Importance of the Donor:Fullerene intermolecular arrangement for high-efficiency organic photovoltaics

Article abstract of DOI:10.1021/ja502985g

The performance of organic photovoltaic (OPV) material systems are hypothesized to depend strongly on the intermolecular arrangements at the donor:fullerene interfaces. A review of some of the most efficient polymers utilized in polymer:fullerene PV devices, combined with an analysis of reported polymer donor materials wherein the same conjugated backbone was used with varying alkyl substituents, supports this hypothesis. Specifically, the literature shows that higher-performing donor-acceptor type polymers generally have acceptor moieties that are sterically accessible for interactions with the fullerene derivative, whereas the corresponding donor moieties tend to have branched alkyl substituents that sterically hinder interactions with the fullerene. To further explore the idea that the most beneficial polymer:fullerene arrangement involves the fullerene docking with the acceptor moiety, a family of benzo[1,2-b:4,5-b]dithiophene-thieno[3,4-c]pyrrole-4,6-dione polymers (PBDTTPD derivatives) was synthesized and tested in a variety of PV device types with vastly different aggregation states of the polymer. In agreement with our hypothesis, the PBDTTPD derivative with a more sterically accessible acceptor moiety and a more sterically hindered donor moiety shows the highest performance in bulk-heterojunction, bilayer, and low-polymer concentration PV devices where fullerene derivatives serve as the electron-accepting materials. Furthermore, external quantum efficiency measurements of the charge-transfer state and solid-state two-dimensional (2D) ¹³C{¹H} heteronuclear correlation (HETCOR) NMR analyses support that a specific polymer:fullerene arrangement is present for the highest performing PBDTTPD derivative, in which the fullerene is in closer proximity to the acceptor moiety of the polymer. This work demonstrates that the polymer:fullerene arrangement and resulting intermolecular interactions may be key factors in determining the performance of OPV material systems.

Full text of DOI:10.1021/ja502985g

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Article
Importance of the donor:fullerene intermolecular
arrangement for high-efficiency organic photovoltaics
Kenneth R. Graham, Clément Cabanetos, Justin P. Jahnke, Matthew N. Idso, Abdulrahman
El Labban, Guy O. Ngongang Ndjawa, Thomas Heumueller, Koen Vandewal, Alberto
Salleo, Bradley F. Chmelka, Aram Amassian, Pierre M. Beaujuge, and Michael D. McGehee
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 Jun 2014
Downloaded from http://pubs.acs.org on June 24, 2014
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Page 1 of 13
Journal of the American Chemical Society
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Importance of the donor:fullerene intermolecular arrangement
for high-efficiency organic photovoltaics
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Kenneth R. Graham, Clement Cabanetos, Justin P. Jahnke, Matthew N. Idso, Abdulrahman
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El Labban, Guy O. Ngongang Ndjawa, Thomas Heumueller, Koen Vandewal, Alberto Salleo,
3
2***
2**
1*
Bradley F. Chmelka, Aram Amassian, Pierre M. Beaujuge, Michael D. McGehee
1
Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, United States.
2
Division of Physical Sciences & Engineering, King Abdullah University of Science and Technology (KAUST), Thu-
3
wal 23955-6900, Saudi Arabia. Department of Chemical Engineering, University of California, Santa Barbara, Santa
Barbara, CA 93106, United States
KEYWORDS: Organic photovoltaic, organic solar cell, charge separation, organic-organic interface
ABSTRACT: The performance of organic photovoltaic (OPV) material systems are hypothesized to depend strongly on
the intermolecular arrangements at the donor:fullerene interfaces. A review of some of the most efficient polymers uti-
lized in polymer:fullerene PV devices, combined with an analysis of reported polymer donor materials wherein the same
conjugated backbone was used with varying alkyl substituents, supports this hypothesis. Specifically, the literature shows
that higher-performing donor-acceptor type polymers generally have acceptor moieties that are sterically accessible for
interactions with the fullerene derivative, whereas the corresponding donor moieties tend to have branched alkyl substit-
uents that sterically limit interactions with the fullerene. To further explore the idea that the most beneficial poly-
mer:fullerene arrangement involves the fullerene docking with the acceptor moiety,
a
family of ben-
zo[1,2
b:4,5
b′]dithiophene−thieno[3,4
c]pyrrole-4,6-dione polymers (PBDTTPD derivatives) was synthesized and test-
ed in a variety of PV device types with vastly different aggregation states of the polymer. In agreement with our hypothe-
sis, the PBDTTPD derivative with a more sterically accessible acceptor moiety and a more sterically hindered donor moie-
ty shows the highest performance in bulk-heterojunction, bilayer, and low-polymer concentration PV devices where full-
erene derivatives serve as the electron accepting materials. Furthermore, external quantum efficiency measurements of
1
3
1
the charge-transfer state and solid-state two-dimensional (2D) C{ H} heteronuclear correlation (HETCOR) NMR anal-
yses support that a specific polymer:fullerene arrangement is present for the highest performing PBDTTPD derivative, in
which the fullerene is in closer proximity to the acceptor moiety of the polymer. This work demonstrates that the poly-
mer:fullerene arrangement and resulting intermolecular interactions may be key factors in determining the performance
of OPV material systems.
remain as to what factors actually make a high performing
OPV material. Some material properties that can lead to
INTRODUCTION
Organic photovoltaics (OPVs) are a promising PV technol-
ogy due to their use of potentially low-cost and non-toxic
materials, high performance in low light conditions, and
their potential for solution processing on inexpensive flexible
high performance are known, such as reasonably high
charge-carrier mobility, broad and strong absorbance, and
appropriate energy levels to form a type II heterojunction.
