Journal of Modern Physics, 2012, 3, 1870-1877
http://dx.doi.org/10.4236/jmp.2012.312235 Published Online December 2012 (http://www.SciRP.org/journal/jmp)
Correlation between the Solution Chemistry to Observed
Properties of CdTe Thin Films Prepared by CBD Method
Laxman Gouda, Yelameli Ramesh Aniruddha, Sheela K. Ramasesha*
Divecha Centre for Climate Change, Indian Institute of Science, Bangalore, India
Received August 28, 2012; revised October 23, 2012; accepted November 7, 2012
ABSTRACT
The promising solar material Cadmium Telluride (CdTe) is successfully deposited on both plain glass and ITO coated
glass substrates. Many variations in composition of the solution used for deposition of the film are made to optimize the
deposition conditions. The bandgap calculated from optical transmission studies is found to be a function of the Cd/Te
atomic ratio in the film. The atomic ratio in the film is a function of the Cd/Te concentration ratio in the solution used in
deposition. Based on several experimental data points an equation involving the Cd/Te atomic ratio in the film and the
chemistry of the solution is deduced.
Keywords: Photovoltaic Cell; Thin Film; CdTe; Atomic Ratio; Chemical Bath Deposition (CBD)
1. Introduction
Non-silicon based thin film solar cells are making a head
way in the solar market. By 2010, the thin film solar cell
production had grown to 13%. The largest share of pro-
duction was the CdTe cells at 6% and at second place
was silicon based thin film solar cells at 5% [1].
CdTe is a direct band gap semiconductor. The reported
room temperature band gap of CdTe is in the range of 1.4
- 1.5 eV [2-4] ideally suited for solar radiation absorp-
tion. Energy conversion efficiency of CdTe/CdS cells has
been increasing continuously over the past few decades.
At present, the best in the class CdTe cell has solar
power conversion efficiency of 17.3% and at the module
level an efficiency of 14.4% is achieved [5]. The theo-
retical calculations by Fahrenbruch et al. [6] indicated
highest achievable efficiency for these cells to be 17%
which is close to what has been achieved experimentally.
Many fabrication techniques have been attempted for
depositing CdTe thin films. Some are controlled atmos-
phere based techniques like sputtering [4,7,8], thermal
evaporation [9-13], e-beam evaporation [14,15], Mo-
lecular-Beam Epitaxy (MBE) [16,17], Metalorganic Che-
mical Vapor Deposition (MOCVD) [18] etc. All these
techniques are extremely expensive and require special-
ized equipment. In addition the size of the sample that
can be coated is also limited because of the inherent in-
homogeneous coating characteristics of the techniques.
Many groups have attempted deposition of CdTe thin
film by three electrode electro-deposition technique [19-
22]. Though larger samples can be prepared by this
method by having larger electrolyte bath and adjusting
the counter electrode surface area, only one sample can
be deposited at a time. The process will not have a high
throughput in terms of thin film preparation.
The Chemical Bath Deposition (CBD) technique has
been widely used for depositing thin films [23,24]. The
advantage of the CBD process for depositing thin films
are, 1) The process is simple and cost effective, does not
require expensive equipment; 2) Large samples and many
samples can be coated at a time. In CdTe/CdS solar cells,
CdS is deposited from chemical bath [11,25]. The CBD
process for CdS is well established [26-28]. However the
chemical bath process for CdTe is still in its infancy.
There have been a few reports in the literature on the
CBD process for preparing CdTe thin films. Padam and
Malhotra [29] deposited CdTe on glass, ITO-coated glass,
Si wafer and mica using solutions of CdCl2 and TeO2 in
alkaline medium along with triethanolamine (TEA) and
hydrazine hydrate. Klochko et al. [30,31] deposited
CdTe thin film in acidic medium using CdSO4 and TeO2.
Deivanayaki et al. [32] used cadmium acetate and TeO2
to deposit CdTe thin films but there is no mention of the
pH of the solution. Garadkar et al. [33] deposited CdTe
thin films using Sodium Tellurosulphite as a source of
tellurium and CdSO4 for obtaining Cd2+ ions. Most of the
depositions have been carried out at temperatures in the
range of 60˚C - 95˚C. In some cases during deposition the
solution was stirred. However, there is no systematic
study of compositional variations of the starting solution
*Corresponding author.
C
opyright © 2012 SciRes. JMP

