Significant improvement of optoelectronic and photovoltaic properties by incorporating thiophene in a solution-processable D-A-D modular chromophore

Article abstract of DOI:10.3390/molecules201219798

Through the incorporation of a thiophene functionality, a novel solution-processable small organic chromophore was designed, synthesized and characterized for application in bulk-heterojunction solar cells. The new chromophore, (2Z,2′Z)-2,2′-(1,4-phenylen

Full text of DOI:10.3390/molecules201219798

Article
Significant Improvement of Optoelectronic and
Photovoltaic Properties by Incorporating
Thiophene in a Solution-Processable D–A–D
Modular Chromophore
Aaron M. Raynor 1, Akhil Gupta 1,2,*, Christopher M. Plummer 1, Sam L. Jackson 1, Ante Bilic 3,
Hemlata Patil 1, Prashant Sonar 4,* and Sheshanath V. Bhosale 1,
*
Received: 6 October 2015 ; Accepted: 25 November 201 5; Published: 4 December 2015
Academic Editor: Sergei Manzhos
1
School of Applied Sciences, The Royal Melbourne Institute of Technology (RMIT) University, GPO Box 2476,
Melbourne Victoria 3001, Australia; aaron.raynor@rmit.edu.au (A.M.R.);
chris.plummer@rmit.edu.au (C.M.P.); sam.jackson@rmit.edu.au (S.L.J.); hemlatap2@gmail.com (H.P.)
Medicinal Chemistry, Monash Institute of Pharmaceutical Sciences, Monash University,
2
Parkville Victoria 3052, Australia
Virtual Nanoscience Lab, Commonwealth Scientific and Industrial Research Organization (CSIRO)
3
Manufacturing, Parkville Victoria 3052, Australia; ante.bilic@csiro.au
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT),
4
GPO Box 2434, Brisbane QLD 4001, Australia
*
Correspondence: akhil.gupta@monash.edu (A.G.); sonar.prashant@qut.edu.au (P.S.);
sheshanath.bhosale@rmit.edu.au (S.V.B.); Tel./Fax: +61-3-9925-2680 (S.V.B.)
Abstract: Through the incorporation of a thiophene functionality, a novel solution-processable
small organic chromophore was designed, synthesized and characterized for application
in bulk-heterojunction solar cells. The new chromophore, (2Z,21Z)-2,21-(1,4-phenylene)bis(3-
(5-(4-(diphenylamino)phenyl)thiophen-2-yl)acrylonitrile) (coded as AS2), was based on a donor–
acceptor–donor (D–A–D) module where a simple triphenylamine unit served as an electron donor,
1,4-phenylenediacetonitrile as an electron acceptor, and a thiophene ring as the π-bridge embedded
between the donor and acceptor functionalities. AS2 was isolated as brick-red, needle-shaped crystals,
and was fully characterized by 1H- and 13C-NMR, IR, mass spectrometry and single crystal X-ray
diffraction. The optoelectronic and photovoltaic properties of AS2 were compared with those of
a structural analogue, (2Z,21Z)-2,21-(1,4-phenylene)bis(3-(4-(diphenylamino)phenyl)-acrylonitrile)
(
AS1). Benefiting from the covalent thiophene bridges, compared to AS1 thin solid film, the AS2
film showed: (1) an enhancement of light-harvesting ability by 20%; (2) an increase in wavelength of
the longest wavelength absorption maximum (497 nm vs. 470 nm) and (3) a narrower optical
band-gap (1.93 eV vs. 2.17 eV). Studies on the photovoltaic properties revealed that the best
AS2-[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)-based device showed an impressive
enhanced power conversion efficiency of 4.10%, an approx. 3-fold increase with respect to the
efficiency of the best AS1-based device (1.23%). These results clearly indicated that embodiment of
thiophene functionality extended the molecular conjugation, thus enhancing the light-harvesting
ability and short-circuit current density, while further improving the bulk-heterojunction device
performance. To our knowledge, AS2 is the first example in the literature where a thiophene unit has
been used in conjunction with a 1,4-phenylenediacetonitrile accepting functionality to extend the
π-conjugation in a given D–A–D motif for bulk-heterojunction solar cell applications.
Keywords:
solution-processable;
bulk-heterojunction devices;
donor–acceptor–donor;
triphenylamine; thiophene; 1,4-phenylenediacetonitrile
Molecules 2015, 20, 21787–21801; doi:10.3390/molecules201219798
www.mdpi.com/journal/molecules

Molecules 2015, 20, 21787–21801
1. Introduction
The development of renewable energy technologies is pivotal for accommodating the ever
increasing energy demands of the modern society. Such technologies are also important for lowering
environmental pollution and greenhouse gas emissions. Towards this objective, many approaches to
harvest solar energy have been investigated. The fabrication of bulk-heterojunction (BHJ) devices is
one such promising strategy that has attracted considerable attention over the past two decades due to
their advantages of being lightweight, low cost and their flexibility in large-area applications [1–7].
Such devices are comprised of an interpenetrating network of organic donor and acceptor domains
that is formed during their fabrication via solution processing. Conventionally, semiconducting
donor polymers such as poly(3-hexylthiophene) (P3HT) and acceptors such as soluble fullerene
derivatives, PC61BM and its C71 analogue (PC71BM), have been used to obtain a deeper understanding
of device design and morphology [
been developed and significant progress has been attained with promising BHJ architecture. Power
conversion efficiency (PCE) values above 10% has been reported with such polymeric donors [14 17].
8–13]. Apart from archetypal P3HT, conjugated polymers have also
–
In the interim, solution-processed small molecular donor-based BHJ devices have also aroused
interest, mainly due to their advantages of well-defined chemical structure, convenient purification
methods, such as simple column chromatography, and monodisperse molecular weight [18–23]. These
advantages allow and encourage researchers to exert efforts for the design and development of small
molecular donors. Recently, immense efforts have been dedicated to developing small molecular-based
solution-processable organic solar cells [1,21] and so far, the highest PCE of 9.95% was achieved by
Kan et al. [24] which is analogous to those of the polymer-based solution-processable devices. Thus, in
view of such reports and the fact that BHJ devices incorporating small molecular donors can compete
with polymer-based devices, there is an overwhelming interest in developing small molecular donors.
Recent years have seen a dramatic surge not only in terms of device efficiency using small
molecular donors but also in their design and efficient synthetic development. A variety of small
molecule donor materials based on donor–acceptor (D–A) combinations such as D–A–D [25
–27],
A–D–A [28 29], D– –A [18 19 30] and star-shaped architectures [31] have been reported. The D–A–D
,
π
, ,
design in particular is one of the most promising and successful modules based on which various donor
and acceptor units have been explored for high-performance solution-processable photovoltaic devices.
A finite number of central accepting units, such as naphthalene diimide [25], diketopyrrolopyrrole [27],
2-pyran-4-ylidenemalononitrile [32] and thiazolothiazole [33] have been reported to suit the D–A–D
module. Not only that the availability and selection of such accepting blocks is limited, it is furthermore
imperative that the target chromophore must possess a low optical band gap, broad absorption profile,
high mobility and appropriately tuned highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels using such blocks. Such requirements do possess
a challenge for an organic chemist who must consider such factors while designing a new chromophore
based on the D–A–D module. Therefore, it is not surprising that there exists an enormous scope for the
design and development of new light-harvesting materials based on the challenging D–A–D module
and is an aspiration for most of the researchers.
In our own studies of small molecule chromophores and charge transport materials based on
a variety of D–A combinations, we have reported examples of successful solution-processable BHJ
photovoltaic devices [18,19,25,34–37]. Furthermore, we are highly interested to extend our efforts
on the design and development of new chromophores that are inspired by the D–A–D module.
In this study, we report the design, facile synthesis and characterization of the optoelectronic and
photovoltaic properties of two small organic chromophores, AS2 and AS1, (shown in Figure 1),
and their direct comparison. Both materials are based on a D–A–D structural motif where a
triphenyl-amine functionality has been chosen as a common donor at both ends of the central
acceptor unit, 1,4-phenylenediacetonitrile (PDA), so as to get symmetrical AS2 and AS1. AS2 is
a structural analogue of AS1 where a thiophene ring has been introduced between the donor and
acceptor functionalities in order to vary the optoelectronic and photovoltaic properties (Figure 1).
21788

