Kronfli_Rosanna_K_201406_MASc_thesis.pdf

A Novel Design for Fully Printed Flexible AC-Driven Powder Electroluminescent Devices on Paper

by

Rosanna Kronfli

A thesis submitted in conformity with the requirements for the degree of Masters of Applied Science

Chemical Engineering and Applied Chemistry University of Toronto

Copyright by Rosanna Kronfli 2014

ii

A Novel Design for Fully Printed Flexible AC-Driven Powder

Electroluminescent Devices on Paper

Rosanna Kronfli

Masters of Applied

Chemical Engineering and Applied Chemistry

University of Toronto

2014

Abstract

ACPEL devices were fabricated onto various paper substrates. The dielectric and phosphor

layers were mask printed, a PEDOT:PSS/SWCNT ink was inkjet-printed for the cathode and a

translucent conductor was applied with a paintbrush for the anode resulting in a maximum

luminance of 8.05 cd/m2 at 300 VAC and 60 Hz. It was found that the conductivity of the

PEDOT:PSS/SWCNT ink on the various paper types was affected by the coating and paper

thickness. Novel ACPEL devices were also fabricated by incorporating paper as the dielectric

layer of the device. The maximum luminance achieved was 7.24 cd/m2 at 300 VAC and 60 Hz. It

is shown that the dielectric constant of the paper and hence the performance of the resulting EL

device may be enhanced by filling the sheet with BaTiO3 and by the surface treatment of the

sheet.

iii

Acknowledgments

I would like to thank my supervisor, Professor Ramin Farnood for his guidance and support

during my project.

Thank you to Peter Angelo for all the training, Jeffrey Castrucci for his help with the luminance

measurements and Alexandra Tavasoli for the work on the filled paper and the SEM.

My time at UofT has been both challenging and rewarding. I would like to thank my labmates

for always lending an ear and the Chem Eng Community for making Wallberg feel like home.

Most importantly, thank you to my parents and sister for their constant and endless love.

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Table of Contents

Acknowledgments .......................................................................................................................... iii

Table of Contents ........................................................................................................................... iv

Nomenclature ............................................................................................................................... viii

List of Tables .................................................................................................................................. x

List of Figures ................................................................................................................................ xi

Chapter 1 ......................................................................................................................................... 1

1 Introduction ............................................................................................................................. 1

1.1 Project motivation and significance ................................................................................. 1

1.2 Hypothesis and objectives ................................................................................................ 3

Chapter 2 ......................................................................................................................................... 4

2 Literature review ..................................................................................................................... 4

2.1 Background on electroluminescence ................................................................................ 4

2.2 ACPEL vs. ACTEL .......................................................................................................... 4

2.2.1 Device structures ....................................................................................................... 4

2.2.2 Comparing performance ........................................................................................... 5

2.3 ACPEL light emission mechanism .................................................................................. 5

2.4 ACPEL device operation and features ............................................................................. 6

v

2.5 Materials in ACPEL devices ............................................................................................ 8

2.5.1 ZnS-based phosphors ................................................................................................ 8

2.5.2 Dielectric materials ................................................................................................... 9

2.5.3 Electrode materials .................................................................................................. 10

2.5.4 Substrates ................................................................................................................ 10

2.6 ACPEL developments .................................................................................................... 11

2.7 Solution deposition techniques ....................................................................................... 11

2.8 Commercial products ..................................................................................................... 13

Chapter 3 ....................................................................................................................................... 14

3 ACPEL devices on paper ...................................................................................................... 14

3.1 Experimental methods .................................................................................................... 14

3.1.1 Device preparation .................................................................................................. 14

3.1.2 Conductivity of PEDOT:PSS/SWCNT ink ............................................................ 16

3.1.3 Device characterization ........................................................................................... 17

3.2 Results ............................................................................................................................ 18

3.2.1 Substrate effects on conductivity ............................................................................ 18

3.2.2 Substrate effects on luminance ............................................................................... 20

3.3 Conclusions .................................................................................................................... 22

Chapter 4 ....................................................................................................................................... 23

vi

4 Novel design for ACPEL devices on paper .......................................................................... 23


4.1.1 Device preparation and characterization ................................................................. 24

4.2 Results ............................................................................................................................ 24

4.2.1 Effect of structure on luminance ............................................................................. 24

4.3 Conclusions .................................................................................................................... 28

Chapter 5 ....................................................................................................................................... 29

5 Novel design for ACPEL devices on BaTiO3-filled paper ................................................... 29


5.1.1 Substrate preparation .............................................................................................. 29

5.1.2 Substrate characterization ....................................................................................... 31

5.1.3 Conductivity of PEDOT:PSS/SWCNT ink ............................................................ 31

5.1.4 Device preparation .................................................................................................. 32


5.2 Results ............................................................................................................................ 32

5.2.1 Substrate characterization ....................................................................................... 32

5.2.2 Effect of substrate on conductivity ......................................................................... 38


5.3 Conclusions .................................................................................................................... 41

vii

Chapter 6 ....................................................................................................................................... 42

6 Summary ............................................................................................................................... 42

6.1 Future work .................................................................................................................... 43

7 References ............................................................................................................................. 44

8 Appendices ............................................................................................................................ 52

8.1 Appendix A: Composite structure .................................................................................. 52

8.2 Appendix B: BaTiO3 vs. dielectric constant .................................................................. 56

8.3 Appendix C: Screen printed devices .............................................................................. 57

8.4 Appendix D: SEM cross-sections of A devices ............................................................. 59

8.5 Appendix E: SEM cross-sections of B, C, D devices .................................................... 61

8.6 Appendix F: SEM cross-sections of filter paper devices ............................................... 66

viii

Nomenclature

AC Alternating current

ACPEL Alternating current driven powder electroluminescent

ACTEL Alternating current driven thin-film electroluminescent

CIE International Commission on Illumination

DC Direct current

EDX Energy dispersive spectrometry

EL Electroluminescence

LEC Light-emitting capacitor

LED Light-emitting diode

L-V Characteristic luminance-voltage

Mw Molecular weight

NC Nanocrystal

PL Photoluminescence

SEM Scanning electron microscopy

TAPPI Technical Association of the Pulp and Paper Industry

VAC Voltage in alternating current

Permittivity

0 Vacuum permittivity

D Permittivity of dielectric

P Permittivity of phosphor

Dielectric constant

Length

Electrical resistivity

Conductivity

A Sample area

b Luminance equation constant

C Capacitance

ix

d Thickness between electrodes

dD Thickness of dielectric layer

dP Thickness of phosphor layer

Ep Applied electric field

L0 Luminance equation constant

L30 Luminance at 30 V above Vth

R Resistance

V Voltage

Vth Threshold voltage

Vtot Total applied voltage

x

List of Tables

Table 1: Substrate characteristics .................................................................................................. 14

Table 2: PEDOT:PSS/SWCNT based ink formulations [41] ....................................................... 15

Table 3: Formulation for dielectric and phosphor resins .............................................................. 16

Table 4: A Devices off and on (~190 VAC) ................................................................................. 21

Table 5: Summary of device A performance ............................................................................. 22

Table 6: Devices B, C, and D off and on (~190 VAC) ................................................................. 24

Table 7: Summary of devices B, C, and D performance .............................................................. 28

Table 8: Dip-coating formulation ................................................................................................. 29

Table 9: Ratio of BaTiO3 nanopowder to micron-sized powder .................................................. 30

Table 10: Filtration solution composition ..................................................................................... 30

Table 11: Coating composition ..................................................................................................... 31

Table 12: Devices off and on (~190 VAC) ................................................................................... 39

Table 13: Summary of filled paper device performance .............................................................. 41

xi

List of Figures

Figure 1: ACPEL schematic cross-section (a) bottom-emission structure, (b) top-emission

structure ........................................................................................................................................... 1

Figure 2: AC-driven EL device structure (a) ACPEL, (b) ACTEL ................................................ 5

Figure 3: Device structure A ......................................................................................................... 14

Figure 4: Conductor with uniform cross-sectional area A ............................................................ 17

Figure 5: SEM micrographs of paper surfaces at x500. (a) Xerox Supergloss; (b) Multicoat; (c)

34 lb Catalyst Electracote Gloss; (d) Xerox Copy paper; (e) 24.6 Catalyst TD Directory; (f)

18.0 Catalyst TD Directory ....................................................................................................... 18

Figure 6: Layer thickness of PEDOT:PSS/SWCNT inkjet-printed onto substrates ..................... 19

Figure 7: Resistance of PEDOT:PSS/SWCNT inkjet-printed onto substrates ............................. 19

Figure 8: Conductivity of PEDOT:PSS/SWCNT inkjet-printed onto substrates ......................... 20

