Novel D-π-A type triphenylamine based chromogens for DSSC: Design, synthesis and performance studies

Article abstract of DOI:10.1039/c6nj01314g

In this work, four novel D-π-A organic dyes (MH, MT, DT, DH) consisting of triphenylamine as an electron donor and 2,4-thiazolidinedione/hydantoin as anchoring groups were designed and synthesized for use in dye-sensitized solar cells (DSSCs). The synthesized dyes were characterized using FT-IR, NMR, mass spectrometry, absorbance and electrochemical measurements. The photophysical, electrochemical and photovoltaic properties of the solar cells based on these dyes were investigated to study the effect of two different anchoring groups on the performance of the dye-sensitized solar cells. The dye consisting of two hydantoin anchoring groups shows the highest conversion efficiency of 2.70% (short-circuit current = 7.60 mA cm^-2, open circuit voltage = 570 mV, and fill factor = 0.62 under standard AM1.5G irradiation) compared to the other designed dyes. The observed results will provide basic information for the future design of sensitizers with different anchoring groups for photovoltaic applications.

Full text of DOI:10.1039/c6nj01314g

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Novel D–p–A type triphenylamine based
chromogens for DSSC: design, synthesis
Cite this:DOI: 10.1039/c6nj01314g
and performance studies†
Behzad Hosseinzadeh,a Alireza Salimi Beni,*b Masoumeh Azari,b Maryam Zarandib
and Marzeih Karamib
In this work, four novel D–p–A organic dyes (MH, MT, DT, DH) consisting of triphenylamine as an electron
donor and 2,4-thiazolidinedione/hydantoin as anchoring groups were designed and synthesized for use in
dye-sensitized solar cells (DSSCs). The synthesized dyes were characterized using FT-IR, NMR, mass
spectrometry, absorbance and electrochemical measurements. The photophysical, electrochemical and
photovoltaic properties of the solar cells based on these dyes were investigated to study the effect of two
different anchoring groups on the performance of the dye-sensitized solar cells. The dye consisting of
two hydantoin anchoring groups shows the highest conversion efficiency of 2.70% (short-circuit current =
7.60 mA cmꢀ2, open circuit voltage = 570 mV, and fill factor = 0.62 under standard AM1.5G irradiation)
compared to the other designed dyes. The observed results will provide basic information for the future
design of sensitizers with different anchoring groups for photovoltaic applications.
Received (in Montpellier, France)
25th April 2016,
Accepted 27th July 2016
DOI: 10.1039/c6nj01314g
www.rsc.org/njc
porphyrins as dye containing metal have achieved a maximum
power conversion efficiency of 13%.6 Metal-free organic sensi-
1. Introduction
In recent years, the quest for renewable energy has received tizers have also shown promise due to their advantages such as
worldwide attention owing to the fast depletion of fossil fuel low cost, high molar extinction coefficient, and facile molecular
reserves and the environmental pollution problems that accom- design. Recently, an unprecedented power conversion efficiency
pany fossil fuel consumption. Solar energy conversion devices of 10.3% was reported for an organic sensitizer,7 indicating that
that convert sunlight into electricity are seen as inevitable with further strategic optimization, metal-free organic sensitizers
future energy candidates due to the unlimited supply of solar may overtake their organometallic counterparts in terms of
energy.1,2
photovoltaic performance. Novel organic sensitizers such as
Dye-sensitized solar cells have been significantly developed triarylamine,8 coumarin,9–11 tetrahydroquinoline,12,13 mero-
as energy conversion devices, due to their competent perfor- cyanine,14,15 cyanine,16,17 phenothiazine,18 indoline,19,20 and
mance and low production costs.3–5 Dye-sensitized solar cells hemicyanine21,22 have been used in DSSCs with good results.
(DSSCs) are the most effective renewable photovoltaic devices A typical structure of metal-free organic sensitizers is a donor–
because of their advantages such as ease of fabrication, short p-bridge–acceptor system (D–p–A). An appropriate donor will
energy payback time, low sensitivity to temperature variation, lead to a good light absorption spectrum and matched energy
flexibility, transparency, efficiency at different angles with low levels with both the conduction band edge of TiO2 and the
light intensity and low emission of CO2, and also they are more redox potential of the electrolyte. Therefore, the choice of a
environmentally friendly than conventional inorganic solar donor moiety and its structural optimization are critical to the
cells. Among the major components of DSSCs, the sensitizer spectral response, charge injection efficiency and stability of
plays a crucial role in achieving a high power conversion sensitizers with high photovoltaic performance. Among the
efficiency and has been extensively studied. To overcome the various organic dyes, triphenylamine (TPA) is still one of the
high cost of ruthenium complexes, metal free organic dyes have most popular donor moieties and the modification of TPA is
been extensively explored in recent years. For example, zinc desirable.23–26 A successful approach has been demonstrated
by incorporating a planar amine with bulky substituents, which
not only could increase the charge-separated state lifetime by
the delocalization of the generated cation over a planar amine
unit but also could avoid dye aggregation.27 With two adjacent
a Department of Chemistry, Mazandaran University, Babolsar, Iran
b Chemistry Department, Yasouj University, Yasouj 75918-74831, Iran.
