Organic Circuits Research Articles

(Invited) Organic Semiconductor, Receptor Material and Circuit Design for Organic Electronic Vapor Sensors and Biosensors

We will present our recent results on the design of semiconducting material, dielectric receptor matrices, and stabilizing and amplifying circuit structures for detection of volatile analytes in the atmosphere and proteins in aqueous solution. Besides our natural interest in designing materials that respond more strongly to analytes of interest than to likely interferents, we also emphasize the use of chemical and electronic amplification methods to increase the ratio of the desired responses to the drift (signal/noise ratio). Printable materials, especially polymers, are emphasized for the responsive layers. The use of multiple sensing elements, typically field-effect transistors (FETs), to create patterns of responses increases the selectivity of the information, either by narrowing the classes of compounds providing the responses, distinguishing time-dependent from dose-dependent responses, and/or increasing the ratio of analyte responses to environmental drifts. As a specific example, using pairs of FETs, in series or in parallel, allows device drifts to be substantially canceled while analyte responses are maintained and even reinforced. This strategy has led to significant improvements in the use of polymer-based FETs to detect pollutant gases such as nitrogen dioxide and ammonia. To increase the stability of systems used to detect analytes in solution, we sometimes separate the sensing surface from the output device in an arrangement known as a remote gate. We show that the output device may be an organic-based or a silicon-based transistor, and can respond to electrochemical potential changes at the sensing surface arising from a variety of chemical interactions. This type of configuration was applied to the understanding of vapor sensors and the effect of receptor polarity on the detection of brain injury biomarker proteins. New details about the mechanisms for sensor responses to analytes can be obtained from remote gate configurations, including connection of interfacial voltages and charge densities in responsive polymer semiconductors.

Flexible short-channel organic transistors and inverter circuits using top-contact and double-gate structure

We demonstrate threshold voltage controllable short-channel transistors and organic inverter circuits with top-contact electrodes on flexible substrates. The source and drain electrodes were thermally evaporated on semiconductor material and patterned using photolithography and a wet etching process. The mobility of the fabricated transistors with channel lengths (LCH) of 3 μm was 0.495 ± 0.037 cm2 V–1 s–1, which is 91% of that of a long-channel transistor with LCH of 50 μm. Finally, switching voltage (VSW) controllable organic inverter circuits are demonstrated with a wide-range tunability of over 1.91 V at a supply voltage (VDD) of 3 V and a gain over 75.8 (V/V).

Transparent and Colorless Polyimides Containing Multiple Trifluoromethyl Groups as Gate Insulators for Flexible Organic Transistors with Superior Electrical Stability

A suitable insulating polymer material that is compatible with the fabrication process of organic transistors and has excellent electrical properties is critically required for the next-generation flexible organic electronics. In this study, using a one-step polymerization method, we synthesized two different solution-processable polyimides (PIs) incorporated with abundant trifluoromethyl groups. Not only were the two resulting PIs-termed 6FDA-6FDAM-PI and 6FDA-TFMB-PI-well soluble in organic solvents, but also they showed transparent and colorless optical properties. The fluorinated PI films showed smooth surface topographies and surface energy values that were appropriate for their use in bottom-gate organic transistors. Organic transistors separately fabricated with 6FDA-6FDAM-PI and 6FDA-TFMB-PI as the gate insulators showed excellent device performance and electrical stability under various testing conditions, especially for pentacene-based devices. The excellent performance of the devices with fluorinated PIs was attributed to the enhanced microstructure of the organic semiconductor and the fluorine-rich characteristic of the underlying gate insulator. Furthermore, organic complementary circuits including the basic logic gates of NOT, NOR, and NAND were demonstrated using these devices.

Zero drive load inverter with high gain and large noise margin based on organic weak epitaxy growth method

We constructed zero drive load inverters by employing two identical unipolar pentacene thin-film transistors, based on the fabrication technology of organic weak epitaxy growth, which exhibited the features of high gains and large noise margins. Pentacene films were grown on a p-6P inducing layer, which reduced the interface disorder and increased the scale of crystalline domains. The pentacene/p-6P transistors showed high-quality saturation characteristics and the negative shift of threshold voltages, which caused large noise margins and the operation range of full swing. Furthermore, the hole mobility was increased to 1.28 cm2 (Vs)−1. As a result, zero drive load inverters exhibited a large gain of up to 30, compared with the gains of only four based on single pentacene transistors. We also discuss the dependence of large gains and device performance. These results demonstrated the fabrication technology of organic weak epitaxy growth were promising and advantageous for achieving high performance organic inverters and logic circuits.

