Reduction of chain interactions in a class of blue fluorene copolymers with adamantane units

The blue color stability and low current efficiency are two of main drawbacks of blue fluorene homo-polymer light emitting diodes. These are caused by interactions or aggregations between fluorene units and by ketone defects in fluorene homo-polymer. In this study, we introduced a bulky adamantane unit into polyfluorene chain, which has a big volume, to reduce interactions between fluorene chain units and to increase blue color stability and current efficiency of the devices. Fluorene-adamantane copolymers were synthesized by Suzuki coupling reaction. The electroluminescence (EL) spectra of the fluorene–adamantane copolymer light emitting devices had clear peaks and the spectra did not change with increasing driving voltages, whereas the EL spectra of poly(9,9-di-n-octyfluorene) (PF8 homo-polymer) light emitting device had a broad peak and the spectrum became broader when the driving voltage was increased. The fluorene–adamantane copolymer blue light emitting devices had more blue emission, higher efficiency, external quantum efficiency and luminance than those of PF8 homo-polymer light emitting device. These were attributed to an effective reduction of interactions between the fluorene units by the bulky adamantane units.