Low-cost Flexible Substrates Research Articles

Low temperature plasma sintering of silver nanoparticles

The fabrication of flexible electronics using the deposition of solution-processed nanomaterials generally requires low-temperature post-processing to optimize functionality. We studied sintering of silver nanoparticle (AgNP) films on glass substrates by applying argon (Ar) plasma to achieve improved electrical conductivity. This process meets the low temperature processing requirements for standard low-cost polymeric flexible substrates. The relationship between plasma parameters (such as power and sintering time) versus sintering results (such as electrical sheet resistance, sintered structure depth, materials composition variation, and film nanostructure) is reported for 23 and 77nm diameter AgNPs. In addition, plasma processing typically induces a small surface thermal effect. We monitored the surface temperatures of the AgNP films in-situ during plasma sintering. By sintering control groups at these monitored surface temperatures using a vacuum oven, we confirmed that the resistivity due to plasma sintering is less than that produced by thermal sintering. Our data show that, the measured lowest resistivities for plasma sintered AgNP films are about only 5 and 12 times greater than the bulk Ag resistivity for 23 and 77nm, respectively.

High voltage and efficient bilayer heterojunction solar cells based on an organic–inorganic hybrid perovskite absorber with a low-cost flexible substrate

A low temperature (<100 °C), flexible solar cell based on an organic-inorganic hybrid CH3NH3PbI3 perovskite-fullerene planar heterojunction (PHJ) is successfully demonstrated. In this manuscript, we study the effects of energy level offset between a solar absorber (organic-inorganic hybrid CH3NH3PbI3 perovskite) and the selective contact materials on the photovoltaic behaviors of the planar organometallic perovskite-fullerene heterojunction solar cells. We find that the difference between the highest occupied molecular orbital (HOMO) level of CH3NH3PbI3 perovskite and the Fermi level of indium-tin-oxide (ITO) dominates the voltage output of the device. ITO films on glass or on the polyethylene terephthalate (PET) flexible substrate with different work functions are investigated to illustrate this phenomenon. The higher work function of the PET/ITO substrate decreases the energy loss of hole transfer from the HOMO of perovskite to ITO and minimizes the energy redundancy of the photovoltage output. The devices using the high work function ITO substrate as contact material show significant open-circuit voltage enhancement (920 mV), with the power conversion efficiency of 4.54%, and these types of extra-thin planar bilayer heterojunction solar cells have the potential advantages of low-cost and lightweight.

Sn-induced low-temperature (~ 150 °C) crystallization of Ge on insulator

Low-temperature formation (~150°C) of Ge films on insulator is investigated for realization of advanced flexible devices. We propose utilization of Sn as catalyst to enhance the crystallization at low-temperatures. By annealing (150–200°C) of a-Ge/Sn stacked structures formed on insulators, the composition distributions of Ge/Sn layers are inverted, and Sn/poly-Ge stacked structures are obtained. The results demonstrate that the crystallization occurs at 150°C, which is slightly below the eutectic temperatures. This Sn-induced crystallization technique is useful to obtain poly-Ge on low-cost flexible substrates (softening temperature: ~200°C).

Highly conductive and transparent aluminum-doped zinc oxide thin films deposited on polyethylene terephthalate substrates by pulsed laser deposition

Highly transparent and conductive aluminum-doped zinc oxide thin films were deposited on low-cost flexible polyethylene terephthalate substrates at room temperature using pulsed laser deposition and the effects of oxygen pressure and film thickness on film properties were investigated. It was found that grain sizes play a greater role only at smaller film thicknesses in affecting carrier mobility. Resistivity changes at larger film thickness can be caused by near surface/interface depletion that affected both mobility and carrier concentration. The inherent film transparency did not change and any reduction in the film transmittance is likely related to a thickness dependent attenuation effect. This means that different transparent conducting oxides should each possess an optimum film thickness, whereby optimized zinc oxide is typically about 100nm. A low resistivity of ~6.6×10−4Ωcm with a high normalized transparency index of >0.9 for a 110±10nm thick room-temperature deposited film was obtained, representing one of the best results obtained to date.

