Green and low cost preparation of cigse thin film by a nanocrystals ink based spin coating method

Abstract This thesis presents our efforts that aim towards a green and low cost prodedure to fabricate CIGSe thin films which would be used as absorber layers for the thin film solar cel

Ph.D Thesis

Green and low-cost preparation of CIGSe thin film

by a nanocrystals ink based spin-coating method

Graduate School of Yeungnam University

School of Chemical Engineering Major in Chemical Engineering

Le Thi Thuy Trang

Advisor: Prof Chinho Park Co-advisor: Prof JaeHong Kim

February 2020

Ph.D Thesis

Green and low-cost preparation of CIGSe thin film

by a nanocrystals ink based spin-coating method

Co-advisor: Prof JaeHong Kim

Presented as Ph.D Thesis

February 2020

Graduate School of Yeungnam University

School of Chemical Engineering Major in Chemical Engineering

Le Thi Thuy Trang

Acknowledgements

I woud like to express my deep gratitude to my advisor, Professor Chinho Park at the School of chemical engineering He has provided me the invaluable contributions in the research work by various profitable discussions His devoted instructions have been my sources of inspiration during Ph.D periods, and have given me the great encouragement to fulfill both the research and writing of this thesis

I deeply thank to Professor JaeHong Kim, who has supported me to complete this Ph.D program

My special thanks also go to Dr Babu, Dr Vasu and Dr Nguyen for their valuable comments and suggestions in my research

I am thankful to the IMSL lab’s members who have ever helped me many things I will remember for their trust and friendship Your sentiments toward me are quite precious

I thank to my family for giving me a wider perspective on things, unlimited forbearance, and the time deprived from them

Last, I gratefully acknowledge Yeungnam university and BK 21 program because of their supports

December, 2019

Le Thi Thuy Trang

Abstract

This thesis presents our efforts that aim towards a green and low cost prodedure to fabricate CIGSe thin films which would be used as absorber layers for the thin film solar cell industry So, an ―ink‖ solution based process, using spin-coating technique, followed by annealing in selenium environment with different temperature programs was utilized to prepare CIGSe thin film

Firstly, CIGSe nanoparticles were synthesized using NaBH4 containing green solvent – ethanol by a sonochemical method The effect of the ultrasonification time intervals on the synthesis was investigated in the range of 2-5 hr, 5 hr was found to be the most suitable time to obtain the expected CuIn0.7Ga0.3Se2 compounds The as-synthesized CIGSe nanoparticles possessed a tetragonal structure with quasi-spherical shape, and the targeted Cu(In0.7Ga0.3)Se2

composition A new reaction scheme is proposed to explain the role of NaBH4 in the plausible reaction paths The non-toxic solvent used in the synthesis with no additional heating makes the developed method cheaper and ―greener‖ than the previously reported methods

Next, three different solvents: 2-propanol, 2-methoxyethanol, and their 2:1 mixture (v/v ratio) were investigated as a dispersion medium for the as-synthesized CIGSe nanocrystals to form a stable ink solution The last one – mixture of 2-propanol : 2-methoxyethanol = 2 : 1 (v/v) – was found to be the most suitable Furthermore, the influences of various annealing modes on the CIGSe grain size and density in the resulting film were also studied The as-prepared CIGSe thin film was around 1 µm thick, possessed a tetragonal structure, and the band gap energy was estimated about ~ 1.3-1.4 eV

The newly developed cheaper and ―greener‖ non-vacuum process was applied successfully from the nanocrystals synthesis to the formation of ―ink‖ solution, and produced

high quality thin films, which opens a new route to the cost-competitive commercialization of CIGSe thin film solar cells

Contents

Acknowledgements i

Abstract v

List of figures viii

Chapter 1: Literature Overview 1

1.1 The material system of Cu(In,Ga)Se 2 quaternary 1

1.1.1 Introduction of the structural property 1

1.1.2 Optical and electrical properties 2

1.2 Overview of solar cell 2

1.2.1 The story of solar cell 2

1.2.2 The operating principle of solar cell 10

1.3 Thin film deposition techniques 10

1.3.1 The vacuum method 11

1.3.1.1 Co-evaporation 11

1.3.1.2 Sputtering (two-step process) 13

1.3.1.3 Chemical vapor deposition (CVD) 14

1.3.2 The non-vacuum method 15

1.3.2.1 The nanoparticles based method 16

1.3.2.2 Direct solution based process 18

1.4 Physical methods for characterizing solids 18

1.4.1 X-ray diffraction 18

Chapter 2: Green and low-cost synthesis of CIGSe nanoparticles using ethanol as a solvent by a sonochemical method - a new approach 31

