Dicyanovinylene-substituted selenophene-thiophene co-oligomers for small-molecule organic solar cells

Article abstract of DOI:10.1021/cm201392c

We report on the design, synthesis, and characterization of a series of terminal dicyanovinylene-substituted quinquechalcogenophenes as light-harvesting small-molecule donor materials for organic solar cells. The spectroscopic, electrochemical, and thermal properties of these pentamers were investigated. The replacement of thiophene unit(s) by selenophene(s) results in a bathochromic shift of the longest wavelength absorption band with concomitant increase of the molar extinction coefficient. Cyclic voltammetry measurements revealed fully reversible oxidation and irreversible reduction processes. The highest occupied and lowest unoccupied molecular orbital (HOMO/LUMO) energy levels were determined from electrochemical measurements and lie in the range of -5.6 and -3.8 eV. Vacuum-deposited bulk-heterojunction solar cells fabricated with the novel chalcogenophenes as donor and C₆₀ as acceptor displayed high open-circuit voltages of up to 1 V, short-circuit currents close to 8 mA?cm^-2, and power conversion efficiencies over 3%.

Full text of DOI:10.1021/cm201392c

ARTICLE
pubs.acs.org/cm
Dicyanovinylene-Substituted SelenopheneꢀThiophene
Co-oligomers for Small-Molecule Organic Solar Cells
Stefan Haid,† Amaresh Mishra,*,† Christian Uhrich,‡ Martin Pfeiffer,‡ and Peter B€auerle*,†
†Institute of Organic Chemistry II and Advanced Materials, University of Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany
‡Heliatek GmbH, Treidlerstrasse 3, 01139 Dresden, Germany
S Supporting Information
b
ABSTRACT: We report on the design, synthesis, and char-
acterization of a series of terminal dicyanovinylene-substituted
quinquechalcogenophenes as light-harvesting small-molecule
donor materials for organic solar cells. The spectroscopic,
electrochemical, and thermal properties of these pentamers
were investigated. The replacement of thiophene unit(s) by
selenophene(s) results in a bathochromic shift of the longest
wavelength absorption band with concomitant increase of the
molar extinction coefficient. Cyclic voltammetry measurements
revealed fully reversible oxidation and irreversible reduction processes. The highest occupied and lowest unoccupied molecular
orbital (HOMO/LUMO) energy levels were determined from electrochemical measurements and lie in the range of ꢀ5.6 and ꢀ3.8
eV. Vacuum-deposited bulk-heterojunction solar cells fabricated with the novel chalcogenophenes as donor and C60 as acceptor
displayed high open-circuit voltages of up to 1 V, short-circuit currents close to 8 mA cmꢀ2, and power conversion efficiencies
3
over 3%.
KEYWORDS: oligothiophene, selenophene, dicyanovinylene acceptor, bulk-heterojunction solar cells, structureꢀproperty
relationship
’ INTRODUCTION
efficiency of 3.4% in planar heterojunction (PHJ) solar cells.15,16
The high VOC obtained in these cells can be ascribed to the
relatively deep-lying highest occupied molecular orbital (HOMO)
energy level at ꢀ5.6 eV versus vacuum. It is well-established that
the VOC is related to the difference between the HOMO energy
level of the donor and the lowest unoccupied molecular orbital
(LUMO) energy level of the acceptor.24,25 Replacement of the
butyl chains by ethyl chains resulted in a decrease in efficiency
(2.5%) due to a lower fill factor (FF).26 Vacuum processing also
allows the fabrication of bulk-heterojunction (BHJ) solar cells by
coevaporation of donor and acceptor components. Very recently,
by use of a nonalkylated DCV-substituted quinquethiophene as
donor in p-i-n-type BHJ devices, record efficiencies of 5.2% were
achieved.12 The BHJ layer was prepared by coevaporation of the
quinquethiophene donor and the C60 acceptor. With this BHJ
structure, the donorꢀacceptor interface area is highly increased,
thus ensuring more efficient charge separation. Furthermore, the
resulting interpenetrating network facilitates charge transport.
While oligo- and polythiophenes are among the best investi-
gated materials in the field of organic electronics,6,27 oligo- and
polyselenophenes were only scarcely investigated. Nevertheless,
they show some promising properties, especially for application
In the past decade, organic solar cells (OSC) have been
intensely investigated because very thin devices can be fabricated
on flexible substrates with low material consumption.1ꢀ4 In the
fields of both small-molecule and polymer solar cells, an en-
ormous increase in power conversion efficiencies has been
achieved in recent years, from under 1% in the beginning to
currently over 7%.5ꢀ13 Very recently, certified power conversion
efficiencies of 8.3% have been independently released for a
tandem cell prepared by the combination of two BHJ devices
based on small molecules by Heliatek GmbH and for solution-
processed polymer solar cells by Konarka.14 Small molecules
offer several advantages compared to their polymeric counter-
parts, such as well-defined structure, efficient functionalization,
and purification. Most importantly in the case of small molecules,
batch variations, which can affect polymers, can be avoided.
Small-molecule organic solar cells (SMOSC) can be prepared
either from solution or by vacuum processing. In the latter case,
very good efficiencies were achieved by use of various small
molecules as light-harvesting electron-donor materials and C60 as
acceptor.12,15ꢀ23 Especially, dicyanovinylene (DCV) -substi-
tuted oligothiophenes showed very promising photovoltaic
properties. In a series of DCV-substituted oligothiophenes with
butyl side chains in the β-positions of the oligothiophene back-
bone, the quinquethiophene (DCV5T) showed the best perfor-
mance with a high open-circuit voltage (VOC) of 1.0 V and an
Received: May 17, 2011
Revised:
September 1, 2011
Published: September 26, 2011
r
2011 American Chemical Society
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in organic solar cells, that is, lower band gaps, red-shifted
absorption, good polarizability, and improved interchain charge
transfer.28ꢀ33 Despite these favorable properties, only very
few polyselenophenes and selenophene-containing copolymers
were tested in polymer solar cells; they showed efficiencies in
the range 0.1ꢀ2.7%.34ꢀ37 There are as well only a few examples
for selenophene-based sensitizers in dye-sensitized solar cells
(DSSC).38,39 Until now, there has been only one example of a
SMOSC using a selenophene-containing donor material.40 In
that report, solution-processed BHJ solar cells were fabricated
from selenophene-substituted diketopyrrolopyrrole derivatives,
giving efficiencies of 1.5%.
In this study, we present the synthesis of three DCV-end-
capped quinquechalcogenophenes with increasing number of
selenophene units from one to three (Figure 1). The mixed
oligomers are compared to parent oligothiophene DCV5T,
which lacks selenophene units. The effects of increasing seleno-
phene content on optical, electrochemical, and thermal proper-
ties are investigated, and good structureꢀproperty relationships
were deduced. Furthermore, the fabrication of vacuum-pro-
cessed BHJ solar cells with these compounds is described, leading
to very promising results.
’ RESULTS AND DISCUSSION
Synthesis. Synthesis of parent DCV5T is described in the
literature.15 Co-oligomers 1ꢀ3 were synthesized in a similar
fashion via palladium-catalyzed cross-coupling reactions. The
synthesis of 1 and 3 (Scheme 1) started with selenophene 4,
which was dibrominated with N-bromosuccinimide (NBS) to
afford 2,5-dibromoselenophene 5 in 84% yield. The latter
compound was coupled with dibutylated thiophene boronic
ester 7 in a Suzuki-type cross-coupling reaction to obtain
dithienylselenophene 8 in 82% yield.
Mixed trimer 8 was diiodinated with iodine/mercury(II)
acetate in chloroform to give diiodinated trimer 9 in a yield of
95%. Pentamer 12a was prepared in 93% yield by a Stille-type
coupling reaction of 9 and tributylstannyl thiophene 10 in
dimethylformamide (DMF) at 75 °C. On the other hand, Pd0-
catalyzed Suzuki cross-coupling reaction of 9 with selenophene
boronic ester 11 gave 12b in 96% yield. Mixed pentamers 12a
and 12b were then further functionalized in a VilsmeierꢀHaack
Figure 1. Investigated chalcogenophenes.
