CHAPTER-III - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/4064/9/09...Thin films of cadmium sulphide (CdS) were deposited onto glass and optically transparent, electrically

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


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


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.


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.


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).


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


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.


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 %


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.


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.


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


92

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)


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.


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


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.


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.


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].


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


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


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.


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


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


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.


104

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