synthesis and characterization of cu2znsnse4¬ thin film prepared by spin coating and selenization...
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a
M.S. Thesis
Synthesis and Characterization of
Cu2ZnSnSe4Thin Film Prepared by Spin
Coating and Selenization from Metal-
Ethanolamine Coordination Compound
Precursor
Graduate School of Yeungnam University
Department of Materials Science and Engineering
Major in Materials Science and Engineering
ERSAN YUDHAPRATAMA MUSLIH
Advisor: Professor KYOO HO KIM
February 2015
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b
Synthesis and Characterization ofCu2ZnSnSe4Thin Film Prepared by Spin
Coating and Selenization from Metal-
Ethanolamine Coordination Compound
Precursor
Advisor: Professor KYOO HO KIM
Presented as M.S. Thesis
February 2015
Graduate School of Yeungnam University
Department of Materials Science and Engineering
Major in Materials Science and Engineering
ERSAN YUDHAPRATAMA MUSLIH
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c
Ersan Yudhapratama Muslihs M.S. Thesis is
approved by
Committee member: Prof. Jae Yeol, Lee
Committee member: Prof. Dang-Hyok, Yoon
Committee member: Prof. Kyoo Ho, Kim
February 2015
Graduate School of Yeungnam University
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e
colloidal based solution. Therefore, metal-ETA solution after diluted in the
ethanol has ability to attach to the substrate strongly and could form a
homogeneous Cu2ZnSnSe4thin film. Besides that, due to ETA could act as chelate
ligand, it may prevent Cu, Zn, and Sn from oxidation when it was coated on the
substrate.
In this work, metal-ETA coordination compound was coated on the soda
lime glass (SLG) substrate by spin coating technique at 2000 rpm for 10 second
coating time using, and 0.2 mL of metal-ETA coordination compound solution.
After coated, continued with heat treatment at 200 oC for 10 minutes, repeated
from the coating process until heat treatment for 5 times, but in the last repetition,
the heat treatment was done for 120 minutes. Moreover, to obtain the Cu2ZnSnSe4
thin film, selenization was done at 550 oC using selenium pellets under Argon
(95%) + H2 (5%) atmosphere for 120 minutes in the tube furnace. By using this
alternative technique, the Cu2ZnSnSe4thin film has been successfully synthesized
which showing better morphology and stronger attach to the substrate and having
suitable optical and electrical properties for solar cell application. Therefore,
Cu2ZnSnSe4 thin film which synthesis by this alternative technique, can be
applied for solar cell application.
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ACKNOWLEDGEMENT
I would like to give my sincerest thanks to my advisor, Professor Kyoo Ho
Kim, who has been support and provides me the opportunity to pursue master
course as an international student at Nano and Thin Film Materials Laboratory,
Yeungnam University. His vision has brought me into an interesting subject that is
important to renewable energy for future generation, particularly in thin film
materials for photovoltaic application.
My personal gratitude goes to my mother, Erna Garnasih, for loves,
supports, prays, and encourages through all my live. And also for my wife, Gina
Nurinnadia, for her loves, patience, prays, and great understanding has help me
ease the the thesis process.
I would also like to thank all the members of Nano and Thin Materials
Laboratory: Muhamad Ikhlasul Amal and Fianty for the valuable discussions, for
their helps and kindly assists from the first time I came to Korea. My thanks are
given to all Indonesian students who studying in Yeungnam University for the
generous friendship, guidance and supports.
Gyeongsan, February 2015
Ersan Yudhapratama Muslih
Nano and Thin Film Materials Laboratory
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TABLE OF CONTENTS
ACKNOWLEDGEMENT
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
CHAPTER I
INTRODUCTION AND MOTIVATION
I.1. General Background
I.2. Review of Solar Technology Development
I.3. Chalcogenide-based Thin Film Solar Cells
I.4. Alternative Material for Thin Film Solar Cells
I.5. Alternative Technique to Synthesis Cu2ZnSnSe4Thin Film Solar Cells
I.6. Research Objectives
CHAPTER II
LITERATURE STUDY
II.1. Principle of Solar Cell
II.2.Cu2ZnSnSe4Thin Film as Absorber Layer on Solar Cells Application
II.3. Principle of Spin Coating Technique
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Solvents by Visual
IV.1.2. Preliminary Observation of Ethanolamine to Metal Salts and the Other
Solvents by Raman Spectroscopy and Fourier Transform Infrared (FT-
IR)
IV.2. Spin Coating Preparation
IV.2.1. Spin Coating Effect
IV.2.2. Heat Treatment Process of Metal-ETA
IV.3. Preparation of Cu2ZnSnSe4Thin Film
IV.3.1.Selenization Atmosphere Effect
IV.3.2.Selenization Time Effect
IV.3.3.Selenization Temperature Effect
IV.4. The Chemical Composition Effect
CHAPTER V
CONCLUSIONS
REFERENCES
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LIST OF FIGURES
Figure 1.1
Figure 1.2
Figure 1.3:
Figure 1.4
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6.
Figure 2.7
Growth prospect for world primary energy demand
Oil production world summary
The best research cell efficiencies of photovoltaic in
laboratory
Cu2ZnSnS4 thin film made by dip-coating and its cross
section
Energy band illustration for PN junction
Energy band diagram of a pn-heterojunction solar cell
The graph of the I-V characteristics of of the p-n junction
when non-illuminated (dark) and illuminated
The structure of chalcopyrite, kesterite and stannite crystal
structures
Ternary phase diagrams of the Cu2SnSe3-SnSe2-ZnSe and
Cu2Se-ZnSe-SnSe2
Compositional ranges of metallic precursor films represented
on the superimposed ternary CuZnSn and the modified
metal chalcogenide phase diagrams
CZT precursor composition map of Cu2ZnSnSe4 solar cells
efficiency
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Figure 2.8
Figure 2.9
Figure 2.10
Figure 2.11
Figure 2.12
Figure 2.13
Figure 2.14
Figure 2.15
Figure 2.16
Figure 3.1
Figure 3.2.
Figure 3.3.
Figure4.1
Figure4.2
Cu2ZnSnSe4thin film solar cells arrangement
Common steps in the Cu2ZnSnSe4 thin film fabrication by
wet process deposition
Optical micrograph of cellular defect pattern found in an
aluminum titanate solgel coating at the center of the silicon
wafer
Schematic illustration of the capillary instability that operates
during the drying stage of spin coating
Structure of ethanolamine
ETA production reaction from ethylene oxide
Inter molecular hydrogen bonding of ETA
Illustration of the [Cu(ETA)2]2-coordination compound
Common bonding modes for carboxylate ligands
Spin coating scheme
Furnace arrangement for selenization
Flow chart and conditions of Cu2ZnSnSe4 thin films
fabrication process
Illustration of metal-ethanolamine coordination compound in
the solvents
Physical and chemical properties ratio scheme of ethanol and
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Figure4.3
Figure4.4
Figure 4.5
Figure 4.6
Figure 4.7
Figure4.8
Figure4.9
Figure 4.10
Figure 4.11
Figure 4.12
ethanolamine composition mixture with metal salts base on
experiment.
Dilution effect of metal-ETA with ethanol
Fourier Transform Infrared (FT-IR) spectrum (a) and Raman
spectrum (b) for ETA and metal-ETA coordination
compounds solution.
Spin coating effect
Physical and chemical properties of organic and inorganic
compounds in the metal-ETA coordination compound
according temperature.
FT-IR spectrum of metal-ETA solution and metal-ETA after
heat treatment.
Raman spectrum for metal-ETA coordination compound after
heat treatment.
Energy Dispersive X-ray (EDX) spectrum of metal-ETA
afterheat treatment.
Cu-Zn-Sn ternary phase diagram at 200
o
C
X-ray diffraction (XRD) pattern of metal-ETA after heat
treatment
FT-IR spectrums of metal-ETA coordination compound (a)
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CHAPTER I
INTRODUCTION AND MOTIVATION
I.1. General Background
Human population is increasing day by day, and likewise for energy
demand, due to energy demand is proportional to population. At 2009, world
energy demand has been reached 12.13 billion tons of oil equivalent (toe) and it is
increasing 1.3% every year. If this condition is continually happening, world
energy demand predicted reaches 16.96 billion toe at 2035 [1].