Scheme 1. Synthesis of Pentamers 1 and 3a
a (a) NBS (2 equiv), DMF, room temperature (rt), 16 h. (b) (i) n-BuLi, THF, 0 °C; (ii) 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. (c)
Pd2(dba)3, HPtBu3BF4, aq K3PO4, THF, 50 °C, overnight. (d) I2/Hg(OAc)2, CHCl3, 0 °C to rt. (e) 2-Tributylstannylthiophene 10, Pd(PPh3)2Cl2,
THF, 75 °C, 3 days. (f) 4,4,5,5-Tetramethyl-2-(selenophen-2-yl)-1,3,2-dioxaborolane 11, Pd2(dba)3, HPtBu3BF4, aq K3PO4, THF, rt, overnight. (g)
POCl3/DMF, DCE, rt, 22 h. (h) Malononitrile, β-alanine, DCE/EtOH, reflux, 24 h.
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formylation with POCl3/DMF in dichloroethane to afford
dialdehydes 13a and 13b in 89% and 94% yield, respectively.
In a final step, the dialdehydes were transformed into the
corresponding dicyanovinylene compounds by Knoevenagel
condensation with malononitrile and β-alanine in a dichloro-
methane/ethanol mixture. Thus, co-oligomers 1 and 3 were
obtained in 79% yield each.
Synthesis of co-oligomer 2 started from 5-bromoseleno-
phene-2-carbaldehyde 14 and tetrabutylated terthiophene 16
(Scheme 2). Aldehyde 1441 was condensed with malononitrile
in a Knoevenagel condensation to afford brominated DCV-
substituted selenophene 15 in 84% yield. Terthiophene 1642
was distannylated by lithiation using n-butyllithium in the
presence of tetramethylethylenediamine (TMEDA) and subse-
quently quenched with trimethyltin chloride. Distannylated
terthiophene 17 was obtained in 83% yield. Final Pd0-catalyzed
Stille-type cross-coupling of 15 and 17 in DMF at 75 °C afforded
mixed pentamer 2 in 67% yield. All DCV-substituted pentamers
could be obtained in high purity by flash chromatography on
silica gel.
end-capped pentamers were investigated by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC)
under an inert atmosphere at a heating rate of 10 °C/min
(Figure 2). The excellent thermal stability of the oligomers was
manifested in their TGA profile, showing decomposition tem-
peratures (Td) with 5% weight loss above 370 °C (Table 1). With
an increase in selenophene units, Td was found to be slightly
decreased from 392 °C for DCV5T to 372 °C for co-oligomer 3.
Nevertheless, this indicates excellent thermal stability suitable for
successful vacuum sublimation. In the DSC curves, one sharp
endothermic peak indicating the melting temperature (Tm) is
observed for each compound. With increasing number of sele-
nophene units, the melting point gradually increases from 203 °C
for DCV5T to 215 °C for co-oligomer 3. All compounds start to
decompose above 360ꢀ370 °C, which is indicated by a broad
exothermic peak in the DSC curves.
Optical Spectroscopy. The optical properties of the DCV-
end-capped pentamers were investigated by UVꢀvis and fluo-
rescence spectroscopy in solution (measured in dichloromethane)
and in thin films (prepared by vacuum sublimation). The data are
summarized in Table 1. Representative spectra for co-oligomer
3 are shown in Figure 3. The spectra of the other pentamers are
similar and are omitted for clarity reasons.
Thermal Properties. For the fabrication of vacuum-processed
solar cells, the thermal stability of the applied donor materials is of
crucial importance. Therefore, the thermal properties of the DCV-
In solution, all compounds showed one major absorption
maximum between 510 and 540 nm, which corresponds to the
πꢀπ* (HOMOfLUMO) transition of the conjugated π-sys-
tem. It is bathochromically shifted compared to unsubstituted
quinquethiophenes or quinqueselenophenes43,44 due to lower-
ing of the LUMO energy by the electron-withdrawing DCV end
groups.
Scheme 2. Synthesis of Compound 2a
The absorption maximum is gradually red-shifted with in-
creasing number of selenophene units from 513 nm for DCV5T
(without selenophenes) to 533 nm for 3 (three selenophenes).
The pronounced red shift upon going from co-oligomer 1 to 2
seems to be due to the DCV-terminated selenophenes intro-
duced at the periphery of the molecules. The selenophene in the
middle of the molecule only leads to a small red shift as shown by
comparison of compounds that differ only in the core seleno-
phene, that is, DCV5T and 1 (shift from 513 to 516 nm) or 2 and
3 (shift from 529 to 533 nm). The molar extinction coefficient
linearly increases with the number of selenophenes (62 600
L molꢀ1 cmꢀ1 for DCV5T to 66 000 L molꢀ1 cmꢀ1 for 3).
a (a) Malononitrile, β-alanine, DCE/EtOH, reflux, 17 h. (b) TMEDA, n-
BuLi, THF, ꢀ78 to 50 °C. (c) Pd(PPh3)4, DMF, 75 °C, 16 h.
3
3
3
3
This correlation is illustrated in Figure 4.
Figure 2. (a) TGA thermograms of DCV-end-capped pentamers 1ꢀ3 and DCV5T measured under N2. Conditions: N2 flow 50 mL/min, heating rate
10 °C/min. (b) DSC curves for mixed selenopheneꢀthiophene oligomers 1ꢀ3 in comparison to DCV5T measured under argon flow at a heating rate
of 10 °C/min.
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Table 1. Optical Data for SelenopheneꢀThiophene Pentamers
solution
thin film
λem(nm)
εmax (L molꢀ1 cmꢀ1
)
λem(nm)
Egopt (eV)c
λabs(nm)
Egopt (eV)d
a
b
compd
Td
Tm
λabs(nm)
3
3
DCV5T
392
384
375
372
203
211
213
215
513
516
529
533
62 600
661
667
675
673
2.02
2.01
1.96
1.93
573
577
589
592
726
735
737
758
1.76
1.74
1.72
1.67
1
2
3
63 600
64 900
66 000
a Decomposition temperature corresponding to 5% weight loss in N2 determined by TGA. b Melting temperature determined from DSC. c Estimated
from the onset of UVꢀvis spectra in DCM. d Estimated from the onset of UVꢀvis spectra of films prepared by vacuum sublimation.
Figure 3. Representative UVꢀvis absorption and fluorescence spectra
Figure 5. UVꢀvis absorption and fluorescence spectra of optimized
of pentamer 3. Solution spectra were measured in dichloromethane.
donorꢀacceptor layers as used in bulk-heterojunction solar cells. Blend
Thin films (30 nm) were prepared by vacuum sublimation.
layers of all four compounds are shown, each with C60 in a mixing ratio of
2:1 (v/v) deposited on a heated substrate (90 °C).
from solution spectra. Also here, the gradual lowering of the band
gap correlates with increasing number of selenophene units in
the mixed oligomers.
In Figure 5, the absorption and emission spectra of the
donorꢀacceptor blend layers are shown as they are used in the
optimized bulk-heterojunction solar cells presented later in
this paper. All four compounds were codeposited with C60
(2:1 by volume) on heated substrate (90 °C). All absorption
spectra show an additional absorption peak at 340 nm and an
increased absorption at around 450 nm compared to the
pristine films, due to the C60 absorption. All four compounds
show an absorption maximum at around 570 nm. In contrast
to the absorption in solution, the blend layer with DCV5T
shows the highest absorbance in this wavelength range. The
Figure 4. Correlation of molar extinction coefficients with number of
absorbance of the co-oligomers is decreased by a factor of
selenophenes.
about 20%. The emission of the blend layer with DCV5T is
reduced by about 2 orders of magnitude compared to the
The fluorescence spectra of the pentamers are structureless
and show a single emission maximum. An increasing number of
selenophenes also leads to a red shift of the emission maximum
from 661 nm for DCV5T to 675 nm for 2 in solution.
