14‐3: High‐Performance Organic‐TFT Circuits Fabricated by All‐Printing Technology on Flexible Plastic Substrates

A self-assembled benzimidazole-copper complex (SABCC) is utilized as the gate insulating material for pentacene organic thin film transistors (OTFTs) fabricated on a flexible polyethylene terephthalate plastic substrate. The fabrication features of SABCC are self-assembling, water-based processing, and selectivity of deposition. The pentacene OTFTs exhibit reasonable device characteristics. Threshold voltage, carrier mobility, on/off current ratio, and subthreshold swing are determined to be ∼−4.2 V, ∼0.08 cm2 V−1 s−1, ∼4×103, and ∼4.4 V/decade, respectively. The water-based fabrication process of the SABCC gate insulator on copper electrodes may provide an opportunity for low cost OTFTs on flexible plastic substrates.

This paper presents a printed organic complementary technology on flexible plastic substrate with high performance N and P-type Organic Thin Film Transistors (OTFTs), based on small-molecule organic semiconductors in solution. Challenges related to the integration of both OTFT types in a common complementary flow are addressed, showing the importance of surface treatments. Data on single devices and elementary complementary digital circuits (inverters and ring oscillators) are presented, demonstrating that a robust and reliable flow with high electrical performances can be established for printed organic devices.

Organic thin-film transistors with record static and dynamic performance fabricated on flexible plastic substrates are presented. The TFTs operate with voltages of 3 V, have channel lengths as small as 0.6 μm and show on/off current ratios as large as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> , subthreshold slopes as steep as 59 mV/decade (at a temperature of 292 K), and signal delays measured in ring oscillators as low as 79 ns per stage. All these represent record figures for organic TFTs, enabled by small lateral device dimensions, a very small gate-dielectric thickness, and a small contact resistance of 29 Ωcm, which is also a record for organic field-effect transistors.

We explore for the first time the direct growth of high quality InAs nanowires (NWs) on flexible plastic substrates by MOVPE. We have recently reported on self-catalyzed growth of InAs NWs using a new low-temperature growth regime, where NWs were obtained at temperatures as low as 280°C. We showed that low-temperature growth is beneficial for a variety process flows and growth substrates where excessive heat is detrimental, particularly for plastic substrates. In this work, we synthesize InAs nanowires in-situ on flexible polyimide substrates without any need for transfer techniques. Presently semiconductor nanowires are routinely grown on high-priced crystalline substrates as it is extremely challenging to grow directly on plastics and flexible substrates due to high temperature requirements and substrate preparation. At the same time plastic substrates can offer many advantages such as extremely low price, light weight, mechanical flexibility, shock and thermal resistance and biocompatibility. We show that the fabricated NWs are optically and electrically active with strong light emission in the infrared range. Overall, we demonstrate that light-emitting InAs nanowires can be synthesized directly on flexible plastic substrates inside a MOVPE reactor, and we believe that our results will further advance the development of the nanowires-based flexible electronic devices.

Self-powered flexible electronics beyond thermal limits

Characteristics of indium–tin oxide thin films grown on flexible plastic substrates at room temperature

Organic semiconductors have great potential for future electronic applications owing to their inherent flexibility, low cost, light weight and ability to easily cover large areas. However, all of these advantageous material properties can only be harnessed if simple, cheap and low-temperature fabrication processes, which exclude the need for vacuum deposition and are compatible with flexible plastic substrates, are employed. There are a few solution-based techniques such as spin-coating and inkjet printing that meet the above criteria. In this paper, we describe a novel all-solution-processed nonvolatile memory device fabricated on a flexible plastic substrate. The source, drain and gate electrodes were printed using an inkjet printer with a conducting organic solution, while the semiconducting layer was spin-coated with an n-type polymer. The charge-trapping layer was composed of spin-coated reduced graphene oxide (rGO), which was prepared in the form of a solution using Hummer’s method. The fabricated device was characterized in order to confirm the memory characteristics. Device parameters such as threshold voltage shift, retention/endurance characteristics, mechanical robustness and reliability upon bending were also analyzed.

