Three horizontal anchor porphyrins for dye-sensitized solar cells: An optical, electrochemical and photovoltaic investigation

Article abstract of DOI:10.1016/j.poly.2016.06.002

Porphyrins are promising DSSC sensitizers focused on novel anchoring. Herein, three horizontal anchor zinc porphyrins have been designed, synthesized, and well-characterized. Their optical, photovoltaic, theory calculation and electrochemical performances

Full text of DOI:10.1016/j.poly.2016.06.002

Accepted Manuscript
Three horizontal anchor porphyrins for dye-sensitized solar cells: An optical,
electrochemical and photovoltaic investigation
Yu Wu, Jia-Cheng Liu, Wen-Bo Guo, Ren-Zhi Li, Neng-Zhi Jin
PII:
S0277-5387(16)30219-4
DOI:
http://dx.doi.org/10.1016/j.poly.2016.06.002
POLY 12040
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
25 March 2016
1 June 2016
Please cite this article as: Y. Wu, J-C. Liu, W-B. Guo, R-Z. Li, N-Z. Jin, Three horizontal anchor porphyrins for
dye-sensitized solar cells: An optical, electrochemical and photovoltaic investigation, Polyhedron (2016), doi: http://
dx.doi.org/10.1016/j.poly.2016.06.002
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Three horizontal anchor porphyrins for dye-sensitized solar cells:
An optical, electrochemical and photovoltaic investigation
a
Yu Wu , Jia-Cheng Liu* , Wen-Bo Guo , Ren-Zhi Li , Neng-Zhi Jin
a b
a
c
d
a
Key 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.
b
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s Republic of
China
c
Key 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
d
Gansu Computing Center, Lanzhou 730030, People’s Republic of China.
*Corresponding authors.
Tel: +869317975636, fax: +869317971989.
E-mail addresses: jcliu8@nwnu.edu.cn (J. C. Liu).
Abstract
Porphyrins are promising DSSC sensitizers focused on novel anchoring. Herein,
three horizontal anchor zinc porphyrins have been designed, synthesized, and
well-characterized. Their optical, photovoltaic, theory calculation and electrochemical
performances in dye-sensitized solar cells were investigated. Absorption spectra
indicated the stronger H-aggregation in ortho carboxylic acid anchor porphyrin
(
o-ZnPA) than that in meta carboxylic acid anchor porphyrin (m-ZnPA) on the
surface of TiO nanoparticles. The photoelectric conversion efficiency studies showed
2
a significantly enhanced photovoltaic behavior for meta carboxylic acid anchor
porphyrin (m-ZnPA) compared to meta tetrazolium anchor porphyrin (m-ZnPTz).
The infrared spectroscopies, theory calculations and the electrochemical impedance
spectroscopies revealed that carboxylic acid anchoring binding modes could increase
the efficiency of electron injection.
Keywords dye-sensitized solar cells, zinc porphyrin dye, horizontal, anchoring
group
1
/ 19

1
. Introduction
Solar energy, as an environmental friendly and continual energy sources, solves the
energy problems worldwide [1-3]. Dye-sensitized solar cells (DSSCs) have attracted
extensive attentions in view of inexpensive materials, easy processing, and minimal
environmental impact [4-6]. As such, it is the most feasible technology for the
conversion of solar energy directly into electricity. In order to improve the
photoelectric conversion efficiency of the dye-sensitized solar cells, selecting the
appropriate photosensitizers play a crucial role. In typical DSSCs, usually using Ru
polypyridyl complex [7], organic sensitizers [8-10] and zinc porphyrin dyes [3,11] as
photosensitizers, respectively. Porphyrins are especially promising DSSC sensitizers
due to their structural can be used to collect solar energy and achieve photoinduced
electron and energy transfer [12-16]. Thus, numerous zinc porphyrin dyes have been
reported based on a donor-π-acceptor (D-π-A) architecture, such as SM315 [17], with
the anchoring group of 4-ethynyl benzothiadiazole benzoic acid attached at the meso
position exhibited remarkably high efficiency (13%).
Generally, the porphyrin sensitizers are linked to the TiO
anchoring groups, such as carboxyl groups, which provide electron injection to the
TiO electrode surfaces. Recently, some researchers made to developing novel
2
semiconductor through
2
anchoring groups, which improve the photoelectric efficiency. For example, salicylic
acid was reported as an anchoring group for zinc porphyrin dyes [18].
8
-hydroxylquinoline was used as an anchoring group for metalloporphyrin sensitized
solar cells [19]. Coutsolelos’s group introduced a series of pyridine into porphyrin
dyes as an anchoring group [20,21]. Hiroshi Imahori’s group reported that tropolone is
a robust anchoring group for porphyrin sensitized solar cells [22]. The above are
single anchoring group, a typical porphyrin dye with four benzoic acids as anchoring
groups was reported by Rochford et al [23]. The four anchoring groups are in the meta
or ortho position on the meso-phenyl rings of the porphyrin macrocycle, which is
called horizontal anchor porphyrin.
In this paper, three horizontal anchor zinc porphyrins (m-ZnPA, o-ZnPA,
2
/ 19

