Inkjet Deposition of Electrolyte: TowardsFully Printed Light-Emitting

Electrochemical Cells

Mattias Lindh

Department of PhysicsUmeå University

September 13, 2013

Master’s thesis, Civilingenjörsprogrammet i Teknisk fysik, Umeå University.Mattias Lindh, mali0136@student.umu.se.

Inkjet Deposition of Electrolyte: Towards Fully Printed Light-Emitting Electrochemical Cellsis a project done in the course Master’s Thesis in Engineering Physics, 30.0 ECTS at theDepartment of Physics, Umeå University.

Supervisor: Andreas Sandström, Department of Physics, Umeå UniversityExaminer: Thomas Wågberg, Department of Physics, Umeå University

Title page coffee photograph from http://www.Free-Photo-Gallery.org

AbstractOrganic electronics is a hot and modern topic which holds great promise for present and future applications.One such application is the light-emitting electrochemical cell (LEC). It can be fully solution processedand driven at low voltage providing light emission from a large surface. Inkjet printers available todaycan print a variety of inks, both solutions and dispersions. The technique is scalable and a quick and easyway to accurately deposit small quantities of material in user definable patterns onto a substrate. This isdesirable to make low cost and efficient optical devices like displays.

In this thesis it has been shown that solid electrolytes, after being dissolved in a liquid solvent, can beinkjet printed into a set of well separated distinct drops with an average maximum thickness of 150 nm.The electrolytes are commonly used in LECs and comprised by poly(ethylene glycol) with molar massesranging from 1 – 35 kg/mol, and potassium trifluoromethanesulfonate (KCF3SO3)—together dissolved incyclohexanone to form an ink. The smallest achieved edge to edge distance between the printed drops was40 μm. Together with a drop diameter of 50 μm it yields a coverage of 24% at a resolution of 280 dpi.

Profiles of dried deposited drops of electrolyte were examined with a profilometer, which showed adistinct coffee ring effect on each drop. In particular, the ridges of the coffee rings were broken into pillarlike shapes, together forming a structure akin to a scandinavian ancient remnant called stone ship. Differentdrop diameters were measured in and between the indium tin oxide samples. The drops’ speeds and sizes atejection from the nozzles seemed unchanged, and wettability is most probably the physical phenomena tolook into in order to understand what generates the differences. Local changes in surface roughness and/orsurface energy, possibly originating from the cleaning process of the samples, is most likely the cause. Noindications towards large differences in surface tension between the printable inks were seen, however theirviscoelastic properties were not measured.

As part of the thesis work a LEC characterization set-up was built. It drives a LEC at constant currentand measures the driving voltage, -current, and luminance over time. The set-up is controlled by a Labviewvirtual instrument and the data exported to a text-file for later analysis. The precision of the luminancemeasurements is ±0.1 cd/m2 for readings < 50 cd/m2, but the accuracy is uncertain.

The conclusion of this thesis is that it is indeed possible to print solid electrolytes dissolved in cyclo-hexanone with an inkjet printer. However, in order to fully understand the spreading and drying of thedrops, studies of the inks’ viscoelastic properties, together with surface roughness and -energy density ofthe substrates, are needed. The largest molar mass of nicely printable poly(ethylene glycol), at an ink con-centration of 10 mg/ml, was 35 kg/mol. This is comparable to the molar mass of an active light-emittingmaterial, “SuperYellow”, often used in LECs. Even though their respective molecular structures are verydifferent, this indicates that inkjet printing of complete LEC-inks, containing both the active material andsolid electrolyte, is feasible. Most probably it would require substantial tuning of the printing parameters.This thesis provides further hope for future fully inkjet printed LECs.

Keywords: Inkjet printing, organic electronics, light-emitting electrochemical cell, solid electrolyte, coffeering, stone ship, wettability

iii

SammanfattningOrganisk elektronik är ett hett och modernt område som lovar gott för nutida och framtida applikation-er. En sådan applikation är ljusemitterande elektrokemiska celler (LECar). Dessa kan tillverkas med lös-ningsmedelsbaserade processer, drivas av låga spänningar samt ge ljusemission från stora ytor. Det finnsidag bläckstråleskrivare som kan skriva ut många olika typer av lösningar och dispersioner. Bläckstråleut-skrift är en metod som kan skalas upp och utgör ett snabbt och enkelt sätt att deponera små mängdermaterial i specifika mönster på ett noggrannt sätt. Detta är efterstävansvärt för att man ska kunna produceraeffektiva optiska komponenter till en låg kostnad, exempelvis skärmar.

I detta examensarbete har det visats att fasta elektrolyter (efter att lösts upp i ett flytande lösningsmedel),som ofta används i LECar, kan skrivas ut med en bläckstråleskrivare till en samling med väl separeradedroppar med en medelmaxtjocklek på 150 nm. Elektrolyterna bestod av poly(etylenglykol) med molmassa1 – 35 kg/mol samt kaliumtrifluormetylsulfonat (KCF3SO3) löst i cyklohexanon till ett bläck. Det minstaavståndet mellan de utskrivna dropparnas kanter som kunde uppmätas var 40 μm. Tillsammans med endroppdiameter på 50 μm ger detta en täckningsgrad på 24 % och en upplösning på 280 dpi.

De deponerade och torkade elektrolytdropparnas profiler undersöktes med en profilometer. Detta visadepå en markerad kafferingseffekt för varje droppe. Dessutom kunde det konstateras att ”kafferingen” varuppbruten till pelarlika former, vilka tillsammans bildar en struktur som påminner om en skandinaviskfornlämning som brukar benämnas skeppssättning. Olika droppdiametrar kunde uppmätas på både enskildaoch olika indiumtennoxidprov. Eftersom både droppformation och dropphastighet vid skrivarmunstyckenavar oförändrade, tyder detta på att det är provens vätbarhet som varierar. Detta kan i sin tur härledas tilllokala skillnader i ytstruktur och/eller ytenergi på provet. Antagligen beror dessa på små avvikelser somuppkommit i rengöringsprocessen som föregår utskrift. Inga indikationer på stora skillnader i ytspänningmellan de bläck som gick att skriva ut fanns, dock mättes inte de viskoelastiska egenskaperna.

Som en del i detta examensarbete byggdes en karaktäriseringsuppställning för LECar. Denna driver enLEC med konstant ström och mäter driftspänning, -ström och LECens luminans över tid. Uppställningenkontrolleras av ett Labviewprogram och data exporteras till en textfil för senare analys. Luminansmät-ningarnas precision är ±0.1 cd/m2 för värden < 50 cd/m2, men noggrannheten är osäker.

Slutsatsen är att det verkligen är möjligt att med en bläckstråleskrivare skriva ut fasta elektrolyter lösta icycklohexanon. För att förstå hur dropparna breder ut sig och torkar på substraten bör man undersöka bläck-ens viskoelastiska egenskaper, samt substratens ytstrukturer och -energier. Den tyngsta poly(etylenglykol)som gick att skriva ut vid 10 mg/ml koncentration, och med uppfyllda droppkriterier, hade molmassan35 kg/mol. Detta är jämförbart med molmassan för ett aktivt och ljusemitterande material, ”SuperYellow”,som ofta används i LECar. Även om deras respektive molekylära strukturer är mycket olika, indikerar dettaatt det borde vara möjligt att skriva ut fullständiga LEC-bläck innehållandes både det aktiva material ochden fasta elektrolyt som behövs. Dock skulle det troligen krävas mycket arbete med skrivarens inställningar.Detta examensarbete inger ytterligare hopp om framtida LECar helt utskrivna med bläckstråleskrivare.

Nyckelord: Bläckstråleskrivare, organisk elektronik, ljusemitterande elektrokemiska celler, fast elektrolyt,kaffering, skeppssättning, vätbarhet

iv

Preface

I have been interested in renewable energy production, advanced technicalities, and fundamental under-standing in general throughout my entire life. When the possibility of working with organic photonics andelectronics arose I realized that it was the path for me. Through measurement problems, contact difficul-ties, and final success—this report hopefully invites you to grasp some of the complexities and difficultiesin experimental polymer physics.

