Slip-stacked perylenediimides as an alternative strategy for high efficiency nonfullerene acceptors in organic photovoltaics

Article abstract of DOI:10.1021/ja508814z

Perylenediimide (PDI)-based acceptors offer a potential replacement for fullerenes in bulk-heterojunction (BHJ) organic photovoltaic cells (OPVs). The most promising efforts have focused on creating twisted PDI dimers to disrupt aggregation and thereby suppress excimer formation. Here, we present an alternative strategy for developing high-performance OPVs based on PDI acceptors that promote slip-stacking in the solid state, thus preventing the coupling necessary for rapid excimer formation. This packing structure is accomplished by substitution at the PDI 2,5,8,11-positions ( headland positions ). Using this design principle, three PDI acceptors, N,N-bis(n-octyl)-2,5,8,11-tetra(n-hexyl)-PDI (Hexyl-PDI), N,N-bis(n-octyl)-2,5,8,11-tetraphenethyl-PDI (Phenethyl-PDI), and N,N-bis(n-octyl)-2,5,8,11-tetraphenyl-PDI (Phenyl-PDI), were synthesized, and their molecular and electronic structures were characterized. They were then blended with the donor polymer PBTI3T, and inverted OPVs of the structure ITO/ZnO/Active Layer/MoO₃/Ag were fabricated and characterized. Of these, 1:1 PBTI3T:Phenyl-PDI proved to have the best performance with J_sc = 6.56 mA/cm², V_oc = 1.024 V, FF = 54.59%, and power conversion efficiency (PCE) = 3.67%. Devices fabricated with Phenethyl-PDI and Hexyl-PDI have significantly lower performance. The thin film morphology and the electronic and photophysical properties of the three materials are examined, and although all three materials undergo efficient charge separation, PBTI3T:Phenyl-PDI is found to have the deepest LUMO, intermediate crystallinity, and the most well-mixed domains. This minimizes geminate recombination in Phenyl-PDI OPVs and affords the highest PCE. Thus, slip-stacked PDI strategies represent a promising approach to fullerene replacements in BHJ OPVs.

Full text of DOI:10.1021/ja508814z

Article
pubs.acs.org/JACS
Slip-Stacked Perylenediimides as an Alternative Strategy for High
Efficiency Nonfullerene Acceptors in Organic Photovoltaics
Patrick E. Hartnett,†,∥ Amod Timalsina,†,∥ H. S. S. Ramakrishna Matte,†,‡,∥ Nanjia Zhou,‡ Xugang Guo,†
Wei Zhao,§ Antonio Facchetti,†,§ Robert P. H. Chang,*,‡ Mark C. Hersam,*,†,‡ Michael R. Wasielewski,*,†
and Tobin J. Marks*,†,‡
‡
†Department of Chemistry and the Materials Research Center, and Department of Materials Science and Engineering and the
Materials Research Center, The Argonne-Northwestern Solar Energy Research Center, Northwestern University, 2145 Sheridan
Road, Evanston, Illinois 60208, United States
§Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, United States
S
* Supporting Information
ABSTRACT: Perylenediimide (PDI)-based acceptors offer a
potential replacement for fullerenes in bulk-heterojunction
(BHJ) organic photovoltaic cells (OPVs). The most promising
efforts have focused on creating twisted PDI dimers to disrupt
aggregation and thereby suppress excimer formation. Here, we
present an alternative strategy for developing high-performance
OPVs based on PDI acceptors that promote slip-stacking in the
solid state, thus preventing the coupling necessary for rapid
excimer formation. This packing structure is accomplished by
substitution at the PDI 2,5,8,11-positions (“headland positions”). Using this design principle, three PDI acceptors, N,N-bis(n-
octyl)-2,5,8,11-tetra(n-hexyl)-PDI (Hexyl-PDI), N,N-bis(n-octyl)-2,5,8,11-tetraphenethyl-PDI (Phenethyl-PDI), and N,N-
bis(n-octyl)-2,5,8,11-tetraphenyl-PDI (Phenyl-PDI), were synthesized, and their molecular and electronic structures were
characterized. They were then blended with the donor polymer PBTI3T, and inverted OPVs of the structure ITO/ZnO/Active
Layer/MoO3/Ag were fabricated and characterized. Of these, 1:1 PBTI3T:Phenyl-PDI proved to have the best performance
with Jsc = 6.56 mA/cm2, Voc = 1.024 V, FF = 54.59%, and power conversion efficiency (PCE) = 3.67%. Devices fabricated with
Phenethyl-PDI and Hexyl-PDI have significantly lower performance. The thin film morphology and the electronic and
photophysical properties of the three materials are examined, and although all three materials undergo efficient charge separation,
PBTI3T:Phenyl-PDI is found to have the deepest LUMO, intermediate crystallinity, and the most well-mixed domains. This
minimizes geminate recombination in Phenyl-PDI OPVs and affords the highest PCE. Thus, slip-stacked PDI strategies
represent a promising approach to fullerene replacements in BHJ OPVs.
100% and fill factors approaching 80%.11−14 However, despite
successes in laboratory scale devices, fullerenes are not an ideal
INTRODUCTION
■
In recent years, the field of organic photovoltaics (OPVs) has
material for production on an industrial scale. They are
expensive to synthesize, difficult to modify chemically, and are
unstable in air.3,15 In contrast, perylenediimide (PDI)
derivatives are an attractive alternative to fullerenes because
they have low-lying LUMO levels, are chemically and
environmentally robust (some have been used as automotive
paint pigments), and have high electron mobilities.16−18
Equally important, they have major advantages when compared
to fullerenes in having large optical transition dipoles in the
solar spectral region, and are synthetically straightforward to
modify, offering a wide range of electronic and solid-state
packing properties.16,18,19 Furthermore, they are inexpensive to
synthesize and purify on a large scale.20
been an area of intense research because these devices offer a
potentially efficient, cost-effective, and scalable approach for
solar energy conversion.1−4 Although OPV efficiencies have
been rapidly increasing, with multiple reports of power
conversion efficiencies (PCE) exceeding 10%, they still lag
behind typical inorganic solar cells.5,6 Nevertheless, OPVs can
be mass produced using solution based roll-to-roll processing
from inexpensive, earth-abundant, and environmentally benign
materials. Therefore, OPVs offer the potential to eclipse
inorganic solar cells in cost per watt and eventually achieve
grid-parity. The vast majority of efficient OPV systems are
based on bulk-heterojunctions (BHJs) of either small molecule
or polymer donors mixed with fullerene-based acceptors to
form interpenetrating networks of electron and hole conducting
pathways.7−10 Fullerenes have many unique charge carrier
capabilities, and these have been exploited in well-optimized
systems to obtain internal quantum efficiencies approaching
Received: August 26, 2014
© XXXX American Chemical Society
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Figure 1. Structures of the PDI acceptors and the donor polymer used in this Article. The headland positions of PDI (C2, C5, C8, and C11) are
denoted by arrows in the PDI8 structure. The possible molecular packing structure of each of the PDIs is also shown. The relative slip angles of
Hexyl-PDI and Phenethyl-PDI are unknown but are hypothesized to follow the trend shown in the figure.
