Why do LEDs and other light emitting solid state devices have a definite lifespan?
Because at elevated temperatures they change. Dopants migrate, crystalline defects merge, insulating layers become charged (under the influence of ionizing radiation), contact metals diffuse on and into the semiconductor.
thank you very much. that would mean that higher energy efficiency of a device would mean not only more energy efficiency in and of itself, but higher total lifetime, correct?
Indeed. When the LED efficacy in Lumens/Watt starts to approach 300, the heat generation falls rapidly. All the energy is coming out as light. At that point you only have phosphor and encapsulant degradation due to high luminous flux. It will still degrade but the life could be measured in the many decades, not years, of continuous operation.
Peak efficacies of lab devices today are approaching 200 Lumens/Watt.
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