Fluorine substituent effect on organic dyes for sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2011.12.013

Two sets of organic dyes containing a stilbene backbone with fluorine substituents were designed for a study on the quantum efficiency of dye-sensitized solar cells (DSSCs). The results revealed that adding a fluorine substituent on the phenyl group ortho to the cyanoacrylate can enhance the light-harvesting performance in comparison with the unsubstituted one. However, when the two ortho-positions were both substituted by fluorine atoms, the performance of DSSCs was substantially reduced. The reason was mainly ascribed to a distortion from a planar geometry caused by steric hindrance. The π-conjugation was therefore disturbed, and the result led to a substantial reduction of the short-circuit photocurrent density (J_sc). Another effect was found that the open-circuit photovoltage (V_oc) of the doubly substituted derivative was lower than that of the mono-substituted one. The more flexible conformation of the difluoro-substituted dyes induced an undesired nonradiative decay, therefore led to a reduction of open-circuit photovoltage. The phenomenon can be verified by electrochemical impendence spectrum. The non-planar geometry was realized by a computation using the density function theory (DFT) model. The slight blue shift of absorption band was also consistent with the calculated transition energy by a time dependent DFT model.

Full text of DOI:10.1016/j.jphotochem.2011.12.013

Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 47–54
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Fluorine substituent effect on organic dyes for sensitized solar cells
Yan-Duo Lin, Tahsin J. Chow∗
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 August 2011
Received in revised form 1 December 2011
Accepted 25 December 2011
Available online 29 December 2011
Two sets of organic dyes containing a stilbene backbone with fluorine substituents were designed for a
study on the quantum efficiency of dye-sensitized solar cells (DSSCs). The results revealed that adding
a fluorine substituent on the phenyl group ortho to the cyanoacrylate can enhance the light-harvesting
performance in comparison with the unsubstituted one. However, when the two ortho-positions were
both substituted by fluorine atoms, the performance of DSSCs was substantially reduced. The reason was
mainly ascribed to a distortion from a planar geometry caused by steric hindrance. The ␲-conjugation
was therefore disturbed, and the result led to a substantial reduction of the short-circuit photocurrent
density (Jsc). Another effect was found that the open-circuit photovoltage (Voc) of the doubly substituted
derivative was lower than that of the mono-substituted one. The more flexible conformation of the
difluoro-substituted dyes induced an undesired nonradiative decay, therefore led to a reduction of open-
circuit photovoltage. The phenomenon can be verified by electrochemical impendence spectrum. The
non-planar geometry was realized by a computation using the density function theory (DFT) model. The
slight blue shift of absorption band was also consistent with the calculated transition energy by a time
dependent DFT model.
Keywords:
Dye-sensitized solar cell
Stilbene
Fluorine substituted spacer
Charge transfer
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
D–S–A dyads, in which one or two fluorine atoms are substituted on
a phenyl group ortho to the cyanoacrylic acid acceptor. The effect of
Developing clean and renewable energy sources has become a
global concern in recent years. Dye-sensitized solar cells (DSSCs)
have attracted considerable attention due to their low manufac-
turing cost in comparison with traditional silicon-based solar cells
[1,2]. Since the pioneering work by Grätzel and co-worker [3], many
efficient transition–metal complexes [4–7] and metal-free organic
dyes [8–18] have been reported. In general, the structure of organic
dyes consists of a triphenylamine (TPA) moiety as an electron-
donating group (D) and a cyanoacrylic acid as an electron acceptor
(A). An intramolecular charge transfer can be induced across a ␲-
conjugated spacer (S) upon photo-excitation. Among a variety of
structural designs, the one using a stilbene as the S group, i.e.,
compound H-P, (Fig. 1) was reported to have achieved the high-
est quantum efficiency of ꢀ = 9.1% [19]. In order to further improve
the efficiency of DSSCs, electron-withdrawing group such as flu-
orine has been incorporated onto S [20–22]. In a recent report it
have been shown that adding a fluorine atom at the ortho-position
to the cyanoacrylic acid can enhance the quantum efficiency, i.e.,
on the structures close resemblance to F-P and H-P. Encouraged by
these results, we report herein the synthesis of two series of organic
adding fluorine substituent is analyzed by comparing their DSSCs
performances with the aid of theoretical computations.
2. Materials and methods
2.1. General information
The 1H and 13C NMR spectra were recorded on a Bruker 400 MHz
spectrometer, and 19F spectra were recorded on a Bruker 300 MHz
spectrometer. Fast atom bombardment (FAB) mass spectra were
recorded on a Jeol JMS 700 double-focusing spectrometer. UV
spectra were measured on a Jasco V-530 double beam spec-
trophotometer. Fluorescence spectra were recorded on a Hitachi
F-4500 fluorescence spectrophotometer. Compounds purchased
from commercial sources were used as received. The syntheses of
compounds 1N [18], 1P [19], 3PH [19], and H-P [19] have been
reported previously.
2.2. Synthesis
The synthetic pathways of the dyes are illustrated in
Scheme 1. Compound 1 was coupled with commercially avail-
able 4-bromobenzaldehyde, 4-bromo-2-fluorobenzaldehyde, and
4-bromo-2,6-difluorobenzaldehyde (i.e., 2H, 2F, and 2FF), respec-
∗
Corresponding author at: 128 Academia Road Sec. 2, Nankang Taipei 115 Taiwan,
ROC. Tel.: +886 2 2789 8637; fax: +886 2 2788 4179.
