Current State and Future Perspectives of Printable Organic and Perovskite Solar Cells

Photovoltaic technology presents a sustainable solution to address the escalating global energy consumption and a reliable strategy for achieving net-zero carbon emissions by 2050. Emerging photovoltaic technologies, especially the printable organic and perovskite solar cells, have attracted extensive attention due to their rapidly transcending power conversion efficiencies and facile processability, providing great potential to revolutionize the global photovoltaic market. To accelerate these technologies to translate from the laboratory scale to the industrial level, it is critical to develop well-defined and scalable protocols to deposit high-quality thin films of photoactive and charge-transporting materials. Herein, the current state of printable organic and perovskite solar cells is summarized and the view regarding the challenges and prospects toward their commercialization is shared. Different printing techniques are first introduced to provide a correlation between material properties and printing mechanisms, and the optimization of ink formulation and film-formation during large-area deposition of different functional layers in devices are then discussed. Engineering perspectives are also discussed to analyze the criteria for module design. Finally, perspectives are provided regarding the future development of these solar cells toward practical commercialization. It is believed that this perspective will provide insight into the development of printable solar cells and other electronic devices.

MXene-Based Micro-Supercapacitors: Ink Rheology, Microelectrode Design and Integrated System

MXenes have shown great potential for micro-supercapacitors (MSCs) due to the high metallic conductivity, tunable interlayer spacing and intercalation pseudocapacitance. In particular, the negative surface charge and high hydrophilicity of MXenes make them suitable for various solution processing strategies. Nevertheless, a comprehensive review of solution processing of MXene MSCs has not been conducted. In this review, we present a comprehensive summary of the state-of-the-art of MXene MSCs in terms of ink rheology, microelectrode design and integrated system. The ink formulation and rheological behavior of MXenes for different solution processing strategies, which are essential for high quality printed/coated films, are presented. The effects of MXene and its compounds, 3D electrode structure, and asymmetric design on the electrochemical properties of MXene MSCs are discussed in detail. Equally important, we summarize the integrated system and intelligent applications of MXene MSCs and present the current challenges and prospects for the development of high-performance MXene MSCs.

Organic Transistors Based on Highly Crystalline Donor-Acceptor π-Conjugated Polymer of Pentathiophene and Diketopyrrolopyrrole

Oligomers and polymers consisting of multiple thiophenes are widely used in organic electronics such as organic transistors and sensors because of their strong electron-donating ability. In this study, a solution to the problem of the poor solubility of polythiophene systems was developed. A novel π-conjugated polymer material, PDPP-5Th, was synthesized by adding the electron acceptor unit, DPP, to the polythiophene system with a long alkyl side chain, which facilitated the solution processing of the material for the preparation of devices. Meanwhile, the presence of the multicarbonyl groups within the DPP molecule facilitated donor-acceptor interactions in the internal chain, which further improved the hole-transport properties of the polythiophene-based material. The weak forces present within the molecules that promoted structural coplanarity were analyzed using theoretical simulations. Furthermore, the grazing incidence wide-angle X-ray scanning (GIWAXS) results indicated that PDPP-5Th features high crystallinity, which is favorable for efficient carrier migration within and between polymer chains. The material showed hole transport properties as high as 0.44 cm2 V-1 s-1 in conductivity testing. Our investigations demonstrate the great potential of this polymer material in the field of optoelectronics.

Defect-Engineered Semiconducting van der Waals Thin Film at Metal-Semiconductor Interface of Field-Effect Transistors

The significance of metal-semiconductor interfaces and their impact on electronic device performance have gained increasing attention, with a particular focus on investigating the contact metal. However, another avenue of exploration involves substituting the contact metal at the metal-semiconductor interface of field-effect transistors with semiconducting layers to introduce additional functionalities to the devices. Here, a scalable approach for fabricating metal-oxide-semiconductor (channel)-semiconductor (interfacial layer) field-effect transistors is proposed by utilizing solution-processed semiconductors, specifically semiconducting single-walled carbon nanotubes and molybdenum disulfide, as the channel and interfacial semiconducting layers, respectively. The work function of the interfacial MoS2 is modulated by controlling the sulfur vacancy concentration through chemical treatment, which results in distinctive energy band alignments within a single device configuration. The resulting band alignments lead to multiple functionalities, including multivalued transistor characteristics and multibit nonvolatile memory (NVM) behavior. Moreover, leveraging the stable NVM properties, we demonstrate artificial synaptic devices with 88.9% accuracy of MNIST image recognition.

Incorporation of Diketopyrrolopyrrole into Polythiophene for the Preparation of Organic Polymer Transistors

Polythiophene, as a class of potential electron donor units, is widely used in organic electronics such as transistors. In this work, a novel polymeric material, PDPPTT-FT, was prepared by incorporating the electron acceptor unit into the polythiophene system. The incorporation of the DPP molecule assists in improving the solubility of the material and provides a convenient method for the preparation of field effect transistors via subsequent solution processing. The introduction of fluorine atoms forms a good intramolecular conformational lock, and theoretical calculations show that the structure displays excellent co-planarity and regularity. Grazing incidence wide-angle X-ray (GIWAXS) results indicate that the PDPPTT-FT is highly crystalline, which facilitates carrier migration within and between polymer chains. The hole mobility of this π-conjugated material is as high as 0.30 cm2 V-1 s-1 in organic transistor measurements, demonstrating the great potential of this polymer material in the field of optoelectronics.

Large-Area Soluble Covalent Organic Framework Oligomer Coating for Organic Solution Nanofiltration Membranes

Covalent organic frameworks (COFs) are a family of engaging membrane materials for molecular separation, which remain challenging to fabricate in the form of thin-film composite membranes due to slow crystal growth and insoluble powder. Here, an additive approach is presented to construct COF-based thin-film composite membranes in 10 min via COF oligomer coating onto poly(ether ether ketone) (PEEK）ultrafiltration membranes. By the virtue of ultra-thin liquid phase and liquid-solid interface-confined assembly, the COF oligomers are fast stacked up and grow along the interface with the solvent evaporation. Benefiting from the low out-plane resistance of COFs, COF@PEEK composite membranes exhibit high solvent permeances in a negative correlation with solvent viscosity. The well-defined pore structures enable high molecular sieving ability (Mw = 300 g mol-1 ). Besides, the COF@PEEK composite membranes possess excellent mechanical integrities and steadily operate for over 150 h in the condition of high-pressure cross flow. This work not only exemplifies the high-efficiency and scale-up preparation of COF-based thin-film composite membranes but also provides a new strategy for COF membrane processing.

Band Engineering Through Pb-Doping of Nanocrystal Building Blocks to Enhance Thermoelectric Performance in Cu3 SbSe4

Developing cost-effective and high-performance thermoelectric (TE) materials to assemble efficient TE devices presents a multitude of challenges and opportunities. Cu3 SbSe4 is a promising p-type TE material based on relatively earth abundant elements. However, the challenge lies in its poor electrical conductivity. Herein, an efficient and scalable solution-based approach is developed to synthesize high-quality Cu3 SbSe4 nanocrystals doped with Pb at the Sb site. After ligand displacement and annealing treatments, the dried powders are consolidated into dense pellets, and their TE properties are investigated. Pb doping effectively increases the charge carrier concentration, resulting in a significant increase in electrical conductivity, while the Seebeck coefficients remain consistently high. The calculated band structure shows that Pb doping induces band convergence, thereby increasing the effective mass. Furthermore, the large ionic radius of Pb2+ results in the generation of additional point and plane defects and interphases, dramatically enhancing phonon scattering, which significantly decreases the lattice thermal conductivity at high temperatures. Overall, a maximum figure of merit (zTmax ) ≈ 0.85 at 653 K is obtained in Cu3 Sb0.97 Pb0.03 Se4 . This represents a 1.6-fold increase compared to the undoped sample and exceeds most doped Cu3 SbSe4 -based materials produced by solid-state, demonstrating advantages of versatility and cost-effectiveness using a solution-based technology.