Figure 9: Luminance of device A on various substrates. Note Multicoat has 2 replicates ........... 22

Figure 10: Device structure B ....................................................................................................... 23

Figure 11: Device structure C ....................................................................................................... 23

Figure 12: Device structure D ....................................................................................................... 24

Figure 13: Luminance of B devices on various substrates (note Multicoat B has 2 replicates) ... 26

Figure 14: Luminance of C devices on various substrates ........................................................... 27

Figure 15: Luminance of D devices .............................................................................................. 27

Figure 16: Device structure D1 ..................................................................................................... 32

xii

Figure 17: Device structure D2 ..................................................................................................... 32

Figure 18: Mass increase of filled paper samples. Dip coated samples at various dipping times;

and filtration samples with BaTiO3 at various % of nanopowder, the balance being micron-sized

powder and with and without sonication for 1 hour. .................................................................... 33

Figure 19: Total ash content of filled paper samples. Dip coated samples at various dipping

times; and filtration samples with BaTiO3 at various % of nanopowder, the balance being

micron-sized powder and with and without sonication for 1 hour. .............................................. 33

Figure 20: Thickness after calendaring. Dip coated samples at various dipping times; and

filtration samples with BaTiO3 at various % of nanopowder, the balance being BaTiO3 micron-

sized powder and with and without sonication for 1 hour. ........................................................... 34

Figure 21: SEM micrograph of Ahlstrom Grade 992 Filter paper before filling, x100 ................ 34

Figure 22: Filtration, unsonicated solution, 80% BaTiO3 nanopowder, at x100 .......................... 35

Figure 23: Filtration, sonicated solution, 80% BaTiO3 nanopowder, at x100 .............................. 35

Figure 24: Filtration, sonicated solution, 20% BaTiO3 nanopowder, at x100 .............................. 35

Figure 25: Back, filtration, sonicated, 80% BaTiO3 nanopowder at x100 ................................... 36

Figure 26: Filtration, sonicated solution, 20% BaTiO3 nanopowder, coated at x100 .................. 36

Figure 27: Filtration, sonication solution, 80% BaTiO3 nanopowder, coated at x100 ................. 36

Figure 28: Dielectric constant of filled paper samples. Dip coated samples at various dipping


micron-sized powder and with and without sonication for 1 hour. .............................................. 37

Figure 29: Thickness and dielectric constant for filtration method samples with and without

coating at various % of nanopowder, the balance being micron-sized powder ............................ 37

xiii

Figure 30: Thickness and conductivity of 10 printed layers of PEDOT:PSS/SWCNT ink.......... 38

Figure 31: Luminance of coated 20% BaTiO3 nanopowder filter paper device. Note that the first

number refers to the structure type while the second refers to the sample replicate. ................... 40

Figure 32: Luminance of coated 80% BaTiO3 nanopowder filter paper device. Note that the first

number refers to the structure type while the second refers to the sample replicate. ................... 40

Figure 33: Composite structure ..................................................................................................... 52

Figure 34: Composite thickness and dielectric constant ............................................................... 52

Figure 35: 50% phosphor composite device luminance ............................................................... 53

Figure 36: 50% phosphor composite device patterned by inkjet printing cathode ....................... 53

Figure 37: SEM cross section of 50% phosphor composite device on Xerox Supergloss, x130 . 54

Figure 38: SEM cross section of 75% phosphor composite device on Xerox Supergloss, x100 . 54

Figure 39: SEM cross section of 25% phosphor composite device on Xerox Supergloss, x100

(note, composite layer not adhered to paper) ................................................................................ 55

Figure 40: Effect of polydispersion on dielectric constant ........................................................... 56

Figure 41: Luminance of Supergloss A screen printed devices. Note 4 replicates are reported. . 57

Figure 42: SEM cross section of Supergloss A, x100 (screen printed) ........................................ 58

Figure 43: SEM cross section of Xerox Supergloss A, x130 ....................................................... 59

Figure 44: SEM cross section of Multicoat A, x100 .................................................................... 59

Figure 45: SEM cross section of Catalyst Electracote A, x130 .................................................... 60

Figure 46: SEM cross section of Xerox Copy paper A, x100 ...................................................... 60

xiv

Figure 47: SEM cross section of 18.0 Catalyst Directory A, x100 ............................................... 60

Figure 48: SEM cross section of Xerox Supergloss D, x100 ....................................................... 61

Figure 49: SEM cross section of Multicoat B, x100 ..................................................................... 61

Figure 50: SEM cross section of Multicoat C, x100 ..................................................................... 62

Figure 51: SEM cross section of Multicoat D, x140 .................................................................... 62

Figure 52: SEM cross section of Catalyst Electracote B, x120 .................................................... 62

Figure 53: SEM cross section of Catalyst Electracote D, x100 .................................................... 63

Figure 54: SEM cross section of Xerox Copy paper C, x100 ....................................................... 63

Figure 55: SEM cross section of Xerox Copy paper D, x100 ...................................................... 63

Figure 56: SEM cross section of 24.6 Catalyst Directory C, x100 ............................................... 64

Figure 57: SEM cross section of 24.6 Catalyst Directory D, x100 ............................................... 64

Figure 58: SEM cross section of 18.0 Catalyst Directory B, x100 ............................................... 64

Figure 59: SEM cross section of 18.0 Catalyst Directory C, x100 ............................................... 65

Figure 60: SEM cross section of 18.0 Catalyst Directory D, x200 ............................................... 65

Figure 61: SEM cross section of 20% BaTiO3 nanopowder, coated, structure D1, x200 ............ 66

Figure 62: SEM cross section 20% BaTiO3 nanopowder, coated, structure D2, x100 ................. 66

Figure 63: SEM cross section of 80% BaTiO3 nanopowder, coated, structure D1, x100 ............ 67

1

Chapter 1

1 Introduction

1.1 Project motivation and significance

In the information age, electronic displays and lighting play an important role. There are many

applications for these devices which results in a large market for their use. Therefore, the

continued research for efficient, low power, low cost, long life, high luminance devices is

inevitable and vital for consumers and the industry [1].

In 1936, zinc sulfide (ZnS) electroluminescence was discovered by Destriau [1]. From this

discovery, AC-driven powder electroluminescent (ACPEL) devices were developed. ACPEL

devices are typically operated between 50 and 1000 Hz under a sine wave [2]. APCEL device

structure consists of a phosphor and dielectric layer sandwiched between two electrodes. Most

commonly, the bottom-emission structure (Figure 1a), consists of a transparent substrate (glass

or polymer) sputtered with indium tin oxide (ITO) [1]. Then a phosphor layer is solution

processed. The phosphor layer usually consists of phosphor particles (20-25 m [3]) dispersed in

an organic binder with a permittivity () typically between 8 to 15 at 1 kHz, such as

polyvinylidene fluoride (PVDF) with = 8 [4]. The dielectric layer consists of a large dielectric

constant material such as cyanoethyl cellulose or glass [1]. Alternatively, a high permittivity

ceramic can either be dispersed as a powder in an organic binder or sputtered as a thin-film. The

cathode, usually aluminium or silver [1], is typically also sputtered. To protect the device, it is

occasionally encapsulated in a resin [4].

(a)

(b)

Figure 1: ACPEL schematic cross-section (a) bottom-emission structure, (b) top-emission

structure

2

The top-emission structure (Figure 1b) uses the same processes, except starting with the cathode

being deposited onto the substrate. This construct allows the use of opaque substrates, which

opens up the opportunity for paper-based ACPEL devices. It was expected that top-emission

devices would outperform bottom-emission devices because of the lower optical loss and lower

average thickness due to the difference in deposition sequences [5]. It was found that the top-

emission device did in fact have a higher luminance of 210 cd/m2, as opposed to the bottom-

emission device which has a of 150 cd/m2, at 150 VAC, 400 Hz [5]. However, these two devices

were not exactly analogous because the bottom-emission device was deposited onto glass

whereas the top-emission device was deposited on paper by passivating the surface with spin-on-

glass (SOG). In [6], (SOG) again was used to eliminate the porosity of the paper, but retain

flexibility. Screen printing was used to deposit the dielectric and phosphor layers of an ACPEL

device on the paper. The cathode and anode (aluminium and ITO) were deposited using DC

sputtering. The maximum luminance achieved was 210 cd/m2 at 150 VAC and 400 Hz. In [7],

inkjet printing and Meyer rod coating have been used in a fully solution processed ACPEL

device onto various paper substrates resulting in a luminance of 40 cd/m2 at 200 VAC and 60

Hz.

