Effect of furan π-spacer and triethylene oxide methyl ether substituents on performance of phenothiazine sensitizers in dye-sensitized solar cells

Article abstract of DOI:10.1039/c9nj01720h

Conjugated π-spacers are used to improve the light harvesting properties of sensitizers for dye-sensitized solar cells. In several recent works, furan has outperformed other popular π-spacers such as thiophene and phenyl. One of the best performing phenothiazine dyes in the literature has no π-spacer, and two polar triethylene oxide methyl ether (TEOME) chains. Herein we report the synthesis and evaluation of three novel sensitizers based on EO3, investigating the effect of introducing a furan linker and stepwise removal of the TEOME chains. The furan linker redshifted the onsets in the IPCE spectra by 50 nm and improved the PCE by up to 29% compared to EO3, despite lowering the V_OC by more than 60 mV. The best sensitizer of the study was AFB-30, with an average PCE of 5.86% (J_SC = 10.41 mA cm^-2, V_OC = 783 mV, FF = 0.73) under 1 sun AM 1.5G illumination.

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Effect of furan π-spacer and triethylene oxide methyl ether
substituents on performance of phenothiazine sensitizers in dye-
sensitized solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
Audun Formo Buene,a Nanna Boholm,b Anders Hagfeldtc and Bård Helge Hoffa*
DOI: 10.1039/x0xx00000x
Conjugated π-spacers are used to improve the light harvesting properties of sensitizers for dye-sensitized solar cells. In
several recent works, furan has outperformed other popular π-spacers such as thiophene and phenyl. One of the best
performing phenothiazine dyes in the literature has no π-spacer, and two polar triethylene oxide methyl ether (TEOME)
chains. Herein we report the synthesis and evaluation of three novel sensitizers based on EO3, investigating the effect of
introducing a furan linker and stepwise removal of the TEOME chains. The furan linker redshifted the onsets in the IPCE
spectra by 50 nm and improved the PCE by up to 29% compared to EO3, despite lowering the VOC by more than 60 mV. The
best sensitizer of the study was AFB-30, with an average PCE of 5.86% (JSC = 10.41 mA cm-2, VOC = 783 mV, FF = 0.73) under
1 sun AM 1.5G illumination.
system which is transferring charge upon excitation. Because
the photon flux in the solar spectrum is finite, increasing the
absorption of the sensitizers is crucial in improving the
Introduction
The introduction of dye-sensitized solar cells made quite a stir
in 1991, when Michael Grätzel and Brian O’Regan published the
remarkably efficient photoelectrochemical cell using light
harvesting sensitizers anchored on mesoporous titania
nanoparticles.1 Since then, a number of great advancements
have been made within the field of dye-sensitized solar cells
(DSSCs). Although still popular, focus has largely moved from
rare earth metal complex sensitizers to fully organic sensitizers.2
Advantages such as higher extinction coefficients and a larger
available toolbox of reliable organic reactions favour the
organic sensitizers.3
photocurrents. A common design strategy to accomplish this is
the introduction of π-spacers.12 A range of spacers have been
reported in the dye literature, ranging from the very simple,
such as
a
single double bond13, various aromatic and
heteroaromatic spacers14-16, to the more complex electron
withdrawing units such as diketopyrrolo-pyrrole17 or
benzothiadiazole.18
Much effort has also been devoted to developing solid-state
DSSCs,19 and one of the most recent advancements is the
discovery of the so-called ‘Zombie cells’ improving on the
already efficient cells based on solid hole conducting materials
such as spiro-OMeTAD.20 Another direction for the DSSC field is
the development of aqueous electrolytes.21 Water has a higher
boiling point than most organic solvents commonly used in
DSSC electrolytes and most salts are excellently soluble in it.
