A-D-A-type S,N-Heteropentacenes: Next-Generation Molecular Donor Materials for Efficient Vacuum-Processed Organic Solar Cells

Article abstract of DOI:10.1002/adma.201402448

A new class of acceptor-substituted S,N-heteropentacenes is developed for vacuum-processed organic solar cells, providing encouraging power conversion efficiencies of up to 6.5%. Atomic force microscopy (AFM) investigations give a direct correlation between the blend film morphology and the photovoltaic parameters, such as short-circuit current density (J_SC) and fill factor (FF).

Full text of DOI:10.1002/adma.201402448

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A–D–A-type S,N-Heteropentacenes: Next-Generation
Molecular Donor Materials for Efficient Vacuum-Processed
Organic Solar Cells
Amaresh Mishra, Dusko Popovic, Astrid Vogt, Hannelore Kast, Tanja Leitner,
Karsten Walzer, Martin Pfeiffer, Elena Mena-Osteritz, and Peter Bäuerle*
Conjugated oligomers, often referred to as “small molecules”,
come more and more into focus as organic semiconductors
in organic solar cells (OSC).[1] Not only do the power conver-
sion efficiencies (PCE), up to 9%, reach those of corresponding
polymer solar cells,[2–4] but also high reproducibility is achieved
due to the defined molecular structure.[5] Besides common pro-
cessing from solution, oligomers furthermore allow processing
by evaporation, which leads to highly controlled and well-
defined layers of high purity.[6] An additional advantage is that
vacuum techniques are solvent-free, while especially the fre-
quently used chlorinated solvents are a critical technical issue
in solution-processing and printing of organic solar cells.[7]
In this respect, so-called A–D–A oligothiophenes (A =
acceptor, D = donor) turned out to be a very promising and
successful class of compounds. Starting from an initial effi-
ciency of 3.4% for single-junction solar cells comprising a
2,2-dicyanovinyl (DCV)-capped quinquethiophene (DCV5T-Bu)
as p-type and fullerene C60 as n-type semiconductor,[8] by opti-
mization of the molecular structure and device preparation, a
PCE of 6.9% was recently achieved with a similar methylated
derivative (DCV5T-Me).[9] Vacuum-processing allows the con-
struction of complex multilayer device structures and PCEs of
tandem cells were continuously improved to 10.7% by Heli-
atek GmbH.[10] This culminated in a certified record efficiency
of 12.0% on 1.1 cm2 by a triple cell construction as well using
A–D–A oligothiophene/C60 subcells.[11]
by an extended band-like π-conjugated backbone comprising
substantial quinoidal character and bond-length equalization in
the ground state. In this novel class of potential donor mate-
rials for OSCs, we were able to combine the favorable proper-
ties of oligothiophenes, which exhibit increased stability due
to a low-lying highest occupied molecular orbitial (HOMO)
energy level, and of oligoacenes, which typically show high
charge-transport mobilities due to a planar, rigid, and highly
conjugated backbone.[13]
In this contribution, we report synthesis, characterization,
and photovoltaic properties of a novel series of DCV-capped
S,N-heteropentacenes (DCV-SN5) 1–3, in which the substituent
at the nitrogen atoms is varied (Figure 1). In comparison to
the nonfused terthiophene parent oligomer DCV3T 4, which
has a rather modest performance in solar cells,[14] a strong red-
shift and intensification of the absorption by keeping suitable
energy levels of the frontier orbitals were obtained. In vacuum-
processed single-junction m-i-p solar cells,[15] efficiencies as
high as 6.5% for propyl derivative 1 as donor and fullerene C60
as acceptor were achieved after optimization of the processing
parameters during device fabrication. Characterization of the
photoactive blends (oligomers 1–3/C60) by atomic force micros-
copy (AFM) gave a clear correlation of the phase separation
with the trend seen in the photovoltaic parameters, which even-
tually depend on the molecular structure: 1 (propyl) >2 (hexyl)
>>3 (tolyl).
