Impact of the different electron-releasing subunits on the dye-sensitized solar cell performance of new triphenylamine-benzimidazole based molecules

Article abstract of DOI:10.1016/j.saa.2015.04.074

New triphenylamine-benzimidazole type small molecules with different electron-releasing groups were designed and synthesized to investigate their photovoltaic performances in dye sensitized solar cells (DSSCs). Their good visible absorptions covering the 400-535 nm in addition to suitable lowest unoccupied molecular orbital (LUMO) energy levels between -3.03 and -3.11 eV make good candidates them for DSSC devices. Fluorescence quenching studies of the dyes with pristine titania support the good electron injection to conduction band of TiO₂. Time resolved measurements of the dyes in solutions indicate the occurence of charge generation during the excited state. One of the used dyes in DSSC devices, TPA5a, carrying a methoxy group in triphenylamine part of the structure, gave much higher power conversion efficiency (PCE) value of 4.31% as compared to the other derivatives. Device fabricated from TPA5a dye gives good external quantum efficiency (EQE) value above 70% at 460 nm. Also, electron impedance spectroscopy (EIS) analysis of the devices gives a good explanation of the understanding of the cell performances.

Full text of DOI:10.1016/j.saa.2015.04.074

Accepted Manuscript
Impact of the different electron-releasing subunits on the dye-sensitized solar
cell performance of new triphenylamine-benzimidazole based molecules
Haluk Dinçalp, Gözde Murat Saltan, Deniz Aykut, Ceylan Zafer
PII:
S1386-1425(15)00546-6
http://dx.doi.org/10.1016/j.saa.2015.04.074
SAA 13625
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
To appear in:
Received Date:
Revised Date:
Accepted Date:
12 November 2014
10 March 2015
22 April 2015
Please cite this article as: H. Dinçalp, G.M. Saltan, D. Aykut, C. Zafer, Impact of the different electron-releasing
subunits on the dye-sensitized solar cell performance of new triphenylamine-benzimidazole based molecules,
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.
2015.04.074
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Impact of the different electron-releasing subunits on the dye-
sensitized solar cell performance of new triphenylamine-
benzimidazole based molecules
Haluk Dinçalp a,*, Gözde Murat Saltan a, Deniz Aykut b, Ceylan Zafer b,*
a Department of Chemistry, Faculty of Arts and Science, Celal Bayar University, Yunus Emre, 45140
Manisa/Turkey
b Solar Energy Institute, Ege University, Bornova, 35100 Izmir/Turkey
* Corresponding author e-mail: haluk.dincalp@cbu.edu.tr (H. Dinçalp); Tel: + 90-236-2013158; Fax: + 90-236-
2412158; ceylan.zafer@ege.edu.tr (C. Zafer); Phone: +90-232-3111245; Fax: +90-232-3886027
ABSTRACT
New triphenylamine-benzimidazole type small molecules with different electron-releasing groups
were designed and synthesized to investigate their photovoltaic performances in dye sensitized solar
cells (DSSCs). Their good visible absorptions covering the 400-535 nm in addition to suitable lowest
unoccupied molecular orbital (LUMO) energy levels between −3.03 and −3.11 eV make good
candidates them for DSSC devices. Fluorescence quenching studies of the dyes with pristine titania
support the good electron injection to conduction band of TiO . Time resolved measurements of the
2
dyes in solutions indicate the occurence of charge generation during the excited state. One of the used
dyes in DSSC devices, TPA5a, carrying a methoxy group in triphenylamine part of the structure, gave
much higher power conversion efficiency (PCE) value of 4.31% as compared to the other derivatives.
Device fabricated from TPA5a dye gives good external quantum efficiency (EQE) value above 70%
1

at 460 nm. Also, electron impedance spectroscopy (EIS) analysis of the devices gives a good
explanation of the understanding of the cell performances.
Keywords: Triphenylamine, Benzimidazole; Time resolved emission spectra, Charge transfer, Dye sensitized
solar cells, Electron impedance analysis
Introduction
Most of the devices/techniques used in the conversion of solar energy into the electricity
could not give satisfactory efficiencies for marketing purposes. Dye-sensitized solar cells
(DSSCs) present good opportunities to the dealers in order to gain much cheaper solar cells
and relatively much higher cell performances as compared to the silicon solar cells.
Nowadays, high conversion efficiency value of 12.7% obtained from a D-π-A porphyrin dye
[1] under incident intensity of AM1.5 solar light for DSSCs allows us to look with hopefully
for the future in ensuring energy demands. In this context, it is necessary to design novel
materials for DSSCs such as D-π-A structures enabling to control an efficient electron transfer
from dye to the TiO2 surface in the working mechanism.
Triphenylamine (TPA) group is chosen as a core group in most of efficient molecular
architectures for DSSCs. Both its good solution-processability arising from three dimensional
structure of benzene rings [2] and also efficient hole-transporting property [3, 4] enable it an
efficient chromophore for DSSC devices. Bäuerle et al. reviewed the metal-free organic dyes
used in DSSCs and reported that the power conversion efficiencies for different type TPA
molecules have been varied between the values of 1.7% and 9.1% [5]. The number of TPA
groups is also important in cell performances. Efficiency was reached to 5.06% when the bis-
TPA substituted dye was used instead of the corresponding mono-TPA substituted dye [6]. In
the literature survey, it is reported a wide variety of substituted groups attached to TPA core
in order to increase cell efficiencies. Researchers have used different TPA motifs which are
2

