Plastic Film Substrates Research Articles

Overcoming Downscaling Limitations in Organic Semiconductors: Strategies and Progress.

Organic semiconductors have great potential to revolutionize electronics by enabling flexible and eco-friendly manufacturing of electronic devices on plastic film substrates. Recent research and development led to the creation of printed displays, radio-frequency identification tags, smart labels, and sensors based on organic electronics. Over the last 3 decades, significant progress has been made in realizing electronic devices with unprecedented features, such as wearable sensors, disposable electronics, and foldable displays, through the exploitation of desirable characteristics in organic electronics. Neverthless, the down-scalability of organic electronic devices remains a crucial consideration. To address this, efforts are extensively explored. It is of utmost importance to further develop these alternative patterning methods to overcome the downscaling challenge. This review comprehensively discusses the efforts and strategies aimed at overcoming the limitations of downscaling in organic semiconductors, with a particular focus on four main areas: 1) lithography-compatible organic semiconductors, 2) fine patterning of printing methods, 3) organic material deposition on pre-fabricated devices, and 4) vertical-channeled organic electronics. By discussing these areas, the full potential of organic semiconductors can be unlocked, and the field of flexible and sustainable electronics can be advanced.

Development of New Generation Electrodes for Registration of Heart Bioelectric Potentials

When conducting electrophysiological studies, electrodes are used to register bioelectrical signals, the correct choice and use of which determine the reliability and diagnostic significance of the data obtained. Recording an electrocardiogram is a standard procedure in medicine, but its monitoring is often limited to 24 hours. This is due to the limited performance of the electrodes. The properties of the skin/electrode interface determine the performance of medical equipment. Therefore, the surface conditions and the composition of the material from which the electrode is made should comply with the requirements of the electrocardiogram recording device. It is important to implement fast transmission of a useful signal with low losses and without artifacts. Modern electrodes using Ag/AgCl have a limited-service life, since their dehydration and surface degradation lead to the formation of various artifacts on the electrocardiogram record. Alternative – dry flexible electrodes. Such electrodes can be based on carbon materials (reduced graphene oxide or a diamond-like coating) on a plastic (film) substrate. The emphasis of modern research is aimed at carrying out work on the development of dry electrodes, which would provide an opportunity to carry out high-quality long-term registration of electrocardiosignals without gels and adhesives.

Dry Printing of Ag–Ni Conductive Particles Using Toner-Type Printed Electronics

Printed electronics are a set of additive manufacturing methods for creating future flexible electronics on thin polymeric sheets. We proposed the toner-type, dry, page-printing of Ag–Ni composite conductive particles on flexible plastic sheets without pre-treatment. No chemical solvents are necessary to compose the inks of the electronic materials used for the toner-type printing, and no chemical treatment is required for the plastic film substrate surface. In addition, multilayer printing is simple when using toner printing because previously printed materials do not need to be resolved; furthermore, composing the thick films of the electronic materials is relatively simple. In this study, we fabricated an Ag–Ni composite toner to improve the fluidity of the toner particles compared to bare Ag particles. We successfully printed IC peripheral circuits at a resolution of 0.20 mm and demonstrated that the actual electrical circuit pattern can be formed using our method.

Artificial neural network approach to predict the lightfastness of gravure prints on the plastic film

AbstractThe lightfastness of prints is an important property for assessing their print stability. The objective of this study is to determine the lightfastness rate of printed films due to long‐time exposure by applying artificial neural network (ANN). Package printing is gradually becoming extremely important because its color and quality increase the marketability of the product. Sometimes it has been observed that the initial print quality is bright and attractive. However, with time, it degrades the exposure of light, water, or other external parameters. Thus, it reduces the marketability if its color degrades before its expiration. Therefore, the lightfastness of prints may be considered for the authenticity or validity of the product. The plastic film substrate is chosen because it has extensive usage in food and other packaging industries. The samples printed in the gravure process are exposed in artificial lightfastness tester BGD 865/A Bench Xenon Test Chamber (B‐SUN) for assessing the lightfastness of prints. The ocean optics spectroradiometer (DH2000BAL) is used to measure the spectrophotometric properties of prints before and after exposure. The obtained reflectance spectra are modeled by applying an ANN technique that is proposed to predict the fading rate of the printed film. The optimal model gives excellent prediction with the minimum mean square error for each color and a correlation coefficient of 0.80 to 0.99. ANN model and a Regression model (assuming first‐order kinetic equation) are compared for predicting the lightfastness properties of prints. The results show that the ANN has better prediction capability than the regression model.

