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Much attention is paid to the efficiency droop of GaN-based light-emitting diodes (GaN-LEDs) but another phenomenon observed on industry-grade devices is still hardly investigated in the literature: the astonishingly low bias measured even at higher current. Hurni et al. report that photons emitted from their blue LEDs have a higher energy than the injected electrons, i.e., the electrical efficiency exceeds unity up to a current density of 75 A/cm2. The suspected reason is the absorption of thermal energy by injected electrons, which is subsequently removed by blue light emission. We have recently reproduced this phenomenon by advanced numerical simulation.
Our analysis reveals the exact contribution of different heat transfer mechanisms. In particular, the heat extraction (Peltier cooling) from the GaN lattice is calculated as electrons and holes climb up the GaN potential barrier before dropping into the InGaN quantum well (see picture). This potential barrier is created in part by the built-in polarization field. Non-polar LEDs exhibit 30% less net Peltier cooling. These simulations will be instrumental in the exploration of cooling strategies for GaN-LEDs (see NUSOD 2016 paper TuB3). For the first time in 2012, electroluminescent cooling of GaSb-based infrared LEDs was demonstrated to deliver a wall-plug efficiency above unity, but only at low power and elevated temperature.
Update 10/16/16: More details are now published in Optical and Quantum Electronics.