Incorporating benzotriazole moiety to construct D-A-π-A organic sensitizers for solar cells: Significant enhancement of open-circuit photovoltage with long alkyl group

Article abstract of DOI:10.1021/cm202226j

Two novel benzotriazole-containing organic dyes based on D-A-π-A configuration, WS-5 with octyl group and WS-8 with methyl group, have been designed and synthesized for use in dye-sensitized solar cells (DSSCs). Compared with the traditional D-π-A sensitizers, the benzotriazole unit as an additional acceptor has several merits: (i) essentially facilitating the electron transfer from the donor to the acceptor/anchor; (ii) conveniently tailoring the solar cell performance with a facile structural modification on 2-position in the benzotriazole unit; and (iii) the nitrogen-containing heterocyclic group of benzotriazole being expected to improve the open-circuit photovoltage. The analysis of controlled intensity modulated photovoltage spectroscopy reveals that the replacement of methyl with octyl group enhances electron lifetime by 4-fold and retards charge recombination rate constant by 4-fold. The two dye-loaded TiO₂ films possess almost the same conduction band position under the same condition, as revealed by the charge densities at open-circuit against open-circuit photovoltage. Therefore, the significant enhancement of open-circuit photovoltage from methyl to octyl group is attributed to the suppressed charge recombination. Under simulated AM1.5G solar light (100 mW cm^-2), the DSSC based on WS-5 produces a short-circuit photocurrent of 13.18 mA cm^-2, an open-circuit photovoltage of 0.78 V, a fill factor of 0.78, corresponding to a power conversion efficiency of 8.02%.

Full text of DOI:10.1021/cm202226j

ARTICLE
pubs.acs.org/cm
Incorporating Benzotriazole Moiety to Construct DÀAÀπÀA Organic
Sensitizers for Solar Cells: Significant Enhancement of Open-Circuit
Photovoltage with Long Alkyl Group
Yan Cui,†,‡ Yongzhen Wu,†,§ Xuefeng Lu,‡ Xi Zhang,‡ Gang Zhou,*,‡ Fohn B. Miapeh,§ Weihong Zhu,*,§ and
Zhong-Sheng Wang*,‡
‡Department of Macromolecular Science, Department of Chemistry and Laboratory of Advanced Materials, Fudan University,
Shanghai 200438, P. R. China
§Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science and Technology, Shanghai 200237, P. R. China
S Supporting Information
b
ABSTRACT: Two novel benzotriazole-containing organic dyes based on
DÀAÀπÀA configuration, WS-5 with octyl group and WS-8 with methyl
group, have been designed and synthesized for use in dye-sensitized solar cells
(DSSCs). Compared with the traditional DÀπÀA sensitizers, the benzotriazole
unit as an additional acceptor has several merits: (i) essentially facilitating the
electron transfer from the donor to the acceptor/anchor; (ii) conveniently
tailoring the solar cell performance with a facile structural modification on
2-position in the benzotriazole unit; and (iii) the nitrogen-containing hetero-
cyclic group of benzotriazole being expected to improve the open-circuit
photovoltage. The analysis of controlled intensity modulated photovoltage
spectroscopy reveals that the replacement of methyl with octyl group enhances
electron lifetime by 4-fold and retards charge recombination rate constant by
4-fold. The two dye-loaded TiO2 films possess almost the same conduction band position under the same condition, as revealed by
the charge densities at open-circuit against open-circuit photovoltage. Therefore, the significant enhancement of open-circuit
photovoltage from methyl to octyl group is attributed to the suppressed charge recombination. Under simulated AM1.5G solar light
(100 mW cmÀ2), the DSSC based on WS-5 produces a short-circuit photocurrent of 13.18 mA cmÀ2, an open-circuit photovoltage
of 0.78 V, a fill factor of 0.78, corresponding to a power conversion efficiency of 8.02%.
KEYWORDS: dye sensitizers, solar cells, benzotriazole, open-circuit photovoltage, charge recombination
’ INTRODUCTION
processes. However, the power conversion efficiencies of organic
sensitizer-based cells are generally lower than those based on
Ru complex sensitizers. The major reason is the relatively low
open-circuit photovoltage (Voc) for metal-free DSSCs, possibly
arising from shorter electron lifetime in comparison to the DSSCs
based on the Ru dyes.5c Recently, Tian et al. have discussed the
effects of charge recombination and electron injection efficiency on
Voc, formulating basic guidelines and strategies for improving Voc
and the overall performance of DSSCs.4c
Among metal-free organic sensitizers, the DÀπÀA structure
is the most common character of these organic dyes,4 which has
been well-demonstrated to extend the absorption spectra, adjust
the HOMO and LUMO levels, and realize the intramolecular
charge separation. Recently, DÀAÀπÀA configuration with
benzothiadiazole,6a,b quinoxaline,6c and cyano6d groups as ac-
ceptors has been designed and attracted more attention because
Since the discovery by Gr€atzel and co-workers in 1991,1 dye-
sensitized solar cells (DSSCs) have attracted extensive attention
in the last two decades because of the efficient conversion of solar
energy to electricity at a low cost.2 Up to date, such cells
employing ruthenium (Ru) polypyridyl complexes as sensitizers
have achieved power conversion efficiencies of 11À12% under
standard global air mass 1.5 (AM1.5G).3 However, in view of
cost and limited availability of Ru complexes, alternative metal-
free organic dyes,4 commonly constructed with donorÀπ-bridgeÀ
acceptor (DÀπÀA) configuration, are strongly desirable be-
cause of the following features:4,5 (i) larger absorption coeffi-
cients (attributed to an intramolecular πÀπ* transition) than
metal complex photosensitizers (attributed to metal-to-ligand
charge transfer), leading to more efficient light-harvesting;
(ii) the feasibilities and varieties in molecular designs for easy control
of their optoelectronic properties; and (iii) no concern on the
limited resource of noble metals, such as ruthenium. To date,
DSSCs based on metal-free organic sensitizers have been receiv-
ing much attention, especially in cost-effective facile preparation
Received: August 1, 2011
Revised:
August 26, 2011
Published: September 13, 2011
r
2011 American Chemical Society
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Scheme 1. Synthetic Routes for Sensitizers WS-5 and WS-8a
Figure 1. Chemical structures of DÀAÀπÀA sensitizers WS-5 and
WS-8 containing low band gap and strong electron-withdrawing unit of
benzotriazole as additional acceptor, and reference dye LS-1.
