Encapsulation Of Organic Light Emitting Diodes Research Articles

39.1: Invited Paper: Study on Filler Film Degradation Mechanism in OLED Encapsulation

Dam & Fill is the main encapsulation method for large-sized top-emitting Organic light-emitting diodes (OLEDs) displays, in which light transmits through filler encapsulation film. Transparency of the filler film is a critical factor in obtaining a high efficiency and high reliability OLED display. In this paper, we study the degradation mechanism of filler film in OLED encapsulation. The results reveal that Ultraviolet (UV) light below 325 nm degrades filler film and reduces the film transmittance, CF film and low temperature effectively mitigate UV light damage to filler film, and high temperature (100 ℃) has no effect on filler film transparency.

Evaluation of encapsulation strategies for solution-processed flexible organic light-emitting diodes

Owing to the sensitivity of organic materials to ambient conditions, encapsulation of organic light-emitting diodes (OLEDs) is critical. In this work, Al2O3 film, deposited by atomic layer deposition (ALD) technique, on flexible polyethylene terephthalate (PET) substrate is used as a barrier film. Al2O3 is also deposited directly on OLED as thin-film encapsulation. Electrical Ca tests are used to measure water vapor transfer rate (WVTR) values, and an optimum WVTR value of 9 *10−4 g/m2/day using 43 nm Al2O3 film has been obtained. The related results on five different encapsulation structures for solution-processed OLEDs: (i) glass to glass encapsulation (G2G) as a reference, (ii) glass to barrier film encapsulation (G2B), (iii) glass to direct thin-film encapsulation (G2D), (iv) barrier to barrier film encapsulation (B2B), and (v) barrier to direct thin-film encapsulation (B2D), are evaluated. The operating half-lifetime values of OLEDs with G2B- and G2D-encapsulation are 54% and 82% of the half-lifetime with G2G encapsulation, respectively. The lifetime of flexible OLED devices with B2D encapsulation is also 34% more than that with B2B encapsulation. Based on the bending stress analysis, the estimated critical bending radius for B2B and B2D encapsulation of flexible OLEDs are 16.83 and 7.07 mm, respectively.

Process Parameter Optimization of a Polymer Derived Ceramic Coatings for Producing Ultra-High Gas Barrier.

Silica is one of the most efficient gas barrier materials, and hence is widely used as an encapsulating material for electronic devices. In general, the processing of silica is carried out at high temperatures, i.e., around 1000 °C. Recently, processing of silica has been carried out from a polymer called Perhydropolysilazane (PHPS). The PHPS reacts with environmental moisture or oxygen and yields pure silica. This material has attracted many researchers and has been widely used in many applications such as encapsulation of organic light-emitting diodes (OLED) displays, semiconductor industries, and organic solar cells. In this paper, we have demonstrated the process optimization of the conversion of the PHPS into silica in terms of curing methods as well as curing the environment. Various curing methods including exposure to dry heat, damp heat, deep UV, and their combination under different environments were used to cure PHPS. FTIR analysis suggested that the quickest conversion method is the irradiation of PHPS with deep UV and simultaneous heating at 100 °C. Curing with this method yields a water permeation rate of 10−3 g/(m2⋅day) and oxygen permeation rate of less than 10−1 cm3/(m2·day·bar). Rapid curing at low-temperature processing along with barrier properties makes PHPS an ideal encapsulating material for organic solar cell devices and a variety of similar applications.

Emerging Biomedical Applications of Organic Light‐Emitting Diodes

AbstractAs solid‐state light sources based on amorphous organic semiconductors, organic light‐emitting diodes (OLEDs) are widely used in modern smartphone displays and TVs. Due to the dramatic improvements in stability, efficiency, and brightness achieved over the last three decades, OLEDs have also become attractive light sources for compact and “imperceptible” biomedical devices that use light to probe, image, manipulate, or treat biological matter. The inherent mechanical flexibility of OLEDs and their compatibility with a wide range of substrates and geometries are of particular benefit in this context. Here, recent progress in the development and use of OLEDs for biomedical applications is reviewed. The specific requirements that this poses are described and compared to the current state of the art, in particular in terms of the brightness, patterning, stability, and encapsulation of OLEDs. Examples from several main areas are then discussed in some detail: on‐chip sensing and integration with microfluidics, wearable devices for optical monitoring, therapeutic devices, and the emerging use in neuroscience for targeted photostimulation via optogenetics. The review closes with a brief outlook on future avenues to scale the manufacturing of OLED‐based devices for biomedical use.