E-mail: salimibeni@mail.yu.ac.ir; Tel: +987412223028
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nj01314g phenyl rings connected by a methylene linker, bridged TPA is a
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Fig. 1 Chemical structures of the reference compound TPACn and the triphenylamine based dyes MH, MT, DH and DT.
potential donor unit in constructing a donor–p–acceptor (D–p–A) electrothermal KSB1N apparatus. Chemicals were purchased
type system.
It is notable that many academic groups have investigated
from Aldrich, Fluka and Merck chemical companies.
metal-free sensitizers, but the design of sensitizers containing
new anchoring groups is in its infancy and is one of the
interesting points in the design of sensitizers.
2.2 Synthesis and characterization of compounds
Scheme 1 shows the synthetic approach for the new chromo-
phores MH, MT, DH and DT. These dyes were synthesized by
well-known reactions such as the Vilsmeier–Haack formylation
reaction and Knoevenagel reaction. The starting triphenylamine
(TPA) was synthesized from phenylamine and iodobenzene in a
nitrogen atmosphere (Cu catalyst, 115 1C, 3.5 h).30 In the next
step, compound 1 and 2 were synthesized via a Vilsmeier reaction.
During the formylation reactions, the number of aldehyde groups
introduced was controlled by varying the amount of POCl3. Then,
Knoevenagel condensation reactions of these aldehydes with
2,4-thiazolidinedione and hydantoin afforded the target dyes
MH, MT, DH and DT in acetic acid using ammonium acetate as
a catalyst.
In this study, in continuation of our investigation to develop
new sensitizers and find novel anchoring groups for sensitizers,28,29
four types of triphenylamine-based organic dyes MH, MT, DH, and
DT (molecular structures are shown in Fig. 1) with two different
anchoring groups were designed and synthesized as sensitizers for
DSSCs. The acceptor/anchoring groups used are 2,4-thiazolidine-
dione and hydantoin. The photophysical, electrochemical, and
photovoltaic properties of the dyes were also investigated.
2. Experimental section
2.1 Equipment and materials
1H and 13C NMR spectra were recorded on a Bruker 400
Ultrashield NMR and CDCl3 or DMSO-d6 was used as a solvent.
2.3 General procedures for the synthesis of MH, MT,
DH and DT
FT-IR spectra were recorded in a KBr matrix using a JASCO Aldehyde (1 mmol) and hydantoin or 2,4-thiazolidinedione
FT-IR-680 plus spectrometer. Elemental analyses were performed (1.5 mmol) were added to 15 mL of glacial acetic acid and
using a Heraeus CHN–O-Rapid analyzer. Mass spectra of the refluxed in the presence of 150 mg of ammonium acetate. After
products were obtained using a HP (Agilent technologies) 5937 cooling to room temperature, the mixture was poured into ice
Mass Selective Detector. Melting points were measured on an water. The solid was filtered and washed with water. Finally, the
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Scheme 1 Synthetic procedure of the organic dyes.
precipitate was purified by column chromatography (hexane/ acetate/n-hexane (1 : 5, v/v) to give a pale yellow solid (yield =
ethyl acetate).
94%). m.p.: 129–131 1C; FT-IR (cmꢀ1): 1690, 1492, 1335, 1308,
2.3.1 Synthesis of 4-(diphenylamino) benzaldehyde (1). 1229, 1157, cmꢀ1. Anal. cacld for C19H15NO: C, 83.49, H, 5.53,
The synthesis of this compound was carried out based on a N, 5.13. 1H NMR (CDCl3-d6, 400 MHz): d (ppm): 9.82 (s, 1H,
previously reported method.31 To a mixture of Ph3N (3 g, CHO), 7.70 (d, J = 8.7 Hz, 2H), 7.36 (t, J = 7.7–8.2 Hz, 4H), 7.21–
12.23 mmol) and DMF (40 mL) at 0 1C was added POCl3 (1.2 mL, 7.17 (m, 6H), 7.04 (d, J = 8.7 Hz, 2H); 13C NMR (CDCl3-d6,
12.87 mmol, 1.05 eq.) dropwise with stirring. The resulting mixture 100 MHz): 189.94, 153.32, 146.58, 132.45, 130.89, 129.37,
was stirred at 95–100 1C under nitrogen for 20 h after which TLC 127.51, 126.25, 120.54; anal. calcd for C19H15NO: C, 83.49,
analysis showed that no starting material remained. The mixture H, 5.53, N, 5.13. Found: C, 83.49, H, 5.57, N, 5.15.
was then cooled to room temperature, poured into ice–water
2.3.2 Synthesis of 4,40-(phenylazanediyl)dibenzaldehyde (2).
(150 mL) and neutralized with 4 M NaOH solution. The solid Compound 2 was prepared using a previously reported method.32
was collected, washed with water, dried, and purified by column To a mixture of Ph3N (1 g, 4.08 mmol) and DMF (8 mL) at 0 1C was
chromatography on neutral alumina using a mixture of ethyl added POCl3 (4 mL, 42.91 mmol, 10.5 eq.) dropwise with stirring.
