High-performance dye-sensitized solar cells based on 5,6-bis-hexyloxy- benzo[2,1,3]thiadiazole

Article abstract of DOI:10.1039/c2jm30427a

New dipolar compounds containing a 5,6-bis-hexyloxy-benzo[2,1,3]thiadiazole entity in the conjugated spacer between the electron donating arylamine and the electron accepting 2-cyanoacrylic acid have been synthesized. These compounds were used as the sensitizers for efficient dye-sensitized solar cells. Compared to the congeners without the hexyloxy chains, the new dyes effectively suppress the dark currents and significantly improve the cell performance. The best conversion efficiency reached ～92% of the N719-based standard DSSC fabricated and measured under similar conditions (AM 1.5 irradiation). The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm30427a

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PAPER
High-performance dye-sensitized solar cells based on 5,6-bis-hexyloxy-benzo
[2,1,3]thiadiazole†
a
a
b
c
a
Hsien-Hsin Chou, Yung-Chung Chen, Hsuan-Jui Huang, Ting-Hui Lee, Jiann T. Lin, Chiitang Tsaic
*
and Kellen Chend
Received 20th January 2012, Accepted 21st March 2012
DOI: 10.1039/c2jm30427a
New dipolar compounds containing a 5,6-bis-hexyloxy-benzo[2,1,3]thiadiazole entity in the conjugated
spacer between the electron donating arylamine and the electron accepting 2-cyanoacrylic acid have
been synthesized. These compounds were used as the sensitizers for efficient dye-sensitized solar cells.
Compared to the congeners without the hexyloxy chains, the new dyes effectively suppress the dark
currents and significantly improve the cell performance. The best conversion efficiency reached ꢀ92%
of the N719-based standard DSSC fabricated and measured under similar conditions (AM 1.5
irradiation).
flexibility. Conjugated segments favoring the quinoid structure
such as planar oligothiophene6 have been found to red shift the
1. Introduction
Dye-sensitized solar cells (DSSCs) have attracted considerable
absorption band effectively. An alternative ingenious approach
research interest for the past two decades and have been
is incorporation of an electron-deficient entity such as cyano
considered to be an attractive alternative to existing silicon-based
group7 and heteroaromatic ring,8 or a segment with intrinsic long
solar cell technology.1 Among sensitizers used for DSSCs,
wavelength absorption such as diketopyrrolopyrrole (DPP),9
€
ruthenium dyes (or Gratzel dyes) are still leading in solar-to-
cyanine,10 boron-dipyrromethene (BODIPY)11 and squaraine12
electricity conversion efficiency and reliability,2 with a record
high efficiency of 11.5%.3 High performance (efficiency > 11%) of
Zn-porphyrin dyes has also been achieved very recently.4 The
limited resource of ruthenium metal and the tedious purification
process of porphyrin dyes may restrict wide application of these
two sensitizers. In contrast, metal-free sensitizers have advan-
tages of low cost and design flexibility, and efficiency as high as
10% has been achieved by Wang’s group.5 Efficient utilization of
photons below a threshold wavelength of ꢀ920 nm is considered
important for decent performance of solar cells. Compared to
ruthenium and porphyrin dyes, metal-free dyes normally absorb
at relatively lower wavelength, which will lead to less efficient
light-harvesting. A viable way to remedy this deficit in solar light
absorption of a dipolar compound is using a stronger electron
donor or acceptor, or a more effective conjugated spacer for
enhancing charge transfer between the donor and the acceptor.
Among these, variation of the spacer has the advantage of design
entities in the spacer.
We have been interested in push–pull type metal-free sensi-
tizers using arylamine as the electron donor and 2-cyanoacrylic
acid as the electron acceptor.7d,8a,13 Though high conversion
efficiency reaching >95% of the DSSC based on N719 (bis(te-
trabutyl-ammonium)-cis-di(thiocyanato)-N,N0-bis(4-carboxylato-
40-carboxylic acid-2,20-bipyridine)ruthenium(II))14 fabricated and
measured under similar conditions was attained, most of the
sensitizers suffer from relatively short absorption wavelengths.
Longer absorption wavelengths were found possible as cyano-
vinyl7d or benzothiadiazole (benzoselenadiazole)8a entity was
implanted in the conjugated spacer. However, the DSSCs based
on these sensitizers exhibited lower efficiencies. Density func-
tional calculation results indicated that some charge trapping at
the electron deficient moiety hampered efficient electron injection
from the sensitizer to the TiO2 nanoparticles. The cell perfor-
mance is improved as the charge trapping is eased.7d After our
work on benzothiadiazole- and benzoselenadiazole-based sensi-
tizers, some reports demonstrated that high performance DSSCs
were possible with benzothiadiazole derived sensitizers if the
spacer was properly tethered with long hydrocarbon chains.8b,c In
continuation of our studies on benzothiadiazole-based sensi-
tizers, we set up to tether two long alkoxy chains at the benzo-
thiadiazole entity considering dual functions of the alkoxy
chains: (1) they may function as electron-donating units to
counteract the electron-deficient character of benzothiadiazole
and help to suppress the charge trapping effect and (2) similar to
aInstitute of Chemistry, Academia Sinica, 11529 Nankang, Taipei, Taiwan.
E-mail: jtlin@chem.sinica.edu.tw; Fax: +886-2-2783-1237; Tel: +886-2-
2789-8523
bDepartment of Chemical Engineering, Tatung University, 10452 Taipei,
Taiwan
cDepartment of Chemistry, Chinese Culture University, 11114 Taipei,
Taiwan
dAU Optronics Corporation, 30078 Hsinchu, Taiwan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm30427a
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J. Mater. Chem., 2012, 22, 10929–10938 | 10929

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long hydrocarbon chains, they may lead to more compact
packing of the dyes on TiO2 and help to suppress dark currents.
Herein, we report the synthesis and physical properties of new
benzothiadiazole-based sensitizers. DSSCs fabricated from these
dyes will also be presented.
