Flexible Polyethylene Naphthalate Substrate Research Articles

Stable organic static random access memory from a roll-to-roll compatible vacuum evaporation process

An organic Static Random Access Memory (SRAM) based on p-type, six-transistor cells is demonstrated. The bottom-gate top-contact thin film transistors composing the memory were fabricated on flexible polyethylene naphthalate substrates. All metallization layers and the p-type semiconductor dinaphtho[2,3-b:2',3'-f] thieno[3,2-b]thiophene were deposited by thermal evaporation. The gate dielectric was deposited in a vacuum roll-to-roll environment at a web speed of 25 m/min by flash-evaporation and subsequent plasma polymerisation of tripropyleneglycol diacrylate (TPGDA). Buffering the TPGDA with a polystyrene layer yields hysteresis-free transistor characteristics with turn-on voltage close to zero. The static transfer characteristic of diode-connected load inverters were also hysteresis-free with maximum gain >2 and noise margin ∼2.5 V. When incorporated into SRAM cells the time-constant for writing data into individual SRAM cells was less than 0.4 ms. Little change occurred in the magnitude of the stored voltages, when the SRAM was powered continuously from a −40 V rail for over 27 h testifying to the electrical stability of the threshold voltage of the individual transistors. Unencapsulated SRAM cells measured two months after fabrication showed no significant degradation after storage in a clear plastic container in normal laboratory ambient.

Bi-stable switching behaviors of ITO/EVA:ZnO NPs/ITO transparent memory devices fabricated using a thermal roll lamination technique

This research reported the transparent non-volatile write-once-read-many-times (WORM) memory behaviors of memory devices based on zinc oxide nanoparticles (ZnO NPs) embedded in an insulating poly(ethylene-co-vinyl acetate) (EVA), sandwiched between two ITO-coated flexible polyethylene naphthalate (PEN) substrates. The memory devices were fabricated employing a thermal roll lamination technique with the laminated-structure of PEN/ITO/EVA:ZnO NPs/ITO/PEN. The average transmittance of the laminated memory device was over 70% for an optical visible range of 400–800 nm. The maximum ON/OFF current ratio for the memory device was about 103. In addition, a reliability study for continuous read operations in a long time memory device is presented. The conductance switching mechanisms of the laminated memory device were analyzed using theoretical models on the basis of the experimental data.

Energy Consumption Estimation of Organic Nonvolatile Memory Devices on a Flexible Plastic Substrate

The energy consumption during the operation of organic nonvolatile memory devices fabricated on a flexible polyethylene naphthalate (PEN) substrate is investigated. For bistable resistive memory devices, the applied external voltage and time are essential factors for switching the memory cell from the OFF to ON state because the amounts of voltage and time determine the applied energy needed to set the memory cell. Using the composite material polyimide (PI) and [6,6]‐phenyl‐C61 butyric acid methyl ester (PCBM) as the active layer of the bistable resistive memory devices on a flexible PEN substrate, nonvolatile unipolar switching behavior and good electrical reliability of PI:PCBM memory devices are observed, and the relationship between the applied energy and the switching characteristics for various applied voltages and times is characterized. The results of the performed experiments show that higher ON state currents are reached as greater set voltages or times are applied, and reliable switching behavior is observed at over ≈10−6 J of applied energy with at least 4 V of applied voltage and 10 ms of pulse time.

Paper No P24: Effect of Mechanical Strain on Charge‐Transfer VT‐Shift Compensation Circuits for Flexible AMOLED Displays

AbstractA 4‐thin‐film transistor (TFT) voltage‐programmed pixel circuit is fabricated using low‐temperature (170°C) hydrogenated amorphous silicon (a‐Si:H) TFTs process on flexible polyethylene naphthalate substrate. The electrical stability of the 4‐TFT circuit was characterized under mechanical strain and flat (unstrained) conditions. The compensated drive current degraded approximately 5% under electrical bias and tensile strain. This result was similar to pixel circuits fabricated at high temperature (300°C) on rigid glass substrates.

