New bithiazole-functionalized organic photosensitizers for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2012.10.002

Five bithiazole-based organic dyes D₁-D₅ containing different electron donor moieties (thiophene, uorene, carbazole, and triarylamine) in the molecular frameworks were synthesized, characterized and applied in dye-sensitized solar cells (DSSCs). The effects of electron-donating moieties of the organic dyes on their photophysical, electrochemical, and photovoltaic properties have been investigated in detail. These dyes exhibit strong charge transfer absorption bands in the visible region. Their redox potential levels were estimated by cyclic voltammetry and found to match well with the charge flow in DSSCs. The combination of broad absorption bands with fairly high extinction coefficients and appropriate redox properties makes these bithiazole-based molecules promising dyes for DSSCs. For solar cell device based on D₄, the maximal monochromatic incident photon-to-current conversion efficiency (IPCE) can reach up to 68.5%, with a short-circuit photocurrent density (J_sc) of 9.61 mA cm^-2, an open-circuit photovoltage (V_oc) of 0.70 V and a fill factor (FF) of 0.70, which results in a power conversion efficiency (PCE) of 4.65% under illumination of an AM 1.5 solar cell simulator.

Full text of DOI:10.1016/j.dyepig.2012.10.002

Dyes and Pigments 96 (2013) 516e524
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New bithiazole-functionalized organic photosensitizers for dye-sensitized solar
cells
,
b
c
,
a
d
Lai-Fan Lai a, Cheuk-Lam Ho a , Yung-Chung Chen , Wen-Jun Wu , Feng-Rong Dai , Chung-Hin Chui ,
*
*
Shu-Ping Huang e, Kun-Peng Guo f, Jiann-T’suen Lin b , He Tian , Shi-He Yang , Wai-Yeung Wong
,
c
f
a,
*
*
a Institute of Molecular Functional Materials, Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong,
PR China
b Institute of Chemistry, Academia Sinica, 128 Academia Road, Section 2, Nankang, Taipei 115, Taiwan
c Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Meilong Road 130, Shanghai, 200237, PR China
d Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, PR China
e Institute for Materials Chemistry and Engineering, and International Research Center for Molecular Systems, Kyushu University, Fukuoka 819-0395, Japan
f Department of Chemistry, The Hong Kong University of Science and Technology, Clearwater Bay, Hong Kong, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Five bithiazole-based organic dyes D1eD5 containing different electron donor moieties (thiophene, flu-
orene, carbazole, and triarylamine) in the molecular frameworks were synthesized, characterized and
applied in dye-sensitized solar cells (DSSCs). The effects of electron-donating moieties of the organic
dyes on their photophysical, electrochemical, and photovoltaic properties have been investigated in
detail. These dyes exhibit strong charge transfer absorption bands in the visible region. Their redox
potential levels were estimated by cyclic voltammetry and found to match well with the charge flow in
DSSCs. The combination of broad absorption bands with fairly high extinction coefficients and appro-
priate redox properties makes these bithiazole-based molecules promising dyes for DSSCs. For solar cell
device based on D4, the maximal monochromatic incident photon-to-current conversion efficiency
(IPCE) can reach up to 68.5%, with a short-circuit photocurrent density (Jsc) of 9.61 mA cmꢀ2, an open-
circuit photovoltage (Voc) of 0.70 V and a fill factor (FF) of 0.70, which results in a power conversion
efficiency (PCE) of 4.65% under illumination of an AM 1.5 solar cell simulator.
Received 22 August 2012
Accepted 3 October 2012
Available online 22 October 2012
Keywords:
Bithiazole
Photosensitizer
Organic dye
Dye-sensitized solar cell
Synthesis
Carbazole
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
and black dye have achieved solar-to-electricity conversion effi-
ciency higher than 10% under AM 1.5 simulator, a value close to an
The most abundant and clean source of energy on earth is solar
energy to date. The solar energy reaching the earth in 1 h is almost
equivalent to the current global annual energy consumption. To
replace the use of fossil fuels and petroleum, the conversion of
sunlight energy into electrical energy is, therefore, seen as one of
the most promising solutions for the energy crisis problem. The
energy of sunlight can be directly converted into electricity by the
photovoltaic effect using a solar cell electrical device [1]. Inorganic
semiconductor solar cells, especially silicon in various forms, have
been developed over several decades and have important appli-
cations [2]. However, silicon materials are not cost effective. In
recent years, Ru(II)-polypyridyl photosensitizers such as N719, N3,
amorphous silicon-based photovoltaic cell [3e6]. As compared to
metal-based Ru(II) dyes, organic compounds can also be used in the
manufacture of dye-sensitized solar cells (DSSCs) [7]. Organic
DSSCs are a low-cost alternative of our renewable energy sources in
the past decades because they have potential to get high power
conversion efficiency (PCE) and their devices are easy to fabricate
[8]. Their high molar extinction coefficients, ease of purification and
flexible structural modifications render them attractive light-
harvesting materials. Recently, Wang and co-workers reported an
impressive PCE of 9.8% with excellent stability for a DSSC based on
a metal-free sensitizer [9]. The performance of DSSCs based on
other metal-free organic dyes has also been remarkably improved
recently by several groups [10]. These researchers have applied
various approaches to tune the HOMO and LUMO energy levels of
the chromophores, where the donor, linker or acceptor moieties
were altered independently. These results suggest that smartly
designed metal-free organic dyes are also highly competitive
candidates as photosensitizers for DSSCs.
* Corresponding authors.
E-mail addresses: clamho@hkbu.edu.hk (C.-L. Ho), wjwu@ecust.edu.cn
(W.-J. Wu), jtlin@chem.sinica.edu.tw (J.-T. Lin), rwywong@hkbu.edu.hk
(W.-Y. Wong).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.10.002

L.-F. Lai et al. / Dyes and Pigments 96 (2013) 516e524
517
The thiazole unit is one of the strongest electron-accepting aza-
heterocycles because it contains one electron-withdrawing
nitrogen atom of imine (C]N) in place of the carbon atom at the
desorption ionization time of flight (MALDI-TOF) mass spectra were
recorded on an Autoflex Bruker MALDI-TOF system. NMR spectra
were measured in CDCl3 or d6-DMSO on a Bruker AM 400 MHz FT-
NMR spectrometer, and chemical shifts were quoted relative to
tetramethylsilane for 1H and 13C nuclei. UVeVis spectra were ob-
tained on an HP-8453 diode array spectrophotometer.
