Two main factors governing the effect of terminal substituents on the properties of thiophene–phenylene co-oligomers are revealed.

Efficient operation of organic electronic devices requires high charge‐carrier mobilities in their active layers, but only several organic semiconductors show confirmed charge‐carrier mobilities exceeding that of amorphous silicon (≈1 cm2 V−1 s−1). Charge transport in high‐mobility organic semiconductor crystals is considerably hindered by non‐local electron‐phonon interaction (NLEPI) transforming dynamic disorder induced by low‐frequency (LF) vibrations into fluctuations of charge transfer integrals. In this work, using two crystals of naphthalene diimide derivatives as an example, LF vibrational modes that strongly modulate the charge transfer integrals are computationally revealed. The importance of the discussed LF modes for limiting the charge‐carrier mobility is justified by analyzing the effect of the dynamic disorder on the charge‐carrier dynamics, estimating the charge‐carrier mobility in the two crystals, and observing quite a good agreement of the latter with the experimental values. Finally, it is shown that the contribution of various modes to the NLEPI correlates with their experimental Raman intensities. As a result, it is suggested that LF Raman spectroscopy can be used for experimental study of NLEPI, which can help with screening organic semiconductors showing high charge‐carrier mobility and promote rational design of such materials.

Organic optoelectronics requires materials combining bright luminescence and efficient ambipolar charge transport. Thiophene-phenylene co-oligomers (TPCOs) are promising highly emissive materials with decent charge-carrier mobility; however, they typically show poor electron injection in devices, which is usually assigned to high energies of their lowest unoccupied molecular orbitals (LUMOs). A widely used approach to lower the frontier orbitals energy levels of a conjugated molecule is its fluorination. In this study, we synthesized three new fluorinated derivatives of one of the most popular TPCOs, 2,2′-(1,4-phenylene)­bis­[5-phenylthiophene] (PTPTP) and studied them by cyclic voltammetry, absorption, photoluminescence, and Raman spectroscopies. The obtained data reveal a positive effect of fluorination on the optoelectronic properties of PTPTP: LUMO levels are finely tuned, and photoluminescence quantum yield and absorbance are increased. We then grew crystals from fluorinated PTPTPs, resolved their structures, and showed that fluorination dramatically affects the packing motif and facilitates π-stacking. Finally, we fabricated thin-film organic field-effect transistors (OFETs) and demonstrated a strong impact of fluorination on charge injection/transport for both types of charge carriers, namely, electrons and holes. Specifically, balanced ambipolar charge transport and electroluminescence were observed only in the OFET active channel based on the partially fluorinated PTPTP. The obtained results can be extended to other families of conjugated oligomers and highlight the efficiency of fluorination for rational design of organic semiconductors for optoelectronic devices.

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.

ones; only a few OSs show reproducible mobilities exceeding that for amorphous silicon (μ ≈ 1 cm 2 V −1 s −1 ), a workhorse of modern thin-film electronics. [1,2] Moreover, reliable mobility measurements in OSs are difficult and time-consuming. For example, the commonly used measurement method with the use of organic field-effect transistors (OFETs) is complicated by many factors, such as the contacts, the architecture, the dielectric, etc., which results in various artifacts and pitfalls. [3] Therefore, focused search for high-mobility OS materials among a huge number of candidates needs an effective approach to charge-carrier mobility estimation prior to its measurements in devices. Importantly, in the context of such a search, one does not necessarily have to predict a value of μ closely enough to the experimental mobility to be further measured: a mobility estimation characterized with a good correlation with experimental μ values for quite a lot of (but not all) OSs also provides a way toward efficient screening of high-mobility OS materials.Two types of OSs show high charge-carrier mobilities: smallmolecule organic crystals (crystalline OSs) and polymers. In what follows, we will focus on crystalline OSs, whose crystal structures resolved from X-ray diffraction data are needed for Further progress in organic electronics demands new highly efficient organic semiconductor (OS) materials. So far, however, band-like charge transport with high mobilities has been reliably demonstrated only in a few OSs, and development of efficient methods for search of high-mobility materials among a plethora of OSs remains extremely important. In the present work, a spectroscopic method is presented for screening of crystalline OSs with efficient charge transport, to be used prior to time-consuming device measurements. Specifically, the work focuses on a physical rationale and an experimental benchmark of a correlation between the intensities of the low-frequency Raman spectrum and the strength of dynamic disorder limiting the charge-carrier mobility in a material. As a result, two physicsinspired spectroscopic descriptors for charge-carrier mobility estimation are suggested, both of which clearly correlate with device mobilities reported for various crystalline OSs. It is anticipated that the spectroscopic method based on these descriptors can serve as a powerful search tool for revealing new high-mobility OS materials.

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...

Conformational space of polyphenylenevinylene oligomers is systematically investigated computationally at energies relevant for room temperature dynamics in a solvent and in a solid state. Our calculations show that optimal oligomer structures are essentially planar. However, lack of a deep minimum at the planar geometry allows for large molecular deformations even at very low temperatures. At larger angles, rotational motion of dihedrals intermix with two orthogonal bending motions of the entire molecule. In a crystalline environment these degrees of freedom intermix with translational and rotational motions, whereas purely intramolecular modes are well separated. The reliability of our calculations is confirmed by an excellent match of the theoretical and experimental Raman spectra of crystalline stilbene in the entire spectral range including the low-frequency part. Obtained results provide important insights into nature of low-frequency vibrations, which play a key role in charge transport in organic semiconductors.

Recently developed ultrathin two-dimensional (2D) organic semiconductor crystals are a promising platform for advanced organic electronic devices. Remarkable quality of such crystals results in charge-carrier mobilities comparable to those of bulk crystals, but their structure and orientation are hard to study because of their extremely small thickness. Here, we applied surface-enhanced Raman spectroscopy (SERS) to investigate the structure of the thinnest 2D single crystalsmonolayers, which are based on thiophene-phenylene co-oligomers: 1,4-bis­(5′-decyl-2,2′-bithiene-5-yl)­benzene and 1,4-bis­(5′-hexyl-2,2′-bithiene-5-yl)­benzene. Their Raman spectra were calculated as a function of the molecule orientation and SERS microscopy maps were acquired. High sensitivity of SERS allowed us to study monolayer single-crystal domains with the optical spatial resolution. Raman anisotropy was used to probe the orientations of single-crystal domains and the molecule orientation within them. Notably, the SERS microscopy detected the presence of a submonolayeramorphous material between the crystalline domains, which is practically inaccessible to optical or conventional atomic force microscopies (AFMs). The submonolayer was also studied by lateral-force AFM, which showed notably higher friction and adhesion. We found that the measured Raman anisotropy significantly reduced by the metal-covered substrate still allowing us to distinguish orientations of molecules in the 2D crystals and in the submonolayer. Anisotropy-sensitive SERS was shown to be promising for studying 2D organic semiconductor crystals.