Flexible and printed organic transistors: From materials to integrated circuits

Abstract This article reports on the recent progress in the research and development of flexible and printed organic thin-film transistor (OTFT) devices, including organic materials, fabrication processes, electronic devices, and integrated circuits, and highlights their application to healthcare sensors. The fabrication process flow for printed OTFT devices is described with various printing methods such as inkjet printing, dispensing, reverse-offset printing, and spin-coating. Silver nanoparticle inks are commonly used to form interconnect and electrode layers with inkjet printing and reverse-offset printing. Highly crystalline small-molecule organic semiconductors are patterned on the printed source and drain electrodes. Various integrated circuits such as inverters, D flip-flops (D-FFs), and operational amplifiers (OPAs) are fabricated based on the printed OTFT devices, and their electrical performance is discussed in detail. The potential applications of printed integrated circuits to biosensors are successfully demonstrated on plastic film substrates, enabling various flexible and printed sensor systems for potential use in the Internet of things (IoT) society.

(Invited) Synthesis of Thermally and Chemically Stable Silver Nanowires for Transparent Electrode

Transparent conductive electrodes (TCEs) are important components of various optoelectronic devices and commonly used in solar cells, touch screens and other display technologies. The most widely used material for industrial applications of TCEs is indium tin oxide (ITO), which has a low sheet resistance of less than 100 Ω sq−1, and high optical transparency of ~90%. However, ITO-based TCEs are relatively expensive owing to the scarcity of the indium supply and the high sputtering processing costs. In addition, ITO films are brittle, which limits their application as flexible and stretchable components in future electronic devices. In order to address these concerns, various emerging materials such as graphene, carbon nanotubes (CNTs), conductive polymers, and metal NWs are being considered as alternatives to ITO. Among these candidate replacements, Group 11 metal NWs, especially silver NWs (Ag NWs) have garnered special interest owing to their excellent electrical and optical properties; and, solution-processability. Ag NWs are usually prepared and suspended in a solution and the resultant colloidal ink is used to coat plastic film or other substrates by spin-coating with Mayer bar or roll-to-roll processing to create transparent conducting sheets. Highly transparent and conductive Ag NW based thin films have been fabricated with a sheet resistance of <20 Ω sq−1 and a transmittance of ∼90%, which is similar to the performance of commercial ITO substrates. However, Ag NWs-based electrodes suffer from inherent limitations that restrict their applicability. One major issue is that Ag NWs are prone to oxidation, which limits their functional processability and lifetime. Coating the Ag NWs with a protective metal oxide (e.g. TiO2, SnO2, ZnO) has the potential to substantially improve the chemical, thermal and mechanical stabilities of Ag NWs. However, the preparation of a metal oxide protection coating usually requires depositing the metal oxide in a vacuum on a prepared film, which is not suitable for roll-to-roll continuous production of films with large areas. Fabrication of highly conductive and air-stable Ag NWs-based electrodes via an all-solution process for flexible and transparent applications remains a challenge. Here, we developed a nucleation mediated method using nanowire seeds for the selective synthesis of uniform Ag NWs via polyol syntheses. The seed-mediated polyol approach gives Ag NWs with high reproducibility (>90% in numerous repetitions of the synthesis), high processing efficiency (grams or more are possible for each synthesis run), and high aspect ratios (>2000), with average dimensions of L> 100 µm and D < 50nm.The Ag@TiO2 NWs subsequently created by surface modification of the Ag NWS have a substantially higher thermal stability than Ag NWs which have the same diameter, and can be annealed to 300 ˚C. Transparent conductive films can be prepared by simple vacuum filtration or by spin coating onto an appropriate filter paper and then by direct transfer to the desired glass or plastic substrate.Therefore, Ag@TiO2 nanowires are ideal candidates for transparent electrodes with good conductivity, high transmittance and low haze, and are demonstrated to be thermally and chemically stable for long-term applications.

