Coherence in Polycrystalline Thin Films of Twisted Molecular Crystals.

Helicoidal crystallites in rhythmically banded spherulites manifest spectacular optical patterns in small molecules and polymers. It is shown that concentric optical bands indicating crystallographic orientations typically lose coherence (in-phase twisting) with growth from the center of nucleation. Here, coherence is shown to increase as the twist period decreases for seven molecular crystals grown from the melt. This dependence was correlated to crystallite fiber thickness and length, as well as crystallite branching frequency, a parameter that was extracted from scanning electron micrographs, and supported by numerical simulations. Hole mobilities for 2,5-didodecyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-C12) measured by using organic field-effect transistors demonstrated that more incoherent boundaries between optical bands in spherulites lead to higher charge transport for films with the same twist period. This was rationalized by combining our growth model with electrodynamic simulations. This work illustrates the emergence of complexity in crystallization processes (spherulite formation) that arises in the extra variable of helicoidal radial twisting. The details of the patterns analyzed here link the added complexity in crystal growth to the electronic and optical properties of the thin films.

Temperature-induced polymorphism of a benzothiophene derivative: reversibility and impact on the thin film morphology.

The identification of polymorphs in organic semiconductors allows for establishing structure-property relationships and gaining understanding of microscopic charge transport physics. Thin films of 2,7-bis(octyloxy)[1]benzothieno[3,2-b]-benzothiophene (C8O-BTBT-OC8) exhibit a substrate-induced phase (SIP) that differs from the bulk structure, with important implications for the electrical performance in organic field effect transistors (OFETs). Here we combine grazing incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM) to study how temperature affects the morphology and structure of C8O-BTBT-OC8 films grown by physical vapor deposition on SiO2. We report a structural transition for C8O-BTBT-OC8 films, from the SIP encountered at room temperature (RT) to a high temperature phase (HTP) when the films are annealed at a temperature T ≥ 90 °C. In this HTP structure, the molecules are packed with a tilt angle (≈39° respect to the surface normal) and an enlarged in-plane unit cell. Although the structural transition is reversible on cooling at RT, AFM reveals that molecular layers at the SiO2 interface can remain with the HTP structure, buried under the film ordered in the SIP. For annealing temperatures close to 150 °C, dewetting occurs leading to a more complex morphological and structural scenario upon cooling, with coexistence of different molecular tilts. Because the molecular packing at the interface has direct impact in the charge carrier mobility of OFETs, identifying the different polymorphs of a material in the thin film form and determining their stability at the interfaces are key factors for device optimization.

Charge Transport in Twisted Organic Semiconductor Crystals of Modulated Pitch.

Many molecular crystals (approximately one third) grow as twisted, helicoidal ribbons from the melt, and this preponderance is even higher in restricted classes of materials, for instance, charge-transfer complexes. Previously, twisted crystallites of such complexes present an increase in carrier mobilities. Here, the effect of twisting on charge mobility is better analyzed for a monocomponent organic semiconductor, 2,5-bis(3-dodecyl-2-thienyl)-thiazolo[5,4-d]thiazole (BDT), that forms twisted crystals with varied helicoidal pitches and makes possible a correlation of twist strength with carrier mobility. Films are analyzed by X-ray scattering and Mueller matrix polarimetry to characterize the microscale organization of the polycrystalline ensembles. Carrier mobilities of organic field-effect transistors are five times higher when the crystals are grown with the smallest pitches (most twisted), compared to those with the largest pitches, along the fiber elongation direction. A tenfold increase is observed along the perpendicular direction. Simulation of electrical potential based on scanning electron microscopy images and density functional theory suggests that the twisting-enhanced mobility is mainly controlled by the fiber organization in the film. A greater number of tightly packed twisted fibers separated by numerous smaller gaps permit better charge transport over the film surface compared to fewer big crystallites separated by larger gaps.

Mobility anisotropy in the herringbone structure of asymmetric Ph-BTBT-10 in solution sheared thin film transistors.

