Electrolyte-Free Dye-Sensitized Solar Cell with High Open Circuit Voltage Using a Bifunctional Ferrocene-Based Cyanovinyl Molecule as Dye and Redox Couple

Article abstract of DOI:10.1021/acs.organomet.8b00104

Functionalization of ferrocenyl moieties with a cyanovinyl system using a unique solid-state synthetic method led to the formation of donor-acceptor type compounds with wide absorption in the visible region. A DSSC study using the ferrocenyl cyanovinyl co

Full text of DOI:10.1021/acs.organomet.8b00104

Communication
Cite This: Organometallics XXXX, XXX, XXX−XXX
Electrolyte-Free Dye-Sensitized Solar Cell with High Open Circuit
Voltage Using a Bifunctional Ferrocene-Based Cyanovinyl Molecule
as Dye and Redox Couple
†
†
‡
§
,∥
,†
A. Ghosh, S. Mishra, S. Giri, S. M. Mobin, A. Bera,* and S. Chatterjee*
†
Department of Chemistry, National Institute of Technology Rourkela, Rourkela, Orissa 769008, India
‡
Theoretical Chemistry Laboratory, Department of Chemistry, National Institute of Technology Rourkela, Rourkela, Orissa 769008,
India
§
Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Madhya Pradesh 452017, India
Department of Physics, Indian Institute of Technology Jammu, Jammu, Jammu and Kashmir 181121, India
∥
*
S Supporting Information
ABSTRACT: Functionalization of ferrocenyl moieties with a cyanovinyl
system using a unique solid-state synthetic method led to the formation of
donor−acceptor type compounds with wide absorption in the visible
region. A DSSC study using the ferrocenyl cyanovinyl compound as a dye
with an electrolyte-free fabrication system showed an unprecedented
open-circuit voltage (VOC) of 763−841 mV. DFT calculations were
carried out to understand an unique electron transfer mechanism for the
DSSC device which may be responsible for the high VOC
.
ntegrating additional functionalities to a functional material
and applying them in useful devices are universal strategies to
band gap that can only absorb a very small part of the solar
11
I
spectrum. Thus, designing a ferrocenyl derivative that can
absorb a significant part of visible light that could not only act as
a photon absorber but also, because of its intrinsic redox
properties, would be able to regenerate the dye after the
design multifunctional materials. Ferrocene-based organo-
metallic compounds have gained a great deal of research
attention because of their interesting properties related to redox
switching, optical nonlinearity, electronic communication,
photogenerated electron has been transferred to the TiO layer
1
−6
2
molecular receptor, sensors, etc.
Recently, substantial
would obviate any additional redox electrolyte in the solar cell
device. In that case, a ferrocenyl photon absorber can play the
dual role of a dye and a redox system.
interest has been given to ferrocenyl molecules containing a
donor−acceptor system because of their unique photophysical
7
−10
and optical properties.
The intrinsic redox property of the
We have been, for some time, working on a variety of
ferrocene-based molecular systems ranging from ferrocenylhy-
drazones to heterobimetallic ferrocenylchalcones, ferrocene-
based cluster compounds, ferrocenyldithiocarboxylates, and
multiferrocenyl entities and studied their electrochemical and
ferrocenyl fragment has made it a potential candidate in dye-
sensitized solar cells (DSSC), mostly as a redox electrolyte to
improve the device efficiency.
Typically, a DSSC comprises a dye-sensitized mesoporous-
TiO -based photoanode, a redox couple electrolyte, and a
2
12
platinum counter electrode. Photons with sufficient energy
excite the dye molecules, followed by a rapid injection of
photophysical properties. While working on such systems, we
came across a range of donor−acceptor type molecules which
showed wide absorption in the visible region and discrete redox
potential. Therefore, we focused our study on the exploration of
some novel ferrocene-based donor−acceptor compounds as
bifunctional molecules to work both as a dye and a redox
electrolyte. Recently, a great deal of research has been focused
on improving the performance of dye-sensitized solar cells
electrons into the conduction band of TiO nanoparticles,
2
thereby oxidizing the dye molecule. The redox couple
electrolyte subsequently reduces the dye molecule to its ground
state while the electrolyte is regenerated at the counter
electrode, resulting in open circuit voltage (VOC) and short
circuit current (J ). Recently, Daeneke et al. reported a DSSC
SC
study using ferrocenyl derivatives as redox electrolytes to
(
DSSC) by using various approaches such as structural
understand the variation of dye regeneration efficiency with the
11
modification of the dye and surface treatment of QDs which
change in ferrocene/ferrocenium redox potential. This and a
handful of other reports showed that ferrocenyl compounds are
mostly used as redox couples and have never been used as
effective sensitizers in DSSC due to their comparatively larger
Received: February 20, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00104
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
show a sharp increase in the absorptivity, sensitization
properties, and power conversion efficiency.