incorporated into the special subunits including thiophene [7-9], rhodanine-3-acetic acid [10],
diketopyrrolopyrrole [11], thieno[3,4-c]pyrrole-4,6-dione [12], benzothiadiazole [13],
carbazole [14], quinoxaline [15], triazine [16] and phenoxazine [17] groups. Wang et al.
reported much higher cell efficiency of 6.6% with a thiophene bridged double D-π-A dye as
compared to its monobranched analogue. Wang also emphasized that the cross shape of the
double D-π-A branched dyes can inhibit charge recombination [18].
Also, hole transporting property of the sensitizer should be more effective than the charge
recombination in dye regenaration. Hsu et al. synthesized 1H-phenanthro[9,10-d]imidazole
dyes as the sensitizers for DSSCs and obtained good efficiencies, ranging from 2.95% to
4.68%. They reported that incorporation of benzimidazole into TPA structure delays charge
recombination of the electrons in the conduction band of TiO2 with the oxidized dyes in
DSSCs [19]. Among the limited number of studies for DSSCs using TPA-benzimidazole
conjugates, Jia and co-workers synthesized two novel TPA- and benzimidazole-based
chromophores and obtained good efficiencies around 2.5% under full sunlight of AM 1.5G.
They also reported that the introduction of benzimidazole group into the TPA increases the
molar extinction coefficients and λmax values [20].
In our study, novel D-A type TPA5a-b and TPA4c organic dyes (Fig. 1a-b) which contain
TPA-benzimidazole hybrid donor and 2-cyanoacetic acid acceptor have been synthesized and
used in DSSCs. Ground state optical properties both in solution and on thin films and excited
state behaviours in two different solvents of polarities have been investigated in detail. The
impact of the electron releasing subunits on the device performance has been also
investigated.
Fig. 1a [21]
Fig. 1b [22], [20], [23], [24, 25]
3

Experimental section
General procedures
1H NMR and C NMR were measured on a Bruker 400 MHz spectrometer. FT-IR spectra
13
were recorded on a Perkin Elmer-Spectrum BX spectrophotometer preparing KBr pellets. The
UV–vis spectra were recorded with a Perkin Elmer Lambda 950 spectrometer in solutions and
on thin films. FLSP 920 Edinburg fluorescence phosphorescence spectrophotometer was used
to determine the fluorescence decay kinetics of the dyes in solution. The Edinburgh
Instruments F900 exponential tail fit method [26] was used for the calculation of decay times
after the single photon timing analysis of the compounds were completed at the excitation
wavelength of 368 nm. Fluorescence decay histograms were recorded in 10000 data channels.
The fitted decay curve was settled by the fitting parameters such as χ2<1.2 goodness of fit
[27].
Electrochemical characterization of the sensitizers were performed using a CH
instruments-Electrochemical Workstation with a standard three-electrode electrochemical
compartment in 100 mM [TBA][PF6] solution in Me-CN as the supporting electrolyte. Ag
and Pt wires were preferred as reference and counter electrodes, respectively. A polished
glassy carbon was used as working electrode in the measurements at the scan rate of 100
mV/s. Oxidation potential of ferrocene-ferrocenium (Fe/Fe+) couple was detected at +0.72 V
for the calculation of the onset values of Ered. HOMO and LUMO energy levels of TPA5a-b
and TPA4c dyes were calculated by the formulas:
onset
ox
onset
ox
E
= −(4.8 + E
), E
= E 0ox − E0ox(ferrocene) , E
= E
+ E [28]
HOMO gap
HOMO
LUMO
Molecular geometries of TPA5a-b and TPA4c dyes were optimized by MM+ force field
using Hyperchem Package Version 8.0 computational software. Semiempirical calculations
4

were performed using ZINDO/S (Single Point/CI) method to illustrate the possible electron
distribution on HOMO and LUMO energy levels of the structures.
Materials
Chloroform, acetonitrile, tetrahydrofuran, sodium borohydride, piperidine, silica gel,
tetrakis(triphenylphosphine)palladium(0), hydrogen bromide solution (47%), triphenylamine,
N-bromosuccinimide, phosphoryl chloride were purchased from Merck Company. Benzene,
sodium carbonate, bromine were purchased from Carlo Erba. 2,1,3-benzothiadiazole,
magnesium sulfate, 3-(N,N-dimethylamino)-phenylboronic acid and cyanoacetic acid were
purchased from Sigma-Aldrich. 3-methoxybenzeneboronic acid, CoCl2·6H2O (Alfa Aesar)
and methanol (Riedel-de-Haën) were used as received. Other organic solvents were analytical
grade and used without further purification.
4,7-dibromo-2,1,3-benzothiadiazole
(BT1)
and
4,7-bis(3-methoxyphenyl)-2,1,3-
benzothiadiazole (BT2) were synthesized according to the reference [29]. (4-
Bromophenyl)diphenylamine (TPA1) precursor was synthesized to the well-known procedure
[30].
DSSC fabrication and characterization
DSSCs were fabricated by coating of the TiO2 nanocrystalline paste onto transparent
conducting oxide coated glass electrode (TCO, Pilkington TEC15, •15_/sq resistance) using
Doctor Blading technique.
The paste is prepared by sol-gel method by hydrolysis of titanium tetraisopropoxide [31].
Ethyl cellulose was added to the paste as a binder and α-terpineol was used as a surfactant. In
order to get good electrical contact between the TiO2 nanoparticles, the photo-anodes were
5

