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Chapter-VII
ANTIMONY-DOPED Cd0.7Ni0.3Se
THIN FILMS: SYNTHESIS
AND CHARACTERIZATION
Chapter-VII
155
7.1 Introduction
The binary and ternary semiconductor thin films are being actively
investigated for their potential to photovoltaic applications. The study of these
materials is important due to the fact that the band gap and lattice parameters
can be varied by changing the cation composition. In our earlier studies, a
detailed discussion of the basics involved in the Cd1-xNixSe thin film
deposition processes and technological requirements for construction and
fabrication of an efficient semiconductor-electrolyte junction solar cell is
given. From these studies, a significant PEC performance has been noticed for
Cd0.7Ni0.3Se thin films. However, the observed performance of the PEC cell
was found to be of smaller magnitude .The loss in conversion efficiency has
been attributed to the several reasons. 1
One of the reasons in the loss in conversion efficiency is due to higher
resistivity of the photoelectrode material. An efficient way to decrease the
resistivity and to improve the properties of semiconducting material is to dope
with a suitable impurity like copper, silver, antimony, bismuth, indium etc.
The dopant has shown to enhance properties in number of host lattices. 2-6
Antimony (Sb) is one of the most extensively studied dopant in the various
host lattices. 7-11
Liu Huiyong 12
pointed out that AgInSbTe thin film with
high antimony content was easy to separate-out antimony crystalline state
firstly when heat-treatment was carried out at low temperature. The reason for
the formation of antimony crystalline state may be the introduction of other
nucleation sites into the network of alloy.
The electronic and optical properties of semiconductors are strongly
influenced by the doping process, which provides the basis for tailoring the
desired carrier concentration and, consequently, the absorption, emission and
transport properties as well. When the density of n-type or p-type doping
becomes sufficiently high, the impurity merges with conduction and valence
band and causes the formation of band tail and band gap shrinkage.13-15
The attempts were made to study the effect of antimony dopant on the
physical, compositional, structural, morphological, optical, thermoelectrical,
Chapter-VII
156
electrical properties and photoelectrochemical properties of optimum
Cd0.7Ni0.3Se composition and the results are presented and discussed in this
chapter.
7.2 Experimental Details
7.2.1 Synthesis of Sb-Doped Cd0.7Ni0.3Se Thin Films
For the synthesis of Cd0.7Ni0.3Se:Sb thin films typical bath was made
in a reactive solution obtained by mixing of, 7 mL cadmium sulphate
octahydrate (0.25M), 3 mL nickel sulphate (0.25M), 8.5 mL tartaric acid
(1M), 14 mL ammonia, 2 mL hydrazine hydrate (50%), 6 mL (5M) NaOH
and 10 mL sodium selenosulphate (0.25M). The varying concentration of
antimony from 0.01 to 1.0 mol % was used. To obtain Sb-doped Cd0.7Ni0.3Se
thin films, the calculated volume of SbCl3 solution was directly added to the
reaction bath. Sodium selenosulphate was prepared by following the method
reported earlier.16
All the chemicals used were of AR grade. The total volume
of the reaction mixture was made to 150 mL by adding double distilled water.
The beaker containing reactive solution was transferred to ice bath of 278 K.
The pH was found to be 12.00 ± 0.05. Four-glass substrates were kept
vertically in a reaction mixture and rotated with a speed of 50±2 rpm. The
temperature of the solution was allowed to rise slowly to room temperatures.
After the deposition for 240 min, the substrates were taken out of the bath,
rinsed with distilled water, dried in air and kept in desiccator. For
photoelectrochemical studies, thin films were deposited on polished stainless
steel substrates.
7.2.2 Characterization of Sb-Doped Cd0.7Ni0.3Se Thin Films
The thickness of all thin films was measured with usual weight
difference method. The structural properties of the ‘annealed’ Sb-doped
Cd0.7Ni0.3Se thin film samples were recorded by a Philips PW-1710 X-ray
diffractometer (XRD) with Cr Kα1 radiation in the 2θ range from 100-100
0.
The electrical conductivity of all the film was measured using a ‘dc’
two-probe method. A quick drying silver paste was applied at the ends of the
film for ohmic contact purpose. For the measurements of conductivity, a
Chapter-VII
157
constant voltage of 30V was applied across the sample. The current was noted
at different temperatures. Maintaining a temperature gradient along the length
of a film and the potential difference between the points separated by a 1cm
was recorded with a digital microvoltmeter. A calibrated thermocouple probe
(chromel-alumel, 24 gauge) with a digital indicator was used to sense the
working temperature.