Although PDIs have been investigated in OPVs for more
than a decade, including in the very first bilayer devices, and
despite their very favorable chemical and electronic properties,
PDIs have historically performed poorly in BHJ solar cells.21,22
One major obstacle has been that many PDI derivatives do not
form good bulk-heterojunction morphologies. Because of their
extended π-surfaces, PDIs are strongly aggregated in the solid
state, and when cast into films form large crystallites that can be
micrometer in length.23 Such behavior translates into BHJ
domains that are too large for efficient exciton splitting. While
exciton lifetimes and diffusion lengths are quite large in some
PDIs, many PDI derivatives tend to form excimers that act as
traps for excitation energy. Furthermore, because typical
exciton diffusion lengths in donor materials are on the order
of 10 nm, large fractions of the excitons are quenched in films
having large domain sizes, before reaching a donor−acceptor
interface for carrier creation.1,24−27
Recent efforts to minimize the formation of such unfavorably
large PDI BHJ domains and rapid excimer formation have
focused on three major strategies, all yielding substantially
improved OPV performance: (1) creating twisted PDI dimers
attached at the imide position via a hydrazine linker, (2)
creating twisted PDI dimers attached at the 1,6,7,12-positions
(“bay positions”, see PDI8 in Figure 1) either directly or with a
spacer, and (3) using bulky “swallow tail” PDI side chains at the
imide position to introduce twisting in the packing
structure.28−32 Specifically, Narayan et al. reported that a PDI
dimer linked through the imide position inhibits excimer
formation, leading to OPV PCEs that are 20× those observed
in the corresponding PDI monomer, and affording a PCE of
3.2%.28,33 Similarly, it has been shown that the direct
attachment of two PDIs through the bay position results in a
dimer in which the individual chromophore π-planes are
twisted ∼70° relative to one another.30 This twist inhibits
strong π-stacking and excimer formation, and yields OPV PCEs
near 6%.34 High performance can also be achieved by adding a
thiophene linker at the bay position, resulting in a 4.03%
efficiency, which can be increased to 4.34% by tuning the active
layer donor−acceptor distribution.31,32 Interestingly, it has also
been demonstrated that by mixing the simple N,N-bis(3-
aminopentyl)PDI, which can be synthesized in good yields in a
single step, with different donor materials, PCEs up to 3% can
be achieved.29,35
Although the strategy of using twisted PDI dimers has
resulted in the highest published PCEs for any nonfullerene
OPVs to date, it is not without limitations. In the solid state,
PDI charge transport is highly anisotropic with high mobilities
along the π-stacking direction and far lower mobilities in the
transverse direction, whereas the high performance of fullerenes
has been attributed to their tendency to isotropically transport
charge in all directions.11,20 Solar cell performance can be
maximized only when charge separation and charge collection
are both efficient. Photogenerated free charge carriers can be
trapped and undergo fast recombination in the absence of a
continuous pathway to the collecting electrode with appreciable
mobility.17,36,37 By disrupting close, ordered PDI packing when
using twisted dimers, electron mobility is also likely
compromised. An alternative approach to the requisite BHJ
nanometer scale morphology while still retaining the electronic
connectivity required for free charge conduction is to use
polymeric acceptors. Several groups have implemented PDI-
containing polymers, which results in seemingly good
morphology and long-range order. However, OPV performance
is not significantly improved in these systems.24,38−42
An ideal BHJ acceptor system should preserve the long-range
π-stacking characteristics of PDI while simultaneously disrupt-
ing excimer formation. Here, we propose a new approach to
addressing this challenge by introducing substitutions at the
“headland” (C2, C5, C8, and C11) positions of PDI8 (see
Figure 1). Excimer formation has been shown to be rapid in H-
aggregated PDI systems, occurring with a time constant as low
as 215 fs, but far slower in J-aggregated systems where the
exciton diffusion length has been measured to be ∼96 nm.27,43
In fact, in cofacial dimers, the excimer formation rate in a slip-
stacked dimer has been shown to be an order of magnitude
slower than that in a completely cofacial dimer.44 Substitution
at the headland positions takes advantage of this because
headland substituted PDIs have been shown to organize in slip-
stacked structures, resulting in the inhibition of excimer
formation, without completely disrupting π-stacking.45 Another
benefit of headland substitutions is that they greatly increase
the solubility of PDI without resulting in a core twist, allowing
solution processability in systems without branched side
chains.46,47 Slip-stacking behavior can be achieved in several
PDIs without core substitution; however, core unsubstituted
systems are generally too insoluble for solution processing
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unless they have bulky solubilizing groups at the N positions
disrupting π-stacking.48,49 For this reason, slip-stacked PDIs
have not been commonly investigated in solution-processed
bulk heterojunction solar cells.
BHJ domains. This minimizes geminate recombination and
affords the highest PCE of 3.67%.
Materials Synthesis and Optoelectronic Properties
Characterization. The three PDIs, Hexyl-PDI, Phenethyl-
PDI, and Phenyl-PDI, were synthesized via Ru-catalyzed C−H
coupling procedures.46,47 Unlike the unmodified PDI8, all three
new PDIs are soluble in common organic solvents including
dichloromethane, chloroform, dichlorobenzene, and toluene.
Optical absorbance spectra of the films cast from chloroform
and cyclic voltamograms (CVs) obtained in dichloromethane
solutions are shown in Figure 2a,b. Data for the three PDIs are
EXPERIMENTAL METHODS
■
Materials Synthesis. All materials were synthesized and
characterized according to modified literature procedures. The details
of the synthesis and characterization are provided in the Supporting
Information.
OPV Device Fabrication. Inverted organic OPV devices with the
structure ITO/ZnO/Active Layer/MoO3/Ag were fabricated accord-
ing to published literature procedures.14 The details of device
fabrication and optimization are summarized in the Supporting
Information.
