Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - HEROIC (High-frequency printed and direct-written Organic-hybrid Integrated Circuits)

Teaser

The HEROIC project aims 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 with maximum operation frequencies of 1 GHz and...

Summary

The HEROIC project aims 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 with maximum operation frequencies of 1 GHz and complementary integrated logic circuits switching in the 10-100 MHz range, 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, suggests that suitably downscaled printed polymer transistors with operation frequencies in the GHz regime, at least three orders of magnitude higher than previously printed polymer devices, are achievable, by addressing the specific challenges set in the HEROIC trans-disciplinary research programme: (i)development of scalable high resolution processes for the patterning of functional inks, where printing will be combined with direct-writing techniques such as fs-laser machining, both in an additive and subtractive approach; (ii)development of printable nanoscale hybrid dielectrics with high specific capacitance, where low-k polymer buffer materials will be combined with solution processable high-k dielectrics; (iii)improvement of the control of charge injection and transport in printed polymer and hybrid semiconductors; (iv)development of advanced printed and direct-written transistors architectures with low parasitic capacitances for high-speed operation. HEROIC will radically advance and expand the applicability of polymer-based printed electronics, thus making it suitable for next generation portable and wearable short-range wireless communicating devices with low power consumption.

Work performed

The HEROIC project is showing very good progress in the period. The main results which can be made public can be viewed online on the project webpage http://www.heroicproject.eu/.
Very promising results have been achieved in the high-resolution direct-writing of electrodes to be used in combination with printing techniques (Work Package 1, WP1). Efforts have been focused over the period to polymer conductors, such as PEDOT:PSS, and Ag nanoparticles AgNP inks, both commercial ones and ad hoc synthesized one, thanks to the collaboration of the IIT Center in Lecce.
The team in the period has been able to master high resolution fs-laser ablation of PEDOT:PSS (T1.2), suitable for the scaling of electrodes of fully polymer high-frequency transistors (Figure 1). It was shown the possibility of achieving high resolution ablated channels down to 12 µm length in between source and drain electrodes, on inkjet printed PEDOT:PSS conductive patterns on PEN plastic (T1.3, Figure 1). This result is in line Milestone 1.1 (High yield laser-ablated sub-micron channels with advanced inks on plastic).

Strong progress also in laser-sintered contacts (T1.2 and T1.3) according to the scheme of Figure 2 have been made, achieving controlled resolution on glass and plastic substrates.
Thanks to the described activities, two main results well beyond the state-of-the-art have been achieved. Result 1: with the use of laser ablated PEDOT:PSS contacts on PEN, a record fT = 4.9 MHz have been achieved in an all-polymer FET realized without the use of any mask by combining printing and direct-writing (T5.1, Bucella et al, All Polymer FETs Direct-Written on Flexible Substrates Achieving MHz Operation Regime, IEEE Transactions on Electron Devices 64 (2017) 1960-1967, DOI: 10.1109/TED.2017.2655943. Result 2: with the use of laser sintered contacts on glass, a record high fT = 20 MHz for a printed polymer based FET, again combining only printing and direct-writing processes. (T5.1, Figure 3, A. Perinot, et al. “Direct-written polymer field-effect transistors operating at 20 MHz”, Scientific Reports 6 (2016) 38941, DOI: 10.1038/srep38941).
The previous result was achievable thanks to a very important finding in the development of high mobility, fast printed polymers FET: through the control of the self-assembling properties of a polymer semiconductor, we unveiled a methodology that allows the alignment over large-area of polymer semiconductors thanks to fast coating processes, compatible with roll-to-roll-techniques (Bucella et al., Nature Communications 6, 2015, Article number: 8394; http://dx.doi.org/10.1038/ncomms9394). The method is based on inducing a high degree of preaggregation in solution, so that shear stress induced with fast-coating is effective in aligning polymer backbones. This process is scalable and orders of magnitude faster than previously reported alignment techniques (T3.3, Figure 4).