However, many systems that display these properties yield
1
–3
9–14
substrates. Critical to the operation of an organic photo-
voltaic (OPV) device is the interface between an electron
donating material and an electron accepting material, where
photogenerated excitons are dissociated and separated into
free charges. It is expected that the molecular arrangement
at this interface and the resulting interfacial energetics play a
major role in exciton dissocation, charge separation, and
only moderate or low performance,
potentially because
the intermolecular arrangements and resulting energy land-
scapes at the donor-acceptor interface are unfavorable for
charge separation. The role that this interfacial arrangement
can play is apparent in the theoretical literature, where cal-
1
5
16,17
culated electron transfer rates, exciton binding energies,
1
8–21
interfacial
energetics,
6,22
and
charge
separation
4
1
charge recombination processes, yet this important role
probabilities
vary dramatically based on the molecular
remains to be clearly established.
arrangement between electron donating and electron accept-
ing molecules. Any of these factors has the potential to
greatly influence the performance of the OPV material sys-
tem; however, probing the intermolecular arrangement is
Although hundreds, or even thousands, of OPV materials
1
,5–9
have been synthesized and studied,
many questions still
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Journal of the American Chemical Society
Page 2 of 13
experimentally difficult and thus its effects on device per-
courages the fullerenes to dock with the polymer in a specific
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formance remain relatively untested.
location. Generally, this specific location appears to be with
the A moiety of the polymer, though in a few select literature
examples the specific location may be with the D moiety.
The development of high-efficiency polymers and small
molecules for PV applications has largely stemmed from the
use of donor-acceptor (D-A) systems, whereby an electron-
rich moiety (“donor”) is covalently bound to an electron-
deficient (“acceptor”) moiety. In these D-A type systems the
change in electron distribution between D and A moieties
upon photoexcitation can vary substantially depending on
the strength of the D and A moieties; though, in most D-A
polymers utilized in OPVs the LUMO tends to be more local-
ized on the A moiety, whereas the HOMO tends to be more
RESULTS AND DISCUSSION
Many of the highest performing polymers reported in the
27–40
literature are highlighted in Figure 1.
The majority of
these polymers have branched alkyl groups on the D moie-
ties and either no or linear alkyl groups on the A moieties.
This combination makes the A moieties more sterically ac-
cessible to the fullerene. For example, if a fullerene (essen-
tially a ball of almost 1 nm diameter) is placed on top of each
polymer backbone in Figure 1, the larger branched alkyl
groups on the D moieties will sterically favor the fullerene to
approach the A moieties. In PTB7 and PTDB2 there are
branched alkyl groups on the A moieties, though these A
moieties remain relatively accessible since the branch point
is distanced from the conjugated backbone by a coplanar
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23
delocalized over the D and A moieties. These D-A com-
pounds, generally either polymers or oligomers, are the elec-
tron donor materials in OPV devices and usually a fullerene
derivative is the electron accepting material; thus, from here
on “polymer” and “fullerene” will be used in reference to the
electron donating and electron accepting materials, respec-
tively, while donor and acceptor will be used in reference to
the electron-rich and -deficient moieties of the electron do-
nating material. Among other reasons, these D-A com-
pounds are advantageous owing to the precise control over
the optical gap and energy levels of the frontier molecular
orbitals that can be achieved through varying the strength of
41
ester group. The phenyl groups in PIDTT-DFBT and
PIDTDTQx are out of plane, thereby providing significant
33
steric hindrance over the center of the D moiety. Some ex-
ceptions of where the A moiety is more sterically hindered
are polymers containing amide groups, such as DPP, isoindi-
go (II), and BTI (see PDPPTPT, P(IID-DTC), and PBTI3T in
Figure 1, respectively). These exceptions may possibly be
explained by a relatively strong interaction between amide
groups and fullerenes that outweighs the steric hindrance
arising from the branched alkyl groups. This strong amide-
fullerene interaction has previously been observed in solu-
tion state studies of fullerenes in or with amide containing
6,24–26
the D and A moieties.
Interestingly, a clear structural
trend emerges in the alkyl substitution pattern for D-A com-
pounds that show the highest performance in OPVs. That is,
the donor moieties tend to have more bulky alkyl substitu-
ents while the acceptor moieties have less bulky or no alkyl
substituents. Herein, by combining literature trends among
OPV materials with a systematic study of a set of specifically
designed polymers, we explore the hypothesis that the high-
est performing polymers have a chemical structure that en-
4
2,43
solvents.
Figure 1. High performing PV polymers and their record power conversion efficiencies as reported in the literature, with red
27–40
and blue substituents off the D and A moieties respectively.
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Journal of the American Chemical Society
A possible alternative explanation for the prevalence of
mer family, the polymer name is color-coded red, black, or
green according to the performance trend that would be
predicted based on the hypothesis that the highest perform-
ing material will result when the A moiety is more sterically
accessible and/or the D moiety more sterically hindered.
Red predicts a less sterically favorable polymer:fullerene ar-
rangement with a more sterically accessible D moiety, black
a more neutral arrangement with sterics not necessarily en-
couraging any particular arrangement, and green a favorable
arrangement with the fullerene being encouraged to dock
with the A moiety. The performance should thus increase
from red to black to green, and as can be seen in Figure 2 and
Table 1 the majority of polymers do follow the predicted
trend.
