Phase Transitions and Anisotropic Thermal Expansion in High Mobility Core‐expanded Naphthalene Diimide Thin Film Transistors

In situ grazing incidence wide‐angle X‐ray scattering (GIWAXS) is used to study the in situ thermal behavior of solution‐processed, high mobility core‐expanded naphthalene diimide thin films. A series of three different molecules is studied where the side‐chain branching position is systematically varied through the use of 2‐, 3‐ and 4‐branched N‐alkyl chains. For all molecules, a number of different phases and their associated phase transitions are observed with heating up to 200 °C. In situ GIWAXS measurements allow following significant variations of packing in each phase including crystalline coherence length, orientation, d‐spacing, and paracrystallinity, as well as, for the first time, thin film thermal expansion coefficients in both the in‐plane and out‐of‐plane direction. Relating these parameters with device measurements of quenched films, a striking correlation is found between high field‐effect mobilities and low in‐plane thermal expansion coefficients. This relationship indicates that high in‐plane thermal expansion coefficients are detrimental to in‐plane charge transport due to the formation of nanoscale defects in the critical first few monolayers upon quenching.     

Space‐Charge‐Stabilized Ferroelectric Polarization in Self‐Oriented Croconic Acid Films

Great progress has been made recently in molecular ferroelectrics with properties even comparable to those of inorganic ferroelectrics. However, it is difficult to develop basic thin films and devices for practical applications since most molecular ferroelectrics are uniaxial. The single polar axes of crystallites inside their films, if available, are usually oriented randomly. These can induce the components without contribution to ferroelectric polarization and a large depolarization electric field to suppress polarization. In this work, it is demonstrated that uniaxial croconic acid films in two‐terminal devices, deposited by thermal evaporation, can show effective ferroelectric polarization and nonvolatile memory switching behavior with small coercive fields of 11–30 kV cm^?1. The polar c‐axes in thick crystalline films (>500 nm) are found to be self‐oriented nearly at a desired direction. With the assistance of trapped charges, stable ferroelectric polarization can be achieved, in spite of the existence of nonferroelectric components. These may pave a way to utilize uniaxial molecular ferroelectrics for various applications, such as gate dielectrics, electrets, and memory devices.     

Uniform π‐System Alignment in Thin Films of Template‐Grown Dicarbonitrile‐Oligophenyls

The ordering and conformational properties of dicarbonitrile‐para‐oligophenyls are studied with complementary methods, namely X‐ray structure analysis, low‐temperature scanning tunneling microscopy, and near‐edge X‐ray absorption fine‐structure spectroscopy. The packing of the functionalized variants differs from their technologically interesting para‐oligophenyl counterparts, both in the bulk crystal phase and in thin films grown by organic molecular beam epitaxy (OMBE) under ultra‐high vacuum conditions on the Ag(111) surface. In the crystal phase, the conformation depends on the number n of phenyl rings, exhibiting an intriguing screw‐like structure in the case of n = 4 at room temperature as well as at 180 K. For OMBE‐grown thin films, the whole series acquires the same type of conformation, characterized by alternately twisted phenyl rings, similar to the pure oligophenyl species. However, for all tested molecules, the orientation of the molecular reference plane is uniform within the entire film and coincides with the surface plane. This contrasts with the herringbone ordering adopted by the phenyl backbones without the carbonitrile groups. Our results demonstrate how the functionalization of moieties with extended conjugated electron systems can help to improve the structural homogeneity in technologically relevant organic thin films.     

