Chapter solar cell devices, infrared optical window, photo

 

Chapter 03

Processing
Techniques

 

 

3.1 Processing of Cadmium Sulphide and Cadmium
Telluride

  Polycrystalline
compound Cadmium telluride (CdTe) is formed from Cadmium and Tellurium. It is
an II-VI semiconductor compound which
is very suitable for thin film solar cell devices, infrared optical window,
photo detectors, and semiconductor devices. 

CdTe is well known for its optical properties
especially 39, 40.

 

3.2 Physical Properties of CdTe

 

Molecular formula

CdTe

Molar mass

240.01 g mol?1

Density

6.2 g/cm3

Melting point

1092 °C

Boiling point

1130 °C

Solubility in other solvents

insoluble

Band gap

1.44 eV (@300 K,
direct)

Refractive index (nD)

2.67 (@10 µm)

Table. 3.1 Physical properties of CdTe

3.2.1. Structure of CdTe

 

It has
a direct band gap of 1.45 eV. The minimal-energy state in the conduction band and the maximal-energy
state in the valence band are each characterized by
a certain crystal momentum (k-vector) in the Brillouin zone. If the
k-vectors are the same, it is called a “direct gap”. If they are
different, it is called an “indirect gap”.

CdTe can absorb more than 90% of the
photons having energy greater than 1.45 eV, that’s why it is well suited for
solar cell applications. CdTe is stable up to 500 oC. CdTe has Lattice
constant of 0.68 nm at 300 K.  The
specific heat capacity is about 210 J/(kg·K) at 293 K.  In solar cell fabrication CdTe is used as a
P-Type semiconductor which has a junction with CdS as an N-Type semiconductor.
Infrared detector material (HgCdTe) is manufactured when (CdTe) is alloyed with
mercury. The very important application of CdTe is a radiation detector used to
detect X-rays, Alpha, Beta and gamma rays, for this purpose CdTe is doped with
Chlorine. CdTe has electron affinity of 4.3eV.

Work function of 5.5 eV for P-Type
CdTe. It can be doped both as P-Type as well as NType semiconductors 41, 42.

 

 

Figure 3.1 .CdTe structure

 

3.3 Defects

   
It has some native defects Cation vacancies (Vcd) can behave as double
accepters and Anion vacancies (VTe) can behave as double donors.
These vacancies can be formed with other extrinsic impurities 43, 44. 

 

3.4 Toxicity

A problem of CdTe is that it is
toxic if ingested and if its powder is inhaled. If properly processed and
manufactured and encapsulated then it may harmless. It is observed that
elemental Cadmium is more toxic than CdTe. It is also studied that CdTe quantum
dots causes extensive reactive oxygen damage to cell membrane, mitochondria,
and cell nucleus. If it is to be used at large scale commercialization of solar
panels then exposure and long term safety of CdTe will be serious issue. So
attempts are being made to overcome all these issues. 

A document hosted
by the U.S. National Institutes of Health dated 2003 discloses that:
“Brookhaven National Laboratory (BNL) and the U.S. Department of Energy (DOE)
are nominating Cadmium Telluride (CdTe) for inclusion in the National
Toxicology Program (NTP). This nomination is strongly supported by the National
Renewable Energy Laboratory (NREL) and First Solar Inc. The material has the
potential for widespread applications in photovoltaic energy generation that
will involve extensive human interfaces. Hence, we consider that a definitive
toxicological study of the effects of long-term exposure to CdTe is a
necessity.”

The BNL has given
the research that CdTe large-scale PV modules have not any health risks not any
threats for environment. These modules can be recycled.  These modules do not produce any pollutants
during their working for long term 45, 46.

 

3.5 Availability

Major issue for the CdTe PV modules is the availability of Tellurium.
Actually Tellurium is also an extremely rare element (1-5) parts per billion in
the crust of Earth.

Manufacturing of CdTe PV solar
panels at large-commercial scale will cause depletion of Tellurium 47.

