Theoretical Analysis of Phonons and Their Influence on Charge Transport in Novel Thienoacene Molecular Crystals

We present two novel thienoacene molecular compounds that comprise four central tetrathienyl cores fused with two naphtyl rings, DN4T and isoDN4T. The straight ‘acene like’ structure of DN4T results in high electron densities on the sulfur atoms while these are absent in isoDN4T that adopts a ‘phene like’ structure. Using Density Functional Theory (DFT), we show that this translates into a reduced molecular reorganization energy, enlarged transfer integrals in the herringbone planes, and as a result larger hole mobility values in DN4T vs isoDN4T molecular crystals.<br/>The carrier mobility in these crystals is known to be limited by low frequency phonons collapsing the wavefunction at room temperature. Remarkably, we find that the transfer integrals in DN4T are rather insensitive to the sliding motion of the aromatic cores, reported to be the ‘killer’ mode in related molecules [1]. The full spectral density, as obtained by combining classical Molecular Dynamic simulations to DFT calculations, reveals a rich structure that points to the phonon modes affecting the most charge transport.<br/>[1] <i>Schweicher, Guillaume, et al. "Chasing the “killer” phonon mode for the rational design of low disorder, high mobility molecular semiconductors." Advanced Materials 31.43 (2019): 1902407</i>