Polyacenes are archetypal π-conjugated systems with important applications in organic semiconductors. This series exhibits four distinct types of layered packing motifs that significantly affect semiconducting properties, yet their structural origins remain elusive. Here, we show that stepwise quantum-chemical optimization applied to a structural model with a minimal and rational set of variables enables systematic reproduction of all the polymorphs. The model starts with a monomolecular layer composed of molecular pairs, where the short axes are arbitrarily positioned and oriented under glide symmetry, while the long axes remain parallel and unshifted. By introducing long-axis inclinations, torsions, and interlayer displacements, we obtain locally stable structures that reproduce the geometries and relative stabilities of all the polymorphs. Potential energy map analysis reveals distinct stabilizing mechanisms specific to each form. These findings clarify the origin of polymorphism in polyacenes and provide insight into how molecular packing influences charge transport. This work establishes a predictive first-principles framework for understanding and designing functional crystalline molecular materials.

A field plate is a grounded metal layer underneath the source electrode in thin-film transistors (TFTs) and was found to significantly reduce pinch-off voltage and enhance intrinsic gain (up to 320) of inkjet-printed organic TFTs. The operating mechanism was investigated through automated fabrication and statistical analysis of over 3000 devices with various channel length (L) and field plate length (L fp). Crucially, in the saturation regime, transconductance and drain current were governed by L fp rather than L. We propose and validate a new theoretical model, supported by device simulations, which demonstrates that pinch-off occurs not at the drain or source, but at the edge of the field plate. This novel mechanism explains the observed low pinch-off voltage and suggests that device performance can be improved through miniaturization, offering a key advantage over conventional high-gain architectures like source-gated transistors. In addition, the field plate enables to control pinch-off voltage simply by layout change, providing functional versatility. Finally, a compact model was developed to facilitate the design of high-performance printed analog circuits, highlighting the potential of these devices for future flexible electronics.

Molecular monolayers at fluid interfaces are generally considered mechanically fragile, collapsing under lateral compression. Here we show that a rationally designed amphiphilic π-conjugated molecule instead forms a monolayer that adapts to mechanical stress. Langmuir films of 3-hydroxyphenyl-7-decyl-[1]benzothieno[3,2- b ][1]benzothiophene (HP-BTBT-C10) accommodate and store compressive stress without collapse. In situ optical imaging reveals that lateral compression converts into reversible out-of-plane deformation via anisotropic molecular reorganization, producing ordered wrinkles. Electron diffractometry and high-resolution electron microscopy show localized multilayer folding while retaining local molecular packing. Intermolecular interaction calculations suggest that hydroxy substitution helps stabilize both the Langmuir monolayer on water and folded multilayer wrinkles, while preserving the strong layer-forming tendency of alkyl-substituted extended π-core molecules. Dynamic measurements based on barrier-oscillation experiments further demonstrate repeatable viscoelastic stress responses mediated by sequential folding and wrinkle reversibility, establishing mechanically adaptive molecular monolayers that bridge conventional Langmuir films and biologically inspired membranes.

The highly lyophobic perfluoropolymer Cytop is widely recognized as an excellent interfacial layer material for organic thin-film transistors (OTFTs), owing to its ability to eliminate interfacial traps and to enable steep subthreshold swing (SS), high carrier mobility, and stable operation. However, its strong lyophobicity poses a major challenge for subsequent solution-based processing, particularly in achieving high-resolution electrode patterning. In this study, we demonstrate that an extended meniscus technique enables direct photolithographic processing of photoresists on pristine Cytop. Uniform photoresist layers were deposited by blade coating under substrate heating, allowing fabrication of short-channel electrodes with lengths below 10 mu m. Bottom-gate, bottom-contact OTFTs incorporating single-crystal organic semiconductor (OSC) thin films of phenyl/alkyl-substituted benzothieno[3,2-b]naphtho[2,3-b]thiophene exhibited steep SS values as low as 70-85 mV dec-1 and field-effect mobilities exceeding 1 cm2 V-1 s-1. The contact resistance, estimated from the channel-length dependence of the transfer characteristics, was approximately 1.5 kQ cm. The gate-voltage dependence indicates that carrier accumulation at the OSC/electrode/dielectric ternary interface contributes to reducing contact resistance. This study represents a significant step toward realizing short-channel OTFTs with high mobility, low operating voltage, and stable performance.

