Correlation between the Solution Chemistry to Observed Properties of CdTe Thin Films Prepared by CBD Method

doi:10.4236/jmp.2012.312235

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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

Email: *[email protected]

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.

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



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





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)

(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

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,



hAhE









 (4)

By plotting





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,



Cd TEA

CdTe TEA

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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