π‐Conjugated Molecules Crosslinked Graphene‐Based Ultrathin Films and Their Tunable Performances in Organic Nanoelectronics

Graphene‐based ultrathin films with tunable performances, controlled thickness, and high stability are crucial for their uses. The currently existing protocols, however, could hardly simultaneously meet these requirements. Using amino‐substituted π‐conjugated compounds, including 1,4‐diaminobenzene (DABNH2), benzidine (BZDNH2), and 5,10,15,20‐tetrakis (4‐aminophenyl)‐21H,23H‐porphine (TPPNH2), as cross‐linkages, a new protocol through which graphene oxide (GO) nanosheets can be anchored on solid supports with a high stability and controlled thickness via a layer‐by‐layer method is presented. A thermal annealing leads to the reduction of the films, and the qualities of the samples can be inherited by the as‐produced reduced GO films (RGO). When RGO films are integrated as source/drain electrodes in OFETs, tunable performances can be realized. The devices based on the BZDNH2‐crosslinked RGO electrodes exhibit similar electrical behaviors as those based on the non‐π‐conjugated compound crosslinked electrodes, while improved performances can be gained when those crosslinked by DABNH2 are used. The performances can be further improved when RGO films crosslinked by TPPNH2 are employed. This work likely paves a new avenue for graphene‐based films of tunable performances, controlled thickness, and high stability.     

The Relationship between Structural and Electrical Characteristics in Perylenecarboxydiimide‐Based Nanoarchitectures

The controlled assembly of the prototypical n‐type organic semiconductor N,N′‐1H,1H‐perfluorobutyl dicyanoperylenecarboxydiimide (PDIF‐CN₂) into ordered nanoarchitectures and the multiscale analysis of the correlation between their structural and their electrical properties is reported. By making use of the Langmuir–Blodgett (LB) technique, monolayers of PDIF‐CN₂ arranged in upright standing molecular packing on different substrates are formed. Postdeposition thermal treatment makes it possible to trigger a reorganization into layered ultrathin crystalline nanostructures, exhibiting structural and photophysical properties similar to those of microscopic crystals obtained by solvent‐induced precipitation. The controlled engineering of these molecular architectures on surfaces enables us to identify both a dependence of the monolayer resistance on the molecular tilt angle in vertical junctions and a pronounced charge‐transport anisotropy with enhanced transport along the π–π stacking direction of the PDI core. While a charge carrier mobility for electrons as high as 10^–2 cm² V^–1 s^–1 is determined in monolayer field‐effect transistors for the in‐plane direction, being the highest yet reported value for a n‐type LB monolayer, the out‐of‐plane mobility measured by conductive atomic force microscopy in multilayered structures is found to be one order of magnitude lower.     

Highly Sensitive Temperature Sensor: Ligand‐Treated Ag Nanocrystal Thin Films on PDMS with Thermal Expansion Strategy

Highly sensitive temperature sensors are designed by exploiting the interparticle distance–dependent transport mechanism in nanocrystal (NC) thin films based on a thermal expansion strategy. The effect of ligands on the electronic, thermal, mechanical, and charge transport properties of silver (Ag) NC thin films on thermal expandable substrates of poly(dimethylsiloxane) (PDMS) is investigated. While inorganic ligand‐treated Ag NC thin films exhibit a low temperature coefficient of resistance (TCR), organic ligand‐treated films exhibit extremely high TCR up to 0.5 K^?1, which is the highest TCR exhibited among nanomaterial‐based temperature sensors to the best of the authors' knowledge. Structural and electronic characterizations, as well as finite element method simulation and transport modeling are conducted to determine the origin of this behavior. Finally, an all‐solution based fabrication process is established to build Ag NC‐based sensors and electrodes on PDMS to demonstrate their suitability as low‐cost, high‐performance attachable temperature sensors.     

High Performance Roll‐to‐Roll Produced Fullerene‐Free Organic Photovoltaic Devices via Temperature‐Controlled Slot Die Coating

Solution‐processed organic photovoltaics (OPVs) have continued to show their potential as a low‐cost power generation technology; however, there has been a significant gap between device efficiencies fabricated with lab‐scale techniques—i.e., spin coating—and scalable deposition methods. Herein, temperature‐controlled slot die deposition is developed for the photoactive layer of OPVs. The influence of solution and substrate temperatures on photoactive films and their effects on power conversion efficiency (PCE) in slot die coated OPVs using a 3D printer‐based slot die coater are studied on the basis of device performance, molecular structure, film morphology, and carrier transport behavior. These studies clearly demonstrate that both substrate and solution temperatures during slot die coating can influence device performance, and the combination of hot substrate (120 °C) and hot solution (90 °C) conditions result in mechanically robust films with PCE values up to 10.0% using this scalable deposition method in air. The efficiency is close to that of state‐of‐the‐art devices fabricated by spin coating. The deposition condition is translated to roll‐to‐roll processing without further modification and results in flexible OPVs with PCE values above 7%. The results underscore the promising potential of temperature‐controlled slot die coating for roll‐to‐roll manufacturing of high performance OPVs.     

