Dicyanovinyl-substituted oligothiophenes: Structure-property relationships and application in vacuum-processed small molecule organic solar cells

Article abstract of DOI:10.1002/adfm.201001639

Efficient synthesis of a series of terminally dicyanovinyl (DCV)-substituted oligothiophenes, DCVnT 1-6, without solubilizing side chains synthesized via a novel convergent approach and their application as electron donors in vacuum-processed m-i-p-type planar and p-i-n-type bulk heterojunction organic solar cells is described. Purification of the products via gradient sublimation yields thermally highly stable organic semiconducting materials in single crystalline quality which allows for X-ray structure analysis. Important insights into the packing features and intermolecular interactions of these promising solar cell materials are provided. Optical absorption spectra and electrochemical properties of the oligomers are investigated and valuable structure-property relationships deduced. Photovoltaic devices incorporating DCVnTs 4-6 showed power conversion efficiencies up to 2.8% for planar and 5.2% for bulk heterojunction organic solar cells under full sun illumination (mismatch corrected simulated AM 1.5G sunlight). The 5.2% efficiency shown here represents one of the highest values ever reported for organic vacuum-deposited single heterojunction solar cells.

Full text of DOI:10.1002/adfm.201001639

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Dicyanovinyl-Substituted Oligothiophenes: Structure–
Property Relationships and Application in Vacuum-
Processed Small-Molecule Organic Solar Cells
Roland Fitzner, Egon Reinold, Amaresh Mishra, Elena Mena-Osteritz,
Hannah Ziehlke, Christian Körner, Karl Leo, Moritz Riede, Matthias Weil,
Olga Tsaryova, André Weiß, Christian Uhrich, Martin Pfeiffer, and Peter Bäuerle*
production on flexible substrates.[1–9] The
field started by utilization of organic small
molecules[2,10–14] and later on by using func-
tional semiconducting polymers,[5,15–20]
resulting in remarkable improvement in
power conversion efficiencies over the
decade from about 1% to over 7%. To date,
functionalized oligo- and polythiophenes
are among the best materials for applica-
tion in organic photovoltaics.[16,21]
The most frequently used material for
small molecule organic solar cells (SMOSC)
prepared by vacuum-evaporation processes
are represented by metal phthalocyanines
(Pc) as electron donor (D) and fullerene
C60 as acceptor (A) with reported efficien-
cies of 5% for a single cell.[22] However,
for outdoor applications, the performance
of these solar cells should be improved in
terms of efficiency along with stability and
costs. One aspect of improving efficiency
is the use of suitable electron donors and
Efficient synthesis of a series of terminally dicyanovinyl (DCV)-substituted
oligothiophenes, DCVnT 1–6, without solubilizing side chains synthesized
via a novel convergent approach and their application as electron donors
in vacuum-processed m-i-p-type planar and p-i-n-type bulk heterojunction
organic solar cells is described. Purification of the products via gradient sub-
limation yields thermally highly stable organic semiconducting materials in
single crystalline quality which allows for X-ray structure analysis. Important
insights into the packing features and intermolecular interactions of these
promising solar cell materials are provided. Optical absorption spectra and
electrochemical properties of the oligomers are investigated and valuable
structure–property relationships deduced. Photovoltaic devices incorporating
DCVnTs 4–6 showed power conversion efficiencies up to 2.8% for planar
and 5.2% for bulk heterojunction organic solar cells under full sun illumina-
tion (mismatch corrected simulated AM 1.5G sunlight). The 5.2% efficiency
shown here represents one of the highest values ever reported for organic
vacuum-deposited single heterojunction solar cells.
acceptors with optimized energy levels to minimize energetic
losses during exciton separation. In this respect, various D-A
and A-A substituted oligothiophenes have been prepared and
tested in SMOSCs.[23–30] We have demonstrated that a photo-
voltage (VOC) of as high as 1.0 V can be obtained by replacing
the frequently used metal Pcs by terminally dicyanovinyl (DCV)-
substituted oligothiophenes bearing butyl side chains (DCV5T-
Bu) for solubility reasons as donor when using fullerene C60 as
acceptor. These solar cells reached power conversion efficien-
cies of up to 3.4%.[24,25,31,35] The high VOC obtained is due to a
high ionization potential (IP) of DCV5T-Bu (IP = –5.6 eV). The
effect of the alkyl chain length, attached at the β-position of the
conjugated backbone on device performance within the series
of DCV-functionalized quinquethiophenes was also established.
Solar cells prepared from these derivatives revealed that the
butyl-substituted oligomers showed higher efficiencies (η =
3.4%) than the ethyl-substituted DCVnT-Et (η = 2.5–3.0%).[33]
The reduced efficiency was mainly due to the reduced fill factor
(FF). The difference in performance was explained by the dif-
ference in packing, by differences in hole-injection between the
hole-transport layer and the oligothiophene as well as a differ-
ence in hole mobility of the oligothiophene derivatives.
1. Introduction
Organic solar cells (OSC) have attracted enormous interest
in recent years, because of their potential towards low-cost
R. Fitzner, Dr. E. Reinold, Dr. A. Mishra, Dr. E. Mena-Osteritz,
Prof. P. Bäuerle
Institute of Organic Chemistry II and Advanced Materials
University of Ulm
Albert-Einstein-Allee 11, 89081 Ulm, Germany
E-mail: peter.baeuerle@uni-ulm.de
H. Ziehlke, C. Körner, Prof. K. Leo, Dr. M. Riede
Institute of Applied Photophysics
TU Dresden, Mommsenstraße 13, 01062 Dresden, Germany
Dr. O. Tsaryova, Dr. A. Weiß, Dr. C. Uhrich, Dr. M. Pfeiffer
heliatek GmbH
Treidlerstr. 3, 01139 Dresden, Germany
Dr. M. Weil
Institut für Chemische Technologien und Analytik
Abteilung Strukturchemie
Technische Universität Wien
Getreidemarkt 9/164-SC, 1060 Vienna, Austria
DOI: 10.1002/adfm.201001639
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Photovoltaic properties of DCVnT-Bu
(n = 4, 5 and 6) with various lengths of the
oligothiophene unit were also investigated.
The VOC decreased from 1.13 V for DCV4T-
Bu to 1.0 V for DCV5T-Bu and to 0.93 V for
DCV6T-Bu, which is ascribed to the decrease
in their IP and an increase in energy of the
highest occupied molecular orbital (HOMO).
Scheme 2. Convergent synthesis of dicyanovinyl-substituted oligothiophenes DCVnT 3–6.
On the other hand, no clear trend was observed for short-circuit
current density (JSC) and fill factor (FF) values. The power con-
version efficiency of DCV4T-Bu was lower compared to DCV5T-
Bu because of its lower JSC and FF, while the small differences
in JSC and FF values between DCV5T-Bu and DCV6T-Bu do not
represent a general trend.[34]
One advantage of the vacuum-processable SMOSCs is that
tandem cells can easily be fabricated. Very recently, by com-
bining in a tandem device two bulk heterojunctions of C60 in
blend layers with an optimized DCVnT-derivative and a novel
pigment with absorption range between 600 and 750 nm devel-
oped by BASF, respectively, a certified efficiency of 6.1% on
2 cm2 active area[36] and shortly after a certified record efficiency
of 8.3% on 1.1 cm2 device have been released.[37]
Herein, we investigate structure–property relationships in a
series of DCVnTs with conjugated chain lengths of one to six
thiophene units (n = 1–6) and without solubilizing alkyl side
chains, well being aware that synthesis and characterization
of longer derivatives might be problematic due to low solu-
bility. However, we expect that these oligomers may show good
packing and thermal stability which might influence the solar
cell performance.
decrease in solubility, reaction of quaterthiophene-dialdehyde
10 resulted in an inseparable mixture of target DCV4T 4 and
the singly reacted intermediate with a DCV-group at one chain
end and an unreacted formyl group at the other.
