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Dissertations Graduate College

12-2012

Gravure Printability of Indium Tin Oxide Nanoparticles on Glass Gravure Printability of Indium Tin Oxide Nanoparticles on Glass

and PET Films for Applications in Printed Electronics and PET Films for Applications in Printed Electronics

Dania Awni Alsaid Western Michigan University, dania_hasan1@yahoo.com

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Recommended Citation Recommended Citation Alsaid, Dania Awni, "Gravure Printability of Indium Tin Oxide Nanoparticles on Glass and PET Films for Applications in Printed Electronics" (2012). Dissertations. 95. https://scholarworks.wmich.edu/dissertations/95

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GRAVURE PRINTABILITY OF INDIUM TIN OXIDE NANOPARTICLES ON GLASS AND PET FILMS FOR APPLICATIONS

IN PRINTED ELECTRONICS

by

Dania Awni Alsaid

A Dissertation Submitted to the

Faculty of The Graduate College in partial fulfillment of the

requirements for the Degree of Doctor of Philosophy

Department of Paper Engineering, Chemical Engineering, and Imaging Advisor: Margaret Joyce, Ph.D.

Western Michigan University Kalamazoo, Michigan

December 2012

GRAVURE PRINTABILITY OF INDIUM TIN OXIDE NANOPARTICLES ON GLASS AND PET FILMS FOR APPLICATIONS

IN PRINTED ELECTRONICS

Dania Awni Alsaid, Ph.D.

Western Michigan University, 2012

The advancements in the field of solution processable electro-active materials and

their ability to be printed on different substrates have led to the evolution of printed

electronics. In this field, electronic components are manufactured with conventional

printing methods. Transparent electrodes made from indium tin oxide (ITO) are part of

many electronic devices. Currently in industry, highly conductive ITO films are prepared

by sputtering. The sputtering and then patterning of ITO films is a sophisticated process

that consumes high energy, generates waste and produces films with limited flexibility.

Therefore, there is a need to investigate processing methods for creating ITO films other

than sputtering. Gravure printing is an excellent option for printing the ITO nanoparticles.

However, very little research has been done to study the gravure printing process for

producing ITO films or the properties of the films after printing and sintering.

First part of this work investigates gravure printability of ITO nanoparticles based

coating on polyethelene terephthalate (PET). A wide range of sheet resistivities and film

thicknesses were obtained by varying the cell diameter and the aspect ratios (AR) of the

engraved cells of the gravure cylinder. The printed films were found to be highly flexible

in comparison to commercially available sputtered ITO films on PET. However, due to

the polymeric binder in the ITO coating, the printed layers were of high resistance. This

finding would limit the possible application of such inks to antistatic and electromagnetic

shielding applications.

The second part of this work summarizes the gravure and inkjet printing of ITO

nanoparticles dispersion without polymeric binder on glass. The printed films were

sintered at high temperature and the results were compared to samples that were post-

processed with a photonic sintering system. The electrical performance, transmission and

surface roughness of the printed ITO films were analyzed. The ability to use photonic

sintering can improve the processing efficiency of these films. Time and energy can be

saved and the process is suitable for roll-to-roll printing. Through this work, the gravure

printing process is shown to hold promise as a new manufacturing process for ITO films.

© 2012 Dania Awni Alsaid

ii

ACKNOWLEDGMENTS

All the praise to God for his guidance in my graduate school and all the bounties

he gave me in this life. My gratitude goes to my beloved parents who raised me with the

love for science, and emphasizing on the importance of education. I can see in them the

true meaning of “unconditional” love. Many thanks to my sisters, Ruba, Lubna, Maram

and my brother Mohammad for their encouragement and the unlimited support. I would

like to express my warmest gratitude and special thanks to my supportive and patient

husband, Hasan, for the confidence he has in me and for all the love. To my two beautiful

children, Jude and Ahmad, it is because of you that I completed this degree. I love you

more than words can say.

My sincere appreciation goes to my advisor, Dr. Margaret Joyce, for her

supervision and assistance during my research. The joy and enthusiasm she has for her

research is a great example she has provided of how successful women in engineering

can be. I learned a lot from the courses you taught me and loved working with you.

I would like to thank the members of my sponsoring committee; Dr. Erika

Rebrosova for training me on the many different instruments in the labs, and for the

advice and sharing her knowledge and expertise in printing., Dr. Marian Rebros for

having answers to my question especially for inkjet and screen printing; Dr. Massood

Atashbar for serving in my committee regardless of his busy schedule. Your all support is

appreciated.

ii

Acknowledgments—Continued

I gratefully acknowledge the PCI department and the chair Dr. Said Abubakr for

the funding sources that made my Ph.D. work possible either as a RA with Dr. Margaret

Joyce or as a TA. I owe special thanks to Dr. Andy Kline for the respect and the trust he

had in me while I was his TA for many years.

Special thanks to Dr. Svell Hill and Maria Nargiello from Evonik Industries for

the technical support and material supply during the work in this dissertation.

To the faculty whom I had classes with, or the opportunity to work with or know

during the last six years, to all my colleagues and friends at WMU, thanks for the kind

memories.

For all my family, relatives and friends who supported me in all my pursuits, I

love you all.

Dania Awni Alsaid

iii

TABLE OF CONTENTS

ACKNOWLEDGMENTS ...................................................................................... ii

LIST OF TABLES .................................................................................................. vii

LIST OF FIGURES ................................................................................................ viii

CHAPTER

1. INTRODUCTION ...................................................................................... 1

2. LITERATURE REVIEW ........................................................................... 3

2.1. Printed Electronics ....................................................................... 3

2.2. Transparent Electrodes in Electronic Devices ............................. 4

2.2.1. Requirements of Transparent Electrodes ...................... 4

2.2.2. Transparent Conductive Oxides .................................... 5

2.2.3. ITO as A Transparent Electrode ................................... 6

2.2.3.1. Advantages of ITO ........................................... 6

2.2.3.2. Sputtering of ITO ............................................. 8

2.2.3.3. ITO on Polymer Substrates .............................. 10

2.2.3.4. Deposition of ITO Nanoparticles ..................... 11

2.2.3.5. Gravure Printing of ITO Nanoparticles ........... 14

2.2.4. Photonic Sintering of Nanoparticles ................................. 16

3. PROBLEM STATEMENT .............................................................................. 19

iv

Table of Contents- Continued

CHAPTER

4. EVALUATION OF ITO NANOPARTICLES FOR THE COATING AND GRAVURE PRINTING ON PET .................................. 21

4.1 Abstract ........................................................................................ 21

4.2. Experimental ................................................................................ 22

4.2.1. ITO Nanoparticles Based Coating ..................................... 22

4.2.2. Deposition of ITO Nanoparticles ....................................... 22

4.2.2.1. Bar Coating ...................................................... 22

4.2.2.2. Gravure Printing............................................... 23

4.2.3. Design of Gravure Plates ................................................... 24

4.2.4. Drying Conditions .............................................................. 25

4.2.5. Substrates .......................................................................... 25

4.2.6. Scratch Resistance Test...................................................... 26

4.2.7. Humidity Control ............................................................... 26

4.2.8. Electrical Testing ............................................................... 26

4.2.9. Optical Transmission ......................................................... 27

4.2.10. Mechanical Assessment of Printed and Sputtered ITO Films ................................................................................ 28

4.2.11. Roughness and Surface Topography ............................... 29

4.3. Results and Discussion ................................................................ 30

4.3.1. Evaluation of Coated LTH2 ITO Based Coating on PET and Glass ........................................................................... 30

v

Table of Contents- Continued

CHAPTER

4.3.2. Gravure Printing of LTH2 ITO for Application in Flexible Electronics ......................................................... 38

4.4. Conclusion .................................................................................... 47

5. GRAVURE PRINTING OF ITO ON GLASS AND PHOTONIC SINTERING OF PRINTED ITO NANOPARTICLES .............................. 48

5.1. Abstract ........................................................................................ 48

5.2 Experimental ................................................................................ 49

5.2.1. ITO Nanoparticles Based Dispersion................................. 49

5.2.2. Gravure Printing................................................................. 49

5.2.3. UV Ozone (UVO) Cleaning............................................... 50

5.2.4. Contact Angle and Surface Energy Measurements .......... 51

5.2.5. Sintering Conditions .......................................................... 52

5.2.6. Roughness and Surface Topography ................................. 56

5.3. Results and Discussions ................................................................ 58

5.4. Conclusion .................................................................................... 70

6. EVALUATION OF ITO NANOPARTICLESFOR PRINTABILITY WITH THE INKJET PRINTER ................................................................. 72

6.1. Abstract ........................................................................................ 72

6.2. Introduction .................................................................................. 73

6.3. Experimental ................................................................................ 76

6.3.1. ITO Nanoparticles Based Dispersion................................. 76

vi

Table of Contents- Continued

CHAPTER

6.3.2. Inkjet Printing .................................................................... 76

6.3.3. Ink Rheology ...................................................................... 77

6.4. Results and Discussions ............................................................... 78

6.5. Conclusion ................................................................................... 90

REFERENCES ....................................................................................................... 91

APPENDICES

A. The Solar Spectral Irradiance ......................................................................... 109

B. Heat Capacity of ITO ..................................................................................... 117

vii

LIST OF TABLES

5-1. Physical and thermal properties of ITO, glass and PET ............................. 65

6-1. Physical properties of the HBS ................................................................... 80

6-2. Fluid properties and Z Number for ITO dispersion .................................... 90

viii

LIST OF FIGURES

2-1. Sheet resistivity of various materials .......................................................... 7

2-2. Illustration of the principle of sputtering vacuum deposition ..................... 8

2-3. Steps of patterning sputtered ITO films ...................................................... 9

2-4. Gravure printing process ............................................................................. 15

4-1. Gravure K-printing proofer ......................................................................... 23

4-2. Comparison of the cross-section for cells with 120 µm diameter for the three AR .................................................................................................... 24

4-3. Engraved areas with different cell diameters at AR=0.16 and 145º engraving diamond...................................................................................... 25

4-4. Schematic of the four-point probe technique .............................................. 27

4-5. Illustration of the basic principle of the spectrophotometer ....................... 28

4-6. Schematic and actual display for mechanical assessment .......................... 29

4-7. Principle of vertical scanning interferometry ............................................. 30

4-8. Sheet resistivity of ITO coatings on glass at different wet film thicknesses ................................................................................................. 31 4-9. Transmission spectrum of ITO coated films on glass at different film thicknesses ................................................................................................. 32

4-10. Effect of drying conditions on sheet resistivity of ITO coatings on glass .. 33

4-11. Dry film thickness of ITO coatings on glass .............................................. 34

4-12. 3-D plot of the coated films on glass with VSI ........................................... 34

4-13. The effect of RH on resistance of ITO coatings on glass ........................... 35

4-14. The effect of RH on resistance of ITO coatings on PET ............................ 36

ix

List of Figures-Continued

4-15. The effect of temperature on the resistance of ITO coatings on glass ........ 36

4-16. The effect of temperature on the resistance of ITO coatings on PET ......... 37

4-17. Scratches of ITO coating on glass and PET ................................................ 38

4-18. Sheet resistivity of gravure printed ITO films at different cell diameters and AR ....................................................................................................... 39

4-19. Thickness of ITO printed films at different cell diameters and AR ........... 40

4-20. 2-D images of the edges of printed ITO films for 100 µm cell diameter at AR=0.26 and AR=0.38 and the cross section of each film ...... 41

4-21. Transmission of LTH2 coating at different sheet resistivities .................... 43

4-22. Transmission spectra of PET, printed ITO, and sputtered ITO .................. 44

4-23. The effect of bending on sheet resistivity of sputtered and printed ITO films ............................................................................................................ 46

4-24. Surface topography of ITO films before and during bending..................... 46

5-1. The AccuPress MicroGravure Printing system at WMU ........................... 50

5-2. The engraved cylinder with the four solid areas and detail of engraved cells at different resolutions in lpi ............................................................... 50 5-3. Equilibrium sessile drop ............................................................................. 52

5-4. The Sinteron 2000, Xenon Corporation ...................................................... 54

5-5. Typical spectrum for type C lamp for Xenon Flash system ....................... 54

5-6. Pulse width with different PFN sets, based on datasets given in the system’s manual .......................................................................................... 55 5-7. Pulse energy with different PFN sets, based on datasets given in the system’s manual .......................................................................................... 56 5-8. The principle of an AFM ............................................................................ 58

x

List of Figures-Continued

5-9. The effect of UV Ozone treatment on the surface energy of glass ............. 59 5-10. Contact angle of ITO on glass before and after UV Ozone treatment on glass............................................................................................................. 59

5-11. The effect of UVO treatment on the contact angle of ITO on glass ........... 60

5-12. Thickness of printed ITO films at different engraving resolutions............. 61

5-13. Sheet resistivity of printed ITO films at different sintering methods ......... 62

5-14. Transmission spectra of printed ITO films on glass (with the base substrates) ............................................................................ 67

5-15. 3-D surface topography of ITO at high temperature and photonic sintering with VSI ....................................................................................... 68

5-16. 2-D surface topography of ITO at high temperature and photonic sintering with VSI ....................................................................................... 69

5-17. Surface topography of printed ITO film sintered at 500 °C with AFM at (10X10) µm2 and (5X5) µm2 .................................................................. 70

6-1. A piezoelectric nozzle .................................................................................. 74

6-2. The DMP and ink cartridge ......................................................................... 77

6-3. Viscosity curve for ITO nanoparticles ........................................................ 79

6-4. Viscosity curves for the HBS ...................................................................... 80

6-5. Rheological properties of PGDA dilutions ................................................. 82

6-6. Non-uniform ink coverage with 30% PGDA at 813 dpi............................. 82

6-7. Wetting of the nozzle’s plate and misdirection of drops with PGDA dilutions ...................................................................................................... 83

6-8. Printed 100 µm lines with 50% PGDA dilution at different resolutions .... 83

6-9. Droplet Formation with 40% ITO dilution ................................................. 84

xi

List of Figures-Continued

6-10. Firing wave forms and images of printed lines at 950 dpi .......................... 85

6-11. Droplet formation with 40% (Butanediol, PGDA, IPA) dilution ............... 86

6-12. Comparison between the rheological properties of the ITO dilution and the silver ink ................................................................................................ 87 6-13. Images of printed features with Butanediol, PGDA and IPA dilution ....... 87

6-14. Effect of filtration on the rheological properties......................................... 88

6-15. Carreau model for the original ITO dispersion ........................................... 89

1

CHAPTER 1

INTRODUCTION

Conventional printing processes are used in printed electronics to deposit

functional inks to create electrical components and devices on various substrates.

Transparent electrodes (TE) can be one layer in many electronic applications such as in

solar cells and light emitting diodes (LED).

TE made from metal oxides (TCO) were shown to have excellent electrical and

optical properties. ITO is the most widely used TCO in industrial manufacturing of

transparent conductive films. Many processes are used to produce ITO films, but

sputtering is the most dominant. Sputtering produces highly dense ITO films over large

areas and therefore subsequent structuring, or patterning, is necessary. Most typically, it

is a batch process involving multiple processing steps some of which require

sophisticated equipments and consume considerable amount of energy and time. In

addition, the patterning of ITO is a subtractive process with considerable material waste.

