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.

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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.