Efficient Polymeric Light-emitting Devices with Aluminum Cathode
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Efficient polymeric light-emitting devices with aluminum cathode X. Y. Deng and K. Y. Wong Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China ABSTRACT By blending poly(ethylene glycol) (PEG) into an electroluminescence (EL) polymer, significantly enhanced EL efficiency in a polymer light–emitting diode (PLED) with aluminum electrode was achieved. An orange-color-emitting PLED with 10 wt% PEG blending achieved device efficiencies exceeding 2.6 cd/A for a wide range of bias voltage, which is more than two orders of magnitude higher than that for a similar PLED without the PEG blending. A similar enhancement was observed by introducing PEG as an ultra-thin interfacial layer between the emissive polymer and the cathode. The enhancement was also observed for several different species of emissive polymers, and for blending with either PEG or poly(ethylene oxide) (PEO) with different molecular weights. The enhanced efficiency was a result of the reduction of electron injection barrier height at the cathode-polymer interface. It is believed that interfacial interaction that is specific to Al plays an important role in the enhancement mechanism. INTRODUCTION Polymer light-emitting diodes (PLEDs) have attracted much interest worldwide since its discovery by Friend and co-workers [1] in 1990. Engineering of the polymer-electrode interfacial properties is crucial to achieve balanced hole and electron injection for high efficiency operations. A low work function metal such as calcium or barium is commonly used as the cathode to reduce the barrier height and to promote electron injection [2], However, low work function metals are highly susceptible to environmental degradation and are difficult to process. From a practical point of view, it would be desirable to be able to employ stable Al metal as the electrode. By inserting an ultra-thin layer of alkali metal fluorides between the Al cathode and the emissive layer, it has been shown that dramatically improved efficiency resulted both in organic light-emitting diodes (OLEDs) [3] as well as in PLEDs [4-6]. In other approaches, soluble metal ionic polymers, surfactants or metal organic compounds that were either spincoated on [7,8] or blended into [9] the emissive polymer also resulted in significant enhancement in efficiency for PLEDs with Al electrode. In all the aforementioned approaches, the metallic atoms contained in the interlayer or in the surfactant additive are believed to be essential to the enhancement mechanism. In an earlier communication [10], we reported that the device efficiency of PLEDs based on poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) using Al electrode was significantly enhanced when MEH-PPV was blended with poly(ethylene glycol) (H(OCH2CH2)nOH, PEG). This finding was unique among the other approaches, since PEG contains no metallic atom in its structure. Further results in achieving high efficiency PLEDs using Al electrodes are described in this report. We found that similar enhancem
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