Solution-based Fabrication Research Articles

Facile Routes To Improve Performance of Solution-Processed Amorphous Metal Oxide Thin Film Transistors by Water Vapor Annealing.

Here, we report on a simple and high-rate oxidization method for producing solution-based compound mixtures of indium zinc oxide (IZO) and indium gallium zinc oxide (IGZO) metal-oxide semiconductors (MOS) for thin-film transistor (TFT) applications. One of the issues for solution-based MOS fabrication is how to sufficiently oxidize the precursor in order to achieve high performance. As the oxidation rate of solution processing is lower than vacuum-based deposition such as sputtering, devices using solution-processed MOS exhibit relatively poorer performance. Therefore, we propose a method to prepare the metal-oxide precursor upon exposure to saturated water vapor in a closed volume for increasing the oxidization efficiency without requiring additional oxidizing agent. We found that the hydroxide rate of the MOS film exposed to water vapor is lower than when unexposed (≤18%). Hence, we successfully fabricated oxide TFTs with high electron mobility (27.9 cm(2)/V·s) and established a rapid process (annealing at 400 °C for 5 min) that is much shorter than the conventional as-deposited long-duration annealing (at 400 °C for 1 h) whose corresponding mobility is even lower (19.2 cm(2)/V·s).

Preformed nanoporous carbon nanotube scaffold-based multifunctional polymer composites.

Multifunctional polymer nanocomposites that simultaneously possess high modulus and strength, high thermal stability, novel optical responses, and high electrical and thermal conductivity have been actively researched. Carbon nanotubes are considered an ideal additive for composites because of their superlative physical, electronic and optical properties. While nanotubes have successfully added electrical conductivity, thermal stability, and novel optical responses to polymers, mechanical reinforcements, although substantial, have been well below any theoretical estimations. Here, we integrated preformed hydrogels and aerogels of individually dispersed nanotubes with polymer to increase elastic modulus of composites according to Halpin-Tsai model up to at least 25 vol % of nanotubes. Our solution-based fabrication method allowed us to create bulk composites with tunable form-factors, and with polymers that were incompatible with nanotubes. Further, in this approach, nanotubes were not covalently linked among themselves and to the polymer, so intrinsic optical, electrical, and thermal properties of nanotubes could be exploited. The optically active nanotubes, for example, added a strain-dependent, spatially resolved fluorescence to these composites. Finally, the nanoporous nanotube networks suppressed the polymer glass transition and extended the mechanical integrity of polymer well above its polymer melting point, and both the nanotubes and polymer remained thermally stable above their decomposition temperatures.

Reversible optical control of conjugated polymer solubility with sub-micrometer resolution.

Organic electronics promise to provide flexible, large-area circuitry such as photovoltaics, displays, and light emitting diodes that can be fabricated inexpensively from solutions. A major obstacle to this vision is that most conjugated organic materials are miscible, making solution-based fabrication of multilayer or micro- to nanoscale patterned films problematic. Here we demonstrate that the solubility of prototypical conductive polymer poly(3-hexylthiophene) (P3HT) can be reversibly "switched off" using high electron affinity molecular dopants, then later recovered with light or a suitable dedoping solution. Using this technique, we are able to stack mutually soluble materials and laterally pattern polymer films by evaporation or with light, achieving sub-micrometer, optically limited feature sizes. After forming these structures, the films can be dedoped without disrupting the patterned features; dedoped films have identical optical characteristics, charge carrier mobilities, and NMR spectra as as-cast P3HT films. This method greatly simplifies solution-based device fabrication, is easily adaptable to current manufacturing workflows, and is potentially generalizable to other classes of materials.

Solution-based fabrication of a highly catalytically active 3D network constructed from 1D metal-organic framework-coated polymeric worm-like micelles.

A 3D network constructed from metal-organic framework composite nanowires with a uniform width and a loose (swollen) structure has been prepared. It contained micro-, meso- and macro-pores, which make the 3D network ideal for use as a catalyst, as evidenced by its high catalytic activity in the Knoevenagel reaction.

