener, K. B.
Crystallographic Study of a Single Crystal to Single Crystal
Photodimerization and Its Thermal Reverse Reaction. Angew. Chem. Int.
Ed. 1993, 32, 1614–1616.
(51) Anthony, J. E. Functionalized Acenes and Heteroacenes for
Organic Electronics. Chem. Rev. 2006, 106, 5028–5048.
(52) All single-crystal structures are available in the SI and from the
CCDC database with the following identification numbers: ITIC-0F:
13
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 14 of 15
(74) Schaumann, E.; Ketcham, R. [2+2]-Cycloreversions. Angew.
Conversion Efficiency of 11.6% by Inheriting the Advantages of Binary
Chem. Int. Ed. Engl. 1982, 21, 225–247.
(75) Kataoka, F.; Shimizu, N.; Nishida, S. Cycloaddition Reaction of
Cells. ACS Energy Lett. 2018, 3, 555–561.
1
2
3
4
5
6
7
8
(95) Zhang, M.; Gao, W.; Zhang, F.; Mi, Y.; Wang, W.; An, Q.;
Wang, J.; Ma, X.; Miao, J.; Hu, Z.; Liu, X.; Zhang, J.; Yang, C. Efficient
ternary non-fullerene polymer solar cells with PCE of 11.92% and FF of
76.5%. Energy Environ. Sci. 2018, 11, 841–849.
(96) Wang, J.; Gao, W.; An, Q.; Zhang, M.; Ma, X.; Hu, Z.; Zhang, J.;
Yang, C.; Zhang, F. Ternary non-fullerene polymer solar cells with an
efficiency of 11.6% by simultaneously optimizing photon harvesting and
phase separation. J. Mater. Chem. A 2018, 6, 11751–11758.
(97) Yu, R.; Zhang, S.; Yao, H.; Guo, B.; Li, S.; Zhang, H.; Zhang,
M.; Hou, J. Two Well-Miscible Acceptors Work as One for Efficient
Fullerene-Free Organic Solar Cells. Adv. Mater. 2017, 29, 1700437.
(98) Jiang, W.; Yu, R.; Liu, Z.; Peng, R.; Mi, D.; Hong, L.; Wei, Q.;
Hou, J.; Kuang, Y.; Ge, Z. Ternary Nonfullerene Polymer Solar Cells with
12.16% Efficiency by Introducing One Acceptor with Cascading Energy
Level and Complementary Absorption. Adv. Mater. 2018, 30, 1703005.
(99) Ma, X.; Gao, W.; Yu, J.; An, Q.; Zhang, M.; Hu, Z.; Wang, J.;
Tang, W.; Yang, C.; Zhang, F. Ternary nonfullerene polymer solar cells
with efficiency >13.7% by integrating the advantages of the materials and
two binary cells. Energy Environ. Sci. 2018, 11, 2134–2141.
(100) Liu, T.; Luo, Z.; Fan, Q.; Zhang, G.; Zhang, L.; Gao, W.; Guo,
X.; Ma, W.; Zhang, M.; Yang, C.; Li, Y.; Yan, H. Use of two structurally
similar small molecule acceptors enabling ternary organic solar cells with
high efficiencies and fill factors. Energy Environ. Sci. 2018, 11, 3275–
3282.
Some
Representative
1-Cyclopropyl-1,3-butadienes
with
Tetracyanoethylene and Reaction of the Resultant Vinylcyclobutanes. An
Easy Vinylcyclobutane-Cyclohexene Rearrangement. J. Am. Chem. Soc.
1980, 102, 711–716.
(76) Hall, H. K. Jr. Bond-Forming Initiation in Spontaneous Addition
and Polymerization Reactions of Alkenes. Angew. Chem. Int. Ed. Engl.
1983, 22, 440–455.
(77) Gu, R.; Flidrova, K.; Lehn, J.-M. Dynamic Covalent Metathesis in
the C=C/C=N Exchange between Knoevenagel Compounds and Imines. J.
Am. Chem. Soc. 2018, 140, 5560–5568.
