Two self-assemblies of Schiff base porphyrins to modify titanium dioxide electrodes for supramolecular solar cells

Article abstract of DOI:10.1007/s11164-014-1781-8

Abstract In this work, two Schiff base porphyrins have been successfully synthesized. The metal-ligand axial coordination assembling strategy has been used to organize two assemblies based on the two porphyrins appended isonicotinic acid ligands. Further, the assemblies were absorbed on the semiconducting TiO₂ electrode surfaces by the carboxylic groups of isonicotinic acid ligands, and their photovoltaic performances were performed under irradiance of 100 mW cm^-2 AM 1.5G sunlight. Photoelectrochemical studies show a significantly enhanced photovoltaic behavior for phenol-based zinc porphyrin assembly compared to pyridine-based zinc porphyrin assembly. The UV-Vis absorption, fluorescence spectra, molecular orbital patterns, and HOMO-LUMO energy gaps of the assemblies were also performed to further understand their photovoltaic features. In addition, the assembled modes of the assemblies immobilized on TiO₂ electrode surfaces were also verified by transmission electron microscopy.

Full text of DOI:10.1007/s11164-014-1781-8

Res Chem Intermed
DOI 10.1007/s11164-014-1781-8
Two self-assemblies of Schiff base porphyrins to modify
titanium dioxide electrodes for supramolecular solar
cells
•
•
•
•
Yu Wu Jia-Cheng Liu Jing Cao Ren-Zhi Li
Neng-Zhi Jin
Received: 20 March 2014 / Accepted: 1 August 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract In this work, two Schiff base porphyrins have been successfully syn-
thesized. The metal–ligand axial coordination assembling strategy has been used to
organize two assemblies based on the two porphyrins appended isonicotinic acid
ligands. Further, the assemblies were absorbed on the semiconducting TiO2 elec-
trode surfaces by the carboxylic groups of isonicotinic acid ligands, and their
photovoltaic performances were performed under irradiance of 100 mW cm-2 AM
1.5G sunlight. Photoelectrochemical studies show a significantly enhanced photo-
voltaic behavior for phenol-based zinc porphyrin assembly compared to pyridine-
based zinc porphyrin assembly. The UV–Vis absorption, fluorescence spectra,
molecular orbital patterns, and HOMO–LUMO energy gaps of the assemblies were
also performed to further understand their photovoltaic features. In addition, the
assembled modes of the assemblies immobilized on TiO2 electrode surfaces were
also verified by transmission electron microscopy.
Keywords Schiff base porphyrin Á Self-assembly Á Solar cells
Electronic supplementary material The online version of this article (doi:10.1007/s11164-014-1781-
8) contains supplementary material, which is available to authorized users.
Y. Wu Á J.-C. Liu Á J. Cao
Northwest Normal University, Lanzhou 730070, People’s Republic of China
J.-C. Liu (&)
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093,
People’s Republic of China
e-mail: jcliu8@nwnu.edu.cn
R.-Z. Li
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China
N.-Z. Jin
Gansu Computing Center, Lanzhou 730030, People’s Republic of China
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Y. Wu et al.
Introduction
Excavation by modern society of fossil fuels leads to global warming, environ-
mental pollution, and other natural disasters, and therefore more and more people
focus on renewable energy sources. Solar energy, as one of the several promising
clean energy sources, could contribute to a stable energy supply and slow down the
ring of these natural disasters. In nature, porphyrin chromophore in chlorophyll can
be used to collect solar energy and achieve photoinduced electron and energy
transfer [1–5]. Inspired by this intriguing natural principle, much effort has been
made to modify the porphyrin unit, with an ultimate goal of using such a natural
feature to construct excellent artificial light-harvesting devices in dye-sensitized
solar cells (DSSC). It is worthy noticed that the porphyrin-based solar cell has
achieved an excellent PCE value with 13 % under simulated air mass 1.5 global
sunlight [6]. Porphyrin-based self-assemblies, as energy/electron/hole-carrier
agents, have also attracted much attention from worldwide scientists and engineers
because they can generate efficient charge separation and transfer the separated
charges to their corresponding electrodes [7–11]. We have successfully designed
and organized a series of assemblies based on N-heterocyclic porphyrin and
acetohydrazide zinc porphyrin derivatives to functionalize the nanostructured TiO2
electrode surfaces [12–18].
