Encapsulation issues in organic light emitting diodes

Chapter Introduction 1.1 Introduction to organic light-emitting diode (OLED) 1.1.1 OLED device Since Tang and Van Slyke first reported high efficiency bi-layer OLED in 1987 [1], much effort has been made to improve their performance The emerging organic lightemitting diode technology holds considerable promise for the smooth panel display industry This is why a number of research institutes and commercial companies have formed collaborations to accelerate the development of suitable mass-production processes and equipment for OLED devices In particular, conjugated polymer OLED technology has attracted much of the interest recently due to its low cost of processing The key to OLED technology is the development of organic semiconductor materials，which is also a crucial starting point for so-called "plastic electronics" During the 1980s, Kodak and UK-based start-up Cambridge Display Technology (CDT) developed displays that formed luminescent images when electric currents were passed through thin layers of organic materials based on small organic molecules and conjugated polymers to generate light In effect, each pixel in the display behaved in the same way as a miniature light emitting diode (LED) 1.1.2 Advantages of OLED over LCD display technology An OLED is an electronic device made by placing a series of organic thin films between two conductors When an electrical current is applied, light is emitted by a process called electroluminescence as electrons and holes injected from the cathode and anode respectively, recombine in the organic light-emitting layer hν Figure 1.1 OLED structure showing injection of electrons from the cathode and holes from the ITO anode, and their recombination in the electroluminescent (EL) layer OLED displays are lightweight, durable, power efficient and ideal for portable applications OLED display fabrication has fewer process steps and also uses lower-cost materials compared to liquid crystal display (LCD) It is believed that OLEDs can replace the current LCD technology in many display applications due to the following performance advantages over LCD: • Greater brightness • Faster response time for full motion video • Wider viewing angles • Lighter weight • Higher power efficiency • Broader operating temperature ranges • Greater cost-effectiveness 1.1.3 Lifetime challenges The lifetime of a display is the number of hours that the display is functional [2] It can be classified into storage lifetime and operational lifetime Storage lifetime denotes how long it would be possible to store the display in the absence of current Operational lifetime is usually defined as the time for the electroluminescence of the OLED to degrade to half its initial brightness (typically 100 cd/m2) Although some of the properties of OLED match, and in some cases, surpass those of current LED’s, the fact remains, however, that the lifetime in these devices needs to be further improved for long-term consumer applications [3] Currently, the only products having displays based on OLEDs are “short-term” commodity items such as cell phones, car stereo systems, and digital camera displays Most recently, Seiko Epson has reported a 40-inch TV screen prototype made from conjugated polymer OLED Considerable research had been done to identify the causes of degradation, which shortened the lifetime of the OLEDs [2] The most prominent morphological change observed in some degrading devices was the de-lamination of the cathode material [4] The de-lamination appeared in electroluminescence micrographs as non-emissive spots These non-emissive areas were sometimes referred to as ‘‘dark spots’’ or ‘‘black spots’’ [5] (although they not look ‘‘black’’ under external illumination, see below) The nonemissive spots were attributed initially to local heating caused by short circuits which led to the formation of pinholes and local ablation [6] or local fusion of the metallic cathode [7] Recent studies [8] revealed that the non-emissive spots had domelike structures termed ‘‘bubbles’’ filled with gases (mostly oxygen) presumably evolved in the course of electrochemical and photo-electrochemical processes in the presence of water vapor [9, 10] According to some recent reports [5, 11], the bubbles originated from pinholes in the metallic electrode when the device was powered up in the presence of atmospheric humidity In a recent report on state-of-the-art devices, the hole-injecting conducting polymer layer was found to be locally doped in the black spot, which pointed to an electrochemical degradation process driven by electrical current in the presence of moisture [12] 1.2 OLED device fabrication process 1.2.1 OLED materials OLED materials for displays can be classified into two main types, each of which has its own distinct fabrication process and unique set of advantages and limitations 1) Small organic molecules This approach involves vacuum batch deposition of small organic molecule material layers onto a glass or silicon backplane The technology is well proven but currently only suits mass-production of small or medium-sized displays up to about 15 inches in diameter The fabrication of larger displays is hindered by the shadow mask that is used to define the micron sized pixels of the display 2) Conjugated polymers The advantage of conjugated polymer light-emitting display (PLED) materials is their solubility in organic solvents, allowing them to be deposited onto a glass or flexible plastic substrate using spin-coating or ink jet printing Conjugated polymer technology enables the fabrication of larger displays as compared to small-molecule OLED, as there is no need for shadow masking or vacuum deposition processing PLED displays can also be operated at lower voltages and are more powerefficient than those