As planned in T3.4, we explored also high-mobility hybrid semiconductors as alternative active media for high frequency printed transistors. In particular, we’ve been testing different polymer-wrapped single-walled carbon nanotube (s-SWCNT) formulations deposited by inkjet printing. The formulations were provided by M. A. Loi, University of Groningen (The Netherlands). In Bucella et al. “Inkjet Printed Single-Walled Carbon Nanotube Based Ambipolar and Unipolar Transistors for High-Performance Complementary Logic Circuits” Advanced Electronic Materials 2 (2016) 1600094, we adopted P3DDT polymer chains to select only semiconducting chirality and, through an enrichment process, a stable formulation in oDCB with high loading (up to 0.2 mg mL−1) has been obtained resulting in a dense network of nanotubes already after a single printing pass. We obtained balanced ambipolar FETs, with maximum saturation mobility of 15 cm2 V−1 s−1.

Final results

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)
From the starting of the project, we improved our record frequency operation for printed and direct-written polymer transistors from a few tens of kHz up to 20 MHz (Figure 6).
Within the international context, we achieved the highest small-signal operative frequency, 20 MHz, for a FET based on a printed polymer which was fabricated without the use of any processing mask and by combining only low-temperature, printing and direct-writing processes (Perinot et al., “Direct-written polymer field-effect transistors operating at 20 MHz” Scientific Reports 6 (2016) 38941). Figure 7 summarizes the measured fT of organic FETs from selected works achieving the best results, which are classified with respect to the fabrication methods adopted for the patterning of the electrodes and for the deposition of the semiconductor (OSC) layer. The record fT reported to date reaches the value of 27.7 MHz for a device based on C60 with a channel length of 2 μm defined by photolithography. Therefore, with our work so far, we are very close to the absolute fT record for organic FETs, but achieved by using scalable coating techniques and laser-based direct-writing methods with a completely mask-less procedure. As a note, any other result close or above 10 MHz has been obtained in Japan.
Importantly, with such an approach we overcome 10 MHz, the threshold set for driving high resolution displays, thus qualifying our results as the base of a future high performance organic electronics technology.

FIGURES CAPTIONS

Figure 1. Sketch and optical microscopy images of the different geometries of source and drain electrodes realized through the fs-ablation direct writing process and using inkjet printed PEDOT:PSS as conducting material. a) Single cut, b) Small finger and c) Interdigitated.

Figure 2. fs-laser sintering process of AgNPs contacts on glass
Figure 3. (left) Fabrication process flow for OFETs integrating laser-sintered electrodes. (right) Measured transconductance and total gate capacitance for a device (Vgs = 30 V, L = 1.75 μm), and determination of the transition frequency.
Figure 4. Bar-coating at 3 m/min of a P(NDI2OD-T2) aligned polymer over large area, achieving both sub-monolayer films (0.5 g/l in mesitylene) and 10-20 nm films (5 g/l in mesitylene).
Figure 5. a) Absorption spectrum of HiPCO:P3DDT in ODCB after enrichment. The chiralities of the SWNTs present in the sample are indicated in red. In the inset, the structure of SWCNT-P3DDT hybrid is displayed. b) Sketch of the inkjet printing process for the deposition of s-SWCNTs solution. The nozzle diameter adopted has an orifice diameter of 60 µm. The spacing between consecutive drops was set at 100 µm and the printing speed was 50 mm/s. c-d) AFM maps of s-SWCNTs networks obtained by: c) one printing pass, d) two printing passes. Scale bars, 1 µm. e) Mobility plot extracted from devices made by increasing the number of printed passes (channel width W = 200 µm and channel length L = 40 µm). The holes mobility (blue lines) and electrons mobility (red lines) in linear regime (empty symbols) and saturation regime (filled symbols) are reported.
Figure 6. Evolution of internal results and results obtained in collaboration from the starting of the HEROIC project.
Figure 7. State-of-the-art for transition frequency for organic transistors, and progress in HEROIC.

Website & more info

More info: http://www.heroicproject.eu/.