2.2. Convergent Syntheses
Due to solubility problems of the longer homologues, only
a series up to the trimer could be reliably synthesized by the
usual linear synthetic approach. Thus, we developed a conver-
gent route to DCV-capped oligothiophenes DCV3T 3 to DCV6T
6 (Scheme 2). Terminal building blocks, DCV-substituted bro-
mothiophene 11[47] and bithiophene 12[48] were reacted with
central components, distannylated mono- and bithiophene 13
and 14,[49–51] in a Stille-type coupling under Pd(PPh3)4-catalysis
in DMF as solvent. Thus, in a [1T+1T+1T]-coupling DCV3T 3
was obtained in 99% yield, DCV4T 4 in 97% by a [1T+2T+1T]-,
DCV5T 5 in 94% by a [2T+1T+2T]-, and finally DCV6T 6 in
82% by a [2T+2T+2T]-route. THF or toluene as solvent resulted
in lower yields and lower purities of the DCVnT raw mate-
rials. The advantage of this method was that the intermediates
coming from the first Stille coupling showed sufficient solu-
bility so that the second coupling worked, whereby the target
DCVnTs precipitated with already high purity. After washing
and extracting with various solvents, the materials were finally
purified by gradient vacuum sublimation.
2. Synthesis of Dicyanovinyl-Functionalized
Oligothiophenes DCVnT
2.1. Linear Syntheses
3. Thermal Properties
Synthesis of the smaller members of the series, DCV1T-DCV3T
1–3, were already described in literature and were prepared
for non-linear optical properties.[38,39] Based on our experience
with alkylated DCVnTs, we firstly synthesized DCV1T-DCV4T
1–4 by a linear approach (Scheme 1). Thus, bi-, ter-, and quater-
thiophene dialdehydes 8–10,[40–43] which were prepared by
Vilsmeier-Haack formylation of the parent oligothiophenes[44–46]
were reacted with malononitrile and ß-alanine as catalyst to
form target DCVnTs 2 and 3 (n = 2, 3) in nearly quantitative
yield and in high purity. DCV1T 1 was obtained from commer-
cially available thiophene-2,5-dicarbaldehyde 7 and malononi-
trile without the use of catalyst. However, due to a dramatic
In order to obtain crystalline material of high purity for structure
determination by XRD methods, gradient vacuum sublimation
(p < 10−5 mbar) was performed for DCVnTs 2–6. Derivatives 2–4
sublimated almost quantitatively at sublimation temperatures
well below their melting points and without significant decom-
position. For DCV5T 5 and DCV6T 6 sublimation temperatures
in the range of the melting points were necessary in order to
obtain sublimation rates allowing for 100 mg scale purification.
Due to material degradation, the sublimation yields for com-
pounds 5 and 6 were lower with around 50%.
The melting points were determined using Differential Scan-
ning Calorimetry (DSC) and used to evaluate the purity of target
compounds 1–6. Purification was repeated until the melting
points remained unchanged for two consecutive purification
steps (Soxhlett extraction, gradient vacuum sublimation). As an
example, analytical data for DCV4T 4 measured after each puri-
fication step are shown in Table 1. The first sublimation of the
raw product, which was filtered off from the reaction mixture
and washed with various solvents, did not yield a significant
improvement in material purity according to elemental analysis.
CN
NC
CH2(CN)2
-alanine
NC
CN
OHC
7-10 (n=1-4)
CHO
S
S
n
n
1-4 (n=1-4)
Scheme 1. Linear synthesis of dicyanovinyl-substituted oligothiophenes
DCVnT 1–4.
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T a b le 1 . Analytical data for DCV4T 4 measured after each subsequent
purification step.
whereas in the case of the odd-numbered oligomers a strong
dipole moment up to 6.5 D was calculated.
Elemental analysis
Melting point
C
H
N
(onset DSC) [°C]
4. Structure Analysis
Calculated
59.73
59.11
59.23
59.60
59.67
2.09
2.38
2.11
2.07
2.04
11.61
11.71
11.36
11.46
11.59
Platelet-like crystals of DCV4T were obtained after a gradient
sublimation process. The dark-red crystals have a metallic
luster and grew larger than 4 mm while the thickness typically
remained far under 0.1 mm. Figure 2a shows a cross-section of
the sublimation tube containing the compound and Figure 2b
an optical micrograph of a crystal. Changes on the crystal ori-
entation induced different behaviors against the absorption of
polarized light (Figure 2b, white arrow): the crystal showed a
stronger light absorption when the long axis is oriented parallel
(shining red-purple) than perpendicular to the polarized light.
Because the transition dipole responsible for the light absorp-
tion in DCV4T is oriented parallel to the long molecular axis,
we conclude that the oligothiophene backbone lies, within 30°
(see Figure 3), parallel to the longer axis of the crystal.
Washed product
1. Sublimation
2. Sublimation
3. Sublimation
317.6
318.1
319.9
319.6
The second sublimation yielded analytically very pure material,
which showed a considerable increase in melting point. A third
sublimation did not significantly change the melting point.
The melting points of compounds 2–6 showed an odd-even-
effect with respect to the number of thiophene units present
in the oligothiophene backbone (Figure 1). The odd-numbered
compounds DCVnT 3 and 5 melted at significantly lower tem-
peratures than the shorter homologues 2 and 4. The melting
point of compound 5 (287 °C) was about 20 °C higher than that
of 3 (265 °C) and the same trend was seen for compounds 4
(320 °C) and 2 (300 °C). However, DCV6T 6 (319 °C) did not
show the expected further increase in melting point with respect
to DCV4T 4. These results may originate from the difference in
symmetry of odd- and even-numbered DCVnTs. Semi-empirical
Austin Model 1 (AM1) geometry optimizations under restricted
Hartree Fock conditions were carried out for all DCVnTs (vide
infra, Section 5). All calculations yielded structures wherein the
terminal DCV acceptor units were arranged such that one of the
cyano carbon atoms was in close proximity to the sulfur atom
of the neighboring thiophene unit (S1…C23 and S2…C20). The
sulfur-carbon interaction locks the DCV-group into one of the
two possible planar configurations with respect to the oligothi-
ophene core. This finding is further confirmed by the single
crystal structure analysis of DCV4T 4 (vide infra). Hence, odd-
numbered oligomers belong to the C2v symmetry point-group
comprising a mirror plane perpendicular to the molecular axis,
whereas the even-numbered homologues have a C2h symmetry.
Consequently, the latter ones do not include a dipole moment,
The crystals belong to the monoclinic space group P21/n,
with 4 molecules in the unit cell, whose parameters are: a =
3.8295(8), b = 27.695(6), c = 22.663(5) Å, β = 91.68(3)°. In
Figure 3 the molecular structure of DCV4T in the crystal
a)
b)
400
even numbered:
C
symmetry
2h
350
300
250
200
odd numbered:
C
symmetry
2v
2
3
4
5 6
Number of thiophene units
Figure 2. Picture of a sectioned sublimation tube containing DCV4T crys-
tals (a) and photograph of a single crystal of DCV4T under the polarized
light microscope (b).
Figure 1. Melting points for DCVnTs 2–6 (determined from the onset of
the melting peak in DSC plots).
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adjacent thiophenes nor typical rotational
disorder (syn/anti conformations).[54,55]
The unit cell containing four molecules
dispose in two centrosymmetric pairs of two
antiparallelly arranged molecules (Figure 4) .
