A novel self-assembly with horizontal anchor porphyrin for supramolecular solar cells

Article abstract of DOI:10.1039/c6ra07834f

In this work, employing the meta-mediated assembling strategy for organizing different metal ions (M, M = Zn²⁺, Cd²⁺ and Co²⁺) horizontal anchor porphyrins (MPA) and two antennas molecules (Lx, x = 1, 2) on the nanostructured TiO₂ electrode surfaces have been prepared, resulting in a ZnPA+Lx assembled approach. Further, the assemblies were absorbed on the semiconducting TiO₂ electrode surfaces by the carboxylic groups of horizontal anchor porphyrin, and their photovoltaic performances in solar cells were performed under an irradiance of 100 mW cm^-2 AM 1.5G solar simulator. The photoelectric conversion efficiency of the solar cells sensitized by the dyes with triphenylamine group antenna molecule (L2) were obviously enhancement when compared to that sensitized by the dye with dimethylamino group antenna molecule (L1). The results were also verified by the transient photovoltage decay, theory calculations and optical performance. In addition, the assembled modes of the assemblies immobilized on TiO₂ electrode surfaces were also verified by transmission electron microscopy.

Full text of DOI:10.1039/c6ra07834f

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A novel self-assembly with horizontal anchor
porphyrin for supramolecular solar cells†
Cite this: RSC Adv., 2016, 6, 60773
a
a
a
ab
c
*
*
Ren-Zhi Li
Yu Wu, Jun-Xiang Zhang, Xiao-Xia Feng, Jia-Cheng Liu,
and Neng-Zhi Jind
In this work, employing the meta-mediated assembling strategy for organizing different metal ions (M, M ¼
Zn2+, Cd2+ and Co2+) horizontal anchor porphyrins (MPA) and two antennas molecules (Lx, x ¼ 1, 2) on the
nanostructured TiO2 electrode surfaces have been prepared, resulting in a ZnPA+Lx assembled approach.
Further, the assemblies were absorbed on the semiconducting TiO2 electrode surfaces by the carboxylic
groups of horizontal anchor porphyrin, and their photovoltaic performances in solar cells were
performed under an irradiance of 100 mW cmꢀ2 AM 1.5G solar simulator. The photoelectric conversion
efficiency of the solar cells sensitized by the dyes with triphenylamine group antenna molecule (L2) were
obviously enhancement when compared to that sensitized by the dye with dimethylamino group
antenna molecule (L1). The results were also verified by the transient photovoltage decay, theory
calculations and optical performance. In addition, the assembled modes of the assemblies immobilized
on TiO2 electrode surfaces were also verified by transmission electron microscopy.
Received 25th March 2016
Accepted 17th June 2016
DOI: 10.1039/c6ra07834f
www.rsc.org/advances
Porphyrin-based self-assemblies, as energy/electron/hole-
carrier agents, have also attracted much attention because
Introduction
they can generate efficient charge separation and carrier of
separated charges in their corresponding photoelectrochemical
devices.13–17 Up to now, a series of surface-fabrication methods
have been reported. Imahori et al. presented a Pd-mediated
stepwise self-assembly of zinc porphyrin on the SnO2 elec-
trode surface.18 D'Souza et al. reported a metal–ligand axial
coordination approach to construct a series of (donor)1–
(donor)2 porphyrin-based assemblies.19 Our recent work mainly
focuses on zinc porphyrin derivatives appended organic acid via
metal–ligand axial coordination attached to the TiO2 electrode
surfaces,20,21 which use of vertical anchors molecular. To the
best our knowledge, the document of organization assemblies
of horizontal anchor porphyrins are very few.