E-mail address: tjchow@chem.sinica.edu.tw (T.J. Chow).
tively, by a Heck-type reaction to yield the corresponding
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.12.013

48
Y.-D. Lin, T.J. Chow / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 47–54
CHO
R1
R2
Pd(OAc)2
CHO
R2
H-P F-P FF-P
H-N F-N FF-N
NCCH2COOH
P(o-tolyl)3
R1
+
NH4OAc
AcOH
NEt3, DMF
N
Br
Ph
Ar
N
Ph
Ar
1 P) Ar = C6H5
N) Ar = C10H7
2 H) R1 = R2 = H
F) R1 = H, R2 = F
FF) R1 = R2 = F
3 PH) Ar = C6H5 , R1 = R2 = H
PF) Ar = C6H5 , R1 = H, R2 = F
PFF) Ar = C6H5 , R1 = R2 = F
NH) Ar = C10H7, R1 = R2 = H
NF) Ar = C10H7, R1 = H, R2 = F
NFF) Ar = C10H7, R1 = R2 = F
Scheme 1. Synthesis of the dyes.
compound of 3. The aldehydes were condensed with cyanoacetic
acid to afford the final products, i.e., the P and N series dyes, via
Knöevenagel reaction in the presence of ammonium acetate.
(m, 3H), 7.31–7.26 (m, 5H), 7.22 (d, 1H, 7.0 Hz), 7.21–7.04 (m,
8H), 6.95 (d, 1H, J = 16.2 Hz). 13C NMR (100 MHz, CDCl3): ı 186.50,
166.41, 163.86, 148.63, 147.23, 146.49, 132.99, 129.66, 129.41,
128.98, 128.02, 124.99, 124.24, 123.60, 122.58, 113.12. 19F NMR
(282 MHz, CDCl3): ı −123.16 (s, 1F). FAB-HRMS (m/z): 393.1529
(M+) (calcd for C27H20FNO: 393.1526).
2.3. General procedures for Heck reaction
A
heterogeneous mixture of 0.28 ml of triethylamine,
1.00 ml of DMF, 1.05 mmol of styrene derivatives, 2 mol% of
Pd(OAc)2/Pd2(dba)3, 4 mol% of P(o-tolyl)3, and 1.00 mmol of aryl
bromide under argon were heated at 80 ◦C for 18 h. The solution
was cooled and then 20 ml of CH2Cl2 was added. The insoluble
residue was filtered off and the filtrate was concentrated in vacuo
to afford a crude product. Further purification was performed by
column chromatography.
2.6. (E)-4-(4-Diphenylaminostyryl)-2,6-difluorobenzaldehyde
(3PFF)
Compound 3PFF were synthesized via the typical Heck reaction
procedure as described above. Purification was performed by silica
gel column chromatograph eluted solvent CH2Cl2/hexane (1/10) to
provide product 3PFF (0.78 g, 42% yield), mp: 74–75 ◦C. 1H NMR
(400 MHz, CDCl3): ı 10.29 (s, 1H), 7.38 (d, 2H, J = 8.7 Hz), 7.31–7.26
(m, 4H), 7.16–7.03 (m, 11H), 6.86 (d, 1H, J = 16.1 Hz). 13C NMR
(100 MHz, CDCl3): ı 183.92, 164.82, 162.22, 149.01, 146.79, 134.18,
129.43, 128.92, 128.23, 125.13, 123.77, 123.14, 122.37, 112.25,
109.34. 19F NMR (282 MHz, CDCl3): ı −116.15 (s, 2F). FAB-HRMS
(m/z): 412.1513 (M+H+) (Calcd for C27H20F2NO: 412.1523).
2.4. General procedures for Knöevenagel condensation reaction
A mixture of aldehyde precursor (1.10 mmol), cyanoacetic
acid (1.30 mmol), and ammonium acetate (0.28 mmol) in acetic
acid (20 ml) was placed in a three-necked flask under nitro-
gen atmosphere and was stirred at 80 ◦C for 18 h. After cooling,
the reaction was extracted with CH2Cl2. The organic layer was
dried over anhydrous MgSO4. The filtrate was concentrated under
reduced pressure. Purification was performed by recrystallization
in CH2Cl2/hexane.
2.7. (E)-4-(4-(Naphthylphenylamino)styryl)benzaldehyde (3NH)
Compound 3NH were synthesized via the typical Heck reaction
procedure as described above. Column chromatography with ethyl
acetate/hexane (1/10) as eluent afforded the yellow solid of 3NH
(0.79 g, 38% yield), mp: 162–163 ◦C. 1H NMR (400 MHz, CDCl3):
ı 9.97 (s, 1H), 7.91 (d, 2H, J = 8.0 Hz), 7.85–7.79 (m, 3H), 7.60 (d,
2H, J = 8.2 Hz), 7.48 (d, 2H, J = 7.7 Hz), 7.39–7.35 (m, 4H), 7.35–7.21
(m, 3H), 7.16–7.00 (m, 2H), 7.00–6.95 (m, 4H). 13C NMR (100 MHz,
CDCl3): ı 191.52, 148.88, 147.73, 143.97, 143.01, 135.32, 134.93,
131.87, 131.17, 130.22, 129.39, 129.25, 128.47, 127.84, 127.32,
2.5. (E)-4-(4-Diphenylaminostyryl)-2-fluorobenzaldehyde (3PF)
Compound 3PF were synthesized via the typical Heck reaction
procedure as described above. Purification was performed by sil-
ica gel column chromatograph eluted solvent CH2Cl2/hexane (1/5)
to provide product 3PF (1.07 g, 54% yield), mp: 88–89 ◦C. 1H NMR
(400 MHz, CDCl3): ı 10.31 (s, 1H), 7.83 (d, 1H, J = 7.6 Hz), 7.41–7.34
COOH
COOH
COOH
COOH
COOH
COOH
NC
NC
NC
F
NC
NC
NC
F
F
F
F
F
N
N
N
N
N
N
H-P
F-P
FF-P
H-N
F-N
FF-N
Fig. 1. The structures of organic dyes.

