CHARACTERIZATION OF CdSe AND CdTe LAYERS GROWN BY ELECTRO DEPOSITION METHOD AND FABRICATION OF A CdTe/CdSe HETERO JUNCTION SOLAR CELL STRUCTURE

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1.1
BACKGROUNDS

1.1 Introduction

The earth
is rich in natural resources however the earth’s natural resources are being
consumed at a tremendous rate. It will only be a matter of time before those
resources are depleted and the population is forced to consider other
alternatives. People can wait until every resource has been consumed or people
can continue to move forward and explore the possibilities of utilizing the
power of something that remains constant –the sun. Therefore there is a drastic
need for a renewable energy source.

The
conventional sources of energy like hydro, thermal and nuclear are able to meet
the current requirements to some extent. But whether the conventional sources
of energy are sufficient to meet the demand in coming centuries remains a
question. The consumption of non-renewable energies has caused a huge damage to
the environment. Electricity generated from fossil fuels has led to high
concentrations of harmful gases like carbon monoxide, carbon dioxide, etc in
the atmosphere. This in turn led to many problems such as ozone layer depletion
and global warming.

Many
hydroelectric plants have been constructed in recent past. This is one of the
better ways of generating electricity with minimum waste products. Construction
of these projects involves a lot of capital to be invested. Moreover most of
the rivers are not perennial and hence cannot generate power continuously
throughout the year. Another disadvantage of hydroelectric plants is their
detrimental effect on the ecosystem.

Construction
of large hydroelectric plants has caused tremors in those areas. The nuclear
energy is another alternative source of energy, which has a potential to
produce many megawatts of power. Nuclear energy if used for constructive
purposes can generate power. On the other hand if it is used for destructive
purposes can be benignant to human life. The option of nuclear energy raises
safety issues like disposal of its waste products. So these factors make many
nations to rethink about constructing nuclear plants.

Hence an
alternative form of energy is an obvious choice to meet the requirements. Thus
there is an immediate need for the development of non-conventional energy
sources. Tidal, solar, geo-thermal and wind are few non-conventional energy
sources. These are inexhaustible energy sources and hence are renewable. They
cause fewer emissions. Their use can reduce pollution. They stand out as a
viable source of clean and limitless energy.

The tidal
energy would be an attractive alternative if it were more reliable. However the
vast regions across the world are covered by land and hence it is not a
feasible option. Windmills do not meet the practical applications of present
day. They can be used only for some specific purpose and also cannot generate
many megawatts of power. 

Sun is the
centre of all the activities in the world and amazingly powerful. It is
calculated that in every second about 6



















 1011 kg of H2
is converted to He in sun. An enormous amount of energy (approx 4

 1020 J) is produced in
this process. This energy is emitted as an electromagnetic radiation. It is
projected that this constant amount of energy can be obtained for at least 1010 years. Hence among
the above renewable energy sources, solar energy is the most readily available
source of energy.

As the demand for more
electric power increases and present power plant reaches end of life, the need
for an environmentally benign means of generating electricity becomes imperative. Conversion of sunlight directly to
electricity using
photovoltaic (PV) is a means of generating electricity
without causing changes to the environment because the fuel consumed is
external to the Earth.

The term
“photovoltaic” comes from the Greek word phos means “light” and
“voltaic” means electrical, from the name of the Italian physicist
Volta, after whom the measurement unit volt is named. The term
“photo-voltaic” has been in use in English since 1849 . A solar cell
or photovoltaic cell is a device that converts light energy into electrical
energy by the photovoltaic effect. Photovoltaics are the field of technology
and research related to the application of solar cells as solar energy.
Sometimes the term solar cell is reserved for devices intended specifically to
capture energy from sunlight, while the term photovoltaic cell is used when the
source is unspecified. Solar cells have many applications. Individual cells are
used for powering small devices such as electronic calculators. Photovoltaic
arrays generate a form of renewable electricity, particularly useful in
situations where electrical power from the grid is unavailable such as in
remote area power systems, Earth-orbiting satellites and space probes, remote
radiotelephones and water pumping applications. Photovoltaic electricity is
also increasingly deployed in grid-tied electrical systems. Photovoltaic
technologies are still in their infancy, but have the potential to play an
important role in meeting electric power needs in the next decades, provided
that PV systems can be made competitive with conventional power generation and
other emerging renewable energy technologies. Present-day crystalline copper
indium di-selenide (CIS) technology is approaching its lower limit in terms of
production costs. Thin-film photovoltaic modules can be produced by economical,
high volume manufacturing techniques, dramatically reducing cost, and are in a
pre-manufacturing development stage. However, unlike crystalline CIS, equipment
for these technologies is largely unique and custom designed. In addition, the
processes involved for making large-­area, high-volume, and thin-film PV
modules are very complex. As a result, the translation of the laboratory
results to large-scale manufacturing has been much more difficult than expected.

The
semiconducting metal chalcogenides represent as interesting class of materials, which are attractive for large-scale
applications because of the easy
availability, and low cost of the
starting materials. In recent years, semiconducting chalcogenide films of
different metals have found worldwide application in various fields of science
and technology. The utilization of these promising semiconducting materials
needs low-cost production and pollution-free techniques. The direct collection
of solar energy involves artificial devices, called solar collectors that are
designed to collect the energy, sometimes through prior focusing of the sun’s
rays. The energy, once collected, is used in a thermal or a photoelectric or
photovoltaic process. In the photovoltaic process, solar energy is converted
directly to electrical energy without
intermediate mechanical devices
(by Photoelectric
Effect). Solar cells
made from thin slices of crystalline silicon,
gallium arsenide, or other semiconductor materials convert solar radiation
directly into electricity. Cells with conversion efficiencies in excess of 30%
are now available [4]. By connecting large numbers of these cells into
modules, the cost of photovoltaic electricity has been reduced to very
low.

The photovoltaic effect of the solar cell operation was
discovered in 1839 by a French physicist, and one out of a family of four
generations of scientists, Alexandre- Edmond Becquerel. He was the father of
Henri Becquerel, a French physicist, Nobel laureate, and one of the discoverers
of radioactivity. However, it was not until 1883 that the first solar cell was
built, by Charles Fritts, who coated the semiconductor selenium with an
extremely thin layer of gold to form the junctions. The device was only around
1% efficient. Russell Ohl patented the modern solar cell in 1946. Sven Ason
Berglund had a prior patent concerning methods of increasing the capacity of
photosensitive cells. The modern age of solar power technology arrived in 1954
when Bell Laboratories, experimenting with semiconductors, accidentally found
that silicon doped with certain impurities was very sensitive to light.

This resulted in the production of the first practical
solar cells with a sunlight energy conversion efficiency of around 6 percent.
Russia launched the first artificial satellite in 1957, and the United States’
first artificial satellite was launched in 1958 using solar cells created by
Peter Iles in an effort spearheaded by Hoffman Electronics. The first spacecraft
to use solar panels was the US satellite Explorer 1 in January 1958. This
milestone created interest in producing and launching a geostationary
communications satellite, in which solar energy would provide a viable power
supply. This was a crucial development which stimulated funding from several
governments into research for improved solar cells.

In 1970 the first
highly effective GaAs heterostructure solar cells were created by Zhores Alferov
and his team in the USSR [9]. Metal Organic Chemical Vapour Deposition (MOCVD)
production equipment was not developed until the early 1980’s, limiting the
ability of companies to manufacture the GaAs solar cell. In the United States,
the first 17% efficient air mass zero (AM0) single-junction GaAs solar cells
were manufactured in production quantities in 1988 by Applied Solar Energy
Corporation (ASEC). The “dual junction” cell was accidentally
produced in quantity by ASEC in 1989 as a result of the change from GaAs on
GaAs substrates to GaAs on Germanium (Ge) substrates. The accidental doping of
Ge with the GaAs buffer layer created higher open circuit voltages,
demonstrating the potential of using the Ge substrate as another cell. As GaAs
single-junction cells topped 19% AM0 production efficiency in 1993, ASEC developed
the first dual junction cells for spacecraft use in the United States, with a
starting efficiency of approximately 20%. These cells did not utilize the Ge as
a second cell, but used another GaAs-based cell with different doping.
Eventually GaAs dual junction cells reached production efficiencies of about
22%. Triple Junction solar cells began with AM0 efficiencies of approximately
24% in 2000, 26% in 2002, 28% in 2005, and in 2007 have evolved to 30% AM0
production efficiency, currently in qualification. In 2007, two companies in
the United States, Emcore Photovoltaics and Spectrolab, produce 95% of the
world’s 28% efficient solar cells.

Interest
arose in solar cells as an alternative energy source for terrestrial
applications in the mid-1970s after the political crisis in the Middle East,
the oil embargo, the realization that fossil fuel sources were limited, and
recently the current political crisis in the Middle East, and the latest war
with Iraq. We peaked in domestic oil production in the 1970’s and as far as
crude oil is concerned, we will never again produce as much domestic oil as we
did at the turn of the century in 2000, even if we drill as hard as we can in
the Artic National Wildlife Refuge and offshore combined. The gap between the
United States oil consumption and production will only continue to widen. The
cost target for electricity from a photovoltaic plant operating for 30 years
was established in 1986 to be equal to about 0.06 US$/kWh. It was estimated
that this requires module efficiencies in the range of 15% to 20% for a flat
panel system and 25% to 30% for a system operating under concentrated sunlight.



Year

Fig.
1.1 World PV production growth.

Photovoltaic’s
has experienced extraordinary growth during the last few years with overall
growth rates between 30% and 40% making further increase of production facilities
and attractive investmen. In 2008, the world-wide photovoltaic industry
delivered some 6,941 MW of photovoltaic generators shown in Fig. 1.1 .

The
major advantages of the use of thin films over bulk materials are;

a) Low
processing costs.

b) Relatively less
material costs.

c) Substrates used
mostly are glass, metal sheet or polymers which are inexpensive.

d) Thin film solar
cells can be produced on large area substrates giving a high output with a low
unit cost.

The major
disadvantages of thin film polycrystalline solar cells are:

a) Thin film
polycrystalline solar cells have less efficiency when compared to its
equivalent counter parts.

b) Polycrystalline
materials have grain boundaries. They have crystal defects. These defects are
the centres of impurities. These defects act as recombination centres.
Recombination of charge carriers results in poorer electronic properties.

c) Reproducibility of
large area uniform films is not assured.

d) Stability of these
cells is another issue to be yet resolved.

1.2
SOLAR CELL STRUCTURE

Currently,
the semiconductor most widely used in solar cells is single-crystal silicon.
Because of the cost involved in producing the bulk material, cells produced by
this method are prohibitively expensive for all but the smallest scale or most
specialised applications (such as on calculators and satellites). Higher efficiencies
have been produced by using single-crystal III-V semiconductors and more
elaborate constructions (e.g. multi-quantum wells), but this advantage has
always been more than offset by the resultant increase in cost. The thin-film cadmium
telluride / cadmium sulfide solar cell has for several years been considered to
be a promising alternative to the more widely used silicon devices. It has several features which
make it especially attractive: 

  • The
    cell is produced from polycrystalline materials and glass, which is a
    potentially much cheaper construction than bulk silicon.
  • The
    chemical and physical properties of the semiconductors are such that the
    polysilicon thin-films can be deposited using a variety of different
    techniques .
  • CdTe
    has a bandgap which is very close to the theoretically-calculated optimum
    value for solar cells under unconcentrated sunlight.
  • CdTe
    has a high absorption coefficient, so that approximately 99% of the
    incident light is absorbed by a layer thickness of only 1µm (compared with
    around 10µm for Si), cutting down the quantity of semiconductor
    required. 

1.2.1 Cell Construction

The
CdTe/CdS solar cell is based around the heterojunction formed between n-type
CdS and p-type CdTe. The basic composition of the cell can be seen in Fig. 1.2.



Fig. 1.2 CdS/CdTe solar cell
structure.

The functions of the different layers are as follows: 

  • Glass The solar cell is produced
    on a substrate of ordinary window glass, because it is transparent, strong
    and cheap. Typically around 2-4 mm thick, this protects the active layers
    from the environment, and provides all devices’ mechanical strength. The
    outer face of the pane often has an anti-reflective coating to enhance its
    optical properties.
  • Transparent
    conducting oxide

    Usually of tin oxide or indium tin oxide (ITO), this act as the front
    contact to the device. It is needed to reduce the series resistance of the
    device, which would otherwise arise from the thinness of the CdS
    layer. 
  • Cadmium
    sulfide
    The
    polycrystalline CdS layer is n-type doped (as CdS invariably is), and therefore
    provides one half of the p-n junction. Being a wide band gap material (Eg
    ~ 2.4 eV at 300K) it is transparent down to wavelengths of around 515 nm,
    and so is referred toas the window layer. Below that wavelength, some of
    the light will still pass through to the CdTe, due the thinness of the CdS
    layer (~ 100 nm). 
  • Cadmium
    telluride
    The
    CdTe layer is, like the CdS, polycrystalline, but is p-type doped. Its
    energy gap (1.5 eV) is ideally suited to the solar spectrum, and it has a
    high absorption coefficient for energies above this value. It acts as an
    efficient absorber and is used as the p side of the junction. Because it
    is less highly doped than the CdS, the depletion region is mostly within
    the CdTe layer. This is therefore the active region of the solar cell,
    where most of both the carrier generation and collection occur. The
    thickness of this layer is typically around 10 µm. 
  • Back
    contact

    Usually of gold or aluminium, the back contact proves a low resistance
    electrical connection to the CdTe. P-type CdTe is a notoriously difficult
    material on which to produce an ohmic contact, and so the junction will
    inevitably display some Schottky diode (rectifying) characteristics. Due
    to its high conductivity, the metal layer needs only be a few tens of
    nanometres in thickness.
  • Since
    the active layers of the device are those on top of the glass substrate,
    this construction is referred to as a superstrate configuration.

1.3 REVIEW OF THE PREVIOUS WORK

Most of the II-VI compounds have drawn
interests of many people in research because they find their applications in optoelectronic
devices, photo-electrochemical cells, thin film transistors, gas sensors,
acousto-optical devices, vidicones , photographic photoreceptors and gamma ray
detectors.

Among II-VI compounds CdTe, HgI2,
CdS, CdSe etc are prominent because of their properties like direct band gap,
high absorption coefficients etc. CdTe, CdS and CdSe find its applications in
photovoltaic devices and HgI2 finds its application in detectors.
Different techniques have been used in depositing these materials and in
fabricating devices. Close space sublimation (CSS), Chemical bath deposition
(CBD), Sputtering, Thermal evaporation, Molecular beam epitaxy (MBE) and Metal
organic molecular beam epitaxy (MOMBE) are some of the vacuum deposition
techniques. Lot of research has been done in past years and literature reflects
the progress made in this field. In this section, a brief review of literature
on CdSe for optoelectronic applications done by various research groups is
mentioned.

Benamar et al.
employed “cathodic electrodeposition” method for depositing polycrystalline
CdSe films on ITO/glass substrates . The process involved a potentiostatic
reduction of CdSe from acid aqueous bath. The aqueous solution had 0.2 M CdSO4
or CdCl2 and 7

 10-4 M H2SeO3 for
simultaneous codeposition of Cd and Se ions. They claimed that when CdSO4 is
used as a source for Cd ions, deposited CdSe had cubic structure. Using CdCl2, both
cubic and hexagonal forms of CdSe was observed. SEM studies showed that modular
spanning of 1- 4µm or less in extent is present.

Nasr et al. conducted
comparative studies on photoelectrochemical behavior of SnO2/CdSe
and OTE/SnO2/CdSe nanocrystalline films. ITO coated glass was used
as an optically transparent electrode (OTE). SnO2 was coated on OTE
and was dried in air on a hot plate. CdSe films were deposited both on OTE and
OTE/SnO2. CdSO4 and SeO2 were chemicals used
in the reaction. They conducted both electrical and optical measurements which
suggested that coupling of OTE/SnO2/CdSe had better performance when
compared to SnO2/CdSe in terms of conversion efficiency and
stability. OTE/SnO2 electrode absorbed light below 400 nm whereas
OTE/SnO2/CdSe absorbed till 700 nm with a peak value at 470 nm.