Molecules 2015, 20, 21787–21801
When compared with the commonly used accepting groups, such as dicyanovinylidene, aromatizable
cyanopyridone, indenedione or oxoindenemalononitrile [38
extended -conjugation over the whole molecular backbone, mainly due to its bidentate nature. It is
notable to mention that the use of the PDA unit for electroluminescent conjugated polymers has been
reported in the literature [40 41] as well as one- and two-photon spectroscopy studies [42 46]. However,
its suitability as an acceptor in small molecular chromophores, particularly D–A–D modular, for BHJ
applications is still unknown [ 21]. This provides an encouragement and some strong incentive for
,39], PDA can be an acceptor of choice for
π
,
–
4,
its investigation. Owing to the exploration of PDA acceptor unit in the D–A–D module, this study
continues our search for the generation of new organic chromophores for BHJ applications.
Figure 1. Molecular structures of the newly designed AS2 and reference AS1 materials investigated in
this study.
2. Results and Discussion
2.1. Design Strategy, Synthesis
The materials AS2 and AS1 were synthesized via the Knoevenagel condensation of appropriate
aldehydes with active methylene groups of the PDA acceptor unit and their chemical structures
were confirmed by 1H- and 13C-NMR, mass spectrometry, and, where possible (AS2 only), by single
crystal X-ray diffraction (XRD). The synthetic methodology for synthesizing AS1 was similar to an
old literature report [42], albeit dissimilar base and solvent were used to deal with a homogeneous
reaction solution. Knoevenagel condensation reaction of an aldehyde with active methylene group is
an efficient way of generating a double bond between a π-bridge and acceptor functionality. The use of
such chemistry is a common strategy to generate organic sensitizers for dye-sensitized solar cells [47].
However, the use of same strategy to develop small molecular chromophores for BHJ applications
is still limited [4,21]. Herein, not only we are demonstrating the Knoevenagel condensation reaction
of PDA acceptor unit but also the fabrication of solution-processable BHJ devices incorporating a
fullerene acceptor (PC61BM) and either AS2 or AS1 as a donor component (Figure 1). To the best of
our knowledge, this is the first time PDA has been used to generate D–A–D modular small molecular
chromophores for BHJ applications. Initial screening of the BHJ devices revealed that greater PCE
was achieved for AS2 (4.10% for AS2 compared with 1.23% for AS1), as confirmed by the increased
short-circuit current density (8.01 mA
¨
cm´2 for AS2 and 3.15 mA cm´2 for AS1), under simulated
¨
AM 1.5 illumination (100 mW¨ cm´2).
Both materials were based on the D–A–D module and the central acceptor moiety was directly
linked to the terminal donor functionalities in order to create a conjugated structure. The development
of these target materials incorporates the use of two identical donor units (triphenylamine) on each
side of the central core, resulting in symmetrical chromophores. Insertion of a planar, conjugated
functionality, such as thiophene in AS2, between the donor and acceptor components of a target
21789

Molecules 2015, 20, 21787–21801
material can provide greater absorption over visible light spectrum when compared with otherwise
structurally similar compounds [38,48]. Moreover, the selection of thiophene over highly aromatic,
conjugating functionalities, such as phenyl, was based on the earlier work reported by Würthner
et al. [49] and Gupta et al. [50] where it has been demonstrated that replacement of a phenyl group
with thiophene can provide significant spectral red-shifts and is advantageous for superior charge
delocalization. As a result, AS2 is deemed to exhibit a large red shift of lambda maximum when
compared with the reference compound AS1. Both of the materials were synthesized per the reaction
shown in Scheme 1 and were purified by simple column chromatography. Brick-red, needle-shaped
crystals, suitable for single crystal XRD, were prepared by diffusing methanol into a dichloromethane
solution of AS2, over approximately three days. However, no crystal growth was observed for AS1
Both materials were synthesized in moderate to high yields (AS2 = 63% and AS1 = 86%) and were
highly soluble in a variety of common organic solvents, for example chlorobenzene, chloroform, and
toluene. The solubility of organic p-type materials is paramount for fabricating solution-processable
BHJ devices and both the materials fulfill this criterion. In fact, the solubility of AS2 was found to be
higher by 50% w/v when compared with AS1.
.
Scheme 1. Reaction scheme for the synthesis of AS2 and AS1.
The precursor aldehyde
to obtain yellow, needle-shaped crystals. Diffraction measurements were performed at 200 K on
a Bruker Apex II CCD diffractometer using Mo K radiation( = 0.71073 Å) The structure were
1 of AS2 was also crystallized by diffusion of methanol into chloroform
α
λ
solved using dual space methods using the program SHELX-2014/7 [51] using the Olex2 1.2 GUI [52],
with anisotropic thermal parameters for all non-hydrogen atoms. All non-hydrogen atoms were
refined anisotropically by full-matric least-squares methods SHELX-2014/7. Molecular drawings were
obtained using Mercury [53]. The utility of
however, its use to generate small molecular chromophores for BHJ applications is seldom
reported [ 21]. This state of affairs encourages us and provides a strong incentive to report its synthesis
and crystal growth strategy.
The compound was crystallized in the monoclinic space group (P 21/c) with four asymmetric
1 for dye-sensitized solar cells has been reported [54],
4
,
1
units in one cell. The thiophene group and the adjacent phenyl groups are planar with pendant
phenyl groups displaced around nitrogen. The packing consist of a two-fold screw axis with centers
of inversion between sulfur molecules as well as a glide plane perpendicular to the thiophene plane.
The packing is dominated by π-π face-to-face stacking between the thiophene and phenyl groups.
The pendant phenyl groups are stabilized by
2.771–3.283 Å as shown in Figure 2.
π-π edge-to-face stacking with distances in the range
21790

