T. Shu et al. / Electrochimica Acta 137 (2014) 700–704
701
Table 1
The photovoltaic parameters of CdS QDSSCs with different electrolytes and counter
electrodes.
samples
J
sc(mA/cm2)
Voc(V)
FF
(%)
polysulfide-Pt
AT-/BAT-Pt
polysulfide CoS
AT-/BAT - CoS
polysulfide -PEDOT
AT-/BAT -PEDOT
3.50
2.61
6.03
4.84
4.35
3.88
0.32
0.48
0.35
0.50
0.42
0.55
0.47
0.55
0.52
0.59
0.66
0.72
0.52
0.70
1.10
1.42
1.20
1.53
reaction with sodium azide. A solution of 0.05 M isothiocyanate
and 0.075 M sodium azide was refluxed for 6 hours. The mixture
was cooled and filtered. Then the filtrate was extracted twice with
ether. The aqueous layer was acidified to pH 2.5 with concentrated
hydrochloric acid to precipitate the corresponding mercaptan com-
pound. This precipitation was filtered, washed with water, and then
dried under vacuum at 40 ◦C for 12 h to get purified mercaptan
compound. After that, the mercaptan was deprotonated by stirring
with an excess of sodium bicarbonate in ethanol for 2 h at room
temperature, and then filtered. The pure sodium thiolate (AT-Na+
or T-Na+) was obtained after evaporation of the solvent in the fil-
trate and recrystallization. The oxidized species (BAT or T2) was
prepared by oxidation of AT or T with hydrogen peroxide. 1 mL of
30% hydrogen peroxide was added dropwise to 0.01 mole of the
corresponding mercaptan compound in 50 mL of ethanol. The mix-
ture was stirred at 30 ◦C for 12 h. The reaction mixture was cooled
to 0 ◦C. The purified oxidized species was obtained after filtration
and recrystallization.
Scheme 1. Structures of the organic redox couples T-/T2 and AT-/BAT.
2. Experimental details
2.1. Materials
All the chemicals were of analytic grade and used without fur-
ther purification. F-doped tin oxide glass FTO (15 ꢀ/square) was
obtained from Pilkington UK. The nanoporous TiO2 film and the
TiO2 scattering layer were prepared using PST-18NR (JGC Cata-
lysts and Chemicals Ltd., Japan) and PST-400 C (JGC Catalysts and
Chemicals Ltd., Japan), respectively.
The AT-/BAT and T-/T2 electrolytes both contained 0.4 M of the
reduced species and 0.1 M of the oxidized species together with
0.4 M 18-crown-6 (18-C-6), 0.05 M LiClO4 in acetonitrile. For com-
parison, the inorganic polysulfide electrolyte with Na2S (0.5 M),
S (0.125 M), and KCl (0.2 M) in a water/methanol (3/7, v/v) solu-
tion was prepared. The QDSSCs were fabricated by sealing the
photoanodes and CEs together in a sandwich configuration with
a 25-m-thick hot-melt polymer (Surlyn, Solaronix).
2.2. Fabrication of CdS QDSSCs
The 10-m-thick nanoporous TiO2 film was prepared by doctor-
blading NR18 paste on FTO glass and then annealed at 500 ◦C
for 30 min. Then, a 4-m-thick scattering layer of 400-nm TiO2
particles (PST-400 C) were doctor-blade on the film and sintered
at 500 ◦C for 30 min. Successive ionic layer adsorption and reac-
tion (SILAR) method was employed to assemble CdS QDs on
the TiO2 film [23]. The final TiO2 film was dipped into a 0.5 M
Cd(NO3)2 ethanol solution for 5 min, rinsed with ethanol, and then
dipped into a 0.5 M Na2S methanol solution for 5 min, rinsed with
methanol. Repeat this procedure for 5 times to get CdS QDs sen-
sitized electrode. The PEDOT film was electropolymerized using
a three-electrode system [14], a FTO glass as the working elec-
trode, a platinum foil as the counter electrode, and Ag/AgCl as
the reference electrode. The solution for electropolymerization
consists of 0.01 M ethylenedioxythiophene and 0.1 M lithium bis-
trifluoromethanesulfonylimide in acetonitrile. The PEDOTHDC CE
and PEDOTLDC CE were prepared by applying a constant poten-
tial until a charge capacities of 100 mC cm−2 and 2 mC cm−2
was reached, respectively. The Pt CE was prepared by thermal
decomposition of hexachloroplatinic acid onto FTO glass. The CoS
CE was prepared according to ref [18]. A aqueous solution (pH
11.0, 75 mL) containing 0.017 M Co(NO3)2, 0.045 M thioacetamide,
0.040 M 3-mercaptopropionic acid was heated at 100 ◦C for 30 min.
The color instantly changed to black, indicated the formation of CoS
nanoparticles. The resulting CoS nanoparticles were precipitated
tate was dispersed in ethanol (4 mL) to get a CoS paste. Evenly apply
a drop of CoS paste (about 0.1 mL) on the FTO glass, dried in an oven
at 100 ◦C for 10 min to obtain a CoS CE.
2.3. Characterization
The scanning electron microscopic (SEM) images were per-
formed using
a Sirion 200 field emission scanning electron
microscope. The photocurrent density-voltage (I-V) characteristics
were measured using a Keithley 2400 source/meter and a New-
port solar simulator (model 91192-1000) under the illumination of
AM1.5 and an intensity 100mW/cm2 which was calibrated with a
standard Si solar cell. A mask with a window of 0.13 cm2 was used
to define the active area of the cell. The incident photon conversion
efficiency (IPCE) was measured using a 150 W xenon lamp (Oriel)
fitted with a monochromator (Cornerstone 74004) and recorded
using a Newport 2931-C power meter. Electrochemical impedance
spectroscopy of the QDSSCs was carried on ZAHNER ENNIUM Elec-
trochemical Workstations in the frequency range 0.1 to 105 Hz with
AC amplitude of 10 mV in the dark. EIS analysis was fitted using
Z-view software.
To compare the performance of the electrolytes and counter
electrodes, the QDSSCs with different electrolytes and counter elec-
trodes were fabricated and their I-V characteristics were measured.
Fig. 1. shows the I-V characteristics of CdS QDSSCs with differ-
ent combinations of electrolytes and counter electrodes. The open
circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) and
conversion efficiency () of the QDSSCs are listed in Table 1. It can
be seen that, the polysulfide-Pt based QDSSC shows a low efficiency
The synthetic routes of the organic AT-/BAT and T-/T2 redox
couples have been described elsewhere [13,14]. The synthesis of
thiolate forms (AT− or T−) started from commercially available
isothiocyanate (phenyl isothiocyanate or methyl isothiocyanate)
which was transformed into the corresponding 1-phenyltetrazol-
5-thiol (AT) or 1-methyltetrazol-5-thiol (T) by cycloaddition