Ramrakhiani studied
the characteristics of zinc doped polycrystalline CdSe films . CdSe films were
first deposited on titanium substrates at room





temperature from a solution of SeO2, CdSO4
and H2SO4 by electrocodeposition method. Zn ions were
then incorporated on CdSe by dipping the film in an aqueous solution of ZnSO4.  Incorporation of Zn ions at the surface
produced favorable states in the band gap which improved the charge transfer
kinetics at the interface thereby reducing the recombination process. Upon heating
these films, Zn diffused through the grain boundaries and reduced the
recombination centers for majority carriers. When some of the Cd atoms were
replaced by Zn atoms, the bandgap of the material increased. Hence Voc
was improved but Isc was reduced to some extent.

Baban et al. studied the structural and
optical characteristics of thermally evaporated CdSe thin films on glass
substrates . The influence of preparation conditions on the fundamental
absorption of CdSe thin films was also studied. Polycrystalline powder of CdSe
was thermally evaporated by a quasi- closed volume technique. Thin film samples
were deposited at various source and substrate temperatures. XRD studies
revealed that the films had a hexagonal wurtzite structure with (002) orientation.
AFM tests confirmed that the grain size was between 20 – 100 nm. They concluded
that as source and substrate temperature increases, crystallite size also
increases.

Padiyan et
al. observed the influence of thickness and substrate temperature on electrical
and photoelectrical properties of vacuum deposited CdSe films on glass substrates.
Optical studies of the films confirmed that the band gap decreases with an
increase in the thickness of the films and substrate temperature. They stated
that the grain size increased with an increase in the substrate temperature.
The band gap energies at different temperatures were observed to vary and they
attributed this to the grain size. By conducting photoelectrical measurements,
they concluded that the dark and light currents for as deposited substrates
increased with an increase in the film thickness and substrate temperature.

Sene et al.
studied the effects of silicontungstic acid on CdSe films grown on polymer
substrates . They employed chemical bath deposition (CBD) method for depositing
films on various substrates like glass, gold, ITO and an organic polymer
material (PMeT (poly (3-methylthiophene))). CdSe deposited by CBD on PMeT
formed a p-n junction. Electropolymerization of CBD CdSe formed a Schottky-type
junction. P-N junction obtained a conversion efficiency of 0.03 % while the
Schottky-type junction had a conversion efficiency of 1.3 %. The presence of
silicontungstic acid in the chemical bath increased the conversion efficiency
of Schottky- type junction to 2.7 %. They stated that the use of
silciontungstic acid resulted in forming highly efficient junctions.

Kale et al.
studied the thickness dependent properties of CdSe thin films deposited by
chemical deposition method [25]. The effect of thickness on electrical,
structural and optical properties was studied. The chemicals used in the
process were CdSO4, Na2SeSO3 and Ammonia. They
concluded that as the deposition temperature decreased from 358 K to 273K,
thickness decreased from 2400 Å to 600 Å and grain size decreased from 80 to 40
Å. As the temperature was increased, dissociation of the complex and the anion,
the rate of release of selenium and thickness of the films increased. Larger
grains were obtained.

Latif
studied the electrical properties of evaporated CdSe thin films. They claimed
that with the increase of deposition temperature, increase in the variation of
the current density with the voltage was observed. They have concluded that for
Al-CdSe-Au thin films at low voltages, current varies exponentially with
voltage. They have further stated that the films had an electron concentration
of 1.1

 1018 cm-3.

Cadmium
telluride was first synthesized by Frerichs in 1947 by reacting cadmium and
telluride vapors in hydrogen gas. Jenny and Bube reported the semiconducting
nature of the material in 1954. They showed that n- and p-type CdTe could be
fabricated by doping CdTe with foreign impurities . It was not until 1956 that
Loferski proposed the use of CdTe as a photo-absorber material .

Pandey et
al. reported the electrodeposition of cadmium telluride on a nickel electrode
from a special bath containing 1M CdCl2, 0.01M TeCl4 and
0.33M KI in ethylene glycol at 160 °C . KI was added to the bath to improve the
deposit adhesion on the substrate and to hinder ion complexation. Steady-state
current-potential and cyclic voltammetry experiments were performed along with
electrolyte ohmic resistance measurements. The experimental results evidenced
the fairly good electrochemical behaviour of this organic electrolyte bath and
its negligible ohmic resistance. Cyclic voltammetry showed that the reduction
of Te(IV) ions occurs via a four-electron process as well as a six-electron
process followed by a chemical reaction. Moreover, during slow-scan cyclic
voltammetry and steady-state current-potential studies, the underpotential
deposition of Cd(II) ions was observed. This behaviour has been attributed to
the negative Gibb’s free energy change involved in the CdTe-forming reaction.

Kosyachenko
et al. reported the requirements imposed on the electrical properties of the
absorber layer in the CdTe based solar cell. They observed that in a CdTe/CdS
structure, with total absorption of radiation and no electrical losses due to
the voltage drop across the bulk part of the CdTe film, the material
resistivity was not exceeded 103 ?-cm. This implied that the Fermi
level should be away from the valance band top by no more than ~0.3eV. It was estimated
that the location of the Fermi level in the bandgap of a semiconductor also
depends on the degree of compensation of impurities with ionization energy of
the impurities in the range 0.05~0.15eV.

Rusu and
Rusu studied the optical behaviour of multilayered CdTe/Cu thin films deposited
by stacked layer methods [32].  They
investigated the effect of Cu-doping on the structure and optical properties of
the CdTe thin films evaporated onto unheated substrates. It was established
that the incorporation of Cu into CdTe films inhibits the formation of Te
aggregates which are commonly observed in such films and improves the
crystalline quality of the films. The value of 1.48 eV obtained for energy band
gap for CdTe/Cu films by optical absorption measurements showed that addition
of copper into the films diminishes the effect of tellurium excess on the band
gap of CdTe films deposited onto unheated substrates. The obtained results
revealed that deposition of alternate layers of CdTe and Cu may be a promising
method for elimination of tellurium excess from CdTe films and for improving of
the physical properties of such films used in optoelectronic device technology.

Douri et
al. reported the optical properties of Al- and Sb- doped CdTe thin films. Pure
CdTe thin films as well as CdTe films doped with various percentages of Al and
Sb were grown on the glass substrate using thermal evaporation technique. Film
composition and deposition parameters were investigated for their bearing of
film optical properties. The films were allowed direct transition with optical
energy gap lying within the range 1.44–1.57 eV. Increasing in substrate
temperature Ts, and dopant percentage concentrations for both Al and
Sb caused a decrease in the optical band gap value. There is increase in the
optical constants, i.e. extinction coefficient ke, and refractive
index n, with increase in substrate temperature and doped percentage
concentrations except for undoped CdTe thin film, where ke decreases
with increasing Ts, and the variation of ?r and ?i
have similar trend as for n and ke, respectively.

The first
CdTe heterojunctions were constructed from a thin film of n-type CdTe material
and a layer of p-type copper telluride (Cu2-xTe), producing ~7% efficient
CdTe based thin film solar cell. However, these devices showed stability
problems similar to those encountered with the analogous Cu2-xS/CdS
solar cell, as a result of the diffusion of copper from the p-type.

Adirovich et
al. first deposited CdTe films on TCO coated glass; this is now used almost
universally for CdTe/CdS solar cells, and is referred to as the superstrate
configuration.

Bonnet and
Rabenhorst had made CdTe/CdS heterojunctions by evaporating CdTe films onto Mo
foils . Their success fueled further interest in CdTe/CdS technology. By 1982,
Kodak researchers Tyan and Albuerne had produced the first 10% efficient solar
cell.

Subsequent
refinement of the same superstrate structure first demonstrated by Adirovich et
al. has culminated with record device efficiencies of over 16%.

1.4
OBJECTIVE OF THE PRESENT RESEARCH WORK

In this work, our aim
is to grow cadmium selenide (CdSe) thin film as a window layer and cadmium
telluride (CdTe) thin film as an absorber layer, and finally to fabricate a CdTe/CdSe/FTO/glass
heterojunction solar cell structure using a simple, easy and low-cost
technique.

To fabricate a
complete solar cell structure, it is important to get the best window and
absorber layers. In this research, we first deposited CdSe and CdTe thin films
onto the FTO-coated glass substrates by low cost electrodeposition (ED) method.
The optimized growth conditions have been chosen for both the CdSe and CdTe
layers by investigating the photoelectrochemical, optical, structural and
morphological properties of the deposited films.

Photo-electrochemical
(PEC) cell measurement has been carried out to determine the conductivity type
of both CdSe and CdTe thin film samples. Transmittance
and absorbance measurements have been perfo
rmed for investigating the
optical properties of the deposited films. Structural
properties have been investigated by X-ray Diffraction (XRD).
Surface morphology of the deposited films has been
investigated using
Scanning electron
microscopy (SEM).
Annealing effect
has also been investigated of the deposited films
.

After getting the
optimum conditions of both CdSe and CdTe layers, the CdTe thin films have been
deposited onto the optimized CdSe films and a CdTe/CdSe/FTO/glass structure
have been developed. Metal contacts have then been applied on the top surface
of the CdTe films of CdTe/CdSe/FTO/glass structure to make a back contact for
solar cell structure, and I-V characteristics of the fabricated structures have
been investigated.

MATERIALS
AND DEPOSITION METHODS

The material presented
in this chapter provides the theory of semiconductor junctions and the
principle of photovoltaic thin film solar cell. It also describes the
properties of the studied materials and explains why they were chosen for this
work. It reviews different deposition methods and explains the reason for
choosing our working method.

2.1
PHYSICAL PRINCIPLE OF PHOTOVOLTAIC ENERGY CONVERSION

The properties of
junctions are the basis of all semiconductor devices, including optoelectronics
ones. The most common junction and hence the one introduced first, is the p-n
junction, which is formed from n- and p-type materials of the same
semiconductor. Theoretically one can expect an improvement in the efficiency of
a solar cell device if it consists of materials with different band gap
energies, which match different parts of the solar spectrum. Another important
system is the metal-semiconductor junction, which is involved in all electronic
device structures and is extremely important for material characterization,
such as DLTS. Full coverage of the theoretical background and derivation of the
equations can be found elsewhere.

2.1.1
The p-n Junction

When a p-semiconductor
and n-semiconductor are placed in intimate contact, holes diffuse from the p to
n regions and electrons from n to p. As a result of this transfer of carriers,
positively charged donor ions are left behind on the n-side of the junction and
negatively charged acceptor ions accumulate on the p-side. The existence of the
positive charges on one side and negative charges on the other side causes an
electric field. This field forces electron to drift from p to n and holes from
n to p, so that a balance is established at which each of the hole and electron
currents is zero. Because a voltage difference, Vbi is formed
between the p and n regions known as the built-in voltage, an energy
difference, qVbi, exists between them. Thus an energy barrier is
established across the depletion region. Consequently the energy bands are bent
and the Fermi levels of the two regions are aligned.



(a)



(b)

Fig.
2.1 Band
diagram of a p-n junction in equilibrium (a) before contact, and (b) after
contact.

The built-in potential
Vbi is given by



Where k is the
Boltzmann constant, T the temperature, q the magnitude of electronic charge, NA
the acceptor concentration in the p-side, ND the donor concentration
in the n side and ni the intrinsic charge carrier density.

The width of the
depletion layer w is given by



Where ? is the
semiconductor permittivity and other symbols have their usual meanings.

2.1.2
The Heterojunction

Photon with energies
greater than the band gap energy generate electrons in the conduction band,
which transform part of their energy into heat until they occupy energy states
at the conduction band edge, which tends to decrease the performance of the
device. In addition photons with energy lower than the band gap energy of the
semiconductor do not contribute to the photocurrent. In order to overcome these
problems, one can form a junction between materials with different band gap
energies, which are optimised for different wavelength ranges of the spectrum.
The material with the larger band gap energy is on the top. Light with energy
less than its band gap but greater than the one of the second semiconductor
passes through the first semiconductor, which act as a window, and is absorbed
by the second semiconductor. Two types of heterojunctions exist and their band
diagrams differ from the one of a classical p-n homojunction.

Figure 2.2  shows the energy band diagram of two isolated
pieces of semiconductors. The two semiconductors have different band gaps Eg,
different permittivities

,
different work functions ?m, and different electron
affinities ?. Work function and electron affinity are defined as that energy
required to remove an electron from the Fermi level EF and
from the bottom of the conduction band EC, respectively, to a
position just outside the material (vacuum level). The difference in energy in
the conduction-band edges in the two semiconductors is represented by ?EC
= (X1-X2)
and that in the valence-band edges by ?EV.



Fig.
2.2 Energy-band
diagram for two isolated semiconductors.

When a junction is
formed between these semiconductors, the energy-band profile at equilibrium for
an n-on-p hetero-junction is shown in Fig. 2.3 . Since the Fermi
level must coincide on both sides in equilibrium and the vacuum level is
everywhere parallel to the band edges and is continuous, the discontinuity in
conduction band edges (?EC) and valence-band edges (?EV)
is invariant with doping in those cases where Eg and ? are
not functions of doping. The total built-in potential Vbi is
equal to the sum of the partial built-in voltage (Vb1 + Vb2),
where Vb1 and Vb2 are the electrostatic
potential supported at equilibrium by semiconductors 1 and 2, respectively.



Fig.
2.3 Energy-band
diagram of a np hetero-junction in thermal equilibrium.

2.1.3
The Metal Semiconductor Junction

Metal semiconductor
contacts show either rectifying or ohmic behaviour. Rectifying contacts permit
current in one direction only, whereas ohmic contacts allow easy current flow
in both directions from metal to semiconductor and from semiconductor to metal.
Whether a contact is rectifying or ohmic depends on the Fermi levels in both
the metal and semiconductor.

2.1.4
Theory of Thin Film Solar Cells

When a semiconductor
is exposed to light, photons with energy h? >Eg create electron–hole
pairs. The internal electric field produced by one of the junctions described
before separates these photo-generated carriers to produce an electric signal.
In an energy band diagram of  n-p hetero-junction
solar cell, the charge carriers generated in the depletion layer or within one
diffusion length from the edge of the depletion region drift under the
influence of the electric field with electrons going into the n region and
holes going into the p region, both contributing to the photo-generated
current.



Fig.
2.4 Energy
band diagram of an n-p hetero-junction under illumination.

2.2PROPERTIES
OF SOLAR CELL MATERIAL

The choice of
materials for photovoltaic conversion is based on a number of requirements
including:

1. A direct band gap
with nearly optimum values for either homo-junction or hetero-junction devices.

2. A high optical
absorption coefficient, which minimizes the requirement for high minority
carrier lengths.

3. The possibility of
producing n- and p-type material, so that the formation of
homo-junction as well as hetero-junction devices is feasible. Generally p-type
material is preferred because electrons in many cases have a higher mobility,
and the materials therefore exhibit a higher minority carrier length. Another
reason is that most suitable window materials have an n- type character, and a p-
type absorber is needed in a hetero-junction device.

4. A good lattice and
electron affinity match with large band gap (window) materials such as CdS,
CdSe or ZnO so that hetero-junctions with low interface state densities can be
formed and device limiting conduction band spikes can be avoided.

These requirements are
fulfilled by a number of II-VI compounds. For photovoltaic applications, only
cadmium and zinc compounds are directly suitable. They are direct band gap
semiconductors, with high absorption coefficients and can be used as thin-film
materials. Cadmium telluride (CdTe) is a leading thin film photovoltaic
material due to its near ideal band gap of 1.45 eV, its high optical absorption
coefficient and availability of different device fabrication methods, for solar
energy conversion. A thin film of CdTe with thickness of approximately 2 ?m
will absorb nearly 100% of the incident radiation .