Fig. 1.1. Growth prospect for world primary energy demand [1].
In other hand, the amount of fossil fuel is decreasing day by days, even it
could be running out. In several years later, the amount of fossil fuel production is
declining, but the fossil fuel price is increasing. Besides that, the fossil fuel
combustion effect also harms to the environment. The resulting impact from fossil
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fuel combustion is a greenhouse gas that led to the global climate change.
Therefore, it is important to develop many kinds of alternative energy which
could reduce to the fossil fuel dependence, one of alternative energy that could be
used for reduce fossil fuel dependence is energy which comes from the sun, or we
called as solar energy.
Fig. 1.2. Oil production world summary [2].
I.2. Review of Solar Technology Development
Sunlight has a huge potential energy, approximately 23.000Tw/year [3].
There is more than enough solar irradiation available to satisfy the worlds energy
demands. On average, each square meter of land on earth is exposed to enough
sunlight to generate 1,700 kWh of energy every year using currently available
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technology. The total solar energy that reaches the Earths surface could meet
existing global energy needs 10,000 times over [4]. Every year, solar cell capacity
is continues to be improved. Even, world solar cell capacity has been predicted
improved until 40 GW/year at 2010 [5].
Nowadays, solar cells can be classified into four generations. The first
generation is silicone base solar cell (poly crystal and single crystal). The second
generation is the thin film generation (CuInSe2, Cu2InGaSe4, CdTe, C2ZnSnS/Se4,
etc.). The third generation is multi-junction, dye sensitized solar cell (DSSC), and
organic solar cell. The fourth generation solar cell is a hybrid solar cell (combine
between inorganic and organic materials) [6,7]. Amongst all many kinds of solar
cells, silicon base solar cell still dominated market share solar cell in the world
because silicon used for the first commercial solar cell, non-toxic, abundantly
available in the earths crust, and silicon photovoltaic modules have shown their
long-term stability over decades in practice [8,9,10]. Figure 1.3 shows many kinds
of solar cells and their efficiency.
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I.3. Chalcogenide-based Thin Film Solar Cells
In order to improve solar cells performance, it is important to develop non
silicon solar cells which having a direct band gap, high efficiency, and easy to
fabricate. One of the non silicon materials in thin film solar cells which having
high efficiency is Cu2InGaSe4. This material consists of copper, indium, gallium,
and selenium, but sometimes selenium could be replaced by sulfur or even mix
both of them [13]. Cu2InGaSe4thin film solar cell is the highest efficiency among
the other single junction chalcogenide thin film solar cell and this efficiency can
be reached due to Cu2InGaSe4has a direct band gap [11]. Therefore, Cu2InGaSe4
thin film can be easier to convert sunlight into the electricity compared indirect
band gap theoretically [14]. Overall, thin film solar cell has many advantages,
such as efficient and high performing materials and reduced significantly costs
due to less material required and easy to modify or combine with other materials
[15].
I.4. Alternative Material for Thin Film Solar Cells
Because Cu2InGaSe4 or CuInSe2 contains non-abundant elements, it
makes CuInSe2and Cu2InGaSe4solar panel price are expensive. However, indium
and gallium can be replaced by zinc and tin because they are cheaper and
abundant elements as well [16]. This replacement will form Cu2ZnSnSe4 as
derivative material from Cu2InGaSe4. Like Cu2InGaSe4, Cu2ZnSnSe4 thin film
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has a direct band gap energy of 1.1-1.7eV [17,18], this band gap is the ideal band
gap for a single junction solar cell and has an absorption coefficient more than 104
cm-1[19]. Therefore, Cu2ZnSnSe4 has potential properties to be a good absorber
layer in the thin film solar cell.
I.5. Alternative Technique to Synthesis Cu2ZnSnSe4Thin Film Solar Cells
In the other hand, thin film absorber layer fabrication consists of two main
processes, there are vacuum and non vacuum process. Vacuum process is known
well process and many kinds of vacuum process can be applied to fabricate thin
film solar cell, such as sputtering, pulsed laser, and thermal evaporation.
Unfortunately, this process is high a cost process due to required expensive
equipment such as vacuum chamber, pump, high purity target, etc. Besides that, in
vacuum process also needs extra cost for maintenance and spare part. Contrary
from vacuum process, non-vacuum process is a relative low cost process due to
no needed expensive equipment and the high cost for maintenance and their spare
part. Besides that, by using non-vacuum process elements composition is easy to
control.
Due to the advantages of the non-vacuum method, our laboratory has been
developed this method for several years. In our laboratory, dip-coating technique
by using water and ethanol as solvent were developed. By using a dip-coating
technique, Cu2ZnSnS4 film has been successfully made from metal salts which
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mixed with thiourea as sulfur source. From this mixture, metal sulfide can be
formed by heat up the mixture at 60 oC, and due to sulfurization process at 500 oC
for 1 hour and then Cu2ZnSnS4was obtained. However, this technique cannot be
applied to synthesis Cu2ZnSnSe4 thin film because selenourea price is very
expensive, 177 USD for only 1 gram (sigmaaldrich.com). It is very huge
difference compared with thiourea which only 1.4 USD for 1 gram
(sigmaaldrich.com), this is one of the reasons why synthesis Cu2ZnSnSe4 thin
film using selenourea by solution process is not an effective technique. Another
reason is because selenourea and thiourea has similar properties which can make a
colloidal system with metal salts in the water and ethanol as solvents. In this
system, the metal selenide particle is inhomogeneous, unstable mixture, and
difficult to attach to the substrate, thats why we need to mixed for 3 hours at
solution preparation process. Even though Cu2ZnSnSe4 can be attached on the
substrate, the metal selenide particles are loose adhesivity with substrate and
could make a porous morphology. As consequences, Cu2ZnSnSe4 thin film by
dip-coating process cannot be applied for solar cell application even though it has
suitable optical and electrical properties.
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Fig. 1.4. Cu2ZnSnS4thin film made by dip-coating and its cross section [20]
In order to develop a simple solution process that can be used for synthesis
Cu2ZnSnSe4 thin film, metal salts should be solved in one solution without the
presence of selenium to avoid metal selenide particles formation. By using single
true solution, it can make a homogeneous particle size, stable and attach strongly
on the substrate. And then, this metal salts solution should be coated on the
substrate by using spin coating, and the last step is the selenization process to
obtain Cu2ZnSnSe4thin film from metal salts solution.
I.6. Research Objectives
This work offers an alternative solution technique to synthesis
Cu2ZnSnSe4thin film by simple, low cost, versatile, and environmentally friendly.
This alternative technique consists of three main steps. The first step was
preparation of metal-ethanolamine coordination compound solution of copper,
zinc, and tin salts with ethanolamine (ETA). The second step was coating the
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metal-ETA coordination compound solution on the substrate by spin coating. And
the third step was selenization to obtain Cu2ZnSnSe4 thin film at prolongtime
from 30 120 minutes and elevated temperature from 250 550 oC under
different atmospheric conditions: Ar (100%) and Ar (95%) + H2 (5%). The
physical properties of Cu2ZnSnSe4thin film of this technique such as structural,
compositional, optical and electrical properties as a function of deposition
parameter were also investigated.
Therefore, the main objectives of this research are followed:
1.
Investigate an alternative technique to synthesis a single phase of
Cu2ZnSnSe4thin film by spin coating and selenization from metal-
ethanolamine coordination compound solution.
2. Determine the optimum condition of alternative technique.
3. Study the structure, optical, electrical properties of Cu2ZnSnSe4thin film
for solar cell application.
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CHAPTER II
LITERATURE STUDY
II.1. Principle of Solar Cell
Solar cells are cells that can convert sunlight into the current electricity
directly using semiconductor material. This conversion took place at the atomic
level which involves two types of semiconductor, p-type and n-type. Both of
semiconductors has different properties, p-type semiconductor contains mostly
free holes and n-type contains mostly free electrons. If both of semiconductors are
merged together, it can make a junction that usually called as PN junction.
