Supramolecular Order of Solution‐Processed Perylenediimide Thin Films: High‐Performance Small‐Channel n‐Type Organic Transistors

N,N′‐1H,1H‐perfluorobutyl dicyanoperylenecarboxydiimide (PDIF‐CN₂), a soluble and air stable n‐type molecule, undergoes significant reorganization upon thermal annealing after solution deposition on several substrates with different surface energies. Interestingly, this system exhibits an exceptional edge‐on orientation regardless of the substrate chemistry. This preferential orientation is rationalized in terms of strong intermolecular interactions between the PDIF‐CN₂ molecules. The presence of a pronounced π–π stacking is confirmed by combining near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS), dynamic scanning force microscopy (SFM) and surface energy measurements. The remarkable charge carrier mobility measured in field‐effect transistors, using both bottom‐ and top‐contact (bottom‐gate) configurations, underlines the importance of strong intermolecular interactions for the realization of high performing devices.     

Electric Field Tuning Molecular Packing and Electrical Properties of Solution‐Shearing Coated Organic Semiconducting Thin Films

Recent improvements in solution‐coated organic semiconductors (OSCs) evidence their high potential for cost‐efficient organic electronics and sensors. Molecular packing structure determines the charge transport property of molecular solids. However, it remains challenging to control the molecular packing structure for a given OSC. Here, the application of alternating electric fields is reported to fine‐tune the crystal packing of OSC solution‐shearing coated at ambient conditions. First, a theoretical model based on dielectrophoresis is developed to guide the selection of the optimal conditions (frequency and amplitude) of the electric field applied through the solution‐shearing blade during coating of OSC thin films. Next, electric field‐induced polymorphism is demonstrated for OSCs with both herringbone and 2D brick‐wall packing motifs in 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene and 6,13‐bis(triisopropylsilylethynyl) pentacene, respectively. Favorable molecular packing can be accessible in some cases, resulting in higher charge carrier mobilities. This work provides a new approach to tune the properties of solution‐coated OSCs in functional devices for high‐performance printed electronics.     

Solution‐Processed Monolayer Organic Crystals for High‐Performance Field‐Effect Transistors and Ultrasensitive Gas Sensors

This work innovatively develops a dual solution‐shearing method utilizing the semiconductor concentration region close to the solubility limit, which successfully generates large‐area and high‐performance semiconductor monolayer crystals on the millimeter scale. The monolayer crystals with poly(methyl methacrylate) encapsulation show the highest mobility of 10.4 cm² V^?1 s^?1 among the mobility values in the reported solution‐processed semiconductor monolayers. With similar mobility to multilayer crystals, light is shed on the charge accumulation mechanism in organic field‐effect transistors (OFETs), where the first layer on interface bears the most carrier transport task, and the other above layers work as carrier suppliers and encapsulations to the first layer. The monolayer crystals show a very low dependency on channel directions with a small anisotropic ratio of 1.3. The positive mobility–temperature correlation reveals a thermally activated carrier transport mode in the monolayer crystals, which is different from the band‐like transport mode in multilayer crystals. Furthermore, because of the direct exposure of highly conductive channels, the monolayer crystal based OFETs can sense ammonia concentrations as low as 10 ppb. The decent sensitivity indicates the monolayer crystals are potential candidates for sensor applications.     

Exciton‐Charge Annihilation in Organic Semiconductor Films

Time‐resolved optical spectroscopy is used to investigate exciton‐charge annihilation reactions in blended films of organic semiconductors. In donor–acceptor blends where charges are photogenerated via excitons, pulsed optical excitation can deliver a sufficient density of temporally overlapping excitons and charges for them to interact. Transient absorption spectroscopy measurements demonstrate clear signatures of exciton‐charge annihilation reactions at excitation densities of ≈10¹⁸ cm^?3. The strength of exciton‐charge annihilation is consistent with a resonant energy transfer mechanism between fluorescent excitons and resonantly absorbing charges, which is shown to generally be strong in organic semiconductors. The extent of exciton‐charge annihilation is very sensitive not only to fluence but also to blend morphology, becoming notably strong in donor–acceptor blends with nanomorphologies optimized for photovoltaic operation. The results highlight both the value of transient optical spectroscopy to interrogate exciton‐charge annihilation reactions and the need to recognize and account for annihilation reactions in other transient optical investigations of organic semiconductors.     

