A novel self-assembly based on double-layer zinc porphyrin sensitizers in supramolecular solar cell

Article abstract of DOI:10.1080/00958972.2018.1551528

Two new self-assemblies based on zinc porphyrin dyes ZnPx-ZnPA (x = 1, 2) were synthesized and used for dye-sensitized solar cells. Capping layer dyes consist of zinc porphyrin (ZnP) as core unit and carbazole (P1), phenothiazine (P2) as electron-donating

Full text of DOI:10.1080/00958972.2018.1551528

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
A novel self-assembly based on double-layer zinc
porphyrin sensitizers in supramolecular solar cell
Yu Wu, Hua Liu, Jia-Cheng Liu, Ren-Zhi Li & Neng-Zhi Jin
To cite this article: Yu Wu, Hua Liu, Jia-Cheng Liu, Ren-Zhi Li & Neng-Zhi Jin (2018): A novel self-
assembly based on double-layer zinc porphyrin sensitizers in supramolecular solar cell, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2018.1551528
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A novel self-assembly based on double-layer zinc porphyrin sensitizers in
supramolecular solar cell
YU WU*a, HUA LIUb, JIA-CHENG LIU*b, REN-ZHI LIc and NENG-ZHI JINd
aResearch Center of New Catalytic Materials of Guizhou Province, School of Chemistry and Chemical Engineering,
Qiannan Normal University for Nationalities, Duyun 558000, People’s Republic of China
bKey Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province, Key Laboratory of
Eco-Environment-Related Polymer Materials of Ministry of Education, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou 730070, 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
dGansu Computing Center, Lanzhou 730030, People’s Republic of China
Abstract
Two new self-assemblies based on zinc porphyrin dyes ZnPx-ZnPA (x=1,2) were synthesized
and used for dye-sensitized solar cells. Capping layer dyes consist of zinc porphyrin (ZnP) as
core unit and carbazole (P1), phenothiazine (P2) as electron-donating group. The dye ZnPA
contains carboxylic group as an anchor porphyrin with upper dyes (denoted as ZnP1 and ZnP2)
formed dyads by coordination bonds of Zn-to-ligand self-assembled devices. The assembly
modes were also verified by transmission electron microscopy (TEM). The influence of the
upper porphyrins on molecular structure as well as photovoltaic performance was investigated
via photophysical and electrochemical studies and calculations. With addition of the
phenothiazine unit, the ZnP2-ZnPA possesses better light-harvesting capability with a
significantly red-shifted Q-band. The photoelectrochemical efficiencies for ZnP2-ZnPA are
better than those of ZnP1-ZnPA ascribed to larger Jsc and Voc.
*Corresponding authors. ywu5@sgmtu.edu.cn (Y. Wu); jcliu8@nwnu.edu.cn (J.C. Liu)
1

Graphical abstract
Keywords: Dye-sensitized solar cells; Zinc porphyrin sensitizers; Self-assembly; Anchoring
porphyrin
1. Introduction
The energy crisis and environmental pollution have generated attention to green renewable
energy. Dye-sensitized solar cells (DSSCs), as one of several promising clean energy sources,
have attracted attention due to their supramolecular photosynthetic architectures to mimic the
photoinduced energy and electron transfer processes [1-5]. Zinc-porphyrin sensitizers composed
of donor-π-acceptors (D-π-A) are promising avenues in DSSCs in terms of good light-harvesting
ability and high efficiency [6, 7]. SM315 [8] is a highly efficient porphyrin sensitizer, but is
difficult to synthesize. Porphyrin-based self-assemblies can generate efficient charge separation
in photoelectrochemical devices [9-14]. Although the self-assembly system simplifies the
chemical synthesis and optimizes the porphyrin structure, our recent work mainly focuses on
2

