Porphyrins modified with a low-band-gap chromophore for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.orgel.2011.12.014

Two novel porphyrin dyes (PMBTZ and PHBTZ) modified with alkyl-thiophene and 2,1,3-benzothiadiazole (BTZ) moieties were designed and synthesized. The optical and electrochemical properties were characterized by UV-visible, fluorescence spectroscopy and cyclic voltammetry. With the introduction of the low-band-gap chromophore onto the porphyrins, the absorption spectra of the two porphyrin dyes in the range of 450-600 nm were broadened and the maximum wavelength was red-shifted compared with P_Zn as expected. The first oxidation potentials (E_ox1) were altered to the negative, which lowered from 1.27 to 1.11 and 1.15 eV, respectively. For a typical solar cell device based on dye PMBTZ, the maximal monochromatic incident photon-to-current conversion efficiency (IPCE) can reach to 65%, with a broad respondent region of 350-800 nm. Under standard global AM 1.5 solar condition, the dye-sensitized solar cell (DSSC) based on the dye PMBTZ showed the best photovoltaic performance: a short-circuit photocurrent density (J_sc) of 14.11 mA/cm², an open-circuit photo voltage (V_oc) of 0.59 V, and a fill factor (ff) of 0.66, corresponding to solar-to-electric power conversion efficiency (η) of 5.46%.

Full text of DOI:10.1016/j.orgel.2011.12.014

Organic Electronics 13 (2012) 560–569
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Porphyrins modified with a low-band-gap chromophore
for dye-sensitized solar cells
Weiping Zhou a, Zhencai Cao a, Shenghui Jiang a, Hongyan Huang a, Lijun Deng a,
b,
Yijiang Liu a, Ping Shen a, Bin Zhao a, Songting Tan a, , Xianxi Zhang
⇑
⇑
a College of Chemistry, Key Laboratory of Advanced Functional Polymeric Materials, Xiangtan University, Xiangtan 411105, PR China
b Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering,
Liaocheng University, Liaocheng 252059, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 October 2011
Received in revised form 16 December 2011
Accepted 28 December 2011
Available online 10 January 2012
Two novel porphyrin dyes (PMBTZ and PHBTZ) modified with alkyl-thiophene and 2,1,3-
benzothiadiazole (BTZ) moieties were designed and synthesized. The optical and electro-
chemical properties were characterized by UV–visible, fluorescence spectroscopy and cyc-
lic voltammetry. With the introduction of the low-band-gap chromophore onto the
porphyrins, the absorption spectra of the two porphyrin dyes in the range of 450–
600 nm were broadened and the maximum wavelength was red-shifted compared with
PZn as expected. The first oxidation potentials (Eox1) were altered to the negative, which
lowered from 1.27 to 1.11 and 1.15 eV, respectively. For a typical solar cell device based
on dye PMBTZ, the maximal monochromatic incident photon-to-current conversion effi-
ciency (IPCE) can reach to 65%, with a broad respondent region of 350–800 nm. Under stan-
dard global AM 1.5 solar condition, the dye-sensitized solar cell (DSSC) based on the dye
PMBTZ showed the best photovoltaic performance: a short-circuit photocurrent density
(Jsc) of 14.11 mA/cm2, an open-circuit photo voltage (Voc) of 0.59 V, and a fill factor (ff) of
Keywords:
Dye-sensitized solar cells
Low-band-gap chromophore
Photovoltaic performance
Porphyrins
0.66, corresponding to solar-to-electric power conversion efficiency (g) of 5.46%.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
But then, with a view to the limited ruthenium resource,
metal-free organic dyes have attracted considerable attention
in recent years owing to their excellent flexibility in terms of
molecular tailoring and high molar absorption coefficients
In recent years renewable energy sources are under
intense study due to global warming and increasing energy
demands. The solar-to-electric energy conversion is likely to
play a key role in replacing fossil fuels. And then, tremen-
dous research efforts have been devoted to dye-sensitized
solar cells (DSSCs) to seek cost-effective and resource-
unlimited photovoltaic devices [1]. To date, the Ru-polypyr-
idyl complexes utilized by Grätzel proved to be the most
efficient TiO2-sensitizers with the N3 dye showing an over-
all photovoltaic cell energy conversion efficiency of 11% [2].
Although this efficiency is half of the traditional silicon
based cells, it is still high enough for practical utility.
due to intramolecular p–
p⁄ transitions. In the past years, var-
ious metal-free dyes such as coumarin [3], indoline [4], pery-
lene [5], merocyanine [6], porphyrin [7], triarylamine [8],
and carbazole [9] have been reported, all of which have ob-
tained power conversion efficiency in the range of 5–9%.
Recently, porphyrins have been recognized as promising
components of molecular electronic and photonic devices
[10] because of their photochemical and electrochemical
stabilities, strong absorbing ability in the visible region
due to p–
p⁄ transitions of the conjugated macrocycle and
high molar extinction coefficients [11]. They show ultrafast
electron injection and slow charge recombination kinetics
which are indistinguishable to those of N3 dye [12], one of
the most efficient photosensitizers ever reported. However,
⇑
Corresponding authors.
E-mail addresses: tanst2008@163.com (S. Tan), zhangxianxi@lcu.e-
du.cn (X. Zhang).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.12.014

W. Zhou et al. / Organic Electronics 13 (2012) 560–569
561
the narrow light absorption spectra of porphyrins, which
2. Experimental details
poorly match to solar light distribution, resulted in inferior
performances as compared with ruthenium polypyridyl
complexes applied in DSSCs. A strategy to solve this prob-
lem is to make the strong and narrow Soret-band broad
and weak Q-bands strong by extension of the porphyrin
2.1. Materials and reagents
All starting materials were purchased from Pacific Chem-
Source and Alfa Aesar. DMF, HCCl3, MeCN and POCl3 were
dried and distilled by accustomed methods. All other sol-
vents and chemicals used in this work were analytical grade
and used without further purification. All chromatographic
separations were carried out on silica gel (200–300 mesh).
p
-system by modifying a b-position [13]. For example,
Seunghun et al. synthesized b,b0-quinoxalino porphyrins
containing carboxylic acid binding groups employed for
dye-sensitized solar cells, which showed broadened and
red-shifted light absorption with the aid of
p-extensions.