m-ZnPTz, shown in Scheme 1) have been designed and synthesized. They include
carboxyl and tetrazole groups as anchoring groups. The optical, electrochemical,
theoretical calculations and the photovoltaic performances of the assemblies are
studied to further understand the behavior of the dye-sensitized solar cells.
Scheme1. Structure of zinc porphyrin dyes
3
/ 19

2
. Experimental Section
2.1 General methods
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 (600 MHz) was measured on a
Varian Mercury Plus-400 spectrometer. An LS1000 solar simulator (Solar Light Com.
-2
Inc., USA) was used to give an irradiance of 100 mW cm (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 λ300) 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(λ) =
hcJsc/eФλ, where h is Planck’s constant, c is the speed of light in a vacuum, e is the
electronic charge, λ is the wavelength (m), Jsc is the short-circuit photocurrent density
-
2
-2
mA cm ), and Ф is the incident radiative flux (mW m ). Photovoltaic performance
(
2
was measured by using a metal mask with an aperture area of 0.1576 cm . A
homemade heating–cooling system was used for temperature dependent J-V
measurements. Cyclic voltammetry trials were conducted using a CHI 1140C
instrument, with a traditional three-electrode system. The working electrode,
reference electrode, and auxiliary electrode was a glassy carbon electrode, Ag/AgCl
and Pt electrode, respectively. The electrochemical impedance spectroscopy (EIS) was
achieved on a Princeton applied research VMP2 multi potentiastat. The frequency of
the AC signal ranged between 0.1 Hz and 100 kHz. The amplitude of the AC signal
was 10mV at room temperature.
2.2 Device fabrication
2
A screen-printed double layer film of interconnected TiO particles was used as the
mesoporous negative electrode. A 7 µm thick transparent layer of 20 nm sized titania
4
/ 19

2
particles were first printed on the fluorine-doped SnO (FTO) conducting glass
electrode and further coated by a 5 µm thick scattering layer of 400 nm sized titania
particles. The film thickness was measured by a benchtop Ambios XP-1 stylus
2
profilometer. The detailed preparation procedures of TiO nanocrystals, pastes for
screen-printing, and nanostructured TiO
24]. A cycloidal TiO
2
film have been reported by Prof. P. Wang
2
electrode (~0.28 cm ) was stained by immersing it into an
[
2
ethanol solution containing ZnPA (2mM) anchoring molecule overnight. Other TiO
2
electrode was stained by immersing it into DMF solution containing ZnPTz (2mM)
anchoring molecule overnight. After being washed with acetonitrile 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, 30
2
mM I in acetonitrile solvent. After all these procedures, the cells were located in the
over for heating posttreatment at 100℃ for 30 min and cooled to room temperature
before photoelectrochemical measurements.
2.3 Synthesis of Porphyrin Dyes
All solvents and reagents were used directly without further purification as
commercially analytical grade. The ZnPA porphyrin dyes were synthesized by the
modified Lindsey method [25,26], especially, it is reacted with different reactants.
Syntheses followed as Scheme1.
5
/ 19

Scheme1. Synthetic protocol for zinc porphyrin dyes
5,10,15,20-tetra(3-cyanophenyl) porphyrin (a)
Syntheses of a was prepared according to literature [26]. 3-cyanophenyl (0.98g,
7.5mmol) was refluxed in a mixture propionic acid (50mL) and 0.52mL of distilled
pyrrole (7.5mmol) was then added dropwise. After complete addition, the reaction
mixture was stirred for another 2h and cooled to room temperature. After 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.
5,10,15,20-tetra[3-(2,3,4,5-tetrazolyl)phenyl] porphyrin (b)
Syntheses of b was prepared according to literature [27]. A solution of
6
/ 19

