Self-Assembled Metallic Nanowire-Based Vertical Organic Field-Effect Transistor

Kleemann H., Günther A.A., Leo K., Lüssem B.

Vertical organic thin-film transistors (VOTFTs) are promising devices to overcome the transconductance and cut-off frequency restrictions of horizontal organic thin-film transistors. The basic physical mechanisms of VOTFT operation, however, are not well understood and VOTFTs often require complex patterning techniques using self-assembly processes which impedes a future large-area production. In this contribution, high-performance vertical organic transistors comprising pentacene for p-type operation and C60 for n-type operation are presented. The static current-voltage behavior as well as the fundamental scaling laws of such transistors are studied, disclosing a remarkable transistor operation with a behavior limited by injection of charge carriers. The transistors are manufactured by photolithography, in contrast to other VOTFT concepts using self-assembled source electrodes. Fluorinated photoresist and solvent compounds allow for photolithographical patterning directly and strongly onto the organic materials, simplifying the fabrication protocol and making VOTFTs a prospective candidate for future high-performance applications of organic transistors.

Chen W., Rinzler A., Guo J.

Poisson and drift-diffusion equations are solved in a three-dimensional device structure to simulate graphene-based vertical field effect transistors (GVFETs). Operation mechanisms of the GVFET with and without punched holes in the graphene source contact are presented and compared. The graphene-channel Schottky barrier can be modulated by gate electric field due to graphene's low density of states. For the graphene contact with punched holes, the contact barrier thinning and lowering around punched hole edge allow orders of magnitude higher tunneling current compared to the region away from the punched hole edge, which is responsible for significant performance improvement as already verified by experiments. Small hole size is preferred due to less electrostatic screening from channel inversion layer, which gives large electric field around the punched hole edge, thus, leading to a thinner and lower barrier. Bilayer and trilayer graphenes as the source contact degrade the performance improvement because stronger electrostatic screening leads to smaller contact barrier lowering and thinning. High punched hole area percentage improves current performance by allowing more gate electric field to modulate the graphene-channel barrier. Low effective mass channel material gives better on-off current ratio.

Azulai D., Cohen E., Markovich G.

Gold/silver nanowires (NWs) of controlled diameters were synthesized from catalytic metal seed particles at the substrate/solution interface. Small seed nanoparticles of three different sizes: ~1 nm (11 gold atoms), ~1.4 nm (~55 gold atoms), and homemade nanoparticles of ~2 nm were used. By varying a single type of seed particle concentration in the growth solution, the NW diameters and morphology could be controlled, between bundles of ultrathin NWs of ~2-3 nm diameter to thicker isolated single NWs with a mean diameter of ~16 nm. In addition, the catalytic reduction rate leading to NW growth was found to be seed size dependent at small seed sizes (

Lemaitre M.G., Donoghue E.P., McCarthy M.A., Liu B., Tongay S., Gila B., Kumar P., Singh R.K., Appleton B.R., Rinzler A.G.

An improved process for graphene transfer was used to demonstrate high performance graphene enabled vertical organic field effect transistors (G-VFETs). The process reduces disorder and eliminates the polymeric residue that typically plagues transferred films. The method also allows for purposely creating pores in the graphene of a controlled areal density. Transconductance observed in G-VFETs fabricated with a continuous (pore-free) graphene source electrode is attributed to modulation of the contact barrier height between the graphene and organic semiconductor due to a gate field induced Fermi level shift in the low density of electronic-states graphene electrode. Pores introduced in the graphene source electrode are shown to boost the G-VFET performance, which scales with the areal pore density taking advantage of both barrier height lowering and tunnel barrier thinning. Devices with areal pore densities of 20% exhibit on/off ratios and output current densities exceeding 10(6) and 200 mA/cm(2), respectively, at drain voltages below 5 V.

Ben-Sasson A.J., Tessler N.

