Horizontal molecular orientation in vacuum-deposited organic amorphous films

Organic amorphous films fabricated by vacuum deposition have been used as essential components in organic light-emitting diodes (OLEDs) because they have the advantages of nanometer-scale surface smoothness and easy controllability of thickness. However, molecular orientation in organic amorphous films has been disregarded for the past 20 years since the beginning of the research on OLEDs. Here, we demonstrate horizontal molecular orientation in neat and doped organic amorphous films and show the general relationship between molecular structures and the molecular orientation. It was found that when molecular structure is linear or planar, the anisotropy of the molecular orientation in films becomes generally large. The results show the vital importance of the horizontal molecular orientation to understand the light emission and the carrier transport in OLEDs. To elucidate the device physics in OLEDs, the molecular orientation in amorphous films should generally be taken into consideration.