(Invited) Photo Induced Effects in OTFTs: Mechanisms and Applications
Authors: Mariucci Luigi , Rapisarda Matteo , Valletta Antonio , Fortunato Guglielmo , Carturan Sara , Branchini Paolo , Calvi Sabrina 
Published: July 2021

The technology based on organic devices and sensors is becoming attractive for a wide range of applications. Indeed, the intrinsic characteristics of the organic materials, as the low-temperature process, flexibility, lightweight, biocompatibility, and even biodegradability, make them the basis for devices for the emerging flexible plastic electronics. Among the different possible applications, the electro-optical properties of organic semiconductors can be exploited for the development of devices for photosensing [1], imaging array [2], and optical memory [3]. The wide number of organic materials with different absorption spectra allows the development of photosensors with large spectral sensitivity or tuned to specific wavelengths. In particular, organic phototransistors (OPTs) are particularly promising since they combine light detection, typical of the photodiodes (OPD), with the intrinsic signal amplification that increases the device sensitivity. The response of the OPTs to light irradiation was widely studied in the last years. The photons with energy above the optical gap generate excitons that can dissociate into free holes and electrons and, in p-type devices, the photo-generated electrons are trapped. Negative trapped charges induce the shift of the flat band voltage as well as the threshold voltage of the OPT resulting in the increase of the drain current [4]. The location and the nature of the defect states are still under investigation. Depending on the organic semiconductor, gate dielectric and device structure, the defects have been identified as interface states between semiconductor and insulators [2], defects into the semiconductors [3], defects induced by the diffusion of oxygen or moisture into the OSC [5]. Trapped charge near the source/drain electrodes can also reduce the injection contact barrier, increasing the drain current [6]. In any case, photo-induced effects appear to be a quite general effect since they are observed in devices with different semiconductor molecules and different gate dielectric (both organic and inorganic). In this presentation, the different mechanisms will be reviewed and the photo-induced effects will be discussed for OTFTs with evaporated and solution-processed semiconductors and different device structures. The high stability of these devices under gate bias stress, allows to investigate in detail the effects induced by the light.
Furthermore, this presentation will show the application of the electro-optical properties of the OTFTs to the development of radiation detectors based on Organic Photo Transistors (OPTs), made on flexible substrate, and coupled with plastic scintillators. The OPTs are based on organic semiconductor dinaphtho [2,3-b:2′,3′-f] thieno[3,2-b] thiophene (DNTT) and have Bottom Gate/Top contact multi-finger device structure in order to simplify the device coupling with scintillator and increase the device area. The channel length, L, and channel width, W, are 100 µm and 2 cm, respectively. The gate, source and drain, and the semiconductor layer (DNTT, 30 nm thick) are thermally evaporated through metal masks. Spin coated layers of Cytop are used as gate dielectric and passivation layer. Thanks to the high quality of the semiconductor and of the dielectric/semiconductor interface, the devices show good electrical characteristics, with field effect mobility of about 1 cm2/Vs, a threshold voltage of the order of -10 V and subthreshold voltage of about 1V/dec. Furthermore, the devices do not show hysteresis of the transfer characteristics and are quite stable under bias stress. The OPTs have been characterized by electrical and optical measurement in order to evaluate the figures of merit parameters of the photodetectors, such as the photo-switching speed, the responsivity, and the photocurrent on/off ratio (also indicated as photosensitivity). The measurements have been carried out under monochromatic light with λ= 450 nm (above the optical gap of the DNTT). At this wavelength, these OPTs are able to detect very low light intensity (of the order of few nW/cm2), with a responsivity of 0.25 A/W and sensitivity of 2x10-2. The OPTs are successfully coupled with siloxane based scintillator.
This work is supported by INFN-CNS5 project "FIRE".
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