Solution Processed Metal Oxide High-κ Dielectrics for Emerging Transistors and Circuits

Performance enhancement of InSnZnO thin-film transistors by modifying the dielectric-semiconductor interface with colloidal quantum dots

Thin film transistors (TFTs) with InSnZnO (ITZO) and Al2O3 as the semiconductor and dielectric layers, respectively, were investigated, aiming to elevate the device performance. Chemically synthesized CuInS2/ZnS core/shell colloidal quantum dots (QDs) were used to passivate the semiconductor/dielectric interface. Compared with the pristine device, the device with the integrated QDs demonstrates remarkably improved electrical performance, including a higher electron mobility and a lower leakage current. Moreover, the integration of QDs largely mitigates hysteresis in the bidirectional transfer characteristics of the device. Improved negative bias stress stability is also observed in the device with QDs. The performance enhancement is ascribed to the reduction of the trap states induced by the defects in Al2O3, and the screening of electrical dipoles at the Al2O3/ITZO interface. This work proposes a new strategy to passivate the semiconductor/dielectric interface, which not only improves TFT performance, but also holds potential for optoelectronic applications.

High-Entropy Oxides: Pioneering the Future of Multifunctional Materials

The high-entropy concept affords an effective method to design and construct customized materials with desired characteristics for specific applications. Extending this concept to metal oxides, high-entropy oxides (HEOs) can be fabricated, and the synergistic elemental interactions result in the four core effects, i.e., the high-entropy effect, sluggish-diffusion effect, severe-lattice-distortion effect, and cocktail effect. All these effects greatly enhance the functionalities of this vast material family, surpassing conventional low- and medium-entropy metal oxides. For instance, the high phase stability, excellent electrochemical performance, and fast ionic conductivity make HEOs one of the hot next-generation candidate materials for electrochemical energy conversion and storage devices. Significantly, the extraordinary mechanical, electrical, optical, thermal, and magnetic properties of HEOs are very attractive for applications beyond catalysts and batteries, such as electronic devices, optic equipment, and thermal barrier coatings. This review will overview the entropy-stabilized composition and structure of HEOs, followed by a comprehensive introduction to the electrical, optical, thermal, and magnetic properties. Then, several typical applications, i.e., transistor, memristor, artificial synapse, transparent glass, photodetector, light absorber and emitter, thermal barrier coating, and cooling pigment, are synoptically presented to show the broad application prospect of HEOs. Lastly, the intelligence-guided design and high-throughput screening of HEOs are briefly introduced to point out future development trends, which will become powerful tools to realize the customized design and synthesis of HEOs with optimal composition, structure, and performance for specific applications.

Layered Deep-UV Optical Crystal KZn₂BO₃Br₂ as a High-κ Dielectric for 2D Electronic Devices

The development of dielectrics with atomic planes and van der Waals (vdW) interfaces is essential for enhancing the performance of 2D devices. However, vdW dielectrics often have smaller bandgaps compared to traditional 3D dielectrics, limiting their options. This study introduces AZBX (AZn₂BO₃X₂, where A = K or Rb, X = Cl or Br), a nonlinear deep-ultraviolet optical crystal, as a quasi-vdW layered dielectric ideal for 2D electronic devices. Focusing on KZBB, it's excellent dielectric properties, including a wide bandgap, high dielectric constant (high-κ), and smooth interfaces are demonstrated. When used as the top gate dielectric in a KZBB/MoS₂ field-effect transistor (FET) with MoS₂ channels and graphene contacts, the device exhibits outstanding performance, with a steep subthreshold swing (≈ 73 mV dec-1), high on/off ratio (≈ 10⁷), negligible hysteresis (0-8 mV), and stable, low leakage current (≈10⁻⁷ A cm- 2) before breakdown. This work expands the 2D material and dielectric landscape and highlights the strong potential of AZBX as high-performance dielectrics.

Molecular Engineering of Coordination Ligand for Multifunctional Sol-Gel Oxides

Here a ligand exchange strategy for synthesizing sol-gel oxides is demonstrated to achieve multifunctionality including direct photolithography, high dielectric strength, and high charge carrier mobility, which is challenging to obtain in such oxides. For this purpose, a series of bidentate ligands with azide termini and ethylene-glycol bridges is synthesized, and these ligands are universally applicable to the synthesis of a variety of dielectric and semiconductor oxides. Optimized photolithography conditions yield a high-quality ZrO2 dielectric film with a high dielectric constant and strength of ≈18 and ≈7 MV cm-1, respectively. Additionally, this strategy is applied to semiconductor oxides such as In2O3 and ZnO, and the all-oxide-patterned solution-processed thin-film transistor (TFT) demonstrates a high charge carrier mobility of ≈40 cm2 V-1 s-1. An oxide TFT array is fully photopatterned on a 4-inch Si wafer; uniform performances are observed across these devices. This study suggests the possibility of realizing multifunctional oxides for application in advanced electronics using simple ligand exchange chemistry.

High-Performance Low-Voltage Thin-Film Transistors: Experimental and Simulation Validation of Atmospheric Pressure Plasma-Assisted Li5AlO4 Metal Oxide Solution Processing

Metal oxide materials processed using solution methods have garnered significant attention due to their ability to efficiently and affordably create transparent insulating layers or active channel layers on various substrates for thin-film transistors (TFTs) used in modern electronics. The key properties of TFTs largely depend on how charge carriers behave near the thin layer at the semiconductor and dielectric interface. Effectively controlling these characteristics offers a straightforward yet effective approach to enhancing device performance. In this study, we propose a novel strategy utilizing atmospheric pressure plasma (APP) treatment to modulate the electrical properties of dielectric thin films and the interfaces between dielectric and semiconductor layers in TFTs processed by using solution methods. Through APP exposure, significant improvements in key TFT parameters were achieved for solution-processed TFTs. Interface states have been reduced from 1013 to 1011 cm-2, and the on/off current ratio has increased from 103 to 106 while maintaining a high field-effect mobility of 34 cm2 V-1 s-1. Additionally, UV-visible spectroscopy and X-ray analysis have confirmed the effectiveness of APP treatment in controlling interface states and traps, leading to overall performance enhancements in the TFTs. Furthermore, our experimental findings have been systematically validated using technology computer-aided design (TCAD) simulations of fabricated TFTs.

Universal Way to Enhance Solution-Processed High-κ Oxide Dielectrics Performance by Sulfate Incorporation

Vacuum-free, solution-processable high-κ-oxide dielectrics are considered to be a key element for emerging low-cost flexible electronics. However, they usually suffer from low breakdown strength and frequency-dependent capacitance, which limit their broader applications. Here, we report a universal way to improve solution-based high-κ oxide dielectric properties (e.g., Al2O3, ZrO2, Ga2O3, Sc2O3, Ho2O3, and Sm2O3) by sulfate incorporation. In-depth characterization shows that sulfate incorporation could reduce hydrogen and oxygen vacancy-related defects in high-κ oxides, thereby improving the dielectric performance. The optimized S-doped high-κ oxides show smooth surface (rms < 0.20 nm), low leakage current (∼10-7 A/cm2@4 MV/cm), excellent dielectric breakdown strength (>10 MV/cm), and stable capacitance-frequency characteristics. Besides, oxide thin-film transistors based on these high-κ dielectrics exhibit excellent performance (e.g., mobility >20 cm2 V-1 s-1, on/off ratio of ∼107, threshold swing of ∼0.14 V dec-1, threshold voltage of ∼0 V, and hysteresis of ∼0.02 V). Thus, this work provides a general approach for the development of high-quality solution-based high-κ oxides for transistor circuitry.

