The effect of additional electron donating group on the photophysics and photovoltaic performance of two new metal free D-π-A sensitizers

Article abstract of DOI:10.1016/j.dyepig.2015.05.028

The synthesis, photophysical, electrochemical and photovoltaic properties of two D-π-A organic sensitizers, having triphenylamine as electron donor (with or without additional electron donating hexyloxy group), carboxylic acid as electron acceptor and ben

Full text of DOI:10.1016/j.dyepig.2015.05.028

Accepted Manuscript
The effect of additional electron donating group on the photophysics and photovoltaic
performance of two new metal free D-π-A sensitizers
An. Margalias, K. Seintis, M.Z. Yigit, M. Can, D. Sygkridou, V. Giannetas, M. Fakis, E.
Stathatos
PII:
S0143-7208(15)00207-7
10.1016/j.dyepig.2015.05.028
DYPI 4791
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 17 February 2015
Revised Date: 2 May 2015
Accepted Date: 22 May 2015
Please cite this article as: Margalias A, Seintis K, Yigit MZ, Can M, Sygkridou D, Giannetas V, Fakis
M, Stathatos E, The effect of additional electron donating group on the photophysics and photovoltaic
performance of two new metal free D-π-A sensitizers, Dyes and Pigments (2015), doi: 10.1016/
j.dyepig.2015.05.028.
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.

ACCEPTED MANUSCRIPT
The effect of additional electron donating group on the photophysics
and photovoltaic performance of two new metal free D-
π-A sensitizers.
a
b
c
d,*
a,b
b
An. Margalias , K. Seintis , M.Z. Yigit , M. Can , D. Sygkridou , V. Giannetas , M.
b,*
a,*
Fakis , E. Stathatos .
a
Electrical Engineering Department, Technological-Educational Institute of Western
Greece, 26334 Patras, Greece
b
Department of Physics, University of Patras, 26500 Patras, Greece
c
Department of Material Science and Engineering, Faculty of Engineering and
Architecture, Izmir Katip Celebi University, Cigli, 35620 Izmir, Turkey
d
Department of Engineering Sciences, Faculty of Engineering and Architecture, Izmir
Katip Celebi University, Cigli, 35620 Izmir, Turkey
*
Corresponding author: Department of Electrical Engineering, Technological-Educational
Institute of Western Greece, GR-26334 Patras, Greece. Phone: +30 2610-369242, Fax: +30
2
*
610-369193, e-mail: estathatos@teiwest.gr (E. Stathatos).
Co-Corresponding author: Department of Engineering Sciences, Faculty of Engineering and
Architecture, Izmir Katip Celebi University, Cigli, 35620 Izmir, Turkey. Phone: +90232 329
3
*
535/3725, e-mail: mustafa.can@ikc.edu.tr (M. Can).
Co-Corresponding author: Department of Physics, University of Patras, 26500 Patras, Greece.
Phone: +30 2610 996794, e-mail: fakis@upatras.gr (M. Fakis).
1

ACCEPTED MANUSCRIPT
Abstract
The synthesis, photophysical, electrochemical and photovoltaic properties of two
D-π-A organic sensitizers, having triphenylamine as electron donor (and hexyloxy as
additional electron donating group), carboxylic acid as electron acceptor and
benzimidazole group as π-bridge, are described in detail. The photophysical
measurements were carried out for the dyes in solutions and on TiO and Al O
3
2
2
nanostructured films using time resolved spectroscopy in the fs-ps and ns timescale. The
electron injection dynamics on a series of TiO nanostructured films sensitized with the
2
two dyes, have shown a faster electron injection for the dye with additional electron
donating group. The photovoltaic characterization has shown a maximum overall
conversion efficiency of 3.4% for dye sensitized solar cells with a quasi-solid state
electrolyte. The performance of the solar cells is explained in terms of the photophysical
properties, electron injection dynamics and local resistance to charge transfer across the
TiO -electrolyte interface measured with electrochemical impedance spectroscopy.
2
Keywords: Donor/π-bridge/acceptor structure, dye sensitized solar cells, photophysics,
electron injection dynamics, impedance spectroscopy.
2

ACCEPTED MANUSCRIPT
1
. Introduction
Third generation dye sensitized solar cells (DSSCs) have been proposed within the
last two decades, as low-cost alternatives to conventional amorphous silicon
photovoltaics owing to the simplicity of their fabrication procedure, practically under
ambient conditions with mild chemical processes [1,2]. The unique design of DSSCs
offers choices for systematically changing the materials of the different parts that
constitute the DSSCs in order to optimize and improve their performance. One of the
main components of the DSSCs is the photosensitizer which is responsible for the visible
and near-infra-red utilization of the solar light since metal oxide semiconductors which
are also employed in the cells, absorb only in the ultraviolet spectral region. At present,
ruthenium complexes and metal porphyrins photosensitizers have an overall solar to
energy conversion efficiency higher than 12% for small size devices [3,4]. However,
metal free organic dyes can also be considered as an alternative choice for the conversion
of solar energy to electricity in DSSCs [5-11]. Pure organic dyes can be prepared
according to cheap synthetic routes or finally obtained from nature as component of
flowers and fruits. The reason for choosing pure organic dyes is that they have certain
advantages over metal based photosensitizers; they are easily modified with common
synthetic procedures, while they exhibit high molar extinction coefficients compared to
metal complexes. Organic dyes with high molar extinction coefficients can be used
advantageously in thin TiO film based solar cells which are mainly required in solid
2
state devices where the mass transport and pore filling are limited affecting the
performance of the cells.
3

ACCEPTED MANUSCRIPT
In recent years, most of the published work on the synthesis of new pure organic
dyes for DSSCs has been focused on the Donor-π-Acceptor (D-π-A) system owing to its
effective photo-induced intramolecular charge transfer characteristics [12-17].
Compounds commonly used as electron donors are derivatives of triphenylamine [18-20],
coumarin [21, 22], carbazole [23, 24] or indoline [25, 26], with the triphenylamine being
the most popular. Thiophene [27], benzene [28] or fluorene [29] fragments act, among
others, as the π-conjugated linker between the electron donor and acceptor. The electron
acceptor, usually a carboxyl group or cyanoacrylic acid fragment, also plays the role of
2
an anchoring group responsible for binding on the TiO mesoporous film surface.
Although a reasonable number of pure organic sensitizers have been reported to date, a
quest for dyes exhibiting high molecular extinction coefficients and additional optimized
properties to meet the criteria needed for efficient DSSCs devices is still ongoing. This is
obvious by the large number of recently published articles in the field [30-36]. In several
cases adding auxiliary electron donating groups coupled to the triphenylamine moiety,
has been shown to improve the properties of the sensitizer by enhancing their
intramolecular charge transfer characteristics and by inducing a bathochromic shift of the
absorption spectrum [37-39]. In addition, introducing auxiliary electron donating groups
can lead to a decreased possibility of π-π aggregation. On the other hand, the addition of
bulky donors can also have detrimental effects by reducing the amount of adsorbed dye
molecules. In a very recent work, we have reported on the effect of additional methoxy
groups coupled to the triphenylamine where an enhanced spectral response was observed
for the methoxy-substituted dye [40]. However, an undesired faster electron-cation
recombination was also observed in these dyes.
4