Abstract Organic molecules with π-conjugated backbones and alkyl side groups show promise for applications as solution-processable high-mobility semiconductors. However, the energy–momentum (E–k) dispersion, essential for band-like transport, has not been experimentally demonstrated. This is mainly due to the surface sensitivity of the standard measurement technique, angle-resolved photoelectron spectroscopy (ARPES), which is impeded by side groups concealing the semiconducting π-backbones beneath the crystalline sample surfaces. In this study, the highest-occupied molecular orbital (HOMO) bands of 2-n-octyl-[1]benzothieno[3,2-b]naphtho[2,3-b]thiophene (mono-2-C8-BTNT) were successfully probed using ARPES with a low excitation photon energy of 8 eV to increase the photoelectron probing depth beyond 2 nm. The in-plane anisotropy and temperature-dependent evolution in the E–k dispersion relations and hole effective mass of the mono-2-C8-BTNT HOMO bands were experimentally demonstrated despite the presence of the side groups.

We report the characteristic domain wall (DW) patterns observed in rare-metal-free plastic/ferroelectric films of [MDABCO][PF6], where [MDABCO]+ is a cage-like molecular ion with a protrusion. In its paraelectric plastic phase, [MDABCO][PF6] adopts a cubic lattice. However, in the ferroelectric phase, the lattice undergoes significant distortion into a right prism (pseudocube) with a kite-shaped quadrilateral base. This distortion results from the alignment of the [MDABCO]+ protrusions parallel to the base planes of the prism lattice, with spontaneous polarization along the (110) direction of the pseudocubic (pc) lattice. This leads to multiaxial ferroelectricity with six possible polarization directions. We demonstrate that two distinct types of thin films, each with unique crystal orientations, can be fabricated depending on the film growth method, resulting in different DW patterns. Films grown from a confined solution layer between top and bottom substrates exhibit the pseudocubic {100}pc plane nearly parallel to the substrate, producing a DW pattern corresponding to two possible in-plane polarization orientations. In contrast, blade-coated films show the diagonal {110}pc plane of the pseudocubic lattice aligned parallel to the substrate surface. These films predominantly feature polarized domains with in-plane polarization components, while thin, streak-like domains with out-of-plane polarization components intersect at a consistent angle of approximately 54 degrees. Furthermore, blade-coated films allow for polarization switching across the entire film surface, generating out-of-plane polarization components. These findings advance our understanding of polarization domain control in plastic/ferroelectrics and highlight their potential for use in printable ferroelectric-based devices.

Crystal and conformational polymorphisms play crucial roles in the physical and chemical properties of materials, impacting their stability, solubility, and bioavailability, which are essential for various applications in pharmaceuticals, materials science, and chemistry. Despite their significance, the structural analysis of these polymorphisms, particularly conformational polymorphisms, remains challenging due to the limited methodology that provides sufficient resolution for microcrystalline variants of polymorphs. Three-dimensional electron diffraction (3D ED) is an emerging technique with significant potential for elucidating the microcrystal structures of functional organic molecules, pharmaceuticals, and biomolecules. Despite this potential, there are limited instances of 3D ED structures for small molecules exhibiting the lowest crystallographic symmetry with a preferred orientation and possibly conformational variations of constituent molecules. A novel organic semiconductor, Ph-anti-benzothieno[5,6-b]benzothieno[3,2-b]thiophene-C10 (antiC10), is one of such examples. We successfully determined the 3D ED structure of this challenging molecule. The antiC10 crystal exhibited the lowest symmetry (space group P1), and the preferred orientations against the grid resulted in a missing cone. These challenges were surmounted by employing a sequential molecular replacement approach with an ab initio-generated search model. The resulting octameric antiC10 structure reveals a two-monolayer architecture and an antiparallel alkyl-interdigitated herringbone configuration in contrast to the all-parallel associations observed in its previously reported isomer. Concurrently, the alkyl chains are intricately interdigitated with each other and positioned between the adjacent π-core strata. Detailed analysis has elucidated the conformational polymorphism in herringbone packing between the two monolayers as well as in intramolecular conformations among monomers. The structure with conformational polymorphism is presumably in a metastable intermediate state, stabilized by twinning. These findings may provide critical insights into the crystallization mechanisms and rational design of organic semiconductors. This research demonstrates that advancements in 3D ED technology and sequential phasing methodologies have enabled the study of previously unreachable structures.

