Spray coated red light supplementary phosphors films on different glass substrates for plant growth lighting

The red spectral band plays a crucial role in plant growth. Due to the significant absorption peaks of chlorophyll A and chlorophyll B in the red wavelength range of 640-660 nanometers, red photons can be effectively absorbed by these pigments, thereby promoting photosynthesis. In previous studies, we synthesized warm white LED lights for plants, but the spectral peak was near orange-yellow, so the effect on promoting photosynthesis was not significant. Since the traditional coating process usually requires high-temperature sintering, and commercial red phosphor powder CaAlSiN 3 : Eu sintered in an air atmosphere would be oxidized and thus reduce the luminous efficiency. Therefore, this work introduced a non-sintering coating strategy, using a curable epoxy-based slurry and spray coating technology to prepare phosphor films without the need for high-temperature treatment, thus avoiding the oxidation of commercial CaAlSiN 3 : Eu phosphor powder under high temperature. In this experiment, four spatial layouts of phosphor powders were prepared. It was found that the layered structure was superior to the mixed structure because it reduced the occurrence of reabsorption, supplemented the red spectral band, and retained the yellow-green emission. Under the excitation of a 30 W LED source, the surface temperature of all samples remained below 90 degrees Celsius, far lower than the thermal quenching temperature of the phosphor. Optical property analysis confirmed that this layered phosphor powder film not only enhanced the supplementary effect of the red spectrum, but also significantly improved the color reproduction index (Ra = 86.5, R9 = 93.5, R14 = 91.9). In the plant growth experiment, the average SPAD value of the lettuce samples irradiated with the optimized film was 40.0, which exceeded the natural light (26.0) and the previous experimental results (29.1), proving the practical application value of the proposed layered optical coating structure in promoting photosynthesis.