Molecules 2015, 20, 21787–21801
Figure 2. Packing of 1 along the b axis.
AS2 co-crystallizes with chloroform in the triclinic space group (P-1) with a center of symmetry
around the central phenyl group. The packing is dominated by face-to-face stacking between
central phenyls and thiophenes, and by edge-to-face -stacking between pendant phenyl groups
π-π
π-π
with the distance 2.772 Å as shown in Figure 3. The details of packing structure, formula, crystal size
of 1 and AS2 are described in Table 1.
Table 1. Details of crystal data and structure refinement parameters of 1 and AS2.
Identification Code
CCDC: 1420377 (1)
CCDC: 1420378 (AS2)
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
C23H17NOS
355.43
200(2)
monoclinic
P21/c
C31H22OS
442.55
200(2)
monoclinic
P21
19.916(3)
6.6680(9)
13.4336(16)
90
96.613(3)
90
1772.1(4)
4
1.332
5.5812(7)
47.120(6)
9.2435(11)
a/Å
b/Å
c/Å
α/˝
90
β/˝
103.343(3)
90
2365.3(5)
γ/˝
Volume/Å3
Z
4
ρcalcg/cm3
1.243
0.158
µ/mm´1
0.194
F(000)
744.0
928.0
Crystal size/mm3
Radiation
0.292 ˆ 0.076 ˆ 0.063
MoKα (λ = 0.71073)
4.118 to 48.588
0.667 ˆ 0.137 ˆ 0.087
MoKα (λ = 0.71073)
3.458 to 67.842
2Θ range for data collection/
Index ranges
´
22
ď
h
ď
23,
´
7
ď
k
ď
7,
´
15
ď
l
ď
15
´8 ď h ď 8, ´73 ď k ď 73, ´14 ď l ď 12
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I ě 2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole/e Å´3
15255
83,300
2883 [Rint = 0.0707, Rsigma = 0.0447]
19,180 [Rint = 0.0616, Rsigma = 0.0596]
19180/1/595
2883/0/235
0.949
1.028
R1 = 0.0412, wR2 = 0.1095
R1 = 0.0720, wR2 = 0.1274
0.17/´0.26
R1 = 0.0659, wR2 = 0.1531
R1 = 0.0972, wR2 = 0.1684
0.29/´0.39
21791

Molecules 2015, 20, 21787–21801
Figure 3. Packing of AS2 along the c axis.
2.2. Optoelectronic Properties
The optical properties of AS2 and AS1 were investigated by measuring their ultraviolet–visible
(UV–Vis) absorption spectra in chloroform solution and in pristine spin-cast films (Figure 4). The
longest wavelength absorption maximum (
nm which was red-shifted by 23 nm when compared with the solution
absorption maximum and extinction coefficient (AS2 = 59,000 M´1 cm´1 AS1 = 49,000 M´1 cm´1
)
λmax) exhibited by AS2 in solution form was at 459
λ
max
of AS1. Both the
¨
;
¨
increased with the insertion of thiophene functionality. With the insertion of thiophene ring we found
enhancement to the peak molar absorptivity of >20% of AS2 compared with AS1. This enhanced profile
allows a larger amount of the solar spectrum to be absorbed, thus exhibiting greater intramolecular
charge transfer (ICT) transition. We observed a similar bathochromic absorption shift in the thin
film spectrum of AS2 compared with that of AS1 (Figure 4). The strong red-shift is attributed to the
extended π-conjugation within the molecular backbone of AS2 that became possible with the insertion
of thiophene functionality. This type of control over the absorption profile through the insertion of a
strongly conjugated unit can help to fine tune optical energy levels, to enhance light harvesting and
BHJ device performance.
Density functional theory (DFT) calculations using the Gaussian 09 suite of programs [55
]
and the B3LYP/6-311 + G(d,p)//B3LYP/6-31G(d) level of theory indicated that the HOMO orbital
densities of both AS2 and AS1 have a major distribution over the whole molecular backbone and
the LUMO densities were delocalized through the central acceptor functionality and adjacent rings
(Figure 5). Inserted thiophene rings in case of AS2 can accommodate LUMO density with almost equal
contribution as the central PDA, thus sparing the adjoining phenyl rings of the donor triphenylamine
for an efficient segregation of HOMO and LUMO densities. Such separation is ideal for the ICT
transition and is attributed to the presence of a strong conjugated unit, of which thiophene is an
example, in the given D–A–D system. Experimental estimation of the HOMO energies was carried out
using photo electron spectroscopy in air (PESA) and the LUMO energies were calculated by adding
the optical band gap (film spectra) to the HOMO values (see Figure 6 for energy level diagram and
Figure S1 [see Supplementary Material] for the PESA curve). Film spectra indicated that insertion
of thiophene reduces the band gap of AS2 by 0.24 eV when compared with AS1. Furthermore, the
estimation of HOMO using PESA revealed that the HOMO energy level of AS2 was raised by 0.18
eV when compared with the HOMO level of AS1. This is in agreement with the DFT calculations
that the presence of thiophene indeed plays a crucial role for: (1) density segregation; (2) tuning the
optical energy levels; and (3) theoretical and experimental band gap reduction. These measurements
21792

