Extrinsic Failure of High-Power Blue LEDs for Horticulture Applications Stressed in Hot Environments

This study examines the reliability of optical materials, specifically silicone lenses, in high-power gallium nitride-based blue light-emitting diodes (LEDs) for horticulture applications. We subjected commercially available LEDs to accelerated stress tests at various temperatures <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left(45^{\circ} \mathrm{C} \text{to} 105^{\circ} \mathrm{C}\right)$</tex> and a current 1.2 times the maximum nominal value. The results indicate a significant decrease in optical power (up to 40%) after prolonged stress, primarily due to the degradation and cracking of the silicone lens. Optical images confirmed a progressive degradation pattern. A second experiment compared a reference silicone with an optimized version. While the reference silicone showed similar degradation, the optimized silicone remained stable for nearly 5,000 hours. Raman spectroscopy revealed the reference silicone to be poly(methyl, phenyl)siloxane, while the optimized one was likely polydimethylsiloxane. The degradation in reference silicone is attributed to a combination of thermal induced chemical decomposition of silicone (possibly enhanced by high-energy blue photons) and thermomecanical stress, leading to depolymerization and cracking. This research highlights the critical role of silicone composition in the long-term reliability of high-power LEDs.