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Quantum efficiency is the ratio of emitted photons to injected electrons. So, how can an electron generate more than one photon ? One possible solution was demonstrated numerically by inserting tunnel junctions into the multi-quantum well (MQW) active region of a GaN-based LED (details). With one tunnel-junction (TJ), each electron gets two chances to generate a photon, because it can tunnel back into the conduction band after the first generation process. Inspired by the numerical demonstration, such GaN-based TJ-MQW LED was recently fabricated for the first time (details). With three tunnel-junctions (picture), each electron gets four chances to generate a photon, enabling quantum efficiencies up to 400%. Due to various loss processes, the practical simulation example results in a peak quantum efficiency of 253% at low power and still more than 100% at high power (details). Now, is this TJ-MQW concept a possible solution to the much debated GaN-LED efficiency droop ? Yes and no. Yes, because the calculated high-power wall-plug efficiency WPE is larger than without tunnel junction (WPE = optical output power divided by electrical input power). No, because the efficiency still droops with stronger current injection – because the increasing carrier density inside the quantum wells always causes increasing carrier losses. However, in my view, the beauty of this TJ-MQW concept lies in the prospect that it enhances the WPE at high power no matter what the actual carrier loss mechanism may be (details).
Update 5/1/15: Latest results are published in Optical and Quantum Electronics.