Correlation between interface-dependent properties and electrical performances in OFETs

One of the fundamental points in determining the electrical performances in organic devices is that, even though they are usually thought as macroscopic devices, their behaviour is strongly driven by interfacial phenomena taking place in the nano scale. The focus of this thesis is the realization and characterization of Organic Field effect Transistors (OFETs) with a particular interest for investigating the influence of the interfaces on the electrical performances of the devices. Indeed, the parameters which mostly influence the electrical behaviour can be divided into to groups: 1) Intrinsic parameters of the material, as molecular packing and island nucleation, where the interfacial phenomena are taking place in the inter-molecular and inter-island scale. 2) Structural properties of the device, where the role of the triple interface metal electrode/organic semiconductor/gate dielectric can, in fact, significantly dictate the electrical behaviour of the device. In conclusion, in this thesis we demonstrated that the electrical and optoelectronic performances of organic semiconductor based devices are strongly influenced on one side by the geometry and architecture of the device itself, on the other hand, by the structural and morphological properties of the employed materials, and that the nanosized interfacial volume between the different materials layers in the device often play a key-role for determining the final properties of the device. In other words, device behaviour is more affected by material boundaries than by intrinsic properties of materials

All-organic, low voltage, transparent and compliant organic field-effect transistor fabricated by means of large-area, cost-effective techniques

The development of electronic devices with enhanced properties of transparency and conformability is of high interest for the development of novel applications in the field of bioelectronics and biomedical sensing. Here, a fabrication process for all organic Organic Field-Effect Transistors (OFETs) by means of large-area, cost-effective techniques such as inkjet printing and chemical vapor deposition is reported. The fabricated device can operate at low voltages (as high as 4 V) with ideal electronic characteristics, including low threshold voltage, relatively high mobility and low subthreshold voltages. The employment of organic materials such as Parylene C, PEDOT:PSS and 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) helps to obtain highly transparent transistors, with a relative transmittance exceeding 80%. Interestingly enough, the proposed process can be reliably employed for OFET fabrication over different kind of substrates, ranging from transparent, flexible but relatively thick polyethylene terephthalate (PET) substrates to transparent, 700-nm-thick, compliant Parylene C films. OFETs fabricated on such sub-micrometrical substrates maintain their functionality after being transferred onto complex surfaces, such as human skin and wearable items. To this aim, the electrical and electromechanical stability of proposed devices will be discussed

Direct X-ray photoconversion in flexible organic thin film devices operated below 1 v

The application of organic electronic materials for the detection of ionizing radiations is very appealing thanks to their mechanical flexibility, low-cost and simple processing in comparison to their inorganic counterpart. In this work we investigate the direct X-ray photoconversion process in organic thin film photoconductors. The devices are realized by drop casting solution-processed bis-(triisopropylsilylethynyl)pentacene (TIPS-pentacene) onto flexible plastic substrates patterned with metal electrodes; they exhibit a strong sensitivity to X-rays despite the low X-ray photon absorption typical of low-Z organic materials. We propose a model, based on the accumulation of photogenerated charges and photoconductive gain, able to describe the magnitude as well as the dynamics of the X-ray-induced photocurrent. This finding allows us to fabricate and test a flexible 2 × 2 pixelated X-ray detector operating at 0.2 V, with gain and sensitivity up to 4.7 × 10^4 and 77,000 nC mGy ^(-1) cm^(-3), respectively

Direct X-ray photoconversion in flexible organic thin film devices operated below 1 v

The application of organic electronic materials for the detection of ionizing radiations is very appealing thanks to their mechanical flexibility, low-cost and simple processing in comparison to their inorganic counterpart. In this work we investigate the direct X-ray photoconversion process in organic thin film photoconductors. The devices are realized by drop casting solution-processed bis-(triisopropylsilylethynyl)pentacene (TIPS-pentacene) onto flexible plastic substrates patterned with metal electrodes; they exhibit a strong sensitivity to X-rays despite the low X-ray photon absorption typical of low-Z organic materials. We propose a model, based on the accumulation of photogenerated charges and photoconductive gain, able to describe the magnitude as well as the dynamics of the X-ray-induced photocurrent. This finding allows us to fabricate and test a flexible 2 × 2 pixelated X-ray detector operating at 0.2 V, with gain and sensitivity up to 4.7 × 10 4 and 77,000 nC mGy 1 cm 3, respectively

Turning an organic semiconductor into a low-resistance material by ion implantation

We report on the effects of low energy ion implantation on thin films of pentacene, carried out to investigate the efficacy of this process in the fabrication of organic electronic devices. Two different ions, Ne and N, have been implanted and compared, to assess the effects of different reactivity within the hydrocarbon matrix. Strong modification of the electrical conductivity, stable in time, is observed following ion implantation. This effect is significantly larger for N implants (up to six orders of magnitude), which are shown to introduce stable charged species within the hydrocarbon matrix, not only damage as is the case for Ne implants. Fully operational pentacene thin film transistors have also been implanted and we show how a controlled N ion implantation process can induce stable modifications in the threshold voltage, without affecting the device performanc

Isotropic contact patterning to improve reproducibility in organic thin-film transistors

