'Donor-free' oligo(3-hexylthiophene) dyes for efficient dye-sensitized solar cells

Article abstract of DOI:10.1039/c5ta09133k

The common trend in designing dyes for use in DSSCs with iodide-based electrolyte is based on a donor-π spacer-acceptor (D-π-A) architecture. Here, we report two 'donor-free' cyanoacrylic end-functionalized oligo(3-hexylthiophene) dyes (5T and 6T). Despite having no donor group, both dyes show reversible first oxidation process. Both 5T and 6T have n-hexyl alkyl chains to retard aggregation at different positions as well as different numbers of thiophene moieties. However, the dyes showed similar absorption properties and redox potentials. The DSSCs based on these dyes give power conversion efficiencies of more than 7%, although a significant difference in the V_OC and FF has been observed. Using electrochemical impedance spectroscopy, this is attributed to the presence of more trap states when 6T attaches to TiO₂ and modifies the surface, mainly affecting the fill factor. Overall, these dyes introduce a new and effective design concept for liquid-electrolyte DSSC sensitisers.

Full text of DOI:10.1039/c5ta09133k

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‘Donor-free’ oligo(3-hexylthiophene) dyes for efficient dye-
sensitized solar cells
Yue Hu,a Aruna Ivaturi, a Miquel Planells,a Chiara L. Boldrini b, Alessio Orbelli Biroli,
b Neil Robertson*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The common trend in designing dyes for use in DSSCs with iodide-based electrolyte is based on a donor - π
spacer - acceptor (D-π-A) architecture. Here, we report two ‘donor-free’ cyanoacrylic end-functionalized
oligo(3-hexylthiophene) dyes (5T and 6T). Despite having no donor group, both dyes show reversible first
oxidation process. Both 5T and 6T have n-hexyl alkyl chains to retard aggregation at different positions as
well as different numbers of thiophene moieties. However, the dyes showed similar absorption properties
and redox potentials. The DSSCs based on these dyes give power conversion efficiencies of more than 7%,
although a significant difference in the VOC and FF has been observed. Using electrochemical impedance
spectroscopy, this is attributed to the presence of more trap states when 6T attaches to TiO2 and modifies
the surface, mainly affecting the fill factor. Overall, these dyes introduce a new and effective design concept
for liquid-electrolyte DSSC sensitisers.
www.rsc.org/
used acceptor which decreases the energy gap between HOMO and
LUMO, thus leading to a red shift of the lowest energy absorption
maximum.10 Various donor groups have been investigated,
Introduction
Over the last 20 years, dye-sensitised solar cells (DSSCs) have
attracted significant interest as low-cost alternatives to
including anthraquinone, boradiazaindacene, carbazole, coumarin,
N,N-diakylaniline, hemicyanine, heteroanthracene, indoline,
merocyanine, tetrahydroquinoline, triarylamine, squaraine,
perylene and polymeric species. 11 The electron-donor group is
usually bulky in order to reduce the electron recombination
between redox electrolyte and the TiO2 surface and to stabilize the
oxidized dye. The influence of the donor size in D - π - A dye was
studied by Yang et al.9 A higher voltage was observed and reported
when a bulky-indoline moiety was introduced as donor into the IQ4
dye.12
conventional photovoltaic technologies. They present a record
efficiency (uncertified) of ~13% under standard reporting
conditions.1, 2 Ruthenium complexes have been designed to work as
sensitisers with standard I-/I3- liquid redox electrolyte, giving a
record certified efficiency of 11.9 ± 0.4 %.3, 4 However, several
challenges limit their development for large scale applications
including material costs, difficulty in purification and environmental
impact. In solid-state dye-sensitised solar cells (ssDSSC) where the
liquid based I-/I3- redox couple is exchanged for a hole conductor,
thinner TiO2 films are needed because of mass transport limitations
or insufficient pore filling. 5-7 Accordingly, metal-free organic
sensitisers with high extinction coefficients have been developed
and the overwhelming trend in designing these dyes is based on a
donor - π spacer - acceptor (D- π -A) architecture.8, 9 In such dyes,
the donor and the π spacer part contain the HOMO of the dye while
the LUMO is distributed on the acceptor part and the anchoring
group. Upon excitation, the electrons move from donor to acceptor
through the π -bridge. This modular design allows organic dyes to
show great diversity and flexibility. Cyanoacrylic acid is a commonly
However, previous study in our group on ssDSSCs using a ‘donor-
free’ sensitiser, cyanoacrylic end-functionalized oligo(3-
hexylthiophene) (5T) and ‘donor- π-acceptor’ sensitiser based on
the same structure (MK-2) showed that without an electron donor
group, a significantly-higher open-circuit voltage (VOC) could be
achieved while the short-circuit current (JSC) remained
approximately the same. This resulted in a significantly higher over-
all efficiency, comparable with the very best dyes for ssDSSCs.
