Accepted Manuscript

Organic sensitizers incorporating 3,4-ethylenedioxythiophene as the conjugatedbridge: Joint photophysical and electrochemical analysis of photovoltaic performance

Wenqin Li, Bo Liu, Yongzhen Wu, Shiqin Zhu, Qiong Zhang, Weihong Zhu

PII: S0143-7208(13)00158-7

DOI: 10.1016/j.dyepig.2013.04.031

Reference: DYPI 3930

To appear in: Dyes and Pigments

Received Date: 22 February 2013

Revised Date: 24 April 2013

Accepted Date: 25 April 2013

Please cite this article as: Li W, Liu B, Wu Y, Zhu S, Zhang Q, Zhu W, Organic sensitizers incorporating3,4-ethylenedioxythiophene as the conjugated bridge: Joint photophysical and electrochemical analysisof photovoltaic performance, Dyes and Pigments (2013), doi: 10.1016/j.dyepig.2013.04.031.

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Organic sensitizers incorporating 3,4-ethylenedioxythiophene as the

conjugated bridge: Joint photophysical and electrochemical analysis of

photovoltaic performance

Wenqin Lia,†, Bo Liub,†, Yongzhen Wua, Shiqin Zhua, Qiong Zhanga, and Weihong Zhua,*

aShanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for

Advanced Materials and Institute of Fine Chemicals, East China University of Science &

Technology, Shanghai 200237, P. R. China. Tel.: (+86) 21-6425-0772. Fax: (+86) 21-

6425-2758. E-mail: whzhu@ecust.edu.cn

bCollege of Chemistry and Material Science, Hebei Normal University, Hebei,

Shijiazhuang 050023, P. R. China.

†These authors contributed equally to this work.

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Abstract

Two novel D-π-A organic dyes containing either an indoline donor or a triphenylamine

donor with each containing the 3,4-ethylenedioxythiophene unit as the conjugated bridge,

were developed for dye-sensitized solar cells. Incorporating 3,4-ethylenedioxythiophene

as the conjugated bridge brings several characteristics, such as broadening absorption

range and increasing molar extinction coefficients, and improving electron injection with

enhancement of the short-circuit photocurrent. The indoline containing dye shows a more

negative oxidation potential and a bathochromic shift in absorption spectra than the

triphenylamine substituted dye, indicative of the more powerful electron-donating

capability of the indoline unit. With coadsorption of chenodeoxycholic acid, the indoline

dye based dye-sensitized solar cell exhibited promising conversion efficiency of 6.05%,

with a short-circuit photocurrent of 13.23 mA cm-2, open circuit voltage of 642 mV, and a

fill factor of 0.711. In the indoline dye system, the dye-sensitized solar cell is workable

with the driving force of 150 mV for the dye regeneration process, paving a road towards

minimizing energy losses in the dye regeneration process.

Keywords: Solar cells, Sensitized dyes, Energy loss, Conjugated bridge, Indoline.

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1. Introduction

Along with increasing energy demands and global warming problems, dye sensitized

solar cells (DSSCs) have become one of the most promising alternatives to the

photovoltaic conversion of solar energy with a reasonable performance-cost ratio [1−12].

Recently a record efficiency exceeding 12.0% has been obtained with zinc porphyrin dyes

[13]. With respect to metal complexes, pure metal-free organic dyes have become

promising due to their high extinction coefficients, facile structural modification and

simple synthesis [1,4]. Generally, for the popular donor-π-acceptor (D-π-A)

configuration, the dye sensitizer should be composed of an electron-donating moiety,

conjugated bridge and electron-accepting/anchoring moiety. Slight variations within these

different sections may cause significant change in photovoltaic performances [14-28].

Recently, we have systematically demonstrated that the incorporation of an additional

acceptor into the conjugated bridge was beneficial for tailoring molecular, optimizing

energy levels and improving photo-stability [5]. However, better focussed research on the

interpretation of photophysical and electrochemical effect on photovoltaic performances

of dyes is still challenge.

As a typical D-π-A sensitizer, the indoline-based dye LS-1 (Fig. 1), employing a 2,5-

thienyl unit as the conjugated bridge, shows low efficiency of 4.5% [15]. Obviously, there

is much room in the optimization of both Jsc and Voc. The 3,4-ethylenedioxythiophene

(EDOT) is well known electron-rich fragment, and has been applied in polymer-based

photovoltaic devices and dye sensitizers as π-conjugated linkers for the sake of improving

the light-harvesting capability [29-37]. Considering that EDOT improves the co-planarity

with neighboring aromatic groups with a preferential expansion of the light-

responsiveness [30], herein we incorporated EDOT into LS-1 to construct sensitizer LS-4

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(Fig. 1) to optimize the light-harvesting capability. Meanwhile, we designed and

synthesized LS-5 (Fig. 1), along with the same conjungated bridge and anchoring group,

but triphenylamine as electron donor, to systematically investigate the effect of electron

donor on photovoltaic performances of DSSCs. Remarkably, with respect to the reference

dyes of LS-1 and LS-5, LS-4 shows several favorable characteristics in light harvesting

and overall conversion efficiency. With preliminary optimization, the LS-4 based DSSC

exhibits a promising conversion efficiency (η) of 6.05%, with a short circuit current

density (Jsc) of 13.23 mA cm–2, open circuit voltage (Voc) of 642 mV, and fill factor (ff) of

0.711 under AM 1.5 illumination (100 mW cm–2). Our results also reveal that for the

indoline-based sensitizer LS-4, the driving force of 150 mV between the highest occupied

molecular orbital (HOMO) and the Nernst potential of I–/I3– redox couple is workable in

the dye regeneration, paving a road towards minimizing energy losses in dye regeneration

process.

