On Organic Thin-film Transistors Research Articles

Performance analysis of OTFT-based SRAM topologies

In terms of mechanical flexibility, organic SRAM offers better designs and a commercially feasible option with the ability to deliver acceptable performance. This paper investigates the implementation of different SRAM topologies based on organic thin film transistors (OTFTs). In this work, a compact spice model is used to simulate pOTFT and nOTFT in LTSpice software. Time delays, power consumption, the power delay product (PDP), and static noise margin (SNM) for read and write operations are calculated, and a comparative analysis of OTFT based 6T, 7T, 8T, and 9T SRAM topologies is performed. Among different topologies, 9T OTFT SRAM cell achieves a 1.67× increase in SNM, compared to conventional 6T OTFT-based SRAM cell. The highest figure of merit value of 9T SRAM cell indicates its suitability for various applications.

Furan functionalized naphthalenediimide semiconductors with different N-alkyl chains for n-type organic thin-film transistor applications

Development of n-type organic semiconductors (OSCs) is necessary in order to develop complementary electronics based on organic thin-film transistors (OTFTs). In this investigation, we designed and synthesized a series of four novel imide- and core-substituted naphthalene diimide (NDI) molecules. The molecular architecture was altered via introduction of different alkyl side chains (octyl and 2-ethylhexyl) to the imide-N positions and 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) to the NDI core. The NDI-TCF derivatives are abbreviated as NDI-TCF1A, NDI-TCF1B, NDI-TCF2A, and NDI-TCF2B, using a “1” or “2” to indicate the number of TCF subunits at the NDI core, and an “A” or a “B” to indicate an octyl or a 2-ethylhexyl alkyl chain at the imide-N position, respectively. All four compounds displayed similar photophysical properties but exhibited different electrochemical properties with lower LUMO levels of −4.25 eV, −4.33 eV, −4.27 eV and −4.35 eV for NDI-TCF1A, NDI-TCF1B, NDI-TCF2A, and NDI-TCF2B, respectively. These LUMO levels suggest a capacity for electron transport. Furthermore, the NDI-TCF chromophores were incorporated into OTFTs and systematically studied on the relationship between the NDI-TCF molecular architecture and electron transport properties. NDI-TCF1A and NDI-TCF1B displayed more consistent OTFT performance than NDI-TCF2A and NDI-TCF2B. NDI-TCF1A and NDI-TCF1B exhibited low threshold voltages (VT) of −0.43 V ± 4.78 V and 6.12 V ± 6.63 V, respectively, electron mobilities (μe) of ca. 3 × 10−5 cm2V−1s−1, and on/off (ION/OFF) ratios of ca. 103. Insofar that TCF facilitates electron transport, the OTFT data suggest that the number of TCF subunits affect OTFT performance via thin-film morphology. The thin-film morphology of NDI-TCF1A, NDI-TCF1B, NDI-TCF2A, and NDI-TCF2B was studied using atomic force microscopy (AFM) technique and supported by X-ray diffraction measurements.

67‐2: Invited Paper: Organic Thin‐Film Transistor Flexible Hybrid Integration for Low‐power Ubiquitous Sensor Systems

This paper firstly introduces a flexible hybrid integration (FHI) architecture based on organic thin‐film transistors (OTFTs) for ubiquitous sensor systems, followed by description of the low power OTFT technologies. Then, progresses on sensor integration based on the OTFTs for FHI are discussed, proving the versatility of the technology.

Exploiting the KPFM capabilities to analyze at the nanoscale the impact of electrical stresses on OTFTs properties

Two different Kelvin Probe Force Microscopy (KPFM) measurement configurations have been combined to evaluate at the nanoscale the effects of an electrical stress on Organic Thin Film Transistors (OTFTs) properties. As an example, Channel Hot Carrier (CHC) degradation has been induced to provoke some damage in the studied devices. The results show that the use of the two KPFM configurations, together with their nanoscale resolution, provides additional information about the damage in the different regions/materials of the devices, allowing to correlate device level characteristics with the nanoscale material properties.

A high performance 3-bit ripple counter based on organic TFTs

We propose a high performance 3-bit negative edge-triggered ripple counter based on organic thin film transistors (OTFTs). All the logic gate circuits used in this work are inverters and NAND circuits based on OTFTs with large zero-VGS load. A clock signal with the voltage range of 0–30 V and a frequency of 12.5 kHz is used for the ripple counter as a clock input. A high output level of ∼27.4 V and a low output level of ∼4 or 5 V are measured at the 2nd and 3rd stages’ output node of the ripple counter. Their frequencies are one quarter and one eighth of the input signal’s frequency. The output signal of the proposed ripple counter changes when its input signal falls to low level from high.