3-position of thiophene. Therefore, a number of
p-conjugated
polymers incorporating bithiazole moieties have been demon-
strated to be promising as new n-type transporting materials, and
have also been used as an acceptor unit for photovoltaic application
[11e13]. Therefore, bithiazole-based materials are conceived to be
attractive photovoltaic materials. However, organic dyes containing
the bithiazole moiety are still not very well studied. On the other
hand, triarylamine-based dyes have been widely used because the
arylamine groups can increase the electron density of donor moiety
and hence improve the dye performance effectively [14]. Also,
compounds involving fluorene and carbazole moieties in the main
chain have been extensively reported and used in photovoltaics
research [15]. Introduction of a thiophene moiety normally allows
a red-shift of the absorption maximum and broadens the spectral
profile of absorption of a particular dye.
2.2. General procedure for the synthesis of compounds 10 and 12e15
Compounds 10 and 12e15 were synthesized by a similar
procedure based on Suzuki coupling reaction between different
arylboronic acid and brominated bithiazole derivatives. The
synthetic procedure for 10 will be described below as an example.
2.2.1. 5-(4,40-Dinonyl-2,20-bithiazol-5-yl)thiophene-2-
carbaldehyde (10)
In a 50 mL round bottom flask, 5-bromo-4,40-dinonyl-2,20-
bithiazole (858 mg, 1.71 mmol) in tetrahydrofuran (40 mL) was
added to a mixture of 5-formylthiophen-2-yl-2-boronic acid (401 g,
2.58 mmol), Pd(PPh3)4 (136 mg, 0.17 mmol) and 2 M Na2CO3
(3.4 mL, 6.89 mmol). The mixture was heated at 80 ꢁC for two days.
Afterwards, water was added and the solution mixture was
extracted with ethyl acetate. The combined organic layer was dried
by Na2SO4 and filtered. The mixture was then concentrated under
reduced pressure. The residue was purified by column chroma-
tography using CH2Cl2 as eluent to get compound 10 as a yellow
Based on the above considerations, five new organic bithiazole-
bridged dye photosensitizers D1eD5 were designed and synthe-
sized with thiophene, triarylamine, carbazole and fluorene units as
the electron donor, bithiazole as a p-conjugated bridge, and a cya-
noacrylic acid moiety as the electron-withdrawing and anchoring
group (Chart 1). The five new dyes have been applied to the
sensitization of nanocrystalline TiO2-based solar cells and the cor-
responding photovoltaic properties, electronic and optical proper-
ties are also presented in this contribution.
solid (445 mg, 0.84 mmol, 49%). 1H NMR (400 MHz, CDCl3):
d
¼ 9.91
(s, 1H, CHO), 7.74e7.73 (d, J ¼ 4.0 Hz, 1H, Ar), 7.27e7.26 (d,
J ¼ 2.8 Hz, 1H, Ar), 7.01 (s, 1H, Ar), 3.00e2.96 (t, J ¼ 7.7 Hz, 2H,
nonyl), 2.83e2.79 (t, J ¼ 7.6 Hz, 2H, nonyl), 1.81e1.72 (m, 4H, nonyl),
1.42e1.31 (m, 24H, nonyl), 0.89e0.85 (m, 6H, nonyl). 13C NMR
2. Experimental
2.1. Materials and reagents
(100 MHz, CDCl3):
d
¼ 182.65 (CHO), 159.99, 159.70, 159.67, 156.65,
143.29, 142.99, 136.80, 127.82,126.18,115.57 (Ar), 37.12, 31.92, 31.56,
30.81, 29.74, 29.57, 29.52, 29.46, 29.35, 29.34, 29.28, 29.25, 27.13,
22.72, 19.77, 14.18 (nonyl). FAB-MS (m/z): 531.3 [M]þ. C29H42N2OS3
(530.85): calcd. C 65.62, H 7.97, N 5.28; found C 65.34, H 8.10, N 5.35.
All the reactions were performed under a nitrogen atmosphere
by using standard Schlenk techniques. Solvents were dried by
standard methods and distilled prior to use. The preparations of
precursor compounds 1e9 are given in the Supporting information
and other chemicals are commercially available and used as
received. Fast atom bombardment (FAB) mass spectra were recor-
ded on a Finnigan MAT SSQ710 system. The matrix-assisted laser
2.2.2. 5-(50-Bromo-4,40-dinonyl-2,20-bithiazol-5-yl)thiophene-2-
carbaldehyde (11)
In the absence of light, NBS (108 mg, 0.067 mmol) was added
dropwise to a solution of 5-(4,40-dinonyl-2,20-bithiazol-5-yl)thio-
phene-2-carbaldehyde (307 g, 0.58 mmol) in a mixture of acetic
acid (30 mL) and chloroform (30 mL) and the entire mixture was
stirred overnight. Then, it was concentrated under reduced pres-
sure and purified by silica gel column using CH2Cl2/n-hexane (2:1,
v/v) as eluent to afford compound 11 as a yellow solid (195 mg,
COOH
COOH
NC
S
NC
S
C9H19
C9H19
N
S
S
N
S
S
C4H9
N
0.32 mmol, 55%). 1H NMR (400 MHz, CDCl3):
d
¼ 9.91 (s, 1H, CHO),
N
C9H19
NC
C9H19
N
D1
7.74e7.73 (d, J ¼ 4.0 Hz, 1H, Ar), 7.27 (s, 1H, Ar), 2.98e2.95 (t,
J ¼ 7.7 Hz, 2H, nonyl), 2.79e2.75 (t, J ¼ 7.6 Hz, 2H, nonyl), 1.80e1.70
(m, 4H, nonyl), 1.43e1.31 (m, 24H, nonyl), 0.89e0.86 (m, 6H, nonyl).
