The effect of direct amine substituted push-pull oligothiophene chromophores on dye-sensitized and bulk heterojunction solar cells performance

Article abstract of DOI:10.1016/j.tet.2013.02.040

The effect of direct amine substitution on the photovoltaic performance of donor-acceptor oligothiophenes in dye-sensitized and bulk heterojunction (BHJ) solar cells was investigated. These dyes have a common donor (di-p-tolylamino), π-bridge (terthiophene), and tunable acceptors to provide new materials for evaluation in dye-sensitized solar cells (DSCs) and BHJ solar cells. They were tested with liquid, ionic liquid, and metal-complex based electrolytes for DSCs and PC₆₁BM for solution processable BHJ solar cells. Direct amine substitution of the oligothiophene resulted in a significant spectral red-shift, reduction in band-gaps and improved photovoltaic performance in DSCs but not for BHJ devices.

Full text of DOI:10.1016/j.tet.2013.02.040

Tetrahedron 69 (2013) 3584e3592
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
The effect of direct amine substituted pushepull oligothiophene
chromophores on dye-sensitized and bulk heterojunction solar cells
performance
,
,1
Akhil Gupta a b, Vanessa Armel b, Wanchun Xiang b, Giovanni Fanchini a
,
Scott E. Watkins a , Douglas R. MacFarlane , Udo Bach c, Richard A. Evans a
,
b
a,
b,
,
*
*
a CSIRO Materials Science and Engineering, CSIRO Future Manufacturing Flagship, Bag 10, Clayton South, 3169 Victoria, Australia
b Department of Materials Engineering, Faculty of Engineering, Monash University, Wellington Road, Clayton, 3800 Victoria, Australia
c Tech Fellow, The Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, 3168 Victoria, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 October 2012
Received in revised form 21 January 2013
Accepted 11 February 2013
Available online 21 February 2013
The effect of direct amine substitution on the photovoltaic performance of donoreacceptor oligothio-
phenes in dye-sensitized and bulk heterojunction (BHJ) solar cells was investigated. These dyes have
a common donor (di-p-tolylamino),
p-bridge (terthiophene), and tunable acceptors to provide new
materials for evaluation in dye-sensitized solar cells (DSCs) and BHJ solar cells. They were tested with
liquid, ionic liquid, and metal-complex based electrolytes for DSCs and PC61BM for solution processable
BHJ solar cells. Direct amine substitution of the oligothiophene resulted in a significant spectral red-shift,
reduction in band-gaps and improved photovoltaic performance in DSCs but not for BHJ devices.
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Keywords:
Donorepeacceptor
Oligothiophene
Bulk heterojunction
Dye-sensitized solar cells
1. Introduction
sufficient absorption of the visible spectrum that a polymer like
P3HT achieves simply by its extended conjugation length. Reported
Organic-based photovoltaic devices (OPVs) are receiving sig-
nificant attention due to their promise as low-cost, flexible power
sources. They can be broadly classified into DSCs based on titanium
dioxide (TiO2) films1 and purely organic devices2 that typically have
a BHJ3 or evaporated bilayer4 active layer. The polythiophene P3HT
has been the chromophore and hole transport material of choice in
BHJ5,6 and various permutations of donoreacceptor oligothio-
phenes have been used in organic-based DSCs.7 In the field of BHJ
OPVs the use of small molecules in place of P3HT has the potential
to simplify synthesis and purification and enable greater control
over energy levels and morphology.8e11
In our own development of small molecule chromophores/
charge transport materials,12 we have sought to design molecules
based on a common core structure that, with simple modifications,
can be used in both BHJ and DSC devices. Small molecules for BHJ
and DSCs generally have a donoreacceptor structure to obtain
donoreacceptor oligothiophenes exclusively use a triarylamine-
type donor such that a phenyl (or related aromatic unit like fluo-
rene) links the amine nitrogen to the thiophene.7,13,14 In this report,
we examine the strategy where the donor amine nitrogen is a direct
substituent on the oligothiophene spacer and its effect on chro-
mophores for applications in DSCs and BHJ devices. The new
chromophores show significant spectral red-shifts, reduction in
energy band-gaps, and superior DSC performance when compared
in parallel with the triphenylamine equivalents. Here we have used
a ditolylamino oligothiophene derivative with the direct sub-
stitution of the amine nitrogen on the oligothiophene and methyl
groups at the p-positions to avoid the possible dimerization of the
phenyl ring.15
We have synthesized three model examples (1, 2, 3) with this
modification along with reference dyes (R1, R2) such that the
length of conjugation between the amine nitrogen and acceptors is
the same for all compounds. The structures are shown in Fig. 1. In
the modified design, the acceptor groups were varied with cyano-
carboxyvinylidene (1) used for DSCs and dicyanovinylidene (2), and
diethylthiobarbituric acid (3) for BHJ devices. The compounds were
synthesized following the method of Marder et al. who studied
derivatives with two thiophene units as non-linear optical
* Corresponding authors. Tel.: þ61 3 9545 2507 (R.A.); tel.: þ61 3 9545 2618
(S.E.); e-mail addresses: scott.watkins@csiro.au (S.E. Watkins), richard.evans@
csiro.au (R.A. Evans).
1
Present address: Department of Physics & Astronomy, University of Western
Ontario, 1151 Richmond Street, London, Ontario N6A 3K7, Canada.
0040-4020/$ e see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.02.040

A. Gupta et al. / Tetrahedron 69 (2013) 3584e3592
3585
Fig. 1. Molecular structures of the investigated newly designed (1, 2, and 3) and reference (R1 and R2) materials.
materials.16 The molecules reported in this work were found to be
highly soluble in a variety of conventional organic solvents, such as
dichloromethane, chloroform, chlorobenzene, and toluene. High
solubility is a desirable feature for the fabrication of solution-
processable OPVs and precludes the need for inclusion of electro-
actively inert alkyl chains into the organic module. To the best of
our knowledge, this is the first time that materials based on
a common structural core have been evaluated in both DSCs and
BHJ photovoltaic devices.