In the search for even stronger absorbers, which at the same
time have good charge-transport properties, are thermally
stable, and sublime nearly without residue, we very recently
introduced fused thiophene-pyrrol-based S,N-heteroacenes and
their acceptor-capped derivatives as next-generation A–D–A
donor molecules.[12] Corresponding DCV-substituted S,N-he-
terohexacene DCV-SN6 showed very intense optical transitions
and exhibited high p-channel charge-carrier mobilities caused
The synthesis of novel A–D–A-type fused S,N-heteropenta-
cenes 1–3 is outlined in Scheme 1. The crucial building block
3,3′,3′′,4′-tetrabromo-2,2′:5′,2′′-terthiophene 7 was synthesized
in a 78% yield by a Negishi-type cross-coupling reaction of
tetrabromothiophene 5 and (3-bromothien-2-yl)zinc(II) chloride
6 in the presence of dichloro[1,1′-bis(diphenylphosphino)ferro-
cene]palladium (Pd(dppf)Cl2) as catalyst.[16] The next step was
a fourfold Buchwald–Hartwig amination reaction of terthio-
phene 7 with different amines in the presence of Pd(dba)2 as
the Pd source, dppf as ligand, and sodium tert-butoxide as base.
The S,N-heteropentacenes 8–10 were obtained in good yields
of 50–60%. In the next step, fused heteroaromatics 8–10 were
converted into corresponding dialdehydes 11–13 by Vilsmeier–
Haack formylation in 75–85% yield after column chroma-
tography. Subsequent Knoevenagel condensation with maloni-
trile gave DCV end-capped target S,N-heteropentacenes 1–3 in
excellent yields of 80–88%.
Dr. A. Mishra, D. Popovic, A. Vogt, H. Kast,
T. Leitner, 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
Dr. K. Walzer, Dr. M. Pfeiffer
Heliatek GmbH
Treidlerstrasse 3, 01139, Dresden, Germany
The thermal properties of 1–3 were investigated using dif-
ferential scanning calorimetry (DSC) (Figure S1, Supporting
DOI: 10.1002/adma.201402448
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are directed out of plane in the opposite direction of the
planar conjugated backbone (Figure 2a,b and Figure S3, Sup-
porting Information). The conjugated backbone of DCV-SN5
1 exhibits bond-length equilibration with an extremely small
averaged difference of 0.01 Å (Table S1, Supporting Informa-
tion). DCV-SN5 1 arranges in perfectly antiparallel stacked
columns of pairs of molecules separated by a distance of
9 Å (Figure 2b). Strong π−π interactions at distances of 3.45 Å
and 3.493 Å stabilize the columns (Figure 2b, and Table S2,
Supporting Information). Perpendicular to the −101 plane,
DCV-SN5 1 interacts with four other molecules via various
cyano groups, which come close to vinylic or heteroaromatic
hydrogens or to sulphur atoms (Figure 2d (green and red lines),
Table S3, Supporting Information).
The absorption spectra of DCV-SN5 derivatives 1–3 in
dichloromethane solution and in thin films are shown in
Figure 3 and compared to the parent oligomer DCV3T 4. All
the data are summarized in Table 1. The fluorescence spectra
of 1–3 and DCV3T 4 are shown in the supporting information
(Figure S5, Supporting Information).
We will discuss the general trends of the optoelectronic prop-
erties of DCV-SN5 1–3 in comparison to DCV3T 4 which can be
derived from the data and give an indication on the influence
of the planarization and rigidification in the fused SN5-sys-
tems. First of all, the maximum absorption and fluorescence of
1–3 in solution and absorption in thin films is strongly shifted
to longer wavelengths and the molar extinction coefficient is
increased by a factor of 1.8–2.3 as compared to DCV3T 4. At
the same time, the optical gap decreased. The influence of the
rigidification of the conjugated π-system in 1–3 can as well be
seen in the large decrease of the Stokes shift compared to 4.
Within the series of the fused DCV-SN5 1–3, there are small dif-
ferences in the optical data of 1 and 2 compared to tolyl deriv-
ative 3 which for 3 indicate a distorsion of the bulky aromatic
Figure 1. Molecular structures of acceptor-substituted S,N-heteropenta-
cenes 1–3 in comparison to non-fused parent oligothiophene DCV3T 4.