ACCEPTED MANUSCRIPT
In this respect, we present here the synthesis and detailed photophysical and solar
cell characterization of two new organic dyes with D-π-A structure (MZ-341 and MZ-
2
2
35, Scheme 1), where the donor is a triphenylamine group which, in the case of MZ-
35, is coupled to additional electron donating hexyloxy groups. The effect of this
additional electron donating groups on the photophysical and solar cell properties of these
dyes is reported. The π-conjugated bridge between donor and acceptor is benzimidazole
derivative. It is the first time that a weak acceptor such as benzimidazole group is used
between donor (triphenylamine group) and acceptor (carboxylic acid) to achieve lower
reduction potential and to facilitate charge transfer. The photophysical properties of the
dyes are studied in dye solution with different solvent polarity by means of steady state
absorption and fluorescence spectroscopy as well as with time resolved fluorescence
spectroscopy in the fs-ps and ns time scale. The injection dynamics are studied for the
dyes adsorbed on the surface of TiO films. Finally, electrochemical impedance
2
spectroscopy (EIS) measurements and electrical characterization of DSSCs employing
2
the new dyes were performed for thin TiO nanocrystalline films (~2 ꢀm) and quasi-solid
state electrolyte while the results are discussed in terms of the photophysical properties of
the dyes and electrochemical impedance measurements.
2
. Experimental Section
2
.1 Dyes and synthetic route
The chemical structures of the newly synthesized sensitizers are presented in
Scheme 1. The electron donor in MZ-341 is a triphenylamine group which in the case of
MZ-235 is coupled to additional electron donating hexyloxy groups. The electron
5

ACCEPTED MANUSCRIPT
acceptor is a carboxylic acid while benzimidazole group is used a π-conjugated linker.
The synthesis of the sensitizers is described below and briefly the procedure is presented
in Scheme 2:
Synthesis of 1-(hexyloxy)- 4-iodophenol (1). A mixture of 4-iodophenol (8.8g,
4
0mmol), potassium carbonate (5.6g, 40mmol), 18-Crown-6 (1g, 4mmol), acetone
(100mL) and1-bromohexane (6.6g, 40mmol) were added in a round bottomed flask and
refluxed with stirring overnight. The reaction was controlled and ended with thin-layer
chromatography (TLC), cooled down to room temperature, filtered and the reaction
mixture was extracted with diethylether (2 x 20mL) and water (2 x 20mL). The organic
phase was separated, dried over sodium sulphate and the solvent was evaporated by
rotary evaporator. The crude product was purified by column chromatography
1
(
dichloromethane/hexane: 1/1, V/V) to obtain a colorless oil (91%, yield). H NMR (400
MHz CDCl ): δ 7.50 (d, 2H), 6.63 (d, 2H), 3.87 (t, 2H), 1.76-1.70 (m, 2H), 1.44-1.39 (m,
3
1
3
2
6
H), 1.34-1.29 (m, 4H), 0.89 (t, 3H). C NMR (400 MHz CDCl ): δ 138.37, 117.18,
3
8.35, 31.84, 29.41, 25.96, 22.87, 14.30.
Synthesis of (4-bromopheny)bis[4-(hexyloxy)phenyl]amine (2). In a round bottomed
flask, a mixture of CuI (0.2g, 1mmol) and phenanthroline (0.18g, 1mmol) were added
and dissolved in toluene (10mL). Dean-Stark aparatus and reflux condenser were set and
the reaction mixture was stirred for half an hour. 1-(hexyloxy)-iodobenzene (5g,
1
6mmol), 4-bromoaniline (1.65g, 9,6mmol), potassium hydroxate (4.8g, 77mmol) and
toluene (20mL) were added and the mixture was refluxed with stirring overnight. The
reaction was controlled and ended with TLC, cooled to the room temperature, filtered
over celite and washed with dichloromethane. The reaction mixture was extracted with
6

ACCEPTED MANUSCRIPT
dichloromethane (3x20mL) and water (3x20mL). The organic phase was separated, dried
over sodium sulphate and the solvent was evaporated by rotary evaporator. The crude
product was purified by column chromatography (dichloromethane/hexane: 1/4, V/V) on
1
silica gel to yield yellow oil (66% yield). H NMR (400 MHz CDCl ): δ 7.21 (d, 2H),
3
7
.00 (d, 2H), 6.8 (d, 6H), 3.91 (t, 4H), 1.78-1.72 (m, 4H), 1.46-1.42 (m, 4H), 1.36-1.32
1
3
(
m, 8H), 0.90 (t, 6H). C NMR (400 MHz CDCl ): δ 168.37, 134.67, 131.33, 130.50,
3
1
28.98, 127.89, 127.24, 125.16, 110.48, 40.43, 40.23, 40.02, 39.81, 39.60.
Synthesis of (4-{bis[4-(hexyloxy)phenyl]amino}phenyl)boronic acid (3). A round
bottomed flask was vacuumed and given argon which provides isolated atmosphere. (4-
bromophenyl)bis[4-(hexyloxy)phenyl]amine (3.8g, 7.3mmol) was dissolved with dry
THF (10mL) and added to the flask. The temperature was set to -80°C with acetone and
dry ice. N-butyl lithium (3.75mL, 7.25mmol) was added dropwise into the flask carefully
and stirred for half an hour. Trimethylborate (8.4mL, 72.6mmol) was added dropwise to
the mixture and stirred overnight. The reaction was controlled and ended with TLC. The
residue was extracted with diethylether (3x30mL) and 1M hydrochloric acid aqueous
solution (3x30mL). The organic phase was separated, dried over sodium sulphate and the
solvent was evaporated by rotary evaporator. The crude product was purified by column
chromatography (ethylacetate/hexane: 3/1, V/V) on silica gel to obtain white solid (74%
1
yield). H NMR (400 MHz d -DMSO): δ 7.68 (s, 2H), 7.57 (d, 2H), 6.97 (d, 4H), 6.85 (d,
6
4
0
1
H), 6.66 (d, 2H), 3.89 (t, 4H), 1.70-1.63 (m, 4H), 1.42-1.36 (m,4H), 1.30-1.26 (m, 8H),
1
3
.85 (t, 6H). C NMR (400 MHz CDCl ): δ 156.03, 150.073, 140.37, 135.87, 127.65,
3
18.08, 116.07, 68.30, 31.66, 29.37, 25.87, 14.53.
7