a Reaction conditions: (i) Pd(PPh3)4, 2 M K2CO3 aqueous solution,
THF, 12 h, 80 °C; (ii) 5-formylthiophen-2-boronic acid, Pd(PPh3)4,
2 M K2CO3 aqueous solution, THF, 12 h, 80 °C; (iii) cyanoacetic acid,
piperidine, acetonitrile, 7 h, 80 °C.
the additional acceptor chromophore is expected to facilitate the
electron transfer from the donor to the acceptor/anchor (i.e.,
cyanoacetic unit). To distinguish our designed structure from the
typical DÀπÀA configuration and emphasize the role of benzo-
triazole unit, we herein refer our dyes to the DÀAÀπÀA
structured dyes. The presence of two acceptors in the nomen-
clature does not mean two discrete, spatially separated LUMO
orbitals present in the molecule. In fact, the LUMO orbital is
delocalized over the AÀπÀA units, which are therefore a single
acceptor unit.
’ EXPERIMENTAL SECTION
Materials. The starting materials of 4,7-dibromo-2-octyl-benzotriazole,10
4,7-dibromo-2-methyl-benzotriazole,11 and 7-bromo-1,2,3,3a,4,8b-
hexahydro-4-(4-methylphenyl)-cyclopent[b]indole12 were synthesized
according to the reported literatures. Tetrahydrofuran (THF) was dried
over 4 Å molecular sieves and distilled under argon atmosphere from
sodium benzophenone ketyl immediately prior to use. All other solvents
and chemicals used for synthesis were purchased commercially, and used
as received without further purification.
Benzotriazole, a very cheap and commercially available ma-
terial, has been employed to construct low band gap copolymers
for bulk-heterojunction solar cells because of its unique electron-
accepting properties.7 However, the utilization of benzotriazole
unit in molecular sensitizers for DSSCs has not been reported so
far to the best of our knowledge. Herein, the benzotriazole unit
was incorporated into indoline based organic dyes to further
develop novel DÀAÀπÀA configuration sensitizers (WS-5 and
WS-8, Figure 1) based on the following considerations: (i) the
strong electron-withdrawing properties of benzotriazole to es-
sentially facilitate the electron transfer from the donor to the
acceptor/anchor; (ii) a facile structural modification on 2-posi-
tion in the benzotriazole unit to tailor their solar cell perfor-
mance; (iii) the absence of sulfur site in benzotriazole, which is
prone to form dye-iodine complexes available for serious charge
recombination, being probably favorable for high Voc;4c,8 and (iv)
like 4-tert-butylpyridine (TBP), the nitrogen-containing hetero-
cyclic group of benzotriazole being expectable to improve Voc.
Actually, many nitrogen-containing heterocyclic derivatives
including imidadole, triazole, pyrimidine, and benzimidazole
have been tested as additives in the electrolyte for the sake of
increasing Voc.9 As expected, an improvement in the electron
distribution of donor unit in WS-5 and WS-8 can be
realized, especially in increasing the photostability of synthetic
intermediates and final sensitizers. Additionally, the controlled
intensity modulated photovoltage spectroscopy was measured
to study the effect of alkyl group on Voc. Compared with WS-8,
the long octyl group introduced to the 2-position of the
benzotriazole unit in WS-5 successfully realized a significant
increase in Voc by 100 mV. Charge density data indicate that
the two dye-loaded TiO2 films possess almost the same conduc-
tion band position under the same condition. Therefore, the
significant enhancement of Voc from methyl to octyl group is
attributed to the suppressed charge recombination. With this
molecular design, the power conversion efficiency of 8.02%
along with a high Voc (0.78 V) was achieved with WS-5-
based DSSC.
Instrumentation. 1H NMR and 13C NMR spectra were recorded
on a Bruker AM-400 instrument with tetramethylsilane (TMS) as
internal standard. High-resolution mass spectroscopy (HRMS) was
performed using a Waters LCT Premier XE spectrometer. The UVÀvis
absorption spectra of the dye solutions and the dye-loaded transparent
films were recorded on a Shimadzu UV-3150PC spectrophotometer.
Electrochemical redox potentials were obtained by cyclic voltammetry
(CV) measurements, which were performed with a computer-controlled
Autolab analyzer using a typical three-electrode electrochemical cell
in a solution of tetrabutylammonium hexafluorophosphate (0.1 M)
in anhydrous acetonitrile at a scan rate of 50 mV sÀ1 at room temperature
under nitrogen. Dye-adsorbed TiO2 film on conductive glass was used as
the working electrode, a Pt wire as the counter electrode, and an Ag/Ag+
electrode as the reference electrode. The potential of the reference
electrode was calibrated with ferrocene, and all potentials mentioned in
this work are against the normal hydrogen electrode (NHE).
The electron lifetimes were measured with intensity modulated
photovoltage spectroscopy (IMVS),13 whereas charge densities at
open-circuit were measured using charge extraction technique.14 IMVS
and charge extraction analysis were carried out on an electrochemical
workstation (Zahner XPOT, Germany), which includes a green light
emitting diode (LED, 532 nm) and the corresponding control system.
The intensity-modulated spectra were measured at room temperature
with light intensity ranging from 0.5 to 45 W/m2, in modulation
frequency ranging from 0.1 Hz to 10 kHz, and with modulation
amplitude less than 5% of the light intensity. Charge densities at open
circuit under white light from a LED were measured to compare the
conduction band positions in different DSSCs.