Moreover, when the n-type semiconductor and p-type semiconductor materials are
merged together, a potential difference occurs between both sides of the PN
junction. This condition makes some of the free electrons from the donor impurity
atoms migrate across to fill up the holes in the p-type region. However, due to the
electrons moved across the PN junction from the n-type region to the p-type
region, the electrons could be paired with holes and causing both to disappear, at
the same time, when electron leaves n-type region, the electrons leave holes
(positive charges) behind. This phenomenon also happens to the holes from p-
type region, when holes moved across the junction to the n-type region, the holes
could be paired with electron and disappear, at the same time the holes leave
electron behind either. As a result, the charge density of the p-type along the
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junction is filled with negatively charged acceptor ions, and the charge density of
the n-type along the junction becomes positive. This area called as depletion layer
or depletion area and the charge transfer of electrons and holes across the PN
junction is known as diffusion. The diffusion makes electron and holes paired
each other along of PN junction.
Fig. 2.1. Energy band illustration for PN junction [21].
In this condition, the total charge on each side of a PN Junction must be a
neutral charge and this condition also called as equilibrium condition. In
equilibrium condition, due to no potential difference, the electricity cannot be
produced (fig.2.2.a). Therefore, it needs energy to break up the electrons and
holes paired particles and makes the electrons and holes moving into the opposite
side. This energy can get from photons in the sunlight.
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Fig. 2.2. Energy band diagram of a pn-heterojunction solar cell: (a) At thermal
equilibrium in dark and (b) under illumination, open circuit conditions. Number 1
and 2 refer to an n-type and a p-type semiconductor ECiand EVito their
conduction and valence bands, respectively. Egiand EFiare the band gaps and
Fermi levels, respectively [22].
When photons striking the solar cells (fig. 2.2.b), it will absorbed by the
semiconductor and allows electrons in the PN junction to be unpaired and
released, this condition generating extra mobile electrons and extra mobile holes
which flow to the n-type and p-type region respectively, this phenomenon called
as photo generation of charge carriers and the resulting separation of positive and
negative charges across the junction called as a potential difference. By
connecting both type of semiconductor to an external circuit, it allows electrons
and holes to flow into the external circuit and form an electrical current which can
be used for electrical devices. The conversion of sunlight to usable electrical
energy has been dubbed the photovoltaic effect. The electricity which produced
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by a photovoltaic device is direct current and can be used directly or stored for
later use.
Solar cells are characterized by current-voltage (IV) measurements in the
dark and under standardized illumination that simulates the sunlight. The most
important parameters that describe the performance of solar cells are maximum
possible delivered energy (Pmp), open circuit voltage (VOC), short circuit current
(ISC), fill factor (FF), and conversion efficiency ().
Fig. 2.3. The graph of the I-V characteristics of the p-n junctionwhen non-
illuminated (dark) and illuminated [23].
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The open-circuit voltage (VOC) is the maximum voltage available from a
solar cell, and this occurs at zero current. The open-circuit voltage corresponds to
the amount of forward bias on the solar cell due to the bias of the solar cell
junction with the light-generated current [24]. Whereas the short-circuit current
(ISC) is the current through the solar cell when the voltage across the solar cell is
zero [25]. The main parameter that determines the solar cell efficiency is the
maximum possible delivered energy (Pmp) which can be defined as a fully
electronic system that varies the electrical operating point of the modules so that
the modules are able to deliver maximum available power. This parameter
obtained from multiplication of Imp and Vmp(Pmp= VmpxImp), which is shown as a
square inside the I-V curve. The next derivative parameter is fill factor (FF) which
can be defined as the ratio of the maximum power from the solar cell to the
product of Vocand Isc. Graphically, the FF is a measure of the squarearea of the
solar cell and is also the area of the largest rectangle which will fit in the I-V
curve [26]. The FFis expressed according to the following equation:
FF =
=
The efficiency is the most commonly used parameter to compare the
performance of one solar cell to another. Efficiency is defined as the ratio of
energy output from the solar cell to input energy from the sun [27]. The efficiency
of a solar cell is determined as the fraction of incident power which is converted
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Fig. 2.4. Chalcopyrite, kesterite and stannite crystal structures [28].
Cu2ZnSnSe4 can be synthesized from metal selenide mixture such as
Cu2Se, ZnSe, and SnSe2. From isothermal section of Cu2Se-ZnSe-SnSe2system
and Cu2SnSe3-ZnSe-SnSe2 system at 670 K, note that Cu2ZnSnSe4 can be
synthesized by mixing three kinds of metal selenide compound such as Cu2Se,
SnSe2, and ZnSe with certain composition. The composition could be
stoichiometric or non-stoichiometric composition. Besides that, Cu2ZnSnSe4also
can be synthesized from mixture among Cu2SnSe3, ZnSe and small amount of
SnSe2or in other words, Cu2ZnSnSe4can be synthesized from mixture between
Cu2SnSe3with ZnSe.
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Fig.2.5. Ternary phase diagrams of (a) Cu2SnSe3-SnSe2-ZnSe and (b) Cu2Se-
ZnSe-SnSe2[29].
Overall, according ternary phase diagrams above, reaction formation of
Cu2ZnSnSe4can happen either through three kinds of metal selenide binary alloys
or two kinds metal selenide of ternary and binary alloy. However, there are still
any probabilities that binary alloys of metal selenide such as Cu2Se and SnSe2
form Cu2SnSe firstly and then form Cu2ZnSnSe4 with ZnSe. Besides synthesis
pathway, physical properties of Cu2ZnSnSe4 also have been studied specifically.
Table 2.1.shows the physical properties of Cu2ZnSnSe4.
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Table 2.1. Physical properties of kesterite Cu2ZnSnSe4[30].
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Besides physical properties, Cu2ZnSnSe4 also has optical and electrical
properties which very important aspects in absorber layer. There are several
important properties which should be posses as absorber layer, such as carrier
concentration, mobility, resistivity and band gap. Table 2.2. shows vary optical
and electrical properties of Cu2ZnSnSe4 from several compositions and
fabrication method.
Table 2.2. Optical and electrical properties of Cu2ZnSnSe4from several works.
Carrier Concentration
(cm3)
Mobility
(cm2/Vs)
Resistivity
(cm)
Band gap
(eV)Reference
1.00 x 1019
21
1.48 1.56[31]
3.11 x 1018
8.28
0.24 1.57[32]
7.96 x 1018
1.3
0.2 1.06 [33]
4.95 x 1015
- 1.60 x 1017
54.22 -84.86
0.47 -
23.27
[34]
4.88 x 1017
- 7.52 x 1017
58.4 -87.1
1.48[35]
1 x 1016
- 1 x 1019
21
0.24[36]
Regardless of the methods, chemical composition of Cu2ZnSnSe4 as
absorber layer has been studied to get high efficiency of solar cells. According
Cu2ZnSnSe4ternary phase diagram, the favorable precursor film composition of
Cu-poor, Zn-rich and Sn-rich (Zn/Sn stoichiometric) for fabricating highly
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efficient kesterite photovoltaic exhibits intermetallic CuZn, Cu6Sn5and elemental
Sn phases [u].
Fig. 2.6. Compositional ranges of metallic precursor films represented on the
superimposed ternary CuZnSn (black) and the modified metal chalcogenide
(red) phase diagrams. Nos. 1, 2, 3, 4 and 5 are Cu2ZnSn(SxSe1x)4, Cu2ZnSn3S8,
Cu4SnS9, Cu2Sn(SxSe1-x)3and Cu2Sn4S9, respectively [37].
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From the chemical composition of Cu2ZnSnSe4, if we mapping the
chemical composition according to Cu/(Zn+Sn) ratio, Zn/Sn ratio and the
efficiency which resulting, it is clearly only CZT precursor which having
chemical composition with Cu/(Zn+Sn) ratio range of 0.7 0.9 and the Zn/Sn
ratio range of 1.1 1.4 (Cu-poor, Zn-rich, Sn-rich) as a product which can give a
high efficiency for solar cells [56].
