Porphyrin sensitizers containing an auxiliary benzotriazole acceptor for dye-sensitized solar cells: Effects of steric hindrance and cosensitization

Article abstract of DOI:10.1016/j.dyepig.2018.03.054

Dye-sensitized solar cells (DSSCs) have attracted intensive attention in developing photovoltaic devices for employing solar energy. For developing panchromatic and efficient porphyrin sensitizers, it has been demonstrated to be an effective approach to i

Full text of DOI:10.1016/j.dyepig.2018.03.054

Accepted Manuscript
Porphyrin sensitizers containing an auxiliary benzotriazole acceptor for dye-sensitized
solar cells: Effects of steric hindrance and cosensitization
Heli Song, Weiqiang Tang, Shuangliang Zhao, Qingyun Liu, Yongshu Xie
PII:
S0143-7208(18)30229-8
10.1016/j.dyepig.2018.03.054
DYPI 6639
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 31 January 2018
Accepted Date: 23 March 2018
Please cite this article as: Song H, Tang W, Zhao S, Liu Q, Xie Y, Porphyrin sensitizers containing
an auxiliary benzotriazole acceptor for dye-sensitized solar cells: Effects of steric hindrance and
cosensitization, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.03.054.
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Porphyrin sensitizers containing an auxiliary benzotriazole
acceptor for dye-sensitized solar cells: effects of steric
hindrance and cosensitization
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Heli Song , Weiqiang Tang , Shuangliang Zhao , Qingyun Liu and Yongshu Xie*
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Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular
Engineering, East China University of Science and Technology, Meilong 130, Shanghai 200237, China.
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School of Chemical Engineering and State Key Laboratory of Chemical Engineering, East China University of
Science and Technology, Shanghai, 200237, China.
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College of Chemistry and Environmental Engineering, Shandong University of Science and Technology,
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Qingdao, P. R. China.
* Corresponding author: Yongshu Xie, E-mail: yshxie@ecust.edu.cn
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Abstract: Dye-sensitized solar cells (DSSCs) have attracted intensive attention in
developing photovoltaic devices for employing solar energy. For developing
panchromatic and efficient porphyrin sensitizers, it has been demonstrated to be an
effective approach to introduce an electron-withdrawing benzothiadiazole unit as an
extra electron acceptor. In contrast, the structurally similar benzotriazole moiety
remains relatively unknown in this respect. In this work, we have synthesized a novel
porphyrin dye containing an extra electron acceptor of benzotriazole. Photophysical
and electrochemical investigations revealed red-shifted absorption and a narrower
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band gap induced by the benzotriazole unit. Thus, a reasonable efficiency of 8.39%
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has been achieved with a J of 17.00 mAñcm and a V of 712 mV. On this basis, to
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investigate the effect of the dihedral angle between the benzotriazole and the adjacent
phenylene ring, one or two methyl groups were attached to the ortho-positions of the
phenylene ring. As a result, the enlarged torsion angles induce poor electronic
coupling between the LUMO of the dyes and TiO , resulting in lower efficiencies of
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6.61% and 3.62%, respectively. With the purpose to further improve the efficiencies,
coadsorption and cosensitization approaches were employed. And the efficiencies
have been successfully elevated to 9.32%, 8.49% and 7.34%, respectively. These
results demonstrate the effectiveness of incorporating an auxiliary benzotriazole
acceptor into porphyrin dyes on achieving extended light-harvesting wavelength
ranges and illustrate the importance of avoiding severe torsion within the acceptor
part of the porphyrin dyes. In contrast to the commonly used cosensitizers with
absorption peaks lying in the absorption valley between the Soret band and the Q
bands of porphyrin dyes (500−600 nm), the utilization of an organic dye with broad
absorption in this work has been demonstrated to be effective for improving the
efficiencies despite the fact that its absorption peak at 450 nm largely overlaps with
the Soret band of the porphyrin dyes. These results provide an alternative approach
for developing cosensitizers for porphyrin dyes.
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Keywords: Dye-sensitized solar cells; Porphyrin; Sensitizer; Benzotriazole; Steric
hindrance; Cosensitization.
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1. Introduction
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Exploitation of new and clean energy sources for future development has
become one of the major challenges in contemporary society. In this respect,
dye-sensitized solar cells (DSSCs) are attractive and promising because of their
relatively efficient conversion of sunlight to electricity with the advantages of
flexibility, easy fabrication and low cost [1,2]. Among the DSSC sensitizers,
porphyrins exhibit advantages of high molar absorption coefficients with tunable
molecular structures and properties [3,4]. In this respect, a number of excellent
porphyrin sensitizers like YD2-o-C8, GY50 and SM315 have been developed with
efficiencies achieved well above 10% [5-7].
Despite the excellent photovoltaic behavior, two inherent drawbacks have been
observed for porphyrin dyes. The first is the intrinsic absorption void in the NIR
region and the absorption valley between the Soret and Q bands in the range of
500-600 nm [8-11]. The other problem is the relatively high tendency for aggregation
induced by the large conjugated framework, which will lead to quenching of the
excited dyes and diminished electron injection [12-14]. To address these problems,
various approaches have been reported. For example, insertion of
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2,1,3-benzothiadiazole (BTD) unit as an auxiliary electron acceptor has been
demonstrated to be effective for enhancing the electronic coupling between the
porphyrin core and the anchoring group, resulting in relatively panchromatic
absorption and photo-response [7,15-19]. On the other hand, attachment of alkoxyl or
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alkyl chains to appropriate positions can improve dye solubility and reduce
aggregation [20-25].
In contrast to the widely used BTD unit, the structurally similar benzotriazole
(BTz) moiety remains relatively unknown in this respect. In fact, in some cases, the
insertion of a BTD unit unfavorably leads to worse photovoltaic performance [26-28],
and BTz might be advantageous in these cases. In addition, the presence of an
additional nitrogen atom facilitates introduction of alkyl chains for suppressing dye
aggregation and charge recombination [29]. Herein, we incorporate a BTz unit as the
auxiliary acceptor to develop a novel porphyrin dye XW33 (Fig. 1), using a
phenothiazine-based donor.
On the other hand, it is an effective approach to develop porphyrin dyes by
borrowing concepts developed for metal-free organic dyes. It has been demonstrated
by Li and coworkers that in a certain series of organic dyes, a more twisted spacer
neighboring to the anchoring group is favorable for improving V and efficiencies
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(Fig. 1) [30]. By considering this background, XW34 and XW35 (Fig. 1) were
synthesized with one or two methyl groups attached at the ortho-positions of the
phenylene ring adjacent to the BTz unit. As a result, XW33-XW35 exhibit power
conversion efficiencies of 8.39%, 6.61% and 3.62%, respectively. These results
indicate that the more twisted spacer is unfavorable for the photovoltaic behavior. To
improve the cell performance, the porphyrin dyes were coadsorbed with
chenodeoxycholic acid (CDCA) and cosensitized with a phenothiazine-based organic
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dye PT-C6 [31], which exhibits a broad absorption peak. Thus, dramatically enhanced
efficiencies of 9.32%, 8.49% and 7.34% were achieved for XW33, XW34, and
XW35 respectively, despite the fact that the absorption peak of PT-C6 is centered at
450 nm, largely overlapping with the Soret bands of the porphyrin dyes. These
successful cosensitization examples indicate that the broad absorption band of PT-C6
can well compensate for the narrow Soret band of the porphyrin dyes.