In the crystal, molecules extend in layers
parallel to the (9 3 21) plane, leaving space
for channels parallel to [100] that represent
approximatly 17% of the overall volume. In
these channels, disordered molecules are
situated (for treatment of the disorder, see
experimental section). The analysis of the
intermolecular short contacts (atom dis-
tances below the van der Waals radii) (Table 3
and Figure 3b) reveals some interesting fea-
tures: firstly, odd short distances between
the hydrogen atoms H2-H11 are observed
(Figure 3b). Moreover, relevant weak non-
classical C-H…N hydrogen bonding inter-
actions between the hydrogen atoms (H7,
H14, H17, and H21) and all nitrogen atoms
lead to interatomic distances below the van
der Waals radii (Figure 2b and Table 3). The
interactions occur with all the neighboring
molecules, consolidating in this way the
crystal packing.[56]
DCV4T does not exhibit a herringbone
structure in the crystal, like observed for
other β-unsubstituted thiophene-based
compounds.[52,53,57] The π-conjugated back-
bone of the molecules in a column are
oriented parallel to one another, as it is
seen in the view from the [101] direction
(Figure 4b). The intermolecular distances,
“d”, are as close as 3.514 Å, indicating a
compact packing and π−π stacking of the
Figure 3. Molecular structure of DCV4T (front and side view) (a). Atomic numbering scheme and
thiophene ring labels (b). Inter- and intramolecular short contact distances are depicted by red
and grey lines, respectively. Torsion angles between adjacent thiophene rings: A–B 176.8°, B–C
176.7°, C–D 178.7°, N4–A 6.4°, D-N1 7.8°.
thiophene backbones. In the perpendic-
ular direction, the molecules (Figure 4a)
are lying in approximate coplanarity with
a small offset between neighbouring mole
structure is depicted. The thiophene units show all trans con-
formation while the DCV groups are in an all cis conformation
relative to the sulfur atoms of the terminal thiophene units.
The molecule evidences an almost planar conformation with
dihedral angles between the thiophenes smaller than 4°. Con-
sidering all atoms in the molecule, the RMS deviation from
planarity is 0.101 Å, with the N atoms of the DCV groups exhib-
iting the largest deviation of up to 0.3 Å. Slight differences
between the terminal DCV groups and the individual C–S bond
lengths (detailed information ESI or CCDC) origin the deviation
of DCV4T from a centrosymmetric molecule. The bond lengths
of the oligothiophene moiety are summarized in Table 2 and the
inter- and intramolecular non-bonding atom distances shorter
than the sum of the corresponding van der Waals radii are col-
lected in Table 3. The values for the thiophenic C–C double
bonds (1.379–1.394 Å), C–C single bonds (1.401–1.412 Å) and
the C–C interring bonds (1.445 Å) are comparable with those
of other oligothiophenes.[52,53] In contrast to several thiophene
compounds, DCV4T showed neither syn conformations of
cules (Δ = 1.206 Å; Figure 4b). The parallel ordering of the π-
conjugated system in DCV4T thus may arise from weak non-
classical hydrogen bonding interactions caused by the terminal
nitrile residues, which act as supramolecular synthons pre-
venting the herringbone arrangement.[56]
Figure 5 shows the overlap of the molecules viewed from the
direction normal to the (9 3 21) plane. The aromatic backbones
are almost completely overlapped, showing small slipped offsets
of 1.3 Å in both, the long molecular axis and the perpendicular
directions. Thus, the coplanarity of the thiophene rings together
with a highly efficient overlap between the molecules of DCV4T
in the crystal, shown by close π−π stacking distances and very
small offsets, evidence the conditions for favorable charge car-
rier transport in DCV4T. From X-ray powder diffraction data on
intensive pestle samples of the DCVnTs can be deduced that
all the compounds indeed exhibit a layered structure with a 2θ
parameter corresponding with distances between 3.2 and 3.9 Å,
which indicates π−π stacking of the molecules in accordance
with the X-ray structure data of DCV4T analyzed in this study.
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T a b le 2 . Bond lengths [Å] in the oligothiophene moiety in DCV4T single
crystals.
N1
C23
N2
C24
C22
C21
C4
C23
C22
C24
C22
C21
C4
1.146(3)
1.436(2)
1.151(3)
1.443(2)
1.366(2)
1.432(2)
1.739(1)
1.390(2)
1.402(2)
1.385(2)
1.730(1)
1.447(2)
1.736(1)
1.377(2)
1.411(2)
1.381(2)
1.734(1)
1.450(2)
C9
S3
C10
C11
C12
S3
1.727(1)
1.384(2)
1.411(2)
1.379(2)
1.735(1)
1.448(2)
1.732(1)
1.387(2)
1.400(2)
1.389(2)
1.745(2)
1.426(2)
1.361(2)
1.440(2)
1.435(2)
1.151(2)
1.149(2)
C9
C10
C11
C12
C12
C13
C13
C14
C15
C16
C16
C17
C18
C18
C19
C20
C13
S4
S1
C4
C3
C14
C15
C16
S4
C3
C2
C2
C1
C1
S1
C1
C5
C17
C18
C19
C20
N3
C5
S2
C5
C6
C6
C7
C7
C8
C8
S2
N4
C8
C9
T a b le 3 . Short inter- (left) and intra- (middle and right) molecular con-
tacts [Å], below the van der Waals radii, in DCV4T single crystals.
C23…C24a)
H2…H11b)
N1…H14c)
N2…H7d)
N3…H17e)
N4…H2b)
C4…C23
S1…C23
S1…C6
S2…C2
S2…C10
S3…C7
S3…C14
S4…C11
S4…C20
S4…N4
3.326
2.367
2.678
2.587
2.481
2.487
3.076
3.078
3.215
3.206
3.224
3.210
3.198
3.227
3.110
3.347
3.072
Figure 4. Molecular packing of DCV4T in the single crystal viewed from
the direction normal to the (9 3 21) plane (a). View from the [101] direc-
tion showing the π−π stacking ([d] of the molecules (b).
C16…C20
Symmetry codes: a) −1 + x, y, z; b) 1 - x, 1 - y, 1 - z; c) −1/2 + x, 1/2 – y, 1/2 + z;
d) −1.5 + x, 1/2 – y, 1/2 + z; e) 4 – x, 1 – y, – z.
5. Optical and Redox Properties
5.1. Steady-State Spectroscopy
One of our objectives was to investigate optical and redox prop-
erties of the DCVnTs 1–6, which are important parameters for
their application in organic solar cells. Absorption and emis-
sion spectra of 1–6 were measured in dichloromethane and are
summarized in Table 4. Figure 6 presents the UV–vis and emis-
sion spectra of DCVnTs 1–5. The molar extinction coefficient of
DCV6T 6 could not be determined due to its limited solubility
in dichloromethane.
Compared to non-functionalized oligothiophenes, lowering of
the lowest unoccupied molecular orbital (LUMO) was achieved
by introduction of electron-withdrawing DCV end groups. This
clearly results in strong absorptions in the visible region. As a
general trend, strong bathochromic shifts of the absorption
maximum assigned to the π−π∗ (HOMO→LUMO) transition of
Figure 5. Overlap of molecules of DCV4T viewed from the direction
normal to the (9 3 21) plane. The molecules in adjacent layers are drawn
in gray gradation.
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T a b le 4 . Spectroscopic data for DCVnTs 1–6. The values in brackets represent shoulders.
λmaxabs [nm]
solution
ε
max [L mol−1 cm−1
solution
]
λmaxem [nm]
solution
λmaxabs [nm]
α
max [105 cm−1
filmb)
]
λmaxem [nm]
DCVnT
Stokes shift
[cm−1 a)
Egopt [eV]
solutionc)
Egopt [eV]
filmd)
]
fi lm
fi lm
1
2
3
4
5
6
406 (427)
457 (481)
495 (521)
518
39000
45500
59300
66800
73300
n.d.e)
460 (439)
508 (540)
564 (601)
612 (651)
663
320
606
2.78
2.42
2.20
2.09
2.02
2.00
1047
1342
1441
1803
528 (574)
560 (521,620)
570
695
670 (718)
767
1.85
1.78
1.69
1.68
1.72
1.86
1.67
530
532
692
579
795
a) Difference between the 0–0 vibronic transitions of the absorption and emission spectra. The 0–0 vibronic transitions were determined by mathematical Gaussian decon-
volution. b) Thin film absorption coefficients were estimated from reflection corrected absorbance curves using the lambert-beer law. c) Estimated using the onset of the
UV–vis spectra in DCM. d) Estimated using the onset of the UV–vis spectra of films prepared by vacuum sublimation. e) Due to the low solubility the molar extinction
coefficient could not be determined.
