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The increasing interest in non-volatile memory devices has extended the exploration towards new materials, such as organic-inorganic hybrids. Devices based on organic semiconductors and embedded metal nanoparticles (MNPs) were found to display resistive bistability, suitable for programmable electronic applications. Different models were developed to explain the resistive switching mechanism occurring in the devices. Charging/ de-charging of MNPs and concomitant resistivity changes was mainly proposed as mechanism, despite the lack of solid experimental evidence.[ ] In this contribution we report on the role of the space-charge field due to charged MNPs in two-terminal devices, via electrical characterization. Devices comprise 4,4-bis[N-(1-naphthyl)-N-phenyl-amino]diphenyl (α-NPD) with embedded gold nanoparticles (AuNPs). The electrical characterization (current vs bias) of the devices was conducted with and without illumination during operation. Due to the energy level alignment of the chosen materials, the AuNPs behave as deep charge carrier traps. The induced space-charge spontaneously sets the device to the low conductivity state. The de-charging of the AuNPs can then be dynamically induced through illumination, setting the device to a high conductivity state.

Despite the ability to optically control the charging state of the AuNPs, the devices do not display any bistability. This finding provides evidence that the commonly proposed MNPs charging/de-charging mechanism can be excluded as the cause for electrical bistability in two-terminal devices, and that other mechanisms, such as filament formation, should be evoked.

Diodes containing a layer of aluminum oxide combined with a layer of π-conjugated polymer show nonvolatile memory effects after they have been electroformed. Electroforming is induced by application high bias voltage close to the limit for dielectric breakdown and can be performed reliably and with high yield on organic-inorganic hybrid diodes with controlled oxide thickness. Here we investigate the initial stage of the electroforming process and show through temperature dependent current-voltage characterization that electrons are trapped in deep traps at the interface between π-conjugated polyspirofluorene polymer and the aluminum oxide.

Characterizing filamentary switching in resistive memories For many organic, inorganic and hybrid memory devices the resistive switching mechanism is well known to rely on filament formation [1]. This implies that localized conductive paths are established between the two terminal electrodes during the forming step. This filaments sustain the current flow when the memory is in the low conductive state and they can be ruptured and possibly re-formed for more than hundreds of I-V cycles. The nature and morphology of filaments has been long time debated especially for organic memories. The filament size, density and formation mechanism have been very challenging to be characterized, and need appropriate experimental techniques. However, filaments in organic memories have been recently identified and characterized by cross-section transmission electron microscopy (TEM), conductive-AFM, AFM-tomography and through depth profile analysis combining Time-of-flight secondary ions mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). In particular, 3D spectroscopic images obtained with ToF-SIMS give access for the first time to filament formation process and rupture mechanism. From these results, a clear picture of the filament(s) dynamics during memory operation can be drawn. In this contribution, recent results showing filaments in memories based on different structures and architectures will be discussed. The memories are based on insulating polymers (polystyrene [2] and poly methyl methacrylate [3]), conductive polymers/nanocomposites (polyera N1400 with metal NPs [4]), and small semiconducting molecules (Tris(8-hydroxyquinolinato)aluminium - Alq3 [5]). The results show that resistive switching clearly involves the inhomogeneous metal diffusion in the organic layer taking place during the top electrode deposition and during memory operation. This may be of great relevance in many other organic electronics applications. REFERENCES [1] S. Nau, S. Sax, E.J.W. List-Kratochvil, Adv. Mater. 2014, 26, 2508–2513. [2] Y. Busby, N. Crespo-Monteiro, M. Girleanu, M. Brinkmann, O. Ersen, J.-J. Pireaux, Organic Electronics 2015, 16, 40–45. [3] C. Wolf, S. Nau, S. Sax, Y. Busby, J.-J. Pireaux, E.J.W. List-Kratochvil (under submission). [4] G. Casula, P. Cosseddu, Y. Busby, J.-J. Pireaux, M. Rosowski, B. Tkacz Szczesna, K. Soliwoda, G. Celichowski, J. Grobelny, J. Novák, R. Banerjee, F. Schreiber, A. Bonfiglio, Organic Electronics, 2015, 18, 17-23. [5] Y. Busby, S. Nau, S. Sax, E.J.W. List- Kratochvil, J. Novak, R. Banerjee, F. Schreiber, J.-J. Pireaux, (under submission)

