Direct observation of the crystallization in EL organic thin films by total reflection X-ray diffractometer

The degradation of amorphous organic electroluminescent (EL) devices is generally assumed due to the crystallization enhanced by the Joule heat. Here, in order to reveal directly the structural changes in organic EL films during an annealing process, we performed first total reflection X-ray diffraction measurements for N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4, 4′-diamine (TPD) films vapor-deposited on SiO₂ glass substrates. Atomic force microscopy (AFM) was also used to observe the morphological modifications through annealing. The X-ray data revealed that the TPD film remained amorphous below 100 °C, followed by a gradual change into crystalline during annealing. Moreover, combining this result with the AFM data, we found that there existed some relationship between morphological change and crystallization in the film.