EL Properties of Polyfluorene Copolymers Containing Hole-Transporting Carbazole Group
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EL Properties of Polyfluorene Copolymers Containing Hole-Transporting Carbazole Group Do-Hoon Hwang1*, Jong-Don Lee1, Moo-Jin Park1 , Ji-Hoon Lee2 and Chang-Hee Lee3 1
Department of Applied Chemistry, Kumoh National Institute of Technology, Kumi 730-701, Korea ([email protected]) 2 Electronic Materials Lab. Samsung Advanced Institute of Technology, P.O. Box 111 Suwon 440-600, Korea 3 Department of Physics, Inha University, Incheon 402-751, Korea
ABSTRACT
An alkoxyphenyl substitutued new polyfluorene derivative, poly(9,9-bis(4’-n-octyloxyphenyl) fluorene) (PBOPF) was synthesized through Ni(0) mediated polymerization. Blue light-emitting PBOPF shows peak PL at 426 nm. No significant excimer emission was observed even after annealing the polymer film at 100 oC for 2h. A series of random copolymers of 2,7-dibromo-9,9bis(4’-n-octyloxyphenyl)) fluorene (BOPF) and 2,7-dibromo-N-(2’-ethylhexyl)carbazole (EHC) were also synthesized through Ni(0) mediated polymerization. Carbazole comonomer was introduced to improve hole-transporting properties of PBOPF. The synthesized poly(BOPF-coEHC)s showed similar UV-visible absorption and PL emission to PBOPF. EL devices were fabricated in an ITO/PEDOT/polymer/Ca/Al configuration. The EL devices using copolymers showed improved device performance than the device using PBOPF homopolymer due to a more balanced charge transport.
INTRODICTION A number of polyfluorene (PF) polymers and their derivatives have been studied since poly(9,9-di-n-hexylfluorene) (PDHF) was first reported as a blue-light emitting polymer [1]. The interest in these polymers arose because they show highly efficient photoluminescence (PL) and electroluminescence (EL), excellent thermal and oxdative stability, and good solubility in common organic solvents [2-4]. A major drawback of PFs, however, is that they show excimer and/or aggregate formation upon thermal annealing or the passage of current, leading to red-
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shifted emission. Continuing efforts have been made to suppress excimer formation in PFs, including copolymerization with anthracene [5], end-capping with a sterically hindered group [6], and introducing sterically hindered substituents at the 9-position of fluorene [7,8]. However, appropriate blue emitting materials that meet the requirements for display applications have yet to be obtained, and further improvements are necessary. Recently we synthesized a new PF derivative, poly(9,9-bis(4’-n-octyloxyphenyl))fluorene (PBOPF) [9]. Alkoxyphenyl groups were introduced on 9-position of fluorene to suppress long wavelength excimer emission. A series of copolymers of 2,7-dibromo-9,9-bis(4’-noctyloxyphenyl)) fluorene (BOPF) and 2,7-dibromo-N-(2’-ethylhexyl)carbazole (EHC) were also synthesized to improve the hole-transporting properties of PBOP
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