Optimizing parylene and photoconductor thickness in indirect conversion amorphous selenium detectors

Amorphous selenium (a-Se) provides an opportunity for a low cost, large area, avalanche photodetector for use in indirect conversion detectors. However, its bandgap of 2.2 eV reduces the response at long wavelengths, specifically the 550 nm green light emitted by CsI:Tl scintillators, limits its application. Incorporating tellurium into the a-Se conversion layer is known to reduce the bandgap and increase sensitivity at these longer wavelengths. Previous studies have demonstrated this effectiveness and have shown that high conversion efficiencies can be achieved despite the reduced carrier mobility and lifetime of Se-Te. This group has proposed utilizing a Se-Te layer in an indirect conversion flat panel detector with 85 um pixel pitch, implementing a parylene hole blocking layer. Results of that work demonstrated the need for optimization of the thickness of those layers to achieve high sensitivity, reasonable leakage, and low lag and ghosting. In this study, we evaluate the effects of varying the parylene layer thickness and the photodetector conversion layer for single pixel Se-Te devices. We find that, while thicker Se-Te and parylene devices achieve low dark current, anticipated signal levels, and low lag, thinner samples suffer from signal loss and residual charge in the device. Varying the thickness of parylene leads to tradeoffs in dark current and residual charge, each of which is important in the performance of the final imager. To make use of parylene as a hole blocking layer, thicker photoconductor and parylene layers must be employed.