Self-organizing mesomorphic diketopyrrolopyrrole derivatives for efficient solution-processed organic solar cells

Article abstract of DOI:10.1021/cm401244x

We describe the synthesis, self-organization, and photovoltaic properties of diketopyrrolopyrrole (DPP)-based mesomorphic small-molecule materials, 3,6-bis{5-(4-alkylphenyl)thiophen-2-yl}-2,5-di(2-ethylhexyl)-pyrrolo[3,4-c] pyrrole-1,4-diones (DPP-TP6 and DPP-TP12), which have strong visible absorption characteristics. The effects of varying terminal alkyl chains on the self-assembling properties and photovoltaic device performances have been studied. With the appropriate ratio of the lengths of the alkyl chains to its rigid DPP core, DPP-TP6 exhibits liquid-crystalline properties and forms well-developed nanostructured bulk heterojunction (BHJ) architectures with a fullerene derivative (PC₇₁BM). Organic solar cells (OSCs) employing BHJ DPP-TP6:PC₇₁BM films show a power conversion efficiency as high as 4.3% with a high open-circuit voltage of 0.93 V and a fill factor of 0.55. These results demonstrate that liquid-crystalline organization to direct molecular self-assembly is a promising strategy for fabricating high-performance solution-processed small-molecule OSCs.

Full text of DOI:10.1021/cm401244x

Article
pubs.acs.org/cm
Self-Organizing Mesomorphic Diketopyrrolopyrrole Derivatives for
Efficient Solution-Processed Organic Solar Cells
Woong Shin,†,§ Takuma Yasuda,*,†,‡,§,⊥,∥ Go Watanabe,⊗ Yu Seok Yang,‡,§ and Chihaya Adachi*,‡,§,⊥
‡
§
†Department of Automotive Science, Department of Applied Chemistry, Center for Organic Photonics and Electronics Research
⊥
(OPERA), and International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka,
Nishi-ku, Fukuoka 819-0395, Japan
∥PRESTO, Japan Science and Technology Agency (JST), Chiyoda-ku, Tokyo 102-0076, Japan
⊗School of Science, Kitasato University, Kitasato Minami-ku, Sagamihara, Kanagawa 228-8555, Japan
S
* Supporting Information
ABSTRACT: We describe the synthesis, self-organization, and
photovoltaic properties of diketopyrrolopyrrole (DPP)-based
mesomorphic small-molecule materials, 3,6-bis{5-(4-
alkylphenyl)thiophen-2-yl}-2,5-di(2-ethylhexyl)-pyrrolo[3,4-c]-
pyrrole-1,4-diones (DPP-TP6 and DPP-TP12), which have
strong visible absorption characteristics. The effects of varying
terminal alkyl chains on the self-assembling properties and
photovoltaic device performances have been studied. With the
appropriate ratio of the lengths of the alkyl chains to its rigid
DPP core, DPP-TP6 exhibits liquid-crystalline properties and
forms well-developed nanostructured bulk heterojunction
(BHJ) architectures with a fullerene derivative (PC71BM). Organic solar cells (OSCs) employing BHJ DPP-TP6:PC71BM
films show a power conversion efficiency as high as 4.3% with a high open-circuit voltage of 0.93 V and a fill factor of 0.55. These
results demonstrate that liquid-crystalline organization to direct molecular self-assembly is a promising strategy for fabricating
high-performance solution-processed small-molecule OSCs.
KEYWORDS: organic solar cells, diketopyrrolopyrrole, bulk heterojunction, self-organization, liquid crystals
dissociation and charge carrier transport in BHJ OSCs.8
However, if the scale of the phase-segregated domains in the
INTRODUCTION
■
Organic solar cells (OSCs) are drawing considerable interest
BHJ layer is smaller than the Coulomb capture radius, it will
for the great promise as a next-generation clean and renewable
energy source.1,2 To date, intensive research efforts have
increase the geminate or bimolecular recombination proba-
bility.8a Therefore, the design of suitable donor molecules,
focused on developing dye-sensitized solar cells,3 low-bandgap
polymer/fullerene bulk heterojunction (BHJ) solar cells,1a−e,4
which are capable of forming nanoscale ordered structures and
and vacuum-deposited small-molecule solar cells.5 Recently,
overall optimum film morphologies, is a prerequisite for high-
solution-processed small-molecule OSCs1f−i,6,7 have been
performance BHJ OSCs. Our intention here is to utilize self-
organization of liquid-crystalline (LC) molecules9 for the
emerging as an attractive alternative to widely studied
polymeric counterparts, offering several promising advantages
such as monodispersity and well-defined structure, easy
purification, and less batch-to-batch variation (better reprodu-
fabrication of well-ordered BHJ architectures in small-molecule
OSCs, to achieve better photovoltaic efficiencies. A few
researches have been conducted on tuning the active layer
morphology by using LC donor molecules.10 As a platform to
cibility). Indeed, high power conversion efficiencies (PCEs)
explore this approach, we have designed and synthesized simple
exceeding 6% have been accomplished thus far for small-
molecule BHJ OSCs,6a−d approaching those with conjugated
diketopyrrolopyrrole (DPP)-based small molecules, DPP-TP6
and DPP-TP12 (Scheme 1), since the incorporation of
multiple flexible chains into the rigid π-conjugated DPP core
can provide highly light-absorbing donor materials forming LC
ordering. Various DPP-based small molecules7 and polymeric
materials4b,11 have been developed to render high charge carrier
polymers. However, overall performance of solution-processed
small-molecule devices still lags behind alternative sources. In
most cases, the small molecular system suffers from inferior film
quality and interconnectivity in the active layer because of their
intrinsic crystallization, which results in low device fill factors.
