Compared to display applications, solid-state lighting demands more of its down-conversion phosphors. These need to perform at elevated temperatures, in close proximity to a bright ultraviolet or blue LED source, and without significant protection from the environment, e.g., suspended in a polymer rather than sealed in glass. The efficiency with which phosphors can convert the LED source into visible light, comprising mixtures, for example, of yellow and red photons, is in part related to whether all down-converted photons are in a spectral range visible to the human eye. Traditional phosphors are broadband, such that a significant percentage of the photons emitted by red-emitting phosphors are unproductive. For optimal white-light LED efficiency, alternative phosphors are sought that possess high quantum yield (QY), high stability, and narrowband red emission. Colloidal semiconductor nanocrystals, or quantum dots (QDs), are candidate down-conversion materials due to their characteristic large absorption cross section (efficient excitation), fast radiative recombination (reduced saturation), and efficient narrowband photoluminescence (higher efficacy LED). That said, conventional QDs suffer from significant self-reabsorption (overlap of absorption and emission wavelengths), rapid energy transfer to non-radiative trap centers, and instability in oxygen-containing environments under photoexcitation. To address the deficiencies of QDs, we have explored the application of ultra-photostable “giant” QDs (gQDs), where the synthetically engineered core/thick-shell nanostructure affords suppression of self-reabsorption and non-radiative processes, as well as long-term stability at room-temperature. Toward meeting the most stringent requirements for QY (&gt;80%) and stability at elevated temperature and high photon flux, we have developed two synthetic variants of the gQD, observing an apparent conundrum – the highest QY gQDs are not the most stable and the most stable gQDs do not possess near-unity QY. Here, I describe the single-nanocrystal-level optical and structural characterizations that have resulted in precise synthesis-structure-properties correlations for the informed design of new ultra-bright and ultra-stable gQDs for efficient solid-state lighting.

Shining a light on ultra-stability in nanomaterials for energy-efficient solid-state lighting

256th National Meeting (2018)

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