Novel D-π-A structured Zn(ii)-porphyrin dyes containing a bis(3,3-dimethylfluorenyl)amine moiety for dye-sensitised solar cells

Article abstract of DOI:10.1039/c2cc31384g

A high solar-to-electricity conversion efficiency of 7.22% was achieved with a short circuit current (J_sc) of 15.30 mA cm^-2, an open circuit voltage (V_oc) of 669 mV and a fill factor (FF) of 0.71 for the 2Flu-ZnP-CN-COOH dye with a multi-functional co-adsorbent, under 100 mW cm^-2 AM 1.5 G simulated light.

Full text of DOI:10.1039/c2cc31384g

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COMMUNICATION
Novel D–p–A structured Zn(II)-porphyrin dyes containing a
bis(3,3-dimethylfluorenyl)amine moiety for dye-sensitised solar cellswz
Min Soo Kang,a Sung Ho Kang,a Sang Gyun Kim,a In Taek Choi,a Jung Ho Ryu,a
Myung Jong Ju,a Daeheum Cho,b Jin Yong Leeb and Hwan Kyu Kim*a
Received 24th February 2012, Accepted 19th March 2012
DOI: 10.1039/c2cc31384g
A high solar-to-electricity conversion efficiency of 7.22% was
achieved with a short circuit current (Jsc) of 15.30 mA cmꢀ2, an
open circuit voltage (Voc) of 669 mV and a fill factor (FF) of 0.71 for
the 2Flu–ZnP–CN–COOH dye with a multi-functional co-adsorbent,
under 100 mW cmꢀ2 AM 1.5 G simulated light.
and an electron-withdrawing carboxyphenylethynyl anchoring
group at the opposite meso-position have exhibited remarkably
high Z values of up to 12.3%.7 The additional electron-donating
group effect of the electron-donating substituent attached to the
core of the p-system in sensitisers has been addressed extensively.6
Liu and co-workers demonstrated that broad absorption
bands and easy intra-molecular charge separation can be
achieved by introducing a cyano-acrylic acid anchoring group
linked through a phenylene bridge to the porphyrin dye, which
eventually improved on the good cell performance of DSSCs.9
In this communication, in an effort to develop a novel porphyrin
sensitiser possessing both highly efficient and long-term stable
properties,8b we report the first synthesis and the optical, electro-
chemical and photovoltaic properties of a push–pull porphyrin
with an electron-donating bis(3,3-dimethylfluorenyl)amine
group at the meso-position and a strongly electron-withdrawing
cyano-acrylic acid anchoring group at the opposite meso-position
linked through a phenylene p-conjugation bridge, as well as the
reference porphyrin with a carboxylic acid moiety for DSSC
(Fig. 1). The detailed synthetic procedure10 and characterization
are shown in the ESI.z
Dye-sensitised solar cells (DSSCs)1 have been widely regarded
as next-generation photovoltaics for providing electricity at
lower expense and with more versatility. Although an overall
light-to-electricity energy conversion efficiency above 11% has
been established with ruthenium-based sensitisers,1b the limited
resources and the environmental issues concerning ruthenium
usage make it necessary to seek alternative dyes. Porphyrins are
some of the most widely studied sensitisers for DSSCs because
of their strongly absorbing Soret bands (400–450 nm) and
moderately absorbing Q bands (550–600 nm).2 Nevertheless,
porphyrin sensitisers based on the light-harvesting properties of
the basic porphyrin core typically have admittedly moderate cell
performance compared with ruthenium polypyridyl complexes.
Imahori and co-workers have reported that this occurs mainly
because of the insufficient light-harvesting ability of the typical
porphyrins used in optimised cells, especially at around 500 nm
and at a wavelength beyond 600 nm,3,4 so that broadening and
red-shifting of Soret and Q bands together with an enhancement of
the light-harvesting capability of the Q bands relative to the Soret
band in porphyrins are straightforward approaches to increasing
their efficiency as sensitisers in DSSCs.3–5 As demonstrated
for other sensitisers in DSSCs, the introduction of both an
electron-donating substituent and an electron-withdrawing
substituent including an anchoring group to a core with a
p-system is also appealing for the modulation of the light-
harvesting properties of sensitisers.6–8 In fact, push–pull porphyrins
with an electron-donating diarylamino group at the meso-position
The UV-visible absorption and steady-state fluorescence
spectra of 2Flu–ZnP–COOH and 2Flu–ZnP–CN–COOH in
THF solution and adsorption onto TiO2 film (3 mm) were
measured (see Fig. S1, ESIz). The peak positions and molar
absorption coefficients (e) of the Soret and Q bands as well as
the wavelengths for the emission maxima are listed in Table 1.
The Soret and Q bands of two Zn(II)-porphyrin dyes became
broader and were shifted toward longer wavelengths compared
with our previously reported Zn(II)-porphyrins2b,11 due to the
a Department of Advanced Materials Chemistry, Korea University,
208 Seochang, Jochiwon, ChungNam 339-700, Korea.
E-mail: hkk777@korea.ac.kr; Fax: +82-41-867-5396;
Tel: +82-41-860-1493
b Department of Chemistry, Sungkyunkwan University,
Suwon 440-746, Korea
w This article is part of the ChemComm ‘Porphyrins and phthalocyanines’
web themed issue.
z Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc31384g
Fig. 1 Molecular structures of Zn(II)-porphyrin dyes.
Chem. Commun., 2012, 48, 9349–9351 9349
c
This journal is The Royal Society of Chemistry 2012