Because of the relatively large exciton binding energy (e.g.,
0.3 eV) and short exciton diffusion length (e.g., 10−20 nm) in
organic semiconductors, a large donor/acceptor (D/A)
interfacial area should be required for efficient exciton
Received: April 17, 2013
Revised: May 17, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/cm401244x | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
a
Scheme 1. Synthesis of DPP-Based Donor Molecules
Figure 1. DSC thermograms of DPP-TP6 and DPP-TP12 at 5 °C
a
Reaction conditions: (i) 1-bromo-2-ethylhexane, K2CO3, DMF; (ii)
NBS, CHCl3; (iii) 2-(4-alkylphenyl)-4,4,5,5-tetramethyl-[1,3,2]-
dioxaborolane, K2CO3, Pd(PPh3)4, THF/H2O.
min−1.
mobility and photovoltaic properties. Nguyen and co-workers
reported DPP-containing small-molecule donors that achieved
up to 4.8% power conversion efficiencies (PCEs) with [6,6]-
phenyl-C71-butyric acid methyl ester (PC71BM) in solution-
processed BHJ devices.7a,b
In this study, we investigate the optoelectronic and self-
organizing properties of DPP-TP6 and DPP-TP12, as well as
their performance as donor materials in solution-processed
OSCs. Furthermore, we here demonstrate that the formation of
well-developed nanostructured domains through LC organ-
ization has a drastic impact on the photovoltaic properties.
Figure 2. (a) UV/vis absorption spectra of DPP-TP6 in CHCl3
solution and an as-spun thin film. (b) Photoelectron yield spectrum of
a DPP-TP6 thin film.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. DPP-TP6 and DPP-
TP12 were synthesized from dithienyl-DPP precursor (1),12
using palladium-catalyzed Suzuki−Miyaura cross-coupling
reactions (see Experimental Section and the Supporting
Information for details). To make donor molecules with a
deeper highest occupied molecular orbital (HOMO) level
suitable for OSC applications, the phenylthiophene-appended
DPP was designed as the low bandgap π-conjugated core and
was tethered with multiple alkyl chains, which enhance
solubility and film-forming properties. The chemical structures
of the final products were ascertained by NMR spectroscopy,
matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) mass spectrometry, and elemental analysis
after purification. As expected, DPP-TP6 and DPP-TP12 are
soluble in common organic solvents such as CHCl3, THF,
toluene, and chlorobenzene.
coated thin film of DPP-TP6 exhibits a red-shifted λmax at 637
nm with a large maximum absorption coefficient of 4.7 × 104
cm−1, and a distinct subpeak at 581 nm. The red-shift of the
lowest vibronic band in the solid state originates from J-type
aggregation,13 because of the excitonic coupling between the
transition dipoles of adjacent molecules. The absorption bands
thus extend from 500 nm to a lower energy level of ∼700 nm in
the thin films of DPP-TP6 and DPP-TP12, and their optical
bandgap is estimated to be 1.8 eV from the absorption edge.
The HOMO energy levels of DPP-TP6 and DPP-TP12 in
the thin films have been determined to be approximately −5.5
eV by photoelectron yield spectroscopy (Figure 2b). The low-
lying HOMO levels and sufficient LUMO offsets (>0.6 eV),
allow these materials to serve as electron donors in
combination with PC71BM as an acceptor material, which has
HOMO and LUMO levels of −6.1 and −4.3 eV, respectively.
The optical data of DPP-TP6 and DPP-TP12 are summarized
in Table 1.
Thermogravimetric analysis (TGA) indicates that DPP-TP6
and DPP-TP12 possess high thermal stability with a 5% weight
loss temperature (Td) greater than 360 °C under N2
atmosphere (Supporting Information, Figure S1). Thermal
behavior of these compounds have been further studied by
differential scanning calorimetry (DSC). As shown in Figure 1,
DPP-TP6 with hexyl terminal chains exhibits two endothermic
peaks at 117 and 170 °C upon heating, corresponding to LC
mesophase and isotropic phase transitions, respectively.13 In
DPP-TP12, the introduction of dodecyl chains on both
terminals results in a decrease of the isotropization temperature
to 128 °C and the LC mesophase disappears.
Optical Properties. The UV/vis absorption spectrum of
DPP-TP6 in CHCl3 solution presents an absorption peak
(λmax) at 602 nm (Figure 2a), which corresponds to the π−π*
(HOMO → LUMO) transition. The structured absorption is
likely intrinsic to a single molecule with restricted intra-
molecular motion. Compared with dilute solutions, a spin-
To clarify how the thermotropic LC behavior can affect the
molecular assembly and optoelectronic properties, the
Table 1. Optical Properties of DPP-Based Materials
absorption λmax (nm)
c
d
d
HOMO
(eV)
LUMO
(eV)
Eg
a
b
compound solution
DPP-TP6 602, 563 637, 581
DPP-TP12 603, 563 651, 592
film
(eV)
−5.50
−5.48
−3.69
−3.68
1.81
1.80
a
b
In CHCl3 solution at 10−4 M. Spin-coated thin film (ca. 100 nm).
c
d
Determined by photoelectron yield spectroscopy. LUMO = HOMO
+ Eg, in which the optical energy gap, Eg, was derived from the
absorption onset of the thin film.
B
dx.doi.org/10.1021/cm401244x | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
absorption spectra of DPP-TP6:PC71BM and DPP-
TP12:PC71BM (1:1, w/w) blend films have been measured
by varying the thermal annealing temperatures (Figure 3). The
Figure 4. Energy-level diagram of BHJ solar cells based on DPP-
TP:PC71BM layer and the calculated HOMO/LUMO distributions of
the π-conjugated core of the donor molecules.
Various donor:PC71BM blend ratios (1:1, 1.5:1, and 2:1, w/w),
film thickness, and postdeposition thermal annealing have been
examined. Table 2 summarizes the photovoltaic parameters of
representative BHJ devices obtained under AM 1.5G
illumination at an intensity of 100 mW cm−2. These devices
provide reasonably high open-circuit voltages (Voc) of ∼0.93 V,
regardless of the D/A weight ratio, which is attributed to a large
difference (∼1.2 eV) between the HOMO level of the donors
and the lowest unoccupied molecular orbital (LUMO) level of
the PC71BM acceptor15 (Figure 4).
