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, Civilingenj¨ orsprogrammet i teknisk fysik, Ume˚ a 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 at the Department of Physics, Ume˚ a University.
Supervisors: Mattias Lindh and Petter Lundberg, Department of Physics, Ume˚ a University
Examiner: Ludvig Edman, Department of Physics, Ume˚ a 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 now be 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, and possibility to be fully solution processed. Today LECs can emit light of various colors, but to be applicable in the lighting industry, white light need to be produced in an efficient way. White light on the other hand, is one of the toughest ”colors” to achieve in an efficient way, and is of particular interest in general lighting applications, where high color-rendering index devices are necessary. In this thesis I show that blue light can be partially converted, into white light, by utilizing the photoluminescence of color conversion layers (CCLs).
Furthermore, I show that a high color-quality white light can be attained by adopting a blue-emitting LEC with a CCL. Particularly, three different color-conversion materials were embedded onto a blue bottom-emitting LEC, to study the resulting spectrum. One of the materials, MEH-PPV, have good absorption compatibility with the electroluminescence of the blue emitters, but the materials photoluminescence do not cover the red to deep-red range of the 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 white LEC with CIE-coordinates x = 0.29, and y = 0.36, color-rendering index = 71, and correlated color temperature = 7200 K. These properties makes it potentially useful in outdoor-lighting applications. 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 green parts of the blue emitters electroluminescence spectrum. Thus it is necessary to combine it with MEH-PPV to be able to absorb all wavelengths from the blue-emitter and get a broad light-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 rendering index = 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-, and
IR-radiation, together with the high color rendering properties make the white LEC a possible
candidate for even the most demanding lighting-applications, such as art galleries, and shop
display windows, together with indoor lighting. In this thesis, I show that the CCLs function
well. However, for the LECs to be worthy competitors, the efficiency and lifetime of the blue
emitter need improvements.
Sammanfattning
Organiska halvledande material har f˚ att en hel del uppm¨ arksamhet de senaste ˚ aren och ˚ aterfinns idag i flera komponentstrukturer. En s˚ adan applikation, ljus-emitterande elektrokemisk cell (LEC), ¨ ar en potentiell utmanare i belysningsbranschen; tack vara sin enkla struktur, l˚ aga driftsp¨ anning, samt m¨ ojlighet till billiga, storskaliga, produktionstekniker. Idag kan LECar emit- tera ljus av flera olika f¨ arger, men f¨ or att vara applicerbar i belysningsbranschen kr¨ avs tillg˚ ang till vitt ljus. Vitt ljus ¨ ar en av de sv˚ araste ”f¨ argerna” att tillverka p˚ a ett effektivt tillv¨ agag˚ angss¨ att, och ¨ ar dessutom av s¨ arskilt intresse i allm¨ anbelysning d¨ ar ett h¨ ogt f¨ arg˚ atergivningstal kr¨ avs. I den h¨ ar uppsatsen konstateras att bl˚ att ljus kan bli delvis konverterat till vitt ljus, med hj¨ alp av f¨ argkonverteringslager.
Dessutom, konstateras att ett h¨ ogt f¨ arg˚ atergivningstal ¨ ar m¨ ojligt att uppn˚ a genom att s¨ atta ett f¨ argkonverteringslager p˚ a en bl˚ aemitterande LEC. I synnerhet s˚ a testades tre olika f¨ argkonverteringsmaterial p˚ a bl˚ aemitterande LECar f¨ or att se hur det p˚ averkade LECens emis- sionsspektrum. Ett av materialen, MEH-PPV, hade bra absorptionskompatibilitet med elektro- luminiscensen fr˚ an den bl˚ a emitteraren, men materialets fotoluminiscens t¨ ackte inte den r¨ oda till djupr¨ oda delarna av spektumet; n¨ odv¨ andiga f¨ or att erh˚ alla h¨ oga f¨ arg˚ atergivningstal (≥80).