Information) and thermogravimetric analysis (TGA) (Figure S2,
Supporting Information). In comparison to the parent DCV3T
4 (265 °C),[17] ring-fusion increased the melting point to 414 °C
for 1, to 296 °C for 2, and to 395 °C for 3 indicating strong
intermolecular interactions in the solid state. All three deri-
vatives displayed high thermal stability with an onset of the
decomposition temperature (Td) at around 400 °C for 1, 353 °C
for 2, 392 °C for 3, respectively, as confirmed by TGA.
Single crystals of DCV-SN5 1 obtained by fractionated sub-
limation crystallized in the monoclinic space group P 21/n
with four equivalent molecules in the unit cell arranging
in antiparallel pairs with a 2-fold screw axis symmetry (a =
7.34121(11), b = 27.6849(4), c = 12.5274(2) Å; α = 90.00°, β =
103.8281°(15), γ = 90.00°) (Figure 2c). Bond lengths, selected
angles, and torsion angles are compiled in Table S1 in the Sup-
porting Information. The two alkyl chains of each molecule
Scheme 1. Synthesis of acceptor-substituted S,N-heteropentacenes 1–3.
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Figure 2. Single-crystal X-ray structure analysis of DCV-SN5 1, individual molecule on the (−12 10 10) plane (a) and packing arrangement in columns of
pairs (b). The packing in the unit cell (c) and intramolecular short contacts between the adjacent individual molecules in layered packing motif viewed
from the direction normal (d) and parallel (e) to the (−1 0 1) plane.
substituents on the nitogen with respect to the conjugated
SN5-backbone. In thin films, the absorbance of DCV-SN5 2
decreased by a factor of about 20% compared to 1 and 3 which
is ascribed to the presence of insulating hexyl substituents. Fur-
thermore, it should be noted that in comparison to the frequently
used ZnPc[18] the film spectra of DCV-SN5 derivatives showed
significant spectral broadening and strong increase in the absorb-
ance and therefore could act as better light harvesters (Figure 3b).
The redox properties of 1–4 were determined by cyclic voltam-
metry in a dichloromethane/tetrabutyl ammonium hexafluoro-
phosphate (0.1 M) electrolyte and the potentials were referenced
against the redox couple ferrocene/ferricenium (Fc/Fc+), which
was set to −5.1 eV vs vacuum[19] in order to consequently calculate
the frontier orbital levels. The data are summarized in Table 1
and representative CVs shown in Figure S6 in the Supporting
Information. The trends seen in the optical measurement are
continued and fused 1–3 showed a much lower first oxida-
tion potential and therefore a destabilized HOMO compared
to parent 4. The first reduction is at more negative potentials
for the former series and the LUMOs are slightly stabilized, in
total however, the electrochemical gap is diminished in agree-
ment with the optical data. Still the HOMO energy levels of 1–3
lie sufficiently low to assure high open circuit voltages (VOC) in
bulk heterojunction solar cells and the LUMOs are sufficiently
high to provide efficient electron transfer to the LUMO of the
acceptor C60 ( ≈ −4.1 eV) used in the solar cells.[20]
We performed quantum chemical DFT calculations on the
fused DCV-SN5 1–3 in comparison to parent DCV3T 4 in order to
see the influence of the rigidification on the electron density dis-
tribution in the respective systems. The reproduction of the fron-
tier orbitals of DCV-SN5 1 and DCV3T 4 are shown in Figure 4.
In contrast to the nonfused system, the electron density distri-
bution in the HOMO of the fused system showed a noticeable
contribution of the S and N heteroatoms.[12,17] Furthermore, a
continuous electron density is visible in the conjugated π-system
in contrast to the classical nodal HOMO distribution of non-
fused DCV3T 4 (Figure 4, bottom). In both cases, the LUMOs
exhibited the classical quinoidal character with the electron den-
sity extended to the acceptor groups (Figure 4, top). The first
electronic transition, which is calculated to be predominantly
HOMO → LUMO revealed stronger oscillator strenght for DCV-
SN5 1 (f = 2.1) compared with non-fused 4 (f = 1.7). High values
of the state dipoles evidence the CT-character of the transitions.
Figure 3. UV–vis spectra of DCV-SN5 1–3 and parent oligomer DCV3T
4 in (a) dichloromethane solution at 25 °C and in (b) 30 nm thick films
deposited by vacuum sublimation on glass substrates. The film spec-
trum of the frequently used zinc phthalocyanine (ZnPc)[18] is shown for
comparison.