5.80; N, 6.13%. HR-MS (MALDI), m/z: 899.2995 ([M + H]+,
1
calcd for C50H51N4O6S3: 899.2970). H NMR (400 MHz, THF-
d8): d 8.92 (d, J ¼ 4.4 Hz, 1H), 8.66 (d, J ¼ 4.4 Hz, 1H), 8.43 (s,
1H), 7.91 (d, J ¼ 4.4 Hz, 1H), 7.55 (d, J ¼ 8.4 Hz, 2H), 7.36 (d,
J ¼ 4.4 Hz, 1H), 7.10 (d, J ¼ 8.0 Hz, 4H), 6.93 (d, J ¼ 8.4 Hz,
2H), 6.92 (d, J ¼ 8.0 Hz, 4H), 4.30 (t, J ¼ 7.2 Hz, 2H), 4.21 (t, J ¼
7.2 Hz, 2H), 3.81 (s, 6H), 2.21–2.05 (m, 4H), 1.58–1.32 (m, 12H),
0.98–0.93 (m, 6H). 13C NMR (101 MHz, THF-d8): d 163.18,
156.54, 153.41, 150.65, 150.60, 150.47, 148.77, 146.95, 145.86,
143.57, 140.43, 137.32, 136.68, 132.95, 131.68, 131.07, 126.68,
126.16, 121.47, 119.98, 119.56, 115.88, 115.36, 114.57, 99.07,
74.70, 74.23, 54.66, 31.80, 31.61, 30.42, 29.94, 25.71, 25.62, 22.60,
13.44.
2. Experimental methods
2.1. General information
Unless otherwise specified, all the reactions were performed
under nitrogen atmosphere using standard Schlenk techniques.
All solvents used were purified by standard procedures, or
purged with nitrogen before use. NMR spectra were recorded on
a Bruker AV-III 400 spectrometer operating at 400.13 MHz or
an AVA 300 spectrometer operating at 300.13 MHz. All chro-
matographic separations were carried out on silica gel (60M,
230–400 mesh). Mass spectra (FAB or MALDI) were recorded
on a VG70-250S mass spectrometer. Elemental analyses were
performed on a Perkin-Elmer 2400 CHN analyzer. Absorption
spectra were recorded on a UV-Vis spectrophotometer (DB-20,
Dynamica). Fluorescence spectra were recorded on a Hitachi
F-4500 spectrophotometer. Voltammetric experiments were
carried out using an electrochemical analyzer (CHI-621B, CH
Instruments, Inc.) at room temperature with a conventional
three-electrode configuration consisting of a platinized working
electrode, platinum counter-electrodes and a non-aqueous Ag/
AgNO3 reference electrode. The photoelectrochemical charac-
terizations on the solar cells were carried out using an Oriel Class
A solar simulator (Oriel 91195A, Newport Corp.). Photocur-
rent–voltage characteristics of the DSSCs were recorded with
a potentiostat/galvanostat (CHI650B, CH Instruments, Inc.) at
a light intensity of 100 mW cmꢁ2 calibrated by an Oriel reference
solar cell (Oriel 91150, Newport Corp.). The monochromatic
quantum efficiency was recorded through a monochromator
(Oriel 74100, Newport Corp.) under short circuit conditions.
Electrochemical impedance spectra (EIS) were recorded for
DSSCs under illumination at open-circuit voltage (VOC) or dark
at ꢁ0.55 V potential at room temperature. The frequencies
explored ranged from 10 mHz to 100 kHz. The TiO2 nano-
particles and N719 were purchased from Solaronix S. A.,
Switzerland.
The syntheses of compounds R1–R4 follow the same proce-
dure as that of compound R5 using appropriate aldehydes.
Compound R1: Yield 15%. Anal. calcd for C44H44N4O4S2: C,
69.81; H, 5.86; N, 7.40. Found: C, 69.70; H, 5.84; N, 7.35%. MS
1
(FAB), m/z: 756.28 ([M+]). H NMR (300 MHz, CDCl3): d 8.86
(d, J ¼ 4.2 Hz, 1H), 8.42 (s, 1H), 7.96 (d, J ¼ 4.2 Hz, 1H), 7.60 (d,
J ¼ 8.7 Hz, 2H), 7.28 (t, J ¼ 7.8 Hz, 6H), 7.19–7.15 (m, 6H), 7.06
(t, J ¼ 7.4 Hz, 2H), 4.29 (t, J ¼ 7.1 Hz, 2H), 3.85 (t, J ¼ 6.6 Hz,
2H), 2.06–1.99 (m, 2H), 1.59–1.23 (m, 16H), 0.91–0.85 (m, 6H).
13C NMR (101 MHz, CDCl3): d 154.23, 152.93, 152.65, 150.44,
148.18, 147.71, 146.11, 142.02, 138.23, 138.09, 136.22, 131.94,
131.82, 129.59, 126.70, 126.56, 125.43, 125.39, 125.26, 123.65,
122.18, 115.99, 75.24, 74.89, 31.82, 30.35, 30.28, 29.92 25.89,
25.84, 22.87, 22.76, 14.29, 14.25. Compound R2: Yield 75%.
Anal. calcd for C55H56N4O4S2: C, 73.30; H, 6.26; N, 6.22.