High Hole Mobility and Thickness‐Dependent Crystal Structure in α,ω‐Dihexylsexithiophene Single‐Monolayer Field‐Effect Transistors

Monolayer‐thickness two‐dimensional layers of α,ω‐dihexylsexithiophene (α,ω‐DH6T) exhibit field‐effect hole mobility of up to 0.032 cm² V^?1 s^?1, higher than previously reported for monolayers of other small‐molecule organic semiconductors. In situ measurements during deposition show that the source‐drain current saturates rapidly after the percolation of monolayer‐high islands, indicating that the electrical properties of α,ω‐DH6T transistors are largely determined by the first molecular monolayer. The α,ω‐DH6T monolayer consists of crystalline islands in which the long axes of molecules are oriented approximately perpendicular to the plane of the substrate surface. In‐plane lattice constants measured using synchrotron grazing‐incidence diffraction are larger in monolayer‐thickness films than the in‐plane lattice constants of several‐monolayer films and of previously reported thick‐film structures. Near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS) reveals that the larger in‐plane lattice constant of single‐monolayer films arises from a larger tilt of the molecular axis away from the surface normal. NEXAFS spectra at the C 1s and S 2p edges are consistent with a high degree of molecular alignment and with the local symmetry imposed by the thiophene ring. The high mobility of holes in α,ω‐DH6T monolayers can be attributed to the reduction of hole scattering associated with the isolation of the thiophene core from the interface by terminal hexyl chains.     

High‐Resolution Near‐Field Optical Investigation of Crystalline Domains in Oligomeric PQT‐12 Thin Films

The structure and morphology on different length scales dictate both the electrical and optical properties of organic semiconductor thin films. Using a combination of spectroscopic methods, including scanning near‐field optical microscopy, we study the domain structure and packing quality of highly crystalline thin films of oligomeric PQT‐12 with 100 nanometer spatial resolution. The pronounced optical anisotropy of these layers measured by polarized light microscopy facilitates the identification of regions with uniform molecular orientation. We find that a hierarchical order on three different length scales exists in these layers, made up of distinct well‐ordered dichroic areas at the ten‐micrometer‐scale, which are sub‐divided into domains with different molecular in‐plane orientation. These serve as a template for the formation of smaller needle‐like crystallites at the layer surface. A high degree of crystalline order is believed to be the cause of the rather high field‐effect mobility of these layers of 10^?3 cm² V^?1 s^?1, whereas it is limited by the presence of domain boundaries at macroscopic distances.     

Block‐Copolymer‐Templated Synthesis of Electroactive RuO2‐Based Mesoporous Thin Films

RuO₂‐based mesoporous thin films of optical quality are synthesized from ruthenium‐peroxo‐based sols using micelle templates made of amphiphilic polystyrene‐polyethylene oxide block copolymers. The mesoporous structure and physical properties of the RuO₂ films (mesoporous volume: 30%; pore diameter: ～30 nm) can be controlled by the careful tuning of both the precursor solution and thermal treatment (150–350 °C). The optimal temperature that allows control of both mesoporosity and nanocristallinity is strongly dependent on the substrate (silicon or fluorine‐doped tin oxide). The structure of the resulting mesoporous films are investigated using X‐ray diffraction, X‐ray photoelectron spectroscopy, and atomic force microscopy. Mesoporous layers are additionally characterized by transmission and scanning electron microscopy and ellipsometry while their electrochemical properties are analyzed via cyclic voltammetry. Thick mesoporous films of ruthenium oxide hydrates, RuO₂ · xH₂O, obtained using a thermal treatment at 280 °C, exhibit capacitances as high as 1000 ± 100 F g^?1 at a scan rate of 10 mV s^?1, indicating their potential application as electrode materials.     

