Investigation of dark current mechanisms in a polyimide blocking layer utilized in an amorphous lead oxide-based x-ray detector

Abstract

Amorphous lead oxide (a-PbO) is considered a promising photoconductor to replace amorphous selenium (a-Se), extending the diagnostic capabilities of direct conversion detectors to dynamic and high-energy applications. A blocking layer structure was required to maintain dark current (DC) to tolerable levels in a-PbO detectors (below 1 – 10 pA/mm² ). Adopting an approach utilized in the development of a-Se technology, here, a polyimide (PI) blocking layer is used to suppress DC in an a-PbO detector. DC was measured as a function of time (DC kinetics) on a single-pixel PI/a-PbO detector prototype. DC decayed below 1 pA/mm2 within two hours of bias application. To optimize the PI/a-PbO detector’s operation, a mathematical model was derived to simulate experimental observed DC decay and probe DC suppression mechanisms. The model shows that DC decay results from an accumulation of charge within the bulk of PI and at the PI/aPbO interface. This screens the electric field inside the PI layer and reduces injection from the ITO. The results of the DC kinetics modelling indicate that the conduction mechanism responsible for DC decay in PI/a-PbO detectors is different from that in the similar PI/a-Se detectors. The Poole-Frenkel model best describes the DC-field dependence in PI/a-Se detectors. In contrast, the Schottky model more accurately describes DC-field dependency in PI/a-PbO detectors. A unique distribution of the trapping states within the PI/a-PbO structure is suggested to cause this difference. In addition, the model reveals that the field redistribution causes the field within a-PbO to increase with time, and therefore charge collection efficiency and temporal performance improve.