Figure 5a depicts the current density−voltage (J−V)
characteristics of as-cast and annealed DPP-TP6:PC71BM
(1:1, w/w) BHJ solar cells. The optimized device based on
the DPP-TP6:PC71BM layer with annealing at 140 °C, exhibits
a short-circuit current density (Jsc) of 8.4 mA cm−2, Voc of 0.93
V, and a fill factor (FF) of 0.55, yielding a power conversion
efficiency (PCE) of 4.3% (Table 2); whereas the as-cast device
gives a much lower PCE of 2.6%. It can be found that for DPP-
TP6-based devices, thermo-responsive LC molecular organ-
ization improves Jsc and FF, significantly increasing device
efficiencies. The substantial increases in Jsc and FF should be
attributed to the reduction of series resistance (Rs) and the
increase of shunt resistance (Rsh) of the devices, upon the LC
phase transition in the active layer. From a physical point of
view, Rs is associated with the conductivity of the semi-
conductors, and thus the carrier mobility in the active layer;
while Rsh is related to the loss of charge carriers via
recombination and leakage.8 Indeed, the dark J−V curves of
the devices (inset of Figure 5a) reveals the effective suppression
of the leakage current under low biases after thermal annealing,
indicating improved diode characteristics. Similar enhancement
in the PCEs upon thermal annealing has been attained for other
DPP-TP6:PC71BM blend ratios as well (Table 2). When the
DPP-TP6:PC71BM blend ratios are changed to 1.5:1 and 2:1,
the Jsc tends to decrease gradually and the Voc remains relatively
constant (Supporting Information, Figure S4).
To address the origin of the enhanced device performances,
we have measured the incident photon-to-current conversion
efficiency (IPCE) spectra for the DPP-TP6-based BHJ devices
(Figure 5b). With annealing at 140 °C, a pronounced
enhancement in photoresponses ranging from 350 to 600
nm, corresponding to the superposition of the absorption of H-
aggregated DPP-TP6 and PC71BM molecules, can be observed.
The IPCE reaches the maximum value at ∼70%, which is
considerably high for small-molecule BHJ solar cells, implying
highly efficient charge extraction. In contrast, negligible
enhancement of the device performances (PCE = 1.1% to
Figure 3. Changes in absorption spectra of (a) DPP-TP6:PC71BM
(1:1, w/w) and (b) DPP-TP12:PC71BM (1:1, w/w) blend thin films
upon thermal annealing.
blend films were deposited by spin-coating from CHCl3
solutions, with a donor concentration of 7−8% (w/v). It
should be noted that the absorption coefficient of the J-
aggregation band at ∼640 nm observed for the as-cast DPP-
TP6:PC71BM film steadily decreases and at the same time a
new absorption band centered around 540 nm emerges, as the
annealing temperature increases to 140 °C (Figure 3a). This
hypsochromic shift of the absorption band is the defining
characteristic of H-type aggregation.14 Therefore, upon thermal
treatment, the J-aggregates composed of the DPP-TP6
molecules eventually transform into the thermodynamically
favored H-aggregates, accompanied by specific stacking changes
within the thin film. Very similar spectral changes have been
observed for pristine DPP-TP6 thin films as a function of
annealing temperature (Supporting Information, Figures S2
and S3). This thermo-responsive reorganization behavior for
DPP-TP6 is in accordance with the appearance of the LC state
from 117 °C upon heating (Figure 1), in which the DPP-TP6
molecules retain at least some mobility. However, for DPP-
TP12:PC71BM films, the initial peak position and absorption
intensity do not change significantly by thermal annealing
(Figure 3b), suggesting that the DPP-TP12 molecules with
longer dodecyl chains predominantly form slipped-stacking J-
aggregates in the thin films over a wide range of temperatures.
The difference in the terminal chain length has a strong impact
on the mesomorphism and self-organizing behavior of the
donor molecules in both the neat and the BHJ thin films.
Solar Cell Performance. Solution-processed BHJ solar
cells have been fabricated using DPP-TP6 and DPP-TP12 as
an electron donor and PC71BM as an acceptor with a standard
device structure (Figure 4): ITO/PEDOT:PSS (40 nm)/
donor:PC71BM blend (90−120 nm)/LiF (1 nm)/Al (100 nm).
C
dx.doi.org/10.1021/cm401244x | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
a
Table 2. Photovoltaic Parameters for Solution-Processed BHJ Solar Cells
donor
D/A ratio (w/w) thickness (nm) annealing temp. (°C) Jsc (mA/cm2) Voc (V)
FF (%)
PCE (%)
Rs (Ω cm2) Rsh (Ω cm2)
DPP-TP6
1:1
112
107
114
94
as-spun
140
7.51 0.26
8.27 0.10
6.65 0.26
7.28 0.08
7.04 0.47
6.24 0.10
3.86 0.11
3.73 0.17
0.92
0.93
0.91
0.93
0.91
0.94
0.91
0.93
38
54
36
51
32
50
31
35
1
2.6 0.1
4.2 0.1
2.1 0.2
3.5 0.1
2.0 0.2
3.0 0.1
1.1 0.1
1.2 0.1
18.9
9.1
566
773
238
1426
213
967
352
451
1:1
1
1.5:1
1.5:1
2:1
as-spun
140
2
1
1
1
2
1
12.8
12.1
36.0
17.2
69.4
85.1
112
101
97
as-spun
140
2:1
DPP-TP12
1:1
as-spun
1:1
108
120
a
Device structure: ITO/PEDOT:PSS/donor:PC71BM blend/LiF/Al, with 4 mm2 illuminated areas. All devices were characterized under the
standard AM 1.5G 1 Sun test conditions. Power conversion efficiencies (PCEs) were derived from the equation: PCE = (Jsc × Voc × FF)/P0, where
Jsc = short-circuit current density, Voc = open-circuit voltage, FF = fill factor, and P0 = incident light intensity. Series resistance (Rs) and shunt
resistance (Rsh) were estimated from the reverse slopes of the J−V curves at zero current and at zero voltage, respectively.
Figure 6. Polarizing optical micrographs (left panels) and AMF height
images (right panels) of DPP-TP6:PC71BM (1:1, w/w) films on ITO-
coated glass substrates: (a, d) as-spun and after thermal annealing at
(b, e) 120 °C and (c, f) 140 °C for 10 min. The scan size for the AFM
images is 5 × 5 μm.
