Photoconductors for medical imaging detectors: materials issues and device designs

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The flat-panel active matrix imagers (FPAMI) are the most successful digital X-ray detectors for medical imaging such as mammography and general radiography. The FPAMIs are generally either phosphor based (indirect conversion) or photoconductor based (direct conversion, presently amorphous selenium, a-Se, based). The a-Se based detectors produce much sharper images than the indirect conversion detectors. Although the FPAMIs have high potential to reduce the radiation dose, they operate far from the fundamental quantum-noise level. The dose levels at quantum-noise limited detectors can be much lower than that used by currently available FPAMIs. Recently, the organic perovskite (e.g., polycrystalline methylammonium lead triiodide) photoconductors have shown good potential to be used in direct conversion detectors for low dose and lost cost medical X-ray imaging detectors. Its X-ray sensitivity is almost an order better than a-Se. The organic perovskite can be uniformly deposited over large area using a printing method. This method is cheaper than the high temperature-process needed to prepare CsI or a-Se, and should reduce the cost of FPAMI system. However, organic perovskite X-ray detectors can't be commercialized unitil overcoming few scientific challenges such as obtaining low dark current, high resolution, free of image lag and ghosting, and long-term material and device stability. Very low-dose medical X-ray imaging could also be achieved by utilizing carrier multiplication (e.g., avalanche process) at a very high electric field in a-Se layer for higher charge signal, which could improve the signal to noise ratio. The applicant's research program will endeavor to develop low-dose and low-cost medical X-ray imaging detectors using organic perovskites and avalanche a-Se. The research objective includes 1) investigation of charge carrier generation and transport mechanisms in organic perovskites photoconductors and a-Se, 2) investigation of appropriate multilayer perovskite detector structure that shows very low dark current together with high resolution and negligible image lag and ghosting, 3) Investigation of carrier transport mechanisms through various layers and hence determination of the optimum design of multilayer structure for a-Se avalanche X-ray detectors, and 4) study of long term stability. The development of novel organic perovskite X-ray detectors in this proposal can make a big difference to medical imaging. It can greatly lower the radiation dose used in fluoroscopy and general X-ray radiography. It will also open the door to a new class of low-cost organic perovskite large-area X-ray sensors and fuel innovation in other X-ray imaging techniques, such as computed tomography. Another novelty of this proposal is to design multilayer a-Se avalanche detector structure for very low dose and high resolution direct conversion X-ray imaging. This research will be beneficial to Canada's R&D efforts in medical digital imaging.