Improved electroluminescence lifetime and efficiency of polymer light- emitting diodes with plasma-treated indium tin ox
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ABSTRACT We 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/m 2) 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.
INTRODUCTION Over the last 6 years, a significant effort has been devoted to the investigation of the operational stability of polymer Light-Emitting Diodes (LEDs) [1-7]. These are promising counterparts of inorganic semiconductors, especially for large area, low cost, and mechanically flexible applications. Unlike inorganic semiconductors, however, polymeric and oligomeric devices are relatively delicate and subject to degradation when operated without appropriate control of environment and driving conditions. The ageing is not governed by the polymer properties alone, but depends on all the device elements, and hence on the interplay of all the processes taking place during operation, from charge injection and transport, to triplet and singlet excitons formation and to their non-radiative or radiative decay. In the first reported investigation of ageing of polymer LEDs [1], we were able to show that ITO/PPV/Ca (PPV = poly(p-phenylene vinylene)) diodes could be operated longer than 1200 hours (corresponding to more than 106 C/cm 2 charge flux), and deduce that diffusion of alkaline-earth cathodes through the polymer was not significant, for metallisation procedures carried out at residual pressures in the range 106/l0.5 mbar, as also confirmed by later investigations [8]. As for the stability of the active material, the Philips group carefully investigated the quantum yield for photobleaching of a PPV derivative and concluded that only 1 in 104 excitation events resulted in polymer degradation, and were then able to rule this out as the lifetime limiting mechanism in their diodes [4]. The finding naturally shifted the emphasis of lifetime investigations to the physics of the interfaces, with substantial improvements obtained upon introduction of low work function cathodes [6, 9], and of a Hole Transport/Injection Layer (HTL). The latter can be a heavily doped conducting polymer such as polyaniline (PANI), or poly(3,4-ethylene dioxythiophene), PEDOT, doped with Poly(styrene sulfonate), PSS, [5-7, 10], phthalocyanines [3], or triphenylamine derivativ
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