Novel cationic water-soluble polyfluorene derivatives with ion-transporting side groups for efficient electron injection in PLEDs

Synthesis of cationic water-soluble polyfluorene derivatives with various side groups, which are used as electron injecting layers in polymer light emitting diodes, is described. Neutral polyfluorene derivatives containing bromo-alkyl terminal groups were synthesized by a palladium catalyzed Suzuki coupling reaction. The bromo-alkyl terminal groups in the neutral polyfluorenes were quaternized by treatment with a trimethyl amine solution. When a high work-function metal such as Ag is used as a cathode in a light emitting diode with an ITO/PEDOT:PSS/MEH-PPV/water-soluble polyfluorene/Ag configuration, effects of these water-soluble polyfluorenes on the device performance were investigated. In the case of poly[(9,9-bis((6′-(N,N,N-trimethylammonium) hexyl)-2,7-fluorene))-alt-(9,9-bis(2-(2-methoxyethoxy)ethyl)-fluorene)] Dibromide (WPF-oxy-F) containing ethylene oxide groups as the electron injecting layer, the electroluminescence efficiency of light emitting devices was significantly enhanced by about two orders of magnitude compared to that of a device without an electron injecting layer because migration of bromide ions via the ethylene oxide side groups led to large space charge. As a result, the injection barrier could be reduced between the emitting layer and Ag cathode resulting high electroluminescence efficiency.