Controlling Ion Motion in Polymer Light-Emitting Diodes Containing Conjugated Polyelectrolyte Electron Injection Layers

The properties and function of an anionic conjugated polyelectrolyte (CPE)-containing ion-conducting polyethylene oxide pendant (PFPEOCO2Na) as electron injection layers (EILs) in polymer light-emitting diodes (PLEDs) are investigated. A primary goal was to design a CPE structure that would enable acceleration of the device temporal response through facilitation of ion motion. Pristine PLEDs containing PFPEOCO2Na exhibit luminance response times on the order of tenths of seconds. This delay is attributed to the formation of ordered structures within the CPE film, as observed by atomic force microscopy. Complementary evidence is provided by electron transport measurements. The ordered structures are believed to slow ion migration within the CPE EIL and hence result in a longer temporal response time. It is possible to accelerate the response by a combination of thermal and voltage treatments that “lock” ions within the interfaces adjacent to PFPEOCO2Na. PLED devices with luminance response times of microseconds, a 105 fold enhancement, can therefore be achieved. Faster luminance response time opens up the application of PLEDs with CPE layers in display technologies.