2.2.1 Properties of
Cadmium Selenide

In recent years,
semiconducting chalcogenide thin films have received much attention due to
their world-wide applications in various fields of science and technology. Cadmium
selenide is known to be a mixed ionic-electronic conductor. CdSe is a promising
photovoltaic material because of its high absorption coefficient and nearly
optimum band gap energy for the efficient absorption of light and conversion
into electrical power. Cadmium selenide in its wurtzite crystal structure is an
important II-VI semiconductor. As a semiconductor CdSe has a band gap of 1.74
eV at 300 K. It is an n-type semiconductor, which is difficult to dope p-type,
however p-type doping has been achieved using nitrogen. CdSe is also being
developed for use in opto-electronic devices, laser diodes, nanosensing, and
biomedical imaging. They are also used being tested for use in high-efficiency
solar cells.

Most of the usefulness
of CdSe stems from nanoparticles. Nanoparticles are just what the name implies,
particles of CdSe that are 1-100 nm (1nm = 10-9m) in size. CdSe
particles of this size exhibit a property known as quantum confinement. Quantum
confinement results when the electrons in a material are confined to a very
small value. Quantum confinement is size dependent, meaning the properties of
CdSe nanoparticles are tunable based on their size.

Due
to its high electronic quality and a fixed optical bandgap of 1.74 eV, cadmium
selenide is chosen as one of the materials that can be used as an absorber
layer in the top cell. It belongs to II-VI group of the periodic table. Cd has
2 electrons in its outer most orbit while Se has 6 electrons. Every Cd atom
transfers its 2 electrons to 6 valence electrons of Se to form CdSe. A vacancy
of a selenium atom frees two electrons of cadmium making CdSe n-type.



Fig. 2.5 The
wurtzite structure of CdSe

Wurtzite structure of CdSe

CdSe
crystallizes either in wurtzite (Hexagonal) or cubic (Zinc blend) structures.
The wurtzite structure of CdSe is shown in Fig. 2.5.

Crystal
properties

Crystal growth method

Seeded vapor phase growth

Crystal growth orientation

(001)

Maximum size

Up to 50mm diameter

Variations

Doped crystals

Crystallographic
properties

Crystallographic structure

Hexagonal

a= 0.43Å nm

c= 0.7021 nm

Defects structure

Color

Dark gray (metallic)

Physical
properties

Molecular formula

CdSe

Density

5.816 g/cm3

Molar mass

191.37g/mol

Melting point

1541 K

Hardness

4 Mohs

Thermal conductivity

3.49 W m-1 K-1

Specific heat capacity

N/A

Dielectric constant

8.35eV

Band gap (@ 300 K)

1.67 eV

Hall mobility

1050 cm2/V/sec

Specific resistivity

1 and 107 ohms-cm

Emission Wavelength

630 nm @ 300 K

Solubility in water

Insoluble

Appearance

Greenish Brown or
dark red solid power

Optical
properties

Transmission range

0.53µm – 15µm (2mm thick)

Refractive index

2.55

2.2.2
Properties of Cadmium Telluride

Cadmium Telluride
(CdTe) is a crystalline compound formed from Cadmium and Tellurium. It is used
as an optical absorber and a solar cell material. It is one of the leading thin
film photovoltaic materials due to their optimum energy band gap (about 1.5 eV)
and light absorption coefficient (>104 cm-1) for
visible solar radiation. It is usually sandwiched with cadmium sulfide to form
a p-n junction photovoltaic solar cell. Thin film CdTe provides a cost
effective solar cell design. CdTe is also applied for electro-optic modulators.
It has the greatest electro-optic coefficient of the linear electro-optic
effect among II-VI compound crystals. CdTe doped with chlorine is used as a
radiation detector for x-rays, gamma rays, beta particles and alpha particles.
CdTe can operate at room temperature allowing the construction of compact
detectors for a wide variety of applications in nuclear spectroscopy . The
properties that make CdTe superior for the realization of high performance
gamma- and x-ray detectors are high atomic number, large bandgap and high
electron mobility ~1100 cm2/V·s, which result in high intrinsic
?? (mobility-lifetime) product and therefore high degree of charge collection
and excellent spectral resolution.

Physical
properties

Molecular
formula

CdTe

Molar
mass

240.01
g/mol-1

Density

5.85
g/cm3

Melting
point

1092 °C

Boiling
point

1130 °C

Solubility
in other solvents

Insoluble

Band
gap

1.44ev
@300K direct

Refractive
index

2.67
(@10 µm)

Lattice
constant

0.648 nm
at 300K

Poisson
Ratio

0.41

Thermal
properties

Thermal
conductivity

6.2
W·m/m2·K at 293 K

Specific heat capacity

210
J/kg·K at 293 K

Thermal
expansion coefficient

5.9×10?6/K
at 293 K

2.3
DEPOSITION METHOD

Thin film can be
prepared by many physical and chemical techniques. In this section, we describe
briefly some of the commonly used methods for depositing large area thin film
on a variety of commercially available substrates. The physical techniques are
vacuum evaporation, sputtering, spraying and painting while vapor deposition,
oxidation, immersion plating, chemical bath deposition and electroplating are called chemical techniques.

2.3.1 Physical Techniques

2.3.1.1
Vacuum Evaporation Method

In
this method, material is first thermally vaporized and then condensed on a
substrate. Among the numerous deposition parameters substrate temperature,
evaporation rate and the vacuum quality affect the microstructure and surface
morphology of the coatings. Thermal evaporation technique allows precise
control over the film thickness. Hence by sequential evaporation with a good
control over thickness multilayer interface stacks of any complexity can be
prepared with great precision.

The
evaporation of a material required to be heated at sufficiently high
temperature to produce the desired vapor pressure. The rate of free evaporation
of vapor atoms from a clean surface of unit area in vacuum is given by the
Langmur-Dushman kinetic theory equation [18]:

Ne=3.513×1021
Pe/(MT)0.5 [molecules cm-2 s-1]

Where
Pe is the equilibrium vapor pressure (in Torr) if
the evaporant under saturated vapor at a temperature T, and M is the molecular
weight of the vapor species. The vapor atoms traverse the medium and are made
to condense on a substrate surface to form a thin film. The rate of
condensation/deposition of the vapor atoms depends on the vapor source
substrate geometry and the condensation coefficient on the surface under given
physical condition.

 The temperature of a material for evaporation
may be raised by direct or indirect heating. The simplest and most common
method is to support the material in a filament or on a boat, which is heated
electrically. Many shapes and sizes of filaments and boats of a number of
materials are commercially available to suit a range of evaporation materials
and applications. The geometrical distribution of vapors from several standard
sources is well known and is described in textbooks.

Multiple evaporation sources
are essential for multilayer technology as well as for obtaining controlled composition alloys/semiconductors. Besides
controlling
the vapor pressure of the constituents, the substrate
temperature provides an important control
over the relative sticking coefficients and thermodynamic stability of the
different vapor species. Thus a multiple source and temperature technique is
ideally suited for preparing
films of well-defined
composition of multi-component
semiconductors and has been used extensively
for IV-VI, II-VI, and III-V compound
films.

 Vacuum evaporation requires a system with a
known vacuum and its residual gas analysis. A diffusion pump backed by a rotary
pump system continues to be the 10-6– 10-8 Torr workhorse
in thin film technology largely because of its modest price, simplicity, and
high speed. By using special diffusion pump oil (e.g., polyphenyl ether), a
cryogenic baffle, and an all-metal system, ultrahigh vacuum (UHV) in the range
10-8 -10-10 Torr are easily obtained.

Vapour species may be
created by kinetic ejection from the surface of a material (called target of
cathode) by bombardment with energetic and nonreactive ions. The ejection
process, known as sputtering, takes place as a result of momentum transfer
between the impinging ions and the atoms of the target surface. The sputtered
atoms are condensed on a substrate to form a film. The major advantage of this
method is that almost any material can be sputter deposited at a high
deposition rate compared to evaporation methods. Moreover advantage of the
sputtering techniques is that the composition of the sputtered film is nearly
the same as that of the method and that the rate of deposition remains constant
with time.

2.3.1.2
Spray Pyrolysis

The spray pyrolysis
technique involves spraying a solution, usually aqueous, containing soluble
salts of the constituent atom of the desired compound onto a heated substrate
maintained at elevated temperatures. The sprayed droplet reaching the hot
substrate surface undergoes pyrolytic (endothermic) decomposition and forms a
single crystallite or a cluster of crystallites of the product. The other
volatile byproducts and the excess solvent escape in the vapor phase. The
substrate provides the thermal energy for the thermal decomposition and
subsequent recombination of the constituent species followed by sintering and
recrystallization of the clusters of crystallites giving rise to a coherent
film. This method is cheaper and more convenient.

2.3.1.3
Painting

This is a simple and
cheap method of producing selective coating. This can be applied directly on
the finished substrate with a brush. The spectral properties of a selective
paint depend upon the optical properties of pigments, the particle size and the
multiple scattering effects within the pigment binder composite.

2.3.2
Chemical Techniques

2.3.2.1 Chemical Vapor
Deposition (CVD)

 In this technique, the formation of the films
is due to the heterogeneous reaction-taking place at or near the substrate
surface. The reactants are in gaseous form and leave a non-volatile product
behind. The nature of the film deposited is strongly influenced by the nature
of the chemical reaction. The deposition takes place generally at atmospheric
pressure. The high temperature of deposition helps in the elimination of stress
and delimitation of the coating.

 Anodization is an electrolytic oxidation
process in which a metal is made the anode in a suitable electrolyte so that
when an electric current is passed through the electrolyte, the metal surface
is converted to its oxide. Depending upon the solvent action of the selected
electrolyte on the anodic oxide and the operating conditions employed a porous
anodic oxide film can be grown on the anode. By carrying out electrolysis of
this anode film in a metallic salt solution, fine metal particles can be
embedded in the pores of the anodic oxide film, giving rise to a black colour
to the coating. The operating parameters like composition of the electrolyte,
temperature and time have to be optimized to get good selectivity. If a
substrate consisting of a noble metal let M1 is immersed in a solution
containing ions of nobler M2, the more noble metal ions displace the
less noble metal at the substrate surface. The reaction likely is

M1
+ M2 ? M2 + M1

The metallic ions
taking part in this exchange reaction can partly get oxidized in the presence
of suitable oxidizing species in the solution. The optical behaviour of coating
is determined by the pH and concentration of the bath and dipping time.

2.3.2.2. Electroplating

Electroplating is one
of the most widely used methods to deposit oxide and sulfides on metals. Black
nickel and black chrome are the two well-known selective absorbers prepared on commercial scale by electroplating.
In the plating process, parameters like pH of the solution, temperature of the
bath, current density and plating time effect the micro structure topology and
composition of the coatings. By varying the
current while plating one can obtain a stack of numerous finely divided
layers with a continuous gradient of composition
resulting in a continuous gradient of
refractive
index.

2.3.2.3 Chemical Bath Deposition (CBD)

Chemical bath
deposition (CBD) is a method of growing thin film of certain materials on a
substrate immersed in an aqueous bath containing appropriate reagents at
temperatures ranging from room temperature to 100°C. The CBD technique was first
used in 1946 to prepare PbS films for infrared applications. It is only recently
[20-23] that large-area and large-scale applications of this technique to
obtain doped and un-doped multi-component semiconductor films of usual,
unusual, and meta-stable structures have necessitated an understanding of the
Physics and Chemistry of the process involved.

According
to the solubility product principle, in a saturated solution of a weakly
soluble compound, the product of the molar concentrations of its ions (each
concentration term being raised to a power equal to the number of ions of that
kind
shown by the formula for the compound), called
the ionic product, is a constant at given
temperature.

The CBD technique
involves the controlled precipitation from solution of a compound on a suitable
substrate. The technique offers many advantages over the more established vapor
phase synthetic routes to semiconductor materials, such as CVD, MBE and spray pyrolysis.
Varying the solution pH, temperature and reagent concentration allies’ factors
such as control of film thickness and deposition rate by varying the solution
pH, temperature and reagent concentration with the ability of CBD to coat large
areas, in a reproducible and low cost process. In CBD, two processes are
traditionally used for film growth: single dip, where the substrate is immersed
in the reaction bath only once, and multiple dips, where the same substrate is
repeatedly coated to obtain thicker film.

A major drawback of
the CBD process is the inefficiency of the process, in terms of the
utilizations of starting materials and their conversion to thin films. The
extent of the heterogeneous reaction on the substrate surface is limited by two
major factors, the competing homogeneous reaction in solution (which results in
massive precipitation in solution) and deposition of material on the CBD
reactor walls.

2.3.2.4
Electrodeposition

The history of
electrodeposition can be traced back to 1833-34 when Faraday enunciated the law
relating the chemical change with the passes of electricity through an
electrolyte. The phenomenon of electrolysis is governed by the following two
laws, (i) the magnitude of chemical change occurring is proportional to the quantity
of electricity passed and (ii) the masses of different species deposited at or
dissolved from electrodes by the same quantity of electricity are in direct
proportion to their chemical equivalent weights.



Fig.
2.6 Schematic
representation of a simple electrodeposition bath.

The
two laws can be combined and expressed mathematically as



Where W is the mass
(in grams) of the substance deposited, I is the current (in amperes), E is the chemical equivalent weight (in
grams), and t is the reaction time (in
seconds). F is a constant called
the Faraday constant, equal to 96500 C and is the amount of charge required to deposit one equivalent of any
ion from a solution.

A simple set-up used
for electrodeposition of an element M can be schematically represented as shown
in Fig. 2.6. It consists of the following components:

i)  
The
electrolyte: This can be an aqueous, non-aqueous or molten solution of a
suitable salt containing the ionic species to be deposited. For good and
reproducible electrodeposition, it is necessary that the electrolyte bath is
sufficiently conducting and stable over the working range of temperature and
potential.

ii)
The
working electrode: this can be a cathode or an anode of a conducting material
over which the electrodeposition is to be carried out.

iii)
The
counter electrode: The counter electrode is also made up of a conducting
material and has a polarity opposite to that of working electrode.

iv)
The
reference electrode: this is normally used to monitor the deposition of the
working electrode. It can be normally hydrogen electrode (NHE), a saturated
calomel electrode (SCE), a silver/silver halide electrode etc.

v)
 The driving force: In electrodeposition, the
driving force is electrical, and can be derived by an external power supply.
The eletrodeposition can be performed under a potentiostatic or a galvanostatic
condition using a DC or a pulsed power supply.

 Although, the electrodeposition of
semiconductor can be performed on an anode (anodic deposition), the latter has
been overwhelmingly preferred. In anodic deposition, it is difficult to get
sufficiently thick films of desired stoichiometry. One can describe a cathodic
electrodeposition process by the following simple reaction:

  Mz+ + ze ? M

If the electrolyte
contains more than one ionic species that can be simultaneously deposited, then
the electrodeposition process for two types of ionic species can be written as,

  M+ +
e ? M

N+
+ e ? N

  Or M+ + N+ + ? MN

Of all these
techniques electrodeposition appears to be more promising materials fabrication
technique. The following reasons can be ascribed to this:

(1) Electrodeposition
is a low cost and high throughput technique requiring a simple setup and
nominal power.

(2) It can be easily
scaled up for the commercial purposes. Electroplating is being used by a number
of industries for corrosion resistant coating, aluminium refining,
microelectronics circuit fabrications etc.

(3) It can yield very
high purity materials. For example Lyons et. al. have reported
electrodeposition of polycrystalline CdTe films which were purer than the ultra
pure crystals bought as 99.9999% materials [25].

(4) It may not require
high purity materials. Purification of starting materials can be accomplished
by a pre-electrolysis in presence of a complexant to prevent  the co-deposition of undesirable impurities.

(5) Fairly high growth
rates can be achieved using electrodeposition. A large number of elements,
binary and ternary semiconductors have been electrodeposited and photovoltaic
devices developed from these films.