 

3.6 CdTe Deposition Techniques

   So many techniques can be used
for the CdTe. It is the versatility of the CdTe because it provides many ways
for its deposition. The method of deposition should be economical, easily
scalable, easy to handle, can give good conversion efficiency of the device.
The method should be easily applicable at large industrial level. The
deposition technique will be preferred which has maximum conversion efficiency,
low cost and have high deposition rate. Many techniques have efficiency up to
12 % in laboratory but not at the industrial level. There should be such kind
of deposition technique that will give good conversion efficiency in laboratory
as well as at industrial level 48.

 

3.6.1 Electrodeposition

The deposition of a substance on an electrode by the process of
electrolysis.  It is a technique in which electric current is passed through a
chemical solution and ionization occurs then these ions deposit on the
substrate. R.K. Sharma et. al deposited CdTe thin films electrochemically from
an acidic aqueous solution which is 
containing 2.5 M CdSO4·8H2O and 120 ppm TeO2.
Temperature was constant at 80 oC and electrolytic bath was at
constant stirring speed of (70-80) rpm (rotations per minute), pH was at 2. In
electrochemical cell Platinum was anode saturated calomel as reference
electrode and CdS coated substrate was working electrode.

 

Figure 3.2. Apparatus of Electrodeposition

 

The deposition rate is about 0.86
?m/hour. The function of saturated calomel electrode is to check the deposition
conditions for the CdTe deposition it was maintained at 0.58 to

0.62 V. Electronic grade H2SO4 was
used to provide H+ ions during deposition 49.

 

3.6.2 Physical Vapor
Deposition

 It is the process in which material is
sublimated from solid or liquid source and condensed upon the substrate, mostly
the whole process is done in a vacuum. The deposition rate for physical vapor
deposition is approximately from 1 to 10 nm per second. This deposition
technique is used for the deposition of alloys, elements and compounds by the
reactive deposition. In reactive deposition a gas environment is used which has
a reaction with the source material (depositing material) to form a compound,
the gas environment may be a nitrogen gas etc. The physical vapor deposition can be categorized as

(i)     Vacuum
evaporation

(ii)   Sputter
deposition 50, 51

 

3.6.2.1
Vacuum Evaporation

 In this process source material is evaporated
thermally inside the vacuum and its vapors are condensed at the substrate. The
vacuum has a benefit of controlling the contaminations and reducing the melting
temperature of the source material. Vacuum also increases the mean free path
for the motion of deposited species. The vacuum required for deposition is from
10-5 torr to 10-10 torr. Thermal evaporation is obtained
by thermal heating sources such as tungsten coils. The vacuum evaporation
technique is used for fabrication of decorative coating, corrosion protective
coatings and electrically conducting films 52.

 

3.6.2.2
Sputter Deposition

 The sputter deposition is done by physical
sputtering. It is the process in which material is not heated thermally so it
is a non vaporizing process. Material from target is ejected and then deposits
onto a substrate. When energetic particles like ions are bombarded on the
sputtering target a plume of material is released and deposits onto a substrate
just like a shower of sand when a golf ball lands in the bunker. The bombarding
particles are usually gaseous ions accelerated from plasma. The sputtering gas
is often an inert gas like Argon gas. The spacing between source and substrate
is less than vacuum deposition. The plasma pressure for sputter deposition is
from 5 mtorr to 30 mtorr. The sputtered particles are thermalized by gas phase
collision before they reach substrate surface.

3.6.3 Atomic Layer Epitaxy

      Atomic layer epitaxy
(ALE) or Atomic Layer Chemical vapor deposition (ALCVD)  now a days 
it is known as  Atomic Layer
Deposition (ALD)  is a technique used for
the production of high quality, thin, solid films of specific crystal
structures or orientations. This method provides very fine control of film
thicknesses to one atomic layer. It has a wide range of applications in areas
such as thin film ceramics, gas sensors, radiation detectors, optical/infrared
filters, surface hardening and fiber optical materials.  The term epitaxy comes from the Greek roots, meaning of word epi, is “above”, and meaning
of  taxis,
meaning “in ordered manner”. It can be translated “to arrange
upon”.                

