DSpace at VNU: The Dynamic Resistance of CdS CdSe ZnS Co-Sensitized TiO2 Solar Cells

Braz J Phys
DOI 10.1007/s13538-014-0266-y

GENERAL AND APPLIED PHYSICS

The Dynamic Resistance of CdS/CdSe/ZnS Co-Sensitized TiO2
Solar Cells
Tung Ha Thanh & Lam Quang Vinh & Huynh Thanh Dat

Received: 25 May 2014 /
# Sociedade Brasileira de Física 2014

Abstract Quantum dots' sensitized solar cells (QDSSCs)
can create the high-performance and low-cost photovoltaic in the future. In this study, we synthesized the film of
TiO2/CdS/CdSe/ZnS photoanodes by successive ionic layer adsorption reaction (SILAR) method. The absorption
spectra, photoluminescent spectra and electrochemical impedance spectra (EIS) of the film TiO2/CdS/CdSe/ZnS
photoanodes show that the structure of energy levels in
the conduction band (CB) of photoanode materials CdS,
CdSe, and ZnS quantum dots (QDs) can absorb a great
number of photons in each region and inject stimulated
electrons quickly into the conduction band (CB) of TiO2.
Furthermore, we also studied the influence of the SILAR
cycles on the dynamic resistance, the lifetime of electrons
in QDSSCs through Nyquist and Bode.
Keywords Counter electrode . Quantum dots . Solar cells

1 Introduction
One of the main reasons for the growing interest in
quantum dots is their use in cheap solar cells, which have
the possibility to increase the thermodynamic conversion
efficiency above the Shockley–Queisser limit [1]. The

T. Ha Thanh (*)
Faculty of Physics, Dong Thap University, Cao Lãnh, Dong Thap
Province, Vietnam
e-mail:
L. Quang Vinh
University of Science, Vietnam National University—HCM City,
Hanoi, Vietnam
H. Thanh Dat
Vietnam National University—HCM City, Hanoi, Vietnam

thermodynamic limit of the light to electric power conversion efficiency, also known as Shockley–Queisser limit, originates from the fact that photons with energies
below the band-gap energy are not absorbed, while photons with energies above the band-gap energy release the
additional energy (Ephoton-Egap) mostly as heat. Thirdgeneration solar cells aim toward conversion efficiencies
beyond the Shockley–Queisser limit through advanced
photovoltaic (PV) concepts such as multijunction cells,
optical up- and downconverters, multiple carrier generation by impact ionization. Their development has been
based on different p–n junctions and the use of quantum
dots (QDs) to replace dyes. Performance above 40 % has
been obtained [2]. In recent years, researchers have discovered the QDs which can create the high performance
of solar cells [3]. QDs can be changed in particle size,
leading to a change in absorption spectrum [4].
Controlling QDs size, we can change their absorption
spectrum. Furthermore, in association with biological
molecules, QDs can transfer charge faster while reducing
losses and helping passivated surface (reduced defect
states) of them. In 1990, Vogel and his colleagues have
used CdS QDs with Pt cathode [5]. However, this is a
new direction in quantum dots sensitized solar cells
(QDSSCs) research. Since then, there have been a large

number of studies such as different QDs replacement,
TiO2 semiconductor materials, electrolyte, and counter
electrodes to enhance photovoltaic performance [6–8].
Lee and his colleagues studied CdSe and CdTe QDs using
Pt counterelectrode with an efficiency of under 1 % [9].
One year later (2008), they went on investigating CdS and
CdSe QDs and improved the performance efficiency to
1.2 %, with the use of polysulfide electrolyte [8].
Meanwhile, Lopez-Luke et al., Mora-Sero et al., Shen
et al., and Tachibana et al. [10–13] synthesized CdS and
CdSe QDs with Pt counterelectrode, but in different


