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Cite this: Chem. Commun., 2014,
50, 13952
Received 6th August 2014,
Accepted 11th September 2014
DOI: 10.1039/c4cc06160h
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Organic dyes with intense light absorption
especially suitable for application in thin-layer
dye-sensitized solar cells†
Alessio Dessı̀,ab Massimo Calamante,ab Alessandro Mordini,ab Maurizio Peruzzini,b
Adalgisa Sinicropi,c Riccardo Basosi,c Fabrizia Fabrizi de Biani,c Maurizio Taddei,c
Daniele Colonna,d Aldo Di Carlo,d Gianna Reginato*b and Lorenzo Zani*b
www.rsc.org/chemcomm
Three new thiazolo[5,4-d]thiazole-based organic dyes have been
designed and synthesized for employment as DSSC sensitizers.
Alternation of the electron poor thiazolothiazole unit with two
propylenedioxythiophene (ProDOT) groups ensured very intense
light absorption in the visible region (e up to 9.41 104 M
1
cm
1
in THF solution). The dyes were particularly suitable for application in
transparent and opaque thin-layer DSSCs (TiO2 thickness: 5.5–6.5 lm,
efficiencies up to 7.71%), thus being good candidates for production
of solar cells under simple fabrication conditions.
Since their discovery in 1991,1 dye-sensitized solar cells (DSSCs)
have been considered a promising alternative to traditional
silicon-containing photovoltaic devices. In a DSSC, the power
conversion efficiency (Z) is strongly dependent on the sensitizer,
since the latter is involved in both the light absorption and
charge transfer processes.2
Traditionally, the most efficient DSSCs were built with
ruthenium complexes as sensitizers,2,3 but their performances
have been surpassed thanks to the use of porphyrin-based dyes4
and, more recently, inorganic lead perovskites.5 Despite that,
employment of organic dyes still offers some potential advantages,
such as the possibility of avoiding the use of expensive and/or
hazardous metals and the capability to harvest light efficiently
due to their high molar extinction coefficients.6 Indeed, organic
sensitizers were reported to give DSSCs with 411% efficiency,7
sometimes even coupled with excellent stability.8 Organic dyes
with a superior light-absorption ability could be particularly
useful for the sensitization of thin TiO2 layers,9 both for the
development of solid-state solar cells10 and the construction of
colored, transparent modules to be applied in building-integrated
photovoltaics (BIPV).11 In addition, the discovery of new sensitizers
providing high efficiencies even under simple fabrication conditions, without deposition of multiple TiO2 layers, application of
surface treatments or excessive use of additives, would be very
important to reduce the current economic and environmental
costs12 of DSSC technology, favoring its commercial exploitation. Therefore, with the aim of finding a class of sensitizers
with the appropriate electronic and physico-chemical properties for application in thin-layer DSSCs, we started a thorough
investigation targeting new organic D–p–A dyes endowed with
innovative heterocyclic cores.
During such studies, we described for the first time sensitizers bearing a thiazolo[5,4-d]thiazole (TzTz) ring as their central
unit,13,14 among which compound TTZ1 (Fig. 1) gave the best
photovoltaic efficiency of 3.53% in the presence of chenodeoxycholic acid (CDCA) as a co-adsorbent.13 The relatively low Z yielded
a
Dipartimento di Chimica ‘‘U. Schiff’’, Università degli Studi di Firenze,
Via della Lastruccia 13, 50019 Sesto Fiorentino, Italy
b
Istituto di Chimica dei Composti Organometallici (CNR-ICCOM), Via Madonna del
Piano 10, 50019 Sesto Fiorentino, Italy. E-mail: [email protected];
Fax: +39 055 5225203; Tel: +39 055 5225245
c
Dipartimento di Biotecnologie, Chimica e Farmacia, Università degli Studi di
Siena, Via A. Moro 2, 53100 Siena, Italy
d
Center for Hybrid and Organic Solar Energy (C.H.O.S.E.), Università di Roma
‘‘Tor Vergata’’, Via del Politecnico 1, 00133 Rome, Italy
† Electronic supplementary information (ESI) available: Experimental details,
synthetic procedures for compounds 2–9 and TTZ3–5, copies of the 1H- and
13
C-NMR spectra of all compounds, Fig. S1–S9, Scheme S1 and Table S1. See DOI:
10.1039/c4cc06160h
13952 | Chem. Commun., 2014, 50, 13952--13955
Fig. 1
Structures of dye TTZ1 and new organic sensitizers TTZ3–5.