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Fig. 1 Molecular structures of porphyrin dyes XW33-XW35, the cosensitizer PT-C6 and
organic dyes LI-85 and LI-86 reported by Li and coworkers.
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2. Experimental section
2.1 Materials and Instrumentation
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All chemical reagents and solvents of analytical grade were purchased and used
without further purification unless otherwise noted. Fluorine-doped SnO conducting
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glass (FTO glass, transparency >90% in the visible range, sheet resistance 15
Ω/square) was purchased from the Geao Science and Educational Co. Ltd of China.
TiO paste (18 NR-T and 18 NR-AO) was purchased from Dyesol Ltd.
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H NMR and C NMR spectra were recorded using a Bruker AM 400
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spectrometer at 298 K with tetramethylsilane (TMS) as the internal standard. High
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resolution mass spectroscopy (HRMS) measurements were performed using a Waters
LCT Premier XE spectrometer. Matrix-assisted laser desorption/ionization
time-of-flight mass spectra (MALDI-TOF-MS) were measured on a Shimadzu-Kratos
model Axima CFR+ mass spectrometer using dithranol as the matrix. UV-Vis
absorption spectra were measured on a Varian Cary 60 spectrophotometer, and
fluorescence spectra were recorded on a Varian Cary Eclipse fluorescence
spectrophotometer. The cyclic voltammograms of the dyes were carried out in
acetonitrile on a Zahner IM6e electrochemical workstation based on a three electrode
system using 0.1 M TBAPF (Adamas) as the supporting electrolyte, the sensitizer
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attached to a nanocrystalline TiO film deposited on the conducting FTO glass as the
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working electrode, a platinum wire as the counter electrode, and a regular saturated
calomel electrode (SCE) as the reference electrode. The scan rate was fixed at 100
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mV s .
Photovoltaic measurements were performed by employing an AM 1.5 solar
simulator equipped with a 300 W xenon lamp (model no. 91160, Oriel). The power of
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the simulated light was calibrated to 100 mW cm using a Newport Oriel PV
reference cell system (model 91150 V). J–V curves were obtained by applying an
external bias to the cell and measuring the generated photocurrent with a model 2400
source meter (Keithley Instruments, Inc. USA). The voltage step and delay time of the
photocurrent were 10 mV and 40 ms, respectively. Action spectra of the incident
monochromatic photon-to-electron conversion efficiency (IPCE) for the solar cells
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were obtained with a Newport-74125 system (Newport Instruments). The intensity of
monochromatic light was measured with a Si detector (Newport-71640). The
electrochemical impedance spectroscopy (EIS) measurements of all the DSSCs were
performed using a Zahner IM6e Impedance Analyzer (ZAHNER-Elektrik GmbH &
CoKG, Kronach, Germany), with the frequency range of 0.1 Hz–100 kHz and the
alternative signal of 10 mV. The ZSimpWin software was used to fit the experimental
EIS data.
2.2 Syntheses of the dyes
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Compounds 1 [32], 2 [33], 5 [34] and 8 [15] were synthesized according to
literature methods.
Compound 3 [35]. To a 250 mL flask was added compound 2 (4.0 g, 19 mmol),
MeOH (100 mL) and KI (623 mg, 3.8 mmol). The mixture was stirred at 50°C before
t-BuOOH (3.9 mL, 70% (aq), 28 mmol) was added dropwise over 30 min. Then the
mixture was refluxed for 24 h. After cooling to room temperature, saturated Na S O
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(aq) was added. The mixture was extracted with ethyl acetate, dried over anhydrous
Na SO and evaporated in vacuo. The residue was purified by column
2
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chromatography on silica gel with petroleum ether / ethyl acetate (15:1, v/v) to afford
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compound 3 as a white solid (3.8 g, 84%). H NMR (400 MHz, CDCl ) δ 7.73 (s, 2H),
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3.90 (s, 3H), 2.46 (s, 6H).
Compound 4 [36]. To a 250 mL three-neck flask was added compound 3 (4.5 g,
18 mmol), bis(pinacolato)diboron (5.2 g, 20 mmol), 1,4-dioxane (60 mL),
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Pd(PPh ) Cl (519 mg, 0.74 mmol) and KOAc (5.5 g, 56 mmol). The mixture was
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stirred at 90°C for 24 h under nitrogen. Water was added and the mixture was
extracted with dichloromethane and washed with water three times. The combined
organic layers were dried over Na SO and evaporated in vacuo. The residue was
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purified by column chromatography with petroleum ether / ethyl acetate (15:1, v/v) to
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give the product as a white solid (4.0 g, 75%). H NMR (400 MHz, CDCl ) δ 7.61 (s,
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2H), 3.89 (s, 3H), 2.43 (s, 6H), 1.40 (s, 12 H).
Compound 6a. To a 100 mL three-neck flask was added compound 5 (4.3 g, 11
mmol), methyl 4-boronobenzoate (1.0 g, 5.6 mmol) and THF (50 mL). Then
Pd(PPh ) (134 mg, 0.116 mmol) and K CO (6 mL, 2 M aq) were added under
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nitrogen. The mixture was refluxed for 12 h under nitrogen. The organic layer was
extracted with dichloromethane, washed with water, dried over Na SO and
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evaporated in vacuo. The residue was purified by column chromatography on silica
gel with petroleum ether / dichloromethane (1:1, v/v) to give the product as a
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colorless oil (1.5 g, 60%). H NMR (400 MHz, CDCl ) δ 8.17 (d, J = 8.8 Hz, 2H,
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phenyl), 8.08 (d, J = 8.4 Hz, 2H, phenyl), 7.67 (d, J = 7.6 Hz, 1H, phenyl), 7.48 (d, J
= 7.6 Hz, 1H, phenyl), 4.79 (t, J = 7.2 Hz, 2H, -NCH -), 3.96 (s, 3H, -OMe), 2.23 –
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2.08 (m, 2H), 1.46 – 1.34 (m, 4H), 1.30 – 1.20 (m, 6H), 0.87 (t, J = 6.8 Hz, 3H). C
NMR (101 MHz, CDCl ) δ 166.89, 144.44, 142.61, 141.01, 129.99, 129.62, 129.60,
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129.19, 128.42, 125.43, 110.55, 57.21, 52.21, 31.71, 30.20, 29.05, 28.95, 26.54, 22.60,
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14.07. HRMS (ESI, m/z): [M+H] calcd for C H BrN O , 444.1287; found,
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444.1290.
