OLED Classic Training Materials

 

　　Introduction to OLED Display Technology

 

　　OLED (Organic Light-Emitting Diode, also known as organic electroluminescent display or organic light-emitting semiconductor) is the English abbreviation for organic light-emitting diode. It is a device that generates electroluminescence through a multilayer organic thin-film structure. Its ease of fabrication and low driving voltage make OLED particularly well-suited for flat-panel display applications. Compared with LCDs, OLED displays are lighter and thinner, offer higher brightness, lower power consumption, faster response times, superior image clarity, greater flexibility, and higher luminous efficiency.

 

 

　　Similar to LCD (Liquid Crystal Display), OLED driver backplanes are also categorized into two types: active-matrix OLED (AMOLED) and passive-matrix OLED (PMOLED). Among them, PMOLED’s driving method is relatively outdated, requiring a full-panel scan; as the panel area increases, the refresh rate drops and the drive current decreases, making it difficult to achieve high resolution, large screen sizes, and high brightness. Consequently, PMOLED is mainly used in lower-end, small-screen applications. In contrast, AMOLED is currently the mainstream technology, employing LTPS-TFT (Low Temperature Poly-Si Thin Film Transistor) to precisely control and drive each pixel. This driving technology is consistent with the TFT-LCD technology widely used in today’s market.

 

 

　　Compared with LCD, the most distinctive feature of OLED is its self-emissive nature, which eliminates the need for a backlight. This characteristic confers numerous advantages: superior color-gamut control and wider viewing-angle performance; significantly higher luminous efficiency, faster response times, higher contrast ratios, and lower power consumption due to the absence of polarization requirements for the light path; the elimination of the backlight effectively reduces weight and enables thinner form factors; moreover, current technologies allow circuitry to be deposited directly onto flexible substrates, rendering the entire OLED display flexible—something LCD cannot achieve. These performance benefits align with many emerging consumer demands, making OLED a rapidly growing next-generation display technology.

 

　　The three color emission mechanisms of OLED displays:

 

 

　　OLED (Organic Light-Emitting Diode), also known as organic electroluminescent display or organic electroluminescence display, is a current-driven organic light-emitting device that emits light through the injection and recombination of charge carriers. The luminous intensity is directly proportional to the injected current. Under the influence of an electric field, holes generated at the anode and electrons generated at the cathode migrate toward the hole-transport layer and the electron-transport layer, respectively, and then further into the emissive layer. When these charge carriers meet in the emissive layer, they form excitons, which excite the luminescent molecules and ultimately produce visible light.

 

　　OLED is a device that generates electroluminescence through a multilayer organic thin-film structure. It is easy to fabricate and requires only a low driving voltage, key characteristics that make OLED particularly well suited for flat-panel display applications. Compared with LCDs, OLED displays are lighter and thinner, offer higher brightness, lower power consumption, faster response times, superior image clarity, greater flexibility, and higher luminous efficiency, thereby meeting consumers’ evolving demands for display technology. An increasing number of display manufacturers worldwide are investing in R&D, significantly accelerating the industrialization of OLED technology. So, as a critical material for OLED displays—OLED organic light-emitting materials—what domestic companies are capable of producing them, how are their products performing in the market, and what is the current state of these companies?

 

　　Currently, AMOLED technology on the market is mainly divided into two types, differing in their color-generation methods. LG and other manufacturers primarily use WOLED (White-OLED) technology, in which the emissive layer emits white light that is then filtered through color filters to produce different colors; this approach is currently employed mainly for medium- and large-sized panels, such as TV displays. In contrast, other manufacturers—including Samsung and domestic panel makers—largely adopt RGB-OLED (Red-Green-Blue OLED) technology, which uses FMM (Fine Metal Mask) precision metal masks for vapor deposition, enabling each pixel to emit one of the three primary colors—red, green, or blue—and then combining these colors through additive color mixing to produce other hues. This technology is currently used predominantly for small- and medium-sized panels, such as those used in smartphones.

 

　　Each of the two approaches has its own advantages and disadvantages. WOLED, because it incorporates an additional color-filter layer, exhibits lower brightness, higher power consumption, and a narrower color gamut that is constrained by the filter, making it less wide than RGB-OLED. The primary drawback of RGB-OLED, on the other hand, lies in the differing lifespans of the three distinct color materials, which can lead to color shift over prolonged use, severely impacting user experience and resulting in a relatively short overall lifespan.

 

 

　　The absence of a backlight allows OLED panels to have a relatively simpler structure with fewer layers.

