The next generation of solid-state semiconductor-based lighting devices may not be LEDs at all—instead, they could be gallium nitride (GaN)-based laser diodes, which have higher efficiency than GaN-based LEDs and also do not suffer from efficiency droop at high power outputs. As with LEDs, the laser diodes would be combined with phosphors to create white light. Engineers at Appotronics (Shenzhen, China), which makes laser-projector displays, and Sun Yat-Sen University (Guangzhou, China) are very interested in these new light sources and are developing the ceramic phosphors that are needed to handle the high intensities produced by laser diodes. They are experimenting with aluminum oxide/yttrium aluminum garnet (YAG):cerium (Al2O3-YAG:Ce) phosphors of varying composition to find the best laser-lighting phosphor for the job.
In general, they found that compositions that had a higher molar ratio of Al2O3 to YAG:Ce made better phosphors in a number of ways. First, due to higher thermal conductivity, the accumulated heat under high-power laser light affected a smaller region in the ceramic; second, the light-extraction efficiency from the ceramic surfaces increases due to higher scattering; and third, the crystal grain size was smaller, which in combination with the higher thermal conductivity reduces cracking. The best phosphor composition produced an output luminescence power of 2 and 11 W, respectively, for an input light power of 5 and 32 W, respectively, at a wavelength of 445 nm, with an almost linear relationship in between. The researchers also noted that the type of sintering additive was important, with tetraethoxysilane (TEOS) producing phosphors with the highest quantum efficiency. Reference: M. Xu et al., Opt. Express (2019); https://doi.org/10.1364/oe.27.00087.