The DSSC based on porphyrin dye exhibited power conver-
sion efficiencies of 5.2% [14]. The Grätzel group and the Offi-
cer group showed that a combination of a conjugated
diethenyl linker at the b-pyrrolic position with a carboxyl-
ate binding group improves cell performance and gives a
cell efficiency of 7.1% [15]. Diau group and the Yeh group re-
ported novel porphyrin molecules with carboxylic acid
binding groups at meso-positions connected with a phenyl-
ethynyl bridge co-adsorbed with chenodioxycholic acid
(CDCA) on TiO2, giving an excellent cell efficiency of 11%
[16]. Theoretical calculations were also used in design and
screening of promising porphyrin sensitizer candidates,
and interpreting the performance differences of porphy-
rin-sensitized solar cells, which provided further assistance
to the experimental researches [17].
In our previous reports, a successful approach was intro-
duced by incorporating thiophene moieties into the organic
framework, which not only achieved high conversion effi-
ciency of 5.14% but also revealed that the linker between
donor and accepter could not be too long [11e]. To further
improve the photovoltaic performance of this system, an
enhanced spectral response of the sensitizer in the red
region is required. In this article, we designed and synthe-
sized two dyes based on porphyrins modified with thio-
phene and 2,1,3-benzothiadiazole (BTZ) moieties as
shown in Fig. 1, which were then applied in DSSCs. Density
functional theory (DFT) calculations of the dyes were also
performed to help interpreting the performances of corre-
sponding solar cells. The introduction of low-band-gap
chromophore in the bridging framework is presumed to
increase the spectral response in the red portion of the solar
spectrum [18]. The alkyl groups on the thiophenes were
aimed to reduce charge recombination and improve the
solubility of the dyes in solutions to some extent.
2.2. Analytical instruments
1H and 13C NMR spectra were recorded with a Bruker
Avance 400 instrument. Element analyzes were measured
by an Elementar Vario EL III element analyzer. UV–visible
spectra of the dyes were measured on a Perkin–Elmer
Lamada 25 spectrometer. The PL spectra were obtained
using Perkin–Elmer LS-50 luminescence spectrometer.
MALDI-TOF mass spectrometric measurements were per-
formed on Bruker Bifiex III MALDI-TOF. Electrochemical re-
dox potentials were obtained by cyclic voltammetry (CV)
using a three-electrode configuration and an electrochem-
istry workstation (ZAHNER ZENNIUM). The working elec-
trode was
a glassy carbon electrode; the auxiliary
electrode was a Pt electrode, and saturated calomel elec-
trode (SCE) was used as reference electrode. Tetrabutylam-
monium hexafluorophosphate 0.1 M was used as
supporting electrolyte in dry DMF. Ferrocene was added
to each sample solution at the end of the experiments,
and was used as an internal potential reference.
2.3. General procedure for preparation of porphyrin-modified
TiO2 electrode and photovoltaic measurements
Fluorine-doped SnO2 conducting glass (FTO) were
cleaned and immersed in aqueous 40 mM TiCl4 solution
at 70 °C for 30 min, then washed with water and ethanol,
sintered at 450 °C for 30 min. The 20–30 nm particles sized
TiO2 colloid blended with 0.1 wt.% magnesium acetate
solution was coated onto the FTO glass by sliding glass
rod method to obtain a TiO2 film of 10–15 lm thickness
after drying. The 200 nm particles sized TiO2 colloid was
coated on the electrode by the same method, resulting in
CH3
CH3
N
CH3
S
S
N
N
N
N
N
N
N
N
N
N
S
S
N
N
S
N
N
S
H3C
Zn
H3C
Zn
COOH
CN
Br
H3C
Zn
COOH
CN
N
CH3
CH3
C6H13
C6H13
PMBTZ
CH3
PHBTZ
CH3
PZn
CH3
Fig. 1. Molecular structures of porphyrin dyes and Pzn
.

562
W. Zhou et al. / Organic Electronics 13 (2012) 560–569
a TiO2 light-scattering layer of 4–6
l
m thickness. The TiO2
2.4.2. 4,7-bis-(4-Methylthiophen-2-yl)benzo[1,2,5]thiadiazole
(2-a)
4,7-Dibromo-2,1,3-benzothiadiazole (1 g, 3.4 mmol)and
electrodes were immersed into each of the 1 mM DMF solu-
tion of the porphyrins at room temperature. After dye
adsorption, the dye-coated electrodes were copiously rinsed
with ethanol. The amounts of the porphyrins adsorbed on
the TiO2 films were determined by measuring absorbance
at the Soret-band of the dye molecules that were dissolved
from the dye-adsorbed TiO2 films into DMF/H2O (4:1) con-
taining 0.1 M NaOH. The photovoltaic measurements were
performed in a sandwich cell consisting of the porphyrin-
sensitized TiO2 electrode as the working electrode and a Pt
foil as the counter electrode. The electrolyte consists of
0.5 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP) in
3-methoxypropionitrile and the efficient irradiated area of
cell was 0.196 cm2. The photocurrent–voltage (J–V) charac-
teristics were measured with a Keithley 2602 Source meter
under the 100 mW cmꢀ2 irradiation of a 500 W Xe lamp
with a global AM 1.5 filter for solar spectrum simulation.
1-a (2.85 g, 6.8 mmol), Pd(PPh3)4 (0.084 g, 0.075 mmol),
were dissolved in toluene (50 mL) and the mixture was re-
fluxed for 48 h. After evaporating the solvent under reduced
pressure, H2O (50 mL) and chloride (50 mL) were added. The
organic layer was separated and dried over anhydrous
MgSO4. The solvent was removed under reduced pressure.