2. Results and discussion
2.1. Design concept and synthetic strategy
All the new materials were based on the De
sign and have a common di-p-tolylamino donor, terthiophene as
-bridge, and various acceptors to generate a set of organic dyes
peA modular de-
Fig. 2. Molar absorptivities of newly synthesized dyes 1e3 (thick lines) vs reference
dyes R1 and R2 (dotted lines) in chloroform solutions.
a
p
that were evaluated as organic semiconductor components for
OPVs. In the modified design, replacement of the phenyl group with
thiophene provided significant spectral red-shifts. This could be
attributed to the lower aromaticity of thiophene, that is, advanta-
geous for superior charge delocalization.17 Furthermore, with the
use of the strong aromatizable diethylthiobarbituric acid acceptor
unit for BHJ design, we could expect bathochromic shifts in the
absorption that might provide a higher molar extinction coefficient
and narrower band gap. Intramolecular charge transfer (ICT) would
be enhanced with the use of an acceptor fragment with increased
acceptor strength, which in-turn would provide for greater light
harvesting and perhaps better positioning of energy levels.
The synthesis involved the reaction of di(p-tolyl)-2-
aminothiophene with n-BuLi and subsequent reaction with tribu-
tyltin chloride to give N,N-di-p-tolyl-5-(tributylstannyl)thiophene-
2-amine. The stannane was then reacted with 50-iodo-2,20-bithio-
phen-5-carbaldehyde using bis(triphenylphosphine)palladium(II)
chloride to provide the 500-formyl-5-(N,N-di-p-tolylamino)-
2,20:50200-terthiophene. This penultimate aldehydic component was
a common synthon, which was condensed with cyanoacetic acid,
malononitrile, and diethylthiobarbituric acid to give compounds 1,
2, and 3, respectively. These new materials were prepared in
moderate to high yields and their chemical structures were con-
firmed by 1H NMR spectroscopy and mass spectrometry.
Fig. 3. Normalized film absorption spectra of newly synthesized dyes 1e3 (thick lines)
vs reference dyes R1 and R2 (dotted lines); R1 and 1 on 1.2
2, 3, and R2 on glass substrates.
mm thick TiO2 film and dyes
bathochromic shifts of solution and film absorption spectra, re-
spectively, of compound 2 when compared with reference com-
pound, R212a that has a phenyl linker between the amine nitrogen
and terthiophene unit. Similarly, the acid derivative 1 displays
16 nm and 22 nm red shifts of solution and film spectra when
compared with the reference dye R1,18 which has a phenyl group
rather than thiophene between a diarylamino nitrogen and cya-
nocarboxydithiophene. The use of the strong acceptor group,
diethylthiobarbituric acid, leads to a more marked red-shift of
46 nm and a 10% enhancement in the peak molar absorptivity of 3
compared with the dicyanovinylidine group in compound 2. We
2.2. Optical properties
The comparative ultravioletevisible (UVevis) spectra of all the
materials measured in solution and as thin films are shown in Figs.
2 and 3, respectively. Direct attachment of the diarylamino donor
group to the terthiophene resulted in 42 nm and 30 nm

3586
A. Gupta et al. / Tetrahedron 69 (2013) 3584e3592
observe the same bathochromic absorption shift in thin film
spectra of 3 compared with 2, a finding that is consistent with our
previous work.12a Of the three groups studied in this work, the use
of a carboxylic acid group (to allow binding to TiO2) causes com-
pounds R1 and 1 to have the highest energy absorbance.
construct DSCs. Identical TiO2 films with a two-layer architecture
(6 m transparent TiO2 with 30 nm average particle diameter plus
a 6 m reflective TiO2 layer with 400 nm average particle diameter)
m
m
were used to construct all cells. Two different types of electrolytes
based on volatile liquid or a non-volatile ionic liquid were used. In
both types of devices dye 1 clearly outperforms reference dye R1,
affording devices with a higher short-circuit current and open-
circuit voltage. As a validation of our fabrication processes we
note that the results for dye 1 are comparable to those of a recent
report of a structurally related dye.19 The 25% improvement (when
using a volatile liquid electrolyte) in the power conversion effi-
ciency (PCE) for devices based on compound 1 compared with
those based on compound R1 is notable. This emphasizes the sig-
nificant effect that small changes in chemical structure can have on
device performance. For practical applications, iodine-based elec-
trolytes, while often enabling good device performance, are afflic-
ted with a number of potential disadvantages, such as their high
volatility, significant coloration, and corrosive nature. Iodine-free,
one-electron outer-sphere redox couples, such as cobalt(II)/(III)
polypyridyl complexes, are promising alternative redox mediators
due to their weak coloration and their compatibility with a wide
range of metal substrates.20 Because of these recent promising re-
ports with cobalt electrolytes,21 it was relevant to test the best
performing dye 1 with this electrolyte. We examined the photo-
2.3. Energy levels
HOMO energies were estimated using Photo Electron Spec-
troscopy in Air (PESA) and the LUMO energies were calculated by
adding the optical band gap to the HOMO values. On the basis of
UVevis spectra and energy level calculations, it was evident that
the modified design of direct infusion helps to reduce the band gap
of 1 and 2 when compared with reference dyes R1 and R2. More-
over, the stronger acceptor in compound 3 further reduces the band
gap when compared with 2. These levels are all in the range re-
quired of donor materials for BHJ devices and dye materials for
DSCs (Figs. 4 and 5). The photo-physical and electronic properties of
all the materials are summarized in Table 1.
voltaic performance of dye
1
in DSCs with [Co(bpy)3]2þ/3þ
(bpy¼2,20-bipyridine) as the redox mediator20 under similar con-
ditions as for conventional, triiodide/iodide based electrolytes. A
notable PCE of 5.2% was obtained with fill factor of 0.75 under
optimized conditions, which represents the first example of a di-
rectly substituted amino-oligothiophene DSC chromophore with
a metal-complex based electrolyte and establishes the compati-
bility between the two novel methodologies. Currentevoltage
(JsceVoc) curves are shown in Fig. 7 and the photovoltaic perfor-
mance data are summarized in Table 2.