Figure 5. (a) J−V characteristics under one sun illumination (inset:
dark J−V curves) and (b) IPCE spectra for BHJ solar cells based on
DPP-TP6:PC71BM (1:1, w/w) blend as-cast and after thermal
The AFM image of the as-cast DPP-TP6:PC71BM blend film
exhibits a small-grained (100−200 nm in size) macrophase-
separated morphology, with a root-mean-square (RMS)
roughness of 12.6 nm (Figure 6d); whereas the pure DPP-
TP6 neat film has a smooth surface (RMS = 1.0 nm,
Supporting Information, Figure S6). This coarse surface
structure of the blend films is ascribed to the low entropy of
mixing between DPP-TP6 and PC71BM during the spin-
coating process. After thermal annealing at 140 °C, the
protruding edges merge into larger homogeneous domains
(approximately tens of micrometers in size), and the surface
morphology of the composite blend film becomes smoother
with a surface roughness of around 4 nm (Figure 6e, f). The
obvious trend of such morphological changes by LC
organization is also observed for other blend ratios. We infer
that thermal annealing allows the LC DPP-TP6 and PC71BM
molecules to diffuse and reorganize into thermodynamically
favorable nanoscale interpenetrating networks in the active
layer. Since the exciton diffusion length of organic materials is
generally about 10−20 nm,8 the well-developed interpenetrat-
annealing at 140 °C for 10 min.
1.2%) has been observed for DPP-TP12:PC71BM devices even
after annealing (Supporting Information, Figure S5). Despite
the similarities in molecular structures and physical properties,
the non-LC DPP-TP12 molecules are unable to reorganize into
an appropriate chromophore packing structure upon heating,
resulting in a relatively low device performance (Table 2).
Thin Film Morphology and Structures. To gain deeper
insight into the variation in the photovoltaic properties of DPP-
TP6:PC71BM BHJ devices by LC organization, the active layer
morphologies have been analyzed using polarizing optical
microscopy (POM) and atomic force microscopy (AFM), as
shown in Figure 6. The morphology of the active material in
the BHJ solar cells significantly affects the device performance.
With increasing annealing temperature, the POM images
indicate the emergence of highly birefringent LC domains
with sizes over several tens of micrometers (Figure 6a−c),
implying the spontaneous formation of well-organized crystal-
lites within the blend films.
D
dx.doi.org/10.1021/cm401244x | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
ing nanostructures with enlarged interfacial D/A areas should
be beneficial to effective exciton dissociation and charge
transport in the BHJ layer. In contrast, for the non-LC DPP-
TP12:PC71BM blends the grains are not interconnected and
the surface morphology of the films is coarse even after thermal
annealing (Supporting Information, Figure S7). There is
weaker molecular ordering within the blend films owing to
the lack of LC characteristics, which agrees with the lower
photovoltaic properties of DPP-TP12, compared with those of
DPP-TP6.
(MD) simulations have been performed on the DPP-
TP6:PC71BM (1:1, w/w) system with a constant temperature
at 140 °C, as displayed in Figure 8 (see also Supporting
X-ray diffraction (XRD) studies clearly manifest the
enhancement of molecular ordering within the DPP-
TP6:PC71BM blend films by LC organization (Figure 7). The
Figure 8. MD snapshot of DPP-TP6:PC71BM (1:1, w/w) blend at
140 °C, after 50 ns. Atoms belonging to PC71BM, the DPP-TP6
conjugated backbone, and the alkyl chains are colored blue, pink, and
gray, respectively. A movie file in the Supporting Information reveals
the 360° view of the three-dimensional structure of the binary blend.
Information). The simulation cell contains 160 molecules of
DPP-TP6 and 128 molecules of PC71BM, and the cell size is
around (x, y, z) = (6.6 nm, 6.6 nm, 8.5 nm) at the equilibrated
state. It is observed that the DPP-TP6 molecules repel the
PC71BM molecules to form a stable phase-segregated structure
after proper relaxation. The formation of zigzag layers of
PC71BM agglomerates can be discerned, which is similar to the
single-crystal structure of PCBM.17
Figure 7. Out-of-plane XRD patterns of DPP-TP6:PC71BM (1:1, w/
w) films on ITO-coated glass substrates: (a) as-spun and after thermal
annealing at (b) 120 °C and (c) 140 °C for 10 min. The peak with a *
denotes reflection from ITO.
Charge Carrier Mobilities. As the enhancement in
photovoltaic properties can also be related to facile charge
transport and collection, the impact of LC organization on
charge carrier mobilities of the semiconductor materials has
been assessed in organic field-effect transistors (OFETs). Top-
contact bottom-gate OFETs have been fabricated by employing
pure DPP-TP6 and blended DPP-TP6:PC71BM (1:1, w/w)
materials. The carrier mobilities are obtained from the source−
drain current versus gate voltage (ID vs VG) plots in well-
resolved saturation regions (Figure 9). As-cast films of the
pristine DPP-TP6 exhibit a hole mobility of 7.0 × 10−4 cm2 V−1
s−1. After thermal annealing at 140 °C, the hole mobility of
DPP-TP6 increases to 4.2 × 10−3 cm2 V−1 s−1 (Figure 9a).
Thermal annealing results in a higher degree of crystallinity of
the DPP-TP6 molecules (cf. Figures 6 and 7), which is
manifested in increased hole mobilities. We have further
investigated the hole mobility of the annealed DPP-TP6 film
by means of the space-charge-limited current (SCLC)
method.18 The SCLC hole mobility is 2−4 × 10−3 cm2 V−1
s−1 (Supporting Information, Figure S8), which is coincident
with that obtained with the OFET, in spite of the difference in
the charge transport lengths between SCLC (ca. 300 nm) and
OFET devices (50 μm). However, it has been found that the
hole mobility of DPP-TP6 decreases to approximately 10−5
cm2 V−1 s−1 when mixed with PC71BM in a 1:1 (w/w) ratio,
which is about 2 orders of magnitude lower with respect to the
pristine DPP-TP6 after annealing.
as-cast DPP-TP6:PC71BM (1:1, w/w) film only shows a weak
broad scattering at 2θ = 5.8°, corresponding to a d-spacing of
1.52 nm, which originates from very small crystalline domains
of DPP-TP6. Once the thin film is annealed to 120 °C, an
additional reflection peak appears at 6.7° (d = 1.32 nm).
