A Study of Semitransparent Cathodes on the Performance of Top-Emitting Polymer Light-Emitting Diodes

This work reports the influence of semitransparent cathodes, which were made of Ca or Al covered with a thin layer of Ag, on the performance of poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV)-based top-emitting polymer light-emitting diodes (T-PLEDs). The physical properties of the semitransparent cathodes alter the optical microcavity of T-PLEDs and adjust the output characteristics of the electroluminescence (EL). The bright, red-emissive, MEH-PPV-based T-PLED, with Ca(10 nm)/Ag(20 nm) as the semitransparent cathode, exhibited 3.6 cd/A, 38.7 nm, and ( x = 0.67, y = 0.33) for the luminous efficiency, full-width at half-maximum of the EL, and Commission International de I'Eclairage coordinate, respectively. The favorable performance can be attributed to the formation of an appropriate microcavity structure, which results in spatial redistribution and enhances the out-coupling of EL emission in the direction of the surface normal.