ACCEPTED MANUSCRIPT
Synthesis
benzo[de]isochromene-1,3-dione (4). In a round bottomed flask a mixture of 6-bromo-
,3-benzoisochromene-1,3-dione (103mg, 0.4mmol), (4-{bis[4-(hexyloxy) phenyl]
of
6-(4-{bis[4-(hexyloxy)phenyl]amino}phenyl)-1H,3H-
1
amino} phenyl) boronic acid (150mg, 0.3mmol) was added and dissolved in 1,2-
dimethoxyethane (15mL). After that, [1,1′-Bis (diphenylphosphino) ferrocene]
dichloropalladium (II) (20mg, 0.022mmol) and potassium carbonate aqueous solution
(1M, 2mL) were added. The system was isolated under argon atmosphere, heated up to
boiling temperature and stirred overnight. The reaction was controlled and ended with
TLC, extracted with, dichloromethane (3x20mL) and water (3x20mL). The organic phase
was separated, dried over sodium sulphate and the solvent was evaporated. The crude
product was purified by column chromatography (dichloromethane/hexane: 1/2, V/V) on
1
silica gel to obtain red solid. H NMR (400 MHz CDCl ): δ 8.62 (m, 2H), 8.54 (dd, 1H),
3
7
4
1
3
.74 (dd, 2H), 7.31(d, 2H), 7.16 (d, 4H), 7.05 (d, 2H), 6.89 (d, 4H), 3.96 (t, 4H), 1.79 (m,
13
H), 1.47 (m, 4H), 1.35 (m, 8H), 0.91 (t, 6H). C NMR (400 MHz CDCl ): δ 156.56,
3
39.89, 134.90, 133.25, 130.65, 129.00, 128.07, 127.73, 127.34, 119.14, 115.91, 68.69,
1
13
1.61, 29.32, 26.00, 23.02, 14.23. The H-NMR and C-NMR spectra of the molecule
appear in supporting information as Fig. S1.
Synthesis of 4-(4-{bis[4-(hexyloxy)phenyl]amino}phenyl)-7-oxo-7H-
benzimidazo[2,1-a]benzo[de]isoquinoline-11-carboxylic acid (5). In a round bottomed
flask mixture of 6-(4-{bis[4-(hexyloxy)phenyl]amino}phenyl)-1H,3H-
benzo[de]isochromene-1,3-dione (70mg, 0.1mmol), 3,4-diaminobenzoic acid (33mg,
.2mmol), zinc acetate (20mg, 0.1mmol) was added and dissolved in pyridine (15mL).
a
0
The mixture was refluxed with stirring overnight. The reaction was controlled and ended
8

ACCEPTED MANUSCRIPT
with TLC. The reaction mixture was neutrealized by addition of 1M hydrochloric acid
aqueous solution and was extracted with dichloromethane (3x20mL) and water
(3x20mL). The organic phase was separated, dried over sodium sulphate and the solvent
was evaporated by rotary evaporator. The crude product was purified by column
chromatorgraphy (dichloromethane/hexane: 9.5/0.5, V/V) on silica gel to obtain red solid.
1
H NMR (400 MHz CDCl ): δ 8.86 - 8.75 (d, 1H), 8.60 (t, 1H), 8.18 (m, 1H), 8.01 (m,
3
1
6
1
2
H), 7.90 (m, 1H), 7.72 (m, 2H), 7.47 (m, 2H), 7.34 (m, 2H), 7.14 (m, 4H), 7.07 (m, 2H),
1
3
.89 (m, 4H), 3.96 (t, 4H), 1.35 (m, 16H), 0.91 (m, 6H). C NMR (400 MHz CDCl
3
): δ
64.14, 156.28, 149.68, 140.53, 130.65, 129.64, 127.41, 119.45, 115.70, 68.67, 31.82,
1
13
9.90, 29.55, 25.98, 22.69, 14.55. The H-NMR and C-NMR spectra of the molecule
appear in the supporting information as Fig. S2.
Synthesis of methyl 4-bromo-7-oxo-7H-benzimidazo[2,1-a]benzo[de]isoquinoline-
1
1-carboxylate(6). A mixture of 6-bromo-1H,3H-benzo[de]isochromene-1,3-dione
28mg, 0.1mmol), methyl 3,4-diaminobenzoate (33mg, 0.2mmol), zinc acetate (20mg,
.1mmol) were added in a round bottomed flask and dissolved in pyridine (15mL). The
(
0
reaction was refluxed overnight and was controlled with TLC. The reaction mixture was
neutrealized by 1M hydrochloric acid aqueous solution and extracted with
dichloromethane (3x20mL) and water (3x20mL). The organic phase was dried with
sodium sulphate and the solvent was evaporated by rotary evaporator. The crude product
was purified by column chromatorgraphy (dichloromethane/hexane: 9.5/0.5, V/V) on
1
silica gel to obtain a yellow solid. H NMR (400 MHz d -DMSO): δ 8.88 (m, 1H), 8.71
6
(t, 1H), 8.56 - 8.46 - 8.45 (m, 1H), 8.20 (m, 1H), 8.09 (m, 1H), 8.08 (m, 1H), 7.98 - 7.91
1
3
(
3
m, 1H), 7.89 - 7.85 (m, 1H), 4.00 (s, 3H). C NMR (400 MHz CDCl ): δ 163.51,148.41,
9

ACCEPTED MANUSCRIPT
1
1
47.31, 133.32,131.96, 131.68, 131.43, 130.34, 129.49, 128.46, 127.62, 127.10, 125.48,
1
13
24.12, 122.50, 116.28, 51.75. The H-NMR and C-NMR spectra of the molecule
appear in the supporting information as Fig. 3S.
Synthesis of methyl 4-[4-(diphenylamino)phenyl]-7-oxo-7H-benzimidazo[2,1-
a]benzo[de]isoquinoline-11-carboxylate (7). A mixture of methyl 4-bromo-7-oxo-7H-
benzimidazo[2,1-a]benzo[de]isoquinoline-11-carboxylate
(123mg,
0.3mmol),
4-
(diphenylamino)phenylboronic acid (90mg, 0.3mmol) were added in a schlenk flask and
dissolved in 1,2-dimethoxyethane (10mL). Then [1,1′-bis (diphenylphosphino) ferrocene]
dichloropalladium (II) (12mg, 0,02mmol) and potassium carbonate aqueous solution
(1M, 2mL) were added to the mixture. The system was isolated under argon atmosphere,
heated up to boiling temperature and stirred overnight. The reaction was controlled and
ended with TLC while the extraction was made with dichloromethane (3x20mL) and
water (3x20mL). The organic phase was dried with sodium sulphate and the solvent was
evaporated. The crude product was purified by column chromatography
1
(
dichloromethane/hexane: 1/2, V/V) on silica gel to obtain orange solid. H NMR (400
MHz CDCl
3
): δ 8.90 - 8.84 (m, 2H), 8.65 -8.64- 8.57 (m, 1H), 8.48 (d, 1H), 8.19 (t, 1H),
7
7
1
1
1
.97- 7.86 (d, 1H), 7.81-7.72 (m, 1H), 7.42 - 7.37 - 7.34 (m, 6H), 7.27 - 7.21(m, 6H),
1
3
.11 (t,2H), 6.93 (t, 2H), 4.00 (s, 3H). C NMR (400 MHz CDCl ): δ 164.31, 163.60,
3
48.33, 147.26, 134.54, 133.64, 133.48, 131.99, 131.82, 131.57, 131.00, 130.79, 129.58,
28.31, 127.94, 127.77, 127.17, 126.79, 125.04, 123.71, 122.68, 122.57, 122.38, 122.27,
1
13
20.95, 120.62, 116.09, 51.66. The H-NMR and C-NMR spectra of the molecule
appear in the supporting information as Fig. 4S.
1
0

