Influence of the additional electron-withdrawing unit in β-functionalized porphyrin sensitizers on the photovoltaic performance of dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2016.12.027

The β-functionalized porphyrin containing an additional electron-withdrawing unit, 2,3-diphenylquinoxaline(DPQ) for LP-5 or 2,1,3-benzothiadiazole (BTD) for LP-6 with different electron-withdrawing abilities, between the porphyrin core and the anchoring group and the reference porphyrin dye (LP-4) have been designed and synthesized for DSCs. The influence of the additional electron-withdrawing units on molecular properties as well as photovoltaic performance of the corresponding DSCs was investigated systematically. Compared with LP-4, the introduction of additional electron-deficient unit at the porphyrin β π-linker in LP-5 and LP-6 decreases the lowest unoccupied molecular orbital (LUMO) energy levels, resulting in the broader absorption spectra and significantly improved IPCE spectra in the region 350–500 nm, which ensures the better light-harvesting properties and the higher short-circuit current density (J_sc). On the other hand, the introduction of additional acceptors of LP-5 and LP-6 induces dye aggregation and reduces the lifetime of the charge–separated states, which decreases the open–circuit voltage (V_oc). Interestingly, the loss in V_oc is overcompensated by the improvement in J_sc. The study provides not only an alternative approach to design novel porphyrin sensitizers, but also an insight into how to manipulate the LUMO energy levels of porphyrin sensitizers via the β-linker modifications for the optimal photovoltaic applications.

Full text of DOI:10.1016/j.dyepig.2016.12.027

Accepted Manuscript
Influence of the additional electron-withdrawing unit in β-functionalized porphyrin
sensitizers on the photovoltaic performance of dye-sensitized solar cells
Futai Lu, Yaqing Feng, Xuexiang Wang, Yanming Zhao, Guang Yang, Jie Zhang, Bao
Zhang, Zhixin Zhao
PII:
S0143-7208(16)30996-2
10.1016/j.dyepig.2016.12.027
DYPI 5652
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 20 October 2016
Revised Date: 24 November 2016
Accepted Date: 4 December 2016
Please cite this article as: Lu F, Feng Y, Wang X, Zhao Y, Yang G, Zhang J, Zhang B, Zhao Z,
Influence of the additional electron-withdrawing unit in β-functionalized porphyrin sensitizers on the
photovoltaic performance of dye-sensitized solar cells, Dyes and Pigments (2017), doi: 10.1016/
j.dyepig.2016.12.027.
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ACCEPTED MANUSCRIPT
Influence of the additional electron-withdrawing unit in
β-functionalized porphyrin sensitizers on the photovoltaic
performance of dye-sensitized solar cells
a,b,1
a,b,c,2
b,6
d,3
b,4
b,5
Futai Lu , Yaqing Feng
, Xuexiang Wang , Yanming Zhao , Guang Yang , Jie
a,c,7,
e,8,
Zhang , Bao Zhang
* and Zhixin Zhao
*
a
School of Chemical Engineering and Technology, Tianjin University
Tianjin 300350, P. R. China
Tel.: +86-022-27892323. Fax: +86-022-27892323
b
Tianjin Co-Innovation Center of Chemical Science and Engineering, Tianjin University,
Tianjin 300072, P. R. China
c
Tianjin Engineering Research Center of Functional Fine Chemicals
d
Department of Application of Chemical Engineering, Lanzhou Petrochemical College of
Vocational Technology, Lanzhou 730060, P. R. China
e
Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for
ptoelectronics Department, School of Optical and Electronic Information, Huazhong
University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, P. R. China
1
5
2
3
4
lufutaitiger@126.com, yqfeng@tju.edu.cn 23205903@qq.com zym84699384@163.com,
,
6
7
8
1
76879931@qq.com, zhjie@tju.edu.cn, baozhang@tju.edu.cn*, zhixin-zhao@hust.edu.cn*
*
Corresponding authors: baozhang@tju.edu.cn
1

ACCEPTED MANUSCRIPT
Abstract
The β-functionalized porphyrin containing an additional electron-withdrawing
unit, 2,3-diphenylquinoxaline(DPQ) for LP-5 or 2,1,3-benzothiadiazole (BTD) for
LP-6 with different electron-withdrawing abilities, between the porphyrin core and
the anchoring group and the reference porphyrin dye (LP-4) have been designed and
synthesized for DSCs. The influence of the additional electron-withdrawing units on
molecular properties as well as photovoltaic performance of the corresponding DSCs
was investigated systematically. Compared with LP-4, the introduction of additional
electron-deficient unit at the porphyrin β π-linker in LP-5 and LP-6 decreases the
lowest unoccupied molecular orbital (LUMO) energy levels, resulting in the broader
absorption spectra and significantly improved IPCE spectra in the region 350-500 nm,
which ensures the better light-harvesting properties and the higher short-circuit
current density (J ). On the other hand, the introduction of additional acceptors of
sc
LP-5 and LP-6 induces dye aggregation and reduces the lifetime of the charge–
separated states, which decreases the open–circuit voltage (V ). Interestingly, the loss
oc
in V is overcompensated by the improvement in J . The study provides not only an
oc
sc
alternative approach to design novel porphyrin sensitizers, but also an insight into
how to manipulate the LUMO energy levels of porphyrin sensitizers via the β-linker
modifications for the optimal photovoltaic applications
.
Keywords: porphyrin; electron-withdrawing unit; additional acceptors; dye-sensitized
solar cells
2

