In this paper, we investigated different organic and inorganic hole-blocking contacts for amorphous selenium (a-Se)-based photodetectors: , , perylene tetracarboxylic bisbenzimidazole (PTCBI), and polyimide (PI). has previously been used as a blocking layer for high-gain a-Se devices. has similar properties to . 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 , 20 nm of , 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 . 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.