MOST DISPLAY SCIENTISTS AND ENGINEERS ARE AWARE of quantum-dot (QD) color converters and their current use and potential in displays. Although this issue of ID focuses mostly on QDs, it is important to remember that another technology exists with an even greater impact from a market value and volume perspective. The largest share of wide color gamut (WCG) displays belongs to phosphor-downconverted blue LEDs, where phosphor research and development during the last decade has led to significant improvements in color gamut while maintaining screen luminance and easy adaptability to LED and LCD supply chains. To spotlight this technology, this article describes the role of narrow-band phosphors in displays and addresses the forward-looking roadmap for approaches using them. We compare the market positions of different color-conversion methods and discuss innovations for narrow-band phosphors and the development of new green- and red-emitting phosphors by GE. These innovations include potential phosphor-QD hybrid solutions as the display industry moves to Rec 2020 and the potential to use narrow-band phosphors as color converters in new display technologies, including microLEDs. Color is emitted or converted in displays in many different ways. We broadly discuss these basic approaches that are used to make red, green, and blue (RGB) light for RGB subpixels: The last 10 years have brought many advances in OLEDs, QDs, and mini- and microLEDs. However, most display screens still use relatively large white InGaN LEDs in an LCD-BLU with a phosphor coating that converts blue LED emission into green, yellow, and red light, depending on the specific phosphors used in the LED package. The first and highest volume of phosphors used in display LEDs for LCD-BLUs are yellow Ce3+−doped garnet (primarily Y3Al5O12 compositions) phosphors, which reach an initial color gamut of 70 to 80 percent as defined by the NTSC in typical LCDs. Phosphor invention and development led to Gen 2 phosphors based on Eu2+−doped phosphors for separate red and green broadband-emitting phosphors to improve color gamut to 80 to 85 percent NTSC with >20 percent losses in screen luminance. In 2014, GE introduced K2SiF6:Mn4+ (abbreviated KSF or PFS for potassium fluorosilicate) narrow-band red, which when combined with β-SiAlON:Eu2+ green further improved gamut to >95 percent NTSC with minimal loss in screen luminance. The switch from less-efficient broadband phosphors to Gen 3 narrow-band phosphors, such as KSF/β-SiAlON:Eu2+ phosphor blends, was relatively seamless across the LED and LCD industry. Once the phosphor blend was switched at the LED package level, it directly went into the supply chain for LCD-BLUs and displays and established commercial benchmarks for WCG LCDs. GE licenses its KSF phosphor technology into the display industry, with LED sales of more than 60 billion KSF-containing LEDs. Nine years later, narrow-band phosphors retain a large commercial value in WCG flat panel displays (FPDs): $8 billion for Gen 3 phosphors compared to around $4 billion for QDs and $40 billion for OLEDs (Fig. 1). While displays using mainly broadband phosphors still represent most of the FPD market, future color specifications such as DCI-P3 and BT.2020 will continue to reduce their market share. Therefore, we focus on the status and progress toward developing future WCG displays using narrow-band phosphors. Overall color management techniques and revenues. QD: quantum dot; RGB: red, green, and blue; UV: ultraviolet; WOLED: white OLED. Source: Hendy Consulting. Products with narrow-band phosphors are in all major display products: TVs, monitors, laptops, tablets, and smartphones. Apple, Dell, Samsung, HP, Asus, Sony, TCL, Microsoft, Lenovo, MSI, and Hisense use narrow-band phosphors, including KSF, to enhance color gamut with minimal sacrifices in luminance, battery life, or cost. Some monitors and laptops with KSF phosphor have 100 percent DCI-P3 color gamut with response times of less than 2 milliseconds (ms), approaching LCD technology's limits. Data show that narrow-band phosphors are the top WCG solution for larger-area screens. Phosphor-based solutions also are strong in other markets (Fig. 2). We discuss the role of different WCG solutions on a unit basis by market for TVs, monitors, notebooks, tablets, and phones. We take the same data and estimate that the narrow-band phosphors have an addressable display market of around $8 billion in 2023 based on display module value, where TVs are addressed as an open cell value (Fig. 3). WCG by market with indicated market penetration rates (units, 000s). The red lines indicate the relevant penetration rates into each of these markets. Source: Hendy Consulting. Comparing addressable market sizes for phosphors and QDs (USD, 000s). MNT: monitor; NB: notebook. Source: Hendy Consulting As display designs