ACCEPTED MANUSCRIPT
1
. Introduction
Dye-sensitized solar cells (DSCs) have attracted considerable attention because of
their relatively high light-to-electricity conversion efficiencies, ease of fabrication and
low production costs [1]. In recent years, significant efforts have been devoted to
improve cell efficiencies [2]. Ruthenium complexes are typical sensitizers in DSCs
that exhibit power conversion efficiency of more than 11% [3]. However, the scarcity,
high cost and the problems with isomerization during the purification may hamper
their widespread application. Inspired by the process of solar energy collection by
photosynthetic cores of bacteria and plants, people have designed and synthesized
numerous porphyrins as the light harvester for applications in DSCs [4]. The intrinsic
advantages of porphyrin-based dyes are the large absorption coefficient of their Soret
and Q-bands in the visible region, versatile modification at the meso and β position of
their core and facile tuning of their optical, photophysical and electrochemical
properties [5-7].
Typical meso disubstituted D–π–A porphyrin sensitizers composed of an
dialkylamine or diarylamine as the donor, and an ethynyl benzoic acid as the accepter,
have been extensively investigated [8-9]. Although the meso-functionalized
porphyrins are featured by inspiring power conversion efficiency (PCE), multistep
synthetic procedures and the corresponding low overall yield are still big challenges
[
10-12]. Tetraarylporphyrins are easily achievable by the direct condensation of
pyrrole and an aryl aldehyde, which could be functionalized at the β-pyrrolic position
by relatively few synthetic steps [13-14]. However, few studies focused on the
3

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β-functionalized porphyrin sensitizers due to the relatively lower PCEs of the
corresponding DSCs, compared to meso-substituted ones. In general, the modification
of meso-tetraphenylporphyrins is mostly focused on the β linkage between the
porphyrin core and the anchoring group as it serves primarily to extend π conjugation,
leading to the enhancement of the molecule’s light harvesting properties. So far,
ethynyl benzene [10], dithienylethylene [11], ethane [15], pentadiene [15], ethenyl
benzene [16] etc, have been successfully utilized as the π linkers. However, their role
in light harvesting is limited. The best performance of β-functionalized zinc porphyrin
ZnBD2 was reported by Kim and co-workers, which contains
a
bis(4-tert-butylphenyl)amino group at the meso position and two 2,4-pentadienoic
acid anchoring groups at the β-pyrrolic positions [16]. This porphyrin dye reaches a
PCE of 8.2%, which is comparable to the performance of a N dye (PCE = 7.7%)
3
under the same condition.
Recently, a new type of organic dye with an additional electron-withdrawing unit
introduced into the π bridge as an internal acceptor has been reported and
characterized as a D-A-π-A architecture, which is believed to optimize the molecule’s
optoelectronic properties via decreasing the lowest unoccupied molecular orbital
(
LUMO) energy levels, resulting in extension of the light absorption spectra and the
improvement of the photovoltaic performance for use in DSCs [17-19]. Inspired by
the success of the D-A-π-A featured organic dyes, the electron-deficient units were
also introduced to the porphyrin framework [20-25], resulting in the D-A-π-A type
porphyrin dye SM315 which is reported to reach a PCE record of 13% for use in
4

ACCEPTED MANUSCRIPT
porphyrin-sensitized solar cells (PSCs) without the requirement of a co-sensitizer [20].
The porphyrin dye SM315 contains an additional electron-withdrawing BTD in the
mesoπlinker (hereafter referred to as meso BTD-porphyrin). Therefore, a rational
design of π linkers makes a great contribution to the development of PSCs. To the
best of our knowledge, so far D-A-π-A type porphyrin sensitizers with the
electron-deficient BTD moiety are all modified at the meso position, and no studies
on the influence of the introduction of an additional electron-withdrawing unit at the
porphyrin β position on the performance of the resultant PSCs have reported.
Taking these points into account, we have designed and synthesized three porphyrin
dyes LP-4, LP-5 and LP-6 which were functionalized at the β-pyrrolic position of
5
,10,15,20-tetraphenylporphyrin. Based on the reference porphyrin dye LP-4 which is
featured with a typical D–π–A configuration containing the ethynyl benzoic acid as
the acceptor moiety, two electron-deficient groups with different
electron-withdrawing abilities, 2,3-diphenylquinoxaline (DPQ) for LP-5 and BTD for
LP-6 were introduced into the π-linker between the anchoring group and the
porphyrin core. The influence of introducing different auxiliary acceptor on the
porphyrins’ optical and electrochemical properties and photovoltaic performance of
PSCs based on these porphyrins are systematically investigated. The study would
shed some light on how to manipulate the LUMO energy levels of porphyrin
sensitizers via the modification of the β-linker for the optimal photovoltaic
applications.
5