 

　　OLED mass-production process

 

　　The OLED manufacturing process is primarily divided into three stages: the backplane stage, the frontplane stage, and the module stage.

 

 

　　The backplane process involves repeatedly depositing thin films, exposing them to light, and etching on glass or flexible substrates to form an LTPS (low-temperature polycrystalline silicon) driving circuit, which provides signal and power inputs to the emissive devices.

 

　　In the front-panel fabrication process, organic light-emitting materials are precisely deposited onto the backplane using vapor-deposition equipment and a high-precision metal mask, then integrated with the driving circuitry to form the light-emitting device, which is subsequently encapsulated in a water- and oxygen-free environment.

 

 

　　After vapor deposition is completed, the module assembly process entails cutting the fully encapsulated panel to the final product dimensions, followed by panel testing, polarizer lamination, IC bonding, touch-panel integration, cover-glass attachment, and final module testing—essentially combining the pixel-array panel, once cut to the appropriate size for downstream applications, with other component materials.

 

 

　　OLED Vapor Deposition Technology (Excerpt from IHS)

 

　　Whether it is an RGB color-separation AMOLED display for small- and medium-sized applications or a WOLED display for large-size applications, the fabrication process remains vacuum vapor deposition. Consequently, in these devices, functional layers such as HIL, HTL, EML (for red, green, and blue colors), ETL, EIL, the cathode, and the charge-generation layer are still deposited continuously on the TFT substrate via vacuum vapor deposition.

 

　　Due to process integration requirements and to prevent cross-contamination, different functional layers must be deposited in separate evaporation chamber chambers, and after deposition is complete, robotic arms are used to transfer the substrates between these chambers.

 

 

　　The principle of vapor deposition can be essentially simplified as a process in which the material is heated to sublimate and then redeposited onto a cooler substrate.

 

　　The evaporation chamber is equipped with an evaporation source, a mask-aligning device, glass substrates, and cooling plates. During organic thin-film evaporation, the substrate bearing the TFTs must first be flipped over; subsequently, the mask is aligned with the substrate using the mask-aligning device; finally, the evaporation source is activated to deposit the film onto the substrate.

 

　　In the entire vapor deposition system, the deposition equipment, shadow masks, and evaporation sources are supplied by different equipment vendors. On small- and medium-sized AMOLED panel production lines, the most widely used deposition equipment is manufactured by Japan’s Cannon Tokki, while the primary suppliers of FMM are DNP and Darwin, among others.

 

　　Introduction to FMM Mask Templates

 

 

　　In the manufacturing process of organic light-emitting diode (OLED) panels, evaporation and chemical vapor deposition (CVD) processes require the use of masks to ensure the precise positioning of evaporated organic materials, metals, or CVD-deposited films on specific areas of the substrate.

 

　　What is a photomask? It is a structure in which various functional patterns are fabricated on a thin-film, plastic, or glass substrate and precisely aligned, for use in the selective exposure of photoresist coatings.

 

　　The mask is an Open Mask (Chinese: kāifàng yǎnmó bǎn) or a Fine Metal Mask (English: Fine Metal Mask; abbreviated as FMM).

 

 

　　Currently, a complete OLED vapor-deposition metal FMM mask consists of a mask frame, a metal mask substrate, support strips, and shading strips. The fabrication process involves first tensioning and securing the shading strips and support strips to the mask frame, followed by tensioning and fixing the mask substrate to the frame.

 

　　The遮挡条 functions to cover the gap between two adjacent mask substrates, while the支撑条 provides structural support for both the mask substrate and the遮挡条. However, in actual production, due to the setting of process parameters and limitations in fabrication accuracy, the遮挡条 can interfere with the normal geometry of the mask substrate, leading to defects such as color mixing and color shift during the vapor deposition process when the mask is used.

 

　　The FMM mask plate primarily consists of two components: the mask frame and the mask sheet. The mask frame serves as a supporting structure that secures and stabilizes the mask sheet, while the mask sheet is laser-welded to the frame. Over time, during repeated evaporation processes, the mask film may sustain damage due to various factors, compromising the precision of deposition and failing to meet the product’s accuracy requirements. Consequently, the mask sheet must be replaced. During replacement, the mask sheet is desoldered, leaving soldering residues on the mask frame. These residues must be ground down before a new mask sheet can be re-soldered. After several such re-soldering cycles, the entire mask frame requires overall grinding; after three or four rounds of grinding, the mask frame must be discarded.

 

 

　　Because the existing mask frame is designed as an integrated unit, any scrapped mask must be discarded in its entirety. Moreover, given the high cost of the mask frame, this results in a significant increase in the running costs associated with its use in production.

