Semiconductor Ink Research Articles

Dissolution of Sn, SnO, and SnS in a Thiol-Amine Solvent Mixture: Insights into the Identity of the Molecular Solutes for Solution-Processed SnS.

Binary solvent mixtures of alkanethiols and 1,2-ethylenediamine have the ability to readily dissolve metals, metal chalcogenides, and metal oxides under ambient conditions to enable the facile solution processing of semiconductor inks; however, there is little information regarding the chemical identity of the resulting solutes. Herein, we examine the molecular solute formed after dissolution of Sn, SnO, and SnS in a binary solvent mixture comprised of 1,2-ethanedithiol (EDT) and 1,2-ethylenediamine (en). Using a combination of solution (119)Sn NMR and Raman spectroscopies, bis(1,2-ethanedithiolate)tin(II) was identified as the likely molecular solute present after the dissolution of Sn, SnO, and SnS in EDT-en, despite the different bulk material compositions and oxidation states (Sn(0) and Sn(2+)). All three semiconductor inks can be converted to phase-pure, orthorhombic SnS after a mild annealing step (∼350 °C). This highlights the ability of the EDT-en solvent mixture to dissolve and convert a variety of low-cost precursors to SnS semiconductor material.

Cosolvent approach for solution-processable electronic thin films.

Low-temperature solution-processable electronic materials are of considerable interest for large-area, low-cost electronics, thermoelectrics, and photovoltaics. Using a soluble precursor and suitable solvent to formulate a semiconductor ink is essential for large-area fabrication of semiconductor thin films. To date, it has been shown that hydrazine can be used as a versatile solvent to process a wide range of inorganic semiconductors. However, hydrazine is highly toxic and not suitable for large-scale manufacturing. Here we report a binary mixed solvent of amine and thiol for effective dispersion and dissolution of a large number of inorganic semiconductors including Cu2S, Cu2Se, In2S3, In2Se3, CdS, SnSe, and others. The mixed solvent is significantly less toxic and safer than hydrazine, while at the same time offering the comparable capability of formulating diverse semiconductor ink with a concentration as high as >200 mg/mL. We further show that such ink material can be readily processed into high-performance semiconducting thin films (Cu2S and Cu2Se) with the highest room-temperature conductivity among solution-based materials. Furthermore, we show that complex semiconductor alloys with tunable band gaps, such as CuIn(S(x)Se(1-x))2 (0 ≤ x ≤ 1), can also be readily prepared by simply mixing Cu2S, Cu2Se, In2S3, and In2Se3 ink solutions in a proper ratio. Our study outlines a general strategy for the formulation of inorganic semiconductor ink for low-temperature processing of large-area electronic thin films on diverse substrates and can greatly impact diverse areas including flexible electronics, thermoelectrics, and photovoltaics.

High-Speed Printing of Transistors: From Inks to Devices

The realization of a high-speed printing technique with high resolution and pattern fidelity is critical to making printed electronics a viable technology for electronics manufacturing. The printing requirements of printed electronics are substantially different that those of graphic arts. To make printed electronics a reality, it is necessary to deliver high resolution, good reproducibility, excellent pattern fidelity, high process throughput, and compatibility with the requisite semiconductor, dielectric, and conductor inks. In this paper, we review the physics of pattern formation from pixelated primitives, such as those that exist during inkjet and gravure printing, and will show how control of drop merging and drying can be used to produce high-fidelity shapes, including lines, squares, and intersections. We additionally discuss the physical underpinnings of gravure printing and inkjet printing, and show how these techniques can be scaled to produce high-fidelity highly scaled patterns, including sub-2 micron features at printing speeds of $\sim$ 1 m/s. Finally, in conjunction with high-performance materials, we describe our realization of high-performance fully printed transistors on plastic, offering high-switching speed, excellent process throughput, and good fidelity over large areas.

All-printed flexible organic transistors enabled by surface tension-guided blade coating.

A combination of surface energy-guided blade coating and inkjet printing is used to fabricate an all-printed high performance, high yield, and low variability organic thin film transistor (OTFT) array on a plastic substrate. Functional inks and printing processes were optimized to yield self-assembled homogenous thin films in every layer of the OTFT stack. Specifically, we investigated the effect of capillary number, semiconductor ink composition (small molecule-polymer ratio), and additive high boiling point solvent concentrations on film fidelity, pattern design, device performance and yields.

32.3:Invited Paper: Demonstration of High Mobility Un‐isolated Corbino OTFT's with Improved On/Off Ratios Suitable for use in Flexible Displays

AbstractPresented are OTFT arrays fabricated from a high mobility organic semiconductor ink wherein the preferred TFTs are corbino‐type devices. As a direct consequence of the corbino design, the resulting TFTs have very high on/off ratios, typically > 107coupled with very low off currents. This corbino approach demonstrates that high mobility OTFTs with superior switching ratios can be obtained without the need for isolation of the organic semiconductor. Hansen solubility parameters and mobility data are reported. This also indicates potential for the commercial use of organic semiconductor materials in the industrial production of OTFT backplanes for flexible AMOLED displays.

