KuQuinones as sensitizers for NiO based p-type dye-sensitized solar cells

Article abstract of DOI:10.1039/c6nj03466g

A new series of KuQuinones (KuQs) have been synthesized and employed as dye-sensitizers for NiO-based p-type dye-sensitized solar cells (p-DSSCs). KuQs are pentacyclic quinoid compounds which are characterized by a fully conjugated structure that is responsible for the strong and broad absorption in the visible spectrum. The HOMO/LUMO states of KuQs considered here have matching energy levels with the upper edge of the NiO valence band and I^?/I₃^? redox potential energy. These features render such compounds suitable for NiO sensitization in p-DSSCs. The new carboxylic acid-substituted KuQ derivatives proposed here differ in the length of the alkyl chain. The JV characteristic curves and the external quantum efficiency spectra have been recorded. The results showed that the performances of KuQ-sensitized cells were similar to that of the benchmark sensitizer erythrosine B (Ery B), despite the lack of electronic conjugation between the anchoring group and the light absorbing unit. This result led us to hypothesize that the photoinduced charge transfer between the excited KuQ dyes and the NiO electrode occurred through space and not via chemical bonds as it usually occurs in these systems. The mechanism of charge transfer through space has been supported by data from IR spectroscopy.

Full text of DOI:10.1039/c6nj03466g

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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KuQuinones as sensitizers of NiO based p-type dye-sensitized
solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
Matteo Bonomo,a† Federica Sabuzi,b† Aldo Di Carlo,c Valeria Conte,b Danilo Dinia* and Pierluca
Gallonib*
DOI: 10.1039/x0xx00000x
A new series of KuQuinones (KuQs) has been synthesized and employed as dye-sensitizers of NiO-based p-type dye-
sensitized solar cells (p-DSSCs). KuQs are pentacyclic quinoid compounds which are characterized by a fully conjugated
structure that is responsible of the strong and broad absorption in the visible spectrum. The HOMO/LUMO states of the
KuQs here considered match their energy levels with the upper edge of NiO valence band and I-/I3- redox potential energy.
These features render such compounds suitable for NiO sensitization in p-DSSC. The new carboxylic acid-substituted KuQs
derivatives here proposed differ for the length of the alkyl chain. The JV characteristic curves and external quantum
efficiency spectra have been recorded. Results showed that performances of KuQs-sensitized cells were similar to that of
benchmark sensitizer erythrosine B (Ery B), despite the lack of electronic conjugation between the anchoring group and
the light absorbing unit. This result led us to hypothesize that the photoinduced charge transfer between excited KuQs
dyes and the NiO electrode occurred through space and not via chemical bonds as usually occurs in these systems. The
mechanism of charge transfer through space has been supported by data of IR spectroscopy.
www.rsc.org/
Introduction
Theoretical conversion efficiencies up to 1.5 times larger than
the respective DSSCs with single photoactive electrode can be
p-type
dye-sensitized
solar
cells
(p-DSSCs)
are
obtained with t-DSSCs.15The most widely used photocathodic
material for p-DSSCs is NiO (a p-type semiconductor) in the
configuration of thin film (thickness < 3.5 µm)16-20 with
nanostructured features and wide bandgap (Eg > 3.2 eV).21 The
open morphology of mesoporous NiO films allows the
anchoring of large amounts of dye (ca. 10-11 moles per cm2 of
electrode),14 with consequent efficient sensitization of the p-
DSSC photocathode. The working principle of a p-DSSC consists
in the electron transfer (ET) from NiO cathode to the
photoexcited dye-sensitizer with successive ET from the
transiently reduced dye to the oxidized form of a redox shuttle
(Fig. 1).22 Beside the studies of NiO electrodes feasibility23-26
and the investigation of alternative p-type semiconductors,27,28
the research on p-DSSCs (and t-DSSCs) has also considered the
preparation of novel organic dyes for progressively performing
devices. Among these, the classes of erythrosines,11,29
coumarines,30-33 triphenylamines,34,35 perylene imides,13,18,36
isoindigo dyes,37 squaraines,38,39 porphyrins,40 BODIPYs,14,41
push–pull conjugated molecules,42,43 diketopyrrolopyrroles,44
and pyridyl metal complexes45-47 have been explored for
improving the effectiveness of hole photoinjection and for
preventing recombination phenomena.48 Moreover, recently
Zhang and Cole investigated the use of different anchoring
groups for n-type DSSC.49 Nevertheless, as far as we are aware,
just few examples of non-conjugated linkers for p-DSSC have
been reported.50 As an example, in 2007 Ooyama and co-
workers developed some carbazole-based fluorescent dyes
photoelectrochemical devices that convert solar radiation into
electrical power through a photoinduced process of redox
reduction.1-4 The most important feature related to the
development of p-DSSCs is the nature itself of the
photoactivated process, i.e. the electrochemical reduction of
an appropriate species following the charge separation at the
light absorbing cathode. In addition to the electrical power, a
photoinduced reduction process can lead also to the formation
of chemicals having a very strong economic and environmental
impact, such as solar generated fuels (H2 in primis),5 as well as
the products of direct CO2 photo-transformation.6-8
Currently, the highest reported efficiency for a NiO based p-
DSSC is 2.5%.9 Interestingly, p-DSSCs can be implemented as
parts of tandem DSSCs (t-DSSCs), where both semiconducting
electrodes are properly sensitized with dyes having
complementary absorption features.10-14
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Moreover, for the first time an alkyl chain longer than propyl
(i.e. octyl) has been used as anchoring unit. The performances
of KuQs photosensitizers of mesoporous NiO cathodes for p-
DSSCs have been tested and compared with the one of the
common benchmark sensitizer, EryB.56
Results and discussion
Synthesis and characterization of the dyes
The novel substituted KuQs derivatives have been synthesized
following a procedure previously reported in the literature.54
The general reaction requires 2-hydroxy-1,4-naphtoquinone as
starting compound, which is dissolved in DMSO in the
presence of an excess of an alkyl bromide, an inorganic base
and a catalytic amount of ferrocene. The one-pot reaction
leads to the formation of a pentacylcic, fully conjugated
Fig. 1. Levels of the electrical potential, E, that participate in the photoinduced
electron transfer process. Separation (1) indicates the width of the open circuit voltage
VOC for a NiO based p-DSSC. The redox couple I3-/I- represents the redox mediator (or
redox shuttle). In the scheme, the dye-sensitizer unit (D) absorbs light for the
photoactivated electron transfer. Dotted arrows (2) and (3) show two possible
processes of electronic recombination (unwanted phenomena) successive to the
charge separation step induced by dye photoactivation.
