An Improved Blue Polymer Light-Emitting Diode by Using Sodium Hydroxide/Ca/Al Cathode

The performance of blue polymer light-emitting diodes (PLEDs) based on poly(9,9-dioctylfluorene) (PFO) is improved by introducing a thin layer of sodium hydroxide (NaOH) between the calcium cathode and the PFO emissive layer. By replacing the commonly used Ca/Al cathode by a NaOH (2.5nm)/Ca (10nm)/Al cathode, the driving voltage is reduced from 8.3V to 5.4V and the light-emitting efficiency is enhanced from 0.46cd/A to 0.72cd/A for achieving a luminance of 500cdm², respectively. Moreover, the device with NaOH/Ca/Al cathode shows a pure blue emission of (0.17, 0.12) at high brightnesses. These improvements are attributed to introduction of a thin layer of NaOH that can lower the interfacial barrier and facilitate electron injection.