It is well accepted that Schiff base molecules are also considered to be the active
materials in polymerization, functional dyes, and pigments in the field of medicine
[19]. Schiff bases have been regarded as dye sensitizers in solar cells, and have
shown a perfect performance [20–22]. So, to understand the performances of the
assemblies with Schiff base porphyrins on the electrode surfaces is an urgent task. In
this paper, two novel Schiff-base zinc porphyrins (ZnPx, x = 1 and 2, shown in
Scheme 1) have been designed and synthesized. Then, a metal-mediated assembling
strategy has been used to construct two new assemblies based on ZnPx appended
isonicotinic acid ligands (A) by the coordination bonds of Zn-to-ligand on the
semiconducting TiO2 electrode surfaces, resulting in a ZnPx-A assembled approach.
The detailed assembly mode is shown in Scheme 1. The UV–Vis absorption spectra,
Scheme 1 The detailed assembly approach of the assemblies on TiO2 electrode surfaces
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Two self-assemblies of Schiff base porphyrins
fluorescence spectra, molecular orbital (MO) patterns, HOMO–LUMO energy gaps,
and the photovoltaic performances of the assemblies are studied to further
understand the behavior of the assemblies-sensitized solar cells.
Experimental
Materials and physical measurements
All solvents and reagents were used directly without further purification as
commercially analytical grade. 5,10,15,20-tetra(para-nitrophenyl)porphyrin (a) was
prepared according to the corresponding literature method [23, 24]. Electronic
absorption spectra were measured on a UV-2550 spectrometer. The Luminescence
spectrum was measured by an LS-55 (PE USA Inc) fluorescence 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 1. Surface
topography of the self-assembly films on TiO2 electrode surface was imaged using
atomic force microscopy (AFM, Nanoscope III, Digital Instruments Co.) in contact
tapping mode. Transmission electron microscopy (TEM) (Hitachi Model H-900)
was prepared to characterize the morphology and particle size distribution. An
LS1000 solar simulator (Solar Light Com. Inc., USA) was used to give an irradiance
of 100 mW cm-2 (the equivalent of one sun at AM 1.5G) at the surface of a testing
cell. The current–voltage characteristics were obtained by applying external
potential bias to the cell and measuring the dark current and photocurrent with a
Keithley model 2602 digital source meter. This process was fully automated using
Labview 8.0. A similar data acquisition system was used to control the incident
photon-to-collected electron conversion efficiency (IPCE) measurement. Under full
computer control, light from a 1,000-W xenon lamp was focused through a
monochromator onto the photovoltaic cell under testing. A computer-controlled
monochromator (Omni k300) was incremented through the spectral range
(300–900 nm) to generate a photocurrent action spectra with a sampling interval
of 10 nm and a current sampling time of 2 s. IPCE is defined by IPCE(k) = hcJsc/
eUk, where h is Planck’s constant, c is the speed of light in a vacuum, e is the
electronic charge, k is the wavelength (m), Jsc is the short-circuit photocurrent
density (mA cm-2), and U is the incident radiative flux (mW m-2). Photovoltaic
performance was measured by using a metal mask with an aperture area of
0.158 cm2. A homemade heating–cooling system was used for temperature-
dependent J-V measurements.
Device fabrication
A screen-printed double-layer film of interconnected TiO2 particles was used as the
mesoporous negative electrode. A 7-lm-thick transparent layer of 20-nm-sized
titania particles were first printed on the fluorine-doped SnO2 (FTO) conducting
glass electrode and further coated by a 5-lm-thick scattering layer of 400-nm-sized
titania particles. Film thickness was measured by a benchtop Ambios XP-1 stylus
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Y. Wu et al.
profilometer. The detailed preparation procedures of TiO2 nanocrystals, pastes for
screen printing, and nanostructured TiO2 film have been reported by Prof. P. Wang
[25]. A cycloidal TiO2 electrode (*0.28 cm2) was stained by immersing it into a
methanol solution containing isonicotinic acid (2 mM) as anchoring molecule
overnight, after removal of the unbound molecules (through three ethanol
washings), then immersing it into a dye solution containing ZnPx (0.2 mM) in
DMF for 1 h, and after this, washed by DMF solution three times. After being
washed with ethanol and dried by air flow, the sensitized titania electrode was
assembled with a thermally platinized FTO electrode. The electrodes were separated
by a 35-lm-thick Bynel (DuPont) hot-melt gasket and sealed up by heating. The
internal space was filled with a liquid electrolyte using a vacuum backfilling 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. After
all these procedures, the cells were located in the oven for heating posttreatment at
100 °C for 30 min and cooled to room temperature before photoelectrochemical
measurements.
Synthesis of porphyrins
The synthesis of b
5,10,15,20-tetra(para-nitrophenyl)porphyrin (a) (1.82 mmol, 1.45 g) was dissolved
in HCl (60 ml) and then stannous (25 mmol, 5.64 g) in HCl under argon. The
mixture was refluxed 1 h at 65 °C. Mixture in an ice bath the reaction was 0.5 h,
and neutralized with aqueous ammonia and filtered, the crude product was column
using chloroform as eluent chromatography to give b. Yield, 19 %.1H NMR
(400 MHz, CDCl3): d 2.74(s, 2H), 5.58(s, 8H), 6.98–7.01(m, 8H), 7.84–7.86(m,
8H), 8.88(s, 8H).