Synthesis of 3a. Indoline borate 2 (Scheme 1) was prepared with
our published method6a from the raw material of 7-bromo-1,2,3,3a,4,8b-
hexahydro-4-(4-methylphenyl)-cyclopent[b]indole (3.5 g, 10.7 mmol)
by a Schlenk technique. The unpurified 2 in THF solution was reacted
with 4,7-dibromo-2-octyl-benzotriazole (1a, 4.70 g, 12.08 mmol) under
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Suzuki coupling reaction using Pd(PPh3)4 (0.20 g, 0.17 mmol) and
K2CO3 aqueous solution (6.0 mL, 2.0 M) as catalysts in THF (25 mL)
for 12 h. After cooling, water was added and the reaction mixture was
extracted with CH2Cl2. The combined organic layer was washed with
H2O and brine, dried over anhydrous Na2SO4, and evaporated under
reduced pressure. The crude product was purified by column chroma-
tography (CH2Cl2/PE = 1/6) on silica gel, and the single substituted
product was obtained as a yellow oil 3a (1.30 g, 2.33 mmol, 19% for two
steps). 1H NMR (400 MHz, CDCl3): δ 7.74À7.77 (m, 2 H, phenyl-H),
7.57 (d, J = 7.7 Hz, 1 H, phenyl-H), 7.34 (d, J = 7.7 Hz, 1 H, phenyl-H),
7.22 (d, J = 8.5 Hz, 2 H, phenyl-H), 7.16 (d, J = 8.4 Hz, 2 H, phenyl-H),
6.99 (d, J = 8.3 Hz, 1 H, phenyl-H), 4.84 (m, 1 H, indoline-CH), 4.77 (t,
J = 7.3 Hz, 2 H, octyl-NCH2-), 3.91 (m, 1 H, indoline-CH), 2.34 (s, 3 H,
indoline-CH3), 2.13 (m, 3 H, octyl-NCH2CH2- and indoline-CH2-),
1.95 (m, 2 H, indoline-CH2-), 1.81 (m, 1 H, indoline-CH2-), 1.66 (m,
1 H, indoline-CH2-), 1.59 (m, 1 H, indoline-CH2-), 1.25 (m, 10 H, octyl-
NC2H4C5H10-), 0.86 (t, J = 6.6 Hz, 3 H, octyl-CH3). 13C NMR (100
MHz, CDCl3): δ 148.34, 144.42, 142.85, 140.25, 135.40, 131.56, 131.50,
129.78, 129.32, 128.18, 126.52, 124.71, 123.06, 120.20, 107.51, 106.99,
69.23, 56.99, 45.46, 35.15, 33.76, 31.73, 30.19, 29.08, 29.00, 26.59, 24.46,
22.61, 20.80, 14.07. HRMS (ESI, m/z): [M]+ calcd for C32H3779BrN4,
556.2202, C32H3781BrN4, 558.2181; found, 556.2205, 558.2196.
Synthesis of 3b. 3b (0.25 g, 0.54 mmol, yield 20%) was obtained as
yellow powder in a similar way with 3a. 1H NMR (400 MHz, CDCl3): δ
7.71À7.73 (m, 2 H, phenyl-H), 7.58 (d, J = 7.7 Hz, 1 H, phenyl-H), 7.34
(d, J = 7.7 Hz, 1 H, phenyl-H), 7.22 (d, J = 8.5 Hz, 2 H, phenyl-H), 7.16
(d, J = 8.4 Hz, 2 H, phenyl-H), 6.99 (d, J = 8.3 Hz, 1 H, phenyl-H), 4.83
(m, 1 H, indoline-CH), 4.57 (s, 3 H, benzotriazole-CH3), 3.91 (m, 1 H,
indoline-CH), 2.34 (s, 3 H, indoline-CH3), 2.08 (m, 1 H, indoline-CH2-),
1.94 (m, 2 H, indoline-CH2-), 1.80 (m, 1 H, indoline-CH2-), 1.66 (m,
1 H, indoline-CH2-), 1.59 (m, 1 H, indoline-CH2-). 13C NMR (100 MHz,
CDCl3): δ 148.41, 144.68, 143.14, 140.24, 135.45, 131.60, 131.51,
129.80, 129.52, 128.14, 126.38, 124.72, 123.27, 120.23, 107.55, 106.80,
69.26, 45.45, 43.65, 35.19, 33.76, 24.46, 20.82. HRMS (ESI, m/z):
[M + H]+ calcd for C25H2479BrN4, 459.1184, C25H2481BrN4, 461.1164;
found, 459.1190, 461.1180.
CDCl3): δ 9.94 (s, 1 H, ÀCHO), 8.13 (d, J = 4.0 Hz, 1 H, thienyl -H),
7.78À7.85 (m, 4 H, thienyl-H and phenyl-H), 7.54 (d, J = 7.6 Hz, 1 H,
phenyl-H), 7.24 (d, J = 8.4 Hz, 2 H, phenyl-H), 7.17 (d, J = 8.3 Hz, 2 H,
phenyl-H), 7.01 (d, J = 8.1 Hz, 1 H, phenyl-H), 4.86 (m, 1 H, indoline-
CH), 4.61 (s, 3 H, benzotriazole-CH3), 3.94 (m, 1 H, indoline-CH), 2.35
(s, 3 H, indoline-CH3), 2.10 (m, 1 H, indoline-CH2-), 1.94 (m, 2 H,
indoline-CH2-), 1.82 (m, 1 H, indoline-CH2-), 1.67 (m, 1 H, indoline-CH2-),
1.60 (m, 1 H, indoline-CH2-). 13C NMR (100 MHz, CDCl3): δ 181.90,
149.40, 147.51, 141.99, 141.30, 140.91, 139.00, 136.22, 134.39, 132.08,
130.62, 128.80, 127.48, 125.78, 125.64, 123.79, 123.72, 121.15,
119.31, 119.22, 106.45, 68.27, 44.59, 43.71, 34.91, 33.06, 23.91,
20.34. HRMS (ESI, m/z): [M + H]+ calcd for C30H27N4OS, 491.1906;
found, 491.1911.