Genom att l¨ agga ett f¨ argkonverteringslager av MEH-PPV, p˚ a en bl˚ a emitterare, framst¨ alldes en kall vit LEC med CIE-koordinater x = 0.29 och y = 0.36, samt f¨ arg˚ atergivningstal = 71 och f¨ argtemperatur = 7200 K, vilket ¨ ar tillr¨ ackligt f¨ or till exempel utomhusbelysning. ¨ Aven poly- mer red unders¨ oktes som f¨ argkonverteringsmaterial. Materialet t¨ ackte det r¨ oda till djupr¨ oda delarna av spektrumet, men saknade ljusabsorption i de gr¨ ona delarna av den bl˚ a emitterar- ens elektroluminiscens-spektrum. D¨ arav var det n¨ odv¨ andigt att kombinera den med MEH-PPV f¨ or att maximera ljusabsorption, fr˚ an den bl˚ a emitteraren, men samtidigt f˚ a ett brett ljusemis- sionsspektrum fr˚ an LECen.
F¨ or att bevara en del av det bl˚ a ljuset, designades en ny komponentstruktur. En topp-
emitterande bl˚ a LEC med dubbla konverteringslager framst¨ alldes med imponerande f¨ argren-
deringsegenskaper. LECen hade f¨ arg˚ atergivningstalet = 89, f¨ argtemperaturen = 6400 K, med
CIE-koordinater x = 0.31, y = 0.33. F¨ arg˚ atergivningstalet ¨ ar det h¨ ogsta rapporterade f¨ or en vit
LEC. Avsaknad av UV-, och IR-str˚ alning tillsammans med LECens f¨ arg˚ atergivnings-egenskaper,
g¨ or att den vita LECen har potential att kunna anv¨ andas ¨ aven f¨ or de mest kr¨ avande belysning-
somr˚ adena s˚ a som i konstgallerier, skyltf¨ onster samt f¨ or inomhusbelysning. I den h¨ ar uppsatsen
visar jag att f¨ argkonverteringslagren fungerar bra, men f¨ or att konkurrera i belysningsbranschen
beh¨ over effektiviteten och livsl¨ angden p˚ a den bl˚ a LECen f¨ orb¨ attras.
“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
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. Without them, 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 Group for all the help I received and all the knowledge they shared. It meant a lot to me. Special thanks to 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, always giving me something to look forward to, and consistently filling me with joy.
Thank you so much, everyone, for being you.
Joel Vedin,
Ume˚ a, Sweden,
18 June, 2016.
Contents
1 Introduction 1
1.1 Background . . . . 1
1.2 Purpose . . . . 2
1.3 Objectives . . . . 2
2 Theory 3 2.1 The light-emitting electrochemical cell . . . . 3
2.1.1 Emitting layer . . . . 3
2.1.2 Electrodes . . . . 4
2.1.3 Operating mechanism of LECs . . . . 5
2.2 Color-conversion layers . . . . 6
2.3 Measures of illumination quality . . . . 7
2.3.1 CIE-coordinates . . . . 7
2.3.2 Correlated color temperature of black body emitters . . . . 7
2.3.3 Color-rendering index of white light . . . . 7
3 Method 9 3.1 Materials . . . . 9
3.1.1 Fluorescent polymers . . . . 9
3.1.2 Electrolyte . . . . 11
3.2 Fabrication of an LEC device . . . . 11
3.2.1 Cleaning . . . . 12
3.2.2 Deposition of the emitting layer . . . . 12
3.2.3 Thermal evaporation of the electrode . . . . 12
3.2.4 Encapsulation . . . . 14
3.2.5 Deposition of the color-conversion layer . . . . 14
3.3 Characterization . . . . 14
3.3.1 Luminance and lifetime . . . . 14
3.3.2 Thickness measurements . . . . 14
3.3.3 Spectroscopic measurements . . . . 14
3.3.4 Illumination quality calculations and photography . . . . 15
4 Results 17 4.1 Properties of the blue emitter . . . . 17
4.2 Bottom emitting LECs employing different CCLs . . . . 18
4.2.1 Super yellow color conversion layer . . . . 18
4.2.2 MEH-PPV color conversion layer . . . . 19
4.2.3 Polymer red color conversion layer . . . . 21
4.3 Characteristics of top-emitting LECs utilizing different CCLs . . . . 22
5 Discussion 27 5.1 Performance of the color-conversion materials . . . . 27
5.2 Whiteness of the top-emitting LECs . . . . 28
5.3 Efficiency and lifetime of the blue emitter . . . . 29
5.3.1 Green peak in polymer blue’s spectrum . . . . 29
5.4 Future studies . . . . 30
6 Conclusion 31
Bibliography 33
Appendix A Super yellow based LEC
Appendix B Special color rendering indices
Chapter 1 Introduction
1.1 Background
Since the invention of organic semiconducting polymers, they have received a lot of attention in the 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 enables cheap continuous process techniques like roll-to-roll printing and coating to be used, in order to achieve thin semiconducting films.
