Progress of M2Si5N8:Eu series in industrial LED phosphors

Abstract

Herein, we provide an overview of the history of research on industrial LED phosphors, focusing on Sr₂Si₅N₈:Eu, which was the first reported nitride phosphor but it is not ideal for industrial LED lighting and displays. While it had desirable red emission and blue excitation properties for LED devices, its thermal stability and working durability were initially poor. The next significant concept appeared during the research on M₂Si₅N₈:Eu for more than 20 years. Understanding photoionization, one of the mechanisms for thermal quenching, has deepened with the new concepts of “trap center-assisted improvement of thermal stability”, “europium-trapped exciton”, which makes the excited electron return from the trap center to the 5d band of europium, “PL intensity induced by the modification of the energy distance between the conduction band and the trap center or the 5d band of Eu²⁺”, etc. Understanding the non-emissive process in the configurational coordinate model (jumping of excited electron over the crossing point between the two excitation and ground state curves), as another mechanism for thermal quenching, has also deepened with the new concepts of “cation disorder-assisted improvement of thermal stability”, “size balance between covalent and ionic parts”, “relation between the number of coordination of N³⁻ to Si⁴⁺ and the structural stability of host crystal or Eu–N bond”, etc. For more than 20 years, the composition of Sr₂Si₅N₈:Eu has been successfully modified to (Sr,Ba)₂Si₅N₈:Eu, and then (Ba,Ca,Sr)₂Si₅N₈:Eu and (Sr,Ba)₂(Al,Si)₅(N,O)₈:Eu having better thermal stability. The above-mentioned fundamental research for clarifying mechanism and finding phenomena is considered to have contributed to the successful industrialization of the 2-5-8 group for LED lighting and displays.