While organic transistors' performances are continually pushed to achieve lower power consumption, higher working frequencies, and higher current densities, a new type of organic transistors characterized by a vertical architecture offers a radically different design approach to outperform its traditional counterparts. Naturally, the distinct vertical architecture gives way to different governing physical ground rules and structural key features such as the need for an embedded transparent electrode. In this paper, we make use of a zero-frequency electric field-transparent patterned electrode produced through block-copolymer self-assembly based lithography to control the performances of the vertical organic field effect transistor (VOFET) and to study its governing physical mechanisms. Unlike other VOFET structures, this design, involving well-defined electrode architecture, is fully tractable, allowing for detailed modeling, analysis, and optimization. We provide for the first time a complete account of the physics underpinning the VOFET operation, considering two complementary mechanisms: the virtual contact formation (Schottky barrier lowering) and the induced potential barrier (solid-state triode-like shielding). We demonstrate how each mechanism, separately, accounts for the link between controllable nanoscale structural modifications in the patterned electrode and the VOFET performances. For example, the ON/OFF current ratio increases by up to 2 orders of magnitude when the perforations aspect ratio (height/width) decreases from ∼0.2 to ∼0.1. The patterned electrode is demonstrated to be not only penetrable to zero-frequency electric fields but also transparent in the visible spectrum, featuring uniformity, spike-free structure, material diversity, amenability with flexible surfaces, low sheet resistance (20-2000 Ω sq(-1)) and high transparency (60-90%). The excellent layer transparency of the patterned electrode and the VOFET's exceptional electrical performances make them both promising elements for future transparent and/or efficient organic electronics.

van de Groep J., Spinelli P., Polman A.

We present a transparent conducting electrode composed of a periodic two-dimensional network of silver nanowires. Networks of Ag nanowires are made with wire diameters of 45-110 nm and a pitch of 500, 700, and 1000 nm. Anomalous optical transmission is observed, with an averaged transmission up to 91% for the best transmitting network and sheet resistances as low as 6.5 Ω/sq for the best conducting network. Our most dilute networks show lower sheet resistance and higher optical transmittance than an 80 nm thick layer of ITO sputtered on glass. By comparing measurements and simulations, we identify four distinct physical phenomena that govern the transmission of light through the networks: all related to the excitation of localized surface plasmons and surface plasmon polaritons on the wires. The insights given in this paper provide the key guidelines for designing high-transmittance and low-resistance nanowire electrodes for optoelectronic devices, including thin-film solar cells. For the latter, we discuss the general design principles to use the nanowire electrodes also as a light trapping scheme.

Barnes T.M., Reese M.O., Bergeson J.D., Larsen B.A., Blackburn J.L., Beard M.C., Bult J., van de Lagemaat J.

Networks made of single-walled carbon nanotubes (SWNTs) and metallic nanowire networks, graphene, and ultra-thin metal films have all been proposed as replacements for transparent conducting oxides (TCOs) in photovoltaic and other applications. However, only limited comparisons of nanostructured networks and TCOs are available. Several common figures of merit that are often used to compare the electrical and optical performance of the transparent contacts are evaluated here, and the merits of each method of comparison are discussed. Calculating the current loss due to absorption in the TCO is the most useful metric for evaluating new materials for use in solar cells with well-defined sheet resistance requirements and known quantum efficiencies. The ‘Haacke’ figure of merit, ΦH, correlates fairly well with current loss and is a good metric for evaluating electro-optical performance for more general applications. The analyses presented here demonstrate that silver nanowire networks are much closer to achieving optimal electrical and optical properties than carbon-based networks.

Yue Y., Liu P., Zhang Z., Han X., Ma E.

Three sets of uniaxial tensile tests have been performed in situ in transmission electron microscopy/high-resolution electron microscopy on Cu nanowires (NWs) to accurately map out the sample size dependence of elastic strain limit. Atomic-resolution evidence was obtained for an exceedingly large recoverable strain (as much as 7.2%) that can be sustained in the lattice of a single-crystalline Cu NW with a diameter of ∼5.8 nm. This ultrahigh elastic strain is consistent with the predictions from molecular dynamics simulations for nanowires and approaches the ideal elastic limit predicted for Cu by ab initio calculations.

McCarthy M.A., Liu B., Donoghue E.P., Kravchenko I., Kim D.Y., So F., Rinzler A.G.

Efficient organic light-emitting transistors use carbon nanotubes as the source electrode.

Chao Y., Chen C., Zan H., Meng H.