first sintered at 450 °C for 1 h and then, immersed in 0.04M TiCl4 solution for 30 min at 70
°C and finally, electrodes were calcinated at 500 °C for 30 min. The thickness of the resulting
layer was measured as 10 µm. The resulting electrodes were cooled down slowly to 100 °C
and then, the electrodes were immersed to 0.3 mM solutions of the photosensitizers in
chloroform for 4 h to obtain a monolayer dye coated mesoporous titania electrode. Electrodes
were rinsed with the same solvent to get rid-off the non-adsorbed dye from the titania surface.
Alumina, Al2O3, was used as control layer with much higher band gap energy than that of
TiO2 layers in fluorescence quenching studies. It was prepared by the hydrolysis of aluminium
tri-sec-butoxide [Al(OC4H9)3, 0.25 mol] in the solutions of water (25 mol) and HCl (0.01
mol). Then, polyethylene glycol (PEG 400) was added to this solution giving the desired
particles in 70% porosity and 7 nm in dimensions. TCO (Pilkington TEC15, •15_/sq
resistance) was coated with Al2O3 nanocrystalline paste using Doctor Blading technique.
Al2O3 coated TCO electrodes were sintered at 450 °C for 1 h.
Platinum catalyst layer on the counter electrode was coated onto the TCO electrode by
thermal reduction of hexachloroplatinic acid to the metallic platinum. DSSC device was
fabricated by placing the counter electrode and dye sensitized TiO2 electrode in sandwich
geometry and sealing with a thermoplastic frame (Surlyn 50 µm thick). DSSC device was
completed after the redox electrolyte [0.6M BMII, 0.1M LiI (99.9%), 0.05M I (99.9%) and
2
0.5M tBP in 3-methoxypropionitrile] was injected into the space between electrode by using
the pre-drilled hole on the counter electrode that was sealed afterwards.
The light-to-electric energy conversion efficiency measurements for the devices with 0.25
cm2 working area were performed by a solar simulator equipped with a 750W metal halide
lamp and AM 1.5G filter. The light intensity was calibrated to 100 mW/cm2 using reference Si
solar cell (Frunhofer ISE Calab). External quantum efficiency (EQE) and electrochemical
impedance spectroscopy (EIS) measurements were used to characterize the DSSC devices.
6

EIS measurements were carried out under reverse bias of Voc condition at −0.65 V in dark
with frequencies in the range of between 0.1 Hz to 1 MHz.
Synthesis
Synthesis of 3,3''-dimethoxy-1,1':4',1''-terphenyl-2',3'-diamine (PDA)
BT2 (0.1 g, 0.287 mmol) was dissolved in 4 mL of EtOH:THF (3:1) solvent mixture and
then, NaBH4 (0.027 g, 0.7175 mmol) and CoCl2.6H2O (6.8 mg, 0.0287 mmol) were slowly
added to this solution. Black solid Co2B was formed instantly and H2S evolution was noted in
a few minutes. The mixture was refluxed under an argon atmosphere for 4 h, then cooled to
room temperature and then filtered to separate the black solid. The solvent was evaporated
and then, 50 mL of water was added to the solution. The organic product was extracted with
Et2O (3×30 mL). The combined organic extracts were dried over MgSO4 and the solvent was
removed. The resulted bisamine product was very air-unstable so that it would be used
immediately in the next step. Yield 93%.
Synthesis of 4,4'-[(4-bromophenyl)imino]dibenzaldehyde (TPA2)
Phosphoryl chloride (0.691 g, 4.45 mmol) was slowly added to 7 mL of DMF in an ice
bath. After the mixture was slowly cooled to room temperature, Vilsmeier reagent is ready to
use. TPA1 (0.580 g, 1.78 mmol) was added to this solution and the mixture was refluxed at
95 °C for 20 h. The reaction mixture was poured into an ice-water and then, neutralized with
2 N NaOH. Mono- and bis-aldehydes precipitated together. The crude mixture was filtered
and then, purified by column chromatography on silica gel using dichloromethane:n-hexane
1
(3:1) as eluent. Yield 55%. FT−IR (KBr pellet, cm− ): 2928 and 2871 (aromatic ν ), 1743,
C−H
1693 (aldehyde ν ), 1588 (aromatic ν ), 1505, 1489, 1319, 1274, 1166, 1007, 827, 520
C=O
C=C
7