The optical absorption measurements were made in the wavelength
range 400-1500 nm by using a Hitachi-330 (Japan) UV-VIS-NIR double
beam spectrophotometer at room temperature. Placing an identical, uncoated
glass substrate in the reference beam made a substrate absorption correction.
The analysis of the spectrum was carried out by computing the values of
absorption at every step of 2 nm. A 250MK-III Stereoscan (USA) scanning
electron microscope (SEM) was used for the microscopic observations. For
photo-electrochemical characterization, antimony doped Cd0.7Ni0.3Se thin
films deposited on stainless steel plates were used as photoanode, sulphide-
polysulpide as electrolyte and CoS treated graphite rod as counter electrode.
I-V, C-V characteristics in dark and power output curves under constant
illumination of 30 mW/cm2 were determined. To determine built-in-potential,
reverse saturation current was measured in the temperature range 303-373 K.
Photo-response was also recorded. The performance parameters such as nd,
Фβ, Isc, Voc, η%, ff%, Rs, Rsh, etc. were computed from the data obtained.
7.3. Results and discussion
7.3.1 Physical Properties and Compositional Analysis
The ‘as deposited’ antimony doped, Cd0.7Ni0.3Se thin films were found
to be uniform and well adherent to the substrate. The color of the films was
found to darken with increase in concentration of Sb. The film thickness
increases from 0.65 to 0.74 µm as doping concentration increases upto 0.075
mol%, thereafter it decreases. The increase in film thickness can be explained
on the basis of substitutional inclusion of antimony ions in the interstitial
position of lattice or in cationic vacancies already present in the host. The
thickness of the film is also dependent on the factors such as deposition
Chapter-VII
158
temperature, deposition time, pH, molar concentration, speed of the rotation,
etc. However, in our investigation, all parameters except doping concentration
were kept at their optimum values and the films were doped with antimony
concentration from 0.0 to1.0 mole %. At higher doping concentration, the
impurity atom might be occupying interstitial sites causing an impurity
scattering and thereby preventing the further growth of the film. 17
The
thickness was measured every 30 minutes and plotted against time of
deposition as shown in Fig. 7.1 for 0.075 mol % antimony concentration. The
nucleation was not observed within first 30 minutes which indicates that the
process requires an induction time for the nucleation on substrate. This also
suggests that the process of deposition follows ion-by–ion growth mechanism
instead of cluster-by-cluster. Growth kinetics for the development of typical
Cd0.7Ni0.3Se:Sb (0.075 mol% Sb) is shown in Fig. 7.2 The figure shows that,
in early stages of growth the deposition varies linearly with deposition
temperature and then decreases after typical temperature.
The film composition was determined by dissolving the known weight
of film material in concentrated HNO3 and analyzing the film by atomic
absorption spectroscopy. From these studies, the actual amount of antimony
entered into the lattice of Cd0.7Ni0.3Se was calculated. The amount of metal
ion is taken in the bath and those obtained from AAS analysis are given in
Table 7.1
7.3.2 X-Ray Diffraction Studies
The X-ray diffractograms of ‘annealed’ antimony-doped Cd0.7Ni0.3Se
thin films deposited on glass substrate are shown in Fig. 7.3. The
crystallographic study reveals that doped samples are polycrystalline in
nature. In the present investigation, Sb-doped Cd0.7Ni0.3Se thin films were
found to be hexagonal in nature. The spectra for pure CdSe [JCPDF Card
No.08-0459] and pure NiSe [JCPDF Card No.75-0610] were used for
identification purpose.
Chapter-VII
159
0
0.3
0.6
0.9
0 1 2 3 4 5 6
Deposition time (hours)
Th
ickn
ess (
µm
)
Fig.7.1 A plot of film thickness versus time for Cd0.7Ni0.3Se: Sb thin
film (0.075 mol% antimony concentration)
Fig.7.2 A plot of film thickness versus temperature for Cd0.7Ni0.3Se: Sb
thin film (0.075 mol% antimony concentration)
Chapter-VII
160
20 40 60 80 100
0
10
20
30
40
50
(203)
(112)(1
03)
(110)(1
02)(1
00)
0.075% Sb
Inte
nsit
y(A
.U.)
2θ2θ2θ2θ0000
(a) (b)
20 40 60 80 100
0
10
20
30
40
50
0.25% Sb
(203)
(112)(103)
(110)
(102)
(100)
2θ2θ2θ2θ0000
Inte
nsit
y(A
.U.)