Incident photon to current conversion efficiency (IPCE) analysis
of the DSC devices, Fig. 8, confirm that the improved device per-
formance is due to compound 1 absorbing further into the red than
compound R1. Compound 1 shows a high IPCE from 400 to 570 nm
with the highest values of 63.3% at 495 nm for the solvent-based
electrolyte, 57.1% at 475 nm with the ionic liquid electrolyte and
a higher maxima of w68% at 500 nm for the cobalt electrolyte.
Photovoltaic devices based on dye 1 using a liquid electrolyte were
found to be stable after keeping them for 30 days in ambient air,
showing excellent photochemical stability, retaining approximately
90% of their initial efficiency.
To explore our hypothesis that compounds with improved ab-
sorbance properties could also be used in other photovoltaic de-
vices, compounds 2 and 3 were used as donor materials in solution-
processed BHJ photovoltaic devices (see Table 2). BHJ architectures
typically deliver higher device power conversion efficiencies by
maximizing the surface area of the interface between the donor
and acceptor materials in the active layer. For all the compounds, 2,
3, and R2, the device structure 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 of 2, 3 or R2 and the solubilized
fullerene PC61BM. This was done by spin coating mixtures of the
appropriate dye with PC61BM in chloroform under ambient con-
ditions in 1:1 ratios without subsequent annealing. Devices based
on compound 2 showed a PCE of 0.79%. This compares with a PCE of
0.88% for compound R2. Devices fabricated using 3 afforded a PCE
of 0.33% (see Table 2). The modest efficiencies for 2 and 3 are most
likely due to charge mobility problems, as suggested by the low fill
factors. The examined charge carrier mobilities in devices using 2
and 3 from their corresponding organic field effect transistors
(OFETs) (see Experimental for details) were found to be very low
with mobilities in the range ꢀ10ꢁ8 cm2/V s. Optimized devices
Fig. 4. Energy diagram of DSC sensitizers based on the results of PESA and thin film
absorption spectra. (Note: this does not represent a device structure).
The electrochemistry experiments revealed that all the newly
synthesized materials 1, 2, and 3 undergo reversible oxidations. For
compound 1, three reversible oxidation potentials were recorded
and are shown in Fig. 6. For 2 and 3, first reversible oxidation po-
tential has been shown. Compounds 2 and 3 are potential donor
materials for BHJ solar cells and the reversible oxidation suggests
their suitability for BHJ devices (see Fig. 6). The estimated energy
levels of 1, 2 and 3 in solution are consistent with those measured in
the solid state by PESA/UV–vis (see Table S1 in supplementary
data).
2.4. Photovoltaic properties
Having shown that the new design strategy results in com-
pounds that absorb more of the visible spectrum of light, the per-
formance of the compounds in photovoltaic devices was evaluated.
In a side-by-side comparison, both the dyes 1 and R1 were used to

A. Gupta et al. / Tetrahedron 69 (2013) 3584e3592
3587
Fig. 5. Energy level diagram depicting the band-gaps of 2, 3, and R2 in comparison to P3HT and PC61BM. (Note: this does not represent a device structure).
Table 1
Photo-physical and electronic properties of all the materials investigated
Dye
Absorption
Emission lmaxa/nm
EHOMO eVc
Eband gap eVd
ELUMO eVe
Solution lmaxa/onset (nm) ε[Mꢁ1 cmꢁ1
]
Film lmaxb/onset (nm)
1
R1
2
3
R2
476/630 [21,330]
460/552 [31,780]
554/685 [28,080]
600/745 [30,700]
512/602 [41,300]
424/602
402/530
572/735
602/785
542/675
761
680
770
816
716
ꢁ5.25
ꢁ5.35
ꢁ5.42
ꢁ5.20
ꢁ5.60
2.05
2.34
1.69
1.58
1.84
ꢁ3.20
ꢁ3.01
ꢁ3.73
ꢁ3.62
ꢁ3.76
a
Absorption and emission spectra were measured in chloroform solutions.
b
Absorption spectra of thin solid films spin-cast on glass substrates from chloroform solutions of 2, 3, and R2. For dye 1 and R1, a 1.2
a 1.5 M dye solution in a chloroform and ethanol mixture (v/v 1:1) containing 25 M chenodeoxycholic acid for 4 h.
HOMO levels of the dyes were measured using PESA on solid films.
mm thick TiO2 film was sensitized using
m
m
c
d
e
Optical band-gaps were estimated from the tangent of the edge of longest wavelength in thin solid films.
LUMO levels were calculated from the optical band-gaps (film) and HOMO levels (ELUMO¼Eband gapþEHOMO).
based on compounds 2 and 3 showed lower Voc values when
compared with R2. These values are consistent with the measured
HOMO values where the more positive HOMOs would predict
position of TiO2 as future work. In addition, it is of interest to see
how this type of modification may affect the performance of
benchmark dyes that are currently providing >10% efficiencies.
Thus the design concept will be further examined in more so-
phisticated and champion dye structures with alternative acceptor
groups suitable for binding to TiO2.
a lower Voc
.
3. Conclusions
In conclusion, we have demonstrated the effect of the direct
connection of donor amines to oligothiophenes for donoreacceptor
small molecules in both DSC and BHJ OPV applications. This was
done in practice by replacing the conventional phenyl linker with
a thiophene group. In a direct comparison, we have shown that the
new design results in substantial red-shifting of the absorption
maxima in the visible region. The device results highlight the su-
perior performance of these direct nitrogen substituted oligothio-
phenes over conventional triaryl amino-oligothiophenes for DSC
applications but not for BHJ devices. The increased efficiency in
DSCs from 4.9% to 6.1% suggests that it is now appropriate to
examine in more detail the correlation of dye-structure, charge
recombination, electron transport, and the conduction band
4. Experimental section
4.1. General methods
4.1.1. Materials. All the reagents and chemicals used, unless oth-
erwise specified, were purchased from SigmaeAldrich Co. The
solvents used for reactions were obtained from Merck Speciality
Chemicals (Sydney, Australia) and were used as such.
4.1.2. Spectroscopic measurements. Unless otherwise specified, all
1H and 13C NMR spectra were recorded using a Bruker AV400
spectrometer at 400 MHz and 100.6 MHz, respectively, or a Bruker
AV200 spectrometer at 200 MHz and 50 MHz, respectively.

