Molecular design of triarylamine dyes incorporating phenylene spacer and the influence of alkoxy substituent on the performance of dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2011.09.019

Molecular design and understanding the structure-property relationship of π-conjugated spacer play pivotal roles in realization of considerable enhancement performance of dye-sensitized solar cells (DSSCs). Three triphenylmaine dyes, namely MX11-13, have

Full text of DOI:10.1016/j.jphotochem.2011.09.019

Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 8–16
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Molecular design of triarylamine dyes incorporating phenylene spacer and the
influence of alkoxy substituent on the performance of dye-sensitized solar cells
Hongyu Han, Mao Liang∗, Kai Tang, Xiaobing Cheng, Xueping Zong, Zhe Sun, Song Xue∗
Department of Applied Chemistry, Tianjin University of Technology, Tianjin 300384, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Molecular design and understanding the structure–property relationship of ␲-conjugated spacer play
pivotal roles in realization of considerable enhancement performance of dye-sensitized solar cells
(DSSCs). Three triphenylmaine dyes, namely MX11-13, have been designed and synthesized, which incor-
porate 2,2,6,6-tetramethylbenzo[1,2-d;4,5-dꢀ]bis[1,3]dioxole (TMBD), phenyl and 1,4-dipropoxybenzene
(DPB) as ␲-conjugated spacer, respectively. The effects of alkoxy substituent upon the photophysical,
electro-chemical characteristics and performance of dye-sensitized solar cells are investigated. For a
typical device, the MX13-based cell affords an overall power conversion efficiency (ꢀ) of 7.02%, with
Received 23 June 2011
Received in revised form
11 September 2011
Accepted 22 September 2011
Available online 29 September 2011
Keywords:
Dye-sensitized solar cell
Photovoltaic
High molar extinction coefficient
Phenylene spacer
short-circuit photocurrent density (JSC), open-circuit voltage (VOC) and fill factor (ff) of 15.5 mA cm−2
,
697 mV and 0.65, respectively.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Most efficient metal-free organic sensitizers are constituted
by donor (D), ␲-conjugated spacer (␲) and acceptor moieties
(A). The ␲-conjugated spacer between the donor and accep-
tor is recognized an important role in determining the overall
conversion efficiency. Thiophene derivatives [9–15] and phenyl
derivatives [26–28] have been incorporated as the conjugated
spacer for tuning the wavelength ranges, absorption capability,
and other characteristics required for DSSCs. It is noted that
the effect of substituting groups of the chromophores on pho-
tovoltaic performance is distinguishable, apart from the impact
of chromophore itself. A successful molecular engineering was
achieved by incorporating long alkyl groups into the thiophene
group, which not only increased the photocurrent but also mini-
mized charge recombination [25]. Another successful substituting
group is alkoxy groups. Using 3,4-ethylenedioxythiophene (EDOT)
as ␲-conjugated spacer, which was connected to triphenylmaine
or functional triarylamine, exhibited high photocurrent and high
open-circuit photovoltage [21,22,29]. Very recently, Liang et al.
reported OR dyes bearing 3,4-propylenedioxythiophene (ProDOT)
linkers with enhanced performance for DSSCs [30]. These results
could be mainly attributed to broadened absorption spectra and
high molar extinction coefficients of organic dyes as a result of
alkoxy substituent effect. In addition, the side chain of both EDOT
and ProDOT is orbicular, guaranteeing the amount of dye-uptake
on the TiO2 film due to small size of the side chain.
There has been considerable interest for development of cheap
and easily accessible renewable energy sources due to increas-
ing energy demands and global warming. Compared to traditional
silicon-based solar cells, dye-sensitized solar cells (DSSCs) are
viewed as promising candidates for renewable clean energy
sources in virtue of their low manufacturing cost and impressive
photovoltaic performance [1]. The DSSCs composed of a meso-
porous semiconductor metal semiconductor [2], photosensitizer
[3], redox electrolyte [4], and a counter electrode are complex
systems. To achieve high solar power conversion efficiency, there-
fore, great efforts are focused on designing and synthesizing
new photosensitizers: Ru-based complexes and organic sensitiz-
ers. The former compounds contain expensive ruthenium metal
and require careful synthesis and tricky purification steps. On
the other hand, organic sensitizers with low cost, ease of struc-
tural tuning, and generally high molar extinction coefficients have
emerged as competitive alternatives to the Ru-based counter-
parts [5]. As a result, various tailor-made organic dyes including
coumarin [6,7], indoline [8], triphenylamine [9–15], dialkylaniline
[16,17], bis(dimethylfluorenyl) aminophenyl [18,19], truxeney-
laminophenyl [20–23] and carbazole1 [24,25] have been reported,
showing promising power efficiency values in the range of 5–10.3%.
However, this strategy of molecular design was not found
in construction of phenylene spacer after
literature. As part of our efforts to investigate the sub-
stituent effect of alkoxy groups, we designed and synthesized a
a search of the
∗
Corresponding authors. Tel.: +86 22 60214250; fax: +86 22 60214252.
E-mail addresses: liangmao717@126.com (M. Liang), xuesong@ustc.edu.cn
(S. Xue).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.09.019

H. Han et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 8–16
9
standard. The redox potential of ferrocene internal reference is
taken as 0.63 V vs NHE. Electrochemical impedance spectroscopy
(EIS) in the frequency range of 100 mHz to 100 kHz was performed
with a PARSTAT 2273 potentiostat/galvanostat/FRA in the dark with
the alternate current amplitude set at 10 mV.