ACCEPTED MANUSCRIPT
Synthesis
of
4-[4-(diphenylamino)phenyl]-7-oxo-7H-benzimidazo[2,1-
a]benzo[de]isoquinoline-11-carboxylic acid (8). Methyl 4-[4-(diphenylamino)phenyl]-7-
oxo-7H-benzimidazo[2,1-a]benzo[de]isoquinoline-11-carboxylate was added in a round
bottomed flask and dissolved in THF:ethanol (1:1) mixture. 1 M KOH solution was
added in the reaction flask and refluxed overnight. The reaction was controlled and ended
with TLC while the organic solvent was evaporated and the mixture was acidified with
1
M HCl. The precipitate was filtered and washed with pure water. The crude product was
purified by column chromatorgraphy (dichloromethane/methanol: 9.5/0.5, V/V) on silica
1
gel to obtain red solid. H NMR (400 MHz CDCl
3
): δ 8.96 - 8.89 (m, 1H), 8.67 - 8.56
(m, 1H), 8.26 (t, 1H), 7.82 (m, 2H), 7.71 (m, 1H), 7.54 (m, 1H), 7.42 - 7.41 (m, 2H), 7.34
13
(
m, 4H), 7.23 (m, 8H), 7.11 (t, 2H). C NMR (400 MHz CDCl ): δ 166.39, 163.96,
3
1
1
63.77, 153.38, 148.43, 147.46, 147.26, 133.49, 131.99, 131.82, 131.58, 130.98, 130.80,
1
29.49, 128.30, 127.76, 126.76, 125.09, 123.71, 122.52, 122.38, 120.95, 116.09. The H-
1
3
NMR and C-NMR spectra of the molecule appear in the supporting information as Fig.
5
S.
2
.2 Materials
All solvents and reagents, unless otherwise stated, were of puriss quality and used
as received. Copper (I) iodide and 1-bromohexane were purchased from Fluka; Acetone,
dichloromethane, toluene, 18-crown-6, phenanthroline, n-butyllithium, 1,2-
dimetoxyetane,
tetrahydrofuran
(THF),
trimethylborate,
[1,1’-
Bis(diphenylphosphino)ferrocene]dichloropalladium(II), 4 bromo naphthalene and 3,4
diaminobenzoic acid were purchased from Sigma-Aldrich. 4-iodophenol and 4-
1
1

ACCEPTED MANUSCRIPT
bromoaniline were purchased from Alfa Aesar. Potassium carbonate and zinc acetate
were purchased from Riedel de Haen.
For fabrication of solar cells, TiO and Al O films: Lithium iodide, iodine, 1-
2
2
3
methyl-3-propylimidazolium
hydrate (H PtCl ),
iodide,
hydrogen
hexachloroplatinate(IV)
ether, 3-
poly(propylene
glycol)bis(2-aminopropyl)
2
6
isocyanatopropyltriethoxysilane, guanidine thiocyanate and all solvents were purchased
TM
2
from Sigma-Aldrich. SnO :F transparent conductive electrodes (FTO, TEC A8), 8
Ohm/square, were purchased from Pilkington NSG Group. Commercial ultra pure
titanium isopropoxide (TTIP, 97%, Aldrich), aluminum tributoxide (AlBuO, 95%,
Aldrich), Triton X-100 (polyoxyethylene-10- isooctylphenyl ether) surfactant (99.8%,
Aldrich) and glacial acetic acid (AcOH, Aldrich) were used to make the precursors for
TiO and Al O sols. The preparation of TiO and Al O films was described in previous
2
2
3
2
2
3
publications [41, 42].
2
.3 Preparation of solar cells
In the construction of the solar cells a quasi-solid state electrolyte was used. This
was chosen as it combines the high ionic conductivity of liquids while it reduces the risk
of leaks minimizing sealing problems in the cells. For the gel electrolyte applied to the
DSSCs, we used a hybrid organic-inorganic material, ICS-PPG230, which was prepared
according to a procedure described in previous publications [43, 44]. Briefly, 0.7 grams
of the functionalized alkoxide precursor ICS-PPG230 were dissolved in 1.6 grams of
sulfolane and 0.8 grams of 3-methoxypropionitrile under vigorous stirring. Then, 0.368 g
AcOH were added followed by 0.3 M 1-methyl-3-propylimidazolium iodide, 0.1 M LiI
1
2

ACCEPTED MANUSCRIPT
2 2
and 0.05 M I in a final molar ratio of AcOH:LiI:MPImI:I = 2.5:0.1:0.3:0.05. Finally
0
.204 g of tert-butyl pyridine and 0.036 g of guanidine thiocyanate was added to the
above mixture. After few hours of stirring, one drop of the obtained sol was placed on the
top of the titania electrode with adsorbed dye molecules and a slightly platinized FTO
counter electrode was pushed by hand on the top. The platinized FTO glass was made by
o
exposing it to a H PtCl solution (5mg/1ml of i-PrOH) followed by heating at 450 C for
2
6
1
0 minutes. The two electrodes tightly stuck together by −Si−O−Si− bonds developed by
the presence of ICS-PPG230. The cell active area for the electrical measurements was
2
0
.28 cm determined by a black mask.
2
.4 Methods
Electrochemical measurements for MZ-341 and MZ-235 dyes were carried out with
CH 440B Instruments potentiometer. FT-IR spectra were recorded on a Thermo
-1
1
Scientific Nicolet iS5 FT-IR Spectrometer using an ATR system (4000–650 cm ). H
1
3
o
and C NMR (Varian-400 MHz) data were recorded at 25 C using CHCl -d and d -
3
6
DMSO as solvents and TMS as an internal standard. The steady state absorption and
fluorescence spectra were taken using a Hitachi U-2900 UV-Vis spectrophotometer and a
Hitachi F-2500 fluorescence spectrophotometer, respectively. The accuracy of the
fluorescence measurements was checked by detecting the emission band of Europium
3
+
5
7
Eu ( D
0
→ F
2
transition) at 613 nm as a reference. The fluorescence spectra of the solid
o
samples were collected using a specific holder which had an inclination angle of 10
assuring the exact same geometry in all measurements and avoiding increased reflections
towards the detector. Unsensitized TiO and Al O films were used as reference samples
2
2
3
1
3

