Light Controls Polymorphism in Thin Films of Sexithiophene

We investigate the influence of light on the growth process and resulting phase coexistence of the organic semiconductor α-sexithiophene (6T). We demonstrate that 6T thin films deposited on potassium chloride (KCl) in dark environments exhibit a bimodal growth, with phase coexistence of both low-temperature (LT) and high-temperature (HT) polymorphs. In contrast, films grown under illumination with 532 nm light at 1.5 W/cm² exhibit an increased purity of the LT phase, while the HT phase growth is slowed down by about a factor of 4. To understand the mechanism behind this optical control, we use in situ X-ray diffraction, atomic force microscopy, optical absorption measurements, as well as first-principles calculations for the optical absorption spectra of the HT and LT phase. We deduce that the phase purification is due to optical heating of the molecular film and lower cohesive energy of the HT phase compared to the LT phase, so that nucleation and growth of the HT phase are significantly reduced by light. On the basis of these findings, we suggest using light as a control parameter in organic molecular beam deposition to grow thin films of enhanced phase purity.