Compound 6b. It was synthesized in a similar way with 6a except that compound
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1 was used instead of methyl 4-boronobenzoate. Colorless oil (yield 61%). H NMR
(400 MHz, CDCl ) δ 8.02 (s, 1H, phenyl), 7.95 (dd, J = 7.6, 1.2 Hz, 1H, phenyl), 7.65
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(d, J = 7.6 Hz, 1H, phenyl), 7.42 (d, J = 8.0 Hz, 1H, phenyl), 7.14 (d, J = 7.6 Hz, 1H,
phenyl), 4.72 (t, J = 7.6 Hz, 2H, -NCH -), 3.95 (s, 3H, -OMe), 2.25 (s, 3H,
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phenyl-CH ), 2.14 – 2.02 (m, 2H), 1.37 – 1.29 (m, 4H), 1.28 – 1.18 (m, 6H), 0.86 (t, J
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= 6.8 Hz, 3H). C NMR (101 MHz, CDCl ) δ 167.04, 143.83, 143.10, 141.48, 136.75,
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131.63, 130.88, 130.31, 129.86, 128.90, 127.01, 109.96, 57.16, 52.16, 31.67, 30.24,
+
29.02, 28.92, 26.48, 22.59, 20.53, 14.06. HRMS (ESI, m/z): [M+Na] calcd for
C H BrN NaO , 480.1263; found, 480.1265.
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Compound 6c. To a 100 mL Schlenk tube was added compound 5 (1.00 g, 2.57
mmol), compound 4 (895 mg, 3.08 mmol) and DMF (30 mL). Then Pd(PPh ) (59 mg,
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0.051 mmol) and K PO (1.36 g, 6.42 mmol) were added under nitrogen. The mixture
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was stirred at 100°C under nitrogen for 9 h and then poured into water and extracted
with dichloromethane. The organic layer was washed with water, dried over
anhydrous Na SO and evaporated in vacuo. The residue was purified by column
2
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chromatography on silica gel with petroleum ether / dichloromethane (3:2, v/v) to
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give the product as a colorless oil (450 mg, 37%). H NMR (400 MHz, CDCl ) δ 7.84
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(s, 2H, phenyl), 7.66 (d, J = 7.6 Hz, 1H, phenyl), 7.02 (d, J = 7.2 Hz, 1H, phenyl),
4.70 (t, J = 7.2 Hz, 2H, -NCH -), 3.94 (s, 3H, -OCH ), 2.11 – 2.04 (m, 2H,
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-NCH CH -), 2.03 (s, 6H, phenyl-CH ), 1.31 – 1.27 (m, 4H), 1.27 – 1.20 (m, 6H),
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0.86 (t, J = 6.4 Hz, 3H). C NMR (101 MHz, CDCl ) δ 167.19, 143.98, 142.92,
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141.10, 136.99, 129.91, 129.58, 129.05, 128.65, 126.73, 109.55, 57.11, 53.44, 52.08,
+
31.63, 30.23, 29.00, 28.87, 26.42, 22.56, 20.58, 14.03. HRMS (ESI, m/z): [M+H]
calcd for C H BrN O , 472.1600; found, 472.1601.
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General synthetic procedure for compounds 7a~7c.
To a 100 mL Schlenk tube was added 6a~6c (1.0 mmol), THF/Et N (24 mL / 12
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mL), Pd(PPh ) Cl (5 mol%) and CuI (5 mol%) and charged with nitrogen. Then the
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mixture was heated at 50°C before trimethylsilylacetylene (4 mmol) was added over
15 min via a syringe. The mixture was allowed to reflux for 12 h. The solvents were
evaporated and the residue was purified by silica gel column chromatography to get
the crude product. To the crude product was added THF/MeOH (20 mL / 20 mL) and
KOH (1 eq, 1M aq). The solution was allowed to stir at room temperature for 1 h.
Then water was added and the product was extracted with dichloromethane. The
organic layers were separated, dried over Na SO and evaporated in vacuo. Silica gel
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column chromatography was performed to obtain the ethynyl product.
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7a. Brownish yellow solid (yield 58%, 2 steps). H NMR (400 MHz, CDCl ) δ
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8.18 (d, J = 8.4 Hz, 2H, phenyl), 8.12 (m, J = 8.4 Hz, 2H, phenyl), 7.67 (d, J = 7.2 Hz,
1H, phenyl), 7.59 (d, J = 7.2 Hz, 1H, phenyl), 4.80 (t, J = 7.2 Hz, 2H, -NCH -), 3.96
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(s, 3H, -OMe), 3.58 (s, 1H, ethynyl), 2.24 – 2.08 (m, 2H), 1.44 – 1.34 (m, 4H), 1.33 –
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1.26 (m, 6H), 0.89 – 0.85 (m, 3H). C NMR (101 MHz, CDCl ) δ 166.88, 145.55,
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142.34, 141.10, 131.17, 129.97, 129.72, 128.54, 124.38, 111.94, 83.26, 57.09, 52.22,
+
31.72, 30.25, 29.06, 28.98, 26.56, 22.61, 14.08. HRMS (ESI, m/z): [M+Na] calcd for
C H N O Na, 412.2001; found, 412.1997.
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7b. Colorless oil (yield 65%, 2 steps). H NMR (400 MHz, CDCl ) δ 8.03 (s, 1H,
3
phenyl), 7.96 (d, J = 8.0 Hz, 1H, phenyl), 7.65 (d, J = 7.2 Hz, 1H, phenyl), 7.44 (d, J
= 8.0 Hz, 1H, phenyl), 7.24 (d, J = 7.2 Hz, 1H, phenyl), 4.73 (t, J = 7.2 Hz, 2H,
-NCH -), 3.95 (s, 3H, -OCH ), 3.56 (s, 1H, ethynyl), 2.26 (s, 3H, phenyl-CH ), 2.13 –
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2.03 (m, 2H, -NCH CH -), 1.37 – 1.29 (m, 4H), 1.28 – 1.17 (m, 6H), 0.86 (t, J = 6.8
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Hz, 3H). C NMR (101 MHz, CDCl ) δ 167.04, 144.87, 142.83, 141.74, 136.75,
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132.69, 131.65, 130.91, 130.29, 129.88, 127.00, 126.05, 111.57, 82.86, 79.42, 57.04,
52.16, 31.68, 30.29, 29.03, 28.94, 26.50, 22.59, 20.54, 14.06. HRMS (ESI, m/z)
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[M+H] calcd for C H N O , 404,2338; found, 404.2337.
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7c. Colorless oil (yield 32%, 2 steps) H NMR (400 MHz, CDCl ) δ 7.85 (s, 2H,
3
phenyl), 7.66 (d, J = 7.2 Hz, 1H, phenyl), 7.11 (d, J = 7.2 Hz, 1H, phenyl), 4.71 (t, J =
7.2 Hz, 2H, -NCH -), 3.94 (s, 3H, -OCH ), 3.56 (s, 1H, ethynyl), 2.11 – 2.04 (m, 2H),
2
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2.03 (s, 6H, phenyl-CH ), 1.32 – 1.26 (m, 6H), 1.24 – 1.18 (m, 4H), 0.85 (t, J = 6.8
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Hz, 3H). C NMR (101 MHz, CDCl ) δ 167.21, 145.01, 142.67, 141.43, 136.92,
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131.90, 131.10, 129.60, 128.67, 125.78, 111.32, 82.67, 79.41, 57.00, 52.08, 31.64,
+
30.29, 29.70, 29.01, 28.90, 26.45, 22.57, 20.55, 14.04. HRMS (ESI, m/z) [M+H]
calcd for C H N O , 418.2495; found, 418.2501.
