IGM. Development of Vapor Deposition Processes for OLEDs. Bachelor Thesis. Prof. Dr.-Ing. N. Frühauf. 28th of September Alexandru Andrei Lungu

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1 IGM Institut für Großflächige Mikroelektronik Institut für Großflächige Mikroelektronik Prof. Dr.-Ing. N. Frühauf Development of Vapor Deposition Processes for OLEDs Bachelor Thesis 28th of September 2014 Author : Supervisor : Co-supervisor : Alexandru Andrei Lungu Jirka Radusch Joaquin Puigdollers Gonzalez

2 i Abstract This bachelor thesis describes the development of vapour deposition processes for the creation of a warm white OLED. An OLED is a layer or a stack of organic materials situated between two electrodes, which emits light when an electric current is injected into the device. The thesis starts with a short introduction about the history of the OLED, its main uses today (lightning and displays) and the disadvantages of the technology, such as lifetime, manufacturing costs and encapsulation. Right after the introduction, a set of basic concepts are introduced with the purpose of facilitating the understanding of the thesis s contents. A short explanation about how a basic OLED stack emits light after an electron and hole recombine, is followed by a list of layers that are common to a modern OLED stack. A list of processes used during the experiments is also provided with a description for each one of them. This part ends with the introduction of some basic concepts about color and photometric quantities used during the thesis to better describe the OLEDs. Once the basic concepts have been introduced, the thesis follows by explaining the setup of the experimental environment. To operate the evaporation machine, a couple of recipes (sets of routines) have been developed for different purposes prior to the experiments. One such purpose is the calculation of the tooling factor for each of the sources, a parameter critical to the whole process to ensure that the thickness of the deposited material is the desired one. The main part of the thesis follows right after that. It describes the development of the OLED stack starting with a simple, basic stack. The first OLEDs created can be greatly improved upon by doping some of the layers to achieve better probability for electron-hole recombination. Doping the layers led to noticeable improvements in lifetime, brightness and efficiency. The next step in further improving the efficiency and changing the color of the OLED to the desired one, is to dope the emitting layer. This decision again led to many improvements related to brightness and efficiency and the discovery of possible current drain near the edges of the OLED. By creating a mask to achieve an insulation layer on the edges, this problem not only has been avoided but some efficiency and brightness improvements have been noticed again. The thesis closes with a short discussion about the results, problems encountered during the development of the thesis and a list of suggestions on how to improve the OLED s capabilities even further.

3 Contents 1 Introduction Uses Problems Motivation Basic concepts Basic OLED concepts Processes used during the experiments Color Photometric quantities The luminous flux The luminous intensity Luminance The setup of the working environment The tooling factor Crucible and material used Problems encountered Recipes The development of the anode structure Structure Developing The basic OLED stack The basic OLED stack ii

4 CONTENTS iii 6 Colored light emitting OLED Luminance Lifetime Electrical characteristics Color White light emitting OLED Lifetime Color Luminance Electrical characteristics The edge problem The final white light emitting OLED Color Lifetime Luminance Electrical characteristics Conclusions What could have been avoided Improvements Bibliography 41 A ITO anode mask 42 B Photoresist layer mask 43 C Tooling factor recipe - sources 1,2,7,8 44 D Tooling factor recipe - sources 3,4,5,6 45 E The simple OLED stack complete recipe 46 F The colored light emitting OLED complete recipe 49 G The white light emitting OLED complete recipe 53

5 List of Figures 1.1 Flexible YOUM display from Samsung World largest light panel from LG Chem Comparison between using the typical LCD backlight and an OLED backlight Emission digram for a basic stack The colors associated with different temperatures Luminous intensity diagram Steradian diagram Diagram of the measurement setup Mechanical assembly inside the evaporation chamber Chemical structure of Alq3 [7] The results of the profilometer after using different speeds of scaning Measurements before and after correcting the tooling factor Typical plot of a deposition rate in SQS Top view of the ITO anode structure Side view diagram after sputtering Side view diagram after spin coating Side view diagram after exposure Side view diagram after development Side view diagram after etching Side view diagram of the anode structure The basic OLED stack The energy levels diagram of the simple OLED stack iv

6 LIST OF FIGURES v 5.3 The lifetime of a simple OLED stack at a constant voltage of 22V Color coordinates and spectrum of the basic OLED stack The doped OLED stack The energy levels diagram of the doped OLED stack The lifetime of the doped OLED stack at a constant voltage of 3.5 V Electrical characteristics of the doped OLED stack The change in color coordinates and spectrum The white OLED stack The energy levels diagram of the white OLED stack Manual control of deposition rates during codeposition The lifetime of the white stack at constant voltage of 3.5V Color coordinates and spectrum of the basic OLED stack The power efficiency of the stack Electrical characteristics of the white OLED stack OLED diagrams The edge problem solution Different types of white color Electrical characteristics of the final OLED stack Comparison between final values and other typical ones A.1 The mask used in the development of the ITO structure B.1 The mask used in the development of the isolation layer

7 List of Tables 3.1 Values of the tooling factor for each source vi

8 Nomenclature Φ v Alq3 CCT CIE EBL EIL ET L HBL HIL HOM O HT L IT O LCD LU M O OLED P ID P LED SI SM OLED Å Luminous flux Tris-(8-hydroxyquinoline)aluminum Correlated color temperature Commission Internationale de l Éclairage Electron blocking layer Electron injection layer Electron transport layer Hole blocking layer Hole injection layer Highest occupied molecular orbital Hole transport layer Indium tin oxide Liquid crystal display Lowest unoccupied molecular orbital Organic light-emitting diode Proportional Integral Derivative Polymer light-emitting diode International System of Units Small molecule light-emitting diode Angstrom vii

9 1. Introduction At the beginning of the twentieth century, a radio engineer named H.J. Round discovered the phenomenon of electroluminescence using a piece of carborundum crystal. He later discovered that a large number of crystals were glowing when a voltage of 110 V was applied [1]. Round s discoveries led to the invention of the light-emitting diode. Decades later, two chemists, Ching Tang and Steven Van Slyke, reported the invention of the organic light-emitting diode (OLED) at the research department of Eastman Kodak, in 1987 [2]. Since then, multiple research and studies have improved the basic OLED stack created by Tang and Slyke lowering the operation voltages to only a couple of volts while having a high enough efficiency to be considered a next generation technology. 1.1 Uses The OLED technology is common today in many commercial applications such as home and public lighting or empowering devices such as smartphones, MP3 players and digital cameras. The OLED displays are thinner, have a better color contrast and they are more lightweight. The versatility of the OLED s substrates can mean that the OLED displays can also be made out of pastic materials, therefore making it possible to create flexible smartphones, high definition, transparent or even elastic displays such as the one depicted in Figure 1.1. Because of the high efficiency and brightness, the OLEDs could in the future replace older technologies such as incandescent lightbulbs or fluorescent tubes. Because the OLEDs can be created on different types of substrates, designers can create eye pleasing objects such as the lamp from Figure 1.2. Its big OLED panels (32x32 cm) are capable of an output of 1000 lm, similar to a 75 W incandescent lamp, but with an efficiency of 60 lm/w. 1

