Polymer light-emitting devices using poly(ethylene oxide) as an electron injecting layer

Abstract

The performance of polymer light emitting devices (PLEDs) based on polyvinyl carbazole (PVK) is improved by introducing a nanoscale interfacial thin layer, made of poly(ethylene oxide) (PEO), between the calcium cathode and the PVK emissive layer. It is believed that the PEO layer plays a key role in enhancing the device performance. In comparison to the device with Ca/Al as the cathode, the performance of the PLED with PEO/Ca/Al cathode, including the driving voltage, luminance efficiency is significantly improved. These improvements are attributed to the introduction of a thin layer of PEO that can lower the interfacial barrier and facilitate electron injection. Polymer light-emitting diodes (PLEDs) have attracted much interest worldwide since their discovery by Friend and co-workers in 1990 [1]. Engineering of the polymer-electrode interfacial properties is crucial to achieve balanced hole and electron injection for high efficiency operations. Low work function metals, such as calcium or barium are widely used as a cathode to facilitate electron injection [2][3][4][5][6][7][8]. These metals are very sensitive to moisture and oxygen and form detrimental quenching sites at areas near the interface between the electroluminescent (EL) layer and the cathode. In addition, metal ions formed at the metal/organic interface tend to migrate into the EL layer, thus, affecting the long term stability of devices [9]. To circumvent these problems, it is desirable to use high work-function metals (such as Al, Ag, or Au) as the cathode because of their better environmental stability and the simplicity of their device fabrication. To improve electron injection from high-work-function metals into the emitting layer, numerous approaches have been attempted [10,11]. For example, by inserting a thin layer of lithium fluoride, or cesium fluoride, between Al and the light emitting layer, the electron-injection ability could be significantly improved [10][11][12]. It was shown by the Cao et al. that the insertion of organic surfactant molecules between Al and the EL polymer could improve device performance up to the level obtained by low-work function metal cathode [11]. Li et al. reported that effective electron injection can be achieved by the insertion of an ultrathin aluminum oxide layer between the Al and the emitting layer [13]. Lee et al. reported that polystyrene sodium sulfonate inserted between MEH-PPV and Al significantly improves electron injection [14]. Guo et al. reported that high performance polymer and small molecule OLEDs with an organic oxide, poly(ethylene glycol) dimethyl ether (PEGDE). Because PEGDE can be evaporated at low temperature compared to other inorganic insulators and improves the device performance as much as the inorganic electron injection layers, it could be a good candidate for an organic electron injection layer [15][16][17]. In this study, the performance of PVK based PLEDs is improved by using PEO/Ca/Al cathode. When a thin PEO (5 nm) buffer layer is introduced between the PVK emissive layer an...

Enhancement in light emission and electrical efficiencies of a silicon nanocrystal light-emitting diode by indium tin oxide nanowires

“…This phenomenon could be depressed through low defect densities in the SiN x film and the design of the emissive structure containing the Si NCs such as the multi-quantum well structure or electron blocking layer generally used in organic-and gallium nitride-based LEDs. [20][21][22] Figure 4 shows the schematic diagrams of the possible photon paths at the interface between the ITO film and surrounding air. The light extraction efficiency is limited by the total internal reflection of the generated light in the Si NCs, which occurs at the ITO film-air interface.…”