A Review on Fabrication Process of Organic Light Emitting Diodes ∗ Arnob

Islam, Mamun Rabbani, † Mehedy Hasan Bappy, Mohammad Abu Raihan Miah and Nazmus Sakib Department of Electrical and Electronic Engineering Bangladesh University of Engineering and Technology Dhaka-1000, Bangladesh E-mail: ∗ [email protected], † [email protected]

Abstract—Light emitting diodes (LEDs) have been dominating over years in the realm of incandescent and fluorescent lighting due to their long life, intensity and power efficiency. But having certain limitations like difficulty and cost of production, they have been being restrained from conspicuous application. Organic Light Emitting Diode (OLED) nowadays is a promising feature over Liquid Crystal Display (LCD) and other lighting appliances, due to their low cost, ease of fabrication, brightness, speed, wide viewing angle, low power consumption and contrast. Edging ahead, OLED also facilitates further LED market permeation. This paper discusses the structure of OLED, working principle and operation, fabrication and recent advances made in the area as ease of fabrication process is the singular most important benefit over alternative technologies.

Index Terms- OLED, PVD, Screen printing, Inkjet printing, In-line fabrication, Roll-to-roll process I. I NTRODUCTION An OLED (organic light-emitting diode) is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. The current goal in optoelectronic engineering to replace conventional lighting sources such as incandescent and fluorescent lighting with more power efficient semiconducting light sources, has already had an impact [1]. The benefits of LED lighting include a reduced ecological footprint on our environment in powering these devices, lower monitory expenditure on energy, self-sustainability, and lower fire risk [2]. It will be shown that OLEDs can be developed to become a companion to LEDs in lighting applications, and eventually replace LCD display technology. The key advantages of OLEDs for flat-panel display applications are their self-emitting property, high luminous efficiency, fullcolour capability, wide viewing angle, high contrast, low power consumption, low weight, potentially large area colour displays and flexibility [3]. II. OLED STRUCTURES The basic OLED structure consists mainly of • Indium tin oxide (ITO) • Hole transport layer • Electron transport layer • Emitting layer • Glass substrate

Fig. 1.

Basic OLED structure

Indium tin oxide (ITO) is commonly used as the anode material. It is transparent to visible light and has a high work function which promotes injection of holes into the HOMO level of the organic layer as shown in Figure 1. In introducing fabrication specifics, it is helpful to present an overview of a basic fabrication process. Indium-tin oxide(ITO) is deposited onto a glass panel by some means, typically vacuum sputtering. The ITO substrate is subsequently cleaned by ultrasound, rinsed, dried, and cleaned by organic solvents. The substrate is then subjected to a surface treatment in which the work function is adjusted to the desired level by exposing to ozone or oxygen. The design choices pertinent to the fabrication of OLEDs is discussed herein. The semiconducting mono-layer or heterostructure is either grown, printed, or deposited by some other means. In the case of a bi-layer diode, layers of organic polymer or small molecules are deposited: the holetransport layer (HTL), followed by the electron-transport layer (ETL)[4]. Other organic or metal layers may be present in other designs. Growth of these layers may take place in ultra high vacuum resistive heat evaporation champers, electron beam deposition, solution dipping, or solution spin-coating.

either be in a singlet state or a triplet state depending on how the spins of the electron and hole have been combined. Statistically three triplet excitation will be formed for each singlet exciton. Decay from triplet states (phosphorescence) is spin forbidden, increasing the time scale of the transition and limiting the internal efficiency of fluorescent devices. Phosphorescent organic light-emitting diodes make use of spin orbit interactions to facilitate inter-system crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and improving the internal efficiency. Fig. 2.

Working principle of OLED

III. W ORKING PRINCIPLE & O PERATION A typical OLED is composed of a layer of organic materials situated between two electrodes, the anode and cathode, all deposited on a substrate. The organic molecules are electrically conductive as a result of delocalization of pi electrons caused by conjugation over all or part of the molecule. These materials have conductivity levels ranging from insulators to conductors, and therefore are considered organic semiconductors. The highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of organic semiconductors are analogous to the valence and conduction bands of inorganic semiconductors. Originally, the most basic polymer OLEDs consisted of a single organic layer. Many modern OLEDs incorporate a simple bi-layer structure, consisting of a conductive layer and an emissive layer. More recent developments in OLED architecture improves quantum efficiency (up to 19using a graded hetero-junction. In the graded hetero-junction architecture, the composition of hole and electron-transport materials varies continuously within the emissive layer with a dopant emitter. The graded hetero-junction architecture combines the benefits of both conventional architectures by improving charge injection while simultaneously balancing charge transport within the emissive region. During operation, a voltage is applied across the OLED such that the anode is positive with respect to the cathode (Figure 2). A current of electrons flows through the device from cathode to anode, as electrons are injected into the LUMO of the organic layer at the cathode and withdrawn from the HOMO at the anode. This latter process may also be described as the injection of electron holes into the HOMO. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole. This happens closer to the emissive layer, because in organic semiconductors holes are generally more mobile than electrons. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of this radiation depends on the band gap of the material, in this case the difference in energy between the HOMO and LUMO.As electrons and holes are fermions with half integer spin, an exciton may

