Thin-film polycrystalline Mg/Zn3P2 Schottky photodiodes were fabricated. Their optical and electrical properties under 1 atm air and 10-5 Torr vacuum conditions were measured. It was observed that the spectral quantum efficiency improved under vacuum. These changes were a result of surface effects. Surface ionic charge at the bulk-surface interface and at the grain boundary induces a surface depletion region which is affected by vacuum operation, hence affecting the photodiode properties.