2.3. Fabrication of DSSCs
TiO2 colloid was prepared according to the literature method
[31], which was used for the preparation of the nanocrystalline
films. The TiO2 paste consisting of 18 wt.% TiO2, 9 wt.% ethyl cellu-
lose and 73 wt.% terpineol was firstly prepared, which was printed
on a conducting glass (Nippon Sheet Glass, Hyogo, Japan, fluorine-
doped SnO2 over layer, sheet resistance of 10 ꢁ/sq) using a screen
printing technique. The thickness of the TiO2 film was controlled
by selection of screen mesh size and repetition of printing. The
film was dried in air at 120 ◦C for 30 min and calcined at 500 ◦C for
30 min under flowing oxygen before cooling to room temperature.
The heated electrodes were impregnated with a 0.05 M titanium
tetrachloride solution in a water-saturated desiccator at 70 ◦C for
30 min and fired again to give a ca. 10 ␮m thick mesoscopic TiO2
film. The TiO2 electrode was stained by immersing it into a dye solu-
tion containing 300 ␮M dye sensitizers (ethanol–dichloromethane
3:1) for 24 h at room temperature. Then the sensitized-electrode
was rinsed with dry ethanol and dried by a dry air flow. Pt catalyst
was deposited on the FTO glass by coating with a drop of H2PtCl6
solution (40 mM in ethanol) with the heat treatment at 395 ◦C for
15 min to give photoanode. The dye-covered TiO2 electrode and
Pt-counter electrode were assembled into a sandwich type cell
according to the literature method [31]. The DSSCs had an active
area of 0.16 cm2 and electrolyte composed of 0.6 M 1,2-dimethyl-
3-n-propylimidazolium iodide (DMPImI), 0.1 M LiI, 0.05 M I2, and
0.5 M tertbutylpyridine in acetonitrile.
Scheme 1. Molecular structures of MX11–13.
new building block, namely 2,2,6,6-tetramethylbenzo[1,2-d;4,5-
dꢀ]bis[1,3]dioxole (TMBD), which shares attractive properties
with EDOT/ProDOT such as high electron-richness and control-
ling dye aggregation arising from nonplanar three-dimensional
(3D) branched structure. Herein we reported the synthesis and
characterization of three triphenylmaine dyes, namely MX11,
MX12 and MX13, that incorporating TMBD, phenyl and 1,4-
dipropoxybenzene (DPB) as ␲-conjugated spacer, respectively. In
particular, the profound impacts of alkoxy substituent groups upon
the photophysical, electro-chemical characteristics and perfor-
mance of dye-sensitized solar cells were investigated. Molecular
structures of these three dyes are shown in Scheme 1.
2. Experimental
2.1. Materials and instruments
The synthetic routes for dyes MX11–13 are shown in Scheme 2.
n-Butyllithium was purchased from Alfa. N,N-Dimethylformamide
was dried over and distilled from CaH2 under an atmosphere
of nitrogen. Phosphorus oxychloride was freshly distilled before
use. Dichloromethane and chloroform were distilled from calcium
hydride under nitrogen atmosphere. Titanium(IV) isopropoxide,
tertbutylpyridine and lithium iodide were purchased from Aldrich.
All other solvents and chemicals used in this work were analytical
grade and used without further purification.
2.4. Characterization of DSSCs
The photocurrent–voltage (J–V) characteristics of the solar cells
were carried out using a Keithley 2400 digital source meter con-
trolled by a computer and a standard AM1.5 solar simulator Oriel
91160-1000 (300 W) SOLAR SIMULATOR 2 × 2 BEAM. The light
intensity was calibrated by an Oriel reference solar cell. The action
spectra of monochromatic incident photon-to-current conversion
efficiency (IPCE) for solar cell were performed by using a com-
mercial setup (QTest Station 2000 IPCE Measurement System,
CROWNTECH, USA).
1H NMR and 13C NMR spectra were recorded on a Bruker AM-
400 spectrometer. The reported chemical shifts were against TMS.
Mass spectra were recorded on a LCQ AD (Thermofinnigan, USA)
mass spectrometer. The melting point was taken on a RY-1 ther-
mometer and temperatures were uncorrected.
2.5. Computational methods
2.2. Photophysical and electrochemical measurements
The geometrical structures of the three dyes were optimized by
performed density functional theory (DFT) calculations and time-
dependent DFT (TDDFT) calculations of the excited states at the
B3LYP/6-31+G(d) level with the Gaussian 03W program package.
The absorption spectra of the dyes either in solution or on
the adsorbed TiO2 films were measured by HITACHI U-3310
spectrophotometer. Adsorption of the dye on the TiO2 surface
was done by soaking the TiO2 electrode in a mixture solution
ethanol–dichloromethane 3:1 solution of the dye (standard con-
centration 3 × 10−4 M) at room temperature for 24 h. Fluorescence
measurements were carried out with a HITACHI F-4500 fluores-
cence spectrophotometer. FT-IR spectra were obtained with a
Bio-Rad FTS 135 FT-IR instrument.