I would like to thank professor Ludvig Edman for giving me the possibility of working in his researchgroup. Also I would like to thank my supervisor Andreas for the patience he has shown, and the knowledgeand experience he so generously has shared with me. The other OPEG members also deserve my sincereappreciation as they have explained many things which I did not understand, helped me out in practise,and given me hints on how to tackle problems when I have needed it the most. I specifically wish to thankNahid for introducing the printer to me in such a nice way, and Mariam for her help with LATEX and layoutof this report.

I also want to express my gratitude to all of my friends and family for the support they give and thejoy they help me fill my spare time with.

Finally, without my home supporter, Marie, I would have had a hard time keeping the rest of my lifeup and running whilst working with this project. Thank you for never once waver in your belief in me.

Mattias LindhUmeå, SwedenSeptember 13, 2013

v

CONTENTS

Contents

1 Introduction 11.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 A brief background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.3.1 Inkjet printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.2 Light-emitting electrochemical cells . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.4 Disposition of this report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Theory 32.1 Inkjet printing techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1.1 Drop ejection mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.2 Performance and limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Drop spreading and drying at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.1 Contact angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.2 Drop spreading and drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 The LEC concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3.1 Constituents explained . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3.2 Working principle of the LEC concept . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Methods and materials 113.1 The FUJIFILM Dimatix Materials Printer 2831 . . . . . . . . . . . . . . . . . . . . . . . 113.2 Electrolyte ink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2.1 Choice of ink solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.2.2 Poly(ethylene glycol) as ion solvating material . . . . . . . . . . . . . . . . . . . 133.2.3 Potassium trifluoromethanesulfonate as salt . . . . . . . . . . . . . . . . . . . . . 133.2.4 Mixing of electrolyte ink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3 Printed component fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.3.1 Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.3.2 Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.4 LEC fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.4.1 SuperYellow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.4.2 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.5 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.5.1 Profilometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.5.2 Photography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.5.3 LEC characterization set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4 Results 214.1 Jetting functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.1.1 Different inks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.1.2 Number of nozzles and print speed . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.2 Printed outcome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.2.1 The printed set of droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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CONTENTS

4.2.2 Individual drop profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224.2.3 Maximal coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.3 Features of the characterization set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.3.1 Contacting a sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.3.2 Data sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5 Discussion and Conclusions 27

6 Bibliography 29

A Voltage to luminance conversion 33

B Labview virtual instrument for data acquisition 35

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CONTENTS

viii

Chapter 1

Introduction

1.1 PurposeThe purpose of this project was to investigate the possibilities of depositing an electrolyte onto an IndiumTin Oxide (ITO) substrate in well defined drops with an inkjet printer. Printed drops of electrolyte could bea step towards fully printed Light-emitting Electrochemical Cells (LECs). However, the electrolyte is onlyone of the essential components in a LEC; an active light emitting material and electrodes also need to beincorporated. Printing of active material and electrodes is outside the scope of this project.

1.2 ObjectivesThe electrolyte to be printed should consist of the ion transporting material poly(ethylene glycol) (PEG),and the salt potassium trifluoromethanesulfonate (KCF3SO3). Objectives, more concise and measurablethan the purpose above, follow in an enumerated list:

1. Do a literature study to acquaint myself with the LEC-concept.

2. Choose and buy four PEGs with different molar masses (M).

3. Find a solvent in which the electrolyte could be dissolved and finally printed in a stable way.

4. Adjust the printer settings and the ink configurations to be able to get a well defined and homoge-neous set of printed drops with: a small radius, edge to edge distance of 10 μm, and a thickness of25 nm. The drop pattern sought for should be at least 2x2 mm2.

During the course of the project, several issues and problems have been identified and alleviated. Forexample a LEC measurement and characterization system had to be designed and realized:

5. Make use of one of the Hamamatsu photodiodes available in the lab to measure luminance of LECsamples in the unit cd/m2, and build a constant current circuit to drive them.

6. Connect this set-up to a National Instrument Labview virtual instrument (VI) in order to collect thesampled data (voltage, current and luminance) over time.

1.3 A brief background

1.3.1 Inkjet printingIn 1951, R. Elmqvist of Siemens Elema in Sweden was awarded a US patent [1], for the design of aMeasuring equipment of the recording type, embodying the idea of inkjet printing: a technique whichallows deposition of ink droplets on a substrate. Today there are several methods of ejecting drops, butthey all have in common that drops can be positioned to form a pattern on the substrate. A benefit in

1

CHAPTER 1. INTRODUCTION

addition to 2-D patterns, is that it can be done without large losses of ink, which is inherent in e.g. the spincoating technique. Both continuous inkjet printers and drop-on-demand printers are available today andused in various applications. Inkjet printers which are compatible with a wide range of ink properties areon the market, enabling printing of different materials. One example is the FUJIFILM dimatix materialsprinter 2831, which have been used in this thesis.

1.3.2 Light-emitting electrochemical cellsThe first report on the LEC concept dates back to an article by Pei et al. in 1995 [2]. They reportedan in situ formation of a dynamic p-n junction, followed by light emission. The main drawbacks of theLECs in the early years, were long turn-on-times and poor lifetimes. Therefore, the LEC concept wasnot developed and investigated in much detail, compared to the enormous field of organic light emittingdiodes (OLEDs), which had a head start, invented in 1987 [3]. OLEDs can be found in many places todaye.g. in smartphone displays. Apparently, some drawbacks are connected to the most common structureof OLEDs—they typically consist of multiple, stacked, and very thin layers of different materials, andtheir performance is sensitive to the respective layer thicknesses. LECs are usually not as sensitive in thisrespect, and it has been shown that they can be manufactured completely by solution processes [4], whichhave the potential of being relatively cheap and easy. Just recently, Zheng et al. interestingly reported ofall-solution processed OLED displays [5].

LECs have previously been fabricated with an inkjet printer, as reported by Mauthner et al. [6]. How-ever, they made use of a different inkjet printer, with a larger orifice of the printhead compared to theFUJIFILM DMP-2831, so water dispersions could be jetted. Their LECs were furthermore made in asurface cell structure, and thus emitting light from a thin line instead of a large surface.

1.4 Disposition of this reportIn order to facilitate reading, this report is divided into several chapters. In this chapter 1 you have got abrief introduction to the historical background of inkjet printing and LECs, and also had the possibility ofgetting to know objectives and goals for the project. Chapter 2 focuses on theory and describes differentareas involved, e.g., materials printing, drop drying on substrates, wettability, and the working principleof LECs. Used methods and materials will be described in chapter 3, and results shown in the subsequentchapter 4. Further discussion of the results along with some conclusions and future prospects will be dealtwith in the finalizing chapter 5.

In appendix A you can find calculations for the conversion from photodiode generated current to mea-sured luminance. In appendix B images of the constructed Labview VI used for data acquisition are to befound.

2

Chapter 2

Theory

2.1 Inkjet printing techniquesInkjet printing techniques have been available for many years. However, under the hood of the printers theconstituents and processes can differ completely.

2.1.1 Drop ejection mechanismsContinuous inkjet

The Continuous InkJet (CIJ) method is based on an ink chamber pressurized by a pump. The ink is con-tinuously pushed in a steady jet through a nozzle. Because of Plateau-Rayleigh instabilities [7] introducedby an acoustic wave, the jetted stream is broken into droplets, minimizing the surface energy, at regularintervals. Most of the drops are collected and recirculated to the ink chamber. However, when a printeddrop is wanted, an applied electric field can deflect it (the drops are charged by an electrostatic field as theyform) from the original path, and it can land on the substrate instead of in the re-circulation circuit. Eventhough this printing technique is fast, it suffers from several drawbacks e.g., need for a high pressure pumpfor ink ejection, an intricate re-circulation system, and complicated hardware needed to implement severalnozzles [8].

Drop-on-demand

Drop-on-Demand (DoD) inkjet printers typically use electro-mechanical piezoelectric pressurization of theink chamber [9]. This means that the ink cavity is pressurized by the crystal, because the latter deformsand changes the volume of the chamber when a voltage is applied, see figure 2.1b. For a more detailedexplanation of the drop ejection phases, see [8, 9].

Another method to eject droplets, Thermal InkJetting (TIJ), was simultaneously developed by bothCanon and Hewlett-Packard. It is commonly used in ordinary desktop inkjet printers—a current pulsethrough a heater in one end of the ink chamber creates a vaporized ink bubble, which pressurizes thechamber and pushes a drop through the nozzle [10]. Low boiling point inks are necessary for the TIJprocess, and water (sometimes with additives such as glycol) is a common ink carrier.