Approaching Theoretical Limits in the Performance of Printed P-Type CuI Transistors via Room Temperature Vacancy Engineering

Development of a novel high performing inorganic p-type thin film transistor could pave the way for new transparent electronic devices. This complements the widely commercialized n-type counterparts, indium-gallium-zinc-oxide (IGZO). Of the few potential candidates, copper monoiodide (CuI) stands out. It boasts visible light transparency and high intrinsic hole mobility (>40 cm2 V-1 s-1 ), and is suitable for various low-temperature processes. However, the performance of reported CuI transistors is still below expected mobility, mainly due to the uncontrolled excess charge- and defect-scattering from thermodynamically favored formation of copper and iodine vacancies. Here, a solution-processed CuI transistor with a significantly improved mobility is reported. This enhancement is achieved through a room-temperature vacancy-engineering processing strategy on high-k dielectrics, sodium-embedded alumina. A thorough set of chemical, structural, optical, and electrical analyses elucidates the processing-dependent vacancy-modulation and its corresponding transport mechanism in CuI. This encompasses defect- and phonon-scattering, as well as the delocalization of charges in crystalline domains. As a result, the optimized CuI thin film transistors exhibit exceptionally high hole mobility of 21.6 ± 4.5 cm2 V-1 s-1 . Further, the successful operation of IGZO-CuI complementary logic gates confirms the applicability of the device.

Boronization: A General Strategy for Rare Earth Oxides with Enhanced High-κ Gate Dielectric Performance

Rare earth oxides (REOs) can be used as high-κ gate dielectrics that are at the core of electronic devices. However, a bottleneck remains with regard to obtaining high-performance REO dielectrics due to the serious hygroscopic issue and high defect states. Here, a general boronization strategy is reported to enhance the high-κ REO gate dielectric performance. Complementary characterization reveals that boronization is capable of reducing oxygen vacancies/hydroxyl defects in REOs and suppressing moisture absorption, leading to the improvement of leakage current, breakdown strength (up to 9 MV/cm), and capacitance-frequency stability. Furthermore, oxide transistors based on boronized REO dielectrics demonstrate state-of-the-art device characteristics with a high mobility of 40 cm2/V s, a current on/off ratio of 108, a subthreshold swing of 82 mV/dec, a hysteresis of 0.05 V, and superior bias stress stability.

High-k Solution-Processed Barium Titanate/Polysiloxane Nanocomposite for Low-Temperature Ferroelectric Thin-Film Transistors

Ferroelectric nanoparticles have attracted much attention for numerous electronic applications owing to their nanoscale structure and size-dependent behavior. Barium titanate (BTO) nanoparticles with two different sizes (20 and 100 nm) were synthesized and mixed with a polysiloxane (PSX) polymer forming a nanocomposite solution for high-k nanodielectric films. Transition from the ferroelectric to paraelectric phase of BTO with different nanoparticle dimensions was evaluated through variable-temperature X-ray diffraction measurement accompanied by electrical analysis using capacitor structures. A symmetric single 200 peak was constantly detected at different measurement temperatures for the 20 nm BTO sample, marking a stable cubic crystal structure. 100 nm BTO on the other hand shows splitting of 200/002 peaks correlating to a tetragonal crystal form which further merged, thus forming a single 200 peak at higher temperatures. Smaller BTO dimension exhibits clockwise hysteresis in capacitance-voltage measurement and correlates to a cubic crystal structure which possesses paraelectric properties. Bigger BTO dimension in contrast, demonstrates counterclockwise hysteresis owing to their tetragonal crystal form. Through further Rietveld refinement analysis, we found that the tetragonality (c/a) of 100 nm BTO decreases at a higher temperature which narrows the hysteresis window. A wider hysteresis window was observed when utilizing 100 nm BTO compared to 20 nm BTO even at a lower loading ratio. The present findings imply different hysteresis mechanisms for BTO nanoparticles with varying dimensions which is crucial in understanding the role of how the BTO size tunes the crystal structures for integration in thin-film transistor devices.

Dielectrics for Two-Dimensional Transition-Metal Dichalcogenide Applications

Despite over a decade of intense research efforts, the full potential of two-dimensional transition-metal dichalcogenides continues to be limited by major challenges. The lack of compatible and scalable dielectric materials and integration techniques restrict device performances and their commercial applications. Conventional dielectric integration techniques for bulk semiconductors are difficult to adapt for atomically thin two-dimensional materials. This review provides a brief introduction into various common and emerging dielectric synthesis and integration techniques and discusses their applicability for 2D transition metal dichalcogenides. Dielectric integration for various applications is reviewed in subsequent sections including nanoelectronics, optoelectronics, flexible electronics, valleytronics, biosensing, quantum information processing, and quantum sensing. For each application, we introduce basic device working principles, discuss the specific dielectric requirements, review current progress, present key challenges, and offer insights into future prospects and opportunities.

Gate Dielectrics Integration for 2D Electronics: Challenges, Advances, and Outlook

2D semiconductors have emerged both as an ideal platform for fundamental studies and as promising channel materials in beyond-silicon field-effect-transistors due to their outstanding electrical properties and exceptional tunability via external field. However, the lack of proper dielectrics for 2D semiconductors has become a major roadblock for their further development toward practical applications. The prominent issues between conventional 3D dielectrics and 2D semiconductors arise from the integration and interface quality, where defect states and imperfections lead to dramatic deterioration of device performance. In this review article, the root causes of such issues are briefly analyzed and recent advances on some possible solutions, including various approaches of adapting conventional dielectrics to 2D semiconductors, and the development of novel dielectrics with van der Waals surface toward high-performance 2D electronics are summarized. Then, in the perspective, the requirements of ideal dielectrics for state-of-the-art 2D devices are outlined and an outlook for their future development is provided.

Azide-functionalized ligand enabling organic-inorganic hybrid dielectric for high-performance solution-processed oxide transistors

We propose a highly efficient crosslinking strategy for organic-inorganic hybrid dielectric layers using azide-functionalized acetylacetonate, which covalently connect inorganic particles to polymers, enabling highly efficient inter- and intra-crosslinking of organic and inorganic inclusions, resulting in a dense and defect-free thin-film morphology. From the optimized processing conditions, we obtained an excellent dielectric strength of over 4.0 MV cm-1, a high dielectric constant of ~14, and a low surface energy of 38 mN m-1. We demonstrated the fabrication of exceptionally high-performance, hysteresis-free n-type solution-processed oxide transistors comprising an In2O3/ZnO double layer as an active channel with an electron mobility of over 50 cm2 V-1 s-1, on/off ratio of ~107, subthreshold swing of 108 mV dec-1, and high bias-stress stability. From temperature-dependent I-V analyses combined with charge transport mechanism analyses, we demonstrated that the proposed hybrid dielectric layer provides percolation-limited charge transport for the In2O3/ZnO double layer under field-effect conditions.