L. GOUDA ET AL. 1871
and the actual deposition procedure. In this paper, results
on the effect of some of the deposition parameters like
the concentration of the complexing agent, deposition
time, and the concentrations of cadmium and tellurium
salts on the quality of the CdTe thin film obtained are
presented.
2. Experimental Details
Cadmium acetate (CA), Cd(CH3COO)2·2H2O was used
as a source for Cadmium and TeO2 for Tellurium in the
solution. Triethanolamine (TEA) is the complexing agent
for Cd and Hydrazine hydrate as the reducing agent to
reduce Te4+ to Te2–. TeO2 was dissolved in hot dilute
Sulphuric acid whereas cadmium acetate (CA) was dis-
solved in deionized water. To CA solution TEA and hy-
drazine hydrate were added. 25% ammonia solution was
used to adjust the pH to 13. TeO2 Solution was added in
the end and the solution was heated to 92˚C in less than
30 min. In the initial experiments the substrates were
immersed after the solution attained the required tem-
perature. But later it was found that the quality of the
film was better when the substrate was introduced before
starting to heat the solution. The slides were taken out of
the solution after a stipulated times of 30, 45 and 60 mins.
The slides were washed in boiling water and sonicated
with acetone to remove the loosely adhering particles.
Plain glass slides and ITO coated glass slides were used
as substrates for the deposition of thin films. The compo-
sitions of the solution used for deposition of the film are
given in Table 1. In addition, time was also a variant.
Table 1. Composition of the solution for different experi-
ments. The Cadmium and Tellurium are in molarity where
as TEA and Hydrazine Hydrate are express ed in milliliters.
Sample Cd Te TEA HH
1 1 0.07 30 7
2 1 0.07 28.5 7
3 1 0.07 25 7
4 1 0.07 24 7
5 1 0.07 23 7
6 1 0.07 20 7
7 1 0.07 18 7
8 0.7 0.05 20 5
9 0.5 0.05 12 5
10 0.7 0.07 25 7
11 0.5 0.07 25 7
12 0.3 0.07 25 7
13 0.5 0.05 32 7
14 0.7 0.07 30 7
15 0.5 0.07 30 7
16 0.3 0.07 30 7
17 0.5 0.07 32.6 7
The characterization of the films was carried out by
recording the X-ray diffraction patterns using Philips
XRD ‘X’PERT PRO diffractometer using Cu-Kα radia-
tion (λ = 1.5418 Å). ULTRA 55, Field Emission Scan-
ning Electron Microscope (Karl Zeiss) was used to study
the particle size and film thickness. The films deposited
on glass substrate were used for the annealing and grain
growth studies. The films were heated in air or inert at-
mosphere to different temperatures. The chemical analy-
sis of the films was carried out by using an Induction
Coupled Plasma Spectrophotometer (ICPOES) Thermo
Scientific iCAP 6500, ICP Spectrometer. The samples
for ICP measurements were prepared by dissolving the
film in minimum amount of dilute acids. Perkin-Elmer
Lamda 35 UV visible spectrophotometer was used to
record the absorption spectrum in the wavelength range
of 400 - 1100 nm. The atomic force microscopic studies
were carried out using Dimension ICON with Scan-
Asyst2 machine. The Seebeck coefficient and resistivity
measurements were carried out using homemade probes
and Keithley meters to measure the voltages.
3. Results and Discussion
The as deposited films when seen with naked eye had a
shining finish. The color of film varied from dark ash
color to deep brown. The film thickness was around 250
- 300 nm. The AFM pictures of as deposited films on
ITO coated and plain glass are shown in Figure 1 and the
roughness profile of the film coated on glass is also
shown. The surface roughness seems higher in the ITO
coated substrate than on glass substrate. On glass sub-
strate the roughness is about ±5 nm.
3.1. Crystallographic Studies
The films deposited on glass substrates were thinner than
the ones deposited on ITO coated glass substrates. The
crystallinity of the films was also different in the two
cases with films on ITO coated glass being more crystal-
line while that on plain glass substrate more amorphous.
The ITO coating may be providing nucleating sites for
the CdTe during deposition where as on a glass surface
such nucleating sites would not be present. Substrate
surface effects on nucleation and growth of thin films
would be critical and such effects are seen in electrode-
position of CdTe thin films [34]. The as-deposited films
on glass substrates did not show any pattern in the XRD.
However, the sample #8 deposited on glass for 60 mins
with subsequent annealing in inert atmosphere did show
the XRD peaks corresponding to the cubic CdTe struc-
ture (JCPDS file No. 15-0770). Figure 2 shows the XRD
pattern of as-deposited and annealed sample.
The XRD pattern of as deposited sample #16 (t = 45
min) on ITO coated glass substrate is shown in Figure 3.