Braz J Phys

electrolyte systems (Na2S, NaOH + Na2S + S) and obtained a better performance efficiency of 2.2 %. From
2009 to 2012, various QDSSCs were studied. Cheng
et al. [14] examined CdS and CdSe co-sensitized TiO2
nanowires and nanorods by using Na2S + Na2SO3 electrolyte, and obtained a high efficiency of 2.41 %.
Although there has been much research in point as mentioned above, no study has been conducted about the
mechanism, processes (combined processes, electron
transport processes in semiconductor films and at junctions, and corrosion of the electrode anode by electrolyte)
or about the resistances on QDSSCs performance.
In this paper, we present our investigation of the photovoltaic based on CdS/CdSe/ZnS photoanodes by SILAR
method [15, 16]. The absorption spectra of CdS/CdSe/
ZnS photoanode greatly extended to the visible region,
while the photoluminescent spectra quickly extinguished.
The reason is that the complex structure of CdS, CdSe,
and ZnS QDs is CBTiO2ZnS are −4.2, −3.6, −3.49, and −3.0 eV [17–19]). So,

photoanodes can absorb a large amount of photons in
each region and put stimulated electrons quickly into
the conduction band of TiO2. Furthermore, we also
Fig. 1 a X-Ray Diffraction of the
TiO2/CdS/CdSe/ZnS in different
electrolytes, b, c TEM images of
TiO2 film and TiO2/CdS/CdSe/
ZnS, and d UV–Vis absorption
spectra of the TiO2 films sensitized by CdS/CdSe/ZnS QDs
show the light absorption behavior of photoanodes changed with
the SILAR cycles of CdS, CdSe,
and ZnS

studied the influence of the number of SILAR deaths,
annealing temperature on the diffusion process, and the
existence of electrons in QDSSCs through Nyquist,
bode spectra.

2 Experiment
2.1 Investigation on sensitized TiO2 films
The films were coated with TiO2 layers by silk-screen
printing, and they were then annealed at 500 °C for
30 min. Their sizes ranged from 10 to 30 nm
(Transmission Electron Microscopy image in Fig. 1a).
The thickness of TiO2 films was around 4 μm measured
by Stylus spectra. Then, the films were dipped in 40mmol TiCl4 solution for 30 min at 70 °C and sintered
at 500 °C for 30 min. The specific surface area of the
mesoporous TiO2 was examined by using N2 adsorption
and desorption isotherms before and after the calculation. The surface area is 120.6 m2 g−1 (measured by
BET devices). This result indicates that the synthesized

material had a wider mesoporous structure.

c

1000
800

a
600
400
200

20

30

40

50

b

d

2

1

0
400

500

600

700

800


Braz J Phys

TiO2/CdS/CdSe/ZnS films were synthesized by SILAR
method as follows: firstly, the TiO2 film was dipped in
0.5 M Cd2+-ethanol solution for 1 min and rinsed with
ethanol. Then, it was dipped for 1 min in 0.5 M S2−methanol solution and rinsed with methanol after being
dried in the air (a cycle SILAR). The number of CdS
QDs was increased by repeating the assembly cycles
from 1 to 5 cycles. Secondly, TiO2/CdS was dipped into
1 M Cd2+-ethanol solution for 1 min at room temperature
and rinsed with ethanol. Then, it was dipped for 1 min in
0.5 M Se2−-aqueous solution and rinsed with pure water
after being dried in the air (a cycle SILAR). The number
of CdSe QDs was increased by repeating the assembly
cycles from one to five. For the ZnS passivation layer,
TiO2/CdS/CdSe films were dipped into 0.1 M Zn2+-solution and 0.1 M S2−-solutions for 1 min and rinsed with
pure water between two dips (a total of 2 cycles).
Finally, they were annealed in a vacuum environment
with different temperatures to avoid oxidation. TiO 2/
CdS/CdSe/ZnS thickness was measured by the Stylus

spectra. The average thicknesses of CdS (3 cycles),
CdSe (3 cycles), and ZnS (2 cycles) were 351.9, 56.1,
and 257.8 nm, respectively. The coating of F− ions was
performed by dipping the TiO2 photoelectrode into a
1 M NH 4 F aqueous solution for 2 min, rinsed with
deionized water [20]. Two layers of F− ions were coated:
the first was coated before the deposition of CdS QDs,
the second after the deposition of three layers of QDs,
and the same for CdSe.

