Fully-printed,all-polymer,bendable and highly transparent complementary logic circuits

In this contribution we show a simple approach for the development of all-polymer based complementary logic circuits fabricated by printing on plastic, at low temperature and in ambient conditions. This is achieved by patterning, with a bottom-up approach, solely synthetic carbon-based materials, thus incorporating earth-abundant elements and enabling in perspective the recycling – a critical aspect for low-cost, disposable electronics. Though very simple, the approach leads to logic stages with a delay down to 30 μs, the shortest reported to date for all-polymer circuits, where each single component has been printed. Moreover, our circuits combine bendability and high transparency, favoring the adoption in several innovative applications for portable and wearable large-area electronics.     

A printing technology combining screen-printing with a wet-etching process for the gate electrodes of organic thin film transistors on a plastic substrate

We have developed a practical printing technology for the gate electrode of organic thin film transistors (OTFTs) by combining screen-printing with a wet-etching process using nano-silver (Ag) ink as a conducting material. An Ag film was deposited onto a PVP (polyvinylphenol)-coated PC (polycarbonate) plastic substrate by screen-printing with nano-Ag ink, where Ag content of 20 wt.% was mixed using a terpineol solvent. Subsequently, the film was cured at 200 °C for 60 min, and then finally wet-etched through patterned positive photo-resist masks. The screen-printed Ag electrode exhibited a minimum line width of ∼5 μm, a thickness of ∼65 nm, and a resistivity of ∼10⁻⁶ Ω cm, producing good geometrical and electrical characteristics for a gate electrode. Additionally, it also provided good step coverage with the PVP dielectric layer, and consequently leakage current between the gate and source/drain electrodes was eliminated. Moreover, the electrical characteristic of the screen-printed Ag electrode was not significantly changed even after a bending test in which the Ag electrodes were bent with a bending radius of 6 mm and 2500 iterations of cyclic bending. OTFTs with the screen-printed Ag electrode produced a saturation mobility of 0.13 cm²/Vs and a current on/off ratio of 1.79 × 10⁶, being comparable to those of an OTFT with a thermally evaporated Al gate electrode.     

High-resolution inkjet-printed organic thin film transistor array with commercially applicable source-drain electrode process

High-resolution inkjet printing of an organic thin film transistor (OTFT) array for mass-production is still regarded as an immature technology due to the difficulty in controlling the dimension of pattern and registry with other layers in commercial large-scale substrates. Especially, in the case of on organic gate insulator (OGI) in an inkjet-printed OTFT array, it is impossible to use plasma pre-treatment of the OGI for the hydrophobicity required for high-resolution inkjet printing of an organic semiconductor (OSC) due to its non-selectivity between organic layers, both inside and outside the channel area. A novel and commercially applicable process of the source-drain (SD) electrode prior to inkjet printing of the OSC in the bottom contact structure not only allowed a selective plasma treatment for high-resolution inkjet printing of OSC on OGI without the extra photolithographic process, but also protected the channel interface from the harmful outcomes of wet or plasma processes. This method enabled uniform electrical characteristics of more than 300 thousand pixels of an OTFT array for a backplane. Based on these results, a 5.7 inch electrophoretic display (EPD), with a high resolution of 140 dots per inch (DPI), on a plastic substrate was successfully demonstrated.     

Inkjet printed silver source/drain electrodes for low-cost polymer thin film transistors

Silver tracks for source/drain (S/D) electrodes in low-cost polymer thin film transistors (TFTs) have been realized through inkjet printing technique, using heavily n-doped silicon wafer with thermally grown silicon dioxide as the substrate and poly(3-hexylthiophene) (P3HT) as the channel material. Spin coating a layer of poly-4-vinylphenol (PVPh) onto the substrate was found to enhance the silver track uniformity and lower the cure temperature (from 300 to 210 °C). The surface roughness of the PVPh film was optimized to improve the device performance. The fabricated P3HT TFT with a channel length of 20 μm exhibited a saturation mobility of 3.5 × 10⁻2 cm²/V/s which was three times higher than that obtained in P3HT TFTs with gold S/D electrodes.     