Tailoring of the PbS/metal interface in colloidal quantum dot solar cells for improvements of performance and air stability

Despite the outstanding advantages of a simple structure and cost-effectiveness of solution-based fabrication, Schottky junction quantum dot solar cells (QDSCs) often demonstrate low open-circuit voltage and power conversion efficiency (PCE) due to insufficient band bending at the QD/metal Schottky junction. Generally, this undesirable result stems from the presence of many defects at the QD/metal interface and the consequent Fermi-level pinning effect. Here, we show how the simple oxidation of PbS QDs at the PbS/metal interface can greatly improve the open-circuit voltage, fill factor, and PCE of Schottky junction QDSCs. On the basis of systematic analysis results using current–voltage characterization, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and light-soaking tests, we reveal that this enhancement originates from reduced interface states at the PbS/metal Schottky junction. Moreover, a significant enhancement of stability of the device is confirmed by the maintenance of >55% of its initial PCE even after 500 hours exposure in air without additional passivation.

Solution-Based Preparation of Graphene-Li2s Composite Cathodes for Lithium/Sulfur and Lithium-Ion Batteries

Fully-lithiated lithium sulfide (Li2S, theoretical capacity 1166 mAhg-1) could be a more promising cathode material than sulfur (S), because it is compatible with safer and more stable Li-free anodes (such as graphite or silicon-based ones, to name a few). Unfortunately, Li2S suffers from low electrical and low ionic conductivities as well as the dissolution and shuttling of lithium polysulfides during cycling, as S cathodes do [1]. Due to the high melting point of commercial Li2S (950°C), the preparation of carbon-Li2S composites by melt infiltration is difficult. The large particle size (30-50 µm) makes it challenging to achieve high capacity utilization of this cathode material. The ball milling has been the most common procedure utilized to reduce the size of commercial Li2S powders to improve their rate performance and capacity utilization. Unfortunately, the ball milling procedure does not allow formation of uniform particles with controlled morphology, uniform carbon coating and good electrochemical performance [2, 3].In this research, we report for the first time a simple and versatile solution-based method to prepare the nano-sized Li2S and graphene-Li2S composites that does not induce release of any dangerous gases [4]. As shown in Figure 1, our finding of the lack of alcoholysis of Li2S in ethanol combined with its relatively high solubility allowed the formation of nanostructured Li2S by simple evaporating of ethanol from a Li2S solution. The pure Li2S powder demonstrates good uniformity of the homogeneously precipitated particles, their crystalline cuboid shape and an average size of ~200 nm. As expected, from SEM results, increasing graphene resulted in the reduction of both the average particle size and degree of the Li2S nanoparticle agglomeration.The addition of graphene and other nanostructured carbons in to the solution allows formation of Li2S-containing composites with enhanced electrical conductivity and structural stability. Higher electrolyte molarity was found to enhance cycle stability of the prepared cathodes in half cells (vs. Li foil) due to the reduced polysulfide dissolution [5-7] and maintaining better mechanical properties of a polyvinylidene fluoride (PVDF) binder in a less swollen state [8]. The high rate performance and good cycle stability (less than 15 % degradation after 200 cycles in Figure 2) of the selected graphene-Li2S nanocomposite cathodes suggest a great promise of the proposed method. The proposed methodology opens the door to solution-based fabrication of a variety of other Li2S-containing composites. Further optimization of Li2S-based composites and the formation of shells around the composite particles are explored to further enhance their performance in cells.

Facile solution-based fabrication of ZnIn2S4 nanocrystalline thin films and their photoelectrochemical properties

Hexagonal phase ZnIn2S4 nanocrystalline thin films are deposited by spin-coating method using an air-stable precursor solution. In the process, metal oxide and hydroxide are used as Zn and In sources to avoid the introduction of any anion impurity in ZnIn2S4 thin films. The rod-like nanocrystalline grains with width and length about 13 ± 3 nm and 26 ± 5 nm grow along c-axis after annealing in sulfur vapor at 500 °C for 2 h. Smooth and compact thin films with different In/Zn ratios are fabricated by controlling the ratio of two metal sources in precursor solution. The flat-band potentials of these n-type ZnIn2S4 thin films are in the range of −0.55 to −0.45 V vs. normal hydrogen electrode. The photoelectrochemical measurement demonstrates the excellent visible light response of ZnIn2S4 thin films. The light current under visible light illumination occupies almost 60% of the light current under full spectrum illumination. The facile solution-based method provides a novel thought to fabricate high-quality thin films of multi metal chalcogenides with excellent photoelectric properties via solving the other metal oxide and hydroxide in the solution.

