Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyElectro-thermal investigation of OLEDsL. Pohl, E. Kollár, Zs. Kohári and A. PoppeDepartment of Electron Devices, Budapest University of Technology and EconomicsH-1111 Budapest, Goldmann tér 3., HungaryAbstract-In our project the final goal is to develop the costeffectiveroll-to-roll technology of fabricating large area, highthroughput OLED devices for intelligent lighting applications.The main achievements of the first six months of the project aredescribed in this paper. An electro-thermal FDM simulator wasdeveloped on the basis of an existing thermal-only simulator.The simulations proved the necessity of building a shuntinggrid of high conductance at the transparent anode of the OLEDdevice. The model validation technique and a method for thethermal qualification of the targeted device were also elaborated.I. INTRODUCTIONResearch until now has mainly been motivated by glassbasedorganic electroluminescence (OLED – Organic LightEmitting Diode) displays [1], where the aspects of increasedcontrast, high viewing angle and response speed are critical.With the increasing luminance and efficiency of OLEDs intelligentlighting applications are becoming increasingly relevant.In our research project called Fast2Light [2] theoverall objective is to develop a novel, cost-effective, highthroughput,roll-to-roll, large area deposition process for fabricatinglight-emitting polymer-OLED foils for intelligentlighting applications.Lighting purpose OLEDs require high power density,however the polymer substrate and the materials used in thedevices have bad electrical and heat transfer properties. Inthis article we will present simulation and measurement resultson some sample OLEDs and introduce the new electrothermalextension of the SUNRED field solver program [3].II. ORGANIC LIGHT EMITTING DEVICESAlthough conventional LEDs based on III-V semiconductors(AlInGaP, InGaN) achieve bright emission with sufficientquantum efficiency for the visible spectral region, theiruse for flat panel displays or large area general lighting applicationsis unlikely due to the fabrication cost and packagingissues. Organic semiconductors, however, show goodcharge carrier transport properties as well as are excellentcandidates for cheap and highly effective alternatives forlarge area applications [4].As shown in Fig. 1. Organic Light Emitting Diodes arethin-film multi-layer devices consisting of a substrate foil,film or plate (rigid or flexible, in our project the target substrateis flexible foil for roll-to-roll technology), an electrodelayer, layers of active materials, a counter electrode layer,and a protective barrier layer. At least one of the electrodesLightFig 1. Typical structure of an OLED device [5].The number of layers may vary.must be transparent to light [5]. Voltage bias is applied onthe electrodes. The voltages are low, from 2.5 to ~ 20 V, butthe active layers are so thin (~10Å to 100nm) that the electricfields in the active layers are very high, in the order of10 5 – 10 7 V/cm.To ensure uniform luminance over the large surface of thetargeted lighting device (60 cm × 60 cm) the voltage distributionmust be (very close to) uniform. Due to the poor conductanceof the anode material this goal can only beachieved by using a grid of some material of high conductance(shunting grid). Copper or silver are possible solutionsfrom technological point of view, the latter makes the productioncost higher. Whichever metal is used the grid wiresare not transparent to light so they decrease the luminance ofthe device (out-coupling efficiency).In order to work out thermal and electro-thermal characterizationstrategy for the Fast2Light project, for the actual initialsimulations and measurements we used a proprietaryThis work has been supported by the ICT-2007.3.2/216641 Fast2LightProject of the Framework 7 program of the EU.Fig 2. Photograph of the investigated OLED device.©EDA Publishing/THERMINIC 2008 235ISBN: 978-2-35500-008-9