Investigation of Hole-Blocking Contacts for High-Conversion-Gain Amorphous Selenium Detectors for X-Ray Imaging

In this paper, we investigated different organic and inorganic hole-blocking contacts for amorphous selenium (a-Se)-based photodetectors: CeO2, TiO2, perylene tetracarboxylic bisbenzimidazole (PTCBI), and polyimide (PI). CeO2 has previously been used as a blocking layer for high-gain a-Se devices. TiO2 has similar properties to CeO2. PTCBI has a higher ionization potential compared to a-Se and has a very low hole mobility. PI is a common insulator in the semiconductor industry. It was found that an 800-nm PI layer reduces the dark current by more than two orders of magnitude in comparison with 30 nm of CeO2, 20 nm of TiO2, and 50 nm of PTCBI. No significant charge trapping was found in the devices consisting of an 800-nm PI layer. Unlike previously reported inorganic hole-blocking contact technology, PI layers further benefit from a simple spin coating fabrication step before evaporation of a-Se. Photodetector samples incorporating the PI layer are tested at high electric fields, and gains reaching 4.4 were observed at an electric field > 80 V/mu m. We conclude that using a PI layer is a promising step in the development of high-conversion-gain detectors for emerging applications in large-area medical diagnostic imaging, crystallography, and nondestructive test.