ACCEPTED MANUSCRIPT
for obtaining the spectra of the sensitized ones. The thickness of the oxide films was
about 2 ꢀm according to cross section scanning electron microscopy images.
The fluorescence dynamics were studied by using a femtosecond time resolved
upconversion system that has been described in details elsewhere [41,42,45,46]. The
excitation beam was the second harmonic of a Ti:Sapphire femtosecond laser, at 400 nm
(80 fs pulse duration) and the average excitation power was less than 3 mW. The
Instrument's Response Function (IRF) was 140 fs, while the samples were dilute
-5
2 2 3
solutions of the two dyes in toluene and THF (10 M concentration), or TiO and Al O
films sensitized with the dyes. All dynamics were detected under magic angle conditions.
For obtaining the long-lifetime (nanosecond) dynamics, a Time Correlated Single Photon
Counting (TCSPC) method has been used. This is based on a Fluotime 200 spectrometer
(Picoquant), where two pulsed diode lasers, at 403 and 470nm, were used for the
excitation and a Microchannel-Plate Photomultiplier (MCP-PMT) was used for the
detection. The IRF of the TCSPC spectrometer was ~80 ps.
For solar cells’ characterization and for obtaining the J-V curves, the samples were
illuminated with Xe light using a Solar Light Co. solar simulator (model 16S-300)
equipped with AM 0 and AM 1.5 direct Air Mass filters to simulate solar radiation at the
2
surface of the earth. The light intensity was kept constant at 1000 W/m measured with a
Newport power meter (843-R). Finally, the J-V curves were recorded by connecting the
cells to a Keithley Source Meter (model 2601A) which was controlled by Keithley
computer software (LabTracer). We made three cells for each dye which were tested
under the same conditions in order to avoid any misleading estimation of their efficiency.
Cell performance parameters, including short-circuit current density (JSC), open circuit
1
4

ACCEPTED MANUSCRIPT
voltage (VOC), maximum power (Pmax), fill factor (FF) and overall cell conversion
efficiency n, were determined from each J-V characteristic curve.
EIS characterization was carried out under illumination using the same Xe light
source that was used for the J-V curves. EIS measurements were performed without the
use of a mask with Metrohm Autolab 3.v potentiostat galvanostat (Model PGSTAT
1
28N) through a frequency range of 100 kHz-0.01 Hz using a perturbation of ±10 mV
over the open circuit potential. Experimental data are presented by scattering symbols
while lines represent the fitted plots obtained using Nova 1.10 software.
3
. Results and discussion
3
.1 Electrochemical properties
The electrochemical properties of the MZ dyes were examined by the cyclic
voltammetric method in a 0.1M solution of Bu NPF in acetonitrile with a scan rate of
4
6
-1
1
00mVs . A glassy-carbon was used as the working electrode, platinum as the counter
electrode and Ag/AgCl as the reference electrode. The oxidation potentials were found at
.89 V and 0.78 V (vs. Fc+/Fc) for MZ-341 and MZ-235, respectively. They are
0
assigned to the oxidation of the triphenylamine donor moiety. The reduction potentials
were found at -1.13 and -1.67 V for MZ-341 and at -1.16 and -1.56 V for MZ-235
respectively. They are assigned to the reduction of the benzimidazole and carboxylic acid
acceptor that can be anchored to the TiO surface. It is worth noting that the presence of
2
electron-donating groups coupled to the triphenylamine backbone reduces the oxidation
potential. HOMO levels for both dyes were calculated from the oxidation potentials while
LUMO level were calculated from the reduction potentials. HOMO levels are located at
1
5

ACCEPTED MANUSCRIPT
5
.18 and 5.30 eV while LUMO levels at 3.24 and 3.27 eV for MZ-235 and MZ-341
respectively. Cyclic voltammograms for both dyes are presented in Fig.1.
3
.2 FT-IR measurements of the MZ dyes
The IR spectra obtained for both dyes are presented in Fig.2. The O−H bonds of the
−
1
dyes resonate at around 3400 cm and they are believed to arise from COOH. The peak
−
1
at around 1700 cm is based on the C=O stretch of the carboxylic acid group. Aromatic
and aliphatic CH groups exhibited characteristic absorption bands around 2900 and 2800
−
1
cm . These three important peaks proved that carboxyl groups are successfully
integrated on MZ dyes.
3
.3 Photophysical properties in solutions and films
Figures 3a and 3b present the absorption and fluorescence spectra of dyes MZ-341
and MZ-235 in toluene and THF dilute solutions, respectively. The photophysical
parameters are summarized in Table 1. The dyes exhibit almost identical absorption
spectra, regardless of the solvent used, with peaks at 441-443 nm for MZ-341 and at 459
nm for MZ-235, denoting a relatively polarity-independent nature of the ground state. A
red-shift (~17-19 nm) of the absorption's peak for MZ-235 compared to MZ-341 in both
solvents is observed. This originates from the hexyloxy substituents on the para position
of the two benzenes of the triphenylamine donor [39, 40]. The addition of these extra
electron donating groups (hexyloxy groups) on the triphenylamine donor reduces the
oxidation potential by giving triphenylamine an enhanced nucleophilic character,
resulting in an increased HOMO level of the dye. This is reflected by a red shift of the
1
6

ACCEPTED MANUSCRIPT
absorption maximum for MZ-235 relative to MZ-341 where these substitution groups are
absent.
On the other hand, the fluorescence spectra in toluene and THF display several
differences in both dyes, with those in toluene being narrower and hypsochromically
shifted compared to those in THF. Specifically, the fluorescence spectra in toluene show
peaks at 547 and 576 nm for MZ-341 and MZ-235, respectively. The corresponding
values in THF are 595 and 613 nm. The bathochromic shift as well as the broader
spectrum in THF compared to toluene, is an indication that the excited state presents an
intense charge transfer character. This is consistent with the non normalized fluorescence
spectra (they are not presented), where MZ-341 and MZ-235 exhibit approximately 10-
fold and 80-fold decreased fluorescence intensity in THF relative to toluene. Finally, a
bathochromic shift (18-29 nm) of the fluorescence peak accompanied by a broader
spectrum in both solvents is evident for MZ-235 with the extra electron donating groups,
indicating that this dye adopts a more relaxed excited state than its analogue MZ-
3
41without the additional donor group.
2 3
The absorption and fluorescence spectra of MZ-341 and MZ-235 on Al O and
TiO substrates are shown in Figures 4a and 4b. In Table 1, the related photophysical
2
results are summarized. Regarding the absorption spectra, the two dyes exhibit broad
peaks at 430-450 nm for MZ-341 and at 460-470 nm for MZ-235. As in the solutions,
MZ-235 exhibits a bathochromic shift of the absorption spectrum compared to MZ-341
due to the hexyloxy groups. In general, the absorption spectra of the dyes on TiO and
2
2 3
Al O films are influenced by possible formation of H- or J- aggregates and by the
deprotonation of the carboxyl group. The formation of H-aggregates as well as the
1
7

