Vacuum-deposited small-molecule organic solar cells with high power conversion efficiencies by judicious molecular design and device optimization

Article abstract of DOI:10.1021/ja301872s

Three new tailor-made molecules (DPDCTB, DPDCPB, and DTDCPB) were strategically designed and convergently synthesized as donor materials for small-molecule organic solar cells. These compounds possess a donor-acceptor-acceptor molecular architecture, in w

Full text of DOI:10.1021/ja301872s

Article
pubs.acs.org/JACS
Vacuum-Deposited Small-Molecule Organic Solar Cells with High
Power Conversion Efficiencies by Judicious Molecular Design and
Device Optimization
Yi-Hong Chen,†,# Li-Yen Lin,‡,# Chih-Wei Lu,† Francis Lin,‡ Zheng-Yu Huang,† Hao-Wu Lin,*,†
Po-Han Wang,‡ Yi-Hung Liu,‡ Ken-Tsung Wong,*,‡ Jianguo Wen,§ Dean J. Miller,§
and Seth B. Darling∥,⊥
†Department of Materials Science and Engineering, National Tsing Hua University, Hsin Chu 30013, Taiwan
‡Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
∥
§Electron Microscopy Center and Materials Science Division and Center for Nanoscale Materials, Argonne National Laboratory,
Lemont, Illinois 60439, United States
⊥Institute for Molecular Engineering, The University of Chicago, Chicago, Illinois 60637, United States
S
* Supporting Information
ABSTRACT: Three new tailor-made molecules (DPDCTB,
DPDCPB, and DTDCPB) were strategically designed and
convergently synthesized as donor materials for small-molecule
organic solar cells. These compounds possess a donor−
acceptor−acceptor molecular architecture, in which various
electron-donating moieties are connected to an electron-
withdrawing dicyanovinylene moiety through another elec-
tron-accepting 2,1,3-benzothiadiazole block. The molecular structures and crystal packings of DTDCPB and the previously
reported DTDCTB were characterized by single-crystal X-ray crystallography. Photophysical and electrochemical properties as
well as energy levels of this series of donor molecules were thoroughly investigated, affording clear structure−property
relationships. By delicate manipulation of the trade-off between the photovoltage and the photocurrent via molecular structure
engineering together with device optimizations, which included fine-tuning the layer thicknesses and the donor:acceptor blended
ratio in the bulk heterojunction layer, vacuum-deposited hybrid planar-mixed heterojunction devices utilizing DTDCPB as the
donor and C70 as the acceptor showed the best performance with a power conversion efficiency (PCE) of 6.6 0.2% (the
highest PCE of 6.8%), along with an open-circuit voltage (Voc) of 0.93 0.02 V, a short-circuit current density (Jsc) of 13.48
0.27 mA/cm2, and a fill factor (FF) of 0.53 0.02, under 1 sun (100 mW/cm2) AM 1.5G simulated solar illumination.
(PC71BM) as the acceptor have reached PCEs of up to
7.4%.7e Furthermore, a remarkable PCE of 8.37% has been
reported for a PTB7-based device by incorporating a thin layer
of alcohol/water-soluble polymer as a cathode interlayer.5d
In contrast, small-molecule organic solar cells (SMOSCs)
featuring small molecules as donor materials have received less
attention due to their inferior performance as compared to that
of solution-processed polymer BHJ solar cells. However,
SMOSCs are still of great interest to the research community
because of the attractive advantages of small-molecule semi-
conductors, including well-defined molecular structures, easier
purification, amenability to large-scale production, and better
batch-to-batch reproducibility. As such, ever-increasing research
endeavors have been dedicated to developing SMOSCs over
the past few years, and efficiencies have steadily improved.8−10
A PCE of 7% has been realized for solution-processed SMOSCs
through the rational molecular design of a donor molecule and
INTRODUCTION
■
Organic solar cells (OSCs) are under intensive interdisciplinary
investigation in both academia and industry worldwide because
of their potential to enable mass production of cost-effective
and flexible solar energy conversion devices.1 Over the past
decade, most research efforts have focused on solution-
processed polymer bulk heterojunction (BHJ) solar cells
based on a blend of p-type polymers and n-type soluble
fullerene derivatives as the active layer.2 Significant progress has
been made in this field, primarily stemming from combined
improvements in material design,3 morphology control,4
interface modification,5 and device engineering.6 To date,
power conversion efficiencies (PCEs) in the range of 6−8%
have been steadily achieved for solution-processed single
polymer BHJ solar cells.5e,6e,f,7 In particular, a series of
semiconducting polymeric donors (i.e., PTBs) composed of
alternating benzodithiophene and ester-substituted thieno[3,4-
b]thiophene units, developed by Yu et al., represents the most
successful example, and the BHJ devices based on PTB7
blended with [6,6]-phenyl-C71-butyric acid methyl ester
Received: February 25, 2012
Published: July 26, 2012
© 2012 American Chemical Society
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a particular active-layer processing method.9n In addition, small-
molecule semiconductors can also be fabricated by vacuum