2
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General synthetic procedure for compounds 9a~9c.
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To a 100 mL Schlenk tube was added compound 7a~7c (0.15 mmol), compound
8 (110 mg, 0.060 mmol) and THF/Et N (30 mL/6 mL). Then Pd(PPh ) (5 mol%) and
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CuI (5 mol%) were added under nitrogen. The mixture was allowed to stir at 60°C for
24 h. The solvents were evaporated and the residue was purified first on a silica gel
column (PE/DCM = 2:3) and then on a preparative silica gel plate (PE/THF = 9:1) to
give the crude product, which was recrystallized from DCM/MeOH to give 9a~9c.
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9a. Dark green tar (yield 40%). H NMR (400 MHz, CDCl ) δ 9.97 (d, J = 4.4
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Hz, 2H, pyrrolic), 9.64 (d, J = 4.4 Hz, 2H, pyrrolic), 8.91 (d, J = 4.4 Hz, 2H, pyrrolic),
8.85 (d, J = 4.4 Hz, 2H, pyrrolic), 8.24 (d, J = 8.4 Hz, 2H, phenyl), 8.19 (d, J = 8.8 Hz,
2H, phenyl), 8.06 (d, J = 7.2 Hz, 1H, phenyl), 7.82 – 7.67 (m, 5H, phenyl), 7.48 (d, J
= 8.8 Hz, 2H, phenyl), 7.41 – 7.34 (m, 2H, phenyl), 7.09 – 6.88 (m, 8H, phenyl), 4.98
(t, J = 7.2 Hz, 2H, -NCH -), 4.04 – 3.92 (m, 7H, -OCH , -OCH - and -NCH -), 3.86 (t,
2
3
2
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J = 6.4 Hz, 8H, -OCH -), 2.42 – 2.31 (m, 2H), 1.97 – 1.87 (m, 2H), 1.85 – 1.75 (m,
2
2H), 1.62 – 1.51 (m, 4H), 1.42 – 1.26 (m, 16H), 1.18 – 1.07 (m, 9H), 1.07 – 0.94 (m,
24H), 0.94 – 0.83 (m, 18H), 0.82 – 0.72 (m, 20H), 0.67 – 0.58 (m, 8H), 0.57 – 0.36
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(m, 24H). C NMR (101 MHz, CDCl ) δ 166.87, 159.97, 158.42, 152.00, 151.40,
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150.59, 150.55, 146.01, 145.09, 143.26, 142.52, 141.26, 135.54, 132.22, 132.03,
131.72, 130.96, 130.80, 130.63, 130.06, 129.82, 129.73, 129.16, 129.02, 128.94,
128.08, 127.39, 125.49, 125.41, 124.70, 124.57, 124.42, 120.96, 118.20, 115.57,
115.48, 115.09, 114.78, 105.24, 101.22, 100.32, 99.74, 95.38, 93.03, 92.27, 68.71,
68.08, 57.06, 52.13, 47.77, 31.82, 31.79, 31.60, 31.51, 30.20, 29.71, 29.45, 29.35,
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29.23, 29.16, 29.14, 29.10, 28.73, 28.64, 26.86, 26.78, 26.70, 25.72, 25.29, 22.65,
22.62, 14.12, 14.07, 14.03. MS (MALDI-TOF, m/z) [M] calcd for C H N O SZn,
1
36 176
8
7
2129.2674; found, 2129.1.
1
9b. Dark green tar (yield 39%). H NMR (400 MHz, CDCl ) δ 9.97 (d, J = 4.4
3
Hz, 2H, pyrrolic), 9.65 (d, J = 4.8 Hz, 2H, pyrrolic), 8.92 (d, J = 4.8 Hz, 2H, pyrrolic),
8.86 (d, J = 4.4 Hz, 2H, pyrrolic), 8.10 – 8.03 (m, 2H, phenyl), 8.03 – 7.96 (m, 1H,
phenyl), 7.80 – 7.74 (m, 2H, phenyl), 7.74 – 7.67 (t, J = 8.4 Hz, 2H, phenyl), 7.59 (d,
J = 8.0 Hz, 1H, phenyl), 7.48 – 7.41 (m, 3H, phenyl), 7.39 – 7.32 (m, 2H, phenyl),
7.05 – 6.88 (m, 8H, phenyl), 4.91 (t, J = 7.2 Hz, 2H, -NCH -), 3.99 – 3.91 (m, 7H,
2
-OCH , -OCH - and -NCH -), 3.86 (t, J = 6.4 Hz, 8H, -OCH -), 2.39 (s, 3H,
3
2
2
2
phenyl-CH ), 2.35 – 2.24 (m, 2H), 1.97 – 1.86 (m, 2H), 1.81 – 1.72 (m, 2H), 1.51 –
3
1.27 (m, 22H), 1.19 – 1.09 (m, 9H), 1.07 – 0.95 (m, 24H), 0.93 – 0.85 (m, 16H), 0.82
1
3
– 0.73 (m, 20H), 0.67 – 0.59 (m, 8H), 0.56 – 0.37 (m, 24H). C NMR (101 MHz,
CDCl ) δ 167.13, 159.96, 158.42, 151.99, 151.43, 150.60, 150.59, 145.40, 145.10,
3
143.27, 143.18, 142.36, 136.90, 135.54, 132.22, 132.04, 131.70, 131.16, 130.95,
130.79, 130.61, 130.47, 130.08, 129.81, 129.62, 128.74, 127.38, 127.02, 126.63,
125.48, 125.41, 124.60, 124.44, 121.06, 118.25, 115.58, 115.39, 115.13, 114.78,
114.65, 105.30, 101.16, 99.79, 99.68, 95.37, 93.11, 91.89, 68.73, 68.08, 57.05, 52.10,
48.28, 47.77, 31.83, 31.75, 31.60, 31.51, 30.26, 29.45, 29.35, 29.23, 29.12, 29.09,
28.72, 28.64, 26.88, 26.69, 25.71, 25.27, 22.63, 20.71, 14.08, 14.04, 14.02. MS
(MALDI-TOF, m/z) [M] calcd for C H N O SZn, 2143.2831; found, 2143.2.
