Ultrathin Organic Semiconductor Research Articles

Stepwise Aggregation Control of PEDOT:PSS Enabled High-Conductivity, High-Resolution Printing of Polymer Electrodes for Transparent Organic Phototransistors.

Electrohydrodynamic (EHD) jet printing is a widely employed technology to create high-resolution patterns and thus has enormous potential for circuit production. However, achieving both high conductivity and high resolution in printed polymer electrodes is a challenging task. Here, by modulating the aggregation state of the conducting polymer in the solution and solid phases, a stable and continuous jetting of PEDOT:PSS is realized, and high-conductivity electrode arrays are prepared. The line width reaches less than 5 μm with a record-high conductivity of 1250 S/cm. Organic field-effect transistors (OFETs) are further developed by combining printed source/drain electrodes with ultrathin organic semiconductor crystals. These OFETs show great light sensitivity, with a specific detectivity (D*) value of 2.86 × 1014 Jones. In addition, a proof-of-concept fully transparent phototransistor is demonstrated, which opens up new pathways to multidimensional optical imaging.

High-Performance Organic Synaptic Transistors with an Ultrathin Active Layer for Neuromorphic Computing.

In recent years, much attention has been focused on two-dimensional (2D) material-based synaptic transistor devices because of their inherent advantages of low dimension, simultaneous read-write operation and high efficiency. However, process compatibility and repeatability of these materials are still a big challenge, as well as other issues such as complex transfer process and material selectivity. In this work, synaptic transistors with an ultrathin organic semiconductor layer (down to 7 nm) were obtained by the simple dip-coating process, which exhibited a high current switch ratio up to 106, well off state as low as nearly 10-12 A, and low operation voltage of -3 V. Moreover, various synaptic behaviors were successfully simulated including excitatory postsynaptic current, paired pulse facilitation, long-term potentiation, and long-term depression. More importantly, under ultrathin conditions, excellent memory preservation, and linearity of weight update were obtained because of the enhanced effect of defects and improved controllability of the gate voltage on the ultrathin active layer, which led to a pattern recognition rate up to 85%. This is the first work to demonstrate that the pattern recognition rate, a crucial parameter for neuromorphic computing can be significantly improved by reducing the thickness of the channel layer. Hence, these results not only reveal a simple and effective way to improve plasticity and memory retention of the artificial synapse via thickness modulation but also expand the material selection for the 2D artificial synaptic devices.

Microstructured Ultrathin Organic Semiconductor Film via Dip-Coating: Precise Assembly and Diverse Applications

ConspectusOrganic semiconductors (OSCs) have led to considerable progress in various fields owing to their intrinsic flexibility, adjustable chemical structures, multifunctionalities, and low processing cost. The optoelectronic properties of OSCs greatly depend on their aggregation state in molecular assemblies. To obtain ideal and reliable optoelectronic performances, a highly controlled assembly method for modulating OSC packing structures is indispensable. In particular, ultrathin OSC microstructures consisting of one to several molecular layers, known as microstructured ultrathin organic semiconductor films (MUOSFs), are of great importance for both fundamental research and applications of OSCs. However, most reported film thicknesses or molecular layer numbers of ultrathin OSC films/microstructures are “average values” rather than “real values” and are not molecularly precise. The lack of an effective and general assembly strategy seriously hinders the progress of MUOSFs. In this Account, we summarize our recent progress in exploring the assembly strategy, the underlying mechanism, and diverse applications of MUOSFs prepared via dip-coating under optimized conditions.Dip-coating, with an intrinsic three-phase contact line, has a great advantage to precisely assemble ultrathin OSC films/microstructures. As an evaporation-controlled method, the assembly processes used in dip-coating are susceptible to multiple factors, such as pulling speed, molecular structures, and solution properties. Under the guidance of our proposed “Balance Principle” for adjusting these factors, uniform and continuous MUOSFs can be obtained over a large area. Under optimized conditions, the number of molecular layers of MUOSFs can be customized with monolayer precision, which is significant for investigating the charge transport mechanism of OSCs. At the same time, the ultrathin and partial coverage characteristics of MUOSFs are beneficial to fabricate flexible and transparent devices for wearable electronics. Notably, the morphologies and coverage of ultrathin microstripes can be well tuned by the pulling speed, which is highly desired for high-performance bio/chemical sensors, because the large specific surface area provides abundant reactive sites.The feasibility and generality of our assembly method demonstrate that dip-coated MUOSFs have excellent advantages for fundamental research and applications of OSC materials, based on which many promising future works around MUOSFs can be extended to more related fields by engineering molecular orientations, packing structures, film morphologies, coverage, etc. The work on the assembly and applications of MUOSFs might offer the potential to promote the progress of organic electronics.

OFET chemical sensors: Chemical sensors based on ultrathin organic field‐effect transistors

AbstractIn the past decades, a variety of organic field‐effect transistor (OFET) based sensors have been reported, holding great promise in physical sensing, chemical sensing, biosensing, medical sensing and functionalized integrated sensor arrays, thanks to the superiorities of OFETs, including fabrication versatility and high sensitivity to external stimulus. However, the sensing performance still needs to be enhanced to meet practical application requirements, especially high sensitivity and short response time. Among all the effective strategies, ultrathin organic semiconductor (OSC) film devices show significantly promoted sensing performance, and the fabrication methods to obtain these ultrathin OSC films are of importance and diversity. Here, a brief review focused on ultrathin (including molecular monolayer) OSC film fabrication methods is presented, including high‐vacuum thermal evaporation, solution‐processable methods, and the approach of combining ultrathin films with heterojunctions. Additionally, applications of ultrathin OFET‐based chemical and biological sensors are summarized. Finally, the outlook and challenges ahead are discussed. We hope this review can offer guidance for the fabrication methods and potential applications of ultrathin OFET‐based chemical and biological sensors in the future. © 2020 Society of Industrial Chemistry

