ИСТИНА

High charge mobility is a prerequisite for efficient operation and commercial viability of organic electronic devices, but only several organic semiconductors (OSCs) have demonstrated reproducible charge mobilities above 1 cm2/(V·s), and a few of them have shown band-like charge transport. Search for new high-mobility OSCs requires deep understanding of the charge transport in these materials, and comparative studies of the structurally close OSCs are extremely useful for this purpose. Recently, high band-like electron mobility of about 7 cm2/(V·s) was reported for crystalline 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F2-TCNQ), while mobilities in the crystals of structurally close molecules, TCNQ and F4-TCNQ, are two orders of magnitude lower [1]. Although it was suggested that efficient transport in F2-TCNQ stems from unusual face-to-face packing [1], the mechanism underlying the impact of the packing motif on the mobility in this crystal family remains unclear. It is commonly considered that charge transport in OSCs is controlled by interplay of two factors: charge delocalization characterized by the transfer integrals Ji between the adjacent molecules, and charge localization resulting from electron-phonon interaction usually described by the reorganization energy λ. Recently, it was also shown that intermolecular vibrations can modulate Ji significantly and disrupt charge delocalization limiting charge transport [2]. The crystal packing motif determines Ji; however, it can also modify intra- and intermolecular vibrations [2]. In the current study, we investigate charge transport in a series of Fn-TCNQ (n=0, 2, 4) crystals by means of combined theoretical and experimental approach. The calculated Ji and λ explain low electron mobility value in F4-TCNQ as compared to the other two crystals; however, these values do not explain drastic difference in electron mobility between TCNQ and F2-TCNQ crystals. Moreover, the calculated large λ for an isolated Fn-TCNQ molecule (about 250 meV) contradicts the observed efficient charge transport in F2-TCNQ, indicating significant intermolecular charge delocalization that can decrease λ. Since this delocalization is very sensitive to the intermolecular vibrations [2], we investigated low-frequency vibrational spectra of the considered crystals using Raman spectroscopy and solid-state density functional theory (DFT). We found that the frequencies of the lowest modes are significantly higher in F2-TCNQ that can be a sign of tighter fixation of the molecules in this crystal. The Bader analysis of the periodic electronic density [3] confirms this hypothesis showing higher energy of the π-π stacking in F2-TCNQ. The correlation between the vibrations frequencies, intermolecular interaction energies and electron mobility is discussed from the viewpoint of the transient localization model [2]. This work was supported by RFBR (project № 16-32-60204).