deposition due to their low molecular weight. One notable
merit of vacuum-deposited SMOSCs is that multilayer tandem
cells can be easily prepared;11 these may constitute one of the
promising approaches to breaking through the 10% theoretical
limit for single OSCs.12 In this context, an exceptionally high
PCE of 10.7% has been released for a tandem device by
Heliatek GmbH.13 Although the performance for single
SMOSCs still lags behind those of single solution-processed
polymer solar cells, the current rapid advances have clearly
demonstrated that they can compete with their polymeric
counterparts. With regard to the structural design for molecular
donors, the symmetrical quadrupolar systems9 represent the
most ubiquitous molecular scaffolds and have successfully
demonstrated excellent photovoltaic performance with PCEs of
up to 6.9%.9m In contrast, the asymmetric dipolar push−pull
systems are relatively less explored,10 among which the
merocyanine dyes10c−h have been investigated in depth by
Scheme 2. Synthetic Route for D-A-A Donors DPDCTB,
DTDCTB, DPDCPB, and DTDCPB
synthesized, in which the ditolylaminothienyl donor moiety
of DTDCTB was respectively replaced by three relatively
weaker electron-donating groups. The properties of this series
of donor molecules (including DTDCTB) were investigated,
and their photovoltaic performance was comprehensively
studied and optimized. Remarkably, vacuum-deposited PMHJ
devices utilizing DTDCPB as the donor and C70 as the
acceptor gave a PCE of 6.6 0.2%, with the highest PCE of up
to 6.8%.
the Wurthner and Meerholz research groups. The correspond-
̈
ing vacuum-processed devices have shown remarkable PCEs of
up to 6.1%.10g From our previous study of dye-sensitized solar
cells, we learned that organic dyes with a donor−acceptor−
acceptor (D-A-A) molecular configuration possess both a
smaller bandgap and lower highest occupied molecular orbital
(HOMO) level than their analogs.14 Consequently, we argued
that such D-A-A-type molecules could show the potential to
concurrently enhance the short-circuit current density (Jsc) and
open-circuit voltage (Voc), as utilized as donors for SMOSCs.
Along these lines, we recently reported two D-A-A-type donor
molecules, DTDCTB and DTDCTP, in which an electron-
donating ditolylaminothienyl moiety was connected to an
electron-withdrawing dicyanovinylene (DCV) moiety through
another electron-accepting 2,1,3-benzothiadiazole (BT) and
pyrimidine block, respectively (Scheme 1).15 In practical
RESULTS AND DISCUSSION
■
The synthesis of the four donor molecules is depicted in
Scheme 2. With the key asymmetric building block, 4-bromo-7-
dicyanovinyl-2,1,3-benzothiadiazole, which was previously
described,15a all target compounds were convergently prepared
in gram quantities and moderate yields through Stille coupling
reactions with triarylamine-based tributylstannyl derivatives 1.
Thermal stabilities and morphological properties of these
compounds were investigated by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC), respec-
tively. The decomposition temperatures (Td) (referring to 5%
weight loss) were in the range of 279−316 °C (Table 1),
indicating good thermal stability, which is an important
prerequisite for device fabrication via vacuum deposition. In
DSC, a melting transition manifested from one sharp
endothermic peak was observed for each compound (Table 1).
The molecular structures and crystal packings of DTDCTB
and DTDCPB were characterized by single-crystal X-ray
crystallography (Figure 1). Suitable crystals for X-ray analyses
were obtained by slow diffusion of orthogonal solvents
(dichloromethane/hexane or methanol). In contrast to
DTDCTB, which displayed an almost coplanar conformation
between the thiophene and BT rings with a dihedral angle of
5.5°, DTDCPB was highly distorted from planarity with a
dihedral angle between the phenylene and BT rings of 24.7°,
due to the presence of ortho−ortho steric interactions. The
highly polar nature of the heteroaryl components leads both
molecules to pack in an antiparallel manner and self-assemble
into centrosymmetric dimers. Such a supramolecular arrange-
ment is similar to those observed in some highly dipolar dyes
Scheme 1. Molecular Structures of D-A-A Donors DTDCTB
and DTDCTP
photovoltaic characterization, vacuum-deposited hybrid pla-
nar-mixed heterojunction (PMHJ) devices based on DTDCTB
and DTDCTP as the donor and C70 as the acceptor exhibited
PCEs as high as 5.81%15a and 6.4%,15b respectively. In addition,
our study also revealed that the bandgaps and HOMO levels of
D-A-A-type molecules can be easily fine-tuned through
structural modification of their donor moieties, where their
HOMOs are mainly populated, and thereby, alteration of donor
moieties has a significant impact on the HOMO level but a
limited effect on the LUMO level.14 Accordingly, we envisioned
that the PCEs of SMOSCs adopting our D-A-A molecules as
donors could be further enhanced through delicately
manipulating a trade-off between an extended spectral coverage
(giving Jsc) and an augmented energy offset between the
HOMO level of the donor and the LUMO level of the acceptor
(giving Voc). In this work, three new donor molecules
