Auxiliary donors for phenothiazine sensitizers for dye-sensitized solar cells-how important are they really?

Article abstract of DOI:10.1039/C9TA00472F

Auxiliary donors are common design motifs for phenothiazine sensitizers for dye-sensitized solar cells. Despite this, there are only a few reports on their overall contribution to the photon-to-electron conversion efficiency. Twelve sensitizers have been prepared and investigated, including ten different auxiliary donors in addition to a control with no auxiliary donor. The various auxiliary donors improved the PCE by a modest 4-11%, and pyrene (AFB-19) was determined to be the most efficient auxiliary donor, with the best cell delivering a PCE of 5.36%. Included in the dye series was also the champion dye within the phenothiazine class. With a reported PCE of 12.1%, it would be an excellent phenothiazine reference dye. The high V_OC of 0.83 V of this dye is worth further investigation, but the absorption and photovoltaic performance in this work does not correlate with the previously reported PCE of 12.1%.

Full text of DOI:10.1039/C9TA00472F

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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JPolueransael odfoMnaotetraiadljsuCshtemmairsgtriynsA
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DOI: 10.1039/C9TA00472F
Journal Name
ARTICLE
Auxiliary donors for phenothiazine sensitizers for dye-sensitized
solar cells – How important are they really?
Audun Formo Buenea, Eline Ekornhol Osea, Ane Garborg Zakariassena, Anders Hagfeldtb, Bård
Received 00th January 20xx,
Accepted 00th January 20xx
Helge Hoffa*
DOI: 10.1039/x0xx00000x
Auxiliary donors are common design motifs for phenothiazine sensitizers in dye-sensitized solar cells. Despite this, there are
only a few accounts of their overall contribution to the photon-to-electron conversion efficiency. A systematic selection of
twelve sensitizers have been prepared, investigating ten different auxiliary donors in addition to a control with no auxiliary
donor. The various auxiliary donors improved the PCE by a modest 4-11%, and pyrene (AFB-19) was determined to be the
most efficient auxiliary donor with the best cell delivering a PCE of 5.36%. Included in the dye series was also the champion
dye within the phenothiazine class. With a reported PCE of 12.1%, this would be an excellent phenothiazine reference dye.
The high VOC of 0.83 V of this dye is worth further investigation, but the absorption and photovoltaic performance in this
work does not support the previously reported PCE of 12.1%.
www.rsc.org/
device characterization, we refer the reader to a number of very
good reviews on the topics.8-11
Introduction
While the metal-based dyes rely on a Metal-to-Ligand-
Dye-sensitized solar cells based on mesoscopic oxide electrodes
are a promising third generation photovoltaic technology first
described by O’Regan and Grätzel in 1991.1 Their major
advantages over traditional solar cell technologies are their
inherent tunability with regards to colour and flexibility. In
addition, the prospect of very low production costs and ease of
production are encouraging aspects. Recently, DSSCs have also
emerged as interesting components in low power devices for
the Internet of Things (IoT)2. Transparent solid state DSSCs are
also the crucial component of a new technology for Focus-
Induced Photoresponse distance measurements, by the
company trinamiX3.
Dye development has been an important driving-force
within the DSSC field. The early dyes were based on chlorophyll-
like porphyrin structures4, 5, as well as ruthenium (II) bipyridine
complexes such as N3 and later N719.6 Recognizing the
limitations for these dyes led to the development of metal-free
dyes, allowing for higher extinction coefficients, tunability and
easier purifications in the dye synthesis.7 For a more complete
description of the workings of a DSSC, dye development and
Charge-Transfer (MLCT) process, the metal-free dyes operates
via an internal charge transfer excitation and often use a design
concept following the donor-π-acceptor/anchor (D-π-A) layout.
Other design concepts without a traditional donor group have
also been reported.12, 13 The most utilized anchoring group is the
cyanoacrylic acid also providing a pulling effect.14 Common π-
spacers are usually based on phenyls15, five-membered
heterocycles16 or fused heterocycles17, while popular donor
20
groups include triarylamines18, phenothiazines19,
coumarins21 among others.
and
The role of the donor group is to provide an inductive
pushing effect helping with charge injection upon excitation.
Donor groups may also be able to carry the positive charge of
the sensitizer, keeping this spatially separated from the TiO2 to
suppress electron recombination and facilitate regeneration
from the redox shuttle. Many metal-free dyes utilize a moiety
called an auxiliary donor, making the dye layout a D-D-π-A
system, as illustrated in Figure 1. These auxiliary donors are
reported to increase the extinction coefficient of the sensitizers
a.Department of Chemistry, Norwegian University of Science and Technology
(NTNU), N-7491 Trondheim, Norway
b.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. E-mail: bard.h.hoff@ntnu.no
Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/x0xx00000x
Figure 1. D-D-π-A structure explained for AFB-19.
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shown in Scheme 1. In addition we have included a dye without
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any auxiliary donor, which happens to bDeOpI:re10v.i1o0u39s/lyC9rTeAp0o0r4t7e2dF
by both Yang et al.34 and Wei et al.35. Finally, a reference dye
from Nagarajan et al.36 was also synthesized in an attempt to
verify their claim of over 12% efficiency as well as serving as a
helpful yardstick for our own sensitizers.