1
cm− . 1H NMR (400 MHz, CDCl3 δ 7.27 ppm): δ = 9.90 (2H, s), 7.79 (4H, d, J = 8.7 Hz), 7.50
13
(2H, d, J = 8.7 Hz), 7.19 (4H, d, J = 8.7 Hz), 7.06 (2H, d, J = 8.7 Hz) ppm. C NMR [100
MHz, CDCl3 δ 77.2 (3 peaks)]: δ = 190.5, 151.7, 144.8, 133.4, 131.8, 131.5, 128.4, 123.2,
119.4 ppm.
Synthesis
of
4-[{4-[4,7-bis(3-methoxyphenyl)-1H-benzimidazole-2-yl]phenyl}(4-
bromophenyl)amino]benzaldehyde (TPA3)
TPA2 (0.059 g, 0.156 mmol) was added to the 15 mL of methanol solution of PDA (0.050
g, 0.156 mmol) and then, the mixture was refluxed for 5 h under an argon atmosphere. Yellow
solids appeared in the reaction medium. After the reaction was completed, the reaction
mixture was evaporated in vacuo to dryness and then, the crude product was purified by
column chromatography on silica gel using dichloromethane:ethyl acetate (5:0.5) as eluent.
Yield 45%. FT−IR (KBr pellet, cm−1): 2928 and 2832 (aromatic ν ), 1690 (aldehyde ν ),
C−H
C=O
1593 (aromatic ν ), 1483, 1363, 1317, 1282, 1225, 1162, 1042, 832, 782, 699 cm−1. 1H NMR
C=C
(400 MHz, CDCl3 δ 7.27 ppm): δ = 9.83 (s, 1H), 8.01 (d, J = 8.7 Hz, 2H), 7.72 (d, J = 8.7 Hz,
2H), 7.45 (t, J = 5.2 Hz, 9H), 7.19 (d, J = 8.7 Hz, 2H), 7.11 (d, J = 8.7 Hz, 2H), 7.05 (d, J =
13
8.7 Hz, 2H), 6.96 (dd, J = 8.7, 2.5 Hz, 4H), 3.89 (s, 6H) ppm. C NMR [100 MHz, CDCl3 δ
77.2 (3 peaks)]: δ = 191.7, 161.1, 153.3, 152.1, 148.6, 146.0, 140.7, 133.9, 132.3, 131.4,
130.8, 129.2, 128.6, 126.8, 126.0, 123.6, 122.1, 121.8, 119.2, 115.3, 113.9, 55.9 ppm.
Synthesis
of
4-[{4-[4,7-bis(3-methoxyphenyl)-1H-benzimidazole-2-yl]phenyl}(3'-
methoxybiphenyl-4-yl)amino]benzaldehyde (TPA4a)
A solution of TPA3 (0.030 g, 0.0482 mmol) in benzene (7 mL) and aqueous 2 M
Na2CO3 (3 mL) were added at 60 °C under an argon atmosphere to a mixture of 3-
methoxybenzeneboronic
acid
(0.010
g,
0.0578
mmol)
and
tetrakis(triphenylphosphine)palladium(0) (catalytic level, ≈3 mg) in ethanol (1 mL). After the
8

mixture was refluxed at 85 °C for 4 h under an argon atmosphere, the reaction mixture was
poured into 50 mL of water and then, extracted with chloroform (3×50 mL). The organic
layer was dried over anhydrous magnesium sulfate and evaporated in vacuo to dryness. The
residue was separated by column chromatography on silica gel (dichloromethane:ethanol
1
5:0.5 (v/v)). Yield 60%. FT−IR (KBr pellet, cm− ): 3288, 2925 and 2832 (aromatic ν ), 1686
C−H
(aldehyde ν ), 1590 (aromatic ν ), 1482, 1436, 1361, 1320, 1293, 1222, 1162, 1048, 831,
C=O
C=C
1
780, 696 cm− . 1H NMR (400 MHz, CDCl3 δ 7.27 ppm): δ = 9.85 (s, 1H), 8.02 (d, J = 8.6 Hz,
2H), 7.74 (t, J = 8.2 Hz, 4H), 7.58 (t, J = 8.2 Hz, 4H), 7.47-7.35 (m, 6H), 7.28-7.14 (m, 8H),
13
6.94 (dd, J = 8.6, 2.5 Hz, 2H), 3.91 (s, 6H), 3.89 (s, 3H) ppm. C NMR [100 MHz, CDCl δ
3
77.2 (3 peaks)]: δ = 191.7, 161.5, 153.6, 152.1, 148.9, 146.1, 143.3, 140.9, 140.5, 139.1,
134.0, 132.3, 131.3, 130.7, 130.2, 129.4, 129.0, 127.3, 126.6, 126.0, 125.4, 123.9, 123.1,
122.5, 122.1, 120.9, 120.3, 115.6, 114.7, 114.1, 113.9, 113.6, 55.8 ppm.
Synthesis
of
4-{{4-[4,7-bis(3-methoxyphenyl)-1H-benzimidazole-2-yl]phenyl}[3'-
(dimethylamino)biphenyl-4-yl]amino}benzaldehyde (TPA4b)
A solution of TPA3 (0.040 g, 0.0588 mmol) in benzene (8 mL) and aqueous 2 M Na2CO3
(4 mL) were added to a mixture of 3-(N,N-dimethylamino)-phenylboronic acid (0.015 g,
0.088 mmol) and tetrakis(triphenylphosphine)palladium(0) (catalytic level, ≈4 mg) in ethanol
(2 mL) at 60 °C under an argon atmosphere. After the mixture was refluxed at 85 °C for 2 h
under an argon atmosphere, the reaction mixture was poured into 50 mL of water and then,
extracted with chloroform (3×50 ml). The organic layer was dried over anhydrous magnesium
sulfate and evaporated in vacuo to dryness. The residue was separated by column
chromatography on silica gel (dichloromethane:n-hexane 5:1 (v/v)). Yield 65%. FT−IR (KBr
1
pellet, cm− ): 3056, 2926 and 2853 (aromatic ν ), 1693 (aldehyde ν ), 1592 (aromatic ν ),
C−H
C=O
C=C
1
1
1485, 1359, 1294, 1227, 1163, 1119, 1048, 839, 778, 736, 696 cm− . H NMR (400 MHz,
9