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1
The resulting mixture was stirred at 95–100 1C under nitrogen 1329, 1284, 1185, 1147, 685, 645. H NMR (DMSO-d6, 400 MHz):
for 6 h, followed by similar workup, and purified by column d (ppm): 12.01 (s, 2H), 7.71 (s, 2H), 7.52 (d, J = 8.0 Hz, 4H), 7.41
chromatography (silica gel, EtOAc–hexane 1 : 4, v/v) to give a (t, J = 8.0 Hz, 2H), 7.24 (t, J = 8.0 Hz, 1H), 7.16 (d, J = 8.0 Hz, 2H),
yellow solid (yield = 71%). m.p. 141–143 1C; FT-IR (cmꢀ1): 1703, 7.10 (d, J = 8.0 Hz, 4H). 13C NMR (DMSO-d6, 100 MHz): 168.4,
1591, 1512, 1499, 1335, 1295, 1223, 1177, 834. 1H NMR (CDCl3- 168.0, 148.5, 145.8, 132.3, 131.7, 130.7, 127.8, 127.0, 126.2, 123.4,
d6, 400 MHz): d (ppm): 9.85 (s, 2H, CHO), 7.77 (d, J = 8.7 Hz, 121.5. Mass: m/z = 499 (M + 1), 91, 66, 43. Anal. calcd for
4H), 7.41 (t, J = 7.6–8.0 Hz, 2H), 7.30 (m, 1H), 7.22–7.19 (m, 2H), C26H17N3O4S2: C 62.51, H 3.43, N 8.41, O 12.81, S 12.84; found:
7.20 (d, J = 8.6 Hz, 4H); 13C NMR (CDCl3-d6, 100 MHz): 190.88, C 62.46, H 3.39, N 8.38, O 12.79, S 12.82.
152.42, 145.82, 131.69, 130.55, 127.16, 127.51, 126.86, 123.64,
anal. calcd for C20H15NO2: C, 79.71, H, 5.02, N, 4.65. Found: C,
2.4 Preparation of TiO2 electrode
79.63, H, 5.07, N, 4.63.
The TiO2 electrode was prepared according to a reported literature
2.3.3 Synthesis of 5-[(Z)-1-(4-diphenylaminophenyl)methyli- procedure.33 An amount of acetic acid (12 g, 0.2 mol) was added all
dene]-2,4-imidazolidinedione (MH). The synthesis of this com- at once to 58.6 mg (0.2 mol) of titanium isopropoxide under
pound was carried out based on general procedures, giving a stirring at room temperature. The modified precursor was stirred
pale yellow solid (yield = 85%); m.p. = 209–211 1C; FT-IR (cmꢀ1): for about 15 min and poured into 290 mL of water as quickly as
3426, 3192, 1746, 1713, 1648, 1588, 1510, 1490, 1383, 1285, 1177, possible with vigorous stirring (700 rpm). A white precipitate was
1
1097, 1017, 879, 754, 696, 543; H NMR (CDCl3-d6, 400 MHz): instantaneously formed. One hour of stirring was required to
d (ppm): 8.55 (s, 1H), 8.43 (s, 1H), 7.27–7.34 (m, 6H), 7.10–7.16 achieve a complete hydrolysis reaction. After adding 4 mL of
(m, 6H), 7. 05 (d, J = 8.8 Hz, 2H), 6.67 (s, 1H). 13C NMR (CDCl3-d6, concentrated nitric acid, the mixture was heated from room
100 MHz): 164.4, 154.7, 148.9, 146.7, 130.3, 129.6, 125.5, 124.8, temperature to 80 1C within 40 min and peptized for 75 min.
124.2, 121.8, 112.9. Mass: m/z 355 (M + 1), 283, 180, 77. Anal. Water was then added to the cooling liquid mixture to adjust the
calcd for C22H17N3O2: C 74.35, H 4.82, N 11.82, O 9.00; found: C volume to a final 370 mL. The resulting mixture was kept in a
74.30, H 4.80, N 11.79, O 8.96.
570 mL titanium autoclave and heated at 250 1C for 12 h.
2.3.4 Synthesis of 5-[(Z)-1-(4-diphenylaminophenyl)methyli- Following this step, 2.4 mL of 65% nitric acid was added and
dene]-1,3-thiazolane-2,4-dione (MT). The synthesis of this the dispersion was treated with a 200 W ultrasonic titanium
compound was carried out based on general procedures, giving probe at a frequency of 30 pulses every 2 s. The resultant
an orange solid (yield = 87%); m.p. 270–272 1C; FT-IR (cmꢀ1): colloidal solution was concentrated with a rotary evaporator
3466, 3181, 1736, 1685, 1582, 1506, 1490, 1329, 1285, 1192, 754, to contain 13 wt% TiO2. Finally, it was triply centrifuged to
1
695, 641. H NMR (CDCl3-d6, 400 MHz): d (ppm): 9.21 (s, 1 H), remove nitric acid and washed with ethanol three times to
7.81 (s, 1 H), 7.33–7.37 (m, 6H), 7.15–7.19 (m, 6H), 7.05 (d, J = produce a white precipitate containing 40 wt% TiO2 in ethanol
8.8 Hz, 2H). 13C NMR (CDCl3-d6, 100 MHz): 167.7, 167.2, 150.3, and only trace amounts of water.
146.3, 134.5, 131.9, 129.7, 126.0, 125.0, 124.8, 120.5, 118.0.