A polymer vertical transistor with an on/off current ratio higher than 10 4 is demonstrated. The proposed space-charge limited transistor (SCLT) uses a metal-grid base containing high-density submicrometre openings to modulate the vertical space-charge-limited current (SCLC). The key to obtaining a high on/off current ratio is to reduce the leakage current of SCLT. In this paper, an improved device structure that isolates the grid metal by using both top and bottom insulating layers is demonstrated. Then, with an identical proposed structure, the geometric design is also found to significantly influence the on/off ratio over 3 orders of magnitude. The competition between the SCLC and the grid to collector leakage current is analysed. Finally, the influence of tetrafluoro-tetracyano-quinodimethane doping on the transistor characteristics is investigated. The results are important for the design of polymer vertical transistors with high on/off ratios. (Some figures in this article are in colour only in the electronic version)

Kumar A., Zhou C.

The search for materials that can replace tin-doped indium oxide (ITO) as the leading transparent conductive electrode (TCE) has intensified significantly in the past few years, motivated by the ever-increasing price of indium. Materials such as carbon nanotube (CNT) films, graphene films, metal nanowire gratings, and random networks have been at the forefront of research in this direction. A paper by Wu et al. in this issue discusses the use of solution-processed graphene as the TCE in organic light-emitting devices. Advantages such as large-scale fabrication at relatively less expense, compatibility with flexible substrates, and improving performance have significantly contributed to their case as potential candidates for TCEs. Demonstrations of various display and photovoltaic devices using TCEs made of these materials, with performances rivaling those employing ITO, have provided the research community with encouragement to explore new materials and to address the associated scientific and technological challenges.

Ben-Sasson A.J., Avnon E., Ploshnik E., Globerman O., Shenhar R., Frey G.L., Tessler N.

We report the design and implementation of a vertical organic field effect transistor which is compatible with standard device fabrication technology and is well described by a self consistent device model. The active semiconductor is a film of C60 molecules, and the device operation is based on the architecture of the nanopatterned source electrode. The relatively high resolution fabrication process and maintaining the low-cost and simplicity associated with organic electronics, necessitates unconventional fabrication techniques such as soft lithography. Block copolymer self-assembled nanotemplates enable the production of conductive, gridlike metal electrode. The devices reported here exhibit On/Off ratio of 104.

Azulai D., Belenkova T., Gilon H., Barkay Z., Markovich G.

The preparation of conductive and transparent gold/silver nanowire mesh films is reported. The nanowires formed after the reduction of the metal ions was triggered and a thin growth solution film was spread on a substrate. Metal reduction progressed within a template of a highly concentrated surfactant liquid crystalline mesostructure formed on the substrate during film drying to form ordered bundles of ultrathin nanowires. The films exhibited metallic conductivity over large areas, high transparency, and flexibility.

Reineke S., Lindner F., Schwartz G., Seidler N., Walzer K., Lüssem B., Leo K.

Light-emitting diodes based on organic materials (known as OLEDs) are emerging as an attractive technology for a variety of lighting and display applications. If the performances of white-light OLEDs are improved, for example, they could be used to produce large-area lighting sources. That will require efficiencies on a par with existing technologies such as fluorescent tubes, which produce around 70 lumens per watt. That benchmark — 90 lumens per watt in fact — has now been achieved with OLEDs that make use of a novel emitter layer structure with high internal quantum efficiency, and high-index glass substrates to boost outcoupling efficiency. Before practical applications are possible, matters of cost, manufacturing methods and longevity need to be addressed, but the goal would be a future light source with a potentially smaller carbon footprint than today's technologies. Light-emitting diodes based on organic materials (known as OLEDs) have a number of attractive qualities that could make them the light sources of choice for the future. Unfortunately until now they have never reached the power efficiencies of fluorescent tubes. Here, the engineering of white OLEDs with power efficiencies at least as high as that of standard fluorescent tubes brings the future a little closer. The development of white organic light-emitting diodes1 (OLEDs) holds great promise for the production of highly efficient large-area light sources. High internal quantum efficiencies for the conversion of electrical energy to light have been realized2,3,4. Nevertheless, the overall device power efficiencies are still considerably below the 60–70 lumens per watt of fluorescent tubes, which is the current benchmark for novel light sources. Although some reports about highly power-efficient white OLEDs exist5,6, details about structure and the measurement conditions of these structures have not been fully disclosed: the highest power efficiency reported in the scientific literature is 44 lm W-1 (ref. 7). Here we report an improved OLED structure which reaches fluorescent tube efficiency. By combining a carefully chosen emitter layer with high-refractive-index substrates8,9, and using a periodic outcoupling structure, we achieve a device power efficiency of 90 lm W-1 at 1,000 candelas per square metre. This efficiency has the potential to be raised to 124 lm W-1 if the light outcoupling can be further improved. Besides approaching internal quantum efficiency values of one, we have also focused on reducing energetic and ohmic losses that occur during electron–photon conversion. We anticipate that our results will be a starting point for further research, leading to white OLEDs having efficiencies beyond 100 lm W-1. This could make white-light OLEDs, with their soft area light and high colour-rendering qualities, the light sources of choice for the future.