All inkjet-printed piezoelectric polymer actuators: Characterization and applications for micropumps in lab-on-a-chip systems

Digital printing technologies are promising as future manufacturing approaches due to their capabilities of highly flexible and additive material deposition on various substrates. In this contribution, all inkjet-printed piezoelectric polymer actuators are presented based on polyvinylidene fluoride trifluoroethylene (P(VDF-TrFE)) and electrodes printed from silver nanoparticle dispersions. The target application for the actuators described here are membrane pumps for microfluidic lab-on-a-chip (LOC) systems. For the first time, all-inkjet-printed P(VDF-TrFE) actuators are reported and the corresponding piezoelectric d₃₁ coefficient is measured. For manufacturing the actuators, a low-cost procedure is employed that consists of only three inkjet printing and post-processing steps where moderate thermal treatments (T_max = 130 °C) are combined with plasma sintering. The processing is therefore compatible with a wide range of temperature sensitive polymer substrates, completely additive and highly flexible. A sandwich-like structure of a piezoelectric P(VDF-TrFE) layer between two silver electrodes is inkjet-printed onto a polyethylene terephthalate (PET) substrate. When a voltage is applied across the piezoelectric layer, the reverse piezoelectric effect will lead to a bending deflection of this unimorph structure. The piezoelectric d₃₁ coefficients are found to be approximately 7 to 9 pm V⁻¹, which allows the generation of significant actuator deflections. For the application in a micropump, flow rates of several 100 μL min⁻¹ are anticipated, which is promising for LOC applications. Most current micropumps are based on actuator elements that are fabricated separately and mounted on a passive membrane. By using all inkjet-printed actuators, as presented here, the joining step is avoided and the benefits of low-cost printed devices are added to the well-developed processing approaches for microfluidic chips.     

Modeling the gate bias dependence of contact resistance in staggered polycrystalline organic thin film transistors

We have modeled the dependence on the gate voltage of the bulk contact resistance and interface contact resistance in staggered polycrystalline organic thin film transistors. In the specific, we have investigated how traps, at the grain boundaries of an organic semiconductor thin film layer placed between the metal electrode and the active layer, can contribute to the bulk contact resistance. In order to the take into account this contribution, within the frame of the grain boundary trapping model (GBTM), a model of the energy barrier E_B, which emerges between the accumulation layer at the organic semiconductor/insulator interface and injecting contact, has been proposed. Moreover, the lowering of the energy barrier at the contacts interface region has been included by considering the influence of the electric field generated by the accumulation layer on the injection of carriers at the source and on the collection of charges from the accumulation layer to the drain contact. This work outlines both a Schottky barrier lowering, determined by the accumulation layer opposite the source electrode, as well as a Poole-Frenkel mechanism determined by the electric field of the accumulation layer active at the drain contact region. Finally it is provided and tested an analytical equation of our model for the contact resistance, summarizing the Poole-Frenkel and Schottky barrier lowering contribution with the grain boundary trapping model.     

一种新型a-IGZO TFT集成栅极驱动电路设计

在传统集成栅驱动电路中采用非晶InGaZnO薄膜晶体管(a-IGZO TFT)后会造成信赖性的降低,经过分析确定原因为驱动TFT阈值电压漂移。本文提出了一种改进的集成栅驱动电路,通过对驱动TFT栅节点电压的稳定控制,获得了较大的驱动TFT阈值电压漂移冗余度(从原来的不到±-3V扩大到±-9V),克服了a-IGZO TFT阈值电压漂移所造成的电路失效,稳定了集成栅驱动电路并延长了液晶显示器面板的寿命。     

A molecular probe layer modified organic thin film transistor on flexible platform for next generation cyanide sensor

