Master’s Thesis in Engineering Physics, 30 ECTS

Utilizing an efficient color-conversion layer for

realization of a white light-emitting

electrochemical cell

Joel Vedin

June, 2016

Master’s thesis, Civilingenjorsprogrammet i teknisk fysik, Umea University.Joel Vedin, jove0027@student.umu.se.

Utilizing an efficient color-conversion layer for realization of a white light-emitting electrochem-ical cell is a project done in the course Master’s Thesis in Engineering Physics, 30.0 ECTS atthe Department of Physics, Umea University.

Supervisors: Mattias Lindh and Petter Lundberg, Department of Physics, Umea UniversityExaminer: Ludvig Edman, Department of Physics, Umea University.

“So much good, so much evil. Just add water.”

–Markus Zusak, The Book Thief

Abstract

Organic semiconducting materials have received a lot of attention in recent years and can nowbe found in many applications. One of the applications, the light emitting electrochemical cell(LEC) has emerged due to its flat and lightweight device structure, low operating voltage, andpossibility to be fully solution processed. Today LECs can emit light of various colors, but to beapplicable in the lighting industry, white light need to be produced in an efficient way. Whitelight on the other hand, is one of the toughest ”colors” to achieve in an efficient way, and is ofparticular interest in general lighting applications, where high color-rendering index devices arenecessary. In this thesis I show that blue light can be partially converted, into white light, byutilizing the photoluminescence of color conversion layers (CCLs).

Furthermore, I show that a high color-quality white light can be attained by adopting ablue-emitting LEC with a CCL. Particularly, three different color-conversion materials wereembedded onto a blue bottom-emitting LEC, to study the resulting spectrum. One of thematerials, MEH-PPV, have good absorption compatibility with the electroluminescence of theblue emitters, but the materials photoluminescence do not cover the red to deep-red range ofthe spectrum. These parts of the spectrum are necessary to obtain high color rendering indices(≥80). A single layer of MEH-PPV adapted onto a blue-emitting LEC, led to a cold whiteLEC with CIE-coordinates x = 0.29, and y = 0.36, color-rendering index = 71, and correlatedcolor temperature = 7200 K. These properties makes it potentially useful in outdoor-lightingapplications. The photoluminescence of another studied color-converting material, polymer red,covers the red to deep-red range of the spectrum but the material lacks absorption in the greenparts of the blue emitters electroluminescence spectrum. Thus it is necessary to combine itwith MEH-PPV to be able to absorb all wavelengths from the blue-emitter and get a broadlight-spectrum out of the device.

In order to preserve a part of the blue light, a new device configuration was designed. It fea-tures a top-emitting blue LEC with a dual-layer CCL which reach an impressive color renderingindex = 89 at a correlated color temperature = 6400 K (CIE-coordinates x = 0.31, y = 0.33).The color-rendering index is the highest reported for a white LEC. The absence of UV-, andIR-radiation, together with the high color rendering properties make the white LEC a possiblecandidate for even the most demanding lighting-applications, such as art galleries, and shopdisplay windows, together with indoor lighting. In this thesis, I show that the CCLs functionwell. However, for the LECs to be worthy competitors, the efficiency and lifetime of the blueemitter need improvements.

iv

Sammanfattning

Organiska halvledande material har fatt en hel del uppmarksamhet de senaste aren och aterfinnsidag i flera komponentstrukturer. En sadan applikation, ljus-emitterande elektrokemisk cell(LEC), ar en potentiell utmanare i belysningsbranschen; tack vara sin enkla struktur, lagadriftspanning, samt mojlighet till billiga, storskaliga, produktionstekniker. Idag kan LECar emit-tera ljus av flera olika farger, men for att vara applicerbar i belysningsbranschen kravs tillgang tillvitt ljus. Vitt ljus ar en av de svaraste ”fargerna” att tillverka pa ett effektivt tillvagagangssatt,och ar dessutom av sarskilt intresse i allmanbelysning dar ett hogt fargatergivningstal kravs. Iden har uppsatsen konstateras att blatt ljus kan bli delvis konverterat till vitt ljus, med hjalpav fargkonverteringslager.

Dessutom, konstateras att ett hogt fargatergivningstal ar mojligt att uppna genom attsatta ett fargkonverteringslager pa en blaemitterande LEC. I synnerhet sa testades tre olikafargkonverteringsmaterial pa blaemitterande LECar for att se hur det paverkade LECens emis-sionsspektrum. Ett av materialen, MEH-PPV, hade bra absorptionskompatibilitet med elektro-luminiscensen fran den bla emitteraren, men materialets fotoluminiscens tackte inte den rodatill djuproda delarna av spektumet; nodvandiga for att erhalla hoga fargatergivningstal (≥80).Genom att lagga ett fargkonverteringslager av MEH-PPV, pa en bla emitterare, framstalldes enkall vit LEC med CIE-koordinater x = 0.29 och y = 0.36, samt fargatergivningstal = 71 ochfargtemperatur = 7200 K, vilket ar tillrackligt for till exempel utomhusbelysning. Aven poly-mer red undersoktes som fargkonverteringsmaterial. Materialet tackte det roda till djuprodadelarna av spektrumet, men saknade ljusabsorption i de grona delarna av den bla emitterar-ens elektroluminiscens-spektrum. Darav var det nodvandigt att kombinera den med MEH-PPVfor att maximera ljusabsorption, fran den bla emitteraren, men samtidigt fa ett brett ljusemis-sionsspektrum fran LECen.

For att bevara en del av det bla ljuset, designades en ny komponentstruktur. En topp-emitterande bla LEC med dubbla konverteringslager framstalldes med imponerande fargren-deringsegenskaper. LECen hade fargatergivningstalet = 89, fargtemperaturen = 6400 K, medCIE-koordinater x = 0.31, y = 0.33. Fargatergivningstalet ar det hogsta rapporterade for en vitLEC. Avsaknad av UV-, och IR-stralning tillsammans med LECens fargatergivnings-egenskaper,gor att den vita LECen har potential att kunna anvandas aven for de mest kravande belysning-somradena sa som i konstgallerier, skyltfonster samt for inomhusbelysning. I den har uppsatsenvisar jag att fargkonverteringslagren fungerar bra, men for att konkurrera i belysningsbranschenbehover effektiviteten och livslangden pa den bla LECen forbattras.

v

“It’s like everyone tells a story about themselves inside their own head. Always. All the time.That story makes you what you are. We build ourselves out of that story.”

–Patrick Rothfuss, The Name of the Wind

vi

Acknowledgement

First and foremost I would like to thank my supervisors, Mattias Lindh and Petter Lundberg,for all the laughters, good discussions, and valuable advices during this master’s thesis. Withoutthem, you would be holding a very different thesis right now.

I would also like to thank all the colleagues in The Organic Photonics and Electronics Groupfor all the help I received and all the knowledge they shared. It meant a lot to me. Special thanksto professor Ludvig Edman for the opportunity to be part of this group, much appreciated.

Finally, I want to thank my friends, beloved, and family for always being there for me, alwaysgiving me something to look forward to, and consistently filling me with joy.

Thank you so much, everyone, for being you.

Joel Vedin,Umea, Sweden,18 June, 2016.

vii

Contents

1 Introduction 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Theory 32.1 The light-emitting electrochemical cell . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1.1 Emitting layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.2 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.3 Operating mechanism of LECs . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Color-conversion layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Measures of illumination quality . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3.1 CIE-coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3.2 Correlated color temperature of black body emitters . . . . . . . . . . . . 72.3.3 Color-rendering index of white light . . . . . . . . . . . . . . . . . . . . . 7

3 Method 93.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.1 Fluorescent polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1.2 Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2 Fabrication of an LEC device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2.1 Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.2.2 Deposition of the emitting layer . . . . . . . . . . . . . . . . . . . . . . . . 123.2.3 Thermal evaporation of the electrode . . . . . . . . . . . . . . . . . . . . . 123.2.4 Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.2.5 Deposition of the color-conversion layer . . . . . . . . . . . . . . . . . . . 14

3.3 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.3.1 Luminance and lifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.3.2 Thickness measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.3.3 Spectroscopic measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 143.3.4 Illumination quality calculations and photography . . . . . . . . . . . . . 15

4 Results 174.1 Properties of the blue emitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.2 Bottom emitting LECs employing different CCLs . . . . . . . . . . . . . . . . . . 18

4.2.1 Super yellow color conversion layer . . . . . . . . . . . . . . . . . . . . . . 184.2.2 MEH-PPV color conversion layer . . . . . . . . . . . . . . . . . . . . . . . 194.2.3 Polymer red color conversion layer . . . . . . . . . . . . . . . . . . . . . . 21

4.3 Characteristics of top-emitting LECs utilizing different CCLs . . . . . . . . . . . 22

5 Discussion 275.1 Performance of the color-conversion materials . . . . . . . . . . . . . . . . . . . . 275.2 Whiteness of the top-emitting LECs . . . . . . . . . . . . . . . . . . . . . . . . . 285.3 Efficiency and lifetime of the blue emitter . . . . . . . . . . . . . . . . . . . . . . 29

5.3.1 Green peak in polymer blue’s spectrum . . . . . . . . . . . . . . . . . . . 29

viii

5.4 Future studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6 Conclusion 31

Bibliography 33

Appendix A Super yellow based LEC

Appendix B Special color rendering indices

ix

Chapter 1

Introduction

1.1 Background

Since the invention of organic semiconducting polymers, they have received a lot of attention inthe electronics industry. One of the areas is the advancement of organic light-emitting diodes(OLEDs), with potential to offer lower manufacturing costs than their inorganic counterpart.This is particularly due to that organic semiconducting polymers are soluble, which enablescheap continuous process techniques like roll-to-roll printing and coating to be used, in order toachieve thin semiconducting films.

Nowadays OLEDs are well established on the market, where they for example are used inmany displays, like TVs and smartphones [1, 2]. Especially interesting are the white OLEDs,due to their application in solid-state lighting, where a new light source needs to replace theinefficient incandecent bulb [3]. However, these devices are often fabricated using several differentlight emitting layers of different colors [4], complicating the production. Another utilized deviceconfiguration is to have several different emitters in the emitting layer [5, 6]. These devices aresensitive to different aging of the light emitting materials [7]. To overcome these disadvantages acolor-conversion layer (CCL) comprising one material can be utilized. Its purpose is to partiallyconvert the emitted light into longer wavelengths, resulting in a broader spectrum.

Another emerging organic light-emitting device, the light-emitting electrochemical cell (LEC),first reported by Pei et al. in 1995 [8], has some advantages over the OLED. The key advantagesare that LECs typically only constist of one emitting layer, where the semiconducting polymeris blended with an electrolyte. The electrolyte makes the devices more durable, which in turnlowers the precision needed in fabrication. While the OLEDs are sensitive to the thickness oftheir constituent layers [9], LECs are generally not. This simplifies deposition of the single layer,and thereby even cheaper solution processes may be used. Additionally, thick layers are not assensitive to surface roughness [8, 10], facilitating production.

One more advantage is that LECs are not as dependent on the electrode work-functions (dueto the electrolyte blend, decribed in section 2.1.1) as OLEDs allowing cheaper and more air-stable electrodes to be used [11]. LECs (and OLEDs) can be operated at very low voltages, closeto the energy gap potential, Eg/e, of the organic semiconductor, where Eg is the semiconductorsenergy gap and e is the elementary charge. However, due to their electrochemical basis, LECshave some drawbacks; such as limited operational lifetime (due to electrochemical instability),and slower turn-on time than OLEDs.

