Combined Experimental and Monte-Carlo Ray-Tracing Approach for Optimizing Light Extraction in LED COB Modules

High-power light-emitting diodes (LEDs) for lighting applications require a high-efficient packaging to optimize their performances. Due to its high thermal dissipation potential, the chip-on-board (COB) technology is widely used for developing high-power lighting sources. In order to optimize the optical properties of such sources and to propose high optically efficient encapsulation geometry, ray-tracing simulations have been performed. The impact of the shape and volume of the silicone encapsulation on the light extraction and on the intensity distribution of the module was derived. Then, simulation results were correlated with experimental measurements on blue light-emitting COB sources. It is shown that a nearly hemispherical encapsulation with a minimal volume of 5 to 10？mm3 for a 1 mm2 LED die is the optimal configuration regarding both the light extraction and the intensity distribution. 1. Introduction High-power light-emitting diodes (LEDs) have attracted a lot of interest in the recent years due to their low-power consumption and long lifetime. However, the integration of these light sources for general lighting applications will require high-efficient packaging with low thermal resistance and compact size [1]. Thus, due to its compactness and its high thermal dissipation potential, the chip-on-board (COB) technology is one of the favourite candidates for developing customized and thermal-efficient LEDs [2]. Beyond the thermal efficiency, one of the key performance criteria of a LED light source is its optical efficiency. This efficiency comprises several components [3]. The first one is the “internal quantum efficiency”. It is defined as the ratio between the number of photons emitted from the active region per second and the number of electrons injected into the LED per second. It describes the LED junction performance. In this paper, we focus on the second component of this efficiency: the “light extraction efficiency”. It is defined as the ratio between the number of photons emitted into free space per second and the number of photons emitted from the active region per second. At the die level, this efficiency is limited by the total internal reflexions which appear at the interface between the semiconductor material and the air due to their refractive index mismatch. It may be improved either by structuring the die surface [4, 5] and/or by packaging the die with dome-shaped encapsulants with a high refractive index [6]. At the package level, the extraction is limited by the losses which may occur during light propagation within the