Further annealing at 140 °C leads to a steep increase of the
intensity of the latter peak, as a result of the formation of H-
aggregates in the LC mesophase. The size of the DPP-TP6
crystallites (D) can be estimated from the full width at half-
maximum (fwhm) of the respective XRD peaks using Scherrer’s
equation16 D = (Kλ)/(β cos θ), where K is the shape factor
(normally assumed to be 0.89), λ is the incident X-ray
wavelength, β is the fwhm in radians of the XRD peak, and θ is
the diffraction angle. Apparently, the average size of the
crystallites of DPP-TP6 increases from 1.0 nm for the as-spun
to 32 nm for the annealed DPP-TP6:PC71BM films.
This observation allows us to correlate the morphology of
the active layer with the resulting photovoltaic properties. The
larger size of the crystallites corresponds to a larger D/A
segregation in the composite films. This nanostructured texture
provides better percolation pathways for the generated charge
carriers, which would be reflected in larger Jsc and FF of the
annealed DPP-TP6:PC71BM device. Molecular dynamics
E
dx.doi.org/10.1021/cm401244x | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
(THF, 80 mL) were added Pd(PPh3)4 (0.075 g, 0.064 mmol) and
aqueous K2CO3 (2.0 M, 5 mL; N2 bubbled before use). The mixture
was vigorously stirred for 24 h at 55 °C. After cooling to room
temperature, the reaction mixture was poured into water, and then
extracted with CHCl3. The combined organic layers were washed with
water, and dried over anhydrous MgSO4. After filtration and
evaporation, the crude product was purified by column chromato-
graphically (SiO2, eluent = CHCl3), recrystallized from CHCl3/
methanol, and dried under vacuum to afford a dark violet solid. This
compound was further purified by recycling preparative gel permeation
1
chromatography (GPC) prior to use (yield = 0.71 g, 52%). H NMR
(500 MHz, CDCl3): δ 8.97 (d, J = 4.5 Hz, 2H), 7.59 (d, J = 8.0 Hz,
4H), 7.43 (d, J = 4.0 Hz, 2H), 7.24 (d, J = 8.0 Hz, 4H), 4.13−4.04 (m,
4H), 2.64 (t, J = 7.8 Hz, 4H), 1.97−1.93 (m, 2H), 1.67−1.61 (m, 4H),
1.41−1.26 (m, 28H), 0.93−0.89 (m, 18H). 13C NMR (125 MHz,
CDCl3): δ 161.80, 150.00, 144.14, 139.90, 136.85, 130.70, 129.22,
128.40, 126.08, 124.03, 108.11, 46.02, 39.27, 35.77, 31.73, 31.30,
30.40, 28.97, 28.61, 23.75, 23.13, 22.61, 14.09, 14.07 10.62. MS
(MALDI-TOF): m/z 844.50 [M]+. Anal. Calcd (%) for
C54H72N2O2S2: C, 76.73; H, 8.59; N, 3.31; found: C, 76.65; H,
8.63; N, 3.32.
1/2
Figure 9. Source−drain current (ID) and ID vs gate voltage (VG)
plots of OFETs based on (a) pure DPP-TP6 film with annealing at
140 °C for p-type operation and (b) DPP-TP6:PC71BM (1:1, w/w)
blend film with annealing at 140 °C for n-type operation. Insets: POM
images of each device (scale bar: 100 μm). Channel length (L) and
width (W) were 50 and 2000 μm, respectively.
Synthesis of DPP-TP12. This compound was prepared in a
fashion similar to DPP-TP6, using 3,6-bis(5-bromothiophen-2-yl)-2,5-
di(2-ethylhexyl)-pyrrolo[3,4-c]pyrrole-1,4-diones (0.97 g, 1.42 mmol),
2-(4-dodecylphenyl)-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane (1.11 g,
2.98 mmol), and Pd(PPh3)4 (0.066 g, 0.057 mmol). The product was
purified by column chromatography (SiO2, eluent = CHCl3) and
recycling preparative GPC to give a dark purple solid (yield =1.23 g,
Likewise, the electron mobilities show a pronounced
dependence on thermal annealing. The OFETs based on as-
spun DPP-TP6:PC71BM films without annealing exhibits a low
electron mobility of 1.6 × 10−5 cm2 V−1 s−1, and after annealing
at 140 °C, the electron mobility abruptly increases to 1.1 ×
10−3 cm2 V−1 s−1 (Figure 9b). These results suggest that the
enhancement of photovoltaic properties for the BHJ DPP-
TP6:PC71BM films is ascribable in part to the improved hole
and electron transport. However, the electron and hole
mobilities still differ by approximately 2 orders of magnitude
in the DPP-TP6:PC71BM BHJ architectures. Therefore, higher
photovoltaic performance could be achieved if one can further
improve the hole mobility of the LC donor materials.
1
85%). H NMR (500 MHz, CDCl3): δ 8.97 (d, J = 4.0 Hz, 2H), 7.59
(d, J = 8.5 Hz, 4H), 7.43 (d, J = 4.0 Hz, 2H), 7.23 (d, J = 8.0 Hz, 4H),
4.13−4.03 (m, 4H), 2.64 (t, J = 7.8 Hz, 4H), 1.97−1.92 (m, 2H),
1.67−1.60 (m, 4H), 1.43−1.26 (m, 52H), 0.93−0.85 (m, 18H). 13C
NMR (125 MHz, CDCl3): δ 161.78, 149.98, 144.13, 139.87, 136.85,
130.68, 129.21, 128.39, 126.07, 124.02, 108.09, 46.01, 39.27, 35.77,
31.93, 31.34, 30.41, 29.67, 29.65, 29.60, 29.51, 29.36, 29.30, 28.60,
23.74, 23.13, 22.70, 14.11, 14.07, 10.61. MS (MALDI-TOF): m/z
1012.65 [M]+. Anal. Calcd (%) for C66H96N2O2S2: C, 78.21; H, 9.55;
N, 2.76; found: C, 78.20; H, 9.45; N, 2.71.
OSC Device Fabrication and Measurements. Prepatterned
ITO-coated glass substrates were cleansed sequentially by sonicating in
detergent solution, deionized water, acetone, and isopropanol for 10
min each, and were then subjected to UV/ozone treatment for 15 min.