Organic thin film transistor (OTFT) based cyanide (CN⁻) sensor can be of an intense interest due to its simple operation, high sensitivity, system on chip (SoC) integration capacity and applicability towards environmental safety. So, mechanical flexibility of such sensors can extend them to conformable, bio-compatible and wearable applications. In our recent work, a novel OTFT based highly sensitive and mechanically flexible platform was developed to detect CN⁻ in aqueous medium. The sensor was fabricated on a flexible polyethylene terephthalate (PET) substrate by simple spin coating and operated at a very low voltage of −3 V. For the sensing element, a chemically active dansyl-dervatized-triazole linked glucopyranosyl conjugate (DTGC) was used as a molecular probe on the semiconducting poly (3-hexylthiophene) P3HT layer in the device structure. The fabricated sensor was characterized electrically while using an aqueous solution of tetrabutylammonium cyanide (TBAC) as an analyte. Significant changes in device characteristics including 0.4 V shift in threshold voltage, 0.2 cm²v⁻¹s⁻¹ increment in mobility and 2 μA increase in drain current were observed for the OTFT based sensor with presence of CN⁻ analyte. The concentration of the CN⁻ was then varied and thus the minimum detection limit was found to be 1 μM. The sensor also exhibited excellent mechanical flexibility with its stable operation even up to a bending with 5 mm of bending radius. The sensing mechanism of the developed sensor has been explained through the possible charge transfer phenomena from the host semiconductor due to a chemical reaction between the probe layer and the analyte. The sensititvity and chemical reactivity of the molecular probe to the CN⁻ were further confirmed from the microscopic studies and the change in the visual appearance (colour) of the probe with the presence of aqueous CN⁻.     

有机薄膜晶体管直流电流-电压模型的研究

通过对有机薄膜晶体管(OTFT)电流-电压特性的研究,建立了一种用于电路模拟的仿真程序(SPICE)的OTFT直流电流-电压模型,所用的参数都可从实验特性曲线中提取。对一种基于并五苯(Pentacene)的底栅顶接触(TC)结构的OTFT的实验曲线进行参数提取,并利用所得的参数与建立的模型进行仿真,得到的输出特性和转移特性曲线与实验结果无论在线性区还是在饱和区都具有较强的一致性,验证了本文所建模型及参数的准确性。建立的模型能够准确描述OTFT的直流特性,可用于有机电路的SPICE仿真。     

Improved pentacene growth continuity for enhancing the performance of pentacene-based organic thin-film transistors

This study proposes an alternative planar bottom-contact (pBC) structure to enhance the electrical performance of pentacene-based organic thin-film transistors (OTFTs). This pBC structure uses a bilayer dielectric to control planarization with a precise etch depth and introduces a bilayer photoresist lift-off method to ensure that planarization produces an optimum flatness. Because of the improved growth continuity of pentacene near the edge of the source/drain electrodes, the contact resistance between the source/drain and the pentacene was reduced significantly, thereby enhancing the electrical performance of OTFTs. The mechanism for the enhanced performance was also verified by a physics-based numerical simulation.     

Silver nanowire-polymer composite electrode for high performance solution-processed thin-film transistors

Silver nanowires (AgNWs)/poly-(3,4-ethylenedioxythiophene/polystyrene sulphonate) (PEDOT:PSS) composite films as conductive electrode for OTFTs were prepared, and their optical and electrical properties were investigated. The conductive composite films used in this study afforded low sheet resistance of <140 Ω/sq and transmittance as high as 70% in the visible region. For the composite film with 0.1 wt.% of AgNWs, contact resistance as low as 2.7 × 10⁴ Ω cm was obtained, as examined by Transfer length model (TLM) analysis, and work function of the corresponding film was 5.0 eV. Furthermore, the composite films were employed as source and drain electrodes for top-gate/bottom-contact organic thin-film transistors (OTFTs) based on solution-processed 5,11-bistriethylsilylethynyl anthradithiophene (TES-ADT) as organic semiconductor, and the resulting device showed high electrical performance with carrier mobility as high as 0.21 cm²/V s.     