10 CHAPTER 1. INTRODUCTION 2 Figure 1.1: Flexible YOUM display from Samsung Source: Figure 1.2: World largest light panel from LG Chem Source:

11 CHAPTER 1. INTRODUCTION Problems Although there are already many applications for the OLED technology, it is still an undeveloped technology and, as is the norm with all new technologies, the manufacture costs are quite high. These costs vary in function of the processes used, but in general they are higher than manufacturing an LCD, for example. However, the rate at which these costs decrease is higher for the OLED technology. Another disadvantage of using OLEDs is the short lifetime of the organic materials. The blue OLED films degrade faster than the red and green ones which leads to a loss in color balance. This can be avoided for instance by adjusting the pixels sizes, but nevertheless, the total lifetime of the OLED has to be improved in order to compete with other present technologies. As for this thesis, the main disadvantage was the lack of encapsulation of the organic layers. Water and oxygen can quickly degrade the layers, therefore is vital to have a good sealing process in order to improve the lifetime of the OLED. 1.3 Motivation The motivation for the thesis came from the continuous need of efficiency improvement in technology. OLEDs have the potential of becoming the next step in the evolution of lightning and displays but because of their high costs and encapsulation issues they are still not fully ready for the market as standalone devices. But in combination with established technologies such as LCDs, the OLEDs can be really useful. This thesis is focused on the creation of the first white OLED using vapour deposition for future LCD backlight projects at the Institute for Large Area Microelectronics at the University of Stuttgart. Past research at the Institute focused on polymer based OLEDs (PLED) which are cheaper than small molecule OLEDs (SMOLED). PLEDs are more suitable for large-screen displays because they can be made in large sheets using spin coating or solution-processing techniques. This can be an advantage and disadvantage at the same time. Solution-processing techniques are commonly employed in PLED fabrication, but they make it harder to create multilayer stacks. SMOLEDs, on the other hand, are created using evaporation techniques, which, although more expensive, facilitates the creation of multilayer stacks. That s why SMOLEDs are more efficient and more bright than PLEDs. Using an OLED backlight reduces the complexity of the LCD screen by eliminating the need to use many components as shown in Figure 1.3. Reducing the number of components decreases the manufacturing time and costs leading to a more affordable high quality screen. Considering that the LCD only uses 10% of the light emitted by the traditional backlight, using an OLED backlight increases the power efficiency and reduces the environmental costs.

12 CHAPTER 1. INTRODUCTION 4 Figure 1.3: Comparison between using the typical LCD backlight and an OLED backlight Source: Therefore the first goal would be the development of the vapor deposition processes to create an OLED to serve for backlight purposes. To do that, first, certain parameters have to be obtained and optimized and afterwards a complete recipe for a multilayer stack has to be created. In the end, this thesis has to prove the viability of the processes and electrically characterize the final device.

13 2. Basic concepts 2.1 Basic OLED concepts The most basic form of OLED is an emitting layer situated between an anode and a cathode. The emission of light happens when an electron and a hole form a high energy molecular state called exciton, which at the end of its lifetime generates a photon [3]. The wavelength of the light emitted is related to the energy of the exciton therefore by changing the molecular structure it is possible to change the color. Figure 2.1: Emission digram for a basic stack The amount of photons emitted is directly related with the amount of excitons created. Hence, by increasing the probability of electron-hole recombination the light emitted would be brighter. To do so, it is really important to control the flow of charges in the OLED and block them in the emitting layer by combining materials with the right HOMO and LUMO levels. The HOMO layer stands for highest occupied molecular orbital and is the equivalent of the valence band in the inorganic semiconductors. It is the orbital possessing the maximum electron energy of all the electron-filled orbitals. If two material with similar HOMO layers 5

14 CHAPTER 2. BASIC CONCEPTS 6 are put together, it is easier for holes to pass from one layer to another. Thus by using the proper HOMO levels it is easier to assure a high flow of holes or block the holes in the emitting layer. The LUMO layer stands for lowest unoccupied molecular orbital and is the equivalent of the conduction band in the inorganic semiconductors. It is the orbital with the lowest electron energy among all the unfilled electron orbitals [3]. The LUMO level is for the electrons, what the HOMO level is for the holes, therefore by using the correct LUMO levels it is possible to ease the flow of the electrons or block them in the emitting layer. More developed OLED stacks use multiple layers to maximize the probability of recombination. Depending on the LUMO and HOMO levels, each of these layers are useful for a certain job: Electron injection layer (EIL). For easier injection of electrones from the cathode to the next layer. Its LUMO level must be lower than the next layer s LUMO level but higher than the cathode s workforce. Electron transport layer (ETL). For electron transportation from the EIL to the next layer. The LUMO level again should be lower than the next layer s LUMO level but higher than EIL s. Electron blocking layer (EBL). To block the electrons in the emitting material. Its LUMO level should be lower than the LUMO level of the emitting material. Hole injection layer (HIL). For easier injection of holes from the anode to the next layer. Its HOMO level must be higher than the next layer s HOMO level but lower than the cathode s workforce. Hole transport layer (HTL). The HTL is for holes what ETL is for electrons. It has to have the HOMO level higher than the next layer s HOMO level but lower than the workforce of the anode. Hole blocking layer (HBL). To block the holes in the emitting material. Its HOMO level should be higher than the HOMO level of the emitting material. Because the LUMO and HOMO levels of the available materials don t always fit perfectly, it is common to use a process called doping to adjust them. Doping is basically the adding of another material in small percentages to change the color of the light emitted or to improve the efficiency by modifying the energy levels. 2.2 Processes used during the experiments During the preparation of the experimental environment and the run of the experiments, a few processes were used that are worth mentioning in order to completely understand this thesis.