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Fig. 1 Molecular structure of the three porphyrin sensitizers
Scheme 1. Synthetic route for three porphyrin sensitizers
2
. Experimental
Materials and characterization. The synthetic routes for LP-4, LP-5 and LP-6 are
shown in the Scheme 1. All reactions dealing with air- or moisture-sensitive
compounds
were
carried
out
using
standard
(3)
Schlenk
techniques.
and
5
,8-Dibromo-2,3-diphenylquinoxaline
β-Bromo-5,10,15,20-tetraphenylporphyinato] zinc(Ⅱ ) (5) were synthesized according
to the literature methods [26, 27].
1
H NMR spectra were recorded on a Bruker AV400 MHz spectrometer in CDCl or
3
THF-d8 with tetramethylsilane as a reference. MALDI-TOF-MS was obtained with a
Bruker Autoflex Tof/Tof III instrument. The UV-Vis spectra of dyes in THF solution
-6
(
3*10 M) were measured using Shimadzu UV-1800 in 10 mm quartz cell
Spectrometer. Cyclic voltammetry (CV) measurements were carried out with a
Chenhua CHI600E electrochemical analyzer at a scan rate of 50 mV/s at room
+
temperature: a glassy carbon electrode, a Pt electrode and an Ag/Ag electrode were
used as the working electrode, counter electrode and reference electrode respectively;
+
Fc/Fc (ferrocene/ferrocenium) redox couple was employed for calibration, and 0.1 M
6

ACCEPTED MANUSCRIPT
tetrabutylammonium perchlorate (TBAP) in anhydrous THF for the supporting
electrolyte.
Fabrication and photovoltaic measurements of DSCs. The photoanode was
prepared by coating commercial nanocrystalline TiO (P25) layer with the working
2
2
area of 0.159 cm and the thickness of 16 ꢀm on the FTO conducting glass using
screen-printing method followed by calcinations at 500 Ⅱ for 30 min. After immersed
into a 40 mM aqueous TiCl solution at 70 Ⅱ for 30 min and washed with water and
4
ethanol, the films were heated again at 500 Ⅱ followed by cooling to the room
temperature and soaked in a porphyrin dye solution (0.3 mM) with EtOH/toluene=1:1
at 25 Ⅱ for 3 h for dye loading onto the TiO film. The redox electrolyte consists of
2
0
.6 M 1-propyl-2,3-dimethylimidazolium iodide, 0.05 M I , 0.1 M LiI and 0.5 M
2
tert-butylpyridine in a mixture of acetonitrile and valeronitrile (volume ratio 85:15).
The porphyrin dye-sensitized TiO electrode, electrolyte and Pt foil as the counter
2
electrode (purchased) were fabricated into a typical sandwich-structured cell. Details
on cell fabrication can be found in our previous publications [28].
The photocurrent density-voltage (J-V) characteristics for PSCs were recorded on
Keithley 2400 Source meter (solar AAA simulator, oriel China, calibrated with a
-2
standard crystalline silicon solar) under simulated AM 1.5 irradiation (100 mW cm ).
2
The irradiated area of the cell was 0.159 cm , the active area of the cell is so small
that no mask can be employed during the measurement. Monochromatic incident
photon-to current conversion efficiency (IPCE) for the solar cells was performed on a
7

ACCEPTED MANUSCRIPT
commercial setup (Q Test Station 2000 IPCE Measurement System, CROWNTECH,
USA). Electrochemical impedance spectra (EIS) were scanned in a frequency range of
0
.1-106 Hz and at an AC amplitude of 10 mV at room temperature with a Chenhua
CHI600E analyzer.
Synthesis of porphyrin sensitizers.
Compound 2: 4-(methoxycarbonyl)phenylboronic acid (121 mg, 0.673 mmol, 1.2
equiv), 4,7-bromo-2,1,3-benzothiadiazole (1) (165 mg, 0.561 mmol), Pd(PPh ) (65
3
4
mg, 0.056 mmol, 10 mol%), Na CO (71 mg, 0.673 mmol, 5 mol%) toluene (5 mL)
2
3
and H O (1 mL) were stirred at 90 °C for 12 hour, upon which no more starting
2
material was detected by TLC. The mixture was poured into H O and then extracted
2
with CH Cl . The organic layer was dried over anhydrous MgSO and the solvent was
2
2
4
removed under reduced pressure. the residues were subjected to column
chromatography using DCM/PE = 1/1 as eluent to afford the desired product 2 as a
1
pale yellow solid (53 mg, yield 27%). H NMR (400 MHz, CDCl ) δ 8.19 (d, J = 8.5
3
Hz, 2H), 7.98 (d, J = 8.5 Hz, 2H), 7.95 (d, J = 7.6 Hz, 1H), 7.63 (d, J = 7.6 Hz, 1H),
1
3
3
.97 (s, 3H); C NMR (100 MHz CDCl ): δ 166.7, 153.9, 152.8, 140.9, 132.8, 132.2,
3
1
29.9, 129.1, 128.7, 127.2, 114.2, 52.3.
Compound 4: A mixture of 5,8-dibromo-2,3-diphenylquinoxaline (3) (330 mg, 0.69
mmol), 4-(methoxycarbonyl)phenylboronic acid (130 mg, 0.71 mmol) and
tetrakis(triphenylphosphine) palladium(0) (12 mg, 0.01 mmol) was dissolved in 10 ml
THF and sodium bicarbonate (75 mg, 0.71 mmol) in 1 mL water was added. After
8

