A strategy to design highly efficient porphyrin sensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1039/c1cc12764k

We designed highly efficient porphyrin sensitizers with two phenyl groups at meso-positions of the macrocycle bearing two ortho-substituted long alkoxyl chains for dye-sensitized solar cells; the ortho-substituted devices exhibit significantly enhanced ph

Full text of DOI:10.1039/c1cc12764k

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COMMUNICATION
A strategy to design highly efficient porphyrin sensitizers for
dye-sensitized solar cellsw
Yu-Cheng Chang,a Chin-Li Wang,b Tsung-Yu Pan,a Shang-Hao Hong,b Chi-Ming Lan,a
Hshin-Hui Kuo,b Chen-Fu Lo,b Hung-Yu Hsu,a Ching-Yao Lin*b and Eric Wei-Guang Diau*a
Received 11th May 2011, Accepted 6th June 2011
DOI: 10.1039/c1cc12764k
We designed highly efficient porphyrin sensitizers with two
phenyl groups at meso-positions of the macrocycle bearing two
ortho-substituted long alkoxyl chains for dye-sensitized solar
cells; the ortho-substituted devices exhibit significantly enhanced
photovoltaic performances with the best porphyrin, LD14,
showing JSC = 19.167 mA cmÀ2, VOC = 0.736 V, FF = 0.711,
and overall power conversion efficiency g = 10.17%.
Because of their excellent light-harvesting property with intense
absorption throughout the visible to the near-IR region,
various porphyrins have been designed and applied for dye-
sensitized solar cells (DSSC).1 Optimization of the device
performance for a push–pull zinc porphyrin (YD2) has
attained power conversion efficiency Z = 11.0%,2 which is a
new milestone for a porphyrin-based DSSC to compete with
Chart 1 Molecular structures of PE1 and the LD11–LD14
porphyrins.
electron injection with an appropriate design for an organic
dye. To tackle this problem, we introduce in this communication a
new concept to design a zinc porphyrin sensitizer with long
alkoxyl chains to protect the porphyrin core for a retarded
charge recombination and also to decrease effectively the dye
aggregation for an efficient electron injection.
the highly efficient ruthenium-based DSSC developed almost
two decades ago. This great performance of YD2 is due to its
superior light-harvesting ability through introduction of a
diarylamino group at the meso-position of the porphyrin ring.3
A similar approach was applied in introducing a pyrene
moiety through a p-conjugated link coupled to a porphyrin
core (LD4) to enhance greatly the light-harvesting ability
beyond 800 nm.4 The open-circuit voltages (VOC) of those
highly efficient porphyrin dyes were reported,3,4 however, to be
significantly less than that of the commonly used ruthenium
dye N719. Mozer et al.5 stated that the significantly diminished
electron lifetime is the main reason accounÀting for the smaller
VOC of porphyrins; they proposed that I3 in the electrolyte
might be attached to the positively charged Zn-center of the
porphyrin core for an efficient electron interception from the
TiO2 surface to occur. Tian and co-workers,6 who reviewed
the photovoltaic results for many organic and ruthenium
sensitizers, concluded that VOC can be improved upon decreasing
the charge recombination and increasing the efficiency of
Chart 1 shows the molecular structures of the proposed
porphyrins based on this design. The core structures are based
on PE1 reported elsewhere,7 with two phenyl substituents
bearing two dodecoxyl (–OC12H25) chains at the ortho-
position of each phenyl group (labeled LD12); the reference
porphyrin (LD11) has three dodecoxyl groups at the meta- and
para-positions of each phenyl group. We designed similarly a
push–pull porphyrin (LD14) based on the structure of LD13
reported elsewhere.8 The syntheses of these porphyrins are
similar to those reported elsewhere;7-9 details of the procedures
are given in ESI.w Fig. S1w shows UV/vis absorption spectra of
the four porphyrins in THF solution; the related spectral
data are summarized in Table S1.w We investigated the
electrochemical properties of these porphyrins using cyclic
voltammetry (CV); the cyclic voltammograms are shown in
Fig. S2w with the resulting oxidation and reduction potentials
summarized in Table S1.w Based on the CV results, a
potential-level diagram showing the HOMO and the LUMO
of each porphyrin is presented in Fig. 1. These CV results
indicate that the porphyrins with long alkoxyl chains attached
at the ortho-positions of the phenyl rings have more negative
reduction and oxidation potentials than those with long
a 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
b Department of Applied Chemistry, National Chi Nan University,
Puli, Nantou Hsien 54561, Taiwan. E-mail: cyl@ncnu.edu.tw;
Fax: +886 49-2917956; Tel: +886 49-2910960 ext. 4152
w Electronic supplementary information (ESI) available: Fig. S1–S10,
Tables S1–S5, and experimental details. See DOI: 10.1039/c1cc12764k
c
8910 Chem. Commun., 2011, 47, 8910–8912
This journal is The Royal Society of Chemistry 2011