Devices using 5T as the sensitiser and spiro-OMeTAD as the hole-
transport material gave an efficiency of 4.4% whereas the ones
using MK-2 as the sensitiser gave only 2.8%. 13
To follow up the above work, in the present study we have tested
-
5T in liquid-state dye-sensitised solar cells using classic I-/I3 redox
couple. Another donor-free dye, 6T, which has one more thiophene
unit than 5T, as well as a simpler synthetic scheme is also designed,
synthesised, characterised and tested. To compare the performance
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of 5T and 6T with the traditional D-π-A dyes, we also fabricated the have high synthetic yield, opening a new strategy of designing
liquid-state dye-sensitised solar cells based on MK2 dye and using I- highly efficient ‘donor-free’ dyes in the future. DSSCs using 6T
/I3- redox couple. The dyes studied in the present work have similar showed significantly higher VOC but lower JSC and FF than those
molecular backbone as MK2 - both use cyanoacrylic acid as an using 5T. By using electrochemical impedance spectroscopy (EIS), it
acceptor group and four thiophene derivatives as a π-spacer. While was found that 6T leads to more trapping states, which may be
MK2 has a terminal carbazole donor group to complete the D-π-A attributed to the extra conjugation or different position of alkyl
structure, 5T and 6T have additional thiophene to elongate the π- chain of 6T than 5T.
system to achieve a molecular size similar to MK2. The best PCE
obtained with MK2, 5T, 6T and N719 are respectively, 5.05%,
7.64%, 7.07% and 8.89% (under the conditions optimized for N719).
Most of the D-π-A dyes exhibiting good efficiencies in liquid-state
dye-sensitised solar cells have complicated structures and have low
synthetic yield.11 However, the donor-free dyes reported in the
present study can be easily synthesised using cross-coupling and
Scheme 1. The structure of 5T and 6T (above) and the synthetic procedure for the synthesis of 6T
Table 1 Photophysical and Electrochemical properties of 5T and 6T
λmaxa / nm (Ɛ ×10-4 / M-1 cm-1)
478 (3.9), 376 (2.4)
Eoxb vs. NHE/ V
1.08
Eredb vs. NHE / V
-1.29
E0-0c / eV
2.15
ΔEelectrochemical / V
E*d vs. NHE / V
-1.07
5T
6T
2.37
2.30
486 (3.9), 401 (3.1)
0.98
-1.32
2.30
-1.32
a Absorption and emission spectra were measured in DCM with a concentration of 2×10-6 M at room temperature. b Potential was
measured in DCM with 0.3 M [TBA][PF6] as electrolyte and calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference and
converted to NHE by addition of 0.63 V. c 0-0 transition energy, E0-0, estimated from the intercept of the normalized absorption and
emission spectra in DCM. d Estimated excited state redox potential, estimated by subtracting E0-0 from the ground state oxidation potential.