D4
COOH
COOH
NC
13C NMR (100 MHz, CDCl3):
d
¼ 182.64 (CHO),159.75,158.94,157.74,
S
S
C9H19
C4H9
156.73, 143.46, 142.73, 136.76, 127.93, 126.45 (Ar), 31.92, 31.62,
31.47, 30.71, 30.20, 29.55, 29.45, 29.40, 29.34, 29.20, 28.80, 22.74,
14.18 (nonyl). FAB-MS (m/z): 608.1 [M]þ. C29H41N2BrOS3 (609.74):
calcd. C 57.13, H 6.78, N 4.59; found C 57.05, H 6.92, N 4.68.
N
S
S
C9H19
N
S
N
C9H19
S
N
C4H9
C9H19
S
D2
COOH
D5
NC
2.2.3. 5-(4,40-Dinonyl-50-(thiophen-2-yl)-2,20-bithiazol-5-yl)
thiophene-2-carbaldehyde (12)
S
C9H19
N
S
Yield: 81%; deep yellow solid. 1H NMR (400 MHz, CDCl3):
S
N
d
¼ 9.79 (s, 1H, CHO), 7.61e7.60 (d, J ¼ 3.9 Hz, 1H, Ar), 7.29e7.27 (m,
C9H19
1H, Ar), 7.15e7.14 (d, J ¼ 4.0 Hz, 1H, Ar), 7.10e7.08 (m, 1H, Ar), 7.00e
6.98 (m,1H, Ar), 2.89e2.81 (m, 4H, nonyl),1.77e1.64 (m, 4H, nonyl),
1.35e1.20 (m, 24H, nonyl), 0.83e0.73 (m, 6H, nonyl). 13C NMR
N
D3
(100 MHz, CDCl3):
143.24, 142.93, 136.71, 132.74, 128.42, 127.80, 127.72, 127.55, 126.68,
d
¼ 182.49 (CHO), 159.36, 156.97, 156.71, 154.75,
Chart 1. The chemical structures of organic dyes D1–D5 in this study.

518
L.-F. Lai et al. / Dyes and Pigments 96 (2013) 516e524
126.34 (Ar), 31.92, 30.83, 30.34, 29.57, 29.56, 29.52, 29.50, 29.45,
29.33, 29.21 (nonyl). FAB-MS (m/z): 612.2 [M]þ. C33H44N2OS4
(612.97): calcd. C 64.66, H 7.24, N 4.57; found C 64.80, H 7.43, N
4.55.
was purified by chromatography on a silica gel column using
CH2Cl2/MeOH (1:1, v/v) as eluent. The mixture was then washed
with methanol to give D1 as a red solid (82 mg, 0.14 mmol, 72%). 1H
NMR (400 MHz, CDCl3):
d
¼ 8.81 (s, 1H, COOH), 8.26 (s, 1H, C]CH),
7.52 (s, 1H, Ar), 6.99 (s, 1H, Ar), 6.86 (s, 1H, Ar), 3.34 (s, 2H, nonyl),
2.74 (m, 2H, nonyl), 1.68e1.67 (m, 4H, nonyl), 1.33e1.20 (m, 24H,
nonyl), 0.88e0.81 (m, 6H, nonyl). MALDI-TOF (m/z): 598.1 [M]þ.
C32H43N3O2S3 (597.89): calcd. C 64.28, H 7.25, N 7.03; found C 64.40,
H 7.32, N 7.20.
2.2.4. 5-(4,40-Dinonyl-50-(4-(phenyl(p-tolyl)amino)phenyl)-2,20-
bithiazol-5-yl)thiophene-2-carbaldehyde (13)
Yield: 81%; orange solid. 1H NMR (400 MHz, CDCl3):
d
¼ 9.90 (s,
1H, CHO), 7.73e7.72 (d, J ¼ 4.0 Hz, 1H, Ar), 7.29e7.25 (m, 5H, Ar),
7.15e7.11 (m, 4H, Ar), 7.07e7.05 (m, 5H, Ar), 3.00e2.97 (t, J ¼ 7.7 Hz,
2H, nonyl), 2.84e2.80 (t, J ¼ 7.6 Hz, 2H, nonyl), 2.34 (s, 3H, Ar-CH3),
1.80e1.76 (m, 4H, nonyl), 1.42e1.22 (m, 24H, nonyl), 0.88e0.84 (m,
2.3.2. D2
Yield: 68%; deep red solid. 1H NMR (400 MHz, d6-DMSO):
6H, nonyl). 13C NMR (100 MHz, CDCl3):
d
¼ 182.58 (CHO), 160.01,
d
¼ 8.03 (s, 1H, C]CH), 7.75e7.73 (m, 2H, Ar), 7.70e7.69 (d,
156.74, 156.68, 153.85, 148.21, 147.32, 144.57, 143.19, 143.17, 136.76,
135.47, 133.73, 130.16, 129.92, 129.36, 127.69, 125.90, 125.67, 124.64,
124.07, 123.28, 121.83 (Ar) 53.43 (AreCH3) 31.91, 31.89, 30.94, 30.81,
29.84, 29.78, 29.70, 29.56, 29.53, 29.48, 29.44, 29.33, 29.31, 29.25,
20.90 (nonyl). FAB-MS (m/z): 787.3 [M]þ. C48H57N3OS3 (788.18):
calcd. C 73.15, H 7.29, N 5.33; found C 73.34, H 7.10, N 5.41.
J ¼ 4.0 Hz, 1H, Ar), 7.46e7.45 (d, J ¼ 3.9 Hz, 1H, Ar), 7.40e7.39 (m,
1H, Ar), 2.97e2.88 (m, 4H, nonyl), 1.71e1.70 (m, 4H, nonyl), 1.34e
1.21 (m, 24H, nonyl), 0.83e0.80 (m, 6H, nonyl). MALDI-TOF (m/z):
680.4 [M]þ. C36H45N3O2S4 (680.01): calcd. C 63.59, H 6.67, N 6.18;
found C 63.69, H 6.78, N 6.22.