Y.-D. Lin, T.J. Chow / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 47–54
49
126.83, 126.54, 126.34, 126.24, 124.93, 124.08, 122.86, 122.64,
120.78. FAB-HRMS (m/z): 426.1867 (M+H+) (calcd for C31H24NO:
426.1858).
133.89, 129.43, 129.01, 128.24, 125.12, 123.75, 123.17, 122.41,
113.73, 109.24, 108.49. 19F NMR (282 MHz, CDCl3): ı −104.87 (s,
2F). FAB-HRMS (m/z): 478.1585 (M+H+) (Calcd for C30H21F2N2O2:
479.1571).
2.8.
(E)-4-(4-(Naphthylphenylamino)styryl)-2-fluorobenzaldehyde
2.12. (E)-3-(4-((E)-4-(Naphthylphenylamino)styryl)phenyl)-2-
(3NF)
cyanoacrylic acid
(H-N)
Compound 3NF were synthesized via the typical Heck reaction
procedure as described above. Purification was performed by silica
gel column chromatograph eluted solvent CH2Cl2/hexane (1/25)
to provide product 3NF (0.53 g, 44% yield), mp: 90–91 ◦C. 1H NMR
(400 MHz, CDCl3): ı 10.30 (s, 1H), 7.90 (d, 2H, J = 8.6 Hz), 7.83–7.80
(m, 2H), 7.48 (q, 2H, J = 7.6 Hz), 7.39–7.31 (m, 5H), 7.23–7.11 (m, 6H),
7.02–6.88 (m, 4H). 13C NMR (100 MHz, CDCl3): ı 186.49, 166.41,
163.85, 149.24, 147.58, 146.60, 142.89, 135.32, 133.11, 131.16,
129.29, 128.95, 128.75, 128.49, 128.06, 127.35, 126.93, 126.60,
126.35, 126.27, 124.03, 123.81, 123.07, 122.86, 122.39, 120.53,
113.03. 19F NMR (282 MHz, CDCl3): ı −123.24 (s, 1F). FAB-HRMS
(m/z): 443.1676 (M+) (calcd for C31H22FNO: 443.1685).
Compound H-N was obtained according to the standard Knöeve-
nagel condensation reaction as described above. Purification was
performed by recrystallization in CH2Cl2/hexane to afford red solid
(1.00 g, 85% yield), mp: 201–202 ◦C. 1H NMR (400 MHz, DMSO-
d6): ı 8.28 (s, 1H), 8.04–8.01 (m, 3H), 7.93 (d, 1H, J = 8.2 Hz),
7.84 (d, 1H, J = 8.4 Hz), 7.73 (d, 2H, J = 8.3 Hz), 7.61–7.37 (m, 7H),
7.30–7.26 (m, 2H), 7.13 (d, 1H, J = 16.3 Hz), 7.05–6.99 (m, 3H),
6.86 (d, 2H, J = 8.6 Hz) ppm. 13C NMR (100 MHz, DMSO-d6): ı
171.89, 163.44, 153.56, 148.14, 147.04, 142.42, 142.23, 134.90,
131.62, 131.28, 130.49, 129.89, 129.42, 128.57, 128.11, 127.30,
126.96, 126.66, 126.32, 124.86, 123.29, 122.67, 122.40, 120.04,
116.37, 101.93 ppm. FAB-HRMS (m/z): 493.1928 (M+H+) (calcd for
2.9. (E)-4-(4-(Naphthylphenylamino)styryl)-2,6-
difluorophenyl)benzaldehyde
(3NFF)
C34H25N2O2: 493.1916).
2.13. (E)-3-(4-((E)-4-(Naphthylphenylamino)styryl)-2-
fluorophenyl)-2-cyanoacrylic acid
(F-N)
Compound 3NFF were synthesized via the typical Heck reaction
procedure as described above. Purification was performed by silica
gel column chromatograph eluted solvent CH2Cl2/hexane (1/25) to
provide product 3NFF (0.66 g, 41% yield), mp: 97–98 ◦C. 1H NMR
(400 MHz, CDCl3): ı 10.28 (s, 1H), 7.91–7.88 (m, 2H), 7.81 (d, 1H,
J = 8.2 Hz), 7.51–7.46 (m, 2H), 7.39–7.32 (m, 4H), 7.27–7.23 (m, 2H),
7.17–7.13 (m, 3H), 7.04–7.02 (m, 3H), 6.94 (d, 2H, J = 8.6 Hz), 6.81
(d, 1H, J = 16.1 Hz). 13C NMR (100 MHz, CDCl3): ı 183.97, 164.86,
162.26, 149.61, 147.41, 146.77, 142.76, 135.32, 134.32, 131.13,
129.34, 128.52, 128.30, 128.05, 127.39, 127.04, 126.65, 126.36,
126.30, 123.97, 123.1862, 122.74, 120.28, 112.18, 109.27. 19F NMR
(282 MHz, CDCl3): ı −116.24 (s, 2F). FAB-HRMS (m/z): 461.1584
(M+) (calcd for C31H21F2NO: 461.1584).
