Interesting, From the article at photonics.com:
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Posted On: 05/04/2018 8:41:39 PM
Interesting, From the article at photonics.com:
QDs in displays today
To date, only two methods of QD implementation have been available in displays: film and edge optic. In both methods, blue LEDs are used as the illumination source, and QDs are dispersed in a polymer that is protected from atmospheric degradation by external packaging. Both provide the important benefits of enhanced efficiency and brightness and an improved color gamut. The differences lie in how they are implemented into the display backlight unit.
Film: Red and green QDs are homogeneously dispersed in a polymer film, which is then placed into the backlight unit along with several other optical films. Light conversion happens over a large area and can be illuminated by LEDs on either the edge or back of the display. Typically, the QD-polymer composites are sandwiched between two expensive barrier films that prevent atmosphere from reaching the QDs and degrading their emissive properties.
Edge optic: QDs are dispersed in a polymer inside of a glass tube on top of the strip of LEDs at the edge of the display. This method uses far fewer QDs than the film option but can only be used with edge-lit displays. Since this method is closer to LEDs, the light flux and temperature is greater and a hermetic glass seal is required to ensure stable operation.
There are many advantages with either implementation strategy, but the main reason film has become the dominate form factor for QDs in displays is the ease of implementation and ability to integrate into nearly any LCD type (curved, edge-lit, backlit, large TV, small tablet).
Film is more expensive than edge and uses more QD material, but the benefits outweigh the drawbacks for most display manufacturers. With the sale of QD Vision to Samsung in 2016, the edge optic became obsolete. All commercial displays that currently incorporate QDs use the film approach. Even though film may be the form factor of choice for now, there are many ways QDs could be integrated into future displays.
Future of QDs in displays
QDs as color filter replacements may be the next big breakthrough for displays. In this approach, red and green QDs are patterned on the subpixel level and act as the color filter and color converter in one step. The blue subpixel would simply pass through the blue light from the LEDs.
If successful, this implementation of QDs could result in dramatically improved efficiency by more than 2× over film implementation and better viewing angle (rivaling OLED), along with the enhanced color that all QD implementations enable6.
However, the challenges to overcome are substantial. One is the redesign of the LCD stack to incorporate the second polarizer in the cell with QDs following. This is a requirement because QDs depolarize light. Subpixels require polarized light to operate, and if QDs are utilized in the same position as current color filters, the LCD will not work. This alone represents a dramatic change to the LCD production process, which must be driven by the panel makers, not the QD component suppliers. The component makers must be able to create extremely thin films with very high QD concentrations such that they absorb nearly 100 percent of the blue light that strikes each subpixel.
In addition, subpixel-level patterning or printing must be implemented, which means very different process requirements for the QD/polymer composites; some require high-temperature curing steps.
Even if all of these issues can be solved, there are additional challenges in light management: preventing unintentional excitation from blue room light, extracting all photons from QDs that emit in all directions, and reducing or eliminating unintentional crosstalk among neighboring subpixels. Look for this technology to be available in the next few years.
On LEDs and microLED: Red and green QDs are dispersed in the silicone encapsulant on top of the blue LED die. This implementation strategy has the benefit of being incredibly simple — a true drop in substitute for phosphors. If both red and green QDs can be engineered to survive in the high-temperature, high-flux environment of LED chips, this approach could lead to simplified production with less QD usage.
Recent success has been reported by Pacific Light Technologies in Lumileds midpower LEDs to create white lighting with high color quality and high efficacy7,8. The QDs used in these devices are specifically engineered to survive the high-stress environment of LED packages.
Unfortunately, this approach has only worked with CdSe because InP is not stable enough to survive on-chip conditions. It is already available as a lighting product with red CdSe QDs, but further improvements are required to improve green QD performance before a commercial product for displays can be introduced.
Summary of successful current implementation and next-generation implementation methods of QDs in displays.
