Transport Properties and Novel Sensing Applications of Organic Semiconducting Crystals

The present thesis is focused on the study of Organic Semiconducting Single Crystals (OSSCs) and crystalline thin films. In particular solution-grown OSSC, e.g. 4-hdroxycyanobenzene (4HCB) have been characterized in view of their applications as novel sensors of X-rays, gamma-rays, alpha particles radiations and chemical sensors. In the field of ionizing radiation detection, organic semiconductors have been proposed so far mainly as indirect detectors, i.e. as scintillators or as photodiodes. I first study the performance of 4HCB single crystals as direct X-ray detector i.e. the direct photon conversion into an electrical signal, assessing that they can operate at room temperature and in atmosphere, showing a stable and linear response with increasing dose rate. A dedicated study of the collecting electrodes geometry, crystal thickness and interaction volume allowed us to maximize the charge collection efficiency and sensitivity, thus assessing how OSSCs perform at low operating voltages and offer a great potential in the development of novel ionizing radiation sensors. To better understand the processes generating the observed X-ray signal, a comparative study is presented on OSSCs based on several small-molecules: 1,5-dinitronaphthalene (DNN), 1,8-naphthaleneimide (NTI), Rubrene and TIPS-pentacene. In addition, the proof of principle of gamma-rays and alpha particles has been assessed for 4HCB single crystals. I have also carried out an investigation of the electrical response of OSSCs exposed to vapour of volatile molecules, polar and non-polar. The last chapter deals with rubrene, the highest performing molecular crystals for electronic applications. We present an investigation on high quality, millimeter-sized, crystalline thin films (10 – 100 nm thick) realized by exploiting organic molecular beam epitaxy on water-soluble substrates. Space-Charge-Limited Current (SCLC) and photocurrent spectroscopy measurements have been carried out. A thin film transistor was fabricated onto a Cytop® dielectric layer. The FET mobility exceeding 2 cm2/Vs, definitely assess the quality of RUB films

Fully Textile X-Ray Detectors Based on Fabric-Embedded Perovskite Crystals

The interest and thrust for wearable ionizing radiation dosimeters are rapidly growing, stimulated by a large number of different applications impacting on humankind, spanning from medicine to civil security and space missions. Lead halide perovskites are considered one of the most promising classes of novel materials for X-ray detectors due to their superior electronic and detection performance coupled with compatibility with solution-based printing processes, allowing fabrication onto flexible substrates. It is reported on fully textile perovskite-based direct X-ray detectors, where the photoactive layer is constituted by a silk-satin fabric functionalized with methylammonium lead bromide perovskite crystals embedded in the textile. The reliability of the proposed fabrication process, based on simple and low-tech deposition techniques adaptable to industrial printing technologies for textiles, is assessed by realizing different detector's architectures that exhibit comparable detection performances. Sensitivity values up to (12.2 +/- 0.6) mu C Gy(-1) cm(-2) and a limit of detection down to 3 mu Gy s(-1) are achieved, and low bias operation (down to 1 V) is demonstrated, validating wearable applications. Further, fully textile pixelated matrix X-ray sensors are implemented and tested, providing the proof of principle for large-area scalability

Radiation Hardness and Defects Activity in PEA2PbBr4 Single Crystals

Metal halide perovskites (MHPs) are low-temperature processable hybrid semiconductor materials with exceptional performances that are revolutionizing the field of optoelectronic devices. Despite their great potential, commercial deployment is hindered by MHPs lack of stability and durability, mainly attributed to ions migration and chemical interactions with the device electrodes. To address these issues, 2D layered MHPs have been investigated as possible device interlayers or active material substitutes to reduce ion migration and improve stability. Here we consider the 2D perovskite PEA2PbBr4 that was recently discussed as very promising candidate for X-ray direct detection. While the increased resilience of PEA2PbBr4 detectors have already been reported, the physical mechanisms responsible for such improvement compared to the standard "3D" perovskites are not still fully understood. To unravel the charge transport process in PEA2PbBr4 crystals thought to underly the device better performance, we adapted an investigation technique previously used on highly resistive inorganic semiconductors, called photo induced current transient spectroscopy (PICTS). We demonstrate that PICTS can detect three distinct trap states (T1, T2, and T3) with different activation energies, and that the trap states evolution upon X-ray exposure can explain PEA2PbBr4 superior radiation tolerance and reduced aging effects. Overall, our results provide essential insights into the stability and electrical characteristics of 2D perovskites and their potential application as reliable and direct X-ray detectors

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 Detection of 5-MeV Protons by Flexible Organic Thin-Film Devices

The direct detection of 5-MeV protons by flexible organic detectors based on thin films is here demonstrated. The organic devices act as a solid-state detector, in which the energy released by the protons within the active layer of the sensor is converted into an electrical current. These sensors can quantitatively and reliably measure the dose of protons impinging on the sensor both in real time and in integration mode. This study shows how to detect and exploit the energy absorbed both by the organic semiconducting layer and by the plastic substrate, allowing to extrapolate information on the present and past irradiation of the detector. The measured sensitivity, S = (5.15 ± 0.13) pC Gy−1, and limit of detection, LOD = (30 ± 6) cGy s−1, of the here proposed detectors assess their efficacy and their potential as proton dosimeters in several fields of application, such as in medical proton therapy

