Revealing the influence of hole injection material's molecular orientation on OLED's performance

Abstract In this study, we introduced N,N′-bis(4-trifluoromethoxybenzyl) naphthalene-1,4,5,8-tetracarboxylic acid di-imide (NDI-BOCF 3 ) and N,N′-Bis(4- trifluoromethoxyphenyl) naphthalene-1,4,5,8-tetracarboxylic acid diimide (NDI- POCF 3 ) as hole injection layers (HILs) in organic light emitting diodes (OLEDs) and investigated the relationship between HIL's molecular orientation and the hole injection characteristics. NDI-BOCF 3 and NDI-POCF 3 have similar optical and electrochemical properties; however, it is quite surprising that both of these materials demonstrated totally different function when used as HILs in OLEDs. NDI-BOCF 3 is observed to enhance the hole injection, in contrast, NDI-POCF 3 hinders hole injection. X-ray Diffraction (XRD), Electrostatic Surface Potential (ESP) calculations and Ultraviolet Photoelectron Spectrometry (UPS) were applied to explore the mechanism of NDI-BOCF 3 and NDI-POCF 3 as HILs. By comparing molecular orientation and electrostatic potential distribution of these two materials, we identified that the change in anode's work function caused by different molecular orientation of HILs results in completely different hole injection properties.