Emissive Displays Rise and Fall – and Rise Again
by Qun (Frank) Yan
n information display is simply an electronic device designed to share information, especially the visual representation of information. An emissive display is an electronic device that converts electric energy to light directly at the individual pixel level. Emissive displays have a long history as people’s information display of choice. The cathode-ray tube (CRT), one of the earliest information displays, was a type of emissive display that dominated the industry for almost 80 years, ever since it became commercially available in 1922. Another type of emissive display, the plasma display, introduced us to the era of large-sized flat-panel TVs. This new form factor was truly a revolution in display technology, as the larger size, flatter format, lighter weight, and better pixel resolution of flat plasma display panels (PDPs), and later LCDs, quickly pushed small-sized, bulky, heavy, and low-resolution CRTs into extinction.
I remember that it was quite exciting when I first became engaged in PDP technology at Plasmaco in Highland, New York, back in 1997. The PDP was a hot topic within the emissive-display community for 50 years – since 1965 – and especially when large plasma TVs entered the marketplace at the end of the last century.
One similarity between CRT and PDP displays is that both emit light through vacuum electronic devices. Lighting through solid-state devices, such as light-emitting diodes (LEDs), has proven to be more energy efficient in terms of converting electric energy into photon energy. Therefore, LCD panels with LED backlighting eventually gained the upper hand against PDPs due to the economy of scale, lower power consumption, and strong marketing – even though plasma TV always won performance shootouts over LCD TV in terms of image quality and video performance. Organic light-emitting-diode (OLED) displays, based on emissive and solid-state display technology, are now gaining some momentum against non-emissive LCD technology in small and flexible formats (such as for smartphones).
OLED technology, which used to be under the emissive display wing at Display Week’s annual technical symposium, became mature enough that it was given its own sessions at Display Week. Without OLEDs, and with PDPs having faded away from the consumer market, emissive displays seemed to be losing visibility at the annual symposium. However, thanks to quantum dots (QDs), emissive displays are once again a “hot” area.
QDs first became a popular topic because they could be applied to LCD backlighting systems. QDs can produce light with a much narrower emission spectrum than phosphor-based light sources, hence providing a more saturated or “pure” color. When employed as part of an LCD backlight system using a blue LED as the stimulus, a much wider color gamut can be achieved than previously possible.
Recent progress in electroluminescent quantum-dot LEDs (QLEDs) has also increased interest in developing QLED devices with a structure similar to that of OLED displays – these could even potentially replace OLED displays. For this reason, I solicited the article, “Quantum-Dot and Quantum-Rod Displays – the Next Big Wave” by Kai Wang and Xiaowei Sun from Southern University of Science and Technology. This article provides a very good review of both photoluminescent and electroluminescent applications of QD materials. I am sure it will help readers better understand the latest developments in this area.
It has been an exciting journey for display-technology evolution over the last 20 years. We have successfully entered the age of thinner and lighter panel displays, the key aspect of the second generation of display technology. As shown in Fig. 1, we are still in the second generation, a period of continuing evolution leading to better pixels (wider color gamut, high dynamic range, higher resolution, and faster frame rate). Currently, LCD technology dominates the display market and OLED technology is challenging LCD technology, especially in terms of small-sized panels and flexible applications.
Although there is still a great deal of work to be done in order to perfect the performance of second-generation display technology, third-generation display technology, with highly realistic interaction, is already on the horizon. One key element of the next-generation displays is that content will “float” in space instead of on the panel surface, which will allow human interaction to occur more naturally. Micro-LEDs are a technology that can very likely achieve this goal.
Micro-LED displays, as the name implies, use individual lighting elements that can be as small as a micrometer – maybe even submicron size in the ot-too-distant future. The very-high-density light rays that can be generated by micro-LEDs may become a key enabler for light-field displays – an exciting technology that can produce true 3D display images in real time. At the Innovation Zone at Display Week 2014, Ostendo showed some promising 3D floating images that used a quantum-photonic-imager (QPI) light-emitting layer and won the I-Zone’s Best Prototype Award. The current development of this technology, however, is still on the scale of 10–50-µm pixels, due to various challenges, and its application is still mainly at the micro- display level.
This issue’s micro-LED review article by Vincent W. Lee, Nancy Twu, and Ioannis (John) Kymissis delivers a comprehensive review of various approaches to micro-LED integration with semiconductor technology. Micro-LED technology could be touching off an era of truly high integration between display and IC technology.
Emissive displays were historically the first dominant technology for information displays. QDs as a wide-color-gamut emissive material have become a major part of LCD backlighting, and QLEDs as emissive display devices can potentially replace another emissive technology — OLEDs. Last of all, emissive micro-LEDs have the potential to bring us into a new era of truly immersive and interactive experience.
I hope this special issue on emissive technology will highlight the current status of emissive displays and generate more positive interest in future emissive-display technology. We are starting to ride the next wave of new emissive-display technology. •
Fig. 1: The display-technology evolution will eventually move to another important emissive technology that will enable the flexible, interactive, and highly realistic displays of the future. The CRT represented the first generation of display technology while PDP, LCD, and OLED represent the second generation. Revolutionary third-generation display technologies are on the horizon.