We carry out the structural design of photonic crystals to enhance the optical absorption of thin-film microcrystalline silicon (mu c-Si) solar cells using two methods. First, by exhaustive search, we choose a structure with the largest absorption within the investigated patterns. Then we employ a sensitivity analysis to finely modulate the structure for further increase of the optical absorption. The obtained mu c-Si solar cell structure with a photonic crystal in this work has more than twice as much optical absorption as that without a photonic crystal.