There are a number of aqueous iodide electrolytes reported in
the literature21, 22, and fully organic redox couple aqueous
electrolytes have also successfully been developed.23 The first
aqueous cobalt electrolyte was reported by Spiccia et al. in
2013.24 Later, cobalt complexes have been reengineered for
increased water solubility avoiding the use of surfactants,
producing very efficient aqueous DSSCs with 5.5% PCE.25
Increasing the efficiency and stability is always a high priority in
solar cell research. For the field of DSSCs, dye development
strives to increase the light harvesting properties and the
development of new semiconducting oxides and redox shuttles
are improving the photovoltages of the devices.4, 5 Fully organic
sensitizers are commonly based a donor-π-acceptor design,
where the donor is an electron rich conjugated system such as
triarylamine6, coumarins7, 8, carbazole9, 10 or phenothiazine11
among many others. The acceptor is electron deficient, often
also acting as the anchoring group, resulting in a push-pull
a. Department of Chemistry, Norwegian University of Science and Technology,
Høgskoleringen 5, NO-7491 Trondheim, Norway
Despite the alluring advantages of aqueous electrolytes, the
best power conversion efficiency of an aqueous DSSC reported
to date is a modest 5.97%, achieved by Lin and coworkers with
the phenothiazine sensitizer EO3.26 This sensitizer incorporates
two hydrophilic TEOME chains, which increases the wetting of
the TiO2, chelates Li+ ions, while also repressing sensitizer
aggregation.27-29 A remarkable efficiency of 9.98% PCE under 1
b. Department of Chemistry, Aarhus University, DK-8000 Aarhus, Denmark
c. Laboratory of Photomolecular Science, Institute of Chemical Sciences and
Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Chemin des
Alambics, Station 6, CH-1015 Lausanne, Switzerland
* Corresponding author. Tel.: +47 73593973, e-mail address: bard.h.hoff@ntnu.no
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x
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Scheme 1 Synthesis of EO3. a) 2, Pd2(dba)3, P(cy)3, NaOtBu, 120 °C, 66%, b) DMF,
POCl3, 80 °C, 63%, c) NBS, r.t., 70%, d) 3, Pd(OAc)2, SPhos, K2CO3, 80 °C, 88%, e)
cyanoacetic acid, piperidine, 80 °C, 62%.
employing the Knoevenagel condensation for converting the
aldehyde into the cyanoacrylic acid anchoring group present in
all these sensitizers.
Fig. 1 Structures of sensitizers AFB-30 to 32, EO3, RK1 and N719.
A route previously used by Iqbal and coworkers lay the
foundations for the synthesis towards the AFB dyes.33
sun AM 1.5G illumination for EO3 with an organic solvent-based
electrolyte was also achieved, despite the very limited
absorption onset of 551 nm.26
A
Buchwald coupling introduced the N-aryl substituents, while a
dibromination with NBS separates this route from the previous
method used for EO3. The brominations are efficient reactions
and the products were obtained as convenient solid materials.
Further, two sequential Suzuki cross couplings are performed.
The first introduces the auxiliary donors in a non-selective
manner also leading to the formation of the dicoupled
byproducts, lowering the yields considerably. The second Suzuki
coupling introduces the furan π-bridge from 5-formyl-2-
furanylboronic acid, while the Knoevenagel condensation is
used for the anchor group installation.
This study is focusing on improving the photophysical
properties of EO3 by introducing a π-spacer between the
phenothiazine donor and the cyanoacrylic acid anchoring
group. In a few recent publications, furan has been reported to
outperform thiophene as a π-spacer12, 30-32. Thus, we chose
furan for extending the conjugation of EO3. We also wanted to
investigate the effect of the TEOME chains, so in addition to
EO3, three furan π-spacer dyes with zero, one and two TEOM-
groups were synthesized, shown in Fig. 1.
Results and Discussion
Synthesis
The synthesis of the literature dye EO3 followed a synthetic
route much the same as the one reported by Lin and
coworkers.26 An overview of the total reaction sequence is
found in Scheme 1. All synthesis details can be found in the ESI.
Compound 5 was prepared through a Buchwald-Hartwig cross
coupling in over 10 g scale. Despite being a highly reliable
reaction, the purification and r
emoval of palladium are the challenges in this step. Further, a
Vilsmeyer-Haack formylation on 5 gave compound 8 in 63%
yield, and brominations with N-bromosuccimide (NBS) gave
compound 9 in 70% yield. In the penultimate step, a Suzuki
coupling was employed to introduce the auxiliary donor in 80%
yield using an aryl boronic ester prepared inhouse (3) before
Fig. 2 Thermogravimetric curves for the four sensitizers measured under air
atmosphere. Inset shows the decarboxylation region.