Intrinsically Stretchable Temperature Sensor Based on Organic Thin-Film Transistors

Achieving reliable electrical performance with stretchability and conformability is a key issue for wearable electronics. In this letter, we demonstrate an intrinsically stretchable temperature sensor element based on organic thin-film transistors (OTFTs). The OTFTs were characterized with respect to their temperature and strain dependence, and subsequently evaluated using differential readout circuit schemes. We considered two circuit variants to accommodate OTFTs with nominally positive or negative threshold voltages. Due to the strain suppression afforded by the differential readout, the intrinsically stretchable temperature sensor element achieves an inaccuracy within 1 °C with uniaxial strains from 0-30 %.

An ultraflexible organic differential amplifier for recording electrocardiograms

Differential amplifiers based on organic thin-film transistors (OTFTs) are attractive for monitoring human vital signs because of their signal amplification and noise elimination capabilities. However, substantial variations in OTFTs lead to the degradation of signal processing performance in circuits and restrict the development of organic differential amplifiers capable of recording weak physiological potentials. Here, we report a 2-μm-thick ultraflexible organic differential amplifier capable of processing physiological signals with high signal integrity and sensitivity. Our approach uses a circuit design and compensation technique that suppress the mismatch among OTFTs to less than a few percent. This leads to a common-mode noise attenuation factor below −12 dB, even during bending to ~50 μm. Using our flexible amplifier, we monitor electrocardiogram signals, where the power of 60 Hz electrical harmonic noise was reduced ~60 times during amplification, yielding electrocardiogram signals with a signal-to-noise ratio of 34 dB. An ultraflexible organic differential amplifier, which is only 2 μm thick and can conform to a person’s skin, can be used to record electrocardiograms with a signal-to-noise ratio of 34 dB.

Precise patterning of organic semiconductors by reactive ion etching

Abstract Minimizing the off-state current is mandatory for the development of low power consumption circuits based on organic thin-film transistors (OTFTs). Small OTFTs which are required for compact circuits suffer from parasitic current pathways through the semiconductor and hence precise patterning of the semiconductor film is essential. Since state-of-the-art patterning techniques such as oxygen based reactive ion etching usually severely damage the organic transistors, usually OTFTs with a large channel width W are discussed in the literature in order to circumvent this problem. Here we demonstrate that precise patterning of organic semiconductor films for OTFT using reactive ion etching is possible without compromising the transistor performance. If adequate etching conditions and appropriate masks are chosen, oxygen radicals do not affect the organic transistor because lateral diffusion as well as vertical bombardment of ions through the etching mask do not occur. With the method presented here, we show small footprint OTFTs with W / L = 25 μm/10 μm and leakage current levels as low as 10 fA/μm which enable organic transistors to be used for demanding electronic applications such as high resolution active-matrix displays or photo detector arrays.

Impact of 2-Ethylhexyl Stereoisomers on the Electrical Performance of Single-Crystal Field-Effect Transistors.

Many organic semiconductors (OSCs) inherit chiral alkyl chains, which ensure the desirable high solubility for solution-processing but may also lead to disorder, inhomogeneous film-formation, as well as interfacial defects due to the presence of mixtures of stereoisomers or diastereomers, which impair their peak performance. Here, single-crystal field-effect transistors (SCFETs) of a diketopyrrolopyrrole-based organic semiconductor with chiral 2-ethylhexyl substituents by sublimation in air and organic ribbon mask method are fabricated. Devices of the mesomer (R/S), both enantiomers (R/R, S/S), as well as mixtures of these three stereoisomers measured under ambient conditions exhibit all appreciable p-channel charge carrier mobilities of > 0.1 cm2 V-1 s-1 despite different packing arrangement in the R/S, R/R (or S/S), and racemate crystal structures. These results suggest a surprising tolerance for isomeric impurities. The highest literature-reported p-channel mobility so far for a diketopyrrolopyrrole-based OSC of 3.4 cm2 V-1 s-1 (Ion /Ioff of 1 × 106 ) is, however, only obtained for the pure R/S mesomer, illustrating the inherent potential of stereochemical purity. These results on SCFETs are further substantiated by studies on organic thin-film transistors (OTFTs) of pure and mixed thin films of the different stereoisomers.

Integration of Biomaterials into Sensors Based on Organic Thin-Film Transistors.

Sensors based on organic thin-film transistors (OTFTs) present various advantages, including high sensitivity and mechanical flexibility, thus possessing potential applications such as wearable devices and biomedical electronics for health monitoring, etc. However, such applications are partially limited by the biocompatibility, biodegradability, and sensitivity to target analytes of OTFT-based sensors, which can be improved by the incorporation of diverse biomaterials. This article presents a brief review from the viewpoint of the type of the integrated biomaterials, including naturally occurring biomacromolecules such as proteins, enzymes, and deoxyribonucleic acid, as well as biocompatible polymers such as polylactide, poly(lactide-co-glycolide), poly(ethylene glycol), cellulose, polydimethylsiloxane, parylene, etc. It is believed that future work in this field should be devoted to the selectivity, sensitivity, and stability improvement as well as the high-level integration and sophistication on the basis of the OTFT-based sensors for physical, chemical, and biological sensing applications.