Fig. 2.7. CZT precursor composition map of Cu2ZnSnSe4solar cells efficiency
[38].
In the solar cells arrangement, Cu2ZnSnSe4has a role as absorber layer in
the thin film solar cell which placed on the Mo back contact. After Cu2ZnSnSe4
layer, placed CdS buffer layer and AZO layer as window layer. Below is
arrangement of Cu2ZnSnSe4thin film solar cells including with each thickness:
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Fig. 2.9. Common steps in the Cu2ZnSnSe4thin film fabrication by wet process
deposition [39].
The first step in Cu2ZnSnSe4 thin film fabrication by spin coating is
deposition or coating step. In deposition step, there are several things that give
effect to the quality of films, such as solution form, surface tension and viscosity.
In the thin film process, sol-gel is usually used as starting material. However,
since sol-gel contains particles which dispersed in the solvent (colloidal system),
if the particles are not homogeneous, it would make the particles spread non-
uniformly on the substrate as consequences it would make non-uniform film
either.
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Fig. 2.11. Schematic illustration of the capillary instability that operates during
the drying stage of spin coating [41].
Evaporation can result in a solvent-depleted surface layer having a
different surface tension than the underlying solution. This condition can be
unstable when the surface layer has a higher surface tension. Note that areas at
fig. 2.11. is labeled high and low both have higher surface tension values
than that of the starting solution but represent random variations in composition
that then lead to lateral fluid motions, building the striation structures [41].
Boiling point is the one of the most considerable things due to in the spin
coating technique involve heat treatment process either which has a purpose to
evaporate or eliminate as much as possible solvent from solution and attached the
desirable particles. Low boiling point is most preferable due to easy to eliminate
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from films. Besides boiling point, wet ability and viscosity also have an important
role in the spin coating process due to both have an effects on the coating ability.
Low wet ability and high viscosity of solution would make the solution thick,
gather, difficult to spread and not coverage the substrate perfectly. Moreover, the
thickness film in the deposition process effected by spin speed, spin time, and
cycle. To determine the film thickness, there are many methods or instruments can
be applied such as using a profilometer or a scanning electron microscope (SEM).
The second step in the spin coating technique is drying or heat treatment
process. As mentioned earlier, the purpose of heat treatment process is to
evaporate or eliminate as much as possible solvent from the film, and attached the
desired substance to the substrate as well. Noted that, in the heat treatment
process, should be done in the right temperature since it is has a possibility to
cause a decomposition of desired substance or possibility to change the chemical
composition of the film.
II.4. Ethanolamine
Ethanolamine or 2-aminoethanol or often abbreviated as ETA or MEA is a
colorless and viscous liquid organic compound which has primary amine and
primary alcohol/hydroxyl as well. ETA has a molecular formula as C2H7NO and
has similarity properties with other amines as a weak base, that is why ETA is
used as a neutralizer for acid gas such as CO2in many industries which produce
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CO2 gas as a side product. ETA also can evaporate cleanly due to ETA has a
simple structure. However, according to material sheet data safety (MSDS), ETA
is a toxic, flammable, corrosive.
Fig. 2.12. Structure of ethanolamine [42].
Ethanolamine is produced by reacting ethylene oxide with aqueous
ammonia, however, in this reaction also produces diethanolamine and
triethanolamine as side products [43]. The reaction of ETA production is shown
below:
Fig. 2.13. ETA fabrication reaction from ethylene oxide [43].
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Physical properties of ETA such as molecular weight, boiling point,
refractive index, solubility, density, viscosity, and surface tension at 298 K and
105Pa is shown in the below:
Table 2.3. Physical properties of ethanolamine [44,45].
Empirical formula C2H7NO RefFormula weight 61.09 g/mol [44]
Boiling point 170 K [44]
Refractive index 1.4541 [44]
Solubility Water: soluble in all proportionsEthanol: soluble in all proportions [44]
Viscosity 18.74mPa.s [45]Surface tension 49.1mN/m [45]
Density 1.01 g/cm3 [45]
Besides physical properties above, since ETA has primary amine and
primary alcohol/hydroxyl sites, it makes ETA has hydrogen bonding either intra
molecular or inter molecular hydrogen bonding. Intra molecular hydrogen
bonding of ETA shown in fig. 2.12, whereas inter molecular of ETA shown in fig.
2.14.
Fig. 2.14. Inter molecular hydrogen bonding of ETA [42].
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The ETA has an ability to make coordination bonding with metal [46].
Coordination bonding between ETA and metal possible happen because of both
amine (NH2) and hydroxyl (OH) in the ETA have free electron pairs, amine has
one free electron pair whereas hydroxyl has two free electron pairs. And then,
they can give one pair of the free electron pairs to the metal atom to use together.
In this condition, ETA acts as a ligand or donor of free electron pair whereas
metal atom acts as center atom or acceptor. Due to in ETA amine and hydroxyl
sites donor their free electron at once, then ETA has probability acts as chelating
agent or bridging agent which connect two metals as a center atom in the
coordination compound system depend on the condition [47]. In the coordination
compound system, ETA molecules have neutral charge and be able to make a
bonding with center atom from two to four atoms of ETA for one center atom
[48,49]. Moreover, in the chemical properties of ETA, besides ability to react or
form a coordination compound, ETA also can be decomposed into CO2and H2O
in a complete combustion, but in particular conditions, ETA may decompose in to
several substances depend on the condition [49]. However, even though ETA has
been decomposed into the other compound, as long as the new compound has a
free electron pair or bonding, it still can make a coordination compound with
metals.
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Table 2.4. ETA degradation under different conditions [49].
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However, if [Cu(ETA)2]2- fabricated from Cu(CH3COO)2 as starting
material, then the solution of [Cu(ETA)2]2- must be contain acetate molecule
which have a possibility to make a bonding with Cu(II) as an anion or even as
ligand due to acetate molecule has carbonyl which have O-donor, if acetate bonds
as an anion, the structure of [Cu(ETA)2]2-is square planar, but if acetate bonds as
ligand, then the structure must be changed into octahedral due to there are only z
axial which provides free space to make a bonding with Cu(II). Acetate has
carboxyl site which is can act as donor free electron pair due to have two O-
donors [51].
Fig. 2.16. Common bonding modes for carboxylate ligands [51].
Unlike Cu, the oxidation number of Zn is only Zn(II) and this
phenomenon is a common phenomenon for the members of the first row of d-
block. The Zn coordination compound can form octahedral, tetrahedral, and
square based pyramidal. Zn with ETA would form a [Zn(ETA)2]2- coordination
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compound which has similar structure as[Cu(ETA)2]2-which described earlier
even though the [Zn(ETA)2]2- coordination compounds has colorless and
diamagnetic which caused by the electronic configuration of Zn2+is d10[50, 52].
Whereas Sn even though not comes from d-block metal, Sn still can make
a coordination compound with ligand due to Sn has a natural inclination to
expand the coordination sphere owing to the availability of vacant d-orbital, thus,
Sn is widely used in metal-organic reaction for many kinds of purposes [53]. Sn
has (II) and (IV) stable oxidation numbers which both oxidation numbers are
stable and commonly used as raw material in the metal-organic reaction. In some
cases, Sn(II) is more preferable than Sn(IV) due to Sn(II) can act as reducing
agent, sothe Sn(II) can prevent oxidation. When Sn(II) make a coordination
compound, Sn(II) can form octahedral, tetrahedral, and trigonal pyramidal
structures. However, Sn(II) with ETA can make an octahedral structures, due to
ligand effect which force the structure of Sn(II) form octahedral structure.
However, in Sn(II) chloride, the presence of Cl- in the mixture solution
probably can give an effect into the structure of coordination compound because
Cl
-
atom can act as a bridging agent to connect two center atoms, whether same or
different center atom. Thus, in the mixture of metal-ETA coordination compound
so many probabilities of formation among metals as center atom, ETA as ligand
and anions the structure of metal-ETA still unknown exactly.
