Symposium F.SF04—Solution-Processed Semiconductors and Devices for Form-Free Displays, Logic and Sensors

Low-voltage, high-performance thin-film transistors (TFTs) have been getting tremendous attention over recent years due to the increasing demand for low-power, portable electronic devices and sensor arrays. However, reducing the power consumption of TFTs is an extremely challenging task [1]. High-κ dielectric materials are thought to play an essential role in applications where a low-power device operation is needed.
In particular, tantalum pentoxide (Ta₂O₅) is a very promising candidate due to the high dielectric constant in the bulk (κ_bulk ~27) and as a thin-film (κ_thin-film ~20). These values are at least two times larger than that of Al₂O₃ (κ_bulk ~9) and five times larger than that of SiO₂ (κ_bulk ~3.9) [2]. As a result, Ta₂O₅ has been abundantly used in electrolytic capacitors, DRAM devices, and recently in solution-processed inorganic semiconductor thin-film transistors as a promising gate dielectric for low-power electronics [3].
whereas high-k insulators introduce undesirable effects at the semiconductor-insulator interface which generally leads to increased trapping which causes a rise of V_TH and SS, as well as lowers the field-effect mobility of charge carriers in TFTs [4]. To overcome both issues in low voltage organic thin-film transistors (OTFTs) passivating of the high-k metal oxides surfaces with self-assembled monolayer (SAM) compounds is routinely used [5]. Although modification of ultra-thin (d ≤ 20 nm) metal oxide gate dielectrics with SAMs is currently a common process in organic TFTs [6], this approach has not yet been explored in metal oxide semiconductor transistors.
In this talk, low-voltage a-IGZO TFTs using solution-deposited, OTS-modified Ta₂O₅ operating at 1 V are presented. The optimised devices display threshold voltages V_TH around 0.4 V, subthreshold swings (SS) below 90 mV/dec, current on/off ratios larger than 10⁵, and field-effect mobility in excess of 2.3 cm²/Vs. The morphology and dielectric properties of both pristine and OTS-treated thin Ta₂O₅ films have been studied. It is shown that the proposed approach has a high potential to enable the design of stable, low-voltage organic semiconductor circuitry in a highly reproducible manner.


References
[1] R. P. Ortiz, A. Facchetti, and T. J. Marks, “High-k organic, inorganic, and hybrid dielectrics for low-voltage organic field-effect transistors,” Chem. Rev., vol. 110, no. 1, pp. 205–239, 2010.
[2] S. Ezhilvalavan and T. Y. Tseng, “Preparation and properties of tantalum pentoxide (Ta2O5) thin films for ultra large scale integrated circuits (ULSIs) application -- A review,” J. Mater. Sci. Mater. Electron., vol. 10, no. 1, pp. 9–31, Mar. 1999.
[3] N. Mohammadian, S. Faraji, S. Sagar, B. C. Das, M. L. Turner, and L. A. Majewski, “One-Volt, Solution-Processed Organic Transistors with Self-Assembled Monolayer-Ta2O5 Gate Dielectrics,” Materials (Basel)., vol. 12, no. 16, p. 2563, 2019.
[4] Q. Wang, W. Wang, S. Jin, H. Zhu, and N. T. Zhang, “Quality-optimized joint source selection and power control for wireless multimedia D2D communication using stackelberg game,” IEEE Trans. Veh. Technol., vol. 64, no. 8, pp. 3755–3769, 2015.
[5] P. Barquinha et al., “Low-temperature sputtered mixtures of high-κ and high bandgap dielectrics for GIZO TFTs,” J. Soc. Inf. Disp., vol. 18, no. 10, pp. 762–772, 2010.
[6] P. Sista et al., “Enhancement of OFET performance of semiconducting polymers containing benzodithiophene upon surface treatment with organic silanes,” J. Polym. Sci. Part A Polym. Chem., vol. 49, no. 10, pp. 2292–2302, 2011.

Two-dimensional (2D) semiconductors have attracted extensive research interest due to their unique size, atomic thickness and band structure. In this respect, transition metal di-chalcogenides (TMDCs) are leading candidates because of strong in-plane chemical bonds and out-of-plane weak interactions. This allows them to be exfoliated into so-called 2D flakes, which can be micrometers wide but less than a nanometer thick. Among them, 2H-WSe₂ as a layered 2D material demonstrates remarkable properties such as low thermal conductivity, high carrier mobility, and direct band-gap, thus offering a suitable platform for next generation nanoscale devices [1-2].
Liquid-phase exfoliation (LPE) of bulk TMDCs can yield high-quality semiconducting flakes dispersed in commonly used solvents. Although a considerable number of research with LPE methods towards 2H-WSe₂ has been conducted, there are lack of reports on controlled organization of these exfoliated flakes on substrates. This step is crucial since produced 2H-WSe₂ flakes tend to restack into a bulk structure via van der Waals interactions, which significantly limit device applications. Langmuir-Blodgett (LB) assembly is an effective and easily integrated approach for arranging 2D flakes into highly ordered single-layer and multilayered films with controllable thickness, well-defined structure, and homogeneity.
Here, we present a cost-effective and facile technique for the controlled processing of 2D 2H-WSe₂ dispersions into uniform close-packed flakes via the LB method. Bulk 2H-WSe₂, synthesized using an atmospheric-pressure chemical vapor deposition (AP-CVD) technique, was exfoliated into mono- to few-layered 2H-WSe₂ flakes using low-boiling solvent (ethanol) at high-frequency bath sonication system. The compression of the liquid-exfoliated WSe₂ flakes on the water sub-phase was used to form a continuous layer, which was subsequently transferred onto a pretreated substrate. UV-Vis, Raman, AFM, HRSEM and HRTEM prove that the as-deposited material is successfully exfoliated from bulk 2H-WSe₂ into mono- to few-layered flakes. Furthermore, we studied the electrical properties of the FIB-fabricated 2H-WSe₂ flake device using a two-probe current versus voltage (I-V) measurements. The proposed technique provides a simple path to assemble structures of solution-processed 2D semiconductors avoiding the challenges related to high cost and high temperature processing for further applications in electronics and optoelectronics.

References
[1] Sahin et.al, Phys. Rev. B, 2013, 87, 165409.
[2] Allain et.al, ACS Nano, 2014, 8, 7180-7185.

Solution processing is an attractive alternative to standard vacuum fabrication techniques for the large-area manufacturing of metal-oxide (MO_x) based electron devices. In this frame, thin-film transistors (TFTs), which serve as the essential electronics building blocks, have to be primarily addressed. Furthermore, when these TFTs are realized on flexible substrates, many potential applications such as displays, chemical sensors, and conformable wearable devices, could be envisioned. However, the need for high-temperature sintering processes (T > 350 °C) has restrained thus far their commercial diffusion. The necessity for temperature-resistant polyimide as a substrate has restrained their field of application mainly to displays (e.g. indium gallium zinc oxide (IGZO) on polyimide). More economic and environmental-friendly substrates such as PET, cellulose, and biopolymers will become essential in future daily life applications requiring cheap and perhaps disposable devices. Among others, smart label sensors for temperature and chemical monitoring of food, realized directly in the packaging could be enabled. Accordingly, the development of a production process compatible with such thermosensitive substrates has to be addressed.

One of the key challenges for the full exploitation of these devices is the reduction of the thermal budget required for the MO_x synthesis. Our approach is based on the use of deep ultraviolet light (DUV) to promote the sol-gel synthesis reaction and therefore reduce the thermal budget of the process. The energetic photons generated by the DUV lamp are used to photo-activated the chemical reactions and initiating the M–O–M bond formation inside the deposited thin film. The quality of the MO_x layer is further enhanced by the heat treatment which eliminates the undesired reaction residuals and densifies the film. Similar photonic processes for solution-processed metal-oxide transistors have been mainly based on indium oxide (InO_x) as semiconductor material. The more convenient chemistry of InO_x facilitates its solution processing at low-temperature. However, the films obtained from these compounds can suffer from unstable behaviour and operation at negative voltages.
We report on TFTs based on a solution-processed indium zinc oxide (IZO) semiconductor utilizing a DUV enhanced curing, which enables a reduction of the processing temperature. A parametric analysis of the curing parameters has been carried out in to investigate the minimal conditions required to achieve performing devices. In this frame, we compared the efficiency of a post-annealing step in with in-situ combined DUV exposure and heating). Furthermore, the curing processes have been investigated using two different DUV light sources evaluating the different outcomes: a single-wavelength (172 nm) emitting excimer lamp and a broader emitting DUV lamp (184 nm/253 nm). The electrical characteristics of the TFT have been extracted and used to establish optimal curing conditions. Notably using DUV, short curing times as low as 10 minutes at 200 °C (e.g. 5 min DUV + 5 min post-annealing @ 200 °C) achieved mobility of 3 cm²/Vs. Higher mobility values (>15 cm²/Vs) have been achieved with prolonged processes (>80 min). While a temperature reduction to 180 °C yielded ~1 cm²/Vs. The effects of the different curing protocols on the chemical composition of the IZO films have been studied using Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy. The IZO semiconductor has been eventually paired with a printed high-k MO_x dielectric film, cured using the same methodology, to realize fully solution-processed MO_x TFTs, with as ultimate goal producing full printed TFTs. The identification of the most effective tool, combined with a fine-tuning of the parameters, allows to the DUV enhanced curing to diminish the thermal budget required to process IZO TFTs, paving the way for its further implementation on temperature-sensitive substrates.

Carbon nanotube thin-film transistors (CNT-TFTs) have attracted intensive attention due to their potential use in various applications, including display backplanes and sensing systems. Device uniformity, which is largely determined by the homogeneity and reproducibility of CNT thin films, is essential for large-scale manufacturing and practical applications. Here, we demonstrate an aerosol-jet printing approach to realizing CNT-TFTs with device characteristics that are among the most uniform of CNT-based transistors to date. The key enabling factors include proper ink formulation, polymer surfactant removal via rapid thermal anneal (RTA), and careful ink temperature control. Terpineol was added as a secondary solvent, and both optical and atomic force microscopy (AFM) reveal that the addition of this nonvolatile and viscous component efficiently suppresses the unwanted coffee-ring effect. The effectiveness of RTA-based polymer removal was supported by AFM results. It was discovered that a lower CNT ink temperature during printing offers higher ink output, increased CNT density, larger field-effect mobility, as well as enhanced device uniformity. This ink temperature-device uniformity relationship was further examined by fabricating batches of CNT-TFTs at a range of different ink temperatures, which revealed that low ink temperature (e.g., 8 °C) not only benefits uniformity of devices printed within a short period of time (~ 70 s), but also improves long-term (~1 h) printing stability. A set of several dozen CNT-TFTs printed from a CNT ink at 8 °C exhibited a mobility of 12.5 ± 0.5 cm² V^-1 s^-1, with a relative standard deviation σ(µ_h) / µ_h as small as 4%. These results unveil the role that ink temperature and other critical processing factors play in aerosol jet printed films and have the potential to address device uniformity issues that have previously plagued the utility of printed CNT thin films.

Organic semiconductors have the advantages of high absorption coefficients and wavelength selectivity compared with those of inorganic materials. As such, organic photodetectors (OPDs) are emerging as the next candidates for commercialized organic electronic devices, organic CMOS image sensors (OCISs). Wavelength-selective OPDs are applicable to the three-dimensional (3D) stacked structure that can replace the existing CMOS image sensors for the expansion of the light-input surface area by maximizing the photons absorbed effectively by each pixel, leading to high sensitivity and resolution since they can absorb selective wavelength from incident light without color filters.
To realize these advantages, systematic approach for molecular design of OPDs and optimized device architecture has been reported by our group recently. Noteworthy, through recent years of research on OPDs, the improvement of charge transport properties is also indispensable for commercialization of high-speed image sensors. Compared to silicon CMOS image sensors, some charges are slowly transported and remain in organic layer, then slow decays of photocurrent induced image lags closely related to remaining charges (RCs) in OCISs.
Here, combined with simulation and experiment, we designed and synthesized novel push-pull type molecules with different moieties by tuning the molecular conformations, which showed improved optical properties as well as charge carrier mobility. According to the improved device performance, we experimentally find out that charge transport properties including RCs are closely related to molecular structure with introduction of carbon-bridge and various types of side group. The effect of molecular conformations on the optoelectronic properties of high-speed OPDs will be discussed in detail.

Metallo Phthalocyanines (MPc) have been widely studied as promising material to develop sensitive gas sensor devices. In particular, its electrical response when exposed to gas analytes encourages its use in chemiresistors. This is so because they show some characteristics that can be exploited for the development of very sensitive gas sensors. When compared with other semiconductors, such as metal oxides, their baseline resistivity is relatively high and can change orders of magnitude when exposed to the sensing gas, which can be easily tuned through the incorporation of substituents in the periphery of the Pc platform. Moreover, there is little difference between their room temperature and higher temperature response. Recent publications report reproducible single digit parts-per-million sensitivities at room temperature, usually when preparing the material at the nanoscale.
This work summarizes advances in the synthesis, solution preparation, and gas sensing applications of CoPc and fluorinated CoPc nanowires and films with thicknesses below 20nm. In this case, room temperature parts-per-billion response is reported. Novel sensors prototypes based on F₁₆CoPc composites activated with Pt nanoclusters show significant reduction in their sensor response time. The report includes a systematic comparison with unsubstituted CoPc, and the response to different reducing and oxidizing gases. These new prototypes will be optimized for the detection of environmental contaminants in areas with high atmospheric relative humidity.