Sn-Based Perovskite Halides for Electronic Devices

Because of its less toxicity and electronic structure analogous to that of lead, tin halide perovskite (THP) is currently one of the most favorable candidates as an active layer for optoelectronic and electric devices such as solar cells, photodiodes, and field-effect transistors (FETs). Promising photovoltaics and FETs performances have been recently demonstrated because of their desirable electrical and optical properties. Nevertheless, THP's easy oxidation from Sn2+ to Sn4+ , easy formation of tin vacancy, uncontrollable film morphology and crystallinity, and interface instability severely impede its widespread application. This review paper aims to provide a basic understanding of THP as a semiconductor by highlighting the physical structure, energy band structure, electrical properties, and doping mechanisms. Additionally, the key chemical instability issues of THPs are discussed, which are identified as the potential bottleneck for further device development. Based on the understanding of the THPs properties, the key recent progress of THP-based solar cells and FETs is briefly discussed. To conclude, current challenges and perspective opportunities are highlighted.

Readily Accessible Metallic Micro-Island Arrays for High-Performance Metal Oxide Thin-Film Transistors

Thin-film transistors using metal oxide semiconductors are essential in many unconventional electronic devices. Nevertheless, further advances will be necessary to broaden their technological appeal. Here, a new strategy is reported to achieve high-performance solution-processed metal oxide thin-film transistors (MOTFTs) by introducing a metallic micro-island array (M-MIA) on top of the MO back channel, where the MO is a-IGZO (amorphous indium-gallium-zinc-oxide). Here Al-MIAs are fabricated using honeycomb cinnamate cellulose films, created by a scalable breath-figure method, as a shadow mask. For IGZO TFTs, the electron mobility (µ_e ) increases from ≈3.6 cm² V^-1 s^-1 to near 15.6 cm² V^-1 s^-1 for optimal Al-MIA dimension/coverage of 1.25 µm/51%. The Al-MIA IGZO TFT performance is superior to that of controls using compact/planar Al layers (Al-PL TFTs) and Au-MIAs with the same channel coverage. Kelvin probe force microscopy and technology computer-aided design simulations reveal that charge transfer occurs between the Al and the IGZO channel which is optimized for specific Al-MIA dimensions/surface channel coverages. Furthermore, such Al-MIA IGZO TFTs with a high-k fluoride-doped alumina dielectric exhibit a maximum µ_e of >50.2 cm² V^-1 s^-1 . This is the first demonstration of a micro-structured MO semiconductor heterojunction with submicrometer resolution metallic arrays for enhanced transistor performance and broad applicability to other devices.

Nano-Scale Ga2O3 Interface Engineering for High-Performance of ZnO-Based Thin-Film Transistors

Thin-film transistor (TFT) is a essential device for future electronics driving the next level of digital transformation. The development of metal-oxide-semiconductor (MOS) TFTs is considered one of the most advantageous devices for next-generation, large-area flexible electronics. This study demonstrates the systematic study of the amorphous gallium oxide (a-Ga₂O₃) and its application to nanocrystalline ZnO TFTs. The TFT with a-Ga₂O₃/c-ZnO-stack channel exhibits a field-effect mobility of ∼41 cm² V^-1 s^-1 and excellent stability under positive-bias-temperature stress. The a-Ga₂O₃/c-ZnO-stack TFT on polyimide (PI) substrate exhibits a negligible threshold voltage shift upon 100k bending cycles with a radius of 3 mm and is very stable under environmental test. The smooth morphology with tiny grains of ∼12 nm diameter with fewer grain boundary states improves the charge transport in Ga₂O₃/ZnO-stack TFT. The existence of amorphous a-Ga₂O₃ in between very thin ZnO layers helps to enhance the heterointerfaces and reduce the defect density in Ga₂O₃/ZnO interface. Therefore, integrating a-Ga₂O₃ in the ZnO channel in stacked TFT can increase mobility and enhance stability for next-generation flexible TFT electronics.

Emerging Synthesis Strategies of 2D MOFs for Electrical Devices and Integrated Circuits

The development of advanced electronic devices is boosting many aspects of modern technology and industry. The ever-increasing demand for advanced electrical devices and integrated circuits calls for the design of novel materials, with superior properties for the improvement of working performance. In this review, a detailed overview of the synthesis strategies of 2D metal organic frameworks (MOFs) acquiring growing attention is presented, as a basis for expansion of novel key materials in electrical devices and integrated circuits. A framework of controllable synthesis routes to be implanted in the synthesis strategies of 2D materials and MOFs is described. In short, the synthesis methods of 2D MOFs are summarized and discussed in depth followed by the illustrations of promising applications relating to various electrical devices and integrated circuits. It is concluded by outlining how 2D MOFs can be synthesized in a simpler, highly efficient, low-cost, and more environmentally friendly way which can open up their applicable opportunities as key materials in advanced electrical devices and integrated circuits, enabling their use in broad aspects of the society.

Recent progress in nanomaterial-based bioelectronic devices for biocomputing system

Bioelectronic devices have received the massive attention because of their huge potential to develop the core electronic components for biocomputing system. Up to now, numerous bioelectronic devices have been reported such as biomemory and biologic gate by employment of biomolecules including metalloproteins and nucleic acids. However, the intrinsic limitations of biomolecules such as instability and low signal production hinder the development of novel bioelectronic devices capable of performing various novel computing functions. As a way to overcome these limitations, nanomaterials have the great potential and wide applicability to grant and extend the electronic functions, and improve the inherent properties from biomolecules. Accordingly, lots of nanomaterials including the conductive metal, graphene, and transition metal dichalcogenide nanomaterials are being used to develop the remarkable functional bioelectronic devices like the multi-bit biomemory and resistive random-access biomemory. This review discusses the nanomaterial-based superb bioelectronic devices including the biomemory, biologic gates, and bioprocessors. In conclusion, this review will provide the interdisciplinary information about utilization of various novel nanomaterials applicable for biocomputing system.

