KEYWORDS: Sensors, Radiation dosimetry, Field effect transistors, X-rays, Solid state electronics, Ionizing radiation, Gamma radiation, Environmental sensing, Control systems
Here, we report on an all-organic solid-state radiation dosimeter patterned onto a plastic substrate that allows for real-time measurements communicated over WiFi. The “sense” area and the conductive traces are made using low-conductivity PEDOT:PSS, and measurements are read out by a low-current op-amp. As the detector is subjected to radiation, the ionized air, substrate, and sense area cause a charge accumulation which is then read out as a voltage from the op-amp. OFETs on either side of the sense area allow for the charge to be cleared, allowing for accurate dose measurement without saturation. Additionally, the inclusion of a PEDOT:PSS ground plane as the first layer on the PEN substrate helps to shield the sensor itself from extraneous static. For X-rays, the limit of detection is approximately 5 mRad/min, and for gamma rays the limit is approximately 5 mRad/hr. Through appropriate control of the clearing OFETs, the device is quickly reset to allow for a continuous measurement.
The development of solid-state radiation dosimeters has been crucial in allowing human workers to thrive while using tools that output ionizing radiation. Here, we report on solid-state tissue-equivalent radiation dosimeters based on PEDOT:PSS. We show reliable measurements of the radiation dosimeters subjected to a wide range of exposure energies and footprints of both X-ray and gamma. In addition, there is a strong indication that the PEDOT:PSS-based devices also give response when introduced to neutron sources. This represents a significant step forward in the production of cheap, reusable radiation dosimeters made with materials of similar radiation cross-section to the human body.
Organic thin-film transistors (OTFTs) are a key technology for enabling novel electronics such as flexible displays, low-cost sensors, or printed RFID tags. Device mobility is the primary figure of merit for OTFTs, but a low contact resistance is critical to achieving marketable performance. The energy level mismatch at the electrode/semiconductor interface hinders charge injection, limiting device performance. This issue has been addressed through chemical treatments with self-assembled monolayers, insertion of metal oxide interlayers, and doping. Here we combine these treatments with modified electrode deposition and post-deposition processing and evaluate the impact on the device properties. Specifically, we alter the contact deposition rate and flame anneal the electrode surface in bottom contact/top gate OTFTs based on the polymer semiconductor indacenodithiophene-co-benzothiadiazole (C16IDT-BT). Tuning the deposition rate leads to larger, flatter grains of gold, increasing the degree of order within the SAM at its surface and creating high work function channels that enhance charge injection. We achieved contact resistances of 200 Ωcm, boosting device mobility up to 10 cm2V-1s-1, a factor of three improvement over previous C16IDT-BT devices in this geometry with the same gate dielectric. We found that flame annealing is effective for further optimizing the gold contact surfaces, increasing grain size by an order of magnitude over those in as-deposited films. Here, a butane torch was passed directly over the contacts and substrate for a short period of time (5 minutes). We determined the impact on device characteristics, including mobility, on/off ratio, subthreshold swing, and threshold voltage.
The electrical performance of organic thin-film transistors (OTFTs) continues to improve, but the effect of electromagnetic radiation on the device performance is still unclear. OTFTs made with solution-processed 5, 11 bis(triethylsilylethynyl) anthradithiophene (diF-TES ADT) in a bottom-gated bottom-contact configuration were fabricated on SiO2 gate dielectric and the interaction of visible light with the semiconducting layer was studied. Monochromatic illumination (λ = 532 nm) that matches the highest absorption band of crystalline diF-TES ADT was used to generate a large number of carriers during device operation. We observe that the OTFTs showed an efficient photocurrent response when incident light of intensity ranging from 1 to 11 µW/µm2 was focused at the center of the channel. Over this range, transfer characteristic curves shifted by up to +14 V as illumination was increased. At an intensity of 1 µW/µm2, the ratio of the number of photons absorbed (and thus excitons generated) to the number of holes measured at the electrode was approximately equal to one. With a five-fold increase of the illumination intensity, we found that the ratio of the excitons generated to the measured charge carriers was an order of magnitude less indicating that the effects of trapping in OFETs has a stronger impact at higher incident power.
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