20 40 60 80 100
0
10
20
30
40
50
(203)
(112)(1
03)
(110)
(102)
(100)
0.75% Sb
Inte
nsit
y(A
.U.)
2θ2θ2θ2θ0000
(c) (d)
20 40 60 80 100
0
20
40
60
1.0 % Sb
(100)
(102)
(110)
(103)
(112)
(203)
In
ten
sit
y (
A.U
.)
2θθθθo
(e)
Fig.7.3 XRD patterns of representative Sb-doped Cd0.7Ni0.3Se thin films
(a) 0.01 mol% Sb (b) 0.075 mol % Sb (c) 0.25mol % Sb (d) 0.75 mol% Sb
(e) 1.0 mol % Sb
20 40 60 80 100
0
20
40
60
0.01% Sb
(112)
(203)
(103)
(110)
(102)
(100)
In
ten
sit
y (
A.U
.)
2θθθθo
Chapter-VII
161
The analysis of spectrum indicated that the films are having
hexagonal structure in the whole range of compositions studied. The analysis
of XRD patterns in terms of hkl planes, interplanar distances, cell size and
lattice parameters has been done by considering hexagonal structure and is
cited in Table 7.2. The most intense reflection observed for all the thin films
were originating from (100) plane. Along with (100) plane, (102) (110) (103)
(112) (203) planes are also observed. The peak intensity and crystallinity of
the films were found to increase with dopent concentration upto 0.075 mol %.
Lattice constants were determined by using following equation;
1/d2
hkl = 4/3 (h2+hk+k
2/a
2) +l
2/c
2 --------------------7.1
It is found that upto 0.075 mol% Sb, the lattice parameter increases smoothly
from 5.3063 to 5.3071 Å for ‘c’ and 3.4794 to 3.4805 for ‘a’ and thereafter
decreases upto 5.3062 Å for ‘c’ and 3.4790 Å for ‘a’ for 1.0 mol% antimony
concentration. The average crystallite size was calculated by resolving the
highest intensity peak (100). The average crystallite size was determined by
using Scherrer’s formula. Upto 0.075 mol% Sb, the particle size increases
from 318 to 377 Å, thereafter decreases upto 246 Å for 1.0 mol% Sb
concentration.
7.3.3 Microscopic Analysis
The SEM micrographs of Cd0.7Ni0.3Se:Sb samples are shown in Fig.
7.4 at 10000X magnification. Cd0.7Ni0.3Se:Sb thin films are seen to be
homogenous, without cracks or pinhole and well cover the glass substrate.
Smooth background that may correspond to some amorphous phase of NiSe
as well as CdSe. The presence of fine background is an indication of one-step
growth by multiple nucleations. The grain size calculated from SEM was
found to tally with those obtained using the XRD. The observed grain size is
listed in Table 7.2.
Chapter-VII
162
(a) (b)
(c) (d)
(e)
Fig.7.4 SEM micrographs of representative Sb-doped Cd0.7Ni0.3Se:Sb thin
films (a) 0.01 mol% Sb (b) 0.075 mol % Sb (c) 0.25mo l% Sb
(d) 0.75 mol% Sb (e) 1.0 mol% Sb
Chapter-VII
163
7.3.4 Optical Studies
The optical properties of as deposited samples have been studied in the
wavelength range from 400-1500 nm without considering losses due to
reflection and scattering at room temperature. The absorption spectra are used
to calculate absorption coefficient, optical band gap and type of transition.
The absorption spectra of representative antimony doped films are shown in
Fig.7.5. For all the compositions, the value of absorption coefficient is high
(α x 104 cm
-1). The value of absorption coefficient depends upon radiation
energy as well as composition of the film. The data were systematically
studied in the vicinity of the absorption edge on the basis of three-dimensional
model. The interpretation of the results can be easily done with the help of
formula derived for three-dimensional crystal. The simplest form of equation
obeyed near and above absorption edge is; 18
α hυ =A(hυ-Eg)n---------------------------------------7.2
where the symbols have their usual meaning. Upto 0.075 mole % Sb, the
absorption is edge shifted towards higher wavelength. This is due to filling of
low lying energy level by conduction electron and segregation of the impurity
along the grain boundary.19
A plot of (αhυ)2
vs. hυ should be a straight line
whose intercept to the x-axis gives the optical band gap. The plots of (αhυ)2
vs. hυ for ‘as deposited’ samples are shown in Fig. 7.6. The linear nature of
plot shows the existence of direct transition. It is observed that the band gap
decreases from 1.61 eV to 1.54 eV as the antimony concentration increases
upto 0.075 mol %. Above 0.075 mol % Sb the band gap increases from 1.54
to 1.64 eV. This is probably due to increased amount of disorder caused by
addition of antimony in the host lattice.20
The variation of band gap with
different concentrations of antimony is shown in Fig. 7.7.
