Material Design of p‐Type Transparent Amorphous Semiconductor,Cu–Sn–I

Transparent amorphous semiconductors (TAS) that can be fabricated at low temperature are key materials in the practical application of transparent flexible electronics. Although various n‐type TAS materials with excellent performance, such as amorphous In‐Ga‐Zn‐O (a‐IGZO), are already known, no complementary p‐type TAS has been realized to date. Here, a material design concept for p‐type TAS materials is proposed utilizing the pseudo s‐orbital nature of spatially spreading iodine 5p orbitals and amorphous Sn‐containing CuI (a‐CuSnI) thin film is reported as an example. The resulting a‐CuSnI thin films fabricated by spin coating at low temperature (140 °C) have a smooth surface. The Hall mobility increases with the hole concentration and the largest mobility of ≈9 cm² V^?1 s^?1 is obtained, which is comparable with that of conventional n‐type TAS.     

Continuous Fabrication of Meter‐Scale Single‐Wall Carbon Nanotube Films and their Use in Flexible and Transparent Integrated Circuits

Single‐wall carbon nanotubes (SWCNTs), especially in the form of large‐area and high‐quality thin films, are a promising material for use in flexible and transparent electronics. Here, a continuous synthesis, deposition, and transfer technique is reported for the fabrication of meter‐scale SWCNT thin films, which have an excellent optoelectrical performance including a low sheet resistance of 65 Ω/? with a transmittance of 90% at a wavelength of 550 nm. Using these SWCNT thin films, high‐performance all‐CNT thin‐film transistors and integrated circuits are demonstrated, including 101‐stage ring oscillators. The results pave the way for the future development of large‐scale, flexible, and transparent electronics based on CNT thin films.     

Eigenschaften amorpher transparenter leitfähiger Oxide (a‐TCOs)

Properties of amorphous transparent conducting oxides (a‐TCOs) In this contribution the optical and electrical properties of amorphous transparent conductive oxides (a‐TCOs) are analysed by means of atomistic material modelling. The investigation of model structures by computer simulations allows a deeper understanding of the physical phenomena. In the case of the a‐TCOs deep energy levels in the electronic band structure, that reduce for example the optical transparency, can be linked to structural defects on the atomic scale. This allows a well‐directed search for strategies which anneal these defects.     

Ultra thin nickel transparent electrodes

Transparent electrodes made of ultra thin metals have recently been demonstrated with performances comparable to those offered by transparent conductive oxides (TCOs), which are traditionally used in applications such as photovoltaic cells, light emitting devices, photodetectors and electro-optical modulators. In this work we report highly uniform, optically transparent and electrically conductive nickel films. Their good performance, combined with low cost and simplicity in processing, make ultra thin Ni films highly competitive, even with respect to the latest developments in TCO technology. Nickel films can be easily incorporated into an industrial process flow and could therefore be an attractive alternative to TCOs in many industrial applications.     

Oxide Semiconductor Thin‐Film Transistors: A Review of Recent Advances

Transparent electronics is today one of the most advanced topics for a wide range of device applications. The key components are wide bandgap semiconductors, where oxides of different origins play an important role, not only as passive component but also as active component, similar to what is observed in conventional semiconductors like silicon. Transparent electronics has gained special attention during the last few years and is today established as one of the most promising technologies for leading the next generation of flat panel display due to its excellent electronic performance. In this paper the recent progress in n‐ and p‐type oxide based thin‐film transistors (TFT) is reviewed, with special emphasis on solution‐processed and p‐type, and the major milestones already achieved with this emerging and very promising technology are summarizeed. After a short introduction where the main advantages of these semiconductors are presented, as well as the industry expectations, the beautiful history of TFTs is revisited, including the main landmarks in the last 80 years, finishing by referring to some papers that have played an important role in shaping transparent electronics. Then, an overview is presented of state of the art n‐type TFTs processed by physical vapour deposition methods, and finally one of the most exciting, promising, and low cost but powerful technologies is discussed: solution‐processed oxide TFTs. Moreover, a more detailed focus analysis will be given concerning p‐type oxide TFTs, mainly centred on two of the most promising semiconductor candidates: copper oxide and tin oxide. The most recent data related to the production of complementary metal oxide semiconductor (CMOS) devices based on n‐ and p‐type oxide TFT is also be presented. The last topic of this review is devoted to some emerging applications, finalizing with the main conclusions. Related work that originated at CENIMAT|I3N during the last six years is included in more detail, which has led to the fabrication of high performance n‐ and p‐type oxide transistors as well as the fabrication of CMOS devices with and on paper.     

