Semiconducting polymers have previously been used as the transduction material in X-ray dosimeters, but these devices have a rather low detection sensitivity because of the low X-ray attenuation efficiency of the organic active layer. Here, we demonstrate a way to overcome this limitation through the introduction of high density nanoparticles having a high atomic number (Z) to increase the X-ray
Thick film ͑Ն5 m thick͒ semiconducting polymer diodes incorporating poly͑triarylamine͒ ͑PTAA͒ have been produced and applied as direct x-ray detectors. Experiments determined that a rectifying diode behavior persists when increasing the thickness of the active layer above typical thin film thicknesses ͑Ͻ1 m͒, and the electrical conduction mechanism of the diodes has been identified. Direct current and photoconductivity measurements on indium tin oxide/ poly͑3,4-ethylenedioxythiophene͒/poly͑styrenesulfonate͒/PTAA/metal diodes confirm that carrier conduction occurs via a Poole-Frenkel mechanism. The energy band structure of diodes ͑having gold or aluminum top electrodes͒ has been elucidated and used to explain the resulting electrical characteristics. Theoretical calculations show that, upon irradiation with x-rays, the diode quantum efficiency increases with increasing polymer film thickness. The diodes produced here display characteristics similar to their thin film analogs, meaning that they may be operated in a similar way and therefore may be useful for radiation dosimetry applications. Upon irradiation, the diodes produce an x-ray photocurrent that is proportional to the dose, thus demonstrating their suitability for direct x-ray detectors. The x-ray photocurrent remains the same in a device after a cumulative exposure of 600 Gy and after aging for 6 months.
Abstract:Semiconducting polymer X-radiation detectors are a completely new family of low-cost radiation detectors with potential application as beam monitors or dosimeters. These detectors are easy to process, mechanically flexible, relatively inexpensive, and able to cover large areas. However, their X-ray photocurrents are typically low as, being composed of elements of low atomic number (Z), they attenuate X-rays weakly. Here, the addition of high-Z nanoparticles is used to increase the X-ray attenuation without sacrificing the attractive properties of the host polymer. Two types of nanoparticles (NPs) are compared: metallic tantalum and electrically-insulating bismuth oxide. The detection sensitivity of 5 µm thick semiconducting poly([9,9-dioctylfluorenyl-2,7-diyl]-co-bithiophene) diodes containing tantalum NPs is four times greater than that for the analogous NP-free devices; it is approximately double that of diodes containing an equal volume of bismuth oxide NPs. The X-ray induced photocurrent output of the diodes increases with an increased concentration of Published in: J. Phys. D: Appl. Phys. 46 (2013) 275102 2 nanoparticles. However, contrary to the results of theoretical X-ray attenuation calculations, the experimental current output is higher for the lower-Z tantalum diodes than the bismuth oxide diodes, at the same concentration of nanoparticle loading. This result is likely due to the higher tantalum NP elecrical conductivity, which increases charge transport through the semiconducting polymer, leading to increased diode conductivity.
Existing inorganic materials for radiation sensors suffer from several drawbacks, including their inability to cover large curved areas, lack of tissue-equivalence, toxicity, and mechanical inflexibility. As an alternative to inorganics, poly(triarylamine) (PTAA) diodes have been evaluated for their suitability for detecting radiation via the direct creation of X-ray induced photocurrents. A single layer of PTAA is deposited on indium tin oxide (ITO) substrates, with top electrodes selected from Al, Au, Ni and Pd. The choice of metal electrode has a pronounced effect on the performance of the device; there is a direct correlation between the diode rectification factor and the metal-PTAA barrier height. A diode with an Al contact shows the highest quality of rectifying junction, and it produces a high X-ray photocurrent (several nA) that is stable during continuous exposure to 50 kV Mo Kα X-radiation over long timescales, combined with a high signal-to-noise ratio with fast response times of less than 0.25 s. Diodes with a low band-gap, 'Ohmic' contact, such as ITO/PTAA/Au, show a slow transient response. This result can be explained by the build-up of space charge at the metal-PTAA interface, caused by a high level of charge injection due to X-rayinduced carriers. These data provide new insights into the optimum selection of metals for Schottky contacts on organic materials, with wider applications in light sensors and photovoltaic devices.
