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OLEDs/LEDs

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In the search for higher efficiency, bright light sources, researchers are investigating how nanophotonics can be used within light emitting diode (LED) and organic light emitting diode (OLED) structures. Nanophotonic elements including textured metallic electrodes, periodic patterning within the active layer, and microscale anti-reflection layers like moth’s eye film can improve the price performance of solid state lighting, and offer long lifetimes, small form factors, high lumens output per watt input, and robust packaging. As a result, LED/OLED structures are of intense research interest for a diverse set of applications including TV screens, cell phones, and other applications. Given the promise of high efficiency solid state sources of quality light, researchers continue to investigate such structures to render them competitive with other sources of visible light including incandescent lamps and fluorescent lighting.

 

A 3D view of an OLED simulation in FDTD Solutions

A 3D view of an OLED simulation in FDTD Solutions

Simulating Planar OLEDs

 

For planar OLEDs/LEDs with no patterning, analytical methods of calculating the optical response are considerably faster than direct simulations of Maxwell’s equations. Lumerical's Stack Optical Solver provides a set of 1D optical stack functions that accurately captures the interference and microcavity effects in multilayer stacks and can be used for rapid analysis of these devices. In most cases, less than one second of calculation time is required to calculate and characterize the optical response of system.

 

See Planar OLEDs for a list of examples using the Stack Optical Solver.

Simulating Patterned OLEDs

 

FDTD Solutions is a high performance optical solver that can capture the effects of wavelength-scale patterning and its impact on the efficiency of the device. This is important for OLED and LED simulations because typical designs include many thin layers of highly dispersive materials and complex wavelength-scale patterning to increase the quantum efficiency. There are 2 methods for increasing the overall efficiency of OLEDs using patterning:

Increase the light extraction efficiency

Increasing the light extraction efficiency of an OLED increases the fraction of emitted power that escapes to the air (or into a desired solid angle). This is essentially a question of redirecting and extracting light to prevent it from being trapped by TIR. In an LED or an OLED, light extraction inefficiencies exist because light generated within a high-index material has difficulty propagating into the surrounding lower-index medium. The constituent layers of the OLED or LED can be textured with micro-scale or nano-scale patterns in order to improve the light extraction efficiency. The texturing leads to:

 

Macroscopic optical effects: this mainly involves patterning or roughening the air/glass interface which can be used to reduce TIR. Also, some authors have concluded that patterning near the emission layers can lead to increased efficiency for the same reason: the light makes multiple round trips from the emission layers to the glass/air interface, and scattering from the grating at the emission layer can change the direction of the reflected light and prevent TIR at the air/glass interface. For example, Peter Vandersteegen, in "Modeling of the Optical Behavior of Organic LEDs for illumination" (Doctoral Thesis, 2008, http://photonics.intec.ugent.be/publications/phd.asp?ID=160), concludes in section 5.4.6 that a substantial efficiency improvement is available for certain designs due to multiple round trip scattering events, but that the efficiency improvement is negligible when these multiple round trip events are ignored. This effect can be studied using FDTD Solutions either by calculating the angular emission of the light in the glass and then combining these results with considerations of the multiple round trip effects or by artificially reducing the thickness of the glass layer such that it can be directly modeled by FDTD and yet remain thick enough to obtain accurate results.

 

Microscopic optical effects: This is typically done with gratings or patterning in and near the emission layers. Ideally these gratings can assist in extracting light that is trapped in the emission layers and extracting it into the glass at angles where it will not suffer TIR at the glass/air interface. We can study this effect directly with FDTD Solutions.

Increase the radiative decay rate

It is possible to increase the radiative decay rate by modifying the environment of the emitting dipoles with the addition of microscopic patterning. Increasing the radiative decay rate can allow for a higher electrical excitation rate. It will also increase the internal quantum efficiency because radiative decay processes can compete more efficiently with non-radiative processes. By relating the radiative decay rate to the power radiated by a dipole source, we can study this effect using FDTD Solutions.

Finally, it should be noted that introducing patterning may have effects on the electrical properties of the device. Modeling these effects is beyond the scope of FDTD Solutions.

 

See Patterened OLEDs for a list of examples using FDTD Solutions.

Interface with Ray Tracing

 

Designing OLED/LED devices requires a combination of nanoscale and macroscale optics. The individual pixels often have sub-wavelength features such as thin dispersive layers scattering structures that require electromagnetic field solvers, while the emission from the macroscopic device requires a ray-based tool. For these applications, one can calculate the angular distribution of a pixel with the Stack Optical Solver or FDTD Solutions, and then load the result into a ray tracing tool in the form of ray sets.

 

See interface with ray tracing for a list of examples using both Lumerical's optical solvers and ray tracing tools.

Application examples

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