15 CHAPTER 2. BASIC CONCEPTS 7 Vapour deposition is a common method of thin-film deposition in which vapour particles are deposited on a substrate where they condensate back to a solid state. The evaporation is done in a vacuum environment to avoid contamination with other particles and to make sure that the layers are uniform. It basically consists in a thermal source that heats up the material (held in a crucible) until its evaporation point. Then the evaporated particles travel directly to a substrate where they condensate. Usually the substrate is rotating to assure even further the uniformity of the deposited layers. The thickness of the layers is controlled using a special computer software and it can be as small as 1 nm. Sputtering is a process in which layers of material are deposited on a substrate using beams of energetic particles. The material that is going to be layered on the substrate is bombarded with energetic particles such as ions. The energy from the collision cause atoms in the material to break from their bonds and deposit themselves on the substrate, forming an uniform layer. Spin coating is another common method of depositing thin-film layers on a surface. First, liquid coating material is deposited in the middle of the substrate. Then, using high rotational speeds, the coating material is spread evenly across the substrate. The thickness of the layer is controlled by how fast and for how long it spins. Spin coating is another process that was used to create OLEDs at the Institute and the results of the thesis will be compared with this technology to consider the viability of this project. Etching is a rather inexpensive process in which an acid is used to disintegrate an unprotected metal. Originally used in decorating armours and guns in the Middle Ages, nowadays it is being used in the manufacturing of printed circuit boards and other electronic devices. 2.3 Color To properly characterize an OLED, parameters such as brightness, color reproduction and luminance uniformity must be taken into account as well. As mentioned before, the goal of this project is to develop an OLED that emits a warm white light. A "warm white" generally means a slightly yellow white but other shades of white can have a different tint, for instance a "cool white" is slightly blue. That is because most white light is not pure white, but contains a tint indicated by the color temperature. Because terms such as "warm" and "cool" are not very precise, a system called Correlated Color Temperature (CCT) is used. Generally the system associates the color appearance of an emitting lamp with the color of the light from a reference source heated up to a particular temperature, measured in Kelvins. In Figure 2.2 it is possible to observe the different colors with which the temperatures are associated. For warm white, the temperature is around 3000 K.

16 CHAPTER 2. BASIC CONCEPTS 8 Figure 2.2: The colors associated with different temperatures The CIE 1931 system was created by Commission Internationale de l Éclairage (CIE) in 1931 to map all the colors that can be perceived by humans using two color coordinates, x and y, and a luminance parameter Y. 2.4 Photometric quantities The energy values that are weighted by the sensitivity of the human eye to the wavelengths of the light are called photometric quantities. Out of all of them the most commonly used and best known are the luminous flux, the luminance and the luminous intensity The luminous flux The luminous flux is the emission rate of light energy corrected, like all the other photometric quantities, for the standardized spectral response of human vision which means that it only counts the visible wavelengths [4]. It is usually used to measure the useful power emitted by a light source. The SI unit of the luminous flux is the lumen (lm). One lumen is the luminous flux of light produced by one candela over a solid angle of one steradian. lm = cd sr (2.1) The luminous intensity The luminous intensity of a point source of light in a given direction is the luminous flux proceeding from the source per unit solid angle [5]. The SI unit of the luminous intensity

17 CHAPTER 2. BASIC CONCEPTS 9 Figure 2.3: Luminous intensity diagram Figure 2.4: Steradian diagram is the candela. Assuming that a point light source emits in a steradian, we can assume that the lumious flux will travel through the areas A 1 and A 2. Because the flux is uniform everywhere (I1 A1=I2 A2), the luminous intensity I1 is higher than I2 because the closer you are to the light source, the brighter it looks. I v (cd) = dφ v(lm) dω (2.2) The solid angle of a cone with apex angle θ, is the area of a spherical cap on a unit sphere. Therefore the luminous flux can be calculated using this formula: Φ v (lm) = I v (cd) 2 π(1 cos(θ)) (2.3) This formula is useful when calculating the luminous flux knowing the luminous intensity and the apex angle Luminance For light sources bigger than one point, another photometric quantity called luminance is used. Considering that an area light source is actually a combination of multiple point sources, the luminance is the measure of the luminous intensity per unit area of light travelling in a given solid angle. The luminance is used to measure the brightness. The SI unit of the luminance is candela per square meter. Relationship between luminous intensity and luminance: For a light surface, using formula 2.2: L = di ds cos(θ) (2.4) L = dφ ds cos(θ) dω (2.5)

18 3. The setup of the working environment All the measurements and manipulations of the OLEDs will be mostly done in a glovebox. A glovebox is a sealed box designed to use and store objects in a different atmosphere, in this case, a nitrogen atmosphere low in oxygen and water. The user can interact with the objects inside using the gloves mounted on the transparent front wall. Because the camera that measures the photometric values is outside the glovebox, the transparent panel reflects part of the light. According to research done in the past [6], the amount of reflected light is 10% of the emitted light. To better characterize the OLEDs, the reflected light must be taken into account in the calculation of the luminance, efficiency and brightness. A diagram of the measurement setup can be observed in Figure 3.1. Figure 3.1: Diagram of the measurement setup 3.1 The tooling factor It is impossible to measure directly the thickness of a layer using only the evaporation chamber therefore a quartz crystal sensor is used to calculate with approximate certainty the amount of material deposited on the substrate. The sensor measures a mass per unit area by measuring the change in frequency of the quartz crystal resonator caused by the addition of a small mass due to deposition at the surface of the acoustic resonator. By listening to the changes in frequency, the machine can measure the thickness of the layer deposited on the sensor. 10

19 CHAPTER 3. THE SETUP OF THE WORKING ENVIRONMENT 11 Because it is impossible to place the sensor on the substrate (unless the sensor is a substrate) there is a difference in the material deposited on the quartz sensor versus the actual material on the substrate. Illustrated in Figure 3.2 is a simple sketch of the mechanical assembly in the evaporation chamber. Figure 3.2: Mechanical assembly inside the evaporation chamber The tooling factor is a correction applied in the measurement of a substrate s thickness by the quartz sensor s reading. Before making a deposition the correct tooling factor must be used to ensure that the thickness of the layer is the desired one. Because a regular OLED is multilayer and for that all working sources in the evaporation chamber had to be used, first it was necessary to calculate the tooling factor for each source. The process to calculate the tooling factor is rather simple. After an initial deposition of a layer with a thickness of T initial and using a random factor (F initial ), a measurement with a profilometer is done to get the measured thickness T measured. After running the test various times to get an average value, the tooling factor is calculated using this formula: F correct = F initial T measured T initial (3.1) To make sure that the calculated tooling factor is correct, the same process should be repeated various times Crucible and material used In the depositions run to obtain the tooling factors of the sources, a crucible made from Aluminium Oxide or alumina (the same material used to produce aluminium metal) was used to hold the test material, Alq3. The ceramic form of Aluminium Oxide is commonly