The pure product 2-a was obtained as orange solid by col-
umn chromatography on silica gel (CH2Cl2/hexane = 1/8).
Yield: 0.86 g, 78%. 1H NMR (CDCl3, 400 MHz, d/ppm): 7.97
(s, 2H, thienyl-H), 7.84 (s, 2H, phenyl-H), 7.05 (s, 2H, thie-
nyl-H), 2.39 (s, 6H, –CH3).
2.4.3. [(3-Methylthiophen-2-yl)benzo[1,2,5]thiadiazol-4-yl]-
4-methylthiophene-2-carbaldehyde (3-a)
In a 100 mL three-necked flask, 2-a (2.1 g, 6.4 mmol),
DMF (0.6 mL, 7.7 mmol), 1,2-dichloroethane 50 mL were
added sequentially under Ar atmosphere and POCl3
(0.7 mL, 7.7 mmol) was added quickly at 0 °C. The mixture
was heated to 80 °C and stirred for 12 h. After cooling to
room temperature, poured into sodium acetate solution
and stirred for an additional 1 h. The solution was extracted
with dichloromethane, and washed with H2O and brine. The
organic layer was dried over anhydrous MgSO4. Solvent was
removed by rotary evaporation, and the residue was puri-
fied by silica gel column chromatography with petroleum
ether/dichloromethane = 1/1 as eluent to yield 3-a as an or-
ange–red solid. Yield: 1.64 g, 72%.1H NMR (CDCl3, 400 MHz,
d/ppm): 10.11 (s, 1H, –CHO), 8.07–8.02 (s, 2H, phenyl-H),
7.97–7.95 (d, 1H, thienyl-H), 7.88–7.86 (d, 1H, thienyl-H),
7.10 (s, 1H, thienyl-H), 2.68 (s, 3H, –CH3), 2.39 (s, 3H, –CH3).
The solar-to-electricity conversion efficiency (g) of the
DSSCs is calculated from short-circuit photocurrent (Jsc),
the open-circuit photovoltage (Voc), the fill factor (ff) and
the intensity of the incident light (Pin) according to the
following equation:
JSCðmA cmꢀ2Þ ꢁ VOCðVÞ ꢁ ff
g
¼
PinðmW cmꢀ2
Þ
The IPCE values are plotted as a function of the excited
wavelength and defined according to the following
equation:
1240 JSCðmA cmꢀ2
Þ
Þ
IPCEðkÞ ¼
ꢁ
kðnmÞ
U
ðmW cmꢀ2
2.4.4. [(4-Methylthiophen-2-yl)benzo[1,2,5]thiadiazol-4-yl]-
4-methylthiophene-2-10,15,20-tris(4-methylphenyl)
porphyrin (4-a)
where Jsc is the short-circuit photocurrent density gener-
ated by monochromatic light, k is the wavelength of inci-
dent monochromatic light, and
intensity.
U is the incident light
In a 250 mL three-necked round-bottomed flask, 3-a
(2.73 g, 7.66 mmol), 4-methylbenzaldehyde (3.17 g,
26.3 mmol) were dissolved in 200 mL propionic acid. The
solution was heated to reflux at 140 °C. Pyrrole (2.44 mL,
30.7 mmol) was added dropwise and stirred for another
2 h. After cooling to room temperature (rt), the solvent
was evaporated. The crude productionwas separated by col-
umn chromatography (petroleum ether/dichlorometh-
ane = 3/1 as an eluent). After recrystallization from
CH3OH, the desired bright purple solid compound 4-a was
obtained. Yield: 0.84 g, 12.1%. 1H NMR (CDCl3, 400 MHz, d/
ppm): 9.11–8.87 (m, 8H, pyrrol-H), 8.50–8.48 (m, 2H, phe-
nyl-H), 8.12–8.05 (m, 10H, phenyl-H), 7.59–7.57 (m, 4H,
aryl-H), 7.08(s, 1H, thienyl-H), 2.72 (s, 9H, –CH3), 2.42 (s,
3H, –CH3), 2.26 (s, 3H, –CH3), ꢀ2.64 (s, 2H, N–H). MALDI-
TOF MS (C57H42N6S3) m/z: calcd for 907.293; found 907.285.
2.4. The detailed experimental procedures and
characterization data
2.4.1. 2-(tri-n-Butylstannyl)-4-methylthiophene (1-a)
Under argon atmosphere, 3-methylthiophene (1.96 g,
20 mmol) and 50 mL freshly distilled dry THF were added
in a 100 mL three-necked flask. n-Butyllithium (8.8 mL,
2.5 M in hexane, 22 mmol) was added dropwise at ꢀ78 °C,
and the solution was stirred for 1 h at ꢀ78 °C, then tributyl-
tin chloride (6 mL, 22 mmol) was added. The mixture was
allowed up to room temperature slowly and stirred for an-
other 24 h. Finally, the mixture was poured into 100 mL
cooled water, and extracted by hexane. The organic layer
was dried over anhydrous MgSO4. Removal of the solvent
by rotary evaporation, a yellow–brown liquid 1-a (7.12 g,
85%) was obtained. The crude product was used in next step
without further purification. 1H NMR (CDCl3, 400 MHz, d/
ppm): 7.20 (s, 1H, thienyl-H), 6.97 (s, 1H, thienyl-H), 2.33
(t, 3H, –CH3), 1.64–1.54 (m, 6H, –CH2), 1.40–1.31 (m, 6H,–
CH2), 1.19–1.08 (m, 6H, –CH2), 0.95–0.90 (m, 9H, –CH3).
2.4.5. [(4-Methylthiophen-5-formyl-2-yl)benzo[1,2,5]thiadia
zol-4-yl]-4-methylthiophene-2-10,15,20-tris(4-methyl
phenyl) porphyrin (5-a)
In a 50 mL three-necked flask, porphyrin 4-a (0.55 g,
0.6 mmol), DMF (1.62 mL, 20 mmol), 1,2-dichloroethane
25 mL were added sequentially under Ar atmosphere and
POCl3 (1.88 mL, 20 mmol) was added quickly at 0 °C. The

W. Zhou et al. / Organic Electronics 13 (2012) 560–569
563
mixture was heated to 80 °C and stirred for 12 h. After
cooling to room temperature, the mixture was poured into
sodium acetate solution and stirred for an additional 1 h.