Synthesis of WS-5. A mixture of aldehyde 4a (0.27 g, 0.46 mmol)
with cyanoacetic acid (43 mg, 0.51 mmol) in acetonitrile (20 mL) was
refluxed in the presence of piperidine (0.5 mL) for 7 h under Argon.
After cooling the mixture was diluted with CH2Cl2, washed by water
and brine, dried over Na2SO4, and evaporated under reduced pressure.
The crude product was purified by column chromatography (CH2Cl2/
methanol =15/1) on silica gel, and yielded the product as a deep red
powder WS-5 (0.20 g, 0.31 mmol, 67%). 1H NMR (400 MHz, DMSO):
δ 8.24 (s, 1 H, =CH-), 8.13 (d, J = 3.7 Hz, 1 H, thienyl-H), 7.73À7.90
(m, 4 H, phenyl-H and thienyl-H), 7.62 (d, J = 7.4 Hz, 1 H, phenyl-H),
7.23 (d, J = 7.7 Hz, 2 H, phenyl-H), 7.18 (d, J = 7.7 Hz, 2 H, phenyl-H),
6.93 (d, J = 8.3 Hz, 1 H, phenyl-H), 4.89 (m, 1 H, indoline-CH), 4.81 (t,
J = 7.2 Hz, 2 H, octyl-NCH2-), 3.88 (m, 1H, indoline-CH), 2.29 (s, 3 H,
indoline-CH3), 2.06 (m, 3 H, octyl-NCH2CH2- and indoline-CH2-),
1.76À1.82 (m, 3 H, indoline-CH2-), 1.62 (m, 1 H, indoline-CH2-), 1.40
(m, 1 H, indoline-CH2-), 1.19 (m, 4 H, octyl-NC2H4C2H4-),1.30 (m, 6 H,
octyl-NC4H8C3H6-), 0.78 (t, J = 5.7 Hz, 3 H, octyl-CH3). 13C NMR
(100 MHz, DMSO): δ 163.89, 147.59, 145.01, 142.15, 141.53, 141.35,
139.46, 136.21, 136.10, 135.13, 130.94, 130.84, 129.73, 128.11, 126.88,
126.02, 124.39, 124.06, 122.08, 119.86, 119.62, 118.79, 106.88, 68.30,
56.17, 44.59, 34.85, 33.09, 31.10, 29.15, 28.44, 28.22, 25.86, 23.93, 21.99,
20.37, 13.84. HRMS (ESI, m/z): [M + H]+ calcd for C40H42N5O2S,
656.3059; found, 656.3058.
Synthesis of 4a. The mixture of 3a (0.80 g, 1.43 mmol), 5-for-
mylthiophen-2-boronic acid (0.24 g, 1.54 mmol), Pd(PPh3)4 (0.16 mg,
0.14 mmol), and K2CO3 aqueous solution (6.0 mL, 2.0 M) in THF
(35 mL) was refluxed for 12 h under argon. After the solution was
cooled, water was added and the reaction mixture was extracted with
CH2Cl2 three times. The combined organic layer was washed with H2O
and brine, dried over anhydrous Na2SO4, and evaporated under reduced
pressure. The crude product was purified by column chromatography
(CH2Cl2/PE = 1/1) on silica gel to obtain the product as a orange red oil
4a (0.23 g, 0.39 mmol, 27%). 1H NMR (400 MHz, CDCl3): δ 9.94 (s,
1 H, ÀCHO), 8.14 (d, J = 4.0 Hz, 1 H, thienyl-H), 7.88 (dd, J = 8.4 Hz,
J = 1.6 Hz, 1 H, phenyl-H), 7.85 (s, 1 H, phenyl-H), 7.78À7.81 (m, 2 H,
phenyl-H and thienyl-H), 7.53 (d, J = 7.7 Hz, 1 H, phenyl-H), 7.24 (d, J =
8.4 Hz, 2 H, phenyl-H), 7.18 (d, J = 8.3 Hz, 2 H, phenyl-H), 7.03 (d, J =
8.4 Hz, 1 H, phenyl-H), 4.86 (m, 1 H, indoline-CH), 4.81 (t, J = 7.2 Hz,
2 H, octyl-NCH2-), 3.93 (m, 1 H, indoline-CH), 2.35 (s, 3 H, indoline-
CH3), 2.18 (m, 2 H, octyl-NCH2CH2-), 2.09 (m, 1 H, indoline-CH2-),
1.95 (m, 2 H, indoline -CH2-), 1.81 (m, 1 H, indoline-CH2-), 1.68 (m, 1 H,
indoline-CH2-), 1.59 (m, 1 H, indoline-CH2-), 1.40 (m, 4 H, octyl-
NC2H4C2H4-), 1.27 (m, 6 H, octyl-NC4H8C3H6-), 0.86 (t, J =
5.7 Hz, 3 H, octyl-CH3). 13C NMR (100 MHz, CDCl3): δ 181.87,
149.37, 147.54, 142.07, 141.37, 140.95, 139.07, 136.32, 134.41, 132.07,
130.66, 128.77, 127.49, 125.78, 125.62, 123.79, 123.73, 121.17, 119.32,
119.27, 106.47, 68.22, 55.82, 44.40, 34.16, 32.68, 30.72, 29.05, 28.08,
27.98, 25.59, 23.42, 21.59, 19.78, 13.04. HRMS (ESI, m/z): [M]+ calcd
for C37H40N4OS, 588.2923; found, 588.2917.