Cyclic voltammetry measurements were performed at room
temperature on a computer controlled LK2005A electrochemical
workstation with Pt-wires as working electrode and counter elec-
trode, Ag/AgCl electrode as reference electrode at a scan rate of
100 mV s−1. Tetrabutylammonium perchlorate (TBAP, 0.1 mol/L)
and MeCN were used as supporting electrolyte and solvent, respec-
tively. The measurements were calibrated using ferrocene as
2.6. The detailed experimental procedures and characterization
data
2.6.1. Synthesis of carbaldehyde 2
2,2,6,6-Tetramethyl-benzo[1,2-d;4,5-dꢀ]bis[1,3]dioxole
[32]
(446 mg, 2 mmol) was added to a round bottom flask under
nitrogen, followed by diethyl ether (25 mL) and TMEDA (1.0 mL,
6.6 mmol). A 2.2 M n-BuLi solution in hexane (3.0 mL, 6.6 mmol)
was slowly added at room temperature. The solution was then
brought to reflux for 1 h. After being re-cooled to room temper-
ature, DMF (1.2 mL, 16 mmol) was slowly added to the cloudy
solution. The colored solution was again brought to reflux for 2 h.
The reaction was cooled to room temperature and 10 mL water

10
H. Han et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 8–16
Scheme 2. Synthetic route toward the MX11–13. (a) TMEDA, n-BuLi, ether, reflux. (b) DMF, reflux. (c) DMF/POCl3, 100 ◦C. (d) NaBH4, EtOH, reflux. (e) PPh3·HBr, CHCl3, reflux.
(f) ButOK, THF, 0 ◦C, 30 min. (g) Carbaldehyde 2, THF, reflux. (h) 1,4-Phthalaldehyde, THF, reflux. (i) 2,5-Dipropoxy-benzene-1,4-dicarbaldehyde, THF, reflux. (j) CNCH2COOH,
CH3CN, piperidine, reflux.
was added. The aqueous layer was extracted by dichloromethane.
The organic layer was combined and dried over MgSO4. The
solution was filtered and evaporated under reduced pressure
to get a salmon pink solid. The solid was purified by column
chromatography on silica gel (petroleum/ethyl acetate = 10:1 as
the eluent) to yield a red solid (292 mg, 53%). Mp: 215–217 ◦C. IR
The combined organic layers were washed with brine and then
dried over MgSO4. The solution was filtered, and concentrated
in vacuo giving a crude oil. The oil was dissolved in 25 mL
of anhydrous chloroform and PPh3HBr (620 mg, 1.81 mmol)
was added. The reaction was then brought to reflux for 1 h
before being re-cooled to ambient temperature. The mixture
was concentrated in vacuo to remove the solvent. The (4-(bis(4-
(hexyloxy)phenyl)amino)benzyl)triphenylphosphonium bromide
was obtained (1.448 g, 1.81 mmol) as a yellow oil.
(KBr): 2998, 2837, 2755, 1686, 1478, 1383, 1313, 1279, 1047 cm−1
.
1H NMR (CDCl3, 400 MHz): ı 10.12 (s, 2H), 1.77 (s, 12H). 13C NMR
(CDCl3, 100 MHz): ı 185.1, 141.3, 122.5, 109.2, 25.9. HRMS (ESI)
calcd for C14H
14O6 (M+H+): 279.0868. Found: 279.0868.
2.6.4. Synthesis of carbaldehyde 6
2.6.2. Synthesis of carbaldehyde 4
The compound 5 (2.50 g, 3.12 mmol) was dissolved in 30 mL of
dry THF under N2 atmosphere and cooled to 0 ◦C. Potassium tert-
butoxide (349 mg, 3.12 mmol) was added slowly while stirring the
solution vigorously. The generated red solution was stirred at 0 ◦C
for another 30 min, and then brought to room temperature. After
that, compound 2 (868 mg, 3.12 mmol) was added in portion and
the mixture was heated to reflux for 5 h. The solution was poured
into ice water and extracted with dichloromethane. The organic
layer was washed with water and dried with MgSO4. The prod-
uct was obtained by a column chromatography (petroleum:ethyl
acetate = 10:1 as the eluent) as a bright yellow solid (786 mg, 35%).
Mp: 145–147 ◦C. IR (KBr): 3422, 2930, 2840, 1687, 1652, 1596,
To a solution of compound 3 (1 g, 2.24 mmol) in anhydrous
DMF (10 mL, 126 mmol) at 0 ◦C under nitrogen atmosphere was
added POCl3 (2 mL, 13.40 mmol) dropwise and stirred for 1 h. Sub-
sequently, the mixture was heated at 90–100 ◦C for 2 h. The mixture
was cooled and poured into an ice-water with vigorous stirring.
After neutralization with NaOH, the mixture was extracted with
ethyl acetate. The organic fractions were combined and dried over
anhydrous MgSO4. The resulting oil was purified by column chro-
matography on silica gel (petroleum:ethyl acetate = 10:1 as the
eluent) to yield a bright yellow oil 4 (856 mg, 85.6%). IR (KBr):
2929, 2859, 1688, 1592, 1562, 1504, 1469, 1390, 1241, 1160,
1028, 826 cm−1
.
1H NMR (400 MHz, CDCl3): ı 9.75 (s, 1H), 7.63
1506, 1448, 1384, 1329, 1243, 1159, 1044, 1005, 831 cm−1 1H NMR
.