Blade Coating Poly(3-hexylthiophene): The Importance of Molecular Weight on Thin-Film Microstructures

Poly(3-hexylthiophene) is one of the most prevalent and promising conjugated polymers for use in organic electronics. However, the deposition of this material in thin films is highly dependent on the process, such as blade coating versus spin coating and material properties such as molecular weight. Typically, large polymer dispersity makes it difficult to isolate the effect of molecular weight without considering a distribution. In this study, we characterize oligothiophenes of exactly 8, 11, and 14 repeat units, which were deposited into thin films by varying blade coating conditions and postdeposition annealing. From synchrotron-based grazing incidence wide-angle X-ray scattering (GIWAXS), scanning transmission X-ray microscopy (STXM) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS), Raman microscopy, optical microscopy, and X-ray diffraction (XRD), it was suggested that higher molecular weight polymers exhibit a fast-forming crystalline polymorph (form-1) while low molecular weight polymers exhibit a slow forming polymorph (form-2) with large domain boundaries. As molecular weight is gradually increased, the polymorph formed transitions from form-1 and form-2, where 11 repeat unit oligomers display both polymorphs. We also found that processing conditions can increase the formation of the form-2 polymorph. We also report improved organic thin film transistor (OTFT) performance when form-1 is present. Overall, oligothiophene polymorph formation is highly dependent on the molecular weight and processing conditions, providing critical insight into the importance of polymer weight control in the development of thin-film electronics based on conjugated polymers.

Ferri-hydrite: A Novel Electron-Selective Contact Layer for InP Photovoltaic and Photoelectrochemical Cells

Solar energy conversion devices with charge-selective contacts are attracting significant research interest as a cost-effective alternative to homojunction counterparts. This study presents a novel approach for fabricating high-performance solar cells based on InP heterojunctions using a solution-processed ferri-hydrite (Fh) electron-selective contact (ESC). The champion cell efficiency of 16.6% is achieved, which is a significant improvement over those from previous studies using other solution-processed ESC materials. X-ray photoelectron spectroscopy measurements showed that the low conduction band offset at the Fh-InP interface facilitated selective transport of photogenerated electrons from InP. Moreover, the Fh electron-selective contact layer provided an excellent photoelectrochemical half-cell water reduction efficiency of 8.4%. The Fh layer not only selectively extracts photogenerated electrons from InP but also simultaneously serves as a surface protection layer, improving the cell's long-term stability. These results demonstrate the potential of Fh as a low-cost and easily fabricated material for use in high-efficiency photovoltaic and photoelectrochemical devices. Our findings pave the way for further improvements in the efficiency of InP heterojunction solar cells by addressing the losses incurred in the cells.

Transparent Conductors Printed from Grids of Highly Conductive Silver Nanosheets

Transparent conductors (TCs) represent key components in many applications from optoelectronic devices to electromagnetic shielding. While commercial applications typically use thin films of indium tin oxide, this material is brittle and increasingly scarce, meaning higher performing and cheaper alternatives are sought after. Solution-processible metals would be ideal owing to their high conductivities and printability. However, due to their opacity to visible light, such films need to be very thin to achieve transparency, thus limiting the minimum resistance achievable. One solution is to print metallic particles in a grid structure, which has the advantages of high tunable transparency and resistance at the cost of uniformity. Here, we report silver nanosheets that have been aerosol jet printed into grids as high-performance transparent conductors. We first investigate the effect of annealing on the silver nanosheets where we observe the onset of junction sintering at 160 °C after which the silver network becomes continuous. We then investigate the effect of line width and thickness on the electrical performance and the effect of varying the aperture dimensions on the optical performance. Using these data, we develop simple models, which allow us to optimize the grid and demonstrate a printed transparent conductor with a transmittance of 91% at a sheet resistance of 4.6 Ω/sq.

Continuous Production of Functionalized Graphene Inks by Soft Solution Processing

The continuous production of high-quality, few-layer graphene nanosheets (GNSs) functionalized with nitrogen-containing groups was achieved via a two-stage reaction method. The initial stage produces few-layer GNSs by utilizing our recently developed glycine-bisulfate ionic complex-assisted electrochemical exfoliation of graphite. The second stage, developed here, uses a radical initiator and nitrogen precursor (azobisisobutyronitrile) under microwave conditions in an aqueous solution for the efficient nitrogen functionalization of the initially formed GNSs. These nitrile radical reactions have great advantages in green chemistry and soft processing. Raman spectra confirm the insertion of nitrogen functional groups into nitrogen-functionalized graphene (N-FG), whose disorder is higher than that of GNSs. X-ray photoelectron spectra confirm the insertion of edge/surface nitrogen functional groups. The insertion of nitrogen functional groups is further confirmed by the enhanced dispersibility of N-FG in dimethyl formamide, ethylene glycol, acetonitrile, and water. Indeed, after the synthesis of N-FG in solution, it is possible to disperse N-FG in these liquid dispersants just by a simple washing-centrifugation separation-dispersion sequence. Therefore, without any drying, milling, and redispersion into liquid again, we can produce N-FG ink with only solution processing. Thus, the present work demonstrates the 'continuous solution processing' of N-FG inks without complicated post-processing conditions. Furthermore, the formation mechanism of N-FG is presented.

Surface Chemistry and Band Engineering in AgSbSe2: Toward High Thermoelectric Performance

AgSbSe₂ is a promising thermoelectric (TE) p-type material for applications in the middle-temperature range. AgSbSe₂ is characterized by relatively low thermal conductivities and high Seebeck coefficients, but its main limitation is moderate electrical conductivity. Herein, we detail an efficient and scalable hot-injection synthesis route to produce AgSbSe₂ nanocrystals (NCs). To increase the carrier concentration and improve the electrical conductivity, these NCs are doped with Sn²⁺ on Sb³⁺ sites. Upon processing, the Sn²⁺ chemical state is conserved using a reducing NaBH₄ solution to displace the organic ligand and anneal the material under a forming gas flow. The TE properties of the dense materials obtained from the consolidation of the NCs using a hot pressing are then characterized. The presence of Sn²⁺ ions replacing Sb³⁺ significantly increases the charge carrier concentration and, consequently, the electrical conductivity. Opportunely, the measured Seebeck coefficient varied within a small range upon Sn doping. The excellent performance obtained when Sn²⁺ ions are prevented from oxidation is rationalized by modeling the system. Calculated band structures disclosed that Sn doping induces convergence of the AgSbSe₂ valence bands, accounting for an enhanced electronic effective mass. The dramatically enhanced carrier transport leads to a maximized power factor for AgSb_0.98Sn_0.02Se₂ of 0.63 mW m^-1 K^-2 at 640 K. Thermally, phonon scattering is significantly enhanced in the NC-based materials, yielding an ultralow thermal conductivity of 0.3 W mK^-1 at 666 K. Overall, a record-high figure of merit (zT) is obtained at 666 K for AgSb_0.98Sn_0.02Se₂ at zT = 1.37, well above the values obtained for undoped AgSbSe₂, at zT = 0.58 and state-of-art Pb- and Te-free materials, which makes AgSb_0.98Sn_0.02Se₂ an excellent p-type candidate for medium-temperature TE applications.