While the additive nature of printing enables the direct deposition and patterning of

materials, thereby offering the advantage of low cost through roll-to-roll processing under

ambient conditions with minimal waste.

Gravure printing is a promising method for the mass production of printed

electronics. It is one of the fastest (up to 3000 ft/min) and highest quality printing method

capable of attaining high resolutions and registration accuracy for relatively low-viscosity

2

inks. It is also known to be advantageous for its high production capacity, stability over

time and the resistance of gravure printing cylinders to aggressive solvents that are used

in some functional inks. In order to print ITO films, the nanoparticles should be dispersed

in a solvent system capable of forming smooth and thin films. The nanoparticles offer

high optical transparency resulting from the low scattering of light.

The objective of this research is to investigate the gravure printability of ITO

nanoparticles and the performance of the printed layers in terms of conductivity,

transmission and roughness. The research is divided into two main parts; Printability of

ITO nanoparticles in a fully formulated coating with low temperature processing on PET

for flexible applications, and the printability of an ITO nanoparticles dispersion on glass

with high temperature and photonic sintering for applications that require high

conductivities. The results showed the possibility of gravure printing ITO nanoparticles

on PET and glass to gain the advantage of direct patterning of the films. When printed on

PET, the ITO films exhibited a high flexibility and high transmission. When printed on

glass, the resulting films were of high conductivity, transmission and smoothness. In

conclusion, the gravure printing of ITO nanoparticles shows promise as a process for the

future replacement of sputtered ITO films.

3

CHAPTER 2

LITERATURE REVIEW

2.1. Printed Electronics

Advancements in the field of active materials and their ability to be printed on

different substrates have led to the evolution of printed electronics. The combination of

the two worlds, electronics and printing, shows incredible promise towards the

development of new methods for the manufacture of electronics, as well as novel

applications. Examples of electronic components and applications that are potentially

compatible with low-cost printing techniques include: organic field effect transistors

(OFET)1, organic thin film transistors (OTFT)2, integrated circuits3,4, capacitors,

photovoltaic devices (PV)5,6,7,8, organic light emitting diodes (OLED), electronic paper,

RFID tags, and more9,10. As the research interests in the area of organic and inorganic

active materials for printed electronics continue to grow, the need for formulated

“functional inks’ increases. These inks can be used in low cost roll-to-roll printing

processes such as flexography, gravure, offset, screen and ink jet printing11. Another

advantage of printed electronics is the ability to fabricate electronic devices on flexible

light weight substrates like plastic films, paper, metal foils or thin glass without losing the

functionality of the devices, which gives them the term “flexible electronics”.

Although graphic printing has been around for many years and is very well

developed, the printing of electronic materials is still quite new, especially when it comes

4

to the use of the gravure, flexography and inkjet printing processes. For printed

electronics, the visual output is not as important as in conventional printing, while the

electronic properties become most important. Solvent choice in functional inks,

wettability between the ink and substrate, and compatibility between the printed layers to

prevent dissolution of one layer with another are critical parameters for printed

electronics. Layer thickness, absence of pin-holes, uniformity of the layer, resolution of

the printed features and registration are crucial in to the functionality of a printed

electronic device11.

2.2. Transparent Electrodes in Electronic Devices

2.2.1. Requirements of Transparent Electrodes

TE are important part of many electronic devices12.Their primary application is as

a thin film on glass or plastics in touch panels displays, liquid crystal displays (LCD),

plasma displays and flat panel displays13,14. They are part of televisions, computer

screens, almost every cell phone, solar cells, biosensors15, and gas sensors16. They are

used in energy-efficient windows (functional glass window for thermal management in

architectural and automotive industry)17, and in smart window applications18,19.

TE must be highly conductive while allowing light to enter the device with

minimum light absorption and reflection20,21. In addition, they have to be

electrochemically stable, and should not cause any degradation of the active materials in

the device22.

5

A transparent electrode in a device provides the electrical contact between the

active layer and the outside world. For an example, in a PV device, the anode collects the

holes and the cathode collects the electrons. The anode is the transparent electrode that

allows the light to enter the device with minimum light absorption and reflection. The

choice of the best electrode material is determined by its electrical resistance, work

function and PV configuration. In addition, the interaction between the semiconductor

and the electrode should be considered when choosing the contact material since it can

affect the open circuit voltage and hence the PV performance20.

2.2.2. Transparent Conductive Oxides

TCO were shown to have excellent electrical and optical properties in about

196523. The first work seems to have been on cadmium (Cd) oxide. It was later followed

by investigations of ITO. A specific advantage of TCO when compared to their noble

metal-based counterparts is the chemical and mechanical stability in ambient

conditions12. The most widely used TCO are ITO24, zinc oxide ZnO25,26, aluminum doped

zinc oxide (AZO- ZnO:Al), Ga-doped ZnO (GZO)27 and fluorine doped tin oxide (FTO-

SnO2:F)28,29. ITO is evolving rapidly owing to its application in optoelectronic

devices30,31. ITO films can be deposited by different technologies; sputtering, evaporation

and chemical vapor deposition (CVD). In sputtering, plasma is set up in a low pressure

of inert and/or reactive gases to deposit the atoms of the raw material onto a substrate.

This method is widely used to make uniform films on glass, polymers, metals, etc14.

Evaporation is another deposition method of ITO32. Here, the raw material is heated in

vacuum and the vapor is transferred to the substrate at a sufficient rate. Sputtering and

6

evaporation are often referred to jointly as ‘‘physical vapor deposition’’ or (PVD)12.

CVD is another deposition technique that uses heat to decompose a vapor of a

‘‘precursor’’ chemical to make a thin film of desired composition33,34. Out of all these

methods, sputtering is the most commonly used deposition technique of ITO in the

industry.

2.2.3. ITO as A Transparent Electrode

2.2.3.1. Advantages of ITO

Indium tin oxide (In2O3:SnO2) is without a doubt the most commonly employed

TE material in solar cells 35,36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59,

OLED60,61,62,63,64, , TFT65, 66 and antistatic conductive coatings67. In addition, ITO is found

in a wide variety of panel displays68, such as panels for airplanes and automotive

industry, consumer electronics; displays for home televisions, video games, and phones.

ITO is known for its high transparency over a broad range of wavelengths, high electrical

conductivity69,70 and good barrier properties towards oxygen71,72. Recently, there have

been a lot of interest to find an alternative to ITO such as carbon nanotubes (CNT) and

poly (3, 4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonate (PSS) but

ITO is still the material of choice for its high conductivity and high transmission. Figure

2-1 shows the range of electrical conductivities of ITO and different materials for

different applications.

7

Figure 2-1. Sheet resistivity of various materials, adapted from73. “Single CNT limit” refers to a network of aligned, infinitely long carbon nanotubes.

Transparent and conducting materials at the same time are rare because the

conductivity and absorption of light energy are both determined by the free electron

density of a given material74. In general, the band gap of a material must be greater than 3

eV (wavelength around 400 nm) so visible light is not absorbed and the material becomes

transparent31. Most organic semiconducting polymers used today in OPV have optical

band gaps around 2 eV and only use the region of the solar spectrum below 650 nm75.

The wide band gap of indium oxide (direct gap of 3.55-3.75 eV)76, and that of ITO (3.3-

3.4 eV)77 is the reason for the remarkable combination of high conductivity and high

transparency in the visible light region.

8

2.2.3.2. Sputtering of ITO

ITO is a heavily doped, n-type semiconductor78,79,80 with high work function (Φ

ITO = 4.4 − 4.7 eV)81 and sheet resistivities as low as 4 to 8 Ω/ are commercially

available. Highly conductive and transparent ITO films have been prepared by sputtering

on glass82,83,84 or polymer substrates85,86,87,88. As mentioned earlier, sputtering utilizes low

pressure plasma of inert and/or reactive gases to deposit the atoms of a raw material

(known as the target) as a uniform film on an adjacent surface (the substrate)12.

Figure 2-2 shows the principle of the sputtering process89.

Figure 2-2. Illustration of the principle of sputtering vacuum deposition, adapted from12,19

The sputtering process occurs in a vacuum chamber and consumes high energy.

Most of the sputtering energy goes into heating the target that must be cooled later89. The

sputtering of ITO can be expensive in roll-to-roll processing of electronic devices.

Another critical issue with sputtering is the waste of material where up to 65% of the

target can be unused90. Since sputtering might vary from manufacturer to manufacturer

and from batch to batch, the chemical history of ITO films varies and this affects the

Plasma

Electrical Source Inert gas

Vacuum Chamber

Target

Deposited thin film

Substrate

work function of the sputtered films.

the anode is on the top of the active laye

the underlying organic layers

roughness on the nanometer scale with

of active layers in the electronic d

PEDOT:PSS is applied to compensate for

electrodes94.

Overall, the sputtering and then microstructuring the ITO film

that adds steps to the manufacturing proc

microstructuring or patterning

achieved by wet etching

photolithography108,109,110

photolithography process

exposing the photoresist to light through a mask to transfer

developing of the pattern and removal of photoresist that was exposed to light, etching

the exposed ITO, and finally removing the remaining photoresist.

Figure 2-3. Steps of patterning sputtered ITO films

9

the sputtered films. In addition, if the device structure was inverted so

the anode is on the top of the active layer, sputtering of ITO is known to cause damage to

the underlying organic layers91. Sputtering also produces films of relatively high

roughness on the nanometer scale with spikes92 that can be problematic for the deposition

of active layers in the electronic device93. Therefore, in many applications,

PEDOT:PSS is applied to compensate for the roughness of the

sputtering and then microstructuring the ITO film95 is a batch process

that adds steps to the manufacturing process and hence increases costs. The

icrostructuring or patterning of sputtered ITO is a subtractive technique generally

achieved by wet etching96,97,98,99,100,101,102,103 or plasma etching104,105

110,111,112. Figure 2-3 illustrates the steps

photolithography process, which includes applying a photoresist to the sputtered ITO,

exposing the photoresist to light through a mask to transfer the desired

pattern and removal of photoresist that was exposed to light, etching

the exposed ITO, and finally removing the remaining photoresist.

Steps of patterning sputtered ITO films, adapted from113,114,115

In addition, if the device structure was inverted so

is known to cause damage to

. Sputtering also produces films of relatively high

that can be problematic for the deposition

in many applications, a layer of

the sputtered ITO

is a batch process

ess and hence increases costs. The

sputtered ITO is a subtractive technique generally

105,106,107 requiring

illustrates the steps utilized in the

photoresist to the sputtered ITO,

the desired pattern,

pattern and removal of photoresist that was exposed to light, etching

115

10

2.2.3.3. ITO on Polymer Substrates

ITO is a ceramic, brittle material116 and while it is possible to sputter ITO

electrodes on flexible plastic substrates such as PET, bending the devices processed on

these substrates can damage the ITO layer causing a reduction in its electrical

properties117,118,119,120,121,122,123. This limits the bending radius and the number of

mechanical cycles that ITO on PET should be subject to71. Fracture strains as low as

1.5% of ITO films can significantly reduce electrical conductivity in flexible

applications124. Chen et al.125 showed the mechanism of electrical failure when sputtered

ITO coatings are bent under tension to be by channeling cracks. Hamasha et al.126

observed these cracks under a cyclic bending of sputtered ITO on PET. They showed

these cracks to be worst when ITO films were bent under combinations of high

temperatures and high humidity. The cracks were also observed by Cairns et al.127.

Cairns reported that the cracks are responsible for the increase in sheet resistance of ITO

films on PET when the tensile strain is increased. A threshold strain for the increase in

resistance was reported to depend on the thickness of sputtered ITO films. The thinner

the sputtered ITO layer on PET, the larger the strain before any cracks can be initiated in

the film. The threshold strain for cracking of thin ceramic films on ductile substrates was

verified by Wang et al.128 to be inversely proportional to the square root of the thickness.

This cracking and brittleness when the sputtered ITO films are strained or bent is

obviously an issue for any application in which flexibility is required. Therefore, it is

useful to look into cost effective approaches for the deposition of more flexible ITO films

such as direct printing of ITO nanoparticles inks.

11

2.2.3.4. Deposition of ITO Nanoparticles

Today, the commonly used continuous printing processes in the graphic printing

industry are offset, gravure, and flexography129. The printing techniques rely on a rotary

cylinder or “image carrier” that deposits an entire pattern of ink on the substrate in a

single pass. The desired patterned is transferred directly from the inked image carrier to

the substrate to eliminate any additional manufacturing steps. The printing presses

operate at high speeds as the substrate is generally fed from roll-to-roll which reduces

cost and increases the manufacturing volume.

The process of printing ITO nanoparticles for application as the first layer can be

attractive since the sintering temperature and solvent choice for the inks does not affect

any subsequent printed layers and are only limited to the substrate. ITO nanoparticles

form smooth and thin films130

and can produce fine features. Printed ITO nanoparticles

on polymer substrates are typically more flexible and less prone to cracking than the

sputtered films131,132.This makes them good candidates for printed TE. They also offer a

high optical transparency resulting from low light scattering, especially when the

nanoparticles are below 30 nm133,134. However, the particle size should be as large as

possible to reduce the scattering of grain boundary and hence improve the electrical

properties. A tradeoff between the optical and electrical properties would favor

nanoparticle size of 20 to 30 nm135. The primary drawback of printing these nanoparticles

is the lower electrical conductivity of the printed films when compared to sputtered ITO.

For ITO, the resulting electrical properties depend on the fabrication method, process

parameters of the given deposition technique and the post-deposition sintering method. If

12

the tradeoffs between the electrical and mechanical properties and the advantages of low

cost printing can be optimized, the printing of ITO nanoparticles may be a viable

alternative for manufacturing TE where the demand for electrical conductivity is less

stringent.

The deposition of ITO nanoparticles from solution on glass or plastic substrates

was reported in the past, where ITO nanoparticles were spin coated136,137,138

, dip

coated139,140,141 and printed with inkjet printing142.

Ederth et al143,144,145 studied the properties of spin coated ITO nanoparticles with

subsequent annealing. They found that the sintering of the nanoparticles could be used to

produce films that had excellent electrical conductivity and optical transparency,

although the performance was not as good as a typical solid ITO films.

Cirpan et al.146 produced ITO films with optical and electrical properties suitable

for LED by spin coating a 30% dispersion of the nanoparticles in ethanol. The coated

ITO on glass was then annealed at 600°C for 3 hours in vacuum. A transmission of 91.6

% was achieved at a film thickness around 600 nm. The ITO nanoparticles films were

used as anode contacts for a polymer LED. The devices with the coated ITO

nanoparticles showed a luminance performance comparable to or even higher than LED

fabricated with commercially available ITO anodes.

