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CHAPTER-III
Wet chemical synthesis of CdS
thin films and their
photoelectrochemical
performance
Chapter-III Wet chemical…………photoelectrochemical performance
81
Wet chemical synthesis of CdS thin films and
their photoelectrochemical performance
3.1. Outline:
Thin films of cadmium sulphide (CdS) were deposited onto glass and
optically transparent, electrically conducting substrates (fluorine doped tin
oxide on glass) via wet chemical synthesis. The as grown CdS thin films
exhibits the particles size in the range of 4-19 nm in diameter. These films
were analyzed for their structural, optical and surface morphological study by
means of UV-visible spectroscopy, X-ray diffraction, X-ray photoelectron
spectroscopy, scanning electron microscopy and atomic force microscopy.
The X-ray diffraction patterns reveal cubic crystal structure. The atomic force
microscopy study revealed a novel egg like morphology of CdS nanoparticles.
Photoelectrochemical (PEC) performances were investigated using two
electrode configurations in polysulphide electrolyte.
CHAPTER
III 3
Chapter-III Wet chemical…………photoelectrochemical performance
82
3.2 Introduction:
A fundamental property of semiconductor is the band gap. Band gap is
energy separation between the filled valence band and empty conduction
band. The band gap of semiconductor is found to increase due to a decrease
in the particle size; this is known as the quantum size effect. This arises
because of the confinement of charge carriers in potential wells of small
dimensions. The recent interests in size quantization effects of nanocrystalline
semiconductors have drawn great attention towards metal-chalcogenide-
based systems in solar cell. Particular interest are CdX and PbX (X = S, Se,
and Te) nanocrystalline thin films, which have relatively small band gaps and
thus are capable of harvesting photons in the visible and infrared region.
Along with light harvesting capabilities the semiconductors have potential
applications in biology, optics, and electronics and transport [1-4]. Several
attempts have been made to synthesize nanocrystalline thin films with various
particle size and shape.
CdS is a typical semiconductor with a direct band gap of 2.42 eV, has
been considerable interests due to its interesting optical properties and its
potential applications in the field of light emitting diodes, solar cells, opto-
electronics devices, photo catalyst, X-ray detectors, solar energy stockings
and in display devices [5-11]. CdS is a common material used as a buffer
layer in the formation of solar cell devices based on CIS, CuInSe2,
CuInGaSe2, CdTe [12-14]. Recently, CdS nanoparticles are applied to
Semiconductor Sensitized Solar Cells (SSSCs) to improve the performance of
wide band gap semiconductor materials [15-17]. In SSSCs, CdS nanoparticle
forms a thin layer on wide band gap semiconductors like ZnO, TiO2 having
nanostructured morphologies like nanorodes, nanotubes, etc. Thin layer
would be helpful for significant improvement in photoresponse in the visible
region. Under light irradiation, excited CdS nanoparticles electrons injected
across the CdS/ZnO or CdS/TiO2 interface. Need for the nanoparticles in
SSSCs or buffer layer in solar cells is of high band gap energy and
transmittance. For these purposes size quantized CdS thin films are achieved.
Size quantized CdS thin films have a suitable conductivity (>1016
carriers/cm3), and sufficient thickness to allow high transmission and good
Chapter-III Wet chemical…………photoelectrochemical performance
83
uniformity to avoid electrical short-circuit effects. The change in band gap in
quantized CdS is due to increase of the conduction band edge because of the
lower effective mass of the electron compared with the hole. This translates
into a decrease in electron affinity and a corresponding change in the band
line up. For thin film CIS/CdS, where the CdS conduction band minimum is
below that of the CIS the decrease in CdS electron affinity due to size
quantization should result in a reduction of the conduction band offset. This in
turn should result in an increase in open circuit voltage (Voc) and reduction in
the interface recombination [18-20].
Particular properties achieved in thin films depend on the deposition
method and the particular conditions of preparation. CdS thin films can be
prepared by chemical, physical and electrochemical methods such as vacuum
evaporation [21], sputtering [22], spray pyrolysis [23], chemical bath
deposition [24], electro-deposition [25], successive ionic layer adsorption and
reaction method (SILAR) [26], screen printing [27]. Of these methods,
chemical bath deposition (CBD) is simple and economic. CBD provides the
suitable way to control the thickness and transparency of thin film. These will
satisfy the requirement of buffer layer in solar cell and thin layer of
nanoparticles on wide band gap semiconductor for charge injection process in
SSSC.