Nowadays OLEDs are well established on the market, where they for example are used in many 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 the inefficient incandecent bulb [3]. However, these devices are often fabricated using several different light emitting layers of different colors [4], complicating the production. Another utilized device configuration is to have several different emitters in the emitting layer [5, 6]. These devices are sensitive to different aging of the light emitting materials [7]. To overcome these disadvantages a color-conversion layer (CCL) comprising one material can be utilized. Its purpose is to partially convert 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 advantages are that LECs typically only constist of one emitting layer, where the semiconducting polymer is blended with an electrolyte. The electrolyte makes the devices more durable, which in turn lowers the precision needed in fabrication. While the OLEDs are sensitive to the thickness of their 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 as sensitive to surface roughness [8, 10], facilitating production.
One more advantage is that LECs are not as dependent on the electrode work-functions (due to 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, close to the energy gap potential, E
g/ e , of the organic semiconductor, where E g is the semiconductors energy gap and e is the elementary charge. However, due to their electrochemical basis, LECs have 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 common
way 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 structure
of a sandwich LEC acting as a CCL. The intention of the CCL is to partially convert the emitted
light of the device so that white light can be attained. CCLs can be added to the LECs with
CHAPTER 1. INTRODUCTION
the same methods as the emitting layer. Thus, device fabrication is not hampered by the extra device layer. This approach is not entirely new, and have been used in both OLED [16, 17] and LEC [18, 19] devices. White LECs employing a CCL have some advantages over single layered white LECs. Degradation is focused on one material, the emitting layer, instead of several as is the case with a single layered white LEC [12]. The latter devices often suffer from bias dependent color-shifts [20, 21]. However, devices using a single emitting-layer have been reported with color-rendering indices (CRIs) of 83 and 84 (100 is perfect score) [13, 21], which is sufficient for indoor-lighting [22]. To be applicable in more demanding situations, where very high-quality lighting is desired, a CRI of about 90 is required [23]. The CRI of a light source is determined by first simulating how well the light source can render 14 test color samples, providing 14 special CRIs. The CRI is then the average of the first eight of these special CRIs. For a light source to be useful in biomedical applications, the ninth special CRI (R 9 ) is of particular interest, as it describes deep-red color-rendering [24]. A device with both high CRI, and R 9 is sought in many commercial applications [25], but a high R 9 is hard to obtain, and commercial white LEDs have typically low R 9 values around 14 [26].
1.2 Purpose
White LECs have previously been constructed with a single emitting layer [12], where different electroluminescent conjugated polymers (CPs) are blended in order to emit white light from the device. White LECs utilizing a CCL have also been reported, but the focus has been directed towards the efficiency of the devices, and not on the whiteness [18, 19, 27]. All reported devices also utilize the same red emitting dye. The purpose of this project is to investigate a slightly different approach, namely using a blue-emitting LEC together with an efficient polymer based CCL, in order to attain quality white light. The CCL is supposed to partially convert the blue light from the device into light of a longer wavelength, allowing a broad-spectrum light emission from the device. The main objective is to see if better color rendering can be attained in an efficient 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 device characteristics of the LEC. In other words get acquainted with the doping process, the different 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 accustomed with 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 light for a long period, at least 200 cd m −2 for an hour, to allow studies directed towards the degradation 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-red spectrum. It should also allow some of the blue light to travel through the layer granting a broad emission spectrum.
5. Design and implement a color-conversion layer in a blue surface emitting LEC for the
realization 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 (depicted
in 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.