(78) Kulchat, S.; Meguellati, K.; Lehn, J.-M. Organocatalyzed and
Uncatalyzed C=C/C=C and C=C/C=N Exchange Processes between
Knoevenagel and Imine Compounds in Dynamic Covalent Chemistry.
Helv. Chim. Acta 2014, 97, 1219–1236.
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
(79) Indeed no reaction between ITIC-4F and dialdehyde 13CHO-
IDTT-13CHO is observed, however this does not confirm the CA/CR
mechanism as a preference for condensation onto the 12CHO-IDTT(12C)-
2F over the dialdehyde in the RC/C mechanism could also explain these
results. See Scheme S5 and Figure S23.
(80) The set of doublets in the 19F{1H} NMR spectrum (Figure 4b)
with ~3% abundance are attributable to ITIC-4F, which is likely produced
from subsequent self-scrambling of the generated ITIC-2F.
(81) We note that H2O does increase the redistribution extent, which
we attribute to the higher dielectric constant of the wet solvent. We
observe the redistribution trend of PhMe < PhCl < 1,2,-dichlorobenzene
which matches the trend in solvent dielectric (Figure S20).
(82) Zhang, H.; Yao, H.; Hou, J.; Zhu, J.; Zhang, J.; Li, W.; Yu, R.;
Gao, B.; Zhang, S.; Hou, J. Over 14% Efficiency in Organic Solar Cells
Enabled by Chlorinated Nonfullerene Small-Molecule Acceptors. Adv.
Mater. 2018, 1800613.
(83) Lee, J.; Ko, S.-J.; Seifrid, M.; Lee, H.; McDowell, C.; Luginbuhl,
B. R.; Karki, A.; Cho, K.; Nguyen, T.-Q.; Bazan, G. C. Design of
Nonfullerene Acceptors with Near-Infrared Light Absorption Capabilities.
Adv. Energy Mater. 2018, 1801209.
(101) Li, Y.; Lin, J.-D.; Liu, X.; Qu, Y.; Wu, F.-P.; Liu, F.; Jiang, Z.-
Q.; Forrest, S. R. Near-Infrared Ternary Tandem Solar Cells. Adv. Mater.
2018, 1804416.
(102) Wu, W.; Zhang, G.; Xu, X.; Wang, S.; Li, Y.; Peng, Q. Wide
Bandgap Molecular Acceptors with
a Truxene Core for Efficient
Nonfullerene Polymer Solar Cells: Linkage Position on Molecular
Configuration and Photovoltaic Properties. Adv. Funct. Mater. 2018, 28,
1707493.
(103) Tang, D.; Wan, J.; Xu, X.; Lee, Y. W.; Woo, H. Y.; Feng, K.;
Peng, Q. Naphthobistriazole-based wide bandgap donor polymers for
efficient non-fullerene organic solar cells: Significant fine-tuning
absorption and energy level by backbone fluorination. Nano Energy 2018,
53, 258–269.
(104) Xue, P.; Xiao, Y.; Li, T.; Dai, S.; Jia, B.; Liu, K.; Wang, J.; Lu,
X.; Han, R. P. S.; Zhan, X. High-performance ternary organic solar cells
with photoresponse beyond 1000 nm. J. Mater. Chem. A. 2018, 6, 24210–
24215.
(105) Zhang, J.; Liu, W.; Chen, S.; Xu, S.; Yang, C.; Zhu, X. One-Pot
Synthesized Electron-Acceptor Composite Enables Efficient Fullerene-
Free Ternary Solar Cells. J. Mater. Chem. A. 2018, 6, 22519–22525.
(106) Bi, Z.; Zhu, Q.; Xu, X.; Naveed, H. B.; Sui, X.; Xin, J.; Zhang,
L.; Li, T.; Zhou, K.; Liu, X.; Zhan, X.; Ma, W. Efficient Quaternary
Organic Solar Cells with Parallel-Alloy Morphology. Adv. Funct. Mater.
2019, 1806804.