Paper is a low cost biodegradable and flexible porous material that is widely used in roll-to-roll

printing [8]. Paper can be customized to achieve the desirable performance by the addition of

fillers and other additives during the paper-making stage or by surface treatment (e.g. coating) in

the post-formed stage [9]. Conversely, paper can only be used in low temperature conditions,

which limits its use in conjunction with high sintering materials. Furthermore, paper is a porous

substrate, so its interaction with other materials that are deposited on it needs to be taken into

consideration. Also, uncoated paper has a rough surface, but this can be circumvented by coating

the surface of paper typically with a blend of pigments and polymeric binders [9]. Lastly,

although paper is flexible and resilient to mechanical shock, it is also adsorptive and susceptible

to tearing, and wrinkling [10]. Finally, paper has been used as a substrate or mechanical

separation in paper batteries [11] or capacitors [12] and as a functional element in transistors and

batteries [8].

3

1.2 Hypothesis and objectives

It is our hypothesis that:

1. Paper itself can act as an effective dielectric material in ACPEL devices. 2. Device performance can be improved by reducing ink penetration into paper.

The objectives are to:

Solution deposit functional ACPEL devices on paper

Increase the dielectric constant of paper

Characterize the devices in terms of luminance, dielectric constant and layer thickness

4

Chapter 2

2 Literature review

2.1 Background on electroluminescence

Electroluminescence (EL) describes the emission of light upon the application of an electric field

[1]. Electroluminescence can be classified as either charge injection EL or high-field EL (or

true electroluminescence [4]). Charge injection electroluminescence explains the mechanism

for light-emitting diodes (LEDs) where light emission occurs upon the application of a low

electric field to a p-n junction in a semiconductor causing radiative recombination of electron

holes and electrons. The colour of the light emitted is dependent on the bandgap of the

semiconductor. High-field electroluminescence in EL devices instead involves a high electric

field being applied to a phosphor material to cause radiative recombination of holes and

electrons. Phosphors have crystalline structures of the host material, which is doped with one or

more metals or halogens (also called activating agents or luminescent centres [1]) [13]. The

choice of dopant(s) and concentration(s) is what controls the characteristic wavelength of the

colour emission. For example ZnS:Cu,Cl emits blue (~460 nm) or green (~520 nm) depending

on the amount of chlorine [1]. These devices can be driven by AC or DC current. DC driven

devices will not be covered. Furthermore, AC-driven EL devices can be constructed as AC thin-

films electroluminescent (ACTEL) devices or AC powder electroluminescent (ACPEL) devices.

These devices are active displays.

2.2 ACPEL vs. ACTEL

2.2.1 Device structures

In an ACTEL device, the phosphor film is sandwiched between two dielectric films to protect

against dielectric breakdown, whereas in ACPEL devices there is only one dielectric layer

(Figure 2). Although both devices are classified as high-field, ACTEL device electric fields

tend to be on the order of 106 V/cm, whereas ACPEL device electric fields tend to be on the

order of 104 V/cm. Therefore, any imperfections in the phosphor film would result in a short-

5

circuit in the device and catastrophic breakdown would occur. Because of this construction, EL

devices are sometimes referred to as light-emitting capacitors (LECs).

a) ACPEL

b) ACTEL

Figure 2: AC-driven EL device structure (a) ACPEL, (b) ACTEL

2.2.2 Comparing performance

ACTEL and ACPEL devices exhibit a highly non-linear luminance-voltage (L-V) characteristic.

In both cases, below the threshold voltage Vth little light is emitted (< 1 cd/m2). Above Vth,

luminance increases sharply with increasing voltage until saturation. Devices are generally

operated at 30 V above Vth (L30). ACTEL devices tend to have a higher luminance and have a

higher turn-on voltage than ACPEL devices. Furthermore, ACTEL devices operate at higher

voltages compared to APTEL devices at the same frequency [1]. Although ACTEL devices are

generally brighter, they are processed in the solid state, which is more energy intensive than

solution processed ACPEL devices. Solution processing methods that are of particular interest

include printing. Printing, unlike conventional etching methods for patterning, reduces material

waste, and is high throughput. Although various types of electrical components are printed [14],

the printed electronics industry is still at the very early stages of its development.

2.3 ACPEL light emission mechanism

The light emission mechanism for ACPEL devices is not widely agreed upon. In 1963, Fischer

proposed ( [15], [16]) that in a ZnS:Cu phosphor particle, the hexagonal structure of the ZnS

phosphor during sintering is transformed to the cubic structure during cooling. In this process,

copper precipitates on the defects in the ZnS particles causing internal CuxS conducting needles.

As a result, hetero-junctions are formed between CuxS precipitates and the ZnS host that

concentrate the electric field tunnelling holes and electrons. When the electric field is reversed,

6

the electrons and holes recombine causing EL emission. Therefore, upon application of electric

field above a threshold value, pairs of small bright points form. When the electric field is

increased, these bright points extend to each other to form comet-shaped emissive regions.

In 2007, Grzeskowiak et al. [17], observed small 1-2 m aligned bright points in ZnS:Cu

particles near the surface. With increasing voltage, the number of bright points increased and the

EL emission was observed on the surface of the particle adjacent to the positive polarity of the

applied field. This means that the electrons accelerate to the surface and radiatively recombine

upon reversal of the electric field at Cu+ sites. Therefore, the near-surface EL emission does not

support Fischers model.

2.4 ACPEL device operation and features

Although ACPEL devices operate in excess of 150 VAC, the current drawn typically is less than

1 mA/cm2 (at < 1 kHz). Therefore ACPEL devices are considered low power devices [18]. It is

understood that changes in operating frequency affects emission wavelength and emission

intensity, whereas changes in operating voltage affects emission intensity, only [19].

Furthermore, increasing frequency results in a proportional increase in luminance and a

proportional decrease in lifetime. Therefore, luminance and lifetime are trade-off characteristics

[1]. The following equation shows the relationship between luminance, L, and applied voltage,

V, where L0 and b are constants determined by the material:

Equation 1

Below the threshold voltage, ACPEL devices can be approximated as two capacitors in series

[20] where, ED is the electric field applied in the dielectric layer; EP is the electric field applied in

the phosphor layer; D is permittivity of the dielectric; and P is the permittivity of the phosphor,

Equation 2

The total applied voltage (Vtot) is divided between each layer [20], where dD is the thickness of

the dielectric layer; and dP is the thickness of the phosphor layer,

7

Equation 3

By rearranging the above equations, the efficiency of the device can be described as [20],

Equation 4

Therefore, to maximize the applied electric field, the thickness of the dielectric layer must be

minimized and the permittivity maximized.

Generally speaking, ACPEL devices are robust, low cost, and can be operated over a wide

temperature and altitude range [21]. Furthermore, they can be fabricated in atmospheric

environmental conditions due to their relative insensitivity to oxygen and humidity [14]. Doped

ZnS is the most common phosphor used in these devices and phosphors can be mixed to tune

colour emission [21].

Since, ACPEL devices are low luminance devices, various improvements have been attempted in

order to increase their luminance. It was found that the addition of 0.1% TiO2 to the phosphor

layer increased the luminance from approximately 325 to 375 cd/m2 at 50 VAC and 26 kHz. It

was hypothesized that TiO2 acts as an oxidizing agent and therefore provides electrons to the

phosphor layer [22]. However, TiO2 is a well-known pigment used for its brightness and high

refractive index. Therefore, the observed increase in luminance could be partly due to reflection

of light by TiO2 particles. Carbon nanotubes can also improve the performance of ACPELs. The

addition of 1% single wall carbon nanotubes (SWCNT) to the phosphor layer has been reported

to increase the luminance from 0 to 35 cd/m2 at 300 VAC and 10 kHz. It was hypothesized that

SWCNT enhanced the local electric field and allowed electron injection to the phosphor particles

[23]. Furthermore, in order to increase the electron injection into the device, SWCNTs have been

added to the dielectric layer. Adding 1.5% SWCNT to the dielectric layer increased the

luminance from approximately 350 to 400 cd/m2 at 1 kHz [24].

8

2.5 Materials in ACPEL devices

2.5.1 ZnS-based phosphors

ZnS is a semiconductor material with a bulk bandgap of 3.58 eV [25]. Phosphor materials are

usually encapsulated in order to prevent decomposition from the application of an electric field

in the presence of moisture [21].

Phosphors with a characteristic emission peaks are usually mixed in order to achieve a white

emitting device. In [26], a single synthesis involving a two-step firing and subsequent milling

process produced 20 m white-emitting ZnS:Mn, Cu, Cl phosphors. An ACPEL device was

screen printed and tested. Although the emission spectra were collected at increasing

concentrations of Cu and Mn ions, voltage and frequency, CIE coordinates were not reported.

