QLED, quantum dot OLED or quantum dot LCD?
As we know, liquid crystal display (LCD) and organic light-emitting diode display (OLED) are two streams for displays now. Quantum dot technology has been ushered in to strengthen the color gamut of LCD to somehow compete with OLED.
However, recently, there is a new kind of display technology called QLED that's hot but creating more confusion.
In early May, the Korean press reported that Samsung would soon develop "QLED." According to news reports, Samsung does not expect active-matrix OLED (AMOLED) to be successful for large-area applications such as flat-panel TVs. Samsung believes that its manufacturing cost for AMOLED would jump exponentially as the display area increases, and that quality issues like burn-in and image sticking of the OLED would also present problems for larger applications. Therefore, Samsung is developing alternative solutions for its future TV lineup, and QLED is emerging as a strong candidate.
Now the question is: what is QLED?
Details of QLED have not been fully revealed. But IHS believes that the "Q" in QLED stands for quantum dot (QD). And then there are two possible types of QLED:
- The first is a converted type of AMOLED that replaces the emission layer with Q materials.
- The second is an advanced LCD that adopts QD as the color filter (C/F) material. And it still uses the LED backlight as the display's light source.
In other words, while LCD is using quantum dot technology mainly on the backlight unit to strengthen the color gamut, OLED can also utilize quantum dot technology to strengthen the light-emitting efficiency.
The first AMOLED type would be a completely self-emissive display that would likely need higher electrical mobility than a typical LCD thin-film transistor (TFT). Therefore, low-temperature poly silicon (LTPS) or oxide semiconductor array substrates would need to be applied instead of typical TFTs. One merit of QLED is a lower investment cost than AMOLED. Samsung may want to introduce QLED rather than AMOLED in its TVs due to the minimum investment and fab revision. However, the first option does not seem adequate.
The second type, the advanced LCD with QD instead of a C/F layer, would have a similar structure to a typical LCD. It would have the same amorphous silicon (a-Si) TFT, liquid crystal, and glass substrates. Only the C/F layer would be deposited with quantum dot instead of dye materials. Therefore, it can be manufactured in a typical LCD fab with minimum equipment revision or investment. At the same time, it offers a superior color gamut. The color gamut of current TFT LCD is limited due to color selectivity of the C/F layer. If the C/F layer is replaced with QDs, the color gamut can have enhanced color selectivity. The QD layer could play a role as a self-emissive layer, and it could also lead to contrast improvements for AMOLED.
This second type has a lower cost with minimum investment and advanced picture quality with higher color gamut and contrast ratio, unlike the first type, which is expected to have higher manufacturing costs. Therefore, the advanced LCD with QD instead of C/F layer seems much closer to the QLED that IHS imagines will come to fruition.
However, there are still many technical development obstacles for QLED.
Put QD in the color filter layer of the LCD rather than in the backlight unit? Not much is known about QD for this type of application. Current QD materials have not been adequately examined for the replacement of the C/F layer, and an optimized QD device structure in new displays has not been presented yet.
Aside from the unknown factors, there is one fundamental item related to the LCD: the polarizer.
The LCD uses the polarization of light, and the LCD cell is sandwiched between two polarizers (bottom and top polarizers). Light from the backlight is polarized by the bottom polarizer when entering the liquid crystal (LC) cell. Its polarization is then controlled by the LC, and the voltage applied to the LC determines if it can pass through the top polarizer (the LCD's on/off).
The C/F layer is placed inside the LCD cell, facing the polarized light. If a QD layer is introduced, it will emit green and red lights by converting blue light from the backlight. The green and red lights are not from the bottom polarizer, but from the QD layer, so it must be non-polarized. Non-polarized light cannot pass through the top polarizer or there would be a malfunction in the LCD's on/off. To avoid this problem, light from the LCD cell must pass through the top polarizer before entering the QD layer. Hence, the top polarizer would need to be placed beneath the QD layer, inside the LCD cell.
Based on this, the necessity for an in-cell polarizer arises.
In-cell polarizers have been studied only in terms of lowering costs for typical LCDs by replacing the top polarizer. But for QLED, as an advanced LCD type with a QD layer, an in-cell polarizer would become a necessity. This could present another big hurdle in developing QLED.
David Hsieh is Director of Analysis & Research within the IHS Technology group
Posted on 13 June 2016
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