Image: University of Cambridge
Quantum-dot LEDs (QD-LEDs) are based on quantum dots (QDs), semiconductor particles just a few nanometers in size. Because of their extremely small size, their behaviour is governed by quantum mechanics. By ‘exciting’ quantum dots with electricity or driving radiation, they can be induced to emit light, the color of which is determined by the size of the QD.
QD-LEDs are already a leading technology in display technology, with companies such as Samsung carrying commercial television ranges based on QD-LEDs. Competition is rife between microLEDs, organic LEDs (OLEDs) and QD-LEDs when it comes to displays. QD-LEDs are not yet widely used in general lighting, and currently only one company offers a commercial QD-LED for lighting-based applications: ams OSRAM. However, recent developments in the technology have suggested QD-LEDs may become important for general lighting applications.
Conventional LEDs have a phosphor layer over a blue-pump LED. These are phosphor-converted LEDs (PC-LEDs). In QD-LEDs, the QDs are also placed on top of an LED chip, in a slurry of silicone encapsulating material. The LED drives the emission process in the QD. There are significant advantages to using QD-LEDs over conventional PC-LEDs. Perhaps most importantly, QDs offer the option of highly-controllable spectral tunability. QD narrow band emmissions enable high levels of spectral tunability while phosphor converted LEDs create broad emissions that are much less tunable.
QDs also provide a specific advantage in the so-called ‘green gap’, an area of the spectrum well known for reduced performance of green LEDs. QDs also offer efficiency advantages for producing red light. Typically, when using phosphor-based LEDs for red light, an inability to control the bandwidth of the output emission spectrum of the phosphor means that some infrared light is emitted. For visible LEDs, this light is effectively wasted. For QD-LEDs, a narrow emission spectrum means that a sharp cut-off can be obtained in the red, providing higher efficiencies in lighting-based applications. QD-LEDs also provide a cost-effective path to very high color rendering LEDs. A recent study by University of Cambridge showed that only 4 QD colors could create QD-LEDs with a CRI of 92.
Remaining challenges for QD-LEDs are:
- Improving efficiency to compete with PC-LEDs,
- Stability and durability of QD, due to “on-chip” thermal stress and environmental degradation of the QDs,
- Higher cost of manufacturing.
Some researchers believe major improvements in QD-LEDs are not far off and could allow QD-LEDs to outcompete PC-LEDs.
You can learn more at ElectroOptics here.
Image: ams OSRAM
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