High-efficient saturated red light-emitting diodes are realized basedon a bilayer of phenyl-substituted poly [p-phenylene vinylene]derivative (P-PPV) and copolymer (PFO-DBT15) of9,9-dioctylfluorene (DOF) and 4,7-di-2-thienyl-2,1,3-benzothiadiazole(DBT). External electroluminescent (EL) quantum efficiency ofPFO-DBT15 is increased from 1.6% for a single-layer device to 4.7%for the bilayer device by insertion of a P-PPV layer between PEDOT(polyethylene dioxythiophene-polystyrene sulfonic acid) and PFO-DBT15at the current density of 35 mA/cm2. The luminescence efficiencyreaches 0.83 cd/A, and the Commission Internationale de I'Eclairagecoordinates (CIE) become nearly x = 0.700 and y = 0.300. Incomparison with the devices from PFO-DBT15 and P-PPV blend films, theP-PPV/PFO-DBT15 bilayer device shows higher EL quantum efficiency andbetter stability under high current density. The improved deviceperformance can be attributed to the charge-confinement effect at theinterface of the P-PPV/PFO-DBT15 bilayer structure.