3588
A. Gupta et al. / Tetrahedron 69 (2013) 3584e3592
Fig. 6. Cyclic-voltammograms in dichloromethane for all the new compounds 1 (up-
per), 2, and 3 (lower) at a sweep rate of 50 mV sꢁ1 indicating three reversible oxidation
potentials for 1 and first reversible oxidation potentials for 2 and 3.
Fig. 7. JsceVoc curves for the optimized BHJ and DSC photovoltaic devices under
simulated sunlight (AM 1.5, 1000 W/m2). For comparison, DSCs comprising dyes 1 and
R1 were fabricated using liquid electrolyte (LE) and ionic liquid electrolyte (ILE), and
compound 1 was tested with a cobalt electrolyte (CE). For the new BHJ dyes 2 and 3,
the photovoltaic devices were fabricated using blends with PC61BM (1:1 wt) and
comparative data is represented in Table 2.
Chemical shifts (d) are measured in parts per million. Thin Layer
Chromatography (TLC) was performed using 0.25 mm thick plates
precoated with Merck Kieselgel 60 F254 silica gel, and visualized
using UV light (254 nm and 365 nm). Melting points were mea-
sured using a Gallenkamp MPD350 digital melting point apparatus
and are uncorrected. Positive ion electron impact (EI) mass spectra
were recorded using a ThermoQuest MAT95XL mass spectrometer
using ionization energy of 70 eV. High resolution mass spectra
(atmospheric-pressure chemical ionization (APCI)) experiments
were carried out on a Thermo Scientific Q-Exactive FTMS, ionizing
by APCI from an ASAP probe.22 UVevis absorption spectra were
recorded using a Hewlett Packard HP 8453 diode array spectrom-
eter. Fluorescence spectra were measured using a PerkineElmer
LS50B fluorimeter.
2 of ferrocene/ferrocenium redox couple. The Fc/Fcþ redox couple
were measured as 0.65 eV, 0.37 eV and 0.38 eV for the corre-
sponding materials 1, 2, and 3, respectively. Respective cyclic-
voltammograms of Fc/Fcþ vs the silver pseudo-reference elec-
trode have been represented in Fig. S1 (see Supplementary data).
4.1.4. Device fabrication and characterization of field effect tran-
sistors. A doped (Nw3ꢂ1017 cmꢁ3) silicon (Si) wafer was used as
Table 2
Comparative dye-sensitized and BHJ solar cells performances
Dye/device
type
Testing
conditions
Voc
(mV)
Jsc
FF
Efficiency
%)
(mA/cm2)
(h
4.1.3. Electrochemical measurements. PESA measurements were
recorded using a Riken Keiki AC-2 PESA spectrometer with a power
setting of 5 nW and a power number of 0.5. Samples for PESA were
1/DSC
LEa
744
652
733
704
665
840
702
731
11.70
9.80
9.55
8.80
9.10
2.60
3.34
1.42
0.73
0.77
0.75
0.78
0.74
0.40
0.34
0.31
6.10
4.90
5.20
4.90
4.50
0.88
0.79
0.33
ILEb
CEc
R1/DSC
LEa
prepared on 1.2 mM TiO2 for 1 and R1 and on cleaned glass sub-
ILEb
strates for 2, 3, and R2. Electrochemical measurements were car-
ried out using a PowerLab ML160 potentiostat interfaced via
a PowerLab 4/20 controller to a PC running EeChem For Windows
Ver. 1.5.2. The measurements were run in argon-purged dichloro-
methane with tetrabutylammonium hexafluorophosphate (0.2 M)
as the supporting electrolyte. The cyclic-voltammograms were
recorded using a standard three electrode configuration with
a glassy carbon (2 mm diameter) working electrode, a platinum
wire counter electrode, and a silver wire pseudo reference elec-
trode. The silver wire was cleaned in concentrated nitric acid fol-
lowed by concentrated hydrochloric acid and then washed with
deionized water. Cyclic-voltammograms were recorded with
a sweep rate of 50 mV sꢁ1. All the potentials were referred to the E1/
R2/BHJ
2/BHJ
3/BHJ
1:1 PC61BMd
1:1 PC61BMd
1:1 PC61BMd
a
1.0 M 1,3-dimethylimidazolium iodide, 0.03 M iodine, 0.1 M guanidinium
thiocyanate, 0.5 M tert-butylpyridine, 0.05 M lithium iodide in a mixture of aceto-
nitrile and valeronitrile (85/15, v/v).
b
1-Ethyl-3-methylimidazolium iodide/1,3-dimethylimidazolium iodide/1-ethyl-
3-methylimidazolium tetracyanoborate/lithium iodide/iodine/N-methylbenzimi-
dazole in molar ratio (12/12/16/1/1.67/4).
c
0.20
M
(Tris(2,20-bipyridyl)cobalt(II) bis(trifluoromethanesulfonyl)imide)
([Co(bpy)3](TFSI)2), 0.06 M [Co(bpy)3](TFSI)3, 0.05 M LiTFSI, 1.00 M tert-butylpyr-
idine (tBP) in acetonitrile.
d
BHJ dyes in blends with PC61BM (1:1 wt). Device structure is ITO/PEDOT:PSS
(38 nm)/active layer/Ca (20 nm)/Al (100 nm) with active layer thickness of about
50 nm.