Optical Transmittance Improvements of Al2O3/TiO2 Multilayer OLED Encapsulation Films Processed by Atomic Layer Deposition

Encapsulation of organic light-emitting diodes (OLEDs) is the only way to prevent degradation due to the penetration of moisture and oxygen. However, the upper encapsulation layer, located in the path through which light from the OLED passes, adversely affects light transmission. In this study, we calculated the optical transmittance by using admittance trajectory simulation and compared the data with actual Al2O3/TiO2 multilayer film processed by atomic layer deposition (ALD). By applying the simulated data on to actual encapsulation layer, we can observe the optical transmittance increase of up to 8% compared to the film of Al2O3/TiO2/Al2O3 structure. Over 90% of optical transmittance was achieved for all the visible wavelength ranges while maintaining water vapor transmission rate (WVTR) of the processed multilayer thin film as low as of 5 × 10−5 g/m2/day, which was measured using MOCON Aquatran 2.

Highly-stable PEN as a gas-barrier substrate for flexible displays via atomic layer infiltration.

Polymer substrates with superior barrier properties are of great importance for the development of highly-stable flexible displays. The atomic layer infiltration (ALI) method has been utilized to integrate nanoscale inorganic materials in the subsurface of commercial PEN substrates, and the in-suit quartz crystal microbalance (QCM) is employed to study the growth behaviour as the process parameters vary, in which the nucleation and infiltration stages have been demonstrated. The O2 plasma pre-treatment prior to Al2O3 infiltration was used to determine its effect on the water vapor transmission rate (WVTR), and significantly improved barrier properties were observed compared to those of the ones without the O2 plasma pre-treatment via the electrical Ca tests, which was attributed to the surface clean and improved film adhesion. The lowest WVTR value measured was 1.28 × 10-5 g m-2 day-1 for the O2 plasma pre-treated PEN substrate coated with 100 ALI cycles, which improved 3-4 orders of magnitude compared to that of the pristine ones. Besides, the infiltrated PEN substrate with O2 plasma pre-treatment exhibited good mechanical stability, with only a slight increase of the WVTR value which was observed after the bending fatigue test with a radius of 5 mm. Furthermore, when applied to the encapsulation of organic light-emitting diodes (OLEDs), the normalized luminance remained above 94% after storage for 800 hours.

Thin-film encapsulation of Al2O3 multidensity layer structure prepared by spatial atomic layer deposition

Aluminum oxide films deposited by spatial atomic layer deposition have been used for thin-film encapsulation of organic light-emitting diodes. In this study, a multidensity layer structure consisting of two Al2O3 layers of different densities was deposited at varying ozone flow rates. The structure improved moisture permeation barrier characteristics, as confirmed by the water vapor transmission rate (WVTR) measurement. The lowest WVTR of the multidensity layer structure was 5.3 × 10−5 g m−2 day−1, which is two orders of magnitude lower than that of a reference single-density Al2O3 layer. This improvement can be attributed to the location mismatch of paths in the film for atmospheric gases, such as oxygen and water, due to different layer densities. High-resolution transmission electron microscopy and x-ray photoelectron spectroscopy confirmed the multidensity layer structure’s superior characteristics as an encapsulation layer due to a location mismatch of the paths for oxidative species between the two layers.

Optimization of Al2O3/TiO2 nanolaminate thin films prepared with different oxide ratios, for use in organic light-emitting diode encapsulation, via plasma-enhanced atomic layer deposition.

Encapsulation is essential for protecting the air-sensitive components of organic light-emitting diodes (OLEDs), such as the active layers and cathode electrodes. Thin film encapsulation approaches based on an oxide layer are suitable for flexible electronics, including OLEDs, because they provide mechanical flexibility, the layers are thin, and they are easy to prepare. This study examined the effects of the oxide ratio on the water permeation barrier properties of Al2O3/TiO2 nanolaminate films prepared by plasma-enhanced atomic layer deposition. We found that the Al2O3/TiO2 nanolaminate film exhibited optimal properties for a 1 : 1 atomic ratio of Al2O3/TiO2 with the lowest water vapor transmission rate of 9.16 × 10(-5) g m(-2) day(-1) at 60 °C and 90% RH. OLED devices that incorporated Al2O3/TiO2 nanolaminate films prepared with a 1 : 1 atomic ratio showed the longest shelf-life, in excess of 2000 hours under 60 °C and 90% RH conditions, without forming dark spots or displaying edge shrinkage.

