Nonvacuum process technology was used to produce Cs3Sb photocathodes on substrates, and in situ panel devices were fabricated. The performance of the devices was characterized by measuring anode current as functions of the devices' operation times. An excitation light source with 475 nm wavelength was used for the photocathodes. The device has a simple diode structure, providing unique characteristics such as a large gap, vertical electron beam directionality, and resistance to surface contamination from ion bombardment and poisoning by outgassing species. Accordingly, Cs3Sb photocathodes function as flat emitters, and the emission properties of the photocathode emitters depend on the vacuum level of the devices. Improvement of current stability has been observed after conducting the electrical conditioning process to remove possible adsorbates on Cs3Sb flat emitters.