Crystallization and Aggregation Processes of Vacuum-Evaporated TPD Films

Abstract

Crystallization of organic layers in small organic light-emitting diode (OLED) devices has a detrimental impact on their performance, such as current density-voltage characteristics. Using scanning tunneling microscopy, we investigated the crystallization process of organic films grown on indium tin oxide substrates. N,-diphenyl-N,-bis(3-methyl-phenyl)-1,-biphenyl-4,-diamine (TPD), typically used as a hole transport layer in OLEDs, was employed as the sample material. As-grown TPD films consisted of separate spheroid-shaped grains. These grains were relatively unstable and they easily transformed into crystallized grains. The crystallization process proceeded grain by grain. Crystallized grains changed to a ball-like shape, suggesting that reconstruction of the molecular stacking structure inside the grains had occurred. Slab optical wave guide spectroscopy revealed that a new absorption band appeared at in the annealed film, in addition to the normal absorption band at . This suggests that annealing changed the molecular stacking structure in an H aggregate fashion, which is energetically stable, resulting in a blueshift in the absorption band. These crystallized grains formed clusters. The growth of these clusters was largely enhanced by heat. However, the dimensions of each crystallized grain remained unchanged. As the clusters grew, they aggregated and caused macroscopic deformation in the film.