Effect of added ionic salt on the quantum efficiency of self-assembled films prepared with poly(p-phenylene vinylene)

Multilayer electroluminescence (EL) devices based on the self-assembled layers of poly(p-phenylene vinylene) (PPV) and poly(sodium 4-styrenesulfonate) (PSS) were prepared and characterized. The increase of recombination region for the radiative decay of polaron excitons in the film device was achieved by two different fabrication methods: increase of the number of bilayers and increase of ionic strength of PSS solution using a salt additive. When simply increasing the number of bilayers, the relative device efficiency was improved proportional to the number of bilayers deposited. In contrast, the addition of NaCl salt to the PSS dipping solution resulted in higher thickness deposition but exhibited low luminescence level as well as low quantum efficiency. We believe that the decrease of the quantum efficiency, despite the increased thickness in the case of added salt in the PSS solution, is caused by the defect formation at the interface between PPV/PSS film and Al electrode. In order to minimize this problem, we inserted one insulating bilayer composed of cationic poly(allylamine hydrochloride) (PAH) and anionic poly(methacrylic acid) (PMA) in direct contact with the Al electrode. Upon this insertion, significant improvement of device efficiency was achieved and the quantum efficiency was shown to be proportional to the total film thickness even in the case of the salt addition scheme.