Defect Tolerance of π-Conjugated Polymer Crystal Lattices and Their Relevance to Optoelectronic Applications

This work seeks to understand the role of structural defects in the polymer chain on the crystallization and crystal lattice of pi-conjugated polymers, which is crucial for being able to predict morphology and performance of pi-conjugated polymer active layers in optoelectronic devices. Such a predictive understanding of device performance has been difficult to establish; however, in this work self-assembled nanowires of poly(3-hexylthiophene) (P3HT) are used to reduce analytical contributions from amorphous domains, allowing a more direct path to probe the effect of regio-defects and chain end-groups on the crystal lattice. By use of P3HT synthesized to have precisely varied defect concentrations, it was demonstrated that these defects can be incorporated into the crystal lattice, particularly regio-defects. However, this incorporation comes with a decrease in electronic properties as a result of diminished short-range order and lattice distortions. Bulky end-groups can be incorporated into the lattice, although there is a preference for their exclusion. However, the use of pi-pi interacting end-groups, such as toluene, is shown to mitigate disruption to the crystal lattice that results from the end group incorporation. In fact, pi-pi interacting end-groups seem to promote long-range order and crystal growth. Additionally, it was found that tuning the molecular weight of the polymer to an integer multiple of the observed width of the crystal lamellae, l(c), can increase the enthalpy of fusion, Delta H-f, for the crystal by as much as 20% by facilitating the exclusion of end-groups from the crystal lattice. These results demonstrate that pi-conjugated polymer crystal lattices have a high tolerance for disruptions to short-range order so that long-range order can be preserved. In addition, this study underscores the need to consider structural defects in polymer chains and their consequences on the crystal lattice during the design and implementation of pi-conjugated polymers.