[Fig. 1]

2. Experimental

2.1. Materials and characterization

All experiments were performed under an argon atmosphere. THF was pre-dried over

4Ǻ molecular sieves and distilled under an argon atmosphere from sodium benzophenone

ketyl. CH2Cl2 used for fabricating solar cells was pretreated with CaH2. Other starting

materials were used as commercially purchased and without any further purification.

1H NMR and 13C NMR spectra were recorded on a Bruker AM 400 spectrometer with

the chemical shifts against TMS, operating at 400 and 100 MHz, respectively. HRMS

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were recorded with a Waters ESI mass spectrometer. FT-IR spectra were recorded in the

region of 400-4000 cm-1 on a Thermo Electron Avatar 380 FT-IR instrument (KBr Discs).

The UV-vis spectra were measured with a Cary 100 spectrophotometer. Cyclic

voltammograms were obtained with a Versastat II electrochemical workstation (Princeton

Applied Research) using a three-electrode cell with a Pt working electrode, a Pt wire

auxiliary electrode, and a regular calomel reference electrode (SCE) in saturated KCl

solution. 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) was used as the

supporting electrolyte in THF. The scan rate was 100 mV s–1. Fc/Fc+ redox couple was

added as an internal potential reference after measurement.

2.2. Theory approach

The ground-state geometries of LS-1, LS-4 and LS-5 were optimized in the gas phase

by DFT with the Gaussian 09 package [38], using the hybrid B3LYP [39] functional and

the 6–31+G(d) basis set. For the TDDFT calculations, performed on the B3LYP

optimized ground-state geometries, the Coulomb attenuating B3LYP (CAM-B3LYP)

approach [40] was used with 6–31+G(d) basis set. Solvent effect was taken into account

into the TDDFT calculations in THF with the non-equilibrium version of the C-PCM

model [41] implemented in Gaussian09 [39].

2.3. Fabrication of dye-sensitized solar cells

The FTO conducting glass was obtained from Touki, Co. TiO2 paste (PST-18NR for 20

nm and PST-400C for 400 nm) was provided by JGC C&C Ltd. 8 µm nanocrystalline

TiO2 electrodes with 4 µm scattering layer were prepared and modified following the

reported procedure [42]. The thickness of the TiO2 film was measured by a surface

profiler (Dektak Co., ltd., Model DAKTAK II). The dye-loaded electrodes were prepared

by dipping TiO2 electrodes (1.2 cm × 0.8 cm) into 0.3 mM dye solution

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(CH2Cl2/THF=9/1) with 2, 5 and 10 mM CDCA for 12 h. To prepare the counter

electrode, the Pt catalyst was deposited on the cleaned FTO glass by coating with a drop

of H2PtCl6 solution (50mM in ethanol solution) with the heat treatment at 500 °C for 30

min. In this work, 0.6 MBMII, 0.1 M LiI, 0.05 M I2, and 0.6 M tert-TBP in acetonitrile

was used as the redox electrolyte.

2.4. Photovoltaic measurements

Photovoltaic measurements employed an AM 1.5 solar simulator equipped with a 150

W xenon lamp (OTENTO-SUN II, Bunkoukeiki Co., Ltd.). The power of the simulated

light was calibrated to 100 mW cm–2 using a reference silicon cell (BS-520, Bunkoukeiki

Co., Ltd.). I–V curves were obtained by applying an external bias to the cell and

measuring the generated photocurrent with a Keithley model 2400 digital source meter.

The voltage step and delay time of photocurrent were 1 mV and 20 ms, respectively. Cell

active area was controlled to be 0.142 cm-2 by a metal mask. The photocurrent action

spectra were measured with the IPCE test system consisting of a model SR830 DSP

Lock-In Amplifier and model SR540 Optical Chopper (Stanford Research Corporation,

USA), a 7IL/PX150 xenon lamp and power supply and a 7ISW301 spectrometer. Light-

intensity dependence of the photocurrent density for LS-1 and LS-4 based devices were

measured under short-circuit conditions using an electrochemical workstation (Zahner

XPOT, Germany), which includes a green light emitting diode (LED, 531 nm) and the

corresponding control system. EIS Nyquist and Bode plot for DSSCs was performed

using a two-electrode system in the dark. The spectra were scanned in a frequency range

of 0.1 Hz–100 kHz at room temperature with applied bias potential set at –700 mV. The

alternate current (AC) amplitude was set at 10 mV.

