X-Ray Detectors with Ultrahigh Sensitivity Based on High Performance Printed Organic Field Effect Transistors

In the last decade organic semiconductors have demonstrated to be excellent candidates for the development of a new class of X-Ray detectors able to fulfil important emerging requirements such as the mechanical flexibility, the possibility to cover large and curved surfaces, low-cost, and low-bias operation. Moreover, they are the only material platform that can offer human tissue equivalence, i.e. due to their chemical composition (i.e. low-Z elements) organic materials have radiation absorption comparable to that of human organs and tissues. This is a highly desirable property for a dosimeter to be employed in the medical field since it avoids complex calibration procedures and the perturbation of the radiation beam when the detector is placed between the radiation source and the patient. In this study we propose a double strategy to achieve ultrahigh sensitivity to X-rays by fully organic direct X-ray detectors. We exploited two different advanced printing techniques to deposit the active layer of the organic X-Ray detectors in a fully controlled way obtaining highly performant Organic Field Effect Transistors (OFETs). This approach allowed us to boost the detecting efficiency by means of both the improvement of the collection/transport properties of the devices and the deep comprehension, identification and control of the active trap states ruling the photoconductive gain mechanism of the thin film-based detectors. We deposited blends of polystyrene (PS) with the organic semiconducting small molecule 1,4,8,11-tetramethyl-6,13-triethylsilylethynyl pentacene (TMTES) by Bar Assisted Meniscus Shearing technique.[1] The molecular structure of this material is similar to TIPS-Pn ((6,13-bis(triisopropylsilylethynyl)-pentacene)), however, the BAMS deposited thin films crystallize in a totally different, and more efficient, crystal packing (i.e., herringbone packing motif). This resulted in OFETs that reached very high mobilities of up to 2.5 cm²V^-1s^-1 and showed a reduced density of interfacial hole traps thanks to vertical segregation of the polystyrene at the semiconductor/dielectric interface and the consequent passivation of defects. In the second case [2] we deposited organic thin films of TIPS-Pn and TIPG-Pn (analogous molecule where the two Si atoms have been substituted with two Ge atoms) by Pneumatic Nozzle Printing. This printing procedure allowed to tune the morphology and packing of the film, to selectively vary key features related to trap states affecting the charge transport and collection and, ultimately, the detecting mechanism. We investigated electrically active traps for minority carriers by Photocurrent Spectroscopy with Optical Quenching. This technique allowed to identify the excitonic peaks which induce the inner amplification mechanism under X-ray exposure. Thanks to the activation of the photoconductive gain effect induced by the simultaneous presence of UV-vis and X-Ray photons, we experimentally assessed for the first time the presence and role of electrical traps for minority carriers responsible for this physical phenomenon. By these strategies, exceptional high sensitivities both for rigid and flexible devices ((4.10 ± 0.05) 10¹⁰ µC Gy^-1 cm^-3 and (9.0 ± 0.4) 10⁷ µC Gy^-1 cm^-3 respectively) for X-ray detection are achieved, which are the highest reported so far for direct X-ray detectors based on tissue equivalent full organic active layer, and higher than most perovskite film-based X-ray detectors. As a proof of concept to demonstrate the high potential of these devices, an X-ray image with sub-millimeter pixel size is recorded employing a 4-pixel array. This work highlights the potential exploitation of high performing OFETs for future innovative large-area and highly sensitive X-ray detectors for medical dosimetry and diagnostic applications.<br/>[1] Tamayo et al., Adv. Electron. Mater. 2022, 2200293<br/>[2] Fratelli et al., Adv. Mater. Technol. 2022, 2200769