Efficient Blue Light-Emitting Diodes Based on Conjugated Polymers with Fluorinated Silole Chemically Doped in Fluorene–Carbazole Main Chain

A series of random conjugated copolymers PFCFS1-20 with 2,5-diphenyl-3,4-bis(3-fluorophenyl)-silole (fluorinated TPS) chemically doped in 2,7-fluorene-2,7-carbazole main chain were successfully synthesized and characterized. Polymers PFCFS1-20 with fluorinated TPS contents from 1 to 20 % possess high weight-average molecular weights between 63.5 and 114.1 kg mol−1. The absorption spectra of THF solutions and films of PFCFS1-20 are very similar, showing peaks around 385 nm and optical band gaps around 2.87 eV. Almost no red-shifts of the absorption peaks from a solution to a solid state film for the four copolymers indicate that their interchain interactions are extremely weak. PFCFS1-20 solutions show blue emissions with very limited intrachain excitation energy transfers. The films of PFCFS1-20 also display blue emissions with main peaks at 449 nm for PFCFS1 and 475 nm for PFCFS20, suggesting that the fluorinated TPS may comprise more twisted phenyl peripherals. Two types of electroluminescence (EL) devices with configurations of ITO/PEDOT:PSS/PFCFS/CsF/Al and ITO/PEDOT:PSS/PFCFS/TPBI/CsF/Al were investigated. It was found that PFCFS1-20 in the two types of EL devices could show blue to sky-blue emissions. Among PFCFS1-20, for EL devices without TPBI layer, PFCFS5 possesses the best device performances with maximum luminous efficiency (LEmax) of 1.46 cd A−1 and maximum external quantum efficiency (EQEmax) of 0.93 %. The insertion of TPBI as the electron transport layer remarkably improves the EL performances. Also PFCFS5 displays the best LEmax of 4.59 cd A−1 and EQEmax of 3.21 %. The latter one is among the best efficiency so far reported for silole-containing polymers. Our results suggest that the fluorinated TPS is an efficient chromophore for constructions of blue-emissive conjugated polymers.