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certain alkyl substitution patterns in high-performing PV
polymers is that these derivatives are more synthetically ac-
cessible. However, an analysis of the PV performance of
multiple polymer families, where the alkyl groups are varied
while the polymer backbone is kept constant, emphasizes the
importance of the polymer-fullerene arrangement. In the
majority of these polymers, the highest performance is
achieved for polymers where the A moiety is more sterically
accessible and the D moiety more sterically hindered. These
materials are shown in Figure 2 and their PV performance
characteristics listed in Table 1. The polymers are compared
only among similar derivatives reported in the same publica-
tion or reported by the same research group, with lines in
Table 1 separating the polymer families. Within each poly-
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Figure 2. Donor polymers incorporated into polymer:fullerene bulk-heterojunction OPVs, wherein alkyl groups with varying
bulkiness were utilized with the same conjugated backbone. Substituents are color coded in red or blue depending on whether
they are off the D or A moiety, respectively. The color of the polymer acronym indicates the relative predicted performance in a
polymer family based on the hypothesis described above. Note that some polymers are renamed from their original work, the
renaming strategy is PXXXYYY, where XXX is the donor moiety(s)’ acronym(s) and YYY is the acceptor moiety’s acronym.
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Table 1. Performance of organic photovoltaic cells for the polymers presented in Figure 2.
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2
Polymer
JSC (mA/cm )
VOC (V)
FF
PCE
Follows predicted
trend?
44
PDPP2FT-C14
14.8
11.2
0.65
0.74
0.62
0.65
0.41
0.64
0.60
0.65
0.61
0.48
6.2
5.0
4.1
Yes
Yes
Yes
Yes
Yes
45
PDPP2FT-EH
46
PSOTT
10.2
12.6
7.36
PSEHTT
5.0
1.44
4
7
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
PTTTPD-C12
PTTTPD-BO
1.59
8.12
7.74
12.8
13.9
12.5
10.7
14.5
15.5
14.1
0.59
0.76
0.62
0.60
0.72
0.58
0.66
0.74
0.70
0.75
0.84
0.59
0.61
0.60
0.89
0.88
0.71
0.54
0.50
0.47
0.66
0.59
0.65
0.58
0.69
0.65
0.61
0.57
3.06
2.26
5.10
5.85
4.76
4.1
Yes
Yes
Yes
No
PCPDTTPD-C12
4
8,49
PTB6
PTB2
PTB3
PTB1
PTB5
No
No
No
4
8
PTB7
PTB4
PCPDTTPD-C8
PCPDTTPD-EH
7.4
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
7.1
50
6.31
5.80
5.9
12.9
16.6
18.6
18.7
13.0
10.7
9.4
0.54
0.60
0.64
0.62
0.653
0.521
0.61
5
C1
1
C2
C3
7.3
6.9
7.6
28
PTDBD2
PTDBD3
4.9
4.1
5
2
PBDTTDPPT-C8HD
PBDTTDPPT-PUC8
5.2
0.62
0.75
0.81
0.92
0.70
0.68
0.68
0.73
0.67
0.59
0.81
0.41
0.52
0.59
0.69
0.43
0.55
0.54
0.52
0.61
1.4
No
5
3
EWC1
5.4
2.2
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Maybe
Yes
EWC2
EWC3
7.1
3.1
7.7
3.7
1
4
PNDTDTBT-EHOEH
10.5
9.36
7.85
7.35
6.64
7.98
5.62
6.97
5.88
10.93
10.67
4.5
PNDTDTBT-EHOC10
PNDTDTBT-EHOC12
PNDTDTBT-EHOC14
PNDTDTBT-EHOC16
0.63
0.58
0.59
0.63
0.461
0.441
0.421
0.421
0.464
0.459
3.9
3.3
3.2
2.8
54
PNDTDTBT-HDC8
2.17
2.01
1.20
1.28
3.00
3.36
PNDTDTBT-HDEH
PNDTDTBT-C8C8
PNDTDTBT-C8C12
PNDTDTBT-C8EH
PNDTDTBT-EHEH
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5
PBDTISi-C6
PBDTISi-C8
PBDTISi-EH
12.59
12.81
12.50
12.30
12.17
10.09
12.8
0.825
0.803
0.834
0.774
0.745
0.769
0.86
0.606
0.623
0.531
0.502
0.472
0.467
0.67
6.29
6.41
5.54
4.77
4.32
3.62
7.4
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
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55
PBDTIGe-C6
PBDTIGe-C8
PBDTIGe-EH
56
PBDTTBO
PBDTFBO
11.2
0.81
0.60
5.4
0
1
2
3
4
5
6
7
8
9
0
1
2
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9
0
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0
All the polymers displayed in Figure 2 have been incorpo-
rated into bulk-heterojunction (BHJ) devices, with either
PC BM or PC BM as the fullerene derivative. Considering
The materials were all synthesized in a similar manner to
yield polymers with comparable impurities and end groups.
Number average molecular weights (M ) are similar at 38, 38,
61
71
n
these are all BHJ devices and the nanoscale morphology will
significantly influence device performance, it is even more
remarkable that the majority of the reported polymers follow
the predicted trend. The two notable exceptions are the DPP
containing PBDTTDPPT derivatives and some of the PTB
derivatives. The reason these materials do not follow the
predicted trend may be due to nanoscale morphology differ-
and 36 kDa for the C14/C14, C14/C8, and EH/C8 derivatives,
respectively, and slightly lower at 21 and 19 kDa for the
C14/EH and EH/EH derivatives, respectively. All polymers
have similar PDIs of 1.7 to 2.0. The lower M of derivatives
n
with EH on the TPD unit may result in some performance
61
variations, though the Mn alone is unlikely to explain the
device results for the variety of device architectures present-
ed herein. As shown in Supporting Information Figure S1,
thin films of all the PBDTTPD derivatives show similar ab-
sorbance spectra with nearly identical optical gaps. These
similarities in absorbance spectra suggest that varying the
alkyl substitution pattern does not significantly affect elec-
tronic structure, molecular planarity, and conjugation length.