20 CHAPTER 3. THE SETUP OF THE WORKING ENVIRONMENT 12 used because of its low cost, strength, and ability to withstand temperatures as high as 1800 o C. Among the small molecules used in OLEDs, Alq3 (tris-(8-hydroxyquinoline)aluminum ) is a really popular material because of the relatively high stability of its structures during operation and its good electrical conductivity. It is commonly used as an electron transporter or green emitter. It is also a lot cheaper than the rest of the materials used during the elaboration of this thesis, which makes it a perfect test material. Figure 3.3: Chemical structure of Alq3 [7] Problems encountered Calculating the tooling factor by measuring the thickness of the Alq3 layer was more complicated than initially believed. After leveling the profilometer to be as perpendicular to the substrate as possible, one must take into account the waviness of the glass. Although the glass has a high roughness, its waviness can be in the order of the micrometers and can influence the readings a lot given that the layers are only 100 nm thick. It must also be taken into account the fact that the material used is really soft. It was observed that the speed of the scan and the weight of the stylus also influence a lot, because when the stylus touches the surface it scratches the material and it starts to accumulate material on the tip. This behavior can be observed in Figure 3.4. For a high speed scan, the stylus accumulates material faster, then jumps over the heap and starts reading correctly again. In a slow scan speed the stylus accumulates less material but it does that in a steady way so the reading shows a higher thickness as time goes on. Changing the stylus to a bigger one would, in theory, fix the problem of accumulating material because the same force applied by a bigger stylus would mean less force on the surface. However this experiment wasn t tried because another stylus wasn t readily available.

21 CHAPTER 3. THE SETUP OF THE WORKING ENVIRONMENT 13 (a) Slow speed scan (b) Fast speed scan Figure 3.4: The results of the profilometer after using different speeds of scaning It was also noticed that after removing the substrates from the evaporation chamber, if they are not held parallel to the ground, the material starts slipping towards the ground and creates an uneven layer. Measuring a couple of 100 nm substrates that were held for a couple of minutes at approximately 30 o relative to the ground, values in the range of nm were obtained. This variation in values can lead to the calculation of a wrong tooling factor and eventually a rerun of the experiment. This issue was solved by running more measurements on a substrate in opposite places and averaging the value. The other problems were avoided by depositing a layer of Aluminium with a thickness of 500 Å over the Alq3 layer to cover the soft material. The Aluminium is a harder material so the readings were a lot more precise. (a) First deposition with the wrong tooling factor (b) The correct thickness after correcting the tooling factor Figure 3.5: Measurements before and after correcting the tooling factor.

22 CHAPTER 3. THE SETUP OF THE WORKING ENVIRONMENT 14 The same process was applied to all the sources until the tooling factor was confirmed for each one. The next table shows the tooling factors for all the sources. Source Tooling factor Table 3.1: Values of the tooling factor for each source 3.2 Recipes The control of the system is done by running a set of routines in a bundle named "recipe" using a specialized software that controls the evaporation machine. These routines run operations such as warming up the sources, opening the shutters (a movable cover for the source) or rotating the substrate. The parameters of the material used are saved in another file called "Dataset", which includes information such as evaporation temperatures, thickness and desired deposition rates. The control software is run in parallel with another software called SQS-242 which receives, interprets and displays the readings from the sensors inside the machine and controls the controller card. To be correctly initiated before evaporation, the SQS-242 software needs the parameters of the material being evaporated therefore the main program sends the Datasets of the material to SQS-242 at the start of the recipe. Because the power supplies of the sources were different, the recipes used to calculate the tooling factors were slightly different as well. For all the sources the recipe was depositing a single 100 nm layer of Alq3, but the steps before evaporation were different. In the case of the sources 1,2 and 8 (source 7 was unavailable), the source is heated up in 4 stages. That is because the material needs to be heated up evenly to prevent overheating of the material on the sides of the crucible and to prevent excessive out gassing. It was thought that a total time of 15 minutes is enough to evenly heat up the material. Before using the sources 3-6, the PID for each material has to be calculated using the Autotune function of the software. The PID controller is a control loop feedback mechanism used here to adjust the evaporation rate. The machine automatically detects which is

23 CHAPTER 3. THE SETUP OF THE WORKING ENVIRONMENT 15 Figure 3.6: Typical plot of a deposition rate in SQS-242 the correct amount of power that it has to use in order to emit the desired rate. The Autotune function calculates the PID control values for a certain deposition rate. It is done by warming up the material close to the selected deposition rate and then clicking "Autotune" in the main software. The sources 3-6 needed to be controlled slightly different than the others because they used a newer version of the power source. These sources had to be preheated from the recipe itself. Organic sources are preheated to a temperature slightly below the evaporation point of the material with the shutter closed, that is why in the first few lines of the recipe the sources are preheated to a certain point and then the control is done by deposition rate using the PID values obtained before. The complete recipes used during the experiments can be seen in the Appendix.

24 4. The development of the anode structure Indium tin oxide (ITO, or tin-doped indium oxide) is a mixture of indium oxide (In 2 O 3 ) and tin oxide (SnO 2 ), typically 90% In 2 O 3, 10% SnO 2 by weight. It is one of the most popular materials to create anode structures in OLEDs because of its high conductivity, stability and transparency. The development of the anode structure is a little more complex and time consuming because it is a combination of the different processes described before, all of them common in the construction of displays. 4.1 Structure The structure of the anode is the one seen in Figure 4.1. This structure was chosen because it avoids short circuits between the anode and the cathode. By having a separate region (the empty region in between) the short circuit is avoided because there is no anode beneath the layers. To create this structure a special mask (available in the Appendix) was made using a common mask editing software called L-edit. You can observe that the mask contains not one but nine structures. This way nine substrates can be created at the same time and then cut into the final substrates at the end to save time and energy. Figure 4.1: Top view of the ITO anode structure 16

25 CHAPTER 4. THE DEVELOPMENT OF THE ANODE STRUCTURE Developing ITO sputtering. The first step in the creation of the anode structure is the deposition of ITO on the glass substrate using sputtering. This creates a thin layer of ITO that covers all the surface of the glass. Figure 4.2: Side view diagram after sputtering Photoresistant spin coating. To create the structure described earlier, the substrate has to be covered first with a photoresistant material using spin coating. After spin coating, the substrate has to be put inside a hot plate to better fix the new layer. This process will again cover all the surface. Figure 4.3: Side view diagram after spin coating Exposure. After designing the mask with L-edit and printing it on the glass, it is possible to insert this new mask in a Mask Aligner and UV Exposure System and use it to shape the photoresist layer into the desired structure. The machine uses ultraviolet exposure to weaken the material in the exposed areas. Figure 4.4: Side view diagram after exposure Developing the photoresist. The next step is removing the weakened photoresistant material from outside the structure. This is done by immersing the substrate for 30s in a special solution which rapidly dissolves the photoresistant material from the exposed areas. The area covered by the mask is harder to remove so it still remains a good of part of it protecting the ITO under it.