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Scheme 2 Synthesis route to AFB dyes. a) Pd2(dba)3, P(cy)3, NaOtBu, 120 °C, b) NBS, r.t., c) For prod. 11/12: Pd(PPh3)2, K2CO3, 80 °C, 13: Pd(OAc)2, SPhos, K2CO3, 80 °C, d)
Pd(OAc)2, SPhos, K2CO3, 80 °C, e) cyanoacetic acid, piperidine, 80 °C.
The melting points of the two dyes with two TEOME chains (EO3 in absorption can be seen for the AFB dyes as compared to EO3
and AFB-32) were observed around 125 °C, while no melting (Fig. 3a). This can only be attributed to the increase in
point was observed for AFB-30 and 31. This suggests the TEOME conjugation from the furan unit. On TiO2 films the differences
chains decrease the crystallinity of the sensitizers. Upon further are most prominent in the intensities of the ICT bands, and the
heating, visible decompositions were observed at the following relative extinction coefficients increased in the order EO3<AFB-
temperatures: 145 °C (AFB-31) < 150 °C (AFB-32) < 170 °C (EO3) 32<AFB-31<AFB-30. The order mirrors the extinction
< 264 °C (AFB-30). We assume the observed decomposition is a coefficients from solution, but the differences are larger. This
decarboxylation because of visible gas formation. The high could be caused by minute variances in thickness of the TiO2
decomposition temperature of AFB-30 was intriguing, and we films, but dye loading could also certainly play a role. The ICT
suspected this to have one of two possible explanations. Either peak position did not change when the dyes were anchored on
the presence of TEOME chains lowers the thermal stability, or TiO2 compared to in solution, suggesting no aggregation issues
the first decarboxylation step was not detected in the manual with any of the dyes. However, a considerable broadening of
melting point analyses of AFB-30. We suspected the latter to be
the case, because the molecular landscape around the
anchoring cyanoacrylic acid is identical for all the AFB dyes, thus
similar decarboxylation behaviours were expected. In order to
conclude, we performed thermogravimetric analyses on all four
dyes under air to investigate the decomposition as a function of
temperature, and the results are plotted in Fig. 2.
A number of decomposition steps are observed for the four
sensitizers in Fig. 2. The weight losses in the first steps are
proportional in size to be attributed to decarboxylation
processes. The decarboxylations are observed around 150 °C for
all the AFB sensitizers while EO3 does not decompose until 225
°C. This indicates the introduction of the furan π-spacer does
affect the thermal stability negatively compared to EO3.
Fortunately, despite the lowered thermal stability, under no
part of the fabrication process or device operation will the
sensitizers be subjected to temperatures in this range.
However, it must be stressed that the chemical environment in
devices cannot be compared to TGA analysis under air, so any
device stability claims would have to be investigated by other
means.
Photophysical properties
The solution and TiO2 film UV/Vis absorption spectra of the
three sensitizers and EO3 are shown in Fig. 3, with the extracted
data in Table 1. In solution, both hyper- and bathochromic shifts
Fig. 3 UV/Vis absorption properties of sensitizers AFB-30 to 32 compared to the
of the ICT band are observed upon the introduction of the furan
π-spacer. In the region 350-400 nm a significant improvement
literature dye EO3. (a) Measured in THF solution (0.02 mM), (b) measured on
screen printed TiO2 films (2.5 μm 18NR-T on TEC10 FTO glass).
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Table 1 Photophysical properties of dyes AFB-30 to 32 and EO3.