Found: C, 73.12; H, 5.86; N, 6.22%. MS (FAB), m/z: 900.3
([M+]). 1H NMR (400 MHz, CDCl3): d 8.87 (d, J ¼ 4.0 Hz, 1H),
8.44 (s, 1H), 7.96 (d, J ¼ 4.0 Hz, 1H), 7.75 (d, J ¼ 8.0 Hz, 1H),
7.70 (s, 1H), 7.67 (d, J ¼ 8.0 Hz, 1H), 7.62 (d, J ¼ 8.0 Hz,
1H), 7.25 (t, J ¼ 8.0 Hz, 4H), 7.09 (m, 5H), 7.05 (d, J ¼ 8.0 Hz,
1H), 7.01 (t, J ¼ 7.2 Hz, 2H), 4.32 (t, J ¼ 6.8 Hz, 2H), 3.80 (t, J ¼
6.8 Hz, 2H), 2.11–1.90 (m, 7H), 1.57–1.46 (m, 5H), 1.37–1.33 (m,
5H), 1.21–1.13 (m, 7H), 0.91 (t, J ¼ 6.8 Hz, 3H), 0.82 (t, J ¼ 6.8
Hz, 3H), 0.48 (t, J ¼ 6.8 Hz, 6H). 13C NMR (101 MHz, CDCl3):
d 170.41, 154.21, 152.95, 152.80, 152.01, 150.34, 149.75, 148.21,
147.75, 146.33, 141.87, 138.34, 136.23, 131.87, 131.21, 130.06,
129.41, 127.39, 125.53, 124.17, 123.73, 122.80, 120.83, 119.42,
18.89, 116.11, 75.24, 74.86, 56.42, 32.81, 31.83, 31.75, 30.33,
25.89, 25.64, 22.87, 22.71, 14.25, 14.14, 8.91. Compound R3:
Yield 75%. Anal. calcd for C46H44N4O4S2: C, 70.74; H, 5.68; N,
7.17. Found: C, 70.54; H, 5.55; N, 7.07%. HR-MS (FAB), m/z:
2.2. Synthesis
1
The synthetic procedures and characterizations of compounds
115a, 2–8 and 915b,15c are described in the ESI†.
780.2810 ([M+], calcd for C46H44N4O4S2: 780.2804). H NMR
(400 MHz, CDCl3): d 8.83 (d, J ¼ 4.0 Hz, 1H), 8.39 (s, 1H), 7.91
(d, J ¼ 4.0 Hz, 1H), 7.47 (d, J ¼ 8.4 Hz, 1H), 7.28 (t, J ¼ 8.0 Hz,
4H), 7.11 (d, J ¼ 8.0 Hz, 4H), 7.07 (t, J ¼ 8.0 Hz, 2H), 7.01 (d,
J ¼ 8.4 Hz, 2H), 4.39 (t, J ¼ 7.8 Hz, 2H), 4.22 (t, J ¼ 7.8 Hz, 2H),
2.05–1.87 (m, 4H), 1.57–1.31 (m, 12H), 0.90–0.85 (m, 6H). 13C
NMR (75 MHz, CDCl3): d 167.77, 157.53, 153.30, 152.88,
149.64, 148.93, 147.99, 147.20, 145.66, 138.11, 136.56, 133.04,
132.16, 129.68, 125.48, 124.10, 121.97, 117.15, 115.98, 115.33,
109.04, 102.67, 81.83, 75.51, 75.15, 31.95, 31.79, 30.70, 30.24,
26.08, 25.83, 22.84, 14.24. Compound R4: 42%. Anal. calcd for
C48H46N4O4S3: C, 68.71; H, 5.53; N, 6.68. Found: C, 68.62; H,
5.49; N, 6.66%. HR-MS (MALDI), m/z: 839.2770 ([M]+, calcd
3-{5-[7-(5-{4-[Bis-(4-methoxy-phenyl)-amino]-phenyl}-thio-
phen-2-yl)-5,6-bis-hexyloxybenzo[1,2,5]thiadiazol-4-yl]-thiophen-
2-yl}-2-cyano-acrylic acid (R5). Compound 9 (0.18 g, 0.22
mmol) and 2-cyanoacetic acid (0.12 g, 1.41 mmol) were dissolved
in a mixture of MeCN (10 mL) and THF (5 mL) under nitrogen.
To the resulting solution were added three drops of piperidine
and then heated under reflux for 14 h. Then volatiles were
removed and the residue was taken up by CH2Cl2. The organic
layer was washed with water, dried over MgSO4, filtered and
pumped dry. Further purification of the crude product via
column chromatography using silica gel (EA/AcOH: 200/1 as the
eluent) gave R5 as a black powder (170 mg, 87%). Anal. calcd for
C50H50N4O6S3: C, 66.79; H, 5.60; N, 6.23. Found: C, 66.67; H,
1
for C48H46N4O4S3: 839.1146). H NMR (400 MHz, THF-d8):
d 8.97 (d, J ¼ 4.4 Hz, 1H), 8.72 (d, J ¼ 4.0 Hz, 1H), 8.47 (s, 1H),
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7.98 (d, J ¼ 4.4 Hz, 1H), 7.67 (d, J ¼ 8.4 Hz, 2H), 7.48 (d, J ¼ 4.0
Hz, 1H), 7.30 (t, J ¼ 7.8 Hz, 4H), 7.10–7.16 (m, 6H), 7.06 (t, J ¼
7.2 Hz, 2H), 4.21–4.34 (m, 4H), 2.03–2.24 (m, 4H), 1.39–1.59 (m,
12H), 0.91–0.97 (m, 6H). 13C NMR (101 MHz, THF-d8):
d 164.25, 154.58, 152.03, 151.88, 151.68, 148.90, 148.69, 147.61,
147.06, 144.71, 138.52, 137.98, 134.07, 133.50, 132.34, 130.32,
129.48, 127.55, 125.65, 124.54, 124.25, 123.30, 120.63, 116.97,
116.77, 100.29, 75.92, 75.46, 68.36, 32.91, 32.74, 31.53, 31.08,
30.51, 26.84, 26.76, 26.55, 23.73, 14.55.
characters, and avoid drawing conclusions from the excitation
energy.
