Pure Deep Blue Light-Emitting Diodes from Alternating Fluorene/Carbazole Copolymers by Using Suitable Hole-Blocking Materials

Abstract

The influences of the carbazole content on the photophysical, electrochemical, and electroluminescent properties of alternating fluorene/carbazole copolymers PFnCz (n = 1, 2, 3) with well-defined chemical structures have been systematically investigated. The incorporation of carbazole units into the polyfluorene (PF) backbone resulted in a blue shift of both the absorption and photoluminescence (PL) emission peaks, improved PL thermal stability, raised HOMO energy levels, and thus facilitated hole injection into the copolymers. Pure deep blue electroluminescence (EL) with narrow fwhms (full width at the half-maximum) (39−52 nm) and negligible low-energy emission bands was successfully achieved from the PFnCz copolymers by using 1,3,5-tris(4‘-fluorobiphenyl-4-yl)benzene (F-TBB) as a hole-blocking layer and Alq3 as an electron injection/transporting layer. This device configuration stabilized the blue emission from the PF derivatives. An efficiency of 0.72 cd/A at a luminance of 100 cd/m2 was obtained even with aluminum metal as the cathode.