A novel precipitation and subsequent firing process was developed in order to produce ZnS:Cu,

ZnS:Cu, Al, and ZnS:Cu, Al, Au phosphors [27]. This process results in phosphors with

improved luminescent properties than commercial materials.

2.5.1.1 Doped ZnS nanoparticles

Quantum dots are inorganic semiconductor nanocrystals (NCs) with a diameter less than 10 nm

which is less than the diameter of a Bohr exciton. Quantum confinement effects result from

spatial confinement of electrons and holes to the dimensions of the material [28].

ZnS quantum dots doped with Cu2+

and Mn2+

were synthesized by chemical precipitation and

inkjet-printed onto photographic quality inkjet paper, photocopy paper, cotton fabric,

polyethylene terephthalate (PET), ITO-coated PET, silicon wafer, and wool. The particle

diameter was 40-80 nm. It was found that photoluminescence (PL) was only preserved when

printing on photo-quality inkjet paper and cotton fabric because the nanoparticles did not absorb

into the substrate [29].

ZnS:Mn NCs have been reported to exhibit high PL efficiencies when smaller than 5 nm, which

makes them suitable for EL devices. A single layer DC-driven EL device using these NCs was

constructed in [30]. The maximum luminance from the device was reported 0.45 cd/m2 at 42 V.

9

Above 42 V, dielectric breakdown occurred resulting in decreased luminance and current

density.

In [31], nearly monodisperse ZnS:Mn NCs were synthesized in an aqueous co-precipitation

method, characterized, dispersed into an organic solvent, and fabricated into an AC driven EL

device. The size of the NCs was controlled by varying the concentration of Zn2+

. The NCs were

estimated to have a diameter of 3-4 nm. This phosphor material was subsequently incorporated

in a ACPEL device. The device was fabricated by depositing ZnO:Al, as a transparent electrode

and Si3N4, as the insulating layer using radio-frequency magnetron sputtering onto glass. The

NCs were incorporated into an organic resin with a solvent to screen print the layer. Finally,

thermal evaporation was used for the Al electrode. The resulting device had a luminance of

approximately 1 cd/m2 at 160 VAC and 5 kHz

2.5.2 Dielectric materials

As mentioned previously, the dielectric layer can either be solution or solid-state processed. SOG

materials and tetraethylorthosilicate (TEOS) were deposited using spin-coating in a double

dielectric ACPEL device [32]. Although this is not the conventional structure for ACPEL

devices, it was found the resulting luminance was 48.9 and 74.5 cd/m2, respectively at 150 VAC

and 400 Hz.

In order to increase the dielectric constant of the dielectric layer, high dielectric ceramics can be

incorporated into the organic binder. Commonly used ceramics include barium titanate (BaTiO3)

( [5], [7], [6]) and silicone dioxide (SiO2) ( [22], [23]). It has been found that nanopowder

BaTiO3 increased the performance of ACPEL devices as opposed to micron-sized powder likely

because of the increased packing density. Dielectric constant increased from 55 for 2 m

particles to 99 for 300 nm particles. Consequently, the luminance of the device fabricated from

the above materials increased from 150 to 182 cd/m2 at 150 VAC and 400 Hz [33]. Another

advantage of using nanoparticle ceramics is the ability to employ inkjet printing as a solution

deposition method ( [34], [35], [36]).

10

2.5.3 Electrode materials

Conventionally, the electrodes are deposited via sputtering with aluminium, silver, or ITO [1].

However these materials require high sintering temperatures (>200C) [37] which is not practical

for some substrates such as paper. Furthermore, although ITO is favourable because of its

conductivity and transparency, it has a brittle nature [38] and has limited processability which

does not make it a suitable electrode for flexible electronics.

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (or PEDOT:PSS) can also be used as

both a transparent ( [39], [7]) and back electrode [7]. Although it has a low conductivity

compared to metals, the conductivity can be enhanced with the addition of SWCNTs and it can

be easily dispersed in aqueous inks for inkjet printing ( [40], [41]). Furthermore, it can be

integrated in flexible electronics.

Another common electrode material is silver and silver nanoparticles. Silver nanoparticles have

been inkjet-printed onto a flexible opaque substrate to create a back electrode for composite

ACPEL devices [42]. Due to the high conductivity of silver, the resulting device had a relatively

high luminance value of 250 cd/m2 Furthermore, line spacing in the inkjet printing process was

investigated as a means to create half-tone, or grey scale, images [14].

2.5.4 Substrates

As mentioned above, common transparent substrates are ITO-coated glass and polymers. Some

common polymers include PET [43] and polyethylene naphthalate [44]. An ACPEL device was

printed onto a PET open-mesh fabric and PEDOT:PSS was inkjet-printed as the transparent

electrode. The resulting device had a luminance of 44 cd/m2 at 400 VAC and 400 Hz which is

lower than the luminance of an analogous device using ITO-glass (96 cd/m2) due to the lower

transmittance (23% vs. 80 % at 550 nm) and higher electrical resistance of PEDOT:PSS

compared to ITO (approx. 550 /sq vs. 15 /sq) [39]. In [45], paper and textiles were used as a

substrate for screen-printed ACPEL devices. Although luminance was not reported, the devices

were functional at 1000 VAC and 1 kHz.

11

2.6 ACPEL developments

A simplified approach to ACPEL devices is to combine the phosphor and dielectric layers into a

composite layer. This composite layer consists of an organic binder, phosphor powder, and

dielectric powder. This composite is sandwiched between two electrodes. The advantages being

that the fabrication steps and costs are reduced. In [46], a rhodamine dye was incorporated into

the composite layer. The blue-green emitting ZnS:Cu phosphor was used to excite red-orange

emitting rhodamine dye resulting in a luminance of 200 cd/m2 at 100 VAC and 400 Hz with this

architecture. The combination of the blue-green and red-orange emissions resulted in a white

emission with CIE coordinates of (0.336, 0.380).

A composite ACPEL device, having a BaTiO3, ZnS-based phosphor and transparent conductive

oxide (In2O3 or SnO2) was compared to a conventional a two-layered ACPEL device, having a

BaTiO3 dielectric and a ZnS phosphor/transparent conductive oxide layer. It was found that the

addition of 1% In2O3 increased the EL intensity by approximately 1.2 times in the composite

device at 220 VAC and 60 Hz, but no additional benefits were observed in the 2-layer device.

The 2-layer device had an EL intensity of approximately 1.4 times that of the composite device

at 220 VAC and 60 Hz [47].

A tandem structure containing two alternating dielectric and phosphor layers was also

investigated. Three different phosphor materials were studied, including phosphor, nano-

phosphor, and nano-phosphor/SWCNT. The resulting ACPEL devices had a luminance of 440,

3370 and 4358 cd/m2, respectively at 230 VAC and 1 kHz [48].

2.7 Solution deposition techniques

Screen printing is a method whereby a mesh is used to meter and distribute the coating material.

This is typically done by using a squeegee to spread the ink over the screen [49], while the

patterned area is defined by a mask. Screen printing is a common method for ACPEL device

deposition ( [26], [6], [45], [47]).

12

In lithography, an intermediate roller called an impression cylinder is used to transfer the ink

from a printing plate cylinder to the substrate. Lithography is a high speed and low-cost per sheet

printing process. Off-set lithography was used to deposit silver nanoparticles in an interdigitated

electrode structure ( [50], [51]). Lithographic printing was also used for the phosphor layer and

in an ACPEL device, and the resulting construct had a luminance of 10 cd/m2 at 270 VAC and

400 Hz [52].

Pad printing is an alternative method for patterning on flat and curved surfaces. In pad printing,

an engraved plate is coated with ink, a silicon pad contacts the plate and transfers the pattern.

The pad then makes contact with the substrate to transfer the pattern. An ACPEL device was

printed onto a ceramic dish with 80 mm radius of curvature [44]. The fabricated ACPEL device

had a luminance of 180 cd/m2 at 200 VAC and 1 kHz.

Inkjet printing is a versatile patterning tool for dispersed inorganic particles ( [53], [54], [55],

[56],) dissolved or dispersed polymers, metal nanoparticles and dissolved organics [57]. Inkjet

printing is considered as a drop-on-demand (DoD) process that allows for the accurate

patternation of controlled amounts (in the order of picolitres) of ink on the substrate. Due to its

flexibility and high throughput nature, inkjet printing promises to be a cost effective technology

for printed electronics.

Doctor blading is a common coating technique where a blade is used to spread a thin layer of

coating material on a substrate. In [58], an ACPEL device was deposited using doctor blading

and operated at 250 VAC and 2 kHz. However, the resulted luminance was not reported by the

authors.