CDCl3 δ 7.27 ppm): δ = 9.84 (s, 1H), 8.05 (d, J = 8.5 Hz, 2H), 7.74 (d, J = 8.5 Hz, 2H), 7.65 –
7.54 (m, 6H), 7.49 – 7.41 (m, 6H), 7.27 – 7.13 (m, 8H), 6.95 (d, J = 8.5 Hz, 2H), 3.89 (s, 6H),
13
3.02 (s, 6H) ppm. C NMR [100 MHz, CDCl3 δ 77.2 (3 peaks)]: δ = 191.5, 161.1, 153.4,
152.1, 151.8, 148.6, 140.1, 133.5, 132.8, 132.7, 132.6, 132.5, 132.0, 130.7, 130.6, 130.2,
129.9, 129.3, 129.2, 129.1, 128.9, 127.2, 127.1, 125.8, 121.6, 121.4, 116.7, 116.1, 112.5,
112.4, 112.1, 111.7, 55.6, 40.9 ppm.
Synthesis of (2E)-3-{4-[{4-[4,7-bis(3-methoxyphenyl)-1H-benzimidazole-2-yl]phenyl}(3'-
methoxybiphenyl-4-yl)amino]phenyl}-2-isocyanoacrylic acid (TPA5a)
A mixture of compound TPA4a (0.0270 g, 0.0381 mmol), cyanoacetic acid (0.004 g,
0.0490 mmol) and piperidine (10 µL, 0.11 mmol) was refluxed in 4 mL of MeCN at 80 °C for
4 h under argon atmosphere. The reaction progress was controlled by TLC. After completion,
the solvent was evaporated. The crude product was purified by column chromatography on
silica gel using dichloromethane:ethanol (4:1) as eluent. Yield 55%. FT−IR (KBr pellet,
1
cm− ): 3436 (O−H stretching), 2936 (aliphatic ν ), 2214 (nitrile ν ), 1593 (carboxylic acid
C−H
C•N
1
1
ν ), 1506, 1483, 1390, 1363, 1323, 1295, 1183, 1047, 834, 785, 696 cm− . H NMR (400
C=O
MHz, DMSO δ 2.49 ppm): δ = 12.65 (s, 1H), 8.31 (d, J = 8.5 Hz, 2H), 7.99 (s, 1H), 7.90 (d, J
= 8.5 Hz, 2H), 7.73 – 7.70 (m, 2H), 7.54 – 7.35 (m, 4H), 7.31 – 7.21 (m, 10H), 7.12 (d, J =
7.8 Hz, 2H), 7.04 (d, J = 8.5 Hz, 2H), 6.94 (t, J = 8.5 Hz, 2H), 3.85 (s, 6H), 3.82 (s, 3H) ppm.
13C NMR [100 MHz, DMSO-d6 δ 40.2 ppm (7 peaks)]: δ = 160.7, 153.4, 151.3, 148.6, 148.5,
146.3, 143.3, 140.7, 140.5, 134.8, 132.8, 132.8, 131.1, 131.1, 130.4, 130.4, 130.1, 129.4,
127.0, 126.9, 126.3, 125.7, 125.7, 124.1, 122.6, 122.2, 122.0, 121.7, 119.8, 115.7, 114.8,
114.6, 113.7, 113.0, 112.9, 55.8 ppm.
Synthesis of (2E)-3-(4-{{4-[4,7-bis(3-methoxyphenyl)-1H-benzimidazole-2-yl]phenyl}[3'-
(dimethylamino)biphenyl-4-yl]amino}phenyl)-2-isocyanoacrylic acid (TPA5b)
10