L. GOUDA ET AL.
JMP
1872
(a) (b)
profile # 1/256 Pt = 13.0 nm Scale = 30.0 nm Y Axis = – 20 μm
(c)
Figure 1. Atomic Force Micrographs of thin film deposited on (a) ITO glass and (b) Plain glass; (c) is the surface profile of
the film deposited on glass.
Figure 3. XRD pattern of sample #16 deposited for 45 mins
on ITO coated glass substrate.
Figure 2. XRD pattern of as deposited and inert gas an-
nealed sample #8.
gles match well with the JCPDS file. The other peaks are
due to ITO (JCPDS 6-416). The structure of CdTe de-
posited is predominantly cubic and reasonably crystalline.
The unit cell parameters calculated using the peak posi-
tions from XRD is 6.4424 Å which is in good agreement
with the reported value [31,34,35].
The main diffraction peaks are at 2 theta of 23.45, 30.47,
35.2, 40.68, 46.3 and 50.84. The peaks at 23.45, 40.68
and 46.3 correspond to diffraction from (111), (220) and
(311) planes of cubic CdTe (JCPDS file No. 15-0770).
Inter planar spacings calculated for these two theta an-

L. GOUDA ET AL. 1873
3.2. Effect of Annealing

12
22
0
n
t
dd Kt
The top down SEMs of the film surface are shown in
Figures 4 and 5. The as deposited films on glass were
amorphous in nature with small grain structure. The film
surface looks clean, homogenous and dense with no pin
holes. Annealing experiments were carried out in air and
in inert atmosphere on sample #8 (t = 60 min). The
grains grow to similar sizes in the two atmospheres. The
grain boundaries are created and the grain growth is oc-
curring through grain boundary movement. In the thin
films annealed in air the grain growth is very clear
whereas in the inert atmosphere annealed samples, up to
about 350˚C, there is slow grain growth and at higher
temperatures, the grain disintegrate into finer particles.
The grain growth trends are shown in Figure 6. In
both atmospheres up to 350˚C, the grains grow at a rate
~0.3 nm/degree. Beyond that temperature, the rate of
growth increases by an order of magnitude in the case of
air annealed samples.
Using grain growth equation [34],
(1)
Expressing K as [36],

–
0exp QRT
KK

(2)
where d is the grain size at time t, K grain growth con-
stant and n is the grain growth exponent. n is estimated to
be 0.5 for shorter annealing times. Thus, by plotting


22
ln dd
0t versus inverse temperature, the activation
energies for grain growth are calculated. For the air an-
nealed sample, an activation energy was 0.08 eV for T <
350˚C and 0.66 eV for 350˚C < T < 500˚C where as for the
inert atmosphere annealed sample Q is found to be 0.16
eV up to 350 C. These values are much lower than the
reported value of 0.99 and 1.17 eV for CdCl2 treated and
untreated CdTe films for a film thickness of 1.4 µm [34].
Activation energy of 2.5 eV is reported for a 2 µm thick
electrodeposited CdTe film [37] that is close to the acti-
vation energy of 2.44 eV for Cd diffusion in CdTe at
minimum Cd vapor pressure. The rate of recrystallization
(a)
1
μ
m
(b)
200nm
(c)
200nm
Figure 4. SEM of air annealed sample # 8 at different temperatures: (a) As-deposited; (b) 150˚C; (c) 500˚C.
(a)
200nm
(b)
100nm
(c)
200nm
Figure 5. SEM of sample #8 annealed in inert atmosphere at different temperatures: (a) 150˚C; (b) 250˚C; (c) 450˚C.
Figure 6. The average grain diameter of particles in the film as a function of annealing temperatures.