(Invited) A New Category of Transparent Conductive Films: Printed Carbon Nanotube Hybrids

Transparent Conductive Films (TCFs) are used in a wide range of commercial applications, including information displays, capacitive touch sensors, solar photovoltaic modules, EMI shielding windows, transparent heaters, etc. There are many materials that are transparent and many materials that are electrically conductive, but there are very few materials that are both. Indium Tin Oxide (ITO) has dominated the TCF materials market for the past four decades, because of its excellent optoelectronic performance (high transparency, low sheet resistance, low haze, and excellent environmental stability). ITO is typically vacuum coated onto glass sheets or flexible plastic film substrates, with circuit patterns formed by laser ablation or chemical etching methods. The market for TCF materials is forecasted to exceed $5 billion by 2022, with more than half of the market being comprised of TCFs on flexible plastic films (vs. glass) by that date. It is important to note that ITO has well-known shortcomings on flexible plastic films. First, ITO is a brittle ceramic, so it can easily crack when flexed. This is a major limitation, as many product designs are moving towards thinner, flexible and 3D-shaped form factors. Second, ITO’s optoelectronic properties are much better on glass, due to the temperature limitations of plastic. This is a major limitation, as nearly all product designs work better with lower sheet resistance. Third, circuit patterning of ITO on flexible plastic films is expensive, due to handling damage (cracking of circuit features). This is a major limitation, as many product designs are moving towards thinner and larger area substrates, which are even more difficult to handle. Thus, there is an unmet market need for an ITO Alternative that addresses the shortcomings of ITO on flexible plastic films. Several TCF material categories have been established, but there has been limited commercial traction… One category is conductive polythiophene materials (PEDOT), which offer the advantages of low-cost circuit patterning (can be printed) and excellent flexibility and thermoformability. However, PEDOT is lacking in two key areas: 1) sheet resistance is higher than ITO on plastic films at the same transparency; 2) environmental stability is questionable, especially during UV and high temperature / humidity aging tests. These two shortcomings have made PEDOT a niche player. A second category is Carbon Nanotubes (CNTs), which offer all the advantages of PEDOT without compromising environmental stability. However, CNT TCFs have even higher sheet resistance than PEDOT at the same transparency. This shortcoming has made CNTs irrelevant for most TCF applications. A third category is Metal Mesh (MM), which are essentially printed (or etched) metal mesh patterns that have large enough open areas to have high transparency (like a window screen) and large enough cross-sectional area of metal to have low sheet resistance. MM TCFs are comprised of Silver &/or Copper. MM offers significantly lower sheet resistance than ITO on plastic films at the same transparency. MM also offers far superior flexibility than ITO. However, MM has is lacking in two key areas: 1) the metal lines comprising the MM are typically visible and this can be distracting – although this can be addressed with fine line printing (line widths ≤ 6µm), the MM cost is much higher; 2) creating TCF circuit patterns is expensive. A fourth category is Silver Nanowires (AgNWs), which offer all the advantages of MM without compromising aesthetics (MM visibility). However, AgNW is lacking in three key areas: 1) environmental stability is questionable, especially during UV and high temperature / high humidity aging tests; 2) robust electrical connections to AgNW are challenging due to the insulating polymer used to encapsulate AgNWs; 3) creating TCF circuit patterns is expensive. Despite their shortcomings, AgNW and MM appear to be gaining the most commercial traction, yet market penetration is still low. CHASM has recently created a new TCF category called Printed CNT Hybrids, which are made by combining CNTs with either AgNW or MM technologies to create a Hybrid solution that is superior to CNT, AgNW or MM technologies alone. CHASM is marketing its portfolio of Printed CNT Hybrids under the AgeNT™ brand. This presentation describes how CHASM combines its distinguished CNT synthesis capabilities (CoMoCAT™ technology) with proprietary ink formulation capabilities (V2V™ technology) to create AgeNT™ printed CNT Hybrid TCFs that are positioned to become a leading ITO Alternative. Figure 1

Acceleration test using combined stresses for flexible solar modules

Production technologies of large-area, light-weight flexible thin film silicon solar cells and modules using 1-m-wide and 3,000-m-long heat-resistant plastic film substrates were developed. A unique monolithic device structure having through-hole contacts formed on a flexible plastic film was also developed. The unique solar cell device structure enables us to cut and connect flexible film cells in order to easily customize the voltage and current specifications of products. Durable solar cell and module designs and effective acceleration tests were established. In particular, an acceleration test that combines damp heat and a current injection test was established. Approximately 3,000 h of damp heat and current injection corresponds to 20 years of outdoor exposure in Japan. In this paper, we propose a pressure cooker and current injection test to achieve a much faster acceleration test. 300 h of pressure cooker and current injection test is approximately equivalent to 3,000 h of damp heat and current injection test and 20 years of outdoor exposure in Japan.

