Synthesis, photophysics, and electroluminescence of high-efficiency saturated red light-emitting polyfluorene-based polyelectrolytes and their neutral precursors

High-efficiency saturated red light-emitting polyelectrolytes and their neutral precursors: aminoalkyl-substituted polyfluorenes (PFN) with different 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DBT) contents were synthesized by Suzuki coupling reaction. Their quaternized ammonium polyelectrolyte derivatives were obtained through a post-polycondensation treatment on the terminal amino groups. The resulting copolymers are soluble in polar solvents. Narrow band-gap (NBG) DBT units incorporated into the polymer backbone act as exciton traps. As a result, all the polymers emit red light specific to the DBT segments under UV excitation in the solid state. It was found out that the PFN–DBT copolymers show good device performance when high work-function metals, such as Al, are used as the electron-injection cathode replacing conventional low work-function cathodes such as Ca and Ba. It has been shown that these polymers also can be used as electron injection layers (ETL) in double layer structures in polymer light-emitting diodes (PLEDs). Double layer devices, with PFN–DBT as an electron injection layer on top of conventional RGB light-emitting polymers, with high work-function metal cathodes emit light exclusively from the RGB polymers and show high external quantum efficiency.