Dark Current modeling and characterization of amorphous lead oxide-based x-ray photoconductive devices for applications in medical imaging

Abstract

High atomic number (Z) polycrystalline and amorphous photoconductors are currently being investigated to extend direct conversion X-ray detectors to real-time and high-energy lowdose applications. Amorphous lead oxide (a-PbO) is one of the most promising photoconductor candidates because of its negligible signal lag and high theoretical X-ray conversion efficiency. However, a-PbO layers are still experimental; PbO technology has been developed to the point where material science and engineering approaches must be applied to make a-PbO detector prototypes suitable for low-dose X-ray imaging. This includes determining the most appropriate a-PbO multilayer detector structures with specially designed blocking layers that will withstand the high electric fields needed for efficient (i.e., complete) collection of X-ray generated charge while maintaining an acceptable dark current (DC) level. DC is a source of noise in the detector structure that degrades the signal-to-noise ratio (SNR) of the detector system in low-exposure applications. Here we investigate the use of polyimide (PI) as a hole-blocking layer. PI blocking layers were proven successful in the only commercially used direct conversion detectors, which are based on layers of photoconductive amorphous selenium (a-Se). Experimentally, PI was shown to have the most suitable electrical and physical properties for our a-PbO technology. In addition, PI has a straightforward application process of spin coating. Therefore, PI was chosen as a hole blocking layer to decrease DC to tolerable levels in an a-PbO-based detector. [...]