5
7
ences, differences in fullerene solubility in the mixed phase,
a sufficiently sterically accessible acceptor moiety in all de-
rivatives, and/or favorable intermolecular interactions be-
tween the A moieties and the fullerene that lead to a pre-
ferred arrangement regardless of alkyl substitution pattern.
The PNDTDTBT-C8EH derivative is listed as “maybe” in the
table due to the bulky EH groups on the thiophenes pointing
towards the central NDT donor moiety, and thereby provid-
ing some steric bulk around that moiety as well as the thio-
phenes. An alternative explanation regarding the role of a
specific polymer:fullerene arrangement, which may not nec-
essarily be fullerene docking with the A moiety, will be pre-
sented later in the manuscript and would encompass some of
the aforementioned exceptions. Importantly, some of the
polymers listed in Figure 2 and Table 1 that have a favorable
alkyl substitution pattern still only have PCEs in the 3-4%
range. These moderate PCEs highlight that an appropriate
alkyl substitution pattern does not a guarantee a high PCE,
as several other properties, such as absorbance, blend film
morphology, and charge-carrier mobilities, must also be op-
timized for a high performing OPV material system.
In this work the family of PBDTTPD derivatives shown in
5
8–60
Figure 3b are compared,
whereby the alkyl groups ap-
pended to the donor and acceptor moieties are either linear
or branched. Varying the alkyl groups between linear C H
8
17
Figure 3. a) Proposed model illustrating how branched alkyl
groups on the donor unit direct the fullerene towards the
acceptor unit in PBDTTPD(EH/C8), b) the PBDTTPD fami-
ly of polymer derivatives designed to sterically favor different
(
C8) or C H (C14) and branched 2-ethylhexyl (EH) groups
14 29
alters steric accessibility to the D and A units. As shown
schematically in Figure 3a, the branched alkyl group is ex-
pected to provide steric hindrance and direct the fullerene to
be closer to the linearly substituted unit. According to our
hypothesis the highest performing derivative will have the
branched EH group on BDT and the linear C8 group on TPD,
referred to as EH/C8. The nomenclature used throughout
this manuscript indicates the alkyl substituents on the BDT
moiety followed by the substituent on the TPD moiety, i.e.
C14/C8 is the derivative where R =C14 and R =C8.
polymer:fullerene arrangements, and schematics of the c) 3-
57,62,63
phase BHJ morphology,
d) bilayer, and e) low polymer
content device architectures.
To experimentally test whether the OPV performance is in
part determined by the molecular arrangement at the poly-
mer:fullerene interface, the series of OPV device types pre-
sented in Figure 3c-e are examined. These include standard
BHJ architectures with PC BM as the electron accepting
1
2
61
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material, bilayer OPV devices with C60 as the electron ac-
details of the nanoscale morphology, including the domain
5
7,64,65
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
cepting material, and low polymer concentration devices
whereby 1 to 8 wt.% of polymer is blended in a PC BM ma-
sizes and fullerene solubility in the mixed phase,
and
these differences are nearly impossible to distinguish from
other factors such as polymer:fullerene intermolecular ar-
rangements. Bilayer device architectures, as shown in Figure
3d, were thus fabricated and tested to eliminate the effects of
nanoscale morphology differences.
61
trix. The bilayer and low-polymer concentration devices are
included to reduce nanoscale morphology effects and mini-
mize the influence of polymer-polymer interactions. In all
device types the EH/C8 derivative, where the branched sub-
stituent is on the BDT (donor) moiety and the linear substit-
uent is on the TPD (acceptor) moiety, outperforms the other
derivatives in terms of JSC and overall PCE.
The results of bilayer (BL) OPV devices with a pure poly-
mer layer and a thermally evaporated pure C60 layer are pre-
sented in Table 2 and Supporting Information Figure S3. To
minimize interfacial mixing between C60 and the polymers,
the substrates were cooled down to ca. 0 °C during C60 depo-
sition. Similar to the BHJ results, the EH/C8 derivative out-
performs the other derivatives with a PCE of 1.5% vs. 1.1% for
the next most efficient EH/EH derivative. The trends in the-
se BL devices are relatively comparable to the trends in the
BHJ devices with a few exceptions. The main exception is
that the C14/EH derivative shows a comparable PCE to the
linearly substituted C14/C14 and C14/C8 derivatives, whereby
in the BHJ devices the JSC and PCE of the C14/EH based OPV
device is only half that of the C14/C14 based device. This
discrepancy between BHJ and BL device trends is most likely
due to nanoscale morphology differences in the BHJ devices.
