(337cy) Twisted Crystalline Morphology and Its Application in Functional Materials

About one third of molecular crystals can spontaneously grow from the melt as banded spherulites with helicoidally twisted fibers.[i] The fraction is higher than one half for binary charge transfer complexes (CTCs)[ii]. Our goal is to understand how this common growth feature affects electrical properties of organic semiconductors. Intensity of crystal twisting depends on crystallization temperature and is characterized by a twisting pitch, a length needed for a fiber to rotate by 180°. Measurements performed on organic field-effect transistors for films of tetracyanoethylene-phenanthrene and tetracyanoethylene-pyrene CTCs demonstrated three times higher charge mobilities for twisted crystals compared to the straight ones. Likewise, semiconductors tetrathiafulvalene (TTF)[iii], 2,5-bis(3-dodecyl-2-thienyl)-thiazolo[5,4-d]thiazole (BDT) [iv] (Figure a-d) and 2,5-didodecyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-C12) (Figure e-h) showed around ten times higher mobilities for twisted crystals with the smallest pitches compared to those with the largest pitches. DFT calculations suggested a negligible role of lattice distortions for the enhanced mobility in twisted crystals. A more plausible explanation relied on the crucial role of fiber organization in the film. A greater number of tightly packed twisted fibers separated by numerous smaller gaps allowed better charge transport over the film surface compared to a smaller number of larger crystallites separated by a fewer number of larger gaps. Simulations of the BDT films conductance based on the actual surface topographies supported this idea (Figure b).

The electronic properties may depend not only the twisting pitch itself but also on the spatial organization of the fibers into coherent bands (Figure e, f). Seven systems analyzed here show that the boundary of banded spherulites is less coherent for the film with larger pitches. Using a statistical approach we demonstrated that as branching intensity decreased, the crystal fiber thickness and its variance increased leading to larger pitch and its variance. This resulted in a less coherent band boundary (Figure g). This approached was generalized for crystallites where branching varied with orientation of the fiber by using numerical simulations (Figure h). The effect of fiber coherence in organic semiconductors films with twisted fibers on optical and electrical properties is under investigation.

Acknowledge

Thanks for The Margaret and Herman Sokol Fellowship for financial support.

Reference

[[i]] A. G. Shtukenberg, X. Zhu, Y. Yang, B. Kahr. Common occurrence of twisted molecular crystal morphologies from the melt. Cryst. Growth Des. 2020, 20, 6186-6197.

[[ii]] Y. Yang, Y. Zhang, C. T. Hu, M. Sun, S. Jeong, S. S. Lee, A. G. Shtukenberg, B. Kahr. Transport in twisted crystalline charge transfer complexes. Chem. Mater. 2022, 34, 1778-1788.

[[iii]] Y. Yang, K. Zong, S. J. Whittaker, Z. An, M. Tan, H. Zhou, A. G. Shtukenberg, B. Kahr, S. S. Lee. Twisted tetrathiafulvalene crystals. Mol. Syst. Des. Eng. 2022, 7, 569-576.

[[iv]] Y. Yang, L. S. de Moraes; C. Ruzie, G. Schweicher, Y. H. Geerts, A. R. Kennedy, H. Zhou, S. J. Whittaker, S. S. Lee, B. Kahr, A. G. Shtukenberg. Charge transport in twisted organic semiconductor crystals of modulated pitch. Adv. Mater. 2022, 34, 2203842.