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CHAPTER III
EXPERIMENTAL DETAILS
III.1. Metal-ETA Coordination Compound Solution Preparation
From the literature study, the optimum composition of Cu2ZnSnSe4 thin
films for the solar cell absorber layer is Cu-poor and Zn rich ratio. Therefore, in
this work, we were choosing Cu-poor and Zn-rich conditions as an initial
composition which made from 4.6793 g of copper(II)acetate monohydrate
(Cu(CH3COO)2.H2O), 3.6583 g of zinc(II)acetate dehydrate
(Zn(CH3COO)2.H2O), and 2.1443 g of tin(II)chloride dihydrate (SnCl2.H2O). All
metal salts were dissolved ultrasonically in the 30 mL of ETA gradually with the
dissolution order was Sn2+, Cu2+, and Zn2+ until the metal-ETA coordination
compound solution color was dark blue transparent. After metal-ETA coordination
compound was formed, then dilute with ethanol until the total volume is 100 mL.
By doing this way, we could get the final concentration of Cu2+, Zn2+, and Sn2+
were 0.0750 M, 0.0500 M, and 0.0500 M respectively with the composition of the
solvents were 30% ETA and 70% ethanol. At the Zn/Sn ratio investigation, only
change the composition of metal-ETA but the composition of solvents were still
the same.
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III.2. Spin Coating Preparation
The spin coating process was done on 1 inch x 1 inch soda lime glass
(SLG) which cleaned in an ultrasonic water bath in acetone, ethanol, and de-
ionized water, for 20 minutes respectively. The metal-ETA coordination
compound solution was flattened as 0.2 mL with a homemade spin coater from
WiseStir MSH-20A magnetic stirrer for 10 second at 2000 rpm. And then heat up
in the tube furnace under air atmosphere at 200 oC for 10 minutes, repeat this step
for 5 cycles and for the last cycle, heat up at 200 oC for 120 minutes until the
black shiny color was obtained.
Fig. 3.1. Spin coating scheme
III.3. Cu2ZnSnSe4Thin Film Preparation
The metal-ETA after heat treatment were annealed into a small glass tube
and this small tube was then placed inside a quartz tube furnace at atmospheric
pressure under constant Ar (95%) +H2 (5%) gasses flow as atmosphere. The
selenization was carried out by elevated the substrates temperature from room
temperature to 550 oC with heating rate as 10 oC/min using Se pellets as selenium
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source and then holding for 120 minutes at peak temperature to obtain
Cu2ZnSnSe4 thin film. After selenization, the sample was cooled to room
temperature under natural condition.
Fig. 3.2. Furnace arrangement for selenization
In order to investigate atmosphere effect in Cu2ZnSnSe4 crystallization,
the metal-ETA was selenization under Ar (95%) + H2(5%) and Ar (100%) at 550
oC. to investigate crystallization of Cu2ZnSnSe4by temperature, the selenization
done by elevated temperature from 250 to 550 oC for 120 minutes under Ar (95%)
+ H2 (5%) atmosphere. Whereas, to investigate the selenization time effect, the
selenization was done under Ar (95%) + H2(5%) and Ar (100%) at 550oC for 30
-120 minutes. All investigation used Se pellets as selenium source.
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Condition Unit
Cu/(Zn+Sn) 0.75 -
Zn/Sn 1.75 - 1.25 -
Cu 0.01310.0169 M
Zn 0.00750.0125 M
Sn 0.0500 M
Solvent ETA:ethanol (30:70) -
Condition Unit
Speed 500 - 2000 rpm
Time 5 - 15 secondCycles 3 - 15 -
Volume 0.2 mL
Heat temp 200oC
Heat time10 every cycles and
120 for the last cyclemin
Condition Unit
Temperature 250 - 550 oC
Time 30 - 120 min
Heating rate 10oC/min
AtmosphereAr (95%) + H2(5%) -
Ar (100%) -
Se source 2 Se pellets pcs
SEMEDX
XRD
UV-Vis
FT-IR
Raman
Fig. 3.3. Flow chart and conditions of Cu2ZnSnSe4thin films fabrication process
SolutionPreparation
Spin CoatingProcess
SelenizationProcess
Analysis
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III.4. Analytical Methods
III.4.1.Microstructural and Compositional Analysis
The micro-structural and cross-sectional analysis of films were determined
under the Scanning Electron Microscope (SEM), Hitachi S-4800, Japan, while
compositional analysis of films was examined by using the Energy Dispersive X-
Ray (EDX, Horiba, Japan) attached to the respective SEM apparatus.
III.4.2.Crystallinity and Phase Investigation
The Crystallinity and structural analysis performed under X-Ray
Diffractometer (XRD), RIGAKU DMAX 2500 Japan with Cu-
K monochrometer, thin film collimator, fixed angle 20 and = 1.5405. The
measurement conditions were performed at 40 kV, 100 mA, scan speed 2
0
with
diffraction angle 2
between 10 and 65o.
III.3.3. Optical Transmittance
The optical properties of the films were observed by means of double slit
Ultraviolet-Visible-Near Infrared (UV-Vis-NIR) Spectrophotometer, Cary 5000,
Varian, USA, at spectral range 300 1500 nm representing ultraviolet-visible
light-far infrared wavelength. The observations focused on films optical
transmittance. Moreover, the organic phase information was obtained from
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CHAPTER IV
SYNTHESIS AND CHARACTERIZATION OF Cu2ZnSnSe4THIN FILM
BY SPIN COATING PPROCESS
IV.1. Preparation of Metal-Ethanolamine Coordination Compound
Solution
IV.1.1. Preliminary Observation of Ethanolamine to Metal Salts and the
Other Solvents by Visual
The preliminary test in this work is very important, because much of
crucial information can be gathered by doing this test. In the literature study, the
advantages of ETA as a solvent has been described. Nevertheless, it is important
to collect data about ETA with metal salts and the other solvents to prove what has
been written in the literature study and to gather information in order to find out
suitable composition of the solution for this technique.
This preliminary test was done as qualitatively by dissolving same amount
of metal salts were dissolved in the same amount of ETA, de -ionized (D.I.) water
and ethanol separately without adding some basic or acid and without heat
treatment. Among the three kinds of solutions, only ETA which be able to dissolve
metal salts completely, due to Cu, Zn, and Sn were making coordination
compound with ETA which is can be dissolved in the ETA itself.
In the other hand, clearly shown that metal salts from acetate can be
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dissolved in the water due to they be able make coordination compounds that
dissolved in the water [44]. Whereas Sn(II)chloride was formed [Sn(OH)Cl](s)
compound which cannot be dissolved in the D.I. water [54]. Moreover, Cu and Zn
in the ethanol were dissolved slightly, whereas Sn in ethanol was dissolved [44].
Table 4.1. Dissolution table of metal salts with D.I. water, ethanol, and ETA.
D.I. water Ethanol ETA
Cu2+
Soluble, clear Slight soluble, dispersed Soluble, clear
Zn2+
Soluble, clear Dispersed Soluble, clear
Sn2+
Dispersed Soluble Soluble, clear
To make sure Cu, Zn, and Sn were making coordination compound with
ETA, after Cu, Zn, and Sn dissolved in ETA, amount of D.I. water and ethanol
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were added into coordination compound solution each coordination compound
solution. However, after adding D.I. water and ethanol, Cu, Zn, and Sn
coordination compound with ETA still shown clear appearance due to each metal
has been formed a coordination compound with ETA which prevented the other
molecules such as hydroxyl from water or ethanol make a bonding with metal.
Therefore, after each metal makes a coordination compound with ETA, even
though adding water or ethanol, it could not form an insoluble compound
anymore, then the next, effect from D.I. water or ethanol addition is only dilution.
However, interaction among inorganic materials as center atom and how the
structure in this condition is difficult to identify exactly, it is need particular
investigation about that.
Fig.4.1. Illustration of metal-ethanolamine coordination compound in the
solvents.
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Table 4.2. Dissolution table of metal-ETA coordination compound with D.I. water
and ethanol.