(DPDCTB, DPDCPB, and DTDCPB; Scheme 2) were
reported by Wurthner and Meerholz et al. and has been
̈
proposed to effectively eliminate molecular dipole moments in
the solid state and, thus, avoid the large energetic disorder that
is thought to be detrimental to efficient charge carrier
transport.10c−h The average distance between two neighboring
BT rings was smaller for DTDCTB (ca. 3.4 Å) than for
DTDCPB (ca. 3.5 Å), accordingly, resulting in a denser
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Table 1. Photophysical, Electrochemical, and Thermal Parameters for DPDCTB, DTDCTB, DPDCPB, and DTDCPB
a
ΔEopt film
E
E
ΔECV
HOMO
g
LUMO
h
Td
(°C)
T
1
1
λabs soln (nm) (ε,
M−1 cm−1
λabs film
b
c
oxd
rede
f
i
m j
compd
)
(nm)
(eV)
(V)
(V)
(eV)
(eV)
(eV)
(°C)
DPDCTB
DTDCTB
DPDCPB
DTDCPB
639 (35856)
663 (41660)
549 (24365)
570 (27445)
670
684
577
595
1.91
1.86
2.14
2.08
0.41
0.35
0.60
0.47
−1.07
−1.09
−1.13
−1.19
1.48
1.44
1.73
1.66
−5.35
−5.30
−5.50
−5.43
−3.44
−3.44
−3.36
−3.35
314
316
279
308
187
234
208
288
a
b
c
Measured in dichloromethane solutions (10−5 M). Thin films prepared by vacuum deposition onto fused-silica substrates. Estimated from the
extinction coefficient maxima of thin films (Figure 4a). Measured in dichloromethane solutions with 0.1 M tetrabutylammonium
d
e
hexafluorophosphate (TBAPF6) as a supporting electrolyte. Measured in tetrahydrofuran solutions with 0.1 M tetrabutylammonium perchlorate
f
g
h
(TBAP) as a supporting electrolyte. Calculated from the difference between Eox1 and Ered1. Determined by UPS. LUMO = HOMO + ΔEopt film.
i
j
Temperature corresponding to 5% weight loss obtained from TGA analysis. Under N2 at a heating rate of 10 °C/min. Temperature obtained from
DSC analysis. Under N2 at a heating rate of 10 °C/min.
Figure 2. Cyclic voltammograms of DPDCPB, DTDCPB, DPDCTB,
and DTDCTB recorded in solution.
Figure 1. X-ray-determined molecular structures and crystal packings
of DTDCTB and DTDCPB.
the reduction of the DCV block, whereas the second wave can
be attributed to the reduction of the BT fragment. In contrast
to the oxidation behaviors, the reduction potentials of these
four molecules were relatively insensitive to the structural
variations of the donor moieties, even though the reduction
potentials of DPDCTB and DTDCTB were slightly less
negative than those of DPDCPB and DTDCPB. Furthermore,
the solid-state thin-film HOMO levels of the four molecules
were determined using ultraviolet photoelectron spectroscopy
(UPS) (Figure S1). The HOMO and corresponding calculated
LUMO levels are summarized in Table 1. The trend observed
in the HOMO levels was in good agreement with that of the
oxidation potentials. Specifically, the donors bearing p-tolyl
substituents and/or thiophene showed less positive oxidation
potentials and possessed higher-lying HOMO levels than their
counterparts. Apparently, the HOMO levels of our D-A-A-type
molecules can be strategically fine-tuned through structural
modification of the donor moieties, and solar cell devices based
on these three new donors are anticipated to offer higher Voc
values than that of the DTDCTB-based device (vide infra).
The UV−vis absorption spectra of the four molecules in
dichloromethane are depicted in Figure 3a. All showed an
intense and broad absorption band in the long-wavelength
region, which was assigned to the ICT transition. Compared to
DPDCPB and DTDCPB, significant bathochromic shifts in
absorption bands together with increases in molar extinction
coefficients (ε) were found for DPDCTB and DTDCTB
because of the electron-rich and fortified quinoidal characters of
thiophene as well as the coplanar conformation between the
thiophene and BT rings, which are all beneficial in lowering the
energy and augmenting the oscillator strength of the ICT
transition. In addition, the absorption peaks of DTDCPB and
DTDCTB were red-shifted by ca. 20 nm as compared to those
packing [d = 1.364 g/cm3 for DTDCTB vs 1.308 g/cm3 for
DTDCPB, Table S1 in the Supporting Information (SI)]. The
degree of bond length alteration (BLA), calculated as the
difference between the bond length of C4−C5 and the average
bond length of C3−C4 and C5−C6 of the invariable BT
acceptor, amounted to 0.001 Å for DTDCTB and 0.030 Å for
DTDCPB (Table S2). The bond length between the
triarylamine donors and the BT acceptor (i.e., bond length of
C6−C7) for DTDCTB was shorter than that for DTDCPB
(1.434 Å vs 1.472 Å). The smaller BLA of the BT acceptor and
the shorter bond length of C6−C7 for DTDCTB indicate that
the incorporation of thiophene at the donor moiety facilitates
π-electron delocalization and imparts an enhanced quinoidal
character to the conjugated backbone. In other words, the
adoption of the ditolylaminothienyl donor moiety promotes
efficient intramolecular charge transfer (ICT) from the donor
side to the acceptor side of DTDCTB, thus leading to the
formation of mesomeric structures (D−A ↔ D+A−).3a,16
Figure 2 shows the cyclic voltammograms of the four donor
molecules recorded in solution. Pertinent electrochemical data
are summarized in Table 1. All four molecules exhibited one
quasi-reversible oxidation wave corresponding to the oxidation
of the diarylaminothienyl or diarylaminophenyl donor moieties.
