A fluctuational electrodynamics model for the optimization of light-extraction efficiency in thin-film light-emitting diodes

Abstract

The rapid development of thin film light-emitting diodes (LEDs) has enabled the enhancement of the light extraction beyond geometrical limits but more quantitative understanding of the underlying optical processes is required to fully optimize the extraction. We present first-principle calculations of the light extraction efficiency and optical energy flow in thin-film LEDs. The presented model generalizes the methods of fluctuational electrodynamics to excited semiconductors and simultaneously accounts for wave optical effects, e.g., interference and near-field coupling as well as the internal absorption of the light-emitting material in determining the rate of light emission and internal dissipation in the optical cavity formed by a planar LED. The calculations show that in structures with a metallic mirror, the emissivity of the active region can approach unity at selected wavelengths, even when the nominal emissivity of the active region is only moderate. However, the results also show that near-field coupling of emission from the active region to the mirror can provide a substantial non-radiative loss channel reducing the maximum light extraction efficiency to 0.67 in our example setup. These losses can be partly compensated by the efficient photon recycling enabled by thick active regions that quench emission to confined modes and thereby reduce parasitic absorption.