7.3.5 Electrical and Thermoelectrical Properties
The electrical conductivity of ‘as deposited’ Sb-doped Cd0.7Ni0.3Se thin
film on non-conducting glass slide was determined by using a ‘dc’ two-probe
method in the temperature range 300-525 K. The variation of log
(conductivity) versus inverse absolute temperature for the cooling curve is
Chapter-VII
164
0
0.1
0.2
0.3
0.4
0.5
0.6
400 600 800 1000 1200 1400
Wavelength (nm)
Ab
so
rb
an
ce
0.01%Sb
0.075%Sb
0.25 %Sb
0.75 %Sb
1.0%Sb
Fig.7.5 Absorption spectra of Sb-doped Cd0.7Ni0.3Se thin films
(doping concentration 0.01, 0.075, 0.25, 0.75, 1.0 mol %, antimony)
Chapter-VII
165
0
0.5
1
1.5
0 1 2 3Photon energy (eV)
( αα ααh
υυ υυ)2
x1
08 (
eV
/cm
)2
0.01%Sb
0
0.5
1
1.5
2
0 1 2 3 4
Photon energy (eV)
( αα ααh
υυ υυ)2
x1
08 (
eV
/cm
)2 0.075 %Sb
(a) (b)
0
0.5
1
1.5
2
2.5
3
3.5
0 1 2 3Photon energy (eV)
( αα ααh
υυ υυ)2
x1
08 (
eV
/cm
)2 0.25 %Sb
0
0.5
1
1.5
2
2.5
0 1 2 3 4Photon energy (eV)
( αα ααh
υυ υυ)2
x1
08 (
eV
/cm
)2
0.75% Sb
(c) (d)
0
0.4
0.8
1.2
1.6
2
0 1 2 3 4Photon energy (eV)
( αα ααh
υυ υυ)2
x1
08 (
eV
/cm
)2
1.0 %Sb
(e)
Fig.7.6 Determination of band gap of Cd0.7Ni0.3Se: Sb thin films (doping
concentration 0.01, 0.075, 0.25, 0.75 and 1.0 mol % antimony)
Chapter-VII
166
1.52
1.54
1.56
1.58
1.6
1.62
1.64
1.66
0 0.2 0.4 0.6 0.8 1
Doping concentration (mol % Sb)
Ba
nd
ga
p (
eV
)
Fig.7.7 Variation of band gap with doping concentration for
Sb-doped Cd0.7Ni0.3Se thin films
shown in Fig.7.8. The electrical conductivity at room temperature increases as
the antimony concentration increases upto 0.075 mol % and there after value
decreases for higher concentration. Up to 0.075 mole % incorporation of Sb in
the lattice results in decrease in boundary potential and improvement in
crystallite size, which results in increase of carrier concentration as well as
mobility.21
As the size of Sb3+
ion is less than that of Cd2+
ion, the
incorporation could cause a scattering process thereby reducing the
mobility.22-23
Above 0.075 mole % Sb the conductivity decreases, as more and
more distortion is observed in the lattice structure, which results in increase in
grain boundary scattering, thereby reducing the carrier mobility. 24-25
The
electrical conductivity variation with temperature during heating and cooling
cycles was found to be different and this shows that the ‘as deposited’ films
undergo irreversible changes due to annealing out of non-equilibrium
irreversible changes defects during first heating. The activation energy is
calculated using exponential form of Arrhenius equation;
σ = σoexp(-Ea/kT)--------------------------------7.3
Where the terms have usual meaning. The activation energy is found to
Chapter-VII
167
-5.5
-4.5
-3.5
-2.5
-1.52 2.5 3
1000/T (K-1
)
log
σσ σσ
0.01% Sb
0.075%Sb
1%Sb
(a)
-5.5
-4.5
-3.5
-2.5
-1.5
2 2.5 3
1000/T (K-1
)
log
σσ σσ
0.1%Sb
0.25%Sb
0.75%Sb
(b)
Fig. 7.8 Plot of log (conductivity) versus 1000/T for Cd0.7Ni0.3Se:Sb thin
films
Chapter-VII
168
decrease upto 0.075 mole % and increase for higher concentration. The value
of activation energy as well as specific conductivity at 300 and 525 K is listed
in Table 7.3.
