Development of D–π–A dye with (pyridiniumyl)alkanesulfonate as electron-withdrawing anchoring group for dye-sensitized solar cell

Accepted Manuscript Development of D-π-A dye with (pyridiniumyl)alkanesulfonate as electron-withdrawing anchoring group for dye-sensitized solar cell Yousuke Ooyama, Ph. D., Takafumi Sato, Toshiaki Enoki, Joji Ohshita PII:

S0143-7208(15)00322-8

DOI:

10.1016/j.dyepig.2015.08.012

Reference:

DYPI 4894

To appear in:

Dyes and Pigments

Received Date: 12 June 2015 Revised Date:

30 July 2015

Accepted Date: 7 August 2015

Please cite this article as: Ooyama Y, Sato T, Enoki T, Ohshita J, Development of D-π-A dye with (pyridiniumyl)alkanesulfonate as electron-withdrawing anchoring group for dye-sensitized solar cell, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.08.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical abstract

50 SAT-1 SAT-2

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30 20 10 0 380

480

580

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IPCE / %

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680

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Wavelength / nm

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ACCEPTED MANUSCRIPT Development of D-π-A dye with (pyridiniumyl)alkanesulfonate as electron-withdrawing anchoring group for dye-sensitized solar cell

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University,

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a

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Yousuke Ooyama,*a Takafumi Sato,a Toshiaki Enokia and Joji Ohshita*a

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Higashi-Hiroshima 739-8527, Japan.

Author to whom correspondence should be addressed: Yousuke Ooyama, Ph. D.

Fax: (+81) 82-424-5494;

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Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University

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E-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract A new D–π–A dye with a (benzo[4,5]thieno[2,3-c]pyridiniumyl)propanesulfonate as an electron-withdrawing anchoring group has been designed and developed as a photosensitizer

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for dye-sensitized solar cells. It was found that the new dye is adsorbed onto the TiO2 electrode through the formation of a bidentate bridging linkage between the sulfonate group

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of the dye and Lewis acid site on the TiO2 surface. The short-circuit photocurrent density of

sensitizer

which

contained

a

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the fabricated solar cell based on the new dye is higher than that of a previous D–π–A dye benzo[4,5]thieno[2,3-c]pyridine

moiety

as

an

electron-withdrawing anchoring group, but the open circuit photovoltage of the solar cell based on the new dye is lower than that measured for the previous analogue. On the basis of

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molecular orbital calculations and electrochemical impedance spectroscopy analysis, the photovoltaic performance of DSSC based on the new D–π–A dye with the

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(pyridiniumyl)propanesulfonate is discussed by taking account of the adsorption states of the

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dye adsorbed on TiO2 electrode.

Keywords: dye-sensitized solar cells; D–π–A dye sensitizer; electron-withdrawing anchoring group; pyridinium group; Brønsted acid site

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1. Introduction Dye-sensitized solar cells (DSSCs) using dye-adsorbed TiO2 electrodes have attracted much

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attention as a new generation of sustainable photovoltaic devices from chemists, physicists, and engineers because of the scientific interest in their construction and operational principles,

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and low cost of production. [1] During the last decade, much effort have been made on the

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development of efficient dye sensitizers to improve the photovoltaic performances of DSSCs.[2-28] In particular, many kinds of donor-acceptor π-conjugated (D–π–A) dyes with a carboxyl group as electron-withdrawing anchoring group have been designed and developed as one of the most promising classes of organic dye sensitizers because of their strong

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photoabsorption properties originating from the intramolecular charge transfer (ICT) excitation from the donor to acceptor moiety in the D–π–A structures.[4-28] The conventional

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D–π–A dye sensitizers with carboxyl group are adsorbed on the TiO2 electrode through the

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bidentate bridging linkage between the carboxyl group of the dye and Brønsted acid sites (surface-bound hydroxyl groups, Ti–OH) on the TiO2 surface. Recently, on the other hand, many researchers have focused on searching new electron-withdrawing anchoring groups as an alternative to the conventional carboxyl groups.[29] Consequently, new types of D–π–A dye

sensitizers

with

a

nitro

group,[30]

aldehyde,[31,

32]

2-(1,1-dicyanomethylene)rhodanine,[33] pyridine,[34-46] or 8-hydroxylquinoline,[47] as an 3