 

　　For example, based on current Mask Frame prices, an Invar Frame costs RMB 2 million per unit, while a SUS Frame costs RMB 200,000. For IK-level production capacity, each Mask Frame must undergo factory polishing once; for 4K-level capacity, one Mask Frame must be scrapped. When considering the entire production line, three Mask Frames need to be scrapped: one for organic vapor deposition, one for metal vapor deposition, and one for CVD. In other words, for every 4K of production capacity generated, a Mask Frame worth RMB 6 million must be written off, resulting in excessively high running costs.

 

 

　　Vacuum thermal evaporation

 

　　The vacuum level in conventional thermal evaporation typically ranges above 10⁻⁴ Pa; the higher the vacuum, the fewer defects in the resulting thin film and the higher the material purity. When organic materials are heated under vacuum, their behavior depends on their specific properties: some first liquefy and then vaporize, while others sublime directly. The vaporized species then depart from the material surface at a certain initial velocity, travel outward, reach the ITO substrate, cool, and deposit to form a thin film.

 

　　When the vacuum level falls below 10⁻⁴ Pa, the vacuum chamber becomes saturated with water molecules, oxygen molecules, and other impurity gases. During the evaporation process, these gas molecules collide with the organic small-molecule materials, severely degrading film quality and potentially leading to reduced device performance or even device failure. In the early stages of OLED research, a two-stage pumping system consisting of a mechanical pump and a molecular pump in tandem was typically employed to achieve and maintain high vacuum levels.

 

　　In recent years, the use of sputter ion pumps following molecular pumps has enabled the attainment of ultra-high vacuum for the fabrication of high-performance OLEDs. Two types of instruments are employed to measure chamber vacuum: thermocouple and resistance gauges, which are used for low-vacuum measurements below 0.1 Pa, and ionization gauges, which are used for high-vacuum measurements above 0.1 Pa. The thickness of functional layers is monitored using oscillating quartz crystals; the typical deposition rates are 0.5–2 Å/s for organic materials and 2–5 Å/s for metals, with layer thicknesses ranging from 80 to 100 nm.

 

 

　　New FMM Area-Source Technology

 

　　During SID Display Week 2018, OLEDON CEO Hwang Chang-Hoon unveiled a new planar-source FMM deposition technology capable of producing AMOLED panels with a pixel density of 2,250 ppi.

 

　　According to public reports, linear light sources currently used in mass production suffer from a limitation: due to a shadowing distance of 3 μm, achieving high resolution results in the overlap of adjacent patterns. However, Hwang states that by employing a planar light source, he has successfully reduced the image’s shadowing distance to 0.18 μm—the smallest shadowing distance ever reported.

 

　　Hwang developed the “X-mixing” technique, in which the host and dopant films are deposited separately on a metal substrate, after which the host and dopant molecules spontaneously diffuse through planar evaporation to form the emissive layer. “It is challenging to control the dopant composition using linear-source evaporation, as this method simultaneously evaporates both the host and dopant materials. However, because the X-mixing technique allows precise control over the composition, it can achieve a material utilization rate four times higher than that of conventional methods.”

 

　　OLED materials

 

 

　　An OLED is a double-injection light-emitting device in which, under the drive of an external voltage, electrons and holes injected from the electrodes recombine in the emissive layer to form bound electron–hole pairs, known as excitons. These excitons then radiatively decay, emitting photons and generating visible light.

 

 

　　The light-emitting process of OLEDs typically involves the following five basic stages:

 

 

 

　　Carrier injection: Under the influence of an external electric field, electrons and holes are injected from the cathode and anode, respectively, into the organic functional layer sandwiched between the electrodes.

 

　　Charge carrier transport: Injected electrons and holes migrate from the electron-transport layer and the hole-transport layer, respectively, toward the emissive layer.

 

　　Carrier recombination: After electrons and holes are injected into the emissive layer, they are bound together by Coulomb forces to form electron–hole pairs, known as excitons.

 

　　Exciton migration: Due to the imbalance in electron and hole transport, the primary exciton-generation region typically does not span the entire emissive layer, leading to diffusive migration driven by concentration gradients.

 

　　Exciton radiative de-excitation emits a photon: the exciton undergoes a radiative transition, emitting a photon and releasing energy.

 

　　An OLED device consists of n-type organic materials, p-type organic materials, a cathode metal, and an anode metal. Electrons (or holes) are injected from the cathode (or anode), transported through the n-type (or p-type) organic materials to the emissive layer—typically an n-type material—and then recombine to emit light.