In spite of the
several outstanding advantages the application of electrodeposition for solar
cell fabrication is hampered due to following reasons:

·
The
electrodeposited semiconductors tend to acquire a coarse and irregular surface
morphology. In many cases the deposits are porous. These features are
undesirable since they increase forward leakage current, cell resistance and
poor photo generated carrier collection efficiency.

·
The
control of stoichiometry in case of compound semiconductor is also difficult.
The problem becomes more critical as one move from binary to ternary systems.

The theoretical
understanding of the electrodeposition process for compound semiconductors is
at an early stage of development. The complete electroplating process can be
broadly divided into the following steps

A)
Transport
of ionic species from the electrolyte bulk to a position near the electrode.

B)  
Discharge
of ionic species leading to a charge transfer reaction.

C)  
Electro-crystallisation
of the deposits.

2.4
PRINCIPLES OF ELECTRODEPOSITION OF SEMICONDUCTORS

We shall elaborate the
general principles leading to electrodeposition of compound semiconductors. For
simultaneous deposition of the elements A and B from the compound AB, it is
necessary that the following conditions are satisfied.

Ø
The
respective electrode potentials for both the components must be equal and more
positive than any other electrode process.

Ø
They
should be capable of depositing independently.

Ø
The
electrolyte should be conducting and stable.

The above remarks are
applicable to both elemental and compound semiconductors. Apart from a few
elemental semiconductor (e.g. Si, Ge, Se, Te) most of the semiconductors of
interest are compound semiconductors. The following three problems are typical
of compound semiconductor electrodeposition.

1.
Most
of the compound semiconductors have at least one metallic (Ga, In, Cd, Cu, Zn,
Hg etc) and one non-metallic (S, Se, P, As etc ) components as in GaAs,
GaP,  InP, CdS, CdSe, CdTe, ZnTe, CdZnTe,
CdHgTe, CuInSe2. Unfortunately the reduction potentials of the non-metals
are widely different from those from the metal ions. The general condition for
co-depositing the different components of the compounds requires that the
constituents have equal reduction potentials. This is difficult to attain for
compound semiconductors.

2.
Most
of the compound semiconductors have very negative Gibb’s free energy. This may
shift the deposition potential of the less noble component to a more positive
value, helping the co-deposition.

3.
Many
compound semiconductors exhibit multiple phases. The possibility of such
multiphase formation may lead to additional complexities in the activity term
controlling the deposition. The various factors, which influence the
electrodeposition of compound semiconductors, can be broadly classified into
two categories:

v  Thermodynamic factors

v  Kinetic factors

2.4.1
Steps Involved in the Electrodeposition Process

Consider a cathodic
electrodeposition reaction involving Mz+ ions. The ion can be in
either of the following forms:

1.
Hydrated
form electrodepositing with the overall cell reaction

Mz+.nH2O
+ ze ? M + nH2O

2.
Complexed
form (with any legand or solvent) electrodepositing with the overall cell
reaction



 + ze ? M + xA 

In general the above
electrodeposition reactions occur in the following successive steps

I.  
Ionic
transport

 
 Discharge

 Breaking up of ion-legand bond (sometimes this
step is skipped)

IV.  
Incorporation
of adatoms onto the substrate followed by nucleation and growth.

The electrodeposition
of II-VI semiconductors is relatively easy and many reports are available on
the electrodeposition from aqueous and nonaqueous baths. In one mechanism Cd2+
gives CdSe by following reaction [26]

H2SeO3
+6H+ + 4e ? H2Se + 3H2O

Cd2+
+H2Se ? CdSe + 2H+

The rate of reduction
of H2SeO3 depends on the concentration of the Se4+
ions. At higher concentration the overall process manifests as a net four
electron reduction of Se4+ to elemental selenium.

2H2Se
+H2SeO3 ? 3Se + 3H2O

In an alternative
mechanism the electrodeposition of CdSe is based on the initial reduction of Cd2+
to Cd0 followed either by the chemical oxidation of Cd by H2SeO3
or the electrochemical reduction of H2SeO3 on the cadmium
surface [27].

3Cd + H2SeO3
+ 4H+ ? CdSe + 3H2O +2Cd2+

Cd + H2SeO3
+4H+ + 4e ? CdSe + 3H2O

Another mechanism
involving a direct six electron reduction of Cd2+ and Se4+
to form CdSe

Cd2+
+ H2SeO3 + 4H+ + 6e ? CdSe + 3H2O

Deposition mechanism
of CdTe can be described by the following reaction [28]

Te4+
+ 4e ? Te E0 =
0.328V (SCE)

Cd2+
+ 2e ? Cd   E0 =
0.68V (SCE)

Cd + Te
? CdTe ?G= -92KJ

PRINCIPLES OF CHARACTERISATION METHODS

The
following investigations are performed in the present research work:

Ø  Photoelectrochemical
(PEC) cell is used to determine the carrier type
of CdSe and CdTe films.

Ø  Optical
properties like transmittance, absorbance
and band gap energy measurement of the CdSe
and CdTe films.

Ø  Structural
properties of CdSe and CdTe films are measured using X-ray diffraction (XRD).

Ø   Surface morphology of the CdSe and CdTe films are measured by Scanning electron microscopy (SEM).

Ø  Current-voltage
Characteristics of CdTe/CdSe/FTO/glass structure are measured.

3.1 PHOTOELECTROCHEMICAL (PEC) CELL MEASUREMENT

PEC
measurements are non-destructive and used to determine the electrical
conductivity type of semiconductor materials [1].





(a)   (b)

Fig. 3.1 Band diagrams of (a) n-type semiconductor-liquid junction, (b) p-type
semiconductor-liquid junction.

When a
semiconducting electrode is immersed in a solution containing a redox couple,
the chemical potential at both electrode and solution must be identified, if no
external field is applied. The bands in the semiconducting bend act so as to
equalise the Fermi level and the redox potential. The direction of bending depends
on the particular system but for n and p-type materials it is usually in the
direction indicated in Fig. 3.1. Illumination of the electrode surface can
bring about promotion of electrons from valence to conduction bands and the
field gradients at the junction will result in separation of the photo
generated electrons and holes, as in the purely solid state case. For upward
curvature as in Fig. 3.1(a) electrons move into the bulk of the semiconductor,
while holes leave the surface to oxidise the anions of the redox couple. If an
external circuit is made with a counter electrode immersed in the solution,
then the electrons will flow from the semiconductor electrode to the counter
electrode to reduce ions in the solution. Fig. 3.1(b) shows the analogous
energy diagram for absorption by the p-type material, from which electrons
leave the surface to reduce the cations of the redox couple. The direction of
the current or the polarity of the voltage measured in the external circuit
gives the conductivity type of the semiconductor.

3.2
OPTICAL CHARACTERIZATION

To determine the optical constants (n and k) of thin films, a large number of experimental techniques are used. A critical
discussion of these techniques is given by
Chopra
. The most commonly employed method involves separate determination of n and k from reflectance and
transmittance measurement on the same film. At a film thickness where the
effects of multiple
reflections are
suppressed, the transmittance T of a film of index n1-ik1
and thickness t
is given by



(3-1)

Where no
is the index of the substrate and the ambient is assumed to be air. A plot of
logT vs t would then yield the value of k1 from the intercept as
well as the slope. If interference and
multiple reflections are neglected, T and R are related by T =
(1-R) exp (-4?k1t/?). When
reflection at the film/substrate interface is taken into account, T = (1-R)2
exp(-4?k1t/?) for no<n1,
ko = 0. Since the absorption coefficient ? is equal to
4?k1t/?, measurement of R and T data on the same film offers the most
convenient method for determining of ?.

Spectrophotometer
is used at different wavelength to determine the value of ?. To determine the nature (direct or
indirect) of the optical transition and the value of the optical gap Eg, the spectral variation of ? can be fitted to the equation ? = A (h?-­Eg)1/2  .

3.2.1 Band Theory of Solids

Any solid
has a large number of bands. In theory, it can be said to have infinitely many
bands (just as an atom has infinitely many energy levels). However, all but a
few lie at energies so high that any electron that reaches those energies
escapes from the solid. These bands are usually disregarded.

Bands have
different widths, based upon the properties of the atomic orbitals from which
they arise. Also, allowed bands may overlap, producing (for practical purposes)
a single large band.



Fig.
3.2 Schematic diagram of
bands in a solid.

Figure 3.2
shows a simplified picture of the bands in a solid that allows the three major
types of materials to be identified: metals, semiconductors and insulators.

Metals contain a band that is partly empty and partly filled
regardless of temperature. Therefore they have very high conductivity.

The
lowermost, almost fully occupied band in an insulator or semiconductor is
called the valence band by analogy with the valence
electrons of individual atoms. The uppermost, almost
unoccupied band is called the conduction band
because only when electrons are excited to the conduction band can current flow
in these materials. The difference between insulators and semiconductors is
only that the forbidden band gap between the
valence band and conduction band is larger in an insulator, so that fewer
electrons are found there and the electrical conductivity is lower. Because one of the main mechanisms for electrons
to be excited to the conduction band is due to thermal energy, the conductivity
of semiconductors is strongly dependent on the temperature of the material.

This band
gap is one of the most useful aspects of the band structure, as it strongly
influences the electrical and optical properties of the material. Electrons can
transfer from one band to the other by means of carrier generation and recombination
processes. The band gap and defect states created in the band gap by doping can be used to create semiconductor devices such as diodes, transistors, laser diodes, solar
cells and others.

In typical
calculations, a single electron is assumed to be in the form of a plane wave
moving, for example, in the x direction with propagation constant k, also
called a wave vector. Solving the Schrödinger equation, the space-dependent
wave function (from this, the average position, energy, and momentum of the
particle can be found) for the electron is

? k
(x) = U (k, x)



(3-2)

Where the function U(k, x) modulates the wave function according to the
periodicity of the lattice.

In such a calculation, allowed values of energy can be plotted versus the
propagation constant k, theoretically
for semiconductors three methods, orthogonalized
plane-wave, the pseudo potential and the k-p method are studied for
the
energy bands of solids. Since the periodicity of most lattices is different in
various directions, the (E, k) diagram must
be plotted for the various crystal directions, and
the full relationship between E and k is a complex
surface, which should be visualized
in three dimensions.

Measurement of Energy Gap:

The following two methods are usually used to determine the energy gap:

1. Electrical method: Conductivity method

2. Optical method: Absorption method

3.2.1.1 Conductivity Method

The electrical conductivity of an intrinsic semiconductor is given by

? = nie(µn+ µp)

Here the
concentration ni increases exponentially with temperature as
equation

ni =
2





If we combine these equations, we may write the conductivity in the form

? = 2e



n+ µp)

(3-3)

or, ? = f (T )

 (3-4)

Where 2e



n+ µp) = f (T) is a function, which weakly depends on temperature, i.e., as a
polynomial. Thus conductivity increases exponentially with
temperature because of the exponential factor in Eq.
(3-4).

At T = 0 K, the conductivity is zero and the sample behaves as an
insulator. At this temperature the resistance of the sample is
high. From Eq. (3-4) we can write

RT
= Ro

(3-5)

Where

RT = The resistance of the semiconductor at temperature
T K.

Ro=

 = The resistance of the
semiconductor at temperature 0 K.

At a
certain temperature to the equation (3-5) becomes



= Ro

 (3-6)

Using
equations (3-5) and (3-6) we can get

Ln

 = Ln

 +



 (3-7)

A plot of
In

versus 

 therefore yield a straight
line whose slope,

, determines the band gap energy, Eg.

In the
early days of semiconductors this was the standard procedure for determining
the energy gap. Now a day, however, optical method is using to measure the
energy gap.

3.2.1.2
Optical Method: Absorption Method

Ionic
crystals exhibit strong absorption and reflection in the infrared region as a
result of the interaction of light with optical photons. Because of the
partially ionic character of their bonds, compound semiconductors such as GaAs,
GaP, etc., should exhibit these properties.

In
fundamental absorption, an electron absorbs a photon (from the incident light),
and jumps from the valence band into conduction band. The photon energy must be
equal to the energy gap, or larger. The frequency must therefore be



 (3-8)

The
frequency ?o =

 is referred to as the absorption edge.

In the
transition process (photon absorption), the total energy and momentum of the
electron-photon system must be conserved. Therefore

E= Ei + hv    (3-9)

kf
= ki + q   (3-10)

where Ei
and Ef are the initial and final energies of the electron in the
valence and conduction bands, respectively, and ki, kare the corresponding electron
momentum.

The vector q is the wave vector for the absorbed photon. The wave
vector in the optical region is negligibly small. The momentum therefore
reduces to

kf =
ki   (3-11)




i.e. the momentum of the electron alone is conserved. This
selection rule means that only vertical transitions in k-space are allowed
between the valence and conduction bands (Fig. 3.3).

Fig. 3.3 The fundamental absorption process in semiconductors.

Calculating
the absorption coefficient for fundamental absorption requires quantum
manipulations. Essentially, these consists of treating the incident radiation
as a perturbation which couples the electron state in the valence band to its
counterpart in the conduction band, and using the technique of quantum
perturbation theory. One then finds that the absorption coefficient has the
form

?d = A(h v – Eg)? (3-12)  

where A is
a constant involving the properties of the bands 1/2 is a parameter, and Eg
is the energy gap.

The
absorption coefficient increases parabolically with the frequencies above the
fundamental edge. The absorption coefficient associated with fundamental
absorption is large about  104 cm-1.



Fig. 3.4 Schematic band diagram for the photoluminescence processes in a direct
gap material (left) and an indirect gap material (right).

Absorption
process occurs in the direct gap semiconductors. The shaded states at the
bottom of the conduction band as shown in Fig. 3.4 and the empty states at the
top of the valence band represent the electrons and holes respectively created
by the absorption of photons with energy ??exc > Eg.
The cascade of transitions within the conduction and valence bands represents
the rapid thermalization of the excited electrons and holes through phonon
emission. In a direct gap material (left) of Fig. 3.4, the conduction band
minimum and the valence band maximum occur at the same k values. Both the
photon absorption and emission (i.e. the electron-hole recombination) processes
can conserve momentum without the assistance of phonons, since the momentum of
the absorbed or emitted photon is negligible compared to the momentum of the
electron. We therefore represent photon absorption and emission processes by
vertical arrows on E-k diagrams. In an indirect gap material (right) of Fig. 3.4,
the conduction band minimum and the valence band maximum occur at different k
values. As a result, to conserve momentum, the photon absorption process must
involve either absorption (indicated by a “+” sign) or emission (indicated
by a “-” sign) of a phonon, while the PL process requires the
emission of a phonon. Since the energy of a phonon (~ 0.01 eV) is much smaller
than the energy of the PL photon, for an indirect gap material, the peak energy
of the PL also roughly reflects its band gap.

3.2.2 Spectrometric Measurement

3.2.2.1 Absorption Phenomenon

When light is incident on a crystalline solid, different optical
phenomena such as reflection, refraction, absorption, transmission, etc. may
occur.

Optical absorption may occur in
two processes: intrinsic and extrinsic. Intrinsic absorption in Fig. 3.5 (a)
corresponds to the raising of an electron from the
valence band to the
conduction band.



Fig. 3.5 (a)
Intrinsic transition, (b) & (c) Extrinsic transition

Extrinsic optical
absorption corresponds to the raising of an electron from the imperfection to
the conduction band as in Fig. 3.5(b), or the raising of an electron from the
valence band to an imperfection in Fig. 3.5(c) [6].

Optical
absorption in solid is the result of any of the following five processes

1) Excitation
of crystal vibration

2) Formation
of excitations

3) Excitation
of free electrons and holes within the allowed band

4) Excitation
of free electron and holes from one band to another of the same type.

5) Excitation
of electrons across the gap from the valence band to conduction band.

Of the
five absorption processes, only the last gives rise to photoconductivity.