Herein, we designed and synthesized horizontal anchor
porphyrins with different metal ions (M, M ¼ Zn2+, Cd2+ and
Co2+) (denoted as MPA, shown in Scheme 1), which selecting
metal-mediums of Zn2+ can construct horizon anchor porphyrin
molecules on the TiO2 electrode surface. Further, such a novel
self-assembly (described as ZnPA+Lx, x ¼ 1, 2) with two antenna
molecule (denoted as Lx, x ¼ 1, 2, shown in Scheme 1) and
horizontal anchor zinc porphyrin by metal–ligand axial coor-
dination was constructed, wherein these assemblies were
immobilized directly on the double-layer nanostructured TiO2
and fabricated into solar cells. The optical properties, topog-
raphy, theoretical calculations, transient photovoltage decay
and the photovoltaic performances of the assemblies are
studied to further understand the behavior of the ZnPA+Lx
based dye-sensitized solar cells.
Dye-sensitized solar cells (DSSCs), as an alternative energy
technology, have received much attention for worldwide scien-
tists and engineers owing to their low costs, ease of fabrication,
and minimal environmental impact.1–5 It is well-known that
porphyrins dyes play a key role in this type of DSSC, in which
they used to collect solar energy, achieve photoinduced electron
and transfer solar energy into electricities.6 Zinc porphyrin dyes
have been reported based on a donor–p–acceptor (D–p–A)
architecture, such as SM315, which exhibited remarkably high
efficiencies up to 13%.7 These porphyrin dyes were linked to
surface of TiO2 through anchoring groups such as carboxyl,8
salicylic acid,9 pyridine,10 tropolone,11 to enable the porphyrin
ring plane almost to be vertical. Additionally, Rochford et al.12
reported a typical horizontal porphyrin with four benzoic acids
as anchoring group to bind onto the TiO2 electrode surface.
aKey Laboratory of Eco-Environment-Related Polymer Materials of Ministry of
Education, Key Laboratory of Polymer Materials of Gansu Province, Key Laboratory
of Bioelectrochemistry & Environmental Analysis of Gansu Province, College of
Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou
730070, People's Republic of China. E-mail: jcliu8@nwnu.edu.cn
bState Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093,
People's Republic of China
cKey Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM),
Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM),
Nanjing Tech University (Nanjing Tech), Nanjing 211816, People's Republic of
China. E-mail: iamrzli@njtech.edu.cn
dGansu Computing Center, Lanzhou 730030, People's Republic of China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra07834f
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Device fabrication
The detailed preparation procedures of TiO2 nanocrystals,
pastes for screen-printing, and nanostructured TiO2 lm have
been reported by Prof. P. Wang.24 A cycloidal TiO2 electrode
(ꢁ0.28 cm2) was stained by immersing it into a methanol
solution containing ZnPA (2 mM) as anchoring molecule over-
night, aer removal of the unbound molecules (through three
ethanol washings), then the electrode was immersed into a dye
solution containing Lx (0.2 mM) in CHCl3/DMF (85/15, v/v) for 1
h, then, washed by acetonitrile solution three times and dried
by air ow. The sensitized titania electrode was assembled with
a thermally platinized FTO electrode. The electrodes were
separated by a 35 mm-thick Bynel (DuPont) hot-melt gasket and
sealed up by heating. The internal space was lled with a liquid
electrolyte using a vacuum backlling system. The electrolyte-
injecting hole on the counter electrode glass substrate, made
with a sand-blasting drill, was sealed with a Bynel sheet and
a thin glass cover by heating. The electrolyte used contained 50
mM LiI, 30 mM I2 in acetonitrile solvent. Aer all these proce-
dures, the cells were located in the oven for heating post
treatment at 100 ꢂC for 30 min and cooled to room temperature
before photoelectrochemical measurements.
Scheme 1 Structural diagram of MPA and antenna molecule (Lx, x ¼ 1, 2).
Experimental section
General methods
All solvents and reagents were used directly without further
purication as commercially analytical grade. 5,10,15,20-
Tetra(3-cyanophenyl) porphyrin (a) was synthesized according
to Lindsey's method with slight modication.22 UV-vis spectra
were measured on a UV-2550 spectrometer. Luminescence
spectrum was measured by an LS-55 (PE USA Inc) uorescence
spectrophotometer at room temperature. 1H NMR (400 MHz)
was measured on a Varian Mercury Plus-400 spectrometer. The
detailed synthesis processes are shown in Scheme 3 and 4.