Quantum dot light-emitting diodes (QLEDs) are considered as next-generation displays due to the outstanding advantages of quantum dots (QDs) such as large color gamut by narrow emission, high brightness, solution-processability and potential long term stability. The performance of QLED is characterized by external quantum efficiency (EQE), which is the multiplication of internal quantum efficiency (IQE) and light out-coupling efficiency (EQE = IQE×light out-coupling efficiency). Recent progress has improved EQE of QLEDs for red, green and blue color to about 20%, which is on the level of commercial phosphorescent organic light-emitting diodes (OLEDs). Considering that the light out-coupling efficiency of a typical multilayer structure device is 20%, it can be seen that the IQE of QLEDs reached 100%. Therefore, research on light out-coupling efficiency or light extraction of QLEDs has been gaining attention to further improve the EQE.
In OLEDs, light is commonly extracted through corrugated structure or photonic crystal like internal light extraction nanostructure. Generally, these nanostructures are located between the multilayer structure and transparent electrode to interact with the target wavelength. However, applying this strategy for enhancement of QLEDs has difficulty even though the structural similarity of the multilayer structure. This is because multilayer structure of QLEDs is fabricated based on solution process while OLEDs are fabricated through vacuum deposition. Therefore, in QLED, the light extraction nanostructure strongly effects the morphology of the multilayer device causing degradation of the operation characteristic. Furthermore, very narrow QDs emission with 30nm of full-width at half maximum (FWHM) suggests the need for more advanced technique with finer and more precise nanostructures.
In this study, we demonstrate a metal oxide (MoO₃) nanostructure fabricated by modifying immersion transfer printing (iTP) method for internal light extraction of QLEDs. MoO₃ was applied as the internal light extraction material on the basis of high refractive index value (n) of 2.1 at 520 nm and semiconducting properties as a hole transport layer in optoelectronic devices. iTP is introduced as the fabrication method for its compatibility with the solution processed QLEDs and capability for forming high resolution MoO₃ nanostructure. Previous reported iTP, which uses hydrophobic particles, was modified to compensate for the incompatibility between hydrophilic MoO₃ particles and PDMS brushed substrates. PDMS-PS co-brush system was applied to control the surface affinity as a way to achieve MoO₃ incorporation onto the Si template. In addition, PDMS/PS bilayer was used as the picking carrier polymer to increase the transfer yield of the MoO₃ nanostructure. As a result, nanoscale line patterns with widths of 1000, 350, and 50 nm in large areas are well demonstrated. Finite-difference time-domain (FDTD) simulations are conducted to help design the precise size and shape of the pattern for light extraction to optimize the QLED internal light extraction. QLED was fabricated based on this design and showed improvement in the performance of the QLED device.

Enhancing device stability is essential to the widespread commercial adoption of organic electronic devices. Interactions of organic semiconductors with their environment often lead to degradation of device performance; therefore, it is important to understand how such devices as organic thin-film transistors and organic photovoltaics are affected by these interactions. This allows scientists and engineers to both choose appropriate applications for these materials and guide the development of new materials with enhanced stability. Polymers are of particular interest due to their capacity to be processed in solution for a new generation highly scalable and inexpensive electronic devices from sensors to solar cells. We have studied the stability of p- and n-type materials of interest for transistor and photovoltaic applications in environments relevant to commercial applications of these materials, including exposure to air, vacuum, and varied temperatures. In all cases, significant degradation could be observed. After 18 days of air exposure for p-type materials P3HT and PBDB-T, a 95% loss in hole mobility (µ_h) was observed for P3HT, while PBDB-T only experienced a µ_h loss of 42%.¹ For n-type polymers studied, a loss in electron mobility (µ_e) of 99% was observed for N2200 exposed to air for two weeks, while an 84% µ_e loss was observed for PIBDFBT-37.² Preliminary studies on polymers in organic photovoltaics indicate similar trends, but the degradation of charge transport properties does not give the full picture. We plan to further explore the impact of polymer degradation and charge transport on the stability of organic photovoltaics and transistors.

1. Brixi, et al., 2018, J. Mater. Chem. C, 6, 11972-11979
2. Brixi, et al., 2020, Sci. Rep., 10, 4014

Nano-characterisation techniques, including confocal microscopy, atomic force microscopy (AFM) and tip enhanced Raman scattering (TERS), rely on the ability of each instrument to resolve nano-scale features without distortions and artefacts, however, reference samples allowing to perform fully quantitative characterisation of their responses are not available. Calibration of lateral and depth resolution of advanced scanning-probe microscopy methods is currently very challenging and calls for the development of nanoscale standards with features in 10-200 nm range with various morphological and optoelectronic characteristics.
We demonstrate the development of solution-based, single-nanowire-based reference samples, where easily identifiable device positions each contain a semiconducting nanowire with a particular set of properties. Key parameters that are considered include: diameter and length, Raman response, photoluminescence and conductivity. The devices are self-assembled from a wide range of solution-processed semiconducting nanowires, including Si, Ge, InAs, InP, GaN, Ge/Si. Individual nanowires are positioned in pre-defined locations using electric-field assisted process, dielectrophoresis (DEP), by applying controllable DEP signal voltage and frequency. Nanowires are characterised, and various response metrics are analysed using conducting-AFM, confocal Raman, TERS and other techniques to establish which nanowire materials provide highly distinguishable responses, necessary for reference standards. Challenges in identifying most suitable nanowire materials, including their spectroscopic characteristics (e.g. Raman) and morphological features (e.g. diameters) are discussed and several reference samples for the characterisation of lateral and vertical resolution of nanoscale scanning probe techniques are suggested.

Copper oxide Cu₂O is an important and well known p-type transition metal oxide semiconductor material, which has already been employed in the fabrication of electronic devices. For example Cu₂O has been used in thin photovoltaic devices, resistive switching, transistors, gas sensors or catalysts. The films were fabricated by Pulsed Laser Deposition from CuO ceramic target by means of Nd:YAG laser operated at 266 nm wavelength. MgO(100) substrates were mounted on substrate holder 5 cm away from the target and maintained at temperature in the range 500-700 °C. Our attention was mainly focused on the influence of the oxygen pressure, which was varied between 10^-5 Pa and 1 Pa, on the structural and following on optical and electrical properties. The investigation of the plasma was carried out by Optical emission spectroscopy. The surface morphology and composition were characterised by AFM and XPS, respectively. The crystalline quality of the films were characterised by means of XRD and following by TEM and HRTEM, which confirmed the epitaxial grown of the films in low oxygen pressure up to 0.1 Pa. When the oxygen pressure exceeded 0.1 Pa the growing films started to be polycrystalline and the CuO phase also appeared. Because we focused on utilization of the Cu2O films as gas sensors, the near ambient pressure photoelectron spectroscopy was carried out to investigation of surfaces composition in the presence of gasses and vapours such as ethanol, hydrogen and NO2.

Conjugated polymers have a wide range of applications in optoelectronic devices and in numerous sensing technologies. Conjugated polymers that have thermally activated optical or electrical responses have garnished interest due to their potential as thermal sensors. These thermochromic properties are often introduced via doping. In this study, we investigate the use of electrochemically doped poly(3-hexylthiophene) (P3HT) thin films as multimodal temperature sensors. P3HT thin films spin coated on indium-tin oxide coated glass substrates are electrochemically doped using tetrabutylammonium perchlorate (TBAP) dissolved in acetonitrile. The pristine P3HT thin films, which are purple in color, exhibit a color change to blue, upon doping. The doped P3HT thin films are then heated under vacuum at 140⁰ C and the color of the thin films reverts from blue to purple indicating thermally activated dedoping. UV-visible absorbance spectra of pristine, doped and dedoped P3HT thin films show that the change in optical absorption properties upon doping is reversed due to thermally activated dedoping. Energy dispersive X-ray spectroscopy (EDS) reveal the presence of dopant molecules (TBAP) distributed throughout the doped P3HT film. Upon dedoping, the EDS maps show almost negligible levels of the dopant molecules. This further proves that heating of doped P3HT thin films triggers dedoping and causes the distinctive thermochromic response, i.e., a color change from blue to purple. The electrical properties of doped and dedoped films are also expected to result in a significantly different current upon application of voltage. This warrants fabrication of a current-based temperature sensor that would perform colorimetrically and electrically.

The increasing diffusion of portable and wearable technologies results in a growing interest in electronic devices having features such as flexibility, lightness-in-weight, transparency and wireless operation. Organic electronics was proposed as a potential candidate to fulfill such needs, in particular targeting pervasive Radio-Frequency (RF) applications. Still, limitations in terms of device performances at RF, particularly severe when large-area and scalable fabrication techniques are employed, have largely precluded the achievement of such an appealing scenario. In this work, we demonstrate a high-frequency rectifier based on fully inkjet printed organic Schottky diodes fabricated through scalable large-area methods on plastic. The inkjet printing of the organic semiconductor, the core of the diode structure, has been carefully optimized in order to obtain a low series resistance, which is mandatory to improve the diode cut off frequency. Moreover, the employed materials in the presented device have been chosen to finely tune the barriers formed at metal-semiconductor interface, in order to increase the rectification ratio of the diode: the obtained value is higher than 10⁶ which is, to the best of our knowledge, the highest value ever reported with a fully printed structure. Furthermore, we show, as a proof-of-concept, how a rectifier circuit integrating the fully printed diode, can successfully supply power to a polymer micro-electronic circuit printed on a plastic foil with a similar process. In particular, by rectifying a 13.56 MHz AC voltage the rectifier enables the correct operation of a D-Flip Flop, which is a fundamental logic building block for serialization elements. The possibility of harvesting electrical power from RF waves and delivering it to a cheap flexible substrate through a simple printed circuitry paves the way to a plethora of appealing distributed electronic applications.

Herein, doping/decoration of gold nanoparticles (AuNPs) within mesoporous silica-titania nanocomposite is
achieved via a facile and co-assembly sol-gel method. Polyethylene glycol is used as a co-structure-directingagent. For sensing analysis, a mixture of organic dyes i.e., bromophenol blue, phenol red, and cresol red is encapsulated in the AuST nanocomposite matrix. FESEM/EDX analysis shows a crack-free surface, porous network and uniform distribution of Au, Ti, Si, along with dye species. FTIR and XRD suggested the heterogeneous chemical bonding and crystallite size 24 nm after encapsulation. AuST nanocomposite shows thermally stable behavior after 450 °C even after co-dyes encapsulation. High surface area 322 ± 2.5 m2/g, pore diameter 30.2 Å, and pore volume 0.24 cm3/g, average surface roughness 10 nm, and refractive index 1.29 is advantageous for good sensing response at pH 1–12. The sensitivity is measured as 10 counts/pH at 441 nm. Moreover, good reversible response, fast response time 2.1 sec in acidic media, and 0.9 sec in basic media is observed.

Stretchable electronics has emerged as a next-generation technology that goes beyond rigid and flexible electronics to enable novel form-free devices and provide advanced user-experiences. Integration of rigid integrated circuit (IC) chips and inkjet-printed circuits on pre-strained elastomers has been regarded as one of the most powerful methods for fabricating stretchable multifunctional systems due to their unique advantages such as a facile and low-cost process, and a high degree of customizability. However, direct inkjet-printing on elastomers is challenging due to their low surface energy that causes dewetting. Furthermore, the super-hydrophobic surface shows poor adhesion with inkjet-printed metal films as well as conductive adhesives for chip bonding, significantly limiting system stability under deformation. Therefore, appropriate surface modification is indispensable for the robust implementation of inkjet-printing-based stretchable hybrid electronic system. Surface modification of elastomers has been realized by oxygen-plasma or ultraviolet/ozone (UVO) treatments. Both methods render the surface of the elastomers wettable by creating a large number of hydrophilic groups on it, where well-defined patterns can be formed by inkjet-printing. However, the methods have several limitations when applied to inkjet-printing on pre-strained elastomers. Plasma-treated elastomers undergo fast hydrophobic recovery due to the molar migration from the bulk to the surface, resulting in unstable wettability, uneven patterns, and poor adhesion. Alternatively, the UVO treatment provides improved stability and uniformity. However, a thick layer of the elastomer from the surface is stiffened by UV energy, which causes a large strain mismatch between the thick stiff surface and soft bulk. Consequently, the pre-stretched elastomer cannot completely return to its original dimension, restricting the maximum stretchability of the system far behind compared to the applied pre-strain. Therefore, a new surface modification strategy that simultaneously offers stable wetting and a high level of adhesion without compromising their stretchability is highly required.
In this work, we demonstrate robust stretchable hybrid electronic system based-on inkjet-printing and hybrid surface modification of polydimethylsiloxane (PDMS). The surface treatment offers stable wettability and great adhesion and preserves the substrate stretchability. Specifically, the pre-strained PDMS (mixed at 10:1) is exposed with oxygen-plasma to form hydroxyl groups and poly-L-lysine (PLL) solution (aqueous solution, 0.1% (w/v) in H₂O) is sequentially drop-casted on the plasma-treated PDMS. After that, the PDMS is rinsed with deionized (DI) water. Abundant amine groups functionalized on the surface of the PDMS enable stable wetting and good adhesion for inkjet-printing process. Furthermore, the modified surface shows remarkable adhesion with conductive epoxy, allowing robust IC chip bonding on the printed stretchable circuits. To investigate the effect of the hybrid surface treatment, we compare three biaxially pre-stretched PDMS substrates that undergo UVO, oxygen-plasma, and oxygen-plasma with PLL treatments, respectively. The plasma-treated PDMS results in uneven patterns and poor adhesion due to its instability and fast hydrophobic recovery. While the UVO-treated PDMS only shows stretchability of 19% when it is pre-stretched ~70%, the PDMS with hybrid surface treatment restores its original dimension, i.e., it shows the same stretchability as the pre-strain value. Finally, a series of stretchable hybrid electronic systems are demonstrated with their mechanical robustness, including stretchable LED displays, which proves the superiority of our hybrid surface treatment. The detailed methods and results will be discussed at the conference.
This research was supported by Samsung Research Funding & Incubation Center of Samsung Electronics under Project Number SRFC-IT1801-07.