Flexible radiation dosimeters have been produced incorporating thick films (>1 μm) of the semiconducting polymer poly([9,9-dioctylfluorenyl-2,7-diyl]-co-bithiophene). Diode structures produced on aluminium-metallised poly(imide) substrates, and with gold top contacts, have been examined with respect to their electrical properties. The results suggest that a Schottky conduction mechanism occurs in the reverse biased diode, with a barrier to charge injection at the aluminium electrode. Optical absorption/emission spectra reveal a band gap of 2.48 eV for the polymer. The diodes have been used for direct charge detection of 17 keV X-rays, generated by a molybdenum source. Using operating voltages of -10 and -50 V respectively, sensitivities of 54 and 158 nC/mGy/cm 3 have been achieved. Increasing the operating voltage shows that the diodes are stable up to approximately -200 V without significant increase in the dark current of the device (<0.2 nA).
Incorporation of an effective additive 1,3-diaminoguanidine monohydrochloride in MAPbI3 film can increase the grain size and reduce trap density, consequently increasing the carrier lifetime and reducing charge recombination, leading to an improved efficiency from 19.1% to 20.3% in a p–i–n structured solar cell.
Abstract:Recently, a new family of low-cost X-radiation detectors have been developed, based on semiconducting polymer diodes, which are easy to process, mechanically flexible, relatively inexpensive, and able to cover large areas. To test their potential for radiotherapy applications such as beam monitors or dosimeters, as an alternative to the use of solid-state inorganic detectors, we present the direct detection of 6 MV X-rays from a medical linear accelerator using a thick film, semiconducting polymer detector. The diode was subjected to 4 ms pulses of 6 MV X-rays at a rate of 60 Hz, and produces a linear increase in photocurrent with increasing dose rate (from 16.7 to 66.7 mGy.s -1). The sensitivity of the diode was found to range from 13 to 20 nC.mGy -1 .cm -3 , for operating voltages from -50 to -150 V, respectively. The diode response was found to be stable after exposure to doses up to 15 Gy. Testing beyond this dose range was not carried out. Theoretical calculations show that the addition of heavy metallic nanoparticles to polymer films, even at low volume fractions, increases the X-ray sensitivity of the polymer film/nanoparticle composite so that it exceeds that for Published in: Physics in Medicine and Biology, 58 (2013) 2 silicon over a wide range of X-ray energies. The possibility of detecting X-rays with energies relevant to medical oncology applications opens up the potential for these polymer detectors to be used in detection and imaging applications using medical X-ray beams. IntroductionOver the past 60 years, radiation oncology apparatus has been developed, based on Linear Particle a lower activation energy is required to produce an ionisation pair (approx. 10 times lower), and the higher material density leads to a higher efficiency (up to 10 3 times greater), making them ideal produced from a medical LINAC. In this case, the attenuation coefficient of the polymer for the high energy X-rays is approx. 30 times less than that for the lower energy X-rays. Methods MaterialsPoly([9,9-dioctylfluorenyl-2,7-diyl]-co-bithiophene) (F8T2, number-average molecular weight (Mn) = 45,000 g.mol -1 , weight-average molecular weight (Mw) = 120,000 g.mol -1 ) was prepared as previously reported. [Theim et. al. 2005 accelerated at 100 rpm/s to 500 rpm and held for 60 s, 3) accelerated at 100 rpm/s to 2000 rpm and held for 30 s, 4) decelerated at 100 rpm/s. This procedure produced a relatively smooth polymer film with a thickness of approx. 10 μm. After this, the films were typically dry to the touch; a short period of drying under atmospheric conditions was however required for some films, before annealing under vacuum at 110°C for 24 h. The thickness of the polymer layers was subsequently measured using a surface profilometer (Dektak 8, Veeco Instruments). To complete the diode, gold (Au) or Al electrodes (100 nm thick, 0.5 x 0.5 cm 2 ), depending on the substrate used, were thermally evaporated onto the F8T2, through a shadow mask, at a pressure of 10 -6 mbar. The diodes were connected to the measurem...
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