The possibility of developing fully organic electronic circuits is critically dependent on the ability to realize a full set of electronic functionalities based on organic devices. In order to complete the scene, a fundamental element is still missing, i.e. reliable data storage. Over the past few years, a considerable effort has been spent on the development and optimization of organic polymer based memory elements. Among several possible solutions, transistor-based memories and resistive switching-based memories are attracting a great interest in the scientific community. In this paper, a route for the fabrication of organic semiconductor-based memory devices with performances beyond the state of the art is reported. Both the families of organic memories will be considered. A flexible resistive memory based on a novel combination of materials is presented. In particular, high retention time in ambient conditions are reported. Complementary, a low voltage transistor-based memory is presented. Low voltage operation is allowed by an hybrid, nano-sized dielectric, which is also responsible for the memory effect in the device. Thanks to the possibility of reproducibly fabricating such device on ultra-thin substrates, high mechanical stability is reported.

The increasing quest to find lightweight, conformable or flexible image detectors for machine vision or medical imaging brings organic electronics into the spotlight for these fields of application. Here were we introduce a unique imaging device concept and its utilization in an organic, flexible detector array with simple passive matrix wiring. We present a flexible organic image detector array built up from non-volatile resistive multi-bit photo-switchable elements. This unique realization is based on an organic photodiode combined with an organic resistive memory device wired in a simple crossbar configuration. The presented concept exhibits significant advantages compared to present organic and inorganic detector array technologies, facilitating the detection and simultaneous storage of the image information in one detector pixel, yet also allowing for simple read-out of the information from a simple passive-matrix crossbar wiring. This concept is demonstrated for single photo-switchable pixels as well as for arrays with sizes up to 32 by 32 pixels (1024 bit). The presented results pave the way for a versatile flexible and easy-to-fabricate sensor array technology. In a final step, the concept was expanded to detection of x-rays.

With the recent improvements in printed devices, it is now possible to build integrated circuit systems out of printed devices. The combination of sensor, logic, and rewritable memory will greatly enhance the functionalities of printed electronics. We have demonstrated integrated sensor tags based on organic complementary circuits patterned by inkjet printing. One example is a temperature threshold sensor tag, wherein if the thermistor temperature exceeds a pre-set threshold, the control circuit generates a pulse to write into a nonvolatile ferroelectric memory cell. The trigger temperature is set by adjusting the bias voltage across the thermistor bridge to match the trigger voltage of the printed threshold circuit, and the threshold temperatures has been tuned between 8 °C and 45 °C with a bias voltage below 30V.

The electronic properties of solution-processable small molecule organic semiconductors (OSCs) have rapidly improved in recent years, rendering them highly promising for various low-cost large area electronic applications such as active matrix displays, radio frequency identification tags, and integrated logic circuits. However, practical applications of organic electronics requires patterned and precisely registered OSC film within the transistor channel region with uniform electrical properties over a large area, a task that remains a significant challenge. Here we present a novel technique known as CONNECT (Controlled OSC NucleatioN and Extension for CircuiTs), which utilizes differential surface energy and solution shearing to simultaneously generate self-patterned and self-registered OSC film within the channel region and with aligned crystalline domains, resulting in low device-to-device variability. We have fabricated transistor density as high as 840 dpi, with a yield of 99%. We have successfully built various logic gates and a 2-bit half adder circuit, demonstrating the practical applicability of our technique for large-scale circuit fabrication. CONNECT was expanded to use with inkjet printed silver electrodes, showing the versatility of our method to accommodate various solution deposition and fabrication methods.