The solution was extracted with dichloromethane, and
washed with H2O and brine. The organic layer was dried
over anhydrous MgSO4. Solvent was removed by rotary
evaporation, and the residue was purified by silica gel
column chromatography with petroleum ether/dichloro-
methane = 1/1 as eluent to yield 5-a (0.24 g, 42.8%) as a
purple–red solid. 1H NMR (CDCl3, 400 MHz, d/ppm):10.14
(s, 1H, –CHO), 9.06–8.87 (m, 8H, pyrrol-H), 8.53 (s, 1H,
aryl-H), 8.14–8.05 (m, 10H, phenyl-H), 7.59–7.57 (m, 5H,
aryl-H), 2.72 (s, 9H, –CH3), 2.71 (s, 3H, –CH3), 2.26 (s, 3H,
–CH3), ꢀ2.65 (s, 2H, N–H). MALDI-TOF MS (C58H42N6S3O)
m/z: calcd for 935.198; found 935.252.
(CDCl3, 400 MHz, d/ppm): 7.18 (s, 1H, thienyl-H), 6.96 (s,
1H, thienyl-H), 2.82–2.80 (t, 2H, –CH2), 1.70–1.62 (m, 2H,
–CH2), 1.60–1.55 (m, 8H, –CH2), 1.36–1.30 (m, 8H, –CH2),
1.11–1.06 (m, 8H, –CH2), 0.93–0.87 (m, 12H, –CH3).
2.4.9. 4,7-bis-(4-Hexylthiophen-2-yl)benzo[1,2,5]thiadiazole
(2-b)
The product 2-b was prepared using the same procedure
for 2-a except that 1-b was used instead of 1-a. Yield:
1.08 g, 68%. 1H NMR (CDCl3, 400 MHz, d/ppm): 7.99 (s, 2H,
thienyl-H), 7.84 (s, 2H, phenyl-H), 7.06 (s, 2H, thienyl-H),
2.71 (t, 4H, –CH2), 1.72–1.60 (m, 8H, –CH2), 1.36–1.22 (m,
8H, –CH2), 0.94 (t, 6H, –CH3).
2.4.10. [(3-Hexylthiophen-2-yl)benzo[1,2,5]thiadiazol-4-yl]-
4-hexylthiophene-2-carbaldehyde (3-b)
2.4.6. [(4-Methylthiophen-5-formyl-2-yl)benzo[1,2,5]
thiadiazol-4-yl]-4-methylthiophene-2-10,15,20-tris
(4-methylphenyl) porphyrin zinc (6-a)
The product 3-b was prepared using the same procedure
for 3-a except that 2-b was used instead of 2-a. Yield: 2.5 g,
78%. 1H NMR (CDCl3, 400 MHz, d/ppm): 10.11(s, 1H, –CHO),
8.07–8.05 (d, 2H, phenyl-H), 7.98–7.96 (d, 1H, thienyl-H),
7.89–7.87 (d, 1H, thienyl-H), 7.11 (s, 1H, thienyl-H), 3.07–
3.03 (t, 2H, –CH2), 2.74–2.70 (t, 2H, –CH2), 1.81–1.70 (m,
4H, –CH2), 1.43–1.36 (m, 12H, –CH2), 0.93–0.92 (t, 6H, –
CH3).
A mixture of 5-a (0.24 g, 0.25 mmol) and Zn (OAc)2
(0.18 g, 1 mmol) in CHCl3 (50 mL) and CH3OH (10 mL)
was refluxed for 4 h. After cooling to room temperature,
the mixture was washed with H2O. The organic layer was
dried over anhydrous MgSO4 and concentrated by rotary
evaporation. A purple with a little green solid of compound
6-a was obtained. Yield: 0.23 g, 93.7%. 1H NMR (CDCl3,
400 MHz, d/ppm): 10.13 (s, 1H, –CHO), 9.06–8.89 (m, 8H,
pyrrol-H), 8.53 (s, 1H, aryl-H), 8.12–8.05 (m, 10H, phe-
nyl-H), 7.59–7.57 (m, 5H, aryl-H), 2.72 (s, 9H, –CH3), 2.26
(s, 3H, –CH3), 2.71(s, 3H, –CH3).
2.4.11. [(4-Hexylthiophen-2-yl)benzo[1,2,5]thiadiazol-4-yl]-
4-hexylthiophene-2-10,15,20-tris(4-methylphenyl) porphyrin
(4-b)
The product 4-b was prepared using the same proce-
dure for 4-a except that 3-b was used instead of 3-a.
(1.02 g, 12.7%). 1H NMR (CDCl3, 400 MHz, d/ppm): 9.10–
8.85 (m, 8H, pyrrol-H), 8.52–8.05 (m, 12H, phenyl-H),
7.81–7.56 (m, 4H, aryl-H), 7.08 (s, 1H, thienyl-H), 2.76–
2.65 (s, 9H, –CH3), 2.64–2.49 (t, 4H, –CH2), 1.77–1.65 (m,
4H, –CH2), 1.41–1.25 (m, 12H, –CH2), 0.54–0.52 (t, 6H, –
CH3), ꢀ2.65 (s, 2H, N–H).
