High-Efficiency Saturated Red Bilayer Light-Emitting Diodes: Comparative Studies with Devices from Blend of the Same Light-Emitting Polymers

High-efficient saturated red light-emitting diodes are realized based on a bilayer of phenyl-substituted poly p-phenylene vinylene derivative (P-PPV) and copolymer (PFO-DBT15) of 9,9-dioctylfluorene (DOF) and 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DBT). External electroluminescent (EL) quantum efficiency of PFO-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-DBT15 at the current density of 35mA/cm². The luminescence efficiency reaches 0.83cd/A, and the Commission Internationale de I'Eclairage coordinates (CIE) become nearly x=0.700 and y=0.300. In comparison with the devices from PFO-DBT15 and P-PPV blend films, the P-PPV/PFO-DBT15 bilayer device shows higher EL quantum efficiency and better stability under high current density. The improved device performance can be attributed to the charge-confinement effect at the interface of the P-PPV/PFO-DBT15 bilayer structure.