Compound F-N was obtained according to the standard Knöeve-
nagel condensation reaction as described above. Purification was
performed by recrystallization in CH2Cl2/hexane to afford red solid
(0.16 g, 76% yield), mp: 134–135 ◦C. 1H NMR (400 MHz, DMSO-d6):
ı 8.29 (s, 1H), 8.21 (t, 1H, J = 7.8 Hz), 8.02 (d, 1H, J = 8.2 Hz), 7.93 (d,
1H, J = 8.2 Hz), 7.83 (d, 1H, J = 8.2 Hz), 7.63–7.38 (m, 9H), 7.30–7.26
(m, 2H), 7.14–7.00 (m, 4H), 6.85 (d, 2H, J = 7.8 Hz) ppm. 13C NMR
(100 MHz, DMSO-d6): ı 171.92, 162.97, 162.68, 160.15, 148.45,
146.93, 145.32, 144.13, 142.15, 134.91, 133.01, 130.49, 129.47,
128.92, 128.79, 128.60, 128.34, 127.35, 127.04, 126.70, 126.35,
123.87, 123.27, 122.94 (d, J = 67.7 Hz), 122.61, 119.83, 117.65 (d,
J = 45.0 Hz), 115.97, 112.61, 104.52 ppm. 19F NMR (282 MHz, DMSO-
d6): ı −114.08 (s, 1F). FAB-HRMS (m/z): 510.1739 (M+) (calcd for
C34H23FN2O2: 510.1744).
2.10. (E)-3-(4-((E)-4-Diphenylaminostyryl)-2-florophenyl)-2-
cyanoacrylic acid
(F-P)
2.14. (E)-3-(4-((E)-4-(Naphthylphenylamino)styryl)-2,6-
difluorophenyl)-2-cyanoacrylic acid
(FF-N)
Compound F-P was obtained according to the standard Knöeve-
nagel condensation reaction as described above. Purification was
performed by recrystallization in CH2Cl2/hexane to afford red solid
(0.37 g, 79% yield), mp: 228–229 ◦C. 1H NMR (400 MHz, CDCl3): ı
8.59 (s, 1H), 8.48 (t, 1H, J = 7.9 Hz), 7.41–7.38 (m, 3H), .7.31–7.26
(m, 7H), 7.20–7.04 (m, 8H), 6.95 (d, 1H, J = 16.1 Hz). 13C NMR
(100 MHz, CDCl3): ı 166.45, 163.95, 161.38, 148.84, 147.16, 146.99,
146.50, 146.40, 133.53, 129.43, 128.19, 125.09, 124.05, 123.71,
122.97, 122.52, 118.03, 115.25, 112.78, 112.55. 19F NMR (282 MHz,
CDCl3): ı −112.79 (s, 1F). FAB-HRMS (m/z): 460.1587 (M+) (Calcd
for C30H21FN2O2: 460.1579).
Compound FF-N was obtained according to the standard
Knöevenagel condensation reaction as described above. Purifica-
tion was performed by recrystallization in CH2Cl2/hexane to afford
red solid (0.39 g, 75% yield), mp: 133–134 ◦C. 1H NMR (400 MHz,
CDCl3): ı 8.31 (s, 1H), 7.90 (d, 2H, J = 8.4 Hz), 7.81 (d, 1H, J = 8.2 Hz),
7.51–7.46 (m, 2H), 7.39–7.32 (m, 4H), 7.27–7.23 (m, 2H), 7.16–7.00
(m, 6H), 6.94 (d, 2H, J = 8.7 Hz), 6.83 (d, 1H, J = 16.1 Hz). 13C NMR
(100 MHz, CDCl3): ı 176.92, 162.39, 159.83, 149.54, 147.42, 145.47,
143.92, 142.76, 135.30, 133.94, 131.11, 130.08, 129.31, 128.31,
127.87, 127.19, 126.64, 126.34, 126.29, 125.69, 123.97, 123.23,
123.04, 122.77, 120.31, 113.84, 105.15, 108.37. 19F NMR (282 MHz,
CDCl3): ı −104.99 (s, 2F). FAB-HRMS (m/z): 528.1652(M+) (calcd
for C34H22F2N2O2: 528.1649).
2.11. (E)-3-(4-((E)-4-Diphenylaminostyryl)-2,6-diflorophenyl)-
2-cyanoacrylic acid
(FF-P)
Compound FF-P was obtained according to the standard
Knöevenagel condensation reaction as described above. Purifica-
tion was performed by recrystallization in CH2Cl2/hexane to afford
red solid (0.58 g, 74% yield), mp: 226–227 ◦C. 1H NMR (400 MHz,
CDCl3): ı 8.33 (s, 1H), 7.39 (d, 2H, J = 8.6 Hz), 7.31–7.26 (m, 5H),
7.15–7.03 (m, 10H), 6.88 (d, 2H, J = 16.1 Hz). 13C NMR (100 MHz,
CDCl3): ı 165.30, 162.45, 159.89, 148.97, 14709, 145.46, 144.11,
2.15. Device fabrication
A thin film of TiO2 (16–18 ␮m thick) was coated on a 0.25 cm2
FTO glass substrate. It was immersed in a THF solution containing
3 × 10−4 M dye sensitizers for 12 h, then rinsed with anhydrous ace-
tonitrile and dried. Another piece of FTO with sputtering 100 nm

50
Y.-D. Lin, T.J. Chow / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 47–54
thick Pt was used as a counter electrode. The active area was
controlled at a dimension of 0.25 cm2 by adhering 60 ␮m thick
polyester tape on the Pt electrode. The photocathode was placed
on top of the counter electrode and was tightly clipped together to
form a cell. Electrolyte was then injected into the seam between
two electrodes. An acetonitrile solution containing LiI (0.5 M), I2
(0.05 M), and 4-tert-butylpyridine (0.5 M) was used as the elec-
trolyte. Devices made of a commercial dye N719 under the same
condition was compared as a reference. The cell parameters were
0.30
0.25
0.20
0.15
0.10
0.05
0.00
H-P
F-P
FF-P
a
obtained under an incident light with intensity 100 mW cm−2
,
which was generated by a 300 W Xe lamp passing through an AM
1.5 filter. The current–voltage parameters of DSSCs were recorded
by a potentiostat/galvanostat model CHI650B (CH Instruments,
USA).