Molecules 2015, 20, 21787–21801
and calculations provide a strong rational for our design strategy that the induction of a conjugated
functionality can indeed improve the optoelectronic properties of a given chromophore. The energy
level diagram advised that the band gaps of these materials are all in the range required of donor
materials for BHJ devices. AS2 optical band gap is somewhat narrower in magnitude than 2.0 eV
measured for the conventional P3HT. The optical and electrochemical properties of both the materials
in solution and film form are summarized in Supplementary Material Table S1.
Figure 4. Normalized absorption spectra of compounds AS2 and AS1 in CHCl3 solutions (upper) and
for pristine as-casted films (lower); (films of AS2 and AS1 were spin-coated at 2000 rpm for 1 min to
give a film thickness of ~70 nm).
Figure 5. Orbital density distribution for the frontier molecular orbitals of AS2 and AS1. DFT
calculations were performed using the Gaussian 09 suite of programs and the B3LYP/6-311 +
G(d,p)//B3LYP/6-31G(d) level of theory. Theoretical HOMO/LUMO energy levels and band-gaps (vs.
Vac scale) are also shown.
21793

Molecules 2015, 20, 21787–21801
Figure 6. Energy level diagram depicting the band gaps of AS2 and AS1 in comparison with P3HT and
PC61BM. Experimental estimation of the HOMO energies was carried out using PESA and the LUMO
energies were calculated by adding the optical band gap (film spectra) to the HOMO values.
Encouraging though the optoelectronic properties are, the compounds must display thermal
stability given the rigorous conditions used in device fabrication such as annealing at temperature
in the excess of 100 ˝C. In line with this requirement and to determine the thermal stability of AS2
and AS1, thermogravimetric analysis (TGA) was conducted. TGA (Supplementary Material Figure S2)
indicated that both AS2 and AS1 are thermally stable and will not degrade during the annealing of
BHJ devices.
After correlating the optoelectronic properties of AS2 and AS1 with those of the conventional
PC61BM acceptor (see Figure 6), we screened their potency as donor materials (p-type) in
solution-processable BHJ devices under simulated sunlight and monochromatic light illumination.
The blend solutions of both the materials and PC61BM were used to cast an active layer on top of
the PEDOT:PSS surface. The BHJ device architecture used was ITO/PEDOT:PSS (38 nm)/active
layer/Ca (20 nm)/Al (100 nm) where the active layer was a solution processed blend of either AS2 or
AS1 and PC61BM. For AS2, a promising PCE of 4.10% was achieved when the film was spin-coated
from a chlorobenzene solution as a 1:1 blend with PC61BM. By contrast, the maximum PCE obtained
for a device based on AS1 was 1.23%, when fabricated under similar conditions. The comparative
current–voltage curves for the optimized blends of AS2 and AS1 with PC61BM are shown in Figure 7.
High boiling solvents such as chlorobenzene are better not only from processing point of view but
also for achieving smoother films without crystallization occurring on the active surfaces. Latter was
particularly true as our efforts to construct BHJ devices using a low-boiling solvent, such as chloroform,
afforded either uneven surfaces or minor cracks on the active surfaces.
The optimized devices based on AS2 exhibited a decreased open circuit voltage (Voc) than the
devices based on AS1. This in fact is consistent with the measured HOMO values where a higher
HOMO for AS2 would predict a lower Voc. On the other hand, the short circuit current density (Jsc)
for the devices fabricated using AS2 was higher than the Jsc extracted from the devices based on
AS1. This was in agreement with the observed bathochromic shift in the absorption spectrum of
AS2 compared with AS1. The photovoltaic cell parameters for AS2-based devices were 0.88 V [open
circuit voltage, Voc], 8.01 mA
cm´2 [current density, (Jsc)], 0.58 [fill factor, (FF)] and 4.10% [power
¨
conversion efficiency, (PCE)]. Initial screening of the BHJ devices based on AS1:PC61BM showed
moderate device performance with a high Voc of 0.90 V, FF of 43% Jsc of 3.15 mA/cm2 and an overall
PCE of 1.23%. Taken as a whole, the insertion of thiophene functionality into the studied D–A–D
21794

Molecules 2015, 20, 21787–21801
structural motif incorporating PDA acceptor functionality resulted in significant enhancement of
Jsc and PCE values by factors >2 and >3, respectively, thus promoting the use of a smaller conjugated
functionality, such as thiophene, as an interesting structural concept for the design and development
of highly efficient BHJ materials. Furthermore, it is notable to mention that AS2 showed optimal
performance with inexpensive PC61BM and simple device architecture, thus providing some strong
incentive to apply the design concept reported in this work to the new generations of BHJ materials.
Figure 7. Current–voltage curves for the optimized devices based on AS2 and AS1 in blends
with PC61BM (1:1 w/w) under simulated sunlight (AM 1.5, 1000 W m
´2). Device structure was:
¨
ITO/PEDOT-PSS (38 nm)/active layer/Ca (20 nm)/Al (100 nm) where the active layers were the blends
of either AS2 or AS1 and PC61BM spun on top of the films of PEDOT:PSS using chlorobenzene solvent.
The incident photon-to-current-conversion efficiency (IPCE) spectra of these BHJ devices are
shown in Figure 8. The IPCE measurement of these BHJ devices was broad spectrum, typically
covering most of the visible range, from 350 to 750 nm. AS2 and AS1 exhibited high plateaus at ~40%
and ~27% for the best BHJ devices respectively. The IPCE spectrum of AS2, which carried a thiophene
functionality, was red-shifted when compared with AS1, a finding that is consistent with the result of
thin film absorption spectrum. The significantly higher peak IPCE of AS2 compared to AS1 indicated
that the superior performance of AS2 can be rationalized in terms of enhanced light-harvesting and
appropriately tuned optical energy levels, thereby corroborating the design principle.
Figure 8. IPCE spectra of AS2 and AS1 with PC61BMblends.
21795

Molecules 2015, 20, 21787–21801
To examine the physical microstructure of the blend surface, we used atomic force microscopy
(AFM) in tapping mode. The actual surface morphology of the blend films of AS2/AS1:PC61BM
(1:1 w/w) is shown in Figure 9. Physically, both the blends were found to be smooth and the root
mean square roughness of 0.41 nm and 0.35 nm was observed for AS2 and AS1 respectively. No
cracks were observed on the film surfaces when the films were spin-casted using chlorobenzene
(3000 rpm). The processing of active films of BHJ devices using a high boiling solvent such as
chlorobenzene is advantageous over low boiling solvent such as chloroform and is in agreement
with the AFM morphology. Our attempts to fabricate BHJ devices using chloroform resulted in very
poor photovoltaic performance. This was mainly due to inferior film quality. Though AS2 exerted
promising PCE in this preliminary work, ample scope still exists to explore device strategies to enhance
PCE. The performance might be improved by (1) using PC71BM or (2) effective interlayer, such as
metal oxide interlayer, which can facilitate the efficient charge extraction, and (3) devising processing
methods, such as use of additives. Work towards some of such strategies is the subject of on-going
work in our laboratories. The discovery of potential materials, such as AS2, exhibiting promising
optoelectronic and photovoltaic properties opens up the way to develop D–A–D modular small organic
chromophores, with the use of PDA acceptor in particular, and paves the way for such materials to be
used for other organic electronic applications.
Figure 9. AFM images of 1:1 blends of AS2 (top) and AS1 (bottom) with PC61BM as-casted from
chlorobenzene solution. Topographic (left) and phase images (right) are shown.
3. Experimental Section
3.1. Materials and Instruments
All reagents and chemicals used, unless otherwise specified, were purchased from Sigma-Aldrich
(Sydney, Australia). The solvents used for reactions were obtained from Merck Speciality Chemicals
(Sydney, Australia) and were used as received. Unless otherwise specified, all 1H- and 13C-NMR spectra
21796