2.5 Fabrication of DSSCs
Mass: m/z = 372 (M + 1), 355, 301; anal. calcd for C22H16N2O2S:
C 70.95, H 4.33, N 7.52, O 8.59, S 8.61; found: C 70.91, H 4.31, All the anode films for the DSSCs were made under the same
N 7.50, O 8.56, S 8.59.
standard conditions33 and are composed of a 12 mm thick
2.3.5 Synthesis of 5-((Z)-1-{4-[4-(2,5-dioxotetrahydro-1H-4- transparent layer (TiO2 with diameter of 20 nm) and a 6 mm
imidazolylidenmethyl)(phenyl)anilino]phenyl}methylidene) 2,4- thick scattering layer (TiO2 nanoparticles with a diameter of
imidazolidinedione (DH). Same as the general procedure but 200 nm). Specifically, a doctor blade technique was utilized to
4,40-(phenylazanediyl)dibenzaldehyde (301 mg, 1 mmol) instead prepare the photoanode (TiO2) films. Then, the electrodes were
of 4-(diphenylamino)benzaldehyde and hydantoin (300 mg, immersed in a 0.3 mM dye bath in acetonitrile : t-butanol
3.0 mmol) was used. The isolated yellow solid is DH, yield: (volume ratio, 1 : 1) solution for these dyes and maintained in
82%, m.p. 215–217 1C. FT-IR (cmꢀ1): 3423, 1755, 1717, 1651, the dark for 18 h. Afterward, the electrodes were rinsed with
1590, 1509, 1389, 1299, 1185, 878, 835, 698. 1H NMR (DMSO-d6, ethanol to remove non-adsorbed dyes and dried in air. To
400 MHz): d (ppm): 11.20 (s, 2H), 10.47 (s, 2H), 7.55 (d, J = 8.0 Hz, prepare the counter electrode, Pt catalyst was deposited on
4H), 7.37 (t, J = 8.0 Hz, 2H), 7.16 (t, J = 8.0 Hz, 1H), 7.10 (d, J = cleaned FTO glass by coating with a drop of H2PtCl6 solution
8.0 Hz, 2H), 6.97 (d, J = 8.0 Hz, 4H), 6.37 (s, 2H). 13C NMR (10 mM in 2-propanol solution) with heat treatment at 400 1C
(DMSO-d6, 100 MHz): 166.0, 156.1, 147.1, 146.5, 131.3, 130.4, for 15 min. The dye-containing TiO2 electrode and Pt counter
127.9, 127.1, 126.0, 125.1, 123.4, 108.6. Mass: m/z 465 (M + 1), electrode were assembled into a sandwich-type cell. The devices
307, 289, 260, 216, 167, 77. Anal. calcd for C26H19N5O4: C 67.09, were completed by filling the electrolyte by pre-drilled holes in
H 4.11, N 15.05, O 13.75; found: C 67.05, H 4.08, N 15.04, the counter electrodes and finally the holes were sealed with a
O 13.73.
2.3.6 Synthesis of 5-((Z)-1-{4-[4-(2,4-dioxo-1,3-thiazolan-5-
ylidenmethyl)(phenyl)anilino]phenyl}methylidene)-1,3-thiazolane-
Surlyn sheet and a thin glass cover by heating.
2.6 Photovoltaic performance measurements
2,4-dione (DT). DT was synthesized by the same procedure as Current density–voltage measurements were performed using
DH, affording an orange coloured solid in 86.6% yield. m.p.: simulated AM 1.5 sunlight with an output power of 100 mW cmꢀ2
246–248 1C. FT-IR (cmꢀ1): 3459, 1733, 2921, 1683, 1580, 1503, Electrochemical impedance spectroscopy (EIS), was measured in
.
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the frequency range of 10ꢀ1–105 Hz under 1 sun bias illumination properties of core electrons were treated with DFT semi-core
under open circuit conditions using computer controlled potentio- pseudoplots in term of the DNP basis set. The convergence energy
stat/galvanostat (IVIUM, Compactstat). Incident photon-to-current tolerance, gradient, and displacement convergence were 1.0 ꢁ
efficiencies (IPCE) were measured with monochromatic incident 10ꢀ5 Ha, 0.004 Ha Åꢀ1, and 0.005 respectively.
light of 1 ꢁ 1016 photon per cm2 under 100 mW cmꢀ2 with bias
light in DC mode (Jarrel Ash monochromator, using a 100 W
3. Results and discussion
3.1 Optical properties
halogen lamp). A calibrated photodiode (Thorlabs) was used to
study the optical properties of layers.