Results and Discussion
Dye synthesis
The synthesis to the building block 3,7-dibromo-10-hexyl-10H-
phenothiazine (1) as well as dyes AFB-8 and AFB-9 has
previously been reported by the authors.37 All detailed synthetic
procedures are given in the ESI, and a general synthesis scheme
for dyes AFB-8, 9, 12-19 is shown in Scheme 1. The various
auxiliary donors were introduced using a Suzuki cross coupling
from their respective aryl boronic acids. This reaction displays
low chemoselectivity, leading to roughly a 1:2:1 distribution of
starting material 1, the desired monocoupled products and the
dicoupled byproducts. The starting material is possible to
recover, and the purifications are usually uncomplicated, so we
have settled for this approach rather than the also fairly popular
route of a monobromination followed by a Suzuki coupling then
another bromination24, 25. If no π-spacer is required, then
formylation followed by bromination is a common approach.23,
In the synthesis of dye 18, the tert-butyldimethylsilyl ether
(TBDMS) protecting group of the naphtholic boronic acid was
cleaved during the Suzuki coupling, and rather conveniently the
Figure 2. Structures of the twelve dyes investigated.
when directly in conjugation with the main chromophore,
leading to higher power conversion efficiencies in DSSCs.22
For the phenothiazine scaffold, some of the most common
auxiliary donors are para-alkoxy phenyls23, 24, ortho- and para-
alkoxy phenyls20, 25, triphenylamines26, 27 and carbazoles.28
Although a number of auxiliary donors have been tested for
the phenothiazine scaffold, only a few reports actually compare
the auxiliary donors to the auxiliary donor-free dye.24, 29, 30 In
these cases the contribution from the auxiliary donors varies
from a 16% decrease to a 14% increase in performance, thus
highlighting the importance of understanding how different
auxiliary donors affect phenothiazine dye performance.
Because the aromatic system of 10H-phenothiazine in the
ground state is bent, the conjugation is broken at the N and S
bridges. This implies that the nature of the auxiliary donor is
predominantly to provide an inductive push in the push-pull
system of D-D-π-A phenothiazine dyes.
However, the auxiliary donor is also the part of the sensitizer
molecule that is most likely to interact with the redox shuttle in
regeneration of the oxidized dye molecule. Therefore, the role
of the auxiliary donor must be more complex than that of a
simple electron donating substituent. Several auxiliary donors
have been designed to increase its interactions with certain
redox shuttles, copper and cobalt complexes in particular.31-33
In order to gain further understanding into the role and
importance of the auxiliary donor in phenothiazine dyes, we
have prepared ten dyes varying only the auxiliary donor. The
rest of the dye molecules remains the same with a thiophene π-
spacer, n-hexyl anti-aggregation chain, the 10H-phenothiazine
core and the cyanoacrylic acceptor and anchoring group. The
selected auxiliary donors for this study includes six different
phenyl-based, three naphthyl-based and one pyrene donor, as
38
deprotected naphthol product 8 was obtained. Deprotection of
TBDMS with mild carbonate bases or palladium(II) is not
unheard of39, 40, but we were positively surprised by the
efficiency of the deprotection, eliminating the need for a
separate deprotection step.
In order to efficiently introduce the thiophene π-spacer, a
borylation-Suzuki two-step approach was used. First the
building blocks with the auxiliary donor moieties were
borylated, following a protocol by Billingsley and Buchwald.41
Without further purification, the crude phenothiazine pinacol
boronic
esters
were
coupled
with
5-bromo-2-
thiophenecarboxaldehyde using Pd(OAc)2 and SPhos in a 1:1
(v/v) mixture of water and 1,4-dioxane at 80 °C. The resulting
aldehydes were then converted into cyanoacrylic acid
anchoring groups via the Knovenagel condensation following
the procedure from Iqbal et al.42 using cyanoacetic acid and
Scheme 1. General synthesis route for dyes AFB-8, 9, 12-19 as well as the
dehalogenation route to AFB-20. a) Suzuki coupling, b) borylation-Suzuki two-step
reaction, c) Pd-catalyzed dehalogenation, d) Knoevenagel condensation. For
details, see ESI.
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DOI: 10.1039/C9TA00472F
Figure 3. (a)-(b): Absorption spectra of all dyes in THF solution (2 × 10-5 M). (a) AFB-8, 9, 12 to 15 have phenyl-based donors, (b) AFB-16 to 18 have naphthyl-based donors,
while AFB-19 and Dye 2 have pyrene donors. Dye AFB-20 has no auxiliary donor. (c)-(d): Absorption spectra of all dyes on TiO2 films (2.5 μm, 18NR-T, GreatcellSolar).
piperidine in acetonitrile. The workup and purification
The monocoupled product 19 obtained after purification
conditions were kept as similar as possible for all the target was debrominated in a palladium catalyzed reaction as reported
molecules, in case any residual reagents or solvents were to by Chen et al.43, using Pd(OAc)2, triphenylphosphine and K2CO3
affect the molecular properties in any way.
in n-butanol at 100 °C to yield compound 20. The Knoevenagel
Sensitizer AFB-20 has previously been prepared through condensation produced sensitizer AFB-20, bearing no auxiliary
monobromination approaches by Yang et al.34 and Wei et al.35 donor.
By using precursors already at hand, we obtained AFB-20
Because the reference sensitizer Dye 2 has no π-spacer
through another route (Scheme 1). Firstly, a Suzuki coupling between the phenothiazine core and the anchoring group, a
catalyzed by the Pd(OAc)2/SPhos system introduced the slightly different synthetic route had to be employed, see
thiophene carbaldehyde in
a non-selective manner. (5- Scheme 2. First, 10H-phenothiazine was alkylated at the 10H
Formylthiophen-2-yl)boronic acid proved less stable than nitrogen atom, then monoformylation by Vilsmeier-Haack
phenyl-based boronic acids, thus a larger excess (1.5 eq.) was reaction followed by a bromination with N-bromosuccinimide
required to obtain comparable conversion.
(NBS). The resulting building block 25 was then coupled with 1-
ethynylpyrene (22) in a Sonogashira cross coupling before the
finishing Knoevenagel reaction installed the anchoring group
using the same conditions as for the other dyes.
Photophysical properties
The UV/Vis absorption spectra of all dyes in THF and anchored
on TiO2 films are shown in Figure 3, with extinction coefficients
and absorption peaks given in Table 1. The absorption of all dyes
display two major peaks, one between 300 and 400 nm
attributed to the π-π* excitation of the aromatic system, while
the other peak is found in the region between 400 and 500 nm,
which is the internal charge transfer (ICT) transition process.