Molecules 2015, 20, 21787–21801
were recorded using a Bruker AV300 spectrometer at 300 and 75 MHz or a Bruker AV400 spectrometer
at 400 and 100 MHz, respectively (Bruker Corporation, Billerica, MA, USA). Chemical shifts (δ) are
reported in parts per million (ppm). Thin-layer chromatography (TLC) was performed using 0.25 mm
thick plates precoated with Kieselgel 60 F254 silica gel (Merck, Darmstadt, Germany) and visualized
using UV light (254 nm and 365 nm). Melting points were measured using a MPD350 digital melting
point apparatus (Gallenkamp, Sanyo, Osaka, Japan) and are uncorrected. High-resolution mass spectra
experiments were carried out on a Q-Exactive FTMS (Thermo Scientific, Bremen, Germany) ionizing
by atmospheric-pressure chemical ionization (APCI) from an ASAP probe. All UV-Vis absorption
spectra were recorded on a Hewlett Packard (HP) 8453 diode array UV-Vis spectrophotometer (Agilent
Technologies, Mulgrave Victoria, Australia). Thin films were spin-coated from chlorobenzene at a
spin speed of 2000 rpm for 1 min onto cleaned glass slides. PESA measurement was recorded using a
Riken Keiki AC-2 PESA spectrometer (RKI Instruments, Union City, CA, USA) with a power setting
of 5 nW and a power number of 0.5. Samples for PESA were prepared on clean glass substrates.
Fabrication and characterization of BHJ devices, and preparation of thin-film transistors has been
reported previously [18,30].
3.2. Synthesis and Characterization of Target Molecules
3.2.1. Synthesis of AS1
1,4-Phenylenediacetonitrile (500 mg, 3.21 mmol) was added to the mixture of
4-(diphenyl-amino)benzaldehyde (1.84 g, 6.74 mmol) in methanol (50.0 mL) at room temperature and
the resulting mixture was heated at reflux overnight. The precipitated solid was collected by filtration,
washed with methanol and dried under vacuum to give 1.85 g (86.3%) of AS1 as an orange powder.
m. p. 242–245 ˝C; HPLC (5% H2O/ACN): 97.6%; IR (solid film, cm´1) 3061, 3035 (Ar -CH str), 2250,
2206 (-CN str), 1580, 1504, 1486 (Ar C=C str), 1330, 1285, 1192, 1179; 1H-NMR (400 MHz, CDCl3):
7.80–7.76 (m, 4 H), 7.68 (s, 4 H), 7.45 (s, 2 H), 7.33–7.28 (m, 8 H), 7.16–7.10 (m, 12 H), 7.05–7.02 (m, 4 H);
δ =
13C-NMR (400 MHz, CDCl3):
δ = 150.2, 146.5, 141.8, 135.1, 130.8, 129.6, 126.2, 126.1, 125.8, 124.5, 120.7,
118.5, 106.6; HRMS (APCI): calculated for C48H35N4 [M+H]+ 667.2856; found 667.2851.
3.2.2. Synthesis of 5-(4-(Diphenylamino)phenyl)thiophene-2-carbaldehyde (1)
A solution of 1,2-dimethoxyethane (DME, 40.0 mL) and 2M Na2CO3 (20.0 mL) is degassed with
nitrogen (N2) for 30 min. To this solution 5-bromothiophene-2-carbaldehyde (191 mg, 1.00 mmol) and
(4-(diphenylamino)phenyl)boronic acid (433 mg, 1.50 mmol) were added and the mixture was heated
˝
at 60 C for 30 min. [Pd(PPh3)4] (110 mg, 0.10 mmol) was added and the resulting mixture was stirred
at 90 ˝C overnight. The reaction mixture was extracted with diethyl ether (3
50 mL). The organic
ˆ
layers were combined, washed with brine (100 mL) and dried over anhydrous magnesium sulfate.
The solvent was evaporated to afford crude yellow solid, which was crystallized from hexane and
chloroform to yield 378 mg (71%) of
cm´1): 3313, 3023, 2794, 1956, 1887, 1720, 1659, 1584, 1527, 1485, 1465, 1324, 1261, 1226, 1177, 1153,
1075, 1054; 1H-NMR (300 MHz, CD2Cl2):
= 9.87 (s, 1H), 7.75 (d, J = 6.8 Hz, 1H), 7.60–7.57 (m, 2H),
7.37–7.31 (m, 5H), 7.71–7.15 (m, 6H), 7.13–7.06 (m, 2H); 13C-NMR (300 MHz, CDCl3):
= 182.9, 154.5,
1
as yellow needle like crystals. m. p. 100–102 ˝C; IR (solid film,
δ
δ
149.5, 147.4, 141.8, 138.1, 129.8, 127.6, 126.5, 125.6, 124.3, 123.4, 122.6; LRMS (ESI; 2% Formic acid): m/z
= 356 (M + H)+; HRMS (APCI): calculated for C23H17NOS [M]+ 355.1031; found 355.1026.
3.2.3. Synthesis of AS2
A solution of sodium ehtoxide was prepared by dissolving one pellet (approx. 200 mg) of sodium
˝
hydroxide (NaOH) in ethanol (EtOH, 20 mL) with mild heating (40 C). To this solution PDA (78 mg,
0.50 mmol) was added and allowed to dissolve. A solution of (390 mg, 1.1 mmol) in EtOH (10 mL)
1
was added dropwise and the resulting solution was refluxed for 6 h. The mixture was allowed to cool
to room temperature and placed in an ice-bath for 30 min. The separated red solid was filtered off,
21797