2.5. Synthesis of compound B

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A solution of n-butyllithium (2.5 M in hexane, 3.1 mL, 7.75 mmol) was added drop-

wise to EDOT (1.0 g, 7.03 mmol) in pre-dried THF (35 mL) at –78 °C under argon

atmosphere. Trimethyl borate (1.3 mL, 9.75 mmol) was added after the mixture was

stirred at –78 °C for 1 h. The reaction mixture was allowed to warm up to room

temperature for 2 h, and then stirred overnight. Without further purification, the mixture

was used for Suzuki cross-coupling. The mixture of A (2.59 g, 7.84 mmol), aqueous

K2CO3 (2 M, 15 mL), Pd(PPh3)4 (20 mg, 0.02 mmol) and THF (30 mL) were heated to

80–90 °C under argon atmosphere for 30 min. The above synthesized borate ester was

added slowly, and refluxed for further 12 h. After cooling to room temperature, the

mixture was extracted with CH2Cl2 (50 mL × 3). The organic portion was combined, and

the solvent was removed by rotary evaporation. The residue was purified by column

chromatography using silica gel (CH2Cl2/petroleum ether = 1/20, v/v) to give a yellow oil.

Yield: 29.2%. 1H NMR (400 MHz, CDCl3, ppm): δ 7.43 (s, 1H), 7.37 (dd, J = 8.4 Hz, J =

1.6 Hz, 1H), 7.12–7.19 (m, 4H), 6.89 (d, J = 8.4 Hz, 1H), 6.19 (s, 1H), 4.74–4.78 (m, 1H),

4.27 (m, 2H), 4.23 (m, 2H), 3.81–3.85 (m, 1H), 2.32 (s, 3H), 2.04 (m, 1H), 1.90–1.92 (m,

2H), 1.79 (m, 1H), 1.63 (m, 1H), 1.52 (m, 1H). 13C NMR (100 MHz, CDCl3, ppm): δ

146.68, 142.13, 140.47, 136.34, 135.20, 131.01, 129.69, 125.54, 123.36, 122.73, 119.62,

107.58, 99.60, 95.25, 69.01, 64.63, 64.51, 45.39, 34.98, 33.80, 24.40, 20.74. HRMS (ESI,

m/z): calcd for C24H24NO2S [M + H]+, 390.1528; found 390.1526.

2.6. Synthesis of compound C

A solution of n-butyllithium (2.5 M in hexane, 2.3 mL, 5.75 mmol) was added drop-

wise to a solution of compound B (1.8 g, 4.62 mmol) in pre-dried THF (35 mL) at –78 °C

under argon atmosphere. Trimethyl borate (0.92 mL, 6.93 mmol) was added after the

mixture was stirred at –78 °C for 1 h. The reaction mixture was allowed to warm up to

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room temperature for 2 h, and then stirred overnight. Without further purification, the

mixture is used for Suzuki cross-coupling. A mixture of 5-bromo-2-

thiophenecarboxaldehyde (1.20 g, 6 mmol), aqueous K2CO3 (2 M, 20 mL), Pd(PPh3)4 (20

mg, 0.02 mmol) and THF (40 mL) were heated to 80–90 °C under argon atmosphere for

30 min. The above synthesized borate ester was added slowly, and refluxed for further 12

h. After cooling to room temperature, the mixture was extracted with CH2Cl2 (50 mL × 3).

The organic portion was combined, and the solvent was removed by rotary evaporation.

The residue was purified by column chromatography using silica gel (CH2Cl2/petroleum

ether = 1/10, v/v) to give an orange red solid 582 mg. Yield: 32.1%. 1H NMR (400 MHz,

CDCl3, ppm): δ 9.83 (s, 1H), 7.64 (d, J = 4.0 Hz, 1H), 7.47 (s, 1H), 7.43 (dd, J = 8.4, 1.8

Hz, 1H), 7.22 (d, J = 4.0 Hz, 1H), 7.16–7.20 (m, 4H), 6.87 (d, J = 8.4 Hz, 1H), 4.79 (m,

1H), 4.34–4.43 (m, 4H), 3.84 (m, 1H), 2.33 (s, 3H), 2.05 (m, 1H), 1.90–1.92 (m, 2H),

1.78 (m, 1H), 1.66 (m, 1H), 1.53 (m, 1H). 13C NMR (100 MHz, CDCl3, ppm): δ 182.36,

147.65, 145.54, 141.01, 140.05, 139.90, 137.13, 136.50, 135.44, 131.70, 129.82, 126.10,

122.88, 122.16, 122.12, 120.30, 120.19, 107.44, 106.85, 69.21, 65.16, 64.54, 45.35, 35.14,

33.71, 24.42, 20.82. HRMS (ESI, m/z): calcd for C29H26NO3S2 [M + H]+, 500.1354;

found 500.1352.

2.7. Synthesis of dye LS-4

Intermediate C (782 mg, 1.56 mmol) and cyanoacetic acid (955.2 mg, 11.23 mmol)

were dissolved in acetonitrile (15 mL) in the presence of piperidine (0.5 mL), and then

refluxed for 8 h. The solvent was removed by rotary evaporation, the residue was purified

by column chromatography on silica (CH2Cl2/MeOH = 10/1, v/v) to give a dark red solid.