The ICT peak of dyes on TiO2 are blueshifted by approximately
10 nm compared to the solution measurements. The absorption
of the two pyrene-containing sensitizers display irregular
features in the spectra likely to stem from separate absorptions
Scheme 2. Synthetic route towards Dye 2. a) Ethynyltrimethylsilane, PdCl2(PPh3)2,
CuI, Et3N, 80 °C, b) NaOH, r.t., c) NaH, 1-bromooctane 66 °C, d) DMF, POCl3, 1,2-
dichloroethane, reflux, e) NBS, r.t., f) PdCl2(PPh3)2, CuI, Et3N, 80 °C, g) cyanoacetic
acid, piperidine, 80 °C.
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DOI: 10.1039/C9TA00472F
Figure 4. HOMO and LUMO levels of all dyes. We take the HOMO levels from the oxidation potentials of the dyes measured with cyclic voltammetry and the LUMO levels were
calculated from the optical band gaps.
within the pyrene moiety. UV/Vis absorption spectra of 1- of the HOMO and LUMO levels of the dyes. All the dyes apart
bromopyrene and 1-ethynylpyrene (ESI, Figure S2) display from AFB-20 and Dye 2 display an Eox value in the range of 0.86-
similar irregular absorption features in the same region.
0.94 V vs. SHE. For AFB-20 and Dye 2, the Eox values were found
All the sensitizers with phenyl-based auxiliary donors have at 1.01 and 1.06 V, respectively. Compared to AFB-20, the
very similar UV/Vis absorption properties. The absorption introduction of these auxiliary donors raises the HOMO-levels
-
maxima of the two dyes with ortho-methoxy substituents on by up to 0.15 eV. While using the I-/I3 redox couple there is still
the phenyl donor are redshifted by about 20 nm compared to sufficient driving force for regeneration, a higher HOMO could
the rest of the selection. However, the optical bandgaps are all give regeneration problems when working with cobalt or
in the range of 2.32-2.36 eV, and the differences in extinction copper redox couples. The di-substituted auxiliary donor dyes
coefficients are also very small. The naphthyl series displays
(AFB-14 and 15) have, by a very small margin, the highest
largely the same properties as the phenyl dyes in terms of positioned HOMO levels. Only slightly below are the mono-
bandgaps, absorption maxima and extinction coefficients. For substituted phenyl and naphthyl-substituted dyes, which are
this series, the triethylene oxide methyl ether containing dye indistinguishable by cyclic voltammetry.
AFB-18 causes a blueshift of the absorption maxima of about 15
nm, however the bandgaps are virtually unchanged. The
spectrum of AFB-20 (no auxiliary donor) reveals an ICT band
Table 1. Photophysical properties of all dyes.
a
d
e
f
Dye
λabs
ε
Em.b
λabsc on
E0-0
Eox
ELUMO
comparable in intensity and position to the dyes with auxiliary
donor, but the π-π* excitation is noticeably less pronounced. It
is therefore from UV/Vis quite clear that the auxiliary donors in
phenothiazine dyes do not significantly increase the ICT
transition.
(nm) (M-1cm-1) (nm) TiO2 (nm) (V)
(V vs
SHE)
(V vs
SHE)
-1.38
-1.43
-1.45
-1.40
-1.45
-1.50
-1.41
-1.40
-1.42
-1.44
-1.35
-1.36
AFB-8
AFB-9
441
439
459
442
457
443
455
450
437
441
436
418
22200
20600
19050
23250
22600
22600
21350
19850
20200
24450
20050
27550
626
637
601
629
618
617
618
629
594
620
624
585
432
441
444
443
444
431
445
440
430
441
426
n/ag
2.32 0.94
2.33 0.90
2.36 0.91
2.32 0.92
2.32 0.87
2.36 0.86
2.34 0.93
2.30 0.90
2.32 0.90
2.36 0.92
2.36 1.01
2.42 1.06
AFB-12
AFB-13
AFB-14
AFB-15
AFB-16
AFB-17
AFB-18
AFB-19
AFB-20
Dye 2
Electrochemical properties
Cyclic voltammetry was performed for all dyes to determine the
oxidation potentials of the sensitizers, and they all displayed a
single reversible oxidation peak. The dyes are sparingly soluble
in acetonitrile, so DMF was selected to ensure all dyes could be
measured in the same solvent. Acetonitrile is the main
component of the DSSC electrolyte, so in measuring the
sensitizers in DMF slightly shifted values may be obtained, but
any difference is assumed to be comparable for all twelve
a Maximum of most red-shifted peak. b Emission when ICT band is excited, in
c
THF solution. Maximum of most red-shifted peak on TiO2 (2.5 μm,
a
significant magnitude.44 Cyclic
d
sensitizers and not of
GreatcellSolar 18NR-T). Calculated from the intersection of the absorption
and normalized emission spectra. e Measured in DMF vs Fc+/Fc, converted to
voltammograms are shown in the ESI (Figure S3 and S4), and the
extracted values for the oxidation potentials vs. SHE are found
in Table 1 while Figure 4 gives a visual summary of the position
V vs. SHE by a conversion factor of 0.624. f Calculated from Eox-E0-0
.
g No clear
peak to assign.
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Table 2. Photovoltaic performance of all dyes under 1 sun AM 1.5G illumination, and from IPCE measurements.