2.4.7. 2-Cyano-3-(3-methyl-5-(4-(10,15,20-tris(4-methyl
phenyl))porphyrinatozinc (II)yl)-3-methyl-5-benzo[1,2,5]
thiadiazol-4-yl)thienyl acrylic acid (7-a)
To a solution of acetonitrile (15 mL) and toluene (5 mL),
porphyrin compound 6-a (0.20 g, 0.20 mmol), cyanoacetic
acid (0.073 g, 0.87 mmol) and piperidine (0.5 mL) were
added sequentially under Ar atmosphere. The mixture was
refluxed for 16 h. After cooling to room temperature, the
solution was extracted with dichloromethane, and washed
with H2O and 0.1 M HCl. The organic layer was dried over
anhydrous MgSO4. Solvent was removed by rotary evapora-
tion, and the residue was purified by silica gel column chro-
matography with methanol/dichloromethane (1/20) as
eluent to yield a purple solid with a little green compound
7-a. Yield: 0.095 g, 42%. 1H NMR (CDCl3, 400 MHz, d/ppm):
9.17–8.97 (m, 8H, pyrrol-H), 8.55 (s, 1H, aryl-H), 8.52 (s,
1H, vinyl-H), 8.13 (m, 10H, phenyl-H), 7.57 (m, 5H, aryl-
H), 2.72–2.60 (m, 15H, –CH3). 13C NMR (DMSO, 400 MHz,
ppm): 152.2, 150.6, 150.2, 150.0, 149.6, 140.6, 138.4,
136.9, 134.5, 134.3, 134.1, 132.6, 132.5, 131.9, 131.7,
131.6, 131.3, 131.1, 139.9, 129.6, 128.1, 127.9, 127.8,
127.6, 127.2, 127.0, 125.6, 125.3, 125.1, 124.7, 124.5,
123.7, 121.6, 121.1, 14.3. MALDI-TOF MS (C61H41N7S3O2Zn)
m/z: calcd for 1063.179; found 1063.181. Anal calcd for
2.4.12. [(4-Hexylthiophen-5-formyl-2-
yl)benzo[1,2,5]thiadiazol-4-yl]-4-hexylthiophene-2-10,15,20-
tris(4-methylphenyl) porphyrin (5-b)
The product 5-b was prepared using the same proce-
dure for 5-a except that 4-b was used instead of 4-a. Yield:
0.25 g, 45.9%. 1H NMR (CDCl3, 400 MHz, d/ppm):10.12 (s,
1H, –CHO), 9.06–8.85 (m, 8H, pyrrol-H), 8.56–8.05 (m,
12H, phenyl-H), 7.81–7.77 (s, 1H, thienyl-H), 7.56–7.55
(m, 3H, aryl-H), 3.08–3.06 (t, 2H, –CH2), 2.96–2.70 (m,
9H, –CH3), 2.57–2.53 (t, 2H, –CH2), 2.06–2.00 (t, 2H, –
CH2), 1.82–1.76 (t, 2H, –CH2), 1.45–1.25 (m, 12H, –CH2),
0.93–0.83 (t, 3H, –CH3), 0.53–0.50 (t, 3H, –CH3), ꢀ2.65–
2.69 (s, 2H, N–H). MALDI-TOF MS (C68H62N6S3O) m/z: calcd
for 1075.406; found 1075.319.
2.4.13. [(4-Hexylthiophen-5-formyl-2-
yl)benzo[1,2,5]thiadiazol-4-yl]-4-hexylthiophene-2-10,15,20-
tris(4-methylphenyl) porphyrin zinc (6-b)
C61H41N7O2S3Zn: C, 68.79; H, 3.85; N, 9.21; S, 9.02%. Found:
C, 68.34; H, 4.05; N, 8.19; S, 8.16%.
The product 6-b was prepared using the same proce-
dure for 6-a except that 5-b was used instead of 5-a. Yield:
0.55 g, 95%. 1H NMR (CDCl3, 400 MHz, d/ppm):10.12 (s, 1H,
–CHO), 9.06–8.85 (m, 8H, pyrrol-H), 8.56–8.05 (m, 12H,
phenyl-H), 7.81–7.77 (s, 1H, thienyl-H), 7.56–7.55 (m, 3H,
aryl-H), 3.08–3.06 (t, 2H, –CH2), 2.96–2.70 (m, 9H, –CH3),
2.4.8. 2-(tri-n-Butylstannyl)-4-hexylthiophene (1-b)
The product 1-b was prepared using the same proce-
dure for 1-a except that 3-hexylthiophene was used in-
stead of 3-methylthiophene. Yield:7.82 g, 85%. 1H NMR

564
W. Zhou et al. / Organic Electronics 13 (2012) 560–569
2.57–2.53 (t, 2H, –CH2), 2.06–2.00 (t, 2H, –CH2), 1.82–1.76
(t, 2H, –CH2), 1.45–1.25 (m, 12H, –CH2), 0.93–0.83(t, 3H, –
CH3), 0.53–0.50 (t, 3H, –CH3).
