Thiazolo[5,4-d]thiazole-based organic sensitizers with strong visible light absorption for transparent, efficient and stable dye-sensitized solar cells

Article abstract of DOI:10.1039/c5ra03530a

A small set of thiazolo[5,4-d]thiazole-based D-π-A organic dyes, endowed with bis-pentylpropylenedioxythiophene (ProDOT) moieties in the π-spacer, was designed with the aid of computational analysis, synthesized and characterized. The presence of bulky an

Full text of DOI:10.1039/c5ra03530a

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Thiazolo[5,4-d]thiazole-based organic sensitizers
with strong visible light absorption for transparent,
efficient and stable dye-sensitized solar cells†
Cite this: RSC Adv., 2015, 5, 32657
Alessio Dessı,ab Massimo Calamante,ab Alessandro Mordini,ab Maurizio Peruzzini,a
`
Adalgisa Sinicropi,c Riccardo Basosi,c Fabrizia Fabrizi de Biani,c Maurizio Taddei,c
a
a
Daniele Colonna,d Aldo di Carlo,de Gianna Reginato and Lorenzo Zani
*
*
A
small set of thiazolo[5,4-d]thiazole-based D–p–A organic dyes, endowed with bis-
pentylpropylenedioxythiophene (ProDOT) moieties in the p-spacer, was designed with the aid of
computational analysis, synthesized and characterized. The presence of bulky and electronrich ProDOT
groups beside the electron poor thiazolothiazole unit induced optimal physico-chemical properties,
including broad and intense visible light absorption. As a consequence, the dyes were particularly
suitable for application in thin layer dye-sensitized solar cells (TiO2 thickness: 3.0–6.5 mm). Small-scale
(0.25 cm2) devices prepared using standard materials and fabrication techniques gave power conversion
efficiencies up to 7.71%, surpassing those obtained with two different reference dyes. Transparent larger
area cells (3.6 cm2) also showed good h values up to 6.35%, not requiring the use of a co-adsorbent, and
retained their initial efficiency over a period of 1000 h storage at 85 ꢀC. These results make this new
family of organic sensitizers promising candidates for successful application in the production of efficient
and stable transparent DSSCs for building-integrated photovoltaics.
Received 26th February 2015
Accepted 30th March 2015
DOI: 10.1039/c5ra03530a
www.rsc.org/advances
sensitizers,4 whose performances were later surpassed by
porphyrin dyes5 and, most recently, lead-containing perov-
Introduction
skites.6 Fully organic sensitizers have also been employed, and
efficiencies higher than 10%,7–9 sometimes even coupled with
excellent stabilities,10 have been observed in such cases. The
latter class of dyes presents some potential advantages
compared to the other sensitizers, such as the possibility of not
using precious or toxic metals, the ability to modify molecular
properties by exible synthetic procedures and the efficient
harvesting of sunlight due to high molar absorptivities.11
In particular, organic dyes with superior light absorption
ability would be especially suitable for the sensitization of thin
TiO2 layers,12 possibly useful for the development of solid-state
DSSCs,13 but also for the construction of colored, transparent
photovoltaic modules. The latter could nd optimal application
in the context of building-integrated photovoltaics (BIPV),14
currently considered one of the most appealing opportunities
for DSSC commercialization.15
Since their discovery in 1991,1 dye-sensitized solar cells (DSSCs)
have been widely regarded as a potential alternative to tradi-
tional silicon-based photovoltaic devices.2,3 In a DSSC, light
harvesting takes place at the photoanode, constituted by a thin
layer of a wide-band gap inorganic semiconductor (most oen
TiO2) sensitized with a suitable dye: therefore, the choice of
sensitizer plays a crucial role in determining the overall
performances of the device.
Traditionally, DSSCs with high power conversion efficiencies
(h) were rst obtained with ruthenium metal-organic
aIstituto di Chimica dei Composti Organometallici (CNR-ICCOM), Via Madonna del
Piano 10, 50019 Sesto Fiorentino (FI), Italy. E-mail: gianna.reginato@iccom.cnr.it;
lorenzo.zani@iccom.cnr.it; Fax: +39 055 5225203; Tel: +39 055 5225245
b
`
Dipartimento di Chimica “U. Schiff”, Universita degli Studi di Firenze, Via della
Lastruccia 13, 50019 Sesto Fiorentino (FI), Italy
Another key point for a wide application of DSSCs has to deal
with the simplication of fabrication techniques. It is well
known that the application of surface treatments on both the
conductive glass and the inorganic semiconductor, the
consecutive deposition of multiple TiO2 layers of different
average particle size and shape,16 or the introduction of several
additives and co-adsorbents both in the sensitizing baths and
cell electrolytes10 are oen required to obtain high efficiency.
The discovery of new sensitizers able to give good performances
c
`
Dipartimento di Biotecnologie, Universita degli Studi di Siena, Chimica e Farmacia,
Via A. Moro 2, 53100 Siena, Italy
dDyepower Consortium, Viale Castro Pretorio 122, 00185 Rome, Italy
e
`
Center for Hybrid and Organic Solar Energy (C.H.O.S.E.), Universita degli Studi di
Roma “Tor Vergata”, Via del Politecnico 1, 00133 Rome, Italy
† Electronic
supplementary
information
(ESI)
available:
Additional
computational data, general synthetic remarks, synthesis and characterization
of all new compounds, additional photoelectrochemical measurements, copies
of the NMR spectra of compounds 7, 15, 19–20, 23, TTZ6 and 7. See DOI:
10.1039/c5ra03530a
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under simplied conditions would allow to reduce the which was considered benecial both for their synthesis as well
economic and environmental costs of DSSCs,17 thus helping to as their purication.
make this technology more sustainable.
For these reasons, we replaced the 3-hexylthiophene units of
In recent years, both we18 and others19 introduced new DSSC TTZ1 with bulkier and more electronrich bis-pentylpropylene-
dyes featuring fused thiazole heterocycles as central electron dioxythiophene (ProDOT) moieties (TTZ3, Fig. 1).22 The design
withdrawing units. Among our compounds, sensitizer TTZ1 of such scaffold was supported by an accurate DFT computa-
(Fig. 1), having a thiazolo[5,4-d]thiazole (TzTz) core,20 provided tional analysis. We also modied the terminal triphenylamine
the best performances, with a maximum h of 3.53% obtained in group by introducing different substituents in para-position
the presence of chenodeoxycholic acid (CDCA) as a co-adsor- (TTZ4–5). The preparation and application of compounds
bent.18a We reasoned that the low efficiency observed for TTZ1 TTZ3–5 have been already reported by us in a recent preliminary
could be attributed to a combination of factors, such as its communication.23 To further modulate the electronic proper-
limited light harvesting ability, its scarce solubility in most ties of the dyes and inuence their photovoltaic performances,
organic solvents and its tendency to form aggregates on TiO2 we decided to alter the conjugate portion of the compounds by
surface, requiring the use of CDCA to improve photocurrents. inserting an electronrich ethylenedioxythiophene (EDOT) ring
Based on these considerations, we designed a new series of in place of the terminal thiophene present in TTZ3–5 (TTZ6),
TzTz-containing sensitizers (see Fig. 1, TTZ3–7), with the and also to modify the terminal donor unit by replacing the
precise aim to increase their visible light absorption and triarylamine group with a phenothiazine moiety (TTZ7).
improve their solubility compared to TTZ1.