2.3.3. D3
2.2.5. 5-(50-(9-Butylcarbazol-3-yl)-4,40-dinonyl-2,20-bithiazol-5-yl)
Yield: 65%; dark red solid. 1H NMR (400 MHz, d6-DMSO):
thiophene-2-carbaldehyde (14)
d
¼ 8.05 (s, 1H, C]CH), 7.71e7.69 (d, J ¼ 4.0 Hz, 1H, Ar), 7.45e7.30
Yield: 80%; orange solid. 1H NMR (400 MHz, CDCl3):
d
¼ 9.90 (s,
(m, 5H, Ar), 7.18e7.16 (d, J ¼ 8.2 Hz, 2H, Ar), 7.10e6.99 (m, 5H, Ar),
6.95e6.93 (m, 2H, Ar), 2.96e2.91 (m, 2H, nonyl), 2.78e2.75 (m, 2H,
nonyl), 2.28 (s, 3H, Ar-CH3), 1.72e1.60 (m, 4H, nonyl), 1.36e1.19 (m,
24H, nonyl), 0.85e0.79 (m, 6H, nonyl). MALDI-TOF (m/z): 854.5
[M]þ. C51H58N4O2S3 (855.23): calcd. C 71.63, H 6.84, N 6.55; found C
71.76, H 6.91, N 6.63.
1H, CHO), 8.17 (d, J ¼ 1.5 Hz,1H, Ar), 8.10e8.08 (d, J ¼ 7.7 Hz,1H, Ar),
7.72e7.71 (d, J ¼ 4.0 Hz, 1H, Ar), 7.56e7.42 (m, 4H, Ar), 7.27e7.25
(m, 2H, Ar), 4.34e4.31 (t, J ¼ 7.2 Hz, 2H, butyl), 3.02e2.98 (t,
J ¼ 7.7 Hz, 2H, nonyl), 2.92e2.88 (t, J ¼ 7.6 Hz, 2H, nonyl), 1.90e1.80
(m, 6H, nonyl and butyl), 1.46e1.21 (m, 24H, nonyl), 0.99e0.95 (t,
J ¼ 14.7 Hz, 3H, butyl), 0.89e0.82 (m, 8H, nonyl and butyl). 13C NMR
(100 MHz, CDCl3):
d
¼ 182.59 (CHO), 160.13, 156.98, 156.68, 153.86,
2.3.4. D4
143.26, 143.13, 140.91, 140.20, 136.78, 136.66, 127.66, 127.03, 126.24,
125.83, 123.16, 122.50, 121.80, 121.36, 120.47, 119.31, 109.01, 108.95
(Ar) 43.04, 31.90, 31.60, 31.16, 30.85, 29.94, 29.67, 29.58, 29.55,
29.52, 29.46, 29.43, 29.32, 29.27, 20.60, 14.12, 13.90 (nonyl and
butyl). FAB-MS (m/z): 751.3 [M]þ. C45H57N3OS3 (752.15): calcd. C
71.86, H 7.64, N 5.59; found C 71.78, H 7.88, N 5.70.
Yield: 73%; deep red solid. 1H NMR (400 MHz, d6-DMSO):
¼ 8.32 (s, 1H, C]CH), 8.22e8.20 (d, J ¼ 7.7 Hz, 1H, Ar), 8.04 (s, 1H,
d
Ar), 7.72e7.45 (m, 6H, Ar), 7.24e7.20 (t, J ¼ 7.4 Hz, 1H, Ar), 4.44e
4.40 (m, 2H, butyl), 2.97e2.84 (t, 4H, nonyl), 1.76e1.70 (m, 6H,
nonyl and butyl), 1.34e1.09 (m, 24H, nonyl), 0.90e0.72 (m, 11H,
nonyl and butyl). MALDI-TOF (m/z): 819.5 [M]þ. C48H58N4O2S3
(819.19): calcd. C 70.38, H 7.14, N 6.84; found C 70.32, H 7.20, N 7.01.
2.2.6. 5-(50-(9,9-Dibutylfluoren-2-yl)-4,40-dinonyl-2,20-bithiazol-5-
yl)thiophene-2-carbaldehyde (15)
2.3.5. D5
Yield: 83%; orange solid. 1H NMR (400 MHz, CDCl3):
d
¼ 9.92 (s,
Yield: 70%; deep orange solid. 1H NMR (400 MHz, d6-DMSO):
¼ 8.06 (s, 1H, C]CH), 7.93e7.86 (m, 2H, Ar), 7.73e7.72 (m, 1H, Ar),
1H, CHO), 7.77e7.73 (m, 3H, Ar), 7.45e7.43 (m, 2H, Ar), 7.37e7.35
(m, 3H, Ar), 7.29e7.26 (m, 1H, Ar), 3.03e2.99 (m, 2H, nonyl),
2.90e2.86 (m, 2H, nonyl), 2.02e1.98 (m, 4H, butyl), 1.82e1.80 (m,
4H, butyl), 1.27e1.24 (m, 30H, nonyl and butyl), 1.11e1.09 (m, 4H,
butyl), 0.69e0.67 (m, 10H, nonyl and butyl). 13C NMR (100 MHz,
d
7.59 (s, 1H, Ar), 7.51e7.48 (m, 3H, Ar), 7.37e7.36 (m, 2H, Ar), 3.03e
2.97 (m, 10H, nonyl and butyl), 2.06e2.02 (m, 4H, nonyl and butyl),
1.63e1.54 (m, 10H, nonyl and butyl), 1.15e1.01 (m, 20H, nonyl and
butyl), 0.64e0.50 (m, 12H, nonyl and butyl); MALDI-TOF (m/z):
874.6 [M]þ. C53H67N3O2S3 (874.31): calcd. C 72.81, H 7.72, N 4.81;
found C 72.90, H 7.97, N 4.90.