The UV-vis absorption spectra of sensitizers MH, MT, DH and
DT are shown in Fig. 2, and their photophysical data are listed
2.7 Theoretical calculation methods
in Table 1. In general, D–p–A organic dyes show two principal
groups of absorption bands: one due to intramolecular charge
transfer (ICT) transitions in the visible region and the other due
to localized p–p* transitions at shorter wavelengths.34 The two
dyes, MT and MH, exhibit one visible absorption band that
appears at 427 and 433 nm, respectively, which can be ascribed
to an efficient charge-separated state produced by an intra-
molecular charge transfer (ICT) between the donor and acceptor
end groups.
As can be seen in Fig. 2, DT and DH exhibit obvious
maximum absorption peaks at two distinct regions. The higher
energy peaks at 385 nm for DT and at 374 nm for DH can be
ascribed to a p–p* transition of the triphenylamine unit and the
lower energy peaks at 446 nm for DT and at 457 nm for DH can
be attributed to the intramolecular charge transfer (ICT)
between the arylamine donating unit and acceptor/anchoring
moiety, thereby producing an efficient charge separated state.35
The molar extinction coefficients (e) of the ICT bands of MH,
MT, DH and DT were 3.37 ꢁ 104, 4.31 ꢁ 104, 5.91 ꢁ 104 and
6.10 ꢁ 104 Mꢀ1 cmꢀ1, respectively (Table 1) which are higher
than that of the standard N719 sensitizer (1.42 ꢁ 104 Mꢀ1 cmꢀ1),36
indicating a better light harvesting ability of these new triphenyl-
amine based metal-free organic dyes than that of classical organo-
metallic dyes. The higher value of e for DH and DT is due to the
extended conjugation with p linker units and two anchoring units.
An increased number of anchoring units leads to a higher e value
due to the large oscillator strength of the charge transfer transition,
according to the Franck–Condon principle.37
The electronic configuration and geometric structure of the dyes
were optimized by density functional theory (DFT) calculations
with the Gaussian 03 program package at the B3LYP/6-31G(d,p)
level of theory. To model the adsorption modes of the dyes on the
TiO2 anatase(101) surface, periodic density functional theory
calculations were carried out using the Dmol3 package as imple-
mented in Material Studio (version 5.5). The TiO2 anatase(101)
surface is modelled using a slab with a thickness of six atomic
layers. Periodic boundary conditions are employed with a surface
supercell of 21.77 ꢁ 15.10 Å2 and a vacuum layer of at least 20 Å to
prevent interaction between adjacent slabs. For these calculations,
we used the generalized gradient approximation (GGA) functional
with Perdew and Wang (PW91) formulation. The electronic
The absorption spectra of triphenylamine dyes adsorbed on
the TiO2 surface are shown in Fig. 3 and the corresponding data
are included in Table 1. When the dyes were attached on TiO2
films, the absorption spectra may shift more or less compared
with those in solution because of strong interactions between
the dyes and semiconductor surface, which can lead to aggregates
Fig. 2 Absorption spectra of the organic dyes in THF solution.
Table 1 Band gap (calculated by DFT/B3LYP), absorption, and electrochemical parameters for organic dyes
(HOMO/LUMO)a
(eV)
Band
gapa
(labsb/nm)/
HOMOd
(vs. NHE) (V)
LUMO f
(vs. NHE) (V)
(e/104 Mꢀ1 cmꢀ1
)
lmaxc/nm
lmaxc/nm
E0–0 (eV)
e
Dye
MH
MT
DH
DT
ꢀ4.82/ꢀ2.81
ꢀ5.00/ꢀ3.05
ꢀ4.92/ꢀ3.13
ꢀ5.20/ꢀ3.49
2.01
1.95
1.79
1.71
427 (3.37)
433 (4.31)
457 (5.91)
446 (6.10)
398
395
410
408
398
395
410
408
1.57
1.70
1.45
1.46
2.65
2.53
2.39
2.43
ꢀ1.08
ꢀ0.83
ꢀ0.94
ꢀ0.97
a
b
B3LYP/6-31G(d) calculated values. Absorptions of charge-transfer transition were measured in THF; e: molar extinction coefficient at lmax
Maximum absorption on TiO2 films. HOMO of dyes measured by cyclic voltammetry in 0.1 M tetrabutylammonium hexafluorophosphate in
.
c
d
e
f
acetonitrile. E0–0 = 1240/l intersection. LUMO was calculated by HOMOꢀE0–0
.
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with the surface which directly reduces the energy of the p* level
and the formation of H-aggregates on the TiO2 surface.39,40 Thus,
a difference in the number and type of anchoring groups in the
dyes may result in different arrangement of the dyes on the TiO2
surface.
3.2 Electrochemical properties
Cyclic voltammetry in MeCN solution was used to determine
the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) of the dyes. The relevant
CV data are presented in Table 1. The results are presented in
Fig. 4 and graphically depicted in Fig. 5.