Figure 3. Summary of successful current implementation and next-generation implementation methods of QDs in displays. Courtesy of QD Vision, Nanosys, and Lumileds.
MicroLEDs have recently become a hot topic as a next-generation display technology. QDs could play a key role in microLED technology because they are a much smaller size than traditional phosphor particles, which are larger than individual microLEDs. To simplify microLED production and integration into large area displays, blue-only microLEDs could be used with red and green QD down-converters to efficiently generate full-color microLED displays.
Electroluminescent: The ultimate display technology — electroluminescent — is a self-emissive technology that does not rely on inefficient down-conversion of blue photons to red and green photons.
Electroluminescent QD displays would utilize the same QDs in previous strategies, but instead of acting as a down-converter, they are electrically stimulated to emit photons and act as QD LEDs. This technology promises dramatic efficiency improvements since there is no wasted light, and it will lower cost by the elimination of many of the optical components found in a traditional LCD.
This approach would result in enhanced efficiency, brightness, viewing angle, and color, and could potentially open the door to flexible displays.
But don’t expect to see this technology in living rooms any time soon. Electro-luminescent QDs will require major improvements in stability and efficiency before they become a viable option. Additionally, they will require patterning at the subpixel level, but many of the advancements in OLED could be adapted for electroluminescent QD displays.
For now, this approach remains an active research project in both academia and industry. If successful, electroluminescent QDs will truly be a disruptive technology in the display industry.
Based on the rapid adoption of QDs in display technology over the last few years, it seems clear that we are likely to continue to see rapid progress in the incorporation of QD technology into commercial displays.
The current approach of QD films is just the beginning, as there are multiple technological innovations on the horizon that incorporate them into LCD technology.
The upcoming revolution of QD-enabled displays will continue to improve color and efficiency and provide an all-around enhanced viewing experience.
Meet the author
Peter Palomaki, Ph.D., is the owner and chief scientist at Palomaki Consulting. He assists clients with technical projects involving quantum dots, nanofabrication, and nanomaterials that enable next-generation optical technologies; email: peter@palomakiconsulting.com.
QDs in displays today
To date, only two methods of QD implementation have been available in displays: film and edge optic. In both methods, blue LEDs are used as the illumination source, and QDs are dispersed in a polymer that is protected from atmospheric degradation by external packaging. Both provide the important benefits of enhanced efficiency and brightness and an improved color gamut. The differences lie in how they are implemented into the display backlight unit.
Film: Red and green QDs are homogeneously dispersed in a polymer film, which is then placed into the backlight unit along with several other optical films. Light conversion happens over a large area and can be illuminated by LEDs on either the edge or back of the display. Typically, the QD-polymer composites are sandwiched between two expensive barrier films that prevent atmosphere from reaching the QDs and degrading their emissive properties.
Edge optic: QDs are dispersed in a polymer inside of a glass tube on top of the strip of LEDs at the edge of the display. This method uses far fewer QDs than the film option but can only be used with edge-lit displays. Since this method is closer to LEDs, the light flux and temperature is greater and a hermetic glass seal is required to ensure stable operation.
There are many advantages with either implementation strategy, but the main reason film has become the dominate form factor for QDs in displays is the ease of implementation and ability to integrate into nearly any LCD type (curved, edge-lit, backlit, large TV, small tablet).
Film is more expensive than edge and uses more QD material, but the benefits outweigh the drawbacks for most display manufacturers. With the sale of QD Vision to Samsung in 2016, the edge optic became obsolete. All commercial displays that currently incorporate QDs use the film approach. Even though film may be the form factor of choice for now, there are many ways QDs could be integrated into future displays.
Future of QDs in displays
QDs as color filter replacements may be the next big breakthrough for displays. In this approach, red and green QDs are patterned on the subpixel level and act as the color filter and color converter in one step. The blue subpixel would simply pass through the blue light from the LEDs.
If successful, this implementation of QDs could result in dramatically improved efficiency by more than 2× over film implementation and better viewing angle (rivaling OLED), along with the enhanced color that all QD implementations enable6.