ACCEPTED MANUSCRIPT
stirring at 70 °C for 4 h under argon, the mixture was poured into water and extracted
with CH Cl . The organic layers were separated, dried over magnesium sulfate and
2
2
filtered. After evaporation of solvent, the residues were subjected to column
chromatography using DCM/PE = 2/1 as eluent, Recrystallization from PE/DCM to
1
give 4 as a yellow solid (172 mg, yield 35%). H NMR (400 MHz, CDCl ) δ 8.20 (d,
3
J = 8.4 Hz, 2H), 8.13 (d, J = 7.9 Hz, 1H), 7.88 (d, J = 8.4 Hz, 2H), 7.76–7.72 (m, 2H),
7
.69 (d, J = 7.9 Hz, 1H), 7.56 (d, J = 6.9 Hz, 2H), 7.48–7.29 (m, 6H), 4.00 (s, 3H);
1
3
C NMR (101 MHz, CDCl ) δ 167.08, 153.11, 152.87, 142.20, 139.08, 139.04,
3
1
1
38.65, 138.41, 138.32, 132.90, 130.80, 130.28, 130.21, 130.05, 129.40, 129.33,
29.24, 128.36, 128.27, 124.13, 52.20.
Compound 6: A mixture of the zinc complex of 5 (1 g, 1.32 mmol),
triisopropylacetylene (0.61 mL, 3.33 mmol), Pd(PPh ) Cl (180 mg, 0.26mmol), CuI
3
2
2
(
76.6 mg, 0.39 mmol), THF (50 mL) and NEt (6 mL) was gently refluxed for 4 h
3
under dinitrogen. The solvent was removed under vacuum. The residue was purified
by column chromatography (silica gel) using DCM/PE = 1/1 to as eluent to give the
1
product 6 (926 mg, 82%) as a purple solid. H NMR (400 MHz, CDCl ) δ 9.22 (s,
3
1
8
1
8
H), 8.97 (d, J = 5.3 Hz, 4H), 8.89 (d, J = 4.7 Hz, 1H), 8.76 (d, J = 4.7 Hz, 1H), 8.27–
.19 (m, 6H), 8.16 (d, J = 7.0 Hz, 2H), 7.85–7.71 (m, 10H), 7.67 (t, J = 7.4 Hz, 2H),
.05–1.22 (m, 21H). MALDI-TOF-MS: m/z calcd for C H N SiZn, 856.29; found,
5
5
48
4
56.40 [M+].
9

ACCEPTED MANUSCRIPT
Compound 7: To a solution of porphyrin 6 (143 mg, 0.167 mmol) in dry THF (20
mL) was added TBAF (0.84 mL, 1M in THF). The solution was stirred at 23Ⅱ for 30
min under dinitrogen. The mixture was quenched with H O and then extracted with
2
CH Cl . The organic layer was dried over anhydrous MgSO and the solvent was
2
2
4
removed under reduced pressure. The residue and methyl-4-iodobenzoate (218 mg,
.835 mmol) were dissolved in a mixture of dry THF (36 mL) and NEt (7 mL) and
0
3
the solution was degassed with dinitrogen for 10 min; Pd (dba) (46 mg, 0.05 mmol)
2
3
and AsPh (102 mg, 0.33 mmol) were added to the mixture. The solution was refluxed
3
for 4 h under dinitrogen. The solvent was removed under reduced pressure. The
residue was purified by column chromatography (silica gel) using DCM/PE = 2/1 as
eluent. Recrystallization from DCM/hexanes to give 7 (70 mg, yield 50%) as a purple
1
solid. H NMR (400 MHz, CDCl ) δ 9.25 (s, 1H), 8.92 (d, J = 7.0 Hz, 4H), 8.88 (d, J
3
=
4.7 Hz, 1H), 8.78 (d, J = 4.7 Hz, 1H), 8.25–8.17 (m, 8H), 7.90 (d, J = 8.4 Hz, 2H),
7
2
8
.82–7.71 (m, 9H), 7.66 (t, J = 7.3 Hz, 2H), 7.63–7.57 (m, 1H), 7.41 (d, J = 8.4 Hz,
H), 3.86 (s, 3H). MALDI-TOF-MS: m/z calcd for C H N O Zn, 834.19; found,
5
4
34
4
2
34.27 [M+].
Porphyrin LP-4: Compound 7 (70 mg, 0.084mmol) was dissolved in THF (30 mL),
MeOH (20 mL) and a solution of NaOH (20 % w/w in water, 8 mL) added. The
solution was heated at 40 °C for 2 hours upon which TLC (silica, DCM) indicated
complete hydrolysis of the ester. The reaction mixture was diluted with DCM (100
mL) washed with water (100 mL), HCl (1M, 120 mL), water (100 mL), the organics
dried (Na SO ) and evaporated. The residue was loaded onto a short column (silica,
2
4
1
0