2.1 Introduction 31

2.2 Experiment details: 32

2.3 Results and discussion 34

Chapter 3: The formation of CIGSe thin film by a nanocrystals ink based spin-coating method 44

3.1 Introduction 44

3.2 Experiment detail 45

3.3 Results and discussion 48

Conclusions 56

References 57

List of puplications 70

List of figures Chapter 1

Fig 1.1: The unit cell of the chalcopyrite lattice structure a) CuInSe2, b) Cu(In,Ga)Se2 1

Fig 1.2: The working of a silicon solar cell 4

Fig 1.3: The structure of a dye sensitized solar cells 6

Fig 1.4: Mechanism of dye sensitized solar cell 7

Fig 1.5: Basic structure or Organic Solar Cell 8

Fig 1 6: The operating principle of solar cell 10

Fig 1 7: Schematic of evaporation system 11

Fig 1 8: Schematic illustration of different coevaporation recipes 13

Fig 1.9: Schematic of sputtering system 14

Fig 1.10: Schematic of chemical vapor deposition system 15

Fig 1.11: Schematic illustration of X-ray generation system 20

Fig 1 12: (a) Section through an X-ray tube (b) An X-ray emission spectrum 20

Fig 1.13: Basic components of a monochromatic XPS system 23

Fig 1.14: Schematic illustration of a raman spectroscopy system 24

Fig 1.15: Schematic illustration of scanning electron microscopy system 26

Fig 1.16: Schematic illustration of Scanning electron microscopy system 28

Fig 1.17: Block diagram of DTA 29

Fig 1.18: Schematic illustration of differential scanning calorimetry system 30

Chapter 2

Fig 2.1: XRD patterns of the CIGSe nanoparticles synthesized with difference ultrasonification

time 36

Fig 2.2: SEM images of CIGSe NPs with ultrasonification time of 2 hr (a), 3.5 hr (b), 4.5 hr (c) and 5 hr (d) 37

Fig 2.3: Raman spectra of as-prepared CuIn0.7Ga0.3Se2 NPs with ultrasonification time of 5 hr 38 Fig 2.4: a) XPS survey spectrum and Cu 2p (b), In 3d (c), Ga 2p (d) , and Se 3d (e ) core-level spectrum of as-synthesized CuIn0.7Ga0.3Se2 nanoparticles 40

Fig 2.5: TEM images of CIGSe nanoparticles with ultrasonification time of 5 hr before (a) and after (b) annealed at 500 oC 41

Fig 2.6: XRD pattern of CuIn0.7Ga0.3Se2 NPs before and after annealed at 500 oC 42

Fig 2.7: Absorption spectrum (a) and plot of (αhν)2 versus photon energy hν (b) of the annealed CuIn0.7Ga0.3Se2 nanoparticles 43

Chapter 3 Fig 3.1: The images of colloidal solutions with different dispersion mediums: propanol (a), 2-methoxyethanol (b) and solvent mixture of 2-propanol and 2-2-methoxyethanol (c) after being kept in 1 month 48

Fig 3 2: The images of the spin coated films by 2-propanol solvent based ink solution (a), and solvent mixture based ink solution (b) 49

Fig 3 3: Cross-section SEM image of CIGSe thin film with the first annealing mode 49

Fig 3 4: Cross-section SEM image of CIGSe thin film with the second annealing mode 51

Fig 3 5: Cross-section SEM image of CIGSe thin film with the third annealing mode 52

Fig 3 6: Surface SEM image of CIGSe thin film with the third annealing mode 53

Fig 3 7: XRD pattern of CIGSe thin film after being annealed with the third annealing mode 54 Fig 3.8: Plot of (αhν)2 versus photon energy hν of CIGSe thin film after being annealed with the third annealing mode 55

Chapter 1: Literature Overview

CuInxGa1-xSe2 (CIGSe) quaternary is a p-type semiconductor material which is a member

of the I-III-VI2 chalcopyrites family [1] It has been known as an effective light-absorbing layer for thin film solar cell industry, and CIGSe thin film solar cells were first studied at the Bell laboratories in 1975 Much efforts have been done on CICSe solar cells, therefore the efficiency

of CIGSe solar cells have reached up to 23.35 %, recently [2]

1.1.1 Introduction of the structural property

The basis for the structure of CIGSe quaternary is the structure of CISe ternary Some indium atoms in unit cell of CISe ternary are replaced by gallium atoms to form unit cell of CIGSe [1]

Fig 1.1: The unit cell of the chalcopyrite lattice structure a) CuInSe2, b) Cu(In,Ga)Se2 [3].