Visually Imperceptible Liquid‐Metal Circuits for Transparent,Stretchable Electronics with Direct Laser Writing

A material architecture and laser‐based microfabrication technique is introduced to produce electrically conductive films (sheet resistance = 2.95 Ω sq^?1; resistivity = 1.77 × 10^?6 Ω m) that are soft, elastic (strain limit >100%), and optically transparent. The films are composed of a grid‐like array of visually imperceptible liquid‐metal (LM) lines on a clear elastomer. Unlike previous efforts in transparent LM circuitry, the current approach enables fully imperceptible electronics that have not only high optical transmittance (>85% at 550 nm) but are also invisible under typical lighting conditions and reading distances. This unique combination of properties is enabled with a laser writing technique that results in LM grid patterns with a line width and pitch as small as 4.5 and 100 µm, respectively—yielding grid‐like wiring that has adequate conductivity for digital functionality but is also well below the threshold for visual perception. The electrical, mechanical, electromechanical, and optomechanical properties of the films are characterized and it is found that high conductivity and transparency are preserved at tensile strains of ≈100%. To demonstrate their effectiveness for emerging applications in transparent displays and sensing electronics, the material architecture is incorporated into a couple of illustrative use cases related to chemical hazard warning.     

Highly transparent and conducting In doped CdO synthesized by sol-gel solution processing

Cadmium oxide (CdO) is a much-studied wide gap semiconductor with an inherent high mobility of?>?100 cm²/Vs, high electron concentration of?>?10²¹ cm^?3 and a wide optical transparency window of?>?1800 nm. These unique properties make CdO a potential transparent conductor for full spectrum photovoltaics. However, in order to achieve optimum material properties for optoelectronic applications, CdO was grown by vacuum-based physical or chemical vapor deposition methods. In this work, we explored the application of a low-cost sol-gel spin coating method to achieve highly conducting and transparent CdO thin films doped with 0–10% In (CdO:In). We find that while as-grown CdO:In films are nanocrystalline/amorphous with a high resistivity of?~?1 Ω-cm, polycrystalline and highly conducting films can be obtained after optimized annealing at?≥?400 °C. However, the electron concentration n saturates at?~?5?×?10²⁰ cm^?3 for In concentration?>?5% (or N_In?~?1.9?×?10²¹ cm^?3). This low activation of In may be attributed to the high density of native defects and/or impurities incorporated in the sol-gel process. With 5% In doping, we obtained a low resistivity of ρ?~?2.5?×?10^–4 Ω-cm and a high mobility μ?~?50 cm²/Vs. These values of σ and µ are better than those reported for other TCOs synthesized by solution processes and comparable to conventional commercial TCOs grown by physical vapor deposition methods. Benefiting from their high mobility, these sol-gel CdO:In films are optically transparent over a wide spectral range up to λ?>?1800 nm, making them promising as transparent conductors for optoelectronic devices utilizing the infrared photons.

     

Highly Crystalline C8‐BTBT Thin‐Film Transistors by Lateral Homo‐Epitaxial Growth on Printed Templates

Highly crystalline thin films of organic semiconductors offer great potential for fundamental material studies as well as for realizing high‐performance, low‐cost flexible electronics. The fabrication of these films directly on inert substrates is typically done by meniscus‐guided coating techniques. The resulting layers show morphological defects that hinder charge transport and induce large device‐to‐device variability. Here, a double‐step method for organic semiconductor layers combining a solution‐processed templating layer and a lateral homo‐epitaxial growth by a thermal evaporation step is reported. The epitaxial regrowth repairs most of the morphological defects inherent to meniscus‐guided coatings. The resulting film is highly crystalline and features a mobility increased by a factor of three and a relative spread in device characteristics improved by almost half an order of magnitude. This method is easily adaptable to other coating techniques and offers a route toward the fabrication of high‐performance, large‐area electronics based on highly crystalline thin films of organic semiconductors.     