OTFT Device Fabrication and Testing. Top-gate bottom-
contact devices were fabricated in a nitrogen glovebox by spin-coating
5 mg/mL CHCl3 semiconductor solutions filtered through 0.2 μm
PTFE filters (2000 rpm) on thermally evaporated gold source/drain
electrodes (∼50 nm thick, glass substrate). This film was baked at 110
°C for 10 min. Next, the Cytop dielectric layer (CTL-809M) was spin
coated at 2000 rpm and the film baked at 110 °C for 10 min. The
device structure was completed by thermal deposition of the Au gate
electrode. All three PDI-based devices were tested in ambient
conditions, and the Phenyl-PDI device was also tested under vacuum.
TFTs based on the PBTI3T:Phenyl-PDI semiconductor blend
(donor:acceptor ratio of 1:1) were also fabricated, and both electron
and hole mobilities were measured in ambient conditions.
Steady-State Spectroscopy. Steady-state optical absorbance
spectra were measured using a Shimadzu UV-1800 spectrometer.
Fluorescence spectra were measured using a Photon Technology
International photon-counting spectrofluorometer. Film samples were
spin-coated on glass coverslips following the same procedure as was
used for the active layers in device preparation. Film fluorescence
measurements were carried out directly on the films, which were
placed at a 20° angle to the excitation beam.
Femtosecond Transient Absorption Spectroscopy (fsTA).
The details of the transient absorption instrumentation and
experimental methodology have been reported previously.45 Briefly,
for visible and near-infrared fsTA, samples were excited at 532 nm with
0.5 μJ/pulse at 1 kHz. The excitation pulse was focused to a 3 mm
diameter spot size for film samples to minimize the excitation density
and avoid singlet annihilation.
GIWAXS. Grazing incidence X-ray scattering (GIWAXS) measure-
ments were carried out at Beamline 8-ID-E of the Advanced Photon
Source at Argonne National Laboratory.50 Detailed experimental and
data analysis procedures are given in the Supporting Information.
Figure 2. Optoelectronic properties of the present acceptor molecules
and donor polymers: (a) Optical absorption spectra of films spun-cast
from chloroform with 0.5% diiodooctane by volume. (b) Solution CVs
of the PDI acceptors in CH2Cl2. (c) Estimated frontier molecular
orbitals of the present PDI acceptors along with PC71BM and donor
polymers.
RESULTS
■
In this contribution, we report the synthesis and character-
ization of three headland substituted PDIs, N,N-bis(n-octyl)-
2,5,8,11-tetra(n-hexyl)-PDI (Hexyl-PDI), N,N-bis(n-octyl)-
2,5,8,11-tetraphenethyl-PDI (Phenethyl-PDI), and N,N-bis(n-
octyl)-2,5,8,11-tetraphenyl-PDI (Phenyl-PDI) (Figure 1), as
potential new acceptors for BHJ OPVs. The neat molecules and
blend films are characterized by UV−vis optical spectroscopy,
cyclic voltammetry, fluorescence spectroscopy, GIWAXS, and
AFM. Next, inverted architecture BHJ OPV devices are
fabricated using the high-efficiency donor polymer PBTI3T
(Figure 1). To understand striking OPV performance variations
across this PDI series, the resulting BHJ blend morphologies
and microstructures are characterized in detail by AFM, TEM,
and GIWAXS, while blend charge separation and recombina-
tion dynamics are examined using photoluminescence quench-
ing and femtosecond transient absorption spectroscopy. It will
be seen that PBTI3T:Phenyl-PDI has the deepest lying
LUMO, intermediate crystallinity, and the most well-mixed
summarized in Table 1, along with that of the benchmark n-
type material PC71BM. The estimated HOMO and LUMO
energy levels of the acceptors and the donor polymer PBTI3T
are also shown in the energy diagram in Figure 2c.
Each of the PDIs undergoes two quasi-reversible reduction
events, and none show any oxidative activity in the scan range.
The reduction potentials of Hexyl-PDI, Phenethyl-PDI, and
Phenyl-PDI are −0.67, −0.77, and −0.85 V vs SCE,
respectively, corresponding to LUMO energies of −4.01,
−3.91, and −3.83 eV. Note that the LUMO energy of
Phenyl-PDI is very similar to that of PC71BM, −4.04 eV. The
optical band gaps (Eg) of Hexyl-PDI, Phenethyl-PDI, and
Phenyl-PDI films follow a trend similar to that of the LUMOs,
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Table 1. Summary of PDI Optoelectronic Properties
a
b
LUMO energy level of the materials was measured via solution CV. HOMO levels were calculated by subtracting the LUMO level from the optical
c
d
band gap. The absorbance peaks (λAbs) of the film. The optical band gap (Eg) calculated from the band edge of the films.
Figure 3. J−V (a) and EQE (b) curves of champion devices of the three PDI acceptors when blended with the donor polymer PBTI3T. About 60
nm thick active layers were cast from chloroform with 0.5% DIO.
with Phenyl-PDI having the smallest gap (618 nm, 2.01 eV)
and Hexyl-PDI the largest (600 nm, 2.07 eV). The absorbance
maxima (λmax) of the three molecules also follow the same
trend. Thus, λmax of Phenyl-PDI is the most red-shifted at 577
nm, and that of Phenethyl-PDI is next at 542 nm. While
Hexyl-PDI also has a peak at 541 nm, its λmax is blue-shifted to
492 nm.
Device Fabrication and Evaluation. Inverted solar cells
of the structure ITO/ZnO/PBTI3T:PDI/MoO3/Ag were
fabricated from PBTI3T:Hexyl-PDI, PBTI3T:Phenethyl-
PDI, and PBTI3T:Phenyl-PDI BHJ blends (Figure 3). The
device performance parameters for the three blends under the
optimized fabrication conditions are summarized in Table 2.