Chapter 2 Theory
This chapter contains the theory regarding the LEC and how it works. It also describes the concept 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 two electrodes (connection points) and an emitting layer. Distinguishing for the LEC is the emitting layer, which in an LEC not only comprises the CP, but also an electrolyte. The electrolyte’s role is to dope the polymer; facilitating hole, and electron injection. Thus, the LEC typically requires 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 emitting layer is situated between an indium tin oxide (ITO) anode, and an aluminium cathode. The transparent 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 one hundred 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 comprise
a blend of a CP and an electrolyte. The CP is responsible for both the light emitting properties
of the device and the transportation of electrons and holes inside the device. The electrolyte, on
the other hand, is responsible for the doping of the CP, and the important formation of electric
double layers. The compatibility between the electrolyte and the CP is important, as it strongly
affects the stability and performance of the device [28].
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-, and double-bonds. The P z orbitals of the carbon atom, who form the π-orbitals, overlap each other extending the π-conjugation along the backbone. Thus, a system of delocalized π-electrons is formed, giving the polymer properties closely related to an inorganic semiconductor [29]. The repeat 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 valence band, and a conduction band. The energy gap in CPs is characterized by the energy difference between 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 energy gap 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 wavelength of 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, energy gap, and solubility with organic solvents [31, 32]. These CPs generally provide good luminescent properties, while still maintaining high electron and hole mobilities. In addition, many CPs have a 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 is solved 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 conducting polymer ensures that ions can move freely in the emitting layer. Typically, the salt LiCF 3 SO 3 (LiTF, lithium trifluoromethanesulfonate), or KCF 3 SO 3 (KTF, potassium trifluoromethanesulf- onate) is used, which can be dissolved in the ion conductor hydroxyl-capped trimethylolpropane ethoxylate (TMPE-OH). KTF has been shown to have higher diffusion rate in TMPE-OH than LiTF, 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 using PEO have been shown to experience phase separation with many commonly used CPs; causing a 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 commonly
used 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 allows
transmission of visible light from the emitting material through the ITO and glass layers. For
the top electrode, aluminium is often chosen as it displays good cathode performance [29] and
high reflectance. The aluminium does not just stop light from leaving at the top of the sandwich
CHAPTER 2. THEORY
LEC, but also reflects most of the light, allowing for almost doubling the luminescence of the device.
2.1.3 Operating mechanism of LECs
The operating mechanism of the LEC is interesting from a physical perspective, since a lot of things take place in the emitting layer. When the device is in open circuit, the mobile ions randomly distribute in the emitting material, depicted in figure 2.3a. It is first when a voltage bias is applied to the electrodes of the LEC things take effect. The external bias shift the energy levels 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 the ions gather at their respective electrodes, see figure 2.3c, thin electric double layers are formed in the interface of the electrodes and the emitting material, resulting in a large potential drop in the double layers. The consequence is a shift in the CP’s HOMO, and LUMO, benefiting charge injection. The charges only have to tunnel through a nanometer thin layer [37], under influence of a large electric field. The remaining ions in the bulk of the emitting layer redistribute and compensate the injected electrons and holes, see figure 2.3d. The emitting layer become electrochemically doped from the injection of charge carriers making the emitting layer p-type doped 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-n
junction [38]. Electrons and holes can now meet in the p-n junction and form an exciton which
in turn decays to a photon, and the device emits light.
CHAPTER 2. THEORY
LUMO
HOMO
(a)
Ano de Catho de –
+ – +
+ – + –
– + – +
(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 open circuit (a). In (b), an external bias is applied over the electrodes and the ions move with the resulting electric field. (c) shows how the electron (and hole) injection is governed by the formed electric double layers. In (d) a p-n junction has formed and excitons 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 to convert its emission. The CCL absorbs the incoming photons, which have larger energy than the CP’s energy gap and excitons are formed. After a short relaxation period these excitons will emit photons with longer wavelengths. It is desired to have a polymer with high photoluminescence quantum yield (PLQY), which is a measure of how many photons the polymer emits per absorbed photon. 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 the light emitted from the emitting layer. This can be controlled by making CCLs with different thickness, 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-emit
light 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 degradation
of the device is solely dependent on one polymer, providing a more color-stable device [40].
CHAPTER 2. THEORY
Glass ITO Emitting Layer
Aluminium
CCL
Figure 2.4: A typical sandwich light-emitting electrochemical cell (LEC) in (a), and a sandwich LEC 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 properties of light. In this section one of the most well used approaches for characterizing white light is described.