ACCEPTED MANUSCRIPT
DCM then 1:9 MeOH/DCM) to afford a brown solid which was further purified by
recrystallization (THF/MeOH) to afford the final product LP-4 as a purple solid (51
1
mg, yield 75%). H NMR (400 MHz, DMSO) δ 9.03 (s, 1H), 8.76 (m, 4H), 8.72 (d, J
=
4.7 Hz, 1H), 8.60 (d, J = 4.7 Hz, 1H), 8.23-8.12 (m, 9H), 7.96 (d, J = 8.4 Hz, 2H),
.85–7.77 (m, 11H), 7.75–7.70 (m, 4H), 7.48 (d, J = 8.4 Hz, 2H). MALDI-TOF-MS:
m/z calcd for C H N O Zn, 820.18; found, 820.24[M+].
7
5
3
32
4
2
Compound 8: A mixture of porphyrin 6 (143 mg, 0.167 mmol), dry THF (20 mL) and
TBAF (0.84 mL, 1M in THF) under the argon atmosphere was stirred at 23 Ⅱ for 30
min. H O was added and the resulting solution was extracted with CH Cl . The
2
2
2
organic layer was dried (MgSO ) and the concentrated under reduced pressure. The
4
residue and 4 (412 mg, 0.835 mmol) were dissolved in a mixture of dry THF (36 mL)
and NEt (7 mL) and the solution was degassed with dinitrogen for 10 min; Pd (dba)
3
3
2
(
46 mg, 0.05 mmol) and AsPh (102 mg, 0.33 mmol) were added to the mixture. The
3
solution was refluxed for 4 h under dinitrogen. The solvent was removed under
reduced pressure. The residue was purified by column chromatography (silica gel)
using DCM/PE = 2/1 as eluent. Recrystallization from DCM/hexanes to give 8 (89
1
mg, yield 48%) as a purple solid. H NMR (400 MHz, CDCl ) δ 9.49 (s, 1H), 8.95 (d,
3
J = 6.9 Hz, 4H), 8.90 (d, J = 4.7 Hz, 1H), 8.80 (d, J = 4.7 Hz, 1H), 8.31 (m, 4H),
8
.27–8.18 (m, 4H), 8.04 (d, J = 8.3 Hz, 2H), 7.94 (d, J = 8.3 Hz, 2H), 7.91–7.72 (m,
2H), 7.73–7.65 (m, 4H), 7.62 (d, J = 6.6 Hz, 2H), 7.44–7.29 (m, 6H).
1
MALDI-TOF-MS: m/z calcd for C H N O Zn, 1114.29; found, 1114.44 [M+].
7
4
46
6
2
1
1

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Porphyrin LP-5: Compound 8 (89 mg, 0.08 mmol) was dissolved in THF (30 mL),
MeOH (20 mL) and a solution of NaOH (20 % w/w in water, 8 mL) was added. The
solution was heated at 40°C for 2 hours upon which TLC (silica, DCM) indicated
complete hydrolysis of the ester. The reaction mixture was diluted with DCM (100
mL) washed with water (100 mL), HCl (1 M, 120 mL), water (100 mL), the organics
dried (Na SO ) and evaporated. The residue was loaded onto a short column (silica,
2
4
DCM then 1:9 MeOH/DCM) to afford a brown solid which was further purified by
recrystallization (THF/MeOH) to afford the final product LP-5 as purple solid (69 mg,
1
yield 78%). H NMR (400 MHz, THF) δ 9.30 (s, 1H), 8.78 (d, J = 9.2 Hz, 4H), 8.73
(
d, J = 4.7 Hz, 1H), 8.65 (d, J = 4.6 Hz, 1H), 8.29–8.19 (m, 4H), 8.15 (dd, J = 5.6, 2.3
Hz, 6H), 7.98 (d, J = 8.2 Hz, 2H), 7.91 (d, J = 7.6 Hz, 1H), 7.85–7.66 (m, 12H), 7.65–
.56 (m, 5H), 7.37 (t, J = 7.4 Hz, 1H), 7.65–7.56 (m, 5H). MALDI-TOF-MS: m/z
calcd for C H N O Zn, 1110.28; found, 1110.37 [M+].
7
7
3
44
6
2
Compound 9: A solution of porphyrin 6 (143 mg, 0.167 mmol) in dry THF (20 mL)
under an argon atmosphere was treated with TBAF (0.84 mL, 1M in THF), then the
resulting solution was stirred at 23 Ⅱ . After half an hour, the mixture was quenched
with H O and then extracted with CH Cl . The organic layer was washed (water and
2
2
2
brine), dried over anhydrous MgSO and concentrated. The residue and 2 (290 mg,
4
0
.835 mmol) were dissolved in a mixture of dry THF (36 mL) and NEt (7 mL) and
3
the solution was degassed with dinitrogen for 10 min; Pd (dba) (46 mg, 0.05 mmol)
2
3
and AsPh (102 mg, 0.33 mmol) were added to the mixture. The solution was refluxed
3
for 4 h under dinitrogen. The solvent was removed under reduced pressure. The
1
2

