OFET Sensors with Poly 3-hexylthiophene and Pentacene as Channel Materials for Ionizing Radiation
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OFET Sensors with Poly 3-hexylthiophene and Pentacene as Channel Materials for Ionizing Radiation Harshil N. Raval1 and V. Ramgopal Rao1 1 Centre of Excellence in Nanoelectronics, Department of Electrical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai – 400 076, India. ABSTRACT Pentacene and poly 3-hexylthiophene (P3HT) are the most promising p-type organic semiconducting materials for fabrication of organic field effect transistors (OFETs). OFETs with aforesaid organic semiconducting materials have been demonstrated as total dose detectors for ionizing radiation, wherein the changes in the electrical characteristic parameters, such as, increase in the OFF current, increase in the ON current, change in the current ratio, shift in the threshold voltage, change in the subthreshold swing, etc., were used as a measure of ionizing radiation dose. Upon exposure to ionizing radiation P3HT based OFET sensor has shown an OFF current sensitivity of 4.4 nA/Gy while pentacene based OFET sensor has shown an OFF current sensitivity of 26.7 nA/Gy for a total of 50 Gy dose of ionizing radiation. Change in the conductivity of the thin-films of pentacene and P3HT were observed and compared using electrostatic force microscopy (EFM) imaging before and after exposure to ionizing radiation. Effects of ionizing radiation on the energy band structures of the organic semiconducting materials, pentacene and P3HT, have been studied using UV-visible spectroscopy. Moreover, analysis of UV-visible spectra of the thin-films suggested the generation of energy states in larger quantity in case of pentacene thin-film as compared to P3HT thin-film upon exposure to the same dose of ionizing radiation. These results confirm the higher sensitivity observed in pentacene OFET sensor as compared to P3HT OFET sensor in terms of the change in electrical parameters. INTRODUCTION Organic semiconducting materials are being explored for various sensing applications as their material properties change upon interaction with various chemicals, gases, chemical vapors and high energy ionizing radiation [1-7]. These organic electronic sensors also offer various advantages, such as, low cost manufacturing and large area coverage with flexible substrates. Pentacene and P3HT are widely used p-type organic semiconducting materials for various organic electronic applications [8-10] and were also reported by us as sensors for ionizing radiation [3-5]. In this work, the change in the conductivity of the thin-films of pentacene and P3HT, upon exposure to ionizing radiation, was studied using EFM imaging technique. Moreover, using UV-visible spectrums of thin-films of these organic semiconducting materials, the generation of energy states upon exposure to ionizing radiation is proposed as the phenomenon responsible for the observed increase in the conductivity. Results of the UV-visible spectra are also used to justify the higher sensitivity of pentacene OFET as compared to P3HT OFET sensor. The chemical structure and details of pentacene and P3HT are shown
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