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by TTZ1 was likely due to a combination of factors, such as its
insufficient light harvesting ability, its limited solubility and its
tendency to form aggregates onto the TiO2 surface.
We reasoned that replacement of the hexylthiophene rings
present in TTZ1 with more electronrich and less aromatic units
should increase the HOMO energies of the resulting molecules,
reducing their HOMO–LUMO gap and leading to a red-shift of
their absorption spectra. Moreover, an improvement in light
harvesting was also expected by alternating such moieties with
the electronpoor TzTz and cyanoacrylic units.15 Finally, introduction of further alkyl chains on the sensitizer backbone was
supposed to improve their solubility and reduce aggregation.
Accordingly, we modified the dye scaffold by introducing two
bis-pentyl propylenedioxythiophene (ProDOT) groups (TTZ3).16
In addition, to modulate the electronic properties of the dyes, we
decided to decorate the donor unit with different electronrich
substituents, namely hexyloxy and hexylthio groups (TTZ4,5).
The structures of the new organic dyes are shown in Fig. 1.
The energy and shape of the frontier molecular orbitals
(FMOs) of compounds TTZ3–5 were computed by means of
Density Functional Theory (DFT) calculations at the B3LYP/
6-31G* level17 (alkyl chains were replaced with methyl groups to
reduce computational effort). As an example, the HOMO and
LUMO orbitals for dye TTZ5 are reported in Fig. 2 and compared
with those previously found for dye TTZ1,13 while the FMOs for
all new dyes are shown in Fig. S1 (ESI†).
In line with our expectations, dyes TTZ3–5 showed smaller
HOMO–LUMO gaps compared to TTZ1, mostly due to HOMO
destabilization. Orbital shapes were similar in all cases, with
the HOMO–1 distributed along the entire conjugated system
and the HOMO and LUMO mostly localized on the donor and
Fig. 2
Frontier molecular orbitals of dyes TTZ112 and TTZ5.
Table 1
Spectroscopic and electrochemical data for dyes TTZ3–5
Dye
lmax abs.a [nm] (e [M
TTZ3
TTZ4
TTZ5
510 (81 400)
518 (86 600)
510 (94 100)
a
THF solution.
b
1
cm 1])
acceptor moieties, respectively. In addition, the absorption
maxima (lamax), oscillator strengths (f) and vertical excitation
energies (Eexc) for TTZ3–5 were also calculated by means of
time-dependent DFT at the PCM/CAMB3LYP/6-31G* level
(Table S1, ESI†).18,19 Results indicated that the new dyes should
show intense absorptions with maxima above 500 nm, stemming from mixed HOMO - LUMO and HOMO 1 - LUMO
transitions.
Supported by the above computational data, we embarked
on the synthesis of the new molecules (see Scheme S1, ESI†).
First, we prepared the central heterocyclic unit by modification
of our microwave-assisted thiazolothiazole synthesis,20 changing
the solvent from nitrobenzene to n-butanol and adjusting the
reaction time and temperature in order to improve substrate
conversion and facilitate product isolation by column chromatography. After electrophilic iodination,13 two consecutive Suzuki
cross-coupling reactions allowed the introduction of the donor
and acceptor moieties. To minimize formation of the symmetrical double coupling products, in the first reaction a strictly
stoichiometric amount of boronic reagent has to be used, and
the reaction has to be stopped before full conversion of the
starting material, resulting in low yields of products 4–6; nevertheless, in all cases a certain amount of unreacted diiodide 3
(30–66%) could be recovered after purification for later reuse.
Furthermore, in the second coupling step, use of MW heating
allowed to shorten reaction times, thus limiting the formation of
byproducts (for example stemming from protodehalogenation
processes). The synthesis was then completed by Knoevenagel
condensation of aldehydes 7–9 with cyanoacetic acid to give
compounds TTZ3–5, which were fairly soluble in various organic
solvents.
