Interface effects on the defect state formation in organic devices

摘要

We have investigated the role of interfaces in the formation of traps in organic devices using poly(9,9-dihexylfluorene-co-N,N-di(9,9-dihexyl-2-fluorenyl)-N-phenylamine) (PF-N-Ph) as an emissive material. The basic structure of the studied diodes is ITO/PEDOT:PSS/PF/M where M is Al or Ca/Al. Trap parameters have been measured by Charge based Deep Level Transient Spectroscopy (Q-DLTS) in diodes having different electrode configurations. Five trap levels have been identified in the basic device structures with activation energies in the range of 0.1 - 0.6 eV and trap densities in the range of 10~(16) - 10~(17) cm~(-3). On the cathode side, no noticeable changes have been observed when changing the electrode from aluminium to calcium. On the anode side, comparing the trap parameters in devices with and without a PEDOT:PSS layer, we show that the hole injection layer introduced new trap levels, which are electron-traps. The density of these traps is of the order of 10~(16) cm~(-3) and their levels are at ～ 0.3 and ～ 0.5 eV from the band edges. The findings confirm and complete quantitatively earlier results by other groups on the role of the PEDOT:PSS /organic interface in the trap formation in OLEDs.