Versatile Host Materials for Both D-A-Type and Multi-Resonance TADF Emitters toward Solution-Processed OLEDs with Nearly 30% EQE

Fabricating solution-processible host material for thermally activated delayed fluorescence (TADF) emitter remains a formidable challenge for organic light-emitting diodes (OLEDs). In this work, two new host materials, namely 3CzAcPy and 9CzAcPy, are found to exhibit high triplet energy levels, high thermal stability, and excellent film morphology from a solution process. An in-depth analysis on the photophysical data and device performance reveals the isomeric effect of the host materials has a significant impact not only on the host properties, but also on the host-dopant interactions and thus the performance of the resulting solution-processed TADF OLEDs. Impressively, the new hosts are proven to be suitable for both donor-acceptor type and multi-resonance TADF emitters, achieving state-of-the-art device performance. By using the new host 9CzAcPy, solution-processed OLED based on a donor-acceptor TADF emitter of DPAC-PCN, a maximum external quantum efficiency (EQE) of 29.5% is achieved, and solution-processed narrowband OLED based on a multiple-resonance TADF emitter of BN-CP1 acquires a maximum EQE of 26.6%. These efficiencies represent the highest values among the solution-processed TADF OLEDs. This study highlights the significance of host-dopant interactions in modulating the electroluminescence performance of TADF emitters, and provides an effective design principle for solution-processible host materials.

High-Mobility Flexible Transistors with Low-Temperature Solution-Processed Tungsten Dichalcogenides

The investigation of high-mobility two-dimensional (2D) flakes beyond molybdenum disulfide (MoS₂) will be necessary to create a library of high-mobility solution-processed networks that conform to substrates and remain functional over thousands of bending cycles. Here we report electrochemical exfoliation of large-aspect-ratio (>100) semiconducting flakes of tungsten diselenide (WSe₂) and tungsten disulfide (WS₂) as well as MoS₂ as a comparison. We use Langmuir-Schaefer coating to achieve highly aligned and conformal flake networks, with minimal mesoporosity (∼2-5%), at low processing temperatures (120 °C) and without acid treatments. This allows us to fabricate electrochemical transistors in ambient air, achieving average mobilities of μ_MoS2 ≈ 11 cm² V^-1 s^-1, μ_WS2 ≈ 9 cm² V^-1 s^-1, and μ_WSe2 ≈ 2 cm² V^-1 s^-1 with a current on/off ratios of I_on/I_off ≈ 2.6 × 10³, 3.4 × 10³, and 4.2 × 10⁴ for MoS₂, WS₂, and WSe₂, respectively. Moreover, our transistors display threshold voltages near ∼0.4 V with subthreshold slopes as low as 182 mV/dec, which are essential factors in maintaining power efficiency and represent a 1 order of magnitude improvement in the state of the art. Furthermore, the performance of our WSe₂ transistors is maintained on polyethylene terephthalate (PET) even after 1000 bending cycles at 1% strain.

All-Solution-Processed Quantum Dot Light-Emitting Diode Using Phosphomolybdic Acid as Hole Injection Layer

In this study, we investigate phosphomolybdic acid (PMA), which allows solution processing of quantum dot light-emitting diodes. With its low cost, easy solution processes, and excellent physical and optical properties, PMA is a potential candidate as the hole injection layer (HIL) in optoelectronic devices. We evaluate the physical and electrical properties of PMA using various solvents. The surface morphology of the PMA film was improved using a solvent with appropriate boiling points, surface tension, and viscosity to form a smooth, pinhole-free film. The energy level was regulated according to the solvent, and PMA with the appropriate electronic structure provided balanced charge carrier transport in quantum dot electroluminescent (QD-EL) devices with enhanced efficiency. A device using PMA dissolved in cyclohexanone was demonstrated to exhibit improved efficiency compared to a device using PEDOT:PSS, which is a conventional solution HIL. However, the stability of PMA was slightly poorer than PEDOT:PSS; there needs to be further investigation.

Two-Dimensional Janus MXene Inks for Versatile Functional Coatings on Arbitrary Substrates

Solution processing of two-dimensional nanomaterial inks guarantees efficient, straightforward fabrication of functional films, coatings, flexible devices, etc. Despite the excellent solution processibility and viscoelasticity of MXene aqueous inks, formulation of nonaqueous MXene inks with great affinity to both hydrophilic and hydrophobic substrates has proven quite challenging, limiting the practical applications of MXenes in printing/coatings on various substrates. Here, MXene surface chemistry is manipulated by asymmetrically grafting polystyrene and further concentrating the flakes into additive-free Janus MXene organic inks. The modified MXene nanosheets exhibit hydrophilicity on one side and hydrophobicity on the other. As a result, Janus MXene nanosheets ensure broad dispersibility in polar and nonpolar solvents, which in turn greatly extends the ink shelf life by slowing down the oxidation kinetics. Janus MXene sheets dispersed in toluene at room temperature remain at 90% of the initial solids after 1 month of storage. Janus surface engineering on MXene flakes guarantees the straightforward formation of uniform yet firm, large-area coatings on hydrophilic or hydrophobic substrates. These coatings demonstrate improved photothermal properties and chemical stability as well as good electromagnetic interference shielding performance. This strategy provides a simple and cost-effective way to promote the performance of MXene electronics in a variety of applications.

High-performance and low-power source-gated transistors enabled by a solution-processed metal oxide homojunction

Cost-effective fabrication of mechanically flexible low-power electronics is important for emerging applications including wearable electronics, artificial intelligence, and the Internet of Things. Here, solution-processed source-gated transistors (SGTs) with an unprecedented intrinsic gain of ~2,000, low saturation voltage of +0.8 ± 0.1 V, and a ~25.6 μW power consumption are realized using an indium oxide In₂O₃/In₂O₃:polyethylenimine (PEI) blend homojunction with Au contacts on Si/SiO₂. Kelvin probe force microscopy confirms source-controlled operation of the SGT and reveals that PEI doping leads to more effective depletion of the reverse-biased Schottky contact source region. Furthermore, using a fluoride-doped AlO_x gate dielectric, rigid (on a Si substrate) and flexible (on a polyimide substrate) SGTs were fabricated. These devices exhibit a low driving voltage of +2 V and power consumption of ~11.5 μW, yielding inverters with an outstanding voltage gain of >5,000. Furthermore, electrooculographic (EOG) signal monitoring can now be demonstrated using an SGT inverter, where a ~1.0 mV EOG signal is amplified to over 300 mV, indicating significant potential for applications in wearable medical sensing and human-computer interfacing.

Entrapped Molecule-Like Europium-Oxide Clusters in Zinc Oxide with Nearly Unaffected Host Structure

Nanocrystalline ZnO sponges doped with 5 mol% EuO_1.5 are obtained by heating metal-salt complex based precursor pastes at 200-900 °C for 3 min. X-ray diffraction, transmission electron microscopy, and extended X-ray absorption fine structure (EXAFS) show that phase separation into ZnO:Eu and c-Eu₂ O₃ takes place upon heating at 700 °C or higher. The unit cell of the clean oxide made at 600 °C shows only ≈0.4% volume increase versus undoped ZnO, and EXAFS shows a ZnO local structure that is little affected by the Eu-doping and an average Eu³⁺ ion coordination number of ≈5.2. Comparisons of 23 density functional theory-generated structures having differently sized Eu-oxide clusters embedded in ZnO identify three structures with four or eight Eu atoms as the most energetically favorable. These clusters exhibit the smallest volume increase compared to undoped ZnO and Eu coordination numbers of 5.2-5.5, all in excellent agreement with experimental data. ZnO defect states are crucial for efficient Eu³⁺ excitation, while c-Eu₂ O₃ phase separation results in loss of the characteristic Eu³⁺ photoluminescence. The formation of molecule-like Eu-oxide clusters, entrapped in ZnO, proposed here, may help in understanding the nature of the unexpected high doping levels of lanthanide ions in ZnO that occur virtually without significant change in ZnO unit cell dimensions.

