DBR Reflector Structure for LED Wavelength Selectivity
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Solution Overview
Problem
Conventional white LED assemblies suffer from reduced light generation efficiency due to unreflected downward-traveling blue light being absorbed by the die-attach adhesive or aluminum core PCB, rather than being reflected back and re-emitted as white light.
Innovation Solution
A reflector structure incorporating a Total Internal Reflection (TIR) layer and a Distributed Bragg Reflector (DBR) structure, along with a reflective metal layer, is used to enhance the reflectivity of blue and yellow light, optimizing the reflectivity for wavelengths between 440 nm and 700 nm, thereby improving the Photon Recycling Efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a single layer of highly reflective metal (e.g., silver) is used as the reflector structure, then the reflectivity for blue light is improved, but contamination and electromigration issues occur reducing reliability
Solution Approach 1:
The patent uses a composite reflector structure consisting of multiple dielectric layers (DBR) with alternating high and low refractive indices, combined with a protective metal layer. This composite structure achieves high reflectivity (R>95%) while the protective metal layer prevents contamination and electromigration of the underlying reflective layers, thus resolving the contradiction between reflectivity and reliability.
Solution Approach 2:
The patent employs a sacrificial protective metal layer (such as aluminum or silver) that is exposed to environmental conditions first, sacrificing itself to protect the underlying DBR structure. This disposable layer takes the hit from contamination and oxidation, preserving the functional integrity of the reflector system over time.
2Reliability
If a DBR structure with multiple dielectric layers is used to improve reflectivity, then reliability is improved, but the structure complexity increases
Solution Approach 1:
The patent applies local quality by creating spatial variation in layer thicknesses within the DBR structure. Specifically, the thickness of dielectric layers is modulated in different regions to achieve wavelength-selective reflectivity enhancement. This allows the reflector to maintain high reliability through the DBR mechanism while managing complexity through localized rather than uniform structural modifications.
3Use of energy by moving object
If the LED active layer emits blue light in all directions, then the light generation efficiency is improved, but a substantial amount of light travels downward and is absorbed instead of being reflected back
Solution Approach 1:
The patent extracts and addresses the downward-traveling light component separately from the overall omnidirectional emission. By placing a specialized reflector structure (DBR with wavelength-selective layers) at the bottom of the LED device, it specifically targets and redirects the downward-propagating blue light back into the active region, converting what would be lost energy into useful light output.
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 novel reflector structure achieves a Photon Recycling Efficiency of over 85% for light in the 500 nm to 700 nm range, significantly improving the overall light emission efficiency of the LED device by effectively reflecting and re-emitting yellow and blue light.
Implementation Method 1
Distributed bragg reflector for reflecting light of multiple wavelengths from an LED
Implementation Method 2
The materials are selected such that the alternating layers have a high index of refraction, and then a low index of refraction, and then a high index of refraction, and so forth down the stack
Implementation Method 3
According to Snell's law, all of the light traveling from a material having a higher index of refraction toward a material having a lower index of refraction at an angle greater than the critical angle will be reflected back into the higher-index-of-refraction material without experiencing any energy loss. This mechanism is known as total internal reflection (TIR).
Implementation Method 4
The phosphor absorbs some of the blue light and fluoresces, thereby re-emitting light of longer wavelengths including green, yellow and red light
Data Source
AI summary
A blue LED device has a transparent substrate and a reflector structure disposed on the backside of the substrate. The reflector structure includes a Distributed Bragg Reflector (DBR) structure having layers configured to reflect yellow light as well as blue light. In one example, the DBR structure includes a first portion where the thicknesses of the layers are larger, and also includes a second portion where the thicknesses of the layers are smaller. In addition to having a reflectance of more than 97.5 percent for light of a wavelength in a 440 nm-470 nm range, the overall reflector structure has a reflectance of more than 90 percent for light of a wavelength in a 500 nm-700 nm range.