Abstract

X-ray photoconductor-based flat-panel X-ray imagers (FPXIs) produce superior X-ray image as compared to scintillator-based detectors and are the commercially available digital X-ray detectors for mammography. These detectors are, at present, under scrutiny for use in general radiography, fluoroscopy, tomosynthesis and portal imaging. Although amorphous selenium (a-Se) is the most successful photoconductor, recently, Hybrid Organic−Inorganic Perovskites (HOIPs) receive much attention for this application because of their good charge transport properties, higher attenuation coefficient and solution-based cheaper process techniques. These detectors need high detective quantum efficiency (DQE) for getting clear X-ray image.

The photoconductor layer thickness plays a very important role in conventional detector structure (i.e., a photoconductor layer is sandwiched between two electrodes where charges are collected in corresponding pixels) on imaging performances. A relatively thicker layer is required for better sensitivity. However, the thicker layer contributes to more noise and signal spreading as carriers must travel a longer distance to reach the electrodes, and thus adversely affects image resolution and DQE). In a folded structure, charge carriers travel perpendicular to the direction of incident X-rays and thus the X-ray quantum efficiency can be improved using a thicker layer without affecting the charge collection by keeping the charge collecting electrodes at a reasonable distance. It is essential to do a comparative analysis on DQE performance of both conventional and folded structure.

This dissertation focuses on developing mathematical model for studying the DQE of both folded and conventional detector structures by incorporating the quantum noise due to random charge carrier trapping in the cascaded linear system model. An analytical expression for the variance of charge collection in folded structure has been developed. The optimum values of photoconductor layer thickness and spacing between electrodes for maximizing the DQE under various material parameters and detector operating conditions are also investigated. The DQE critically depends on the detector thickness. As a result, the optimum DQE can be even below 0.3 for certain values of material and device parameters in conventional structure. On the contrary, the folded structure provides more design flexibility for achieving high DQE (even higher than 0.7) by adjusting the distance between electrodes without compromising the quantum efficiency. Since the folded structure is more complex than the conventional structure, one should prefer the folded structure if the photoconductor possesses relatively low linear attenuation coefficient for higher energy X-rays together with poor charge carrier transport properties. Moreover, it has also been found that the poly MAPbI3 detector shows significantly better DQE performance than the Amorphous Selenium Detector.