High‐Mobility Air‐Stable Solution‐Shear‐Processed n‐Channel Organic Transistors Based on Core‐Chlorinated Naphthalene Diimides

High charge carrier mobility solution‐processed n‐channel organic thin‐film transistors (OTFTs) based on core‐chlorinated naphthalene tetracarboxylic diimides (NDIs) with fluoroalkyl chains are demonstrated. These OTFTs were prepared through a solution shearing method. Core‐chlorination of NDIs not only increases the electron mobilities of OTFTs, but also enhances their air stability, since the chlorination in the NDI core lowers the lowest unoccupied molecular orbital (LUMO) levels. The air‐stability of dichlorinated NDI was better than that of the tetrachlorinated NDIs, presumably due to the fact that dichlorinated NDIs have a denser packing of the fluoroalkyl chains and less grain boundaries on the surface, reducing the invasion pathway of ambient oxygen and moisture. The devices of dichlorinated NDIs exhibit good OTFT performance, even after storage in air for one and a half months. Charge transport anisotropy is observed from the dichlorinated NDI. A dichlorinated NDI with ?CH₂C₃F₇ side chains reveals high mobilities of up to 0.22 and 0.57 cm² V^?1 s^?1 in parallel and perpendicular direction, respectively, with regard to the shearing direction. This mobility anisotropy is related to the grain morphology. In addition, we find that the solution‐shearing deposition affects the molecular orientation in the crystalline thin films and lowers the d(001)‐spacing (the out‐of‐plane interlayer spacing), compared to the vapor‐deposited thin films. Core‐chlorinated NDI derivatives are found to be highly suitable for n‐channel active materials in low‐cost solution‐processed organic electronics.     

Quantifying the Energy Barriers and Elucidating the Charge Transport Mechanisms across Interspherulite Boundaries in Solution‐Processed Organic Semiconductor Thin Films

Grain boundaries act as bottlenecks to charge transport in devices comprising polycrystalline organic active layers. To improve device performance, the nature and resulting impact of these boundaries must be better understood. The densities and energy levels of shallow traps within and across triethylsilylethynyl anthradithiophene (TES ADT) spherulites are quantified. The trap density is 7 × 10¹⁰ cm^?2 in devices whose channels reside within a single spherulite and up to 3 × 10¹¹ cm^?2 for devices whose channels span a spherulite boundary. The activation energy for charge transport, E_A, increases from 34 meV within a spherulite to 50–66 meV across a boundary, depending on the angle of molecular mismatch. Despite being molecular in nature, these E_A’s are more akin to those found for charge transport in polymer semiconductors. Presumably, trapped TES ADT at the boundary can electrically connect neighboring spherulites, similar to polymer chains connecting crystallites in polymer semiconductor thin films.     

New Configuration of Solid‐State Neutron Detector Made Possible with Solution‐Based Semiconductor Processing

The unique benefit of solution‐based fabrication of solid‐state p‐n junctions is demonstrated for radiation detection. In particular, an in situ inorganic semiconductor synthesis and film deposition facilitates a novel neutron detector configuration consisting of a host inorganic semiconductor matrix impregnated with a guest neutron sensitizing material. Spectroscopic investigations of the structural order of the top detector active layer indicate that it consists of interpenetrating networks of the host semiconductor nanocrystals and sensitizing guest material that self‐assemble during film formation. The host semiconductor network exhibits a good charge transport as evidenced by steady‐state photoconductivity measurements. The detectors developed indicate high sensitivity to ionizing radiation and a demonstrated ability to detecting thermal neutrons.     