Thin film absorption and emission spectra are red-shifted by
about 60ꢀ80 nm and significantly broadened compared to the
corresponding solution spectra. This red shift is mainly caused by
planarization and ordering of the molecules in the solid state,
resulting in greater interchromophoric interactions and better
packing in thin film. The optical band gaps (Egopt), which are
calculated from onset of thin film absorption spectra, were
1.76ꢀ1.67 eV and about 0.25 eV lower than the values calculated
pristine DCV5T layer. Nevertheless, the photoluminescence
is not completely quenched when the blend layer is deposited
on heated substrate, which is a hint for phase separation of
donor and acceptor leading to optimized charge carrier
transport in the blend layer. In general, a fraction of the
excitons generated in the larger domains of DCV5T are not
able to reach the C60 interface, thus leading to inefficient
quenching of the DCV5T luminescence.18 In contrast, the
luminescence of the blend layers with co-oligomers 1ꢀ3 is
almost completely quenched by C60. This behavior points
toward a lower degree of donorꢀacceptor phase separation in
the blend layer compared to DCV5T.
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Cyclic Voltammetry. Redox properties were determined by
cyclic voltammetry in dichloromethane with TBAPF6 (tetrabutyl-
ammonium hexafluorophosphate) as supporting electrolyte (0.1 M).
The cyclovoltammograms are shown in Figure 6 and the data are
summarized in Table 2. Additionally, the redox properties of thin
films prepared by drop-casting from dichloromethane solution
onto a platinum electrode were determined by cyclic voltamme-
try in acetonitrile with TBAPF6 as supporting electrolyte (0.1 M)
(see Figure S1 in Supporting Information). No significant
change in the redox properties was observed with increasing
number of selenophene units in the oligomers. All compounds
showed two reversible oxidation waves at around 0.60 and 0.85 V,
respectively, which indicate the formation of stable radical
cations and dications as expected for pentamers. In the negative-
potential regime, an irreversible reduction wave between ꢀ1.36
and ꢀ1.50 V was observed, which is assigned to the reduction of
the DCV end groups. The oxidation potentials of co-oligomers 1
and 3 containing core selenophene units are somewhat lower
than those of their thiophene counterparts DCV5T and 2,
reflecting the lower electronegativity of selenium.
HOMO energies of all pentamers lie with slight variations
between ꢀ5.61 and ꢀ5.57 eV, and the LUMO energy levels
slightly vary from ꢀ3.74 to ꢀ3.80 eV. The HOMO energies in
thin films lie between ꢀ5.63 and ꢀ5.68 eV, and the LUMO
energies are in the range of ꢀ3.75 to ꢀ3.90 eV. The energy levels
in solution and in thin films are in good agreement. All oligomers
have relatively low-lying HOMO energy levels as a result of
incorporation of the DCV acceptor end groups, which are
beneficial for achieving a high VOC in solar cells. These values are
very similar to DCV5T, which lacks the selenophene moieties,
and should be well suited for incorporation into SMOSCs.
The electrochemical band gaps (EgCV) obtained with these co-
oligomers in solution are gradually decreased with increasing
number of selenophene units (1.87 eV for DCV5T to 1.77 eV for
co-oligomer 3). In thin films, the band gaps lie in the same range,
but the observed trend is not so clearly visible. The electro-
chemical band gaps are in good agreement with the optical band
gaps (Egopt) determined from solution or thin-film spectra.
Solar Cell Characterizations. The DCV-end-capped thiopheneꢀ
selenophene pentamers and DCV5T were used as electron-donor
materials in coevaporated bulk-heterojunction solar cells with
fullerene C60 as electron acceptor. The cells were prepared by
vacuum sublimation following the m-i-p construction principle
reported in the literature.46
HOMO and LUMO energy levels of all compounds were
calculated from the onset of first oxidation and reduction waves,
respectively, and data are presented in Table 2. In solution, the
The general device structure of the investigated solar cells is
shown in Figure 7. An indium tin oxide (ITO) -coated glass
substrate was first covered with a layer of C60 (15 nm), and then a
donorꢀacceptor bulk-heterojunction layer (20 nm) was depos-
ited by coevaporation of the respective mixed pentamer (donor)
and C60 (acceptor). All devices were heated to a substrate
temperature of approximately 90 °C during the deposition of
the mixed layer. We expected a stronger phase separation when
Figure 6. Cyclic voltammograms of mixed selenopheneꢀthiophene
oligomers, measured in DCM/TBAPF6 (0.1 M), c ≈ 1 ꢁ 10ꢀ3
mol Lꢀ1, 295 K, scan rate = 100 mV sꢀ1
.
Figure 7. General device architecture of investigated BHJ solar cells.
3
3
Table 2. Electrochemical Data for SelenopheneꢀThiophene Pentamers and DCV5Ta
solution
thin film
compd
Eox1 (V)
Eox2 (V)
Ered1 (V)
HOMOb (eV)
LUMOb (eV)
HOMOc (eV)
LUMOc (eV)
EgCV solnd
EgCV filmd
DCV5T
0.58
0.57
0.60
0.55
0.87
0.86
0.89
0.83
ꢀ1.50
ꢀ1.48
ꢀ1.36
ꢀ1.45
ꢀ5.61
ꢀ5.60
ꢀ5.63
ꢀ5.58
ꢀ3.74
ꢀ3.75
ꢀ3.80
ꢀ3.80
ꢀ5.67
ꢀ5.68
ꢀ5.64
ꢀ5.63
ꢀ3.75
ꢀ3.90
ꢀ3.89
ꢀ3.85
1.87
1.85
1.83
1.77
1.92
1.78
1.75
1.78
1
2
3
a Measured in DCM-TBAPF6 (0.1 M), [compound] = 10ꢀ3 mol Lꢀ1, 25 °C, V = 100 mV sꢀ1, vs Fc+/Fc. b Measured in dichloromethane solution.
3
3
c Drop-cast films on platin electrode measured in acetonitrile solution; set Fc+/Fc; EHOMO = ꢀ5.1 eV (ref 45). d Calculated from the difference between
E
onset, red1 and Eonset, ox1.
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Figure 8. (a) JꢀV characteristics of bulk-heterojunction solar cells with DCV-end-capped pentamers as donor materials. (b) Corresponding external
quantum efficiency spectra of BHJ solar cells.
Table 3. Photovoltaic Parameters of Bulk-Heterojunction Solar Cells Containing DCV-End-Capped SelenopheneꢀThiophene
Pentamers and DCV5T
incident light
compd
JSC (mA cmꢀ2
)
VOC (V)
FF (%)
saturationa
intensity (mW cmꢀ2
)
JSC,ap,100b (mA cmꢀ2
)
η (%)
3
3
3
DCV5T
7.0
5.5
7.0
5.4
1.02
0.99
1.0
43
48
39
46
1.17
1.14
1.26
1.23
99
92
7.9
6.5
7.5
5.8
3.47
3.09
2.93
2.53
1
2
3
101
105
0.95
a Defined as J(ꢀ1 V)/JSC. b Measured with mask.
the substrate is heated during the deposition, providing better
charge transport within the percolation pathways of the mixed
layer.18,20 This layer was covered with 5 nm of undoped and
50 nm of doped 9,9-bis[4-(N,N-bis-biphenyl-4-ylamino)phenyl]-
9H-fluorene (BPAPF) as hole transport layer. The undoped
BPAPF layer was introduced to avoid direct contact between the
active layer and the doped BPAPF layer, which otherwise could
lead to quenching of excitons by the dopants.18 The thickness of
the doped BPAPF layer was chosen to have the active layer
approximately in the interference maximum. NDP9 (10 wt %)
was used as p-dopant, and another layer of NDP9 (1 nm) was
used between the hole transport layer (HTL) and the top Au
electrode to facilitate charge extraction. The dopant material
NDP9 was purchased from Novaled AG, Dresden, Germany. It
can be used for doping of organic materials with low-lying
HOMO energy levels and was chosen due to better stability
and processability. The blend ratio of donor to acceptor is 2:1 by
volume, which was found to be optimal for previously reported
DCV derivatives.12,20
under illumination are in the range 0.95ꢀ1.02 V. The obtained
high VOC can be ascribed to the large energy difference between
the HOMO of the donor and the LUMO of the C60 acceptor.