5
7
1
,10,15,20-tetra(3-cyanophenyl) porphyrin a (0.6g, 0.84mmol), NaN
3
(0.48g,
.34mmol), NH Cl (0.36g, 6.74mmol), and anhydrous DMF (50mL) was heated at
4
3 4
20℃ for 2 days. Additional NaN (0.34g, 5.18mmol) and NH Cl (0.22g, 4.1mmol)
were added to push the reaction, which monitored by TLC until the reaction was
complete. The DMF was evaporated under reduced pressure then cold H O (20mL)
was added. The resulting solution was acidified with 6 N HCl then CH Cl (20mL)
2
2
2
was added. The resulting precipitate was collected and dried under vacuum to provide
b in 80% yield as a green solid, which was used in the metalation step without further
purification.
5,10,15,20-tetra(2-cyanophenyl) porphyrin (c)
Syntheses of c was prepared according to literature. Synthesis method was similar
to 5,10,15,20-tetra(3-cyanophenyl) porphyrin. Yield, 10%
General Procedure for a and c Metalation
3
The free base a and c porphyrins (0.4mmol) was dissolved in CHCl (80mL). A
solution of Zn(OAc)
solution in one portion, and the reaction mixture was refluxed 4h. The solvents were
removed in vacuo leaving a purple solid, which was dissolved in CH Cl and washed
successively with 5% aqueous NaHCO and water. The organic layer was dried over
Na SO , and the solvent was removed in vacuo. The metaloporphyrins were obtained
in 80% yields following silica gel chromatography with dichloromethane/ethyl acetate
2
2
•2H O (1.6mmol) in MeOH (10mL) was added to the porphyrin
2
2
3
2
4
(
95:5) to afford pure metalloporphyrins.
General Procedure for metalloporphyrins alkaline hydrolysis
2 5
The metalloporphyrins were added 5M NaOH (10mL), then add 8~10mL C H OH,
and the reaction mixture heated at 80℃ for 48h, with moist pH paper detecting
whether the reaction of ammonia discharged there, until no ammonia gas evolution
reaction. Diluted with water, filtration of impurities, and the filtrate neutralized with
6M HCl to precipitate appears. Allowed to stand overnight, with sand core funnel
filtration, and dried to obtain the product m-ZnPA and o-ZnPA. Yield, 73%
1
m-ZnPA H NMR (600MHz, DMSO-d
6
) δ13.2(s, 4H), 8.86~8.83(s, 10H),
+
.46~8.33(m, 8H), 8.23~8.12(m, 4H), 6.83 (s, 2H). MS (ESI, m/z): [M+H] calcd for
8
49 31 4 8
C H N O Zn, 867.2; found, 868.1
7
/ 19

Zn-5,10,15,20-tetra[3-(2,3,4,5-tetrazolyl)phenyl] porphyrin (m-ZnPTz)
A solution of e (0.4mmol) and Zn(OAc) •2H O (2mmol) in anhydrous DMF (50mL)
2
2
was heated at reflux for 4~5h. The reaction mixture was allowed to cool to room
temperature then DMF was evaporated under reduced pressure. The crude product
1
was silica gel chromatography to give m-ZnPA2. Yield, 70% H NMR (600MHz,
DMSO-d
6
) δ 8.65(d, 4H), 8.41~7.97(m, 8H), 6.67~5.71(m, 10H), 5.30(s, 2H) MS
+
ESI, m/z): [M+H] calcd for C48
(
H
28
N
20Zn, 848.2; found, 869
8
/ 19

3
. Results and Discussion
3.1. Optical and Electrochemical Properties
In order to studied the electronic transition of these dyes. The UV-Vis absorption
spectra of these dyes in DMF solutions are shown in Figure 1. All three dyes exhibited
the features typical of the porphyrin ring, with absorptions at 400~480nm for intense
Soret band (corresponding to the π-π* transitions) and weak Q bands at 550~630nm
(
corresponding to the energy vibration transition). Compared to the absorption spectra
of m-ZnPA and o-ZnPA, the spectra of o-ZnPA and m-ZnPA red-shifted (~40nm),
indicating that the o-ZnPA are being high steric hindrance, π-π stacking interactions
lead to extend the absorption to longer wavelengths. Both bands of m-ZnPTz
red-shifted for ca. 5nm relative to those of m-ZnPA, which can be rationalized by the
stronger conjugation effect of the tetrazole group respect to the carboxylic acid. There
are three porphyrin dyes absorbed on the TiO
S1. The absorption spectra of three dyes are broadened may be presence of
aggregations on the TiO surfaces. Comparison the solution spectra (Figure 1) show
2
thin film (2.5µm) are shown in Figure
2
that slightly red-shifted, which is beneficial for improving light-harvesting.
Figure1. UV-vis absorption spectra of three porphyrin dyes in DMF solution
Figure 2 showed the fluorescence spectra of three porphyrin dyes in DMF. Dyes
emission spectra were very similar. o-ZnPA is slightly blue-shifted than that of
m-ZnPA, which is o-ZnPA being high steric hindrance. Notedly, the spectra (Figure
S2) reveal blue shifts and broadening compared to those solution fluorescence spectra
(
Figure2), ascribed to the occurrence of J-aggregations of dye molecules on the TiO
2
9
/ 19

