An important strategy for realizing flexible electronics is to use solution-processable materials that can be directly printed and integrated into high-performance electronic components on plastic. Although examples of functional inks based on metallic, semiconducting and insulating materials have been developed, enhanced printability and performance is still a challenge. Printable high-capacitance dielectrics that serve as gate insulators in organic thin-film transistors are a particular priority. Solid polymer electrolytes (a salt dissolved in a polymer matrix) have been investigated for this purpose, but they suffer from slow polarization response, limiting transistor speed to less than 100 Hz. Here, we demonstrate that an emerging class of polymer electrolytes known as ion gels can serve as printable, high-capacitance gate insulators in organic thin-film transistors. The specific capacitance exceeds that of conventional ceramic or polymeric gate dielectrics, enabling transistor operation at low voltages with kilohertz switching frequencies.

Expanding applications for microelectronics in large-area sensor arrays, disposable sensor tapes, timeϪtemperature smart labels, radio frequency identification tags, and roll-up displays 1Ϫ4 motivate efforts to integrate electronics onto flexible plastic, paper, or metal substrates. A principal strategy for achieving flexible electronics is to employ graphic arts methods such as flexographic or ink-jet printing to pattern metallic, semiconducting, and insulating inks onto foils and paper. 5Ϫ10 Liquid phase printing offers the potential for high-throughput roll-toroll or sheet-to-sheet processing of electronics on large-area substrates, facilitating applications where large areas are necessary (e.g., displays) and also potentially translating into low production cost. Yet the challenge for printed electronics is to achieve high-performance circuits. The inherently low carrier mobilities of many printable organic or nanoparticle-based semiconductors lead to reduced transistor switching frequencies and high circuit supply voltages. Alternative strategies in which silicon chips are bonded to flexible substrates (by transfer printing or pick-andplace methods) are also attractive because they benefit from the superior electronic properties of silicon and the very advanced state of silicon microelectronics technology.11 In a competitive environment, the success of liquid phase printed electronics depends on substantial performance improvements, in particular, the development of faster, lower power printed circuits. Figure 1a displays a summary of reported signal delay times versus supply voltages for ring oscillator circuits based on organic semiconductors and carbon nanotube (CNT) arrays. It is evident that for nonprinted organic ring oscillators (open blue symbols) signal delays of 1Ϫ10 s have been achieved but only for supply voltages of 10Ϫ100 V, 12Ϫ24 while for supply voltages in the range of 4Ϫ10 V, the delay is above 10 s for the fastest circuits, with most displaying Ͼ1 ms switching times.25Ϫ28 Reports of printed ring oscillators are less common (solid green symbols in Figure 1a), and these circuits have generally required tens of volts to achieve switching times on the order of 1 ms.29Ϫ32 Such large voltages are not practical for many potential applications of flexible electronics where power will be supplied by thin-film batteries or radio frequency fields. Very recently, unipolar, p-type electrolyte-gated ring oscillator circuits have been demonstrated that indeed operate at very low

The fabrication and characterization of printed ion‐gel‐gated poly(3‐hexylthiophene) (P3HT) transistors and integrated circuits is reported, with emphasis on demonstrating both function and performance at supply voltages below 2 V. The key to achieving fast sub‐2 V operation is an unusual gel electrolyte based on an ionic liquid and a gelating block copolymer. This gel electrolyte serves as the gate dielectric and has both a short polarization response time (<1 ms) and a large specific capacitance (>10 µF cm−2), which leads simultaneously to high output conductance (>2 mS mm−1), low threshold voltage (<1 V) and high inverter switching frequencies (1–10 kHz). Aerosol‐jet‐printed inverters, ring oscillators, NAND gates, and flip‐flop circuits are demonstrated. The five‐stage ring oscillator operates at frequencies up to 150 Hz, corresponding to a propagation delay of 0.7 ms per stage. These printed gel electrolyte gated circuits compare favorably with other reported printed circuits that often require much larger operating voltages. Materials factors influencing the performance of the devices are discussed.

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Organic semiconductor single crystals gated with electrolytes exhibit a pronounced maximum in channel conductance at hole densities >10(13)   cm(-2). The cause is a strong decrease in the hole mobility with increasing charge density, which is explained in terms of a percolation model that incorporates trapping of holes by ions at the semiconductor-electrolyte interface. In the case of rubrene crystals, the peak channel conductance occurs at hole densities near 3 × 10(13)  cm(-2). The magnitude of the effect will be large for semiconductors with low dielectric constants and narrow bandwidths, and thus is likely to be a general phenomenon in organic semiconductors gated with electrolytes.

The transition from early adulthood to the elder is marked by functional and structural brain transformations. Many previous studies examined the correlation between the functional connectivity (FC) and aging using resting‐state fMRI. Results showed that the changes in FC are linked to aging as well as the cognitive ability changes. However, some researchers proposed that the FC is not static but dynamic changes during the resting‐state fMRI scan. In this study, we examined the correlation between the resting‐state dynamic functional network connectivity and age using resting scan data of 434 subjects. The results suggested: (a) age is negatively associated with variability of dynamic functional network connectivity state; (b) the dwell time of each age range spends in each state is different; (c) the dynamic graph metrics curve of each age ranges is different and 19–30 age range has the largest average global efficiency and average local efficiency; (d) some dynamic functional network connectivity measures were correlated to the certain cognitive ability. Overall, the results suggested the changes in dynamic functional network connectivity measures may be a characteristic of the aging process and in further investigations may provide a deep understanding of the aging process.

We report direct measurement of the electrochemical potential at organic semiconductor/gate dielectric interfaces in printed polymer transistors employing a gel electrolyte as the gate insulator. An oxidized silver wire reference electrode was embedded into the gel electrolyte, and its potential relative to the grounded source contact was measured simultaneously with the transistor transfer characteristics. The referenced turn-on voltages of transistors based on three common polymer semiconductors [(poly-3-hexylthiophene, poly(3,3‴-didodecylquaterthiophene), and poly(9,9′-dioctylfluorene-co-bithiophene)] were found to correlate with the reported highest occupied molecular orbital levels (ionization potentials) for these materials. Further, analysis of the transfer characteristics revealed a negative differential transconductance regime at high gate-induced carrier densities (∼1015 cm−2), which we attribute to a combination of band filling and a mobility lowering effect.

The authors report the fabrication and characterization of tetracene single-crystal field-effect transistors (FETs) utilizing an air or vacuum gap as the gate dielectric. The linear mobility of the device can be as high as 1.6cm2∕Vs in air, with a subthreshold slope lower than 0.5VnF∕decadecm2. By changing the orientation of the same crystal on the air-gap substrate, surface charge transport along different crystallographic directions was measured. There is pronounced anisotropy in the mobility; temperature dependent measurements show the mobility is activated (in contrast to air-gap FETs based on rubrene) and that the activation energy is independent of transport direction. Gate electrode displacement current was also recorded for these devices, allowing accurate determination of the gate induced surface charge and the fraction of trapped charge.