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Table 1. Optical and electrochemical properties of S,N-heteropentacenes 1–3 and DCV3T 4.[17]
CVc)
Oligomer
Egopt sola)
[eV]
Stokes shift
Egopt film a)
E0
[V]
E0
[V]
E0
red1
[V]
HOMOb) LUMOb)
Eg
λ
abs sol
[nm]
ε
λ
[nm]
em sol
λ
abs film
[nm]
ox1
ox2
[cm−1
]
[eV]
[eV]
[eV]
[eV]
[L mol−1
cm−1
]
1
2
3
4
582
582
575
495
140 000
139 100
108 800
59 300
1.99
2.01
2.04
2.20
616
612
605
564
opt
714
579
595, 637
594, 630
579, 624
528, 574
1.69
1.73
1.75
1.85
0.61
0.62
0.67
1.06
1.22
1.23
1.25
–
–1.43
–1.50
–1.57
–1.25
–5.64
–5.65
–5.70
–6.09
–3.78
–3.79
–3.79
–3.90
1.86
1.86
1.91
2.19
691
1047
a)Estimated using the onset of the UV–vis spectra, Eg = 1240/λonset
;
b)Estimated from the onset of the respective redox waves, Fc/Fc+ value set to −5.1 eV vs vacuum;
c)Calculated from Eg = ELUMO −EHOMO
.
far lowest for the tolyl counterpart (0.7%). The reasonable va-
lues indicate that excitons are efficiently separated at the D-A
interface and readily transported through the bulk. The satu-
ration (defined as J( −1 V)/JSC) close to unity, i.e., a weak vol-
tage bias dependence of the current in reverse direction, deter-
mined for the alkyl derivatives 1 and 2 indicates that excitons
are efficiently separated at the D–A interface and readily trans-
ported through the bulk.[9,17] The FF, however, decreases from
0.70 for oligomer 1 to 0.49 for oligomer 2. This points to an
impaired transport in the active mixed layer of 2 and C60 leading
to an increased voltage-dependent charge-carrier recombination
between 0 V and VOC. The longer alkyl chain length of 2 and,
consequently, a higher intermolecular distance of donor mole-
cules in the bulk could supply a possible explanation for this
behaviour. In contrast to oligomers 1 and 2, the 3-based device
showed a much lower performance. The low FF of 0.40 and the
saturation of 1.59 suggest poor charge carrier transport. The
JSC is lower than in the 1- and 2-based devices despite similar
chemical and energetic structures, as well as similar absorp-
tion. One possible explanation for this behaviour could be the
trapping of charge carriers on the tolyl group leading to voltage
independent recombination.
Figure 4. Representative HOMO/LUMO electron-density distribution of
DCV-SN5 propyl derivative 1 and DCV3T 4 as reference.
In order to understand the influence of the substituents
at the amino groups in DCV-SN5 1–3, bulk heterojunction
solar cells with m-i-p-type (metal-intrinsic-p-doped) device
architecture[6,15] were prepared by vacuum deposition. The
DCV-SN5 1–3 were used as the donor (D) and C60 as the
acceptor (A) of the bulk heterojunction. The layer sequence in
the device consisted of ITO/C60 (15 nm)/DCV-SN5 1–3: C60
(20 nm)/BPAPF (10 nm)/BPAPF:NDP9 (45 nm, 10 wt%)/NDP9
(1 nm)/Au (50 nm). The D:A ratio was 1:1 by volume and the
two materials were coevaporated at a substrate temperature of
90 °C, which is important for the crystalline growth of the blend
layer and enhanced fill factors.[21] The BPAPF p-doped with
NDP9 (10 wt%) was used as the hole transport layer (HTL), on
top of which another 1 nm thick layer of NDP9 was deposited
to facilitate charge extraction. 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.[22] The J–V characteristics of the
solar cells are shown in Figure 5a and summarized in Table 2.
Corresponding EQE-spectra are depicted in Figure 5b.