zinc porphyrin derivatives appended with organic acid via metal-ligand axial coordination
attached to the TiO2 electrode surfaces [15-18].
Aromatic amine derivatives are electron-rich compounds and are widely used in
optoelectronics [19]. Researchers commonly used arylamine derivatives as donors of
photosensitizers in dye-sensitized solar cells [20]. Common electron donors are triphenylamine,
triarylamine, indole, phenothiazine and carbazole. Carbazole and phenothiazine groups with
strong photoelectric properties are good donors. In this paper, two kinds of electron donors are
used as the axis coordination porphyrin and the anchoring porphyrin to construct a
supramolecular dye-sensitized solar cell.
We synthesized two new self-assemblies (described as ZnPx-ZnPA, x=1,2) cells. Two
zinc porphyrins substituted at the meso-positions with different units (denoted as ZnPx, x=1,2,
shown in scheme 1) and anchor porphyrin (denoted as ZnPA, shown in scheme 1) by
metal-ligand axial coordination were immobilized directly on the double-layer nanostructured
TiO2 and fabricated into solar cells. To probe into the impact of different donor groups on the
photovoltaic performance of their corresponding cells, two different donor units (carbazole and
phenothiazine) of these assemblies were introduced. The optical properties, topography,
theoretical calculations, electrochemical impedance spectroscopy and the photovoltaic
performances of the assemblies are studied to further understand these sensitized solar cells.
The assembly processes of ZnPx-ZnPA on TiO2 surface are as follows: a porphyrin
molecule (ZnPA) as anchoring group was immobilized on the TiO2 electrode surface through
carboxylic groups, then the desired ZnPx dye was bound to the anchoring group through axial
coordination from Zn(II) ions of ZnPx and the N of ZnPA. The detailed assembly approach is
shown in scheme 2.
3

Scheme 1. Structural diagram of ZnPx and anchoring porphyrin (ZnPA).
Scheme 2. The detailed assembly approach of these assemblies on TiO2 electrode surface.
4

2. Experimental
2.1. Materials and physical measurements
All solvents and reagents were used without purification as commercially analytical grade.
Compounds a and b were prepared according to the corresponding literature method [21].
Electronic absorption spectra were measured on a UV-2550 spectrometer. The luminescence
spectrum was measured by a LS-55 (PE USA Inc.) fluorescence spectrophotometer at room
1
temperature. H NMR (400 MHz) spectra were measured on a Varian Mercury Plus-400
spectrometer. Surface topography of the self-assembled films on TiO2 electrode surface was
imaged using transmission electron microscopy (TEM) (Hitachi Model H-900). Electrochemical
impedance spectroscopy (EIS) was achieved on a Princeton applied research VMP2 multi
potentiostat. The frequency of the AC signal ranged between 0.1 Hz and 100 kHz. The amplitude
of the AC signal was 10 mV. The syntheses processes are shown in scheme 3.
5

Scheme 3. The synthetic routes of the zinc porphyrin ZnPx (x=1,2).
2.2. Synthesis
2.2.1. Synthesis of c. To a solution of a (0.24 g, 1 mmol) and b (0.3 g, 1 mmol) in
dichloromethane (100 mL) was added trifluoroacetic acid (TFA) (0.12 mL, 1.5 mmol) dropwise
under argon. After reacting for 30 min, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
(0.57 g, 2.5 mmol) was added and the mixture was stirred for an additional 1 h. The solvent was
6

evaporated at reduced pressure and the residue was redissolved in toluene. The solution was
treated with a second portion of DDQ (0.57 g, 2.5 mmol) and refluxed for another 1 h. The
toluene was removed at reduced pressure and the crude product was purified by column
chromatography on silica gel using dichloromethane/petroleum ether as the eluent to afford the
desired porphyrin as a purple solid (13% yield) and reacted with Zn(OAc2)·2H2O to give c in 83%
yield.
To a solution of the porphyrin obtained above (0.242 g, 0.2 mmol) was dissolved in
CH2Cl2 (20 mL). A solution of Zn(OAc)2·2H2O (0.17 g, 0.8 mmol) in MeOH (5 mL) was added
to the porphyrin solution in one portion, and the mixture was refluxed for 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 phase was dried over Na2SO4,
filtered, and evaporated to dryness to give the desired porphyrin c. 1H NMR (400 MHz, DMSO)
δ 8.86 (s, 4H), 8.17 (d, J =7.8 Hz, 4H), 8.04 (d, J=7.2 Hz, 4H), 7.52 (d, J=9.6 Hz, 4H), 7.42-7.34
(m, 4H), 4.21-4.16 (m, 4H), 3.88 (s, 12H), 1.88-1.79 (m, 4H), 1.66-1.58 (m, 4H), 1.08-1.01 (m,
6H), -3.00 (s, 2H). HRMS (ESI, m/z): [M+H]+ calcd for C56H52Br2N4O6, 1036.22; found 1037.3.
2.2.2. Synthesis of ZnP1. A mixture of the porphyrin c (0.11 g, 0.1 mmol), carbazole (0.13 g,
0.8 mmol), Cu (32 mg, 0.5 mmol) and K2CO3 (83 mg, 0.6 mmol) was refluxed in DMF (10 mL)
under a nitrogen atmosphere for 24 h. TLC monitors the disappearance of reactants and the end
of the reaction. After removing solvent by evaporation a purple solid remained, which was
dissolved in CHCl3/H2O and washed successively. The organic phase was dried over Na2SO4,
filtered, and evaporated to dryness to give the crude product. The residue was treated by column
chromatography on silica gel, and the main product was collected to give the desired porphyrin
1
ZnP1 (65% yield). H NMR (400 MHz, DMSO) δ 8.48 (d, J=4.7 Hz, 4H), 8.26 (d, J=4.7 Hz,
4H), 8.04 (d, J=8.7 Hz, 4H), 7.85 (d, J=8.7 Hz, 4H), 7.76-7.52 (m, 6H), 7.39-7.21 (m, 14H), 4.32
(t, J=6.8 Hz, 4H), 3.95 (s, 12H), 2.02-1.93 (m, 4H), 1.67 (dd, J = 15.0, 7.5 Hz, 4H), 1.10 (t,
J=7.4 Hz, 6H) HRMS (ESI, m/z): [M+H]+ calcd for C80H66N6O6Zn, 1270.43; found 1271.41.
2.2.3. Synthesis of ZnP2. A solution of phenothiazine (39.8 mg, 0.2 mmol) and KOH (16.8 mg,
6 mmol) in DMSO (100 mL) was stirred 30 min at room temperature under argon. After the
addition of porphyrin d (0.11 g, 0.1 mmol), stirring was continued for 3 h at room temperature
7