The synthesis of c
b (0.14 mmol, 0.094 g) and Zn(CH3COO)2 (1.08 mmol, 0.198 g) were dissolved a
mixture of methanol and chloroform under argon. The mixture was refluxed 4 h at
65 °C. After completion of the reaction, the solvent was removed, the crude product
by column chromatography with dichloromethane to give c. Yield, 65 %. 1H NMR
(400 MHz, d6-DMSO): d 5.46(s, 8H), 6.95–6.97(m, 8H), 7.79–7.81(m, 8H), 8.83(s,
8H).
The synthesis of ZnP1
c (0.133 mmol, 0.0,979 g) and 2-pyridinecarboxaldehyde (1.2 ml) were dissolved in
DMF (20 ml) under argon. The mixture was refluxed 24 h at 80 °C. After the
reaction was cooled to room temperature, water was added to give the crude product
that was filtered, washed with water and methanol, and the cake was washed and
1
dried to obtain ZnP1. Yield, 46 %. H NMR (400 MHz, d6-DMSO): d 2.73(s, 4H),
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Two self-assemblies of Schiff base porphyrins
7.61–7.62 (m, 4H), 7.78–7.80 (m, 8H), 8.05–8.09 (m, 4H), 8.82–8.90 (m, 8H),
8.35–8.36 (m, 4H), 8.99–9.10 (m, 4H). ESI–MS: m/z 1,093 [M ? H]?.
The synthesis of ZnP2
ZnP2 was synthesized in a similar procedure as ZnP1 except that 2-pyridinecar-
1
boxaldehyde (1.2 ml) was replaced by salicylaldehyde (1.2 ml). Yield, 43 %. H
NMR (400 MHz, d6-DMSO): d 7.50 (s, 4H), 7.83 (s, 12H), 7.85–7.87 (m, 8H),
8.21–8.23 (m, 8H), 8.91 (s, 8H), 9.31–9.34 (m, 4H), 13.31 (s, 4H). ESI–MS: m/z
1,153 [M ? H]? (Scheme 2).
a
b
c
Scheme 2 Synthesis of the porphyrins
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Y. Wu et al.
Results and discussion
The assembled processes of the assemblies on TiO2 surfaces were as follows: a
molecule as anchoring group was immobilized on the TiO2 electrode surface
through carboxylic groups; then the desired dye of ZnPx was bound on the a unit by
the coordination-bonded from porphyrin central Zn(II) ion of ZnPx and the N atom
of A. The detailed assembly approach is shown in Scheme 1.
In order to further demonstrate the Schiff base porphyrins efficiently immobilized
on TiO2 electrode surfaces, the surface morphologies of the assemblies immobilized
on the TiO2 nanoparticles were performed using a transmission electron microscope
(TEM). Compared to the length of two ends of the assembly obtained by
computational simulation (Fig. 1) and the film thickness identified under TEM
analysis (Fig. 2), we can observe that the distances between the two ends of
assemblies (*1.9 nm for ZnP1-A and *1.8 nm for ZnP2-A) are equal to the film
thicknesses from the direct TEM observations, indicating that the assembled mode
of the assembly ZnPx-A sensitized on TiO2 electrode surfaces should be described
as shown in Scheme 1 and Fig. 1.
The parameters of UV–Vis absorption and fluorescence spectra of ZnPx in DMF
solution and ZnPx-A absorbed on the TiO2 thin films (2.5 lm) are summarized in
Table S1. As shown in Fig. 3, all dyes exhibit the maxima of the spectra
characteristic of porphyrin attributed to p–p* transitions in the range of
400–480 nm for strong Soret (B) bands and 550–630 nm for Q bands. Comparison
of the film with the solution spectra show that the absorption bands of films are
slightly red-shifted and considerably broadening B bands from those of solution
spectra. The broaden absorption bands may be assigned to the presence of porphyrin
aggregations on the TiO2 surfaces [12, 26]. It is worthy noticed that the absorption
maxima of ZnP2-A are slightly red-shifted than those of ZnP1-A, signifying the
existence of enhanced photovoltaic performance for ZnP2-A-sensitized solar cell.
