Concept: Thin-film transistor
Organic thin-film transistors (OTFTs) can be fabricated at moderate temperatures and through cost-effective solution-based processes on a wide range of low-cost flexible and deformable substrates. Although the charge mobility of state-of-the-art OTFTs is superior to that of amorphous silicon and approaches that of amorphous oxide thin-film transistors (TFTs), their operational stability generally remains inferior and a point of concern for their commercial deployment. We report on an exhaustive characterization of OTFTs with an ultrathin bilayer gate dielectric comprising the amorphous fluoropolymer CYTOP and an Al2O3:HfO2 nanolaminate. Threshold voltage shifts measured at room temperature over time periods up to 5.9 × 105 s do not vary monotonically and remain below 0.2 V in microcrystalline OTFTs (μc-OTFTs) with field-effect carrier mobility values up to 1.6 cm2 V-1 s-1. Modeling of these shifts as a function of time with a double stretched-exponential (DSE) function suggests that two compensating aging mechanisms are at play and responsible for this high stability. The measured threshold voltage shifts at temperatures up to 75°C represent at least a one-order-of-magnitude improvement in the operational stability over previous reports, bringing OTFT technologies to a performance level comparable to that reported in the scientific literature for other commercial TFTs technologies.
In this article, we report only 10 atomic layer thick, high mobility, transparent thin film transistors (TFTs) with ambipolar device characteristics fabricated on both a conventional silicon platform as well as on a flexible substrate. Monolayer graphene was used as metal electrodes, 3-4 atomic layers of h-BN were used as the gate dielectric, and finally bilayers of WSe2 were used as the semiconducting channel material for the TFTs. The field effect carrier mobility was extracted to be 45 cm(2)/(V s), which exceeds the mobility values of state of the art amorphous silicon based TFTs by ∼100 times. The active device stack of WSe2-hBN-graphene was found to be more than 88% transparent over the entire visible spectrum and the device characteristics were unaltered for in-plane mechanical strain of up to 2%. The device demonstrated remarkable temperature stability over 77-400 K. Low contact resistance value of 1.4 kΩ-μm, subthreshold slope of 90 mv/decade, current ON-OFF ratio of 10(7), and presence of both electron and hole conduction were observed in our all two-dimensional (2D) TFTs, which are extremely desirable but rarely reported characteristics of most of the organic and inorganic TFTs. To the best of our knowledge, this is the first report of all 2D transparent TFT fabricated on flexible substrate along with the highest mobility and current ON-OFF ratio.
Trialkylgermyl functionalization allows development of high-performance soluble small-molecule organic semiconductors with mobilities greater than 5 cm(2) V(-1) s(-1) . Spray-deposited organic thin-film transistors show a record mobility of 2.2 cm(2) V(-1) s(-1) and demonstrate the potential for incorporation in large-area, low-cost electronic applications.
Organic semiconductors with higher carrier mobility and better transparency have been actively pursued for numerous applications, such as flat-panel display backplane and sensor arrays. The carrier mobility is an important figure of merit and is sensitively influenced by the crystallinity and the molecular arrangement in a crystal lattice. Here we describe the growth of a highly aligned meta-stable structure of 2,7-dioctylbenzothieno[3,2-b]benzothiophene (C8-BTBT) from a blended solution of C8-BTBT and polystyrene by using a novel off-centre spin-coating method. Combined with a vertical phase separation of the blend, the highly aligned, meta-stable C8-BTBT films provide a significantly increased thin film transistor hole mobility up to 43 cm(2) Vs(-1) (25 cm(2) Vs(-1) on average), which is the highest value reported to date for all organic molecules. The resulting transistors show high transparency of >90% over the visible spectrum, indicating their potential for transparent, high-performance organic electronics.
A silver molecular ink platform formulated for screen, inkjet and aerosol printing is presented. A simple formulation comprising silver neodecanoate, ethyl cellulose and solvent provides improved performance vs established inks yet with improved economics. Thin, screen printed traces with exceptional electrical (< 10 mΩ/□/mil or 12 μΩ·cm)) and mechanical properties are achieved following thermal or photonic sintering, the latter having never been demonstrated for silver salt based inks. Low surface roughness, sub-micron thicknesses and linewidths as narrow as 41 μm outperform commercial ink benchmarks based on flakes or nanoparticles. These traces are mechanically robust to flexing and creasing (less than 10% change in resistance) and bind strongly to epoxy-based adhesives. Thin traces are remarkably conformal, enabling fully printed metal-insulator-metal (MIM) band-pass filters. The versatility of the molecular ink platform enables an aerosol jet compatible ink that yields conductive features on glass with 2X bulk resistivity and strong adhesion to various plastic substrates. An inkjet formulation is also used to print top source/drain contacts and demonstrate printed high mobility thin film transistors (TFT) based on semiconducting single walled carbon nanotubes. TFTs with mobility values of ~25 cm(2)V(-1)sec(-1) and current on/off ratios > 10(4) were obtained, performance similar to evaporated metal contacts in analogous devices.
