We report the improvement of the electroluminescence efficiency of light-emitting diodes (LEDs) based on polyfluorene blends, upon insertion of a thin film of poly(p-phenylene vinylene), PPV, between a hole-injection layer of poly(3,4-ethylene dioxythiophene), doped with polystyrene sulfonic acid, and the polyfluorenes emissive layer. For LEDs using a blend of poly(9,9′-dioctylfluorene), with 5 wt % of the green emitter poly(9,9′-dioctylfluorene-altbenzothiadiazole), and calcium cathodes, the efficiency increases from 2.1 to 4.1 cd/A upon insertion of such a PPV layer. We propose that such an improvement is mainly due to the electron-blocking effect of the PPV layer, leading to improved charge carriers balance within the emissive layer.

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