Dye
λabsa (nm)
ε
Em.b (nm) λabsc on TiO2 (nm)
Rel. ε on
TiO2
1.00
1.70
1.36
E0-0d (eV)
E0-0e (eV)
Eoxf (V)
ELUMOg (V)
(M-1cm-1)
14448
20634
19844
19385
EO3
446
449
454
451
584
603
605
600
451
446
454
451
2.38
2.38
2.39
2.39
2.36
2.32
2.32
2.32
1.07
0.96
0.93
0.93
-1.31
-1.42
-1.46
-1.46
AFB-30
AFB-31
AFB-32
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a Maximum of most red-shifted peak. b Emission when ICT band is excited, in THF solution. c Maximum of most red-shifted peak on TiO2 (2.5 μm, 18NR-T). d Calculated
f
from the intersection of the absorption and normalized emission spectra in solution. e Calculated from the absorption onset in solution. Measured vs. Fc+/Fc on
stained TiO2 electrodes in acetonitrile with 0.1 M LiTFSI, converted to V vs. SHE by 0.624 V. Scan rate 20 mV s-1. g Calculated from Eox-E0-0
.
methodology being more closely related to actual device
operating conditions. Reversible oxidation peaks were recorded
for all the sensitizers. However, as the stability of the oxidized
sensitizers usually is very limited, a fading of the signal is
observed upon multiple cycles especially at low scan rates.
The oxidation potential of the literature dye EO3 was found at
1.07 V vs. SHE, only 30 mV away from the reported value of 1.04
V vs. NHE in solution. Li and coworkers further developed EO3
by closing one of the triethylene oxide methyl ether chains into
a 12-crown-4 ether (DCE1).35 The reported photophysical
properties of DCE1 are very similar to the values obtained for
EO3 in this work.
Fig. 4 Energy level diagram displaying the bandgap size and position of HOMO and
LUMO of EO3 and the AFB sensitizers.
the absorption was observed, shifting the absorption onset
towards the red by about 50 nm.
From the emission spectra (Fig. S2, ESI), a peak shift of 16-21 nm
is observed when introducing the furan moiety. Despite the
shifted peak positions, when the emission spectra are
normalized to the absorption spectra, virtually identical optical
bandgaps are obtained for all the four sensitizers from the
absorption-emission intersection. We expected the increased
conjugation from the furan moieties to lower the bandgaps.
Optical bandgaps can alternatively be obtained by the
absorption onset estimated from where the tangent to the ICT
peak crosses the baseline. When using this method an optical
bandgap of 2.36 eV is obtained for EO3, and 2.32 eV for all three
AFB dyes, see Table 1.
Fig. 5 J-V curves of the best device in each parallel. Corresponding dark current
measurements included as dashed lines.
Electrochemical properties
Cyclic voltammetry of the sensitizers adsorbed on TiO2 was
measured in acetonitrile with 0.1
M LiTFSI supporting
electrolyte with a scan rate of 20 mV s-1. The electrodes had no
blocking layer, a 6 μm screen printed layer of 18 NR-T TiO2 paste
on FTO glass, no scattering layer and no CDCA in the staining
solutions. The cyclic voltammograms can be found in the ESI
(Fig. S4). The E1/2 value of Fc+/Fc vs. Ag/AgCl was found at 0.2216
V. The subsequent measurements are given in Table 1 vs. SHE
by the conversion constant of 624 mV.34 We find the described
method to be a highly efficient way of measuring the oxidation
potentials of sensitizers, with the added benefit of the
Fig. 6 IPCE spectra of dyes EO3, AFB-30 to 32 with A6141 I-/I3- electrolyte.
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As the oxidation potential of EO3 is 1.07 V vs. SHE, it is clear that Looking at dye AFB-30 compared to AFB-31 and 32, it appears
the introduction of the furan π-spacer lifts the HOMO levels the presence of the TEOME chains reduces dye loading by up to
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DOI: 10.1039/C9NJ01720H
significantly, as presented visually in Fig. 4. The extracted 40%. While similar dye loading values for the two dyes with two
oxidation potentials of AFB-31 and AFB-32 are indistinguishable TEOME chains (EO3 and AFB-32) were expected, higher values
by cyclic voltammetry at 0.93 V vs SHE, while the value for AFB- were measured for AFB-32. A possible explanation could be the
30 is slightly lower at 0.96 V vs SHE. AFB-30 is the only sensitizer staining conditions favouring the dyes with furan π-spacer over
without any TEOME chains, and the difference in oxidation EO3. The variance in dye loading in-part explain the efficiency
potential could arise from the interactions of these moieties differences. Likely, a more careful optimisation of CDCA
with the lithium cations present in the supporting electrolyte, concentration could increase the dye loading and efficiency of
marginally raising the oxidation potentials of AFB-31 and AFB- the devices based on TEOME containing dyes.