All the solar cells showed high open-circuit voltages (VOC) of
around 0.95 V. These values are due to the low-lying HOMOs
and typical for DCV-substituted oligothiophenes.[8,17] With res-
pect to the other photovoltaic parameters, the short-circuit cur-
rent density (JSC), fill factor (FF), saturation, EQE, and series
resistance (RS), the alkylated derivatives 1 and 2 clearly outper-
formed arylated DCV-SN5 3 which as well reflects the findings
for the optoelectronic properties. The results were further sup-
ported by characterization of the photoactive blend layer by
AFM. In total, a very good efficiency of 5.6% was found for
propyl derivative 1, followed by the hexylated 2 (3.7%), and by
At this point it is noteworthy, that parent, nonbridged oli-
gothiophene DCV3T 4 and C60 did not lead to an efficient
exciton separation into charge-carrier pairs at the interface
because of the low HOMO offset between the two materials
leading to a poor solar cell performance.[14]
Because of the promising solar-cell performance of propyl
derivative 1, we further optimized and investigated the influ-
ence of layer thickness and substrate heating temperature on
the device performance. The data are summarized as well in
Table 2 and shown in Figure S7 in the Supporting Information.
Whereas only marginal changes in VOC and fill factors were
observed by increasing the photoactive layer thickness from
20 nm to 30 nm, the JSC values were improved using the
thicker layers. Irrespective of the film thickness, a continuous
increase in the FF was observed with increasing substrate tem-
perature from 70 °C to 90 °C reaching a maximum value of ca.
0.70, showing that exciton and charge separation efficiencies
become less field-dependent with increasing temperature.
The best performance of a DCV-SN5 1:C60 BHJ solar cell
was finally achieved with a 30 nm active layer deposited at
90 °C substrate temperature exhibiting a mismatch-corrected
JSC value of 10.2 mA cm−2, a VOC of 0.92 V, and a FF of 0.69.
This translates to 6.5% efficiency, which is among the best
values for vacuum-processed single junction cells. It should be
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respect to efficiency. The improvement in JSC is in good agree-
ment with the external quantum efficiency (EQE) measure-
ments. The maximum EQE value for the most efficient device
is close to 70% at 600 nm covering a broad spectral range from
300–700 nm. The improvement in device performance by
increasing the substrate temperature is attributed to a change
in the mixed layer morphology and resulting stronger phase
separation induced by the applied substrate heating.[21,23] All
devices at higher temperature showed a low photocurrent sat-
uration value of ca. 1.05 indicating efficient charge separation
and extraction.[17]
In this respect, the surface morphology of 20 nm thick
1–3:C60 photoactive blend layer prepared at 90 °C substrate
temperature were investigated by atomic force microscopy
(AFM) (Figure 6). Strong differences were seen for the DCV-
SN5 derivatives 1–3 and are clearly a consequence of the dif-
ferent N-substitution. The surface of the 1:C60 blend layer is
uniformly composed of grains with a diameter of 30–50 nm
at an average height of 14 nm (Figure 6a and Figure S8 in the
Supporting Information) indicating better pathways for charge
transport. The analysis of the corresponding phase images
showed relevant phase shifts up to 74° (Figure 6d) and the
histogram over several phase images discloses two well dif-
ferentiated maxima of the phase-shift at 14° and 56°, which
we attribute to the acceptor and donor homophases.[24–26]
The phase images revealed 20–30 nm isolated islands of the
donor phase, which cover almost 25% of the surface area. The
acceptor regions (low phase-shift) formed an interconnected
network of thin fibrils whereas the mixed phase (ca. 50% of
the surface area) appeared to surround the donor islands and is
as well in contact with the acceptor network (Figure S9a, Sup-
porting Information).
The surface of 2:C60 blend layers showed a less pro-
nounced grain structure with a small average height of 2.6 nm
(Figure 6b and Figure S8 in the Supporting Information). The
phase images revealed a much finer structure, in which only
very few and small islands of 1–10 nm (ca. 5% of the area)
correspond to the donor phase and over 50% of the surface
is covered by thick interconnected low phase-shift acceptor
regions (Figure 6e and Figure S9b in the Supporting Informa-
tion). The mixed areas in the case of 2:C60 are forming islands
mostly in contact with the acceptor network.