As shown in Figure 6b the emission maximum under-
goes a marked bathochromic shift with increasing number of
thiophene units from 1 (λem = 460 nm) to 6 (λem = 692 nm).
This is further ascribed to the extended conjugation. Sim-
ilar to the trends seen in the absorption spectra, the vibronic
transitions become less prominent with increasing number
of thiophene units from 1 to 4. Structureless emission bands
were observed for DCVnTs 5 and 6. Moreover, with increasing
number of thiophene units from 1 to 6, a gradual increase in
Stokes shift (1: 320 cm−1 , 6: 1803 cm−1 ) was observed. This is an
indication of significant structural changes between the ground
and the excited state. The lack of vibronic structure in the solu-
tion spectra of higher oligomers is a further hint for torsional
disorder in the ground state.
Compared to the solution spectra, absorption and emis-
sion spectra of DCVnT films are significantly broadened and
red-shifted (Figure 7). The reason for this red-shift in thin film
absorption should be due to coplanarization and ordering of the
molecules in the bulk, suggesting better packing in thin films.
DCV4T 4 showed a considerably broad and structured absorp-
the conjugated π-system were observed with increasing number
of thiophene units, confirming full conjugation of the π-system.
DCV1T 1 showed a structured absorption band with a maximum
at 406 nm and shoulders at 387 and 427 nm. This absorption
band shifts to lower energy, becomes broad and loses structure
as the number of thiophene units increases from 2 (457 nm) to
6 (532 nm). Furthermore, the molar extinction coefficients were
found to increase with increasing number of thiophene units
from 1 (ε = 39 000 L mol−1 cm−1 ) to 5 (ε = 73 300 L mol−1 cm−1 ).
8
1
a)
7
6
5
4
3
2
1
0
2
3
4
5
tion band with a maximum at 560 nm compared to 3 (λabs
=
528 nm), 5 (λabs = 570 nm) and 6 (λabs = 579 nm). This may
indicate further structural reorganization of 4 in the bulk due to
pronounced intermolecular π–π interactions.
300
400
500
600
700
In solution, the optical band gap (Egopt) of the DCVnTs
decreases from 2.78 for 1 to 2.0 eV for 6 with increasing
number of the thiophene units and is well in the range of semi-
conducting oligothiophenes. In thin films, the optical band
gaps are lowered by about 0.33–0.35 eV compared to the gaps
obtained from solution spectra.
Wavelength [nm]
1
b)
1.0
0.8
0.6
0.4
0.2
0.0
2
3
4
5
We further compared these non-alkylated DCVnT compounds
1–6 to some previously reported alkyl-substituted DCVnTs.[25,31,34]
Due to the planarization of the molecular backbone, the absorp-
tion maxima of non-alkylated DCVnTs are slightly red-shifted
concomitant with a large increase in molar extinction coeffi-
cients compared to the alkyl-substituted DCVnTs.
400
500
600
700
800
900
5.2. Electrochemical Characterization
Wavelength [nm]
The redox properties of DCVnTs 1–3 were measured in 0.1 M
CH2Cl2/TBAPF6 solutions. Due to the low solubility of longer
Figure 6. Electronic absorption (a) and emission spectra (b) of the
DCVnT-series 1–5 in dichloromethane.
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Figure 7. Normalized absorption and emission spectra of DCVnTs 3–6 in thin films (30 nm) prepared by vacuum evaporation onto quartz substrates.
5.3. Quantum Chemical Calculations
DCVnTs 4–6 in dichloromethane the electrochemical charac-
terizations were carried out in tetrachloroethane/TBAPF6 at
80 °C (Figure 8). All data are summarized in Table 5. DCV1T
1 showed two reversible one-electron reduction processes at
−0.87 and −1.21 V, respectively. DCVnTs 2–6 showed two quasi
reversible reduction waves which are positively shifted with
increasing number of thiophene units. DCVnT 2 and 3 showed
a single reversible oxidation wave at 1.44 and 1.06 V, respec-
tively, within the measured electrochemical window, indicating
the formation of stable radical cations. In contrast, DCVnTs 4 to
6 showed two reversible oxidation waves corresponding to the
formation of stable radical cations and dications, which is typ-
ical for oligothiophenes.
The HOMO and LUMO energies of all compounds were cal-
culated from the onset values of the first oxidation and reduc-
tion waves using the standard approximation that the Fc/Fc+
HOMO level is −5.1 eV versus vacuum.[58] These data are com-
piled in Table 5 and graphically shown in Figure 9. Due to the
low lying HOMO energies DCVnTs 2 (–6.5 eV) and 3 (–6.09 eV)
showed n-type character. On the contrary, the HOMO ener-
gies of DCVnTs 4 to 6 (–5.85 to −5.43 eV) are within the
required range for p-type semiconducting materials ensuring
high open circuit voltages (VOC) in heterojunction solar cells
by using C60 as acceptor, because VOC typically depends on
the difference between the HOMO level of the donor and the
LUMO level of the acceptor. The HOMO/LUMO energy levels
of C60 are also presented in Figure 9. With growing number
of thiophene units from DCV2T 2 to DCV6T 6 the HOMO
energy levels significantly increase, whereas the LUMO
energy levels remain relatively unaffected. In total, this leads
to a lowering of the gap. The calculated electrochemical gaps
(EgCV) for all compounds are in good agreement to the optical
gaps (Egopt) .
The semi-empirical Austin Model 1 (AM1) method under
restricted Hartree Fock conditions was used to analyze the geom-
etry and electron distribution of the frontier orbitals for all DCV
6
5
0.4 μA
4
-2.0 -1.6 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2
E [V] vs. Fc/Fc+
Figure 8. Differential pulse voltammograms of DCVnTs 4–6 (from
bottom to top) measured in tetrachloroethane–TBAPF6 (0.1 M), scan rate
100 mV s−1 , potentials versus Fc/Fc+, measured at 80 °C.
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T a b le 5 . Electrochemical data and molecular orbital energies of the DCVnTs 1–6.
DCVn T
E0ox1 [V]a)
E0ox2 [V]a)
E0red1 [V]a)
−0.87
E0red2 [V]a)
−1.21
HOMO [eV]b)
LUMO [eV]b)
Egcv [eV]c)
1
−4.30
−4.05
−3.90
−3.87
−3.73
−3.87
2
1.44
1.06
0.84
0.64
0.46
−1.12
−1.23
−6.50
−6.09
−5.85
−5.62
−5.43
2.45
2.19
1.98
1.89
1.56
3
−1.25
−1.43
4[d]
5[d]
6[d]
1.20
0.93
0.77
−1.41
−1.52
−1.37
−1.51
−1.39
−1.52
a) Measured in DCM-TBAPF6 (0.1 M), [dye] = 10−3 mol⋅L−1 , 25 °C, V = 100 mV·s−1 , versus Fc+/Fc; b) set Fc+/Fc EHOMO = −5.1 eV; c) Calculated from the difference between
the values of Eonsetred1 and Eonsetox1; d) Measured in tetrachloroethane at 80 °C.
compounds. DCV5T 5 is shown as a representative structure in
Figure 10. As illustrated there, the electron density distribution
of the HOMO for all molecules is mainly located on the oligothi-
ophene backbone, whereas the electron density of the LUMO is
distributed over the whole molecule with higher density on the
terminal thiophene and DCV units. With increasing number of
thiophene units, it can also be seen that the electron density of
the LUMO is reduced at the central thiophene unit(s). This distri-
bution of HOMO-LUMO energies may facilitate charge transfer
from the donors to the C60 acceptor unit in OSCs.