3. Results and discussion
3.1. Synthesis of the materials
Five new benzothiadiazole-based dyes R1–R5 are shown in
Fig. 1. Dye S1 reported by us previously8a is also included for
comparison. The synthetic routes of these compounds exten-
sively utilize palladium-catalyzed Stille coupling20 and Suzuki
coupling,21 as depicted in Scheme 1. Compound 1 first underwent
palladium-catalyzed Stille coupling reactions with stannane-
functionalized 2-thiophenyl[1,3]dioxolane. Subsequent removal
of the acetal protecting group under acidic conditions yielded
aldehyde-containing benzothiadiazoles (2). Second Stille
coupling reactions of 2 with arylamino stannanes gave 7–9 as
dark red powders in good yields. Compounds 5–6 were obtained
via different approaches. Suzuki (compound 3, 37%) or Stille C–
C coupling (compound 4, 69%) of 1 with arylamines gave the
bromobenzothiadiazole intermediates 3–4. Further reactions
with aldehyde-containing thienyl derivatives using either Suzuki
(compound 5, 11%) or Stille (compound 6, 69%) coupling gave
the aldehydes 5–6 as orange to red powders. Further conversion
of 5–9 to dipolar dyes R1–R5 was achieved via conventional
Knoevenagel condensation22 with 2-cyanoacetic acid. These dyes
have good solubility in common organic solvents such as tetra-
hydrofuran, methylene chloride and acetonitrile.
2.3. Assembly and characterization of DSSCs
The photoanode used was the TiO2 thin film (12 mm of 20 nm
particles as the absorbing layer and 6 mm of 400 nm particles as
the scattering layer) coated on an FTO glass substrate16 with
a dimension of 0.5 ꢂ 0.5 cm2, and the film thickness measured by
a profilometer (Dektak3, Veeco/Sloan Instruments Inc., USA). A
platinized FTO produced by thermopyrolysis of H2PtCl6 was
used as a counter-electrode. The TiO2 thin film was dipped into
the THF solution containing 3 ꢂ 10ꢁ4 M dye sensitizers for at
least 12 h. After rinsing with THF, the photoanode adhered to
a polyester tape of 60 mm thickness and a square aperture of
0.36 cm2 was placed on top of the counter-electrode and tightly
clipping them together to form a cell. The electrolyte was then
injected into the space and the cell was sealed with Torr Sealꢀ
cement (Varian, MA, USA). The electrolyte was composed of
0.5 M lithium iodide (LiI), 0.05 M iodine (I2), and 0.5 M 4-tert-
butylpyridine that was dissolved in acetonitrile.
3.2. Physical properties
2.4. Quantum chemistry computation
Fig. 2 displays the absorption and emission spectra of the dyes in
THF, and the data are listed in Table 1. In contrast to S1 and R1
which have three prominent absorption bands, the other dyes
have two prominent absorption bands in the range of 300–
650 nm. The band with absorption maximum below 400 nm and
mixed with shoulders is attributed to more localized p–p*
transition. The one with absorption maximum above 454–
536 nm is due to more localized p–p* transition possessing
intramolecular charge transfer character. This is evidenced from
the computation results (vide infra) and comparison of R2 with 6,
in which the 2-cyanoacrylic acid entity of R2 is replaced by
a formyl group (Fig. 3). The absorption wavelength decreases as
The computations were performed with Q-Chem 4.0 software.17
Geometry optimization of the molecules was performed using
a hybrid B3LYP functional and 6-31G* basis set. For each
molecule, a number of possible conformations were examined
and the one with the lowest energy was used. The same functional
was also applied for the calculation of excited states using time-
dependent density functional theory (TD-DFT). There exist
a number of previous works that employed TD-DFT to char-
acterize excited states with charge-transfer character.18 In some
cases underestimation of the excitation energies was seen.19
Therefore, in the present work, we use TD-DFT to visualize the
extent of transition moments as well as their charge-transfer
the acceptor becomes weaker (strength: CHO
< CH]
Fig. 1 The structures of the dyes.
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Scheme 1 (a) (i) ‘‘Stannyl reagent’’, cat. Pd(PPh3)2Cl2, DMF, 90 ꢃC and (ii) 4 N HCl, r.t., (b) ‘‘stannyl reagent’’, cat. Pd(PPh3)2Cl2, DMF, 80 ꢃC, (c)
‘‘boronic reagent’’, cat. Pd(PPh3)4, 2 M aq. Na2CO3, EtOH, toluene, 85 ꢃC and (d) NCCH2CO2H, cat. piperidine, MeCN, 90 ꢃC.
C(CN)(COOH)). Compound R1 (466 nm, 3 ¼ 20 100 Mꢁ1 cmꢁ1
)
and its neighboring aryl groups (vide infra), which results in
twisting between them and jeopardizes charge transfer from the
donor to the acceptor. R4 exhibits hypsochromic and hypo-
chromic shifts of the absorption compared with its congener
without the two alkoxy chains,23 which can also be rationalized
by the same reason. Compared to R1, compound R2 absorbs at
a shorter wavelength but with higher extinction coefficient
(454 nm, 3 ¼ 39 600 Mꢁ1 cmꢁ1). This is consistent with our
previous observation on fluorene-based sensitizers.7d Insertion of
an ethynyl entity bridging between benzothiadiazole and phenyl
groups of R1 helps to alleviate the steric congestion and elongate
the conjugation length of the molecule. Consequently, R3
(478 nm, 3 ¼ 37 400 Mꢁ1 cmꢁ1) has significant bathochromic and
hyperchromic shifts in absorption compared to R1, despite the
poorer orbital overlap of energetically mismatched sp and sp2
orbitals.24 Compound R4 (501 nm) has absorption wavelength
longer than that of R2 (454 nm) by ꢀ50 nm. This can be ratio-
nalized by two reasons: (1) compared to a phenyl ring, there is
much less steric congestion between a thiophene ring and the
neighboring 5,6-bishexyloxy-benzo[2,1,3]thiadiazole entity (see
Computation section, vide infra), which provides more effective
conjugation and (2) the smaller resonance energy of the thio-
phene moiety compared with benzenoid moieties facilitates
charge transfer from the donor to the acceptor.25 The presence of
two methoxy groups renders the arylamino entity in R5
a stronger donor than that in R4. Consequently, R5 (536 nm)
absorbs at a longer wavelength than R4 because of the stronger
charge transfer character in the former. Compound R5 also has
the longest absorption wavelength among all with onset
absorption extending to ꢀ650 nm. The absorption maximum on
the TiO2 surface (Table 1) of the dyes follows the same trend as
that measured in solution: R5 (518 nm) > R4 (489 nm) > S1
(480 nm) > R3 (460 nm) > R1 (455 nm) > R2 (441 nm). However,
the spectrum of each compound is blue shifted compared to the
exhibits both hypsochromic and hypochromic shifts in absorp-
tion compared with its hexyloxy-free congener, S1 (491 nm, 3 ¼
27 500 Mꢁ1 cmꢁ1).8a Apparently, the presence of the two alkoxy
chains results in steric congestion between the benzothiadiazole
Fig. 2 Absorption and emission spectra of the dyes recorded in THF.
