A simple acrylic acid functionalized zinc porphyrin for cost-effective dye-sensitized solar cells

Article abstract of DOI:10.1039/c2cc33337f

A simple zinc porphyrin with an acrylic acid at the meso position exhibits an energy conversion efficiency of 5.1%, demonstrating its potential for cost-effective dye-sensitized solar cells.

Full text of DOI:10.1039/c2cc33337f

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COMMUNICATION
A simple acrylic acid functionalized zinc porphyrin for cost-effective
dye-sensitized solar cellsw
Hongshan He,*a Ashim Gurung,a Liping Sia and Andrew G. Sykesb
Received 9th May 2012, Accepted 8th June 2012
DOI: 10.1039/c2cc33337f
A simple zinc porphyrin with an acrylic acid at the meso position
exhibits an energy conversion efficiency of 5.1%, demonstrating
its potential for cost-effective dye-sensitized solar cells.
Dye-sensitized solar cell (DSC) is an electrochemical device.
One major component is the dye molecules that are coated on
the surface of titanium dioxide (TiO2) nanoparticles for light
harvesting and electron injection into the TiO2 conduction
band.1 The energy conversion efficiency of the device relies
largely upon the nature of the dye.2 Recent studies have shown
Fig. 1 Synthesis of DMPZn-C2-COOH. Reaction conditions: (a) methyl
acrylate, Pd(OAc)2, Ph3P, toluene, 80 1C; (b) KOH, EtOH, 60 1C.
porphyrins to be promising alternatives to conventional
ruthenium(II) dyes due to the abundance of carbon sources
and their benign nature in the environment.3–5 Diau et al.6,7
found that a porphyrin with a 4-ethylnylbenzoic acid at its
meso position is an excellent platform for high efficiency
DSCs. They8,9 found that introducing bulky tert-butyl groups
into phenyl groups increased the energy conversion efficiency
from 2.5% to 3.29% and further addition of diphenylamino
groups to the porphyrin ring increased the efficiency to
6.15%.10 In 2010, Bessho et al.11 added two long alkyl groups
to diphenylamino groups and an efficiency of 11% was
obtained. In 2011, Yella et al.12 further replaced the tert-butyl
group by long alkoxide groups and a record efficiency of
12.3% was achieved. However, sophisticated structures require
multistep synthesis in an inert and dry environment, which
could potentially increase the overall cost of devices.
Fig. 2 Electron density distribution profiles of LUMO and HOMO
of DMPZn-C2-COOH before (A) and after (B) being adsorbed on the
[Ti2O2(OH)2(H2O)4]2+ model. The calculations were carried out under
vacuum using the 631-g(d) basis set and B3LYP functional (see ESIw
for details).
In quest of simple and efficient dyes, we designed a porphyrin
(denoted DMPZn-C2-COOH) with an acrylic acid group at its
meso position, as shown in Fig. 1. In this porphyrin, COOH
functions as an anchoring group and the CQC bond as a
spacer. Density functional theory (DFT) calculations showed
that electrons in the frontier molecular orbital delocalized from
the porphyrin ring to the acrylic acid group and the electron
densities at frontier molecular orbitals were considerably high,
as shown in Fig. 2. The energy level of LUMO is À2.4761 eV,
which is higher than that of the conduction band of TiO2
nanoparticles (B4.2 eV).13 When a binuclear titanium model
compound, [Ti2O2(OH)2(H2O)4]2+, was used to mimic the
TiO2 nanoparticles,14 it was found that two O atoms from
acrylic acid bind to two neighboring Ti atoms. HOMO is
located in the porphyrin ring, whereas LUMO is delocalized to
Ti atoms, indicating a strong electronic coupling between
porphyrin and TiO2 nanoparticles. Based on these results,
we synthesized DMPZn-C2-COOH and an energy conversion
efficiency of 5.1% was achieved. Considering its simple synthesis,
this porphyrin can serve as a new platform for the construction
of excellent dyes for cost-effective DSC devices.
a Center for Advanced Photovoltaics, South Dakota State University,
Brookings, SD 57007, USA. E-mail: Hongshan.he@sdstate.edu;
Fax: +1 605 688 4401; Tel: +1 605 688 6962
b Department for Chemistry, University of South Dakota, Vermillion,
SD 56069, USA
w Electronic supplementary information (ESI) available: Synthetic
procedures, H NMR, CIF of DMPZn–Br, dye loading, fluorescence
1
study of Al2O3 films, I–V and IPCE of cells using a cocktail method.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c2cc33337f
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7619–7621 7619