(d, J = 8.00 Hz, 2H), 7.12 (d, J = 8.40 Hz, 4H), 6.89 (d, J = 8.40 Hz, 4H),
6.85 (d, J = 8.00 Hz, 2H), 3.96 (t, J = 6.32 Hz, 4H), 1.82–1.75 (m, 4H),
1.48–1.45 (m, 4H), 1.36–1.34 (m, 8H), 0.93 (t, J = 6.36 Hz, 6H). 13C
NMR (100 MHz, CDCl3): ı 190.2, 157.0, 154.1, 138.6, 131.4, 128.0,
127.7, 116.7, 115.6, 68.4, 31.6, 29.4, 25.8, 22.6, 14.0. HRMS (ESI)
calcd for C31H39NO3 (M+H+): 474.3008. Found: 474.3009.
(CDCl3, 400 MHz): ı 10.04 (s, 1H), 7.48 (d, J = 16.4 Hz, 1H), 7.35 (d,
J = 8.4 Hz, 2H), 7.06 (d, J = 8.8 Hz, 4H), 6.89 (d, J = 8.4 Hz, 2H), 6.86
(d, J = 8.8 Hz, 4H), 6.84 (d, J = 16.4 Hz, 1H), 3.95 (t, J = 6.5 Hz, 4H),
1.80–1.75 (m, 16H), 1.48–1.45 (m, 4H), 1.36–1.34 (m, 8H), 0.92 (t,
J = 6.6 Hz, 6H). 13C NMR (CDCl3, 100 MHz): ı 185.1, 155.8, 149.2,
140.7, 140.3, 138.2, 135.9, 129.0, 127.8, 126.8, 120.1, 119.8, 115.3,
114.3, 112.8, 104.6, 76.7, 68.3, 31.6, 30.92, 29.3, 25.9, 25.8, 22.6,
14.0. HRMS (ESI) calcd for C45H53NO7 (M+H+): 720.3900. Found:
720.3905.
2.6.3. Synthesis of triarylamine 5
Carbaldehyde 4 (856 mg, 1.81 mmol) was added to a round
bottom flask, followed by ethanol (15 mL) and NaBH4 (137 mg,
3.62 mmol). Subsequently, the solution is then brought to reflux
for 1 h. The reaction was cooled to room temperature and
was quenched by saturated aqueous ammonium chloride solu-
tion. The aqueous layer was extracted with dichloromethane.
2.6.5. Synthesis of MX11
To
a solution of compound 6 (100 mg, 0.14 mmol) and
cyanoacetic acid (16 mg, 0.19 mmol) was added acetonitrile (5 mL),
dichlormethane (2.5 mL) and piperidine (100 ␮L). The solution was

H. Han et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 8–16
11
refluxed for 5 h. After cooling the solution, the solvent was removed
in vacuo. Dichloromethane was added. The organic layer was sep-
arated and washed 3 times with water, dried over anhydrous
MgSO4, and filtered. The pure product was obtained by silica gel
chromatography (dichloromethane:methanol = 10:1 as the eluent)
to give a red powder (76 mg, 69.0%). Mp: 187 − 189 ◦C. IR (KBr):
3422, 2918, 2849, 1593, 1566, 1506, 1241, 1150, 1120, 1021,
800 cm−1 1H NMR (400 MHz, CDCl3): ı 7.79 (s, 1H), 7.33 (d,
.
J = 8.7 Hz, 2H), 7.31 (d, J = 16.0 Hz, 1H), 7.00 (d, J = 8.7 Hz, 4H), 6.88 (d,
J = 8.8 Hz, 4H), 6.75 (d, J = 16.0 Hz, 1H), 6.72 (d, J = 8.5 Hz, 1H), 3.95
(t, J = 6.6 Hz, 4H), 2.17 (s, 2H), 1.80–1.75 (m, 16H), 1.48–1.45 (m,
4H), 1.36–1.34 (m, 8H), 0.92 (t, J = 6.6 Hz, 6H). 13C NMR (100 MHz,
DMSO): ı 156.0, 149.0, 140.7, 140.0, 138.7, 136.1, 128.1, 127.5,
119.8, 119.1, 116.0, 115.9, 79.6, 68.2, 67.5, 31.5, 29.2, 26.0, 25.7,
22.5, 14.3. HRMS (ESI) calcd for C48H54N2O8 (M+H+): 787.3958.
Found: 787.3953.
Fig. 1. Absorption spectra of MX11–13 in CH2Cl2 (1 × 10−5 M).
2.6.6. Synthesis of carbaldehyde 7
The product was synthesized according to the procedure for syn-
thesis of 6; yield 30%. IR (KBr): 3434, 2928, 2363, 1685, 1590, 1506,
J = 16.4 Hz, 1H), 7.15 (d, 1H), 7.00 (d, J = 8.7 Hz, 4H), 6.88 (d, J = 8.7 Hz,
4H), 6.73 (d, J = 8.4 Hz, 2H), 4.08 (t, J = 6.3 Hz, 2H), 3.94–3.87 (m, 6H),
1.80–1.74 (m, 4H), 1.68-1.61 (m, 4H), 1.37–1.35 (m, 4H), 1.28–1.20
(m, 8H), 1.01 − 0.98 (m, 6H), 0.87 − 0.84 (m, 6H). 13C NMR (100 MHz,
DMSO): ı 164.5, 157.2, 156.0, 153.4, 149.8, 149.1, 145.9, 139.9,
138.2, 132.6, 132.1, 132.0, 131.9, 129.1, 128.9, 127.5, 119.8, 116.2,
111.7, 110.3, 70.9, 70.6, 68.2, 53.0, 31.8, 29.5, 25.7, 22.6, 22.5, 22.5,
14.3, 11.1. HRMS (ESI) calcd for C48H58N2O6 (M+H+): 759.4373.