1
37 178
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7
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289
9c. Dark green tar (yield 35%). H NMR (400 MHz, CDCl ) δ 9.96 (d, J = 4.8
3
Hz, 2H, pyrrolic), 9.65 (d, J = 4.8 Hz, 2H, pyrrolic), 8.91 (d, J = 4.8 Hz, 2H, pyrrolic),
8.85 (d, J = 4.8 Hz, 2H, pyrrolic), 8.08 (d, J = 7.2 Hz, 1H, phenyl), 7.91 (s, 2H,
phenyl), 7.80 – 7.74 (m, 2H, phenyl), 7.71 (t, J = 8.4 Hz, 2H, phenyl), 7.51 – 7.45 (m,
2H, phenyl), 7.40 – 7.35 (m, 2H, phenyl), 7.31 (d, J = 7.2 Hz, 1H, phenyl), 7.05 –
6.92 (m, 8H, phenyl), 4.88 (t, J = 7.2 Hz, 2H, -NCH -), 4.03 – 3.93 (m, 7H, -OCH ,
2
3
-OCH - and -NCH -), 3.86 (t, J = 6.4 Hz, 8H, -OCH -), 2.31 – 2.22 (m, 2H), 2.17 (s,
2
2
2
6H, phenyl-CH ), 1.96 – 1.88 (m, 2H), 1.85 – 1.76 (m, 2H), 1.48 – 1.41 (m, 4H), 1.40
3
– 1.32 (m, 10H), 1.32 – 1.26 (m, 6H), 1.19 – 1.08 (m, 9H), 1.08 – 0.95 (m, 24H), 0.94
13
– 0.83 (m, 18H), 0.83 – 0.74 (m, 20H), 0.67 – 0.58 (m, 8H), 0.57 – 0.35 (m, 24H). C
NMR (101 MHz, CDCl ) δ 167.30, 159.97, 158.43, 152.00, 151.45, 150.62, 145.51,
3
145.12, 143.28, 143.03, 142.05, 137.18, 135.56, 132.23, 132.08, 131.74, 130.97,
130.80, 130.63, 130.41, 130.09, 129.83, 129.42, 128.98, 128.71, 127.39, 126.29,
125.49, 125.42, 124.61, 124.45, 121.04, 118.23, 115.59, 115.44, 115.13, 114.80,
114.33, 105.33, 101.21, 99.77, 99.39, 95.41, 93.03, 91.87, 68.75, 68.09, 57.02, 52.05,
47.78, 31.82, 31.72, 31.60, 31.52, 30.27, 29.43, 29.33, 29.22, 29.11, 29.07, 28.70,
28.63, 26.89, 26.70, 26.65, 25.72, 25.26, 22.63, 20.70, 14.08, 14.04, 14.03. MS
(MALDI-TOF, m/z) [M] calcd for C H N O SZn, 2157.2987; found, 2157.3.
1
38 180
8
7
General synthetic procedure for XW33~XW35.
To a Schlenk tube was added compound 9a~9c (0.025 mmol), THF (20 mL) and
LiOH·H O (42 mg, 1.0 mmol) in water (2 mL) and then charged with nitrogen. The
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mixture was stirred at 60°C for 17 h under nitrogen. Water was added and the product
was extracted with dichloromethane. The organic layer was separated and dried over
Na SO . The solvents were evaporated and the residue was purified on a preparative
2
4
silica gel plate (DCM/MeOH = 15:1) to obtain the crude product, which was
recrystallized from DCM/MeOH to give XW33~XW35.
1
XW33. Dark green powders (yield 75%). H NMR (400 MHz, CDCl :
3
DMSO-d = 1:2) δ 12.78 (br, 1H), 9.86 (d, J = 4.4 Hz, 2H, pyrrolic), 9.52 (d, J = 4.8
6
Hz, 2H, pyrrolic), 8.77 (d, J = 4.8 Hz, 2H, pyrrolic), 8.71 (d, J = 4.4 Hz, 2H, pyrrolic),
8.34 (d, J = 8.4 Hz, 2H, phenyl), 8.16 (d, J = 8.4 Hz, 2H, phenyl), 8.09 (d, J = 7.2 Hz,
1H, phenyl), 7.94 (d, J = 7.2 Hz, 1H, phenyl), 7.80 (dd, J = 10.0 Hz, J = 1.6 Hz, 1H,
1
2
phenyl), 7.75 – 7.66 (m, 3H, phenyl), 7.48 (d, J = 8.8 Hz, 2H, phenyl), 7.42 – 7.31 (m,
2H, phenyl), 7.14 – 6.98 (m, 6H, phenyl), 6.93 (d, J = 8.4 Hz, 2H, phenyl), 5.02 (t, J =
6.8 Hz, 2H), 3.95 (t, J = 6.4 Hz, 4H), 3.87 (t, J = 5.6 Hz, 8H), 2.36 (t, J = 7.2 Hz, 2H),
1.85 (t, J = 6.8 Hz, 2H), 1.78 – 1.70 (m, 2H), 1.63 – 1.43 (m, 10H), 1.39 – 1.27 (m,
16H), 1.17 – 1.06 (m, 10H), 1.03 – 0.88 (m, 46H), 0.78 – 0.69 (m, 16H), 0.65 – 0.53
(m, 16H), 0.47 – 0.30 (m, 9H). MS (MALDI-TOF, m/z) [M] calcd for
C H N O SZn, 2115.2518; found, 2115.2.
135 174
8
7
1
XW34. Dark green powders (yield 92%). H NMR (400 MHz, CDCl :
3
DMSO-d = 1:2) δ 12.86 (br, 1H, -COOH), 9.85 (d, J = 4.8 Hz, 2H, pyrrolic), 9.52 (d,
6
J = 4.4 Hz, 2H, pyrrolic), 8.74 (d, J = 4.8 Hz, 2H, pyrrolic), 8.69 (d, J = 4.4 Hz, 2H,
pyrrolic), 8.24 – 8.20 (m, 1H, phenyl), 8.09 (d, J = 7.2 Hz, 1H, phenyl), 7.93 (d, J =
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8.0 Hz, 1H, phenyl), 7.80 (d, J = 8.0 Hz, 1H, phenyl), 7.75 – 7.66 (m, 3H, phenyl),
7.61 – 7.45 (m, 4H, phenyl), 7.44 – 7.34 (m, 2H, phenyl), 7.19 – 7.00 (m, 6H, phenyl),
6.93 (d, J = 8.8 Hz, 2H, phenyl), 4.93 (t, J = 6.2 Hz, 2H, -NCH -), 4.01 – 3.92 (m, 4H,
2
-OCH - and –NCH -), 3.86 (t, J = 5.6 Hz, 8H, -OCH -), 2.36 (s, 3H, phenyl-CH ),
2
2
2
3
2.31 – 2.21 (m, 2H), 1.88 – 1.77 (m, 2H), 1.76 – 1.68 (m, 2H), 1.53 – 1.39 (m, 8H),
1.38 – 1.29 (m, 10H), 1.29 – 1.23 (m, 7H), 1.14 – 1.06 (m, 8H), 1.04 – 0.94 (m, 24H),
0.93 – 0.84 (m, 22H), 0.80 – 0.70 (m, 20H), 0.63 – 0.48 (m, 16 H), 0.41 – 0.29 (m,
8H). MS (MALDI-TOF, m/z) [M] calcd for C H N O SZn, 2129.2674; found,
1
36 176
8
7
2129.3.
