Exploring the potential of ellipsometry for the characterisation of electronic, optical, morphologic and thermodynamic properties of polyfluorene thin films

Abstract

We have used spectroscopic ellipsometry to characterise thin films of poly(9,9-dioctylfluorene) (PFO) spin coated on different substrates. Analysis of the ellipsometry data suggests that a model based on critical points of zero order (i.e. excitons) provides an appropriate framework to describe the data. It is found to be very accurate and to result in low standard deviations and a low correlation among the fitting parameters compared to other models, such as the Harmonic Oscillator Approximation or the Model Dielectric Function. We have used this exciton model to study the dielectric function of PFO: uniaxial anisotropy was found with the optical axis perpendicular to the plane of the substrate. We demonstrate, however, that conventional variable angle spectroscopic ellipsometry (VASE) in reflection mode is not sensitive enough to accurately establish the film birefringence, and that other techniques, such as interference enhancement VASE need to be used. We also demonstrate the use of ellipsometry for measuring the glass and crystalline phase transition temperatures of PFO and report the dependence of the polymer refractive index on temperature: the large d n/d T of PFO could be used to add functionality to optoelectronic/photonic devices.