Yield: 57.2%. IR (KBr, cm-1): 2212 (C≡N), 1685, 1256. 1H NMR (400 MHz, DMSO-d6,

ppm): δ 8.28 (d, J = 4.1 Hz, 1H), 7.81 (d, J = 3.1 Hz, 1H), 7.48 (s, 1H), 7.40 (dd, J = 8.4,

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1.8 Hz, 1H), 7.32 (d, J = 4.0 Hz, 1H), 7.19 (m, 4H), 6.85 (d, J = 8.4 Hz, 1H), 4.87 (m,

1H), 4.48 (m, 2H), 4.41 (m, 2H), 3.83 (m, 1H), 2.28 (s, 3H), 2.05 (m, 1H), 1.77 (m, 2H),

1.65 (m, 1H), 1.55 (m, 1H), 1.37 (m, 1H). 13C NMR (100 MHz, DMSO-d6, ppm): δ

164.43, 146.70, 142.26, 141.08, 140.50, 139.31, 137.43, 136.70, 135.31, 133.54, 130.88,

129.74, 125.51, 122.37, 122.20, 121.77, 119.58, 118.72, 118.05, 117.75, 106.92, 105.78,

68.20, 65.21, 64.48, 44.43, 34.82, 33.01, 23.91, 20.33. HRMS (ESI, m/z): calcd for

C32H27N2O4S2 [M – H]–, 567.1412; found 567.1412.

2.8. Synthesis of compound F

The synthetic method was similar to that of compound C, and purified by column

chromatography on silica (CH2Cl2/petroleum ether = 1/20, v/v) to give an orange solid

582 mg. Yield: 35.2%. 1H NMR (400 MHz, CDCl3, ppm): δ 9.85 (s, 1H), 7.65 (d, J = 4.0

Hz, 1H), 7.59 (d, J = 8.4 Hz, 2H), 7.27–7.25 (m, 5H), 7.12 (d, J = 7.6 Hz, 4H), 7.07–7.03

(m, 4H), 4.44–4.43 (m, 2H), 4.37–4.35 (m, 2H). 13C NMR (100 MHz, CDCl3, ppm): δ

182.49, 147.33, 147.13, 145.02, 140.80, 140.42, 137.57, 137.02, 129.37, 127.14, 125.90,

124.71, 123.31, 123.16, 122.60, 118.59, 65.11, 64.61. HRMS (ESI, m/z): calcd for

C29H22NO3S2 [M + H]+, 496.1041; Found 496.1044.

2.9. Synthesis of dye LS-5

The synthetic method was similar to that of dye LS-4, and purified by column

chromatography on silica (CH2Cl2/MeOH = 10/1, v/v) to afford a red solid. Yield: 62.4%.

IR (KBr, cm-1): 2215 (C≡N), 1683, 1258. H NMR (400 MHz, DMSO-d6, ppm): δ 8.16 (s,

1H), 7.64 (d, J = 3.2 Hz, 1H), 7.58 (d, J = 8.4 Hz, 2H), 7.34–7.28 (m, 5H), 7.09–7.02 (m,

6H), 6.96 (d, J = 8.4 Hz, 2H), 4.45–4.39 (m, 4H). 13C NMR (100 MHz, DMSO-d6, ppm):

δ 146.71, 146.25, 140.06, 139.97, 137.91, 136.35, 134.31, 129.58, 126.81, 125.65, 124.23,

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123.41, 122.83, 122.75, 115.93, 107.41, 65.13, 64.56. HRMS (ESI, m/z): calcd for

C32H23N2O4S2 [M + H]+, 563.1099; found 563.1098.

3. Results and discussion

3.1. Synthesis

Considering the high conductivity and excellent coplanarity with the neighboring

aromatic ring [29-31], an additional EDOT unit was incorporated as a conjugated bridge

into the traditional D-π-A configuration to afford the novel sensitizers LS-4 and LS-5. As

illustrated in Scheme 1, the electron-rich segment EDOT was attached to the donor

moiety by conventional Suzuki cross-coupling reaction with a relatively high yield to

afford intermediates B and E. Aldehyde intermediates C and F were also synthesized by a

Suzuki cross-coupling and Knoevenagel reaction to access the target sensitizers. The

reference dye LS-1 was obtained according to our earlier publication [15]. The chemical

structures of all compounds were fully characterized by 1H and 13C NMR spectroscopy

and HRMS (see experimental section).

[Scheme 1]

3.2. Optical and Electrochemical Properties

The absorption spectra of dyes LS-1, LS-4 and LS-5 in tetrahydrofuran (THF) and

adsorbed onto 3 µm transparent TiO2 films are depicted in Fig. 2, and the corresponding

data are listed in Table 1. As a typical D-π-A system, the three dyes exhibit a typical

strong absorption band in the range of 400–650 nm with maximum absorption

coefficients of 3.15–4.45 × 104 M–1cm–1 in THF, arising from the typical intramolecular

charge transfer (ICT) process between the indoline/triphenylamine donor and the

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cyanoacetic acid acceptor moiety. Compared with LS-1, incorporation of an additional

EDOT unit in LS-4 significantly red-shifted the absorption band by 56 nm, revealing that

EDOT can efficiently assure the electronic coupling between the donor and acceptor

moieties. With respect to LS-5, the introduction of an indoline donor in LS-4 shifted the

absorption peaks by 48 nm, indicative of the priority of indoline in electron-donating

ability. When adsorbed onto thin transparent TiO2 films (3 µm), the absorption peaks for

LS-1, LS-4 and LS-5 were each blue shifted to a different extent, with bands located at

420, 486 and 458 nm, respectively. As known, when anchored onto nanocrystalline TiO2

surface, the deprotonation and H-aggregation of sensitizers always results in a blue-shift

of absorption maxima, while J-aggregates always result in a red-shift [43-46]. To

quantize the absorption variation caused by deprotonation, another set of photophysical

analysis was carried out in the presence of excessive triethylamine. As shown in Fig. 2c,

the absorption peaks of LS-1, LS-4 and LS-5 were blue-shifted to 442, 474, 455 nm,

respectively, upon sufficient deprotonation. Hence the blue-shift in absorption peak upon

adsorption onto TiO2 film with these dyes may ascribe to the synergy effects of

deprotonation and aggregation.

