STUDIES ON PHOTOCONDUCTING PROPERTIES
OF PURE AND MIXED COMPOSITE OF
TRANSITION METAL CHALCOGENIDES
A MINOR RESEARCH PROJECT
REFERENCE OF SANCTION-NO. FILE 47-1997/11 (WRO)
DATED 22.02.2012
XI PLAN
SUBMITTED TO
UNIVERSITY GRANTS COMMISSION (WRO), PUNE.
BY
DR. AJALKAR B.D.
M.Sc., Ph.D.
DEPARTMENT OF CHEMISTRY
SHIVRAJ COLLEGE OF ARTS, COMMERCE
AND D.S.KADAM SCIENCE COLLEGE,
GADHINGLAJ, DIST.-KOLHAPUR.
(M.S.)
September-2015
CHAPTER – I
Nearly every aspect of the modern world is built upon the availability of
abundant and relatively inexpensive energy. Our industrial development, abundance
of material goods, food supply, agricultural practices and nearly all the conveniences
of our everyday lives depend on a ready supply of energy. Use of energy has
accelerated over time due to an increase in human population and discovery of new
technologies for utilizing the energy. The source of most of our energy demand has
been the non-renewable fossil fuels such as coal, oil and natural gas. This is
unfortunate from a long-term point of view, as these precious resources that took
millions of years for nature to produce are being depleted in a century or also due to
high rate of utilization by the world community. The type of growth in energy
resource consumption that we have witnessed throughout the last century is
exponential and is characterized by the existence of a doubling time, during which the
energy consumption doubles then redoubles in another time interval equal to doubling
time and so on. As because the present non-renewable energy sources (fossil fuels) are
both finite and non-renewable over dependence on these resources could lead to
catastrophic consequences. It is now time for us to understand that a large fraction of
the current worlds problems are not just a result of the economic or political failures
but instead, are related to the world's diminishing natural resources. The solution lies
in conversation and a change from our reliance on non-renewable resources to
SOLAR ENERGY and other renewable resources.
The sun, which mankind has regarding as the source of light and energy since
time immemorial, is gigantic, inexhaustible, continuous, and free from pollution and
permanent source of energy. The most tantalizing feature of earth receives about 1.5 x
1021 watt-hours of solar radiation annually. This huge population of this planet, but it
is only about one two billion of the sun's massive outpouring energy about 3.9 x 1020
MW. The solar radiation occupies the electromagnetic spectral range from 2000 A 0 to
20.000 A0 and out of this 47% energy is distributed in the visible region, 8% in the
ultraviolet and 45% in the infrared region. The desert areas of the world extend over
about 20 million square kilometers. This land neither grows food nor supports
population in any way, but will fulfill about 400 times the world's present energy
need, if solar energy incident is put to use by any means. This obvious superfluity of
energy with no known way of collecting more than a tiny fraction of it, makes solar
energy research both extraordinarily challenging and maddeningly frustrating. Five
major technologies to trap solar energy are being developed. These are 1) solar
thermal ii) biomass iii) wind energy system iv) tidal energy and v) solar photovoltaic.
These systems, which convert radiant energy directly into electrical energy, called as
solar cells, generally use higher energy portion of the solar spectrum.
Chemical methods are the most important tools for growth of the thin films
owing to their versatility for deposition of large number of elements and compounds
at relatively low temperatures. Both in the form of vitreous and crystalline layers,
with high degree of perfection and purity, thin films can be deposited with required
stoichiometry. Large or small and even or uneven surfaces of all types, conducting or
insulating substrate surface can be coated with relative ease.
The processes are
economical and have been industrially exploited to a large scale. The various types of
deposition processes involved are
(i) Chemical Vapour Deposition (CVD)
Organometallic Vapour Deposition (MOCVD) ii) Spray pyrolysis
Electrodeposition
iv) Anodization
iii)
v) Screen printing, vi) Chemical Batch
Deposition and vii) Arrested Precipitation Technique (APT)
A great deal of experimental research is underway for trapping the solar energy
by employing a p-n junction cell, much stress is being given for the development of
the polycrystalline thin film photovoltaic. These are being used popularly in weather
and communication satellites.