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the thiophene backbone and is more intense for 6T. (Figure 2). This
assignment was confirmed by measuring UV-vis spectra in different
solvents [See the Supporting Information Figure S2 (a)]. In a mixture
of acetonitrile and butanol, the lowest energy absorption band of
both compounds blue shifted significantly while the higher energy
band remained at the same wavelength, indicating the charge
transfer nature of the lowest energy absorption band. Figure 2 also
gives the absorption spectra of 5T and 6T on 3 μm transparent TiO2
films. Both spectra broadened, implying the dyes form Herring-
bone aggregates on TiO2 films. 19 6T showed wider absorption than
5T on TiO2 film because of its extra conjugation. This has also been
seen by Masumoto, et al.20, who studied the optical properties of
thiophene-based oligomers and found the effective conjugation
length of oligothiophenes was six thiophene rings.
Results and discussion
Synthesis
The synthesis of oligo(3-hexylthiophene)s and poly(3-
hexylthiophene) has been reported using several routes and the
challenge is to separate the high-purity monodisperse regioregular
oligo(3-hexylthiophene)s in reasonable quantities. The previous
work in our group was based on extending the oligomer stepwise
by one unit per cross-coupling followed by activation of the newly
formed chain end to give 5T.14 This approach was part of a study to
Photoluminescence was also studied in solution [see the Supporting
Information Figure S2 (b)]. 5T and 6T both showed very broad
emission response, covering the visible and even near-IR region. 5T
showed a bigger Stokes-shift than 6T (5973 cm-1 and 2908 cm-1,
respectively).
prepare shorter oligomers including 1 – 5 thiophene units14
,
however it is not very efficient when the longer chains of 5T and 6T
are specifically desired. In this work, 6T was therefore synthesised
using a ‘Fibonacci Route’ reported by Heeney. 15 The regioregularity
of 6T is proven by the presence (brominated precursor) and absence
(stannylated precursor) of 3-bond H-H coupling of the terminal
protons in NMR. Also, 6T has a sharp melting point of 132℃. The
synthetic route is shown in Scheme 1. The synthesis of the oligomer
(3HT)n was achieved by the coupling of Br-(3HT)n-x and (3HT)x-SnBu3.
Thus, (3HT)n with n=1,2,3,5,8,13 and 21 can be obtained more
efficiently than adding one thiophene unit per cross-coupling. Also,
this strategy allowed easy purification by vacuum distillation of
solvent and column chromatography. Finally, in this work, a
thiophene unit functionalized with a cyanoacrylic end group was
added to enable appropriate attachment to the TiO2. We note that
this Fibonacci route leads to alkyl chains in a different position. The
alkyl chains for 5T are at the 3-positions of the thiophene, however,
for 6T, they are at the 2-positions. Alkyl chains were attached to
the thiophene unit to avoid strong π- π interaction,16 which has
been proven to lead to a decreased electron injection in the solar
17
cells.
Furthemore, alkyl chains play a role in reducing charge
recombination between electrons in TiO2 and the redox
electrolyte18
.
Optical Properties
Optical properties of 5T and 6T were studied in dichloromethane at
2 × 10-6 M. For both dyes, the lowest energy absorption band is
around 480 nm. The absorption band positions and their extinction
coefficients are summarized in Table 1. The addition of thiophene
unit and extension of π-system does not lead to significant changes
in either the position of the lowest energy absorption band or the
extinction coefficient of this band but has a stronger influence on
the higher energy band, which corresponds to a π- π* transition of
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Figure 2 UV-vis spectra of 5T and 6T at 2 × 10-6 M in DCM (a) and 3
Figure 3 Energy level schemes for the Kohn-Sham orbitals of 5T and
6T, including selected Kohn-Sham orbitals and the HOMO-LUMO
energy gap.
