By Ruiqing (Ray) Ma
In the 20 years since I entered the display field, I have never felt as much excitement about the future of display technology as I do today. One of the most exciting recent trends is the re-emergence of inorganic emissive technology. For example, at this year’s Display Week, the two most-attended seminars were about microLEDs and quantum dots. In this special issue, we have compiled three excellent articles to cover the topic of inorganic emissive materials, with the focus on quantum dots and emerging metal-halide perovskites.
The emergence of quantum dots coincided with OLED succeeding in smartphones and entering the TV market several years ago. Quantum dot wasn’t involved in the fight between OLED and LCD for smartphone displays because that battle wasn’t about color. OLED succeeded because it possesses properties that are uniquely important to mobile devices: 1) power consumption is insensitive to high resolution; 2) thin, curved, or flexible form factors; and 3) fast switching for augmented-reality/virtual reality applications. In 2013, for the first time, OLED exceeded LCD in display resolution in smartphones, which more or less concluded the battle between the two technologies for that application. The same year also witnessed the start of the next battle, with the introduction of 55-in. OLED TVs. Was OLED going to take over the TV market too?
For large-format TV, the three factors critical to mobile applications are not as important. The competition is more about color, perceived brightness, contrast, viewing angle, and cost. In response to OLEDs entering the TV market six years ago LCD took the offensive in 2013 by bringing in quantum dots that claimed the advantage in, surprisingly, color. This was done by replacing the white LED backlight with a yellow (red plus green) QD film placed in front of a blue LED backlight. Because the spectra of QDs are narrower than those of typical inorganic phosphors and OLED emitters, the color of QD-based LCDs is more saturated, and remains saturated at high luminance levels.
QDs, QLEDs, and Perovskites
Since then, QD has established itself as the key enabler for wide color gamut (WCG). Its next move gets a lot more interesting. In the article “A New Frontier for Quantum Dots in Displays,” researchers at Nanosys describe the use of a QD color-conversion (QDCC) layer to generate red and green colors at the front of the pixels, only electrically driving the blue color. By default, this configuration provides better color because of QD. But more important, by replacing the inefficient color filter with high-efficiency color-conversation QD materials, the display becomes more efficient. In addition, QDCC is a platform technology that works well with most other display technologies – LCD, OLED, and microLED. In fact, it provides additional benefit to each of those technologies. Remember the viewing-angle problem of LCDs? Gone. How about the patterning challenge for OLEDs? Gone too. The advantage for microLED is even more convincing – think about how much easier operations will be if one only needs to handle/transfer/drive one type of microLED instead of all three colors.
The battle between OLED and LCD for TV will likely be a long one, because neither technology holds significant advantage, and each still requires some solutions to fix shortcomings. This prompts the question: is there a “perfect” technology that can combine the advantages of both LCD and OLED? Surprisingly, the answer is yes. In “The Dawn of QLED for the FPD Industry,” Dr. Cass Xiang and his colleagues at TCL report their latest progress in printing QD-based electroluminescent devices. QLED has basically all the advantages of OLED plus a more saturated color and a low-cost printing process. Of course, serious challenges still exist that need to be addressed before this technology can be commercialized, but good progress is being made by various groups around the world, including the TCL team.
What makes the field of inorganic emissive materials even more exciting is that quantum dot is not the only game in town. In the article “Metal-Halide Perovskites: Emerging Light-Emitting Materials,” authors Lianfeng Zhao and Barry P. Rand from Princeton University report on their work on metal-halide perovskite-based light-emitting devices. There are a lot of similarities between metal-halide perovskites and quantum dots, together with some unique challenges. As direct-bandgap, defect-tolerant semiconductors, perovskites present intriguing possibilities and could find their roles in future displays.
Like two railroad tracks, LCD and OLED have been able to maintain their own identities while paralleling each other closely. With quantum dots, they start to collide and interact. If we add microLED and other display technologies to the mix, I can see many possibilities for future displays. I can’t wait to find out where we’ll be when we revisit this topic in a couple of years. Until then, enjoy reading these articles.
Ruiqing (Ray) Mahas served and continues to serve in many capacities for SID and Display Week. He is currently director for QD devices at Nanosys. He can be reached at firstname.lastname@example.org. •