Large-area Processing Research Articles

N-Type Molecular Photovoltaic Materials: Design Strategies and Device Applications.

The use of photovoltaic technologies has been regarded as a promising approach for converting solar energy to electricity and mitigating the energy crisis, and among these, organic photovoltaics (OPVs) have attracted broad interest because of their solution processability, flexibility, light weight, and potential for large-area processing. The development of OPV materials, especially electron acceptors, has been one of the focuses in recent years. Compared with fullerene derivates, n-type non-fullerene molecules have some unique merits, such as synthetic simplicity, high tunability of the absorption and energy levels, and small energy loss. In the last 5 years, organic solar cells based on n-type non-fullerene molecules have achieved a significant breakthrough in the power conversion efficiency from approximately 4% to over 17%, which is superior to those of fullerene-based solar cells; meanwhile, n-type non-fullerene molecules have created brand new opportunities for the application of OPVs in some special situations. This Perspective analyzes the key design strategies of high-performance n-type molecular photovoltaic materials and highlights instructive examples of their various applications, including in ternary and tandem solar cells, high-efficiency semitransparent solar cells for power-generating building facades and windows, and indoor photovoltaics for driving low-power-consumption devices. Moreover, to accelerate the pace toward commercialization of OPVs, the existing challenges and future directions are also reviewed from the perspectives of efficiency, stability, and large-area fabrication.

Thiol functionalized polymer submicron particles prepared by soap‐free emulsion polymerization and their adsorption of lead ions in water

AbstractOxidization, thiol‐ene click addition, and inhibition of thiols make it hard to prepare thiol‐containing particles from free radical polymerization. Using methyl methacrylate (MMA) and a newly synthesized polymerizable 2‐(((2‐[pyridin‐2‐yldisulfanyl]ethoxy)carbonyl)amino) ethyl acrylate (PDSECAE) as monomers, the P(MMA‐co‐PDSECAE) submicron particles (SPs) were first prepared by soap‐free emulsion polymerization, in which the thiol was protected in the form of pyridyl disulfide group from side reactions. The thiol functionalized P(MMA‐co‐PDSECAE)‐SH SPs were obtained by post‐treating the SPs with dithiothreitol (DTT), and then used to adsorb Pb2+ in water. Results showed that in the preparation of these SPs, the initiator type, the initial pH and the PDSECAE dosage played an important role in the stability of polymerization and the resulted emulsions, and the colloid properties as well as the content of thiol groups of the P(MMA‐co‐PDSECAE)‐SH SPs. The P(MMA‐co‐PDSECAE)‐SH SPs had nearly 10 times greater Pb2+ adsorption capacity than the corresponding PMMA SPs or P(MMA‐co‐PDSECAE) SPs under optimized experimental conditions, and the adsorption capacity of Pb2+ increased with PDSECAE dosage and the DTT reduction time. Due to the relatively large specific surface area and easy preparation process, the P(MMA‐co‐PDSECAE)‐SH SPs have potential application value in the adsorption of heavy metal ions.

System for Fabrication of Large-Area Roll Molds by Step-and-Repeat Liquid Transfer Imprint Lithography.

The effective production of nanopatterned films generally requires a nanopatterned roll mold with a large area. We report on a novel system to fabricate large-area roll molds by recombination of smaller patterned areas in a step-and-repeat imprint lithography process. The process is accomplished in a method similar to liquid transfer imprint lithography (LTIL). The stamp roll with a smaller area takes up the liquid resist by splitting from a donor substrate or a donor roll. The resist is then transferred from a stamp roll to an acceptor roll and stitched together in a longitudinal and, if necessary, in a circumferential direction. During transfer, the nanostructured resist is UV-exposed and crosslinked directly on the acceptor roll. The acceptor roll with the stitched and recombined stamp patterns is ready to be used as a large-area roll mold for roll-based imprinting. A system for this purpose was designed, and its operation was demonstrated taking the example of an acceptor roll of 1 m length and 250 mm diameter, which was covered by 56 patterned areas. Such a system represents an elegant and efficient tool to recombine small patterned areas directly on a large roll mold and opens the way for large-area roll-based processing.