We utilize 25% of our total electricity consumption for lighting and display applications. To address this consumption, several efficient lighting technologies have been targeted. Inorganic solid-state lighting together with organic light-emitting diodes (OLEDs) are the candidates spearheading the lighting revolution of the 21st century. However, while OLEDs have shown commercial success in rigid display applications, a number of challenges remain to be overcome in order to fulfil their full potential.
Among these are the continued quest for stable emitter materials and to improve strategies for the outcoupling of light from the device. Additionally, energy, temperature and cost efficient processing methods need to be targeted in order to fabricate the transparent conductive electrodes (TCEs) needed for device operation.
Today, transparent conductive oxides, and in particular indium tin oxide (ITO), represent the dominant class of TCEs. ITO combines high transparency (>85% in the visible wavelength range) with low sheet resistance (ca. 10 Ω sq^-1) when deposited on glass. As a result, most optoelectronic devices utilize ITO as the transparent electrode. However, ITO has some inherent limits in its range of applications. It is mechanically unstable upon bending, while its scarcity translates to a volatile and high price. It is therefore desirable to replace ITO with other transparent conductive electrodes [1].
Of particular interest are low temperature processes (<130 °C), since these are compatible with the use of temperature-sensitive and low cost flexible substrates, such as polyethylene terephthalate (PET). In this contribution we highlight our recent work on the design, fabrication and characterization of a number of different low temperature processes using, among others, inkjet printing and spray coating to produce TCEs [2,3]. These approaches are based on Ag and Cu nanoparticle, particle-free and nanowire inks. We will show that the developed ITO-free electrodes on PET show superior optical, electrical and mechanical performance compared to PET/ITO reference samples.

[1] Hermerschmidt et al., Adv. Mater. Technol. 4 (2019) 1800474.
[2] Hermerschmidt et al., Adv. Mater. Technol. 3 (2018) 1800146.
[3] Kinner et al., Phys. Status Solidi RRL (2020) 2000305.

In many professions, ionizing radiation is either a by-product of some process or is itself utilized, posing a significant hazard to those in the surrounding area. Workers will usually be required to wear a personal radiation dosimeter which measures and integrates the cumulative radiation dose experienced such that either prolonged or flash exposures are recorded and analyzed for the safety of the wearer. For personal dosimeter badges, the sensing element is usually made of relatively high-Z elements which absorb radiation differently than the lighter elements found in the human body.^1,2 An all-organic radiation dosimeter could more closely approximate the effect of radiation on biological tissues. Another added benefit of such organic-based devices is that they can be made quite cheaply and on a lightweight, flexible substrate. These factors together with the burgeoning IoT systems could allow for a more robust detector network in areas relevant to homeland security. Here, we report on an all-organic radiation dosimeter using the conducting polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)³ as both the detector region and conductive traces patterned on a polyethylene napthalate (PEN) substrate. Ionizing radiation incident on a central PEDOT:PSS pad induces a static charge in the air, the PEDOT:PSS itself, and the underlying PEN substrate that is measured as a voltage by the connected op-amp voltage follower.^4–6 Two organic field-effect transistors (OFETs) with the PEDOT:PSS source and drain, respectively, connected to the central PEDOT:PSS region allow for an initial biasing of the central pad, as the semiconductor 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) functions as two very large resistors. The OFETs allow for a very small amount of current to flow through the ungated semiconductor in order to place the central pad at a desired voltage, from which any deviations are measured. To clear the charge from the central pad, one or both of the OFETs can be turned on through the respective PEDOT:PSS gate electrode underneath a parylene dielectric. We show reproducible and reliable measurements of the radiation dosimeters subjected to a wide range of exposure energies of both X-ray and gamma rays using wide and focused beams resulting in an approximate detection limit of 5 mRad/hour. In addition, there is very strong evidence that the PEDOT:PSS-based devices can detect neutrons from a shielded and unshielded beryllium-encased plutonium source at a dose of 620 mRad/hour. This represents a significant step forward in the production of cheap, reusable radiation dosimeters made with materials of similar radiation cross-section to the human body.

1. Beckerle, P. & Ströbele, H. Charged particle detection in organic semiconductors. Nucl. Instruments Methods Phys. Res. Sect. A 449, 302–310 (2000).
2. Fraboni, B. et al. Organic Semiconducting Single Crystals as Next Generation of Low-Cost, Room-Temperature Electrical X-ray Detectors. Adv. Mater. 24, 2289–2293 (2012).
3. Schrote, K. & Frey, M. W. Effect of irradiation on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) nanofiber conductivity. Polymer. 54, 737–742 (2013).
4. Bardash, M. An organic semiconductor device for detecting ionizing radiation on a cellular level. Organic Semiconductors in Sensors and Bioelectronics III (Vol. 7779, p. 77790F). International Society for Optics and Photonics. 77790F (2010). doi:10.1117/12.861999.
5. Bardash, M. Solid state tissue equivalent detector, main component for a light-weight tissue equivalent microdosimeter. U.S. Patent 8,350,225 (2013).
6. Cavallari, M. R., Bardash, M. & Kymissis, I. Fully organic solid state tissue equivalent radiation dose (SSTED) detector (Conference Presentation). in Organic and Hybrid Sensors and Bioelectronics XII (eds. Shinar, R., Kymissis, I. & List-Kratochvil, E. J.) 4 (SPIE, 2019). doi:10.1117/12.2530701.

Electronic skin that can map pressure distribution on arbitrary surfaces has gained tremendous significance for its wide range of use in various fields such as skin prosthesis, artificial nerve systems for soft robotics, and advanced human-machine interfaces. To keep pace with these trends, many studies have focused on reporting high-performance pressure sensors and their passive-matrix array using novel materials and structures. Among various types of pressure sensors, capacitive pressure sensors with engineered microstructures offer significant potential owing to their high sensitivity, fast response time, and great mechanical flexibility. However, previous works on capacitive pressure sensors mainly focused on the performance of a single sensor, and their application to a high-resolution array was challenging because of their complex structure and fabrication process. Therefore, reported sensor arrays using capacitive pressure sensors generally showed low spatial fidelity due to the crosstalk among pixels and low pixel density. Furthermore, to exploit the capacitance change in practical applications, the circuit requires a capacitance-to-current converter that can limit the design freedom and make the system unstable on complex 3-dimensional surfaces. Therefore, device-scale integration between capacitive pressure sensors and thin-film transistors(TFTs) is highly desirable for capacitance-to-current conversion and further active-matrix applications. Although a few studies have reported pressure-sensitive thin-film transistors, they usually suffer from complex fabrication processes and limited flexibility due to their sophisticated structures such as sidewall spacers to make air gaps in dielectric layers.
In this work, we propose a facile and large-area compatible method for pressure-sensitive TFTs (PSTFTs) and their application to flexible sensor arrays. Our PSTFT is based on all-inkjet-printed single-wall carbon nanotube(SWCNT) TFTs, where a silver nanowire (AgNW) gate electrode with nanostructured surface morphology softly touch the top surface of the dielectric layer. The fabrication process of the PSTFT is fairly simple, highly customizable, and cost-effective. The SWCNT active channel, silver (Ag) source and drain electrodes, and poly(4-vinylphenol) (PVP) dielectric layer were sequentially inkjet-printed on a surface-engineered polyethylene naphthalate (PEN) substrate. The AgNW gate electrode was then formed on another flexible substrate by spray-coating. The 3D nanostructures in the AgNW random network form nano-scale air gaps between the gate electrode and dielectric. These nanoscale air gaps are key to the capacitance modulation in the PSTFTs. When the vertical pressure is applied to the top gate electrode, as the air gaps disappear and the contact area between the gate electrode and dielectric increased, the capacitance between the gate electrode and active layer drastically increases without any supporting spacer. Our PSTFT showed well-split transfer curves with on-off ratios of from 1 to 1.3*10⁴ according to the applied pressure. When the pressure reached 700 kPa, the TFT showed 1.3*10⁴ on/off ratio. We further demonstrate the feasibility of our approach by mounting the highly conformable sensor array on human skin to read out the change of the pressure such as finger touch and joint flexure. This study provides a new pathway to map pressure distribution using capacitive pressure sensors without any crosstalk among adjacent pixels, which can be further applied to any type of deformable substrates with potential applications such as skin-mounted device, deforming soft robotics, and display. The detailed methods and results will be discussed later.
This work was supported by the Technology Innovation Program (No.20008801, Development of muscular function management solution based on electronic skin with EMG IMU and Strain sensor) funded By the Ministry of Trade, Industry & Energy(MOTIE, Korea).

In this presentation we report the development of novel semiconductors, as well as thin-film engineering, for flexible and stretchable organic and inorganic (electrochemical)transistors and sensors. In particular we show that “ultra-soft” polymers comprising naphthalenediimide units co-polymerized with “rigid” and “flexible” organic units can change how charge transport is affected by mechanical stress, demonstrating that polymer backbone composition is more important that film degree of texturing. Furthermore, molecular design of polymers enables plasticization of a small molecule semiconductor film used in thin-film transistors. In addition, we report new “soft” transistor architectures using porosity as key element enhancing mechanical flexibility and tuned charge transport. Finally, metaloxide semiconductor nanostructures are fabricated in a very simple way and enable TFT and several types of sensors which can be monolithically integrated. These devices can better sense analytes, intercalate ions, and be chemically doped.

The general trend in semiconductor fabrications is the low thermal budget because of limitations on the minimum device dimension or the substrate material. For example, in IC fabrication, the process temperature can change the dopant redistribution, which directly affects the junction and interface properties. In large area flat panel displays, the glass substrate can only tolerate a low temperature, i.e., below the softening temperature. Since the solution process method can deposit thin films at a low temperature, it is compatible with various rigid or flexible substrates. Currently, there are many efforts in applying the solution process method to fabricate semiconductor products.

Among many types of low temperature prepared devices, thin film transistors (TFTs) are probably most popular because they have been critical components in the flat panel displays for 3 decades [1]. TFTs can also be used in a broad range of non-display products, such as chemical, gaseous, solution, bio, magnetic, and optical sensors as well as imagers, memories, and circuit drivers [2,3]. In TFTs, the semiconductor layers can be made of small or large organic molecules or inorganic silicon, metal oxide, or compound materials. The dielectric layer can also be made of organic or inorganic materials. However, in spite of many reports on solution process fabricated TFTs, no commercial products are available on the market. In this paper, a detailed analysis of some critical issues in this kind of device will be presented. For the device performance part, the bulk and interface defects, which may be of chemical or physical nature, are key factors to be improved. For the reliability part, the long-term stability, such as shifts of characteristics under the operation condition, will be addressed. Many disadvantageous material properties in the low temperature solution processed thin films and stacked structures can be improved with a low thermal budget pulsed rapid thermal annealing process [4,5]. A discussion on this method will be included in this presentation, too.

[1] Y. Kuo, ECS Interface, 22(1), 55-60 (2013).
[2] Y. Kuo, Amorphous Silicon Thin Film Transistors, Kluwer Academic Publishers, Norwell, MA, 2004.
[3] Y. Kuo, Polycrystalline Silicon Thin Film Transistors, Kluwer Academic Publishers, Norwell, MA, 2004.
[4] Y. Kuo and C. H. Lin, ECS 237th Meeting, Abstract # 130648, May 10-15, 2020.
[5] Y. Kuo, and C.-C. Lin, MRS Proc. Symp., 1426, 269-274 (2012).

Organic semiconductors have attracted considerable interest for spectrally-selective light sensing (i.e., narrowband photodetection), which is relevant to manifold applications—e.g., colorimetry, healthcare monitoring, biology, chemical fingerprinting.¹ The attractiveness of organic semiconductors for narrowband photodetection is especially due to the capability of many such compounds to absorb light precisely over the spectral range of interest, thereby enabling filterless narrowband-absorption-type organic photodetectors (OPDs). However, it has been generally challenging to date to realize such OPDs while achieving good spectral selectivity. This has been primarily due to the widespread use of fullerenes as electron acceptors, which bring along a detrimental shallow absorption tail through the visible range.² Additionally, while solution-based methods have been recognized as key to developing low-cost and/or free-form photodetectors and imagers, narrowband-absorption-type OPDs comprising solution-based active layers have thus far struggled to deliver high photoconversion efficiency.³

In this study we show that non-fullerene acceptors (NFAs) provide an attractive route to high-performance solution-processed OPDs with narrowband-absorption-type capability. We discuss the viability and general applicability of this approach by presenting the NFA-based solutions we have developed for narrowband-absorption-type OPDs selectively responding in the green range and the far-red range. In particular, we have studied two solution-processible NFAs: a benzodithiophene-based compound with narrowband absorption in the far-red range (solid-state spectral absorbance width of 132 nm) and a subphthalocyanine-based compound with narrowband absorption in the green range (solid-state spectral absorbance width of 80 nm). In both cases, we have explored solution-deposited photoactive layers consisting of bulk-heterojunctions in which the NFAs are combined with donors that are either transparent or that also selectively absorb in the target spectral range.

By tailoring our solution-processed NFA-based photoactive layers, we have achieved narrowband-absorption-type photodetection with cutting-edge performance. In self-powered operation, our NFA-based OPDs for the far-red range deliver the highest specific detectivity to date (1.4×10¹³ Jones) of all narrowband-absorption-type far-red-selective OPDs, and concurrently achieve the narrowest spectral width (141 nm) of all solution-processed implementations.⁴ Moreover, our green-selective NFA-based OPDs deliver a spectral with of 130 nm with an external quantum efficiency up to 40%, which is the highest to date for green-selective solution-processed narrowband-absorption-type OPDs. Our OPDs additionally show a linear response over an optical power range greater than four orders of magnitude, and exhibit a fast response compatible with a wealth of relevant applications. The versatility of our NFA-based narrowband-absorption-type approach marks an important step in the realization of solution-processible OPDs towards high-performance and low-cost colour sensors and imagers, and additionally motivate dedicated synthetic efforts in NFA research targeting narrowband OPD applications.

1 V. Pecunia, Organic Narrowband Photodetectors: Materials, devices and applications, Institute of Physics Publishing, Bristol, UK, 2019.
2 R. D. Jansen-van Vuuren, A. Armin, A. K. Pandey, P. L. Burn and P. Meredith, Adv. Mater., 2016, 4766–4802.
3 V. Pecunia, J. Phys. Mater., 2019, 2, 042001.
4 K. Xia, Y. Li, Y. Wang, L. Portilla and V. Pecunia, Adv. Opt. Mater., 2020, 1902056.