Devices incorporating the blends of the three different
acceptors show marked variations in PCE. The highest PCE
of 3.67% is obtained from a PBTI3T:Phenyl-PDI “champion
cell” where the average PCE is 3.60%. In contrast, the PCE of a
champion PBTI3T:Phenethyl-PDI device is 1.20% with an
average efficiency of 1.15% over four devices, while the PCE of
a champion PBTI3T:Hexyl-PDI cell is lower at 0.65%, with an
average of 0.58%. Interestingly, while the open circuit voltages
(Voc) of all three devices types are large and similar (1.016−
1.076 V), the short circuit current densities (Jsc) and fill factors
(FF) of the Phenethyl-PDI and Hexyl-PDI devices are
significantly lower than those of the Phenyl-PDI devices. All
three blends respond to light within a similar wavelength range,
300−685 nm. However, PBTI3T:Phenyl-PDI has the highest
conversion effeciency at all wavelengths with EQEs between
∼35% and ∼45% throughout the 350−750 nm range, with a
peak EQE of 46% attained at 570 nm. In contrast, the highest
EQE attained by PBTI3T: Phenethyl-PDI is 17% at 550 nm,
while the highest attained by PBTI3T:Hexyl-PDI is only 11%
at 540 nm (Figure 3).
TFT devices were fabricated to measure the carrier mobility
in pristine films of the PDI materials as well as in the champion
PBTI3T:Phenyl-PDI blend. The results are summarized in
Table 3, and I/V curves are shown in the Supporting
Information (Figure S7). Importantly, the electron mobilities
for the pristine films follow the performance trend observed for
the OPV blends, with the Phenyl-PDI-based TFTs exhibiting
the highest mobility (2.4 × 10−3 cm2 V−1 s−1 in ambient and 2.8
× 10−3 cm2 V−1 s−1 in vacuum) followed by the Phenethyl-PDI
(4.2 × 10−4 cm2 V−1 s−1 in ambient) and then by the Hexyl-
PDI (1.0 × 10−4 cm2 V−1 s−1 in ambient) devices. The electron
mobility of Phenyl-PDI dropped substantially in the blend,
a
Table 2. Solar Cell Performance of Champion Cells
b
acceptor
Voc [V]
Jsc [mA/cm2]
FF [%]
PCE [%]
Hexyl-PDI
1.076
1.016
1.024
1.51
2.44
6.56
39.29
48.48
54.59
0.65 (0.58)
1.20 (1.15)
3.67 (3.60)
Phenethyl-PDI
Phenyl-PDI
a
Active layers were ∼60 nm thick with a donor:acceptor ratio of 1:1
and were cast from chloroform solutions that contained 0.5% DIO by
volume. Average PCE of four cells is given in parentheses.
b
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a
rough (3.7 nm RMS) and GIWAXS images still display discrete
reflections. Phenethyl-PDI and PBTI3T form very smooth
films and show broad amorphous peaks in the GIWAXS
patterns. When the PDIs are blended with the donor polymer,
the resulting films become significantly smoother than the
corresponding neat PDI films. However, as in the case of the
neat films, Hexyl-PDI forms the roughest and Phenethyl-PDI
forms the smoothest films. The blend films also lack the sharp
GIWAXS reflections present in the neat PDI films, indicating
loosening of local PDI crystalline packing.
Table 3. OTFT Performance
μe (cm2 V−1 s−1) ambient
μh (cm2 V−1 s−1
)
(vacuum)
ambient
Hexyl-PDI
1.0 × 10−4
4.2 × 10−4
2.4 × 10−3 (2.8 × 10−3
4.2 × 10−5
Phenethyl-PDI
Phenyl-PDI
)
PBTI3T:Phenyl-PDI
1.7 × 10−2
a
All devices were top-gate bottom-contact and were fabricated and
tested as described in the Experimental Methods.
The GIWAXS data can be further analyzed by resolving into
constituent horizontal (in-plane or xy) and vertical (out-of-
plane or z) linecuts, as shown in Figure 5a,b. In both horizontal
and vertical directions, for each of the films, two sets of
reflections are evident. The first set at 0.2 Å−1 < q < 0.6 Å−1 (or
10−31 Å), identified in the figure by arrows, corresponds to the
polymer and the PDI lamellar stacking planes that are formed
due to side-chain interactions. The second set around q = 1.7
Å−1 (or 3.7 Å), as highlighted by the square brackets,
corresponds to the reflection planes that are formed by the
π-stacking interactions of the polymer and the PDIs. The π-
stacking peaks that appear in the out-of-plane direction
correspond to the face-on orientation of the stacks on the
substrates.51 When molecules and polymers orient face-on on
while the hole mobility of PBTI3T was comparable to those
reported for the same polymer backbone.14
Film Morphology and Crystallinity. Possible relation-
ships between thin film properties and the solar cell
performance were investigated by studying the morphology
and crystallinity of the neat polymer and PDI films and the
polymer:PDI BHJ blend films. AFM and 2D GIWAXS images
are shown in Figure 4a,b, respectively. Among the PDIs, Hexyl-
PDI is by far the most crystalline, displaying the largest
crystalline domains (1−2 μm), greatest surface roughness (4.2
nm RMS), and the most intense GIWAXS scattering peaks.
Phenyl-PDI films are of intermediate crystallinity; it does not
form micrometer-sized crystallites, but the surface is still rather
Figure 4. (a) AFM images of neat and blend films. RMS roughnesses for the neat films of Hexyl-PDI, Phenethyl PDI-, Phenyl-PDI, and PBTI3T
are 4.2, 1.6, 3.7 and 0.6, respectively. The RMS roughnesses of the PBTI3T:Hexyl-PDI, PBTI3T:Phenethyl-PDI, and PBTI3T: Phenyl-PDI blends
are 2.3, 0.8, and 1.0 nm, respectively. (b) 2D GIWAXS images of the neat and the blend films.
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Figure 5. In-plane (a) and out-of-plane (b) GIWAXS linecuts of the neat PBTI3T, Hexyl-BTI, Phenethyl-PDI, and Phenyl-BTI films and of the
donor:acceptor blends.
Figure 6. TEM images of (a) PBTI3T:Hexyl-PDI, (b) PBTI3T:Phenethyl-PDI, and (c) PBTI3T:Phenyl-PDI BHJ blend films.
the substrate surface, their side chains typically interdigitate and
lie parallel to the surface. This results in a corresponding side-
chain interaction peak in the horizontal direction.48 PBTI3T
exhibits a face-on π-stacking peak at qz = 1.73 Å−1 and a
corresponding lamellar peak at qxy = 0.27 Å−1. This corresponds
to a π-stacking distance of 3.63 Å and a backbone stacking
distance of 23.3 Å, metrics that are in very close agreement with
reported values for similar systems.14 Among the neat PDI
films, Phenethyl-PDI displays a clear π-stacking peak (qz = 1.66
Å−1, d = 3.78 Å), whereas the Hexyl-PDI and Phenyl-PDI films
exhibit several crystalline reflections in the π-stacking region.
