Highly Sensitive,Photocontrolled, Organic Thin‐Film Transistors Using Soluble Star‐shaped Conjugated Molecules

The highly photosensitive characteristics of organic thin‐film transistors (OTFTs) made using soluble star‐shaped oligothiophenes with four‐armed π‐conjugation paths, 4(HPBT)‐benzene and 4(HP3T)‐benzene molecules having a relatively high quantum yield, are reported. 4(HPBT)‐benzene‐based organic phototransistors (OPTs) exhibited high photosensitivity (～2500–4300 A W^?1) even with low optical powers (～6.8–30 µW cm^?2) at zero gate bias. The measured photosensitivity of the devices was much higher than that of inorganic single‐crystal Si‐based phototransistors, as well as that of other OPTs reported earlier. With the highly photosensitive characteristics of the 4(HPBT)‐benzene‐based OPTs, a high ratio of the on and off current switching of ～4 × 10⁴ with low optical power and low gate bias was observed. The slow relaxation of the photoinduced charges and charge‐trapping phenomena at the interface could lead to a reproducible memory operation for 4(HPBT)‐benzene‐based OPTs.     

Influence of Thiol Self‐Assembled Monolayer Processing on Bottom‐Contact Thin‐Film Transistors Based on n‐Type Organic Semiconductors

The performance of bottom‐contact thin‐film transistor (TFT) structures lags behind that of top‐contact structures owing to the far greater contact resistance. The major sources of the contact resistance in bottom‐contact TFTs are believed to reflect a combination of non‐optimal semiconductor growth morphology on the metallic contact surface and the limited available charge injection area versus top‐contact geometries. As a part of an effort to understand the sources of high charge injection barriers in n‐channel TFTs, the influence of thiol metal contact treatment on the molecular‐level structures of such interfaces is investigated using hexamethyldisilazane (HMDS)‐treated SiO₂ gate dielectrics. The focus is on the self‐assembled monolayer (SAM) contact surface treatment methods for bottom‐contact TFTs based on two archetypical n‐type semiconductors, α,ω‐diperfluorohexylquarterthiophene (DFH‐4T) and N,N′bis(n‐octyl)‐dicyanoperylene‐3,4:9,10‐bis(dicarboximide) (PDI‐8CN₂). TFT performance can be greatly enhanced, to the level of the top contact device performance in terms of mobility, on/off ratio, and contact resistance. To analyze the molecular‐level film structural changes arising from the contact surface treatment, surface morphologies are characterized by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The high‐resolution STM images show that the growth orientation of the semiconductor molecules at the gold/SAM/semiconductor interface preserves the molecular long axis orientation along the substrate normal. As a result, the film microstructure is well‐organized for charge transport in the interfacial region.     

n‐Type Quinoidal Oligothiophene‐Based Semiconductors for Thin‐Film Transistors and Thermoelectrics

Organic electronic devices have gained immense popularity in the last 30 years owing to their increasing performance. Organic thin‐film transistors (OTFTs) are one of the basic organic electronic devices with potential industrial applications. Another class of devices called organic thermoelectric (OTE) materials can directly transform waste heat into usable electrical power without causing any pollution. p‐Type transistors outperform n‐type transistors because the latter requires a lower orbital energy level for efficient electron injection and stable electron transport under ambient conditions. Aromatic building blocks can be utilized in constructing n‐type semiconductors. Quinoidal compounds are another promising platform for optoelectronic applications because of their unique properties. Since their discovery in 1970s, quinoidal oligothiophene‐based n‐type semiconductors have drawn considerable attention as candidates for high‐performance n‐type semiconductors in OTFTs and OTEs. Herein, the development history of quinoidal oligothiophene‐based semiconductors is summarized, with a focus on the molecular design and the influence of structural modification on molecular packing and thus the device performance of the corresponding quinoidal oligothiophene‐based semiconductors. Insights on the potential of quinoidal oligothiophenes for high‐performance n‐type OTFTs and OTEs are also provided.     

