Franz-Peter Wenzl

A comprehensive study on the parameters effecting color conversion in phosphor converted white light emitting diodes

Due to the light scattering processes that take place within the color conversion elements (CCE) of phosphor converted light-emitting diodes (LEDs) and the rather different emission characteristics of the blue LED and the converted light, which have to be matched by the scattering processes, a better understanding of the underlying physical aspects is indispensable for device optimization. We give, based on optical ray-tracing, a comprehensive survey on the parameters that effect color conversion and light scattering within the CCEs of phosphor converted LEDs. Studies range from variations of the geometrical (height, width) to the compositional (concentration of the phosphor in the matrix material, differences of the refractive indices of the matrix and the phosphor materials, phosphor particle size) parameters and identify their respective impacts on the color temperatures and the luminous efficacies of the respective LEDs.

Impact of extinction coefficient of phosphor on thermal load of color conversion elements of phosphor converted LEDs

Abstract Besides their direct impact on the respective correlated color temperature, the extinction coefficient and the quantum efficiency of the phosphor also have tremendous impact on the thermal load of the color conversion elements of phosphor converted LEDs under operation. Because of the low thermal conductivity of the silicone matrix in which the phosphor particles are typically embedded, the by far highest temperatures within the LED assembly are reached within the color conversion element. Based on a combined optical and thermal simulation procedure we show that in particular a larger value for the extinction coefficient might have a beneficial impact on the resulting thermal load.

Multi-scale simulation of an optical device using a novel approach for combining ray-tracing and FDTD

Optimizing the properties of optical and photonic devices calls for the need to control and manipulate light within structures of different length scales, ranging from sub-wavelength to macroscopic dimensions. Working at different length scales, however, requires different simulation approaches, which have to account properly for various effects such as polarization, interference, or diffraction: at dimensions much larger than the wavelength of light common ray-tracing techniques are conveniently employed, while in the (sub-)wavelength regime more sophisticated approaches, like the socalled finite-difference time-domain (FDTD) technique, are used. Describing light propagation both in the (sub-)wavelength regime as well as on macroscopic length scales can only be achieved by bridging between these two approaches. Unfortunately, there are no well-defined criteria for a switching from one method to the other, and the development of appropriate selection criteria is a major issue to avoid a summation of errors. Moreover, since the output parameters of one simulation method provide the input parameters for the other one, they have to be chosen carefully to ensure mathematical and physical consistency. In this contribution we present an approach to combine classical ray-tracing with FDTD simulations. This enables a joint simulation of both, the macro- and the microscale which refer either to the incoherent or the coherent effects, respectively. By means of an example containing one diffractive optical element (DOE) and macroscopic elements we will show the basic principles of this approach and the simulation criteria. In order to prove the physical correctness of our simulation approach, the simulation results will be compared with real measurements of the simulated device. In addition, we will discuss the creation of models in FDTD based on different analyze techniques to determine the dimensions of the DOE, as well as the impact of deviations between these different FDTD models on the simulation results.

A Simulation Procedure Interfacing Ray-Tracing and Finite-Difference Time-Domain Methods for a Combined Simulation of Diffractive and Refractive Optical Elements

A simulation procedure which enables integrated simulation of optical devices including both refractive and diffractive optical elements at different length scales is presented. The approach uses the Poynting vector to interface between a Ray-tracing and a finite-difference-time-domain tool for a step by step simulation of both the light propagation through a laser-written diffractive grating structure and its measurement by an appropriate setup. These simulated results are in great accordance with experimentally determined ones. Furthermore, the impact of parameter variations is analyzed and discussed in detail.

Investigation of thermal properties of power LED illumination assemblies

The reliability and long-term stability of solid-state lighting devices strongly depend on successful thermal management. It is well known that junction temperature instability negatively impacts the lightning properties of LED illuminators. However, for prediction of life time and longterm stability the temperature distribution also inside the color converter must be known. A deeper understanding of these demanding thermal issues is provided in our paper on the base of measurements and simulations also taking into account the spatial power loss distribution due to absorption of light and Stokes shift within the color converter. For this purpose 3-dimensional models were set-up and examined to thermally characterize high-power LED assemblies. Two different types of set-ups were investigated. Moreover, an efficient solution is presented where a junction-to-case thermal resistance below 10 K/W is achieved.

White light quality of phosphor converted LEDs from a phosphor materials perspective of view: an evaluation based on combined thermal and optical simulations

Color constancy and color maintenance are key issues in the context of the utilization of light-emitting diodes (LEDs) for general lighting applications. For a systematic approach to improve the white light quality of phosphor converted LEDs and to fulfill the demands for color temperature reproducibility and constancy, it is imperative to understand how compositional, optical and thermal properties of the color conversion elements (CCE), which typically consist of a phosphor particles embedded in a transparent matrix material, affect the correlated color temperature of a white LED source. Based on a combined optical and thermal simulation procedure, in this contribution we give a comprehensive discussion on the underlying coherences of light absorption, quantum efficiency and thermal conductivity and deduce some strategies to minimize the temperature increase within the CCE in order to maintain acceptable color variations upon device operation.