When blended with the PDIs, the polymer π-stacking peak is
evident at qz = 1.67 Å−1 (d = 3.76 Å), with the stacking distance
slightly larger than that in neat PBTI3T films.
In contrast to what is observed for face-on stacking, the
present peaks at qxy ≈ 1.7 Å−1 and 0.2 Å−1 < qz < 0.6 Å−1
represent molecules oriented edge-on to the substrate surface.
Comparing the face-on and edge-on metrics gives a semi-
quantitative measure of the relative orientation of the
crystallites in the film. The stacking distances and correlation
lengths from Scherrer analysis of the lamellar stacking peaks of
the PDI and PBTI3T neat and blend films are summarized in
Supporting Information Table S2.52 The neat PBTI3T films
exhibit a preferential face-on orientation with a face-on to edge-
on (f:e) ratio of 2.2:1.0, in agreement with the literature.14 The
neat Phenethyl-PDI films show predominantly amorphous
features but also exhibit a clear π-stacking peak with an f:e ratio
of 2.1:1.0. Furthermore, the neat Phenyl-PDI and Hexyl-PDI
films show relatively strong crystallinity as compared to the
Phenethyl-PDI and the PBTI3T neat films, and the correlation
lengths of the Phenyl-PDI and Hexyl-PDI films are greater
than 10 nm, as compared to correlation lengths of ∼5 nm for
Phenethyl-PDI and PBTI3T. However, preferential orienta-
tion is difficult to quantify due to large number of reflections in
the π-stacking region. Although orientation was not deter-
mined, the neat films of Phenyl-PDI and Hexyl-PDI do show a
strong preferential orientation as demonstrated by well-defined
reflections, whereas the scattering patterns for Phenethyl-PDI
and PBTI3T are far more isotropic. Furthermore, the scattering
data for Phenyl-PDI match a simulated powder pattern based
on the reported crystal structure (Supporting Information
Figure S4).45 This means that the Phenyl-PDI molecules retain
their slip-stack packing behavior within the larger crystalline
domains of the corresponding films.
Note also that the PBTI3T components of all of the present
BHJ blend films exhibit enhanced face-on orientation, similar to
the behavior previously observed for PBTI3T:fullerene
blends.14 In fact, the PBTI3T edge-on π-stacking reflection
cannot be observed in any of the blend film data. Scherrer
analysis of the lamellar peaks in the blend films reveals that the
correlation lengths of the PDIs decrease upon the addition of
the polymer in all three cases, whereas the PBTI3T correlation
length is approximately unchanged in the PBTI3T:Phenyl-PDI
and the PBTI3T:Hexyl-PDI blends but is increased in the case
of PBTI3T:Phenethyl-PDI.
The nanoscale morphology of the donor:acceptor blends was
also investigated by TEM (Figure 6). The PBTI3T:Phenyl-
PDI films appear to be very homogeneous with small crystalline
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Figure 7. (a) Normalized optical absorbance spectra of the donor:acceptor blend films and (b) photoluminescence (PL) spectra of the neat and
blend films.
Figure 8. Global fits of fsTA data for PBTI3T:Hexyl-PDI, PBTI3T:Phenethyl-PDI, and PBTI3T:Phenyl-PDI blend films at different time intervals
(a−c); and the kinetic fits of the PDI anion absorbance in the three blends between 700 and 900 nm (d).
grains (∼2−5 nm in size), uniformly covering the entire scan
area. In contrast, the PBTI3T:Hexyl-PDI films appear less
homogeneous and show features extending ∼100 nm. In
contrast, the PBTI3T:Phenethyl-PDI films are devoid of any
large features but exhibit large amorphous regions scattered
over the scan area.
Charge Separation and Charge Recombination Dy-
namics. Photoluminescence (PL) experiments (Figure 7)
reveal that each of the present three PDIs almost entirely
quenches the fluorescence of PBTI3T BHJ films. Quenching
efficiencies of 99%, 97%, and 89% are measured for the
PBTI3T:Hexyl-PDI, PBTI3T:Phenethyl-PDI, and PBTI3T:-
Phenyl-PDI blend films, respectively. Such low PL quantum
yields suggest that excitons in each of the blends travel to the
donor:acceptor interface and are separated with high efficiency.
The charge separation and recombination behavior of the
three blends were further investigated with femtosecond
transient absorption spectroscopy (fsTA). Transient spectra
and recombination rates for Hexyl-PDI, Phenethyl-PDI,
Phenyl-PDI, and PBTI3T neat films, as well as the transient
spectra for the blend films, are provided in the Supporting
Information (Figures S5,S6). Global fitting of the fsTA data via
singular value decomposition (SVD) methods reveals the
existence of three species in each of three blends. The species
associated spectra are shown in Figure 8a−c. The first species
decays with a time constant of 3−7 ps and is assigned to
PBTI3T excitation. The second species decays with a time
constant of 200−370 ps and is assigned to a population of
trapped geminate radical pairs, which undergo rapid charge
recombination. The third and the final species lives for the
length of the experiment and is assigned to fully separated
charge carriers. Kinetic fits of the PDI anion region for the
blend films are also shown in Figure 8d and support the time
constants extracted from the global fitting.
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Figure 9. Diffraction-derived crystal structure of Phenyl-PDI showing the close slip-stacked packing.44 The slip angle along the long axis of the PDI
core is 47°. The crystal structures of Phenethyl-PDI and Hexyl-PDI could not be determined because diffraction-quality crystals could not be
grown.
