Sodium hydroxide/aluminum bilayer cathode for enhancing the performance of blue polymer light-emitting diodes

Sodium hydroxide/aluminum (NaOH∕Al) bilayer cathode has been developed for improving the performance of blue polymer light-emitting diodes (PLEDs) based on poly(9,9-dioctylfluorene). In comparison to the control device with Ca∕Al as the cathode, the performance of the PLED with NaOH∕Al cathode, including driving voltage, light-emitting efficiency, and color purity, is significantly improved. For achieving a luminance of 500cd∕m2, the driving voltage is reduced from 8.3to5.3V and the light-emitting efficiency is increased from 0.46to0.67cd∕A. Moreover, the device with NaOH∕Al cathode showed a pure blue emission with Commission Internationale de l’Eclairage coordinates of (0.17, 0.09) as compared to that of (0.18, 0.13) for control device operated at same condition. These improvements are attributed to the use of an efficient bilayer NaOH∕Al electron injection contact that also prevents the formation of Ca-induced fluorenone occurred in PLEDs with Ca∕Al cathode.