Fabrication of a Metal Roller Mold with Nanoimprinted Pattern Using Thermal Nanoimprint Lithography

In this work, fabrication of a metal roller mold with nanoimprinted pattern was demonstrated. To get metal nanopattern on a metal roller mold, thermal nanoimprint lithography (TNIL) and subsequent electroforming process were conducted. A poly(methyl methacrylate) (PMMA) nanopattern was fabricated by TNIL process using a polydimethylsiloxane (PDMS) soft stamp on a bare PMMA film. An optimal experimental condition of TNIL process was investigated for large area, uniform PMMA nanopatterning. As a result, large area PMMA line patterns with 200 nm linewidth were fabricated by large-area TNIL process. Electroforming process on the PMMA nanopatterned film resulted in nickel (Ni) nanopattern with a linewidth of 200 nm from the PMMA line pattern. A large area (360 mm by 730 mm in width and length) Ni stamp for a roller mold was fabricated by laser cutting and tiling process of the multiple electroformed Ni stamps. We successfully fabricated a Ni roller mold with feature size of 200 nm in linewidth by attaching the large area Ni stamp to the surface of a roller body.

Laser-based surface pre-treatment for metal-plastic hybrids using a new process strategy

Abstract In order to join metal-plastic hybrid parts so as to achieve lightweight constructions, innovative joining technologies are essential. For this, thermal joining is a promising approach due to the absence of any joining agent. The joint strength can be increased by surface pre-treatment of the metal with continuous wave laser radiation. Large joining areas, however, are challenging in terms of productivity, efficiency and geometrical flexibility when using conventional beam trajectories. A more promising approach is the use of continuous beam trajectories, which can be easily combined with on-the-fly processing. More precisely, a circularly oscillating laser focal spot is geometrically independent of every possible in-plane load direction, and causes lower thermal distortion of the metal sheet than using conventional beam trajectories. Therefore, the circular beam oscillation was characterized for the processing of large areas. In this context, the resulting topography of the metal was analyzed with respect to the surface roughness and a design criterion was mathematically derived and experimentally validated. Additionally, the joint strength was compared to the strength resulting from conventional beam trajectories. For verification, steel substrates were joined with a carbon-fiber-reinforced thermoset. The laser-based pre-treatment using the circularly oscillating laser focal spot led to a similar joint strength, while being more productive when combined with on-the-fly processing, compared to conventional structuring trajectories. Consequently, the processing time can be significantly reduced.

Parallel, Multi-Material Electrohydrodynamic 3D Nanoprinting.

Direct mass-transfer via liquid nanodroplets is one of the most powerful approaches for additive micro/nanofabrication. Electrohydrodynamic (EHD) dispensing has made the delivery of nanosized droplets containing diverse materials a practical reality; however, in its serial form it has insufficient throughput for large-area processing. Here, a parallel, nanoscale EHD method is developed that offers both improved productivity and material diversity in 3D nanoprinting. The method exploits a double-barreled glass nanopipette filled with material inks to parallelize nanodripping ejections, enabling a dual 3D nanoprinting process. It is discovered that an unusual electric field distribution created by cross talk of neighboring pipette apertures can be used to steer the microscopic ejection paths of the ink at will, enabling on-demand control over shape, placement, and material mixing in 3D printed nanostructures. After thorough characterizations of the printing conditions, the parallel fabrication of nanomeshes and nanowalls of silver, CdSe/ZnS quantum dots, and their composites, with programmed designs is demonstrated. This method is expected to advance productivity in the heterogeneous integration of functional 3D nanodevices in a facile manner.