In addition to the previously reported data, herein we will
We supposed that replacement of the simple hexyl chains disclose a full account describing the computational validation
present in TTZ1 with two alkoxy substituents could reduce the of compounds TTZ3–7, their detailed synthesis as well as their
aromatic stabilization of the thiophene rings, thus slightly complete characterization. We will also report their use as
increasing the HOMO energies and decreasing the HOMO– sensitizers for thin-layer DSSCs under a variety of conditions
LUMO gaps of the resulting molecules, ultimately leading to a (opaque/transparent semiconductor layers, presence/absence of
red-shi of their absorption spectra. Moreover, a spectral red- additives, different layer thicknesses and surface areas,
shi and an improvement in light harvesting were expected different electrolytes), and compare their performances with
by alternating the electronrich thiophene moieties with the those of different reference sensitizers. Finally, the results of
electron withdrawing thiazolothiazole and cyanoacrylic units.21 stability studies conducted under controlled conditions, as well
Finally, placement of further alkyl chains on the sensitizers as those of electrochemical impedance spectroscopy investiga-
backbone was regarded as a way to improve their solubility, tions, will be discussed.
Results and discussion
Computational analysis
Before starting our synthetic efforts, we analysed compounds
TTZ3–7 computationally. In the calculations, the alkyl chains
present on the molecules were replaced by simple methyl
groups to reduce the computational burden, without affecting
the length and nature of the conjugate system.
Initially, structures were optimized in vacuo by means of DFT
calculations at the B3LYP/6-31G* level;24 the energy and shape
of their frontier molecular orbitals (FMOs) were then compared
with those previously found for dye TTZ1, which was used as a
reference. Computed values of the HOMO–LUMO energy gaps
for the TTZ3–7 structures having both ProDOT rings in the most
stable “chair” conformation are reported in Fig. 2, while the
wave function plots of the FMOs are reported in Fig. S1 (ESI†).
Dye TTZ3 showed a reduced HOMO–LUMO gap compared to
TTZ1 (2.11 eV vs. 2.17 eV), mostly due to HOMO destabilization.
The HOMO energy increased even further and the frontier
orbital gap decreased (1.95 eV) when the terminal triphenyl-
amine unit was decorated with alkoxy substituents (TTZ4) to
increase its electron donating character. In the case of dye TTZ5,
alkylthio substituents were introduced on the donor moiety,
since it was shown that their use could lead to DSSCs with
improved Voc and h compared to the oxygenated counterparts
(for a discussion, see below).25 The presence of such functional
Fig. 1 Structure of previously described sensitizer TTZ1 and of new
dyes TTZ3–7.
groups had a similar, but smaller effect on the HOMO energy,
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Fig. 2 Computed (B3LYP/6-31G*) HOMO–LUMO energy gaps of compounds TTZ1 and TTZ3–7.
resulting in a FMO gap of 2.00 eV. Compound TTZ7, bearing a dithiooxamide under microwave activation (Scheme 1): initially,
phenothiazine donor moiety,26 had a HOMO–LUMO gap we carried out this transformation under the optimized condi-
comparable to that found for dye TTZ3, with a value of 2.09 eV. tions already established in our previously reported MW-
Finally, in the case of compound TTZ6, the presence of an EDOT assisted synthesis of thiazolothiazoles (nitrobenzene solution,
group on the terminal portion made the molecule more elec- 150 ꢀC, 30 min).29 However, we quickly realized that, due to the
tronrich, in turn causing a signicant LUMO destabilization presence of the long alkyl chains on the ProDOT moieties, the
without a big change of the HOMO energy. As a result, the FMO product would not precipitate from the reaction mixture, thus
gap in this compound was larger than in dyes TTZ3–5,7, and complicating its separation from nitrobenzene. For this reason,
closer to that found for TTZ1 (2.17 vs. 2.18 eV). For all we changed the solvent to n-BuOH30 (facilitating chromato-
compounds, orbital shapes were similar, with the HOMO ꢁ 1 graphic purication of 2) and briey optimized reaction
distributed along the entire conjugated system and the HOMO temperature and time, ultimately obtaining the desired
and LUMO mostly localized on the donor and acceptor moie- heterocyclic product in moderate yield.
ties, respectively (Fig. S1†).
Aer electrophilic halogenation with N-iodosuccinimide, the
To learn more on the electronic transitions of the new resulting bis-iodide 3 was subjected to Pd-catalyzed Suzuki
organic sensitizers, the absorption maxima (lamax), oscillator cross-coupling with boronic acids/esters 8–9, 12 and 15 to
strengths (f) and vertical excitation energies (Eexc) in the most install the donor moieties. To minimize formation of the
stable conformation were assessed by TD-DFT calculations in symmetrical double-coupling products,
a
stoichiometric
THF at the CAM-B3LYP/6-31G*27 level (Table S1†). For all dyes amount of boronic reagent had to be employed, and the reac-
the excitation process in the visible region results mostly from tion had to be stopped before reaching full conversion, result-
mixed HOMO / LUMO and HOMO ꢁ 1 / LUMO transitions. ing in low yields of products 4–7; despite that, a signicant
Interestingly, although TTZ6 was the compound with the amount of unreacted diiodide 3 could usually be recovered aer
highest computed HOMO–LUMO gap, its excitation comprised chromatographic purication for further reuse. Due to the
a larger H / L fraction compared to the other dyes, resulting in lower reactivity of boronic esters 10 and 15, an alternative
a slightly red-shied absorption maximum. In agreement with protocol using [Pd(dppf)Cl2] as the catalyst and KF as the base
our expectations, computational results indicated that the new was employed instead of the classical procedure with Pd(PPh3)4.
dyes should all have absorption maxima higher than 500 nm,
Whereas boronic acid 8 is commercially available, and ester
red-shied of approx. 20–30 nm relative to TTZ1; in addition, 9 was obtained via a reported procedure,31 compound 12 was
the higher oscillator strengths found for TTZ3–7 suggested the prepared by the route shown in Scheme 1. First, Pd-catalyzed
possibility of more intense light absorption.
