Synthesis and characterization of three organic dyes with various donors and rhodanine ring acceptor for use in dye-sensitized solar cells

Article abstract of DOI:10.1007/BF03246061

A series of new organic dyes comprising carbazole, iminodibenzyl, and phenothiazine moieties as the electron donors and rhodanine ring as the electron acceptor/anchoring groups were designed and developed for use in dye-sensitized solar cells. HOMO and LUMO energy level tuning was achieved by varying the carbazole, iminodibenzyls and phenothiazine donors. This was evidenced by spectral and electrochemical experiments and density functional theory calculations. Electrochemical studies indicated that the phenothiazine unit was much more effective in lowering the ionization potential than were the iminodibenzyl and carbazole units. The phenothiazine dye shows a solar-energy-to-electricity conversion efficiency (η) of 4.87%; the carbazole and iminodibenzyl dyes show η of 2.54% and 3.52%, respectively. These findings reveal that using carbazole, iminodibenzyl and phenothiazine donors as light-harvesting sensitizers are promising candidates for dye-sensitized solar cells.

Full text of DOI:10.1007/BF03246061

J. Iran. Chem. Soc., Vol. 7, No. 3, September 2010, pp. 707-720.
JOURNAL OF THE
Iranian
Chemical Society
Synthesis and Characterization of Three Organic Dyes with Various Donors and
Rhodanine Ring Acceptor for Use in Dye-Sensitized Solar Cells
a
a
a
a
a
b,c
d
a,b
T.Y. Wu , M.H. Tsao , F.L. Chen , S.G. Su , C.W. Chang , H.P. Wang , Y.C. Lin and I.W. Sun *
a
Department of Chemistry, National Cheng Kung University, Tainan, Taiwan
b
Sustainable Environment Research Center, National Cheng Kung University, Tainan 701, Taiwan
Department of Environmental Engineering, National Cheng Kung University, Tainan, Taiwan
Institute of Environmental Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
Received 22 August 2009, Accepted 29 October 2009)
c
d
(
A series of new organic dyes comprising carbazole, iminodibenzyl, and phenothiazine moieties as the electron donors and
rhodanine ring as the electron acceptor/anchoring groups were designed and developed for use in dye-sensitized solar cells.
HOMO and LUMO energy level tuning was achieved by varying the carbazole, iminodibenzyls and phenothiazine donors. This
was evidenced by spectral and electrochemical experiments and density functional theory calculations. Electrochemical studies
indicated that the phenothiazine unit was much more effective in lowering the ionization potential than were the iminodibenzyl
and carbazole units. The phenothiazine dye shows a solar-energy-to-electricity conversion efficiency (ꢀ) of 4.87%; the carbazole
and iminodibenzyl dyes show ꢀ of 2.54% and 3.52%, respectively. These findings reveal that using carbazole, iminodibenzyl and
phenothiazine donors as light-harvesting sensitizers are promising candidates for dye-sensitized solar cells.
Keywords: Dye-sensitized solar cells, Organic dyes, Donor, Absorption, Electrochemistry, Rhodanine
INTRODUCTION
high molar extinction coefficient, tunable absorption spectral
response from the visible to the near infrared (NIR) region,
environmental friendliness and inexpensive production
techniques. Since 2000 AD, an increasing number of studies
have been devoted to the development of organic dyes [4-7].
Reasonable photoconversion efficiencies have been reached
with some push-pull type organic chromophores [8]. Recently,
some organic coumarin [9], indoline [10], merocyanine [11]
and hemicyanine dyes [12] have shown impressive
photovoltaic performance. Most of the organic sensitizers
applied in DSSC act as both the electron donors (D) and the
electron acceptors (A) linked by a ꢀ-conjugation bridge, which
is called the D-ꢀ-A structure [13]. However, many organic
dyes exhibit low conversion efficiency in DSSCs due to the
formation of dye aggregates on the semiconductor surface.
The growing demand for energy and global warming
necessitate a greater focus on renewable energy sources. A
dye-sensitized solar cell (DSSC) using Ru complexes as a
photosensitizer was first reported by O’Regan and Grätzel in
1
991 [1]. Its low-cost and easy preparation makes DSSC one
of the most promising photovoltaic cells. Ruthenium
polypyridyl complexes are the most promising sensitizers for
DSSCs, achieving power conversion efficiencies (3) !10% [2]
and good long-term stability [3]. Although the most efficient
sensitizers to date are ruthenium complexes, organic dyes have
been attracting attention because of their ease of synthesis,
*Corresponding author. E-mail: iwsun@mail.ncku.edu.tw