Surface topography of the self-assembly lms on TiO2 electrode
surface was imaged using transmission electron microscopy
(TEM) (Hitachi Model H-900). The current density–voltage (J–V)
and incident photon-to-collected electron conversion efficiency
(IPCE) were measured under an irradiance of 100 mW cmꢀ2 (AM
1.5G) by using a metal mask with an aperture area of 0.158 cm2.
The details of photoelectrochemical measurements were illus-
trated in our previous article.21 Transient photoelectrical
examinations were tested on Autolab-PGSTAT302N electro-
chemical workstation equipped with ADC10M high-speed
module.23
Synthesis of dyes
General procedure for free base porphyrin (a). 3-Cyano-
phenyl (0.98 g, 7.5 mmol) was reuxed in a mixture propionic
acid (50 mL) and 0.52 mL of distilled pyrrole (7.5 mmol) was
then added dropwise. Aer complete addition, the reaction
mixture was stirred for another 2 h and cooled to room
temperature. Aer evaporation under reduced pressure of the
solvent to dryness, the solid residue was dissolved in methylene
chloride and chromatographed on aluminum oxide. Elution
with methylene chloride to give a in 18% yield as a purple solid,
which was used in the metalation step without further
characterization.
Scheme 2 The assembled approach of the assemblies on TiO2 electrode surfaces.
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Scheme 3 Synthesis protocol for anchoring molecule.
ltrate neutralized with 6 M HCl to precipitate appears. Allowed
to stand overnight, with sand core funnel ltration, and dried to
obtain the product ZnPA, CdPA and CoPA. Yield, 73%.
ZnPA. 1H NMR (600 MHz, DMSO-d6) d 13.2 (s, 4H), 8.86–8.83
(s, 10H), 8.46–8.33 (m, 8H), 8.23–8.12 (m, 4H), 6.83 (s, 2H). MS
(ESI, m/z): [M + H]+ calcd for C49H31N4O8Zn, 867.2; found, 868.1.
General procedure for antenna molecule dyes L1 and L2.
Syntheses of antenna molecule were prepared according to the
corresponding literature method.25 Aldehydes and 4-amino-
pyridine equimolar amounts were mixed in 50 mL toluene
under stirring at heating (80–100 ꢂC) for 4 h. Aer completion of
the reaction, the reaction was cooled to room temperature,
acetic-acid were added the reaction. The reaction mixture was
ltered and recrystallized from methanol. Yields: 86–90%.
Scheme 4 Synthesis protocol for antenna molecule.
General procedure for a metalation (b). The free base
a porphyrins (0.4 mmol) was dissolved in CHCl3 (80 mL). A
solution of M(OAc)2$2H2O (1.6 mmol) (M: Zn(OAC)2$2H2O;
Cd(OAc)2$2H2O; Co(OAc)2$2H2O) in MeOH (10 mL) was added
to the porphyrin solution in one portion, and the reaction
mixture was reuxed 4 h. The solvents were removed in vacuo
leaving a purple solid, which was dissolved in CH2Cl2 and
washed successively with 5% aqueous NaHCO3 and water. The
organic layer was dried over Na2SO4, and the solvent was
removed in vacuo. The metalloporphyrins were obtained in 80%
1
L1. H NMR (600 MHz, DMSO-d6) 3.32 (6H, s), 5.16 (1H, s),
7.26–7.88 (8H, m). MS (ESI, m/z): [M + H]+ calcd for C14H15N3,
225.2; found, 226.1.
L2. 1H NMR (600 MHz, DMSO-d6) 5.16 (1H, s), 6.60–6.68 (6H,
m), 7.26–7.88 (10H, m). MS (ESI, m/z): [M + H]+ calcd for
C
14H15N3, 349.2; found, 350.1.
yields
following
silica
gel
chromatography
with
dichloromethane/ethyl acetate (95 : 5) to afford pure
metalloporphyrins.