The short-circuit densities (JSC,ap,100) showed values in the
range 7.9ꢀ5.8 mA cmꢀ2. The values match with the EQE
3
spectra shown in Figure 8b within experimental error. The
BHJ devices showed broad external quantum efficiency (EQE)
covering from 400 to 750 nm, which are in good agreement with
the thin-film absorption spectra. The difference in JSC values seen
in the JꢀV curve is further reflected in the EQE spectra, with the
DCV5T device showing the best EQE value of about 60% at
around 600 nm. The increased JSC value for DCV5T is further
corroborated with the thin-film spectra of the donor/C60 blend
layers, where DCV5T displayed higher absorbance compared to
the selenophene-containing derivatives. Both compounds (1 and 3)
with a selenophene substitution in the center of the molecule
showed a significant decrease in JSC,ap,100. This effect has already
been observed by comparing absorption in solution and the
absorption of the blend layers. One possible explanation is a
lower degree of phase separation compared to DCV5T, leading
to a lower degree of crystallinity in the DCV pentamer phase and
thus to an increased intramolecular disorder, resulting in a
decrease of absorption in the blended film.
Figure 8a shows the JꢀV characteristics of BHJ solar cells. The
active area of the solar cells was approximately 6 mm2. The
photovoltaic parameters of these BHJ devices are summarized in
Table 3. Since the correct active area of especially small organic
solar cells is difficult to determine, the JꢀV characteristics of each
solar cell were measured with an aperture mask of 2.76 mm2 as well.
In Table 3, the saturation of the photocurrent with reverse bias
is also given. The saturation is defined by the current density
under illumination at ꢀ1 V bias divided by JSC. The devices
showed saturation values between 1.14 and 1.26, which indicates
that there is some recombination at short-circuit condition and in
reverse bias. A saturation value close to unity corresponds to
efficient charge separation and extraction, while a larger value
hints at recombination losses and field-dependent dissociation of
From this measurement, JSC,ap,100 (normalized to 100 mW cmꢀ2
)
3
and the power conversion efficiency (η) were calculated. The
mismatch between the spectrum of the sun simulator and
AM1.5G was not taken into account.
All devices measured under dark showed good rectification
behavior. The open-circuit voltages (VOC) obtained with all devices
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charge carriers. Devices with compounds 2 and 3 with seleno-
phenes placed in direct contact with the DCV end group showed a
significant increase in saturation. This voltage-dependent recom-
bination of photogenerated charge carriers might be attributed
either to a degradation in charge carrier transport in the blend layer
or to a decrease in the probability of exciton separation at the
donorꢀacceptor interface.
internal standard. The splitting patterns are designated as follows:
s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet).
The assignments are SeꢀH (selenophene protons), ThꢀH (thiophene
protons), ꢀCHO (aldehyde protons), DCV-H (dicyanovinylene pro-
tons), Bu (butyl protons). Melting points were determined on a Mettler
Toledo DSC 823e and were not corrected. Elemental analyses were
performed on an Elementar Vario EL. Thin-layer chromatography was
carried out on aluminum plates, precoated with silica gel, Merck Si60
F254. Preparative column chromatography was performed on glass
columns packed with silica gel, Merck Silica 60, particle size 40ꢀ
43 μm. EI mass spectra were recorded on a Varian Saturn 2000 GC-
MS and MALDI-TOF mass spectra on a Bruker Daltonics Reflex III.
Optical measurements in solution were carried out in 1 cm cuvettes
with Merck Uvasol-grade solvents. Absorption spectra were recorded on
a Perkin-Elmer Lambda 19 spectrometer, and corrected fluorescence
spectra were recorded on a Perkin-Elmer LS 55 fluorescence spectrom-
eter. Cyclic voltammetry experiments were performed with a computer-
controlled Autolab PGSTAT30 potentiostat in a three-electrode single-
compartment cell with a platinum working electrode, a platinum wire
counterelectrode, and an Ag/AgCl reference electrode. All potentials
were internally referenced to the ferrocene/ferrocenium couple.
Thin Film and Device Fabrication. Thin films and heterojunction
solar-cell devices were prepared by thermal vapor deposition in ultrahigh
vacuum at a base pressure of 10ꢀ7 mbar onto the substrate at room
temperature. Thin films for absorption and emission measurements
were prepared on quartz substrates, solar cells on tin-doped indium
oxide (ITO) coated glass (Thin Film Devices; sheet resistance of
The efficiency decreased from 3.5% to 2.5% with increasing
number of selenophenes. In the case of pentamers 1 and 3, the
decrease in η is caused by the lower JSC, while in the case of 2, the
decrease in η is primarily caused by the lower fill factor (FF).
Recently, McCulloch and Durrant and co-workers35,47 reported a
broad EQE spectrum for regioregular poly(3-hexylselenophene)/
PC61BM device relative to the regioregular poly(3-hexyl-
thiophene)/PC61BM device, while the peak EQE is significantly
lower for the former. The result was ascribed to the relatively
lower internal photocurrent generation efficiencies and poor
charge transport of the former at the donorꢀacceptor interface.
’ CONCLUSION
In conclusion, we have prepared a series of novel A-D-A
quinquechalcogenophenes for small-molecule organic solar cells.
The co-oligomers/pentamers differ by the number and position
of selenophene units in the π-conjugated backbone. Structures
and purity were fully characterized by NMR, mass, and elemental
analysis. The pentamers were further compared with oligothio-
phene DCV5T, which lacks the selenophene unit. All of the new
co-oligomers exhibit excellent thermal stability. The increase in
the number of selenophene units resulted in a bathochromic shift
in the absorption spectra, thus lowering the optical band gap. A
slight variation in the HOMO/LUMO energy levels determined
from cyclic voltammetry resulted in a lowering of the band gap
from 1.87 eV for DCV5T to 1.77 eV for co-oligomer 3. Organic
BHJ solar cells were fabricated by vacuum processing with these
co-oligomers as electron donor and C60 as the electron acceptor.
Under standard AM1.5G conditions, the highest PCE, ∼3.5%,
was obtained for DCV5T. Despite broader and more intense
absorption, Se-containing co-oligomers unexpectedly showed
slightly lower performance compared to DCV5T in SMOSCs
but still in a good range of 2.5ꢀ3.1%. The PCEs gradually
decrease with increasing selenophene units. Photoluminescence
study of blend layers pointed toward a lower degree of donorꢀ
acceptor phase separation for 1-3 in comparison to DCV5T. This
study has shown the influence of molecular design on the device
performance. The lowering of the optical band gap and HOMO/
LUMO energies of A-D-A-type oligomers by varying the sub-
stitution pattern in the π-conjugated backbone is not the only
criterion to obtain a good solar cell. There is still room for further
optimization by carefully considering the charge-transport prop-
erties, charge separation, and dissociation at the donorꢀacceptor
interface, and significant advances are anticipated in the near
future for the design of this class of p-type semiconductors.
30 Ω squareꢀ1). Layer thicknesses were determined during evaporation
3
by using quartz crystal monitors calibrated for the respective material. The
thin films prepared for absorption and emission measurements are
approximately 30 nm thick. Thin film absorption spectra were recorded
on a Shimadzu UV-2101/3101 UVꢀvis spectrometer. The thin film
emission spectra were recorded with an Edinburgh Instruments FSP920
fluoresence spectrometer. Bulk-heterojunction solar cells were prepared
layer by layer without breaking the vacuum. The layer structure of the
bulk-heterojunction solar cells is as follows: ITO/15 nm C60/20 nm
blend layer of respective pentamer and C60 (ratio 2:1 in volume)
prepared by coevaporation deposited on heated substrate (90 °C)/
5 nm BPAPF/50 nm BPAPF doped with 10 wt % NDP9 (Novaled AG,
Dresden, Germany)/1 nm NDP9/50 nm gold. The dopant material
NDP9 was purchased from Novaled AG, Dresden, Germany, and is
available by directly contacting the company.