Molecules 2015, 20, 21787–21801
washed with cold EtOH (50.0 mL) followed by cold hexane (50.0 mL), and dried under high vacuum
˝
at 40 C. The solid was crystallized using CHCl3/hexane to afford 574 mg of AS2 (63%) as a brick-red
solid. m. p. >240 ˝C (decomposed at 240 ˝C); IR (solid film, cm´1) 3674, 2988, 2208 (-CN str), 1578,
1
1486, 1440, 1421, 1324, 1271, 1238, 1179, 1064, 922, 829. H-NMR (300 MHz, CD2Cl2):
δ = 7.77–7.76
(m, 3H), 7.65–7.63 (m, 1H), 7.60–7.59 (m, 2H), 7.36–7.31 (m, 5H) 7.18–7.15 (m, 5H), 7.12–7.08 (m, 3H);
13C-NMR (300 MHz, CDCl3)
= 149.9, 148.6, 147.1, 136.1, 134.8, 134.7, 134.4, 129.4, 127.0, 126.7, 126.0,
δ
124.9, 123.6, 122.8, 122.7, 118.1, 105.7; LRMS (MALDI-TOF): m/z = 830.229; HRMS (APCI): calculated
for C56H39N4S2 [M + H]+ 831.2611; found 831.2602.
3.2.4. X-ray Crystallography
CCDC’s 1420377 for
crystallographic data for this paper.
1
and 1420378 for AS2 contain the supplementary
These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk).
4. Conclusions
In conclusion, we have demonstrated the first use of the PDA acceptor functionality in conjunction
with a thiophene unit for the design and development of a BHJ chromophore, AS2, where PDA was
used in the D–A–D modular arrangement. The incorporation of this strong conjugating thiophene unit
helped to improve light-harvesting, photocurrent density and PCE of AS2 when compared with an
analogue, AS1. The incorporation of the thiophene functionality was of clear benefit in improving the
BHJ performance and indicates a potential to be broadly applicable in the design and development of
future high performance BHJ chromophores.
Supplementary Materials: Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/
20/12/19798/s1.
Acknowledgments: S.V.B. (RMIT) acknowledges financial support from the Australian Research Council (ARC),
Australia, under a Future Fellowship Scheme (FT110100152). The CSIRO Division of Materials Science and
Engineering, Clayton, Victoria is acknowledged for providing support through a visiting fellow position (A.G.).
P.S. is thankful to the ARC Future Fellowship Scheme (FT130101337) at Queensland University of Technology,
Brisbane, Queensland.
Author Contributions: A.M.R. synthesis, characterization, properties of materials and fabrication of devices; S.L.J
and C.M.P.: single crystal structure determinations; A.G. and H.P. device fabrication and characterization; A.B.:
performance of DFT calculation; P.S: results and discussion part, and S.V.B.: design, supervision and analysis of
data. All the authors contributed for the manuscript preparation.
Conflicts of Interest: The authors declare no conflict of interests.
References
1.
Chen, Y.; Wan, X.; Long, G. High performance photovoltaic applications using solution-processed small
molecules. Acc. Chem. Res. 2013, 46, 2645–2655. [CrossRef] [PubMed]
2.
Duan, C.; Huang, F.; Cao, Y. Recent development of push–pull conjugated polymers for bulk-heterojunction
photovoltaics: Rational design and fine tailoring of molecular structures. J. Mater. Chem. 2012, 22,
10416–10434. [CrossRef]
3.
4.
5.
6.
Günes, S.; Neugebauer, H.; Sariciftci, N.S. Conjugated polymer-based organic solar cells. Chem. Rev. 2007
107, 1324–1338. [CrossRef] [PubMed]
Lin, Y.; Li, Y.; Zhan, X. Small molecule semiconductors for high-efficiency organic photovoltaics.
Chem. Soc. Rev. 2012, 41, 4245–4272. [CrossRef] [PubMed]
Huang, Y.; Kramer, E.J.; Heeger, A.J.; Bazan, G.C. Bulk heterojunction solar cells: Morphology and
performance relationships. Chem. Rev. 2014, 114, 7006–7043. [CrossRef] [PubMed]
Heegar, A.J. 25th Anniversary article: Bulk heterojunction solar cells: Understanding the mechanism of
operation. Adv. Mater. 2014, 26, 10–28. [CrossRef] [PubMed]
,
21798