Gross et al.147 prepared transparent electrodes of ITO nanoparticles in ethanol by

using doctor blading on glass. The effect of annealing temperature and time on electrical,

optical and morphological properties was investigated. A sheet resistivity of 150 Ω/ was

13

achieved with annealing at 550 °C. The conductivity was improved with higher annealing

temperatures to reach 50 Ω/ at 1000 °C for 60 min.

Jeong et al.148 showed the successful printing of ITO nanoparticles on glass at

30% solids in ethanol with a piezoelectric inkjet printer. The ITO films were rapidly

annealed at a temperature of 450 °C in a nitrogen/oxygen mixture. The patterned ITO

electrodes with an average transmittance of 84.14% and sheet resistance of 202.7 Ω/

were fabricated in an organic solar cell that had performance characteristics with an open

circuit voltage (VOC) of 0.57 V, fill factor (FF) of 0.44, and power conversion efficiency

(PCE) of 2.13%.

ITO nanoparticles were also printed with an inkjet printer by Hong et al149 using

30 wt% ITO ink in an alcohol solvent system. ITO films were heat treated at 600 °C

under nitrogen atmosphere to achieve sheet resistivity of 455 Ω/ and optical

transmittance of 90%.

Hong et al.150 showed a linear relationship between the nanoparticles

concentration and the electrical resistance of inkjet printed ITO films for concentrations

below 10% of the ITO nanoparticles.

Kim et al.151 investigated the effect of ITO nanoparticles concentration on the

electrical and the optical characteristics of inkjet printed ITO films. A dramatic reduction

of the resistance was observed when the concentration of the nanoparticles in the solvent

was increased from 10 to 20%.

14

2.2.3.5. Gravure Printing of ITO Nanoparticles

Although successful printing of ITO nanoparticles was achieved by inkjet

printing, gravure printing should be considered for the mass production of ITO films.

Gravure printing is one of the fastest (up to 3000 ft/min)152

and highest quality printing

methods capable of attaining high resolution and registration accuracy for relatively low-

viscosity inks. It is also known to be advantageous for its high production capacity,

stability over time and the resistance of printing cylinders to aggressive solvents that are

used in some functional inks153,154,155,156,157. In gravure, the image is made of cells

engraved into a copper surface which is then chromed. Prior to printing, ink is applied to

the cylinder and the excess ink is removed with a doctor blade. The ink in the recessed

cells is transferred to the substrate by means of an impression cylinder which applies

pressure to allow the substrate to come in contact with the engraved cylinder (Figure 2-

4). Ink release from the engraved cells depends on the volume (cell diameter and cell

depth) and overall shape of the cells158. Therefore, the method of engraving the image

carrier is an important factor determining the quality of the printed films. For graphic

printing, electromechanical engraving is the dominant engraving process. In this process,

the imaging information is digitally transferred by a computer, which signals an

engraving head with a diamond stylus to create the desired image areas159. The gravure

cylinder rotates during the engraving process at a constant speed. The diamond stylus

oscillates at high frequencies and penetrates the cylinder to directly cut out the cells159.

The angle of the cells can be altered to produce elongated or compressed cells. The cells

are never completely emptied during the gravure printing process. The amount of

transferred ink depends on many factors. These include ink viscosity, cell diameter, cell

15

depth and sidewall angle, a ratio of cell depth to cell diameter (referred to as aspect ratio

or AR), surface energy, and ink-surface interactions160,161. Ink viscosity plays an

important role during ink transfer and ink leveling. Viscosity of the ink depends on the

particles concentration, particle size, binder, dispersant and other additives used in ink

formulation162. Higher viscosity inks typically are more shear resistant and thus transfer

less to the substrate. Printing inks are mostly viscoelastic substances that show a time-

dependent elastic and viscoelastic response under shear stressing163,164.

Higher AR and larger cell opening create more cell volume. A small cell has a

larger surface to volume ratio than a large cell. This results in a stronger adhesion force

between the cell wall and fluid in a small than in a large cell. Therefore, it is expected

that smaller cells empty proportionally less ink than larger ones165. The amount of the ink

transferred also depends on ink rheology. More fluid inks will transfer more readily.

Furthermore, inks tend to adhere more to substrates with high surface energy, which

helps to release more ink from the cells.

Figure 2-4. Gravure printing process

16

Puetz, et al.166,167 showed successful deposition of ITO nanoparticles inks on PET

by gravure printing and then UV curing. The properties of ITO films were comparable

with that obtained by other wet deposition techniques, such as spin coating. Optical

transmission up to 88 % and sheet resistance of typically 3 to 10 KΩ/ were obtained for

films with thicknesses ranging between 300 to 1000 nm. Lower sheet resistivity was

reached by additional treatment with forming gas or nitrogen. Though sheet resistivities

were higher than that of sputtered ITO on PET (factor of 50 to 100), the faster deposition,

lower temperature, lower cost and higher flexibility of the printed films could be of

advantage where moderate sheet resistance is tolerable.

Another attempt to print ITO nanoparticles by gravure process was successfully

achieved by Neff131. The thickness of the printed films ranged from 300 nm to 1300 nm

with around 90 to 300 nm roughnesses. The films had high resistivity and low

transparency. However, Neff stated that sheet resistance is most highly correlated to the

ratio of ITO relative to ink binder, and transparency is most highly correlated to the

thickness and roughness of the ITO films. Therefore, optimizing ink formulation

increases conductivity and reducing solvent evaporation rate can produce smoother layers

and hence improved transparency.

2.2.4. Photonic Sintering of Nanoparticles

Oxygen vacancies are important in ITO films for the improvement of electrical

performance. Each oxygen vacancy in ITO contributes two free electrons. This creates an

impurity band that overlaps the conduction band to produce a degenerate semiconductor17

with a high level of doping. To form these oxygen vacancies, coated or printed ITO

17

nanoparticles films are usually sintered with high temperatures and sometimes post

annealed in an atmosphere of a forming gas (N2/H2) as reducing agent67,168,169. The

reducing environments encourage the formation of oxygen vacancies and hence improve

the electrical properties72. The high temperature sintering is only applicable to the glass

samples since the polymer substrates are limited by their glass transmission temperatures.

One option for sintering the nanoparticles without the need of the high temperature is by

photonic sintering from a light source.

To understand what is happening in photonic sintering, it is important to know

that the surface to volume ratio of nanoparticles is larger than that in the bulk, drastically

altering their thermodynamic properties. According to the Gibbs-Thomson effect, the

vapor pressure of a small particle is inversely related to the surface curvature and hence

the radius of the particle170. This causes a reduction of the melting point of nanoscaled

material when compared to the bulk171, a phenomenon known as the “melting point

depression”172. Equation 2.1 describes this phenomenon and shows the melting point to

decrease with a decrease in particle size:

= ∞1 − ∆∞

(2.1)

where Tm(∞),∆H(∞), and ρs are the bulk melting temperature, the bulk latent

heat of fusion, and the solid phase density, respectively. r represents the radius of a

spherical particle and Tm(r) is the melting point of a particle with radius r. is the

solid–liquid interfacial energy.

18

Because of this melting point depression, any absorbed photonic energy – from a

light source- raises the temperature of the nanoparticles in very short time. This causes

rapid sintering and bonding of the nanoparticles at room temperature173,174,175.

In photonic sintering, the energy is delivered in the form of light pulses or

“flashes” from a flash lamp system to convert the nanoparticles to a continuous film. The

energy delivered to the flash lamp and the pulse width can be adjusted to the sintering

needs of the deposited materials. By carefully controlling the energy intensity of the

lamp, the pulse length, as well as the number of flashes, the temperature inside the

printed film can be controlled without any excess energy dissipating into the substrate,

hence not damaging the underlying substrate. This is especially advantageous for

polymer films176. The pulses have different speeds depending on the applied voltage to

the lamp and the pulsing system. The shorter the pulse duration, the quicker the sintering

process will be. The flashes can be less than a millisecond in duration, with high energies

that are pulsed at room temperature. Since the sintering occurs in fractions of seconds, the

unit for photonic sintering can be installed in series for roll-to-roll printing and curing.

The sintering system also allows for instant on-off energy that would be harder to achieve

with continuous systems. Photonic sintering has been shown to sinter nanosized silver

175,176,177 and nanosized copper178 but research on its use with other nanoparticles has

been limited. Therefore, the need exists for more research to investigate the possibility of

photonic sintering of ITO nanoparticles on glass or flexible substrates.

19

CHAPTER 3

PROBLEM STATEMENT

The ability to directly pattern ITO films by gravure printing the nanoparticles can

be of a great advantage to the industry. For some applications, it can eliminate the energy

intensive sputtering step and the need for expensive microstructuring of the sputtered

films. Gravure printing offers the advantages of high volume, reduced waste, and roll-to-

roll printing of transparent electrodes. The greatest challenge is to achieve electrical

performance of the printed films similar to that of the sputtered ITO. By optimizing the

gravure printing parameters, the formulation of ITO nanoparticles, and the sintering

process, printed ITO films can provide an alternative solution to the manufacturing of

ITO based transparent electrodes. To date, there are only few reports focusing on the

gravure printing of ITO nanoparticles. This research extends the previous ITO print

studies by examining the parameters of gravure printing in more details. It also

investigates the possibility of photonic sintering of gravure printed ITO nanoparticles. In

order to achieve that, the work was divided into multiple studies;

Study 1: Evaluation of a fully formulated ITO coating with a binder system by bar

coating on PET and glass. The coated samples were evaluated in terms of electrical

performance, transmission, curing conditions and scratch resistance. The ITO coating

was also gravure printed on PET with a lab scale printer. Gravure printing parameters,

more specifically, the variation of the gravure cell diameter, the AR, and engraving

20

resolution were studied in order to determine the effects these parameters have on

properties of the printed ITO layer on PET (electrical, optical and surface properties).

Moreover, the printed films were assessed for mechanical flexibility and compared to that

of sputtered ITO films.

Study 2: Evaluation of a solvent-based ITO nanoparticles dispersion and its

printability on glass using a sheet-fed AccuPress Gravure System. The effect of

engraving resolution and post sintering conditions on the properties of the printed ITO

layer (electrical, optical and surface properties) were studied and compared. Photonic

sintering, which is currently used for metallic nanoparticles, was applied in this study to

investigate the possibility of sintering printed ITO nanoparticles by this method. The

process of photonic sintering can greatly benefit the printed electronics field because it

offers reductions in sintering time, energy and cost in addition to its suitability for roll-to-

roll processing.

Study 3: The printability of ITO nanoparticles dispersion was investigated with a

piezoelectric inkjet printer. The effect of process parameters, like waveform, frequency,

voltage and resolution is described in details. Formulation, viscosity and surface tension

of the dispersion were optimized for inkjet printing. The Z number, which predicts

printability in inkjet printing, was calculated and compared to the numbers published in

the literature.

21

CHAPTER 4

EVALUATION OF ITO NANOPARTICLES FOR THE COATING AND GRAVURE PRINTING ON PET

4.1. Abstract

The possibility to directly pattern ITO layers at ambient conditions by printing has

many benefits. Printing, being an additive process, would greatly reduce the amount of

energy, labor and material used by the current manufacturing processes to deposit and

pattern ITO. In this work, the gravure printability of ITO nanoparticles on PET was

studied. The nanoparticles were first coated to characterize the material in terms of

conductivity, transmission, and stability at different humidity and temperatures. The

material was then printed on PET with a lab scale gravure printer. A wide range of sheet

resistivities and film thicknesses were obtained by varying the specifications of the

gravure cells. From the regression analysis of the results, a good estimation of sheet

resistivity of the printed films at different gravure cell volumes and AR was achieved.

The films also showed transparency above 95% in the visible light region. The printed

films were assessed for mechanical flexibility and compared to commercially available

sputtered ITO on PET. The electrical performance of printed ITO layers was not

deteriorated with bending in contrast to the sputtered films. Therefore, printed ITO

nanoparticles can be of great benefit for applications in flexible electronics and utilization

in the field of energy efficiency on bendable substrates.

22

4.2. Experimental

4.2.1. ITO Nanoparticles Based Coating

The material evaluated in this study was a cellulosic based ITO nanoparticles

coating (VP Disp. ITO LTH2) from Evonik Industries. According to the manufacturer,

the ITO content is approximately 30% in mainly 2-isopropoxy ethanol. The cellulosic

binder in the formulated ITO nanoparticles contributes to the smoothness, uniformity and

flexibility of the printed films. The coating was formulated from a highly crystalline

nanostructured powder (VP ITO TC8).

4.2.2. Deposition of ITO Nanoparticles

4.2.2.1. Bar Coating

Before printing, the material was bar coated on PET and glass to evaluate its

performance in terms of scratch resistance, electrical performance and transparency. Bar

applicators capable of depositing different wet film thicknesses were used to deposit the

ITO coating onto the PET film and the glass substrates. The coatings were initially cured

at a low temperature of 120 °C for 1 hour according to the supplier’s recommendations.

The resistivities of the coated films were measured on the same day of application and

one week later. All samples were exposed to visible light during the one week

measurement period. According to the material supplier, the sheet resistivity was

expected to drop after one week then stabilize.

23

4.2.2.2. Gravure Printing

The printing experiments were performed on lab-scale gravure K-Printing Proofer

by Testing Machines Inc (Figure 4-1). The K-Printing Proofer requires a small amount of

ink per print (about 2 ml) and gives a good indication of what to expect on a large-scale

printing press. The K-Printing Proofer has the capability of adjusting the printing speed

with a numbered dial. The speed corresponding to each speed number was calculated by

measuring the printing time and the distance for the impression roll to travel across the

engraved plate.

Figure 4-1. Gravure K-printing proofer

24

4.2.3. Design of Gravure Plates

The independent variables in these experiments are cell diameter and AR. In order

to test the effect of these variables on print quality, gravure plates were engraved

electromechanically (by SGS International, Inc.) with three different AR (0.16, 0.26, and

0.38) and cell diameters ranging from 55 to 120 µm. Figure 4-2 shows the cross section

of the 120 µm diameter cell for the three AR. Figure 4-3 shows the microscopic images

of the engraved cells for the AR = 0.16 at various cell diameters.

Figure 4-2. Comparison of the cross-section for cells with 120 µm diameter for the three AR

25

(8.5/55) (10.2/65) (12.6/80)

(15.8/100) (18.9/120)

Figure 4-3. Engraved areas with different cell diameters at AR=0.16 and 145º engraving diamond. The number below each image represents the values in µm for cell depth/cell diameter.

4.2.4. Drying Conditions

After printing, the films were initially placed in a hot air oven at 120 ºC for 1

hour. The results were then compared to those obtained by a new drying condition. In this

drying condition, the samples were placed in the oven at 50 ºC and the temperature was

gradually increased to 120 °C for a total drying time of 75 minutes.

4.2.5. Substrates

PET (Melinex ST505 from DuPont Teijin Films) with a thickness of 125 µm was

used for printing. For the mechanical assessment part of this study, a commercially

available sputtered ITO on PET film was used.

26

4.2.6. Scratch Resistance Test

A diamond tip that generates a controlled scratch on the surface of the sample was

used to measure the scratch resistance of the ITO coatings. Using a Friction/Peel Tester, a

constant load was applied to the diamond tip to drag it across the surface of the samples.