The less-positive oxidation potentials of DTDCPB and
DTDCTB relative to DPDCPB and DPDCTB can be
rationally ascribed to the presence of the stronger electron-
donating p-tolyl substituents, while DPDCTB and DTDCTB
showed less positive oxidation potentials than DPDCPB and
DTDCPB due to the electron-rich nature of thiophene. On the
other hand, two reversible reduction waves were observed in
the cathodic potential regime. The first wave can be assigned to
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Figure 3. Absorption spectra of DPDCTB (squares), DTDCTB
(circles), DPDCPB (triangles), and DTDCPB (diamonds) (a) in
dichloromethane solutions and (b) as vacuum-deposited thin films.
Figure 4. (a) Optical constants of DPDCTB (squares), DTDCTB
(circles), DPDCPB (triangles), and DTDCPB (diamonds) thin-film
spectra. (b) Ordinary (in-plane) and extraordinary (out-of-plane)
optical constants of DPDCPB and DTDCPB thin-film spectra.
of DPDCPB and DPDCTB by virtue of the stronger electron-
donating nature of the p-tolyl substituents. On the other hand,
the thin-film absorption bands of these molecules were
broadened and red-shifted in comparison to those observed
in solution (Figure 3b), likely due to the intermolecular π−π
stacking of the molecules in the solid state as manifested in the
crystal packings.
molecules with linear π-systems.17 This uniaxial anisotropy
property generally contributes to stronger absorption for
horizontally polarized incident light and therefore may favor
high Jsc (vide infra).19
Optical constants (refractive index, n, and extinction
coefficient, k) of vacuum-deposited thin films of the four
molecules, determined by the combination of reflection and
transmission ellipsometry, are shown in Figure 4a.17 The trend
in the maximum extinction coefficients is well coincident with
that in the absorption maxima. Both DPDCTB and DTDCTB
thin films can be modeled as isotropic layers and show high kmax
values of ∼1 that are among the highest reported for organic
solar-active materials.18 However, the optical isotropic model
cannot be adopted to describe the behavior of the two
phenylene-containing molecules (DPDCPB and DTDCPB)
thin-films. Therefore, the DPDCPB and DTDCPB films were
treated as uniaxially anisotropic with the optical axis along the
surface normal. That means the optical constants are
distinguished by those for the ordinary (in-plane) polarization,
no + iko and those for the extraordinary (out-of-plane)
polarization, ne + ike.17 The uniaxial model can be successfully
fitted to the experimental data, and the extracted optical
constants are shown in Figure 4b. The larger ko relative to ke for
DPDCPB and DTDCPB indicates preferred orientations
parallel to the surface plane upon vacuum deposition onto
substrates, as the transition dipole moment of the lowest energy
excited state is usually along the molecular backbone for
In common with the photovoltaic characterization of
DTDCTB,15a the vacuum-deposited PMHJ structure was
adopted in this study,8a,c where the active layers were composed
of a BHJ layer sandwiched between a homogeneous donor and
acceptor layer. This kind of device architecture can provide not
only efficient photon harvesting but also good photogenerated
charge carrier transport to the respective electrodes. Fullerene
C
70 was chosen as the electron-accepting counterpart owing to
its broader and larger extinction coefficients relative to C60.
More importantly, the C70-based PMHJ devices always
demonstrated higher PCEs than C60-based PMHJ devices in
our previous investigations.15,19,20 Generally, the devices were
fabricated with a configuration of ITO/MoO3 (5 nm)/donor (7
nm)/donor:C70/C70 (7 nm)/2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline (BCP) (10 nm)/Ag (150 nm). In these
devices, the MoO3 thin film acts as the hole-transporting
layer, and the BCP thin film serves as the electron-transporting
layer with an exciton blocking character. The optimal
thicknesses of the active layers (the donor layer, donor:C70
BHJ layer, and C70 layer) are dependent on the optical field
distribution as well as exciton diffusion lengths and carrier
recombination rates in the thin films.15 Practical devices with
various thicknesses for active layers were fabricated and
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measured. The optimized thicknesses of the BHJ layer in
DPDCTB-, DTDCTB-, DPDCPB-, and DTDCPB-based
devices were found to be 50, 40, 40, and 40 nm, respectively.