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Fig. 1 Potential-level diagram of the LD11–LD14 porphyrins, the
TiO2 conduction band, and the redox mediator.
alkoxyl chains attached at meta- and para-positions of the
phenyl rings. As a result, the LUMO levels of LD12 and
LD14 are above those of LD11 and LD13, respectively.
Such information further implies that the ortho-substituted
porphyrins might have larger VOC and more rapid electron
injection than their meta-substituted counterparts, to enhance
the photovoltaic performance of the corresponding devices, as
discussed below.
Fig. 2 (a) Current–voltage curves and (b) IPCE action spectra of
devices made of LD11–LD14 porphyrins.
Table 1 Photovoltaic parameters of TiO2 films sensitized with por-
phyrins LD11–LD14 under simulated AM-1.5G illumination (100
mW cmÀ2) and an active area of 0.16 cm2
Dye
JSC/mA cmÀ2
VOC/mV
FF
Z (%)
The LD11–LD14 porphyrins were fabricated into DSSC
devices according to procedures reported elsewhere.4 Fig. 2a
and b show current–voltage characteristics and IPCE action
spectra of the corresponding devices, respectively; the resulting
photovoltaic parameters are summarized in Table 1. Both VOC
and JSC of LD12 are significantly greater than those of LD11,
leading to 55% enhancement of the overall power conversion
efficiency for the two devices (7.43 vs. 4.78%). The amounts of
dye loading were determined to be 96 and 153 nmol cmÀ2 for
LD12 and LD11, respectively, which show a trend opposite to
their JSC values (13.235 vs. 9.735 mA cmÀ2). As expected,
JSC becomes significantly improved on incorporating the
p-conjugated phenylethynyl dimethylamino group for both
LD14 and LD13 (19.167 and 18.438 mA cmÀ2), and VOC of
the former is also greater than that of the latter (0.736 vs. 0.697 V).
Both the enhanced VOC and JSC of the LD14 device yield a
remarkable power conversion efficiency, Z = 10.17%.
LD11
LD12
LD13
LD14
9.735
13.235
18.438
19.167
674
741
697
736
0.728
0.758
0.727
0.721
4.78
7.43
9.34
10.17
The results indicate that there exists an apparent potential
shift of ECB for each system: ECB of LD12 and LD14 were
increased by 34 and 24 meV compared to that of LD11 and
LD13, respectively, explaining the enhancement of VOC of the
corresponding devices shown in Fig. 2. The observed increased
ECB of the ortho-substituted devices is consistent with the
raised ELUMO of the corresponding porphyrins shown in
Fig. 1.
To compare the charge recombination kinetics at the elec-
trolyte/TiO2 interface for the LD11/LD12 system and the
LD13/LD14 system, respectively, Fig. 4a and b show semi-
logarithmic plots of electron lifetime (tR) vs. VOC. Because of
the shifts of ECB for the LD12 and LD14 devices, a fair
comparison of the degree of charge recombination should be
made at the same reference potential level. As a result, the
potential scales of tR were decreased by 34 and 24 mV for
LD12 and LD14, respectively, so that the electron lifetimes
could be compared at the same ‘equivalent common conduc-
tion band’ level.12 The shifted tR vs. VOC plots of the LD12
and the LD14 devices are indicated as dashed lines in Fig. 4a
and b, respectively. After corrections for the shifts of the
conduction band edge of TiO2 for both systems, we still
observed tR for the ortho-substituted porphyrins to be greater
than those for their reference compounds; the discrepancy of
tR between LD13 and LD14 is particularly notable. These
results indicate that the ortho-substituted long alkoxyl chains
The electron-transport kinetics10 were measured with seven
white-light intensities as bias irradiation (powers in a range
10–70 mW cmÀ2). The intensities of the probe pulse at 655 nm
were appropriately attenuated so that the perturbation
characteristic of the probe was maintained. For each measure-
ment the probe pulse was focused on the device with the bias
irradiation; a quick shutter in the probe path was triggered to
block the probe beam so as to determine the zero of reaction
duration. The resulting transients are shown in Fig. S3–S10
(ESIw); the corresponding kinetic parameters, determined
from fitting the transients according to a procedure introduced
previously,10 are summarized in Tables S2–S5.w Fig. 3a and b
show plots of chemical capacitance (Cm) vs. VOC for the
LD11/LD12 and the LD13/LD14 systems, respectively; a
characteristic exponential rise feature of Cm as a function of
À
VOC was obtained for each plot. As Cm is proportional to
11
play a key role in preventing the approach of I3 in the
the density of states of TiO2 at each VOC
,
the plots shown
electrolyte to the surface of TiO2 so as to retard the electron
interception in the electrolyte/TiO2 interface. In addition to
the increased ECB, the slower charge recombination is another
in Fig. 3 provide direct information about the shift of
the conduction band edge of TiO2 (ECB) upon dye uptake.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8910–8912 8911