Iterative-SGLRT for Multiple-Scatterer Detection in SAR Tomography

This letter introduces a multiple-scatterer detection method in synthetic aperture radar tomography (TomoSAR), named iterative sequential generalized likelihood ratio test (iterative-SGLRT). In this technique, the number of scatterers is sequentially decided by the generalized likelihood ratio test (GLRT) pixel by pixel, after iteratively estimating the parameters. It is a good tradeoff of the aforeproposed methods of sup-GLRT and fast-sup-GLRT on accuracy and efficiency. Simulated comparisons showed that iterative-SGLRT outperformed fast-sup-GLRT on the performances of detection probability and accuracy without substantial computation time increase, and compared with sup-GLRT, its performance loss could be negligible with computational burden greatly reduced. In addition, both iterative-SGLRT and sup-GLRT have been applied to the TerraSAR-X data set over Shenzhen city. The 3-D reconstruction of the test site and the separation of the overlaid scatterers have been achieved. In addition, verification using light detection and radar (LiDAR) indicated a root-mean-square error (RMSE) of $ {0.1\rho _{s}}$ for both methods of the height estimated. Accordingly, iterative-SGLRT is very suitable for large urban area processing for its super-resolution, high efficiency, and robustness.

Nanosecond laser-based high-throughput surface nanostructuring (nHSN)

We present a novel nanosecond laser-based high-throughput surface nanostructuring (nHSN) process that can simultaneously create random nanostructures and attain desirable surface chemistry over large-area metal alloy surfaces. nHSN consists of two sequential steps: (1) nanosecond laser texturing (NLT) and (2) chemical immersion treatment (CIT). NLT step in water confinement (wNLT) does not generate topological patterns but preconditions the metal surface chemically and mechanically. Our analysis shows that surface nanostructuring results from a combined effect of chemical etching and attachment of functional groups during the CIT phase of nHSN. A proper silane reagent can be selected for the CIT phase to achieve the desired surface wetting behavior, while laser parameters can also be adjusted during the NLT phase to finely tune the nanostructuring mechanism. nHSN nanostructures with fluorosilane chemistry repel water, while those with cyanosilane chemistry attract water. Extreme wettability including superhydrophobicity and superhydrophilicity is assessed for multiple engineering metal alloys including aluminum, steel and titanium alloys. Compared with existing ultrashort laser-based surface-texturing methods, the nHSN laser scan time represents a significant improvement in processing efficiency and enables a practical throughput for large-area processing of engineering alloys.

Effects of laser-induced periodic surface structures on the superconducting properties of Niobium

It is well known that the use of ultrashort (fs) pulsed lasers can induce the generation of (quasi-) periodic nanostructures (LIPSS, ripples) on the surface of many materials. Such nanostructures have also been observed in sample’s surfaces irradiated with UV lasers with a pulse duration of 300 ps. In this work, we compare the characteristics of these nanostructures on 1-mm and on 25-μm thick niobium sheets induced by 30 fs n-IR and 300 ps UV pulsed lasers. In addition to conventional continuous or burst mode processing configurations, two-dimensional laser beam and line scanning modes have been investigated in this work. The latter allows the processing of large areas with a more uniform distribution of nanostructures at the surface. The influence of the generated nanostructures on the superconducting properties of niobium has also been explored. For this aim, magnetic hysteresis loops have been measured at different cryogenic temperatures to analyse how these laser treatments affect the flux pinning behaviour and, in consequence, the superconductor’s critical current values. It was observed that laser treatments are able to modify the superconducting properties of niobium samples.

Surface Assembly Strategy for the Fabrication of MoS2 Thin-Film Patterns

Molybdenum disulfide (MoS2) is a novel material with remarkable properties that is widely investigated for future applications in electronics, sensors, and smart materials. In this respect, a low cost, easy process, large-area mass production process is needed to obtain diverse structures and surface patterns of MoS2. Here, we demonstrate an effective MoS2 patterning process using substrate surface modification that can solve the difficulties encountered in previous studies. This technique utilizes the sulfur vacancies introduced during chemical exfoliation of MoS2 flakes. The MoS2 patterning is performed via surface modification with thiol molecules on the substrate in pre-designed shapes. The thiol (???SH) group required to bond MoS2 to the surface-modified substrate using (3-mercaptopropyl)trimethoxysilane was confirmed by FT-IR and the patterned MoS2 was confirmed by Raman shift. Through this process, a gas sensor was fabricated and its feasibility was confirmed to show its applicability to various applications MoS2.

The Effect of Crystalline Aluminum Oxide on Device Performance of Solution-Processed Indium-Gallium-Zinc-Oxide Thin-Film Transistors Operating at Low Voltage.