300
400
500
600
Wavelength (nm)
2.16. Computation methods
The computations were performed with the Gaussian 03 pro-
gram package [23]. The geometry was optimized by using B3LYP
(Becke three parameters hybrid functional with Lee–Yang–Perdew
correlation functionals) with the Pople 6-31G(d,p) atomic basis set
[24,25]. The excitation transition of dyes were calculated using
time-dependent density functional theory (TDDFT) calculations
with B3LYP/6-31G(d,p).
0.30
0.25
0.20
0.15
0.10
0.05
0.00
b
H-N
F-N
FF-N
3. Results and discussion
3.1. Absorption spectra
The absorption spectra of both the P and N series compounds
in THF are shown in Fig. 2, the related photophysical and elec-
trochemical data are summarized in Table 1. All dyes displayed
two major absorption bands at 300–350 nm and 400–450 nm.
The former is attributed to the ␲–␲* transition, and the latter
is assigned to the intramolecular charge transfer (ICT) transition
from the triarylamine donors to the cyanoacrylic acid, as con-
firmed by the negative solvatochromic shifts in more polar solvents.
This phenomenon may be ascribed to the deprotonation of the
carboxylic group, which weakens the electron-withdrawing abil-
ity of the acceptor (see Figs. S1 and S2 in Supplementary data)
[26,27]. The absorption maxima for monofluoro-substituted dyes,
i.e., F-N and F-P, are not much different from those without
the fluorine substituent. However, the absorption maxima for
the difluoro-substituted dyes, i.e., FF-N and FF-P, exhibit a slight
blue shift. It is likely caused by the steric hindrance between
the two ortho-fluorine atoms and the cyanoacrylic acid, which
twists the ␲-conjugation out of plane. The blue shift transition
complies well with the result of computation according to the
time-dependent density functional theory (TDDFT) (Table S1). The
molar extinction coefficients of these dyes were in the range
of 2.67–2.80 × 104 M−1 cm−1, which is higher than that of N719
300
400
500
600
Wavelength (nm)
Fig. 2. Absorption spectra of (a) dye P (b) dye N in the THF solutions.
(1.41 × 104 M−1 cm−1), revealing a good light harvesting nature. In
Fig. 3 the spectra of dyes adsorbed on the TiO2 surface are shown,
and the corresponding data are collected in Table 1. The absorption
maxima of these dyes display a blue-shift in comparison with those
in solution. This blue shift is attributed to the deprotonation of car-
boxylic acid upon attaching to the TiO2 surface, a similar effect to
the negative solvatochromic shift in polar solvents [26,27].
3.2. Electrochemical properties
The redox potentials of all compounds were measured by
cyclic voltammetry (CV) in THF solution containing 0.1 M tetra-
butylammonium hexafluorophosphate (TBAPF6) as the supporting
electrolyte, Ag/AgNO3 as the reference electrode, and a standard
ferrocene/ferrocenium redox couple as an internal standard. The
Table 1
Photophysical and electrochemical data.
a
b
c
d
Compd
ꢁabs (nm)
ꢁmax (ε × 10−4, dm3 mol−1 cm−1
)
ꢁfl (nm)
E0–0 (nm)/(eV)
Eoxe/V
ELUMOf/V
H-P
F-P
FF-P
H-N
F-N
433 (400)
436 (399)
422 (380)
434 (406)
427 (407)
421 (398)
2.71
2.67
2.80
2.73
2.69
2.76
607
615
620
611
600
611
2.50
2.51
2.55
2.51
2.52
2.55
1.15
1.19
1.20
1.20
1.21
1.19
−1.35
−1.32
−1.35
−1.31
−1.31
−1.36
FF-N
a
Absorption maximum in THF, and that on TiO2 in parentheses.
Extinction coefficient in THF.
Maximum of fluorescence in THF.
b
c
d
e
f
Onset of the absorption spectra in THF.
Oxidation potential measured vs Fc+/Fc in THF were converted to normal hydrogen electrode (NHE) by addition of +0.63 V.
Energy of LUMO estimated by Eox − E0–0
.

Y.-D. Lin, T.J. Chow / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 47–54
51
Table 2
Fluorescence quantum yields (˚f), fluorescence decay times (ꢂf), rate constants for
fluorescence decay (kf), and nonradiative decay (knr) for dyes P and N in THF.
a
H-P
F-P
FF-P
1.0
0.5
0.0
a
b
Compd
˚
f
ꢂf (ns)
kf (108 s−1
)
knr (108 s−1
)
H-P
F-P
FF-P
H-N
F-N
0.22
0.19
0.16
0.21
0.22
0.17
2.29
2.42
2.33
2.12
2.53
2.15
0.96
0.79
0.69
0.99
0.87
0.79
3.41
3.35
3.61
3.73
3.08
3.86
FF-N
a
kf = ˚f/ꢂf.
knr = (1 − ˚f)/ꢂf.
b
The fluorescence quantum yields (˚f) and lifetimes (ꢂf) of dyes P
400
500
600
and N in THF are listed in Table 2, in which the ␶f values were deter-
mined at the emission maxima. All decay curves were fitted well
into a single-exponential equation. The results showed that the val-
ues of knr for difluoro-substituted compounds, i.e., FF-P and FF-N,
are greater than those of other dyes. It is also worthy of mentioning
that the fluorescence lifetime of all dyes, i.e., around 2.12–2.53 ns,
was significantly longer than the time needed for effective electron
transfers from the dyes to the TiO2 conductive band [28].
