LED components today are complex systems with many optical, electrical, thermal, mechanical and transport interactions. These interactions imply to consider the LED component as a system, in order to identify potentials and remove deficits and improve the LED. Extensive research and development of last year’s allows to reach a high efficacy for LEDs – in the general lighting – of over 200 lm/W, and become commodity in the lighting market. In order to achieve higher efficacies, considering all mentored interactions is necessary. This talk gives an overview of system architecture of LED component today and shows projections into the future.
For Solid State Lighting devices thermal management has a great impact on the performance. Therefore, it is necessary
to analyze thermal and optical behavior simultaneously due to the mutual interaction between the two. For a remote
phosphor configuration the dependence of the device performance on thermal conductivity and refractive indices of
phosphor’s host material is investigated. Even though the temperature range inside the phosphor can be dramatically
reduced by choosing the right material, the overall performance suffers from the low extraction efficiency of light due to
the high refractive index of the chosen material.
Starting with a seminal paper by Forbes [1], orthogonal polynomials have received considerable interest as descriptors of
lens shapes for imaging optics. However, there is little information on the application of orthogonal polynomials in the
field of non-imaging optics. Here, we consider fundamental cases related to LED primary and secondary optics. To make
it most realistic, we avoid many of the simplifications of non-imaging theory and consider the full complexity of LED
optics. In this framework, the benefits of orthogonal polynomial surface description for LED optics are evaluated in
comparison to a surface description by widely used monomials.
Photonic crystals (PhCs) are known to diffract guided modes in a light-emitting diode into the light extraction cone
according to Bragg´s law. The extraction angle of a single mode is determined by the phase match between the guided
mode and the reciprocal lattice vector of the PhC. Hence, light extraction by PhCs enables strong beam-shaping if the
number of guided modes can be kept to a minimum. InGaN thin-film micro-cavity light-emitting diodes (MCLEDs)
with photonic crystals (PhCs) emitting at 455 nm have been fabricated. The GaN layer thickness of the processed
MCLEDs with a reflective metallic p-contact was 850 nm. One and two-dimensional PhCs were etched 400 nm into the
n-GaN to diffract the guided light into air. The farfield radiation pattern was strongly modified depending on the lattice
type and lattice constant of the PhC. Two- six- and twelve-fold symmetry was observed in the azimuthal plane from 1D
lines, hexagonal lattices and Archimedean A7 lattices, respectively. The emission normal to the LED surface was
enhanced by up to 330% compared with the unstructured MCLEDs. The external quantum efficiency was enhanced by
80% for extraction to air. The flux from PhC-MCLEDs in a radial lens was 15.7 mW at 20 mA and 36% external
quantum efficiency was measured at 3 mA. High order diffraction was found to contribute significantly to the
enhancements in efficiency and directionality. The experimental results are compared with FDTD simulations.
Keywords: light-emitting diodes, photonic crystal, cavity, InGaN
Photonic crystals are known to enhance the extraction efficiency of LEDs and simultaneously shaping the emission
pattern. In order to determine the radiation pattern we developed a model based on coupled mode theory that takes into
account the lattice pattern, etch depth and the mode distribution. From a basic geometrical consideration a fundamental
limit for the directionality is predicted. The calculations fit well to experimental data obtained from green InGaN LEDs
incorporating a hexagonal PhC revealing a maximum directionality of 31% within 30°. Additional FDTD simulations
were performed for determining quantitatively the extraction efficiency of PhC LEDs compared to LEDs with a
roughened surface. Despite its lower overall extraction efficiency, the PhC LED outperforms a standard LED with
surface roughening within an acceptance angle of 34° due to the higher directionality of the extracted light.
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