Sekitani T., Zaitsu K., Noguchi Y., Ishibe K., Takamiya M., Sakurai T., Someya T.

By using state-of-the-art printing technologies and functional inks, we have demonstrated organic nonvolatile flexible random-access-memory matrices with a nondestructive read-out capability and a time-continuous current output; these functionalities have not been simultaneously achieved even by silicon-based conventional memory. A memory cell comprising three transistors becomes possible with inkjet printing and other solution-based processes, which can use ferroelectric copolymer ink comprising poly(vinylidenefluoride-co-trifluoroethylene) and insulating ink comprising polyimide precursors properly within the planer plastic substrate. A large ldquo1 : 0rdquo current ratio of 10 5 is observed in air when it is annealed at 135degC , which is sufficiently low to be compatible with many plastic substrates. When stored in air, the ldquo1 : 0rdquo ratio was still 10 4 after 15 days and 10 3 after 5 months, which is sufficient for practical applications. Furthermore, human-scale communication sheets were manufactured as the first demonstration utilizing large-area organic memories.

Pyo G., Heo S.J., Kim D., Yu M., Kim J., Cha S., Kwon H., Jang J.E.

Chen L., Wang Y., Zhang J., Chen L., Zhai J., Song J.

Zhong S., Chen K., Wang S., Manshaii F., Jing N., Wang K., Liu S., Zhou Y.

Harnessing the unique attributes of metal-based nanowires (MNWs), such as their adaptability, high aspect ratio and conductivity, this review elucidates their burgeoning role as a distinct class of nanomaterials poised to revolutionize sensor technologies. We provide an in-depth examination of MNW assembly methods, highlighting procedural details, foundational principles and performance metrics. Manufacturing electrochemical biosensors and field-effect transistor (FET) biosensors by MNWs offers advantages such as enhanced sensitivity, improved signal-to-noise ratios and increased surface area for efficient biomolecule immobilization. MNWs contribute to precise and reliable biosensing platforms, optimizing the performance of these devices for various applications, such as diagnostics and environmental monitoring. Electrochemical biosensors are noted for their speed, cost-effectiveness, ease of use and compatibility with compact instrumentation, offering potential for precise biomarker quantification. Meanwhile, FET biosensors demonstrate the potential for early-stage biomarker identification and pharmaceutical applications with nanoscale materials like MNWs, thereby enhancing their detection capabilities. Additionally, we explore the prospects of integrating machine learning and digital health with MNWs in electrical biosensing, charting an innovative path for future advancements in this field. This advancement is facilitated by their electronic properties, compact design and compatibility with existing technologies. We expect this review to highlight future trends and challenges in the use of MNWs for biosensors.

Ponomarenko Vladimir P., Popov Victor, Shuklov Ivan, Ivanov Victor V., Razumov Vladimir F.

Photosensing based on colloidal quantum dots (CQDs) is a rapidly developing area of infrared photoelectronics. The use of colloidal quantum dots markedly simplifies the manufacture, decreases the restrictions to the pixel pitch of the photosensitive elements, and reduces the production cost, which facilitates the wide use of IR sensors in various technological systems. This paper is the first exhaustive overeview of the architectures, methods of manufacturing and basic properties of photonic sensors based on colloidal quantum dots of compounds of Group II, IV and VI elements. Characteristic features of the synthesis and roles of the ligands and CQD morphology in the design of photosensors are considered in detail. The structures of photoresistive, photodiode and phototransistor elements based on HgTe, HgSe, PbS and PbSe CQDs, which are sensitive in various spectral ranges, are described. The main parameters of the most advanced optoelectronic devices based on colloidal quantum dot structures are presented. The key trends in the development of this area are analyzed.The bibliography includes 361 references.