(Invited) Wearable Printed Pressure Sensors with Ferroelectric Polymer for Healthcare Applications

<Introduction> Flexible printed organic electronics have various advantages over conventional inorganic electronics. These organic devices such as sensors and actuators can be fabricated on a plastic thin-film substrate with low-temperature printing processes[1,2]. Especially, physical sensors based on a ferroelectric polymer have a great potential in the development of wearable and disposable health-care devices such as pressure sensors. As the ferroelectric polymer for the sensors, poly(vinylidene difluoride trifluoroethylene) [P(VDF–TrFE)] (Piezotech, 62-010, VDF:TrFE molar ratio of 75:25) is the most promising material because of high ferroelectricity and piezoelectricity[3]). Moreover, the P(VDF–TrFE) is compatible with printing processes because of its solubility in several organic solvents[4,5]. <Results and Discussions> A representative printed physical sensor based on P(VDF–TrFE) was fabricated on a flexible plastic film substrate by screen printing method. PEDOT:PSS (Heraeus, Clevios SV 4 Stab) was used for the top and bottom electrodes. Thicknesses of the P(VDF–TrFE) layer and the electrodes were 2 µm and 1 µm, respectively. The P(VDF–TrFE) layer could generate the voltage of 50 mV by applied pressure of 50 kPa[5]. The generated voltage clearly depended on the thickness of P(VDF–TrFE) layer. Our printed sensor exhibited high sensitivity in the low pressure region, which is indicating the potential application to wearable sensors which monitoring health condition of human. In order to monitor the health condition of a volunteer, the sensor was attached to the skin near the neck using a skin-compatible adhesive patch. We have succeeded in the detection of the pulse rate with 55 pulse per minute. These results indicate that our P(VDF–TrFE) based sensor is significantly promising for wearable health-care applications. The detection of the pulse rate from volunteer was authorized by the Ethics Committee of Yamagata University (authorization code: 29-2). <Acknowledgement> This study was partially supported by the Japan Science and Technology Agency (JST). T. S. would like to thank all member of Research Center for Organic Electronics in Yamagata University, Piezotech and Arkema corporation. <References> [1] K. Fukuda et al., Sci. Rep., 3, 2048 (2013). [2] S. Conti et al., Adv. Mater. Technol., 2, 1600212 (2017). [3] D. Thuau et al., J. Mater. Chem. C, 5, 9963 (2017). [4] G. Knotts et al., Appl. Phys, Lett., 104, 233301 (2014). [5] T. Sekine et al., Sci. Rep., 8, 4442 (2018).

VUV-Assisted Photochemical Sol-Gel Processing of Metal Oxide Thin Films

Metal oxides such as TiO2, ZnO, SnO2 are useful in many of energy devices because of their high chemical stability, n-type semiconductivity and photoelectrochemical properties. Various methods of solution processing of their thin films are known, such as chemical bath deposition, electrodeposition and sol-gel reactions. Typically, high temperatures need to be applied for crystallization to achieve good properties. The need of high temperature however limits the choice of substrates to heat-resistant materials. Also, crystallization often results in pinholes and cracks to damage the performance of devices. We have developed an alternative sol-gel method to obtain ultra-smooth thin films of various metal oxides at room temperature, by employing photochemical decomposition of coated precursor by vacuum ultraviolet (VUV) light irradiation. The method is compatible with conductive plastic film substrates, so that multiple applications in thin film devices such as transparent conductive layer, light emitter, buffer layer in solar cells and thin-film encapsulation (TFE) as an example shown in Fig. 1 can be envisioned. Figure 1

Fully printed and flexible ferroelectric capacitors based on a ferroelectric polymer for pressure detection