Amphiphilic Poly(p‐phenylene)s for Self‐Organized Porous Blue‐Light‐Emitting Thin Films

Micro‐ and nanostructuring of conjugated polymers are of critical importance in the fabrication of molecular electronic devices as well as photonic and bandgap materials. The present report delineates the single‐step self‐organization of highly ordered structures of functionalized poly(p‐phenylene)s without the aid of either a controlled environment or expensive fabrication methodologies. Microporous films of these polymers, with a honeycomb pattern, were prepared by direct spreading of the dilute polymer solution on various substrates, such as glass, quartz, silicon wafer, indium tin oxide, gold‐coated mica, and water, under ambient conditions. The polymeric film obtained from C₁₂PPPOH comprises highly periodic, defect‐free structures with blue‐light‐emitting properties. It is expected that such microstructured, conjugated polymeric films will have interesting applications in photonic and optoelectronic devices. The ability of the polymer to template the facile micropatterning of nanomaterials gives rise to hybrid films with very good spatial dispersion of the carbon nanotubes.     

Ultralong Room‐Temperature Phosphorescence from Amorphous Polymer Poly(Styrene Sulfonic Acid) in Air in the Dry Solid State

Polymer‐based room‐temperature‐phosphorescent (RTP) materials are attractive alternatives to low‐molecular‐weight organic RTP compounds because they can form self‐standing transparent films with high thermal stability. However, their RTP lifetimes in air are usually short (<≈0.4 s). Here, the simple organic amorphous polymer, poly(styrene sulfonic acid) (PSS), exhibits an ultralong RTP lifetime in air when desiccated. The maximum lifetime is 1.22 s, which is three times that of previously reported RTP amorphous organic polymers. The lifetime can be controlled by the PSS molecular weight and by the ratio of sulfonic acid groups introduced into the polymer. The dry polymers should enable unprecedented molecular engineering in organic molecule‐based optoelectronic devices because of the self‐standing and thermal stability attributes.     

Phase‐Separation‐Induced Micropatterned Polymer Surfaces and Their Applications

We report a route to fabricate micropatterned polymer films with micro‐ or nanometer‐scale surface concavities by spreading polymer solutions on a non‐solvent surface. The route is simple, versatile, highly efficient, low‐cost, and easily accessible. The concavity density of the patterned films is tuned from 10⁶ to 10⁹ features cm^–2, and the concavity size is controlled in the range from several micrometers to less than 100 nm, by changing the film‐forming parameters including the polymer concentration, the temperature of the non‐solvent and the interactions between polymer, solvent, and non‐solvent. We further demonstrate that these concavity‐patterned films have significantly enhanced hydrophobicity, owing to the existence of the surface concavities, and their hydrophobicity could be controlled by the concavity density. These films have been used as templates to successfully fabricate convex‐patterned polymer films, inorganic TiO₂ microparticles, and NaCl nanocrystals. Their other potential applications are also discussed.     

Strongly Anisotropic Thermal Conductivity of Free‐Standing Reduced Graphene Oxide Films Annealed at High Temperature

Thermal conductivity of free‐standing reduced graphene oxide films subjected to a high‐temperature treatment of up to 1000 °C is investigated. It is found that the high‐temperature annealing dramatically increases the in‐plane thermal conductivity, K, of the films from ≈3 to ≈61 W m^?1 K^?1 at room temperature. The cross‐plane thermal conductivity, K_⊥, reveals an interesting opposite trend of decreasing to a very small value of ≈0.09 W m^?1 K^?1 in the reduced graphene oxide films annealed at 1000 °C. The obtained films demonstrate an exceptionally strong anisotropy of the thermal conductivity, K/K_⊥ ≈ 675, which is substantially larger even than in the high‐quality graphite. The electrical resistivity of the annealed films reduces to 1–19 Ω □^?1. The observed modifications of the in‐plane and cross‐plane thermal conductivity components resulting in an unusual K/K_⊥ anisotropy are explained theoretically. The theoretical analysis suggests that K can reach as high as ≈500 W m^?1 K^?1 with the increase in the sp² domain size and further reduction of the oxygen content. The strongly anisotropic heat conduction properties of these films can be useful for applications in thermal management.     

