Abstract

ABSTRACT
X-ray sensitivity and resolution of hybrid perovskite X-ray imaging detectors
Afazul Hoq
The flat panel detectors (FPD) are digital detectors that are utilized extensively in medical applications such as digital chest radiography and mammography. The detectors are subjected to electromagnetic radiation to produce digital images of human internal organs. The electromagnetic radiation (X-ray) generates electron-hole pair in the photoconductor layer. The photogenerated electron-hole pairs drift under the influence of an applied electric field and generate a photocurrent. The performance of detectors essentially depends upon the selection of photoconductors. Over the years, x-ray imaging detectors with a-Se are widely used because of its uniform deposition, moderate sensitivity, good image resolution, and considerably low dark current. However, the medical diagnostic applications of a-Se get restricted due to high ionization energy, relatively low absorption coefficient. The hybrid organic perovskite has ionization energy an order lower than a-Se, good absorption coefficient, and superior charge transport properties that make hybrid organic perovskite a suitable candidate to replace the commercially used a-Se detectors.
In this thesis, the sensitivity of hybrid organic perovskite-based detectors under different operation conditions has been critically analyzed. The change of sensitivity is not significant for single exposure medical diagnostic X-ray applications. However, the sensitivity of perovskite-based detectors starts to deteriorate due to repeated and large exposures. The electric field across the photoconductor thickness becomes non-uniform due to the accumulation of trapped charge carriers. The loss of carriers due to bimolecular recombination leads to a decline of charge collection efficiency which results in an overall reduction of sensitivity. A physics-based model has been implemented to analyze the x-ray sensitivity under repeated exposures and different operating conditions. The model included the coupled continuity, trapping rate, and Poisson’s and these equations are solved simultaneously. All possible recombination processes and traps are considered in the model with appropriate assumptions. The results of the model are compared with experimental values. It is vital to investigate the loss of image resolution of a hybrid organic perovskite detector. The loss of image resolution of organic perovskite material is mainly due to k-fluorescence reabsorption, charge carrier trapping, and charge carrier diffusion. A physics-based model has been used to study the reduction of the modulation transfer function for different spatial frequencies. The work in this thesis identifies the main reasons for the ghosting effect under repeated exposures and the loss of image resolutions.