Light-emitting diodes (LEDs) are semiconductor devices that convert electricity into light. The use of white light emitting diodes (WLEDs) is particularly interesting due to their wide array of applications.
Current approaches to develop WLEDs include mixing individual LEDs emitting red, green and blue colours to produce white light or by using phosphor materials to convert monochromatic light from a blue or ultraviolet (UV) LED to broad-spectrum white light. However, the latter approach suffers from lower efficiency compared to normal LEDs and poor colour rendering index (CRI).
Combining the near-UV LED and the tri-colour phosphors may produce high-quality WLEDs, but the sulphide-based red phosphors have poor absorbance at the near-UV region and lack chemical stability. Hence, more efficient phosphors are needed.
Of late, there have been investigations into new and novel families for high-performance phosphors applicable to WLEDs, cathode ray tubes, plasma display panels, flat panel displays and fluorescent lamps. One of these studies is being conducted at the National Institute for Interdisciplinary Science and Technology (NIST) in India.
The research group is focusing on the development of novel, stable and efficient phosphors that can be effectively excited in the near UV or the blue region for use in generating white light in phosphor-converted white LEDs. Various host matrices, such as alkaline sulphides, lanthanide oxysulphides, bromates, phosphates, and oxides have been used to synthesise the primary colour-emitting phosphors, containing Ce3+ (blue), Tb3+ (green) and Eu3+ (red).
Oxide phosphors, which are currently used in WLEDs and flat panel displays, are attractive due to their superior chemical and thermal stability. Working on phosphate and oxide-based hosts, the research team developed new red and green emitting phosphors, AYP2O7.5:RE3+ (A = Ca and Sr; Eu and Tb), with xenotime type structure, under near-UV irradiation. The research also investigated a novel powellite-based red-emitting phosphor (CaLaNbMoO8:xEu3+).
They found that the material exhibits enhanced red emission of Eu3+ under the excitation of near-UV and blue light, making it a potential red phosphor in WLEDs. So far the research is focused mainly on binary or ternary oxides in most of the systems, and research in complex oxides is limited. “Our group believes the complex oxides play a major role in achieving better properties in most of the applications,” says Prabhakar Rao, one of the researchers.
Self-absorption is one of the main challenges in phosphor-based WLEDs that leads to deteriorating quality of whiteness and unstable shades of white light. The self-absorption causes the phosphors that emit longer wavelength colours (blue) to absorb shorter wavelengths colours (green and red) produced by other materials. The research team is tackling this issue in two ways – discovering phosphors with full colour emission and minimising self-absorption.
Research on high performance phosphors is important for the development of various applications. As commercial WLEDs in lighting applications remain in competition with cheaper conventional lighting sources such as light bulbs and florescent tubes, it will be interesting to see if using the new phosphor materials will enhance the performance of WLEDs. It is expected that energy efficiency and lower cost will be the main performance criteria to achieve more competitive WLEDs.
For more information contact Patrick Cairns, Frost & Sullivan, +27 (0)21 680 3274, patrick.cairns@frost.com, www.frost.com
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