To evaluate the mechanism of electron transfer and the
reduction of the dye cation back to its original state for the next
cycle, cyclic voltammograms were recorded in acetonitrile
solution containing tetrabutylammonium hexafluorophosphate
Fig. 3 Absorption spectra of the four dyes absorbed on TiO2 film (0.1 M) as the supporting electrolyte, TiO2 films stained with
electrodes.
sensitizer as the working electrode and Pt as the counter electrode
with a scan rate of 100 mV sꢀ1. The potentials measured vs. Fc/Fc+
was converted into normal hydrogen electrode (NHE) potentials.41
of the dyes forming on the semiconductor surface, such as
The ground oxidation potentials (Eox) correspond to the
H-aggregation for the blue shift or J-aggregation for the red highest occupied molecular orbitals (HOMO) while the lowest
shift. The absorption spectra on TiO2 are broadened and the unoccupied molecular orbitals (LUMO) were obtained from the
maximal absorption peaks for MH, MT, DH and DT on the TiO2 values of Eox and the zero–zero band gaps (E0–0) estimated from
film are observed at 398, 395, 410, and 408 nm, respectively. the onset of the UV-visible absorption spectra. The HOMO and
They are blue shifted by 29, 38, 36, and 49 nm compared with LUMO levels of the dyes are collected in Table 1. Apparently,
the solution spectra, respectively. This is ascribed to deproto- the HOMO levels of all dye molecules are lower than the iodide/
nation of the dyes or the formation of H-aggregation on the triiodide redox potential value (0.4 V vs. NHE), indicating that
semiconductor surface.38 Hypsochromic shifts on the TiO2 the oxidized dyes could be efficiently regenerated by the
surface were assigned to the interaction of the anchoring group electrolyte. Their LUMO levels are estimated from the HOMOs
Fig. 4 Cyclic voltammograms of the four dyes.
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throughout the whole structure, and the LUMO orbital is
mainly located at the linker and extends through the anchoring
unit. Obviously, the HOMO and LUMO perfectly show the
donor–acceptor character of the triphenylamine and anchoring
groups. The availability of sufficient electron density at the
anchoring group in the LUMO results in good electronic
coupling between the excited electrons of the sensitizer in
the LUMO with the vacant d-orbitals of TiO2 where they are
attached to fabricate the photoanode. This indicates facile
intramolecular charge separation in these molecules and results
in facile electron injection after photoexcitation. In the case of
MH and DH dyes, the LUMO levels were majorly distributed in
the hydantoin moiety. This is because of the strong electron
withdrawing nature of the N atom compared with the S atom in
thiazolidinedione. It is suggested that the N atom increases the
electron withdrawing nature of the hydantoin which also induces
intramolecular charge transfer from the donor to the acceptor
group under illumination compared with thiazolidinedione.
The results indicated that the HOMO–LUMO excitation
induced by light irradiation can effectively move the electron
distribution from the TPA moiety to the 2,4-thiazolidinedione/
hydantoin moiety. Photoinduced electrons can be efficiently
transferred from the dye to the TiO2 surface by this electron
separation.
Fig. 5 The schematic energy levels of MH, MT, DH and DT based on
absorption and electrochemical data.
and the energy gaps (E0–0), and the values are higher than the
conduction band (CB) of the TiO2 anode (0.5 V vs. NHE),
suggesting an efficient driving force for electron injection from
the LUMO of the dyes to the CB band of TiO2 semiconductor.
The LUMO level order of the di-anchoring dyes was different
from those of mono-anchoring dyes possibly due to the con-
jugation length and the electron withdrawing nature of the
acceptor of the synthesized dyes. Consequently, although
the additional anchoring group makes the HOMO level of the
di-anchoring group slightly more positive, the HOMO–LUMO
gap decreased and the absorption spectra were red-shifted.
Therefore, the four dyes are feasible for application in DSSCs.
3.4 Photovoltaic device performance
Fig. 7 shows the current density–voltage ( J–V) characteristics of
solar devices based on the four dyes. Photovoltaic parameters
including the short circuit photocurrent density ( Jsc), open
circuit voltage (Voc), fill factors (FF) and power conversion
efficiency (PCE) are summarized in Table 2. According to
Fig. 7, it is clear that the photovoltaic performances of the
DSSCs can be clearly affected by the anchoring group in the dye
molecules. Power conversion efficiency (PCE) of the DSSCs
based on the synthesized dyes increases in the order of MT
(0.99%) o DT (1.42%) o MH (1.75%) o DH (2.70%). The DSSC
based on the DH dye shows better properties with an open
3.3 Molecular orbital calculations
To gain further insight into the distinct difference in the
photovoltaic performance of these dyes, DFT calculations were
carried out at the B3LYP/6-31G(d) level for the geometry opti-
mization and the calculated results are displayed in Table 1. As
shown in Fig. 6, the HOMO orbitals of four dyes are delocalized
Fig. 6 The frontier molecular orbitals of the HOMO and LUMO levels
calculated with B3LYP/6-31G(d) of the synthesized dyes.
Fig. 7 Photocurrent density vs. voltage for DSSCs based on dyes under
AM1.5G simulated solar light (100 mW cmꢀ2).
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Table 2 The performance parameters of the dye-sensitized solar cells
Fig. 8 plots the incident photon-to-electron conversion effi-
ciencies (IPCE) which are functions of the incident wavelength
for the corresponding DSSCs of the resulting organic dyes on
TiO2 films. It should be noted that the IPCE values increase in
the order of DT o MTo MH o DH. The onset of IPCE for the
DH device was ca. 700 nm. IPCE values higher than 90% were
observed in the range of 430–520 nm with a maximum value of
95% at 480 nm for the device based on DH, while MH showed
about 78% at 400 nm. The IPCE spectra of the DSSC based on
MT is slightly lower than that of the DSSC based on MH.
