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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