[Fig. 2]

Prior to the fabrication of DSSCs, cyclic voltammograms were recorded on the three

dyes in THF to evaluate the possibility of electron injection from the excited dye to the

conduction band of TiO2 and dye regeneration by the redox species I–/I3– (shown in Fig.

3). The HOMO and LUMO levels of these dyes are summarized in Table 1. The first

oxidation potentials in THF solution, corresponding to HOMO values, were measured as

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0.69, 0.55 and 0.76 V vs. normal hydrogen electrode (NHE) for LS-1, LS-4 and LS-5,

respectively. Generally, the Nernst potential of I–/I3– redox couple is regarded as 0.4 V vs.

NHE. That is, the driving force for regenerating LS-4 is only 0.15 V, which may cause a

debate whether the value is sufficient for efficient dye regeneration [47]. Cyclic

voltammetry was further performed using TiO2 film deposited FTO glass (fluorine doped

SnO2, sheet resistance: 7 Ω/square, transmission, 90% in the visible region) sensitized by

LS-4 as a working electrode to identify the value. As summarized in Table 1, the HOMO

value for LS-4 coated onto TiO2 film is calculated to be 0.55 V. That is, the driving force

for regeneration of the LS-4 cation was measured to be ~150 mV both in solution and on

TiO2 film. The LUMO values of three dyes are calculated to be –1.49, –1.33 and –1.32 V

coated onto TiO2 film according to the equation of LUMO = HOMO – E0–0. All these

data are sufficiently more negative than the conduction band (ECB) of TiO2, indicating

that the electron injection process from the LUMO orbital of these sensitizers to the

conduction band of TiO2 is energetically permitted [48,49]. It is noteworthy that, the

oxidation potential of LS-4 shifts cathodically by 0.21 V with respect to that of LS-5,

further demonstrating the priority in electron-donating ability of indoline moiety with

respect to triphenylamine.

[Table 1]

[Fig. 3]

It is well-established that the electronic structure is also an important factor on

photovoltaic performance. To gain insight into the frontier orbital configurations and

structural parameters of these dyes, density functional theory (DFT) calculations were

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performed with the Gaussian 09 package [38]. As shown in Fig. 4, for EDOT based

sensitizers LS-4 and LS-5, the dihedral angle formed between EDOT and thiophene

plane was computed to be 0.7° and 0.2°, respectively. These data imply that there is a

good coplanarity between the EDOT and thiophene segments. To evaluate the absorption

spectra in Fig. 2, time-dependent density functional theory (TD-DFT) calculations were

performed in THF, and the calculated transitions in terms of compositions, energies and

oscillator strengths are reported in Table 2. From the analysis of the TD-DFT

eigenvectors, the experimental absorption band in the visible region can be assigned to

the predominated transitions from HOMO to LUMO orbital, with a composition for LS-1

(86%), LS-4 (75%) and LS-5 (71%) in terms of molecular orbital contributions.

[Table 2]

[Fig. 4]

As shown in Fig. 5, the HOMO orbital for three dyes is delocalized over the whole

structure, whereas the LUMO is mainly populated on the conjugated bridge and acceptor

unit. As well known, that the adsorption of these dyes onto TiO2 films is realized via the

binding manner between the –COOH and TiO2. That is, the electron transition from the

donor moiety to the anchoring group originated by the absorption in visible region is

beneficial to the electron injection into the conduction band of the TiO2 film. Notably, in

both the HOMO and LUMO orbitals of LS-4 and LS-5, the electron density located at the

EDOT fragment is significant (Fig. 5), indicating that the beneficial effects in electron

delocalization throughout the whole structure.

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[Fig. 5]

3.3 Photovoltaic Performances

Fig. 6 shows the action spectra of monochromatic incident photon-to-electron

conversion efficiency (IPCE) for DSSCs based on LS-1, LS-4 and LS-5 coadsorbed with

2 mM chenodeoxycholic acid (CDCA) measured with the wavelength of 300–800 nm,

using 0.6 M 1-butyl-3-methylimidiazolium iodide (BMII), 0.1 M LiI, 0.05 M I2, and 0.6

M tert-butylpyridine (4-TBP) in acetonitrile as the redox electrolyte. The IPCE onsets for

three dyes are observed at around 705, 790 and 730 nm, respectively, in the order of LS-1

< LS-5 < LS-4. Notably, the IPCE profile of indoline-based sensitizer LS-4 is much

broader than those of LS-1 and LS-5, which is in consistent with the above-mentioned

absorption characteristics.