4.2.7. Humidity Control

The conditions were controlled via a Caron Environmental Test Chambers,

Models 6030.

4.2.8. Electrical Testing

Resistivity is considered the most basic parameter of any conductor or

semiconductor material. Sheet resistivity was measured by the four-point probe

technique. In this technique, four probes spaced equally apart from one another are

brought in contact with the surface of the material. A current is driven through the two

outer probes (I). The potential difference (∆V) developed between the two inner probes

was measured with a Keithley 2602-SourceMeter. For a material of thickness

significantly lower than the probe spacing (s), the surface or sheet resistivity (Rsh) in

(Ω/) is then calculated according to equation 4-1179. Figure 4-4 shows a schematic view

of the four-point probe technique.

= ∆ (4-1)

27

Figure 4-4. Schematic of the four-point probe technique

4.2.9. Optical Transmission

Optical transmittance spectrum curves were obtained with a LAMBDA 35

UV/Vis Spectrophotometer in the wavelength range of 290 nm to 1100 nm. A

spectrophotometer is an instrument that measures the amount of light -electromagnetic

radiation- that passes through a medium. The radiation is characterized by its wavelength

in units of nm. The UV/Vis Spectrophotometer uses the light in the visible and adjacent

(near ultraviolet (UV) and near infrared (NIR)) range. The energy of the electromagnetic

radiation interacts with atoms in a discrete way to give the material an absorption or

emission characteristics180. In absorption spectrophotometer, the wavelength from a light

source is isolated into individual wavelengths by a diffraction grating. The light then

passes through the sample where it is either absorbed or transmitted depending on its

chemical composition181. The transmitted light (or photons) will then reach a detector to

be read directly. The amount of light absorbed by the sample (A) can be calculated from

the transmittance (T) as follows181:

= −!"# (4-2)

V -I +I

s s s

28

The basic parts of a spectrophotometer are a light source, a diffraction grating and a

detector as shown in Figure 4-5.

Figure 4-5. Illustration of the basic principle of the spectrophotometer

4.2.10. Mechanical Assessment of Printed and Sputtered ITO Films

Both sputtered films and printed ITO nanoparticles on PET were cut to the same

size and clamped at the end between two electrical contacts. The electrical resistance was

monitored as the bending radius (radius of curvature) was changed at a constant rate

(Figure 4-6). Bending creates tensile stress on the top (printed) side and compressive

stress on the bottom side. Images of the samples under bending were taken with a digital

camera. The images were imported into Adobe Illustrator CS5 to find the radii of

curvatures.

Diffraction grating

Detector

Sample Light source

29

Figure 4-6. Schematic (top) and actual display (bottom) for mechanical assessment

4.2.11. Roughness and Surface Topography

The topography of the surface was measured with an optical profilometer. The

values of the measured topography parameters depend on the size of the scanned area.

Both the film thickness and roughness in this experiment was measured using a RST Plus

White Light Interferometer by WYKO (now Bruker). Since the optical profilometry is a

non-contact method, it is non-destructive and provides more repeatable measurements

without damaging the surface of the sample. Different configurations of scanning

interferometer may be used to measure macroscopic objects with different degree of

roughness. The vertical scanning white light interferometry (VSI) can measure up to 500

Sample

Electrical contact Electrical contact

Radius of curvature

ITO sample

30

µm surface height variations and gives a height resolution of approximately 3 nm182. The

interferometer consists of a reference mirror and a beam splitter. The beam splitter

produces interference fringes when the light reflected from the mirror recombines with

the light reflected off the sample182. In this technique, the surface height of an object is

measured by scanning in the Z-direction. A three dimensional image can be extracted by

evaluating the degree of coherence, fringe visibility, between the sample and the

reference mirror183. Figure 4-7 shows schematic representation of the principle behind the

VSI.

Figure 4-7. Principle of vertical scanning interferometry, adapted from182,184

4.3. Results and Discussion

4.3.1. Evaluation of Coated LTH2 ITO Based Coating on PET and Glass

The bar coated films on glass substrate visually appeared more uniform and

transparent than the films on PET substrate. In comparison to the glass substrate, the ITO

Reference mirror

Camera

White light source

Reflected light Surface

Beam splitter

coated PET had very high sheet resistivit

1.5 and 2 mils). Figure 4

different thicknesses. As the wet film thickness increased

average light transmission of each coating in the visible light range is shown as a

percentage above each coating thic

Figure 4-8. Sheet resistivity of ITO coatings on glass at different wet film thicknesses. Average light transmission in the visible region is shown above each thicknessthe base substrate).

Spectral curves for the coatings in

All coated films showed high transmission in the VIS (290 to 390 nm) and absorption in

both UV (290 to 390 nm) and

the Solar Spectral Irradiance: AM 1.5 availabl

A). The spectrum shows the highest power efficiency at the visible light region

makes it the region of interest for light transmission.

0

2000

4000

6000

8000

10000

12000

14000

16000

She

et

Re

sist

ivit

y (Ω

/)

1 2 3 4

94.4 %

1 2 3 41 2 3 41.5 2 3 4

31

very high sheet resistivities especially at low wet film thickness

1.5 and 2 mils). Figure 4-8 shows the sheet resistivity of the ITO films on glass at

different thicknesses. As the wet film thickness increased, sheet resistivity decreased. The

average light transmission of each coating in the visible light range is shown as a

percentage above each coating thickness.

. Sheet resistivity of ITO coatings on glass at different wet film thicknesses. Average light transmission in the visible region is shown above each thickness

curves for the coatings in the (UV-VIS-NIR) are shown in Figure 4

All coated films showed high transmission in the VIS (290 to 390 nm) and absorption in

(290 to 390 nm) and NIR (750 to 1100 nm) spectra. The curves are compared to

the Solar Spectral Irradiance: AM 1.5 available on the surface of the earth

The spectrum shows the highest power efficiency at the visible light region

makes it the region of interest for light transmission.

1

Wet Film Thickness (mils)

1 2 3 4

94.4 %

90 %

88.7 %

1 2 3 4

84.3 %

1 2 3 41.5 2 3 4

After 1 Weak

Same Day

especially at low wet film thicknesses (0.5,

ITO films on glass at

sheet resistivity decreased. The

average light transmission of each coating in the visible light range is shown as a

. Sheet resistivity of ITO coatings on glass at different wet film thicknesses. Average light transmission in the visible region is shown above each thickness (without

NIR) are shown in Figure 4-9.

All coated films showed high transmission in the VIS (290 to 390 nm) and absorption in

(750 to 1100 nm) spectra. The curves are compared to

e on the surface of the earth185 (Appendix

The spectrum shows the highest power efficiency at the visible light region, which

%

After 1 Weak

Same Day

32

Figure 4-9. Transmission spectrum of ITO coated films on glass and PET at different film thicknesses (without the base substrate).

As mentioned above, the samples were initially dried at 120 °C for one hour. To

allow more time for the ink to level before annealing, a new set of coated glass samples

were put in an oven at 50 °C and the temperature was increased gradually to 120 °C and

dried for a total time of 75 minutes. A significant improvement in conductivity was

achieved with the new drying condition, as shown in Figure 4-10. This could be

attributed to the better film forming and less shrinkage of the ITO films in comparison to

those treated with a sudden increase in the drying temperature. Therefore, this drying

procedure was applied to all future coated or printed samples prepared from this ITO

coating.

0

0.5

1

1.5

2

2.5

0

10

20

30

40

50

60

70

80

90

100

200 400 600 800 1000

Wavelenght (nm)

Spe

ctra

l Irr

ad

ian

ce (

W/m

2.n

m)

Tra

nsm

issi

on

(%

)

1.5 on Glass

2

3

4

3 on PET

AM 1.5 Spectrum

Wet film thickness (mils)

33

Figure 4-10. Effect of drying conditions on sheet resistivity of ITO coatings on glass

The dry film thickness was measured using White Light Interferometry. Figure 4-

11 shows the wet film thickness and its corresponding dry film thickness on glass. A 3-D

plot of the dried films confirms the uniformity of the ITO coating on glass especially at 3

mils of wet film thickness, as shown in Figures 4-12.

0

1000

2000

3000

4000

5000

6000

7000

8000

1

She

et

Re

sist

ivit

y (Ω

/)

Wet Film Thickness (mils)

2 3 4

Drying @ 120 °C/1 hr

Gradual increase in

drying temperature

34

Figure 4-11. Dry film thickness of ITO coatings on glass

1.5 2 3 4

Figure 4-12. 3-D plot of the coated films on glass with VSI

The effects of relative humidity (RH) and temperature on the sheet resistivity of

the ITO coated glass and PET samples were investigated. The coated samples were first

conditioned for 24 hours at 50 % RH and 25°C. Later, the temperature was kept constant

while humidity changed with time. The conditions were controlled via a Caron

Environmental Test Chamber. Figure 4-13 and 4-14 show an increase in the relative

resistance of ITO on glass and PET with an increase in RH. It was also found that the

coating showed a hysteresis effect of resistance with RH changes. The increase in

0

1

2

3

4

5

6

7

0.5 1 1.5 2 2.5 3 3.5 4 4.5

Dry

Fil

m T

hic

kn

ess

(µ

m)

Wet Film Thickness (mils)

35

resistivity is related to the cellulose content in the binder system of the LTH2 coating.

The crystalline structure in cellulose has the ability to absorb or release moisture

depending on the RH186, a phenomenon known by “Hygroscopy”. With higher RH, the

moisture uptake increases which causes the swelling of the cellulosic binder that

inversely affects the electrical properties of the coating. A similar experiment was conducted to study the effect of temperature on the

relative resistance of ITO coatings. The RH was kept constant at 40 %. The samples were

first conditioned at 23° C and 40% RH for 24 hours. Figure 4-15 and 4-16 show an

increase in the relative resistance of ITO on glass and PET with the increase in the

temperature of the chamber. A hysteresis effect of resistance was also observed when

exposing the ITO coating to different temperatures. Therefore, it is important to keep the

ITO coating at a controlled temperature and humidity until the final device structure has

been encapsulated with a barrier coating to humidity and oxygen.

Figure 4-13. The effect of RH on resistance of ITO coatings on glass. Data in red represents the hysteresis effect

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

40 50 60 70 80

Re

lati

ve C

ha

nge

in R

esi

sta

nce

Relative Humidity (%)

36

Figure 4-14. The effect of RH on resistance of ITO coatings on PET. Data in red represents the hysteresis effect

Figure 4-15. The effect of temperature on the resistance of ITO coatings on glass. Data in red represents the hysteresis effect

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

40 45 50 55 60 65 70 75 80

Re

lati

ve C

ha

nge

in R

esi

sta

nce

Relative Humidity (%)

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

15 20 25 30 35 40 45 50 55

Re

lati

ve C

ha

nge

in R

esi

sta

nce

Temperature (°C)

37

Figure 4-16. The effect of temperature on the resistance of ITO coatings on PET. Data in red represents the hysteresis effect

Images of the scratch on glass and PET were taken with an ImageXpert image

analysis system (Figure 4-17). The images show wider and more continuous scratches on

the glass than those on PET. The deformability of PET, in addition to its surface

chemistry, improve the adhesion and the cohesion forces between the ITO nanoparticles

that make the ITO coatings on PET more resistant to scratches than the coatings on glass.

0.6

1

1.4

1.8

2.2

2.6

3

3.4

3.8

4.2

4.6

15 20 25 30 35 40 45 50 55

Re

lati

ve C

ha

nge

in R

esi

sta

nce

Temperature (°C)

38

Glass

PET

Figure 4-17. Scratches of ITO coating on glass and PET

4.3.2. Gravure Printing of LTH2 ITO for Application in Flexible Electronics

Since the LTH2 ITO showed excellent adhesion and high transmission on PET,

the next step was to try gravure printing the coating on PET for electronics where

flexibility and low processing temperatures are required. The material was printed with

the gravure K-Proofer at different AR as described before. The printing speed was kept

constant during printing at around 60 ft/min. Figure 4-18 shows the sheet resistivity

results for the printed ITO films. It can be seen that the sheet resistivity decreases with

increasing cell diameter and closely follows a power function fit. This trend is the same

for all tested AR, with the AR=0.38 resulting in the lowest sheet resistivities. The high

sheet resistivity at small cell diameters is most likely due to the very thin and nonuniform

ink coverage deposited by these cells.

39

Figure 4-18. Sheet resistivity of gravure printed ITO films at different cell diameters and AR

When printing multilayer structures, smoothness is important as each layer serves

as the printing surface for the subsequent layer. The solid content of some electronic inks

are quite low and the dry thickness is much thinner than the wet film thickness. As a

result, defects and protrusions are reproduced and perhaps amplified in subsequent layers.

A critical issue with rough surfaces in transparent electrodes is the excessive light

scattering and low absorption. The most important parameter is the root mean square

roughness (Rrms) which is the average of peaks and valleys of a material’s surface profile

around the mean roughness. The Rrms gives a good general description of the height

variations in the surface. However, care should be taken when analyzing the Rrms data as

a “surface with a few high-amplitude features may have the same rms as a one with many

low-lying features”187. Roughness of the printed films was measured at 454 µm x 600 µm

scale. There was no significant correlation between the thickness and the roughness of the

10

100

1000

10000

40 60 80 100 120 140

She

et

Re

sist

ivit

y (K

Ω/

)

Cell Diameter (μm)

AR=0.16

AR=0.26

AR=0.38

40

films. The roughness values ranged from 15 nm to 40 nm, which are acceptable when

compared to the roughness of PET used in this work, which was around 13 nm.

Furthermore, the roughness of the printed films is lower than that of the sputtered PET,

which was around 75 nm. The thickness of printed films was measured for all tested

conditions. The results are presented in Figure 4-19. Although the AR = 0.26 has a lower

cell volume than the AR = 0.38, the difference in film thickness between them was not

statistically significant. However, more uniform ink coverage was observed for the films

printed at the AR = 0.38 (Figure 4-20), which could explain the lower sheet resistivity of

the films for the AR = 0.38 than that for the AR = 0.26. The figure also shows the cross

section and thickness of each film.