We report on the fabrication and demonstration of fully printed ferroelectric capacitors using poly(vinylidene fluoridetrifluoroethylene) [P(VDF–TrFE)]. The printed ferroelectric capacitors were primarily fabricated by ink-jet printing on a thin plastic film substrate. The annealing process for the P(VDF–TrFE) layer was optimized from the viewpoints of surface morphology and crystallinity. A good ferroelectric polarization–electric field loop and piezoelectricity in the P(VDF–TrFE) were achieved for the printed ferroelectric capacitors. We have succeeded in the detection of a weak pressure of 150 mbar using the printed ferroelectric capacitor, which is an indication of a potential application to health-care biosensors. These results were realized by the optimization of the annealing temperature for the P(VDF–TrFE) layer.

Ultrathin flexible memory devices based on organic ferroelectric transistors

Here, we demonstrate ultrathin, flexible nonvolatile memory devices with excellent durability under compressive strain. Ferroelectric-gate field-effect transistors (FeFETs) employing organic semiconductor and polymer ferroelectric layers are fabricated on a 1-µm-thick plastic film substrate. The FeFETs are characterized by measuring their transfer characteristics, programming time, and data retention time. The data retention time is almost unchanged even when a 50% compressive strain is applied to the devices. To clarify the origin of the excellent durability of the devices against compressive strain, an intermediate plane is calculated. From the calculation result, the intermediate plane is placed close to the channel region of the FeFETs. The high flexibility of the ferroelectric polymer and ultrathin device structure contributes to achieving a bending radius of 0.8 µm without the degradation of memory characteristics.

X-Ray Detector-on-Plastic With High Sensitivity Using Low Cost, Solution-Processed Organic Photodiodes

We made and characterized an X-ray detector on a 25- $\mu \text{m}$ -thick plastic substrate that is capable of medical-grade performance. As an indirect conversion flat panel detector, it combined a standard scintillator with an organic photodetector (OPD) layer and oxide thin-film transistor backplane. Using solution-processed organic bulk heterojunction photodiode rather than the usual amorphous silicon, process temperature is reduced to be compatible with plastic film substrates, and a number of costly lithography steps are eliminated, opening the door to lower production costs. With dark currents as low as 1 pA/mm $^{2}$ and sensitivity of 0.2 A/W the OPD also meets functional requirements: the proof-of-concept detector delivers high-resolution, dynamic images at 10 frames/s, and 200 pixels/in using X-ray doses as low as $3~\mu $ Gy/frame.

Organic Phototransistors With All-Polymer Bulk Heterojunction Layers of p-Type and n-Type Sulfur-Containing Conjugated Polymers

All-polymer phototransistors were fabricated using both glass and flexible plastic film substrates by employing bulk heterojunction channel layers of p-type polymer (P3HT) and n-type polymer (THBT-ht). The devices could detect the entire visible light because the n-type polymer could sense photons in the deep red parts (>650 nm). The responsivity of devices was higher at the lower light intensity, while it could be controlled by varying the gate and/or drain voltages. Similar performances were measured for flexible all-polymer phototransistors with a bottom-source/drain and top-gate electrode configuration.

Numerical analysis of acoustic impedance microscope utilizing acoustic lens transducer to examine cultured cells

A new technique is proposed for non-contact quantitative cell observation using focused ultrasonic waves. This technique interprets acoustic reflection intensity into the characteristic acoustic impedance of the biological cell. The cells are cultured on a plastic film substrate. A focused acoustic beam is transmitted through the substrate to its interface with the cell. A two-dimensional (2-D) reflection intensity profile is obtained by scanning the focal point along the interface. A reference substance is observed under the same conditions. These two reflections are compared and interpreted into the characteristic acoustic impedance of the cell based on a calibration curve that was created prior to the observation. To create the calibration curve, a numerical analysis of the sound field is performed using Fourier Transforms and is verified using several saline solutions. Because the cells are suspended by two plastic films, no contamination is introduced during the observation. In a practical observation, a sapphire lens transducer with a center frequency of 300MHz was employed using ZnO thin film. The objects studied were co-cultured rat-derived glial (astrocyte) cells and glioma cells. The result was the clear observation of the internal structure of the cells. The acoustic impedance of the cells was spreading between 1.62 and 1.72MNs/m3. Cytoskeleton was indicated by high acoustic impedance. The introduction of cytochalasin-B led to a significant reduction in the acoustic impedance of the glioma cells; its effect on the glial cells was less significant. It is believed that this non-contact observation method will be useful for continuous cell inspections.