Buchwald–Hartwig reaction between 1-bromo-4-hexylth-
iobenzene32a and aniline afforded triarylamine 10, which was
then brominated with N-bromosuccinimide (NBS) to yield
compound 11.32b Finally, conversion of the latter to the corre-
sponding boronic ester 12 was accomplished via Pd-catalyzed
borylation, which proceeded with higher yield than the previ-
ously reported Li-mediated procedure.32b
Synthesis of compounds TTZ3–7
The starting point of the synthesis of dyes TTZ3–7 was consti-
tuted by aldehyde 1, which was prepared in four steps following
a reported procedure.28 Compound 1 was converted to the cor-
responding thiazolo[5,4-d]thiazole (2) by reaction with
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Scheme 1 Reagents and conditions: (a) dithiooxamide (0.5 eq.), n-BuOH, MW, 120 ꢀC, 2.5 h then chloranil (0.25 eq.), THF, reflux, 10 min, 42%; (b)
N-iodosuccinimide (2.5 eq.), CHCl3/AcOH, rt, 16 h, 93%; (c) 8 or 9 (1.0 eq.), Pd(PPh3)4 (10 mol%), Na2CO3 (5.0 eq.), toluene, 100 ꢀC, 16 h, 18–29%;
(d) 12 or 15 (1.0 eq.), [Pd(dppf)Cl2] (10 mol%), KF (5.0 eq.), toluene/EtOH, 78 ꢀC, 5–16 h, 12–18%; (e) aniline (0.33 eq.), Pd2(dba)3$CHCl3 (5 mol%),
dppf (10 mol%), tBuONa (1.33 eq.), toluene, reflux, 20 h, 80% (f) N-bromosuccinimide (1.0 eq.), CHCl3, rt, 4 h, 98% (g) [Pd(dppf)Cl2] (10 mol%), bis-
pinacolatodiboron (1.5 eq.), AcOK (3.0 eq.), N,N-DMF, 80 ꢀC, 16 h, 60–83%; (h) PyHBr3 (0.85 eq.), CH2Cl2/CH3CN, rt, 30 min, 60%.
Reagent 15 was also prepared through a bromination/ moiety through a cross-coupling/Knoevenagel condensation
catalytic borylation sequence starting from known phenothia- sequence, with both processes proceeding in good to high
zine 13.33
yields (Scheme 2). In the case of compounds TTZ3–5 and
However, initial electrophilic halogenation with typical TTZ7 the rst reaction was a Suzuki coupling of iodides 4–7
reagents such as NBS and Br2 under various conditions took with commercial boronic acid 21: such transformation was
place with limited success due to either lack of reactivity, or activated by microwaves to shorten reaction times and limit
formation of bis-brominated derivatives. For this reason, we the formation of by-products (for example deriving from
examined alternative reaction conditions, and ultimately protodehalogenation reactions). Stille cross-coupling of
employed
a sub-stoichiometric amount of pyridinium intermediate 4 with EDOT-containing stannane 23 was
bromide perbromide34 to obtain a mixture of 13 and 14 in a employed instead for dye TTZ6. Stannane 23 was prepared
1 : 3 molar ratio, which could be used as such in the starting from known aldehyde 22 (ref. 35) by protection of the
following step. Pd-catalyzed borylation of bromide 14 with carbonyl group with trimethylorthoformate followed by
bis(pinacolato)diboron then took place uneventfully, lithiation of the resulting dimethylacetal and treatment with
producing boronic ester 15 in good yield aer chromato- tributyltin chloride; at the end of the reaction, work-up with
graphic separation from unreacted 13.
an aqueous solution of KHSO4 led to deprotection of the
Aer preparation of intermediates 4–7, the synthesis of formyl group, allowing isolation of compound 23.
dyes TTZ3–7 was completed by introduction of the acceptor
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Scheme 2 Reagents and conditions: (a) 21 (1.5 eq.), [Pd(dppf)Cl2] (5 mol%), KF (6.0 eq.), toluene/MeOH, MW, 70 ꢀC, 30 min, 57–74%; (b) 23
(1.5 eq.), Pd(PPh3)4 (5 mol%), toluene, 110 ꢀC, 24 h, 74%; (c) cyanoacetic acid (10.0 eq.), NH4OAc (4.0 eq.), toluene/AcOH, 110 ꢀC, 6 h, 84–97%; (d)
CH(OMe)3 (1.5 eq.), DDQ (5 mol%), CHCl3/MeOH, 50 ꢀC, 2 h then n-BuLi (1.5 eq.), Bu3SnCl (2.0 eq.), THF, ꢁ78 ꢀC to rt, 18 h, 67%.
Characterization of dyes TTZ3–7
In THF solution, all dyes showed broad and intense absorptions
in the visible region (Fig. 3), with maxima in the 510–521 nm
range, thus displaying a bathochromic shi of ca. 40–50 nm
compared to TTZ1.18a In agreement with the results of our
computational analysis (see above), dyes TTZ4 and TTZ6 showed
the most red-shied absorption maxima; more generally,
experimental excitation energies were quite close to the calcu-
lated values, with differences of at most 0.1 eV.
All dyes exhibited uorescence in THF solution: therefore,
optical band gaps (E0–0) could be obtained from the intersection
of the normalized absorption and emission spectra (Fig. S2†),
and were comprised in the 2.14–2.25 eV range (Table 1), with
TTZ4 and TTZ6 having the smallest E0–0 values. Such values
were in good agreement with those derived from the corre-
sponding Tauc plots,36 which were found to be in the 2.12–
2.16 eV range and in the same relative order (Fig. S3†).
When the dyes were attached to nanocrystalline TiO2, they
exhibited slightly broader and blue-shied absorption spectra
(21–27 nm), probably due to deprotonation of the carboxylic
Fig. 3 UV-Vis spectra of dyes TTZ3–7 in THF solution. Concentra-
tions: TTZ3, 8.47 ꢂ 10ꢁ6 M; TTZ4, 8.89 ꢂ 10ꢁ6 M; TTZ5, 8.96 ꢂ
10ꢁ6 M; TTZ6, 7.61 ꢂ 10ꢁ6 M; TTZ7, 9.57 ꢂ 10ꢁ6 M.
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Table 1 Spectroscopic and electrochemical data for dyes TTZ3–5
lmax absa [nm] (3 [Mꢁ1 cmꢁ1])
lmax ema [nm]
lmax abs. on TiO2 [nm]
Eoxb [V]
Eox [V]
E0–0d [eV]
G [107 mol cmꢁ2
]
c
*
Dye
TTZ3
TTZ4
TTZ5
TTZ6
TTZ7
510 (81 400)
518 (86 600)
510 (94 100)
521 (94 500), 548 (85 100)
513 (96 700)
587
601
573
636
596
484
491
487
498
492
1.02
0.77
0.91
0.93
0.85
ꢁ1.23
ꢁ1.42
ꢁ1.33
ꢁ1.21
ꢁ1.37
2.25
2.19
2.24
2.14
2.22
0.99
1.08
1.19
1.25
1.75
THF solution. b Potential vs. NHE. c Calculated from Eox ꢁ E0–0
.