10932 | J. Mater. Chem., 2012, 22, 10929–10938
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Table 1 Electrochemical parameters of the dyesa
lem
(nm)
l
(nm)
nst
(cmꢁ1
TiO2
Dye
labs (3 ꢂ 10ꢁ4 Mꢁ1 cmꢁ1
)
)
E1/2 (DEp)b (mV)
HOMO/LUMO (eV)
E0–0c (eV)
E0–0*c (V)
R1
R2
R3
R4
466 (2.01), 408 (1.37), 307 (2.26)
454 (3.96), 345 (4.60), 310 sh (3.70)
478 (3.74), 344 (3.63), 2.98 sh (2.52)
501 (2.55), 371 (2.76), 346 sh (2.40),
305 (1.88)
619
614
577
631
455
441
460
489
5304
5739
3590
4112
606 (169), ꢁ1379 (i)
493 (104), ꢁ1470 (i)
627 (191), ꢁ1434 (i)
458 (126), ꢁ1368 (i)
ꢁ5.71/ꢁ3.42
ꢁ5.59/ꢁ3.26
ꢁ5.73/ꢁ3.42
ꢁ5.56/ꢁ3.39
2.29
2.33
2.31
2.17
ꢁ0.98
ꢁ1.14
ꢁ0.98
ꢁ1.01
R5
S1
536 (3.07), 422 sh (1.63), 352 (2.60)
491 (2.75), 388 (1.63), 309 (2.84)
NA
650
518
480
NA
1533
282 (127), ꢁ1345 (110)
ꢁ5.38/ꢁ3.40
1.98
2.12
ꢁ1.00
583 (143), ꢁ1210 (i)
ꢁ5.68/ꢁ3.56
ꢁ0.84
a
All experiments were conducted in THF. labs: absorption maximum; 3: extinction coefficient; lem: absorption wavelength; lTiO : thin film absorption on
2
TiO2; nst: Stokes shift. b Scan rate, 100 mV sꢁ1; electrolyte, [(n-Bu)4N]PF6; DEp is the separation between the anodic and cathodic peaks; i: irreversible.
Potentials are quoted with reference to the internal ferrocene standard. The HOMO and LUMO energies are calculated using the formula HOMO ¼ 5.1
+ (E1/2 ꢁ EFc) and LUMO ¼ HOMO ꢁ Egap, where 5.1 refers to the energy level of ferrocene in vacuo. c The optical bandgap (E0–0) was derived from the
intersection of the absorption and emission spectra; the bandgap E0–0*: the excited state oxidation potential vs. NHE.
Fig. 3 Absorption spectra of 6 and R2 recorded in THF.
spectrum in the solution. This can be attributed to the deproto-
nation of the carboxylic acid,13b which was also confirmed by
Fig. 4 Cyclic voltammograms of the compounds recorded in THF.
measuring the absorption of the dyes in the presence of Et3N.
Except for R5, the new dyes emit in the range between 577 and
631 nm and the emission spectra are shown in Fig. 2b. Large
Stokes shift (3590–5739 cmꢁ1) observed in these compounds can
the electron-deficient benzothiadiazole moiety will deplete the
electron density of the arylamine and therefore increase the
oxidation potential of the arylamine. The higher oxidation
potential of R1 than S1 is attributed to the electron-withdrawing
influence of the hexyloxy chains (vide infra), which further
decreases the electron density of the benzothiadiazole moiety.
The effect of the benzothiadiazole moiety on the arylamine is
alleviated as the spatial distance between the arylamine and the
benzothiadiazole moiety increases, i.e., R1 vs. R2. The more
planar conformation of R3 apparently increases the electronic
communication between the arylamine and the benzothiadiazole
moiety. Consequently, R3 has the highest oxidation potential.
The first oxidation potential was used to calculate the HOMO
energy level referenced to ferrocene at ꢁ5.1 eV.26 This potential
and the optical band gap were used to obtain the LUMO energy
level. Except for R5, irreversible reduction waves were observed
for all the dyes. The excited state potentials (E0–0*, see Table 1) of
the dyes were estimated from the first oxidation potential and the
zero–zero excitation energy, E0–0. These values (ꢁ0.84 to
ꢁ1.14 V vs. NHE) are more negative than the conduction band
edge of the TiO2 electrode (ꢁ0.5 V vs. NHE),1a which ensures
favorable electron injection from the excited dye to TiO2. Dye
be attributed to the charge transfer nature of the excited state.
More planar conformation of the lowest emission excited state
may also contribute to the Stokes shift. For example,
compounds with a larger twisted conformation at the ground
state (vide infra), R1 and R2, exhibit larger Stokes shift. In
comparison, the least sterically congested R3 has the smallest
Stokes shift among all.
