LED Light Extraction Layer with Gradient Refractive Index
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Solution Overview
Problem
Conventional light-emitting diodes (LEDs) suffer from reduced light extraction efficiency due to the large difference in refractive indices between the semiconductor layers and the transparent substrate, causing light with incident angles greater than 35 degrees to be totally reflected and confined within the device.
Innovation Solution
A light-emitting device with a bonding layer having a gradually changed refractive index is introduced, increasing the critical angle for light emission beyond 35 degrees, thereby reducing total reflection and enhancing light extraction efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional LED structure with direct interface between semiconductor layer and transparent substrate is used, then the device structure is simple, but the light extraction efficiency is reduced due to total internal reflection at the interface
Solution Approach 1:
A bonding layer with gradually changed refractive index is introduced between the semiconductor light-emitting stack and the transparent substrate. This intermediate layer acts as a refractive index gradient medium that progressively transitions light from the high refractive index semiconductor material to the lower refractive index substrate, reducing total internal reflection and improving light extraction efficiency by approximately 15%.
2Device complexity
If the critical angle is limited to less than 35 degrees due to refractive index difference, then the interface design is straightforward, but the far-field angle is restricted to about 117 degrees
Solution Approach 1:
The bonding layer employs a gradual refractive index change from the semiconductor interface toward the substrate, transforming the abrupt refractive index step into a continuous gradient. This parameter transition expands the critical angle beyond 35 degrees and increases the far-field emission angle to greater than 120 degrees at 50% light intensity, enabling broader light extraction without complicating the interface design.
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 solution increases light extraction efficiency by about 15% and expands the far-field angle to greater than 120 degrees at 50% light intensity, allowing more light to be extracted from the device.
Implementation Method 1
the bonding layer has a gradually changed refractive index, and each of critical angles at the bonding layer and the transparent substrate for the light emitted from the light-emitting layer towards the transparent substrate is larger than 35 degrees
Implementation Method 2
the difference of the refractive indexes between the transparent substrate 10 and the first semiconductor layer 12 is large, the critical angle θc between the transparent substrate 10 and the first semiconductor layer 12 is less than 35 degrees. When light emitted by the light-emitting layer 14 travels from the first semiconductor layer 12 to the transparent substrate 10, the incident angle of light must be within 35 degrees for light to exit. Light with an incident angle more than 35 degrees is totally reflected at the interface
Data Source
AI summary
A light-emitting device is disclosed and comprises: a transparent substrate; a semiconductor light-emitting stack on the transparent substrate, wherein the semiconductor light-emitting stack comprises a first semiconductor layer close to the transparent substrate, a second semiconductor layer away from the transparent substrate, and a light-emitting layer capable of emitting a light disposed between the first semiconductor layer and the second semiconductor layer; and a bonding layer between the transparent substrate and the semiconductor light-emitting stack, wherein the bonding layer has a gradually changed refractive index, and each of critical angles at the bonding layer and the transparent substrate for the light emitted from the light-emitting layer towards the transparent substrate is larger than 35 degrees.