Though the photovoltaic technology is well
established, its higher cost is the major hindrance against its large-scale utilization. As
an alternative to the above problem, the photoelectrochemical solar cells (PESC'S),
which convert light into electricity when a junction between a semiconducting
electrode and a liquid electrolyte is illuminated, have attracted the scientific
community.
The physical and electrochemical properties of the semiconductor
electrode play a major role in deciding the conversion efficiency of these cells. The
photoelectrochemical cells were fabricated using [Mo(S1-xSex)2] as an active
photoelectrode, Iodide/polyiodide as an electrolyte and graphite rod as a counter
electrode.
The as fabricated cells were characterized through the I-V and C-V
characteristics in dark, power output, curves under constant illumination, photo,
spectral and were analysed to evaluate the various cell parameters (viz. junction
quality factor in dark and in light, flat band potential, form factor, series and shunt
resistance, densities of donor levels etc.).
CHAPTER – II
Thermoelectricity:
In this chapter, we have briefly reviewed the thermoelectric phenomena and the
thermoelectrics. Since thermoelectrics is a direct energy conversion between heat and
electric power, it has various advantages. It can get some energy back from waste
heat, and can cool materials without an exchange media like a freon gas. Thus this
technology has attracted a renewed interest from the viewpoint of increasing needs for
environment friendly energy source. In the last decade, new thermoelectric materials
have been searched extensively, some of which have better thermoelectric properties
than the conventional thermoelectric materials.
From the viewpoint of basic science, the thermoelectric power is an entropy (or
heat) carried by an electron. This is more or less controversial terminology, because
the entropy and heat are a concept in macroscopic world, whereas the electron is a
concept in the microscopic worlds, which will give a new insight or direction to
condensed matter science.
To summarize, the features of a new semiconductor material with high ZT will
likely be
i)
A high symmetry crystal structure (high degeneracy of the band extrema
near the Fermi level) with a large number of heavy elements per unit cell
(low кph).
ii)
Small electronegativity differences between the elements in the
compound (high mobility or long scattering time).
iii)
Alloying or ‘rattling’ to further reduce the lattice thermal conductivity.
iv)
At least one high effective mass.
v)
The ability to dope the material to a high carrier density of about 1019
carriers/cm3.
Photoconductivity:
The chalcogenide materials are one of the most widely known families of
amorphous semiconductors; they find vast applications in optical fibres, memory
devices, reversible phase change optical recording etc., and study of these materials is
becoming a great field of interest. Selenium is selected because of its wide
commercial applications, its device applications like switching, memory, and
xerography made it attractive. It also exhibits an interesting property of reversible
phase transformation. This property makes it very useful in optical memory devices.
But in the pure state it has disadvantages because of its short lifetime and low
sensitivity. This problem can be overcome by alloying Se with some impurity atoms
(Mo Bi, Te, Ge, Sb, As etc.), which give higher sensitivity, higher crystallization
temperature and smaller ageing effects. The addition of antimony to chalcogenide
glasses is generally accompanied by a marked change in their electrical and
photoelectrical properties. Though it is possible to find some papers in the literature
dealing with the effect of the addition of Sb on electrical and photoelectrical
properties of chalcogenide glasses , to the best of our knowledge, there is no thorough
study dealing with the replacement of Sb by Bi on dark and photoconductivity of
Sb2Se3 thin films. The addition of another element in binary systems has been quite
useful in improving some of the properties of glassy semiconductors. Through
addition of third element stabilize the structure, which makes ternary system more
stable thermally, the density of defect states is increased, which affects the
photoconductive properties. In the present work, the effect of addition of bismuth, on
the photoconductivity of amorphous/crystalline Mo2-xBixS3 (x = 0.0, 0.02, 0.04, 0.06,
0.08 and 0.10) thin films deposited by arrested precipitation technique is investigated.
