Design and characterization of alkoxy-wrapped push-pull porphyrins for dye-sensitized solar cells

Article abstract of DOI:10.1039/c2cc31111a

Three alkoxy-wrapped push-pull porphyrins were designed and synthesized for dye-sensitized solar cell (DSSC) applications. Spectral, electrochemical, photovoltaic and electrochemical impedance spectroscopy properties of these porphyrin sensitizers were we

Full text of DOI:10.1039/c2cc31111a

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Cite this: Chem. Commun., 2012, 48, 4368–4370
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COMMUNICATION
Design and characterization of alkoxy-wrapped push–pull porphyrins
for dye-sensitized solar cellswz
Teresa Ripolles-Sanchis,a Bo-Cheng Guo,b Hui-Ping Wu,c Tsung-Yu Pan,c Hsuan-Wei Lee,b
Sonia R. Raga,a Francisco Fabregat-Santiago,a Juan Bisquert,*a Chen-Yu Yeh*b and
Eric Wei-Guang Diau*c
Received 15th February 2012, Accepted 9th March 2012
DOI: 10.1039/c2cc31111a
Three alkoxy-wrapped push–pull porphyrins were designed and
synthesized for dye-sensitized solar cell (DSSC) applications.
Spectral, electrochemical, photovoltaic and electrochemical
impedance spectroscopy properties of these porphyrin sensitizers
were well investigated to provide evidence for the molecular
design.
Porphyrins are promising candidates as highly efficient sensitizers
for dye-sensitized solar cells (DSSC) because of their superior
light-harvesting ability in the visible region.1–3 Recent advances
on the development of a porphyrin sensitizer (YD2-o-C8) with
Chart 1 Molecular structures for YD20–YD22 porphyrin dyes.
co-sensitization of an organic dye (Y123) using a cobalt-based
electrolyte attained a power conversion efficiency of 12.3%,4
which is superior to those developed based on Ru complexes5
with two methoxyl substitutes and YD22 has a phenylamino
group with two n-butyl chains. On the other hand, YD20 and
and becomes a new milestone in this area. The key structural
YD21 dyes have the same donor group but the cyanoacrylic
feature on molecular design of a highly efficient porphyrin
acid was used as an anchoring group in YD21. This approach
mimics the molecular design of an organic dye7 having the
sensitizer is to bear with long alkoxyl chains in the ortho-
positions of the meso-phenyls so as to effectively envelope the
acrylonitrile group with strong electron-pulling power to act as
porphyrin ring to reduce the degree of dye aggregation for a
an efficient acceptor for the porphyrin dye.
higher electron injection yield and to form a blocking layer for a
better charge collection yield.6 In the present study, we further
The details for the syntheses, optical and electrochemical
characterizations of YD20–YD22 are given in ESI.z These
design three porphyrin sensitizers (YD20–YD22, Chart 1) based
porphyrin dyes were fabricated into DSSC devices for photo-
on the structure of YD2-o-C8 but with extended p-conjugation in
order to enhance the light-harvesting ability. Basically all of them
voltaic and electrochemical impedance spectroscopy (EIS)
characterizations. Fig. 1a and b show the J–V curves and
have the same ortho-substituted porphyrin core with two
the corresponding Incident Photon to Current Conversion
phenylethynyl (PE) groups acting as a p-bridge in the meso-
position of the ring. YD20 and YD22 dyes have the acceptor
Efficiency (IPCE) action spectra for the YD20–YD22 devices,
respectively; the obtained photovoltaic parameters and the amounts
group (ethynylbenzoic acid) the same as that of YD2-o-C8 but
of dye-loading are summarized in Table 1. The results indicate that
with different donor groups: YD20 has a triphenylamino group
the short-circuit current densities (JSC) exhibit a trend YD20 4
YD22 4 YD21 and the open-circuit voltages (VOC) display a trend
a Photovoltaics and Optoelectronic Devices Group,
´
´
Departament de Fısica, Universitat Jaume I, 12071 Castello, Spain.
E-mail: bisquert@fca.uji.es; Tel: +34-964-38-7540
b Department of Chemistry and Center of Nanoscience &
Nanotechnology, National Chung Hsing University, Taichung 402,
Taiwan. E-mail: cyyeh@dragon.nchu.edu.tw;
YD20 4 YD22 B YD21; the overall power conversion efficiencies
(Z) show the same order as JSC, which is consistent with the
variations of the IPCE action spectra showing the same order.
As a result, YD20 has the highest JSC (17.43 mA cmꢀ2) and VOC
(676 mV), which yields the greatest Z (8.1%) among the three
porphyrins under investigation. Even though the cyanoacrylic
substitute makes YD21 a slight red shift in the absorption spectrum
(Fig. S1, ESIz), the floppy feature of the CQC double bond might
tilt the molecules adsorbed on TiO2 film to significantly decrease its
IPCE values and the corresponding current density. However,
YD20 and YD22 have the same anchoring group and very similar
Fax: +886-4-22862547; Tel: +886-4-22852264
c Department of Applied Chemistry and Institute of Molecular
Science, National Chiao Tung University, Hsinchu 30010, Taiwan.
E-mail: diau@mail.nctu.edu.tw; Fax: +886-3-5723764;
Tel: +886-3-5131524
w This article is part of the ChemComm ‘‘Porphyrins and Phthalocyanines’’
web themed issue.
z Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc31111a
c
4368 Chem. Commun., 2012, 48, 4368–4370
This journal is The Royal Society of Chemistry 2012

