LED/OLED Light Extraction Efficiency

Optimizing the light extraction efficiency of LED/OLEDs using FDTD Solutions

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.

Philips Logo"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.
- Dr. H. Greiner, Philips

Step 1: Construct the FDTD Solutions model of the LED multilayer, including photonic crystal texturing

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.  The LED layer structre is constructed from simulation primitives and the materials contained within the material database, while the photonic crystal texturing is arrayed from a single cylinder primitive.  The response of the LED structure can be measured using a number of simulations using dipoles with various orientations with respect to the photonic crystal lattice.

FDTD Solutions with LED light extraction model
Three-dimensional model of an LED solid state emitter with photonic crystal texturing, with the dipole excitation as shown.

Step 2: Model the far-field light emitted from the unpatterned LED for the unoptimized layer structure

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, all achieved in a single simulation by using the dispersive materials from which the LED structure was constructed.

LED light extraction with an unoptimized layer structure
Here the far field field intensity is shown for an unoptimized layer structure without microstructured patterning.  The light emitted at >40 degrees will be trapped inside the structure owing to total internal reflection (TIR).

Step 3: Add microscale texturing to redirect and increase the light emitted from the LED

Adding photonic crystal-like microstructure to the LED design allows for the redirection of light emitted from the LED structure. This allows for the LED designer to increase the efficiency with which the extraction efficiency can be increased by directing light that would otherwise be trapped due to TIR to be emitted. Proper design of the microstructure patterning can be used to constructively scatter the emitted radiation in a favoured direction tailored to the specific end application.

LED light extraction once the layer structure has been optimized
This far field image for the structure incorporating microtexturing shows that more radiation is directed upwards from the LED.
add photonic crystal texturing to LED to redirect and enhance light extraction
Plotting the ratio of the light emitted in the photonic crystal LED to the unpatterned LED shows that the amount of enhancement obtained varies strongly with wavelength.

Step 5: Watch the movie and gain operational insight into LED light extraction

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 that much of the light is trapped by total internal reflection in the layer structure without photonic crystal texturing
Note the vertically-scattered radiation present in the LED with the textured layer compared to the untextured structure