Synthesis and Electroluminescence properties of Polyfluorene derivatives for light-emitting diodes

Abstract

New single-polymer electroluminescent system containing three individual emission species of polyfluorenes as a blue host, 2,1,3-benzothiadiazole unit as a green dopant and phenylene derivative unit as a red dopant on the main chain were synthesized and characterized. All polymerizations were carried out by palladium(0) catalyzed Suzuki coupling reaction with equivalent molar ratio of the diboronic ester monomer to the dibromo monomers under dry argon protection. The structure and properties of these polymers were characterized using UV-vis spectroscopy, GPC, Photoluminescence (PL), and Electroluminescence (EL) spectroscopy. The synthetic polymers, Polyfluorene [PF], Poly(fluorene-benzothiadiazole-pyridine) [PF-BT-PD], were soluble in common organic solvents and easily spin-coated onto the indium-tin oxide (ITO)-coated glass substrates. The optical band gaps of polymers of 2.97 eV and 2.55 eV corresponding to PF and PF-BT-PD, respectively were calculated from the onset of UV-vis spectra. Light-emitting diodes (LEDs) with ITO/PEDOT:PSS/polymer/LiF/Al configuration were fabricated, and the devices using copolymers showed red shift EL spectra relative to that of PF. The fabricated light-emitting devices showed good performance in terms of turn on voltage and electroluminescence. The new PF-BT-PD copolymer was successfully employed as emissive polymer layer in polymer light-emitting devices (PLEDs). The turns on voltages of copolymers were lower than that of PF. By changing the molar ratios of these monomer units, the electroluminescence spectra can be adjusted to white-light emission.