Waveforms

Conversely to the CIJ technique, DoD printers eject drops only when one is wanted. It is done by apply-ing a voltage across the piezoelectric crystal in a characteristic time and amplitude pattern—a so calledwaveform.

Waveforms can be divided into several different phases, each one linked to a process in the nozzleconnected to the ink chamber. Basically though, only two phases are necessary: one fill phase, and onepressurization- and ejection phase. Nonetheless, additional phases are often needed for damping, relaxationof the internal flows, and improved jetting performance, see figure 2.1a for a basic example of a waveform.

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CHAPTER 2. THEORY

Each phase has a given amplitude (voltage) and duration. In order to get good jetting performance—withfast, round, and stable droplets without tails—the total waveform duration, number of phases, and theindividual phases’ amplitudes, durations, and slew rates (slopes), need to be manually adjusted.

(a) Basic waveform (b) Ink chamber pressurization

Figure 2.1: In figure 2.1a the simplest possible waveform is shown. I is the fill phase, and II is thepressurization- and ejection phase. II is connected to figure 2.1b. As the piezoelectric crystalIII deforms, the pressure in the chamber increases and ink flow out from the reservoir IV & V,and a drop is ejected.

2.1.2 Performance and limitations

The cartridges used in this thesis have printheads with nozzles that give droplets of 10 pl nominal volume.This is specified by the manufacturer, which also state that the volume can change somewhat dependingon the ink characteristics and the printer settings [9]. To measure the droplet volume accurately is a cum-bersome process and large uncertainty can be introduced when using volatile inks such as many organicsolvents.

Piezoelectric DoD printers are often able to print inks with varying properties, as long as these fallwithin certain ranges. Still, it has been shown [8] that the specified values are no absolutes. With somecareful tuning and manual adjustments of the waveform and inkjet settings, even inks outside of the regimefor a functional fluid can be printed, although the print speed and -stability can be affected. It is worthto mention that the surface tension of water (which is commonly used as ink carrier in ordinary desktopinkjet printers) often is much larger than the specified maximum value, which will render water based inksdifficult to print with this technique. However, many organic solvents fall into the printable regime and itsvicinity.

Beads-on-a-string

Non-newtonian effects on drop formation, such as Beads-On-A-String (BOAS), see figure 2.2, can occurfor inks containing PEG. Yan et.al. [11] observed a BOAS effect when printing PEG with M = 300 kg/moldissolved in water to a concentration of 0.01wt%. Mauthner et.al. [6] observed BOAS for LEC-inks withPEG with M = 100 kg/mol, but not when using a lower M = 35 kg/mol. BOAS is destructive to goodquality printing, and hinders fast and stable jetting. Bath et.al. [12] showed that the BOAS phenomenondepends on four different interacting forces: capillary, viscous, elastic, and inertial. Even though the inksolvent in itself is newtonian and in the printable regime, small additions of e.g. PEG, can completelychange the jetting functionality of the ink and give rise to non-newtonian effects [13].

4

2.2. DROP SPREADING AND DRYING AT A GLANCE

Figure 2.2: This figure shows what the non-newtonian liquid effect known as “beads-on-a-string” can looklike in everyday life. Beads on a string of saliva have formed due to the highly extensible prop-erties of the mucopolysaccharide polymers in saliva [12]. Beads can in the same manner formon the tail of an ejected ink droplet as a result of the long and extensible chains of poly(ethyleneglycol).

2.2 Drop spreading and drying at a glance

2.2.1 Contact angleIt is now 207 years since Young described the static relations between energies of different interfaces, andthe contact angle of a liquid drop on a flat solid surface [14]. The relation known as Youngs equation (2.1)can be used to describe if and how a drop will spread on a surface. It relates the contact angle θC withthe interfacial tensions γIJ, where I and J denotes the phases; S, G, and L stands for Solid, Gas and Liquidrespectively. See figure 2.3a for a clearer view of the different terms. The liquid – gas interfacial tension isoften called surface tension (N/m), whilst the solid – liquid ditto is denoted surface energy density (J/m2).Note that there are no dimensional contradiction them in between.

cos(θC) =γSG − γSL

γLG(2.1)

Youngs equation was originally derived from the concept of local forces. Attractive forces between similarmolecules (like in a liquid) are known as cohesive, and between different types of molecules (like betweenthe solid and the liquid phases) they are known as adhesive. If the contact line (i.e. the line where the solidand liquid phases meet the third substance, gas) is kept in a fixed position, or pinned as it is usually called,force balance between cohesive- and adhesive forces will yield equation (2.1).

If we consider the case γSG − γSL < γLG, the drop will tend to spread completely on the surface andθC = 0. This corresponds to complete wetting. In the opposite case, where γSL − γSG < γLG, the drop willinstead strive to keep its shape and remain a perfect sphere, with θC = 180. However, the latter scenariois a theoretical limit not yet achieved in reality. Even for the example of water droplets on fluorinatedsurfaces with very low wettability, maximum equilibrium contact angles of only about 156 are reported[15]. Materials which make contact angles with water < 90 are considered hydrophilic, and materialswhich make contact angles with water > 90 are considered hydrophobic.

Another thing which greatly can affect the wettability of a surface is the surface roughness. The wellknown example of a water droplet on a lotus leaf can be seen in figure 2.3b, showing superhydrophobicitywith a contact angle of almost 150.

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CHAPTER 2. THEORY

(a) The contact angle of a drop on a solid. (b) Waterdroplet on a lotus leaf

Figure 2.3: The contact angle of a drop on a solid, and interfacial tension terms, are shown in figure 2.3a.This figure also depicts the profile of a spherical cap, image from [16]. Measured angle of awater droplet on a lotus leaf is shown in figure 2.3b, image from [17].

There is a difference between complete wetting as described above, and impregnation. While completewetting is hard to achieve on a smooth solid, impregnation (of for example sponges) is common. If thescale of the surface roughness is right, it can act as a sort of 2-D sponge and liquid quickly be drawn outto cover the surface due to the adhesive forces. If however the surface roughness has another scale (like inthe case of the lotus flower), opposite effects such as depicted by figure 2.3b can occur [18].

2.2.2 Drop spreading and drying

Typically, printed droplets are small (ejected 10 pl drops should have a diameter of roughly 25 μm if theyare assumed to be spherical), and according to Quère [18] it is then a reasonable assumption that each dropon a surface, immediately after deposition, has the shape of a spherical cap, see figure 2.3a. The most naïveway of imagining what happens to the profile when it dries, is to simply decrease the height of the cap bya factor matching the concentration of solids in the drop. This would give a dry “drop” which is thickest inthe middle and thins out radially.

However, drops and films rarely dry in that simple manner. Usually dried drops of a dispersion orsolution have a distinct ring or ridge along the perimeter. This was reported by Deegan et.al. as the coffeering effect [19]. They explained further that if the contact line is pinned (see section 2.2.1) and θC > 0,basic requirements for coffee ring effects are met. With a pinned contact line the drop cannot shrink whenthe liquid evaporates, and an outward, capillary, and convective flow inside the drop must compensate forthe evaporated liquid at, and close to, the edge, see figure 2.4. This flow obviously carries the solids along,and a close packed ring or ridge forms at the edge. Often, coffee rings are undesirable, and several ways ofsuppressing the coffee ring effect have been proposed [20, 21, 22]. These methods are however intricate,and the most basic properties to keep track of in order to have any control of the drying process, are thefluid properties of the ink, surface energies of the solid substrates, and their roughnesses. As indicated byRouth [23], a larger contact angle should suppress the ring formation somewhat. Nevertheless, it is thespatial distribution of evaporative flux which really determines the internal fluid motions in the drop, andhence the shape and profile of the deposited solids [24].

6

2.3. THE LEC CONCEPT

Figure 2.4: This figure outlines the physics behind the coffee ring formation. If the contact line is pinned(and the edge of the drop therefore fixed in position), and the drop liquid evaporates, the result-ing internal- , radial- , outward flow carries the solids to the contact line. The solids are left ina ring- or ridge like fashion when the liquid has evaporated: a coffee ring.

