Enhancement Performance of Organic Light-Emitting Diodes Through Self-Assembled Monolayer-Modified ITO Anode

Herein, three self-assembling materials (SAMs), (pyridine-4-ylmethyl)phosphonic acid (PyPA), benzylphosphonic acid (BnPA), and pentafluorobenzyl phosphonic acid (F5BnPA), are selected as hole-injecting layers in organic light-emitting diodes (OLEDs) and the effects of the aromatic ring of SAMs on indium tin oxide (ITO) work function (WF), hole injection, and device performances are investigated. After assembling them on ITO, the X-ray photoemission spectroscopy data suggest that three samples have a similar binding mode, and the ultraviolet photoemission spectroscopy shows the effective change of ITO WF gradually enlarged along the order of PyPA, BnPA, and F5BnPA, resulting in increase in hole-injecting capability in turn. The calculated molecule dipole of pyridine is 2.65 Debye, clearly larger than that of phenyl (0.32 Debye) and pentafluorophenyl (1.88 Debye), but PyPA presents minimum ITO WF change. Such differentiated WF changes are not only contributed by the molecular dipole of aromatic ring but also their titling angles. In OLEDs based on tris(8-hydroxyquinoline) aluminum (III) (Alq(3)), the device modified by PyPA affords the highest external quantum efficiency (EQE) of 1.45% and current efficiency (CE) of 4.62 cd A(-1). However, in the device using 9-[4-(4,6-diphenyl-1,3,5-triazine-2-yll)phenyl]-N,N,N ',N '-tetraphenyl-9H-carbazole-3,6-diamine as emitters, the OLEDs modified by F5BnPA show the best EQE of 24.85% and CE of 81.88 cd A(-1).