Estimated from the intersection of normalized absorption and emission spectra.
a
d
acid group on the semiconductor surface (Fig. 4).37 The
amount of dyes attached to TiO2 was measured through the
desorption method, by exposing electrodes sensitized with
TTZ3–7 (similar to those later used for DSSC fabrication) to a
0.1 M KOH solution in THF/MeOH until complete discolor-
ation, and measuring the absorbance of the resulting solu-
tion. The highest density was measured for TTZ7, but all dyes
exhibited similar values in the 0.99–1.75 ꢂ 10ꢁ7 mol cmꢁ2
range.
Small-scale photovoltaic measurements
Aer characterization, compounds TTZ3–7 were employed as
sensitizers in the construction of small-scale test DSSCs
(0.25 cm2 active surface, see Fig. 6a), aiming to evaluate their
general level of performance and determine which one had the
most promising photovoltaic properties.
In view of the very high molar extinction coefficients, sug-
gesting the possibility of efficient light-harvesting, we reasoned
that the new dyes could be conveniently used in DSSCs having a
thinner TiO2 layer compared to typical devices. For this reason,
two series of cells were assembled: a rst series with a trans-
parent photoanode of 5.5 mm thickness (obtained with
commercial Dyesol 18NR-T paste), and a second series with an
opaque electrode of 6.5 mm thickness (Dyesol 18NR-AO).
We decided to keep device fabrication as simple as possible,
as we reasoned that the efficiency data obtained under such
simplied conditions could serve as a useful starting point to
assess the potential of the new dyes in view of possible, more
challenging, large-scale applications. In particular, no blocking
layer of TiO2 was deposed either on the conductive glass
substrate or on the nanocrystalline semiconductor layer by
treatment with aq. TiCl4, and no light-scattering layer was
employed in photoanode fabrication. Furthermore, differently
from previous studies on thin-layer DSSCs,39,40 a commercial
Ground-state oxidation potentials (Eox) of all compounds
were measured by means of cyclic voltammetry in CH2Cl2
solution (Fig. S4†). The strong electrondonating character
resulting from introduction of the alkoxy groups on the tri-
arylamine portion of TTZ4 was reected by its relatively low
Eox value (0.77 eV vs. NHE); interestingly, also the presence of
the terminal phenothiazine unit had a similar effect, with
TTZ7 displaying a much lower oxidation potential compared
to TTZ3. However, all the dyes had Eox values positive enough
to ensure regeneration by the iodide/triiodide redox
shuttle.38 Excited state oxidation potentials were obtained
*
from the expression Eox ¼ Eox ꢁ E0–0, and were always found
to be more negative than the conduction band edge of TiO2
(approx. ꢁ0.5 V vs. NHE), conrming the possibility of
smooth electron injection from the dye to the semiconductor
during solar cell operation.
ꢁ
electrolyte solution based on the Iꢁ/I3 redox couple (Dyesol
HPE) was used in the cells, since it usually gives optimal results
with ruthenium sensitizers and is therefore still the most used
for practical applications such as BIPV.
For both types of devices, performances obtained with the
new sensitizers were compared with those of standard organic
dye D5 (ref. 41 and 42) and Ru-based sensitizer Z907
(Fig. S5†).43,44 Typical J/V curves measured on the transparent
solar cells are shown in Fig. 5a, while the corresponding IPCE
spectra are displayed in Fig. 5b and the relevant photovoltaic
parameters are reported in Table 2.
In the case of transparent cells, dyes TTZ3–7 gave efficiency
values between 4.85 and 7.39%, with TTZ5 and TTZ7 being the
best two sensitizers. Remarkably, all new thiazolo[5,4-d]thiazole
dyes gave better efficiencies than standard organic dye D5, and
the two most efficient compounds surpassed even the perfor-
mance provided by ruthenium sensitizer Z907 (h 6.35–7.39% vs.
5.51%). This result was particularly signicant, since Z907 is
one of the sensitizers of choice for applications in BIPV.45 All
Fig. 4 Normalized UV-Vis spectra of dyes TTZ3–7 adsorbed on TiO2. new compounds gave broad IPCE spectra with onsets around
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Fig. 5 Typical J/V curves (a) and IPCE spectra (b) for transparent cells built with dyes D5, Z907 and TTZ3–7.
Table 2 Photovoltaic parameters for DSSCs built with dyes D5, Z907 and TTZ3–7a
Transparent DSSCsb
Opaque DSSCsc
Dye
CDCAd
Jsc [mA cmꢁ2
]
Voc [V]
FF [%]
h [%]
CDCAd
Jsc [mA cmꢁ2
]
Voc [V]
FF [%]
h [%]
D5
Z907
TTZ3
ꢁ
ꢁ
ꢁ
+
9.60
13.50
12.79
15.62
12.18
14.27
16.20
16.59
12.29
10.68
14.99
15.97
0.624
0.686
0.687
0.697
0.669
0.692
0.716
0.717
0.646
0.640
0.686
0.694
63
60
61
60
60
59
63
59
63
57
62
60
3.78
5.51
5.41
6.55
4.85
5.85
7.39
7.08
5.01
3.90
6.35
6.61
ꢁ
ꢁ
ꢁ
+
11.04
13.85
13.65
16.52
12.99
15.18
16.05
18.33
13.00
13.86
15.26
17.22
0.619
0.687
0.671
0.683
0.665
0.675
0.721
0.709
0.644
0.646
0.680
0.689
58
58
60
58
57
55
60
59
56
56
59
58
3.99
5.61
5.45
6.54
4.93
5.71
6.91
7.71
4.70
5.03
6.08
6.94
TTZ4
TTZ5
TTZ6
TTZ7
ꢁ
ꢁ
+
+
ꢁ
+
ꢁ
+
ꢁ
ꢁ
+
+
ꢁ
+
ꢁ
+
a
Average values of three devices; active surface area 0.25 cm2. b TiO2 layer thickness 5.5 mm. c TiO2 layer thickness 6.5 mm. d “ꢁ“: without CDCA; “+”:
with 1 mM CDCA added in the sensitizing bath (0.1 mM dye in THF).
700 nm (Fig. 5b), in agreement with their UV-Vis spectra ndings (compare the HOMO isodensity plots of TTZ4 and
recorded on TiO2: in particular, the IPCE of dye TTZ5 was TTZ5 in Fig. S1,† and consider the higher Jsc and Voc recorded
superior to Z907 in the 370–480 and 550–660 nm regions, for dye TTZ5).
consistent with its higher photocurrent.