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residue was purified by column chromatography (silica gel) using DCM/PE = 2/1 as
eluent. Recrystallization from DCM/hexanes to give 9 (73 mg, yield 45%) as a purple
1
solid. H NMR (400 MHz, CDCl ) δ 9.42 (s, 1H), 8.92 (d, J = 8.0 Hz, 4H), 8.88 (d, J
3
=
4.7 Hz, 1H), 8.77 (d, J = 4.7 Hz, 1H), 8.31–8.17 (m, 8H), 8.14 (d, J = 8.4 Hz, 2H),
.08 (d, J = 8.3 Hz, 2H), 7.76 (t, J = 7.5 Hz, 10H), 7.65–7.56 (m, 3H), 7.46 (t, J = 7.5
Hz, 1H), 3.91 (s, 3H). MALDI-TOF-MS: m/z calcd for C H N O SZn, 968.19;
8
6
0
36
6
2
found 968.28 [M+].
Porphyrin LP-5: Compound 9 (73 mg, 0.076 mmol) was dissolved in THF (30 mL),
MeOH (20 mL) and a solution of NaOH (20 % w/w in water, 8 mL) was added. The
solution was heated at 40°C for 2 hours upon which TLC (silica, DCM) indicated
complete hydrolysis of the ester. The reaction mixture was diluted with DCM (100
mL) washed with water (100 mL), HCl (1 M, 120 mL), water (100 mL), the organics
dried (Na SO ) and evaporated. The residue was loaded onto a short column (silica,
2
4
DCM then 1:9 MeOH/DCM) to afford a brown solid which was further purified by
recrystallization (Et O/MeOH) to afford the final product LP-6 as a purple solid (59
2
1
mg, yield 81%). H NMR (400 MHz, THF) δ 9.26 (s, 1H), 8.82 (d, J = 7.3 Hz, 4H),
8
.77 (d, J = 4.7 Hz, 1H), 8.68 (d, J = 4.7 Hz, 1H), 8.26-8.15 (m, 12H), 7.95 (d, J = 7.4
Hz, 1H), 7.81-7.70 (m, 9H), 7.69–7.60 (m, 3H), 7.55 (t, J = 7.5 Hz, 1H).
MALDI-TOF-MS: m/z calcd for C H N O SZn, 954.17; found, 954.24 [M+].
5
9
34
6
2
1
3

ACCEPTED MANUSCRIPT
. Results and discussion
3
3
.1 Synthesis
The two key intermediate 2 and 4 were prepared by a Suzuki coupling reaction. In the
next step, these electron-withdrawing unit were attached to the porphyrin core through
a triple bond by a two–step Sonogashira coupling reaction. Finally, the
Zn(ꢀ)-porphyrin sensitizers LP-4, LP-5 and LP-6 were obtained via hydrolysis. The
dyes were obtained as a purple solid and soluble in most organic solvents, such as
THF, DMF and DMSO.
3
.2 Photophysical properties
The UV-vis spectra of the metalloporphyrins LP-4, LP-5 and LP-6 in THF solvent
are shown in Fig. 2a, and the corresponding absorption maxima and extinction
coefficients for the Soret- and Q-bands are listed in Table 1. The absorption spectrum
of LP-4 displays three absorption peaks at 435 nm, 566 nm, and 603 nm, which are
assigned as the π–π* transitions of the conjugated system and intramolecular charge
transfer (ICT) transitions of the D-π-A conjugated backbone [29]. The introduction of
DPQ and BDT acceptor units had a significant impact on the absorption spectra of
LP-5 and LP-6, most evident by the splitting of the Soret band, resulting in a shoulder
at 464 nm (for LP-5) and 471 nm (for LP-6) next to the maximum Soret band,
respectively, but the absorption intensity are significantly decreased. Furthermore, the
Q-bands of LP-5 and LP-6 were slightly red-shifted to 605 nm and 606 nm,
respectively. It is believed that spectrum broadening and red-shift can be attributed to
the increasing of the π-conjugation and the facilitated electron transfer from the
1
4

ACCEPTED MANUSCRIPT
porphyrin to the acceptor (ICT) due to the introduction of additional
electron-withdrawing acceptor. Both the splitting and red-shifting of the absorbance
maxima for LP-5 and LP-6 were consistent with those previously reported meso
BTD-porphyrin [20]. However, compared to LP-5, LP-6 has similar absorption
properties but the Soret band is more red-shifted, which suggests that the larger the
electron withdrawing nature of the additional acceptor unit, the greater red-shift in the
photoresponse, which is also consistent with the results obtained for the metal free
organic dyes by Tian group [30]. Furthermoreꢁthere is a broadened absorption spectra
of the porphyrin dyes absorbed on the TiO film compared with the dyes in the
2
solution especially for Soret band (Fig. 2b), indicating the formation of J-aggregates
[
31]. The fluorescent emission spectra of LP-4, LP-5 and LP-6 were obtained in THF
solution, and the major emission bands observed at 616, 619 and 618 nm,
respectively. The Stoke shifts for LP-4, LP-5 and LP-6 were 13 nm, 14 nm and 12
nm, respectively, which are smaller compared to metal-free organic dyes, suggesting a
smaller energy required for geometrical reorganization of porphyrin dyes at
photoexcited state [32-33].
Fig. 2 UV-visible absorption spectra of compound LP-4, LP-5 and LP-6 (a) in THF
solutions and (b) on transparent TiO films (4 um).
2
Table 1 Photophysical properties of the three sensitizers
3
.3 Electrochemical properties
1
5