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Table 1 Photovoltaic parameters of DMPZn-C2-COOH sensitized
solar cells
Thickness
(mm)
Timea
(h)
JSC
Ratiob (mA cmÀ2
VOC
(V)
Z
(%)
)
FF
6
6
6
6
12
12
4
4
8
12
4
4
1 : 0.0
1 : 0.1
1 : 0.1
1 : 0.1
1 : 0.0
1 : 0.1
6.67
7.37
6.48
6.62
9.22
9.40
650
623
640
632
767
753
0.700 3.02
0.713 3.27
0.698 2.89
0.710 2.96
0.723 5.12
0.736 5.21
a
b
The dye immersion time. The molar ratio of porphyrin to BET dye.
Fig. 3 Absorption spectra of DMPZn-C2-COOH in CH3CN solution
and on the TiO2 nanoparticle film.
The DMPZn-C2-COOH was prepared by bromination of
5,10-mesitylporphyrin zinc using NBS in THF at room
temperature, followed by an Heck reaction between the resulting
5-bromo-10,20-mesityporphyrin zinc (DMPZn-Br) and methyl
acrylate in toluene at 80 1C and subsequent hydrolysis in ethanol
in the presence of KOH at 60 1C. The overall yield was B70%
(for synthetic details, see ESIw). The porphyrin ring and the
carboxylic acid group were at the trans position.15 Two mesityl
groups most likely align to the porphyrin ring plane with torsion
angles very close to 901, as evidenced by single-crystal X-ray
diffraction analysis of DMPZn-Br (Fig. S1, ESIw).
The absorption spectra of DMPZn-C2-COOH are shown in
Fig. 3. In acetonitrile, a Soret band was observed at 426 nm
and two Q bands were observed at 557 and 607 nm. When
adsorbed on the TiO2 nanoparticle film, the peak positions
shifted 2 to 3 nm to the longer wavelength. It was found that
the ratio of intensities at 610 and 560 nm was increased from
0.47 in solution to 0.8 on the TiO2 film, indicating strong
electronic coupling between the porphyrin ring and the TiO2
conduction band and alteration of porphyrin geometry.16 The
compound has strong fluorescence at 623 nm with a lifetime of
B1.83 ns, which is much longer than the lifetime of electron
injection (on the fs scale) from the porphyrin to the TiO2
conduction band. This ensures that the electron injection is
kinetically favorable.
Fig. 4 The J–V curves of DMPZn-C2-COOH-sensitized solar cells
under 4 h dye immersion time. The optical images are DMPZn-
C2-COOH/BE on the TiO2 film with molar ratio 1 : 0 (A), 1 : 0.1(B),
1 : 0.5 (C), 1 : 1(D) and just TPPZn-COOH (E). The active area was
0.16 cm2 and the light intensity was 100 mW cmÀ2
.
Photovoltaic performance of DMPZn-C2-COOH was
tested in TiO2 nanoparticle (NP)-based solar cells. The cells
were fabricated as we described previously.17 In all experiments,
TiO2 NP films with certain thickness were immersed in dye
solution (B0.3 mM) in ethanol at room temperature for 4, 8
and 12 h. The final cells were then tested under AM1.5G
conditions. The obtained photovoltaic parameters are listed in
Table 1. Typical J–V and IPCE (incident photon to charge
carrier efficiency) curves of cells with 4 h immersion time are
shown in Fig. 4 and 5. It was found that the optimum dye
immersion time was 4 h. The dye loading density did increase
when the immersion time increased to 8 and 12 h (see Fig. S3
and Table S1 in ESIw), the efficiency decreased, which can be
ascribed to the formation of aggregates, preventing the injection of
electrons into the conduction band of TiO2. An energy conversion
efficiency of 5.12% was obtained when the thickness of the
TiO2 NP film increased from 6 mm to 12 mm. Under the same
conditions, a benzoic acid modified porphyrin, TPPZn-COOH,
exhibited an energy conversion efficiency of 1.8%.18
Fig. 5 The IPCE of DMPZn-C2-COOH-sensitized solar cells.