Wu et al.
They are also associated with low operational stability due
solution was warmed to 75 °C, treated portionwise with
potassium tert-butoxide (4.264 g, 38 mmol), and then refluxed
for 24 h. After 150 ml of water was added, the mixture was
extracted with chloroform (75 ml). Crude oils obtained by
removing the solvent were purified by column
chromatography (silica gels, n-hexane/ethyl acetate: 20/1 as
to the formation of unstable radical species during redox
reaction cycles. Organic dyes have a pushꢀpull structure
similar to that of nonlinear optical molecules, which are
composed of a donor, conjugated bridge, and acceptor. Many
researchers have attempted to create organic dyes through the
structural modification of the donor [14], conjugated bridge
1
eluent) to give 2b as a white solid (4.76 g, 61%). H NMR
[15], or acceptor unit [16]. Thus, we are interested in the
(CDCl ), 14(ppm): 1.16 (t, 3H, CH ), 3.17 (m, 4H, CH ), 3.80
3
3
2
structural modification of the organic dyes’ donor unit to
develop efficient organic dyes which would not aggregate.
Our second objective is to study the effect of such structural
changes on DSSC efficiency.
(m, 2H, CH ), 6.92 (m, 2H, ar), 7.12 (m, 6H, ar). Elem. Anal.
2
Calcd. for C H N: C, 86.05%; H, 7.67%; N, 6.27%. Found:
16
17
C, 85.96%; H, 7.73%; N, 6.20%.
11-Ethyl-3-formyliminodibenzyl (3b). POCl (7.4 g, 48
3
Iminodibenzyl is a diphenylamine wherein two ortho
mol) was added dropwise to 4.3 g (58 mmol) of freshly
positions are joined by
a
dimethylene bridge. For
distilled N,N-dimethylformamide at 0 °C in nitrogen
phenothiazine sensitizer, the two ortho positions of the
diphenylamine are joined by an electron-donating sulfur atom.
No DSSC dyes with iminodibenzyl sensitizers have been
reported. Phenothiazine sensitizer is incorporated in this study
in order to compare the structure impact between dimethylene
bridge and sulfur atom. In this work, metal-free dyes with
carbazole, iminodibenzyl, and phenothiazine sensitizers are
reported and their optical, electrochemical, photovoltaic
properties and structural relationships are investigated in
detail.
atmosphere. A solution of 11-ethyl-iminodibenzyl (2b: 3.35 g,
15 mmol) in N,N-dimethylformamide was added dropwise to
POCl /DMF complex at 30 °C. The reaction mixture was
3
stirred at 80 °C for 4 h. When the reaction finished (TLC
monitoring), the reaction mixture was cooled to room
temperature and poured into ice water. The obtained mixture
was neutralized with NaOH until pH ꢁ7-8. The precipitates
were separated by filtration and washed with methanol. The
crude product was purified by column chromatography
1
(eluent:ethylacetate-hexane, 1:6). Yield of 3b: 3.2 g (85%). H
NMR (CDCl ), 14(ppm): 1.07 (t, 3H, CH ), 3.11 (m, 4H, CH ),
3
3
2
EXPERIMENTAL
3.85 (m, 2H, CH ), 7.00-7.08 (m, 1H, ar), 7.19-7.28 (m, 4H,
2
ar), 7.57 (s, 1H, ar), 7.64-7.67 (m, 1H, ar), 9.78 (s, 1H, CHO).
Elem. Anal. Calcd. for C17 17NO: C, 81.24%; H, 6.82%; N,
Chemicals
H
All starting materials were purchased from Aldrich (St.
Louis Missouri), Lancaster (Karlsruhe), TCI (Tokyo), and
Acros (Geel) and used as received. Dimethylformamide
5.57%. Found: C, 81.04%; H, 6.90%; N, 5.46%.
2-(5-((11-Ethyl-iminodibenzyl-2-yl)methylene)-4-oxo-2-
thioxothiazolidin-3-yl)acetic acid (R2). To a stirred 3b
(
DMF), dichloromethane, and POCl were distilled over CaH2
(1.005 g, 4 mmol) in CH COOH (30 ml) were added
3
3
under N2 atmosphere. Tetrabutylammonium perchlorate
TBAP) was purchased from Alfa Aesar (Karlsruhe), and
,2-dimethyl-3-propylimidazolium iodide (DMPImI) was
synthesized and purified according to a procedure in the
literature [17].
rhodanine-3-acetic acid (0.794 g, 4.15 mmol) and ammonium
acetate (0.125 g, 1.625 mmol). Then the mixture was heated to
120 °C and the reaction continued for 2 h at the said
temperature. Then the reaction mixture was allowed to cool to
room temperature. The solid was collected by filtration and
washed with water thoroughly. After drying in air, the crude
product was purified by column chromatography on silica gel
with CH Cl /methanol (10:1, v/v) as eluent to give R2 in 83%
(
1
Synthetic Procedure for Carbazole, Iminodibenzyl,
and Phenothiazine-Containing Dyes (R1-R3)
2
2
1
11-Ethyl-iminodibenzyl (2b). Iminodibenzyl (1b: 6.834
yield. H NMR (CDCl
3
), 14(ppm): 1.08 (t, 3H, CH ), 3.11 (m,
3
g, 35 mmol), iodoethane (5.93 g, 38 mmol), and 80 ml of
DMF were added to a 50-ml two-necked glass reactor. The
4H, CH ), 3.86 (m, 2H, CH ), 4.73 (s, 2H, CH ), 7.04 (m, 1H),
7.19-7.28 (m, 4H), 7.36 (s, 1H), 7.45 (d, 1H), 7.74 (s, 1H).
2
2
2
7
08

