A COMPARISON OF HOLE BLOCKING ELECTRON-TRANSPORT POLYMERS IN ORGANIC LEDS

Three main-chain aromatic polyethers with different electroactive hete rocyclic moieties, 1,4-quinoxaline, 1,3,4-oxadiazole and 1,3,5-triazin e, have been synthesized. The polymers are amorphous with glass transi tion temperatures above 200 degrees C. The polymers with these high el ectron affinity units were used as hole blocking/electron transport la yers (HBETL) in light-emitting diodes (LEDs) having the HBETL casted o n top of a hole transport/emitting PPV layer. In order to compare the influence of the different polyethers on the LED characteristics, thre e multilayer devices (ITO/PPV/HBETL/Al) with different HBETLs were inv estigated. Relative to the single layer PPV device, quantum efficienci es were improved by two orders of magnitude in all multilayer devices and power efficiency was increased using poly(quinoxaline ether) as HB ETL. To investigate the electrochemical behavior of the three HBETLs, cyclic voltammetry measurements were carried out and the HOMO/LUMO ene rgy values determined from redox potentials were used to understand th e hole blocking property. Lowering the onset voltage using the poly(qu inoxaline ether) as HBETL in two-layer devices is compatible with the high electron affinity of this polymer.