Just like OLEDs, white LECs are also desired due to their potential in solid-state lighting.Several different approaches can be utilized to acquire white light from LECs. The most commonway is to have a single emitting layer [12], comprising a blend of different luminescent materials[13, 14, 15]. Another approach is to apply a photoluminescent phosphor onto the device structureof a sandwich LEC acting as a CCL. The intention of the CCL is to partially convert the emittedlight of the device so that white light can be attained. CCLs can be added to the LECs with

1

CHAPTER 1. INTRODUCTION

the same methods as the emitting layer. Thus, device fabrication is not hampered by the extradevice layer. This approach is not entirely new, and have been used in both OLED [16, 17] andLEC [18, 19] devices. White LECs employing a CCL have some advantages over single layeredwhite LECs. Degradation is focused on one material, the emitting layer, instead of severalas is the case with a single layered white LEC [12]. The latter devices often suffer from biasdependent color-shifts [20, 21]. However, devices using a single emitting-layer have been reportedwith color-rendering indices (CRIs) of 83 and 84 (100 is perfect score) [13, 21], which is sufficientfor indoor-lighting [22]. To be applicable in more demanding situations, where very high-qualitylighting is desired, a CRI of about 90 is required [23]. The CRI of a light source is determined byfirst simulating how well the light source can render 14 test color samples, providing 14 specialCRIs. The CRI is then the average of the first eight of these special CRIs. For a light source tobe useful in biomedical applications, the ninth special CRI (R9) is of particular interest, as itdescribes deep-red color-rendering [24]. A device with both high CRI, and R9 is sought in manycommercial applications [25], but a high R9 is hard to obtain, and commercial white LEDs havetypically low R9 values around 14 [26].

1.2 Purpose

White LECs have previously been constructed with a single emitting layer [12], where differentelectroluminescent conjugated polymers (CPs) are blended in order to emit white light from thedevice. White LECs utilizing a CCL have also been reported, but the focus has been directedtowards the efficiency of the devices, and not on the whiteness [18, 19, 27]. All reported devicesalso utilize the same red emitting dye. The purpose of this project is to investigate a slightlydifferent approach, namely using a blue-emitting LEC together with an efficient polymer basedCCL, in order to attain quality white light. The CCL is supposed to partially convert the bluelight from the device into light of a longer wavelength, allowing a broad-spectrum light emissionfrom the device. The main objective is to see if better color rendering can be attained in anefficient way by using polymers in a CCL configuration, adopted to the LEC

1.3 Objectives

The purpose can be split up into the following main objectives:

1. Do a background literature study to get familiar with the operating mechanism and devicecharacteristics of the LEC. In other words get acquainted with the doping process, thedifferent parts of the LEC and how they contribute to the LEC mechanism.

2. Fabricate yellow-emitting LEC devices, according to a standard recipe, to get accustomedwith the production procedure, and the equipment used to produce the devices. Yellow-emitting LECs are today the easiest to fabricate and get stable emission from.

3. Fabricate blue-emitting LEC devices, according to a standard recipe, emitting stable lightfor a long period, at least 200 cd m−2 for an hour, to allow studies directed towards thedegradation and efficiency of the color-converting materials.

4. Identify color-conversion materials absorbing light in the blue spectrum. The color-convers-ion materials should be photoluminescent with light-emission in the yellow to deep-redspectrum. It should also allow some of the blue light to travel through the layer grantinga broad emission spectrum.

5. Design and implement a color-conversion layer in a blue surface emitting LEC for therealization of a functioning color-conversion device. The device should emit white light,with CIE coordinates in the white region of the CIE 1931 chromaticity diagram (depictedin figure 2.5). A CRI over 70 for at least 1 hour, and a luminance of over 100 cd m−2

during that hour, should be acquired by the LEC.

2

Chapter 2

Theory

This chapter contains the theory regarding the LEC and how it works. It also describes theconcept of color-conversion layers and how light can be characterized in respect to its quality.

2.1 The light-emitting electrochemical cell

In order to emit light from an OLED or an LEC, three things are necessary. These are twoelectrodes (connection points) and an emitting layer. Distinguishing for the LEC is the emittinglayer, which in an LEC not only comprises the CP, but also an electrolyte. The electrolyte’srole is to dope the polymer; facilitating hole, and electron injection. Thus, the LEC typicallyrequires fewer layers, as hole-, and electron-injection layers are unnecessary.

A typical LEC configuration is the sandwich cell, depicted in figure 2.1, where the emittinglayer is situated between an indium tin oxide (ITO) anode, and an aluminium cathode. Thetransparent ITO allows light to exit from the emitting layer, through the ITO/glass layers.

Glass

Emitting layer

Aluminium cathode

ITO anode

Figure 2.1: A typical sandwich LEC with its different layers. Each layer is generally about onehundred nanometers thick (the schematic is not to scale).

2.1.1 Emitting layer

The emitting layer of a polymer LEC, where a lot of the physics take place, typically comprisea blend of a CP and an electrolyte. The CP is responsible for both the light emitting propertiesof the device and the transportation of electrons and holes inside the device. The electrolyte, onthe other hand, is responsible for the doping of the CP, and the important formation of electricdouble layers. The compatibility between the electrolyte and the CP is important, as it stronglyaffects the stability and performance of the device [28].

3

CHAPTER 2. THEORY

Conjugated polymers

Conjugated polymers are organic macromolecules (large molecules composed of several mono-mers), with an extended π-conjugation along its backbone chain of alternating single-, anddouble-bonds. The Pz orbitals of the carbon atom, who form the π-orbitals, overlap each otherextending the π-conjugation along the backbone. Thus, a system of delocalized π-electrons isformed, giving the polymer properties closely related to an inorganic semiconductor [29]. Therepeat unit of polyacetylene, one of the simplest polymers, is shown in figure 2.2.

CPs have energy gaps closely related to the inorganic counterpart, which have a valenceband, and a conduction band. The energy gap in CPs is characterized by the energy differencebetween their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular or-bital (LUMO), which in turn are determined by the structure of the CP’s repeat units (HOMO,and LUMO represents the CP’s counterpart to the inorganic semiconductors valence-, and con-duction band, respectively). By increasing the number of repeat units in a chain, the energygap can be decreased, as each repeat unit add an extra energy level to the HOMO and LUMO[30]. The energy gap of the CP and the wavelength of the emitted light are related by

E =hc

λ, (2.1)

where E is the photon energy, h is Planck constant, c is the speed of light, and λ is the wavelengthof the light.

C

C

( )n

Figure 2.2: The repeat unit of polyacetylene.

Typical CPs are different derivatives of poly-(p-phenylene vinylene) (PPV) and poly(spirobi-fluorene), where the side groups can be replaced to acquire the desired color emission, energygap, and solubility with organic solvents [31, 32]. These CPs generally provide good luminescentproperties, while still maintaining high electron and hole mobilities. In addition, many CPs havea strong photoluminescence in their undoped state [33].

Electrolyte

The CP needs to be doped to increase electron and hole transport in the emitting layer. This issolved by blending the CP with an electrolyte, allowing the emitting layer to be doped, in situ,under a voltage bias. A typical LEC electrolyte consists of a salt and an ion transporting poly-mer. The salt provide the mobile ions in the emitting layer of the LEC, while the ion conductingpolymer ensures that ions can move freely in the emitting layer. Typically, the salt LiCF3SO3

(LiTF, lithium trifluoromethanesulfonate), or KCF3SO3 (KTF, potassium trifluoromethanesulf-onate) is used, which can be dissolved in the ion conductor hydroxyl-capped trimethylolpropaneethoxylate (TMPE-OH). KTF has been shown to have higher diffusion rate in TMPE-OH thanLiTF, while still maintaining a high efficiency [34]. There are some advantages with TMPE-OH,compared to the more traditional ion transporter polyethylene oxide (PEO), where LECs usingPEO have been shown to experience phase separation with many commonly used CPs; causinga rougher surface on the emitting layer and lowering the stability of the devices [35].

2.1.2 Electrodes

Two electrodes are required to operate an LEC, one cathode and one anode. ITO is commonlyused as the bottom electrode in sandwich LECs due to its good combination of low sheet resist-ance and high optical transparency to visible radiation [36]. The transparency of the ITO allowstransmission of visible light from the emitting material through the ITO and glass layers. Forthe top electrode, aluminium is often chosen as it displays good cathode performance [29] andhigh reflectance. The aluminium does not just stop light from leaving at the top of the sandwich

4

CHAPTER 2. THEORY

LEC, but also reflects most of the light, allowing for almost doubling the luminescence of thedevice.

2.1.3 Operating mechanism of LECs

The operating mechanism of the LEC is interesting from a physical perspective, since a lot ofthings take place in the emitting layer. When the device is in open circuit, the mobile ionsrandomly distribute in the emitting material, depicted in figure 2.3a. It is first when a voltagebias is applied to the electrodes of the LEC things take effect. The external bias shift the energylevels of the emitting material and the introduced electric field redistribute the mobile ions,driving the cations (anions) towards the cathode (anode), as illustrated in figure 2.3b. As theions gather at their respective electrodes, see figure 2.3c, thin electric double layers are formedin the interface of the electrodes and the emitting material, resulting in a large potential dropin the double layers. The consequence is a shift in the CP’s HOMO, and LUMO, benefitingcharge injection. The charges only have to tunnel through a nanometer thin layer [37], underinfluence of a large electric field. The remaining ions in the bulk of the emitting layer redistributeand compensate the injected electrons and holes, see figure 2.3d. The emitting layer becomeelectrochemically doped from the injection of charge carriers making the emitting layer p-typedoped near the anode and n-type doped close to the cathode. As the applied voltage is increased,to or above the CP’s energy gap, more electrons and holes will be injected at the electrodes.The n-, and p-type doped regions in the emitting layer grow, until they meet and form a p-njunction [38]. Electrons and holes can now meet in the p-n junction and form an exciton whichin turn decays to a photon, and the device emits light.

5

CHAPTER 2. THEORY

LUMO

HOMO

(a)

Anode

Cat

hode

–

+

–

+

+

–

+

–

–

+

–

+

(b)

–

+

–

+

–+

–

+

–

+

–

+

(c)

––

–

+

++

+

– +

+–

–

(d)

p-n junction

––

–

+

++

–

–

–

+

+

+

+ Cation

– Anion

Hole

Electron

Photon

Figure 2.3: Simplified sketch of the working principle of an LEC. First when the device is opencircuit (a). In (b), an external bias is applied over the electrodes and the ions movewith the resulting electric field. (c) shows how the electron (and hole) injection isgoverned by the formed electric double layers. In (d) a p-n junction has formed andexcitons form and release photons, and we thereby get light.

2.2 Color-conversion layers

To convert light from one wavelength to another, a color-conversion layer (CCL) can be utilized.It can be made of a photoluminescent CP and is most often applied on a light-emitting device toconvert its emission. The CCL absorbs the incoming photons, which have larger energy than theCP’s energy gap and excitons are formed. After a short relaxation period these excitons will emitphotons with longer wavelengths. It is desired to have a polymer with high photoluminescencequantum yield (PLQY), which is a measure of how many photons the polymer emits per absorbedphoton. Due to concentration quenching, the PLQY is often lower in solid films than in solution,caused by a higher polymer concentration in the film [39].

To attain a broad emission spectrum from the LEC, it is not desired to convert all of thelight emitted from the emitting layer. This can be controlled by making CCLs with differentthickness, as a thin layer will transmit more photons from the emitting layer out of the device.Figure 2.4 demonstrate how the CCL partially converts light from the emitting layer and re-emitlight of a different wavelength. The biggest advantage of using a CCL to attain a broad light-spectrum, instead of having an emitting layer with several different CPs, is that the degradationof the device is solely dependent on one polymer, providing a more color-stable device [40].

