Solution-processed conjugated-polymer thin films have been investigated to apply for printed, large-area, and lightweight electronic devices. The control of aggregate structure of conjugated polymer is important to enhance the performance of their devices, because aggregate structure affects optical, charge transport, and mechanical properties. Poly(3-hexylthiophene) (P3HT) has been used as a prototypical conjugated polymer to study processing-structure-property relationships. A P3HT film is composed of mixture of H- and J-aggregates of P3HT, where H- and J-aggregates have side-by-side and head-to-tail structures, respectively. We have reported the structure and charge transport property of P3HT and hybrid films with respect to orientation, deposition technique, side chain flexibility, and uniaxial alignment. Here, we show a one-pot method to prepare a P3HT film composed of high purity H-aggregate up to ≈99 % using a solution process of drop casting of P3HT solution onto low-wettability substrate at room temperature. The component ratio of H- and J-aggregates was estimated by analyzing photoluminescence spectra of P3HT films. It is found that the component ratio is changed by three orders of magnitude by controlling solvent evaporation process using different substrates. From the results of contact angle, vibrational Raman spectra, and grazing-incidence X-ray diffraction as a function of the component ratio, the selective formation of H-aggregate is attributed to the increase of interchain π-orbital overlapping and the decrease of intrachain backbone planarity of P3HT molecules. Such a pure H-aggregate film exhibited 6-fold hole density in comparison to a conventional film, according to the results of transient photoconductivity measurements. References:  D. Kajiya, S. Ozawa, T. Koganezawa, and K. Saitow, J. Phys. Chem. C 119, 7987 (2015).  D. Kajiya and K. Saitow, Nanoscale 7, 15780 (2015).  D. Kajiya, T. Koganezawa, and K. Saitow, J. Phys. Chem. C 120, 23351 (2016).  M. Imanishi, D. Kajiya, T. Koganezawa, and K. Saitow, Sci. Rep. 7, 5141 (2017).