3.2.2.2
Excitation of Electrons

The
necessary condition for the absorption of a photon and the formation of a
electron-hole pair is the condition h???E, where ?E is the width of the
forbidden energy gap in the semiconductor. The conversion of energy must be
satisfied. If the electron with momentum p is transferred from valance band to
conduction band of momentum p’ by absorbing a photon of momentum (h?/c)s then,

 pp
= (h?/c)s (3-14)

where s is unit vector in
the direction of motion of the photon. The photon momentum is negligible and so equation can be
written as,

pp = 0   (3-15)

i.e. only those transitions are allowed where momentum of the electron
undergoing transition remain fixed. Allowed transitions correspond to vertical
or direct transition.

 If k and k
are the wave vectors of the electron at valence band and of the electron transferred to the conduction band, then the
equation becomes

*
?k – ?k‘ = 0

*
 i.e. k’ = k   (3-16)

In addition to the vertical optical transition where momentum does not
change we have the non-vertical of indirect transition where wave vector k
changes.

Indirect
transition is impossible with a collision only between two bodies (i.e. the photon energy and the electron), because the
law of conservation of wave number
will be violated. So, it is assumed
that there is a probability of three-body collision involving a photon, an electron and a photon. In this interaction the
electron absorbs
the main part of the photon energy and changes the wave
number at the expense of photons, i.e., by interacting with lattice.

4210

Fig.
3.6 Direct
and indirect transitions.

The process described above may be expressed as follows:

k’=k ± K? (3-17)

Where K is the phonon absorbed or released during the transfer process.
The relations that governs the intrinsic band edge are

??1t = ?Eoo

??1t= (?EoT ± ??phonon )     (3-18)

where, ?Eoo,
band gap determined from spectrophotometric
measurement,
?EoT, band gap
determined from thermal measurement, ?
?phonon, energy corresponding absorbed of emitted phonon during the process.

It has been shown that the absorption coefficient
?
near the intrinsic absorption edge is approximately given by,

  ? ? (?? ?Eo)x

? = constant (?? ?Eo) x   (3-19)

For direct transition (allowed) x =1/2

For direct transition (forbidden) x = 3/2

For indirect transition (allowed) x = 2

For indirect transition (forbidden) x = 3 .

3.2.2.3 Absorption by Charge Carriers

When light penetrates into a semiconductor material the interaction
between the incoming and the charge carriers (holes and electrons) may
lead to absorption of light. The carriers
are accelerated; thereby their energy increases at the expose of the lattice
converting then light energy ultimately to heat energy (lattice vibration). The
phenomenon can be derived by chemical
electro-dynamics. The absorption
coefficient for such interaction is given by

? = 4?/cn?
   
(3-20)

where n = refractive index of the material, c
= velocity of light,
? = conductivity of the material and ? is frequency dependent parameter.

3.2.2.4 Absorption Spectrum

The attenuation of light transmitted through a distance x of a material
is

  I(x) =1(0) e?x      (3-21)

Where I (x) is the intensity of light after it passes a distance x of
the medium, k is the absorption coefficient.

The dependence of absorption coefficient on the frequency ?(?) or on the wavelength ?(?) is known as the absorption spectrum of
solid. The relation between the absorption coefficient,
measure of probability of photon absorption cross-section
?(?) and the
concentration of absorption centres N
can be written as:

?(?) = ?(?) N    (3-22)

For a semiconductor containing absorption centres of different nature
with its own effective cross-section

?i(?) = ?i(?) Ni (3-23)

The obtained absorption co-efficient is the sum of partial absorption
co-efficients

? = ? ?i (?) = ? ?i(?) Ni = ?(?)     (3-24)

If R is the reflectivity (R=IR/Io) then equation
(3-19) becomes

I (x) = Io
(1-R) e?x (3-25)

Where IR is
the intensity if the reflected ray.

We define transmission T as T=(IT/Io) where, IT is the intensity if transmitted beam. If there is no reflection the intensity of transmitted beam after
a distance x is,

I(x) =Io e?x

or, T = (IT/Io) = [I (x)/Io] = e?x

or,   ln (1/T) = ?x = A  (3-26)

where A is called the absorbance.

If we have the reflectivity R, then the modified equation of absorbance
would be

A = ? x = ln 1 + (1 + 4c2R2/2c)
1/2   (3-27)

Where c =
T/(1-R)2 and T =
I (x)/ Io

3.3 Structural Characterization

3.3.1
X-ray Diffraction (XRD)

X-ray diffraction
is the most precise technique for studying the crystal structure of solids,
generally requiring no elaborate sample preparation and is essentially non-destructive.
Thin surface films, up to about 1000 Å thick, can be investigated using X-ray
diffraction,. Thicker films can be characterized by reflection high-energy
electron diffraction (RHEED). Analysis of the diffraction patterns obtained by
these techniques and comparison with standard ASTM data can reveal the
existence of different crystallographic phases in the film, their relative
abundance, the lattice parameters, and any preferred orientations. From the
width of the diffraction line, it is possible to estimate the average grain
size in the film.




Fig. 3.7 Schematic
diagram of X-ray diffraction process.

The term
structure encloses a variety of concepts, which describe on various scales, the
arrangement of the building blocks of materials. On an atomic scale, one deals
with the crystal structure, which is defined by the crystallographic data of
the unit cell. These data contain the shape and dimensions of the unit cell and
the atomic position within its Bravias structure. They are obtained by
diffraction experiments. On a coaster scale, one deals with the microscopic
observations of the microstructure, which characterizes the size, shapes and
mutual arrangements of individual crystal grains. It also includes the
morphology of the surface of the materials. Microstructure and surface
morphology observation of coatings, which are too thick for direct transmission
also depend heavily on the high resolving power of electron microscopy.
Suitable technique is surface replication and scanning electron microscopy.
Frequently one has to determine whether a given deposit is a single crystal or
polycrystalline either with a random distribution of orientation with respect
to the coating plane. For a single crystal coating, it is important to know its
orientation relationship with respect to the substrate. X-ray diffraction is a
suitable tool to determine the crystal structure of any unknown materials,
whether the sample is a single crystal or poly crystals, either with a random
distribution of orientations or with a preferred orientation with respect to
the film plane.

The
technique of XRD is based on Bragg’s law

N? = 2dsin?

where n is
the order of diffraction, ? is the wavelength of the x-rays, d is the distance
between two neighbouring planes, ? is the angle between the incident x-ray and
the crystalline plane (in radians).

As shown
in Fig. 3.7 when a monochromatic X-ray beam of wavelength ? is incident on the
lattice planes of a crystal at an angle ?, diffraction occurs only when the
distance travelled by the rays reflected from successive planes differs by a
complete number n of wavelengths. In polycrystalline materials, several d
values satisfy Bragg’s law by varying the angle ?.



Fig.
3.8 Schematic of the X-ray powder diffractometer.

The x-ray source provides a polychromatic radiation produced from
a copper target source. The monochromator positioned in front of the detector
is tuned to select only the CuK? emission line from the source. The system can
then be viewed as operating with a monochromatic source whose wavelength is
1.5406Ao. The sample is rotated around an axis. As the scanning
proceeds, the detector is rotated around the same axis to detect the diffracted
beam. The instrument is fully computer controlled. The x-rays detected from the
sample surface have been diffracted by families of planes, according to Bragg’s
law. The result of XRD measurements is a diffractogram, which is a plot of the
intensity (number of counts) versus the angle. The different phases in the
crystal (from peak positions), phase concentrations
(from peak heights), crystallite sizes (from peak widths) and amorphous content
(from background hump) can be deduced from a diffractogram.

3.4 Surface morphological characterization

The
optical and electrical properties of thin films are very sensitively influenced
by the crystallographic and micro-structural characteristics of the film. Similarly,
the structural features of the interface at the film also affect the electronic
behaviour of the photovoltaic materials. Several techniques have been developed
which provide image of the morphological, crystallographic, and defect
structure of materials component.

3.4.1 Scanning Electron Microscopy
(SEM)

SEM utilizes an electron beam to scan and produce a
magnified image of the specimen sample. SEM is similar to optical microscopy;
the exception is that electrons are used instead of photons. Much larger
magnifications are, therefore, possible with a SEM since electron wavelengths
are much smaller than photon wavelengths and the depth of field is much larger.



Fig.3.9: Schematic representation of SEM
operation.

The SEM provides the investigator with a highly
magnified image of the surface of a material that is very similar to what one
would expect if one could actually see the
surface visually. This tends to simplify image interpretations considerably,
but reliance on intuitive reactions to SEM images can, on occasion, lead to
erroneous results. The resolution of the SEM can approach a few nm and it can
operate at magnifications that are easily adjusted.

Not only is topographical information produced in the
SEM, but information concerning the composition near surface regions of
the material is provided as well.
In the SEM, a source of electrons is focused (in vacuum) into a fine probe that
is rastered over the surface of the specimen. The main operation of the
instrument are shown in Fig. 3.9.

An electron beam is generated by an electron gun at the
top of the column. This beam is focused by two magnetic lenses and deflected by
two scanning coils before hitting the sample in a very small spot. To avoid
undue scattering and absorption of the electrons, the column and sample chamber
is operated under vacuum. When the electrons penetrate into the sample, several
processes take place as they are being absorbed. Some are reflected (back
scattered electron, BSE) out of the sample and are collected by a suitable
detector. These electrons can be used to obtain information on the mean atomic
number in the part of the sample from which they originate. Secondary electron
(SE), with very low energy, are also released from the sample from an area
close to the spot where the primary electrons enter. Collected by a suitable
detector, these are used to obtain information on the topography of the sample.
Thus, this signal is used to modulate the intensity on a viewing screen, which
is scanned synchronously with the primary beam in the column. Thereby an image
is generated on the viewing screen with high brightness in areas with a strong
signal from the detector, and darker areas, where weak signal are detected. To
select an area of investigation, the sample is mounted on a special stage, with
allows translation, tilt and rotation of the sample. This stage can be
controlled from outside the vacuum chember. An energy dispersive x-ray (EDX)
spectrometer is also mounted within the equipment.

3.5
Device Characterization

Two methods are used
to characterize the solar cells

·
Current-voltage
(I-V) measurements

·
Capacitance-voltage
(C-V) measurements

Several electrical
properties of the devices can examine, such as the fill factor FF, short circuit current Isc, open circuit voltage Voc, energy conversion
efficiency ?, series resistance Rs,
shunt resistance Rsh,
ideality factor n, barrier height ?b,
depletion width w and doping concentrations ND
or NA, by using these
methods. In our work we have used only I-V measurements.

3.5.1
Current-voltage measurements

 The solar cell conversion efficiency relates
to:

Ø  Reflection – some of
the incident light are reflected from the surface of the cell;

Ø  Wavelength – some of the
light reaching the cell, have wavelength outside the spectral response of the
cell and not produce electron-hole pairs;

Ø  Recombination – of the
electron-hole pairs created, some are recombined before diffusing to the
junction.



Fig. 3.10 Eequivalent circuit of
a PV solar cell.

The energy conversion
process involves photo-generation and charge separation. A photovoltaic solar
cell is basically a semiconductor diode. The semiconductor material absorbs the
incoming photons and converts them to electron-hole pairs. In this photo-generation
step, the decisive parameter is the band gap energy Eg of the
semiconductor. In an ideal case, no photons with an energy h? < Eg
will contribute to photo-generation, whereas all photons with an energy h?
> Eg will each contribute the energy Eg
to the photo-generated electron-hole pair, with the excess energy (h? – Eg)
being very rapidly lost because of thermalization.

In the second step of
the energy conversion process, charge separation, the photogenerated
electron-hole pairs are separated, with electrons drifting to one of the
electrodes and holes drifting to the other electrode, because of the internal
electric field created by the diode structure of the solar cell. The
performance of a solar cell under illumination can be completely described by
the current-voltage dependence. If we consider a typical current-voltage curve
of a pn-junction diode in the
dark and under illumination as shown in Fig. 3.11, we can characterize three
parameters that give a complete description of the electrical behaviour: short-circuit
current, Isc
, open-circuit voltage, Voc, and the fill factor, FF. These three parameters are sufficient
to calculate the energy conversion
efficiency ? of the solar cell.



Fig. 3.11 Current-voltage
characteristics of a pn-junction solar cell.

The short-circuit
current Isc, which obtained for Voc = 0, is
equal to the light generated current, Isc = Il, if
the series resistance Rs is zero. A finite series resistance Rs
reduces the short-circuit current. The open-circuit voltage Voc,
which obtained for I = 0, is determined by the ratio Il/Is
and thus by the absorption and light-generation processes and the
efficiency with which the charge carriers reach the depletion region. In the
ideal case where Isr = Rs = 0 and Rsh
=
?, then:



The performance of the
solar cell is eventually determined by the fraction of the total power of
incident light that can be converted into electrical power. Under illumination,
the junction is forward biased and the external load resistance determines an
operating point on the current-voltage curve. The electrical power output P
= IV
is equal to the area of the rectangle. In general, the solar cell will
be operated under conditions that give the maximum power output. The maximum
possible area Pmax = VmaxImax for a
given current voltage curve determines the fill factor FF, which is defined
by



FF is larger the more
“square-like” the current voltage curve is. Typically, it has a value of 0.7 to
0.9 for cells with a reasonable efficiency. The three parameters Voc,
Isc, and FF are sufficient to calculate the
energy-conversion efficiency ? of the solar cell, which is defined by



.

EXPERIMENTAL DETAILS

In
this chapter we deal with the various experimental apparatus and experimental
procedure those are
used for the optical, structural and surface morphological characterization of
cadmium selenide and cadmium telluride thin
films and electrical characterization of CdTe/CdSe/FTO/glass heterojunction
solar cell structure. Sample preparation was carried out using
electrodeposition technique. Optical measurement was carried out to investigate
optical properties (such as transmittance, absorbance and band gap) of the
deposited films at Bangladesh Council for Scientific and Industrial Research
(BCSIR), Dhaka. XRD measurement was performed to identify the phases present in
the deposited films at Bangladesh Atomic Energy Centre.
SEM measurement was carried out to investigate the
surface morphology of the deposited films at Centre of Excellence, Dhaka
University.

4.1
EXPERIMENTAL APPARATUS

4.1.1
Apparatus for Sample Preparation

4.1.1.1
Apparatus for Electrodeposition



Fig.
4.1
Experimental Set up for Electrodeposition System

Electrodeposition
of CdSe and CdTe were carried out using a three electrode potensiostatic system
by BASI EPSILON instrument. The Epsilon system requires a serial port connection
or USB port connection to the Pentium III or better computer with 128MB RAM,
50MB available hard drive space running window 98, 2000 or XP, the BAS C3 Cell
stand, which is the group of wires that connects the epsilon to the electrode
of the electrochemical cell.

The
link between the PC and the epsilon are automatically established. The status
of the link is displayed in the CS Link Dialog box, which is disappear,
once connection has been established. The following dialog box represent
starting of epsilon.



Fig.
4.2
CS Link Dialog box.

All electrochemical cells require at least two
electrodes, since the potential of a given electrode can only be measured
relative to another electrode, the potential of which must be constant (a
reference electrode). In potentiometric measurements (such as measurement of
pH), there is no current through the cell, and these two electrodes are
sufficient (it should be noted that many pH and ion-selective electrodes used
in potentiometric measurements are combination electrodes – both electrodes are
contained within the same body). However, in a cyclic voltammetry experiment,
an external potential is applied to the cell, and the current response is
measured. Precise control of the external applied potential is required, but
this is generally not possible with a two electrode system, due to the
potential drop across the cell due to the solution resistance (potential drop
(E) = current (i) x solution resistance (R)) and the polarization of the
counter electrode that is required to complete the current measuring circuit.
Better potential control is achieved using a potentiostat and a three electrode
system, in which the potential of one electrode (the working electrode) is controlled
relative to the reference electrode, and the current passes between the working
electrode and the third electrode (the auxiliary/counter electrode).

The
electrodeposition bath consists of an electrolyte containing metal ions, a
working electrode with substrate, on which the deposition is desired, a counter
electrode. When a current flow through the electrolyte, the cations and anions
move towards the cathode and anode, respectively and may deposit on the
electrodes after undergoing a charge transfer reaction.