pentadienyl rings of the ferrocene moiety (Figure S6). To
understand the scope for the use of red mud in general reactions
related to Knoevenagel condensation, a hydrazone-based
derivative (3) containing a methylene active group was prepared
using a modified solid-state procedure. Cyanoacetylhydrazone
(3) was then reacted with ferrocenylmonoaldehyde using a
solid-supported reaction process in the presence of red mud to
obtain the two new ferrocene-based hydrazone compounds
13
In this report, we describe a novel synthetic method to
prepare donor−acceptor type ferrocenylcyanovinyl compounds
using a superior solvent-free synthesis technique and study their
photovoltaic characteristics by fabricating a DSSC device
without using any additional redox electrolyte. We anticipated
that the ferrocenyl-cyanovinyl-based donor−acceptor systems
will absorb photons within a wide spectrum due to their donor−
acceptor properties and in addition will also act as a ferrocenyl-
based redox couple. Indeed, the electrolyte-free dye-sensitized
solar cell gave an excellent open circuit voltage of 0.84 V, which
is higher than the open circuit voltage achieved in a conventional
5
5
[{(η -C H )Fe(η -C H )CHC(CN)C(O)NHNC(CH )-
5
5
5
4
3
5
5
Ph}] (4) and [{(η -C H )Fe(η -C H )}{CHC(CN)C(O)-
5
4
5
4
NHNC(CH )Ph} ] (5) (Scheme 1). Compounds 4 and 5
3
2
have been characterized by IR, NMR and mass spectroscopic
techniques (Text S1).
13c,g−k
ruthenium-based DSSC.
The electrochemical properties of compounds 1−5 have been
examined in acetonitrile solution using 0.1 M tetrabutylammo-
nium perchlorate (TBAP) salt by cyclic voltammetry (CV) and
differential pulse voltammetry (Figure S7). Compound 1,
involving a ferrocenyl moiety linked to one cyanovinyl group,
exhibited a reversible redox process at +0.61 V vs SCE due to the
Fe(II)/Fe(III) redox couple, while compound 2 with
disubstituted cyanovinyl chains at the two cyclopentadienyl
rings showed the redox process at much higher potential (+0.87
V vs SCE), making the molecule difficult to oxidize. However,
when the cyanovinyl ester group in 1 was replaced with
cyanovinylhydrazone in compound 4, the redox process
becomes slightly easier, showing the Fe(II)/Fe(III) couple at
0.57 V vs SCE. The same trend has been observed with
compound 5, which contains two cyanovinylhydrazone chains
Use of solid inorganic supports for solvent-free organic
reactions has attracted global attention due to their ability to
perform reactions under mild conditions and easy product
14,15
isolation methods.
Solid-state Knoevenagel condensation
involving an organic substrate on an inorganic support such as
Al O , SiO , KF/Al O , KOH/SiO , etc. have been well studied,
2
3
2
2
3
2
while that involving organometallic moieties has not been
15,16
explored extensively.
