The first principles methods applied to enhance a performance of new photoconductive materials in X-ray imaging

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The goal of this program is to investigate the capabilities of the novel photoconductors for application in direct conversion X-ray imaging technology. The diagnostic imaging market in Canada is currently dominated by technology which utilizes a two-step approach: scintillators are used to convert the X-ray signal to light and then it is converted to the electrical signal by photoconductor. Withdrawing scintillators from this scheme would allow for a reduction in the signal loss and improvement of the spatial resolution being impaired by light scattering in scintillators. The success of this new direct conversion approach relies on finding novel photoconductive materials that have both the capacity to provide the high sensitivity to X-ray and compatibility with flat panel display technology. The compatibility is the primary component that limits our choice to amorphous and polycrystalline materials as these are the only available materials that provide uniform deposition over a large area of imaging matrixes at the required low temperature. To cover the whole range of the X-ray imaging applications, two potential candidates are considered: amorphous Se (a-Se) for mammographic energy range (20 keV) and high atomic number polycrystalline PbO for diagnostic energy range (60-120 keV). The amorphous and polycrystalline materials are disordered media, which are known to suffer from poor transport properties due to low charge mobilities and charge trapping on defects. While for a-Se the effect of trapping on photoconductivity is well discussed, the transport properties of polycrystalline PbO are unknown but the low charge mobility is believed to be caused by trapping. To suppress the trapping some material science and engineering solutions have to be applied, but this requires an understanding of fundamental properties, nature of defects and peculiarities of the charge transport. Another big issue of disordered materials is their structural instability: PbO compound degrades upon exposure to air, while a-Se is metastable with respect to crystalline Se and as such it experiences structural modifications upon exposure to light or X-ray. Any structural transformations are highly undesired as they result in permanent degradation of material properties, thus shortening utilization lifetime. Through the application of the first-principles methods we can gain the fundamental insight into the atomistic processes that drive behaviour of materials and to determine what affects their performance in different applications and suggest ways how to improve their technology. The computing technology has advanced simulations to the point where materials can be "grown virtually" with their properties predicted/manipulated theoretically before being created in a lab. Therefore, in this research program the first-principles methods are applied to model polycrystalline PbO and a-Se with focus on understanding of the mechanisms of carrier trapping and structural degradations of photoconductors. Such understanding is essential to provide a feedback for technology optimization and to develop the material science solutions directed on suppression of the parasitic effects degrading the photoconductor performance. It is expected to facilitate promotion of the Direct-conversion Flat-panel X-ray detector technology into the healthcare industry in Canada: our ultimate goal is to achieve a reduction of the radiation exposure of medical personals by enhancing performance of photoconductors. The collaboration with the experimental Advanced Imaging group at Thunder Bay Regional Research Institute (TBRRI) working on fabrication and commercialization of X-ray detectors, provides the unique opportunity to combine theory and experiment.