26 CHAPTER 4. THE DEVELOPMENT OF THE ANODE STRUCTURE 18 Figure 4.5: Side view diagram after development Etching. To finally shape the structure, the ITO around it must be removed too. Now that the photoresist is gone, the ITO is removed by immersing the substrate in HBr at 45 o C for 2 minutes. This process completely removes all the unnecessary ITO and gives the shape of the anode structure. Figure 4.6: Side view diagram after etching Dissolving the photoresist. The last step is to remove the layer of photoresistant material that covers the structure. Submersing the substrates in acetone and propanol for 20 minutes is enough to remove it. Figure 4.7: Side view diagram of the anode structure After all the steps, the ITO anode is finally deposited on the substrates in the desired structure.

27 5. The basic OLED stack It was mentioned in the introduction that after first discovering the electroluminance of certain materials, research was conducted to improve the ammount of energy used to dissipate light. By using a thin film of an organic material between a cathode and an anode, less energy had to be used to emit a higher amount of light, creating so the first OLED stack. 5.1 The basic OLED stack The light emitted by an OLED is created in the emitting layer by the recombination of an electron and a hole. Sometimes, to improve the emission efficiency or to change the color of the emitted light, the material used for this layer is doped with another material known as dopant. In a simple OLED stack the emitting material is not doped. Although the first structure is functional, it can be improved upon by adding additional layers. By making it easier to transfer electrons and holes from the cathode and anode to the emitting layer, efficiency is increased and energy consumption is reduced. These layers are called the hole transport layer (HTL) and the electron transport layer (ETL). The HTL is used to transport holes from the anode to the emitting layer. The material is usually selected so that the HOMO level is between the workforce of the anode and the HOMO level of the emitting material. The hole transport materials are also sometimes used as electron blockers and are designed with a wider gap to assure that the excitation energy of the emitting layer is not lost in the transportation layer. The ETL is similar to the HTL only that it transports electrons injected from the cathode to the emitting layer. The LUMO level is usually between the workforce of the cathode and the emitting material and is also designed with a wider gap to block exciton and hole movement and improve the emission efficiency. Aligning these layers between the anode and the cathode creates the basic OLED stack as seen in Figure 5.1. To better understand why the ETL and HTL improve the injection of charges to the emitting layers take a look at the energy level diagram. In the diagram it can be seen 19

28 CHAPTER 5. THE BASIC OLED STACK 20 Cathode ETL EMT HTL Anode Substrate Figure 5.1: The basic OLED stack the different LUMO and HOMO levels for the materials used in the OLED and how the electrons and holes pass from one energy level to another. The holes are injected from the anode and pass to the HTL, which reduces the potential barrier between the emitting material and the anode. To maximize the number of holes injected in the emitting material, the HOMO level of the HTL should be as close as possible to the average value of the anode workforce and the HOMO level of the emitting material. In this case the HOMO level of the HTL is really good. Anode HTL EML ETL Cathode LUMO HOMO Figure 5.2: The energy levels diagram of the simple OLED stack The electrons are injected from the cathode to the ETL, which should reduce the potential barrier between the cathode and the emitting layer. Similar to the HTL, the ETL should have a LUMO level as close to the average value of the cathode workforce and the LUMO level of the emitting material. Although in this case the LUMO level is not optimum and

29 CHAPTER 5. THE BASIC OLED STACK 21 suggests the need of doping this layer later to improve the efficiency and the lifetime. A high barrier would mean that a higher voltage would be necessary to inject electrons in the OLED which would decrease the efficiency significantly. The low lifetime of the OLED was confirmed later in experiments. As seen in Figure 5.3 a basic stack with the layers described before that was working at a voltage a lot lower than the maximum brightness operation point didn t last for more than 3 minutes. The resistance of the ITO combined with the high voltage needed to inject the electrons increase the temperatures at the edges of the OLED enough for the layers to melt, therefore for the OLED to break. Lowering the operation voltage is a must for the lifetime to improve. Luminance (cd/m²) Time (s) Figure 5.3: The lifetime of a simple OLED stack at a constant voltage of 22V Although the lifetime was short, it was still long enough to be possible to get some color and efficiency measurements. Starting with the former, it was noticed that the color that was emitted was light green. The exact color coordinates in the CIE 1931 color space can be seen in Figure 5.4a. Taking a look at the spectrum representation of the emitted light, it can be seen that the dominant wavelength is 523 nm and that the curve contains mostly only green colors. The fact that the color of the OLED was other than white was hardly surprising since the OLED had only one layer of emitting material which was not even doped. The main goal of the thesis is to obtain an OLED that emits warm white light. For that to happen, the OLED should have different emitting layers composed of different materials or an emitting material tuned using dopants.

30 CHAPTER 5. THE BASIC OLED STACK 22 Spectral radiance Radiance (Watts/sr/m²) Wavelength (nm) (a) The CIE 1931 color space (b) The radiance spectrum Figure 5.4: Color coordinates and spectrum of the basic OLED stack The other qualitative parameters that have to be taken into account to fully characterize the OLED are the brightness and the efficiency. The basic OLED stack was fairly bright with a luminance at its maximum measured point of 834 cd/m 2 when connected to a voltage of 22 V and a 0.25 A current. To put this value in context, the typical values of luminance for a LCD display range only from 200 to 250 cd/m 2 [8]. Although the brightness was surprisingly good, the efficiency was a lot less impressive. Assuming that the luminous flux is uniform and by using the formulas discussed in section 2.4 it is easy to calculate the efficiency. Knowing that the area of the OLED is 9.6 cm 2, the luminous intensity is: I v = L v A OLED = = 0.8 cd (5.1) Assuming that the OLED is emitting light in a perfect semisphere, the apex angle would be 90 o. Using this apex angle in formula 2.3, we can calculate the total luminous flux as: Φ v = I v 2 π = 5.05 lm (5.2) This luminance was obtained when the OLED was connected to 22 V and 0.25 A, which means that it consumed a total power of 5.5 W. Therefore the efficiency is only lm/w. Chasing the goal of obtaining an efficient warm white OLED at this point after analysing the results, there are only two ways to continue. One would be doping the emitting layer with a material that would change the color of the OLED from light green to warm white. The other would be to continue improving the efficiency, and therefore the brightness, by doping the HTL and the ETL. The second option was chosen because doping the HTL and

31 CHAPTER 5. THE BASIC OLED STACK 23 the ETL would also change the color of the emitted light, since there would be more layers and other materials included in the stack.