compound,
where
two
molecules
of
the
hydroxynaphtoquinone are condensed with one molecule of
the alkyl bromide. Unexpectedly, the reaction proceeds with
the loss of one carbon atom, apparently from the alkyl
bromide; however, a full understanding of the mechanism is
still not achieved. By changing the nature of the alkyl bromide,
a small library of KuQuinone derivatives has been already
synthesized: compounds differ in side chain length or
functional group. In the perspective of employing KuQs as dye-
sensitizers in p-type DSSC, the synthesis of KuQs with a
carboxylic acid as terminal group bound on the alkyl side chain
was mandatory in order to covalently link the organic dye to a
metal oxide electrode. In this context, several attempts to
obtain the desired compounds using carboxylic acid-
functionalized alkylbromide (such as 6-bromohexanoic acid or
11-bromoundecanoid acid) as alkyl bromide source have been
performed, but no products were obtained. Likely, in the
reaction conditions adopted, the intra-molecular substitution
reaction between the carboxylate group and the alkyl bromide
portion occurs, making the reagent unsuitable for the
reaction.57 Alternatively, KuQs containing a carboxylic acid
group could be easily obtained by hydrolysis of the
corresponding esters (Scheme 1). Specifically, ethyl 6-
bromohexanoate and ethyl 11-bromoundecanoate have been
synthesized and subsequently used as reagents in KuQs
synthesis, leading to 1-(3-ethoxycarbonylpropyl)KuQuinone
and for the first time a through space ET has been suggested.51
This result provided a new strategy for dyes-design: in fact,
inspired by such approach Hao et al. synthesized a new class of
D-p-A dyes (η
= 3.7 %)52 and quite recently, they reported the
employment of similar dyes as sensitizers for NiO-based p-type
DSSC.53
Here we report for the first time the adoption of a novel class
of organic dyes, called KuQuinones (KuQs),54 as sensitizers in p-
type DSSCs. These compounds are able to harvest light in the
visible region of the spectrum, due to their pentacyclic and
totally conjugated structure, which makes them suitable dyes
in solar-energy conversion devices.55 In this work, properly
substituted KuQuinones derivatives, which structures are
reported in Fig. 2, have been synthesized, with the aim to
anchor them on NiO electrodes. It is important to note how
these KuQs employed non conjugated linkers as anchoring
groups (Fig. 2).
(KuQ3CO2Et)
and
1-(8-ethoxycarbonyloctyl)KuQuinone
(KuQ8CO2Et), respectively. KuQ3CO2Et and KuQ8CO2Et have
been obtained in good yields if compared with the other KuQs
derivatives previously synthesized.54 Moreover, the adopted
purification procedures resulted much easier than the other,
because of the formation of lower amounts of undesired side
products.
KuQCH3
KuQ3CO2H
KuQ8CO2H
Scheme 1. Synthesis of KuQuinones
Fig. 2. Structures of the new KuQs employed as dye-sensitizers in this study.
2 | J. Name., 2012, 00, 1-3
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Hydrolysis of the ester groups led to the formation of the We outline here that KuQ3CO2H LUMO energy is slightly lower
corresponding carboxylic acids in very high yields. than that of KuQCH3 and KuQ8CO2H. This evidence is quite
KuQ3CO2H and KuQ8CO2H showed very low solubility in all important for p-type DSSC: if the energy level of LUMO is
conventional organic solvents, except in THF. UV-vis positioned far from the upper edge of the valence band of the
absorption spectra recorded in this solvent are reported in Fig. semiconductor, the undesired phenomenon of recombination
3. They have been compared with the absorption spectrum of is minimized, thus an energetically distant LUMO entails lower
1-methylKuQuinone (KuQCH3) in toluene, which synthesis was recombination rate.
