White is a complicated colour. This is because what we perceive as white light is a combination of all the different colours in the visible region of the electromagnetic spectrum. However, not all white light is quite the same. Some lights we perceive as warmer or cooler than others, depending on the exact balance of three primary colours used. Dr Enguo Chen at Fuzhou University has been exploring some of the most complex facets of white balance for displays, including those faced by new quantum dot technologies.
The perfect display should be capable of rendering any colour, with excellent contrast and brightness at the highest resolution. Each of these desirable characteristics poses its own unique technical challenge. While display technologies have made huge leaps forward in recent years, we are still far from achieving all of this, particularly in a device that is energy efficient, lightweight and cheap.
Display panels are made up of tiny units, called pixels. A modern computer monitor may have tens of thousands of these, each with the capability of generating combinations or independent emission of red, green and blue light. By balancing and combining these colours, a single pixel can generate a subset of colours, known as a gamut. The wider the gamut, the greater the range of colours that can be generated.
The numbers of pixels in a display contributes to its resolution. A single pixel has a finite size and so a display with a given area can only contain so many pixels. A high pixel density generally gives a good resolution producing a realistic, smooth image. Low numbers of pixels lead to graphics like those found in old arcade games, where the objects on screen are clearly made up of large ‘blocks’ of colour.
All of these factors make display architectures very challenging to create. Firstly, you need chemical compounds capable of generating large amounts of light in the colours you want in a way that can be controlled on demand. Secondly, you need to find how to combine all of these compounds in a small area and all of this needs to be highly reproducible and accurate for mass manufacturing. Thirdly, as Dr Enguo Chen at Fuzhou University has shown, you need to worry about the properties of the light itself. Although device structures are so small that they are invisible to our eye, they are large relative to the visible wavelengths of light. Over these microscopic distances, light can be focused and defocused, which in turn also affects the colour mixing in the display.
Dr Chen and his team have been working on ways to calculate and understand the behaviour of different colours of light in colour-converted quantum dot pixels and develop an approach that anyone can use to calculate the right distribution of the three-coloured subpixels in their display architecture to achieve the widest gamut and best colour reproduction in their colour-converted display.
Quantum dots – magic chemistry
At the heart of any display are molecules or materials that luminesce. This means that when they are supplied with energy in some form, be it through electrical charge or by absorbing light from another source, the molecules will start to emit light of certain colours. There are many different families of compounds capable of luminescence. Some are even found in nature, such as the organic molecules in fireflies that allow them to glow at night.
Some of the most widely used display technologies make use of organic light emitting diodes (OLEDs). These are films of organic molecules that luminesce when an electrical current is applied. OLEDs are popular as they can be easy to print and fashioned into flexible materials, for wearable and foldable displays.
However, they also have their downsides. OLEDs do not always have excellent brightness and can sometimes be problematic and moisture-sensitive when they are used in displays. This is why there is so much excitement around the possibilities offered by quantum dot displays.
Quantum dots are a type of nanoparticle that exhibit some strange effects due to their small size (see Figure 1). The electronic structure of the quantum dot, which dictates what wavelengths they absorb and emit, more closely resembles that of an atom than a molecule. This electronic structure can be tuned easily by changing the size and shape of the dots, which makes it possible to control exactly what colours they emit.
Dr Chen and his colleagues have been developing some new quantum dots and corresponding pixellation approach that are perfect for incorporating into display devices. Here, they offer much brighter colours than is achievable with OLEDs and the possibility to extending the colour gamut far beyond what would be otherwise achievable.
Building a device
What Dr Chen does is take some of these quantum dots to create what is known as a pixelated quantum dot colour conversion film (QDCCF) – one of the most exciting technologies for next-generation, high-pixel-density, full-colour displays. These QDCCFs each have their own blue mini-LED backlight to generate the light to excite the quantum dot layers so they luminesce (see Figure 2). Then they have a diffuse plate to control the light to the quantum dots and on top, they have the final quantum dot layer.