until the end of the reaction. Removal of solvent by evaporation left a purple solid, which was
dissolved in CHCl3/H2O and washed successively. The organic phase was dried over Na2SO4,
filtered and evaporated to dryness to give the crude product. The residue was treated by column
chromatography on silica gel, and the main product was collected to give the desired porphyrin
1
ZnP2 (62% yield). H NMR (400 MHz, DMSO) δ 8.66 (d, J=79.0 Hz, 8H), 8.08 (d, J=6.5 Hz,
2H), 7.96 (s, 2H), 7.26 (d, J=7.6 Hz, 2H), 7.17 (d, J=7.8 Hz, 2H), 7.09 (d, J=6.8 Hz, 2H), 6.92
(dd, J=27.5, 7.4 Hz, 6H), 6.70 (dd, J=29.6, 7.3 Hz, 6H), 6.01-5.61 (m, 6H), 3.90 (d, J =42.8 Hz,
4H), 3.49 (s, 12H), 2.12 (t, J=16.1 Hz, 4H), 1.59-1.42 (m, 4H), 1.27-0.99 (m, 6H). HRMS (ESI,
m/z): [M+H]+ calcd for C80H66N6O6S2Zn, 1334.38; found 1335.4.
2.3. Device fabrication
The preparation procedures of TiO2 nanocrystals, pastes for screen-printing, and nanostructured
TiO2 film have been reported by Prof. P. Wang [22]. A cycloidal TiO2 electrode (~0.28 cm2) was
stained by immersing it into a CHCl3/EtOH (10/90, v/v) solution containing ZnPA (2 mM) as
anchoring molecule overnight. After removal of the unbound molecules (through three ethanol
washings), then the electrode was immersed into a dye solution containing ZnPx (x=1,2)
(0.2 mM) in CHCl3/DMF (85/15, v/v) for 1 h, then, washed by acetonitrile solution three times
and dried by air flow. The sensitized titania electrode was assembled with a thermally platinized
FTO electrode. The electrodes were separated by a 35-μm-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 and 30 mM I2 in acetonitrile solvent. After all these
procedures, the cells were located in the oven for heating at 100 C for 30 min and cooled to
room temperature before photoelectrochemical measurements.
3. Results and discussion
In order to demonstrate the zinc porphyrin assemblies are efficiently immobilized on the TiO2
electrode surfaces; we used a transmission electron microscope (TEM) and computational
calculations, the method well-established [23, 24]. Figure 1 shows the optimized molecular
structure of TiO2 nanoparticles modified with ZnPx-ZnPA. The molecule heights were
8