Hierarchical Nanoscale Structuring of Solution-Processed 2D van der Waals Networks for Wafer-Scale, Stretchable Electronics

Two-dimensional (2D) semiconductors are promising for next-generation electronics that are lightweight, flexible, and stretchable. Achieving stretchability with suppressed crack formation, however, is still difficult without introducing lithographically etched micropatterns, which significantly reduces active device areas. Herein, we report a solution-based hierarchical structuring to create stretchable semiconducting films that are continuous over wafer-scale areas via self-assembly of two-dimensional nanosheets. Electrochemically exfoliated MoS₂ nanosheets with large lateral sizes (∼1 μm) are first assembled into a uniform film on a prestrained thermoplastic substrate, followed by strain relief of the substrate to create nanoscale wrinkles. Subsequent strain-relief cycles with the presence of soluble polymer films produce hierarchical wrinkles with multigenerational structures. Stretchable MoS₂ films are then realized by curing an elastomer directly on the wrinkled surface and dissolving the thermoplastic. Three-generation hierarchical MoS₂ wrinkles are resistant to cracking up to nearly 100% substrate stretching and achieve drastically enhanced photoresponsivity compared to the flat counterpart over the visible and NIR regimes, while the flat MoS₂ film is beneficial in creating strain sensors because of its strain-dependent electrical response.

High-Yield Ti3 C2 Tx MXene-MoS2 Integrated Circuits

It is very challenging to employ solution-processed conducting films in large-area ultrathin nanoelectronics. Here, spray-coated Ti₃ C₂ T_x MXene films as metal contacts are successfully integrated into sub-10 nm gate oxide 2D MoS₂ transistor circuits. Ti₃ C₂ T_x films are spray coated on glass substrates followed by vacuum annealing. Compared to the as-prepared sample, vacuum annealed films exhibit a higher conductivity (≈11 000 S cm^-1 ) and a lower work function (≈4.5 eV). Besides, the annealed Ti₃ C₂ T_x film can be patterned through a standard cleanroom process without peeling off. The annealed Ti₃ C₂ T_x film shows a better band alignment for n-type transport in MoS₂ channel with small work function mismatch of 0.06 eV. The MoS₂ film can be uniformly transferred on the patterned Ti₃ C₂ T_x surface and then readily processed through the cleanroom process. A large-area array of Ti₃ C₂ T_x MXene-MoS₂ transistors is fabricated using different dielectric thicknesses and semiconducting channel sizes. High yield and stable performance for these transistor arrays even with an 8 nm-thick dielectric layer are demonstrated. Besides, several circuits are demonstrated, including rectifiers, negative-channel metal-oxide-semiconductor (NMOS) inverters, and voltage-shift NMOS inverters. Overall, this work indicates the tremendous potential for solution-processed Ti₃ C₂ T_x MXene films in large-area 2D nanoelectronics.

van der Waals SWCNT@BN Heterostructures Synthesized from Solution-Processed Chirality-Pure Single-Wall Carbon Nanotubes

Single-wall carbon nanotubes in boron nitride (SWCNT@BN) are one-dimensional van der Waals heterostructures that exhibit intriguing physical and chemical properties. As with their carbon nanotube counterparts, these heterostructures can form from different combinations of chiralities, providing rich structures but also posing a significant synthetic challenge to controlling their structure. Enabled by advances in nanotube chirality sorting, clean removal of the surfactant used for solution processing, and a simple method to fabricate free-standing submonolayer films of chirality pure SWCNTs as templates for the BN growth, we show it is possible to directly grow BN on chirality enriched SWCNTs from solution processing to form van der Waals heterostructures. We further report factors affecting the heterostructure formation, including an accelerated growth rate in the presence of H₂, and significantly improved crystallization of the grown BN, with the BN thickness controlled down to one single BN layer, through the presence of a Cu foil in the reactor. Transmission electron microscopy and electron energy-loss spectroscopic mapping confirm the synthesis of SWCNT@BN from the solution purified nanotubes. The photoluminescence peaks of both (7,5)- and (8,4)-SWCNT@BN heterostructures are found to redshift (by ∼10 nm) relative to the bare SWCNTs. Raman scattering suggests that the grown BN shells pose a confinement effect on the SWCNT core.

Conformal Encapsulation of Silver Nanowire Transparent Electrodes by Nanosized Reduced Graphene Oxide Leading to Improved All-Round Stability

Solution-processed silver nanowire (AgNW) networks are promising as next-generation transparent conductive electrodes due to their excellent optoelectronic properties, mechanical flexibility, as well as low material and processing costs. However, AgNWs are prone to thermally induced fragmentation and chemical degradation, necessitating a conformal protective coating typically achieved by low-throughput methods such as sputtering or atomic layer deposition. Herein, we report a facile all-solution-based approach to synthesize a conformally coated AgNW network by nanosized reduced graphene oxide R(nGO). In this method, probe ultrasonication is used to obtain nanosized GO, which is coated on AgNWs by a layer-by-layer approach and then chemically treated to form R(nGO)/AgNW. We show that our transparent electrode has excellent transmittance (85-92%) and sheet resistance (17.5 Ω/sq), combined with outstanding thermal and electrothermal stability, thanks to the conformal nature of the R(nGO) film, and demonstrate its use as a transparent heater with a high maximum temperature. This, in conjunction with improved long-term chemical and mechanical bending stability of R(nGO)/AgNW, indicates that our newly developed process represents an effective and low-cost strategy to improve the overall operational resilience of metal nanowire-based transparent conductive electrodes.

A Personalized Electronic Tattoo for Healthcare Realized by On-the-Spot Assembly of an Intrinsically Conductive and Durable Liquid-Metal Composite

Conventional electronic (e-) skins are a class of thin-film electronics mainly fabricated in laboratories or factories, which is incapable of rapid and simple customization for personalized healthcare. Here a new class of e-tattoos is introduced that can be directly implemented on the skin by facile one-step coating with various designs at multi-scale depending on the purpose of the user without a substrate. An e-tattoo is realized by attaching Pt-decorated carbon nanotubes on gallium-based liquid-metal particles (CMP) to impose intrinsic electrical conductivity and mechanical durability. Tuning the CMP suspension to have low-zeta potential, excellent wettability, and high-vapor pressure enables conformal and intimate assembly of particles directly on the skin in 10 s. Low-cost, ease of preparation, on-skin compatibility, and multifunctionality of CMP make it highly suitable for e-tattoos. Demonstrations of electrical muscle stimulators, photothermal patches, motion artifact-free electrophysiological sensors, and electrochemical biosensors validate the simplicity, versatility, and reliability of the e-tattoo-based approach in biomedical engineering.

Liquid-Phase Exfoliation of Magnetically and Optoelectronically Active Ruthenium Trichloride Nanosheets

α-RuCl₃ is a layered transition metal halide that possesses a range of exotic magnetic, optical, and electronic properties including fractional excitations indicative of a proximate Kitaev quantum spin liquid (QSL). While previous reports have explored these properties on idealized single crystals or mechanically exfoliated samples, the scalable production of α-RuCl₃ nanosheets has not yet been demonstrated. Here, we perform liquid-phase exfoliation (LPE) of α-RuCl₃ through an electrochemically assisted approach, which yields ultrathin, electron-doped α-RuCl₃ nanosheets that are then assembled into electrically conductive large-area thin films. The crystalline integrity of the α-RuCl₃ nanosheets following LPE is confirmed through a wide range of structural and chemical analyses. Moreover, the physical properties of the LPE α-RuCl₃ nanosheets are investigated through electrical, optical, and magnetic characterization methods, which reveal a structural phase transition at 230 K that is consistent with the onset of Kitaev paramagnetism in addition to an antiferromagnetic transition at 2.6 K. Intercalated ions from the electrochemical LPE protocol favorably alter the optical response of the α-RuCl₃ nanosheets, enabling large-area Mott insulator photodetectors that operate at telecommunications-relevant infrared wavelengths near 1.55 μm. These photodetectors show a linear photocurrent response as a function of incident power, which suggests negligible trap-mediated recombination or photothermal effects, ultimately resulting in a photoresponsivity of ≈2 mA/W.