32. For the I-/I3- electrolyte, the HOMO and LUMO positions are
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suitable for efficient electron injection and dye regeneration to Inspired by the high performance of EO3 in aqueous DSSC, we
take place.
tested the four sensitizers in devices with the aqueous cobalt
electrolyte [Co(bpy-pz)2]2+/3+ reported by Ellis et al.25 The
electrolyte composition was 0.13 M [Co(bpy-pz)2]Cl2, 0.06 M
[Co(bpy-pz)2]Cl3 and 0.8 M 1-methylbenzimidazole (MBI) in
deionized water. J-V curves and photovoltaic parameters are
reported in the ESI (Fig. S5 and Table S1). The average
efficiencies were, in ascending order: EO3 (0.17%) < AFB-32
(0.18%) < AFB-31 (0.40%) < AFB-30 (0.57%). The open-circuit
voltages were considerably lower than the ones previously
reported for the [Co(bpy-pz)2]2+/3+ electrolyte, and AFB-30 was
again the most efficient sensitizer. The low efficiency could be a
Photovoltaic performance
The photovoltaic performance of the devices fabricated with
the novel sensitizers, EO3 and the references N719 and RK136
was evaluated. Data from J-V sweeps under 1 sun AM 1.5G
illumination and from incident photon-to-current conversion
efficiency (IPCE) measurements are given in Table 2.
The photovoltaic measurements using the I-/I3 electrolyte
-
A614137 show that the introduction of the furan π-spacer
increases the current, but at a considerable cost in the voltage.
Similar behaviour in photovoltage has been observed previously
in studies on phenothiazine dyes with and without π-spacers.31,
38 A direct comparison between AFB-32 and EO3, only differing
in the furan π-spacer, shows an increased performance by 16%.
Among the AFB dyes, AFB-30 is the most efficient at 5.86% PCE,
comparable in performance to the literature dye RK1 previously
delivering over 10% PCE.36 In the dark current measurements,
no leaking could be observed from any of the devices, which
-
confirms the recombination pathway from the TiO2 to I3 is a
slow process. From the IPCE measurements (Fig. 6) significant
differences can be seen between EO3 and the AFB dyes. The
effect of the furan π-spacer can be seen clearly as increased
power conversion in the region around 400 nm and an
extension of the IPCE curves of about 60 nm towards longer
wavelengths. A clear difference is observed in the absorption
onsets between EO3 and the AFB dyes in the IPCE spectra,
suggesting the difference in optical bandgaps should be larger
than those obtained from the UV/Vis-emission intersection
method. The photocurrent response of the devices is redshifted
compared to the photophysical measurements of the dyes. Part
of this shift can be explained by the broadening of the
absorption peak observed when the sensitizers were anchored
on TiO2, while the remaining difference must arise from
interactions with the device electrolyte. Upon integration of the
IPCE spectra, short-circuit current density estimates were
obtained, which were within 8% of the JSC values from the J-V
sweeps under 1 sun illumination.
Dye loading measurements by desorption was performed in 40
mM tetrabutylammonium hydroxide in stabilized THF. For AFB-
30, the dye loading is nearly identical to the reported value for
the thiophene analogue RD-SC, recently reported by Liao et al.39
Fig. 7 (a) Charge extraction values measured at different light intensities, (b)
electron lifetime measurements corrected for the observed conduction band
shifts from (a) and then plotted versus the extracted charges at the same
potentials. Devices fabricated with A6141 organic electrolyte.
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Table 2 Photovoltaic performance under 1 sun AM 1.5G illumination of dyes AFB-30, 31 and 32, compared to EO3, RK1 and N719. Averages of three devices.