The VOC values for the BL devices are 0.07 to 0.10 V lower
than the BHJ devices, which is likely the result of replacing
PC BM with with the more electronegative C60, as demon-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
BHJ devices of the polymer:fullerene blends were all opti-
mized for film thickness, polymer:fullerene ratio, and solvent
additives for each individual polymer. All polymers showed
similar optimized conditions, and therefore results are pre-
sented with identical fabrication conditions. As evident in
Table 2 and the J-V curves included in the Supporting Infor-
mation, the EH/C8 derivative outperforms the other materi-
als in BHJ devices, with a 40 to 170% higher JSC and a 1 to 38%
higher FF vs. the other derivatives. As a result the PCE is
6
.0% for the EH/C8 derivative as compared to 3.8% for the
next highest performing derivative. The PCE of 6.0% for the
EH/C8 derivative is lower than can be achieved with PC BM
71
and comparable to the previous value reported with PC BM
61
5
9
and no solvent additives of 6.3%.
The higher performance
for the EH/C8 derivative is in agreement with the proposed
model, though it is difficult to make any definitive conclu-
sions from these trends given the large number of variables
contributing to the performance of a BHJ device. For exam-
ple, the performance of a BHJ device depends strongly on the
6
1
66
strated previously for bilayer cells.
Table 2. Performance of organic photovoltaic cells based on BHJ and BL architectures with standard deviations
indicated.
2
Material
C14/C14
C14/C8
C14/EH
EH/C8
Architecture
JSC (mA/cm )
VOC (V)
FF
PCE (%)
3.8±0.1
BHJ
BHJ
BHJ
BHJ
BHJ
BL
6.3±0.1
0.94±0.01
0.90±0.02
0.97±0.01
0.94±0.01
0.96±0.03
0.84±0.02
0.80±0.01
0.85±0.01
0.87±0.01
0.89±0.03
0.65±0.07
0.53±0.02
0.48±0.03
0.66±0.01
0.50±0.03
0.63±0.02
0.66±0.01
0.58±0.05
0.66±0.01
0.58±0.02
6.9±0.3
3.6±0.2
9.6±0.2
6.6±0.4
1.3±0.1
3.3±0.2
1.7±0.2
6.0±0.3
3.2±0.3
EH/EH
C14/C14
C14/C8
C14/EH
EH/C8
0.70±0.03
0.63±0.02
0.70±0.08
1.5±0.1
BL
1.2±0.1
BL
1.5±0.3
BL
2.7±0.3
2.1±0.1
EH/EH
BL
1.1±0.1
To decrease the effects of polymer-polymer interactions
present in the BHJ and bilayer devices, low polymer concen-
ences in performance should be primarily related to how the
polymers interact with the fullerene.
tration devices with 1 to 8 wt.% polymer in PC BM were fab-
61
In these low polymer concentration devices the best per-
forming polymer is again the EH/C8 derivative. This deriva-
tive shows approximately twice the JSC and PCE as the other
derivatives in the 1,2,4, and 8% polymer devices as shown in
Figure 4 and the J-V curves in the Supporting Information.
The relatively large standard deviations across multiple de-
67
ricated and characterized. As shown schematically in Fig-
ure 3e, it is expected that predominantly isolated polymer
strands are present as opposed to polymer aggregates. With
the polymer backbones all being identical, and presumably
minimal polymer-polymer interchain interactions, the differ-
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vice sets may be attributed to variations in PC BM batch and
61
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
uncertainties associated with the low polymer concentra-
tions (±15%). To probe whether or not differences in device
performance can be attributed to a difference in the number
of excitons reaching a polymer-fullerene interface, photolu-
minescence spectra of the blend films were measured. At a
polymer concentration of 2% the fullerene emission is 34 ±
5% quenched for the C14/EH, EH/C8, and EH/EH blends
and 26 ± 5% quenched in the C14/C14 and C14/C8 blends, as
shown in Supporting Information Figure S6. This similar
quenching behavior indicates performance differences are
not likely due to an increased number of excitons reaching a
polymer:fullerene interface. With similar exciton quenching
behavior it is also likely that at this low polymer concentra-
tion there are not significant differences in the amount of
polymer aggregation, which is expected to be minimal. At
higher polymer concentrations of 4 and 8% some aggrega-
tion does occur as observed through transmission electron
microscopy (TEM) and grazing incident wide angle X-ray
scattering (GIWAXS), see Supporting Information, though
this does not change the trend in device performance.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
To determine whether the performance differences were in
part due to differences in non-geminate charge recombi-
nation dynamics, the charge-carrier lifetimes for non-
geminate recombination were measured by transient photo-
68
voltage (TPV) as described by Shuttle, et al. These TPV
measurements were performed on PV devices with 2% poly-
mer:98% PC61BM, Supporting Information Figure S5, as in
these devices polymer aggregation is minimized as compared
to the 4 and 8% polymer devices. At one sun light intensity
all polymers show a very similar lifetime of ca. 3 µs, with no
correlation between short circuit current and charge carrier
lifetime. Furthermore, as a function of carrier density the
charge-carrier lifetime τ is almost identical for EH/C8 and
EH/EH. These similar bimolecular recombination lifetimes
indicate that differences in PV performance cannot be ex-
plained by differences in non-geminate recombination rates,
thus leaving differences in geminate recombination as the
most likely factor in differentiating the PV performance.
With the same conjugated backbone, similar emission
quenching behavior, similar bimolecular recombination dy-
namics, and predominantly polymer:fullerene interactions
present, the most straightforward explanation for the differ-
ences in device performance is that the probability of charge
separation or geminate recombination is dependent on the
interfacial polymer:fullerene arrangement. Therefore, these
results support the hypothesis that the highest performance
of the EH/C8 derivative originates from favorable docking of
the fullerene with the A moiety of the polymer.