Found: 759.4366.
1473, 1240, 1072 cm−1 1H NMR (DMSO, 400 MHz): ı 9.93 (s, 1H),
.
7.83 (d, J = 16.4 Hz, 2H), 7.73 (d, J = 8.6 Hz, 2H), 7.43 (d, J = 8.4 Hz, 2H),
7.37 (d, J = 16.4 Hz, 1H), 7.11 (d, J = 16.4 Hz, 1H), 6.99 (d, J = 8.6 Hz,
4H), 6.87 (d, J = 8.6 Hz, 4H), 6.73 (d, J = 8.6 Hz, 2H), 3.89 (t, J = 5.6 Hz,
4H), 1.69–1.59 (m, 4H), 1.39–1.35 (m, 4H), 1.31–1.26 (m, 8H), 0.91
(t, J = 6.6 Hz, 6H). 13C NMR (DMSO, 100 MHz): ı 192.5, 167.4, 156.0,
149.2, 144.2, 139.9, 135.0, 132.3, 132.2, 131.9, 130.5, 129.1, 128.4,
127.4, 126.9, 124.5, 119.0, 115.9, 68.1, 65.5, 31.8, 30.5, 29.5, 25.7,
22.4, 19.1, 14.3, 14.0. HRMS (ESI) calcd for C39H45NO3 (M+H+):
576.3477. Found: 576.3481.
3. Results and discussion
3.1. UV–vis absorption/emission spectra
2.6.7. Synthesis of MX12
The product was synthesized according to the procedure for syn-
thesis of MX11, giving a salmon pink powder in 64.0% yield. Mp:
As Fig. 1 presents, we first recorded the UV–vis absorption spec-
tra of MX11–13 dissolved in dichloromethane so as to preliminarily
evaluate the substituent influence on the light capture capacity,
and the detailed parameters are summarized in Table 1. The three
dyes display two strong absorption bands at around 280–380 nm
and 400–600 nm, which mainly stem from the intramolecular
charge-transfer transition. MX11 featuring four alkoxy substituents
on the pheneylene has a maximum molar absorption coefficient
(ε) of 55 × 103 M−1 cm−1, which is remarkably higher than the
corresponding value of MX12 (21 × 103 M−1 cm−1) and MX13
(32 × 103 M−1 cm−1) in the visible region. The maximum absorp-
tion coefficient dependence on molecular structure is pronounced.
In general, those possessing electron donor groups on the spacer
displayed high values of ε, while those possessing withdrawing
groups displayed low values [33]. It is evident that the maxi-
mum molar absorption coefficient of organic dyes is enhanced with
the increasing alkoxy substituent groups. The maximum absorp-
tion peaks (ꢂmax) of MX11–13 were observed in visible region
at 453, 453 and 472 nm, respectively. The maximum absorption
peak dependence on dye structure is also pronounced. Those pos-
sessing large conjugation length [34] or large extent of electron
delocalization [35] displayed high maximum absorption peaks. The
geometry of molecular structure plays an important role in the
absorption peak as well [30,36]. Minor changes in the geometry
of molecular structure may result in different electron delocaliza-
tion. For the MX12 and MX13, the dihedral angles of phenylene
spacer and cyanoacetic acid are 3.4◦ and 1.5◦ (Fig. 2), respectively,
resulting in similar planar conjugating system. Therefore, a red shift
of MX13 relative to MX12 is mainly derived from more delocaliza-
tion over an entire conjugated system due to alkoxy substituent
groups. In the case of MX11, the dihedral angle between the TMBD
and cyanoacetic acid is 45.8◦ (Fig. 2), giving a more twisted con-
figuration than that of MX12-13. If there are no alkoxy substituent
groups on the phenylene spacer, MX11 will exhibit a blue-shifted
187–189 ◦C. IR (KBr): 3416, 1664, 1499, 1438, 1390, 1111, 559 cm−1
.
1H NMR (400 MHz, DMSO): ı 7.89 (s, 1H), 7.87 (d, J = 8.4 Hz, 2H), 7.65
(d, J = 8.4 Hz, 2H), 7.45 (d, J = 8.7 Hz, 2H), 7.31 (d, J = 16.3 Hz, 1H), 7.09
(d, J = 16.3 Hz, 1H), 7.03 (d, J = 8.8 Hz, 4H), 6.92 (d, J = 8.8 Hz, 4H),
6.75 (d, J = 8.7 Hz, 2H), 3.95 (t, J = 6.4 Hz, 4H), 1.73 − 1.66 (m, 4H),
1.43 − 1.37 (m, 4H), 1.32 − 1.28 (m, 8H), 0.89 (t, J = 6.8 Hz, 6H). 13
C
NMR (100 MHz, DMSO): ı 163.6, 155.9, 148.9, 147.0, 140.4, 140.0,
132.3, 130.6, 130.3, 128.8, 128.2, 127.4, 126.8, 124.9, 120.1, 119.3,
116.0, 68.1, 39.4, 31.5, 29.2, 25.7, 22.5, 14.4. HRMS (ESI) calcd for
C42H46N2O4 (M+H+): 643.3536. Found: 643.3530.