CIGSe crystals have the tetragonal chalcopyrite lattice structure which drived from the cubic structure of the group IV semiconductor The unit cell is corresponds to two stacked zincblende unit cells, where copper, indium and gallium atoms are regularly ordered Each group

I (Cu) and group III (In or Ga) atom is tetrahedrally bonded to four group VI atoms (Se), and

each Se atom is coordinated is tetrahedrally coordinated to 2 group I atoms and 2 group III atoms

[1] The lattice parameters of CuInxGa1-xSe2 are a=b= 5.696, value of c depend on value x composition of Ga element [4]

1.1.2 Optical and electrical properties

Optical properties

CIGSe semiconductors possess high optical absorbtion coefficient α > 105

cm-1, and relatively low direct band gap energy value in the range 1.0-1.7 eV, resulting from the hybridization of Cu d-orbitals and Se p-orbitals in the valence bands [5,6]

The band gap energy of CuIn1-xGaxSe2 compounds depend on the Ga/Ga+In

concentration ratio, but also by the Cu content The optical band gap energy of CuInSe2

is about 1.0 eV when In atoms are gradually substituted by gallium atoms, the band gap can be increase to 1.68 eV, which is the optical band gap energy of CuGaSe2 [1] The band gap energies of the CuIn1-xGaxSe2 quaternary with different compositions from x= 0 to 1 are calculated based on an empirical ―bowing equation‖ [7,8]:

cm2/Vs with polycrystalline films [9,10]

1.2 Overview of solar cell

In 1839, a French physicist-Alexander Edmond Becquerel discovered the technology directly producing electricity from the solar energy [11] This was the beginning of the solar cell technology-which open a new gate for the renewable energy technology

Solar cells are often divided into three main caregories which are called generations up to recent years:

- The first generation: Monocrystalline silicon (mono c-Si), polycrystalline silicon (poly c-Si) and armophous silicon cells

- The second generation: amorphous silicon (a-Si), CdTe and CuInS/CIGS

- The third generation: DSSCs, Perovskite and organic The first generation

The first silicon solar cell with an efficiency of 6% was studied at Bell laboratories and reported by Chapin, Fuller and Pearson in 1954 [12] One year later, the first commercial solar cells with 2% efficient were produced by Hoffman Electronics corporation using wafers of silicon (monocrystalline silicon) The following success of this technology was mostly founded on the further development of the Czochalski method in the 1940s enabling the production of high purity silicon [13], leading to the efficiency of these solar cells currently reaching to 26.7% [14] However, the cost of fabricating monocrystalline silicon solar cells are high because of the purification process

of bulk In 1958, T Mandelkorn created n-on-p silicon solar cells which were better suited for space with more resistance to radiation damage at U.S Signal Corps Laboratories, and these cells were applied to a space satellite ― Vanguard 1‖ – a first solar powered satellite launched at this year with a 0.1W, 100 cm2 solar panel [15]

Different from monocrystalline silicon solar cells, the solar cells made from polycrystalline silicon or amorphous silicon are much cheaper because they are much less pure than monocrystalline silicon, therefore the way slicon made is easier and simpler However, the highest efficient solar cell made from polycrystalline silicon is recorded of 22.3% [14], lower monocrystalline solar cell

Fig 1.2 shows structure of a silicon solar cell A silicon solar cell typically is a sandwich of two layers : the first layer is a positive layer ( p-type silicon ) which usually made by doping silicon with boron to form extra holes in the lattice The second layer is a negative layer ( n-type silicon ) which usually made by doping silicon with phosphorus to gain extra electrons in the lattice

Fig 1.2: The working of a silicon solar cell [16].