CDCl3):
d
¼ 182.61 (CHO), 159.92, 157.33, 156.76, 154.39, 151.34,
150.96, 143.26, 143.14, 141.50, 140.29, 136.78, 136.04, 130.03, 128.21,
127.76, 127.61, 126.98, 126.07, 123.69, 122.97, 122.01, 119.98 (Ar)
55.14, 40.18, 31.91, 30.86, 29.96, 29.58, 29.55, 29.53, 29.52, 29.47,
29.32, 26.03, 23.08, 13.86 (nonyl and butyl). FAB-MS (m/z): 806.4
[M]þ. C50H66N2OS3 (807.27): calcd. C 74.39, H 8.24, N 3.47; found C
74.55, H 8.39, N 3.62.
2.4. Computational methods
All calculations were performed using the Gaussian 03 program
package [16]. DFT at the gradient-corrected correlation functional
level PBE1PBE was used to optimize the ground state geometries of
D1eD5. The PBE1PBE functional gave the best results in the simu-
lations of UVeVis absorption properties by comparison of different
functionals (B3LYP, BHandHLYP, and MPWB1K) [17]. On the basis of
the ground-state optimized geometries, 50 singlet excited states
were calculated to determine the vertical excitation energies using
the TDDFT method at the PBE1PBE level. The 6e31G (d, p) effective
core potential basis set was applied for all atoms.
2.3. Synthesis of organic dyes D1eD5
Dyes D1eD5 were synthesized by a similar procedure following
the reaction between the carbaldehyde derivatives and cyanoacetic
acid in the presence of piperidine. A typical procedure is given for D1.
2.3.1. D1
5-(4,40-Dinonyl-2,20-bithiazol-5-yl)thiophene-2-carbaldehyde
(100 mg, 0.19 mmol) in 4 mL of dry chloroform, cyanoacetic acid
(32.1 mg, 0.38 mmol) and piperidine (0.1 mL) were added. The
mixture was heated at reflux temperature overnight. Afterwards,
the mixture was concentrated under reduced pressure. The residue
2.5. Assembly and characterization of DSSCs
The TiO2 nanoparticles and reference compound N719 were
purchased from Solaronix, S.A., Switzerland. The photoanode used

L.-F. Lai et al. / Dyes and Pigments 96 (2013) 516e524
519
C9H19
was the TiO2 thin film (12
layer and 6
mm of 20 nm particles as the absorbing
N
S
S
O
O
ii
i
m
m of 400 nm particles as the scattering layer) coated
C9H19 CH2Br
N
C9H19 CH3
C9H19
on FTO glass substrate with a dimension of 0.5 ꢂ 0.5 cm2 [18]. The
film thickness was measured by a profilometer (Dektak3, Veeco/
Sloan Instruments Inc., USA). A platinized FTO produced by ther-
mopyrolysis of H2PtCl6 was used as a counter electrode. The TiO2
8
7
C9H19
Br
N
S
S
(HO)2B
CHO
S
2
iii
thin film was dipped into the THF solution containing 3 ꢂ 10ꢀ4
M
N
C9H19
9
iv
dye sensitizers for at least 12 h. After rinsing with THF, the pho-
COOH
toanode adhered with a polyester tape of 60 mm in thickness and
CHO
with a square aperture of 0.36 cm2 was placed on top of the counter
electrode and they were tightly clipped together to form a cell.
Electrolyte was then injected into the space and then the cell was
sealed with the Torr Seal cement (Varian, MA, USA). The electrolyte
was composed of 0.5 M lithium iodide (LiI), 0.05 M iodine (I2), and
0.5 M 4-tert-butylpyridine dissolved in acetonitrile.
NC
S
C9H19
N
S
S
S
v
C9H19
N
S
N
C9H19
S
N
10
C9H19
D1
Scheme 2. Synthesis of D1: (i) Br2, methanol; (ii) dithiooxamide, ethanol; (iii) NBS,
DMF; (iv) Pd(PPh3)4, Na2CO3 (2 M, aq), THF; (v) cyanoacetic acid, piperidine, dry CHCl3.
The photoelectrochemical characterizations on the solar cells
were carried out using an Oriel Class A solar simulator (Oriel
91195A, Newport Corp.). Photocurrent-voltage characteristics of
the DSSCs were recorded with a potentiostat/galvanostat (CHI650B,
CH Instruments, Inc.) at a light intensity of 100 mW cmꢀ2 calibrated
by an Oriel reference solar cell (Oriel 91150, Newport Corp.). The
monochromatic quantum efficiency was recorded through
a monochromator (Oriel 74100, Newport Corp.) at short-circuit
condition. The intensity of each wavelength was in the range of
1e3 mW cmꢀ2. Electrochemical impedance spectra were recorded
for DSSCs under illumination at open-circuit voltage (Voc) at room
temperature. The frequencies explored range from 10 mHz to
100 kHz.
obtained by Suzuki coupling reaction between
9 and 5-
formylthiophen-2-yl-2-boronic acid 2. Knoevenagel condensation
of the aldehyde 10 and cyanoacetic acid in the presence of piperi-
dine gave the desired dye D1. For the donor-substituted dyes D2e
D5, bromination of compound 11 gives the corresponding
bromide which reacts with an appropriate arylboronic acid via
Suzuki coupling to afford intermediates 12e15. The syntheses of
the dyes were then completed by the same procedure as that for D1
and they were isolated in moderate yields. The structures of all the
dyes were verified by NMR spectroscopy and MALDI-TOF mass
spectrometry. The dyes are orange or orange-red in color and are
readily soluble in common organic solvents such as THF, CH2Cl2,
CHCl3 and DMSO.
3. Results and discussion
3.1. Materials synthesis
3.2. Photophysical properties
All of the new dyes have been synthesized according to some
classical organic transformations and the synthetic routes are
outlined in Schemes 1e3. The dyes are composed of an aromatic
donor moiety, a conjugated bridge of a bithiazole unit and 1-
cyanoacrylic acid as the electron acceptor and anchoring group to
the TiO2. The key material 4,40-dinonyl-2,20-bithiazole 8 was ob-
tained firstly by bromination of 2-undecanone with bromine to
form 1-bromo-undecan-2-one, which then underwent cycloaddi-
tion with dithiooxamide via the Hantzsch thiazole synthesis [19].