A. Gupta et al. / Tetrahedron 69 (2013) 3584e3592
3589
than 50 ppm. Aqueous solutions of PEDOT/PSS (HC Starck, Baytron
P AI 4083) were filtered (0.2 m RC filter) and deposited onto glass
m
substrates in air by spin coating (Laurell WS-400B-6NPP lite single
wafer spin processor) at 5000 rpm for 60 s to give a layer having
a thickness of 40 ꢃ 5 nm. The PEDOT/PSS layer was then annealed
on a hotplate in a glove box at 145 ꢄC for 10 min. For OPV devices,
the newly synthesized organic p-type materials and PC61BM (Nano-
C) were separately dissolved in individual vials by magnetic stir-
ring. Blend ratios and solution concentrations were varied to op-
timize device performance, see below. The solutions were then
combined, filtered (0.2 mm RC filter), and deposited by spin coating
(SCS G3P spin coater) onto the ITO-coated glass substrates inside
a glove box (with H2O and O2 levels both <1 ppm). Film thicknesses
were determined on identical samples using a Dektak 6M Profil-
ometer. The coated substrates were then transferred (without
exposure to air) to a vacuum evaporator inside an adjacent
nitrogen-filled glove box. Samples were placed on a shadow mask
in a tray. The area defined by the shadow mask gave device areas of
exactly 0.2 cm2. Deposition rates and film thicknesses were moni-
tored using a calibrated quartz thickness monitor inside the vac-
uum chamber. Layers of calcium (Ca) (Aldrich) and aluminum (Al)
(3 pellets of 99.999%, KJ Lesker) having thicknesses of 20 nm and
100 nm, respectively, were evaporated from open tungsten boats
onto the active layer by thermal evaporation at pressures less than
2ꢂ10ꢁ6 mbar. Where used, C60 (Nano-C) and 2,9-Dimethyl-4,7-
diphenyl-1,10-phenanthroline (Aldrich) were evaporated from
alumina crucibles. A connection point for the ITO electrode was
made by manually scratching off a small area of the active layers. A
small amount of silver paint (Silver Print II, GC Electronics, part
number: 22-023) was then deposited onto all of the connection
points, both ITO and Al. The completed devices were then encap-
sulated with glass and a UV-cured epoxy (Summers Optical, Lens
Bond type J-91) by exposing to 365 nm UV light inside the glove box
for 10 min. The encapsulated devices were then removed from the
glove box and tested in air within 1 h. Electrical connections were
made using alligator clips. The OPV devices were tested using an
Oriel solar simulator fitted with a 1000 W xenon lamp filtered to
give an output of 100 mW/cm2 at simulated AM 1.5. The lamp was
calibrated using a standard, filtered silicon (Si) cell from Peccell
Limited, which was subsequently cross-calibrated with a standard
reference cell traceable to the National Renewable Energy Labora-
tory. The devices were tested using a Keithley 2400 source meter
controlled by Lab-view software.
Fig. 8. IPCE curves of DSC devices sensitized with 1 and R1 based on a volatile liquid
electrolyte (LE) (up) and non-volatile ionic liquid electrolyte (ILE) (down); besides 1
with cobalt electrolyte (CE).
a substrate and as gate electrode. Discrete bottom contact OFETs
were fabricated on thermally grown smooth silicon dioxide (SiO2)
(230 nm). Interdigitated source and drain electrodes were photo-
lithographically patterned from a 50 nm sputtered gold layer.
Channel length, L, of the devices was 20 mm and the channel width,
W, was 2 mm. The SiO2 layer was first cleaned with acetone, then
with 2-propanol and finally treated with UV ozone. Device fabri-
cation was completed by spin coating the organic semiconductor
layers at 1500 rpm in the glove box from solutions in chloroform.
The active layers were consisting of pristine compounds. Without
any further treatment, completed devices were transferred (under
air-free conditions) to another glove box fitted to probe the devices
using a probe station. Electrical measurements were carried out
using a Keithley 2612 dual channel SMU. The reproducibility of the
OFET preparation procedure was high with a confidence interval for
the extracted mobility of ꢃ15% of the values, which have been
presented. The carrier mobility values presented in this work were
generally averaged from measurements done on at least four de-
vices on the same substrate.
4.1.6. Device fabrication and characterization of dye-sensitized pho-
tovoltaic devices
4.1.6.1. Materials. The [Co(bpy)3]2þ and [Co(bpy)3]3þ complexes
forming the redox couple were synthesized according to the liter-
ature23 with slight modifications. One equivalent of CoCl2$6H2O
and 3.3 equiv of 2,20-bipyridine were dissolved in a minimum
amount of methanol, the solution was stirred at reflux for 1 h. An
excess of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was
then added to the solution to precipitate the cobalt complex. Af-
terward, the solid was filtered, washed with water and diethyl
ether, and dried under vacuum for 24 h. Oxidation of Co2þ complex
was performed by adding a slight excess amount of NOBF4 to the
acetonitrile solution of the complex. The solvent was removed by
rotary evaporation. The complex was redissolved in acetonitrile and
a 10 fold excess of LiTFSI was added to precipitate the product,
which was then filtered, washed with water and diethyl ether, and
dried under vacuum for 24 h. These two compounds were used
without further purification.
4.1.5. Device fabrication and characterization of BHJ photovoltaic
devices. Indium tin oxide (ITO)-coated glass (Kintek, 15
U/,) was
cleaned by standing in a stirred solution of 5% (v/v) Deconex 12PA
detergent at 90 ꢄC for 20 min. The ITO-coated glass was then suc-
cessively sonicated for 10 min each in distilled water, acetone, and
isopropanol. The substrates were then exposed to a UV-ozone clean
at room temperature for 10 min. UV/ozone cleaning of glass sub-
strates was performed using a Novascan PDS-UVT, UV/ozone
cleaner with the platform set to maximum height. The intensity of
the lamp was greater than 36 mW/cm2 at a distance of 10 cm. At
ambient conditions, the ozone output of the UV cleaner is greater
4.1.6.2. Working electrode preparation. The glass substrate used
was 4 mm thick 10
U ,
ꢁ1 conductive FTO (fluorine doped tin oxide)

3590
A. Gupta et al. / Tetrahedron 69 (2013) 3584e3592
glass with high transparency in the visible range purchased from
Nippon sheet glass. Mesoporous 4ꢂ4 mm TiO2 films were printed
using a commercial semi automatic screen printer and commercial
30 nm TiO2 screen printing paste (PST-30NR, JGC Catalysts and
Chemicals). The scattering paste was prepared from 400 nm parti-
cles mixed 5 to 1 by weight with 30 nm particles and addition of 3.5
parts ethyl cellulose and 30.5 parts terpineol. After a first sintering
process (500 ꢄC), a TiCl4 treatment was applied by immersing the
films in a 40 mM aqueous TiCl4 solution and heating at 70 ꢄC for
30 min in a water saturated atmosphere. Prior to device assembly,
the films were sintered again (500 ꢄC), cooled to approximately
80 ꢄC, and immersed in the dye solution. The dyed films were rinsed
with absolute ethanol to remove non-attached dye from the films.