IV. S TEPS IN FABRICATION In general OLEDs are fabricated in a class 1000 cleanroom to produce results with as high a consistency as possible. However, OLEDs are relatively tolerant to dust, as it is insulating and generally only stops the device working where the dust has landed on the surface [5]. In this section, a generalized fabrication process is discussed. There are six basic steps in the fabrication process from the substrate to devices ready for use. These are described below A. Substrate cleaning Preparing the ITO surface for coating simply consists of sonicating the substrates in a sodium hydroxide (NaOH) solution to remove the photoresist, followed by a rinse in deionized (DI) water and blow dry. The first step is to load the substrates into the cleaning rack such that they all have the same orientation. The loaded substrate rack is then placed in a beaker and submerged in a 10% solution of NaOH in water. The substrates are then sonicated to remove the photoresist. Depending upon the power and temperature of the sonicator the photoresist may either dissolve or de-laminate as sheets. The time that it takes for this to occur will depend on the ultrasonic bath used as well as the temperature. After sonication the substrates should be thoroughly rinsed with water to wash away the photoresist. To ensure that they is no residual layer of photoresist present they should be put back in the ultrasonic bath in a fresh NaOH solution for about the same time again. Following this second sonication, the substrate should be again rinsed thoroughly with water and keep immersed in water until ready to blow dry to avoid contamination by dust. B. Applying PEDOT:PSS PEDOT:PSS is a common hole injection layer material [6]. The chemical name of it is poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate). Getting a high quality PEDOT:PSS film is critical for effective device performance and is often the most difficult part of device fabrication. PEDOT:PSS requires a pristine and hydrophilic surface in order to coat properly, which should have been achieved with the cleaning routine above. It is also critical to ensure that the active areas have not come into contact with any other surfaces as this will affect how well the ITO will spin. For typical use in OLEDs, the PEDOT:PSS are spin coated at 5000 rpm for 30 seconds

to produce a film thickness of around 40 nm. To minimise material use this can be done by pipetting 20 to 30 µL into the middle of a spinning substrate. After spinning has completed visually inspect the PEDOT:PSS films for defects and for best performance discard any substrates with imperfections near the active pixels. After spin coating, the PEDOT:PSS should be wiped off the cathode with a cotton bud soaked in DI water. Then the substrates are placed either in a box with the lid closed to avoid dust settling on devices, or if kept in air for more than a few minutes place directly on a hotplate. C. Applying active layer The active layer can be applied either in air or in a glovebox with little difference in performance provided exposure time and light levels are minimised. Pipetting 20 µL of the solution onto a substrate spinning at 2000 rpm should provide a good even coverage with approximately the desired thickness. The substrate needs to be spun until dry, which is typically only a few seconds. Following spin coating, the samples can be solvent or thermally annealed if desired. For the OLED reference solution thermal annealing is recommended to be done at 80◦ C for 10 minutes. Before cathode deposition, the cathode strip needs to be wiped clean. Finally, the substrates need to be placed face down in the evaporation shadow mask with the cathode strip at the wide end of the apertures. D. Cathode evaporation Typically, aluminium of 100 nm is evaporated at a rate of around 1.5 A/s, but thinner cathodes (50 nm) have also been used with no decrease in initial performance noted. Calcium evaporation is relatively trivial as it melts at low temperatures, however it can only be used effectively in conjunction with a glovebox otherwise degradation occurs. E. Annealing After cathode deposition, thermal annealing can be performed if required. Annealing at a temperature of approximately 150◦ C for 15 minutes gives optimal performance. F. Encapsulation Encapsulating the devices protects them against degradation by oxygen and moisture once removed from the glovebox. True encapsulation for lifetimes of thousands of hours requires the use of glass welding technology and/or getter layers of calcium. V. M ETHODS OF FABRICATION The most common methods of fabricating OLEDs are • Physical vapor deposition • Screen printing • Inkjet printing • In-line fabrication • Roll to roll process These methods are described below.