Fig. 7 presents I-V curves of the cells measured at 100 mW cm–2 under simulated 1.5

air mass global solar light, and the resulting parameters such as Jsc, Voc, ff and overall

conversion efficiency (η) are summarized in Table 3. Under optimized conditions, the

DSSCs based on LS-4 exhibited a maximum solar energy to electricity conversion

efficiency of 6.05% with photovoltaic parameters (Jsc = 13.23 mA cm–2, Voc = 642 mV, ff

= 0.711), while for LS-5, the optimized efficiency of 5.15% was obtained with the

photovoltaic parameters (Jsc = 11.68 mA cm–2, Voc = 638 mV, ff = 0.696). Generally,

CDCA is often introduced as coadsorbent to break up dye aggregation due to its steric

structure [50, 51]. In our devices, TiO2 films were immersed into the dye solution with

the existence of different concentrations of CDCA to screen the optimized performance

of solar cells. As listed in Table 3, without coadsorption of CDCA, the solar cells based

on LS-4 exhibited slightly lower photocurrent density with respect to LS-5 despite

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broader absorption spectra and higher molar absorption coefficient. When coadsorbed

with 2 mM CDCA, the performance of LS-4 based DSSCs increased dramatically.

Undoubtedly, the presence of CDCA in the dye layer affects the state of aggregation of

the sensitizer. Dye aggregation should be avoided in DSSCs since it not only decreases

electron injection efficiency (low Jsc), but also facilitates charge recombination (low Voc)

between injected electrons and oxidized species near TiO2 surface.

[Fig. 6]

Considering that the driving force for regenerating pure LS-4 on TiO2 is only ~150 mV

according to aforementioned electrochemical measurements, we began to wonder if the

coadsorption of CDCA influences the energy levels of LS-4 on the TiO2 electrode, and

hence accelerating the dye regeneration process. We further measured the oxidation

potential of CDCA/LS-4 coadsorbed TiO2 electrode by cyclic voltammetry, and

estimated the HOMO level of LS-4 upon coadsorption. The results indicated that there is

no obvious change in HOMO orbital for LS-4 upon coadsorption with CDCA. That is,

we can rule out the coadsorption effect on the dye regeneration driving force. Accordingly,

we ascribe the significant enhancement in photovoltaic performance to the favorable

packing of dye molecules by efficiently avoiding aggregation with the assistance of the

co-adsorbent. Meanwhile, we can identify that the driving force for regenerating with the

value of 150 mV is workable for such indoline based organic sensitizers [47], which is

much smaller than that of Ru-sensitizers (generally greater than 500 mV) [4]. Such a

small regenerating driving force is caused by the essential up-shifting in HOMO level due

to the contribution of indoline and of EDOT. If the uplifted HOMO level and narrowed

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molecular energy gap in LS-4 could be efficiently transferred to extra-photocurrent, the

higher photovoltaic performance can be expected. As shown in Fig. 2b, the incorporation

of EDOT fragment in LS-4 red-shifted the absorption onset from 600 nm to almost 760

nm with respect to LS-1. Theoretically, such an increase can result in about 50% higher

photocurrent [52]. However, the experimental Jsc for LS-4 is only enhanced by 20% with

respect to LS-1 under the optimized conditions (11.04 and 13.23 mA cm-2 for LS-1 and

LS-4, respectively), which is far less than the expected 50% increment.

[Fig. 7]

[Table 3]

Light-intensity dependence of the photocurrent density for LS-1 and LS-4 based

devices were measured under short-circuit conditions (Fig. 8) [10]. Within the light

intensity range of 0.5−70 W m–2, the photocurrent density of LS-1 was increased linearly

with the enhancement in light intensity. As expected, the corresponding photocurrent

density of LS-4 is higher in this range, which can be attributed to the higher light-

harvesting coefficients of LS-4 than that of LS-1 (Fig. 2b). However, the photocurrent

density of LS-4 did not increase linearly along with the light intensity, and the Jsc value

was close to that of LS-1 at 70 W m–2 (Fig. 8). Even under the weak light intensity range,

the curve slope of LS-4 decreased gradually. Accordingly, it is conceivable that the loss

in Jsc will become more serious under the simulated light condition (AM 1.5, 100 mW

cm–2 or 1000 W m–2) for cell device evaluation. Given that the photoanode, counter

electrode and electrolyte are all the same for both LS-1 and LS-4 based devices, the

electron transport in TiO2 as well as mass transport in electrolyte should not be the reason

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for the nonlinear increase of Jsc vs. light intensity in the LS-4 based cell. We can expect

that the possible reason might result from its slightly high HOMO level with thereafter

small regeneration driving force. Accordingly, the 150 mV driving force for regenerating

LS-4 is workable but may be less efficient, especially under high light-intensity

conditions. The further modulation of energy levels is desirable with molecular design.

[Fig. 8]

3.4 Electrochemical Impedance Spectroscopy (EIS)

As listed in Table 4, the Voc for LS-4 and LS-5 is a little higher than that for LS-1

irrespective of the addition of CDCA. To further shed light on the phenomenon, EIS was

carried out with the liquid-electrolyte based DSSCs co-sensitized with 2 mM CDCA.

Typical EIS Nyquist plots and Bode phase plots measured under dark conditions are

shown in Fig. 9. Furthermore, the equivalent circuit presented in Fig. 9c was used to

analyze the reaction resistance of the DSSCs and corresponding parameters are listed in

Table 4 [53-57]. As shown in Fig. 9, the radius of the semicircle in the intermediate

frequency regime of the Nyquist plot shows the dye-sensitized TiO2/redox species (I–/I3–)

interface charge transfer resistance. Obviously, the radius in the region for LS-4 and LS-5

based DSSCs is larger than that for LS-1, indicative of higher charge recombination

resistance for LS-4 and LS-5 based DSSCs. Again, the electron lifetimes for

recombination can be estimated from the peak frequency of charge transfer with RCT in

EIS Bode plot, τe = 1/(2πf). For three sensitizers, the peak frequency decreased in the

order of LS-1 > LS-5 > LS-4, and the electron lifetime enhanced in reverse with the

calculated value of 4.15, 5.90 and 6.16 ms, respectively (Table 4). Apparently, the

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incorporation of EDOT is very slightly beneficial to the enhancement of electron lifetime

or the repression of charge recombination. It is consistent with their observed Voc, that is,

only a small increase by 20-30 mV in Voc was found with respect to LS-1 (Table 3).