High-performance organic electronic devices require organic semiconductor (OSC) thin films to possess desired microstructures. An effective approach is use of graphene as a template for controlling growth behavior of OSC crystals. Unlike on ordinary substrates, the growth modes of OSCs on graphene are not only determined by the graphitic surface structure but also electronic characteristics of graphene. In this talk, I will discuss how OSC molecules assemble on graphene surface under the influence of graphene’s doping effect. Also I will discuss a result that the OSC films grown on a graphene template under an optimized condition provide favorable vertical and lateral transport pathways for charge carriers and excitons in organic transistor and photovoltaic devices

Controlled morphology evolution via directed assembly has played a central role in the development of modern electronic, optical and clean energy materials. In comparison to conventional ‘hard’ materials, organic electronics can be easily processed into diverse form factors by low-cost, high-throughput methods such as roll-to-roll printing and 3D printing. The printing conditions intimately couple with the assembly process and sensitively modulate the solid-state properties in the fabricated devices. A major challenge in this field lies in controlling the nucleation, growth, aggregation and alignment of conjugated polymers during solution printing and coating, which critically impact the printed device performance by orders of magnitude. The rapid printing process creates a complex environment with coupled physics that drive the polymer assembly far from equilibrium. Yet little is known of the non-equilibrium molecular assembly pathways and the printing physics so essential to tuning molecular conformation and multiscale morphology and the resulting intrachain and interchain charge transport properties of printed organic semiconductors.

In our work, we elucidate fundamental printing physics through finite element simulations and free energy modeling. By doing so, we are able to decipher non-equilibrium assembly pathways and control multi-scale assembly of semiconducting polymers. We further establish processing-structure-property relationship by performing printing experiments, morphology and device characterizations. We observe unexpected flow-induced morphology and electronic transition that accompanies change in printing regimes. We elucidate that printing flow in a moving, drying meniscus can drastically alter the polymer assembly pathways by flow-induced conformation change. We further design bioinspired assembly processes learning from biosystems, allowing molecules to put themselves together cooperatively into highly ordered structures otherwise not possible. High degree of morphology control from molecular to device scale further enables new insights into charge transport properties of semiconducting polymers and realizes advanced electronic devices such as printed 2D monolayer devices and ultrasensitive chemical sensors.

The implementation of different organic optoelectronic devices in a single architecture may represent a winning approach for the development of compact, highly-sensitive and -selective sensors.
Here, the monolithic integration of organic photodiodes (OPDs) onto organic light-emitting transistors (OLETs) is demonstrated, discussed and validated as a new platform for biosensing. While OLETs combine electrical switching characteristics with light generation capability, the deposition of OPDs directly onto the source electrodes of the OLETs provides the light-sensing ability to the whole multifunctional system, as a result of the spectral compatibility between OLETs and OPDs.
The potential of the platform is demonstrated in a real sensing application by depositing a nanoplasmonic grating on the top side of the encapsulating glass of the OLET+OPD chip. The nanoplasmonic thin film, which has been suitably designed and developed, has the ability to change the spectral reflectivity if a variation of the refractive index occurs on the top surface.
The effectiveness of the new detection scheme is validated by proving that the OPD photocurrent, generated as a consequence of the light emitted by the OLET and then reflected by the nanoplasmonic grating, changes by a factor 10^-9 A when exposing the sensing surface from water to alcoholic solutions at different concentrations.

In recent decades, a rising interest in flexible electronic devices has led to an increasing request for novel fabrication technologies for a large set of possible applications such as prosthetics, human-robot interaction, and rehabilitation. In this field, one of the major goals is the fabrication of flexible electronic devices by means of unconventional and highly compliant materials. In particular, organic materials have attracted significant interest in the scientific and industrial community due to their intrinsic mechanical characteristics and to the possibility to process them at very low temperatures at low costs, enabling new potential applications not achievable with inorganic counterparts.
In this work, we demonstrate that is possible to fabricate an all solution processed organic field effect transistor with a total thickness lower than 150 nm, which is the thinnest organic transistor ever fabricated, by adopting an approach based on solution-assisted delamination of freestanding ultra-thin layers of a well-known polymer insulator, namely poly(vinyl formal) (PVF). Thanks to the peculiar characteristics of the employed materials, the device shows an excellent transparency, together with an extremely high level of conformability. In fact, the proposed device is able to sustain bending to extremely small curvature radii and to conform on complex 3D surfaces, such as human skin, which makes it non-invasive and completely imperceptible, opening to a multitude of possible applications of our approach in several fields, from wearable electronics to implantable-electronics. The device fabrication relies entirely on solutions process techniques (such as spin-coating and ink-jet printing) and can be potentially up-scaled to an industrial size. Moreover, the devices show very reliable electrical performances, with mobility up to 0.18 cm²/Vs, and a very good reproducibility, which are requirements particularly challenging when large-area fabrication techniques and solution processed materials are employed.

Carrier mobility in conjugated polymers continues to increase with recent reports of field-effect mobilities exceeding 10 cm²/V.s. Charge transport is intrinsically dependent on processes occurring across multiple lengthscales. In order to access order parameters at the molecular scale we use charge modulation spectroscopy combined with theory. This technique allows us to further differentiate field-induced and doping-induced charges and study how their delocalization depends on local structure and disorder. Furthermore, we study the mesoscale organization of polymers using new techniques in the transmission electron microscope. By combining 4D STEM and HRTEM we are able to study the microstructure across a range of length-scales in real space and reciprocal space. These techniques are used on homopolymers and donor-acceptor copolymers and allow to extract information about the microstructure that is typically not visible by inspection. Such multiscale studies of microstructure are instrumental in guiding our understanding of charge transport in conjugated polymers.

Thermoelectric energy conversion is one of the key technologies for energy harvesting devices to convert waste heat into electric power, and vice versa. Particularly, for IoT (Internet of Things) society based on wearable electronics, flexible large-scale thermoelectric conversion devices are highly required. Therefore, as the active semiconductors of these devices, organic conducting polymers are strong candidates due to their low-temperature solution processability, compatibility with large-area deposition techniques, and intrinsic robust mechanical properties. However, the thermoelectric transport mechanism of these materials is still unclear due to their unique disordered nature, in particular the poor interconnectivity between crystalline domains. Here, we investigated their thermoelectric and carrier transport properties using the electrochemical doping technique and successfully maximized their performances [1].

In thermoelectric energy conversion devices, the Seebeck coefficient (= thermopower, S), which is the proportional constant to the voltage generation against an induced temperature gradient, is a significant factor in designing thermoelectric devices. Importantly, according to the Mott equation, S is proportional to the energy derivative of the electronic density of states at around Fermi energy; therefore, it is critically important to control the energy band filling. Conventionally, the band filling is tuned by chemical doping and, in most materials, it is very difficult to establish precisely controllable doping methods. Another important factor for thermoelectric energy conversion devices is the thermoelectric power factor S²σ (σ is conductivity). It should be strongly emphasized that the power factor has to be optimized to maximize the electric power output of thermoelectric devices. Consequently, this is a key parameter for applications. However, it is widely known that there is a trade-off between S and σ in terms of carrier density, n. Although σ is almost linearly proportional to n, S decreases with increasing n. Therefore, again, it is necessary to maximize S²σ by tuning n.

To overcome this limit, we developed electrochemical doping technique [2-6] and combined this technique with thermoelectric measurements [1, 7-12]. In particular, we applied this method into semicrystalline PBTTT and donor-acceptor copolymers [1]. As the results, we successfully maximized their power factor and clarified the behind physics.

[1] H. Tanaka, T. Takenobu, et al., Sci. Adv., 6, eaay8065 (2020)
[2] K. Matsuki, J. Pu, and T. Takenobu, Adv. Funct. Mater., 1908641 (2020)
[3] Q. Liu, T. Takenobu, et al., Adv. Electron. Mater., 6, 1901414 (2020)
[4] Y. Kawasugi, T. Takenobu, et al., Sci. Adv., 5, eaav7282 (2019)
[5] M.-H. Chiu, T. Takenobu, et al., Adv. Matter., 31, 1900861 (2019)
[6] J. Pu, T. Takenobu, et al., ACS Nano, 13, 9218 (2019)
[7] K. Kanahashi, J. Pu, and T. Takenobu, Adv. Energy Mater., 1902842 (2019)
[8] K. Kanahashi, T. Takenobu, et al., npj 2D Mater. Appl. 3, 44 (2019)
[9] J. Pu, T. Takenobu, et al., Phys. Rev. B 94, 014312 (2016)
[10] Shimizu, T. Takenobu, et al., SMALL 12, 3388 (2016)
[11] Y. Kawasugi, T. Takenobu, et al., Appl. Phys. Lett. 109, 233301 (2016).
[12] K. Yanagi, T. Takenobu, et al., Nano Lett. 14, 6437-6442 (2014)

Organic vertical transistors with conductive-network electrodes composed of carbon- or metal-based nanowires or meshes have been gaining increasing attention in recent years and several comprehensive reviews have been published [1, 2]. Differing from planar field-effect transistors (FETs) or thin-film transistors (TFTs), the conduction of the carriers in such devices occurs through vertical channels that are controlled by the gate field from the gaps between the nanowires or from the holes within the metal films [3]. Because of their submicron channel length, vertical transistors based on organic semiconductors (e.g., polymers or small molecules) with carbon-nanotube electrodes or metal meshes exhibit a large current density that can be used to drive light-emitting diodes [4]. The device structure also show great potential on flexibility, high degree of intergration. In addition, the layer-by-layer structure can be deposited by state-of-the-art printing technologies in industrial printing lines established for organic light-emitting diodes. However, the devices lack concise physical images to understand the operations and explicit design rules to achieve the necessary performance, such as sharp subthreshold swing, a large on: off ratio and and saturation in the high drain voltage (V_D).
Here, we develop a device theory with concise physical images, which are generally applicable for devices with organic or inorganic semiconductors. The simplified solution of Poisson’s equation reveals that the electrostatic potential at the semiconductor-dielectric interface is controlled by both the gate and drain field, behaving like a plucked string. The spacing between electrodes and the capacitance ratio between dielectrics and semiconductors are critical for achieving strong gate tunability of the interfacial potential, which can be maximized to achieve a sharp turn-on property toward the Boltzmann limit in the subthreshold regime. Above the threshold, the conduction channels in devices with Schottky contacts can change from the “L type” to “I type”, or vice versa, during scanning and the current-voltage relations can be well described by modifying classical transistor equations. The derived theories and equations agree well with the numerically simulated devices and reported experiments, revealing the physical images and providing explicit rules for designing, fabricating, and characterizing such transistors.
[1] J. Liu, Z. Qin, H. Gao, H. Dong, J. Zhu, and W. Hu, Vertical organic field-effect transistors, Adv. Mater. 29, 1808453 (2019).
[2] H. Kleemann, K. Krechan, A. Fischer, and K. Leo, A review of vertical organic transistors, Adv. Funct. Mater. 1907113 (2020).
[3] X. Fang, C.-H. Lin, Y.-T. Sun, H.-T. Chin, H.-W. Zan, H.-F. Meng, S.-F. Horng, and L. A. Wang, A solvent-free lift-off method for realizing vertical organic transistors with low leakage current and high ON/OFF ratio, Org. Electron. 31, 227 (2016).
[4] F. M. Sawatzki, D. H. Doan, H. Kleemann, M. Liero, A. Glitzky, T. Koprucki, and K. Leo, Balance of Horizontal and Vertical Charge Transport in Organic Field-Effect Transistors, Phys. Rev. Appl. 10, 034069 (2018).
[5] C. Liu, Z. Chen, K. Huang, S. Hu, X. Liang, and J. Chen, Vertical Transistors with Conductive-Network Electrodes: A Physical Image and What It Tells. Phys. Rev. Appl. 13, 054066. (2020)

Responsive modulation of neural networks is increasingly being used to treat patients with auditory-neurological disorders and neuropsychiatric diseases. Yet, current technology burdens neurostimulation tools with bulky, non-biocompatible electrical components that require rigid encapsulation for long-term implantation in body. Recently, we created a novel transistor architecture (internal ion-gated organic electrochemical transistors; IGT) that can be an efficient building block for integrated bioelectronics. These transistors include all the key features required for safe, efficient, and prolonged use of transistors in biological environments: i) they are made out of biocompatible and stable materials; ii) they are soft and conformable; iii) they show high speed and amplification to detect potentially low-amplitude ionic signals of the body; iv) they can perform certain computations.
Here, I am presenting the vision of our newly founded lab towards designing, based on that emerging technology, and developing novel fully implantable, contained and responsive neural interface devices that will allow long-term acquisition and closed-loop manipulation of neural circuits with high spatiotemporal resolution over extended period of time to reveal neural dynamics in the auditory-neurological pathway.

In this talk, I will focus on the high performance monolayer organic crystal and utilize it to develop an OFET with record high current density. The semiconductor is based on the solution processable DNTT derivatives and the deposition method is solution shearing. Among the four derivates, the best performance for the contact resistance is 25 ohm-cm at V_DS = -1 V, under a gate bias of -85V. The monolayer crystal could conduct unprecedently high density of current up to 19 μA/μm per channel width which is corresponding to 1.2 MA/cm² normalized by channel cross-section area. This current density is significantly higher than the reported values of other OFETs and also comparable with the inorganic transistors. Operating the OFETs with a high current density, however, would induce significant heating effect and a temperature rise of 110 K is obtained in the channel area. This inevitable wasted heat in the high power operation would lead to the thermal damage in the OFETs especially at the metal/organic contact interface. To suppress the thermal damage, a reduced voltage pulse width down to 10 μs and duty cycle down to 1% would still operate the device properly. Our work suggests that (i) monolayer crystal is a favored configuration in lowering the contact resistance; (ii) molecularly thin semiconductor could still conduct enormous amount of current; and (iii) thermal management is critical for future studies on OFETs, especially those with outstanding performance. Lastly, I will demonstrate how can use this high current density OFETs to drive different electronic devices such as LEDs and buzzers.

Printing organic small molecule semiconductors by solution shearing for the fabrication of highly performing organic field-effect transistors (OFETs) is currently of major technological interest. However, two challenging issues remain unsolved in order to implement these devices in real applications: reproducibility and long-term stability. The blending of the organic semiconducting molecules (OSC) with insulating binding polymers has proved to be an efficient route to ensure the formation of homogenous films over large areas with high device-to-device reproducibility. However, since molecules are bound together through weak van der Waals interactions, they are prone to polymorphism. Slight differences between polymorphs can lead to dramatic changes in their mobilities. Further, some technologically desirable polymorphs are metastable but can degrade with time to the most thermodynamically stable form, thus impacting device performance.
Recently, we have shown that the use of blends of OSCs with polymers by Bar-Assisted Meniscus Shearing (BAMS) gives rise to highly crystalline films.[1] In addition, the control of the deposition parameters (coating speed and temperature) as well as the modification of the ink formulation can be used as tools to tune the thin films morphology and the formation of thermodynamic and Kinetic polymorphs.[2] In a recent publication, we have also demonstrated that the blending approach can also be exploited to trap metastable polymorphs, which can lead to devices with both improved performance and long-term stability.[3]
The devices fabricated with this methodology have been applied for the development of X-ray detectors [4], biosensors[5] and for recording the activity of cells[6].

