Understanding the origin of efficiency losses is key to developing the ultimate solid-state light source.Indium gallium nitride (InGaN) has great potential in LEDbased applications becasue of its ability to emit in the shortwavelength region, from near-UV to green. The emitting region is deposited as a quasi-2D quantum-well (QW) heterostructure composed of InGaN and gallium nitride. The past decade has seen tremendous progress in both epitaxial (or crystaline-layer) growth and InGaN LED design.We recently demonstrated a wall-plug efficiency (i.e., the conversion of electrical to optical power as measured from the line source to the resulting emission) of about 60% for a blue ThinGaN R LED. 1 The chip-level light-extraction efficiency of such devices has reached values in excess of 80%. However, the internal quantum efficiency for light generation by electron-hole carrier recombination remains lower. Even worse, the internal efficiency usually peaks around current densities considerably below the operating current and decreases monotonously towards higher currents, a phenomenon frequently called 'droop.' Understanding and reducing droop is crucial to reach the ultimate efficiency of InGaN-based LEDs. Various mechanisms that may cause this effect have been suggested, including carrier escape, 2 losses due to dislocations, 3 and the Auger effect. 4,5 In our work, we validate that the efficiency drop at high current is QW internal. We compare temperature-and excitationpower-density-dependent resonant photoluminescence (PL) to electroluminescence (EL) using a green-emitting InGaN single-QW (SQW) LED structure. We measure PL and EL on the same ThinGaN chip. For PL excitation, a 405nm laser optically excites electron-hole pairs well below the band-gap energy of the GaN barrier. 6 This enables us to reliably isolate QW-internal losses from losses due to parasitic carrier recombination outside the QW. Our experimental data strongly supports QW-internal loss. Figure 1. Internal efficiency of a green-emitting single-quantum-well (SQW) LED measured by electroluminescence (EL, empty boxes) compared to that measured in a resonant-photoluminescence (PL) experiment (solid boxes). Both experiments were performed at 300 (black) and 4K (red). Carrier generation and recombination in EL and PL are shown schematically in the conduction-and valence-band (CB/VB) diagrams in the insets. a.u.: Arbitrary units.All significant trends of the EL efficiency are followed perfectly by the resonant-PL efficiency (see Figure 1).We can reproduce the dependence of the internal efficiency on current density, J, using a simple rate-equation model of the form J ∼ A × n + B × n 2 + C × n 3 , where n is the QW carrier density and A, B, and C are coefficients for nonradiative, radiative, and (nonradiative) Auger-like recombination, respectively. This model can describe the emission characteristics of greenemitting InGaN-based LED over a wide current-density range (see Figure 2). We thus identify a high-density QW-internal Auger-like process as the culpri...