ACCEPTED MANUSCRIPT electron-withdrawing anchoring group have been developed. Among them, we have demonstrated that the D–π–A dye sensitizers with a pyridyl group are predominantly adsorbed on the TiO2 electrode through the coordinate bonding between the pyridyl group of the dye

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and Lewis acid site (exposed Tin+ cations) on the TiO2 surface.[34-39] More recently, we have developed the D–π–A dye sensitizer with (pyridiniumyl)alkanesulfonate, which exhibits

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the photoabsorption band in a longer wavelength region than the corresponding D–π–A dye

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sensitizers with pyridyl groups.[48]

In this work, to ensure the usefulness of the (pyridiniumyl)alkanesulfonate as a stronger electron-withdrawing anchoring group leading to the bathochromic shift of the photoabsorption band, we have designed and developed the D–π–A dye sensitizer SAT-2

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with a (benzo[4,5]thieno[2,3-c]pyridiniumyl)propanesulfonate moiety (Scheme 1). Its optical and electrochemical properties, adsorption states on TiO2 nanoparticles, and photovoltaic

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performance in a DSSC of SAT-2 were investigated. It was found that the dye SAT-2 is

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adsorbed onto the TiO2 electrode through the formation of the bidentate bridging linkage between the sulfonate group of the dye and Lewis acid site on the TiO2 surface. The short-circuit photocurrent density (Jsc) of DSSC based on the dye SAT-2 is higher than that of the D–π–A dye sensitizer SAT-1[39] with a benzo[4,5]thieno[2,3-c]pyridine as an electron-withdrawing anchoring group, because the light-harvesting efficiency (LHE) of SAT-2 in the long-wavelength region is higher than that of SAT-1. On the other hand, the 4

ACCEPTED MANUSCRIPT open-circuit photovoltage (Voc) of DSSC based on the dye SAT-2 is lower than that of SAT-1. On the basis of molecular orbital calculations and electrochemical impedance spectroscopy (EIS) analysis, the photovoltaic performance of DSSC based on the D–π–A dye with the

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(pyridiniumyl)propanesulfonate is discussed by taking account of the adsorption states of the

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dye adsorbed on TiO2 electrode.

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2. Experimental section

Melting points were measured with a Yanaco micro melting point apparatus MP model. IR spectra were recorded on a Perkin Elmer Spectrum One FT-IR spectrometer by ATR method. High-resolution mass spectral data were acquired on a Thermo Fisher Scientific LTQ Orbitrap

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XL. 1H NMR spectra were recorded on a Varian-400 (400 MHz) FT NMR spectrometer with tetramethylsilane as an internal standard. Absorption spectra were observed with a Shimadzu

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UV-3150 spectrophotometer and fluorescence spectra were measured with a HORIBA

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FluoroMax-4 spectrofluorometer. Cyclic voltammetry (CV) curves were recorded in aceonitrile/Bu4NClO4 (0.1M) solution for SAT-1 and DMF/Bu4NClO4 (0.1M) solution for SAT-2, respectively, with a three-electrode system consisting of Ag/Ag+ as reference electrode, Pt plate as working electrode, and Pt wire as counter electrode by using a AMETEK Versa STAT 4 potentiostat. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of SAT-1 and SAT-2 were 5

ACCEPTED MANUSCRIPT evaluated from the spectral analyses and the CV data. The HOMO energy level was evaluated from the E1/2ox. The LUMO energy level was estimated from the E1/2ox and an intersection of absorption and fluorescence spectra, which correspond to the energy gap between the HOMO

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and the LUMO. Electrochemical impedance spectroscopy (EIS) for DSSCs in the dark under a forward bias of −0.60 V with a frequency range of 10 mHz to 100 kHz was measured with a

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AMETEK Versa STAT 3.