Some limitations of this generation : Silicon possesses an indirect band gap with a low absorption coefficient, so it is not a ideal material for a light conversion Besides, the silicon absorber layers require a high purity and a thickness up to around 200 µm, therefore the cost of fabrication is high and the product is less visual aesthetic

The second generation

The second generation solar cells were studied with the purpose of reducing limitations of the first generation solar cells mentioned above The advantage of this generation is that Semiconductor materials used in this generation possess direct band gap

The second generation solar cells consist of absorber layers only 1 to 4 µm thick, therefore they are called as thin film solar cells These layers are much more thinner than silicon solar cells, so they have more visual aesthetic Absorber layers are deposited onto

a large, inexpensive substrate such as glass, polymer or metal

The second generation solar cells have a lower fabricating cost than the first generation solar cell, however, the efficiencies of cells are lower, i.g amorphous Si (a-Si) solar cell (9.5 % ), CdTe/CdS solar cell (19.6%) and CIGSe solar cell (23 %) [14]

Besides, these generation solar cells also exist another drawbacks such as using semiconductor materials which are either becoming rare or highly toxic

The structure of the second generation solar cell still relies on a p-n junction design as the first generation solar cells Therefore, it works as the first generation solar cells

The third generation

The third generation solar cells emerged with efforts aiming to minimize the fabricating costs of first generation solar cells and improve toxicity materials of second generation solar cells These solar cells aren’t designed relying on the p-n junction as the previous generation solar cells

Dye sensitized solar cells (DSSCs)

Dye sensitized solar cells (DSSCs) are also called Gratzel cells which

low-cost solar cells, however, the conversion efficiencies are still quite low, around 11.9 % [14]

The basic structure of DSSC is shown in Fig 1.3 It consists of anode (photoelectrode), cathode (counter electrode), a dye sensitizer and a redox electrolyte The counter electrode is created by coating a thin layer of carbon, platinum or graphite on a conducting plate ( the conducting plate was made of glass plate on which a conducting layer of indium or fluoride doped tin oxide is deposited) The anode is also prepared by coating a thin layer of TiO2 on conducting plate This plate is then immersed in a dye sensitizer The anode and cathode are then joined and sealed with a redox electrolyte inside

Fig 1.3: The structure of a dye sensitized solar cells [18].

The working principle of DSSCs is shown in Fig 1.4

 When sunlight shines on the cell, photosentizer absorbs photon and electrons are excited from ground level D to excited state D*

These excited electrons are moved to conduction band of TiO2 by oxidizing the dye sensitizer D+:

The limitation of these solar cells are using liquid electrolytes, which cause term stability because of organic solvent evaporation and leakage, difficulty in sealing device and electrode corrosion.

short-Organic or polymer solar cells

These solar cells use thin films ( ~100 nm) of organic semiconductors including polymers such as copper phthalocyanin, polyphenlylene vinylene, carbon fullerene derivatives The basic structure of a organic or polymer cell is described in Fig 1.5The active layer include electron acceptor and electron donor materials, they are sandwiched between two metallic conductors When the semiconductor material absorbs a photon, a electron-hole pair is formed The charge trends to remain bound in the form of a exciton and is separated when the exciton diffuses to the donor-acceptor interface The advantages of these solar cells : absorber layers are materials having the quite high optical absorption coefficient Besides, the organic/polymer solar cells are inexpensive, light weight and flexible The disadvantages of organic/polymer solar cells : the efficiencies of these cells are relatively low ( around 11% ) [14] and they have low stability

Fig 1.5: Basic structure or Organic Solar Cell [20]

Quantum dot solar cells

Quantum dot solar cells includes Schottky CQD solar cells and bulk heterojunction QD solar cells

- Schottky CQD solar cells: the quantum dot films could act as both absorbers and as the charge transport medium These solar cells work relying on illumination through a transparent ohmic contact to a p-type CQD film which formed rectifying junction with a Shallow work function metal and conversion efficiencies of cells reached over 3 %

- Bulk heterojunction QD solar cells: Different to Schottky CQD solar cells, these cells use a monolayer of molecular absorbers on a wide band gap semiconductor matrix, the depleted heterojunction architecture applies a highly doped n-type metal oxide in a p-n heterojunction with a p-type CQD film The CQD films have 50-400 nm thick

Perovskite solar cells

Perovskite solar cells are a new family of solar cells, belongs to the third generation The general formular of perovskite is ABX3, where A is monovalent, B is divalent ions and X is either O, C, CN or a halogen CH3NH3PbI3, CH3NH3PbBr3 are the most common perovskites applied for solar cells

Perovskite solar cells are studied basing on dye-sensitized solar cells Therefore, the solar cell structural of perovskite solar cell is similar to DSSCs, but absorber layer in perovskite solar cells is replaced by CH3NH3PbI3, CH3NH3PbBr3 compound instead of Dyes as in DSSSc

1.2.2 The operating principle of solar cell

- When sunlight shines on the cell, photon in sunlight hit the solar panel and are absorbed by semiconducting materials