Synthesis and Characterization of Three Organic Dyes
Elem. Anal. Calcd. for C H N O S : C, 62.24%; H, 4.75%;
washed with water thoroughly. After drying in air, the crude
2
2
20
2
3 2
N, 6.60%; S, 15.11%. Found: C, 62.14%; H, 4.72%; N, 6.58%;
S, 15.02%.
product was purified by column chromatography on silica gel
with CH Cl /methanol (10:1, v/v) as eluant to give R3 in 89%
2
2
1
1
0-Ethyl-phenothiazine (2c). Phenothiazine (1c: 11.94 g,
yield. H NMR (DMSO-d ), 14(ppm): 1.33 (t, 3H, CH ), 3.99
6
3
6
0 mmol), iodoethane (10.92 g, 70 mmol), and 80 ml of DMF
(m, 2H), 4.73 (s, 2H), 6.97-7.25 (m, 5H), 7.40 (s, 1H), 7.49 (d,
1H), 7.78 (s, 1H). Elem. Anal. Calcd. for C H N O S : C,
56.05%; H, 3.76%; N, 6.54%, S, 22.45%. Found: C, 55.90%;
H, 3.76%; N, 6.52%, S, 22.37%.
2-(5-((9-Ethyl-carbazol-3-yl)methylene)-4-oxo-2-thioxot
hiazolidin-3-yl) acetic acid (R1). 9-Ethyl-carbazole (2a) and
9-ethyl-3-formylcarbazole (3a) were synthesized following the
procedures used for compounds 2b and 3b, respectively. To a
were added to a 50-ml two-necked glass reactor. The solution
was warmed to 75 °C, treated portionwise with potassium
tert-butoxide (7.86 g, 70 mmol), and then refluxed for 24 h.
After 150 ml of water was added, the mixture was extracted
with chloroform (75 ml). Crude oils obtained by removing the
solvent were purified by column chromatography (silica gels,
n-hexane/ethyl acetate: 20/1 as eluent) to give 2c as a white
solid (11.32 g, yield: 83%). MP: 103-104 °C. 1H NMR
20
16
2
3 3
stirred 3a (0.893 g, 4 mmol) in CH COOH (30 ml) were added
3
(
6
CDCl ), 1 (ppm): 1.42 (t, 3H, CH ), 3.92 (m, 2H, CH ),
.85-6.88 (m, 4H, ar), 7.11-7.16 (m, 4H, ar). Elem. Anal.
rhodanine-3-acetic acid (0.794 g, 4.15 mmol) and ammonium
acetate (0.125 g, 1.625 mmol). Then the mixture was heated to
120 °C and the reaction continued for 2 h at the said
temperature. Then the reaction mixture was allowed to cool to
room temperature. The solid was collected by filtration and
washed with water thoroughly. After drying in air, the crude
product was purified by column chromatography on silica gel
with CH Cl / methanol (10:1, v/v) as eluant to give R1 in 95%
3
3
2
Calcd. for C H NS: C, 73.97%; H, 5.76%; N, 6.16%; S,
1
14
13
4.11%. Found: C, 73.83%; H, 5.73%; N, 6.13%, S: 14.10%.
0-Ethyl-3-formylphenothiazine (3c). POCl3 (14.8 g,
.097 mol) was added dropwise to 8.6 g (0.116 mol) of freshly
1
0
distilled N,N-dimethylformamide (molar ratio 1:1.2) at 0 °C in
nitrogen atmosphere. A solution of 6.8 g (0.03 mol) of
N-ethyl-phenothiazine in N,N-dimethylformamide was added
2
2
1
yield. H NMR (CDCl ), 1 (ppm): 1.35 (t, 3H, CH ), 4.51 (m,
3
3
dropwise to POCl /DMF complex at 30 °C. The reaction
2H, CH ), 4.77 (s, 2H, CH ), 7.31 (t, 1H), 7.55 (t, 1H),
3
2
2
mixture was stirred at 80 °C for 4 h. When the reaction
finished (TLC monitoring), the reaction mixture was cooled to
room temperature and poured into ice water. The obtained
mixture was neutralized with NaOH until pH =ꢁ7-8. The
precipitates were separated by filtration and washed with
methanol. The crude product was purified by column
7.70-7.84 (m, 3H), 8.08 (s, 1H), 8.30 (d, 1H), 8.51 (s, 1H).
Elem. Anal. Calcd. for C H N O S : C, 60.59%; H, 4.07%;
N, 7.07%; S, 16.18%. Found: C, 60.34%; H, 4.15%; N, 6.88%;
S, 15.97%.
20
16
2
3 2
Preparation of Dye-Sensitized TiO2 Electrode
(photoanode) and Counter Electrode
chromatography (eluent:ethylacetate-hexane, 1:6). Yield of 3c:
1
5
3
.36 g (70%). H NMR (CDCl ), 14(ppm): 1.45 (t, 3H, CH ),
F-doped tin oxide (FTO) glass plates (3 mm thickness, 7 ꢁ
3
3
-
2
.98 (m, 2H, CH ), 6.88-6.98 (m, 3H, ar), 7.09-7.19 (m, 2H,
cm ) were first cleaned ultrasonically in a cleaning detergent
solution for 15 min, and rinsed with water and ethanol. The
FTO electrodes were then immersed into 40 mM TiCl4
(aqueous) at 70 °C for 30 min and rinsed with water and
2
ar), 7.57-7.65 (m, 2H, ar), 9.80 (s, 1H, CHO). Elem. Anal.
Calcd. for C H NOS: C, 70.56%; H, 5.13%; N, 5.49%, S,
15
13
12.56%. Found: C, 70.49%; H, 5.26%; N, 5.37%, S, 12.38%.
2
-(5-((10-Ethyl-phenothiazin-3-yl)methylene)-4-oxo-2-t
ethanol. Two kinds of TiO paste, containing nanocrystalline
2
hioxothiazolidin-3-yl) acetic acid (R3). To a stirred 3c (1.021
(~25 nm) TiO (Degussa P25, paste A) and submicroparticle
2
g, 4 mmol) in CH COOH (30 ml) were added rhodanine-3-
TiO (500 nm, paste B), respectively, were prepared using a
3
2
acetic acid (0.794 g, 4.15 mmol) and ammonium acetate
previously reported procedure [18,19]. To prepare paste A,
commercial titania powder (3 g, Degussa P25) was ground in a
mortar with a small amount of water (1 ml) containing
(0.125 g, 1.625 mmol). Then the mixture was heated to 120 °C
and the reaction continued for 2 h at the said temperature.
Then the reaction mixture was allowed to cool to room
temperature. The solid was collected by filtration and
acetylacetone (0.1 ml). After the TiO particles were dispersed,
2
the paste was diluted with 3 ml water under continued
7
09