Gravure‐printed PEDOT:PSS on flexible PEN substrate as ITO‐free anode for polymer solar cells

In this work, highly conductive and transparent films based on poly(3,4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) doped with dimethyl sulfoxide (DMSO) were printed onto a flexible substrate, the polyethylene naphthalate (PEN), using the gravure printing method. Gravure‐printed DMSO‐PEDOT:PSS ink suitably modified with isopropanol was developed and used as anode in flexible solar cells replacing the commonly used anode, that is, Indium Tin Oxide (ITO). Several inks were prepared and characterized in terms of viscosity and surface energy and tested by gravure printing. The wettability of the PEN substrate was investigated through contact angle measurements. For comparison, the anodic conductive polymer was also spin coated on the PEN substrate using an ink suitable for this technique. The films produced with both the techniques, gravure printing and spin coating, each using a proper ink, were characterized in terms of structure and physical–chemical properties. The printed layers were employed as anodes into ITO free‐polymer solar cells based on a bulk heterojunction of Poly({4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl}{3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno [3,4‐b]thiophenediyl}) (PTB7) and [6,6]‐phenyl C70‐butyric acid methyl ester ([70]PCBM). The performances of the devices with printed and spin coated polymeric anode were compared. The cell with the spin‐coated anode reached a power conversion efficiency of 3% while the corresponding device with the printed anode a value around 2%. POLYM. COMPOS., 36:1104–1109, 2015. © 2015 Society of Plastics Engineers

Features of flexible transparent conducting films based on polyaniline-carbon nanotube composite

We propose a new concept in the formation of transparent conducting films based on single-walled carbon nanotubes (SWNTs) and polyaniline (PANI) on a flexible polyethylene naphthalate substrate. It is established that the resistance of SWNT-PANI composite films decreases to less than half as compared to pure nanotubes, while the transparency is retained. Mechanisms responsible for a change in the conductivity of composite molecular systems are discussed based on differences in the transport of charge carriers in nanotubes and the polymer with allowance for their interaction.

Bias-stress stability of low-voltage p-channel and n-channel organic thin-film transistors on flexible plastic substrates

The bias-stress stability of low-voltage organic p-channel and n-channel thin-film transistors (TFTs) based on five promising organic semiconductors and fabricated on flexible polyethylene naphthalate (PEN) substrates has been investigated. In particular, it has been studied to which extent the bias-stress-induced decay of the on-state drain current of the TFTs is affected by the choice of the semiconductor and by the gate-source and drain-source voltages applied during bias stress. It has been found that for at least some of the organic p-channel TFTs investigated in this study, the bias-stress stability is comparable to that of a-Si:H and metal-oxide TFTs, despite the fact that the organic TFTs were fabricated at significantly lower process temperatures, which is important in view of the fabrication of these devices on plastic substrates.

Selective deposition of graphene sheets on a flexible substrate by a nonuniform electric field

This study reports on the precise positioning of high-quality graphene sheets on a flexible polyethylene naphthalate substrate using a dielectrophoretic (DEP) force. Positive DEP assembly is carried out using a 100 kHz frequency AC signal with a peak-to-peak voltage of 10 V. The presence and quality of the graphene flakes aligned around the prepatterned electrodes are characterized by scanning electron microscopy, optical microscopy, and micro-Raman spectroscopy. Current–voltage measurements were also used to evaluate the electrical properties of these flexible devices under varying compressive (−) and tensile (+) strain conditions up to ±0.6%, where the currents decreased with increasing strains.

Organic Electronics: Printed, Flexible, Organic Nano‐Floating‐Gate Memory: Effects of Metal Nanoparticles and Blocking Dielectrics on Memory Characteristics (Adv. Funct. Mater. 28/2013)

High-performance, top-gated, organic, nanofloating gate memory (NFGM) devices fabricated using spatially well-distributed metallic nanoparticles as a floating gate and dielectrics with long charge relaxation properties are reported by Kang-Jun Baeg, Yong-Young Noh, Dong-Yu Kim, and co-workers on page 3503. The organic NFGM devices show wide memory windows, a high on/off current ratio, and quasi-permanent retention characteristics. The 256-bit NFGM array is also fabricated successfully on a flexible polyethylene naphthalate (PEN) substrate.

