Ernő Kollár

Nonlinear electro-thermal modeling and field-simulation of OLEDs for lighting applications I: Algorithmic fundamentals

Large area OLEDs aimed at lighting applications should provide homogeneous luminance-homogeneity is one of the quality metrics of such devices. Local light generation depends on both the local temperature and the local voltage drop across the light emitting polymer(s) in the device. Therefore the thermal and electrical engineering of OLEDs aimed at lighting applications is critical. Due to the large area of these devices the coupled electrical and the thermal simulation problem is of distributed nature. Electrical characteristics of organic semiconductor materials used in OLED devices are highly nonlinear, and their nonlinear temperature-dependence is significant. In our present approach to distributed electro-thermal field simulation we address special needs of OLEDs, which is not yet the case with widely used, commercially available simulation tools. In this paper we present the latest version of our SUNRED electro-thermal field solver algorithm capable of handling coupled, non-linear electro-thermal problems. The new features of the algorithm are demonstrated by modeling some research OLED samples available to us in the Fast2Light project-this way simulation results are compared against measured data.

A procedure to correct the error in the structure function based thermal measuring methods

In this paper a methodology is presented to correct the systematic error of structure function based thermal material parameter measuring methods. This error stems from the fact that it is practically impossible to avoid parallel heat-flow paths in case of forced one-dimensional heat conduction. With the presented method we show how to subtract the effect of the parallel heat-flow paths from the measured structure function. With this correction methodology the systematic error of structure function based thermal material parameter measuring methods can be practically eliminated. Application examples demonstrate the accuracy increase obtained with the use of the method.

Thermal behavior of acousto-optic devices: Effects of ultrasound absorption and transducer losses

In the present paper we analyze the electric and acoustic losses in acousto-optic devices, especially in their ultrasonic transducers and the related thermal effects. We include electric and acoustic losses into the classical electric equivalent model of the transducer, to explain the characteristics of the measured electric and thermal behavior. Measured temperature distributions on the acousto-optic crystal faces serve visualization of the conversion efficiency of the radio-frequency input to bulk acoustic waves. We show that the pronounced acoustic frequency dependence of the temperature distribution is in correlation with the frequency dependent losses in the transducer and in the bulk. We also demonstrate experimentally the effectiveness of our active and passive heat removing and temperature stabilization methods.

Improvements of the variable thermal resistance

A flat mounting unit with electronically variable thermal resistance has been presented in the last year. The design was based on a Peltier cell and the appropriate control electronics and software. The device is devoted especially to the thermal characterization of packages, e.g. in dual cold plate arrangements. Although this design meets the requirements of the static measurement we are intended to improve its parameters as the settling time and dynamic thermal impedance and the range of realized thermal resistance. The new design applies the heat flux sensor developed by our team as well, making easier the control of the device. This development allows even the realization of negative thermal resistances.

Design of a Static TIM Tester

Testing the thermal properties of thermal interface material (TIM) has been a big challenge for decades. Recent development trends made this challenge even bigger, as now the values that have to be measured are extremely small. In this paper, we present a newly developed TIM tester equipment that is targeting to overcome all the problems that present industrial TIM testing methods face. The main idea behind our design is to use the capabilities of microelectronics in order to make small sized sensors both for temperature and heat-flux sensings. This way it is possible to place these sensors in the closest proximity of the measured sample. This paper presents details of all the technical solutions of the newly developed static TIM tester that is capable to measure R th of unit area values in the order of 0.01 K cm 2 / W with good accuracy. Special attention is made to analyze and eliminate the possible sources of measurement inaccuracy. A number of measurement examples prove the usability of the developed measuring instrument.