- When a photon of light is absorbed by one of these atoms in the N-yype it will dislodge an electron, creating a free electron and a hole The free electron and hole has sufficient energy to jump out of the depletion zone

- If a wire is connected from the cathode (N-type) to the anode (P-type) electrons will flow through the wire

- The electron is attracted to the positive charge of the P-type material and travels through the external load creating a flow of electric current

- The hole created by the dislodged electron is attracted to the negative charge of type material and migrates to the back electrical contact

N As the electron enters the PN type from the back electrical contact it combines with the hole restoring the electrical neutrality

Fig 1.6: The operating principle of solar cell [21]

1.3 Thin film deposition techniques

A wide variety of deposition techniques have been applied to make CIGSe thin film These could be sorted by two main methods: The vacuum method and the non-vacuum method

1.3.1 The vacuum method

1.3.1.1 Co-evaporation

The simpler, vacuum evaporation method, is shown in the figure 1.7 The system operates under a high vacuum (10-6 torr or better) Material from the evaporation source is converted into the gaseous phase by heating or electron bombardment, and gaseous material then deposits on the substrate and its surroundings as a film Various substrate materials are used, depending on the subsequent application of the film that is to be deposited

Fig 1.7: Schematic of evaporation system

The one-stage process

Roughing Pump

This is the simplest process for preparation of CIGSe absorber layer All elements are deposited simultaneously from separated sources at constant evaporation rates and substrate temperature (Fig 1.8) That is a reason lead to the film is always Cu poor which grown from this process possessed columnal grains typically less than 1 µm wide and the grains sizes trend to be smaller near the back contact

The two-stage process

The two-stage process has been originally studied by Mickelsen and Chen [22,23]

In this deposition process, the bulks of the film were grown from a Cu-rich precursor layer at a substrate temperature in the range of 400-450 oC (stage 1), followed by a Cu-poor precursor layer deposited at 500-550 oC (stage 2) until the overall composition is copper deficient [24]

Films grown by the two-stage process have larger grain sizes in comparirion with the single stage process It is explained that a Cu-rich precursor layer deposited in the first stage formed CIGSe quaternary and CuxSe binary impurities, which were assumed to

be liquid at high temperature (~ 500 oC) Therefore, the film containing CIGSe and

grain size in film

The 3-stage process

This is a sequential process first proposed by Kessler et al [25] The CIGSe solar cells reaching the highest efficiency to date were fabricated with absorber layers grown with the three-stage process This process was studied by Kessler consisting of three stages:

- Stage 1: An (In,Ga)xSey precursor layer is deposited at low substrate temperature of about 330 oC

- Stage 2: Copper and Selenium elements are co-evaporated at a higher substrate temperature of around 500-560 oC until a Cu/In+Ga ratio of 1.15 at the endpoint to extend the grain size The increased substrate temperature load

to an alloying of the element and the formation of the chalcopyrite phase

- Stage 3: a small amount of gallium, indium and selenium are co-evaporated at the same substrate temperature of stage 2 until the required overall copper deficiency of the film ( Cu/In+Ga ~ 0.84-0.88) is obtained

The absorber layers prepared with this process have a smooth surface and more crystallinity

Fig 1 8: Schematic illustration of different coevaporation recipes[26]

1.3.1.2 Sputtering (two-step process)

Sputtering is a common physical vapor deposition used by manufacturers of semiconductors This technique is possible to produce alloys of precise composition Sputtered

films exhibit excellent uniformity, density, purity and adhesion Sputtering method was first applied to prepare CuInSe2 thin film by Chu et al [27] However, only the metals were deposited

by sputtering while selenium elements were incorporated in the second step Therefore, this process is of them called the two-step process

The operation: The apparatus used for sputtering is outlined in the figure Basically, it consists of a bell jar which contains a reduced pressure-10-1 to 10-2 torr-of an inert gas, argon or xenon This gas is subjected to a potential drop of several kilovolts creating a glow discharge from which positive ions are accelerated towards the cathode(target) These high energy ions remove material from the cathode which then condenses on the surroundings, including the substrates to be coated, which are placed in a suitable position relative to the cathode The mechanism of sputtering or removal of material from the cathode, involves the transfer of momentum from the gaseous ions to the cathode in such a way that atoms or ions are ejected from the cathode

Fig 1.9: Schematic of sputtering system[28]

1.3.1.3 Chemical vapor deposition (CVD)

Chemical vapor deposition is a chemical process used to produce high quality, high

performance, solid materials The process is often used in the semiconductor industry to produce thin films

