Effects of Buffer Layer Treatments on the Characteristics and Performances of OLEDs

The authors demonstrated the performance of Organic Light Emitting Diodes (OLEDs) with hexylphosphonic acid (PA) or UV-ozone treatment on their molybdenum trioxide (MoO3) anode buffer layers. The OLEDs with a PA treated (5 mM @1 h) MoO3 layer have lower turn-on voltage and low current efficiency roll-off under high operating current. The hole-only device (ITO/MoO3 with PA or UV-ozone treatment/NPB/Al) was fabricated to calculate the active energy via temperature dependent I-V measurement. When the devices were operated at high temperatures, the activation energy of the UV-ozone treated, and untreated hole-only devices became nonlinear. However, the activation energy of the PA treated devices had a more stable performance at high temperatures. The interfacial resistance of the untreated hole-only devices and the PA and UV-ozone treated devices were calculated by Admittance Spectroscopy (AS) to be 204.5, 363, and 450 O. These results indicated that the PA treated devices reduced the activation energy, surface energy mismatch, and interface resistance to enhance the efficiency of carrier injection, which in turn lowers the turn on voltage. Moreover, PA treatment produces a long carbon chain and smooths the morphology of MoO3 to prevent roll-off phenomenon of OLEDs under high operating current.