Wu et al.
grinding and a surfactant, Triton X-100 (0.05 ml), was added
to facilitate the spreading of the colloid on the substrate. Paste
tetrabutylammonium perchlorate (TBAP) as the supporting
electrolyte. The working electrode was a glassy carbon
electrode, the auxillary electrode was a Pt wire, and the
A was sonicated for about 24 h at 28 °C. TiO film (12 ,m)
2
+
-1
was prepared by spin coating. After paste A was dried at 125
reference electrode was Ag/Ag . The scan rate was 100 mV s
o
C, ~4 ,m thick paste B was spin-coated on top of it. The
and the temperature was 25 °C. Ferrocene was added to each
sample solution at the end of the experiments. The
electrodes coated with TiO pastes were gradually heated (5
2
-
1
+
°
C min ) under airflow up to 450 °C for 30 min followed by
ferrocenium/ferrocene (Fc/Fc ) redox couple was used as an
treating with 40 mM TiCl , rinsed with water and ethanol, and
internal potential reference. The potential versus SCE in
acetonitrile was calibrated according to a procedure published
by Matsui et al. [20]. The photovoltaic measurements of the
DSSCs were performed using a Newport M-66907 450 W
xenon light source through an infrared blocking filter and a
Keithley 2400 digital source meter linked to a computerized
control and data acquisition system. The light intensity was
1000 W m under an AM 1.5 light source. Cell temperatures
were kept at 25 °C during the illumination. Light intensity was
calibrated using a mono-Si reference solar cell (PVM134).
The incident photon-to-current conversion efficiency (IPCE)
as a function of excitation wavelength was measured using an
incident light 300 W xenon lamp. A 300W xenon arc lamp
solar simulator (#91160A, Oriel) with an AM 1.5 Globe filter
(#59044, Oriel) was used to measure the I-V characteristics of
the quasi-solid-state DSSC. The illumination was fixed at 100
mW cm-2 using a reference solar cell and meter (#91150,
Oriel).
4
sintered again at 450 °C for 30 min. The electrodes with an
active area of 0.5 cm × 0.5 cm were immersed in a 1:1
-
4
acetonitrile and isoporpanol solution containing 3 × 10 M of
the dye. The dye-adsorbed TiO films were then rinsed with
2
dry ethanol, and dried in air. A counter electrode was prepared
by sputtering a thin platinum layer (50 nm) on an FTO
-
2
substrate. The thicknesses of the TiO films were determined
2
by profilometry. Platinum sputtering was carried out using a
Hitachi E 1045 instrument. The platinum layer thickness on
the glass substrates was estimated using the amount of
sputtered platinum recorded on a quartz thickness monitor.
DSSC Assembly
The dye-adsorbed TiO electrode and Pt-counter electrode
2
were assembled into a sandwich-sealed type cell by heating
them with hot-melt ionomer film (25 ,m thickness, Solaronix)
as a spacer. A drop of electrolyte solution: 0.1 M LiI, 0.05 M
I2, 0.6 M DMPII and 0.5 M tert-butyl pyridine (TBP) in
acetonitile (ACN)) was injected through a hole in the counter
electrode, which was then sealed with hot-melt ionomer film
and glass. The electrolyte was introduced into the cell and
sealed with AB epoxy for 30 min. The working area of the
ꢁ
Computation Methods
R1-R3 are the carbazole, iminodibenzyl, and
phenothiazine-containing dyes, respectively. The geometric
and electronic properties of R1-R3 were obtained using the
Gaussian 03 program package [21]. The calculation was
optimized using B3LYP (Becke three parameter hybrid
function with Lee-Yang-Perdew correlation functions) with
the Pople 6-31+g(d) atomic basis set. The excitation
transitions of R1, R2, and R3 were calculated using
time-dependent density functional theory (TD-DFT)
calculations with B3LYP/6-31+g(d). Molecular orbitals were
visualized using Gaussview.
2
electrode was 0.25 cm . ꢁ
Characterization of the Dyes and Photovoltaic
Measurements of the Solar Cells
1
The H spectra were obtained on a Bruker 400 MHz
FT-NMR. Chemical shifts are reported in ppm relative to
tetramethylsilane 1 units. The absorption spectra of the dyes in
solution and adsorbed on TiO films were recorded on a Cary
2
100 UV-Vis spectrophotometer. Fluorescence measurements
were carried out using a Hitachi F-4500 fluorescence
spectrophotometer. Cyclic voltammetry was performed using
an electrochemical workstation (CH instruments Inc., CHI,
RESULT AND DISCUSSION
Synthesis and Structure of Sensitizers
model 750A) and conducted in CH CN solution using 0.1 M
In this study, three donor-acceptor '-conjugated dyes with
3
7
10

Synthesis and Characterization of Three Organic Dyes
a carboxyl group were synthesized; the amine derivatives
acted as the electron donors while a rhodanine-3-acetic acid
moiety acted as the anchoring group for attachment on the
metal oxide and as the electron acceptor. The two components
were connected by a '-conjugated methane unit. The
electron-donating nature and structural variations of the amine
unit and its electronic properties were studied.
the dihedral angle between the phenyl unit and dimethylene
bridge inside the iminodibenzyl unit (-4) was 126 . This
o
could be attributed to the incorporation of a dimethylene
bridge between two phenyl units increasing the steric
hindrance, increasing the dihedral angles of -1 and -4.
Dihedral angles may affect the stability and suitability of dyes
as photosensitizers. Planar carbazole unit readily causes
intermolecular 'ꢀ' stacking interactions. The incorporation of
dimethylene bridge and sulfur units to iminodibenzyl and
phenothiazine-based photosensitizer (R2 and R3), respectively,
reduces the coplanarity of photosensitizers, and minimizes the
formation of dye aggregates on the semiconductor surface.
The dihedral angle (-5) of the R3 phenothiazine sulfur atom
As is shown in Fig. 1, R1-R3 are the carbazole,
iminodibenzyl,
and
phenothiazine-containing
dyes,
respectively. The aldehydes (3a, 3b, 3c) were prepared by
treating N-ethylcarbazole, N-ethyliminodibenzyl, and N-
ethylphenothiazine with POCl in DMF as previously reported
3
by Lai et al. [22]. The final reaction for R1, R2, and R3 was
the condensation of the respective aldehyde with
rhodanine-3-acetic acid by the Knoevenagel reaction in the
presence of ammonium acetate. The optimized structures of
R1, R2 and R3 were studied to compare the structure
differences between carbazole, iminodibenzyl, and whereas
o
is 99.2 , which is smaller than that of the phenothiazine
o
nitrogen atom (-1: 122.1 ). This may be attributed to the
structure difference between nitrogen and sulfur atoms;
nitrogen atom has a lone pair whereas sulfur atom has two
lone pairs.
Fig. 1. Synthesis of the carbazole, iminodibenzyl and phenothiazine substituted dyes.
phenothiazine photosensitizers (Fig. 2). The dihedral angles
Optical Properties
(
-1) of the R1 carbazole ring, the R2 iminodibenzyl ring, and
The UV-Vis and emission spectra of R1, R2, and R3 in
CH CN are shown in Fig. 3 and the 0 are listed in Table 1,
o
o
o
the R3 phenothiazine ring were 108.7 , 123.7 , and 122.1 ,
respectively, implying that the addition of a dimethylene
bridge and sulfur units significantly increases the dihedral
angle of nitrogen atoms. The dihedral angle between two
3
max
together with the UV-Vis spectra of the corresponding dyes
adsorbed on TiO2 film. All dyes show broad absorption
spectra ranging from 350 to 800 nm. The absorption spectrum
o
phenyl units inside the carbazole unit (-4) was 106.4 ;
of R1 in CH CN has two distinct absorption bands at around
3
711