[1] I. Temiño, F.G.Del Pozo, A.Murugan, S. Galindo, J. Puigdollers, M. Mas-Torrent, Adv. Mater. Technol. 2016, 1,1600090.
[2] S. Galindo, A. Tamayo, F. Leonardi, M. Mas-Torrent, Adv. Funct. Mater., 2017, 27,1700526.
[3] T. Salzillo, A. Campos, A. Babuji, R. Santiago, S. T. Bromley, C. Ocal, E. Barrena, R. Jouclas, C. Ruzie, G. Schweicher, Y. H. Geerts, M. Mas-Torrent, Adv. Funct. Mater., 2020, 2007115.
[4] I. Temiño, L. Basiricò, I. Fratelli, A. Tamayo, A. Ciavatti, M. Mas-Torrent, B. Fraboni, Nature Communications 2020, 11:2136.
[5] S. Ricci, S. Casalini, V. Parkula, M. Selvaraj, D. Deniz, P. Greco, F. Biscarini, M. Mas-Torrent, Biosensors and Bioelectronics 2020, 167, 112433.
[6] A. Kyndiah, F. Leonardi, C. Tarantino, T. Cramer, R. Millan-Solsona, E. Garreta, N. Montserrat, M. Mas-Torrent, G. Gomila, Biosensors and Bioelectronics 2020, 150, 111844.

The first [BEDT-TTF = bis(ethylenedithio)- tetrathiafulvalene based quasi-twodimensional organic superconductor β-(BEDT-TTF)_²I₃ was first reported back in 1984.^[1] Soon it became clear that ion radical salts (IRSs) derived from BEDT-TTF exhibit tuneable electronic band structures; therefore, such molecules are excellent building blocks for engineering a rich and diverse family of organic crystalline metals and semiconductors. Electronic band structures of BEDT-TTF-based molecular conductors originate from ordered arrangements, such as stacks and layers, leading to metallic charge-transfer salts with partially filled bands.^[2]
One interesting characteristic of BEDT-TTF-based crystalline conductors is the very deformable molecular and crystal structure with strong electron−electron and electron−phonon couplings. Thanks to this, their anisotropic electronic structures exhibit many fascinating electronic and structural phase transitions caused by lattice deformations, which can be controlled by external stimuli such as light, temperature, strain, pressure, and humidity, among others. Nevertheless, it is necessary to engineer these crystals into a proper material for sensing applications. This was done by forming polycrystalline layers of IRSs, derived from BEDTTTF-based conductors, in nanocomposite bilayer (BL) films a strategy that allows combining electrical properties of IRSs with classical properties of insulating polymers, like flexibility, transparency, and solution processability.
Developing smart materials that can respond to an external stimulus is of major interest in artificial sensing devices able to read information about the physical, chemical and/or biological changes produced in our environment. Additionally, if these materials can be deposited or integrated on flexible, transparent substrates, their appeal is greatly increased. Such properties can be further tuned by choosing the nature of the IRSs enabling high sensitivity towards strain, pressure, temperature or even contactless radiation sensing i.e. bolometers.^[3,4] In a very recent example, bilayer films, composed of conducting polycrystalline layers of two dimensional BEDT-TTF-IRSs, hydroresistive sub-micron sized crystals on top of a polymeric host matrix permit to electrically monitor relative humidity in a stable and fully reversible fashion.^[5] This sensor platform enables the combination of high electrical performance of single crystals with processing properties of polymers towards a simple, low-cost and highly sensitive platform for applications in robotics, biomedicine and human health care.

[1] E. B. Yagubskii, I. F. Shchegolev, V. N. Laukhin, P. A. Kononovich, M. V. Kartsovnik, A. V. Zvarykina, L. I. Buravov, JETP Lett. 1984, 39,
12.
[2] J. M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser, H. H. Wang, A. M. Kini, M.-H. Whangbo, Organic Superconductors
(Including Fullerenes): Synthesis, Structure, Properties and Theory; Prentice Hall: Englewood Cliffs, NJ, 1992.
[3] E. Laukhina, R. Pfattner, L. R. Ferreras, S. Galli, M. Mas-Torrent, N. Masciocchi, V. Laukhin, C. Rovira, J. Veciana. Advanced Materials,
2009, 21, 1-5.
[4] R. Pfattner, V. Lebedev, E. Laukhina, S. Chaitanya Kumar, A. Esteban-Martin, V. Ramaiah-Badarla, M. Ebrahim-Zadeh, F. Pelayo García
de Arquer, G. Konstantatos, V. Laukhin, C. Rovira, J. Veciana. Advanced Electronic Materials, 2015, 1, 1500090.
[5] R. Pfattner, E. Laukhina, L. Ferlauto, F. Liscio, S. Milita, A. Crespi, V. Lebedev, M. Mas-Torrent, V. Laukhin, C. Rovira, J. Veciana, ACS
Applied Electronic Materials 2019, 1, 1781.

While lithography provides unparalleled control over dimensions, additive manufacturing techniques enable unique design flexibility. Here we consider the merits of hybrid fabrication involving the combined use of lithography and various additive manufacturing techniques in bioelectronics. We examine examples ranging from implantable sensors to neuromorphic transistors and discuss various fabrication strategies, showing the hybrid approaches offer several advantages.

Organic electronics has been developed for many novel applications towards large area and flexible electronics, but it has been traditionally considered only for low frequency applications. However roadmaps towards GHz operation have been recently proposed, exploiting improved electronic properties and optimized electronic device architectures. Such frequency range would be appealing for example to enable local area networks interconnecting large-area distributed and wearable sensors. Towards such applications, manufacturing processes comprising high-throughput printing techniques would be very appealing, although limitations in spatial resolution and in the control of electronic properties add a further obstacle to an already very challenging roadmap. In this contribution I will report on our progress in developing printed electronic devices capable of operating at radio-frequencies, targeting the High-Frequency range. In particular, I will show direct-written polymer field-effect transistors (FETs) operating above 20 MHz at low-voltage on plastic substrates, and fully inkjet printed diodes enabling rectifiers on plastic at 13.56 MHz. Finally, I will show recent results on FETs targeting the Ultra-High Frequency range, where local communication would be feasible.

2-Dimensional materials (2DMs) are very attractive building blocks for the next generation electronics, which require low cost and flexible devices that can be easily integrated onto substrates such as paper and plastic. The atomically thin nature of 2DMs enables mechanical flexibility, high sensitivity, easy functionalization, and allows one to stack together different 2DMs to fabricate multifunctional heterostructure-based devices.
In this talk I will show a general formulation approach to make water-based, defect-free and biocompatible 2DM-based inks suitable for fabrication of a wide range of fully printed devices, such as photodetectors, capacitors and transistors, on paper and plastic [1-3].
Furthermore, printed 2DM-based devices can be easily coupled with printed RFID tags made of graphene, enabling the development of a wireless and battery-free multisening platform on paper [4-6]. Finally, the inks can be easily combined with polycrystalline 2DMs produced by chemical vapour deposition, allowing quick fabrication of complex circuits on paper, such as high-gain inverters, logic gates and current mirrors [7].
If time allows, I will also introduce a new C-based, highly resilient and flexible substrate for printed electronics, specifically designed for applications that involve repeated strain, where paper cannot be used because the incresing fatigue in the cellulose substrate leads to failure of the device. We will show that this highly resiliant substrate can sustain over 50k bending cycles without failure, in contrast to paper, which starts failing already after 5k cycles [5].

[1] McManus et al, Nature Nano, 12, 343 (2017)
[2] Worsley et al, ACS Nano, 2018, DOI: 10.1021/acsnano.8b06464
[3] Lu et al, ACS nano 13 (10), 11263 (2019)
[4] Leng et al, 2D Materials 7 (2), 024004 (2020)
[5] Worsley et al, submitted
[6] Casiraghi et al, Carbon 129, 462 (2018)
[7] Conti et al, arXiv preprint arXiv:1911.06233

The promises of a printed electronics revolution continue to be hindered by challenges in the performance, reproducibility, and broad utility of printed devices. Inks from nanoscale materials have received increasing attention for their potential to overcome some of these hurdles, particularly in yielding direct-printed thin films meeting target electrical performance, air stability, and process compatibility. While many commercial options now exist for conducting inks, accessibility of versatile and robust semiconducting and insulating inks remains a challenge. In this talk, the recent progress on, and benefits of, semiconducting carbon nanotube (CNT) inks will be reviewed, particularly in the context of direct-write, aerosol jet-printed electronic devices. Proper control of ink properties and printing process, including factors such as ink temperature and printing time, can yield semiconducting CNT thin films with relatively high mobility, uniformity, and reproducibility. In addition, two attractive options for direct-write printed insulating inks will be discussed: hexagonal boron nitride (hBN) and crystalline nanocellulose (CNC). It will be shown how the appropriate design of these nanomaterial-based inks can enable the printing of fully recyclable electronics, where the constituent nanomaterials can be reclaimed and reused after initial printing and use in electronic devices.

Easy-to-fabricate, low-power and low-voltage thin-film-transistors (TFTs) that allow robust (complementary/complementary-like) circuit integration are highly sought-after for the development of off-the-grid smart sensors for the Internet of Things (IoT) ecosystem (1). Semiconducting single walled carbon nanotubes (sc-SWCNT) are a promising semiconductor material for TFTs with high field-effect mobilities and capable of low-voltage operation (2). However, fabricating sc-SWCNT TFT electronics with complementary (CMOS) characteristics typically requires complex fabrication processes, due to the several materials solutions (e.g. selective doping, different contact metals, multiple capping layers) required to this end, which also result in extra process steps (3). In this talk, we present our findings on the capability of ambipolar semiconducting single-walled carbon nanotube network (sc-SWCNTN) TFTs to deliver electronics that is easy-to-fabricate, low-power, (ultra-)low-voltage, and complementary-like. Our ambipolar TFTs comprise a sc-SWCNTN active layer deposited and patterned via aerosol-jet printing—with a maximum processing temperature of 120 °C—atop self-assembled monolayer (SAM)/AlO_x hybrid nanodielectrics. These TFTs exhibit symmetric ambipolar characteristics with balanced electrons and hole mobilities in the neighborhood of 10 cm² V^-1 s^-1, along with an onset voltage near 0 V and threshold voltages of < 1 V, all which enables these devices to operate within a voltage range of ~ 1 V.

The significance of our ambipolar sc-SWCNTN devices for low-voltage TFT electronics is assessed both experimentally and theoretically. We firstly discuss the impact of two key TFT parameters—subthreshold slope and flatband voltage—on the voltage and power requirements of circuits based on our ambipolar sc-SWCNTN TFTs. Moreover, by connecting our sc-SWCNTN TFTs in CMOS fashion, we demonstrate experimentally that the resultant inverters and NAND gates deliver CMOS-like performance with positive noise margins at supply voltages (V_DD) as low as 0.5 V and up to 2 V. On the high end of this V_DD range, inverter gates achieve gains > 80 V/V and bandwidths exceeding 100 kHz. Additionally, on the low end of this V_DD range, inverters are able to operate with subnanowatt static power dissipation, which is well aligned with the power requirements of off-the-grid applications. Importantly, in addition to its cutting-edge capabilities, our CMOS-like integration comes with the inherent advantage of using one single semiconductor for both n-channel and p-channel conduction, which allows a streamlined fabrication process compared to alternative approaches reported in the literature (e.g., involving dopants, different capping layers, different source and drain contact materials). In summary, we demonstrate that the combination of printed sc-SWCNTs and hybrid SAM nanodielectrics provides an attractive route to the realization of ambipolar sc-SWCNT TFT CMOS-like circuits for low-voltage/(ultra-)low power electronics.

References
1. J. A. Cardenas, J. B. Andrews, S. G. Noyce, A. D. Franklin, Carbon nanotube electronics for IoT sensors. Nano Futur. 4, 012001 (2020).
2. P. Prakash, K. Mohana Sundaram, M. Anto Bennet, A review on carbon nanotube field effect transistors (CNTFETs) for ultra-low power applications. Renew. Sustain. Energy Rev. 89, 194–203 (2018).
3. Y. Yang, L. Ding, J. Han, Z. Zhang, L.-M. Peng, High-Performance Complementary Transistors and Medium-Scale Integrated Circuits Based on Carbon Nanotube Thin Films. ACS Nano. 11, 4124–4132 (2017).

The past decades have witnessed a commendable progress in the development of materials for low-cost flexible electronics. New products that are beyond reach with current technologies can soon become a mainstay in our lives if these materials reach the necessary standards for performance and stability. The development of processing techniques that can address the manufacturability – scalability – sustainability – performance balance represents the next key step in the integration of these materials into next-generation electronic devices. In this presentation, I will introduce the use of laser printing, a rapid, scalable, environmentally friendly and low-cost manufacturing technique for deposition and patterning of various functional materials on flexible substrates. The first example focuses on organic semiconductors, and I will discuss the fabrication and characterization of organic thin-film transistors placed on plastic and paper. Aerosol spray laser lithography was used to define the electrodes, in conjunction with different metal inks: the pattern was created using a regular toner, which was subsequently selectively removed. We created a grid of transistor devices with variable channel lengths and widths and obtained good charge carrier mobilities and an excellent tolerance to bending. Next, I will describe the use of laser printing for the fabrication of metal halide perovskite films. The electrical properties of the laser printed films were comparable with those of the spin-coated layers, despite the fact that the microstructure consists of randomly oriented crystallites. The current-voltage characteristics exhibit negligible hysteresis and the electrical properties of the films are very stable under ambient conditions due to the fact that a vertical separation of the toner components results in an encapsulating the perovskite layer by the other toner components. Our work introduces an exciting new avenue for the fabrication of stable perovskite devices that is scalable and devoid of hazardous solvents.