in the solid state. The intermolecular packing motifs proposed
in Figure 1 for two of the molecules, the columnar structure of
PDI8 and slip-stacked structures of Phenyl-PDI, are derived
from their known crystal structures. The crystal structures of
Phenethyl-PDI and Hexyl-PDI could not be obtained as they
did not form diffraction quality crystals but instead formed long
very fine fibers (∼1 μm in diameter and several hundred
micrometers in length), suggesting a strong interaction in one
direction. This is likely a result of the strong π-stacking
interactions commonly observed in PDI derivatives.55 On the
basis of the structures and the absorption spectra of these
molecules, we hypothesize that the relative slip angles in the
PDIs are as shown in Figure 1, with the Phenyl-PDI being the
most slipped, the Phenethyl-PDI having an intermediate slip
angle, and finally the Hexyl-PDI being the least slipped. The
effect of crystal packing on the optical absorption of PDI has
been well studied, and calculations by Engels et al. suggest that
a longitudinal displacement of 3.5 Å (as observed in the crystal
structure of Phenyl-PDI) should result in an increasing band
gap with decreasing slip angle.56This trend in the slip angle can
be explained in terms of the steric bulk of the different headland
substituents. Thus, the encumbrance of the phenyl ring is
greater than that of the phenethyl group, which in turn is
greater than that of an n-hexyl group. It is important to note
that despite the change in molecular packing, hypothesized here
to decrease the slip angles, both Phenethyl-PDI and Hexyl-
PDI exhibit some vibronic structure in steady-state fluorescence
(Figure 7). These vibronic structures are characteristic of
monomer-like fluorescence and are not observed in systems
with rapid excimer formation, where broad fluorescence spectra
with larger Stokes shifts are observed.57 Additionally, despite
the decreased slip angle in Phenethyl-PDI and Hexyl-PDI, the
excited-state lifetimes of all three films are greater than 100 ps,
which should be sufficient for long distance exciton diffusion.
These results suggest that all three molecules successfully
inhibit excimer formation in the solid state. We propose that
this is the result of a slip-stacked packing geometry. This
mechanism is consistent with that observed in solution for
dimers and solution aggregates.27,44 Slip-stacking has also been
observed in crystals of “bare” PDIs without substituents on the
headland or bay positions, but as far as we know the effect of
DISCUSSION
■
PDIs can be synthetically modified at the imide N-positions,
the “bay” positions, or the “headland” positions. The most
commonly studied PDI derivatives are substituted at the imide
and the bay positions. This is because substitution at the
headland positions was difficult until the development of
efficient Ru-catalyzed C−H activation methodologies.46,47
Unlike substitution in the imide N positions, which has a
minimal effect on PDI electronic properties, and substitution in
the bay positions, where controlling electron-donating proper-
ties can be complicated by a substituent-induced core-twisting,
PDI substitution at the headland positions can result in large
electronic properties changes while maintaining core planar-
ity.18
Phenyl-PDI crystallizes in a slip-stacked motif, which is
preserved in vapor-deposited45 and, as this work shows,
solution-deposited thin films (Figure 9, Supporting Information
Figure S4). Photoexcitation of these films results predominantly
in singlet exciton fission with small amounts of competing
excimer formation. Singlet exciton fission is undesirable in
OPVs that are not specifically designed to take advantage of it
because the resultant triplet excitons are quite low in energy
and therefore unable to undergo efficient charge separation.
However, the rate of singlet exciton fission in these films is slow
and the rate of excimer formation is even slower, resulting in
singlet exciton lifetimes on the order of 200 ps, which is
significantly longer than in systems with rapid excimer
formation such as in most PDI films.27,53 Furthermore, the
rate of charge separation in BHJs is generally much faster than
200 ps, meaning that headland substituted PDIs may provide a
potential solution to the undesirable excimer formation
observed in other PDIs. Headland substituted PDIs have
been used in solar cells before; however, in that case, an N-
swallowtail substituent was used to disrupt slip-stacking
behavior, thus resulting in a packing motif with very little π-
overlap.46,47 This structural limitation is consistent with the
modest PCE of 0.50% observed in BHJ OPVs fabricated with
that material.54 Additionally, that system used alkyl substituents
in the headland positions that, as shown here, raise the LUMO
energy, which may disfavor charge separation.
The steady-state optical absorption spectra of the present
PDI thin films also reveal interesting information about packing
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packing geometry in these systems on the rate of excimer
formation has not been systematically investigated.
a similar donor polymer, containing an identical core but
slightly solubilizing side chains, blended with PC71BM.14 The
PBTI3T:Hexyl-PDI blends afford low Jsc and low FF (1.51
mA/cm2 and 39.29%) and consequently a low PCE of 0.65%.
The PBTI3T:Phenethyl-PDI blends afford a slightly higher
PCE of 1.20% because of slightly higher Jsc (2.44 mA/cm2) and
FF (48.48%) metrics. The PCE afforded by PBTI3T:Phenyl-
PDI is even higher, up to 3.67%, because of its larger Jsc (6.56
mA/cm2) and FF (54.59%). Interestingly, all three PDI
derivatives exhibit high and very similar Voc’s despite having
different LUMO levels. This is likely a result of increased
nonradiative recombination and resistance in the PBTI3T:-
Phenethyl-PDI and the PBTI3T:Hexyl-PDI blends.63,64 This
is supported by the fact that the slope of the J−V curve near
open-circuit conditions is steep in the case of the
PBTI3T:Phenyl-PDI blend and is less steep in the case of
the PBTI3T:Phenethyl-PDI blend and even less so in the
PBTI3T:Hexyl-PDI blend.
As shown in the Results, the TFT electron mobility of our
PDIs (μPhenyl‑PDI > μPhenetyl‑PDI > μHexyl‑PDI) follows the same
OPV performance trends and is comparable to or exceeding
those of other nonfullerene acceptors reported in the
literature.65 This trend in mobility could explain the observed
trend in fill factor observed as fill factor has been shown to be
closely related to the mobility of the active layer materials and
to bimolecular recombination.63,66,67 Additionally, although the
pristine electron mobility is comparable to the hole mobility in
pristine PBTI3T, we observe a significant decrease in electron
mobility in the blend film. This likely results in a charge
transport imbalance, which limits efficiency improvement and
provides a target for further optimization.
While the PL quenching experiments suggest that the three
PDIs reported here can efficiently separate charge, this does not
explain the differences in OPV performance between the three
blends. In fact, the OPV performance trend that would be
expected from the PL quenching results is opposite of what is
observed experimentally. The trend must therefore reflect the
following: (1) differing blend morphologies, (2) differing blend
charge recombination features, or (3) some combination of the
differing blend morphology and charge recombination
characteristics. Here, we discuss these issues in detail.
The morphology/microstructure of the present blends was
studied further by AFM, GIWAXS, and TEM. The AFM images
reveal that all three films are quite smooth with RMS
roughnesses between 0.8−2.3 nm, with PBTI3T:Hexyl-PDI
being the roughest and the PBTI3T:Phenethyl-PDI being the
smoothest. It is of particular note that these films are
significantly smoother than the corresponding neat PDI films.