A mixed solution of organic semiconductors Ph-BTBT-Cn with different alkyl chain length (n) forms a highquality molecular bilayer by suppressing layer-by-layer stacking when the molar fraction of the longer chains (chi L) is 0.1-0.6. In this study, we performed molecular dynamics simulations to investigate the dynamics of alkyl chains in the mixed bilayer. The order parameters and dihedral angles of the alkyl chains were analyzed as a function of chi L. The results revealed that increasing chi L enhances the ordering of the longer alkyl chains, thereby reducing their torsional motion. A stochastic model of the number of free surplus chains explains the molar fraction dependence of film morphology.

Organic molecules with a rigid, π-conjugated core (π-core) and flexible alkyl chains (C n ) naturally exhibit liquid crystal (LC) phases, promoting self-assembly of quasi–two-dimensional semiconducting layered crystals. However, particular roles of rigid and flexible parts in layer formations remain elusive. Here, we demonstrate formation of an unprecedented superlayer cocrystal phase via a unique smectic LC phase in the equimolar melt mixture of symmetrically distinct molecules. The molecules used are a monoalkylated [(π-core)-C n ] using 2-octyl[1]benzothieno[3,2- b ][1]benzothiophene (mono-C 8 -BTBT) and a dialkylated [C n -(π-core)-C n ] using 2,7-dioctyl[1]benzothieno[3,2- b ][1]benzothiophene (di-C 8 -BTBT). Thermal analyses show that the superlayer cocrystal is exclusively induced at the equimolar mixture via melt crystallization from the LC phase. X-ray structure analysis reveals a reversible C n -(π-core)-C n ···(π-core)-C n stacking arrangement in the superlayer cocrystal, where π-cores and alkyl chains form nearly independent layers. Notably, this melt crystallization allows solvent-free fabrication of semiconductive polycrystalline films for excellent thin-film transistors. These findings pave the way for tailoring a quasi–two-dimensional structure in LC materials toward molecular electronics.

The selection of gate dielectric materials is critical in organic thin-film transistors (OTFTs), as carrier transport occurs at the semiconductor/gate dielectric interface, directly impacting device performance. In this study, we systematically investigate how gate dielectric surfaces affect both carrier transport and injection at the contacts in model inverted-coplanar or bottom-gate bottom-contact (BGBC), single-crystal OTFTs. We examine ten gate dielectric surfaces with relatively high hydrophobicity and classify their influence into three groups based on chemical structure and surface energy: (i) fluorinated polymers, (ii) silane coupling agents (SCAs) without phenyl rings, and (iii) hydrocarbons containing phenyl rings. Among these, OTFTs using fluorinated polymers (low surface energy) exhibited the highest mobility and the smallest subthreshold swing (SS), while devices with high surface energy dielectrics displayed reduced mobility or degraded subthreshold characteristics. The transfer-line method (TLM) and Kelvin probe force microscopy (KFM) revealed that contact resistance is highly sensitive to the gate dielectric, with the highest resistance observed in devices using phenyl-containing hydrocarbons with high surface energy. Importantly, when the effects of contact resistance are excluded, all the devices exhibited high intrinsic mobility. These findings highlight the pivotal role of gate dielectric surface properties in determining both carrier transport and injection, offering key insights for optimizing the performance of BGBC-type OTFTs.

Here, we systematically investigated the effects of the thiophene-fused isomer and the end-cap substitution on high-layered crystallinity, film formability, and field-effect transistor characteristics in pi-extended and alkylated organic semiconductors (OSCs). We developed four kinds of unsymmetric rod-like OSCs based on syn-/anti-isomers of benzothieno-[6,5-b]-/benzothieno-[5,6-b]-benzothieno-[3,2-b]-thiophene (BTBTT) with phenyl/alkyl substitutions with different alkyl chain lengths: synCn and antiCn (n = 6, 10). The layered molecular packing motifs of the compounds are distinct from the thiophene orientation of isomeric pi-cores but are unaffected by the alkyl chain length. The synCn forms a bilayer-type layered herringbone (b-LHB) packing composed of head-to-head arrangement of unidirectionally aligned molecular layers showing high-layered crystallinity and high carrier mobility over 10 cm2 V-1 s-1. By contrast, the antiCn forms an antiparallel alkyl-interdigitated herringbone (aai-HB) structure in which the respective pi-core layer is composed of alternating antiparallel alignment of pi-cores and the alkyl chains are interdigitated with each other between the adjacent pi-core layers. The latter shows relatively poor crystalline-film formability and moderate carrier mobility. Dispersion-corrected density functional theory calculations of intermolecular interaction energy reveal that the overall shape of the rigid pi-core components is crucial for achieving unidirectionally aligned and closely packed 2D pi-core layers and that the flexible end-cap substituents strengthen and balance the layered crystallinity. The findings will be crucial for designing and developing the highly layered crystalline and high-performance OSCs.