2.1. Synthesis

3-(7-(5-(4-(Bis(4-ethoxyphenyl)amino)phenyl)thiophen-2-yl)benzo[4,5]thieno[2,3-c]pyridin-2 -ium-2-yl)propane-1-sulfonate, SAT-2: A solution of SAT-1[39] (0.05 g, 0.083 mmol) and

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1,3-propane sultone (0.015 g, 0.12 mmol) in DMF (5 mL) was stirred at 90 ˚C for 15 days under an argon atmosphere. After concentrating under reduced pressure, the resulting residue

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was washed by ethyl acetate (3 × 20 mL) to give SAT-2 (0.037 g, yield 62 %) as a dark red

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solid; m.p. 250–252 °C; IR (ATR): ν˜ = 1590, 1503, 1194, 1030 cm-1; 1H NMR (400 MHz, [D6]DMSO) δ = 1.33 (t, J = 6.8 Hz, 6H), 2.32 (t, J = 7.2 Hz, 2H), 3.2–3.4 (m, 2H, overlapping peak of dissolved water in DMSO-d6), 3.99–4.04 (m, 4H), 4.82 (t, J = 6.4 Hz, 2H), 6.79 (d, J = 8.8 Hz, 2H), 6.93 (d, J = 8.8 Hz, 4H), 7.07 (d, J = 8.8 Hz, 4H), 7.47 (d, J = 4.0 Hz, 1H), 7.55 (d, J = 8.8 Hz, 2H), 7.84 (d, J = 4.0 Hz, 1H), 8.08 (d, J = 8.4 Hz, 1H), 8.67 (s, 1H), 8.76 (d, J = 8.4 Hz, 1H), 8.99 (d, J = 6.4 Hz, 1H), 9.04 (d, J = 6.4 Hz, 1H), 9.85 (s, 1H) ppm. 6

ACCEPTED MANUSCRIPT HRMS (ESI): m/z (%): calcd for C40H36N2O5S3Na [M+Na+] 743.16786; found 743.16876.

2.2. Computational methods

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The semi-empirical calculation was carried out with the WinMOPAC Ver. 3.9 package (Fujitsu, Chiba, Japan). Geometry calculation in the ground state was made using the AM1

routine

(keyword

EF).

INDO/S

(intermediate

neglect

of

differential

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following

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method. The geometry was completely optimized (keyword PRECISE) by the eigenvector

overlap/spectroscopic) calculation was performed using single excitation full SCF/CI (self-consistent field/configuration interaction), which included the configuration with one electron excited from any occupied orbital to any unoccupied orbital, where 225

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configurations were considered [keyword CI (15 15)].

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2.3. Preparation of DSSCs

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The TiO2 paste (JGC Catalysts and Chemicals Ltd., PST-18NR) was deposited on a fluorine-doped-tin-oxide (FTO) substrate by doctor-blading, and sintered for 50 min at 450 ºC. The 9 µm thick TiO2 electrode was immersed into 1.0 mM dye (SAT-1) solution in THF or 0.1 mM dye (SAT-2) solution in chloroform for 15 hours enough to adsorb the dye sensitizers. The DSSCs were fabricated by using the TiO2 electrode (0.5×0.5 cm2 in photoactive area) thus prepared, Pt-coated glass as a counter electrode, and a solution of 0.05 M iodine, 0.1 M 7

ACCEPTED MANUSCRIPT lithium iodide, and 0.6 M 1,2-dimethyl-3-propylimidazolium iodide in acetonitrile as electrolyte. The photocurrent-voltage characteristics were measured using a potentiostat under a simulated solar light (AM 1.5, 100 mW cm-2). IPCE spectra were measured under

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monochromatic irradiation with a tungsten-halogen lamp and a monochromator. Absorption spectra of the dyes adsorbed on TiO2 nanoparticles were recorded on the dyes-adsorbed TiO2

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film in the transmission mode with a calibrated integrating sphere system.

3. Results and discussion 3.1. Synthesis

D–π–A dye SAT-2 with (benzo[4,5]thieno[2,3-c]pyridiniumyl)propanesulfonate was

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3.2. Optical properties

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synthesized from D–π–A dye SAT-1[39] and 1,3-propan sultone (Scheme 2).

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The photoabsorption and fluorescence spectra of SAT-1 and SAT-2 in DMSO are shown in Fig. 1a. The dye SAT-2 in DMSO shows the absorption maximum (λmaxabs) at 465 nm, which is assigned to the intramolecular charge-transfer (ICT) excitation from the electron donor moiety (triphenylamino group) to the electron acceptor moiety (benzothienopyridinium). Thus, the λmaxabs for the ICT band of SAT-2 occurs at a longer wavelength by 53 nm than that of SAT-1, but the molar extinction coefficient (ε = 24700 M–1 cm–1) for the ICT band of SAT-1 8

ACCEPTED MANUSCRIPT is lower than that (45000 M–1 cm–1) of SAT-2. The corresponding fluorescence bands for the two dyes occur at 607 nm (Table 1). The absorption spectra of SAT-1 and SAT-2 adsorbed on TiO2 film are shown in Fig. 1b. The absorption spectrum of SAT-2 adsorbed on TiO2 film

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is broadened but the absorption maximum shows a hypsochromic shift compared with that of SAT-2 in DMSO, although the absorption maximum of SAT-1 adsorbed on TiO2 is similar to

3.3. Electrochemical properties

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that of SAT-1 in DMSO.