Dye
Dye loading
(10-7 mol cm-2)
IPCE JSC (mA cm-2)b
JSC
VOC (mV)
FF
PCE (%)
(mA cm-2)
EO3
AFB-30
AFB-31
AFB-32
RK1
0.86
2.83
2.07
1.64
-
6.68
10.91
9.71
9.92
-
7.17 ± 0.19
10.41 ± 0.34
8.95 ± 0.24
9.30 ± 0.26
9.48 ± 0.26
12.82 ± 0.04
829 ± 9.17
783 ± 6.56
764 ± 19.31
763 ± 4.51
778 ± 2.39
808 ± 4.40
0.78 ± 0.01
0.73 ± 0.01
0.73 ± 0.01
0.75 ± 0.02
0.79 ± 0.01
0.74 ± 0.03
4.53 ± 0.23
5.86 ± 0.11
4.91 ± 0.06
5.25 ± 0.21
5.81 ± 0.19
7.54 ± 0.32
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N719
-
-
a
Electrolyte composition 0.03 M I2, 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.1 M guanidinium thiocyanate (GuSCN) and 0.5 M tert-butylpyridine in
acetonitrile:valeronitrile (85:15, v:v). b Obtained by integration of the IPCE spectrum over the 1 sun AM 1.5G spectrum.
result of mismatched energy levels or unfavourable dye- 45 minutes before being further cleaned in a UV/O3 cleaner for
electrolyte interactions, but the phenomenon has not been 15 minutes. A TiO2 blocking layer was deposited with
further investigated.
hydrothermal deposition by immersion in an aqueous 40 mM
TiCl4 solution for 45 minutes at 70 °C then rinsed with deionized
Attempting to further unnest the behaviour of these dyes, water and ethanol, repeated once. The mesoporous titania
charge extraction and electron lifetime measurements were nanoparticle paste (18NR-T, GreatCellSolar) was screenprinted
performed on the devices, see Fig. 7. The electron densities onto the FTO glass with a 54T mesh then dried at 125 °C for 5
obtained from the charge extraction measurements (Fig. 7a) minutes. Two layers of 18NR-T was deposited, followed by a
were comparable for all four dyes, with the highest values found scattering layer (WER2-O, GreatCellSolar) printed with the same
for AFB-32. Also from these measurements, it is clear that dye screen. The total thickness of the active layer was measured to
EO3 delivers a much higher VOC than the AFB dyes. This could be be 10 μm by profilometer. The titania electrodes were then
the result of
a conduction band shift, higher electron sintered on a programmable hotplate with the ramping profile
concentration or longer electron lifetimes. The charge of 125, 250, 375, 450 and 500 °C for 5, 5, 5, 15 and 15 minutes
extraction values (QOC) are direct measurements of the electron with 5 minutes ramping time between each temperature step.
concentration in the TiO2, and any conduction band shift may TiCl4 post treatment was performed on the electrodes by
be found from the charge extraction measurements as a immersion in aqueous 40 mM TiCl4 solution for 30 minutes at
horizontal shift of one curve relative to another for the same 70 °C, followed by rinsing with deionized water and ethanol. The
QOC value. However, in order to determine the contribution of photoanodes were completed by sintering on a hotplate at 500
electron lifetime to the photovoltage, one has to correct for the °C for 45 minutes.
conduction band shift and compare the electron lifetime
measurements for the same charge densities, i.e. QOC values, as Counter electrodes were fashioned from TEC15 FTO glass, cut
described previously40 and shown in Fig. 7b.
to size and holes for electrolyte filling were drilled with a
diamond drill bit. The FTO slides were then cleaned with
Very little difference is found for the QOC values for the four ultrasonication in Hellmanex solution, deionized water, ethanol
sensitizers, but a large relative conduction band shift of and finally in acetone, each for 15 minutes. A solution of H2PtCl6
approximately 80 mV is observed between AFB-32 and EO3. (10 mM in iso-propanol, 5 μL/cm2) was dropcast on the
From Fig. 7b we observe only small differences in electron individual electrodes and then they were fired at 400 °C for 15
lifetimes following the order of AFB-30 < EO3 < AFB-31 < AFB- minutes to leave only the catalytic layer of platinum.