Fig. 1 (a) Current vs. voltage characteristics of DSSC devices prepared
with YD20 (black), YD21 (red), and YD22 (green) under illumination
of simulated AM 1.5 full sunlight (100 W cmꢀ2) with an active layer of
0.16 cm2 and (b) the corresponding action spectra for the efficiency of
incident photon-to-current conversion (IPCE).
Table 1 Photovoltaic parameters of porphyrin-based dye-sensitized
solar cells (active layer 0.16 cm2) under 100 mW cmꢀ2 light illumination
(AM 1.5 G) for YD20–YD22
Fig. 2 (a) Capacitance, (b) transport resistance, (c) recombination
resistance, and (d) diffusion length of YD20–YD22 dyes in DSSC
plotted with respect to the Fermi level voltage (ꢀVF) with removing
the effect of series resistance.
Dye loading/
nmol cmꢀ2
JSC
/
VOC
mV
/
Z
(%)
Dye
mA cmꢀ2
FF
YD20
YD21
YD22
161
132
134
17.43
12.05
14.87
676
631
634
0.686
0.721
0.700
8.1
5.5
6.6
all the TiO2 conduction bands remain almost unchanged for the
three dyes as obtained from the capacitance data.
To understand the origin of the small differences in the VOC
found for the three different dyes it is needed to analyze the
behavior of the recombination resistance in Fig. 2c. In previous
studies,8,10 when comparing the recombination resistance of
different samples it has been found that the higher the value of
absorption spectra (Fig. S1, ESIz), therefore, the differences in
IPCE and photocurrent are related to the effect of the donor
groups. Note that the decrease in the IPCE occurs at a nearly
constant level for all the wavelengths of the spectra for YD21
compared to YD20. Thus, the loss of electrons is independent
of the energy of the absorbed photons. Transport and injection
losses may be considered for the decrease in IPCE, which is
discussed in the following.
Rrec, the larger the VOC, while only very large changes in photo-
current produce small variations in VOC. The results here match
very well with this analysis: as it can be seen in Fig. 2c, YD20 has
the larger recombination resistance and VOC, whereas YD21 and
Dye loading measurements yielded 161, 132, and 134 nmol
cmꢀ2 for YD20, YD21 and YD22, respectively. The changes in
JSC between the dyes with the same anchoring group, YD20
and YD22, may be understood in terms of the different
amounts of loaded sensitizer. Further explanation is needed
for sample YD21 as the decrease in JSC is larger despite the
amount of dye loading in the cell is the same as for YD22.
Electrochemical Impedance Spectroscopy was used to
complete the analysis of injection and to gain insight into
the transport and charge losses characteristics of the DSSC
with the different dyes.8 From the fitting of impedance
spectra of the DSSC at different applied potentials under 1
sun illumination, we obtained the chemical capacitance (Cm),
transport resistance in the TiO2 (Rtr), recombination resistance
(Rrec), as a function of the Fermi level voltage (VF) shown in
Fig. 2a, b, and c, respectively. Other contributions to the total
resistance of the cell such as diffusion, counter electrode and FTO
resistances were grouped as series resistance (Rs). The effect of Rs in
the applied potential (Vapp) was removed to obtain the VF that
may be calculated through VF = Vapp ꢀ jRs. From the plot of
Cm vs. ꢀVF shown in Fig. 2a, the position of the conduction band
edge of TiO2 (Ec) may be estimated as reported elsewhere.9
Through these calculations, we estimated that for YD20 Ec E
ꢀ0.48 V vs. NHE, while for YD21 Ec was displaced +4 mV and
YD22 ꢀ10 mV. Data from transport resistance shown in Fig. 2b
also provide very small displacements in Ec, corroborating that
YD22 have similar values of Rrec showing almost the same VOC
.
Data from Rrec and Rtr may be used to calculate the
diffusion length (Ln) in TiO2 film shown in Fig. 2d as8
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Ln ¼ L Rrec=Rtr
ð1Þ
where L is the film thickness (15 mm) represented as a dashed
curve in Fig. 2d. The Ln values exhibit a systematic trend with
the order YD20 4 YD22 4 YD21 with those of YD20 and
YD22 reaching values greater than their film thickness whereas
those of YD21 being significantly smaller than the film thickness.
This implies that the YD21 device suffers from a poorer collection
efficiency of injected electrons what produces the extra decrease in
JSC found for this sample.
The small differences found for the position of the conduction
band edge (Ec) may also help to fine tune the roles of the linker in
these Zn–porphyrin dyes. If the Fermi level potential is shifted the
amounts found for the displacement of Ec, it is possible to compare
the recombination resistance of the DSSC at the potential level
with the same number of injected electrons. To do this we define
the potential at the equivalent conduction band position8
Vecb = VF ꢀ DEc/e
(2)
where e is the electron charge and DEc = Ec ꢀ Ec,ref, for which
E
c,ref is the position of the conduction band of YD20. Based on
Chem. Commun., 2012, 48, 4368–4370 4369
c
This journal is The Royal Society of Chemistry 2012