General Synthesis and Solution Processing of Metal-Organic Framework Nanofibers

By virtue of their extraordinarily high surface areas, ordered pore structures, various compositions, and rich functionality, metal-organic frameworks (MOFs) are of great interest in diverse fields such as gas separation, sensing, catalysis, energy, environment science, and biomedicine. However, the difficulty in processing MOF crystals and controlling the MOF superstructure is emerging as a critical issue in their application. Herein, it is reported that a robust template, i.e., nanofibrillated cellulose (NFC), can be used for the synthesis of MOF materials with 1D nanofiber morphology. NFC@MOF core-shell nanofibers with a uniform network structure and high aspect ratios can be prepared by use of this template. The small crystal size, flexibility, and good dispersity of the NFC@MOF nanofibers make it convenient for the macroscale assembly and solution processing of MOF materials. A proof-of-concept study is demonstrated wherein freestanding MOF nanofiber membranes represent good performance in applications of water treatment and heterogeneous catalysis reaction. This general synthesis and solution-processing strategy may herald a new era in promoting the industrial application of MOFs.

Contact Conductance Governs Metallicity in Conducting Metal Oxide Nanocrystal Films

Although colloidal nanoparticles hold promise for fabricating electronic components, the properties of nanoparticle-derived materials can be unpredictable. Materials made from metallic nanocrystals exhibit a variety of transport behavior ranging from insulators, with internanocrystal contacts acting as electron transport bottlenecks, to conventional metals, where phonon scattering limits electron mobility. The insulator-metal transition (IMT) in nanocrystal films is thought to be determined by contact conductance. Meanwhile, criteria are lacking to predict the characteristic transport behavior of metallic nanocrystal films beyond this threshold. Using a library of transparent conducting tin-doped indium oxide nanocrystal films with varied electron concentration, size, and contact area, we assess the IMT as it depends on contact conductance and show how contact conductance is also key to predicting the temperature-dependence of conductivity in metallic films. The results establish a phase diagram for electron transport behavior that can guide the creation of metallic conducting materials from nanocrystal building blocks.

Processing of Lead Halide Perovskite Thin Films Studied with In-Situ Real-Time X-ray Scattering

Lead halide perovskites have been of paramount interest for solution-processable solar cells, reaching power conversion efficiencies larger than 25%. In this spotlight, we will provide a systematic overview of the influence of different solution-based processing routes of lead halide perovskites on their phase transformation and conversion as revealed through in-situ X-ray-scattering experiments. These experiments were performed in conditions closely mimicking thin film processing methods and conditions used for thin film solar cell device fabrication and therefore provide critical information about the mechanism of the phase transformation, its onset, the kinetics, as well as the emergence and disappearance of various (meso)phases along the way. The measurements capture the overall solidification and conversion process of lead halide perovskite inks into solid films via so-called one-step and two-step spin-coating processes as well as blade coating and hot casting. Processing routes are applied to films based on basic components as well as mixtures of different anions and cations, solvents, and antisolvents, all of which deeply affect the thin film microstructure and morphology of the light-absorbing semiconductor and associated solar cell devices.

Room-Temperature Postannealing Reduction via Aqueous Sodium Borohydride and Composition Optimization of Fully Solution-Processed Indium Tin Oxide Films

Solution-processed transparent conductive oxides offer the advantages of low-cost, high-throughput fabrication of electronic devices compared to the specific requirements of vacuum deposition techniques. However, adapting the current state of the art to ink deposition calls for optimization of the precursor ink composition and the postdeposition process. Solution processing of indium tin oxide films can be accomplished at reduced temperatures (250-400 °C) by annealing soluble precursor metal salts together with a fuel/oxidizer, causing an exothermic reaction with elevated local temperatures. Following layer-by-layer cycles of deposition and annealing, a postprocessing step is required via heating (300 °C) under a 5% H₂ reducing atmosphere. To address the discrepancy between the versatility of ink deposition and the limitations of controlled atmosphere postprocessing, here we investigate the effects of postprocess dipping in aqueous sodium borohydride at room temperature as an alternative, which allows for a completely solution-based process from ink to film. In addition to postprocessing, the solution composition was also optimized by removing the fuel additive and by adjusting the In/Sn content. Indium tin oxide (ITO) films were spin-coated and annealed in air at 250, 300, and 400 °C and characterized by UV/vis spectroscopy to obtain optical transmittance, atomic force microscopy to obtain film thickness and surface morphology, and a Hall effect system for electrical parameters. Additional data from X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) indicate that crystallinity is affected by the reducing environment. Results revealed an order-of-magnitude improvement of the Haacke figure of merit (FOM) from 4.3 × 10^-4 Ω^-1, 382 Ω/□ sheet resistance (R_s), and 84% transmittance (%T) for the traditional 9:1 In/Sn precursor ink with fuel additive followed by 300 °C of 5% H₂-furnace post-treatment compared to that of the optimized fully solution-processed 8.5:1.5 In/Sn ink without fuel followed by an ambient air at 25 °C dipping in aqueous sodium borohydride, leading to 3.0 × 10^-3 Ω^-1 FOM, 84.5 Ω/□ R_s, and 87%T including the glass substrate.

Bendable Polycrystalline and Magnetic CoFe2O4 Membranes by Chemical Methods

The preparation and manipulation of crystalline yet bendable functional complex oxide membranes has been a long-standing issue for a myriad of applications, in particular, for flexible electronics. Here, we investigate the viability to prepare magnetic and crystalline CoFe₂O₄ (CFO) membranes by means of the Sr₃Al₂O₆ (SAO) sacrificial layer approach using chemical deposition techniques. Meticulous chemical and structural study of the SAO surface and SAO/CFO interface properties have allowed us to identify the formation of an amorphous SAO capping layer and carbonates upon air exposure, which dictate the crystalline quality of the subsequent CFO film growth. Vacuum annealing at 800 °C of SAO films promotes the elimination of the surface carbonates and the reconstruction of the SAO surface crystallinity. Ex-situ atomic layer deposition of CFO films at 250 °C on air-exposed SAO offers the opportunity to avoid high-temperature growth while achieving polycrystalline CFO films that can be successfully transferred to a polymer support preserving the magnetic properties under bending. Float on and transfer provides an alternative route to prepare freestanding and wrinkle-free CFO membrane films. The advances and challenges presented in this work are expected to help increase the capabilities to grow different oxide compositions and heterostructures of freestanding films and their range of functional properties.

All-Solution-Processed Van der Waals Heterostructures for Wafer-Scale Electronics

2D van der Waals (vdW) materials have been considered as potential building blocks for use in fundamental elements of electronic and optoelectronic devices, such as electrodes, channels, and dielectrics, because of their diverse and remarkable electrical properties. Furthermore, two or more building blocks of different electronic types can be stacked vertically to generate vdW heterostructures with desired electrical behaviors. However, such fundamental approaches cannot directly be applied practically because of issues such as precise alignment/positioning and large-quantity material production. Here, these limitations are overcome and wafer-scale vdW heterostructures are demonstrated by exploiting the lateral and vertical assembly of solution-processed 2D vdW materials. The high exfoliation yield of the molecular intercalation-assisted approach enables the production of micrometer-sized nanosheets in large quantities and its lateral assembly in a wafer-scale via vdW interactions. Subsequently, the laterally assembled vdW thin-films are vertically assembled to demonstrate various electronic device applications, such as transistors and photodetectors. Furthermore, multidimensional vdW heterostructures are demonstrated by integrating 1D carbon nanotubes as a p-type semiconductor to fabricate p-n diodes and complementary logic gates. Finally, electronic devices are fabricated via inkjet printing as a lithography-free manner based on the stable nanomaterial dispersions.