In the present study, we describe the synthesis and characterization of
nanocrystalline CdS thin films via CBD technique. The films deposited at
various time intervals at low temperature (less than 1000C) using low
precursor concentration. Band gap up to 2.98 eV, an increase of 0.56 eV over
the bulk band gap of CdS, have been obtained for these CdS thin films. The
prepared thin films are further investigated for their structural, surface
morphological and optical properties. The photoelectrochemical (PEC)
performance such as J-V characteristics in dark and under illumination,
photovoltaic output, spectral response and transient response of prepared
films are studied. Finally, the performance parameters like short circuit
current density (JSC), open circuit voltage (VOC), series resistance (RS), shunt
resistance (RSh), junction ideality factor (nd) under illumination, fill factor (FF)
and photo-conversion efficiency (η) are discussed.
Chapter-III Wet chemical…………photoelectrochemical performance
84
3.3. Experimental:
3.3.1 Substrate Cleaning:
Cleaning of the substrates for thin film depositions is most important
factor. It affects the adherence, smoothness and uniformity of the film. The
techniques to be adopted for cleaning depend on nature of substrate, degree
of cleanness required and nature of contaminants to be removed. The
common contaminants are grease, adsorbed water, air borne dust and oil
particles. Cleanness is the process of breaking the bonds between substrates
and contaminants without damaging the substrates. There are various
methods to supply energy for breaking such bonds, such as heating,
bombarding by ions scrubbing etc. The following cleaning procedure was
used for glass substrates.
1) The substrates were first washed with the neutral detergent solution
labogent and then with the doubled distilled water.
2) The substrates were boiled in chromic acid for few minutes.
3) NaOH treatment was given to remove the acidic contaminants.
4) The substrates were again washed with double distilled water.
5) Lastly, the substrates were ultrasonically cleaned.
6) Drying of the substrates were done in the vapor of the alcohol
3.3.2. Preparation of FTO coated conducting glass substrates:
The fluorine doped tin oxide (FTO) conducting coatings were prepared
by spray pyrolysis technique using pentahydrated stannic chloride (SnCl4-
5H2O) (purity 98%) and ammonium fluoride (NH4F) (purity 95%) as precursor
salts. The solution was prepared in double distilled water and sprayed through
a specially designed glass nozzle onto ultrasonically cleaned glass
substrates. The deposition parameters like solution concentration, spray rate,
nozzle to substrate distance (NSD), carrier gas flow rate, etc. were kept
constant at optimized values. The FTO coated conducting glass substrates,
with 90–95% transparency and sheet resistance 10–15 Ωcm2, were prepared.
Chapter-III Wet chemical…………photoelectrochemical performance
85
These FTO coated conducting glass substrates were further used for
deposition of ZnO thin films.
3.3.3. Preparation of CdS thin film:
All chemical were purchased from s. d. fine-chemicals and used without
any further purification. The cadmium sulfate (CdSO4·H2O) and thiourea
(H2N⋅CS⋅NH2) were used as cadmium (Cd) and sulphur (S) precursors.
Ammonia (NH3) was used as complexing agent.
For the deposition of nanostructured CdS thin film, 1mM CdSO4
solution was prepared in 15 ml double distilled water, then 2M liquor NH3
solution was added. This clear solution was kept under stirred condition for 10
min. Then, 15 ml, 1mM thiourea is added to above solution. Bath temperature
was optimized at 70oC. The CdS thin films were deposited by dipping the
substrates in to the above solution for 50, 100,150, 200, 250 and 300 min,
and samples are denoted as C1, C2, C3, C4, C5 and C6 respectively. The
deposited CdS films were rinsed with double distilled water, and allowed to
dry at room temperature, in ambient air. Fig. (3.1) shows the schematic
diagram of CBD method for the deposition of CdS thin films. Thicknesses of
the thin films of the samples were found to in the range 549 to 1389 Å.