Design of Pentacene-Based Organic Field-Effect Transistor for Low-Frequency Operational Transconductance Amplifier

Mounting electronics circuits on a plastic flexible substrate are pertinent for biosensing applications due to their resilient nature, minimal processing conditions, lightweight and low cost. Organic Field-Effect Transistors (OFET)-based amplifier for flexible biosensors have been proposed in this paper. To design flexible biosensing circuits, Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) with Polycyclic Hydrocarbon is a suitable choice. It is a big challenge to build an organic circuit using graphene electrode due to its poor performance of [Formula: see text]-type OFET, therefore it is advisable to use Pentacene as [Formula: see text]- and [Formula: see text]-type Organic semiconductors. Pentacene being one among the foremost totally investigated conjugated organic molecules with a high application potential because the hole mobility in OFETs goes up to 0.2[Formula: see text]cm2/(Vs), which exceeds that of amorphous silicon. In biosignal process, the first and most important step is to amplify the biosignal for further processing. Operational Transconductance Amplifier (OTA) plays an essential role in biological signal measuring instruments like EEG, ECG, EMG modules which measure the heart, muscle and brain activities. The OTA designed using this OFET is adaptable for flexible sensor circuits and also it derives the transconductance of 67 which is similar to silicon OTA. The amplifier designed here gives unit gain of 42[Formula: see text]dB with a frequency of 195[Formula: see text]Hz which is suitable for low-frequency biosignal processing applications.

Hysteresis-Free, High-Performance Polymer-Dielectric Organic Field-Effect Transistors Enabled by Supercritical Fluid.

Organic field-effect transistors (OFETs) are of the core units in organic electronic circuits, and the performance of OFETs replies critically on the properties of their dielectric layers. Owing to the intrinsic flexibility and natural compatibility with other organic components, organic polymers, such as poly(vinyl alcohol) (PVA), have emerged as highly interesting dielectric materials for OFETs. However, unsatisfactory issues, such as hysteresis, high subthreshold swing, and low effective carrier mobility, still considerably limit the practical applications of the polymer-dielectric OFETs for high-speed, low-voltage flexible organic circuits. This work develops a new approach of using supercritical CO2 fluid (SCCO2) treatment on PVA dielectrics to achieve remarkably high-performance polymer-dielectric OFETs. The SCCO2 treatment is able to completely eliminate the hysteresis in the transfer characteristics of OFETs, and it can also significantly reduce the device subthreshold slope to 0.25 V/dec and enhance the saturation regime carrier mobility to 30.2 cm2 V−1 s−1, of which both the numbers are remarkable for flexible polymer-dielectric OFETs. It is further demonstrated that, coupling with an organic light-emitting diode (OLED), the SCCO2-treated OFET is able to function very well under fast switching speed, which indicates that an excellent switching behavior of polymer-dielectric OFETs can be enabled by this SCCO2 approach. Considering the broad and essential applications of OFETs, we envision that this SCCO2 technology will have a very broad spectrum of applications for organic electronics, especially for high refresh rate and low-voltage flexible display devices.

Organic-based inverters: basic concepts, materials, novel architectures and applications.

While organic materials have demonstrated industry-leading performances in a wide array of electronic applications (including OLEDs and OPVs), their use for integration into electronic circuits has been so far limited, in spite of their potential for portable, flexible, light-weight, low-cost applications. However, recent advances in organic (semi)conductors and novel designs in organic field-effect transistors and hybrid systems have reaffirmed the potential of organic logic circuits. This review article provides an overview of organic-based inverter operation and considers all aspects of such circuits including their active layer, processing methods, hybrid organic/inorganic inverters, novel architectures and potential applications.

Tuning the Charge Carrier Polarity of Organic Transistors by Varying the Electron Affinity of the Flanked Units in Diketopyrrolopyrrole‐Based Copolymers

AbstractFine‐tuning of the charge carrier polarity in organic transistors is an important step toward high‐performance organic complementary circuits and related devices. Here, three new semiconducting polymers, namely, pDPF‐DTF2, pDPSe‐DTF2, and pDPPy‐DTF2, are designed and synthesized using furan, selenophene, and pyridine flanking group‐based diketopyrrolopyrrole cores, respectively. Upon evaluating their electrical properties in transistor devices, the best performance has been achieved for pDPSe‐DTF2 with the highest and average hole mobility of 1.51 and 1.22 cm2 V−1 s−1, respectively. Most intriguingly, a clear charge‐carrier‐polarity change is observed when the devices are measured under vacuum. The pDPF‐DTF2 polymer exhibits a balanced ambipolar performance with the µh/µe ratio of 1.9, whereas pDPSe‐DTF2 exhibits p‐type dominated charge carrier transport properties with the µh/µe ratio of 26.7. Such a charge carrier transport change is due to the strong electron‐donating nature of the selenophene. Furthermore, pDPPy‐DTF2 with electron‐withdrawing pyridine flanking units demonstrates unipolar n‐type charge transport properties with an electron mobility as high as 0.20 cm2 V−1 s−1. Overall, this study demonstrates a simple yet effective approach to switch the charge carrier polarity in transistors by varying the electron affinity of flanking groups of the diketopyrrolopyrrole unit.