A. Physical vapor deposition Physical Vapor Deposition (PVD) is a group of vacuum coating techniques used to deposit thin films of various materials on different surface.This technique is based on the formation of vapor of the material to be deposited as a thin film. The material in solid form is either heated until evaporation (thermal evaporation) or sputtered by ions (sputtering).It is also possible to bombard the sample with an ion beam from an external ion source.Thermal vapor evaporation of small molecules is carried out on glass surface.Multicolor displays are made by properly matched shadow masks for depositing RGB emitting material [7]. 1) Physical Vapor Deposition Technologies:: There are two technologies which are often used for physical vapor deposition (PVD). Physical vapor deposition is done by thermal evaporator. Here, the material is heated to attain gaseous state. Besides, Electron Beam Evaporator is also used. Another method is Sputtering which is carried out under high vacuum condition. Here plasma as the particle source is used to strike the target [8][9]. 2) Thermal evaporator: Thermal evaporator uses an electric resistance heater to melt the material and raise its vapor pressure to a useful range. This is done in a high vacuum environment.An electron beam evaporator fires a high energy beam from an electron gun to boil a small spot of the material [8] 3) Sputtering: Sputtering is a physical process whereby atoms in a solid target material are ejected into the gas phase due to bombardment of the material by energetic ions.The ions for the sputtering process are supplied by the plasma that is induced in the sputtering equipment. Sputtering relies on a plasma (usually a noble gas, such as argon) to knock material from a surface [8]. 4) Advantages: Determination of the thickness of the film can easily be determined using Physical Vapor Deposition. Films can be deposited at a high rate without heating substrate and leaving surface undamaged due to low energy atoms. Moreover, two dimensional combinational arrays of OLEDs can be easily fabricated [8]. 5) Disadvantages: The main disadvantage of this process is cost-ineffectiveness. The deposit thickness varies for large or multiple substrate. moreover, the alloy compound which is deposited cannot be accurately controlled. Inefficient use of material discourages the use of this process [8]. B. Screen printing Screen printing is a commonly used technique for fast, inexpensive deposition of dye films over large areas. In addition, screen printing allows patterning to easily define which areas of the substrate receive deposition. It is mainly used industrially. The essential components of a screen printing process consist of a cloth of interwoven threads. Cloth is stretched tightly in a frame. A patterned mask is prepared on the stretched cloth in the frame. Ink is poured onto the top surface of the cloth. A substrate onto which the ink is to be printed is placed underneath the framed cloth so that it

does not directly contact the bottom surface of the cloth. A squeegee spreads the ink lightly over the patterned open cloth area without pressing down on the substrate. This fills the openings in the cloth with ink. Then the squeegee presses the cloth from the top against a substrate underneath and by sliding horizontally over the surface, squeezes out the ink in the open cloth areas onto the substrate, leaving the printed pattern. This remains wet first. So then the printed image is dried. This can be repeated many times by replacing the substrate and printing a new [10]. A variety of cloth types is available. Polyester is common; nylon cloth and metal cloth are also made. The specific limits we have found to our process apply to polyester cloth; however, nylon and metal cloth will give essentially similar results. Mesh count is the number of threads per inch in the cloth. The Theoretical Ink Volume is the volume of ink in all mesh openings per unit area of substrate. This volume is the thickness of the ink deposit as if the ink were coating the substrate below the open cloth as a uniform, continuous layer. A high tension is maintained on the cloth to keep it from sagging in the screen. A higher mesh count cloth gives both higher print definition and lower theoretical ink volume, but the mesh opening and percent open area decrease. In general, the printed layers of light-emitting polymer lamp construction need to be as thin as possible which entails using higher mesh count screens with lower theoretical ink volume values [10]. In a typical single layer white OLED fabrication by screen printing method ITO (Indium Tin Oxide) glasses are ultrasonically cleaned, followed by rinsing with deionized water, trichloroethylene, acetone and methanol. The cleaned ITO glasses are patterned via a standard micro lithographic process. HCl (37%, Aldrich) is used as the etchant for the ITO. For the surface treatment of the ITO, the patterned ITO glasses were treated by oxygen plasma for some minutes as RMS roughness is lower in plasma treated ITO than bare ITO glasses. The pinholes are also reduced due to plasma treatment. For white OLED, DPVBi(4,4-bis(2,2-diphenylvinyl)1,1biphenyl, 99.95% purity, Gracel), -NPD (N,N-diphenylN,N-bis(1-naphthyl)-1,1 biphenyl-4,4 diamine, 99.95% purity, Gracel) and rebrene(99.96% purity, Gracel) are dissolved in a previously prepared solution of polystyrene in chlorobenzene. The solution is then screen printed using mask. Then LiF and Al layer is deposited to form OLED device [11]. 1) Advantages: Screen printing has been known to be as versatile, simple, fast, cost-effective coating techniques. It does not require expensive vacuum technology, as is the case with physical vapor deposition. It can also be applied to versatile surface shapes and sizes. 2) Limitations: It has been believed that screen printing is not suitable for deposition of thin films with less than 100 nm thickness. But Pardo et al. [12] demonstrated the use of screen printing in the deposition of an organic active layer having a thickness of several tens of nanometers which opened a new possibility that the screen printing process could be utilized for fabricating the OLEDs and it has already been done successfully by many.