In addition, the donor:C70 blended volume ratio in the BHJ
layer was also systematically fine-tuned (Figure S2−5 and Table
S3−6). The devices fabricated with a donor:C70 ratio of 1:1.6 in
the BHJ layer gave the best results, except for the
DPDCPB:C70 device that showed the best performance at a
ratio of 1:1.
Device performance measurements were carried out under
ambient atmosphere using 100 mW/cm2 AM 1.5G simulated
solar illumination. Spectral mismatch-corrected current den-
sity−voltage (J−V) characteristics of the donor:C70 (1:1.6) and
DPDCPB:C70 (1:1) PMHJ devices are shown in Figure 5a, and
(1:1.6) device, which was among the highest values ever
reported for OSCs. Interestingly, the Jsc value of the
DPDCTB:C70 device was slightly lower than that of the
DTDCPB:C70 device, even though DPDCTB exhibited larger
extinction coefficients and a smaller band gap than those of
DTDCPB, which can possibly be attributed to beneficial
impact from the uniaxial anisotropy character of DTDCPB.
The external quantum efficiency (EQE) spectra of the PMHJ
solar cells, shown in Figure 5b, were used to calculate the
mismatch factors.21 Both EQE spectra of the DPDCTB:C70
and DTDCTB:C70 devices exhibited high plateaus of ∼50% (or
even higher) throughout the UV−vis range and extend to the
near-IR region, whereas the DTDCPB:C70 cell delivered the
highest EQEs over the UV−vis range among this series of
devices. The calculated Jsc values obtained by integrating the
EQE spectra with the standard AM 1.5G solar spectrum
matched the measured Jsc values within 3−5%. It is worthwhile
to mention that the structural modulation of the donor
moieties in these D-A-A donors and consequent shift of their
HOMO levels and adjustment of their bandgaps clearly lead to
a trade-off between the Voc and Jsc values. Remarkably, the
DTDCPB:C70 device exhibited the best performance through
striking a balance between the photovoltage and photocurrent,
thus resulting in a PCE of 6.6 0.2% (the highest of 6.8%)
with a Voc of 0.93 0.02 V, Jsc of 13.48 0.27 mA/cm2, and fill
factor (FF) of 0.53 0.02. The DTDCPB:C70 (1:1.6) device
was certified by the Photovoltaic Metrology Laboratory,
Industrial Technology Research Institute (ITRI) to have a
PCE of 6.68% (see certification report in SI).
To gain insight into the differences in the performance of
DTDCPB:C70 PMHJ devices at various blended ratios, carrier
mobilities and thin film morphologies were examined. The
carrier mobilities were measured using the space charge-limited
current (SCLC) method. The hole and electron mobilities for
the DTDCPB:C70 (1:1.6) blended film were 6.49 × 10−5 and
4.32 × 10−4 cm2 V−1 s−1 at 700 (V/cm)0.5, respectively, which
were higher and more balanced than those for the
DTDCPB:C70 (1:1) film (Figure S6 and Table S7), thus
contributing to the higher device performance for the
DTDCPB:C70 (1:1.6) device. Although significant achieve-
ments have already been realized in our system, further
improvements in PCEs are still in high demand. In this respect,
the moderate FF values of ∼0.5 obtained in our devices
represent a potential area for further optimization to obtain
even better performance. Specifically, the shunt resistances
(Rshunt) in our devices, calculated from the inverse slopes of the
J−V curves in the fourth quadrant, are several orders of
magnitude lower than those in optimized polymer solar cells
(Table 2).4b,22 In general, both a high Rshunt and a low series
resistance (Rseries) are desirable for producing high FF, and a
low Rshunt actually stems from more charge recombination and
leakage current.23 Accordingly, increasing hole mobility and
thus realizing more balanced carrier transport, to improve the
Rshunt and the resulting FF values, will be a future research
focus.24
Figure 5. (a) Mismatch-corrected J−V characteristics (under 1 sun,
AM 1.5G illumination) and (b) EQE spectra of DPDCTB:C70 (1:1.6)
(squares), DTDCTB:C70 (1:1.6) (circles), DPDCPB:C70 (1:1.6)
(triangles), DTDCPB:C70 (1:1.6) (diamonds), and DPDCPB:C70
(1:1) (stars) PMHJ solar cells.