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

Highly efficient surface-emitting semiconductor lasers exploiting quasi-crystalline distributed feedback photonic patterns

Abstract: Quasi-crystal distributed feedback lasers do not require any form of mirror cavity to amplify and extract radiation. Once implemented on the top surface of a semiconductor laser, a quasi-crystal pattern can be used to tune both the radiation feedback and the extraction of highly radiative and high-quality-factor optical modes that do not have a defined symmetric or anti-symmetric nature. Therefore, this methodology offers the possibility to achieve efficient emission, combined with tailored spectra and controlled beam divergence. Here, we apply this concept to a one-dimensional quantum cascade wire laser. By lithographically patterning a series of air slits with different widths, following the Octonacci sequence, on the top metal layer of a double-metal quantum cascade laser operating at THz frequencies, we can vary the emission from single-frequency-mode to multimode over a 530-GHz bandwidth, achieving a maximum peak optical power of 240 mW (190 mW) in multimode (single-frequency-mode) lasers, with record slope efficiencies for multimode surface-emitting disordered THz lasers up to ≈570 mW/A at 78 K and ≈720 mW/A at 20 K and wall-plug efficiencies of η ≈ 1%

Molecular Weight Tuning of Organic Semiconductors for Curved Organic-Inorganic Hybrid X-Ray Detectors

Curved X-ray detectors have the potential to revolutionize diverse sectors due to benefits such as reduced image distortion and vignetting compared to their planar counterparts. While the use of inorganic semiconductors for curved detectors are restricted by their brittle nature, organic-inorganic hybrid semiconductors which incorporated bismuth oxide nanoparticles in an organic bulk heterojunction consisting of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) are considered to be more promising in this regard. However, the influence of the P3HT molecular weight on the mechanical stability of curved, thick X-ray detectors remains less well understood. Herein, high P3HT molecular weights (>40 kDa) are identified to allow increased intermolecular bonding and chain entanglements, resulting in X-ray detectors that can be curved to a radius as low as 1.3 mm with low deviation in X-ray response under 100 repeated bending cycles while maintaining an industry-standard dark current of mu C Gy(-1) cm(-2). This study identifies a crucial missing link in the development of curved detectors, namely the importance of the molecular weight of the polymer semiconductors used

Tissue Equivalent Curved Organic X-ray Detectors Utilizing High Atomic Number Polythiophene Analogues

Organic semiconductors are a promising material candidate for X-ray detection. However, the low atomic number (Z) of organic semiconductors leads to poor X-ray absorption thus restricting their performance. Herein, the authors propose a new strategy for achieving high-sensitivity performance for X-ray detectors based on organic semiconductors modified with high –Z heteroatoms. X-ray detectors are fabricated with p-type organic semiconductors containing selenium heteroatoms (poly(3-hexyl)selenophene (P3HSe)) in blends with an n-type fullerene derivative ([6,6]-Phenyl C71 butyric acid methyl ester (PC70BM). When characterized under 70, 100, 150, and 220 kVp X-ray radiation, these heteroatom-containing detectors displayed a superior performance in terms of sensitivity up to 600 ± 11 nC Gy−1 cm−2 with respect to the bismuth oxide (Bi2O3) nanoparticle (NP) sensitized organic detectors. Despite the lower Z of selenium compared to the NPs typically used, the authors identify a more efficient generation of electron-hole pairs, better charge transfer, and charge transport characteristics in heteroatom-incorporated detectors that result in this breakthrough detector performance. The authors also demonstrate flexible X-ray detectors that can be curved to a radius as low as 2 mm with low deviation in X-ray response under 100 repeated bending cycles while maintaining an industry-standard ultra-low dark current of 0.03 ± 0.01 pA mm−2

Morphology and mobility as tools to control and unprecedentedly enhance X-ray sensitivity in organic thin-films

Organic semiconductor materials exhibit a great potential for the realization of large-area solution-processed devices able to directly detect high-energy radiation. However, only few works investigated on the mechanism of ionizing radiation detection in this class of materials, so far. In this work we investigate the physical processes behind X-ray photoconversion employing bis-(triisopropylsilylethynyl)-pentacene thin-films deposited by bar-assisted meniscus shearing. The thin film coating speed and the use of bis-(triisopropylsilylethynyl)-pentacene:polystyrene blends are explored as tools to control and enhance the detection capability of the devices, by tuning the thin-film morphology and the carrier mobility. The so-obtained detectors reach a record sensitivity of 1.3 · 104 µC/Gy·cm2, the highest value reported for organic-based direct X-ray detectors and a very low minimum detectable dose rate of 35 µGy/s. Thus, the employment of organic large-area direct detectors for X-ray radiation in real-life applications can be foreseen.I.F., L.B., A.C., and B.F. acknowledge funding from INFN through the CSN5 FIRE project. This work was also funded by the ERC StG 2012-306826 e-GAMES and the DGI (Spain) project FANCY CTQ2016-80030-R. The authors also thank the Generalitat de Catalunya (2017-SGR-918), the Networking Research Center on Bioengineering, Biomaterials, and Nanomedicine (CIBER-BBN), and the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centers of Excellence in R&D (SEV-2015-0496). I.T. and A.T. are enrolled in the Materials Science PhD program of Universitat Autònoma de Barcelona and acknowledge FPU fellowship from the Spanish Ministry.Peer reviewe