We investigated the optimum thickness of the p-, i-, and n-layers to improve transmittance of the p-layer, the absorbance of the i-layer and the recombination rate of the n-layer. a-Si:H-based solar cells with different thicknesses for each layer were fabricated using plasma enhanced chemical vapor deposition (PECVD) and their performance parameters were compared. The optical properties of the intrinsic layer (i-layer) and the optimum hydrogen content in the i-layer were investigated and analyzed using UV/Vis/NIR and FT-IR, respectively. The optimum thicknesses of the p-, the i-, and the n-layer represented about 300, 2000, and 600 angstrom, respectively. The maximum absorbance and the minimum transmittance were measured at an i-layer thickness of 2000 angstrom in the wavelength range between 400 and 800 nm. The FT-IR measurements showed the optimum hydrogen content in the i-layer was about 10.1 at.%. We verified that the thickness of each layer and the hydrogen content had greatly influenced the electrical properties of the fabricated cells.