Results and discussion
The preparation processes of the assemblies on TiO2 surfaces
were as follows: a porphyrin molecule (ZnPA) as anchoring
group was immobilized on the TiO2 electrode surface through
carboxylic groups: then the antenna molecule of Lx was boun-
ded on the porphyrin by axially coordination to the central Zn(II)
ion in the anchoring porphyrin ZnPA via the N atom of Lx. The
detailed assembly approach is shown in Scheme 2.
General procedure for metalloporphyrins alkaline hydrolysis
(c). The metalloporphyrins were added 5 M NaOH (10 mL), then
add 8–10 mL C2H5OH, and the reaction mixture heated at 80 ꢂC
for 48 h, with moist pH paper detecting whether the reaction of
ammonia discharged there, until no ammonia gas evolution
reaction. Diluted with water, ltration of impurities, and the
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Surface topography
In order to further demonstrate the zinc porphyrins are effi-
ciently immobilized on the TiO2 electrode surfaces, the surface
morphologies of the assemblies immobilized on the TiO2
nanoparticles were performed using a transmission electron
microscope (TEM).18,26 With respect to the comparison of the
height of two ends of the dyad obtained by computational
simulation (Fig. 1) and the lm thickness identied by TEM
analysis (Fig. 2). As shown in Fig. 1, the thicknesses of the self-
assembly systems as ZnPA+L1 and ZnPA+L2 are 1.9 nm and 2.1
nm respectively, which are in agreement with the results of TEM
shown in Fig. 2, indicating that the assembled mode of the
assembly ZnPA+Lx sensitized on TiO2 electrode surfaces should
be described are shown in Scheme 2 and Fig. 1.
Fig.
3
UV-vis absorption spectra of ZnPA in DMF solution and
UV-visible and uorescence spectral properties
ZnPAꢀf, ZnPA+Lx on TiO2 thin films.
The UV-vis absorption spectra of ZnPA, ZnPAꢀf and ZnPA+Lx
are shown in Fig. 3 and the corresponding spectroscopic data
are summarized in Table 1.
Table 1 The peak wavelength of the UV-vis absorption and fluores-
cence spectra of ZnPA in DMF solution and ZnPAꢀf, ZnPA+Lx on the
TiO2 thin films
As expected, zinc porphyrin dyes exhibit a typical intense
absorbing Soret band within 400–480 nm and moderate
absorbing Q bands in a range of 550–630 nm, which are
attributed to the p–p* electron transition. In DMF solution,
ZnPA showed an intense absorption peak for the Soret band at
425 nm and weak absorption speak for the Q bands at 558 nm
and 597 nm. On TiO2 lms, the dyes showed a slight red shi
and considerably broadening as compared to those in solution.
Dye
Absorptiona lmax/nm
Emissiona lmax/nm
ZnPA
425, 558, 597
428, 557, 595
427, 563, 601
430, 560, 598
608, 654
596, 655
594, 651
596, 652
ZnPAꢀf
ZnPA+L1
ZnPA+L2
a
The absorption and emission data were measured in DMF. Excitation
wavelengths/nm: 420.
The broaden absorption bands of ZnPA+Lx may be assigned to
the presence leads to the formation of dyes aggregates.27
Furthermore, the Q bands of the self-assemblies display slightly
red-shied than that of anchoring porphyrin ZnPA immobilized
on TiO2 thin lm (ZnPAꢀf). That indicates the assembly dyes
enhanced the photoelectronic responses.
Fig. 4 showed the uorescence spectra of ZnPA in DMF
solution and ZnPAꢀf, ZnPA+Lx absorbed on the TiO2 thin lms
(2.5 mm). The emission spectrum of ZnPA+L2 is slightly red-
shied than that of ZnPA+L1, which is consistent with the
variation of the UV-vis absorption spectra. When jointing Lx
onto TiO2 surface via anchoring porphyrin ZnPA, the spectra of
ZnPAꢀf and ZnPA+Lx are weakened and blue shied. These
reveal blue shis compared to those of ZnPA, ascribed to the
occurrence of J-aggregations of porphyrin molecules on the
TiO2 lms.28 The weakened uorescence intensities indicated
the existence of electronic coupling behavior between Lx and
Fig. 1 Molecular modeling of ZnPA+Lx on TiO2 electrode surfaces.
anchoring
porphyrin
through
Zinc-to-ligand
axial
coordination.29,30
Theoretical calculations and energy levels
To provide the electronic structure of the dyes, their geometries
and energies were fully optimized by DFT using the Gaussian 09
program at the B3LYP/LANL2DZ level.31,32 The molecule orbital
Fig. 2 TEM images of TiO2 nanoparticles modified with ZnPA+Lx.