The thermoelectric power generated by the samples was measured in
the 300-500 K temperature range. In thermoelectric power measurements, the
open circuit thermovoltage generated by the sample when a temperature
gradient is applied across a length of the sample was measured using a digital
microvoltmeter. The temperature difference between the two ends of the
sample causes transport of carriers from the hot to cold end, thus creating an
electric field, which gives rise to thermovoltage across the ends. The
thermovoltage generated is directly proportional to temperature gradient
maintained across the semiconductor ends. From the sign of the potentiometer
terminal connected at the cold end, one can deduce the sign of predominant
charge carriers.26
In our investigation, the positive terminal was connected to
the cold end; therefore, the films exhibit p-type conductivity. The temperature
dependence of thermoelectric power is shown in Fig.7.9. The thermoelectric
power is seen to increase up to 0.075 mol% Sb, and there after it is seen to
decrease. The carrier density and mobility were determined for all the samples
using equations mentioned in section 5.4.4. The plot of carrier density against
temperature is shown in Fig. 7.10. The carrier concentration decreases with
doping concentration up to 0.075 mol% Sb, thereafter it increases for higher
doping level. However, mobility increases with doping concentration up to
0.075 mol% Sb, thereafter decreases. The height of the potential barrier at the
grain boundary can be determined from the plot of log µT1/2
versus 1000/T.
Fig.7.11 shows such variations for some representative samples. The height of
potential barrier at the grain boundary decreases with antimony concentration
up to 0.075 mol%, but increases further for higher doping level.
Chapter-VII
169
0
10
20
30
40
50
60
300 350 400 450 500 550
Temperature (K)
Th
erm
oe
lec
tric
po
we
r (
µµ µµV
/K)
0.01% Sb
0.1% Sb
0.075%Sb
0.75%Sb
1%Sb
0.25%Sb
Fig.7.9 The temperature dependence of thermoelectric power for
Cd0.7Ni0.3Se: Sb thin films.
Chapter-VII
170
2
2.5
3
3.5
4
4.5
300 350 400 450 500 550
Temperature (K)
Carrie
r c
oncentr
ati
on (
n)X
10
19
19
19
19
x=0.01
x=0.075
x=0.1
x=0.75
x=0.25
x=1.0
Fig. 7.10 The temperature dependence of carrier density for Cd0.7Ni0.3Se: Sb
thin films
Chapter-VII
171
7.3.6 PEC Studies
7.3.6.1 I-V, C-V Characteristics in Dark
To know the nature of charge transfer across the photoelectrode
electrolyte interface, Butler-Volmer relation was used. The value of β was
greater than 0.5, confirming the rectifying nature of the interface. The current
I developed is maximum for 0.075 mol% Sb doping concentration. The
junction ideality (nd) was determined from the plot of log I versus V for all
samples. Fig.7.12 shows variation of log I with V of sample. The junction
ideality factor varies with the doping concentration. The junction ideality
factor decreases from 4.64 to 2.67 as the doping concentration increases up to
0.075 mol% Sb.(Table 7.4). For higher level of doping concentration, the
junction ideality factor increases. The lower value indicates the less trap
density at the interface.27
To determine the flat band potential, C-V curve
plots are constructed and are shown in Fig.7.13 The flat band potential (Vfb) is
found to increase from 710 to 763 mV, with doping concentration upto 0.075
mol% Sb and thereafter decreases for higher doping level. This is because
antimony creates new donor level, which shifts Fermi level in upward
direction, thereby increasing the amount of band bending and hence flat band
potential increases. At higher doping concentration, a pinning of Fermi level
may decrease the flat band potential.28-29
The plots deviate from linearity and
indicate that the junction is graded type23
7.3.6.2 Power Output Curves
To study the various power output characteristics of Sb doped
Cd0.7Ni0.3Se, PEC cells were examined under 30 mW/cm2 intensity. The
power output curve for representative doping concentration is shown in
Fig.7.14. From the figure, it is observed that Voc and Isc increases with
increase in doping concentration up to 0.075% Sb and thereafter they
decrease. As the doping concentration increases 0.075% Sb, Voc as well as Isc
is found to increase from 260 to 275 mV and 161 to 186 µA/cm2
respectively.(Table 7.4). The increase in the Voc and Isc directly affects the
Chapter-VII
172
-5
-4.5
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3
1000/T (K-1
)
log
µµ µµT
1/2
0.01% Sb
0.075% Sb
0.1% Sb
0.25%Sb
0.75% Sb
1.0% Sb
Fig. 7.11 A plot of log µT1/2
versus 1000/T for Cd0.7Ni0.3Se: Sb thin films
Chapter-VII
173
-6
-5
-4
-3
-2
0 50 100 150 200 250 300
Voltage (mV)
log
I
0.01% Sb
0.075% Sb
0.25% Sb
0.75% Sb
1.0% Sb
Fig.7.12 Variation of log I versus V of Cd0.7Ni0.3Se: Sb cells.