μm transparent on TiO2 (b)
Electrochemical properties and DFT calculations
Electrochemical properties for 5T and 6T were investigated by cyclic
voltammetry (CV) and square-wave voltammetry (SWV). Despite
the fact that 5T and 6T don’t have donor groups, both the dyes
showed a reversible first oxidation process (Supporting Information,
Figure S3 and S4). The values against NHE are presented in Table 1.
The addition of one more thiophene unit in 6T shifts the oxidation
peak to less positive potential; 5T shows the first oxidation
potential at 1.08V whereas 6T shows it at 0.98V (see Supporting
Information Table S1-S3). They are both higher than that of the
iodide/triiodide (0.40 V vs. NHE) redox electrolyte energy level,
guaranteeing good driving force for the dye regeneration. Little
difference was observed for the reduction processes (5T: -1.29 V,
6T: -1.32 V vs. NHE). This is because the LUMO is localized mainly at
the cyanoacrylic moiety and is little affected by the length of the
thiophene chain. The electrochemical results were supported by
DFT calculations. The energy level schemes for the Kohn-Sham
orbitals of 5T and 6T, including selected Kohn-Sham orbitals and the
HOMO-LUMO energy gap are shown in Figure 3. For both 5T and
6T, the HOMO is distributed along the conjugated π-system and the
LUMO is located on the cyanoacetic acid unit through the
Photovoltaic Performance of DSSCs
The photovoltaic performance of the DSSCs based on
iodide/triiodide redox electrolyte using the 5T and 6T dyes was
analysed as a function of dye soaking time of 3-24 hours and
compared with DSSCs using N719. (Supporting information Figure
S4-S6). The best performance for all the dyes was observed for the
devices soaked for 24 hours. To compare the performance of 5T
and 6T with the traditional D-π-A dyes, DSSCs based on MK2 dye
were also fabricated.The J-V characteristics of the best cells are
shown in Figure 4 and the corresponding device characteristics
along with the estimated amount of dye loaded on 18 µm TiO2
photoelectrodes soaked in the corresponding dyes for 24 hours are
given in Table 2. N719 has the highest JSC because it exhibits the
widest absorption spectrum. The IPCE of DSSCs using 5T and 6T (see
Supporting Information Figure S8) exceeded 60% over most of
visible light region from 400 nm to 650 nm, with a maximum of
~80% at around 500 nm and the lowest values were between 650-
700 nm. N719 showed better performance in this region. The slight
shift in the absorption onset of the 6T as compared to 5T on TiO2
[Figure 2 (b)] is also evident from the shift in the IPCE (Figure S8)
and results in a higher Voc for 6T compared to 5T (Table 2). The
absorbed amount of dye increased slightly with increasing the
number of thiophene. This can be understood interms of increased
π- π interaction with expanding π-conjugation. 6T would form a
thicker aggregate on TiO2 surface due to increased number of
thiophene and since the adsorbed dye amount of the former is
larger than the 5T it would in turn result in slower diffusion of redox
species in the mesoporous TiO2 leading to increased charge transfer
resistance (as observed in the EIS measurements discussed later)
leading to lower FF. A higher dye loading of the 6T compared to 5T
on TiO2 (Table 2), however, doesn’t lead to a higher short circuit
photocurrent as might be expected. Hagberg et al. observed similar
thiophene, allowing good charge separation and charge
directionality after photo-excitation and preventing back
regeneration of the dye with the injected electrons. 21 Time-
dependent DFT calculation was performed with a dichloromethane
polarisable continuum model (PCM)22 using the CAM-B3LYP
functional23. The result is consistent with the experimental UV/Vis
and is shown in Supporting Information Table S1-S2.