2.3 The LEC concept

2.3.1 Constituents explained

Π -conjugated polymers

The light-emitting layer in a polymer LEC is usually a π-conjugated polymer. The π-conjugation refers aring of sp2 hybridized carbon atoms. The sp2 hybridization is a specific binding configuration, in whichoverlapping p-orbitals allow for de-localization of π-electrons, which make the polymer electrically con-ducting. Adding many repeat units into a large and sometimes complex polymer, spreads the energies ofthe different electronic states. For large enough number of rings, the shifts will result in something similarto energy bands, although they actually consist of separate levels [25]. The level of the highest occupiedmolecular orbital (HOMO), of the polymer in question, then corresponds to the edge of the valence bandin an inorganic semiconductor. In the same way, the lowest unoccupied molecular orbital (LUMO) corre-sponds to the edge of the conductance band, and the gap between them obviously constitute a band-gap[26]. The wavelength spectrum of the emitted light will correspond to the energy differences between statesin the bands, as governed by the well known equation (2.2), where E is energy, h is Plancks constant, c isthe speed of light and λ is the wavelength.

E =hcλ

(2.2)

Electrolyte

An electrolyte is usually a liquid solution in which ions can move and thereby transport charge. Themost basic example is probably ordinary table salt, sodium chloride (NaCl), dissolved in water. In solidstate LECs on the other hand, the electrolyte can per definition not be liquid [27]. Instead of water, apolymer like PEG can be used in combination with an alkalic salt such as KCF3SO3 or LiCF3SO3 (lithiumtrifluoromethanesulfonate). PEG works as solid1 solvent for either salt [28], and together they compriseelectrolytes which are commonly used in solid state polymer LECs [27, 29, 30]. The lithium cation, Li+,has smaller ionic mobility in PEG matrices than the potassium cation, K+. This is often attributed to a lowerelectronegativity or charge density [31]. The wavefunction of the valence electron is more spatially spread,and the ion can thus move more easily in the PEG matrix. LiCF3SO3 has still been shown to perform well

1PEG can appear as liquid, soft- or hard solid at room temperature depending on its molar mass. Long chains, with high molarmass, tend to be harder, and shorter chains waxy or even liquid.

7

CHAPTER 2. THEORY

in LECs together with the ionic solvent trimethylolepropane ethoxylate (TMPE) [29, 32]. It has also beenused together with PEG, functionalized with methacrylate endgroups, reaching good performance [33].

Electrodes

In order to contact a LEC and drive it, two electrodes are required. In case a sandwich cell (see section 3.4for further explanation) is to be built, the bottom electrode needs to be transparent. ITO is often chosen asit has a comparably good combination of sheet conductance and transparency. Unfortunately, ITO is brittleand contains expensive and exhaustible indium [34]. Thus, other materials are needed in the long run andfor flexible devices. A possible candidate is graphene, which has been deployed successfully as bottomcathode in LECs [35, 36, 37].

The top electrode can for example be a conducting metal or oxide. It is not unusual to use aluminiumdeposited by vacuum evaporation, as described in section 3.4.2. It has been used in many studies and seemto match the properties of the PEG:KCF3SO3 electrolyte [38]. In addition, it is highly reflective, and underthe assumptions of total reflection and that no light is reabsorbed in the active layer, the luminance of thesample can be doubled.

2.3.2 Working principle of the LEC conceptWhen a voltage bias is applied to the electrodes of a LEC, some interesting physics take place. From theoriginal no bias state, see figure 2.5a, where there is no actual difference between the two sides of thedevice (for simplicity we assume both electrodes are made of identical metals), the levels of the electrodeworkfunctions are shifted, see figure 2.5b. Even if the voltage is high, the current through the device willbe very small because the widths of the energy barriers are too thick for tunnelling required for hole- andelectron injection. Electrons and holes at the cathode and anode respectively, attract positive and negativeions from the active layer electrolyte. The ions migrate to gather close to each electrode and form electricdouble layers (EDLs). The large concentration of ions screens the electric field which results in band bend-ing, which decreases the widths of the potential energy barriers [39], see figure 2.5c. Tunnelling is therebyfacilitated and the electron and hole injections into the HOMO and LUMO levels increase significantly[40]. Injected holes and electrons oxidise and reduce the π-conjugated polymer which can then be seen asp- and n-doped. As more holes and electrons are injected, more of the material is doped and the dopingfronts propagate towards each other. Injected electrons and holes can pairwise form an electrically neutralquasiparticle called an excition. When the doped fronts are close enough for the individual components inthis short lived state to meet, it recombines and sends out a photon carrying the excess energy, see figure2.5d [41, 42]. As long as the photon does not react with anything and is not reflected back into the LEC atthe active layer – glass interface, light is emitted.

8

2.3. THE LEC CONCEPT

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9

CHAPTER 2. THEORY

10

Chapter 3

Methods and materials

3.1 The FUJIFILM Dimatix Materials Printer 2831In the OPEG group [43] at Umeå University, a FUJIFILM Dimatix Materials Printer 2831, capable ofprinting inks with a wide range of fluid properties [9], is available, see figure 3.1. The main reason for usinga materials printer, is that it enables the user to print inks of his or her own choice. User fillable cartridgesand adjustable jetting functionalities, see section 3.3.2, allows for precise and controlled deposition of arelatively well known amount of material. Substrates are held in place on the platen by a vacuum pump.The printer is controlled by a user friendly computer interface, in which all different printer settings can beadjusted.

Figure 3.1: The FUJIFILM Dimatix Materials Printer 2831 used in this project. It easily fits in a fume hoodwhich is convenient when working with organic solvents.

The temperature of both the printhead, see figure 3.2b, and the substrate platen can be adjusted sep-arately: the printhead temperature in order to alter the properties of the ink and facilitate jetting of highviscosity inks, and temperature of the substrate platen to control the drying of the film or drops. For adetailed specification of functional fluid properties, see [9].

Cartridges (Dimatix reference number DMC-11610) with polypropylene containers, see figure 3.2a,were used as the liquid crystal polymer (LCP) containers, also available from FUJIFILM, are made formore aggressive solvents. Large molecules were to be printed, and cartridges delivering droplets withnominal volume of 10 pl were used, because they have larger orifices than the 1 pl printheads which are

11

CHAPTER 3. METHODS AND MATERIALS

also available. The printheads have 16 nozzles which are separated by 254 μm. By rotating the printhead,the inter drop distance can be set down to 5 μm, corresponding to a maximum resolution of 5080 dpi [9].

(a) User fillable cartridge container (b) Cartridge printhead with connections

Figure 3.2: Pictures showing some different components of the printer cartridges. They consist of a con-tainer (a) and a printhead (b). The small dark rectangle directly above the white tape on theprinthead holds a line with 16 nozzles, separated by 254 μm.

3.2 Electrolyte inkThe inks used in this study were made up by cyclohexanone (Sigma Aldrich, 99.5%), as ink solvent, andPEG (Sigma-Aldrich, BioUltra M = 103 to 6 ·105 g/mol) and KCF3SO3 (Sigma-Aldrich) as ion trans-porting material and salt of the electrolyte respectively. For images of the molecular structure of the inkcomponents, see figure 3.3.

(a) Cyclohexanone(b) Potassium trifluoromethanesul-

fonate (c) Poly(ethylene glycol)

Figure 3.3: The molecular structure, displayed by skeletal formulas, of the ink components. Cyclohex-anone in 3.3a, is the ink solvent. Potassium trifluoromethanesulfonate in 3.3b, is the salt ofthe electrolyte. In subfigure 3.3c, poly(ethylene glycol), which is the ion solvating material, isshown.

3.2.1 Choice of ink solventCyclohexanone is a relatively harmless solvent, as can be seen in its Material Safety Data Sheet [44]. Ithas properties (e.g., viscosity, surface tension, and boiling temperature), which according to the printeruser manual [9], almost match the specifications for printable inks. In addition it dissolves both PEGand KCF3SO3 and should therefore be a good candidate as ink solvent. During the solvent choice it wasthen assumed that addition of electrolyte would not change the properties significantly. This might notnecessarily be the case, see section 2.1.2.

A few other solvents with reasonable properties (e.g., water, 1-propanol, propan 2-ol, chlorobenzene,and tetrahydrofuran) were also tested, but all of them failed in either printing performance in combination

12

3.3. PRINTED COMPONENT FABRICATION

with PEG and KCF3SO3, or to simply dissolve the electrolyte components.