Bithienopyrroledione‐Based Copolymers,Versatile Semiconductors for Balanced Ambipolar Thin‐Film Transistors and Organic Solar Cells with V oc > 1 V

Conjugated polymer semiconductors P1 and P2 with bithienopyrroledione (bi‐TPD) as acceptor unit are synthesized. Their transistor and photovoltaic performances are investigated. Both polymers display high and balanced ambipolar transport behaviors in thin‐film transistors. P1‐ based devices show an electron mobility of 1.02 cm² V^?1 s^?1 and a hole mobility of 0.33 cm² V^?1 s^?1, one of the highest performance reported for ambipolar polymer transistors. The electron and hole mobilities of P2 transistors are 0.36 and 0.16 cm² V^?1 s^?1, respectively. The solar cells with PC₇₁BM as the electron acceptor and P1/P2 as the donor exhibit a high V _oc about 1.0 V, and a power conversion efficiency of 6.46% is observed for P1‐ based devices without any additives and/or post treatment. The high performance of P1 and P2 is attributed to their crystalline films and short π–π stacking distance (<3.5 Å). These results demonstrate (1) bi‐TPD is an excellent versatile electron‐deficient unit for polymer semiconductors and (2) bi‐TPD‐based polymer semiconductors have potential applications in organic transistors and organic solar cells.     

Diverse Organic Field‐Effect Transistor Sensor Responses from Two Functionalized Naphthalenetetracarboxylic Diimides and Copper Phthalocyanine Semiconductors Distinguishable Over a Wide Analyte Range

Naphthalenetetracarboxylic diimide derivatives (octyl “8” NTCDI, dimethylaminopropyl “DMP” NTCDI) and copper phthalocyanine (CuPc) are used to form a diverse organic field‐effect transistor (OFET) sensor array. CuPc and 8‐NTCDI are p‐channel and n‐channel semiconductors, respectively, showing expected and opposing responses to analytes. DMP‐NTCDI, on the other hand, because of its ionizable side chain, shows response directions and magnitudes that are not correlated to those of the other two. The result is a distinct response pattern and unambiguous recognition ability for individual analytes. The differences are even more dramatic if the time evolution of the responses is considered. The three‐response patterns obtained from representative polar, nonpolar, acidic, and basic vapors are all different, showing the potential for this approach in rapid, low‐cost electronic detection of volatile compounds.     

高性能顶栅结构有机薄膜晶体管

采用六联苯(p-6P)和氧钒酞菁(VOPc)作为有源层材料,利用弱外延生长技术制备有机薄膜晶体管(OTFT)。在相同的工艺条件下制备了顶栅结构(top-gate)和底栅结构(bottom-gate)两种器件构型,发现两种不同结构的OTFT器件特性存在较大的差异,top-gate OTFT的迁移率比bottom-gate OTFT高很多。在顶栅结构的器件构型中获得了较高的器件特性参数,迁移率达到1.6cm2/V.s。研究了弱外延生长技术应用在两种不同器件构型中的差异,并解释了顶栅结构OTFT迁移率较高的原因。     

High‐Performance and Stable Organic Thin‐Film Transistors Based on Fused Thiophenes

A series of new organic semiconductors for organic thin‐film transistors (OTFTs) using dithieno[3,2‐b:2′,3′‐d]thiophene as the core are synthesized. Their electronic and optical properties are investigated using scanning electron microscopy (SEM), X‐ray diffraction (XRD), UV‐vis and photoluminescence spectroscopies, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The compounds exhibit an excellent field‐effect performance with a high mobility of 0.42 cm² V^–1 s^–1 and an on/off ratio of 5 × 10⁶. XRD patterns reveal these films, grown by vacuum deposition, to be highly crystalline, and SEM reveals well‐interconnected, microcrystalline domains in these films at room temperature. TGA and DSC demonstrate that the phenyl‐substituted compounds possess excellent thermal stability. Furthermore, weekly shelf‐life tests (under ambient conditions) of the OTFTs based on the phenyl‐substituted compounds show that the mobility for the bis(diphenyl)‐substituted thiophene was almost unchanged for more than two months, indicating a high environmental stability.     