2.3 Fabrication of QDSSCs
The structure of QDSSCs was designed by the Surlyn
between photoanodes and counterelectrodes at 170 °C.
The electrolyte was filled from a hole made on the counter electrode. The active area of QDSSCs was 0.38 cm2.
The polysulfide electrolyte was 0.5 M Na2S, 0.2 M S, and
0.2 M KCl in Milli-Q ultrapure water/methanol (7:3 by
volume).

SourceMeter via a simulated Air mass 1.5 standard (AM
1.5) sunlight with an output power of 100 mW/cm2 produced by a solar simulator (Solarena, Sweden).

3 Results and Discussions
QDSSCs used these QDs to replace dye in DSSCs. So,
we studied the stability of the photoanodes in different
electrolyte for the examined photovoltaic. Figure 1a
shows the XRD of TiO2/CdS/CdSe/ZnS photoanode in

a

6

Current density (mA/cm2)

2.2 Investigation on TiO2/CdS/CdSe/ZnS films

TiO2/CdSe at 1 hour
TiO2/CdSe at 10 hour

5

TiO2/CdSe at 18 hour
TiO2/CdSe at 20 hour
TiO2/CdSe at 24 hour

4

b

3
2
1
0
0.0

0.1

0.2

0.3

Voltage (V)

c

2.4 Characterizations
The morphology of the investigated samples was observed by means of TEM. The crystal structure was analyzed with an X-ray diffractometer (Philips, PANalytical
X’pert, CuKα radiation). The absorption properties of the
samples were investigated with a diffuse reflectance UV–
Vis spectrometer (JASCO V-670). Photocurrent–voltage
measurements were performed on a Keithley 2400

Fig. 2 a–c The J–V curves of the QDSSCs with different photoanodes
under one sun illumination


Braz J Phys
Table 1 Photovoltaic performance parameters of QDSSCs based on
TiO2/CdS and TiO2/CdSe photoanodes
Solar cells

JSC (mA/cm2)

VOC (V)

Fill factor
FF

Efficiency
η (%)

TiO2/CdS at 1 h
TiO2/CdS at 2 h
TiO2/CdS at 3 h
TiO2/CdS at 5 h
TiO2/CdSe at 1 h
TiO2/CdSe at 10 h
TiO2/CdSe at 18 h
TiO2/CdSe at 20 h
TiO2/CdSe at 24 h

0.763
1.87
2.23
1.7
0.256
0.59
2.08
5.47
2.13

0.276
0.38
0.294
0.22
0.31
0.32
0.33
0.33
0.29

0.255
0.242
0.34
0.3
0.25
0.24
0.27
0.31
0.24

0.054
0.17
0.22
0.12
0.02
0.046
0.184
0.575
0.15

different electrolytes. It is clear that the photoanode was
oxidized with I−/I3− electrolyte. Two new peaks appeared
at 34 and 52° positions after the photoanode was immersed in I−/I3− electrolyte. Moreover, there were the
quenched diffraction peaks of the photoanode at 30, 48,
and 54.5°. Meanwhile, the sample immersed in S 2−/Sn2−
electrolytes did not change the structure. Therefore, in
this study, we decided to select the S2−/Sn2− electrolyte
to produce QDSSCs. Detailed morphological features
and crystallinity of the pure TiO2 and TiO2/CdS/CdSe/
ZnS photoanodes were investigated with a TEM image.

Figures 1b, c show a TEM image of pure TiO2 and TiO2/
CdS/CdSe/ZnS photoanodes conducted with the SILAR
cycle numbers of CdS, CdSe, and ZnS at 3, 3, and 2,
respectively. We can see that QDs covered the surface of
TiO2 nanoparticles. It shows that the mean diameter of
QDs is from 2 to 5 nm. The results from TEM demonstrate that the SILAR method is an efficient TiO2 strategy for obtaining the covering QDs on the TiO2 surface.
We know that the optical TiO 2 /CdS/CdSe/ZnS
photoanode was important for conducted photovoltaic.

