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Device Fabrication and Characterization of OFET-RAD for Dosimetric Response of Megavoltage X-Rays

A Zeidell1, T Ren1,2*, D Filson1, H Haneef1, J Bourland1,2, J Anthony3, O Jurchescu1, (1) Wake Forest University, Winston-Salem, NC (2) Wake Forest School of Medicine, Winston-Salem, NC (3) University of Kentucky Center for Applied Energy Research, Lexington, KY

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Diodes and MOSFET detectors are made of high atomic number materials which are non-tissue equivalent and provide limited spatial dosimetry resolution. This study aims to develop and characterize the dosimetry response of a tissue-equivalent, organic field-effect transistor (OFET-RAD).

Methods: 2,8-Difluoro-5,11-bis (triethylsilylethynyl) (diF-TES-ADT) semiconductor film was deposited by spin coating to fabricate OFETs. The OFET-RAD samples were placed at depth 1.5 cm inside a 30 x 30 x 11.5 cm³ custom Solid-Water™ phantom. The samples were irradiated at SSD=100 cm by a standard 10 x 10 cm² field from an Elekta Versa HD™ linear accelerator with accumulated dose levels between 0 and 100 cGy with a 20 cGy increment, between 100 cGy to 1100 cGy with a 200 cGy increment. The threshold voltages were extracted from the square root of the drain current with respect to the gate-source voltage.

Results: The threshold voltage shift of the FETs was observed as a function of cumulative dose. Confirmed by capacitance measurements, the dielectric is unaffected by megavoltage x-ray irradiation. The charge carrier mobility remains unchanged as a function of dose. No significant chemical changes in the organic semiconductor film were observed from nuclear magnetic resonance, gas chromatography, UV-Vis, and thin-layer chromatography.

Conclusion: OFET-RAD have been fabricated and their radiation response evaluated. In the 0 to 100 cGy range, positive shifts of the threshold voltage were recorded indicating the electron trapping. In the 100 cGy to 1000 cGy range, negative shifts are recorded as a result of spatial disorder introduced in the organic semiconductor layer. OFET-RAD can be fabricated over a large area, are flexible, and can be conformal to a curved surface over a large area. Additionally, devices are mechanically robust, and can withstand bending radii of up to 9 mm.

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Funding Support, Disclosures, and Conflict of Interest: The device characterization was performed with support from grants from the National Science Foundation through Grants DMR-1386 1627925 and ECCS- 1810273.

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