In THF solution, the new dyes showed broad and intense
absorptions in the visible region (Fig. S2, ESI†) with maxima in
the 510–518 nm range, red shifted of ca. 40 nm compared to
TTZ1,13 and extremely high molar absorptivities, reaching up to
9.41 104 M 1 cm 1 in the case of TTZ5 (Table 1). Optical band
gaps (E0-0), obtained from the intersection of the normalized
absorption and emission spectra in THF (Fig. S3, ESI†), were
comprised in the 2.19–2.25 eV range. Attachment of the dyes to
nanocrystalline TiO2 caused further broadening and a slight
blue-shift of their absorption spectra, a phenomenon that
was attributed to deprotonation of the carboxylic acid groups
(Fig. S4, ESI†).21
The density of dyes attached to the semiconductor was
measured via the desorption method, and was found to be
slightly higher for TTZ5 compared to the other two sensitizers
(up to 1.19 10 7 mol cm 2). Ground-state oxidation potentials
(Eox), measured by means of cyclic voltammetry (Fig. S5, ESI†),
lmax em.a [nm]
lmax abs. on TiO2 [nm]
Eoxb [V]
587
601
573
484
491
487
1.10
0.85
0.99
potential vs. NHE. c Calculated from Eox
This journal is © The Royal Society of Chemistry 2014
E0-0.
d
Eox*c [V]
1.15
1.34
1.25
E0-0d [eV]
G [107 mol cm 2]
2.25
2.19
2.24
0.99
1.08
1.19
Estimated from the intersection of normalized absorption and emission spectra.
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were more positive than the redox potential of the I /I3 couple
(+0.4 V vs. NHE), indicating possible regeneration of the sensitizers.
Finally, excited state oxidation potentials, obtained from the expression Eox* = Eox E0-0, are more negative than the conduction band
edge of TiO2 (approx. 0.5 V vs. NHE).
Photovoltaic performances of dyes TTZ3–5 were assessed by
fabrication of thin-layer DSSCs, which differed only for the nature
of the TiO2 layer employed. A first series of devices featured a
transparent semiconductor layer with a thickness of 5.5 mm,
while a second series of cells contained an opaque TiO2 layer
with a thickness of 6.5 mm. In view of the possible practical
Fig. 3 Typical J/V curves (a) and IPCE spectra (b) for transparent cells built
with dyes D5, Z907 and TTZ3–5.
Table 2
application of thin-film DSSCs mentioned above, we kept device
fabrication as simple as possible using only commercially available materials. In particular, different from other studies on thinlayer DSSCs,22,23 we did not perform any superficial treatment
(e.g. treatment with aq. TiCl4) nor on the conductive substrate nor
on the semiconductor layer, and we did not employ any lightscattering layer in the photoanode. In addition, we decided to use
the I /I3 redox couple, since that is the one that works best with
ruthenium sensitizers and is therefore routinely used for BIPV
applications.
For both types of devices, performances obtained with the
new sensitizers were compared with those of standard organic
dye D524 and Ru-based sensitizer Z907.25 Typical J/V curves
obtained for the transparent devices are shown in Fig. 3a,
together with the corresponding IPCE spectra (Fig. 3b), while
the relevant photovoltaic parameters are reported in Table 2.
Transparent cells built with the new dyes gave efficiencies
between 4.85% and 7.39%, with TTZ5, bearing a hexylthiosubstituted triphenylamine moiety, being the best sensitizer.
The sulphur-containing donor unit was introduced since it was
reported to enhance dye regeneration and increase Voc compared
to its oxygenated analogue,26 a result confirmed by our data
(compare Voc of TTZ5 with that of TTZ4). On the other hand, the
lower photocurrents measured for TTZ3-4 compared to TTZ5 could
be caused either by the inferior light-harvesting ability of TTZ3
(lowest e, lmax on TiO2 and G among all dyes) or by inefficient dye
regeneration for TTZ4 (Eox less than 0.5 V higher than the redox
potential of the I /I3 couple). Finally, the relatively low fill factors
observed for all cells (59–63%) were likely due to the small
thickness of the TiO2 layer in combination with the absence of a
blocking layer, which probably favoured electron recombination
between the conductive substrate and the electrolyte.