The devices were sealed with a platinized FTO counter electrode
using a hot-melt (Surlyn, DuPont). The devices were then filled
with an electrolyte through a hole in the counter electrode. The
hole was then sealed with a Surlyn disk and a thin glass to avoid the
leakage of electrolyte.
4.2. Synthesis
All the new compounds 1, 2, and 3 were synthesized and char-
acterized. The synthetic strategy is represented in Scheme 1. Syn-
thesis of 50-iodo-2,20-bithiophene-5-carbaldehyde and R2 has been
reported previously.12a Dye R1 was synthesized as per the literature
procedures.18
4.2.1. Synthesis of N,N-di-p-tolylthiophen-2-amine (4). 2-Iodothio-
phene (1.10 mL, 10.1 mmol) was added to the mixture of di-p-tol-
ylamine (2.00 g, 10.1 mmol), sodium tert-butoxide (1.46 g,
15.2 mmol), and palladium acetate (246 mg, 1.10 mmol) in o-xylene
(30.0 mL) followed by the addition of tri-tert-butylphosphine
(1.30 mL) at room temperature and the resulting suspension was
refluxed overnight. Reaction mixture was filtered through Celite
(3.00 g) and the solvent was evaporated off to get a crude oil, which
was subjected to column chromatography on silica (5% ethyl ace-
tate/hexane) to yield the title compound 4 (1.40 g, 49.5%) as
a greenish yellow oil. NMR data of compound 4 matches with the
reported literature;16 Rf (5% ethyl acetate/hexane) 0.38; dH
(400 MHz, CD3COCD3) 7.12e7.10 (5H, m), 6.99e6.96 (4H, m),
6.92e6.90 (1H, m), 6.67e6.66 (1H, m), 2.29 (6H, s).
4.1.6.3. Counter electrode preparation. Counter electrodes were
prepared from 3 mm thick 15
tin oxide) glass purchased from Nippon glass. A solution of chlor-
oplatinic acid hexahydrate (10 mM) in ethanol was doctor bladed
onto the pre-drilled substrate and pyrolyzed for 15 min at 450 ꢄC.
U ,
ꢁ1 conductive FTO (fluorine doped
4.1.6.4. Electrolyte fabrication. All electrolytes were prepared
using anhydrous solvents, deoxygenated by freeze-drying and
stored in a nitrogen glove box. Iodide, lithium iodide and LiTFSI
were purchased from commercial suppliers in high purity and used
as received. tert-Butylpyridine (tBP) was distilled under nitrogen
atmosphere and then stored in the glove box.
4.1.6.5. Cell assembly. The dried working electrode and counter
electrode were assembled using a 25
mm thick Surlyn gasket
4.2.2. Synthesis of 500-(N,N-di-p-tolylamino)-200,50-bithiophene-20,5-
bithiophene-2-carboxaldehyde (6). Compound 4 (1.20 g, 4.30 mmol)
dissolved in anhydrous tetrahydrofuran (20.0 mL) was treated drop
wise with n-butyllithium (1.60 M, 4.00 mL, 6.45 mmol) at ꢁ78 ꢄC
under nitrogen. The solution was stirred for 30 min and tributyl-
stannyl chloride (1.82 g, 5.60 mmol) was added. Reaction mixture
was allowed to warm to room temperature and stirred overnight.
Water was added (30.0 mL) and the product was extracted into
dichloromethane. The organic layer was separated, washed with
water followed by brine, and dried over anhydrous sodium sulfate.
The solvent was evaporated to afford crude 5 as a brown oil, which
was used as such for the next step. The 70% yield of 5 was roughly
estimated according to 1H NMR data. To a degassed solution of 5
(10.0 g, 17.6 mmol) and 50-iodo-2,20-bithiophene-5-carbaldehyde
(5.64 g, 17.6 mmol) in toluene (100 mL) was added bis(-
triphenylphosphine)palladium(II) chloride (445 mg, 0.63 mmol) at
room temperature under argon. The reaction mixture was refluxed
overnight and the solvent was removed under reduced pressure to
get a crude orange solid, which was subjected to column chroma-
tography on silica (10% ethyl acetate/hexane) to yield the title
compound 6 (5.00 g, 60.3%) as a deep orange shiny solid; mp
(Solaronix) of the dimensions 5ꢂ6 mm. A pneumatic finger was
used to apply pressure while heating through the counter elec-
trode. The cells were then vacuum backfilled through a predrilled
hole in the counter electrode. The backfilling hole was sealed at
150 ꢄC using a square of aluminum backed Surlyn prepared by
melting 25 m
m Surlyn onto aluminum foil at 100 ꢄC.
4.1.6.6. IeV Characterization. A sun simulator (Oriel) fitted with
a filtered 1000 W xenon lamp was used to provide simulated solar
irradiation (AM1.5, 1000 W mꢁ2). Currentevoltage curves were
recorded using a Keithley 2400 source meter. The output of the
light source was adjusted using a calibrated silicon photodiode
(Peccell Technologies). The photodiode was fitted with a color filter
provided by the supplier, to minimize the optical mismatch be-
tween the calibration diode and the dye-sensitized solar cells. The
light intensity was adjusted by using a filter wheel equipped with
a series of mesh filters.
4.1.6.7. IPCE measurement. IPCE was measured using a 150 W
xenon lamp (Oriel) fitted with a monochromator (Cornerstone 260)
as a monochromatic light source. The illumination spot size was
chosen to be slightly smaller than the active area of the DSC test
cells. IPCE photocurrents were recorded under short circuit con-
ditions using a Keithley 2400 source meter. The monochromatic
photon flux was quantified by means of a calibrated silicon pho-
todiode (Peccell Technologies).
