Link to this article in Chinese 钙钛矿: 提高显示屏亮度的新材料

Colour conversion materials like perovskites and quantum dots (QDs) are often considered purely in the context of increasing colour gamut in displays. However, materials with narrower emission spectra can also boost display brightness and power efficiency. In this article, we look at the potential of perovskites to increase brightness in a direct comparison with InP QDs, and we explore how brightness will relate to colour performance in future display designs.

The background

Underlying the entire world of color-converting materials is the metric of color purity. Colour conversion materials (perovskites, QDs, and phosphors) all have a unique emission spectrum. The narrowness (width) of this emission spectrum dictates the color purity of the image, the narrower the better.

The emission wavelength of perovskite materials is determined by their chemical composition. This is in contrast to QDs which rely on quantum confinement to define the emission wavelength. The quantum confinement approach requires QDs to be manufactured to incredibly precise tolerances because a 1 nm variation in size can change the emitted colour of a green quantum dot to blue or orange. The inevitable variation of QD particle sizes leads to a distribution of emission wavelengths and therefore to a broader spectrum. Perovskite nanocrystals, however, can vary from nanometres to microns in size and will still emit pure light over an extremely narrow wavelength range. (See Colour by Chemistry: Why perovskites are simpler to make than quantum dots.)

Figure 1 compares the emission spectra of Helio’s perovskites with a typical InP QD implementation. Green perovskites from Helio emit with a peak Full Width Half Maximum (FWHM) of <25 nm, while InP green emission is typically >35 nm.

Figure 1: Helio’s perovskite emission compared to InP QD emission. The QLED spectrum (black) is a measured QD component from a commercial display containing InP QDs.

This raises two important questions: 1) What colour performance and brightness benefits does the narrower emission of perovskites provide and 2) How can the colour performance and brightness of the display be optimized for a particular display application?

The analysis

Human perception of light is described by the “Luminosity Response Function” which tells us that our eyes are most sensitive to green photons at 555 nm [1]. The closer an emitter comes to this wavelength, the brighter the display will measure (all else being equal). However, using a green emitter at 555 nm is rarely the best choice as it will adversely impact color gamut. The best green wavelength to maximize Rec2020 color gamut is 532 nm. The model developed at Helio tells us where in that range we can achieve the best of both worlds; wide color gamut with optimized brightness.

Let us assume for a moment that 90% of Rec2020 color gamut is a reasonable target for next generation displays. As it turns out, very few displays approach this metric today, but perovskites will help enable future displays to achieve this target. The output of Helio’s model which is summarized in Fig. 2 for three different gamuts (80%, 85%, and 90% Rec2020) can tell us how to maximize brightness with a range of different green emitter peak widths. Specific peak positions are not captured in this figure but are available upon request [2].

Figure 2. Modeled data for green emitters showing relative brightness at the optimal peak wavelength (y-axis) for peak widths ranging from 40nm to 20nm. The data are split into three groups based on the specified coverage of Rec2020 color gamut.

What does this data tell us about how the narrower emission of perovskites drives improved colour gamut and brightness?

Conclusion 1: Up to 10% boost in brightness

Within each color gamut group the brightness increases when the peak width decreases. In the case of 90% Rec2020 an emitter with 25 nm FWHM such as Helio’s perovskites shows a brightness increase of almost exactly 10% compared to a 37 nm FWHM emitter such as InP QDs. A 10% increase in brightness translates directly into a 10% reduction in power consumption at a given brightness which is of great importance to consumers and industry regulators.

Conclusion 2: Up to 10% gain in colour gamut

If we compare the current state of the art InP emitter (37 nm FWHM) at 80% Rec2020 to Helio’s perovskites (25 nm FWHM) at a color gamut of 90%, we can see that the Rec2020 colour gamut can be increased by 10% at almost the same brightness. On the other hand, any increase in Rec2020 which is not achieved by introducing narrower emission will noticeably reduce brightness.

The summary

The narrower emission of Helio’s perovskite materials provides a valuable option to panel makers to increase display performance. We have shown here how Helio’s perovskites can achieve up to 10% increase in brightness without a colour gamut penalty, or a 10% increase colour gamut without a brightness penalty (or a combination of the two) when compared with InP QDs. 

Panel and device makers are constantly striving to increase brightness and reduce the power consumption of displays whether they are LCD, QLED or OLED including QD-OLED. This is a ‘brightness battle’ where product designers are squeezing out every photon they can with tweaks to the LEDs, backlights, and algorithms. In this context, perovskites provide a unique opportunity to boost brightness by 10% compared with InP QDs whilst also bringing the other unique advantages of perovskites including ultra-thin colour conversion layers (see our previous articles below).

Don't miss our other blogs in this series....

In previous articles, we have described how colour conversion is driving innovation in TV displays, introduced the unique advantages of perovskites, highlighted the importance of the high optical absorbance of perovskites and looked at how perovskites can increase performance and manufacturing simplicity when applied to in-pixel architectures.

References:

1. https://en.wikipedia.org/wiki/Luminous_efficiency_function
2. ICDT paper 55.6 "High performance perovskite nanocrystals and photoresists for in-pixel colour conversion", presented by Helio on April 3, 2023 in Nanjing, China.