Figure 4. JSC (a) and PCE (b) of polymer:PC BM solar cells
61
where the concentration of polymer was varied from 1 to 8%
by weight relative to the total solids concentration. Error
bars represent standard deviations of several device sets
made with varying batches of PC BM.
61
One method of directly probing the polymer:fullerene in-
terfacial energetics is through the charge-transfer (CT) state
absorbance, where the CT state is an intermolecular state
69
formed between the polymer and fullerene. This CT state
can be probed with sensitive absorbance or EQE measure-
ments, as shown in Figure 5. With interfacial energetics de-
termined in part by intermolecular interactions, e.g. dipolar
19
and quadrupolar, the CT state absorbance is sensitive to the
molecular arrangement at the polymer:fullerene interface.
Figure 5 shows the CT region of the EQE spectra and fits to
the CT band for the devices with polymer concentrations of 2
69,70
and 4%, where the data is fit with equation 1:
−ꢈꢀꢉꢊ + ꢄ − ꢀꢋꢌ
ꢂ
ꢀ
ꢁꢀ ∝
exp ꢇ
ꢍꢎꢎꢎꢎꢎꢎꢎꢎꢎꢎꢎꢈ1ꢋ
4
ꢄꢅꢆ
ꢀ
√4ꢃꢄꢅꢆ
Here, k is the Boltzmann constant, f is a term that accounts
for the internal quantum efficiency, number of CT states, and
electronic coupling, ECT is an effective energy of the CT state
and λ is related to the width of the CT absorbance band.
More specifically, λ contains a reorganization energy term
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(
λ0) that applies to a particular CT state environment and an
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
energetic disorder term that reflects heterogeneities in CT
state molecular arrangement and environment resulting
from the restrictive, inhomogeneous solid-state film. λ0 is
the difference in energy between a vertically excited CT state
with the same nuclear coordinates as the lowest energy
ground CT state and the energy minimum of the excited CT
state after environmental (low frequency) and internal (high
69,71,72
frequency) reorganization,
where in traditional liquid
systems the environmental term is due to solvent reorganiza-
tion. In the solid state, where inhomogeneous environments
exist and nuclear positions are more constrained, the inter-
molecular arrangement and environment around each
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
unique CT pair can give rise to significant differences in ab-
73,74
sorbance and emission characteristics.
In a film consist-
ing of many unique CT state environments these differences
lead to greater optical peak widths and thereby increases in
λ. Thus, in the polymer:PC BM films analyzed here λ results
61
Figure 5. EQE spectra of the CT region, with fits to equation
indicated with solid lines, for polymer concentrations of a)
2 and b) 4%.
from internal reorganization, environmental reorganization,
and energetic disorder components. With the energy of a CT
1
1
7,75
state depending on the molecular arrangement,
energetic
disorder and λ will be reduced if a more specific molecular
arrangement exists. From the fits of the data shown in Fig-
ure 5, we indeed find large differences in λ. For example, λ
As evident from Figure 5 and the Supporting Information,
EH/C8 in PC BM shows the narrowest CT bands with λ of
61
0.06 and 0.08 eV for the 2 and 4% devices, respectively. This
narrower CT band for the EH/C8 derivative is in agreement
with a more specific and consistent arrangement between
polymer and fullerene, as a broader distribution of poly-
mer:fullerene arrangements would increase the energetic
distribution of CT states and increase λ. Displaying a slightly
higher λ of 0.10 eV for the 2% polymer concentration device
is the C14/C14 derivative. When a branched chain is present
on the TPD unit in EH/EH, λ is even higher at 0.13 eV for the
2% blend. A potential reason for this trend is that the EH
group on TPD limits the otherwise energetically favorable
TPD interaction with the fullerene, thus leading to a broader
distribution of polymer:fullerene intermolecular arrange-
ments. Another potential explanation for the decreased λ
with the linear substituted TPD derivatives is that the fuller-
ene is closer to the polymer backbone, as smaller center-
center distances between molecules in a CT complex will
for the 4% EH/C8:PC BM blend is 0.08 eV vs. 0.21 eV for the
61
4
% EH/EH:PC BM blend
61
To confirm that energetic disorder contributes to λ, we in-
vestigate a system with a known low amount of disorder and
a well-defined and specific polymer:fullerene arrangement,
76
i.e. thePBTTT:PC BM bimolecular crystal. Analysis of the
71
CT region of EQE spectra for the PBTTT:PC BM bimolecular
71
crystal shows one of the narrowest CT absorbance bands of
all BHJ systems, with λ of 0.12-0.13 eV (Supporting Infor-
69,77,78
mation) compared to 0.2-0.4 eV for other BHJ systems.
This narrow CT absorbance band is indeed consistent with a
more specific polymer-fullerene conformation.
71
reduce the environmental reorganization term. When the
polymer concentration is increased to 8% the CT bands are
broader for all polymers, with λ values of 0.2 to 0.3 eV. This
broadening of the CT band most likely results from polymer
aggregation occurring at these higher concentrations, where
the aggregate regions of varying order would result in a wide
spread of energy states. The narrower band at low concen-
tration is another indicator that the majority of the polymers
are more dispersed and minimally aggregated at low concen-
trations. The fact that all device types show the highest per-
formance for the EH/C8 derivative, regardless of the degree
of polymer aggregation, is in agreement with the hypothesis
that a specific polymer:fullerene arrangement leads to im-
proved charge separation.