A quality photocatalytic activity of an amorphous titanium oxide film achieved using the selectively photochemical etching and its application for the organic light emitting diode encapsulation

In this study, a rapid, cost-effective, and room-temperature achieved selectively photochemical etching (SPCE) process was developed to improve the photocatalytic activity of a PECVD-deposited amorphous titanium oxide (a-TiOx) film. The a-TiOx film treated by the SPCE process showed a photocatalytic activity comparable to that of a high-temperature prepared TiOx film. As examined from the surface roughness and morphologies, optical transmittance, and chemical bond configurations, the mechanism responsible for the significant enhancement on the photocatalytic activity was ascribed to the formation of the nano-textures with the increase in the surface acidity due to the incorporating of fluorine ions. The SPCE-treated a-TiOx film with the quality photocatalytic activity was then applied to encapsulate the blue organic light emitting diode (OLED) to result in the surface with self-cleaning function. A super-hydrophilic surface was available using the internal light irradiation. In addition, the OLED encapsulated by the SPCE-treated a-TiOx film also was beneficial for enhancing the light-extraction efficiency as a consequence of its notably nano-textured surface.

Thin film encapsulation for organic light-emitting diodes using inorganic/organic hybrid layers by atomic layer deposition.

A hybrid nanolaminates consisting of Al2O3/ZrO2/alucone (aluminum alkoxides with carbon-containing backbones) grown by atomic layer deposition (ALD) were reported for an encapsulation of organic light-emitting diodes (OLEDs). The electrical Ca test in this study was designed to measure the water vapor transmission rate (WVTR) of nanolaminates. We found that moisture barrier performance was improved with the increasing of the number of dyads (Al2O3/ZrO2/alucone) and the WVTR reached 8.5 × 10−5 g/m2/day at 25°C, relative humidity (RH) 85%. The half lifetime of a green OLED with the initial luminance of 1,500 cd/m2 reached 350 h using three pairs of the Al2O3 (15 nm)/ZrO2 (15 nm)/alucone (80 nm) as encapsulation layers.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-0857-8) contains supplementary material, which is available to authorized users.

Flexible Encapsulation of Organic Light-Emitting Diode Using the LiF/polymer/LiF Buffer and Metal Foil

New simple and reliable encapsulation using the inorganic/organic/inorganic buffer and metal foil was studied on a red phosphorescent organic light-emitting diode (PhOLED) for flexible electronics. The LiF/polymer(AZ1512-photoresist)/LiF and Cu foil were used as the materials of the inorganic/organic/inorganic layers and a metal cover in the configuration of encapsulation, respectively. To make the process simple in the fabrication of red PhOLED, bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2) was used in common as a host material of emissive layer and an electron transporting material of electron transport layer.The fabricated device before encapsulation showed a maximum quantum efficiency of 15.3%, color coordinates of (0.67, 0.32) and a luminance of 5,100 cd/m2 at 8V. Luminance change was little found in the process of flexible encapsulation. Luminance degradation of the device encapsulated with the buffer layers of LiF/AZ1512-photoresist/LiF and Cu foil according to the passage of time was very stable during a long term after one week and comparable to that of the device encapsulated with a conventional glass lid.

Optimization of Al2O3 Films Deposited by ALD at Low Temperatures for OLED Encapsulation

The properties of Al2O3 encapsulation layers prepared by atomic layer deposition were investigated as a function of film thickness. By using a short pumping gas time (PGT) and a low viscosity gas, high-quality films for the encapsulation of organic light-emitting diodes were obtained. The encapsulation properties could be improved by decreasing the PGT and using a low viscosity gas as the precursor at a low temperature. The water vapor transmission rate of the film could reach up to 4 × 10–3g/m2/day. In addition, atomic force microscopy images and investigations of the refractive index showed that the film properties did not change substantially with increasing thickness. Furthermore, by optimizing the PGT, the total deposition time could be effectively reduced.

Separation Characteristics of an LMPA-Polymer Mixture for the Encapsulation of Organic Light-Emitting Diode

An encapsulation method using the separation of low melting point alloy (LMPA) and a polymer from an LMPA-polymer mixture is proposed. Sn-Bi alloy and epoxy were used for the mixture. Separation was conducted using a two-step heat treatment of 160 °C for 5 min and 180 °C for 5 min. We found that the flux contained in the Sn-Bi solder played an important role in the separation process. A Cu sacrificial layer was introduced using the sputtering method under the Sn-Bi layer with thickness variations of 1500, 3000, and 6000 A. Using the 6000-A-thick Cu layer, the LMPA formed a well-separated and continuous sealing line. This double-line structure shows good potential for use as a sealing line for organic light-emitting diode encapsulation.