We present highly power-efficient flexible organic light-emitting diodes by combining nanostructured metallic transparent conductor on plastic substrates, leading to a power efficiency over 160 lm/W with angular color stability. Flexible organic light-emitting diodes (OLEDs) hold great promise for future bendable display and curved lighting applications. One key challenge of high-performance flexible OLEDs is to develop new flexible transparent conductive electrodes (TCEs) with superior mechanical, electrical and optical properties. Indium-tin-oxide (ITO) electrode is the most widely used TCE in optoelectronic devices due to its excellent electrical conductivity and light transmission, but the high-temperature fabrication and the brittle nature under repeated bending condition hinder its application on flexible plastic substrates. An additional drawback of using ITO in flexible OLEDs on plastic substrates is the limited light outcoupling efficiency due to the severe trapping loss of the internally emitted photons. Various materials and structures have been proposed to function as flexible TCEs. Metal-dielectric composite electrode (MDCE) has been regarded as an effective TCE for flexible devices in terms of mechanical flexibility, electrical conductivity, optical transparency, and large-area film uniformity. Whilst MDCE may be an ideal candidate to replace ITO, particularly in flexible devices on plastic substrates, two key challenges should be overcome. First, forming an ultrathin metal film (≤ 10 nm) in a MDCE for improving the optical transmission is extremely difficult due to the dewetting problem with an isolated granular morphology, causing the adverse degradation in optical transmittance and electrical conductance with particle plasmon absorption and severe disconnection. Second, an optical microcavity effect is inevitable with the use of a planar MDCE structure, leading to the spectral and angular dependence of the emission characteristics. Herein, we demonstrate a new strategy to achieve a powerful transparent conductive electrode on plastic substrate that combines a quasi-random nanostructured optical coupling layer and an ultrathin metal alloy conduction layer [1,2]. The optimum electrical conductivity, optical manipulation capability, and high tolerance to mechanical bending are realized in this composite electrode, which is favorable for the fabrication of ITO-free flexible OLEDs with state-of-the-art performance on low-refractive-index plastic substrate. The angularly and spectrally independent boost in light outcoupling of white emission is obtained by minimizing the waveguide mode, metallic electrode-related microcavity effect and surface plasmonic loss due to the integrated quasi-random outcoupling structure in the composite electrode[3-5]. The resulting white flexible OLED exhibits the high enhancement in efficiency, e.g., external quantum efficiency ~70% and power efficiency over 160 lm/W. In addition, this composite electrode has a scalable manufacturing potential in large-area flexible electronic systems. Fig. 1. Device structure and performance of flexible white OLEDs. (a-c) Comparison of different transparent electrodes: (a) Optical properties. Inset is the schematic illustration of flexible OLED structure with NMDCE on plastic substrate. (b) Sheet resistance as a function of nominal film thickness. (c) Mechanical stability under continuous bending test. (d-f) Performance characteristics of white flexible OLEDs: (d) Current density and luminance as a function of driving voltage. (e) Current and power efficiencies as a function of luminance. (f) Normalized EL spectra. Inset is the photograph of white-emission flexible OLED. (g-i) Simulations of normalized cross-section intensity field distributions (at 520 nm) with propagation of OLEDs on (g) ITO, (h) MDCE and (i) NMDCE using FDTD method.

Inorganic semiconductor nanowires, known for their exceptional electronic properties and mechanical flexibility, are widely regarded as the ideal 1D channel materials for creating high‐performance flexible electronics. In this work, the integration of ordered arrays of silicon nanowire (SiNW) field effect transistors (FETs) directly onto flexible plastic substrates is showcased. The self‐aligned crystalline SiNW multi‐channels are first grown through an in‐plane solid–liquid–solid mechanism on rigid substrates, and then efficiently transferred in‐batch onto flexible polyethylene terephthalate (PET) plastics. The FETs constructed on these transferred SiNW channels exhibit outstanding performance, with a high on/off current ratio of &gt;105, a low subthreshold swing of 175 mV dec−1, and remarkable mechanical stability that can endure an extremely small bending radius of 0.5 mm for 1000 cycles. Furthermore, inverter logics are also successfully demonstrated on plastic substrates, highlighting a prominent routine for scalable integration of high‐quality SiNW channels in the development of low‐cost, high‐performance flexible displays and wearable electronics.

This letter reports the fabrication of PbZr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (PZT) thin-film capacitors on flexible plastic substrates. The PZT film was formed on a wafer using a sol-gel method and transferred to a thin plastic substrate using an elastomeric stamp. The PZT film on the plastic substrate showed a well-saturated hysteresis loop with a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> of ~ 20μC/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of ~1.1 V at a supplied voltage of 3 V, which are similar to those observed for PZT films on rigid wafers, as well as stable operation under an 8-mm bending radius. These characteristics suggest promising applications in flexible electronic systems.

