Early failure of high-power white LEDs for outdoor applications under extreme electrical stress: Role of silicone encapsulant

Solid state lighting systems have deeply penetrated the market and are widely available. White light emitting diodes (LEDs) are based on a gallium nitride blue-emitting chip and a phosphor-based layer is used to convert blue radiation to white light, in combination with an optical system to enhance light extraction. High temperatures can be detrimental for LED reliability, resulting in a gradual or catastrophic failure. Therefore, the design of such devices, especially in relation to use cases that require operation at high current and temperature levels, needs to be based on reliability considerations. In this work we analyze the robustness of high power white LEDs submitted to driving currents exceeding the nominal levels by means of electrical and optical characterization, as well as on failure analysis techniques. The results highlight a significant degradation of the main optical parameters (luminous flux, color coordinates, correlated color temperature, …). Device cross sectioning highlighted a heavy darkening of the silicone material in the overstressed devices, and a delamination of the phosphors. Raman spectroscopy analysis highlighted an enhancement in luminescence of degraded samples. Such degradation was ascribed to the high temperature reached by the LEDs during the stress, that caused the degradation of the silicone lens; the process can be enhanced by the low efficiency of the phosphors and LEDs at high temperatures, resulting in a stronger heating of the devices.