6.2: Invited Paper: A New Automated Manufacturing Line of All‐Printed TFT Array Flexible Film

AbstractIn the printed electronics field, it has been needed to realize the manufacturing technologies for inventing mass production phase. We JAPERA, which was organized 2011 as a NEDO project to develop the manufacturing technologies of large area flexible printed electronics, established the automated and continuously operated manufacturing line for which produced the all‐printed TFT array flexible film. We operated the offset printer, inkjet printer, screen printer, and slit‐die coater. Specialized ink materials, for example Ag nano particle ink for gate electrode and source/drain electrode, highly purified polymer ink for gate insulator, and organic semiconductor ink for TFT active layer have been developed. We have been acquiring successful results of TFT array flexible film by high yield. The applications of our THT array flexible film for making a display device or a sensor device are presented.

Automated Continuously-Manufacturing Line of All-Printed Organic TFT Array Flexible Film

In the printed electronics field, it has been needed to realize the manufacturing technologies for inventing mass production phase. We JAPERA, which was organized 2011 as a NEDO project to develop the manufacturing technologies of large area flexible printed electronics, established the automated and continuously operated manufacturing line for which produced the all-printed TFT array flexible film. We operated the offset printer, inkjet printer, screen printer, and slit-die coater. Specialized ink materials, for example Ag nano particle ink for gate electrode and source/drain electrode, highly purified polymer ink for gate insulator, and organic semiconductor ink for TFT active layer have been developed. We have been acquiring successful results of TFT array flexible film by high yield. The applications of our THT array flexible film for making a display device or a sensor device are presented.

Microstructural Control over Soluble Pentacene Deposited by Capillary Pen Printing for Organic Electronics

Research into printing techniques has received special attention for the commercialization of cost-efficient organic electronics. Here, we have developed a capillary pen printing technique to realize a large-area pattern array of organic transistors and systematically investigated self-organization behavior of printed soluble organic semiconductor ink. The capillary pen-printed deposits of organic semiconductor, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS_PEN), was well-optimized in terms of morphological and microstructural properties by using ink with mixed solvents of chlorobenzene (CB) and 1,2-dichlorobenzene (DCB). Especially, a 1:1 solvent ratio results in the best transistor performances. This result is attributed to the unique evaporation characteristics of the TIPS_PEN deposits where fast evaporation of CB induces a morphological evolution at the initial printed position, and the remaining DCB with slow evaporation rate offers a favorable crystal evolution at the pinned position. Finally, a large-area transistor array was facilely fabricated by drawing organic electrodes and active layers with a versatile capillary pen. Our approach provides an efficient printing technique for fabricating large-area arrays of organic electronics and further suggests a methodology to enhance their performances by microstructural control of the printed organic semiconducting deposits.

Self‐Organization of Inkjet‐Printed Organic Semiconductor Films Prepared in Inkjet‐Etched Microwells

AbstractThe high‐precision deposition of highly crystalline organic semiconductors by inkjet printing is important for the production of printed organic transistors. Herein, a facile nonconventional lithographic patterning technique is developed for fabricating banks with microwell structures by inkjet printing solvent droplets onto a polymer layer, thereby locally dissolving the polymer to form microwells. The semiconductor ink is then inkjet‐printed into the microwells. In addition to confining the inkjet‐printed organic semiconductor droplets, the microwells provide a platform onto which organic semiconductor molecules crystallize during solvent evaporation. When printed onto the hydrophilic microwells, the inkjet‐printed 6,13‐bis(triisopropylsilylethynyl) pentacene (TIPS_PEN) molecules undergo self‐organization to form highly ordered crystalline structures as a result of contact line pinning at the top corner of the bank and the outward hydrodynamic flow within the drying droplet. By contrast, small crystallites form with relatively poor molecular ordering in the hydrophobic microwells as a result of depinning of the contact line along the walls of the microwells. Because pinning in the hydrophilic microwells occurred at the top corner of the bank, treating the surfaces of the dielectric layer with a hydrophobic organic layer does not disturb the formation of the highly ordered TIPS_PEN crystals. Transistors fabricated on the hydrophilic microwells and the hydrophobic dielectric layer exhibit the best electrical properties, which is explained by the solvent evaporation and crystallization characteristics of the organic semiconductor droplets in the microwell. These results indicate that this technique is suitable for patterning organic semiconductor deposits on large‐area flexible substrates for the direct‐write fabrication of high‐performance organic transistors.

