Solution-Processed, Large Area, Pixelated Direct-Detection Radiation Detectors

Flatpanel Xray detector technologies are desired to replace Xray film, but available detector materials have a number of limitations, such as low efficiency, low sensitivity, high noise, small format, high cost, long product development times, and/or expensive infrastructure. Directdetection (photoconductive) materials are theoretically favored over indirect (scintillator) materials, but have yet to achieve high Xray absorption, ionization and charge collection. Present photoconductive materials are also not highly suitable for manufacturing, including e.g. uniform largearea deposition. A directdetection nanocrystal (NC)based pixelated array detector technology is being developed, taking advantage of semiconductor NCs abilities for Xray absorption and charge transport. The NCs can convert Xray photons to charge carriers, and when NC films are implemented with pixelated TFT or CMOS readout circuits, the resulting detectors allow the formation of versatile digital signals. In this way, a variety of custom Xraysensitive materials can be synthesized from chemical precursors and cast from solution onto a common electronic readout circuit design, including largearea designs. Lead iodide NCs were synthesized and optimized using various ligand coatings. The NCs morphology and optoelectronic properties were characterized as a function of synthesis conditions. NC films were coated onto simple detector structures and characterized; photoconductive and photovoltaic detectors were both demonstrated. The films showed very good responsivity and detective efficiencies, as well as linear Xray response under biases up to 50 VDC. Temporal response and stability were also studied. To support manufacturability of PbI2 NC films, the NC synthesis will be developed and scaled using an automated continuousflow microreactor, making available larger volumes of NCs having known size, shape, phase, and purity. Ligandswapping processes will be further developed to support NC deposition, as well as the methods of depositing the films on existing TFT and CMOS pixelated readout circuits. Working largearea Xray detectors will be fabricated, demonstrated, and characterized. Commercial Applications and Other Benefits: The technology supports the fabrication of largearea detectors with high pixel counts, instant digitization, realtime feedback, elimination of toxic film processing, and reduced recurring costs. When realized, these benefits will broadly address needs in various fields such as materials research, archaeology, biology, medicine, and nondestructive testing. The innovation will allow wider, less expensive, lowerdose Xray imaging, made available to a larger portion of the worldwide population.