Thin films of the organic semiconductor Ph-BTBT-10 and blends of this material with polystyrene have been deposited by a solution shearing technique at low (1 mm s-1) and high (10 mm s-1) coating velocities and implemented in organic field-effect transistors. Combined X-ray diffraction and electrical characterisation studies prove that the films coated at low speed are significantly anisotropic. The highest mobility is found along the coating direction, which corresponds to the crystallographic a-axis. In contrast, at high coating speed the films are crystallographically less ordered but with better thin film homogeneity and exhibit isotropic electrical characteristics. Best mobilities are found in films prepared at high coating speeds with the blended semiconductor. This work demonstrates the interplay between the crystal packing and thin film morphology and uniformity and their impact on the device performance.

Molecular Disorder in Crystalline Thin Films of an Asymmetric BTBT Derivative.

The molecule 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10) is an organic semiconductor with outstanding performance in thin-film transistors. The asymmetric shape of the molecule causes an unusual phase behavior, which is a result of a distinct difference in the molecular arrangement between the head-to-head stacking of the molecules versus head-to-tail stacking. Thin films are prepared at elevated temperatures by crystallization from melt under controlled cooling rates, thermal-gradient crystallization, and bar coating at elevated temperatures. The films are investigated using X-ray diffraction techniques. Unusual peak-broadening effects are found, which cannot be explained using standard models. The modeling of the diffraction patterns with a statistic variation of the molecules reveal that a specific type of molecular disorder is responsible for the observed peak-broadening phenomena: the known head-to-head stacking within the crystalline phase is disturbed by the statistic integration of reversed (or flipped) molecules. It is found that 7-15% of the molecules are integrated in a reversed way, and these fractions are correlated with cooling rates during the sample preparation procedure. Temperature-dependent in situ experiments reveal that the defects can be healed by approaching the transition from the crystalline state to the smectic E state at a temperature of 145 °C. This work identifies and quantifies a specific crystalline defect type within thin films of an asymmetric rodlike conjugated molecule, which is caused by the crystallization kinetics.

Chasing the "Killer" Phonon Mode for the Rational Design of Low-Disorder, High-Mobility Molecular Semiconductors.

Molecular vibrations play a critical role in the charge transport properties of weakly van der Waals bonded organic semiconductors. To understand which specific phonon modes contribute most strongly to the electron-phonon coupling and ensuing thermal energetic disorder in some of the most widely studied high-mobility molecular semiconductors, state-of-the-art quantum mechanical simulations of the vibrational modes and the ensuing electron-phonon coupling constants are combined with experimental measurements of the low-frequency vibrations using inelastic neutron scattering and terahertz time-domain spectroscopy. In this way, the long-axis sliding motion is identified as a "killer" phonon mode, which in some molecules contributes more than 80% to the total thermal disorder. Based on this insight, a way to rationalize mobility trends between different materials and derive important molecular design guidelines for new high-mobility molecular semiconductors is suggested.

Insight from electron density and energy framework analysis on the structural features of Fx-TCNQ (x = 0, 2, 4) family of molecules.

In this study, the nature and characteristics of the intramolecular and intermolecular interactions in crystal structures of the fluoro-substituted 7,7,8,8-tetracyanoquinodimethane (TCNQ) family of molecules, i.e. Fx-TCNQ (x = 0, 2, 4), are explored. The molecular geometry of the reported crystal structures is directly dependent on the degree of fluorination in the molecule, which consequently also results in the presence of an intramolecular N[triple-bond]C...F-C π-hole tetrel bond. Apart from this, the energy framework analysis performed along the respective transport planes provides new insights into the energetic distribution in this class of molecules.

Substrate-Induced Phase of a Benzothiophene Derivative Detected by Mid-Infrared and Lattice Phonon Raman Spectroscopy.

