HIGHLY EFFICIENT AND THERMALLY STABLE ORGANIC LIGHT-EMITTING DIODES

摘要

Thermal stability and long lifetime of molecular organic light-emitting diodes (MOLEDs) are key issues for their use in flat panel displays. Such an application requires them to maintain device characteristics, e.g. Quantum efficiency and emission spectrum at different operating temperatures. Degradation of MOLEDs upon heating might be attributed to the morphological instability of the hole transport layer. For instance, the widely used hole transport material, N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD), has a relatively low glass transition temperature (Tg) near 63°C and is easy to recrystallize at room temperature. Hence, hole transport materials with high Tg offer an effective approach for improving the thermal stability of MOLEDs. On the other hand, the electroluminescence (EL) quantum efficiency of a device using an undoped tris-(8-hydroxyquinolinato) aluminum (Alq_3) as the emitting layer decreases rapidly with increasing temperature. The decrease of EL efficiency can be partially attributed to the decrease of the photoluminescence (PL) quantum yield of Alqs with increasing temperature. In this study, we demonstrate thermally stable and efficient MOLEDs based on the combination of double hole transport layers with a high Tg, and an emitting layer doped with a fluorescent dye. Using hole transport materials with Tg above 100°C, the devices show considerably stable emission up to 110°C without failure. When Alq_3 is doped with a quinacridone derivative and is used as the emitting layer, the EL efficiency at room temperature is enhanced and is sustained up to 110°C. Improvements of EL efficiency and its temperature dependence are discussed. Results on device lifetime measurements are also presented.