QD II
Yesterday we spent some time breaking out a bit of detail as to the Samsung Display QD/OLED project and today we follow that up with a bit more discussion about quantum dots.. But before we delve into the applications for quantum dots, there is the question of the market for these nanostructures, and as with many technologies, there are discrepancies and disagreements as to the size of the market. A quick look at some of the market estimates from industry reports shows a disagreement on market growth, likely based on the problem of inclusion that plagues much industry research. While the past year estimates are fairly similar, future expectations are considerably varied among the more publicize estimates. Rather than deal with the inconsistencies of such market data, which can include the use of quantum dots in medical and laser applications, our focus is on the use of quantum dots in consumer electronics.
The primary current application for quantum dots in the CE space is for QD Enhancing Film, a sheet consisting of top and bottom polyethylene films (PET) that act as barriers and a layer of a polymer that contains a suspension of quantum dots. The sheet is placed between the backlight and the liquid crystal system in an LCD display as an add-in, or can be molded into a replacement for the display’s light guide or diffuser, a translucent sheet that helps to evenly spread the white light from the backlight across the entire display. While LEDs are used in most LCD display backlights the white light they produce contains a broad spectrum of colors.
In displays without quantum dots, after the light passes through the liquid crystal, it reaches a color filter that is composed of red, green, and blue dots made of phosphors, materials that glow (luminescence) when exposed to radiant energy. These phosphors eliminate much of the light energy contained in the white light in order to narrow the color to one of the red, green, or blue components that make up a display pixel. For example, while a red phosphor dot emits red light, it blocks all other colors and therefore reduces the amount of light energy reaching the user. Phosphors also have other limitations in that they are not very precise in terms of the light they let through, and with the broad spectrum of light coming from the LED backlight, a red phosphor might also allow some reddish orange or bluish red light to pass.
By inserting the quantum dot film before the light reaches the color filter, the dots can take the white light and ‘convert’ much of the broad color components, passing on a more pure red, green and blue to the phosphors, which then would be blocking less color energy. As an alternative, instead of a white LED backlight, a blue LED backlight can be used, with red and green quantum dots converting 2/3 of the light to those colors and letting the blue light pass through to the color filter, again ‘purifying’ the colors before they reach the color filter, with both alternatives producing more vibrant colors without the high cost of more ‘precise’ phosphors.
As we covered the use of quantum dots in Samsung Display’s new QD/OLED TV display system, we look a bit further out on the quantum dot timeline, with a focus on Micro-LEDs. Not to be confused with Mini-LEDs, which are small but relatively easily produced LEDs, Micro-LEDs are generally under 100µm (0.004”) and are produced using typical MOCVD processes, however due to their size, transferring the requisite number of Micro-LEDs from a production wafer to a display substrate can be a monumental task, given that a 4K display would require 8.29m such Micro-LEDs of each of three colors, for a total of 24.88m Micro-LEDs, each having to be removed from the production wafer and placed on the display substrate to precisely match the driving circuitry. As we have discussed previously, the transfer times involved, even using specially designed mass transfer tools, is quite long and therefore expensive.
That said there is a bigger problem for Micro-LEDs. As each of the three color LEDs (RGB) are produced on their own wafers, there tends to be an interim step where each color’s LED die are removed from the production wafer and placed on a temporary substrate in order to be arranged in RGB sequence. Once all three color Micro-LEDs have been placed on the temporary substrate in the correct order, they are again moved in sequence to the display substrate, further adding to TAC time and cost. In order to eliminate this interim step, quantum dots can be used to simplify the transfer process. In such a case only one color Micro-LED is needed, typically blue, which are moved directly from the production wafer to the display substrate, covering the entire display. Red and green quantum dots are then applied to 2/3 of the blue Micro-LEDs, converting their light from blue to green and red, and eliminating the need for three separate Micro-LED production wafers.
This concept also helps to solve another problem facing Micro-LEDs, which is one of consistency. When LEDs are produced there are both variations in light output and color, which tend to get worse as the size of the LED gets smaller. The potential solution is to test each Micro-LED, measuring luminance and color, and creating a ‘wafer map’ that tells the transfer tool to skip those that don’t meet minimum specifications, but even with this time consuming testing and mapping step, there are still significant variations across the wafer that would affect the final product. Quantum dots can be ‘pumped’ by a relatively broad light spectrum, meaning that the variations in blue color that might occur during Micro-LED production, will not affect the ability of the red and green quantum dots to shift the blue to those primaries, avoiding the necessity of creating a ‘quality’ map, only looking for those Micro-LEDs that don’t work at all, creating a higher quality Micro-LED display than might be created using red, green, and blue Micro-LEDs and no QDs.