Chapter-VII
174
0
1
2
3
4
5
6
7
200 300 400 500 600 700 800
Voltage (mV vs.SCE)
1/C
22 22*10
8(F
-2cm
4)
0.075% Sb
0.01% Sb
0.25% Sb
0.75% Sb
1.0% Sb
Fig. 7.13 The C-V plots of Cd0.7Ni0.3Se: Sb cells
Chapter-VII
175
conversion efficiency and fill factor. An improvement in conversion
efficiency and fill factor has been noted for 0.075 mol% Sb concentration.
The increase in the Isc might be due to decreased photoelectrode resistance
and an increased absorbance by the material itself, while the enhancement in
Voc could be correlated to the increase in flat band potential, barrier height
and partly due to improved grain structure of the material itself. 27, 30
The
resistance of the cell was found to decrease upto 0.075mol% Sb concentration
and then increase for higher doping level.
7.3.6.3 Barrier-height Measurement
The barrier height for Cd0.7Ni0.3Se thin film doped with antimony, were
determined by measuring the reverse saturation current through the junction at
various temperature from 363 K to 303 K. From the plot of reverse saturation
current (Io) against 1000/T, value of barrier height was determined. Fig.7.15
shows the plot of log (Io/T2) with 1000/T. The non-linearity of plots in higher
temperature regions can be attributed to Pool-Frankel type of conduction
mechanism. The barrier height was found to be increase from 0.180 to 0.194
eV, as the doping concentration was increased upto 0.075 mol% Sb and then
it decreased thereafter for higher doping concentration.
7.3.6.4 Photo-response
The photoresponse of all Cd0.7Ni0.3Se thin film doped with antimony,
were measured in the illumination intensity range of 10-50 mW/cm2. Fig.7.16
shows plot of Voc against illumination intensity. The open circuit voltage
deviates from the linearity at high illumination. This is due to presence of
surface states at the interface and unequal distribution of electrons and holes
in the space charge region. Fig.7.17 shows plot of Isc against illumination
intensity. The short circuit current varies almost linearly with incident light
intensity. The lighted ideality factor (nL) were determined for all the cells by
plotting the Voc and log Isc. The plot of Voc with log Isc is shown in Fig.7.18.
The different performance parameters such as nd, Фβ, Isc, Voc, η%, ff%, Rs,
Rsh, Vfb, nL are included in the Table 7.4 for different Cd0.7Ni0.3Se:Sb thin
films.
Chapter-VII
176
0
50
100
150
200
0 50 100 150 200 250 300
Voltage (mV)
Cu
rren
t ( µµ µµ
A/
cm
2)
0.01%Sb
0.075%Sb
0.25%Sb
0.75% Sb
1.0%Sb
Fig 7.14 Power output curves for Cd0.7Ni0.3Se: Sb photoelectrodes
-10
-9.5
-9
-8.5
-8
-7.5
2.6 2.8 3 3.2 3.4
1000/T (K-1
)
log
(I o
/T2)
0.01%Sb
0.075%Sb
0.25%Sb
0.75%Sb
1.0%Sb
Fig.7.15 variation of log Io/T2 for Cd0.7Ni0.3Se: Sb cells
Chapter-VII
177
50
150
250
350
450
10 20 30 40 50
Light intensity (mW/cm2)
Voc (m
V)
0.25%Sb
0.075%Sb
0.75%Sb
0.01% Sb
1.0%Sb
Fig.7.16 Variation of Voc with illumination intensity Cd0.7Ni0.3Se: Sb cells
0
100
200
300
10 20 30 40 50
Light intensity (mW/cm2)
I sc (
µµ µµA
/cm
2)
0.01%Sb
0.075%Sb
0.25%Sb
0.75% Sb
1.0%Sb
Fig. 7.17 Variation of Isc with illumination intensity Cd0.7Ni0.3Se: Sb cells
Chapter-VII
178
-5
-4.5
-4
-3.5
-3
50 100 150 200 250 300 350
Voc (mV)
log
Is
c
1.0%Sb
0.75% Sb
0.25%Sb
0.075%Sb
0.01%Sb
Fig. 7.18 Plot of log Isc versus Voc for Cd0.7Ni0.3Se: Sb cells
Chapter-VII
179
Table 7.1: Compositional analysis of Cd0.7Ni0.3Se: Sb thin films
Sr.