trends in a series of organic chromophores when the π-conjugation
between the donor (triphenylamine moiety) and the acceptor
(cyanoacetic acid moiety) were systematically extended.24 A similar
trend was also observed by Barea et al. in n-Thiophene absorbers
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when increasing the conjugation length from 2 to 6.21 Such a
decrease in JSC was attributed to the differences in dye binding and
orientation affecting injection efficiency.21,24 Overall, the best PCE
efficiencies obtained were N719 (8.94%), 5T (7.64%) and 6T
on the best cells under illumination from a LED at different bias
potentials in the frequencies between 1MHz and 0.1Hz. The data
were fitted using the transmission line model. 26, 27 The equivalent
circuit is shown in Figure S9. From the fittings, charge transfer
resistance (RCT), electron transport resistance (Rtrans) and chemical
(7.07%). 5T has a lower PCE than N719 mainly due to a loss in VOC
,
which is not unusual in organic dyes. A significant decrease in the fill capacitance of TiO2 (CChem) were extracted. The electron lifetime
factor is observed when comparing the devices using 6T and others. and electron transport time were calculated according to the
In contrast, the best PCE efficiency obtained in DSSCs based on MK2 equations: τe = RCT * CChem and τtrans = Rtrans * CChem, respectively.
(processed under conditions optimised for N719) is 5.05%, with Jsc
of 17.64 mA.cm-2, Voc of 0.64 V and FF of 45.0%. The best PCE
value reported in literature for MK2 sensitised DSSCs is 8.3% with
Figure 5 shows the charge transfer resistance and the chemical
capacitance of the TiO2 as a function of voltage and the electron
lifetime as a function of capacitance. Although it is increasingly
Jsc of 15.22 mA.cm-2, Voc of 0.73 V and FF of 75.0%.25 The higher Jsc common to use the chemical capacitance as a reference for the
and lower FF observed in the present study could be related to the
photoelectrode thickness of 18 µm in contrast to the 16 µm
reported in literature. Whereas the lower Voc is related to the
electrolyte composition, especially the iodine concentration of
0.2M in the former in contrast to 0.03M used in the present study.
relative position of the conduction band edge in different devices,
Barnes, et al.28 have highlighted the need to analyse both charge
and transport data in addition to simple electron lifetime
measurements when drawing conclusions about DSSC behaviour,
for the reason that some dyes may modify the distribution of trap
states as well as shifting the semiconductor conduction band. Based
on this knowledge, we first examine the relationship between
transport time and capacitance. The longer transport time of 6T
over 5T indicates more trap states in the presence of 6T (Figure 5
(a)) also consistent with the higher capacitance for 6T (Figure 5 (b)).
A low FF is generally ascribed to high series resistance, 29,30 which
includes the redox reaction resistance at the counter electrode, the
resistance of electron transport by ions in the electrolyte and
electron transport through the TiO2. The longer transport time for
6T indicates a higher resistance for transport, which leads to a
lower FF and a slightly lower JSC for 6T than the other dyes. This may
be caused by the extra conjugation of 6T compared to 5T, or the
different position of the alkyl chains which modifies the TiO2 surface
differently. The longer electron lifetime for 6T over 5T [Figure 5(c)],
despite the higher number of traps, corresponds with the
experimental Voc which is higher for 6T. In contrast, 5T and N719
have similar trap states, so any different device behaviour can be
attributed to a shift of conduction band edge, possibly related to
the different protonation state of N719 compared with 5T and 6T.
From Figure 5 (c) N719 has much longer electron lifetime than 5T
and this leads to a final VOC value trend of N719>6T>5T.
Figure 4 J-V characteristics and IPCEs of the DSSCs using N719, 5T
and 6T under dark and under AM1.5 simulated sunlight of
illumination of 100 mW cm-2.
Table 2 Photovoltaic performance for the champion cells measured
under AM 1.5 illumination. The photoelectrodes were soaked in the
dye solution for 24 hours.