Rational Design of Organic Semiconductors for Texture Control and Self‐Patterning on Halogenated Surfaces

Understanding the interactions at interfaces between the materials constituting consecutive layers within organic thin‐film transistors (OTFTs) is vital for optimizing charge injection and transport, tuning thin‐film microstructure, and designing new materials. Here, the influence of the interactions at the interface between a halogenated organic semiconductor (OSC) thin film and a halogenated self‐assembled monolayer on the formation of the crystalline texture directly affecting the performance of OTFTs is explored. By correlating the results from microbeam grazing incidence wide angle X‐ray scattering (μGIWAXS) measurements of structure and texture with OTFT characteristics, two or more interaction paths between the terminating atoms of the semiconductor and the halogenated surface are found to be vital to templating a highly ordered morphology in the first layer. These interactions are effective when the separating distance is lower than 2.5 d_w, where d_w represents the van der Waals distance. The ability to modulate charge carrier transport by several orders of magnitude by promoting “edge‐on” versus “face‐on” molecular orientation and crystallographic textures in OSCs is demonstrated. It is found that the “edge‐on” self‐assembly of molecules forms uniform, (001) lamellar‐textured crystallites which promote high charge carrier mobility, and that charge transport suffers as the fraction of the “face‐on” oriented crystallites increases.     

Vertically Segregated Structure and Properties of Small Molecule–Polymer Blend Semiconductors for Organic Thin‐Film Transistors

A comprehensive structure and performance study of thin blend films of the small‐molecule semiconductor, 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene (diF‐TESADT), with various insulating binder polymers in organic thin‐film transistors is reported. The vertically segregated composition profile and nanostructure in the blend films are characterized by a combination of complementary experimental methods including grazing incidence X‐ray diffraction, neutron reflectivity, variable angle spectroscopic ellipsometry, and near edge X‐ray absorption fine structure spectroscopy. Three polymer binders are considered: atactic poly(α‐methylstyrene), atactic poly(methylmethacrylate), and syndiotactic polystyrene. The choice of polymer can strongly affect the vertical composition profile and the extent of crystalline order in blend films due to the competing effects of confinement entropy, interaction energy with substrate surfaces, and solidification kinetics. The variations in the vertically segregated composition profile and crystalline order in thin blend films explain the significant impacts of binder polymer choice on the charge carrier mobility of these films in the solution‐processed bottom‐gate/bottom‐contact thin‐film transistors.     

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.     

Ultrathin,Organic, Semiconductor/Polymer Blends by Scanning Corona‐Discharge Coating for High‐Performance Organic Thin‐Film Transistors

A new thin‐film coating process, scanning corona‐discharge coating (SCDC), to fabricate ultrathin tri‐isopropylsilylethynyl pentacene (TIPS‐PEN)/amorphous‐polymer blend layers suitable for high‐performance, bottom‐gate, organic thin‐film transistors (OTFTs) is described. The method is based on utilizing the electrodynamic flow of gas molecules that are corona‐discharged at a sharp metallic tip under a high voltage and subsequently directed towards a bottom electrode. With the static movement of the bottom electrode, on which a blend solution of TIPS‐PEN and an amorphous polymer is deposited, SCDC provides an efficient route to produce uniform blend films with thicknesses of less than one hundred nanometers, in which the TIPS‐PEN and the amorphous polymer are vertically phase‐separated into a bilayered structure with a single‐crystalline nature of the TIPS‐PEN. A bottom‐gate field‐effect transistor with a blend layer of TIPS‐PEN/polystyrene (PS) (90/10 wt%) operated at ambient conditions, for example, indeed exhibits a highly reliable device performance with a field‐effect mobility of approximately 0.23 cm² V^?1 s^?1: two orders of magnitude greater than that of a spin‐coated blend film. SCDC also turns out to be applicable to other amorphous polymers, such as poly(α‐methyl styrene) and poly(methyl methacrylate) and, readily combined with the conventional transfer‐printing technique, gives rise to micropatterned arrays of TIPS‐PEN/polymer films.     

