Essential Differences of Organic Films at the Molecular Level via Vacuum Deposition and Solution Processes for Organic Light-Emitting Diodes

This paper presents an in-depth understanding of the essential differences of organic small-molecule thin films at the molecular level via vacuum deposition and solution processes for organic light-emitting diodes (OLEDs). Synchrotron-based two-dimensional grazing incidence X-ray diffraction has been used to investigate the essential difference. The result reveals that tris(4-carbazoyl-9-ylphenyl)amine (TCTA) molecules show highly oriented arrangements, that is, face-to-face π–π stacking, in vacuum-deposited films, unlike the randomly arranged molecules in spin-coated films. The face-to-face π–π stacking behavior of the molecules in a vacuum-deposited TCTA film leads to higher hole-transport mobility, which is the essential reason for the higher efficiency of a vacuum-deposited OLED compared with that of a solution-processed counterpart, consistent with the calculation results.