3.3.4 Characterization of CdS thin films:
The structural properties of the CdS thin films were studied with X-ray
diffraction (XRD) using an X-ray diffractometer (Philips, PW 3710, Almelo,
Holland) operated at 25 kV, 20 mA with CuKα radiation (1.5407 Å). The
Fourier transform Raman (FT-Raman) spectra of the films were recorded in
the spectral range of 250–1000 cm−1 using FT-Raman spectrometer (Bruker
MultiRAM, Germany) that employs Nd:YAG laser source with an excitation
wavelength 1064 nm and resolution 4 cm-1. The UV–Visible absorbance
spectra of CdS thin films were recorded using a UV–visible
spectrophotometer (UV3600, Shimadzu, Japan).
Chapter-III Wet chemical…………photoelectrochemical performance
86
Figure (3.1): Schematic experimental set up of CBD method and reaction
mechanism for deposition of CdS thin films
The surface morphology of the films were examined by analyzing the
scanning electron microscopy (SEM) (Model JEOL-JSM-6360, Japan),
operated at 20 kV. Field emission scanning electron microscope (FESEM
Model: JSM-6701F) was employed for closer insight into the CdS morphology.
The thickness of the resulting CdS thin film was estimated using surface
profiler (Ambios XP-1). The surface morphology and surface roughness of the
films was observed by using atomic force microscopy (AFM, Digital
Instrument, nanoscope III) operated at room temperature. The chemical
composition and valence states of constituent elements were analyzed by X-
ray Photoelectron Spectroscopy (XPS, Physical Electronics PHI 5400, USA)
with monochromatic Mg-Kα (1254 eV) radiation source. The J-V
characteristics were measured using Semiconductor Characterization System
SCS-4200 Keithely, Germany using two electrode configurations.
3.4. Results and discussion
3.4.1 X-Ray Diffraction study:
Fig. (3.2) shows the XRD patterns of the as deposited CdS thin film
Chapter-III Wet chemical…………photoelectrochemical performance
87
samples C1, C2, C3, C4, C5 and C6 on soda-lime glass for various film
thicknesses. The presence of broad XRD peaks is an indication of small
crystallite size in the nanorange, affirming the nanocrystalline nature of the
CdS thin films. The comparison of observed XRD patterns with the standard
(JCPDS data 80-0019) confirms the formation of CdS having cubic crystal
structure with diffraction peaks (111), (200), (220) and (311) planes at 39.90,
46.34, 67.67 and 81.03 degree respectively. The X-ray analysis revealed that
all the samples of CdS thin films are polycrystalline in nature with cubic crystal
structure. There is no any impurity peaks were originated from the samples
shows the purity of samples. The lattice parameter ‘a’ of the CdS is
determined from the analysis of the X-ray diffraction pattern and is estimated
from the formula for cubic system.
2
222
2
1
a
lkh
d
++=
………….. (3.1)
The mean values of a= 5.80 Å is in good agreement with the reported value
a=5.82 Å.
The mean crystallite size of CdS was calculated using the Debye–
Scherrer formula
θβ
λ
cos)111(
kD = ……………….. (3.2)
where λ = 1.5406Å , k is the dimensionless constant (0.95), β is the corrected
broadening of the diffraction line measured at half of its maximum intensity
(taken in radians by multiplying a factor of π/360) and D200 the diameter of
crystallite and θ is diffraction angle. The crystallite size of the CdS samples
C3, C5 and C6 are found to be 15.29, 20.73 and 24.90 nm respectively.
Chapter-III Wet chemical…………photoelectrochemical performance
88
Figure (3.2) XRD spectrum of CdS samples- (a) C1 (b) C2 (c) C3 (d) C4 (e) C5
and (f) C6
3.4.2 UV-visible Spectroscopy study:
Due to the nanocrystalline thin films, UV-Vis spectroscopy has become
an effective tool in determining the size and optical properties. Fig. (3.3)
shows the room temperature optical absorption spectrum of the all CdS
samples (C1 to C6) recorded in the range of 280-620 nm without taking into
account of scattering and reflection losses. As the size of the semiconductor
particle decreases to the nanoscale the absorption peaks of the prepared CdS
samples appear blue shifted compared with that of bulk CdS. It can be easily
understood that quantum confinement effect is present in the prepared CdS
thin films. Fig. (3.4) shows the optical transmittance of CdS samples C1 to C6.
The CdS samples are optically transparent in the range of 55 to 75 %
Chapter-III Wet chemical…………photoelectrochemical performance
89
Figure (3.3): Absorption spectrum of C1 to C6 CdS samples.
