Optical and morphological investigations of non-homogeneity in polyfluorene blends

We report investigations of organic light-emitting diodes with an active layer consisting of poly(9,9-dioctylfluorene), PFO, blended with a green emitting derivative (F8BT). We used indium tin oxide (ITO) anodes with and without a hole transporting layer, consisting of doped poly(3,4-ethylene dioxythiophene). We also studied the incorporation of an oxadiazole based hole-blocking/electron transporting layer, HBL, namely 2-(4-biphenylyl)-5-butylphenyl-1,3,4-oxadiazole, PBD), between the emissive layer and the Ca/Al cathodes. The presence of this layer reduces the range of radiative recombination in a region closer to the HBL interface, and therefore, allows investigation of depth-dependent structural non-uniformities of the emissive layer. We find marginal alterations of the electroluminescence spectra depending on the weight concentration of the F8BT in the range 5-25%. Efficient energy transfer from the host (PFO) to the guest (F8BT) ensures that the emission spectra are dominated by the green component. However, we find that blue emission from the PFO host is much more intense for the structures incorporating the HBL than for those without it. We propose that phase separation phenomena of the blend induced by the spin-coating of the PBD-based layer, are responsible for the higher weight of the blue component. (C) 2001 Elsevier Science B.V. All rights reserved.