3.2.2 Poly(ethylene glycol) as ion solvating material

PEG is known to dissolve KCF3SO3, and has previously been used in production of LECs, see e.g. [25].The molar mass of the PEG is more or less a measure of how many repeat units of ethylene glycol there arein each polymer chain, see figure 3.3c. The viscosity of a solution with PEG is, contrary to the assumptionstated in the previous section, heavily dependent on both its concentration and molar mass [11, 13]. There-fore, and as no facilities for measuring the ink properties directly were available, several different molarmasses of the solid solvent were to be tried in the printer, to see if any would work.

3.2.3 Potassium trifluoromethanesulfonate as salt

The ions in the electrolyte are potassium cations (K+) and trifluoromethanesulfonate (triflate) anions(CF3SO−

3 ). Together they form the salt KCF3SO3, see figure 3.3b. The ions can move in a PEG ma-trix and thus they together form an electrolyte. To remove as much residual moist as possible from thereceived salt, it was dried over night in a vacuum oven at 150 C before use. Thereafter the salt was storedin a nitrogen filled glove box with typical levels of H2O- and O2 vapour < 1 ppm.

3.2.4 Mixing of electrolyte ink

The complete mixing of the electrolyte inks was conducted in ambient air atmosphere. PEG was placedin a glass vial and weighed. A clean magnetic stirrer was added to each glass vial. Thereafter cyclohex-anone was added with a syringe to get a concentration of 10 mg/ml. The lid was closed and the vial puton a magnetic hotplate at 50 C, for at least 12 hours. The same procedure was followed for all the dif-ferent PEGs. KCF3SO3 was easily dissolved in cyclohexanone and no heating or magnetic stirring wastherefore necessary; the glass vial was simply shaken and left to rest for at least 12 hours after addition ofcyclohexanone.

The master blends described above were used to mix the final inks in new glass vials achieving a massratio of PEG : KCF3SO3 = 4 : 1, see table 3.1. For convenience, the inks were named ink 1 – 6. They differin choice of PEG and the concentration (for ink 5 and 6). Before filling the printers cartridges, the inkswere degassed and filtered through a 0.2 μm poly(tetrafluoroethylene) (PTFE) filter to remove any largeparticles which would clog the nozzles.

Table 3.1: The differences between the different inks used in this project. All inks contain the alcalic saltKCF3SO3, and cyclohexanone as the ink carrier.

Name PEG (kg/mol) Concentration (mg/ml)Ink 1 1 10.0Ink 2 2 10.0Ink 3 10 10.0Ink 4 35 10.0Ink 5 100 0.10Ink 6 600 0.02

3.3 Printed component fabrication

3.3.1 Cleaning

The ITO substrates were first cleaned in an ultrasonic bath in acetone at 20 C for 30 minutes. The acetonewas then exchanged for propan 2-ol, and the substrates cleaned again in the same way. A petri dishcontaining the substrates was subsequently put in an oven at 120 C for at least 12 hours.

13

CHAPTER 3. METHODS AND MATERIALS

3.3.2 PrintingA custom made waveform was developed for each cyclohexanone based ink used in this thesis. In additionfine tuning of each waveform was generally needed if the cartridge was replaced or any conditions, e.g. am-bient temperature, was changed. The viscoelastic properties of cyclohexanone together with PEG requiregreat care when developing and optimizing the waveform, see figure 3.4. Due to the special properties ofthe inks, the waveform allows only for low frequency jetting. This decreases print speed and printability[9].

Figure 3.4: The slow jetting waveform used for inks 1 – 4.

The slow jetting waveform in figure 3.4 was used to jet inks 1 – 4. Ink 5 and ink 6 needed waveformswith larger voltage differences and faster dynamics to be jettable, see also section 4.1.1. Temperatures ofthe printhead and platen, and the voltages used for each ink, are all found in table 3.2.

Table 3.2: Printer settings for the different inks.

Ink Printhead temp. ( C) Platen temp. ( C) Waveform Nozzle voltage (V)1 30 30 slow 302 30 30 slow 203 30 30 slow 244 30 30 slow 275 40 40 peg100k 306 50 50 peg600k 22

3.4 LEC fabricationA working LEC emits light in the active layer between the electrodes as described in section 2.3.2. To getemission from a surface rather than a single point or line, LECs can be made in a sandwich architecture,see figure 3.5, as opposed to a surface structure. To be able to test the characterization set-up, sandwichLECs were fabricated by standard techniques.

14

3.4. LEC FABRICATION

Figure 3.5: The sandwich architecture of a light-emitting electrochemical cell, shown in this figure, allowsemission of light from the surface covered by both electrodes and the active layer. Note thatthe use of a transparent electrode, in this case indium tin oxide (ITO), is necessary for light tobe able to escape the sandwich structure.

3.4.1 SuperYellowSuperYellow (SY, Merck, Livilux PDY-132) is a complex, large π-conjugated co-polymer, see figure 3.6,with a band gap of 2.7 eV [38]. It has a light-emission mainly in the yellow – green range. It was chosento be the active (light-emitting) polymer in the LECs used in this thesis for test of the LEC characterizationset-up. SY has been shown to work well with e.g. PEG:KCF3SO3 electrolyte [25, 38] among several others.

Figure 3.6: The molecular structure of SuperYellow. Image from [45].

3.4.2 FabricationThe ITO substrates were cleaned as described in section 3.3.1. Master solutions of PEG, KCF3SO3, and SYwere made following the procedure described in section 3.2.4. A new ink was mixed and a mass proportionof SY:PEG:KCF3SO3 = 1 : 0.1 : 0.03 was achieved. The ink was then further diluted with cyclohexanoneto a concentration of 7.5 mg/ml, a magnetic stirrer was added, the vial wrapped in aluminium foil, andfinally placed on a magnetic hotplate at 50 C for 12 hours.

Spincoating

When the ink was prepared and the substrates clean and dry, the ink was deposited on the ITO via spincoat-ing. It is a procedure where an excessive amount of ink is placed on the substrate, whereupon the spincoater(Spin 150, APT GmbH), see figure 3.7, rotates the substrate at high speed (typically 500 – 4000 rpm). Dueto the centrifugal force the liquid is evenly distributed over the substrate and forms a thin film; excessive inkis expelled over the edges of the substrate. Depending on the surface tension and -energy density of the ink

15

CHAPTER 3. METHODS AND MATERIALS

and substrate surface respectively, the thickness of the resulting film can be adjusted via rotational speedand duration. After the ink was deposited onto the substrates, the samples were moved into a nitrogen filledglove box, and placed on a hotplate at 50 C for at least 12 hours.

Figure 3.7: The SPIN 150 spincoater used to deposit thin films of ink on a substrate.

Deposition of electrode by vacuum evaporation

To complete the structure needed for a LEC, an electrode was evaporated onto the active layer. Aluminiumwas chosen as it has previously been shown to work well with the specific LEC components used in thesamples [38]. Evaporation of metals and other solids is not straightforward, but requires some complexmachinery. The material is put in a tungsten “boat” connected in series to a powerful power supply. Whena current (typically up to 220 A for aluminium) flows through the boat it is heated by resistive energylosses. When the boat is hot enough the material evaporates, see figures 3.8a and 3.8b, and deposits onthe sample placed above. The use of “vacuum” (a low pressure, about 5 ·10−6 hPa) has several purposes:decreasing heat spreading by convection and thus delaying heating of the sample, and to make sure thealuminium vapour deposits evenly.

An aluminium layer of about 100 nm was deposited onto the samples through a shadow mask. Theshape of the electrodes can be seen as the fingers in figure 3.8c. Each finger is a separate LEC unit. TheLECs were thereby complete in the sense that they would emit light if properly connected to a powersupply.

