Design, Synthesis and Modeling of Luminescent Ceramics for Application in Solid State Lighting

University Of California-San Diego, La Jolla CA

Investigators

Abstract

NON-TECHNICAL DESCRIPTION: Lighting accounts for 22% of the total US electrical energy use, which translates to $50 billion per year spent on lighting accompanied with 130 million tons of carbon emitted into the atmosphere from fossil fuel plants. Solid state lighting, based on blue-emitting light emitting diodes with a luminescent powder (phosphor), has emerged as highly efficient, long lasting light sources to replace incandescent and fluorescent lighting. The phosphor market in the US for white-emitting LEDs is currently $500M/year and is expected to reach $1B/year by 2015. Identifying new phosphors in a reliable and systematic way with high quantum efficiency and thermal stability is crucial for these new energy saving devices. Nanophosphors represent an exciting opportunity to reduce light scattering, thereby improving the extraction efficiency. This work is interdisciplinary and spans the fields of materials science, optical properties, chemistry, and atomic scale modeling that involves both experiments and modeling. Graduate students are trained in sophisticated electron microscopy techniques, theoretical and computational methods. Diversity efforts are continued and strengthened. The involvement of students from a Hispanic-serving Institution and a middle/high school that serves low-income students are included. New classes are developed for the graduate curriculum. TECHNICAL DETAILS: Solid state lighting, based on blue-emitting (450 nm) light emitting diodes (LEDs) with a luminescent powder (phosphor), has emerged as highly efficient, long lasting light sources to replace incandescent and fluorescent lighting. New diodes that emit in the near UV (370-410 nm) have recently been recognized as chips that could improve the extraction efficiency of the light source. This new development requires the discovery of new phosphor systems in the nano-sized range to fully exploit this new technology. The phosphors used in this work are wide band gap materials (hosts) that contain a small amount of activator (rare-earth element). Depending on the host:activator combination, colors across the visible spectrum can be obtained. This project aims at validating the following three hypotheses: (1) using an empirical approach combined with first principles modeling, new high quantum efficiency, thermally stable phosphors can be identified for near UV LED white-emitting light sources, (2) the low quantum efficiency of nanosized phosphors can be determined and perhaps overcome and (3) modeling using a combination of semi-local and hybrid density functional theory will provide insight on the mechanisms for photon absorption and emission of new phosphor systems, the phase, chemical and thermal stability and on the quantum efficiency of nanosized phosphors. These hypotheses are tested by conducting by a variety of experimental and computational tasks: (1) the design of phosphors in which the excitation energy lies in the near UV spectral range of 370-410 nm, (2) the design of phosphors in which sensitization is via the excitation of complex functional groups, (3) using a combination of analytical tools, a systematic approach will be conducted to evaluate the factors behind the low quantum efficiency of nanosized phosphors (< 200 nm). Surface and bulk analyses will identify the local environment of the activator and the traps that quench the luminescence and (4) a hierarachy of first principles methods are used to investigate the phase stability, aqueous stability, thermal stability and electronic structure of the phosphor materials to be synthesized and tested experimentally. The theoretical calculations are used to guide and interpret experiments and also to guide which compositions and structures of the phosphor materials are most promising for UV LED applications.