The lower efficiencies recorded with the other dyes could be
The improved performance of dye TTZ5 compared to the due to a combination of several factors, including lower light-
other sensitizers could be attributed to the presence of an harvesting efficiency (TTZ3, lowest molar absorptivity and dye
alkylthio-substituted triphenylamine moiety as the donor density on TiO2), insufficient dye regeneration (TTZ4, highest
portion of the molecule. A conjugated dye bearing such donor Eox among all dyes) and increased recombination (TTZ6, larger
group was investigated in detail by Berlinguette et al. and, as HOMO coefficients on the terminal part of the molecule, see
mentioned above, was found to induce superior photocurrent Fig. S1†).
and photovoltage in DSSCs compared to its oxygenated
Data obtained for opaque cells were generally consistent
analogue:25 such effect was attributed to a 25-fold enhancement with those recorded for transparent devices (Table 2 and Fig. S6
of the dye regeneration rate constant which, according to the and 7†), with TTZ5,7 being once again the two best sensitizers,
authors, could either be due to formation of an adduct between albeit with slightly reduced efficiency values (h 6.08–6.91%).
iodide and the “so” sulphur atoms, or simply result from the Short-circuit current, open-circuit voltage and ll factor values
greater HOMO coefficients found on the S atoms compared to were similar in the two classes of cells, reecting the use of
the O atoms. The latter observation is supported also by our analogous fabrication conditions. The relatively low ll factors
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observed for all cells (55–63%) were likely due to the small
Again, for each electrode thickness, two series of cells
thickness of the TiO2 layer employed, combined with the were prepared, either by addition of CDCA in the sensitizing
absence of a blocking layer, probably favouring electron bath or not. The well-known organic dye D35 (Fig. S5†) was
recombination through direct contact between the conductive selected as a reference,46 since this compound has recently
substrate and the electrolyte solution.
been established worldwide as a reference organic sensitizer
To verify if the tridimensional arrangement of alkyl chains for DSSCs, due to its good performances coupled with high
on the ProDOT moieties22 was sufficient to suppress aggregation transparency and excellent stability.47 Typical J/V curves
of the new dyes on TiO2 surface, we also built two series of cells obtained for the CDCA-free and CDCA-containing strip cells
for which photoanode staining was carried out in the presence are shown in Fig. 7a and b, respectively, while the relevant
of 1 mM chenodeoxycholic acid (CDCA, Table 2 and Fig. S8 photovoltaic parameters are reported in Tables 3 and 4.
and 9†). In both device classes, an increase in Jsc was observed
Although DSSCs with a thicker TiO2 layer gave slightly
for all sensitizers, with the sole exception of TTZ6, perhaps due improved performances compared to those with a thinner
to the steric bulk of its terminal EDOT ring (minimizing electrode, differences in efficiency were rather small.
aggregation even in the absence of CDCA), while the largest Predictably, thicker devices produced a higher photocurrent
enhancement was displayed by dye TTZ3 (+21–22%). This time, as a result of the larger amount of dye adsorbed on the
the best result was provided by TTZ5-containing opaque cells, anode, but at the same time their open-circuit voltages were
which gave an average efficiency of 7.71% with a remarkable smaller, probably due to higher electron recombination. Fill
photocurrent of 18.33 mA cmꢁ2
.
factors of these devices were generally lower than those of
small-scale cells due to the higher series resistance usually
observed for larger DSSCs.48 However, it was really remark-
able that all the three dyes TTZ3–5 yielded more efficient
devices than standard sensitizer D35, with the best result
provided once again by TTZ5 (6.35% average efficiency for
the 5 mm device). The superior efficiencies were mostly due to
the higher photocurrents of the thiazolo[5,4-d]thiazole-
containing dyes compared to D35; such difference could be
explained considering their wider (and in the case of TTZ5
also higher) IPCE spectra, see Fig. S10–12.†
To our surprise, and in contrast to what was observed in
the case of small-scale cells, the effect of CDCA on the
performance of the larger-area devices (Table 4 and Fig. 7b)
was that of giving lower efficiencies compared to the CDCA-
free strip cells, with the sole exception of the 5 mm device
built with dye TTZ4. In particular, no general improvement
in photocurrent was observed in this second series of cells,
highlighting a lower tendency of the dyes to aggregate under
these conditions. Accordingly, we tentatively assume that
when the standard 0.1 mM dye solution is used to sensitize
the larger electrodes, a more homogeneous distribution of
the sensitizer is achieved, minimizing stacking and other
aggregation phenomena and thus eliminating the potential
benets brought by the co-adsorbent.
Fabrication and characterization of strip DSSCs
Following the promising results obtained with small-scale
DSSCs (0.25 cm2), we decided to build larger area (3.6 cm2)
strip cells (Fig. 6b), aiming to investigate how the increase in
active surface could affect device efficiency and stability, in view
of a possible scale-up of the system. Accordingly, we also
introduced some changes in device fabrication. First, electrodes
were prepared with two different thicknesses of transparent
TiO2, namely 3 and 5 mm, in order to minimize semiconductor
employment and enhance transparency; secondly, a different
commercial electrolytic solution (Dyesol HSE) was used, still
based on the Iꢁ/I3ꢁ redox couple, which is designed to improve
device stability at the cost of maximum performance. Finally,
only dyes TTZ3–5 were used in these experiments, due to their
easier preparation procedure.
At this stage, the stability of the strip cells fabricated with
dyes TTZ3–5 (Fig. 8) was also tested. A new series of 3 mm-
thick devices was built and their power conversion efficien-
cies measured under standard illumination over a period of
approx. 1000 h, during which they were stored in the dark at
85 ꢀC. All DSSCs showed excellent stability during this period
of time, with nal efficiencies comparable to the initial ones
(for TTZ4-5 a slight increase was even observed within the
rst 72 h). In particular for compound TTZ5 the efficiency
was steadily around 6% for the entire duration of the
experiment, consistent with the results described above.
Interestingly, while the short circuit current density had a
small increase at the beginning of the experiment followed
by a slow decrease, an opposite pattern was observed for the
open circuit voltage, with an initial drop followed by a
Fig. 6 (a) Small-scale dye-sensitized solar cells (active area 0.25 cm2)
and (b) strip DSSCs (active area 3.6 cm2) used in this study.
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Fig. 7 Typical J/V curves for transparent strip cells built with dyes D35 and TTZ3–5 in the absence (a) and in the presence (b) of CDCA. Hollow
symbols: 3 mm devices; full symbols: 5 mm devices.
Table 3 Photovoltaic parameters for strip DSSCs built with dyes D35
and TTZ3–5a
Dye
Thickness [mm]
Jsc [mA cmꢁ2
]
Voc [V]
FF [%]
h [%]
D35
TTZ3
3
3
5
3
5
3
5
9.93
14.65
15.70
13.14
13.51
15.78
17.00
0.758
0.693
0.678
0.682
0.667
0.732
0.705
54
53
52
52
52
53
53
4.06
5.40
5.58
4.70
4.71
6.20
6.35
TTZ4
TTZ5
a
Average values of two devices; active surface area 3.6 cm2.
Table 4 Photovoltaic parameters for strip DSSCs built with dyes D35
and TTZ3–5 with the addition of CDCAa
Dye
Thickness [mm]
Jsc [mA cmꢁ2
]
Voc [V]
FF [%]
h [%]
D35
TTZ3
3
3
5
3
5
3
5
9.52
13.80
16.31
12.62
14.31
15.29
16.38
0.754
0.696
0.667
0.690
0.677
0.722
0.711
55
47
50
52
52
54
50
3.93
4.52
5.48
4.55
5.04
6.03
5.88
TTZ4
TTZ5
a
Average values of two devices; active surface area 3.6 cm2; 1.0 mM
CDCA was added to the electrode sensitization bath.