Bar‐Coated Organic Thin‐Film Transistors with Reliable Electron Mobility Approaching 10 cm2 V−1 s−1

AbstractThe realization of high‐performance n‐type organic thin‐film transistors (OTFTs) via solution processing remains challenging, while these devices are considered highly desirable as basic elements for organic circuits. Apart from the designing of semiconducting materials, the controlling of thin‐film microstructures using specific processing techniques is also effective to enhance OTFT performance. In this work, bar coating—a meniscus‐guided and high throughput production compatible technique—is utilized to orient a diketopyrrolopyrrole‐based conjugated polymer (P4FTVT‐C32) for uniaxially aligned thin film with large crystalline grains, assisted by polyethylenimine ethoxylated interlayer. The anisotropic molecular arrangement of the bar‐coated thin films is confirmed by X‐ray diffraction, atomic force microscopy, and polarized UV−vis absorption analyses. OTFTs based on the oriented thin films exhibit remarkable charge transport anisotropy. High saturation and linear mobility up to 9.38 and 8.35 cm2 V−1 s−1 along with high on/off ratio of 105–106 is achieved when the electric field is parallel to the coating direction. These are the highest reliable electron mobilities obtained from solution‐processed OTFTs, considering their high reliability factor >80%. These results demonstrate that the electron mobility of conjugated polymers can be significantly improved by proper solution processing at optimized interfaces.

A Large Anisotropic Enhancement of the Charge Carrier Mobility of Flexible Organic Transistors with Strain: A Hall Effect and Raman Study.

Utilizing the intrinsic mobility–strain relationship in semiconductors is critical for enabling strain engineering applications in high‐performance flexible electronics. Here, measurements of Hall effect and Raman spectra of an organic semiconductor as a function of uniaxial mechanical strain are reported. This study reveals a very strong, anisotropic, and reversible modulation of the intrinsic (trap‐free) charge carrier mobility of single‐crystal rubrene transistors with strain, showing that the effective mobility of organic circuits can be enhanced by up to 100% with only 1% of compressive strain. Consistently, Raman spectroscopy reveals a systematic shift of the low‐frequency Raman modes of rubrene to higher (lower) frequencies with compressive (tensile) strain, which is indicative of a reduction (enhancement) of thermal molecular disorder in the crystal with strain. This study lays the foundation of the strain engineering in organic electronics and advances the knowledge of the relationship between the carrier mobility, low‐frequency vibrational modes, strain, and molecular disorder in organic semiconductors.

Understanding the Meniscus‐Guided Coating Parameters in Organic Field‐Effect‐Transistor Fabrications

AbstractMeniscus‐guided coating (MGC) is mainly applicable on the soluble organic semiconductors with strong π–π overlap for achieving single‐crystalline organic thin films and high‐performance organic field‐effect‐transistors (OFETs). In this work, four elementary factors including shearing speed (v), solute concentration (c), deposition temperature (T), and solvent boiling point (Tb) are unified to analyze crystal growth behavior in the meniscus‐guided coating. By carefully varying and studying these four key factors, it is confirmed that v is the thickness regulation factor, while c is proportional to crystal growth rate. The MGC crystal growth rate is also correlated to latent heat (L) of solvents and deposition temperature in an Arrhenius form. The latent heat of solvents is proportional to Tb. The OFET channels grown by the optimized MGC parameters show uniform crystal morphology (Roughness Rq < 0.25 nm) with decent carrier mobilities (average µ = 5.88 cm2 V−1 s−1 and highest µ = 7.68 cm2 V−1 s−1). The studies provide a generalized formula to estimate the effects of these fabrication parameters, which can serve as crystal growth guidelines for the MGC approach. It is also an important cornerstone towards scaling up the OFETs for the sophisticated organic circuits or mass production.

Electrohydrodynamic-Jet (EHD)-Printed Diketopyrrolopyroole-Based Copolymer for OFETs and Circuit Applications.

