Harald Ade1

1, Department of Physics and Organic and Carbon Electronics Lab (ORaCEL), North Carolina State University, Raleigh, North Carolina, United States

Although it is well known that intra- and intermolecular ordering greatly impact the electronic and optoelectronic properties of semiconducting polymers, the interrelationship between ordering of alkyl sidechains and conjugated backbones has not yet been fully detailed. A recent discovery clearly demonstrated that the tendency of the side-chains and the backbone to order is not synergistic [1], rather a competition was observed in six representative semiconducting polymers. The ordering of the respective polymer components is monitored by NEXAS, UV-vis spectroscopy, as well as diffraction and exhibits distinct signatures. Overall, a vertically layered multilayer nanostructure with ordered sidechain layers alternating with disordered backbone layers is observed as the equilibrium structure at room temperature, with sidechain ordering within the alkane layer exhibiting a coherence lengths >70 nm. In contrast, the side-chain are typically very amorphous, even in materials that show lamellar diffraction peaks. The long-range sidechain ordering has been exploited as a transient state to fabricate PBnDT-FTAZ films with an atypical edge-on texture and 2.5x improved OFET mobility. Overall, these observations imply that a new way towards molecular design needs to be pursued, which could possibly produce significantly improved ordering and thereby improve electronic and optoelectronic properties. We will review past and ongoing research towards perfect polymer crystals and monolayers [2] and propose the use of a design perspective that views the backbone and side-chain similar to classic semiconducting heterostructure bulk epitaxy. In other words, we need to design materials as completely ordered crystals and not partially ordered sublattices.

[1] J. H. Carpenter, M. Ghasemi, E. Gann, I. Angunawela, S. J. Stuard, J. J. Rech, E. Ritchie, B. T. O’Connor, J. Atkin, W. You, D. M. DeLongchamp, H. Ade, Advanced Functional Materials, https://doi.org/10.1002/adfm.201806977 (2019)
[2] M. Li, D. K. Mangalore, J. Zhao, J. H Carpenter, H. Yan, H. Ade, H, Yan, K. Müllen, P. WM Blom, Wojciech Pisula, Dago M Leeuw, Kamal Asadi, Nature communications 9, 451 (2018)