Nanocrystal Quantum Dots for Use in White LED Lighting Systems (2004)
A quantum-dot LED on a slide.
Testing a quantum dot LED in a thermal chamber.
White LEDs are created by combining inorganic phosphors, commonly YAG:Ce phosphor, with nitride-based blue LEDs. However, it is difficult to achieve a tailored-spectrum white light with available phosphors. Semiconductor nanocrystals, known as quantum dots, may provide an alternative. Quantum dots behave like phosphors, but they can be tuned to radiate any color simply by changing the physical size of the dot. Because the solid-state lighting community is aiming to develop energy-efficient white LEDs with certain spectral characteristics, researchers are now looking to quantum dots as a new resource. In this study, LRC researchers characterized the photometric and optical characteristics of cadmium selenide (CdSe) quantum dots as a first step toward identifying methods to incorporate these nanocrystals in white LED systems.
EXPERIMENT AND RESULTS
Quantum dots (QDs) that emit monochromatic light at 520 nm and 620 nm wavelengths were tested to understand their photoluminescence properties and their potential as a down-conversion material for white LEDs.
Different concentrations of QDs were tested for their efficiency. In high concentrations, QDs absorb much of their own radiant power and light output, reducing their efficiency and causing a peak wavelength shift to a longer wavelength. In this study, an optimum concentration was found to maximize the efficiency.
An experiment also was conducted to understand the absorption of the emission from smaller QDs by larger QDs when two sizes are mixed together. However, because more than one factor was introduced in the experiment, the results were inconclusive.
Finally, to compare the luminous efficiency and thermal sensitivity of the QDs with that of the YAG:Ce phosphor, samples of both were dissolved in the same type of binding material (epoxy) and placed on microscope slides. The results showed that the efficiency of the QDs was much lower than that of the YAG:Ce phosphor. However, these were limited results from small batches of tested QDs, which had relatively low quantum yields.
Additionally, it was found that QDs exhibit a higher thermal sensitivity than YAG:Ce phosphor.
In one experiment, researchers measured the photoluminescence properties of quantum dots mixed into a chemical solution at different concentrations. The left graph shows relative light output as a function of quantum dot concentration when excited by 390 nm and 470 nm wavelength radiations. The right graph shows that the peak wavelength shifts to a longer wavelength when concentration increases. By selecting the optimum concentration, researchers can maximize the radiant power and efficiency of a quantum-dot LED.
A series of studies to optimize the factors affecting QD efficiency, such as quantum yield, excitation wavelength, excitation intensity, and path length, are needed before reaching a final conclusion. It appears, however, that with improved quantum efficiency these types of QD can become useful in the future for white LEDs.
Lighting Research Center