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applications9a,16 because of their great steric hindrance preventing
molecular aggregation. For porphyrin-sensitized solar cells, Hupp
and co-workers17 reported that ortho-substituted porphyrins with
two anchoring groups showed a device performance slightly less
than that of an N719 dye. In the present work, such a molecular
design was applied for a push–pull porphyrin (LD14) to attain an
exceptional cell performance, Z = 10.2%. According to the crystal
structure of an ortho-substituted porphyrin reported by Nikiforov
et al.,18 we expect that the four dodecoxyl chains in the devices
play a key role in wrapping the porphyrin core in a shape that
prevents dye aggregation effectively and forms a blocking layer
on the TiO2 surface. The net effect is to increase ELUMO
of the molecule, to up-shift ECB of TiO2 upon dye uptake,
and to retard charge recombination in the electrolyte/TiO2 inter-
face for an enhanced device performance. Work is in progress to
incorporate this concept of molecular design for other porphyrins
to attain even better device performance.
Fig. 3 Plots of chemical capacitance (Cm) vs. open-circuit voltage
(VOC) showing the exponential distribution of the density of states of
the TiO2 potential for comparisons of (a) LD11 with LD12 and (b)
LD13 with LD14.
National Science Council of Taiwan and Ministry of
Education of Taiwan, under the ATU program, provided
support for this project.
factor accounting for the enhanced VOC, as we observed for
the LD12 and LD14 devices.
Notes and references
A strategy to improve VOC for organic dyes has always been an
issue attracting many profound investigations. For example,
Tian and co-workers,13 who studied cone-shaped organic dyes,
observed an enhanced VOC for the dye with a methoxyl group;
they speculated that the increased ELUMO might produce deep
electron injection for retarded charge recombination. Mori and
co-workers14 found only a little influence on the shift of ECB for
devices made of organic dyes and concluded that the smaller
VOC of organic sensitizers relative to those of Ru complexes
is due mainly to their smaller electron lifetimes. Hagfeldt and
co-workers15 investigated triphenylamino organic dye-based solar
cells and made a similar conclusion: varied VOC reflect varied
electron lifetimes rather than varied positions of ECB. For zinc
porphyrin sensitizers, we observed significant decreases of ECB for
porphyrins with a long phenylethynyl link.9 In the present work,
not only the contribution of charge recombination but also the
increased ECB through dye uptake should be considered for the
observed VOC enhancement.
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Fig. 4 Plots of electron lifetime (tR) vs. open-circuit voltage (VOC
)
showing the degree of charge recombination for comparisons of (a)
LD11 with LD12 and (b) LD13 with LD14. The dashed lines show the
data of LD12 (a) and LD14 (b) devices with a potential displacement
according to the shifts indicated in Fig. 3.
c
8912 Chem. Commun., 2011, 47, 8910–8912
This journal is The Royal Society of Chemistry 2011