Efficient polymer white-light-emitting diodes

Polymer white-light-emitting diodes (WPLEDs) with bilayer structures were fabricated by the spin-coating method. The device structure used here is indium-tin-oxide/polyethylene-dioxythiophene (ITO/PEDOT)/blend1/blend2/Ba/Al, whose blend1 consists of poly(N-vinylcarbazole) (PVK), a copolymer of 9,9-diotylfluorene, and 4,7-di(3-hexyl-thien-2-yl)-2,1,3-benzothiadiazole (PFO-DHTBT), and blend2 is the blend of polyhedral oligomeric silsesquioxane-terminated poly (9,9-dioctylfluorene) (PFO-poss) and phenyl-substituted PPV derivative (P-PPV). The copolymer PFO-DHTBT emits red light. The blend2 gives off both green and blue emission bands in the electroluminescence (EL) spectrum. The relative intensity of the green and blue bands depends on the blend ratio. The EL spectrum of the device can be controlled by adjusting the weight ratios in the blend1 and blend2, respectively. At the weight ratios of 100:3 (PVK to PFO-DHTBT) in blend1 and 100:1 (PFO-poss to P-PPV) in blend2, and by keeping the thickness of the two blend films to 40 nm, the white fluorescent light emission with CIE coordinates of (0.33, 0.32) is achieved in a large voltage range, the maximal external quantum and luminance efficiencies of 3% (ph/eh) and 4.4 cd/A are obtained at 6.1 V, and the maximal luminance is 6300 cd/m(2) at 10 V. (c) 2005 American Institute of Physics.