Photovoltaic Characterization. JꢀV and EQE measurements were
carried out in a glovebox with nitrogen atmosphere. JꢀV characteristics
were measured on a source-measure unit (Keithley SMU 2400) and
an AM 1.5G sun simulator (KHS Technical Lighting SC1200). The
intensity was monitored with a silicon photodiode (Hamamatsu S1337),
which was calibrated at Fraunhofer ISE. The mismatch between the
spectrum of the sun simulator and the AM 1.5G spectrum was not taken
into account. For well-defined active solar-cell areas, aperture masks
(2.76 mm2) were used. Simple EQE measurements were carried out by
use of the sun simulator in combination with color filters for mono-
chromatic illumination. The illumination intensities were measured with
a silicon reference diode (Hamamatsu S1337).
Materials. Tetrahydrofuran (THF, Merck) was dried under reflux
over sodium/benzophenone (Merck). Dimethylformamide (DMF,
Merck) was dried under reflux over phosphorus pentoxide (Merck).
Dichloromethane (DCM), CHCl3, n-hexane, and diethyl ether were
purchased from Merck and distilled prior to use. All synthetic steps were
carried out under an argon atmosphere (except Knoevenagel con-
densations). Selenophene was purchased from Molekula, HP(tBu)3BF4
from Alfa Aesar, and potassium carbonate from ABCR. n-Butyllithium
’ EXPERIMENTAL SECTION
Physical Measurements and Instrumentation. NMR spectra
were recorded on a Bruker AMX 500 (1H NMR, 500 MHz; 13C NMR,
125 MHz) or an Avance 400 spectrometer (1H NMR, 400 MHz; 13
C
NMR, 100 MHz) at 25 °C unless otherwise noted. Chemical shift values
(δ) are expressed in parts per million with residual solvent protons
(1H NMR, δH = 7.26 for CDCl3; 13C NMR, δC = 77.0 for CDCl3) as
(1.6 N in n-hexane) and Pd2(dba)3 CHCl3 were purchased from Acros.
3
N-Bromosuccinimide (NBS), iodine, mercury(II) acetate, phosphoryl
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Chemistry of Materials
ARTICLE
chloride, and β-alanine were purchased from Merck. Malononitrile, tri-
methyltin chloride, 2-isopropoxy-4,4,5,5-tetramethyl[1,3,2]dioxaborolane
(ITDB), and 2-(tributylstannyl)thiophene 10 were purchased from Aldrich.
5-Bromoselenophene-2-carbaldehyde41 and 3,4,300,400-tetrabutyl-2,20:
50,200-terthiophene42 were synthesized according to literature procedures.
Syntheses. 2,5-Dibromoselenophene (5). In the absence of light,
selenophene 4 (4.02 g, 30.7 mmol) was dissolved in dry DMF (25 mL)
and the solution was degassed. NBS (10.88 g, 61.2 mmol) was added in
four portions within 30 min, and afterward the orange solution was stirred at
room temperature (rt) for 18 h. The reaction mixture was poured into
iceꢀwater (100 mL) and extracted with DCM (3 ꢁ 100 mL). The
combined organic phases were washed with brine (100 mL) and water
(2 ꢁ 100 mL) and dried over Na2SO4. Removal of the solvent yielded
10.1 g of an orange liquid, which was purified by column chromatog-
raphy (silica; n-hexane). The pure product (7.46 g, 25.8 mmol, 84%) was
obtained as a colorless liquid. 1H NMR (400 MHz, CDCl3) δ = 7.00 (s,
2H, H-3,4). 13C NMR (100 MHz, CDCl3) δ = 132.98 (C-3,4), 115.60
(C-2,5). MS (CI) m/z = 290 [M + H+] (calcd for C4H2Br2Se, 288.83).
2,5-Bis(3,4-dibutylthien-2-yl)selenophene (8). In a Schlenk tube and
under argon, 2,5-dibromoselenophene 5 (3.99 g, 13.8 mmol) and 2-(3,4-
dibutylthien-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 7 (10.4 g,
32.3 mmol) were dissolved in degassed THF (40 mL). Pd2(dba)3
(243 mg, 266 μmol, 2 mol %) and HPtBu3BF4 (154 mg, 545 μmol, 4 mol
%) were added, the solution was degassed again and 20 mL (40.0 mmol)
of an aqueous K3PO4 solution (2 M) was added. The solution was
stirred at 50 °C for 22 h. THF was mostly evaporated, 50 mL of water
was added, and the mixture was extracted with DCM (3 ꢁ 50 mL). The
combined organic layers were dried over Na2SO4 and the solvent was
removed in vacuo. The crude product was purified by column chroma-
tography (flash silica, n-hexane) to obtain the pure product 8 (5.86 g,
11.3 mmol, 82%) as an off-white solid, mp 56 °C. 1H NMR (400 MHz,
CDCl3) δ = 7.21 (s with Se satellites, 3JSeꢀH = 5.2 Hz, 2H, SeꢀH), 6.85
(s, 2H, ThꢀH), 2.71 (t, 3J = 7.8 Hz, 4H, α-Bu), 2.54 (t, 3J = 7.8 Hz, 4H,
α-Bu), 1.69 ꢀ 1.62 (m, 4H, β-Bu), 1.58 ꢀ 1.50 (m, 4H, β-Bu),
1.49ꢀ1.40 (m, 8H, γ-Bu), 0.98 (t, 3J = 7.4 Hz, 6H, δ-Bu), 0.96
(t, 3J = 7.4 Hz, 6H, δ-Bu). 13C NMR (100 MHz, CDCl3) δ = 143.61,
141.70, 138.50, 133.26, 127.81, 118.94, 32.76, 31.84, 28.93, 27.64, 23.01,
22.69, 14.02, 13.92. Anal. Calcd for C28H40S2Se: C, 65.71; H, 7.76; S,
12.34. Found: C, 65.91; H, 7.69; S, 12.22.
dropwise during 20 min. The yellow solution was stirred at ꢀ78 °C for
1 h. ITDB (4.47 g, 24.1 mmol) was added dropwise during 20 min and
the turbid mixture was allowed to slowly warm to rt. It became yellow
and clear and was stirred at rt for 16 h. The solvent was removed in
vacuum (not to dryness) and a saturated NH4Cl solution (30 mL) was
added. It was extracted with DCM (3 ꢁ 30 mL), and the combined
organic layers were washed with brine (50 mL) and water (50 mL) and
dried over Na2SO4. Removal of the solvent afforded the crude product as
an orange oil, which quickly crystallized. The resulting yellowish crystals
were dried under high vacuum and 5.60 g (21.8 mmol, 94%) of pure
product was obtained. 1H NMR (400 MHz, CDCl3) δ = 8.37 (dd with
Se satellites, 3J = 5.2 Hz, 4J = 0.8 Hz, 2JHꢀSe = 45.2 Hz, 1H, SeꢀH-5),
7.95 (dd with Se satellites, 3J = 3.6 Hz, 4J = 0.8 Hz, 2JHꢀSe = 13.6 Hz,
3
3
1H, SeꢀH-3), 7.46 (dd, J = 5.2 Hz, J = 3.6 Hz, 1H, SeꢀH-4), 1.35
(s, 12H, ꢀCH3). 13C NMR (100 MHz, CDCl3) δ= 139.56, 137.63, 130.96,
84.09, 24.77. MS (CI) m/z = 258 [M + H+] (calcd for C10H15BO2Se,
257.00). Anal. Calcd for C10H15BO2Se: C, 46.73; H, 5.88. Found: C,
46.66; H, 5.83.
2,5-Bis(3,4-dibutyl-2,20-bithien-5-yl)selenophene (12a). In a Schlenk
tube and under argon, 2,5-bis(3,4-dibutyl-5-iodothien-2-yl)selenophene
9 (1.41 g, 1.84 mmol) and 2-(tributylstannyl)thiophene 10 (2.76 g, 7.18
mmol) were dissolved in dry degassed THF (20 mL). Pd(PPh3)2Cl2
(51.5 mg, 73.5 μmol, 4 mol %) and cesium fluoride (550 mg, 3.62 mmol)
were added, and the solution was degassed and stirred at 75 °C for 90 h.