previously reported.54
The energy values of the HOMO-LUMO states calculated with
All the spectra were characterized by two intense bands DFT are reported in Table 1. An experimental confirmation of
between 420 and 600 nm with molar extinction coefficients up the validity of such theoretical results is given by cyclic
to 15000 M-1cm-1. Another absorption band with lower voltammetries of these dyes dissolved in dichloromethane (see
intensity, was detected between 350 and 400 nm; this band SI). Specifically, the energy level of LUMO has been calculated
becomes much more intense upon deprotonation of enol by measuring the reduction potential (Ered) of each dye, while
oxygen.55 The partial blue shift of the novel synthesized KuQs the energy level of HOMO (that is higher than 2 V) has been
with respect to the absorption spectrum of KuQCH3 is probably estimated according to the following equations:60
due to solvent properties, which can affect the spectral
profiles of similar compounds.57
E (HOMO) = E (LUMO) - Eg
(1)
The energy levels of HOMO-LUMO orbitals have been
determined with density functional theory (DFT) and B3LYP
hybrid functional with 6-311G dp basis set.59 HOMO and LUMO
orbitals of KuQuinone dyes are shown in Fig. 4.
ͥͦͨͦ
Eg = hν = #ꢀ Ɣ ϋ
(2)
ϋ
-¬±£²
where Eg corresponds to the optical energy gap that can be
calculated through UV-visible spectroscopy, using λonset as the
longest absorption wavelength.
For all compounds, both HOMO and LUMO show electronic
conjugation due to π-electrons. Specifically, HOMO-orbitals
are centered in the middle region of the pentacyclic systems
and they are extended up to the carbonyl groups, where the
electronic density is much higher. Moreover, they are partially
delocalized on the alkyl side chain. In this regard, the long C
sp3 chain in KuQ8CO2H does not allow the carboxylic acid
group to electronically interact with the pentacyclic core. For
this reason, the scheme of the energies of the frontier orbitals
is very similar to that of KuQCH3. In KuQ3CO2H, the HOMO
orbital is extended along the short alkyl side chain, up to the
carboxylic acid, which can interact with the pentacyclic system
thus lowering the orbital energy. Concerning the LUMO-level,
these orbitals are distributed all over the π-system, showing a
complete delocalization. The position of the LUMO level is not
influenced by the side chain length and the presence of
carboxylic acid group but it is exclusively dependent from the
conjugated system.
For all compounds the energy values of HOMO and LUMO
theoretically and experimentally calculated show
a
discrepancy comprised between 0.5 and 0.85 eV, as commonly
observed for similar comparisons.60a,c,61 Unlike the theoretical
calculations - where HOMO and LUMO energies relative to
KuQ3CO2H were slightly lower than those of KuQCH3 and
KuQ8CO2H - experimental data show comparable energies for
all compounds.
All the KuQs derivatives here proposed have frontier orbitals
with appropriate energy in relation to the position of the
upper edge of NiO valence band and I-/I3 redox energy level.
-
This makes our compounds suitable for NiO sensitization and
their consequent application as dyes in the corresponding p-
DSSC.
Table 1. HOMO-LUMO energy levels for KuQs dyes.
Theoretically calculateda
Experimentally calculated
HOMO
(eV)
LUMO
(eV)
Eg
HOMO
(eV)
LUMO
(eV)b
-4.39
Dye
(eV)
2.21
-6.60
KuQ3CO2H
KuQ8CO2H
KuQCH3
-6.61
-6.30
-6.32
-3.80
-3.52
-3.50
2.19
2.17
-6.56
-6.51
-4.37
-4.34
a
Frontier orbitals energy levels have been estimated through DFT calculations
using B3LYP hybrid functional with 6-311G dp basis set; energies are referred to
the vacuum.
b
Values have been calculated using cyclic voltammetry conducted in a 0.1 M
solution of TBAP in CH2Cl2 at a scan rate of 100 mV/s. A standard calomel
electrode (SCE) was used as the reference electrode, a platinum wire as counter
electrode and a platinum disk as working. Data in the table are referred to the
NHE potential, set 4.44 eV below the vacuum level. Specifically, energy of LUMO
level has been calculated using the following equation: ELUMO= -e [Ered + 4.4].60
Fig. 3. UV-vis absorption spectrum of KuQ3CO2H (red line) and KuQ8CO2H (orange line)
in THF and KuQCH3 in toluene (blue line).
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a)
b)
c)
Fig. 4. Representation of HOMO (front and side view, on the left) and LUMO (front and side view, on the right) orbitals for KuQ3CO2H (a), KuQ8CO2H (b) and KuQCH3 (c).
The presence of such voids in the NiO electrode surface
NiO characterization
ensures a high contact area between the sensitized layer of
NiO thin films have been prepared as deposits on a FTO-
NiO and the liquid electrolyte throughout the whole thickness
covered glass panel through screen-printed technique (see
of the oxide film. Since NiO is an electroactive material,
Experimental).
another important consequence of its nanoporosity is the
Screen-printed NiO presented an open and scale-like
direct proportionality between thickness and current
morphology with cavities and apertures having a linear size in
exchanged by the electrode. The crystal structure, as well as
the order of few hundreds of nanometers (Fig. 5).
the surface morphology and the electrical connectivity
between nanostructured units affect the device behavior since
they are all responsible of the charge transport between NiO
and the supporting substrate.62 Previous studies demonstrated
that in NiO-based p-DSSC the optimum film thickness is
approximatively 2,5 µm.26
NiO has been spectrophotometrically characterized before the
assembly in the p-DSSC (Fig. 6). The transmittance of NiO in
the visible spectrum was quite high, thus indicating the good
transparency of the electrode. Subsequently, NiO electrodes
were functionalized with the novel synthesized dyes in order
to test their possible application in p-type DSSC. In general,
optimal dyes need to be chemisorbed on the oxide surface in
Fig. 5. SEM image of NiO electrode.