Wavelength (nm)
b
H-N
F-N
FF-N
1.0
0.5
0.0
3.4. Photovoltaic performance of DSSCs
The optimized incident photo-to-current conversion efficien-
cies (IPCE) data of the dyes, obtained by using a sandwich cell
comprising I2 (0.05 M), LiI (0.5 M), and tert-butylpyridine (0.5 M)
in acetonitrile as the redox electrolyte, are shown in Fig. 4. It
is anticipated that the incorporation of tert-butylpyridine may
400
500
600
100
Wavelength (nm)
a
H-P
F-P
FF-P
Fig. 3. Absorption spectra of (a) dye P (b) dye N adsorbed on the TiO2 films.
80
60
40
20
0
potential level of highest occupied molecular orbital (HOMO) was
estimated from the first oxidative wave (Eox) as listed in Table 1.
The results reveal that the HOMO levels of the dyes are all more pos-
itive than the I¯/I3¯ redox couple (∼0.4 V versus NHE) and thus the
regeneration of the oxidized dyes is thermodynamically feasible.
The oxidation potentials of fluorine substituted dyes are found to be
close to the corresponding unsubstituted ones (Table 1). The low-
est unoccupied molecular orbital (LUMO) levels of the dyes were
deduced from the first oxidation potential and the 0–0 transition
energy (E0–0) measured at the onset of the absorption spectrum.
The LUMO levels of doubly substituted derivatives, i.e., FF-P and
FF-N, are more negative than those of the mono-substituted ones,
i.e., F-P and F-N. Nevertheless, their LUMO levels are more negative
than the conduction band edge of the TiO2 (−0.5 V versus NHE),
which assure that the electron injection process is energetically
favourable.
400
500
600
700
800
Wavelength (nm)
100
80
60
40
20
0
b
H-N
F-N
FF-N
3.3. Fluorescence quantum yields and lifetime
The excited-state energy could be dissipated either through a
radiative emission or through a nonradiative decay, i.e., torsional
motion about a specific bond. In the present work the amount of
non-radiative decay in the difluoro-substituted compounds was
a crucial factor influencing the efficiency of DSSCs [18]. To ver-
ify the significance of non-radiative decay, kinetic parameters of
the excited state are measured. From the fluorescence quantum
yields and the lifetimes, the rate constant of nonradiative decay
(knr = (1 − ˚f)/ꢂf) can be deduced.
400
500
600
700
800
Wavelength (nm)
Fig. 4. The IPCE plots of DSSCs made with (a) dyes P and (b) dyes N.

52
Y.-D. Lin, T.J. Chow / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 47–54
50
a
F-N
FF-N
40
30
20
10
0
0
20
40
60
80
100
120
140
160
Z' (ohm)
30
25
20
15
10
5
b
F-N
FF-N
0
1
10
100
1000
10000
100000
Frequency (Hz)
Fig. 6. The electrochemical impedance spectra of (a) Nyquist plots, and (b) Bode
phase plots for the DSSCs made with F-N and FF-N.
Fig. 5. Current–voltage plots for the DSSCs made with (a) dyes P and (b) dyes N
under AM 1.5 G simulated solar light (100 mW cm−2).
difluorophenyl and cyanoacrylate moieties, which reduced the effi-
ciency of ␲-conjugation.
have an effect of decreasing the dark current, thus enhancing the
open-circuit photovoltage. The IPCE maxima of H-P and F-P were
measured to be 83% and 80%, respectively, while those of H-N and
F-N were 78% and 81%. The IPCE spectra of FF dyes were relatively
low, e.g., 42–56% at 440 nm, compared with the unsubstituted ones.
The current–voltage (J–V) curves of the DSSCs under standard
global AM 1.5 G irradiation is shown in Fig. 5. The open-circuit
photovoltage (Voc), short-circuit photocurrent density (Jsc), fill fac-
tor (ff), and solar-to-electrical energy conversion efficiencies (ꢀ)
are summarized in Table 3, where N719 is included for a com-
parison. The overall performances of F-P and F-N were slightly
better than those of the corresponding H-P and H-N. For FF
dyes, the Jsc values are substantially lower than those of H and
F dyes. It was ascribed to the twisted conformation between the
The EIS of F and FF were performed in the dark under a
forward bias to study the interfacial charge transfer processes.
The small semicircle in the Nyquist plot (<20 ꢃ at the Z axis
in Fig. 6a) corresponds to the electron transfer processes at the
Pt/electrolyte interface, while the large semicircle (>20 ꢃ at the
Z axis) corresponds to the electron transfer processes at the
TiO2/dye/electrolyte interface. The effectiveness of the dyes can be
mainly reflected by the radius of the large semicircle. As indicated
in Fig. 6 the radius for FF-N is significantly smaller than that of
F-N, expressing its higher charge recombination rate in the dark.
A larger dark current normally leads to a smaller value of Voc
,
which is consistent with the observed performance of the device.
A similar result can also be detected by the Bode plots. The higher-
frequency peak (>100 Hz in Fig. 6b) represents the electron transfer
processes at the Pt/electrolyte interface, while the lower frequency
peak (<100 Hz) represents the electron transfer processes at the
TiO2/dye/electrolyte interface. From Fig. 6b it is clear that the posi-
tion of high-frequency peaks for the two dyes did not change much.
However, the lower frequency peak of F-N device shifted to a lower
frequency than that of the FF-N device, an indication of a longer
lifetime in the former [29,30].