32. Considering the lower charge densities and electron lifetime
values of EO3 we would expect the actual difference in Staining solutions had a concentration of 0.5 mM of dye and 5
photovoltage to be lower than 80 mV. From the J-V sweeps, a mM of chenodeoxycholic acid. For the AFB dyes, a mixture of
shift of 66 mV is found, thus confirming the conduction band THF and acetonitrile (57:43, v/v) was used, while absolute
shift is the main contributor to the high VOC for this dye.
ethanol was used for RK1 and a mixture of tert-butanol and
acetonitrile (1:1, v/v) for N719. The electrodes were stained for
20 hours at room temperature before being rinsed in
acetonitrile for 2 minutes, dried under air and sealed with the
counter electrodes in a drybox using 25 μm thick Surlyn gaskets.
The A6141 electrolyte contained 0.03 M I2, 0.6 M 1-butyl-3-
Experimental
Synthesis
All details on the synthesis and characterization of the
sensitizers in this work can be found in the ESI.
methylimidazolium iodide (BMII), 0.1
M
guanidinium
thiocyanate (GuSCN) and 0.5 tert-butylpyridine in
M
acetonitrile/valeronitrile (85:15, v/v), and was injected by
vacuum backfilling. The filling hole was sealed with Surlyn and a
glass disk, and the protruding edges of the working and counter
Device fabrication
FTO glass (NSG10, Nippon Sheet Glass) was cut to size and
cleaned with Deconex 21 (2 g/L in H2O) in an ultrasonic bath for
6 | J. Name., 2012, 00, 1-3
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C.-P. Lee, R. Y.-Y. Lin, L.-Y. Lin, C.-T. Li, T.-C. Chu,VSie.-wSA.rSticulenO, Jn.linTe.
electrodes were covered in soldering tin from an ultrasonic
soldering iron for lowered sheet resistance.
Lin and K.-C. Ho, RSC Adv., 2015, 5, 23810-23825.
DOI: 10.1039/C9NJ01720H
Y. Hong, Z. Iqbal, X. Yin and D. Cao, Tetrahedron, 2014, 70,
6296-6302.
A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595-6663.
Y. Saygili, M. Stojanovic, N. Flores-Díaz, S. M. Zakeeruddin, N.
Vlachopoulos, M. Grätzel and A. Hagfeldt, Inorganics, 2019, 7,
30.
A. Mahmood, Solar Energy, 2016, 123, 127-144.
K. D. Seo, H. M. Song, M. J. Lee, M. Pastore, C. Anselmi, F. De
Angelis, M. K. Nazeeruddin, M. Grätzel and H. K. Kim, Dyes
Pigm., 2011, 90, 304-310.
K. Hara, Z.-S. Wang, T. Sato, A. Furube, R. Katoh, H. Sugihara,
Y. Dan-oh, C. Kasada, A. Shinpo and S. Suga, J. Phys. Chem. B,
2005, 109, 15476-15482.
N. Koumura, Z.-S. Wang, M. Miyashita, Y. Uemura, H.
Sekiguchi, Y. Cui, A. Mori, S. Mori and K. Hara, J. Mater. Chem.,
2009, 19, 4829-4836.
Device characterization
Current-voltage characteristics were measured under 1 sun AM
1.5G illumination from a solar simulator (Oriel, USA, 450 W)
connected to a Keithley 2400. A black metal mask with an
aperture of 0.16 cm2 was used for all the measurements.
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Incident
photon-to-current
conversion
efficiency
measurements (IPCE) were measured on an Arkeo-Ariadne
(Cicci Research s.r.l) with a 300 W Xenon lamp, from 350 to 750
nm. Electron lifetime and charge extraction measurements
were performed with the Dyenamo Toolbox (Dyenamo,
Sweden).
8
9
10 K. Kakiage, Y. Aoyama, T. Yano, T. Otsuka, T. Kyomen, M. Unno
and M. Hanaya, Chem. Commun., 2014, 50, 6379-6381.
11 R. M. El-Shishtawy, J.-D. Decoppet, F. A. M. Al-Zahrani, Y. Cao,
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12 S. H. Kim, H. W. Kim, C. Sakong, J. Namgoong, S. W. Park, M. J.
Ko, C. H. Lee, W. I. Lee and J. P. Kim, Org. Lett., 2011, 13, 5784-
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13 H. Tian, X. Yang, R. Chen, Y. Pan, L. Li, A. Hagfeldt and L. Sun,
Chem. Commun., 2007, 3741-3743.