(84) Cheng, P.; Wang, J.; Zhang, Q.; Huang, W.; Zhu, J.; Wang, R.;
Chang, S.-Y.; Sun, P.; Meng, L.; Zhao, H.; Cheng, H.-W.; Huang, T.; Liu,
Y.; Wang, C.; Zhu, C.; You, W.; Zhan, X.; Yang, Y. Unique Energy
Alignments of a Ternary Material System toward High-Performance
Organic Photovoltaics. Adv. Mater. 2018, 1801501.
(85) Zhang, J.; Yan, C.; Wang, W.; Xiao, Y.; Lu, X.; Barlow, S.;
Parker, T. C.; Zhan, X.; Marder, S. R. Panchromatic Ternary Photovoltaic
Cells Using
a Nonfullerene Acceptor Synthesized Using C–H
Functionalization. Chem. Mater. 2018, 30, 309–313.
(86) Khlyabich, P. P.; Burkhart, B.; Thompson, B. C. Efficient Ternary
Blend Bulk Heterojunction Solar Cells with Tunable Open-Circuit
Voltage. J. Am. Chem. Soc. 2011, 133, 14534–14537.
(87) Savoie, B. M.; Dunaisky, S.; Marks, T. J.; Ratner, M. A. The
Scope and Limitations of Ternary Blend Organic Photovoltaics. Adv.
Energy Mater. 2015, 5, 1400891.
(88) Schwarze, M.; Tress, W.; Beyer, B.; Gao, F.; Scholz, R.;
Poelking, C.; Ortstein, K.; Günther, A. A.; Kasemann, D.; Andrienko, D.;
Leo, K. Band structure engineering in organic semiconductors. Science,
2016, 352, 1446–1449.
(89) Eastham, N. D.; Dudnik, A. S.; Harutyunyan, B.; Aldrich, T. J.;
Leonardi, T. J.; Manley, E. F.; Butler, M. R.; Harschneck, T.; Ratner, M.
A.; Chen, L. X.; Bedzyk, M. J.; Melkonyan, F. S.; Facchetti, A.; Chang,
R. P. H.; Marks, T. J. Enhanced Light Absorption in Fluorinated Ternary
Small-Molecule Photovoltaics. ACS Energy Lett. 2017, 2, 1690–1697.
(90) Street, R. A.; Davies, D.; Khlyabich, P. P.; Burkhart, B.;
Thompson, B. C. Origin of the Tunable Open-Circuit Voltage in Ternary
Blend Bulk Heterojunction Organic Solar Cells. J. Am. Chem. Soc. 2013,
135, 986–989.
(91) Lu, L.; Kelly, M. A.; You, W.; Yu, L. Status and prospects for
ternary organic photovoltaics. Nat. Photonics 2015, 9, 491–500.
(92) Bartesaghi, D.; Perez Idel, C.; Kniepert, J.; Roland, S.; Turbiez,
M.; Neher, D.; Koster, L. J. Competition between Recombination and
Extraction of Free Charges Determines the Fill Factor of Organic Solar
Cells. Nat. Commun. 2015, 6, 7083.
(93) Liu, X.; Yan, Y.; Yao, Y.; Liang, Z. Ternary Blend Strategy for
Achieving High-Efficiency Organic Solar Cells with Nonfullerene
Acceptors Involved. Adv. Funct. Mater. 2018, 1802004.
(107) Salleo, A.; Kline, J. R.; DeLongchamp, D. M.; Chabinyc, M. L.
Microstructural Characterization and Charge Transport in Thin Films of
Conjugated Polymers. Adv. Mater. 2010, 22, 3812–3838.
(108) 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, 5488–5519.
(109) Murgatroyd, P. N. Theory of space-charge-limited current
enhanced by Frenkel effect. J. Phys. D: Appl. Phys. 1970, 3, 151–156.
Insert Table of Contents artwork here:
(94) Hu, Z.; Zhang, F.; An, Q.; Zhang, M.; Ma, X.; Wang, J.; Zhang,
J.; Wang, J. Ternary Nonfullerene Polymer Solar Cells with a Power
14
ACS Paragon Plus Environment

Page 15 of 15
Journal of the American Chemical Society
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
15
ACS Paragon Plus Environment