By using quantum dots to color convert a single color Micro-LED display, a number of processes can be eliminated or simplified, reducing production time and cost. While such a display system would not be purely RGB Micro-LED, using quantum dots would make the production of small pitch Micro-LED displays a far more feasible task in a mass production environment, and while there are still many production problems that need to be solved before Micro-LED displays can be mass produced at prices that are within the budget of most consumers, the use of QDs would eliminate a number of potentially large bottlenecks and push forward the Micro-LED timeline. More tomorrow.
In displays without quantum dots, after the light passes through the liquid crystal, it reaches a color filter that is composed of red, green, and blue dots made of phosphors, materials that glow (luminescence) when exposed to radiant energy. These phosphors eliminate much of the light energy contained in the white light in order to narrow the color to one of the red, green, or blue components that make up a display pixel. For example, while a red phosphor dot emits red light, it blocks all other colors and therefore reduces the amount of light energy reaching the user. Phosphors also have other limitations in that they are not very precise in terms of the light they let through, and with the broad spectrum of light coming from the LED backlight, a red phosphor might also allow some reddish orange or bluish red light to pass.
By inserting the quantum dot film before the light reaches the color filter, the dots can take the white light and ‘convert’ much of the broad color components, passing on a more pure red, green and blue to the phosphors, which then would be blocking less color energy. As an alternative, instead of a white LED backlight, a blue LED backlight can be used, with red and green quantum dots converting 2/3 of the light to those colors and letting the blue light pass through to the color filter, again ‘purifying’ the colors before they reach the color filter, with both alternatives producing more vibrant colors without the high cost of more ‘precise’ phosphors.
As we covered the use of quantum dots in Samsung Display’s new QD/OLED TV display system, we look a bit further out on the quantum dot timeline, with a focus on Micro-LEDs. Not to be confused with Mini-LEDs, which are small but relatively easily produced LEDs, Micro-LEDs are generally under 100µm (0.004”) and are produced using typical MOCVD processes, however due to their size, transferring the requisite number of Micro-LEDs from a production wafer to a display substrate can be a monumental task, given that a 4K display would require 8.29m such Micro-LEDs of each of three colors, for a total of 24.88m Micro-LEDs, each having to be removed from the production wafer and placed on the display substrate to precisely match the driving circuitry. As we have discussed previously, the transfer times involved, even using specially designed mass transfer tools, is quite long and therefore expensive.
That said there is a bigger problem for Micro-LEDs. As each of the three color LEDs (RGB) are produced on their own wafers, there tends to be an interim step where each color’s LED die are removed from the production wafer and placed on a temporary substrate in order to be arranged in RGB sequence. Once all three color Micro-LEDs have been placed on the temporary substrate in the correct order, they are again moved in sequence to the display substrate, further adding to TAC time and cost. In order to eliminate this interim step, quantum dots can be used to simplify the transfer process. In such a case only one color Micro-LED is needed, typically blue, which are moved directly from the production wafer to the display substrate, covering the entire display. Red and green quantum dots are then applied to 2/3 of the blue Micro-LEDs, converting their light from blue to green and red, and eliminating the need for three separate Micro-LED production wafers.
This concept also helps to solve another problem facing Micro-LEDs, which is one of consistency. When LEDs are produced there are both variations in light output and color, which tend to get worse as the size of the LED gets smaller. The potential solution is to test each Micro-LED, measuring luminance and color, and creating a ‘wafer map’ that tells the transfer tool to skip those that don’t meet minimum specifications, but even with this time consuming testing and mapping step, there are still significant variations across the wafer that would affect the final product. Quantum dots can be ‘pumped’ by a relatively broad light spectrum, meaning that the variations in blue color that might occur during Micro-LED production, will not affect the ability of the red and green quantum dots to shift the blue to those primaries, avoiding the necessity of creating a ‘quality’ map, only looking for those Micro-LEDs that don’t work at all, creating a higher quality Micro-LED display than might be created using red, green, and blue Micro-LEDs and no QDs.
By using quantum dots to color convert a single color Micro-LED display, a number of processes can be eliminated or simplified, reducing production time and cost. While such a display system would not be purely RGB Micro-LED, using quantum dots would make the production of small pitch Micro-LED displays a far more feasible task in a mass production environment, and while there are still many production problems that need to be solved before Micro-LED displays can be mass produced at prices that are within the budget of most consumers, the use of QDs would eliminate a number of potentially large bottlenecks and push forward the Micro-LED timeline. More tomorrow.
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