A mixture of compound TPA4b (0.040 g, 0.055 mmol), cyanoacetic acid (0.006 g, 0.071
mmol) and piperidine (10 µL, 0.11 mmol) was refluxed in 7 mL of MeCN at 80 °C for 2 h
under argon atmosphere. The reaction progress was controlled by TLC. After completion, the
solvent was evaporated. The crude product was purified by column chromatography on silica
1
gel using dichloromethane:ethanol (4:1) as eluent. Yield 70%. FT−IR (KBr pellet, cm− ): 3429
(O−H stretching), 2924, 2852 (aliphatic ν ), 2214 (nitrile ν ), 1598 (carboxylic acid ν ),
C−H
C•N
C=O
1
1
1505, 1485, 1385, 1360, 1322, 1293, 1227, 1183, 1047, 834, 784, 696 cm− . H NMR (400
MHz, DMSO δ 2.49 ppm): δ = 12.65 (s, 1H), 8.31 (d, J = 8.7 Hz, 2H), 7.96 (s, 1H), 7.88 (d, J
= 8.3 Hz, 2H), 7.72 – 7.67 (m, 2H), 7.54 – 7.41 (m, 4H), 7.28 – 7.22 (m, 10H), 7.11 (d, J =
8.7 Hz, 2H), 7.01 (s, 1H), 6.93 (t, J = 8.3 Hz, 2H), 6.72 (d, J = 8.3 Hz, 1H), 3.84 (s, 6H), 2.94
(s, 6H) ppm. 13C NMR [100 MHz, DMSO-d6 δ 40.3 ppm (7 peaks)]: δ = 160.4, 153.3, 152.1,
150.3, 148.6, 146.0, 145.9, 141.3, 140.7, 140.5, 138.8, 135.6, 134.8, 134.7, 133.0, 132.3,
131.1, 130.6, 130.4, 130.0, 129.3, 127.7, 127.0, 126.6, 126.3, 125.3, 122.8, 122.0, 121.7,
119.2, 115.7, 115.6, 114.7, 113.7, 111.4, 55.7 ppm.
Synthesis of (2E)-3-{4-[{4-[4,7-bis(3-methoxyphenyl)-1H-benzimidazole-2-yl]phenyl}(4-
bromophenyl)amino]phenyl}-2-isocyanoacrylic acid (TPA4c)
A mixture of compound TPA3 (0.050 g, 0.0735 mmol), cyanoacetic acid (0.008 g, 0.0945
mmol) and piperidine (10 µl, 0.11 mmol) was refluxed in 3 mL of MeCN at 80 °C for 3 h
under argon atmosphere. The reaction progress was controlled by TLC. After completion, the
solvent was evaporated. The crude product was purified by column chromatography on silica
1
gel using dichloromethane:ethanol (4:1) as eluent. Yield 60%. FT−IR (KBr pellet, cm− ): 3429
(O−H stretching), 2923, 2852 (aliphatic ν ), 2218 (nitrile ν ), 1588 (carboxylic acid ν ),
C−H
C•N
C=O
1
1484, 1436, 1323, 1293, 1182, 1119, 1047, 833, 696 cm− . 1H NMR (400 MHz, DMSO δ 2.49
ppm): δ = 12.67 (s, 1H), 8.30 (dd, J = 13.8, 8.7 Hz, 2H), 7.90 (dd, J = 13.8, 8.7 Hz, 4H), 7.69
– 7.53 (m, 8H), 7.52 – 7.39 (m, 2H), 7.32 – 7.17 (m, 5H), 7.02 (t, J = 9.8 Hz, 1H), 6.95 (d, J =
11

13
8.2 Hz, 1H), 3.85 (s, 6H) ppm. C NMR [100 MHz, DMSO-d6 δ 39.9 ppm (7 peaks)]: δ =
159.9, 159.5, 139.9, 139.7, 133.7, 132.4, 131.9, 131.8, 130.4, 130.3, 129.6, 129.2, 129.1,
126.5, 125.9, 125.6, 123.8, 123.5, 121.4, 121.1, 115.0, 114.2, 113.9, 113.1, 55.4 ppm.
Results and discussion
Excited state properties of the dyes in solutions and on thin films
Normalized UV–vis absorption spectra of TPA5a-b and TPA4c dyes in chloroform and on
thin films are shown in Fig. 2a and Fig. 2b, respectively. The spectroscopic parameters are
also summarized in Table 1. The absorption spectra of the dyes in chloroform display two
distinct absorption peaks around 359 and 428 nm. The narrow absorption bands around 359
nm can be attributed to the π−π* electronic transition and the other broad bands with shoulder
extending up 520 nm region are assigned to the intramolecular charge transfer (ICT) between
the TPA donor and the cyanoacrylic acid acceptor [9, 32-34]. When the dyes are adsorbed
onto the TiO2 films, the absorption spectra give a broad band at maximum wavelength of 428
nm. This observation may be related to the aggregation of the dyes on the semiconductor
surface [22]. When TiO2 cover is replaced by the alumina (Al2O3) coating, the absorption
spectra of the dyes are shifted to the blue region as compared to those on TiO2 surface because
of the formation of H-type aggregates [35]. Visible absorption maximum of TPA4c dye gives
a blue shift of 10 nm in chloroform as compared to that of in toluene solution. This behaviour
can be explained by the increase in band gap energy resulted from the interaction between the
bromine group of TPA4c dye and the chloride atoms of the solvent. While TPA4c dye shows
higher molar extinction coefficients than that of the others in chloroform solution, this value
for TPA5a dye is higher than that of other coated dyes on TiO2 surface. This means that the
12

strongest solar light harvesting capacity could be expected from TPA5a dye causing the much
higher cell efficiency than that of the others in DSSC device which will be fabricated later.
Fig. 2a
Fig. 2b
When the dyes are excited at 365 nm in solutions, they exhibit emission maxima between
547 and 557 nm in chloroform (Fig. 3a). In toluene, TPA5a dye gives a red shift of 18 nm
with respect to the chloroform solution. Fig. 3b shows the fluorescence emission spectra of
the dyes adsorbed onto the TiO surface. All of the bands were red-shifted and broadened as
2
compared to those of the dyes in solutions. Studied dyes give large Stoke's shift values which
can be attributed to the delocalization of the excited energy along TPA-benzimidazole
conjugate then forming ICT state. In order to understand electronic interaction between the
dyes and TiO2-coated substrates, we examined the fluorescence quenching measurements on
thin film. Fig. 3c illustrates the fluorescence emission change of TPA5a dye when adsorbed
on alumina- or TiO2-coated glass substrates at the excitation wavelength of 400 nm.
Significant energy losses in its emission intensity can be attributed to the good electron
transfer from LUMO level of dye to the conduction band of TiO2. However, no quenching
behaviour was observed when TiO2 coating was replaced by alumina possesing a high band
gap value of ≈9eV, which is beyond the possible electron transfer limits from the dye to the
alumina.
Fig. 3a
Fig. 3b
Fig. 3c
Table 1
The fluorescence lifetimes of the dyes were measured using time-correlated single-photon-
counting system at the collected emission wavelength of 570 nm in solutions as shown in Fig.
4 and summarized in Table 2. In toluene, the time profile shows biexponential decay
13

