Tuning the randomization of lamellar orientation in poly(3-hexylthiophene) thin films with substrate nano-curvature

Abstract

We report the effect of substrates with uniform nanocurvature on the structure of deposited thin films of high molecular weight regio-regular poly(3-hexylthiophene) (P3HT). The curvature-induced effects on the in-plane and out-of-plane crystalline structure and the lamellae orientation were investigated using synchrotron Grazing Incidence X-ray Diffraction (GIXRD) and Atomic Force Microscopy (AFM).It was found that the substrate curvature influences both the lamellar persistence length and the 3D orientation. In particular, AFM results showed that the lamellar persistence length is reduced as the surface nano-curvature increases. GIXRD experiments showed a diffraction ring for the (100) peak for regio-regular P3HT thin films onto all the nano-curved surfaces with respect to the flat one, corresponding to an overall random lamellar orientation for nano-curved films. Semi-quantitative determinations of the degree of randomization show that it increases with the substrate nano-curvature. Finally, dark conductivity measurements of regio-regular P3HT thin films showed again an increase of about one order of magnitude with curvature, with a threshold value of 0.014 nm−1, closely following the same changing trend of the lamellar orientational and morphological parameters. This last effect, which indeed was not observed for amorphous regio-random P3HT thin films deposited onto substrates with the same nanocurvature, suggests the strong correlation of the dark conductivity increase to the peculiar curvature-induced lamellar features (i.e., reduced persistence length and increased orientation randomness) arrangement in regio-regular P3HT thin films. A phenomenological model is proposed to account for the observed structural and electrical effects of nano-curved substrates.