View Article Online
DOI: 10.1039/C9TA00472F
Dye
AFB-8
AFB-9
IPCE JSC (mA cm-2)a
8.76
JSC (mA cm-2)
9.12 ± 0.15
8.89 ± 0.07
9.03 ± 0.21
9.40 ± 0.48
9.32 ± 0.32
8.65 ± 0.07
9.90 ± 0.30
9.46 ± 0.14
8.75 ± 0.45
9.00 ± 0.44
8.21 ± 0.05
8.91 ± 0.07
13.20 ± 0.13
VOC (V)
FF
PCE (%)
0.78 ± 0.01
0.78 ± 0.00
0.75 ± 0.01
0.76 ± 0.00
0.75 ± 0.00
0.79 ± 0.00
0.73 ± 0.01
0.74 ± 0.00
0.78 ± 0.00
0.78 ± 0.00
0.76 ± 0.00
0.83 ± 0.01
0.76 ± 0.01
0.67 ± 0.00
0.70 ± 0.02
0.69 ± 0.01
0.70 ± 0.02
0.68 ± 0.00
0.71 ± 0.02
0.68 ± 0.01
0.70 ± 0.01
0.70 ± 0.02
0.72 ± 0.02
0.72 ± 0.01
0.70 ± 0.02
0.70 ± 0.01
4.76 ± 0.00
4.81 ± 0.13
4.67 ± 0.10
4.95 ± 0.15
4.76 ± 0.19
4.84 ± 0.19
4.90 ± 0.17
4.90 ± 0.14
4.79 ± 0.11
5.00 ± 0.20
4.51 ± 0.08
5.16 ± 0.03
6.98 ± 0.10
8.73
8.84
9.28
9.49
8.62
10.02
9.47
8.91
9.16
AFB-12b
AFB-13
AFB-14
AFB-15
AFB-16
AFB-17
AFB-18
AFB-19
AFB-20
Dye 2
7.97
8.55
12.97
N719
a Obtained by integration of the IPCE spectrum over the 1 sun AM 1.5 G spectrum. b Average values of two cells.
All the oxidation potentials should be sufficiently higher 77% and the PCE was improved by 62%. By looking at Figure 7,
-
than the redox potential of the I-/I3 electrolyte (0.35 V vs. it is evident that a sensitizer delivering this impressive JSC will
NHE45) in order for efficient regeneration to take place. The need an absorption onset above 760 nm, corresponding to a
calculated LUMO levels of the dyes (Eox – E0-0) are all found from bandgap smaller than 1.63 eV.46 In addition, integration of the
-1.35 to -1.50, giving all sensitizers sufficient driving force for IPCE spectra published by Nagarajan et al.36 yields a more
the charge injection process into TiO2 (-0.5 V vs. SHE) to be modest 7 mA cm-2, and an estimated absorption onset from the
favorable.
published absorption spectra on TiO2 of 605 nm supports a
maximum short-circuit current of 13 mA cm-2. The reported
open-circuit voltage is also remarkably high, considering the
reported electrolyte contains 0.5 M LiI. The presence of LiI in
the electrolyte reduces the VOC by lowering the Fermi level of
Photovoltaic performance
The DSSC devices were fabricated in parallels of three. The J-V
curves, measured under 1 sun AM 1.5G illumination (100 mW
cm-2) and the incident-photon-to-current conversion efficiency
(IPCE) spectra are shown in Figure 5 and 6, respectively. When
comparing AFB-20 to the rest of the dyes, it is clear that the use
of an auxiliary donor improves the PCE by 4-11% (7% average).
The most efficient dyes, Dye 2 and AFB-19, also display a higher
dye loading compared to AFB-20 (ESI, Table S1). By grouping the
efficiencies of the phenyl-based (AFB-8
based (AFB-16 17 18) and pyrene (AFB-19) auxiliary donors,
the average PCE values are 4.80, 4.86 and 5.00%, respectively.
The series comprised of AFB-8 AFB-16 and AFB-19 also display
, 9, 12-15), naphthyl-
,
,
,
the same trend. The efficiencies increase from 4.76% for phenyl
to 4.90% for naphthyl and 5.00% for the pyrene auxiliary donor.
We must however stress that the statistical basis for these
claims is marginal, as the overall performance differences
between the auxiliary donors are relatively small.
From the IPCE spectra in Figure 6 it is clear that Dye 2 is an
efficient light harvesting molecule, but has the narrowest IPCE
spectrum of the dyes in this study. The no auxiliary donor dye
AFB-20 also suffers from lower absorption in the region above
500 nm, resulting in a lower overall performance. Not
surprisingly, the rest of the sensitizers have very similar IPCE
spectra.
The sensitizer Dye 2, is previously characterized with an
impressive JSC of 24.2 mA cm-2 and VOC of 846 mV.36 By the
addition of 50 eq. of chenodeoxycholic acid (CDCA) to the
staining solutions, the authors reported a 6% decrease in dye
loading from 5.51 × 10-7 to 5.20 × 10-7 mol cm-2, JSC increased by
Figure 5. Current-voltage plots for the best device within each parallel.
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Table 3. CDCA optimization study for sensitizer AFB-19. The concentrations of
DOI: 10.1039/C9TA00472F
CDCA in the staining solutions were 0, 0.5, 2.5, 5.0 and 10.0 mM. The results are
averages of three cells.
CDCA
(eq.)
CDCA
conc.
(mM)
0
0.5
2.5
Jsc
Voc
(mV)
FF
PCE
(%)
(mA cm-2)
0
1
5
10
20
8.11 ± 0.16 758 ± 6.9 0.67 ± 0.01 4.12 ± 0.07
8.59 ± 0.23 763 ± 9.4 0.70 ± 0.02 4.61 ± 0.21
9.16 ± 0.08 753 ± 4.5 0.70 ± 0.01 4.82 ± 0.11
9.73 ± 0.23 764 ± 6.0 0.71 ± 0.01 5.25 ± 0.11
9.66 ± 0.09 758 ± 7.8 0.70 ± 0.01 5.14 ± 0.04
5.0
10.0
Figure 8. DSSC characteristics for various CDCA concentrations ranging from zero
to 20 equivalents for sensitizer for AFB-19. Average values from three devices.
using photoanodes with 4
μm 30NR-D TiO2 paste + 4 μm
Figure 6. IPCE spectra for the best device within each parallel.