Figure (3.4): Optical transmittance of CdS samples C1 to C6
CdS is a direct band gap semiconductor with a band gap of 2.42 eV in
the bulk form [28]. The band gap value (Eg) is simply determined by using the
graph of absorption coefficient against wavelength (equation 2.13, Chapter II).
The band gap values were obtained as 2.98, 2.82, 2.75, 2.67, 2.59 and 2.42
eV for samples C1 to C6 respectively.
Chapter-III Wet chemical…………photoelectrochemical performance
90
Figure (3.5): Plot of (αhν)2 against hν and arrows shows the band gap
3.4.2.1 Particle size determination:
The size of nanoparticles can be estimated using theoretical models
such as effective mass approximation [29-30] and hyperbolic band model [31].
3.4.2.1. a) Effective mass approximation (EMA)
The EMA is based on the assumption of parabolic relation between the
electron energy E and the wave vector k. Effective mass approximation has
been used for the determination of particle size which relate the change in
band gap energy to the radius of the particle
2 2
2 * *
0
1 1 1.8
8 4
nano bulk
g g
e h
h eE E
r m m rπεε
≅ + + −
…………. (3.3)
where, Egnano = the band gap energy of the CdS nanoparticle as
determined from the UV-vis absorbance spectrum; Egbulk = band gap energy
of bulk CdS at room temperature (2.42 eV); h= Planck’s constant; e= charge
on electron; r= radius of the particle; me* = effective mass of conduction band
electron in CdS and mh* = effective mass of valence band hole in CdS.
Chapter-III Wet chemical…………photoelectrochemical performance
91
3.4.2.1. b) Hyperbolic band model (HBA)
For larger sizes of the nanocrystallites the EMA gives a good
description of the size dependence of band gap. However, it grossly
overestimates the change in the band gap for smaller nanocrystals. One
possible cause of breakdown of EMA may be the assumption of parabolic
energy bands of the form h2k2/2me which is accurate only for small values of
k. To overcome the shortcomings of parabolic band approximations the
hyperbolic band model (HBA) has been proposed. The main improvement of
the hyperbolic band model over Brus model is the inclusion of the effect of
electron and hole band non-parabolicity. Hyperbolic model has been
proposed to explain the change of energy band gap as function of particles
radius as shown to be used to calculate the size dependent optical band gap
of CdS clusters.
2
( ) ( ) 28
g g nano g bulk
hE E E
MR∆ ≅ − = ………………….. (3.4)
where, M= effective mass of the system. For the CdS, M=1.919 10-31 kg.
Table (3.1): The band gap energy values with particle size for all CdS
samples
3.4.3 XPS analysis:
Fig. 3.6 (a) shows the survey spectrum of the CdS sample C6. No
peaks of other elements except Cadmium (Cd), Sulphur (S), Carbon (C), and
Oxygen (O) are observed.
Particle size (nm) Sr. No.
Sample Deposition time (min)
Thickness ( Å )
Band gap (eV)
By effective mass
approximation
By hyperbolic
band model 1 C1 50 549 2.98 3.51 3.57 2 C2 100 585 2.82 4.60 4.72 3 C3 150 734 2.75 6.57 6.68 4 C4 200 867 2.67 7.61 7.72 5 C5 250 1100 2.59 8.37 8.46 6 C6 300 1389 2.44 18.80 18.92
Chapter-III Wet chemical…………photoelectrochemical performance
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Figure (3.6 a, b and c): XPS survey of CdS thin film sample C6
The C and O peaks stem mainly from the atmospheric contamination due
exposure of the sample to air. An unambiguous presence of the Cd3d doublet
signal at 404 eV and 419 eV clearly shows the formation of CdS. Fig. 3.4 (b-c)
depicts narrow range scans for the Cd and S peak region of the same
samples. The binding energies obtained in the XPS analysis have been
corrected taking into account the specimen charging and by referring to C1s
at 284.88 eV. The two peak structure in Cd 3d core level arises from the spin-
orbit interaction with the Cd 3d5/2 peak position at 403.75 eV and the 3d3/2 at
410.48 eV. It is clear from the spectral graph that, Cd 3d exhibits narrow, well
defined feature for doublet structure. This suggests that, specifically Cd atoms
appear to bond to S atoms. The XPS binding energies of Cd 3d3 at 404.14 eV
and the S 2p at 160.89 eV are indicative of the CdS chemistry. The peak
originated at 1111.04 eV is due to the auger electron of Cd [32].