Area-Selective Chemical Doping on Solution-Processed MoS2 Thin-Film for Multi-Valued Logic Gates

Multi-valued logic gates are demonstrated on solution-processed molybdenum disulfide (MoS₂) thin films. A simple chemical doping process is added to the conventional transistor fabrication procedure to locally increase the work function of MoS₂ by decreasing sulfur vacancies. The resulting device exhibits pseudo-heterojunctions comprising as-processed MoS₂ and chemically treated MoS₂ (c-MoS₂). The energy-band misalignment of MoS₂ and c-MoS₂ results in a sequential activation of the MoS₂ and c-MoS₂ channel areas under a gate voltage sweep, which generates a stable intermediate state for ternary operation. Current levels and turn-on voltages for each state can be tuned by modulating the device geometries, including the channel thickness and length. The optimized ternary transistors are incorporated to demonstrate various ternary logic gates, including the inverter, NMIN, and NMAX gates.

Defect Engineering in Solution-Processed Polycrystalline SnSe Leads to High Thermoelectric Performance

SnSe has emerged as one of the most promising materials for thermoelectric energy conversion due to its extraordinary performance in its single-crystal form and its low-cost constituent elements. However, to achieve an economic impact, the polycrystalline counterpart needs to replicate the performance of the single crystal. Herein, we optimize the thermoelectric performance of polycrystalline SnSe produced by consolidating solution-processed and surface-engineered SnSe particles. In particular, the SnSe particles are coated with CdSe molecular complexes that crystallize during the sintering process, forming CdSe nanoparticles. The presence of CdSe nanoparticles inhibits SnSe grain growth during the consolidation step due to Zener pinning, yielding a material with a high density of grain boundaries. Moreover, the resulting SnSe-CdSe nanocomposites present a large number of defects at different length scales, which significantly reduce the thermal conductivity. The produced SnSe-CdSe nanocomposites exhibit thermoelectric figures of merit up to 2.2 at 786 K, which is among the highest reported for solution-processed SnSe.

Two-Step Synthesis of Bismuth-Based Hybrid Halide Perovskite Thin-Films

Lead halide perovskites have been revolutionary in the last decade in many optoelectronic sectors. Their bismuth-based counterparts have been considered a good alternative thanks to their composition of earth-abundant elements, good chemical stability, and low toxicity. Moreover, their electronic structure is in a quasi-zero-dimensional (0D) configuration, and they have recently been explored for use beyond optoelectronics. A significant limitation in applying thin-film technology is represented by the difficulty of synthesizing compact layers with easily scalable methods. Here, the engineering of a two-step synthesis in an air of methylammonium bismuth iodide compact thin films is reported. The critical steps of the process have been highlighted so that the procedure can be adapted to different substrates and application areas.

Solubility of Hybrid Halide Perovskites in DMF and DMSO

Solution methods remain the most popular means for the fabrication of hybrid halide perovskites. However, the solubility of hybrid perovskites has not yet been quantitively investigated. In this study, we present accurate solubility data for MAPbI₃, FAPbI₃, MAPbBr₃ and FAPbBr₃ in the two most widely used solvents, DMF and DMSO, and demonstrate huge differences in the solubility behavior depending on the solution compositions. By analyzing the donor numbers of the solvents and halide anions, we rationalize the differences in the solubility behavior of hybrid perovskites with various compositions, in order to take a step forward in the search for better processing conditions of hybrid perovskites for solar cells and optoelectronics.

The Importance of Surface Adsorbates in Solution-Processed Thermoelectric Materials: The Case of SnSe

Solution synthesis of particles emerges as an alternative to prepare thermoelectric materials with less demanding processing conditions than conventional solid-state synthetic methods. However, solution synthesis generally involves the presence of additional molecules or ions belonging to the precursors or added to enable solubility and/or regulate nucleation and growth. These molecules or ions can end up in the particles as surface adsorbates and interfere in the material properties. This work demonstrates that ionic adsorbates, in particular Na⁺ ions, are electrostatically adsorbed in SnSe particles synthesized in water and play a crucial role not only in directing the material nano/microstructure but also in determining the transport properties of the consolidated material. In dense pellets prepared by sintering SnSe particles, Na remains within the crystal lattice as dopant, in dislocations, precipitates, and forming grain boundary complexions. These results highlight the importance of considering all the possible unintentional impurities to establish proper structure-property relationships and control material properties in solution-processed thermoelectric materials.

Conductive, Acid-Doped Polyaniline Electrospun Nanofiber Gas Sensing Substrates Made Using a Facile Dissolution Method

A novel dissolution method that allows for the total solvation of high-concentration, high-molecular-weight polyaniline (PANi) doped with (+)-camphor-10-sulfonic acid (CSA) is reported. Preparation of 12-16 wt % 65,000 Da PANi solutions in N,N-dimethylformamide is achievable using a simple one-pot method. Doped polyaniline solutions in common organic solvents were processed into nanofibers using a convenient single-nozzle electrospinning technique. The electrospinning of PANi-CSA into nanofibrous membranes generated substrates that were subsequently employed in colorimetric gas sensing. These substrates demonstrated linearity of response upon exposure to 50-5500 ppm ammonia at ambient (50 ± 10% RH) and high (80% RH) humidity.

Thiol-Amine-Based Solution Processing of Cu2 S Thin Films for Photoelectrochemical Water Splitting

Cu₂ S is a promising solar energy conversion material owing to its good optical properties, elemental earth abundance, and low cost. However, simple and cheap methods to prepare phase-pure and photo-active Cu₂ S thin films are lacking. This study concerns the development of a cost-effective and high-throughput method that consists of dissolving high-purity commercial Cu₂ S powder in a thiol-amine solvent mixture followed by spin coating and low-temperature annealing to obtain phase-pure crystalline low chalcocite Cu₂ S thin films. After coupling with a CdS buffer layer, a TiO₂ protective layer and a RuO_x hydrogen evolution catalyst, the champion Cu₂ S photocathode gives a photocurrent density of 2.5 mA cm^-2 at -0.3 V vs. reversible hydrogen electrode (V_RHE ), an onset potential of 0.42 V_RHE , and high stability over 12 h in pH 7 buffer solution under AM1.5 G simulated sunlight illumination (100 mW cm^-2 ). This is the first thiol-amine-based ink deposition strategy to prepare phase-pure Cu₂ S thin films achieving decent photoelectrochemical performance, which will facilitate its future scalable application for solar-driven hydrogen fuel production.

Ohmic-Contact-Gated Carbon Nanotube Transistors for High-Performance Analog Amplifiers

The growing demand for ubiquitous data collection has driven the development of sensing technologies with local data processing. As a result, solution-processed semiconductors are widely employed due to their compatibility with low-cost additive manufacturing on a wide range of substrates. However, to fully realize their potential in sensing applications, high-performance scalable analog amplifiers must be realized. Here, ohmic-contact-gated transistors (OCGTs) based on solution-processed semiconducting single-walled carbon nanotubes are introduced to address this unmet need. This new device concept enables output current saturation in the short-channel limit without compromising output current drive. The resulting OCGTs are used in common-source amplifiers to achieve the highest width-normalized output current (≈30 µA µm^-1 ) and length-scaled signal gain (≈230 µm^-1 ) to date for solution-processed semiconductors. The utility of these amplifiers for emerging sensing technologies is demonstrated by the amplification of complex millivolt-scale analog biological signals including the outputs of electromyography, photoplethysmogram, and accelerometer sensors. Since the OCGT design is compatible with other solution-processed semiconducting materials, this work establishes a general route to high-performance, solution-processed analog electronics.