Healable,Transparent, Room‐Temperature Electronic Sensors Based on Carbon Nanotube Network‐Coated Polyelectrolyte Multilayers

Transparent and conductive film based electronics have attracted substantial research interest in various wearable and integrated display devices in recent years. The breakdown of transparent electronics prompts the development of transparent electronics integrated with healability. A healable transparent chemical gas sensor device is assembled from layer‐by‐layer‐assembled transparent healable polyelectrolyte multilayer films by developing effective methods to cast transparent carbon nanotube (CNT) networks on healable substrates. The healable CNT network‐containing film with transparency and superior network structures on self‐healing substrate is obtained by the lateral movement of the underlying self‐healing layer to bring the separated areas of the CNT layer back into contact. The as‐prepared healable transparent film is assembled into healable transparent chemical gas sensor device for flexible, healable gas sensing at room temperature, due to the 1D confined network structure, relatively high carrier mobility, and large surface‐to‐volume ratio. The healable transparent chemical gas sensor demonstrates excellent sensing performance, robust healability, reliable flexibility, and good transparency, providing promising opportunities for developing flexible, healable transparent optoelectronic devices with the reduced raw material consumption, decreased maintenance costs, improved lifetime, and robust functional reliability.     

In Situ Electro‐Mechanical Experiments and Mechanics Modeling of Fracture in Indium Tin Oxide‐Based Multilayer Electrodes

Indium Tin Oxide (ITO) films are widely used as transparent electrodes in electronic displays and solar cells. However, the small fracture strain of brittle ITO films poses significant challenge to their applications in flexible electronics devices that often undergo large deformation. Inspired by recent development of inorganic/organic hybrid permeation barriers for flexible electronics, we design and fabricate ITO‐based multilayer electrodes with enhanced electro‐mechanical durability. In situ electro‐mechanical experiments of five structural designs of ITO‐based multilayer electrodes are performed to investigate the evolution of crack density and the corresponding variance of electrical resistance of such electrodes. A coherent mechanics model is established to determine the driving force for crack propagation in the ITO layer in these electrodes. The mechanics model suggests that a top protective polymeric coating above and an intermediate polymeric layer below the ITO layer can effectively enhance the mechanical durability of the ITO electrodes by reducing the crack driving force up to 10‐folds. The modeling results offer mechanistic understanding of the in situ experimental measurements of the critical fracture strains of the five types of ITO‐based multilayer electrodes. The findings in this work provide quantitative guidance for the material selection and structural optimization of ITO‐based multilayer transparent electrodes of high mechanical durability.     

Solution Combustion Synthesis: Low‐Temperature Processing for p‐Type Cu:NiO Thin Films for Transparent Electronics

Low‐temperature solution processing opens a new window for the fabrication of oxide semiconductors due to its simple, low cost, and large‐area uniformity. Herein, by using solution combustion synthesis (SCS), p‐type Cu‐doped NiO (Cu:NiO) thin films are fabricated at a temperature lower than 150 °C. The light doping of Cu substitutes the Ni site and disperses the valence band of the NiO matrix, leading to an enhanced p‐type conductivity. Their integration into thin‐film transistors (TFTs) demonstrates typical p‐type semiconducting behavior. The optimized Cu_5%NiO TFT exhibits outstanding electrical performance with a hole mobility of 1.5 cm² V^?1 s^?1, a large on/off current ratio of ≈10⁴, and clear switching characteristics under dynamic measurements. The employment of a high‐k ZrO₂ gate dielectric enables a low operating voltage (≤2 V) of the TFTs, which is critical for portable and battery‐driven devices. The construction of a light‐emitting‐diode driving circuit demonstrates the high current control capability of the resultant TFTs. The achievement of the low‐temperature‐processed Cu:NiO thin films via SCS not only provides a feasible approach for low‐cost flexible p‐type oxide electronics but also represents a significant step toward the development of complementary metal–oxide semiconductor circuits.     

Electrical,optical, and structural properties of thin films with tri-layers of AZO/ZnMgO/AZO grown by filtered vacuum arc deposition

Transparent conductive oxides (TCO) are indispensable as front electrode for most of thin film electronic devices such as transparent electrodes for flat panel displays, photovoltaic cells, windshield defrosters, transparent thin film transistors, and low emissivity windows. Thin films of aluminum-doped zinc oxide (AZO) have shown to be one of the most promising TCOs. In this study, three layered Al-doped ZnO (AZO)/ZnMgO/AZO heterostructures were prepared by filtered cathodic arc deposition (FCAD) on glass substrates. The objective is to find a set of parameters that will allow for improved optical and electrical properties of the films such as low resistivity, high mobility, high number of charge carriers, and high transmittance. We have investigated the effect of modifications in thickness and doping of the ZnMgO inner layer on the structural, electrical, and optical characteristics of the stacked heterostructures.     