Synthesis of WS-8. WS-8 (60 mg, 0.11 mmol, yield 68%) was
obtained as deep red powder in similar way with WS-5: 1H NMR
(400 MHz, DMSO): δ 13.83 (br, 1 H, ÀCOOH), 8.46 (s, 1 H, =CH-),
8.18 (d, J = 3.4 Hz, 1 H, thienyl-H), 8.04 (d, J = 3.3 Hz, 1 H, thienyl-H),
7.90À7.94 (m, 3 H, phenyl-H), 7.68 (d, J = 7.6 Hz, 1 H, phenyl-H), 7.24
(d, J = 8.0 Hz, 2 H, phenyl-H), 7.19 (d, J = 8.0 Hz, 2 H, phenyl-H), 6.95
(d, J = 8.2 Hz, 1 H, phenyl-H), 4.90 (m, 1 H, indoline-CH), 4.61 (s, 3 H,
benzotriazole-CH3), 3.90 (m, 1 H, indoline-CH), 2.29 (s, 3 H, indoline-
CH3), 2.08 (m, 1 H, indoline-CH2-), 1.85 (m, 3 H, indoline-CH2-), 1.63
(m, 1 H, indoline-CH2-),1.42(m,1H,indoline-CH2-). 13C NMR (100 MHz,
DMSO): δ 163.70, 148.14, 147.78, 145.66, 142.37, 141.74, 139.92, 139.40,
135.21, 135.10, 131.59, 130.95, 129.76, 128.30, 126.94, 125.83, 124.86,
124.46, 122.10, 119.73, 119.25, 116.90, 115.79, 106.91, 68.36, 44.57, 43.72,
34.91, 33.07, 23.94, 20.37. (ESI, m/z): [M + H]+ calcd for C33H28N5O2S,
558.1964; found, 558.1962.
Fabrication of Dye-Sensitized Solar Cells. Fluorine-doped
SnO2 glass (15 Ω/sq, transmittance 80À85%, Nippon Sheet Glass,
Japan) substrates were cleaned in a detergent solution by an ultrasonic
bath, washed with acetone and water, and then dried using N2 current.
Nanocrystalline TiO2 films, 14 μm, consisting of a 7 μm transparent
layer (∼ 20 nm nanoparticles) and 7 μm scattering layer (∼ 100 nm
particles)15 in thickness, were prepared using a screen printing tech-
nique, followed by sintering at 525 °C under an air flow. After cooling,
the TiO2 films were impregnated in a 0.05 M aqueous TiCl4 solution for
30 min at 70 °C, and then rinsed with deionized water. The TiCl4-
treated TiO2 films were annealed at 450 °C for 30 min, and then cooled
to 120 °C before immersed into the dye solution (0.3 mM in chloro-
form:ethanol = 3:7) for 16 h to allow the dye molecules to adsorb onto
Synthesis of 4b. 4b (0.08 g, 0.16 mmol, yield 29%) was obtained as
orange red powder in a manner similar to that of 4a. 1H NMR (400 MHz,
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the TiO2 surface. After the adsorption of dyes, the electrodes were rinsed
with chloroform and acetonitrile, respectively. The resulting photoelec-
trode and the Pt-counter electrode were assembled into a sealed
sandwich solar cell with a thermoplastic frame (Surlyn 30 μm thick).
Redox electrolyte (0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide,
0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine in acetonitrile) was
introduced through a hole in the counter electrode via suction through
another drilled hole. Finally, the two holes were sealed using another hot
melt Surlyn film covered with a thin glass slide.16
Photovoltaic Measurements. The current densityÀvoltage (JÀV)
characteristics of the DSSCs were measured by recording JÀV curves
using a Keithley 2400 source meter under the illumination of AM1.5G
simulated solar light coming from an AAA solar simulator (Newport-
94043A equipped with a 450 W Xe lamp and an AM1.5G filter). The
DSSCs were fully covered with a black mask with an aperture area of
0.2304 cm2 during measurements. The incident light intensity was
calibrated with a standard silicon solar cell (Newport 91150). Action
spectra of the incident monochromatic photon-to-electron conversion
efficiency (IPCE) for the solar cells were obtained with an Oriel-74125
system (Oriel Instruments). The intensity of monochromatic light was
measured with a Si detector (Oriel-71640).
’ RESULTS AND DISCUSSION
Design and Synthesis. In DÀπÀA configuration of sensiti-
zers, the tuning of donor and acceptor units has been well
demonstrated to extend the absorption spectrum, adjust the
HOMO and LUMO levels, and realize the intramolecular charge
separation. As well demonstrated, the indoline unit has more
powerful electron-donating capability than that of common
triphenylamine.6a,e As shown in Figure 1, for the design of sen-
sitizers WS-5 and WS-8, we employed indoline unit as the donor,
cyanoacetic acid as the acceptor/anchor, and a thiophene moiety
as the conjugation bridge. Additionally, an electron-withdrawing
benzotriazole unit was incorporated between the donor and π-
conjugation unit. We have demonstrated that the additional
acceptor unit can facilitate the electron migration from the donor
to the cyanoacetic acetic unit.6a,d However, the Voc is relatively
low as compared to the reported efficient DSSCs. Considering
that 4-tert-butylpyridine (TBP), imidadole, pyrimidine and ben-
zimidazole have been employed as additives in the electrolyte to
increase Voc, here the nitrogen-containing heterocyclic group of
benzotriazole unit can be expectable to enhance Voc. As well-
known, the strong intermolecular πÀπ interaction may lead to
interfacial charge recombination, thus resulting in a decrease in
Voc. To reduce the intermolecular πÀπ interaction, an alkyl
group was conveniently linked to the 2-position of benzotriazole
unit. As depicted in Scheme 1, the syntheses of WS-5 and WS-8
were started from 4,7-dibromo-2-methyl-benzotriazole and 4,
7-dibromo-2-octyl-benzotriazole, respectively.10,11 Two asym-
metrical Suzuki coupling17 with indoline borate 2 and 5-for-
mylthiophen-2-boronic acid, respectively, afforded monoaldehyde-
substituted precursors 4a and 4b. Because of the low oxidation
potential, the bromo-substituted indoline was extremely photo-
sensitive and deteriorated in a short time under day light and air
with color change from very pale yellow to dark green. We
checked the deteriorated chemical with traditional thin layer
chromatography, showing at least two new byproduct compo-
nents. By contrast, after connecting the electron-withdrawing
acceptor unit of benzotriazole group, the resulting indoline
intermediates became very stable for half a year without
distinct color change. Therefore, the design of DÀAÀπÀA
Figure 2. (a) Absorption spectra of WS-5, WS-8, and reference LS-1 in
CH2Cl2 solutions. (b) Absorption spectra of WS-5 and WS-8 anchored
onto TiO2 films (1.0 μm in thickness).