The CVD method is developing into an extremely important way of making high purity thin films and coatings for industrial applications, especially in electronics, as well as for fundamental scientific research Conceptually, it is simple; precursor molecules containing the elements of interest are decomposed in the gas phase and the products deposit as thin films on every available object in the vicinity

There are various acronyms used to describe variations on the CVD technique; a commonly-used one is MOCVD which refers to the metal-organic nature of the precursors In many applications, such as the fabrication of multilayer semiconductor devices, it is necessary for the deposited films to have the correct structural orientation and to be coherent with the underlying layer Hence vapor phase epitaxy is essential in the growth mechanism

1.3.2 The non-vacuum method

Fig 1.10: Schematic of chemical vapor deposition system

Fig 1.11: Schematic of chemical vapor deposition system

1.3.2.1 The nanoparticles based method

The characteristic of this method is that the films are made from the nanoparticles containing ink These nanoparticles were synthesized in a isolated step and dispersed then

in a suitable solvent to form ink This ink is coated onto substrate by various methods such as spin-coating, dip casting, doctor blade, chemical spray, drop casting…

Some main method to synthesize nanoparticles as following:

- Hot inject and heat up methods: The both methods use organic ligands to control the nucleation and growth kinetics and they act as capping agents to provide colloidal stability However, In the hot inject method, the purpose is

to separate nucleation from growth Many articles reported about synthesis of nanoparticles by this method such as: Tang et al [29] used oleylamine solution of selenium element at 80 oC to inject into other precursors Ahmadi

et al.[30] used hexadecyleamine as coordinating solvent to synthesize CIGSe nanoparticles Li et al [31] synthesized CISe nanoparticles using dedecylthiol solvent as organic ligand and trioctylphosphine (TOP) as the solvent to dissolve selenium element nuclei of CuInSe2 appeared immediately after selenium solution injected, etc In the heat up method, the gradual heating of all precursor results in simultaneous nucleation, growth leading to form more polydisperse nanoparticles Similar to hot inject method, a variety of nanoparticles were synthesized by this approach: Chiang et al.[32] used oleylamine as a capping agent to synthesize CIGSe nanoparticles at the gram scale In 2014, Singh et al [33] report a large scale synthesis of CIGSe nanocrystals by heat up method using dodecylamine as a coordinating solvent

- Solvothermal: This method associates with the heating of solution containing precursors at high temperature and pressure during which the equilibrium is varied with temperature The solution containing precursors is heated in a autoclave ( a sealed vessel) in which the pressure and temperature exceed the atmostphere pressure and the boiling point of the solvent Some reports relate

to this method such as: Chun et al [34] synthesized CIGS nanoparticles at a reaction temperature of 280 oC for 36 h Li et al [35] heated a solution consisting of ethylendiamine and precursors ( Se, CuCl2, InCl3) in autoclave

at 180 oC for 15 h to obtain CISe nanocrystals Gu et al.[36] conducted experiment at a reaction temperature of 230 oC for 24 h through a solvothermal method to synthesized CIGSe nanoparticles, etc

- Hydrothermal method: this method is similar to sovothermal method However, the hydrothermal method uses aqueous solution containing precursors to synthesize nanocrystals Wu et al.[37] synthesized CuInSe2 in

2011 at 180oC for 1 h Shim et al synthesized CuInSe2 at 200oC for 12 h using acetic acid as a mineralizer [38] Ramkumar et al synthesized CuInSe2

at 150oC for 2 h using ethylendiamine as a capping agent [39]

- Polyol method: this approach was studied in the 1980s by Fievet et al [40] for preparation of micron and submicron size metal particles This method involves the preparation of metallic powders by reduction of inorganic compounds in liquid polyols ( ethylene glycol, diethylene glycol, triethylene glycol…) The role of these polyols: at first, it acts as a solvent to dissolve the solid precursors due to its rather high dielectric constant, then reduce the

metallic cation to the metallic form in the liquid phase, the homogeneous nucleation and growth of the metallic phase perform in this stage Palchick et

al [41] employ this method to synthesize CuInSe2 nanoparticles in triethylene glycol Wu et al synthesized CIGSe in tetra ethylene glycol at 280oC [42]