Role of Fluoride Doping in Low-Temperature Combustion-Synthesized ZrOx Dielectric Films

Zirconium oxide (ZrO_x) is an attractive metal oxide dielectric material for low-voltage, optically transparent, and mechanically flexible electronic applications due to the high dielectric constant (κ ∼ 14-30), negligible visible light absorption, and, as a thin film, good mechanical flexibility. In this contribution, we explore the effect of fluoride doping on structure-property-function relationships in low-temperature solution-processed amorphous ZrO_x. Fluoride-doped zirconium oxide (F:ZrO_x) films with a fluoride content between 1.7 and 3.2 in atomic (at) % were synthesized by a combustion synthesis procedure. Irrespective of the fluoride content, grazing incidence X-ray diffraction, atomic-force microscopy, and UV-vis spectroscopy data indicate that all F:ZrO_x films are amorphous, atomically smooth, and transparent in visible light. Impedance spectroscopy measurements reveal that unlike solution-processed fluoride-doped aluminum oxide (F:AlO_x), fluoride doping minimally affects the frequency-dependent capacitance instability of solution-processed F:ZrO_x films. This result can be rationalized by the relatively weak Zr-F versus Zr-O bonds and the large ionic radius of Zr⁺⁴, as corroborated by EXAFS analysis and MD simulations. Nevertheless, the performance of pentacene thin-film transistors (TFTs) with F:ZrO_x gate dielectrics indicates that fluoride incorporation reduces I-V hysteresis in the transfer curves and enhances bias stress stability versus TFTs fabricated with analogous, but undoped ZrO_x films as gate dielectrics, due to reduced trap density.

Solid-Ionic Memory in a van der Waals Heterostructure

Defect states dominate the performance of low-dimensional nanoelectronics, which deteriorate the serviceability of devices in most cases. But in recent years, some intriguing functionalities are discovered by defect engineering. In this work, we demonstrate a bifunctional memory device of a MoS₂/BiFeO₃/SrTiO₃ van der Waals heterostructure, which can be programmed and erased by solely one kind of external stimuli (light or electrical-gate pulse) via engineering of oxygen-vacancy-based solid-ionic gating. The device shows multibit electrical memory capability (>22 bits) with a large linearly tunable dynamic range of 7.1 × 10⁶ (137 dB). Furthermore, the device can be programmed by green- and red-light illuminations and then erased by UV light pulses. Besides, the photoresponse under red-light illumination reaches a high photoresponsivity (6.7 × 10⁴ A/W) and photodetectivity (2.12 × 10¹³ Jones). These results highlighted solid-ionic memory for building up multifunctional electronic and optoelectronic devices.

Expeditiously Crystallized Pure Orthorhombic-Hf0.5Zr0.5O2 for Negative Capacitance Field Effect Transistors

Ultralow-power logic devices are next-generation electronics in which their maximum efficacies are realized at minimum input power expenses. The integration of ferroelectric negative capacitors in the regular gate stacks of two-dimensional field-effect transistors addresses two intriguing challenges in today's electronics; short channel effects and high operating voltages. The complementary-metal-oxide-semiconductor-compatible Hf_0.5Zr_0.5O₂ (HZO) is an excellent ferroelectric material crystallized in a noncentrosymmetric o-phase. The present work is the first to utilize pulsed laser deposition (PLD)-grown phase-pure ferroelectric HZO to achieve steep slope negative capacitance (NC) in field effect transistors (FETs). A dual-step growth strategy is designed to achieve phase-pure orthorhombic HZO on silicon and other conducting substrates. The room-temperature PLD-grown amorphous HZO is allowed to crystallize using rapid thermal annealing at 600 °C. The polycrystalline orthorhombic HZO is further integrated with atomic layer deposition-grown HfO₂ to achieve a stable NC transition. The stack is further integrated into the molybdenum disulfide channel to achieve steep switching and a hysteresis-free operation of the resulting FETs. The subthreshold swings of the FETs are 20.42 and 26.16 mV/dec in forward and reverse bias conditions, respectively.

New Opportunities for High-Performance Source-Gated Transistors Using Unconventional Materials

Source-gated transistors (SGTs), which are typically realized by introducing a source barrier in staggered thin-film transistors (TFTs), exhibit many advantages over conventional TFTs, including ultrahigh gain, lower power consumption, higher bias stress stability, immunity to short-channel effects, and greater tolerance to geometric variations. These properties make SGTs promising candidates for readily fabricated displays, biomedical sensors, and wearable electronics for the Internet of Things, where low power dissipation, high performance, and efficient, low-cost manufacturability are essential. In this review, the general aspects of SGT structure, fabrication, and operation mechanisms are first discussed, followed by a detailed property comparison with conventional TFTs. Next, advances in high-performance SGTs based on silicon are first discussed, followed by recent advances in emerging metal oxides, organic semiconductors, and 2D materials, which are individually discussed, followed by promising applications that can be uniquely realized by SGTs and their circuitry. Lastly, this review concludes with challenges and outlook overview.

Reviving the "Schottky" Barrier for Flexible Polymer Dielectrics with a Superior 2D Nanoassembly Coating

The organic insulator-metal interface is the most important junction in flexible electronics. The strong band offset of organic insulators over the Fermi level of electrodes should theoretically impart a sufficient impediment for charge injection known as the Schottky barrier. However, defect formation through Anderson localization due to topological disorder in polymers leads to reduced barriers and hence cumbersome devices. A facile nanocoating comprising hundreds of highly oriented organic/inorganic alternating nanolayers is self-coassembled on the surface of polymer films to revive the Schottky barrier. Carrier injection over the enhanced barrier is further shunted by anisotropic 2D conduction. This new interface engineering strategy allows a significant elevation of the operating field for organic insulators by 45% and a 7× improvement in discharge efficiency for Kapton at 150 °C. This superior 2D nanocoating thus provides a defect-tolerant approach for effective reviving of the Schottky barrier, one century after its discovery, broadly applicable for flexible electronics.

Oligothiophene Phosphonic Acids for Self-Assembled Monolayer Field-Effect Transistors

Semiconducting self-assembled monolayers (SAMs) represent highly relevant components for the fabrication of organic thin-film electronics because they enable the precise formation of active π-conjugates in terms of orientation and layer thickness. In this work, we demonstrate self-assembled monolayer field-effect transistors (SAMFETs) composed of phosphonic acid oligomers of 3-hexylthiophene (oligothiophenes-OT) with systematic variations of thiophene repeating units (5, 10, and 20). The devices exhibit stable lateral charge transport with increased mobility as a function of thiophene unit counts. Importantly, our work reveals the packing and intermolecular order of varied-chain-length SAMs at the molecular scale via X-ray reflectivity (XRR) and quantitative X-ray photoelectron spectroscopy (XPS). Short oligomers (OT5-PA and OT10-PA) arrange almost perpendicular to the substrate, forming highly ordered SAMs, whereas the long-chain OT20-PA exhibits a folded structure. By tuning the molecular order in the monolayers via the SAM substitution reaction, the OT20-PA devices show a tripling in mobility.