2.3.1 CIE-coordinates
To the normal human eye it is easy to distinguish if a light is red, blue, or any other commonly occurring color. In computers, it is also easy to translate different wavelengths of light into particular colors. However, it is tricky to convert a large range of wavelengths into the color perceived by the human eye, and how natural the color is perceived. In order to link these together the Commission internationale de l’´ eclairage (CIE) defined the CIE 1931 standard observer [41], which chromaticity diagram can be seen in figure 2.5. The CIE 1931 color space use two dimensions (x and y) to describes the chromaticity of a light and one dimension to describe its brightness (Y ). Chromaticity describe the color-quality of a light without respect to its luminance. The CIE-coordinates are calculated from the spectrum with the help of three color-matching functions, one for each cone cell in the eye, which can be found in [42]. Detailed calculations 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 the temperature of a black body radiator [42]. A cold white light-emitter will have a blueish white color, while a warm white light-emitter is perceived as orange-reddish white. The flames in a campfire for example are often perceived as blue in the middle where they are hottest, and orange-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 above 5000 K. The CCT of a light emitter is determined by the emitters coordinates (u, and v) in the CIE 1960 color space [43]. The point in the Planckian locus, see figure 2.5, that is closest to the light source’s coordinates provides the CCT of the light source [42]. The locus represents the color path an incandescent black body would take when increasing its temperature, from deep-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 the
visible spectrum, compared to a natural light source [42]. For example, a yellow light-emitter
illuminating 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 reveal
yellow colors. Consequently, these light sources will have a poor CRI value. In a camera flash,
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 a standardized 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 to zero in litterature [44]). A light source with CRI of 100 represents a perfect black body while a light 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 temperature is 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 can compare how well the light source renders its colors, compared to a black body radiator. The 14 test-colors provide 14 special CRIs (R i ) of the light source, where only a spectrum of the source is 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 a high strong red special CRI, R 9 [47]). The general CRI of a light source is calculated as the mean of the first eight special color rendering indices. A list of the fourteen special CRIs is found 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).
Chapter 3 Method
In this chapter the different materials, used in this work, are explained. Furthermore it contains a thorough description of how LECs are fabricated and how different parts of the LEC were characterized.
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 and electrodes also play an important role in the LEC. In this section the utilized materials are listed, 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 electroluminescent material, when aiming for a wide spectrum, using CCLs. It is a conjugated co-polymer with blue to blue green light-emission. The molecular structure of polymer blue is illustrated in figure 3.1a. Polymer blue was used in the emitting layer, for all devices employing a CCL, where it was blended with KTF and TMPE-OH, using cyclohexanone as the solvent (molecular structure in figure 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-based
co-polymer with green to yellow photoluminescence. It was used both in the emitting layer of
the first LECs fabricated in this project (see first objective in section 1.3, and result in appendix
A), 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 in
the deep-blue to blue spectrum [49], super yellow could be suitable to use as CCL together with
CHAPTER 3. METHOD
a blue-emitting LEC. Cyclohexanone can be used to dissolve super yellow up to a concentration of 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 polymer blue’s emission spectrum [50], making it ideal as a color-converting material for attaining white light from a polymer blue based LEC. The molecular structure of MEH-PPV is depicted in figure 3.3a, it is soluble (at least to 20 mg mL −1 ) in toluene (figure 3.3b), and have previously been 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-PPV can provide, polymer red 1 (PR, Livilux SPR-001 L05, Merck GmbH, Germany) was used. It is a fluorescent co-polymer with light absorption in the deep-blue to blue region [49], making it suitable 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.
1
No molecular structure was available from the provider.
CHAPTER 3. METHOD
3.1.2 Electrolyte
Ion-transporter
To allow for ionic motion in the emitting layer, the ionic solvent/ion conducting material hydroxyl-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 ion transporters [53].
(a) Hydroxyl-capped trimethylolpropane ethoxylate
(b) Potassium
trifluoromethanesulfonate Figure 3.4: The molecular structure of the electrolyte components. Hydroxyl-capped
trimethylolpropane ethoxylate (a) is the ion-transporter and Potassium trifluoromethanesulfonate (b) is the salt.
