Synthesis of oxadiazole-based polymers containing a carbazole-vinylene or fluorene-vinylene group and their hole-injection/transport behavior in light-emitting diodes

摘要

A series of conjugated (poly{N-(2-ethylhexyl)-3,6-carbazole-vinylene-alt- [(2,5-bisphenyl)-1,3,4-oxadiazole]}) and nonconjugated (poly{N-(2-ethylhexyl)-3, 6-carbazole-vinylene-alt-[(2,5-bisphenol)-1,3,4-oxadiazole]}) and poly{9,9-dihexyl-2,7-fluorene-vinylene-alt-[(2,5-bisphenol)-1,3,4-oxadiazole]}) polymers containing oxadiazole and carbazole or fluorene moieties in the polymer backbone were synthesized with a multiple-step procedure. The properties of the polymers, including the photophysical and electrochemical characteristics, could be fine-tuned by adjustment of the components or structures in the polymer chains. The polymers were used to examine the hole-injection/transport behavior as hole-injection/hole-transport layers in double-layer indium tin oxide (ITO)/polymer/aluminum tris(8-hydroxyquinoline)/LiF/Al devices by the determination of their energy levels. The effects of the polymers in these devices on the charge-transport behavior were compared with a control device fabricated with poly(ethylenedioxythiophene) (PEDOT)-poly(styrene sulfonate) (PSS). Devices containing the synthesized polymers showed comparable adhesion to the ITO anode and good hole-injection/transport performance. In addition, they exhibited higher electroluminescence over an identical range of current densities than the control device. This was attributed to the prevention of radiative exciton quenching caused by the PEDOT-PSS interfaces and the improvement of electron/exciton blocking due to the higher electron affinity of the synthesized polymers.