Enhanced light output in bilayer light-emitting diodes with film thickness variations

Electroluminescence (EL) and the dynamics of charge carrier recombination in the PVK/poly(9,9′-dihexylfluorene-2,7-divinylene-m-phenylenevinylene-stat-p-phenylenevinylene)(PDHFPPV) bilayer EL devices, which have various film thickness ratios, were studied. The indium-tinoxide (ITO)/(PVK/PDHFPPV bilayer)/Li:Al devices showed more enhanced color quality compared with those of single and blended devices. The current–voltage–luminescence characteristics of the devices, which have a different PVK/PDHFPPV thickness ratio, are systematically studied and compared with a best PVK:PDHFPPV blended device. The EL light output of bilayer devices can be improved remarkably by controlling PVK and PDHFPPV polymer thickness. The ITO/(PVK (60 nm)/PDHFPPV (50 nm) bilayer)/Li:Al device showed ∼50 times improved light output compared with that of best blended device.