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

Medical Applications of Tissue-Equivalent, Organic-Based Flexible Direct X-Ray Detectors

The aim of this study is to assess direct X-ray detectors based on organic thin films, fabricated onto flexible plastic substrates, and operating at ultra-low bias (<1 V), for different medical applications. With this purpose, flexible fully organic pixelated X-ray detectors have been tested at the imaging beamline SYRMEP (SYnchrotron Radiation for MEdical Physics) at the Italian synchrotron Elettra, Trieste. The detectors' performance has been assessed for potential employment both as reliable wearable personal dosimeters for patients and as flexible X-ray medical imaging systems. A spatial resolution of 1.4 lp mm−1 with a contrast of 0.37 has been evaluated. Finally, we validate the detector using X-ray doses and energies typically employed for actual medical radiography, and using X-ray beam pulses provided by a commercial dental radiography system, recording a sensitivity of 1.6 × 105 μC Gy−1 cm−3 with a linear response with increasing of the dose rates and a reliable signal to 100 ms X-rays pulses

Roadmap on printable electronic materials for next-generation sensors

The dissemination of sensors is key to realizing a sustainable, ‘intelligent’ world, where everyday objects and environments are equipped with sensing capabilities to advance the sustainability and quality of our lives—e.g., via smart homes, smart cities, smart healthcare, smart logistics, Industry 4.0, and precision agriculture. The realization of the full potential of these applications critically depends on the availability of easy-to-make, low-cost sensor technologies. Sensors based on printable electronic materials offer the ideal platform: they can be fabricated through simple methods (e.g., printing and coating) and are compatible with high-throughput roll-to-roll processing. Moreover, printable electronic materials often allow the fabrication of sensors on flexible/stretchable/biodegradable substrates, thereby enabling the deployment of sensors in unconventional settings. Fulfilling the promise of printable electronic materials for sensing will require materials and device innovations to enhance their ability to transduce external stimuli—light, ionizing radiation, pressure, strain, force, temperature, gas, vapours, humidity, and other chemical and biological analytes. This Roadmap brings together the viewpoints of experts in various printable sensing materials—and devices thereof—to provide insights into the status and outlook of the field. Alongside recent materials and device innovations, the roadmap discusses the key outstanding challenges pertaining to each printable sensing technology. Finally, the Roadmap points to promising directions to overcome these challenges and thus enable ubiquitous sensing for a sustainable, ‘intelligent’ world

Influence of device geometry in the electrical behavior of all organic ambipolar field effect transistors

Microcontact printing has been used for realizing all organic, highly flexible, ambipolar field effect transistors based on a pentacene-C60 heterojunction. The contacts were patterned using poly(ethylenedioxythiophene): poly(styrenesulfonate) as conductive polymer. We have compared three different device architectures, bottom contact, top contact, and middle contact, and demonstrated in the last case that a perfect balance between n- and p-type mobilities may be obtained, being source ad drain directly interfaced with both semiconductors. This led to a significant improvement of the electrical performances, which, in turn, allowed the realization of all organic complementary inverters