Molecules 2015, 20, 21787–21801
7.
8.
9.
Scharber, M.C.; Sariciftci, N.S. Efficiency of bulk-heterojunction organic solar cells. Prog. Polym. Sci. 2013, 38,
1929–1940. [CrossRef] [PubMed]
Boudreault, P.-L.T.; Najari, A.; Leclerc, M. Processable Low-Bandgap Polymers for Photovoltaic Applications.
Chem. Mater. 2011, 23, 456–469. [CrossRef]
Facchetti, A.
π-Conjugated polymers for organic electronics and photovoltaic cell applications. Chem. Mater.
2011, 23, 733–758. [CrossRef]
10. Gupta, A.; Watkins, S.E.; Scully, A.D.; Singh, T.B.; Wilson, G.J.; Rozanski, L.J.; Evans, R.A. Band-gap tuning
of pendant polymers for organic light-emitting devices and photovoltaic applications. Synth. Met. 2011, 161,
856–863. [CrossRef]
11. He, Y.; Li, Y. Fullerene derivative acceptors for high performance polymer solar cells. Phys. Chem. Chem. Phys.
2011, 13, 1970–1983. [CrossRef] [PubMed]
12. Laquai, F.; Andrienko, D.; Mauer, R.; Blom, P.W.M. Charge carrier transport and photogeneration in
P3HT:PCBM photovoltaic blends. Macromol. Rapid Commun. 2015, 36, 1001–1025. [CrossRef] [PubMed]
13. Dang, M.T.; Hirsch, L.; Wantz, G.; Wuest, J.D. Controlling the morphology and performance of
bulk heterojunctions in solar cells. Lessons learned from the benchmark poly(3-hexylthiophene):
[6,6]-phenyl-C61-butyric acid methyl ester system. Chem. Rev. 2013, 113, 3734–3765. [CrossRef] [PubMed]
14. You, J.; Chen, C.-C.; Hong, Z.; Yoshimura, K.; Ohya, K.; Xu, R.; Ye, S.; Gao, J.; Li, G.; Yang, Y. 10.2% Power
conversion efficiency polymer tandem solar cells consisting of two identical sub-cells. Adv. Mater. 2013, 25,
3973–3978. [CrossRef] [PubMed]
15. You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-C.; Gao, J.; Li, G.; et al.
A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. Commun. 2013, 4. [CrossRef]
[PubMed]
16. Lu, L.; Zheng, T.; Wu, Q.; Schneider, A.M.; Zhao, D.; Yu, L. Recent advances in bulk heterojunction polymer
solar cells. Chem. Rev. 2015. [CrossRef] [PubMed]
17. Guo, X.; Baumgarten, M.; Muellen, K. Designing
π-conjugated polymers for organic electronics.
Prog. Polym. Sci. 2013, 38, 1832–1908. [CrossRef]
18. Gupta, A.; Ali, A.; Bilic, A.; Gao, M.; Hegedus, K.; Singh, B.; Watkins, S.E.; Wilson, G.J.; Bach, U.; Evans, R.A.
Absorption enhancement of oligothiophene dyes through the use of a cyanopyridone acceptor group in
solution-processed organic solar cells. Chem. Commun. 2012, 48, 1889–1891. [CrossRef] [PubMed]
19. Kumar, R.J.; Churches, Q.I.; Subbiah, J.; Gupta, A.; Ali, A.; Evans, R.A.; Holmes, A.B. Enhanced photovoltaic
efficiency via light-triggered self-assembly. Chem. Commun. 2013, 49, 6552–6554. [CrossRef] [PubMed]
20. Li, Y.; Guo, Q.; Li, Z.; Pei, J.; Tian, W. Solution processable D-A small molecules for bulk-heterojunction solar
cells. Energy Environ. Sci. 2010, 3, 1427–1436. [CrossRef]
21. Mishra, A.; Bäuerle, P. Small molecule organic semiconductors on the move: Promises for future solar energy
technology. Angew. Chem. Int. Ed. 2012, 51, 2020–2067. [CrossRef] [PubMed]
22. Liu, J.; Walker, B.; Nguyen, T.-Q. Solution-processed molecular bulk heterojunction solar cells. In Organic
Photovoltaics, 2nd ed.; Brabec, C.J., Scherf, U., Dyakonov, V., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA:
Weinheim, Germany, 2014; pp. 95–137.
23. Roncali, J.; Leriche, P.; Blanchard, P. Molecular materials for organic photovoltaics: Small is beautiful.
Adv. Mater. 2014, 26, 3821–3838. [CrossRef] [PubMed]
24. Kan, B.; Zhang, Q.; Li, M.; Wan, X.; Ni, W.; Long, G.; Wang, Y.; Yang, X.; Feng, H.; Chen, Y. Solution-processed
organic solar cells based on dialkylthiol-substituted benzodithiophene unit with efficiency near 10%. J. Am.
Chem. Soc. 2014, 136, 15529–15532. [CrossRef] [PubMed]
25. Patil, H.; Gupta, A.; Bilic, A.; Jackson, S.L.; Latham, K.; Bhosale, S.V. Donor-acceptor-donor modular small
organic molecules based on the naphthalene diimide acceptor unit for solution-processable photovoltaic
devices. J. Electron. Mater. 2014, 43, 3243–3254. [CrossRef]
26. Sonar, P.; Williams, E.T.; Singh, S.P.; Manzhos, S.; Dodabalapur, A. A benzothiadiazole end capped
donor-acceptor based small molecule for organic electronics. Phys. Chem. Chem. Phys. 2013, 15, 17064–17069.
[CrossRef] [PubMed]
27. Walker, B.; Tamayo, A.B.; Dang, X.-D.; Zalar, P.; Seo, J.H.; Garcia, A.; Tantiwiwat, M.; Nguyen, T.-Q. Nanoscale
phase separation and high photovoltaic efficiency in solution-processed, small-molecule bulk heterojunction
solar cells. Adv. Funct. Mater. 2009, 19, 3063–3069. [CrossRef]
21799