were selected as donor materials in organic solar cells using C60
as electron acceptor. All investigated structures are based on the
m-i-p (planar heterojunction) and p-i-n concept (bulk hetero-
junction) and made by vacuum deposition.[60]
6.1. Planar Heterojunction Solar Cells
For initial characterization, a planar heterojunction structure
was chosen. This gives a well defined interface between donor
and acceptor and allows for the investigation of the different
electronic properties of the molecules in solar cells without
extra effects due to bulk heterojunction morphologies. For all
DCVnTs 4–6, the layer sequence consisted of 15 nm C60 evap-
orated on ITO coated glass, 6 nm DCVnT, 5 nm of undoped
and 50 nm of p-doped 9,9-bis{4-[di-(p-biphenyl)aminophenyl]}
fluorene (BPAPF) as hole transport layer (HTL). The undoped
BPAPF layer was introduced to avoid direct contact between the
active layer and the doped BPAPF layer that otherwise could lead
to quenching of excitons by the dopants.[30] The thickness of the
doped BPAPF layer was chosen to have
6. Fabrication and Characterization of DCVnTs in
Organic Solar Cells
The optical and redox properties of the molecules showed a sig-
nificant change in absorption (see Figure 6 and 7), as well as in
the energy levels along the DCVnT series. These changes are
expected to significantly affect the properties of solar cells based
on DCVnTs. Given the energy levels, compounds DCVnT 4–6
the active layer approximately in the inter-
ference maximum. For p-doping, NDP9
-3.0
was used at 10 wt% and another 1 nm
thick layer of NDP9 was used between the
HTL and the Au top electrode, to facilitate
charge extraction. The complete stack is
shown in Figure 11.
-3.5
-3.73
-3.87
-3.90
-3.87
-4.05
-4.08
-4.0
-4.30
-4.5
In Figure 12 the mismatch corrected
current-voltage (J–V) characteristics of the
flat heterojunction solar cells for DCV4T
4 (device A), DCV5T 5 (device B) and
DCV6T 6 (device C) are depicted. The cor-
responding solar cell parameters derived
from these curves are summarized in
Table 6.
-5.0
-5.5
-5.43
-5.62
-6.0
-6.5
-7.0
-7.5
-5.85
-6.09
-6.30
-6.50
Device
A
(DCV4T 4) showed
a
-7.08
VOC of 0.97 V and the smallest JSC of
2.9 mA cm−2 . The FF was rather low with
a value of 42% and the saturation, which
is defined as J(−1V)/JSC, had a value of 1.3.
The overall efficiency η was determined
to 1.2%. Device B (DCV5T 5) showed the
same VOC as device A, but JSC improved
C60
1
4
6
2
5
3
Figure 9. Representation of HOMO/LUMO energies of the DCVnTs 1–6 derived from electrochem-
ical data and compared to C60. The HOMO/LUMO energy levels of C60 are taken from ref.[59] and
were determined from the onset of the oxidation and reduction waves measured in 0.1 M TBAPF6/
tetrachloroethane. The HOMO energy value for 1 was determined from the optical gap using the
equation EHOMO = Egopt + ELUMO
.
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(a)
6
4
a)
4
5
6
2
0
-2
-4
-6
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Figure 10. Representative frontier orbitals of DCV5T 5 obtained by semi-
empirical AM1-calculation (carbons in black, nitrogens in blue, sulfurs in
yellow and hydrogens are omitted for clarity reasons).
Voltage [V]
(b)
Energy [eV]
to 5.1 mA cm−2 and the FF to 52% resulting in an efficiency
of 2.6%. In device C (DCV6T 6) VOC dropped to 0.91 V,
but JSC remained with 4.8 mA cm−2 nearly similar to device
B. FF reached a maximum of 64% and the resulting efficiency
was 2.8%.
4 3.63.2 2.8 2.4
2
1.6
0.5
0.4
0.3
0.2
0.1
0.0
b)
4
5
6
All devices showed a high VOC, as VOC for flat heterojunc-
tion devices is determined by the effective gap, i.e., the ener-
getic distance between the LUMO of the acceptor C60 and the
HOMO of the donor.[61] However, the shift of HOMO levels
with increasing thiophene chain length is only partially reflected
in the decrease of VOC. Devices A and B exhibited the same
VOC and the difference in VOC for devices B and C (0.06 V) is
smaller than what is expected from electrochemical measure-
ments of the energy levels (about 0.2 V, see Table 6) and from
TD-DFT calculations of the energy levels (calculated difference:
0.14 V).[34] The main reason for the differences is that both
latter values were obtained for single molecules, whereas the
values shift due to molecular packing in thin film. Still, the
difference between optical gap and eVOC at one sun intensity
gives values between 0.7 and 0.8 eV, which is on the lower side
in comparison to other organic materials combinations. The
smallest observed difference between effective gap and eVOC is
about 0.4 eV, close to what is expected from the splitting of the
quasi Fermi levels.
300
400
500
600
700
800
Wavelength [nm]
Figure 12. a ) J–V characteristics of planar heterojunction solar cells con-
taining DCVnTs 4 (squares), 5 (pentagons) and 6 (hexagons) as donor
materials. b) The corresponding EQE spectra were used to calculate the
mismatch factor for the J–V characteristics.
450 nm) to the photocurrent can be identified. The shift in thin
film absorption of the donors with increasing oligothiophene
chain length (see Table 4) is reflected in the behaviour of the
EQE. There is a red-shift in EQE peak from 560 to 580 nm for
device A to B, whereas there are only small differences with
a small blue-shift by less than 10 nm between devices B and
C, but not as significant as the differences of optical gaps in
thin film might suggest. Correspondingly, JSC for devices B
and C are comparable, with A being much lower. In the EQE
spectra it can be seen, that for all DCVnTs 4–6 the majority of
the current is coming from the donor materials. However, the
EQE peak of C60 increases with increasing chain length. This
indicates that the separation efficiency of excitons generated on
the C60 into free charge carriers grows with increasing chain
length, as expected from the changes in energy levels. This is
also reflected in FF and saturation. FF continuously increases
from DCV4T 4 (42%) to DCV6T 6 (64%), showing that exciton
and charge separation efficiencies become less field-dependent
with increasing chain length. Since the charge carrier mobili-
ties in DCVnTs are expected to be comparable and given only
6 nm of donor material, differences in FF are unlikely to be
due to transport limitations. Differences in FF may occur due
to injection barriers at the BPAPF interface and this should
The differences in JSC can be explained by the different
absorption of the compounds and is reflected in the EQE spectra
shown in Figure 12b, in which the contribution of the donor
DCVnT 4–6 (≈400 to 800 nm) and of the acceptor C60 (≈300 to
Figure 11. Stack structure of the solar cells with a planar heterojunctions
of DCV4T-DCV6T 4–6 and C60
.
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T a b le 6 . Photovoltaic parameters of flat heterojunction solar cells containing DCVnTs 4, 5 and 6.
Device
VOC [V]
0.97
FF [%]
42
Saturation
Cell area [mm2]
J
SC [mA cm−2
]
η [%]
1.2
Intensity [mW cm−2
]
A (DCV4T 4 )
B (DCV5T 5)
C (DCV6T 6)
2.9
5.1
4.8
100
106
101
1.3
1.2
1.2
2.96
2.83
2.96
0.97
52
2.6
0.91
64
2.8
especially influence device A employing DCV4T 4 because of
its low HOMO level. However, the J–V characteristics for device
A showed no signs of an S-kink, a feature characteristic for
injection barriers.
The behaviour of the saturation is consistent with differ-
ences in exciton and charge carrier separation. A saturation
value close to unity corresponds to efficient charge separation
and extraction while a larger value hints for recombination
losses and field dependent dissociation of charge carriers. For
DCV4T 4, exciton separation appears to be at the limit, but it
is becoming sufficient for DCV5T 5 and DCV6T 6, although
the peak values for all compounds do not differ much. With
values between 1.2 and 1.3, the saturation shows that there
is some recombination at JSC-condition and in reverse bias,
the increase of current without saturating shows that fur-
ther charge carriers can be extracted with increasing elec-
tric field in the device and it also indicates the presence of a
photoshunt.