Perovskites on Ice: An Additive-Free Approach to Increase the Shelf-Life of Triple-Cation Perovskite Precursor Solutions

The development of stable perovskite precursor solutions is critical if solution-processable perovskite solar cells (PSCs) are to be practically manufacturable. Ideally, such precursors should combine high solution stability without using chemical additives that might compromise PSC performance. Here, it was shown that the shelf-life of high-performing perovskite precursors could be greatly improved by storing solutions at low-temperature without the need to alter chemical composition. Devices fabricated from solutions stored for 31 days at 4 °C achieved a champion power conversion efficiency (PCE) of 18.6 % (97 % of original PCE). The choice of precursor solvent also impacted solution shelf-life, with DMSO-based solutions having enhanced solution stability compared to those including DMF. The compositions of aged precursors were explored using NMR spectroscopy, and films made from these solutions were analysed using X-ray diffraction. It was concluded that the improvement in precursor solution stability is directly linked to the suppression of an addition-elimination reaction and the preservation of higher amounts of methylammonium within solution.

Solution-Processed Stretchable Ag2 S Semiconductor Thin Films for Wearable Self-Powered Nonvolatile Memory

Compared with the large plastic deformation observed in ductile metals and organic materials, inorganic semiconductors have limited plasticity (<0.2%) due to their intrinsic bonding characters, restricting their widespread applications in stretchable electronics. Herein, the solution-processed synthesis of ductile α-Ag₂ S thin films and fabrication of all-inorganic, self-powered, and stretchable memory devices, is reported. Molecular Ag₂ S complex solution is synthesized by chemical reduction of Ag₂ S powder, fabricating wafer-scale highly crystalline Ag₂ S thin films. The thin films show stretchability due to the intrinsic ductility, sustaining the structural integrity at a tensile strain of 14.9%. Moreover, the fabricated Ag₂ S-based resistive random access memory presents outstanding bipolar switching characteristics (I_on /I_off ratio of ≈10⁵ , operational endurance of 100 cycles, and retention time >10⁶ s) as well as excellent mechanical stretchability (no degradation of properties up to stretchability of 52%). Meanwhile, the device is highly durable under diverse chemical environments and temperatures from -196 to 300 °C, especially maintaining the properties for 168 h in 85% relative humidity and 85 °C. A self-powered memory combined with motion sensors for use as a wearable healthcare monitoring system is demonstrated, offering the potential for designing high-performance wearable electronics that are usable in daily life in a real-world setting.

Low-Temperature Graphene-Based Paste for Large-Area Carbon Perovskite Solar Cells

Carbon perovskite solar cells (C-PSCs), using carbon-based counter electrodes (C-CEs), promise to mitigate instability issues while providing solution-processed and low-cost device configurations. In this work, we report the fabrication and characterization of efficient paintable C-PSCs obtained by depositing a low-temperature-processed graphene-based carbon paste atop prototypical mesoscopic and planar n-i-p structures. Small-area (0.09 cm²) mesoscopic C-PSCs reach a power conversion efficiency (PCE) of 15.81% while showing an improved thermal stability under the ISOS-D-2 protocol compared to the reference devices based on Au CEs. The proposed graphene-based C-CEs are applied to large-area (1 cm²) mesoscopic devices and low-temperature-processed planar n-i-p devices, reaching PCEs of 13.85 and 14.06%, respectively. To the best of our knowledge, these PCE values are among the highest reported for large-area C-PSCs in the absence of back-contact metallization or additional stacked conductive components or a thermally evaporated barrier layer between the charge-transporting layer and the C-CE (strategies commonly used for the record-high efficiency C-PSCs). In addition, we report a proof-of-concept of metallized miniwafer-like area C-PSCs (substrate area = 6.76 cm², aperture area = 4.00 cm²), reaching a PCE on active area of 13.86% and a record-high PCE on aperture area of 12.10%, proving the metallization compatibility with our C-PSCs. Monolithic wafer-like area C-PSCs can be feasible all-solution-processed configurations, more reliable than prototypical perovskite solar (mini)modules based on the serial connection of subcells, since they mitigate hysteresis-induced performance losses and hot-spot-induced irreversible material damage caused by reverse biases.

Solution-Processed MoS2 Film with Functional Interfaces via Precursor-Assisted Chemical Welding

Molybdenum disulfide (MoS₂) presents fascinating properties for next-generation applications in diverse fields. However, fully exploiting the best properties of MoS₂ in largescale practical applications still remains a challenge due to lack of proper processing methods. Solution-based processing can be a promising route for scalable production of MoS₂ nanosheets, but the resulting assembled film possesses an enormous number of interfaces that significantly compromise the intrinsic electrical properties. Herein, we demonstrate the solution processing of MoS₂ and subsequent precursor-assisted chemical welding to form defective MoS_2-x at the nanosheet interfaces. The formation of defective MoS_2-x significantly reduces the electrical contact resistances, and thus the chemically welded MoS₂ film exhibits more than 2 orders of magnitude improved electrical conductivity. Furthermore, the chemical welding provides MoS_2-x interface induced additional defect originated functionalities for diverse applications such as broadband photodetection over the near-infrared range and improved electrocatalytic activity for hydrogen evolution reactions. Overall, this precursor-assisted chemical welding strategy can be a facile route to produce high-quality MoS₂ films with low-quality defective MoS_2-x at the interfaces having multifunctionalities in electronics, optoelectronics, and electrocatalysis.

Organic Electronics Picks Up the Pace: Mask-Less, Solution Processed Organic Transistors Operating at 160 MHz

Organic printed electronics has proven its potential as an essential enabler for applications related to healthcare, entertainment, energy, and distributed intelligent objects. The possibility of exploiting solution-based and direct-writing production schemes further boosts the benefits offered by such technology, facilitating the implementation of cheap, conformable, bio-compatible electronic applications. The result shown in this work challenges the widespread assumption that such class of electronic devices is relegated to low-frequency operation, owing to the limited charge mobility of the materials and to the low spatial resolution achievable with conventional printing techniques. Here, it is shown that solution-processed and direct-written organic field-effect transistors can be carefully designed and fabricated so to achieve a maximum transition frequency of 160 MHz, unlocking an operational range that was not available before for organics. Such range was believed to be only accessible with more performing classes of semiconductor materials and/or more expensive fabrication schemes. The present achievement opens a route for cost- and energy-efficient manufacturability of flexible and conformable electronics with wireless-communication capabilities.

Thin-Film Engineering of Solution-Processable n-Type Silicon Phthalocyanines for Organic Thin-Film Transistors

Metal and metalloid phthalocyanines are an abundant and established class of materials widely used in the dye and pigment industry as well as in commercial photoreceptors. Silicon phthalocyanines (SiPcs) are among the highest-performing n-type semiconductor materials in this family when used in organic thin-film transistors (OTFTs) as their performance and solid-state arrangement are often increased through axial substitution. Herein, we study eight axially substituted SiPcs and their integration into solution-processed n-type OTFTs. Electrical characterization of the OTFTs, combined with atomic force microscopy (AFM), determined that the length of the alkyl chain affects device performance and thin-film morphology. The effects of high-temperature annealing and spin coating time on film formation, two key processing steps for fabrication of OTFTs, were investigated by grazing-incidence wide-angle X-ray scattering (GIWAXS) and X-ray diffraction (XRD) to elucidate the relationship between thin-film microstructure and device performance. Thermal annealing was shown to change both film crystallinity and SiPc molecular orientation relative to the substrate surface. Spin time affected film crystallinity, morphology, and interplanar d-spacing, thus ultimately modifying device performance. Of the eight materials studied, bis(tri-n-butylsilyl oxide) SiPc exhibited the greatest electron field-effect mobility (0.028 cm² V^-1 s^-1, a threshold voltage of 17.6 V) of all reported solution-processed SiPc derivatives.