2.6.8. Synthesis of carbaldehyde 8
The product was synthesized according to the procedure for
synthesis of 6; yield 28%. IR (KBr): 2932, 2871, 1728, 1590, 1505,
1240 cm−1 1H NMR (DMSO, 400 MHz): ı 10.31 (s, 1H), 7.47
.
(d, J = 16.2 Hz, 1H), 7.44 (s, 1H), 7.39 (d, J = 8.6 Hz, 2H), 7.25 (d,
J = 16.2 Hz, 1H), 7.18 (s, 1H), 7.03 (d, J = 8.6 Hz, 4H), 6.91 (d, J = 8.6 Hz,
4H), 6.75 (d, J = 8.6 Hz, 2H), 4.13 (t, J = 6.3 Hz, 2H), 3.97–3.89 (m, 6H),
1.79–1.75 (m, 4H), 1.67–1.61 (m, 4H), 1.41–1.34 (m, 4H), 1.30–1.27
(m, 8H), 1.03–0.98 (m, 6H), 0.92–0.85 (m, 6H). 13C NMR (DMSO,
100 MHz): ı 188.4, 167.4, 156.3, 156.0, 150.2, 149.1, 139.9, 135.1,
133.1, 132.2, 132.0, 129.1, 128.2, 127.5, 123.4, 119.1, 115.9, 111.2,
110.1, 70.8, 70.5, 68.1, 65.5, 31.5, 30.9, 29.2, 25.7, 22.3, 19.1, 14.4,
14.0, 13.4, 11.0. HRMS (ESI) calcd for C45H57NO5 (M+H+): 692.4315.
Found: 692.4314.
2.6.9. Synthesis of MX13
The product was synthesized according to the procedure for
synthesis of MX11, giving a fuchsia powder in 55.0% yield.
Mp: 193–196 ◦C. IR (KBr): 3422, 2929, 2362, 1637, 1508, 1385,
1072 cm−1 1H NMR (400 MHz, DMSO): ı 8.51 (s, 1H), 7.4 (d,
.
J = 16.4 Hz, 1H), 7.38 (d, J = 8.4 Hz, 2H), 7.36 (s, 1H), 7.25 (d,

12
H. Han et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 8–16
Fig. 2. Optimized geometrical configuration of MX11–13.
Fig. 3. Absorption spectra of MX11–13 on TiO2 film.
Fig. 4. Cyclic voltammogram of MX11–13.
absorption relative to MX12. However, the maximum absorption
peak of MX11 is the same as that of MX12. This result indicated
that extent of electron delocalization is enhanced with increased
alkoxy substituent groups on the phenylene spacer. Therefore, the
introduction of alkoxy substituent groups in ␲ spacer favors the
light harvesting of organic dyes.
3.2. Electrochemical properties
To obtain and understand the molecular orbital energy levels,
cyclic voltammetry (CV) was employed to measure the ground-
state oxidation potential (Eox) of the dyes in acetonitrile solution.
Representative cyclic voltammograms are shown in Fig. 4. The
first quasi-reversible one-electron oxidation wave is taken as the
highest occupied molecular orbital (HOMO) level. The excited-
state potential (Eox*), reflecting the LUMO level of the dye, can
be derived from the ground-state oxidation potential and the
zero–zero excitation energy (E0–0) according to the following
equation: Eox* = Eox − E0–0. As depicted in Table 1, the HOMO
levels of dyes MX11 and 13 (0.81 V vs NHE) are lower than
that of MX12 (0.83 V vs NHE) because of the presence of the
Upon adsorption onto transparent mesoporous TiO2 films
(3 ␮m), MX11–13 show an evident hypsochromic effect com-
pared with that measured in solution (Fig. 3), indicating
a
weaker electron-withdrawing capability of the carboxylatetita-
nium assembly than that of the carboxylic acid [37]. Emission
maxima of dyes MX11–13 can be found at 500–700 nm when
the dyes are excited at their respective absorption bands at
400–600 nm (Table 1).
Table 1
Optical properties and electrochemical properties of three dyes.
a
Dye
ꢂmax/nm (ε/103 M−1 cm−1
)
ꢂmax/nmb
ꢂint/nmc
E0–0/eVd
Eox/Ve
Eox*/Vf
MX11
MX12
MX13
453 (37)
453 (14)
472 (21)
605
626
620
541
537
556
2.29
2.31
2.23
0.81
0.83
0.81
−1.48
−1.48
−1.42
a
Absorption spectra of dyes measured in DCM (1.0 × 10−5 M).
Emission spectra of dyes measured in DCM (1.0 × 10−5 M)
b
ε is the extinction coefficient at ꢂmax of absorption.
c
The intersect of the normalized absorption and the emission spectra (ꢂint).
d
e
f
E0–0 values were calculated from: E0–0 = 1240/ꢂint
Eox (vs NHE) of the dyes in acetonitrile was measured by cyclic voltammogram.
Eox* (vs NHE) was calculated from Eox* = Eox − E0–0
.
.