In order to investigate the electrochemical properties of SAT-1 and SAT-2, the cyclic voltammetry (CV) curves were recorded in aceonitrile/Bu4NClO4 (0.1M) solution for SAT-1

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and DMF/Bu4NClO4 (0.1M) solution for SAT-2, respectively. The CV curve of the two dyes are shown in Fig. 2. The reversible oxidation waves were observed at 0.27 V for SAT-1 and

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0.34 V for SAT-2, respectively, vs. ferrocene/ferrocenium (Fc/Fc+) (Table 1). The

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corresponding reduction waves appeared at 0.20 V for SAT-1 and 0.28 V for SAT-2, respectively, thus showing that the oxidized states of the two dyes are stable. The HOMO energy level vs. the normal hydrogen electrode (NHE) was evaluated from the half-wave potential for oxidation (Eox1/2 = 0.24 V for SAT-1 and 0.31 V for SAT-2). The HOMO energy level was 0.87 V for SAT-1 and 1.03 V for SAT-2, respectively, vs. NHE. This result shows that the HOMO energy levels of the two dyes are more positive than the I3–/I– redox potential 9

ACCEPTED MANUSCRIPT (0.4 V), and thus this ensures an efficient regeneration of the oxidized dyes by electron transfer from the I3–/I– redox couple in the electrolyte. The LUMO energy level of SAT-2 was estimated from the Eox1/2 and an intersection (525 nm; 2.36 eV) of absorption and

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fluorescence spectra in DMSO, that is, the LUMO energy level was obtained through eq. – [E0-0 – HOMO], where E0-0 transition energy is an intersection of absorption and fluorescence

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spectra corresponding to the energy gap between the HOMO and LUMO. The LUMO energy

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level was –1.84 V for SAT-1[39] and –1.33 V for SAT-2, respectively. Evidently, the LUMO energy levels of the two dyes are higher than the energy level (Ecb) of the CB of TiO2 (–0.5 V), suggesting that an electron injection to the CB of TiO2 is thermodynamically feasible. Thus, this result indicates that the HOMO and the LUMO energy levels of SAT-2 are lower than

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those of SAT-1, but from SAT-1 to SAT-2 the lowering of the LUMO energy level is larger than that of the HOMO energy level. Consequently, it was revealed that the bathochromic

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shift of the ICT absorption band for SAT-2 relative to SAT-1 is attributed to the stabilization

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of the LUMO energy level by the introduction of (pyridiniumyl)propanesulfonate with a stronger electron-withdrawing ability than pyridyl group, resulting in a decrease in the HOMO–LUMO band gap.

3.4. Theoretical calculations In order to examine the HOMO and the LUMO of SAT-1 and SAT-2, the molecular 10

ACCEPTED MANUSCRIPT orbitals of the two dyes were calculated using semiempirical molecular orbital (MO) calculations (AM1, INDO/S). The MO calculations indicate that the HOMO is mostly localized on the triphenylamino moiety containing a thiophene ring for each of the dyes, and

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the LUMO is mostly localized on the benzothienopyridine moiety containing a thiophene ring for SAT-1 and benzothienopyridinium moiety for SAT-2, respectively (Fig. 3). Accordingly,

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the MO calculations reveal that the dye excitations upon light irradiation induce a strong ICT

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from the triphenylamino moiety to the benzothienopyridine or benzothienopyridinium moiety.