However, the challenges to overcome are substantial. One is the redesign of the LCD stack to incorporate the second polarizer in the cell with QDs following. This is a requirement because QDs depolarize light. Subpixels require polarized light to operate, and if QDs are utilized in the same position as current color filters, the LCD will not work. This alone represents a dramatic change to the LCD production process, which must be driven by the panel makers, not the QD component suppliers. The component makers must be able to create extremely thin films with very high QD concentrations such that they absorb nearly 100 percent of the blue light that strikes each subpixel.
In addition, subpixel-level patterning or printing must be implemented, which means very different process requirements for the QD/polymer composites; some require high-temperature curing steps.
Even if all of these issues can be solved, there are additional challenges in light management: preventing unintentional excitation from blue room light, extracting all photons from QDs that emit in all directions, and reducing or eliminating unintentional crosstalk among neighboring subpixels. Look for this technology to be available in the next few years.
On LEDs and microLED: Red and green QDs are dispersed in the silicone encapsulant on top of the blue LED die. This implementation strategy has the benefit of being incredibly simple — a true drop in substitute for phosphors. If both red and green QDs can be engineered to survive in the high-temperature, high-flux environment of LED chips, this approach could lead to simplified production with less QD usage.
Recent success has been reported by Pacific Light Technologies in Lumileds midpower LEDs to create white lighting with high color quality and high efficacy7,8. The QDs used in these devices are specifically engineered to survive the high-stress environment of LED packages.
Unfortunately, this approach has only worked with CdSe because InP is not stable enough to survive on-chip conditions. It is already available as a lighting product with red CdSe QDs, but further improvements are required to improve green QD performance before a commercial product for displays can be introduced.
Summary of successful current implementation and next-generation implementation methods of QDs in displays.
Figure 3. Summary of successful current implementation and next-generation implementation methods of QDs in displays. Courtesy of QD Vision, Nanosys, and Lumileds.
MicroLEDs have recently become a hot topic as a next-generation display technology. QDs could play a key role in microLED technology because they are a much smaller size than traditional phosphor particles, which are larger than individual microLEDs. To simplify microLED production and integration into large area displays, blue-only microLEDs could be used with red and green QD down-converters to efficiently generate full-color microLED displays.
Electroluminescent: The ultimate display technology — electroluminescent — is a self-emissive technology that does not rely on inefficient down-conversion of blue photons to red and green photons.
Electroluminescent QD displays would utilize the same QDs in previous strategies, but instead of acting as a down-converter, they are electrically stimulated to emit photons and act as QD LEDs. This technology promises dramatic efficiency improvements since there is no wasted light, and it will lower cost by the elimination of many of the optical components found in a traditional LCD.
This approach would result in enhanced efficiency, brightness, viewing angle, and color, and could potentially open the door to flexible displays.
But don’t expect to see this technology in living rooms any time soon. Electro-luminescent QDs will require major improvements in stability and efficiency before they become a viable option. Additionally, they will require patterning at the subpixel level, but many of the advancements in OLED could be adapted for electroluminescent QD displays.
For now, this approach remains an active research project in both academia and industry. If successful, electroluminescent QDs will truly be a disruptive technology in the display industry.
Based on the rapid adoption of QDs in display technology over the last few years, it seems clear that we are likely to continue to see rapid progress in the incorporation of QD technology into commercial displays.
The current approach of QD films is just the beginning, as there are multiple technological innovations on the horizon that incorporate them into LCD technology.
The upcoming revolution of QD-enabled displays will continue to improve color and efficiency and provide an all-around enhanced viewing experience.
Meet the author
Peter Palomaki, Ph.D., is the owner and chief scientist at Palomaki Consulting. He assists clients with technical projects involving quantum dots, nanofabrication, and nanomaterials that enable next-generation optical technologies; email: peter@palomakiconsulting.com.
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