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One of the key challenges in the design of LEDs is to optimize their efficiency.
In an LED or OLED, light extraction inefficiencies exist owing to the difficulty in light generated within
a high-index material having difficulty propagating into the surrounding medium - usually air - owing to
total internal reflection. However, the thicknesses of the constituent layers of the LED can be optimized
and layers can be textured with micro-scale or nano-scale patterns in order to improve the light extraction
efficiency.
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"I simulate 15X15X3 μm3 LED pixels including metallic backplace on a 20 node cluster. Prior
to FDTD Solutions, this was not possible. Lumerical provides extremely fast single and multi-processor
software to speed your design efforts.
H. Greiner, Philips
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Step 1: Construct the FDTD Solutions model of the LED multilayer, including photonic crystal texturing
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The layout editor shows all of objects contained in the simulation project. Objects can be moved and resized
with simple mouse movements. The windows show the top, side, end and perspective views to assist with
visualization.
- construct the LED layer structure by using the object align functions
- add photonic crystal-like microstructure by using the cylinder primitive and the array function
- orange box shows the extent of the computation volume and the PML boundaries
- yellow planes show transmission monitors
- use a x-oriented dipole source to model the recombination of an electron-hole pair in the active layer; the
blue arrow indicates the polarization direction
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Step 2: Model the far-field light emitted from the unpatterned LED for the unoptimized layer structure
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Many integrated analysis functions facilitate data visualization and analysis. Here, field intensity data emitted
out of the top of the LED structure is projected to the far-field using built-in, fully-vectorial near to far field
projection tools. Performing this analysis in the frequency-domain allows for the measurement of the CW light
extraction efficiency at multiple frequencies - in a single simulation.
- far field visualization shows angular divergence of signal
- here, the far field field intensity is shown for an unoptimized layer structure, without microstructured
patterning
- light emitted at >40 degrees will be trapped inside the structure owing to total internal reflection (TIR),
suggesting that this structure has yet to be optimized
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Step 3: Perform a parameter sweep using the integrated scripting environment to optimize the LED layer structure
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Use built-in scripting routines to render complicated analysis simple. Use a custom-designed script to vary the
layer thickness of the active LED layer to optimize the light contained with a 40 degree emission cone.
- selecting the far field image with the greatest fractional power radiated upward yields an
optimum active LED layer thickness
- the far field image now shows that more radiation is directed upwards from the LED relative to the unoptimized
plot above
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Step 4: Add microscale texturing to redirect and increase the light emitted from the LED
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To the optimized layer structure above, we add photonic crystal-like microstructure to the LED design. This allows
for the LED designer to redirect and increase the efficiency with which the extraction efficiency can be increased.
Proper design of the microstructure patterning can be used to constructively scatter the emitted radiation in a
favoured direction.
- The far field intensity plot shows the intensity of the central lobe is stronger and more tightly distributed
than the structure without microstructure patterning
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Step 5: Watch the movie and gain operational insight into LED light extraction
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unpatterned LED - click to play movie
patterned LED - click to play movie
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To better understand the origin of the light extraction efficiency improvement, use the built-in movie monitor
in FDTD Solutions to capture the field dynamics of the simulation.
- Here, we investigate the extraction gain for a vertically-oriented dipole
- Note the vertically-scattered radiation present in the LED with the textured layer compared to the untextured
structure
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