Organic single crystals as low power, room temperature electrical X-ray detectors
Organic semiconductors are promising materials for several applications, ranging from thin film transistors (TFT), to light emitting diodes (LED), to solar cells and sensors. As detectors for ionizing radiation, organic semiconductors have so far received less attention, their main use being in scintillators, which convert ionizing radiation into visible photons, and in photodiodes, which turn visible photons coming from a scintillator into an electrical signal. Only few examples of direct conversion of the ionizing radiation into an electrical signal have so far been reported, and they were always related to semiconducting or conducting polymer thin films, or charge-transfer conducting organic crystals, based on the presence, inside the detector, of metallic electrodes exposed to the ionizing radiation. |
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Figure 2: (a) switching response under x-rays beam of a 4HCB-based device at different voltages; (b) plot of the device response (ΔI = ION-IOFF) vs. dose rate along the vertical axis of a 4HCB-based device, held at different operating voltages, with the metal (Ag) electrodes exposed to the x-rays beam; (c) plot of the device response ΔI vs. the applied voltage for 4HCB-based devices. Black squares: metal (Ag) electrodes shielded from the x-rays beam; red circles: the same device after irradiation with an overall x-rays dose of 2.1 kGy; blue triangles: device tested after an aging period of 1 month; green stars: response of a 4HCB-based all-organic device, contacted with PEDOT:PSS electrodes; (d) micrograph of an all-organic device based on 4HCB.
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To assess the radiation hardness under x-rays of the tested OSSCs, a 4HCB-based detector was exposed to a massive amount of radiation (yielding a total dose of 2.1 kGy). After that, the device was again tested under an on/off switching x-rays beam, showing that the previous heavy x-rays exposure did not affect the overall device performance (Fig. 2c, black squares: response before the x-rays high dose exposure; red circles: response after the exposure). In addition, the overall device performances did not vary even for aged devices (Fig. 2c, blu triangles).
Finally, the intrinsic response of the OSSC to the x-rays was measured by realizing a fully organic, flexible detector, using poly(dimethylsiloxane), PDMS, as a substrate, and PEDOT:PSS, a conducting polymer blend, as the electrodes material. The performances of this device were comparable to those of another one fabricated using Ag electrodes shielded from x-rays (thus not contributing with secondary electrons to the overall detection mechanism; see Fig. 2c, black squares vs. green stars).
All-organic devices (like those shown in Fig. 2d), thanks to their low cost, ease of fabrication and potentially widespread availability, can represent a very interesting technological development in the field of ionizing radiation detectors.
This research was conducted by the following team:
- Beatrice Fraboni, Andrea Ciavatti , Francesco Merlo, Luca Pasquini, Anna Cavallini – Dipartimento di Fisica, Università di Bologna, Italy
- Alberto Quaranta - Dipartimento Ingegneria dei Materiali e Tecnologie Industriali (DIMTI), Università di Trento, Italy
- Annalisa Bonfiglio - Dipartimento Ingegneria Elettrica ed Elettronica, Università di Cagliari
- Alessandro Fraleoni-Morgera - Sincrotrone Trieste – Organic OptoElectronics Lab
Reference
Beatrice Fraboni, Andrea Ciavatti , Francesco Merlo, Luca Pasquini, Anna Cavallini, Alberto Quaranta, Annalisa Bonfiglio, and Alessandro Fraleoni-Morgera, “Organic Semiconducting Single Crystals as Next Generation of Low-Cost, Room-Temperature Electrical X-ray Detectors”,Advanced Materials, 24, 2289 (2012), doi: 10.1002/adma.201200283.