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KuQ8CO2H occurs through the carboxylic group bound on the
side chain (Fig. 8).
ATR-FTIR spectroscopy clearly confirmed the formation of such
bond. In fact, IR spectra of pure KuQ3CO2H and KuQ8CO2H
show a well defined peak respectively at 1708 and 1695 cm-1,
which are ascribable to the C=O stretching peak in the
carboxylic acid group (see Supp. Info). Once adsorbed on NiO
such signal totally disappear. This experimental evidence
unambiguously indicates that the bond between NiO surface
and KuQs occurs through the carboxylic anchoring group53,64
(Fig. 9), thus confirming the bidentate bridging structure
reported in Fig. 8.64
Fig. 6. Transmittance spectra of: non-functionalized NiO bare (black line), EryB-
functionalized NiO (purple line), KuQ3CO2H-functionalized NiO (red line), KuQ8CO2H-
functionalized NiO (orange line) and KuQCH3-functionalized NiO (blue line).
order to be strongly anchored on the oxide.
In this regard, KuQs derivatives having a carboxylic group on
the side chain have been prepared (Fig. 2). Moreover, looking
at the general structure of KuQuinones, it can be noticed that
the enol group and the vicinal carbonyl oxygen are at the
appropriate distance for coordinating transition metal ions,
such as nickel ions on the electrode surface. This geometry of
coordination allowed the binding of similar organic
compounds, such as 4-hydroxyanthraquinone derivatives, to
Ti4+ ions in TiO2 electrodes, generating devices having energy
conversion efficiencies up to 1.5%.63 Likewise, the sensitization
of NiO through the coordination of KuQs by nickel cations is
expected, with the opportunity to bind also KuQCH3 to the NiO
electrode surface (Fig. 7). However, it can be observed that
transmittance spectra of the KuQuinones adsorbed on NiO
electrodes were only slightly different from the reference
electrode (Fig. 6). In particular, if compared to the sharp, well
defined peak obtained with EryB dye, KuQ3CO2H and
KuQ8CO2H spectra showed a very broad peak in the region
between 460 and 630 nm, which probably indicates the
presence of aggregates. In fact, it is already known that the
planar, pentacyclic and fully conjugated structure of KuQs may
favor aggregation phenomena on electrode surfaces.55
Conversely, transmittance spectrum of KuQCH3 on NiO was
comparable with the non-functionalized NiO, indicating that
the expected coordination between KuQuinones and NiO
through enol and carbonyl oxygen atoms did not occur. This
evidence clearly makes the KuQCH3 dye unsuitable for DSSC
application. On the other hand this finding constitutes a
further confirmation that the chemisorption of KuQ3CO2H and
Fig. 8. Representation of KuQnCO2H anchored on NiO.
Fig. 9. FT-IR spectra of KuQ3CO2H (on the top) and KuQ8CO2H (on the bottom) pure
(black line) and on NiO (red line). NiO background has been subtracted from each
spectrum.
Fig. 7. Expected KuQCH3 coordination to NiO.
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Table 2. Key parameters deducted by the JV curves. Errors have been calculated on the
KuQs-sensitized p-DSSC
average of 3 similar devices.
The KuQ3CO2H and KuQ8CO2H sensitized NiO electrodes have
been employed as photocathodes in p-type DSSCs. The
performances obtained with KuQs-sensitized DSSC have been
compared with the one of EryB-sensitized DSSC. The JV curves
and the relative photoelectrochemical parameters are
reported in Figure 10 and Table 2, respectively. Open circuit
potential (VOC) values were very similar one to each other. The
higher value of 98.7 mV has been measured for KuQ8CO2H. At
a structural level, the higher VOC measured in the KuQ8CO2H
sensitized cell is consistent with the longer side chain that
Dye
VOC (mV)
JSC (mA/cm2)
FF (%)
η (%)
Ery B
90.6±0.8
92.2±0.9
99.0±1.0
0.74±0.06
0.74±0.04
0.63±0.04
35.0±0.6
35.9±0.3
36.6±0.3
0.023±0.001
0.025±0.002
0.022±0.001
KuQ3CO2H
KuQ8CO2H
as we could expect, although the latter one contains a longer
alkyl side chain (5 sp3 carbon atoms). In this context, it should
be considered that the long side chain may be able to bend on
itself: in this condition the KuQ core of KuQ8CO2H may be
approximately at the same distance of KuQ3CO2H from the
oxide surface with consequent generation of devices with
similar efficiency. This explanation should be considered as
tentative and some different experiments have to be
performed in order to properly prove it; nevertheless it is
strongly consistent with the reported results (see IR analysis).
Parameters obtained by JV curves were in agreement with
those obtained by IPCE (Incident photon-to-current conversion
efficiency) analysis (Fig. 11).
produces
a strong effect of electrode passivation, thus
reducing recombination phenomena.