6

CHAPTER 2. THEORY

Glass

ITO

Emitting Layer

Aluminium

CCL

Figure 2.4: A typical sandwich light-emitting electrochemical cell (LEC) in (a), and a sandwichLEC employing a color-conversion layer (CCL) in (b).

2.3 Measures of illumination quality

There are many different approaches that can be utilized to characterize different propertiesof light. In this section one of the most well used approaches for characterizing white light isdescribed.

2.3.1 CIE-coordinates

To the normal human eye it is easy to distinguish if a light is red, blue, or any other commonlyoccurring color. In computers, it is also easy to translate different wavelengths of light intoparticular colors. However, it is tricky to convert a large range of wavelengths into the colorperceived by the human eye, and how natural the color is perceived. In order to link thesetogether the Commission internationale de l’eclairage (CIE) defined the CIE 1931 standardobserver [41], which chromaticity diagram can be seen in figure 2.5. The CIE 1931 color spaceuse two dimensions (x and y) to describes the chromaticity of a light and one dimension todescribe its brightness (Y ). Chromaticity describe the color-quality of a light without respectto its luminance. The CIE-coordinates are calculated from the spectrum with the help of threecolor-matching functions, one for each cone cell in the eye, which can be found in [42]. Detailedcalculations are also listed in the book.

2.3.2 Correlated color temperature of black body emitters

Correlated color temperature (CCT) is defined as the color of the light corresponding to thetemperature of a black body radiator [42]. A cold white light-emitter will have a blueish whitecolor, while a warm white light-emitter is perceived as orange-reddish white. The flames ina campfire for example are often perceived as blue in the middle where they are hottest, andorange-red at the outer colder parts. The same holds for light sources. A warm white light-emitter will have a CCT of about 2700-3000 K, while the CCT for a cold emitter will rise above5000 K. The CCT of a light emitter is determined by the emitters coordinates (u, and v) in theCIE 1960 color space [43]. The point in the Planckian locus, see figure 2.5, that is closest tothe light source’s coordinates provides the CCT of the light source [42]. The locus representsthe color path an incandescent black body would take when increasing its temperature, fromdeep-red to blueish white [42]. A more detailed description of how to calculate the coordinates,and the CCT, can be found in [42].

2.3.3 Color-rendering index of white light

Color-rendering index (CRI) specifies how true a light source can reveal different colors in thevisible spectrum, compared to a natural light source [42]. For example, a yellow light-emitterilluminating a blue object will not reveal the blue color of the object as well as a blue light-emitter. The same applies in the opposite direction as a blue light-emitter will poorly revealyellow colors. Consequently, these light sources will have a poor CRI value. In a camera flash,

7

CHAPTER 2. THEORY

emittance of all visible colors are desired, to be able to perceive all colors in the captured photo.In this case a light source with high CRI is convenient.

The CRI of a light source is determined by comparing its color-rendering properties with astandardized black body radiator. The black body radiator is defined to have a CRI of 100,which corresponds to perfect color-rendering properties. CRI is typically ranged from 0 to 100,even if negative values are possible by the definition (negative CRIs are often rounded up tozero in litterature [44]). A light source with CRI of 100 represents a perfect black body while alight source with CRI of 0 will make all colors look the same under illumination [42].

To calculate the CRI of a light source, one starts by looking at its CCT. If the temperatureis lower than 5000 K a black body is used as reference, otherwise CIE standard illuminant D [45]with the corresponding CCT is used. By utilizing 14 test-color samples (listed at [46]) one cancompare how well the light source renders its colors, compared to a black body radiator. The 14test-colors provide 14 special CRIs (Ri) of the light source, where only a spectrum of the sourceis required, as reference data is available in most colorimetry books (for example Color science[42]). The first eight color-samples are used when calculating the general CRI of a light source,while the remaining six are for special purposes (illuminating red meat for example requires ahigh strong red special CRI, R9 [47]). The general CRI of a light source is calculated as themean of the first eight special color rendering indices. A list of the fourteen special CRIs isfound in Appendix B, and complete special CRI calculations are found in [42].

Figure 2.5: CIE 1931 chromaticity diagram with a roughly encircled white light region (dashed)and the planckian locus (solid line).

8

Chapter 3

Method

In this chapter the different materials, used in this work, are explained. Furthermore it containsa thorough description of how LECs are fabricated and how different parts of the LEC werecharacterized.

3.1 Materials

Several different materials are necessary for the fabrication of an LEC. It is not just the CPs,responsible for the properties of the emitted light, that are of importance. The electrolyte andelectrodes also play an important role in the LEC. In this section the utilized materials arelisted, together with their properties and their importance for the LEC device.

3.1.1 Fluorescent polymers

Polymer blue (PB, Livilux SPB-02T, Merck GmbH, Germany) is a suitable electroluminescentmaterial, when aiming for a wide spectrum, using CCLs. It is a conjugated co-polymer withblue to blue green light-emission. The molecular structure of polymer blue is illustrated in figure3.1a. Polymer blue was used in the emitting layer, for all devices employing a CCL, where it wasblended with KTF and TMPE-OH, using cyclohexanone as the solvent (molecular structure infigure 3.1b).

(a) Polymer blue (b) Cyclohexanone

Figure 3.1: The repeat units of polymer blue (a) and the solvent cyclohexanone (b), respectively.

Super yellow (SY, Livilux PDY-132, Merck GmbH, Germany) is a conjugated PPV-basedco-polymer with green to yellow photoluminescence. It was used both in the emitting layer ofthe first LECs fabricated in this project (see first objective in section 1.3, and result in appendixA), and as a color-conversion layer. Super yellow is fluorescent with a high PLQY (60%), [48],making it applicable to use as an effective color-conversion material. With light absorption inthe deep-blue to blue spectrum [49], super yellow could be suitable to use as CCL together with

9

CHAPTER 3. METHOD

a blue-emitting LEC. Cyclohexanone can be used to dissolve super yellow up to a concentrationof 20 mg mL−1. The molecular structure of super yellow is presented in figure 3.2.

Figure 3.2: Repeat units of the conjugated co-polymer super yellow.

The CP poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV, LT-S931,Luminescence Technology Corp., Taiwan) is a PPV polymer with orange to red photolumines-cence. MEH-PPV is a fluorescent polymer with light absorption in the whole region of polymerblue’s emission spectrum [50], making it ideal as a color-converting material for attaining whitelight from a polymer blue based LEC. The molecular structure of MEH-PPV is depicted infigure 3.3a, it is soluble (at least to 20 mg mL−1) in toluene (figure 3.3b), and have previouslybeen used as CCL in OLEDs [51, 52].

(a) MEH-PPV (b) Toluene

Figure 3.3: Repeat units of the polymer MEH-PPV (a) and the solvent toluene (b), respectively.

In order to attain light-emission in the deeper red spectrum (650-700 nm) than MEH-PPVcan provide, polymer red1 (PR, Livilux SPR-001 L05, Merck GmbH, Germany) was used. It isa fluorescent co-polymer with light absorption in the deep-blue to blue region [49], making itsuitable as color-converting material for attaining white light from a polymer blue based LEC.It is soluble (at least to 30 mg mL−1) in toluene.

1No molecular structure was available from the provider.

10

CHAPTER 3. METHOD

3.1.2 Electrolyte

Ion-transporter

To allow for ionic motion in the emitting layer, the ionic solvent/ion conducting materialhydroxyl-capped trimethylolpropane ethoxylate (TMPE-OH, Sigma-Aldrich) was used, see fig-ure 3.4a. TMPE-OH has been shown to be less prone to side reactions than previously used iontransporters [53].

(a) Hydroxyl-capped trimethylolpropaneethoxylate

(b) Potassiumtrifluoromethanesulfonate

Figure 3.4: The molecular structure of the electrolyte components. Hydroxyl-cappedtrimethylolpropane ethoxylate (a) is the ion-transporter and Potassiumtrifluoromethanesulfonate (b) is the salt.

Salt

Potassium trifluoromethanesulfonate (KTF, CF3SO3K, Aldrich) is a salt comprising potassiumcations K+, and trifluoromethanesulfonate anions (CF3SO−

3 ). Together with TMPE-OH, KTFforms the solid-state electrolyte where the ions can move freely with the aid of the TMPE-OHmolecules. Before usage, the salt was dried at 190°C in a vacuum oven to get rid of excess moist,as it might damage the CPs.

3.2 Fabrication of an LEC device

LECs with three different device structures were fabricated. The first design, shown in figure2.1, was used to determine the characteristics of the LEC when the CCL is excluded. The secondsetup, shown in figure 3.5a, shows a bottom emitting LEC with a CCL. The purpose of the CCLis to partially convert the electroluminescent light from the emitting layer, so that light with abroader spectrum is emitted from the device. Finally the third design, see figure 3.5b, uses athin transparent silver layer as anode (15 nm thick), allowing light to pass through both the topand bottom of the LEC. With this device configuration half of the light never travels throughthe CCL, and is directed out of the device, while the other half is directed towards the CCL. Byfabricating CCLs thick enough to absorb as much as possible of the blue light, directed towardsit, about half of the blue light produced by the LEC will be converted. This allows a broadrange of wavelengths to be emitted from the top of the device thanks to the reflective aluminiumlayer underneath the CCL.

Since the CPs are sensitive to oxygen and water, all device fabrications took place in aglovebox with a nitrogen environment ([O2] < 5 ppm, [H2O] < 2 ppm).

11

CHAPTER 3. METHOD

Glass

ITO

Emitting layer

Aluminium

CCL

(a)

Glass

ITO

Emitting layer

Silver

CCL

Aluminium

(b)

Figure 3.5: Schematic illustration of two different device configurations. The left figure showa bottom emitting LEC with a color-conversion layer (CCL) while the right figurerepresents a top emitting LEC employing a CCL and an aluminium layer, wherethe latter works as a reflector.

3.2.1 Cleaning

Small particles, on the surface of the ITO coated glass substrates, often leads to bad emittinglayer films. Examples of bad layers are particle spots, or comet shaped regions with no (orlittle) emitting material. Cleaning the substrates is thus necessary to get a stable LEC device.Consequently, an ultrasonic bath was used to clean the ITO substrates in acetone for 30 minutesat 30°C. Afterwards, the cleaning procedure was repeated using isopropyl alcohol instead ofacetone. Finally the substrates were dried in an oven for at least 4 hours at 120°C before theywere placed in a nitrogen filled glovebox.

3.2.2 Deposition of the emitting layer

CPs, TMPE-OH, and KTF were separately dissolved in cyclohexanone at a concentration of 10mg mL−1 and stirred on a magnetic hot plate until completely dissolved (>8 hours) at 50°C.The emitting material was acquired by blending the three master solutions at the mass ratio{CP:TMPE-OH:KTF} = {1:0.1:0.03}, and stirring the produced solution on a magnetic hotplate for at least 5 hours. The thin emitting layer was formed by applying the emitting materialsolution to the ITO substrate, which was then spin-coated with a SPS Spin 150 to remove excesssolution.

Spin-coating is a technique used in order to deposit thin uniform films to small flat substrates,see figure 3.6. The emitting layer substance is first applied to the substrates surface, see figure3.6a, when the spin-coater is at rest. By applying a high rotational speed (generally 800-4000rpm) the excess substance is forced off the substrates sides by the centrifugal force, see figure3.6b-c. After rotating for a short time (typically 10-60 seconds) a thin uniform emitting layerfilm will be formed on top of the substrate, as shown in figure 3.6d. The final film thicknessmainly depend on the viscosity of the substance and the interaction between the substance andthe surface of the substrate. It can however be affected by altering the duration, rotational speed,initial acceleration, and the concentration of the emitting material, the film thickness could becontrolled. After the emitting layer had been spin-coated on the substrate, it was heated at 50°Con a magnetic hotplate for at least 8 hours. The heat removes the remaining solvent from theemitting layer, forming a dry film. Heating at a low temperature for an extended time ensuresthat the polymer chains are not damaged by the heat, while allowing closer packed polymerchains to form over a longer time. All emitting layers in this work was spin-coated at 2000 rpm,for 60 seconds, with acceleration 2000 rpm s−1.

