Phosphor downconverters remain a high-priority research area for LEDs. Current state-of-the-art phosphors have several drawbacks, which may include material supply issues, poor color rendering, low efficiency, and limited chemical/thermal stability. Quantum dots (QDs) offer another potential pathway, but at the level of single dots they exhibit non-emissive properties such as blinking and significant self-absorption. These conventional QD behaviors limit efficiency — especially in the solid state — and useful application as downconverter materials.

Los Alamos National Laboratory (LANL) has developed a new “giant” QD (gQD) that addresses the deficiencies of both phosphors and conventional QDs. Like conventional QDs, LANL’s technology offers color tunability in the red spectral window, which is key for creating warm-white LEDs, and narrow emission peaks that improve efficacy by allowing for less light emission beyond the visible wavelength, thus reducing the amount of wasted energy. Importantly, the technology has unique attributes of being non-blinking emitters as well as efficient absorbers that don’t exhibit significant self-absorption. Both of these critical properties result from intentional “nanoscale engineering” of the QD structure.

With the help of DOE funding, LANL has advanced quantum yields of thick-shell (>6 nm) cadmium-selenide/cadmium sulfide red-emitting gQDs to >80%. The team has also developed a method to study variables relevant to device-level lifetime testing at the single-QD level — time, flux, temperature, and humidity. LANL’s method involves a novel approach to assess quantum yield and to perform other sophisticated optical analyses for individual gQDs under the stress conditions faced by downconverter materials in LEDs for solid-state lighting. This is very important because it’s enabling unprecedented insight into how synthetically controlled nanoscale structure determines both initial efficiency and lifetime stability in response to high flux, temperature, and humidity.  Its use is speeding up the experimental development of high-efficiency, high-stability gQD-based LEDs.

So far, LANL has shown clear correlations between single-dot performance and device-level performance. Namely, gQDs that exhibit 100% recovery in emission efficiency following single-dot-level exposure to high flux and temperature (15 W/mm2 and 100°C) also afford the most stable gQD LEDs (no quantum-yield drop and no blue shifting in emission color after >400 hours of high-temperature-operating-life testing, compared to at least 70% quantum-yield drop and 5 nm blue shift in the case of “non-giant” QDs). The single-dot performance is then further correlated with gQD structural and optical analyses. In this way, the new method is enabling more rapid design and development of advanced gQD formulations. (September 2016)

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