Table 2 Photovoltaic performance parameters of QDSSCs
based on TiO2/CdS/CdSe/ZnS
photoanodes

Solar cells

TiO2/CdS(1)/CdSe(3)/ZnS(2)
TiO2/CdS(2)/CdSe(3)/ZnS(2)
TiO2/CdS(3)/CdSe(3)/ZnS(2)
TiO2/CdS(4)/CdSe(3)/ZnS(2)
TiO2/CdS(5)/CdSe(3)/ZnS(2)
TiO2/CdS(3)/CdSe(1)/ZnS(2)
TiO2/CdS(3)/CdSe(2)/ZnS(2)
TiO2/CdS(3)/CdSe(4)/ZnS(2)
TiO2/CdS(3)/CdSe(5)/ZnS(2)
TiO2/CdS(3)/CdSe(3)/ZnS(1)

Figure 1d shows the UV–Vis absorption spectra of
photoanodes measured after each cycle of SILAR. As
expected, the absorbance increased with the increasing
cycles of CdS and CdSe. However, only the absorption

spectra with SILAR cycles of TiO 2 /CdS(3)/CdSe(3)/
ZnS(2) photoanode show the best deposition. Under
550-nm wavelength region, an increase in absorption
was due to more CdS loaded on TiO2/CdS/CdSe/ZnS
film. From 550 to 629 nm, a higher deposition degree of
CdSe on TiO2/CdS/CdSe/ZnS electrode resulted in the
shift of the absorption peak toward the red region. The
sizes of QDs were consistent with the sizes measured
from the TEM images. A higher absorption was thus
obtained because the absorption spectrum of ZnS
complemented those of CdSe and CdS QDs.
Furthermore, ZnS acted as a passivation layer to protect
CdS and CdSe QDs from photocorrosion [21].
We conducted a set of QDSSCs based on TiO2/CdS,
TiO2/CdSe, and TiO2/CdS/CdSe/ZnS photoanodes with
polysulfide electrolyte. Figures 2a, b present the J–V
curves of QDSSCs based on TiO2/CdS and TiO2/CdSe
photoanodes with different deposition times (an active area
of 0.38 cm2) at AM 1.5 (100 mW/cm2). The best power
conversion efficiencies of QDSSCs based on TiO2/CdS
and TiO2/CdSe photoanodes were obtained with the deposition times at 3 and 20 h, respectively. Lower power
conversion efficiencies were obtained from the QDSSCs
with deposition times less than 3 h (for CdS) and 20 h (for
CdSe) or more than 3 h (for CdS) and 20 h (for CdSe). The
QDSSCs based on TiO2/CdS (TiO2/CdSe) show an opencircuit voltage (Voc) of 0.294 V (0.33 V), a short-circuit
current density (Jsc) of 2.23 mA/cm2 (5.47 mA/cm2), fill
factor (FF) of 0.34 (0.31), and an energy conversion efficiency (η) of 0.22 % (0.575 %). These are in line with
those reported in [22–24].
The TiO 2 /CdS/CdSe/ZnS co-sensitized solar cells
demonstrated a better performance (1.52 %) than the

JSC (mA/cm2)

2.18
4.28
4.79
5.73
3.05
6.05
4.21
3.30
2.08
7.03

VOC (V)

0.29
0.54
0.76
0.39
0.45
0.356
0.55
0.48
0.33
0.39

Fill factor

Efficiency

FF

η (%)

0.35
0.37
0.41
0.31
0.32
0.256
0.38
0.31
0.27
0.26

0.22
0.86
1.52
0.68
0.45
0.55
0.88
0.50
0.18
0.73


Braz J Phys

Fig. 3 The energy level alignment of QDSSCs [30]

(1)_TiO2/CdS(1)/CdSe(3)/ZnS(2)
(2)_TiO2/CdS(2)/CdSe(3)/ZnS(2)

1200

(3)_TiO2/CdS(3)/CdSe(3)/ZnS(2)
(4)_TiO2/CdS(4)/CdSe(3)/ZnS(2)

Z'' (Ohm)

(5)_TiO2/CdS(5)/CdSe(3)/ZnS(2)

(5)

900

(4)

a

(1)

600

300
(2)
(3)

0
0

500

1000

1500

2000

Z' (Ohm)

b

TiO2/CdS(3)/CdSe(1)/ZnS(2)
TiO2/CdS(3)/CdSe(2)/ZnS(2)