3.2.3 Thermal evaporation of the electrode

The final step of the LEC fabrication was to deposit the top electrode on the emitting layer. Itis done by thermally evaporating aluminium or silver in a vacuum chamber with the pressurep < 4 ·10−6 Pa. It is a procedure where an evaporant is placed in a tungsten boat. When a highcurrent (150-220 A depending on material) is passing through the boat, the boat is joule heated.

12

CHAPTER 3. METHOD

Figure 3.6: The figure show the four main steps of the spin-coating mechanism. (a) demonstratea thick emitting layer applied to an ITO substrate, situated on top of a spin-coater chuck. (b) show how the emitting layer is altered, directly after the chuckis rotationally accelerated. In (c), the substrate is rotated at high speed, forcingemitting material off the sides by the centrifugal force. (d) show the final, thinemitting layer, formed on top of the substrate.

After a high enough temperature is reached the material is evaporated, making the materialattach to the above surface, see figure 3.7. By placing a shadow mask directly in front of thesubstrates, different electrode patterns can be formed. The vacuum chamber remove particlesfrom the atmosphere around the evaporant and the substrate, when evaporating. As a result,the evaporated material stand a lower risk of colliding with other particles before reaching thesubstrate. By reducing the amount of collisions, a more homogeneous electrode is acquired.

Figure 3.7: An illustration of the evaporation principle. Current is flowing through the tungstenboat and heat is accumulated by the boats electric resistance. The evaporant in theboat is then heated until metal vapor is formed, which subsequently gather on thesubstrate placed above the boat. A shadow mask placed in front of the substrateallows electrode patterns to be made.

13

CHAPTER 3. METHOD

3.2.4 Encapsulation

During operation the fabricated LEC is not stable in air and must be encapsulated, to preventside reactions with oxygen and water, before it can be operated outside the nitrogen environmentin a glove box. A second purpose with the encapsulation is to prevent damaging the LEC withthe spin-coater chuck, e.g. when spin-coating was performed on both sides of the ITO/glasssubstrate. To encapsulate the device, a UV curable epoxy (Ossila Limited) was applied to thedesired surface. By attaching a thin piece of glass to the epoxy and UV curing it for 10 minutesan air stable LEC was acquired. A more detailed description of the encapsulation technique isdescribed in [54].

3.2.5 Deposition of the color-conversion layer

The CCL was applied to the LEC in a similar way as the emitting layer, described in section3.2.2. The CP was dissolved in toluene and stirred on a magnetic hot plate at 50°C for at least 8hours. The CCL was completed by spin-coating (2000 rpm, 2000 rpm s−1, 60 s) the CP solutionon an encapsulated LEC surface and heating it at 50°C for at least 8 hours. For the top emittingLECs a 100 nm aluminium layer was evaporated on the CCL’s surface to reflect the light backtowards the emitting layer.

3.3 Characterization

3.3.1 Luminance and lifetime

The luminance and lifetime of the bottom-emitting LECs were measured with an OLED lifetimetester (M6000 PMX OLED Lifetime Tester, McScience). It is an easy to use test system wherethe LECs are mounted in a jig unit with a built in photodiode (MC9600 Panel Mounting Unit,McScience). The jig unit is connected to the constant current generator in the lifetime tester.The photodiode measures the mounted LEC’s luminance as a function of time and the lifetimeand brightness of the device are recorded. The lifetime was defined as the time between theLEC’s turn-on time, when it first reached brightness >100 cd m−2, and the time where thebrightness dropped under 100 cd m−2 again. The lifetime tester was calibrated with a luminancemeter (LS-110, Konica Minolta) so that the brightness is measured within 5% of the brightnessmeasured by the luminance meter.

For the top-emitting LECs, the brightness was measured with a calibrated photodiode, im-plemented with an eye response filter (S9219-01, Hamamatsu Photonics). A constant currentwas generated by an Agilent U2781A mounted with an Agilent U2722A source measure unit(Agilent technologies).

3.3.2 Thickness measurements

A stylus profilometer (Dektak XT, Bruker) was used to determine the thickness of the spin-coated layers on the LECs. The profilometers stylus is lowered on top of the layer surface.By moving the stylus laterally across the surface, at a specified contact force, surface thicknessvariations can be measured over the travel distance. By gently scraping off a line across a spin-coated surface, the thickness can be measured by moving the stylus perpendicularly over theline.

3.3.3 Spectroscopic measurements

To measure the electroluminescence, a fiber-optic spectrometer (USB2000+, Ocean optics) wasused. Inside the spectrometer, the light is first diffracted by a grating. By directing the diffractedlight into different detectors, the intensity of the light can be measured at different wavelengths.The spectral response of the spectrometer is however not uniform, and this internal responsemust be compensated, when doing fluorescence measurements. Therefore a relative irradiancemeasurement procedure was used, in which a tungsten halogen lamp (HL-2000-HP-232R, Ocean

14

CHAPTER 3. METHOD

Optics) was used as reference source. The collected spectra was stored using the softwareSpectaSuite (Ocean optics).

The absorption of the polymer films was measured with a UV/VIS spectrophotometer(Lambda 35, PerkinElmer). To measure the absorption, a light source is directed into a gratingmonochromator. The light is splitted into two beams, one reference and one for the sample.A photo-detector measures the output from the two beams, providing the transmittance of thefilm as the measured signal divided by the reference. Quarts substrates were used as base forthe polymer films.

A fluorescence spectrometer (LS 45, PerkinElmer) was used to measure the photolumin-escence of the spin-coated polymer films. The spectrophotometer sends a beam of light intoa monochromator to select light of a specified wavelength. If the specified wavelength rangeis within the polymer’s absorption range, the polymer is excited and its fluorescence can bemeasured with a photo-detector. The polymer films were spin-coated on quarts substrates.

3.3.4 Illumination quality calculations and photography

The CIE-coordinates, CRI, and CCT parameters of white light emission were calculated inMatlab (R2014b) with a slightly modified code attained from Lighting research center [55]. Thecode was tested with data from various light sources found at Designing with LEDs [56]. Thecalculated CRI values were within 0.3 units from the stated values, while CCT were within 100K from the stated temperature.

The photographs were taken with a Canon EOS 500D equipped with a Sigma EX 150/2,8DG HSM Macro lens. Photographs have been adjusted with respect to brightness and contrastusing GIMP 2.8.8.

15

Chapter 4

Results

This chapter presents the achieved results during this work. The first, and prerequisite, objectivewas to create a stable super yellow LEC with significant brightness for a long duration (see section1.3). The result from this study can be found in appendix A. The following sections cover theresults from the remaining objectives, with focus on the design and implementation of CCLs(see objective 5).

4.1 Properties of the blue emitter

In figure 4.1, the brightness and voltage of the blue emitter, operated at a constant currentdensity of j = 7.5 mA cm−2, is shown. The turn-on time and lifetime of the device was 29seconds and 21 hours, respectively, while the maximum brightness achieved was 320 cd m−2.Initial voltage of the LEC was 7.2 V which dropped to its lowest point after 35 minutes whenthe device reached 5 V. The peak efficiency of the device was 4.3 cd A−1. The thickness of boththe emitting and aluminium layers was 100 nm.

Time (hours)0 5 10 15 20

Brig

htne

ss (

cd m

-2)

0

50

100

150

200

250

300

350

Vol

tage

(V

)

0

2

4

6

8

10

12

14

BrightnessVoltage

Figure 4.1: Lifetime of the polymer blue based LEC. The solid blue line shows how the bright-ness varies over time. The red dashed line displays the voltage development duringthe lifetime of the device.

17

CHAPTER 4. RESULTS

4.2 Bottom emitting LECs employing different CCLs

The results presented in this section all have the device structure shown in figure 3.5a, where theblue emitter in section 4.1 was utilized. The different subsections present the obtained resultsafter using the additional fluorescent polymers, described in section 3.1.1, as CCL.

4.2.1 Super yellow color conversion layer

Super yellow was the first studied color-conversion material, where two different CCL thicknesseswere tested: 20 nm and 65 nm spin-coated from a 5 and 6 mg mL−1 solution. With super yellowas CCL most of the light absorption took place in the blue part of the electroluminescencespectrum of polymer blue, as displayed in figure 4.2. The green parts of the spectrum wasalmost unabsorbed by the CCL, which photoluminescence re-emitted green to yellow light. Thiscan be seen in figure 4.2, where the spectrum of the blue emitter, adopting a 65 nm thick superyellow CCL, is depicted. In figure 4.3 a trend can be seen that when applying a super yellowCCL to the blue emitter, the resulting light will shift from blue-green, to yellow-green. A thickerCCL convert more light, increasing yellow light output at the expense of blue light. Lack of lightin the orange to red part of the spectrum (and deep-blue) caused low CRI values. Specifically,CRI of 47 and 50 was obtained using CCLs with thickness 20 and 65 nm, respectively.

Wavelength (nm)350 400 450 500 550 600 650 700

Abs

orba

nce

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1EL: PBEL: CCL (SY, 65 nm)PL: SYAbsorption: SY

Nor

mal

ized

Inte

nsity

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Figure 4.2: Absorption (black dotted) and photoluminescence (magenta dash-dotted) of superyellow (SY) films. The figure also present the electroluminescence (EL) of thepolymer blue (PB) based emitter (blue line), and the blue emitter employing a 65nm thick super yellow CCL (red dashed).

18

CHAPTER 4. RESULTS

Figure 4.3: CIE-coordinates for the electroluminescence (EL) of a polymer blue (PB) basedemitter (blue ring), employing a 20 nm (white diamond), or a 65 nm (red haxagram)thick super yellow (SY) CCL. The CIE-coordinates of the photoluminescence (PL)of super yellow is marked with a cyan star.

4.2.2 MEH-PPV color conversion layer

To increase the CRI and get a broader emission spectrum, MEH-PPV was tested in a CCL.MEH-PPV is able to absorb light in the whole electroluminescence spectrum of polymer blue,as shown in figure 4.4. The absorption together with the orange-red photoluminescence ofMEH-PPV, see figure 4.4, resulted in broader emission spectrum of the devices, compared tothe devices utilizing super yellow CCLs (see figure 4.2 for comparison). Using a 75 nm thickMEH-PPV CCL, the electroluminescence of polymer blue was partially converted into a broadlight-emission ranging from blue to orange-red wavelengths, depicted in figure 4.4. The whitelight-emitting device had a CRI of 67, a CCT of 4900, and a good balance between absorptionand photoluminescence, as about half of the blue-green light was converted by the CCL (see themagenta dashed line in figure 4.4). The CIE-coordinates of the device is marked in figure 4.5,and all of the first 14 special CRIs can be found in table B.1. Two additional CCL thicknesseswere also tested, which CRI and CCT values are listed in table 4.1, their CIE-coordinates areplotted in figure 4.5. The 200 nm thick CCL absorbed most of the blue to green light and adevice with almost only MEH-PPV photoluminescence was acquired, giving a orange-red light.With the third CCL, a 40 nm thin MEH-PPV layer, the CRI was increased to 71 but a colderwhite light was achieved with CCT = 7200 K.

Table 4.1: Color-rendering index (CRI) and correlated color temperature (CCT) of polymerblue based LECs employing different MEH-PPV CCL thicknesses, together with theink concentration used during fabrication.