The effect of temperature on the dc dark conductivity (σ), the photosensitivity (σph/σ),
the activation energy for dc conduction ( E) and activation energy of photoconduction
( Eph) of Mo2-xSbxS3 (x = 0.0, 0.02, 0.04, 0.06, 0.08 and 0.10) thin films is also
investigated.
CHAPTER –III
The deposition bath was prepared in 100 ml beaker by the addition of 20 ml
0.05M Mo-TEA complex, 30 ml 0.05M Thiourea and rest is distilled water to make
total volume 80 ml. Thoroughly cleaned glass substrates were mounted on a specially
designed substrate holder. The substrate holder was attached to constant speed gear
motor and rotated at a speed of 50±2 rpm. The temperature of the bath was maintained
at 45oC. The final pH of the reaction mixture was made 10±0.2 by the addition of
ammonia. After deposition period of three hours, samples were taken out, washed
with distilled water and kept in dark desiccators.
Pure molybdenum selenide (MoSe2) thin films were prepared by using 20 ml
0.30M Mo TEA complex and 30 ml 0.30M sodium selenosulphite solutions in a 100
ml container and rest is water to make 80 ml total volume. The conditions of operative
parameters are temperature (20 oC), pH (10.5) and period of deposition is 6 hour.
Mixed Mo2-xSbxSe3 and Mo2-xSbxSe3 thin films were prepared by varying
concentrations of bismuth and antimony. Amount and concentration of sodium
selenosulphite was kept constant for all composition of Mo2-xSbxSe3 and Mo2-xSbxSe3
series. The concentration of Bi3+ and Sb3+ were varied in a volume stoichiometric ratio
so as to obtain various compositions of Bi2-xSbxSe3 and Sb2- xBixSe3 films. The
composition and nomenclature of Mo2-xSbxSe3 and Mo2-xSbxSe3 film series.
The arrested precipitation technique is found to be most convenient method for
deposition of mixed Mo2-xSbxS3 and Mo2-xSbxSe3 thin films.
Conclusion:
The deposited films are compact, homogeneous, very adherent and with mirror
aspect. The adherence of the films to the substrate was so good that the films were not
peeled or scraped off with stainless steel knives.
CHAPTER – IV
The mixed thin films of Mo-xSbxS3 and Mo2-xSbxSe3 are grown by arrested
precipitation technique. The optimum condition, growth mechanism and physical
properties of the as grown thin films are discussed in Chapter III. These films are used
for chemical compositional analysis by ESP and EDS method.
Conclusion:
The combinatorial Mo2-xSbxS3 and Mo2-xSbxSe3 thin films of varying
composition were quantitatively analyzed using extractive spectrophotometry (ESP)
and energy dispersive X-ray microanalysis (EDS) techniques. It is found that results
obtained by extractive spectrophotometry were in good agreement with the results
obtained by energy dispersive X-ray microanalysis. The important conclusions drawn
on the basis of experimental results are as follows.
1.
The proposed technique is sensitive, selective, rapid, low cost which can be
used for microanalysis of semiconducting materials.
2.
The quantity of elements initially taken and experimental results are in good
agreement.
3.
This method gives average percentage of the constituents of the semiconductor
films.
4.
The results on Mo2-xSbxS3 thin films shows almost perfect stoichiometry.
CHAPTER – V
The XRD pattern of Mo2-xSbxS3 with various Sb contents (x =0, 0.02, 0.04,
0.06, 0.08, 0.1) deposited at 45 oC. The peaks become broader and some peaks
appeared at high θ values with increasing Sb doping. This is conclusive evidence for
new ordering. Careful observation on the pattern of the deposits shows that some lines
were remained unchanged in the pattern and some new lines appeared in the pattern.
