synthesis and characterization of cu2znsnse4¬ thin film prepared by spin coating and selenization...

8/9/2019 Synthesis and Characterization of Cu2ZnSnSe4 Thin Film Prepared by Spin Coating and Selenization from Metal-Ethanolamine Coordination Compound Prec

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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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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


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



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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

i

ii

v

vi

1

1

2

5

5

6

8

10

10

15

22


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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

40

45

47

47

50

58

61

68

70

82

91

95

102


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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.

synthesis and characterization of cu2znsnse4¬ thin film prepared by spin coating and selenization...

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