194e197 ꢄC; Rf (10% ethyl acetate/hexane) 0.30; IR (liquid film, cmꢁ1
)
3063, 3028, 2918, 1657, 1505, 1465, 1448, 1291, 1218, 1044, 794, 731;
dH (400 MHz, CDCl3) 9.82 (1H, s), 7.63e7.62 (1H, m), 7.20e7.19 (1H,
m), 7.17e7.16 (1H, m), 7.09e7.05 (8H, m), 6.95e6.94 (1H, m),
6.91e6.90 (1H, m), 6.45e6.44 (1H, m), 2.31 (6H, s); dC (400 MHz,
CDCl3) 182.5, 153.3, 147.3, 145.3, 141.3, 140.3, 137.6, 133.6, 133.3,130.1,
127.8, 127.1, 123.8, 123.6, 123.5, 123.3, 118.2, 21.0; LRMS (EI): Mþ,
471.1; HRMS (EI): Mþ, found 471.0792. C27H21NO32S3 requires
471.0780.
4.1.6.8. Device fabrication. The dye was tested in photovoltaic
devices using standard titanium dioxide with a 6
parent layer of 30 nm sized particles and a 6 m thick scattering
mm thick trans-
m
layer of 400 nm sized particles on top of the transparent layer. A
blocking layer TiO2 of few nm was deposited prior to the screen
printing of the transparent layer. The films were sintered for 30 min
at 500 ꢄC and cooled down to 80 ꢄC before immersion of the films in
a solution containing 0.3 mM dye with 10 mM chenodeoxycholic
acid as co-adsorbant in a 1:1 mixture of chloroform and ethanol.
4.2.3. Synthesis of (2Z)-3-[5-(5-{5-[bis(4-methylphenyl)amino]thio-
phen-2-yl}thiophen-2-yl)thiophen-2-yl]-2-cyanoprop-2-enoic acid
(1). Compound 6 (400 mg, 0.850 mmol) was dissolved in chloro-
form (20.0 mL) in a 100 mL RB flask at room temperature. Cyano-
acetic acid (217 mg, 2.55 mmol) was added in one portion followed
by the addition of piperidine (0.084 mL, 0.850 mmol) at room

A. Gupta et al. / Tetrahedron 69 (2013) 3584e3592
3591
Scheme 1. Synthesis of new compounds 1, 2, and 3.
temperature. The resulting red solution was refluxed overnight.
Solvent was removed and the residue was dissolved in chloroform,
washed with 2 M hydrochloric acid, water, and brine. The organic
layer was dried over anhydrous Na2SO4 and recovered to get crude
oil, which was subjected to column chromatography on silica (10%
methanol/chloroform) to yield the title compound 1 (200 mg,
43.5%) as a deep red solid; mp 248e252 ꢄC; Rf (10% methanol/
chloroform) 0.27; IR (liquid film, cmꢁ1) 3049 (br), 3026, 2953, 2928,
2856, 2218, 1653, 1604, 1581, 1506, 1394, 1243, 1168, 1036, 825, 791;
dH (400 MHz, CD3SOCD3) 8.11 (1H, s), 7.66e7.65 (1H, m), 7.40e7.36
(2H, m), 7.15e7.12 (6H, m), 7.02e7.00 (4H, m), 6.47e6.46 (1H, m),
2.27 (6H, s); dC (400 MHz, CD3SOCD3) 163.7, 152.1, 144.6, 141.3,
140.6, 137.8, 136.6, 135.4, 133.2, 133.0, 130.1, 127.1, 126.8, 124.3,
123.9, 123.8, 123.1, 119.1, 118.1, 108.5, 79.2, 20.4; HRMS (APCI):
[MꢁCO2þH]þ, found 495.1015. C29H23N232S3 requires 495.1018.
was added in one portion followed by the addition of pyridine
(0.342 mL, 4.25 mmol) at room temperature under nitrogen. The
resulting purple solution was stirred overnight and water (100 mL)
was added to the reaction mixture. The organic layer was separated,
washed with water, brine, and dried over anhydrous sodium sul-
fate. Solvent was removed in vacuo and the crude material was
subjected to column chromatography on silica (20% ethyl acetate/
hexane) to yield the title compound 2 (1.20 g, 54.4%) as a dark black
powder; mp 170e174 ꢄC; Rf (20% ethyl acetate/hexane) 0.38; IR
(liquid film, cmꢁ1) 2920 (br), 2200, 1730, 1566, 1457, 1433, 1250,
840; dH (400 MHz, CD2Cl2) 7.75 (1H, s), 7.61e7.60 (1H, m), 7.32e7.31
(1H, m), 7.23e7.22 (1H, m), 7.13e7.06 (8H, m), 7.02e7.01 (1H, m),
6.97e6.96 (1H, m), 6.44e6.43 (1H, m), 2.32 (6H, s); dC (400 MHz,
CD2Cl2) 154.5, 150.6, 149.6, 145.5, 142.1, 141.1, 134.5, 133.6, 132.7,
130.5, 128.8, 127.2, 124.7, 124.6, 124.1, 123.8, 117.8, 115.1, 114.3, 75.8,
21.1; LRMS (EI): Mþ, found 519.1; HRMS (EI): Mþ, found 519.0890.
C30H21N3S3 requires 519.0892.
4.2.4. Synthesis of 2-{[5-(5-{5-[bis(4-methylphenyl)amino]thiophen-
2-yl}thiophen-2-yl)thiophen-2-yl]methylidene}propanedinitrile
(2). Compound 6 (2.00 g, 4.25 mmol) was dissolved in chloroform
(50.0 mL) in a 100 mL RB flask and malononitrile (1.40 g, 21.3 mmol)
4.2.5. Synthesis of 5-{[5-(5-{5-[bis(4-methylphenyl)amino]thiophen-
2-yl}thiophen-2-yl)thiophen-2-yl]methylidene}-1,3-diethyl-2-

3592
A. Gupta et al. / Tetrahedron 69 (2013) 3584e3592
2. Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.;
Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498e500.
3. Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270,
1789e1791.
4. Tang, C. W. Appl. Phys. Lett. 1986, 48, 183e185.
5. Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater. 2005, 15,
1617e1622.
6. Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat.
Mater. 2005, 4, 864e868.