Inverted coplanar or bottom-gate bottom-contact (BGBC)-type thin-film transistors (TFTs) present several advantages for the manufacture and application of organic TFTs, although serious difficulties are encountered when trying to achieve sufficiently high performance. Recently, it was demonstrated that both high mobility and ideal on-off switching are attainable in BGBC-type printed organic TFTs with highly clean semiconductor-gate dielectric interfaces. However, an unknown channel material dependence in the device performance is found. Here, we show that the stability of semiconductor/metal/dielectric ternary interfaces is a crucial factor in the operation of BGBC-type organic TFTs. We fabricate single-crystal organic semiconductor (OSC) films with various numbers of layers using two different materials (phenyl/alkyl-substituted benzothieno[3,2-b]benzothiophene and phenyl/alkyl-substituted benzothieno[3,2-b]naphtho[2,3-b]thiophene) on highly lyophobic Cytop gate dielectric surfaces. The transfer characteristics exhibit notable time-dependent degradation, which clearly depends on the material, layer number, and encapsulation. Kelvin-probe force microscopy measurements reveal that the degradation is ascribed to contact resistance at the source electrodes, while it can be more suppressed in multilayer (two or more layers) OSCs. Atomic force microscopy and in-plane x-ray diffraction profiles present signs of the transformation in single molecular bilayer OSCs laid on the electrodes. The results suggest the importance of the quality of the OSC layer at ternary interfaces, providing a clue for improving the performance of BGBC-type organic TFTs.

Some rodlike organic molecules exhibit exceptionally high layered crystallinity when composed of a link between π-conjugated backbone (head) and alkyl chain (tail). These molecules are aligned side-by-side unidirectionally to form self-organized polar monomolecular layers, providing promising 2D materials and devices. However, their interlayer stacking arrangements have never been tunable, preventing the unidirectional arrangements of molecules in whole crystals. Here, it is demonstrated that polar/antipolar interlayer stacking can be systematically controlled by the alkyl carbon number n, when the molecules are designed to involve effectively weakened head-to-head affinity. They exhibit remarkable odd-even effect in the interlayer stacking: alternating head-to-head and tail-to-tail (antipolar) arrangement in odd-n crystals, and uniform head-to-tail (polar) arrangement in even-n crystals. The films show excellent field-effect transistor characteristics presenting unique polar/antipolar dependence and considerably improved subthreshold swing in the polar films. Additionally, the polar films present enhanced second-order nonlinear optical response along normal to the film plane. These findings are key for creating polarity-controlled optoelectronic materials and devices.

Unique flip-flop motion of liquid-crystal organic semiconductor molecules responsible for phase transition was revealed by molecular dynamics simulations.

It is critical to understand molecular ordering processes in small-molecule organic semiconductor (OSC) films in optimizing electronic device applications, although it is difficult to observe and investigate the ordering characteristics at a mesoscopic or device scale. Here, we report that friction force microscopy (FFM) allows visualizing the ordering transformation process from a thermodynamically metastable phase to a stable phase at a mesoscopic scale. We utilized 2-octyl-benzothieno[3,2-b]naphtho[2,3-b]thiophene (2-C8-BTNT) as a typical highly layered-crystalline OSC. We found that the friction force between an AFM tip and spin-coated OSC films significantly depends on whether local film states are in metastable monolayer phase or stable bilayer-type herringbone (b-LHB) phase that exhibits high carrier mobility. The formation of the stable b-LHB phase leads to lower friction than the metastable monolayer phase, clearly visualizing the molecular order. Force map (Fmap) analysis indicates that the lower friction in the b-LHB phase should be associated with the reduction of interfacial adhesion force. Notably, the observed results demonstrate that the spin-coated thin film changes from continuous film with the monolayer phase to rugged microcrystal grains with the b-LHB phase when left at ambient conditions. By contrast, an appropriate post-thermal annealing process facilitates the phase transformation without inducing such morphological changes. The technique provides a unique and effective tool for revealing the relationship between processing conditions and device performance in polycrystalline OSC films.