consisting of a major contribution (66%) at around 3.78 ns and a minor contribution (34%) of
the long decay component at around 8.06 ns for TPA5a dye. Similar contributions were
detected for TPA5b and TPA4c dyes in both solvents. The shortest decay components define
the decay profile of excited state fluorescence of benzimidazole subunits mixed with TPA
main cores. Time constants for similar TPA structures are within the range of 2-3 ns in the
literature [36-38]. Such large shifts of the emission spectra to longer wavelengths in less polar
toluene solutions could not be explained by only solvatochromic effect. This result supports
the formation of charge-separated (CS) state in solutions. The longest decay profiles can be
attributed to the CS process via the donor and acceptor sites of the molecules. Fluorescence
quantum yields of TPA5a-b dyes are higher than that of TPA4c in toluene. TPA4c dye gains
its highest yield value in more polar chloroform solution. It can be understood from Table 2
that changes in fluorescence quantum yields are consistent with the relative amplitude values
for the generation of ICT complex in the studied solvents. Higher percentage of contribution
(α2) for TPA5a-b and TPA4c dyes in the studied solvents, the more increase in the
corresponding fluorescence quantum yields of the sensitizers.
Fig. 4
Table 2 [39]
CV measurements and molecular orbital calculations
Fig. 5 gives the CV graphs of TPA5a-b and TPA4c dyes and Table 3 summarizes the
redox potentials with band gaps of the studied dyes. TPA5a, TPA5b and TPA4c dyes exhibit
only one oxidation potentials at E°ox = 1.38, 1.30 and 1.32 eV, respectively. The easier
oxidation of TPA5b dye is assigned to the presence of aniline group having more electron
releasing capacity than that of other substituents. LUMO energy levels of TPA5a, TPA5b
and TPA4c dyes are calculated from the reduction onset values and found to be about −3.11,
14

−3.03 and −3.06 eV for, respectively. These values are fairly suitable level for injecting
electron from LUMO excited level of dye to the CB band of TiO2 surface.
Fig. 5
Table 3
Fig. 6 shows the electronic distribution on HOMO and LUMO orbitals for the
geometrically optimized structure of the sensitizers. Electronic distribution of HOMO level of
the compounds is densily localized on the benzimidazole side. In the first excited state,
electronic cloud moves from benzimidazole to the acceptor acrylic acid side through TPA
group. This pathway is good hint for an efficient electron transfer from LUMO level of the
dyes to the CB of titania.
Fig. 6
Photovoltaic properties of DSSCs and electrochemical impedance spectroscopy analysis
Fig. 7 shows the current-voltage (J-V) characteristics of DSSCs based on TPA5a-b and
TPA4c dyes adsorbed on TiO2 and Table 4 gives the photovoltaic parameters of the devices.
In DSSC devices TPA5a, TPA5b and TPA4c dyes exhibited cell efficiencies of 4.31%,
2.65% and 2.57%, respectively. DSSC based on organic dye TPA5a yielded a short-circuit
current (Jsc) of 9.03 mA cm−2, an open-circuit voltage (Voc) of 740 mV, and a fill factor (ff) of
0.64. The methoxy groups attached to the TPA can enhance the electron delocalization and
therefore optimize the electron donation ability of the sensitizer. Also, Fig. 8 compares the
EQE spectra of the devices fabricated with the synthesized dyes. DSSC based on TPA5a dye
yields the highest and broadest EQE curve in the graph with the value of 71% at 460 nm
among the devices using the other sensitizers.
Fig. 7
Table 4
15

Fig. 8
Fig. 9 shows the Nyquist plots for DSSCs based on TPA5a-b and TPA4c dyes which were
measured under reversed bias of Voc condition at −0.65 V in dark with frequency of 0.1 Hz to
1 MHz. Electrochemical impedance spectroscopy (EIS) dedicates the interfacial charge
recombination kinetics of the DSSCs. Generally, DSSCs exhibit three semi-circles in a
Nyquist plot. The high frequency semi-circle which is related to charge transfer resistance
arises from the double layer of electrolyte/TCO electrodes. The low frequency semi-circle is
related to diffusion resistance of the redox species in the electrolyte which is called Warburg
Impedance. Finally, the larger middle semi-circle is related to the charge recombination
resistance (Rrec) of TiO2/Dye/Electrolyte interface arising from charge recombination from
TiO2 CB to the dye HOMO and redox couple oxidation potential. Larger Rrec indicates much
slower charge recombination which increases Jsc and Voc.
Since the semicircle for Nernst diffusion within the electrolyte was overlapped by the
intermediate frequency large semicircle, only two semicircles are observed in the Nyquist
plots of the presented work. Nyquist plots offer the enhancement of recombination resistance
(Rrec) in the order of TPA4c<TPA5b<TPA5a, as well-correlated with increasing efficiencies.
Fig. 9
Fig. 10 proposes a suitable electron injection pathway from the excited sensitizer to the CB
band of titania from donor TPA main core.
Fig. 10
Conclusion
In summary, we have synthesized a series of new metal-free DSSC sensitizers carrying
triphenylamine-benzimidazole group as donor and cyanoacrylic acid acceptor bridged with a
methine linker. The formation of CT complex could be estimated by means of time resolved
16

