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High-frequency printed and direct-written Organic-hybrid Integrated Circuits

Periodic Reporting for period 4 - HEROIC (High-frequency printed and direct-written Organic-hybrid Integrated Circuits)

Reporting period: 2019-10-01 to 2020-03-31

The HEROIC project aimed at filling the gap between the generally low operation frequencies of printed, organic flexible electronics and the high-frequency regime, by demonstrating polymer-based field-effect transistors capable of operating at radio-frequencies, fabricated by means of printing and direct-writing scalable processes in order to retain low temperature manufacturability of cost-effective large area electronics on plastic. The recent development of semiconducting polymers with mobilities in the range of 1 to 10 cm2/Vs, and even higher in the case of aligned films, suggested that suitably downscaled printed polymer transistors with operation frequencies above 100 MHz, could be achievable. HEROIC has succesfully demonstrated polymer field-effect transistors operating at 160 MHz, fabricated with a scalable, mask-less approach, by combining bar-coating and fs-laser sintering. This sets now the new record in terms of maximum operation frequency for organic transistors, and improves of 1000x the previous record devices based on solution-processed organic semiconductors. Such result makes it possible now to describe a roadmap to solution processed GHz electronics, something that was not even thought possible before HEROIC. This technology, in which EU has a solid leadership at the moment, have a strong potential for unlocking applications such as addressing electronics of flexible high-resolution displays and large-area imagers, as well as wireless communication for large-area printed sensors.
Main Achievements.
HEROIC achieved the current world-record for what regards the highest frequency of transition fT of an organic transistor, demonstrating a direct-written device, based on a printed polymer, capable of operating at 160 MHz.[1] This is the very first report demonstrating that an organic transistor can operate above 100 MHz, and represents a clear milestone towards signal transmission with organic circuits as well as more intense computation capabilities of the same.
In detail, in WP1 the combination of solution based processes, in particular inkjet printing and bar-coating, combined with fs-laser machining has been extensively investigated achieving two main results: demonstration of mask-less fabricated, all-organic, flexible polymer transistors with high resolution ablated channels operating at 4.9 MHz at 40 V,[2] and sintering of micron scale metallic electrodes for polymer transistors with an operation frequency up to 20 MHz at 30 V.[3] In WP2, first a hybrid approach based on a stack of a thin coated polymer and a physically deposited parylene layer was developed, to demonstrate low voltage transistors on plastic, operating down to 2 V with excellent electrical characteristics.[4] Then, fully solution-based approaches, to enable printability, were studied and developed, obtaining a very versatile multi-stack of a cross-linked low-k polymer and a high-k polymer, enabling low-voltage transistors.[5] Among the most relevant results of WP3, it was first developed a method to uniaxially align polymer semiconductors with a roll-to-roll compatible coating techniques, bar-coating.[6] A dedicated review paper was published on the topic.[7] Relevant results in structure-transport property relationship were obtained, with the rationalization of the microstructural origin of temperature independent transport in polymer field-effect transistors,[8] and in the understanding of the relationship between specific aggregation in polymer films and the relative balance in holes and electrons transport properties.[9] The results obtained in WP4 enabled the possibility to characterize high-frequency organic transistors with S-parameters,[10] veyr rarely adopted in the case of organic devices. All the efforts in HEROIC activities converged in WP5. The starting point in the group were fully printed organic transistors operating at around 25 kHz at 40 V.[11] In 5 years, HEROIC demonstrated direct-written polymer transistors operating at 160 MHz when biased at 40 V, currently the absolute record for organics, and polymer transistors fabricated on plastic achieving 22 MHz at 12 V,[12] the record for devices fabricated on plastic. This makes it possible now to describe a roadmap to solution processed GHz electronics.[13]

Exploitation.
Thanks to the results obtained with fs-laser writing of downscaled organic transistors, it was possible to successfully propose a technology transfer project under the call POR FESR 2014-2020 of Regione Lombardia. Such project, called iLabel, is worth in total more than 5 M€, is currently ongoing and targets the development of innovative, market-ready eco-friendly packaging electronic labels.

Dissemination.
HEROIC results were presented at several leading international conferences (MRS, E-MRS). HEROIC main achievements were covered also with posts on social media (LinkedIn, Facebook), and with outreach events (Milan Digital Week, Bergamo Scienza, and others). A very successful workshop was organized in Milan in 2017, with 15 leading scientists invited from EU, Korea, US, Saudi Arabia.
Figure 1 gives an immediate overview of the progress achieved in HEROIC in terms of maximum frequency of transition fT for transistors based on solution-processed organic semiconductors. Red symbols indicate works led by the HEROIC team,[14] dark symbols results in which the HEROIC team supported research led by other international groups. [15-18]

Figure 2 summarizes the measured fT of organic transistors from selected international works achieving the best results (blue symbols),[19-30] comprising all technologies, including devices with lithography patterned electrodes and evaporated semiconductors. After a first fast increase in fT from 2007 to 2011, when 27.7 MHz were achieved with a device based on an evaporated small molecule and a channel length of 2 μm defined by photolithography. Then a sort of plateau is present up to 2018. HEROIC started in 2015, targeting high-frequency with transistors based on printed semiconductors, fabricated without the use of any processing mask and by combining only low-temperature, printing and direct-writing processes. With a fast improvement, HEROIC reached fT values achieved with other technologies, and then surpassed them setting the new absolute record for organic transistors at 160 MHz.

REFERENCES
[1] manuscript under review 2020.
[2] IEEE TED 2017, 64, 1960.
[3] Sci. Rep. 2016, 6, 38941.
[4] Adv. Electron. Mater. 2018, 4, 1800340.
[5] manuscript in preparation 2020.
[6] Nat. Commun. 2015, 6, 8394.
[7] Adv. Mater. 2018, 30, 1705463.
[8] Nat. Commun. 2019, 10, 3365.
[9] Nat. Commun. 2019, 10, 5226.
[10] IEEE EDL. 2019, 40, 953.
[11] Org. Electron. 2015, 20, 132.
[12] manuscript under review 2020.
[13] Adv. Funct. Mater. 2020, 30, 1907641.
[14] Adv. Sci. 2018, 1801566.
[15] Adv. Electron. Mater. 2015, 1, 1500024.
[16] Appl. Phys. Lett. 2016, 108, 023302.
[17] IEDM, 1-5 Dec. 2018, 2018.
[18] AM-FPD, 2-5 July 2019, 2019.
[19] Nature Nanotechnol. 2007, 2, 784.
[20] IEDM, 7-9 Dec. 2009, 2009.
[21] Jpn. J. Appl. Phys. 2011, 50, 01BC01.
[22] Org. Electron. 2013, 14, 1656.
[23] Adv. Mater. 2014, 26, 2983.
[24] Adv. Mater. Interf. 2014, 1, 1300124.
[25] Org. Electron. 2015, 20, 119.
[26] Adv. Electron. Mater. 2015, 1, 1500155.
[27] Sci. Rep. 2018, 8, 7643.
[28] Sci. Adv. 2018, 4, eaao5758.
[29] Org. Electron. 2018, 54, 40.
[30] Sci. Adv. 2020, 6, eaaz5156.
Progress of HEROIC in terms of the maximum frequency of transition
Global progress in the frequency of transition of organic transistors