The presence of a substrate-induced polymorph of 2,7-dioctyloxy[1]benzothieno[3,2-b]benzothiophene is probed in microscopic crystals and in thin films. Two experimental techniques are used: lattice phonon Raman and IR spectroscopy. The bulk crystal and substrate-induced phase have an entirely different molecular packing, and therefore, their Raman spectra are characteristic fingerprints of the respective polymorphs. These spectra can be unambiguously assigned to the individual polymorphs. Drop-cast and spin-coated thin films on solid substrates are investigated in the as-prepared state and after solvent-vapor annealing. Because Raman spectroscopy is less sensitive with decreasing film thickness, IR spectroscopy is shown to be a more feasible tool for phase detection. The surface-induced phase is mainly present in the as-prepared thin films, whereas the bulk phase is present after solvent-vapor annealing. This result suggests that the surface-induced phase is a metastable polymorph.

Rotator side chains trigger cooperative transition for shape and function memory effect in organic semiconductors.

Martensitic transition is a solid-state phase transition involving cooperative movement of atoms, mostly studied in metallurgy. The main characteristics are low transition barrier, ultrafast kinetics, and structural reversibility. They are rarely observed in molecular crystals, and hence the origin and mechanism are largely unexplored. Here we report the discovery of martensitic transition in single crystals of two different organic semiconductors. In situ microscopy, single-crystal X-ray diffraction, Raman and nuclear magnetic resonance spectroscopy, and molecular simulations combined indicate that the rotating bulky side chains trigger cooperative transition. Cooperativity enables shape memory effect in single crystals and function memory effect in thin film transistors. We establish a molecular design rule to trigger martensitic transition in organic semiconductors, showing promise for designing next-generation smart multifunctional materials.

DFT-Assisted Polymorph Identification from Lattice Raman Fingerprinting.

A combined experimental and theoretical approach, consisting of lattice phonon Raman spectroscopy and density functional theory (DFT) calculations, is proposed as a tool for lattice dynamics characterization and polymorph phase identification. To illustrate the reliability of the method, the lattice phonon Raman spectra of two polymorphs of the molecule 2,7-dioctyloxy[1]benzothieno[3,2-b]benzothiophene are investigated. We show that DFT calculations of the lattice vibrations based on the known crystal structures, including many-body dispersion van der Waals (MBD-vdW) corrections, predict experimental data within an accuracy of ≪5 cm-1 (≪0.6 meV). Due to the high accuracy of the simulations, they can be used to unambiguously identify different polymorphs and to characterize the nature of the lattice vibrations and their relationship to the structural properties. More generally, this work implies that DFT-MBD-vdW is a promising method to describe also other physical properties that depend on lattice dynamics like charge transport.

Unraveling Unprecedented Charge Carrier Mobility through Structure Property Relationship of Four Isomers of Didodecyl[1]benzothieno[3,2-b][1]benzothiophene.

The structural and electronic properties of four isomers of didodecyl[1]-benzothieno[3,2-b][1]benzothiophene (C12-BTBT) have been investigated. Results show the strong impact of the molecular packing on charge carrier transport and electronic polarization properties. Field-induced time-resolved microwave conductivity measurements unravel an unprecedented high average interfacial mobility of 170 cm(2) V(-1) s(-1) for the 2,7-isomer, holding great promise for the field of organic electronics.

Organic ferroelectric/semiconducting nanowire hybrid layer for memory storage.

Ferroelectric materials are important components of sensors, actuators and non-volatile memories. However, possible device configurations are limited due to the need to provide screening charges to ferroelectric interfaces to avoid depolarization. Here we show that, by alternating ferroelectric and semiconducting nanowires over an insulating substrate, the ferroelectric dipole moment can be stabilized by injected free charge carriers accumulating laterally in the neighboring semiconducting nanowires. This lateral electrostatic coupling between ferroelectric and semiconducting nanowires offers new opportunities to design new device architectures. As an example, we demonstrate the fabrication of an elementary non-volatile memory device in a transistor-like configuration, of which the source-drain current exhibits a typical hysteretic behavior with respect to the poling voltage. The potential for size reduction intrinsic to the nanostructured hybrid layer offers opportunities for the development of strongly miniaturized ferroelectric and piezoelectric devices.

Structural Evolution of an Organic Semiconducting Molecule onto a Soft Substrate.