Subsequently, this compound was monobrominated by N-bromo-
succinimide (NBS) to give 9. Then the key intermediate 10 was
The UVeVis absorption spectra of the sensitizers D1eD5 in THF
at room temperature are displayed in Fig. 1 and the data are
collected in Table 1. All of the dyes give two distinct absorption
bands: one relatively weak band in the near UV region at around
270e300 nm can be attributed to the pep* transition and the other
stronger band at around 400e450 nm is caused by the intra-
molecular charge transfer (ICT) between the different donors and
the cyanoacrylic acid anchoring moiety, producing an efficient
charge separated state [19,20]. The spectra of all the dyes are quite
similar, but those with stronger donor group D2eD5 are red-shifted
O
ii
i
CHO
(HO)2B
CHO
S
S
S
O
1
2
C4H9
C4H9
N
C4H9
N
N
iii
iv
B(OH)2
4
3
Br
C4H9
C4H9
C4H9
C4H9
v
vi
+ C H Br
Br
Br
B(OH)2
4
9
5
6
Scheme 1. Synthetic routes for various boronic acid derivatives: (i) p-toluenesulfonic acid, glycol, benzene; (ii) n-BuLi, B(OMe)3, HCl; (iii) NBS, CHCl3; (iv) n-BuLi, B(OMe)3; (v)
NaOH, DMSO; (vi) n-BuLi, B(OMe)3.

520
L.-F. Lai et al. / Dyes and Pigments 96 (2013) 516e524
Table 1
CHO
CHO
Absorption and PL data of D1eD5 in THF at 298 K.
S
S
C9H19
ii
C9H19
Br
N
S
S
i
Dye
lmax in
THF (nm)
Molar extinction
lmax on TiO2
film (nm)
lem in
THF (nm)
N
S
S
coefficients (Mꢀ1 cmꢀ1
)
Ar(OH)2
N
10
N
C9H19
C9H19
D1
D2
D3
D4
D5
263, 413
289, 425
301, 432
265, 296, 428
264, 296, 427
1590, 5140
3490, 13,450
17,190, 25,820
19,817, 26,850, 36,788
8560, 8030, 28,560
399
406
416
420
415
502
534
560
539
530
11
COOH
CHO
NC
S
S
iii
C9H19
Ar
N
S
C9H19
Ar
N
S
S
S
N
C9H19
N
coefficients of D3eD5 are even higher than N3 dye
(1.6 ꢂ 104 Mꢀ1 cmꢀ1) [4]. The higher molar extinction coefficients of
the organic dyes allow a correspondingly thinner nanocrystalline
film so as to avoid the decrease of the film mechanical strength.
This also benefits the electrolyte diffusion in the film and reduces
the recombination possibility of the light-induced charges during
transportation [22].
C9H19
12: Ar =
13: Ar =
D2: Ar =
S
S
Me
Me
N
Fig. 2 depicts the absorption spectra on TiO2 film and photo-
luminescence (PL) spectra in THF solution of the new dyes. Their
absorption spectra on TiO2 film are broadened due to the interac-
tion of the anchoring group of organic dyes with the TiO2 electrode
which was commonly observed for other organic dyes [23,24]. In
addition, all of the dyes exhibit a broadened absorption spectrum
and a slight blue shift of 14 nm (D1), 19 nm (D2), 16 nm (D3), 8 nm
(D4) and 14 nm (D5) in wavelength as compared to that measured in
THF solution. This broadening favors the light harvesting of the
solar cells and thus increases the photocurrent response region
with a rise of short-circuit current density (Jsc) [23]. The blue shift is
due to the formation of H-aggregate on the TiO2 electrode [25].
Such a slight shift indicates that the aggregates of dyes are not
obvious which may be explained by the notion that the introduc-
tion of long alkyl chains of bithiazole unit inhibits the formation of
aggregates to some extent [26]. The dyes show emissions when
they are being excited. Similar to their UVeVis absorption spectra,
the most red-shifted peak is shown at around 560 nm for D3 which
has triarylamine as an electron-donating group while the most
blue-shifted peak is shown at around 502 nm by the sensitizer D1
without an electron-donating group (Table 1).
N
D3: Ar =
C4H9
C4H9
N
N
14: Ar =
15: Ar =
D4: Ar =
D5: Ar =
C4H9
C4H9
C4H9
C4H9
Scheme 3. Synthesis of D2eD5: (i) CHCl3/acetic acid, overnight; (ii) Pd(PPh3)4, Na2CO3
(2 M, aq), THF; (iii) cyanoacetic acid, piperidine, dry CHCl3.
in comparison to that of D1, which could be ascribed to the presence
of more electron-rich moieties to produce more extended
p-
conjugation system. The absorption peak maximum of the dye
ranks in the order: D3 (432 nm) > D4 (428 nm) z D5 (427 nm) > D2
(425 nm) > D1 (413 nm). The increase in conjugation length and the
increased electron density associated with the fluorenyl group in
D5, carbazolyl group in D4 and triarylamine group in D3 lead to
a larger bathochromic shift of the absorption maximum [21]. The
molar extinction coefficients of the ICT bands for D1eD5 were also
investigated in THF and increase in the order of
D4 > D5 > D3 > D2 > D1, which suggests that D4 should have the
best light-harvesting ability. Noticeably, the molar extinction
All of the dyes are photoluminescent at room temperature
(Table 1) and the fluorescence peak maximum ranges from 502 to
560 nm. Similar to the absorption spectra, the presence of electron-
releasing groups would shift the emission maximum to the red and
the most electron-donating triarylamine unit would give the most
red-shifted peak at 560 nm for D3.