Engineering Copper Iodide (CuI) for Multifunctional p-Type Transparent Semiconductors and Conductors

Developing transparent p-type semiconductors and conductors has attracted significant interest in both academia and industry because metal oxides only show efficient n-type characteristics at room temperature. Among the different candidates, copper iodide (CuI) is one of the most promising p-type materials because of its widely adjustable conductivity from transparent electrodes to semiconducting layers in transistors. CuI can form thin films with high transparency in the visible light region using various low-temperature deposition techniques. This progress report aims to provide a basic understanding of CuI-based materials and recent progress in the development of various devices. The first section provides a brief introduction to CuI with respect to electronic structure, defect states, charge transport physics, and overviews the CuI film deposition methods. The material design concepts through doping/alloying approaches to adjust the optoelectrical properties are also discussed in the first section. The following section presents recent advances in state-of-the-art CuI-based devices, including transparent electrodes, thermoelectric devices, p-n diodes, p-channel transistors, light emitting diodes, and solar cells. In conclusion, current challenges and perspective opportunities are highlighted.

Solution synthesis and dielectric properties of alumina thin films: understanding the role of the organic additive in film formation

Alumina thin films are synthesized by combustion synthesis of mixtures of aluminium nitrate (ALN) and methylcarbazate (MCZ). The interdependence of the ratio of oxidizer and reducing agent on composition, microstructure and electronic properties of the resulting oxide layers is investigated. The dielectric and insulating behaviour is improved by addition of different amounts of MCZ (MCZ : ALN = 0.67 or 2.5). In this way films (thickness ∼140 nm) with a dielectric constant κ of 9.7 and a dielectric loss tan δ below 0.015 can be achieved. Medium concentrations of MCZ (MCZ : ALN = 1.0 or 1.5) lead to films with lower performance, though. Our studies indicate two opposing effects of the organic additive. Removal of organic residues during film formation as combustion gases is potentially detrimental. Larger amounts of MCZ, however, cause condensation reactions in the precusor mixture, which improve the microstructure. The porosity of the films can be sucessfully analyzed by positron annihilation liftetime studies. In this way the impact of the organic ligand sphere on the resulting microstructure can be quantified. Samples prepared from ALN alone exhibit mesopores and also larger micropores. In contrast, the formation of mesopores can be inhibited by addition of MCZ.

Sub-thermionic, ultra-high-gain organic transistors and circuits

The development of organic thin-film transistors (OTFTs) with low power consumption and high gain will advance many flexible electronics. Here, by combining solution-processed monolayer organic crystal, ferroelectric HfZrO_x gating and van der Waals fabrication, we realize flexible OTFTs that simultaneously deliver high transconductance and sub-60 mV/dec switching, under one-volt operating voltage. The overall optimization of transconductance, subthreshold swing and output resistance leads to transistor intrinsic gain and amplifier voltage gain over 5.3 × 10⁴ and 1.1 × 10⁴, respectively, which outperform existing technologies using organics, oxides and low-dimensional nanomaterials. We further demonstrate battery-powered, integrated wearable electrocardiogram (ECG) and pulse sensors that can amplify human physiological signal by 900 times with high fidelity. The sensors are capable of detecting weak ECG waves (undetectable even by clinical equipment) and diagnosing arrhythmia and atrial fibrillation. Our sub-thermionic OTFT is promising for battery/wireless powered yet performance demanding applications such as electronic skins and radio-frequency identification tags, among many others.

Exploiting negative capacitance effects in organic thin-film transistors (OTFTs) is advantageous for enhancing device performance. Here, the authors report solution-processed sub-thermionic OTFTs and circuits with ferroelectric hafnium oxides that show ultra-low power and ultra-high gain.

Investigation of the physical, optical, and chemical properties of phase segregated AlCoOx thin films from a novel hexol-type cluster

The use of a novel inorganic nanoscale cluster (Al[(μ-OH)₂Co(NH₃)₄]₃(NO₃)₆) was investigated for its utility as a precursor for AlCoOx films. Mixed-metal aluminum and cobalt oxide thin films were solution deposited from the novel cluster solution via the spin-coating method on Si (100) and quartz substrates. The films were annealed at increasing temperatures up to 800 °C, and characterization of these films via TEM and XRD confirms binary Co₃O₄ crystalline phase present in an amorphous Al₂O₃ network. Films are relatively smooth (R_rms < 4 nm), polycrystalline, and demonstrate a tunable optical response dominated by Co₃O₄ with two electronic transitions.

Conductive Polymer-Assisted Metal Oxide Hybrid Semiconductors for High-Performance Thin-Film Transistors

Metal oxide semiconductors doped with additional inorganic cations have insufficient electron mobility for next-generation electronic devices so strategies to realize the semiconductors exhibiting stability and high performance are required. To overcome the limitations of conventional inorganic cation doping to improve the electrical characteristics and stability of metal oxide semiconductors, we propose solution-processed high-performance metal oxide thin-film transistors (TFTs) by incorporating polyaniline (PANI), a conductive polymer, in a metal oxide matrix. The chemical interaction between the metal oxide and PANI demonstrated that the defect sites and crystallinity of the semiconductor layer are controllable. In addition, the change in oxygen-related chemical bonding of PANI-doped indium oxide (InO_x) TFTs induces superior electrical characteristics compared to pristine InO_x TFTs, even though trace amounts of PANI are doped in the semiconductor. In particular, the average field-effect mobility remarkably enhanced from 15.02 to 26.58 cm² V^-1 s^-1, the on/off current ratio improved from 10⁸ to 10⁹, and the threshold voltage became close to 0 V actually from -7.9 to -1.4 V.

Highly Reliable Organic Field-Effect Transistors with Molecular Additives for a High-Performance Printed Gas Sensor

Organic semiconductors (OSCs) are promising sensing materials for printed flexible gas sensors. However, OSCs are unstable in the humid air, which limits the realization of gas sensors for multiple usages. In this paper, we report a facile and effective way to improve the air stability of an OSC film to realize multiple reversibly used printed gas sensors by adding molecular additives. The tetracyanoquinodimethane (TCNQ) or 4-aminobenzonitrile (ABN) additives effectively prevent adsorption of moisture from the air on the OSC layer, thereby providing a stable gas sensor operation. The organic field-effect transistor (OFET)-based indacenodithiophene-co-benzothiadiazole with TCNQ or ABN shows highly reliable ammonia (NH₃) gas sensing up to 10 ppm in air, with 23.14% sensitivity, and the gas sensor signal can recover up to 100%. In particular, the stability of gas detection is greatly improved by the additives, which can be performed in the air for 16 days. The result indicates that the elimination of moisture trapped in OSCs with molecule additives is critical in the improvement of device air/operational stabilities and the achievement of high-performance OFET-based gas sensors.

