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

该计划的目标是研究新型光导体在直接转换x射线成像技术中的应用能力。加拿大的诊断成像市场目前由采用两步方法的技术主导：使用闪烁体将x射线信号转换为光，然后通过光电导体将其转换为电信号。将闪烁体从该方案中移除将允许减少信号损失并改善因闪烁体中的光散射而受损的空间分辨率。这种新的直接转换方法的成功依赖于寻找新型光导材料，这种材料既能提供对x射线的高灵敏度，又能与平板显示技术兼容。兼容性是限制我们选择非晶和多晶材料的主要因素，因为这些是唯一可以在所需的低温下在大面积成像基质上提供均匀沉积的材料。为了涵盖整个x射线成像应用范围，考虑了两种潜在的候选材料：用于乳房x射线成像能量范围（20 keV）的无定形Se （a-Se）和用于诊断能量范围（60-120 keV）的高原子序数多晶PbO。非晶和多晶材料是无序介质，由于电荷迁移率低和缺陷上的电荷捕获，它们的输运性能很差。虽然捕获对a-Se光电导率的影响已经得到了很好的讨论，但多晶PbO的输运性质尚不清楚，但低电荷迁移率被认为是由捕获引起的。为了抑制捕获，必须应用一些材料科学和工程解决方案，但这需要了解电荷输运的基本性质、缺陷的性质和特性。无序材料的另一个大问题是它们的结构不稳定性：PbO化合物暴露在空气中会降解，而a-Se相对于晶体Se是亚稳态的，因此它在暴露于光或x射线下会经历结构改变。任何结构转变都是非常不希望的，因为它们会导致材料性能的永久性退化，从而缩短使用寿命。通过应用第一性原理方法，我们可以获得对驱动材料行为的原子过程的基本见解，并确定在不同应用中影响其性能的因素，并提出改进其技术的方法。计算技术已经先进到可以在实验室中创建之前，通过理论上预测或操纵材料的特性来“虚拟生长”的地步。因此，在本研究项目中，将第一性原理方法应用于多晶PbO和a-Se的建模，重点了解载流子捕获和光导体结构降解的机制。这种理解对于为技术优化提供反馈和开发针对抑制降低光导体性能的寄生效应的材料科学解决方案至关重要。预计它将促进直接转换平板x射线探测器技术在加拿大医疗保健行业的推广：我们的最终目标是通过提高光导体的性能来减少医务人员的辐射暴露。与雷湾地区研究所（TBRRI）的实验先进成像小组合作，致力于x射线探测器的制造和商业化，提供了将理论和实验结合起来的独特机会。

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.