Flexible organic thin-film transistors based on poly(3-hexylthiophene) films for nitrogen dioxide detection

Flexible nitrogen dioxide (NO2) gas sensors based on organic thin-film transistors (OTFTs) were developed in this work, in which conductive indium tin oxide (ITO) coated polyethylene naphthalene-2,6-dicarboxylate (PEN) was designed for the flexible substrates, and poly methyl methacrylate (PMMA) and poly(3-hexylthiophene) (P3HT) was spin-coated as the gate dielectric layer and spray-deposited as the active layer, respectively. The effects of PMMA concentrations and P3HT solution volume on performances of flexible OTFTs were systematically investigated. The results showed that the optimized flexible OTFT exhibited high field-effect mobility (9.51×10–3 cm2/(V s) at a gate bias of–50 V) and excellent response characteristics, including high sensitivity (up to 0.169 ppm–1), good repeatability and selectivity. The flexibility of the developed OTFT sensor was also investigated, and the results showed that the electrical and gas-sensing properties were affected by the bending cycles, thus further work should be done for improving the flexibility of the sensor. This work provides an effective approach in developing high performance flexible OTFT NO2 gas sensor.

15.2: Invited Paper: Organic LCDs on TAC film: low‐cost, area‐scalable flexible displays with glass‐like optical performance

Flexible Organic LCD (OLCD) is a new display technology, developed by FlexEnable, based on organic thin‐film transistors (OTFTs) on plastic substrates. With optical performance being one of the most important attributes of any display, achieving glass‐like optical performance on plastic is a key requirement for mass market adoption.We explain how the use of low temperature processes (below 100°C) not only allows the use of plastic substrates (TAC) with near ideal optical properties, but also brings cost, yield and scalability advantages in manufacturing compared to flexible display approaches using inorganic TFTs that require much higher processing temperatures.

Effect of Vertical Annealing on the Nitrogen Dioxide Response of Organic Thin Film Transistors.

Nitrogen dioxide (NO2) sensors based on organic thin-film transistors (OTFTs) were fabricated by conventional annealing (horizontal) and vertical annealing processes of organic semiconductor (OSC) films. The NO2 responsivity of OTFTs to 15 ppm of NO2 is 1408% under conditions of vertical annealing and only 72% when conventional annealing is applied. Moreover, gas sensors obtained by vertical annealing achieve a high sensing performance of 589% already at 1 ppm of NO2, while showing a preferential response to NO2 compared with SO2, NH3, CO, and H2S. To analyze the mechanism of performance improvement of OTFT gas sensors, the morphologies of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) films were characterized by atomic force microscopy (AFM) in tapping mode. The results show that, in well-aligned TIPS-pentacene films, a large number of effective grain boundaries inside the conducting channel contribute to the enhancement of NO2 gas sensing performance.

A Printed Organic Amplification System for Wearable Potentiometric Electrochemical Sensors

Electrochemical sensor systems with integrated amplifier circuits play an important role in measuring physiological signals via in situ human perspiration analysis. Signal processing circuitry based on organic thin-film transistors (OTFTs) have significant potential in realizing wearable sensor devices due to their superior mechanical flexibility and biocompatibility. Here, we demonstrate a novel potentiometric electrochemical sensing system comprised of a potassium ion (K+) sensor and amplifier circuits employing OTFT-based pseudo-CMOS inverters, which have a highly controllable switching voltage and closed-loop gain. The ion concentration sensitivity of the fabricated K+ sensor was 34 mV/dec, which was amplified to 160 mV/dec (by a factor of 4.6) with high linearity. The developed system is expected to help further the realization of ultra-thin and flexible wearable sensor devices for healthcare applications.

Organic thin-film transistors with over 10 cm2/Vs mobility through low-temperature solution coating

ABSTRACTRecent studies on organic thin-film transistors (OTFTs) have reported high mobility values, but many of them showed non-ideal current–voltage characteristics that could lead to the overestimation of the mobility values. In this study, the non-ideal transistor behavior was briefly investigated by considering the effect of charge injection, and a method of overcoming the effect was developed. Correspondingly, various charge injection layers were developed, and their effects on the modification of metal contacts, including work function tuning and interfacial doping, were studied. The materials that had been coated formed a good metal-semiconductor interface through fine manipulation in the wetting and dewetting of the selected liquid. With such electrodes, the OTFTs were fabricated at room temperature and exhibited almost ideal transistor behavior in terms of the current–voltage characteristics, featuring high (over 10 cm2/Vs) field-effect mobility.