A thin layer (∼40 nm) of PEDOT:PSS (Clevios P VP Al 4083) was
spin-coated onto the precleaned ITO substrate at 2000 rpm for 60 s,
and then baked at 150 °C for 10 min under air. The photoactive layer
was deposited by spin-cast (1000 rpm for 60 s) from a CHCl3 solution
containing 7−8 mg/mL of a donor and a respective amount of
PC71BM, after passing through a 0.45 μm poly(tetrafluoroethylene)
filter. The thickness of the active layers was 90−120 nm, measured
with a Dektak profilometer. Finally, a 1-nm thick LiF and 100-nm
thick Al layers were thermally evaporated on top of the active layer
under high vacuum, through a shadow mask defining an active device
area of 0.04 cm2. The current density−voltage (J−V) curves of
photovoltaic devices were measured using a Keithley 2400 source
measure unit in air under AM 1.5G solar illumination at 100 mW cm−2
(1 sun) using a Bunko-keiki SRO-25GD solar simulator and IPEC
measurement system, calibrated with a standard Si solar cell.
OFET Device Fabrication and Measurements. OFET devices
were fabricated in a top-contact configuration on heavily doped n-type
Si wafers with 300-nm thick thermally grown SiO2. The SiO2/Si
substrates were pretreated with a piranha solution at 90 °C for 0.5 h,
and then copiously cleaned with sonication in deionized water,
acetone, and isopropanol for 10 min each, followed by UV/ozone
treatment for 15 min. An organic semiconductor layer was spin-coated
onto the substrate from a CHCl3 solution with a donor concentration
of 4% (w/v), at 1000 rpm for 60 s under N2 atmosphere. The devices
were completed by evaporating 50-nm thick Au (or Al) source and
drain electrodes through a shadow mask, where the source−drain
channel length (L) and width (W) were 50 and 2000 μm, respectively.
Current−voltage characteristics of the OFETs were measured using an
CONCLUSIONS
■
We have synthesized and characterized solution-processable
diketopyrrolopyrrole-based photovoltaic molecules, DPP-TP6
and DPP-TP12, that show an intense absorption in the visible
region from 400 to 700 nm, with a small optical bandgap of 1.8
eV and a deep HOMO level of −5.5 eV. The use of these
molecules as electron donors has been demonstrated in
solution-processed BHJ solar cells with PC71BM as the electron
acceptor. We have investigated the effect of LC properties on
photovoltaic performance of the BHJ devices based on DPP-
TP6:PC71BM and DPP-TP12:PC71BM films, in which the
molecular packing and thin film morphologies are strongly
affected by the length of the terminal alkyl chains. The Jsc and
FF have increased significantly for the DPP-TP6-based devices
through the LC organization process, and the best-performing
device has generated a high PCE of 4.3% with a Voc of 0.93 V,
Jsc of 8.4 mA cm−2, and FF of 0.55 without use of any additives.
The induction of LC characteristics to direct molecular self-
assembly represents an effective strategy for designing donor
molecules. It is anticipated that further enhancement of
photovoltaic performances could be realized by a combination
of fine structural tuning of DPP-based materials and precise
control of their LC organization processing. Further studies
along this line are underway.
EXPERIMENTAL SECTION
■
Synthesis of DPP-TP6. To a stirred solution of 3,6-bis(5-
bromothiophen-2-yl)-2,5-di(2-ethylhexyl)-pyrrolo[3,4-c]pyrrole-1,4-
diones (1.10 g, 1.61 mmol) and 2-(4-hexylphenyl)-4,4,5,5-tetramethyl-
[1,3,2]dioxaborolane (0.98 g, 3.38 mmol) in dry tetrahydrofuran
F
dx.doi.org/10.1021/cm401244x | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Agilent B1500A semiconductor parameter analyzer under an inert
atmosphere at room temperature. Field-effect mobilities (μ) were
calculated in the saturation regime of the ID using the following
equation: ID = (W/2L)μ Ci(VG − Vth)2, where ID is the source−drain
current, Ci is the capacitance per unit area of the gate dielectric (11.5
nF/cm2), VG is the gate voltage, and Vth is the threshold voltage.
Darling, S. B. J. Am. Chem. Soc. 2012, 134, 13616. (c) Chiu, S.-W.; Lin,
L.-Y.; Lin, H.-W.; Chen, Y.-H.; Huang, Z.-Y.; Lin, Y.-T.; Lin, F.; Liu,
Y.-H.; Wong, K. T. Chem. Commun. 2012, 48, 1857. (d) Fitzner, R.;
Reinold, E.; Mishra, A.; Mena-Osteritz, E.; Ziehlke, H.; Korner, C.;
̈
Leo, K.; Riede, M.; Weil, M.; Tsaryova, O.; Weiβ, A.; Uhrich, C.;
Pfeiffer, M.; Bauerle, P. Adv. Funct. Mater. 2011, 21, 897. (e) Fitzner,
̈
R.; Mena-Osteritz, E.; Mishra, A.; Schulz, G.; Reinold, E.; Weil, M.;
Korner, C.; Ziehlke, H.; Elschner, C.; Leo, K.; Riede, M.; Pfeiffer, M.;
ASSOCIATED CONTENT
* Supporting Information
Experimental details, additional data and spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
̈
■
Uhrich, C.; Bauerle, P. J. Am. Chem. Soc. 2012, 134, 11064.
̈
S
(f) Steinmann, V.; Kronenberg, N. M.; Lenze, M. R.; Graf, S. M.;
Hertel, D.; Meerholz, K.; Burckstummer, H.; Tulyakova, E. V.;
̈
̈
Wurthner, F. Adv. Energy Mater. 2011, 1, 888.
̈
(6) For example: (a) Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C.
J.; Bazan, G. C.; Heeger, A. J. Nat. Mater. 2012, 11, 44. (b) van der
Poll, T. S.; Love, J. A.; Nguyen, T.-C.; Bazan, G. C. Adv. Mater. 2012,
24, 3646. (c) Liu, X.; Sun, Y.; Perez, L. A.; Wen, W.; Toney, M. F.;
Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2012, 134, 20609.
(d) Zhou, J.; Wan, X.; Liu, Y.; Zuo, Y.; Li, Z.; He, G.; Long, G.; Ni, W.;
Li, C.; Su, X.; Chen, Y. J. Am. Chem. Soc. 2012, 134, 16345. (e) Zhou,
J.; Wan, X.; Liu, Y.; Long, G.; Wang, F.; Li, Z.; Zuo, Y.; Li, C.; Chen, Y.
