LED Diffusing Surface Light Extraction via Refractive Index Mismatch
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Solution Overview
Problem
Light emitting devices face inefficiencies in light extraction due to the critical angle limitation when light travels from a higher refractive index epitaxial layer to a lower refractive index medium, leading to internal reflection and absorption.
Innovation Solution
A light emitting device with a diffusing surface and a transparent adhesive layer is used, where the light emitting stack and the adhesive layer have different refractive indices, allowing for increased light extraction by diffusing light that would otherwise be restricted by the critical angle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If light travels from a higher refractive index epitaxial layer to a lower refractive index medium, then light extraction is possible, but light with incident angle larger than critical angle is totally reflected and absorbed, reducing light extraction efficiency
Solution Approach 1:
The patent applies curvature by forming spherical or hemispherical microlens structures on the light emitting surface. These curved microstructures modify the light path and reduce total internal reflection by changing the incident angles of light rays at the interface between the high refractive index epitaxial layer and the low refractive index external medium, thereby improving light extraction efficiency
Solution Approach 2:
The patent segments the light emitting surface into multiple discrete microlens structures distributed across the surface. Each microlens acts as an independent light extraction element, collectively improving overall light extraction efficiency while maintaining the device's functional integrity
2Loss of energy
If a diffusing surface is formed in the light emitting stack, then light extraction efficiency is improved by reflecting and diffusing light, but the device structure becomes more complex
Solution Approach 1:
The patent merges the diffusing surface function directly into the light emitting stack structure itself, rather than adding a separate diffusing layer. The diffusing surface is formed as an integral part of the light emitting device structure, combining light emission and light diffusion functions in a single integrated component, thus improving light extraction without proportionally increasing device complexity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The diffusing surface enhances light extraction efficiency by reflecting and diffusing light that was previously absorbed, improving the overall light emitting efficiency of the device.
Implementation Method 1
when a light beam generated from a light emitting diode (LED) travels across an interface from a material of a higher index of refraction to a material of a lower index of refraction, the angle between the incident light beam and the reflected light beam must be equal or less than 2θc for the light to be emitted out. It means that when the light generated from the LED travels from an epitaxial layer having a higher index of refraction to a medium having a lower index of refraction, such as a substrate, air and so on, a portion of the light will be refracted into the medium, and another portion of the light with an incident angle larger than the critical angle will be reflected to the epitaxial layer of the LED.
Implementation Method 2
when a light is directed from one material with an index of refraction n1 towards another material with an index of refraction n2, the light will be refracted if its incident angle is smaller than a critical angle θc
Data Source
AI summary
A light emitting device having a transparent substrate, a light emitting stack, and a transparent adhesive layer is provided. The light emitting stack is disposed above the transparent substrate and comprises a diffusing surface. The transparent adhesive layer is disposed between the transparent substrate and the diffusing surface of the light emitting stack; an index of refraction of the light emitting stack is different from that of the transparent adhesive layer.