3. Results and discussion
3.1. Synthesis and characterization
The two dyes have been synthesized according to sev-
eral classical reactions. The synthetic strategy is shown in
Scheme 1. Taking PMBTZ as example, the dye was synthe
sized as following steps. First, 2-(tri-n-butylstannyl)-4-
methylthiophene 1-a, was obtained from 3-methylthioph-
ene in the presence of n-BuLi and tributyltin chloride. Then
the compound 3-a was obtained by two steps: (1) 4,7-bis-
(4-methylthiophen-2-yl)benzo[1,2,5]thiadiazole (2-a) was
synthesized by Stille coupling with 4,7-dibromo-2,1,3-ben-
zothiadiazole and 1-a; (2) then 2-a reacted via Vilsmeier
formylation reaction with POCl3 and DMF in 1,2-dichloro-
ethane to get 3-a. The key intermediate 4-a was obtained
by Adler-Longo method with 3-a and 4-methylbenzalde-
hyde and pyrrole. Subsequently, 4-a was converted into
compound 5-a by Vilsmeier formylation. Finally, com-
pound 5-a was treated with Zn(OAc)2 to get 6-a, and then
reacted with cyanoacetic acid by Knoevenagel reaction to
give PMBTZ. And PHBTZ was synthesized by the same pro-
cedure as that for PMBTZ except that 3-hexylthiophene
was used instead of 3-methylthiophene. The structures of
2.4.14. 2-Cyano-3-(3-hexyl-5-(4-(10,15,20-tris(4-methyl
phenyl))porphyrinatozinc (II)yl)-3-hexyl-5-benzo[1,2,5]
thiadiazol-4-yl)thienyl acrylic acid (7-b)
The product 7-b was prepared using the same procedure
for 7-a except that 6-b was used instead of 6-a. Yield: 0.12 g,
34.8%. 1H NMR (CDCl3, 400 MHz, d/ppm): 9.06–8.85 (m, 8H,
pyrrol-H), 8.57 (s, 1H, vinyl-H), 8.38–8.05 (m, 12H, phenyl-
H), 7.81–7.77 (s, 1H, thienyl-H), 7.56–7.55 (m, 3H, aryl-H),
3.08–3.06 (t, 2H, –CH2), 2.96–2.70 (m, 9H, –CH3), 2.57–
2.53 (t, 2H, –CH2), 2.06–2.00 (t, 2H, –CH2), 1.82–1.76 (t,
2H, –CH2), 1.45–1.25 (m, 12H, –CH2), 0.93–0.83(t, 3H, –
CH3), 0.53–0.50 (t, 3H, –CH3). 13C NMR (DMSO, 400 MHz,
ppm): 152.2, 150.8, 150.2, 150.0, 149.6, 140.1, 138.2,
137.0, 134.8, 134.6, 134.4, 132.6, 132.5, 132.1, 131.9,
131.8, 131.6, 131.3, 130.0, 129.9, 128.0, 127.9, 127.8,
127.7, 127.2, 127.1, 125.6, 125.5, 125.1, 124.7, 124.5,
123.9, 121.9, 121.1, 34.2, 31.4, 29.4, 28.8, 25.0, 22.1, 14.3.
MALDI-TOF MS (C71H61N7S3O2Zn) m/z: calcd for 1203.332;
found 1203.325. Anal calcd for C71H61N7O2S3Zn: C, 70.76;
H, 5.06; N, 8.14; S,7.97%. Found: C, 69.21; H, 5.28; N, 7.25;
S, 7.26%.
Scheme 1. The synthetic strategy of monomers and the two dyes. Reagents and conditions: I: (1) n-BuLi, THF, ꢀ78 °C, (2) tributyltin dichloride; II:
Pd(PPh3)4, toluene; III: POCl3, DMF, 1,2-dichloroethane; IV: pyrrole, propionic acid; V: Zn(OAc)2, CH3OH, CHCl3; VI: cyanoacetic acid, piperdine, CH3CN.

W. Zhou et al. / Organic Electronics 13 (2012) 560–569
565
and Q-bands was improved evidently. On the other hand,
the blue-shift of PHBTZ compared to PMBTZ can be readily
interpreted by the molecular modeling study of the two
sensitizers. The optimized structures of the two porphyrin
dyes calculated at density functional B3LYP/LANL2DZ level
were shown in Fig. 3. The ground state structure of PMBTZ
and PHBTZ possess twist of 103.4° and 96.0° between the
porphyrin macrocycle and the thienyl unit. And the dihe-
dral angles of the thienyl and the BTZ units of the two dyes
are 0.9° and ꢀ0.3°. Relatively, PMBTZ possesses more delo-
calization over an entire conjugated system than PHBTZ.
Further, the UV–visible absorption spectra of the two por-
phyrin dyes adsorbed onto TiO2 films are described in
Fig. 4. Both the two dyes show broad absorption spectra,
in the range of 350–800 nm. A slight red-shift from 423
to 442 nm was found due to the J-aggregation on TiO2 elec-
trode. Such a broadening of the absorption spectra is due to
an interaction between the dyes and TiO2 [19]. When the
two sensitizers were adsorbed on the TiO2 electrode, the
color of PMBTZ based film is darker than that of PHBTZ,
which predicts more adsorbed amounts. Further, it is obvi-
ous that the molar extinction coefficient of PMBTZ is larger
than that of PHBTZ in the range of 350–650 nm when ad-
sorbed on TiO2 thin film, which is correspondence with the
color of the two films. According to the above, PMBTZ is
expected to gain better IPCE and Jsc value.
250000
200000
150000
100000
50000
0
2-a
PMBTZ
PHBTZ
P
Zn
300
400
500
600
700
Wavelength (nm)
Fig. 2. Absorption spectra of porphyrin dyes and 2-a in CHCl3 solutions.
the two dyes were verified by 1H NMR, 13C NMR, and MAL-
DI-TOF mass spectra.
3.2. Optical properties
Fig. 2 showsthe absorptionspectraof sensitizers (PMBTZ
and PHBTZ), reference PZn and low-band-gap chromophore
(2-a) measured in CHCl3 solutions. The data are listed in Ta-
ble 1. The compound 2-a exhibits two strong absorption
The fluorescence data of the dyes are listed in Table 1
and the emission spectra is presented in Fig. 5. The fluores-
cence spectrum of PZn exhibits emission with two peaks
(kem = 598, 645 nm) arising from the porphyrin core. The
maximum emission peaks of PMBTZ and PHBTZ are 661
and 656 nm, respectively. And a shift of the maximum
fluorescence emission at longer wavelengths is observed
as the conjugation increased. Furthermore, the peaks of
the two dyes at 598 nm are weaker than that of PZn, which
bands at 313 nm (
e
= 2.6 ꢁ 104 Mꢀ1 cmꢀ1) and 458 nm
(
e
= 1.4 ꢁ 104 Mꢀ1 cmꢀ1). PZn exhibits a strong but narrow
absorption peak located at 420 nm and weak Q-bands at
547 (
e
= 7 ꢁ 103 Mꢀ1 cmꢀ1) and 586 nm, which are attrib-
uted to
p–
p⁄ electron transition. After modification, the syn-
thesized porphyrin dyes obtain improved spectra as shown
in Fig. 2, which is broadened in Soret-bands and strength-
ened in Q-bands.
