摘要： The light emitting layer (EML) is generally prepared by mixing host and dopant to realize organic light emitting diode (OLED). However, phase separation is often observed during the fabrication process to prepare OLEDs depending on the structure of the host materials. In particular, phase separation due to π-π stacking is frequently observed during thermal annealing for the solution process. The annealing process is required for solvent removal and complete relaxation of the molecule. Hence, the materials with a high glass transition temperature (Tg) are ideal because phase separation occurs due to π-π stacking during annealing process if Tg is too low. To understand this phenomenon, we compared two host materials with similar molecular weights but different three-dimensional connectivity, which causes different rotational freedom. Then, we investigated the effect on the device properties depending on the annealing conditions. In both materials, when the annealing temperature rises above 120 °C, the dopant completely escaped from the EML. However, the material that does not disturb the molecular stacking order by annealing due to its limited free rotation through internal bond shows much better device characteristics even after annealing at a higher temperature than Tg. The results show that interdiffusion at the interface and unstable internal density distribution with annealing temperature are responsible for the device degradation behavior.