A high-yielding evaporation-based process for organic transistors based on the semiconductor DNTT

We report on the performance of organic thin film transistors manufactured in an all-evaporated vacuum roll-to-roll process. We show that dinaphtho [2,3-b:2′,3′-f] thieno[3,2-b]thiophene (DNTT) is a suitable semiconductor material for deposition onto a flash evaporated polymer insulator layer to make bottom-gate top-contact transistors. Significantly, in batches of 90 transistors, the process approached a 100% yield of high mobility transistors with high on/off ratios and low gate-leakage. By contrast, a solution-deposited insulator layer led to significant gate leakage in a high proportion of transistors leading to poor yield. The performance of DNTT devices is shown to be superior to that of previously reported pentacene devices. Transistor performance is further enhanced by inclusion of a low-polarity surface modification, such as polystyrene, to the acrylate. The devices show good environmental stability but we demonstrate also that they can be in-line encapsulated with an acrylate and a SiOx overlayer without damaging the underlying transistor. Finally, a first demonstration is made of organic vapour jet printing of the DNTT to manufacture transistors with a high semiconductor deposition rate.     

Surface modification of printed silver electrodes for efficient carrier injection in organic thin-film transistors

The modification of printed silver electrode surfaces for use as the bottom-contact electrodes of organic thin-film transistors (OTFTs) is reported. Printed silver electrodes fabricated using the surface photoreactive nanometal printing (SuPR-NaP) technique are inevitably covered with an inert surface layer of alkylamines that is originally used for encapsulation of the silver nanoparticles (AgNPs). However, it may act as a built-in protective layer against carrier injections. We demonstrate that a simple vapor exposure method is sufficient for converting the protective layer into a layer that assists carrier injection. As modifiers, we used various types of fluorinated benzenethiols that exhibit a stronger coordination with the silver surfaces than the alkylamimes. We detected the chemical conversion from alkylamine encapsulation to thiol coordination by surface enhanced Raman spectroscopy (SERS) and evaluated the improvement in the carrier injection using a transfer length method (TLM) for the OTFTs. Among the modifiers, the pentafluorobenzenethiol (PFBT) treatment significantly improves the device performance and stability of the OTFTs.     

Optimization of electrohydrodynamic-printed organic electrodes for bottom-contact organic thin film transistors

In this study, we investigate the optimization of printed (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) as source/drain electrodes for organic thin film transistors (OTFTs) through electrohydrodynamic (EHD) printing process. The EHD-printed PEDOT:PSS electrodes should fulfill the prerequisites of not only high conductivity but also optimum surface tension for successful jetting. The conductivity of PEDOT:PSS was dramatically enhanced from 0.07 to 352 S/cm by the addition of dimethylsulfoxide (DMSO). To use the DMSO-treated PEDOT:PSS solution in the EHD printing process, its surface tension was optimized by the addition of surfactant (Triton X-100), which was found to enable various jetting modes. In the stable cone-jet mode, the patterning of the modified PEDOT:PSS solution was realized on the surface-functionalized SiO₂ substrates; the printed line widths were in the range 384 to 81 μm with a line resistance of 8.3 × 10³ Ω/mm. In addition, pentacene-based OTFTs employing the EHD-printed PEDOT:PSS as source and drain electrodes were found to exhibit electrical performances superior to an equivalent vacuum-deposited Au-based device.     

Modulation of surface solubility and wettability for high-performance inkjet-printed organic transistors

The surface solubility and wettability of photosensitive layers of polystyrene (PS) were engineered to evaluate its effect on the crystalline microstructure and film morphology of inkjet-printed 6,13-bis(triisopropylsiylethynyl) pentacene (TIPS-pentacene). UV exposure proved to be a simple and effective method for modulating the solubility of PS films with controllable crosslinking. As compared with the untreated PS film, the film morphology of the printed semiconductor on the UV-irradiated crosslinked PS films was significantly optimized. The optimal degree of crosslinking and solubility of the PS film were achieved by UV irradiation at a dose of 0.417 J cm⁻². Field-effect transistors fabricated using well-organized crystals on the optimal crosslinked PS film exhibited a maximum mobility of 0.48 cm² V⁻¹ s⁻¹ and an average value of 0.19 cm² V⁻¹ s⁻¹. The performance is clearly superior to that of devices prepared on a pristine PS film (0.02 cm² V⁻¹ s⁻¹).     