Table 1 Photophysical, electrochemical and photovoltaic performance data of Zn(II)-porphyrin dyes
Photovoltaic performance datad
Potentials and energy level
Eoxb/V E0–0c/V
Eox
Absorption lmaxa/nm
(e/Mꢀ1 cmꢀ1
Emission
lmax
ꢀ
(vs. NHE) (vs. NHE) E0–0d/V
Jsc/mA cmꢀ2 Voc/mV FF Z (%)
a
Dye
)
2Flu–ZnP–CN–COOH 420(201508), 570(8065), 671
625(10271)
0.89
0.89
1.93
1.94
ꢀ1.04 without CDCA 10.037
579
628
669
558
580
708
0.75 4.37
0.72 6.64
0.71 7.22
0.77 3.16
0.73 4.7
with CDCA
with HC-A
14.588
15.303
2Flu–ZnP–COOH
419(253788), 569(9169), 667
623(10273)
ꢀ1.05 without CDCA 7.371
with CDCA
11.104
16.738
N719
0.73 8.64
a
b
Absorption and emission spectra were measured in THF solution. The oxidation potentials of dyes were measured in THF with 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) with a scan rate of 50 mV sꢀ1 (vs. NHE). c E0–0 was determined from the intersection of normalized
absorption and emission spectra. d The concentration was maintained at 2 ꢁ 10ꢀ4 M in ethanol solution, with 0.4 mM chenodeoxycholic acid (CDCA) or in
ethanol/THF (2 : 1) solution with 0.1 mM HC-A as a coadsorbent, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), and 0.1 M LiI, 0.05 M I2, 0.5 M
4-tert-butylpyridine (TBP) in dry acetonitrile as electrolyte. Performances of DSSCs were measured within a 0.16 cm2 working area using a metal mask.
directly connected chemical structure between the electron-rich
N atom and the porphyrin unit, as expected.12 The absorptions at
800 nm for the dyes adsorbed onto a TiO2 film are not consistent
with the IPCE spectrum. These absorptions can be attributed to
absorption of light by the dye cation due to the introduction of
the diarylamino groups on the porphyrin.5b The zero–zero
excitation energies (E0–0) were determined to be 1.93 eV for
2Flu–ZnP–CN–COOH and 1.94 eV for 2Flu–ZnP–COOH,
respectively, through the intersection of the normalised
absorption and emission spectra (Fig. S2, ESIz). The oxidation
potentials of porphyrins were measured by cyclic voltammetry
(see Fig. S5, ESIz) and are also summarised in Table 1. The
injection potential (ES+/S*) was calculated by subtracting the
excitation energy (E0–0) from the first oxidation potential (ES+/S
Fig. 3 The incident photon-to-current conversion efficiency (IPCE)
)
spectra for DSSCs based on Zn(II)-porphyrin dyes.
of the sensitiser. Both porphyrin dyes showed the same ES+/S
values of +0.89 V vs. NHE, which are low enough to be below
the redox potential of Iꢀ/I3ꢀ to ensure efficient regeneration. The
injection potential (ꢀ1.04 V vs. NHE for 2Flu–ZnP–CN–COOH
and ꢀ1.05 V vs. NHE for 2Flu–ZnP–COOH) is also energetically
favourable because of the more negative LUMO values above the
conduction band edge of TiO2 (commonly recognised as ꢀ0.5 V vs.
NHE13). The current density–voltage (J–V) curves and the incident
photon to current conversion efficiency (IPCE) spectra of DSSCs
incorporated with porphyrins are shown in Fig. 2 and 3, respectively.
The relevant parameters are also listed in Table 1. The detailed
cell fabrication procedure is presented in the ESI.z As illustrated
in Fig. 3, the IPCE spectrum of 2Flu–ZnP–CN–COOH-based
cells with co-adsorbents such as CDCA or HC-A (see Fig. S8,
ESIz) showed an onset as long as 740 nm and steeply
reached nearly a plateau value close to 70% from around
650 nm up to 400 nm, demonstrating efficient light harvesting
in the visible region. Interestingly, the IPCE response of the
2Flu–ZnP–CN–COOH-based device with the multi-functional
co-adsorbent HC-A14 was enhanced compared to that with
CDCA. We previously reported that both Jsc and Voc of
organic dye-based DSSCs could be dramatically enhanced by
introducing a multi-functional co-adsorbent due to the light-
harvesting effect at shorter wavelength regions and efficient
retardation of charge recombination.14,15 However, in the case
of 2Flu–ZnP–COOH-based cells, the spectrum onset is shifted
toward shorter wavelength regions around 700 nm with a
reduced IPCE value. The 2Flu–ZnP–CN–COOH based-cell
without CDCA showed similar behaviour to 2Flu–ZnP–COOH.
This indicates that the introduction of a cyano-acrylic acid
electron accepting group promoted efficient intra-molecular
charge separation. As we know, the aggregation of dye could
decrease both Jsc and Voc and the use of CDCA could
effectively suppress dye aggregation.14–16 When CDCA is not
introduced into the device, rapid charge recombination
between neighbouring dye molecules occurs so that intra-
molecular charge separation may be interfered with due to
rapid migration of excited state electrons.15 Such an enhanced
spectral response induced by the efficient intra-molecular
charge separation and use of the multi-functional co-adsorbent
HC-A resulted in an overall photovoltaic performance of
Fig. 2 Photocurrent density vs. voltage curves for DSSCs based on
Zn(II)-porphyrin dyes and N719 under AM 1.5 G simulated solar light
(100 mW cmꢀ2) and TiO2 film thickness and active area of 20 (12 + 8)
mm and 0.16 cm2, respectively.
c
9350 Chem. Commun., 2012, 48, 9349–9351
This journal is The Royal Society of Chemistry 2012