Figure 4-19. Thickness of ITO printed films at different cell diameters and AR

100

150

200

250

300

350

400

450

500

40 60 80 100 120 140

Th

ick

ne

ss o

f P

rin

ted

Fil

ms

(nm

)

Cell Diameter (μm)

AR=0.38

AR=0.26

AR=0.16

41

Figure 4-20. 2-D images of the edges of printed ITO films for 100 µm cell diameter at AR=0.26 and AR=0.38 (top) and the cross section of each film (bottom)

Regression analysis was performed for sheet resistivity results at different cell

diameters and AR. The regression results provide a predictive tool when designing

gravure cylinders to achieve desired sheet resistivities for certain applications. Equations

4-3 through 4-5 show the best model fit for the relationship between the sheet resistivity

(Y) in KΩ/ and cell diameter (x) in µm with R2 of 0.96, 0.98 and 0.99 for AR = 0.16,

0.26, and 0.38, respectively:

42

Y = 2E+07 x-2.39, for AR=0.16 (4-3)

Y = 467697 x-1.68, for AR=0.26 (4-4)

Y = 221813 x-1.65, for AR=0.38 (4-5)

The mechanisms of electrical conductivity and optical transmission are very much

interdependent. Excellent electrical properties can be achieved by printing thicker films,

but often at the expense of transmission. Transmissions above 95% (without the base

substrate) were obtained for the printed samples. The difference in light transmission was

not significant in the range of the printed thicknesses. The transparency was dramatically

affected at much larger thicknesses obtained with the bar coating of the same ITO ink on

glass as shown in Figure 4-21. The thickness of the ITO coatings on glass ranged from

1.4 to 6.5 µm with sheet resistivities ranging from 12 to 2 kΩ/. It should be noted, that

some of the current applications utilize transparent electrodes with sheet resistivities

around 100 Ω/sq for flat panel displays and 1000 Ω/sq for touch screens with 80%

transparency at 550 nm73. The printed and coated ITO layers made with LTH2 coating

are more suitable for applications in electromagnetic shielding and antistatic coatings

rather than display applications.

43

Figure 4-21. Transmission of LTH2 coating at different sheet resistivities (without the base substrate)

ITO nanoparticles were also printed at a larger thickness using a solid plate with

72 µm cell depth and 242 µm cell diameter (100 line/inch (lpi)). The sheet resistivity was

around 23.5 KΩ/ with a measured thickness around 600 nm. The transmission of these

printed ITO films was compared to the transmission of sputtered ITO on PET. The

sputtered films had a sheet resistivity of 80 Ω/, a measured thickness of around 100 nm

and a roughness of around 75 nm. The transmission of both, printed and sputtered films,

was measured and compared to that of PET to show how the transparency was affected

by the printing or the sputtering of ITO (Figure 4-22). Although the thickness of the

printed films was larger than the sputtered ones (around 6 times), the printed films

showed higher transmission in the visible light region. The comparison also shows the

difference in transparency between the PET used in sputtering and the sputtered films to

be much larger than that between the PET used for printing and the printed ITO films.

50

55

60

65

70

75

80

85

90

95

100

1 10 100 1000

Tra

nsm

issi

on

at

55

0 n

m (

%)

Sheet Resistivity (KΩ/)

Gravure on PET

Bar coating on glass

44

Figure 4-22. Transmission spectra of PET, printed ITO, and sputtered ITO (both printed and sputtered ITO are measured with the base substrates)

Brittle coatings can limit the applicability of thin-film electronic devices intended

for processing or use under bending. Therefore, the printed ITO films (at 600 nm) were

assessed for mechanical flexibility and compared to sputtered ITO films on PET. For the

mechanical assessment, both samples were cut to the same size and clamped between two

electrical contacts. The electrical resistance was monitored as the bending radius (radius

of curvature) was changed. The radius of curvature was estimated from images of the

samples taken during testing.

Figure 4-23 shows the relative change in electrical resistance with bending for the

sputtered and printed ITO films. The electrical resistance of the sputtered ITO film

deteriorated very rapidly while the printed films retained their electrical performance,

30

40

50

60

70

80

90

100

300 350 400 450 500 550 600 650 700

Tra

nsm

issi

on

(%

)

Wavelenght (nm)

PET for sputtering

PET for printing

Printed ITO

Sputtered ITO

45

even at high radii of curvature. Figure 4-24 shows the change in the topography of the

samples before and during bending using the White Light Interferometry to generate

images. Continuous channeling cracks were observed for the sputtered films during

bending while the surface of the printed samples resembles a wrinkled texture. This

continuity of the crack in the sputtered films is a barrier of electron movement and

explains the increase in resistance while bending. The cracks were also observed by Chen

et al.125, Hamasha et al.126 and Cairns et al.127 when sputtered ITO films were strained.

The figure also shows the common spikes that can be seen on sputtered ITO surface

which are responsible for the relatively high roughness of the films. For the printed ITO

nanoparticles, the cellulosic binder in the formulated ink holds the nanoparticles together

and makes the film stretchable and highly flexible. It is important for transparent

conductors to withstand mechanical flexing for devices being used and/or processed

under flexing such as rollable displays, lighting and solar cells, credit and smart cards,

etc.

46

Figure 4-23. The effect of bending on sheet resistivity of sputtered and printed ITO films

Figure 4-24. Surface topography of ITO films before and during bending

0.1

1

10

100

0 2 4 6 8 10 12

Re

lati

ve

Ch

an

ge

in

Re

sist

an

ce (

R/R

˳)

Radius of Curvature (mm)

Sputtered ITO

Printed ITO

Spikes

Crack

47

4.4. Conclusion

This study demonstrated the successful gravure printing of thin, electrically

conductive and transparent ITO nanoparticles based electrodes. A fully formulated

cellulosic based ITO nanoparticles ink was used for coating and printing. The effect of

RH and temperature on the electrical performance of ITO was examined. A wide range of

sheet resistivites and film thicknesses were obtained by varying the gravure cell diameter

and AR. Transparencies above 90% in the visible light region were obtained for the

printed films. Regression analysis of the results provided a good estimation of sheet

resistivity of the printed films at different gravure cell volumes and AR. This would help

to determine engraving specifications for the printing cylinder to obtain a desired

performance of printed ITO films. With an appropriate adjustment of the gravure printing

parameters, different sheet resistivity and optical transparency values can be obtained.

The printed films were also assessed for mechanical flexibility and compared to

commercially available sputtered ITO films on PET. The electrical performance of

printed ITO nanoparticles did not significantly deteriorate with bending in contrast to the

sputtered films. The material used for this study offers low temperature processing of

ITO films at ambient temperature and could eliminate the need for energy intensive

vacuum process. However, the sheet resistivity of printed ITO films would need to be

improved in order for this material to be a viable alternative to sputtered ITO in major

printed electronics applications such as displays or photovoltaics.

48

CHAPTER 5

GRAVURE PRINTING OF ITO ON GLASS AND PHOTONIC SINTERING OF PRINTED ITO NANOPARTICLES

5.1. Abstract

Currently, ITO films are prepared by the expensive process of sputtering. The

sputtering and then patterning the ITO film is a batch process that adds manufacturing

steps, hence increases costs. The gravure printing of ITO nanoparticles, as a direct

patterning process, eliminates the need for the sputtering and etching to pattern ITO

films. In this study, the ITO nanoparticles were printed with an AccuPress MicroGravure

Printing system. Photonic sintering has proven to sinter nano silver and nano copper

particles. Therefore, the printed films were sintered with this approach to examine the

possibility of eliminating the prolonged thermal sintering of ITO nanoparticles. For

comparison, the printed ITO films on glass were sintered by the two methods; thermally

at 500 °C and photonically with intensive light pulses. The photonic sintering was

performed using a Sinteron 2000 intensive pulsed light system manufactured by Xenon

Corporation. Applying 80 pulses accomplished sintering within a total time interval of

100 seconds. The transmission of sintered films with the photonic approach was around

88% with a sheet resistivity of 1 KΩ/. Higher conductivities were achieved but at the

expense of transmission. PET substrate was able to withstand this pulsing condition,

which shows the possibility of sintering printed ITO on flexible substrates. The

49

combination of direct patterning of ITO by gravure and post-processing with photonic

sintering improves the efficiency of manufacturing ITO by saving on energy and time.

5.2. Experimental

5.2.1. ITO Nanoparticles Based Dispersion

A 30% ITO dispersion from Evonik Industries (VP Disp ITO TC8 DDAA) in

diacetone alcohol was evaluated for printability on glass.

5.2.2. Gravure Printing

The ITO dispersion was printed on glass with an AccuPress MicroGravure

Printing system from Ohio Gravure Technologies (Figure 5-1). The press is designed for

the tight tolerances and high accuracy required for printed electronics. The gravure

cylinder was engraved electromechanically with four solid areas, each with different

engraving resolutions of 120, 160, 200, 250 lpi. Figure 5-2 shows the difference in cell

opening at the different engravings resolutions. The press was running at a printing speed

of 1.2 m/s for all the printing trials.

Figure 5-1. The AccuPress MicroGravure

120

Figure 5-2. The engraved cylinder with the four solid areas (top) andcells at different resolutions in lpi (bottom)

5.2.3. UV Ozone (UVO)

A UVO Cleaner by Jelight Company Inc

to printing. The device uses a UV lamp to split oxygen (O

50

AccuPress MicroGravure Printing system at WMU

160 200

2. The engraved cylinder with the four solid areas (top) and detail of engraved at different resolutions in lpi (bottom)

(UVO) Cleaning

by Jelight Company Inc. was used to clean the glass sheets

. The device uses a UV lamp to split oxygen (O2) molecules from an air stream

detail of engraved

was used to clean the glass sheets prior

) molecules from an air stream

51

to atoms (O-). The atoms attach to other oxygen molecules (O2), forming ozone (O3) that

attacks contaminants on the surface of the substrate. The glass sheets were first cleaned

with isopropanol (IPA) and then placed in the UVO cleaner for 10 minutes.

5.2.4. Contact Angle and Surface Energy Measurements

The surface energy of a substrate is an important parameter that affects wetting,

spreading of a liquid, and adhesion between liquid and substrate. Contact angle is directly

related to the surface energy of the substrate that quantitatively describes the wetting of

solid surfaces by liquids. The higher wetting behavior is related to smaller contact angles.

To examine the effect of the UVO cleaning of glass on the surface energy, a First Ten

Angstroms (FTÅ200) was used to measure the contact angle of ITO before and after the

cleaning.

The most widely used technique in the determination of the contact angle is the

sessile drop technique188. The contact angle of a sessile drop created by a liquid on a solid

surface is described by the Young- Laplace189 equation (5-1). The equilibrium sessile

drop is demonstrated in Figure 5-3.

$%. cos * = $+ −$% (5-1)

where * is the contact angle

$% is the surface/interfacial tension at the liquid-vapor interface

$+ is the surface/interfacial tension at the solid‐vapor interface

$% is the surface/interfacial tension at the solid‐liquid interface

52

Figure 5-3. Equilibrium sessile drop, adapted from189

Owens and Wendt190 developed an equation to calculate the surface energy of

solids for systems where the liquid and solid interact by hydrogen bonding as well as

dispersion forces. The equation is based on the measurement of contact angles with water

and methylene iodide and described as the following189:

1 + cos *$% = 2/$01 $%1 + 2/$02 $%2 (5-2)

where D indicates the dispersive and P indicates the polar component of surface energy.

5.2.5. Sintering Conditions

All printed ITO films were first cured in a conventional oven at 120 °C for 1 hour.

The films were then sintered by two methods; conventional high temperature sintering of

ITO nanoparticles recommended by the supplier of the ink and photonic sintering with

intensive pulsed light. The high temperature sintering was performed in a tube oven at

500°C for 2 hours. The films were first placed in the oven at room temperature and the

temperature was raised to 500 °C at a rate of 5 °C/minute. The second method was used

to examine the possibility of photonic sintering the ITO nanoparticles with high energy

pulsed light from a flash lamp system. The system used for this purpose was a Sinteron

θ

Vapor

Liquid

Solid

γLV

γLS

γSV

53

2000 unit, Xenon Corporation (Figure 5-4). The Sinteron 2000 provides a high energy,

pulsed light for photonic sintering of conductive nanoparticles inks on heat sensitive

substrates. The Xenon flash lamp produces a broadband spectrum of light from deep

ultraviolet (UV) to infrared (IR) as in Figure 5-5. The energy in the system is compressed

over short-duration pulses to deliver high peak power. The peak power results in a deeper

penetration of light into the material191. The flashes are pulsed at room temperature with

an energy output up to 2,000 J and pulse duration of 2 ms192. The Sinteron 2000 has four

bays that house the power supply; the controller and two pulse forming network (PFN)

bays containing two PFNs each. The PFN sets can be changed by an easy adjustment of

the connection of the system. Each PFN set has different pulse periods and energy levels.

Figure 5-6 shows the four different pulse widths that can be selected at each PFN set. The

energy levels of the flash lamp can be controlled by adjusting the voltage on the system.

Figure 5-7 illustrates the energy into the flash lamp based on the voltage setting. The

printed ITO samples on glass were placed exactly one inch below the center of the lamp

to be exposed to the same amount of energy for each trial.

54

Figure 5-4. The Sinteron 2000 @ WMU

Figure 5-5. Typical spectrum for type C lamp for Xenon Flash system191

55

10

Figure 5-6. Pulse width with different PFN sets, based on datasets given in the system’s manual192

56

Figure 5-7. Pulse energy with different PFN sets, based on datasets given in the system’s manual192

5.2.6. Roughness and Surface Topography

The surface roughness and thickness of ITO films were characterized with

WYKO RST Plus White Light Interferometer. For higher resolution topography of the

surface, ITO films were scanned using an Atomic Force Microscopy (AFM) from

ThermoMicroscopes Autoprobe CP Research System. The AFM was operated in a

“tapping mode”. The AFM is a high resolution microscope that provides a 3D image of

a surface on a nanoscale. The microscope comprises a probe with a very sharp tip

supported by a cantilever. When the tip comes close to the surface of a sample, an

interaction of atomic force is produced193. The force between the tip and the sample

57

surface can be of various kinds such as; covalent bonding, ionic bonding, metallic

bonding, hydrogen bonding, and van der Waals forces194. Due to this interaction forces,

the cantilever is deflected or oscillated (depending on the mode) and the tip is displaced.

The displacement of the tip is detected by the reflection of a laser beam off the cantilever,

which is captured by an optical system. The tip of the probe scans the surface in the x and

y direction. The x and y scans, in addition to the displacement of the probe in the z

direction, creates a three dimensional image of the surface topography193. The resolution

in the x-y direction ranges from 0.1 to 1.0 nm while in the Z direction, the resolution is in

the atomic range of 0.01 nm193. AFM operates in number of modes. In general, imaging

modes are divided into static (contact) mode and dynamic (non-contact or tapping) mode.

In the contact mode, the force between the tip and surface is kept constant during

scanning through a feedback loop. This maintains a constant deflection of the cantilever

and a constant tip–sample separation193. Because of the contact between the tip and

surface, the sample is often destroyed or even pushed out of the field by the tip. In the

non-contact mode, a stiff cantilever oscillates while the tip is scanning the surface. The

tip lightly taps the sample without touching it. The tip-sample separation is maintained

constant by using a constant oscillation of the cantilever193. The tapping in the non-

contact mode protects both the sample and the tip from being damaged. Figure 5-8 shows

the basic principle of an AFM.