Figure 4 displays the fluorescence spectra of ZnPx in DMF solution and ZnPx-A
absorbed on the TiO2 thin films (2.5 lm). The emission spectrum of ZnP2-A is
slightly red-shifted than that of ZnP1-A, which is consistent with the variation of the
UV–Vis absorption spectra. Notedly, the spectra of ZnPx-A reveal blue shifts
Fig. 1 Molecular modeling of the assemblies on the TiO2 electrode surfaces
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Two self-assemblies of Schiff base porphyrins
Fig. 2 TEM images of TiO2 nanoparticles modified by the assemblies
0.8
ZnP1
ZnP2
0.6
ZnP1-A
ZnP2-A
0.4
0.2
0.0
400
450
500
550
600
650
Wavelength/nm
Fig. 3 UV–Vis spectra of ZnPx in DMF solution and ZnPx-A on TiO2 thin films
400
ZnP1
ZnP2
ZnP1-A
ZnP2-A
300
200
100
0
560
580
600
620
640
660
680
Wavelength/nm
Fig. 4 Fluorescence spectra of ZnPx in DMF solution and ZnPx-A on TiO2 thin films
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Y. Wu et al.
compared to those of ZnPx, ascribed to the occurrence of J-aggregations [12] of
porphyrin molecules on the TiO2 films.
To gain insight into the electron density distribution within the frontier and other
close-lying of the assemblies, we performed calculations with density functional
theory (DFT) using the Gaussian 09 program [27] at the B3LYP/LanL2DZ level. As
shown in Fig. S1, the MO patterns of HOMOs are mainly localized at porphyrin
cores with slight delocalization to the neighboring donor units, and the LUMOs
show electronic distribution on A groups. These features suggest the presence of
excellent electron-separated status for these assemblies-sensitized devices. Figure 5
reveals the energy-level diagram of these assemblies and the conducting band (CB)
TiO2. The energy gap between the HOMO and LUMO is 2.32 eV for ZnP1-A, and
2.25 eV for ZnP2-A. The LUMO levels of the assemblies are above that of the
conducting band (CB) of TiO2, indicating that the LUMO levels of the assemblies
should all be capable of the electron injection to the CB of TiO2.
The assemblies were fabricated into DSSC devices and measured their respective
photovoltaic performances under irradiance of 100 mW cm-2 AM 1.5G sunlight.
The photocurrent action spectra of the assemblies-sensitized solar cells in the
wavelength range of 300–800 nm are shown in Fig. 6. The behaviors of
photocurrent responses roughly resemble corresponding UV–Vis spectra, especially
for the features of broadened spectra. The maximum value of ZnP2-A device over
20 % is significantly larger than that of the ZnP1-A device, suggesting that the
phenol unit may serve as a potential donor group to ensure the excellent
performance of the corresponding zinc porphyrin dye for DSSC. This behavior is
also consistent with the result reported by our work [17]. As shown in Fig. 7, the
short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc), fill factor
(FF) and overall conversion efficient (g) are found to be 1.20 mA/cm2, 0.27 V, 0.65,
and 0.21 % for ZnP2-A device. Due to the faint color of sensitized electrodes and
the smaller UV–Vis absorbing intensities, ZnPx-A-sensitized cells show the lower
photovoltaic performances compared to the high-efficiency DSSCs [28, 29].
Fig. 5 Orbital energy levels of
the assemblies, TiO2, and
Dye
LUMO
-2
-3
-4
-5
ZnP2-A
2.25
ZnP1-A
2.32
electrolyte I-/I3
-
TiO
2
CB
-
-
I /I
3
Redox
HOMO
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Two self-assemblies of Schiff base porphyrins
20
15
10
5
ZnP1-A
ZnP2-A
0
400
500
600
700
λ/nm
Fig. 6 Photocurrent action spectra of ZnPx-A-sensitized solar cells
1.5
1.0
ZnP2-A
ZnP2-A(d)
0.5
0.0
0.00
0.06
0.12
0.18
0.24
0.30
Voltage/V
Fig. 7 J-V characteristics of ZnP2-A-sensitized SC measured under irradiance of 100 mW cm-2 AM
1.5G sunlight (ZnP2-A) and in the dark condition [ZnP2-A(d)]
Conclusions
In summary, we have successfully designed and obtained two new self-assemblies
based on two Schiff base porphyrins bound to isonicotinic acid by metal–ligand
axial coordination on the nanostructured TiO2 electrode surfaces. The UV–Vis
absorption, fluorescence spectra, and the photovoltaic performances of these
assemblies are also prepared. Our results reveal significantly enhanced photovoltaic
behavior for phenol-based zinc porphyrin assembly. This feature indicates the
phenol entity may be used as a potential substituent applied into the sensitizers. To
improve the solar cells’ efficiency of such an assembly, our further study about the
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Y. Wu et al.
assemblies based on porphyrin acid derivatives serving as the anchoring groups is
on process.
Acknowledgments The National Natural Science Foundation of China (No. 20871099) 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.
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