Fig. 8 Stability test conducted with 3 mm-thick DSSCs sensitized with
dyes TTZ3–5 over the course of 1000 h. Ageing in the dark at 85 ꢀC.
subsequent recovery; similarly, ll factors increased slightly
during the experiment. These results indicate that sensi-
tizers of the thiazolothiazole series are promising candidates
for the potential fabrication of large-scale thin-lm DSSC
modules due to their good performances coupled with high
photochemical stability.
Electrochemical impedance spectroscopy
In the nal part of our studies, we analyzed more in detail the
charge transfer processes taking place in the cells by means of
electrochemical impedance spectroscopy (EIS).49 Measurements
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for TTZ3–5-containing strip cells (5 mm) were performed in the resistance of the cell (Rs), the charge transfer resistance at the
dark at ꢁ0.60 V forward bias over the 0.1 Hz–100 KHz frequency TiO2-dye-electrolyte interface (Rrec) and the charge transfer
range: the corresponding Nyquist plots (i.e. the plots showing resistance at the counter electrode (RCE). Here, while Rs and RCE
the minus imaginary part of the impedance ꢁZ00 vs. the real part were comparable for all dyes (reecting the similar fabrication
Z0 when sweeping the frequency) are shown in Fig. 9a, while conditions and the same materials used in the counter elec-
Bode phase plots (i.e. the plots showing the current/voltage trode), Rrec values increased in the order TTZ4 (22.0 U) < TTZ3
phase shi vs. the frequency) are shown in Fig. 9b.
(30.2 U) < TTZ5 (37.2 U), suggesting that for the latter dye a
In the Nyquist plots, due to the use of a liquid electrolyte, two slower charge recombination was taking place at the photo-
semicircles were observed: the small semicircle at higher anode. Considering the Bode phase plots, the frequency of the
frequencies corresponds to charge transfer processes at the Pt– peaks observed in the middle-frequency domain can be used to
electrolyte interface, while the larger semicircle at lower evaluate the electron lifetime within the semiconductor (se),50
frequencies derives from the charge transfer processes at the providing another indication of the charge recombination rate.
TiO2–dye–electrolyte interface. Several parameters can be In agreement with the previous measurements, we found that
obtained by tting the spectra with a known electrochemical electron lifetimes increased in the order TTZ4 (15.1 ms) < TTZ3
model (see the equivalent circuit in Fig. 9c), namely the series (17.5 ms) < TTZ5 (23.2 ms), conrming that electron recombi-
nation was more efficiently prevented by the dye with the
alkylthio-terminal chains. As recently pointed out in other
studies,51 electron lifetime, inuenced by factors such as
molecular size and dye adsorption behaviour, exerts a strong
inuence on cell photovoltage. Indeed, also here, the trend in se
values was in good agreement with Voc values measured for the
cells and reported in Table 4.
Finally, we also measured EIS spectra at different potentials
for all three dyes (see Fig. S13† for the corresponding Nyquist
plots). As expected, with a decrease in the bias potential from
ꢁ0.80 to ꢁ0.45 V the interfacial charge recombination process
gave rise to a progressively larger semicircle in the mid-
frequency region; at high potential, both interfacial charge
transfer- and transport resistances within the TiO2 became
smaller due to the increased electron density, and a small
feature at the lowest applied frequencies became visible as an
ꢁ
effect of the diffusion of the I3 ions in the electrolyte.
Conclusions
In this work, ve new thiazolo[5,4-d]thiazole-based organic dyes
(TTZ3–7) were synthesized and characterized with the aim to
use them as sensitizers in thin-layer DSSCs (TiO2 thickness #
6.5 mm). The new compounds displayed broad and intense
absorption spectra in the visible region, with impressive molar
absorptivities up to 9.67 ꢂ 104 Mꢁ1 cmꢁ1. Small-scale solar cells
built with TTZ3–7, both transparent and opaque, gave good
power conversion efficiencies (h up to 7.71%), which in the case
of TTZ5 and TTZ7 were clearly superior to those obtained with
standard Ru-dye Z907. Larger-scale strip cells (active area:
3.6 cm2), featuring thin lms of transparent TiO2 (3–5 mm) and a
high stability electrolyte, gave efficiencies in line with those
obtained with the smaller devices, with TTZ5 being once again
the best sensitizer (h up to 6.35%). Remarkably, in this case
employment of CDCA was not necessary, since addition of the
co-adsorbent to the sensitizing bath did not lead to any
improvement in cell efficiencies compared to the original
devices. Even more importantly, all the dyes examined at this
stage (TTZ3–5) led to solar cells with very high stability, as
ꢀ
shown by a 1000 h storage test at 85 C, at the end of which
Fig. 9 Electrochemical impedance spectra for compounds TTZ3–5
efficiencies resulted practically unchanged compared to initial
measured in the dark at ꢁ0.60 V forward bias: (a) Nyquist plot; (b) Bode
phase plot; (c) equivalent circuit.
ones.
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Finally, the charge transfer processes within the cells were
RSC Advances
The devices underwent photovoltaic characterization by using a
investigated by means of electrochemical impedance spec- AM 1.5G solar simulator equipped with a Xenon lamp (KHS Solar
troscopy. These measurements indicated that dye TTZ5 was Constant 1200). The measurements were performed with a power
able to minimize electron recombination from the semi- of incoming radiation of 100 mW cmꢁ2. J/V curves were obtained
conductor to the oxidized electrolyte more efficiently than by applying an external bias to the cell and measuring the gener-
the other sensitizers, resulting in a longer charge carrier ated photocurrent with a Keithley model 2400 digital source-meter,
lifetime and higher Voc. Such data, coupled with the obser- under the control of dedicated LabTracer 2.0 soware. A black
vation reported by Berlinguette et al. that dyes with alkylthio- shading mask was used to avoid overestimation of the measured
substituted triphenylamine donor units are more efficiently parameters. IPCE spectra were measured with a dedicated appa-
regenerated compared to their oxygenated or unsubstituted ratus built with the following components: Newport model 70612
analogs,25 could contribute to explain the superior photo- Xenon lamp (150 W), Cornerstone 130 1/8 m monochromator and
voltaic performances exhibited by dye TTZ5 in comparison Keithley model 2400 digital source-meter.
with the other sensitizers described in this study.
Electrochemical impedance spectroscopy (EIS) measure-
We are currently trying to expand our library of ments were performed in the dark using a AUTOLAB 302N
thiazolothiazole-based sensitizers and develop a synthetic potentiostat (Metrohm) equipped with a Nova electrochemical
procedure suitable for their large-scale preparation. The dyes interface system, working at ꢁ0.80 V, ꢁ0.60 V and ꢁ0.45 V
will then be used in the fabrication of transparent, thin lm forward bias. The spectra were recorded over a frequency range
DSSC modules to be subjected to even more demanding of 10ꢁ1 Hz to 105 Hz with an amplitude of 10 mV. Data tting
efficiency and stability measurements. The results of such was carried out using the EC-Lab soware (V9.46).
investigations will be reported in due course.
