Abstract
Humans perceive color with the help of photosensitive tissues found at the back of their eyes. The rod and cone cells present in the human retina act as photoreceptors that detect the incoming light and transmit signals to the brain with the assistance of optical nerves. Typically, under photopic vision, the human eye has maximum sensitivity for visible light, corresponding to the greenish-yellow region of the visible spectra. Phosphor plays an indispensable role in modern-day lighting applications and a coating of phosphor can be found in almost all modern fluorescent and solid-state lighting devices. Furthermore, it is very difficult to achieve high efficiency direct green emission from LED’s, a challenge often referred to as the “Green Gap” by the industry; consequently, synthesizing efficient green-emitting phosphors is crucial for the lighting industry. Inspired and motivated by the above facts, the primary objective of my research is to synthesize a phosphor that would generate an intense green emission upon blue or ultraviolet (UV) excitation.
The Europium (Eu2+) doped Strontium Thiogallate (SrGa2S4) phosphor (SrGa2S4:Eu2+) is a well-known high luminescence efficiency phosphor that emits green light in the 530-550 nm range when excited by blue or ultraviolet light in the 200-450 nm range. However, the biggest challenge with this material is its rapid luminescence degradation when left under ambient condition for several weeks. This limits the use of this phosphor for LED and display devices. Coating the surface with protective barrier layers has been used to provide better stability. However, they are not adequate for shelf-life of 20-25 years which is necessary for lighting and display applications. The origin of the luminescence degradation can be traced to the presence of multiphase crystallites in the synthesized powder using the traditional solid-state growth method. The multiphase crystallites consist of compositions that have poor luminescence efficiency and high reactivity with oxygen. In this research, a new synthesis process has been developed to reduce or eliminate the unwanted low-efficiency phases from the phosphor powder matrix. Followed by the solid-state synthesis, a high-temperature flux crystal growth was used. Phosphors generated using the high-temperature flux growth step exhibit enhancement in luminescence efficiency by 100-150% compared to the solid-state synthesized phosphors. In addition, luminescence degradation has been found to be significantly reduced. Phosphors left under ambient conditions for months showed minimal change in luminescence efficiency. An intense green luminescence with an emission peak at 535 nm was found in the synthesized phosphors indicating that the primary phase of the phosphor composition was unaltered by the flux growth step. An additional benefit of the process used in this research is the elimination of hazardous Hydrogen Sulfide gas that is being used in the industry for synthesizing these phosphors. The synthesis process used in this research is based on solid-state starting materials. Future research efforts are necessary to optimize the flux growth temperature and crystal growth rate (growth duration) to study the trade-off between crystal phases and luminescence efficiency and degradation (shelf-life) of this compound. The effect of higher flux growth temperature on the decomposition of the high-efficiency thiogallate phase needs to be researched and understood.;
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