1 2 0 0 1 0 0 0 8 0 0 6 0 0 4 0 0 2 0 0 0
( a )X P S o f C d S
S2p
S2sC
1s
Cd3
d
O1s
Cd3
p3
Cd3
p1
CdM
N2
Cou
nts
/s
B i n d i n g E n e r g y ( e V )
166 164 162 160 158 156
(b)S2p
Counts
/s
Binding Energy (eV)416 414 412 410 408 406 404 402 400
Cd3
d5
Cd3
d3
(c)
Cou
nts
/s
Binding Energy (eV)
Chapter-III Wet chemical…………photoelectrochemical performance
93
3.4.4 Scanning electron microscopy:
Scanning electron microscopy (SEM) is a suitable method for the
analysis of nanostructured thin films.
Figure (3.7): SEM images of CdS sample C1, C2, C5 and C6
Fig.3.7 represents the SEM micrograph of as-grown CdS samples, onto glass
substrate fabricated by CBD at 70oC. It was observed from the images that
the as-deposited CdS films were homogeneous and covered the substrate
well without crakes. The SEM image shows that surface is smooth and well
covered with CdS film. It can be seen that nanoparticles are grouped and form
large grain. The morphology of CdS semiconductor materials depends on the
deposition time. The nanoparticles have diameter in the range of 55–80nm.
We believe that the formation of nanoparticles from the CdS clusters is by
dissolution–condensation (recrystallization) process at ultra low concentration
of precursor. As the deposition time was varied the nanoparticles sizes were
increases.
Chapter-III Wet chemical…………photoelectrochemical performance
94
3.4.5 Atomic Force Microscopy:
Atomic force microscopy (AFM) was employed to characterize the 3-D
and 2-D surface morphology of the CdS thin films. Treatment on the AFM
data allows quantitative information to be extracted on surface roughness.
The surface roughness is expressed in the terms of the root-mean-square
(RMS) value. Fig. (3.8) and (3.9) shows the AFM images of sample C2 and
C6. Its corresponding SEM image is also shown in fig.3.7. The AFM images
were taken at different spots on the films in order to obtain a representative
image of the surface. The AFM images clearly show the egg like morphology
of CdS nanoparticles. It can be seen that small spherical particles are grouped
and form large grain. The surface roughness of the CdS sample C2 and C6
are found to be 8.19 and 84.92 nm. The unique structural features may be
advantageous, as this morphology provides high surface area for efficient
permeability of electrolytes into the inner structure while maintaining a high
surface area for enhanced surface activities, as compared to that of the bulk
CdS structure obtained at room temperature. The roughness of the samples is
increases as the deposition time increase which is beneficial for PEC
performance.
Figure (3.8): Two dimensional and three dimensional AFM images of CdS
sample C2
Chapter-III Wet chemical…………photoelectrochemical performance
95
Figure (3.9): Two dimensional and three dimensional AFM images of CdS
sample C6
3.4.6. Photoelectrochemical characterization:
For the photoelectrochemical characterization of the CdS thin film
samples C1 to C6, all the measurements were performed in an electrolyte of 1
M polysulfide (Na2S-NaOH-S) in a two-electrode arrangement of following
configuration:
Glass/FTO / CdS/Na2S-NaOH-S/G
In thin film configuration of photoelectrochemical cell, CdS thin film
deposited on FTO acts as a working electrode (active area ~ 1.0 cm2), and G
is graphite, which acts as a counter electrode. The J-V characteristics were
measured by a SCS-4200 unit in the dark and under illumination at 28
mW/cm2
3.4.6.1 J-V characteristics:
Fig. (3.10) shows the J-V characteristics of all the CdS samples (C1 to
C6). The J-V characteristic in the dark resembles diode-like characteristics for
the PEC cells fabricated with all the samples. Under illumination, shifting of
the J–V curve in the fourth quadrant of the graph suggests that the electrons
are the majority carriers, confirming the n-type conductivity of CdS thin films.
Chapter-III Wet chemical…………photoelectrochemical performance
96
Figure (3.10): J-V characteristics of CdS thin film samples C1 to C6
The use of nanocrystalline in place of single crystal is desired from the
realistic point, utilizing nanocrystalline thin film semiconductor in PEC cell is
the absence of space charge layer at the electrode–electrolyte interface.
