Synthesis and characterization of diporphyrin sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1039/b917316a

Novel porphyrin dimers with broad and strong absorption in the visible and/or near IR regions have been synthesized; the meso-meso-linked porphyrin dimer (YDD1) exhibited the best photovoltaic performance with power conversion efficiency 5.2% under AM 1.5

Full text of DOI:10.1039/b917316a

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Synthesis and characterization of diporphyrin sensitizers for
dye-sensitized solar cellsw
Chi-Lun Mai,a Wei-Kai Huang,b Hsueh-Pei Lu,b Cheng-Wei Lee,a Chien-Lan Chiu,a
You-Ren Liang,a Eric Wei-Guang Diau*b and Chen-Yu Yeh*a
Received (in Cambridge, UK) 21st August 2009, Accepted 4th November 2009
First published as an Advance Article on the web 24th November 2009
DOI: 10.1039/b917316a
Novel porphyrin dimers with broad and strong absorption in
the visible and/or near IR regions have been synthesized; the
meso–meso-linked porphyrin dimer (YDD1) exhibited the best
photovoltaic performance with power conversion efficiency 5.2%
under AM 1.5G one solar illumination.
electrochemical and photovoltaic properties of four porphyrin
dimers; their molecular structures show diverse connectivity
between the two porphyrin macrocyles, as displayed in
Scheme 1. Details of their synthetic procedures appear in the
ESIw.16–19
Absorption spectra of these porphyrin dyes are shown in
Fig. 1; the corresponding spectral properties are listed in
Table S1 (ESIw). All porphyrin dimers exhibit a much broader
absorption than that of reference compound YD0. Compound
YDD0 shows split Soret bands in the range 400–500 nm, and
red shifts and broadening of the Q bands due to interporphyrin
electronic coupling. Dimer YDD1 also exhibits a split Soret
band ascribed to excitonic coupling. The absorption spectra of
YDD2 and YDD3 exhibit a typical feature for fused porphyrin
dimers with three major bands. Bands I and II of YDD2 and
YDD3 feature a range across almost the entire visible region.
Bands III appears at 756 and 845 nm for YDD3, whereas those
for YDD2 are much wider and range from 900 to 1300 nm.
The molar absorption coefficients at the maximum absorption
wavelengths for these dimers are all large and fall in the range
0.3–2.9 Â 105 dm3 molÀ1 cmÀ1. Fluorescence spectra of YDD2
and YDD3 were unobtainable because of the sensitivity limit
of our detector for this region, but they are expected to fall in
the near IR region.20
The development of clean and renewable energy sources
reflects the limited fossil resources and severe environmental
problems caused by their combustion. Infinite and inexhaustible
solar energy is a key resource to meet a rapidly increasing
global demand for energy. Dye-sensitized solar cells (DSSC)
are promising devices to generate clean energy as an alter-
native to the traditional solar cells based on silicon. A typical
DSSC comprises dye-sensitized nanocrystalline films of TiO2
and an iodide/triiodide mediator.1 The greatest efficiency (Z)
for the conversion of solar to electric energy for a DSSC
is 410% based on ruthenium polypyridine complexes.1b In
view of the cost and environmental concerns about ruthenium
dyes, organic dyes have attracted attention because of their
diversity, the facile modification of their molecular structures,
their intense absorption and cheap production.2–11
To generate a large photocurrent response, organic dyes in
an efficient DSSC must have broad and intense absorption in
the visible and near IR regions.12 Porphyrin sensitizers are
dominant candidates for this purpose because of their intense
absorption in Soret and Q bands to harvest solar energy
efficiently in a broad spectral region,13 but the existence of a
gap between the Soret and Q bands in monomeric porphyrins
limits their cell performances. Porphyrin arrays linked with
conjugated acetylene bridges exhibit strong electronic coupling
between porphyrin rings, resulting in splitting of the Soret
band and broadening of the Q bands.14 Electronic absorption
spectra of meso–meso-linked porphyrin arrays and their
corresponding doubly and triply fused porphyrin arrays also
show wide absorption covering the visible and near IR
region.15 By dint of such spectral features, these porphyrin
arrays are prospectively efficient sensitizers for application
in DSSC. Here we report the synthesis and the spectral,
The reduction and oxidation potentials of these porphyrin
dyes are summarized in Table S1 (ESIw). The cyclic
voltammogram of YDD0 shows two 1-eÀ reversible oxidations
at E1/2 = +0.89 and +1.08 V (Fig. S1, ESIw). The first and
second oxidations are cathodically shifted by 0.15 and 0.40 V
relative to those of YD0, reflecting elevated energy levels due
a Department of Chemistry, National Chung Hsing University,
Taichung 402, Taiwan. E-mail: cyyeh@dragon.nchu.edu.tw;
Fax: +886 4-2286-2547; Tel: +886 4-2285-2264
b Department of Applied Chemistry and Institute of Molecular
Science, National Chiao Tung University, Hsinchu 300, Taiwan.
E-mail: diau@mail.nctu.edu.tw; Fax: +886 3-572-3764;
Tel: +886 3-513-1524
w Electronic supplementary information (ESI) available: Syntheses,
characterizations, devices, measurements, supplementary Table S1
and Fig. S1–S4. See DOI: 10.1039/b917316a
Scheme 1 Molecular structures of porphyrin dyes.
Chem. Commun., 2010, 46, 809–811 | 809
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Fig. 3 (a) Current–voltage characteristics of DSSC devices with
Fig. 1 Calibrated absorption spectra of YD0 and YDD0–YDD3 in