Recent Developments in Functional Thin Films

Thin film technology is a significant focus of academic, governmental, and industrial research efforts. The economic impact of thin film use is significant and is growing at an impressive rate. BCC Research assigned the thin film global market a value of $9.3 billion in 2011; it is anticipated to reach a value of $14.9 billion by 2016. 1 Drivers of thin film research at this time include lowering the processing cost and environmental burden associated with thin film processing. 2 In addition, efforts are underway to develop thin films on flexible substrates for use in medical prostheses and in energy harvesting devices. Functional thin films have other potential biomedical applications, including use in drug delivery. 3 A search using Thomson Reuters’s Web of Knowledge indicates that the number of published items involving functional thin films has steadily increased each year since 1994; more than 1150 papers were published on this topic and nearly 30,000 citations involved this topic in 2012. 4 In this issue of JOM, the development of thin films for biomedical, corrosion resistance, electronic, optical, radiation detection, thermal barrier, and tribological applications is described. In ‘‘Improved Mobility and Transmittance of Room-Temperature Deposited a-IGZO Films with Low-Temperature Post-Fabrication Anneals,’’ T.L. Alford et al. demonstrate radio frequency (RF) sputtering of amorphous indium gallium zinc oxide onto flexible poly ethylene naphthalate substrates. Annealing the film at low temperatures in an oxygen atmosphere and in vacuum is shown to modify the optical and electrical properties of the thin films. In ‘‘Inkjet Printing of Amphotericin B onto Biodegradable Microneedles using Piezoelectric Inkjet Printing,’’ R.D. Boehm et al. demonstrate modification of the surfaces of Gantrez (ISP Investments, Inc., Wilmington, DE) 169 BF microneedles with amphotericin B using piezoelectric inkjet printing. A radial diffusion assay is used to demonstrate the activity of these drug-loaded microneedles against the yeast Candida parapsilosis. In ‘‘Effect of LowTemperature Microwave Processing and Copper Content on the Properties of Ag-Cu Thin Film Alloys,’’ S. Das and T.L. Alford examine the use of microwave annealing for enhancing the structural and electrical properties of silver-copper thin films. Their work indicates that copper oxide encapsulation layers are formed by microwave annealing. In ‘‘Reflectance Spectroscopy of Functional Ag-Cu Thin Films: Correlation of Reflectivity with Cu Content,’’ S. Das and T.L. Alford evaluate the use of magnetron sputtering to co-sputter pure silver and copper targets. The electrical mobility and optical reflectivity properties of silver-copper thin films on silicon substrates are evaluated. In ‘‘Thermo-mechanical

Controllable vertically aligned ZnO nanorods on flexible polyethylene naphthalate (PEN) substrate using chemical bath deposition synthesis

Zinc oxide (ZnO) nanorods were successfully grown on polyethylene naphthalate substrates with a seed layer using a wet chemical bath deposition method at a low temperature. Using various precursor concentrations, the diameter, length, and density of the ZnO nanorods were controlled, and their optical and crystallinity properties were investigated. X-ray diffraction and field emission scanning electron microscopy were used to examine the structure and morphology of the ZnO nanorods. The obtained ZnO nanorods were hexagonal and grew vertically from the substrate in the (002) direction along the c-axis. The low compressive strain values confirmed the high-quality crystal structure of the synthesized ZnO nanorods. A 0.050 M precursor concentration resulted in nanorods with a uniform diameter along their entire length and diameters ranging from 10 nm to 40 nm. The photoluminescence results indicated that the ZnO nanorods grown using a 0.050 M precursor concentration exhibited the sharpest and most intense PL peaks in the UV range compared with the other samples. Therefore, the precursor concentration considerably influenced the growth of the ZnO nanorods. These ZnO nanorods can be greatly applied for the development of flexible, elastic electronic, and optoelectronic devices.

