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.