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Fig. 6 Orbital energy-levels of the assemblies, TiO2 and electrolyte Iꢀ/I3
.
ꢀ
Fig. 4 Fluorescence spectra of ZnPA in DMF solution and ZnPAꢀf,
ZnPA+Lx on TiO2 thin films.
bands of porphyrin, roughly analogous to their corresponding
UV-vis spectra. The maximum IPCE value of ZnPA+Lx devices
approaching 70%, which is signicantly larger than that of
anchor porphyrin (ZnPA) device and also outperforms those of
others assemblies reported by D'Souza and co-workers.19 Thus,
their onset wavelength thresholds (Fig. 7) lie in the order of
ZnPA < ZnPA+L1 < ZnPA+L2, which is in consistent with the
sequence of photocurrent Jsc (vide infra).
The photocurrent–voltage (J–V) curves of DSSCs with ZnPA,
ZnPA+L1, and ZnPA+L2 were shown in Fig. 8 and the photo-
voltaic parameters were presented in Table 2. The open-circuit
voltages of the cell sensitized by ZnPA higher than that of
ZnPA+Lx, which the assembly cells possess the dye aggregation.
Owing to the antenna elements retarded the dye regeneration
and then reduced the electron lifetime of the conducting band
of titania accordingly, thus this presence could be partially
responsible for the reduction of Voc. To be more specic, the Jsc
values within 3.23–4.08 mA cmꢀ2 for ZnPA+Lx are higher than
that of 2.33 mA cmꢀ2 for ZnPA; this may be ascribed to the
broadened absorption caused by the enhanced electron-
donating ability of the antenna elements relative to the
(MO) patterns of HOMO and LUMO of dyes were shown in
Fig. 5. The electrons of the HOMO levels for all the dyes mainly
distribute over the porphyrin center framework, while the
LUMO orbitals are markedly delocalized over the porphyrin
macrocycle turn to the anchoring group. Fig. 6 reveals the
energy-level diagram of these assemblies and conducting band
(CB) TiO2. The LUMO levels of these assemblies above that of
the CB of TiO2, and the HOMO levels below that of the elec-
trolyte Iꢀ/I3ꢀ. These results conrm assemblies should be effi-
cient to inject electrons to the CB of TiO2 upon excitation and
the corresponding oxidized molecule can be efficiently reduced
by the electrolyte.33
Photovoltaic performance of DSSCs
The assemblies ZnPA+Lx were sensitized on the TiO2 lms to
serve as working electrodes of DSSC devices for the photovoltaic
characterization under irradiance of 100 mW cmꢀ2 AM 1.5G
solar simulator. As shown in Fig. 7, the photocurrent action
spectra of ZnPA+Lx reveals the energy conversion of B and Q
Fig. 5 Molecular orbitals of ZnPA, ZnPA+L1 and ZnPA+L2.
Fig. 7 IPCE action spectra of porphyrin-sensitized solar cells.
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Fig. 8 J–V curves of porphyrin-sensitized solar cells.
Table 2 Photovoltaic performance of DSSCs based on porphyrin dyes
Dye
Jsc (mA cmꢀ2
)
Voc (V)
FF (%)
h (%)
ZnPA
ZnPA+L1
ZnPA+L2
2.33
3.23
4.08
0.38
0.35
0.36
69.5
63.2
64.5
0.61
0.71
0.95
anchor porphyrin in ZnPA. The highest Jsc of 4.08 mA cmꢀ2
achieved for ZnPA+L2 due to the presence of the triphenylamine
group. As a result, ZnPA+L1 and ZnPA+L2 demonstrate DSSC
efficiencies of 0.71% and 0.95%, respectively, higher than that
of ZnPA.
