Illumination of Conjugated Polymers Reduces Nucleation Probability and Slows Down Crystal Growth Rate

Using the set-up of an optical microscope, we have examined the influence of illumination with white light on the crystallization behavior in molten films of poly(3-(2,5-dioctylphenyl)thiophene) and poly(3-hexylthiophene). We observed a reduction in nucleation density and crystal growth rate induced by illumination. The amount of this reduction increased with the increase in light intensity. Melting samples previously crystallized under illumination and recrystallizing them in the dark showed full reversibility of the crystallization behavior, demonstrating that these changes in the crystallization behavior induced by illumination were not permanent. We tentatively suggest that absorption of photons by the polymer induced chain stiffening, possibly causing a reduction of polymer diffusivity, which, in turn, slows down crystal growth rate and reduces nucleation probability. We expect that many other conjugated polymers will show a similar impact of illumination on crystallization.