3.3. Electrochemical properties
The electrochemical properties of these dyes were examined by
the cyclic voltammetric method. The corresponding electro-
chemical data are compiled in Table 1. Fig. 4 and S1† show the
cyclic and differential pulse voltammograms of the dyes,
respectively. The first quasi-reversible oxidation wave is attrib-
uted to the oxidation of the arylamino group, and this potential
decreases in the order of R3 (627 mV) > R1 (606 mV) > S1
(583 mV) > R2 (493 mV) > R4 (458 mV) > R5 (282 mV). R5 has
the lowest oxidation potential because the oxidized arylamine
can be stabilized by the two electron-releasing methoxy groups as
well as the electron excessive thiophene ring nearby. In contrast,
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Table 2 Performance parameters of DSSCs fabricated with the dyesa
b
Rrec (U)
b
Rct (U)
Dye
VOC (V)
JSC (mA cmꢁ2
)
h (%)
FF
Adsorption (mol cmꢁ2
)
R1
R2
R3
R4
R5
S1
N719
0.68
0.68
0.70
0.67
0.67
0.58
0.73
12.0
11.3
12.7
15.2
11.9
13.5
16.1
5.57
5.34
6.22
6.72
5.36
5.01
7.41
0.68
0.69
0.70
0.66
0.68
0.64
0.64
2.64 ꢂ 10ꢁ7
1.17 ꢂ 10ꢁ7
1.18 ꢂ 10ꢁ7
2.75 ꢂ 10ꢁ7
4.16 ꢂ 10ꢁ7
3.71 ꢂ 10ꢁ7
350
326
386
285
335
60
23.6
20.1
17.1
16.6
25.8
16.7
16.1
560
a
Experiments were conducted using TiO2 photoelectrodes with approximately 18 mm thickness and 0.25 cm2 working area on the FTO (15 U sqꢁ1
substrates. Charge recombination resistances (Rrec) and charge transfer resistances (Rct) were obtained each from the radius of the intermediate
frequency semicircle in Fig. 8a and b, respectively.
)
b
regeneration is also ensured due to the more positive oxidation
potential of the dyes (1.33–0.98 V vs. NHE) than that of the Iꢁ/
ꢁ
I3 redox couple (ꢀ0.4 V vs. NHE).1a
3.4. Device performances
The devices fabricated with the dyes R1–R5 and S1 were tested
for solar cell applications. The performance parameters are
shown in Table 2. Fig. 5 and 6 display the photovoltage–current
curves and photon-to-current conversion efficiencies (IPCEs) of
the cells, respectively. Compared to congeners without hexyloxy
chains at the benzothiadiazole moiety,8a,d,23 DSSCs based on R1–
R5 exhibited significantly higher VOC values (0.67–0.70 V). For
Fig. 6 IPCE plots for the DSSCs.
example, the VOC value of an R1-based cell is higher than that of
an S1-based cell (0.58 V). This may be attributed to the
suppression of dark current (vide infra, Fig. 7). Long hydro-
carbon chains tethered at the conjugated spacer between the
donor and the acceptor were suggested to blockade close contact
of electrolytes with the TiO2 surface, thus leading to smaller dark
current.8b,27 Our observations are similar to Ko’s report on the
benzothiadiazole chromophore.8b Ko reported that incorpora-
tion of the hydrocarbon chain at the inward side of the two
thiophene rings linked to the central benzothiadiazole signifi-
cantly increased the VOC value due to suppression of dark
current. In our case, the two hexyloxy chains are tethered at the
benzothiadiazole moiety. It is important to note that benzo-
thiadiazole dyes8c having long hydrocarbon chains tethered at
the outward side of the two thiophene rings linked to the central
Fig. 7 J–V curves of DSSCs in the dark.
benzothiadiazole have VOC values below 0.56 V. The
hydrocarbon chains tethered at the phenyl rings of the arylamine
moiety cannot effectively suppress the dark current, either.23
Very high concentration of deoxycholic acid (DCA) was added
as the co-adsorbent (0.3 mM and 80 mM dye) for optimization of
the cell performance. In our case, the performance of DSSCs
based on R4 with added co-adsorbent (chenodeoxycholic acid,
CDCA) is inferior to that of DSSC without CDCA (see Fig. S2
and Table S1†). As more CDCA is added, the decreased dye
density on the TiO2 surface will lead to lower current density and
consequently lower cell efficiency.
The DSSCs of R1–R3 show higher conversion efficiencies than
the standard device N719 in the region ca. 350–500 nm. The
highest conversion efficiency reaches ꢀ80% at 475 nm in the case
Fig. 5 J–V curves of DSSCs based on the dyes.
10934 | J. Mater. Chem., 2012, 22, 10929–10938
of R3. The device R4 shows impressively broadened IPCE
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spectra, nearly the same spectral coverage as N719 (onset at
ꢀ750 nm), and flattened conversion efficiency of 60–70% in the
region 400–650 nm. The best result of device R4 has a JSC of
15.20 mA cmꢁ2 (same as N719), a VOC of 0.72 V and an overall
efficiency of 6.72%. The conversion efficiency reaches 92% of
standard N719 device (7.41%) and surpasses those of benzo-
thiadiazole-containing congeners such as JK-69 (ref. 8b) by Ko
and BzTCA8c by Pei. Though a high efficiency of 7.62% was
reported for the benzothiadiazole-containing cyanine dye,10 the
conversion efficiency of DSSC only reached ꢀ80% of the N719
cell fabricated under the same condition. Despite the long
absorption wavelength (590 nm) of the cyanine dye, its DSSC has
a lower VOC (0.54 V) than ours. There is ꢀ20% drop in the JSC
value from R4 (15.2 mA cmꢁ2) to R5 (11.9 mA cmꢁ2), though the
two cells have a similar VOC value of 0.67 V. We speculate that
the high lying HOMO of R5 results in less efficient regeneration
of the oxidized dye after electron injection due to the diminished
gap between the HOMO and the energy level of the redox couple.