Therefore, to understand the wider
scope of solid-supported reactions, we identified red mud (RM)
as a possible support due to its metal oxide content and alkalinity
and explored its potential in organometallic Knoevenagel
reactions. Red mud (RM) is a toxic solid waste produced in
the process of alumina extraction and gives rise to major
environmental pollution. Thus, a solid-phase reaction of
ferrocenylaldehyde and 1,1′-ferrocenyldicarboxyaldehyde with
cyanoethylacetoacetate was investigated using red mud (RM) as
a solid support. Equivalent amounts of the reactants were added
to solid red mud powder, and the reaction mixture was
thoroughly mixed and stirred at room temperature for 3 h. After
the reaction, the mixture was extracted in ethyl acetate solvent
and dried under vacuum. TLC showed the formation of a pink
product which was purified by short-column chromatography
for spectroscopic characterization and further study. Character-
ization using infrared, NMR, and mass spectroscopic techniques
attached to the Cp rings; it shows an oxidation potential at E1/2
=
0.76 V vs SCE. Incorporation of the ferrocenyl substituent
within the cyanovinyl ester fragment significantly influences the
donor−acceptor and absorption properties of 1 and 2. The
solution-based UV−visible absorption spectra of 1 and 2 exhibit
major bands in the 320, 335, 380, 425, 510, and 521 nm regions
(Figure S13). The thin film absorption spectra were obtained by
depositing the compound on a mesoporous-TiO -coated glass
2
substrate by dip-coating. The absorption spectra for both
compounds reveal wide absorption in the visible region in the
400−700 nm region, as shown in Figure S14. The onset
absorption edges of 1 and 2 are 703 and 679 nm, respectively,
corresponding to optical band gaps of 1.8 eV.
5
5
revealed the formation of 1-[{(η -C H )Fe(η -C H )CH
5
5
5
4
5
C(CN)CO Et}] (1) and 1,1′-[{Fe(η -C H )CHC(CN)-
2
5
4
CO Et} ] (2) in high yields as shown in Scheme 1 (Text S1).
2
2
The absorption spectra of 1 and 2 were correlated by TD-
DFT calculations and are consistent with the experimental data
Scheme 1. Synthesis of Compounds 1, 2, 4 and 5
(
Figure S9). A corresponding energy level diagram has been
constituted with the help of redox potential data and thin film
absorption spectra, as given in Figure 1a.
The wide absorption properties in the visible region and the
measured diode characteristics (Figure S10) prompted us to
evaluate the DSSC performance of compounds 1 and 2 as
bifunctional molecules to work as both a dye and a redox couple
system. TiO /[1]- and TiO /[2]-based mesoporous hetero-
2
2
junctions were used to study the electrolyte-free sensitized solar
cells. A schematic representation of the TiO /[1]/Pt meso-
2
porous heterojunction is shown in Figure S15. A complete pore
filling of the mesoporous TiO was achieved by dipping the
2
meso-TiO -coated FTO into a 10 mM solution of 1, as shown in
2
A single crystal, grown from a dichloromethane/n-hexane
solvent mixture at −10 °C, was used for a single-crystal X-ray
diffraction study. The molecular structure of 2 confirms the
presence of a disubstituted ferrocenyl framework with a partially
staggered conformation having a torsional angle of 50.84°
between the two chains. Both of the chains containing a
cross-sectional FESEM images (Figure S11). Nonlinear diode
like characteristics of TiO /[1],[2]/Pt in the current density vs
2
voltage (J−V) curve under dark conditions confirms the
formation of a heterojunction at the meso-TiO /[1] and
2
meso-TiO /[2] interfaces (Figure S10). Photovoltaic perform-
2
ances of these mesoporous heterojunction solar cells were
2
−
CHC(CN)CO Et unit are attached to two different cyclo-
measured in air at 1 sun illumination (100 mW/cm ), and
2
B
DOI: 10.1021/acs.organomet.8b00104
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
using 1 and 2 in an electrolyte-free system, we conducted DFT
calculations on neutral and oxidized species. The calculation
reveals an electron transfer mechanism involving a ground state
(
1, 2) which on photoexcitation goes to states 1*, 2* and
subsequently injects electrons to the TIO CB, oxidizing the
molecule to 1 , 2 . The calculated MOs of the optimized 1 , 2
2
+
+
+
+
are energetically more stable than the platinum work function
and become reduced by the Pt cathode to regenerate the dye (1,
2
). Figure S16 shows a tentative schematic representation of the
electron transfer. It has been understood that the presence of an
optimized oxidized state restricts the recombination process and
also helps in easier reduction of the dye using the Pt cathode.
