High Sensitivity Flexible X-Ray Detectors Based on Printed Perovskite Inks

Metal halide perovskites are rapidly emerging as active materials in low-cost high-performing optoelectronic devices. The success is driven by their long carrier diffusion lengths, low trap densities, high mobilities and low-temperature solution-processability that combine the high performance of traditional inorganic semiconductors with the low-cost, large area scalable, printing technologies typical of organic semiconductors. The presence of heavy atoms and the high crystal density make them ideal for direct detection of ionizing radiation. Since the first reports on direct X-ray detection in perovskite single crystals in 2013, the great thrust put forward by the scientific community resulted in solution-grown high purity, large and thick perovskite single crystals detectors that surpass state-of-the-art radiation detectors based on Silicon and CZT. Despite top performance of perovskite single crystals, they do not fully exploit the solution processability on flexible plastic substrates. Flexible X-ray detectors are low-cost and lightweight devices that can be operated at low voltages and strongly limit the use lead-based toxic materials. Up to now, flexible direct X-ray detectors have been developed employing mainly organic materials or hybrid blends and few preliminary results have been reported on thin-film perovskite-based X-ray detectors. [1]<br/>We develop a direct X-ray detector based on the photo-conducting properties of printed micrometers-thick film of methylammonium lead triiodide (MAPbI₃) microcrystals, passivated with an organic layer of PCBM. The deposition methods here employed allow to target the possibility of scaling-up the process with large-area compatible techniques, thanks both to the ease of print of the active material on different kind of substrates and to the synthesis methods, that strongly exclude high boiling point- and toxic-organic solvents, here replaced by benign solvents. We used the bar coating printing technique that, by realizing multiple depositions, permitted us to obtain deposited films of tunable thickness up to tens of microns, maximizing the film thickness to enhance radiation absorption, while maintaining a good layer uniformity and high electrical performance.[2] The devices showed limited hysteresis, stability in time and good radiation hardness over total dose of several Grays. The X-ray response is sharp, repeatable over multiple cycles and scales with the bias. The photocurrent increment (ION - IOFF) is linear with the impinging dose rate in the tested X-ray energy range above, thus demonstrating the capability of the detectors to be employed as reliable real-time direct dosimeters under a wide X-ray energy range (40 -150kVp) and for a wide dose rate range, spanning from µGy/s to mGy/s. The sensitivity reaches the remarkable top value of 2270 µC Gy^-1 cm^-2. The sensitivity is the highest value among all the films, perovskite-based, which are compatible with large area and flexible substrates. Noteworthy, such high sensitivity values are competitive also with others high performing perovskite-based detectors that generally have much thicker active layer. Time response of 40 ms and limit of detection result comparable with the typical values reported for polycrystalline perovskite films.<br/>Finally, we validated the flexibility of our devices, assessing the real-time X-ray response under bending stress and under X-ray. The devices can reliable operate down to bending radius of 0.5 mm. Therefore, the here reported devices allow to move a step forward, passing from conformable to strongly flexible real-time direct radiation detectors, operating a low-voltage and apt to be fabricated with large-area scalable processes [3].<br/>[1] S. Yakunin et al., Nat Photon, 9, 7, 444–449 (2015)<br/>[2] V. Venugopalan et al, Chem (2019), 5, 868.<br/>[3] A. Ciavatti et al, Adv. Funct. Mater. 2021, 2009072.