Physical properties of active materials built up from small molecules are dictated by their molecular packing in the solid state. Here we demonstrate for the first time the growth of n-channel single-crystal field-effect transistors and organic thin-film transistors by sublimation of 2,6-dichloro-naphthalene diimide in air. Under these conditions, a new polymorph with two-dimensional brick-wall packing mode (β-phase) is obtained that is distinguished from the previously reported herringbone packing motif obtained from solution (α-phase). We are able to fabricate single-crystal field-effect transistors with electron mobilities in air of up to 8.6 cm(2) V(-1) s(-1) (α-phase) and up to 3.5 cm(2) V(-1) s(-1) (β-phase) on n-octadecyltriethoxysilane-modified substrates. On silicon dioxide, thin-film devices based on β-phase can be manufactured in air giving rise to electron mobilities of 0.37 cm(2) V(-1) s(-1). The simple crystal and thin-film growth procedures by sublimation under ambient conditions avoid elaborate substrate modifications and costly vacuum equipment-based fabrication steps.
Carbon nanotube thin film transistors (CNT-TFTs) are regarded as promising technology for active matrix pixel driving circuits of future flat panel displays (FPD). For FPD application, unipolar thin film transistors (TFTs) with high mobility (μ), high on-state current (ION), low off-current (IOFF) at high source/drain bias and small hysteresis are required simultaneously. Though excellent values of those performance metrics have been realized individually in different reports, the overall performance of previously reported CNT-TFTs has not met the above requirements. In this paper, we found that yttrium oxide (Y2O3) capping is helpful in improving both IONand μ of CNT-TFTs. Combining Y2O3capping and Al2O3passivation, unipolar CNT-TFTs with high ION/IOFF(>107) and low IOFF(∼pA) at -10.1 V source/drain bias, and relatively small hysteresis in the range of -30 V to +30 V gate voltage were achieved, which are capable of active matrix display driving.
In this paper, we introduce a transparent fingerprint sensing system using a thin film transistor (TFT) sensor panel, based on a self-capacitive sensing scheme. An armorphousindium gallium zinc oxide (a-IGZO) TFT sensor array and associated custom Read-Out IC (ROIC) are implemented for the system. The sensor panel has a 200 × 200 pixel array and each pixel size is as small as 50 μm × 50 μm. The ROIC uses only eight analog front-end (AFE) amplifier stages along with a successive approximation analog-to-digital converter (SAR ADC). To get the fingerprint image data from the sensor array, the ROIC senses a capacitance, which is formed by a cover glass material between a human finger and an electrode of each pixel of the sensor array. Three methods are reviewed for estimating the self-capacitance. The measurement result demonstrates that the transparent fingerprint sensor system has an ability to differentiate a human finger’s ridges and valleys through the fingerprint sensor array.
Conjugated polymer extraction (CPE) has been shown to be a highly effective method to isolate high purity semiconducting single-walled carbon nanotubes (sc-SWCNTs). In both literature reports and industrial manufacturing, this method has enabled enrichment of sc-SWCNTs with high purity (≥ 99.9%). High selectivity is typically obtained in non-polar aromatic solvents, yet polar solvents may provide process improvements in terms of yield, purity and efficiency. Using a novel amphiphilic fluorene-alt-pyridine conjugated copolymer with hydrophilic side chains we have investigated the enrichment of sc-SWCNTs in polar solvents. Various conditions such as polymer/SWCNT ratio, solvent polarity, solvent dielectric constant, as well as polymer solubility and SWCNT dispersability were explored in order to optimize the purity and yield of the enriched product. Herein, we provide new insights on CPE by demonstrating that a conjugated polymer having a hydrophobic backbone and hydrophilic oligo(ethylene oxide) side chains provides near full recovery (95%) of sc-SWCNTs using a multi-extraction protocol. High purity is also obtained and differences in chiral selectivity compared to analogous hydrophobic systems were confirmed by optical absorption and Raman spectroscopy, as well as photoluminescence excitation mapping (PLE). Taking into consideration the solvent dielectric constant, polarity index as well as polymer solubility and SWCNT dispersability provides new insights and a better understanding of structure-property effects on sc-SWCNT enrichment. The resulting hydrophilic SWCNT dispersions demonstrate long-term colloidal stability, making them suitable for ink formulation and high performance thin film transistors (TFT) fabrication.
In this paper, we demonstrate high performance and hysteresis-free solution-processed indium-gallium-zinc-oxide (IGZO) thin film transistors (TFTs) and high-frequency-operating 7-stage ring oscillators using a low-temperature photochemically activated Al2O3/ZrO2 bilayer gate dielectric. It was found that the IGZO TFTs with single layer gate dielectrics such as Al2O3, ZrO2 or sodium-doped Al2O3 exhibited large hysteresis, low field-effect mobility or unstable device operation owing to the interfacial/bulk trap states, insufficient band offset or a substantial number of mobile ions present in the gate dielectric layer, respectively. In order to resolve these issues and to explain the underlying physical mechanisms, a series of electrical analyses for various single and bilayer gate dielectrics was carried out. It is shown that compared to single layer gate dielectrics, the Al2O3/ZrO2 gate dielectric exhibited high dielectric constant of 8.53, low leakage current density (~10-9 A cm-2 at 1 MV cm-1) and stable operation at high frequencies. Using the photochemically activated Al2O3/ZrO2 gate dielectric, 7-stage ring oscillators operating at an oscillation frequency of ~334 kHz with propagation delay of < 216 nanoseconds per stage were successfully demonstrated on a polymeric substrate.