Microstructural studies of organic light-emitting devices by Monte Carlo simulation of two-dimensional triangles

The fast degradation of organic light-emitting devices (OLEDs) remains as the main obstacle to the commercialization of OLEDs. Among the failure mechanisms proposed, crystallization of the amorphous Alq₃ film that leads to the quenching of electroluminescence plays a crucial role and is little understood. Because in situ studies of the Alq₃ layer, with the probing of thin organic film by, for example, x-ray diffraction, are very difficult, if not impossible, Monte Carlo simulation is therefore conducted. The molecular motion of Alq₃ is simulated by two-dimensional triangles which interacted with a square-well potential in an isothermal-isobaric ensemble. Simulated results show the structural relaxation of Alq₃ from amorphous to crystalline upon thermal annealing. To impede this ordering process, quenched impurities of various shapes were added to the system. It is found that impurities of circular shape and of low aspect ratio have relatively the highest disordering effect. This is in good agreement with the experimental results. In addition, the preferred geometry of the Alq₃ system was also examined.