Solid-state nuclear magnetic resonance (NMR) spectros-
copy complements the EQE measurements of the CT states
by providing molecular-level insights on the interactions of
the fullerene with the donor and acceptor moieties in the
conjugated
polymers. Specifically, solid-state two-
1
3
1
dimensional (2D)
C{ H} heteronuclear correlation
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(
HETCOR) NMR measurements exploit through-space di-
To gain insight into the polymer-fullerene intermolecular
arrangement, 8 wt. % polymer, 92% PC61BM blends were
1
3
1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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5
5
5
5
5
5
5
6
pole-dipole couplings of locally proximate (< 1 nm) C and H
nuclei to correlate their isotropic chemical shifts. This pro-
vides direct information on intra- and key intermolecular
interactions among the chemically distinct polymer and full-
erene moieties shown in Figure 6a and b, which depicts the
PC BM functional groups (orange), the linear alkyl chains
1
3
1
probed with 2D C{ H} HETCOR NMR. Although some ag-
gregation is evident at 8 wt. % polymer, these higher polymer
1
3
1
concentrations are necessary to maximize the 2D C{ H}
HETCOR signal arising from polymer-fullerene interactions.
1
3
1
For the 8 wt.% EH/C8 in PC BM blend, the 2D C{ H}
61
61
HETCOR spectrum acquired at room temperature also yields
well-resolved correlated signals, most of which reflect the
same intramolecular contributions as for the neat compo-
nents. Importantly, however, additional 2D intensity correla-
tions are observed in Figure 6c that directly establish inter-
molecular interactions between the PC BM and EH/C8 pol-
(blue), the branched EH alkyl groups (red), and the polymer
1
3
1
backbone (brown). In particular, the 2D C{ H} HETCOR
spectra of the neat EH/C8 and C14/EH conjugated polymers
1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
and PC BM (Supporting Information) yield well-resolved H
61
13
and C signals that are confidently assigned to the specific
61
13
polymer and PC BM moieties, labeled in Figure 6a, as indi-
61
ymer moieties. Specifically, C signals associated with the C
60
1
13
cated above the respective 1D H and C MAS spectra in Fig-
ure 6c.
fullerene group at 140-148 ppm are strongly correlated with
1
the H signals at ca. 1.2 ppm and ca. 8.3 ppm (Figure 6c, red
1
arrows) associated with the alkyl and aromatic H moieties of
1
the polymer, respectively. It is noteworthy that the only H
atoms on the polymer backbone are associated with carbon
site 2 of the BDT unit, each of which is adjacent to a TPD
moiety. Similar correlated intensities are observed in the 2D
1
3
1
C{ H} spectrum (Supporting information, Figure S9) of the 8
wt.% C14/EH in PC BM blend. These 2D intensity correla-
61
tions unambiguously establish the close (< 1 nm) proximities
of the C60 moieties of the PC BM molecules to the polymer
61
backbone for both of the EH/C8 and C14/EH heterojunction
blends, as required for efficient charge transfer.
The 2D NMR results furthermore suggest that the type and
placement of the alkyl groups influence the local configura-
tions of the PC BM moieties near the polymer backbone. In
61
1
3
particular, the C signals at 31 ppm and 14 ppm that are asso-
ciated with carbon atoms b-f and h, respectively, of the linear
C8 alkyl chains on the TPD acceptor moiety are strongly cor-
1
related (Figure 6c, blue arrows) with the H signals at 2-3
ppm from moieties 2’-4’ of the PC BM functional group
61
13
(Figure 6c, blue band). While the C signal at 31 ppm con-
tains overlapping signals from carbon atoms b-f of the linear
C8 alkyl chain and δ of the branched EH alkyl groups, most
13
of this signal intensity appears to arise from the C atoms of
the C8 chains (5/TPD moiety), as opposed to those of the EH
groups (2/BDT moiety). This is evident from comparison
13
1
with the 2D C{ H} HETCOR spectrum (Supporting infor-
mation) for the 8 wt% C14/EH in PC BM blend, in which
61
significantly greater intensity is observed in the correlated
13
signals at 31 ppm in the C dimension and at 2-3 ppm in the
1
H dimension. This is consistent with the greater population
13
of interior C atoms of the linear C14 chains (22/BDT moie-
ty) relative to the δ-type carbon atoms in the branched EH
alkyl groups (1/TPD moiety) and also compared to the
Figure 6. Molecular structures of (a) EH/C8 and PC BM,
61
with their respective moieties labeled and (b) schematic
showing red and blue arrows that indicate intermolecular
13
EH/C8 blend. Moreover, the association of the C signals at
31 ppm with linear alkyl moieties is also evidenced by the
different relative intensities of the weaker correlated signals
interactions between the conjugated polymer and PC BM
61
moieties that are consistent with the 2D NMR intensity cor-
13
1
13
1
relations. (c) Solid-state 2D C{ H} dipolar-mediated hetero-
nuclear correlation (HETCOR) NMR spectrum acquired at
at 14 ppm ( C) and 2-3 ppm ( H), which are consistent with
the comparable populations of moieties ꢀ, h, and n in the two
blends (Supporting Information Figure S13). These results
collectively indicate that, in both blend materials, the
PC BM functional groups interact to greater extents with the
room temperature for an 8 wt% EH/C8 in PC BM blend un-
61
der MAS conditions of 12.5 kHz, with an 8-ms CP contact
13
1
1
time. 1D C{ H} CPMAS and single-pulse H MAS spectra are
shown along the top horizontal axis and the left vertical axis,
respectively.