Rapid and Reliable Formation of Highly Densified Bilayer Oxide Dielectrics on Silicon Substrates via DUV Photoactivation for Low-Voltage Solution-Processed Oxide Thin-Film Transistors

In this research, we report the rapid and reliable formation of high-performance nanoscale bilayer oxide dielectrics on silicon substrates via low-temperature deep ultraviolet (DUV) photoactivation. The optical analysis of sol-gel aluminum oxide films prepared at various concentrations reveals the processable film thickness with DUV photoactivation and its possible generalization to the formation of various metal oxide films on silicon substrates. The physicochemical and electrical characterizations confirm that DUV photoactivation accelerates the efficient formation of a highly dense aluminum oxide and aluminum silicate bilayer (17 nm) on heavily doped silicon at 150 °C within 5 min owing to the efficient thermal conduction on silicon, resulting in excellent dielectric properties in terms of low leakage current (∼10^-8 A/cm² at 1.0 MV/cm) and high areal capacitance (∼0.4 μF/cm² at 100 kHz) with narrow statistical distributions. Additionally, the sol-gel bilayer oxide dielectrics are successfully combined with a sol-gel indium-gallium-zinc oxide semiconductor via two successive DUV photoactivation cycles, leading to the efficient fabrication of solution-processed oxide thin-film transistors on silicon substrates with an operational voltage below 0.5 V. We expect that in combination with large-area printing, the bilayer oxide dielectrics are beneficial for large-area solution-based oxide electronics on silicon substrates, while DUV photoactivation can be applied to various types of solution-processed functional metal oxides such as phase-transition memories, ferroelectrics, photocatalysts, charge-transporting interlayers and passivation layers, etc. on silicon substrates.

Ultraviolet Light-Densified Oxide-Organic Self-Assembled Dielectrics: Processing Thin-Film Transistors at Room Temperature

Low-temperature, solution-processable, high-capacitance, and low-leakage gate dielectrics are of great interest for unconventional electronics. Here, we report a near room temperature ultraviolet densification (UVD) methodology for realizing high-performance organic-inorganic zirconia self-assembled nanodielectrics (UVD-ZrSANDs). These UVD-ZrSAND multilayers are grown from solution in ambient, densified by UV radiation, and characterized by X-ray reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and capacitance measurements. The resulting UVD-ZrSAND films exhibit large capacitances of >700 nF/cm² and low leakage current densities of <10^-7 A/cm², which rival or exceed those synthesized by traditional thermal methods. Both the p-type organic semiconductor pentacene and the n-type metal oxide semiconductor In₂O₃ were used to investigate UVD-ZrSANDs as the gate dielectric in thin-film transistors, affording mobilities of 0.58 and 26.21 cm²/(V s), respectively, at a low gate voltage of 2 V. These results represent a significant advance in fabricating ultra-thin high-performance dielectrics near room temperature and should facilitate their integration into diverse electronic technologies.

Nanocluster-Based Ultralow-Temperature Driven Oxide Gate Dielectrics for High-Performance Organic Electronic Devices

The development of novel dielectric materials with reliable dielectric properties and low-temperature processibility is crucial to manufacturing flexible and high-performance organic thin-film transistors (OTFTs) for next-generation roll-to-roll organic electronics. Here, we investigate the solution-based fabrication of high-k aluminum oxide (Al₂O₃) thin films for high-performance OTFTs. Nanocluster-based Al₂O₃ films fabricated by highly energetic photochemical activation, which allows low-temperature processing, are compared to the conventional nitrate-based Al₂O₃ films. A wide array of spectroscopic and surface analyses show that ultralow-temperature photochemical activation (<60 °C) induces the decomposition of chemical impurities and causes the densification of the metal-oxide film, resulting in a highly dense high-k Al₂O₃ dielectric layer from Al-13 nanocluster-based solutions. The fabricated nanocluster-based Al₂O₃ films exhibit a low leakage current density (<10⁻⁷ A/cm²) at 2 MV/cm and high dielectric breakdown strength (>6 MV/cm). Using this dielectric layer, precisely aligned microrod-shaped 2,7-dioctyl[1]benzothieno [3,2-b][1] benzothiophene (C8-BTBT) single-crystal OTFTs were fabricated via solvent vapor annealing and photochemical patterning of the sacrificial layer.

High-Intensity Ultrasound-Assisted Low-Temperature Formulation of Lanthanum Zirconium Oxide Nanodispersion for Thin-Film Transistors

The process complexity, limited stability, and distinct synthesis and dispersion steps restrict the usage of multicomponent metal oxide nanodispersions in solution-processed electronics. Herein, sonochemistry is employed for the in situ synthesis and formulation of a colloidal nanodispersion of high-permittivity (κ) multicomponent lanthanum zirconium oxide (LZO: La₂Zr₂O₇). The continuous propagation of intense ultrasound waves in the aqueous medium allows the generation of oxidant species which, on reaction, form nanofragments of crystalline LZO at ∼80 °C. Simultaneously, the presence of acidic byproducts in the vicinity promotes the formulation of a stable as-prepared LZO dispersion. The LZO thin film exhibits a κ of 16, and thin-film transistors (TFTs) based on LZO/indium gallium zinc oxide operate at low input voltages (≤4 V), with the maximum mobility (μ) and on/off ratio (I_on/I_off) of 5.45 ± 0.06 cm² V^-1 s^-1 and ∼10⁵, respectively. TFTs based on the compound dielectric LZO/Al₂O₃ present a marginal reduction in leakage current, along with enhancement in μ (6.16 ± 0.04 cm² V^-1 s^-1) and I_on/I_off (∼10⁵). Additionally, a 3 × 3 array of the proposed TFTs exhibits appreciable performance, with a μ of 3-6 cm² V^-1 s^-1, a threshold voltage of -0.5 to 0.8 V, a subthreshold swing of 0.3-0.6 V dec^-1, and an I_on/I_off of 1-2.5 (×10⁶).

Electrospun Yb-Doped In2O3 Nanofiber Field-Effect Transistors for Highly Sensitive Ethanol Sensors

Enhancing the reliability and sensitivity of gas sensors based on FETs has been of extensive concern for their practical application. However, few reports are available on nanofiber FET gas sensors fabricated by the electrospinning process. In this work, ethanol gas sensors based on Yb-doped In₂O₃ (InYbO) nanofiber FETs are fabricated by a simple and fast electrospinning method. The optimized In₂O₃ nanofiber FETs with a doping concentration of 4 mol % show a better electrical performance, including a high mobility of 6.67 cm²/Vs, an acceptable threshold voltage of 3.27 V, and a suitable on/off current ratio of 10⁷, especially the enhanced bias-stress stability. When employed in ethanol gas sensors, the gas sensors exhibit enhanced stability and improved sensitivity with a high response of 40-10 ppm, which is remarkably higher than that of previously reported ethanol gas sensors. Moreover, the InYbO nanofiber FET sensors also demonstrate a low limit of detection of 1 ppm and improved sensing performance ranging from sensitivity to the ability of selectivity. This work opens up a new prospect to achieve highly sensitive, selective, and reliable ethanol gas sensors using electrospun Yb-In₂O₃ nanofiber FETs with improved stability.