7. Mishra, A.; Fischer, M. K. R.; Bauerle, P. Angew. Chem., Int. Ed. 2009, 48,
2474e2499.
8. Burckstummer, H.; Kronenberg, N. M.; Gsanger, M.; Stolte, M.; Meerholz, K.;
Wurthner, F. J. Mater. Chem. 2010, 20, 240e243.
9. Liu, Y. S.; Wan, X. J.; Yin, B.; Zhou, J. Y.; Long, G. K.; Yin, S. G.; Chen, Y. S. J. Mater.
Chem. 2010, 20, 2464e2468.
10. Mishra, A.; Bauerle, P. Angew. Chem., Int. Ed. 2012, 51, 2020e2067.
11. Winzenberg, K. N.; Kemppinen, P.; Fanchini, G.; Bown, M.; Collis, G. E.;
Forsyth, C. M.; Hegedus, K.; Singh, T. B.; Watkins, S. E. Chem. Mater. 2009, 21,
5701e5703.
12. (a) Gupta, A.; Ali, A.; Bilic, A.; Gao, M.; Hegedus, K.; Singh, B.; Watkins, S. E.;Wilson,
G. J.; Bach, U.; Evans, R. A. Chem. Commun. 2012, 1889e1891; (b) Gupta, A.; Ali, A.;
Singh, T. B.; Bilic, A.; Bach, U.; Evans, R. A. Tetrahedron 2012, 68, 9440e9447.
13. Thomas, K. R. J.; Hsu, Y.-C.; Lin, J. T.; Lee, K.-M.; Ho, K.-C.; Lai, C.-H.; Cheng, Y.-M.;
Chou, P.-T. Chem. Mater. 2008, 20, 1830e1840.
sulfanylidene-1,3-diazinane-4,6-dione (3). A solution of compound 6
(980 mg, 2.08 mmol) and diethylthiobarbituric acid (1.25 g,
6.24 mmol) in methanol (100 mL) was heated to reflux for 4 h and
the reaction mixture was cooled to ambient temperature. The solid
that formed in the reaction was filtered off and washed with
methanol to obtain the title compound 3 (1.20 g, 88.5%) as a black
fluffy solid; mp 210e215 ꢄC; IR (liquid film, cmꢁ1) 2920 (br), 1732,
1456, 1330e1130 (br), 840; dH (400 MHz, CD2Cl2) 8.58 (1H, s),
7.81e7.80 (1H, m), 7.44e7.43 (1H, m), 7.34e7.33 (1H, m), 7.13e7.07
(8H, m), 7.04e7.03 (1H, m), 7.00e6.99 (1H, m), 6.44e6.43 (1H, m),
4.58e4.53 (4H, m), 2.33 (6H, s), 1.33e1.26 (6H, m); dC (400 MHz,
CD2Cl2) 179.4, 161.4, 160.5, 154.7, 154.5, 149.1, 148.1, 145.5, 142.2,
136.3,134.5,133.8,130.5,128.8,127.5,125.0,124.7,124.1,123.9,117.9,
110.0, 44.3, 43.5, 21.1, 12.8, 12.6; LRMS (EI): Mþ, found 653.1; HRMS
(EI): Mþ, found 653.1305. C35H31N3O232S4 requires 653.1294.
Acknowledgements
This research was funded through the Flexible Electronics
Theme of the CSIRO Future Manufacturing Flagship and was also
supported by the Victorian Organic Solar Cell Consortium (Victo-
rian Department of Primary Industries, Sustainable Energy Re-
search and Development Grant, Victorian Department of Business
and Innovation, Victoria’s Science Agenda Grant and the Australian
Solar Institute). We are thankful to Dr. Jo Cosgriff and Dr. Carl
Braybrook for MS analysis.
14. Xia, P. F.; Feng, X. J.; Lu, J. P.; Tsang, S.-W.; Movileanu, R.; Tao, Y.; Wong, M. S.
Adv. Mater. (Weinheim, Ger.) 2008, 20, 4810e4815.
15. Sreenath, K.; Suneesh, C. V.; Kumar, V. K. R.; Gopidas, K. R. J. Org. Chem. 2008,
73, 3245e3251.
16. Bedworth, P. V.; Cai, Y.; Jen, A.; Marder, S. R. J. Org. Chem. 1996, 61,
2242e2246.
€
17. Wurthner, F.; Wortmann, R.; Matschiner, R.; Lukaszuk, K.; Meerholz, K.;
DeNardin, Y.; Bittner, R.; Brauchle, C.; Sens, R. Angew. Chem., Int. Ed. 1997, 36,
2765e2768.
18. Zhang, F.;Luo, Y.-H.; Song, J.-S.; Guo, X.-Z.;Liu, W.-L.; Ma, C.-P.; Huang, Y.;Ge, M.-F.;
Bo, Z.; Meng, Q.-B. Dyes Pigm. 2009, 81, 224e230.
19. Fischer, M. K. R.; Wenger, S.; Wang, M. K.; Mishra, A.; Zakeeruddin, S. M.;
€
Supplementary data
Gratzel, M.; Bauerle, P. Chem. Mater. 2010, 22, 1836e1845.
20. (a) Bai, Y.; Yu, Q. J.; Cai, N.; Wang, Y. H.; Zhang, M.; Wang, P. Chem. Commun.
2011, 4376e4379; (b) Daeneke, T.; Kwon, T. H.; Holmes, A. B.; Duffy, N. W.;
Bach, U.; Spiccia, L. Nat. Chem. 2011, 3, 211e215.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2013.02.040.
21. Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.;
€
Diau, E. W. G.; Yeh, C. Y.; Zakeeruddin, S. M.; Gratzel, M. Science 2011, 334,
629e634.
References and notes
22. McEwen, C. N.; McKay, R. G.; Larsen, B. S. Anal. Chem. 2005, 77, 7826e7831.
23. Feldt, S. M.; Gibson, E. A.; Gabrielsson, E.; Sun, L. C.; Boschloo, G.; Hagfeldt, A. J.
Am. Chem. Soc. 2010, 132, 16714e16724.
€
1. O’Regan, B.; Gratzel, M. Nature 1991, 353, 737e740.