M-ETA coordination
compound solution
M-ETA coordination
compound solution + D.I.
water
M-ETA coordination
compound solution +
ethanol
Cu2+
clear, dark blue keep clear, dark blue keep clear, blue
Zn2+
clear, transparent keep clear, transparent keep clear, transparent
Sn2+
clear, transparent keep clear, transparent keep clear, transparent
The ETA can dissolve all metal salts completely, in the other side, ETA has
high surface tension. This property makes ETA difficult to attach to the substrate
and difficult to evaporate as well. Therefore, needs an addition solvent that can
dilute metal-ETA coordination compound, having low surface tension and volatile
as well, that is the reason why ethanol was added into metal-ETA coordination
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compound as solvent as well.
Fig.4.2. Physical and chemical properties ratio scheme of ethanol and
ethanolamine composition mixture with metal salts base on experiment.
In this experiment, metal-ETA coordination compound solution could
form if minimum ETA:ethanol composition percentage ratio was 30%:70%. If
ETA more than 30%, it makes the solution has high surface tension, the metal-
ETA coordination compound could not spread well, and of course difficult to
evaporate. Thus, too high of surface tension could make inhomogeneous and
uncovered area on the substrate. Whereas if the ETA less than 30%, although it
has a low surface tension and volatile, it makes a colloidal or suspension systems
which could make an inhomogeneous particle size and difficult to attach to the
substrate. Thus, the optimum ETA:ethanol composition percentage ratio was
30%:70%.
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Fig. 4.3. Dilution effect of metal-ETA with ethanol (a) 0%, (b) 30%, (c) 50%, and
(d) 70%.
IV.1.2.Preliminary observation of ethanolamine to metal salts and the other
solvents by Raman Spectroscopy and Fourier Transform Infrared
(FT-IR)
Besides by visual, preliminary test also done by instruments such as by
Fourier Transform Infrared (FT-IR) and Raman spectroscopy. From FT-IR and
Raman spectroscopy analysis, both of the results showing same tendencies, there
was no significant change between pure ETA spectrums with metal-ETA
spectrums due to in the metal-ETA coordination compound solution contains a lot
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of ETA as solvent. As consequences, reduction vibration from OH and NH2sites
from ETA as chelate ligand which was undetectable due to make a bonding with
metal which act as central atom.
Fig. 4.4. Fourier Transform Infrared (FT-IR) spectrum (a) and Raman spectrum
(b) for ETA and metal-ETA coordination compounds solution.
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From observation by visual and instrumental, the reactions that can be
proposed are:
Cu2++ xETA [Cu(ETA)x]2+
Zn2++ xETA [Zn(ETA)x]2+
Sn2++ xETA [Sn(ETA)x]2+
The reaction that can be proposed in the mixture:
[Cu(ETA)x]y++ [Zn(ETA)x]
y++ [Sn(ETA)x]y+ [CuZnSn(ETA)x]
y+
IV.2. Spin Coating Preparation
IV.2.1. Spin Coating Effect
As described in the literature study, the function of spin in the coating
process is for flattening the samples on the substrate until the desired thickness of
the film is achieved, therefore, spin coating commonly effects to the thin film
thickness. There are several variations in the spin coating technique: spin speed,
coating time, and amount of cycles. Therefore, it is important to find the optimum
condition to make desired thickness of thin film. However, due to in the metal -
ETA not only contains Cu, Zn, and Sn as the main compiler of Cu2ZnSnSe4thin
film, but also contains amorphous carbon as residue which can decrease from
initial state due to evaporation of sample. Therefore, the desired thickness before
annealing process should be thicker than the desired thickness of the final
product.
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The spin speed of the substrate affects to the centrifugal force, therefore,
higher of speed could make a thinner thin film. In this work, speed in the coating
was limited to the performance of the spinner which has a maximum speed at
2000 rpm. Besides spin speed, coating time also effect to the thickness, longer
time of spin, more thin film can get, due to by prolonging the coating time it can
maximize unattached liquid on the substrate to leave. Meanwhile, amount of
cycles in the spin coating could increase the thin film thickness due to accumulate
of the metal-ETA which produced every cycle.
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Fig.4.5. Spin coating effect: speed (a), coating time (b), and cycles (c) effect to the
thickness.
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IV.2.2. Heat Treatment Process of Metal-ETA
As described in the previous chapter, metal-ETA was made from Cu, Zn,
and Sn coordination compound with ETA which deposited by spin coating and
heating treatment. Heat treatment has a goal to eliminate all organic compounds
from solvents and the other reagent which is has potential to form a carbon
residue in the thin film. Besides that, heat treatment also can evaporate Sn in the
precursor due to Sn has a low melting point compared than Cu and Zn. As
consequences, the chemical composition should be changed due to too a high
temperature of heat treatment. So that, it is important to find out the optimum
temperature to remove as much as possible organic compounds from metal-ETA
without change the chemical composition as can as possible.
The coordination compound of ETA such as Cu-ETA and Zn-ETA start to
decompose at 150 oC and 175 oC respectively, whereas ETA boiling point is 175
oC either [55]. Then, at 180 oC, inter-metallic binary alloy such as CuxZny and
CuxSny are starting to form [56], meanwhile melting point of Sn is 230oC, in
other words, temperature for heat treatment cannot beyond than 230 oC. Thus, 200
o
C was chosen as optimum condition due to at 200
o
C, it was sufficient
temperature to decomposing metal-ETA coordination compound, form inter-
metallic binary alloy of Cu, Zn, and Sn, but still under than Sn melting point, so it
may prevent chemical composition change any further.
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Fig. 4.6. Physical and chemical properties of organic and inorganic compounds
of metal-ETA coordination compound according temperature.
During heat treatment, organic compound in the metal-ETA was
evaporated, whether it comes from solvents or ligand in the coordination
compound. If the organic compound comes from solvents, it was evaporating, but
if the organic compound comes from ligand, it was decomposing. Both things can
be detected by FT-IR spectrophotometer.
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cm-1and 2869 cm-1were exhibited for hydrocarbon (C-H) bonding.
After one cycle of heat treatment, metal-ETA shows some peaks even
though some peaks were changed, it could be shifted, decreased the intensity or
even disappeared. This condition was evidence that ETA in the metal-ETA was
decomposed partially. Since ETA has amine and hydroxyl as active sites, both
sites could react and make many kinds of compound during heat treatment
[55,49]. However, due to metal-ETA was made from a mixture and so many
possibilities of structures from ETA if decompose partially, it is difficult to
recognize exactly what state is it. However, there were conspicuous peaks which
still could identify, those were the broad peak at 3422 cm-1 which exhibit or
hydroxyl site without indicating the presence of hydrogen bonding from solvent
anymore, it means all solvent has been removed completely. Peak at 2215 cm-1
was specifically represented of nitrile group, peaks at 2932 and 2869 cm -1were
represented of C-H bonding, shoulder peak at 1700 cm-1 was representative of
C=O bonding and 1058 cm-1was representative of C-N bonding.
After 120 minute heat treatment in the last cycles, almost every organic
compound was removed, but there were still few peaks can be detected. There
were only two peaks still detected at 1627 cm-1and 3430 cm-1. Peak at 1627 cm-1
is indicated as free alkenes (ethylene) which consist of carbon with sp2
hybridization [57,58]. However, due to ethylene could make a bonding with
metals as ligand by using orbital, this condition makes ethylene can stay longer
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in the metal-ETA, therefore, FT-IR still detects the presence of ethylene. This
phenomenon has been described in the literature study. Whereas, peak at 3430
cm-1 is indicated as small amount of hydroxide bonding which probably exist in
the amorphous carbon. To confirm this state, Raman spectroscopy analysis also
done, due to Raman spectroscopy is the instrument that widely and commonly
used for analysis carbon. At Raman spectrum for metal-ETA, clearly seen D and
G peaks at 1350 cm-1 and 1585 cm-1 respectively that indicate as amorphous
carbon [59]. This amorphous carbon probably comes from the incomplete
combustion reaction of organic compounds such as acetate as anion.
Fig. 4.8. Raman spectrum for metal-ETA after heat treatment.
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Besides organic compounds, metal-ETA also contains inter-metallic
compounds such as Cu, Zn, and Sn as raw material for Cu2ZnSnSe4fabrication.