Direct junction temperature measurement in high-power LEDs

The light quality and long-term stability of phosphor converted light-emitting diodes (LEDs) for luminaires depend on the temperature distribution inside the LED chip and the color conversion element. Therefore, a reliable and accurate method to establish the LEDs junction temperature is required to further improve and optimize high quality LED luminaires. In this paper we describe the development and application of an innovative junction temperature measurement method which is based on a precise and universally applicable calibration procedure, allowing to use the calibrated LED itself as a temperature sensor under the respective operation condition of interest. This method is based on an extremely fast pulse measurement procedure allowing to record pairs of forward current and voltage drop values periodically every microsecond starting from the first microsecond of a pulse. From these experiments we reap two kinds of result: (1) Independent of the pulse shape in our experiments we observe a constant relation of current-to-voltage drop which we interpret as a constant junction conductivity. Depending on the type of LED (but independent of the packaging technology) we obtain a constant junction conductivity throughout several ten microseconds which we understand as a proof that the junction temperature did not change during this very first pulse phase. (2) The junction conductivity obtained in this moment is a measure for the junction temperature so that a calibration can be made by comparison with an independent steady-state temperature measurement made at zero-current condition. The method has been successfully applied to thermally characterize high-power LED modules as a 3 × 3 LED arrays with color conversion glob tops built-up on an insulated metal substrate (IMS).

Predicting solutions toward improved high power white LED light sources: a combined theoretical and experimental study

To compete with and to surpass the performance of traditional lighting systems, white LED development is still facing the necessity of further improvements. An important topic that has to be addressed in this context is the spatial homogeneity of the white light emitted, an issue that is directly associated with the geometry and the composition of the color conversion elements (CCE) in phosphor converted LEDs. In order to avoid the need for experimental realization and inspection of a large number of different configurations and compositions, optical simulation provides a time- and cost saving alternative. In this contribution we discuss a simulation procedure which allows us to predict optimized solutions for the CCEs in white LED light sources. The simulation process involves the set-up of a model for the blue emitting LED chip and the implementation of a multitude of different geometries and compositions of individual CCEs on top of the chip. Since the light is scattered within the CCEs, the respective scattering model, which considers the phosphor particle size distribution and the phosphor weight fraction is of particular importance. In the final sequence of the modeling procedure color uniformity is checked by monitoring the irradiance distributions both for the blue LED light and the yellow converted light separately on a detector. From a comparison of the simulation results for a significant number of different layouts we can deduce the impact of the individual materials parameters and predict optimized CCEs which are finally compared with real device set-ups in order to verify the accuracy of the simulation procedure.

Aluminum-nanodisc-induced collective lattice resonances: Controlling the light extraction in organic light emitting diodes

We examine aluminum-nanodisc-induced collective lattice resonances as a means to enhance the efficiency of organic light emitting diodes. Thus, nanodisc arrays were embedded in the hole transporting layer of a solution-processed phosphorescent organic blue-light emitting diode. Through extinction spectroscopy, we confirm the emergence of array-induced collective lattice resonances within the organic light emitting diode. Through finite-difference time domain simulations, we show that the collective lattice resonances yield an enhancement of the electric field intensity within the emissive layer. The effectiveness for improving the light generation and light outcoupling is demonstrated by electro-optical characterization, realizing a gain in a current efficiency of 35%.

Adjusting the emission color of organic light-emitting diodes through aluminum nanodisc arrays

Abstract. With the introduction of phosphorescent and thermally activated delayed fluorescence emitter materials, organic light-emitting diodes (OLEDs) with internal quantum yields of up to 100% can be realized. Still, light extraction from the OLED stack is a bottleneck, which hampers the availability of OLEDs with large external quantum efficiencies. Many different strategies to enhance the outcoupling of the light have been suggested, for instance, the use of collective lattice resonances induced by arrays of plasmonic nanodiscs. Here, we investigate the usability of these nanodisc arrays to tune the emission color of an organic blue-emitting material. By means of extinction and photoluminescence spectroscopy, we show a correlation of the sharp features observed in extinction with a selective fluorescence enhancement. At the same time, the nanodisc array also modifies the microcavity of an OLED stack. For one exemplarily lattice constant of an aluminum nanodisc array directly integrated into an OLED stack, we show that a combination of these effects allows the modification of the emission color from CIE1931 (x,y) chromaticity coordinates of (0.149, 0.225) to (0.152, 0.352). Importantly, the OLED exhibited a similar emission color modification under optical as well as electrical excitation.