2000

TiO2/CdS(3)/CdSe(3)/ZnS(2)

(5)

TiO2/CdS(3)/CdSe(4)/ZnS(2)
TiO2/CdS(3)/CdSe(5)/ZnS(2)

Z'' (Ohm)

TiO2/CdS (0.22 %) and TiO2/CdSe QDSSC (0.575 %)

(shown in Tables 1 and 2) [25]. This suggests that the
charge injection from CdSe conduction level to TiO2
conduction level may not be effective, due to the quasiFermi levels of CdSe being lower than that of TiO2 [26].
However, the quasi-Fermi level of CdS quantum dots
was higher than that of the TiO2 layer [27], and it is
expected to improve the charge injection from CdSe to
TiO2. Moreover, a ZnS coating formed a potential barrier
between QDs and the electrolyte, which blocked the
electrons in the CB from QDs to the electrolyte and
reduced the defect states in QDs [28]. So, the electron
density in the conduction band of QDs increased and the
enhanced JSC was obtained. And thus, a high performance was obtained. In addition, with the increasing
electron density in the conduction band of QDs, the
quasi-Fermi level correspondingly increased and consequently, VOC = (EFn–EFo)/(−q) increased (Fig. 3). With
the combination of CdS, CdSe, and ZnS, the CdS
Fermi energy level was higher than that of TiO2, and
beneficial effects were conferred to the coupled QDSSCs
system. It is evident that the parameters of the coupled
QDSSCs were influenced by CdS/CdSe/ZnS cosensitization cycles [29]. This is because CdS, CdSe,
and ZnS QDs led to the quasi-Fermi level alignment
and it resulted in a cascade energy level structure in the
order of CB TiO2 introduction of a CdS layer between TiO 2 and CdSe
elevated the conduction band edge of CdSe, making a
higher driving force for the injection of stimulated electrons out of the CdSe layer [25]. Moreover, the photocurrent density might be enhanced with QDs loaded by
means of increasing coating cycles [28].
To further investigate the dynamic resistance of
QDSSCs, the electrochemical impedance spectra (EIS)
under illuminated conditions for QDSSCs with different
CdS and CdSe SILAR cycles were carried out to research the charge transfer process [29]. Figures 4a–c

show the Nyquist plots of TiO 2 /CdS and TiO 2 /CdSe

(4)

1000

c

(1)

(2)

0

(3)

1000

2000

Z' (Ohm)

3000

4000

d

Fig. 4 a, b Nyquist and Bode impedance plots of EIS spectra measured
under the illuminated conditions for QDSSCs with CdS SILAR cycles

from one to five layers and c, d CdSe SILAR cycles from one to five
layers


Braz J Phys
Table 3 The resistance and lifetime obtained from the EIS measurements of TiO2/CdS/CdSe/
ZnS for different CdS SILAR
cycles

No

Sample

RS (Ω)

Rct2 (Ω)

Rct1 (Ω)

τ (ms)

1
2
3
4
5

TiO2/CdS(1 layer)/CdSe(3)/ZnS(2)
TiO2/CdS(2 layers)/CdSe(3)/ZnS(2)
TiO2/CdS(3 layers)/CdSe(3)/ZnS(2)

TiO2/CdS(4 layers)/CdSe(3)/ZnS(2)
TiO2/CdS(5 layers)/CdSe(3)/ZnS(2)

21.9
33.9
38.1
35.4
26

351
333
83.5
1930
16,100

1,570
158
9.21
2330
59,100

4.9
3.2
3.2
1.6
1.8

QDSSCs when SILAR cycles of CdS changed from one
to five. The EIS illustrated two semicircles at high frequency and low frequency. The small semicircle was due
to the resistance against movement of charge at Pt/

