New fluorene derivatives for blue electroluminescent devices: Influence of substituents on thermal properties, photoluminescence, and electroluminescence

A new series of fluorene derivatives with different substituents at the C-2,7- and C-9 positions has been synthesized and characterized with respect to their luminescence and thermal properties. It was found that substitutions at the C-2,7 positions significantly affected the thermal properties and optical properties, whereas substitutions at the C-9 position mainly affected the thermal properties of the fluorene derivatives. Organic light-emitting diodes (OLEDs) with a structure of indium tin oxide (ITO)/copper phthalocyanine (CuPc) (15 nm)/4,4' bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) (50 nm)/fluorene derivative (30 nm)/tris(8-quinolinolato)aluminum (Alq(3)) (50 nm)/Mg/Ag (200 nm) were fabricated. OLEDs based on the naphthyl-substituted fluorenes gave saturated blue emission centered at 452 nm, while those based on anthryl-substituted fluorenes showed a blue-green emission with a wider spectrum. OLEDs based on 2,7-dipyrenyl-9'9-dimethylfluorene (DPF) exhibited blue emission with a maximum efficiency of 4.8 cd/A and CIE coordinates of x = 0.17 and y = 0.24. A similar DPF-based device with an additional hole block layer exhibited better blue emission with a maximum efficiency of 5.2 cd/A and CIE coordinates of x = 0.15 and y = 0.21, which are among the best values ever reported for blue OLEDs without a dopant based on fluorescence molecules. The results show that fluorenes with suitable substitutents can act as blue-emitting materials with a high thermal stability and high electroluminescence efficiency.