Structural Rearrangement of Organic Semiconductor Molecules with an Asymmetric Shape in Thin Films

Abstract

2-Decyl-7-phenyl [1]benzothieno [3,2-b][1]benzothiophene (Ph-BTBT-C 10 ) exhibits excellent performances as an active layer in organic thinfilm transistors, and its performances are greatly influenced by the molecular packing, i.e., the crystalline polymorphs. This compound has the so-called thinfilm phase in a vapor-deposited film, which is different from the single-crystal structure (the bulk phase). In this work, thin films of Ph-BTBT-C 10 are prepared by spin coating, and the effect of aging on the film structure is investigated by high-resolution infrared (IR) spectroscopy in combination with X-ray diffraction. The results show that the as-spun film has the same thin-film phase as the vapor-deposited film and that the thin-film phase is rapidly converted to the bulk phase by aging at room temperature. In addition, the spectral analysis also reveals conformational changes in the decyl chain during the crystalline transition process. This work highlights the importance of using IR spectroscopy with a high-wavenumber resolution for structural analysis of molecular thin films.

Accessing the electronic structure of liquid crystalline semiconductors with bottom-up electronic coarse-graining

“…While BTBT molecules form antiparallel cofacial π–π stacking in the smectic A phase, a herringbone-like structure with offset π–π stacking is observed in the smectic E phase, reminiscent of the crystal structure observed in the benzothienobenzothiophene family. 47–55,59–66 These features are consistent with CG structure factor analyses and the characterization of the nematic order parameter as a function of cutoff radius, as depicted in Fig. 2b and c.…”

“…Our simulation results align with these recent findings. 59–66 Specifically, the transition from antiparallel cofacial π–π stacking 133 to offset π–π stacking (also known as nanosegregated stacking) 134,135 within the monolayers from smectic A to smectic E, accompanied by a considerable increase in the tilt angle between the principal director and the layer normal, may serve as a precursor to the formation of the lipid-like bilayer structure observed in the crystal phase. 65,66…”

“…38–43 While the manipulation of liquid crystal (LC) phases in OSCs for enhanced device performance is a common theme throughout the community, 44–46 recent work has explored morphology-dependent conductivity in asymmetric benzothienobenzothiophene-based compounds, a promising type of LC-forming OSCs used in field-effect transistors. 47–66 These compounds exhibit a remarkable combination of attributes, including high charge mobility, superior solubility and processability, robust thermal durability, and the capability to regulate molecular orientation. While layered smectic phases with head-to-head bilayer (lipid-like) structures have been reported in the asymmetric benzothienobenzothiophene family, recent work by Han et al introduced a derivative, 2-(4-methoxyphenyl)-7-octyl-benzothienobenzothiophene (BTBT), 56,57 that exhibited a rare nematic phase (long-range order without layered structure) when inserted into rubbed planar anchoring sandwich cells.…”

“…However, the mechanisms underlying phase transitions between crystals and different smectic phases or the formation of the nematic phase remain unclear. 59–66 Importantly, large changes in the electronic conductivities were observed in transitioning benzothienobenzothiophene-based molecules through different LC phases. For traditional multiscale computational methods, the analysis of such conductivity trends would represent an effort warranting state-of-the-art computing resources; slow relaxation times would necessitate CG modeling, which would need to be connected with all-atom (AA) backmapping and molecular dynamics, followed by ad nauseam QC characterization at the ∼10–100 nm scale to compute morphology dependent electronic properties.…”

Friction Force Mapping of Molecular Ordering and Mesoscopic Phase Transformations in Layered-Crystalline Organic Semiconductor Films

“…To fully use the highly layered OSCs, it is necessary to investigate and understand the conversion of the metastable phase to a more ordered phase by thermal treatments. It is effective to first identify the crystal structure of thin-film phases by X-ray diffraction, although it does not allow the discrimination of each phase spatially within the films. − Polarization dependence of the electronic absorption spectrum allows us to identify each phase, although the spatial resolution is limited by the optical diffraction limit . Atomic force microscopy (AFM) enables enough high spatial resolution, but it is not easy to distinguish phase states from topographic images, as they seem to be continuous without grain boundaries in the case of the annealed films, though detailed step-height measurements may allow for discrimination of these phases. − ,, Therefore, no useful technique is known to be effective for investigating local phase changes in the OSC films.…”