Synthesis and Processing of Monodisperse Oligo(fluorene‐co‐bithiophene)s into Oriented Films by Thermal and Solvent Annealing

A series of oligo(fluorene‐co‐bithiophene)s, OF2Ts , have been synthesized and characterized to investigate the effects of oligomer length and pendant aliphatic structure on glassy‐nematic mesomorphism. The OF2Ts comprising more than one repeat unit and their polymer analogue, PF2T , carrying 52 number‐average repeat units, possess the highest occupied molecular orbital energy level at ?5.3 ± 0.2 eV, but the anisotropic field‐effect mobilities increase with the oligomer length. Spin coating from high‐boiling chlorobenzene with and without subsequent exposure to saturated chlorobenzene vapor constitute solvent‐vapor annealing and quasi‐solvent annealing, respectively. Solvent‐vapor annealing yields monodomain glassy‐nematic films in which OF2Ts are aligned as well as with thermal annealing across a 2 cm diameter. Quasi‐solvent annealing, however, amounts to kinetically trapping a lower orientational order than solvent‐vapor or thermal annealing. While amenable to thermal annealing at elevated temperatures, PF2T shows no alignment at all following either strategy of solvent annealing.     

Synthesis,Structure, Electronic State,and Luminescent Properties of Novel Blue‐Light‐Emitting Aryl‐Substituted 9,9‐Di(4‐(di‐p‐tolyl)aminophenyl)fluorenes

Novel blue‐light‐emitting fluorene derivatives 5a–c and 7a–c containing bulky and highly emissive groups, namely pyrene, 10‐phenylanthracene‐9‐yl and 10‐(4′‐diphenylaminophenyl)anthracene‐9‐yl groups, as well as hole‐injecting/transporting triarylamines were synthesized. Single crystals of compounds 5a , 5c , 7a , and 7c were grown and their crystal structures were determined by X‐ray diffraction. The four fluorene derivatives have nonplanar molecular structures, which reduce the intermolecular interaction and the likelihood of molecular aggregation or excimer formation. No unwanted long‐wavelength emission was observed in the photoluminescence (PL) spectra of the 5a–c and 7a–c thin films. Their PL spectra reveal excellent thermal stability after annealing treatment under air and ambient light. All of the six compounds show high fluorescence quantum yields and outstanding thermal stabilities. The 2‐aryl and 2,7‐diaryl substituents at the fluorene molecule have a significant effect on the photophysical properties and the thermal characteristics. The six compounds show almost the same energy levels for the highest occupied molecular orbitals (HOMOs) of about ?5.20 eV, which allows effective hole injection. The C2‐ and C7‐aryl substituents play a relatively less‐important role in the HOMO energy levels, which depend mainly on the triphenylamino groups at the C9 position. The molecular orbitals, excitation energy, and emission energy were calculated to explain the real origin of their photophysical characteristics. The HOMOs are mainly localized on the triphenylamino groups at the C9 position, while the lowest unoccupied molecular orbitals (LUMOs) have a significant orbital density at the C2‐ and/or C7‐aryl substituents. Pure‐blue‐light‐emitting diodes based on 2,7‐diaryl‐9,9‐di(triarylamino)fluorenes were fabricated.     

