This Small Business Innovation Research Phase I project seeks to greatly improve the light output of white-light light-emitting diodes (LEDs) using fluorescent nanoparticles or quantum dots (QDs) made from silicon. The most efficient and economical design for a contemporary white LED is based on a blue LED chip which excites a yellow-emitting rare-earth phosphor. This particular mix of blue and yellow hues produces a cool and bluish-white light which cannot faithfully render some colors in illuminated objects, especially in the case of red tones. The proposed solution places red-color-enhancing silicon-QDs (SiQDs) in the beam-path of these white LEDs; by absorbing some of the blue light output and re-emitting red light, the SiQDs complement the absent red spectrum, and thus improve the overall color rendering performance. The company has thus far established baseline methodologies for the scalable production of brightly luminescing SiQDs. This Phase I project will enable the company to improve the performance of said SiQDs, and also establish their long-term performance stability.
The broader impact/commercial potential of this project is to develop SiQDs that can ultimately replace current rare-earth-derived phosphors. China has a virtual monopoly over global rare-earth supply, and is enacting draconian restrictions on extraction and exportation to capitalize on its position. The cost of rare-earth phosphors used in packaged white-light LEDs is thus set to soar because of the dwindling supply and increasing market demand for additional rare-earth phosphors (such as europium-doped nitrides or sulfides) to generate reds for premium LED offerings. Hence, an earth-abundant and cost-effective alternative phosphor material is desperately needed. Semiconductor QDs are considered a promising solution due to their wide-range wavelength-tunability and high photoluminescence quantum yield. Because of the inert nature and the abundance of silicon, SiQDs can provide a non-toxic, high-stability, and lower cost solution, in comparison with established heavy-metal cadmium-chalcogenide QD materials (such as CdSe QDs). The project will serve as the first step in the development of a repertoire of products which permit the creation of all visible light wavelengths via SiQD-phosphors, allowing white-light LEDs to be free from the need for rare-earth- and cadmium-based phosphors. This will in turn make LED lighting more aesthetically attractive, affordable, and environmentally friendly.
The goal of this NSF SBIR Phase I project is to develop low-cost and eco-friendly silicon quantum dot (SiQD) phosphors as a substitute for the conventional phosphors derived from rare-earth elements (REEs) for lighting applications based on light-emitting diodes (LEDs). Contemporary white-light LEDs are predominantly of the design that mixes the blue light emitted from blue LED chips with the yellow light that is generated by a yellow-emitting REE phosphor, which is excited by said blue light. To improve the color-rendering, especially in red colors, a red-emitting REE phosphor, is added to the yellow-emitting phosphor, tuning the original "cool white" into "warm white". However, such red-emitting phosphors are the most expensive of phosphors, costing up to three times the price of the yellow-emitting phosphors. As a result, the company plans to first commercialize its red-emitting SiQD-phosphors as a point of entry into the LED lighting market. During the course of the project, the company has greatly improved the photoluminescence quantum yield (PLQY; the measure of color-conversion efficiency of a phosphor) of their red-emitting SiQD-phosphors from 20% to 51%, at 365 nm excitation. As a proof-of-concept, the red-emitting SiQD-phosphors were applied in white-light LEDs, and the resulting warm white light had a color-rendering quality close to that of an ideal incandescent light source. More broadly, China has a virtual monopoly over global rare-earth supply, and has recently enacted draconian restrictions on extraction and exportation to capitalize on its position. Since then, the cost for REE phosphors has risen significantly, most severely impacting the prices of red-emitting REE phosphors that are required for premium warm white-light LED lamps. Furthermore, rare-earth bearing ores are naturally laced with radioactive constituents, like uranium and thorium; rare-earth extraction often causes radioactive contamination about mining sites, and rare-earth processing inevitably generates concentrated radioactive wastes. Hence, cost-effective phosphors without supply-restrictions and severe environmental concerns are desperately sought by the global lighting industry. Semiconductor quantum dots (QDs) are considered promising solutions due to their wide-range color-tunability (emission color is tuned by the QD size), high PLQY, and not using any REE. Because of the inert nature and the abundance of silicon, a derivative of common sand, SiQDs can provide a low-cost and non-toxic phosphor solution, in contrast to the more established CdSe QDs which suffer from heavy-metal cytotoxicity. The company strives to develop a full spectral-repertoire of SiQD-phosphors, thus allowing white-light LED manufacturers to be free from the need for REE and CdSe phosphors, making current LED lighting more attractive, affordable, and more health- and environmentally-friendly.