Figure 13. Layer sequence of BHJ solar cells D,E, and F incorporating a
blend layer of DCV5T 5 and C60.
a)
10
5
D
E
F
6.2. Bulk Heterojunction Solar Cells
In the following, we present three bulk heterojunction (BHJ)
solar cells comprising a blend layer of DCV5T 5 and C60 with
layer thicknesses of 20, 30 and 40 nm. The layer structure and
the preparation process of the BHJ solar cells were optimized
regarding efficiency and stability. In comparison to the planar
heterojunction solar cells described above, an additional layer
of n-doped C60 was used to stabilize the contact to the ITO and
aluminum was used as top contact instead of gold providing
better reflection characteristics and less parasitic absorption.
Additionally, a p-doped layer of spiro-NPB was introduced in
contact to the p-doped BPAPF layer. The blend ratio of DCV5T
5 and C60 is 2:1 in volume, what was found to be optimal for
alkylated DCVnT compounds.[62] The blend layer was deposited
while heating the substrate to 90 °C leading to a more crystal-
line growth of the blend layer and enhanced fill factors com-
pared to deposition at room temperature.[63,64] The exact layer
sequence is depicted in Figure 13.
Figure 14a shows the J–V characteristics of BHJ solar cells
D, E, and F with blend layer thicknesses of 20, 30, and 40 nm,
respectively. The active area of the solar cells was 6.5 mm2. In
order to exactly determine the solar cell efficiencies, all three
cells were measured with an aperture mask of 5.06 mm2. The
photovoltaic parameters of the three BHJ solar cells are sum-
marized in Table 7.
0
-5
-10
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Voltage [V]
Energy [eV]
4 3.63.2 2.8 2.4
2
1.6
1.0
0.8
0.6
0.4
0.2
0.0
b)
4
5
6
300
400
500
600
700
800
Wavelength [nm]
Figure 14. a ) J–V characteristics of BHJ solar cells D (20 nm), E (30 nm)
and F (40 nm) comprising a blend layer of DCV5T 5 and C60 (blend ratio 2:1
in volume) with different thicknesses. b) EQE spectra of BHJ solar cells D, E
and F were used to calculate the mismatch factor and to calculate the cor-
rected short current density J sc,corr from the measured J–V characteristics.
All of the J–V characteristics measured in the dark showed
good rectifying behavior and similar series resistance at volt-
ages above VOC, while VOC of all cells measured under illumina-
tion is almost constant (0.96 V); this is in good agreement with
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T a b le 7 . Photovoltaic parameters of bulk-heterojunction solar cells containing DCV5T 5 with blend layer thicknesses of 20, 30 and 40 nm.
Device
THICKNESS [nm]
VOC [V]
0.96
FF [%]
61
Saturation
1.06
J
SC [mA cm−2
]
η [%]
3.9
Intensity [mW cm−2
]
D
E
F
20
30
40
6.9
9.0
102.8
102.5
102.3
0.95
55
4.6
1.10
11.1
0.97
49
5.2
1.10
VOC of the planar heterojunction solar cell (device B, 0.97 V)
prepared with the same donor material shown above.
Repeated gradient vacuum sublimation gave thermally highly
stable and pure organic semiconducting donor materials. An
odd-even effect of the melting points was found and assigned
to the symmetry of the molecules. An X-ray structure analysis
of tetramer DCV4T 4 revealed the importance of the DCV
groups in the molecular packing in the bulk by providing inter-
molecular C-N and H-N interactions and a nearly ideal π−π
stacking as a prerequisite for good charge transport. Charac-
terization of the optical properties in solution and in thin films
brought the expected red-shift, increase in absorptivity, and
lowering of the gap with increasing chain length and number
of thiophene units in the conjugated backbone. Determination
of the redox potentials by cyclic and differential pulse voltam-
metry resulted in the similar trend, i.e., the HOMO energies
increase with increasing chain length, whereas the LUMO
values stay nearly unaffected. The longer oligomers DCVnT 4–6
were implemented in vacuum-processed planar heterojunction
organic solar cells and mismatch-corrected efficiencies of 1.2–
2.8% have been achieved under full sun illumination. The opti-
mization of bulk heterojunction solar cells using a 40 nm active
layer of DCV5T 5 and C60 in a 2:1-ratio resulted in a mismatch-
corrected value of 5.2% for a 5.06 mm2-sized and masked
device. The 5.2% efficiency reported here represents one of
the highest values ever reported for organic vacuum-deposited
single junction solar cells and clearly shows the advantage of
a higher internal interface in bulk heterojunctions compared
to planar heterojunctions. Thus, the structural fine-tuning of
these A-D-A-type oligothiophenes led to highly efficient mate-
rials with sufficiently good electronic and charge transport
properties, which are crucial prerequisites in bulk heterojunc-
tion devices.
JSC steadily and significantly increased with increasing blend
layer thickness indicating that we were indeed dealing with
a BHJ where the complete volume of the blend contributes to
the photocurrent. However, with increasing layer thickness a
decrease in FF from 61% to 49% was observed. This shows that
the charge carrier collection efficiency decreases with increasing
layer thickness in the voltage regime around the maximum
power point (MPP). The saturation of photocurrent with reverse
bias for all cells is almost constant (1.1). This shows that the
exciton and charge carrier dynamics at negative voltages are only
weakly influenced by the blend layer thickness variations and we
already collect most of the geminate carrier pairs at zero bias.
In Figure 14b, the EQE spectra of the three BHJ solar cells
are shown. The spectral shape of the EQEs remained almost
unchanged with changing blend layer thickness. The spectra
showed a maximum at around 580 nm, a shoulder at about
450 nm and a local maximum at 350 nm. The maximum at
580 nm is in good agreement with the thin film absorption
spectrum of DCV5T 5 (Figure 7) and the EQE spectrum of
device B (Figure 13). The shoulder around 450 nm stems from
the additional C60 in the blend layer. From the EQE spectra,
the mismatch factor is calculated to account for the mismatch
between sun simulator and the AM1.5 spectrum. The corrected
illumination intensities are 103 mW cm−2 for device D and E
and 102 mW cm−2 for device F. The mismatch corrected illu-
mination intensities were used to calculate the conversion effi-
ciencies. The mismatch corrected conversion efficiency (ηcor
)
increased with increasing blend layer thickness from 3.9% at
20 nm to 5.2% at 40 nm. The efficiency sublinearly increased
with the blend layer thickness mostly due to the decrease of
FF. This rapid decrease of FF already at moderate blend layer
thickness indicates lower mobility values in the blends in spite
of the favorable crystal packing for DCVnTs described above.
Obviously, the crystalline order seems to be at least partly lost
in the blends. Another problem may be a lack of nanoscale
phase separation and thus a lack of C60 nanocrystallites in
the blends. The details of the blend layer morphology require
further detailed studies and optimization. Nevertheless, the
5.2% efficiency after mismatch correction shown here already
represents one of the highest values ever reported for organic
vacuum-deposited single junction solar cells.
8. Experimental Section
Instruments and Spectroscopic Measurements: 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), at 25 °C or
100 °C. Chemical shift values (δ) are expressed in parts per million using
residual solvent protons (1H NMR, δH = 2.50 for DMSO-d6 and δH
=
5.93 for tetrachloroethane-d2; 13C NMR, δC = 74.20 tetrachloroethane-d2)
as internal standard. The splitting patterns are designated as follows:
s (singlet), d (doublet) and m (multiplet). The assignments are ThH
(thiophene protons) and DCVH (dicyanovinyl protons). Melting points
were determined using a Mettler Toledo DSC 823e and were not
corrected. Elemental analyses were performed on an Elementar Vario EL.
CI mass spectra were recorded on a Finnigan MAT SSQ-7000.
7. Conclusion
X-ray diffraction data of DCV4T were collected on a Bruker APEX-II
CCD area detector diffractometer using graphite-monochromated
Mo Kα radiation. Correction for absorption (multi-scan method) was
applied. The structure was solved by direct methods and refined on F2
with the SHELXTL program package.[65] All non-H atoms were refined
A novel series of terminally acceptor (dicyanovinyl)-substituted
oligothiophenes, DCVnT 1–6, without solubilising side chains
has been synthesized via a convergent approach in high yields.