A novel OFET-based biosensor for the selective and sensitive detection of lactate levels.

Biosensors based on organic field effect transistors (OFETs) are one of the more promising device applications in organic electronics. However, OFET-based biosensors are still in their early stages of development compared to other electrochemical biosensors. This study is the first to report on an extended-gate type organic field effect transistor (OFET) for lactate detection in aqueous media. Here, the extended-gate electrode of the OFET was modified with layers of a lactate oxidase and a horseradish peroxidase osmium-redox polymer on a flexible plastic film substrate for an enzymatic redox reaction of lactate. The device exhibited both high selectivity and sensitivity. The limit of detection (LOD) and the limit of quantification (LOQ) were estimated to be 66 nM and 220 nM, respectively, which are the sufficient detection limit for practical sensor applications. The obtained results confirm that extended-gate type OFET devices are applicable to enzyme-based biosensors for detecting lactate levels.

An anion sensor based on an organic field effect transistor.

We propose an organic field effect transistor (OFET)-based sensor design as a new and innovative platform for anion detection. OFETs could be fabricated on low-cost plastic film substrates using printing technologies, suggesting that OFETs can potentially be applied to practical supramolecular anion sensor devices in the near future.

15.1: Invited Paper: Printed Organic TFT Array and Integrated Circuits

AbstractA fully‐printed organic thin‐film transistor (OTFT) array (30times30) and integrated circuits including logic and ring oscillator circuits based on pseudo‐CMOS inverter were fabricated on plastic film substrates by using printing methods. The printed OTFT array exhibited excellent, uniform performance with a high mobility of 2 cm2/Vs. The printed flip‐flop circuit operated well with a delay time of 3.5 ms. The printed three‐stage ring oscillator operated at 400 Hz and 40V. These fabrication techniques are applicable to flexible displays, RFID tags and biosensors.

Printed pressure sensor array sheets fabricated using poly(amino acid)-based piezoelectric elements

We report on the device characteristics of a sheetlike flexible pressure sensor array consisting of poly(amino acid)-based piezoelectric elements fabricated on a plastic-film substrate by a printing technique. Poly(amino acid)-based piezoelectric elements were fabricated using poly(amino acid) as the piezoelectric material, silver paste ink as the electrodes, and polyimide sheet as the substrate. When dropping the load of 200 g from a height of 8.5 cm to the piezoelectric element, +1.7 and −1.4 V were observed owing to the displacement of the dipole moment in the poly(amino acid) film. We confirmed that pressures from 0 to 10,000 N were successfully measured from the change in the capacitance of the piezoelectric element, which underwent mechanical strain caused by the pressure application. The flexible pressure sensor element exhibited no noticeable degradation after repeated pressure tests at 10,000 N. Moreover, the spliced large-area pressure sensor sheet successfully detected human-stepping pressure and monitored positional information.

Conducting polymer/in-situ generated platinum nanoparticle nanocomposite electrodes for low-cost dye-sensitized solar cells

Here we report new low-cost nanocomposite counter electrodes for dye-sensitized solar cells (DSSCs), which were prepared by in-situ generation of platinum (Pt) nanoparticles inside poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solutions at room temperature. The Pt nanoparticles (Ptnp) were evenly dispersed in the PEDOT:PSS films and their size was 20∼40nm. The PEDOT:PSS-Ptnp films were well coated on substrates, and their optical transmittance was slightly better than conventional thermally-reduced Pt electrode. The short circuit current density of DSSCs with the PEDOT:PSS-Ptnp films was almost comparable to that with the conventional Pt film. The thickness control experiment disclosed that the series resistance of the PEDOT:PSS-Ptnp films is mainly responsible for the low fill factor. We found that the Ptnp was enriched on the surface of the nanocomposite films when the spin-coating speed was high, which has been assigned as an additional reason for the improved device performance. Hence further improvement in device performance is expected by employing high conductivity PEDOT:PSS materials. Finally, we expect that the present method can contribute to the fabrication of low-cost DSSCs using flexible plastic film substrates at low temperatures.