Amphipathic Side Chain of a Conjugated Polymer Optimizes Dopant Location toward Efficient N-Type Organic Thermoelectrics

There is no molecular strategy for selectively increasing the Seebeck coefficient without reducing the electrical conductivity for organic thermoelectrics. Here, it is reported that the use of amphipathic side chains in an n-type donor-acceptor copolymer can selectively increase the Seebeck coefficient and thus increase the power factor by a factor of ≈5. The amphipathic side chain contains an alkyl chain segment as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer does not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. As confirmed by kinetic Monte Carlo simulations with the host-dopant distance as the only variable, a reduced Coulombic interaction resulting from a larger host-dopant distance contributes to a higher Seebeck coefficient for a given electrical conductivity. Finally, an optimized power factor of 18 µW m^-1 K^-2 is achieved in the doped polymer film. This work provides a facile molecular strategy for selectively improving the Seebeck coefficient and opens up a new route for optimizing the dopant location toward realizing better n-type polymeric thermoelectrics.

Probing the Dielectric Effects on the Colloidal 2D Perovskite Oxides by Eu3+ Luminescence

Solution-processing technique of two-dimensional (2D) materials requires the knowledge of the dielectric effects on mutual interaction of each component and the layer structure variation. This research uses the magnetic dipole (MD) transition of intentionally doped Eu³⁺, which is dependent on the dielectric environments, as an optical probe to study the dielectric effects on the colloidal charge-bearing [Ca_1.8Eu_0.1Na_0.1Nb₃O₁₀]^- perovskite nanosheets (NSs) in various solvents. Results reveal that the solvent molecules with longer alkyl chain could more easily impact the ligands on the surface of the NSs, leading to a weaker interaction between the ligands and the NSs as well as less distortion of Eu³⁺ site (Ca²⁺ site) at the inner layer of the NSs. The large-sized ligands would impede the stacking of the NSs, while H⁺ would make the H⁺-modified NSs restack more easily. With the assistance of density functional theory (DFT) simulation, it is found that the ligands or the dielectric solvents could distort or relax the surface covalent polyhedra [NbO₆]^7- to a larger extent than the inner polyhedra. Small-sized ligands and a large thickness with more atomic layers of the NSs can resist structural variation caused by solvents. The acquired knowledge in this research benefits the understanding of the solution-processing technique for industrial application of 2D materials.

High-Sensitivity Acoustic Molecular Sensors Based on Large-Area, Spray-Coated 2D Covalent Organic Frameworks

2D covalent organic frameworks (2D COFs) are a unique materials platform that combines covalent connectivity, structural regularity, and molecularly precise porosity. However, 2D COFs typically form insoluble aggregates, thus limiting their processing via additive manufacturing techniques. In this work, colloidal suspensions of boronate-ester-linked 2D COFs are used as a spray-coating ink to produce large-area 2D COF thin films. This method is synthetically general, with five different 2D COFs prepared as colloidal inks and subsequently spray-coated onto a diverse range of substrates. Moreover, this approach enables the deposition of multiple 2D COF materials simultaneously, which is not possible by polymerizing COFs on substrates directly. When combined with stencil masks, spray-coated 2D COFs are rapidly deposited as thin films larger than 200 cm² with line resolutions below 50 µm. To demonstrate that this deposition scheme preserves the desirable attributes of 2D COFs, spray-coated 2D COF thin films are incorporated as the active material in acoustic sensors. These 2D-COF-based sensors have a 10 ppb limit-of-quantification for trimethylamine, which places them among the most sensitive sensors for meat and seafood spoilage. Overall, this work establishes a scalable additive manufacturing technique that enables the integration of 2D COFs into thin-film device architectures.

Solution-Processed Quasi-Two-Dimensional/Nanoscrystals Perovskite Composite Film Enhances the Efficiency and Stability of Perovskite Light-Emitting Diodes

Solution-processed quasi-two-dimensional (Q-2D)/colloidal perovskite nanocrystals (PNCs) perovskite composite films are first prepared as the emitting layers of perovskite light-emitting diodes (PeLEDs). The subsequent multi-spin-coating of PNCs not only fills the gully-like fluctuations of the nanocrystal pinning-prepared Q-2D perovskite films and decreases their surface roughness but also transforms the bilayer perovskite nanosheets into multilayer ones, thus improving the charge transport and reducing the hole-injection barrier in the composite films. More importantly, the bromide vacancies and Pb defects in the Q-2D perovskites are removed via Br^- supply and Pb-OOC-R interaction, in which the Br ions and COO^- groups (from oleic acid) come from the PNC solution, and the radiation recombination is significantly enhanced. Based on the Q-2D/PNCs perovskite composite emitter, the PeLEDs achieve a maximum luminescence of ∼2.0 × 10⁴ cd/m² and a peak current efficiency of 27.5 cd/A, showing 175 and 337% enhancements compared to the control device with the pristine Q-2D perovskite emitter. The lifetime for the luminance decaying to 50% of the initial intensity increases by a factor of 13.8, demonstrating that the device stability is also improved by the Q-2D/PNCs perovskite composite film.

High-Performance and Reliable Lead-Free Layered-Perovskite Transistors

Perovskites have been intensively investigated for their use in solar cells and light-emitting diodes. However, research on their applications in thin-film transistors (TFTs) has drawn less attention despite their high intrinsic charge carrier mobility. In this study, the universal approaches for high-performance and reliable p-channel lead-free phenethylammonium tin iodide TFTs are reported. These include self-passivation for grain boundary by excess phenethylammonium iodide, grain crystallization control by adduct, and iodide vacancy passivation through oxygen treatment. It is found that the grain boundary passivation can increase TFT reproducibility and reliability, and the grain size enlargement can hike the TFT performance, thus, enabling the first perovskite-based complementary inverter demonstration with n-channel indium gallium zinc oxide TFTs. The inverter exhibits a high gain over 30 with an excellent noise margin. This work aims to provide widely applicable and repeatable methods to make the gate more open for intensive efforts toward high-performance printed perovskite TFTs.

Metal Oxide Nanosheets as 2D Building Blocks for the Design of Novel Materials

Research into 2-dimensional materials has soared during the last couple of years. Next to van der Waals type 2D materials such as graphene and h-BN, less well-known oxidic 2D equivalents also exist. Most 2D oxide nanosheets are derived from layered metal oxide phases, although few 2D oxide phases can be also made by bottom-up solution syntheses. Owing to the strong electrostatic interactions within layered metal oxide crystals, a chemical process is usually needed to delaminate them into their 2D constituents. This Review article provides an overview of the synthesis of oxide nanosheets, and methods to assemble them into nanocomposites, mono- or multilayer films. In particular, the use of Langmuir-Blodgett methods to form monolayer films over large surface areas, and the emerging use of ink jet printing to form patterned functional films is emphasized. The utilization of nanosheets in various areas of technology, for example, electronics, energy storage and tribology, is illustrated, with special focus on their use as seed layers for epitaxial growth of thin films, and as electrochemically active electrodes for supercapacitors and Li ion batteries.