Research interest in thin-film transistor (TFT)-based flexible electronics for next-generation foldable and rollable displays is increasing, centering on flexible plastic substrates such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyimide (PI). For flexible electronics using plastic substrates, it is important to select suitable semiconductor materials that can be deposited at low substrate temperature. Metal oxide (MO) semiconductors such as indium-gallium-zinc oxide (IGZO), zinc oxide (ZnO) are attracting attention as suitable candidates for plastic substrate electronics because of their higher mobility and optical transmittance than hydrogenated amorphous silicon (a-Si:H). In addition, since it can be processed at a lower temperature than low-temperature polysilicon (LTPS), it can be easily applied to flexible plastic substrates. One of the greatest advantages of MO is that it can be easily fabricated by conventional semiconductor processing methods such as sputtering, atomic laser deposition, and solution processes. Among them, solution-processed deposition method is particularly advantageous in terms of manufacturing efficiency because it does not require a separate vacuum system, and can efficiently utilize raw materials, so the fabrication cost is inexpensive. In particular, spray pyrolysis can guarantee environmental stability, high mobility, and large area production than other solution processes. In this work, we developed the high-performance, flexible a-IGZO/Al2O3 TFTs using spray pyrolysis fabricated on the flexible PI substrate. In a previous study, we developed a method to reduce curling and electrostatic discharge (ESD) damage to PI substrates by using a mixture of carbon nanotubes (CNTs) and graphene oxide (GO) between carrier glass and PI substrate. Therefore, it was possible to mechanically detach the TFTs on PI substrate and handle them easily even after detachment. The spray-pyrolyzed Al2O3 film is one of the high-k dielectric materials, which can lower the operating voltage, so the low-power consumption of the TFT backplane can be realized. The flexible spray-pyrolyzed a-IGZO/Al2O3 TFT exhibits the field-effect mobility (µFE) of 9.75 cm2 V−1 s−1, threshold voltage (VTH) of −0.8 V, subthreshold swing (SS) of 0.29 V dec−1, and on/off current ratio of 6.2 × 107. Even without TFT passivation layer, it shows environmental stability. The TFTs show robust electrical performance even under tensile strain with a bending radius of 1 mm. In addition, it showed stable electrical characteristics even in the repeated bending test of 30,000 cycles under compressive strain with a bending radius of 1 mm. This is due to fewer defects resulting from the smooth interface of the a-IGZO film and the Al2O3 film deposited through spray pyrolysis. Therefore, spray-pyrolyzed a-IGZO/Al2O3 TFT fabricated on PI substrate is a promising candidate for next-generation foldable display application. Figure 1

Flat panel displays made on plastic substrates are envisioned for use in certain commercial and military systems because they are more rugged and lightweight than displays made on glass substrates. High information content can be attained for such displays using an active matrix array of thin film transistors (TFTs) for the pixels and high current TFTs for the drivers. In this work the fabrication of high performance polysilicon TFTs on flexible plastic substrates is presented along with corresponding electrical characteristics. Plastic substrates pose severe temperature constraints on the fabrication process. To overcome electrical characteristics. Plastic substrates pose sever temperature constraints on the fabrication process. To overcome these constraints, our group at LLNL has used low temperature silicon, oxide, and aluminum thin film deposition steps and pulsed excimer laser processing to perform the TFT channel crystallization and the source/drain doping. Sheet resistance values below 1k(Omega) /$DAL are obtained using our laser doping technique for 900 angstrom thick polysilicon films. Our n-channel polysilicon TFT electrical performance on plastic shows mobilities up to 50 cm<SUP>2</SUP>/V-sec and ON current to OFF current ratios of up to 1 X 10<SUP>6</SUP> for gate voltages from -1 to +35 V.

A process for the fabrication of organic thin-film transistors (TFTs) with channel lengths as short as 0.5 μm on flexible plastic substrates has been developed. The TFTs are fabricated in the bottom-gate, top-contact (inverted staggered) architecture and employ vacuum-deposited small-molecule organic semiconductors and a low-temperature-processed gate dielectric that is sufficiently thin to allow the TFTs to operate with voltages of about 2 to 3 V. For a channel length of 0.5 μm, the TFTs have effective field-effect mobilities of 0.08 cm2/Vs (p-channel TFTs) and 0.03 cm2/Vs (n-channel TFTs), on/off current ratios of at least 106, and subthreshold swings of about 140 mV/decade. For 11-stage complementary and unipolar ring oscillators based on TFTs with a channel length of 1 μm, signal propagation delays per stage as short as 6.6 μs and 420 ns have been measured at a supply voltage of 3 V.

Below-one-volt organic thin-film transistors with large on/off current ratios

Integration of a graphene ink as gate electrode for printed organic complementary thin-film transistors