In polymeric semi-conducting materials, both molecular and crystalline orientations determine optical, electronic and opto-electronic properties in thin films since these properties are by essence highly anisotropic. Therefore, it is of interest to control the contact plane of crystalline domains in the active layers as it determines, for instance, the direction of facile charge transport in the cases of OPV and OFET. Mechanical rubbing of polymer films has been widely used in the liquid crystal display industry to prepare oriented alignment layers of polyimides. We successfully applied this fast orientation method to a large palette of thiophene and fluorene based pi-conjugated systems i.e. semi-conducting homopolymers, donor-acceptor alternated copolymers and block co-oligomers.[1] Highly oriented films are obtained without the use of alignment layer. Both, the temperature of the films during rubbing and the molecular weight distribution of the polymer determine the level of orientation.[2] In this work, we demonstrate the precise control of lamellar periodicity by varying the rubbing and annealing temperatures of poly(3-alkythiophene) (P3HT and P3BT) thin films. The nanostructure, observed by HR-TEM, can be clearly correlated to the optical properties of the rubbed thin films. Higher processing temperatures affect the lamellar structure of the polymers causing a longer exciton delocalization and reducing therefore the excitonic coupling constant in a P3AT crystal. The effect of lamellar thickness on charge transport properties is also discussed in this presentation.
[1] Orienting semi-conducting pi-conjugated polymers, M. Brinkmann, L. Hartmann, L. Biniek, K. Tremel and N. Kayunkid, Macromolecular Rapid. Comm. 2014, 35, 9. [2] High-Temperature Rubbing: A Versatile Method to Align π-Conjugated Polymers without Alignment Substrate, L. Biniek, S. Pouget, D. Djurado, E. Gonthier, K. Tremel, N. Kayunkid, E. Zaborova, N. Crespo-Monteiro,
O. Boyron, N. Leclerc, S. Ludwigs, and M. Brinkmann, Macromolecules 2014, 47, 3871.