Long-range exciton diffusion facilitates efficient exciton harvesting in a lightly-doped organic semiconductor crystal.

Dynamic disorder manifested in fluctuations of charge transfer integrals considerably hinders charge transport in high-mobility organic semiconductors. Accordingly, strategies for suppression of the dynamic disorder are highly desirable. In this study, we suggest a novel promising strategy for suppression of dynamic disorder—tuning the molecular electrostatic potential. Specifically, we show that the intensities of the low-frequency (LF) Raman spectra for crystalline organic semiconductors consisting of π-isoelectronic small molecules (i.e. bearing the same number of π electrons)—benzothieno[3,2-b][1]benzothiophene (BTBT), chrysene, tetrathienoacene (TTA) and naphtho[1,2-b:5,6-b′]dithiophene (NDT)—differ significantly, indicating significant differences in the dynamic disorder. This difference is explained by suppression of the dynamic disorder in chrysene and NDT because of stronger intermolecular electrostatic interactions. As a result, guidelines for the increase of the crystal rigidity for the rational design of high-mobility organic semiconductors are suggested.

Recent theoretical studies have shown that charge transport in high-mobility organic semiconductors is limited by low-frequency vibrations because of strong non-local electron–phonon interaction. Here we investigate two high-electron-mobility organic semiconductors with similar molecular structures but considerably different crystal packings, TCNQ and F₂-TCNQ, and reveal the relationship between the experimental low-frequency Raman spectra and the calculated contributions of various vibrational modes to the electron–phonon interaction. We suggest that the combination of Raman spectroscopy with solid-state DFT is a powerful tool for probing electron–phonon interaction and focused search for high-mobility organic semiconductors.

Owing to combination of chemical and thermal stability, favorable molecular packing, and efficient electron transport, naphthalene diimide derivatives (NDIs) are promising materials for n-channel organic field effect transistors (OFETs). For tuning the properties of n-conductive organic semiconductors, as well as for improvement of their air stability, fluorination is a frequently used approach. In this study, we demonstrate how very small modification of the molecular structure – fluorine substitution in the p-position of the phenyl rings of N,N′-diphenyl-NDI (Ph-NDI) – dramatically changes the crystal packing but almost does not affect electron transport. We show that this two-atom modification of Ph-NDI changes the molecular packing motif from π-stacking to a herringbone one, in contrast with usually observed improvement of π-stacking with fluorination. This unexpected behavior is mainly attributed to changes in the electrostatic potential of the phenyl rings as a result of fluorination, which alters their relative orientation and modifies the packing of the NDI cores. Nevertheless, though the herringbone packing is typically considered as less favorable for charge transport, the theoretical electron mobility is slightly higher in the fluorinated Ph-NDI. The results obtained improve the understanding of the relationship between the molecular and crystal structures of organic semiconductors and their impact on charge transport, which is of key importance for rational design of high-mobility materials for organic electronics.

Properties of the organic semiconductors can be finely tuned via changes in their molecular structure. However, the relationship between the molecular structure, molecular packing, and (opto)electronic properties of the organic semiconductors to guide their smart design remains elusive. In this study, we address computationally and experimentally the impact of subtle modification of a thiophene–phenylene co-oligomer CF₃-PTTP-CF₃ on the molecular properties, crystal structure, charge transport, and optoelectronic properties. This modification consists in the substitution of two C–H atom pairs by N atoms in the thiophene units and hence converting them to thiazole units. A dramatic effect of the N-substitution on the crystal structure—the crossover from the herringbone packing motif to π-stacking—is attributed to significant changes in the molecular electrostatic potential. The changes in the molecular and crystal structure resulting from the N-substitution clearly reveal themselves in the Raman spectra. The increase of the calculated electron mobility in the corresponding crystals as a result of the N-substitution is rationalized in terms of the changes in the molecular and crystal structure. The charge transport, electroluminescence, and photoelectric properties are compared in thin-film organic field-effect transistors based on CF₃-PTTP-CF₃ and its N-substituted counterpart. An intriguing similarity between the effects of N-substitution in the thiophene rings and fluorination of the thiophene–phenylene oligomer is revealed, which is probably associated with a more general effect of electronegative substitution. The obtained results are anticipated to facilitate the rational design of organic semiconductors.