The increase in the number of lines is not due to improvement in the crystallinity but
due to introduction of new plane spacings, caused by nonuniform distoration as a
result of doping.There are no JCPDS standard data available for different composition
of Mo2-xSbxS3. Hence the plane indices are obtained by comparing the intensities and
positions of the peaks with those of Bi2Se3 and Sb2Se3 which are given by JCPDS file
no. 77-2016 and 15-0861. The formation of solid solution is expected because both
materials crystallises in orthorhombic structure. The observed ‘d’ values for MoS2 and
Mo2-xSbxS3 thin films in comparison with the JCPDS data. The observed‘d’ values are
in good agreement with the standard values for the orthorhombic structure of MoS2
and MoSe2.
The scanning electron micrograph of top view of the four typical Mo2-xSbxSe
thin films in the as grown condition at 5000X magnification. The SEM micrographs
shows well adherent, smooth and uniform film surface without cracks under low
magnification which accounts high mechanical stability of the samples. We have
observed significant variation in the morphological properties of the films for the
variation of Sb doping.
The as-deposited thin films have been measured to ascertain the noncrystalline/crystalline nature of the material. The presence of only broad features and
the absence of any sharp peaks in the diffractograms are taken as evidence for the
amorphous nature of the MoSe3 and Mo1.98Sb0.02Se3 thin film samples. An amorphous
structure is expected since deposition is carried out at low temperature (20 oC). The
diffusion background and broad peak appearing at low angles are due to amorphous
glass substrate. The XRD peaks for Mo1.92b0.08Se3 and Mo1.90Sb0.10Se3 films indicate
polycrystalline nature. The plane indices are obtained by comparing the intensities and
positions of the peaks with those of MoSe3 and Bi2Se3 which are given by JCPDS file
no. 75-1462 and 77-2016. The observed d-values are in good agreement with the
standard d-values for the orthorhombic structures of MoS2and MoSe2. The observed
d-values for Mo2-xSbxSe3 films in comparison with the JCPDS data. The hkl indexes
are shown above the diffraction in Fig. 5.4B. The crystallite size has been estimated
from Scherrer formula, using the full-width at the half-maximum of the most intense
diffraction peak. It was observed that with increasing Bi doping concentration in the
film, the crystallite size and crystallinity of MoSe3 increases.
The morphology of these films was analysed in order to obtain a higher rate of
grain nucleation and a more refined grain structure. SEM photographs in the Fig.
5.5B. were taken at 20,000X magnification to observe the surface morphology. No pin
holes or cracks could be observed for all the samples. The grains shows agglomerated
morphology and appear very homogeneous. The micrographs shows compact
structure composed of single type small densely packed crystals. The SEM shows
increase in number of grains with increase in Bi content (x) which is confirmed from
increase in film thickness. The surface of the substrate was not covered completely
upto X= 0.06.
CHAPTER – VI
The temperature dependence of the dc conductivity through the temperature
range (300 to 500 K) suggests that there are two types of conduction channels that
contribute to the conductivity. At higher temperatures, the conduction is due to band
conduction through extended states, where the values of activation energy increase
with increasing Sb content. At low temperatures, conduction takes place through VRH
in localized states near the Fermi level.
While the values of the thermopower and thermal conductivity are encouraging,
the poor electrical conductivity of Mo2-xSbxS3 S3 thin films inhibits its application as a
thermoelectric material. Since the specific conductivity of
Mo2-xSbxS3 thin films at
room temperature is too low for thermoelectric applications, enhancing the charge
carrier concentration via doping would be required. So to make these materials useful
thermoelectric, further research must be necessary to increase their electrical
conductivity significantly while preserving the reasonably large thermopower and low
thermal conductivity.
Temperature dependence of conductivity in dark as well as in presence of light
shows that, conduction is through a thermally activated process in both cases. The
activation energy is found to decrease with increase in Sb content, indicating shift of
the Fermi level with Bi content. The photosensitivity (σph/σ) increases with Bi content.
*****