This quantum dot layer consists of three distinct regions of red, green, and blue subpixels. The light emitted from each of these will mix to generate the range of colours we see on the display, but how does Dr Chen know how to arrange these different subpixels to get the best colour mixing?
Displays are assessed on their colour gamut, which essentially describes the possible spectral combinations achievable with the red, green and blue subpixels. However, this only accounts for the spectral mixing of the light. What it neglects is any consideration of how the focusing of the spatially separated subpixels may contribute to the appearance of the display.
By using advance lithography techniques that allow for very precise etching and machining of devices, Dr Chen has been able to explore how the positioning of the subpixels affects the display quality and develop rigorous descriptions to calculate the exact white balance point for a pixel to have the optimal display properties (see Figure 3). This makes it possible to exploit fully the achievable colour gamut with highly-tuneable quantum dots and avoid any colour losses that would arise from imperfections in the device construction.
Widening the gamut
Having precise models for calculating conversion efficiencies and the light transmissions of various colours is key for streamlining the device manufacturing and testing process. Without Dr Chen’s models, it would be necessary to trial hundreds or thousands of different combinations of subpixel placements to find, by trial and error, the optimum combination. Now, this modelling can all be performed on a computer with enough accuracy that what gets made is already optimised.
With new quantum dots making it possible to generate bright new colours, and Dr Chen’s models for both understanding device design for QDCCFs, these technologies are moving towards being in a display near you: bigger, brighter, and flashier than ever before.
This is a good question. Photoluminescence and electroluminescence are two ways to drive pixelated quantum dot colour conversion films in displays. We have been working on the photoluminescence route for several years and are pleased to see that this technology is getting more mature. Recently, the main challenge on photoluminescence should be the conversion efficiency improvement with the display pixel size shrinkage. In terms of electroluminescence, quantum dot displays still have a long way to go. At this stage, long-term stability is the most important hurdle to overcome for actual displays.
- Chen, E., Lin, J., Yang, T., Chen, Y., Zhang, X., Ye, Y., … Guo, T. (2021). Asymmetric Quantum-Dot Pixelation for Color-Converted White Balance. ACS Photonics. https://doi.org/10.1021/acsphotonics.1c00596
- Lin, S., Tan, G., Yu, J., Chen, E., Weng, Y., Zhou, X., … Guo, T. (2019). Multi-primary-color quantum-dot down-converting films for display applications. Optics Express, 27(20), 28480. https://doi.org/10.1364/oe.27.028480
- Xie, H., Chen, E., Ye, Y., Xu, S., & Guo, T. (2020). Highly Stabilized Gradient Alloy Quantum Dots and Silica Hybrid Nanospheres by Core Double Shells for Photoluminescence Devices. Journal of Physical Chemistry Letters, 1428–1434. https://doi.org/10.1021/acs.jpclett.9b03578
- Xu, S., Yang, T., Lin, J., Shen, Q., Li, J., Ye, Y., … Guo, T. (2021). Precise theoretical model for quantum-dot color conversion. Optics Express, 29(12), 18654. https://doi.org/10.1364/oe.425556
Dr Enguo Chen explores complex facets of white balance for quantum dot displays.
- National Natural Science Foundation of China (Grant No. 62175032)
- National Key Research and Development Program of China (2017YFB0404604)
- Fujian Science and Technology Key Project (2020H4021)
- Fuzhou Key Scientific and Technological Project (2020-Z-14)
- Project from Mindu Innovation Laboratory (2020ZZ111)
- Top Victory Electronics (Fujian) Co., Ltd.
- Guangdong Poly Opto-Electronics Co., Ltd.
- Zhangzhou Malata Technology Co., Ltd.
Enguo Chen is currently an associate professor at Fuzhou University. His research interests mainly include optical design of optical systems and emerging display technologies. He has published over 80 papers in academic journals and conference proceedings, and owned over 30 authorised Chinese invention patents.
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