High dielectric constant (high-k) materials have been extensively investigated for low-voltage operating electronics. In recent years, solution-processed high-k dielectrics have been of technological interests in low fabrication cost, large area process and good film quality, compared to the vacuum-process technology. In this paper, we demonstrate solution-processed aluminum oxide (Al2O3) dielectrics for high performance solution-processed indium-gallium-zinc-oxide (IGZO) thin-film transistors (TFTs) operating at low voltage. The material and electrical properties of Al2O3 dielectrics fabricated at different post-annealing temperatures were analyzed by atomic force microscopy, scanning electron microscopy, X-ray diffraction and capacitance-voltage measurements. We also investigate the effect of crystalline Al2O3 dielectrics on the device performance of solution-processed IGZO TFTs. It is concluded that improved interfacial characteristics of crystalline Al2O3 dielectrics enhance the device performance of solution-processed IGZO TFTs operating at 3 V.

Flexible Ultralow-Power Sensor Interfaces for E-Skin

Thin-film electronics has hugely benefitted from low-cost processes, large-area processability, and multifunctionality. This has not only stimulated innovation in display and sensor technology but has also demonstrated great potential for the integration of components for human–machine interfaces. For electronics to be deployed as sensor interfaces and signal processing, the quest for low power is compelling due to the inherently limited battery lifetime. This review will present the state of the art in thin-film electronics and demonstrate examples of low-cost printable transistors and biosensors that are flexible/stretchable for wearable and other applications. Ultralow-power design for thin-film transistors will be discussed from the standpoint of reducing both operating voltage and operating current, taking into account the challenges in meeting frequency requirements. Compact models for circuit design will be reviewed along with new insights into ultralow-power transistors and high-gain amplifier circuits. Finally, a concept for an integrated system comprising sensors and interfacing circuits will be demonstrated, which has the potential to enable batteryless operation.

(Invited) Organic Thermoelectric Materials and Devices

Within the last 3 years, thermoelectric technologies have seen renewed global deployment providing unique solutions to pressing thermal problems. Today thermoelectric technologies are helping to ensure vaccines viably reach the most remote locations in the world, are providing personal comfort in wearable devices addressing medical circulatory conditions, and are being designed for integration into electric vehicles thereby reducing the global warming contributions of refrigerants in conventional air conditioning systems. The majority of these technologies have resulted from innovative engineers utilizing conventional materials in niche applications, however, additional societal contributions can be achieved by leveraging the large-area processability and abundance of polymer-based thermoelectrics. Unfortunately, the material performance of polymer-based thermoelectric materials is lacking compared to their inorganic counterparts. To circumvent this challenge, composites or hybrid organic-inorganic materials are often employed in functional devices. Furthermore, while there is an abundance of p-type materials to select from, there are few air-stable n-type thermoelectric materials. Metal coordinated polymers (or metallo-organic polymers) are one promising class of thermoelectric materials where a metal (or semimetal) atom is present along the polymer backbone. This structure produces periodic centers of high electron density, which, when coupled to the vibrational modes (e.g., vibrons or phonons), could result in appreciable power factors. Both air-stable p-type and n-type metal coordinated polymers can be readily synthesized from abundant materials and are promising scalable alternatives to inorganics that leverage solution processing. This talk will first provide a motivating overview of emerging thermoelectric technologies being developed. Next, this talk will discuss progress in synthesizing and controlling the thermoelectric properties of poly(nickel-ethenetetrathiolate) (i.e., Ni-ETT) and poly(nickel-tetrathiooxalate) (i.e., Ni-TTO), which are both air-stable n-type thermoelectric materials containing nickel along the polymer backbone. Next, this talk will discuss our work studying poly(3-alkyl-chalcogenophene), where the isovalent series of thiophene, selenophene, and tellurophene were investigated. Finally, this talk will introduce new thermoelectric device architectures, specifically printable and knittable devices, that are enabled by these materials. Throughout this talk emphasis will be placed on the engineering and scaling challenges, which have been overlooked in the pursuit of high performance materials, but are most critical to realizing thermoelectric devices.

