Temperature-induced aggregation evolution in poly(3-hexylthiophene) spun films: a UV-vis absorption and Franck-Condon analysis study.

Aggregates of conjugated polymers, which are larger assemblies than individual polymer chains, critically influence the structure-function relationships in polymer-based optoelectronic devices. In this work, we investigate the temperature-dependent optical response of poly(3-hexylthiophene-2,5-diyl) (P3HT) spun films using ex situ UV-vis absorption spectroscopy combined with Franck-Condon (FC) vibronic analysis. Films were subjected to step-wise thermal annealing from 25 °C to 300 °C in 10 °C increments and subsequently measured at room temperature, thereby capturing the kinetically trapped morphological states corresponding to each annealing condition. The absorption spectra show progressive broadening and reduced vibronic resolution with increasing annealing temperature. From the FC model, we derive four structural parameters i.e., aggregate fraction, exciton bandwidth, conjugation length, and site energetic disorder. The derived parameters indicate a distinct transition threshold near 200 °C, above which the aggregate fraction decreases from ∼43% to ∼33%, while the exciton bandwidth broadens from 64 to 135 meV. Concurrently, the 0-0 electronic transition shifts from 2.071 to 2.121 eV, suggesting a reduction in the effective conjugation length from ∼45 to ∼15 monomers. These trends are consistent with thermally enhanced molecular mobility and the progressive disruption of ordered interchain packing. Although the extracted parameters are model-dependent, they provide a consistent quantitative framework for interpreting the thermal response of P3HT aggregates and may help guide processing strategies for optimizing conjugated polymer thin films in electronic devices.