THF was evaporated, water (50 mL) was added, and the mixture was
extracted with DCM (3 ꢁ 50 mL). The combined organic phases were
washed with water (50 mL) and dried over Na2SO4 and the solvent was
removed. The crude product was purified by column chromatography
(flash silica, hexane/DCM 9:1). The pure product (1.18 g, 1.73 mmol,
94%) was obtained as an orange solid, mp 102 °C. 1H NMR (400 MHz,
CDCl3) δ = 7.32 (d, 3J = 5.2 Hz, 2H, ThꢀH), 7.25 (s, 2H, SeꢀH), 7.15
4
3
4
(d, J = 2.8 Hz, 2H, ThꢀH), 7.07 (dd, J = 5.2 Hz, J = 3.6 Hz, 2H,
ThꢀH), 2.75ꢀ2.68 (m, 8H, α-Bu), 1.63ꢀ1.39 (m, 16H, β-Bu + γ-Bu),
0.98 (t, 3J = 7.2 Hz, 6H, δ-Bu), 0.96 (t, 3J = 7.6 Hz, 6H, δ-Bu). 13C NMR
(100 MHz, CDCl3) δ = 140.93, 140.20, 139.74, 136.10, 132.10, 129.84,
127.90, 127.36, 125.83, 125.31, 33.01, 32.90, 28.11, 27.84, 23.10, 23.00,
13.90, 13.87. MS (MALDI-TOF) m/z [M+] = 684.2 (calcd for
C36H44S4Se, 684.15). Anal. Calcd for C36H44S4Se: C, 63.22; H, 6.48;
S, 18.75. Found: C, 63.34; H, 6.37; S, 18.63.
2,5-Bis(3,4-dibutyl-5-iodothien-2-yl)selenophene (9). 2,5-Bis(3,4-
dibutylthien-2-yl)selenophene 8 (2.89 g, 5.56 mmol) was dissolved in
dry chloroform (100 mL) and the solution was degassed. At 0 °C,
mercury(II) acetate (3.72 g, 11.7 mmol) was added, and the solution was
stirred at 0 °C for 15 min and then overnight at rt. The orange solution
was cooled back to 0 °C and iodine (2.96 g, 11.7 mmol) was added. The
solution became greenish blue; it was stirred at 0 °C for 30 min and then
at rt for 4 h. The resulting solid was filtered off and washed thoroughly
with chloroform, and the filtrate was washed with sodium disulfite
solution (100 mL). The aqueous phase was extracted with chloroform
and DCM. The combined organic layers were dried over Na2SO4 and
the solvent was removed in vacuum. The crude product was purified by
column chromatography (flash silica, n-hexane). The pure product 9
(4.06 g, 5.26 mmol, 95%) was obtained as a yellow solid, mp 96 °C. 1H NMR
(400 MHz, CDCl3) δ = 7.15 (s, 2H, SeꢀH), 2.73 (virtual t, 4H, α-Bu),
2.53 (virtual t, 4H, α-Bu), 1.55ꢀ1.39 (m, 16H, β-Bu + γ-Bu), 0.98 (t, 3J =
7.0 Hz, 6H, δ-Bu), 0.94 (t, 3J = 7.2 Hz, 6H, δ-Bu). 13C NMR (100 MHz,
CDCl3) δ = 147.59, 140.91, 138.39, 138.13, 128.22, 74.29, 33.05, 32.08,
31.05, 28.40, 22.93, 22.86, 13.93, 13.86. MS (MALDI-TOF) m/z [M+] =
772.0 (calcd for C28H38I2S2Se, 771.97). Anal. calcd for C28H38I2S2Se: C,
43.59; H, 4.96; S, 8.31. Found: C, 43.44; H, 4.88; S, 8.44.
2,5-Bis[3,4-dibutyl-5-(selenophen-2-yl)thien-2-yl]selenophene
(12b). 2,5-Bis(3,4-dibutyl-5-iodothien-2-yl)selenophene 9 (1.47 g, 1.90
mmol) and 4,4,5,5-tetramethyl-2-(selenophen-2-yl)-1,3,2-dioxaboro-
lane 11 (1.23 g, 4.79 mmol) were dissolved in dry THF (22 mL) and
the solution was degassed. Pd2(dba)3 CHCl3 (39 mg, 37.7 μmol, 2 mol
3
%) and HPtBu3BF4 (21.8 mg, 75.1 μmol, 4 mol %) were added, the
solution was degassed again and 5.8 mL (11.6 mmol) of a degassed 2 M
aqueous K3PO4 solution were added. The reaction mixture was stirred at
rt overnight. THF was mostly distilled off, the residue was redissolved in
DCM, and water (50 mL) was added. The organic layers were separated
and the aqueous layer was extracted with DCM (2 ꢁ 50 mL). The
combined organic layers were dried over Na2SO4 and the solvent was
removed. The crude product was purified by column chromatography
(flash silica, hexane/DCM 9:1) to obtain product 12b (1.42 g, 1.82
1
mmol, 96%) as an orange solid, mp 110 °C. H NMR (400 MHz,
CDCl3) δ = 8.00 (dd with Se satellites, 3J = 4.4 Hz, 4J = 2.4 Hz, 2JHꢀSe
=
47.0 Hz, 2H, SeꢀH), 7.32ꢀ7.28 (m, 4H, SeꢀH), 7.25 (s with Se
3
satellites, JHꢀSe = 5.2 Hz, 2H, SeꢀH), 2.75ꢀ2.67 (m, 8H, α-Bu),
1.62ꢀ1.41 (m, 16H, β-Bu + γ-Bu), 0.97 (t, 3J = 7.2 Hz, 6H, δ-Bu), 0.96
(t, 3J = 7.2 Hz, 6H, δ-Bu). 13C NMR (100 MHz, CDCl3) δ = 141.01,
140.87, 139.89, 139.83, 132.23, 132.11, 130.95, 129.82, 127.96, 127.90,
33.00, 32.92, 28.16, 27.91, 23.10, 23.02, 13.90, 13.87. MS (MALDI-TOF)
m/z [M+] = 778.1 (calcd for C36H44S2Se3, 778.04). Anal. Calcd for
C36H44S2Se3: C, 55.59; H, 5.70; S, 8.25. Found: C, 55.79; H, 5.52; S, 8.29.
4,4,5,5-Tetramethyl-2-(selenophen-2-yl)-1,3,2-dioxaborolane (11).
Selenophene 4 (3.04 g, 23.2 mmol) was dissolved under argon in dry THF
(30 mL) and the solution was cooled to ꢀ78 °C. At this temperature,
n-butyllithium (1.6 M in hexane, 14.8 mL, 23.7 mmol) was added
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Chemistry of Materials
ARTICLE
50,500-(Selenophene-2,5-diyl)bis(30,40-dibutyl-2,20-bithiophene-5-car-
baldehyde) (13a). DMF (1.29 g, 17.6 mmol) and phosphoryl chloride
(2.70 g, 17.6 mmol) were dissolved under argon in 12 mL of dichloro-
ethane, and the solution was stirred at rt for 2 h. 2,5-Bis(3,4-dibutyl-2,20-
bithien-5-yl)selenophene 12a (600 mg, 0.88 mmol) was dissolved under
argon in dichloroethane (15 mL) and the Vilsmeier reagent was added. The
solution turned from yellow to deep red. It was stirred at rt for 21 h.
Saturated NaHCO3 solution (50 mL) was added, and the mixture was
stirred for 1.5 h at rt. It was extracted with DCM (3 ꢁ 50 mL), the
combined organic layers were washed with water (50 mL) and dried over
Na2SO4, and the solvent was removed. The crude product was purified by
column chromatography (flash silica, traces of monoaldehyde eluted with
DCM, dialdehyde eluted with DCM/EtOAc 95:5). The pure product (578
mg, 0.78 mmol, 89%) was obtained as a red solid, mp 96 °C. 1H NMR (400
MHz, CDCl3) δ = 9.89 (s, 2H, ꢀCHO), 7.71 (d, 3J = 4.0 Hz, 2H, ThꢀH),
7.30 (s, 2H, SeꢀH), 7.25 (d, 3J = 4.0 Hz, 2H, ThꢀH), 2.80ꢀ2.71 (m, 8H,
α-Bu), 1.62ꢀ1.43 (m, 16H, β-Bu + γ-Bu), 0.98 (t, 3J = 7.2 Hz, 12H, δ-Bu).
