Walking the Route to GHz Solution‐Processed Organic Electronics: A HEROIC Exploration

Abstract

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.

Organic Electronics Picks Up the Pace: Mask‐Less, Solution Processed Organic Transistors Operating at 160 MHz

“…Such performance should also be obtained with the sole use of mask‐less and scalable fabrication processes, in order to retain the manufacturability edge of organic devices. [ 15 ]…”

“…[ 19 ] This aspect is intertwined with the need for reduction of the capacitive parasitism related to the gate‐to‐source and gate‐to‐drain geometrical overlap, which, in the frame of the current‐crowding injection model, also affects charge‐injection in a non‐trivial way. [ 15 ] Finally, the design of efficient strategies for the dissipation of the generated heat becomes of paramount importance in order to prevent the destructive breakdown of the device and to allow for continuous‐mode operation: downscaled OFETs with channel lengths in the order of the µm, sustaining a current per unit width in excess of 1 mA mm −1 and voltages in the range of few tens of volts, need to dissipate efficiently a power density in the range 10 to 100 Wmm −2 , which can easily lead to thermal breakdown of the device. The latter is not surprising, considering that the constituting materials, in particular plastic substrates, are characterized by a very low thermal conductivity, making heat dissipation highly inefficient.…”

“…The total gate capacitance, at first order, can be estimated as follows: where L ov is the geometrical overlap between gate and source (or drain) electrode and 2 d , for low‐overlap structures of the kind presented here, accounts for the contribution of the fringing field in the form of an “equivalent overlap length”, equal to the thickness of the dielectric d . [ 15 ]…”

Coherent Electron Transport in Air‐Stable, Printed Single‐Crystal Organic Semiconductor and Application to Megahertz Transistors

“…Furthermore, the sufficiently low I D of 10 −12 A at zero V G (Figure S3, Supporting Information) is promising for low‐power devices. f T is given by [ 14,43 ] where μ eff denotes the effective carrier mobility. Taking the μ eff at L = 6 µm to be 0.58 cm 2 V −1 s −1 (Figure S3, Supporting Information), the theoretical f T is 2.0 MHz, which is almost consistent with our experimental results.…”

Organic Thin Film Transistors in Mechanical Sensors

“…[ 38 ] Given that there have been multiple reports of device mobilities around 10 cm 2 V s ‐1 , the roadmap provided by Zschieschang and others indicates that future studies should center on not only contact resistance but device geometries as well. [ 39,40 ]…”