Thus, the existence and amount of Cu, Zn, and Sn also obviously could be
detected by using Energy Dispersive X-ray (EDX).
Fig. 4.9. Energy Dispersive X-ray (EDX) spectrum of metal-ETA after heat
treatment.
However, besides detecting existence of Cu, Zn, and Sn, EDX also detects
the existence of carbon, oxygen, and chloride which trapped in the metal-ETA as
residue. Those residues were come from the incomplete decomposition of organic
compounds such as ligand and anions which deposited along Cu, Zn, and Sn in
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the metal-ETA. The existence of carbon and oxygen in the metal-ETA also match
with FT-IR and Raman spectroscopy data, only chloride which could not detected
by FT-IR and Raman but detected by EDX. Moreover, inter-metallic compound in
the metal-ETA such as Cu, Zn, and Sn could make inter-metallic binary alloy such
as CuZn and CuSn even though in 200 oC [56]. According to Cu-Zn-Sn ternary
phase diagram, with composition of Cu, Zn, and Sn after heat treatment as
43.79%, 29.25%, and 26.96% respectively, then, the metal-ETA has Sn, CuZn,
and CuSn phases.
Fig. 4.10. Cu-Zn-Sn ternary phase diagram at 200oC [56].
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The existence of inter-metallic binary alloy obviously detected by x-ray
diffraction (XRD) pattern which shows two peaks. Both peaks probably exhibit
inter-metallic binary alloy peaks such as CuxZnyand CuxSny[60,61,62]. Both of
inter-metallic compounds are possible to form below 200 oC [56,61]. However,
there was no evidence for ZnxSnycompound due to preferential reaction between
zinc and tin with copper [56]. Moreover, in the XRD pattern of metal-ETA also
detected amorphous a small broad peak belong to amorphous carbon at 2 20-38.
Fig. 4.11. X-ray diffraction (XRD) pattern of metal-ETA after heat treatment.
From FT-IR spectrum, Raman spectrum, EDX spectrum, XRD pattern and
Cu-Zn-Sn ternary phase diagram, the reactions that can be proposed are:
[CuZnSn(ETA)x]y+ CuxZny+ CuxSny+ Sn + Organic residue (C,O,Cl)
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IV.3. Preparation of Cu2ZnSnSe4Thin Film
Cu2ZnSnSe4 thin film was obtained by selenization of metal-ETA
coordination compound under Ar (95%) +H2(5%) atmosphere at 550oC for 120
min. Before selenization, FT-IR spectrum of metal-ETA coordination compound
shows the presence of amorphous carbon peak, but after selenization, FT-IR
spectrum of Cu2ZnSnSe4 was completely changed without showing amorphous
carbon peaks anymore, and shows high of transmittance due to the Cu2ZnSnSe4
does not have absorbance in the infrared area.
Fig. 4.12. FT-IR spectrums of metal-ETA coordination compound (a)
before and (b) after selenization at 550oC for 120 min under Ar (95%) + H2(5%)
atmosphere.
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From the XRD pattern of metal-ETA coordination compound before and
after selenization obviously shown different. According to the Cu2ZnSnSe4XRD
pattern reference (ICDD: 00-052-0868), the peaks of metal-ETA coordination
compound after selenization was matched with Cu2ZnSnSe4pattern reference.
Fig. 4.13. The XRD pattern of Cu2ZnSnSe4selenization (a) before and (b) after
selenization at 550oC for 120 min under Ar (95%) + H2(5%) atmosphere.
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Fig. 4.14. Raman spectrums of Cu2ZnSnSe4selenization (a) before and (b) after
selenization at 550 oC for 120 min under Ar (95%) + H2(5%) atmosphere.
Besides XRD pattern, Raman spectrum after selenization of metal-ETA
coordination compound also shows identical peak for Cu2ZnSnSe4 at 171 cm-1,
193 cm-1, and 233 cm-1 without presence of secondary phase and amorphous
carbon as well.
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IV.3.1.Selenization Atmosphere Effect
One of the major problems in the Cu2ZnSnSe4 thin film fabrication using
organic solution technique is appearance of carbon residue in the bottom of
Cu2ZnSnSe4 thin film due to incomplete combustion of organic compound.
Incomplete combustion could be appeared due to insufficient amount of oxygen in
the atmosphere to make a complete reaction of organic compound. Contrary, to
grow Cu2ZnSnSe4in the annealing process, it needs an inert atmosphere to avoid
oxidation reaction between metals and oxygen in the atmosphere. However, in
this technique, presence of amorphous carbon as residue was probably caused
from the organic compound such as acetate as anion, whereas ethylene was
evaporate easily, thus, could not leave any traces as carbon residue. In selenization
under inert atmosphere, acetate could not decompose completely due to
insufficient of oxygen and due to amorphous carbon removal reaction only
depends on the spillover reaction, thus, amorphous carbon existence is avoidable
[63]. Besides that, in the inert atmosphere, amorphous carbon on the bottom of
Cu2ZnSnSe4 could not be removed due to trapped by Cu2ZnSnSe4 formation.
Meanwhile, reaction of Cu2ZnSnSe4 formation in inert atmosphere only depends
on the Se vapor which reacts with Cu, Zn, and Sn to form metal selenide. Further,
metal selenides such as CuxSey and SnxSey was merged and formed Cu2SnSe3.
Finally, Cu2SnSe3 was merged with ZnSe to form Cu2ZnSnSe4, this reaction
mechanism is well known reaction [64].
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In the other hand, H2 also has an important role in the removing organic
compound such as acetate which contribute to the presence of oxygen and
amorphous carbon in the metal-ETA. In the metal-ETA, metal oxide can be
formed from metal-ETA coordination compound decomposition, due to metal in
the metal-ETA coordination compound was directly bonded with oxygen from
ETA and acetate. Moreover, oxygen from metal oxide can react with H2directly
or with H2Se and form MSe and H2O which can be used for removing amorphous
carbon which still remains as residue. Besides that, non-catalytic reaction also can
occur at this condition, and gives the double effect of carbon residue removal.
This reaction also called as a carbon gasification reaction which could occur in
mild low-temperature [73]. Moreover, the presence of ethylene as ligand also can
be removed with hydrogen by hydrogenation reaction, therefore, ethylene could
not act as ligand anymore.
The XRD pattern of Cu2ZnSnSe4 thin film which annealed under Ar
(95%) +H2(5%) atmosphere shows narrow peaks without showing the secondary
phases existence, due to effect of H2which can react with Se form H2Se which
could eliminate the secondary phases during annealing process, whereas the XRD
pattern of Cu2ZnSnSe4 thin film which annealed under Ar (100%) atmosphere
shows a small peak probably belong to secondary phases such as CuxSey or
SnxSey, and also shows slight broadened peak which indicates probably non-
uniform phase of the thin film due to small amount of secondary phases existence.
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Fig. 4.16. The XRD pattern of Cu2ZnSnSe4selenization under (a) Ar (100%) and
(b) Ar (95%) + H2(5%) atmosphere.
Scanning electron microscopy (SEM) image of Cu2ZnSnSe4 which
annealed under Ar (100%) was smaller grain size and non -uniform morphology
compared than Cu2ZnSnSe4thin film which annealed under Ar (95%) + H2(5%)
atmosphere and it was matched by the XRD pattern. Moreover, cross-sectional
image by Scanning Electron Microscope (SEM), also clearly shows that
Cu2ZnSnSe4thin film which annealed by Ar (100%) still having carbon residue in
the bottom of Cu2ZnSnSe4layer due to Cu2ZnSnSe4thin film synthesis reaction
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only depends on non-catalytic reaction between Se with metal-ETA which occur
on the surface because Se vapor cannot infiltrate too deep until the bottom of
metal-ETA. Meanwhile, carbon residue process only depends on the carbon
monoxide synthesis reaction in the high temperature which only effective for
carbon residue in the surface area. As consequences, some carbon residue which
existed on the bottom was trapped by Cu2ZnSnSe4layer which grows on the top
of carbon residue.