Towards solution-processed ambipolar hybrid thin-film transistors based on ZnO nanoparticles and P3HT polymer

Solution-processed n-channel oxide semiconductor thin-film transistors (TFTs) were fabricated using zinc oxide (ZnO) nanoparticles. Polycrystalline fused-ZnO nanoparticle films were produced by spin-coating ZnO nanosphere dispersions following by a subsequent heat treatment. The solution-processable semiconductor ink based on ZnO was prepared by dispersing the synthesized ZnO nanospheres in isopropanol mixed with ethanolamine to various concentrations from 20 to 80mg/mL. Such concentration dependence on morphology and microstructure of thin films was studied on spin-coated ZnO films by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Spin-coated ZnO films involved as active layers in transistor configuration delivered an almost ideal output characteristic (Id–Vd) with an electron mobility up to 3×10−2cm2/Vs. As a p-channel semiconductor, a poly(3-hexylthiophene) (P3HT) solution-processable ink was deposited by spin-coating on top of closely packed ZnO nanoparticles-based films to form an uniform overlying layer. A hybrid (inorganic–organic) interface was formed by the direct contact between ZnO and P3HT leading to carrier redistribution. Such solution-processed hybrid thin-film transistors delivered in air well balanced electron and hole mobilities as 3.9×10−5 and 2×10−5cm2/Vs, respectively.

Lateral multijunction photovoltaic cells

In this work, features of solar cells of lateral type were analyzed. The authors offered a design of a monolithic compact solar module with cells electrically connected in series and with a dispersion element (holographic grating). Simulation of a multistage converter was carried out. It has been shown that with increasing the number of cells n the maximum limited efficiency  increases. Its maximum values up to  ≈ 53.6% are reached for n = 15 (in the case of perfectly matched semiconductors and conditions AM0). With further increase of n , the efficiency decreases. For a set of concrete semiconductors  ≈ 45% the maximum efficiency for AM0 conditions may be achieved, when the number of cells equals 4. It has been shown that the calculation results agree with experimental data. The possibility of technical implementation of solar cells with using inkjet printers was investigated, too. Briefly discussed have been the properties of these printers, as well as metal, semiconductor and dielectric inks.

Rheological and Drying Considerations for Uniformly Gravure‐Printed Layers: Towards Large‐Area Flexible Organic Light‐Emitting Diodes

AbstractPrinting organic semiconductor inks by means of roll‐to‐roll compatible techniques will allow a continuous, high‐volume fabrication of large‐area flexible optoelectronic devices. The gravure printing technique is set to become a widespread process for the high throughput fabrication of functional layers. The gravure printing process of a poly‐phenylvinylene derivative light‐emitting polymer dissolved in a two solvent mixture on poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is studied. The surface tensions, contact angles, viscosities, and drying times of the formulations are investigated as a function of the solvent volume fraction and polymer concentration. The properties of the ink grant a homogeneous printed layer, suitable for device fabrication, when the calculated film leveling time is shorter than a critical time, at which the film has been frozen due to loss of solvent via evaporation. The knowledge obtained from the printing process is applied to fabricate organic light‐emitting diodes (OLEDs) on flexible substrates, yielding a luminance of ≈5000 cd m−2.

How to print gadgets [inkjet printing

One company, NthDegree, claims to have invented a ground-breaking method of transferring specific semiconductor properties to a chosen substrate using additive printing and inorganic semiconductor inks. These inks are made of microscopic functional devices such as diodes, transistors and other passive components. These are still built using traditional wafer fabrication technology - there's no revolutionary fab in a box' product just yet. By using these tiny semiconductor devices, the company's printing process creates functional inks that can be printed using standard high-speed printing presses.

Determining the coating speed limitations for organic photovoltaic inks

To determine the output capability of present organic photovoltaic (OPV) materials, it is important to know the theoretical maximum coating speeds of the used semiconductor formulations. Here, we present a comprehensive investigation of the coating stability window of several prototype organic semiconductor inks relevant for organic solar cells. The coating stability window was first determined experimentally by a sheet to sheet coater at velocities of up to 10m/min. A numerical simulation model based on the “Coating Window Suite 2010” software was established to give insight into the coating stability limitations at higher coating velocities. An analysis of PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)] in a water/isopropyl alcohol mixture as well as P3HT:PCBM [poly(3-hexylthiophene-2,5-diyl):Phenyl-C61-butyric acid methyl ester] in chlorobenzene, o-xylene and tetrahydronaphthalene showed the possibility of coating speeds up to 60m/min. The simulation further revealed the maximum coating head distances for a given wet film thickness. Finally, we show a solar-cell with slot-die coated PEDOT:PSS and P3HT:PCBM-layer based on the parameters obtained by the simulated data, which exhibits reasonable performance.

