Anisotropy of the optical and electrical properties of highly-oriented polyfluorenes

Polyfluorenes (PFs) are conjugated polymers with exciting optical and electronic properties and potential applications in optoelectronics. Due to the liquid-crystalline character of most polyfluorene homopolymers, thin layers with a large degree of in-plane alignment can be fabricated with these polymers. Upon appropriate substitution of the polyfluorene backbone, layers with an optical anisotropy in absorption and polarization ratios in emission of more than twenty have recently been fabricated. When included in an organic light-emitting diode structure, highly-polarized blue electroluminescence with an integrated polarization ratio of up to 22 at a brightness of 800 cd/A was realized. These devices represent potential candidates for the illumination of LC displays. Investigations on the anisotropy of the charge carrier mobility revealed that the mobility is highly anisotropic, with only a very small mobility perpendicular to the polymer chains. This is in agreement with expectations based on the results of electron-microscopy studies, which established a structure in which the charge-conducting PF backbone is surrounded by a coaxial, insulating mantle of hydrocarbon side chains. Polyfluorene chains can thus be considered as semiconducting wires, which contain the insulation within their chemical structure.