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