High Photoconductive Responsivity in Solution‐Processed Polycrystalline Organic Composite Films

We demonstrate a novel approach for enhancing photoconductive responsivity (R) using a solution‐based organic semiconductor composite that yields R approaching 25 AW^?1, which is two to three orders of magnitude higher than the R in films comprising a single molecular component. We present extensive studies of photoconductivity, photoluminescence, and crystalline structural order that elucidate the mechanisms underlying this high photoconductive responsivity. The high R is found to arise from high photoconductive gain (82) due to a long mobile hole lifetime stemming from a prolonged occupation of electrons in deep traps generated at interfacial regions between the molecular crystallites.     

Gate Capacitance‐Dependent Field‐Effect Mobility in Solution‐Processed Oxide Semiconductor Thin‐Film Transistors

Solution‐processed oxide semiconductors (OSs) used as channel layer have been presented as a solution to the demand for flexible, cheap, and transparent thin‐film transistors (TFTs). In order to produce high‐performance and long‐sustainable portable devices with the solution‐processed OS TFTs, the low‐operational voltage driving current is a key issue. Experimentally, increasing the gate‐insulator capacitances by high‐k dielectrics in the OS TFTs has significantly improved the field‐effect mobility of the OS TFTs. But, methodical examinations of how the field‐effect mobility depends on gate capacitance have not been presented yet. Here, a systematic analysis of the field‐effect mobility on the gate capacitances in the solution‐processed OS TFTs is presented, where the multiple‐trapping‐and‐release and hopping percolation mechanism are used to describe the electrical conductivity of the nanocrystalline and amorphous OSs, respectively. An intuitive single‐piece expression showing how the field‐effect mobility depends on gate capacitance is developed based on the aforementioned mechanisms. The field‐effect mobility, depending on the gate capacitances, of the fabricated ZnO and ZnSnO TFTs clearly follows the theoretical prediction. In addition, the way in which the gate insulator properties (e.g., gate capacitance or dielectric constant) affect the field‐effect mobility maximum in the nanocrystalline ZnO and amorphous ZnSnO TFTs are investigated.     

Effect of Non‐Chlorinated Mixed Solvents on Charge Transport and Morphology of Solution‐Processed Polymer Field‐Effect Transistors

Using non‐chlorinated solvents for polymer device fabrication is highly desirable to avoid the negative environmental and health effects of chlorinated solvents. Here, a non‐chlorinated mixed solvent system, composed by a mixture of tetrahydronaphthalene and p­‐xylene, is described for processing a high mobility donor‐acceptor fused thiophene‐diketopyrrolopyrrole copolymer (PTDPPTFT4) in thin film transistors. The effects of the use of a mixed solvent system on the device performance, e.g., charge transport, morphology, and molecular packing, are investigated. p‐Xylene is chosen to promote polymer aggregation in solution, while a higher boiling point solvent, tetrahydronaphthalene, is used to allow a longer evaporation time and better solubility, which further facilitates morphological tuning. By optimizing the ratio of the two solvents, the charge transport characteristics of the polymer semiconductor device are observed to significantly improve for polymer devices deposited by spin coating and solution shearing. Average charge carrier mobilities of 3.13 cm² V^?1 s^?1 and a maximum value as high as 3.94 cm² V^?1 s^?1 are obtained by solution shearing. The combination of non‐chlorinated mixed solvents and the solution shearing film deposition provide a practical and environmentally‐friendly approach to achieve high performance polymer transistor devices.     