The shape and the peak positions of the spectra are
analogous for PMBTZ and PHBTZ. The two dyes both exhi-
is accounted for the extent of
p-conjugation that causing
effective intramolecular electron transfer.
bit strong Soret – at 424 (
e
= 1.19 ꢁ 105 Mꢀ1 cmꢀ1) and
423 nm (
e
= 2.35 ꢁ 105 Mꢀ1 cmꢀ1), and moderate Q-bands.
After modification with the low-band-gap chromophore on
the linker, the Soret-bands of the two dyes are much
broader and red-shifted than PZn. It is obvious that the
spectrum of PMBTZ at the Soret-band with a shoulder is
broader than that of PHBTZ which is attributed to the J-
aggregation. And the Q-bands of the two dyes are much
stronger than PZn. Corresponding molar extinction coeffi-
cients of the two dyes at Q-bands are 2.5 ꢁ 104 and
3.8 ꢁ 104 Mꢀ1 cmꢀ1, respectively, which is 3–5 folder than
PZn. And the absorption in the range between Soret-band
3.3. Electrochemical properties
We employed cyclic voltammetry to determine the re-
dox potentials of the two porphyrins, and the electrochem-
ical reactions of the two dyes were measured in DMF
containing 0.1 M tetrabutylammonium hexafluorophos-
phate as a supporting electrolyte. The cyclic voltagram is
shown in Fig. 6 and the electrochemical data are summa-
rized in Table 2. The first oxidation potentials (Eox1) of the
two dyes, corresponding to the HOMO energies of the dyes,
Table 1
UV–visible, PL spectral data of porphyrin dyes.
a
Dyes
Soret/Q-band(s)a
(
e
,105 Mꢀ1 cmꢀ1
)
Soret/Q-band (f)b
Cc /10ꢀ8 mol cmꢀ2
kem (nm)d
kint (nm)e
PMBTZ
PHBTZ
PZn
424 (1.19), 571 (0.25)
423 (2.35), 552 (0.38)
420 (2.21), 547 (0.07)
448, 580
442, 583
2.4
0.85
661, 726
612, 656
598, 645
594
577
448
a
b
c
Absorption spectra measured in CHCl3 solution.
Absorption spectra obtained on TiO2 film.
Amount of the dyes absorbed on TiO2 film.
Wavelengths for emission spectra in CHCl3 solution by exciting at Soret-wavelength.
Measured by the intercept of the normalized absorption and emission spectra.
d
e

566
W. Zhou et al. / Organic Electronics 13 (2012) 560–569
Fig. 3. The optimized structures of (a) PMBTZ and (b) PHBTZ calculated at density functional B3LYP/LANL2DZ level.
PMBTZ
PHBTZ
PZn
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.0
0.8
0.6
0.4
0.2
0.0
PMBTZ
PHBTZ
400
500
600
700
800
550
600
650
700
750
800
Wavelength (nm)
Wavelenth (nm)
Fig. 4. UV–visible absorption spectra of porphyrins adsorbed on TiO2
films.
Fig. 5. Normalized PL spectra of porphyrin dyes in CHCl3.
obtained from the intersection of absorption andꢂemission
spectra the absorption edge [20]. The calculated E0—0 values
are ꢀ0.98 and ꢀ1.00 eV, respectively. They are both more
negative than the conduction edge (ꢀ0.50 V vs. NHE) of
TiO2, which is compatible with electron injection from the
excited state of the dye to the conduction band (CB) of
TiO2. As described above, the energy levels for the two dyes
are 1.11 and 1.15 eV, respectively. They are both at poten-
tials greater than that of the Iꢀ=I3ꢀ couple, which assure
regenerations of the oxidized states. The reduction oxida-
tion potentials are obtained from the relation Ered = Eox1
ꢀ
E0–0, in which Eox1 is the first oxidation potential of a
porphyrin dye and E0–0 is the zero–zero excitation energy

W. Zhou et al. / Organic Electronics 13 (2012) 560–569
567
In order to gain further insight into the molecular orbi-
tal spatial distribution, the frontier molecular orbitals of
the dyes PMBTZ and PHBTZ were also simulated based
on density functional theory (DFT) calculations at B3LYP/
LANL2DZ level. Full geometry optimizations and fre-
quency/intensity calculations of the novel porphyrin sensi-
tizers were performed at B3LYP level using the LANL2DZ
basis set. None of the frequency calculations generated
imaginary frequencies, indicating that the optimized
geometries were true energy minima. All calculations were
performed using the Gaussian03 program [22] on the IBM
P690 system in the Shandong Province High Performance
Computer Center. The spatial distributions of the HOMO
and LUMO of the dyes are shown in Fig. 7. It is obvious that
the HOMO is localized mainly on the porphyrin cycle and
PMBTZ
PHBTZ
Fc
Fc
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Potential (V)
Fig. 6. Cyclic voltammograms of the porphyrin dyes.
extended along the p-conjugated bridge to the central re-
gion of the molecule. On the other hand, the LUMO is local-
ized mainly along the cyanoacrylic unit through thienyl
and BTZ units. We notice that the HOMO–LUMO excitation
induced by light irradiation could move the electron distri-
bution from porphyrin cycle to the cyanoacrylic acid moi-
ety via thienyl and BTZ bridges, and the photo-induced
electron transfer from the dyes to the TiO2 electrode can
occur efficiently by the HOMO–LUMO transition.
Table 2
Electrochemical data for porphyrin dyes and driving forces for electron-
transfer processes on the TiO2.
a
b
c
d
e
Dyes
E0–0
(eV)
Eox1
(V)
Ered
(V)
D
(eV)
Ginj
D
(eV)
Greg
PMBTZ 2.09
PHBTZ 2.15
PZn
1.11
1.15
1.230
ꢀ0.98
ꢀ1.00
ꢀ1.290
ꢀ0.48
ꢀ0.50
ꢀ0.61
ꢀ0.65
2.77
a
Determined from the intercept of the normalized absorption and
3.4. Photovoltaic properties of porphyrin-sensitized TiO2 cells
emission spectra.
b
First oxidation potentials (vs. NHE).