Fill Factor (FF) values were quite similar for all the dyes. The
most important difference has been observed for the short
circuit current density (JSC): in fact EryB and KuQ3CO2H devices
exceeded the value of 0.7 mA/cm2. This is probably due to the
higher dye loading if compared with KuQ8CO2H. Even though
Ery B and KuQ3CO2H induced the same current density in the
p-DSSC, the mechanisms of charge transfer appeared different.
In fact, for Ery B sensitizer, as for other well known dyes, the
photoproduced electron is transferred from the dye to the
From figure 11 two different contributions can be highlighted:
an intense peak (about 8-10%) between 320 and 450 nm,
which is due to the NiO bare (mostly from Ni3+ centers) and a
second signal that is due to the photoexcitation of the dyes at
wavelengths higher than 500 nm. The latter peak is specific of
the dye: as already reported in trasmittance spectra, Ery B
peak was quite sharp whereas the KuQs ones were broad.
It is mandatory to stress how both peaks contribute to the
overall cell efficiency. Interestingly, KuQ8CO2H and KuQ3CO2H
showed very similar IPCE spectra, although the latter has a
higher overall efficiency than KuQ8CO2H. Furthermore, from
IPCE spectra it is evident that the photoelectrochemical
contribution of NiO became lower upon sensitization. In
particular, KuQ8CO2H showed higher APCE (Absorbed Photon-
to-current Conversion Efficiency, not shown) but lower LHE
(Light Harvesting Efficiency) if compared with KuQ3CO2H.
semiconductor surface through
electronically conjugated bonds.
a conjugated system of
On the contrary, by using KuQuinones dyes, such mechanism is
not practicable. In fact, the anchoring group is linked to the
acceptor unit through a fully saturated alkyl chain, therefore
an alternative but equally efficient mechanism occurs. Hence,
the quantomechanic effect of tunneling is expected.
In particular, when the alkyl chain is sufficiently short, as for
KuQ3CO2H dye, the electron acceptor unit of the dye is quite
close to the NiO surface, thus making the ET from the dye to
the NiO surface a feasible event.
Clearly, the quantum yield of such phenomenon is strongly
dependent on the length of the alkyl chain: a shorter chain
should imply higher quantum yields. Interestingly, current
intensities of KuQ3CO2H and KuQ8CO2H were not as different
Fig. 11. IPCE spectra of complete p-DSSC sensitized with different dyes: EryB (purple
line), KuQ3CO2H (red line), KuQ8CO2H (orange line). In black has been reported the
IPCE spectra of a undyed NiO-based device.
Fig. 10. JV curves for complete p-DSSC sensitized with: EryB (purple line), KuQ3CO2H
(red line), KuQ8CO2H (orange line).
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This is probably due to the decrease of the Ni3+ centers that Interestingly, all JV curves showed similar slopes to the ones
constitute the preferential sites of dye anchoring. The more generated by pristine NiO. This finding led us to conclude that
pronounced decrease of the IPCE peak typical of NiO in KuQs these dyes do not have a catalytic or quenching effect for NiO
−
spectra with respect to ERY B system is due to the fact that towards the dark reduction process I3 +2e−→3I−. Thus,
KuQs absorb light also between 300 and 400 nm in a non triiodide reduction occurs exclusively on NiO bare (and
effective way for charge photoexcitation.
eventually from the exposed FTO) when no light is absorbed by
Moreover, control experiments have been carried out in the the KuQs sensitized system.46,66
absence of light (Table 3 and Fig. 12) to evaluate the dark
current. Under the adopted experimental conditions, the dark
Conclusions
cathodic current was probably due to the dark reduction of
−
triiodide to iodide, I3 +2e−→3I− at the NiO/dye/electrolyte
In this work, new KuQuinone derivatives having a carboxylic
interface and, eventually, at the FTO/electrolyte interface. The
latter interface can participate in the electrochemical process
if the NiO layer is not compact, thus not preventing the effect
of shunting between FTO substrate and the electrolyte.14,65
In the present work, the deposition of a compact NiO layer for
the prevention of shunting was not considered. Therefore,
shunting can occur at the TCO/electrolyte interface if the first
layer of nanoporous NiO (which is in direct contact with FTO)
does not uniformly cover the transparent conductive coating.
This evidence may imply that part of the surface charge in the
cathode of the sensitized cell can be generated either from a
process of dark charge transfer between dye and NiO
substrate or from an anion physisorption process, depending
on the nature of the anchored sensitizer and on its surface
concentration.
acid terminal group and differing in side chain length have
been synthesized. Desired products have been obtained in
good yields if compared with the previously reported synthetic
procedures of differently substituted KuQs.54 Due to their
favorable spectroscopic properties, such as the intense
absorption in the visible region of the spectrum and their
calculated HOMO-LUMO energy levels, these organic
compounds have been anchored on NiO to test their
application as sensitizers in p-type DSSCs. Results highlighted
that KuQuinones-sensitized cells showed
a
better
photoelectrochemical performance than Ery B, a conventional
benchmark dye-sensitizer for p-DSSCs. In particular, fill factor
(FF) values were analogous for all the tested dyes, while
current density values (JSC) were similar for Ery B and
KuQ3CO2H. The latter sensitizer was loaded in larger amount
with respect to KuQ8CO2H, because of the smaller chain
length. Despite the lack of electronic conjugation between the
anchoring unit and the light-absorbing unit, the photoinduced
charge transfer occurred through space and not through the
conjugated linker, as expectable. These findings indicate that
the new dyes here proposed are particularly interesting
photosensitizers for NiO-based p-type DSSCs. Obviously, the
synthesis of novel KuQuinone derivatives possessing the light
absorbing pentacyclic unit directly linked with the anchoring
group through a conjugated π-linker is expected to improve
the effectiveness of the charge transfer processes with
consequent beneficial effects on the overall performance of
the device.
