Linear side chains in benzo[1,2-b:4,5-b′]dithiophene-thieno[3,4-c] pyrrole-4,6-dione polymers direct self-assembly and solar cell performance

Article abstract of DOI:10.1021/ja400365b

While varying the size and branching of solubilizing side chains in π-conjugated polymers impacts their self-assembling properties in thin-film devices, these structural changes remain difficult to anticipate. This report emphasizes the determining role that linear side-chain substituents play in poly(benzo[1,2-b:4,5-b′]dithiophene-thieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers for bulk heterojunction (BHJ) solar cell applications. We show that replacing branched side chains by linear ones in the BDT motifs induces a critical change in polymer self-assembly and backbone orientation in thin films that correlates with a dramatic drop in solar cell efficiency. In contrast, we show that for polymers with branched alkyl-substituted BDT motifs, controlling the number of aliphatic carbons in the linear N-alkyl-substituted TPD motifs is a major contributor to improved material performance. With this approach, PBDTTPD polymers were found to reach power conversion efficiencies of 8.5% and open-circuit voltages of 0.97 V in BHJ devices with PC₇₁BM, making PBDTTPD one of the best polymer donors for use in the high-band-gap cell of tandem solar cells.

Full text of DOI:10.1021/ja400365b

Communication
pubs.acs.org/JACS
Linear Side Chains in Benzo[1,2‑b:4,5‑b′]dithiophene−
Thieno[3,4‑c]pyrrole-4,6-dione Polymers Direct Self-Assembly and
Solar Cell Performance
Clement Cabanetos,† Abdulrahman El Labban,† Jonathan A. Bartelt,‡ Jessica D. Douglas,§
́
William R. Mateker,‡ Jean M. J. Frechet,†,§ Michael D. McGehee,‡ and Pierre M. Beaujuge*,†
́
†King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia
‡Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States
§Department of Chemistry and Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California
94720, United States
S
* Supporting Information
chains4 are expected to direct the polymer self-assembly in thin
ABSTRACT: While varying the size and branching of
solubilizing side chains in π-conjugated polymers impacts
their self-assembling properties in thin-film devices, these
structural changes remain difficult to anticipate. This
report emphasizes the determining role that linear side-
chain substituents play in poly(benzo[1,2-b:4,5-b′]-
dithiophene−thieno[3,4-c]pyrrole-4,6-dione)
(PBDTTPD) polymers for bulk heterojunction (BHJ)
solar cell applications. We show that replacing branched
side chains by linear ones in the BDT motifs induces a
critical change in polymer self-assembly and backbone
orientation in thin films that correlates with a dramatic
drop in solar cell efficiency. In contrast, we show that for
polymers with branched alkyl-substituted BDT motifs,
controlling the number of aliphatic carbons in the linear N-
alkyl-substituted TPD motifs is a major contributor to
improved material performance. With this approach,
PBDTTPD polymers were found to reach power
conversion efficiencies of 8.5% and open-circuit voltages
of 0.97 V in BHJ devices with PC71BM, making
PBDTTPD one of the best polymer donors for use in
the high-band-gap cell of tandem solar cells.
films. Thus, changes in the size and branching of the pendant
groups alter the π−π stacking and the lamellar distances
between polymer backbones, which has a significant effect on
the charge transport properties and in turn the device
performance.4,5 However, the factors that determine polymer
crystallite orientation in thin films, implying a preferential
backbone orientation relative to the device substrate, remain a
matter of some debate.3a,6 In particular, structural changes
induced by the side-chain pattern are difficult to anticipate,
specifically when several types of substituents are appended
along the polymer backbone. Ultimately, it is expected that the
ability to direct backbone orientation through material design
will provide access to improved efficiencies in thin-film devices.