Dye
Jsc / mA
cm-2
Voc
/ V
FF /
%
Efficiency
/ %
Dye loading /
× 107 mol cm-2
3.04
N719 17.48
0.68
0.62
0.67
74.7
72.0
62.8
8.89
5T
6T
17.15
16.85
7.64
5.09
7.07
5.38
Electrochemical impedance spectroscopy (EIS)
In order to understand the reasons behind the observed
photovoltaic performance as a function of thiophene chain length,
detailed impedance spectroscopy measurements were performed
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Experimental
Synthetic procedure
Materials. All reagents were purchased from either Sigma-Aldrich
or Alfa-Aesar and were used as received without further
purification. Tributyl(3,4',4'',4''',4''''-pentahexyl[2,2':5',2'':5'',2''':5''',
2''''-quinquethiophen]-5-yl)-stannane was synthesised according to
literature15 and the detailed experimental method is in the
Supporting Information.
Synthesis of 4',4'',4''',4'''',4'''''-pentahexyl-[2,2':5',2'':5'',2''':5''',
2'''':5'''',2'''''-sexithiophen]-5-carbaldehyde Tributyl(3,4',4'',4''',4''''-
pentahexyl[2,2':5',2'':5'',2''':5''',2''''-quinquethiophen]-5-yl)-
stannane (800 mg, 0.71 mmol) and 5-bromo-2-
thiophenecarboxaldehyde (100 mg, 0.52 mmol) were dissolved in
anhydrous toluene (10 ml) and bubbled with N2 for 30 mins.
Tetrakis(triphenylphosphine) palladium (Pd(PPh3)4) (40 mg, 0.035
mmol) was added and the solution was bubbled with N2 for another
10 mins before the solution was heated to 90
℃ for 72h. The
organic solvents were removed by vacuum, then the crude product
was purified by column chromatography (SiO2, hexane) to give a red
oil (290 mg, 59% yield). 1H NMR (500 MHz, CDCl3): 9.48(s, 1H),
7.31(s, 1H), 7.10 (s, 1H), 7.07 (s, 1H), 6.99 (m, 1H), 6.98 (s, 1H), 6.82
(d, J= 4.0Hz, 1H), 6.77 (d, J= 4.0Hz, 1H), 6.60 (m, 1H), 2.87~2.68 (m,
8H), 2.4 (t, J=7.7Hz, 2H), 1.73-1.62 (m, 10H), 1.28-1.15(m, 30H),
0.91-0.84 (m, 15H)
Synthesis of 2-cyano-3-[4',4'',4''',4'''',4'''''-pentahexyl[2,2':5',2'':5'',
2''':5''',2'''':5'''',2'''''-sexithiophen]-5-yl]acrylic acid (6T) 4',4'',4''',
4'''',4'''''-pentahexyl-[2,2':5',2'':5'',2''':5''',2'''':5'''',2'''''-sexithiophen]
-5-carbaldehyde (290 mg, 0.31 mmol) and cyanoacetic acid (40 mg,
0.46 mmol) were dried under vacuum. Then ethanol (2.5 ml) was
added and the solution was bubbled with N2 for 10 mins. Piperidine
(132 mg, 1.55 mmol) was added and the reaction was heated up to
Figure 5 (a) Transport time plotted against capacitance of the TiO2.
(b) Charge transfer resistance and chemical capacitance of the TiO2
plotted against the voltage. (c) Electron lifetime plotted against
capacitance of the TiO2.
