Irreversible Thermodynamic Bound for the Efficiency of Light-Emitting Diodes

Energy-efficient lighting is an important goal, but what is the ultimate limit that we can expect from our technology, and how close are we? Thermodynamic analyses of LED efficiency have been discussed in the reversible case. Incorporating the concept of passive optical extraction, the authors propose an $i\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}e$ model for LED operation, for a more realistic view of theoretical efficiency. Even so, the maximum wall-plug efficiency is $u\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}d$ as emission intensity diminishes, and in the range useful for indoor lighting, output can significantly exceed the electrical input power, as the LED cools the room while it shines.