Tech-Talks BREGENZ - Wolfgang Nemitz, Scientist Joanneum Research

LED professional: Congratulations Mr. Nemitz, on the LED professional Scientific Award handed to you here at the LpS 2015. In your excellent paper you talk about simulating phosphors to generate white light. Can you tell us how you got started?

Wolfgang Nemitz: My work mainly concerned the incorporation of temperature dependent optical parameters into an existing simulation model. The important thing for the optical simulation of color converters that are made from phosphor particles embedded in silicone is to have valid scatter models for the blue light as well as the yellow light. Based on this, we can simulate in which part of the converter the blue light is absorbed and where the yellow light is reemitted.

LED professional: How is the basic architecture of the simulation model constructed?

Wolfgang Nemitz: We can consider the color conversion layer to be a block or volume. This volume has the characteristic of a material like silicone with embedded phosphor particles. There are scattering models for the phosphor particles based on parameters like a minimum, maximum and average size and a standard deviation for each size. In addition, the concentration, the refraction index and the quantum efficiency of the phosphor are defined.

LED professional: Are these parameters adequately known for the phosphors that are used?

Wolfgang Nemitz: You have to determine the optical parameters - mostly experimentally - but afterwards you can adapt the model. For instance, in experiments with fabricated color conversion layers we determined the refractive indexes with ellipsometry. Furthermore, we determined the temperature dependent luminescence intensity of the phosphor in the laboratory. We were also able to measure the thermal conductivity of very thin phosphor layers and apply the results to the entire simulation model. In the model we assume certain statistic scattering within the conversion layer.

LED professional: The wavelength in which the phosphor particle reacts should be relatively precise. The primary emission of the blue LED scatters relatively wide. Does that mean that the entire system of LED and phosphor always has to be known for the system simulation?

Wolfgang Nemitz: That is absolutely right. We use an LED chip for blue light as the basis for the simulations of the white light. We checked the measurements and simulations for a specific LED and examined that type of LED very carefully. If we take a different LED as a basis or maybe a different color converter, then the examinations have to be done again.

LED professional: How precisely do the measurement results correspond to the simulation results on the basis of your model?

Wolfgang Nemitz: The better you know the basic data (the optical parameters, the characteristics of the LED chips, the shape of the color converter, the shape of the optics and the thermal properties), the more precise the results will be.

LED professional: Can you stay within two MacAdams ellipses if you take all requirements, like a 45°-80° angle into account?

Wolfgang Nemitz: All the silicones used have temperature dependent characteristics, whereby the color conversion layers can be arranged neatly. An optimal arrangement can be found for the phosphor material as well as additional silicone layers in order to keep the color coordinates within two MacAdams ellipses.

LED professional: What key findings have your studies shown over the years?

Wolfgang Nemitz: We actually just recently made a crucial discovery. The color conversion layer doesn’t have the same temperature throughout. The temperature in the lower areas (close to the chip surface) is lower, whereas the temperature of the upper surface is much higher. That was well known up until now. What we discovered is that there is a connection between the different temperatures and the various aspects or parameters of white light emission from an LED. When, for example, the maximum temperature is examined in order to determine the risk for materials degradation, one has to consider the temperature at the upper surface of the color conversion layer. This surface temperature can be determined with simulations or infrared thermography. In order to make prognoses on the color coordinates, one has to know the temperatures of those regions of the color conversion layer that are near the LED chip. We showed that these are the areas of the color converter for which the phosphor particles mainly contribute to the emission. When running an LED on 1 A we measured a surface temperature of about 300°C and a temperature of only 140°C close to the LED chip. The conversion layer was about 250 μm thick in this set-up.

LED professional: Why is the outside layer so much hotter than the inner zone?

Wolfgang Nemitz: The heat generated by the blue LED can be dissipated easily to the PCB (and a cooling plate beneath). Unfortunately, the Stoke’s shift losses which occur upon color conversion produces heat in the color conversion layer. When the quantum efficiency of phosphor is less than 1, additional heat is produced and this heat can’t be dissipated because of the poor thermal conductivity of silicone. Of course there are approaches trying to improve heat dissipation from the color conversion layer. In this regard phosphor ceramics or crystal phosphors play an important role because they have fundamentally better thermal characteristics.

LED professional: At the LpS we heard that in the future LEDs will be built into materials, ceilings and other things. Could simulations for embedded lighting also be carried out?

Wolfgang Nemitz: We have already worked together with partners to integrate light into ceilings using appropriate materials in the model. In any case, it is crucial to understand the thermal and optical characteristics of the materials.

LED professional: When evaluating the papers for the LED professional Scientific Award, the advisory board paid special attention to the practical relevance of the papers. Why do you think your paper received such high marks for this point?

Wolfgang Nemitz: The practical relevance is that we can now differentiate between the various temperatures that have to be taken into account for the observation of various effects of Phosphor Coated (PC) LEDs like surface temperature in case you want to make estimates of the risk of material degradation. There is now a simulation model available that runs optical and thermal simulations iteratively. For example, if we start with the optical simulation at room temperature this will be based on certain scatter characteristics of the color converter and this results in a distribution of blue light absorption. In turn, this is the basis for the thermal simulation because blue light absorption determines where, taking into account the quantum efficiency and the Stoke’s shift losses, the heat is produced inside the color converter. And the temperature distribution from the thermal simulation is used for a new optical simulation because the blue light absorption distribution changes with different temperatures and therefore, in turn, has to be calculated again on the base of the temperature distribution. The result can be repeatedly entered in a new thermal simulation.

LED professional: Is this research result unprecedented?

Wolfgang Nemitz: As a rule, in the LED community one mainly talks about thermographic measurements of maximum temperatures or outside temperatures. Lower temperatures near the LED chip are not taken into account or are not known because they can’t be determined by infrared thermography. In addition to being able to determine the risk of material degradation, now the color coordinates can be predicted. These are unprecedented results.

LED professional: Can you tell us about the research team at Joanneum Research?

Wolfgang Nemitz: Our team is made up of ten to twelve researchers. In the area of thermal simulations we work together with Professor Johann Nocolics’ group from the Institute for Sensor and Actuator Systems at the Vienna University of Technology.

LED professional: What do your future research plans look like?

Wolfgang Nemitz: There is still a lot to examine. For high temperatures we still need additional methods for characterization. In this high temperature regime the measurements are a bit more difficult because we have to eliminate several effects.

LED professional: This is your fourth year at the LpS in Bregenz. What is your opinion of the fifth anniversary conference and expo?

Wolfgang Nemitz: The LpS event is continually improving. The program is very appealing and compact. I find it very positive that new aspects of lighting are taken up every year. This year I found the design exhibition very interesting. Here you can see how the focus has changed over the years. New aspects are coming to the forefront and are presented very nicely. All in all, a great development for the LpS event in Bregenz.

LED professional: What do you say about the exhibition?

Wolfgang Nemitz: What I noticed is that the exhibition has grown continuously and there are more people here every year. Since our research is very application related and we work with many industry partners, the LpS is a very good platform for us to make contacts and also to intensify the contacts we have. The award that we won this year also has a very good publicity effect. Our clients that are here see that we work very well on a scientific basis. And that makes us very happy.

LED professional: Thank you very much for the interview. It was very interesting. Again, congratulations!

Wolfgang Nemitz: Thank you.