Thickness (nm) CRI CCT (K) Conc. (mg mL−1)

40 71 7200 5

75 67 4900 6

200 39 2000 10

Thickness (nm) CRI CCT (K) Conc. (mg mL−1)

40 71 7200 5

75 67 4900 6

200 39 2000 10

19

CHAPTER 4. RESULTS

Wavelength (nm)400 450 500 550 600 650 700 750

Abs

orba

nce

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1EL: PBEL: CCL (MEH-PPV, 75 nm)PL: MEH-PPVAbsorption: MEH-PPV

Nor

mal

ized

Inte

nsity

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Figure 4.4: Absorption (black dotted), and photoluminescence (PL, red dash-dotted) spectrumof MEH-PPV films. The electroluminescence (EL) of a polymer blue LEC (bluesolid line) employing a 75 nm thick MEH-PPV CCL (magenta dashed) is alsodisplayed.

Figure 4.5: CIE-coordinates of the electroluminescence (EL) from a polymer blue (PB) basedLEC (blue circle) adopting a 40 (yellow diamond), 75 (white star), and 200 (magentasquare) nm thick MEH-PPV CCL. The CIE-coordinates of the photoluminescence(PL) of MEH-PPV is marked with a green hexagram.

20

CHAPTER 4. RESULTS

4.2.3 Polymer red color conversion layer

Deep-red components of the spectrum was acquired with polymer red as CCL. It mainly absorbslight in the blue part of the electroluminescence spectrum of polymer blue, leaving the green lightuntouched. This results in a transform from blue-green light (polymer blue electroluminescence)to green red as the thickness of the polymer red CCL increased. Figure 4.6 show the resultingspectrum from a 150 nm thick CCL applied to the blue emitter. The device had CRI = 67,CCT = 6000 K, and a good balance between absorption of blue light and re-emission of redlight, which can be observed in the magenta dashed line in figure 4.6, where the blue and redwavelength peak are similar in height. The CIE-coordinates of the device is marked in figure4.7, together with two devices employing a 90, and 200 nm thick polymer red CCL. In thechromaticity diagram a trend can be seen. By adapting the blue-emitter with a polymer redCCL, the light is converted from blue-green to green-red. By using thicker CCLs a blend of thegreen electroluminescence of polymer blue, and the red photoluminescence of polymer red wasacquired. The resulting CRI and CCT of the different devices is presented in table 4.2.

Table 4.2: Color-rendering index (CRI) and correlated color temperature (CCT) values of poly-mer blue based LECs employing different polymer red CCL thicknesses, together withthe ink concentration used during fabrication.

Thickness (nm) CRI CCT (K) Conc. (mg mL−1)

90 55 6900 10

150 67 6000 20

200 66 4400 30

Thickness (nm) CRI CCT (K) Conc. (mg mL−1)

90 55 6900 10

150 67 6000 20

200 66 4400 30

Wavelength (nm)400 450 500 550 600 650 700 750 800

Abs

orba

nce

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1EL: PBEL: CCL (PR, 150 nm)PL: PRAbsorption: PR

Nor

mal

ized

Inte

nsity

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Figure 4.6: Normalized absorption (black dotted) and photoluminescence (PL, red dashed)spectrum of polymer red (PR) films. The emission spectrum of polymer blue (PB,solid blue) with a 150 nm thick polymer red CCL (magenta dash-dotted) is alsodepicted.

21

CHAPTER 4. RESULTS

Figure 4.7: CIE-coordinates for the electroluminescence (EL) of the polymer blue (PB) emitter(blue circle), utilizing a 90 (yellow diamond), 150 (magenta square), and 200 (whitestar) nm thick polymer red (PR), together with the photoluminescence (PL) ofpolymer red (cyan hexagram).

4.3 Characteristics of top-emitting LECs utilizing differ-ent CCLs

Polymer red absorbed too much of the blue light, while leaving the green light almost untouched.To approach this problem, top-emitting devices were fabricated, which design is found in figure3.5b. The new device configuration enabled half of the blue light to escape the device, withoutpassing through a CCL, allowing polymer red to absorb the remaining half. The excess greenlight (which polymer red do not absorb) was absorbed by MEH-PPV to enhance the orange-redpart of the spectrum.

The spectrum of two top-emitting devices using both MEH-PPV and polymer red as CCLare plotted in figure 4.8. The MEH-PPV layer was spin-coated first and polymer red was spin-coated on top of it. Device 1 (red dashed) was not cleaned after CCL encapsulation, while device2 (black dash-dotted) was. The CRI and CCT of the devices are listed in table 4.3. Noteworthyis the white light from device 2, which showed a CRI value of 89. Its special CRIs can be foundin table B.2 in appendix B, where the device have a score of over 85 in all of the first eighttest samples. The device had CCT = 6400 K and CIE-coordinates within 0.1 units from thePlanckian locus (x = 0.31, y = 0.33), described in section 2.3.2. Device 1 had a lower CRI (81)and higher CCT (8500) than device 2.

A device using the same materials in the CCL but positioned in the opposite order wasalso tested. It had CRI of 81 with a higher CCT of 8900 K. All mentioned devices had CIE-coordinates almost on the Planckian locus which can be seen in figure 4.9, where all CIE-coordinates are marked.

A single layer of MEH-PPV was also adapted to the top-emitter. Just like the bottomemitting devices using only MEH-PPV, this device configuration gave a slightly lower CRI of73, compared to devices utilizing both MEH-PPV and polymer red. The CRI-coordinates werecloser to the Planckian locus (see figure 4.9) when using the top-emitter instead of the bottom-emitter. Its spectrum is plotted (magenta dotted) in figure 4.8 where a large peak at λ = 580can be seen.

22

CHAPTER 4. RESULTS

Wavelength (nm)400 450 500 550 600 650 700 750

Nor

mal

ized

Inte

nsity

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

EL: PBEL: CCL (MEH-PPV & PR 1)EL: CCL (MEH-PPV & PR 2)EL: CCL (PR & MEH-PPV)EL: CCL (MEH-PPV)

Figure 4.8: Elctroluminescence (EL) spectrum of polymer blue (PB) through a silver electrode(blue line), together with top-emitting LECs employing different CCLs. The reddashed line show a top-emitting device using MEH-PPV and polymer red (PR) asCCL (device 1). The same structure, fabricated with an extra cleaning step, isdisplayed by the black dash-dotted line (device 2). A device using the same CCLmaterials, spin-coated in the opposite order is depicted with a solid purple line withtriangles. The magenta dotted line show the spectrum of a top-emitting deviceemploying a MEH-PPV CCL.

Figure 4.9: CIE-coordinates of the top-emitting devices utilizing a dual layer of MEH-PPV andpolymer red (yellow diamond, device 1, and white star, device 2), together withthe photoluminescence (PL) of MEH-PPV (green triangle), and polymer red (cyanhexagram). A top-emitting device adapting a CCL of MEH-PPV and polymer red,positioned in the opposite order is marked with a white circle. The CIE-coordinatesof a top-emitting device utilizing a MEH-PPV CCL is marked with a magentasquare. The blue circle denote the electroluminescent (EL) CIE-coordinates ofpolymer blue (PB), through a semi-transparent silver electrode.

23

CHAPTER 4. RESULTS

Table 4.3: Color-rendering index (CRI), and correlated color temperature (CCT) of top-emitting LECs utilizing different CCL designs. When two CCL materials are listed,the former was spin-coated first. The materials ink concentration, that was usedduring fabrication, is listed in the same order.

CCL design CRI CCT (K) Conc. (mg mL−1)

MEH-PPV & Polymer red 1 81 8500 10 and 30

MEH-PPV & Polymer red 2 89 6400 10 and 30

Polymer red & MEH-PPV 81 8900 30 and 10

MEH-PPV 73 5300 10

CCL design CRI CCT (K) Conc. (mg mL−1)

MEH-PPV & Polymer red 1 81 8500 10 and 30

MEH-PPV & Polymer red 2 89 6400 10 and 30

Polymer red & MEH-PPV 81 8900 30 and 10

MEH-PPV 73 5300 10

The performance of device 1, over time, can be seen in figure 4.10 and 4.11. The spectrumof the device is rather stable, where some small variations can be seen in the orange to redrange of the spectrum (figure 4.10). In figure 4.11 the brightness and voltage of the device isplotted against time. The device was operated at a constant current density of j = 14.3 mAcm−2, where the maximum brightness, 123 cd m−2, was reached after 7 minutes with the deviceoperating at 9 V. The peak efficiency attained was 0.86 cd A−1, turn-on time was 3 minutes,and the device lived for almost 20 minutes. The lowest voltage, 6.8 V, was reached after about30 seconds.

Wavelength (nm)400 450 500 550 600 650 700

Nor

mal

ized

Inte

nsity

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Initial15 min30 min60 min90 min120 min

Figure 4.10: Time-dependence of the relative spectrum for a top-emitting LEC, employing aMEH-PPV, and polymer red CCL under operation.

24

CHAPTER 4. RESULTS

Time (minutes)0 20 40 60 80 100 120

Brig

htne

ss (

cd m

-2)

0

20

40

60

80

100

120

Vol

tage

(V

)

0

3

6

9

12

15

18

21BrightnessVoltage

Figure 4.11: Time-dependence of brightness and voltage for an operating, top-emitting LECemploying a MEH-PPV, and polymer red CCL.

To visualize how well device 2 could interpret different colors, a CRI chromaticity diagramwas illuminated by the LEC. A photograph showing the result is found in figure 4.12. The insetin the figure show a picture of the LEC under operation, where a uniform white light can beobserved.

Figure 4.12: Photograph of the white LEC illuminating a CIE chromaticity diagram. The insetin the top right corner show a picture of the device.

25

Chapter 5

Discussion

In this chapter, the performance of the CCLs will be discussed, together with how they contrib-uted to white light in the different device structures. Additionally, polymer blue and how wellit performs as a blue emitter will be discussed.

5.1 Performance of the color-conversion materials

Three different color-conversion materials were tested during this work. In this section thedifferent materials, and how they perform as color converters, will be discussed.

Super yellow was the first studied CCL material in this work, which result can be foundin section 4.2.1. The measured absorption and photoluminescence spectrum of super yelloware consistent with previously reported spectra [48]. Due to the green peak in polymer blue’selectroluminescence, and lack of deep-blue light in the blue emitter, super yellow was rather poorat broadening the spectrum. By increasing the CCL thickness, more blue light was absorbed,and the output light was transformed from blue-green to yellow-green. Hence, a white LEC couldnot be constructed by using a super yellow CCL with a polymer blue based LEC. If a deeperblue emitter had been used (instead of the blue-green used in this work), super yellow might beuseful, due to its deeper blue absorption properties. A thin super yellow CCL is pretty effectiveat converting deep-blue to blue light into green-yellow as seen in section 4.2.1. However, at leastone extra CCL material is necessary to obtain wavelengths in the red part of the spectrum (seefigure 4.3), as the trend in the CIE chromaticity diagram do not show any tendency to acquireCIE-coordinates close to the Planckian locus.

To obtain a broader emission spectrum from the LEC and get closer towards white light,MEH-PPV with orange-red photoluminescence was chosen to replace super yellow in the CCL.With blue-green electroluminescence from the emitter, and orange-red photoluminescence fromthe CCL, the idea was to fabricate LECs with CIE-coordinates closer to the Planckian locus,then possible by utilizing super yellow as CCL (see figure 4.3).