 

　　Typically, in the fabrication of OLED devices, an ITO layer is first sputtered onto a glass substrate to serve as the anode. Subsequently, p-type and n-type organic materials, along with a low-work-function metal cathode, are sequentially deposited via vacuum thermal evaporation. However, organic materials are highly susceptible to reactions with moisture and oxygen, which can lead to the formation of dark spots and result in device failure to emit light. Therefore, once vacuum deposition is complete, the device must undergo encapsulation under an environment that is completely free of moisture and oxygen.

 

 

　　Between the cathode metal and the anode ITO, the commonly used device structure today is generally divided into five layers. As shown in the figure, starting from the ITO side and proceeding outward, the layers are: hole-injection layer, hole-transport layer, emissive layer, electron-transport layer, and electron-injection layer.

 

　　OLED materials primarily comprise emissive materials and base materials, which together account for approximately 30% of the material cost of an OLED display. Emissive materials for OLEDs mainly include red-host/guest materials, green-host/guest materials, and blue-host/guest materials, among others.

 

　　General-purpose OLED materials primarily include the electron-transport layer (ETL), electron-injection layer (EIL), hole-injection layer (HIL), hole-transport layer (HTL), hole-blocking layer (HBL), and electron-blocking layer (EBL). As device architectures are optimized, the variety of these materials continues to expand. Among them, organic emissive-layer materials and transport-layer materials are the key components of OLEDs.

 

 

　　The formation of the OLED emissive material layer involves three major steps: first, chemically synthesizing organic intermediates or crude monomers from raw chemical materials; second, sublimating these intermediates into OLED-grade monomers; and third, further synthesizing pre-sublimation or sublimation-grade materials, which are then deposited onto the substrate by panel manufacturers via vapor deposition to form the OLED organic emissive layer.

 

　　OLED packaging technology

 

　　Organic Light-Emitting Diodes (OLEDs) boast a fully solid-state structure, high brightness, wide viewing angles, fast response times, a broad operating temperature range, and the ability to enable flexible displays, making them one of the most competitive and promising next-generation display technologies today. The cathode materials and organic emissive layers used in OLED devices are highly sensitive to water vapor and oxygen; to achieve the design life of 10,000 hours, the permeation rates of water and oxygen must be kept below 5 × 10⁻⁶ g/m²·day and 10⁻³ g/m²·day, respectively. A water permeation rate of 10⁻⁶ g/m²·day means that, over a 24-hour period, only a single drop of water would be allowed to penetrate an area equivalent to six soccer fields (as shown in Figure 1). To effectively prevent water and oxygen from degrading OLED devices, researchers have developed a variety of encapsulation technologies, including getter-and-dispenser encapsulation, frit sealing, and TFE encapsulation.

 

 

　　Figure 1. Water Permeability Requirements for OLED Devices

 

　　1. Getter & Dispenser Packaging

 

　　To block water and oxygen, the earliest encapsulation approach involved applying a bead of sealant around the perimeter of the OLED device, then covering the device with a glass lid, and finally curing the sealant to complete the encapsulation. The corresponding structural diagram is shown in Figure 2.

 

 

　　Figure 2. Sealant Encapsulation Structure

 

　　Since sealants are predominantly organic materials, their water vapor permeability is limited to only 10⁻¹ g/m²·day, which is insufficient to meet product requirements. Therefore, a desiccant (getter) must be applied concurrently with the sealant coating. A desiccant, also known as an getter, primarily functions to absorb moisture within the sealed cavity after the sealant has been applied and the assembly is sealed. The main components of a getter are calcium oxide (CaO) and strontium oxide (SrO), and the chemical mechanism by which it absorbs water vapor is described by the following equation:

 

　　CaO + H2O → Ca(OH)2

 

　　SrO + H2O → Sr(OH)2

 

　　The encapsulation structure of the getter and dispenser is shown in Figure 3. Although a small amount of water can permeate the sealant, it will be absorbed by the getter within the encapsulation structure, thereby ensuring that the OLED device remains protected from degradation.

 

 

　　Figure 3. Packaging Structure of the Getter and Dispenser

 

　　The getter-and-dispenser encapsulation process is illustrated in Figure 4. Given that the desiccant has a thickness of 0.1–0.3 mm, recesses must be etched into the encapsulation glass to accommodate the desiccant (getter attach). Following desiccant attachment, a sealing bead is dispensed around the perimeter of the display using a dispenser (seal dispenser). Simultaneously, OLED devices are fabricated on the TFT substrate (EV). The completed OLED devices and the encapsulation glass are then assembled under controlled vacuum conditions, during which the sealing material bonds and cures the two glass substrates together. The sealing material fully encloses the periphery of the OLED device, thereby forming a hermetically sealed package structure.