32 6. Colored light emitting OLED Because of the potential barrier between the anode and the HTL and especially the one between the cathode and the ETL, many electrons and holes are not injected in the OLED leading to poor efficiency and lifetime. By adding a small amount of an another material, the LUMO and HOMO levels of the hosts in HTL and ETL can be tweaked to better fit between the workforces of the anode and the cathode and the energy levels of the emitting material. Keeping a part of the old layers undoped, they can still be used as electron/hole blocking layers, now called Electron Blocking Layer and the Hole Blocking Layer. A diagram of the new stack can be seen in Figure 6.1. Cathode ETL HBL EMT EBL HTL Anode Substrate Figure 6.1: The doped OLED stack The problem with this strategy is that the amount of dopant can be really small compared to that of the host and a perfect ratio between them, even if calculated, can be hard to achieve in a real world scenario. Even so, it is possible to illustrate the theoretical effects of the dopants on the host materials as it can be seen in Figure 6.2. The HOMO and LUMO levels of the ETL and HTL should now be closer to the workforce of the anode and cathode which would make it easier to inject holes and electrons into the OLED. 24

33 CHAPTER 6. COLORED LIGHT EMITTING OLED 25 Anode HTL EBL EML HBL ETL Cathode LUMO HOMO Figure 6.2: The energy levels diagram of the doped OLED stack The new stack has indeed proven to be a big upgrade to the basic one showing big improvements in lifetime, efficiency and brightness. 6.1 Luminance The new layers behaved as desired and increased the number of electrons and holes injected into the emitting material, which increased the electron-hole recombination leading to a higher brightness. The highest luminance measured (considering the light reflected by the glovebox s glass as well) was cd/m 2. Applying the same formulas as in the case of the basic OLED stack, the maximum measured luminous intensity of the doped OLED is 1.78 candela which is more than double than before. Continuing to assume that the apex angle is 90 o, the luminous flux is then: Φ v = I v 2 π = 11.2 lm (6.1) 6.2 Lifetime In terms of lifetime, the improvement was considerable. Lowering the potential barrier led directly to a significant decrease in the operating voltages, therefore the temperature problem encountered before has been avoided. Because of this the OLED was able to be constantly emitting light for an average of 30 minutes, which is approximately 10 times longer than before. During the lifetime, the OLED lost 40% of its luminance.

34 CHAPTER 6. COLORED LIGHT EMITTING OLED 26 Luminance (cd/m²) Time (s) Figure 6.3: The lifetime of the doped OLED stack at a constant voltage of 3.5 V 6.3 Electrical characteristics The threshold voltage was lowered to 2 V from around 17 V and the voltage at which the maximum brightness is achieved, to 3.5 V from 22 V. This means that the power used by the OLED has been reduced as well. At 3.5 V the current used by the OLED was 0.79 A resulting in a power consumption of 2.77 W, a lot smaller compared with the previous one of 5.5 W. Even though the rest of the voltages are plausible, the threshold voltage is suspiciously low which may indicate a slight misreading or misconfiguration in the electrical readings of the measuring software. But because the software is proprietary it couldn t be checked if the suspicion was correct. Knowing the luminous flux and the power, the power efficiency is 4.04 lm/w which is more than 4 times better than the previous one. The previous calculated values are in the case of the highest measured brightness which is not the most efficient in terms of power consumption. To highest efficiency achieved during the measurements was lm/w measured at 2,5 V when the OLED needed only 0.47 W to emit with a luminance of cd/m 2, a value similar to the maximum luminance achieved with the basic OLED stack. Comparing in this case how much power the two different stacks need to emit the same amount of luminous flux, the doped OLED needs 11.7 times less power.

35 CHAPTER 6. COLORED LIGHT EMITTING OLED 27 Voltage vs Current Voltage vs Luminance Current (A) Luminance (cd/m²) Voltage (V) Voltage (V) Luminous flux vs Power Voltage vs Power efficiency Luminous flux (lm) Power efficiency (lm/w) Power (W) Voltage (V) 6.4 Color Figure 6.4: Electrical characteristics of the doped OLED stack Along with improvements in efficiency, doping the transport layers led to visual changes as well. The color of the emitted light shifted towards blue from the previous light green, a fact that can be easily noticed in the following CIE diagram. By comparing the emission spectrum of the basic and the doped stack it can also be observed that the spectrum is slightly moved to the left, in the blue wavelengths area, with

36 CHAPTER 6. COLORED LIGHT EMITTING OLED 28 the maximum radiance around 504 nm. Normalized radiance Legend Basic stack Doped stack Wavelength (nm) (a) The CIE 1931 color space (b) The radiance spectrum Figure 6.5: The change in color coordinates and spectrum Now that the efficiency of the OLED has been significantly improved, it is time to dope the emitting layer to once again shift the emission spectrum, this time from blue to warm white.

37 7. White light emitting OLED There are numerous ways of doping the emitting material or organizing the stack of the OLED that will result in the emission of white light. But considering the limited number of sources available in the evaporation chamber and due to the expensive price of the organic materials used in OLEDs, the simplest way was chosen. The solution was doping the emitting material with two different dopants to emit different tones of color, whose combined resulting color closely resemble the white color. With the new layers included, the final stack looks like the one depicted Figure 7.1. Cathode ETL HBL EMT2 EMT1 EBL HTL Anode Substrate Figure 7.1: The white OLED stack Doping the emitting material not only changes the color, but by using the right dopants, it is possible to modify the potential barriers between the emitting layer and the blocking layers. Doing so would improve the efficiency of the OLED a step further by allowing more electrons and holes to be injected in the emitting layer but also blocking more of them from escaping. The energy diagram of the new stack would look like in Figure 7.2. The only inconvenient is that again, the percentages of doping to achieve the correct color could be really small. In a real scenario, combined with numerous software bugs in the controlling of the deposition rates, the doping process proves to be an arduous job. This led to slight differences in the color coordinates of the different OLED samples because of 29

38 CHAPTER 7. WHITE LIGHT EMITTING OLED 30 Anode HTL EBL EML1 EML2 HBL ETL Cathode LUMO HOMO Figure 7.2: The energy levels diagram of the white OLED stack the difficulty of maintaining the correct rate when the control of the deposition rate had to be done manually. Figure 7.3 is a real screenshot of the rates of deposition of the host material and the dopant, in which is possible to observe the high variation of the deposition rate when the sources are controlled manually. Figure 7.3: Manual control of deposition rates during codeposition In some cases the correct deposition rate was as low as the measuring resolution of the software. A good solution proved to be changing the tooling factor of the particular sources to trick the machine into thinking that the rate is a lot higher.

39 CHAPTER 7. WHITE LIGHT EMITTING OLED Lifetime Because one of the main reasons that caused the basic OLED to die has been fixed by doping the transport layers in the doped OLED, the lifetime of the white OLED should be similar to the doped one because the transport and blocking layers are the same. This was indeed proved correct by experimental tests and the results can be seen in Figure 7.4. Normalized luminance (%) Legend Doped stack White stack Time (s) Figure 7.4: The lifetime of the white stack at constant voltage of 3.5V After a thousand seconds, the white OLED appears to be losing less brightness than the doped one. Because the percentage of doping is slightly different all the time (because of manual control) and that there is a possibility of having particles and defects on the substrate that can cause failure, it is hard to know which is the cause of the slightly faster degradation in the doped OLED. 7.2 Color The doping percentages chosen for the emitting layers have proven to be almost correct. The emitted color turned out to be cool white, but not warm white. Using the CIE coordinates, it is hard to notice that the white light that the OLED is emitting was closer to blue than orange, but looking at the spectrum plot, there is a bigger resemblance with the spectrum of a cool white light.