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Cyclic voltammertic (CV) data for LP-4, LP-5 and LP-6 are shown in Table 2 and
Fig. 3. The highest occupied molecular orbital (HOMO) energy levels are located at
-5.15, -5.22 and -5.23 eV, respectively. These values are all more negative than the
-
-
energy level of I /I redox (-4.9 eV) guaranteeing efficient dye regeneration [34]. It is
3
shown that the HOMO energy levels of LP-5 and LP-6 were more negative than that
of LP-4. Interestingly, these results are different from those of the meso
BTD-porphyrin [20], in which the HOMO energy levels remain unchanged after
introducing an additional electron withdrawing unit into the meso π-linker. The
LUMO energy levels (ELUMO) of the three dyes were determined to be -3.08, -3.17
and -3.19 eV, respectively, which are significantly lower than the conduction band
edge energy level of the TiO electrode (-3.9 eV), ensuring an efficient injection
2
process from the excited state of the dyes into the TiO electrode [35]. Compared to
2
LP-4, LP-5 and LP-6 containing DPQ and BTD units in the porphyrin frame show
more negative LUMO levels and the decreased energy gap. Hence, the electronic
transition from HOMO to LUMO orbitals in LP-5 and LP-6 was facilitated, which is
consistent with an enhanced ICT character, and the red-shifting observed in the
absorption spectrum of LP-5 and LP-6. Furthermore, a comparison between the
energy levels of LP-5 and LP-6 suggests that with the increasing of the
electron-withdrawing capability of the additional auxiliary acceptor (BTD>DPQ),
both the HOMO and LUMO levels and the LUMO-HOMO energy gap are decreased
[
36].
1
6

ACCEPTED MANUSCRIPT
Fig. 3 Energy levels (eV) of HOMO and LUMO molecular orbitals for the three
sensitizers
Table 2 Electrochemical data for the porphyrin sensitizers
3
.4 Computational Analysis
We performed theoretical calculations based on density functional theory (DFT) at
the B3LYP/6-31G level to investigate the frontier molecular orbitals and the
geometrical structures of LP-4, LP-5 and LP-6. As shown in Fig. 4, the HOMO of
the three porphyrin dyes lies predominantly on the porphyrin core. Compared with
that of LP-4, the LUMO of LP-5 and LP-6 shows a significant shift through the
acceptor due to the presence of additional electron-withdrawing units (DPQ and
BTD), and a more significant shift is observed for LP-6 with a stronger
electron-withdrawing BTD group. These results are consistent with those obtained in
the experiment and could explain the greater charge transfer characteristics of LP-5
and LP-6. Furthermore, in the optimized configurations, the dihedral angel between
o
the porphyrin core and the neighboring benzene ring in LP-4 is 3 , however, upon the
introduction of additional electron-withdrawing units, the dihedral angels between the
o
o
porphyrin core and the DPQ and BTD unit are 23 and 6 , respectively, and the
dihedral angels between the DPQ and BTD unit and the neighboring benzene ring are
o
o
4
5 and 37 , respectively. Compared to LP-6, the more significant molecular twist of
LP-5 will decrease the effective conjugation length and weaken the charge transfer
interaction, which explains the relatively blue-shifted absorption spectra of LP-5.
1
7

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Fig. 4 Calculated frontier molecular orbitals for the porphyrin dyes.
3
.5 Photovoltaic performance of DSSCs
LP-4, LP-5 and LP-6 were used to fabricate solar cells using an iodine electrolyte
-2
and measured under illumination condition (AM 1.5G 100 mW cm ). The
photocurrent density-voltage (J-V) curves are shown in Fig. 5a and the corresponding
detailed parameters are collected in Table 3. The short circuit current (J ) of the PSCs
sc
-2
-2
fabricated with LP-4 and LP-5 was 7.60 mA cm and 9.03 mA cm with overall
conversion efficiencies of 4.02% and 4.47%, respectively. The LP-6 sensitized cell
-2
gave a J of 11.47 mA cm , V of 0.71 V, and FF of 0.74, corresponding to an
sc
oc
overall conversion efficiency of 6.14%. The highest PCE of LP-6 reached about 80%
of the N-719 based cell measured under the same condition. To clarify the influence
of the introduction of an electron withdrawing unit into the β π-linker of the dyes on the
device performances, we have measured the amount of dyes absorbed on TiO (Table
2
-7
-2
3
). As shown in Table 3, the dye loading results (1.41*10 mol cm for LP-4,
-7
-2
-7
-2
1
.66*10 mol cm for LP-5, and 1.59*10 mol cm for LP-6) increased with the
introduction of additional withdrawing units. Therefore, one reason for the enhanced
Jsc values for LP-5 and LP-6 could be their increased dye loading amounts. It is
believed that another reason for the greater J achieved for LP-5 and LP-6 might be
sc
the enhanced intramolecular charge transfer and the broader absorption spectra due to
the presence of electron-withdrawing units (DPQ and BTD) between the porphyrin
core and anchoring group. Furthermore, the cathodic cyclic voltammetry test
1
8

