Developers of quantum dot technologies for television and mobile device displays are working feverishly to develop new solutions that will greatly improve the efficiencies, cost and picture quality of next-generation televisions based on quantum dot light and color enhancements.
During the recent QLED and Advanced Display Summit sponsored by Samsung in Hollywood, Peter Palomaki, principal of Palomaki Consultants and former engineer with quantum dot resource QD Vision, said that television and quantum dot companies like Samsung, Nanosys, Nanoco and others are trying to improve performance by bringing quantum dots farther out front in the LCD stack.
The approach will improve efficiencies and various aspects of picture performance, and is being explored for use in quantum dot Color Filters and in Electroluminescent quantum dot (or self-emitting quantum dot) technologies.
“It is not going to be easy,” Palomaki warned.
In the next couple of years, quantum dots placed in color filters used for liquid crystal displays will enhance brightness and efficiency, widen viewing angles (because quantum dots can emit in all directions without being polarized), remove the need for each pixel to be on 100 percent of the time because of where the quantum dots are positioned in front of the stack, and enable cooler temperatures because the quantum dots are farther away from the LED heat source helping their longevity, Palomaki said.
Among the obstacles in delivering quatum dot color filters is the need to use an in-cell polarizer. Today white light comes through one polarizer in the liquid crytal before it hits a color filter. Polarized light is transmitted through the color filter. But quantum dots de-polarize light and the second polarizer layer is not going to do its job and light will just come through.
What’s needed is to swap the positions of the quantum dot layer with polarizer layer, which is non-trivial in the production process of the liquid crystal layer, he stressed.
The second problem is full-blue light absorption. In order for quantum dots to work as color filters or color converters, small percentages of blue light cannot be allowed to leak through the subpixels without losing the color accuracy that is a strength of quantum dots.
This requires packing enough quantum dots into a very small area to absorb 99%-plus of the blue light. That is a challenge because there are only a limited half length on the order of 5 or 10 microns of thickness to do that and certain types of quantum dots are better at that than others.
A second future approach to quantum dot display use is on-chip, meaning any form factor where the quantum dots are placed directly on top of an LED chip, like MicroLED, no matter what size the LED is.
A classic example is to use a macroscopic LED as an edge-lit or back-lit solution and instead of a phosphor, use a red or green quantum dot solution.
Any form factor for a quantum chip design is going to be challenged on three major levels. 1) high temperatures of the LEDs themselves make it hard for the quantum dots to survive; 2) really high light flux, with manufacturers moving to brighter and brighter LEDs every year and quantum dots struggle to keep up in that environment because they tend to create faster and faster high light flux; 3) atmospheric exposure from LEDs using silicone encapsulant and silicone polymers are not known for protecting things from exposure to oxygen which can diffuse very easily through silicones.
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Examples of quantum dots on chip technologies that have been successful so far include Lumileds and PLT lighting technologies that are now generally available commercially, but not ready for prime time in display applications.
For use with MicroLEDs, quantum dots will need to be highly concentrated on each LED subpixel (for red and green), which raises additional problems for patterning when placing reds and greens so closely together.
Palomaki said electroluminescent displays have been a pipe dream for quantum dots for more than a decade. In such applications the blue photons emitted by LEDs are substituted with electrons as the quantum dots act as the LEDs themselves.
The benefits of such a technology include throwing out the whole highly inefficient LCD optical stack, including the polarizers, liquid crystal layers, etc. The display becomes simplified, brighter and much more efficient.
Naturally, developing the technology has been anything but simple. Regardless, he said, there are some examples today including full color Active Matrix Quantum Dot LED displays from BOE, which are being used as computer monitors and 5-inch cellphone-like displays using QD-LED.
The reason that there are only short examples is that the lifetimes are pretty abysmal for most types of quantum dots. It is no where near being commercially viable yet, Palomaki predicted.
So far, long life times have been achieved for electrolumiescent red quantum dots, significantly shorter for green quantum dots, and lifespans of only a few minutes for blue quantum dots, and that was using cadmium-based material, which is a toxic and no longer used for commercial displays in many countries.
Solutions based on indium phosphide solutions have even worse life expediencies, Palomaki said, although work continues toward this wholly grail, which could produce some of the best television display quality yet achieved.
By Greg Tarr
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