Extremely bendable thin-film encapsulation of organic light-emitting diodes

We report on an extremely bendable moisture barrier for the thin-film encapsulation of organic light-emitting diodes (OLEDs). Hybrid barriers with various dyads of alternating aluminum oxide (Al2O3) and plasma-polymerized layers, which are utilizable for the thin-film encapsulation of flexible OLEDs, were prepared by atomic layer deposition and plasma chemical vapor deposition, respectively. When the total thickness of Al2O3 was fixed at 20 nm, an ultimate 200-dyad multilayer barrier showed change of less than 20% in water vapor transmission rate from its initial value of the order of 10−4 g/m2/day, even after 10 000 times of bending with a bending radius of 5 mm.

A study of ultraviolet-curable organic/inorganic hybrid nanocomposites and their encapsulating applications for organic light-emitting diodes

UV-curable organic/inorganic hybrid nanocomposites have been successfully synthesized with polymer monomers (tetramethoxysilane (TMOS)/3-glycidoxypropyl-trimethoxysilane (GPTMS)) nano-fillers (silica/alumina), curing agents (bisphenol A (BPA)), and photoinitiators (triaryl sulfonium hexafluoroantimonate (TSH)) by in situ polymerization. Their thermal stability, coefficient of thermal expansion (CTE), adhesion strength, and gas barrier capability have also been investigated with thermogravimetry analysis (TGA), thermomechanical analysis (TMA), micro-computer universal testing machine and moisture penetration tests. Experimental results indicate that nano-fillers are completely homogeneously dispersed in the polymer matrices and the increase of nano-fillers in the nanocomposites causes the raise of decomposition temperature ( T d) as well as gas resistance and the reduction of CTE as well as adhesion strength. Because lab-made organic/inorganic hybrid nanocomposites have been discovered to exhibit excellent gas barrier properties, we have also applied them for the encapsulation of organic light-emitting diodes (OLEDs) and flexible OLEDs. With lab-made nanocomposite d, the lifetimes of OLEDs and flexible OLEDs can be successfully lengthened to 92 and 36 h, respectively, whereas those of OLEDs and flexible OLEDs without encapsulation are 11 and 7 h, respectively.

Thin Film Encapsulation of Light-Emitting Diodes withPhotopolymerized Polyacrylate and Silver Films

A thin film encapsulation of organic light-emitting diodes (OLEDs)is investigated with a multi-layer stack ofpolyacrylate-Ag-polyacrylate-Ag-polyacrylate-Ag-polyacrylate (PAPAPAP).It is shown that the fabrication of polyacrylate films by a wet processdoes not affect the electroluminescent (EL) characteristics of thedevices and polyacrylate films together with the silver layers canperform to minimize oxygen and water diffusion into the organiclight-emitting device. The structure ofpolyacrylate(20 μm)-Ag(200 nm)-polyacrylate(20 μm)-Ag(200 nm)-polyacrylate(20 μm)-Ag(200 nm)-polyacrylate(20 μm)is demonstrated to enhance dramatically the lifetime of OLEDs.

Cellulose acetate and PVDC used as protective layers for organic diodes

In order to increase the lifetime of organic light emitting diodes (OLEDs), it is necessary to prevent the emitter from degradation induced by oxygen contamination from the contact with ambient air. Encapsulation of OLEDs by sealed glass is efficient but not adapted for flexible devices. In this study, we have investigated the behaviour of two polymeric materials used as a protecting layer: cellulose acetate (CA) and polyvinylidene chloride (PVDC). These materials were deposited onto active polymer layers such as poly( p-phenylene vinylene) (PPV) and poly(2-methoxy-5-(2′ethyl-hexyloxy) p-phenylene vinylene) (MEH-PPV). The optical properties (UV-Vis, Raman, IR) of the luminescent polymers were studied as a function of time, up to 21 or 28 days, and compared to those of bare films. The results clearly show the efficiency of the protecting layers. For PPV thin films, both protecting materials delay the effect of air contact, the better efficiency was obtained using CA films. In contrast, MEH-PPV is less sensitive to oxidation than PPV and no clear conclusion could be drawn on the efficiency of the encapsulating layers.