Low-Temperature Fabrication of Hole Blocking Layers for Large-Area, Flexible Amorphous Selenium UV and X-Ray Detectors

Amorphous selenium (a-Se) is a promising material for low-cost, large-area, flexible detectors. Its bandgap of 2.2 eV and relatively low threshold for impact ionization make it ideal for X-ray and UV detection, with a myriad of possible applications in direct and indirect detection for medical imaging, crystallography, and high energy physics. Large-area fabrication is achieved by the use of thermal evaporation, in which selenium can be rapidly deposited to achieve targeted thicknesses. It is easily scaled for commercial production and is already in use in direct-conversion digital flat panel detectors for mammography. However, issues in stability and architecture limit progress in detector development and implementation.<br/>Amorphous selenium is known to crystallize over time; in addition, a-Se presents photodarkening (PD) and photocrystallization (PC) effects under extended exposure times. A blocking layer is also required for reducing hole injection and achieving fields required for avalanche. Previous works have shown that the use of a flexible interface, such as polyimide, can reduce or mitigate PD and PC effects; it has also been shown to serve as an excellent blocking layer. Unfortunately, the high temperature required for polyimide fabrication prevents its use with most transparent, flexible substrates such as PET. And while it is possible that the substrate alone can serve as the flexible host for the a-Se layer, a blocking layer is still required. A suitable hole blocking layer must be capable of low-temperature fabrication, prevent charge injection, have a high breakdown field, and meet the flexibility required to prevent crystallization.<br/>In this work, we investigate several candidate materials that meet these requirements. Amorphous selenium devices are fabricated on PET and glass substrates to compare the performance of flexible devices to traditional rigid substrates. Devices are characterized by dark-currents, UV-Vis light responses, time of flight measurements for hole and electron mobilities, conversion efficiency and gain. In addition, accelerated lifetime tests will demonstrate the ability of these materials to create a more stable Se layer.