13C NMR (100 MHz, CDCl3) δ = 182.61, 146.19, 142.68, 142.14, 140.93,
140.54, 136.87, 134.23, 128.94, 128.61, 126.90, 32.92, 32.53, 28.14, 28.04,
23.05, 23.01, 13.86. MS (MALDI-TOF) m/z [M+] = 739.5 (calcd for
C38H44O2S4Se, 740.14). Anal. Calcd for C38H44O2S4Se: C, 61.68; H, 5.99;
S, 17.33. Found: C, 61.79; H, 5.98; S, 17.51.
2,20-{5,50-[5,50-(Selenophene-2,5-diyl)bis(3,4-dibutylthien-5,2-diyl)]bis-
(selenophene-5,2-diyl)}bis(methan-1-yl-1-ylidene)dimalononitrile
(3). 5,50-[5,50-(Selenophene-2,5-diyl)bis(3,4-dibutylthien-5,2-diyl)]-
diselenophene-2-carbaldehyde 13b (236 mg, 0.283 mmol), malononi-
trile (58.2 mg, 0.881 mmol), and β-alanine (2.20 mg, 24.7 μmol) were
dissolved in a dichloroethane/ethanol mixture (20 mL, 1:1 v/v), and the
solution was refluxed for 24 h. The solvent was evaporated and the
residue was purified by column chromatography (flash silica, DCM).
The pure product 3 (208 mg, 0.224 mmol, 79%) was obtained as a green
solid with a metallic gleam, mp 215 °C. 1H NMR (400 MHz, CDCl3)
7.84 (s, 2H, DCVꢀH), 7.82 (d, 3J = 4.4 Hz, 2H, SeꢀH), 7.43 (d, 3J = 4.4
Hz, 2H, SeꢀH), 7.35 (s, 2H, SeꢀH), 2.81ꢀ2.73 (m, 8H, α-Bu), 1.63ꢀ
1.44 (m, 16H, β-Bu + γ-Bu), 1.00 (t, 3J = 7.2 Hz, 6H, δ-Bu), 0.99 (t, 3J =
7.2 Hz, 6H, δ-Bu). 13C NMR (100 MHz, CDCl3) δ = 154.59, 152.97,
143.98, 143.40, 141.19, 140.99, 138.99, 135.72, 131.36, 129.03, 127.29,
114.39, 114.02, 75.50, 32.86, 32.33, 28.60, 28.11, 23.05, 13.87, 13.80. MS
(MALDI-TOF) m/z [M+] = 930.5 (calcd for C44H44N4S2Se3, 930.05).
Anal. Calcd for C44H44N4S2Se3: C, 56.83; H, 4.77; N, 6.03. Found: C,
56.94; H, 4.77; N, 5.97.
5,500-Bis(trimethylstannyl)-3,4,300,400-tetrabutyl-2,20:50,200-terthiophene
(17). 3,4,300,400-Tetrabutyl-2,20:50,200-terthiophene 16 (2.00 g, 4.23 mmol)
was dissolved under argon in absolute THF (12 mL) and absolute hexane
(7 mL), and TMEDA was added. The solution was stirred at rt for 30 min.
At ꢀ78 °C, n-BuLi (1.6 M in hexane, 6.25 mL, 10.0 mmol) was added
dropwise within 30 min; the solution was warmed to rt and then stirred
at 50 °C for 25 min. A solution of trimethyltin chloride (2.00 g, 10.0
mmol) in absolute THF (7 mL) was added at ꢀ78 °C, and the mixture
was stirred at rt for 3 days. Saturated NH4Cl solution (120 mL) was
added and the mixture was extracted with diethyl ether (3 ꢁ 150 mL);
the combined organic layers were dried over MgSO4, and the solvent
was removed. After drying in high vacuum, product 17 (2.82 g, 3.50
mmol, 83%) was obtained as an orange oil. 1H NMR (400 MHz, CDCl3)
δ = 7.03 (s, 2H, ThꢀH), 2.73 (t, 4H, 3J = 7.9 Hz, α-Bu), 2.55 (t, 4H, 3J =
7.5 Hz, α-Bu), 1.58ꢀ1.38 (m, 16H, β-Bu + γ-Bu), 0.97 (t, 6H, 3J = 7.2
Hz, δ-Bu), 0.97 (t, 6H, 3J = 7.2 Hz, δ-Bu), 0.39 (s, 18H, Me). 13C NMR
(100 MHz, CDCl3) δ = 151.29, 139.86, 136.79, 136.52, 125.45, 34.96,
33.02, 31.51, 27.73, 23.19, 23.11, 13.99, 13.89, 7.81. The product was
used without further purification.
5,50-[5,50-(Selenophene-2,5-diyl)bis(3,4-dibutylthien-5,2-diyl)]dis-
elenophene-2-carbaldehyde (13b). DMF (945 mg, 12.9 mmol) and
phosphoryl chloride (1.99 g, 13.0 mmol) were dissolved under argon in
10 mL of dichloroethane, and the solution was stirred at rt for 2 h. 2,5-
Bis[3,4-dibutyl-5-(selenophen-2-yl)thien-2-yl]selenophene 12b (502 mg,
0.65 mmol) was dissolved under argon in 15 mL of dichloroethane and
the Vilsmeier reagent was added. The solution turned from yellow to
deep red. It was stirred at rt for 22 h. Saturated NaHCO3 solution
(50 mL) was added and the mixture was stirred for 2 h at rt. It was
extracted with DCM (3 ꢁ 50 mL), the combined organic layers were
washed with water (50 mL) and dried over Na2SO4, and the solvent was
removed. The crude product was purified by column chromatography
(flash silica, traces of monoaldehyde eluted with DCM, dialdehyde
eluted with DCM/EtOAc 95:5). The pure product 13b (507 mg, 0.61
1
mmol, 94%) was obtained as a red solid, mp 107 °C. H NMR (400
3
MHz, CDCl3) δ = 9.78 (s with Se satellites, JHꢀSe = 4.4 Hz, 2H,
3
3
ꢀCHO), 7.94 (d, J = 4.2 Hz, 2H, SeꢀH), 7.43 (d, J = 4.2 Hz, 2H,
SeꢀH), 7.31 (s, 2H, SeꢀH), 2.78ꢀ2.71 (m, α-Bu), 1.63ꢀ1.43 (m, 16H,
β-Bu + γ-Bu), 0.98 (t, 3J = 7.0 Hz, 12H, δ-Bu). 13C NMR (100 MHz,
CDCl3) δ = 183.98, 151.11, 148.21, 142.50, 140.90, 140.65, 139.83,
134.22, 131.66, 128.62, 128.03, 32.93, 32.57, 28.31, 28.10, 23.08, 23.06,
13.87, 13.86. MS (MALDI-TOF) m/z [M+] = 834.5 (calcd for
C38H44O2S2Se3, 834.03). Anal. Calcd for C38H44O2S2Se3: C, 54.74;
H, 5.32; S, 7.69. Found: C, 54.81; H, 5.32; S, 7.78.
2-[(5-Bromoselenophen-2-yl)methylene]malononitrile (15). 5-Bro-
moselenophene-2-carbaldehyde 14 (2.15 g, 9.04 mmol), malononitrile
(1.50 g, 22.7 mmol), and β-alanine (53.0 mg, 0.59 mmol) were dissolved
in ethanol (30 mL). The solution was stirred at 60 °C for 15 h and then
cooled to rt. The resulting solid was filtered off, washed with ethanol, and
dried in high vacuum to obtain the pure product (2.16 mg, 7.56 mmol,
84%) as yellow crystals, mp 183 °C. 1H NMR (400 MHz, CDCl3) δ =
7.83 (d with Se satellites, 4J = 0.6 Hz, 3JSeꢀH = 8.1 Hz, DCVꢀH), 7.59
(dd, 3J = 4.3 Hz, 4J = 0.6 Hz, SeꢀH-3), 7.43 (d, 3J = 4.3 Hz, SeꢀH-4).