This result argues that the polymer has weakened the otherwise
long-range order in the Hexyl-PDI and Phenyl-PDI inter-
backbone packing, in agreement with the GIWAXS-derived
PDI correlation lengths, which decrease in both the qz and the
qxy directions of the blends. Not only does the PBTI3T break
up the large PDI crystallites and consequently generate very
smooth films, it does so while still retaining significant face-on π
stacking. This combination provides two very advantageous
factors for solar cell performance.14,68,69 Detailed analysis of the
morphology of the blends at the nanometer scale via TEM
(Figure 5) provides a better understanding of the observed
device performances. The TEM image of the PBTI3T:Hexyl-
PDI blend film shows features ∼100 nm in length, signifying
poor mixing and BHJ morphology. Additionally, it appears that
the PBTI3T:Phenethyl-PDI blend segregates into a nonuni-
The solar cell performance of many different donor−
acceptor blends, including those with PDI-based acceptors,
has been optimized using processing additives such as 1,8-
diiodooctane (DIO).9,14,29,31,58,59 In the present study, small
amounts of DIO (0.5% by volume) significantly enhance the
performance of the PBTI3T:Phenyl-PDI blend OPVs,
probably because of increased microstructural order in the
blend. However, increasing the DIO concentration further
results in the formation of very large (>100 nm, Supporting
Information Figure S3) PDI crystallites and consequently
lowered OPV performance. Solar cell performance data for
different active layer DIO concentrations as well as the AFM,
TEM, and GIWAXS images of the films are summarized in the
Supporting Information. Device performance was also opti-
mized for active layer thickness. The highest performance was
obtained for 60 nm thick films. Thinner films result in higher
fill factors (more efficient current collection) but decreased
absorbance and hence yielded lower short-circuit currents.
Thicker films afford lower fill factors and lower currents,
presumably because of increased resistance.
Besides a favorable morphology, another important consid-
eration in the design of a PDI-based OPV acceptor is the
electron affinity, or LUMO energy. Charge separation in
organic solar cells is driven by the energy offset between the
donor and acceptor LUMO levels.60,61 Literature reported
high-performance PDI-based solar cells often have moderate to
large LUMO−LUMO offsets between the donor and acceptors
(0.6−0.8 eV).31,33 While such a strategy might assist charge
separation, it is not without drawbacks because it results in
energy loss in the form of Voc depression. Obtaining efficient
charge separation without significant Voc losses is ideal. Recent
literature reports show that a low driving force is sufficient for
efficient charge separation in fullerene-based system by
exploiting “hot excitons” and/or by delocalization of charge
carriers.61,62 The present study shows that headland-substituted
PDIs are capable of undergoing rapid charge separation in a
BHJ blend and that they have easily tailorable energetics and
slip-stacked packing structures. The LUMO level can be
predictably tuned by exploiting the electron-donating strength
of the various substituents because of the strong electronic
coupling between the PDI core and headland substituents.
Interestingly, the LUMO energy of Phenyl-PDI determined
electrochemically is very similar to that of PC71BM (−4.01 and
−4.04 eV, respectively). This suggests that, as far as energy level
requirements are concerned, Phenyl-PDI should function well
as an alternative acceptor to fullerenes because the difference in
the energy between the LUMO orbitals of the donor and that
of the acceptor is a measure of the driving force for charge
separation.62 The LUMO energy of PBTI3T is −3.77 eV, so
ΔGCS can be roughly approximated as −0.24, −0.14, and −0.06
eV for Phenyl-PDI, Phenethyl-PDI, and Hexyl-PDI, respec-
tively.14 Although this should be sufficient driving force in the
case of PBTI3T:Phenyl-PDI, it is low for PBTI3T:Phenethyl-
PDI and PBTI3T:Hexyl-PDI.
In terms of OPV performance dependence on acceptor, we
observe significant enhancement on going from Hexyl-PDI →
Phenethyl-PDI → Phenyl-PDI. This trend is solely due to
increases in Jsc and FF because Voc for all three blends is very
similar (∼1.02−1.076 V). It is of particular note that the Voc’s
afforded by the three PDIs reported in this work are
significantly higher (by over 0.22 V) than that obtained with
I
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form morphology on the nanometer scale, and contains
numerous diffuse dark domains that are ∼100 nm across.
These large domains have low contrast in the TEM, suggesting
that they are likely mixed domains. Considering that we
observe quantitative charge separation in the films, it seems
likely that these domains are too finely mixed, resulting in
charge trapping. In contrast to these results, the PBTI3T:-
Phenyl-PDI TEM image reveals a homogeneous and well-
mixed blend with a uniform distribution of light and dark
domains. These domains are only a few nanometers in size and
are probably individual Phenyl-PDI-rich and PBTI3T-rich
domains. The presence of these more pure domains could offer
an explanation as to why there is less geminate recombination
in the PBTI3T:Phenyl-PDI blends. It is possible that fine-
tuning of the film processing conditions may improve the
morphology of both the PBTI3T:Hexyl-PDI and the
PBTI3T:Phenethyl-PDI blends; however, we decided to
maintain the processing conditions constant to limit the
number of variables considering the wide variety of processing
conditions available.
Transient absorption spectroscopy is a powerful technique to
interrogate charge generation and recombination phenomena
in BHJ films.70,71 To begin, the spectra of neat PBTI3T films
show ground-state bleaching, stimulated emission, and a very
strong, broad, excited-state absorbance in the NIR centered at
1250 nm. Furthermore, this PBTI3T excited-state absorption
decays with a time constant of ∼45 ps. The PDI neat films are
characterized by much weaker excited-state absorptions
centered at 700 nm, which decay in several hundred
picoseconds (Supporting Information Figure S5). In marked
contrast, the blend films show clear evidence of rapid charge
separation. Thus, at very early times, the transient spectra of the
blends resemble the PBTI3T excited state, characterized by
strong NIR absorption. This excited-state absorption rapidly
decays with time constants of 3.8, 3.0, and 6.9 ps for the
PBTI3T:Hexyl-PDI, PBTI3T:Phenethyl-PDI, and PBTI3T:-
Phenyl-PDI blends, respectively. This decay is accompanied by
a rise at 750 nm, which is likely due to combined loss of
stimulated emission from the PBTI3T and the formation of a
broad PDI anion absorption feature, consistent with the radical
anion absorbance of PDI solution-phase aggregates and
covalent dimers (Supporting Information Figure S6).72,73
There is also an absorption in the NIR centered at 950 nm,
which is characteristic of the PDI anion and remains after the
decay of the PBTI3T excited-state absorption.74 For these
reasons, the time constants are assigned to charge separation in
the blend films. Given that PBTI3T has an excited-state
lifetime of 45 ps, these time constants result in charge
separation yields of 92%, 94%, and 87% for PBTI3T:Hexyl-
PDI, PBTI3T:Phenethyl-PDI, and PBTI3T:Phenyl-PDI
films, respectively. This result confirms that charge separation
is responsible for the large extent of PL quenching in the blend
films.