Vertically Stacked CVD-Grown 2D Heterostructure for Wafer-Scale Electronics

This paper demonstrates, for the first time, wafer-scale graphene/MoS2 heterostructures prepared by chemical vapor deposition (CVD) and their application in vertical transistors and logic gates. A CVD-grown bulk MoS2 layer is utilized as the vertical channel, whereas CVD-grown monolayer graphene is used as the tunable work-function electrode. The short vertical channel of the transistor is formed by sandwiching bulk MoS2 between the bottom indium tin oxide (ITO, drain electrode) and the top graphene (source electrode). The electron injection barriers at the graphene-MoS2 junction and ITO-MoS2 junction are modulated effectively through variation of the Schottky barrier height and its effective barrier width, respectively, because of the work-function tunability of the graphene electrode. The resulting vertical transistor with the CVD-grown MoS2/graphene heterostructure exhibits a current density exceeding 7 A/cm2, a subthreshold swing of 410 mV/dec, and an on-off current ratio exceeding 103. The large-area synthesis, transfer, and patterning processes of both semiconducting MoS2 and metallic graphene facilitate construction of a wafer-scale array of transistors and logic gates such as NOT, NAND, and NOR.

Hybrid graphene metasurface for near-infrared absorbers.

We experimentally demonstrated an amorphous graphene-based metasurface yielding near-infrared super absorber characteristic. The structure is obtained by alternatively combining magnetron-sputtering deposition and graphene transfer coating fabrication techniques. The thickness constraint of the physical vapor-deposited amorphous metallic layer is unlocked and as a result, the as-fabricated graphene-based metasurface absorber achieves near-perfect absorption in the near-infrared region with an ultra-broad spectral bandwidth of 3.0 µm. Our experimental characterization and theoretical analysis further point out that the strong light-matter interaction observed is caused by localized surface plasmon resonance of the metal film's particle-like surface morphology. In addition to the enhanced light absorption characteristics, such an amorphous metasurface can be used for surface-enhanced Raman scattering applications. Meanwhile, the proposed graphene-based metasurface relies solely on CMOS-compatible, low cost and large-area processing, which can be flexibly scaled up for mass production.

Eco‐Compatible Solvent‐Processed Organic Photovoltaic Cells with Over 16% Efficiency

Recent advances in nonfullerene acceptors (NFAs) have enabled the rapid increase in power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, this progress is achieved using highly toxic solvents, which are not suitable for the scalable large-area processing method, becoming one of the biggest factors hindering the mass production and commercial applications of OPVs. Therefore, it is of great importance to get good eco-compatible processability when designing efficient OPV materials. Here, to achieve high efficiency and good processability of the NFAs in eco-compatible solvents, the flexible alkyl chains of the highly efficient NFA BTP-4F-8 (also known as Y6) are modified and BTP-4F-12 is synthesized. Combining with the polymer donor PBDB-TF, BTP-4F-12 shows the best PCE of 16.4%. Importantly, when the polymer donor PBDB-TF is replaced by T1 with better solubility, various eco-compatible solvents can be applied to fabricate OPV cells. Finally, over 14% efficiency is obtained with tetrahydrofuran (THF) as the processing solvent for 1.07 cm2 OPV cells by the blade-coating method. These results indicate that the simple modification of the side chain can be used to tune the processability of active layer materials and thus make it more applicable for the mass production with environmentally benign solvents.

Mg(OH)2 nano-sheet decorated MgO micro-beams by electron beam irradiation for thermochemical heat storage

Abstract MgO/Mg(OH)2 based thermal energy storage materials are studied. MgO micro-beams were extensively formed on the surface of the millimeter size MgO pellets by electron beam irradiation, and Mg(OH)2 flakes were synthesized in large amounts by inducing reaction with water molecules on the increased MgO surface area. It was confirmed that the mass reduction tendency of Mg(OH)2 flakes synthesized in large quantities was remarkably increased in the course of decomposition into MgO and H2O through an endothermic process. It is expected that this process can be applied to various thermal energy storage systems in the future because there is no dispersion and fine powder, and large-area processing is possible.

