Systematic Control of Nucleation Density in Poly(3-hexylthiophene) Thin Films

Abstract

While molecular ordering via crystallization is responsible for many of the impressive optoelectronic properties of thin-fi lm semiconducting polymer devices, crystalline morphology and its crucial infl uence on performance remains poorly controlled and is usually studied as a passive result of the conditions imposed by fi lm deposition parameters. A method for systematic control over crystalline morphology in conjugated polymer thin fi lms by very precise control of nucleation density and crystal growth conditions is presented. A precast poly(3-hexylthiophene) fi lm is fi rst swollen into a solution-like state in well-defi ned vapor pressures of a good solvent, while the physical state of the polymer chains is monitored using in situ UV–vis spectroscopy and ellipsometry. Nucleation density is selected by a controlled deswelling of the fi lm or by a self-seeding approach using undissolved crystalline aggregates that remain in the swollen fi lm. Nucleation densities ranging successively over many orders of magnitude are achieved, extending into the regime of spherulitic domains 10 to 100 m in diameter, a length scale highly relevant for typical probes of macroscopic charge transport such as fi eld-effect transistors. This method is presented as a tool for future systematic study of the structure-function relation in semicrystalline semiconducting polymers in a broad range of applications.