scattering paste, and counter electrodes with a catalytic PEDOT
layer deposited by electropolymerization.48 A moderate PCE of
2.27% was obtained for the devices. J-V spectra and data are
given in the ESI (Figure S5 and Table S2). The performance was
likely affected by the HOMO level position of AFB-19, only 50
mV below the redox potential of the Cu(tmby) electrolyte
reported at 0.87 V.49 Photovoltages of up to 1.1 V are common
for this electrolyte50, but our devices delivered a VOC of only 910
mV, indicating an insufficient driving force for the dye
regeneration could be the issue. A solution could be to change
to a redox couple with a more negative oxidation potential, such
as El-Shishtawy et al. showed for similar compounds with a
cobalt (II/III) electrolyte.51
TiO2, and the VOC obtained for the same sensitizer in this work
was 830 mV without any addition of LiI. Based on the
insufficient absorption properties, the lack of a reference
sensitizer and the inconsistencies between the published IPCE
and J-V data, we suspect a calibration error or similar to be the
cause of the impressive 12.1% PCE reported by Nagarajan et al.
As stated by Christians et al.47 for the field of perovskite solar
cells, any difference larger than 20% between JSC values from J-
V sweeps under 1 sun and JSC from IPCE integration should call
for further experimental support or explanation. There should
be no reason for this not to be considered best practice within
the DSSC field as well.
The best dye from the preliminary screen with the iodide
electrolyte was AFB-19. As pyrenes are prone to π-stacking52,
and as our best sensitizer had the pyrene auxiliary donor, we
were eager to see how dependent the performance was on the
presence of the anti-aggregation additive chenodeoxycholic
acid (CDCA) in the staining solutions. Five new sets of devices
were fabricated with zero, 1, 5, 10 and 20 molar equivalents of
CDCA (relative to the sensitizer) in the staining solution,
corresponding to 0, 0.5, 2.5, 5 and 10 mM of CDCA. All other
variables were unchanged from the initial efficiency screen, and
the results are summarized in Table 3. As shown in Figure 8, no
addition of CDCA yields the lowest JSC and affects the fill factor
negatively. We have no other explanation to this behavior than
undesirable intermolecular π-π interactions between adjacent
sensitizers. The VOC varies by only 11 mV over the entire
concentration range, meaning the conduction band edge of
TiO2 is not affected, nor the injected electron density. The JSC is
the crucial variable to the differences in PCE, and a slight drop
The best sensitizer, AFB-19, was also tested with the copper
electrolyte, Cu(II/I)(tmby)2TFSI2/1. The devices were fabricated
Figure 7. By integration of the AM 1.5G solar spectrum the theoretical maximum
JSC attainable from
a device can be calculated and plotted against the
corresponding sensitizer absorption onset. 46 The arrows indicate the maximum JSC
for a dye with an absorption onset of 605 nm, and the required onset to achieve a
JSC of 24 mA cm-2.
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comparing lines from the sensitizers in Figure 9aViaetw tAhrtieclesOanmlinee
DOI: 10.1039/C9TA00472F
charge density, it is possible to estimate the relative conduction
band shift caused by each sensitizer from the horizontal shift of
each line. Consequently, the conduction band edge of Dye 2 is
shifted upwards by 45 mV relative to AFB-12
.
By correcting the VOC values by the conduction band edge
shift observed in Figure 9a, a new set of lifetime curves can be
obtained. By exponential regression, the lifetime values for the
potentials of the charge extraction measurements can be
calculated. Finally, by plotting the calculated lifetimes against
the QOC values one is able to compare the lifetime
measurements of the sensitizers at the same electron density,
see Figure 9b.53
The effect of the auxiliary donor on the lifetime can be seen
when comparing AFB-20 to AFB-12. The presence of the 2-
methoxyphenyl auxiliary donor in AFB-12 doubles the lifetime
compared to AFB-20, meaning the recombination is slowed by
the introduction of the auxiliary donors. This could be highly
useful if other redox couples more prone to recombination are
of interest for similar sensitizers. Because the lifetime of Dye 2
is similar to both AFB-12 and AFB-19, we interpret the observed
VOC shift to be predominantly caused by the conduction band
shift and not by higher electron lifetimes.
The overall assessment of auxiliary donors for
phenothiazine sensitizers is that they do provide only a
moderate performance enhancement, through a slight increase
in absorption and increased electron lifetimes. Because they are
not in conjugation with the anchoring group, their contribution
is limited to an inductive nature. In turn, this could be why the
observed effects are only moderate. On the other hand,
auxiliary donors could be used for tuning the properties not
directly related to absorption, such as interactions with the
redox couple, adjacent sensitizers or other electrolyte additives.
If the use of copper or cobalt redox couples is desired for
phenothiazine sensitizers, the synthetic efforts should be
placed on tailoring the HOMO level, lowering the LUMO level
and increasing the molar extinction coefficients.
Figure 9. a) Charge extraction measurements at different light intensities, b)
electron lifetime measurements corrected for conduction band shift relative to
AFB-12 in a), then plotted against the extracted charge for the same potentials.
in JSC is observed when increasing the amount of CDCA from 10
to 20 equivalents, resulting in the peak PCE to be found at 10
equivalents. The peak PCE value for AFB-19 in this
concentration screening was 5.25% and with a smaller standard
deviation compared to the same series in the preliminary
screen, corresponding to a 27% increase in performance
compared to 0 mM CDCA. As the other auxiliary donors are
considered less prone to aggregation, lower concentrations of
CDCA could be favorable, but further investigation would be
needed for each sensitizer.
Conclusions
In an attempt to understand the role of the auxiliary donor in
phenothiazine sensitizers for DSSCs, we synthesized a selection
of dyes varying only this moiety. Twelve sensitizers were
evaluated, including ten different auxiliary donors, a reference
sensitizer with no auxiliary donor and
a “champion”
In order to illuminate the high VOC observed for Dye 2 we
conducted electron lifetime and charge extraction experiments
phenothiazine dye (Dye 2).