Spin coating is a widely used technique that relies on centrifugal force to spread a coating

material into a thin film [49]. Spin coating has been reported for the fabrication of a composite

ACPEL device with a maximum luminance of 111 cd/m2 at 150 VAC and 400 Hz [43].

13

2.8 Commercial products

Several companies in the past and present have worked on the commercialization of ACPEL and

ACTEL devices. They include Elumin8 Systems and Luminous Media [46], iFire Technology

Corp [59], CeeLite Technologies, E-Lite Technologies, Dante Technologies, LimeLite

Technologies and Planar Systems.

Despite significant progress in recent years, to date there is no paper based fully printed flexible

ACPEL device commercially available in the market. Such a device could find wide applications

in smart packaging, sensors, smart textiles, etc. In this thesis, we present novel fully printed

flexible ACPEL designs and examine their performance.

14

Chapter 3

3 ACPEL devices on paper

3.1 Experimental methods

3.1.1 Device preparation

In this chapter, a paper-based fully printed ACPEL is investigated using the top-emission

structure as shown in Figure 3. This structure will hereafter be referred to device structure A. A

composite structure, where the phosphor (GG65/PVDF) and dielectric (BaTiO3/PVDF) are

combined into one resin layer, was also considered (Appendix A).

Figure 3: Device structure A

Table 1 shows the six substrates that have been chosen. Xerox Supergloss is a coated paper that

retains ink on the surface instead of absorbing into the paper fibres unlike Xerox Copy paper

which is uncoated. Therefore, it logically follows that device A on Xerox Supergloss should

perform better than the Xerox Copy paper. The Multicoat paper [60], is a non-commercial coated

paper that has superior ink retention. Coated and uncoated paper from Catalyst Paper

Corporation were used because of their thinness (

15

Multicoat [60] Coated 125 88

34 lb Catalyst Electracote Gloss

Coated 50 55

Xerox Copy paper Uncoated 72 100

24.6 Catalyst TD Directory Uncoated 40 71

18.0 Catalyst TD Directory Uncoated 29 53

Ten 1.2 cm x 1.2 cm pixels were patterned by using a piezoelectric inkjet printer (Dimatix-

Fujifilm DMP2831) to deposit the cathode with a PEDOT:PSS/SWCNT ink [41] [40] [61].

During printing the platen was heated to 60 C. The PEDOT:PSS/SWCNT ink formulation that

was used [41] contained 1.3 w/w% PEDOT:PSS and 0.04 w/w% single walled carbon nanotubes

dispersed in water, as well as other additives for optimized jetting (Table 2). All reagents were

used as received and supplied by Sigma-Aldrich Canada, except where specified. Since the

PEDOT:PSS/SWCNT ink has a relatively low conductivity [62], 10 layers of the conductive ink

were printed at a drop spacing of 25 m for the cathode. Immediately after inkjet printing, the

electrodes were dried on a hotplate in air at 120C. The anode was applied with a paint brush

using DuPont LuxPrint 7164 translucent conductor (10 k/sq/25m). DuPont 7102 carbon

conductor was applied as contact points.

Table 2: PEDOT:PSS/SWCNT based ink formulations [41]

w/w % Component

34 PEDOT:PSS dispersion (1.3% in water)

10 DMSO

17 Glycerol

0.5 Sodium lauryl sulfate

0.5 Surfynol DF-110D Defoamer (Air Products)

10 SWCNT suspension

28 Water

The dielectric layer was deposited by mask-printing. A 20 cm x 2 cm rectangle was cut out of an

acetate sheet and used as a stencil. A polymer blend dissolved in dimethyl acetamide (DMAc)

was used as a binder in the dielectric resin (Table 3). Polyvinylidene fluoride (PVDF) was

chosen for its dielectric properties [4]. It is important to note that PVDF and BaTiO3

nanocomposites exhibit dielectric relaxation where the dielectric constant decreases at high

frequencies [63]. Polyvinylpyrrolidone (PVP) and poly(methyl methacrylate-co-ethyl acrylate)

16

(PMMAEA) were added to decrease the hydrophobicity of PVDF [64]. BaTiO3 nanopowder was

added to increase the dielectric properties [1]. The use of

17

Figure 4: Conductor with uniform cross-sectional area A

Conductivity, (Equation 5), can be calculated as the inverse of resistivity, (Equation 6) by

measuring resistance, R, with a digital multimeter (Equus 4320).

Equation 5

Equation 6

1 to 10 layers of the PEDOT:PSS/SWCNT ink were inkjet-printed on all 6 substrates in 1 cm x 1

cm squares. DuPont 7102 carbon conductor (20-30 /sq/mil) bus bars were applied to two sides

with a silver contact point on top (Conductive silver pen, Polysciences, Inc.). A scalpel was used

to cross-section the sample perpendicular to the direction of printing. The cross-section was

placed vertically on modelling clay and examined under an OMAX MD827S30L microscope

equipped with a digital camera [40] to determine ink layer thickness. Images were stacked using

Combine Z software and analyzed using ImageJ.

3.1.3 Device characterization

The thickness of each resin layer was determined by examining the cross-section under SEM.

Energy dispersive spectrometry (EDX) was performed with the SEM to confirm the presence of

the elements in the layers. The devices were tested at 60 Hz using a Chroma 61501 AC Power

Source and luminance was measured using a Minolta LS-110 luminance meter. All devices were

considered for this measurement except for 24.6 Catalyst Directory.

A

18

3.2 Results

3.2.1 Substrate effects on conductivity

The six substrates vary in roughness, porosity, ink absorbency, thickness, and composition as

discussed above. Conductivity of the PEDOT:PSS/SWCNT electrode and therefore the device

performances should reflect these properties. Coated sheets will retain the PEDOT:PSS/SWCNT

ink on the surface, increasing conductivity and resulting luminance. Figure 5 shows SEM

micrographs of the six paper types. All coated sheets were smooth with some defects while the

uncoated sheets had uneven surfaces and open structures.

Figure 5: SEM micrographs of paper surfaces at x500. (a) Xerox Supergloss; (b) Multicoat;

(c) 34 lb Catalyst Electracote Gloss; (d) Xerox Copy paper; (e) 24.6 Catalyst TD

Directory; (f) 18.0 Catalyst TD Directory

In Figure 6, it can be seen that the coated sheets (Xerox Supergloss, Multicoat and Catalyst

Electracote) resulted in thinner electrodes. Multicoat had the lowest thickness and therefore can

be expected to result in the highest conductivity. The uncoated sheets (Xerox Copy paper, 24.6

Catalyst Directory and 18.0 Catalyst Directory) had thicker electrodes because the ink penetrated

into the paper. In particular, 24.6 Catalyst Directory had the largest thickness and can be

expected to result in the lowest conductivity. The 18.0 Catalyst Directory paper had comparable

ink thickness to the coated sheets, possibly due to its thinness which limits the ink penetration.

a) c) b)

d) e) f)

19

Figure 6: Layer thickness of PEDOT:PSS/SWCNT inkjet-printed onto substrates

In Figure 7, the resistance of the PEDOT:PSS/SWCNT ink on the substrates is shown. As can be

seen, the Multicoat paper had the lowest resistance as expected, followed by Catalyst Electracote

and Xerox Supergloss. The uncoated Catalyst Directory sheets performed similarly in terms of

resistance and Xerox Copy paper has the highest resistance.

Figure 7: Resistance of PEDOT:PSS/SWCNT inkjet-printed onto substrates

0

10

20

30

40

50

60

70

80

0 2 4 6 8 10 12

Th

ick

nes

s (

m)

Layers Xerox Supergloss Multicoat Catalyst Electracote

Xerox Copy paper 24.6 Catalyst Directory 18.0 Catalyst Directory

0.1

1.0

10.0

100.0

1000.0

0 2 4 6 8 10

Res

ista

nce

(K

)

Layers Multicoat Xerox Supergloss

Catalyst Electracote Xerox Copy paper

24.6 Catalyst Directory 18.0 Catalyst Directory

20

In Figure 8, it can be seen that the Multicoat paper results in superior conductivity. The surface

of the Multicoat paper is coated with a blend of Kaolin clay and a styrene-butadiene latex binder,

which creates a low porosity and smooth surface [60]. Aside from the Multicoat paper, Catalyst

Electracote had the highest conductivity. It is expected that Xerox Supergloss would also be a

high performing substrate but in fact it had the lowest overall conductivity. This could be due to

the coating material mixing in with the PEDOT:PSS/SWCNT ink and lowering the conductivity.

As mentioned above 18.0 Catalyst Directory paper was the thinnest sheet and therefore the ink

penetration was limited to the thickness of the paper and resulted in the highest conductivity of

the uncoated sheets followed by 24.0 Catalyst Directory and Xerox Copy paper.

