Improved electroluminescence lifetime and efficiency of polymer light- emitting diodes with plasma-treated indium tin oxide anodes

ABSTRACTWe studied the influence of various surface treatments of indium-tin oxide anodes on the operational stability of high-efficiency green-emitting polymer light-emitting diodes, fabilicated with a doped poly(3,4-ethylene dioxythiophene) PEDOT hole transport layer, a polyfluorene-based emissive layer, and Ca-Al cathodes. The anodes were modified by physical (oxygen-plasma), chemical (aquaregia), or combined treatments. Oxygen-plasma improves the stability under constant current over all the other anodes, with half-brightness lifetimes (initial brightness, 200 cd/m2) two to five times longer than for untreated samples, and 1000 times longer than for aquaregia ones. We derive two major indications for optimisation of PLEDs. First, thermal management of the diode is of the uppermost importance. Second, the ITO anode and in general the electrical properties of the hole-injecting contact are crucial to device operation, even in the presence of a hole transport layer.