Wu et al.
(
a)
(
b)
(
c)
Fig. 2. Optimized geometric parameters (dihedral angle) of (a)
o
o
o
R1: Y1 = 108.7 , Y2 = 125.4 , Y3 = 125.8 , Y4 =
o
o
o
o
1
06.4 , Y5 = 134.0 , Y6 = 106.6 , Y7 = 134.0 ; (b)
o
o
o
R2: Y1 = 123.7 , Y2 = 119.0 , 3 = 117.2 , Y4 =
o
o
o
o
1
=
=
26 , Y5 = 115.9 , Y6 = 119.1 , Y7 = 120.9 ; Y8
116.4 , Y9 = 111 and (c) R3: Y1 = 122.1 , Y2
118.4 , Y3 = 118.7 , Y4 = 120.6 , Y5 = 99.2 ,
o
o
o
Fig. 3. Absorption spectra of R1 (–ꢂ), R2 (…), and R3 (---)
a) in CH CN and (b) absorption on TiO film. (c)
Emission spectra of R1 (–ꢂ), R2 (…), and R3 (---).
o
o
o
o
(
3
2
o
Y6 = 120.4 .
7
12

Synthesis and Characterization of Three Organic Dyes
Table 1. Absorption and Emission Properties of Dyes
Absorption
Emission
Stokes shift
Dye
abs (nm)b
) (M cm ) (at 0abs
22260 (442 nm)
26583 (438 nm)
29521 (460 nm)
1
1
)
0
abs (nm) (on TiO
b
)
0
em (nm)b
531
556
(nm)a
89
118
182
ꢀ
ꢀ
0
2
R1b
R2
R3
288, 442
438
298, 356, 460
445
449
469
642
aStokes shift = PL(soluion)/nm - UV(solution)/nm. Absorption and emission spectra were measured in CH
b
3
CN solution.ꢁ
2
88 and 442 nm, respectively. The weak absorption peaks at
is due to its stronger intramolecular charge transfer because
phenothiazine is a stronger electron-donating ring than
iminodibenzyl. The molar extinction coefficients of R1, R2,
around 288 nm correspond to the 'ꢃ'* electron transition of
the conjugated molecule; the strong absorption peaks at
around 442 nm can be assigned to an intramolecular charge
transfer between the carbazole-based donor and the rhodanine-
1
1
1
1
ꢀ
ꢀ
ꢀ
ꢀ
and R3 are 22260 M cm (at 442 nm), 26583 M cm (at
1
1
ꢀ
ꢀ
438 nm), and 29521 M cm (at 460 nm), respectively; they
3
-acetic acid [3], providing efficient charge-separation at the
are larger than that of N3 (cis-bis(isothiocyanato)bis(2,2'-
1
1
ꢀ
ꢀ
excited state. Under similar conditions, the R2 sensitizer had
absorbance peak at 438 nm representing a slight hypsochromic
shift relative to that of R1, implying that the incorporation of
dimethylene bridge interrupts the conjugation between two
phenyl groups.
bipyridyl-4,4'-dicarboxylato)-ruthenium(II)) (14200 M cm )
[25], indicating that the novel dyes have good light harvesting
ability.
As is shown in Fig. 3c, the PL (photoluminescence)
maximum of R1 is located at 531 nm in CH CN, whereas that
3
Comparing the maximum absorption wavelength of R1-R3
of R2 shifted to 556 nm, and that of R3 shifted to 642 nm. R3
exhibited a relatively large Stokes shift, which could be
attributed to the geometrically relaxed structure of the
phenothiazine center upon excitation [26].
in CH CN solution, R3 (460 nm) shows significant red shift
3
relative to R1 (442 nm) and R2 (438 nm) which can be
attributed to the extra electron-donating sulfur atom in R3.
Red-shifting in the absorption spectra helps the utilization of
solar light. When the R1 sensitizer was adsorbed on the TiO2
surface, there was a slight red shift from 442 to 445 nm (Table
Electrochemical Properties
Cyclic voltammetry was used to determine the redox
potentials of the dyes in order to estimate the driving force of
the photoinduced electron injection process. To determine the
possibility of electron transfer from the excited dye molecules
1
), implying that dyes adsorbed on the TiO surface had partial
2
J-type aggregates. However, the stronger red shift of R2 (438
nm ꢃ 449 nm) compared to that of R1 (442 ꢃ 445 nm) from
the solution to TiO film could imply that the iminodibenzyl-
to the conduction band of the TiO electrode, redox potentials
2
2
containing sensitizer had a higher tendency to form surface
of R1, R2, and R3 were obtained using cyclic voltammetry.
The HOMO energy level was estimated by using the onset
anodic potentials Eox (vs. NHE). The LUMO energy level was
calculated by “E -E ”, where E is determined from the
binding with TiO than that of carbazole-containing sensitizer,
2
as chelating to metal cation, it may enhance the intramolecular
electron transfer (ICT), leading to spectrum shift. Under
similar conditions, the R3 sensitizer showed a slight red shift
ox 0-0
0-0
intersection of absorption and emission spectra.
Usually, the energy level of the ground state (E0 ) is
+
from 460 to 469 nm after being adsorbed on the TiO surface.
2
D /D
The absorption spectra of R1-R3 on the TiO electrode are
estimated from its equilibrium redox potential (E ). The
ferrocenium/ferrocene (Fc/Fc ) redox couple was used as an
2
ox
+
broader than those in CH CN. The broadening of the
3
absorption spectra is due to an interaction between the dyes
internal potential reference. Figure 4 shows the cyclic
and TiO [23-24]. The absorption redshift of R3 relative to R2
voltamograms of R1, R2, and R3 in acetonitrile. The
2
7
13