Salt
Potassium trifluoromethanesulfonate (KTF, CF 3 SO 3 K, Aldrich) is a salt comprising potassium cations K + , and trifluoromethanesulfonate anions (CF 3 SO − 3 ). Together with TMPE-OH, KTF forms the solid-state electrolyte where the ions can move freely with the aid of the TMPE-OH molecules. 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 figure 2.1, was used to determine the characteristics of the LEC when the CCL is excluded. The second setup, shown in figure 3.5a, shows a bottom emitting LEC with a CCL. The purpose of the CCL is to partially convert the electroluminescent light from the emitting layer, so that light with a broader spectrum is emitted from the device. Finally the third design, see figure 3.5b, uses a thin transparent silver layer as anode (15 nm thick), allowing light to pass through both the top and bottom of the LEC. With this device configuration half of the light never travels through the CCL, and is directed out of the device, while the other half is directed towards the CCL. By fabricating CCLs thick enough to absorb as much as possible of the blue light, directed towards it, about half of the blue light produced by the LEC will be converted. This allows a broad range of wavelengths to be emitted from the top of the device thanks to the reflective aluminium layer underneath the CCL.
Since the CPs are sensitive to oxygen and water, all device fabrications took place in a
glovebox with a nitrogen environment ([O 2 ] < 5 ppm, [H 2 O] < 2 ppm).
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 show a bottom emitting LEC with a color-conversion layer (CCL) while the right figure represents a top emitting LEC employing a CCL and an aluminium layer, where the latter works as a reflector.
3.2.1 Cleaning
Small particles, on the surface of the ITO coated glass substrates, often leads to bad emitting layer films. Examples of bad layers are particle spots, or comet shaped regions with no (or little) 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 minutes at 30 °C. Afterwards, the cleaning procedure was repeated using isopropyl alcohol instead of acetone. Finally the substrates were dried in an oven for at least 4 hours at 120 °C before they were 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 10 mg 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 hot plate for at least 5 hours. The thin emitting layer was formed by applying the emitting material solution to the ITO substrate, which was then spin-coated with a SPS Spin 150 to remove excess solution.
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 figure 3.6a, when the spin-coater is at rest. By applying a high rotational speed (generally 800-4000 rpm) the excess substance is forced off the substrates sides by the centrifugal force, see figure 3.6b-c. After rotating for a short time (typically 10-60 seconds) a thin uniform emitting layer film will be formed on top of the substrate, as shown in figure 3.6d. The final film thickness mainly depend on the viscosity of the substance and the interaction between the substance and the 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 be controlled. After the emitting layer had been spin-coated on the substrate, it was heated at 50 °C on a magnetic hotplate for at least 8 hours. The heat removes the remaining solvent from the emitting layer, forming a dry film. Heating at a low temperature for an extended time ensures that the polymer chains are not damaged by the heat, while allowing closer packed polymer chains 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. It
is done by thermally evaporating aluminium or silver in a vacuum chamber with the pressure
p < 4 · 10 −6 Pa. It is a procedure where an evaporant is placed in a tungsten boat. When a high
current (150-220 A depending on material) is passing through the boat, the boat is joule heated.
CHAPTER 3. METHOD
Figure 3.6: The figure show the four main steps of the spin-coating mechanism. (a) demonstrate a 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 chuck is rotationally accelerated. In (c), the substrate is rotated at high speed, forcing emitting material off the sides by the centrifugal force. (d) show the final, thin emitting layer, formed on top of the substrate.
After a high enough temperature is reached the material is evaporated, making the material attach to the above surface, see figure 3.7. By placing a shadow mask directly in front of the substrates, different electrode patterns can be formed. The vacuum chamber remove particles from 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 the substrate. 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 tungsten
boat and heat is accumulated by the boats electric resistance. The evaporant in the
boat is then heated until metal vapor is formed, which subsequently gather on the
substrate placed above the boat. A shadow mask placed in front of the substrate
allows electrode patterns to be made.