58

Figure 5-8. The principle of an AFM, adapted from195,196

5.3. Results and Discussions

To understand the effect of UVO cleaning on the wettability and spreading of ITO

on glass, the surface energy and contact angle were measured before and after cleaning of

the glass sheets. Since the effect of UVO on surface energy decays with time after the

treatment197, both surface energy and contact angle were measured right after the sheets

were cleaned. The printing was also performed immediately after the UVO treatment on

glass. Due to the ozone effect, the surface energy of the glass increased with higher

dispersive component of the energy, Figure 5-9. Because of the increase in the surface

energy, the contact angle of ITO on glass dropped from around 19° to 5° for better

spreading on glass (Figure 5-10 and 5-11). Since the printed features are solid areas

Laser

Feedback electronic loop

Photodetector

XYZ Scanner

Sample

Cantilever Tip

59

rather than fine features, a lower contact angle would be favored to improve the

wettability, and reduce the voids and defects for better film uniformity.

Figure 5-9. The effect of UVO treatment on the surface energy of glass

Figure 5-10. Contact angle of ITO on glass before (left) and after (right) UVO treatment on glass (t >20 s)

0

10

20

30

40

50

60

70

80

Before UV Ozone After UV Ozone

Surf

ace

En

erg

y (d

yne

s/cm

)

Surface Energy

Dispersive Energy

Polar Energy

60

Figure 5-11. The effect of UVO treatment on the contact angle of ITO on glass

The resolution of gravure cells is one factor in determining the thickness of any

printed film. As discussed before in Chapter 4, and currently published in literature198, the

rheology of the material is another factor that affects the transfer of ink to a substrate, the

leveling of the ink and the variation in ink film thickness. The average thickness of the

printed ITO films at different resolution is presented in Figure (5-12). As shown, the

maximum thickness was below 1.2 µm.

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30

Co

nta

ct A

ngl

e (

°)

Time (s)

Before UV Ozone

After UV Ozone

61

Figure 5-12. Thickness of printed ITO films at different engraving resolutions

The comparison between the two sintering conditions was based on the electrical

performance, transmission and surface topography of the ITO films. For best results, all

printed ITO films were first dried in an air oven at 120 °C for 1 hour before sintering to

allow the evaporation of the solvent. For photonic sintering, the energy to the flash lamp

was controlled by changing the system’s voltage. The highest level of energy was applied

with 3 KV at 2000 J/pulse and 2 ms duration of the pulse. The number of pulses needed

to achieve good conductivity of ITO was determined by trial and error experiments to the

point that the printed films were not damaged or the transmission was not dramatically

affected. The optimal number of pulses for this experiment was 80 pulses flashing at a

total time of 100 seconds. With more pulsing, the sheet resistivity was reduced but the

films would turn to a brownish color. Figure 5-13 shows a comparison of sheet

resistivities achieved from the conventional high temperature sintering of ITO and the

photonic sintering. Higher conductivities were obtained for ITO films treated by the high

0

200

400

600

800

1000

1200

1400

1600

100 120 140 160 180 200 220 240 260 280

Th

ick

ne

ss o

f P

rin

ted

Fil

ms

(nm

)

Engraving Resolution (lpi)

62

temperature sintering process than the photonic method. However, a sheet resistivity as

low as 1000 Ω/ was obtained with the Sinteron system in less than 2 minutes, which is a

dramatic saving in time and is suitable for some applications such as touch panels. The

sheet resistance of the ITO after photonic sintering increased to double its original value

after many weeks of storage in air. This is a common problem in many electronic

applications when the deposited films are exposed to oxygen and humidity. Puetz et al.133

observed the same effect with storing UV-cured ITO films in air as opposed to the

samples that were kept under inert conditions and for which the resistivity stayed stable.

Therefore, it is important to keep the photonically sintered ITO films under vacuum or a

controlled inert condition until the final device structure has been encapsulated with a

barrier coating to protect it from moisture and oxygen.

Figure 5-13. Sheet resistivity of printed ITO films at different sintering methods

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

400 600 800 1000 1200 1400

She

et

Re

sist

ivit

y (Ω

/)

Thickness of Printed Films (nm)

Oven sintering @ 500 C

Photonic Sintering

63

The temperature of the sintered film with the Sinteron system was measured

immediately after sintering with an infrared thermometer gun. The temperature of the

film on glass was around 250 °F (121 °C). PET sheets were exposed to the same sintering

conditions. The PET used was a heat-stabilized film that gives excellent dimensional

stability at temperatures up to 302 °F199. The PET films were able to withstand the 80

pulses with a final temperature of 235 °F after sintering. This shows the possibility of

photonic sintering the printed ITO nanoparticles on flexible polymer substrates.

As discussed before in section 2.2.4, due to the phenomenon of the melting point

depression, nanoparticles materials melt at much lower temperatures than the bulk

material. Once the photonic energy is absorbed into the film, it is converted to a thermal

energy to melt the nanoparticles. Performing energy balances around the system and

based on the “first law of thermodynamics”200, the thermal energy causes enthalpy

changes in the nanoparticles. First, it raises the temperature of the nanoparticles to the

melting point and then melts the nanoparticles. The amount of energy consumed to melt

the nanoparticles depends on the latent heat of fusion associated with the phase change in

the material. Theoretically, the rise in the temperature of the film (∆T) is proportional to

the heat energy (Q). In general for an open system, Q is related to temperature as the

following:

3 = ∆4 =56 − 7 = 56∆ (5-3)

where ∆4 is the change in enthalpy associated with the temperature rise

56 is the heat capacity

64

7 is the ambient temperature

T is the final temperature

The relation between the energy density of photonic sintering and the temperature

rise in thin films was investigated by Kim et al201. The thickness of inkjet printed copper

nanoparticles was incorporated by Kim in the energy balance equation to find the

temperature rise while sintering. The relation was written as follows:

8 = − 7 = 9:;<

(5-4)

where E is the energy density (J/cm2)

ρ is the density of material

t is the film thickness

Here, E should be equal to the energy Q divided by the cross section of the

radiating area. Equation 5-4 was based on the assumption that all of the light energy was

absorbed into the copper layer and the polyethylene substrate without scattering and

reflection. Since printed ITO films had a transmission around 90%, the ITO film and the

substrate should absorb about 4.5% of the light energy based on equation (4-2).

Therefore, if energy of 1 J/cm2 radiated into the film is taken as a basis to calculate the

temperature rise, and then based on Equation 5-4, ∆T should be 134.55, 0.35, and 2.61 °C

for ITO films, glass, and PET respectively. The physical and thermodynamics properties

of ITO, glass and PET films used to calculate ∆T and the temperature rises are

summarized in Table 5-1. This high temperature rise of ITO films when compared to that

of glass and PET explains the extremely fast sintering of metallic nanoparticles films

65

without damaging the substrate. For ITO films, to obtain good resistivities after sintering,

80 pulses were flashed as opposed to only few flashes for copper and silver nanoparticles.

One explanation for this is that the rate of pulses per second is limited by the system

energy level and duration of the pulses. This might cause the temperature to drop

between the pulses hence reducing the accumulated temperature of the films. Another

explanation is the high transmission of printed ITO films that the majority of the energy

at each pulse is not totally absorbed. This requires more pulsing in contrast to the

sintering mechanism of dark copper nanoparticles where the energy is assumed to be

totally absorbed by the film201. Still, with 100 s of pulsing, the ability to sinter ITO films

by this approach reduces both the time and cost of heating used in the conventional

thermal sintering of printed ITO nanoparticles. Certainly, the tradeoff between the

electrical performance, cost and time savings by the photonic sintering should be further

optimized to gain the full advantage of the combined process of gravure printing of ITO

nanoparticles and photonic sintering.

Table 5-1. Physical and thermal properties of ITO, glass and PET

Material Density (g/cm3)

Cp (J/g.°C) Thickness of film (µm)

Calculated ∆T (°C)

ITO 7.2 0.387* 1.20 134.55

Glass 2.38 202 0.768202 700 0.35

PET 1.38 1201 125 2.61

* Cp of ITO was estimated from Kopp’s rule200 based on the heat capacities of In, Sn203, and Oxygen as shown in Appendix B

66

Figure 5-14 shows the transmission spectra of printed ITO films obtained by

printing at different engraving resolutions and sintered under different conditions. The

printed films on glass had excellent transmission in the visible light region. The

difference in transmission of the ITO films printed at different resolutions was not

significant, even though film thickness decreased with increasing engraving resolution.

The transmission spectra of the films sintered by the high temperature and photonic

sintering were also very comparable.

67

Figure 5-14. Transmission spectra of printed ITO films on glass (with the base substrates). The top image with the high temperature sintering and the bottom with the photonic sintering of ITO films, numbers in legend are in lpi

The average roughness of glass used for printing (Ravg) was 5.75 nm and the Rrms

was 7.86 nm. There was no significant relation between the thickness of the printed films

and the roughness. The average roughness of the printed films ranged from 12 to 18 nm

0

10

20

30

40

50

60

70

80

90

100

220 420 620 820 1020

Tra

nsm

issi

on

(%

)

Wavelength (nm)

Glass

250

200

160

120

0

10

20

30

40

50

60

70

80

90

100

220 420 620 820 1020

Tra

nsm

issi

on

(%

)

Wavelength (nm)

Glass

250

200

160

120

68

and the Rrms ranged from 16 to 20 nm for both the high temperature and photonic

sintering methods. This exceptional smoothness obtained with the printing of the ITO

nanoparticles added to the advantages of the printing of transparent electrodes. The

topography of ITO films by the two sintering methods was comparable, as shown in

Figure 5-15 and Figure 5-16.

Figure 5-15. 3-D surface topography of ITO at high temperature (top) and photonic sintering (bottom) with VSI

69

Figure 5-16. 2-D surface topography of ITO at high temperature (top) and photonic sintering (bottom) with VSI

To observe the topography of the printed ITO films at higher resolutions, AFM

scans were taken at (10X10 µm2) and (5X5 µm2) for the thermally sintered samples, The

Rrms for the 10X10 µm2 was 111 A> and for the 5X5 µm2 was around 81.3 A> , Figure 5-17.

70

Figure 5-17. Surface topography of printed ITO film sintered at 500 °C with AFM at (10X10) µm2 (left) and (5X5) µm2 scans (right)

5.4. Conclusion

ITO nanoparticles were successfully printed on glass with an AccuPress Gravure

System. Different sheet resistivities and excellent light transmissions were achieved by

varying the engraving resolutions of the gravure cells. An enhancement in the surface

energy and wettability of the ITO dispersion on glass was obtained by the treatment of

the substrate with a UVO cleaner. Two methods were applied for sintering the printed

ITO films. Sheet resistivity of 415 Ω/ was achieved through the use of high temperature

sintering. Printed films were also sintered with photonic energy by the use of a pulsed

light system at a total time of 100 s. The sheet resistivities with the photonic sintering

were higher than that achieved by high temperature sintering, but are still acceptable in

some applications such as touch panels. An optical transmission above 88% was

obtained, regardless of the sintering approach of ITO films. The sintering method didn’t

have a significant effect on surface roughness or transmission. The capability of photonic

71

energy to sinter the ITO nanoparticles without damaging the substrate was explained with

theoretical calculations. The results showed the need for prolonged thermal treatments

may be avoided with photonic sintering, which has been shown previously to sinter silver

and copper nanoparticles, and now shown to be applicable towards ITO nanoparticles. In

general, the photonic sintering process will add a lot to the field of printed electronics not

only for the energy and time savings, but also for its ability to be installed in series with a

roll-to-roll printing process. In addition to the advantage of directly patterning TE by

gravure printing under ambient conditions and high speeds. A more efficient use of

material with printing can be realized in comparison to the sputtering and the

photolithography process.

72

CHAPTER 6

EVALUATION OF ITO NANOPARTICLES FOR PRINTABILITY WITH AN INKJET PRINTER

6.1. Abstract

The objective of this study was to examine the printability of ITO nanoparticles

using an inkjet printing as a non-impact printing (NIP) process. Inkjet printing is a simple

and inexpensive process that eliminates the need for vacuum patterning techniques.

Because of the non-impact nature, waste and contamination are minimized. After the

successful printing of the VP Disp ITO TC8 DDAA with the AccuPress MicroGravure

Printing System, the dispersion was examined for printability with a piezoelectric

FujiFilm Diamtix Material Printer. The firing waveform, frequency and voltage were

adjusted each time to obtain a uniform and continuous ejection of droplets. The drops’

spacing, and hence resolution in dot/inch (dpi), were examined to enhance the print

quality. The dispersion was diluted with different solvent systems to adjust both the

surface tension and rheology of the ink for better ink jetting. Although good jetting was

obtained by altering the process parameter, the instability of jetting was a problem. The Z

number of fluid properties was calculated for the initial dispersion and compared to the Z

values that are available in literature. In summary, the study shows the importance of

knowing the viscosity at the jetting shear rate or the “infinite-rate viscosity” to predict the

possibility of jetting stable droplets through the calculations of the Z number.

73

6.2. Introduction

Ink jet printing has become an important technology for many applications such

as organic electronics and nanotechnology. It has the ability to deposit small volumes of

solutions or suspensions (picoliters) in a well defined pattern. It is a simple and direct

printing process that eliminates the need for expensive vacuum patterning techniques.

Furthermore, inkjet printer is a non impact printing process, thus waste and

contamination are minimized204,205,206. Inkjet printed materials must be processable at low

temperature which enables the deposition of ink on flexible, inexpensive plastic

substrates207.

In a piezoelectric inkjet system, an ink droplet is generated by the mechanical

displacement in the ink channel rather than heating and vaporization204. The system uses

a piezoelectric material that flexes when receiving an electrical pulse depending on the

imaging signal. This creates pressure forces in the ink chamber which expels the ink

droplet out of the nozzle208. A droplet is ejected when the amount of kinetic energy

transferred from the system to the droplet is larger than the surface energy needed to form

a droplet205. At the end of the pulse, the ink is no longer pumped. It detaches from the

nozzle tip driven by the surface tension. Figure 6-1 shows the piezoelectric element and

drop jetting from the inkjet nozzles.

74

Figure 6-1. A piezoelectric nozzle, the PZT stands for Lead Zirconate Titanate, a piezoelectric ceramic material209

High frequencies of the jetting waveforms can result in an earlier wave not

decaying completely and then interacting with the next pressure wave. This results in

chaotic droplet ejection from the print head. With low frequencies, droplets are ejected at

low rates, which reduces the production efficiency205. Therefore, the jetting frequency

should be optimized for each ink till the droplets are jetted at reasonable speed.

The velocity and volume of the droplets depends on the amount of kinetic energy

transferred by varying the driving voltage205. Ink viscosity and surface tension are crucial

parameters in the piezoelectric system. High viscosity inks have a great damping effect

on the pressure propagation responsible for jetting the droplets. Therefore, the viscosity

must be low enough to allow droplet jetting and the ink channel to be refilled quickly.

The surface tension of the ink is responsible for the spherical shape of the droplet exiting

the nozzle. The surface tension must be high enough and the pressure low enough, to

hold the ink in the nozzle without dripping210.

The shear rates in microdrop ejectors are very high and in the 106 s-1 range211.

Firing frequency, voltage, jetting waveform, and resolution in dpi must be perfectly

matched to achieve the best prints on the substrate. The challenge for inkjet inks is to

PZT/Si Bimorph

Nozzle plate

75

have much lower viscosities than those for gravure or flexography inks and also maintain

good stability. This can be challenging especially for conductive inks in which the

particles tend to sediment.

