Characterization of Optically Sensitive Amorphous Selenium Photodetector at High Electric Fields

We investigated the electrical conduction in indium tin oxide/polyimide (PI)/amorphous selenium/Au device structures, operated at high fields (up to 40 V/μm). Devices having different PI thicknesses (0.4-2.5 μm) with the same thickness of a-Se (16 μm) were fabricated, and the dark and photocurrent transient behavior of each device under different biasing voltages was tested. The results indicate that an optimal thickness of PI (~1 μm) was necessary to limit the dark current to below 10 pA/mm² while achieving an ON/OFF ratio of 10⁴, a result not achievable in devices without the PI layer. Furthermore, time-of-flight (TOF) measurements at low electric fields (~10 V/μm) were carried out to measure the exact voltage drop within the PI and a-Se films for the same structure. Based on these TOF measurements, the electric field within the PI layer was determined to reach 138 V/μm to provide efficient electrical conduction for photogenerated electrons.