Zn-Zn porphyrin dimer-sensitized solar cells: Toward 3-D light harvesting

Article abstract of DOI:10.1021/ja9057713

(Figure Presented) Zn-Zn porphyrin dimers have been incorporated into thin dye-sensitized solar cells (DSSCs) to boost their light harvesting efficiency. The photoexcited dimers show efficient and fast electron injection into TiO ₂ indicating t

Full text of DOI:10.1021/ja9057713

Published on Web 10/09/2009
Zn-Zn Porphyrin Dimer-Sensitized Solar Cells: Toward 3-D Light Harvesting
Attila J. Mozer,*,† Matthew J. Griffith,† George Tsekouras,† Pawel Wagner,† Gordon G. Wallace,†
Shogo Mori,‡ Kenji Sunahara,‡ Masanori Miyashita,‡ John C. Earles,§ Keith C. Gordon,§ Luchao Du,|
Ryuzi Katoh,| Akihiro Furube,| and David L. Officer*,†
Intelligent Polymer Research Institute, ARC Centre for Excellence for Electromaterials Science, UniVersity of
Wollongong, Wollongong 2522, Australia, Department of Fine Materials Engineering, Shinshu UniVersity,
Nagano, 386-8567, Japan, MacDiarmid Institute for AdVanced Materials and Nanotechnology, Department of
Chemistry, UniVersity of Otago, Dunedin, New Zealand, and National Institute of AdVanced Industrial Science and
Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Japan
Received July 15, 2009; E-mail: davido@uow.edu.au; attila@uow.edu.au
The emulation of the extraordinary photosynthetic light harvest-
ing apparatus has inspired researchers to investigate the assembly
and chemistry of a wide variety of covalent and noncovalent
porphyrin arrays,1 but surprisingly there have been very few studies
of their use in solar cells.2 While the enhancement of solar-to-
electrical energy conversion efficiency by porphyrin arrays has been
theoretically demonstrated,3 there is little experimental data to
support this. In this communication, we demonstrate for the first
time that each porphyrin in an array can contribute to current
generation in a solar cell.
In the dye-sensitized solar cell (DSSC), light is absorbed by a
pigment chemically bound to titanium dioxide, where charge
separation occurs.4 Since the cross section for photon absorption
of most pigments is smaller than the geometric area occupied on
the surface, light absorption by a pigment monolayer is small.5 To
circumvent this, nanostructured semiconductor electrodes with a
surface roughness factor (internal surface area normalized to the
geometric area) on the order of a 1000 have been used. Incorporat-
ing multichromophore light harvesting arrays with increased
absorption cross sections would have the advantage that (i) novel
electrode structures with smaller internal surface areas, such as
nanotubes,6 nanowires,7 or large porosity mesoscopic structures
could be used or (ii) efficient DSSCs with thinner dye-sensitized
films using ionic liquid8 or solid state electrolytes could be
developed.9
each porphyrin dimer acts as two noninteracting electronic entities,
corresponding to the constituent porphyrin monomers P12 (red and
black components of P10 with a COOH linker) and P199 (blue
and black components of P18), which we have also synthesized
for comparison and are shown in Figure 1S along with the
appropriate orbitals. The frontier molecular orbitals of P10 and P18
(Figure 1) are comprised of spatially equivalent P12 and P199
orbitals (Figure S1). The LUMOs of both P10 and P18 correspond
to the LUMO of P199, while the HOMOs of both P10 and P18
correspond to the HOMO of P12. Furthermore, the calculated orbital
energies for P10 and P18 are largely unperturbed relative to the
energies of corresponding monomer orbitals; i.e. they resemble the
superposition of the P12 and P199 orbitals (Figures S2 and S3,
respectively). This may be due to the 68° dihedral angle between
the planes of the two porphyrin monomers within each dimer that
inhibits conjugation and electronic communication between the
subunits.
The simplest example of a light harvest porphyrin array that could
fulfill these requirements is a porphyrin dimer. In 2000, Koehorst
et al. reported the first enhanced spectral response of porphyrin
dimer-sensitized TiO2 films.10 Subsequently, we investigated the
use of a variety of porphyrin arrays in DSSCs but did not obtain
any evidence for improved photocurrent generation.11 Here, we have
designed new porphyrin dimers, comprising two efficient monopo-
rphyrin dyes linked in either a linear anti (P10) or a 90° syn (P18)
fashion, representing simple building blocks of linear or branched
3-D multichromophore arrays. Using femtosecond transient absorp-
tion (TA) and absorbed photon-to-current conversion efficiency
(APCE) measurements, we show ultrafast, highly efficient electron
injection from the photoexcited state of the porphyrin dimers into
TiO2. The results clearly show the contributions of photon absorp-
tion by both porphyrin chromophores leading to efficient DSSCs
using thin TiO2 electrodes.