Encapsulation

LECs are very sensitive to air and water [46], which causes side reactions in the active layer and delamina-tion of the structure. To be able to test the LEC characterization set-up in air, the samples need protectionfrom the ambient atmosphere. This issue was resolved by encapsulating the samples with glass and epoxy,

16

3.5. CHARACTERIZATION

(a) (b) (c)

Figure 3.8: A schematic description of the evaporation process, 3.8a, with the individual components ex-plicitly marked, 3.8b. Subfigure 3.8c shows an encapsulated light-emitting electrochemicalcell with “fingers” as aluminium top electrodes. Silver paste is painted on the electrode edgesto facilitate contacting. The dashed line represents the extension of the cover glass used forencapsulation.

following the full cover encapsulation technique further described in [46, 47]. The technique was easy,but required a steady hand and careful handling. A drop of UV-curable epoxy was placed on top of theLEC sample. A small and thin cover glass (see figure 3.8c) was then placed on top of the drop and theepoxy spread to the edges of the glass due to gravity and capillary forces. Care was needed not to coverthe contact points of the LEC with either glass or epoxy, since it would then be impossible to contact theelectrodes. Finally the sample was placed under a UV-lamp and the epoxy was cured for 90 minutes.

After the encapsulation, the LEC could be taken out of the glove box and be used in the ambient airatmosphere without degrading quickly due to side reactions and delamination of its structure.

3.5 Characterization

3.5.1 Profilometer

A profilometer (Bruker, Dektak XT) was used to measure the thicknesses of spincoated films and theprofiles of dried printed drops. A profilometer is a machine which probes a surface with a stylus, applyinga constant force. By moving the sample in the horizontal x-direction and keeping the applied stylus forceconstant, a 2-D image of the surface can be attained. By using multiple sweeps and moving the substratein the y-direction a 3-D image of the surface can actually be produced.

The profilometer is a great complement to an atomic force microscope. An important difference isthat profilometry is a measurement technique which require mechanical contact to the sample—possiblydestroying the measured film—whilst an AFM can be run in modes which are non-invasive. AFM mea-surements are for speed reasons, and stage dimensions, seldom performed for larger areas than 10x10 μm2.A profilometer on the other hand handles larger areas or substrates in the mm2 range. Although it is a muchfaster method, it is in contact, and cannot measure the profile as accurately as an AFM.

The profile of the printed drops of electrolyte could be measured after a thin layer (50 nm) of aluminiumwas deposited on top. Without the aluminium, even the smallest stylus force (10 μN) was large enough forthe stylus to penetrate the electrolyte and smear it out, see figure 3.9a. After aluminium had been depositedon top, the default 30 μN stylus force was used without any problems and illustrative pictures could beproduced, see figure 3.9b.

17

CHAPTER 3. METHODS AND MATERIALS

(a) The electrolyte is too soft. (b) Good resulting image.

Figure 3.9: Dried printed drops of the “solid” electrolyte were too soft to be accurately measured by theprofilometer, and the electrolyte was smeared out, 3.9a. If the printed drops instead were cov-ered by a thin layer of aluminium prior to profiling, it worked much better and illustrativeimages could be produced, 3.9b.

3.5.2 Photography

A Canon EOS 60D DSLR camera with a Canon EF 50 mm f1.8 lens was used for documentation of theproject and different set-ups. Some images of the drops on substrate are taken with the printer built-infiducial camera. The settings of this camera are not easily adjusted and the pictures therefore of limitedquality. Pictures of jetted drops are taken by a built in high speed video recorder and are for the samereason of suboptimal quality.

3.5.3 LEC characterization set-up

A set-up for measuring the luminance, driving voltage and -current was built and used to characterize thefabricated LECs. Materials needed were bought from ELFA Distrelec. It was desirable to keep the costsof the set-up down, as well as having the option to choose what finger to drive, see figure 3.8c. Apart fromthat, an on/off switch was added for extra security when contacting the sample. Different driving currentscan be set using a screwdriver and sampled data monitored in real time as well as saved to a text-file.

Constant current generation

When a LEC is driven by constant current, the voltage is initially high and drops when the active layer isdoped since the conductivity increases. As it has been shown that high initial voltage improves the generalperformance of LECs [38], the samples were to be driven by constant current. This is often easily done byusing a good, but expensive, source measuring unit (SMU), e.g. a Keithley 2400 SCS. However, since itwas desirable to keep the costs of this set-up down a National Semiconductors LM334Z TO92 adjustablecurrent source (which costs roughly 20 SEK) was used instead. Together with a potentiometer (MultiturnCermet Trimming Potentiometer 67W) which cost about 10 SEK, and a power supply, the current could beadjusted in the mA range as desired.

Luminance measurement

Luminance is not a straightforward thing to measure. First one must realize how luminance is defined andwhy it should be the measure for brightness of a LEC. Luminance is actually the luminous intensity perunit area for a given solid angle, and the unit is candela per square meter (cd/m2) [48]. Luminous intensity(cd) in the first case is a wavelength weighted measure of the power emitted from a source in a specificdirection per steradian, see figure 3.10. This means that the luminous intensity is conserved with respect tothe distance to the source, since the steradian changes accordingly. In that sense also luminance uncouplesdistance to the sample, and is therefore a good measure of the brightness of a LEC sample emitting from asurface.

18

3.5. CHARACTERIZATION

Figure 3.10: The extracted cone represents a steradian. As distance to the source increases, so does thesurface area of the bottom of the cone to stay 1/4π of the total surface of the sphere. Imagefrom [49].

A Hamamatsu photodiode (S9219-01) with an eye response filter, was used to measure wavelengthweighted power of the emitted light. The photodiode outputs a current depending on the power of theincident filtered light. This current is very small and therefore converted to voltage and heavily amplifiedto simplify the measurement. The conversion and amplification is done by an active circuit, with an op-erational amplifier and a gain resistance, driven by ±15 VDC. The voltage signal can be converted to ameasure of luminance via calculations found in appendix A.

Data acquisition

The signals from the measurement set-up were sampled by a National Instruments DAQ6009 device con-nected to a PC via a USB port. The DAQ was managed, and the data presented and saved, by a LabviewVI written for this specific purpose, see Appendix B. The DAQ6009 is limited to ±10 VDC. If the sampleis driven by higher voltage than 10 V (which might be the case in the beginning of a constant current mea-surement since the power supply can deliver 15 V) the DAQ will simply output the maximum collectablevoltage, which is approximately 10.4 V. Depending on the quality of the VDC power supplies, AC noisecan appear in the measurements. To some extent 50 Hz noise is cancelled in the software by averagingover a certain number of periods, but it might still show in the results. Therefore care should be taken whenchoosing power supplies to both the photodiode and the sample.

19

CHAPTER 3. METHODS AND MATERIALS

20

Chapter 4

Results

4.1 Jetting functionality

4.1.1 Different inks

Using the slow jetting waveform, see section 3.3.2 and figure 3.4, inks 1 – 4 could be jetted at concentration10 mg/ml. Even if individual waveforms were developed for each single ink, the printed results were notbetter (except for ink 5 and 6) than with the slow jetting waveform, although the printing could be donea bit quicker. Due to BOAS effects, see figure 2.2, ink 5 could be jetted only when it was diluted toa concentration of 0.1 mg/ml (100 times less than the original ink) and an ink specific waveform wasdeveloped and used. Ink 6 could not be jetted—the viscosity was much increased and the cohesive forcesin the ink droplets were so strong that ejected drops actually were retracted to the printhead during therefill phase—no pinch off was observed. The drop ejection and retraction could seen in “real time” at closeinspection of the high speed video recorders slow motion output. The “ejected” drops of ink 6 are shown infigure (4.1). Due to difficulties with the printing characteristics, both ink 5 and ink 6 were excluded fromfurther studies.

4.1.2 Number of nozzles and print speed

In general all the remaining inks (1 – 4) could be jetted in a nice way with several nozzles at the same time.Utilizing several nozzles of course speeds up the printing process remarkably, but it also makes the resultmore uncertain. The inks work fairly well, but during long printouts some irregularities in the printingcharacteristics can occur for some of the nozzles; droplet speed, direction deviations or simply total orpartial clogging can distort or destroy the printed pattern. Even though many nozzles actually could beused, and the print speed thereby increased up to a factor of 16, only one was used to increase repeatability.This was not really a problem since only samples with a surface area < 1 cm2 was printed, and the timeit took with one active nozzle (keeping drop to drop distance large enough for separated drops) was in theorder of minutes.

To maintain good printing performance, and decrease the risk of misaligned print nozzles and clogging,the print frequency was kept at about 2 kHz. This is indeed a very low frequency (according to the usermanual from FUJIFILM [9]), which indicates that the ink formulations were not perfectly suited for theprinter.