61
linear C8 or C14 alkyl chains of the polymers, compared to
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the branched EH alkyl groups. As the polymer alkyl chains
It remains difficult to fully design OPV materials, in part
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
are covalently bonded to either the TPD or BDT moieties of
the polymer backbone, the strong intensity correlations be-
because some of the molecular factors influencing charge
separation are not yet known. The literature and work pre-
sented here suggests that a preferred intermolecular ar-
rangement exists in high-performing OPV polymer:fullerene
systems, where the fullerene is generally docked with the
electron accepting moiety of the polymer. Identification of
this trend helps to establish why certain alkyl substitution
patterns have led to successful polymers and will aid in the
more directed design of future materials. For example, new
materials should likely be designed with the electron accept-
ing moiety of the polymer being more sterically accessible
and the electron donating moiety more sterically hindered,
and/or acceptor moieties that interact favorably with the
fullerene should be identified and utilized. Observation of
this trend paves the way for further theoretical and experi-
mental studies, including determination of the intermolecu-
lar energy landscape, how this energetic landscape effects
charge separation, what intermolecular interactions lead to a
favorable energy landscape, and how factors such as interfa-
cial energetic disorder influence charge separation. Deter-
mination of these factors will prove critical in guiding the
design of future high-efficiency OPV material systems.
tween the linear alkyl chains and PC BM functional groups
61
suggest that PC BM species are in closer proximity to moie-
61
ties of the polymer backbone that have linear alkyl chains, as
opposed to those with branched EH groups. The 2D NMR
analyses thus establish close (<1 nm) proximities of the
PC BM molecules with the polymer backbone and the linear
61
alkyl chains, which provide evidence of preferential interac-
tions of the PC BM species and the TPD acceptor moieties in
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
61
the EH/C8 blend, as depicted in the schematic diagram of
Fig. 6b.
Overall, the collective data presented here demonstrate
that PBDTTPD derivatives perform better in OPV devices
when the fullerene is closer to the electron accepting TPD
moiety. One explanation of why this polymer:fullerene ar-
rangement is beneficial is that the resulting intermolecular
interactions create a favorable energy landscape, either the
result of a partial charge transfer, dipole-induced dipole, or
quadrupolar interactions. Theoretically it has been shown
that the interface dipole, as well as the energetics of mole-
cules near the interface, can vary by hundreds of meV de-
4
,18,19,21,79,80
pending on the donor:fullerene arrangement.
Var-
ASSOCIATED CONTENT
iations in energy levels, combined with how charges are sta-
bilized or destabilized by induced dipoles and quadrupolar
interactions, can lead to near zero electron-hole binding
Supporting Information. Experimental details and proce-
dures, synthetic details, current-voltage and PV performance
characteristics, EQE data, TEM images, GIWAXS plots, 2D
13
1
energies
with
certain
interfacial
molecular
C{ H} HETCOR NMR data, photoluminescence data. This
1
6,18,19
arrangements.
These energetic shifts arising from in-
material is available free of charge via the Internet at
http://pubs.acs.org.
termolecular interactions may be a key factor in providing an
energetic driving force for charges to move away from the
polymer:fullerene interface, or out of the mixed phase in a 3-
AUTHOR INFORMATION
Corresponding Authors
57,62,63
phase BHJ system.
Another potential explanation is
that the wavefunction overlap leads to high rates of charge
transfer and low rates of charge recombination when the
fullerene is in closer proximity to the acceptor unit. Both
lower rates of charge recombination and interfacial energetic
offsets have been shown with Monte Carlo simulations to
*mmcgehee@stanford.edu, **pierre.beaujuge@kaust.edu.sa,
***aram.amassian@kaust.edu.sa
Author Contributions
8
1,82
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript.
increase the probability of charge separation,
and it is
indeed possible that either or both of these factors may ex-
plain the performance trend observed for the PBDTTPD de-
rivatives. Yet another potential explanation is that when the
fullerene docks with a specific part of the polymer, either
donor or acceptor moiety, the energetic disorder in both the
polymer and fullerene sites are reduced. Decreased energetic
disorder should increase the probability of charge separation,
as the local charge-carrier mobilities will remain high and
there will not be energetic barriers between lower and higher
energy sites. In this explanation of a specific intermolecular
arrangement leading to decreased energetic disorder, which
results in an improved probability for charge separation, it
may not be as critical as to what the arrangement is as long
as it is consistent. These potential explanations highlight the
need for further theoretical and experimental studies to
bring about a better molecular level understanding of the
variables influencing charge separation and OPV perfor-
mance.
ACKNOWLEDGMENT
This publication was supported by the Center for Advanced
Molecular Photovoltaics (Award No KUS-C1–015–21) and was
made possible by King Abdullah University of Science and
Technology (KAUST). K.R.G. and A.A. acknowledge SABIC
for a post-doctoral fellowship. G.O.N.N., K.R.G., M.D.M.,
and A.A. acknowledge the Office of Competitive Research
Funds for a GRP-CF award. T.H. gratefully acknowledges a
"
9
DAAD Doktorantenstipendium" and the SFB
53 "Synthetic Carbon Allotropes". Use of the Stanford Syn-
chrotron Radiation Lightsource, SLAC National Accelerator
Laboratory, is supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences under Con-
tract No. DE-AC02-76SF00515. The NMR experiments were
conducted in the Central Facilities of the UCSB Materials
Research Laboratory supported by the MRSEC program of
the U.S. NSF under award no. DMR-1121053. The work at
UCSB was supported by the USARO through the Institute for
CONCLUSION
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09-D-0001. The authors also thank Dr. Chad Risko and Prof.
Jean-Luc Brédas for helpful discussions.
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