Frequency-Agile Low-Temperature Solution-Processed Alumina Dielectrics for Inorganic and Organic Electronics Enhanced by Fluoride Doping

The frequency-dependent capacitance of low-temperature solution-processed metal oxide (MO) dielectrics typically yields unreliable and unstable thin-film transistor (TFT) performance metrics, which hinders the development of next-generation roll-to-roll MO electronics and obscures intercomparisons between processing methodologies. Here, capacitance values stable over a wide frequency range are achieved in low-temperature combustion-synthesized aluminum oxide (AlO_x) dielectric films by fluoride doping. For an optimal F incorporation of ∼3.7 atomic % F, the F:AlO_x film capacitance of 166 ± 11 nF/cm² is stable over a 10^-1-10⁴ Hz frequency range, far more stable than that of neat AlO_x films (capacitance = 336 ± 201 nF/cm²) which falls from 781 ± 85 nF/cm² to 104 ± 4 nF/cm² over this frequency range. Importantly, both n-type/inorganic and p-type/organic TFTs exhibit reliable electrical characteristics with minimum hysteresis when employing the F:AlO_x dielectric with ∼3.7 atomic % F. Systematic characterization of film microstructural/compositional and electronic/dielectric properties by X-ray photoelectron spectroscopy, time-of-fight secondary ion mass spectrometry, cross-section transmission electron microscopy, solid-state nuclear magnetic resonance, and UV-vis absorption spectroscopy reveal that fluoride doping generates AlOF, which strongly reduces the mobile hydrogen content, suppressing polarization mechanisms at low frequencies. Thus, this work provides a broadly applicable anion doping strategy for the realization of high-performance solution-processed metal oxide dielectrics for both organic and inorganic electronics applications.

Facile Photo-cross-linking System for Polymeric Gate Dielectric Materials toward Solution-Processed Organic Field-Effect Transistors: Role of a Cross-linker in Various Polymer Types

Energy-efficient solution-processed organic field-effect transistors (OFETs) are highly sought after in the low-cost printing industry as well as for the manufacture of flexible and other next-generation devices. The fabrication of such electronic devices requires high-functioning insulating materials that are chemically and mechanically robust to avoid lowering insulating properties during the device fabrication process or utilization of devices. In this study, we report a facile, fluorinated, UV-assisted cross-linker series using a fluorophenyl azide (FPA), which reacts with the C-H groups of a conventional polymer. This demonstrates the application of the cross-linked films in OFET gate dielectrics. The effects of the cross-linkable chemical structure of the FPA series on the cross-linking chemistry, photopatternability, and dielectric properties of the resulting films are investigated for low/high-k or amorphous/crystalline polymeric gate dielectric materials. The characteristics of insulating layers and behavior of OFETs containing these cross-linked gate dielectrics (for example, leakage current density (J), hysteresis, and charge trap density) depend on the polymer type. Furthermore, an organic-based complementary inverter and various printable OFETs with excellent electrical characteristics are successfully fabricated. Thus, these reported cross-linkers that enable the solution process and patterning of well-developed conventional polymer dielectric materials are promising for the realization of a more sustainable next-generation industrial technology for flexible and printable devices.

Proton Conducting Perhydropolysilazane-Derived Gate Dielectric for Solution-Processed Metal Oxide-Based Thin-Film Transistors

Perhydropolysilazane (PHPS), an inorganic polymer composed of Si-N and Si-H, has attracted much attention as a precursor for gate dielectrics of thin-film transistors (TFTs) due to its facile processing even at a relatively low temperature. However, an in-depth understanding of the tunable dielectric behavior of PHPS-derived dielectrics and their effects on TFT device performance is still lacking. In this study, the PHPS-derived dielectric films formed at different annealing temperatures have been used as the gate dielectric layer for solution-processed indium zinc oxide (IZO) TFTs. Notably, the IZO TFTs fabricated on PHPS annealed at 350 °C exhibit mobility as high as 118 cm² V^-1 s^-1, which is about 50 times the IZO TFTs made on typical SiO₂ dielectrics. The outstanding electrical performance is possible because of the exceptional capacitance of PHPS-derived dielectric caused by the limited hydrolysis reaction of PHPS at a low processing temperature (<400 °C). According to our analysis, the exceptional dielectric behavior is originated from the electric double layer formed by mobile of protons in the low temperature-annealed PHPS dielectrics. Furthermore, proton conduction through the PHPS dielectric occurs through a three-dimensional pathway by a hopping mechanism, which allows uniform polarization of the dielectric even at room temperature, leading to amplified performance of the IZO TFTs.

Amorphous Tin Oxide Applied to Solution Processed Thin-Film Transistors

The limited choice of materials for large area electronics limits the expansion of applications. Polycrystalline silicon (poly-Si) and indium gallium zinc oxide (IGZO) lead to thin-film transistors (TFTs) with high field-effect mobilities (>10 cm²/Vs) and high current ON/OFF ratios (I_On/I_Off > ~10⁷). But they both require vacuum processing that needs high investments and maintenance costs. Also, IGZO is prone to the scarcity and price of Ga and In. Other oxide semiconductors require the use of at least two cations (commonly chosen among Ga, Sn, Zn, and In) in order to obtain the amorphous phase. To solve these problems, we demonstrated an amorphous oxide material made using one earth-abundant metal: amorphous tin oxide (a-SnO_x). Through XPS, AFM, optical analysis, and Hall effect, we determined that a-SnO_x is a transparent n-type oxide semiconductor, where the SnO₂ phase is predominant over the SnO phase. Used as the active material in TFTs having a bottom-gate, top-contact structure, a high field-effect mobility of ~100 cm²/Vs and an I_On/I_Off ratio of ~10⁸ were achieved. The stability under 1 h of negative positive gate bias stress revealed a Vth shift smaller than 1 V.

Synthesis, oxide formation, properties and thin film transistor properties of yttrium and aluminium oxide thin films employing a molecular-based precursor route

Combustion synthesis of dielectric yttrium oxide and aluminium oxide thin films is possible by introducing a molecular single-source precursor approach employing a newly designed nitro functionalized malonato complex of yttrium (Y-DEM-NO21) as well as defined urea nitrate coordination compounds of yttrium (Y-UN 2) and aluminium (Al-UN 3). All new precursor compounds were extensively characterized by spectroscopic techniques (NMR/IR) as well as by single-crystal structure analysis for both urea nitrate coordination compounds. The thermal decomposition of the precursors 1-3 was studied by means of differential scanning calorimetry (DSC) and thermogravimetry coupled with mass spectrometry and infrared spectroscopy (TG-MS/IR). As a result, a controlled thermal conversion of the precursors into dielectric thin films could be achieved. These oxidic thin films integrated within capacitor devices are exhibiting excellent dielectric behaviour in the temperature range between 250 and 350 °C, with areal capacity values up to 250 nF cm-2, leakage current densities below 1.0 × 10-9 A cm-2 (at 1 MV cm-1) and breakdown voltages above 2 MV cm-1. Thereby the increase in performance at higher temperatures can be attributed to the gradual conversion of the intermediate hydroxy species into the respective metal oxide which is confirmed by X-ray photoelectron spectroscopy (XPS). Finally, a solution-processed Y x O y based TFT was fabricated employing the precursor Y-DEM-NO21. The device exhibits decent TFT characteristics with a saturation mobility (μ sat) of 2.1 cm2 V-1 s-1, a threshold voltage (V th) of 6.9 V and an on/off current ratio (I on/off) of 7.6 × 105.

