The Future Is Bright for Emissive Technologies
by Stephen P. Atwood
Welcome to the final issue for 2018, and one that is especially forward-looking, as you can see from our cover. Throughout the year, we have heard a lot of discussion about emissive materials and their potential. They were a major theme at Display Week 2018, and they are clearly supported by a great deal of R&D spending around the world.
Emissive technologies, organic and inorganic, in various forms, have been around since the birth of information displays. CRTs used phosphors, which are inorganic emissive compounds that often utilized rare-earth metals, to make their images. Plasma displays also used phosphors, and there have been many other examples, such as vacuum-fluorescent and thin-film electroluminescent displays. Many of these displays worked in certain applications, but for various reasons lacked the ability to scale in terms of size (larger or smaller), resolution, optical performance, or other aspects that limited their commercial life. And yes, I know it’s rather glib to write off 100 years of CRT technology as having “limited” commercial life. But as we know today, there are a multitude of product embodiments such as tablets, smartphones, and large-screen TVs that were never going to be achieved with CRTs. Hence the generational hunger in our industry for something more – the same hunger that continues to drive researchers toward even better solutions today. As you will learn in this issue, many hope to find those solutions through entirely new classes of emissive technologies.
One-Day Conference from the LA Chapter
However, before we dive into the emissive articles for this issue, I want to draw your attention to the terrific upcoming program on “Selecting and Customizing Display Products” being hosted by the Los Angeles Chapter of the Society for Information Display on February 8, 2019. I’m told this is the chapter’s 15th year organizing this annual program, which proves time and again to be one of the most useful and practical technical seminars in SID’s regional portfolio. I have attended several of these programs in the past, including ones on advanced television, LCD modules, and LEDs. I referred to the notes from the 2008 LED conference many, many times while designing ruggedized backlights over the years that followed. The organizing team in LA includes many well-known industry veterans, including this year’s conference chair Larry Iboshi. The speaker lineup looks to be stellar as well. In our SID News department you can read more about the details of the event. And let’s be honest: what’s not to like about a trip to LA in early February, especially to spend a day at the Costa Mesa Country Club?
Emissive Progress
Our guest editor for this issue is Ruiqing (Ray) Ma, who is currently working as the director for QD devices at Nanosys. Ray has served both as a guest editor and contributing author for ID many times and continues to be one of our go-to experts for emissive technology. In his guest editorial for this issue, Ray expresses renewed excitement about the prospects for emissive technology to help define the future of displays. Whether through the further merging of quantum dots (QDs) and LCDs or through completely new emissive display architectures, the options are becoming more viable for breakthrough product designs. Thanks to Ray for all his hard work in developing our technical lineup for this month.
Our first Frontline Technology feature is “A New Frontier for Quantum Dots in Displays.” The use of QDs in backlight systems to enhance color spectrum for LCD TVs is widely known and well accepted in the marketplace. But QDs can be used in many other ways. Authors Ernest Lee, Chunming (Kevin) Wang, Jeff Yurek, and Ray Ma from Nanosys discuss the fundamentals of a “quantum-dot color-conversion (QDCC) layer” that can be integrated into today’s LCDs as well as added to OLED structures for further enhancement of LCD performance within the subpixels and as part of a direct-emitting structure. Advantages include higher optical efficiency, wide color gamut due to very narrow tunable emission spectra, and manufacturability that is easy to adapt to existing processes.
Of course, building an emissive element from QD material and utilizing electrical excitation directly are intriguing goals for researchers. Getting very narrow spectral emission at high luminance levels without the burden of primary and then secondary emission seems like an obvious next step. Authors Chaoyu Xiang, Weiran Cao, Yixing Yang, Lei Qian, and Xiaolin Yan from TCL Corporate Research describe this research roadmap in their Frontline Technology feature, “The Dawn of QLED for the FPD Industry.” A QLED is simply an LED that is combined with QD material to make a highly tunable narrow-band emitter. It sounds easy, but various challenges do exist, including lifetimes and efficiencies when QDs are driven by electrical vs. optical excitation. However, the authors describe several promising developments that begin to overcome these issues and outline how researchers in various organizations are getting some promising results. Hats off to all the people who are working on this very promising technology.
Another emissive technology showing promise is “Metal-Halide Perovskites: Emerging Light-Emitting Materials,” described in detail by authors Lianfeng Zhao and Barry P. Rand from Princeton University. Similar to QDs, perovskites can emit light by optical excitation through downconversion of shorter wavelengths to longer wavelengths, or by direct electrical excitation with the color of the emitted light determined by the materials’ bandgap energy. They are also highly tunable, producing very narrow color spectra at high efficiency. The authors describe their work in researching perovskite LEDs, achieving luminance and external quantum efficiency (EQE) values approaching those of much more mature OLED materials. These new materials are made in low-temperature solution processes that may be friendlier to flexible substrates than other materials and use inorganic raw materials that are in abundance. I think this is an emissive technology that has real promise for our industry.
Speaking of the industry as a whole and the relative maturity that organic LED tech has achieved, it is fun to see a resurgence of inorganic technology. While our first three features take on the core technology, the fourth, from author and analyst Paul Semenza, attempts to answer the broader question: “Can MicroLEDs and Quantum Dots Revitalize Inorganic Displays?” Paul addresses the state of the art for building various-size displays directly from microLEDs – which is not trivial, considering the challenge of assembling wafer-scale semiconductors into large addressable arrays for TVs and such. He also walks us through a broader overview of the various potential QD embodiments and what lies ahead in terms of opportunities and challenges. I think you will enjoy this balanced and very insightful perspective of both topics.
One notable company that has turned its development focus to microLEDs is Plessey in the UK. You’ve probably heard of Plessey Semiconductors for years, due to the company’s many technical innovations, but most recently, it has been leveraging its considerable experience in gallium-nitride (GaN) on silicon to develop a variety of microLED illuminators and microLED display products. GaN on silicon has several advantages over other wafer approaches such as sapphire because of better thermal performance, lower inherent cost, and potential for larger scalability. In this case, size does matter, because building any large display from these wafers involves either transferring the individual emitting devices or somehow assembling the full wafers into addressable arrays for displays. Our own Jenny Donelan interviewed Myles Blake, business information director for Plessey, and discussed the company’s plans for this technology as well as some of its overall history. I think you will find it very interesting reading in this issue’s Business of Displays Q&A.
In addition to our technical and business features this month, I’m pleased to have the second of what will (hopefully) become a number of excellent columns from SID President Helge Seetzen. In this installment of President’s Corner, “SID Needs YOU,” Helge talks about how SID is organized and run, from the chapters, through the program committees, and ultimately to the executive board. The organization relies entirely on volunteers, and most of this effort starts at the local chapter levels. So, if you have not been to an SID local chapter meeting lately I would strongly encourage you to go, and if you can volunteer your time to help organize and manage activities, that would be even better. It’s made a huge difference in the trajectory of my career and I’m sure it will have a positive impact on yours as well.
With that and our regular news features, it’s a wrap. As we bid goodbye to 2018, I truly hope this year was as rewarding and successful for you as it was for me. I hope you had time to spend with loved ones and time to enjoy some leisure as well as your work. I also want to wish everyone safe and happy holidays. May you find peace, comfort, and joy during this holiday season. Cheers and best wishes! •