With respect to NiO bare, all the sensitized devices showed
higher values of open circuit potential and larger current
densities. Moreover, the largest values of potential were
combined to the largest values of current density.
Table 3. Parameters obtained by the dark JV measurement.
Dye
NiO bare
Ery B
KuQ3CO2H
KuQ8CO2H
JSC (mA/cm2)
Voc (mV)
-0.007
0.048
-0.015
0.076
-0.008
0.051
-0.012
0.064
Experimental
Materials
All commercial reagents and solvents were purchased from
Sigma-Aldrich or Fluka, at the highest degree of purity
available and were used without any further purification.
Anhydrous terpineol was used as a mixture of enantiomers.
FTO-covered glass panel (item no. TCO227) was purchased
from Solaronix, HSE commercial liquid electrolyte from Dyesol
and the liquid resin (3035B) from Threebond.
Instrumentation
Fig. 12. JV curves in the dark: NiO bare (black line), EryB (purple line), KuQ3CO2H (red
1H NMR spectra were recorded with a Bruker Avance 300 MHz
instrument, using CDCl3 as solvent. HRMS analyses have been
line), KuQ8CO2H (orange line).
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ARTICLE
Journal Name
performed with LCQ Duo instrument (ThermoQuest, San Jose, Synthesis
CA, USA). Gas chromatographic analyses were carried out with
General synthetic procedure. KuQuinone derivatives have
been synthesized according to a previously reported literature
procedure.54 Glassware was dried under nitrogen before the
use. In a typical experiment, 1 g (5.75 mmol) of 2-hydroxy-1,4-
naphthoquinone and 2.5 g (8 mmol) of anhydrous Cs2CO3 were
insert in a 50 ml round flask. 12 mmol of the properly
substituted alkyl bromide were added, together with 62 mg
(0.33 mmol) of sublimated ferrocene. Reagents were dissolved
in 22 ml of spectrophotometric grade DMSO (kept overnight
over anhydrous K2CO3 prior to use). The mixture was kept
under stirring at 114°C for 41 hours, then diluted with 100 ml
of dichloromethane and filtered. The filtrate was then washed
with water or brine (2× 500 ml), dried over anhydrous Na2SO4
and filtered. Solvent was removed under reduced pressure and
the brown powder obtained was than purified by
chromatography. The isolated purple compound was then
repeatedly crystallized from dichloromethane−pentane.
a Varian 3900 instrument equipped with a FID 1770 detector
and a 30 m Supelco SPB-5 column (0.25 mm diameter and 0.25
μm internal film). UV-visible absorption spectra were recorded
on a Shimadzu 2450 spectrometer equipped with the UV
Probe 2.34 program. The thickness (l) of the NiO film was
estimated with a Dektat 150® profilometer from Veeco. The
imaging of the surface morphology of mesoporous NiO was
realized with an Auriga Zeiss instrument. The measurements of
optical transmittance of the sensitized NiO electrodes were
made with a double ray spectrometer (UV-2550 by Shimazdu).
The cyclic voltammetries of the solutions of KuQs were
recorded with a Palmsens potentiostat. A standard calomel
electrode (SCE) was used as the reference electrode, a
platinum wire as counter electrode and a platinum disk as the
working electrode. Measurements have been performed in a
0.1 M solution of tetrabutylammonium perchlorate (TBAP,
crystallized from ethyl acetate) in anhydrous dichloromethane
at a scan rate of 100 mV/s. ATR-FTIR spectra have been
obtained as sum of 128 scans at a resolution of 4 cm-1 using a
Thermonicolet iS50-FTIR instrument. Photoelectrochemical
characterizations were carried out using the solar simulator
Solar Test 1200 KHS (class B) at 1000 W/m-2 with artificial solar
spectrum AM 1.5 G. The IPCE curves were recorded using a
1-methylKuQuinone (KuQCH3). The reaction was carried out
according to the reported general procedure, using 1-
bromopropane as alkyl bromide reagent. The brown powder
obtained was purified with a chromatography column on SiO2,
using CH2Cl2 as eluent. 60 mg (0.17 mmol) of KuQCH3 as a
purple powder have been obtained (6.1 % yield). 1H NMR in
−7.74 (m,
CDCl3: δ 18.11 (s, 1H), δ 8.30−8.25 (m, 4H), δ 7.80
4H), δ 2.98 (s, 3H). HRMS (ESI-) m/z: [M−H]− calcd. for C22H11O4
computer controlled set-up consisting of
a Xe lamp
(Mod.70612, Newport) coupled to a monochromator
339.0657; found 339.0642.
(Cornerstone 130 from Newport) and a Keithley 2420 as light-
source meter.