During
the cathodic deposition, there are continuous fluctuations of the cathode
potential owing to the change of the cell parameters (concentrations of ions of
the electrolyte, over voltages, conductivity of the substrate). Hence, these
changes will reduce the quality of the deposited layer. A typical arrangement
used to overcome this problem consists of a three electrode cell. This
technique is called potentiostatic electrolysis and involves establishing a
constant potential on the working electrode with respect to the reference
electrode by means of an electronic potentiostat. The counter electrode is
typically a platinum wire that provides a surface for a redox reaction to
balance the one occurring at the surface of the working electrode, and does not
need special care, such as polishing. In order to support the current generated
at the working electrode, the surface area of the auxiliary electrode must be
equal to or larger than that of the working electrode.

There
are 3 electrode leads and 1 grounded (shielding) lead in the BASI C3 Cell stand
to connect 3 electrodes. The colour code is:

Ø  Black
covered wire: Working electrode lead

Ø  Red
covered wire: Auxiliary electrode lead

Ø  White
covered wire: Reference electrode lead

Ø  Bare
or black wire: Earth ground connector

A
plastic mounting lug near the end of the cell cable as shown in Fig. 4.3
provides relief by preventing movement of the line or cell.



Fig.
4.3.
Cell (electrode) end of cell lead.

The
electrodeposition was carried out in a thoroughly clean and dried glass
electrodeposition cell. Ag/AgCl electrode was used as a reference electrode and
carbon electrode and FTO coated glass substrate was used as a working electrode,
platinum as a counter electrode. All the electrodes were held in proper position
and substrates were joined in the working electrode with the help of Teflon seals
as shown in Fig. 4.4.

























Substrate Working
Electrode


Ag/AgCl Reference
Electrode



Platinum Counter
Electrode









Fig.
4.4.
Electrodeposition system.

4.1.2
Apparatus for Carrier Type Determination

To
determine the electrical conductivity type of CdSe and CdTe layers, a simple
photoelectrochemical (PEC) cell was used [2]. The CdSe/FTO/glass substrate or
CdTe/FTO/glass and a carbon rod were partially immersed in a 10% NaCl solution,
and these two electrodes were connected to a digital voltmeter. The photo-voltage,
created with white light illumination, was estimated by measuring the voltage
under dark and illuminated conditions. Observation of few milli-volts as the
open circuit voltage is an indication of the formation of a good Schottky
barrier at the solid/liquid junction. The experiment consists of an arrangement
shown in Fig. 4.5. If the voltage difference between the dark and illumination
conditions is found to be positive on digital voltmeter, then the sample is
p-type; whereas if the photo-voltage difference is found to be negative, the
sample is n-type.






































Digital
Voltmeter









Anode



10% NaCl Solution



Carbon rod



Cathode



Sample














Fig. 4.5 Experimental set-up for
identifying the carrier type

(PEC
Measurement set-up)

4.1.3 Apparatus for
Optical Characterization





Fig. 4.6
UV-Visible spectrophotometer with holder.

The
optical measurements were performed using a UV-1601V,
UV-visible
spectrophotometer
(Shimadzu Corp., Japan) in Bangladesh Council for Scientific and Industrial
Research (BCSIR)
. This
spectrophotometer was used to measure the relative transmittance
and absorbance of as-deposited and annealed CdSe
and CdTe thin films. Wave length range was selected from 350 nm to 1100 nm.
After finishing base line correction, a sample and FTO-coated glass substrate
were placed vertically on the
thin
film holder provides by the spectrophotometer, being illuminated by a
monochromatic beam of light. Using the absorption spectra, the band gap
values for the films were estimated.

4.1.4
Apparatus for Structural Characterization

4.1.4.1 XRD Apparatus

The X-ray diffraction
(XRD) method was used to investigate the structural properties of the CdSe and
CdTe films at Bangladesh Atomic Energy Centre, (BAEC). The diffraction pattern
was recorded using a Philips PW 3040 X’ Pert PRO XRD system with Cu-K?
radiation using the wavelength of 1.5406 ?, operated at 60 kV and 55 mA, with
high temperature attachment up to 1600


 with the scanning angular range 10° ? 2? ? 90°
to get possible fundamental peaks for each sample. The XRD machine was totally
computer controlled and all the data were stored in the hard disk memory of the
computer for further analysis.



Fig.
4.7
Experimental
set up of X’Pert PRO XRD system.

4.1.5.
Apparatus for surface morphological characterization

4.1.5.1 Scanning
Electron Microscopy (SEM) Apparatus



Fig.
4.8.
Experimental arrangement of SEM apparatus.

Scanning Electron Microscopy (SEM) measurement was performed at Centre
of Excellence, University of Dhaka.
The JEOL JSM-6490LA
(Analytical Scanning Electron Microscope) apparatus was used to carry out the
SEM measurement. The JSM-6490LA is a high-performance, scanning electron
microscope with an embedded energy dispersive X-ray analyzer (EDS) developed by
JEOL which allows for seamless observation and EDS analysis. The take-off-angle
for the JSM-6490LA is 35°, with an analytical working distance of 10mm. The
microscope has a high resolution of 3.0nm. The low vacuum mode (which can be
accessed by the click of a mouse), allows for observation of specimens which
cannot be viewed at high vacuum due to excessive water content or due to a
non-conductive surface. Standard automated features include Auto Focus/Auto
Stigmator, Auto Gun (saturation, bias and alignment), and Automatic Contrast
and Brightness.

4.1.6
Apparatus for Metal Contacts on CdTe thin films

4.1.6.1
Apparatus for Vacuum Coating System



Fig.
4.9

Experimental Set-up of vacuum coating system.

Metal (Al) contacts
were prepared on CdTe films of CdTe/CdSe/FTO/glass substrate by vacuum
evaporation method with base pressure 10-5 Torr at Semiconductor
Technology and Research centre (STRC), D.U. This is completely a coating unit
consisting of a rotary pump and oil diffusion pump capable of producing a
vacuum down to a pressure of 10-5 Torr or less. The chamber was
equipped with a double-pass glass shield. The diffusion pump is separated from
the main chamber by gate valve.

4.1.7
Apparatus for Measurement of I-V Characteristics

The block diagram of
the circuit arrangement to measure the current-voltage characteristics of
CdTe/CdSe/FTO/glass heterojunction solar cell structure is shown in Fig. 4.10.










Power Supply



CdTe/CdSe/FTO/glass Structure




Digital multimeter








Fig.
4.10
Block
diagram of the equivalent circuit for I-Vmeasurements.

Contact problem is one
of the most significant problems in thin film research. To get ohmic contact,
fine Cu-wires were connected to the film with silver paste. In our experiment,
we used direct method to study I-V characteristics of the samples. Steady voltage
was applied in the range 0.1-3.0 V across the electrodes by a laboratory dc
power supply (GPR-3020, GW Taiwan); the current through the sample was measured
by a digital multimeter at Semiconductor Technology Research Centre (STRC),
D.U.

ELECTRODEPOSITION
AND CHARACTERISATION OF Cdse films

The details
experimental procedure of electrodeposition of CdSe thin film on FTO/glass
substrate and characterization of photoelectrochemical, optical, structural and
morphological properties of CdSe thin films have been presented in this
chapter. The annealing effect of the films has also been discussed.

5.1
PREPARATION OF THE SUBSTRATE

FTO-coated glass
substrates were used as substrates. Poor adhesion and non-uniform films are
common problems when depositing films onto smooth surface. So in the deposition
of thin film, substrate cleaning is very important since the contaminated
surface provides nucleation sites facilitating the growth which results
non-uniform film growth. Before depositing CdSe layers, substrates were boiled
using soapy distilled water for around 20 minutes and then rinsed with
distilled water. To eliminate grease and other oily substances, substrates were
rinsed with acetone, and it is then boiled again in distilled water for about
20 minutes. The substrates were then cleaned using ultrasonic cleaning bath for
about 15 minutes in distilled water prior to film deposition.

5.2
ELECTRODEPOSITION OF CdSe THIN FILMS

5.2.1
Cyclic Voltammetry

10-15 ml of the
ferricyanide test solution was added to the cell vial and placed in the cell
holder. Glassy carbon was polished following the polishing instruction in the
polishing kit. Then glassy carbon, platinum wire and Ag/AgCl reference
electrode were placed in the cell. Cyclic voltammetry experiments were then
performed using scanning potentiostat and a data acquisition system. The
working electrode (glassy carbon) potential was repeatedly scanned at a rate of
100 mV/s between two predetermined potential limits (from 0 mV to 750/800 mV
then 0 mV) and corresponding cell current was recorded.



Fig. 5.1. Change Parameters dialog
box for cyclic voltammetry.

It was thus possible
to select the potential range (-450 mV to -550 mV) favouring the formation of a
highly photo responsive semiconducting deposit under the ambient condition.



Fig. 5.2. A typical cyclic voltammogram  showing the important parameters.

5.2.2
CdSe Flim Preparation

For preparation of
cadmium selenide thin film by electrodeposition technique the deposition bath
consisted of an aqueous solution of CdCl2.H2O and H2SeO3.  0.2 M Cadmium chloride was mixed with 0.0007 M
selenous acid in 100 ml distilled water in a beaker and kept it stirring until
the total solid dissolved with the distilled water. The beaker was kept in the
bath in room temperature. Sulfuric acid was mixed with the solution to control
the pH value of the solution.

 The deposition was then carried out onto cleaned
conducting glass substrates in the electrodeposition cell, comprising of a
platinum counter electrode, a Ag/AgCl aqueous reference electrode and carbon
working electrode. Potential values were entered in mV and time values were
entered in minutes into changes parameter dialog box for performing controlled
potential electrolysis. During the electrodeposition, the electrolyte was
continuously stirred at a moderate speed with the help of a magnetic stirrer
and teflon coated paddle. The electrodeposition was allowed to continue for
about 10-30 minutes at a potentiostatic condition. The samples were prepared
within the voltage range of -450 mV to -500 mV and best quality samples were
observed within this voltage range. Immediately after the deposition, the
deposited CdSe films were soaked in warm distilled water to remove traces of
the solvent and were subsequently dried in air. A lot of samples were prepared
by varying applied voltage and deposition time with same pH value and
concentration of CdCl2.H2O and H2SeO3.
After getting photoelectrochemical, optical, structural and morphological measurements,
the as-deposited CdSe samples were then annealed at 200


 for 30 minutes in air ambient to investigate
the annealing effect.

5.2.3 Heat Treatment

Annealing in air plays an important role for improving CdSe thin film
solar cells through the passivation of the grain boundaries and the increase of
minority carrier diffusion length. It was also common observation that air
annealing of as-deposited chalcopyrite films leads to an increase of the
conductivity and improvement of crystalline quality. CdSe thin films were
annealed at 200

 for 30
minutes in air ambient.









-450 mV,
20 min.   -500 mV, 10min. -500 mV, 15min. -500 mV, 20min.







-500 mV,
10min. -500 mV, 10min. -500 mV, 20min.



Fig. 5.3 Samples of
CdSe electrodeposited at different condition.

5.2.4
Chemical Reaction

Cadmium selenide films have been formed according to
the following over-all reaction .

3Cd +  H2SeO3
+ 4H+ ? CdSe + 2Cd 2+ + 3H2O

In this mechanism the first step is the reduction of
H2SeO3 to selenium on the surface of the substrate
according to the reaction

H2SeO3 + 4e + 4H+
? Se0 + 3H2O

Which is at once followed by a successive reduction
processes

H2SeO3 + 4H+ + 6e
?H2Se + 3H2O

And lastly, formation of cadmium selenide takes
place according to the chemical reaction

H2Se + Cd2+ ? CdSe + 2H+

5.3
RESULTS AND DISCUSSION FOR CdSe  THIN
FILMS

5.3.1
Determination of Carrier Type of CdSe Thin Films

To
determine the Carrier type of CdSe layers, a simple photoelectrochemical (PEC)
cell was used . The CdSe/FTO/glass and a carbon rod were partially immersed in
a 10% NaCl aqueous solution, and these two electrodes were connected to a
digital voltmeter. The photovoltage, created with white light illumination, was
estimated by measuring the voltage under dark and illuminated conditions.
Table-5.1 below shows the obtained results for five typical as-deposited samples
of CdSe films. A negative photovoltage was demonstrating the n-type character
of the material. It can be concluded that the as-deposited CdSe samples are
n-type in electrical conductivity.

Table-5.1: The summery of typical PEC measurements for as-deposited
CdSe layers grown on FTO/glass substrates.

Deposition condition of CdSe films

Vdark

(mV)

Vlight

(mV)

?V

(mV)

Comments

1. -450 mV, 20 min

-180.7

-198.6

-17.9

n-type

2.-500 mV, 10 min

-387.7

-400.3.

-12.6

n-type

3.-500 mV, 20 min

-140.3

-196.7

-56.4

n-type

4.-500 mV, 10 min

-348.2

-376.1

-27.9

n-type

5.3.2
Optical Characterization of CdSe Films  

5.3.2.1 Transmittance Measurements

 Figure 5.4(a) and 5.4(b) represent the
variation of transmittance T (%) at wavelength range 350-900 nm of different
as-deposited and annealed CdSe films prepared at different conditions.
Transmittance is obtained to be about 0.9-95% in the wavelength range 350-900
nm. The spectra show that the transmittance increases with wavelength, which
may be due to absorption by carrier in the degenerate films. It is observed
that as-deposited CdSe films have very low transmittance, 0-5%, in the
visible-UV region; low to moderate transmittance, 5-60% in the visible range
and moderate to higher transmittance, 60-90%, in the visible-infrared region as
shown in Fig. 5.4a. The transmittance is significantly increased upon annealing
the CdSe films at 200

 for 30 minutes. It is
observed that annealed CdSe films have very low transmittance, 0-20%, in the
visible-UV region; low to moderate transmittance, 20-70% in the visible range
and moderate to higher transmittance, 70-95%, in the visible-infrared region as
shown in Fig. 5.4b. This higher transmittance of CdSe films makes it suitable
for use as a window material.





Fig. 5.4; Variation of transmittance (T%) with wavelength ? (nm)
of CdSe thin films for: (a) as-deposited, (b) annealed at 200


 for 30 min.

5.3.2.2 Absorbance Measurement

 Figure 5.5(a) and 5.5(b) show the variation of
absorbance with wavelength range 350-900 nm for different as-deposited and
annealed samples respectively. It is observed that the absorbance of the CdSe
films increases continually from the near-infrared towards the visible region,
which makes this material suitable for use in infrared detectors [3, 4]. The
films become totally absorbing at 350 nm. It is observed that as-deposited CdSe
films have very high absorbance, 4.8-2.0 in the visible-UV region; high to
moderate absorbance, 2.0-0.5 in the visible range and moderate to lower
absorbance, 0.5-0.0, in the visible-infrared region as shown in (Fig. 5.5a).
After annealing CdSe films show significant changes observed in absorbance
spectra. It is observed that annealed CdSe films have very high absorbance,
3.5-1.8 in the visible-UV region; high to moderate absorbance, 1.8-0.2 in the
visible range and moderate to lower absorbance, 0.2-0.0, in the
visible-infrared region as shown in (Fig.5.5b). The as-deposited film has the
highest absorbance within the UV region of the absorbance spectrum and there is
a slight lack of trend in the absorbance values displayed in the figure, caused
by the film annealed at 200


in
air ambient.

5.3.2.3 Band Gap Measurements

CdSe thin
film deposited at optimized preparative parameters on FTO coated glass was
characterized by optical absorption technique. The band gap energy Eg
for CdSe films was determined by plotting absorbance versus wavelength, ? (nm)
graphs. Extrapolating the straight line portion of the curve in wavelength axis
gives the values of band gap energy Eg as shown in Fig. 5.6. The estimated values
of the direct energy band gap, lie in the range of 1.64 eV – 1.93 eV. Upon
annealing the sample at 200


 for 30 minutes in air ambient the band gap energy
reduced from 1.93 eV for the as-deposited to 1.68 eV for the CdSe film
deposited at -450 mV deposition potential,
which agrees well with the
standard value reported for bulk CdSe material [5]. The decrease
of band gap upon
annealing is presented in Table-5.2 and some of the graphs are shown in Fig.
5.6.
The
high band gap values exhibited by CdSe thin films together with low absorbance
in the visible-infrared region makes the film ideal for use as window layer in
solar cell application. The decrease of band gap upon annealing indicates the
improvement of crystalline quality of the deposited films.