Towards low-cost flexible substrates for nanoplasmonic sensing

Plasmonic nanostructures have played a significant role in the field of nanotechnology due to their unprecedented ability to concentrate light at the nanometre scale, which renders them precious for various sensing applications. The adsorption of plasmonic nanoparticles and nanostructures onto solid substrates in a controlled manner is a crucial process for the fabrication of nanoplasmonic devices, in which the nanoparticles amplify the electromagnetic fields for enhanced device performance. In this perspective article we summarize recent developments in the fabrication of flexible nanoplasmonic devices for sensing applications based on surface enhanced Raman scattering (SERS) and localized surface plasmon resonance (LSPR) shifts. We introduce different types of flexible substrates such as filter paper, free-standing nanofibres, elastomers, plastics, carbon nanotubes and graphene, for the fabrication of low-cost flexible nanoplasmonic devices. Various techniques are described that allow impregnation of such flexible substrates with plasmonic nanoparticles, including solution processes, physical vapour deposition and lithographic techniques. From the discussion in this Perspective, it is clear that highly sensitive and reproducible flexible plasmonic devices can currently be fabricated on a large scale at relatively low-cost, toward real-world applications in diagnostics and detection.

An electronic nose on flexible substrates integrated into a smart textile

Fabrication costs of gas sensors are decreasing due to the use of low cost flexible polymer substrates and solution based fabrication processes. This development creates new application possibilities for gas sensors and allows the installation into objects of our daily life, such as textiles. Challenges with the integration of electronic components on flexible substrates arise mainly due to strain introduced by bending the substrate. This plays an important role especially during weaving as strong mechanical forces bend flexible electronic devices to radii below 1 mm. To demonstrate that it is feasible to integrate low cost gas sensors into a textile while preserving the functionality, we developed an electronic nose system for textile integration. The electronic nose consists of four carbon black/polymer gas sensors fabricated on a flexible polymer substrate. After fabrication the substrate was cut into yarn like strips and integrated into a textile to create a smart textile with gas sensing functionalities. We used this smart textile to detect different solvent vapors and classified them accordingly. It was possible to detect single solvents such as acetone until a minimal concentration of 50 ppm. The influence of bending on sensors was studied, signal changes introduced by bending could be separated from changes due to solvent exposure. Bending of single strips to a radius below 1 mm did not alter the sensor functionality. The presented work shows the feasibility of the process in combination with textile integration.

Low Temperature Plasma Sintering of Silver Nanoparticles for Potential Flexible Electronics Applications

ABSTRACTDeposition of solution-processed functional materials generally requires additional post-processing to optimize the functionality of the material. We study sintering of Ag nanoparticle (NP) (with average diameter 77nm) deposits for improved electrical conductivity, with emphasis on Argon plasma methods compatible with the low temperature requirements of regular low-cost flexible polymer substrates. The relationship between plasma parameters (such as power and treatment time) versus sintering results (sintered structure depth, film continuity and electrical sheet resistance) will be reported. According to our efforts so far, we have achieved the electrical resistivity of the sintered film at about 20 times greater than the value of bulk silver using a process compatible with the low temperature requirements of common flexible polymer substrates.

Screen printed RFID antennas on low cost flexible substrates

Recently, more and more studies are carried out in the field of printed RFID tags. It is connected with rapid development of new electronic technology, i.e. printed electronics which utilizes printing techniques, like screen printing, inkjet, flexography or gravure, for production of electronic components. This method is on one hand environmentally friendly because it allows eliminating wastes emerging during etching process used commonly in electronics. On the other hand, components can be printed on low cost flexible substrates, like foil or paper. These two factors cause that such products are cheap and can be competitive with their equivalents used currently. In this study, investigations of RFID tag antennas working in UHF frequency range made with screen printing technique are described. Conductive polymer pastes containing silver nanopowder, silver flakes or carbon nanotubes were used for antenna fabrication. Each of them was deposited on foil and paper. Properties of printed antennas were investigated by return loss measurements performed in the frequency range 0.5 ÷ 1.5 GHz. Achieved results were compared with simulation carried out in CST Microwave Studio. Antenna surface profile was checked using optical profilometer or metallographic microscope. Its mechanical tests were also conducted. The obtained results showed that the best candidate for antenna printing on flexible substrate was the paste with silver nanopowder because it combined high conductivity and high mechanical durability.