Figure 1. Chemical structures and calculated HOMO/LUMO orbitals
(B3LYP/3-21g*) for porphyrin dyes.
In their UV/visible spectra (Figure 2), the dimers show an
asymmetrically broadened Soret band and the molar extinction
coefficients (ꢀ) of their Q-bands are nearly double those of the
monomers (Figure 2). Their absorption spectra are a superposition
of the monomers and no additional spectral features are observed,
indicating negligible interaction between the two porphyrin moieties
in the ground state. This is supported by the redox properties
investigated by cyclic voltammetry, which show similar oxidation
onset potentials (Table S1).
Figure 1 shows the HOMO-LUMO orbitals obtained by DFT
calculation for P10 and P18 dimers. DFT calculations show that
† University of Wollongong.
‡ Shinshu University.
§ University of Otago.
| AIST.
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C O M M U N I C A T I O N S
length and was attributed to electron recombination reactions
through space.13 No difference between the longer dimer and shorter
monomer is observed here. There is, however, an additional signal
at <10 ps for the dimer. We have measured similarly fast TA signals
for porphyrin films on SiO2 substrates that may be attributed to
delocalized excited states in, for example, dye aggregates.
Although O’Regan and Gra¨tzel report a high quantum efficiency
for a trimeric ruthenium complex dye in their original Nature paper,4
to the best of our knowledge Figure 3 is the first spectroscopic
report of efficient and fast charge injection of photoexcited organic
multichromophore units bound to TiO2.
Table 1. Photovoltaic Parameters and Dye Uptake of Porphyrin
Dyes
Figure 2. Molar extinction coefficients of P10 (solid line), P18 (dash),
Jsc
[mA cm-2
Voc
[mV]
Eff.
[%]
DYE
FF
Ua
P199 (dot), and P12 (dash dot) measured in solution.
]
P10
P18
P199
P12
N719
6.87
6.78
6.25
5.10
5.51
700
710
670
620
750
0.66
0.65
0.67
0.66
0.70
3.2
3.1
2.8
2.1
2.9
1.2
1.2
1.5
1.2
0.9
Figure 3 shows visible-pump/IR-probe femtosecond transient
absorption (TA) signals for TiO2 films sensitized with P10 porphyrin
dimer and bottom monomer P199 in a redox-containing electrolyte
(0.1 M lithium iodide LiI, 0.6 M 1,2-dimethyl-3-propylimidazolium
iodide DMPImI, 0.5 M 4-tert-butylpyridine TBP, and 0.05 M I2 in
acetonitrile). The samples were excited at 532 nm and probed at
3440 nm, with a time-delayed IR probe beam. At 3440 nm photons
are primarily absorbed by electrons in the TiO2 in the absence of
dye aggregates.12 Therefore, electron injection from the photoex-
cited dye is directly monitored. The TA signals were divided by
the absorbed pump intensity 1-10-A, where A is the absorbance
of the dye-sensitized films at the pump wavelength, which allows
the comparison of both the injection yield and the injection kinetics.
The TA signal was determined for N719 sensitized films in
air. The signal maximum of the N719 samples indicated by the
red line was taken as 100% injection yield. The risetime of the TA
features of the porphyrin-sensitized films is within the time
resolution of the setup (250 fs), and no significant difference
between the monomer and the dimer can be distinguished. The
signal magnitude after 10 ps is similar for both porphyrin dyes,
and it is ∼70% of the N719-TiO2 signals. The >50% quantum yield
for the porphyrin dimer clearly demonstrates photoinduced electron
injection from both photoexcited porphyrins within the porphyrin
dimers.
P10*
8.04
715
0.65
3.8
1.2
a Dye uptake × 10-8 mol cm-2 µm-1
.
To achieve efficient charge injection for the chromophore not
directly linked to the electron acceptor, the photoexcitation is either
transferred to the first chromophore (by energy or electron trans-
fer)10 or it may also directly inject an electron. Due to the fast
risetime of the signals in Figure 3, we cannot distinguish which
mechanism operates. The dye uptake measurements (Table 1) show
that the porphyrin monomer and dimers are absorbed onto TiO2 in
similar amounts and near the dye uptake maximum ((1.2-1.5) ×
10-8 mol cm-2 µm-1) for full surface coverage. This is surprising
given that it might appear at first sight that P18 should occupy
twice the surface area given its angular conformation. Electron
injection and recombination kinetics are quite similar for both P10
and P18 (not shown), which suggests that the constituent porphyrins
of each dimer are situated at a comparable distance from the TiO2
surface. Therefore, we propose that P10 and P18 bind in a similar
fashion as illustrated in Figure 4, which accounts for both the similar
surface coverage and their near-identical kinetics.
Figure 4. Schematic representation of P10 and P18 dimer arrangement on
the TiO2 surface.