Vieira D.H., Nogueira G.L., Merces L., Bufon C.C., Alves N.

AbstractProposals for new architectures that shorten the length of the transistor channel without the need for high‐end techniques are the focus of very recent breakthrough research. Although vertical and electrolyte‐gate transistors are previously developed separately, recent advances have introduced electrolytes into vertical transistors, resulting in electrolyte‐gated vertical field‐effect transistors (EGVFETs), which feature lower power consumption and higher capacitance. Here, EGVFETs are employed to study the charge transport mechanism of spray‐pyrolyzed zinc oxide (ZnO) films to develop a new photosensitive switch concept. The EGVFET's diode cell revealed a current‐voltage relationship arising from space‐charge‐limited current (SCLC), whereas its capacitor cell provided the field‐effect role in charge accumulation in the device's source perforations. The findings elucidate how the field effect causes a continuous shift in SCLC regimes, impacting the switching dynamics of the transistor. It is found ultraviolet light may mimic the field effect, i.e., a pioneering demonstration of current switching as a function of irradiance in an EGVFET. The research provides valuable insights into the charge transport characterization of spray‐pyrolyzed ZnO‐based transistors, paving the way for future nano‐ and optoelectronic applications.

Xie Y., Ding C., Jin Q., Zheng L., Xu Y., Xiao H., Cheng M., Zhang Y., Yang G., Li M., Li L., Liu M.

AbstractWith the rapid development of advanced technologies in the Internet of Things era, higher requirements are needed for next‐generation electronic devices. Fortunately, organic thin film transistors (OTFTs) provide an effective solution for electronic skin and flexible wearable devices due to their intrinsic features of mechanical flexibility, lightweight, simple fabrication process, and good biocompatibility. So far considerable efforts have been devoted to this research field. This article reviews recent advances in various promising and state‐of‐the‐art OTFTs as well as related integrated circuits with the main focuses on: (I) material categories of high‐mobility organic semiconductors for both individual transistors and integrated circuits; (II) effective device architectures and processing techniques for large‐area fabrication; (III) important performance metrics of organic integrated circuits and realization of digital and analog devices for future smart life; (IV) applicable analytical models and design flow to accelerate the circuit design. In addition, the emerging challenges of OTFT‐based integrated circuits, such as transistor uniformity and stability are also discussed, and the possible methods to solve these problems at both transistor and circuit levels are summarized.

Park T., Kim M., Lee E.K., Hur J., Yoo H.

AbstractOrganic semiconductors have great potential to revolutionize electronics by enabling flexible and eco‐friendly manufacturing of electronic devices on plastic film substrates. Recent research and development led to the creation of printed displays, radio‐frequency identification tags, smart labels, and sensors based on organic electronics. Over the last 3 decades, significant progress has been made in realizing electronic devices with unprecedented features, such as wearable sensors, disposable electronics, and foldable displays, through the exploitation of desirable characteristics in organic electronics. Neverthless, the down‐scalability of organic electronic devices remains a crucial consideration. To address this, efforts are extensively explored. It is of utmost importance to further develop these alternative patterning methods to overcome the downscaling challenge. This review comprehensively discusses the efforts and strategies aimed at overcoming the limitations of downscaling in organic semiconductors, with a particular focus on four main areas: 1) lithography‐compatible organic semiconductors, 2) fine patterning of printing methods, 3) organic material deposition on pre‐fabricated devices, and 4) vertical‐channeled organic electronics. By discussing these areas, the full potential of organic semiconductors can be unlocked, and the field of flexible and sustainable electronics can be advanced.

Maria de Andrade D., Merces L., Nawaz A., Bof Bufon C.C.

Nawaz A., Merces L., Ferro L.M., Sonar P., Bufon C.C.