Continuous, roll-to-roll (R2R) printing processes are attractive for manufacturing of flexible electronics. Two common challenges with R2R are reaching small feature sizes and achieving precise registration of multiple functional layers in devices such as transistors. This presentation will cover our efforts to overcome these issues using Self-Aligned Capillarity-Assisted Lithography for Electronics (or SCALE). SCALE has two steps. In the first step, R2R imprinting is used to create a multilevel open network of reservoirs, capillaries and device structures into a UV-curable coating deposited on a flexible substrate. In the second step, electronically functional inks are then delivered sequentially into the reservoirs on this imprinted substrate by inkjet printing, and capillarity pulls inks into capillaries and device structures. Since the imprint stamps are fabricated from high-resolution, Si master patterns created by photolithography, the features created in the imprint resin are also high-resolution, at the micron scale. The single imprinting step also simultaneously creates all the structural features needed to control the flow and deposition of multiple inks, resulting in self-alignment of multiple materials in complex multilayer devices.

To-date we have developed SCALE methods to create conductive networks, resistors, capacitors, diodes and transistors. For example, to manufacture high aspect ratio (thickness/width) SCALE conductive networks, we used a combination of inkjet delivery of a silver particle-free ink into reservoirs of the imprinted substrate, followed by capillary flow and thermal annealing to convert the ink to a silver seed layer and lastly electroless copper plating to build conductor thickness in capillary channels. By optimizing processing conditions and understanding the fundamentals of capillary flow, narrow (e.g., 10 μm), high aspect ratio (>1), low resistance (~1 ohm per cm of conductor length) embedded conductive traces were created. This presentation will show advances in device architecture and performance, and explore the key processing steps. Special attention will be given to continuous roll-to-roll processes.

Organic photodiodes (OPDs) are particularly well suited for future optical sensing technologies in the fields of mobile, wearable and skin-like electronics as they enable chemically tunable optoelectronic performance and fabrication by digital printing techniques on mechanically compliant substrates.⁽¹⁾ However, OPDs have typically utilized active layers developed for photovoltaic applications exhibiting broad absorption range and lacking the spectral selectivity necessary in fields like imaging or communication which require optical detectors that can distinguish between different wavelengths. To solve this, common approaches have relied on device engineering methods which result in devices with increased fabrication complexity.

This work introduces a general solution for inkjet-printing wavelength-selective bulk-heterojunction (BHJ) photodetectors through engineering of the ink formulation and demonstrate its potential for Visible Light Communication (VLC).⁽²⁾ Non-fullerene acceptors (NFAs) are incorporated in a transparent semiconductor polymer donor matrix to narrow and tune the response in the visible range without optical filters or light-management techniques. In this approach, the device spectral response solely depends on the choice of the NFA while the polymer donor dictates the rheological properties of the ink. Thus, this approach effectively decouples the optical response from the viscoelastic ink properties, simplifying process development. A thorough morphological and spectroscopic investigation of the novel BHJ systems finds excellent charge-carrier dynamics enabling responsivities up to 230 mA W⁻¹. This state-of-the-art response combined with high bandwidths >1.5 MHz, allows effective application in a VLC system. In this system, the complementary color-selectivity of the devices enables successful demultiplexing of simultaneously transmitted optical signals without the need of any additional optical filters or light-management techniques.

(1) N. Strobel, M. Seiberlich, R. Eckstein, U. Lemmer, G. Hernandez-Sosa, Organic photodiodes: printing, coating, benchmarks, and applications. Flex. Print. Electron. 4, 043001 (2019).
(2) N. Strobel, N. Droseros, W. Köntges, M. Seiberlich, M. Pietsch, S. Schlisske, F. Lindheimer, R. R. Schröder, U. Lemmer, M. Pfannmöller, N. Banerji, G. Hernandez-Sosa, Color-Selective Printed Organic Photodiodes for Filterless Multichannel Visible Light Communication. Adv. Mater. 32, 1908258 (2020).

The potential of the screen printing method for large-scale production of organic electrochemical transistors (OECTs), combining high production yield with low cost, is here demonstrated. Fully screen printed OECTs of 1 mm² area, based on poly(3,4-ethylenedioxythiophene) doped with poly(styrensulfonate) (PEDOT:PSS), have been manufactured on flexible PET substrates. The goal of this project effort has been to explore and develop the printing processing to enable high yield and stable transistor parameters, targeting miniaturized digital OECT circuits for large scale integration (LSI). Of the 760 OECTs manufactured in one batch on a PET sheet, only 2 devices were found malfunctioning, thus achieving an overall manufacturing yield of 99.7 %. A drain current ON/OFF ratio at least equal to 400 was applied as the strict exclusion principle for the yield, motivated by proper operation in LSI circuits. This consistent performance of low-footprint OECTs allows for the integration of PEDOT:PSS-based OECTs into complex logic circuits operating at high stability and accuracy.

Narrow bandgap polymers are used to realize infrared photodetectors that is relevant to a variety of applications including environmental monitoring and medical diagnosis. To achieve high detectivity in detectors, it is critical to reduce the device noise. This talk presents experimental and modeling studies on the noise current in exemplar organic bulk heterojunction photodiodes, with 10 donor–acceptor combinations spanning wavelength between 800 and 1600 nm. A significant reduction of the noise and higher detectivity were found in devices using non-fullerene acceptors (NFAs) in comparison to those using fullerene derivatives. The low noise in NFA blends was attributed to a sharp drop off in the distribution of bandtail states, as revealed by variable-temperature density-of-states measurements. Taking disorder into account, we developed a general physical model to explain the dependence of thermal noise on the effective bandgap and bandtail spread.

The narrow bandgap polymers are also Faradaic materials for energy storage and present opportunities to create energy dense supercapacitors. The vast majority of conducting polymers are p-type, and here we present promising n-type polymers that are stable when operating within a large potential window (3V), with a best-in-class energy densities (30.4 Wh/kg at a 1 A/g discharge rate), and a long cycle life (90% capacitance retention after 5000 cycles) critical to energy storage and management. This work demonstrates the application of a new class of stable and tunable redoxactive material to realize high-endurance energy storage devices for flexible printed systems.

Recently, metal-halide perovskite materials are considered as a promising candidate for lasing applications. Compared to 3D and other low-dimensional perovskites, quasi-2D perovskites are a better gain media due to its unique properties such as large binding energy and self-assembled quantum wells. Specifically, efficient energy funneling from low-dimensional domains to high-dimensional domains will not only enhance the radiative recombination but also facilitate population inversion.
In this presentation, we will discuss the fundamental requirements for population inversion in perovskites in the context of material dimensionality and morphology. We will also present our recent work on laser cavity design and processing for optimum laser performance.

The conversion of light energy into electricity and chemical fuels through photoelectrochemical (PEC) cells represents a powerful strategy for sustainable fuel and chemical generation, environmental remediation (i.e., pollutant degradation), advanced analytical systems (i.e., chemical sensors) for environmental and biological monitoring, as well as innovative self-powered photodetectors. In particular, aqueous PEC cells, including water splitting ones, are emerging for the development of cheap, easily fabricated, environmentally friendly self-powered photodetectors with high spectral responsivity. In this context, two-dimensional (2D) materials, including either single- and few-layer flake forms, are continually attracting utmost interest as potential advanced photo(electro)catalysts. Recently, group-III and group-IV transition metal monochalcogenides, which can be exfoliated in 2D form due to their low cleavage energy (typically < 0.5 J m^-2), have been theoretically predicted to be water splitting photocatalysts. In fact, their 2D nature intrinsically guarantees that the charge carriers are directly photogenerated at the interface with the electrolyte, where redox reactions take place before they recombine. Moreover, their electronic structure can be tuned by controlling the number of the layers to fulfil the fundamental requirements for water splitting photocatalysts, i.e.: 1) conduction band minimum (CBM) energy (E_CBM) > reduction potential of H⁺/H₂ (E(H⁺/H₂)); 2) valence band maximum (VBM) energy (E_VBM) < reduction potential of O₂/H₂O (E(O₂/H₂O)). Among them, low-cost and environmentally friendly layered gallium sulfide (GaS), gallium selenide (GaSe) and germanium selenide (GeSe) are promising materials candidate for optoelectronic devices due to their properties: tunable electronic structure, strong visible-light absorbance, significant photoresponse due to direct-bandgap transitions, photoferroelectricity and environmental stability. Here, we report the first experimental characterization of the PEC water splitting activity of single-/few-layer flakes of GaS, GaSe and GeSe produced in form inks by scalable liquid-phase exfoliation (LPE) approach in non-toxic solvents. The as-produced dispersions were deposited by spray-coating technique to conceive solution-processed self-powered PEC-type photodetectors. The PEC behaviour of MCs-based photoelectrodes was evaluated in different aqueous media, ranging from acidic to alkaline solutions: 0.5 M H₂SO₄ (pH 0.3), 1 M Na₂SO₄ (pH 6), 1 M KCl (pH 6.5), 1 M KOH (pH 14) under different illumination wavelengths in the visible spectral range, namely 455, 505 and 625 nm. Moreover, due to its large band gap, GaS was also tested under UV radiation (275 nm) obtaining an UV photodetector without using any visible light filter. Instead, GaSe and GeSe photoelectrodes show a responsivity up to ~0.16 A W^–1 and ~0.32 A W^–1, respectively, upon 455 nm illumination at light intensity up to 63.5 µW cm^-2. The obtained performances are superior to those of several self-powered and low-voltage solution-processed photodetectors, approaching the ones of self-powered commercial UV-Vis photodetectors. Our results can open the way towards the use of 2D metal monochalcogenides in innovative PEC systems, (bio)sensors and other innovative optoelectronics devices.

Multispectral and polarimetric imaging provides invaluable information undetectable by the human eye, leading to significant advancements in fields that include biomedical imaging, astronomy, and agriculture. Current methods for snapshot multispectral and polarization imaging require an array of adjacent pixels to create a super-pixel, where each pixel serves to measure a subset of the optical information. However, this super-pixel suffers from significant spatial and sampling error. To overcome these limitations, we introduce a new detector strategy inspired by stomatopods, or mantis shrimp. Mantis shrimp display one of the most intricate vision systems in nature, featuring 16 types of photoreceptor cells capable of perceiving light ranging from UV to far-red, as well as polarized light. We refer to our sensor as a Stomatopod-Inspired Multispectral and POLarization (SIMPOL) sensor. The SIMPOL sensor leverages two key technologies: (1) polarization sensitive semitransparent organic photovoltaics (P-OPVs), and (2) folded retarder (FR) films. The FRs enable finely tuned polarization rotation for different wavelengths of light. The P-OPVs then preferentially sense light polarized with a specific orientation. By pairing the P-OPVs and FRs, we create a versatile sensing platform for narrow optical bandwidth detection and polarization sensing. The SIMPOL detector design consists of a cascading series of P-OPVs and FRs and share many features of the mantis shrimp eye, enabling the realization of multispectral and polarization sensing in a single pixel for the first time.

The function of the SIMPOL sensor is critically dependent on the performance of the P-OPVs, which are required to be highly polarized, and highly transparent along the non-absorbing electric field orientation. The polarization sensitivity of these devices is achieved by orienting the polymer semiconductors in the plane of the film. Here, we introduce a strategy of high temperature rubbing on a low molecular weight all-polymer bulk heterojunction film to achieve a high degree of in-plane alignment. We demonstrate a blend of PBnDT-FTAZ and P(NDI2OD-T2) subjected to the high temperature rubbing results in dichroic ratios of over 18 and photocurrent ratios of over 7 when under orthogonally polarized illumination. This represents the most polarization sensitive OPVs demonstrated to date. The SIMPOL sensor consists of six stacked P-OPVs enabling the detection of four spectral channels with a spectral resolution of up to 16.9 nm, while simultaneously measuring the linear polarization state of the light. We demonstrate the potential of the SIMPOL detector by imaging a scene that includes broad spectral and polarization features, representing a significant advancement in spectral and polarization imaging.

Recently, the development of the existing flat panel display technology has reached an extreme level, interest in the new display form factor has been increased more than ever. Since those researches began, foldable display, which has been successful in establishing itself in the mobile display market since 2019. In additon to the design aspect, it succeeded in receiving the customer’s choice in terms of functionality, such as the expansion of the screen, and is expected to continue to develop in the future. Rollable display technology is also expected to be applied to various products in near future beyond the research level.
In the last decade, stretchable display technologies have attracted much attention as a next step after foldable and rollable displays. A stretchable display capable of realizing a free-form that is impossible with exisiting display technology is expected to be applied as a field of new products. Free-form display, as a high potential and challenging research field, have brought interest to the issue of how to make stretchable behavior that offer both durable and reliable characteristics with conventional and/or newly developed electronic materials. Although there were several efforts to make intrinsic stretchable display based on soft materials with stretchable electronics, the resolution of pixel images and long-life stablity should be improved.
We report on the latest advances in stretchable display based on OLED technology and discuss the recent progress in the design of stretchable display by comparing the key design consideration in terms of substrate, materials, and geometries. We will also introduce the freeform stretchable display based on low temperature poly-silicon (LTPS) active-matrix OLED (AMOLED) technology. This research has introcuced a stretchable display that operates normally without degradation of image quality even after convex or concave static formed shaping using a low-temperature vacuum thermoforming process. In addition, repetitive deformation was applied in a vertical direction to stretch the center of the display panel while the panel was in operation. After applying tensile elongation up to 50,000 cycles using a repetitive stretching machine, no significant difference was observed in the color coordinate measurement depending on the deformation.