[Fig. 9]

[Table 4]

4. Conclusions

In summary, we have introduced additional EDOT as binary spacer into organic D-π-A

dyes. Our results reveal that incorporation of EDOT as a conjugated bridge essentially

facilitates electron delocalization over the whole molecule, and realizes the enhancement

of light-harvesting ability with a predominant increase in Jsc. With respect to

triphenylamine-based dye LS-5, the indoline unit shows a priority in electron-donating

ability, resulting in a more negative HOMO value and significant bathochromic shift in

absorbance action for LS-4. As demonstrated with EIS analysis, the EDOT fragment is

very slightly favorable to the charge recombination resistance, exhibiting little

contribution to Voc. Notably, our results reveal that the driving force of ~150 mV is

workable to indoline-based dye regeneration process, resulting in an overall efficiency of

6.05%.

Acknowledgements

This work was supported by NSFC/China, National 973 Program (2013CB733700), the

Oriental Scholarship, the Fundamental Research Funds for the Central Universities

(WK1013002), SRFDP 20120074110002, STCSM (10dz2220500), and the Open

Funding Project of State Key Laboratory of Luminescent Materials and Devices (SCUT).

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Table captions

Table 1 Photophysical and electrochemical properties of sensitizers LS-1, LS-4 and LS-5

in THF solution and adsorbed onto 3 µm transparent TiO2 films.

Table 2 Calculated TDDFT (CAMB3LYP) excitation energies for the lowest transition

(ev, nm), oscillator strengths (f), composition in terms of molecular orbital contributions,

and experimental absorption maxima

Table 3 Effect of CDCA on photovoltaic performances of LS-1, LS-4 and LS-5.

Table 4 Parameters obtained by fitting the impedance spectra of DSSCs based on LS-1,

LS-4 and LS-5 via the equivalent circuit.

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Table 1 Photophysical and electrochemical properties of sensitizers LS-1, LS-4 and LS-5

in THF solution and adsorbed onto 3 µm transparent TiO2 films.

Dyes LS-1 LS-4 LS-5

λmaxa / nm in THF

(ε / M–1 cm–1)

462

(31500)

518

(44500)

470

(34600) Absorption

λmaxb / nm onto TiO2 420 486 458

in THFc 0.69 0.55 0.76 HOMO (V) vs. NHE)

onto TiO2 filmd 0.68 0.55 0.76

E0–0e (V) 2.17 1.88 2.08

LUMO f (V) –1.49 –1.33 –1.32

[a] Absorption peaks (λmax) and molar extinction coefficients (ε) were measured in THF. [b] λmax was obtained with

adsorption onto 3 µm nanocrystalline TiO2 film deposited on FTO glass. [c] The HOMO was obtained in THF with

ferrocene as internal reference. [d] HOMO was obtained from the formal oxidation potentials adsorbed onto TiO2 film

deposited on FTO glass with ferrocene as external reference. [e] E0–0 was derived from the wavelength at 10%

maximum absorption intensity for the dye-loaded 3 µm nanocrystalline TiO2 film. [f] The LUMO was calculated with

the equation of LUMO = HOMO (on TiO2 film) – E0–0.

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Table 2 Calculated TDDFT (CAMB3LYP) excitation energies for the lowest transition

(ev, nm), oscillator strengths (f), composition in terms of molecular orbital contributions,

and experimental absorption maxima.

Compounds State Composition a E (eV, nm) f

LS-1 S1 86% H L 2.66 (466.0) 1.35

LS-4 S1 75% H L 2.43 (508.0) 1.82

LS-5 S1 71% H L 2.54 (487.8) 1.78

a H = HOMO, L = LUMO

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Table 3 Effect of CDCA on photovoltaic performances of LS-1, LS-4 and LS-5.

Dyes CDCA Voc/mV Jsc/mA cm–2 ff η (%)

0a 607 7.86 0.651 3.10

2 mMb 609 11.04 0.676 4.55

5 mMc 612 8.40 0.692 3.56

LS-1

10 mMd 615 7.80 0.701 3.36

0a 632 10.04 0.701 4.45

2 mMb 642 13.23 0.711 6.05

5 mMc 637 12.11 0.703 5.42

LS-4

10 mMd 633 9.70 0.706 4.34

0a 633 11.19 0.688 4.88

2 mMb 638 11.68 0.696 5.15

5 mMc 632 8.99 0.683 3.88

LS-5

10 mMd 635 8.33 0.685 3.62

[a-d] TiO2 films were immersed in dye solution under various coadsorption concentrations of CDCA.