Ethyl 11-bromoundecanoate. 6 g of 11-bromoundecanoic acid
(23 mmol) were dissolved in 200 ml of absolute ethanol, with
the addition of catalytic amount of 37% HCl solution. The
reaction mixture was kept under stirring at 40°C and checked
NiO paste preparation and deposition
The preparation of the NiO paste constituting the precursor for by GC analysis. Subsequently, it was diluted with 300 ml of
the screen-printing step, has been carried out following a water and extracted with diethyl ether (3 × 250 ml). Organic
previously reported procedure.38 The aspect of innovation in phase was dried over anhydrous Na2SO4 and filtered. The
the modified method adopted in this work is the use of solvent was evaporated under reduced pressure and a pale
preformed NiO nanospheres (max diameter=50 nm) instead of yellow liquid has been obtained (6.1 g, 10 mmol, 90% yield). 1H
˗3.40 (t, 2H), δ
common NiO chemical precursors, such as NiCl2 or Ni(OH)2, NMR in CDCl3: δ 4.18˗4.10 (q, 2H), δ 3.45
˗1.82 (m, 2H), δ 1.68˗1.60 (m, 4H), δ
which generally render the purification and sintering processes 2.34˗2.28 (t, 2H), δ 1.91
much more laborious.17
1.46˗1.25 (m + t, 13H).
The obtained paste was spread over a FTO-covered glass panel Ethyl 6-bromohexanoate. 4.3 g of 6-bromohexanoic acid 97%
previously cleaned by an ultrasonic bath in acetone for 10 (22 mmol) were dissolved in 200 ml of absolute ethanol, and 1
minutes and in ethanol for 10 minutes. The paste was spread ml of 37% HCl solution was added. The reaction mixture was
via screen-printing through a 90.48 T mesh screen on dried kept under stirring at 40°C and checked by GC analysis.
FTO/glass panels. After a pre-drying period of 15 minutes at Afterwards, it was concentrated, diluted with Et2O and
100°C in oven (WHT 5/120 from Welland), the temperature extracted with water (2 × 200 ml). Organic phase was dried
was gradually increased with a ramp of 15°C/min up to 450 °C. over Na2SO4 and filtered and the solvent was evaporated
The screen-printed slurry of NiO nanoparticles was kept at the under reduced pressure to give a pale yellow liquid (4.4 g, 20
highest temperature for 30 minutes to complete the sintering. mmol, 90% yield). 1H NMR in CDCl3: δ 4.18˗4.10 (q, 2H), δ
˗2.30 (t, 2H), δ 1.94˗1.84 (m, 2H), δ
After this period, the resulting mesoporous film of NiO was 3.45˗3.40 (t, 2H), δ 2.35
˗1.43 (m, 2
cooled down to room temperature and a 2 µm thick film of 1.72˗1.62 (m, 2H), δ 1.54
H), δ 1.25
˗1.24 (t, 3H).
NiO was obtained. Furthermore, at the adopted temperature 1-(8-ethoxycarbonyloctyl)KuQuinone
(KuQ8CO2Et).
The
of sintering, the binders were completely combusted, so they reaction was carried out with ethyl 11-bromoundecanoate as
were removed as volatile products of thermolysis from the alkyl bromide and according to the general procedure
oxide layer. The resulting oxide has generally non previously described. The brown powder was purified by plug
stoichiometric features and a more appropriate formula for chromatography (SiO2, eluent CH2Cl2); the isolated purple
describing it is NiOx.
product (184 mg, 0.36 mmol, 13% yield) was then crystallized
8 | J. Name., 2012, 00, 1-3
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ARTICLE
from dichloromethane-hexane and washed with pentane. 1H As a comparison, NiO photocathodes have been functionalized
−7.71
NMR in CDCl3: δ 18.16 (s, 1H), δ 8.29−8.25 (m, 4H), δ 7.84
also with the commercially available dye Ery B, using 0.2 mM
solution in ethanol. All screen-printed NiO samples were
2H), δ 1.68−1.36 (m, 12H + H2O), δ 1.29−1.24 (t, 3H). HRMS sensitized at room temperature for 16 hours and then washed
−2.27 (t,
(m, 4H), δ 4.17−4.10 (q, 2H), δ 3.49−3.44 (t, 2H), δ 2.32
(ESI-) m/z: [M−H]− calcd. for C32H30O6 509.19696; found with the same solvent used in the dipping phase. The screen-
509.19686.
printed NiO photocathode and the platinized FTO counter
sandwich
1-(8-carboxyloctyl)KuQuinone (KuQ8CO2H). 49 mg of electrode were assembled together in
a
KuQ8CO2Et (0.096 mmol) were dissolved in 8 ml of THF. configuration using Bynel® thermoplastic polymeric film having
Afterwards, 2 ml of NaOH 25% in MeOH were added and the the double function of being spacer and sealant. After
system was kept under stirring for one hour and checked by sandwiching the electrodes, the triiodide/iodide HSE
TLC analysis (SiO2, eluent CH2Cl2). Reaction mixture was diluted commercial liquid electrolyte was injected inside the cell by
with 100 ml of NH4Cl 10%, neutralized with HCl 1 M and vacuum backfilling technique. The hole for electrolyte injection
extracted with CH2Cl2. Organic phase was dried over Na2SO4, was sealed with a liquid resin (3035B) which becomes a hard
filtered and the solvent was removed under reduced pressure. paste upon UV curing treatment. The photoactive area of the
KuQ8CO2H has been obtained as a purple powder (44 mg, samples was 0.25 cm2.