Water-based nanoparticulate solar cells using a diketopyrrolopyrrole donor polymer

Organic photovoltaic devices with either bulk heterojunction (BHJ) or nanoparticulate (NP) active layers have been prepared from a 1 : 2 blend of (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}) (PDPP-TNT) and the fullerene acceptor, ([6,6]-phenyl C71-butyric acid methyl ester) (PC70BM). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) have been used to investigate the morphology of the active layers of the two approaches. Mild thermal treatment of the NP film is required to promote initial joining of the NPs in order for the devices to function, however the NP structure is retained. Consequently, whereas gross phase segregation of the active layer occurs in the BHJ device spin cast from chloroform, the nanoparticulate approach retains control of the material domain sizes on the length scale of exciton diffusion in the materials. As a result, NP devices are found to generate more than twice the current density of BHJ devices and have a substantially greater overall efficiency. The use of aqueous nanoparticulate dispersions offers a promising approach to control the donor acceptor morphology on the nanoscale with the benefit of environmentally-friendly, solution-based fabrication.

Solution-Based Fabrication and Characterization of a Logic Gate Inverter Using Random Carbon Nanotube Networks

Semiconducting carbon nanotubes (CNTs) are considered as one of the most promising candidates to replace silicon in future nano-electronics. Single-walled carbon nanotubes (SWNTs) have been used as an active channel material in field-effect transistors (FETs). The nanotube-based circuits show great potential in future electronics and computer technology. Integrating SWNT FETs to form logic gates-the basic units of integrated circuits (ICs)-needs both p- and n-type SWNT FETs. However, without doping, annealing, or other special treatment, the as-obtained SWNT FETs are typically p-type. Here we report a SWNT-based logic device-a logic gate inverter (or a NOT gate)-using simple fabrication methods. The critical components of the inverter including a p-type SWNT FET and an n-type SWNT FET are fabricated using low-cost materials and an easy-to-control solution-based process. The introduction of polyethylenimine (PEI), a polymer with high electron-donating ability, to the device successfully converts the p-type FET to an n-type device. The resulting devices are air-stable outside a vacuum or an inert environment. Electrical characterization of these devices demonstrates that both p-type and n-type FETs produce typical field effects and the resulting logic gate inverter exhibits satisfactory switching characteristics. We believe that the combination of the simple fabrication methods, easy conversion of the transistors, and satisfactory logic gate switching performance can influence fundamental research in nano-materials and practical applications of nano-electronics.

Fabrication of Highly Transparent and Conductive Indium–Tin Oxide Thin Films with a High Figure of Merit via Solution Processing

Deposition technology of transparent conducting oxide (TCO) thin films is critical for high performance of optoelectronic devices. Solution-based fabrication methods can result in substantial cost reduction and enable broad applicability of the TCO thin films. Here we report a simple and highly effective solution process to fabricate indium-tin oxide (ITO) thin films with high uniformity, reproducibility, and scalability. The ITO films are highly transparent (90.2%) and conductive (ρ = 7.2 × 10(-4) Ω·cm) with the highest figure of merit (1.19 × 10(-2) Ω(-1)) among all the solution-processed ITO films reported to date. The high transparency and figure of merit, low sheet resistance (30 Ω/sq), and roughness (1.14 nm) are comparable with the benchmark properties of dc sputtering and can meet the requirements for most practical applications.