identified to be 2.5 nm for ZnP1-ZnPA and 2.6 nm for ZnP2-ZnPA modified TiO2, respectively.
The molecule heights almost match to the layer thicknesses obtained by TEM displayed in
figure 2. This indicates joining porphyrin ZnPx onto the TiO2 surface through axial coordination
with anchoring porphyrin (ZnPA).
The UV-Vis absorption spectra of zinc porphyrin dyes are depicted in figure 3. Owing to
similar structures, all the zinc porphyrin dyes in CHCl3 solution exhibit the intense Soret bands
(400-450 nm) and moderate Q bands (520-620 nm) from the π-π* electron transition [25]. Both
bands of ZnP2 have small red shift of the absorption maximum compared with ZnP1, which can
be rationalized by the stronger electron-donating ability of the phenothiazine than the carbazole.
On TiO2 films, the absorption spectra of the two dyes showed a slight red shift and considerable
broadening as compared to those in solution. Especially, ZnP2-ZnPA Q band is red-shifted
ca. 25 nm relative to those of ZnP1-ZnPA, which is beneficial for improving light-harvesting.
Generally, stronger absorption intensity results in a higher light harvesting efficiency (LHE).
LHE quantifies the capability of devices to absorb photons as a function of the extinction
coefficient of the dye and the concentration and amount of dye-molecules adsorbed onto the TiO2
films [26]. Figure 4 shows a sequence of ZnP2-ZnPA>ZnP1-ZnPA. Such variation in LHE is
mainly attributed to the difference in their respective absorption spectra. Notably, ZnP2-ZnPA
has higher light harvesting efficiency than ZnP1-ZnPA, which is the key factor for higher short
circuit current density (Jsc).
Figure 5 shows the fluorescence spectra of ZnPx in CDCl3 solution and ZnPx-ZnPA on
the TiO2 films. ZnPx-ZnPA shows blue-shift to ZnPx, ascribed to the occurrence of
J-aggregations of porphyrin molecules on the TiO2 films [24]. Especially, the ZnP2-ZnPA is
slightly red-shifted from ZnP1-ZnPA, consistent with variation of the UV-vis absorption spectra.
The parameters of UV-Vis absorption and fluorescence spectra of ZnPx in solution and
ZnPx-ZnPA on TiO2 thin film are summarized in table S1.
To provide further insight into the effect of varying the molecular structures on electron
distribution in the frontier molecular orbitals, density functional theory (DFT) calculations were
employed using the Gaussian 09 program at the B3LYP/LANL2DZ level [27, 28]. The
molecular orbital (MO) patterns of HOMO and LUMO of dyes are shown in figure S1. The
electrons of the HOMO levels for all the dyes mainly localized on the core of the ZnPx
9

porphyrin framework and donor units, while the LUMO orbitals are predominantly delocalized
over the anchoring porphyrin, signifying the presence of excellent electron-separation for these
dyad-sensitized devices.
Figure 6 shows the energy levels of zinc porphyrin dyes that were acquired by DFT
calculations. All of the LUMOs are higher than the conduction edge of TiO2 nanoparticles,
indicating that electron injection from the excited states of the dyes to the conduction band of
TiO2 was energetically favorable. The HOMO levels are below the oxidation potential of
-
electrolyte I-/I3 . These results confirm that electron injection from LUMO to the conducting
band is sufficient. ZnP1-ZnPA contain carbazole moieties with the HOMO level very close to
the iodide/triiodide redox potential value (figure 6), which may result in poor dye regeneration
efficiency and poor photovoltaic behavior (vide infra).
The photovoltaic properties of the devices sensitized by ZnPx-ZnPA were evaluated with
an iodine electrolyte. The incident photon-to-current conversion efficiency (IPCE) spectra for
dyad dyes exhibit a broad spectral response compared with anchor porphyrin ZnPA (figure 7),
indicating that the double-layer structure of the assembly (ZnPx-ZnPA) can efficiently convert
visible light into photocurrent. To further verify the assembly was effectively immobilized on
TiO2 surface, the following control experiment based on the assembly of dye ZnPx physical
absorption directly adsorbed on the TiO2 electrode surface (without anchor porphyrin ZnPA)
was performed as well. The ZnPx device reveals little photoelectric response, indicating the
anchor porphyrin ZnPA can efficiently bind the ZnPx dye on the TiO2 surface via Zn-to-ligand
axial coordination approach. As shown in figure 7, the photocurrent spectra of ZnPx-ZnPA
reveals the energy conversion of B and Q bands of porphyrin, roughly analogous to their
corresponding UV-Vis spectra. More impressively, ZnP2-ZnPA contains an additional
phenothiazine group and has more light-harvesting, and thus, it shows a much higher IPCE value
approaching 68%. Thus, the onset wavelength thresholds lie in the order
ZnPA<ZnP1-ZnPA<ZnP2-ZnPA, which is consistent with the sequence of photocurrent
Jsc (vide infra).
10