films [28]. Table 1 summarizes the optical data of three porphyrin dyes. Three
porphyrin dyes on TiO film digital and presented in Table S1.
2
The electrochemical properties of these porphyrin dyes were investigated by
cyclic voltammetry and presented in Table 1. The oxidation potentials (Eox) of dyes
were +1.19 V for m-ZnPA, +1.16 V for o-ZnPA and +1.19 V for m-ZnPTz, that were
-
-
far below the redox potential of I /I
excitation energies (E0-0) were determined to be 2.18 eV for m-ZnPA, 2.05 eV for
o-ZnPA and 2.14 eV for m-ZnPTz, respectively. The injection potentials (Es+/s*
subtracted the E0-0 from the Eox of the sensitizers was much higher than the potential
of the conducting band of TiO nanoparticles (0.5V) [29].
3
(0.4V) to ensure efficient regeneration. The
)
2
Table1. Absorption/Emission and Electrochemical Properties of Porphyrin Dyes
a
a
b
c
d
dye
absorption
emission
em (nm)
E
0-0
E
ox
E
s+/s*
λmax (nm)
λ
eV
(V vs NHE)
1.19
(V vs NHE)
-0.99
m-ZnPA
o-ZnPA
425, 558, 597
465, 603, ----
430, 562, 601
608, 654
606, 656
610, 650
2.18
2.05
2.14
1.16
-0.89
m-ZnPTz
1.19
-0.95
a
b
Absorption and emission spectra were measured in DMF solution at room temperature.
E
0-0 was determined
c
from the intersection of normalized absorption and emission spectra. The oxidation potentials of dyes were
-
1
d
6
measured in DMF with 0.1M TBAPF with a scan rate of 100mVs (vs NHE). Es+/s* was calculated by Eox-E0-0.
Figure2. Fluorescence spectra of three porphyrin dyes in DMF solution
10 / 19

3.2. FT-IR
2
The functionalization of TiO thin film with three dyes was showed by infrared
spectroscopies (Figure S3). All solid samples evidenced spectra bands from the
-1
pyrrole C-H, C=C, and C=N stretches (over 700~1500cm ) of the porphyrin typical
spectra [30-32]. Samples of m-ZnPA and o-ZnPA showed a strong band of the
-1
carboxylic acid stretching peak (1690~1700cm ). For all two tetraacid porphyrins
bound to TiO films showed the disappearing of the carboxylic acid stretching peak,
2
which concomitant appearance of asymmetric and symmetric stretching modes of the
-
v (CO
2
) [33]. This indicated carboxylate groups through chelating or bidentate
surface [34-37]. Samples of m-ZnPTz allowed a tetrazole
blinding modes on the TiO
2
-
1
N-N vibrational stretching peak located 1005cm , which adsorption on TiO
2
films
showed 1025cm . This appearance confirmed that the interaction of tetrazole group
with TiO films through the conjugated heterocycle [38].
.3. Theoretical Calculations and Energy Levels
To gain a better insight into the structure changes affect the photophysical
-1
2
3
properties and energy levels of frontier molecular orbital, we performed calculations
with DFT using the Gaussian 09 program [39] at the B3LYP/LanL2DZ level. As
shown in Figure S4, 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. It’s
worth noting that the LUMO+2 levels for all of distribute anchoring group. The
features suggest the presence of excellent electron-separated status for these devices.
-
-
The energy-levels diagram of three dyes relative to the TiO
are shown in Figure 3. All of the LUMO are higher than the conduction band edge
CB) of TiO (-4.0eV), indicating that efficient electron injection into TiO conduction
band from the excited states [40]. The HOMO levels are more negative than the
2 3
and the electrolyte I /I
(
2
2
-
electrolyte I /I
-
3
, suggest that electron injection process could thermodynamically
-
accept electrons from I to regenerate the dyes. Notedly, the energy gap between the
HOMO and LUMO is almost equal. It is obvious that m-ZnPA is lowest
energy-levels, which agree with carboxylic acid groups are effective anchor groups.
11 / 19