By using a 40 nm thick MEH-PPV CCL, a white LEC with CIE-coordinates closer to thePlanckian locus was acqired, presented in figure 4.5, with CRI = 71 and CCT = 7200 K. Themoderate CRI limits the LEC to outdoor and portable lighting-applications [57]. A warmerwhite (4900 K) LEC was achieved with a 75 nm thick MEH-PPV CCL, but at the expense ofa slight decrease in CRI. With the 75 nm thick CCL a good balance between absorption andphotoluminescence was found and it was concluded that a further increase in CRI would be hardto attain with this device configuration. The reason is that the photoluminescence of MEH-PPVdo not cover the deep-red spectrum (or the deep-blue), which are necessary to increase the CRI.A trend in the chromaticity diagram (see figure 4.5) can be identified where the light is convertedfrom blue-green to white and finally orange-red by increasing the thickness of the MEH-PPVCCL. How the lack of red to deep-red spectrum affects the CRI can be observed in table B.1,where all special CRIs, closely related to red, have low scores.

27

CHAPTER 5. DISCUSSION

The results in section 4.2.2 show that MEH-PPV can be used by itself to obtain moderateCRI values from a polymer blue LEC. Both of the mentioned devices appear white to the eye,where the device using a 40 nm thick CCL had a slight tint of blue in its appearance. Followingthis argumentation, a thicker MEH-PPV CCL (say 90 - 125 nm) would give a warmer whitelight, but the color-rendering properties would at the same time probably be compromised.

The LECs adapting MEH-PPV CCLs have a broad emission spectrum, but fail to cover thewavelengths representing the red to deep-red colors, as presented in figure 4.4. Polymer redwas utilized to broaden the emission into the deep-red, and the measured photoluminescenceand absorption spectrum (depicted in figure 4.6) of the material is in accordance with previousstudies [58]. The absorption of polymer red is however mainly focused in the blue part of polymerblue’s electroluminescence spectrum, and its green light is almost left unabsorbed. Even if CRIvalues of 66 and 67 was obtained, they appeared green to the eye, and the spectrum showeda large green emission peak at 525 nm, see figure 4.6. Consequently, an emitter with deeperblue electroluminescence should be utilized to improve CRI and appearance of LECs adoptingpolymer red CCLs. In the case of a deeper blue emitter, polymer red might perform well togetherwith super yellow in a CCL, as both polymer red and super yellow have a large light absorptionpeak in the deep-blue range of the spectrum.

Looking at the chromaticity diagram, in figure 4.5, a trend from blue-green light to green-redcan be seen as the polymer red CCL thickness was increased. This is due to the loss of bluelight as mentioned before. To prevent this trend and still utilize the red photoluminescence ofpolymer red, a new device structure is necessary, where some of the blue light is not absorbedby the CCL.

5.2 Whiteness of the top-emitting LECs

The top-emitting LEC (device configuration in figure 3.5) was invented to preserve the bluelight from the polymer blue based LEC, and still being able to utilize polymer red as CCL. Inthis section, the whiteness of the top-emitting LECs, utilizing different CCL configurations, isdiscussed.

All tested top-emitting devices had CRI values larger than 70, as shown in section 4.3.They also had CIE-coordinates almost on the Planckian locus, inside the white region of theCIE chromaticity diagram. The CCL configuration with polymer red spin-coated on top ofa MEH-PPV layer was especially good, as it got a remarkable CRI of 89 with near daylightchromaticity at CCT 6400 K [59]. To date the achieved CRI is the highest reported for whiteLECs [12], and the CIE-coordinates (x = 0.31, y = 0.33) of the white LEC are within 0.01 unitsfrom the Planckian locus. Consequently, the color rendering properties of the device are morethan satisfactory for indoor-lighting [22], and sufficient for very high color-rendering-qualitylighting [23]. In figure 4.12 the color rendering properties of the LEC is shown by illuminating achromaticity diagram. All colors in the diagram appear true to the eye, which is in accordancewith the high CRI. The device did not only have a high score in the first 8 special CRIs, used tocalculate its CRI, the remaining 6 actually had values of over 70 (table B.2) and 4 even scoredover 90. The important R9 color sample [60], which show how good the emitted light rendersskin tones, scored over 95. This result show that deep-red rendering of the device is almostperfect, making it useful in biomedical applications, like surgery [24]. The lack of UV-, andIR-radiation provide even further applications e.g. lighting in art galleries, where non-visiblelight can damage valuable objects [61].

The time dependence graph (see figure 4.10) show that the emitted spectrum is probablyonly dependent on the brightness of the device, and subsequently only the blue-emitter. Theinitial spectrum look quite similar to the spectra measured at, and after an hour (where thebrightness is similar), which indicate that the CCL materials do not degrade over time. Thisdisplays the advantage of only using one type of emitter in the emitting layer as some parts of thespectrum, in devices utilizing several materials in the emitting layer, often degrade faster thanothers, which can be seen in [62]. The emitting layer in the LEC contain the only electricallyactivated polymer, making it more prone to degradation, than the CCL. Dark spots for example,

28

CHAPTER 5. DISCUSSION

often appear in the emitting layer of non-encapsulated LECs [54], though seldom occur in CCLs.Even if the device is considered dead at brightness < 100 cd m−2 it still produced stable whitelight at the end of operation.

The top-emitting LECs using solely MEH-PPV, or a dual-layer of MEH-PPV and polymerred layers spin-coated in reverse order, showed less impressive results than device 2 in section4.3. With the double layer device, a CRI of 81, and CIE-coordinates x = 28, and y = 32 wasattained. However, with a CCT of 8900 K it has limited usage in indoor lighting applications[63]. One hypothesis to the limited CRI and higher CCT is that too much polymer red wasremoved when MEH-PPV was spin-coated on top of it. This can be seen in figure 4.8, wherelower intensities are observed in the deep-red parts of the spectrum (compared to the blue toorange parts). The top-emitter using only MEH-PPV as CCL had a lower CRI of 73 but warmer-white light at 5300 K. The reduced CRI diminish the amount of applications to outdoor-lighting,as mentioned before.

The major cause for the short lifetime of the top-emitters is the silver cathode. Replacingaluminium with silver in the top electrode leads to an increase in the cathodes work function,which should have little to no impact on the LECs efficiency. The decreased stability is probablyexplained by operation at a higher current, in turn needed because of low transparency of thesilver cathode. Higher operating currents often lead to shorter device lifetimes [64].

Two LECs utilizing a combination of MEH-PPV and polymer red (spin-coated in that order)are reported in section 4.3, one was cleaned after CCL encapsulation and the other was not. Adifference in their emission spectrum can be observed in figure 4.8, where the cleaned devices havelarger peaks in the orange to red wavelength range. A speculation is that the device without theextra cleaning step might have accumulated some particles on its surface, affecting its stability.During the fabrication process the devices were kept in a cleaned petri dish whenever possible,but dust particles might still find its way to the devices. The extra cleaning step is favourableto reduce the risk of particles in the emitting layer, and thereby short circuits.

Another hypothesis to the emission difference is that CCLs with different morphology betweeneach other might have formed. Polymer red was spin-coated on top of MEH-PPV in both casesbut when applying it to the MEH-PPV film, it will start to dissolve it. Spin-coating as fast aspossible after applying polymer red is thus crucial to keep most of the MEH-PPV layer intact.Even if both fabrication processes were performed as similar as possible, the CCLs might differbetween the devices.

5.3 Efficiency and lifetime of the blue emitter

Polymer blue formed uniform emitting layer films together with TMPE-OH and KTF. Thelifetime of the device, 23 h, is almost as long as previously reported by Tang et al. (25 h, [13]).However, the peak brightness reached 320 cd m−2, which is about 100 more than the reportedbrightness. The efficiency is lower at 4.3 cd A−1 compared to the previously reported 5.6 cdA−1. The difference in efficiency can probably be explained by the current density that was usedto power the devices. Lower current densities often provide higher efficiencies, at the expense ofbrightness. The reported devices were driven at j = 3.8 mA cm−2 which is about half as largeas the current density used to drive the blue emitters in these experiments (j = 7.5 mA cm−2).

5.3.1 Green peak in polymer blue’s spectrum

Another reason to the reduced efficiency might stem from the solvent used to dissolve thematerial. Previously reported LECs using polymer blue in the emitting layer [13, 65] do notshow an additional peak at 525 nm, which is reported here. The big difference between theirdevices and the ones reported here is the solvent. In the previous works tetrahydrofuran, whichhas a lower boiling point and is not as polar as cyclohexanone, was used. One suggestion isthat the polarity of the solvents affect how the conjugated polymers interact and form a film,as some solvent might tangle up the polymers while others make the polymers form uniformsolutions. As result small clusters of twisted polymers might form in the solutions instead ofthe wanted homogeneous solution of stretched-out, individual polymers. A possible consequence

29

CHAPTER 5. DISCUSSION

is that electron, and hole transport is hampered, in the polymer chains, blocking paths to thelower energy conjugations along the polymer chains [66]. This might also be caused by the lowerboiling point of THF (66° C versus 155° C in cyclohexanone). In this case the polymers mightnot be given enough time to relax and form a uniform crystalline film structure, which againmight hamper electron-, and hole transport in the polymers [67].

In this work, where a broad emission is desired, the extra green peak is advantageous. FewerCCL materials are necessary due to the broad emission of polymer blue, simplifying devicefabrication. Hence, cyclohexanone was used as solvent in all experiments when spin-coatingthe emitting layer, see section 3.1.1. The green light also improves the absorption match withMEH-PPV (see figure 4.4), and made it possible to use two materials in the CCL, with differentabsorption spectra.

5.4 Future studies

To investigate if any device improvements are possible for the white LEC, some suggestions forfuture studies are brought up in this section.

Studies targeted at improving the lifetime and efficiency of the device should be consideredfirst and foremost, as they are the main culprit. The 15 nm thick silver electrode is not trans-parent enough for the purpose, and might react with the emitting layer, causing the rapidvoltage increase in the device during operation. Consequently, a new electrode material shouldbe investigated to improve both the transparency, and the stability of the device.

A CRI of 89 was achieved in this study but some further investigations should be consideredin order to further increase the color-rendering properties. The first suggestion is to study adeeper blue emitter. It would not only be able to enhance the color quality of the device butwould also improve the photoluminescence of polymer red, which absorption spectrum is in thedeep-blue to blue region of the visible spectrum. A downside of replacing the emitting materialis that it will probably not emit in the green part of the spectrum. This can be solved byutilizing super yellow in the CCL, which just like polymer red absorb light in the deep-blue toblue part of the spectrum, but re-emit light in the green to yellow region. A second suggestionfor CRI improvement is to investigate the CCL in the top-emitting LECs. A lot is unclear aboutthe morphology between the dual-layered CCLs. It would be interesting to look further intothis and see if they can be fabricated in an improved way so that even whiter light is emittedfrom the device (e.g. altering the ink concentrations, or spin-coat speed). In that study thephotoluminescent quantum yield of the CCL should also be investigated to see exactly howefficient the color-conversion is.

30

Chapter 6

Conclusion

To summarize, it has been shown that it is possible to realize cold white light, from an LEC, usinga CCL consisting of MEH-PPV, or a double layer of MEH-PPV and polymer red. Specifically,a cold white LEC with CIE-coordinates (x = 0.31, y = 0.33), CRI = 89, and CCT 6400 K wasrealized using the dual-layers, which to date is the highest reported CRI of a white LEC. Thedevice had a 3 minutes turn-on time, and lifetime of almost 20 minutes. In these devices thelifetime is mainly dependent on the stability of the emitter, and not on the CCL materials, sincethey are not electrically activated.

Out of the objectives, listed in section 1.3, all objectives except for one were fulfilled. Neithera luminance of over 100 cd m−2, nor a CRI of over 70 was gained for an hour, which was statedin objective number 5, although a device with CRI far exceeding the goal of 70 was acquired.To reach the demands in objective 5, a more stable top-electrode is needed, which preferably ismore transparent than the silver electrode used in the top-emitting devices.