No.
Mole % of
Sb in
Cd0.7Ni0.3Se
Bath content in ppm Film content in ppm
Cd Ni Se Sb Cd Ni Se Sb
01 0.0 786.87 172.77760 ---- 210.13 47.20 218.00 ------
02 0.01 786.87 172.77 760 0.283 207.12 45.46 201.20 0.0745
03 0.025 786.87 172.77 760 0.708 212.66 46.70 205.41 0.1913
04 0.05 786.87 172.77 760 1.418 201.76 44.51 195.68 0.3639
05 0.075 786.87 172.77 760 2.124 223.12 49.36 216.48 0.6068
06 0.1 786.87 172.77 760 2.837 218.57 47.99 211.15 0.7881
07 0.25 786.87 172.77 760 7.094 204.23 44.84 213.59 2.0864
08 0.5 786.87 172.77 760 14.18 219.57 48.80 186.14 3.4586
09 0.75 786.87 172.77 760 21.28 209.16 47.12 180.95 5.0667
10 1.0 786.87 172.77 760 28.36 214.31 45.10 188.56 9.1483
Chapter-VII
180
Table 7.2: Crystallographic parameters of Cd0.7Ni0.3Se: Sb thin films
Mole % of Sb
in
Cd0.7Ni0.3Se
Observed
‘d’ values
(Ǻ)
Std. ‘d’
values(Ǻ)
hkl Grain Size(Ǻ) Cell
Parameters
(Ǻ) NiSe
(H)
CdSe
(H)
XRD SEM
0.0 3.555
2.398
2.055
1.851
1.737
1.374
3.169
2.039
1.830
1.549
1.509
1.183
3.720
2.554
2.151
1.980
1.834
1.456
100
102
110
103
112
203
318 322 a=3.4794
c=5.3063
0.01 3.557
2.399
2.056
1.853
1.739
1.376
3.169
2.039
1.830
1.549
1.509
1.183
3.720
2.554
2.151
1.980
1.834
1.456
100
102
110
103
112
203
300 325 a=3.4796
c=5.3064
0.025 3.559
2.401
2.058
1.855
1.741
1.378
3.169
2.039
1.830
1.549
1.509
1.183
3.720
2.554
2.151
1.980
1.834
1.456
100
102
110
103
112
203
326 333 a=3.4798
c=5.3065
0.05 3.562
2.403
2.061
1.858
1.745
1.380
3.169
2.039
1.830
1.549
1.509
1.183
3.720
2.554
2.151
1.980
1.834
1.456
100
102
110
103
112
203
358 364 a=3.4801
c=5.3067
0.075 3.565
2.407
2.062
1.858
1.748
1.385
3.169
2.039
1.830
1.549
1.509
1.183
3.720
2.554
2.151
1.980
1.834
1.456
100
102
110
103
112
203
377 384 a=3.4805
c=5.3071
Chapter-VII
181
0.1 3.562
2.405
2.063
1.860
1.746
1.383
3.169
2.039
1.830
1.549
1.509
1.183
3.720
2.554
2.151
1.980
1.834
1.456
100
102
110
103
112
203
349 355 a=3.4800
c=5.3069
0.25 3.560
2.403
2.060
1.858
1.744
1.382
3.169
2.039
1.830
1.549
1.509
1.183
3.720
2.554
2.151
1.980
1.834
1.456
100
102
110
103
112
203
301 307 a=3.4797
c=5.3067
0.50 3.558
2.400
2.058
1.856
1.744
1.380
3.169
2.039
1.830
1.549
1.509
1.183
3.720
2.554
2.151
1.980
1.834
1.456
100
102
110
103
112
203
281 286 a=3.4794
c=5.3066
0.75 3.556
2.394
2.056
1.852
1.739
1.378
3.169
2.039
1.830
1.549
1.509
1.183
3.720
2.554
2.151
1.980
1.834
1.456
100
102
110
103
112
203
277 270 a=3.4792
c=5.3063
1.0 3.553
2.393
2.054
1.849
1.736
1.375
3.169
2.039
1.830
1.549
1.509
1.183
3.720
2.554
2.151
1.980
1.834
1.456
100
102
110
103
112
203
246 250 a=3.4790
c=5.3062
Chapter-VII
182
Table 7.3: Optical and electrical parameters of Cd0.7Ni0.3Se: Sb thin films
Sr.