Fig. 3.

Inkjet Mechanism

C. Inkjet printing Ink jet printing is another way to deposit the organic layers, especially organic polymers. In this method we can use simply an inkjet printer. Organic layers are sprayed onto substrates like ink sprayed on paper during printing. For example, there may be three ink cartridges and three nozzles enabling the printer to print three different colours simultaneously (Figure 3). As the printer head scans the page and the piezoelectric materials are pulsed, ink is squirted from the nozzles onto the page. The only modification to the ink-jet printer for printing OLEDs was to replace the ink cartridges with polymer solutions. Different colors are achieved with different layer materials. For example, if green is desired it is common to use the combination Mq3, where M is a Group III metal and q3 is 8-hydroxyquinolate. Blue is achieved by using Alq2OPh and red is done with perylene derivatives. Organic solutions used here are a solution of hole transport layer and emissive layer organic materials. When using polymers, ink-jet technology is commonly used. We can use an electron transport material layer for better device efficiency [13]. One of the advantages of inkjet printing is a non-contact deposition method by which contamination is minimized. It allows OLEDs to be printed on very large films at reduced cost. Besides fabrication on flexible substrates has become possible using this ink-jet printing. There exist problems with pinholes in an ink-jet printing layer. When using small molecule layers, we cannot use this method. We have to use evaporative technique [13]. D. In-line fabrication In-line fabrication is a mass process technique. Vertical in-line tool operates with continuous substrate flow. Linear sources of depositing organic and metallic materials are used in this process. In this process in-line sources are used where material is deposited from a linear tube (as opposed to the point sources that are more commonly used in OLED manufacture), It improves material usage by a factor of 10. Cheaper mass production technique and excellent thickness homogeneity can be achieved by this process. Deposition

stability is excellent in this method. Complicated stack structures can be implemented using in-line fabrication. Deposition rate and throughput are high. This process can handle large substrate [2]. E. Roll to roll process Roll to Roll (R2R) processing could revolutionize the fabrication of OLED flexible flat panel displays. The prerequisite of this method is flexible substrate, so that the substrate can be rolled. We can divide this process into three parts [2] • Deposition • Patterning (soft lithography, inkjet printing) • Packaging VI. C OMPARISON OF DIFFERENT FABRICATION PROCESSES Among all above discussed fabrication techniques of OLED, it is not an easy task to evaluate which fabrication technique is the best. Actually, the choice of a fabrication technique of OLED depends on some factors like growth quality requirement, amount of production, materials used in the fabrication etc.The most primitive fabrication process is the costly physical vapour deposition technique. Screen printing can be an alternative of this technique. It is cost effective and versatille fabrication process. But thin film less than 100nm is difficult to grow by screen printing. The non-contact deposition method, ink-jet printing is a fabrication process for contamination free growth on flexible substrate. For mass production of OLED, in-line fabrication is definitely the most suitable process. But in near future, roll-to-roll process will become the most compact fabrication technique especially for flexible substrate. VII. C ONCLUSION Recently OLEDs are winning over LEDs for various advantages like, low power consumption, low diving voltage, wide viewing angle, high contrast and high color gamut. Besides these, the most advantageous part of OLED is that it can be made thin and flexible. So it is well used in many portable devices and roll up gazettes. The fabrication methodologies of organic-light emitting diodes have been discussed. It is the ease of fabrication and introduction of established manufacturing technologies, not associated previously with the electronics industry that has gained OLED its notable scientific attention. In the coming decade we will come to be familiar with affordable flexible display and fabric technologies made possible by current and future diligent research. The true advantages will be realized when the individual advances are combined together in the same product. R EFERENCES [1] L. Hung and C. Chen, “Recent progress of molecular organic electroluminescent materials and devices,” Materials Science and Engineering: R: Reports, vol. 39, no. 5, pp. 143–222, 2002. [2] M. M. Salleh, T. Hasnan, T. Azis, S. Sepeai, and M. Yahaya, “Fabrication of organic light emitting diodes (oleds) for flat panel displays,” Berkala Ilmiah MIPA, vol. 17, no. 3, 2008. [3] B. Geffroy, P. le Roy, and C. Prat, “Organic light-emitting diode (oled) technology: materials, devices and display technologies,” Polymer International, vol. 55, no. 6, pp. 572–582, 2006.

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