In this report, we examined the effect of linear side chain
substitutions in poly(benzo[1,2-b:4,5-b′]dithiophene−thieno-
[3,4-c]pyrrole-4,6-dione) (PBDTTPD) polymers (Figure 1) on
both material self-assembly and solar cell performance. Earlier
work showed that some of the highest open-circuit voltages
(VOC) (ca. 0.9 V) and fill factors (FF) (ca. 70%) can be
achieved in conventional BHJ solar cells using PBDTTPD as
the donor and phenyl-C61-butyric acid methyl ester (PCBM)
lassically induced by alkyl substituents appended to the π-
Cconjugated main chain, the solution processability of
polymer donors and their intimate mixing with fullerene
acceptors allow for the preparation of efficient bulk
heterojunction (BHJ) solar cells.1 Beyond the film-forming
properties, nanoscale ordering in the BHJ active layer governs
the material and device performance.1c,2 Ordering aspects
include (i) backbone-to-backbone self-assembly, (ii) prefer-
ential backbone orientation relative to the device substrate (i.e.,
“face on” vs “edge on”), (iii) paracrystalline disorder, (iv)
formation of cocontinuous donor and acceptor domains and
less-ordered mixed phases, and (v) ordering at the mesoscale
(i.e., extended crystallinity). The degree of structural order
accounts for the density of ordered domains across the BHJ,
while the correlation lengths and domain sizes may vary.2c
Both the molecular structure of the motifs in the π-
conjugated main chain3 and the pattern of solubilizing side
Figure 1. Synthesis of PBDTTPD(R1/R2) derivatives bearing alkyl
side chains with various lengths and branching.
Received: January 12, 2013
Published: March 8, 2013
© 2013 American Chemical Society
4656
dx.doi.org/10.1021/ja400365b | J. Am. Chem. Soc. 2013, 135, 4656−4659

Journal of the American Chemical Society
Communication
as the acceptor.4a,7 In particular, the high VOC and FF of
PBDTTPD make it an outstanding candidate for use in the
high-band-gap solar cell of tandem solar cells.8 In this system,
TPD substituents with various sizes and branching impart
distinct molecular packing distances and varying degrees of
nanostructural order in thin films. These structural variations
have been found to correlate with solar cell efficiency, with
power conversion efficiencies (PCEs) ranging from 4% to ca.
7%.4a Other TPD-based polymer derivatives have shown PCEs
of >7%9 and >8% in inverted devices.10
Since π-conjugated polymers appended with solubilizing
linear n-alkyl solubilizing substituents can show reduced π−π
stacking distances between backbones and improved nano-
structural order in BHJ solar cells relative to their branched-
alkyl-substituted counterparts,4a two PBDTTPD polymers with
comparable number-average molecular weights (Mn = 36/38
kDa) and polydispersity indexes (PDI ≈ 2) were synthesized:
one with n-tetradecyl (C14)-substituted BDT and n-octyl (C8)-
substituted TPD motifs [denoted as PBDTTPD(C14/C8)]
and the other with branched 2-ethylhexyl (2EH) and C8
substituents [denoted as PBDTTPD(2EH/C8)]. Synthetic
details and characterization data are given in the Supporting
Information (SI). In earlier work, we showed that for linear
alkyls, a greater number of aliphatic carbons is needed to impart
solubilizing properties comparable to those obtained with
shorter branched alkyl substituents (e.g., linear C14 sub-
stituents were shown to be appropriate alternatives to branched
2EH side chains).11
and 1,8-diiodooctane (DIO) have previously been shown to
help optimize the blend morphology in BHJ polymer solar cells
with PCBM.11b,12 Here, CN proved more effective than DIO in
blends of PBDTTPD(2EH/C8) with PC71BM. In contrast,
solar cells made from PBDTTPD(C14/C8) using the same
processing conditions showed dramatic decreases in both JSC
(9.1 mA/cm2) and FF (42%); the latter could be improved to
53% using CN, albeit at the expense of JSC (8.3 mA/cm2).
Overall, the PCE of the optimized devices did not exceed 4.1%,
corresponding to a nearly 2-fold reduction in solar cell
performance. To explore a possible correlation between the
higher alkyl content in PBDTTPD(C14/C8) and material
performance, a PBDTTPD derivative with n-dodecyl (C12)-
substituted BDT motifs [PBDTTPD(C12/C8)] was synthe-
sized following the same experimental protocol (see the SI).
However, shortening the linear substituents on the BDT motifs
resulted in greatly reduced solubility, thus limiting the polymer
yield. Solar cells made from PBDTTPD(C12/C8) showed
notably reduced JSC (6.8 mA/cm2), FF (51%), and PCE (av
3.1%), while devices cast from CN-containing blends showed
even lower PCEs (av 2.5%). The current density−voltage (J−
V) curves and external quantum efficiency (EQE) spectra of the
optimized devices are shown in Figure 2a,b. The decreases in
T h i n -fi l m B H J s o l a r c e l l s w i t h o p t i m i z e d
PBDTTPD:PC71BM blend ratios (1:1.5 w/w) were fabricated
from chlorobenzene (CB). The standard device architecture
ITO/PEDOT:PSS/PBDTTPD:PC71BM/Ca/Al was used
throughout the study. Similar device fabrication and opti-
mization procedures were used for all of the PBDTTPD
derivatives (see the SI).