Molecules 2015, 20, 21787–21801
28. Loser, S.; Bruns, C.J.; Miyauchi, H.; Ortiz, R.O.P.; Facchetti, A.; Stupp, S.I.; Marks, T.J.
A naphthodithiophene-diketopyrrolopyrrole donor molecule for efficient solution-processed solar cells.
J. Am. Chem. Soc. 2011, 133, 8142–8145. [CrossRef] [PubMed]
29. Yong, W.; Zhang, M.; Xin, X.; Li, Z.; Wu, Y.; Guo, X.; Yang, Z.; Hou, J. Solution-processed
indacenodithiophene-based small molecule for bulk heterojunction solar cells. J. Mater. Chem. A 2013
1, 14214–14220. [CrossRef]
,
30. Gupta, A.; Ali, A.; Gao, M.; Singh, T.B.; Bilic, A.; Watkins, S.E.; Bach, U.; Evans, R.A. Small
molecules containing rigidified thiophenes and a cyanopyridone acceptor unit for solution-processable
bulk-heterojunction solar cells. Dyes Pigm. 2015, 119, 122–132. [CrossRef]
31. Yuan, M.-S.; Wang, Q.; Wang, W.-J.; Li, T.-B.; Wang, L.; Deng, W.; Du, Z.-T.; Wang, J.-R. Symmetrical and
asymmetrical (multi)branched truxene compounds: Structure and photophysical properties. Dyes Pigm.
2012, 95, 236–243. [CrossRef]
32. Li, Z.; Dong, Q.; Li, Y.; Xu, B.; Deng, M.; Pei, J.; Zhang, J.; Chen, F.; Wen, S.; Gao, Y.; et al. Design and synthesis
of solution processable small molecules towards high photovoltaic performance. J. Mater. Chem. 2011, 21,
2159–2168. [CrossRef]
33. Shi, Q.; Cheng, P.; Li, Y.; Zhan, X. A solution processable D-A-D molecule based on thiazolothiazole for high
performance organic solar cells. Adv. Energy Mater. 2012, 2, 63–67. [CrossRef]
34. Gupta, A.; Hangarge, R.V.; Wang, X.; Alford, B.; Chellapan, V.; Jones, L.A.; Rananaware, A.; Bilic, A.; Sonar, P.;
Bhosale, S.V. Crowning of dibenzosilole with a naphthalenediimide functional group to prepare an electron
acceptor for organic solar cells. Dyes Pigm. 2015, 120, 314–321. [CrossRef]
35. Patil, H.; Gupta, A.; Alford, B.; Ma, D.; Privér, S.H.; Bilic, A.; Sonar, P.; Bhosale, S.V. Conjoint use
of dibenzosilole and indan-1,3-dione functionalities to prepare an efficient non-fullerene acceptor for
solution-processable bulk-heterojunction solar cells. Asian J. Org. Chem. 2015. [CrossRef]
36. Patil, H.; Zu, W.X.; Gupta, A.; Chellappan, V.; Bilic, A.; Sonar, P.; Rananaware, A.; Bhosale, S.V.;
Bhosale, S.V. A non-fullerene electron acceptor based on fluorene and diketopyrrolopyrrole building blocks
for solution-processable organic solar cells with an impressive open-circuit voltage. Phys. Chem. Chem. Phys.
2014, 16, 23837–23842. [CrossRef] [PubMed]
37. Raynor, A.M.; Gupta, A.; Patil, H.; Bilic, A.; Bhosale, S.V. A diketopyrrolopyrrole and benzothiadiazole based
small molecule electron acceptor: Design, synthesis, characterization and photovoltaic properties. RSC Adv.
2014, 4, 57635–57638. [CrossRef]
38. Gupta, A.; Ali, A.; Singh, T.B.; Bilic, A.; Bach, U.; Evans, R.A. Molecular engineering for panchromatic
absorbing oligothiophene donor-
[CrossRef]
π-acceptor organic semiconductors. Tetrahedron 2012, 68, 9440–9447.
39. Kronenberg, N.M.; Deppisch, M.; Würthner, F.; Lademann, H.W.A.; Deing, K.; Meerholz, K. Bulk
heterojunction organic solar cells based on merocyanine colorants. Chem. Commun. 2008, 48, 6489–6491.
[CrossRef] [PubMed]
40. Grimsdale, A.C.; Chan, K.L.; Martin, R.E.; Jokisz, P.G.; Holmes, A.B. Synthesis of light-emitting conjugated
polymers for applications in electroluminescent devices. Chem. Rev. 2009, 109, 897–1091. [CrossRef]
[PubMed]
41. Halliday, D.A.; Burn, P.L.; Friend, R.H.; Bradley, D.D.C.; Holmes, A.B. Determination of the average molecular
weigth of poly(p-phenylenevinylene). Synth. Met. 1993, 55, 902–907. [CrossRef]
42. Pond, S.J.K.; Rumi, M.; Levin, M.D.; Parker, T.C.; Beljonne, D.; Day, M.W.; Brédas, J.-L.; Marder, S.R.;
Perry, J.W. One- and two-photon spectroscopy of donor
´
acceptor donor distyrylbenzene derivatives:
´
Effect of cyano substitution and distortion from planarity. J. Phys. Chem. A 2002, 106, 11470–11480. [CrossRef]
43. Maurin, M.; Vurth, L.; Vial, J.-C.; Baldeck, P.; Marder, S.R.; van der Sanden, B.; Stephan, O. Pluronic organic
fluorescent probes for two-photon vascularisation imaging. Nonlinear Opt. Quantum Opt. 2010, 40, 175–181.
44. Fang, H.-H.; Chen, Q.-D.; Yang, J.; Xia, H.; Gao, B.-R.; Feng, J.; Ma, Y.-G.; Sun, H.-B. Two-photon
pumped amplified spontaneous emission from cyano-substituted oligo(p-phenylenevinylene) crystals with
aggregation-induced emission enhancement. J. Phys. Chem. C 2010, 114, 11958–11961. [CrossRef]
45. Yan, Z.-Q.; Yang, Z.-Y.; Wang, H.; Li, A.-W.; Wang, L.-P.; Yang, H.; Gao, B.-R. Study of aggregation induced
emission of cyano-substituted oligo(p-phenylenevinylene) by femtosecond time resolved fluorescence.
Spectrochim Acta A Mol Biomol Spectrosc. 2011, 78, 1640–1645. [CrossRef] [PubMed]
21800

Molecules 2015, 20, 21787–21801
46. Maurin, M.; Stephan, O.; Vial, J.-C.A.; Marder, S.R.; van der Sanden, B.P.J. Deep in vivo two-photon imaging
of blood vessels with a new dye encapsulated in pluronic nanomicelles. J. Biomed. Opt. 2011, 16, 1–6.
[CrossRef] [PubMed]
47. Xiang, W.; Gupta, A.; Kashif, M.K.; Duffy, N.; Bilic, A.; Evans, R.A.; Spiccia, L.; Bach, U. Cyanomethylbenzoic
acid: An acceptor for donor–π–acceptor chromophores used in dye-sensitized solar cells. ChemSusChem
2013, 6, 256–260. [CrossRef] [PubMed]
48. Bobe, S.R.; Gupta, A.; Rananaware, A.; Bilic, A.; Bhosale, S.V.; Bhosale, S.V. Improvement of optoelectronic
and photovoltaic properties through the insertion of a naphthalenediimide unit in donor–acceptor
oligothiophenes. RSC Adv. 2015, 5, 4411–4415. [CrossRef]
49. Würthner, F.; Wortmann, R.; Matschiner, R.; Lukaszuk, K.; Meerholz, K.; DeNardin, Y.; Bittner, R.; Brauchle, C.;
Sens, R. Merocyanine dyes in the cyanine limit: A new class of chromophores for photorefractive materials.
Angew. Chem. Int. Ed. 1997, 36, 2765–2768. [CrossRef]
50. Gupta, A.; Armel, V.; Xiang, W.; Fanchini, G.; Watkins, S.E.; MacFarlane, D.R.; Bach, U.; Evans, R.A. The
effect of direct amine substituted push-pull oligothiophene chromophores on dye-sensitized and bulk
heterojunction solar cells performance. Tetrahedron 2013, 69, 3584–3592. [CrossRef]
51. Sheldrick, G.M. A short history of SHELX. Acta. Cryst. 2008, 64, 112–122. [CrossRef] [PubMed]
52. Dolomanov, O.V.; Bourhis, L.J.; Gildea, R.J.; Howard, J.A.K.; Puschmann, H. OLEX2: A complete structure
solution, refinement and analysis program. J. Appl. Cryst. 2009, 42, 339–341. [CrossRef]
53. Sheldrick, G.M. Crystal structure refinement with SHELXL. Struct. Chem. 2014, 71, 3–8.
54. Hagberg, D.P.; Marinado, T.; Karlsson, K.M.; Nonomura, K.; Qin, P.; Boschloo, G.; Brinck, T.; Hagfeldt, A.;
Sun, L. Tuning the HOMO and LUMO energy levels of organic chromophores for dye sensitized solar cells.
J. Org. Chem. 2007, 72, 9550–9556. [CrossRef] [PubMed]
55. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.;
Mennucci, B.; Petersson, G.A.; et al. Gaussian 09, revision D.01; Gaussian Incorporation: Wallingford, CT,
USA, 2013.
Sample Availability: Samples of the compounds S10 and S11 are available from the authors.
©
2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons by Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
21801