The development of flexible and conformable devices, whose performance can be maintained while being continuously deformed, provides a significant step towards the realization of next-generation wearable and e-textiles applications. Organic field-effect transistors (OFETs) are particularly interesting for flexible and lightweight products, because of their low-temperature solution processability, and the mechanical flexibility of organic materials that endows OFETs the natural compatibility with plastic and biodegradable substrates. Here, an in-depth review of two competing flexible OFET technologies, planar and vertical OFETs (POFETs and VOFETs, respectively) is provided. The electrical, mechanical, and physical properties of POFETs and VOFETs are critically discussed, with a focus on four pivotal applications (integrated logic circuits, light-emitting devices, memories, and sensors). It is pointed out that the flexible function of the relatively newer VOFET technology, along with its perspective on advancing the applicability of flexible POFETs, has not been reviewed so far, and the direct comparison regarding the performance of POFET- and VOFET-based flexible applications is most likely absent. With discussions spanning printed and wearable electronics, materials science, biotechnology, and environmental monitoring, this contribution is a clear stimulus to researchers working in these fields to engage towards the plentiful possibilities that POFETs and VOFETs offer to flexible electronics. This article is protected by copyright. All rights reserved.

Zhang H., Li H., Wang F., Song X., Xu Z., Wei D., Zhang J., Dai Z., Ren Y., Ye Y., Ren X., Yao J.

The synthesis of organic-inorganic hybrid perovskites has greatly facilitated the development of high-performance optoelectronic devices such as photovoltaics, light-emitting diodes, lasers, and photodetectors. We report a CH 3 NH 3 PbI 3-x Cl x vertical-structure field-effect phototransistor (VFEPT) using Au/Ag NWs as transparent source electrode (TSE) and exhibiting wideband absorption in the 380 to 800nm range. Due to the ultra-short channel (78 nm) of VFEPT, multiple circulating holes in laser-induced electron-hole pairs, thus achieving excellent photoconductivity. We further tested the photoelectric performance of VFEPT, which showed that the device had the responsivity (R) is 82.99A/W, the external quantum efficiency (EQE) is 3.3 ×10 4 % and the specific detectivity (D*) is 1.86×10 12 Jones at V G =-2V, V SD =-5V. When V G =2.5 V, V SD =5 V, R reaches 109.15 A/W, EQE reaches 3.3 ×10 4 % and D* reaches 2.45×10 12 Jones. Organic-inorganic hybrid perovskites provide a new approach for high-performance vertical-structured ultra-short channel perovskite photodetectors.

Sun Y., Wang Y., Yuan Q., He N., Wen D.

Neuromorphic technology is the next stage in the evolution of high-performance computing, with its ability to dramatically improve data processing and learning. Hence, the exploration of synaptic electronic devices with multiple excitation modes is the main area of concern. In this work, a vertical organic ferroelectric synaptic transistor (VOFST) capable of achieving the synaptic plasticity is demonstrated, profit by its nanoscale channel length and unique working principle, which exhibits excellent gate modulation capability and good synaptic properties. Based on the excellent tunability of VOFST in various excitation modes, a “Morse code” decoding scheme and the application of the signal identification of over-threshold are proposed. Most importantly, the internal carrier dynamics modulated by VOFST's ferroelectric polarization induction enables the transistor to directly process the temporal information for image recognition tasks. This work guides the development of synaptic devices and provides a platform to realize the neuromorphic functions in electronic field.

Cai S., Zhu H., Jiang C., Wang X., Xu S., Lv W., Sun L., Peng Y.

Vertical organic phototransistors with embedded source electrode (Emb-VOPTs) have the advantage of low dark current due to the blockage of dark injection from the source electrode surface. Among the function layers of Emb-VOPTs, the source electrode plays an important role in providing effective modulation of the electric field in the channel by the gate voltage. By fabrication of the source electrode of Emb-VOPTs, complex patterning techniques using lithography or self-assembly processes are often required which impedes future large-area production. Therefore, development for simpler and cheaper technology is of urgent need. In this letter, we report on fabrication technology for Emb-VOPTs based on vacuum deposition of metal/insulator strips with shadow mask as the source electrode/barrier layer. With this simple and reliable technology, small molecular Emb-VOPTs can be fabricated with solely vacuum thermal deposition without the need of additional pattern processes. With vacuum deposited Al strips as the source electrode, LiF strips as barrier layer and copper phthalocyanine (CuPc) as the photoactive organic channel layer, reasonable good performance was achievable, a photoresponsivity of 293 mA/W, an external quantum efficiency ( EQE ) of 56%, and a specific detectivity of $1.15\times10$ 11 Jones were obtained. By replacing CuPc with C 60 (50 nm)/CuPc(50 nm) heterojunction bilayer, a much higher improved performance at $V_{g}=-100$ V and $P_{int}=0.4$ mW/cm 2 , $R$ of 5078 mA/W, EQE of 969%, $D^\ast $ of $7.78\times10$ 11 Jones, are achieved. The present results demonstrate unambiguously that vacuum deposited metal/insulator strips as the source electrode/barrier layer could be the competitive candidate for low- cost production of Emb-VOPTs.