Controllable Molecular Doping and Charge Transport in Solution‐Processed Polymer Semiconducting Layers

Here, controlled p‐type doping of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV) deposited from solution using tetrafluoro‐tetracyanoquinodimethane (F4‐TCNQ) as a dopant is presented. By using a co‐solvent, aggregation in solution can be prevented and doped films can be deposited. Upon doping the current–voltage characteristics of MEH‐PPV‐based hole‐only devices are increased by several orders of magnitude and a clear Ohmic behavior is observed at low bias. Taking the density dependence of the hole mobility into account the free hole concentration due to doping can be derived. It is found that a molar doping ratio of 1 F4‐TCNQ dopant per 600 repeat units of MEH‐PPV leads to a free carrier density of 4 × 10²² m^?3. Neglecting the density‐dependent mobility would lead to an overestimation of the free hole density by an order of magnitude. The free hole densities are further confirmed by impedance measurements on Schottky diodes based on F4‐TCNQ doped MEH‐PPV and a silver electrode.     

有机半导体薄膜的光学和电学性质研究

采用真空热蒸镀技术制备了NPB有机半导体薄膜和单层夹心结构器件,通过透射谱测量研究了薄膜的光学能隙、折射率和消光系数等光学性质,结果表明有机半导体薄膜具有直接带隙半导体的光学性质,并且其折射率色散性质遵循单振子模型.另外,通过分析器件的电流-电压特性研究了薄膜的电导率、载流子迁移率和载流子浓度等电学性质.这些实验结果对于有机光电子器件的结构设计具有一定的参考价值.     

Porous Organic Field‐Effect Transistors for Enhanced Chemical Sensing Performances

The thin‐film structures of chemical sensors based on conventional organic field‐effect transistors (OFETs) can limit the sensitivity of the devices toward chemical vapors, because charge carriers in OFETs are usually concentrated within a few molecular layers at the bottom of the organic semiconductor (OSC) film near the dielectric/semiconductor interface. Chemical vapor molecules have to diffuse through the OSC films before they can interact with charge carriers in the OFET conduction channel. It has been demonstrated that OFET ammonia sensors with porous OSC films can be fabricated by a simple vacuum freeze‐drying template method. The resulted devices can have ammonia sensitivity not only much higher than the pristine OFETs with thin‐film structure but also better than any previously reported OFET sensors, to the best of our knowledge. The porous OFETs show a relative sensitivity as high as 340% ppm^?1 upon exposure to 10 parts per billion (ppb) NH₃. In addition, the devices also exhibit decent selectivity and stability. This general and simple strategy can be applied to a wide range of OFET chemical sensors to improve the device sensitivity.     

Detection of X‐Rays by Solution‐Processed Cesium‐Containing Mixed Triple Cation Perovskite Thin Films

Materials and technology development for designing innovative and efficient X‐ray radiation detectors is of utmost importance for a wide range of applications ranging from security to medical imaging. Here, highly sensitive direct X‐ray detectors based on novel cesium (Cs)‐based triple cation mixed halide perovskite thin films are reported. Despite being in a thin film form, the devices exhibit a remarkably high X‐ray sensitivity of (3.7 ± 0.1) µC Gy^?1 cm^?2 under short‐circuit conditions. At a small reverse bias of 0.4 V, the sensitivity further increases by orders of magnitude reaching a record value of (97 ± 1) µC Gy^?1 cm^?2 which surpasses state‐of‐the‐art inorganic large‐area detectors (a‐Se and poly‐CZT). Based on detailed structural, electrical, and spectroscopic investigations, the exceptional sensitivity of the triple cation Cs perovskite is attributed to its high ambipolar mobility‐lifetime product as well as to the formation of a pure stable perovskite phase with a low degree of energetic disorder, due to an efficient solution‐based alloying of individual n‐ and p‐type perovskite semiconductors.     