The structural organization and evolution of the organic semiconducting molecule 2,7-dioctyloxy[1]benzothieno[3,2-b]-benzothiophene on a soft matrix is studied. Thin films of a blend formed from polystyrene and the molecule were prepared by spin-coating onto silicon substrates, which were subsequently studied by using a combination of microscopy and scattering techniques. The organic semiconducting molecule segregated to the surface and developed a phase with a different structure to the bulk, as in the case of a substrate induced phase observed previously. Under a solvent vapor annealing procedure, the growth of micrometer-sized tetragonal crystals onto the polymer surface was observed, which was not evidenced for the silicon substrates.

Investigation of the Qx -Qy Equilibrium in a Metal-Free Phthalocyanine.

Phthalocyanines (Pcs) have attracted a lot of interest as small molecules for organic electronics. However, some excited-state properties of metal-free phthalocyanines, as for example, the dynamics of the transition between the nondegenerate Qx and Qy states in a metal-free phthalocyanine, have not been fully established. This effect results in a blue-shifted shoulder with low intensity in the Pc fluorescence spectrum. This shoulder was suggested to be related to emission from the more energetic Qy state. By using ultrafast femtosecond transient absorption, we have found a clear equilibrium between the Qx and Qy state of metal-free phthalocyanines in solution.

Bulky end-capped [1]benzothieno[3,2-b]benzothiophenes: reaching high-mobility organic semiconductors by fine tuning of the crystalline solid-state order.

A series of bulky end-capped [1]benzothieno[3,2-b]benzothiophenes (BTBTs) are developed in order to tune the packing structure via terminal substitution. A coupled theoretical and experimental study allows us to identify 2,7-di-tert-butylBTBT as a new high-performance organic semiconductor with large and well-balanced transfer integrals, as evidenced by quantum-chemical calculations. Single-crystal field-effect transistors show a remarkable average saturation mobility of 7.1 cm(2) V(-1) s(-1) .

Optically switchable transistors by simple incorporation of photochromic systems into small-molecule semiconducting matrices.

The fabrication of multifunctional high-performance organic thin-film transistors as key elements in future logic circuits is a major research challenge. Here we demonstrate that a photoresponsive bi-functional field-effect transistor with carrier mobilities exceeding 0.2 cm(2) V(-1) s(-1) can be developed by incorporating photochromic molecules into an organic semiconductor matrix via a single-step solution processing deposition of a two components blend. Tuning the interactions between the photochromic diarylethene system and the organic semiconductor is achieved via ad-hoc side functionalization of the diarylethene. Thereby, a large-scale phase-segregation can be avoided and superior miscibility is provided, while retaining optimal π-π stacking to warrant efficient charge transport and to attenuate the effect of photoinduced switching on the extent of current modulation. This leads to enhanced electrical performance of transistors incorporating small conjugated molecules as compared with polymeric semiconductors. These findings are of interest for the development of high-performing optically gated electronic devices.

Substrate-induced phase of a [1]benzothieno[3,2-b]benzothiophene derivative and phase evolution by aging and solvent vapor annealing.

Substrate-induced phases (SIPs) are polymorphic phases that are found in thin films of a material and are different from the single crystal or "bulk" structure of a material. In this work, we investigate the presence of a SIP in the family of [1]benzothieno[3,2-b]benzothiophene (BTBT) organic semiconductors and the effect of aging and solvent vapor annealing on the film structure. Through extensive X-ray structural investigations of spin coated films, we find a SIP with a significantly different structure to that found in single crystals of the same material forms; the SIP has a herringbone motif while single crystals display layered π-π stacking. Over time, the structure of the film is found to slowly convert to the single crystal structure. Solvent vapor annealing initiates the same structural evolution process but at a greatly increased rate, and near complete conversion can be achieved in a short period of time. As properties such as charge transport capability are determined by the molecular structure, this work highlights the importance of understanding and controlling the structure of organic semiconductor films and presents a simple method to control the film structure by solvent vapor annealing.

X-ray structural investigation of nonsymmetrically and symmetrically alkylated [1]benzothieno[3,2-b]benzothiophene derivatives in bulk and thin films.