Flexible organic light-emitting diodes on a polyestersulfone (PES) substrate using Al-doped ZnO anode grown by dual-plasma-enhanced metalorganic deposition system

This study proposes flexible organic light-emitting diodes (OLEDs) grown on polyestersulfone (PES) using Al-doped zinc oxide (AZO) as the anode, fabricated by the dual-plasma-enhanced chemical vapor deposition (DPEMOCVD) system. The experimental results including crystalline structure, optical, and electrical characteristics indicate that the quality of AZO films grown on PES depends on the deposition temperature and Al content. The optimal deposition temperature and Al content for AZO film are 185 C and 2.88at%, respectively. Further increasing or decreasing the deposition temperature and Al content degrades the quality of AZO films. The optimal AZO film deposited on the PES substrate was used as the anode for flexible OLED. It shows a similar performance compared to OLEDs using commercial indium–tin-oxide (ITO) as the anode on glass, and represents enhanced characteristics to that of the commercial ITO anode on a flexible polyethylene naphthalate (PEN) substrate. This indicates that the DPEMOCVD-deposited AZO film on the PES substrate can be the anode for flexible OLEDs.

Widely Applicable Coinage Metal Window Electrodes on Flexible Polyester Substrates Applied to Organic Photovoltaics

The fabrication, exceptional properties, and application of 8 nm thick Cu, Ag, Au, and Cu/Ag bilayer electrodes on flexible polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) substrates is reported. These electrodes are fabricated using a solvent free process in which the plastic surface is chemically modified with a molecular monolayer of thiol and amine terminated alkylsilanes prior to metal deposition. The resulting electrodes have a sheet resistance of ≤14 Ω sq⁻¹, are exceptionally robust and can be rapidly thermally annealed at 200 °C to reduce their sheet resistance to ≤9 Ω sq⁻¹. Notably, annealing Au electrodes briefly at 200 °C causes the surface to revert almost entirely to the {111} face, rendering it ideal as a model electrode for fundamental science and practical application alike. The power conversion efficiency of 1 cm² organic photovoltaics (OPVs) employing 8 nm Ag and Au films as the hole-extracting window electrode exhibit performance comparable to those on indium-tin oxide, with the advantage that they are resistant to repeated bending through a small radius of curvature and are chemically well-defined. OPVs employing Cu and bilayer Cu:Ag electrodes exhibit inferior performance due to a lower open-circuit voltage and fill factor. Measurements of the interfacial energetics made using the Kelvin probe technique provide insight into the physical reason for this difference. The results show how coinage metal electrodes offer a viable alternative to ITO on flexible substrates for OPVs and highlight the challenges associated with the use of Cu as an electrode material in this context.

Study of optical and electrical properties of ZnO/Cu/ZnO multilayers deposited on flexible substrate

Transparent conducting oxide thin films have been extensively studied because they are essential elements in optoelectronic applications. In this communication, we report transparent conductive ZnO/Cu/ZnO multilayer films prepared by RF sputtering of ZnO and DC sputtering of Cu on flexible polyethylene naphthalate substrates. The optical and electrical properties of the multilayers were studied for different thicknesses of Cu and ZnO layers. Controlling the thickness of the ZnO layer for a specific Cu thickness affects the transmittance of the multilayer. Electrical properties are influenced by the thickness of ZnO and Cu layers. Sheet resistance of the stack rises with the decrease of Cu thickness and the increase of ZnO thickness. Repeated bending of the stack, after an initial 20% increase of sheet resistance, has no significant effect on the electrical performance of the multilayers.

Design and implementation of electrostatic micro-actuators in ultrasonic frequency on a flexible substrate, PEN (polyethylene naphthalate)

We present an electrostatic actuator fabricated on a flexible polyethylene naphthalate (PEN) substrate, which emits acoustic waves at ultrasonic frequencies. The MEMS actuator has a suspended parylene diaphragm which consists of 2–6mm diameter and 6μm gap between the diaphragm and substrate. Driving circuitry consists of voltage controlled oscillator (VCO) and output buffer chain, and was fabricated by Arizona State University's Flexible Display Center-IC process. The fabricated actuator emits ultrasonic waves at 25kHz, and acoustic sound pressure of 27dB SPL (sound pressure level) driven by the hydrogenated amorphous silicon (a-Si:H) circuitry.