In this study, the color-conversion properties of three different materials were tested. Superyellow was tested as CCL but it did not works well with the electroluminescence of polymerblue, due to the already green light in its emission spectrum. The polymer MEH-PPV was alsotested as CCL. It performed well together with polymer blue and a device with CRI of 73 wascreated. Despite this, MEH-PPV lacked deep-red wavelengths in its photoluminescence. Thus,using it together with a deep-red photoluminescent material would be advantageous. The thirdand last tested color-conversion material was polymer red. It was good at converting the blueelectroluminescence of polymer blue into red to deep-red light but lacked absorption in the greenparts of the spectrum. Therefore, using only polymer red in a CCL, white light-emission is hardto attain. Anyway, polymer red works well together with MEH-PPV, in a dual-layered CCL,and they complete each other in the orange to deep-red spectrum. If a deeper blue emitteris used instead of polymer blue, a combination of super yellow and polymer red will probablyperform well in terms of converting the blue light to high-quality white light.

Finally, even if the devices have great color rendering properties, for color-converted whiteLECs to fully compete in the lighting industry, their brightness and lifetime need to be addressed.The results presented in this thesis is certainly a step in the right direction, and I will end withan old saying— “Out of little acorns grow huge oaks”.

31

Bibliography

[1] “OLED TV: introduction and market status.” http://www.oled-info.com/oled-tv. Ac-cessed: 2016-05-31.

[2] “Mobile phones and smartphones with OLED screens.” http://www.oled-info.com/oled devices/mobile phones. Accessed: 2016-05-31.

[3] “Benchmarking home energy savings from energy-efficient lighting.” http://www.cmhc.ca/odpub/pdf/65830.pdf. Accessed: 2016-05-31.

[4] S. Reineke, F. Lindner, G. Schwarz, N. Seidler, K. Waltzer, B. Lussem, and K. Leo, “Whiteorganic light-emitting diodes with fluorescent tube efficiency,” Nature, no. 459, p. 234, 2009.

[5] Q. Wang, J. Ding, D. Ma, Y. Cheng, L. Wang, and F. Wang, “Manipulating charges andexcitons within a single-host system to accomplish efficiency/CRI/color-stability trade-offfor high-performance OWLEDs,” Advanced Materials, vol. 21, no. 23, pp. 2397–2401, 2009.

[6] Q. Wang, J. Ding, D. Ma, Y. Cheng, and L. Wang, “Highly efficient single-emitting-layerwhite organic light-emitting diodes with reduced efficiency roll-off,” Applied Physics Letters,vol. 94, no. 10, 2009.

[7] M. Nagai, High performance color conversion polymer films and their application to OLEDdevices, pp. 217–238. InTech, 2010.

[8] Q. Pei, G. Yu, C. Zhang, Y. Yang, and A. J. Heeger, “Polymer light-emitting electrochemicalcells,” Science, no. 269, pp. 1086–1088, 1995.

[9] J. Leger, “Organic electronics: The ions have it,” Advanced Materials, vol. 20, no. 4, pp. 837–841, 2008.

[10] G. Yu, Q. Pei, and A. J. Heeger, “Planar light-emitting devices fabricated with luminescentelectrochemical polyblends,” Applied Physics Letters, vol. 70, no. 8, pp. 934–936, 1997.

[11] L. Edman, The Light-Emitting Electrochemical Cell: Utilizing Ions for Self-Assembly andImproved Device Operation, pp. 893–918. WILEY-VCH Verlag & Co. KGaA, 2011.

[12] H.-C. Su and C.-Y. Cheng, “Recent advances in solid-state white light-emitting electro-chemical cells,” Israel Journal of Chemistry, vol. 54, no. 7, pp. 855–866, 2014.

[13] S. Tang, J. Pan, H. Buchholz, and L. Edman, “White light-emitting electrochemical cell,”ACS Applied Materials & Interfaces, vol. 3, no. 9, pp. 3384–3388, 2011. PMID: 21793518.

[14] S. Tang, H. A. Buchholz, and L. Edman, “White light from a light-emitting electrochemicalcell: Controlling the energy-transfer in a conjugated polymer/triplet-emitter blend,” ACSApplied Materials & Interfaces, vol. 7, no. 46, pp. 25955–25960, 2015. PMID: 26536909.

33

BIBLIOGRAPHY

[15] H.-C. Su, H.-F. Chen, P.-H. Chen, S.-W. Lin, C.-T. Liao, and K.-T. Wong, “Efficient solid-state white light-emitting electrochemical cells based on phosphorescent sensitization,” J.Mater. Chem., vol. 22, pp. 22998–23004, 2012.

[16] Y. B. Yuan, S. Li, Z. Wang, H. T. Xu, and X. Zhou, “White organic light-emitting diodescombining vacuum deposited blue electrophosphorescent devices with red surface color con-version layers,” Opt. Express, vol. 17, pp. 1577–1582, Feb 2009.

[17] S. Chen and H.-S. Kwok, “Top-emitting white organic light-emitting diodes with a colorconversion cap layer,” Organic Electronics, vol. 12, no. 4, pp. 677 – 681, 2011.

[18] H.-B. Wu, H.-F. Chen, C.-T. Liao, H.-C. Su, and K.-T. Wong, “Efficient and color-stablesolid-state white light-emitting electrochemical cells employing red color conversion layers,”Organic Electronics, vol. 13, no. 3, pp. 483 – 490, 2012.

[19] G. R. Lin, J. R. Cheng, C. W. Wang, M. Sarma, H. F. Chen, H. C. Su, C. H. Chang, andK. T. Wong, “Solid-state white light-emitting electrochemical cells based on scattering redcolor conversion layers,” J. Mater. Chem. C, vol. 3, pp. 12492–12498, 2015.

[20] L. He, J. Qiao, L. Duan, G. Dong, D. Zhang, L. Wang, and Y. Qiu, “Toward highly efficientsolid-state white light-emitting electrochemical cells: Blue-green to red emitting cationiciridium complexes with imidazole-type ancillary ligands,” Advanced Functional Materials,vol. 19, no. 18, pp. 2950–2960, 2009.

[21] H.-C. Su, H.-F. Chen, Y.-C. Shen, C.-T. Liao, and K.-T. Wong, “Highly efficient double-doped solid-state white light-emitting electrochemical cells,” J. Mater. Chem., vol. 21,pp. 9653–9660, 2011.

[22] M. C. Gather, A. Kohnen, and K. Meerholz, “White organic light-emitting diodes,” Ad-vanced Materials, vol. 23, no. 2, pp. 233–248, 2011.

[23] P. C. Hung and J. Y. Tsao, “Maximum white luminous efficacy of radiation versus colorrendering index and color temperature: Exact results and a useful analytic expression,”Journal of Display Technology, vol. 9, pp. 405–412, June 2013.

[24] M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G. O. Mueller, L. Zhou, G. Harbers,and M. G. Craford, “Status and future of high-power light-emitting diodes for solid-statelighting,” Display Technology, Journal of, vol. 3, no. 2, pp. 160–175, 2007.

[25] G. Cheng, M. Mazzeo, S. D’Agostino, F. Della Sala, S. Carallo, and G. Gigli, “Pure whitehybrid light-emitting device with color rendering index higher than 90,” Optics letters,vol. 35, no. 5, pp. 616–618, 2010.

[26] P.-P. Dai, C. Li, X.-T. Zhang, J. Xu, X. Chen, X.-L. Wang, Y. Jia, X. Wang, and Y.-C.Liu, “A single Eu2+-activated high-color-rendering oxychloride white-light phosphor forwhite-light-emitting diodes,” Light Sci Appl, vol. 5, p. e16024, Feb 2016. Supplement-ary information available for this article at http://www.nature.com/lsa/journal/v5/n2/suppinfo/lsa201624s1.html.

[27] J.-S. Lu, H.-F. Chen, J.-C. Kuo, R. Sun, C.-Y. Cheng, Y.-S. Yeh, H.-C. Su, and K.-T.Wong, “Efficient solid-state white light-emitting electrochemical cells employing embeddedred color conversion layers,” J. Mater. Chem. C, vol. 3, pp. 2802–2809, 2015.

[28] S. Tang, J. Mindemark, C. M. G. Araujo, D. Brandell, and L. Edman, “Identifying keyproperties of electrolytes for light-emitting electrochemical cells,” Chemistry of Materials,vol. 26, no. 17, pp. 5083–5088, 2014.

[29] P. Matyba, Polymer Light-Emitting Electrochemical Cells, Utilizing Doping for Generationof Light. PhD thesis, Umea University, 2011.

34

BIBLIOGRAPHY

[30] A. Ajayaghosh, “Donor-acceptor type low band gap polymers: polysquaraines and relatedsystems,” Chem. Soc. Rev., vol. 32, pp. 181–191, 2003.

[31] R. Cernini, X. C. Li, G. Spencer, A. Holmes, S. Moratti, and R. Friend, “Electrochemicaland optical studies of ppv derivatives and poly(aromatic oxadiazoles),” Synth. Met, no. 84(1-3), pp. 359–360, 1997.

[32] Y. Wu, J. Li, Y. Fu, and Z. Bo, “Synthesis of extremely stable blue light emittingpoly(spirobifluorene)s with suzuki polycondensation,” Organic Letters, vol. 6, no. 20,pp. 3485–3487, 2004. PMID: 15387529.

[33] Q. Pei, “Light-emitting polymers,” Material Matters, vol. 2, no. 3, pp. 26–28, 2007.

[34] Y. Hu and J. Gao, “Cationic effects in polymer light-emitting electrochemical cells,” AppliedPhysics Letters, vol. 89, no. 25, 2006.

[35] T. Wagberg, P. R. Hania, N. D. Robinson, J. H. Shin, P. Matyba, and L. Edman, “On thelimited operational lifetime of light-emitting electrochemical cells,” Advanced Materials,vol. 20, no. 9, pp. 1744–1749, 2008.

[36] H. S. Kim, M.-G. Kim, Y.-G. Ha, M. G. Kanatzidis, T. J. Marks, and A. Facchetti, “Low-temperature solution-processed amorphous indium tin oxide field-effect transistors,” J. Am.Chem. Soc., no. 131 (31), pp. 10826–10827, 2009.

[37] P. Matyba, K. Maturova, M. Kemerink, N. D. Robinson, and L. Edman, “The dynamicorganic p–n junction,” Nature Materials, vol. 8, no. 8, pp. 672 – 676, 2009.

[38] E. M. Lindh, Bilayer Light-Emitting Electrochemical Cells for Signage and Lighting Applic-ations. Licentiate thesis, Umea University, 2016.

[39] K. D. Chaudhuri, “Concentration quenching of fluorescence in solutions,” Zeitschrift furPhysik, vol. 154, no. 1, pp. 34–42, 1959.

[40] J. Silver and R. Withnall, Color Conversion Phosphors for LEDS, pp. 75–109. John Wiley& Sons, Ltd, 2008.

[41] CIE, (1932), Commission Internationale de l’Eclairage Proceedings, 1931. Cambridge:Cambridge University Press, 1932.

[42] G. W. Wyszecki and W. S. Stiles, Color Science. Concepts and Methods, Quantitative Dataand Formulas. New York: John Wiley & Sons, Inc., 1967.

[43] D. MacAdam and D. Lewis, “Projective transformations of I.C.I. color specifications,”JOSA, vol. 27, no. 8, pp. 294–299, 1937.

[44] M. Wood, “CRI— what does it really mean? examining the color rendering index and how itis used.” http://www.mikewoodconsulting.com/articles/Protocol%20Winter%202010%20-%20CRI%201.pdf. Accessed: 2016-06-01.

[45] C. Poynton, Digital Video and HD: Algorithms and Interfaces. The Morgan KaufmannSeries in Computer Graphics, Elsevier Science, 2003.

[46] “CIE colorimetric and colour rendering tables, disk D002, rel 1.3.” http://www.cie.co.at/publ/abst/d002.html. Accessed: 2016-05-10.