Chem. Mater. 2011, 23, 4666. (f) Liu, Y.; Wan, X.; Wang, F.; Zhou, J.;
Long, G.; Tian, J.; Chen, Y. Adv. Mater. 2011, 23, 5387. (g) He, G.; Li,
Z.; Wan, X.; Liu, Y.; Zhou, J.; Long, G.; Zhang, M.; Chen, Y. J. Mater.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yasuda@cstf.kyushu-u.ac.jp (T.Y.), adachi@cstf.
kyushu-u.ac.jp (C.A.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Precursory
Research for Embryonic Science and Technology (PRESTO)
(No. 10111) from JST, Grant-in-Aid for Young Scientists (B)
(No. 23750218) and Young Scientists (A) (No. 25708032),
and Grant-in-Aid for the Funding Program for World-Leading
Innovation R&D on Science and Technology (FIRST) from
JSPS. We would like to thank Mr. Jun Yun Kim and Mr. Sae
Youn Lee for technical support. T.Y. is grateful for the financial
support from the Murata Science Foundation, the Fujifilm
Award in Synthetic Organic Chemistry, and Kyushu University
Interdisciplinary Programs in Education and Projects in
Research Development. G.W. thanks the Research Center for
Computational Science, Okazaki, Japan, for generous permis-
sion of MD simulations. C.A. and T.Y. gratefully acknowledge
the support of the International Institute for Carbon Neutral
Energy Research (WPI-I2CNER), sponsored by MEXT, Japan.
́ ̂
Chem. 2012, 22, 9173. (h) Bura, T.; Leclerc, N.; Fall, S.; Leveque, P.;
Heiser, T.; Retailleau, P.; Rihn, S.; Mirloup, A.; Ziessel, R. J. Am. Chem.
Soc. 2012, 134, 17404. (i) Huang, Y.; Li, L.; Peng, X.; Peng, J.; Cao, Y.
J. Mater. Chem. 2012, 22, 21841. (j) Shang, H.; Fan, H.; Liu, Y.; Hu,
W.; Li, Y.; Zhan, X. Adv. Mater. 2011, 23, 1553. (k) Wei, G.; Wang, S.;
Sun, K.; Thompson, M. E.; Forrest, S. R. Adv. Energy Mater. 2011, 1,
184.
(7) For DPP-based small-molecule OSCs, see: (a) Walker, B.;
Tamayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, J. H.; Garcia, A.;
Tantiwiwat, M.; Nguyen, T.-Q. Adv. Funct. Mater. 2009, 19, 3063.
(b) Liu, J.; Walker, B.; Tamayo, A.; Zhang, Y.; Nguyen, T.-Q. Adv.
Funct. Mater. 2013, 23, 47. (c) Loser, S.; Bruns, C. J.; Miyauchi, H.;
Ortiz, R. P.; Facchetti, A.; Stupp, S. I.; Marks, T. J. J. Am. Chem. Soc.
2011, 133, 8142. (d) Loser, S.; Miyauchi, H.; Hennek, J. W.; Smith, J.;
Huang, C.; Facchetti, A.; Marks, T. J. Chem. Commun. 2012, 48, 8511.
(e) Lee, O. P.; Yiu, A. T.; Beaujuge, P. M.; Woo, C. H.; Holcombe, T.
́
W.; Millstone, J. E.; Douglas, J. D.; Chen, M. S.; Frechet, J. M. J. Adv.
Mater. 2011, 23, 5359. (f) Tamayo, A. B.; Dang, X.-D.; Walker, B.;
Seo, J.; Kent, T.; Nguyen, T.-Q. Appl. Phys. Lett. 2009, 94, 103301.
(g) Tamayo, A. B.; Walker, B.; Nguyen, T.-Q. J. Phys. Chem. C 2008,
112, 11545. (h) Lin, Y.; Cheng, P.; Li, Y.; Zhan, X. Chem. Commun.
2012, 48, 4773.
(8) (a) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736.
(b) Brabec, C. J.; Heeney, M.; McCulloch, I.; Nelson, J. Chem. Soc.
Rev. 2011, 40, 1185. (c) Peumans, P.; Yakimov, A.; Forrest, S. R. J.
REFERENCES
■
(1) For reviews, see: (a) Dennler, G.; Scharber, M. C.; Brabec, C. J.
Adv. Mater. 2009, 21, 1323. (b) Beaujuge, P. M.; Frec
Chem. Soc. 2011, 133, 20009. (c) Li, G.; Zhu, R.; Yang, Y. Nat.
Photonics 2012, 6, 153. (d) Gunes, S.; Neugebauer, H.; Sariciftci, N. S.
Chem. Rev. 2007, 107, 1324. (e) Facchetti, A. Chem. Mater. 2011, 23,
733. (f) Walker, B.; Kim, C.; Nguyen, T.-Q. Chem. Mater. 2011, 23,
470. (g) Mishra, A.; Bauerle. Angew. Chem., Int. Ed. 2012, 51, 2020.
(h) Roncali, J. Acc. Chem. Res. 2009, 42, 1719. (i) Lloyd, M. T.;
Anthony, J. E.; Malliaras, G. G. Mater. Today 2007, 10, 34.
(2) Organic Photovoltaics: Materials, Device Physics, and Manufactur-
ing Technologies; Brabec, C.; Dyakonov, V.; Scherf, U., Eds.; Wiley-
VCH: Weinheim, Germany, 2008.
́
het, J. M. J. J. Am.
̈
Appl. Phys. 2003, 93, 3693. (d) Kippelen, B.; Bred
́
as, J.-L. Energy
̈
Environ. Sci. 2009, 2, 251.
(9) (a) Pisula, W.; Zorn, M.; Chang, J. Y.; Mullen, K.; Zentel, R.
̈
Macromol. Rapid Commun. 2009, 30, 1179. (b) Sergeyev, S.; Pisula,
W.; Geerts, Y. H. Chem. Soc. Rev. 2007, 36, 1902. (c) Kato, T.; Yasuda,
T.; Kamikawa, Y.; Yoshio, M. Chem. Commun. 2009, 729. (d) Yasuda,
T.; Ooi, H.; Morita, J.; Akama, Y.; Minoura, K.; Funahashi, M.;
Shimomura, T.; Kato, T. Adv. Funct. Mater. 2009, 19, 411. (e) Yasuda,
T.; Shimizu, T.; Liu, F.; Ungar, G.; Kato, T. J. Am. Chem. Soc. 2011,
133, 13437.