Jsc = 15.30 mA cmꢀ2, Voc = 669 mV, FF = 0.71 and
Z = 7.22%.
candidate for highly efficient DSSCs. EIS measurements also
showed that charge recombination at the TiO2/dye/electrolyte
interface can be efficiently suppressed by intra-molecular
charge separation as well as co-adsorption of HC-A, thus
resulting in higher Voc.
We carried out density-functional theory (DFT) calculations to
investigate the structures and charge transfer properties employing
Becke’s three parameterised Lee–Yang–Parr (B3LYP) exchange
correlation functional and 6-31G* basis sets using a suite of
Gaussian 09 programmes.17 The molecular orbital shapes of
2Flu–ZnP–COOH and 2Flu–ZnP–CN–COOH show a remarkable
difference of charge transfer (see Fig. S6, ESIz). The highest
occupied molecular orbital (HOMO) shapes of both porphyrins
were very similar. However, the lowest unoccupied molecular
orbital (LUMO) shapes of the two porphyrins are very different.
The electrons in the LUMO of 2Flu–ZnP–COOH were mainly
distributed over the porphyrin moiety. On the other hand, the
electrons in the LUMO of 2Flu–ZnP–CN–COOH were mainly
distributed over the cyano-acryl substituted phenyl moiety, which is
responsible for the charge transfer.
This research was supported by the New & Renewable
Energy of the Korea Institute of Energy Technology Evaluation
and Planning (KETEP) with grants funded by the Korean
Ministry of Knowledge Economy (No. 20103060010020), the
World Class University program funded by the Ministry of
Education, Science and Technology through the National
Research Foundation of Korea (R31-2011-000-10035-0) and by
the Converging Research Center Program through the Ministry
of Education, Science and Technology (2010K000973).
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porphyrin dyes on the J–V characteristics, electrochemical
impedance spectroscopy (see Fig. S7 and Table S1, ESIz)
was performed under dark conditions at a forward bias of
ꢀ0.6 V.14,15 The larger semicircle at intermediate frequencies
represents the interfacial charge transfer resistance (Rct) at the
dyed TiO2/electrolyte interface. The fitted Rct values of
2Flu–ZnP–CN–COOH-based devices (155.6 O for 2Flu–ZnP–
CN–COOH/HC-A, 40.94 O for 2Flu–ZnP–CN–COOH/CDCA)
are much higher than those of a 2Flu–ZnP–COOH-based device
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2Flu–ZnP–CN–COOH with HC-A was greater than with
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the immersion conditions. Nevertheless, despite the high dye
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In conclusion, two kinds of push–pull Zn(II)-porphyrin dyes
with an electron-donating bis(3,3-dimethylfluorenyl)amine
group at the meso-position were successfully designed and
synthesised. The results of calculations and experiments clearly
demonstrate that efficient intra-molecular charge separation of
D–p–A Zn(II)-porphyrin can be induced by inserting a strongly
electron accepting cyano-acrylic acid substituent, resulting in
relatively high overall energy conversion efficiency. The IPCE
response and J–V characteristics were further enhanced by
co-adsorption of HC-A instead of CDCA, making it a promising
17 M. J. Frisch, et al., Gaussian 09, Revision B.01, Gaussian Inc.,
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9349–9351 9351

Showcasing research from Prof. Hwan Kyu Kim’s
laboratory, Department of Chemistry, Korea
University, Korea
As featured in:
Novel D–π–A structured Zn(II)-porphyrin dyes containing a
bis(3,3-dimethylfluorenyl)amine moiety for dye-sensitised
solar cells
The first synthesis and the photovoltaic properties of a
push-pull porphyrin with an electron-donating
bis(3,3-dimethylfluorenyl)amine group at the meso-position and
a strongly electron-withdrawing cyanoacrylic acid anchoring group
at the opposite meso-position, linked through a phenylene
π-conjugation bridge, are described.
See Han Kyu Kim et al.,
Chem. Commun., 2012, 48, 9349.
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