Phase Separation Dynamics in an Annealed Polymer:Fullerene Blend Analyzed using the Critical Point Model of the Permittivity

In this work we show for the first time how spectroscopic ellipsometry can be used to analyze the dynamics of morphological changes that take place during the annealing of a film of two bench-mark materials used for organic solar cells that are the most reported on in the literature, poly(3-hexyl-thiophene 2.5-diyl) (P3HT) and [6,6] phenyl C61 Butyric acid methyl ester (PCBM). Morphological changes within the homopolymer and the blend during annealing process manifest themselves in the optical spectra of the film and are qualitatively attributed to the structural changes. In contrast, we demonstrate that parametric description using the critical point model gives additional information that was previously only accessible by x-ray diffraction based techniques. We analyzed the dimensionality of the excitons as extracted from the model and found a one-to-one correspondence with observed peaks in the x-ray spectra. We show that a preferable morphology for the organic blend, in the sense of the highest dimension of excitons, was found after 20 minutes of annealing treatment at 140°C. Our results indicate to the immense potential of using a macroscopic measurement technique, ellipsometry, to analyze the dynamic of microscopic changes in the active layer morphology.