The introduction of auxiliary donors marginally improved
the absorption properties, raised the HOMO levels by up to 0.15
eV and improved the PCE by 4-11%, predominantly due to
increased JSC values. From electron lifetime and charge
extraction measurements, it was also established that the
addition of an auxiliary donor increased the electron lifetime by
a factor of two. Among the auxiliary donors, the efficiency
followed the order: phenyl < naphthyl < pyrene, with average
PCE’s of 4.76%, 4.90% and 5.00%. However, the overall
performance differences between the auxiliary donors are
within the limits of the standard deviations of the individual
on a new set of devices. Also included were the most (AFB-19
)
and least (AFB-12) efficient dyes of the study in addition to the
no auxiliary donor dye (AFB-20).
The VOC of a device is normally dictated by the position of
the TiO2 Fermi level and the redox potential of the redox couple.
Further, the Fermi level position of the TiO2 is in turn dependent
on both the conduction band edge and the electron
concentration. From charge extraction measurements at
different light intensities, one obtains a range of extracted
charges with corresponding open circuit potentials. By
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measurements. The effect of chenodeoxycholic acid minutes was performed, followed by rinsing in deiVoienwizAertdiclewOantliener
DOI: 10.1039/C9TA00472F
concentration in the staining solution for AFB-19 revealed that and ethanol.
a 10-fold molar excess was the most favorable concentration.
Platinum counter electrodes were fabricated from TEC-8
The best device of the study was sensitized with AFB-19 FTO glass (GreatcellSolar). Holes were drilled with a diamond
delivering a photon-to-electron conversion efficiency of 5.36% drill bit, and a 10 mM H2PtCl6 platinum precursor solution in 2-
with a JSC = 9.99 mA cm-2, VOC = 765 mV and FF = 0.70.
propanol was dropcast (5 μL/cm2) onto the FTO and heated at
The phenothiazine sensitizer with the highest reported PCE 400 °C for 15 minutes. Prior to assembly, the counter electrodes
-
of 12.1% with an I-/I3 electrolyte was also evaluated.36 Although were dried by hot air gun at 400 °C for 25 minutes.
the dye had a high VOC, the performance in terms of PCE was
Due to varying indoor temperatures in arctic Norway,
not significantly different from the other phenothiazine dyes in staining of electrodes was done overnight in a 30 °C cabinet.
this study. Furthermore, integration of the published IPCE Prior to staining the electrodes were annealed by hot air gun at
spectra implies that the PCE values reported by Nagarajan et al. 480 °C for 25 minutes, and upon cooling to 80 °C they were
are twice as high as physically possible under 1 sun AM 1.5G immersed in the staining solutions. A dye concentration of 0.5
illumination. We strongly encourage the practice of integrating mM was used along with 10 equivalents of CDCA unless
the IPCE spectra and the use of reference sensitizers within this specified otherwise. N719 was stained from absolute ethanol
field to avoid such errors in the future.
while the organic dyes were stained from a mixture of
acetonitrile and stabilized THF (43:57, v/v), which equates to a
dielectric constant of 20, previously reported as the optimal
value for dye staining.54 After a staining time of 18 hours, the
electrodes were rinsed in acetonitrile for 2 minutes before
being sealed with the counter electrodes using a Surlyn gasket
(60 μm, Solaronix) in a drybox. Heating from a 50 W PTC heat
element for 2 × 20 seconds provided efficient sealing.
Experimental section
Materials and reagents
Unless specifically mentioned, all reactions were performed
under inert atmosphere with chemicals used as received from
Sigma Aldrich. The experimental procedure for the synthesis of
the sensitizers can be found in the ESI.
The A6141 electrolyte contained 0.60
M 1-butyl-3-
methylimidazolium iodide, 0.03 M I2, 0.10 M guanidinium
thiocyanate and 0.50 M tert-butylpyridine in a mixture of
acetonitrile and valeronitrile (85:15, v/v).6 The electrolyte was
injected by vacuum backfilling before the hole was sealed with
Surlyn and a circular glass cover disc. The electrodes were
painted with conductive silver paint (Electrolube, SCP) before
characterization.
Characterization
NMR analyses (1H and 13C) were recorded on either a Bruker 400
or 600 MHz spectrometer. Infrared absorption (IR) spectra were
recorded on a Bruker Alpha FTIR spectrometer in attenuated
total reflection (ATR) mode. UV/vis spectra were recorded on a
Hitachi U-1900 UV/Vis-spectrophotometer in quartz cuvettes
with 10 mm light path. Fluorescence spectrophotometry was
carried out on a Varian Cary Eclipse in quartz cuvettes. HRMS
analyses were performed on a Waters Synapt G2-S Q-TOF
instrument using either ASAP or ESI ionization.
Device characterization
J-V characteristics of the DSSCs were obtained under 1 sun AM
1.5G illumination from a Sciencetech SP300B solar simulator
calibrated with a Newport Reference Cell (91150V), connected
to a Keithley 2450 sourcemeter. The J-V scan rate was 12 mVs-
1, scanning from short-circuit to open-circuit. A circular mask
with an aperture of 0.238 cm2 was used for all J-V
measurements. IPCE measurements were recorded from a
device assembled from a halogen lamp (Ocean Optics HL-2000)
and a monochromator (Spectral Products CM110), connected
to the Keithley 2450 and programmed in LabView. The DSSCs
and the NIST traceable calibrated photodiode (Thorlabs,
FDS100-CAL) were masked with an aperture of 0.049 cm2.
Charge extraction and electron lifetime measurements were
performed using the Dyenamo Toolbox. For the charge
extraction measurements, the devices were illuminated by a
white LED for 1 second at open circuit. When the light is
switched off, the device is short-circuited and the extracted
charge integrated. It is repeated for several light intensities by
adjusting the LED bias from 50 to 450 mA in 50 mA increments.
In the electron lifetime measurements, the LED light intensity
was modulated giving an AC response signal from the device
from which the electron lifetimes were elucidated.