Table 3
DSSCs performance parameters of dyes.a
Dye
Rel loadingb
Voc (mV)
Jsc (mA cm−2
)
ff
ꢀ (%)
N719
H-P
F-P
FF-P
H-N
F-N
0.89
1.00
1.02
0.97
0.99
1.02
0.96
0.76
0.66
0.67
0.61
0.68
0.67
0.64
14.59
11.12
11.20
8.89
9.99
10.84
9.21
0.63
0.68
0.69
0.67
0.68
0.65
0.68
7.00
4.98
5.19
3.62
4.58
4.73
4.01
3.5. Molecular orbital calculation
FF-N
a
Experiments were conducted with TiO2 photoelectrodes with approximately
The electronic structures of the dyes were computed by DFT
at B3LYP/6-31G(d,p) level with full geometry optimization. The
optimized geometries of H-P and F-P adopt a nearly planar
16 ␮m thickness and 0.25 cm2 working area on the FTO (8 ꢃ/sq.) substrates.
b
The relative amount of dye absorption on TiO2 relative to H-P
(2.1 × 10−6 mol/cm2).

Y.-D. Lin, T.J. Chow / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 47–54
53
Fig. 7. Optimized geometry of H-P (top), F-P (middle) and FF-P (bottom), and the electronic maps of Frontier orbitals.
conformation, particularly between cyanoacrylic acid group and
the adjacent phenyl ring (Fig. 7). The two fluorine substituents
on the phenyl ring of FF-P induce a steric hindrance, thus a tor-
tional angle of ca. 32◦ was twisted between the planes containing
the cyanoacrylic acid group and the adjacent phenyl ring. The dis-
torted nonplanar geometry of FF-P therefore reduces the degree of
␲-conjugation, and results to a blue-shift in the absorption spectra
with respect to that of H-P. The calculated electron distributions
of the frontier molecular orbitals for H-P, F-P and FF-P are shown
in Fig. 7, and that for H-N, F-N and FF-N are shown in Fig. S3. In
all the plots it shows that the HOMOs are mainly populated on the
TPA moiety, while the LUMOs are on the cyanoacrylic acid group.
The lowest energy transition from the TPA to the cyanoacrylic acid
induces a charge separation along the long axis of the molecules.
The electronic transitions were calculated by TDDFT for the low-
est 10 singlet excitations on H-P, F-P and FF-P. The transitions
with oscillator strength above 0.1 are summarized in Table S1.
The lowest energy transitions, which correspond to charge trans-
fers, exhibit high values of oscillator strength (0.93–0.97). The
results correlated well with the large extinction coefficients in their
absorption spectra.
between the difluorophenyl group and the cyanoacrylate moieties.
The twisted geometry reduced the efficiency of ␲-conjugation,
therefore reduced substantially the value of Jsc. The flexible confor-
mation also resulted to a faster rate of charge recombination, which
was evidenced by EIS and led to a lower value of Voc. These results
reveal that the advantage of adding electron-withdrawing sub-
stituents on the acceptor side may be voided by a slight distortion
of molecular geometry. A delicate balance between the electronic
and structure effect is necessary to achieve a high performance of
DSSCs.
Acknowledgments
Financial supports were provided by the Institute of Chemistry,
Academia Sinica, and the National Science Council of Taiwan.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2011.12.013.
References
4. Conclusions
[1] A. Hagfeldt, M. Grätzel, Light-induced redox reactions in nanocrystalline sys-
tems, Chem. Rev. 95 (1995) 49–68.
[2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, M. Grätzel, Dye-sensitized solar cells,
Chem. Rev. 110 (2010) 6595–6663.
[3] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dye-
sensitized colloidal TiO2 films, Nature 353 (1991) 737–740.
[4] M.K. Nazeeruddin, A. Kay, L. Rodicio, R. Humphry-Baker, E. Müller, P.
Liska, N. Vlachopoulos, M. Grätzel, Conversion of light to electricity
by cis-X2Bis(2,2(-bipyridyl-4,4(-dicarboxylate)ruthenium(II) charge-transfer
sensitizers (X = C1−, Br−, I−, CN−, and SCN−) on nanocrystalline TiO2 electrodes,
J. Am. Chem. Soc. 115 (1993) 6382–6390.
A series of organic dyes containing fluoro-substituents on a
phenyl group ortho to the cyanoacrylate acceptor were compared.
The mono-fluorinated dyes showed similar Voc and ff values to
the non-substituted ones, yet showed larger Jsc values due to the
extension of conjugation. As a result, the mono-fluorinated dyes
exhibited better quantum efficiencies. In contrast, the difluori-
ated dyes showed a lower performance due to a twisted geometry

54
Y.-D. Lin, T.J. Chow / Journal of Photochemistry and Photobiology A: Chemistry 230 (2012) 47–54
[5] M.K. Nazeeruddin, P. Péchy, T. Renouard, S.M. Zakeeruddin, R. Humphry-Baker,
[20] B.-S. Chen, D.-Y. Chen, C.-L. Chen, C.-W. Hsu, H.-C. Hsu, K.-L. Wu, S.-H. Liu, P.-
T. Chow, Donor–acceptor dyes with fluorine substituted phenylene spacer for
dye-sensitized solar cells, J. Mater. Chem. 21 (2011) 1937–1945.
[21] D.-Y. Chen, Y.-Y. Hsu, H.-C. Hsu, B.-S. Chen, Y.-T. Lee, H. Fu, Organic dyes with
remarkably high absorptivity; all solid-state dye sensitized solar cell and role
of fluorine substitution, Chem. Commun. 46 (2010) 5256–5258.
P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G.B. Deacon,
C.A. Bignozzi, M. Grätzel, Engineering of efficient panchromatic sensitizers for
nanocrystalline TiO2-based solar cells, J. Am. Chem. Soc. 123 (2001) 1613–1624.