The dark currents of DSSCs are shown in Fig. 7. The cell of S1
has significantly higher dark currents than others, supporting our
earlier observations that the introduction of alkoxy chains in R1–
R5 blockades the close contact of the electrolytes with the TiO2
surface. There is no obvious difference in the dark currents
between the cells of R1–R5 and that of N719. Electrochemical
impedance was measured under dark and illumination condi-
tions.28 The Nyquist plots of DSSCs with different dyes in the
dark and under a forward bias of 55 mV are shown in Fig. 8a. The
charge recombination resistance (Rrec) at the TiO2 surface
deduced by fitting curves from the range of intermediate
frequency (1 to 102 Hz) using Z-View software decreases in the
order of N719 (560 U) > R3 (386 U) > R1 (350 U) > R5 (335 U) >
R2 (326 U) > R4 (285 U) > S1 (60 U). Accordingly, the charge
recombination rate increases in the order of N719 < R3 < R1 < R5
< R2 < R4 < S1. This is to some extent consistent with the order of
decreasing VOC: N719 (0.73 V) > R3 (0.70 V) > R1 (0.68 V) z R2
(0.68 V) > R5 (0.67 V) z R4 (0.67 V) > S1 (0.58 V). The signifi-
cantly smaller charge recombination resistance in S1 than in Rn
(n ¼ 1–5) further substantiates the importance of dark current
suppression upon incorporation of the alkoxy chains in this series
of dyes. In order to have a fair comparison of the conversion
efficiencies among different dyes, the dye adsorption densities (in
units of 10ꢁ7 mol cmꢁ2) on TiO2 were also checked: R5 (4.16) > S1
(3.71) > R4 (2.75) > R1 (2.64) > R3 (1.18) z R2 (1.17) (Table 2). It
is worth noting that R1 has higher conversion efficiency than S1 in
spite of its inferior light harvesting efficiency and the lower dye
adsorption density. In comparison, the DSSC of R4 has both
higher VOC and JSC values than that of S1 because of its more
efficient dark current suppression and light harvesting. Informa-
tion on electron generation and transport inside TiO2 was
obtained from electrochemical impedance measured under illu-
mination. Upon illumination of 100 mW cmꢁ2 under open circuit
conditions, the radius of the intermediate frequency semicircle (1–
20 Hz) in the Nyquist plot (Fig. 8b) and hence electron transport
resistance decrease in the order of R5 (25.8 U) > R1 (23.6 U) > R2
(20.1 U) > R3 (17.1 U) > S1 (16.7 U) > R4 (16.6 U) > N719
(16.1 U). The lower electron transport resistance will assist elec-
tron collection and improve the cell efficiency. It is interesting to
note that the dye without the alkoxy chains (S1) has the smallest
electron transport resistance.
3.5. Theoretical approach
Density functional calculations performed at the B3LYP/6-31G*
level of theory have been carried out in order to correlate the
structures of the sensitizers with their cell performance. Fig. 9
shows the ground-state geometries of the dyes with dihedral
Fig. 8 Electrochemical impedance spectra (Nyquist plots) of DSSC for
Fig. 9 Dihedral angles between the neighboring units or between the
dyes measured (a) in the dark and (b) under illumination.
aromatic ring and 2-cyanoacrylic acid.
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angles between two neighboring conjugated segments indicated.
There is significant twist of the 5,6-bis-hexyloxybenzo[2,1,3]
thiadiazole and benzo[2,1,3]thiadiazole entities from co-
planarity with the phenyl (or fluorenyl) entity, and the dihedral
angle of the former (R1 or R2) is ꢀ10ꢃ larger than that of the
latter (S1). In comparison, the dihedral angle between the thienyl
entity with 5,6-bis-hexyloxybenzo[2,1,3]thiadiazole or benzo
[2,1,3]thiadiazole entity is smaller than 15.1ꢃ. As expected, the
dihedral angle between the 5,6-bis-hexyloxy-benzo[2,1,3]thia-
diazole and the ethynyl-attached phenylene entity is 6.1ꢃ only.
Fig. S3 and S4 (see ESI†) show the frontier orbitals of the dyes
and their corresponding energy states, respectively. The trend of
the calculated HOMO energies, R5 (ꢁ4.69 eV) > R2 (ꢁ4.91 eV)
> R4 (ꢁ4.96 eV) > S1 (ꢁ5.04 eV) z R1 (ꢁ5.04 eV) z R3
(ꢁ5.04 eV), is roughly consistent with that obtained from the
electrochemical studies. The slight inconformity may stem from
gas-phase calculation which excludes the possible influence of the
interaction between tethering hexyloxy chains and solvent. As
expected, compound R5 possessing two electron donating
methoxy groups has the highest HOMO level among all. Though
one of the hexyloxy chains in the new dyes has contribution to
the HOMO, the other hexyloxy chain appears to have greater
contribution to the LUMO (vide infra) in an anti-bonding
fashion. The overall effect of the hexyloxy chain can thus be
considered as resonance electron acceptor. Deviation of the
benzothiadiazole moiety from planarity with its neighboring
aromatic rings will further reduce the electronic communication
between the benzothiadiazole moiety and arylamine. As a result,
R1 (dihedral angle: 42.2ꢃ) has the same HOMO level as S1,
possibly because of the larger dihedral angle (42.2ꢃ) of R1
compared with S1 (34.0ꢃ). R1 and R2 have similar dihedral
angles (R1: 42.2ꢃ; R2: 45.1ꢃ), and the higher HOMO energy level
of R2 is attributed to the diminished influence of the benzo-
thiadiazole moiety on arylamine due to the longer spacer (fluo-
rene) between the two. The thiophene ring between the
benzothiadiazole moiety and arylamine appears to raise the
HOMO energy levels of R4 and R5 above that of R1. With nearly
planar conformation between the benzothiadiazole moiety
and arylamine in R3, the benzothiadiazole moiety is expected to
have stronger influence on arylamine and lead to a low HOMO
energy level.
