Synthesis and characterization of indeno[1,2-6] fluorene-based white light-emitting copolymer

An indenofluorene-based copolymer containing blue-, green-, and red light-emitting moieties was synthesized by Suzuki polymerization and examined for application in white organic light-emitting diodes (WOLEDs). Tetraoctylindenofluor-ene (IF), 2,1,3-benzothiadiazole (BT), and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (DBT) derivatives were used as the blue-, green-, and red-light emitting structures, respectively. The number-average molecular weight of the polymer was determined to be 25,900 g/mol with a polydispersity index of 2.02. The polymer was thermally stable (T _d = ∼398 °C) and quite soluble in common organic solvents, forming an optical-quality film by spin casting. The EL characteristics were fine-tuned from the single copolymer through incomplete fluorescence energy transfer by adjusting" the composition of the red/green/blue units in the copolymer. The EL device using the indenofluorene-based copolymer containing 0.01 mol % BT and 0.02 mol % DBT units (PIF-BT01-DBT02) showed a maximum brightness of 4088 cd/m² at 8 V and a maximum current efficiency of 0.36 cd/A with Commission Internationale de L'Eclair-age (CIE) coordinates of (0.34, 0.32). The EL emission of PIF-BT01-DBT02 was stable with respect to changes in voltage. The color emitted was dependent on the thickness of the active polymer layer; layer (∼60 nm) too thin was unsuitable for realizing WOLED via energy transfer.