Modeling and analysis of lead-free perovskite photoconductors for direct conversion x-ray imaging detectors

Abstract

In a direct conversion digital X-ray image detector, incident X-rays are converted tochargeandreadoutusingathinfilmtransistor(TFT)arrayforstorageanddisplay.Thedigital X-ray imaging system offers better image quality, reduced X-ray exposure, andthe possibility for real-time imaging compared to the traditional analog system.Thechoiceofthephotoconductoranddesignforoptimumoperationiscrucialforthesupe-rior performance of the detector. In this thesis, a numerical model of X-ray sensitivityassumingaperturbedelectricfieldduetothebulkchargetrappingandionaccumulationnearthephotoconductor/metalinterfacehasbeendeveloped.Thecontinuityequations,trappingrateequations,andPoisson’sequationaresimultaneouslysolvedbynumericalmethod.The continuity equations are solved to calculate charge concentration con-sidering the perturbed electric field.The free carrier concentration and electric fielddistribution are used to determine the photocurrent.The integration of the photocur-rent is the collected charge. The X-ray sensitivity of a photoconductor is the collectedchargeperunitareaperunitexposureofradiation.Thedevelopednumericalmodelhasbeen applied to Cs2AgBiBr6 (CABB) based X-ray detectors. CABB possesses impres-siveoptoelectronicpropertiessuitableforX-raydetectionwithahighlystablestructureand free from toxic Pb. The modeling works in this thesis identify the important fac-torsthatlimitthedetectorperformancesuchasthemobility-lifetimeproductofchargecarriers, ion accumulation, and charge carrier injection from electrodes.The optimaldevicethicknessforanAu/CABB/AuX-raydetectorisestimatedtobe∼350and550µm for 30 keV X-ray energy. It has been found that the optimum dose for maximumsensitivityis∼4.5-7µGyfor30keVX-rayenergy.Inchestradiology,itisdiscoveredthat CABB has higher sensitivity than a-Se by 32% for an applied electric field of 5V/µm. In mammography, CABB has higher sensitivity than a-Se by 5.58% for an ap-pliedelectricfieldof5V/µm.ThisinvestigationcanultimatelyleadtothereductionofpatientexposureindifferentdiagnosticmedicalX-rayimaging.