H. Han et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 8–16
13
Table 2
Photovoltaic performance of DSSCs sensitized with the three dyes.a
Dye
JSC/mA cm−2
VOC/mV
ff
ꢀ/%
MX11
MX12
MX13
10.6
15.0
15.5
681
720
697
0.67
0.64
0.65
4.84
6.91
7.02
a
Photovoltaic performances of DSSCs were measured under irradiation of
AM1.5G simulated solar light (100 mW cm−2) at room temperature with a 0.16 cm2
working area.
electron-donating alkoxy substituent groups. They are all more
positive than the Nernst potential of I−/I3 redox couple (0.4 V vs
−
NHE), ensuring regeneration of the oxidized dyes by I− after elec-
tron injection. The LUMO levels of MX13 (−1.42 V vs NHE) is more
positive than that of dyes MX11 and MX12 (−1.48 V vs NHE), nar-
rowing the HOMO–LUMO energy gaps and resulting in red shift
of absorption spectra. They are all negative than the conduction
band of TiO2 (ECB, −0.5 V vs NHE), which provided sufficient driv-
ing forces for electron injection. Therefore, the HOMO–LUMO levels
of MX11–13 are suitable for DSSCs.
Fig. 6. Current–potential (J–V) curves for the DSSCs based on MX11–13 under AM1.5
irradiation (top) and in the dark (bottom).
conditions, MX11 gave JSC of 10.6 mA cm−2, VOC of 681 mV, and ff
of 0.67, corresponding ꢀ of 4.84%.
3.3. Photovoltaic performance
3.4. Dependence of short-circuit photocurrent densities on the
interfacial charge recombination, LUMO levels and dye
aggregation.
The incident monochromatic photon-to-current conversion
efficiency (IPCE) measurements of DSSCs based on MX11–13 using
the redox electrolyte consisting of 0.6 M DMPImI, 0.05 M I2, 0.1 M
LiI, and 0.5 M TBP in acetonitrile are shown in Fig. 5. The IPCE spec-
tra of MX12 and MX13 exceed 75% in the visible spectral region
from 400 to 580 nm, reaching their maximum of 84% and 82% at
480–500 nm, respectively. Considering the light absorption and
scattering loss by the conducting glass, the broad and high pho-
tocurrent action spectrum indicates high light harvesting yield in
these devices [34]. MX13 exhibits a slightly broader IPCE absorp-
tion relative to MX12, which is in good accord with the preceding
absorption measurements. MX11 containing TMBD unit exhibits a
relatively lower IPCE relative to those of the MX12–13.
As shown in above, the substituent effect of alkoxy groups on
photovoltaic performance of DSSCs are obvious. When the pheneyl
is displaced by DBP containing two alkoxy groups, the JSC rises to
15.5 mA cm−2, leading to a higher efficiency of MX13. In contrast,
introduction of TMBD as spacer (MX11) does not lead to a further
enhanced JSC (10.6 mA cm−2) relative to that of MX13. To compre-
hend the significant JSC variation, factors contributing to IPCE need
to be considered. IPCE is expressed by the light harvesting efficiency
(LHE) and product of absorbed photon-to-current conversion effi-
ciency (APCE) [38]:
Photovoltaic characteristics of the DSSCs based on the three dyes
under AM1.5G condition (100 mW cm−2) are displayed in Table 2
and the J–V curves are plotted in Fig. 6. The short-circuit photocur-
rent densities (JSC), open-circuit photovoltages (VOC), and fill factors
IPCE(ꢂ) = LHE × APCE = LHE × ꢀcol × ˚inj
where APCE should be divided into two terms: the overall charge
collective efficiency (ꢀcol) and the overall electron injection effi-
ciency (˚inj).
(ff) of DSSCs sensitized by MX12 and MX13 are 15.0 mA cm−2
,
720 mV, 0.64 and 15.5 mA cm−2, 697 mV, 0.65, respectively, yield-
ing overall conversion efficiencies (ꢀ) up to 6.91% and 7.02%,
respectively. We attribute the excellent efficiencies to both broad
and high IPCE spectra of MX12 and MX13. Under the same
Light-harvesting efficiency: The LHE at the maximum absorption
wavelength can be estimated using the following equation [39]:
LHE = 1 − 10−A. Where A is the absorbance of the dye on TiO2 at the
maximum wavelength. According to our measurements, all the A
values for the three dyes on the TiO2 film (10 ␮m) are over 3, indi-
cating similar LHE values for these dyes. Therefore, any difference
in the photocurrent among DSSCs based on these dyes could be
attributed to the difference of ꢀcol and ˚inj. Several factors including
the interfacial charge recombination, LUMO levels and dye aggre-
gation were discussed to scrutinize the overall charge collective
efficiency and the overall electron injection efficiency in DSSCs,
which could make a critical contribution to the IPCE summit.
Charge collection efficiency: To probe the possible impact of
alkoxy substituents upon the interfacial charge recombination at
the TiO2/dye/electrolyte interface, the electrochemical impedance
spectroscopy (EIS) is measured for the DSSCs based on the three
dyes. Typical EIS Nyquist plots and Bode phase plots measured
in the dark under a forward bias of −0.7 V are shown in Fig. 7.
The equivalent circuit [22] presented in Fig. 7 was used to fit
the experimental data of all of the samples. Rs are the series
resistance accounting for the transport resistance of the TCO and
the electrolyte. Cꢃ and RCT are the chemical capacitance and the
charge recombination resistance at the TiO2/electrolyte interface,
respectively. CPt and RPt are the interfacial capacitance and charge
Fig. 5. IPCE spectra for DSSCs based on MX11–13.

14
H. Han et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 8–16
Fig. 7. EIS for DSSCs based on MX11–13 measured in the dark under −0.7 V bias
displayed in the form of Nyquist plots and Bode phase plots (insert); top right corner
is the equivalent circuit used to fit the impedance spectra.