Figure 8: Conductivity of PEDOT:PSS/SWCNT inkjet-printed onto substrates

3.2.2 Substrate effects on luminance

Using the SEM micrographs, the thickness of the dielectric and phosphor layers were estimated

(Appendix D).

As can be seen in Table 4, the coated papers tended to luminesce more evenly than uncoated

papers. Additionally higher basis weight papers were less wrinkled and therefore had a more

even distribution of materials and subsequently a more even luminescence.

0

100

200

300

400

500

0 2 4 6 8 10 12

Co

nd

uct

ivit

y (

S/m

)

# of Layers

Xerox Supergloss Multicoat Catalyst Electracote

Xerox Copy paper 24.6 Catalyst Directory 18.0 Catalyst Directory

21

Table 4: A Devices off and on (~190 VAC)

Device Off On, ambient light On, darkened room

Supergloss A

No image available

Multicoat A

Catalyst Electracote A

Copy paper A

26.4 Catalyst

Directory A

Not considered

18.0 Catalyst

Directory A

As seen in Figure 9, coated papers had higher luminance than uncoated papers. This is likely

because of a better retention of the ink and resin on the surface of substrates. Multicoat and

Xerox Supergloss had the highest luminance and lowest turn-on voltage. Although Catalyst

Electracote paper produced a high conductivity cathode layer, it resulted in a low luminance

device. On the other hand, 18.0 Catalyst Directory paper that had a comparable cathode

conductivity to the Catalyst Electracote, resulted in a higher luminance. This could be due to a

thicker phosphor layer, and thinner dielectric layer. Furthermore, low basis weight papers tended

to wrinkle when the PEDOT:PSS/SWCNT ink and resins were applied and dried. This uneven

surface could cause an uneven distribution of resin material and therefore a lower luminance.

22

Figure 9: Luminance of device A on various substrates. Note Multicoat has 2 replicates

3.3 Conclusions

As can be seen in Table 5, in general, coated papers performed better than uncoated papers.

Although 18.0 Catalyst Directory paper is uncoated, the phosphor layer was more than twice as

thick as the phosphor layer in other A devices and explains its high luminance. Higher luminance

devices tend to have a thinner dielectric layer than low luminance devices. Furthermore, thicker

dielectric layers resulted in a higher turn-on voltage.

Table 5: Summary of device A performance

Substrate Dielectric

layer thickness

(m)

Phosphor layer

thickness (m) Turn-on voltage

(VAC)

Maximum

luminance

(cd/m2)

Xerox Supergloss 31 19 50 5.68

Multicoat 46 24 50 8.05

Catalyst Electracote 100 47 75 3.16

Xerox Copy paper 113 46 100 0.97

24.6 Catalyst Directory Not considered

18.0 Catalyst Directory 63 104 50 7.56

0.0

2.0

4.0

6.0

8.0

10.0

0 50 100 150 200 250 300

Lum

inan

ce (

cd/m

2 )

Voltage (VAC)

Supergloss A Multicoat A1 Multicoat A2

Electracote A Copy paper A 18.0 Catalyst Directory A

23

Chapter 4

4 Novel design for ACPEL devices on paper

Paper is a weak dielectric compared to the dielectric materials that are generally used in ACPEL

devices. The novel device structures presented in this chapter not only use the paper substrate as

mechanical support, but also incorporate the paper as a functional layer of the device. The goal is

to move towards the elimination of the dielectric layer and optimize the performance of these

devices. In this chapter, three novel constructs were examined as described below. In device

structure B, a dielectric resin (BaTiO3/PVDF resin) was coated onto the paper to off-set the low

dielectric constant of paper (Figure 10). We can expect a lower device performance than in

device A because of the larger overall dielectric layer thickness (paper and BaTiO3/PVDF

resin). This dielectric layer is expected to have a lower dielectric constant than the

BaTiO3/PVDF layer alone. Furthermore, the BaTiO3/PVDF resin was applied directly to the

paper with no PEDOT: PSS/SWCNT layer to act as a barrier.

In device structure C (Figure 11), the BaTiO3/PVDF resin was applied before the PEDOT:

PSS/SWCNT electrode was printed on the substrate. This could increase the conductivity of the

cathode, because of the hydrophobic nature of the resin which would retain ink on the surface.

The phosphor layer (GG65/PVDF) was applied directly on the paper.

Figure 10: Device structure B

Figure 11: Device structure C

In structure D (Figure 12), paper was used as is. Although paper is not a strong dielectric, this

structure reduces the thickness of the dielectric layer and hence allows a higher electric field in

the phosphor layer.

24

Figure 12: Device structure D


4.1.1 Device preparation and characterization

The same six substrates as in Chapter 3 were investigated in this chapter. Devices were prepared

and characterized in the same way as in Section 3.1. For papers that are coated, the

PEDOT:PSS/SWCNT layer was printed on the coated side of the paper. All devices were

considered for detailed analysis except Xerox Supergloss for structures B and C; Catalyst

Electracote for structure C; Xerox Copy paper for structure B; and 24.6 Catalyst Directory for

structure B in order to simplify the results.

4.2 Results

4.2.1 Effect of structure on luminance

Using the SEM micrographs as provided in Appendix E, the thickness of the dielectric layer

(where applicable) and phosphor layers were estimated.

As can be seen in Table 6, thinner sheets result in a higher luminance device. The brightest

devices tend to be D devices, followed by C and then B.

Table 6: Devices B, C, and D off and on (~190 VAC)

Structure Off, front Off, back On, ambient light On, darkened

room

Supergloss B Not considered

C Not considered

D

No luminance

25

Multicoat B

C

D

Catalyst

Electracote

B

C Not considered

D

Xerox Copy

paper

B Not considered

C

D

24.6

Catalyst

Directory

B Not considered

C

D

18.0

Catalyst

Directory

B

26

C

D

Figure 13 shows the luminance results from B devices. 18.0 Catalyst Directory has the highest

performance, possibly because it is the thinnest substrate.

Figure 13: Luminance of B devices on various substrates (note Multicoat B has 2 replicates)

Figure 14 shows the luminance of C devices. 18.0 Catalyst Directory and Copy paper exhibit the

highest luminance. Multicoat and 24.6 Catalyst Directory resulted in the lowest luminance.

Multicoat was the thickest of these substrates and therefore it was expected to have a lower

luminance. The low luminance of the 24.6 Catalyst Directory sheet could be a result of the low

conductivity of the PEDOT:PSS/SWCNT ink on that substrate.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 50 100 150 200 250 300 350

Lum

inan

ce (

cd/m

2)

Voltage (VAC)

Multicoat B1 Multicoat B2 Catalyst Electracote B 18.0 Catalyst Directory B

27

Figure 14: Luminance of C devices on various substrates

Figure 15 shows the luminance of D devices. The uncoated substrates (Xerox Copy paper, 18.0

and 24.6 Catalyst Directory) resulted in a higher luminance than the Multicoat sheet. Although

the conductivity of the PEDOT:PSS/SWCNT ink was less on these sheets, they were thinner

substrates compared to the Multicoat paper.

Figure 15: Luminance of D devices

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 50 100 150 200 250 300 350

Lum

inan

ce (

cd/m

2 )

Voltage (VAC)

Multicoat C Xerox Copy paper C

18.0 Catalyst Directory C 24.6 Catalyst Directory C

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 50 100 150 200 250 300 350

Lum

inan

ce (

cd/m

2 )

Voltage (VAC)

Multicoat D Electracote D Xerox Copy paper D

18.0 Catalyst Directory D 24.6 Catalyst Directory D

28

4.3 Conclusions

As can be seen in Table 7, device D generally had the highest luminance and lowest turn-on

voltage for each substrate. For B and C devices, higher luminance values were achieved when

the dielectric layer was thinner. Furthermore, in general thicker phosphor layers resulted in

higher luminance.

Table 7: Summary of devices B, C, and D performance

Substrate Structure Dielectric

layer thickness

(m)

Phosphor layer

thickness (m) Turn-on

voltage

(VAC)

Maximum

luminance at 300

VAC (cd/m2)

Xerox

Supergloss

B Not considered

C Not considered

D n/a 209 No luminance

Multicoat B 100 80 225 0.18

C 111 24 150 0.56

D n/a 30 125 0.85

Catalyst

Electracote

B 40 51 100 2.27

C Not considered

D n/a 22 75 1.68

Xerox Copy

paper

B Not considered

C 100 48 75 3.16

D n/a 83 50 7.24

24.6 Catalyst

Directory

B Not considered

C 79 100 175 0.18

D n/a 117 75 4.1

18.0 Catalyst

Directory

B 35 35 100 5.1

C 41 69 100 3.15

D n/a 81 75 5.67

29

Chapter 5

5 Novel design for ACPEL devices on BaTiO3-filled paper

It is common practice to add fillers to the papermaking furnish to improve paper performance. In

this study BaTiO3 powder was considered as filler to improve the dielectric properties of paper.