We report the employment of an electrohydrodynamic-jet (EHD)-printed diketopyrrolopyrrole-based copolymer (P-29-DPPDTSE) as the active layer of fabricated organic field-effect transistors (OFETs) and circuits. The device produced at optimal conditions showed a field-effect mobility value of 0.45 cm2/(Vs). The morphologies of the printed P-29-DPPDTSE samples were determined by performing optical microscopy, X-ray diffraction, and atomic force microscopy experiments. In addition, numerical circuit simulations of the optimal printed P-29-DPPDTSE OFETs were done in order to observe how well they would perform in a high-voltage logic circuit application. The optimal printed P-29-DPPDTSE OFET showed a 0.5 kHz inverter frequency and 1.2 kHz ring oscillator frequency at a 40 V supply condition, indicating the feasibility of its use in a logic circuit application at high voltage.

Analytical modeling for static and dynamic response of organic pseudo all-p inverter circuits

This paper presents the performance analysis of an all-p-organic pseudo-inverter circuit using dual gate organic thin film transistors. The proposed inverter design has shown significantly high performance in terms of noise margin, gain and propagation delay, leading to the design of more robust digital circuits that, too, exhibit augmented performance. The parameters of the all-p-organic pseudo-inverter are compared with those of zero-Vgs load logic (ZVLL) and dynamic load logic based conventional inverters, and a substantial improvement is found for the novel combination of a dual gate flexible TFT with a pseudo-design. Performance parameters were deeply analyzed, and we observed that the noise margin is improved by 42.8% as compared to ZVLL based conventional inverters. A bootstrap technique was implemented to further improve the performance and reduce the threshold voltage drop. The performance parameters were analyzed mathematically and compared with simulated values. The static as well as dynamic characteristics of organic pseudo-all-p inverter, with and without bootstrap technique, were observed. Static power consumption of the organic pseudo-all-p inverter was estimated. In this way, improvement of noise margin by the bootstrap circuit of the organic pseudo-inverter is characterized. The reduced threshold voltage applied to design of low power circuits enables longer power backup for various applications.

Precise Positioning of Organic Semiconductor Single Crystals with Two-Component Aligned Structure through 3D Wettability-Induced Sequential Assembly.

Highly ordered organic semiconductor single-crystal (OSSC) arrays are ideal building blocks for functional organic devices. However, most of the current methods are only applicable to fabricate OSSC arrays of a single component, which significantly hinders the application of OSSC arrays in integrated organic circuits. Here, we present a universal approach, termed three-dimensional (3D) wettability-induced sequential assembly that can programmatically and progressively manipulate the crystallization locations of different organic semiconductors at the same spatial position using a 3D microchannel template, for the fabrication of the two-component OSSC arrays. As an example, we successfully prepared two-component, bilayer structured OSSC arrays consisting of n-type N,N'-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide and p-type 6,13-bis(triisopropylsilylethynyl)pentacene microbelts. The bicomponent OSSCs show ambipolar carrier transport properties with hole and electron mobilities of 0.342 and 0.526 cm2 V-1 s-1, respectively. Construction of complementary inverters is further demonstrated based on the two-component OSSCs. The capability of integration of multicomponent OSSC arrays opens up unique opportunities for future high-performance organic complementary circuits.

Machine Learning for Understanding the Relationship between the Charge Transport Mobility and Electronic Energy Levels for n‐Type Organic Field‐Effect Transistors

AbstractOrganic field‐effect transistors (OFETs) have received considerably more attention than inorganic‐based field‐effect transistors for use in next generation of organic circuits. There are a number of variables, for example, the ordering of the OFETs, their energy levels, and the material used for source/drain electrodes, that influence the magnitude of charge transport mobility. Importantly, a suitable energy level match between highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy level and work function of the electrodes may have a large influence on the measured mobility. An informatics approach, specifically use of machine learning, is proposed for charge transport mobility prediction. Gradient Boosting and Random Forest regression algorithms are used to model previous experimental datasets and HOMO and LUMO energy levels of n‐type materials are optimized using expected machine‐learning methods. The results reveal that Random Forest model benefits the functional analysis of n‐type OFETs in three ways: 1) it provides better understanding of current n‐type organic materials, 2) it may guide the choice of n‐type organic materials and conducting electrodes, and 3) it measures the tradeoffs between the charge transport mobility and electronic energy levels for n‐type OFETs.

Multisubstituted Azaisoindigo-Based Polymers for High-Mobility Ambipolar Thin-Film Transistors and Inverters.

