Influence of the dopant concentration on the morphology of hole-transporting alignment layers based on a polyimide matrix

Investigations on hole-transporting alignment layers (HTALs) consisting of a polyimide matrix doped with hole-transporting materials (HTMs) at different concentrations by means of low-voltage scanning electron microscopy and atomic force microscopy are reported. These layers were recently used as HTALs for liquid crystalline polyfluorenes in polarized light-emitting diodes. For HTM concentrations below 15 wt%, phase-separation was found to be not significant, and the layer characteristics were dominated by the stiff polyimide matrix. These layers aligned polyfluorene very well, resulting in polarization ratios in electroluminescence of more than 20. On the contrary, the morphology was substantially altered at higher dopant concentrations. Moreover, microgrooves became visible after rubbing, indicating that the degree of imidization of the polyimide matrix was reduced. As a result, increasing the concentration of the HTMs above a certain level resulted in a dramatic decrease of the aligning ability of the HTAL.