based on small organic molecules 1.2.2 Display fabrication OLED displays can be made using four simple steps: 1) Substrate preparation The first step is the preparation of the substrate, also known as the backplane Indium tin oxide (ITO) coated glass is the substrate used for passive matrix display fabrication ITO sputter deposition is carried out in a vacuum chamber and film patterning in a class 100 yellow room 2) OLED fabrication The next step is the fabrication of the OLED part of the display This step involves the spin-coating or printing, deposition of the hole transport layer (HTL), electroluminescence (EL) layer and electron transportation layer (ETL) Finally, the cathode electrode is deposited by a thermal evaporation process 3) Encapsulation To protect the OLEDs from being exposed to water vapor and oxygen, the display is hermetically sealed in a protective package This is essential to maximize the display's performance and lifetime 4) Assembly Finally, the driving circuits that drive each pixel are wired up to the display An example of such a display fabricated in IMRE in our work using a blue-emitting polymer is shown in Fig 1.2 Figure 1.2 IMRE matrix blue OLED display (100 × 32 pixels) 1.3 Objective of the thesis OLED technology offers the prospect of realizing flexible displays on plastic substrates using roll-to-roll manufacturing processes One of the biggest challenges to the OLED display industry is not competition from the incumbent LCD industry but from water vapor and oxygen The materials used in small organic molecule and conjugated polymer OLED are vulnerable to degradation by oxygen and water vapor, which can trigger early failure Sealing of the OLEDs from atmospheric oxygen and water vapor is typically accomplished with a glass or metal lid attached to the display substrate using a bead of ultraviolet (UV) cured epoxy Such an encapsulation technique is not viable for a flexible display, since low-moisture permeability epoxies are rigid The obvious solution, a plastic or thin film encapsulation is non trivial Plastics are permeable materials, often with holes many microns in diameter that allow water vapor and oxygen to permeate through The barrier specification required for OLED displays is unclear, since the mechanism of long-term degradation is still a subject of debate [13] However, it is clear that a stable OLEDs requires a moisture barrier which transmits < 10-5 g/m2/day of water vapor [14] All conventional vapor deposited barrier films are therefore inadequate for OLED applications by several orders of magnitude Normally an organic and inorganic composite structure is used in OLED encapsulation The quality of encapsulation depends on the materials, surface topography and the adhesion between the organic and inorganic layers This thesis focuses on issues in encapsulating organic light-emitting diodes with thin films Other than the quality of the encapsulation film, the surface roughness is also a factor limiting the stability and the efficiency of the OLEDs [13] Reducing the surface roughness of the ITO reduces the area of adsorption sites for water vapor and thereby extends the lifetime of the OLED device Better encapsulation could also be obtained with reduced surface roughness Reactive ion etch (RIE) planarization of the indium tin oxide (ITO) substrate, and associated changes in electrical, optical properties and morphology in the device are thus investigated in this thesis Understanding the relationship between the surface roughness and lifetime of the OLED devices, allows improvements in the manufacturing process to be made to achieve better stability of the organic layers and the cathode With improvements in encapsulation and efficiency of OLED devices, mass commercial OLED production would be possible 1.4 Organization of the thesis Chapter reviews current research interest in lifetime and encapsulation of OLED Oxygen and water vapor are the two main known agents leading to degradation of OLED devices Chapter describes the experimental techniques we used to fabricate the OLED devices, measure the ITO surface parameters after RIE treatment and evaluate the various fabrication processes relating to encapsulation of OLEDs Chapter investigates the planarization effect of ITO surface using RIE Reducing the surface roughness improves OLED electrical performance and also reduces the water vapor permeation through the interface between ITO and the encapsulation film Chapter describes laser ablation as a method to realize 300-micron sized pixels This method avoids the formation of photo-resist mushroom structures which traps water vapor The micron sized spikes along the cathode runners formed during laser ablation however poses a challenge to thin film encapsulation Some methods to address these post ablation spikes are discussed in this chapter Chapter describes the use of multi-layer encapsulation to improve the barrier properties to oxygen and water vapor Subsequently the barrier film was used to encapsulate conjugated polymer and small organic molecule OLED devices to investigate the effect of barrier film encapsulation on device lifetime Chapter summarizes the thesis and briefly describes possible future work 10 ... adhesion between the organic and inorganic layers This thesis focuses on issues in encapsulating organic light- emitting diodes with thin films Other than the quality of the encapsulation film,... polymer light- emitting display (PLED) materials is their solubility in organic solvents, allowing them to be deposited onto a glass or flexible plastic substrate using spin-coating or ink jet printing... recombine in the organic light- emitting layer hν Figure 1.1 OLED structure showing injection of electrons from the cathode and holes from the ITO anode, and their recombination in the electroluminescent