3.3. Electrochemical properties
To investigate the electron transfer from the excited dye mole-
cule to the conductive band (Ecb) of TiO2, cyclic voltammetry was
performed for the dyes in thin films on glassy carbon electrode
measured in 0.1 M tetrabutylammonium hexafluorophosphate
solution in acetonitrile at a scan rate of 50 mV sꢀ1. Under these
conditions, the E1/2 of ferrocene was 0.07 V vs Ag/AgCl. The results
are summarized in Table 2. All of the dyes are redox stable and
showed quasi-reversible oxidation and reduction waves [27]. The
HOMO and LUMO energy levels of the dyes were measured from
the onset oxidation and reduction potentials. The negative shifts of
the Eox were observed for D2eD5 versus D1, leading to a decrease of
their energy gaps between the HOMO and LUMO as compared to
D1. That could be attributed to the increased electron density and
D
1.0
0.8
0.6
0.4
0.2
0.0
1
D
2
D
3
D
4
D
5
the larger extension of their p-conjugated systems than that of D1.
300
400
λ / nm
500
600
It is clear that the HOMO level of D3 is the highest among the dyes,
which is in agreement with the order of the electron-donating
ability of the triarylamine donor [21].
Fig. 1. Normalized absorption spectra of the new dyes measured in THF at 298 K.

L.-F. Lai et al. / Dyes and Pigments 96 (2013) 516e524
521
a
b
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
300
400
500
600
700
800
λ / nm
λ / nm
c
d
1.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
300
400
500
600
700
800
λ / nm
λ/ nm
e
1.0
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
300
400
500
λ / nm
600
700
800
Fig. 2. Absorption spectra in THF (solid line) and on TiO2 film (dotted line), and PL spectra (dashed line) in THF of (a) D1, (b) D2, (c) D3, (d) D4 and (e) D5 at 298 K.
From Table 2, both the HOMO and LUMO levels of these dyes
match well with the energy requirement as efficient photosensi-
tizers. Since the reduction waves correspond to the reduction of the
anchoring cyanoacrylic acid group in each case, the LUMO levels of
D1eD5 only span a narrow range of ꢀ3.50 to ꢀ3.72 eV. These values
are much higher than the lower bound level of the conduction band
of TiO2 (ꢀ4.4 eV), indicating that the efficiency of charge injection
from the excited sensitizer molecule to the TiO2 conduction band is
viable [28]. The HOMO energy levels of the sensitizers are calcu-
lated to be ꢀ5.43 to ꢀ5.98 eV. These values are lower than that of
the Iꢀ/Iꢀ3 pair (ꢀ4.9 eV), ensuring effective sensitizer regeneration
process, so that the oxidized dyes could be regenerated by the
reduced part (Iꢀ) in the electrolyte to guarantee an efficient charge
separation in the photovoltaic devices [29,30].
3.4. Computational studies
Time-dependent density functional theory (TDDFT) calculations
were carried out for D1eD5 in order to gain better insights into the
electronic and spectroscopic properties as well as the nature of
absorption bands for these complexes. The density functional
theory (DFT) method at the gradient-corrected correlation func-
tional level PBE1PBE was used to optimize the ground state
geometries of D1eD5 [17]. On the basis of the ground-state opti-
mized geometry, 50 singlet excited states for D1eD5 were calcu-
lated at the PBE1PBE/6e31G (d, p) levels with the TDDFT approach.
As shown from the orbital profiles in Figs. S1eS5 (see
Supporting information), the HOMOs of D1eD5 are mainly located
Table 2
Electrochemical properties of D1eD5.
Dye
Eox/eVa
HOMO/eVb
Ered/eVa
LUMO/eVc
Eg/eV
D1
D2
D3
D4
D5
1.25
1.01
0.70
0.79
0.97
ꢀ5.98
ꢀ5.74
ꢀ5.43
ꢀ5.52
ꢀ5.70
ꢀ1.16
ꢀ1.21
ꢀ1.22
ꢀ1.23
ꢀ1.17
ꢀ3.57
ꢀ3.72
ꢀ3.51
ꢀ3.50
ꢀ3.56
2.41
2.02
1.92
2.02
2.14
a
Onset oxidation and reduction potentials.
HOMO ¼ ꢀe(Eox þ 4.73) (eV).
b
c
LUMO ¼ ꢀe(Ered þ 4.73) (eV).

522
L.-F. Lai et al. / Dyes and Pigments 96 (2013) 516e524
at the
p-conjugated thiazole ligands and the electron-donating
80
60
40
20
0
group. Especially for D4, the HOMO level is highly located at the
triarylamino unit. The LUMOs of D1eD5 have amplitudes mostly on
the anchoring 2-cyano-3-(5-ethynylthiophen-2-yl)acrylic acid unit.
As indicated from Table S1 (Supporting information), the calculated
lowest-energy absorption (from the HOMOþ1/LUMO transition
(98e100%)) can be primarily assigned to the ICT transition from the
electron-donating group to the electron-accepting 2-cyano-3-(5-
ethynylthiophen-2-yl)acrylic acid unit. It was revealed that the
N719
D
1
D
2
D
3
D
4
D
5
HOMO
/ LUMO excitation transferring electrons from the
electron-donating group to the anchoring ligand can facilitate
electron injection from the excited state of the sensitizer to TiO2.
The TDDFT calculations suggest that addition of the electron-
donating group, especially the triarylamino group, can elevate the
HOMO energy level (D4 (HOMO: ꢀ5.20 eV) > D5 (ꢀ5.74 eV) z D3
(ꢀ5.76 eV) > D2 (ꢀ5.88 eV) > D1 (ꢀ6.09 eV)), resulting in the
enhancement of the absorption ability of the corresponding
compound by reducing the energy gap for the HOMO / LUMO
transition.
400
500
600
700
800
λ / nm
Fig. 3. Photocurrent action spectra of the TiO2 electrode sensitized by D1eD5.
Conversely, by comparing D3 with D4 and D5, there is a dramatic fall
in both Jsc and Voc values in D3, which is ascribed to the lower IPCE
value of D3. The decreased IPCE values suggest that there is an
inefficient regeneration of the oxidized dye and accordingly a lower
Jsc (6.85 mA cmꢀ2) value was observed. These results agree well
with the corresponding IPCE spectra.
For the overall light to electricity conversion efficiency of D1eD5,
dye D4 exhibited the best
solar light condition, which reached 64% with respect to that of an
N719-based device fabricated under similar fabrication conditions.