5.1.1 Substrate preparation

Impregnation of the substrate was carried out by two methods: dip-coating and filtration. In the

dip-coating procedure, the sheets were dipped into a suspension of BaTiO3 nanopowder and

polymer. In the filtration method, various solutions of BaTiO3 nano- and micron- sized powder

were passed through filter paper to fill the pores.

5.1.1.1 Dip-coating

24.6 Catalyst TD Directory was impregnated with a solution containing BaTiO3 nanopowder

and poly(methyl methacrylate) (PMMA) The formulation is shown in Table 8. Dispersion was

achieved by sonicating for 1 hour. Samples were dipped into the solution for 5, 10, and 15

minutes and then heated flat on a hotplate in air at 120 C. The solution was not refreshed

between samples. The methyl methacrylate was polymerized in situ by heating after dip-coating.

Table 8: Dip-coating formulation

w/w% Component

5 BaTiO3 nanopowder (

30

5.1.1.2 Filtration

Filtration was used to impregnate Ahlstrom Grade 992 Filter paper (cellulose, 46 gsm, 150 m

thickness, particle retention 43 m). Samples were cut into 15 cm diameter circles and placed in

a Noram sheet former. Nanoparticle BaTiO3 (

31

Table 11: Coating composition

w/w % Component

89.5 Hydrocarb 90 (Omya Canada)

10 Styronal ND 656 (BASF)

0.5 Dispex N40 V (BASF)

The coating was applied by Meyer rod (gauge 0.003 in) and left to dry at 50 C in air for 2

minutes. The coated filter paper was then calendared once at 100 kPa and 50 C.

5.1.2 Substrate characterization

The thickness of the sheets was measured using a TMI 49-61 Micrometer before and after

impregnation and after calendering. The samples were also weighed before and after

impregnation and calendaring to determine the BaTiO3 loading. To confirm that the mass

increase was due to BaTiO3 loading, the ash content of samples was also measured by

incineration at 500 C in a Barnstead Thermolyne FB1400 furnace for 1 hour. SEM micrographs

were taken before and after filtration to examine the structure of the neat papers and the

distribution of filler material on the surface of the sheets after impregnation.

Dielectric constant () was measured with an Agilent U1701B Handheld Capacitance Meter

through the sheet by applying silver electrodes with a silver conductive pen and treating the

sample as a parallel plate capacitor [65]. Then was calculated using [66]:

Equation 7

By measuring capacitance C, the thickness of the dielectric layer between the electrodes d, the

sample area A and knowing the vacuum permittivity 0, the dielectric constant can be

calculated. These samples were prepared specifically for this measurement and were separate

from the EL devices.

5.1.3 Conductivity of PEDOT:PSS/SWCNT ink

The conductivity was estimated by printing 10 layers of the PEDOT:PSS/SWCNT ink using the

same procedure as Section 3.1.2.

32

5.1.4 Device preparation

In this chapter, EL devices were prepared according to the D structure as described earlier.

Since the dip-coated sheets did not dry, they were not considered a viable option and therefore

were not used for device preparation. For the filtration method, EL devices were prepared with

coated and uncoated sheets containing two levels of BaTiO3 nanopowder, namely 20% and 80%.

Two configurations were investigated (Figure 16 and Figure 17).

Figure 16: Device structure D1

Figure 17: Device structure D2


The thickness of the phosphor layer and paper was determined by examining the cross-section

under SEM. The devices were tested using the same procedure as section 3.1.3.

5.2 Results

5.2.1 Substrate characterization

As can be seen in Figure 18, the mass increase for the dip-coated samples was 10 times higher

than that of the filtered samples, however, the higher mass was mainly due to the higher moisture

content of these samples. Furthermore, results show that in the filtration method, sonication

improves the overall retention of BaTiO3. Also, it is important to note that in the filtration

method, although the amount of BaTiO3 that was added equaled to 50% of the mass of the filter

paper, less than 30% of this material was retained in all cases.

33

Figure 18: Mass increase of filled paper samples. Dip coated samples at various dipping


micron-sized powder and with and without sonication for 1 hour.

When the mass increase was compared to the total ash content (Figure 19), it can see seen that

the amount of inorganic material (BaTiO3) for the dip-coated samples was less than the amount

in the filtration method. It is important to note that the paper itself had approximately 3 w/w %

ash content. The mass increase of the dip-coated sheets was an exaggerated value because the

samples were saturated with methyl methacrylate and PEG 300.

Figure 19: Total ash content of filled paper samples. Dip coated samples at various dipping



0%

20%

40%

60%

80%

100%

120%

140%

160%

5 min 10 min 15 min 0%

nano

20%

nano

50%

nano

80%

nano

100%

nano

0%

nano

20%

nano

50%

nano

80%

nano

100%

nano

dip-coated filtration, unsonicated solutions filtration, sonicated solutions

Mas

s In

crea

se

0%

5%

10%

15%

20%

25%

30%

5 m

in

10

min

15

min

un

trea

ted

0%

nan

o

20

% n

ano

50

% n

ano

80

% n

ano

10

0%

nan

o

un

trea

ted

0%

nan

o

20

% n

ano

50

% n

ano

80

% n

ano

10

0%

nan

o

un

trea

ted

dip-coated filtration, unsonicated solution filtration, sonicated solution

Ash

Conte

nt

(w/w

%)

34

Figure 20 shows that the loading of BaTiO3 does not appreciably affect the thickness of the

sample after calendaring compared to the untreated case. Furthermore, dip-coating and filtration

with and without sonication result in a sheet with comparable thickness In addition, the final

thickness of impregnated sheets was comparable to that of the commercial papers used in

previous chapters.

Figure 20: Thickness after calendaring. Dip coated samples at various dipping times; and

filtration samples with BaTiO3 at various % of nanopowder, the balance being BaTiO3


SEM micrographs of filter paper before and after impregnation are provided in Figure 21, Figure

22, and Figure 23. As can be seen, the filter paper had an uneven surface and an open structure of

pores.

Figure 21: SEM micrograph of Ahlstrom Grade 992 Filter paper before filling, x100

0

20

40

60

80

100

5 m

in

10

min

15

min

un

trea

ted

0%

nan

o

20

% n

ano

50

% n

ano

80

% n

ano

10

0%

nan

o

un

trea

ted

0%

nan

o

20

% n

ano

50

% n

ano

80

% n

ano

10

0%

nan

o

un

trea

ted


Thic

knes

s (

m)

35

After filtration, these pores were filled with BaTiO3. The white areas in Figure 22 and Figure 23

indicate the presence of BaTiO3 as confirmed by SEM-EDX. As predicted, sonication improved

distribution and filling of BaTiO3 into the filter paper. Even though the sonicated samples

appeared to be more uniform than the unsonicated samples, they still contained unfilled regions

which could create undesirable variations in the dielectric properties of the sheet. Furthermore,

the non-uniformities in the BaTiO3 loading will affect the uniformity of the device luminance.

Samples containing 20% nanopowder (Figure 24) had a more even distribution of BaTiO3 on the

surface compared to the 80% nanopowder case. Since the amount of BaTiO3 retained was similar

for both sheets, this result suggests BaTiO3 nanopowder more readily penetrated and filled the

pores of the filter paper.

Figure 22: Filtration, unsonicated

solution, 80% BaTiO3 nanopowder,

at x100

Figure 23: Filtration, sonicated


at x100

Figure 24: Filtration, sonicated solution, 20% BaTiO3 nanopowder, at x100

The back of the sample was also investigated (Figure 25). From this image, it can be seen that

the filter paper was not fully filled with respect to its thickness.

100 m

36

Figure 25: Back, filtration, sonicated, 80% BaTiO3 nanopowder at x100

Figure 26 and Figure 27 show the micrographs of filled samples after coating. As can be seen,

the surface was more uniform and smooth than the uncoated case.

Figure 26: Filtration, sonicated


coated at x100

Figure 27: Filtration, sonication


coated at x100

Figure 28 shows the dielectric constant of the various samples. The dip-coated method yields the

highest dielectric constant, but also has a lot of variation. Although the amount of ash was

comparable throughout all the methods, the addition of PEG 300 and methyl methacrylate could

be contributing to the increase in dielectric constant. The unsonicated filtration method yielded a

substrate with a very low dielectric constant in contrast to the sonicated filtration method. This

could be due to the uneven distribution of BaTiO3 in the paper in the unsonicated case. The

highest and most reliable value occurs for samples filled with the 20% nanopowder in the

sonicated filtration method.