Dahal D., Paudel P.R., Kaphle V., Radha Krishnan R.K., Lüssem B.

Organic field-effect transistors (OFETs) have shown great potential for applications that require low temperature deposition on large and flexible substrates. To increase their performance, in particular a high transconductance and transit frequency, the transistor channel length has to be scaled into the submicrometer regime, which can be easily achieved in vertical organic field effect transistors (VOFETs). However, despite high performance observed in VOFETs, these transistors usually suffer from short channel effects like weak saturation of the drain current and direct source-drain leakage resulting in large off currents. Here, we study the influence of the injection barrier at the source electrode on the OFF currents, on/off ratio, and transconductance of vertical OFETs. We use two semiconducting materials, 2,6-diphenyl anthracene (DPA), and C60 to vary the injection barrier at the source electrode and are able to show that increasing the Schottky barrier at the source electrode can decrease the direct source/drain leakage by 3 orders of magnitude. However, the increased injection barrier at the source electrode comes at the expense of an increased contact resistance, which in turn will decrease its transconductance and transit frequency. With the help of a 2D drift-diffusion simulation we show that the trade-off between low off currents and high transconductance is inherent to the current VOFET device setup and that new approaches have to be found to design VOFETs that combine good switching properties with high performance.

Amadi E.V., Venkataraman A., Papadopoulos C.

Abstract Self-assembly offers unique possibilities for fabricating nanostructures, with different morphologies and properties, typically from vapour or liquid phase precursors. Molecular units, nanoparticles, biological molecules and other discrete elements can spontaneously organise or form via interactions at the nanoscale. Currently, nanoscale self-assembly finds applications in a wide variety of areas including carbon nanomaterials and semiconductor nanowires, semiconductor heterojunctions and superlattices, the deposition of quantum dots, drug delivery, such as mRNA-based vaccines, and modern integrated circuits and nanoelectronics, to name a few. Recent advancements in drug delivery, silicon nanoelectronics, lasers and nanotechnology in general, owing to nanoscale self-assembly, coupled with its versatility, simplicity and scalability, have highlighted its importance and potential for fabricating more complex nanostructures with advanced functionalities in the future. This review aims to provide readers with concise information about the basic concepts of nanoscale self-assembly, its applications to date, and future outlook. First, an overview of various self-assembly techniques such as vapour deposition, colloidal growth, molecular self-assembly and directed self-assembly/hybrid approaches are discussed. Applications in diverse fields involving specific examples of nanoscale self-assembly then highlight the state of the art and finally, the future outlook for nanoscale self-assembly and potential for more complex nanomaterial assemblies in the future as technological functionality increases.

Pyo G., Lee G.J., Lee S., Yang J.H., Heo S.J., Choi G.H., Cha S., Jang J.E.

The vertical thin film transistor (VTFT) has several advantages over the planar thin film transistor, such as a high current density and low operating voltage, because of the structural specificity. However, it is difficult to realize transistor operation in a VTFT because of the structural limitation that the gate field is blocked. As a solution, the conductivity modulation of a graphene electrode is studied with a micro-hole structure as a gate field transfer electrode. The micro-hole array pattern in the graphene allows better penetration of the gate field to junction and the work function to be modulated. Moreover, the patterning induces a doping effect on the graphene which results in a high barrier at the p–n junction and improves the conductivity in the device operation. The optimum performance is shown at 5 µm hole size and 30% hole ratio by analyzing the hole size and the area ratio. The proposed structure shows about 20 times higher on-current than a planar transistor with a same active area. Compared to a VTFT using simple graphene working function modulation, the proposed structure has an on-state current that is ten times higher and off-state current that is reduced 50%, and therefore has an improved on–off ratio.