A detailed structural study of the bulk and thin film phases observed for two potential high-performance organic semiconductors has been carried out. The molecules are based on [1]benzothieno[3,2-b]benzothiophene (BTBT) as conjugated core and octyl side groups, which are anchored either symmetrically at both sides of the BTBT core (C8-BTBT-C8) or nonsymmetrically at one side only (C8-BTBT). Thin films of different thickness (8-85 nm) have been prepared by spin-coating for both systems and analyzed by combining specular and grazing incidence X-ray diffraction. In the case of C8-BTBT-C8, the known crystal structure obtained from single-crystal investigations is observed within all thin films, down to a film thickness of 9 nm. In the case of C8-BTBT, the crystal structure of the bulk phase has been determined from X-ray powder diffraction data with a consistent matching of experimental and calculated X-ray diffraction patterns (Rwp = 5.8%). The packing arrangement of C8-BTBT is similar to that of C8-BTBT-C8, that is, consisting of a lamellar structure with molecules arranged in a "herringbone" fashion, yet with lamellae composed of two head-to-head (or tail-to-tail as the structure is periodic) superimposed molecules instead of only one molecule for C8-BTBT-C8. As for C8-BTBT-C8, we demonstrate that the same phase is observed in bulk and thin films for C8-BTBT whatever the film thickness investigated.

Photophysical properties and electronic structure of retinylidene-chlorin-chalcones and analogues.

Synthetic chlorins can accommodate diverse substituents about the macrocycle perimeter. Simple auxochromes (e.g., vinyl, acetyl, phenyl) allow systematic tuning of spectral and photophysical features. More extensive spectral tailoring may be achieved by using more potent, highly conjugated substituents that themselves bring new absorption into a target spectral region, if deleterious excited-state quenching processes can be avoided. To explore such an expanded substituent space, herein the spectral and photophysical properties of four chlorin-chalcones are reported. The molecules are free base and zinc chlorins with substituents at the 13-position that include a chalcone and an extended chalcone derived by reaction of the 13-acetylchlorin with benzaldehyde and all-trans-retinal, respectively. Measurements of the spectral and photophysical properties (Φf, τs, kf, kic, kisc) are accompanied by density functional calculations that examine the characteristics of the frontier molecular orbitals. The chlorin-chalcones in nonpolar (toluene) and polar (dimethylsulfoxide) media exhibit bathochromically shifted (and intense) Qy absorption bands. The presence of the retinylidene group adds new absorption in the blue-green region where the chlorins are typically transparent; excitation in this region leads to quantitative formation of the chlorin Qy excited state. The spectral properties generally correlate with substituent effects on the frontier MOs. The four chlorin-chalcones in the solvent toluene have high fluorescence yields (0.24-0.30) and multi-nanosecond singlet excited-state lifetimes (3.7-8.4 ns), in addition to the added absorption imparted by the chalcone moiety. Collectively, the studies reported herein provide insight into the fundamental properties of chlorins and illustrate the utility of chalcones as a means of both tuning and augmenting the spectral properties of these chromophores.

Order, viscoelastic, and dielectric properties of symmetric and asymmetric alkyl[1]benzothieno[3,2-b][1]benzothiophenes.

The morphology, the viscoelastic, the dielectric properties and the dynamics of phase transformation are studied in symmetrically and asymmetrically substituted alkyl[1]benzothieno[3,2-b][1]benzothiophenes (C8-BTBT) by X-ray scattering, rheology, and dielectric spectroscopy. The interlayer spacing reflects the molecular and supramolecular ordering, respectively, in the symmetrically and asymmetrically substituted BTBTs. In the asymmetric BTBT, the core layer is double in size with a broader network of intermolecular interactions though the increased S-S contacts that is prerequisite for the development of high performance OFET devices. Two crystal states with elastic and viscoelastic responses were identified in the symmetric compound. In contrast, the SmA phase in the asymmetric compound is a viscoelastic solid. A path-dependent dielectric environment with a switchable dielectric permittivity was found in both compounds by cooling below 0 °C with possible implications to charge transport. The kinetics of phase transformation to the crystalline and SmA phases revealed a nucleation and growth mechanism with rates dominated by the low activation barriers.