      It is a method that is based on sequential use of gas phase chemical
process. Mostly in (ALE) two chemicals are used called precursors. Chemical
reaction takes place between these precursors with surface one at a time in
sequential manner. Using ALE alternating monolayers of two different elements
can be deposited onto a substrate. Dr. Tuomo Suntola invented
this technique in 1977, at the University of Helsinki in Finland. In this
technique material amount deposited in each cycle is constant. This method
provides crystalline and uniform films especially if very thin film is
required. Laboratory of advanced energy systems, Finland used ALE to grow both
CdS and CdTe layers in a single process. ALE reactor with four individually
controlled sources to control inside solid reactants, and has number of sources
for external inert gas flow and liquid reactants.

The spacing between two substrates is 1-3 mm.

   
CdS films were deposited by using elemental Cd and S powders as
reactants. Temperature is (300-500) oC. Temperature for CdTe is (350
-400) oC.  For the fabrication
of CdTe elemental Cd and Te is used. All layers were deposited in one run at
the same reaction temperature 53.

 

 3.6.4 Molecular Beam Epitaxy

It is a fabrication
of crystalline thin film epitaxially above the other crystalline substrate with
the beam of molecules or atoms.  Molecular beam epitaxy (MBE) method was
invented by J.R Arthur and Alfred Y.Cho in 1960s at Bell Telephone Laboratories

USA. This method required an ultra high vacuum up to
(10-8 pa) for the film deposition.

The rate of deposition is very slow
which is 1 ?m/hr. The materials are evaporated and reach the substrate
individually then on the wafer reaction take place between these vapors. This
method can give high-purity epitaxial layers of compound semiconductors.

The word “beam” shows that
sublimated atoms of the material do not interact with each other and with
vacuum chamber until reach the wafer, due to long mean free paths of atoms If
crystalline film is fabricated on the substrate of the same material is called
Homoepitaxy. So this epitaxy is done with only single material.

Heteroepitaxy is performed with
different materials. In Heteroepitaxy crystalline thin film is fabricated on
the substrate of a different material. Example gallium nitride (GaN) on
Sapphire, aluminium gallium indium phosphide (AlGaInP) on Galium arsenide
(GaAs).  The disadvantage of this method
is that it is very expensive.

In MBE technique vapor phase, liquid phase and solid
phase methods can be used 54.

 

3.6.5 Close Spaced
Sublimation

          It is a process for a thin film
deposition of materials in a vacuum. The material is sublimated by heating and
its vapors condensed onto a substrate which is placed above the source
material. The basic phenomenon of thin film deposition based on dissociation at
high temperature. 

                         CdTe ? Cd
+ Te

Before the fabrication of thin film
by close spaced sublimation the substrate need a proper cleaning by Acetone,
Isopropyl alcohol, rinse with distilled water and ultrasonic cleaning. Quality
of thin films, material transport and deposition rate depends upon the
following parameters 55, 56.

 

3.6.5.1
Source and Substrate Temperature

   
It is observed that high substrate temperature of CdTe provides good
performance of solar cell devices. Resistivity of CdTe decreases by increasing
the substrate temperature and grain size reduces by increasing substrate
temperature. Deposition rate also improved at high source temperature 57.

3.6.5.2
Spacing between Source and Substrate

  
The spacing between source and substrate is inversely proportional to
the rate of deposition.

 

3.6.5.3   Vacuum Level

    Vacuum level for CdS and CdTe deposition
is from 10-3 to 10 -5 mbar.

 

3.6.5.4
Source Stoichiometry

   Composition
of material is an important for thin film quality.

 

 3.6.5.5 Annealing

Annealing of the
films provides improvement of surface morphology and it reduces roughness of
surface of CdS and CdTe films. Recombination centers reduce by annealing.
Crystallinity of film also improved by annealing. Open circuit voltage also
increased by increasing annealing temperature. Spectral response especially in
the range of 500 to 600 nm also improved by annealing temperature.  Hence efficiency of solar cell can be
improved 58-60.

