Quantum Dots, Quantum Mechanics & Displays

What Are Quantum Dots?

Quantum dots (QDs) are tiny crystals in the nano-meter range. They consist of a cluster of semiconductor atoms, often surrounded by an additional semiconductor layer. An outer functional polymer or lipid coating enables coupling with proteins, oligonucleotides (short DNA or RNA molecules), antibodies, or other molecules (Figure 1). Measuring between 2 and 10nm – roughly 10 to 150 atoms – they exhibit remark-able optical and electrical properties.

These characteristics can be tuned by adjusting particle size, material, and composition. These features make quantum dots interesting for a wide range of applications, including optical sensors, biomonitoring, drug delivery, solar cells, displays, and even photocatalysis (light-driven chemical reactions). In 2023, Alexei Ekimov, Louis Brus, and Moungi Bawendi were awarded the Nobel Prize in Chemistry for their three decades of pioneering research on quantum dots.

Quantum Dots – Applications in Displays

What makes quantum dot technology so interesting for displays is photoluminescence. Quantum dots can absorb light and re-emit it at a different wavelength. The color of the emitted light depends directly on the particle size – the smaller the dot, the shorter the wavelength. This is due to the fact that very small particles, only a few nanometers in size, provide less space for electron oscillation, directly affecting optical properties. Using InP (indium phosphide) and ZnS (zinc sulfide) quantum dots, 6nm dots emit long-wave red light, 3nm dots emit green light, and 2nm dots emit short-wave blue light. Typically, a blue light source with a wavelength of 450nm is used.

Comparing TFT-LCD Technology with New Quantum Dot TFTs

In general, a conventional TFT display uses a backlight. White LEDs produce light that passes through color filters, creating a colored image. White LEDs consist of a blue LED chip covered with YAG phosphor (yttrium aluminum garnet), which emits white light. The downside of this technology is that the resulting colors are not as pure and vivid. This is due to the wide emission spectrum, causing colors to interfere with each other.

In a quantum dot display, however, blue LEDs replace the white ones for the backlight. The QD layer is placed directly on the light-guide. When simulated by the blue LEDs, the quantum dots emit vibrant red and green light, which combines with the blue light to produce white light. As in a regular LCD, the final image is formed by passing
the light through a color filter.

Comparing OLED Displays with Quantum Dot OLED Displays

It’s important to note that »OLED« here always refers to an AMOLED display, which is a display with a TFT layer (active matrix) that controls the individual organic LEDs for each pixel. In standard OLED displays, white OLEDs are used for the pixels or subpixels. Like in TFT displays, color is generated using a color filter. The differ-ence from TFT-LCDs is that OLED displays also make use of the white light emitted by the OLEDs.

When integrating quantum dot technology, the QD layer and color filter are placed directly above the light source – in this case, the OLEDs. White OLEDs are replaced by blue ones, and the quantum dots emit red and green light when excited by the blue light, while the blue is used directly.

Advantages of Quantum Dot Displays

Wider Color Gamut: Integrating quantum dots expands a display’s color gamut, allowing it to render a broader range of colors. This enables manufacturers to reproduce colors that are closer to what we see in nature. For a better understanding, the color gamut refers to the full range of colors perceivable by the human eye. In the CIE color system, colors are represented mathematically, producing a tongue-shaped diagram showing saturation and hue (wavelength). One example is the sRGB (standard red-green-blue) color gamut, which defines the colors that can be displayed on a digital device (e.g., a monitor). Quantum dots can expand a display’s sRGB gamut by 35-45%.