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anisotropically. The thiophene H-atoms were placed in calculated
positions and refined as riding. The structure contains accessible voids
with a volume of ca. 400 Å3. These voids represent approximately 17%
of the overall volume and are filled with heavily disordered molecules,
discernible from the smeared electron density (maximum electron
density within the voids 4.2 e−Å−3 ). This disorder could not be resolved
during refinement; electron density associated with peaks in these voids
was eventually omitted by applying the SQUEEZE option in PLATON.[66]
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 determined by taking these spectra as well as
the spectral responses of the reference diode and the sample (from EQE
measurements) into account. For well defined active solar cell areas,
aperture masks were used.
Reagents and Chemicals: Dimethylformamide (Merck) was first
refluxed over P4O10 and distilled, then refluxed over BaO and distilled
again. All synthetic steps were carried out under argon atmosphere.
n-Butyl lithium (1.6 N in n-hexane) and CuCl2 were purchased from
Merck. Malononitrile, trimethyltin chloride, 2-isopropoxy-4,4,5,5-
tetramethyl[1,3,2]dioxaborolane,2,5-dibromothiophene,5-bromothiophene-
2-carboxaldehyde and thiophene-2,5-dicarbaldehyde 7 were purchased
from Aldrich. 2,2′-Bithiophene was synthesized from lithiated
thiophene by oxidative coupling with CuCl2 in 80% yield.[46] Ter- and
quaterthiophene were prepared from 2,5-dibromothiophene and 5,5′-
dibromo-2,2′-bithiophene[67] using Suzuki-type coupling with 2-(thien-2-
yl)-4,4,5,5-tetramethyl[1,3,2]dioxaborolane[68] in yields of 96% and 85%,
respectively. The oligothiophenes gave dialdehydes 8–10 by Vilsmeier-
Haack formylation.[40–43] Compounds 11 and 12 were synthesized
from 5-bromothiophene-2-carboxaldehyde and 5-bromo-5′-formyl-2,2′-
bithiophene[43] by reaction with malononitrile in yields of 70% and 93%,
respectively.[47,48] Distannylated compounds 13 and 14 were synthesized
according to literature[49–51] and recrystallized before use.
DCV4T: C24H10N4S4, Mr
= 482.60, dark-red needle with metallic
lustre, 0.55 × 0.12 × 0.06 mm3, monoclinic, P21/n, a = 3.8295(8) Å,
b = 27.695(6) Å, c = 22.663(5) Å, β = 91.68(3)°, V = 2402.5(8) Å3,
Z = 4, μ = 0.414 mm−1 , dx = 1.334 g·cm−3 , T = 100 K. 36948 reflections
collected (θmax = 35.0°) and merged to 10230 independent data (Rint
=
0.046); final R indices (I > 2σ(I)): R1 = 0.0488, wR2 = 0.1176. Detailed
crystallographic data for structure DCV4T has been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication
CCDC 786021.
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
spectrometer. 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 counter electrode, and an Ag/AgCl reference electrode. All potentials
were internally referenced to the ferrocene/ferrocenium couple.
Synthesis of 2,2′-[Thien-2,5-diylbis(methane-1-yl-1-ylidene)]dimalono-
nitrile 1: A solution of thiophene-2,5-dicarbaldehyde 7 (400 mg,
Thin Film and Device Fabrication: Thin films and heterojunction solar
cell devices were prepared by thermal vapor deposition in ultra high
vacuum at a base pressure of 10−8 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, USA, sheet resistance of
30 Ω sq.−1 ). Layer thicknesses were determined during evaporation 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 UV–vis spectrometer. The thin film emission
spectra were recorded with a Edinburgh Instruments FSP920 fluoresence
spectrometer. The flat heterojunction solar cells were prepared layer
by layer without breaking the vacuum and were encapsulated after
processing using a glass cover under nitrogen atmosphere. The layer
structure of the flat heterojunction solar cells is as follows: ITO; 15 nm
C60 (Bucky, USA); 6 nm DCVnT 4–6; 5 nm 9,9-bis[4-(N,N-bis-biphenyl-
4-yl-amino)phenyl]-9H-fluorene (BPAPF, Lumtec, Taiwan; ionization
potential (IP) measured −5.6 eV); 50 nm BPAPF doped with p-dopant
NDP9 (Novaled AG, Germany) with a doping concentration of 10 wt%;
1 nm NDP9; 50 nm Au. BPAPF and C60 were purified at least twice by
vacuum gradient sublimation prior to sample preparation; the dopant
was used as received. NDP9 is used because of better processability,
but doping of materials with low lying HOMO can be achieved in a
comparable way using WO3.
The Layer Structure of the Bulk Heterojunction Solar Cells: ITO; 5 nm
C60 (Bucky, USA) n-doped by coevaporation with NDN1 (Novaled AG,
Germany) (2 wt%); 15 nm C60; 20 to 40 nm blend layer of DCV5T 5
and C60 (ratio 2:1 in volume) prepared by coevaporation deposited
on heated substrate (90 °C); 5 nm BPAPF; 10 nm BPAPF doped with
NDP9 (10 wt%); 30 nm N,N’-bis(naphthalen-1-yl)-N,N’-bis(phenyl)-9,9-
spirobifluorene (spiro-NPB, Lumtec, Taiwan) doped with NDP9 (10 wt%);
1 nm NDP9; 100 nm aluminium. Instead of acridine orange base (AOB)
as n-dopant for C60, NDN1 is used because of better processability, but
a comparable doping performance.
Photovoltaic Characterization: EQE measurements for the
heterojunction solar cell devices were conducted at a custom-made setup
utilizing a xenon arc lamp, a monochromator (Newport Cornerstone) and
a lock-in amplifier (Signal Recovery 7265 DSP). A silicon reference diode
(Hamamatsu S1337) was used to calibrate the setup. J–V characteristics
were measured using an automated setup consisting of a source-
measure unit (Keithley SMU 2400) and an AM 1.5G sun simulator (KHS
2.85 mmol) and malononitrile (766 mg, 11.6 mmol) in ethanol (35 mL)
was stirred at room temperature for 10 min. The precipitate was filtered
off and recrystallized from methanol to give 1 (654 mg, 2.77 mmol, 97%)
as yellow needles. Mp 272 °C (DSC). 1H NMR (tetrachloroethane-d2,
100 °C, δ ppm) 7.87 (s, 2H, DCVH), 7.79 (s, 2H, ThH). 13C NMR
(tetrachloroethane-d2, 100 °C, δ ppm) 148.52, 142.05, 136.49, 112.83,
112.25, 84.58. CI mass 237 [M+H+] (calcd. for C12H4N4S 236.02).
Elemental analysis for C12H4N4S: calcd. C, 61.01; H, 1.71; N, 23.71%;
found: C, 61.10; H, 1.80; N, 23.58%.
Synthesisof2,2′-[2,2′-Bithien-5,5′-diylbis(methane-1-yl-1-ylidene)]dimalon-
onitrile 2: A solution of 2,2′-bithiophene-5,5′-dicarbaldehyde 8 (1.00 g,
4.50 mmol), malononitrile (891 mg, 13.5 mmol) and β-alanine (25 mg,
0.062mmol)inamixtureofdichloroethane(110mL)andethanol(110mL)
was stirred at 60 °C for 22 h. The reaction mixture was allowed to
cool to room temperature, the precipitate filtered off and washed with
ethanol. The crude product was washed with hot dichloromethane to
provide DCV2T 2 (1.42 g, 4.45 mmol, 99%) as red solid. The compound
was further purified by gradient vacuum sublimation. Mp 300 °C
(DSC).1H NMR (DMSO–d6, δ ppm) 8.71 (s, 2H, DCVH), 7.95 (d, 2H,
J = 4.09 Hz, ThH), 7.89 (d, 2H, J = 4.07 Hz, ThH). CI mass 319 [M+H+]
(calcd. for C16H6N4S2 318.00). Elemental analysis for C16H6N4S2: calcd.