A novel approach for improving reproducibility of Organic Field-Effect Transistors electrical performances is proposed. The introduction of isotropic features in the layout of source and drain electrodes is employed to minimize the impact of randomly-distributed crystalline domains in the organic semiconductor film on the reproducibility of basic electrical parameters, such as threshold voltage and charge carrier mobility. A significant reduction of the standard deviation of these parameters is reported over a statistically-relevant set of devices with drop-casted semiconductor, if compared with results obtained in a standard, interdigitated transistor structure. A correlation between electrodes patterning and proposed result is demonstrated by deepening the analysis with the contribution of meniscus-assisted semiconductor printing, in order to precisely control the growth direction of crystals

Optimization of organic field-effect transistor-based mechanical sensors to anisotropic and isotropic deformation detection for wearable and e-skin applications

Flexible electronics represent a viable technology for the development of innovative mechanical sensors. This paper reports a detailed study of electro-mechanical performances of Organic Field-Effect Transistor-based sensor, investigating the role of source-drain electrodes layout in combination with organic semiconductor morphology obtained by different patterning methods. Two different sensor structures, with interdigitated and spiral-shaped source and drain electrodes, are employed together with solution-processed organic semiconductors deposited by drop-casting or patterned by means of meniscus-guided printing. This technique allows the orientation of crystalline domains to specific directions, and was employed to provide anisotropic or isotropic semiconductor patterns onto the transistor’s channel area. The different device configurations are tested as strain gauges and tactile sensors, by imposing anisotropic surface strain or complex deformations by means of custom-made, 3D-printed indenters. A wise choice of device structure and semiconductor patterning allows optimizing sensing performances as a response to specific deformations: interdigitated devices with crystalline domains aligned along the channel length direction are ideal strain gauges, while sensors with spiral-shaped electrodes in combination with isotropic semiconductor patterning are preferential for reproducing the sense of touch, which deals with the transduction of more complex deformation patterns. These results pave the way to the development of innovative sensors in the field of flexible bioengineering, in particular for the development of wearable and e-skin applications for joint motion monitoring and tactile sensing

Spray-Coated, Magnetically Connectable Free-Standing Epidermal Electrodes for High Quality Biopotential Recordings

Acquiring biopotentials from the surface of the body is a common procedure both in the clinical practice and in non-clinical applications as sport and human- machine interfaces. To avoid bulky recording systems and to allow optimal long-term measurements, several tattooable solutions were recently developed, aiming at high-quality and imperceptible electrodes. However, a seamless connection with epidermal electrodes still represents one of the biggest challenges in this field. In this paper, we propose a simple and efficient approach for the fabrication of free-standing epidermal electrodes that can be contacted using small magnetic connectors, thus directly tackling this issue. The proposed electrodes are fabricated using a conductive ink based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) deposited by spray coating, and can be easily contacted using magnetic connectors without disrupting their conformability, thanks to the presence of ferrite nanoparticles integrated within the thin film itself. These electrodes have been successfully employed for the detection of different biopotentials, namely electrocardiogram, electromyogram and electro-oculogram, demonstrating excellent performances for the detection of biosignals from delicate body parts, such as the face, thus demonstrating the effectiveness of the approach for the development of a new generation of magnetically connectable epidermal electrodes for critical biopotentials monitoring

A Modular BLE-Based Body Area Network Embedded into a Smart Garment for Rescuers Real-Time Monitoring in Emergency Scenarios

In this work, we present a prototype of a smart technical underwear for first responders involved in searchand-rescue operations, to be worn under the rescuer’s professional uniform. Polymer-based electrodes able to detect ECG and EMG signals, and organic transistor for joint angles estimation are embedded into the smart garment. The technical underwear implements a body sensor network of BLE nodes able to acquire, process in real-time and transmit electrophysiological and biomechanical data from the sensors to a custom Android app on the rescuer’s smartphone. The app geolocates the data by using the information of the GPS integrated into the smartphone and sends them to the control center for remote monitoring. The system features high modularity, as the rescuer can adopt a subset of sensors depending on the specific operative context, without any app configuration

Unveiling the significance of adduct formation between thiocarbonyl Lewis donors and diiodine for the structural organization of rhodanine-based small molecule semiconductors

Rhodanine vinyl bithiophene (BTR) was synthesized and characterized both spectroscopically and structurally. The reaction of BTR with molecular iodine led to the 1 : 1 “spoke” adduct BTR·I2, formed by interaction of the rhodanine thiocarbonyl group with a diiodine (I2) molecule. The elongation of the I–I bond in the adduct with respect to solid-state I2 and the Raman response in the low-energy region (ν = 150 cm−1) clearly indicate BTR·I2 to be a weak CT adduct. Hybrid-DFT calculations showed that the adduct formation narrowed the HOMO–LUMO gap in BTR·I2 as compared to BTR, while the extended network of secondary interactions, including type-I halogen bonds (XB), results in the formation of an extended 3D network. As a consequence, the room temperature conductivity of BTR·I2 increased with respect to BTR, testifying for a more efficient molecular packing for charge percolation, with the formation of charge carriers in the crystal being facilitated by the presence of I2. It is worth noting that the single-crystal junction device operates at room temperature, in air, and no variation of conductivity over time was observed, indicating that no loss of diiodine occurred during measurements. These results clearly indicate the formation of thiocarbonyl–diiodine CT adducts and their potential as a solid additive for modulating the organization of small molecule semiconductors