Polyol Reduction: A Low-Temperature Eco-Friendly Solution Process for p-Channel Copper Oxide-Based Transistors and Inverter Circuits

An optimized polyol reduction method was proposed for p-type Cu_xO deposition using a spin-coating method at low temperatures. The film characterizations and the electrical properties of integrated thin-film transistors (TFTs) were investigated as a function of annealing temperature and polyol type. The Cu_xO TFTs derived from propylene glycol showed the optimal performance with an average field-effect hole mobility of 0.15 cm² V^-1 s^-1, an on/off current ratio of ∼10⁴, and a threshold voltage of -7 V at a low temperature of 220 °C. Considering few investigations on the operational/air stability of solution-processed p-type oxide TFTs, we carried out systematical studies and revealed different roles of O₂ and H₂O on the device performance. The low activation energy (0.16 eV) for hole transport and high voltage gain (37) of the composed complementary inverter highlight great potential for the construction of all oxide-based transparent flexible electronics and circuits.

Effect of Annealing Temperature for Ni/AlOx/Pt RRAM Devices Fabricated with Solution-Based Dielectric

Resistive random access memory (RRAM) devices with Ni/AlO_x/Pt-structure were manufactured by deposition of a solution-based aluminum oxide (AlO_x) dielectric layer which was subsequently annealed at temperatures from 200 °C to 300 °C, in increments of 25 °C. The devices displayed typical bipolar resistive switching characteristics. Investigations were carried out on the effect of different annealing temperatures for associated RRAM devices to show that performance was correlated with changes of hydroxyl group concentration in the AlO_x thin films. The annealing temperature of 250 °C was found to be optimal for the dielectric layer, exhibiting superior performance of the RRAM devices with the lowest operation voltage (<1.5 V), the highest ON/OFF ratio (>10⁴), the narrowest resistance distribution, the longest retention time (>10⁴ s) and the most endurance cycles (>150).

Toward Robust Photoelectrochemical Operation of Cuprous Oxide Nanowire Photocathodes Using a Strategically Designed Solution-Processed Titanium Oxide Passivation Coating

To date, TiO₂ films prepared by atomic layer deposition are widely used to prepare Cu₂O nanowire (NW)-based photocathodes with photoelectrochemical (PEC) durability as this approach enables conformal coating and furnishes chemical robustness. However, this common approach requires complicated interlayers and makes the fabrication of photocathodes with reproducible performance and long-term stability difficult. Although sol-gel-based approaches have been well established for coating surfaces with oxide thin films, these techniques have rarely been studied for oxide passivation in PEC applications, because the sol-gel coating methods are strongly influenced by surface chemical bonding and have been mainly demonstrated on flat substrates. As a unique strategy based on solution processing, herein, we suggest a creative solution for two problems encountered in the conformal coating of surfaces with oxide layers: (i) how to effectively prevent corrosion of materials with hydrophilic surfaces by simply using a single TiO₂ surface protection layer instead of a complex multilayer structure and (ii) guaranteeing perfect chemical durability. A Cu(OH)₂ NW can be easily prepared as an intermediate phase by anodization of a Cu metal, where the former inherently possesses a hydrophilic hydroxylated surface and thus, enables thorough coating with TiO₂ precursor solutions. Chemically robust nanowires are then generated as the final product via the phase transformation of Cu(OH)₂ to Cu₂O via sintering at 600 °C. The coated NWs exhibit excellent PEC properties and a stable performance. Consequently, the perfect chemical isolation of the Cu₂O NWs from the electrolyte allows a remarkable PEC operation with the maintenance of the initial photocurrent for more than one day.

Scalable Fabrication of Highly Crystalline Organic Semiconductor Thin Film by Channel-Restricted Screen Printing toward the Low-Cost Fabrication of High-Performance Transistor Arrays

Control over the morphology and crystallinity of small-molecule organic semiconductor (OSC) films is of key importance to enable high-performance organic optoelectronic devices. However, such control remains particularly challenging for solution-processed OSC devices because of the complex crystallization kinetics of small-molecule OSC materials in the dynamic flow of inks. Here, a simple yet effective channel-restricted screen-printing method is reported, which uses small-molecule OSCs/insulating polymer to yield large-grained small-molecule OSC thin-film arrays with good crystallization and preferred orientation. The use of cross-linked organic polymer banks produces a confinement effect to trigger the outward convective flow at two sides of the channel by the fast solvent evaporation, which imparts the transport of small-molecule OSC solutes and promotes the growth of small-molecule OSC crystals parallel to the channel. The small-molecule OSC thin-film array produced by screen printing exhibits excellent performance characteristics with an average mobility of 7.94 cm² V^-1 s^-1 and a maximum mobility of 12.10 cm² V^-1 s^-1 , which are on par with its single crystal. Finally, screen printing can be carried out using a flexible substrate, with good performance. These demonstrations bring this robust screen-printing method closer to industrial application and expand its applicability to various flexible electronics.

Polymer Doping Enables a Two-Dimensional Electron Gas for High-Performance Homojunction Oxide Thin-Film Transistors

High-performance solution-processed metal oxide (MO) thin-film transistors (TFTs) are realized by fabricating a homojunction of indium oxide (In₂ O₃ ) and polyethylenimine (PEI)-doped In₂ O₃ (In₂ O₃ :x% PEI, x = 0.5-4.0 wt%) as the channel layer. A two-dimensional electron gas (2DEG) is thereby achieved by creating a band offset between the In₂ O₃ and PEI-In₂ O₃ via work function tuning of the In₂ O₃ :x% PEI, from 4.00 to 3.62 eV as the PEI content is increased from 0.0 (pristine In₂ O₃ ) to 4.0 wt%, respectively. The resulting devices achieve electron mobilities greater than 10 cm² V^-1 s^-1 on a 300 nm SiO₂ gate dielectric. Importantly, these metrics exceed those of the devices composed of the pristine In₂ O₃ materials, which achieve a maximum mobility of ≈4 cm² V^-1 s^-1 . Furthermore, a mobility as high as 30 cm² V^-1 s^-1 is achieved on a high-k ZrO₂ dielectric in the homojunction devices. This is the first demonstration of 2DEG-based homojunction oxide TFTs via band offset achieved by simple polymer doping of the same MO material.

Effects of Environmental Water Absorption by Solution-Deposited Al2O3 Gate Dielectrics on Thin Film Transistor Performance and Mobility

In recent years, many solution-processed oxide transistors have been reported with mobility rivaling or exceeding their vacuum-deposited counterparts. Here, we show that water absorption from the environment by solution-processed dielectric materialsexplains this enhanced mobility. By monitoring the water content of Al₂O₃, ZrO₂, and bilayer dielectric materials, we demonstrate how water absorption by the dielectric affects electrical characteristics in solution-processed metal oxide transistors. These effects, including capacitance-frequency dispersion, counterclockwise hysteresis in transfer curves, and high channel mobility, are elucidated by electron transfer between the gate/channel and trap states within the band gap of the dielectric created by the water.