Hydrogenated amorphous silicon (a-Si:H) is widely adopted in thin-film electronics due to its compatibility with large-area deposition, low fabrication temperatures, and cost-effectiveness. In recent years, a-Si:H photodiodes have gained interest for integration into Lab-on-Chip (LoC) systems, where they are used to detect light signals generated during biomolecular recognition events. However, a key concern in such applications is light-induced degradation (LID), which can affect the reproducibility and stability of the photodiode’s response during continuous operation. This study investigates the light stability of a-Si:H photodiodes with varying intrinsic layer thicknesses (0.4 μm, 1.25 μm and 2.5 μm), exposed to monochromatic light at 450 nm, 550 nm, and 620 nm—wavelengths relevant to fluorescence and chemiluminescence detection. Devices were fabricated using conventional thin-film microfabrication techniques and a comprehensive characterization was conducted, including quantum efficiency (QE) measurements and photocurrent monitoring under continuous illumination for one hour. Despite the relatively high illumination intensities used (up to 2.3 μW), no appreciable degradation in photocurrent or QE was observed for any of the devices or wavelengths tested. These results indicate a high level of photoresponse stability across a range of device thicknesses and operational wavelengths. The findings confirm the suitability of a-Si:H photosensors for long-term use in LoC systems and support their application in sensitive and repeatable optical biosensing platforms.