the ATU program. JB acknowledges support by projects from
Ministerio de Ciencia e Innovacion (MICINN) of Spain
´
(Consolider HOPE CSD2007-00007, MAT2010-19827), and
Generalitat Valenciana (PROMETEO/2009/058). SRR thanks
financial support from Bancaixa foundation under project
Innova 11I272. CYY and EWGD acknowledge support
by projects from National Science Council of Taiwan and
Ministry of Education of Taiwan, under the ATU program.
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In conclusion, although the concept for molecular design
with the cyanoacrylic acid acceptor has been widely applied in
highly efficient organic dyes,7 such an approach does not work
well for the porphyrin sensitizers as demonstrated herein. The
greater performance in the YD20 device than the other two
devices is attributed to its rigid structural feature for a larger
amount of dye-loading, which combined with the higher
recombination resistance and diffusion length yields to larger
JSC and VOC. Modification of the porphyrin structure with
extended p-conjugation for better light harvesting is feasible to
boost up the device performance in the near future.
This work was partially supported by National Science
Council of Taiwan and Ministry of Education of Taiwan, under
Sanchis, H.-P. Wu, L.-L. Li, C.-Y. Yeh, E. W.-G. Diau and
J. Bisquert, J. Phys. Chem. C, 2011, 115, 10898–10902.
c
4370 Chem. Commun., 2012, 48, 4368–4370
This journal is The Royal Society of Chemistry 2012