Wu et al.
the hole transporting ability of the photosensitizer than that of
carbazole unit. The first oxidation peak of R3 (0.68 V) was a
reversible one electron reaction. Both the Eonset (ox) and the first
oxidation potentials of R3 were less positive than those of R2,
indicating that the phenothiazine units were much more
effective in lowering the ionization potential than
iminodibenzyl units. Moreover, introducing a phenothiazine
group into the photosensitizer framework (R3) resulted in a
largely negative shift of the HOMO level, to 0.52 V vs. NHE.
The HOMO levels of R1 and R2 (1.01 and 0.85 V vs. NHE,
respectively) were more positive than that of R3, indicating
more efficient dye regeneration for R1 and R2. The LUMO
level of R3 (-1.74 V vs. NHE) was more negative than those
of R1 and R2 (-1.54 and -1.61 V vs. NHE, respectively),
indicating that R3 dye had more efficient electron injection.
Figure 5 shows the schematic energy diagram of these
Fig. 4. Cyclic voltamogram of R1 (–ꢂ), R2 (…), and R3 (---)
in acetonitrile.
dye-sensitized TiO electrodes. All the excited state oxidation
2
potentials (LUMO) of R1 (-1.54 V), R2 (-1.61 V), and R3
electrochemical data are listed in Table 2. When R1 was swept
(-1.74 V) were more negative than the conduction band edge
in the anodic direction, there was an obvious oxidation peak at
of TiO
2
(-0.5 V (vs. NHE)) [27]. Provided that an energy gap
CB) of 0.2 eV is necessary for
1
.66 V, and two small oxidation peaks at 1.12, 1.25 V.
(between dye LUMO and TiO
2
However, there was no reduction peak, indicating that the
electrochemical behavior in the anodic direction was
irreversible. For R2, the oxidation was also irreversible, with
peak appearing at 0.96 V. The Eonset (ox) and the oxidation peak
of R2 were lower than those of R1, which could be attributed
to the relatively higher electron density on the nitrogen atoms
of R2, allowing it to be more easily oxidized than carbazole
derivatives. It is evident that the iminodibenzyl unit enhanced
efficient electron injection [28], the driving force is sufficient
for efficient charge injection. Thus, the electron injection
process from the excited dye molecule to the TiO conduction
2
band and the subsequent dye regeneration are energetically
permitted. Figure 5 also indicates that the energy levels of the
ground state (HOMO) of R1 (1.01 V), R2 (0.85 V), and R3
-
-
(0.52 V) are sufficiently more positive than the I
/I redox
3
potential (0.42 V (vs. NHE)) [27], indicating that the oxidized
Table 2. Electrochemical Properties and Band Gaps of R1~R3 Dyes
+
b,d
Dye
Epa
Eonset(ox) measured
Eonset(ox) vs. E(S /S*) (V)
E0-0
(eV) c
2.55
2.46
2.26
LUMO vs. NHE
(eV)
Egap
(V)
1.04
1.11
1.24
EFOC (V)a
0.72
vs. NHE
1.01
e
in CH CN
3
R1
R2
R3
1.12, 1.25, 1.66
0.96
0.68, 1.4, 1.86
0.98
0.82
-1.54
-1.61
-1.74
0.56
0.85
0.49
0.23
0.52
a
+
b
+
E
FOC = 0.26 V vs. Ag/Ag . The ground-state oxidation potentials E(S /S) were measured in acetronitrile containing 0.1 M
tetrabutylammonium perchlorate as supporting electrolyte using a glassy carbon working electrode, a Pt counter electrode,
+
c
and a Ag/Ag reference electrode. The E value was estimated from the cross-section of absorption and emission spectra.
0
-0
d
+
+
e
+
The excited-state oxidation potential E(S /S*) was calculated from E(S /S) - E0-0. Egap is the energy gap between E(S /S*)
of the dye and the conduction band level of TiO (-0.5V vs. NHE).ꢁ
2
7
14