[47] H. Inoue, T. Yamaguchi, S. Seo, H. Seo, K. Suzuki, T. Kawata, and N. Ohsawa, “En-hancement of emission efficiency in white OLED device by highly efficient narrow spectrumred-emission material,” Journal of the Society for Information Display, vol. 23, no. 1, pp. 7–13, 2015.

35

BIBLIOGRAPHY

[48] S. Gambino, A. K. Bansal, and I. D. Samuel, “Photophysical and charge-transporting prop-erties of the copolymer superyellow,” Organic Electronics, vol. 14, no. 8, pp. 1980 – 1987,2013.

[49] H.-J. Lee, B. R. Lee, S.-W. Choi, and M. H. Song, “Highly circularly polarized white lightusing a combination of white polymer light-emitting diode and wideband cholesteric liquidcrystal reflector,” Opt. Express, vol. 20, pp. 24472–24481, Oct 2012.

[50] B. Munkhbat, H. Pohl, P. Denk, T. A. Klar, M. C. Scharber, and C. Hrelescu, “Perform-ance boost of organic light-emitting diodes with plasmonic nanostars,” Advanced OpticalMaterials, 2016.

[51] Y.-H. Ho, D.-W. Huang, Y.-T. Chang, Y.-H. Ye, C.-W. Chu, W.-C. Tian, C.-T. Chen, andP.-K. Wei, “Improve efficiency of white organic light-emitting diodes by using nanospherearrays in color conversion layers,” Opt. Express, vol. 20, pp. 3005–3014, Jan 2012.

[52] Q.-j. Qi, X.-m. Wu, Y.-l. Hua, M.-s. Dong, and S. Yin, “White organic light emitting deviceswith a color conversion layer,” Optoelectronics Letters, vol. 6, no. 4, pp. 245–248, 2010.

[53] S. Tang and L. Edman, “Quest for an appropriate electrolyte for high-performance light-emitting electrochemical cells,” The Journal of Physical Chemistry Letters, vol. 1, no. 18,pp. 2727–2732, 2010.

[54] A. Asadpoordarvish, A. Sandstrom, S. Tang, J. Granstrom, and L. Edman, “Encapsulatinglight-emitting electrochemical cells for improved performance,” Applied Physics Letters,vol. 100, no. 19, 2012.

[55] “Lighting research center.” http://www.lrc.rpi.edu/programs/nlpip/lightinganswers/lightsources/scripts/NLPIP LightSourceColor Script.m. Accessed: 2016-05-10.

[56] “Designing with LEDs, light spectrum charts data.” http://www.designingwithleds.com/light-spectrum-charts-data/. Accessed: 2016-05-11.

[57] M. O. Holcomb, R. Mueller-Mach, G. O. Mueller, D. Collins, R. M. Fletcher, D. A. Steiger-wald, S. Eberle, Y. K. Lim, P. S. Martin, and M. Krames, “The LED lightbulb: are wethere yet? progress and challenges for solid state illumination,” in Lasers and Electro-Optics, 2003. CLEO ’03. Conference on, pp. 4 pp.–, June 2003.

[58] M. Ullah, K. Tandy, S. D. Yambem, M. Aljada, P. L. Burn, P. Meredith, and E. B. Namdas,“Simultaneous enhancement of brightness, efficiency, and switching in RGB organic lightemitting transistors,” Advanced Materials, vol. 25, no. 43, pp. 6213–6218, 2013.

[59] A. Borbely, A. Samson, and J. Schanda, “The concept of correlated colour temperaturerevisited,” Color Research & Application, vol. 26, no. 6, pp. 450–457, 2001.

[60] G. He, J. Xu, and H. Yan, “Spectral optimization of warm-white light-emitting diode lampwith both color rendering index (CRI) and special CRI of R9 above 90,” AIP Advances,vol. 1, no. 3, 2011.

[61] N. Kimura, K. Sakuma, S. Hirafune, K. Asano, N. Hirosaki, and R.-J. Xie, “Extrahigh colorrendering white light-emitting diode lamps using oxynitride and nitride phosphors excitedby blue light-emitting diode,” Applied Physics Letters, vol. 90, no. 5, 2007.

[62] C.-Y. Cheng, C.-W. Wang, J.-R. Cheng, H.-F. Chen, Y.-S. Yeh, H.-C. Su, C.-H. Chang,and K.-T. Wong, “Enhancing device efficiencies of solid-state white light-emitting electro-chemical cells by employing waveguide coupling,” J. Mater. Chem. C, vol. 3, pp. 5665–5673,2015.

[63] Y. Shi, Y. Wang, Y. Wen, Z. Zhao, B. Liu, and Z. Yang, “Tunable luminescenceY3Al5O12:0.06Ce3+, xMn2+ phosphors with different charge compensators for warm whitelight emitting diodes,” Opt. Express, vol. 20, pp. 21656–21664, Sep 2012.

36

BIBLIOGRAPHY

[64] H.-C. Su, F.-C. Fang, T.-Y. Hwu, H.-H. Hsieh, H.-F. Chen, G.-H. Lee, S.-M. Peng, K.-T.Wong, and C.-C. Wu, “Highly efficient orange and green solid-state light-emitting electro-chemical cells based on cationic IrIII complexes with enhanced steric hindrance,” AdvancedFunctional Materials, vol. 17, no. 6, pp. 1019–1027, 2007.

[65] S. Tang, A. Sandstrom, J. Fang, and L. Edman, “A solution-processed trilayer electro-chemical device: Localizing the light emission for optimized performance,” Journal of theAmerican Chemical Society, vol. 134, no. 34, pp. 14050–14055, 2012. PMID: 22862541.

[66] M. Zheng, F. Bai, and D. Zhu, “Photophysical process of MEH-PPV solution,” Journal ofPhotochemistry and Photobiology A: Chemistry, vol. 116, no. 2, pp. 143 – 145, 1998.

[67] S. Arnautov, E. Nechvolodova, A. Bakulin, S. Elizarov, A. Khodarev, D. Martyanov, andD. Paraschuk, “Properties of MEH-PPV films prepared by slow solvent evaporation,” Syn-thetic Metals, vol. 147, no. 1–3, pp. 287 – 291, 2004. Supramolecular approaches to organicelectronics and nanotechnology. Proceedings of Symposium F. E-MRS Spring Meeting.

37

Appendix A

Super yellow based LEC

The LECs with super yellow as conjugated polymer in the active layer, together with TMPE-OH,and KTF had an efficiency of 5.7 cd A−1. The lifetime of the device, driven at 0.3 mA, is shownin figure A.1 where the brightness and voltage is plotted versus time. The turn-on time for thedevice was 165 seconds and the maximum brightness achieved was 420 cd m−2. The lowestvoltage, 3 V, occurred after 4.3 hours. The lifetime of the device was 10 days. A brightnessand voltage drop can be seen after about 2 days. This is probably due to an inhomogeneity likea dust particle in the active layer that is burned off. The active layer was spin-coated from acyclohexanone solution with concentration 7 mg mL−1 resulting in a layer thickness of 60 nm.

Time (hours)0 50 100 150 200

Brig

htne

ss (

cd m

-2)

0

50

100

150

200

250

300

350

400

450

500

Vol

tage

(V

)

0

2

4

6

8

10

12

14

BrightnessVoltage

Figure A.1: Lifetime of the super yellow based LEC. The solid blue line shows how the bright-ness varies over time. The green dashed line displays the voltage developmentduring the lifetime of the device.

Appendix B

Special color rendering indices

Table B.1: The fourteen test color samples (TCS), their approximate Munsell color, and appear-ance under daylight are listed together with the special color rendering indices (Ri)of the LEC employing a 75 nm MEH-PPV CCL. Negative values are transformedto zero.

TCS Appr. Munsell Appearence under daylight Ri

TCS01 7,5 R 6/4 Light greyish red 59.72

TCS02 5 Y 6/4 Dark greyish yellow 83.79

TCS03 5 GY 6/8 Strong yellow green 85.73

TCS04 2,5 G 6/6 Moderate yellowish green 54.44

TCS05 10 BG 6/4 Light bluish green 62.74

TCS06 5 PB 6/8 Light blue 79.46

TCS07 2,5 P 6/8 Light violet 74.51

TCS08 10 P 6/8 Light reddish purple 37.16

TCS09 4,5 R 4/13 Strong red 0

TCS10 5 Y 8/10 Strong yellow 66.83

TCS11 4,5 G 5/8 Strong green 49.69

TCS12 3 PB 3/11 Strong blue 48.61

TCS13 5 YR 8/4 Light yellowish pink 66.14

TCS14 5 GY 4/4 Moderate olive green 90.22

TCS Appr. Munsell Appearence under daylight Ri

TCS01 7,5 R 6/4 Light greyish red 59.72

TCS02 5 Y 6/4 Dark greyish yellow 83.79

TCS03 5 GY 6/8 Strong yellow green 85.73

TCS04 2,5 G 6/6 Moderate yellowish green 54.44

TCS05 10 BG 6/4 Light bluish green 62.74

TCS06 5 PB 6/8 Light blue 79.46

TCS07 2,5 P 6/8 Light violet 74.51

TCS08 10 P 6/8 Light reddish purple 37.16

TCS09 4,5 R 4/13 Strong red 0

TCS10 5 Y 8/10 Strong yellow 66.83

TCS11 4,5 G 5/8 Strong green 49.69

TCS12 3 PB 3/11 Strong blue 48.61

TCS13 5 YR 8/4 Light yellowish pink 66.14

TCS14 5 GY 4/4 Moderate olive green 90.22

APPENDIX B. SPECIAL COLOR RENDERING INDICES

Table B.2: The fourteen test color samples (TCS), their approximate Munsell color, and ap-pearance under daylight are listed together with the special color rendering indices(Ri) of the MEH-PPV & PR 2 device.

TCS Appr. Munsell Appearence under daylight Ri

TCS01 7,5 R 6/4 Light greyish red 95.63

TCS02 5 Y 6/4 Dark greyish yellow 91.42

TCS03 5 GY 6/8 Strong yellow green 86.52

TCS04 2,5 G 6/6 Moderate yellowish green 85.33

TCS05 10 BG 6/4 Light bluish green 92.88

TCS06 5 PB 6/8 Light blue 90.56

TCS07 2,5 P 6/8 Light violet 85.66

TCS08 10 P 6/8 Light reddish purple 87.56

TCS09 4,5 R 4/13 Strong red 95.33

TCS10 5 Y 8/10 Strong yellow 80.67

TCS11 4,5 G 5/8 Strong green 90.50

TCS12 3 PB 3/11 Strong blue 74.89

TCS13 5 YR 8/4 Light yellowish pink 94.25

TCS14 5 GY 4/4 Moderate olive green 92.36

TCS Appr. Munsell Appearence under daylight Ri

TCS01 7,5 R 6/4 Light greyish red 95.63

TCS02 5 Y 6/4 Dark greyish yellow 91.42

TCS03 5 GY 6/8 Strong yellow green 86.52

TCS04 2,5 G 6/6 Moderate yellowish green 85.33

TCS05 10 BG 6/4 Light bluish green 92.88

TCS06 5 PB 6/8 Light blue 90.56

TCS07 2,5 P 6/8 Light violet 85.66

TCS08 10 P 6/8 Light reddish purple 87.56

TCS09 4,5 R 4/13 Strong red 95.33

TCS10 5 Y 8/10 Strong yellow 80.67

TCS11 4,5 G 5/8 Strong green 90.50

TCS12 3 PB 3/11 Strong blue 74.89

TCS13 5 YR 8/4 Light yellowish pink 94.25

TCS14 5 GY 4/4 Moderate olive green 92.36