 

 

　　Figure 4. Process Flow Diagram for Getter and Dispenser Packaging

 

　　Getter & Dispenser packaging has two drawbacks:

 

　　1. The desiccant has a certain thickness, necessitating the formation of grooves on the glass substrate, which increases the substrate’s thickness and thereby hinders the miniaturization and thinning of the product.

 

　　2. Desiccants are typically opaque materials; if applied on top of an OLED device, they would obstruct light transmission, making their use in top-emitting OLED devices highly challenging.

 

　　2. Frit Encapsulation

 

　　To overcome the drawbacks of Getter & Dispenser encapsulation, researchers have developed a material that can be coated directly onto glass and, upon curing, exhibits outstanding water- and oxygen-barrier properties: glass frit sealant. Glass frit sealant is a composite consisting of glass powder and solvents; the encapsulation method employing this material is known as frit encapsulation, with the encapsulation structure illustrated in Figure 5. In an OLED device, the top layer is the encapsulating glass, the bottom layer is the TFT substrate glass, and the periphery is sealed with glass frit sealant. Because the cured glass frit sealant achieves glass-like, excellent water- and oxygen-barrier performance, this encapsulation architecture is widely used in mobile phones, tablets, and other consumer electronics.

 

 

　　Figure 5. Frit Packaging Structure

 

　　The process flow for Frit Seal encapsulation is illustrated in Figure 6. First, a glass adhesive is applied to the encapsulation glass using either dispensing or screen printing. The encapsulation glass is then subjected to high-temperature baking (Frit Seal Bake) to volatilize the solvent in the adhesive, leaving behind only solid glass powder. Since solid glass powder lacks adhesion and cannot bond with the TFT glass, a peripheral ring of dummy UV adhesive is dispensed around the outer edge of the encapsulation glass using a dummy UV dispenser. During the assembly process, the TFT glass bearing the OLED device is aligned and bonded to the encapsulation glass, and the dummy UV adhesive is cured to ensure that the OLED device is completely surrounded by the glass adhesive. Subsequently, the solid glass powder must undergo laser sintering to remelt and re-solidify, thereby achieving water- and oxygen-barrier properties comparable to those of glass.

 

 

　　Figure 6. Flowchart of the Frit Packaging Process

 

　　The primary drawback of Frit encapsulation is that, after the glass powder melts and solidifies, its hardness and brittleness become comparable to those of glass, which inherently precludes its use in bent or curved components.

 

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　　TFE packaging

 

　　With the continuous advancement of OLED display technology, curved and foldable products are constantly being introduced, thereby progressively raising the requirements for OLED packaging technologies. Currently, the predominant packaging approach for curved mobile devices is TFE (Thin Film Encapsulation). TFE packaging originated from a multi-layer organic–inorganic thin-film encapsulation technology developed by the U.S. company Vitex Systems, also known as Barix encapsulation; its encapsulation structure is illustrated in Figure 7.

 

 

　　Figure 7. Barix Package Structure Diagram

 

　　The TFE encapsulation structure has been simplified based on the Barix encapsulation architecture. The typical TFE encapsulation structure used in mainstream products is shown in Figure 8: an inorganic water-barrier layer 1 is directly deposited over the OLED device; an organic planarization layer is then formed on top of this inorganic layer; a second inorganic water-barrier layer 2 is subsequently applied over the organic planarization layer; and finally, a cover film is laminated across the entire screen surface for protection.

 

 

　　Figure 8. TFE Package Structure

 

　　The process flow diagram for TFE encapsulation is shown in Figure 9. First, the OLED device (EV) is fabricated on the TFT glass. Upon completion of the OLED device fabrication, the first inorganic water-barrier layer (Inorganic 1) is deposited. After Inorganic 1 is completed, an organic planarization layer (Organic) is formed on top using either dispensing or inkjet printing. The organic planarization layer material can be cured by UV exposure or thermal curing. Once curing is complete, the second inorganic water-barrier layer (Inorganic 2) is deposited on top of the organic planarization layer, followed by the application of a cover film.

 

 

　　Figure 9. TFE Packaging Process Flowchart

 

　　TFE encapsulation strikes an optimal balance between water- and oxygen-barrier performance and bendability, making it the mainstream encapsulation technology for today’s OLED mobile devices.