Synthesis and Characterization of Three Organic Dyes
Fig. 5. Schematic energy level diagram for a DSSC based on dyes.
dye formed after electron injection into the conduction band of
bridge of the iminodibenzyl unit. The electrons of R3 are
-
TiO could accept electrons from I ions in the electrolyte
homogeneously distributed in the phenothiazine unit. The
LUMO electron density geometry distributions of R1-R3 are
located over the rhodanine-3-acetic acid group through the
right phenyl group and the nitrogen atom of carbazole,
iminodibenzyl and phenothiazine units, respectively. At the
excited state (LUMO) with light illumination for R1-R3 dyes,
intramolecular charge transfer occurs, resulting in electron
movement from the donor group to the acceptor groups
(rhodanine-3-acetic acid group). Furthermore, the location of
2
thermodynamically. Therefore, these dyes could be candidates
for sensitizers in DSSCs.
Molecular Orbital Calculations
The geometries of the organic dyes were optimized using
density functional theory (DFT) calculations at a B3LYP/6-
3
1ꢂG (D) level, as shown in Fig. 6. The frontier MOs of
R1-R2 reveal that HOMO-LUMO excitation moves the
electron density distribution from the carbazole and
iminodibenzyl moiety to the rhodanine-3-acetic acid moiety;
however, the frontier MOs of R3 reveal that the electrons at
the ground state (HOMO) are homogeneously distributed in
both the electron donor group and the methylene bridge and
rhodanine-3-acetic acid moiety. Comparing the frontier MOs
of R1 and R2 at the ground state (HOMO), the electrons of R2
are dominantly distributed in the benzene ring and the nitrogen
atom; however, there are few electrons in the dimethylene
the LUMO at the side of the TiO surface and the HOMO at
2
the opposite end of the molecule make the electron injection
into the semiconductor easier and prevent back regeneration of
the dye with the injected electrons.
We modelled the electronic structure of sensitizers both in
vacuo and in CH CN solution. It was found that the inclusion
3
of solvation effects did not lead to a qualitative change in the
electronic structure, even though smaller HOMOꢀLUMO gaps
were calculated in CH CN as compared to the gas phase. The
3
7
15

Wu et al.
Fig. 6. Computed isodensity surfaces of HOMO and LUMO orbitals of R1, R2, and R3.
origins of these electronic absorptions were found by
transfer in the extended charge-transfer state [29]. The
calculated absorption maxima for the excited states seem to be
in good agreement with the measured values, supporting the
band assignments in the UV solution plot.
calculating the singlet electronic transitions with the TDDFT
method in Gaussian 03W program suite. Calculations show
that these visible bands are mainly attributed to the electronic
transition from the highest occupied molecular orbitals
Photovoltaic Performance
(
(
HOMOs) to the lowest unoccupied molecular orbitals
LUMOs). Table 3 shows the calculated absorption maxima in
DSSCs were prepared and compared to investigate the
relationships between the sensitizing behavior of R1-R3 dye
molecules and their structures. The incident photon-to-current
conversion efficiency (IPCE) of a DSSC was estimated to be
between 400 and 800 nm. The IPCE values were measured
according to the following equation [30]:
the visible region, their oscillator strengths, and their
compositions for R1-R3. To analyze the photophysical
properties of these dyes, we performed TD-DFT calculations
of the lowest 10 singlet-singlet excitations of dyes in CH CN
3
solutions. The three transitions of R1-R3 with oscillator
strengths (f) above 0.05 are summarized in Table 3. Compared
with the experimental data, the considerable blue-shift of the
absorption maximum from the calculations is related to the
self-interaction error in TD-DFT arising from the electron
ꢀ
2
1
240(eVꢁnm) Jsc(mAꢁcm )
IPCE(0) ꢂ
ꢀ2
0
(nm)
6(mWꢁcm )
where 0 is the wavelength and 64is the power of the incident
7
16

Synthesis and Characterization of Three Organic Dyes
Table 3. Comparison of Calculated TD-DFT Excitation Energies (eV, nm), Oscillator Strengths (f), Assignment of Molecular
Orbital Contributions and Character, and Experimental Absorption Band Maxima of Dyes
Excited
state
Est
(eV)
Est
(nm)
ꢄ
Exp
(eV)
Exp
(nm)
Dye
f
Composition
0.65135ꢅ(HOMO ꢃ LUMO)
.65219ꢅ(HOMO-2 ꢃ LUMO) -0.10005ꢅ(HOMO-1 ꢃ
1
0.9156 2.7072 457.97 2.81
442
0
2
0.2068ʳ 3.5985ʳ 344.55ʳ
LUMO+2) +0.11737ꢅ(HOMO ꢃ LUMO+2)
R1
0
.54447ꢅ(HOMO-5 ꢃ LUMO) +0.32952ꢅ(HOMO-4 ꢃ
3
0.1195ʳ 4.2929ʳ 288.81ʳ 4.31
288
438
LUMO) +0.12395ꢅ(HOMO-1 ꢃ LUMO+2) -0.11414ꢅ(HOMO
ꢃ LUMO+1)
1
2
0.7783ʳ 3.1809ʳ 389.78ʳ 2.83
0.2756ʳ 2.2543ʳ 549.98ʳ
0.64708ꢅ(HOMO-1 ꢃ LUMO)
0.68084ꢅ(HOMO ꢃ LUMO)
R2
R3
0
.13954ꢅ(HOMO-7 ꢃ LUMO) +0.59457ꢅ(HOMO-6 ꢃ
3
0.1113ʳ 4.1741ʳ 297.03ʳ
LUMO) -0.21827 (HMOM-5 ꢃ LUMO) -0.18699ꢅ(HOMO-1
ꢃ LUMO+1)
0
.11165ꢅ(HOMO-2 ꢃ LUMO) +0.65586ꢅ(HOMO-1 ꢃ
LUMO)
.63479ꢅ (HOMO-2 ꢃ LUMO) +0.16037ꢅ(HOMO ꢃ
1
2
0.5985ʳ 3.1452ʳ 394.2ʳ 2.70
0.1053ʳ 3.5465ʳ 349.6ʳ 3.49
460
356
0
LUMO+2)
-0.11214ꢅ(HOMO-7 ꢃ LUMO) +0.29511ꢅ(HOMO-6 ꢃ
LUMO) +0.41153 (HOMO-5 ꢃ LUMO) -0.23046ꢅ(HOMO-4
ꢃ LUMO) -0.21544ꢅ(HOMO-1 ꢃ LUMO+1)
3
0.0686ʳ 4.1977ʳ 295.36ʳ 4.16
298
+0.14252ꢅ(HOMO ꢃ LUMO+2) +0.23590ꢅ(HOMO ꢃ
LUMO+3) +0.10978ꢅ(HOMO ꢃ LUMO+4)
radiation per unit area.
Figure shows action spectra of monochromatic
solar-energy-to-electricity conversion efficiency (ꢀ) of the
7
DSSCs is calculated from short-circuit current (J ), the
sc
incident-to current conversion efficiencies (IPCEs) for DSSCs
using R1-R3. The IPCEs of R1, R2, and R3 are 43-58% in the
spectral range from 460 to 520 nm, and R3 reaches its
maximum of 58% at 500 nm. When reflection and absorption
losses of the FTO glass substrate are considered, the net
photon-to-electron conversion efficiencies of the three dyes
open-circuit photovoltage (V ), the fill factor (FF), and the
oc
intensity of the incident light (P ) according to the following
in
equation [30]:
ꢀ
2
[
J (mAꢁcm )][V (V)][FF]
sc oc
3
ꢂ
ꢀ2
P (mWꢁcm )
in
(
R1, R2, and R3) would almost exceed 60% in their spectral
Under the standard global AM 1.5 solar condition, the R1
ranges. The decrease of the IPCE above 750 nm in the
long-wavelength region is attributed to the decrease of light
harvesting for these dyes.
A typical photocurrent-photovoltage (I-V) curve for cells
based on R1, R2, and R3 is shown in Fig. 8. The detailed
photovoltaic parameters are summarized in Table 4. The
-
2
cell had a J of 6.62 mA cm , V of 587 mV, and a fill factor
sc
oc
of 0.65, corresponding to an overall conversion efficiency of
.54%. Under similar conditions, the photovoltaic parameters
J , V and ꢀ) of cells with the R2 sensitizer were 7.46 mA
2
(
sc
-
oc
2
cm , 625 mV, and 3.52%, respectively, and those of the R3
7
17