With the rapid aging of Japanese society, how to increase quality of life (QoL) while controlling rising medical expenses has become an urgent issue. To solve this difficult issue, the acquisition and utilization of biological information using new technologies such as wearable devices is more expected. In particular, self-care and home medical care, in which patients and their families are responsible for their own health, are considered as one of the way to solving the issues of a super-aging society. Indeed, wearable sensors that can constantly monitor health conditions and home-use blood pressure monitors with communication functions are being introduced to the market one after another to prepare for the advent of a self-care era.
On the other hand, when designing a new insurance system or incentive system using biometric information from wearable sensors, it is important issue how to confirm whether data measured at home is the patient's own. Furthermore, the risk of patient mix-ups must be reduced as more wearable devices come to be used in hospitals and welfare facilities in the future. Therefore, measuring vital signs simultaneously with biometric authentication of the user is an urgent issue.
We have developed a sheet-type image sensor that enables high-resolution and high-speed reading. This sheet-type image sensor can take the high-resolution image of fingerprints and veins used for biometric authentication. In addition, the same sheet-type imager can measure the pulse wave which is one of the vital signs, and its distribution.
Although there are many reports of sheet-type image sensors, it has not been achieved both high-resolution imaging and high-speed readout, and static biometric data and dynamic vital signs cannot be measured by one sheet-type image sensor. This is because high-sensitivity photodetectors and high-speed switching elements could not be integrated on a polymer substrate without damaging of the switching elements.
The developed sheet-type image sensor is fabricated by densely integrating a high-efficiency readout circuit using an active matrix of a low-temperature polysilicon thin film transistor and a photodetector that uses a highly efficient organic semiconductor as a photosensitive layer. The resolution of the image sensor achieves 508 dots per inch (dpi) required for fingerprint authentication, and the organic photodetector consists of bulk hetero structure organic layer which has high photosensitivity to near infrared light with a wavelength of 850 nanometers (external quantum efficiency of 50% or more). It it easy to integrate the sheet-type image sensor into equipment and attach it to curved surfaces due to the thickness of the polymer base material is 10 micrometers, and the total thickness of the sheet-type image sensor is 15 micrometers (Figure 1). By developing a process technology that integrates photodetectors and thin-film transistors without damage to each other, it has become possible to realize a sheet-type image sensor that achieves both high-resolution imaging and high-speed reading.
Evaluation of vein and fingerprint images taken by this sheet-type image sensor showed that the contrast difference of the veins was less than 5% compared to images using a general CMOS imager. We confirmed that the conformable imager has high image quality equivalent to that of conventional CMOS imagers.
Since the sheet-type image sensor is thin and bendable, it can be easily integrated into wearable devices, and it is possible to measure health condition and perform biometric authentication at the same time. As a result, it is expected that prevention of “spoofing” and patient from being mixed up.

In last decades, the research on flexible logic circuits based on thin-film transistors (TFTs) have been increasingly growing as technologies tend to stabilize enabling application-oriented research. Although backplane applications such as active matrix displays and 2-D imager arrays are the main driver for technology developments, logic applications also benefit from increased performance. In this invited presentation, I will elaborate on several design requirements for TFT-based logic circuits and link them to technology and material requirements.

The focus of this work will be in the direction of flexible IoT applications, ranging from radio-frequency communication chips to sensor readout circuitry to monitor vital parameters. All demonstrated flexible integrated circuits have been realized in a dual-gate self-aligned a-IGZO TFT technology. The main advantage of a self-aligned transistor is the strongly reduced parasitic overlap capacitance between gate and source-drain contacts, enabling faster circuit blocks consuming less power. Consequently, we have demonstrated a flexible IGZO-based NFC communication tag connecting to an NFC-enabled smartphone [1]. In recent work, we have been updating our design libraries with substantially lower power consumption and realized a capacitive tag that communicates directly with the touchscreen of a smartphone as an enabler for the Internet-of-Everything [2]. This tag has been made self-powered by integrating a flexible Perovskite photovoltaic cell that captures the energy from the smartphone screen which powers the chip.

The dual-gate self-aligned transistor provides a designer with an additional benefit, namely the presence of an extra gate. The backgate can be employed for several different purposes: to increase the transconductance, to improve the output resistance or to control the threshold voltage. Most of these design considerations are mainly interesting for analogue circuits, for example to improve the gain of an operational amplifier. As a third case in this presentation, I will discuss our recent achievements on a flexible ECG patch with NFC compatibility for healthcare monitoring patches [3]. Finally, as the newest technology developments move in the direction of increased charge carrier mobilities, matching p-types and low-temperature polysilicon and oxides (LTPO), I will elaborate on the future enabling opportunities for TFT-based logic circuits and applications.

References
[1] K. Myny, “The development of flexible integrated circuits based on thin-film transistors”, Nature Electronics 1, pp. 30-39 (2018)
[2] N. Papadopoulos, et al.; “Touchscreen tags based on thin-film electronics for the Internet of Everything”, Nature Electronics 2, pp. 606-611 (2019)
[3] M. Zulqarnain, et al.; “A flexible ECG patch with NFC compatible RF communication”, accepted to Flexible Electronics (npg)

Acknowledgement – I would like to thank my coworkers at imec and TNO/Holst centre; M. Zulqarnain and E. Cantatore from TUe for their valuable contributions to this work. Part of this work has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program under grant agreement No 716426 (FLICs project).

Development of a human-interactive display that enables the simultaneous sensing, visualisation, and memorisation of a magnetic field has attracted significant attention because it offers control and easy access to information that is rarely available. This study demonstrates a magneto-interactive electroluminescent display patchable on skin, which is capable of sensing, visualising, and storing magnetic field information, thereby enabling 3D motion tracking. For this purpose, a magnetic field-dependent conductive gate is employed in an alternating current electroluminescent (EL) display, which is used to produce non-volatile and rewritable magnetic field-dependent EL display. By constructing mechanically flexible arrays of non-volatile magneto-interactive EL displays, a skin-patchable and pixelated platform is realised. The unique feature of magnetic field varying along the z-axis enables the 3D motion tracking (monitoring and memorisation) on 2D pixelated display. This 3D motion tracking display is successfully used as a non-destructive surgery-path guiding display, wherein a pathway for a surgical robotic arm with a magnetic probe is visualised and recorded on a display patched on the abdominal skin of a rat, thereby helping the robotic arm to find an optimal pathway without damage incurred on the path to the surgery spot.

Oxide thin-film transistors (TFTs) have attracted much attention for next-generation electronics because of their superior performance compared with conventional a-Si TFTs such as high mobility and optical transmittance in the visible light. Particularly, solution process is a promising technique for the oxide TFT fabrication method for being a low-cost method with high composition controllability, high throughput, and various application. In this study, we propose various researches to improve the electrical and optical performances in oxide TFTs. Furthermore, we suggest a solution-processed oxide absorption layer (SAL) to enable detection of visible light for oxide phototransistors.

Voltage-Based Ambi-Ionic Migration (VAM) for Modulating Active Layer
The voltage-based ambi-ionic modulation (VAM) technique allows specific ions to migrate into the active layer by applying bias, easily modulating the active layer. Also, the potassium superoxide (KO_²) solution is employed in oxide thin-film transistors (TFTs) as a source of potassium (K⁺) ions and highly reactive superoxide radical (O₂^*−) ions. By applying external bias, the K⁺ and O₂^*- ions rapidly migrate into the active layer, directly changing its electrical properties and chemical composition. Through the VAM technique, various metal-oxide-based devices such as oxide TFTs can perform better.

Multifunctional Heterogeneous Organic (MHO) Passivation Layer for Oxide TFT
We propose a highly flexible and reliable multifunctional heterogeneous organic (MHO) passivation layer composed of stacked diketopyrrolopyrrole (DPP)-polymer and parylene-C films for improving the stability of oxide TFTs under various ambient environments and mechanical stress. The presented MHO passivation layer leads to high-performance oxide TFTs by: (1) protecting them from external reactive molecules, (2) blocking light to the oxide layer, and (3) improving their electrical characteristics. As a result, oxide TFTs with MHO passivation layer exhibit higher stability in ambient environment and under light. Moreover, since the MHO passivation layer can be deposited at room temperature and exhibits high mechanical stability, it could be practical in the fabrication of flexible/wearable devices.

Selective modulation of electrical characteristics by electro-hydro-dynamic printing
A simple fabrication method for homojunction-structured Al-doped indium–tin oxide (ITO) thin-film transistors (TFTs) using an electro-hydro-dynamic (EHD) jet-printed Al₂O₃ passivation layer with a specific line (W_Al2O3) is proposed. After EHD jet-printing, the specific region of ITO where Al₂O₃ is printed changes from a conducting film to a semiconducting film while Al₂O₃ acts as passivation layer. The channel length of the fabricated TFTs is defined by W_Al2O3 which is easily changed with varying EHD jet printing conditions, replacing masks with varying patterns. Consequently, the proposed approach is promising as a low-cost and flexible manufacturing system for multi-item small-lot-sized production of Internet of Things devices.

Solution-Processed Oxide Absorption Layer (SAL) for Visible-Light Detecting Phototransistor
We propose a solution-processed oxide absorption layer (SAL) for visible-light (532 and 635 nm) detection for indium−gallium−zinc oxide (IGZO) phototransistors. The SAL was deposited onto the sputtered IGZO using precursor solutions composed of the same atomic configuration of sputtered IGZO, resulting in superior interface characteristics. We artificially generated sub-gap states in the SAL using a low temperature (200°C) annealing, minimizing the degradation of the electrical characteristics of TFT. These sub-gap states in SALs improved the photoelectron generation under long-wavelength visible-light despite the wide bandgap of IGZO (over 3.0 eV). As a result, the IGZO phototransistors with SALs have both superior optoelectronic characteristics and high optical transparency.

Hybrid organic-inorganic perovskites are promising materials for the application in solar cells and light-emitting diodes. However, the basic current-voltage behavior for electrons and holes is still poorly understood in these semiconductors due to their mixed electronic-ionic character. To develop an experimentally-validated numerical device model, it is therefore necessary to isolate individual physical phenomena. We investigate the dynamics of ion motion in methyl ammonium lead iodide (MAPbI₃) by impedance spectroscopy and electric displacement as a function of frequency. The displacement response is fully reproduced by a numerical device model that enables us to determine the frequency-dependent dielectric constant, the ion concentration and the ion diffusion coefficient. These validated ion dynamics are applied to analyze space-charge-limited electron and hole currents in MAPbI₃. We demonstrate that the frequency dependence of the permittivity plays a crucial role in the analysis of space-charge-limited currents and their dependence on voltage scan rate. Our mixed electronic-ionic device model accurately reproduces the current-voltage characteristics of single-carrier devices, showing that in MAPbI₃ transport of electrons dominates over holes.

Facile solution-processing of metal oxide thin-film transistors (TFTs) is a promising alternative to conventional vacuum-processing and offers various advantages, such as low cost, large-area fabrication capability, and process simplicity.¹ The patterning of such films is necessary for device integration; however, most solution-processing is still conducted using traditional photolithographic patterning processes. A simple and reliable direct patterning method is a key technological requirement to maximize the advantages of solution-processing.² We have previously reported a direct patterning method for an aqueous In-Ga-Zn oxide (IGZO) precursor.³
A direct patterning method to obtain high-performance carbon-free oxide TFTs using In-Zn oxide (IZO) and In-oxide (InO) will be presented. Direct patterning is achieved by selective photoreaction of water molecules as a precursor solvent and nitrate ligands under ultraviolet irradiation without organic photosensitive additives. This environmentally-friendly chemical etching process uses a non-toxic organic acid (citric acid), followed by annealing at 350 °C, to obtain carbon-free oxide films of IZO and InO. TFTs with these oxide films on SiO₂ dielectrics that were fabricated by the direct patterning method and with sputtered Mo electrodes exhibited high mobilities of 11.5±1.6 cm²/Vs for IZO and 39.0±1.9 cm²/Vs for InO. The InO TFTs with solution-processed dielectrics using siloxane materials also exhibited good switching characteristics and a mobility of more than 30 cm²/Vs. X-ray reflectivity and X-ray diffraction analysis of the InO film density and crystallinity indicated denser and larger crystallinity in the photo-patterned oxide films. Efficient photooxidation by optimization of the deposition conditions and precursor concentrations during UV exposure resulted in higher quality solution-processed oxide films.

1. E. Fortunato, P. Barquinha and R. Martins, Adv Mater 24 (22), 2945-2986 (2012).
2. H. S. Lim, Y. S. Rim and H. J. Kim, Sci Rep 4, 4544 (2014).
3. M. Miyakawa, M. Nakata, H. Tsuji and Y. Fujisaki, Sci Rep 8 (1) (2018).

While lithography provides unparalleled control over dimensions, additive manufacturing techniques enable unique design flexibility. Here we consider the merits of hybrid fabrication involving the combined use of lithography and various additive manufacturing techniques in bioelectronics. We examine examples ranging from implantable sensors to neuromorphic transistors and discuss various fabrication strategies, showing the hybrid approaches offer several advantages.

High-performance organic electronic devices require organic semiconductor (OSC) thin films to possess desired microstructures. An effective approach is use of graphene as a template for controlling growth behavior of OSC crystals. Unlike on ordinary substrates, the growth modes of OSCs on graphene are not only determined by the graphitic surface structure but also electronic characteristics of graphene. In this talk, I will discuss how OSC molecules assemble on graphene surface under the influence of graphene’s doping effect. Also I will discuss a result that the OSC films grown on a graphene template under an optimized condition provide favorable vertical and lateral transport pathways for charge carriers and excitons in organic transistor and photovoltaic devices

Carrier mobility in conjugated polymers continues to increase with recent reports of field-effect mobilities exceeding 10 cm²/V.s. Charge transport is intrinsically dependent on processes occurring across multiple lengthscales. In order to access order parameters at the molecular scale we use charge modulation spectroscopy combined with theory. This technique allows us to further differentiate field-induced and doping-induced charges and study how their delocalization depends on local structure and disorder. Furthermore, we study the mesoscale organization of polymers using new techniques in the transmission electron microscope. By combining 4D STEM and HRTEM we are able to study the microstructure across a range of length-scales in real space and reciprocal space. These techniques are used on homopolymers and donor-acceptor copolymers and allow to extract information about the microstructure that is typically not visible by inspection. Such multiscale studies of microstructure are instrumental in guiding our understanding of charge transport in conjugated polymers.

Recently, metal-halide perovskite materials are considered as a promising candidate for lasing applications. Compared to 3D and other low-dimensional perovskites, quasi-2D perovskites are a better gain media due to its unique properties such as large binding energy and self-assembled quantum wells. Specifically, efficient energy funneling from low-dimensional domains to high-dimensional domains will not only enhance the radiative recombination but also facilitate population inversion.
In this presentation, we will discuss the fundamental requirements for population inversion in perovskites in the context of material dimensionality and morphology. We will also present our recent work on laser cavity design and processing for optimum laser performance.

