Common high-performance semiconducting polymers are not amorphous but semi-para-crystalline

TL;DR

This study reveals that common high-performance semiconducting polymers are semi-para-crystalline rather than amorphous, with their charge transport properties depending more on crystal connectivity than on overall order.

Contribution

It introduces the concept of semi-para-crystallinity and a method to measure para-crystallinity in semiconducting polymers, resolving previous characterization contradictions.

Findings

Long-range charge transport depends on crystal interconnection.

Degree of para-crystallinity correlates with device performance.

Microstructure analysis links morphology to electronic properties.

Abstract

Precise determination of the solid-state microstructure of semiconducting polymers is of paramount importance for the further development of these materials in various organic electronic technologies. Yet, prior characterization of the ordering of semiconducting polymers often resulted in conundrums in which X-ray scattering and microscopy yielded seemingly contradicting results. Here, based on fast scanning calorimetry, we introduce for the first time the concept of the semi-para-crystallinity and measurement of the degree of para-crystallinity (ordered volume/mass fraction) in a set of materials that previously eluded understanding. In combination with lattice distortion determination within para-crystals (g-parameter from X-ray scattering) and nanomorphology, the complete solid-state microstructure is correlated with device properties. Our data show that the long-range charge carrier transport in these materials is more sensitive to the interconnection of para-crystal units than to the amount of structural order itself.