XW35. Dark green powders (yield 70%). H NMR (400 MHz, CDCl :DMSO-d
1
3
6
= 1:2) δ 12.65 (br, 1H, -COOH), 9.86 (d, J = 4.4 Hz, 2H, pyrrolic), 9.53 (d, J = 4.4 Hz,
2H, pyrrolic), 8.78 (d, J = 4.4 Hz, 2H, pyrrolic), 8.72 (d, J = 4.8 Hz, 2H, pyrrolic),
8.11 – 8.06 (m, 2H, phenyl), 7.84 – 7.78 (m, 2H, phenyl), 7.76 – 7.66 (m, 3H, phenyl),
7.49 (d, J = 8.4 Hz, 2H, phenyl), 7.44 – 7.32 (m, 3H, phenyl), 7.15 – 6.99 (m, 6H,
phenyl), 6.94 (d, J = 8.4 Hz, 2H, phenyl), 4.90 (t, J = 6.4 Hz, 2H, -NCH -), 4.02 –
2
3.93 (m, 4H, -NCH - and –OCH -), 3.88 (t, J = 5.6 Hz, 8H, –OCH -), 2.31 – 2.21 (m,
2
2
2
2H), 2.14 (s, 6H, phenyl-CH ), 1.92 – 1.81 (m, 2H), 1.80 – 1.70 (m, 2H), 1.55 – 1.40
3
(m, 8H), 1.39 – 1.30 (m, 10H), 1.29 – 1.24 (m, 7H), 1.19 – 1.08 (m, 10H), 1.07 – 0.89
(m, 44H), 0.82 – 0.71 (m, 20H), 0.68 – 0.53 (m, 16H), 0.48 – 0.35 (m, 8H). MS
(MALDI-TOF, m/z) [M] calcd for C H N O SZn, 2143.2831; found, 2143.3.
1
37 178
8
7
PT-C6. It was synthesized according to literature procedures [31] as dark red
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332
333
334
335
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337
powders. H NMR (400 MHz, CDCl ) δ 8.08 (s, 1H), 7.91 (dd, J = 8.4, 2.0 Hz, 1H,
3
phenyl), 7.67 (d, J = 2.0 Hz, 1H, phenyl), 7.44 (d, J = 8.4 Hz, 2H, phenyl), 7.32 (dd, J
= 8.4, 2.0 Hz, 1H, phenyl), 7.25 (d, J = 2.0 Hz, 1H, phenyl), 6.97 – 6.83 (m, 4H,
phenyl), 3.99 (t, J = 6.8 Hz, 2H), 3.89 (t, J = 7.2 Hz, 2H), 1.89 – 1.74 (m, 4H), 1.53 –
1.43 (m, 4H), 1.40 – 1.32 (m, 8H), 0.94 – 0.86 (m, 6H).
2.3 Fabrication of the solar cells
338
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346
347
348
349
350
351
352
The FTO conducting glass was washed with a detergent solution, deionized
water, acetone and ethanol successively for 20 minutes under ultrasonication before
use. The preparation of the TiO electrodes and fabrication of the cells were adapted
2
from that reported by Grätzel and co-workers [37]. The TiO photoanode was
2
prepared by repetitive screen-printing. Afterwards, the TiO films were heated with a
2
programmed procedure at 275°C for 5 min, 325°C for 5 min, 375°C for 5 min, 450°C
for 15 min, and 500°C for 15 min. The resulting layers were post-treated with 40 mM
TiCl solution at 70°C for 30 min and sintered once again at 450°C for 30 min. Then
4
they were immersed in a 0.1 mM solution of the porphyrin dyes in a mixture of
CHCl and ethanol (2:3, v/v) for 12 h at 25°C for dye uptake. For coadsorption,
3
chenodeoxycholic acid was added directly into the porphyrin dye solutions. For
cosensitization, the coadsorbed photoanode was taken out and rinsed with ethanol,
and then immersed in a 0.3 mM solution of PT-C6 in a mixture of t-BuOH and
acetonitrile (1:1, v/v) for 12 h. For the counter electrode, H PtCl in 2-propanol was
2
6
evenly distributed on FTO glass through spin coating, and the cathode was heated at
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359
400°C for deposition of platinum. Eventually, the two electrodes were sealed with
thermoplastic Surlyn, and an electrolyte solution was injected through one hole in the
counter electrode to complete the fabrication of the sandwich-type solar cells. The
electrolyte was composed of 0.1
M
LiI, 0.05
M
I2, 0.6
M
1,
2-dimethyl-3-propyl-imidazolium iodide (DMPII) and 0.5 M 4-tert-butylpyridine
(TBP) in acetonitrile.
2.4 Theoretical calculation details
360
361
362
363
364
We employed density functional theory (DFT) calculations to optimize the
ground state geometries of the sensitizers, using the hybrid B3LYP functional [38,39]
and the 6-31G* basis set [40]. For zinc atoms, the Los Alamos effective core potential
basis set (LANL2DZ) was used [41]. All calculations were carried out using the
Gaussian09 program package [42].
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365 3. Results and discussion
O
O
O
O
B
B
O
Pd(PPh ) Cl , KOAc
3
2
2
Br
COOMe
B
COOMe
O
1
,4-dioxane, 90℃
1
1
2
) n-BuLi, THF, -78℃
) DMF, rt
O
O
i)
ii)
Br
Br
Br
CHO
B
COOMe
Br
COOMe
2
3
4
C8H17
C8H17
C8H17
N
N
N N
N
N N
R1
R1
N
N
iv), v)
iii)
Br
Br
Br
COOMe
COOMe
R2
R2
5
6a~6c
7a~7c
C6H13O
C6H13O
C12H25O
OC12H25
C12H25O
OC12H25
C8H17
N
S
N
N
R1
R2
N
N
N
S
vi)
N
N
N
N
Zn
N
Br
N
C6H13
Zn
COOMe
N
C6H13
C12H25O
OC12H25
C12H25O
OC12H25
8
9a~9c
C6H13O
C12H25O
OC12H25
C8H17
N
N
N
R1
R2
S
vii)
N
N
N
N
Zn
COOH
N
C6H13
C12H25O
OC12H25
a, XW33: R1 = R2 = H
b, XW34: R1 = H, R2 = CH3
c, XW35: R1 = R2 = CH3
XW33~XW35
3
3
66
67
Scheme 1. Synthetic routes for XW33-XW35. Reaction conditions: i) KI, t-BuOOH, MeOH; ii)
bis(pinacolato)diboron, Pd(PPh ) Cl , KOAc, 1,4-dioxane; iii) for 6a and 6b: methyl
3
3
3
3
68
69
70
71
3
2
2
4-boronobenzoate or 1, Pd(PPh ) , K CO , THF/H O; for 6c: 4, Pd(PPh ) , K PO , DMF; iv)
3
4
2
3
2
3 4
3
4
Pd(PPh ) Cl , CuI, trimethylsilylacetylene, THF/Et N; v) KOH (aq), THF/MeOH; vi) Pd(PPh ) ,
3
2
2
3
3 4
CuI, THF/Et N; vii) LiOH·H O, THF/H O.
3
2
2
372
3.1 Syntheses of the porphyrin dyes
373
374
375
376
377
The synthetic routes for porphyrin dyes XW33-XW35 (Fig. 1) are depicted in
Scheme 1. The key intermediates 1 and 4 were successfully obtained via Pd-catalyzed
Miyaura coupling reactions under optimized conditions in high yields (>70%).
Afterwards, they were connected to benzotriazole through Suzuki-coupling reactions.
Subsequent Sonogashira coupling reactions yielded the ethynyl-substituted acceptor
1
9

ACCEPTED MANUSCRIPT
moieties, which were respectively attached to intermediate 8 to accomplish the
porphyrin dye framework. Final hydrolysis of the esters gave rise to the target
products. All the key intermediates and target dyes were fully characterized with
NMR and MS (Fig. S4-S30).
