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.