3.5. Photovoltaic performance of DSSCs
DSSCs with an effective area of 0.25 cm2 were fabricated by
using our new compounds adsorbed on nanocrystalline TiO2 as the
sensitizers. The DSSCs with different dyes were measured under
AM 1.5 solar light condition. The losses of light reflection and
absorption by the conducting glass were not corrected. The device
performance statistics such as short-circuit current density (Jsc),
open-circuit voltage (Voc), fill factor (FF) and photovoltaic conver-
h of 4.65% under standard global AM 1.5
The lower
h is largely due to the lower Jsc of the sensitizer as
compared to N719.
sion efficiency (h) are summarized in Table 3.
Electrochemical impedance spectroscopy (EIS) is a powerful
technique of characterizing the important interfacial charge
transfer processes in a DSSC. The EIS spectra were measured in the
dark to elucidate correlation of Voc with charge transfer resistance
(Rct) of the devices. The EIS spectra of DSSCs based on D3, D4, D5 and
N719 under a forward bias of 0.055 V in the dark are shown in Fig. 5.
The middle-frequency semicircle in the EIS spectra represents the
charge transfer resistance between the TiO2 surface and the elec-
trolyte. Efficient suppression of the charge transfer resistance leads
to a larger semicircle. The charge transfer resistance is the largest
Fig. 3 shows the action spectra of incident photon-to-current
conversion efficiency (IPCE) for DSSCs using D1eD5. All of the dyes
can efficiently convert visible light to photocurrent in the region
from 400 nm to 600 nm. The IPCE performance of the DSSCs with D4
is higher than the others due to its best light-harvesting ability. Its
IPCE reaches a maximum 68.5% at 459 nm which is comparable to
that of N719 (70.0%) in the region from 400 to 530 nm, although the
IPCE of D4 is lower than N719 in the wavelength region of 530e
700 nm, indicating that D4-sensitized TiO2 electrode can generate
the highest conversion yield among all the dyes studied.
for N719 (648 U) and the smallest for D3 (133 U), indicating that the
charge recombination rate also decreases in the same order. The
results obtained here are roughly consistent with the trend
Fig. 4 shows the currentevoltage (JeV) characteristics of DSSCs
fabricated with these bithiazole-containing photosensitizing dyes
(D1eD5) under simulated sunlight illumination. Relative to D1, the
photovoltaic performance can be significantly improved by intro-
ducing an electron-donating group such as thiophene, triarylamine,
carbazole and fluorene in the molecule in D2eD5. 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. D4 has the highest light-harvesting
efficiency and consequently an improved Jsc (9.61 mA cmꢀ2) due
to the largest molar extinction coefficient among all the dyes. The
light-harvesting efficiency of D5 is lower than that of D4 and this
may be attributed to the smaller molar extinction coefficient.
observed for the Voc and
h
value. The Nyquist plots of DSSCs with
12
10
8
D1
D2
D3
D4
D5
6
Table 3
4
Photovoltaic performance of DSSCs with D1eD5 and N719 under the AM 1.5 solar
illumination.
Dye
Voc/mV
Jsc/mA cmꢀ2
FF
h
/%
2
D1
D2
D3
D4
D5
N719
605
583
625
695
655
745
2.57
6.49
6.85
9.61
7.08
15.3
0.76
0.68
0.70
0.70
0.67
0.64
1.18
2.58
2.98
4.65
3.12
7.29
0
0.0
0.2
0.4
0.6
0.8
Voltage / V
Fig. 4. JeV curves of the DSSC devices under AM 1.5G simulated sunlight illumination.

L.-F. Lai et al. / Dyes and Pigments 96 (2013) 516e524
523
a cyanoacrylic acid as the anchoring group, and a less common
bithiazole ring as the bridge were synthesized and used for the
fabrication of DSSCs. The performance of the photovoltaic devices
depend significantly on the nature and strength of the electron-
donating end group along the conjugated main. Among all of the
dyes examined, DSSCs based on D4 exhibited the best overall light
to electricity conversion efficiency of 4.65% under AM 1.5 irradia-
tion (100 mW cmꢀ2). All these results reveal that these metal-free
bithiazole-based organic dyes are promising molecular materials in
the development of high-performance DSSCs.
600
500
400
300
200
100
0
D3
D4
D5
N719
Acknowledgements
0
200
400
600
800
1000
1200
L.-F. Lai and C.-L. Ho contributed equally to this work. This
work was supported by the University Grants Committee Areas
of Excellence Scheme (AoE/P-03/08), Hong Kong Research Grants
Council (HKBU203011) and Hong Kong Baptist University (FRG2/
10-11/101). We also thank the National Natural Science Founda-
tion of China (NSFC) (project number: 21029001) for financial
support.
Z' (Ohm)
Fig. 5. Impedance plots of the DSSCs based on sensitizers D3, D4, D5 and N719
(measured at 55 mV in the dark).
20
D3
Appendix A. Supplementary data
D4
16
D5
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2012.10.002.
N719
12
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8
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D3, D4, D5 and N719 under light illumination are depicted in Fig. 6.
All the measurements were carried out with an irradiation of AM
1.5 solar simulator at Voc scanning the frequencies from 10 mHz to
100 kHz. Three semicircles were observed in the Nyquist plots. The
small and large semicircles, respectively located in the high- and
middle frequency regions, are assigned to the charge transfer at the
Pt/electrolyte and TiO2/dye/electrolyte interfaces, respectively [31].
Another small semicircle, which should have appeared at the low
frequency region, is overlapped by the middle frequency large
semicircle. The Rct value on the TiO2 surface, which is related to the
charge recombination rate between injected electron and electron
acceptor (Iꢀ3 ) in the electrolyte, is estimated by the large semicircle
width. A large Rct means the small charge recombination rate and
vice versa. As found in Fig. 6, the values of Rct of D3eD5 based DSSCs
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(33
U
for D3, 19
U
for D4 and 32
U
U
for D5, respectively) are larger
). This means that the D3eD5
than that in the case of N719 (16
ꢀ ꢁ
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