The deployment of organic semiconducting materials for radiation detection is an emerging and highly attractive area of materials science research. These organic materials offer the enticing vision of technologies created from low-cost materials that can be printed on-demand with a range of different tailored optoelectronic functionalities. An explosion in the number of available materials, improved functionality of materials, and sophistication of solution-based device fabrication techniques for organic semiconductors in recent years have led to considerable opportunities for the utilization of organic materials in the detection of ionizing radiation. While the potential of organic semiconducting materials for low-cost radiation detection is clear, transitioning these printable materials to a commercial reality presents a significant scientific challenge. In this work, we provide a comprehensive analysis of the use of organic semiconductors for radiation detection. We discuss the fundamental physics of these materials and how their conduction mechanisms, including charge generation and charge transport, differ significantly from established inorganic semiconductors. Various strategies employed to control the nanostructure in organic semiconductors to optimize charge generation and transport for radiation detection are discussed. We provide insights into the strategies employed to fabricate organic semiconducting devices at industrially relevant scales using roll-to-roll solution processing and finally discuss existing examples of organic semiconducting materials utilized in the radiation detection arena.
The development of novel detectors for dosimetry in advanced radiotherapy modalities requires materials that have a water-equivalent response to ionizing radiation such that characterization of radiation beams can be performed without the need for complex calibration procedures and correction factors. Organic semiconductors are potentially an ideal technology in fabricating devices for dosimetry due to tissue-equivalence, mechanical flexibility and relatively cheap manufacturing cost. The response of a commercial organic photodetector (OPD), coupled to a plastic scintillator, to ionizing radiation from a linear accelerator and orthovoltage x-ray tube has been characterized to assess its potential as a dosimeter for radiotherapy. The radiation hardness of the OPD has also been investigated to demonstrate its longevity for such applications.Methods: Radiation hardness measurements were achieved by observing the response of the OPD to the visible spectrum and 70 keV x-rays after pre-exposure to 40 kGy of ionizing radiation. The response of a pre-irradiated OPD to 6 MV photons from a linear accelerator in reference conditions was compared to a non-irradiated OPD with respect to direct and indirect (RP400 plastic scintillator) detection mechanisms. Dose-rate dependence of the OPD was measured by varying the surface-to-source distance between 90 cm and 300 cm. Energy dependence was characterized from 29.5 keV to 129 2 keV with an x-ray tube. The percentage depth dose (PDD) curves were measured from 0.5 cm to 20 cm and compared to an ionization chamber. Results:The OPD sensitivity to visible light showed substantial degradation of the broad 450nm to 600 nm peak from the donor after irradiation to 40 kGy. After irradiation, the spectral shape has a dominant absorbance peak at 370 nm, as the acceptor better withstood radiation damage. Its response to x-rays stabilized to 30% after 35 kGy, with a 0.5% difference between 770 Gy increments. The OPD exhibited reproducible detection of ionizing radiation when coupled with a scintillator. Indirect detection showed a linear response from 25 cGy to 500 cGy and constant response to dose-rates from 0.31 Gy/pulse to 3.4 x10 -4 Gy/pulse. However, without the scintillator, response increased by 100% at low dose rates.Energy independence between 100 keV and 1.2 MeV advocates their use as a dosimeter without beam correction factors.A dependence on the scintillator thickness used during a comparison of the PDD to the ionizing chamber was identified.A 1 mm thick scintillator coupled with the OPD demonstrated the best agreement of ±3%. Conclusions:The response of OPDs to ionizing radiation has been characterized, showing promising use as a dosimeter when coupled with a plastic scintillator. The mechanisms of charge transport and trapping within organic materials varies for visible and ionizing radiation, due to differing properties for direct and indirect detection mechanisms and observing a substantial decrease of sensitivity to the visible spectrum after 40 kGy. This study proved that OP...
A new printable organic semiconducting material combination as a tissue equivalent photodetector for indirect X‐ray detection is demonstrated in this work. The device exhibits a higher optical‐to‐electrical conversion efficiency than any other reported printable organic systems for X‐ray photodetection while also operating efficiently with zero applied bias. Complete X‐ray detectors fabricated by coupling the photodiode with a plastic scintillator are among the first flexible and fully tissue equivalent X‐ray detectors capable of operating without external bias. The response to X‐rays is energy independent between 50 keV and 1.2 MeV, with a detection sensitivity equivalent to inorganic direct X‐ray detectors and one of the fastest temporal responses ever reported for organic X‐ray detectors. The materials can be printed into arrays with a pixel pitch of 120 μm, providing 2D spatial detection. The devices are found to be highly stable with respect to time, mechanical flexing, and large (5 kGy) radiation doses. The new materials and fully tissue equivalent X‐ray detectors reported here provide stable, printable, flexible, and tissue equivalent detectors with a high radiolucency that are ideally suited for wearable applications, where simultaneous monitoring and high transmission of the X‐ray absorbed dose into the human body is required.
There is growing interest in the development of novel materials and devices capable of ionizing radiation detection for medical applications. Organic semiconductors are promising candidates to meet the demands of modern detectors, such as low manufacturing costs, mechanical flexibility, and a response to radiation equivalent to human tissue. However, organic semiconductors have typically been employed in applications that convert low energy photons into high current densities, for example, solar cells and LEDs, and thus existing design rules must be re-explored for ionizing radiation detection where high energy photons are converted into typically much lower current densities. In this work, we report the optoelectronic and X-ray dosimetric response of a tissue equivalent organic photodetector fabricated with solution-based inks prepared from polymer donor poly(3-hexylthiophene) (P3HT) blended with either a non-fullerene acceptor (5Z,5′Z)-5,5′-((7,7′-(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (o-IDTBR) or a fullerene acceptor, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Indirect detection of X-rays was achieved via coupling of organic photodiodes with a plastic scintillator. Both detectors displayed an excellent response linearity with dose, with sensitivities to 6 MV photons of 263.4 ± 0.6 and 114.2 ± 0.7 pC/cGy recorded for P3HT:PCBM and P3HT:o-IDTBR detectors, respectively. Both detectors also exhibited a fast temporal response, able to resolve individual 3.6 μs pulses from the linear accelerator. Energy dependence measurements highlighted that the photodetectors were highly tissue equivalent, though an under-response in devices compared to water by up to a factor of 2.3 was found for photon energies of 30–200 keV due to the response of the plastic scintillator. The P3HT:o-IDTBR device exhibited a higher stability to radiation, showing just an 18.4% reduction in performance when exposed to radiation doses of up to 10 kGy. The reported devices provide a successful demonstration of stable, printable, flexible, and tissue-equivalent radiation detectors with energy dependence similar to other scintillator-based detectors used in radiotherapy.
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