Table 1. Photophysical and Electrochemical Properties of
Sensitizers WS-5 and WS-8
absorption
λmax
ε
λmax on HOMO E0À0
LUMO
(V) d
dye (nm) a (MÀ1 cmÀ1
)
TiO2 (nm) (V)b
(eV) c
a
WS-5 496
WS-8 495
19200
17200
438
428
0.94
0.92
2.12
2.09
À1.18
À1.17
a Absorption peaks (λmax) and molar extinction coefficients (ε) were
measured in CH2Cl2 (∼1 Â 10À5 M). b The formal oxidation potentials
(vs NHE) in acetonitrile were internally calibrated with ferrocene, and
taken as the HOMO. c E0À0 was estimated from the wavelength at 10%
maximum absorption intensity for the dye-loaded TiO2 film. d The
LUMO was calculated with the expression of LUMO = HOMO À E0À0
.
configuration is beneficial to not only the synthetic process,
but also the photostability in the DSSC operation. Finally, the
obtained precursors were converted to sensitizers WS-5 and WS-
8 by Knoevenagel condensation18 with cyanoacetic acid by reflux
in acetonitrile in the presence of piperidine, which were well
characterized by 1H NMR, 13C NMR, and HRMS. The obtained
dyes are deep red in solid state, and can be dissolved in
dichloromethane, THF, and toluene.
UVÀVis Absorption Properties. The UVÀvis absorption
spectra of two sensitizers in CH2Cl2 solutions (1 Â 10À5 M)
are shown in Figure 2a, and the corresponding properties are
summarized in Table 1. The absorption spectra of dyes WS-5 and
WS-8 display the maximum absorption wavelength at 496 (ε =
1.92 Â 104 MÀ1 cmÀ1) and 495 nm (ε = 1.72 Â 104 MÀ1 cmÀ1),
respectively, due to the intramolecular charge transfer (ICT)
from the donor to the acceptor. The almost identical absorption
maxima and extinction coefficients are not difficult to be
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Figure 4. Frontier orbitals of WS-5 and WS-8.
Figure 3. Cyclic voltammograms of the dye-loaded TiO2 films (6 μm
thick, 0.25 cm2 in area) for WS-5 and WS-8. Note that the current
intensity of WS-5 is decreased by around 50% with respect to WS-8.
understood as WS-5 and WS-8 have similar chemical structures
except for the alkyl chain length attached at the benzotriazole
unit. Compared with reference dye LS-1 (Figure 1) whose
absorption peak is located at 483 nm (2.01 Â 104 MÀ1 cmÀ1),
the introduction of benzotriazole unit into the molecular frame
can redshift the ICT absorption peak by 12À13 nm (Figure 2a).
Moreover, in WS-5 and WS-8, the additional absorption band
appearing at around 400 nm and the apparent red-shift in the
absorption threshold are beneficial to the light harvesting.
Consequently, the benzotriazole unit can efficiently decrease
the band gap and optimize energy levels, thus resulting in the
longer responsive wavelength region and higher light-harvesting
efficiency.
Figure 5. IPCE action spectra for DSSCs based on WS-5 and WS-8.
are calculated to be À1.18 and À1.17 V, respectively, indicating
a sufficient driving force for electron injection from oxidized
organic dyes to TiO2 films.20 The energy levels for the two dyes
are also listed in Table 1.
Figure 2b shows the absorption spectra of the organic sen-
sitizers anchored onto a transparent nanocrystalline TiO2 film
(1.0 μm). After adsorption onto the TiO2 films, WS-5 and WS-8
exhibit absorption peaks hypsochromically shifted to 438 and
428 nm, respectively, which is mainly ascribed to the deprotona-
tion of the carboxylic acid and the different polarity of medium.19
Compared with the hypsochromic shift of 58 nm for WS-5,
the hypsochromic shift for WS-8 is more remarkable (67 nm),
probably due to the stronger πÀπ interaction in WS-8 than in
WS-5 because the longer alkyl chain can weaken the πÀπ
interaction. Furthermore, the absorption intensity of WS-5
loaded TiO2 film is slightly lower than that for WS-8. Obviously,
the lower dye adsorption amount for WS-5 on TiO2 surface
stems from the larger molecular size and weaker intermolecular
πÀπ interaction of WS-5 with octyl group than WS-8 with
methyl group. This is further proven by desorbing organic dyes
from the TiO2 surface. The surface concentration of WS-5 is
4.7 Â 10À8 mol cmÀ2 μmÀ1, whereas that for WS-8 increased to
It should be noted that the current intensity of WS-5 is
decreased by around 50% with respect to WS-8 (Figure 3),
which is much higher than the absorption decrease from WS-8 to
WS-5 (by 10%). It is suggestive that the decreased adsorption
amount cannot be the only cause for the decreased current
intensity. As we have discussed elsewhere,21 the initial oxidation
of dye molecules followed by intermolecular electron hopping
across the TiO2 nanoparticle surface is one accepted mechanism
for charge transfer in working electrode.22 Therefore, the forma-
tion of πÀπ stacking in WS-8 allows facile intermolecular
electron hopping as evidenced by the strong current signal.
Upon the incorporation of octyl group in WS-5, the long alkyl
chain suppresses the intermolecular interaction among the dye
molecules, which results in the weakened electron hopping between
each other and the significant decrease in current intensity.