- Sonochemical method: This technical is considered as suitable for preparations of nanoparticles This method relies on a acoustic cavitation phenomenon, i.e the formation, growth, and implosive collapse of a vast amount of vapour cavities in a liquid under ultrasound irradiation leading to form localized hot spots where the reaction could take place Cha et al [43]

utilized this approach to synthesize CIGSe nanoparticles using toxic hydrazine solvent Badgujar et al successfully synthesized CuIn0.7Ga0.3Se2 nanoparticles by sonochemical reaction of relatively non-toxic aqueous metal salt precursors with selenourea [44]

1.3.2.2 Direct solution based process

Different from nanoparticles based process, the thin films made from this process are directly deposited by a nanoparticles free solution which contains metal salts, metal chalcogenides or metal organic precursors Chalcogens are either present in this solution or supplied in the post-annealing step The direct solution based process include various approachs such as electrodeposition, chemical spray pyrolosis, doctor blade method,

1.4 Physical methods for characterizing solids

There are many physical methods for investigating the structures of solids In this dissertation, some methods are described:

1.4.1 X-ray diffraction

X-ray diffraction is able to determine the precise atomic positions and therefore the bond lengths and angles of molecules within a single crystal are identified X-ray diffraction has its limitations and only gives an overall, average picture of a structure; it cannot usually identify localized defects or define the positions of small quantities of dopants However, when it is possible to use X-ray diffraction, it is extremely powerful; the results are very accurate, giving bond lengths to a few tens of picometres

 Generation of X-rays

In 1985, the German physicist Wilhelm Röntgen discovered X-rays, and he was awarded the first Nobel Prize in Physics about this invention in 1901

The generation of X-ray is described as shown as Fig 1.11 A tungsten filament

is electrically heated, emitting electrons which are accelerated by a high potential difference (20-50 kV) to strike a anode (metal target) which is water cooled The electrons from the inermost K shell of the metal target are knocked out by these bombarding electrons creating vacancies which are filled by electrons descending from the shells above The decrease in energy leads to appear radiation Therefore, the anode

emits a continous spectrum of white X-radiation, in which the intense X-ray peaks are

Kα, Kβ lines (Fig 1.12) Electrons descend from the L shell giving the Kα line, and electrons from the M shell giving the Kβ line The frequencies of the Kα and Kβ lines are

at 154.18 pm and 71.07 pm, respectively

Fig 1.11: Schematic illustration of X-ray generation system[46]

Fig 1.12: (a) Section through an X-ray tube (b) An X-ray emission spectrum[46]

 Diffraction of X-ray

In 1912, Maxvon Laue used a crystal of copper sulfate as the diffraction grating which brang him the Nobel Prize in Physics in 1914 This discovery has been taken note by W.H and W.L Bragg They conducted experiments on using X-ray crystal diffraction as a way to determine structure of crystals, and the crystal structure of NaCl was first determined in 1913 W.L Bragg noted that X-ray diffraction behaves like a ―reflection‖ from the planes of the atoms within the crystal and that only at specific orientations of the crystal with respect to the source and detector are X-rays ―reflected‖ from the planes

There are three X-ray diffraction methods:

Laue method:

 transmission Laue method: X-ray pass through the crystal and scattering

Film for record the data set behind of the crystal Some scattering beam transmit the crystal

 Back-reflection Laue method: Film that have tiny hole set in front of the

crystal X-ray pass through tiny hole on the film and diffracted behind of the crystal It came back to film and film record the data

Rotating-crystal method: Setting the crystal vertical with X-ray, Place the cylindrical film around the crystal and rotate it with setting axis Setting the axis of crystal and film become same

Powder method: Make minute size of sample and radiate X-ray It is same as single crystal state that rotating every axis

1.4.2 Spectroscopy

X-ray photoelectron spectroscopy (XPS):

XPS is known as electron spectroscopy for chemical analysis (ESCA) This technique is widely used for analyzing the surface chemistry of materials It can

determine the elemental composition , empirical formular, chemical state and electronic state of the elements within a material

XPS spectra are recorded by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy and number of electrons that emitted from the top 1-10 nm of material being analyzed A photoelectron spectrum is recorded by counting ejected electrons over a range of electron kinetic energies Peaks appear in the spectrum from atoms emitting electrons of a particular characteristic energy The energies and intensities of the photoelectron peaks enable identification and quantification of all surface elements ( except Hydro)

Fig 1.13: Basic components of a monochromatic XPS system [47]

Raman spectroscopy

Raman spectroscopy is widely applied to determine vibrational modes of modecules, therefore moleculars can be identified This technique based on inelastic scattering of monochromatic light, usually from a laser source