Phenyl-PDI blends, respectively (Figure 8a−c). Kinetic fits of
the PDI anion peak from 700 to 800 nm give lifetimes similar
to those from the SVD analysis (Figure 8d). The blends also
have some charge separated population that persists for the
length of the experiment. The major difference between the
three blends is the yield of long-lived charge separation. These
yields track well with the overall PV performance of the BHJ
materials and are 19%, 27%, and 36% for PBTI3T:Hexyl-PDI,
PBTI3T:Phenethyl-PDI, and PBTI3T:Phenyl-PDI, respec-
tively. The short-lived population most likely decays as a result
of geminate pair recombination, while the long-lived population
is most likely nongeminate.36 This suggests that, although a
large percentage of excitons undergo charge transfer in all of
the films, many are trapped in low-lying geminate states rather
than fully dissociating into free charge carriers, which are
collected at the cell electrodes.
There are two possible explanations for why the different
PDI acceptors in this study afford different free carrier yields.
The first concerns the differing morphologies at the donor−
acceptor blends. The second concerns the increased driving
force for dissociation of bound charge transfer states (CT) into
fully separated states (CS). Although all three materials resulted
in smooth films with moderate crystallinity, the TEM shows
there are striking differences in the nanostructures of the
blends. Specifically, while the PBTI3T:Phenyl-PDI blend is
composed of small domains with high contrast in the TEM, the
PBTI3T:Hexyl-PDI and PBTI3T:Phenethyl-PDI blends are
composed of much larger domains with lower contrast (Figure
6). This could be a result of mixing of the PDI into the polymer
domains of these blends and vice versa, which could result in
the inability of CT states to delocalize and fully separate into
free carriers.
Interface morphology has been proposed to underlie
subnanosecond charge recombination in all-polymer solar
cells, and it has been demonstrated that the donor−acceptor
spacing at the interface can have significant effects on charge
separation efficiency.36,75 It has also been demonstrated, for
cases when PDI is blended with certain donor polymers, that
the barrier for charge separation is greater for PDI as an
acceptor than for PCBM.76 It is possible that this barrier is
diminished in PDI systems having bulkier substituents, which
maintain greater donor−acceptor separations at the BHJ
interface.
It is also possible that excess energy in the initial CT state is
used to overcome the charge separation barrier.77 In this case, it
would be logical that the system with the maximum free energy
difference would give the highest yield of separation.
Specifically, the estimated driving forces for charge separation
are −0.24, −0.14, and −0.06 eV for Phenyl-PDI, Phenethyl-
PDI, and Hexyl-PDI, respectively. This means that the CT
state PBTI3T:Phenyl-PDI blend has the most excess energy
and therefore the most favorable probability to fully separate.
This result could also be explained by increased band-
broadening for the Phenyl-PDI as compared to the other
acceptors. Kazmaier and Hoffman predicted that band
broadening qualitatively correlates to the energy and the
band broadness of an absorption in the solid state.49 Because
the Phenyl-PDI absorption is the most red-shifted and
broadest of the three systems, this result suggests the possibility
of increased band broadening, which would lead to increased
stabilization of charge carriers.
The charge recombination rates of the three BHJ blend films
are important for understanding why the OPV performance of
these materials differs so greatly. All three blends exhibit
multiexponential charge recombination. Global fitting of the
fsTA data via singular value decomposition (SVD) reveals the
existence of three species in each of the blends. Furthermore,
the SVD shows that there are two major recombination rates,
one fast and one slow. The fast recombination time constants
from the SVD analysis are 345, 285, and 370 ps for the
PBTI3T:Hexyl-PDI, PBTI3T:Phenethyl-PDI, and PBTI3T:-
Charge separation in organic solar cells is driven by an
energy offset between the donor and the acceptor LUMO
J
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levels.60,61 High-performance PDI-based solar cells reported in
the literature tend to have large LUMO−LUMO offsets of 0.6−
0.8 eV.31,33 While such a strategy may favor charge separation,
it has the disadvantage that it results in energy loss in the form
of Voc lowering. Obtaining a high degree of charge separation
without significant Voc losses is ideal. Here, we are able to
minimize the LUMO−LUMO offset while achieving high Voc’s
of ∼1.0 V
an Office of Science User Facility operated for the U.S.
Department of Energy (DOE) Office of Science by Argonne
National Laboratory, was supported by the U.S. DOE under
Contract No. DE-AC02-06CH11357. SEM and TEM measure-
ments were performed at the EPIC facility, located in
NUANCE Center at Northwestern University. NUANCE
Center is supported by the NSF-MRSEC (DMR-1121262),
Keck Foundation, the State of Illinois, and Northwestern
University.
CONCLUSIONS
■
REFERENCES
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excimer formation while preserving good π-stacking for charge
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decreases with increasing steric bulk in the headland position
and with increasing driving force for charge separation. BHJ
devices employing these three acceptors afford high Voc’s (>1.0
V) and relatively high fill factors. The best performing BHJ
blend, PBTI3T:Phenyl-PDI, affords a PCE of 3.67%. Detailed
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useful strategy for designing high-performance nonfullerene
OPV acceptors.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details including synthetic methods, electro-
chemical characterization, GIWAXS characterization, and
transient characterization of the neat films. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
r-chang@northwestern.edu
m-hersam@northwestern.edu
m-wasielewski@northwestern.edu
t-marks@northwestern.edu
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Author Contributions
∥P.E.H., A.T., and H.S.S.R.M. contributed equally.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported in part by the Argonne-Northwestern
Solar Energy Research (ANSER) Center, an Energy Frontier
Research Center funded by the U.S. Department of Energy
(DOE), Office of Science, Office of Basic Energy Sciences,
under award number DE-SC0001059 (P.E.H. and M.R.W. for
PDI synthesis and optical measurements; H.S.S.R.M. for device
fabrication; N.Z. for TEM measurements) and by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences under Award Number DE-FG02-08ER46536 (A.T. for
device fabrication and characterization). H.S.S.R.M. thanks the
Indo-US Science & Technology Forum (IUSSTF) for a
Postdoctoral fellowship. Use of the Advanced Photon Source,
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