We present a theoretical study of crystalline and amorphous silicon thin-film solar cells with a periodic pattern on a sub-micron scale realized in the silicon layer and filled with silicon dioxide right below a properly designed antireflection (AR) coating. The study and optimization of the structure as a function of all the photonic lattice parameters, together with the calculation of the absorption in a single layer, allows to identify the different roles of the periodic pattern in determining an increase of the absorbance. From one side, the photonic crystal and the AR coating act as impedance matching layers, thus minimizing reflection of incident light over a particularly wide range of frequencies. Moreover a strong absorption enhancement is observed when the incident light is coupled into the quasi guided modes of the photonic slab. We found a substantial increase of the short-circuit current when the parameters are properly optimized, demonstrating the advantage of a wavelength-scale, photonic crystal based approach for patterning of thin-film silicon solar cells.