The primary driver for the development of organic thin-film transistors (TFTs) over the past few decades has been the prospect of electronics applications on unconventional substrates requiring low-temperature processing. A key requirement for many such applications is high-frequency switching or amplification at the low operating voltages provided by lithium-ion batteries (~3 V). To date, however, most organic-TFT technologies show limited dynamic performance unless high operating voltages are applied to mitigate high contact resistances and large parasitic capacitances. Here, we present flexible low-voltage organic TFTs with record static and dynamic performance, including contact resistance as small as 10 Ω·cm, on/off current ratios as large as 1010, subthreshold swing as small as 59 mV/decade, signal delays below 80 ns in inverters and ring oscillators, and transit frequencies as high as 21 MHz, all while using an inverted coplanar TFT structure that can be readily adapted to industry-standard lithographic techniques.

Despite the large body of research conducted on organic transistors, the transit frequency of organic field-effect transistors has seen virtually no improvement for a decade and remains far below 1 GHz. One reason is that most of the research is still focused on improving the charge-carrier mobility, a parameter that has little influence on the transit frequency of short-channel transistors. By examining the fundamental equations for the transit frequency of field-effect transistors and by extrapolating recent progress on the relevant device parameters, a roadmap to gigahertz organic transistors is derived.

Organic printed electronics has proven its potential as an essential enabler for applications related to healthcare, entertainment, energy, and distributed intelligent objects. The possibility of exploiting solution‐based and direct‐writing production schemes further boosts the benefits offered by such technology, facilitating the implementation of cheap, conformable, bio‐compatible electronic applications. The result shown in this work challenges the widespread assumption that such class of electronic devices is relegated to low‐frequency operation, owing to the limited charge mobility of the materials and to the low spatial resolution achievable with conventional printing techniques. Here, it is shown that solution‐processed and direct‐written organic field‐effect transistors can be carefully designed and fabricated so to achieve a maximum transition frequency of 160 MHz, unlocking an operational range that was not available before for organics. Such range was believed to be only accessible with more performing classes of semiconductor materials and/or more expensive fabrication schemes. The present achievement opens a route for cost‐ and energy‐efficient manufacturability of flexible and conformable electronics with wireless‐communication capabilities.

Limited charge carrier mobility of organic semiconductors, especially for solution-processed polymer thin films, has typically relegated organic electronics to low-frequency operation. Nevertheless, thanks to a steady increase in electronic properties of organics, much higher operation frequencies are feasible, suggesting a possible and appealing scenario where lightweight, cost-effective, and conformable electronics can integrate both sensing and radio-frequency transmitting functionalities, which are the key to unlock pervasive networks of distributed sensors revolutionizing human-environment interaction. Few years ago, it was suggested that gigahertz (GHz) field-effect transistors could be achievable even with solution-based processes. This was the basis for the European Research Council project high-frequency printed and direct-written organic-hybrid integrated circuits (HEROIC), which in the last few years investigated such unexplored path. Here, the authors report their vision toward the achievement of radio-frequency organic electronics mainly with solution-based and scalable processes, with reference to the experience of the HEROIC project and to some of the most notable literature examples. The authors show that the achievement of solution-processable organic field-effect transistors with GHz operation is indeed feasible, but requires considering a carefully revised scenario in which the main role is played by charge injection, together with the geometric overlap, the capacitive parasitism associated to fringing and some constraints on the dielectric layer thickness.

An optimized direct arylation polycondensation (DAP) protocol for the synthesis of a novel naphthalene diimide (NDI) 2,2′‐bithiazole (2‐BTz) copolymer (PNDI‐2‐BTz) is presented. The regioselective C–H activation of 2‐BTz at the 5‐positions allows for the synthesis of fully regioregular and homocoupling‐free PNDI‐2‐BTz of high molecular weight in less than 1 h in quantitative yield. Complete end group assignment shows functionalities according to monomer structures or to nucleophilic substitution, and allows for the reliable determination of absolute molecular weight. Compared to the well‐known bithiophene analog PNDIT2, an exceptionally high thermal stability, a hypsochromically shifted charge transfer absorption band and a lower‐lying LUMO energy level is found, making PNDI‐2‐BTz an interesting candidate for applications in organic electronic devices. In contrast to the selective and high yielding C–H activation of 2‐BTz at the 5‐position, the regioisomer 5,5′‐bithiazole is inactive under a variety of conditions.

Thanks to recent progress in terms of materials properties, polymer field-effect transistors (FETs) operating in the MHz range can be achieved. However, further development toward challenging frequency ranges, for a field accustomed to slow electronic devices, has to be addressed with suitable device design and measurements methodologies. In this letter, we report n-type FETs based on a solution-processed polymer semiconductor where the critical features have been realized by a large-area compatible direct-writing technique, allowing to obtain a maximum frequency of transition of 19 MHz, as measured by means of Scattering Parameters (S-Parameters). This is the first report of solution-processed organic FETs characterized with S-Parameters.

Organic field effect transistors are considered among the key enablers of flexible, wearable, lightweight and portable electronics. The possibility to fabricate polymer-based devices from solution with high throughput processes makes organic electronics suitable for low cost mass-production of new and diverse applications. Nevertheless, the investigation of the dynamic behaviour of these devices, i.e. their operational frequency, is still limited and the reported operational speed remains low, especially on flexible substrates, limiting their applicability in many circuitry applications. In this work, we report downscaled p-type polymer field effect transistors (FETs), fabricated on both rigid and flexible polymer substrates, by a combination of coating and direct-writing techniques, achieving radio-frequency operation. FETs with the shortest channel of 1.2 µm can operate up to 22 MHz at a relatively low bias voltage of −12 V, as measured by scattering parameters. Such a transition frequency (f T ) is the highest so far recorded for organic transistors fabricated on flexible substrates. In terms of f T normalized by the applied gate-to-source voltage (f T /V GS ), the achieved 1.83 MHz V −1 is the highest for solution-processed organic transistors on plastic. This result paves the way to the implementation of polymer radio-frequency devices with cost-effective fabrication processes on flexible substrates.

The development of organic thin-film transistors (TFTs) for high-frequency applications requires a detailed understanding of the intrinsic and extrinsic factors that influence their dynamic performance. This includes a wide range of properties, such as the device architecture, the contact resistance, parasitic capacitances, and intentional or unintentional asymmetries of the gate-to-contact overlaps. Here, we present a comprehensive analysis of the dynamic characteristics of the highest-performing flexible organic TFTs reported to date. For this purpose, we have developed the first compact model that provides a complete and accurate closed-form description of the frequency-dependent small-signal gain of organic field-effect transistors. The model properly accounts for all relevant secondary effects, such as the contact resistance, fringe capacitances, the subthreshold regime, charge traps, and non-quasistatic effects. We have analyzed the frequency behavior of low-voltage organic transistors fabricated in both coplanar and staggered device architectures on flexible plastic substrates. We show through S-parameter measurements that coplanar transistors yield more ideal small-signal characteristics with only a weak dependence on the overlap asymmetry. In contrast, the high-frequency behavior of staggered transistors suffers from a more pronounced dependence on the asymmetry. Using our advanced compact model, we elucidate the factors influencing the frequency-dependent small-signal gain and find that even though coplanar transistors have larger capacitances than staggered transistors, they benefit from substantially larger transconductances, which is the main reason for their superior dynamic performance.