Flexible microwave PIN diodes and switches employing transferrable single-crystal Si nanomembranes on plastic substrates

This paper reports the realization of flexible RF/microwave PIN diodes and switches using transferrable single-crystal Si nanomembranes (SiNM) that are monolithically integrated on low-cost, flexible plastic substrates. High frequency response is obtained through the realization of low parasitic resistance achieved with heavy ion implantation before nanomembrane release and transfer. The flexible lateral SiNM PIN diodes exhibit typical rectifying characteristics with insertion loss and isolation better than 0.9 dB and 19.6 dB, respectively, from DC to 5 GHz, as well as power handling up to 22.5 dBm without gain compression. A single-pole single-throw (SPST) flexible RF switch employing shunt-series PIN diode configuration has achieved insertion loss and isolation better than 0.6 dB and 22.9 dB, respectively, from DC to 5 GHz. Furthermore, the SPST microwave switch shows performance improvement and robustness under mechanical deformation conditions. The study demonstrates the considerable potential of using properly processed transferrable SiNM for microwave passive components. Future investigations on transferrable SiNMs will lead to eventual realization of monolithic microwave integrated systems on low-cost flexible substrates.

Design and Development of Radio Frequency Identification (RFID) and RFID-Enabled Sensors on Flexible Low Cost Substrates

This book presents a step-by-step discussion of the design and development of radio frequency identification (RFID) and RFID-enabled sensors on flexible low cost substrates for UHF frequency bands. Various examples of fully function building blocks (design and fabrication of antennas, integration with ICs and microcontrollers, power sources, as well as inkjet-printing techniques) demonstrate the revolutionary effect of this approach in low cost RFID and RFID-enabled sensors fields. This approach could be easily extended to other microwave and wireless applications as well. The first chapter describes the basic functionality and the physical and IT-related principles underlying RFID and sensors technology. Chapter two explains in detail inkjet-printing technology providing the characterization of the conductive ink, which consists of nano-silver-particles, while highlighting the importance of this technology as a fast and simple fabrication technique especially on flexible organic substrates such as Liquid Crystal Polymer (LCP) or paper-based substrates. Chapter three demonstrates several compact inkjet-printed UHF RFID antennas using antenna matching techniques to match IC's complex impedance as prototypes to provide the proof of concept of this technology. Chapter four discusses the benefits of using conformal magnetic material as a substrate for miniaturized high-frequency circuit applications. In addition, in Chapter five, the authors also touch up the state-of-the-art area of fully-integrated wireless sensor modules on organic substrates and show the first ever 2D sensor integration with an RFID tag module on paper, as well as the possibility of 3D multilayer paper-based RF/microwave structures.

Plastics' key role in energy-efficient building

Plastics-based solutions are widespread in the building industry, as the benefits they brought are multiple: insulation, protection from wind and weather, easy refurbishment, attractive new design or efficient energy management. Plastic pipes for heating and water have proved to be superior to conventional piping systems (more flexible, rust-free, anti-scale deposits, etc.) for a long time. Window frame profiles made of polyvinyl chloride (PVC), one of the most widely used plastic in building industry, provide better thermal insulation, making them suitable for use in passive houses as the energy consumption is reduced dramatically. Other polymer systems may also improve energy management for facades, walls and roofs. Composites or sandwich elements made of expanded polystyrene (EPS) filled with graphite particles or polyurethane (PU) rigid foams are used on walls or on roofs, as well as below the ceiling to provide climatic regulation. The surface of front doors, window frames and facades can be coated with UV-resistant acrylonitrile styrene acrylate (ASA) films so as to guarantee long-term protection against weathering. Roof glazing with thermal protection made of polymethyl methacrylate (PMMA) or polycarbonate (PC) multi-wall sheets enables interior spaces to stay pleasantly cool without having to dispense with daylight, and reduces the cost for heating and cooling. Moreover, as the solar heating market is steadily growing, the very promising field of large area, light-weight organic photovoltaic panels has emerged for plastics. Polyvinyl fluoride (PVF) films have been used successfully for the backsheet of photovoltaic modules thanks to their weather and UV-resistance, and high barrier properties against moisture. Light-resistant thermoplastic polyurethanes (TPU) are expected to permit to switch from batch-manufacturing to continuous production, lowering expenses accordingly. Furthermore, new efficient manufacturing methods for large-surface modules on flexible low-cost substrates are under development. Finally, intensive research efforts are currently going on into semiconducting organic materials with high thermal and photochemical resistance, which are intended to replace today's silicon, whereby they absorb sunlight and convert it into electric power. At last, organic Phase Change Materials (PCM) such as HDPE or paraffin compounds provide a new horizon for passive thermal energy storage in bioclimatic architecture, and will contribute to make buildings even more energy-efficient in the future.