Figure3. Energy-level diagram of three porphyrin dyes
3.4. Photovoltaic Performance of DSSCs.
The photovoltaic properties of the devices sensitized by three porphyrin dyes were
-
2
tested by iodine electrolyte and under irradiance of 100mW cm AM 1.5G sunlight.
The incident photo-to-current conversion efficiency (IPCE) spectra of the new
porphyrin dyes sensitized devices in the wavelength range 300-700nm can be seen in
Figure 4. The order of IPCE values of dyes is as follows:
m-ZnPA>m-ZnPTz>o-ZnPA. The IPCE of dyes reveal the energy conversion of
Soret and Q bands of porphyrin, roughly analogous to their corresponding UV-Vis
spectra. Compared with m-ZnPA, the introduction of additional tetrazole group in
m-ZnPTz slightly red-shift. The phenomenon is consistent with the UV-Vis
absorption spectrum. More impressively, o-ZnPA has a weak broadening of
460nm~500nm. However, o-ZnPA sensitized cell reveal lowest IPCE, assigned to the
dye-aggregation on the TiO electrode surface [41]. Therefore, o-ZnPA J-V curve and
2
electrochemical impedance spectroscopy were not investigation. The detailed data of
short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc) and fill factor
(
FF) for these cells are shown in Table S2 and Figure 5. The Jsc increased in the order
of m-ZnPTz<m-ZnPA, which this trend was also consistent with IPCE spectra.
Introduction of the tetrazolium as anchor groups is favorable for extending its
2
conjugation length and preventing the dye-aggregation on the TiO electrode surface,
thus m-ZnPTz shows the highest open voltage (Voc) of 453mV. Whereas, m-ZnPA
sensitized device shows highest short-circuit photocurrent density (Jsc) and total
photoelectronic conversion efficiency (η). The performance of these dyes with the
12 / 19

carboxylic acid group as the anchoring group is superior than that with the tetrazolium
group as the anchoring group.
Figure4. IPCE of DSSCs sensitized by three porphyrin dyes
Figure5. J-V characteristics of DSSCs sensitized by three porphyrin dyes
3.6. Electrochemical Impedance Spectroscopy
To further understand the interfacial charge transportation in the DSSC devices was
investigated by electrochemical impedance spectroscopy, which was carried out at
different applied potentials under dark conditions [42-45]. Figure 6 showed the
Nyquist plots of the DSSCs base on dyes m-ZnPA and m-ZnPTz. Nyquist plots have
three semicircles, and the large semicircle at intermediated frequency reflected the
2
interfacial charge transfer resistance (Rct) at the TiO /dye/electrolyte interface. The
fitted Rct values of m-ZnPTz-based devices are much larger than m-ZnPA-based
devices. In other words, the larger Rct meant a slower charge recombination rate; as a
13 / 19

result, the electron recombination at the TiO
efficiently suppressed, leading to a higher Voc for DSSCs sensitized with m-ZnPTz
23,46].
2
/dye/electrolyte interface can be
[
Figure6. Electrochemical impedance spectroscopy (EIS) of DSSCs based on three porphyrin dyes
14 / 19

4
. Conclusion
In conclusion, three new dyes of horizontal anchor zinc porphyrins using carboxyl
and tetrazole as anchoring groups have been designed and synthesized. The optical,
photovoltaic and electrochemical performance in dye-sensitized solar cells were
investigated. Our results reveal enhanced photovoltaic behavior for m-ZnPA
porphyrin dye. The carboxyl horizontal anchor porphyrin could enhance the efficiency
2
electron injection from dyes to the TiO nanoparticles. To improve the photoelectric
conversion efficiency of use m-ZnPA as horizontal anchor groups for self-assembling,
which is our future research and the results will be reported in due course.
Acknowledgments
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.
Supplementary material
Supplementary figure and data associated with this article can be found, in the
online version.
15 / 19

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Graphical Abstract-Pictogram
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Graphical Abstract
Horizontal anchor zinc porphyrins have promising DSSC sensitizes. The UV-vis
absorption spectra, fluorescence spectra, and the photovoltaic performance of these
photosynthesis devices are described. Our results reveal that these DSSC devices have
impressive photoelectric conversion efficiency, especially for the assembly based on
carboxylic group horizontal anchor porphyrin.
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