Organic Thin‐Film Transistors with Anodized Gate Dielectric Patterned by Self‐Aligned Embossing on Flexible Substrates

An upscalable, self‐aligned patterning technique for manufacturing high‐ performance, flexible organic thin‐film transistors is presented. The structures are self‐aligned using a single‐step, multi‐level hot embossing process. In combination with defect‐free anodized aluminum oxide as a gate dielectric, transistors on foil with channel lengths down to 5 μm are realized with high reproducibility. Resulting on‐off ratios of 4 × 10⁶ and mobilities as high as 0.5 cm² V^?1 s^?1 are achieved, indicating a stable process with potential to large‐area production with even much smaller structures.     

Core‐Brominated Tetraazaperopyrenes as n‐Channel Semiconductors for Organic Complementary Circuits on Flexible Substrates

Organic thin‐film transistors (TFTs) are prepared by vacuum deposition and solution shearing of 2,9‐bis(perfluoroalkyl)‐substituted tetraazaperopyrenes (TAPPs) with bromine substituents at the aromatic core. The TAPP derivatives are synthesized by reacting known unsubstituted TAPPs with bromine in fuming sulphuric acid, and their electrochemical properties are studied in detail by cyclic voltammetry and modelled with density functional theory (DFT) methods. Lowest unoccupied molecular orbital (LUMO) energies and electron affinities indicate that the core‐brominated TAPPs should exhibit n‐channel semiconducting properties. Current‐voltage characteristics of the TFTs established electron mobilities of up to μ_n = 0.032 cm² V^?1 s^?1 for a derivative which was subsequently processed in the fabrication of a complementary ring oscillator on a flexible plastic substrate (PEN).     

Highly Efficient Microscopic Charge Transport within Crystalline Domains in a Furan‐Flanked Diketopyrrolopyrrole‐Based Conjugated Copolymer

Elucidating the interrelation between the molecular structure and charge transport properties in conjugated polymer thin films is an essential issue in developing the design principle of high‐performance polymer materials for application in organic electronics. In particular, the backbone planarity is suggested to be a key element that governs the transport performance, especially in recently developed donor–acceptor (D–A)‐type copolymers exhibiting high mobility, whereas the direct evaluation of the intrinsic transport performance, usually realized only within the small crystalline domains, is difficult by using conventional macroscopic measurements. Here, it is demonstrated that a D–A type copolymer, PDPPF‐DTT, which consists of furan‐flanked diketopyrrolopyrrole (DPP) and dithienothiophene (DTT) units in the conjugated backbone, exhibits a highly efficient charge transport performance within the crystalline domains with a remarkably low activation energy of less than 8 meV, based on microscopic measurements using field‐induced electron spin resonance spectroscopy. This high transport performance is primarily caused by the high backbone planarity realized by introducing furan‐flanked DPP and fused dithienothiophene units, which is demonstrated from the density functional theory calculations. This result provides a microscopic indication of the effectiveness of the present molecular design to produce a planar backbone and realize highly efficient charge transport performance.     

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

Organic semiconductors have sparked interest as flexible, solution processable, and chemically tunable electronic materials. Improvements in charge carrier mobility put organic semiconductors in a competitive position for incorporation in a variety of (opto‐)electronic applications. One example is the organic field‐effect transistor (OFET), which is the fundamental building block of many applications based on organic semiconductors. While the semiconductor performance improvements opened up the possibilities for applying organic materials as active components in fast switching electrical devices, the ability to make good electrical contact hinders further development of deployable electronics. Additionally, inefficient contacts represent serious bottlenecks in identifying new electronic materials by inhibiting access to their intrinsic properties or providing misleading information. Recent work focused on the relationships of contact resistance with device architecture, applied voltage, metal and dielectric interfaces, has led to a steady reduction in contact resistance in OFETs. While impressive progress was made, contact resistance is still above the limits necessary to drive devices at the speed required for many active electronic components. Here, the origins of contact resistance and recent improvement in organic transistors are presented, with emphasis on the electric field and geometric considerations of charge injection in OFETs.