Micro‐/Nanostructured Highly Crystalline Organic Semiconductor Films for Surface‐Enhanced Raman Spectroscopy Applications

The utilization of inorganic semiconductors for surface‐enhanced Raman spectroscopy (SERS) has attracted enormous interest. However, despite the technological relevance of organic semiconductors for enabling inexpensive, large‐area, and flexible devices via solution processing techniques, these π‐conjugated systems have never been investigated for SERS applications. Here for the first time, a simple and versatile approach is demonstrated for the fabrication of novel SERS platforms based on micro‐/nanostructured 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C8‐BTBT) thin films via an oblique‐angle vapor deposition. The morphology of C8‐BTBT thin films is manipulated by varying the deposition angle, thus achieving highly favorable 3D vertically aligned ribbon‐like micro‐/nanostructures for a 90° deposition angle. By combining C8‐BTBT semiconductor films with a nanoscopic thin Au layer, remarkable SERS responses are achieved in terms of enhancement (≈10⁸), stability (>90 d), and reproducibility (RSD < 0.14), indicating the great promise of Au/C8‐BTBT films as SERS platforms. Our results demonstrate the first example of an organic semiconductor‐based SERS platform with excellent detection characteristics, indicating that π‐conjugated organic semiconductors have a great potential for SERS applications.     

Enhancement of Anomalous Hall Effect via Interfacial Scattering in Metal‐Organic Semiconductor Fex(C60)1−x Granular Films Near the Metal‐Insulator Transition

Ferromagnetic metal‐insulator granular films suffer from superparamagnetism, which causes a decrease in the values and temperature stabilities of the anomalous Hall effect (AHE). In this work, organic semiconductor (OSC) fullerene (C₆₀), instead of the traditional inorganic insulators, is used as the matrix and a series of Fe_x(C₆₀)_1?x (x = 0.58–0.91) granular films are fabricated. By utilizing the strong metal/OSC interfacial hybridization, the temperature stability of both magnetization and AHE is significantly improved, and the disordered scattering and consequently the anomalous Hall coefficient is enhanced. The saturated anomalous Hall resistivity of Fe_0.58(C₆₀)_0.42 is 74 µΩ cm at 300 K, which is over three times larger than that of Fe_0.59(SiO₂)_0.41 granular film, and it remains 63 µΩ cm at 2 K. The anomalous Hall coefficient of Fe_0.58(C₆₀)_0.42 is 9.9 × 10^?8 Ω cm G^?1, which is four orders larger than that of pure Fe and larger than most of the existing inorganic granular films. The roles of the intergrain Coulomb interaction, skew‐scattering, side‐jump, and intrinsic mechanism in AHE are evaluated. These results indicate that the organic materials have clear advantages in developing anomalous Hall devices.     

Gold‐Nanoparticle‐Doped TiO2 Semiconductor Thin Films: Optical Characterization

A simple procedure for creating titania sol–gel‐based semiconductor thin films is described. Gold nanoparticles are doped homogeneously into the precursor mixture and the particles are homogeneously distributed in the resultant films when prepared using spin‐coating. The effects of particle loading and annealing temperature on the optical properties of the resultant films are characterized. Ellipsometry, X‐ray diffraction, atomic force microscopy, and surface plasmon spectroscopy are used to monitor the crystallization and porosity changes during film synthesis.     

Underlying Mechanism of Inkjet Printing of Uniform Organic Semiconductor Films Through Antisolvent Crystallization

An underlying mechanism is reported for the formation of highly uniform crystalline organic semiconductor films by the double‐shot inkjet printing (IJP) technique utilizing antisolvent crystallization. It is demonstrated that the ability to form uniform films with this technique can be attributed to the unique nature of the initial contact dynamics between the chemically different microdroplets before occurrence of solute crystallization. Experiments are conducted systematically where a single microdroplet is over‐deposited by the IJP technique on a chemically different sessile droplet, for ten kinds of pure and miscible solvent combinations. The subsequent behavior is observed by high speed camera. The initial contact dynamics can be classified into three dramatically different cases that are respectively referred to as wetting, dewetting, and sinking. These phenomena are unique to microdroplets and the conditions for the occurrence of each type of phenomenon can be consistently explained by the fact that the initial contact dynamics are driven by the difference of surface tension of the liquids. Among the three kinds of dynamics, the wetting phenomenon creates a thin solution layer on the antisolvent droplet surface and can be used thus to manufacture uniform semiconductor films, where the coffee ring effect can be eliminated.