sensitizers YD0-YDD3 under illumination of simulated AM1.5 full
sunlight (100 mW cmÀ2) with active area 0.16 cm2; (b) Corresponding
action spectra for the efficiency of incident photon-to-current conversion.
CH2Cl2/pyridine (100/1).
to strong interporphyrin interaction in YDD0. Compound
YDD1 exhibits two overlapping oxidation waves at
E
1/2 = +0.99 and +1.17 V, corresponding to 1-eÀ abstraction
(IPCE) of porphyrin-based DSSC; the corresponding photo-
voltaic parameters are summarized in Table S1 (ESIw). Both
porphyrin dimers YDD0 and YDD1 perform similarly to the
reference compound (YD0), but the fused porphyrins (YDD2
and YDD3) exhibit poor cell performance. In particular, the
photocurrents of YDD2 and YDD3 are small, consistent with
the potential feature shown in Fig. 2. For YDD2, essentially
no injected electrons were observed (Fig. 3b), because the
energy level of LUMO is substantially lower than the CB edge
of TiO2. For YDD3, a small response in the IPCE action
spectrum corresponds to the contribution of broad Bands
I and II of the fused porphyrin, but no injected electrons were
observed for the broad Band III in region 700–900 nm. We
infer that electron injection from the excited states of YDD3 to
TiO2 competed with energy relaxation from higher excited
states (Bands I/II) to the lowest excited state (Band III), and
there was insufficient kinetic energy for the electrons to inject
from Band III of YDD3 to the CB of TiO2.
from each porphyrin ring. The cyclic voltammogram of YDD2
displays two oxidations at E1/2 = +0.69 and +1.03 V, which
are cathodically shifted by 0.30 and 0.14 V, respectively,
relative to those of meso–meso-linked dimer YDD1. Similar
electrochemical behavior was observed for fused porphyrin
YDD3. The CV results of fused porphyrins show elevated
HOMO and decreased LUMO energy levels, caused by
extended p-conjugation over the two porphyrin macrocycles,
consistent with results obtained from DFT calculations
(Fig. S2, ESIw).
Fig. 2 shows energy levels of YDD0–YDD3, with YD0 for
comparison. The HOMO levels were derived from the first
oxidation potentials of the porphyrins and the LUMO levels
from the difference of the HOMO level and the absorption
threshold of each porphyrin. The HOMO levels of these
porphyrin dimers are all more positive than the oxidation
potential for IÀ/I3À, indicating that dye regeneration might be
feasible for all sensitizers; the LUMO energy levels of only
YDD0 and YDD1 are more negative than the conduction-
band (CB) edge of TiO2, whereas those of YDD2 and YDD3
are not negative enough for effective injection of the excited-
state electrons into the CB of TiO2.
Integrating the IPCE over the AM 1.5G solar spectrum
yields a calculated JSC similar to the collected value for devices
of YD0, YDD0 and YDD1 (Fig. S3, ESIw), confirming the
accuracy of the current–voltage results shown in Fig. 3a. As
shown there, the short-circuit photocurrent density (JSC) of
YDD1 is greater than that of YD0, whereas the open-circuit
voltage (VOC) of the former is smaller than the latter; the net
effect produces a slightly greater power-conversion efficiency
of the former than that of the latter (Z = 5.23 vs. 5.14%). The
fact that the JSC value of YDD1 was significantly greater than
that of YD0 is understood to be due to the effective excitonic
coupling of the two porphyrin macrocycles in the dimer,
whereas such a character was absent in the monomer. As a
result, the IPCE spectrum of YDD1 exhibits a flat response
over the entire visible region whereas that of YD0 shows a
large gap between the Soret and Q bands (Fig. 3b). In contrast,
the overall efficiency of YDD0 was significantly smaller than
that of YDD1 (4.07 vs. 5.23%) because of the smaller JSC and
Fig. 3a and b show the current–voltage characteristics and
efficiencies of conversion of incident photons to current
V
OC values of the former. Even though the IPCE spectrum of
YDD0 shown in Fig. 3b displays a broad feature covering
spectral range 400–800 nm, the IPCE values of YDD0 were
much smaller than those of YDD1 in the region of spectral
response.
Fig. 2 Schematic energy levels of porphyrins YD0 and YDD0–YDD3
based on absorption and electrochemical data in ESIw.
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We notice that the absorption coefficients of YDD0 are, in
general, greater than those of YD0 and YDD1 (Fig. 1), but the
values of IPCE of the former are substantially lower than
those of the latter (Fig. 3b). As the results of dye-loading
experiments (Fig. S4, ESIw) show similar amounts of dye
molecules (B75 nmol cmÀ2) being sensitized on TiO2 films
for YD0-YDD1, the smaller external quantum efficiencies of
YDD0 relative to those of YD0 or YDD1 are thus inferred to
arise from the efficiency of electron injection into TiO2, which
was smaller for the former than for the latter. There are two
reasons that explain these effects; one is that the LUMO
energy level of YDD0 is much lower than that of YD0 or
YDD1, which might impede electron injection for the former.
The other is that the nearly planar structure of YDD0
facilitates p-conjugation between two porphyrin macrocycles
and provides a decreased driving force to push electrons
toward TiO2; this explanation is consistent with the frontier
orbital pictures shown in the supplementary Fig. S2 (ESIw).
The planar geometry of YDD0 might also facilitate the
formation of dye aggregates that significantly decrease the
efficiency of electron injection.
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