13C NMR (100 MHz, CDCl3) 153.23, 142.45, 141.94, 134.11, 131.93,
113.49, 113.39, 78.44. MS (CI) m/z = 286 [M+] (calcd for C8H3BrN2Se,
285.99). Anal. Calcd for C8H3BrN2Se: C, 33.60; H, 1.06; N, 9.80. Found: C,
33.75; H, 1.13; N, 10.00.
2,20-[50,500-(Selenophene-2,5-diyl)bis(30,40-dibutyl-2,20-bithien-50,5-
diyl)]bis(methan-1-yl-1-ylidene)dimalononitrile (1). 50,500-(Selenophene-2,
5-diyl)bis(30,40-dibutyl-2,20-bithiophene-5-carbaldehyde) 13a (275 mg,
0.372 mmol), malononitrile (73.4 mg, 1.11 mmol), and β-alanine (4.00 mg,
44.9 μmol) were dissolved in a dichloroethane/ethanol mixture (25 mL,
1:1 v/v), and the solution was refluxed for 24 h. The solvent was
evaporated and the residue was purified by column chromatography
(flash silica, DCM). The pure product 1 (279 mg, 0.334 mmol, 90%) was
obtained as a green solid with a metallic gleam, mp 211 °C. 1H NMR
(400 MHz, CDCl3) δ = 7.77 (s, 2H, DCVꢀH), 7.69 (d, 3J = 4.4 Hz, 2H,
ThꢀH), 7.34 (s, 2H, SeꢀH), 7.29 (d, 3J = 4.4 Hz, 2H, ThꢀH),
2.84ꢀ2.79 (m, 4H, α-Bu), 2.77ꢀ2.72 (m, 4H, α-Bu), 1.63ꢀ1.44 (m,
16H, β-Bu + γ-Bu), 1.00 (t, 3J = 7.2 Hz, 6H, δ-Bu), 0.99 (t, 3J = 6.8 Hz,
6H, δ-Bu). 13C NMR (100 MHz, CDCl3) δ = 149.89, 148.35, 144.11,
139.17, 133.96, 129.00, 128.60, 126.14, 75.80, 32.87, 32.41, 28.05, 23.04,
23.01, 13.86. MS (MALDI-TOF) m/z [M+] = 836.5 (calcd for
C44H44N4S4Se, 836.16). Anal. Calcd for C44H44N4S4Se: C, 63.21; H,
5.30; N, 6.70. Found: C, 63.15; H, 5.15; N, 6.78.
5,500-Bis[5-(2,2-dicyanovinylene)selenophen-2,5-diyl]-3,4,300,400-tet-
rabutyl-[2,20:50,200]terthiophene (2). In a Schlenk tube, DCV-substituted
bromoselenophene 15 (322 mg, 1.13 mmol) and distannylterthiophene
17 (300 mg, 0.376 mmol) were dissolved under argon in dry DMF
(4 mL), and the mixture was degassed. Pd(PPh3)4 (4 mg, 3 μmol) was
added, and the mixture was degassed again and then stirred at 75 °C for 3
days. Water (30 mL) was added and the mixture was extracted with
DCM (30 mL). The organic layer was dried over Na2SO4, and the
solvent was removed in vacuo. The crude product was purified by
column chromatography (silica, DCM/n-hexane 4:1). The pure product
2 (223 mg, 0.253 mmol, 67%) was obtained as a green solid, mp 213 °C.
1H NMR (CDCl3, 400 MHz) δ = 7.84 (s with Se satellites, 3JSeꢀH = 9.0
Hz, 2H, DCVꢀH), 7.82 (d, 3J = 4.5 Hz, 2H, SeꢀH), 7.43 (d, 3J = 4.5 Hz,
4443
dx.doi.org/10.1021/cm201392c |Chem. Mater. 2011, 23, 4435–4444

Chemistry of Materials
ARTICLE
2H, SeꢀH), 7.19 (s, 2H, ThꢀH), 2.83ꢀ2.73 (m, 8H, α-Bu), 1.63ꢀ1.55
(m, 8H, β-Bu), 1.54ꢀ1.45 (m, 8H, γ-Bu), 1.00 (t, 3J = 7.5 Hz, 6H, δ-
Bu), 0.99 (t, 3J = 7.5 Hz, 6H, δ-Bu). 13C NMR (CDCl3, 125 MHz, 320
K) δ = 154.61, 152.79, 143.82, 142.99, 141.60, 138.94, 136.17, 133.36,
131.51, 127.55, 127.10, 114.25, 113.92, 76.13, 32.79, 32.45, 28.59, 28.01,
23.04, 22.99, 13.78, 13.73. (MALDI-TOF) m/z [M+] = 884.2 (calcd for
C44H44N4S3Se2, 884.1). Anal. Calcd for C44H44N4S3Se2: C, 59.85; H,
5.02; N, 6.35. Found: C, 59.97; H, 5.14; N, 6.47.
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(21) Mishra, A.; Uhrich, C.; Reinold, E.; Pfeiffer, M.; B€auerle, P. Adv.
Energy Mater. 2011, 1, 265–273.
’ ASSOCIATED CONTENT
(22) Steinberger, S.; Mishra, A.; Reinold, E.; Levichkov, J.; Uhrich,
C.; Pfeiffer, M.; B€auerle, P. Chem. Commun. 2011, 47, 1982–1984.
(23) Steinberger, S.; Mishra, A.; Reinold, E.; M€uller, C. M.; Uhrich,
C.; Pfeiffer, M.; B€auerle, P. Org. Lett. 2011, 13, 90–93.
(24) Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.;
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S
Supporting Information. Ten figures, showing plots of
b
cyclic voltammograms on thin films and H NMR, 13C NMR,
1
and MALDI-TOF mass spectra of oligomers 1ꢀ3. This material
is available free of charge via the Internet at http://pubs.acs.org/.
(25) Scharber, M. C.; M€uhlbacher, D.; Koppe, M.; Denk, P.;
Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789–794.
(26) Schulze, K.; Riede, M.; Brier, E.; Reinold, E.; B€auerle, P.; Leo, K.
J. Appl. Phys. 2008, 104, No. 074511.
(27) Mishra, A.; Ma, C.-Q.; B€auerle, P. Chem. Rev. 2009, 109,
1141–1276.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail amaresh.mishra@uni-ulm.de (A.M.) or peter.baeuerle@
uni-ulm.de (P.B.).
(28) Nakanishi, H.; Inoue, S.; Otsubo, T. Mol. Cryst. Liq. Cryst. 1997,
296, 335–348.
’ ACKNOWLEDGMENT
(29) Heeney, M.; Zhang, W.; Crouch, D. J.; Chabinyc, M. L.;
Gordeyev, S.; Hamilton, R.; Higgins, S. J.; McCulloch, I.; Skabara,
P. J.; Sparrowe, D.; Tierney, S. Chem. Commun. 2007, 5061–5063.
(30) Patra, A.; Wijsboom, Y. H.; Zade, S. S.; Li, M.; Sheynin, Y.;
Leitus, G.; Bendikov, M. J. Am. Chem. Soc. 2008, 130, 6734–6736.
(31) Zade, S. S.; Zamoshchik, N.; Bendikov, M. Chem.—Eur. J. 2009,
15, 8613–8624.
(32) Hollinger, J.; Jahnke, A. A.; Coombs, N.; Seferos, D. S. J. Am.
Chem. Soc. 2010, 132, 8546–8547.
(33) Zade, S. S.; Zamoshchik, N.; Bendikov, M. Acc. Chem. Res. 2011,
44, 14–24.
We thank the German Ministry of Education and Research
(BMBF) for financial support in the framework of Project
OPEG 2010.
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