the corresponding solar cell characteristics derived from these
curves are summarized in Table 2.21 As expected, the Voc values
obtained in these devices are consistent with the magnitudes of
the HOMO levels of the donors in spite of the various
donor:C70 blended ratios. The DPDCPB:C70 device gave a
maximum Voc value of 1.00 0.02 V, reflecting the fact that
DPDCPB possesses the lowest HOMO level. Moreover, the
differences in Voc values were closely correlated to those in the
HOMO levels. On the other hand, the Jsc values generally
followed the trend in donor bandgaps, with a maximum value
To extract more information from the BHJ layer, surface
morphologies of the donor:C70 (1:1.6) and DPDCPB:C70
(1:1) blended films were analyzed by tapping-mode atomic
force microscopy (AFM). Although the AFM images revealed
the existence of phase separation (Figure S7), there were no
decisive variations useful to account for the achieved device
performance. Therefore, transmission electron microscopy
(TEM) and scanning transmission electron microscopy
of 15.08
0.30 mA/cm2 generated in the DTDCTB:C70
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Table 2. Photovoltaic Parameters of PMHJ Solar Cells under AM 1.5G Simulated Solar Illumination at an Intensity of 100 mW/
cm2
device type
Jsc (mA/cm2)
Voc (V)
FF
PCE [highest] (%)
Rseries (Ω·cm2)
Rshunt (Ω·cm2)
DPDCTB:C70 (1:1.6)
DTDCTB:C70 (1:1.6)
DPDCPB:C70 (1:1.6)
DTDCPB:C70 (1:1.6)
DPDCTB:C70 (1:1)
DTDCTB:C70 (1:1)
DPDCPB:C70 (1:1)
DTDCPB:C70 (1:1)
12.99 0.42
15.08 0.30
11.28 0.29
13.48 0.27
12.56 0.45
14.72 0.25
11.45 0.23
12.72 0.40
0.88 0.02
0.79 0.02
1.00 0.02
0.93 0.02
0.85 0.02
0.79 0.02
1.00 0.02
0.92 0.02
0.49 0.01
0.48 0.01
0.45 0.02
0.53 0.02
0.42 0.02
0.49 0.01
0.49 0.01
0.53 0.01
5.6 0.2 [5.8]
5.7 0.1 [5.8]
5.0 0.1 [5.1]
6.6 0.2 [6.8]
4.5 0.2 [4.7]
5.7 0.1 [5.8]
5.5 0.1 [5.6]
6.2 0.2 [6.4]
8.3
13.5
27.0
10.6
19.8
7.9
234.5
187.0
322.8
413.0
174.6
205.9
396.3
418.8
15.4
12.5
chemical synthesis were purified by distillation. All chemical reactions
were carried out under an argon or nitrogen atmosphere.
(STEM) were applied to provide morphological information
for the DTDCPB:C70 (1:1) and (1:1.6) blended films. STEM
high-angle annular dark-field (HAADF) imaging revealed
contrast variations in both the DTDCPB:C70 (1:1.6) and
(1:1) films (shown in Figure S8), even though the
morphological details were markedly different between these
samples. The 1:1 blend sample exhibited a less distinctly
resolved domain structure and smaller features compared with
the 1:1.6 system. The lower PCE of the 1:1 device may be due
to inferior charge-transport pathways as a result of
discontinuous domain structures through the active layer
thickness, which is expected to result in increased nongeminate
charge carrier recombination.25 Ongoing studies are aimed at
deciphering the spatially resolved composition of the domains,
which will likely result in new insights into the photovoltaic
processes in these materials.
Synthesis of 2-{[7-(4-N,N-ditolylaminophenylen-1-yl)-2,1,3-
benzothiadiazol-4-yl]methylene}malononitrile (DTDCPB). A
mixture of 4-bromo-7-dicyanovinyl-2,1,3-benzothiadiazole (3.49 g, 12
mmol), 4-(N,N-ditolylamino)-1-(tri-n-butylstannyl)phenylene (1d)
(8.1 g, 14.4 mmol), and PdCl2(PPh3)2 (421 mg, 0.6 mmol) in
anhydrous toluene (60 mL) was stirred and heated to 110 °C under
argon for 5 h. After the reaction mixture was cooled to room
temperature, the solvent was removed by rotary evaporation, and the
crude product was purified by column chromatography on silica gel
with dichloromethane/hexane (v/v, 4:3) as eluent, to afford DTDCPB
as a black solid (4.43 g, 76%), mp 288 °C (DSC); IR (KBr) ν: 3031,
2918, 2858, 2218, 1592, 1505, 1471, 1328, 1264, 1189, 1102, 946, 826
1
cm−1; H NMR (CDCl3, 400 MHz, δ) 8.82 (s, 1H), 8.78 (d, J = 7.6
Hz, 1H), 7.95 (d, J = 9.2 Hz, 2H), 7.84 (d, J = 7.6 Hz, 1H), 7.15−7.09
(m, 10H), 2.36 (s, 6H); 13C NMR (CDCl3, 100 MHz, δ) 154.4, 152.6,
152.5, 150.0, 143.9, 140.0, 134.0, 130.7, 130.4, 130.0, 127.3, 125.7,
125.5, 121.1, 120.1, 113.9, 113.1, 81.8, 21.1; HRMS-FAB+ (m/z): calcd
for C30H21N5S, 483.1518; found, 483.1516.