378
379
380
381
382
3.2 Absorption and electrochemical properties
3
83
84
3
Fig. 2 Absorption spectra of XW33-35 and cosensitizer PT-C6 in THF.
385
386
387
388
389
390
391
392
393
394
The UV-vis absorption spectra of XW33-XW35 in THF are shown in Fig. 2, and
the corresponding data are summarized in Table 1. The three porphyrin dyes exhibit
intense Soret bands with the maxima at 469 nm, 464 nm and 462 nm, respectively,
and corresponding moderate Q bands at 675 nm, 672 nm and 670 nm, respectively.
The successive blue-shifts of the absorption bands from XW33 to XW35 indicate a
deteriorated conjugation, which can be easily understood from the increasing steric
hindrance associated with the increasing number of methyl groups. Consistently, the
estimated energy band gaps (E ) of XW34 and XW35 are 0.01 eV and 0.02 eV
0
-0
larger than that of XW33, respectively (Table 1). It is noteworthy that the Soret and Q
bands of XW33 are both 10 nm red-shifted compared to those observed for our
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0

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396
397
398
399
400
401
402
403
404
405
406
407
previously reported analogue XW10 which does not contain the benzotriazole unit
[15]. This observation implies that the incorporation of the benzotriazole unit
successfully enhances the push-pull effect and thus achieves a favorable extended
spectrum. For the cosensitizer PT-C6, it exhibits a smaller absorption coefficient,
However, its absorption peak centered around 450 nm is very broad, which may
compensate for the relatively narrow Soret bands of the porphyrin dyes in this region.
To evaluate the feasibility of electron injection and dye regeneration processes in
terms of energy levels, cyclic voltammetry tests were conducted (Fig. S3), and the
estimated energy levels are illustrated in Fig. 3 and Table 1. Apparently, with the
increase of steric hindrance from XW33 to XW35, slightly elevated HOMOs and
LUMOs were observed. Fortunately, all the three porphyrin dyes have LUMO levels
(-0.91 V ~ -0.95 V) higher than the TiO conduction band (~ -0.5 V), and HOMO
2
-
-
levels (0.91 V ~ 0.89 V) lower than the I /I (~ 0.4 V) redox shuttle, indicating
3
4
08
09
feasibilities of both electron injection and dye regeneration processes.
4
Table 1. Absorption and electrochemical data of XW33-XW35.
a
d
*
Absorption λmax /nm
Eox /V
Eox /V
TiO
max
2
b
c
Dye
λ
/nm
E0-0 /eV
3
-1
-1
(ε/10 M cm )
(vs NHE) (vs NHE)
XW33 469 (427.5), 675 (126.9)
XW34 464 (397.3), 672 (102.1)
XW35 462 (435.5), 670 (109.0)
470
465
1.82
1.83
1.84
0.91
0.90
0.89
-0.91
-0.93
-0.95
461
a
b
c
-6
4
4
4
4
4
4
4
10
11
12
13
14
15
16
Absorption maxima in THF solutions (2 × 10 M).
Absorption maxima on transparent TiO films.
2
Estimated from the intersection wavelength (λinter) of the normalized UV-vis absorption and
fluorescence spectra using the equation E0-0 = 1240/λinter
d
.
Eox was measured in acetonitrile using 0.1 M tetra-n-butylammonium hexafluorophosphate
(TBAPF ) as the electrolyte (working electrode: FTO/TiO /dye; reference electrode: SCE; counter
6
2
+
electrode: Pt) which was calibrated with ferrocene/ferrocenium (Fc/Fc ) as an external reference.
2
1

ACCEPTED MANUSCRIPT
e
*
417
Estimated from the equation Eox
=
Eox
-
E0-0
.
4
4
18
19
Fig. 3 Schematic energy level diagrams of XW33-XW35.
420
3.3 Photovoltaic performance
421
422
423
424
425
426
427
428
429
430
431
432
433
Based on the results shown in the preceding section, it is anticipated that
XW33-XW35 can be employed as DSSC dyes. Thus, devices were fabricated firstly
using the individual porphyrin dyes and then coadsorbed with CDCA. The results are
shown in Table 2 and Fig. 4. The V values of devices based on the three individual
oc
dyes vary in a small range (704 mV ~ 717 mV), in contrast to the results of Li’s
system where the Voc values are dramatically improved by the increased steric
hindrance [30]. Electrochemical impedance spectroscopy (EIS) results under dark
conditions reveal only slightly increased electron lifetimes of XW34 and XW35 in
comparison to XW33 (Fig. 4d), implying that the distortion-induced retardation of
charge recombination is not as obvious as that observed in Li’s system. On the other
hand, drastically different J values were observed for devices based on the three
sc
-2
dyes. A relatively high J of 17.00 mAñcm was achieved for XW33, while the
sc
-2
-2
corresponding values were drastically decreased to 13.28 mAñcm and 7.43 mAñcm
2
2

ACCEPTED MANUSCRIPT
434
435
436
437
438
439
440
441
442
443
444
445
for XW34 and XW35, respectively. This large difference was unambiguously
reflected in their IPCE spectra (Fig. 4b) with successively lowered IPCE plateaus. As
a result, the achieved power conversion efficiencies were found to be in a descending
order of 8.39% > 6.61% > 3.62% for XW33-35, respectively. Notably, the efficiency
achieved for XW33 is higher than that obtained for our previously reported individual
dye XW11 (7.8%) and comparable to XW10 (8.6%) [15], and is also higher than that
of 5.05% obtained for the reported similar sensitizer DHC-1 [43]. These results
clearly indicate the effectiveness of incorporating the benzotriazole unit and the
validity of the molecular design. On the other hand, the decreasing efficiencies
obtained for XW34 and XW35 show a trend opposite to that observed for the dyes
reported by Li and coworkers [30], which will be discussed in more details in section
3.4.
4
4
4
4
46
47
48
49
Table 2. Photovoltaic parameters of the solar cells sensitized by XW33-XW35 in the absence or
-2
presence of CDCA under simulated AM 1.5 G full sunlight (100 mW·cm ). The active area is
2
0.12 cm . The data are based on five parallel devices. The last column shows the dye loading
amounts.
Dyes
Voc
Jsc
FF
PCE
Dye loading
-
2
-2
[
mV]
[mA·cm ]
[%]
[%]
[nmol·cm ]
XW33
XW33 + CDCA
XW34
712 ± 1
721 ± 2
717 ± 2
729 ± 2
704 ± 1
719 ± 3
17.00 ± 0.29
18.71 ± 0.46
13.28 ± 0.09
14.85 ± 0.55
7.43 ± 0.12
8.24 ± 0.71
69.3 ± 0.3
68.7 ± 0.8
69.5 ± 0.4
68.5 ± 1.0
69.3 ± 0.6
69.4 ± 1.0
8.39 ± 0.12
9.27 ± 0.15
6.61 ± 0.06
7.42 ± 0.19
3.62 ± 0.05
4.11 ± 0.29
114
97
120
89
XW34 + CDCA
XW35
96
XW35 + CDCA
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Fig. 4 a) J-V curves, b) IPCE spectra, c) capacitance and d) electron lifetimes of the DSSCs
based on XW33-XW35 in the absence or presence of CDCA coadsorption.
