Nanocrystal-Based Phosphors with Enhanced Lifetime Stability

One of the main roadblocks to higher efficiencies for warm-white LED light sources is spectrally-wide red-emitting bulk phosphors, which emit a significant amount of their energy either in the far red or infrared where the eye’s response is poor or zero. Consequently, employing these phosphors (along with bulk green-yellow emitting ones) will result in desired warmer light sources, however, at the expense of lower efficiency. Colloidal nanocrystals have been touted for years as a solution to this problem; however, they still suffer from unwanted quenching of the quantum efficiency and increases in their spectral width at the elevated temperatures and excitation levels that are attained during operation of high power LEDs. We recently formed a new and novel class of high efficiency non-toxic nanocrystals which overcome these long-standing problems with nanocrystals. Prior to product introduction, the nanocrystals need to overcome their main limitation (which is common to all nanocrystals) of insufficient long-term stability. We will deal with this issue by modifying the structure of the nanocrystals, thus highly reducing the degree of nanocrystal oxidation which is the main source of their poor lifetime stability. The work will consist of optimizing the nanocrystal composition, while maintaining the nanocrystal’s very good spectral and quantum efficiency properties at elevated temperature and optical flux. Accelerated lifetime testing will be performed on both nanocrystal- based films and LEDs containing the red-emitting nanocrystals, in addition to green-yellow emitting bulk phosphors. The main goal of the Phase I work is to have less than 10% intensity loss after 500 hours of accelerated testing for both nanocrystal-containing films and LEDs. Given these results, Phase II activities will further optimize the LED lifetimes into the desired range of tens of thousands of hours and complete any required scale up of the synthetic process. By combining the advantaged optical performance with the required product lifetime specification, our non-toxic on chip nanocrystals will be direct replacements for current bulk phosphors, thus enabling LED efficacy to further increase by up to a factor of 1.4, while improving the lighting color quality and enabling custom light sources. These performance advantages will enable both commercial and consumer customers to lower their general lighting energy costs, obtain higher quality lighting, and reduce their carbon footprint.