Novel Electroluminescent Conjugated Polyelectrolytes Based on Polyfluorene

Alternating copolymers poly(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene) (P1) and poly(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-1,4-phenylene (P3) were synthesized by the palladium-catalyzed Suzuki coupling reaction and their quaternized ammonium polyelectrolyte derivatives (P2, P4) were obtained through a postpolymerization treatment on the terminal amino groups. The resulting conjugated polyelectrolytes (P2, P4) are soluble in polar solvents such as methanol, DMF, and DMSO while P4 is a water-soluble blue-emitting conjugated polyelectrolyte. The electrochemical and photophysics properties of the resulting copolymers were fully investigated. Both the neutral amino-functionalized polyfluorenes (P1, P3) and the quaternized ammonium salt functional conjugated polyelectrolytes derivatives (P2, P4) were used as the emitting layers in device fabrication. All these polymers show even higher external quantum efficiencies (QE) with the high work-function metal cathode such as Al than with the low work-function (Ba) cathode. The maximal external quantum efficiencies of the diodes are respectively 0.38%, 0.16%, 0.07%, and 0.09% for P1, P2, P3, and P4 with an Al cathode. A possible mechanism of self-assembly of dipole alignment of polar polymer in the polymer/cathode interface was proposed. We have shown that such polymers can be used as an electron-injection layer, which can significantly enhance device performance of light-emitting polymers with high work-function metals such as Al. ITO/PEDT/MEHPPV/P1/Al devices show an external quantum efficiency greater than 2%, as high as that by using a Ba/Al cathode.