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After coadsorption with CDCA, devices based on all the three dyes showed
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slightly improved V and obviously improved J , resulting in elevated efficiencies of
oc
sc
9.27%, 7.42% and 4.11%, respectively. EIS measurements indicated prolonged
electron lifetimes induced by coadsorption with CDCA (Fig. 4d). Meanwhile, dye
loading amounts were estimated by comparing absorption of desorbed dye solutions
with that of their standard solutions. As expected, the competing adsorption of CDCA
caused diminished porphyrin dye uptake (Table 2). In view of the improved V and
oc
prolonged electron lifetimes, it can be concluded that the presence of CDCA is
favorable for forming a more compact adsorption layer on TiO and thus reduces
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charge recombination, and CDCA is also favorable for suppressing the dye
aggregation on the TiO film.
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3.4 Theoretical analysis
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In order to reveal the underlying reason for the strikingly different photovoltaic
performance of the three dyes, density functional theory (DFT) calculations were
employed to obtain the optimized ground state geometries of the dyes, and the
calculated frontier molecular orbitals of HOMO and LUMO are shown in Fig. 5. As
expected, the increasing steric hindrance induced by the introduced methyl groups
causes dramatically aggravated torsion between BTz and the adjacent phenylene ring,
which can be clearly evidenced by the increasing dihedral angles of 30.5°, 48.3° and
77.2° for XW33-XW35, respectively. Meanwhile, the LUMO electron distributions
are also seriously affected. As can be clearly observed in Fig. 5, while the LUMO
electrons of XW33 are reasonably distributed over the porphyrin core and the whole
acceptor part including the anchoring carboxylic group, those of XW34 and XW35
seem to be separated from the anchoring group because of the relatively large dihedral
angles. Especially for XW35, there is very little electron distribution on the
carboxylic anchor, which is indicative of poor electronic coupling between the LUMO
of XW35 and the TiO , resulting in poor electron injection [44], which may account
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for the drastically lowered Jsc values observed for XW34 and XW35. This
observation is in sharp contrast to the results reported by Li and coworkers [30].
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Fig. 5 Calculated frontier molecular orbitals and the optimized geometries of XW33-XW35.
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3.5 Cosensitization
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4
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87
Table 3. Photovoltaic parameters for the cosensitized solar cells under simulated AM 1.5 G full
-2
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sunlight (100 mW·cm ). The active area is 0.12 cm . The data are based on five parallel devices.
The last two columns show the dye loading amounts.
Dyes
Voc
Jsc
FF
PCE
Porphyrin
loading
PT-C6
-
2
[
mV]
[mA·cm ]
[%]
[%]
loading
-
2
-2
[
nmol·cm ] [nmol·cm ]
PT-C6
735 ± 2 13.71 ± 0.29 68.6 ± 1.3
6.91 ± 0.12
9.32 ± 0.09
8.49 ± 0.12
7.34 ± 0.05
–
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137
166
191
XW33 + CDCA + PT-C6 733 ± 3 18.93 ± 0.11 67.2 ± 0.5
XW34 + CDCA + PT-C6 737 ± 3 17.12 ± 0.21 67.3 ± 0.7
XW35 + CDCA + PT-C6 740 ± 4 14.50 ± 0.11 68.6 ± 0.5
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Fig. 6 a) J-V curves, b) IPCE spectra, c) capacitance and d) electron lifetimes of devices based
on XW33-35 in the absence or presence of PT-C6 as the cosensitizer.
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To further improve the photovoltaic performance, especially to compensate for
the intrinsic drawback of XW34 and XW35, a stepwise cosensitization method was
carried out using a previously reported metal-free organic dye PT-C6 [31], which was
selected in this work because of its relatively high V of 735 mV as well as its broad
oc
absorption. In addition, it is easily synthesized and the possibility of using it as a
cosensitizer for porphyrin dyes has been elucidated [45]. As shown in Table 3 and Fig.
6, all the cosensitized devices exhibit further improved V and J as compared with
oc
sc
those coadsorbed with CDCA. As a result, improved PCEs of 9.32%, 8.49% and
7.34% are achieved for XW33-XW35, respectively. It is noteworthy that more
pronounced enhancement of efficiencies was observed for XW34 and XW35 as
compared with XW33. To further insight into these results, dye loading amounts were
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estimated by desorbing the dyes from TiO photoanodes into basic THF solutions
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followed by measuring the absorbance. Although the dye-loading amounts reveal a
dominating role of PT-C6 instead of the XW porphyrin dyes in the cosensitization
systems, the role of the porphyrin dye is still indispensable because typical porphyrin
characters in the long wavelength range of 650 ~ 750 nm can be clearly observed in
the IPCE spectra (Fig. 6b). In addition, EIS analysis indicates further prolonged
electron lifetimes upon cosensitization (Fig. 6d). Hence, the finally achieved
efficiencies are higher than those based solely on PT-C6 or the corresponding
porphyrin dyes, which may be ascribed to the densely packed dye adsorbing layer and
the broad absorption peak of PT-C6, although its peak largely overlaps with the Soret
band of the porphyrin dyes.
513 4. Conclusions
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In conclusion, porphyrin sensitizers XW33-XW35 featuring an alkyl-substituted
benzotriazole unit as the auxiliary acceptor have been designed, synthesized and used
as DSSC sensitizers, among which XW33 exhibits the highest performance of 8.39%.
Introduction of the electron-withdrawing benzotriazole unit induces red-shifted
absorption, which is favorable for light-harvesting. The steric hindrance associated
with the ortho-methyl groups in XW34 and XW35 induces enlarged torsion angles,
leading to slightly prolonged electron lifetimes and retarded charge recombination.
However, the aggravated torsion induces poor electronic coupling between the LUMO
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electrons and TiO , resulting in poor electron injection, and low J values. As a result,
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sc
lower efficiencies of 6.61% and 3.62% were obtained for XW34 and XW35,
respectively. By coadsorption with CDCA and cosensitization with
a
phenothiazine-based organic dye PT-C6, elevated efficiencies of 9.32%, 8.49% and
7.34% were achieved for XW33-XW35, respectively.
These results demonstrate the effectiveness of incorporating a benzotriazole unit
as the auxiliary acceptor in extending the absorption, and elucidate the importance of
avoiding severe torsion within the acceptor part of the porphyrin dyes. Furthermore,
cosensitization with the spectrally broad and high-photovoltaged cosensitizer PT-C6
is effective in achieving higher efficiencies despite the fact that its absorption peak at
450 nm largely overlaps with the Soret band of the porphyrin dyes, which is in
contrast to most commonly used cosensitizers whose absorption peaks lie in the
absorption valley between the Soret band and the Q bands of porphyrin dyes. These
results provide further insight into developing efficient cosensitizers for porphyrin
dyes.
537 Acknowledgment
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541
This work was financially supported by NSFC (21472047, 21772041,
U1707602), the Program for Professor of Special Appointment (Eastern Scholar,
GZ2016006) at Shanghai Institutions of Higher Learning, and the Fundamental
Research Funds for the Central Universities (WK1616004, 222201717003).
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