Printed electronics has been considered for a wide range of novel applications requiring non-conventional properties such as portability, flexibility and wearability and has opened the prospect of economically convenient mass production of electronic devices, owing to the suitability of the fabrication techniques and materials for low-cost, high-throughput industrial production. However, the optimization of some relevant figures of merit still has to be addressed appropriately to credibly enable the implementation of many real applications. Primarily, a transistor operation frequency suitable for more demanding applications must be guaranteed (e.g. more than 10 MHz in the case of RFID-based item-tracking systems or driving circuitry for flexible displays). Secondly, the correct operation of circuits should be achieved at a bias voltage compatible with batteries or energy-harvesting devices. In addition, such goals should be conveyed through a low-cost, industrially scalable production flow.
Here, we fabricated flexible low-voltage, high-frequency Organic Field-Effect Transistors (OFETs) exhibiting a transition frequency of 3 MHz at a bias voltage of 10 V. Such devices were realized only via the use of solution-based techniques (i.e. inkjet printing, femtosecond laser sintering, bar-coating), identifying a process flow which is suitable for the future scale-up to mass production. The high-frequency performance of the realized devices is ascribed to the combination of the high-resolution of the laser direct-writing of electrodes with the optimal charge transport properties of bar-coated poly[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene) (P(NDI2OD-T2)), yielding a charge mobility of 0.3 cm2/Vs (at a bias of 10 V) in devices with a channel length of 1.5 µm and with 1.4-µm-wide electrodes. Concurrently, the low-voltage operation was achieved thanks to a solution-processed dielectric stack integrating a low-k and a high-k material, which results in a specific capacitance of 39 nF/cm2 with a gate current leakage below 10 nA/cm2.
The proof of the feasibility of such low-voltage, high-frequency devices with the use of scalable solution-based techniques is a significant step forward towards the adoption of printed electronics in applications in the fields of smart tagging, wearable devices, large-area sensors and displays.