CONCLUSIONS
■
Synthesis of 2-{[7-(5-N,N-diphenylaminothiophen-2-yl)-
2,1,3-benzothiadiazol-4-yl]methylene}malononitrile
(DPDCTB). The synthetic procedure was similar to that of DTDCPB,
except that the eluent for column purification was dichloromethane/
hexane (v/v, 1:1). DPDCTB was isolated as a black solid (63%), mp
187 °C (DSC); IR (KBr) ν: 3034, 2218, 1579, 1532, 1487, 1296, 1059,
In summary, three new D-A-A-type donor molecules
(DPDCTB, DPDCPB, and DTDCPB) with various donor
moieties were synthesized in a facile fashion by taking
advantage of the asymmetric building block, 4-bromo-7-
dicyanovinyl-2,1,3-benzothiadiazole. X-ray structure analyses
revealed the tendency of these highly dipolar molecules to pack
in an antiparallel manner and self-assemble into centrosym-
metric dimers in the solid state. The adoption of thiophene-
based donors facilitated π-electron delocalization and the
formation of quinoidal mesomeric structures over the
conjugated backbone. Compared to DTDCTB, bearing the
strongest ditolylaminothienyl donor moiety among this series
of molecules, the three new molecules possessed lower HOMO
levels and exhibited hypsochromic shifts in absorption.
Through judicious molecular design and device optimizations,
including fine-tuning the layers’ thicknesses as well as the
blended ratio in the BHJ layer, vacuum-deposited PMHJ
devices based on this series of molecules as donors and C70 as
the acceptor showed PCEs in excess of 5.6%. Notably, the
DTDCPB-based devices achieved an impressively high PCE of
6.8% through striking a balance between the photovoltage and
the photocurrent. We believe that our molecular design
concept paves a new avenue for developing high-performance
donor materials for SMOSCs. Further engineering of molecular
structures and morphology optimization, such as substrate
heating and postannealing in order to boost device
efficiencies,26 are currently underway.
1
756 cm−1; H NMR (CDCl3, 400 MHz) δ 8.69 (s, 1H), 8.64 (d, J =
8.0 Hz, 1H), 8.22 (d, J = 4.0 Hz, 1H), 7.65 (d, J = 8.0 Hz, 1H), 7.40−
7.36 (m, 4H), 7.32−7.29 (m, 4H), 7.21 (t, J = 7.2 Hz, 2H), 6.61 (d, J =
4.0 Hz, 1H); 13C NMR (CDCl3, 100 MHz) δ 159.7, 154.5, 151.7,
150.7, 146.4, 133.7, 132.5, 130.8, 129.6, 127.3, 125.3, 124.7, 121.5,
119.4, 116.2, 114.4, 113.6, 79.4; HRMS-FAB+ (m/z): calcd for
C26H15N5S2, 461.0769; found, 461.0760.
Synthesis of 2-{[7-(4-N,N-diphenylaminophenylen-1-yl)-
2,1,3-benzothiadiazol-4-yl]methylene}malononitrile
(DPDCPB). The synthetic procedure was similar to that of DTDCPB,
except that the eluent for column purification was dichloromethane/
hexane (v/v, 1:1). DPDCPB was isolated as a black solid (49%), mp
208 °C (DSC); IR (KBr) ν: 3047, 2223, 1584, 1524, 1474, 1370, 1336,
1270, 1192, 1044, 931, 821 cm−1; 1H NMR (CDCl3, 400 MHz) δ 8.83
(s, 1H), 8.79 (d, J = 7.6 Hz, 1H), 7.98 (d, J = 8.8 Hz, 2H), 7.86 (d, J =
7.6 Hz, 1H), 7.35−7.31 (m, 4H), 7.22−7.18 (m, 6H), 7.13 (t, J = 7.2
Hz, 2H); 13C NMR (CDCl3, 100 MHz) δ 154.4, 152.6, 152.5, 149.6,
146.6, 139.9, 130.7, 130.5, 129.4, 128.2, 125.8, 125.5, 124.2, 121.4,
121.3, 113.9, 113.1, 82.2; HRMS-FAB+ (m/z): calcd for C28H17N5S,
455.1205; found, 455.1204.