ACCEPTED MANUSCRIPT
deprotonation of the carboxyl group would lead to a hypsochromic shift while J-
aggregates would lead to a bathochromic one. In our case, the absorption peaks in films
do not show significant differences compared to the corresponding values in solutions.
This means that the influence of the above mentioned effects may be canceled. However,
in films, the absorption spectra display a long tail towards the red-edge indicative of the
inhomogeneity and the formation of aggregates.
2 3 2
Interestingly, the fluorescence spectra of MZ-341 and MZ-235 on Al O and TiO
exhibit peaks at almost similar wavelengths namely 580-590 nm (Table 2). These peaks
are red-shifted compared to the peaks in toluene but they are blue-shifted compared to the
values in THF. The latter indicates that the emitting state in films has a less intense
charge transfer character than in THF which possibly indicates deprotonation of the
carboxylate group where the carboxylate anion has a decreased electron accepting
capability relative to its neutral form (higher LUMO level).
The fluorescence dynamics of MZ-341 and MZ-235 have been studied by means of
fs time resolved spectroscopy in toluene and THF solutions and on TiO
substrates on a temporal range from -0.5 up to 500 ps and in specific occasions up to
000 ps. Besides, for lifetime measurements in the ns timescale, a Time Correlated single
2 2 3
and Al O
1
Photon Counting (TCSPC) technique has been used. Figures 5a and 5b show the
dynamics of the two dyes in toluene and THF obtained by the fs spectroscopy, while
figure 5c shows the dynamics obtained by TCSPC technique. The fluorescence transients
by means of fs spectroscopy have been detected at various emission wavelengths within
the whole emission spectrum of the dyes. In Figures 5a and 5b, the dynamics are shown
1
8

ACCEPTED MANUSCRIPT
at the peaks of emission spectra of the two compounds in each solvent while figures S6
and S7 show the dynamics at all wavelengths.
The dynamics of the two dyes exhibit substantial differences. However, both dyes
in THF decay reasonably faster relative to toluene, confirming the lowering of the LUMO
level towards a stronger coupling in the excited state and therefore an enhanced charge
transfer character. In order to analyze the dynamics, a global fitting method has been
employed using a multi-exponential function convoluted with the IRF. Thus, common
decay times have been given at all wavelengths while the pre-exponential factors have
been determined as a function of wavelength. The results are summarized in Table 2 for
the dynamics at the peaks of the emission spectra and in Tables S1 - S4 for all the
detected wavelengths.
Regarding MZ-341 in toluene, two short time constants equal to 1.7 and 5.5 ps
were found. Both time constants exhibit positive pre-exponential factors at short
wavelengths which become negative at long ones, thus leading to a transient relaxation of
the emission spectrum (Table S1). In THF, time constants of 0.63 and 1.0 ps were found
which also refer to a relaxation of the emission towards low-energies as their pre-
exponential factors become negative at long wavelengths (Table S2). The above
mentionned behavior of the short-time dynamics is common in charge trasfer dyes and is
due to ICT formation and solvation of the polar excited state [15,47]. MZ-341 in THF
also exhibits a decay component of 5 ps with positive amplitudes at all wavelengths
which possibly indicates the existence of a non-radiative mechanism depleting the
excited state. On a longer timescale, MZ-341 in both solvents exhibits a slow decay
component of ~4.2-4.4 ns (Fig. 5c and Table 3) which was found the same upon both 403
1
9

ACCEPTED MANUSCRIPT
and 470 nm excitation and is considered as the lifetime of the emitting state. This decay
lifetime was determined by means of TCSPC technique. The similarity of the lifetimes in
both solvents indicates that the emitting state of MZ-341 is the same in both solvents.
Apart from the 4.2-4.4 ns lifetimes a shorter decay time of 0.85 ns has also been found
for MZ-341 in toluene. This time is too slow to be considered as a relaxation time and
could be tentatively ascribed to the emission from an intermediate unrelaxed state.
The dynamics of MZ-235, having the additional electron donating groups, also
show fast decay at short wavelengths while as the detection is shifted to longer ones the
decay is slower and rise components are also observed (Figure S7). The spectral
relaxation for MZ-235 in toluene occurs with components of 1.1 and 4.0 ps respectively
while in THF a single relaxation component of 0.4 ps is observed (Tables S3 and S4).
MZ-235 in toluene exhibits a fast non-radiative decay of 26 ps while at long timescales a
slow decay component of 3.2 ns is observed which is however accompanied by a shorter
one of 0.5 ns. Both ns decay components were found by TCSPC (Table 3). As in the case
of MZ-341, they can be associated to the lifetime of the relaxed excited state and possibly
to an unrelaxed state.
It is noted that the fluorescence intensity of MZ-235 in THF totally vanishes within
the first 500 ps showing a lifetime of 70 ps as obtained by the global fitting analysis. This
fast decay in THF was also accompagied by a significant drop of the fluorescence
intensity in this solvent relative to toluene. Such an abrupt decrease of the lifetime as the
solvent polarity increases has been also observed in the past for other dipolar dyes and
has been attributed to the formation of a Twisted Intramolecular Chrge Transfer state
(TICT) [48-50]. In general, apart from a fast decay of the excited state, the presence of a
2
0

ACCEPTED MANUSCRIPT
TICT state shifts the emission spectrum to longer wavelengths (or cause the appearance
of an intense shoulder at long wavelengths) and decreases the emission efficiency
because of the conical intersection of the TICT state and the ground state surfaces which
allows the population to transfer to the ground state non-radiatively. In order to test
whether a TICT is formed when MZ-235 is disolved in THF, we prepared solutions of
MZ-235 in THF after adding polystyrene (PS). This polymer was chosen because it
disolves easily in THF and it is non-polar [51]. Various solutions were prepared by
adding PS until they became highly viscous (concentration of polystyrene up to 40
mg/ml). By increasing the viscosity of the solutions, the rotational freedom of MZ-235 is
restricted and thus the formation of a TICT should be hindered. Therefore, if the decrease
of the lifetime in THF is due to the formation of a TICT, the fluorescence dynamics
should become slower in the viscous solutions. In Figure 5b, the fluorescence decay of
MZ-235 in THF with 40 mg/ml polystyrene is shown. It is obvious that the decay is
identical to that without PS. In addition, the steady state fluorescence spectrum of MZ-
2
35 in THF/PS solution has been detected and is shown in Figure 3b. Similar to the
dynamics, the spectrum of MZ-235 in THF/PS is found identical to the neat THF
solutions. Therefore, it is highly unlikely that a TICT is formed and the decrease of the
lifetime in THF can be ascribed to the fast, mostly non-radiative, decay of the ICT state
due to the polar solvent molecules.
Figure 6 presents the fluorescence transients of dyes MZ-341 and MZ-235 on
Al O and TiO nanocrystalline films. The dynamics on TiO are significantly faster than
2
3
2
2
on Al
2 3 2
O indicating efficient electron injection from the dye's LUMO to the TiO
conduction band [52-54]. After fitting the results with a three exponential function, the
2
1