Figure 5. J–V characteristics (a) and EQE spectra (b) of m-i-p bulk-het-
erojunction solar cells prepared with DCV-SN5 1–3 as donor and C60 as
acceptor (active area 6.4 mm2).
noted that the devices were standard test structures for mate-
rial comparison purposes without optimisation for microoptic
cavity effects. Also, the Au counter electrode is suboptimal with
Table 2. Photovoltaic parameters of BHJ-devices of DCV-SN5 derivatives 1–3:C60 (blend ratio 1:1). Photovoltaic parameters of BHJ devices com-
prising 1:C60 (1:1) under variation of thickness and substrate temperature. Active area 6.4 mm2.
DCV-SN5 Substrate temp.
Thickness
[nm]
JSC
[mA cm−2
VOC
[V]
FF
PCE
[%]
Sat.a)
EQE @ 600 nm
[%]
RS
[Ω cm2]
[°C]
]
3
2
1
1
1
1
1
1
90
90
90
80
70
90
80
70
20
20
20
20
20
30
30
30
1.94
7.97
8.43
8.53
8.60
10.20
9.60
9.60
0.95
0.95
0.94
0.96
0.94
0.92
0.95
0.94
0.40
0.49
0.70
0.61
0.36
0.69
0.59
0.42
0.7
3.7
5.6
5,0
2.9
6.5
5.4
3.8
1.59
1.09
1.05
1.06
1.20
1.05
1.06
1.13
17
62
63
58
62
70
66
72
19.6
4.3
3.0
3.2
4.1
3.1
3.6
4.0
a)Defined as J( −1 V)/JSC
.
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Figure 6. Tapping mode AFM topography (top) and phase (bottom) images (1 µm × 1 µm) of blend films (1:1 w/w deposited at 90 °C substrate tem-
perature) of 1:C60 (a,d), 2:C60 (b,e) and 3:C60 (c,f).
Finally, the AFM topography images of the surface of 3:C60
showed an almost featureless surface with few large aggregates
(25–50 nm, height of up to 27 nm) (Figure 6c and Figure S8
in the Supporting Information). The very small phase shifts
(<5°) evidence the lack of phase separation in this blend
(Figure 6f). Therefore, fast recombination of the gener-
ated charges should occur due to lack of percolation path-
ways, which is consistent with the low FF (0.40) and JSC
(2.8 mA cm−2) of the device.
which combine increased stability of oligothiophenes with the
rigid and extended π-conjugated backbone of oligoacenes, high-
performance donor materials for organic solar cells have been
designed and prepared. The transfer of the concept to larger
S,N-heterooligoacenes and other ring-fused conjugated systems
is under way in our laboratory.
Supporting Information
In conclusion, we have reported the synthesis and charac-
terization of novel acceptor-substituted S,N-heteropentacenes
DCV-SN5 1–3 as next-generation A–D–A-type molecular donors
for vacuum-processed organic solar cells. The three deriva-
tives structurally differ by their alkyl substituent at the amino
nitrogen (propyl/hexyl/tolyl). Single-crystal X-ray structure ana-
lysis of DCV-SN5 1 gave deeper insight into the specific inter-
molecular interactions and multiple nonbonding short con-
tacts, which are responsible for the packing with strong π–π
interaction and multidirectional electronic coupling. The optical
and electrochemical properties have been studied to determine
the optical absorption and energy levels. Single-junction m-i-p
type solar cells fabricated with propyl derivative 1 as donor and
fullerene C60 as acceptor gave the best PCE of 6.5% in the series
due to enhanced photocurrent (JSC) and fill factor (FF). The
devices were optimized by varying substrate temperature and
film thickness. The alkyl substitution in the series of DCV-SN5
1–3 strongly influences the device performance, which we
attribute to the very different blend morphology and phase
separation, as evidenced by AFM-techniques. Our investigation
demonstrates that with this novel class of fused heterocycles,
Supporting Information is available from the Wiley Online Library or
from the author. The Supporting Information includes experimental
details, synthesis, and analytical data of the oligomers, device fabrication
details and characterization, DSC plots, TGA plots, cyclic voltammetry,
and AFM measurements.
Acknowledgements
This work was supported by the Ministry for Education and Research
(BMBF, program LOTsE 03EK3505G). The authors thank Novaled
GmbH for providing NDP9.
Received: June 2, 2014
Revised: July 15, 2014
Published online:
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