The mode of intermolecular arrangements is crucial in determining organic semiconductor (OSC) device performance, although the method for controlling the arrangements has not yet been established. Here it is demonstrated that mixing OSC molecules with different lengths of substituted alkyl chains allows the emergence of a distinct high-mobility polymorphic phase in alkylated layered crystalline OSCs. The pristine OSC molecules of para-tolyl/alkylated-[1]benzothieno[3,2-b][1]benzothiophene (pTol-BTBT-Cn) at n = 6 and 8 both exhibit a low-mobility isomorphous phase in the crystalline film states. By contrast, a distinct polymorphic phase showing much higher mobility emerges as a result of equimolar mixing of pTol-BTBT-Cn at n = 6 and 8 during the film-coating process. The mixing-induced phase presents tenfold enhanced mobility compared with pure compounds, accompanied by a drastic polymorphic transformation. The emerging technique of electron plays a significant role in stabilizing the mixing-induced phase. The analysis of polarized absorption spectra indicates that a metastable liquid-crystalline (LC) phase is formed prior to the crystallization, implying that the formation of the mixing-induced phase is promoted by the intralayer molecular diffusion motion within the metastable LC phase. The finding of the controllable nature of layered crystallinity should lead to high-end printed electronics.

The experimental mobilities of organic semiconductors, 2-Cn-BTNT (Figure 1), depend on the alkyl chain length n, while the mechanism is unclear. In this study, we elucidate the relationship between alkyl chain length and carrier mobility by classical force field (FF) and density functional theory (DFT) calculations. We analyzed the interactions between an alkyl chain and surrounding molecules and executed NVT molecular dynamics simulations at room temperature and ambient pressure by FF calculations. Transfer integrals were calculated using the DFT method with the coordinates after NVT simulations. As a result, alkyl chain length affects the lattice constant, thermal motion of the alkyl chain and transfer integrals and gives the difference of mobilities.

The attraction between pi-conjugated planar electron donor and acceptor molecules that form many stable charge-transfer (CT) complexes has been explained by quantum chemical CT interactions, although the fundamental origin remains unclear. Here, we demonstrate the mechanism of CT complex formation by potential energy map analysis for TTF-CA and BTBT-TCNQ, using energy decomposition of intermolecular interaction by symmetry-adapted perturbation theory (SAPT) combined with coupled cluster calculation. We find that the source of attraction between donor and acceptor molecules is ascribed primarily to the dispersion force and also to the electrostatic force. In contrast, the contribution of CT interactions to the attractive forces is minimal. We demonstrate that the highly directional feature of the exchange repulsion force, coupled with the attractive dispersion and electrostatic forces, is crucial in determining the intermolecular arrangements of actual CT crystals. These findings are key for understanding the unique structural and electronic properties of pi-conjugated CT complexes. The attraction between pi-conjugated planar electron donor and acceptor molecules within charge-transfer (CT) complexes has been explained by quantum chemical CT interactions, but its fundamental origins remain unclear. Here, the authors combine symmetry-adapted perturbation theory with coupled cluster calculations to probe the mechanism of CT complex formation in crystals, finding that dispersion and electrostatic forces are dominant, with significant directional exchange repulsion.

The experimental mobilities of organic semiconductors, 2-Cn-BTNT (Figure 1), depend on the alkyl chain length n, while the mechanism is unclear. In this study, we elucidate the relationship between alkyl chain length and carrier mobility by classical force field (FF) and density functional theory (DFT) calculations. We analyzed the interactions between an alkyl chain and surrounding molecules and executed NVT molecular dynamics simulations at room temperature and ambient pressure by FF calculations. Transfer integrals were calculated using the DFT method with the coordinates after NVT simulations. As a result, alkyl chain length affects the lattice constant, thermal motion of the alkyl chain and transfer integrals and gives the difference of mobilities.