Preliminary comparison of organic electret-style devices on silicon and flexible plastic substrates as real-time ionizing radiation dosimeters

The use of organic electronics for radiation sensing applications has become increasingly popular in recent years. The inherently low atomic number of the organic components of these devices is cited as motivation for their exploration as tissue-equivalent dosimeters. In this work we fabricate organic thin film transistors (OTFTs) with a polymer electret layer as part of the gate dielectric stack on flexible polyethylene terephthalate (PET) substrates to get a lower average Z, lighter weight, and flexible device. The response of these devices is evaluated by comparing to the same device made on silicon substrates that we have made in the past. To evaluate dose-rate response these devices were irradiated with a 6 MV photon beam with dose rates of 60-600 MU/min. Energy dependence was evaluated by irradiating with 100, 180, and 300 kVp photon beams from an orthovoltage unit and 6 and 10 MV beams from a linear accelerator. Real-time readout was achieved by measuring the drain current with the gate and drain voltage at -15 V. Silicon and PET devices both demonstrated excellent linearity with dose rate. Si devices had a sensitivity of 79.7 nA/Gy and PET devices had a sensitivity of 33.4 nA/Gy. At 100 kVp Si and PET both had an increase in signal compared to at 6 MV of 2.46 and 4.76 times respectively. The larger energy dependence in PET devices is likely due to the high atomic number of the Indium-Tin-Oxide and gold layers. These devices on PET need improvements in sensitivity and energy dependence to achieve similar or better performance than their Si counterparts.