Acknowledgements
Experimental
The authors thank the “Ente Cassa di Risparmio di Firenze”
Transparent photoanodes for small-scale DSSCs were foundation (“IRIS” project) and MIUR (“Progetto Premiale 2011:
prepared by screen-printing a commercial TiO2 paste (Dyesol Produzione di Energia da Fonti Rinnovabili”) for nancial
18NR-T) on a 8 U sqꢁ1 conductive glass substrate (Piꢀlking- support. A. S. and R. B. thank CINECA and CREA (Colle Val
ton), and by sintering the resulting electrodes at 520 C for d’Elsa, Italy) for the availability of high performance computing
30 minutes. Aer sintering, the thickness of the semi- resources and nancial support. In addition, the authors wish
conductor layer was measured by means of a proler (Dektak to thank Dr Francesca De Rossi (C.H.O.S.E., Rome, Italy) for
150, Veeco) and determined to be 5.5 mm. Opaque photo- performing the EIS measurements.
anodes were obtained following an analogous procedure, but
using a different commercial titania paste (Dyesol 18NR-AO);
their nal thickness was 6.5 mm. In both cases, the electrode
Notes and references
active area was 0.25 cm2. Counter electrodes were obtained
by screen printing a commercial platinum-containing paste
(Chimet) on pre-drilled conductive glass plates and by
heating at 420 ꢀC for 15 minutes. TiO2 photo-electrodes were
sensitized by overnight immersion at rt into the appropriate
dye solution (0.1 mM in THF for dyes TTZ3–7, 0.2 mM in
EtOH for dye D5, 0.3 mM in EtOH for dye Z907), either in the
absence or in the presence of 1 mM chenodeoxycholic acid
(CDCA). Aer sensitization, the anodes were rinsed with
EtOH and deionized water, and then dried. A TiO2-sensitized
photoanode and a Pt counter electrode were assembled into a
sealed sandwich-type cell using a 25ꢁmm hot-melt Surlyn®
gasket (Solaronix). A drop of the Iꢁ/I3 containing commer-
cial HPE electrolyte solution (Dyesol) was placed on the
drilled hole on the back of the counter electrode and was
driven into the cell by vacuum backlling. The hole was
nally sealed by using additional sealing lm and a small
glass cover. Fabrication of strip cells (3.6 cm2 active area) was
carried out following the same procedure, except that only
transparent photoanodes were prepared, in a thickness of
3 and 5 mm, and a different electrolyte (Dyesol HSE) was used
to ll the cells. Sensitization of the photoanodes with stan-
dard sensitizer D35 was performed using a 0.3 mM solution
of the dye in 1-methoxy-2-propanol.
1 B. O'Regan and M. Gratzel, Nature, 1991, 353, 737–740.
2 Dye-sensitized solar cells, ed. K. Kalyanasundaram, EPFL
Press, Lausanne, 2010.
¨
3 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595–6663.
4 Recent examples: (a) C. Y. Chen, M. Wang, J. Y. Li,
N. Pootrakulchote, L. Alibabaei, C. H. Ngoc-le, J. D. Decoppet,
J. H. Tsai, C. Gratzel, C. G. Wu, S. M. Zakeeruddin and
M. Gratzel, ACS Nano, 2009, 3, 3103–3109; (b) Y. Numata,
S. P. Singh, A. Islam, M. Iwamura, A. Imai, K. Nozaki and
L. Han, Adv. Funct. Mater., 2013, 23, 1817–1823.
5 L.-L. Li and E. W.-G. Diau, Chem. Soc. Rev., 2013, 42, 291–304.
6 (a) H. J. Snaith, J. Phys. Chem. Lett., 2013, 4, 3623–3630; (b)
S. Kazim, M. K. Nazeeruddin, M. Gratzel and S. Ahmad,
Angew. Chem., Int. Ed., 2014, 53, 2812–2825.
7 M. Zhang, Y. Wang, M. Xu, W. Ma, R. Li and P. Wang, Energy
Environ. Sci., 2013, 6, 2944–2949.
8 J.-H. Yum, T. W. Holcombe, Y. Kim, K. Rakstys, T. Moehl,
J. Teuscher, J. H. Delcamp, M. K. Nazeeruddin and
M. Gratzel, Sci. Rep., 2013, 3, 2446.
9 J. Yang, P. Ganesan, J. Teuscher, T. Moehl, Y. J. Kim, C. Yi,
P. Comte, K. Pei, T. H. Holcombe, M. K. Nazeeruddin,
¨
¨
¨
¨
¨
J. Hua, S. M. Zakeeruddin, H. Tian and M. Gratzel, J. Am.
Chem. Soc., 2014, 136, 5722–5730.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 32657–32668 | 32667

View Article Online
RSC Advances
Paper
`
10 K. Kakiage, Y. Aoyama, T. Yano, T. Otsuka, T. Kyomen, 29 A. Dessı, M. Calamante, A. Mordini, L. Zani, M. Taddei and
M. Unno and M. Hanaya, Chem. Commun., 2014, 50, 6379–
6381.
11 Y. Ooyama and Y. Harima, ChemPhysChem, 2012, 13, 4032–
4080.
12 S. De Sousa, C. Olivier, L. Ducasse, G. Le Bourdon, L. Hirsch
and T. Toupance, ChemSusChem, 2013, 6, 993–996.
13 T. Miyasaka, J. Phys. Chem. Lett., 2011, 2, 262–269.
14 S. Yoon, S. Tak, J. Kim, Y. Jun, K. Kang and J. Park, Build.
Environ., 2011, 46, 1899–1904.
G. Reginato, RSC Adv., 2014, 3, 1322–1328.
30 R. Ziessel, A. Nano, E. Heyer, T. Bura and P. Retailleau,
Chem.–Eur. J., 2013, 19, 2582–2588.
31 L. Chen, B. Zhang, Y. Cheng, Z. Xie, L. Wang, X. Jing and
F. Wang, Adv. Funct. Mater., 2010, 20, 3143–3153.
32 (a) C. G. Bates, R. K. Gujadhur and D. Venkataraman, Org. Lett.,
2002, 4, 2803–2806; (b) K. C. D. Robson, B. D. Koivisto and
C. P. Berlinguette, Inorg. Chem., 2012, 51, 1501–1507.
33 Y.-H. Seo, W.-H. Lee, J.-H. Park, C. Bae, Y. Hong, J.-W. Park
and I.-N. Kang, J. Polym. Sci., Part A: Polym. Chem., 2012,
50, 649–658.
