On the Growth, Structure and Dynamics of P3EHT Crystals
We employ X-Ray diffraction, NMR and UV-vis spectroscopy techniques to shed light on the structure, molecular mobility and crystallization of a prototypical semiconducting polymer poly(3-(2’ethylhexyl)thiophene) (P3EHT). Grazing-incidence X-ray diffraction (GIXD) and high-field 1H-1H double-quantum NMR reveal a tilt of the thiophene rings along the main chains, while high resolution 13C solid-state NMR methods show the presence of highly mobile side-chains. Taken together, the data reveal a high amount of free volume between adjacent chains in the crystalline phase, most likely owing to the additional steric hindrance from the branched ethyl group of the hexyl side chains. Additionally, the aforementioned methods allow us to confirm that crystallization in P3EHT occurs in a one-dimensional fashion along the polymer backbone. An extended model based on the Avrami principles is used to fit the experimental results to extract key parameters, such as nucleation rate and density, that govern these crystallization processes. The results show significantly reduced rates of crystallization in thin P3EHT thin films due to polymer chain confinement effects.