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OLED device (see Fig. 2.) provided by a project partner. Thedevice was realized on glass substrate. Individual OLEDpixels and a larger pixel array were available on the demodevice. The size of the large device was approximately3.3x2.1cm 2 .III. SIMULATION NEEDSThe poor electrical conductance of the anode layer and thelarge area of the targeted lighting device (60 cm × 60 cm)raise the need for electrical simulation, to predict the voltagedrop over the large surface.The electrical simulations aim at finding the optimal gridgeometry that can ensure the uniform voltage but does notreduce luminance by more than a few percent.For the electrical simulation of large area OLEDs conventionallumped electrical circuit models are not appropriate; adistributed approach must be applied. In its physical nature,the electrical simulation problem resembles thermal simulation.The electrical potential distribution can be studied usinga thermal simulation tool utilizing the electrical-thermalanalogy. 1W dissipation corresponds to 1A electrical current.In this case, 1K/W thermal resistance represents 1Ω ofelectrical resistance and the simulated temperature correspondsto the potential distribution, consequently, 1K temperaturedifference corresponds to 1V of potential drop.Thermal simulation is also a must in OLEDs mainly becauseheat-sensitive organic materials are applied.Thermal simulations aim at examining the temperaturedistribution over the surface and inside the layer structure toensure the correct functioning of the device by avoiding e.g.hot spot formation, thus avoiding local overheatibg whichmay result in dark dots. The predicted surface temperaturedistribution can be verified by IR thermal measurement. Thisvalidation is in progress.The principle of the IR validation is the following. Thesurface of the OLED device must be coated with a lightabsorbingpaint for the IR measurement. This paint absorbsthe light emitted by the OLED as well and it will further heatup the surface. This elevated temperature will be measured.To calculate the temperature map of the surface under normaloperating conditions (without coating), the overall energyefficiency should also be measured. After that two simulationsof the OLED structure are required with different excitations:one with the total electrical power and one with theinefficient (dissipating) power. If the thermal map simulatedwith the total power matches the measured temperature distribution,then the simulation with the reduced power willgive the required temperature map. This method covers twogoals at once: validating the model and predicting the temperaturedistribution of the device.These together suggest that a distributed electro-thermalsimulation would be the best approach for simulation ofOLEDs. The main objectives of the simulation are the following:• proper prediction of the voltage drop in the large areaOLEDs to allow design of appropriate shunting nets,24-26 September 2008, Rome, Italy• to calculate joule heating in the OLEDs,• and based on the calculated dissipation map to end upwith a temperature distribution of the large areaOLEDs.The above electrical and thermal simulations can be carriedout either by consecutive electrical and thermal simulations(feeding the result of the electrical one into the thermalsimulation), or by a coupled electro-thermal simulation. Thefirst approach seems easier with commercially availabletools. Any finite element method (FEM) or finite differencemethod (FDM) based tool with thermal or electrical field simulationcapability can be suitable for this purpose.The electro-thermal approach requires a dedicated simulationtool but gives more accurate results due to taking intoconsideration temperature dependent electrical effects aswell.In either case one of the most crucial issues is to estimatethe overall energy efficiency of the OLED device correctly.To meet the simulation needs of OLEDs we decided to extendthe SUNRED thermal simulation algorithm to accountfor joint electrical and thermal simulation.For electrical simulation of such structures that containthin wires it is essential to know the current-flux values aswell to be able to check the possibility of electro-migration.This is another strong argument for using the SUNRED algorithmsince when originally developed, it was already optimizedfor accurate calculation of flux as well [6].The original SUNRED algorithm (successive network reduction)considers the finite difference model of the thermalsystem by means of thermal resistor/capacitor networks anduses network theory to reduce the number of nodes (thus, thenumber of unknowns) to treat during the actual equation solutionprocess. The successive network reduction results in afinal model where there are nodes at the boundaries of thesimulation model – resembling the boundary element method,also widely used in field solvers. For the distributedelectrical problems the same approach is well applicable, soit was straightforward for us, that for the electro-thermal simulationof OLED devices the SUNRED algorithm is a goodchoice.IV. ELECTRO-THERMAL SIMULATION IN SUNREDThe SUNRED algorithm has been developed for thermalfield simulation [3] at the Department of Electron Devices(DED) of BME, and later it has been completed by an electro-staticextension [7]. (Until recently, a commercial versionof the program was also provided by MicReD Ltd.)Electro-thermal simulation required a major revision ofour algorithm: while thermal and electro-static problems canbe described by scalar fields, electro-thermal problems requirethe computation of two dimensional vector fields.Electro-thermal fields can be described by four partial differentialequations [8]. The original model contains Seebeckand Peltier effects and Joule-heating. OLED modeling requiresJoule heating only.©EDA Publishing/THERMINIC 2008 236ISBN: 978-2-35500-008-9