While molecular ordering via crystallization is responsible for many of the impressive optoelectronic properties of thin-film semiconducting polymer devices, crystalline morphology and its crucial influence on performance remains poorly controlled and is usually studied as a passive result of the conditions imposed by film deposition parameters. A method for systematic control over crystalline morphology in conjugated polymer thin films by very precise control of nucleation density and crystal growth conditions is presented. A precast poly(3-hexylthiophene) film is first swollen into a solution-like state in well-defined 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 film or by a self-seeding approach using undissolved crystalline aggregates that remain in the swollen film. Nucleation densities ranging successively over many orders of magnitude are achieved, extending into the regime of spherulitic domains 10 to 100 mu m in diameter, a length scale highly relevant for typical probes of macroscopic charge transport such as field-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.