Under these circumstances, photogenerated charge carriers can move in both
directions may be recombine readily with the hole or leak out at the electrolyte
interface instead of flowing through the external circuit [33]. Apart from this,
the efficiency in CdS-polysulphide PEC cells is limited because of the wide
band gap of CdS and strong absorption by the electrolyte. However this
experiment has not been done for efficiency consideration since efficiency of
a PEC cell depends on various parameters such as thickness of
photoelectrode, annealing, etc.
3.4.6.2 Ideality Factor:
The ideality factor is a fitting parameter that describes how closely the
diode's behavior matches with the behavior predicted by theory. The ideality
factor is determined under forward bias and is normally found to be in
between 1 to 2 depending up on the relation between diffusion current and
recombination current.
Chapter-III Wet chemical…………photoelectrochemical performance
97
Figure (3.11): Graph of ideality factor of CdS thin film samples C1 to C6
The ideality factor becomes 1 when the p-n junction of the diode is an infinite
plane and no recombination occurs within the space-charge region. When
recombination current is more than diffusion current then ideality factor
becomes 2. The ideality factor ‘nd’ of prepared CdS films is determined from
following diode equation (eq.3.5) as,
)1(/ −= kTnqV
odeII ------------- (3.5)
where, Io is the reverse saturation current, V is forward bias voltage, k is
Boltzmann's constant, T is ambient temperature in Kelvin and nd is an ideality
factor. Fig. (3.11) shows the graph of ideality factors for the CdS samples C1
to C6. The ideality factor was found to be 1.76, 1.63, 1.28, 1.58, 1.84 and 2.14
for the sample C1 to C6 under the light illumination respectively. The ideality
factor for the sample C6 is found to be greater than 2 indicates recombination
current is more. The higher value of nd is indicative of the series resistance
effect, surface states and the charge carrier recombination at the
semiconductor–electrolyte interface. These factors reduce the ideality of
devices [34].
Chapter-III Wet chemical…………photoelectrochemical performance
98
3.4.6.3 Photovoltaic output characteristics:
Photovoltaic output characteristic for CdS thin film samples C1 to C6
shown in Fig. (3.12) The open circuit voltage Voc found to be from 182 to 273
mV and short-circuit current Jsc are 39 to 347 µA respectively for sample C1 to
C6. The FF is determine from the following relation
scoc
c
scoc
m
JV
EA
IV
PFF
×
××=
×=
η …………………… (3.6)
where, Pm is the maximum power of the solar cell and is the
multiplication of maximum current density (Jmax) and maximum voltage (Vmax).
The value of Jmax and of Vmax can be extracted from a PEC solar cell.
The fill factor (FF) is directly affected by the values of the cells series
(Rs) and shunt (Rsh) resistance. Increasing the Rsh and decreasing the Rs will
lead to higher fill factor, thus resulting in greater efficiency. The FF and the
light to electricity conversion efficiency (η) along with the Jsc and Voc is given
in table 3.2.
-50 0 50 100 150 200 250 300
0
100
200
300
400
Cu
rren
t D
en
sit
y (
µµ µµA
/cm
2)
Photovoltage (mV)
Dark C1 C2 C3 C4 C5 C6
Figure (3.12): Output characteristics of CdS Samples C1 to C6
Chapter-III Wet chemical…………photoelectrochemical performance
99
Table (3.2): PEC performance of CdS thin films samples C1 to C6
3.4.6.4 Transient photoresponse:
In the photoelectrochemical solar cell the electron-transfer kinetics play
a major role in determining the energy conversion efficiency of the solar cells
[34, 35]. Transient photoresponse is a powerful tool to study the electron
lifetime in solar cells as a function of the photovoltage (Voc); the open-circuit
voltage-decay (Voc) technique. Corresponding to the level of injection of
minority carriers Voc decay curve has three distinct regions as high,
intermediate and low level. Following the technique as reported by Zaban et
al. [36] open-circuit voltage-decay measurements were performed by
monitoring the Voc transient during relaxation from an illuminated quasi-
equilibrium state to the dark equilibrium, see Figure 5. When the 28mW
illumination on a PEC cell at open circuit is interrupted, the excess electrons
are removed due to recombination, with the photovoltage decay rate directly
related to the electron lifetime by the following expression:
1−
−=
dt
dV
e
Tk ocBnτ …………………… (3.7)
The thermal energy is given by kBT, e is the positive elementary charge,
and dVoc/dt is the derivative of the open circuit voltage transient. Appropriate
use of eq 3.7 assumes that the recombination is linear with a first-order
dependence on electron concentration and that electron recombination occurs
only with the electrolyte.