Dye
JSC (mA cmꢀ2
)
VOC (V)
FF
PCE (%)
MH
MT
DH
DT
TPAC1
TPAC2
4.29
2.98
7.60
5.19
5.81
7.34
0.60
0.51
0.57
0.50
0.59
0.56
0.67
0.65
0.62
0.55
0.68
0.67
1.75
0.99
2.70
1.42
2.31
2.73
circuit voltage of 0.57 V, a short circuit photocurrent density of However, the IPCE spectrum of DT is much lower, with a
7.60 mA cmꢀ2 and a fill factor of 0.62 corresponding to an maximum IPCE of 36% at 470 nm.
overall light to electricity conversion efficiency of 2.70%, which
is observable in Fig. 7.
The relatively higher maximum IPCE for DH compared with
the other three dyes is well in agreement with the tendency of
The Jsc of the DSSCs increased in the order of MT o MH o
DT o DH. Double anchoring units in the dye are favorable for
electron injection and hence photocurrent generation. The Jsc is
related to the molar extinction coefficient of the dye molecule,
in which a higher molar extinction coefficient has a better light
harvesting ability and yields a higher Jsc. DH has the highest
light harvesting efficiency and consequently an improved Jsc
(7.60 mA cmꢀ2) due to its largest molar extinction coefficient,
broader absorption spectrum and highest IPCE value among all
the dyes.
The increased Jsc value of DH compared with other three
dyes mainly derives from its relatively better light harvesting
ability (broad absorption spectrum and high molar extinction
coefficient) which is reflected in its better IPCE spectrum.
Voc values increased in the order of DT o MT o DH o MH.
The cells fabricated with the di-anchoring dyes showed lower
Voc values compared to those with the mono-anchoring dyes.
For both DT and DH dyes, the Voc decreases slightly with the
increase of the conjugation length compared to those with the
mono-anchoring dyes.
The performance of triphenylamine based sensitizer TPAC1
and TPAC2 with 2-cyanoacetic acid acceptor were also investi-
gated under the same conditions for comparison. The results
are shown in Fig. 7. Under the same conditions, the TPAC1 and
Fig. 8 IPCE spectra of MH, MT, DH and DT.
TPAC2 sensitized cells gave Jsc values of 5.81 and 7.34 mA cmꢀ2
,
Voc of 0.59 and 0.56 V and FF of 0.68 and 0.67, corresponding to
Z values of 2.31% and 2.73%, respectively. The results for
TPAC1 and TPAC2 are comparable to the literature reported
values. For example, in 2008, TPAC1 was reported to show a Z of
2.47% and in 2008 and 2014, the efficiency of TPAC2 was
reported to be 3.01 and 2.86%, respectively.42,43 Comparing
the performances of TPAC2 with four dyes, apparently 2-cyano-
acetic acid could enhance the power conversion efficiency,
which is mainly attributed from the Jsc values. The results
indicate that the more dye that is adsorbed on the TiO2 surface,
the better Jsc that is observed. The cell fabricated with the
dianchoring dye TPAC2 showed lower Voc values compared to
those with the mono-anchoring dye TPAC1. The dianchoring dyes
transfer more protons to the TiO2 surface upon adsorption.44
Thus, the high concentration of the protons changes the TiO2
surface to a more positive state, lowering the conduction band
edges of TiO2 and lowering Voc.45,46
Fig. 9 Electrochemical impedance spectra ((a) Nyquist plot and (b)
equivalent circuit for (a)) of DSSCs for the four dyes.
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UV-vis absorption properties. IPCE spectra of the cells with the
Electrochemical impedance spectroscopy (EIS) is performed
di-anchoring dyes are broader than those with mono-anchoring to elucidate the interfacial charge recombination processes
dyes, promoting higher Jsc values. This result indicates that the in DSSCs based on the four dyes under dark conditions. The
di-anchoring dyes with broad and red-shifted spectra could Nyquist plots have two semicircles. The larger semicircle at
inject more electrons.
lower frequencies corresponds to the charge transfer processes
Fig. 9 shows the electrochemical impedance spectroscopic at the TiO2/dye/electrolyte interface, while the smaller semicircle
(EIS) analysis of the cells which were recorded using a set up at higher frequencies corresponds to the charge transfer processes at
whose equivalent circuit is presented in Fig. 9(b). The results of the Pt/electrolyte interface. Specifically, the small circles were almost
EIS data fitting are listed in Table 3. Rct, t and Cm correspond to similar in all dye based DSSCs due to the use of the same counter
the charge transfer process occurring at the TiO2/dye/electrolyte electrode and electrolyte. On the other hand, there was a sub-
interface, electron life time and chemical capacitance, respectively. stantial difference in the large semicircles, which indicates that
CPE ct-T and CPE ct-P are the parameters of constant phase charge transfer behavior between TiO2 and dye or between dye
element charge transfer (CPE ct).