Device fabrication
TEC-10 FTO glass sourced from Sigma Aldrich was washed with
Deconex 21 solution (2 g/L H2O) in an ultrasonic bath for 45
minutes before being rinsed with deionized water and ethanol.
UV/O3 cleaning for 15 minutes (Novascan PSD PRO-UV T6) was
done to remove organic contaminants from the FTO glass. A
TiO2 blocking layer was deposited by two hydrothermal
depositions in 40 mM aqueous TiCl4 solution at 70 °C for 30
minutes.
Two active layers of TiO2 (18NR-T, GreatcellSolar) and one
scattering layer (WER2-0, GreatcellSolar) was screen printed
(mesh 53T, active area 0.238 cm2), then placed in a levelling
chamber with ethanol for 3 minutes before drying on a hotplate
for 6 minutes at 125 °C between each print. The electrodes were
sintered in a programmable furnace (Nabertherm LT 9/12) with
the ramping program of 125, 250, 325, 450, 500 °C for 5, 5, 5,
15 and 15 minutes with 10 minutes ramping between each step.
The total TiO2 thickness was measured to be 16.5 μm (11 μm +
5.5 μm) using a profilometer (Veeco, Dektak 150). Finally, a post
treatment in 40 mM aqueous TiCl4 solution at 70 °C for 30
8 | J. Name., 2012, 00, 1-3
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21 K. D. Seo, H. M. Song, M. J. Lee, M. Pastore, C. Anselmi, F. De
Conflicts of interest
There are no conflicts to declare.
Angelis, M. K. Nazeeruddin, M. Grätzel and H.VKie.wKAirtmicl,e ODnylienes
DOI: 10.1039/C9TA00472F
Pigm., 2011, 90, 304-310.
22 A. Venkateswararao and K. R. J. Thomas, in Solar Cell
Nanotechnology, 2013, pp. 41-96.
23 Y. Hua, S. Chang, D. D. Huang, X. Zhou, X. J. Zhu, J. Z. Zhao, T.
Acknowledgements
Chen, W. Y. Wong and W. K. Wong, Chem. Mater., 2013, 25
,
2146-2153.
The authors would like to thank the NV-faculty and the
Department of Chemistry at NTNU for financial support, Dr.
Susana Villa Gonzalez for HRMS analyses and Roger Aarvik for
technical support. Associate professor Solon Economopoulos is
thanked for helpful tips for cyclic voltammetry, Sihai Luo for
SEM imaging and Dr. James Bannock for programming our IPCE
setup. The Research Council of Norway is acknowledged for the
support to the Norwegian Micro- and Nano-Fabrication Facility,
NorFab (project number 245963/F50) and the Norwegian NMR
Platform (project number 226244/F50).
24 H.-H. Gao, X. Qian, W.-Y. Chang, S.-S. Wang, Y.-Z. Zhu and J.-Y.
Zheng, J. Power Sources, 2016, 307, 866-874.
25 X. Yang, J. Zhao, L. Wang, J. Tian and L. Sun, RSC Adv., 2014, 4,
24377-24383.
26 W. Wu, J. Yang, J. Hua, J. Tang, L. Zhang, Y. Long and H. Tian,
J. Mater. Chem., 2010, 20, 1772-1779.
27 J. Li, W. Wu, J. Yang, J. Tang, Y. Long and J. Hua, Sci. China
Chem., 2011, 54, 699-706.
28 D. Muenmart, N. Prachumrak, R. Tarsang, S. Namungruk, S.
Jungsuttiwong, T. Sudyoadsuk, P. Pattanasattayavong and V.
Promarak, RSC Adv., 2016,
29 X. Zhang, F. Gou, J. Shi, H. Gao, C. Xu, Z. Zhu and H. Jing, RSC
Adv., 2016, , 106380-106386.
6, 38481-38493.
6
30 S. Wang, H. Wang, J. Guo, H. Tang and J. Zhao, Dyes Pigm.,
2014, 109, 96-104.
References
1
2
B. O'Regan and M. Grätzel, Nature, 1991, 353, 737-740.
M. Freitag, J. Teuscher, Y. Saygili, X. Zhang, F. Giordano, P.
Liska, J. Hua, S. M. Zakeeruddin, J.-E. Moser, M. Grätzel and A.
Hagfeldt, Nat. Photonics, 2017, 11, 372-378.
31 E. Mosconi, J.-H. Yum, F. Kessler, C. J. Gómez García, C.
Zuccaccia, A. Cinti, M. K. Nazeeruddin, M. Grätzel and F. De
Angelis, JACS, 2012, 134, 19438-19453.
32 P. Gao, Y. J. Kim, J.-H. Yum, T. W. Holcombe, M. K.
3
O. Pekkola, C. Lungenschmied, P. Fejes, A. Handreck, W.
Hermes, S. Irle, C. Lennartz, C. Schildknecht, P. Schillen, P.
Schindler, R. Send, S. Valouch, E. Thiel and I. Bruder, Focus-
Induced Photoresponse: a novel optoelectronic distance
measurement technique, 2017.
Nazeeruddin and M. Grätzel, J. Mater. Chem. A, 2013, 1, 5535-
5544.
33 S. M. Feldt, E. A. Gibson, E. Gabrielsson, L. Sun, G. Boschloo
and A. Hagfeldt, JACS, 2010, 132, 16714-16724.
34 Y. S. Yang, H. D. Kim, J.-H. Ryu, K. K. Kim, S. S. Park, K.-S. Ahn
and J. H. Kim, Synth. Met., 2011, 161, 850-855.
4
5
A. Kay and M. Grätzel, J. Phys. Chem., 1993, 97, 6272-6277.
S. Cherian and C. C. Wamser, J. Phys. Chem. B, 2000, 104
,
35 H. Wei, J. Shen, Y. Liu, T. Huang, Q. Zhang, J. Zhao and X. Zhao,
Dyes Pigm., 2018, 149, 789-795.
3624-3629.
6
M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G.