As shown in Table 1, solar cells fabricated from the control
polymer PBDTTPD(2EH/C8) achieved PCEs of ca. 7%,
combining a high FF of 68%, and a large VOC of 0.96 V. In
parallel, devices cast from blends containing 5% (v/v) of the
processing additive 1-chloronaphthalene (CN) showed im-
proved short-circuit current densities (JSC = 12.5 mA/cm2) and
reached PCEs of 7.5%. Small-molecule additives such as CN
Figure 2. (a) Characteristic J−V curves of BHJ solar cells fabricated
from PBDTTPD(2EH/C8), PBDTTPD(C14/C8), and PBDTTPD-
(C12/C8) under AM1.5G illumination (100 mW/cm2). (b) EQE
spectra of the BHJ devices. (c) GIXS patterns of (left) PBDTTPD-
(2EH/C8) and (right) PBDTTPD(C14/C8) in optimized BHJs with
PC71BM. The scattering intensity is plotted on a logarithmic scale and
normalized in each GIXS pattern.
Table 1. Photovoltaic Performance of the PBDTTPD(R1/
R2) Derivatives in Standard BHJ Devices with PC71BM
a
PCE [%]
b
R1/R2
CN
JSC [mA/cm2] VOC [V]
FF
av
max
2EH/C8
N
Y
10.8
12.5
9.1
0.96
0.93
0.90
0.93
0.92
0.89
0.97
0.97
0.96
0.96
0.68
0.65
0.42
0.53
0.51
0.45
0.71
0.70
0.57
0.62
6.9
7.3
3.3
3.8
3.1
2.5
7.1
8.3
4.5
6.3
7.0
7.5
3.4
4.1
3.2
2.6
7.3
8.5
4.8
6.6
C14/C8
C12/C8
2EH/C7
2EH/C6
N
Y
EQE (<50% at 550 nm) are consistent with the dramatic drops
in JSC observed in the solar cells made from the all-linear-alkyl-
substituted polymers PBDTTPD(C14/C8) and PBDTTPD-
(C12/C8).
Grazing-incidence X-ray scattering (GIXS) data can be used
to correlate polymer side-chain patterns, nanostructural order
in thin films, and device performance effectively.4 Using GIXS,
we examined PBDTTPD(2EH/C8) and PBDTTPD(C14/C8)
both as neat polymer films (see the SI) and in optimized BHJs
with PC71BM (Figure 2c); all of the films were prepared from
CB solutions containing 5% (v/v) CN. The GIXS data show
that the two polymers adopt distinct orientations in thin films.
8.3
N
Y
6.8
6.5
N
Y
10.6
12.6
8.7
N
Y
11.1
a
Optimized devices with a polymer:PC71BM ratio of 1:1.5 (w/w) were
used. All of the devices were solution-cast from chlorobenzene (CB).
b
Devices were prepared from blends without (N) or with (Y) 5% (v/
v) 1-chloronaphthalene (CN) as a processing additive.
4657
dx.doi.org/10.1021/ja400365b | J. Am. Chem. Soc. 2013, 135, 4656−4659

Journal of the American Chemical Society
Communication
The scattering pattern of PBDTTPD(2EH/C8) shows a partial
arc at q ≈ 1.76 Å−1, characteristic of π−π stacking, and
corresponding to a spacing of ca. 3.6 Å. The peak intensity is
particularly pronounced in the out-of-plane direction (qxy ≈ 0),
indicating that the polymer backbones adopt a preferential “face
on” orientation relative to the substrate (π−π stacking “out of
plane”). Meanwhile, the partial arc shape of the peak suggests
that a fraction of the polymer aggregates are somewhat
misaligned with respect to the substrate. Predominantly “face-
on” polymer orientations are commonly associated with higher
solar cell performance.3a,4a,11a,13 In contrast, the GIXS pattern
of PBDTTPD(C14/C8) shows a nearly isotropic ring of
scattering intensity at q ≈ 1.79 Å−1, which points to the absence
of a preferential π−π stacking orientation relative to the
substrate. These intrinsic differences in backbone self-assembly
and polymer orientation in thin films correlate with the drastic
variations in material performance observed in BHJ solar cells
with PC71BM. In parallel, it should be noted that other
morphological parameters not discussed here, such as the
relative degree of crystallinity of the polymers and the purity of
the domains across the active layers, may also contribute to the
variations in BHJ efficiency.
