Semiconductors are active materials able to directly convert high energy photons or other particles into electronic signals and are used to realize x-ray detectors, imagers or dosimeters. Our research targets novel materials that combine efficient optoelectronic conversion with tease of material processing over large areas and on flexible substrates at low cost. Indeed, several applications of high commercial interest, spanning from citizens’ security, to industrial and to medical diagnostics, require thin, conformable sensor panels, for a large-area determination of the incoming radiation dose and energy distribution.
Organic semiconducting materials possess appealing features (e.g. ease of deposition over large areas by means of solution-coating, low-cost techniques potentially onto flexible substrates) for innovative ionizing radiation detection applications. Moreover, the typical density of organic molecules closely resembles that of human tissues, making them very interesting for medical X-ray direct dosimetry applications.
We investigate the direct X-ray photoconversion in organic semiconducting single crystals and in polymer thin-films blended either with π-conjugated small molecules or inorganic high-Z nanocomponents to enhance the radiation stopping power of the material.
Recently, we reported about direct X-ray photo-conversion processes in micro-crystalline, solution processed, thin films deposited by drop casting onto flexible plastic substrates. This class of materials is characterized by an unexpected high X-ray sensitivity justified by photo-modulation of the semiconductor conductivity due to charge accumulation during X-ray exposure, resulting in a photoconductive gain effect.
Microelectronic dosimeters offer the prospect of monitoring ionizing radiation exposure at real time and with small geometric form factors. However, traditional microelectronic materials such as crystalline silicon cannot easily meet the application requirements of microelectronic dosimeters for two reasons: First, they do not absorb sufficient amounts of high energy radiation to be sensitive as they contain mostly elements with a rather low or medium atomic number Z. Second, as crystalline materials, they are fabricated on rigid wavers and cannot be patterned onto flexible plastic foils.
In order to overcome these limitations our research team has been investigating amorphous oxide semiconductors as novel material for microelectronic dosimeters. Our research partner from the Universidade Nova di Lissbon employs such semiconductors to fabricate thin film transistors on flexible plastic foils. Oxide semiconductors based on Indium Gallium Zink Oxide (IGZO) have the right structural and electronic properties to achieve fast transport of electrons as a n-type semiconductor even in the case of an amorphous, that is disordered microstructure. This opens the way to deposit the IGZO semiconductor by physical methods on large flexible substrates instead of relying on crystalline growth. For our research on microelectronic dosimeters, we combined the IGZO transistors with a high-Z oxide dielectric that absorbs the ionizing radiation and builds up a space charge layer. The combined device structure, that we call radiation sensitive oxide field effect transistor (ROXFET), is shown in the Figure to the left. By introducing the amorphous oxide based semiconductor, we achieve a microelectronic dosimeter that outperforms similar silicon based devices by an order of magnitude in sensitivity achieving the detection of radiation doses above 100 microgray. In addition we demonstrate that the dosimeter can be patterned in arrays on flexible plastic foil. The fast electronic transport properties of IGZO make it also possible to integrate the ROXFET in a passive RFID circuit. With support from the French company Tagsys-RFID we created the first wireless radiation sensor that is operated without a battery. Its detection mechanism is so energy efficient, that a radiofrequency signal as generated by an RFID reader provides enough energy to operate the sensor and to send back to the reader the information on the sensor irradiation status.
Hybrid organic-inorganic perovskites have been recently proposed as alternative materials for X- and γ-photon direct detection, thanks to their high Z constituent atoms, e.g. Pb in lead-halide perovskite, combined with a high charge mobility. Recently3, the first results on a thin-film direct X-ray detector based on solution-processed methylammonium lead triiodide (MAPbI3,) perovskite, has beem reported, with sensitivity values up to 25 μC mGy−1cm−3.
Recent related publications
Ciavatti, A.; Cramer, T.; Carroli, M.; Basiricò, L.; Fuhrer, R.; De Leeuw, D. M.; Fraboni, B., Dynamics of direct X-ray detection processes in high-Z Bi2O3nanoparticles-loaded PFO polymer-based diodes, «APPLIED PHYSICS LETTERS», 2017, 111, pp. 1 - 5
Basiricò, Laura; Ciavatti, Andrea; Cramer, Tobias; Cosseddu, Piero; Bonfiglio, Annalisa; Fraboni, Beatrice, Direct X-ray photoconversion in flexible organic thin film devices operated below 1 v, «NATURE COMMUNICATIONS», 2016, 7, pp. 1 - 9
Cramer, Tobias; Fratelli, Ilaria; Barquinha, Pedro; Santa, Ana; Fernandes, Cristina, D'Annunzio, Franck; Loussert, Christophe, Martins, Rodrigo; Fortunato, Elvira; Fraboni, Beatrice, Passive radiofrequency x-ray dosimeter tag based on flexible radiation-sensitive oxide field-effect transistor, «SCIENCE ADVANCES», 2018, 4, 6, eaat1825