Fig. 10 Differences in the Mulliken charge of the dyes upon vertical
excitation.
changes in Mulliken charges of the dyes for the S0 / S1 and S0
/ S2 transitions. The hexyloxy-substituted benzothiadiazole of
R1–R5 appears to be a negative charge collector, as revealed by
pronounced accumulation of the Mulliken charge (Table S2†)
upon photo-excitation to S1 (BtzO6: ꢁ0.45 to ꢁ0.54) and S2
(BtzO6: ꢁ0.34 to ꢁ0.40) states, while the electron-donating
arylamine carries most of the positive charges, both in S1 (Am:
0.40 to 0.57) and S2 (Am: 0.25 to 0.44) states. In comparison, the
Mulliken charges of S1 are as the following: ꢁ0.52, Btz (S1);
ꢁ0.32, Btz (S2); 0.55, Am (S1); and 0.43, Am (S2). Our previous
results indicated that incorporation of electron-deficient entities
in the conjugated spacers may lead to electron trapping and
jeopardize the electron injection unless the electron-deficient
moiety was placed at appropriate positions.7d The product of the
Mulliken charge and the oscillator strength (i.e., taking the
absorption coefficient into consideration, f ꢂ D|e|) was checked
for all dyes, and we found no solid evidence that the benzo-
thiadiazole moiety of S1 accumulated more negative charge
densities when compared to that of R1–R5. The hexyloxy chains
turn out to accumulate some negative charge densities based on
the Mulliken charge analysis of S0 / S1 transitions (see the O6
fragment in Fig. 10 and Table S2†). Therefore, the hexyloxy
chains act as excited state resonance acceptors. Though electron-
trapping was not eased even after incorporation of the hexyloxy
chains at the benzothiadiazole moiety, significant increment in
open-circuit voltage (ꢀ0.1 V or more compared to the dye with
non-modified benzothiadiazole) has been achieved. Moreover,
both high open-circuit voltage and high short-circuit current are
possible (R4) as the spacer is appropriately designed.
TDDFT calculations of the dyes at the same level were also
performed (see Experimental section), and the results are listed in
Table S2 (see ESI†). Among all dyes, the lowest-energy excita-
tions to the S1 state mainly originate from coupling between
HOMO and LUMO orbitals (Fig. S3†). The HOMO mainly
consists of arylamine, extending towards the acceptor side in
different degrees, depending on the spacer. The differences
between electron density distribution of their HOMO and
LUMO orbitals strongly indicate the prominent charge-transfer
character. The oscillator strength (f ¼ 0.31–0.88) of this series of
dyes also echoes the measured absorption coefficients. The
changes in Mulliken charges of the dyes for the S0 / S1, S0 / S2
and S0 / S3 transitions were grouped into several segments,
diphenylamine (Am), bis(4-methoxyphenyl)amine (Am’), fluo-
rene (Flu), phenylene (Ph), ethyne (y), benzothiadiazole (Btz) or
5,6-bis-hexyloxy-benzo[2,1,3]thiadiazole (BtzO6), thiophene (T)
and 2-cyanoacrylic acid (Ac), to estimate the extent of charge
separation upon excitation (Table S2†). Fig. 10 displays the
4. Conclusions
In summary, new metal-free dyes with a 5,6-bis-hexyloxy-benzo
[2,1,3]thiadiazole unit in the conjugated spacer have been
synthesized and tested for dye-sensitized solar cell applications.
Though there is significant charge transfer from arylamine to the
acceptor, 2-cyanoacrylic acid, partial charge accumulation was
also observed at both benzo[2,1,3]thiadiazole and hexyloxy units.
Consequently, these dyes exhibit good light-harvesting proper-
ties in the UV-Vis region. Compared to the congeners without
two hexyloxy chains at the benzo[2,1,3]thiadiazole moiety, these
sensitizers exhibited much higher open-circuit voltages due to
effective suppression of the dark currents. High conversion
10936 | J. Mater. Chem., 2012, 22, 10929–10938
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Commun., 2011, 47, 2874; (e) Y. Shi, R. B. M. Hill, J.-H. Yum,
€
M. K. Nazeeruddin, Angew. Chem., Int. Ed., 2011, 50, 6619; (f)
T. Maeda, Y. Hamamura, K. Miyagana, N. Shima, S. Yagi and
H. Nakazumi, Org. Lett., 2011, 13, 5994.
efficiencies have been achieved with these dyes, and the best
performance of the cells reaches >90% of the N719-based cell
fabricated and measured under similar conditions.
A. Dualeh, S. Barlow, M. Gratzel, S. R. Marder and
13 (a) M.-S. Tsai, Y.-C. Hsu, J. T. Lin, H.-C. Chen and C.-P. Hsu,
J. Phys. Chem. C, 2007, 111, 18785; (b) K. R. Justin Thomas,
Y.-C. Hsu, J. T. Lin, K.-M. Lee, K.-C. Ho, C.-H. Lai,
Y.-M. Cheng and P.-T. Chou, Chem. Mater., 2008, 20, 1830; (c)
Y.-S. Yen, Y.-C. Hsu, J. T. Lin, C.-W. Chang, C.-P. Hsu and
D.-J. Yin, J. Phys. Chem. C, 2008, 112, 12557; (d) J. T. Lin,
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Org. Lett., 2008, 11, 97; (e) M. Velusamy, Y.-C. Hsu, J. T. Lin,
C.-W. Chang and C.-P. Hsu, Chem.–Asian J., 2010, 5, 87; (f)
C.-H. Chen, Y.-C. Hsu, H.-H. Chou, K. R. Justin Thomas,
J. T. Lin and C.-P. Hsu, Chem.–Eur. J., 2010, 16, 3184; (g)
H.-H. Chou, C.-Y. Hsu, Y.-C. Hsu, Y.-S. Lin, J. T. Lin and
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Acknowledgements
We thank Prof. C.-P. Hsu in Institute of Chemistry, Academia
Sinica for her helpful discussions. We acknowledge the support
of the Academia Sinica (AC) and NSC (Taiwan), AUO
Optronics Corp. and the Instrumental Center of Institute of
Chemistry (AC).
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