Fig. (3.13) represents photo-voltage rise and decay curve of PEC cell
Sample Jsc µA/cm2
Voc mV
nd Rs Ω
Rsh kΩ
FF Efficiency η %
C1 39 182 1.76 18.6 4.16 0.22 0.005 C2 133 116 1.63 21.8 1.14 0.26 0.014 C3 209 168 1.28 22.31 0.96 0.30 0.037 C4 265 158 2.14 32.14 0.81 0.29 0.043 C5 285 244 1.84 24.20 1.12 0.30 0.073 C6 347 273 1.58 27.3 1.22 0.32 0.108
Chapter-III Wet chemical…………photoelectrochemical performance
100
of CdS samples C1 to C6. The open circuit voltage, Voc, of the PEC cell was
found to persist for some time after the light is cut off. From the fig. 3.13 it is
clear that as the deposition time was increases the CdS nanoparticles exhibit
superior recombination characteristics, with the longer lifetimes indicating
fewer recombination centers in the nanostructured CdS thin films.
Figure (3.13): Photovoltage-decay measurement of a CdS samples.
Chapter-III Wet chemical…………photoelectrochemical performance
101
3.6 3.7 3.8 3.9 4.0 4.1 4.20.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
lo
g V
oc
log t
C1 C2 C3 C4 C5 C6
Fig. (3.14) Plot of Log (Voc) versus Log (t) for photovoltaic decay curve
The log (Voc) versus log (t) plot was found to be linear (fig. 3.14) obeying the
relation.
bococ tVtV
−×= )0()( ……………… (3.8)
where, Voc (O) and Voc (t) are the open circuit voltages at t=0 and t
second, respectively, and ‘b’ is the decay constant. The linearity of the plot
suggests that the kinetics involved in the voltage decay process is of the
second order.
3.4.6.5 Spectral response studies:
Because of the wavelength dependence of the absorption coefficient
one expects the shorter wavelengths to be absorbed closer to the surface
while the longer wavelengths are absorbed deep in the bulk. Surface
recombination will therefore be more important for short wavelengths while
recombination in the quasi-neutral region is more important for long
wavelengths. Spectral response study of the PEC cells was carried out by
measuring Jsc as a function of wavelength (λ). Before the measurement, PEC
Chapter-III Wet chemical…………photoelectrochemical performance
102
cell was kept in dark for some time and the response was measured using
progression from longer wavelength to shorter wavelengths.
Fig. (3.12) shows the variation of Jsc with wavelength (λ) for the PEC
cell formed for C1 to C6. From spectral response graph, it is observed that,
Jsc increases with λ attains maximum value and then decreases. The
decrease in photocurrent on shorter wavelength side is attributed to the
absorption of light in the electrolyte and high surface recombination [37]. The
decrease in photocurrent on longer wavelength side is might be due to the
transition between the defect levels [38]. Using spectral responses at maxima,
the bandgap can be estimated. The bandgap values are in the range of 2.72
to 2.48eV for the sample C1 to C6.
Figure (3.15): Spectral photo-response of CdS Samples C1 to C6
Chapter-III Wet chemical…………photoelectrochemical performance
103
3.5. Conclusions:
Size quantized CdS thin films have been successfully grown by simple
chemical bath deposition (CBD) method. For the growth of size quantized
CdS thin film low concentration precursor solutions were used. The absorption
studies revealed a strong blue shift in spectra which indicates the quantum
size effect. The as grown CdS thin films shows the particles sizes in the range
of 4-19 nm in diameter. Photoelectrochemical (PEC) performances were
investigated using two electrode configurations in polysulphide electrolyte.
The maximum of efficiency of CdS is found to be 0.1%. The simplicity of this
technique, it is believed, will establish promising opportunities in the
processing of nanostructured materials with enhanced surface activity for
various applications, including solar cells.
Chapter-III Wet chemical…………photoelectrochemical performance
104
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