ACCEPTED MANUSCRIPT
suggested that the LUMO energy levels of LP-5 and LP-6 are more negative than the
-
-
energy level of I /I redox when compared to that of LP-4, providing sufficient
3
regeneration driving force. Overall, the introduction of the electron-withdrawing BTD
-2
leads to a J value as high as 11.47 mA cm for LP-6-based cell which is
sc
significantly greater than that of the cell sensitized by LP-4. However, compared with
that of LP-6 based cell, , the J value of LP-5 cell dropped significantly, which is
sc
believed to be originated from the smaller electron-withdrawing nature of the
additional acceptor.
It is also found the V values achieved in LP-4 and LP-6-based cells are greater
oc
than that of LP-5 cell. The J-V curves in dark as shown in Fig. 5a suggest that the
dark current in PSCs based on LP-4 and LP-6 is smaller than that of LP-5, indicating
that the increase in V for LP-4 and LP-6-based cells was achieved by suppressing
oc
the injected electron recombination rather than negative shift in CB of TiO2.
The incident photo-to-current conversion efficiency (IPCE) spectra of LP-4,
LP-5 and LP-6 were performed to further understand the correlation between the
absorption and J values (Fig. 5b). Compared to LP-4, it can be seen that both LP-5
sc
and LP-6 with the additional electron-withdrawing unit have higher and extended
absorption from 350 to 520 nm which is consistent with the splitted Soret band
absorptions of LP-5 and LP-6 shown in Fig. 2a. Among three dyes, the IPCE value of
LP-6 is the highest and broadest, and over 50% IPCE from 350 to 650 nm achieved.
The highest IPCE value and broadest absorption range of LP-6 ensured the highest Jsc
1
9

ACCEPTED MANUSCRIPT
value and the best cell performance.
Electrochemical impedance spectroscopy (EIS) is a powerful technique of
characterizing the important interfacial charge transfer and carrier transportation
process in DSCs. The Nyquist plots (Fig. 6a) show the middle semicircle in the
middle-frequency region, representing the resistance of charge transfer from TiO to
2
the electrolyte. The larger middle semicircles for LP-4 based DSCs mean that it is
more difficult for charge transfer from TiO to the electrolyte for the device, which is
2
consistent with the smaller dark current for LP-4 dye observed in J-V test. As we
know, the photovoltage of a DSC is intrinsically influenced by the degree of electron
recombination reaction [37-38]. Thus, the EIS Nyquist results are consistent with the
Voc values listed in Table 3. These results indicate that the introduction of
electron-withdrawing units (BTD and DPQ) into the π-spacer of LP-4 can facilitate
the charge recombination, which result in the decrease the loss of the photovoltage of
the device, which is consistent with results obtained from the BTD-meso porphyrin
dye[20-21].
In the Bode phase plots (Fig. 6b), the frequency peak is indicative of the
charge-transport process of injected electrons in TiO . The electron lifetime (τ )
2
e
calculated through the equation τ =1/(2πωmax) (ωmax is the maximum frequency in
e
the Bode plot) can further explain the Voc changes for LP-4, LP-5 and LP-6
-sensitized solar cells [39-40]. The ωmax of the three dyes were found to be in the
order LP-4 < LP-6 < LP-5, and thus the electron lifetime were to increase in the order
2
0

ACCEPTED MANUSCRIPT
LP-5 < LP-6 < LP-4. Thus, the relatively highest Voc of LP-4 could be further
explained. However, the loss of V is overcompensated by a gain in J , resulting in
oc
sc
the superior performance of the LP-5 and LP-6 dyes.
Fig. 5 (a) The J-V curves and (b) the IPCE profiles of LP-4, LP-5 and LP-6 sensitizers
Fig. 6 Nyquist plots (a) and Bode-phase plots (b) of electrochemical impedance
spectra measured at a forward bias of - 0.6 V under dark conditions for the DSSCs.
Table 3 Photovoltaic parameters of porphyrin-sensitized solar cells
4
. Conclusions
In summary, two different electron-deficient units (DPQ for LP-5 and BTD for
LP-6) were first introduced ino the π-linker at the β-pyrrolic position of
,10,15,20-tetraphenylporphyrin. It was found that the introduction of auxiliary
5
acceptor exhibits enhanced light-harvesting efficiency and relatively higher J values.
sc
Howeverꢁthe DSCs based on LP-5 and LP-6 display shorter electron lifetime and
facilitated charge recombination rate, resulting in a lower V values compared to the
oc
reference dye LP-4 without the additional electron-withdrawing unit. Interestingly,
the decreased Voc was overcompensated by a gain in J , resulting in the superior
sc
performance, the optimized PCE of LP-6 reached to 6.14% with a V of 710 mV, a
oc
-2
J of 11.47 mA cm and a FF of 0.74. This finding provides an alternative way to
sc
2
1

ACCEPTED MANUSCRIPT
design and synthesize efficient and novel porphyrin sensitizers via the modification of
porphyrin β π-linkers for photovoltaic applications. Further studies on the molecular
design for the photovoltaic devices with simultaneously improved J and V values
sc
oc
are also in progress.
Acknowledgement
This work was supported by National Natural Science Foundation of China (No.
1476162). The acknowledgment is also extended to China International Science and
2
Technology Project (No. 2012DFG41980ꢁS2016G3413).
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