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Table 4 Parameters obtained by fitting the impedance spectra of DSSCs based on LS-1,

LS-4 and LS-5 via the equivalent circuit.

cells LS-1 LS-4 LS-5

Rh/Ohm cm–2 19.58 20.53 19.55

RCT/Ohm cm–2 21.66 42.07 38.02

n1 0.4391 0.8955 0.8623

Cµ/µF cm–2 493.7 472.9 422.9

RPt/Ohm cm–2 15.73 7.577 10.5

n2 0.9993 0.7022 0.6525

CPt/µF cm–2 387.5 145.4 232.4

τe /ms 4.15 6.16 5.90

Note: Equivalent circuit of the DSSC consisting of TiO2/dye/electrolyte and Pt/electrolyte interface (Fig. 9c); Rh, RCT,

RPt are the series resistance of Pt and FTO glass, charge-transfer resistance at Pt/electrolyte and at TiO2/dye/electrolyte

interface, respectively; Cµ and CPt are the constant phase element for the TiO2/dye/electrolyte and Pt/electrolyte

interface, respectively. n presents the degree of surface inhomogeneity; τe is calculated from the through the relation τe

=1/(2πf).

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Figure Captions:

Fig. 1. Chemical structures of sensitizers LS-1, LS-4 and LS-5.

Fig. 2. Absorption spectra of LS-1, LS-4 and LS-5 measured in THF (a), adsorbed onto 3

µm transparent TiO2 films (b), and in THF with the existence of excessive triethylamine

(c).

Fig. 3. (a) Cyclic voltammetry plots of LS-1, LS-4 and LS-5 measured in THF with

ferrocene as internal reference; (b) Schematic diagram of energy levels of TiO2

conduction band, dyes coated on TiO2 films and I–/I3– redox couple.

Fig.4. Optimized ground-state geometries for LS-1, LS-4 and LS-5.

Fig. 5. Calculated frontier orbitals of dyes LS-1, LS-4 and LS-5 (isodensity = 0.020 a.u.).

Fig. 6. IPCE action spectra of DSSCs based on LS-1, LS-4 and LS-5, coadsorbed with 2

mM CDCA.

Fig. 7. Photocurrent–voltage characteristics of DSSCs sensitized by LS-1, LS-4, and LS-

5.

Fig. 8. Light-intensity dependence of the photocurrent density for LS-1 and LS-4 based

devices under short-circuits conditions.

Fig. 9. EIS Nyquist (a), Bode (b) plots and equivalent circuit (c) for DSSCs based on LS-

1, LS-4 and LS-5 measured under dark.

Scheme 1. Synthetic routes of LS-4 and LS-5: (i) (a) n-BuLi, B(OCH3)3; (b) THF, K2CO3,

Pd(PPh3)4, Reflux for 12 h. (ii) Cyanoacetic acid, piperidine, acetonitrile, reflux for 8 h.

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N SN

S

O O

SCOOH

NC

LS-4LS-1

COOH

NC

S

O O

SCOOH

NC

LS-5

N

Fig. 1. Chemical structures of sensitizers LS-1, LS-4 and LS-5.

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Fig. 2. Absorption spectra of LS-1, LS-4 and LS-5 measured in THF (a), adsorbed onto 3

µm transparent TiO2 films (b), and in THF with the existence of excessive triethylamine

(c).

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Fig. 3. (a) Cyclic voltammetry plots of LS-1, LS-4 and LS-5 measured in THF with

ferrocene as internal reference; (b) Schematic diagram of energy levels of TiO2

conduction band, dyes coated on TiO2 films and I–/I3– redox couple.

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Fig. 4. Optimized ground-state geometries for LS-1, LS-4 and LS-5.

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Fig. 5. Calculated frontier orbitals of dyes LS-1, LS-4 and LS-5 (isodensity = 0.020 a.u.).

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Fig. 6. IPCE action spectra of DSSCs based on LS-1, LS-4 and LS-5, coadsorbed with 2

mM CDCA.

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Fig. 7. Photocurrent–voltage characteristics of DSSCs sensitized by LS-1, LS-4, and LS-

5.

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Fig. 8. Light-intensity dependence of the photocurrent density for LS-1 and LS-4 based

devices under short-circuits conditions.

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Fig. 9. EIS Nyquist (a), Bode (b) plots and equivalent circuit (c) for DSSCs based on LS-

1, LS-4 and LS-5 measured under dark.

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N

Br

NS

O O

NS

O O

S CHO LS-4

iii

i

N

B(OH)2

i

S

O O

Ni

S

O O

S

N

CHOLS-5

ii

E

A B C

D F

Scheme 1. Synthetic routes of LS-4 and LS-5: (i) (a) n-BuLi, B(OCH3)3; (b) THF, K2CO3,

Pd(PPh3)4, Reflux for 12 h. (ii) Cyanoacetic acid, piperidine, acetonitrile, reflux for 8 h.

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Organic sensitizers incorporating 3,4-ethylenedioxythiophene as the

conjugated bridge: Joint photophysical and electrochemical analysis

of photovoltaic performance

Wenqin Lia,†, Bo Liub,†, Yongzhen Wua, Shiqin Zhua, Qiong Zhanga, and Weihong

Zhua,*

Research highlights

> 3,4-Ethylenedioxythiophene was introduced as the conjugated bridge in organic

sensitizers.

> Light-harvesting ability was enhanced with a notable increase in Jsc.

> Joint photophysical and electrochemical analysis of conjugated bridge on

photovoltaic performances.

> The driving force of ~150 mV is workable for the indoline-based dye regeneration

process, resulting in an overall efficiency of 6.05%.