Brightness is one of the key properties influencing display quality and is one area where LCD technology is and will remain superior to its premium competitor, OLED. In combination with a blue LED backlight, red- and green-emitting inorganic phosphors can provide an on-chip or remote film solution to the generation of white light.
A similar approach is known from QD-enhanced LED displays, in which quantum dot (QD) films exploit the color-converting properties of these semiconductor nanoparticles to generate specific wavelengths of red and green light.
However, the hybrid phosphor-QD display is still to reach market readiness. The hybrid phosphor-QD display is a disruptive technology with the potential to offer best-on-the-market brightness and color reproduction in the same package.
This has its origin in the distinct optical properties of the red and green components. Essential to optimal display brightness is a combination of materials with:
- High quantum efficiency (QE)
- Distinct emission and excitation wavelengths of the individual species, thereby preventing self-absorption of the emitted light
- Distinct emission and excitation wavelengths between the combined red and green materials, such that light emitted by one material is not absorbed by the other
This is not easily realized in practice. However, recent developments have yielded two materials that together are closer to the above ideal than has been previously encountered, namely red KSF phosphor and green perovskite quantum dots (pQDs).
These two materials can be combined to make color conversion films for blue LED illumination like that shown below in Figure 1a.
Figure 1a. A KSF – pQD color conversion film overlaid on a blue LED backlight. Image Credit. Avantama AG
Figure 1b demonstrates the absorption and emission behavior of each color conversion component of the example film above. Emitting in the red region of the spectrum, KSF exhibits minimal self-absorption or overlap with the green QD emission.
Whilst self-absorption is typically present to some extent for QDs, by using Cd-free perovskite QDs with high absorption and QE approaching 100% the effect is hardly observed.
Figure 1b. Emission spectrum (white solid line) of white light generated by a green perovskite QD and red KSF film illuminated with a blue LED backlight. Overlaid (dashed line) are the absorption spectra of the KSF (red) and pQDs (green) for the same range of wavelengths. Image Credit. Avantama AG
An additional advantage of KSF and perovskite QDs is that both have an extremely narrow full width at half maximum (FWHM) compared to alternative color conversion materials.
Achieving exceptional color purity and accurate color reproduction is impossible without narrow emission profiles, and as such, it is a key strategy in achieving Rec. 2020 coverage.
Rec. 2020 is a particularly wide color gamut designed for UHD-TV with the goal of bringing color reproduction in displays closer than ever before to what the human eye can perceive.
Depicted in Figure 2, it contains a large area of coverage in the region of the green primary (532 nm) and thus the choice of material for green emission is crucial.
Figure 2. Boundaries of the Rec. 2020 color space shown as a triangle on the CIE 1931 color space. Image courtesy of Sakuramboderivative work GrandDrake - File:CIExy1931.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=21864661.
The importance of green light on the perception of image quality can be understood by considering the sensitivity of the eye to light of different wavelengths (Figure 3).
The human eye is most sensitive to light corresponding to green or greenish-yellow tones, with maximum sensitivity at 555 nm.
Where the eye should receive a wavelength of light to which it is 50% less sensitive than it is at its maximum, twice as much power would be required for a person to perceive the same brightness as light from the same source at 555 nm.
Figure 3. The varying sensitivity of the eye to different wavelengths of light. Image Credit. Avantama AG
It may be noted from the position of the Rec. 2020 color space that a shift of the green apex to wavelengths higher than the green primary at 532 nm results in a reduction of the total color space covered.
As such, a compromise must often be made depending on device requirements whether the wavelength of green emission should maximize the number of available colors or optimize brightness.
Generally, battery-driven devices such as notebooks and tablets will sacrifice color reproduction by choosing green emission of a slightly higher wavelength to improve display brightness, thereby minimizing power consumption by the display.
However, AC mains-driven devices such as monitors and televisions are much less restricted in this regard and a slightly lower green wavelength can transform the image to lifelike scenes.
Amongst the different types of QDs available, perovskite QDs possess unique optical characteristics that mean they are the only route to achieving Rec. 2020 color space in consumer displays.
They also allow facile tuning of emission wavelength through small changes in composition, meaning they can be easily adapted to fit applications of both battery and mains-driven devices whilst still providing the intrinsic advantages of high absorption, efficiency and narrow peak breadth discussed previously.
Combined with red KSF phosphor, green-emitting pQDs allow the production of stunning displays with lifelike image reproductions previously unattainable within the consumer market.
This material pairing reaches new levels of color purity, display brightness and efficiency and will soon be entering the market to bring content to life for an even more immersive viewing experience.
This information has been sourced, reviewed and adapted from materials provided by Avantama AG.
For more information on this source, please visit Avantama AG.