The absorbed photon-to-current conversion efficiencies (APCE) of the dyes related to
electron injection and charge collection efficiency are calculated by using the equation APCE (λ)
= IPCE (λ) / LHE (λ) and the results are depicted in figure S2. APCE values about 70-80% are
achieved for all the assembly dyes, suggesting an effective charge injection process [29, 30].
APCE lower than 100% may be attributed to the charge recombination between the
photo-injected electrons and the oxidized species in the electrolyte.
The photocurrent-voltage (J-V) characteristics of the DSSCs based on ZnPx-ZnPA are
shown in figure 8, based on ZnTPP-ZnPA are shown in figure S3 and the photovoltaic
parameters are presented in table S2. To be more specific, compared to the anchor porphyrin
ZnPA based cell, the solar cells based on the bilayer structured chromophores displayed a
3.8-4.1 mA cm-2 increased current density due to their favorable light harvesting capability. The
highest Jsc of 4.10 mA cm-2 is achieved for ZnP2-ZnPA due to the donating ability of the
phenothiazine unit. ZnTPP-ZnPA based cell has lower Jsc compared to the ZnPx-ZnPA based
devices. This indicates that carbazole and phenathiozine have better light harvesting ability. The
open-circuit voltages of the cell sensitized by ZnPA is higher than that of ZnPx-ZnPA; the
ZnPx-ZnPA devices possess dye aggregation. Anchor porphyrin of ZnPA was reported in our
previous work [31]. The dark current of the porphyrin dye based solar cells was also measured to
investigate the charge recombination behavior at the TiO2 surface. As shown in figure 8, the dark
current of the solar cells fabricated with assembled dyes at a lower potential bias suggest that the
ZnPx-ZnPA based solar cells have higher electron recombination rate than anchor porphyrin
ZnPA based cells. In particular, the fast electron recombination rate of the ZnP1-ZnPA cell is
mainly responsible for relatively lower efficiencies.
Interfacial charge transfer in DSSCs was measured by electrochemical impedance
spectroscopy (EIS) under dark conditions at a forward bias of -0.6 V [32, 33]. The Nyquist plots
(a), Bode phase plots (b) and equivalent circuit are shown in figure 9. The RS, RCT, Rele and C
represent series resistance, charge transfer resistance, Nernst diffusion resistance and double
layer capacitance, respectively. Normally, a Nyquist plot features three semicircles, and the
11

larger semicircle at the intermediate frequency reflected the charge transfer resistance (Rct) at the
TiO2/dye/electrolyte interface [34]. The charge transfer resistance value of the ZnP2-ZnPA
based cell is higher than that of the ZnP1-ZnPA sensitized cell, indicating that the charge
recombination of ZnP2-ZnPA is more serious. The larger Rct value will suppress the charge
recombination between the photoinduced electrons and oxides in the electrolyte, giving a higher
Voc [35, 36]. Usually, a high charge transfer resistance leads to a long electron lifetime.
ZnP2-ZnPA based devices have longer electron lifetime compared to ZnP1-ZnPA based cells
(figure 7b), in agreement with HOMO-LUMO levels and photocurrent responses.
4. Conclusion
Porphyrin sensitizers containing carbazole (ZnP1) and phenothiazine (ZnP2) electron donors
were synthesized, fully characterized, and appended to anchoring porphyrin fabricating two new
self-assembled dye-sensitized solar cells. In these dyes, the self-assemblies have improved
photovoltaic behavior compared with anchoring porphyrin ZnPA. Compared with sensitizer
ZnP1 which contains a carbazole group linked to the meso-position of the porphyrin framework,
introducing phenothiazine unit as a strong electron-donor to the upper porphyrin can achieve
broadened light absorption and IPCE response, resulting in higher Jsc and photoelectric
conversion efficiency. These results optimizing the upper porphyrin sensitizer structures have
been demonstrated to be effective for further investigation in supramolecular solar cells.
Acknowledgements
The National Natural Science Foundation of China (No. 21461023), Gansu Provincial Natural
Science Foundation of China (No. 1606RJZA110) and Research on high-level talents of Qiannan
Normal University for Nationalities (No. qnsyrc201703) have 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.
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Figure 1. Molecular modeling of ZnPA+Lx on TiO2 electrode surfaces.
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Figure 2. TEM images of TiO2 nanoparticles modified with ZnPx-ZnPA.
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Figure 3. UV−Vis absorption spectra of ZnPx in CDCl3 solution and ZnPx-ZnPA on TiO2 thin
films.
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Figure 4. Light harvesting efficiency of the self-assembled TiO2 electrodes.
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Figure 5. Fluorescence spectra of ZnPx in CDCl3 solution and ZnPx-ZnPA on TiO2 thin films.
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Figure 6. Orbital energy levels of the assemblies, TiO2 and electrolyte I-/I3 .
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Figure 7. IPCE action spectra of porphyrin-sensitized solar cells.
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Figure 8. J-V characteristics of porphyrin-sensitized solar cells.
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Figure 9. EIS spectra of DSSCs based on porphyrin dyes: (a) Nyquist plots in the dark; (b) Bode
phase plots in the dark.
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