As the LED industry faces significant overcapacity and declining prices, much talk is devoted to micro-LEDs, with LED manufacturers citing all sorts of statistics and timelines as to when real micro-LED displays will be available. Without delving into the pluses and minuses of using very small LEDs as self-emissive pixels, one of the biggest selling points for such a technology is brightness. LCD displays are bright, driven by their LED backlights, but color filters, the sheet containing the colored dots that the LCD backlights illuminate, reduce a significant proportion of that light output, and OLED TVs, which use those same color filters, are even more hampered by them. This makes the idea of millions of individual RGB LEDs seem quite attractive, given they need no color filter.
In order for OLED displays to compete in the display arena, particularly in the TV space, material suppliers like Universal Display (OLED), Idemitsu Kosan (5019.JP), Merck (MRK) and others have continued to upgrade OLED stack materials to increase their light efficiency. While this has led to increased brightness with each successive generation of materials, OLED displays are still criticized for not being as bright as their LCD counterparts, even in OLED smartphone displays that do not use color filters. So with all of this OLED material R&D, why are have OLED displays been unable to beat LCD display brightness?
The answer, or at least part of it, is light extraction, and while much of OLED material research is focused on keeping the ‘excitrons’ from extinguishing other nearby excitrons before they are ‘glowing’, even when this is accomplished, only 20% to 30% of the light generated is actually emitted from the device. This is due to a characteristic known as TIR, which says that when light moves from a material with a high refractive index to one with a low refractive index, TIR occurs and much of the light turns to heat. While this sounds like the part of physics that caused significant somnolence in High School or college, it is actually quite simple and can be summarized as light moving from one medium (OLED materials) to one with a higher density (air), much of the light is reflected back into the original material, particular light emanating from the materials at a high angle.
This means that finding ways to change that high angle as the light leaves the device and enters the air can improve the amount of light that the user sees. Sounds good, but in order to do that without a large lens that will blend all the pixels, there has to be a light extraction device for every pixel. Given the millions of pixels and sub-pixels in current OLED displays, this presents a big problem, but one that is not insurmountable, and a number of materials are now being used to create what amounts to a lens over each pixel.
These materials, which sit between the glass or thin-film encapsulation layer of an OLED device and the ITO conductive layer, contain nanoparticles combined with polymer materials that can create ‘lenses’ which are expected to improve light extraction efficiency for OLED devices by at least 50%, and can be applied to a number of other large applications, particularly in lighting. As with many technologies, scaling lab processes to mass production levels has proven to be challenging however a small US company, Pixelligent (pvt) seems to have been able to produce these materials in volume, having been funded in part, by grants from the DOD and DOE, and most recently by Japanese Display chemical producer Tokyo Ohka Kogyo (4186.JP) and ink-jet printer producer Kateeva (pvt). As these materials continue to be refined and are integrated into OLED device design and manufacturing, they have the ability to continue to improve OLED displays and lighting, and for a company like Kateeva, the leader in ink-jet deposition, they represent another layer of materials that can be applied along with current encapsulation organic and inorganic materials and could broaden the potential market for OLED oriented ink-jet printing while improving OLED characteristics.