40 CHAPTER 7. WHITE LIGHT EMITTING OLED 32 Legend Final Doped Basic Wavelength (nm) (a) The CIE 1931 color space (b) The radiance spectrum Figure 7.5: Color coordinates and spectrum of the basic OLED stack 7.3 Luminance The maximum brightness measured during the tests was cd/m 2 at 8.5 V and almost 1 A, maximum current injected by the power supply. This means that the efficiency at the maximum brightness was 3.28 lm/w. The maximum efficiency achieved was lm/w at 3 V and A for a luminance of 60 cd/m 2. Power efficiency (lm/w) Voltage (V) Figure 7.6: The power efficiency of the stack

41 CHAPTER 7. WHITE LIGHT EMITTING OLED Electrical characteristics Analysing the plots from Figure 7.7, the most curious fact is that the operating voltages appear to have increased after doping the emitting layer. The new threshold voltage is 2.5 V and the maximum brightness is achieved at 8 V. This doesn t appear to make much sense because the transport layers and the doping percentages for the transport layers theoretically have not changed after altering the emitting layer. However, in a real environment, it is almost impossible to keep exactly the same percentages for every new batch of OLEDs. Voltage vs Current Voltage vs Luminance Current (A) Doped stack White stack Luminance (cd/m²) Doped stack White stack Voltage (V) Voltage (V) Luminous flux vs Power Voltage vs Power efficiency Luminous flux (lm) Doped stack White stack Power efficiency (lm/w) Doped stack White stack Power (W) Voltage (V) Figure 7.7: Electrical characteristics of the white OLED stack

42 CHAPTER 7. WHITE LIGHT EMITTING OLED 34 Because the rates of deposition and the thickness of the layers are sometimes really low, a high variation in the deposition speed of a source can produce layers with different doping percentages. In the case seen in Figure 7.3 assuming that the deposition speed of the dopant material was constant at 0.1 A/s (the speed is so low that its variations are not important) then the percentage of doping is always 5 ± 1.25%. Another explanation would be that in the first measurements of the doped OLED there were errors and the readings were better than in reality. 7.5 The edge problem The continuous drop in efficiency seen in Figure 7.6 as the voltage was increased, was caused by the lack of uniformity on the edges and the differences in thickness. Because the organic materials used were soft, the edges did not have the form of a step like the metals usually have, instead the edge looked more like a curvy steep. This means that on the edges the aluminium is a lot closer to the ITO or even in direct contact in some parts. This creates shortcuts on the edges and a drain of current which increases the brightness and the temperature close to the edges and ultimately leads to the melting of the layers and the death of the OLED. Aluminium Edge thickness Normal thickness ITO (a) OLED diagram as seen from above (b) OLED diagram as seen from the side Figure 7.8: OLED diagrams The solution for this was rather simple. By isolating the edges with an available photoresistant material the shortcuts would be avoided. To do this, a new mask was developed using a mask editing software to create the isolating structures on top of the ITO. The steps to develop the isolating structure are the same as some of the steps used to develop the ITO structure (spin coating, UV exposure and developing), therefore both of the structures are developed one after another in the same session using aligning masks to make sure that they are properly aligned.

43 CHAPTER 7. WHITE LIGHT EMITTING OLED 35 (a) Photoresist mask on top of anode (b) OLED diagram as seen from the side Figure 7.9: The edge problem solution 7.6 The final white light emitting OLED Color Unsurprisingly, there were almost no changes in the color of the light emitted other than the ones triggered by the variations in the doping percentage. For LCD backlight purposes the type of white is not really an important issue, but it may be for lightning where different purposes and personal tastes influence more. Because the project is more focused towards the backlight applications, the fact that the final color is not warm white but cool white is considered more of a doping percentage optimization problem rather than a failure. Figure 7.10: Different types of white color

44 CHAPTER 7. WHITE LIGHT EMITTING OLED Lifetime After doping the transport layers, the potential barrier stopped being the cause for a short lifetime of the OLED and the particles became the main threat to it. Because of this, it was impossible to measure the real improvement in lifetime that the solution for the edge problem allowed. In spite of that it was noticed that the OLEDs could function longer at higher voltages and high luminance Luminance Because of the isolating layer, the current drain that produced high temperatures on the edges didn t materialise again. This led to a more even luminance across the surface of the OLED although the influence of the ITO resistance was now more visible. The ITO superficial resistance is directly related to the distance between the starting point of the stack and a certain point within the stack. The bigger that distance is, the higher the superficial resistance. As the resistance increases, the current passing through decreases, which ultimately leads to a loss in the quantity of emitted light. To avoid this, the surface of the OLED should be reduced. The highest luminance achieved was an excellent cd/m 2 at 10 V and almost 1 A which is the maximum current allowed by the power supply. This means that the efficiency at the maximum brightness was 7,58 lm/w. The maximum efficiency measured was achieved at 4 V and A and was lm/w for a luminance of 678 cd/m 2. The doped OLED at its most efficient point was emitting almost 800 cd/m 2 using 0.47 W. The final OLED was using a similar amount (0.49 W) to radiate cd/m 2 with an efficieny of lm/w. Therefore by using practically the same amount of energy the white OLED is emitting cd/m 2 more, which is an improvement of 42.13% Electrical characteristics As for color, there practically wasn t any change in the voltage-current curve because the potential barriers were the same as before, although an improvement was noticed after applying the photoresist mask, in luminance (13%) and power efficiency (20%).