Fig. 9 (a) Plots of charges extracted from a dye-grafted titania film
TPD
1/2
(QCE) versus Voc; (b) plots of electron half-lifetimes (t ) as a function
of QCE
.
Charge extraction and transient photovoltage decay
To examine the interfacial energetic and dynamic origins of the
Voc variation,34 charge extraction (CE) and transient photo-
voltage decay (TPD) measurements were performed on the
assembly devices with ZnPA+L1 and ZnPA+L2. As illustrated in
Fig. 9a, the dye alternation from ZnPA+L1 to ZnPA+L2 does not
affect the conduction-band edge and the distribution of trap
states of titania with respect to the electrolyte.35 However at
a given QCE, photo-injected electrons in titania, the electron half
lifetime (tT1/P2D) for the ZnPA+L1 device is much shorter than the
ZnPA+L2 cell (Fig. 9b), mirroring the same tendency of Voc.
Table 3 Photovoltaic performance of DSSCs based on MPA series
dyes
Dye
Jsc (mA cmꢀ2
)
Voc (V)
FF (%)
h (%)
ZnPA
CdPA
CoPA
2.33
0.54
0.41
0.38
0.23
0.20
69.5
61.6
51.0
0.61
0.07
0.04
conclusion, when the metal-mediums of Zn can construct
horizon anchor porphyrin molecules on the TiO2 electrode
surface to improve electron injection efficiency and preserve the
photoexcited energy.15,36
The effect of different metal-mediums in solar cells
To further selecting suitable metal-mediums for anchor
porphyrin (MPA), the performances of the MPA series dyes were
investigated by photophysical and photovoltaic methods
(Fig. S1–S6†), and the photovoltaic parameters were displayed
Conclusion
in Table 3. The cell performance of ZnPA demonstrated the In summary, the horizontal anchor porphyrins with different
highest photoelectric conversion efficiency than others. As seen metal ions and antennas molecules have been designed and
in Table 3, the devices of ZnPA display signicantly enhanced synthesized. The zinc ion could be used successfully for hori-
conversion efficiency, roughly consistent with the variation of zontal anchor porphyrin. Further, we have successfully
HOMO–LUMO gaps, UV-vis and uorescence spectra. In designed and obtained a novel type assembly based on
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antennas molecules appended horizontal anchor porphyrin 14 M. S. Choi, T. Yamazaki, I. Yamazaki and T. Aida, Angew.
(ZnPA) via metal–ligand axial coordination and modied the Chem., Int. Ed., 2004, 43, 150–158.
nano-structured TiO2 electrode surface. The optical, photovol- 15 M. Morisue, S. Yamatsu, N. Haruta and Y. Kobuke, Chem.–
taic, transient photovoltage decay and theory calculations of Eur. J., 2005, 11, 5563–5574.
these assemblies are also prepared. In addition, the assembly 16 C. B. Kc, K. Stranius, P. D'Souza, N. K. Subbaiyan,
devices afford more effective transmission channel of the
charge separated state than mono-horizontal anchor porphyrin,
H. Lemmetyinen, N. V. Tkachenko and F. D'Souza, J. Phys.
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¨
which is exhibits a better photovoltaic performance. This work 17 C. Siegers, J. Hohl-Ebinger, B. Zimmermann, U. Wurfel,
¨
provides a basis for further investigation in supramolecular
solar cells.
R. Mulhaupt, A. Hinsch and R. Haag, ChemPhysChem,
2007, 8, 1548–1556.
18 A. Kira, T. Umeyama, Y. Matano, K. Yoshida, S. Isoda,
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Acknowledgements
The National Natural Science Foundation of China (No.
21461023) has supported this work. We are very grateful to Prof.
Peng Wang (Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences) for supplying device fabrication
and measurement of solar cells. We also acknowledge the
support of Gansu Computing Center of china.
19 N. K. Subbaiyan, C. A. Wijesinghe and F. D'Souza, J. Am.
Chem. Soc., 2009, 131, 14646–14647.
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