15 H. Zervos, Dye Sensitized Solar Cells (DSSC/DSC) 2013-2023:
Technologies, Markets, Players, IDTechEx Report, 2012, can
be found at: http://www.idtechex.com/research/reports/dye- 34 C. Djerassi and C. R. Scholz, J. Am. Chem. Soc., 1948, 70, 417–
sensitized-solar-cells-dssc-dsc-2013-2023-technologies-markets-
players-000345.asp.
418.
35 A. K. Mohanakrishnan, A. Hucke, M. A. Lyon,
M. V. Lakshmikantham and M. P. Cava, Tetrahedron, 1999,
55, 11745–11754.
¨
16 S. Ito, T. N. Murakami, P. Comte, P. Liska, C. Gratzel,
¨
M. K. Nazeeruddin and M. Gratzel, Thin Solid Films, 2008,
516, 4613–4619.
17 M. L. Parisi, S. Maranghi and R. Basosi, Renewable
Sustainable Energy Rev., 2014, 39, 124–138.
36 (a) J. Tauc, Mater. Res. Bull., 1968, 3, 37–46, application on
molecular dyes: (b) C. Coluccini, N. Manfredi,
M. M. Salamone, R. Ruffo, M. G. Lobello, F. De Angelis
and A. Abbotto, J. Org. Chem., 2012, 77, 7945–7956.
`
18 (a) A. Dessı, G. Barozzino Consiglio, M. Calamante,
G. Reginato, A. Mordini, M. Peruzzini, M. Taddei, 37 Y. J. Chang and T. J. Chow, J. Mater. Chem., 2011, 21, 9523–
A. Sinicropi, M. L. Parisi, F. Fabrizi de Biani, R. Basosi, 9531.
R. Mori, M. Spatola, M. Bruzzi and L. Zani, Eur. J. Org. 38 G. Boschloo and A. Hagfeldt, Acc. Chem. Res., 2009, 42, 1819–
Chem., 2013, 1916–1926; (b) L. Zani, G. Reginato, 1826.
A. Mordini, M. Calamante, M. Peruzzini, M. Taddei, 39 Z. Wang, M. Liang, L. Wang, Y. Hao, C. Wang, Z. Sun and
A. Sinicropi, M. L. Parisi, F. Fabrizi de Biani, R. Basosi, S. Xue, Chem. Commun., 2013, 49, 5748–5750.
A. Cavallaro and M. Bruzzi, Tetrahedron Lett., 2013, 54, 40 X. Wang, J. Yang, H. Yu, F. Li, L. Fan, W. Sun, Y. Liu,
3944–3948.
Z. Y. Koh, J. Pan, W.-L. Yim, L. Yan and Q. Wang, Chem.
Commun., 2014, 50, 3965–3968.
41 D. P. Hagberg, T. Edvinsson, T. Marinado, G. Boschloo,
A. Hagfeldt and L. Sun, Chem. Commun., 2006, 2245–2247.
19 W. Zhang, Q. Feng, Z.-S. Wang and G. Zhou, Chem.–Asian J.,
2013, 8, 939–946.
20 (a) D. Bevk, L. Marin, L. Lutsen, D. Vanderzande and
W. Maes, RSC Adv., 2013, 3, 11418–11431; (b) L. Zani, 42 D. P. Hagberg, T. Marinado, K. M. Karlsson, K. Nonomura,
M. Calamante, A. Mordini and G. Reginato, in Targets in
Heterocyclic Systems, ed. O. A. Attanasi and D. Spinelli,
P. Qin, G. Boschloo, T. Brinck, A. Hagfeldt and L. Sun, J.
Org. Chem., 2007, 72, 9550–9556.
`
Societa Chimica Italiana, Rome, 2014, vol. 17, pp. 87–124.
43 L.-P. Heiniger, P. G. O'Brien, N. Soheilnia, Y. Yang,
¨
´
21 B. G. Kim, K. Chung and J. Kim, Chem.–Eur. J., 2013, 19,
5220–5230.
N. P. Kherani, M. Gratzel, G. A. Ozin and N. Tetrault, Adv.
Mater., 2013, 25, 5734–5741.
22 Y. Liang, B. Peng, J. Liang, Z. Tao and J. Chen, Org. Lett., 44 P. Wang, S. M. Zakeeruddin, J. E. Moser, M. K. Nazeeruddin,
¨
2010, 12, 1204–1207.
T. Sekiguchi and M. Gratzel, Nat. Mater., 2003, 2, 402–407.
`
23 A. Dessı, M. Calamante, A. Mordini, M. Peruzzini, 45 L. Schmidt-Mende, J. E. Kroeze, J. R. Durrant,
¨
A. Sinicropi, R. Basosi, F. Fabrizi de Biani, M. Taddei,
D. Colonna, A. Di Carlo, G. Reginato and L. Zani, Chem.
Commun., 2014, 50, 13952–13955.
M. K. Nazeeruddin and M. Gratzel, Nano Lett., 2005, 5,
1315–1320.
46 D. P. Hagberg, X. Jiang, E. Gabrielsson, M. Linder,
T. Marinado, T. Brinck, A. Hagfeldt and L. Sun, J. Mater.
Chem., 2009, 19, 7232–7238.
24 (a) A. D. J. Becke, J. Chem. Phys., 1993, 98, 5648–5652; (b)
C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens.
Matter Mater. Phys., 1988, 37, 785–789; (c) P. J. Stephens, 47 Dye D35 is commercially available from Dyenamo AB
F. J. Devlin, C. F. Chabalowski and M. J. Frisch, J. Phys.
Chem., 1994, 98, 11623–11627.
(Sweden). For a list of references regarding applications of
such dye, see: http://www.dyenamo.se/dyenamo_dyes.php.
25 K. C. D. Robson, K. Hu, G. J. Meyer and C. P. Berlinguette, J. 48 Y. Huang, S. Dai, S. Chen, C. Zhang, Y. Sui, S. Xiao and L. Hu,
Am. Chem. Soc., 2013, 135, 1961–1971. Appl. Phys. Lett., 2009, 95, 243503.
26 H. Tian, X. Yang, R. Chen, Y. Pan, L. Li, A. Hagfeldt and 49 F. Fabregat-Santiago, G. Garcia-Belmonte, I. Mora-Sero and
L. Sun, Chem. Commun., 2007, 3741–3743. J. Bisquert, Phys. Chem. Chem. Phys., 2011, 13, 9083–9118.
27 T. Yanai, D. Tew and N. Handy, Chem. Phys. Lett., 2004, 393, 50 Q. Li, L. Lu, C. Zhong, J. Huang, Q. Huang, J. Shi, X. Jin,
51–57. T. Peng, J. Qin and Z. Li, Chem.–Eur. J., 2009, 15, 9664–9668.
28 B. C. Thompson, Y.-G. Kim, T. D. McCarley and 51 L. Yang, Z. Zheng, Y. Li, W. Wu, H. Tian and Z. Wang, Chem.
´
J. R. Reynolds, J. Am. Chem. Soc., 2006, 128, 12714–12725.
Commun., 2015, 51, 4842–4845.
32668 | RSC Adv., 2015, 5, 32657–32668
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