Gravure printing is a very promising method for printed electronics because it combines high throughput with high resolution. Recently, printed lines with 2μm resolution have been demonstrated at printing speeds on the order of 1m/s. Here we build on these results to study how more complex patterns can be printed that will ultimately lead to printed circuits. We study how the drag-out effect leads to proximity effects in gravure when multiple lines are printed close to each other. Drag-out occurs as the doctor blade passes over the roll surface to remove excess ink from the land areas in between the cells that make up the pattern. In addition to this desirable removal of excess ink, some ink from the cells also wicks up the doctor blade and is removed from the cells. This ink is subsequently deposited on the land area behind the cells leading to characteristic drag-out tails. If multiple lines, oriented perpendicular to the print direction, are printed close to each other, the ink that has wicked up the doctor blade from the first line will affect the drag-out process for subsequent lines. Here we show how this effect can be used to enhance print quality of lines as well as how it can deteriorate print quality. Important variables that will determine the regime for printing optimization are ink viscosity, printing speed, cell size, cell spacing and relative placement of lines. Considering these factors carefully allows one to determine design rules for optimal printing results.

For decades, the electronics industry has been accurately described by Moore’s Law, where the march towards increasing density and smaller feature sizes has enabled continuous cost reductions and performance improvements. With flexible electronics, this perpetual scaling is not foreseen to occur. Instead, the industry will be dominated by Wright’s Law, first proposed in 1936, where increasing demand for high volumes of product will drive costs down. We have demonstrated thin film based circuitry compatible with flexible substrates with high levels of functionality designed for such a high volume industry. This includes a generic 8-bit microprocessor totaling more than 3.5k TFTs operating at 2.1 kHz. We have also developed a post fabrication programming technique via inkjet printing of conductive spots to form a one-time programmable instruction generator, allowing customization of the processor for a specific task. The combination demonstrates the possibility to achieve the high volume production of identical products necessary to reap the benefits promised by Wright’s Law, while still retaining the individualization necessary for application differentiation. This is of particular importance in the area of item level identification via RFID, where low cost and individualized identification are necessary. Remotely powered RFID tags have been fabricated using an oxide semiconductor based TFT process. This process is compatible with the post-fabrication printing process to detail individual identification codes, with the goal of producing low cost, high volume flexible tags. The goal is to produce tags compatible with existing NFC communication protocols in order to communicate with readers that are already ubiquitous in the market.

A process for the fabrication of integrated circuits based on bottom-gate, top-contact organic thin-film transistors (TFTs) with channel lengths as short as 1 µm on flexible plastic substrates has been developed. In this process, all TFT layers (gate electrodes, organic semiconductors, source/drain contacts) are patterned with the help of high-resolution silicon stencil masks, thus eliminating the need for subtractive patterning and avoiding the exposure of the organic semiconductors to potentially harmful organic solvents or resists. The TFTs employ a low-temperature-processed gate dielectric that is sufficiently thin to allow the TFTs and circuits to operate with voltages of about 3 V. Using the vacuum-deposited small-molecule organic semiconductor 2,9-didecyl-dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (C10 DNTT), TFTs with an effective field-effect mobility of 1.2 cm2/Vs, an on/off current ratio of 107, a width-normalized transconductance of 1.2 S/m (with a standard deviation of 6%), and a signal propagation delay (measured in 11-stage ring oscillators) of 420 nsec per stage at a supply voltage of 3 V have been obtained. To our knowledge, this is the first time that megahertz operation has been achieved in flexible organic transistors at supply voltages of less than 10 V. In addition to flexible ring oscillators, we have also demonstrated a 6-bit digital-to-analog converter (DAC) in a binary-weighted current-steering architecture, based on TFTs with a channel length of 4 µm and fabricated on a glass substrate. This DAC has a supply voltage of 3.3 V, a circuit area of 2.6 × 4.6 mm2, and a maximum sampling rate of 100 kS/s.

