High optical anisotropy in thin films of polyfluorene and its affect on the outcoupling of light in typical polymer light emitting diode structures

The anisotropic optical response of thin films of the conjugated polymer PF2/6am5 (polyfluorene) is investigated using spectroscopic ellipsometry. The influence of the highly dispersive and anisotropic character of refractive index, found in this work for the ${\mathrm{\pi –\pi }}^{*}$ transition region, on the outcoupling efficiency of light emitting devices is calculated by applying a simple ray optics model. Polymer light emitting diodes are constructed using PF2/6am4 and PtOEP doped PF2/6am4 (0.5%, 2%, and 4%) as an active layer and their external quantum efficiency (EQE) measured. Electroluminescence spectra of the doped devices are shifted to red wavelengths. The effect of transferring the emission away from high dispersive wavelengths on the outcoupling efficiency is determined and internal quantum efficiency (IQE) values for all devises are calculated. It is shown, that the increase in IQE by doping with PtOEP of up to 80% is completely accountable to the higher photoluminescence quantum yield of the dye. Accounting properly for the anisotropic optic properties of thin spun cast polymer films shows that the simple ${\text{2n}}^2$ relationship between IQE and EQE yields a large overestimate of the former.