3.5. FTIR spectra

In order to elucidate the adsorption states of SAT-1 and SAT-2 on TiO2 nanoparticles, we

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measured the FTIR spectra of the dye powders and the dyes adsorbed on TiO2 nanoparticles (Fig. 4). In our previous study, it was found that the dye SAT-1 is adsorbed on the TiO2

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through hydrogen bonding of the pyridyl group at Brønsted acid sites on the TiO2 surface.[39]

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When the dye SAT-2 was adsorbed on the TiO2 surface, the S=O stretching vibrations (1190 and 1030 cm–1) of the sulfonate group significantly weakened; this observation indicates that the dye SAT-2 is adsorbed on the TiO2 through the formation of the bidentate bridging linkage between the sulfonate group of the dye and Lewis acid site on the TiO2 surface.[49, 50] Consequently, the hypsochromic shift and the broadening of absorption band for the dye SAT-2 on adsorbed TiO2 film relative to that of the dye solution may be attributed to the 11

ACCEPTED MANUSCRIPT electrostatic

interaction

and

the

binding

mode

between

the

(benzo[4,5]thieno[2,3-c]pyridiniumyl)propanesulfonate of SAT-2 and TiO2 surface.

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3.6. Dye-sensitized solar cells

The DSSCs based on SAT-1 and SAT-2 were fabricated by using the dye-adsorbed TiO2

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electrode (9 µm), Pt-coated glass as a counter electrode, and an acetonitrile solution with

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iodine (0.05 M), lithium iodide (0.1 M), and 1,2-dimethyl-3-propylimidazolium iodide (0.6 M) as an electrolyte. The photocurrent–voltage (I–V) characteristics were measured under simulated solar light (AM 1.5, 100 mW cm–2). The incident photon-to-current conversion efficiency (IPCE) spectra and the I–V curves are shown in Fig. 5. The IPCE maximum (42%

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at 483 nm) of DSSC based on the dye SAT-2 is red-shifted compared with that (40% at 428 nm) of SAT-1 (Fig. 5a), which is in good agreement with the absorption spectra of the dyes

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adsorbed on TiO2 film. However, there is little difference in the maximum IPCE value

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between the two dyes. The I–V curves show that the Jsc value of SAT-2 (4.20 mA cm-2) is higher than that of SAT-1 (3.12 mA cm-2). Consequently, the relatively high Jsc value of SAT-2 is attributed to the fact that the LHE of SAT-2 in the long-wavelength region is higher than that of SAT-1, resulting in a higher IPCE of SAT-2 in the long-wavelength region. On the other hand, the Voc value of SAT-2 (472 mV) is lower than that of SAT-1 (548 mV). As the result, the solar energy-to-electricity conversion yield (η) of SAT-2 (1.19%) is similar to 12

ACCEPTED MANUSCRIPT that of SAT-1 (1.11%). Thus, electrochemical impedance spectroscopy (EIS) analysis was performed to investigate the difference in the Voc value between the two dyes, that is, we used the EIS to study the electron recombination process in DSSCs in the dark under a forward

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bias of −0.60 V with a frequency range of 10 mHz to 100 kHz. The large semicircle in the Nyquist plot (Fig. 6a), which corresponds to the midfrequency peaks in the Bode phase plots,

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represents the charge recombination between the injected electrons in TiO2 and I3– ions in the

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electrolyte, that is, the charge-transfer resistances at the TiO2/dye/electrolyte interface. The Nyquist plots show that the resistance value of the large semicircle for DSSCs based on SAT-2 (26 Ω) is similar to that of DSSCs based on SAT-1 (22 Ω). The electron recombination lifetime (τe) expressing the electron recombination between the injected

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electrons in TiO2 and I3– ions in the electrolyte, extracted from the angular frequency (ωrec) at the midfrequency peak in the Bode phase plot (Fig. 6b) using τe = 1/ωrec, is 10 ms for DSSCs

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based on SAT-2, which is nearly equivalent to the τe (8 ms) for DSSC based on SAT-1.

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Consequently, the EIS analysis suggests that the major reason for the difference in Voc value between the two dyes is not the charge recombination between the injected electrons in TiO2 and I3– ions in the electrolyte, but the shift of the Ecb of TiO2 by the differences in the direction and the magnitude of dipole moment and the binding modes between the two dye on TiO2 surface [4, 5, 7, 8], that is, the direction and the magnitude of dipole moment and the binding modes for SAT-2 on TiO2 surface may lead to a positive shift of the Ecb of TiO2, 13

ACCEPTED MANUSCRIPT which is responsible for the low Voc value of SAT-2.

