Blue light-emitting diodes from mesogen-jacketed polymers containing oxadiazole units

A series of novel mesogen-jacketed polymers, poly{2,5-bis[(5-tert-butylphenyl)-1,3,4-oxadiazole]styrene} (P-Ct) and poly{2,5-bis[(5-alkoxyl-phenyl)-1,3,4-oxadiazole]styrene} (P-OCn, n is the number of carbons in the alkoxyl groups, n = 6, 8, 10, 12, 14), were incorporated into polymer light-emitting device as the light-emitting layers. All of the polymers had excellent solubility in common organic solvents. The UV and PL spectra for P-Ct were 312 and 400 nm while for P-OCn they were around 328 and 420 nm in the film. There was no red shift in the PL spectrum of P-Ct after heat treatment which maybe related to the jacketed structure. All of the polymers showed high PL quantum yields in THF solution. From the cyclic voltammetry measurement at room temperature, HOMO levels of the polymers were varied from -5.66 to -6.12 eV, LUMO levels were varied from -2.45 to -2.81 eV. Furthermore, the influence of the alkoxyl lengths in four different device configurations was compared. For the device configuration c: ITO/PVK/P-OC8/Ca/Ag, the maximum luminance and maximum external quantum efficiency were obtained for P-OC8 which maybe related to the easier electron injection and transport. Efficiency of P-OC8 was not improved with device d which proved the electron transporting characteristics of P-OC8.