Improved efficiency of light-emitting diodes based on polyfluorene blends upon insertion of a poly(p-phenylene vinylene) electron- confinement layer

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 $\mathrm{poly}{\text{(9,9}}^{\prime }-\mathrm{dioctylfluorene}\mathrm{),}$ with 5 wt % of the green emitter $\mathrm{poly}{\text{(9,9}}^{\prime }-\mathrm{dioctylfluorene}\mathrm{-alt}\mathrm{benzothiadiazole}\mathrm{),}$ 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.