Wu et al.
Table 4. Photovoltaic Performance of DSSCs Based on R1,
R2, and R3 Dyes
Dye
Voc
Jsc
Fill factor
(ff)
34
-
2
(
mV)
(mA cm )
6.62
7.46
10.34
(%)
2.54
3.52
4.87
R1
R2
R3
587.27
624.95
660.55
0.65
0.75
0.71
a
Measured under irradiation of AM 1.5 G simulated solar light
2
ꢀ
(
0
100 mW cm ) at room temperature, 10 ,m film thickness,
2
b
4
.25 cm working area. The concentration of dye is 2 ꢃ 10ꢀ
M in CH CN and 0.6 M tetrabutylammonium iodide (TBAI),
3
0
.1 M LiI, 0.05 M I , 0.6 M DMPII, and 0.5 M 4-tert-
2
butylpyridine (TBP) in dry acetonitrile (ACN) as electrolyte.
Fig. 7. The incident photon-to-current conversion efficiency
spectra for DSSCs based on R1 (–ꢂ), R2 (…), and
R3 (---).
large Voc is also a prerequisite for higher power conversion
efficiencies of phenothiazine substituted dyes.
Mechanism of Photon-to-Current Conversion in the
DSSCs
Figure 9 shows the schematic energy diagram for a DSSC
based on the R2 photosensitizer. A thin film of electrolyte
-
-
solution (I /I dissolved in an organic solvent) is sandwiched
3
between the TiO electrode and another transparent conducting
2
electrode (counter electrode). The R2 photosensitizer is
attached to very small particles (nanocrystalline structure) of
titanium dioxide where it acts as a light absorber. Based on the
electrochemical
properties
and
photocurrent-voltage
characteristics, the relevant electron-transfer process for the
R2 sensitized TiO electrode involves the following processes.
2
First, the dye is excited from the ground state to the excited
state by the absorption of incident photon flux due to the
intramolecular 'ꢀ'* transition. Second, the excited electrons
Fig. 8. Current densityꢀvoltage characteristics for R1 (–ꢂ),
R2 (…), and R3 (---) DSSCs under illumination of
are injected into the conduction band of the TiO electrode
2
-
2
simulated solar light (AM 1.5, 100 mW cm ).
immediately before quenching by emission and consequent
formation of the oxidized dye. Third, the injected electrons in
the conduction band of TiO are transported toward the Pt
2
-
2
sensitizer were 10.34 mA cm , 661 mV, and 4.87%,
respectively. The greater efficiency of the R3 sensitizer is
probably due to the strong electron-donating ability of the
phenothiazine unit when electrons transfer from the
phenothiazine unit to the rhodanine-3-acetic acid group. A
counter electrode through the external load. Fourth, the iodide
is regenerated by the reduction of triiodide at the Pt counter
electrode. Finally, the oxidized dyes are reduced to the ground
state by accepting electrons from iodide anions that have been
oxidized to triiodide anions. The process leads to photovoltaic
7
18

Synthesis and Characterization of Three Organic Dyes
Fig. 9. Schematic energy diagram for a DSSC based on the R2 photosensitizer, I-/I
-
redox electrolyte, TiO anode,
3
2
and Pt cathode.
performance in the solar cell.
corresponding to an overall conversion efficiency of 4.87%
under 100 mW cm irradiation. Accordingly, dyes based on
2
ꢀ
CONCLUSIONS
carbazole, iminodibenzyl and phenothiazines are promising
candidates for DSSCs.
Three dyes containing carbazole, iminodibenzyl, and
phenothiazine units, respectively, were designed and
synthesized as the sensitizers for DSSCs applications.
Iminodibenzyl is a diphenylamine wherein two ortho positions
are joined by a dimethylene bridge. Phenothiazine sensitizer
contains extra electron-donating sulfur in addition to nitrogen.
The optical, electrochemical, and photovoltaic performances
were affected by structural modifications. The LUMO values
of R1 (-1.54 V), R2 (-1.61 V), and R3 (-1.74 V) are more
ACKNOWLEDGEMENTS
The authors would like to thank the National Science
Council of the Republic of China for financially supporting
this project under grant: 96-2113-M-006-014-MY3.
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