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Bedja, R. K. Gupta and A. Islam, J. Phys. Chem. C, 2017, 121,
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15 R. Li, X. Lv, D. Shi, D. Zhou, Y. Cheng, G. Zhang and P. Wang, J.
Phys. Chem. C, 2009, 113, 7469-7479.
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Wu, S.-H. Liu, P.-T. Chou and Y. Chi, J. Mater. Chem., 2011, 21,
1937-1945.
17 X. Sun, Y. Wang, X. Li, H. Agren, W. Zhu, H. Tian and Y. Xie,
Chem. Commun., 2014, 50, 15609-15612.
18 S. Haid, M. Marszalek, A. Mishra, M. Wielopolski, J. Teuscher,
J.-E. Moser, R. Humphry-Baker, S. M. Zakeeruddin, M. Grätzel
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21 F. Bella, C. Gerbaldi, C. Barolo and M. Grätzel, Chem. Soc. Rev.,
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22 F. Bella, S. Galliano, M. Falco, G. Viscardi, C. Barolo, M. Grätzel
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23 H. Tian, E. Gabrielsson, P. W. Lohse, N. Vlachopoulos, L. Kloo,
A. Hagfeldt and L. Sun, Energy Environ. Sci., 2012, 5, 9752-
9755.
Conclusion
Three new sensitizers based on the efficient literature dye EO3
-
were synthesized and evaluated with an I-/I3 electrolyte. The
photophysical and photovoltaic properties were improved and
the best sensitizer, AFB-30, achieved an average PCE of 5.86%,
an improvement of 29% compared to EO3. The introduction of
furan also shifted the HOMO levels towards more negative
potentials, which could affect regeneration efficiency
negatively when using cobalt or copper electrolytes with more
positive Eredox than the I-/I3 shuttle. Finally, the VOC values
-
obtained for the sensitizers were significantly reduced by the
insertion of the furan moiety and a drop in VOC of 46-66 mV
compared to EO3 was attributed to a relative conduction band
shift. We also established through TGA measurements that the
introduction of the furan π-spacer lowered the decomposition
temperature compared to no π-spacer, but the decomposition
temperatures are still sufficiently high for successful device
fabrication and operation. These results indicate that there still
is potential to improve even the best sensitizers. Lastly, we find
that working on the phenothiazine scaffold is a careful
balancing act, because you may improve the absorption
properties, but at a considerable cost in photovoltage.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
24 W. Xiang, F. Huang, Y.-B. Cheng, U. Bach and L. Spiccia, Energy
Environ. Sci., 2013, 6, 121-127.
Dr. Nick Vlachopoulos is acknowledged for very helpful
instructions on the cyclic voltammetry measurements, and
Nikolai Helth Gaukås for the TGA analyses. The support from the
Research Council of Norway to the Norwegian Micro- and Nano-
Fabrication Facility, NorFab (project number 245963/F50) and
the Norwegian NMR Platform (project number 226244/F50) is
highly appreciated.
25 H. Ellis, R. Jiang, S. Ye, A. Hagfeldt and G. Boschloo, Phys.
Chem. Chem. Phys., 2016, 18, 8419-8427.
26 R. Y.-Y. Lin, F.-L. Wu, C.-T. Li, P.-Y. Chen, K.-C. Ho and J. T. Lin,
ChemSusChem, 2015, 8, 2503-2513.
27 J.-H. Yum, S.-J. Moon, C. S. Karthikeyan, H. Wietasch, M.
Thelakkat, S. M. Zakeeruddin, M. K. Nazeeruddin and M.
Grätzel, Nano Energy, 2012, 1, 6-12.
28 S. Panagiotakis, E. Giannoudis, A. Charisiadis, R. Paravatou,
M.-E. Lazaridi, M. Kandyli, K. Ladomenou, P. A. Angaridis, H. C.
Bertrand, G. D. Sharma and A. G. Coutsolelos, Eur. J. Inorg.
Chem., 2018, 2018, 2369-2379.
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Sentence highlighting the novelty of the work:
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Synthesis and characterization of three phenothiazine dyes inspired by literature dye EO3
improving absorption properties and photovoltaic performance in DSSCs.
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137x47mm (300 x 300 DPI)