First reduction potentials approximated from Eox1 and E0–0 (vs. NHE).
c
The dye-sensitized solar cells were constructed using
P25 TiO2 film based on the two dyes. Details for the
fabrication of DSSCs are described in Section 2. The device
performance parameters under simulated AM 1.5 irradia-
tion (100 mW cmꢀ2) are collected in Table 3 and the pho-
tovoltaic characteristics of DSSCs based on the two dyes
are shown in Fig. 8.
d
Driving forces for electron injection from the porphyrin excited sin-
glet state (Eꢂox) to the conduction band of TiO2 (ꢀ0.5 V vs. NHE).
e
Driving forces for regeneration of the porphyrin radical cation by
Iꢀ=Iꢀ3 redox couple (+0.5 V vs. NHE).
both fulfill the requirement for effective electron injection
and dye regeneration in a DSSC system.
Fig. 7. Molecular orbital spatial distributions of the porphyrin dyes.

568
W. Zhou et al. / Organic Electronics 13 (2012) 560–569
Table 3
ing to overall conversion efficiencies of 5.46% and 4.67%,
Photovoltaic performance of DSSCs for the two dyes.
respectively. As we expected, the short circuit photocur-
rent density (Jsc) for DSSCs based on PMBTZ is larger than
that of PHBTZ under the same conditions. To demonstrate
the absorptivity of the two high molar extinction coeffi-
cient sensitizers on thin-film devices, we also determined
the adsorbed dye amounts on the thin films. The total
Dyes
Jsc (mA/cm2)
Voc (V)
ff
g (%)
PMBTZ
PHBTZ
14.11
10.86
0.59
0.61
0.66
0.71
5.46
4.67
amounts (C) of the porphyrins adsorbed on the TiO2 films
were determined by measuring the absorbance of the dyes
that were dissolved from the dye-modified TiO2 films into
20
10
DMF/H2O = 4:1 containing 0.1 M NaOH. The
C values of the
porphyrins absorption in DMF are listed in Table 1. The ad-
sorbed amounts of PMBTZ (2.4 ꢁ 10ꢀ8 mol/cm2) is larger
than that of PHBTZ (0.85 ꢁ 10ꢀ8 mol/cm2), which is corre-
spondence with the absorption spectra of the dyes ad-
sorbed on TiO2 thin films and the IPCE values discussed
previously. The result is attributed to the introduction of
the long hexyl chain to the thiophene segment, which
blocks the absorption of the dye to the TiO2 and increases
the distances between the adjacent dye molecules.
To gain further investigation of the device performances
of the two dyes, a dark current test was conducted. And the
current–voltage curves obtained under dark conditions
were shown in Fig. 8. The curves showed onsets that were
obviously more positive for the DSSCs based on PMBTZ
than that of PHBTZ. It indicates that the back-electron-
transfer process corresponding to the reaction between
the conduction-band electrons in the TiO2 and Iꢀ3 ion in
the electrolyte occurs more easily under dark condition
in DSSCs based on PHBTZ than that of PMBTZ.
0
-10
-20
-30
PMBTZ
PHBTZ
PMBTZ-dark
PHBTZ-dark
0.0
0.2
0.4
Voltage (V)
0.6
0.8
Fig. 8. Current density–voltage characteristics for DSSCs based on the
dyes under illumination and dark condition.
70
PMBTZ
PHBTZ
60
50
40
30
20
10
0
4. Conclusions
In summary, we have developed a new class of porphyrin
dyes featuring donor–acceptor architecture that incorporat-
ing low-band-gap chromophore. They are successfully ad-
sorbed on nanocrystalline anatase TiO2 particles, and
subsequently efficient dye-senstitized solar cells have been
fabricated. As expected, the introduction of the low-band-
gap moieties negated the Eox1 of the two dyes and narrowed
the energy gap, which has positive effect on improving the
light-harvest ability of the dyes. The impressive perfor-
mance dye-sensitized solar cell based on PMBTZ exhibits a
maximum incident photon-to-current conversion efficiency
(IPCE) value of 65%, with a Jsc of 14.11 mA/cm2 and a Voc of
0.59 V. Finally, it seems that the modification with the
low-band-gap chromophores in the bridge of the porphyrin
dyes at meso-position is an effective strategy to improve the
spectra. Furthermore, methyls on the thienyls are more
effective in reducing the charge recombination of the in-
jected electrons with triiodide ions. The inherent mecha-
nism would be studied in following investigation.
400
500
600
700
800
Wavelength (nm)
Fig. 9. IPCE plots for the DSSCs based on the two porphyrin dyes.
The incident photo-to-current conversion efficiency
(IPCE) plots for the DSSCs based on the two porphyrin dyes
are showed in Fig. 9. The two dyes both respond in the
broad range of 350–800 nm and show the maximum IPCE
values around 440 nm, which is accordant with the spectra
of the dyes adsorbed on the TiO2 films. It’s worthy noting
that the response of the dyes modified with low-band-
gap chromophore in the range of 450–550 nm is much
stronger than porphyrin dyes reported by our group previ-
ously [11e,21]. Further the IPCE values of PMBTZ is much
higher than PHBTZ in the whole broad range which may
be resulted from the bigger
C
value, thus may lead to a rel-
Acknowledgements
atively larger photocurrent in DSSC. As shown in Table 3,
the PMBTZ- and PHBTZ-sensitized solar cells gave the
short circuit photocurrent densities (Jsc) of 14.11 and
10.86 mA cmꢀ2, open circuit voltages (Voc) of 0.59 and
0.61 V and the fill factors (ff) of 0.66 and 0.71, correspond-
This work was supported by the National Nature Sci-
ence Foundation of China (No. 50973092, 51173154,
21171084), and the National Basic Research Program of
China (No. 2011CBA00701), and the Specialized Research

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