One of the key trends across electronics manufacturing is reducing the use of potentially harmful materials in consumer devices. Although we tend to hear more about this related to ICs and PCBs, it is equally applicable for display technologies, and display manufacturers need to be aware of RoHS restrictions regarding the use of cadmium-based quantum dots (QDs). Using cadmium may risk an organization’s reputation and potentially result in future liabilities from customer exposure, operator exposure, recycling concerns and a toxic legacy.
As an example, in September 2015, the European Parliament voted to ban the extended use of cadmium in display and lighting applications. Accordingly, the use of cadmium and other heavy metals in electronic and electrical equipment are restricted in the EU under Directive 2011/65/EU, and similar restrictions are being adopted across the world.
The current exemption in Clause 39 of the aforementioned European Directive to allow cadmium-based QDs was recently reviewed, but the EU Parliament has made it clear that EU consumers do not want devices that use cadmium. Key EU member states, environmental groups and consumers are opposed, and we believe the EU Commission is very likely to decide to end Exemption 39 in 2017.
In addition, controls and restrictions are increasing in Asia, with China implementing new RoHS regulations of its own, with Taiwan and Singapore expected to follow suit. Furthermore, major global display manufacturers are moving away from cadmium-based QDs and committing to adopt cadmium-free QDs technology.
Figure 1: Wide Color Gamut & Quantum Dot Display Market Tracker - H1 2017, IHS
What Are Quantum Dots?
Quantum dots are very small semiconductor particles, with optical and electronic properties that differ from those of larger particles. Many types of QDs will emit light of specific frequencies if electricity or light is applied to them, and these frequencies can be precisely tuned by changing the dots' size, shape and material. This makes them ideal for use in display applications, where they offer significant color enhancements, improved color saturation, and a wider color palette that allows them to achieve lifelike color implementation.
QDs have 100 to 10,000 atoms. Particles at sizes below 30nm exhibit a drastic difference from the bulk1 state in terms of optical absorption, energy of exciton (an electron-hole pair combined by coulomb force in semiconductor or insulating materials), and electron-hole pair recombination. A good example is the quantum confinement effect, where a particle size determines the quantum confined energy state—an electron in a box.
QDs with the same materials can emit different colors (photoluminescence (PL) or electroluminescence (EL) depending on the particle size.
- Bulk material Eg: defines maximum PL wavelength (λ)
- Particle gets smaller → Eg increases & PL λ decreases (blue shift)
Figure 2: Chart on left shows the electron density of states vs. the material dimension change, while the photo on the right shows the effective band gap of spherical QDs relative to particle diameter.
Figure 3: Examples of increased color gamut with quantum dots, Source: Dow
Figure 4: Color gamut comparison, Source: Dow
Figure 5: Quantum Dot Film Schematic
Color Filter and On-Chip Applications
Achieving image quality that better reflects the colors found in nature requires an improved color gamut, which is a major driver of display technology. QD on-chip applications lower the QD load per TV, leading to a reduction in the total ownership cost, and color filter applications enable higher system efficiency and color gamut.
The color filter features high system efficiency and color gamut, but it requires solutions that can achieve high-temperature process stability (180°C, 230°C) and concentration quenching2.
Figure 6: Quantum dot color filter illustration
The on-chip application can reduce QD loading, but it requires high-temperature (~150°C) and photon flux stability.
Figure 7: Quantum dot on-chip application illustration
QDs vs. OLED
QDs have an FWHM emission peak below 50nm, whereas OLED’s is typically higher than 50nm. As an inorganic material, QDs can have better thermal stability (joule heating) and a longer expected lifetime. Even in the same composition, QDs vary in color depending on the diameter, and have a similar expected lifetime for all colors, whereas OLED shows different lifetimes by color. Spin statistics are not restricted for QDs; hence 100% equivalent quantum efficiency (EQE) can be achieved.
Figure 8: Color performance of OLEDs compared to QDs
The Cadmium-Free Approach
Fundamentally, QDs will increasingly be used because they deliver richer, more vivid color to electronic displays. Dow is committed to a cadmium-free approach, and has focused on developing TREVISTA™ Cadmium-Free Quantum Dots (CFQD™), which deliver the benefits of QD technology without the drawbacks of cadmium. When incorporated into a display film, our QDs are energized by the light source, emitting deeper reds, brighter greens and a wider color palette.
|| Development in progress
||QDs placed in thin film, covering entire display surface
||QDs placed directly within LED package, which is coupled to light guide
||QDs placed within color filters
||QDs placed between ETL/HTL layers
|| Easy integration and lowest temp/photon flux
|| Reduction in total amount of QD per TV set
|| Higher system efficiency & color gamut than film and on-chip solutions
|| Higher system efficiency and color gamut than OLED
|| Low (20~45°C)
Process T high (~230°C)
Figure 9: Dow’s cadmium-free QD product roadmap
Dow’s perspective is that future displays must meet the needs of customers who wish to comply with RoHS. As such, we are focused on developing cadmium-free QD materials that are free from the EU RoHS-regulated element, and are toxic heavy-metal-free and rare-earth metal-free. Cadmium-free QD displays will soon be widely available, and are expected to dominate the market with their strong performance, safety and environmental benefits.