4.2 Printed outcome

4.2.1 The printed set of droplets

All of the printed ink drops left electrolyte on the substrate in a drop pattern when they dried. The dropspreading on the substrates was slightly different depending on what ink was used. The dried drop diame-

21

CHAPTER 4. RESULTS

Figure 4.1: After dilution to a concentration of 0.02 mg/ml “droplets” of ink 6 could be ejected. It re-quired a very aggressive, individually developed, waveform. In the end no drops landed on thesubstrate as the ejected droplets were retracted into the nozzles.

ters ranged from just over 50 μm to about 120 μm. The largest drops almost spread to overlap and henceform a film instead of well separated drops.

The minimum drop to drop distance achieved, still maintaining a good pattern with separated drops, was90 μm for ink 3 and 4, see figure 4.2, which corresponds to a resolution of 280 dpi. Counting backwardsfrom the drop diameter (50 μm), this relates to an edge to edge distance of 40 μm. The printed dropssometimes spread out and dried in a peculiar way. The edge to edge distance then decreased accordingly,since the drop to drop distance was held constant. However these drops did not have an even distributionof electrolyte and were usually not as circular and distinct as the ordinary ones, see figure 4.2.

4.2.2 Individual drop profiles

The individual drop profiles could be examined with a profilometer after aluminium had been evaporatedon top, see section 3.5.1.

The typical dried drop pattern and -profiles for ink 4 are visualized in figures 4.3 and 4.4. A coffee ringeffect described in section 2.2.2 is apparent, but electrolyte is actually distributed over the whole drop areaand not all of the material is gathered at the edges. Furthermore, the drops have not only formed a ridgealong the contact line during drying. In addition it can be observed that the coffee ring ridge is broken, andthat small pillars have formed along the contact line creating a structure very akin the famous stone shipAle’s Stones in Kåseberga, Sweden, see figure 4.5. The highest pillars measured were about 600 nm, andthe average maximum height of the pillars were about 150 nm.

4.2.3 Maximal coverage

By measuring the diameter of the printed drops with the printer built in fiducial camera and knowing thedrop to drop distance, the coverage in percent is given by equation (4.1), where C is coverage, r is the dropradius and a is the drop to drop distance. The geometry of the printed set resembles a square lattice. The

22

4.3. FEATURES OF THE CHARACTERIZATION SET-UP

(a) Ink 4 printed with 90 μm drop to drop distance. (b) Ink 4 with irregularly spread drops.

Figure 4.2: This figure shows two very different print results. 4.2a represents a nice printout with dense,well defined and even drops. 4.2b on the other hand shows a bad printout with misalignednon-circular drops, sometimes overlapping.

Figure 4.3: Image of dried electrolyte drops from ink 4, printed at a drop to drop distance of 150 μm. Thehighest pillars the set are about 600 nm high. Each drop has a distinct coffee ring.

unit cell area is then just a2 and the area of each drop is π · r2.

C =π · r2

a2 (4.1)

A maximal coverage amounting to 24% for separated individual drops with good drop profiles (as canbe seen in figure 4.4) was achieved for ink 3 and ink 4. It is far from the theoretical maximum of 78.5%for r = a/2, or the coverage given by 10 μm edge to edge distance distance resulting in C = 54.5%, whichwas stated as a goal, see objective 4 in section 1.2.

4.3 Features of the characterization set-upThe set-up, see figure 4.7, was successfully built using the equipment and materials described in section3.5.3. The constant current generator did not really live up to the expectations as the current deviates up to5% depending on the resistance of the LEC sample. Still, the current is actually measured and the variationscan therefore easily be compensated for.

The photodiode was sensitive to the quality of the DC input power, and the initial thought of using twobattery chargers for driving the diode was discarded due to high levels of (AC) noise. Possibly, a couple ofwell matched capacitors in the circuit could decrease the noise level, but instead two ordinary laboratoryVDC power supplies were used, and the noise levels of the signal decreased significantly.

23

CHAPTER 4. RESULTS

Figure 4.4: This figure shows a vertical cross-section of the dried drop pattern of ink 4. The coffee ringeffect is apparent as the two highest peaks, but it is also evident that not all of the material isgathered at the edges.

4.3.1 Contacting a sample

A holder in the set-up contacts each finger of the sample. Via a turn knob outside the box, voltage can beapplied to any of the fingers of the LEC (see figure 3.8c). The holder is not ideal, since it is often difficultto place the LEC such that it makes good contact to all the separate fingers. The use of silver paste oneach electrode increases the functionality, but does not alleviate the actual problem and is no guarantee forperfect operation.

4.3.2 Data sampling

The Labview VI built shows both the raw input signals and the processed data in real time graphs. A buttonenables or disables logging of the measurements to a text-file. Thus the way is paved for more detailedfuture analyses.

Figure 4.5: The stone ship Ale’s stones in Kåseberga, Sweden. Even if the scales differ by six orders ofmagnitude, the resemblance between this structure and the one formed by the dried electrolytedrops in figure 4.3 is striking. Picture from [50].

24

4.3. FEATURES OF THE CHARACTERIZATION SET-UP

(a) With open hood. (b) Hood closed and photodiode visible.

Figure 4.6: These figures show the LEC characterization set-up. In subfigure 4.6a the interior is shown,with connections and components. In subfigure 4.6b the hood is closed and the steel housing,containing the photodiode and its active circuit, mounted on the outside of the box, is visible.

It is difficult to make a VI with all these features without losing some processing time for each iterationof the software. Tiny time discrepancies can in the end add up to an observable shift. To observe itwhen running the VI continuously is however not easy; the shift has not been measured. The VI can runcontinuously for at least 72 hours without crashing, but it is hard to predict what a maximum running timeis, if there is any. Several LEC samples, see figure 4.7a, were made and tested in the set-up in order tocontrol its functionality and test limitations. Data from one such test is shown in figure 4.7b, providingevidence for the functionality of the set-up. The luminance data was calibrated with a luminance meter andadjusted accordingly. The voltage and current was controlled with a multimeter.

(a) (b)

Figure 4.7: In subfigure 4.7a a photograph of the light emission from a light-emitting electrochemical cellis shown. Subfigure 4.7b show measurement data from the same cell. The data has beenexported to a text-file, and is visualized with the help of another program. This plot exemplifiesthe functionality of the characterization set-up.

An image of the front panel of the VI is found in appendix B, and shows the environment in which themeasurement set-up is operated. The code, or block diagram, for the VI is also presented.

25

CHAPTER 4. RESULTS

26

Chapter 5

Discussion and Conclusions

The results in section 4.1 and 4.2, show that it is indeed possible to inkjet print a substrate with drops ofelectrolyte. Ink 1 – 5 and hence a wide range of molar masses of the solid solvent PEG (1 – 100 kg/mol)could be jetted. For ink 1 – 4 with PEG molar masses ranging from 1 – 35 kg/mol the printed drops weredistinct, separated, and could be characterized with the profilometer, see figure 4.3. The difficulties to printink 5 due to BOAS effects, and the lack of the same effects for ink 4, is in good agreement with findingsby Mauthner et al. [6]. Ink 6 could not be printed, and as indicated by figure 4.1, the change of viscoelasticproperties of the ink, due to addition of PEG with large molar mass, is most likely the cause.

Slight differences between drop sizes of the different inks were observed, see section 4.2.1. These canhypothetically be ascribed to different viscoelastic properties of the inks which effects the drop formationand drying process, see section 2.2.2.

Upon inspection of a newly printed sample, it was sometimes noted that the diameter of the dropsvaried—between individual samples, and sometimes between different drops on the same sample. Theworst irregularities could be remedied by filling up the level of ink in the cartridge if it was low, butsome variation in and between samples remained. The jetting and drop formation could be inspected viathe printer built in high speed video recorder. No discrepancies were found, and there are then spatialvariations in and between the substrates left to consider. As pointed out in section 2.2, surface roughnessand surface energy density play a big role in the wettability and hence the drop diameter and drying process.I suggest that differences in the ITO substrates’ surface morphology, together with small deviations in thecleaning processes, are responsible for the changing drop diameters and shapes on the ITO samples. Theseproperties could of course be measured, but that is outside the scope of this project. It is widely known in thescientific community that surface energy density and -roughness (both can to some extent be manipulated)affects contact angles, and hence drop diameters, which in turn supports my suggestion. The respectivesurface tension of the inks should play a big role, but no large differences in drop diameter were foundbetween the inks. This indicates that the surface tension is not heavily dependent on the molar mass ofPEG, even if the viscosity definitely is [11, 13].

