Thickness dependent absorption spectra in conjugated polymers: Morphology or interference?

The thickness dependence of the absorption spectrum of spin-coated films of poly[2-(2(')-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] has been studied using reflectivity and variable angle spectroscopic ellipsometry measurements. It is found that, for films with thicknesses in the range of 18-178 nm, a single set of optical constants is sufficient to simulate accurately all the experimental data used, including the absorption spectra, independently of the film thickness or the processing conditions. Thus, the observed changes in the absorption spectrum with thickness can be fully accounted for by reflectivity and interference effects alone without the need to invoke morphology differences between films.