Poly(ethylene oxide)-functionalized Al cathodes of tunable electron-injection capabilities for efficient polymer light-emitting diodes

Poly(ethylene oxide)-functionalized aluminum (Al) cathodes of tunable electron-injection capabilities are reported in this study for the fabrication of high-performance polymer light-emitting diodes (PLEDs). The composite cathodes are prepared by thermally co-evaporating poly(ethylene glycol) dimethyl ether (PEGDE, M_nca. 2000) polymer with Al metal simultaneously in vacuum as the cathode buffer. We attribute the formation of the carbide-like/Al complex as characterized by X-ray photoelectron spectroscopy (XPS) to the instant chemical reactions between the ethylene oxide functional groups with Al, which markedly enhances the device performance. The electron-injection capabilities of PEGDE:Al/Al cathodes are correlated with the contents of the carbide-like/Al complex in the buffer layer and can, therefore, be modulated by varying the co-evaporation ratios of PEGDE and Al. The luminous efficiency of green polyfluorene-based PLEDs applying an PEGDE:Al (2:1)/Al cathode is of 10.46 cd A^-1, which is much higher than that of ∼0.04, 5.19, 6.40 cd A^-1 for PLEDs with Al, PEGDE/Al and calcium/Al cathodes, respectively. This research leads to an alternative cathode design, free of salts, additives, complicated synthesis, and of tunable electron-injection capabilities to balance the opposite charge carriers for fabricating high-performance PLEDs.