A New Facile Route to Flexible and Semi‐Transparent Electrodes Based on Water Exfoliated Graphene and their Single‐Electrode Triboelectric Nanogenerator

Wearable technologies are driving current research efforts to self‐powered electronics, for which novel high‐performance materials such as graphene and low‐cost fabrication processes are highly sought.The integration of high‐quality graphene films obtained from scalable water processing approaches in emerging applications for flexible and wearable electronics is demonstrated. A novel method for the assembly of shear exfoliated graphene in water, comprising a direct transfer process assisted by evaporation of isopropyl alcohol is developed. It is shown that graphene films can be easily transferred to any target substrate such as paper, flexible polymeric sheets and fibers, glass, and Si substrates. By combining graphene as the electrode and poly(dimethylsiloxane) as the active layer, a flexible and semi‐transparent triboelectric nanogenerator (TENG) is demonstrated for harvesting energy. The results constitute a new step toward the realization of energy harvesting devices that could be integrated with a wide range of wearable and flexible technologies, and opens new possibilities for the use of TENGs in many applications such as electronic skin and wearable electronics.     

还原氧化石墨烯基纳米银线透明导电薄膜研究进展

透明导电薄膜已广泛应用于印刷电子领域,传统的透明导电薄膜氧化铟锡（ITO）因其高脆性低柔韧性而不能满足高速发展的柔性电子行业;纳米银线（AgNWs）和石墨烯均具有良好光学性能、导电性能以及机械性能,使其能成为制备透明导电薄膜的理想材料。综述了近年来还原氧化石墨烯（rGO）基AgNWs透明导电薄膜的研究进展。介绍了柔性导电薄膜的关键参数及rGO/AgNWs透明导电薄膜的成膜工艺;归纳了影响rGO/AgNWs透明导电薄膜光电性能的主要因素和相关研究;阐述了rGO/AgNWs透明导电薄膜在印刷电子领域的应用现状,并展望了rGO/AgNWs透明导电薄膜的未来发展趋势。     

Wafer‐Scale Patterning of Reduced Graphene Oxide Electrodes by Transfer‐and‐Reverse Stamping for High Performance OFETs

A wafer‐scale patterning method for solution‐processed graphene electrodes, named the transfer‐and‐reverse stamping method, is universally applicable for fabricating source/drain electrodes of n‐ and p‐type organic field‐effect transistors with excellent performance. The patterning method begins with transferring a highly uniform reduced graphene oxide thin film, which is pre‐prepared on a glass substrate, onto hydrophobic silanized (rigid/flexible) substrates. Patterns of the as‐prepared reduced graphene oxide films are then formed by modulating the surface energy of the films and selectively delaminating the films using an oxygen‐plasma‐treated elastomeric stamp with patterns. Reduced graphene oxide patterns with various sizes and shapes can be readily formed onto an entire wafer. Also, they can serve as the source/drain electrodes for benchmark n‐ and p‐type organic field‐effect transistors with enhanced performance, compared to those using conventional metal electrodes. These results demonstrate the general utility of this technique. Furthermore, this simple, inexpensive, and scalable electrode‐patterning‐technique leads to assembling organic complementary circuits onto a flexible substrate successfully.     

Low temperature (< 100 °C) deposited P-type cuprous oxide thin films: Importance of controlled oxygen and deposition energy

With the emergence of transparent electronics, there has been considerable advancement in n-type transparent semiconducting oxide (TSO) materials, such as ZnO, InGaZnO, and InSnO. Comparatively, the availability of p-type TSO materials is more scarce and the available materials are less mature. The development of p-type semiconductors is one of the key technologies needed to push transparent electronics and systems to the next frontier, particularly for implementing p-n junctions for solar cells and p-type transistors for complementary logic/circuits applications. Cuprous oxide (Cu₂O) is one of the most promising candidates for p-type TSO materials. This paper reports the deposition of Cu₂O thin films without substrate heating using a high deposition rate reactive sputtering technique, called high target utilisation sputtering (HiTUS). This technique allows independent control of the remote plasma density and the ion energy, thus providing finer control of the film properties and microstructure as well as reducing film stress. The effect of deposition parameters, including oxygen flow rate, plasma power and target power, on the properties of Cu₂O films are reported. It is known from previously published work that the formation of pure Cu₂O film is often difficult, due to the more ready formation or co-formation of cupric oxide (CuO). From our investigation, we established two key concurrent criteria needed for attaining Cu₂O thin films (as opposed to CuO or mixed phase CuO/Cu₂O films). First, the oxygen flow rate must be kept low to avoid over-oxidation of Cu₂O to CuO and to ensure a non-oxidised/non-poisoned metallic copper target in the reactive sputtering environment. Secondly, the energy of the sputtered copper species must be kept low as higher reaction energy tends to favour the formation of CuO. The unique design of the HiTUS system enables the provision of a high density of low energy sputtered copper radicals/ions, and when combined with a controlled amount of oxygen, can produce good quality p-type transparent Cu₂O films with electrical resistivity ranging from 10² to 10⁴ Ω-cm, hole mobility of 1-10 cm²/V-s, and optical band-gap of 2.0-2.6 eV. These material properties make this low temperature deposited HiTUS Cu₂O film suitable for fabrication of p-type metal oxide thin film transistors. Furthermore, the capability to deposit Cu₂O films with low film stress at low temperatures on plastic substrates renders this approach favourable for fabrication of flexible p-n junction solar cells.     