ACCEPTED MANUSCRIPT
2
time constants for MZ-341 were found 1 (0.37), 5.8 (0.42) and 44 ps (0.21) on TiO and
3
.1 (0.44), 17.6 (0.36) and 156 ps (0.20) on Al O . The corresponding time constants for
2 3
MZ-235 were found 0.6 (0.53), 4.7 (0.34) and 48 ps (0.13) on TiO and 2 (0.46), 23.3
2
(
0.32) and 387 ps (0.22) on Al O . Table 4 summarizes the results. The observed faster
2 3
decay of MZ-235 on TiO compared to MZ-341 is ascribed to the presence of the
2
hexyloxy group onto the triphenylamine moiety, which enhances its electron donating
2 3
strength. The same dye on Al O exhibits a faster component within the first few
picoseconds compared to MZ-341, which is also ascribed to its enhanced electron
donating efficiency and can be possibly due to electron injection towards trap states in
Al O . However, at longer timescales a slower decay is evident for MZ-235 on Al O
3
2
3
2
(τ3=387 ps for MZ-235 vs. 156 for MZ-341). This difference in the long lifetimes is
ascribed to the reduced aggregation of MZ-235 due to the elongated hexyloxy
substituents. This reduced aggregation leads to a decreased possibility for quenching and
thus to an increased lifetime of MZ-235 on Al O .
2
3
3
.4 Solar cells performance
The current-voltage (J-V) characteristic curves of the quasi-solid state DSSCs under
one sun illumination using MZ-341 and MZ-235 newly synthesized dyes as sensitizers,
are presented in Figure 7a. For each sensitizer, we prepared three cells which were tested
under the same conditions. The electrical characteristics are presented in Table 5 and
concern the average values obtained from the cells. In all cases, the active area of the
2
cells was 0.28 cm . The quasi-solid state DSSCs constructed with MZ-341 showed an
overall efficiency of 3.4 % slightly better than that obtained for MZ-235 (3.0 %). The
2
2

ACCEPTED MANUSCRIPT
open circuit voltage for both dyes was measured around 660 mV while the short circuit
2
current density was 6.83 and 6.45 mA/cm for MZ-341 and MZ-235 respectively. Since
the short circuit current value is rather low, we believe that this is ascribed to two factors:
The low extinction coefficient value and the ineffective visible light exploitation.
Besides, the relatively lower values for electrical characterisitcs obtained for MZ-235 can
be explained in terms of lower absoprtion of the sensitized TiO photoelectrode compared
2
to that measured for MZ-341 as it can be seen in the inset of Fig. 7a. However, the
devices showed good stability when subjected to 1 sun light irradiance from a Xe lamp
after 300 h of continuous visible light soaking. The cells were all unsealed and were used
without any further treatment after the electrolyte was gelled. The cells with the two dyes
were also examined as efficient photoelectrodes in DSSCs measuring the dark current
suppression. Figure 7b shows that the dark current density in the cells does not have
substantial differences however, the electron leakage, in the cells made with the dye MZ-
2
35 is higher compared to the cells made with MZ-341. This corroborates with the
slightly reduced overall performance of the MZ-235 DSSCs under light exposure.
Figure 8 shows the Nyquist plots obtained from the cells with the two dyes. The
first semicircle corresponds to the Pt/electrolyte interface, R . The charge transfer
pt
resistance at the counter electrode (R ) is represented as a semicircle in the impedance
pt
spectra and as a peak in the Bode phase angle plot. The resistance element related to the
response in the intermediate frequency represents the charge transport at the
TiO /dye/electrolyte interface (R ) and shows diode like behavior. The semicircle at the
2
tr
low frequency, which is attributed to the diffusion in the electrolyte (Rdif), was small and
not well formed indicating a fast diffusion. Finally, the intercept of the horizontal axis
2
3

ACCEPTED MANUSCRIPT
stands for the resistance of the sheet resistance of the FTO substrate and the contact
resistance of the FTO/TiO (R ) [55,56].
2
h
The total series resistance of the cell can be calculated using equation (1)
R = R + R + Rdif
(1)
S
h
pt
The semicircles at the Nyquist plots are obtained from a frequency scan through a wide
range of high, intermediate and low frequency. Thus, the Bode phase plots, shown in Fig.
9
, also feature three characteristic frequency peaks corresponding to each of the Nyquist
characteristic peaks. The equivalent circuit which we used to fit the experimental data is
presented as an inset in Figure 8. For electrodes having a rough surface, the capacitance
element in the equivalent circuit is replaced by a constant phase element (CPE, Q) which
depends on the parameters Y and N. It is possible to convert a CPE element, which is in
o
parallel with a resistance, to a pseudo capacitance using equation (2)

1

1

−1
N
 N

Cpseudo = YO ⋅ R
(2)
The parameters obtained by fitting the data in Fig. 8, are presented in Table 6. The cells
with MZ-341 and MZ-235 dyes exhibit a quite similar behavior of the charge transport
resistance at the TiO /dye/electrolyte interface. However, an increased value for the total
2
series resistance for the cells employing MZ-235 dye is obtained, which agrees with the
slightly smaller energy conversion efficiency of these cells.
Bode plots obtained from cells with both dyes are presented in Figure 9. From these
plots in the medium-frequency range, the characteristic frequency is associated with the
electron transport in the mesoscopic TiO
2
film and the back reaction at the
electrodes for both cells
TiO /electrolyte interface. The electron lifetime in TiO
2
2
2
4

ACCEPTED MANUSCRIPT
employing MZ-235 and MZ-341 was calculated using equation (3), where f is the
characteristic frequency of the peak from the Bode phase plots [57].
1
τ n
=
(3)
2⋅π ⋅ f
The electron lifetime for the cells with MZ-235 was calculated to be 2.8 ms which was
slightly lower than the one obtained for MZ-341 (3.5 ms). This is also an additional
evidence for the better performance of cells employing MZ-341 as photosensitizer.
Moreover, the low and high frequency peak observed in the Bode plots correspond to
triiodide diffusion in the electrolyte and charge transfer at the counter electrode,
respectively. There are no significant changes at least at high frequency peaks implying
that no unexpected reaction had occurred within the electrolyte and counter electrode for
the two dyes.
Overall, by comparing the photodynamics and solar cell performance of the two
sensitizers, it is concluded that MZ-235, with the additional electron donating groups,
exhibits enhanced intramolecular charge transfer properties and faster electron injection.
Taking into account the fact that this dye exhibits a better spectral response than MZ-341,
it was expected to show a higher solar cell efficiency than MZ-341, without the
additional electron donor. However, the opposite is observed. The smaller solar cell
efficiency of the DSSCs prepared with MZ-235 is in agreement with the EIS results such
as higher total series resistance and smaller electron lifetime in TiO electrodes compared
2
to the cells made with MZ-235. Another tentative explanation of the smaller performance
in the hexyloxy substituted sensitizer is a faster recombination of injected electrons with
the dye cations since the hexyloxy groups are expected to increase the positive charge on
the triphenylamine moiety of the dye [40].
2
5