CHAPTER 3. METHOD
3.2.4 Encapsulation
During operation the fabricated LEC is not stable in air and must be encapsulated, to prevent side reactions with oxygen and water, before it can be operated outside the nitrogen environment in a glove box. A second purpose with the encapsulation is to prevent damaging the LEC with the spin-coater chuck, e.g. when spin-coating was performed on both sides of the ITO/glass substrate. To encapsulate the device, a UV curable epoxy (Ossila Limited) was applied to the desired surface. By attaching a thin piece of glass to the epoxy and UV curing it for 10 minutes an air stable LEC was acquired. A more detailed description of the encapsulation technique is described 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 section 3.2.2. The CP was dissolved in toluene and stirred on a magnetic hot plate at 50 °C for at least 8 hours. The CCL was completed by spin-coating (2000 rpm, 2000 rpm s −1 , 60 s) the CP solution on an encapsulated LEC surface and heating it at 50 °C for at least 8 hours. For the top emitting LECs a 100 nm aluminium layer was evaporated on the CCL’s surface to reflect the light back towards 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 lifetime tester (M6000 PMX OLED Lifetime Tester, McScience). It is an easy to use test system where the 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 lifetime and brightness of the device are recorded. The lifetime was defined as the time between the LEC’s turn-on time, when it first reached brightness >100 cd m −2 , and the time where the brightness dropped under 100 cd m −2 again. The lifetime tester was calibrated with a luminance meter (LS-110, Konica Minolta) so that the brightness is measured within 5% of the brightness measured 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 current was 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 thickness variations 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 the line.
3.3.3 Spectroscopic measurements
To measure the electroluminescence, a fiber-optic spectrometer (USB2000+, Ocean optics) was used. Inside the spectrometer, the light is first diffracted by a grating. By directing the diffracted light 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 response
must be compensated, when doing fluorescence measurements. Therefore a relative irradiance
measurement procedure was used, in which a tungsten halogen lamp (HL-2000-HP-232R, Ocean
CHAPTER 3. METHOD
Optics) was used as reference source. The collected spectra was stored using the software SpectaSuite (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 grating monochromator. 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 the film as the measured signal divided by the reference. Quarts substrates were used as base for the 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 into a monochromator to select light of a specified wavelength. If the specified wavelength range is within the polymer’s absorption range, the polymer is excited and its fluorescence can be measured 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 in Matlab (R2014b) with a slightly modified code attained from Lighting research center [55]. The code was tested with data from various light sources found at Designing with LEDs [56]. The calculated CRI values were within 0.3 units from the stated values, while CCT were within 100 K from the stated temperature.
The photographs were taken with a Canon EOS 500D equipped with a Sigma EX 150/2,8
DG HSM Macro lens. Photographs have been adjusted with respect to brightness and contrast
using GIMP 2.8.8.
Chapter 4 Results
This chapter presents the achieved results during this work. The first, and prerequisite, objective was to create a stable super yellow LEC with significant brightness for a long duration (see section 1.3). The result from this study can be found in appendix A. The following sections cover the results 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 current density of j = 7.5 mA cm −2 , is shown. The turn-on time and lifetime of the device was 29 seconds 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 when the device reached 5 V. The peak efficiency of the device was 4.3 cd A −1 . The thickness of both the emitting and aluminium layers was 100 nm.
Time (hours)
0 5 10 15 20
Brightness (cd m
-2)
0 50 100 150 200 250 300 350
Voltage (V)
0 2 4 6 8 10 12 14
Brightness VoltageFigure 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 during
the lifetime of the device.
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 the blue emitter in section 4.1 was utilized. The different subsections present the obtained results after 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 thicknesses were tested: 20 nm and 65 nm spin-coated from a 5 and 6 mg mL −1 solution. With super yellow as CCL most of the light absorption took place in the blue part of the electroluminescence spectrum of polymer blue, as displayed in figure 4.2. The green parts of the spectrum was almost unabsorbed by the CCL, which photoluminescence re-emitted green to yellow light. This can be seen in figure 4.2, where the spectrum of the blue emitter, adopting a 65 nm thick super yellow CCL, is depicted. In figure 4.3 a trend can be seen that when applying a super yellow CCL to the blue emitter, the resulting light will shift from blue-green, to yellow-green. A thicker CCL convert more light, increasing yellow light output at the expense of blue light. Lack of light in 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
Absorbance
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
EL: PB
EL: CCL (SY, 65 nm) PL: SY
Absorption: SY
Normalized Intensity
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 super
yellow (SY) films. The figure also present the electroluminescence (EL) of the
polymer blue (PB) based emitter (blue line), and the blue emitter employing a 65
nm thick super yellow CCL (red dashed).