L. GOUDA ET AL. 1874
was higher with the presence of CdCl2 compared to
without CdCl2. It was concluded that the grain growth in
these films should be Cd diffusion limited. The films
deposited in the present study, from XRD pattern, does
not show a <111> preferred orientation as in the electro-
deposited films. The activation energy for grain growth is
also dependent on the film thickness because the rate of
grain growth is inversely proportional to the film thick-
ness [38]. Thus, in these thin films (<300 nm) grown from
solution, the grain growth may be driven by the grain
boundary diffusion rather than by Cd diffusion.
3.3. Optical Bandgaps and Film Compositions
Typical optical absorption spectrum of the CdTe film is
shown in Figure 7. Optical absorbance was determined
from the variation of the optical transmission with wave-
length (λ). The absorbance α is related to the optical
bandgap through the relation [39],

–n
g
A
hE h



–
 (3)
where A is a constant, hν is the energy of incident radia-
tion, Eg is the bandgap of the material and n is the expo-
nent. The value of n depends on the nature of transition
and in the case of CdTe, since the transition is known to be
direct allowed (15 – 1) [40,41], n is taken to be 0.5. By
simplifying the above equation,

2
g
hAhE 


2
h
  (4)
By plotting

 h
as a function of

and ex-
trapolating the straight line part of the plot to energy co-
ordinate, it is possible to determine the optical bandgap
of CdTe. Optical bandgap varied from about 1.25 to 1.78
eV for various samples and thus it is dependent on the
deposition conditions as seen in Figure 8.
The effect of Cd/Te molar concentration ratio in the
solution used for the deposition of thin films on the
bandgap is studied. Figure 9 shows the variation of
bandgap as a function of the Cd/Te ratio used in the solu-
tion for depositing the films with 25 mL TEA and 30 mL
TEA. Two different deposition times are considered, for
Wavelength/nm
Figure 7. Absorbance spectrum of sample #5.
Figure 8. Optical absorption as a function of energy of ra-
diation; samples (1) #6 (deposition time t = 30 min); (2) #3 (t
= 30 min); (3) #5 (t = 30 min); (4) #17 (t = 45 min); (5) #15 (t
= 60 min) and (6) #3 (t = 60 min).
Figure 9. Variation of bandgap with ratio of Cd/Te ions in
the deposition bath. The open triangles are for solutions
with 30 mL TEA, open diamonds for solutions with 25 mL
TEA. The solid lines are least square fit lines and the bro-
ken line is an indicator for bandgap of 1.45 eV. The solid
triangle and diamond correspond to the experimental data
carried out using calculated compositions from the fitted
line.
25 mL TEA experiments 45 min data and for 30 mL TEA
60 min data are used. As the Cd/Te ratio decreases the
bandgap also decreases. From these plots the Cd/Te ratio
that is required to obtain a bandgap of 1.45 eV is calcu-
lated. The bandgaps of the CdTe films obtained with these
calculated Cd/Te ratios are shown as solid triangles and
diamonds in the figure and these data points agree well
with calculated numbers from the fitted lines. Thus, the
chemistry of the solution used for the deposition of the
thin film plays a major role in controlling the optical
properties of the deposited film.
The Cd/Te atomic ratio (AR) in the thin films is de-
termined from ICP measurements. The AR of the film
depends on the thin film deposition conditions. It is not
just the Cd/Te ratio in the solution used for deposition
but also on the amount of TEA. TEA is used as a com-
plexing agent for Cd ions. The role of TEA is to prevent
the precipitation of Cd(OH)2 under alkaline conditions.
Cd-TEA complex on dissociation through the reverse
reaction facilitates the controlled release of Cd2+ ions.
These ions then combine with the Te2– ions present in the

L. GOUDA ET AL. 1875
solution to form CdTe. The stoichiometry of the depos-
ited films can thus be controlled by varying the concen-
tration of the complexing agent [42]. The role of hydra-
zine hydride is to reduce Te4+ ions in TeO2 to Te2–. The
reaction may be described as,

2
Cd TEA
CdTe TEA
n
n
2
2
Te 2OH
H O




AR 3.687 
A 0.209Cd
16Time



 (5)
In Figure 10, the bandgap is plotted as a function of
the Cd/Te atomic ratio in the film as determined by ICP.
The straight line is fitted through all the points on the
plot and the equation is,

BandgapeV2.308 (6)
There is a relation between the solution chemistry and
the atomic ratio in the film. There are many components
involved in tuning the solution chemistry. In order to
obtain a relationship between the various components of
the solution and the AR of the film the quality control
statistical tool, Minitab, was used. The parameters that
were used to obtain the relation were the Cd, Te, TEA
and HH content in the solution in addition to the time of
deposition. The relationship that was obtained,
AR1.07 0.00433TE
0.11Te0.004