Solar Cell Fabrication and Testing. Fullerene C70 and BCP were
subjected to purification at least once by temperature-gradient
sublimation before use in this study. The organic and metal oxide
thin films as well as metal electrodes were deposited on indium tin
oxide (ITO)-coated glass substrates in a high-vacuum chamber with
base pressure ∼1 × 10−6 Torr. The sheet resistance of ITO was ∼15
Ω/sq. The deposition was performed at a rate of 2−3 Å/s with the
substrate held at room temperature. Thicknesses were monitored
using a crystal oscillator during deposition and were verified later with
spectroscopic ellipsometry. The active area of the cells had an average
EXPERIMENTAL SECTION
■
Synthesis and Materials. All chemicals and reagents were used as
received from commercial sources without purification. Solvents for
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dx.doi.org/10.1021/ja301872s | J. Am. Chem. Soc. 2012, 134, 13616−13623

Journal of the American Chemical Society
Article
size of 2.5 mm2 (intersection area between Ag cathode and ITO
anode) and were carefully measured device-by-device using a
calibrated optical microscope. Devices were encapsulated using a
UV-cured sealant (Epowide EX, Everwide Chemical Co.) and a cover
glass under an anhydrous nitrogen atmosphere after fabrication and
were measured in air. Current density−voltage characteristics were
measured with a SourceMeter Keithley 2636A under illumination of
AM 1.5G solar light from a xenon lamp solar simulator (Abet
Technologies). The incident light intensity was calibrated as 100 mW/
cm2. Taking into account the spectral mismatch between the solar
simulator used in this work and the AM 1.5G reference spectrum (IEC
60904) as well as the mismatch of the photoresponse spectral coverage
of the reference cell (KG5-filtered Si diode) and the test cells, the
mismatch factors were calculated to be 1.006 (DPDCTB:C70 (1:1.6)),
1.014 (DTDCTB:C70 (1:1.6)), 0.994 (DPDCPB:C70 (1:1.6)), 0.997
(DTDCPB:C70 (1:1.6)), and 0.993 (DPDCPB:C70 (1:1)).21 Short-
circuit current densities were all corrected by mismatch factors. The
devices were measured without a mask (the Jsc of cells with and
without a mask had been examined, and the difference was always <3%
due to the low conductive hole/electron transporting layers used in
this study). The deviation values were obtained from device-to-device
variations of 4−8 devices. The external quantum efficiency spectra
were taken by illuminating chopped monochromatic light with a
continuous-wave bias white light (from a halogen lamp) on the solar
cells. The photocurrent signals were extracted with a lock-in technique
using a current preamplifier (Stanford Research Systems) followed by
a lock-in amplifier (AMETEK). The EQE measurement was fully
computer-controlled, and the intensity of the monochromatic light was
calibrated with an NIST-traceable optical power meter (Ophir
Optronics). Absorption spectra were acquired with a spectrometer
(PerkinElmer). Organic films for photoelectron spectroscopy and
ellipsometry measurements were vacuum deposited onto fused-silica
substrates. The HOMO levels of molecules were acquired with a
photoelectron spectrometer (Riken Keiki Co. Ltd.). Ellipsometry
measurements were carried out with a J. A. Woollam Inc. V-VASE
variable-angle spectroscopic ellipsometer. The anisotropic optical
constants of a sample were determined by the combination of
reflection and transmission ellipsometry, which in principle is simpler
and eliminates the risk of sample-to-sample variation.17 The simulation
program was coded with Matlab software (The MathWorks, Inc.) and
performed with a dual-core Intel-CPU personal computer. AFM
images were taken with a Veeco Nanoscope 3100 atomic force
microscope. For accurate comparisons, the film preparation conditions
for the AFM measurements were the same as those for device
fabrication. TEM samples were vacuum-deposited active films on
poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PE-
DOT:PSS) precoated Si wafers, which then were transferred to
holey carbon grids by dissolving the PEDOT:PSS layer in deionized
water. TEM and HAADF imaging were performed on an FEI Titian
TEM with a Cs and Cc image corrector developed by CEOS GmbH
(Heidelberg, Germany). To study these thin films, 200 kV was used,
and no obvious beam damage was observed. HAADF images were
collected using an inner cutoff angle of about 70 mrad to increase
scattering intensity from light elements. The electron and hole
mobilities for the DTDCPB:C70 blended films were determined using
the SCLC method. The hole-only device was configured as follows:
ITO/MoO3 (1 nm)/DTDCPB:C70 (100 nm)/MoO3 (10 nm)/Al (80
nm), while the electron-only device was configured as follows: ITO/
Mg (5 nm)/DTDCPB:C70 (100 nm)/Ca (5 nm)/Al (80 nm).
Current density−voltage characteristics of SCLC devices were also
measured with a SourceMeter Keithley 2636A.
donor:C70 (1:1.6) and DPDCPB:C70 (1:1) blended films,
TEM and STEM images for DTDCPB:C70 (1:1.6) and
DTDCPB:C70 (1:1) blended films, copies of 1H and 13C
NMR spectra, and CIF for DTDCPB. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
hwlin@mx.nthu.edu.tw; kenwong@ntu.edu.tw
■
Author Contributions
#These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to acknowledge financial support from
the National Science Council of Taiwan (NSC 98-2112-M-007-
028-MY3, 98-2119-M-002-007-MY3) and the Low Carbon
Energy Research Center, National Tsing Hua University. Use
of the Electron Microscopy Center (EMC) for Materials
Research and the Center for Nanoscale Materials (CNM) at
Argonne National Laboratory was supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, under contract no. DE-AC02-06CH11357.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Crystal data and selected bond lengths for DTDCTB and
DTDCPB, UPS spectra of all donors, device optimization for
all donors, a mobility diagram for DTDCPB:C70 (1:1.6) and
DTDCPB:C70 (1:1) blended films, calculation details of
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