In stark contrast, in PBDTTPD derivatives with branched-
alkyl-substituted BDTs, such as PBDTTPD(2EH/C8), replac-
ing branched side chains by linear ones on the TPD motifs does
not significantly affect the preferential “face on” orientation of
the polymers in thin films.4a In our effort to improve the
material performance further while maintaining the same
preferential polymer orientation and overall degree of structural
order, we hypothesized that a fine modulation of the number of
aliphatic carbons in the N-alkyl-substituted TPD motifs may be
one of the remaining keys to improving the device PCE. It is
worth noting that each methylene in the solubilizing substituent
appended to TPD affects the ratio of insulating material versus
active conjugated units and that reducing their number may be
beneficial. On this basis, derivatives with n-heptyl (C7)- and n-
hexyl (C6)-substituted TPD motifs [PBDTTPD(2EH/C7) and
PBDTTPD(2EH/C6), respectively] were synthesized using the
same experimental protocol (see the SI). As shown in Table 1,
BHJ solar cells fabricated from PBDTTPD(2EH/C7) achieved
a PCE of 7.3% (av 7.1%), combining a high JSC (10.6 mA/cm2),
a slightly higher VOC (0.97 V), and a high FF (71%).
Importantly, devices cast from blends containing 5% (v/v)
CN showed an even higher JSC (12.6 mA/cm2) while
maintaining a high VOC and FF, with PCEs as high as 8.5%
(av 8.3%). The several batches of PBDTTPD(2EH/C7) made
for the purpose of this demonstration all showed average PCEs
of >7.5%, which are consistently higher than the maximum
PCEs obtained with the control polymer PBDTTPD(2EH/
C8). However, further shortening of the n-alkyl chain on TPD
led to greatly reduced polymer solubility, and solution
processing of PBDTTPD(2EH/C6) proved to be difficult.
Optimized BHJ solar cells cast from CN-containing blends
showed lower JSC (11.1 mA/cm2), FF (62%), and PCE (av
6.3%). Significantly reduced JSC (8.7 mA/cm2), FF (57%), and
PCE (av 4.5%) were obtained without processing additives,
pointing to inherent solution-processing limitations of the
polymer. The J−V curves and EQE spectra of optimized
devices are shown in Figure 3a,b. The comparably broad and
efficient EQE responses for PBDTTPD(2EH/C8) and
PBDTTPD(2EH/C7) (>60% over the range 370−630 nm
and peaking at ca. 70% at 550 nm) are in agreement with the
high JSC of the solar cell devices. The EQE losses observed in
Figure 3. (a) Characteristic J−V curves of BHJ solar cells fabricated
from PBDTTPD(2EH/C8), PBDTTPD(2EH/C7), and PBDTTPD-
(2EH/C6) under AM1.5G illumination (100 mW/cm2). (b) EQE
spectra of the BHJ devices.
the range 450−630 nm for PBDTTPD(2EH/C6) are
consistent with the reduced JSC and PCE (av 6.3%) of the
optimized BHJ solar cells. At a first level of analysis,
PBDTTPD(2EH/C8), PBDTTPD(2EH/C7), and
PBDTTPD(2EH/C6) showed comparable GIXS patterns
both in neat polymer films and optimized BHJs with
PC71BM (see the SI). Quantitative X-ray analyses2c may
provide further insight into the subtle variations of structural
order across the BHJs of these polymer donors with PCBM as
the acceptor.
In summary, we have shown that substitutions of linear side
chains in PBDTTPD polymers can greatly impact the polymer
self-assembling properties and solar cell device efficiency. On
the one hand, n-alkyl substitutions on the BDT motifs trigger a
critical change of preferential polymer orientation in thin films,
resulting in a dramatic drop in BHJ device PCE (<4.2%). On
the other hand, in polymers with branched-alkyl-substituted
BDTs, a fine modulation of the number of aliphatic carbons in
the linear N-alkyl-substituted TPD motifs does not significantly
affect the preferential backbone orientation, yet this approach
shows to be a main key to improving the device performance.