 (7)
Under the concentration ranges the experiments are
carried out, the amount of hydrazine hydrate did not have
a major contribution in determining the Cd/Te atomic
ratio in the film.
In order to validate all the correlations that are ob-
tained from the experimental data fitting, the straight line
fits of Figure 9 were used to calculate the required
Cd/Te concentration ratio in solution to obtain a band
gap of 1.45 eV. For example, with 25 mL TEA and 0.07
M Tellurium in the solution the cadmium that is re-
quired according to the data fit is 1.11 M. Experiment
was carried out with these conditions and the bandgap of
the resulting film was found to be 1.48 eV. From the fit
of data plotted on Figure 10, the AR required to obtain a
Figure 10. Band gap as a function of the Cd/Te atomic ratio
in the thin film. The solid line is the trend line and the dot-
ted broken line corresponds to the band gap of 1.45 eV.
bandgap of 1.45 eV is 0.97, AR predicted by equation 7
for the above experimental conditions is 0.94 and the AR
measured on the above film is 0.99. Thus, the chemistry
of the solution can be related to the bandgap of the film
obtained through the Cd/Te atomic ratio in the thin film.
3.4. Electrical Properties
Electronically, CdTe exhibits amphoteric semiconducting
behavior. By suitably doping CdTe it is possible to make
it a n and p-type conductor. Intrinsically, Cd deficiency
makes CdTe a p-type conductor while with Te vacancies
it becomes a n-type conductor. The Cd deficiency state
lies close to the upper edge of the valence band while Te
vacancy state lies close to the lower edge of the conduc-
tion band in CdTe. When Cd/Te ratio approaches 1 the
resistivity is maximum [13]. The electrical resistivity of
some of the films was in the mega ohm range.
The sign of the Seebeck coefficient is an indicator of
the nature of charge carriers in the material. The as de-
posited film had a positive Seebeck coefficient that in-
creased in to higher value with air annealing at 350˚C as
reported in Table 2. This could be due to partial grain
growth and grain boundary development. Also, there
may be some amount of oxidation taking place at the
grain boundaries. When annealed in inert atmosphere at
350˚C, the Seebeck coefficient of the film increased to
much smaller extent than when annealed in air.
4. Conclusions
Cadmium Telluride thin films have been successfully
deposited from solution on both glass and ITO coated
glass substrates. The grain morphology of the as depos-
ited films on ITO glass was found to be more crystalline
than on plain glass. XRD pattern corresponding to cubic
CdTe is obtained only after annealing in inert atmosphere
for the films deposited on plain glass. The grain growth
studies of these films have been studied by annealing
them in air as well as in inert atmosphere.
The optical studies in the UV-Visible radiation range
show that the bandgap varies from about 1.26 eV to over
1.8 eV depending on the composition of the film. Cd/Te
atomic ratio in the film is correlated with the bandgap
and also with the Cd/Te component ratio in the starting
solution that is used for deposition of the films. The ef-
Table 2. Seebeck coefficient of sample #2 after annealing in
air and inert atmosphere at 350 C.
Sample Seebeck Coefficient, μVK–1
As deposited 7.3
Air annealed@350 C 348
Inert atmosphere annealed@350 C 15.2

L. GOUDA ET AL. 1876
fect of TEA is also studied in detail and it is found that
optimum amount of TEA is essential for obtaining the
right Cd/Te atomic ratio in the film. A relation correlat-
ing the chemical composition of the solution to the Cd/Te
atomic ratio in the film is deduced. Thus, it is shown that
the chemistry of the solution used for deposition of the
film determines the optical properties of the film pro-
duced.
The Seebeck coefficient measurements confirmed the
p-type conduction in the films.
5. Acknowledgements
The authors thank Divecha Centre for Climate Change
for the financial support to carry out this project and Prof.
J. Srinivasan for all encouragement during the course of
this work. We thank Dr. H. N. Vasan for providing labo-
ratory space to carry out experiments without this help
the work would not have taken shape. We also would
like to thank Dr. Vasantacharya and Mr. Jarali for help
with electrical and optical measurements. We acknowl-
edge the help of Mr. Sathyanarayana and Prof. T. N.
Guru Row in SEM and XRD characterization.
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