Poly(3-hexylthiophene) (P3HT) is recognized as a conjugated polymer with a high optical excitation efficiency. Given this characteristic, preaggregation processes leveraging P3HT’s optical excitation have been extensively explored. In this study, we present a distinct preaggregation mechanism that depends on the wavelength of the irradiated light. The study involves irradiating the P3HT solution with various lights (red, green, blue, and UV) and subsequently analyzing the morphology of both the solution and the film through absorbance measurement, microscopy, and X-ray diffraction. The results indicate that irradiation with red and UV light enhances the charge transport properties of the P3HT films due to increased aggregation. Conversely, green and blue light irradiation results in diminished electrical performance and aggregation. Contrary to conventional expectations, our findings suggest that more than one mechanism may coexist in inducing preaggregation, depending on the wavelength. Specifically, UV light irradiation appears to increase the likelihood of excitation of tilted chains, thereby enhancing polymer linearity and promoting preaggregation. In contrast, red light irradiation predominantly induces excitation within the polymer crystals through π–π stacking interactions rather than along the polymer chains. This study provides that different excitation mechanisms are activated depending on the wavelength of the incident light and provides a systematic analysis of the distinct aggregation patterns observed.