ACCEPTED MANUSCRIPT
3
. Conclusions
In conclusion, two new triphenylamine based organic dyes with or without
additional electron donating hexyloxy groups, having benzimidazole moiety as -bridge
π
are synthesized and examined photophysically and as sensitizers in thin TiO film based
2
DSSCs. The dye with the hexyloxy groups, MZ-235, attached to the triphenylamine has
shown a bathochromic shift of its absorption spectrum thus exhibiting a better coverage
of the visible spectrum. In addition, it has shown faster intramolecular charge transfer
2
state formation and faster electron injection toward TiO as it was found through time
resolved fluorescence spectroscopy in the fs-ps and in the ns timescale. However, the dye
without additional groups attached to triphenylamine, MZ-341, proved to be more
efficient in solar cell performance while a maximum overall performance of 3.4% was
obtained for a quasi-solid state DSSC. The better performance of MZ-341 was explained
in terms of higher absorbance on TiO photoelectrode compared to the MZ-235 dye and
2
also examined through electrochemical impedance spectroscopy measurements.
Acknowledgements
Prof. E. Stathatos would like to acknowledge that his research has been co-financed by
the European Union (European Social Fund – ESF) and Greek national funds through the
Operational Program "Education and Lifelong Learning" of the National Strategic
Reference Framework (NSRF) - Research Funding Program: Thales.
Prof. M. Fakis would also like to acknowledge that this work was supported by
Grant Ε.028 from the Research Committee of the University of Patras (Programme K.
Karatheodori).
2
6

ACCEPTED MANUSCRIPT
Dr. M. Can gratefully acknowledge the supports from of Izmir Katip Celebi University
Grants Commission (Project Number: 2013-2-FMBP-17).
References
[
[
[
1] Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-sensitized solar cells.
Chem Rev 2010;110:6595-663.
2] Gong J, Liang J, Sumathy K. Review on dye-sensitized solar cells (DSSCs):
Fundamental concepts and novel materials. Renew Sustain En Rev 2012;16:5848-60.
3] Yella A, Lee HW, Tsao HN, Yi C, Chandiran AK, Nazeeruddin MK et al. Porphyrin-
Sensitized Solar Cells with Cobalt (II/III)–Based Redox Electrolyte Exceed 12 Percent
Efficiency. Science 2011;334:629-34.
[
[
[
[
4] Gao F, Wang Y, Zhang J, Shi D, Wang M, Humphry-Baker R, et al. A new
heteroleptic ruthenium sensitizer enhances the absorptivity of mesoporous titania film
for high efficiency dye-sensitized solar cell. Chem Commun 2008;:2635-7.
5] Kumaresan D, Thummel RP, Bura T, Ulrich G, Ziessel R. Color Tuning in New
Metal-Free Organic Sensitizers (Bodipys) for Dye-Sensitized Solar Cells. Chem Eur J
2
009;15:6335-9.
6] Guo K, Yan K, Lu X, Qiu Y, Liu Z, Sun J, et al. Dithiafulvenyl Unit as a New Donor
for High-Efficiency Dye-Sensitized Solar Cells: Synthesis and Demonstration of a
Family of Metal-Free Organic Sensitizers. Org Lett 2012;14:2214-7.
7] Duan T, Fan K, Zhong C, Chen X, Peng T, Qin J. A new class of organic dyes
containing b-substituted 2, 20-bithiophenene unit as a p-linker for dye-sensitized solar
2
7

ACCEPTED MANUSCRIPT
cells: Structural modification for understanding relationship of structure and
photovoltaic performances. J of Power Sources 2013;234:23-30.
[
[
[
8] Matano Y, Hayashi Y, Nakano H, Imahori H. N,S,P-. Hybrid Donor–π–Acceptor
Organic Dyes for Dye-Sensitized Solar Cell: Synthesis, Optical Properties, and
Photovoltaic Performances. Heteroatom Chem 2014;25:533-47.
9] Hagberg DP, Marinado T, Karlsson KM, Nonomura K, Qin P, Boschloo G, et al.
Tuning the HOMO and LUMO energy levels of organic chromophores for dye
sensitized solar cells. J Org Chem 2007;72:9550-6.
10] Mishra A, Fischer MKR, Bauerle P. Metal-Free Organic Dyes for Dye-Sensitized
Solar Cells: From Structure: Property Relationships to Design Rules. Angew Chem Int
Ed 2009;48:2474-99.
[
11] Yen YS, Chou HH, Chen YC, Hsu CY, Lin JT. Recent developments in molecule-
based organic materials for dye-sensitized solar cells. J Mater Chem 2012;22:8734-47.
12] Gao F, Wang Y, Shi D, Zhang J, Wang M, Jing X, et al. Enhance the Optical
[
2
Absorptivity of Nanocrystalline TiO Film with High Molar Extinction Coefficient
Ruthenium Sensitizers for High Performance Dye-Sensitized Solar Cells. J Am Chem
Soc 2008;130:10720-8.
[
13] Babu DD, Gachumale SR, Anandan S, Adhikari AV. New D-π-A type indole based
chromogens for DSSC: Design, synthesis and performance studies. Dyes Pigments
2
005;112:183-91.
[14] Jiang S, Fan S, Lu X, Zhou G, Wang ZS. Double D–π–A branched organic dye
isomers for dye-sensitized solar cells. J Mater Chem A 2014;2:17153-64.
2
8

ACCEPTED MANUSCRIPT
[15] Bairu SG, Mghanga E, Hasan J, Kola S, Rao VJ, Bhanuprakash K, et al. Ultrafast
Interfacial Charge-Transfer Dynamics in a Donor-π-Acceptor Chromophore Sensitized
TiO Nanocomposite. J Phys Chem C
2013;117:4824-35.
2
[
16] Dryza V, Nguyen JL, Kwon TH, Wong WWH, Holmes AB, Bieske EJ.
Photophysics and aggregation effects of a triphenylamine-based dye sensitizer on
metal-oxide nanoparticles suspended in an ion trap. Phys Chem Chem Phys 2013;
1
5:20326-32.
17] Sobuś J, Burdziński G, Karolczak J, Idígoras J, Anta JA, Ziółek M. Comparison of
TiO and ZnO solar cells sensitized with an indoline dye: Time resolved laser
spectroscopy studies of partial charge separation processes. Langmuir 2014;30: 2505-
2.
[
2
1
[
[
[
[
18] Yu X, Ci Z, Liu T, Feng X, Wang C, Ma T, et al. Influence of different electron
acceptors in organic sensitizers on the performance of dye-sensitized solar cells. Dyes
Pigments 2014;102:126-32.
19] Shen P, Liu YJ, Huang XW, Zhao B, Xiang N, Fei JJ, et al. Efficient triphenylamine
dyes for solar cells: Effects of alkyl-substituents and π-conjugated thiophene unit.
Dyes Pigments 2009;83:187-97.
20] Wan Z, Jia C, Duan Y, Zhou L, Lin Y, Shi Y. Phenothiazine–triphenylamine based
organic dyes containing various conjugated linkers for efficient dye-sensitized solar
cells. J Mater Chem 2012;22:25140-7.
21] Hara K, Wang ZS, Sato T, Furube A, Katoh R, Sugihara H, et al. Oligothiophene
containing coumarin dyes for efficient dye-sensitized solar cells. J Phys Chem B
2
005;109:15476-82.
2
9