Quantum dot light-emitting devices

Abstract

Fully Solution-Based Fabrication of Flexible Light-Emitting Device at Ambient Conditions

Organic light-emitting devices (OLEDs) have long been perceived as a low-temperature, low-cost technology that can be fabricated through solution-based processes. However, how to achieve all-solution-processed OLEDs has been a challenge. Here, we report the fabrication of fully solution processed polymer light-emitting electrochemical cells (PLECs) as an alternative emissive device by spin-coating, rod-coating, and/or blade-coating at ambient conditions. The all-solution-based device, which can be fabricated under ambient air and in large-size, exhibits good flexibility and uniform light emission. The entire fabrication process of the PLEC, include the formation of electrodes, emissive polymer layer, and substrate, are carried out by solution processing under ambient conditions. Moreover, it is notable that employing a solution-processed and transparent SWCNT/AgNW bilayer composite electrode as anode, this fully solution processed PLEC shows even higher device performance than the conventional control dev...

High-performance polymer light emitting diodes with interface-engineered graphene anodes

Recently, graphene-based organic light emitting diodes (OLEDs) were successfully demonstrated using graphene as anodes. However, the graphene electrodes have not been utilized for polymer light emitting diodes (PLEDs) yet, although the simpler device structure and the solution-based fabrication process of PLEDs are expected to be more advantageous in terms of time and cost. Here we demonstrate high-performance polymer light emitting diodes (PLEDs) with simple two-layer structures using interface-engineered single-layer graphene films as anodes. The single-layer graphene synthesized by chemical vapor deposition methods was transferred onto a glass substrate utilizing an elastic stamp, and its work function was engineered by varying the duration and the power of ultraviolet ozone (UVO) treatment. Thus, we were able to optimize the contact between silver electrodes and the graphene anodes, leading to the considerable enhancement of light-emitting performance.

Solution-Based Fabrication of Carbon Nanotube Gas Sensor by Using Dielectrophoresis and Spin-Column Chromatography

Single-walled carbon nanotubes (SWCNTs) gas sensor has attracted a great deal of attention because of their remarkable properties. The sensor response is attribute to the semiconducting CNT whose electronic properties depend on its chirality. The authors have previously found that the sensor response increased by using separated semiconducting SWCNTs from a mixture with metallic one. Since the electronic structure (metallic or semiconducting) of CNTs is governed by their chirality, a chirality-selective fabrication of CNT gas sensor is essential to improve their performance. In this study, we proposed chirality-based separation of semiconducting SWCNTs by using spin-column chromatography. Pristine CNT suspension was separated into three fractions that had different chiralities of semiconducting SWCNTs. Separated semiconducting CNTs of each fraction were used for fabrication of three CNT gas sensors by dielectrophoresis. Comparison of these sensor responses to NO2 revealed that sensor response depended on the chirality.

Solution-based fabrication of a graphene–ZnO nanocomposite

Graphene-based composites represent a new class of materials with potential for many applications. Graphene can be attached to a metal, a semiconductor, or any polymer. In this work, our approach was to attach graphene to a well-known semiconductor, ZnO. We synthesized graphene–ZnO composites by a simple, low-cost, environmentally friendly solvothermal method, carrying out the reaction in different conditions in order to discover the optimum condition, and also to obtain a high-quality product. Our research demonstrated that the optimum temperature to obtain a high-quality product is 180 °C for 20 h. All obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy, electron dispersion spectrometry, X-ray photoelectron spectrometry, Raman spectroscopy, Fourier transform infrared spectrometry, UV–visible spectrophotometry, and thermogravimetric analysis. The XRD confirmed that the crystal structure of the ZnO in the nanocomposite was wurtzite type. The prepared composite was stable to 800 °C with its 80 % weight.

A comparative analysis of thin-film transistors using aligned and random-network carbon nanotubes

The purpose of this project is to investigate the characterization of carbon nanotube (CNT) thin-film transistors based on two solution-based fabrication methods: dielectrophoretic deposition of aligned CNTs and self-assembly of random-network CNTs. The electrical characteristics of aligned and random-network CNT transistors are studied comparatively. In particular, the selection effect of metallic and semiconducting CNTs in the dielectrophoresis process is evaluated experimentally by comparing the output characteristics of the two transistors. Our results demonstrate that the self-assembly method produces a stronger field effect with a much higher on/off ratio (Ion/Ioff). This phenomenon provides evidence that the metallic CNTs are more responsive to dielectrophoretic forces than their semiconducting counterparts under common deposition conditions. In addition, the nanotube–nanotube cross-junctions in random-network CNT films create additional energy barriers and result in a reduced electric current. Thus, additional consideration must be applied when using different fabrication methods in building CNT-based electronic devices.