CHAPTER 4. RESULTS
Figure 4.3: CIE-coordinates for the electroluminescence (EL) of a polymer blue (PB) based emitter (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 of MEH-PPV, see figure 4.4, resulted in broader emission spectrum of the devices, compared to the devices utilizing super yellow CCLs (see figure 4.2 for comparison). Using a 75 nm thick MEH-PPV CCL, the electroluminescence of polymer blue was partially converted into a broad light-emission ranging from blue to orange-red wavelengths, depicted in figure 4.4. The white light-emitting device had a CRI of 67, a CCT of 4900, and a good balance between absorption and photoluminescence, as about half of the blue-green light was converted by the CCL (see the magenta 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 thicknesses were also tested, which CRI and CCT values are listed in table 4.1, their CIE-coordinates are plotted in figure 4.5. The 200 nm thick CCL absorbed most of the blue to green light and a device 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 colder white light was achieved with CCT = 7200 K.
Table 4.1: Color-rendering index (CRI) and correlated color temperature (CCT) of polymer blue based LECs employing different MEH-PPV CCL thicknesses, together with the ink 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
CHAPTER 4. RESULTS
Wavelength (nm)
400 450 500 550 600 650 700 750
Absorbance
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
EL: PB
EL: CCL (MEH-PPV, 75 nm) PL: MEH-PPV
Absorption: MEH-PPV
Normalized Intensity
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) spectrum of MEH-PPV films. The electroluminescence (EL) of a polymer blue LEC (blue solid line) employing a 75 nm thick MEH-PPV CCL (magenta dashed) is also displayed.
Figure 4.5: CIE-coordinates of the electroluminescence (EL) from a polymer blue (PB) based
LEC (blue circle) adopting a 40 (yellow diamond), 75 (white star), and 200 (magenta
square) nm thick MEH-PPV CCL. The CIE-coordinates of the photoluminescence
(PL) of MEH-PPV is marked with a green hexagram.
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 absorbs light in the blue part of the electroluminescence spectrum of polymer blue, leaving the green light untouched. 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 resulting spectrum 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 red light, which can be observed in the magenta dashed line in figure 4.6, where the blue and red wavelength peak are similar in height. The CIE-coordinates of the device is marked in figure 4.7, together with two devices employing a 90, and 200 nm thick polymer red CCL. In the chromaticity diagram a trend can be seen. By adapting the blue-emitter with a polymer red CCL, the light is converted from blue-green to green-red. By using thicker CCLs a blend of the green electroluminescence of polymer blue, and the red photoluminescence of polymer red was acquired. 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 with the 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
Absorbance
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
EL: PB
EL: CCL (PR, 150 nm) PL: PR
Absorption: PR
Normalized Intensity
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 also
depicted.
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 (white star) nm thick polymer red (PR), together with the photoluminescence (PL) of polymer 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 figure 3.5b. The new device configuration enabled half of the blue light to escape the device, without passing through a CCL, allowing polymer red to absorb the remaining half. The excess green light (which polymer red do not absorb) was absorbed by MEH-PPV to enhance the orange-red part of the spectrum.
The spectrum of two top-emitting devices using both MEH-PPV and polymer red as CCL are 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 device 2 (black dash-dotted) was. The CRI and CCT of the devices are listed in table 4.3. Noteworthy is the white light from device 2, which showed a CRI value of 89. Its special CRIs can be found in table B.2 in appendix B, where the device have a score of over 85 in all of the first eight test samples. The device had CCT = 6400 K and CIE-coordinates within 0.1 units from the Planckian 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 was also 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 bottom
emitting devices using only MEH-PPV, this device configuration gave a slightly lower CRI of
73, compared to devices utilizing both MEH-PPV and polymer red. The CRI-coordinates were
closer 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 = 580
can be seen.
CHAPTER 4. RESULTS
Wavelength (nm)
400 450 500 550 600 650 700 750
Normalized Intensity
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
EL: PB
EL: CCL (MEH-PPV & PR 1) EL: CCL (MEH-PPV & PR 2) EL: CCL (PR & MEH-PPV) EL: CCL (MEH-PPV)