An Inkjet‐Printed Field‐Effect Transistor for Label‐Free Biosensing

A flexible, biological field‐effect transistor (BioFET) for use in biosensing is reported. The BioFET is based on an organic thin‐film transistor (OTFT) fabricated mainly by inkjet printing and subsequently functionalized with antibodies for protein recognition. The BioFET is assessed for label‐free detection of a model protein, human immunoglobulin G (HIgG). It is characterized electrically to evaluate the contribution of each step in the functionalization of the OTFT and to detect the presence of the target protein. The fabrication, structure, materials optimization, electrical characteristics, and functionality of the starting OTFT and final BioFET are also discussed. Different materials are evaluated for the top insulator layer, with the aim of protecting the lower layers from the electrolyte and preserving the BioFET electrical performance.     

Load‐Controlled Roll Transfer of Oxide Transistors for Stretchable Electronics

A stretchable and transparent In‐Ga‐Zn‐O (IGZO) thin film transistors with high electrical performance and scalability is demonstrated. A load‐controlled roll transfer method is realized for fully automated and scalable transfer of the IGZO TFTs from a rigid substrate to a nonconventional elastomeric substrate. The IGZO TFTs exhibit high electrical performance under stretching and cyclic tests, demonstrating the potentiality of the load‐controlled roll transfer in stretchable electronics. The mechanics of the load‐controlled roll transfer is investigated and simulated, and it is shown that the strain level experienced by the active layers of the device can be controlled to well below their maximum fracture level during transfer.     

Tailoring Morphology and Structure of Inkjet‐Printed Liquid‐Crystalline Semiconductor/Insulating Polymer Blends for High‐Stability Organic Transistors

Inkjet printing of semiconducting polymers is desirable for realizing low‐cost, large‐area printed electronics. However, sequential inkjet printing methods often suffer from nozzle clogging because the solubility of semiconducting polymers in organic solvents is limited. Here, it is demonstrated that the addition of an insulating polymer to a semiconducting polymer ink greatly enhances the solubility and stability of the ink, leading to the stable ejection of ink droplets. This bicomponent blend comprising a liquid‐crystalline semiconducting copolymer, poly(didodecylquaterthiophene‐alt‐didodecylbithiazole) (PQTBTz‐C12), and an insulating commodity polymer, polystyrene, is extremely useful as a semiconducting layer in organic field‐effect transistors (OFETs), providing fine control over the phase‐separated morphology and structure of the inkjet‐printed film. Tailoring the solubility‐induced phase separation of the two components leads to a bilayer structure consisting of a polystyrene layer on the top and a highly crystalline PQTBTz‐C12 layer on the bottom. The blend film is used as the semiconducting layer in OFETs, reducing the semiconductor content to several tens of pictograms in a single device without degrading the device performance. Furthermore, OFETs based on the PQTBTz‐C12/polystyrene film exhibit much greater environmental and electrical stabilities compared to the films prepared from homo PQTBTz‐C12, mainly due to the self‐encapsulated structure of the blend film.     

Metallo‐Hydrogel‐Assisted Synthesis and Direct Writing of Transition Metal Dichalcogenides

Two dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted interest for their compelling nanoscale new properties and numerous potential applications including fast optoelectronic devices, ultrathin photovoltaics, and high‐performance catalysts. Large‐scale growth of uniform TMDC materials is essential for investigating their physics and for their integration into devices. However, the wafer scale deposition of TMDCs on arbitrary nonselective substrates is still beyond the current state‐of‐the‐art. In this article, a method to synthesize layered TMDCs (MoS₂ and WS₂) at the wafer‐scale by sulfurization of transition metal ions (Mo⁵⁺ and W⁶⁺) in a gelatin template (metallo‐hydrogel) is reported. This process is adaptable to versatile substrates, including amorphous silicon oxide, high‐temperature quartz, and silicon. Although the products are nominally few layer materials, direct band photoluminescent (≈1.8 eV), similar to single‐ or decoupled multilayer MoS₂ is observed. Finally, the solution‐based deposition enables contact printing of TMDC channels to be useable for device applications including thin film transistors with printed silver contacts using the same process.