0.090 mmol, 95% yield). Due to the low solubility of the
product 1H NMR analysis was not practicable. HRMS (ESI-) m/z: Devices Characterization
[M−H]- calcd. for C30H26O6 481.1657; found 481.1728.
The relationship between the characteristic parameters of the
1-(3-ethoxycarbonylpropyl)KuQuinone (KuQ3CO2Et). The
diode-like JV curves is η=[(JSC*VOC)*FF]/Iin in which the term Iin
indicates the intensity (in W∙m-2) of the radiation incident on
reaction was carried out using ethyl 6-bromohexanoate as
alkyl bromide and according to the general procedure. The
the electrochemical photoconversion device. The IPCE spectra
product has been purified by plug chromatography (SiO2,
directly provide the percentage of incident photons converted
eluent CH2Cl2); the isolated purple powder (173 mg, 0.40
into electrons, at a given wavelength. IPCE (sometimes
mmol, 14% yield) was crystallized from dichloromethane-
denominated external quantum efficiency, EQE) is defined as
hexane and then washed with pentane. 1H NMR in CDCl3: δ
IPCE(λ)=[JSC(λ)/e]/Φin(λ)] where e represents the elementary
18.20 (s, 1H), δ 8.27˗8.22 (m, 4H), δ 7.79˗7.71 (m, 4H), δ
charge, and Φin(λ) the flux of photons with energy h(c/λ) which
4.15˗4.11 (q, 2H), δ 3.53˗3.50 (t, 2H), δ 2.50˗2.47 (t, 2H), δ
2.09˗2.04 (m, 2H), δ 1.25˗1.23 (t, 3H). HRMS (ESI-) m/z: [M−H]−
impinge the unit surface of illuminated device per unit time (in
photons m-2s-1). In the definition of photonic energy the
calcd for C27H20O6 439.1187; found 439.1207.
symbols h, c and λ indicate the Planck’s constant, the speed of
1-(3-carboxylpropyl)KuQuinone (KuQ3CO2H). 45 mg of
light and the radiation wavelength, respectively. The defined
KuQ3CO2Et (0.10 mmol) were dissolved in 50 ml of THF.
concept
of
IPCE
leads
to
the
relationship
where
IPCE(λ)=LHE(λ)*Φinj(λ)*ηCC=LHE(λ)*Φinj(λ)*Φesc(λ)*ηtr,40
Afterwards, 5 ml of a saturated solution of NaOH in MeOH
were added and the system was kept under stirring overnight
and checked by TLC analysis (SiO2, eluent CH2Cl2). After
neutralization with 0.1 M HCl, a purple precipitate has been
obtained. 39.2 mg of the product have been obtained as a
purple powder (0.095 mmol, 95% yield). HRMS (ESI-) m/z:
[M−H]− calcd. for C25H16O6 411.0874; found 411.0898.
LHE(λ) is the light harvesting efficiency of the dye at a given
wavelength λ, Φinj(λ) is the efficiency of charge injection from
the excited dye to the semiconductor substrate at the
wavelength of absorption λ (also called hole injection quantum
yield), Φesc(λ) is the escape quantum yield, i.e. the probability
that
a photoinjected hole has to escape from direct
recombination with the reduced dye (also Φesc is wavelength-
dependent), ηtr is the probability the hole has to avoid all
Theoretical calculations
Computational calculations were performed using Gaussian 09 processes of recombination and reaches the current collector,
rev. A.02.59 Geometry optimization was carried out using and ηCC is the charge collection efficiency which refers to the
density functional theory (DFT) and B3LYP hybrid functional efficiency of the transfer of the photoinjeted charge from the
with 6-31G+dp basis set, while HOMO-LUMO energy levels site of photogeneration localized at the semiconductor/dye
have been calculated with 6-311G dp basis set in the vacuum.
interface, to the surface of the current collector (typically the
semiconductor/TCO interface). The LHE(λ) is related to the
absorbance A(λ) at a given wavelength (λ) of the dye-sensitizer
Devices assembly
in the anchored state by the relationship LHE(λ)=1- 10-A(λ)
.
Platinized counter electrodes were prepared by screen printing
platinum paste Ch01 from Chimet through a 100T mesh screen
onto FTO-coated glass.67 Sensitization of screen-printed NiO
photocathodes was carried out through 16 hours dipping in a
solution of the corresponding dye: KuQCH3 (0.3 mM in
toluene), KuQ8CO2H (0.3 mM in THF), KuQ3CO2H (0.2 mM in
THF). THF was used as solvent to dissolve KuQ8CO2H and
KuQ3CO2H dyes due to their very low solubility in other
conventional organic solvents.
Acknowledgements
P.G. thanks the project "Ricerca Scientifica d'Ateneo 2015 from
University of Rome Tor Vergata, SMART project" for financial
support. D.D. acknowledges the financial support from the
University of Rome “LA SAPIENZA” through the program
Ateneo 2012 (Protocol No. C26A124AXX). Dr. Claudia Mazzuca
This journal is © The Royal Society of Chemistry 20xx
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KuQuinones have been used for the first time as dyes in NiO-based p-type DSSCs.