Fig. 9. J–V curves of DSSCs sensitized by MX11 containing different concentration
of CDCA as coadsorbent under AM1.5 irradiation.
we have not been able to show the iodine binding to oxygen,
according to the results of EIS and dark line, it seems that there
is an electrostatic attraction of I3 by oxygen atoms in MX11 and
MXD13, leading to a faster charge recombination reaction relative
to MX12.
transport resistance at the Pt/electrolyte interface, respectively.
The larger semicircle at lower frequencies represents the RCT at
the TiO2/dye/electrolyte interface. The fitted RCT increases in the
order of MX11 (54 ꢁ) < MXD13 (82 ꢁ) < MXD12 (151 ꢁ), which is
consistent with the sequence of VOC values in the devices. In the
range of potentials studied (Fig. 8), the fitted RCT also increases in
the order of MX11 < MXD13 < MXD12. The smaller RCT value means
the electron recombination from the conduction band to the elec-
trolyte occurring more easily. This result can be cross-checked by
measuring the J–V characteristics of the DSSCs in the dark (Fig. 6).
Clearly, electron recombination in device will decrease the ꢀcol, thus
leading to lower IPCE. Therefore, the limited ꢀcol in device based on
MX11 is one of the reasons for the lower IPCE.
−
Electron injection efficiency: It is known that, an insufficient driv-
ing force for electron injection is possible to induce poor ˚inj. LUMO
levels of MX11, MX12 and MX13 are estimated to be −1.48, −1.48,
and −1.42 V vs NHE, respectively (Table 1). We find that the driving
forces for electron injection (Eox* − ECB) are very similar for MX11
and MX12, but the two dyes show quite different IPCE maxima.
Furthermore, the two dyes have identical binding groups and sim-
ilar electronic structure, suggesting that electronic coupling to CB
states will be similar [42]. Therefore, it is not the LUMO level that
limits the IPCE of MX11.
To estimate the dye aggregation on TiO2 film, we have mea-
sured the amounts adsorbed on the TiO2 film. By comparing the
absorbance change of a dye solution (300 ␮M) before and after dye
up-taking with a titania film, the surface coverages (ꢄ ) of MX11,
MX12 and MX13 anchored on TiO2 film were determined to be
5.08 × 10−10, 2.29 × 10−10 and 2.12 × 10−10 mol cm−2, respectively.
More intense light-absorption could be expected since the adsorp-
tion amount of MX11 is about 2.1-fold higher than that of MX12 and
MX13. However, the IPCE maximum of MX11 is fell behind that of
MX12 and MX13, which could be attributable to disadvantageous
intermolecular energy transfer as a result of aggregate formation
[30,43,44]. It seems that intermolecular ␲–␲ interaction in MX11
is stronger than that in MX12 and MX13, which may arise from the
large rigid conjugation structure of TMBD.
These results also indicate that recombination of conduction
band electrons to the electrolyte occurs more easily upon the sub-
stitution with the alkoxy groups. In a study on the VOC of DSSCs
using ruthenium dyes, O’Regan et al. have shown that a change
of oxygen to sulfur in two equivalent positions in a Ru-based dye
structure gives a 2-fold increase in the recombination rate and
a 20–30 mV loss in VOC [40]. Their results strongly support that
binding of iodine to the dye is a key element of the recombina-
tion process in DSSCs. In a study of interfacial electron-transfer
kinetics in metal-free organic dye-sensitized solar cells, Mori
−
et al. supposed that the indirect electrostatic attraction of I3 by
negatively charged atoms in the dyes (e.g.,
N atom in the
cyanoacrylic acid group) increases the local concentration of
−
I3 in the vicinity of TiO2, facilitating dark reaction [41]. Although
It is well known that unfavorable dye aggregation on the semi-
conductor surface could be avoided through optimization of the
molecular structure of the dye or by addition of coadsorbers
(i.e. chenodeoxycholic acid (CDCA)) that prevent aggregation and
enhance the ˚inj [43]. The effect of CDCA concentration in the dye
bath on the performance of MX11-based DSSCs was investigated
by raising [CDCA] from the originally employed 1–10 mM (Fig. 9).
Upon coadsorption with 1 mM CDCA, the JSC increased from 10.6 to
12.7 mA cm−2, resulting in accordingly improved ꢀ values (6.02%).
The increased JSC is attributed to enhancing ˚inj resulting from rel-
atively independent dye molecules arraying on TiO2 surface. When
[CDCA] exceeded 3 mM, the JSC dropped continuously because of
the decreasing of the dye adsorption amount. Under the same con-
ditions, CDCA has no obvious influence on the JSC of DSSCs based on
MX12 and MX13 (1 mM CDCA, MX12: JSC = 15.3 mA cm−2; MX13:
JSC = 15.6 mA cm−2). Though the optimized efficiency of MX11 is
still fell bellow the efficiency of MX12 and MX13, it could be
Fig. 8. Interfacial charge transfer resistance RCT fitted from impedance spectra under
a series of applied potentials.

H. Han et al. / Journal of Photochemistry and Photobiology A: Chemistry 225 (2011) 8–16
15
Acknowledgments
We are grateful to the National 863 Program (2009AA05Z421),
the National Natural Science Foundation of China (21003096) and
the Tianjin Natural Science Foundation (09JCZDJC24400) for finan-
cial supports.
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