For hydrogenated amorphous silicon (a-Si:H) thin film PIN diodes fabricated using plasma-enhanced chemical vapor deposition, the effects of the n^+ layer deposition power, the existence of an aluminum reflector layer, and the incident light direction were investigated. Optical properties of photodiodes, e.g. open-circuit voltage, short-circuit current density, fill factor, efficiency, external quantum efficiency, series resistance, and shunt resistance, with respect to the aforementioned fabrication parameters, were investigated. Three monochromatic LED lights with different wavelengths, as well as a broad spectrum solar simulator, were used as the illumination sources in this research. Plasma power influenced the n^+ film growth rate and resistivity, which affected the overall light detection efficiency in PIN diodes. The absorption of incident light was influenced by the band gap energies of the doped layers of a-Si:H and by the difference in mobility and lifetime of electrons and holes that were photogenerated in the intrinsic layer. The effect of device structure, fabrication conditions, light source, and incident light direction on the performance of a-Si:H PIN diodes was explored in this experimental study. The growth rates and resistivities of doped a-Si:H thin films were found to correlate with the amount of RF power supplied during their deposition. The transmittance of n-type a-Si:H was also dependent on deposition power, and the effects varied depending on the wavelength of incident light exposure. Including a metallic reflector layer in the diode structure always improved its efficiency, especially for long wavelength light, which is not effectively absorbed by a-Si:H. Generally, exposing the diodes to incident light on the p-side yielded better performance, due to the shorter required drift distance for photogenerated holes compared to n-side illumination.