Controlling morphology and phase of pyrite FeS2 hierarchical particles via the combination of structure-direction and chelating agents

Pyrite FeS2 hierarchical microparticles consisting of cubic-like crystals with well defined {100} faceted cubic textures were successfully synthesized using a simple hydrothermal method with the presence of ethylenediamine (en) and polyvinylpyrrolidone (PVP). The roles of PVP and en in the synthesis process were elucidated by the investigation of morphology and crystalline phases of the products under different reaction conditions and by the analysis of atomic structures of the pyrite (100) and (111) surfaces with different termination layers. While en chelates with Fe2+ and reacts with S8 of the reaction agents to help with dispersion and nucleation as well as the role of reducing agent, PVP acts as both a phase regulator due to its reducing ability and a surface capping agent to bind the iron atoms at the FeS2 (111) surface and direct the formation of {100} facets. It was found that the oxidation state of iron ions and the species of anions in the iron reagents also played important roles in the formation of crystalline phase and morphology of the products. The pyrite hierarchical microparticles could be utilized as a new semiconductor ink for solution processing of photovoltaic devices and photodetectors as well as cathode materials for lithium batteries.

Effects of Size on the Optical Properties of Organic Semiconductors: Copper (II) Phthalocyanine Nanoparticles

Organic materials are of great interest for use as electronic and optoelectronic devices. Currently, solar cells are in limited use, due to their relatively high cost. Solar cells based on organic semiconductors are promising as low-cost alternatives to current technologies; for example, low cost printed solar cells may be possible. However, significant frequency ranges in the solar spectrum are not absorbed. Expanding the absorption-spectrum bandwidth for the organic materials by varying the band gap can lead to improved efficiency for solar cells based on organic semiconductor inks. Here we test the possibility of using nanoparticles with different sizes to increase the absorption efficiency.Copper Phthalocyanine (CuPc) nanoparticles have been prepared by a Liquid–Liquid Interface Recrystallization Technique (LLIRCT), and deposited on substrates using a dip-coater at room temperature. The size of the nanoparticles is measured by transmission electron microscopy (TEM). Particle size and morphology are mainly determined by the preparation time. Optical proprieties were measured using UV-VIS spectroscopy from 350 to 1000 nm. The spectra show a shift in the peak positions as the particles become smaller. The band gap increases with the particle size.

Inkjet Printing of TIPS‐PEN on Soluble Polymer Insulating Films: A Route to High‐Performance Thin‐Film Transistors

We present an approach to inkjet print high-performance organic transistors by printing the organic semiconductor ink on a thin, continuous, and solvent-absorbing layer of insulating material. The ink spreading is effectively controlled by local dissolution of the layer, and during drying the characteristic circular morphology with high rims and inner plateau forms.

Ink Jet printable organic semiconducting materials and formulations – Combining printability, high performance and air stability

Fabrication of thin film transistors (TFT's) for display applications by IJ printing involves many individual but interrelated processes from the design of organic semiconductor materials (OSCs), development of stable formulations that can be jetted reliably, the design of patterned substrates together with electrodes and other structures, pre and post treatment processes to ensure correct semiconductor and dielectric layer formation together with optimization of the deposition and drying process. Each of these steps has to be designed to fit the complete manufacturing process, independently developed and then optimized for the total device.In this paper we describe some aspects of this development cycle leading to commercially available ink jet printable organic semiconductor and dielectric inks using polymers and small molecules for TFT fabrication in displays and other device applications including:What is required from OSC materials and processes?How can we IJ print and process OSC's?Examples of IJ printed TFT performance we obtain.

Fabrication of a solution-processed thin-film transistor using zinc oxide nanoparticles and zinc acetate

We have fabricated a solution-processed ZnO thin-film transistor without vacuum deposition. ZnO nanoparticles were prepared by the polyol method from zinc acetate, polyvinyl pyrrolidone, and diethyleneglycol. The solution-processable semiconductor ink was prepared by dispersing the synthesized ZnO in a solvent. Inverted stagger type thin-film transistors were fabricated by spin casting the ZnO ink on the heavily doped Si wafer with 200 nm thick SiO2, followed by evaporation of Cr/Au source and drain electrodes. After the drying and heat treatment at 600 ∘C, a relatively dense ZnO film was obtained. The film characteristics were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). In order to obtain the electrical properties of the solution-derived transistor, the on–off ratio, threshold voltage, and mobility were measured.

Bendable GaN high electron mobility transistors on plastic substrates

A procedure for fabricating flexible forms of high electron mobility transistors (HEMTs) supported on plastic substrates is described. The process uses a combination of conventional top-down, wafer scale fabrication protocols to define a printable form of ultrathin, device quality multilayer AlGaN∕GaN single crystalline microstructures—a so-called microstructured semiconductor ink—and soft-lithographic printing methods to effect their registered transfer to a plastic substrate. These procedures yield high performance, bendable HEMT arrays that are mechanically durable—ones with effective transconductances exceeding nearly all reported forms of printed thin-film transistors.