Oxide thin-film transistors (TFTs) have attracted much attention for next-generation electronics because of their superior performance compared with conventional a-Si TFTs such as high mobility and optical transmittance in the visible light. Particularly, solution process is a promising technique for the oxide TFT fabrication method for being a low-cost method with high composition controllability, high throughput, and various application. In this study, we propose various researches to improve the electrical and optical performances in oxide TFTs. Furthermore, we suggest a solution-processed oxide absorption layer (SAL) to enable detection of visible light for oxide phototransistors.

Voltage-Based Ambi-Ionic Migration (VAM) for Modulating Active Layer
The voltage-based ambi-ionic modulation (VAM) technique allows specific ions to migrate into the active layer by applying bias, easily modulating the active layer. Also, the potassium superoxide (KO_²) solution is employed in oxide thin-film transistors (TFTs) as a source of potassium (K⁺) ions and highly reactive superoxide radical (O₂^*−) ions. By applying external bias, the K⁺ and O₂^*- ions rapidly migrate into the active layer, directly changing its electrical properties and chemical composition. Through the VAM technique, various metal-oxide-based devices such as oxide TFTs can perform better.

Multifunctional Heterogeneous Organic (MHO) Passivation Layer for Oxide TFT
We propose a highly flexible and reliable multifunctional heterogeneous organic (MHO) passivation layer composed of stacked diketopyrrolopyrrole (DPP)-polymer and parylene-C films for improving the stability of oxide TFTs under various ambient environments and mechanical stress. The presented MHO passivation layer leads to high-performance oxide TFTs by: (1) protecting them from external reactive molecules, (2) blocking light to the oxide layer, and (3) improving their electrical characteristics. As a result, oxide TFTs with MHO passivation layer exhibit higher stability in ambient environment and under light. Moreover, since the MHO passivation layer can be deposited at room temperature and exhibits high mechanical stability, it could be practical in the fabrication of flexible/wearable devices.

Selective modulation of electrical characteristics by electro-hydro-dynamic printing
A simple fabrication method for homojunction-structured Al-doped indium–tin oxide (ITO) thin-film transistors (TFTs) using an electro-hydro-dynamic (EHD) jet-printed Al₂O₃ passivation layer with a specific line (W_Al2O3) is proposed. After EHD jet-printing, the specific region of ITO where Al₂O₃ is printed changes from a conducting film to a semiconducting film while Al₂O₃ acts as passivation layer. The channel length of the fabricated TFTs is defined by W_Al2O3 which is easily changed with varying EHD jet printing conditions, replacing masks with varying patterns. Consequently, the proposed approach is promising as a low-cost and flexible manufacturing system for multi-item small-lot-sized production of Internet of Things devices.

Solution-Processed Oxide Absorption Layer (SAL) for Visible-Light Detecting Phototransistor
We propose a solution-processed oxide absorption layer (SAL) for visible-light (532 and 635 nm) detection for indium−gallium−zinc oxide (IGZO) phototransistors. The SAL was deposited onto the sputtered IGZO using precursor solutions composed of the same atomic configuration of sputtered IGZO, resulting in superior interface characteristics. We artificially generated sub-gap states in the SAL using a low temperature (200°C) annealing, minimizing the degradation of the electrical characteristics of TFT. These sub-gap states in SALs improved the photoelectron generation under long-wavelength visible-light despite the wide bandgap of IGZO (over 3.0 eV). As a result, the IGZO phototransistors with SALs have both superior optoelectronic characteristics and high optical transparency.

In last decades, the research on flexible logic circuits based on thin-film transistors (TFTs) have been increasingly growing as technologies tend to stabilize enabling application-oriented research. Although backplane applications such as active matrix displays and 2-D imager arrays are the main driver for technology developments, logic applications also benefit from increased performance. In this invited presentation, I will elaborate on several design requirements for TFT-based logic circuits and link them to technology and material requirements.

The focus of this work will be in the direction of flexible IoT applications, ranging from radio-frequency communication chips to sensor readout circuitry to monitor vital parameters. All demonstrated flexible integrated circuits have been realized in a dual-gate self-aligned a-IGZO TFT technology. The main advantage of a self-aligned transistor is the strongly reduced parasitic overlap capacitance between gate and source-drain contacts, enabling faster circuit blocks consuming less power. Consequently, we have demonstrated a flexible IGZO-based NFC communication tag connecting to an NFC-enabled smartphone [1]. In recent work, we have been updating our design libraries with substantially lower power consumption and realized a capacitive tag that communicates directly with the touchscreen of a smartphone as an enabler for the Internet-of-Everything [2]. This tag has been made self-powered by integrating a flexible Perovskite photovoltaic cell that captures the energy from the smartphone screen which powers the chip.

The dual-gate self-aligned transistor provides a designer with an additional benefit, namely the presence of an extra gate. The backgate can be employed for several different purposes: to increase the transconductance, to improve the output resistance or to control the threshold voltage. Most of these design considerations are mainly interesting for analogue circuits, for example to improve the gain of an operational amplifier. As a third case in this presentation, I will discuss our recent achievements on a flexible ECG patch with NFC compatibility for healthcare monitoring patches [3]. Finally, as the newest technology developments move in the direction of increased charge carrier mobilities, matching p-types and low-temperature polysilicon and oxides (LTPO), I will elaborate on the future enabling opportunities for TFT-based logic circuits and applications.

References
[1] K. Myny, “The development of flexible integrated circuits based on thin-film transistors”, Nature Electronics 1, pp. 30-39 (2018)
[2] N. Papadopoulos, et al.; “Touchscreen tags based on thin-film electronics for the Internet of Everything”, Nature Electronics 2, pp. 606-611 (2019)
[3] M. Zulqarnain, et al.; “A flexible ECG patch with NFC compatible RF communication”, accepted to Flexible Electronics (npg)

Acknowledgement – I would like to thank my coworkers at imec and TNO/Holst centre; M. Zulqarnain and E. Cantatore from TUe for their valuable contributions to this work. Part of this work has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program under grant agreement No 716426 (FLICs project).

In this presentation we report the development of novel semiconductors, as well as thin-film engineering, for flexible and stretchable organic and inorganic (electrochemical)transistors and sensors. In particular we show that “ultra-soft” polymers comprising naphthalenediimide units co-polymerized with “rigid” and “flexible” organic units can change how charge transport is affected by mechanical stress, demonstrating that polymer backbone composition is more important that film degree of texturing. Furthermore, molecular design of polymers enables plasticization of a small molecule semiconductor film used in thin-film transistors. In addition, we report new “soft” transistor architectures using porosity as key element enhancing mechanical flexibility and tuned charge transport. Finally, metaloxide semiconductor nanostructures are fabricated in a very simple way and enable TFT and several types of sensors which can be monolithically integrated. These devices can better sense analytes, intercalate ions, and be chemically doped.

2-Dimensional materials (2DMs) are very attractive building blocks for the next generation electronics, which require low cost and flexible devices that can be easily integrated onto substrates such as paper and plastic. The atomically thin nature of 2DMs enables mechanical flexibility, high sensitivity, easy functionalization, and allows one to stack together different 2DMs to fabricate multifunctional heterostructure-based devices.
In this talk I will show a general formulation approach to make water-based, defect-free and biocompatible 2DM-based inks suitable for fabrication of a wide range of fully printed devices, such as photodetectors, capacitors and transistors, on paper and plastic [1-3].
Furthermore, printed 2DM-based devices can be easily coupled with printed RFID tags made of graphene, enabling the development of a wireless and battery-free multisening platform on paper [4-6]. Finally, the inks can be easily combined with polycrystalline 2DMs produced by chemical vapour deposition, allowing quick fabrication of complex circuits on paper, such as high-gain inverters, logic gates and current mirrors [7].
If time allows, I will also introduce a new C-based, highly resilient and flexible substrate for printed electronics, specifically designed for applications that involve repeated strain, where paper cannot be used because the incresing fatigue in the cellulose substrate leads to failure of the device. We will show that this highly resiliant substrate can sustain over 50k bending cycles without failure, in contrast to paper, which starts failing already after 5k cycles [5].

[1] McManus et al, Nature Nano, 12, 343 (2017)
[2] Worsley et al, ACS Nano, 2018, DOI: 10.1021/acsnano.8b06464
[3] Lu et al, ACS nano 13 (10), 11263 (2019)
[4] Leng et al, 2D Materials 7 (2), 024004 (2020)
[5] Worsley et al, submitted
[6] Casiraghi et al, Carbon 129, 462 (2018)
[7] Conti et al, arXiv preprint arXiv:1911.06233

The promises of a printed electronics revolution continue to be hindered by challenges in the performance, reproducibility, and broad utility of printed devices. Inks from nanoscale materials have received increasing attention for their potential to overcome some of these hurdles, particularly in yielding direct-printed thin films meeting target electrical performance, air stability, and process compatibility. While many commercial options now exist for conducting inks, accessibility of versatile and robust semiconducting and insulating inks remains a challenge. In this talk, the recent progress on, and benefits of, semiconducting carbon nanotube (CNT) inks will be reviewed, particularly in the context of direct-write, aerosol jet-printed electronic devices. Proper control of ink properties and printing process, including factors such as ink temperature and printing time, can yield semiconducting CNT thin films with relatively high mobility, uniformity, and reproducibility. In addition, two attractive options for direct-write printed insulating inks will be discussed: hexagonal boron nitride (hBN) and crystalline nanocellulose (CNC). It will be shown how the appropriate design of these nanomaterial-based inks can enable the printing of fully recyclable electronics, where the constituent nanomaterials can be reclaimed and reused after initial printing and use in electronic devices.

The past decades have witnessed a commendable progress in the development of materials for low-cost flexible electronics. New products that are beyond reach with current technologies can soon become a mainstay in our lives if these materials reach the necessary standards for performance and stability. The development of processing techniques that can address the manufacturability – scalability – sustainability – performance balance represents the next key step in the integration of these materials into next-generation electronic devices. In this presentation, I will introduce the use of laser printing, a rapid, scalable, environmentally friendly and low-cost manufacturing technique for deposition and patterning of various functional materials on flexible substrates. The first example focuses on organic semiconductors, and I will discuss the fabrication and characterization of organic thin-film transistors placed on plastic and paper. Aerosol spray laser lithography was used to define the electrodes, in conjunction with different metal inks: the pattern was created using a regular toner, which was subsequently selectively removed. We created a grid of transistor devices with variable channel lengths and widths and obtained good charge carrier mobilities and an excellent tolerance to bending. Next, I will describe the use of laser printing for the fabrication of metal halide perovskite films. The electrical properties of the laser printed films were comparable with those of the spin-coated layers, despite the fact that the microstructure consists of randomly oriented crystallites. The current-voltage characteristics exhibit negligible hysteresis and the electrical properties of the films are very stable under ambient conditions due to the fact that a vertical separation of the toner components results in an encapsulating the perovskite layer by the other toner components. Our work introduces an exciting new avenue for the fabrication of stable perovskite devices that is scalable and devoid of hazardous solvents.

Continuous, roll-to-roll (R2R) printing processes are attractive for manufacturing of flexible electronics. Two common challenges with R2R are reaching small feature sizes and achieving precise registration of multiple functional layers in devices such as transistors. This presentation will cover our efforts to overcome these issues using Self-Aligned Capillarity-Assisted Lithography for Electronics (or SCALE). SCALE has two steps. In the first step, R2R imprinting is used to create a multilevel open network of reservoirs, capillaries and device structures into a UV-curable coating deposited on a flexible substrate. In the second step, electronically functional inks are then delivered sequentially into the reservoirs on this imprinted substrate by inkjet printing, and capillarity pulls inks into capillaries and device structures. Since the imprint stamps are fabricated from high-resolution, Si master patterns created by photolithography, the features created in the imprint resin are also high-resolution, at the micron scale. The single imprinting step also simultaneously creates all the structural features needed to control the flow and deposition of multiple inks, resulting in self-alignment of multiple materials in complex multilayer devices.

To-date we have developed SCALE methods to create conductive networks, resistors, capacitors, diodes and transistors. For example, to manufacture high aspect ratio (thickness/width) SCALE conductive networks, we used a combination of inkjet delivery of a silver particle-free ink into reservoirs of the imprinted substrate, followed by capillary flow and thermal annealing to convert the ink to a silver seed layer and lastly electroless copper plating to build conductor thickness in capillary channels. By optimizing processing conditions and understanding the fundamentals of capillary flow, narrow (e.g., 10 μm), high aspect ratio (>1), low resistance (~1 ohm per cm of conductor length) embedded conductive traces were created. This presentation will show advances in device architecture and performance, and explore the key processing steps. Special attention will be given to continuous roll-to-roll processes.

Narrow bandgap polymers are used to realize infrared photodetectors that is relevant to a variety of applications including environmental monitoring and medical diagnosis. To achieve high detectivity in detectors, it is critical to reduce the device noise. This talk presents experimental and modeling studies on the noise current in exemplar organic bulk heterojunction photodiodes, with 10 donor–acceptor combinations spanning wavelength between 800 and 1600 nm. A significant reduction of the noise and higher detectivity were found in devices using non-fullerene acceptors (NFAs) in comparison to those using fullerene derivatives. The low noise in NFA blends was attributed to a sharp drop off in the distribution of bandtail states, as revealed by variable-temperature density-of-states measurements. Taking disorder into account, we developed a general physical model to explain the dependence of thermal noise on the effective bandgap and bandtail spread.

The narrow bandgap polymers are also Faradaic materials for energy storage and present opportunities to create energy dense supercapacitors. The vast majority of conducting polymers are p-type, and here we present promising n-type polymers that are stable when operating within a large potential window (3V), with a best-in-class energy densities (30.4 Wh/kg at a 1 A/g discharge rate), and a long cycle life (90% capacitance retention after 5000 cycles) critical to energy storage and management. This work demonstrates the application of a new class of stable and tunable redoxactive material to realize high-endurance energy storage devices for flexible printed systems.