45 CHAPTER 7. WHITE LIGHT EMITTING OLED 37 Voltage vs Luminance Voltage vs Luminance Luminance (cd/m²) Photoresist No photoresist Luminance (cd/m²) Photoresist No photoresist Voltage (V) Voltage (V) Luminous flux vs Power Voltage vs Power efficiency Luminous flux (lm) Photoresist No photoresist Power efficiency (lm/w) Photoresist No photoresist Power (W) Voltage (V) Figure 7.11: Electrical characteristics of the final OLED stack

46 8. Conclusions Creating a simple stack, unsurprisingly, proved itself not to be good enough. The differences between energy levels stop the flow of electrons and create the need for high voltages. The high temperatures close to the contacts soon melt the materials with low melting points destroying the OLED really fast. In the short lifetime of 3 minutes, the simple OLED stack was radiating at its maximum 5.03 lumens (less than half the luminous flux of a candle which radiates one candlepower over 4π radians, which means a flux of lm) with a maximum efficiency not reaching 1 lm/w. The low lifetime and efficiency made it obvious that the adding of additional layers to lower the potential barrier using dopants was necessary. Adding dopants not only changed the color to blue from green but also drastically improved the OLED. The lifetime was increased up to 10 times depending always on the particles that were on the substrate, and the luminous flux was doubled to 11.2 lm. The doping proved to be a good solution but also hard to implement in real life because of the low percentages of doping and deposition rates. The last step in the developing of a white OLED, was to dope the emitting materials to achieve the desired tone of white. By doping the emitting layers, the potential barriers are also reduced in order to boost the electron-hole injection in the emitting material, thus improving so the efficiency and luminance even further. It was noticed that on the edges of the OLED there was a current drain because of the shortcuts created there by of uniformity of the distance between the anode and the cathode. To fix this, a mask was designed to create an insulating layer on the edges to better separate the anode and the cathode. The maximum luminous flux achieved was lm, a great improvement over what was achieved before. The maximum efficiency achieved was better than the efficiency of a normal incandescent 60 W light bulb, even if the luminance and the lifetime is a lot smaller. However, compared with a modern LED backlight, the OLEDs created are not even close to the efficiency, luminance or lifetimes achieved. This is to expected given the disparity between the conditions in which the modern backlights and this thesis OLEDs are manufactured. 38

47 CHAPTER 8. CONCLUSIONS 39 Figure 8.1: Comparison between final values and other typical ones 8.1 What could have been avoided Although during the thesis there have not been many things that went wrong, there were some time delays that could have been avoided. To start, during the lengthy process of calculating the tooling factor for each source in the machine, the author of the thesis should have seeked help faster with the measuring machinery. The machine runs on very old software and proved really unstable in the hands of an inexperienced user and gave wrong readings which led to incorrect tooling factors for the evaporation machine. These wrong tooling factors ultimately led to unnecessary extra depositions, a waste of time and material. With the help of the experienced staff, the machine was properly configured and started to measure correctly. Another software-related problem encountered was in the evaporation machine, which also runs on old and proprietary software. Many bugs encountered along the way were slow to be fixed by the manufacturing company and led to weeks worth of wasted time, and at one point, the continuity of the whole thesis as it is was in danger. Because the materials used in the OLEDs were new and still experimental, many measures of protecting the intellectual property of the university and the manufacturing company had to be taken. This also led to a time loss because of the delays caused by lengthy administration processes which could have easily been avoided. Avoiding all of these problems would have given more time to create more OLEDs, which was highly needed at the end. Some improvements could also have been tested and maybe the results would have been better. 8.2 Improvements To improve the results further, there are a couple of changes that come to mind. First, to try to fix the particle problem. After doping the transport layer, the most obvious problem that killed most of the OLEDs has been the existance of unwanted particles on the substrates. They were most likely originating from inside the evaporation room from the

48 CHAPTER 8. CONCLUSIONS 40 residues left after calculating the tooling factors. Because of time constraints there wasn t enough time to clean the evaporation room of Alq3 residues, process which can take a lot of time. The residues from the source shutters have been observed contaminating the organic materials, and even if they have been removed it is impossible to say if they were all gone. A second fix for the particle problem would be to divide the substrate in smaller pixels. If by chance there is a particle on the substrate it would only kill that pixel and not the whole OLED. This improvement would be quite effective for boosting the luminance and the power efficiency too. When having a big surface, the ITO resistance is playing an important role and reduces the brightness when it gets further from the contact, but it should be avoided by using smaller pixels. This strategy is also used in the industry where any new ground breaking result is obtained from really small OLEDs (2x2 mm, efficiency: 156 lm/w at 1,000 cd/m 2 luminance [9]). Another possible improvement would be warming up the substrate before deposition. Warming it up to a sufficiently high temperature would eliminate the water residues from the substrate. Water is one of the biggest threats to organic materials because in contact with it the materials degrade faster. There is some research already done on the preparation of the substrate before the ITO deposition [10]. Because of the limited number of sources, the recipe was limited to 7 materials. But by using more materials, the OLED stack can be changed by adding new layers. A type of OLED called stacked OLED works by separating the emitting layers by additional blocking and transport layers and has been proved to be more effective [11]. This should improve the number of electron-hole recombinations in each layer and improve the luminance and efficiency. By applying the improvements mentioned earlier and others which were not thought of, the results could be improved even further. The vapour deposition process, created and tweaked during the development of the thesis, proved itself better than other technologies used for OLED creation in the past at the Institute such as spin coating. Together, this thesis, the final recipe and the evaporation parameters will be valuable assets for any new research based on vapor deposition for the creation of highly efficient OLEDs. After seeing continuous progress and good results in the context in which the experiments were carried, the final conclusions are that the project is viable and has proved itself useful for future projects in LCD backlight at the Institute.

49 Bibliography [1] E. Schubert, Light-emitting Diodes. Cambridge University Press, [2] C. Tang and S. A. VanSlyke, Organic electroluminescent diodes, Applied Physics Letters, vol. 51, pp , Sep [3] T. Tsujimura, OLED Display Fundamentals and Applications. Wiley Series in Display Technology, Wiley, [4] W.-C. Cheng and M. Pedram, Power minimization in a backlit tft-lcd display by concurrent brightness and contrast scaling, Consumer Electronics, IEEE Transactions on, vol. 50, pp , Feb [5] D. Fink and H. Beaty, Standard Handbook for Electrical Engineers. McGraw-Hill Engineering & Technology Management, McGraw-Hill., [6] S. Hergert, Aufbau von OLED-Displays und ihre Verkapselung [7] Z. R. Li and H. Meng., Organic Light-emitting Materials and Devices. CRC/Taylor & Francis, Pag [8] I. Fujitsu Microelectronics America, Fundamentals of liquid crystal displays how they work and what they do., Last checked: Sept [9] T. Nozawa, Nec announces oled device with world s highest efficiency., March Last checked: Sept [10] S. So, W. Choi, C. Cheng, L. Leung, and C. Kwong, Surface preparation and characterization of indium tin oxide substrates for organic electroluminescent devices, Applied Physics A, vol. 68, no. 4, pp , [11] T.-W. Lee, T. Noh, B.-K. Choi, M.-S. Kim, D. W. Shin, and J. Kido, Highefficiency stacked white organic light-emitting diodes, Applied Physics Letters, vol. 92, pp , Jan

50 A. ITO anode mask Figure A.1: The mask used in the development of the ITO structure 42

51 B. Photoresist layer mask Figure B.1: The mask used in the development of the isolation layer 43

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