Remarkably, all new dyes gave higher efficiencies than standard
organic dye D5, and compound TTZ5 even outperformed Ru-based
sensitizer Z907 (Z 7.39% vs. 5.51%). Such a result was particularly
significant, since Z907 is one of the sensitizers of choice for
applications in BIPV, thanks to its excellent oxidative and anchoring
stability.27 All compounds gave broad IPCE spectra with onsets
above 700 nm, in agreement with their UV-Vis spectra recorded on
TiO2: in particular, the IPCE of dye TTZ5 was superior to Z907 in the
370–480 and 550–660 nm regions, consistent with its higher Jsc. The
data obtained for opaque cells (Fig. S6 and S7, ESI†) confirmed
Photovoltaic parameters for DSSCs built with dyes D5, Z907 and TTZ3–5a
Transparent DSSCsb
Dye
CDCAd
D5
Z907
TTZ3
+
TTZ4
+
TTZ5
+
Opaque DSSCsc
Jsc [mA cm 2]
Voc [V]
ff [%]
Z [%]
9.60
13.50
12.79
15.62
12.18
14.27
16.20
16.59
0.624
0.686
0.687
0.697
0.669
0.692
0.716
0.717
63
60
61
60
60
59
63
59
3.78
5.51
5.41
6.55
4.85
5.85
7.39
7.08
CDCAd
+
+
+
Jsc [mA cm 2]
Voc [V]
ff [%]
Z [%]
11.04
13.85
13.65
16.52
12.99
15.18
16.05
18.33
0.619
0.687
0.671
0.683
0.665
0.675
0.721
0.709
58
58
60
58
57
55
60
59
3.99
5.61
5.45
6.54
4.93
5.71
6.91
7.71
a
Average values for three devices, measured under AM 1.5G simulated solar irradiation (incident power 100 mW cm 2). 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).
13954 | Chem. Commun., 2014, 50, 13952--13955
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those seen for transparent devices, with TTZ5 being once again
the best sensitizer (Z 6.91%). In general, Voc and Jsc values were
similar in the two classes of cells, but fill factors were slightly
lower in opaque devices, likely due to differences in the TiO2
layer morphology.
To verify whether the bis-pentyl substituted ProDOT units,
with their tri-dimensional arrangement of alkyl chains,16 were
indeed effective in reducing dye aggregation, we also built two
series of cells with the addition of 1 mM CDCA in the sensitizing
bath (Fig. S8 and S9, ESI†). Interestingly, an increase in Jsc was
observed in all cases, with TTZ3 showing the largest enhancement.
This could be explained considering that TTZ3 lacks the terminal
alkyl chains found in TTZ4,5, and should thus be more prone to
aggregation (consistent with its lower solubility). More generally,
Z values varied by 4–21% in the presence of the co-adsorbent
(Table 2), which was less than that previously observed with
TTZ1,12 suggesting that in this case CDCA has a smaller impact
on the dye aggregation state on TiO2. The best result was
obtained when CDCA was used in TTZ5-containing opaque cells,
providing an average efficiency of 7.71% with an impressive Jsc of
18.33 mA cm 2.
In conclusion, we have designed and synthesized three new
thiazolo[5,4-d]thiazole-based organic dyes (TTZ3–5) with the
aim of using them as sensitizers in thin-layer DSSCs (TiO2
thickness r 6.5 mm). Gratifyingly, the new compounds displayed
broad and intense absorption spectra in the visible region, with
exceptional molar absorptivities up to 9.41 104 M 1 cm 1. Solar
cells built with TTZ3–5, both transparent and opaque, gave good
power conversion efficiencies (up to 7.71%), which in the case of
TTZ5 were clearly superior to those obtained with standard Ru-dye
Z907. Further studies are currently underway to gain insight into
the charge transfer dynamics of dyes TTZ3–5, understand the
origin of the superior performances recorded with TTZ5, prepare
new and improved sensitizers and, finally, test the efficiency and
stability of the corresponding DSSCs under optimized conditions.
The authors thank Regione Toscana (‘‘Fotosensorg’’ project),
the ‘‘Ente Cassa di Risparmio di Firenze’’ foundation (‘‘IRIS’’
project) and MIUR (‘‘Progetto Premiale 2011: Produzione di
Energia da Fonti Rinnovabili’’) for financial support. A. S. and
R. B. thank CINECA and C.R.E.A. (Colle Val D’Elsa, Siena, Italy)
for the availability of high performance computing resources.
D. C. and A. D. C. acknowledge the support of Regione Lazio
and MIUR (PRIN 2010 ‘‘DSSCX’’ project).
This journal is © The Royal Society of Chemistry 2014
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