Growth and ellipsometric studies on C60 thin films for solar cell applications

C 60 have been used as an important active layer in organic solar cells and require elaborate understanding of its thin film properties. In this work, we present detailed morphological and ellipsometric studies on C60 thin films for understanding its growth and optical characteristics. At the initial stages of growth, C60 was found to form nanocrystalline granular domains in three-dimensional growth mode that coalesce to form large islandlike structures. As the film grows, comparatively smooth surfaces are obtained by uniform distribution of the nanocrystalline domains. Ellipsometric studies required the use of a bilayer structural model. The applicability of crucial model dielectric functions including Classical-Oscillator model, Forouhi–Bloomer model, and Tauc–Lorentz model for extraction of optical constants [Ñ(E)=n(E)+ik(E)] was investigated. The optical constants extracted from the three models are reported. Tauc–Lorentz model was found to be suitable model dielectric function as compared to the other models. Parameters including transition energy, amplitude, and broadening corresponding to five optical transitions, as well as the optical band gap of C60, are extracted. The optical constants are further used to model growth of the C60 film by studying evolution of the film and the roughness layers in the ellipsometric bilayer model.