(3) For example: (a) O’Regan, Gratzel, M. Nature 1991, 353, 737.
̈
(b) Gratzel, M. Acc. Chem. Res. 2009, 42, 1788. (c) Yella, A.; Lee, H.-
̈
W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau,
E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Gratzel, M. Science 2011,
334, 629.
̈
(10) (a) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons,
̈
̈
E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (b) Wong,
(4) For example: (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.;
Heeger, A. J. Science 1995, 270, 1789. (b) Dou, L.; You, J.; Yang, J.;
Chen, C.-C.; He, Y.; Murase, S.; Moriarty, T.; Emery, K.; Li, G.; Yang,
Y. Nat. Photonics 2012, 6, 180. (c) Small, C. E.; Chen, S.; Subbiah, J.;
Amb, C. M.; Tsang, S. W.; Lai, T. H.; Reynolds, J. R.; So, F. Nat.
Photonics 2012, 6, 115. (d) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.;
Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649.
(5) For example: (a) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183.
(b) Chen, Y.-H.; Lin, L.-Y.; Lu, C.-W.; Lin, F.; Huang, Z.-Y.; Lin, H.-
W.; Wang, P.-H.; Liu, Y.-H.; Wong, K.-T.; Wen, J.; Miller, D. J.;
W. W. H.; Subbiah, J.; Puniredd, S. R.; Purushothaman, B.; Pisula, W.;
Kirby, N.; Mullen, K.; Jones, D. J.; Holmes, A. B. J. Mater. Chem. 2012,
̈
22, 21131. (c) Li, J.; Kastler, M.; Pisula, W.; Robertson, J. W. F.;
Wasserfallen, D.; Grimsdale, A. C.; Wu, J.; Mullen, K. Adv. Funct.
̈
Mater. 2007, 17, 2528. (d) Lincker, F.; Heinrich, B.; De Bettignies, R.;
́ ́
Rannou, P.; Pecaut, J.; Grevin, B.; Pron, A.; Donnio, B.; Demadrille, R.
J. Mater. Chem. 2011, 21, 5238. (e) Bu, L.; Guo, X.; Yu, B.; Qu, Y.; Xie,
Z.; Yan, D.; Geng, Y.; Wang, F. J. Am. Chem. Soc. 2009, 131, 13242.
(f) Sun, Q.; Dai, L.; Zhou, X.; Li, L.; Li, Q. Appl. Phys. Lett. 2007, 91,
G
dx.doi.org/10.1021/cm401244x | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
253505. (g) Dao, Q. D.; Hori, T.; Fukumura, K.; Masuda, T.;
Kamikado, T.; Fujii, A.; Shimizu, Y.; Ozaki, M. Appl. Phys. Lett. 2012,
101, 263301.
(11) For representative DPP-based conjugated polymers, see:
(a) Nielsen, C. B.; Turbiez, M.; McCulloch, I. Adv. Mater. 2013, 25,
1859. (b) Qu, S.; Tian, H. Chem. Commun. 2012, 48, 3039. (c) Wienk,
M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J. Adv. Mater. 2008, 20,
2556. (d) Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk,
M. M.; Turbiez, M.; de Leeuw, D. M.; Janssen, R. A. J. J. Am. Chem.
Soc. 2009, 131, 16616. (e) Woo, C. H.; Beaujuge, P. M.; Holcombe, T.
́
W.; Lee, O. P.; Frechet, J. M. J. J. Am. Chem. Soc. 2010, 132, 15547.
(f) Bronstein, H.; Chen, Z.; Ashraf, R. S.; Zhang, W.; Du, J.; Durrant, J.
R.; Tuladhar, P. S.; Song, K.; Watkins, S. E.; Geerts, Y.; Wienk, M. M.;
Janssen, R.A. J.; Anthopoulos, T.; Sirringhaus, H.; Heeney, M.;
McCulloch, I. J. Am. Chem. Soc. 2011, 133, 3272. (g) Li, W. L.;
Roelofs, W. S. C.; Wienk, M. M.; Janssen, R. A. J. J. Am. Chem. Soc.
2012, 134, 13787.
(12) (a) Tamayo, A. B.; Tantiwiwat, M.; Walker, B.; Nguyen, T.-Q. J.
Phys. Chem. C 2008, 112, 15543. (b) Stas, S.; Sergeyev, S.; Geerts, Y.
Tetrahedron 2010, 66, 1837.
(13) Upon cooling from the isotropic phase, DPP-TP6 tends to form
kinetically favored J-aggregates predominantly. The H-type aggrega-
tion state should thus be a monotropic mesophase.
(14) Kirkus, M.; Wang, L.; Mothy, S.; Beljonne, D.; Cornil, J.;
Janssen, R. A. J.; Meskers, S. C. J. J. Phys. Chem. A 2012, 116, 7927.
(15) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.;
̈
Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789.
(16) Cullity, B. D.; Stock, S. R. Elements of X-Ray Diffraction, 3rd ed.;
Prentice Hall: Upper Saddle River, NJ, 2001.
(17) Rispens, M. T.; Meetsma, A.; Rittberger, R.; Brabec, C. J.;
Sariciftci, N. S.; Hummelen, J. C. Chem. Commun. 2003, 2116.
(18) (a) Blom, P. W. M.; de Jong, M. J. M.; Vleggaar, J. J. M. Appl.
Phys. Lett. 1996, 68, 3308. (b) Bozano, L.; Carter, S. A.; Scott, J. C.;
Malliaras, G. G.; Brock, P. J. Appl. Phys. Lett. 1999, 74, 1132. (c) Kim,
J. Y.; Yasuda, T.; Yang, Y. S.; Adachi, C. Adv. Mater. 2013, 25, 2666.
(d) An, Z.; Yu, J.; Jones, S. C.; Barlow, S.; Yoo, S.; Domercq, B.; Prins,
P.; Siebbeles, L. D. A.; Kippelen, B.; Marder, S. R. Adv. Mater. 2005,
17, 2580.
H
dx.doi.org/10.1021/cm401244x | Chem. Mater. XXXX, XXX, XXX−XXX