and technologies become more diverse, we should consider a forward-looking technology innovation roadmap. This market analysis presents on-chip narrow-band phosphors and remote QDs as two separate technologies, but markets eventually will choose designs that best meet future display requirements, including hybrid approaches. For example, Samsung, TCL, and Hisense TVs have combined phosphors with QD solutions. While narrow-band phosphors primarily are used as part of an LED package and deposited on-chip, they also can be used in remote part films—either with other phosphors or with QD downconverters in mixed-material films. Combining narrow-band phosphors with QDs can potentially provide different functionalities, manufacturing ease, and cost positions for displays. Outside of LCD-BLUs, color conversion in direct-emitting architectures is becoming more relevant for microLED applications. These new applications for narrow-band phosphors require materials’ development to optimize display performance and manufacturability. KSF phosphor technology was commercialized in both lighting and display applications in 2014.1 These products initially used larger LED packages (e.g., 3,030 or 3,528—the two most common in display applications), with 25–30-micron particle-size KSF phosphor powders mixed into silicone with a green phosphor that was deposited directly onto blue InGaN LEDs. KSF phosphors have an optimal narrow red-emission profile and have proven chemically robust to enable on-chip packaging. Since 2014, GE has developed new types of KSF phosphor to meet the color-conversion needs of the ever-evolving display market (Fig. 4 and Table 1). Phosphor technology innovation involves both adapting current phosphor compositions for different package types and display form factors, and developing new phosphor compositions to improve phosphor response times or color gamut. Evolution of KSF phosphor commercialized in multiple form factors and architectures. Source: GE Research The trend toward smaller LEDs (e.g., miniLEDs and microLEDs) requires smaller particle-size KSF phosphors (Fig. 4 and Table 1).2 Next-generation KSF phosphors, with three times higher absorbance and 10 times smaller size relative to 2014, currently are being used in commercial display products both on-chip and in remote film architectures. KSF phosphors have shown on-chip reliability; they maintain performance under high temperature and flux conditions without the need for encapsulation barriers for moisture or oxygen protection. The benefits of miniLED backlights with full-array local dimming can be achieved using films with KSF and other green downconverters. Although KSF can be used directly on-chip in typical LED packages, these films enable a wider selection of narrow-band green downconverters (including QDs) that may not have on-chip reliability (degrading under high blue flux, heat, or oxygen) but improve color quality and screen brightness. One example of commercializing KSF-containing films is the miniLED-based stack in the Apple tablet and monitor, which won the 2022 SID Display of the Year Award. Two new types of red-emitting phosphors can deliver narrow red emission with >30 percent faster photoluminescent decay time (tau) and improved luminance. Next-generation display architectures will need thinner and more efficient solutions than those available today through conversion films or smaller LED packages. Those solutions will require higher light-absorption capabilities and efficiencies, particularly a suitable red primary for the BT.2020 standard. KSF phosphors meet the color requirement with a 630-nm peak wavelength blue-conversion capability. This can serve future ultrahigh-definition TV color-rendering needs.2 When discussing the use of KSF technology in thinner luminescent layers, external quantum efficiency (EQE) versus thickness measurements in relevant form factors are used to understand potential design and product performance. The EQE is proportional to the amount of light absorption at 450 nm (LED or OLED) and the internal quantum efficiency (IQE; defined as the red photons detected divided by the blue photons absorbed). It is inversely proportional to loss mechanisms that decrease red light output, such as self-absorption losses. Patented technology has been developed to decrease particle size and minimize agglomeration while improving absorbance and ink, film, printing, and processing capabilities.3-5 This effort has led to 10 to 20 percent EQE improvements over the past two years and the ability to print thinner films (Fig. 5). We continue to improve the inherent phosphor properties, ink formulations, and printing procedures to push EQE curves toward the top left of Fig. 5, with high EQEs for the thinnest possible films. Competitive zone based on film thickness for color-conversion applications. Source: GE Research. When