Blue LED with more than 100% quantum efficiency
Energy band diagram (red) and light emission profile (blue) of the proposed LED design.
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.