Surface tension and ink rheology should be optimized to guarantee good drop

formation and prevent the wetting of the nozzles’ outlet face (nozzle plate). This wetting

results in formation of aerosol or spray and misdirection of the droplets.

Droplet formation and spreading in inkjet printing depends on group of fluid

properties that are described by a dimensionless number, the Z number212,213. Fromm214

defined the Z number as the following. The Z number is also equivalent to the inverse of

the Ohnesorge number (Oh)

? = @AB/Dƞ

=OhGH (6-1)

where d is the diameter of the printing orifice

ƞ is the viscosity

ρ is the density

γ is the surface tension of liquid

The Z number can predict the droplet formation and gives a good indication of the

stability of ink jetting. If the Z number is low, the viscosity dominates and a large

pressure drop is needed for droplet jetting. If the Z number is high, then satellite droplets

are produced. Fromm214 predicted Z to be larger than 2 for droplet formation while Derby

et al.215 predicted Z to be in the range between 1 and 10. In practice, a Z number much

76

larger than 10 still generates a printable droplet as long the satellites drop joins the main

droplet before reaching the substrate205.

6.3. Experimental

6.3.1. ITO Nanoparticles Based Dispersion

A 30% ITO dispersion from Evonik Industries (VP Disp ITO TC8 DDAA) in

diacetone alcohol was evaluated for printability with an inkjet printer. The surface

tension was measured with the FTA to be around 25 dynes/cm.

6.3.2. Inkjet Printing

The dispersion was evaluated for printability with a FujiFilm Dimatix Material

Printer (DMP 2800). The DMP system uses a piezoelectric crystal that flexes when

receiving an electrical pulse depending on the imaging signal. A 10 pL cartridge with a

16-nozzle print head was attached to the printer. Figure 6-2 shows the DMP and the ink

cartridge. The cartridge has a nominal nozzle diameter of 21 µm. Prior to printing, the

ITO dispersion was filtered with a 450 nm filter and degassed for 30 minutes in an

ultrasonic bath. Electrodes ranging in line widths from 50 to 500 µm were printed in

parallel to the print direction (inkjet head movement).

77

Figure 6-2. The DMP (left) and ink cartridge (right)

According to the manufacturer’s recommendations, for the ink to achieve

optimum performance with the Dimatix, the following ink characteristics should be

met209:

• Viscosity should be between 10 and 12 cp at operating temperature.

• Surface tension should be between 28 and 33 dynes/cm.

• Specific gravity greater than 1 is recommended.

• Degassing to remove any dissolved gas that inhibits jetting

• Filtration, if particle size allows, it is recommended to filter all fluids to 0.2 µm.

6.3.3. Ink Rheology

Printing inks are mostly viscoelastic substances that show a time-dependent

elastic and viscoelastic responses under shear stress. Ink rheological behavior is

important to study to understand the ink flow, ink transfer and ink leveling.

An AR 2000 Advanced Rheometer (TA Instruments) was used to study the

rheological behavior of the ITO dispersion. Low shear rates on the instrument can be

nozzles

78

related to the storage conditions of the ink, while high shear rates can provide

information about the performance of the ink when printing on a press.

6.4. Results and Discussions

As mentioned earlier, the challenge for inkjet inks is to have much lower

viscosities than those for gravure or flexography ink and also to maintain good stability

without dripping from the nozzles. The viscosity curve of the ITO dispersion is shown in

Figure 6-3 with comparison to the rheology of silver ink that prints well with the DMP.

The viscosity of the ITO dispersion was too high and not suitable for jetting with the

DMP. The high viscosity of the ink dissipates the kinetic energy through the fluid so no

droplets can eject from the nozzles. The surface tension of the ink is also important for

inkjet printing as it’s responsible for the spherical shape of the droplet exiting the nozzle.

Both the surface tension and ink rheology should be optimized to guarantee good droplet

formation and prevent the wetting of the nozzles’ plate. Therefore, the ITO dispersion

was diluted with various high boiling point solvents (HBS) in an attempt to meet the

viscosity requirement while preventing nozzle clogging due to ink drying which can

occur with low boiling point solvents. The surface tension and flow properties were

measured for all dilutions before printing. Firing frequency, voltage, jetting waveform,

and resolution in dpi must be perfectly matched to achieve the best prints on the

substrate. These variables were adjusted each time to be able to jet uniform droplets of

the dilutions. Three compatible HBS were added at different percentages and

combinations to the ITO dispersion. These HBS are 1-phenoxy-2-propanol (PPH),

propylene glycol diacetate (PGDA) and butanediol. The percentages of the dilutions

79

represent the ratio of the added solvent to the total mass of the new dilution. Table 6-1

shows a summary of the boiling points and surface tension (ST) for all the solvents. A

comparison between the rheological (flow) properties between the three solvents is

shown in Figure 6-4.

Figure 6-3. Viscosity curve for ITO nanoparticles

0.1000 1.000 10.00 100.0 1000shear rate (1/s)

1.000E-3

0.01000

0.1000

1.000

10.00

visc

osity

(Pa.

s)

ITO

Inkjet NanoSilver

80

Table 6-1. Physical properties of the HBS

Solvent BP (°C) ST

(dynes/cm)

PPH 243 37.6

PGDA 190 32.9

1,4-Butanediol 235 39.6

Figure 6-4. Viscosity curves for the HBS

PGDA –with the lowest viscosity- was added at 25, 35, 40 and 50% of the new

dilutions. The rheological properties are shown in Figure 6-5. The dilution at the 25%

was not jettable. It is possible that the viscosity of the ink was still too high to fill the

0.1000 1.000 10.00 100.0 1000shear rate (1/s)

1.000E-3

0.01000

0.1000

1.000

10.00

visc

osity

(Pa.

s)

1,4 Butanediol

PPH

PGDA

81

nozzles. The temperature of the cartridge was raised to reduce the viscosity but still no

jetting was observed.

The 35% PGDA dilution was jettable at room temperature. However, after a while,

the velocity of the ink was not stable; some nozzles would not fire, or some droplets

would drift during flight. This instability could explain the missing dots and non-

uniformity in the prints as shown in Figure 6-6. Same observations and results were

obtained with the 40 % PGDA dilution. The wetting of the nozzles’ plate was observed

for all PGDA dilutions. Figure 6-7 shows spray formation and misdirection of the

droplets as a result of this plate wetting. Continuous ink coverage and sharper edges were

obtained by printing with a 50% PGDA dilution in ITO, Figure 6-8. The 50% dilution

had a low viscosity resulting in excessive ink spreading especially at high resolutions of

1270 and 2000 dpi. Any missing droplets due to non-jetting of the nozzles were

compensated for with the ink spreading. However, the width gain of the printed lines was

almost double the designed line width.

82

Figure 6-5. Rheological properties of PGDA dilutions

Figure 6-6. Non-uniform ink coverage with 30% PGDA at 813 dpi

0.1000 1.000 10.00 100.0 1000shear rate (1/s)

1.000E-3

0.01000

0.1000

1.000

10.00

visc

osity

(Pa.

s)ITO w 25% PGDA

ITO w 35% PGDA

ITO w 40% PGDA

ITO w 50% PGDA

83

Figure 6-7. Wetting of the nozzle’s plate (left) and misdirection of drops with PGDA dilutions (right)

1016 1270 2000

Figure 6-8. Printed 100 µm lines with 50% PGDA dilution at different resolutions in (dpi)

Next, the ITO dispersion was diluted with two solvents at 50% (50% PPH and

50% PGDA). It was thought that the PPH would raise the surface tension to the original

dispersion while the PGDA would lower the viscosity. The instability of inkjetting was

still a problem.

A combination of three solvents was added to the ITO dispersion at 50% (25%

PPH, 37.5% PGDA, and 37.5% IPA). The thought was that the PGDA and PPH would

help in maintaining a high surface tension of the ink while the IPA would increase the

evaporation rate to slow spreading and improve the uniformity of the printed film. The

jetting observed with this dilution was the best obtained so far with no ink spreading

around the nozzles (Figure 6-9). However, when printed, the prints again were not

uniform with missing dots. A 40% dilution with the same ratio of solvents was printed.

As observed before, the droplets initially jetted uniformly and responded well to the

Nozzle

84

waveform and voltage but with time the jetting became unstable or eventually stopped.

The waveform was adjusted to improve the jetting and hence the uniformity of the print.

Figure 6-10 shows the effect of the waveform adjustment on the printability of the 40%

dilution. A new 40% dilution with different ratios of solvents (19% PPH, 42% PGDA,

and 39% IPA) was printed, however, the same problem with instability was still present.

Figure 6-9. Droplet Formation with 40% ITO dilution (25% PPH, 37.5% PGDA, and 37.5% IPA)

85

Waveform 1 Waveform 2

50 µm Lines

Figure 6-10. The effect of the firing waveform (top) on the printability at 950 dpi (bottom)

0

2

4

6

8

10

12

14

16

0 50 100 150 200

Ad

dre

ss

Firing Voltage (v)

Waveform 2

Waveform 1

86

An attempt to print a 40% dilution of (20.5% Butanediol, 43% PGDA, and 36.5%

IPA) showed good droplet formation (Figure 6-11). The rheological properties of the

dilution were again compared to that of silver ink that previously had shown good

printability by the DMP. Figure 6-12 shows the ITO dilution to have higher viscosity at

low shear rates than the silver ink. Therefore, it was difficult to jet the ink at the

beginning. The ink cartridge was heated from around 28 to 50 ˚C. Raising the

temperature lowered the viscosity for the ink to respond to the jetting waveform. Again,

the stability of the jetting with time was still a problem that affected the uniformity of the

prints as shown in Figure 6-13.

Figure 6-11. Droplet formation with 40% (Butanediol, PGDA, IPA) dilution

87

Figure 6-12. Comparison between the rheological properties of the ITO dilution and the silver ink

50 µm 100 µm Solid area

Figure 6-13. Images of printed features with Butanediol, PGDA and IPA dilution at 813 dpi

The rheological properties for all dilutions were measured before filtration. To

examine the effect of filtration on these properties, the rheology of a 50% dilution of

butanediol and IPA was measured before and after filtration. No significant difference in

0.01000 0.1000 1.000 10.00 100.0 1000shear rate (1/s)

1.000E-3

0.01000

0.1000

1.000

10.00

visc

osit

y (P

a.s)

ITO w 40% (Butanediol+PGDA+IPA) NanoSilver-Inkjet

88

the shear thinning behavior of the two dilutions, especially at high sheer rates, was

observed with filtering the dilution as shown in Figure 6-14.

Figure 6-14. Effect of filtration on the rheological properties

Although good droplet jetting was obtained by altering the surface tension,

viscosity and waveforms of the jetting dilutions, the instability of the jetting was the main

problem in obtaining uniform and continuous prints. As mentioned previously, the shear

rate in the microdrop ejectors are in the 106 s-1 range211. The response of the original ITO

dispersion and all dilutions to this jetting shear rate was not possible to predict with the

AR Rheometer. For the Z number, the infinite-rate viscosity is needed to predict the

0.01000 0.1000 1.000 10.00 100.0 1000shear rate (1/s)

0.01000

0.1000

1.000

visc

osity

(Pa

.s)

ITO w 50% (Butanediol+IPA)Filtered ITO w 50% (Butanediol+IPA)

89

possibility of proper and stable jetting. Therefore, a Carreau model was applied to the

rheology curve of the original ITO dispersion to predict the infinite-rate viscosity. The

Carreau model216 is a mathematical expression that describes the shear thinning behavior

of the fluid. The model is excellent for numerical simulations of flow processes at low

and high shear rates. Figure 6-15 shows the rheology of the original ITO dispersion and

the Carreau model curve. The infinite-rate viscosity with the Carreau model was 0.025

Pa.s. The Z number was then calculated for the original ITO dispersion using equation 6-

1 and the predicted infinite-rate viscosity. Table 6-2 shows a summary of the dispersion’s

fluid properties and the calculated Z value.

Figure 6-15. Carreau model for the original ITO dispersion

0.01000 0.1000 1.000 10.00 100.0 1000 10000shear rate (1/s)

0.01000

0.1000

1.000

10.00

100.0

1000

visc

osit

y (P

a.s)

ITO

Carreau model of ITO

a: zero-rate viscosity: 435.8 Pa.s

b: infinite-rate viscosity: 0.02495 Pa.s

90

Table 6-2. Fluid properties and Z number for ITO dispersion

Surface tension

(dynes/cm)

Density (g/cm3)

Infinite-rate viscosity (Pa.s)

Nozzle diameter for 10 pL cartridge

(µm)

Z number

25 1.3 0.025 21 1.045

The calculated Z number was around 1, which is lower than the predicted value

by Fromm214 and in the low range predicted by Derby et al215. The infinite-rate viscosity

for all the dilutions would be very close to the one of the original ITO dispersion. The

low value of the Z number means the viscosity of the ITO dispersion dominated in these

experiments. It also explains the instability of the jetting for the ITO dispersions.

Therefore, the Z number has shown again its importance in predicting fluid jetting in

inkjet printers.

6.5. Conclusion

This work shows the importance of fluid viscosity in piezoelectric inkjet printing.

Since droplet ejection depends on the propagation of the waveform, the viscosity should

be suitability low to prevent any damping effect on the waveform. The challenge was to

adjust the surface tension and viscosity of ITO nanoparticles printed successfully with

gravure to the requirements of the piezoelectric inkjet printing. Good jetting was obtained

by the adjustment of the dispersion properties; however, the stability of the jetting with

time remained a problem in this study. The viscosity at the jetting shear rate or the

“infinite-rate viscosity” is important to know to predict the stability of jetting through the

calculations of the Z number.