100 m 100 m

37

Figure 28: Dielectric constant of filled paper samples. Dip coated samples at various

dipping times; and filtration samples with BaTiO3 at various % of nanopowder, the

balance being micron-sized powder and with and without sonication for 1 hour.

The dielectric constant was also investigated for the coated sheets as shown in Figure 29. The

coated filter papers were compared to the uncoated filter paper and untreated filter paper. The

dielectric constant values in Figure 29 are higher than Figure 28, the reason for this difference is

not clear and requires additional tests. As can be seen, the coating increased the thickness of the

sheets and subsequently decreases the dielectric constant of the sheet. It could be expected that

the luminance of the coated sample containing 20% BaTiO3 nanopowder will be higher than that

containing 80% BaTiO3 nanopowder because it had a higher dielectric constant and lower

thickness.

Figure 29: Thickness and dielectric constant for filtration method samples with and

without coating at various % of nanopowder, the balance being micron-sized powder

-10

0

10

20

30

40

50

60

70

80

90

5 min 10 min 15 min 0%

nano

20%

nano

50%

nano

80%

nano

100%

nano

0%

nano

20%

nano

50%

nano

80%

nano

100%

nano


Die

lect

ric

Con

stan

t

0

20

40

60

80

100

Untreated 20% nano 20% nano,

coated

80% nano 80% nano,

coated

Thickness (m) Dielectric constant

38

5.2.2 Effect of substrate on conductivity

Figure 30 shows the thickness and conductivity of 10 printed layers of the PEDOT:PSS/SWCNT

ink. As can be seen, coating increased ink retention and decreased the thickness of the ink layer.

As a result, the coated sheets were more than 50 times more conductive than the uncoated sheets.

Furthermore, the coated sheets containing 20% BaTiO3 nanopowder was more conductive than

the one containing 80% BaTiO3 nanopowder. The conductivity of the PEDOT:PSS/SWCNT ink

on the coated sheet containing 20% BaTiO3 nanopowder was less than half of that of the

Multicoat sheet. It is also important to note that both the Multicoat and coated sheets containing

20% BaTiO3 nanopowder had comparable thicknesses (about 60 and 90 m respectively),

however the dielectric constant of the later sample was higher, because it was filled with BaTiO3.

Figure 30: Thickness and conductivity of 10 printed layers of PEDOT:PSS/SWCNT ink


Table 12 shows the various devices that were fabricated. The SEM device cross-sections and

thicknesses of each layer for these devices are provided in Appendix F. As can be seen, the

coated sheets had a higher ink retention indicated by the darker blue colour of the

PEDOT:PSS/SWCNT ink. Furthermore, when the PEDOT:PSS/SWCNT ink was printed

directly onto the filter paper as in the coated structure 1 case, the ink penetration can be seen

through the sheet. This is expected because filter paper is a poor substrate for ink jet printing.

-50

0

50

100

150

200

250

300

80% nano, uncoated 80% nano, coated 20% nano, coated

Thickness (m) Conductivity (S/m)

39

Table 12: Devices off and on (~190 VAC)

Paper type BaTiO3

ratios

Phosphor

layer

thickness

(m)

Off, front Off, back On, ambient

light

On, darkened

room

Filled and

coated

structure 1

20:80 50

Filled and

coated

structure 2

20:80 59

Filled 80:20 Unknown

No luminance

Filled and

coated

80:20 24

Figure 31 and Figure 32 show the luminance results of the filled and coated paper devices.

Figure 31 shows the luminance of the devices printed on the samples containing 20% BaTiO3

nanopowder. As can be seen, there is little variation with device structure. Although the ink

retention and therefore conductivity was higher when the PEDOT:PSS/SWCNT ink was printed

directly onto the coating layer (structure 2), both structures resulted in a comparable luminance.

Since the filter paper is highly porous, it is possible that the coating fully penetrated into the

pores, resulting in both sides of the filter paper being coated. Therefore, the ink retention would

be improved on either side of the sheet. Furthermore, the phosphor layer in both cases was

comparable (Table 12), which would explain the similar luminance.

40

Figure 31: Luminance of coated 20% BaTiO3 nanopowder filter paper device. Note that the

first number refers to the structure type while the second refers to the sample replicate.

Figure 32 shows the luminance of the devices printed on coated samples containing 80% BaTiO3

nanopowder. As can be seen, the luminance was less than half the luminance of the coated 20%

BaTiO3 nanopowder sheets. This can be attributed to the lower dielectric constant of the former

sample. Furthermore, the 20% BaTiO3 nanopowder sheet had a lower thickness.

Figure 32: Luminance of coated 80% BaTiO3 nanopowder filter paper device. Note that the

first number refers to the structure type while the second refers to the sample replicate.

0.0

1.0

2.0

3.0

4.0

5.0

0 50 100 150 200 250 300 350

Lu

min

an

ce (

cd/m

2)

Voltage (VAC)

1_1 1_2 2_1 2_2

0.0

0.5

1.0

1.5

2.0

0 50 100 150 200 250 300 350

Lu

min

an

ce (

cd/m

2)

Voltage (VAC)

1_1 1_2

41

5.3 Conclusions

Table 13 shows that increasing the dielectric constant of the paper can increase the luminance of

the device. Furthermore, compared to commercial papers, the luminance of the filled paper was

higher. Therefore, the performance of EL devices prepared according to structure D can be

improved by enhancing the properties of the paper. Lastly, coating increased ink retention which

increased conductivity and therefore luminance of the device.

Table 13: Summary of filled paper device performance

Device Dielectric layer

thickness (m) Dielectric

constant of

dielectric layer

Phosphor layer

thickness (m) Turn-on

voltage

(VAC)

Maximum

luminance at 300

VAC (cd/m2)

20% nano,

coated, D1

110 31 50 100 3.89

20% nano,

coated, D2

100 31 59 100 3.95

80% nano,

coated, D1

110 12 24 75 1.55

42

Chapter 6

6 Summary

Fully printed ACPEL devices were fabricated on paper in a top-emission structure referred to as

structure A. Six commercial papers were studied: Xerox Supergloss, Multicoat, Catalyst

Electracote, Xerox Copy paper, 24.6 Catalyst Directory and 18.0 Catalyst Directory. These

substrates were chosen for their thickness and surface properties. The phosphor layer is deposited

by mask printing a resin (phosphor dispersed in PVDF). The dielectric layer is deposited by

mask printing a resin (BaTiO3 dispersed in PVDF). Inkjet printing was used to pattern the

cathode using a PEDOT:PSS/SWCNT ink. The anode, DuPont LuxPrint 7164, was applied

using a paint brush. It was found that luminance increased with increasing conductivity of the

PEDOT:PSS/SWCNT ink, increasing phosphor layer thickness and decreasing dielectric layer

thickness. For device structure A, the Multicoat sheet was found to have the highest

performance due to its superior ink retention and conductivity of the PEDOT:PSS/SWCNT ink.

Paper was then used as both the dielectric layer and mechanical support in ACPEL devices. In

structures B and C, the BaTiO3 dielectric resin was coated onto the paper, but in structure D no

BaTiO3 resin was used. For devices with structure B and C, it was important for the dielectric

resin layer to be thin, because with the addition of the paper, the overall dielectric layer resulted

in lower performing devices. Similarly for D devices, thicker sheets resulted in lower

performance. Overall, D devices outperformed B and C devices and a thicker phosphor layer

resulted in a higher luminance.

Although paper is an acceptable dielectric layer, the dielectric constant can be increased with the

addition of BaTiO3. This was accomplished by filtering BaTiO3 nanopowder and micron-sized

powder into Ahlstrom Grade 992 Filter paper. A simple CaCO3-based coating was used to

increase ink retention. Compared to other ratios, it was found that filtration of a 20%

nanopowder and 80% micron-sized BaTiO3 had the highest dielectric constant. Furthermore, the

paper thickness and conductivity of PEDOT:PSS/SWCNT ink was comparable to commercial

papers.

43

6.1 Future work

Although this work showed that ACPEL devices could be fully printed ACPEL onto paper, the

luminance can be improved. This can be accomplished by changing the device preparation,

materials or operating conditions. Firstly, the anode was painted on, which resulted in significant

variation within and amongst the samples. This could be addressed by printing the anode layer.

Similarly, the phosphor and dielectric layers were deposited by mask printing which resulted in

varying layer thicknesses. A method where the resin layers can be better metered (such as screen

printing) would be beneficial to opt

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