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A Guide to the Evolution of Quantum Dot Displays

Are quantum dot acronyms confusing you? Have you heard of QDEF, QDEL or QDCF and wondered what they are? In this whitepaper, we will help you understand the various QD technology implementations and the engineering behind each.

What are quantum dot displays?

Quantum dot (QD) displays are devices that use fluorescent semiconductor nanocrystals (a.k.a quantum dots) as a part of panel architecture to produce monochromatic light to deliver tunable primary colors and increase screen efficiency and performance.

This can be achieved in two fundamental ways:

  1. Photoluminescent (PL) or photo-emissive – where quantum dots are activated by the light source, such as in LED-backlit liquid crystal displays.
  2. Electroluminescent (EL) or electro-emissive – where quantum dots are embedded in each pixel and are activated and controlled via an electric current application.

Quantum dot displays offer a range of benefits, including:

  • High dynamic range (HDR) support due to their high peak brightness properties
  • Ability to achieve the closest proximity to BT 2020 color space because of the superior color saturation and wide color gamut output
  • Low power consumption and increased efficiencies
  • Ability to maximize color volume and contrast ratio for the optimal viewing experience

Read more about the benefits of the quantum dot technology here.

History of quantum dots
Source: Samsung Newsroom

Let's unpack what the difference is between photoluminescence (PL) and electroluminescence (EL) approaches to quantum dot implementation in displays.

Types of QD displays

I. Photoluminescent QD displays (QD-PL)

Today’s prevalent technologies use the QD particles in their photoluminescence (PL) mode, where the light emission is triggered by LED backlight. These displays are referred to as QD-PL type displays.

There are a number of ways to implement this QD-PL configuration:

1.   In-chip
2.   On-chip
3.   On-panel (on-surface)
  a.   QDEF
  b.   QD glass
  c.   QD color filter

In-chip (QD Dot)

This implementation is achieved through embedding the quantum dot particles in the chip by mixing QDs with commercial resin. Because of the chip proximity to the light emitting diode (LED), quantum dots are exposed to extremely high temperatures, which directly affects QD stability and reliability.

Another challenge in this arrangement is the compatibility between a quantum dot surface that is easily damaged by water and humidity, and the resin, leading to what's referred to as poisoning effect and QD agglomeration. For these reasons, this technology is not commercially viable when it comes to the display industry.

On-chip (QD Rail)

Another way of arranging quantum dots within the display panel is on-chip, where QDs are placed within cylindrically-shaped QD-polymer composite – referred to as a "quantum rail" – adjoining the backlight. In this case, even with encapsulation processes and backlight redesign, quantum dots are still too close to the source of heat to sustain performance.

Quantum rail diagram
Source: Samsung Display Co.
On-panel (on-surface)

Quantum Dot Enhancement Film (QDEF)

QDEF on-surface QD arrangement uses film-shaped QD-polymer located between two barrier films on the top of the light-guide plate and before the color filter in a display. QDEF panels use blue LED and red and green quantum dots.

By placing the QD particles away from light source, the risk of the heat exposure is eliminated. However, this approach requires a large number of quantum dot particles, dictating a relatively high fabrication cost.

QD-glass

An alternative on-surface approach to quantum dot arrangement is implementing a QD layer on the top of the glass light-guide plate. This provides the benefits of the QDEF arrangement while avoiding the design gap, allowing for slimmer SETs with thickness under 5 mm.

QD glass panels are more cost effective and are cadmium-free.

QDEF vs QD glass panels
Source: Samsung Display Co.

On-pixel (QDCF)

One of the recent developments in quantum dot application in displays is QD color filters (QDCF), where quantum dot particles are integrated into photoresist and then patterned to replace the colored dyes in sub-pixels.

QD color filters have only red and green quantum dot layers as well as clear sub-pixel to pass the blue light coming from the blue LED. The difference from the conventional color filter model here is that quantum dots act like active elements, and instead of the color filter blocking the light, QDCF is converting it. In this case, quantum dots are farther away from the LED, so exposure to the high temperature is reduced, resulting in a lower light flux.

QDCF approach yields the following benefits:

  • Wider viewing angles as in this arrangement QDs are placed closer to the screen and they emit light in all directions
  • Broader color gamut as quantum dots emit pure, tunable light
  • Thinner displays as there are fewer components to QDCF panels

Brighter displays at lower power consumption with about 50% increase in display efficiency, as quantum dots pass significantly more light than traditional color filters.

II. Electroluminescent QD displays (QD-EL)

Electroluminescence approach to quantum dot utilization is based on quantum dots integration in each pixel, where they generate light at the desired wavelength controlled by the electric current. These displays are referred to as QD-EL type displays.

Electroluminescent quantum dot display technology
Source: Samsung Display Co.

When this electroluminescent mechanism is employed, quantum dot material is placed between the anode and cathode with each sub-pixel containing red, green, and blue QDs.

QD-EL color gamut
Source: Samsung Display Co.

This approach offers significant benefits:

  • Exceptionally wide color gamut – as quantum dots emit light in a narrow spectrum and are finely tunable
  • High contrast ratio – since each pixel can be controlled
  • High brightness at low power consumption – as no backlight, no liquid crystals layer, and no color filter will be required
  • Ultra-high resolution
  • No burn-in with non-organic material
  • Design flexibility – as there is no backlight, this mechanism makes the technology available for application in flexible, foldable, rollable, and transparent displays
  • Lower manufacturing cost comparing to OLEDs – patterning or ink jet printing of QDs instead of using expensive and slow evaporation equipment

This new technology comes with a range of challenges. You can read more about the engineering challenges and possible solutions here.

To summarize, here is the snapshot of the various QD technologies:

Quantum dot technologies compared
Source: Samsung Display Co.