Fabrication of a Completely Transparent and Highly Flexible ITO Nanoparticle Electrode at Room Temperature

We report the fabrication of a highly flexible indium tin oxide (ITO) electrode that is completely transparent to light in the visible spectrum. The electrode was fabricated via the formation of a novel ITO nanoarray structure, consisting of discrete globular ITO nanoparticles superimposed on an agglomerated ITO layer, on a heat-sensitive polymer substrate. The ITO nanoarray spontaneously assembled on the surface of the polymer substrate by a simple sputter coating at room temperature, without nanolithographic or solution-based assembly processes being required. The ITO nanoarray exhibited a resistivity of approximately 2.3 × 10(-3) Ω cm and a specular transmission of about 99% at 550 nm, surpassing all previously reported values of these parameters in the case of transparent porous ITO electrodes synthesized via solution-based processes at elevated temperatures. This novel nanoarray structure and its fabrication methodology can be used for coating large-area transparent electrodes on heat-sensitive polymer substrates, a goal unrealizable through currently available solution-based fabrication methods.

Microwave transmission in graphene oxide

We investigated the radio-frequency transmission properties of reduced graphene oxide (GO) sheets including contact effects with the metal electrodes. GO sheets were prepared by dielectrophoresis and their structural characteristics were analyzed by x-ray photoelectron spectroscopy and Raman spectroscopy. The contact resistance was much higher than the intrinsic resistance over the entire frequency range, thus the contact resistance was considered as a dominant component of impedance in the radio-frequency regime. In the radio-frequency regime, GO sheets showed a drastic decrease in impedance based on a consistent decrease in the intrinsic and contact resistance. These results support the potential of GO as a radio-frequency interconnector with a solution-based fabrication method.

Solution-based fabrication of p-channel and n-channel field-effect transistors using random and aligned carbon nanotube networks

We report both p-channel and n-channel single-walled carbon nanotube (SWNT) film-effect transistors (FETs) using low-cost materials and easy-to-control procedures. The introduction of polyethylenimine (PEI), which has high electron-donating ability, successfully realizes the conversion from p-channel FETs to n-channel devices. The resulting n-channel devices are air-stable outside a vacuum or an inert environment. Instead of conventional chemical vapor deposition method, here in this project, p-channel SWNT FETs are fabricated using two solution-based processes. One method is using layer-by-layer self-assembly to create SWNT random networks and the other is based on dielectrophoresis-aligned SWNTs. By coating a polyethylenimine (PEI) layer on the surface, the transistor demonstrates typical n-channel characteristics. The electrical characteristics of random-network and aligned SWNT transistors are studied comparatively. Our results demonstrate that both p-channel and n-channel devices based on the self-assembly method produce stronger field effects with much higher on/off ratios (Ion/Ioff). The combination of fabrication and conversion methods reported here can lead to the development of more complicated SWNT-based devices such as complementary logic gates, which require both p- and n-channel transistors.

Macroporous foam of reduced graphene oxides prepared by lyophilization

In this study, we demonstrate that water-soluble grafted polymer polyfluorene-grafted-poly(sulfobetainemetahcrylate) (PF-g-PSBMA) allows chemically reduced GO sheets dispersed in aqueous solution due to its ability to stably disperse hydrophobic rGO sheets. Based on the stable rGO/PF-g-PSBMA dispersion, macroporous graphene foam can be simply obtained by lyophilization process (freeze-drying). The resulting composite foam exhibits three-dimensional macroporous structure with high specific surface area and can easily re-dissolve into water. Also, the stable rGO/PF-g-PSBMA dispersion allows solution-based fabrication of thin-film graphene devices. Moreover, PF-g-PSBMA can be removed from rGO foam or film by thermal annealing, and the conductivity of rGO film can be greatly enhanced likely due to its annealing ability.