 

 

 

3.6.5.6
Diffusion

The
particles movement from higher concentration to lower concentration is called
diffusion. The distance particles can travel without any collision is called
mean free path. Diffusion of one kind of particles into other kind of material
can change its characteristics like a semiconductor material can be converted
into N-

Type or a P-Type. The mathematical
description can be explained by the Fick’s law 

      According
to Fick’s first law   

 

 

 

   j is the atomic flux  per unit area per unit time, D is the
diffusion co-efficient   

 (dc/dx) is the
concentration gradient of the vapor 61.                                                

     

3.7 Processing of Cadmium
Sulphide

3.7.1 Chemical Bath
Deposition (CBD)

It is very
easy process for the fabrication of CdS thin films on ITO glass. For the
fabrication of thin film solar cell we need a very thin film up to (60 -80) nm,
which is not easily possible by using close spaced sublimation process. It is
very suitable process especially for solar cell point of view.  It is a process in which substrate is placed
in a hot chemical solution stirring vigorously for specific time, positive and
negative ions will reach and meet on the substrate and thin film is grown. The
advantage of this technique is that neither vacuum and nor very high
temperature is required for CBD 62.  

 

3.8 Fabrication of CdS Thin
Film by Chemical Bath Deposition   

The process is very simple, these are steps given
below

  3.8.1 Cleaning

   
Substrate is cleaned by using detergent then rinsed in distilled water
then again clean with acetone and rinsed distilled water then clean with
isopropyl alcohol (IPA).

 3.8.2
Preparation of the chemical solutions

                  The solution will be prepared
in distilled water according to the given molarities.

Molar mass of CdCl2                           183.32 g/mol

Molar mass of KOH                                  56.0 g/mol

Molar mass of NH4NO3                                80 g/mol

Molar mass of 
thiourea                               
76 g/mol

 

 

Mass required making 1000 ml solution

Mass = (Mgiven X Vreq X Mol
mass)/1000

M = morality given

Vreq = required volume

Mass of CdCl2 required making 1000 ml

(0.02X500X183.22)/1000 = 1.832 grams

 Note: 

      If Hydrated cadmium chloride CdCl2
(2.5H2O) is available then mass of

CdCl2 required for making 1000 ml will be
(2.283 g/1000 ml)

Mass of KOH required making 1000 ml solution

(0.5X1000X56)/1000 = 28 grams

Mass of NH4NO3 required making
1000 ml solution

(1.5X500X80)/1000=60 grams

Mass of Thiourea (NH2)2CS
required making 1000 ml solution

(0.2 X 76 X 1000)/1000 = 15.2 grams

 

3.8.3 Procedure

 

   The procedure is very simple for the
CBD. Clean beaker and substrate carefully

 Add chemical
solutions according to the amount given below

                
Cadmium chloride       CdCl2                     0.02M (80 ml)

                
Ammonium Nitrate      NH4NO3                 1.5M   (80 ml)

               
Potassium Hydroxide    KOH                      0.5M  (200 ml)

These three solutions are added to
the beaker. The substrates are fixed inside the solution with the help of a
substrate holder. The beaker is placed on the hot plate with magnetic stirrer
inside. A pH meter and thermometer is also dipped in the solution to measure pH
and temperature respectively. Provide heat to solution up to temperature of 75 oC.
The solution is provided stirring continuously throughout the experiment. 

      When
temperature reached at 75 oC add the thiourea about 0.2M (80 ml).

 As thiourea is added to the solution the
reaction starts suddenly. So thiourea is the last component which is added to
the solution. So deposition of CdS film starts when the thiourea is added.
Films are retired from the solution according to the specific deposition time
and rinsed immediately with distilled water into an ultrasonic cleaner.  

 

Figure 3.3.CBD Apparatus

 

The CdS deposited films with
pale-yellow color are obtained. The CdS film is deposited on both sides of the
ITO glass. So film from glass side is cleaned by using 10% Hydrochloric acid
(HCl) solution. It needs much care when using HCl acid on glass side for cleaning
because drops of HCl can remove film from ITO side as well. After deposition
these CdS films will be annealed at 400 oC for 30 minutes 63, 64.

 

 

 

 CdS23

60

2.29

CdS
24

127

2.36

CdS
25

170

2.66

CdS
20

257

2.39

Table 4.4 Thickness and energy gaps of CdS
CBD

      

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