75
℃ for 6 h. Then, the mixture was poured into 100 ml of aqueous
HCl 1M and extracted with diethyl ether. The crude was purified by
size exclusion column chromatography (Biobeads S-X3, DCM) to
give a red solid (210 mg, 67% yield). 1H NMR (500 MHz, CDCl3):
8.32(s, 1H), 7.70(d, J= 4.2Hz, 1H), 7.29 (s, 1H), 7.25(d, J= 4.0Hz, 1H),
7.04 (s, 1H), 7.01-6.99(m, 2H), 6.98(s, 1H), 6.93(s, 1H), 2.84-2.76(m,
8H), 2.64(t, 7.7Hz, 2H), 1.75-1.64(m, 10H), 1.48-1.32(m, 30H), 0.96-
0.90(m, 15H). MS ESI (m/z): [M]+ calcd 1009.41500. Found:
Conclusions
Two ‘donor-free’ cyanoacrylic end-functionalized oligo(3-
hexylthiophene) dyes (5T and 6T) have been synthesised, and
this dye design used for the first time as effective sensitizers
for liquid-state dye-sensitized solar cells with I-/I3- redox
couple, giving power conversion efficiency of 7.64% and
7.07%, respectively. 6T showed a significantly higher VOC than
5T but lower JSC and FF. By using electrochemical impedance
spectroscopy (EIS), it was found that 6T has more trap states,
attributed to the extra conjugation or different position of
alkyl chain of 6T. These dyes can be easily synthesised using
cross-coupling and showed reversible first oxidation processes
even without donor groups. Overall, this opens up a new
strategy for future dye design with the potential for simpler
dyes and also new insights into optimised sensitizer design and
performance.
1009.41166. Anal. Calc. for C58H75NO2S6: C 68.93, H 7.48, N 1.39.
Found: C 69.01, H 7.55, N 1.37%. Melting point: 132
.
Methods
Chemical characterisation. 1H NMR spectra were recorded on
Bruker Advance 500 spectrometer. The deuterated solvents are
indicated; chemical shifts, δ, are given in ppm, referenced to TMS,
standardized by the solvent residual signal (1H). Coupling constants
(J) are given in hertz (Hz). MS were recorded on ThermoElectron
MAT 900 using electrospray ionization (ESI) technique. Elemental
analyses were carried out by Stephen Boyer at London
Metropolitan University using a Carlo Erba CE1108 Elemental
Analyser.
Electrochemical characterisation. All cyclic voltammetry and square
wave voltammetry measurements were carried out in anhydrous
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CH2Cl2 using 0.3 M [TBA][PF6] electrolyte in a three-electrode assembled in the sandwich mode where the two electrodes were
system. The solution was purged with N2 prior to measurement. stuck together using thermoplastic (Surlyn). The electrolyte used
The working electrode was a Pt disk. The reference electrode was was a mixture of 1-butyl-3-methylimidazolium iodide (1.3307 g, 1
Ag/AgCl and the counter electrode was a Pt rod. All measurements M), I2 (0.0381 g, 0.03 M), LiI (0.067 g, 0.1 M), 4-tert butylpyridine
were made at room temperature using an μAUTOLAB Type III (0.3380 g, 0.5 M), guanidine thiocyanate (0.059 g, 0.1 M) in
potentiostat, driven by the electrochemical software GPES. Cyclic acetonitrile (4.25ml) and valeronitrile (0.75ml). To compare the
voltammetry (CV) measurements used scan rates of 0.1, 0.2, 0.4, performance of 5T and 6T with the traditional D-π-A dyes, we also
0.6, 0.8 and 1V/s. Square wave voltammetry (SWV) experiments fabricated the DSSCs based on MK2 dye and using the same redox
were carried out at a step potential of 4 mV, a square wave couple mentioned above. The electrolyte was introduced by
amplitude of 25 mV and a square wave frequency of 15 Hz, giving a vacuum suction into the hole and the hole was sealed by cover
scan rate of 40 mV/s. Ferrocene was used as internal standard in glass. The J-V measurements were carried out using a computer-
each measurement and potentials are quoted versus NHE against controlled digital source meter (Keithley 2400) under simulated
which ferrocene/ferrocenium was observed at 0.63 V.