Bias stress effects in pentacene thin-film transistors with poly(methyl methacrylate) gate insulator

ABSTRACTWe demonstrated a bias stress effect on organic thin-film transistors (OTFTs) that causes device degradation, including a large shift in the threshold voltage and hysteresis in the transfer characteristics. Specifically, we analyzed the electrical characteristic variations in pentacene TFTs with a poly(methyl methacrylate) gate insulator under different bias stress conditions. We found that bias stress between the gate electrode and source/drain electrodes affects the charge transport properties, whereas bias stress between the source and drain electrodes influences the charge injection properties.

High-Speed Plastic Integrated Circuits: Process Integration, Design, and Test

In this paper we propose the design and measurement of functional analog and digital circuits with a response time below 1 ms, based on organic thin film transistors (OTFTs) fabricated by means of contact photolithography and self-alignment procedures. An adapted amorphous Silicon TFT compact model is used both in analytic equations and in DC Spice simulations for the design of simple organic circuits. Digital circuits such as inverters and logic gates are demonstrated with DC gains of almost 19 dB. Two analog circuits are also shown: first a differential amplifier with an open loop DC gain of 10 dB and a gain-bandwidth of 3 kHz, and second, a source coupled latch comparator tested with an input frequency of 1 kHz and a clock frequency of 10 kHz. These simple circuits comprised of only a few OTFTs are fabricated directly on flexible plastic sheets and therefore are ideal front-end interfaces for the control and fast read-out of flexible sensors.

Highly Sensitive Flexible NH3 Sensors Based on Printed Organic Transistors with Fluorinated Conjugated Polymers.

Understanding the sensing mechanism in organic chemical sensors is essential for improving the sensing performance such as detection limit, sensitivity, and other response/recovery time, selectivity, and reversibility for real applications. Here, we report a highly sensitive printed ammonia (NH3) gas sensor based on organic thin film transistors (OTFTs) with fluorinated difluorobenzothiadiazole-dithienosilole polymer (PDFDT). These sensors detected NH3 down to 1 ppm with high sensitivity (up to 56%) using bar-coated ultrathin (<4 nm) PDFDT layers without using any receptor additives. The sensing mechanism was confirmed by cyclic voltammetry, hydrogen/fluorine nuclear magnetic resonance, and UV/visible absorption spectroscopy. PDFDT-NH3 interactions comprise hydrogen bonds and electrostatic interactions between the PDFDT polymer backbone and NH3 gas molecules, thus lowering the highest occupied molecular orbital levels, leading to hole trapping in the OTFT sensors. Additionally, density functional theory calculations show that gaseous NH3 molecules are captured via cooperation of fluorine atoms and dithienosilole units in PDFDT. We verified that incorporation of functional groups that interact with a specific gas molecule in a conjugated polymer is a promising strategy for producing high-performance printed OTFT gas sensors.

Enhanced Sensitivity of Gas Sensor Based on Poly(3-hexylthiophene) Thin-Film Transistors for Disease Diagnosis and Environment Monitoring.

Electronic devices based on organic thin-film transistors (OTFT) have the potential to supply the demand for portable and low-cost gadgets, mainly as sensors for in situ disease diagnosis and environment monitoring. For that reason, poly(3-hexylthiophene) (P3HT) as the active layer in the widely-used bottom-gate/bottom-contact OTFT structure was deposited over highly-doped silicon substrates covered with thermally-grown oxide to detect vapor-phase compounds. A ten-fold organochloride and ammonia sensitivity compared to bare sensors corroborated the application of this semiconducting polymer in sensors. Furthermore, P3HT TFTs presented approximately three-order higher normalized sensitivity than any chemical sensor addressed herein. The results demonstrate that while TFTs respond linearly at the lowest concentration values herein, chemical sensors present such an operating regime mostly above 2000 ppm. Simultaneous alteration of charge carrier mobility and threshold voltage is responsible for pushing the detection limit down to units of ppm of ammonia, as well as tens of ppm of alcohol or ketones. Nevertheless, P3HT transistors and chemical sensors could compose an electronic nose operated at room temperature for a wide range concentration evaluation (1–10,000 ppm) of gaseous analytes. Targeted analytes include not only biomarkers for diseases, such as uremia, cirrhosis, lung cancer and diabetes, but also gases for environment monitoring in food, cosmetic and microelectronics industries.

Formaldehyde Detection Based on P3HT/InSnO Composite Thin Film Transistor

Formaldehyde, a colorless and pungent-smelling gas, had been confirmed be a huge threat to people health. The detection of formaldehyde was necessary and important at room temperature. Sprayed P3HT/InSnO composite film based on organic thin film transistors (OTFT) was fabricated to detect formaldehyde at room temperature in this paper. The results showed that P3HT/ InSnO-OTFT showed better response and recovery to HCHO compared with P3HT-OTFT at room temperature.