Ambipolar semiconducting materials have great potential in complementary-like organic logic circuits. Accessing such logic circuits demands balanced hole and electron mobilities. However, the lack of ambipolar high-mobility polymer semiconductors with balanced charge carrier-transporting properties precludes the rapid development of organic logic circuits. In this context, structural modification of semiconductor materials to enhance the electron/hole transport is of great urgency. Herein, a multifunctionalization strategy is used to achieve this goal. Combined electron-withdrawing moieties involving fluorine and pyridinic nitrogen atoms can not only reduce the frontier molecular orbital energies but also planarize the polymer backbone, demonstrating synergetic effects on the control over the carrier injection process at the metal-semiconductor interface and microstructure-sensitive charge transport in the channel. A balanced ambipolar behavior with electron/hole mobilities of 3.88/3.44 cm2 V-1 s-1 was observed, and complementary-like inverters with high gains of greater than 200 were achieved. Microstructure and thin-film morphology were characterized to further reveal the relationship between device performances and macroscopic observables. This multifunctionalization strategy bodes well for developing new ambipolar semiconducting materials.

Static and Dynamic Response Comparison of Printed, Single- and Dual-Gate 3-D Complementary Organic TFT Inverters

A dual-gate configuration has been introduced to organic thin-film transistors (TFTs) for full-depletion behavior to enhance the transconductance, subthreshold slope, and drain current on–off ratio. In this letter, we show how those dual-gate effects influence the static and dynamic responses of three-dimensional (3-D) complementary organic logic gates in comparison to their single-gate counterpart. For the comparative study, 3-D inverters and ring oscillators were fabricated by printing directly on plastic foil in both single- and dual-gate configurations. In the static inverter operation, the fully depleted characteristics of the dual-gate TFTs lead to an increase in voltage gain and a dramatic decrease in standby power consumption. In the dynamic ring oscillator operation, the two configurations have comparable gate delays because the use of an additional gate increases not only the drain current but also the load input gate capacitance. Our findings demonstrate the performance of an inverter that can represent the overall operation of complementary logic gates consisting of pull-up and pull-down networks, thereby providing significant insight into the printed organic TFT circuit design using dual gating effects.

Two-dimensional organic single-crystalline p-n junctions for ambipolar field transistors

Two-dimensional single-crystalline p-n junctions of organic semiconductors (pn-2DCOSs) show great potential in organic logic circuits due to their single crystal nature and excellent ambipolar charge transport. However, there are only few reports on pn-2DCOSs because it is difficult to obtain such highly ordered structure in p-n junction. Herein, a novel and effective solution processing method of secondary transfer technology based on the facile drop casting is used to fabricate devices of pn-2DCOSs based on C8-BTBT (p-type) and TFT-CN (n-type) successfully. The high-performance ambipolar field transistors based on such ultrathin pn-2DCOSs with several molecular layers thickness show well-balanced ambipolar charge transport behaviors with hole mobility as high as 0.43 cm2 V−1 s−1 and electron mobility up to 0.11 cm2 V−1 s−1, respectively. This work is essential for studying the intrinsic properties of organic p-n junctions and achieving high performance in organic complementary circuits.

Solution‐Processed Polymer Thin‐Film Memristors with an Electrochromic Feature and Frequency‐Dependent Synaptic Plasticity

Emerging organic synaptic devices are promising in exploring soft and biocompatible neuromorphic computing systems. By means of a simple and low‐cost solution process, the polymer thin‐film memristors are designed and characterized. The polymer memristors have a bilayer structure composed of an ion‐rich semiconducting layer and an ion‐collecting conducting layer, where the ion redistribution depending on device operation history has an impact on carrier concentration and results in the memristive characteristics. A visible electrochromic feature is observed during the electrical measurements, supporting the proposed Li+ ion drift/diffusion mechanism. The polymer memristors show the continuous and reversible conductivity tuning with a temporary memory effect, which enables the successful emulation of the frequency‐dependent synaptic plasticity. The polymer thin‐film memristors may offer a strategy for developing building blocks of organic neuromorphic circuits.

Monotype Organic Dual Threshold Voltage Using Different OTFT Geometries

It is well known that organic thin film transistor (OTFT) parameters can be shifted depending on the geometry of the device. In this work, we present two different transistor geometries, interdigitated and Corbino, which provide differences in the key parameters of devices such as threshold voltage (VT), although they share the same materials and fabrication procedure. Furthermore, it is proven that Corbino geometries are good candidates for saturation-mode current driven devices, as they provide higher ION/IOFF ratios. By taking advantage of these differences, circuit design can be improved and the proposed geometries are, therefore, particularly suited for the implementation of logic gates. The results demonstrate a high gain and low hysteresis organic monotype inverter circuit with full swing voltage at the output.