Synthesis and characterization of white light-emitting polyfluorene-based copolymers containing new red iridium complex in the main chain

We report the synthesis of single chain white light-emitting polyfluorene (PF)-based copolymers containing a novel red iridium complex ((CVz-PhQ)2IrdbmBr), benzothiadiazole (BT), and 2,5-bisphenyl-1,3,4-oxadiazole (OXD) segments. The resulting copolymers were highly soluble in common organic solvents and could be easily spin-coated onto an indium-tin oxide (ITO)-coated glass substrate to obtain high quality optical thin films. The weight-average molecular weight (Mw) and polydispersity of the copolymers were (17.5–36.3) × 104 and 1.54–1.65, respectively. We fabricated white polymer light-emitting diodes (WPLEDs) in ITO/PEDOT:PSS/PVK/copolymer/CsF/Ca/Al, both with and without the poly(9-vinylcarbazole) (PVK) interlayer. By carefully controlling the concentrations of the low-energy-emitting species in the resulting copolymers, white light-emission with contributions from each of the three primary colors was achieved. By incorporating 2 mol% of an electron transporting material, OXD, as a comonomer into the copolymer backbone and inserting a PVK interlayer, the external quantum efficiency (EQE) and luminance efficiency (LE) of the WPLEDs were enhanced. The turn-on voltages of the copolymers were in the range of 5.5–7.5 V, and the maximum EQE and LE were 1.79% and 1.89 cd/A with CIE coordinates of (0.32, 0.34), which are close to the standard for white light emission.