Theoretical Approach. To gain insight into the geometrical
and electronic properties of the resulted sensitizers, density
functional calculations were conducted using the Gaussian 03
program package at the B3LYP/6-31G* level. It is clearly seen
from Figure 4 that the HOMO is distributed along the DÀAÀπ
system and the LUMO is evenly delocalized across the entire
AÀπÀA system. The well-overlapped HOMO and LUMO
orbitals on the benzotriazole unit indirectly suggest the well
inductive or withdrawing electron tendency from indoline donor
unit to the cyanoacetic acid unit. After photoexcitation, the
electrons in the organic dyes loaded on nanocrystalline TiO2
surface could be successively transferred from the donor to the
benzotriazole unit, and then transferred to the cyanoacetic acid
5.5 Â 10À8 mol cmÀ2 μmÀ1
.
Electrochemical Properties. To investigate the oxidation
potential of organic dyes and thermodynamically evaluate the
possibility of sensitizer regeneration, cyclic voltammetry was
carried out in a typical three-electrode electrochemical cell with
TiO2 films stained with sensitizer as the working electrode
(Figure 3). The formal oxidation potentials, taken as the HOMO
levels of sensitizers WS-5 and WS-8, are calculated to be 0.94 and
0.92 V (vs. NHE, same below), respectively, which are more
positive than IÀ/I3À redox couples (∼ 0.4 V), indicating that the
reduction of oxidized dyes with IÀ ions is thermodynamically
feasible. Correspondingly, estimated from the band gap5e derived
from the wavelength at 10% maximum absorption intensity for
the dye-loaded TiO2 film, the LUMO levels of WS-5 and WS-8
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Figure 7. Charge density at open circuit as a function of Voc for DSSCs
with WS-5 and WS-8. Three identical devices were tested in each case
with standard deviation less than 1%.
Figure 6. JÀV characteristics of DSSCs based on WS-5 and WS-8
measured under illumination of simulated AM1.5G solar light
(100 mW cmÀ2).
subunit, and finally into TiO2. The well-delocalized LUMO over
the AÀπÀA units indicates that the additional acceptor of
benzotriazole unit could facilitate the electron transfer from
the donor to the acceptor/anchor (i.e. cyanoacetic unit).
Solar Cell Performance. Figure 5 shows the IPCE action
spectra of the DSSCs based on sensitizers WS-5 and WS-8. The
onsets of IPCE for both dyes are above 700 nm, which are
consistent with the absorption onsets in the magnified absorp-
tion spectra (Figure 2b). It can be found that both WS-5 and WS-
8 based DSSCs display a platform in the spectral range of 400 À
580 nm with maximum IPCE over 80%. Taking the reflectance
and absorption by the conductive glass into account, the max-
imum IPCE can be regarded as unity. Without coadsorption, the
two dyes exhibit efficient photoelectric conversion, in contrast to
our previously reported dye WS-2, which produces maximum
IPCE below 60% without coadsorption and above 80% with
coadsorption.6a
The current densityÀvoltage (JÀV) curves of the DSSCs
based on WS-5 and WS-8 under simulated AM1.5G irradiation
(100 mW cmÀ2) are displayed in Figure 6, where short-circuit
photocurrent (Jsc), open-circuit photovoltage (Voc), fill factor
(FF), and power conversion efficiency (η) can be determined.
The DSSC based on WS-8 produces η of 6.74% (Jsc = 13.39
mA cmÀ2, Voc = 0.68 V, FF = 0.74). We previously reported
a benzothiadiazole dye (WS-2),6a which produced η of 5.35%
(Jsc = 12.90 mA cmÀ2, Voc = 0.61 V, FF = 0.68) under the same
condition. Evidently, WS-8 is better in terms of solar cell
performance, especially in Voc, than WS-2 without coadsorption.
Although Voc of WS-2 based DSSC was improved to 0.65 V with
coadsorption, it was still lower than that of WS-8 based DSSC
without coadsorption. When a hexyl group was linked to the
thiophene unit in WS-2, the obtained Voc of 0.67 V was much
lower than that (0.78 V) for WS-5. These results indicate that
incorporation of benzotriazole unit into the dye skeleton is
advantageous to high Voc. Actually, many nitrogen-containing
heterocyclic derivatives including 4-tert-butylpyridine (TBP),
imidadole, triazole, pyrimidine, and benzimidazole have been
tested as additives in the electrolyte for the sake of increasing
Voc.9 Here, considering the very similar configuration between
WS-2 and WS-8, the incorporated nitrogen-containing hetero-
cyclic group of benzotriazole in WS-8 would be responsible for
the increased Voc. Additionally, because it is considered that
sulfur atom is prone to form dyeÀiodine complexes available for
serious charge recombination,8 the absence of sulfur atom in
benzotriazole unit for WS-8 with respect to the benzothiadiazole
for WS-2 might be beneficial to an increase in electron lifetime,
which lacks evidence so far and needs to be investigated in the
future. The detailed reasons why WS-8 generates higher Voc than
WS-2 will be clarified in another research. When the methyl
group in dye WS-8 is replaced with octyl group (WS-5), a
significant enhancement of Voc, a mild increase in FF, and similar
Jsc are observed. As a result, η of 8.02% (Jsc = 13.18 mA cmÀ2
,
Voc = 0.78 V, FF = 0.78) was achieved for WS-5 based DSSC.
Many cells were tested for the two dyes, and always WS-5 based
DSSC produces higher η and Voc than WS-8-based DSSC.
Remarkably, the distinct enhancement of Voc from short
methyl group to octyl chain is realized in this work. The Voc
for WS-5 (0.78 V) is much higher than that (0.67 V) for the
benzothiadiazole dye with a long alkyl chain under the same
condition. This suggests that the incorporation of benzotriazole
unit into the dye might be expectable to overcome the major
limitation to the relatively low Voc for metal-free DSSCs.
The generation of Voc is related to the conduction band
position of TiO2 and the charge recombination rate in DSSCs.