Viscardi, P. Liska, S. Ito, B. Takeru and M. Grätzel, JACS, 2005,
127, 16835-16847.
36 B. Nagarajan, S. Kushwaha, R. Elumalai, S. Mandal, K.
Ramanujam and D. Raghavachari, J. Mater. Chem. A, 2017, 5,
10289-10300.
7
8
9
T. Horiuchi, H. Miura, K. Sumioka and S. Uchida, JACS, 2004,
126, 12218-12219.
A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595-6663.
N. Robertson, Angew. Chem. Int. Ed., 2006, 45, 2338-2345.
37 A. F. Buene, N. Uggerud, S. P. Economopoulos, O. R. Gautun
and B. H. Hoff, Dyes Pigm., 2018, 151, 263-271.
38 R. Y.-Y. Lin, T.-M. Chuang, F.-L. Wu, P.-Y. Chen, T.-C. Chu, J.-S.
Ni, M.-S. Fan, Y.-H. Lo, K.-C. Ho and J. T. Lin, ChemSusChem,
2015, 8, 105-113.
10 A. Mishra, M. K. Fischer and P. Baüerle, Angew. Chem. Int. Ed.,
2009, 48, 2474-2499.
39 N. S. Wilson and B. A. Keay, Tetrahedron Lett., 1996, 37, 153-
156.
11 M. Pazoki, U. B. Cappel, E. M. J. Johansson, A. Hagfeldt and G.
Boschloo, Energy Environ. Sci., 2017, 10, 672-709.
12 Y. Hu, A. Abate, Y. Cao, A. Ivaturi, S. M. Zakeeruddin, M.
Grätzel and N. Robertson, J. Phys. Chem. C, 2016, 120, 15027-
15034.
40 N. S. Wilson and B. A. Keay, Tetrahedron Lett., 1997, 38, 187-
190.
41 K. L. Billingsley and S. L. Buchwald, J. Org. Chem., 2008, 73
,
5589-5591.
42 Z. Iqbal, W.-Q. Wu, Z.-S. Huang, L. Wang, D.-B. Kuang, H. Meier
and D. Cao, Dyes Pigm., 2016, 124, 63-71.
43 J. Chen, Y. Zhang, L. Yang, X. Zhang, J. Liu, L. Li and H. Zhang,
Tetrahedron, 2007, 63, 4266-4270.
44 H. Svith, H. Jensen, J. Almstedt, P. Andersson, T. Lundbäck, K.
Daasbjerg and M. Jonsson, J. Phys. Chem. A, 2004, 108, 4805-
4811.
45 G. Boschloo and A. Hagfeldt, Acc. Chem. Res., 2009, 42, 1819-
1826.
46 H. J. Snaith, Adv. Funct. Mater., 2010, 20, 13-19.
47 J. A. Christians, J. S. Manser and P. V. Kamat, J. Phys. Chem.
Lett., 2015, 6, 852-857.
48 H. Ellis, N. Vlachopoulos, L. Häggman, C. Perruchot, M. Jouini,
13 Y. Hu, A. Ivaturi, M. Planells, C. L. Boldrini, A. O. Biroli and N.
Robertson, J. Mater. Chem. C, 2016, 4, 2509-2516.
14 L. Zhang and J. M. Cole, ACS Appl. Mater. Interfaces, 2015, 7,
3427-3455.
15 D. Joly, L. Pellejà, S. Narbey, F. Oswald, J. Chiron, J. N. Clifford,
E. Palomares and R. Demadrille, Sci. Rep., 2014, 4, 4033.
16 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-
5787.
17 H. N. Tsaoꢀ, C. Yiꢀ, T. Moehl, J.-H. Yum, S. M. Zakeeruddin, M.
K. Nazeeruddin and M. Grätzel, ChemSusChem, 2011,
594.
4, 591-
18 A. Mahmood, Solar Energy, 2016, 123, 127-144.
G. Boschloo and A. Hagfeldt, Electrochimica Acta, 2013, 107
45-51.
,
19 Z.-S. Huang, H. Meier and D. Cao, J. Mater. Chem. C, 2016, 4,
2404-2426.
49 Y. Saygili, M. Söderberg, N. Pellet, F. Giordano, Y. Cao, A. B.
Muñoz-García, S. M. Zakeeruddin, N. Vlachopoulos, M.
Pavone, G. Boschloo, L. Kavan, J.-E. Moser, M. Grätzel, A.
Hagfeldt and M. Freitag, JACS, 2016, 138, 15087-15096.
20 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.
This journal is © The Royal Society of Chemistry 20xx
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Page 10 of 12
ARTICLE
Journal Name
50 P. Ferdowsi, Y. Saygili, S. M. Zakeeruddin, J. Mokhtari, M.
Grätzel, A. Hagfeldt and L. Kavan, Electrochimica Acta, 2018,
265, 194-201.
View Article Online
DOI: 10.1039/C9TA00472F
51 R. M. El-Shishtawy, J.-D. Decoppet, F. A. M. Al-Zahrani, Y. Cao,
S. B. Khan, M. S. Al-Ghamdi, B. G. Alhogbi, A. M. Asiri, S. M.
Zakeeruddin and M. Grätzel, New J. Chem., 2018, 42, 9045-
9050.
52 R. Flamholc, D. Plażuk, J. Zakrzewski, R. Métivier, K. Nakatani,
A. Makal and K. Woźniak, RSC Advances, 2014, 4, 31594-
31601.
53 Y. Hao, H. Tian, J. Cong, W. Yang, I. Bora, L. Sun, G. Boschloo
and A. Hagfeldt, ChemPhysChem, 2014, 15, 3476-3483.
54 H. Ozawa, M. Awa, T. Ono and H. Arakawa, Chem. Asian. J.,
2012, 7, 156-162.
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Sentence highlighting the novelty of the work:
A comprehensive study into auxiliary donors for phenothiazine sensitizers for DSSCs, and investigation
of a suspiciously efficient phenothiazine literature dye.