Thus, N-heptyl-substituted TPD-based polymers were found to
reach PCEs of 8.5% in standard BHJ devices with PC71BM, a
significant improvement over their N-octyl-substituted counter-
parts, for which PCEs of ca. 7.5% can be achieved. This
substantial increase in device performance makes PBDTTPD
one of the best polymer donors for use in the high-band-gap
cell of tandem solar cells.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic details, monomer and polymer characterizations,
device fabrication protocols, and additional GIXS patterns. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
pierre.beaujuge@kaust.edu.sa
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge financial support under Baseline
Research Funding from KAUST. Part of this work was
supported by the Center for Advanced Molecular Photovoltaics
(CAMP) (Award KUS-C1-015-21) made possible by KAUST.
The authors thank KAUST Analytical Core Laboratories for
4658
dx.doi.org/10.1021/ja400365b | J. Am. Chem. Soc. 2013, 135, 4656−4659

Journal of the American Chemical Society
Communication
(11) (a) Yiu, A. T.; Beaujuge, P. M.; Lee, O. P.; Woo, C. H.; Toney,
mass spectrometry and elemental analyses and Dr. Michael
Toney, Dr. Kristin Schmidt, and Dr. Christopher Tassone for
their support with the GIXS experiments. Portions of this
research were carried out at the Stanford Synchrotron
Radiation Lightsource User Facility, operated by Stanford
University on behalf of the U.S. Department of Energy, Office
of Basic Energy Sciences.
M. F.; Frec
H.; Beaujuge, P. M.; Holcombe, T. W.; Lee, O. P.; Frec
Am. Chem. Soc. 2010, 132, 15547.
́
het, J. M. J. J. Am. Chem. Soc. 2012, 134, 2180. (b) Woo, C.
́
het, J. M. J. J.
(12) (a) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.;
Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6, 497. (b) Lee, J. K.; Ma,
W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G.
C.; Heeger, A. J. J. Am. Chem. Soc. 2008, 130, 3619. (c) Hoven, C. V.;
Dang, X.-D.; Coffin, R. C.; Peet, J.; Nguyen, T.-Q.; Bazan, G. C. Adv.
Mater. 2010, 22, E63.
REFERENCES
■
(13) Guo, J.; Liang, Y.; Szarko, J.; Lee, B.; Son, H. J.; Rolczynski, B.
S.; Yu, L.; Chen, L. X. J. Phys. Chem. B 2010, 114, 742.
(1) (a) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009,
21, 1323. (b) Thompson, B. C.; Frec
2008, 47, 58. (c) Beaujuge, P. M.; Frec
́
het, J. M. J. Angew. Chem., Int. Ed.
́
het, J. M. J. J. Am. Chem. Soc.
2011, 133, 20009. (d) Zhou, H.; Yang, L.; You, W. Macromolecules
2012, 45, 607.
(2) (a) Salleo, A.; Kline, R. J.; DeLongchamp, D. M.; Chabinyc, M. L.
Adv. Mater. 2010, 22, 3812. (b) Facchetti, A. Chem. Mater. 2011, 23,
733. (c) Rivnay, J.; Mannsfeld, S. C. B.; Miller, C. E.; Salleo, A.; Toney,
M. F. Chem. Rev. 2012, 112, 5488.
(3) (a) Zhang, X.; Richter, L. J.; DeLongchamp, D. M.; Kline, R. J.;
Hammond, M. R.; McCulloch, I.; Heeney, M.; Ashraf, R. S.; Smith, J.
N.; Anthopoulos, T. D.; Schroeder, B.; Geerts, Y. H.; Fischer, D. A.;
Toney, M. F. J. Am. Chem. Soc. 2011, 133, 15073. (b) Rieger, R.;
Beckmann, D.; Mavrinskiy, A.; Kastler, M.; Mullen, K. Chem. Mater.
̈
2010, 22, 5314. (c) Rieger, R.; Beckmann, D.; Pisula, W.; Steffen, W.;
Kastler, M.; Mullen, K. Adv. Mater. 2010, 22, 83. (d) Guo, X.; Zhou,
̈
N.; Lou, S. J.; Hennek, J. W.; Ponce Ortiz, R.; Butler, M. R.;
́
Boudreault, P.-L. T.; Strzalka, J.; Morin, P.-O.; Leclerc, M.; Lopez
Navarrete, J. T.; Ratner, M. A.; Chen, L. X.; Chang, R. P. H.; Facchetti,
A.; Marks, T. J. J. Am. Chem. Soc. 2012, 134, 18427. (e) Scharber, M.