Fig. 5.5; Variation of absorbance with wavelength ? (nm) of CdSe
thin films for: (a) as-deposited, (b) annealed at 200


 for 30 min.

 



Fig. 5.6 Estimation of band gap values of as-deposited and annealed
CdSe films deposited at conditions: (a) -450 mV, 20 min (b) -500 mV, 10 min (c)
-500 mV, 20 min.

Table-5.2
Deposition Condition and Corresponding Band Gap of CdSe Thin Flims:

CdSe

Sample
no.

Deposition
Condition

Band
Gap Eg

(eV)

Time

(min)

Potential

(mv)

Temperature

(

)

As-deposited

(eV)

Annealed

(eV)

Standard

(eV)

1.

20

-450

25

1.93

1.68

1.71

2.

10

-500

25

1.87

1.65

3.

20

-500

25

1.91

1.71

4.

10

-500

25

1.87

1.64

5.3.3 Structural Characterization of CdSe Films

5.3.3.1 XRD Measurement

CdSe thin films may
grow with either sphalerite cubic (zinc-blende type) or the hexagonal (wurzite-type)
structure. The X-ray
diffraction (XRD) method was used to investigate the structural properties of
the CdSe films.
The X-ray diffractograms of CdSe thin films were
scanned in the 2? range of 10–80o
. Fig. 5.7(a) and 5.7(b)
shows the X-ray diffraction spectra of as-deposited and annealed CdSe thin
film. Several well defined peaks are observed in the XRD pattern.
The XRD analysis
reveals that the films are polycrystalline, and the sharp peaks are identified
as (111), (220), (400), (331) and (440) planes of CdSe. The matching of
observed d-values with standard ones depicted in Table-5.3 confirms the
formation of the CdSe material. The lattice constant ‘a’ were very close to the
standard value of 6.0635 A ?. The low intense reflection peak at 25.74


 corresponds to cubic (111) plane, which agrees
with the standard values . The low intensity peaks observed in the XRD pattern
of the sample under study shows that the films are coarsely fine crystallites
or nano-crystalline. The broad hump in the displayed pattern is due to the
amorphous glass substrate and also possibly due to some amorphous phase present
in the CdSe thin films. The intensity of the peaks of as-deposited films became
stronger after annealing it at 200


 for 30 min indicates the improvement of
crystalline quality due to annealing.





Fig.5.7. XRD spectra of (a) as-deposited and (b) annealed CdSe
thin film.

Table-5.3: Comparison of standard and observed “d” values for
electrodeposited CdSe thin films on FTO.

Observed
d-spacing

(Å)

Standard
d-spacing

(Å)

(hkl)
planes

Lattice
constant

‘a’ (Å)

3.5045

3.5100

111

6.0699

2.1465

2.1490

220

6.0712

1.5084

1.51

400

6.033

1.8307

1.8330

331

6.0717

1.0675

1.07

440

6.0386

5.3.4 Surface Morphological Characterization of CdSe Films

5.3.4.1 Scanning Electron Microscopy (SEM) Measurement

Figure
5.8 shows
the SEM micrographs of CdSe thin films deposited at different potential. The
image shows that
it consists of closely packed grains. The scanning
electron microscopy (SEM) study for CdSe films on the FTO-coated glass
substrates reveal
the
uniform distribution of spherical grains over total coverage of the substrate
with a compact and fine-grained morphology. The average grain size was
estimated and it was found that the grain size increased as the deposition
potential was increased



 

(a)   (b)



 

(c)   (d)

Fig.
5.8 SEM
images of CdSe films deposited at the conditions: (a & b) -450 mV, 20 min
(c & d) -500 mV, 15min with different resolutions.

ELECTRODEPOSITION
AND CHARACTERIZATION OF CdTe FILMS

The details experimental procedure of electrodeposition of CdTe thin film
on FTO/glass substrates and characterization of photoelectrochemical, optical,
structural and morphological properties of CdTe thin films have been presented
in this chapter. The annealing effect of the film has also been discussed.

6.1 PREPARATION OF THE
SUBSTRATE

FTO-coated glass substrates were used as substrates. Poor adhesion and
non-uniform films are common problems when depositing films onto smooth
surface. So in the deposition of thin film, substrate cleaning is very
important since the contaminated surface provides nucleation sites facilitating
the growth which results non-uniform film growth. Before depositing CdTe
layers, substrates were boiled using soapy distilled water for around 20
minutes and then rinsed with distilled water. To eliminate grease and other
oily substances, substrates were rinsed with acetone, and it is then boiled
again in distilled water for about 20 minutes. The substrates were then cleaned
using ultrasonic cleaning bath for 15 minutes in distilled water. Finally the
substrates were kept in acetone to eliminate water drop from it prior to dip in
the electrodeposition bath.

6.2 ELECTRODEPOSITION
OF CdTe THIN FILMS

6.2.1 Cyclic
Voltammetry

10-15 ml of the ferricyanide test solution was added to the cell vial and
placed in the cell holder. Glassy carbon was polished following the polishing
instruction in the polishing kit. Then glassy carbon, platinum wire and Ag/AgCl
non-aqueous reference electrode were placed on the cell. Cyclic voltammetry
experiments were then performed using scanning potentiostat and a data acquisition
system. The working electrode (Glassy carbon) potential was repeatedly scanned
at a rate of 100 mV/s between two predetermined potential limits (from 0 mV to
750/800 mV then 0 mV) and corresponding cell current was recorded.



Fig. 6.1.
Change Parameters dialog box for cyclic voltammetry.

It was thus possible to select the potential range (-50 mV to -300 mV)
favouring the formation of a highly photo responsive CdTe layer deposit under
the ambient condition.



Fig. 6.2. A typical cyclic voltammogram showing the important parameters.

6.2.2 CdTe Film
Preparation

For preparation of cadmium telluride thin film by electrodeposition
technique the deposition bath consisted of a non-aqueous solution of CdCl2,
CdI2 and TeCl4. 1.0 M Cadmium chloride was mixed with
0.05 M Cadmium iodide in 100 ml ethylene glycol solvent in a beaker and kept it
stirring approximately for 24 hours until the total raw materials dissolved
into ethylene glycol. The beaker was kept in the bath at 120

 temperatures
with stirring 120 times per minute. 0.0005 M TeCl4 was then added in
the bath.

 The deposition was then carried
out onto cleaned conducting glass substrates in the electrodeposition cell comprising
of a platinum counter electrode, a Ag/AgCl non-aqueous reference electrode and
FTO/glass working electrode. Potential values were entered in mV and time
values were entered in minutes into changes parameter dialog box for performing
controlled potential electrolysis. During the electrodeposition, the
electrolyte was continuously stirred at a moderate speed with the help of a
magnetic stirrer and teflon coated paddle. The electrolyte temperature was
maintained constant at 130

. The electrodeposition was allowed to continue for
about 30-60 minutes at a potentiostatic condition. The samples were grown
within the voltage range of -50 mV to -300 mV and best quality samples were
observed to grow within this voltage range. Immediately after the deposition,
the grown CdTe films were soaked in warm ethylene glycol and then rinsed in warm
distilled to remove traces of the solvent and were subsequently dried in blowing
air. A lot of samples were prepared by varying applied voltage and deposition
time keeping fixed temperature (130


) and fixed concentration of CdCl2 and
CdI2. After getting photoelectrochemical, optical, structural and
morphological measurement, the as-deposited CdTe samples were then annealed at
200


 for 30 minutes
in air ambient to investigate the annealing effect.

6.2.3 Heat Treatment

Annealing in air plays an important role for improving CdTe thin film
solar cells through the passivation of the grain boundaries and the increase of
minority carrier diffusion length. It was also common observation that air
annealing of as-deposited chalcopyrite films leads to an increase of the p-type
conductivity and improve of crystalline quality [1]. CdTe thin films were
annealed at 200

 for 30
minutes in air ambient.



Fig.6.3 Sample of electrodeposited CdTe at different condition.

6.2.4 Chemical
Reaction

Deposition mechanism
of CdTe can be described by the following reaction

Te4+
+ 4e ? Te

Cd2+
+ 2e ? Cd

Cd + Te
? CdTe

6.3 RESULTS AND
DISCUSSION FOR CdTe THIN FILMS

6.3.1 Determination of
Carrier Type of CdTe Thin Films

To determine the Carrier type of CdTe layers, a
simple photo-electrochemical (PEC) cell was used. The CdTe/FTO/glass and a
carbon rod were partially immersed in a 10% NaCl aqueous solution, and these
two electrodes were connected to a digital voltmeter. The photovoltage, created
with white light illumination, was estimated by measuring the voltage under
dark and illuminated conditions. Table-6.1 below shows the obtained results for
five typical as-deposited samples of CdTe films. A positive photovoltage was
demonstrating the p-type character of the material. It can be concluded that
the as-deposited CdTe samples are p-type in electrical conductivity. Testing of electrical conductivity shows that, the films are generally
p-type, although the signals are very low, which could be related to the
presence of a dead layer near the surface of the films or to the existence of
an opposing potential barrier within the layer to the junction formed at the
semiconductor/electrolyte interface.

Table 6.1:
The summery of typical PEC measurements for
as-deposited CdTe layers grown on FTO/glass substrates.

Deposition condition for CdTe films

Vdark (mV)

Vlight (mV)

?V (mV)

Comments

1.-150 mV, 60 min

-231.8

-198.6

+30.2

p-type

2.-200 mV, 60 min

-370.7

-337.4

+33.3

p-type

6.3.2
Optical Characterization of CdTe Films

6.3.2.1 Transmittance
Measurements

 Figure 6.4.a
and 6.4.b represent the variation of transmittance T (%) with wavelength in the
range 350-1100 nm of as-deposited and annealed CdTe films prepared at different
conditions. Transmittance is obtained to be about 24-6% in the wavelength range
350-1100 nm. The spectra show that the transmittance decreases with wavelength.
In the near infrared region, the transmittance is very low. It is
observed that as-deposited CdTe films have very high transmittance, 25-35%  in the visible-UV region; high to moderate
transmittance, 10-25% in the visible range and moderate to lower transmittance,
2-25% in the visible-infrared region as shown in Fig. 6.4a. The transmittance
is significantly increased upon annealing the CdTe films at 200


 for 30 min due
to slight removal of the films after annealing. It is observed that annealed
CdTe films have high transmittance, 30-90% in the visible-UV region; high to
moderate transmittance, 10-30% in the visible range and moderate to lower
transmittance, 5-25% in the visible-infrared region as shown in Fig. 6.4b. This
lower transmittance of CdTe films in visible-infrared region makes it suitable
for use as an absorber material.





Fig.6.4; Variation of Transmittance (T%) with wavelength ?
(nm) of CdTe thin films for: (a) as-deposited, (b) annealed at 200


 for 30 min.

6.3.2.2 Absorbance Measurement

 Figure 6.5a and 6.5b shows the variation of
absorbance with wavelength range 500-1100 nm for different as-deposited and
annealed samples respectively. It is observed that the absorbance of the CdTe
films decreases continually from the near-infrared towards the visible region.
The films show higher absorbance at the range of 900-1100 nm. It is observed that
as-deposited CdTe films have very low absorbance, 0.3-0.75 in the visible
region; moderate to higher absorbance, 0.3-1.15 in the infrared region as shown
in Fig. 6.5a. After annealing CdTe films show improved absorbance spectra as
shown in Fig. 6.5b. After annealing CdTe films show slight changes in absorbance
indicate the improvement of crystal quality upon annealing. It is observed that
annealed CdTe films have very low absorbance, 0.3-1.3 in the visible region;
moderate to higher absorbance, 0.3-2.8 in the infrared region. This higher
absorbance of CdTe films in visible-infrared region makes it suitable for use
as an absorber layer.

6.3.2.3 Band Gap Measurements

CdTe thin
film deposited at optimized preparative parameters on ITO coated glass was
characterized by optical absorption technique. The band gap energy Eg
for CdTe films was determined by plotting absorbance versus wavelength, ? (nm)
graphs. The
linear aspect of curve from Fig.6.6
indicates that absorption in the
high absorption range takes place through direct band-to-band

transitions
[2]
. Extrapolating the straight line portion of the curve in wavelength
axis gives the value of band gap energy (Eg). Deposition condition
and corresponding band gap values are given in Table-6.2.
The estimated values
of the direct energy band gap, lie in the range of 2.0eV – 1.75eV. Upon
annealing the sample at 200


 for 30 minutes in air ambient, the band gap energy
reduced from 1.80eV for the as-deposited to 1.71eV for the annealed CdTe film
deposited at -150 mV, from 2.0eV for the as-deposited to 1.75eV for the
annealed CdTe film deposited at -200 mV.
Which agree well with the standard value reported for
bulk CdTe material [3].

The decrease of
band gap upon annealing is presented in Table-6.2 and some of the graphs are
shown in Fig. 6.6. The decrease of band gap upon annealing indicates the
improvement of crystalline quality of the deposited films. The low band gap
values exhibited by CdTe thin films together with high absorbance in the
infrared region makes the film ideal for use as absorber layer in solar cell
application.





Fig. 6.5; Variation of absorbance with wavelength ? (nm) of CdTe
thin films for: (a) as-deposited, (b) annealed at 200


 for 30 min.





Fig. 6.6 Estimation of band gap of (a) as-deposited and (b) annealed
CdTe film deposited at -200 mV.

Table.6.2:
Deposition
Condition and Corresponding Band Gap of CdTe Thin Flims:

CdTe

Sample
no.

Deposition
Condition

Band
Gap Eg

(eV)

Time

(min)

Potential

(mV)

Temperature

(

)

As-deposited

(eV)

Annealed

(eV)

Standard

(eV)

1.

60

-150

120

1.80

1.71

1.5

2.

60

-200

120

2.0

1.75

6.3.3
Structural Characterization of CdTe Films

6.3.3.1
XRD Measurements

Figure 6.7 shows the
typical diffraction pattern for as deposited and annealed CdTe films. The sharp
diffraction peak observed at 28.30o correspond to (111) planes of
the cubic CdTe structure [4]. XRD peaks at 2? equal to 28.30o, 41o,
46.54o, 57.57o. 63.48o, 68.39o
corresponds to the reflections from (111), (220), (311), (400), (331), (422)
planes respectively. No diffraction peaks associated with metallic Cd, Te or
other compounds were observed. This indicates that respective layered
structures present a single phase with highly oriented CdTe crystallites with
the (111) planes parallel to the substrate. The [111] direction is the close-packing
direction of the zinc-blende structure and this type of ordering is often
observed in polycrystalline CdTe films grown on amorphous substrates [5]. The XRD pattern reveals that the deposited films
are polycrystalline in nature as reported earlier [6]. The (111) peak
corresponds to phase of polycrystalline structure of CdTe. The strong and sharp
diffraction peaks indicate the formation of well crystallined sample. It can be
seen that the major peak (111) is strongly dominating the other peaks. The
intensity of the peaks of as-deposited CdTe films became stronger after
annealing it at 200

 for 30 min indicates the improvement of
crystalline quality due to annealing.





Fig. 6.7 XRD pattern of (a) as-deposited and (b) annealed
CdTe thin film electrodeposited at-150 mV.

6.3.4
Morphological Characteristics of CdTe Films

6.3.4.1
Scanning Electron Microscopy (SEM) Measurement

The SEM photographs of
the annealed CdTe thin films are shown in Fig. 6.8.a-d. No pinholes or creaks
are seen for these samples. The annealing of the film at 200

 for 30 min improves the grain structure. The
film after annealing shows smooth and uniform crack free surface with granular-shaped
identical grains with almost equal dimension spread all over the surface. The
average grain size was found to be 2.5 ?m.