No. Mole % of Sb in
Cd0.7Ni0.3Se
Band
Gap
(eV)
Specific conductivity
(Ω cm)-1
Thickness
(µm)
Activation
energy
(eV) At 300K At 525K
1 0.0 1.61 1.04 x 10-5
7.1x 10-3
0.64 0.530
2 0.01 1.58 1.0 x 10-5
3.2 x 10-2
0.66 0.502
3 0.025 1.57 9.95 x 10-6
2.4 x 10-2
0.70 0.503
4 0.05 1.56 9.66 x 10-6
8.5x 10-3
0.72 0.495
5 0.075 1.54 8.88 x 10-5
5.5 x 10-2
0.74 0.493
6 0.1 1.56 9.25 x 10-6
1.2 x 10-2
0.73 0.512
7 0.25 1.58 9.1 x 10-6
1.6 x 10-2
0.70 0.524
8 0.5 1.59 9.52 x 10-6
1.1 x 10-2
0.68 0.521
9 0.75 1.61 9.8 x 10-6
9.4 x 10-3
0.66 0.501
10 1.0 1.64 0.10 x 10-6
8.7 x 10-3
0.65 0.509
Chapter-VII
183
Table 7.4: PEC cell performance parameters of Cd0.7Ni0.3Se: Sb
photoelectrode
Sr.
No.
Mole % of
Sb in Cd0.7Ni0.3Se
Voc
(mV)
Isc
(µA/cm2)η % ff %
Фβ
(eV)
Vfb
(mV)
Rsh
(Ω)
Rs
(Ω) nL nd
01 0.0 260 161 0.65 48.12 0.180 710 543 826 3.21 4.64
02 0.01 261 165 0.67 49.18 0.185 728 529 805 3.06 3.07
03 0.025 259 171 0.68 50.27 0.187 734 509 774 2.94 3.21
04 0.05 266 179 0.71 50.98 0.190 743 482 733 2.86 3.36
05 0.075 275 186 0.75 53.59 0.194 763 458 697 2.71 2.67
06 0.1 270 183 0.62 51.08 0.187 751 468 713 3.67 2.88
07 0.25 265 181 0.56 47.83 0.180 735 475 723 4.56 3.11
08 0.5 259 173 0.50 40.51 0.177 722 502 764 4.89 3.38
09 0.75 251 165 0.35 38.17 0.173 715 529 805 5.12 3.62
10 1.0 225 145 0.23 35.38 0.170 709 596 908 5.60 4.09
Chapter-VII
184
7.4 Conclusions
Antimony doped Cd0.7Ni0.3Se thin films have been deposited by
chemical bath deposition method using tartarate bath at 298 K. The antimony
donor atoms were found to dissolve substitutionally in the lattice of
Cd0.7Ni0.3Se upto a certain range of doping concentration. The films grow
highly oriented in the hexagonal phase. The crystallinity and particle size
were found to increase with antimony concentration upto 0.075 mol %
whereas for higher values of antimony, the material shows decreased
crystallinity. The grain size calculated by SEM tallies with the particle size
calculated by XRD. The absorption study shows presence of direct band gap
transition. The band gap decreases from 1.61 to 1.54 eV as the doping
concentration increases from 0.0 to 0.075 mol % whereas for higher values of
antimony, the band gap increases. The specific conductance at room
temperature for all the films was found to be of the order of 10-6
(Ω cm)-1
. The
electrical conductivity study indicated presence of only one conduction
mechanism. The conductivity increases while activation energy decreases
upto 0.075 mol% Sb. The open circuit voltage, build-in-potential, short-circuit
current, efficiency, fill factor, flat band potential have been found to increase
up to 0.075 mol% Sb concentration. The shunt resistance, series resistance,
junction ideality factor decrease upto 0.075 mol% Sb concentration.
Chapter-VII
185
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