and electrolyte was significantly altered, which is likely due to the
surface modifications with different dyes. The charge recombina-
tion resistance at the TiO2 surface, Rrec, could be estimated from
the radius of the middle semicircle in the Nyquist plot, and the
larger Rrec meant a slower charge recombination rate. The calcu-
lated resistance values (Rrec) are 20.38 O for MH, 42.85 O for MT,
12.61 O for DH and 32.02 O for DT, respectively.
Table 3 EIS parameters obtained from modelling the EIS results
) )
Dye Rct (O cm2) CPE ct-T (10ꢀ4 CPE ct-P t (ms) Cm (102 mF cmꢀ2
MH 20.38
MT 41.85
DH 12.61
DT 32.02
3.82
0.50
3.88
0.52
0.93
0.88
0.80
0.85
24.38 11.96
4.33
7.38
2.87
1.03
5.85
0.90
The increased Rrec values imply the retardation of charge
recombination between the injected electron into the TiO2 and
Fig. 10 Frontier orbitals of the dye–TiO2 complexes.
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oxidized species (I3ꢀ) in the electrolyte, further showing the 4. Conclusion
superiority of the special configuration of MT.
In conclusion, we have demonstrated a design strategy and syn-
thesis of novel D–p–A type organic sensitizers, in which triphenyl-
amine dyes are utilized as the donor and 2,4-thiazolidinedione/
hydantoin as the acceptor. The influence of different anchoring
groups on the photophysical, electrochemical, and photovoltaic
properties was studied. Computational and experimental results
support that electron transfer from the rich p-system of the donor
group to the acceptor side could give good spectral response and
cell efficiency in dye sensitized solar cell devices.
The introduction of the additional anchoring group extended
the conjugation of the dyes, leading to red-shifting of their
absorption spectra. The di-anchoring dyes transferred more elec-
trons to the TiO2 electrode, increasing the Jsc value. On the other
hand, the di-anchoring dyes transferred more protons to TiO2.
This lowered the Fermi level of the TiO2 and the Voc of the cells. As
a result, the dye containing two hydantoin acceptor unit delivers
the best performance among the dyes studied, exhibiting a power
conversion efficiency of 2.70%, with Jsc of 7.60 mA cmꢀ2 and Voc of
570 mV under simulated AM 1.5 irradiation (100 mW cmꢀ2).
This result indicates that compared to 2,4-thiazolidinedione
and hydantoin as an acceptor/anchor, a triphenylamine and
hydantoin based device opens up a new avenue for researchers
towards dye sensitized solar cells.
By fitting the EIS curves, another important parameter for
DSSCs, electron lifetime (t), could be extracted from the
chemical capacitances (Cm) and RCT using t = Cm ꢁ Rct. The
fitted t increases in the order of DT (2.87 ms) o MT (4.33 ms)
o DH (7.38 ms) o MH (24.38 ms).
Also, these results obtained from EIS measurements give an
explanation for the difference in open circuit voltage for the
three dyes. Generally, the influence of dye on VOC has mostly
been attributed to electron lifetime, which is related to thꢀe
tendency of electron recombination in TiO2 with the Iꢀ/ I3
redox couple in the electrolyte.47 The VOC values of the four dyes
increase in the order of DT (500 mV) o MT (510 mV) o DH
(570 mV) o MH (600 mV). The results agree well with the
t values of the devices.
3.5 Adsorption of dyes on TiO2 anatase(101) surface
The optimized geometries of dye/TiO2 systems are shown in Fig. 10
and the corresponding bond distances as well as adsorption
energies are provided in Table 4. As shown in Table 4, all the dyes
show a rather strong adsorption onto the TiO2 surface, with
binding distances of 2.13, 2.15, 2.14–2.15 and 2.32–2.21 Å for
MH, MT, DH and DT, respectively.
The adsorption energy, Eads is calculated using the expression
Eads = E(slab+dye) ꢀ Eslab ꢀ Edye
Acknowledgements
where Eslab represents the energy of the clean slab, Edye repre-
sents the anchored dye in the gas phase, and E(slab+dye) is the
total energy of the complex of the slab with anchored dye.
A negative value of Eads o 0 indicates stable adsorption. As
shown in Table 4, the adsorption energy (Eads) of each dye/TiO2
complex was calculated to be ꢀ1.94, ꢀ0.90, ꢀ2.82 and ꢀ1.43 eV
for MH, MT, DH and DT, respectively, which are sufficiently
large to allow the assumption that the dye molecules are
chemisorbed on the surface of TiO2.
The HOMOs and LUMOs of dye–TiO2 complexes were also
studied. There was electron distribution mostly delocalized on
dye molecules in the HOMO, whereas the LUMO showed the
injected electron delocalized dominantly on TiO2 surface.
These results indicate that efficient electron injection from
the LUMO of dyes to the CB of TiO2 substrate can be performed
through hydantoin/2,4-thiazolidinedione acceptor groups,
which implies that intermolecular charge transfer (CT) transition
between dye and TiO2 occurred. These results show that direct
charge transfer is the dominant mechanism for the charge-
injection process within the dye–TiO2 complex.
We would like to thank Yasouj University for the financial
support (Research Council Grant).
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