 

(a)   (b)



 

(c)   (d)

Fig.
6.8
SEM images of CdTe films deposited at (a & b) -150 mV and (c & d) -200
mV deposition potential with different resolution.

CdTe/CdSe SOLAR CELL STRUCTURE AND ITS CHARACTERIZATION

This is the first time we took the attempt to fabricate a solar cell
structure. To my knowledge, we are the first group in Bangladesh who could
fabricate the thin film photovoltaic solar cell. CdTe/CdSe/FTO/glass solar
cell is fabricated in our lab. Three main steps have been presented to complete
the device.

Ø  The etching of CdSe films.

Ø  Electrodeposition of CdTe films on etched CdSe films.

Ø  The etching of CdTe/CdSe/FTO/glass surface.

Ø  Formation of metal contacts on CdTe/CdSe/FTO/glass structure.

7.1 WET CHEMICAL
ETCHING

Wet chemical etching was used in device processing in order to clean the
surfaces from contaminants (like oxides), to remove damaged surface layers, to
modify surface composition of the CdSe films and to passivate their surface
states. Electrodeposited and annealed CdSe thin film samples were etched for 30
seconds in commonly used etchants. The chemical etchant solution was NaOH and
Na2S2O3.5H2O in distilled water.
The chemically etched CdSe samples were rinsed in acetone to eliminate water
drops from the samples. The samples were then immediately transferred to the
deposition bath to minimize further oxidation.

7.2 ELECTRODEPOSITION
OF CdTe FILMS ON CdSe FILMS

For preparation of cadmium telluride thin film on CdSe films, the deposition
bath consisted of a non-aqueous solution of 1.0 M CdCl2, 0.05 M CdI2
and 0.0005 M TeCl4. Ag/AgCl reference electrode, platinum counter
electrodes were used in the deposition bath. CdSe/FTO/glass substrates were
attached to the carbon rod and were used as a working electrode. The deposition
was then carried out at an optimum condition of -150 mV deposition potential,
at for 20 minutes. This optimum condition was found by characterizing the
optical, morphological and structural properties of electrodeposited CdTe films
on FTO-coated glass substrates.

During the electrodeposition, the electrolyte was continuously stirred at
a moderate speed with the help of a magnetic stirrer and teflon coated paddle.
The electrolyte temperature was kept constant at 130

. The electrodeposition was allowed to continue for
about 10-20 minutes at a potentiostatic condition at a voltage of -150 mV. Immediately
after the deposition, CdTe/CdSe/FTO/glass structure was soaked in warm ethylene
glycol to remove traces of the solvent and rinsed in warm distilled water and subsequently
dried in air.

7.3 CdTe/METAL
CONTACTS

A stable back-contact that is not significantly rectifying is essential
for good performance and long-term stability of CdTe/CdSe/FTO/glass solar
cells. The formation of a low resistance, low barrier back-contact is one of
the most challenging aspects in the fabrication of high performance CdTe-based
solar cells. In general, metal-to-semiconductor contacts can behave either as a
rectifying (Schottky) or as an ohmic contact depending on the characteristics
of the interface. For a p-type semiconductor with band-gap Eg and
electron affinity ?, and a metal with work function ?m, an ohmic
metal/semiconductor contact is formed when

?m> Eg + ?

and a rectifying
contact is formed when

?m< Eg + ?

CdTe is a p-type semiconductor with a high electron affinity (? = 4.5 eV)
and high band gap (1.45 eV), and thus a metal with a high work–function is
required to make an ohmic contact to CdTe. Most metals, however, do not have
sufficiently high work-functions and therefore form Schottky-barrier contacts
to CdTe absorber layers.

7.3.1 CdTe/Aluminium Contacts
Formation Using Vacuum Evaporation Method

In this work the formation of metal contact was achieved by vacuum
evaporation method. Metal that was used to form a contact was aluminium whose
work function is 4.28 eV. The samples were mounted on mask after etching for 20
seconds in the etchants and small pieces of metallic aluminium were fitted into
the tungsten filament that is connected with a high current source for directly
evaporated into the vacuum chamber. When necessary pressure 10-5 Torr
in vacuum is attained, the metallic aluminium was then heated by passing a
current gradually through the tungsten filament. Aluminium was evaporated from
a tungsten heater and aluminium is found to deposit on CdTe thin layer. After aluminium
deposition the device was taken away from the vacuum chamber and arrangement
was made for the measurement of I-V characteristics.



Fig.7.1 CdTe/CdSe/FTO/glass substrate structure

7.3.2 Electrical Characterization
of CdTe/CdSe/FTO/glass Structure

Schottky barriers on
CdTe and CdSe on CdTe p-n junctions have rectifying current vs. voltage (I-V) characteristics. The barrier
height and ideality factor can be determined for a Schottky barrier from its I-V characteristics. I-V characteristics of a p-n junction
can be used to investigate surface effects, generation and recombination of
carriers in the depletion region, and series resistance effects among others.
The theoretical barrier height for a Schottky contact is the difference between
the metal work function and the semiconductor electron affinity. The
experimentally determined barrier height often deviates from this ideal
behaviour. It is dominated by the semiconductor surface states and is found to
be independent of the metal work function .

The current density
vs. voltage for a Schottky barrier can be expressed as



Where JS
is the saturation current density and n is the ideality factor [1]. The
ideality factor is very close to unity at low doping and at high temperature
but can deviate from this when doping is increased or the temperature is
lowered [2]. The Shockley ideal p-n junction diode equation follows the same
form as above.

7.3.2.1 I-V Measurement

The current voltage behaviour was recorded across FTO and Al/CdTe
contact. The dark current-voltage characteristics of the Al/CdTe/CdSe/FTO/glass
structure are shown in Fig. 7.2. In forward direction of the applied bias, the
current is found to increase. The forward current exhibited a sluggish increase
till 0.8 volt beyond which a sharp threshold was observed.



Fig. 7.2 Current vs. Voltage behaviour of CdTe/CdSe heterojunction solar cell Structure.

The I-V behavior of the Al contacts on CdTe fillms showed the anticipated
rectifying behavior indicating a p-type CdTe film. There is a considerable
series resistance effect at large forward bias. This behavior is consistent
with the large numbers of defects indicated by the carrier densities of CdTe
found via Schottky barrier studies. Current vs. voltage measurements confirmed
the ability to deposit a p-n junction by electrodepositing a CdTe thin film on
a previously electrodeposited n-type CdSe film.

CONCLUSIONS AND FURTHER WORK

8.1
CONCLUSIONS

This is the first time
electrodeposition process has been carried out using three electrode systems
which had not been developed before in our laboratory. It was shown that CdSe
films could successfully be grown on FTO/glass substrate using
electrodeposition method. PEC measurement showed n-type conductivity of CdSe
films.

Optical measurements
showed that CdSe films have higher transmittance and very low absorbance in the
near infrared region. Variation of transmittance is obtained to be about 0.9-95% in the
wavelength range 350-900 nm. It is observed that as-deposited CdSe films have
very low transmittance, 0-5%, in the visible-UV region; low to moderate
transmittance, 5-60% in the visible range and moderate to higher transmittance,
60-90%, in the visible-infrared region. The transmittance is significantly
increased upon annealing the CdSe films at 200


 for 30 minutes. It is
observed that annealed CdSe films have very low transmittance, 0-20%, in the
visible-UV region; low to moderate transmittance, 20-70% in the visible range
and moderate to higher transmittance, 70-95%, in the visible-infrared region.
This higher transmittance of CdSe films makes it suitable for use as a window
material. It is observed that as-deposited CdSe films have very high
absorbance, 4.8-2.0 in the visible-UV region; high to moderate absorbance,
2.0-0.5 in the visible range and moderate to lower absorbance, 0.5-0.0, in the
visible-infrared region. CdSe films show significant changes in absorbance upon
annealing. It is observed that annealed CdSe films have very high absorbance,
3.5-1.8 in the visible-UV region; high to moderate absorbance, 1.8-0.2 in the
visible range and moderate to lower absorbance, 0.2-0.0, in the
visible-infrared region.
The
estimated values of the direct energy band gap lie in the range of 1.64 eV –
1.93 eV. Upon annealing the sample at 200


 for 30 minutes in air ambient the band gap energy
reduced from 1.93 eV for the as-deposited to 1.68 eV for the CdSe film
deposited at -450 mV deposition potential,
which agrees well with the
standard value reported for bulk CdSe material.
The decrease of band gap upon
annealing indicates the improvement of crystalline quality of the deposited
films.

The high band gap
values exhibited by CdSe thin films together with low absorbance in the
visible-infrared region makes the film ideal for use as window layer in solar
cell application.

CdSe films are found
to be polycrystalline and the XRD pattern is dominated by 5 peaks related to
the diffraction by the atomic planes (111), (220), (400), (331), (440) of the
cubic structure. The intensity of the peaks of as-deposited films became
stronger after annealing it at 200

 for 30 min indicates the improvement of
crystalline quality due to annealing.

CdSe layer are smooth,
void free and formed of densely packed grains, uniformly distributed, well
oriented as observed by SEM measurement. These results are of prime importance
for the growth of good quality window layer and achievement of high efficiency
photovoltaic hetero-junction devices.

CdTe films grown by
electrodeposition method are characterized in order to investigate their
electrical, optical, structural and surface morphological properties. PEC
measurement showed that the films are generally p-type, although the signals
are very low, which could be related to the presence of a dead layer near the surface
of the films or to the existence of an opposing potential barrier within the
layer to the junction formed at the semiconductor/electrolyte interface.

Optical measurements
showed that CdTe films have lower transmittance and very high absorbance in the
near infrared region. Variation of transmittance is obtained to
be about 24-6% in the wavelength range 350-1100 nm. The spectra show that the
transmittance decreases with wavelength.
It is observed that as-deposited CdTe films have very
high transmittance, 25-35%  in the
visible-UV region; high to moderate transmittance, 10-25% in the visible range
and moderate to lower transmittance, 2-25% in the visible-infrared region. The
transmittance is significantly increased upon annealing the CdTe films at 200


 for 30 min due
to slight removal of the films after annealing. It is observed that annealed
CdTe films have high transmittance, 30-90% in the visible-UV region; high to
moderate transmittance, 10-30% in the visible range and moderate to lower
transmittance, 5-25% in the visible-infrared. The variations of absorbance are
obtained with wavelength range 500-1100 nm for different as-deposited and
annealed samples. It is observed that the absorbance of the CdTe films
decreases continually from the near-infrared towards the visible region. The
films show higher absorbance at the range of 900-1100 nm. It is observed that
as-deposited CdTe films have very low absorbance, 0.3-0.75 in the visible
region; moderate to higher absorbance, 0.3-1.15 in the infrared region. After
annealing CdTe films show improved absorbance spectra. After annealing CdTe
films show slight changes in absorbance indicate the improvement of crystal
quality upon annealing. It is observed that annealed CdTe films have very low
absorbance, 0.3-1.3 in the visible region; moderate to higher absorbance,
0.3-2.8 in the infrared region
.
The estimated values of the direct energy band gap, lie in the range of 1.75eV
– 2.0eV. Upon annealing the sample at 200


 for 30 minutes in air ambient, the band gap energy
reduced from 1.80eV for the as-deposited to 1.71eV for the annealed CdTe film
deposited at -150 mV, from 2.0eV for the as-deposited to 1.75eV for the annealed
CdTe film deposited at -200 mV. The decrease of band gap upon annealing
indicates the improvement of crystalline quality of the deposited films. The
low band gap values exhibited by CdTe thin films together with high absorbance
in the infrared region makes the film ideal for use as absorber layer in solar
cell application.

CdTe films are found
to be polycrystalline and the XRD pattern is dominated by 6 peaks related to
the diffraction by the atomic planes (111), (220), (311), (400), (331) and (422)
of the cubic structure. The strong and sharp diffraction peaks indicate the
formation of well crystallined sample. It is observed that the major peak (111)
is strongly dominating the other peaks. The intensity of the peaks of
as-deposited CdTe films became stronger after annealing it at 200

 for 30 min indicates the improvement of
crystalline quality due to annealing.

From SEM, the CdTe films
appeared dense and showed crack free surfaces with regular granular shaped
grains. All these properties make this material suitable for using it as an
absorber layer in solar cell structure.

After getting the good
quality of CdSe (window) and CdTe (absorber) layers, a fabrication of CdTe/CdSe
heterojunction solar cell have been performed. Metal (Al) contacts on CdTe film
surface was the last step for the completion of CdTe/CdSe/FTO/glass solar cell
structure. Al metal contact has been carried out on CdTe/CdSe/FTO/glass
structure.

From I-V
characterization, the current voltage
behaviour was recorded across FTO and Al/CdTe contact. The dark current-voltage
characteristics of the Al/CdTe/CdSe/FTO/glass structure are observed. In
forward direction of the applied bias, the current is found to increase. The
forward current exhibited a sluggish increase till 0.5 volt beyond which a
sharp threshold was observed. I-V behaviour of the Al contacts on CdTe fillms
showed the anticipated rectifying behaviour indicating a diode character with
p-type CdTe film. Current vs. voltage measurements confirmed the ability to
deposit a p-n junction by electrodepositing a CdTe thin film on a previously
electrodeposited n-type CdSe film.

8.2
FURTHER WORK

CdTe/CdSe/FTO/glass
solar cells structure grown by electrodeposition at Department of Physics,
University of Dhaka have showed encouraging results but a lot of works still
has to be done to further develop the cells.

Several electrical
properties of the CdTe/CdSe/FTO/glass solar cell structure should be examined,
such as the fill factor FF, Short
circuit current Isc, open
circuit voltage Voc,
energy conversion efficiency ?,
series resistance Rs.
Shunt resistance Rsh,
ideality factor n, barrier height ?b, depletion width w, and doping concentration Nd or Na.

The photosensitivity
of CdTe/CdSe/FTO/glass solar cell structure was not found during the I-V
characterization originating mainly due to the poor ohmicity of the Al/CdTe
contact. Lower contact resistance can be achieved by electrodepositing an over
layer of suitable material on CdTe.

Deep level transient
spectroscopy (DLTS) could be used to evaluate the number of levels due to
impurities or defects within the band gap of the CdSe and CdTe materials, as
these levels usually have detrimental effects on electronic devices. In solar
cell devices they act as stepping-stones for recombination and generation
processes and reduce the photo-current generated by photovoltaic activity.

It has been observed
during this project that the samples electrodeposited from the same bath using
similar conditions seemed to be different. It is not surprising as the
concentrations in the bath are very low and change with the deposition of the
films. Some more experiments will therefore be needed to see how the bath age
can influence the properties of the films (compositional, morphological,
crystalline, electrical and optical properties) and the device performance.

CdTe/CdSe/FTO/glass
structures are very small devices, not suitable for commercialisation purposes.
The development of larger device is on early stage and one will have to improve
the uniformity of the films to reproduce the performance of smaller devices. To
achieve this target, one may have to use an anode of the same size of the
substrate to have more uniform deposition of window and absorber layer on the
total TCO plate area. The influence of annealing process will also have to be
studied more carefully.

Modern Schottky
barrier and hetero-junction theory should be applied to the design and analysis
of other types of p-n hetero-junction thin film solar cells. Future use
of this theory in conjunction with a new semiconductor requires knowledge of
the electron affinity, ?s, ionization potential, IPS,
high-frequency dielectric constant,

,
and charge neutrality level,


 in order to estimate appropriate barrier
heights. ?s, IPS and


should
be estimated experimentally, while


can
be evaluated either experimentally from Schottky barrier trends using numerous
metals [1], or via band structure calculations [2, 3]. Once these semiconductor
parameters are accurately assessed, it may be possible to design and evaluate
thin film solar cells, as well as other types of semiconductor devices.

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