91

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109

Appendix A

The Solar Spectral Irradiance

Table A. The Solar Spectral Irradiance: AM 1.5 Power at UV, VIS & NIR Light Wavelength

(nm) Power

(W/m2.nm) Wavelength

(nm) Power

(W/m2.nm) Wavelength

(nm) Power

(W/m2.nm)

1100 0.46113 1071 0.58304 1042 0.63406

1099 0.48161 1070 0.57178 1041 0.63379

1098 0.47709 1069 0.55428 1040 0.63366

1097 0.54805 1068 0.58561 1039 0.63802

1096 0.47731 1067 0.58637 1038 0.63361

1095 0.49363 1066 0.5833 1037 0.63664

1094 0.5112 1065 0.59461 1036 0.64323

1093 0.48417 1064 0.59722 1035 0.64348

1092 0.55048 1063 0.58815 1034 0.64136

1091 0.55722 1062 0.59782 1033 0.63751

1090 0.52656 1061 0.58694 1032 0.64852

1089 0.54856 1060 0.60073 1031 0.64799

1088 0.56159 1059 0.58453 1030 0.65092

1087 0.53685 1058 0.60286 1029 0.64687

1086 0.52496 1057 0.6091 1028 0.65398

1085 0.56163 1056 0.61055 1027 0.65318

1084 0.54775 1055 0.61242 1026 0.65625

1083 0.56603 1054 0.61048 1025 0.65727

1082 0.55773 1053 0.61302 1024 0.65077

1081 0.54973 1052 0.61487 1023 0.6549

1080 0.56519 1051 0.61862 1022 0.6498

1079 0.5721 1050 0.61802 1021 0.66107

1078 0.57076 1049 0.62206 1020 0.65839

1077 0.57175 1048 0.62559 1019 0.6488

1076 0.58141 1047 0.62008 1018 0.67263

1075 0.56054 1046 0.61078 1017 0.66237

1074 0.5878 1045 0.62712 1016 0.66976

1073 0.57086 1044 0.63067 1015 0.66676

1072 0.58194 1043 0.62773 1014 0.67848

110

Table A-Continued

Wavelength

(nm) Power

(W/m2.nm) Wavelength

(nm) Power

(W/m2.nm) Wavelength

(nm) Power

(W/m2.nm)

1013 0.67556 975 0.55536 937 0.15453

1012 0.67667 974 0.5417 936 0.15267

1011 0.68034 973 0.57081 935 0.2369

1010 0.67695 972 0.64629 934 0.13604

1009 0.67742 971 0.67059 933 0.23459

1008 0.68407 970 0.59689 932 0.28386

1007 0.68463 969 0.6448 931 0.3854

1006 0.67047 968 0.61301 930 0.40679

1005 0.6414 967 0.47353 929 0.51792

1004 0.68056 966 0.47377 928 0.55363

1003 0.69086 965 0.47469 927 0.73684

1002 0.68498 964 0.43242 926 0.65875

1001 0.70013 963 0.47585 925 0.66621

1000 0.69159 962 0.41664 924 0.67571

999 0.69455 961 0.43481 923 0.69698

998 0.69481 960 0.39685 922 0.65584

997 0.69555 959 0.35294 921 0.73004

996 0.70409 958 0.43453 920 0.69657

995 0.70673 957 0.25591 919 0.69161

994 0.70891 956 0.30996 918 0.55612

993 0.69325 955 0.32204 917 0.6835

992 0.70597 954 0.39988 916 0.54099

991 0.70833 953 0.32442 915 0.6355

990 0.68843 952 0.25418 914 0.5874

989 0.70391 951 0.45563 913 0.58906

988 0.69059 950 0.13944 912 0.64508

987 0.69489 949 0.46514 911 0.6258

986 0.70553 948 0.25911 910 0.58553

985 0.64734 947 0.3501 909 0.65912

984 0.6929 946 0.18409 908 0.61091

983 0.62915 945 0.34729 907 0.59833

982 0.65102 944 0.26987 906 0.72502

981 0.67058 943 0.26289 914 0.5874

980 0.56941 942 0.38192 913 0.58906

979 0.60046 941 0.35071 912 0.64508

978 0.57903 940 0.44411 911 0.6258

977 0.60084 939 0.37591 910 0.58553

976 0.53872 938 0.18962 909 0.65912

111

Table A-Continued

Wavelength

(nm) Power

(W/m2.nm) Wavelength

(nm) Power

(W/m2.nm) Wavelength

(nm) Power

(W/m2.nm)

908 0.61091 870 0.89933 832 0.82961

907 0.59833 869 0.88252 831 0.85666

906 0.72502 868 0.8914 830 0.8493

905 0.76337 867 0.85066 829 0.86135

904 0.78862 866 0.78882 828 0.7887

903 0.64483 865 0.89487 827 0.91178

902 0.625 864 0.90956 826 0.8653

901 0.56162 863 0.92934 825 0.89752

900 0.69429 862 0.92367 824 0.86619

899 0.51461 861 0.91648 823 0.62576

898 0.67137 860 0.91764 822 0.88081

897 0.6231 859 0.92081 821 0.92292

896 0.71265 858 0.92094 820 0.79899

895 0.75956 857 0.92059 819 0.83844

894 0.79413 856 0.90343 818 0.76299

893 0.81412 855 0.84746 817 0.78984

892 0.84688 854 0.79 816 0.77171

891 0.86255 853 0.8954 815 0.82927

890 0.86078 852 0.89942 814 0.83681

889 0.87041 851 0.90454 813 0.93236

888 0.85899 850 0.829 812 0.94882

887 0.84799 849 0.91447 811 0.97273

886 0.84515 848 0.9201 810 0.97488

885 0.87913 847 0.91947 809 0.97353

884 0.86859 846 0.94447 808 0.99844

883 0.86494 845 0.94226 807 1.0018

882 0.86787 844 0.91434 806 1.0122

881 0.84563 843 0.93171 805 0.9727

880 0.87434 842 0.92411 804 0.99452

879 0.87144 841 0.93626 803 0.98288

878 0.88607 840 0.94124 802 1.0011

877 0.88948 839 0.92783 801 0.99978

876 0.88625 838 0.92585 800 0.98859

875 0.86204 837 0.93493 799 1.0048

874 0.87451 836 0.90135 798 1.0245 873 0.88999 835 0.92917 797 1.0057

872 0.89887 834 0.86608 796 0.98985

871 0.88671 833 0.88622 795 1.0066

112

Table A-Continued

Wavelength

(nm) Power

(W/m2.nm) Wavelength

(nm) Power

(W/m2.nm) Wavelength

(nm) Power

(W/m2.nm)

794 1.0101 756 1.1185 718 0.93873

793 1.0015 755 1.1321 717 1.0081

792 1.0084 754 1.1353 716 1.1548

791 1.0179 753 1.121 715 1.1428

790 1.0045 752 1.1265 714 1.1823

789 1.0359 751 1.1224 713 1.1719

788 1.0399 750 1.1273 712 1.1856

787 1.053 749 1.1286 711 1.1934

786 1.0656 748 1.1323 710 1.1954

785 1.0649 747 1.1389 709 1.188

784 1.0602 746 1.1381 708 1.1839

783 1.0672 745 1.1404 707 1.1875

782 1.0714 744 1.1408 706 1.1925

781 1.0662 743 1.1316 705 1.1989

780 1.0687 742 1.1084 704 1.1864

779 1.0803 741 1.1078 703 1.1567

778 1.0754 740 1.1119 702 1.15

777 1.0764 739 1.0855 701 1.1489

776 1.0827 738 1.1184 700 1.1636

775 1.0801 737 1.0978 699 1.1721

774 1.08 736 1.0994 698 1.1961

773 1.0797 735 1.1101 697 1.2151

772 1.0802 734 1.1261 696 1.1513

771 1.0716 733 1.0901 695 1.1538

770 1.0646 732 1.052 694 1.1318

769 1.0347 731 0.97702 693 1.1446

768 1.0222 730 1.0294 692 1.1516

767 0.89574 729 0.95503 691 1.1201

766 0.7508 728 0.94968 690 1.0746

765 0.63377 727 0.98988 689 1.026

764 0.49885 726 0.98638 688 1.0195

763 0.35217 725 0.94741 687 0.88285

762 0.63491 724 0.96305 686 1.2174

761 0.14328 723 1.0423 685 1.2454

760 0.24716 722 1.1281 684 1.2445 759 1.0932 721 0.98967 683 1.2526 758 1.1246 720 0.8994 682 1.2662 757 1.1176 719 0.84274 681 1.2601

113

Table A-Continued

Wavelength (nm)

Power (W/m2.nm)

Wavelength (nm)

Power (W/m2.nm)

Wavelength (nm)

Power (W/m2.nm)

680 1.265 641 1.297 606 1.3353

679 1.2629 640 1.2962 605 1.3418

678 1.2737 639 1.3238 604 1.3439

677 1.2669 638 1.3292 603 1.3205

676 1.2786 637 1.3204 602 1.2928

675 1.2639 640 1.2962 601 1.3123

674 1.2742 639 1.3238 600 1.3278

673 1.276 638 1.3292 599 1.3145

672 1.2651 637 1.3204 598 1.3142

671 1.2829 636 1.2768 597 1.3303

670 1.2853 635 1.3065 596 1.326

669 1.3032 634 1.2907 595 1.287

668 1.281 633 1.311 594 1.3029

667 1.2767 632 1.2327 593 1.3086

666 1.286 631 1.2799 592 1.29

665 1.2871 630 1.2589 591 1.3171

664 1.2647 629 1.2758 590 1.2316

663 1.2539 628 1.2328 589 1.1582

662 1.2518 627 1.3022 588 1.3403

661 1.2618 626 1.2655 587 1.3708

660 1.2668 625 1.2667 586 1.3409

659 1.2586 624 1.2751 585 1.3737

658 1.254 623 1.2793 584 1.3845

657 1.1218 622 1.2882 583 1.3872

656 1.0727 621 1.3359 582 1.3729

655 1.222 620 1.3299 581 1.3518

654 1.2807 619 1.3292 580 1.3455

653 1.295 618 1.3228 579 1.323

652 1.2558 617 1.2744 578 1.304

651 1.3071 616 1.2906 577 1.3452

650 1.2299 615 1.3254 576 1.3118

649 1.2234 614 1.2783 575 1.3225

648 1.2625 613 1.3182 574 1.3527

647 1.2744 612 1.337 573 1.3595

646 1.2797 611 1.317 572 1.3534

645 1.317 610 1.3237 571 1.281

644 1.3074 609 1.3292 570 1.324

643 1.313 608 1.3392 569 1.3228

642 1.2995 607 1.3434 568 1.3554

114

Table A-Continued

Wavelength

(nm) Power

(W/m2.nm) Wavelength

(nm) Power

(W/m2.nm) Wavelength

(nm) Power

(W/m2.nm)

567 1.3222 528 1.3508 489 1.2492

566 1.2823 527 1.1795 488 1.3252

565 1.3555 526 1.3479 487 1.2235

564 1.3466 525 1.3859 486 1.0918

563 1.3731 524 1.3962 485 1.3457

562 1.3225 523 1.2976 484 1.3492

561 1.3885 522 1.376 483 1.3742

560 1.3118 521 1.3452 482 1.3899

559 1.2885 520 1.3349 481 1.3836

558 1.3613 519 1.2222 480 1.3825

557 1.3321 518 1.2605 479 1.3586

556 1.3651 517 1.1017 478 1.3839

555 1.3883 516 1.3514 477 1.3392

554 1.3802 515 1.3385 476 1.3299

553 1.3533 514 1.3003 475 1.3755

552 1.3923 513 1.3277 474 1.3304

551 1.3639 512 1.4125 473 1.3144

550 1.3648 511 1.3753 472 1.3661

549 1.3752 510 1.3497 471 1.2975

548 1.3331 509 1.385 470 1.2749

547 1.3717 508 1.321 469 1.3247

546 1.3536 507 1.3548 468 1.3178

545 1.3657 506 1.4153 467 1.2616

544 1.3971 505 1.3598 466 1.319

543 1.3493 504 1.2682 465 1.2905

542 1.3714 503 1.3597 464 1.3055

541 1.2595 502 1.2991 463 1.3452

540 1.3096 501 1.299 462 1.3392

539 1.3558 500 1.3391 461 1.3255

538 1.3896 499 1.3429 460 1.2791

537 1.3229 498 1.3421 459 1.2859

536 1.4292 497 1.3788 458 1.2946

535 1.3701 496 1.3548 457 1.3213

534 1.3491 495 1.4238 456 1.3088

533 1.259 494 1.3402 455 1.2655

532 1.4094 493 1.3719 454 1.273

531 1.4348 492 1.2818 453 1.1854

530 1.3598 491 1.3435 452 1.2822 529 1.4142 490 1.3968 451 1.3376

115

Table A-Continued

Wavelength (nm)

Power (W/m2.nm)

Wavelength (nm)

Power (W/m2.nm)

Wavelength (nm)

Power (W/m2.nm)

450 1.2881 411 0.9077 372 0.46506

449 1.2409 410 0.8091 371 0.47628

448 1.2422 409 0.94717 370 0.51666

447 1.2257 408 0.88488 369 0.47244

446 1.0766 407 0.84545 368 0.45319

445 1.1992 406 0.85878 367 0.48869

444 1.1537 405 0.87849 366 0.49508

443 1.1823 404 0.89849 365 0.4181

442 1.164 403 0.88211 364 0.40472

441 1.0859 402 0.91387 363 0.40005

440 1.0993 401 0.87691 362 0.3535

439 0.95753 400 0.83989 361 0.34278

438 0.99368 399 0.80408 360 0.3924

437 1.1306 398 0.63944 359 0.3065

436 1.1061 397 0.31882 358 0.27936

435 1.007 396 0.56443 357 0.29527

434 0.91653 395 0.60096 356 0.35627

433 0.9905 394 0.3678 355 0.3914

432 1.0628 393 0.35502 354 0.38523

431 0.63779 392 0.58656 353 0.32975

430 0.70134 391 0.62634 352 0.32674

429 0.87766 390 0.58457 351 0.34603

428 0.94625 389 0.50121 350 0.32913

427 0.9355 388 0.4635 349 0.28864

426 0.96667 387 0.47343 348 0.29306

425 0.99312 386 0.44984 347 0.30318

424 0.96182 385 0.48638 346 0.29132

423 0.96531 384 0.36689 345 0.27854

422 0.99499 383 0.32648 344 0.25352

421 1.0067 382 0.41958 343 0.30857

420 0.88467 381 0.54424 342 0.29121

419 0.96354 380 0.49751 341 0.27932

418 0.92392 379 0.52616 340 0.29659

417 0.96392 378 0.60314 339 0.27096

416 0.98628 377 0.50014 338 0.25321

415 0.95569 376 0.47218 337 0.21767

414 0.92134 375 0.41087 336 0.23813

413 0.92951 374 0.38651 335 0.26477

412 0.96686 373 0.42833 334 0.23823

116

Table A-Continued

Wavelength (nm)

Power (W/m2.nm)

Wavelength (nm)

Power (W/m2.nm)

333 0.24263 295 3.22E-06

332 0.24508 294 9.46E-07

331 0.22835 293 1.58E-07

330 0.26192 292 4.08E-08

329 0.23297 291 3.9E-09

328 0.19773 290 5.15E-10

327 0.21834

326 0.20868

325 0.15504

324 0.14852

323 0.11623

322 0.122

321 0.13414

320 0.11277

319 0.10971

318 0.095815

317 0.09302

316 0.067088

315 0.073686

314 0.065266

313 0.058323

312 0.050898

311 0.045392

310 0.027826

309 0.022298

308 0.020753

307 0.015246

306 0.01015

305 0.008934

304 0.005097

303 0.003733

302 0.001457

301 0.000919

300 0.000456

299 0.000203

298 0.000111

297 3.32E-05

296 1.47E-05

117

Appendix B

Heat Capacity of ITO

B. Estimation of Heat Capacity (Cp) of ITO based on Kopp’s Rule

Cp (In2O3:SnO2)= 2 x Cp(In) + 5 x Cp(O) + Cp(Sn) =

= 2 x (26.74) + 5 x (17) + (27.112) = 165.59 J/(mol.°C)

= 0.387 J/(g.°C)