Semiconducting polymers in radiation detectors
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. In a collaboration between the Soft Matter Group and researchers in the Centre for Nuclear and Radiation Physics at Surrey, radiation sensors that use semiconducting polymers in the active layer have been developed. The team reported the first polymeric X-ray detector in 2007 (Boroumand et al., Applied Physics Letters, 91 (2007) 033509 (3 pages)).
Since 2007, the sensitivity of the detectors have been increased as a result of research efforts. Blending the polymer with semiconducting small molecules (pentacene) was shown to increase the charge carrier mobility.
The crystalline pentacene provides a fast route for charge carriers and leads to enhanced performance of the X-ray sensor. Time-of-flight charge-carrier mobility measurements of this blend were made by the team. When pentacene was added, the mobility increased by a factor of 17. This higher mobility led to a four-fold increase in the X-ray sensitivity. To learn more, see : Akarin Intaniwet, Joseph L. Keddie, Maxim Shkunov, and Paul J. Sellin (2011) “High charge-carrier mobilities in blends of poly(triarylamine) and TIPS-pentacene leading to better performing X-ray sensors,” Organic Electronics, 12(11), 1903-1908.
Another strategy , used by the team, is to add nanoparticles of heavy elements, which absorb more of the X-radiation, rather than allowing it to pass through the detector. When bismuth oxide (Bi2O3) nanoparticles (Z = 83 for Bi) are added to a semiconducting polymer in the active layer of an X-ray detector the sensitivity increased by approximately 2.5 times compared to the plain PTAA sensor. These results indicate that the addition of high-Z nanoparticles improves the performance of the dosimeters by increasing the X-ray stopping power of the active volume of the diode. Because the Bi2O3 has a high density, it can be used very efficiently, achieving a high weight fraction with a low volume fraction of nanoparticles. The mechanical flexibility of the polymer is not sacrificed when the inorganic nanoparticles are incorporated. More information is found in A. Inaniwet et al., (2012) Nanotechnology 23, 235502 (7pp).
In an X-ray sensor, a semiconducting polymer is sandwiched between two contacts, such as Al and indium tin oxide. A field is put across the polymer layer. The when X-radiation is absorbed in the polymer, charge carriers are generated and a photocurrent flows. The X-ray photocurrent is proportion to the X-ray dose.
Photograph of a PTAA-Bi2O3 film after peeling off of an ITO/glass substrate. The polymer retains its mechanical flexibility after the addition of the inorganic nanoparticles.
(A) An SEM image (scale bar = 3 mm) and (B) EDX elemental analysis of the cross-section of a sample with 60 wt.% Bi2O3 on an ITO substrate. The materials are represented as follows: PTTA is red; Bi2O3 is green; and ITO is violet. The two white lines are drawn to designate the top and bottom edges of the cross-section.