[6] C.-Y. Chen, J.-G. Chen, S.-J. Wu, J.-Y. Li, C.-G. Wu, K.-C. Ho, Multifunctional-
ized ruthenium-based supersensitizers for highly efficient dye-sensitized solar
cells, Angew. Chem. Int. Ed. 47 (2008) 7342–7345.
[7] J.-F. Yin, J.-G. Chen, Z.-Z. Lu, K.-C. Ho, H.-C. Lin, K.-L. Lu, Toward Optimiza-
tion of oligothiophene antennas: new ruthenium sensitizers with excellent
performance for dye-sensitized solar cells, Chem. Mater. 22 (2010) 4392–4399.
[8] Y.J. Chang, T.J. Chow, Dye-sensitized solar cell utilizing organic dyads containing
triarylene conjugates, Tetrahedron 65 (2009) 4726–4734.
[9] Y.J. Chang, T.J. Chow, Triaryl linked donor acceptor dyads for high-performance
dye-sensitized solar cells, Tetrahedron 65 (2009) 9626–9632.
[10] D.-Y. Chen, K.-Y. Cheng, M.-L. Ho, I.-C. Wu, M.-W. Chung, H. Fu, P.-T. Chou, A
new recognition concept using dye sensitized solar cell configuration, Chem.
Commun. 47 (2011) 985–987.
[11] B.-S. Chen, Y.-J. Chen, P.-T. Chou, A new type of donor–acceptor dye bridged by
the bidentate moiety; metal ion complexation enhancing DSSC performance, J.
Mater. Chem. 21 (2011) 4090–4094.
[12] A. Mishra, M.K.R. Ficher, P. Bäuerle, Metal-free organic dyes for dye-sensitized
solar cells: from structure: property relationships to design rules, Angew.
Chem. Int. Ed. 48 (2009) 2474–2499.
[13] T.H. Kwon, V. Armel, A. Nattestad, D.R. MacFarlane, U. Bach, S.J. Lind,
Dithienothiophene (DTT)-based dyes for dye-sensitized solar cells: synthesis
of 2,6-dibromo-DTT, J. Org. Chem. 76 (2011) 4088–4093.
[14] Y. Numata, I. Ashraful, Y. Shirai, L. Han, Preparation of donor–acceptor type
organic dyes bearing various electron-withdrawing groups for dye-sensitized
solar cell application, Chem. Commun. 47 (2011) 6159–6161.
[22] Y.J. Chang, T.J. Chow, Highly efficient triarylene conjugated dyes for sensitized
solar cells, J. Mater. Chem. 21 (2011) 9523–9531.
[23] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheese-
man, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P.
Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Strat-
mann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich,
A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari,
J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Ste-
fanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith,
M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. John-
son, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Gaussian Inc.,
Pittsburgh, PA, 2003.
[24] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange,
J. Chem. Phys. 98 (1993) 5648–5652.
[25] R. Ditchfield, W.J. Hehre, J.A. Pople, Self-consistent molecular-orbital methods.
IX. An extended gaussian-type basis for molecular-orbital studies of organic
molecules, J. Chem. Phys. 54 (1971) 724–728.
[26] Z. Wang, Y. Cui, Y. Dan-oh, C. Kasada, A. Shinpo, K. Hara, Thiophene-
functionalized coumarin dye for efficient dye-sensitized solar cells: electron
lifetime improved by coadsorption of deoxycholic acid, J. Phys. Chem. C 111
(2007) 7224–7230.
[27] H.-Y. Yang, Y.-S. Yen, Y.-C. Hsu, H.-H. Chou, J.T. Lin, Organic dyes incorporating
dithieno[3,2-b:2(,3(-d]thiophene moiety for efficient dye-sensitized solar cells,
Org. Lett. 12 (2010) 16–19.
[15] H. Im, S., Kim, C, Park S.-H. Jang, C.-J. Kim, K., Kim, N.-G. Park, C. Kim, High perfor-
mance organic photosensitizers for dye-sensitized solar cells, Chem. Commun.
46 (2010) 1335-1337.
[16] S.-H. Lin, Y.-C. Hsu, J.T. Lin, C.-K. Lin, J.-S. Yang, Isotruxene-derived cone-shaped
organic dyes for dye-sensitized solar cells, J. Org. Chem. 75 (2010) 7877–7886.
[17] Y.-D. Lin, C.-T. Chien, S.-Y. Lin, H.-H. Chang, C.-Y. Liu, T.J. Chow, Meta versus
para substituent effect of organic dyes for sensitized solar cells, J. Photochem.
Photobiol. A 222 (2011) 192–202.
[28] S.E. Koops, B.C. O’Regan, P.R.F. Barnes, J.R. Durrant, Parameters influencing the
efficiency of electron injection in dye-sensitized solar cells, J. Am. Chem. Soc.
131 (2009) 4808–4818.
[18] Y.-D. Lin, T.J. Chow, Geometrical effect of stilbene on the performance of organic
dye-sensitized solar cells, J. Mater. Chem. 21 (2011) 14907–14916.
[19] S. Hwang, J.H. Lee, C. Park, H. Lee, C. Kim, C. Park, M.-H. Lee, W. Lee, J. Park, K.
Kim, N.-G. Park, C. Kim, A highly efficient organic sensitizer for dye-sensitized
solar cells, Chem. Commun. (2007) 4887–4889.
[29] M. Adachi, M. Sakamoto, J. Jiu, Y. Ogata, S. Isoda, Determination of parame-
ters of electron transport in dye-sensitized solar cells using electrochemical
impedance spectroscopy, J. Phys. Chem. B 110 (2006) 13872–13880.
[30] Q. Wang, J. Moser, M. Grätzel, Electrochemical impedance spectroscopic anal-
ysis of dye-sensitized solar cells, J. Phys. Chem. B 109 (2005) 14945–14953.