C.; Koppe, M.; Gao, J.; Cordella, F.; Loi, M. A.; Denk, P.; Morana, M.;
Egelhaaf, H.-J.; Forberich, K.; Dennler, G.; Gaudiana, R.; Waller, D.;
Zhu, Z.; Shi, X.; Brabec, C. J. Adv. Mater. 2010, 22, 367.
(4) (a) Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.;
́
Beaujuge, P. M.; Frechet, J. M. J. J. Am. Chem. Soc. 2010, 132, 7595.
(b) Mei, J.; Kim, D. H.; Ayzner, A. L.; Toney, M. F.; Bao, Z. J. Am.
Chem. Soc. 2011, 133, 20130.
(5) (a) Beaujuge, P. M.; Pisula, W.; Tsao, H. N.; Ellinger, S.; Mullen,
̈
K.; Reynolds, J. R. J. Am. Chem. Soc. 2009, 131, 7514. (b) Beaujuge, P.
M.; Tsao, H. N.; Hansen, M. R.; Amb, C. M.; Risko, C.; Subbiah, J.;
Choudhury, K. R.; Mavrinskiy, A.; Pisula, W.; Bred
́
as, J.-L.; So, F.;
Mullen, K.; Reynolds, J. R. J. Am. Chem. Soc. 2012, 134, 8944.
̈
(c) Coffin, R. C.; Peet, J.; Rogers, J.; Bazan, G. C. Nat. Chem. 2009, 1,
657. (d) Ko, S.; Verploegen, E.; Hong, S.; Mondal, R.; Hoke, E. T.;
Toney, M. F.; McGehee, M. D.; Bao, Z. J. Am. Chem. Soc. 2011, 133,
16722. (e) Yang, L.; Zhou, H.; You, W. J. Phys. Chem. C 2010, 114,
16793.
(6) (a) Rivnay, J.; Steyrleuthner, R.; Jimison, L. H.; Casadei, A.;
Chen, Z.; Toney, M. F.; Facchetti, A.; Neher, D.; Salleo, A.
Macromolecules 2011, 44, 5246. (b) Rivnay, J.; Toney, M. F.; Zheng,
Y.; Kauvar, I. V.; Chen, Z.; Wagner, V.; Facchetti, A.; Salleo, A. Adv.
Mater. 2010, 22, 4359. (c) Steyrleuthner, R.; Schubert, M.; Howard, I.;
Klaumunzer, B.; Schilling, K.; Chen, Z.; Saalfrank, P.; Laquai, F.;
̈
Facchetti, A.; Neher, D. J. Am. Chem. Soc. 2012, 134, 18303.
(7) (a) Aïch, B. R.; Lu, J.; Beaupre,
Electron. 2012, 13, 1736. (b) Hoke, E. T.; Vandewal, K.; Bartelt, J. A.;
Mateker, W. R.; Douglas, J. D.; Noriega, R.; Graham, K. R.; Frechet, J.
́
S.; Leclerc, M.; Tao, Y. Org.
́
M. J.; Salleo, A.; McGehee, M. D. Adv. Energy Mater. 2013, 3, 220.
(c) Bartelt, J. A.; Beiley, Z. M.; Hoke, E. T.; Mateker, W. R.; Douglas,
J. D.; Collins, B. A.; Tumbleston, J. R.; Graham, K. R.; Amassian, A.;
́
Ade, H.; Frechet, J. M. J.; Toney, M. F.; McGehee, M. D. Adv. Energy
Mater. 2013, 3, 364.
(8) Beiley, Z. M.; McGehee, M. D. Energy Environ. Sci. 2012, 5, 9173.
́
(9) Chu, T.-Y.; Lu, J.; Beaupre, S.; Zhang, Y.; Pouliot, J.-R.; Zhou, J.;
Najari, A.; Leclerc, M.; Tao, Y. Adv. Funct. Mater. 2012, 22, 2345.
(10) (a) Amb, C. M.; Chen, S.; Graham, K. R.; Subbiah, J.; Small, C.
E.; So, F.; Reynolds, J. R. J. Am. Chem. Soc. 2011, 133, 10062.
(b) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S.-W.; Lai,
T.-H.; Reynolds, J. R.; So, F. Nat. Photonics 2012, 6, 115.
4659
dx.doi.org/10.1021/ja400365b | J. Am. Chem. Soc. 2013, 135, 4656−4659