Which of these materials is most commonly used to create photoconductive devices?
Why Photoconductivity happens when incoming photons carry more energy than a material's bandgap, knocking electrons from the valence band into the conduction band and creating electron-hole pairs that lower the material's resistance. Only semiconductors have a bandgap in the right range for visible and near-infrared light, so crystalline semiconductors such as cadmium sulfide, silicon and germanium are what photocells, photodiodes and phototransistors are made from. Metals already have free electrons, and insulators have gaps too wide for light to bridge.
Watch out Argon is a noble gas used in discharge lamps and welding shielding, not a solid-state light sensor, and the plastic-sounding compound is not a photoconductor at all.
Light in, resistance down - that only works in a semiconductor with a bandgap. Think CdS photocell.