Electro-Optical Properties of a Polymer Dispersed and Stabilized Cholesteric Liquid Crystals System Constructed by a Stepwise UV-Initiated Radical/Cationic Polymerization

Polymer-dispersed liquid crystal (PDLC) and polymer-stabilized liquid crystal (PSLC) are two typical liquid crystal (LC)/polymer composites. PDLCs are usually prepared by dispersing LC droplets in a polymer matrix, while PSLC is a system in which the alignment of LC molecules is stabilized by interactions between the polymer network and the LC molecules. In this study, a new material system is promoted to construct a coexistence system of PDLC and PSLC, namely PD&SChLC. In this new material system, a liquid-crystalline vinyl-ether monomer (LVM) was introduced to a mixture containing cholesteric liquid crystal (ChLC) and isotropic acrylate monomer (IAM). Based on the different reaction rates between LVM and IAM, the PD&SChLC architecture was built using a stepwise UV-initiated polymerization. During the preparation of the PDS&ChLC films, first, the mixture was irradiated with UV light for a short period of time to induce the free radical polymerization of IAMs, forming a phase-separated microstructure, PDLC. Subsequently, an electric filed was applied to the sample for long enough to induce the cationic polymerization of LVMs, forming the homeotropically-aligned polymer fibers within the ChLC domains, which are similar to those in a PSLC. Based on this stepwise UV-initiated radical/cationic polymerization, a PD&SChLC film with the advantages of a relatively low driving voltage, a fast response time, and a large-area processability is successful prepared. The film can be widely used in flexible displays, smart windows, and other optical devices.

(Invited) Morphological Engineering for Polymer-Based Electrochemical Sensors

Bioelectronic devices incorporating polymeric materials into their active region has recently emerged as a driver for the next-generation sensors and healthcare technologies. A rapidly growing interest in this new application of versatile organic semiconductors and conducting polymers has been witnessed during the last few years, shaping a distinct field of ‘organic (or plastic) bioelectronics’. Low-temperature, large-area processability (e.g. printing), excellent biocompatibility, chemical tunability, and mechanical softness are some of the major reasons that organic-based bioelectronics can be a promising option for unconventional wearable and implantable platforms. However, only a small number of materials have shown promising performances up to now, and the systematic understanding has been lacking on the general processing-structure-property relationships of relevant polymers. In this presentation, a facile solution-based post-deposition morphological modification technique is reported, which successfully applied to the model bioelectronic polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. Besides the intended solid-state structural transformation, a multitude of beneficial effects on the performances of electrochemical transistor devices (a fundamental sensing element in bioelectronics) made of the engineered materials were clearly observed and systematically analyzed by a number of experimental tools. Most importantly, the nanoscale chain re-arrangements and induced long-range crystallinity gave rise to the substantial compositional changes that eventually led to the improved anti-swelling property, larger volumetric capacitance, and unprecedentedly high thermal and environmental stability. It is therefore expected that both the scientific understanding and application potentials underlined in this study will serve as a practical guideline for the development of viable organic bioelectronics technologies.

Efficient Enhancement of Electron Transport and Collection Capability in PTB7:PC71 BM-based Solar Cells Enabled by Sulforhodamine Cathode Interlayers.

Development of low-cost water-/alcohol-soluble interfacial materials is a crucial issue to promote the commercialization of polymer solar cells (PSCs). Herein, two derivatives of low-cost rhodamine, called sulforhodamine 101 (SR101) and sulforhodamine B (SRB), are applied as cathode interfacial layers (CILs) to effectively improve the charge-carrier transportation and collection, reduce the work function (WF) of Al counter electrode, and decrease the series resistance and charge recombination in the PSCs. As a result, SR101-based devices show excellent performance with the highest power conversion efficiency (PCE) of 9.10 %, superior to that of both the control devices with MeOH/Al and Ca/Al. Notably, sulforhodamine is commercially available with low cost and great solution-processability. This work demonstrates that sulforhodamine has a great potential as a CIL material,which is suitable for the large-area fabrication process and commercialization of highly efficient PSCs.