One significant contribution to enhanced photovoltaic
high performance of DMPZn-C2-COOH is its broader light
absorption. Fig. 5 shows the IPCE of DMPZn-C2-COOH
and TPPZn-COOH. Under the same conditions, DMPZn-
C2-COOH and TPPZn-COOH showed similar IPCE at the Soret
band; however, DMPZn-C2-COOH exhibited an increased and
red-shifted (B30 nm) IPCE compared to that of TPPZn-COOH.
We believe that this is the major contribution to its high
performance.19,20 The effective electronic coupling between
DMPZn-C2-COOH and TiO2 nanoparticles may also play a
significant role for better photovoltaic performance as revealed by
theoretical calculations shown in Fig. 2. The short distance
between the porphyrin ring and the surface of TiO2 NPs may
also make a great contribution. In geometry-optimized structure,
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7620 Chem. Commun., 2012, 48, 7619–7621
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time, see Fig. S4–S9 and Table S2 in ESIw), only a marginal
increase in energy conversion efficiency (from 5.12% to
5.21%) was achieved. From IPCE, as shown in Fig. 5, it is
quite clear that photons between 450 and 550 nm are picked up
by BET and the fluorescence study on the Al2O3 nanoparticle film
also confirmed efficient energy transfer from BET to porphyrin
(Fig. S10–S12 in ESIw), an IPCE drop between 550 and 650 nm
was observed. This drop could be ascribed to the replacement of
DMPZn-C2-COOH by BET as evidenced by the appearance of a
Soret band in BET solution.
Fig. 6 Proposed adsorption models of DMPZn-C2-COOH (A) and
TPPZn-COOH (B) on the surface of TiO2 nanoparticles.
the distance between the Ti atom and the edge of the porphyrin is
B6.12 A, which is shorter than that in benzoic acid modified
porphyrin TPPZn-COOH (B8.58 A). Therefore, it is most likely
that DMPZn-C2-COOH binds to the TiO2 with two mesityl
groups parallel to the surface of the TiO2 NPs, which widens the
distance between two porphyrin molecules, as shown schematically
in Fig. 6. As a result, density of DMPZn-C2-COOH on the
TiO2 surface decreased (0.67 Â 10À10 and 1.24 Â 10À10 mol cmÀ2
for DMPZn-C2-COOH and TMPZn-COOH,14 respectively);
however, the parallel alignment of mesityl groups provides an
effective shielding to electron recombination with an electrolyte,
which is supported by their different electrochemical impedance
spectroscopy (EIS).21 The Nyquist plots of their EIS obtained at
an applied bias of À0.60 V in the dark are shown in Fig. 7. It was
found that the electron lifetime and recombination resistance
of DMPZn-C2-COOH-sensitized cells (25 ms and 796.8 O)
were larger than those of TPPZn-COOH-sensitized solar cells
(21 ms and 688.9 O). The longer lifetime of DMPZn-C2-COOH
most likely comes from lower electron recombination between
TiO2/electrolyte interfaces.
In summary, a simple porphyrin dye with an acrylic acid at
the meso position exhibited 5.1% energy conversion efficiency,
which is promising for cost-effective dye-sensitized solar cells.
It is expected that integration of a donor group and bulky groups
to the porphyrin framework will lead to better performance.
We are currently working in these directions and results will be
reported in due course.
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Fig. 7 Nyquist plots of DMPZn-C2-COOH and TPPZn-COOH
sensitized cells under bias of À0.60 V. The inset is an equivalent
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7619–7621 7621