Operating principle: When a beam of light (laser source) hits a sample, photons are absorbed by the sample and scattered The majority of the scattered light has the frequency as the incident light and are called as Rayleigh scattering (elastic scattering) However, a tiny of the light scatters at the frequency different from the incident light and are known as Raman scattering ( inelastic scattering ) The shift in frequency between the incident light and Raman scattered light is equal to the frequency of a vibration of the scattering molecular, and hence it gives the information about the vibrational structure of the substance Therefore, Raman scattered lights are collected and then recorded by a detector In the contrary, The Rayleigh scattered lights are filtered out by either a notch filter or a band pass filter

Fig 1.14: Schematic illustration of a raman spectroscopy system[48]

Electron microscopy

Electron microscopy has been widely applied for characterizing solids to study morphology, structure and size, to examine defects and to determine the distribution of elements

The basic principle of the electron microscope is that a beam of accelerated electrons is used instead of visible light Because of the wave particle duality of electrons, they behave like electromagnetic radiation and, at high energies, have very short wavelength (λ)

The electron beam is produced by heating a tungsten filament, a lanthanum hexaboride (LnB6) crystal or from a field emission gun (FEG), which uses a cathode of either tungsten or zirconium oxide The beam is focused by magnetic coil magnets in a high vacuum to a fine spot Detection can be by scintillation counter, film or CCD

Here, I introduce 2 techniques: Scanning electron microscopy and transmission electron microscopy

- Scanning electron microscopy:

This technique is applied to describe a map of surface topography of materials such as catalysts, minerals and polymer It is useful for looking at particle size, crystal morphology, magnetic domains, and surface defects

SEM work as shematic diagram Fig 1.15 Electrons are produced at high voltage

by a electron gun in vacuum chamber, accelerated down passed through a combination of lenses and aperture to generate a finely focused beam of electrons which impact on the atoms of the sample’s surface As these electrons interact with the sample, they produce back scattered electrons (the reflection of electrons by elastic scattering), the secondary electrons from inelastic scattering, characteristic X-

ray (the production of secondary electrons) An SEM image is mainly recorded from the secondary electrons which collected by detector The number of electrons determines the brightness of the image The samples should be coated with platinium

or similar substance, electrical thing to prevent charge building up on the surface

Fig 1.15: Schematic illustration of scanning electron microscopy system [49]

- Transmission electron microscopy

Transmission electron microscopy is a powerful tool for material science, it is applied to observe the features such as structure, size and morphology of very small specimens It was first developed by German Scientists Max Knoll and Ernst Ruska

in 1931 and has evolved over the years to become a common technique that is used

globally in science and engineering to look at micro and nanoparticles This technique can be used to observe samples at a much higher magnification and resolution than can be achieved with a light microscope because wavelength of an electrons is much smaller than that of light It also provides higher resolution images than a scanning electron microscopy due to a very high energy beam of electrons passed through a very thin specimen

The operating principle: The high energy electrons are generated at a potential in the range 100-300 KV, accelerated to two condenser lenses to form a beam of electrons This high energy beam of electrons then passes through the ultrathin sample, some of the electrons hit the atoms at the surface and are deflected (the deflected electrons may be elastically or inelastically scattered), whereas those which transmit through the sample are collected by objective lense and projector lense onto

a detector to produce images

Fig 1.16: Schematic illustration of transmission electron microscopy [50]

Thermal analysis

- Differential thermal analysis

When a substance changes phase, it will produce either an absorption or

an evolution of heat, and the differential thermal analysis technique works basing on this theory The difference in temperature between the sample and

a reference material is monitored and recorded against time

The sample is placed in one chamber, and the reference sample that doesn’t change phase over the temperature range of the setted temperature program is placed in another chamber Both chambers are placed

symmetrically in a furnace The furnace is controlled under a temperature program If the sample has exothermic reactions or endothermic reaction, the difference in temperature between the sample and reference sample will appear

- Differential scanning calorimetry

This technique is applied to determine the amount of heat released

by the sample when the temperature is increased or decreased at a controlled temperature program

Briefly, the sample and reference are placed into separate chambers with an equal amount Each chamber is heated by a separate source in a way that their temperatures are always equal This is accomplished through the use of thermocouples The temperature of each chamber is constantly monitored and if a temperature difference is detected, then amount of heat will be added to the cooler chamber to

compensate for the difference Amount of heat used to maintain equivalent temperatures is recorded as a function with respect to the temperature

Fig 1.18: Schematic illustration of differential scanning calorimetry system[52]