Effect of In-Ga-Zn-O active layer channel composition on process temperature for flexible oxide thin-film transistors

In-Ga-Zn-O (IGZO)-channel oxide thin-film transistors (TFTs) were fabricated on flexible polyethylene naphthalate (PEN) substrates. A lamination and delamination procedure was established that allowed easy handling of the PEN substrate during fabrication. In order to fabricate high-performance flexible IGZO TFTs at lower than normal process temperatures, a 2:1:2 (In:Ga:Zn) IGZO channel composition was proposed. The field-effect mobility, threshold voltage, and subthreshold swing of the fabricated IGZO TFTs were found to be approximately 7.83 cm2 V−1 s−1, 1.93 V, and 0.24 V/decade, respectively, even when a final heat treatment was conducted at a temperature as low as 150 °C. The stability characteristics of the devices were also examined under gate bias stress and constant current stress conditions.

Low temperature fabrication of high performance and transparent Pt counter electrodes for use in flexible dye-sensitized solar cells

High performance Pt counter electrode is prepared by using vacuum thermal decomposition at a relatively low (120°C) temperature on a flexible polyethylene naphthalate substrate coated with indium-doped tin oxide for use in flexible dye-sensitized solar cells. The obtained Pt counter electrode shows a good chemical stability, high light transmittance, and high electrocatalytic activity for the I −3 /I− redox reaction. The energy conversion efficiency of a flexible dye-sensitized solar cell based on the prepared Pt counter electrode and a TiO2/Ti photoanode reaches 5.14% under a simulated solar light irradiation with intensity of 100 mW cm−2.

Electrospun conductive polyaniline–polylactic acid composite nanofibers as counter electrodes for rigid and flexible dye-sensitized solar cells

Conductive polyaniline doped with 10-camphorsulfonic acid (PANI·CSA) blended with polylactic acid (PLA) composite films are directly deposited on rigid fluorine-doped tin oxide and flexible indium tin oxide-coated polyethylene naphthalate substrates by using a simple electrospinning method. The scanning electron microscope and atomic force microscope images show that the PANI·CSA–PLA film is assembled from about 200 nm-diameter nanofibers, with a generally uniform thickness of about 2 μm. The three-dimensional porous fibrous PANI·CSA–PLA films indicate good electrocatalytic performance for I3−/I− electrolyte, demonstrated by electrochemical impedance spectra and cyclic voltammetry measurements. The photoelectric conversion efficiency of the dye-sensitized solar cells (DSCs) firstly based on such rigid and flexible PANI·CSA–PLA counter electrodes achieves 5.3% and 3.1% under 1 sun illumination of 100 mW cm−2 (AM 1.5), respectively, which is close to that of sputtered Pt-based DSCs. In addition, the rigid PANI·CSA–PLA-based DSC demonstrates good stability. As the PANI·CSA–PLA counter electrodes can be obtained in a short time at room temperature, they are potentially applicable in large-area DSCs and suitable for flexible DSCs.

High-Gain Fully Printed Organic Complementary Circuits on Flexible Plastic Foils

We present several fully printed organic complementary circuits using n- and p-type organic thin-film transistors. n-Type and p-type devices are developed on a flexible polyethylene-naphthalate substrate. All organic layers are deposited using a low-cost screen-printing technique. The inverters show a high gain and a switching point at exactly VDD/2. A seven-stage voltage-controlled oscillator (VCO) is designed with an organic output buffer, using the n- and p-type organic transistors. This VCO oscillates at a frequency of 186 Hz. Finally, two complementary differential amplifiers with high gain and large bandwidth are presented. The amplifiers only draw a 1-μA current from a 40-V power supply.

CF4 plasma treatment-assisted inkjet printing for color pixel flexible display

In this paper we report a MEMS flexible display device based on the color filter Fabry–Perot interferometer and fabricated on a transparent and flexible polyethylene naphthalate substrate. Targeting easy processing, fast evolution and reduced fabrication steps, inkjet printing is selected as a promising technology. CF4 plasma surface treatment parameters' influence has been investigated through a design of experiment protocol. Important contact angle increase has led to pattern resolution between 50 and 100 µm depending on solutions and substrate nature. Finally, the designed device presents three primary color pixels with satisfying color purity (CIE 1931 chromaticity diagram—red: x = 0.52, y = 0.36; blue: x = 0.13, y = 0.20; green: x = 0.25, y = 0.57).