electrolyte (Rct1) and FTO/TiO2 interface. Meanwhile,
the large semicircle was due to a resistance against the
electron diffusion in the TiO2 and the charge recombination resistance at the TiO2/QDs/electrolyte interface
(Rct2) and against the inner diffusion in an electrolyte
(Zw). With Fit & Simulator software, we fitted for EIS of
all samples and the values of Rs, Rct1, and Rct2 are listed
in Tables 3 and 4. Rs is a set of resistance to the charge
transfer at Ag/FTO/TiO2 front contact and Ag/FTO/Pt
back contact; Rs values are obtained at about 38.1 Ω
for the best photoanodes. The result shows that the
applied technique is significant.
We can find in Fig. 4a–c that the radius of the semicircles increased when SILAR cycles of CdS or CdSe were
under three layers or over three layers. Compared with the
other photoanodes, the TiO 2 /CdS(3)/CdSe(3)/ZnS(2)
photoanode exhibited a smaller Rct1 and Rct2 (9.21 and
83.5 Ω) and larger lifetime (3.2 ms) (shown in Tables 3
and 4). Also, it shows a fast electron transfer at TiO2/QDs/
electrolyte interface and reduction in recombination as a
ZnS coating protected CdS/CdSe QDs [31]. With increasing SILAR cycles of CdS and CdSe over three layers, the
dynamic resistance increased. This is because the amount
of CdS and CdSe loaded more on TiO2/CdS/CdSe/ZnS,
which indicated the increasing recombination in
photoanodes. The results show that the photogenerated
electrons were captured by the defect states in QDs. So

Table 4 The resistance and lifetime obtained from the EIS measurements of TiO2/CdS/CdSe/
ZnS for different CdSe SILAR
cycles

the electron transfer was more diffusive hindrance, which

increased charge recombination and back transport
reaction.
Figures 4b–d indicate the Bode plots of the TiO2/
CdS and TiO2/CdSe QDSSCs when SILAR cycles of
CdS changed from one to five. We find that the frequency peak of the charge transport process isshifted to
the lowest frequency region corresponding to TiO 2 /
CdS(3)/CdSe(3)/ZnS(2) photoanode. Therefore, it indicates that the lifetime of the electron in the TiO 2 /
CdS(3)/CdSe(3)/ZnS(2) film increases, which favors
the electron transfer with less diffusive hindrance.
Hodes and co-workers found that the lifetime of electron transfer into TiO2 reached 10−12 s (very fast) and
that of electron recombination got 10 −6 s [32]. It is
shorter than the lifetime of electrons in the conduction
band of QDs (calculated in this paper about 3.2 ms). So
the charge transfer in the conduction band increased.

4 Conclusions
We successfully fabricated the QDSSCs based on the TiO2/
CdS, TiO2/CdSe, and TiO2/CdS/CdSe/ZnS photoanode. The
TiO2/CdS/CdSe/ZnS co-sensitized solar cells demonstrated a
better performance (1.52 %) than those of the TiO2/CdS
(0.22 %) and TiO2/CdSe QDSSC (0.575 %). This is because
a ZnS coating formed a potential barrier between QDs and the
electrolyte, which blocked the electrons in the conduction
band from QDs to the electrolyte and reduced the defect states
in QDs. The dimension of the semicircles increased when

No

Sample

RS (Ω)

Rct2 (Ω)

Rct1 (Ω)

τ (ms)

1
2
3
4
5

TiO2/CdS(3)/CdSe(1 layer)/ZnS(2)
TiO2/CdS(3)/CdSe(2 layers)/ZnS(2)
TiO2/CdS(3)/CdSe(3 layers)/ZnS(2)
TiO2/CdS(3)/CdSe(4 layers)/ZnS(2)
TiO2/CdS(3)/CdSe(5 layers)/ZnS(2)

28.6
67
38.1
24.8
21.3

1,670
1,190
83.5
385

415

106
268
9.21
179
243

5.9
4.9
3.2
1.7
1.8


Braz J Phys

SILAR cycles of CdS and CdSe were under three layers or
over three layers. Compared with the other photoanodes, the
TiO2/CdS(3)/CdSe(3)/ZnS(2) photoanode exhibited a smaller
Rct1 and Rct2 (9.21 and 83.5 Ω) and larger lifetime (3.2 ms).
The results show a fast electron transfer at TiO2/QDs/electrolyte interface and a reduction in recombination when a ZnS
coating protected CdS/CdSe QDs [26].
Acknowledgments This work was supported by Vietnam National
University by the name of the project: B 2012-18-5TD, the University
of Science of Ho Chi Minh City and Dong Thap University.

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