An Assessment of µ-Czochralski, Single-Grain Silicon Thin-Film Transistor Technology for Large-Area, Sensor and 3-D Electronic Integration

Single-grain (SG) thin-film transistors (TFTs) fabricated inside location-controlled silicon grains using the mu-Czochralski method are benchmarked for analog and RF applications. Each silicon grain is defined by excimer laser recrystallization of polysilicon. Thin-film transistors may be fabricated in this manner on silicon or low-cost flexible plastic substrates as processing temperatures remain below 350degC, making the SG-TFT a potential enabling technology for large-area highly integrated electronic systems or systems-in-package with low manufacturing cost. Operational amplifier and voltage reference circuits of varying complexity were designed and measured in order to evaluate the effects of channel position and processing variation on analog circuits. A two-stage telescopic cascode operational amplifier fabricated in an experimental 1.5 mum SG-TFT technology demonstrates a DC gain of 55 dB (unity-gain bandwidth of 6.3 MHz), while a prototype CMOS voltage reference with a power supply rejection ratio (PSRR) of 50 dB is also demonstrated. With fT comparable to single-crystal MOSFETs of comparable gate length, the SG-TFT can also enable RF circuits for wireless applications. A 12 dB gain RF cascode amplifier with on-chip inductors and operating in the 433 MHz ISM band is demonstrated. Excellent agreement with simulations is attained using a modified BSIM-SOI model extracted from measurements of experimental SG-TFT devices.

The effect of bonding force on the electrical performance and reliability of NCA joints processed at a lowered temperature

The effect of the bonding force during flip chip-on-flex (FCOF) assembly on the electrical performance of the nonconductive adhesive (NCA) interconnects was investigated in this study, under the precondition of a reduced processing temperature in order to minimize thermally-induced damage to the low-cost flexible substrates. Pressure cooker tests (PCT) were performed to assess the reliability performance of the adhesive joints in high temperature and high humidity conditions. The assembly process was modified and the processing temperature and the bonding force were adjusted according to the experimental results to enable the use of low cost substrates, such as poly(ethylene terephthalate) (PET) materials in smart card fabrication.

Excimer laser annealing of silicon nanowires

Nanowires can potentially be used with low-cost flexible plastic substrates for applications such as large-area displays and sensor arrays. However, high temperature processing steps such as thermal annealing that are incompatible with plastic substrates are still a major hindrance. Laser annealing permits localized energy input without affecting the underlying substrate and can help overcome this problem. In this study, the excimer laser annealing of silicon nanowires is demonstrated to be an efficient means of activating implanted dopants. The optical absorption of the nanowires is discussed and the effect of parameters such as fluence and number of pulses is investigated.

High‐Performance Organic Single‐Crystal Transistors on Flexible Substrates

The electronic properties of organic single crystals have been intensely studied for well over 40 years. Until recently, organic single-crystal field-effect transistors have generated results that are comparable to and sometimes better in performance than hydrogenated amorphous silicon. Organic thin-film transistors are being actively pursued for a broad area of electronic applications, but their charge-carrier mobilities are limited by structural imperfections (i.e., grain boundaries) and impurities. Organic single crystals, on the other hand, have been limited to charge-transport studies mainly because the fabrication of single-crystal transistors poses a technological challenge. Novel methods for fabricating single-crystal devices include the flip-crystal technique, elastomeric stamp platforms, and freestanding devices, where the source–drain electrodes, dielectric, and gate are all fabricated onto the crystal surface. For the most part, a relatively thick and rigid single crystal is employed (5–500 lm thick). Because the fragility makes them difficult to handle, their use has been restricted to simple and basic devices and wide-ranging applications in sensors or plastic transistors for flexible electronics have not yet been possible. Thus, there is a strong need for the development of mechanically flexible, nondestructive, single-crystal devices with prospective applications in organic electronics while maintaining the intrinsic properties and characteristics of organic single crystals. We demonstrate field-effect transistors fabricated from thin and conformable organic single crystals. We report on proofof-concept “flexible” organic single-crystal field-effect transistors with performance exceeding those of previously reported organic thin-film flexible devices. Rubrene single-crystal devices constructed on low-cost flexible substrates (Fig. 1b) yielded mobilities as high as 4.6 cm V s and on/off ratios of approximately 10.