4. Conclusion have

designed

and

developed

a

D–π–A

dye

sensitizer

SAT-2

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We

with

(benzo[4,5]thieno[2,3-c]pyridiniumyl)propanesulfonate as a stronger electron-withdrawing

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anchoring group for dye-sensitized solar cells (DSSCs). The photoabsorption band of SAT-2

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occurs at a longer wavelength than that of D–π–A dye sensitizer SAT-1 with benzo[4,5]thieno[2,3-c]pyridine. It was found that the dye SAT-2 is adsorbed onto the TiO2 electrode through the formation of the bidentate bridging linkage between the sulfonate group of the dye and Lewis acid site (exposed Tin+ cations) on the TiO2 surface. The short-circuit

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photocurrent density (Jsc) of DSSC based on the dye SAT-2 is higher than that of the previously described D–π–A dye sensitizer SAT-1. Consequently, this work indicates that the

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(pyridiniumyl)alkanesulfonate would be expected to be one of the most promising

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electron-withdrawing anchoring group for the efficient D–π–A dye sensitizers, although the further study to improve the Voc value of DSSC based on D–π–A dye sensitizer with the (pyridiniumyl)alkanesulfonate is necessary.

Acknowledgements This work was supported by Grants-in-Aid for Scientific Research (B) from the Japan 14

ACCEPTED MANUSCRIPT Society for the Promotion of Science (JSPS) KAKENHI Grant Number 15H03859 and

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Kumagai Foundation for Science and Technology.

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Figure captions

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Scheme 1. Chemical structures of D–π–A dye sensitizers SAT-1 and SAT-2.

Scheme 2. Synthesis of D–π–A dye sensitizer SAT-2.

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Fig. 1. Absorption (–) and fluorescence (···) spectra of SAT-1 and SAT-2 in DMOS. (b)

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Absorption spectra of SAT-1 and SAT-2 adsorbed on TiO2 film.

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Fig. 2. Cyclic voltammograms of (a) SAT-1 in acetonitrile containing 0.1 M Bu4NClO4 and (b) SAT-2 in DMF containing 0.1 M Bu4NClO4. The arrow denotes the direction of the potential scan.

Fig. 3. (a) HOMO and (b) LUMO of SAT-1 and SAT-2 by semiempirical molecular orbital calculations (AM1, INDO/S). 24

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Fig. 4. FTIR spectra of the dye powders and the dyes adsorbed on TiO2 nanoparticles for

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SAT-2.

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Fig. 5. (a) IPCE spectra and (b) I–V curves of DSSCs based on SAT-1 and SAT-2.

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Fig. 6. (a) Nyquist plots and (b) Bode phase plots of DSSCs based on SAT-1 and SAT-2.

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Scheme 1

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Scheme 2

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SAT-1

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Wavelength / nm

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ε / M-1cm-1

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(a)

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E / V vs. Fc / Fc+

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powder on TiO2

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Table 1. Optical and electrochemical data, HOMO and LUMO energy levels, and DSSC performance parameters of SAT-1 and SAT-2. λmaxabs/nm HOMO LUMO Voc Jsc λmaxfl E1/2ox Dye IPCEmax ff d η (%)d (ε /Vb /Vc /Vc /mA cm-2d /mVd /nma -1 -1 a /M cm ) 0.24 0.87 –1.84 40%@428 nm 3.12 548 0.67 1.11 SAT-1 412 (45000) 612 0.31 1.03 –1.33 42%@483 nm 4.20 478 0.60 1.19 SAT-2 465 (24700) 606 In DMSO. b Half-wave potentials for oxidation (E1/2ox) vs. Fc/Fc+ were recorded in CH3CN/Bu4NClO4 (0.1M) solution for SAT-1 and DMF/Bu4NClO4 (0.1M) solution for SAT-2, respectively. c vs. Normal hydrogen electrode (NHE). d The 9 µm thick TiO2 electrode was immersed into 1.0 mM dye (SAT-1) solution in THF or 0.1 mM dye (SAT-2) solution in chloroform. Under a simulated solar light (AM 1.5, 100 mW cm-2).

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Highlights • We have developed D–π–A dye with (benzothienopyridiniumyl)propanesulfonate for

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DSSCs.

• The D–π–A dye is adsorbed onto the TiO2 electrode by the sulfonate group.

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• The (pyridiniumyl)alkanesulfonate is one of the useful electron-withdrawing anchor.