C, 60.36; H, 1.90; N, 17.60%; found: C, 60.46; H, 1.96; N, 17.43%.
Synthesis of 2,2′-[2,2′:5′,2”-Terthien-5,5”-diylbis(methane-1-yl-1-ylidine)]
dimalononitrile 3 by Linear Synthesis: A solution of 2,2′:5′,2′-terthiophene-
5,5′′-dicarbaldehyde 9 (500 mg, 1.64 mmol), malononitrile (651 mg,
9.9 mmol) and β-alanine (18 mg, 0.2 mmol) in
a mixture of
dichloroethane (80 mL) and ethanol (80 mL) were stirred under reflux
overnight. The reaction mixture was allowed to cool to room temperature,
the precipitate filtered off and washed thoroughly with ethanol to provide
compound 3 (652 mg, 1.63 mmol, 99%) as deep red solid.
Synthesis
1-ylidine)]dimalononitrile
of
2,2′-[2,2′:5′,2”-Terthien-5,5”-diylbis(methane-1-yl-
by Convergent Synthesis: mixture of
3
A
2,5-bis(trimethylstannyl)thiophene 13 (25 mg, 0.061 mmol), 2-[(5-
bromothiophen-2-yl)methylene]malononitrile 11 (37 mg, 0.13 mmol)
and tetrakis-(triphenylphosphine)-palladium(0) (4 mg, 0.003 mmol) was
combined in toluene (5 mL) in a screw cap bottle and was agitated at
110 °C for 3 h. After cooling, the resulting precipitate was filtered off
and washed several times with cold toluene, methanol and n-hexane.
After drying, the product (24 mg, 0.060 mmol, 99%) was obtained as
deep red solid. The compound was further purified by gradient vacuum
sublimation. Mp 265 °C (DSC).1H NMR (DMSO-d6, δ ppm) 8.65 (s,
©
908 wileyonlinelibrary.com
2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2011, 21, 897–910

www.afm-journal.de
www.MaterialsViews.com
[1] G. A. Chamberlain, Solar Cells 1983, 8, 47.
2H, DCVH), 7.91 (d, 2H, J = 4.16 Hz, ThH), 7.73 (s, 2H, ThH), 7.73 (d,
2H, 3.95 Hz, ThH). CI mass 401 [M+H+] (calcd. for C20H8N4S3 399.99).
Elemental analysis for C20H8N4S3: calcd. C, 59.98; H, 2.01; N, 13.99%;
found: C, 59.79; H, 2.16; N, 13.96%.
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2001, 11, 15.
Synthesis of 2,2′-[2,2′:5′,2”:5′′,2′′′-Quaterthien-5,5”‘-diylbis(methane-1-
yl-1-ylidine)]dimalononitrile 4: A mixture of 5,5′-bis(trimethylstannyl)-2,2′-
bithiophene 14 (750 mg, 1.52 mmol), 2-[(5-bromothien-2-yl)methylene]
malononitrile 11 (766 mg, 3.20 mmol) and tetrakis-(triphenylphosphine)
palladium(0) (88 mg, 0.08 mmol) in degassed DMF (40 mL) was stirred
at 80 °C for 4 h. After cooling, the resulting precipitate was filtered
off and washed several times with methanol, n-hexane and acetone.
After drying, the product (714 mg, 1.48 mmol, 97%) was obtained as
a black solid. The compound was further purified by gradient vacuum
sublimation. Mp 320 °C (DSC).1H NMR (DMSO-d6, 100 °C, δ ppm) 8.53
(s, 2H, DCVH), 7.93 (d, 2H, J = 4.15 Hz, ThH), 7.65 (d, 2H, J = 3.96 Hz,
ThH), 7.63 (d, 2H, J = 4.12 Hz, ThH), 7.52 (d, 2H, J = 3.96 Hz, ThH).
CI mass 483 [M+H+] (calcd. for C24H10N4S4 481.98). Elemental analysis
for C24H10N4S4: calcd. C, 59.73; H, 2.09; N, 11.61%; found: C, 59.67; H,
2.04; N, 11.59%.
Synthesis of 2,2′-[2,2′:5′,2”:5′′,2′′′:5′′′,2′′′′-Quinquethien-5,5”‘‘-diylbis(met
hane-1-yl-1-ylidine)]dimalononitrile 5: A mixture of 2,5-bis(trimethylstannyl)
thiophene13(400mg,0.98mmol),5-[(5′-bromo-2,2’-bithien-5-yl)methylene]
malononitrile 12 (659 mg, 2.05 mmol) and tetrakis(triphenylphosphine)-
palladium(0) (56 mg, 0.05 mmol) in degassed DMF (25 mL) was stirred
at 80 °C for 8 h. After cooling, the resulting precipitate was filtered off and
washed several times with methanol, n-hexane and acetone. After drying,
the product (517 mg, 0.92 mmol, 94%) was obtained as a black solid.
The compound was further purified by gradient vacuum sublimation. Mp
287 °C (DSC).1H NMR (DMSO-d6, 100 °C, δ ppm) 8.52 (s, 2H, DCVH),
7.92 (dd, 2H, J = 0.41 Hz, J = 4.15 Hz, ThH), 7.63 (d, 2H, J = 3.95 Hz,
ThH), 7.61 (d, 2H, J = 4.13 Hz, ThH), 7.45 (s, 2H, ThH) 7.44 (d, 2H, J =
3.95 Hz, ThH). CI mass 565 [M+H+] (calcd. for C28H12N4S5 563.97).
Elemental analysis for C28H12N4S5: calcd. C, 59.55; H, 2.14; N, 9.92%;
found: C, 59.52; H, 2.24; N, 9.82%.
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Synthesis of 2,2′[2,2′:5′,2”:5′′,2′′:5′′’2′′′′:5′′′′,2′′′′-Sexithien-5,5”‘‘‘-diylbis
(methane-1-yl-1-ylidine)]dimalononitrile 6:
A
mixture of 5,5′-bis
(trimethylstannyl)-2,2′-bithiophene 14 (1.0 g, 2.03 mmol), 5-[(5′-bromo-
2,2′-bithien-5-yl)methylene]malononitrile 12 (1.371 g, 4.27 mmol) and
tetrakis(triphenylphosphine)palladium(0) (118 mg, 0.10 mmol) in
degassed DMF (50 mL) was stirred at 80 °C for 20 h. After cooling,
the resulting precipitate was filtered off and washed several times with
methanol. The crude product was extracted in a Soxhlett extractor
with chlorobenzene for 8 days. The extract was cooled to room
temperature and the precipitate was filtered off to yield the product
(1.078 g, 1.67 mmol, 82%) as a black solid. The compound was further
purified by gradient vacuum sublimation. Mp 319 °C (DSC). 1H NMR
(tetrachloroethane-d2, 100 °C, δ ppm) 7.69 (s, 2H, DCVH), 7.66 (d, 2H,
J = 4.01 Hz, ThH), 7.34 (d, 2H, J = 3.90 Hz, ThH), 7.27 (d, 2H, J =
4.10 Hz, ThH), 7.16–7.18 (m, 4H, ThH), 7.14 (d, 2H, J = 3.81 Hz, ThH).
CI mass 647 [M+H+] (calcd. for C32H14N4S6 645.95). Elemental analysis
for C32H14N4S6: calcd. C, 59.42; H, 2.18; N, 8.66%; found: C, 59.62; H,
2.37; N, 8.82%.
Crystallographic data (excluding structure factors) for the structure(s)
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no. CCDC-
786021.
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Acknowledgements
We would like to thank the German Research Foundation (DFG) for
financial support in the framework of special program SPP 1355 and
the Ministry for Education and Research (BMBF) in the framework of
program OPEG 2010.
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Received: August 6, 2010
Published online: January 27, 2011
©
wileyonlinelibrary.com
Adv. Funct. Mater. 2011, 21, 897–910
2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
909

www.afm-journal.de
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Adv. Funct. Mater. 2011, 21, 897–910