Synthesis, Characterization, and Electroluminescence of Polyfluorene Copolymers with Phenothiazine Derivative; Their Applications to High-Efficiency Red and White PLEDs

We have synthesized new light-emitting copolymers by introducing the phenothiazine derivative, 4,7-bis(6-bromo-3-thiophenyl-10-n-butylphenothiazine)-2,1,3-benzothiadiazole (BPTR), into the main chain of a homopolymer based on 2,7-dibromo-9,9-bis(4-octyloxy-phenyl) fluorene (BOPF). The polymers, designated as poly{9,9-bis-(4-octyloxyphenyl)-fluorene-2,7-diyl-co-4,7-bis(3-thiophenyl-10-n-butylphenothiazine-6-yl)-2,1,3-benzothiadiazole} (PFPTRs) were synthesized through palladium-catalyzed Suzuki coupling reactions and were found to be thermally stable and readily soluble in common organic solvents. The photoluminescence (PL) emission spectra of the copolymers were highly red-shifted as the fraction of BPTR units in the copolymer was increased. Light-emitting devices were fabricated using the ITO/PEDOT:PSS/polymer/Balq/LiF/Al configuration with the PFPTR polymers as the emitting layer. The electroluminescence (EL) device based on PFPTR 1 showed nearly pure red emission [CIE coordinate values (x, y), (0.62, 0.36)] that was very close to the standard red (0.66, 0.34) demanded by the National Television System Committee. Furthermore, its maximum external quantum efficiency and maximum current efficiency were 2.3% and 1.70 ed/A at 10 V, respectively. Our device results for PFPTR 1 are among the highest values yet reported for saturated red polymer emitters. Moreover, the EL emissions of PFPTR 01 and PFPTR 025 afforded white electroluminescence with CIE coordinates of (0.27, 0.40) and (0.41, 0.37) respectively, which appeared to be near-white to the naked eye.