Solar cell efficiencies of DSSCs based on porphyrin-sensitized,
transparent 2.5 µm TiO2 films (Solaronix-T) are shown in Table 1.
The short circuit current Jsc of both dimers is significantly higher
than that of N719 and up to 10% higher than that of the P199
porphyrin dye. We can also say that no major difference between
the P10 and P18 dimer is observed under these similar fabrication
conditions. As has been observed previously for porphyrin dyes,
the open circuit voltage Voc of all the porphyrin DSSCs is lower
than that of N719 due to shorter electron lifetimes.14 However, the
Voc of the dimers is improved by up to 80 mV compared to the
Figure 3. Fs-TA of porphyrin-sensitized TiO2 films excited by 150 fs pulses
at 532 nm and monitored at 3440 nm in the redox containing electrolyte.
Signals were divided by the absorbed pump intensity. N719 signals obtained
in air are also displayed.
Unlike the N719-TiO2 signal, the porphyrin-sensitized films show
prominent signal decay attributed to electron recombination reaction
with the dye cation radical on the 10 ps-to-ns time scale. A ps
component of recombination kinetics has been observed in a series
of oligo(phenylethylnyl)-linked Zn-porphyrins with a longer binder
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C O M M U N I C A T I O N S
Figure 3, which clearly competes with the dye cation regeneration
process, which has been shown to occur on the ns to µs time scale.
The fs-TA spectra as well as the IPCE/APCE measurements
independently confirm that both porphyrin chromophores contribute
to charge injection.
In conclusion, we have successfully synthesized porphyrin
dimers, where both porphyrin component chromophores show
efficient electron injection into TiO2. DFT calculations suggest that
the two component porphyrins of each dimer do not significantly
interact in their ground state. By incorporating the porphyrin dimers
into DSSCs, we have achieved up to 70% APCE. Surprisingly, no
major difference in dye uptake, injection efficiency, or device
performance has been observed between the linear or angled dimer,
suggesting both of these building blocks could, in principle, be used
to construct larger 3-D multichromophore light harvesting arrays
with efficient solar energy conversion.
Figure 5. Light harvesting (LHE), incident photon-to-current conversion
(IPCE), and absorbed photon-to-current conversion (APCE) efficiency of
P10 (red) and P199 (black)-sensitized TiO2 solar cells.
Acknowledgment. Financial support from the Australian Re-
search Council, DEST and the MacDiarmid Institute for Advanced
Materials and Nanotechnology is gratefully acknowledged.
monoporphyrin dyes. This suggests that the closely packed, bulky
porphyrin dimers have a surface blocking effect, preventing the
approach of the electron-accepting species of the electrolyte to the
TiO2 interface.
Supporting Information Available: Synthesis of compounds,
UV-visible spectroscopy, cyclic voltammetry, DFT calculations and
geometry optimization, dye solar cell fabrication, APCE calculations.
This material is available free of charge via the Internet at http://
pubs.acs.org.
We have observed that the performance of the dimer-sensitized
DSSC is highly dependent on the dye uptake conditions. Optimiza-
tion of the fabrication parameters on a 3 µm thick TiO2 film have
yielded an improved 3.8% P10 DSSC (P10* in Table 1; N719 and
P199 was 3.4% and 3.1%, respectively). These results on thinner
TiO2 films (2.5 µm) clearly demonstrate the advantage of multi-
chromophore light harvesting arrays, where doubling the dye
absorption coefficient results in significantly improved light harvest-
ing efficiency (LHE), although this will not affect the maximum
achieveable efficiency of a fully optimized thick film DSSC, whose
efficiency is dominated by the spectral coverage rather than
absorption coefficient. It will, however, be beneficial for solid state
and quasi-solid state DSSCs or other dye-sensitized photoelectro-
chemical devices in which the total internal surface area for dye
uptake is limited.
The LHE (defined as the fraction of absorbed photons to incident
photons on the sample) of the best dimer-sensitized TiO2 film (P10*)
is ∼20% higher than that of the P199 monomer due to the similar
dye uptake (Table 1) and the much higher molar extinction
coefficient (Figure 5) demonstrating the clear advantage of the
concept used in these thin devices. The incident photon-to-current
conversion efficiency (IPCE) of the dimer, which includes contribu-
tion from light harvesting, electron injection, and charge collection,
is ∼10% larger than that of P199 in the Q-band spectral region. It
is above 50%, which can only be achieved if both porphyrin units
contribute to charge injection. The absorbed photon-to-current
conversion (APCE) efficiency (calculated as IPCE/LHE; see SI)
in the absence of charge collection losses (note that thin TiO2 films
were used) is determined by the charge injection efficiency. The
APCE values are ∼70 - 80% for both P10 and P199, consistent
with the TA measurements. The <100% APCE may be attributed
to the observed fast component of the recombination kinetics in
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