Ferroelectric poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), is increasingly used in organic non-volatile memory devices, e.g., in ferroelectric field effect transistors (FeFETs). Here, we report on FeFETs integrating nanoimprinted arrays of P(VDF-TrFE) nanowires. Two previously-unreported architectures are tested, the first one consisting of stacked P(VDF-TrFE) nanowires placed over a continuous semiconducting polymer film; the second one consisting of a nanostriped blend layer wherein the semiconducting and ferroelectric components alternate regularly. The devices exhibit significant reversible memory effects, with operating voltages reduced compared to their continuous film equivalent, and with different possible geometries of the channels of free charge carriers accumulating in the semiconductor.

The continuous advancements in printed electronics make nowadays feasible the design of printed circuits which enable meaningful applications. Examples include ultra-low cost sensors embedded in food packaging, large-area sensing surfaces and biomedical assays. This paper offers an overview of state-of-the-art digital and analog circuit blocks, manufactured with a printed complementary organic TFT technology. An analog to digital converter and an RFID tag implemented exploiting these building blocks are also described. The main remaining drawbacks of the printed technology described are identified, and new approaches to further improve the state of the art, enabling more innovative applications are discussed.

Due to the increasing demand for storage capacity in various electronic gadgets like mobile phones or tablets, new types of non-volatile memory devices have gained a lot of attention over the last few years. Especially multilevel conductance switching elements based on organic semiconductors are of great interest due to their relatively simple device architecture and their small feature size.

Since organic semiconductors combine the electronic properties of inorganic materials with the mechanical characteristics of polymers, this class of materials is suitable for solution based large area device preparation techniques. Consequently, inkjet based deposition techniques are highly capable of facing preparation related challenges. By gradually replacing the evaporated electrodes with inkjet printed silver, the preparation related influence onto device performance parameters such as the ON/OFF ratio was investigated with IV measurements and high resolution transmission electron microscopy. Due to the electrode surface roughness the solvent load during the printing of the top electrode as well as organic layer inhomogeneity’s the utilization in array applications is hampered. As a prototypical example a 1diode-1resistor element and a 2×2 subarray from 5×5 array matrix were fully characterized demonstrating the versatility of inkjet printing for device preparation.

New, energy-saving, efficient and cost-effective processing technologies such as 2D and 3D inkjet printing (IJP) for the production and integration of intelligent components will be opening up very interesting possibilities for industrial applications of molecular materials in the near future. Beyond the use of home and office based printers, "inkjet printing technology" allows for the additive structured deposition of photonic and electronic materials on a wide variety of substrates such as textiles, plastics, wood, stone, tiles or cardboard. Great interest also exists in applying IJP in industrial manufacturing such as the manufacturing of PCBs, of solar cells, printed organic electronics and medical products. In all these cases inkjet printing is a flexible (digital), additive, selective and cost-efficient material deposition method. Due to these advantages, there is the prospect that currently used standard patterning processes can be replaced through this innovative material deposition technique. A main issue in this research area is the formulation of novel functional inks or the adaptation of commercially available inks for specific industrial applications and/or processes.

In this contribution we report on the design, realization and characterization of novel active and passive inkjet printed electronic devices including circuitry and sensors based on metal nanoparticle ink formulations and the heterogeneous integration into 2/3D printed demonstrators. The main emphasis of this paper will be on how to convert scientific inkjet knowledge into industrially relevant processes and applications.

Due to the required established field to separate photogenerated electrons and holes, the current- voltage (I-V) characteristic in almost all photovoltaic devices in dark is an exponential curve. Upon illumination, the shape of the curve remains almost the same, but the current shifts due to the photocurrent. Also, because of the lack of any storage mechanism, the I-V curve returns to the dark characteristic immediately after light cessation. Here, we are reporting a case study performed on a photo-electric memory effect in an organic bulk hetrojuction device made of ZnO nanowires as the electron transport layer under ambient conditions and within a sealed transfer box filled with nitrogen. The I-V characteristic in dark and light showed a unique change from a rectifying response in dark to a resistive behavior in light. Additionally, after light cessation, a memory effect was observed with a slow transition from the resistive to rectifying response same as the original dark characteristic. The memory effect and its I-V characteristics were tested for the two cases. For practical applications as a photo memory device, further experiments are required to gain a better understanding of the mechanism behind the observed memory effect for the two different cases.