Stretchable and Tunable Microtectonic ZnO‐Based Sensors and Photonics

The concept of realizing electronic applications on elastically stretchable “skins” that conform to irregularly shaped surfaces is revolutionizing fundamental research into mechanics and materials that can enable high performance stretchable devices. The ability to operate electronic devices under various mechanically stressed states can provide a set of unique functionalities that are beyond the capabilities of conventional rigid electronics. Here, a distinctive microtectonic effect enabled oxygen‐deficient, nanopatterned zinc oxide (ZnO) thin films on an elastomeric substrate are introduced to realize large area, stretchable, transparent, and ultraportable sensors. The unique surface structures are exploited to create stretchable gas and ultraviolet light sensors, where the functional oxide itself is stretchable, both of which outperform their rigid counterparts under room temperature conditions. Nanoscale ZnO features are embedded in an elastomeric matrix function as tunable diffraction gratings, capable of sensing displacements with nanometre accuracy. These devices and the microtectonic oxide thin film approach show promise in enabling functional, transparent, and wearable electronics.     

Lateral Monolayer MoSe2–WSe2 p–n Heterojunctions with Giant Built‐In Potentials

2D transition metal dichalcogenides (TMDs) have exhibited strong application potentials in new emerging electronics because of their atomic thin structure and excellent flexibility, which is out of field of tradition silicon technology. Similar to 3D p–n junctions, 2D p–n heterojunctions by laterally connecting TMDs with different majority charge carriers (electrons and holes), provide ideal platform for current rectifiers, light‐emitting diodes, diode lasers and photovoltaic devices. Here, growth and electrical studies of atomic thin high‐quality p–n heterojunctions between molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂) by one‐step chemical vapor deposition method are reported. These p–n heterojunctions exhibit high built‐in potential (≈0.7 eV), resulting in large current rectification ratio without any gate control for diodes, and fast response time (≈6 ms) for self‐powered photodetectors. The simple one‐step growth and electrical studies of monolayer lateral heterojunctions open up the possibility to use TMD heterojunctions for functional devices.     

Highly Flexible Hybrid CMOS Inverter Based on Si Nanomembrane and Molybdenum Disulfide

2D semiconductor materials are being considered for next generation electronic device application such as thin‐film transistors and complementary metal–oxide–semiconductor (CMOS) circuit due to their unique structural and superior electronics properties. Various approaches have already been taken to fabricate 2D complementary logics circuits. However, those CMOS devices mostly demonstrated based on exfoliated 2D materials show the performance of a single device. In this work, the design and fabrication of a complementary inverter is experimentally reported, based on a chemical vapor deposition MoS₂ n‐type transistor and a Si nanomembrane p‐type transistor on the same substrate. The advantages offered by such CMOS configuration allow to fabricate large area wafer scale integration of high performance Si technology with transition‐metal dichalcogenide materials. The fabricated hetero‐CMOS inverters which are composed of two isolated transistors exhibit a novel high performance air‐stable voltage transfer characteristic with different supply voltages, with a maximum voltage gain of ≈16, and sub‐nano watt power consumption. Moreover, the logic gates have been integrated on a plastic substrate and displayed reliable electrical properties paving a realistic path for the fabrication of flexible/transparent CMOS circuits in 2D electronics.     

Chemically Derived Graphene Oxide: Towards Large‐Area Thin‐Film Electronics and Optoelectronics

Chemically derived graphene oxide (GO) possesses a unique set of properties arising from oxygen functional groups that are introduced during chemical exfoliation of graphite. Large‐area thin‐film deposition of GO, enabled by its solubility in a variety of solvents, offers a route towards GO‐based thin‐film electronics and optoelectronics. The electrical and optical properties of GO are strongly dependent on its chemical and atomic structure and are tunable over a wide range via chemical engineering. In this Review, the fundamental structure and properties of GO‐based thin films are discussed in relation to their potential applications in electronics and optoelectronics.