Synthesis and Characterization of Carbazole-Fluorene-Silole Copolymers as Efficient Green Light Emitting Diodes

A novel series of soluble polyfluorene-carbazole-silole-containing polymers (PF-Cz-S) were prepared through the Suzuki coupling reaction; and, the chemical structures were characterized by elemental analysis and NMR. Optoelectronic properties including UV absorption, electrochemistry, photoluminescence (PL), and electroluminescence (EL) of the copolymers were investigated. The copolymers exhibit main absorption peaks at 359 nm for solutions and 354 nm for films with combined contributions from the carbazole and fluorene units. Compared with the solution PL, complete PL excitation energy transfer from the wide band gap segment to a narrow band gap silole unit was achieved by film PL even at a lower silole content. The optical band gaps for PF-Cz-S are between 2.68 and 2.78 eV and the HOMO levels are -5.65 eV, which are contributed by 3,6-carbazole and fluorene blocks. Light-emitting devices fabricated from these polymers display a maximum external quantum efficiency of 0.44 % with an indium-tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) (PEDOT)/PF-Cz-S/CsF/Al configuration whereas an ITO/PEDOT/PCz-F-S/1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI)/CsF/Al with TPBI hole-blocking layer device configuration boosts the efficiency to 2.43 %. The TPBI hole-blocking layer largely improved the efficiency of exciton formation from which the highly efficient green EL from the silole units could be displayed.