AM1.5G irradiation from a solar simulator (92250 A, Newport, USA)
Optical characterisation. Solution UV-Visible absorption spectra under 1 sun equivalent (100 W/m2) condition. A light mask was
were recorded using Jasco V-670 UV/Vis/NIR spectrophotometer used on the DSSCs, so the illuminated active area of DSSC was fixed
controlled using the SpectraManager software. Photoluminescence to 0.126 cm2. Incident photon-to-current conversion efficiency
(PL) spectra were recorded with Fluoromax-3 fluorimeter controlled (IPCE) measurements were carried out with a Bentham PVE300
by the ISAMain software. All samples were measured in a 1 cm cell photovoltaic spectral response system controlled by BenWin+
at room temp. with dichloromethane as a solvent. Concentration of software, light intensity was measured by a calibrated silicon
2×10-6 M were used for UV/Visible and PL.
photodiode (300-1100 nm).
Computational details. The molecular structures were optimised in
vacuum, using the software Avogadro31 to enter the starting
geometry. Then the structure was optimised in dichloromethane,
using the optimised structure from vacuum. All calculations were
carried out using the Gaussian 09 program32 with the hybrid B3LYP
functional23 and the standard 6-31G(d) basis set. Time-dependent
DFT calculations (TD-DFT) were performed using Gaussian 09
program with a dichloromethane polarisable continuum model
(PCM)22 using CAM-B3LYP functional. The 20 lowest singlet
electronic transitions were calculated and processed with the
GaussSum software package.33
Electrochemical impedance spectroscopy (EIS). Electrochemical
impedance spectroscopy (EIS) was performed using the AUTOLAB
PGSTAT30 potentiostat controlled by FRA software, version 4.9.007.
The cells were measured in a frequency range between 7 MHz and
0.02 Hz with forward bias potentials controlled by the intensity of
LED lights. The resulting impedance spectra were analysed with Z-
view software (Scribner Associate Inc) version 3.3a on the basis of
the two channel transmission line model.
Acknowledgements
Solar cell fabrication. To make the working electrodes, fluorine
doped tin oxide (FTO) coated glass was washed using 2% Hellmanex
solution and then by sonication in distilled water, acetone and
methanol. The glasses were further cleaned by UV-ozone cleaner
for 20 mins. The cleaned glass was pre-treated with a solution of
TiCl4 (40mM in water) at 80 ℃ for 30 mins, rinsed with water and
ethanol and sintered at 450 ℃ for 30 mins. The working electrodes
were screen printed using commercial TiO2 paste (DSL 18NR-T,
Dyesol) as the transparent layer and (DSL 18NR-AO, Dyesol) as the
scattering layer. The total thickness of the TiO2 layer was
maintained at 18µm and the area of the layer was 0.28 cm2. The
films were dried at 125℃ for 6 mins and gradually heated over an
hour to 510℃ and annealed for 10 mins using a programmable
hotplate. The films were allowed to cool to room temperature and
post-treated with TiCl4 (40 mM in water) at 80 ℃ for 30 mins and
sintered at 500 ℃ for 30 mins. When the temperature dropped to
about 80-100 ℃, the working electrodes were put into 0.5mM dye
solution in acetonitrile: t-butanol (1:1) and soaked for different time
durations (3-24 hours). The electrodes were removed and washed
with the same solvent mixture, to remove any unbound dye
molecules. Counter electrodes were prepared by drilling a hole into
pre-cut FTO glass. The FTO glass pieces were washed with distilled
water, 0.1M HCl in ethanol and finally acetone. The electrodes were
heated at 400 ℃ for 15 mins both before and after a layer of
Platisol (Solaronix) was applied. The solar cells were then
We thank China Scholarship Council, the University of Edinburgh
and the EPSRC Apex project (EP/M023532/1) for financial support.
Open data: http://dx.doi.org/10.7488/ds/279. C.L.B. and A.O.B.
thank Regione Lombardia and Fondazione Cariplo for financial
support and the use of instrumentation purchased through the
“SmartMatLab Centre” project.
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This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Journal of Materials Chemistry A
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DOI: 10.1039/C5TA09133K
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Journal of Materials Chemistry A
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Easily synthesized ‘donor-free’ dyes work as efficient sensitisers for dye-sensitised solar
cells.