Kinetically Controlled Crystallization in Conjugated Polymer Films for High‐Performance Organic Field‐Effect Transistors

Ordering of semiconducting polymers in thin films from the nano to microscale is strongly correlated with charge transport properties as well as organic field‐effect transistor performance. This paper reports a method to control nano to microscale ordering of poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2)) thin films by precisely regulating the solidification rate from the metastable state just before crystallization. The proposed simple but effective approach, kinetically controlled crystallization, achieves optimized P(NDI2OD‐T2) films with large polymer domains, long range ordered fibrillar structures, and molecular orientation preferable for electron transport leading to dramatic morphological changes in both polymer domain sizes at the micrometer scale and molecular packing structures at nanoscales. Structural changes significantly increase electron mobilities up to 3.43 ± 0.39 cm² V^?1 s^?1 with high reliability, almost two orders of enhancement compared with devices from naturally dried films. Small contact resistance is also obtained for electron injection (0.13 MΩ cm), low activation energy (62.51 meV), and narrow density of states distribution for electron transport in optimized thin films. It is believed that this study offers important insight into the crystallization of conjugated polymers that can be broadly applied to optimize the morphology of semiconducting polymer films for solution processed organic electronic devices.     

Direct Synthesis of Large‐Scale WTe2 Thin Films with Low Thermal Conductivity

Large‐scale, polycrystalline WTe₂ thin films are synthesized by atmospheric chemical vapor reaction of W metal films with Te vapor catalyzed by H₂Te intermediates, paving a way to understanding the synthesis mechanism for low bonding energy tellurides and toward synthesis of single‐crystalline telluride nanoflakes. Through‐plane and in‐plane thermal conductivities of single‐crystal WTe₂ flakes and polycrystalline WTe₂ thin films are measured for the first time. Nanoscale grains and disorder in WTe₂ thin films suppress the in‐plane thermal conductivity of WTe₂ greatly, which is at least 7.5 times lower than that of the single‐crystalline flakes.     

Enhancing the Charge Transport in Solution‐Processed Perylene Di‐imide Transistors via Thermal Annealing of Metastable Disordered Films

The introduction of side chains in π‐conjugated molecules is a design strategy widely exploited to increase molecular solubility thus improving the processability, while directly influencing the self‐assembly and consequently the electrical properties of thin films. Here, a multiscale structural analysis performed by X‐ray diffraction, X‐ray reflectivity, and atomic force microscopy on thin films of dicyanoperylene molecules decorated with either linear or branched side chains is reported. The substitution with asymmetric branched alkyl chains allows obtaining, upon thermal annealing, field‐effect transistors with enhanced transport properties with respect to linear alkyl chains. Branched chains induce molecular disorder during the film growth from solution, effectively favouring 2D morphology. Post‐deposition thermal annealing leads to a structural transition towards the bulk‐phase for molecules with branched chains, still preserving the 2D morphology and allowing efficient charge transport between crystalline domains. Conversely, molecules with linear chains self‐assemble into 3D islands exhibiting the bulk‐phase structure. Upon thermal annealing, these 3D islands keep their size constant and no major changes are observed in the organic field effect transistor characteristics. These findings demonstrate that the disorder generated by the asymmetric branched chains when the molecule is physisorbed in thin film can be instrumental for enhancing charge transport via thermal annealing.     

Room‐Temperature Self‐Organizing Characteristics of Soluble Acene Field‐Effect Transistors

We report on the room‐temperature self‐organizing characteristics of thin films of the organic small‐molecule semiconductor triethylsilylethynyl‐anthradithiophene (TES‐ADT) and its effect on the electrical properties of TES‐ADT‐based field‐effect transistors (FETs). The morphology of TES‐ADT films changed dramatically with time, and the field‐effect mobility of FETs based on these films increased about 100‐fold after seven days as a result of the change in molecular orientation from a tilted structure in the as‐prepared film to a well‐oriented structure in the final film. We found that the molecular movement is large enough to induce a conformational change to an energetically stable state in spin‐coated TES‐ADT films, because TES‐ADT has a low glass‐transition temperature (around room temperature). Our findings demonstrate that organic small‐molecule semiconductors that exhibit a low crystallinity immediately after spin‐coating can be changed into highly crystalline structures by spontaneous self‐organization of the molecules at room temperature, which results in improved electrical properties of FETs based on these semiconductors.