measurements and steady-state optical studies. Computational and experimental results
support that the electron transfer from rich π-system of the donor group to the acceptor side
could give good spectral responses and cell efficiencies in DSSC devices. In particular, the
highest power conversion efficiency obtained for TPA5a dye with 4.31% remarks the
significant contribution of methoxy group to electron delocalization for whole structure.
Acknowledgement
We acknowledge the Research Council of Celal Bayar University with the project number
2010 050 for their financial contribution to our work.
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Figure Legends
Fig. 1. Synthetic pathways of the (a) primary molecule PDA: (i) 2,1,3-benzothiadiazole,
Br2/HBr, 120 °C, 95%; (ii) 3-methoxybenzeneboronic acid, 2M Na2C03, Pd(PPh3)4,
EtOH/benzene, 85 °C, 85%; (iii) NaBH4/CoCl2·6H2O, Et-OH/THF, 80 °C [21], 93% and, (b)
the target molecules: (i) NBS, DMF, 25 °C, 16 h, 79%; (ii) POCl3, DMF, 100 °C [22], 20 h,
55%; (iii) PDA, MeOH, reflux, [20] 45%; (iv) 3-methoxybenzeneboronic acid, 2M Na2CO3,
Pd(PPh3)4, Et-OH/benzene, 85 °C, 16 h, 60% for TPA4a; 3-(N,N-dimethylamino)-
phenylboronic acid, 2M Na2CO3, Pd(PPh3)4, Et-OH/benzene, 85 °C, 16 h, 65% for TPA4b
[23]; (v) cyanoacetic acid, piperidine, Me-CN, 80 °C, [24, 25] 60%; (vi) cyanoacetic acid,
piperidine, Me-CN, 80 °C [24, 25], 55-70%.
Fig. 2. (a) Normalized UV–vis absorption spectra of TPA5a-b and TPA4c dyes in
chloroform solution, and (b) UV–vis absorption spectra of TPA5a-b and TPA4c dyes on
alumina or TiO2-coated glass substrates.
20

Fig. 3. Normalized fluorescence emission spectra of TPA5a-b and TPA4c dyes in chloroform
solution (λexc = 365 nm), and (b) fluorescence emission spectra of TPA5a-b and TPA4c dyes
on TiO2-coated glass substrates. (c) Fluorescence emission change of TPA5a dye when
adsorbed on alumina- or TiO2-coated glass substrates (λexc = 400 nm).
Fig. 4. Fluorescence decays of TPA5a dye in toluene and chloroform solutions (λdetection = 570
nm, time increments in toluene and chloroform are about 8.2 and 5.2 ps, respectively).
Fig. 5. Comparison of cyclic voltammograms of TPA5a-b and TPA4c dyes on glassy carbon
working electrode in 0.1 M [TBA][PF6]/Me-CN (Scan rate: 100 mV s─1).
Fig. 6. Illustration of theoretically calculated electron distribution on HOMO and LUMO
energy levels of optimized structure for TPA5a-b and TPA4c dyes.
Fig. 7. Current-voltage (J-V) characteristics of DSSCs based on TPA5a-b and TPA4c dyes
adsorbed on TiO2.
Fig. 8. EQE spectra of DSSCs based on TPA5a-b and TPA4c dyes.
Fig. 9. EIS Nyquist plots for DSSCs based on TPA5a-b and TPA4c dyes.
Fig. 10. Schematic explanation of possible electron transfer from the sensitized TPA5a dye to
the titania surface in DSSC device.
Table Legends
Table 1
Optical properties of TPA5a-b and TPA4c dyes in toluene and chloroform solutions, and on TiO2-coated glass
substrates (λexc = 365 nm for solutions and 400 nm for thin films).
* also indicates the long-wavelength absorptions of the dyes.
Table 2
Fluorescence decay times (τf (ns)), associated relative amplitudes (αi (%)) and fluorescence quantum yieldsa of
TPA5a-b and TPA4c dyes in toluene and chloroform solutions (λexc = 365 nm was used for quantum
calculations).
21

a Fluorescence quantum yields have been determined using quinine sulfate dihydrate (λexc = 365 nm, ΦF = 0.546,
0.1 N H2SO4) [39].
Table 3
Cyclic voltammetry data and molecular orbital energies of TPA5a-b and TPA4c dyes with respect to the
vacuum level.
a
E0–0 transitions were estimated from the intersection of the absorption and emission spectra attached to a
nanocrystalline TiO2 film.
Table 4
Photovoltaic data of DSSCs based on TPA5a-b and TPA4c dyes.
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26

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