Nano-confined crystallization of organic ultrathin nanostructure arrays with programmable geometries

Fabricating ultrathin organic semiconductor nanostructures attracts wide attention towards integrated electronic and optoelectronic applications. However, the fabrication of ultrathin organic nanostructures with precise alignment, tunable morphology and high crystallinity for device integration remains challenging. Herein, an assembly technique for fabricating ultrathin organic single-crystal arrays with different sizes and shapes is achieved by confining the crystallization process in a sub-hundred nanometer space. The confined crystallization is realized by controlling the deformation of the elastic topographical templates with tunable applied pressures, which produces organic nanostructures with ordered crystallographic orientation and controllable thickness from less than 10 nm to ca. 1 μm. The generality is verified for patterning various typical solution-processable materials with long-range order and pure orientation, including organic small molecules, polymers, metal-halide perovskites and nanoparticles. It is anticipated that this technique with controlling the crystallization kinetics by the governable confined space could facilitate the electronic integration of organic semiconductors.

Photocurrent Engineering of Silicon Nanowire Field‐Effect Transistors by Ultrathin Poly(3‐hexylthiophene)

AbstractPhotoresponse on the silicon nanowire (SiNW) and organic semiconductor interfaces embedding an insulating barrier is understood by a photogating effect associated with charge separations. Still elusive one is when the thickness of organic semiconductor is decreased down to a few molecular layers, where the photoresponse can be strongly altered by the spatial confinement of photoinduced carriers. In this work, the photoresponse modulation of SiNW field‐effect transistors coated with an ultrathin organic semiconductor poly(3‐hexylthiophene) (P3HT) is reported, where the P3HT layer thickness is changed by an order of magnitude. In the absence of laser illumination, the P3HT interface on SiNW slightly decreases the SiNW channel current, irrespective of the P3HT thickness. Upon the laser illumination, the thick case of P3HT layers (≈100 nm) supports the dissociation of photoinduced electrons and holes, hence the photogating effect largely decreases the SiNW channel current. When the P3HT layer thickness is decreased down to the P3HT hole diffusion length (≈8.5 nm), the photoinduced P3HT holes screen out the P3HT interface states, effectively increasing the SiNW channel current up to the intrinsic level. The method of engineering the organic semiconductor thickness suggests a complementary optoelectronic operation, verifying the controllable photoresponse within a compact nanodevice.

Ambipolar Organic Field‐Effect Transistors Based on a Dual‐Function, Ultrathin and Highly Crystalline 2,9‐didecyldinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene (C10‐DNTT) Layer

AbstractUltrathin organic semiconductor thin films have great value in the investigation of carrier transport behavior in organic field‐effect transistors (OFETs). Here, the dual solution shearing (DSS) method is adopted to deposit ultrathin and closely packed 2,9‐didecyldinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]thiophene (C10‐DNTT) films. This smooth, ultrathin and highly crystalline DSS‐processed C10‐DNTT layer can be utilized in p–n stacking bilayer ambipolar OFETs. It has two major functions in the bilayer OFET: (i) it acts as the p‐channel active layer; and (ii) it serves as the growth template of the upper n‐type semiconductor layer. The closely packed alkyl side chains of the C10‐DNTT molecules behave like long alkyl chains in the self‐assembled monolayer and can assist the packing and orientation of the following layer. The F16CuPc layer grown on DSS‐processed C10‐DNTT shows crystallized lamellae structure. The smooth C10‐DNTT surface can suppress the trap states and enhance the charge transfer between the p–n layer interface. The drain‐source current (IDS) in the p‐channel and n‐channel shows a threefold and fivefold increase compared with two‐step thermal evaporation. These findings demonstrate the potential of using solution‐processed ultrathin organic semiconductor in multilayer organic electronics, which cannot be easily achieved by the conventional thermal evaporation approach.

Growth of Ultrathin Organic Semiconductor Microstripes with Thickness Control in the Monolayer Precision

Organische Halbleiter: Das Wachstum ultradünner (<15 nm), kontinuierlicher und mikrostrukturierter organischer Hableiter(OSC)-Filme mit definierten Monoschichten ist wichtig für die organische Elektronik. Dendritische OSC-Mikrostreifen aus einheitlichen und definierten Mono- bis Hexaschichten (ca. 1.6 bis 12 nm; siehe Bild) wurden über einen Tauchbeschichtungsprozess hergestellt.

Growth of Ultrathin Organic Semiconductor Microstripes with Thickness Control in the Monolayer Precision

Organic semiconductors: Growth of ultrathin (<15 nm), continuous, and microstructured organic semiconductor (OSC) films at monolayer precision is important for organic electronics and a challenging task. Uniform monolayer to hexalayer (ca. 1.6–12 nm; see picture) dendritic OSC microstripes at monolayer precision are grown by a dip-coating process.

High Performance Field‐Effect Ammonia Sensors Based on a Structured Ultrathin Organic Semiconductor Film

High performance organic field-effect transistor (OFET)-based ammonia sensors are demonstrated with ultrathin (4-6 molecular layers) dendritic microstripes of an organic semiconductor prepared via dip-coating. These sensors exhibit high sensitivity, fast response/recovery rate, good selectivity, low concentration detection ability, and reliable reversibility, as well as stability. Such a performance represents great progress in the field of OFET-based sensors.