Fig. 4.17. SEM images of (a) Cu2ZnSnSe4which selenized under Ar (100%)
atmosphere, (b) cross-sectional image of Cu2ZnSnSe4which selenized under Ar
(100%), (c) SEM image of Cu2ZnSnSe4which selenized under Ar (95%) + H2
(5%) and (d) cross-section image of Cu2ZnSnSe4which selenized under Ar (95%)
+ H2(5%).
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Fig. 4.19. The (a) cross-sectional image and (b) its EDX spectrum of CZTSe thin
film for 120 minutes at 550oC and under Ar 100% atmosphere.
From selenization effect data, the proposed reaction pathway for growth of
Cu2ZnSnSe4thin film and amorphous carbon removal reactions are:
Cu2ZnSnSe4formation reaction under Ar (100%) atmosphere:
CuxZny+ CuxSny+ xSe CuxSey+ ZnSe + SnxSey
xSn + ySe SnxSey
CuxSey+ SnxSey+ Se Cu2SnSe3
Cu2SnSe3+ ZnSe Cu2ZnSnSe4`
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Cu2ZnSnSe4formation reaction under H2(5%) addition atmosphere:
H2+ Se H2Se
CuxZny+ CuxSny+ xH2Se CuxSey+ ZnSe + SnxSey+ xH2
xSn + ySe SnxSey
CuxSey+ SnxSey+ Se Cu2SnSe3
Cu2SnSe3+ ZnSe Cu2ZnSnSe4
Carbon removal reaction under Ar (100%) atmosphere:
C (amorphous) + O CO
Carbon removal reaction under H2(5%) addition atmosphere:
MO + H2Se MSe + H2O
H2+ O H2O
H2O + C (amorphous) CO + H2
C2H4+ H2C2H6
IV.3.2.Selenization Time Effect
In this work, the effect of selenization time was investigated the structure
by XRD and the morphology by SEM. From the XRD pattern, shows Cu2ZnSnSe4
has been formed for 30 minutes selenization at 550 oC under Ar (95%) + H2(5%)
atmosphere, however, by prolonging selenization time, it could increase the
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crystallinity which indicated from the increasing intensity of the XRD pattern.
Fig. 4.20. XRD pattern of Cu2ZnSnSe4by prolonging selenization times for (a) 30,
(b) 60, (c) 90, and (d) 120 minutes at 550oC under Ar (95%) + H2(5%)
atmosphere.
SEM images show increasing of Cu2ZnSnSe4grain size and by prolonging
the selenization time also give sufficient time for carbon residue to evaporate
completely.
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formed at 250 oC even though in small amount [74]. At this temperature, probably
SnSe also occur from naked Sn with selenium vapor, however, peaks belong to
CuxZny and CuxSny binary alloy in the metal-ETA as starting materials still
dominant. At elevated at 350 oC, not only binary and ternary compounds have
been occurred, but also probably quaternary compound [74]. Binary compounds
which occurred were CuxSey, SnxSey, ZnSe, CuxZny and CuxSny for ternary
compound there was CuSnSe3and for quaternary compound was Cu2ZnSnSe4. At
this temperature, CuSnSe3 and Cu2ZnSnSe4 probably have been occurred even
though in the small amount [74]. At 400 425 oC shows all inter-metallic binary
alloys have been disappeared and remains metal selenide as binary, ternary or
quaternary. At 450 oC and 550 oC, there are five peaks at 2 = 17.35o, 27.13o,
36.11o, 45.09o, and 53.45owhich can be indexed to (101), (112), (211), (204/220),
and (312/116) respectively. These peaks are probably belongs to ZnSe (ICDD: 01-
088-2345), Cu2SnSe3 (ICDD: 01-089-1879) or Cu2ZnSnSe4 peaks (ICDD: 00-
052-0868).
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Fig. 4.22. XRD pattern of metal-ETA at elevated selenization temperatures under
Ar (95%) + H2(5%) for 120 min.
In XRD, difficult to distinguish the secondary phase existence such as
ZnSe and Cu2SnSe3in the Cu2ZnSnSe4thin film due to have similar 2 value with
Cu2ZnSnSe4. However, ZnSe, CuSnSe3 and Cu2ZnSnSe4could be distinguished
by Raman spectroscopy.
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Fig. 4.23. Raman spectrum of (a) metal-ETA before and after elevated
selenization temperatures at (b) 250oC, (c) 350
oC, (d) 450
oC, (e) 550
oC under
Ar (95%) + H2 (5%) for 120 min.
The Raman spectroscopy spectrum from elevated temperature shows the
development of sample from metal-ETA coordination compound to the
Cu2ZnSnSe4 thin film. At metal-ETA shows amorphous spectrum and there was
no peak occurs. After selenization at 250o
C, one peak was occur at 258 cm-1
, this
peak probably belong to metal selenide compounds such as CuxSeyor probably as
amorphous selenide. At 350 oC, shows metal selenide compound such as SnSe
(132 and 150 cm-1), ZnSe (200 and 250 cm-1) and CuSe (260 cm-1). At 450 oC,
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there are few small peaks at 171 cm-1and 193 cm-1which indicated Cu2ZnSnSe4
was started to formed, but still has low crystallinity. At 550 oC, Raman spectrum
shows obvious spectrum of Cu2ZnSnSe4at 171 cm-1, 193 cm-1, and 233 cm-1. At
this temperature, the spectrum indicates the Cu2ZnSnSe4have high crystallinity
without any secondary phases.
Fig. 4.24. SEM images of (a) metal-ETA before and after elevated selenization
temperatures at (b) 250oC, (c) 350
oC, (d) 450
oC, (e) 550
oC and (f) its cross-
section under Ar (95%) + H2(5%) for 120 min.
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The metal-ETA shows have smooth and small grain of morphology. At
250 oC shows obvious some hexagonal that indicated to CuxSeycompound and at
this condition. At 350 oC, selenium binary compounds such as CuSe, ZnSe, and
SnSe have clearly seen, however, CuxSey, ZnSe, and SnxSey have hexagonal,
tetragonal, and thread like shape respectively. At 450 oC, shows the small grain
size of Cu2ZnSnSe4, however, after the elevated selenization temperature until
550 oC, the grain size of Cu2ZnSnSe4 was increased, and the Cu2ZnSnSe4 has
been formed well with high crystallinity. This result is consistent with XRD and
Raman spectrum results for elevated selenization temperature at 450 oC and 550
oC. At 550 oC, Cu2ZnSnSe4 has been completely formed with thickness
approximately 1300 nm. Below is arrow phase diagram as summarize of
temperature effect and change phases in the elevated temperature:
Fig. 4.25. Cu2ZnSnSe4arrow phase diagram.
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At elevated temperature, not only effect to the inter-metallic compound,
but also effected to the organic compound and anions which donates residue such
as chloride and carbon. Both of residues were disappeared gradually by elevated
temperature. By energy dispersive x-ray (EDX) spectrum, chloride and carbon
residues could be detected.
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Fig. 4.26. EDX spectrums of (a) metal-ETA before and after elevated selenization
temperatures at (b) 250oC, (c) 350
oC, (d) 450
oC, and (e) 550
oC under Ar (95%)
+ H2 (5%) for 120 min.
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From EDX spectrums shows chloride, carbon and oxygen as residues were
disappeared gradually by elevated temperature during selenization under Ar
(95%) + H2(5%) atmosphere for 120 min. besides by using EDX, investigation
about carbon residue also done by Raman spectroscopy. Raman spectroscopy is
the common apparatus to investigate about carbon, whether in amorphous,
crystalline, or diamond states. By elevated temperature, carbon residue as
amorphous carbon was disappeared gradually, besides that, the amorphous carbon
also could be disappeared due to H2addition effect. From Raman spectrums at
500 -4000 cm-1, clearly shows degradation of amorphous carbon and increasing
of crystallinity simultaneously. The degradation of amorphous carbon indicates
from D and G peak which keep decreasing by elevated temperature and the
crystallinity could be seen from declining the intensity of spectrums, more low
intensity of spectrum from Cu2ZnSnSe4 thin film at 500 - 4000 cm-1, indicates
higher crystallinity of that Cu2ZnSnSe4.
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