White LEDs
The development of high brightness white LEDs has opened up several important new markets for LEDs. Their use as backlights for small LCD displays has emerged as the first main market. This is currently a billion dollar market and is set to grow as white LEDs are used in ever-larger displays. Further into the future, white LEDs will start to impact on the lighting industry. Currently LEDs account for a negligible amount of this market, however, the use of solid-state lighting (SSL) is predicted to enter a period of rapid growth driven by the substantial energy savings offered by these devices.
Currently the majority of white LEDs on the market consist of a blue LED combined with a yellow phosphor and luminous efficiencies are around 40 lm/W. However the white light produced using this approach is bluish and lacks warmth. In order to improve the colour, new emitters need to be developed. With current technology it is difficult to produce new phosphors that span the whole gamut of colours and can be excited by blue/UV light.
Research at Sharp laboratories of europe (sle)
At SLE we are investigating a new class of phosphors called colloidal quantum dots (QDs). These are small semiconductor nanocrystals (2-10 nm diameter) prepared by a wet chemical synthesis. The unique property of these phosphors is that the colour of light they emit can be tuned by simply altering the size of the nanocrystals.
Figure 1: Fluorescence image of QD/polymer composites
Chemical Synthesis
Some commercially available QDs contain undesirable materials such as cadmium, so at SLE we are developing synthetic strategies to prepare materials based on III-V semiconductors. This work is carried out in SLE’s fully equipped chemistry facility. QDs prepared in this way are coated with a shell of organic capping groups allowing solubility in a range of organic solvents and polymer resins for integration with LEDs.
Figure 2: A chemist preparing quantum dots at SLE
Characterisation
Before integrating the quantum dots with an LED, we first measure the fluorescence efficiency of the quantum dots. The fluorescence efficiency of colloidal quantum dots is affected by a wide variety of factors such as the material they are made of, their size, and their emission colour. A low efficiency can be an indication that the synthesis of the quantum dots has not been optimal, and there are a lot of defects quenching the emission.
The efficiency of fluorescence is measured by determining the absolute photoluminescence quantum yield (PLQY). This gives a measure of the number of photons emitted per absorbed photon. We can do this measurement with the quantum dots in solution or we can do a more realistic solid-state measurement with the QDs dispersed in a polymer matrix. The solid-state measurement is carried out in an integrating sphere, so light emitted at all angles is collected.
Figure 3: SLE’s integrating sphere
Once the quantum dots are incorporated into an LED, a state of the art LED measurement system is used to characterise the quantum dot LEDs. This gives us detailed information about the performance of the LEDs such as the total flux emitted by the LED and how the colour of the LED is changes with the measurement angle.