The proposed electron transfer process also reveals that the role
of the redox couple for the regeneration of dye in the usual
DSSC has been served here by the dye itself due to the presence
of an intrinsic ferrocenyl system. Therefore, the bifunctionality
of molecules 1 and 2, to serve as redox electrolytes in addition to
absorber dyes, make the systems potential candidates for
harvesting solar energy. In summary, a novel synthetic method
with a high yield of the product has been introduced to prepare
some ferrocene-based donor−acceptor molecules that can
absorb in visible light within a wide energy range. The presence
of both a ferrocene moiety and a cyanovinyl chain with wide
absorption properties prompted us to fabricate a unique solar
cell device without any additional redox couple. The electrolyte-
free dye-sensitized solar cell gave an unprecedented open circuit
voltage of 0.76 V for 1 and 0.84 V for 2. Our current effort has
been focused on the fabrication of an electrolyte-free solar cell
device using long-chain donor−acceptor ferrocenyl derivatives
to increase the efficiency of the photovoltaic cell.
Figure 1. (a) Energy level diagram, (b) J-V characteristics of the
heterojunction solar cells under light using 1 and 2 as the absorbers.
typical J−V characteristics are plotted in Figure 1b and their
characteristic parameters are given in Table 1. Under 1 sun
Table 1. Photovoltaic Parameters Derived from the J−V
Characteristics of the Device
JSC (mA/cm2)
compound
VOC (mV)
FF (%)
η (%)
1
2
0.41
0.039
763
841
35
28
0.10
9.18 × 10−3
ASSOCIATED CONTENT
Supporting Information
■
illumination, a short-circuit photocurrent density (J ) of 0.41
*
S
SC
2
mA/cm , open-circuit voltage (VOC) of 763 mV, and fill factor
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00104.
(
FF) of 35% were observed for 1-based devices. A higher VOC
value of 841 mV was observed, and a short-circuit photocurrent
density, J , of 39 μA/cm was achieved for the device fabricated
using compound 2. The study shows that a higher VOC has been
obtained with an electrolyte-free DSSC device in comparison to
2
SC
Characterization data, crystallographic data, calculation
details, and experimental section (PDF)
−
13j,17
the reported performance using I /I as redox electrolyte.
2
Accession Codes
In order to ascertain the nature of the orbitals involved during
DSSC performances, DFT calculations using a LANL2DZ basis
set at the B3LYP level were performed on the optimized
structures of the compound and their oxidized species. DFT
calculations were carried out for compounds 1 and 2 in the gas
phase and were fully optimized without any constraint. The
calculation reveals that the HOMO for both compounds 1 and 2
is mainly localized on the ferrocenyl fragment, while the LUMO
is largely distributed on the cyanoethyl acetate chain (Tables S2
and S5). A natural electron population analysis (NPA) was also
CCDC 1563814 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
S.C.: tel, +91 661 2462656; fax, +91 661 2472926; e-mail,
■
*
−
performed on 1 and 2 as well as on their respective 1e -oxidized
saurav@nitrkl.ac.in.
A.B.: e-mail, ashok.bera@iitjammu.ac.in.
species. The natural electron spin populations (α−β) at the iron
+
+
*
atom in 1 and 2 species are 1.25494 and 1.29594, respectively,
which indicate that during oxidation the electron is released
mainly from the ferrocenyl iron center. The gross natural
electron spin populations of the cyanovinyl ester fragment in the
oxidized species of 1 and 2 are 0.631 au and 0.514, 0.628 au,
respectively, which reveal significant charge delocalization
within the cyanovinyl fragment on oxidation. The calculations
also reveal two different spin densities for the two cyanovinyl
ester chains in compound 2, suggesting unsymmetrical α, β spins
between the two cyanovinyl chains (Table S1). To correlate the
experimentally observed VOC data for the DSSC, fabricated
ORCID
S. M. Mobin: 0000-0003-1940-3822
S. Chatterjee: 0000-0002-6290-4342
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.C. is grateful to the Department of Science and Technology
(SERB), Government of India, for research funding (No. SB/
C
DOI: 10.1021/acs.organomet.8b00104
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
S1/IC-30/2013). A.B. is thankful to DST-INSPIRE for financial
support.
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Chem. 2015, 794, 88−95. (f) Mathur, P.; Chatterjee, S.; Avasare, V. D.
Adv. Organomet. Chem. 2007, 55, 201−277.
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DOI: 10.1021/acs.organomet.8b00104
Organometallics XXXX, XXX, XXX−XXX