To understand the significant improvement of the Voc from
WS-8 to WS-5-based DSSC, first, the relative conduction band
position in the DSSCs based on WS-5 and WS-8 was investi-
gated. According to the method developed by Frank and co-
workers,23 the movement of conduction band contributes to the
change in Voc at constant photoinduced charge density (Q)
which was measured with charge extraction technique14 under
illumination of a white light from LED. Three identical devices
were tested in each case with standard deviation less than 1%.
Figure 7 displays the charge density at open circuit as a function
of Voc for DSSCs with WS-5 or WS-8. It can be found that for
both DSSCs based on WS-5 and WS-8, the Voc increases linearly
with the logarithm of Q. The two curves for the corresponding
DSSCs have the same slope (125 mV) and coincide with each
other. At fixed Q, the almost same Voc for both DSSCs indicates
the identical conduction band position for two cases. Therefore,
the conduction band edge is not influenced when the alkyl in
benzotriazole is changed from methyl to octyl.
Because the introduction of a longer alkyl chain into the dye
molecule does not induce the movement of the conduction band
of TiO2, the significant improvement of Voc from WS-8 to WS-5
based DSSC should be attributed to the repression of charge
recombination, which is related to electron lifetime (τ). Figure 8
shows the electron lifetime as a function of charge density at open
circuit for the DSSCs based on WS-5 and WS-8, respectively.
The electron lifetime was obtained from the frequency at the top
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Chemistry of Materials
ARTICLE
relation with the same exponent of 2.5. Again, the same slope or
exponent suggests the same recombination mechanism for both
DSSCs. Moreover, at fixed Q, Jr for WS-8-based DSSC is about 4
times larger than that for WS-5 based DSSC, indicating that
the recombination rate constant is thus reduced by 4-fold
when methyl is replaced with octyl group. The retarded charge
recombination rate constant will reduce current or electron loss
at open circuit. When more charge is accumulated in TiO2, Fermi
level moves upward and Voc gets larger. The measured charge
density at open circuit under one sun illumination for WS-5 is
about 2-fold higher than that for WS-8. According to the relation
of QÀVoc (Figure 7), the Voc gain arising from the increased Q is
calculated to be about 90 mV, explaining why WS-5 based DSSC
produces 100 mV higher Voc than the WS-8-based DSSC.
Figure 8. Electron lifetime as a function of charge density at open
circuit. Electron lifetime was measured with IMVS under different
intensities of a 532 nm light from LED, and charge densities were
measured with charge extraction technique under the same light
intensities for IMVS measurement. Three identical cells were tested in
each case with standard deviation less than 4%.
’ CONCLUSIONS
In summary, we have successfully designed and synthesized
two novel benzotriazole-containing organic sensitizers with DÀ
AÀπÀA configuration. Replacing the methyl group with octyl
group does not bring about significant changes in absorption
spectrum and energy levels. However, the octyl group suppresses
charge recombination rate constant by 4-fold as compared to the
methyl group. As a result, without any coadsorption, WS-5-based
DSSC achieved η of 8.02% with Voc higher than WS-8 by
100 mV. Because of incorporation of benzotriazole unit into the
dye skeleton, Voc as high as 0.78 V was obtained. Our studies
clearly demonstrate the relation between molecular structure and
device performance. These findings will pave a new way to design
new efficient dye sensitizers.
’ ASSOCIATED CONTENT
Figure 9. Charge recombination current as a function of charge density
at open circuit. Q was measured under illumination of a 532 nm light
from LED. Jsc measured at the same light intensity for Q measurement
was approximately taken as Jr because of the negligible charge recombi-
nation at short circuit. Three identical cells were tested in each case with
standard deviation less than 5%.
S
Supporting Information. Discussion on the low yields of
b
the Suzuki coupling reaction using 5-formylthiophen-2-boronic
acid and characterization of the sensitizers WS-5 and WS-8 by 1H
NMR, 13C NMR, and HRMS. This material is available free of
charge via the Internet at http://pubs.acs.org.
À1
’ AUTHOR INFORMATION
of the semicircle (fmin) in IMVS by the relation τ = (2πfmin
)
.
The charge density at open circuit under the same light intensity
for IMVS measurement was obtained from charge extraction.
Three identical cells were tested in each case with standard
deviation less than 4%. As shown in Figure 8, the electron lifetime
decreases with charge density following a power law relation with
the same slope, suggesting the same recombination mechanism.
At a fixed charge density, the electron lifetime for the DSSC
based on WS-5 is larger than that of WS-8 based DSSC by
around 4-fold. The long octyl group is more effective than the
short methyl group to block I3À ions (or iodine) approaching the
TiO2 surface for charge recombination, and thus WS-5 shows
much longer electron lifetime than WS-8.
Finally, to analyze the effect of alkyl chains on charge recom-
bination rate, charge recombination current (Jr) at open circuit is
plotted against charge density (Q, Figure 9). At open circuit and a
fixed light intensity, the recombination current density Jr equals
the charge-injection current density. However, neither of them
can be directly measured, but can be estimated from the mea-
surement of Jsc. At short-circuit, Jr is taken as Jsc in value under the
same light intensity because of the negligible charge recombi-
nation.23 It is evident from Figure 9 that the recombination
current for both cases increases with Q following a power law
Corresponding Author
*E-mail: zhougang@fudan.edu.cn; zs.wang@fudan.edu.cn. Fax:
(+86)21-5163-0345; whzhu@ecust.edu.cn. Fax: (+86)21-6425-
2758.
Author Contributions
†These authors contributed equally to this work.
’ ACKNOWLEDGMENT
This work was financially supported by the National Basic
Research Program (2011CB933302) of China, the National
Natural Science Foundation of China (20971025, 90922004,
and 50903020), the Shanghai nongovernmental international
cooperation program (10530705300), Shanghai Leading Academic
Discipline Project (B108), the Oriental Scholarship, the Funda-
mental Research Funds for the Central Universities (WK1013002),
and Jiangsu Major Program (BY2010147).
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