benchmarking the current films, the ideal situation would be high EQEs for the thinnest possible film. However, for all luminescent materials, there are trade-offs between layer thickness and EQE, with three regions to consider (Fig. 6). In the region for thinnest films, the ability of a luminescent material to absorb blue light dominates EQE. Here, the higher absorption coefficient of QDs may make that technology more appealing, where the thinnest films are required; however, the caveat is that a thin QD film may need encapsulation layers, which could lead to a higher effective thickness.6 The second region, above ∼8-micron film thickness, is where submicron KSF phosphor has stronger absorption and matches QD EQE. The third region, defined by thicker films, is where self-absorption losses become significant and lead to lower EQEs versus thickness. For these thicker films, the self-absorption of InP QDs results in a decrease in EQE, whereas KSF phosphors without self-absorption losses maintain an IQE >90 percent after being cast and cured into a film.7 Choosing a specific downconverter in films depends on the display architecture and design. KSF-based films generally have advantages where thicker films are acceptable, with the goal being to attain a white point instead of a red point (so not all blue light needs to be absorbed), or in applications where encapsulation is not permitted. Fundamental strengths of narrow-band phosphor solutions. Source: GE Research. Beyond adapting to different display architectures, it also is possible to improve the color gamut for KSF-containing displays by combining KSF with a narrower green downconverter versus β-SiAlON:Eu2+. Combining KSF and green QDs would give the narrowest green and red emissions to maximize color gamut and have high dynamic range and peak luminance. There are two (relatively) straightforward ways to implement this combination. First, KSF and green QDs can be used together in a down-converting film similar to other green phosphor and KSF films. Alternately, KSF also can be used on-chip for a magenta LED (blue and red) combined with a remote green QD film. Multiple companies are pursuing architectures that combine a green-emitting remote perovskite QD film used with KSF phosphor to achieve up to 95 percent BT.2020 gamut. GE is developing narrow-band green phosphors that, when combined with KSF red-emitting phosphor, will increase the color gamut by >5 percent versus the standard KSF/β-SiAlON:Eu2+ phosphor blends.8 These phosphors are being tested in various packages and architectures to understand how they can fit into evolving LED and display products. Fig. 6 illustrates how narrow-band phosphors hold inherent advantages for WCG applications. Further innovation and development will build upon those advantages for use on-chip in traditional LED packages, in remote sheets for miniLED systems, and in inks for future microLED displays. These innovations will help to meet display industry needs today and into the foreseeable future. In looking at color-management materials, phosphors still hold a major share in the display industry. The market is dominated by mature broadband phosphors, representing more than $70 billion of displays. In WCG displays, narrow-band phosphors continue to have proven performance advantages and the largest market share. While phosphor technology often is perceived as being mature with limited innovation potential, GE has defined a broad R&D agenda for integrating phosphor technology into novel display types—especially miniLED and microLED color conversion. GE also is addressing higher EQE at any film thickness, the speed of photoluminescent decay, the delivery of phosphors in resists and inkjet print formulations, and the development of a new green phosphor. As the market experiments with combining approaches, the winner is the consumer who receives more colorful and immersive displays. Ian Hendy is the CEO of Hendy Consulting Limited, a management consulting company focused on the display industry. The firm offers custom business advice to players in the display value chain on market entry, growth, M&A, product and technology strategy, and business planning. He may be reached at [email protected]. James (Jim) Murphy is a principal scientist at GE Research, where he serves as the program manager for display technology and LED phosphors. He received a PhD in physical chemistry from the University of Colorado Boulder and has over 20 years of experience in developing optical materials for displays, lighting, medical imaging, and solar technologies. He may be reached at [email protected]. Anant Setlur is a principal scientist at GE Research. He received a PhD from Northwestern University. His work at GE focuses on materials invention and development for lighting, displays, medical imaging, and aerospace applications.