I conclude that all objectives except number four, see section 1.2, and six have been completely fulfilled.It was stated in objective number four that the drops should be deposited with an edge to edge distance of10 μm. This was not reached as the drops then had the tendency to overlap and merge, forming one bigdrop (or film) instead of two separated small drops. The closest edge to edge distance achieved with wellseparated drops, was 40 μm, for inks 3 and 4. Moreover, it was stated that the thickness of each dried dropshould be 25 nm. Mainly because of the coffee ring effect (further explained in section 2.2.2)—usuallyleaving much of the material at the edges of the drop forming high ridges, or in this case pillars in a stoneship structure, see section 4.2.2—this could not be accomplished. The maximum height of the pillars wasmeasured to be about 600 nm, whilst the average height was about 150 nm: 6 times what was aimed for.

It is interesting to note and worth to mention that techniques to control the size and geometry of inkjet-printed drops are being developed. Ueda et al. indicates that 2-D arrays of superhydrophilic and super-hydrophobic polymers can be used to keep the diameter of drops constant “regardless” of their volume[51]. How to combine these properties with suitable electrodes for LECs remains an open question, but itis intriguing to think of what possibilities these techniques entail.

Objective number six was actually fulfilled, but accuracy of the built in timer in the Labview sampling

27

CHAPTER 5. DISCUSSION AND CONCLUSIONS

VI remains to be investigated. Furthermore, no stress test of the set-up was performed, from which amaximum running time could have been extracted. The precision of the luminance measurements aregood, with deviations in the order of 0.1 cd/m2 for luminances < 50 cd/m2, however the accuracy is notcalibrated in much detail and needs to be adjusted with some reference measurement for more correctreadings. A light emitting sample with a well known luminance could be used. Another way to performthe calibration and adjustment would be to measure a very stable and bright LEC-sample with a hand heldKonica luminance meter, put the same sample in the set-up, and adjust the calibration factor in the softwareaccordingly. This was done, but lack of bright and stable calibration samples decreased the validity of theadjustment and the confidence in the absolute values obtained; there is still room for improvement.

The results from this thesis work show that a solid electrolyte, dissolved in cyclohexanone, can beprinted with a drop-on-demand inkjet printer. Electrodes made of silver nanoparticles can also be printedas shown in [8], and only the active material still remains to be printed in order to get a fully printed LEC.The molar mass of the PEG in ink 4 was 35 kg/mol which is comparable to the molar mass of SY. Evenif the molecular structure of SY and PEG show few similarities, see figures 3.3c and 3.6, I note that ink4 was printed without any big problems thus indicating that future fully Dimatix inkjet-printed LECs areconceivable, and indeed even plausible.

28

Chapter 6

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[31] Latini G, Winroth G, Brovelli S, McDonnell SO, Anderson HL, Mativetsky JM, Samori P, Ca-cialli F. 2010. Enhanced luminescence properties of highly threaded conjugated polyelectrolytes withpotassium counter-ions upon blending with poly(ethylene oxide). Journal of Applied Physics , 107(12).

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[36] Matyba P, Yamaguchi H, Eda G, Chhowalla M, Edman L, Robinson ND. 2010. Graphene andmobile ions: The key to all-plastic, solution-processed light-emitting devices. ACS Nano , 4 (2):637–642.

[37] Matyba P, Yamaguchi H, Chhowalla M, Robinson ND, Edman L. 2011. Flexible and metal-freelight-emitting electrochemical cells based on graphene and pedot-pss as the electrode materials. ACSNano , 5 (1):574–580.

[38] Sandström A, Matyba P, Edman L. 2010. Yellow-green light-emitting electrochemical cells withlong lifetime and high efficiency. Applied Physics Letters , 96 (5).

[39] Matyba P, Maturova K, Kemerink M, Robinson ND, Edman L. 2009. The dynamic organic p-njunction. Nature Materials , 8 (8):672–676.

[40] Edman L. 2005. Bringing light to solid-state electrolytes: The polymer light-emitting electrochem-ical cell. Electrchimica Acta , 50 (19):3878–3885. 9th International Symposium on Polymer Elec-trolytes (ISPE-9), Mragowo, POLAND, AUG 22-27, 2004.

[41] Dick D, Heeger A, Yang Y, Pei Q. 1996. Imaging the structure of the p-n junction in polymerlight-emitting electrochemical cells. Advanced Materials , 8 (12):985–987.

[42] Pei Q, Yang Y, Yu G, Zhang C, Heeger A. 1996. Polymer light-emitting electrochemical cells:In situ formation of a light-emitting p-n junction. Journal of The American Chemical Society , 118(16):3922–3929.

[43] Organic Photonics and Electronics Group at Umeå University. Web page. http://www.physics.

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[47] Asadpoordarvish A. 2010. Glass encapsulated light-emitting electrochemical cells. Master’s thesis.Umeå University.

[48] Chaves J. 2008. Introduction to nonimaging optics. CRC Press.

[49] Steradian figure. Web page. http://commons.wikimedia.org/wiki/File:Steradian.jpg,retrieved 130716.

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CHAPTER 6. BIBLIOGRAPHY

32

Appendix A

Voltage to luminance conversion

The measurement set-up which was built make use of a Hamamatsu photodiode (S9219-01) with an eye re-sponse filter. It was connected to an amplifier with a gain resistance of 117 MΩ. The subsequent calculationof the conversion factor for the set-up follow the procedure outlined in [52].

The wavelength dependent brightness B(λ ) in cd is given by equation (A.1),

B(λ ) = 683 · k(λ ) ·P(λ ) (A.1)

where k(λ ) is the eye-response function (which can be set equal to one because an eye-response filter isincorporated in the photodiode) and P(λ ) is the incident power at wavelength λ . Placing a detector of areaA a distance R from the source means that it will cover A/R2 steradians, see figure 3.10 for an image of asteradian. Total emitted power is therefore,

P(λ ) = Pdiode(λ )R2

A. (A.2)

Inserting equation (A.2) in (A.1) and integrating over all wavelengths yields equation (A.3), which relatesthe brightness of the source to the power incident on the photodiode.

∫∞

0B(λ )dλ =

∫∞

0683 · k (λ ) R2

APdiode (λ )dλ =

683 ·R2

A

∫∞

0Pdiode (λ )dλ (A.3)

By developing the integral we get equation (A.4).

Btot =683 ·R2

APtot

diode (A.4)

Via parameters in table A.1, and applying Ohm’s law, the total luminance of the sample can be calculatedfrom a simple voltage measurement, see equations (A.5) and (A.6).

Table A.1: Parameters for the LEC characterization set-up. Used for calculating the luminance of a sample.

Parameter Abbreviation Value UnitGain resistance Rgain 117 MΩ

Detector area A 0.1296 cm2

Distance to sensor R 3.3 cmSensor responsivity X 0.22 A/WEmitting area Aemit 0.117 cm2

Btot =683 ·R2

APtot

diode =683 ·R2

A ·XUdiode

Rgain(A.5)

33

APPENDIX A. VOLTAGE TO LUMINANCE CONVERSION

Dividing equation (A.5) with the emitting area gives the luminance in cd/m2 of the sample as in (A.6),where C is the conversion factor (in cdA−1V−1) of the photodiode.

Ltot =683 ·R2

A ·Aemit ·X ·RgainUdiode =C ·Udiode (A.6)

The result can be checked by comparing the set-up readings with a Konica brightness meter availablein the lab, and adjusting a correction parameter in the software to compensate for measurement errors ofdistance and sizes of areas.

34

Appendix B

Labview virtual instrument for dataacquisition

Here a figure (B.1) showing the frontpanel of the Labview VI is found. It controls the data sampling andthe LEC characterization set-up, and shows both the sampled data in real time and the measured propertiesin graphs.

Figure B.1: This is the front panel of the data acquisition VI, made to sample measurement data of thesample cell operation over time.

Also a figure showing the block diagram of the Labview VI is presented (B.2). The block diagram isthe code and the heart of a Labview VI. It is a graphical code which visualizes the data flow of the program.

35

APPENDIX B. LABVIEW VIRTUAL INSTRUMENT FOR DATA ACQUISITION

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36