László Pohl

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.

Fast field solver for the simulation of large-area OLEDs

Lighting purpose organic light-emitting devices need special engineering because of the high electrical and thermal requirements of the operation. Our electro-thermal field simulation software is better to satisfy these special demands than the widely used commercial tools. This article surveys the special simulation needs of lighting purpose OLEDs, presents the electro-thermal extension of the FDM-based SUNRED thermal field simulator and the significant algorithmic changes for speed up the program and make it more flexible. The simulation of an existing OLED closes the paper.

Multithreading and Strassen’s algorithms in SUNRED field solver

Complex structures can be well modeled by simulation using appropriate field solvers; however the investigation of detailed models is a time demanding process even on the latest computers. This article surveys the new developments resulting in execution time reduction of the thermal and electro thermal field solver that takes a finite differences method (FDM) based model as input. This tool is based on the vectorized version of SUNRED algorithm. One part of the developments is architectural optimization: multithreading, memory and cache system optimization, the other part is the implementation of matrix multiplication and inversion acceleration algorithms. The result is a significantly faster field solver program.

Advancing the thermal stability of 3D ICs using logi-thermal simulation

3D-ICs have emerged in the past few years. While they solve a large number of problems related to scaling, they also create new ones. Removing the heat from the layers far from the cooling facilities is a great challenge still under intensive research. This paper shows how logi-thermal simulation can be used to predict the operation parameters of large digital systems realized in 3D-ICs. The method can be effectively used to guide place-and-route algorithms and to find the thermal bottlenecks.

How thermal environment affects OLEDs' operational characteristics

In recent years great effort has been put into development of organic light emitting diodes (OLEDs) worldwide. Among many concerns of developers is heat-removal from the thin film structure of the active layers of OLEDs which are typically realized on low thermal conductivity substrates such as glass or polymer foils. The other issue is to provide the OLEDs with transparent, yet high electrical conductivity electric power supply structure, therefore metallic shunting grids are added to the layer stack of OLEDs. These two major issues necessitate self-consistent electro-thermal simulation of large area OLEDs in which the temperature dependent I-V characteristics of the light emitting polymer layers are also considered. In this paper we discuss details of our 2.5D field-solver algorithm extended with such capabilities which was developed for the European funded Fast2Light project. The paper also presents a measurement and simulation example for a glass-based research OLED sample.

Vertical natural convection models and their effect on failure analysis in electro-thermal simulation of large-surface OLEDs

Abstract Besides classical inorganic LEDs, intelligent light sources can be also based on organic LEDs. OLEDs function as surface light sources and manufacturing of large area light sources is feasible with OLED technologies. Despite their lower luminous efficacy their other properties make OLEDs still an attractive option especially in high end indoor applications. In natural convection environment the temperature difference in the same OLED panel can reach 20–30 °C which can result in up to 30–40% difference in current density and thus, in the luminance. This difference in temperature and current density leads to differential ageing of the organic materials. CFD simulation is the obvious way to investigate natural convection environments but integration of a CFD solver in an OLED simulator may be difficult and the solution times are high. As a possible workaround to this problem, in this paper the application of five natural convection models for vertical plates in an electro-thermal field solver based OLED simulator as thermal boundary condition are presented. Steady state and transient simulation results of a free-standing 50 × 50 mm2 active surface OLED, surrounded by still air, are compared with measurement results. A typical failure type of OLEDs is thermal runaway caused by e.g. manufacturing problems, operational damages or overcurrent. The paper presents the effect of the natural convection model on the overcurrent caused thermal runaway simulation results.

Modelling of the thermoelectrical performance of devices based on VO 2

By reaching the limits of conventional silicon-based integrated circuits, more and more effort is done to develop new devices for integrated circuits. A promising structure is based on the semiconductor-to-metal phase change of vanadium-dioxide at about 67°C. In these circuits the information is carried by combined thermal and electrical currents. Thermal effects cannot be separated so well in thermal-electronic circuits as electrical effects in electronic circuits thus, accurate distributed electrothermal simulation is mandatory. This paper presents three VO2 material models, the algorithmic extension of an electrothermal field simulator to be able to handle the hysteresis of VO2 and the modelling of VO2 based devices. The paper compares measured and simulated device characteristics.