Light trapping by a periodically chirped grating structure in thin-film amorphous silicon solar cells is investigated. A thin-film solar cell based on a periodically chirped grating structure is designed. The optical performance of the solar cells is evaluated by examining the integrated absorption, the short-circuit current density, and their angular independence. Further, enhanced absorption compared with a planar reference structure is also demonstrated. A qualitative physical explanation of this broadband absorption enhancement, which results from the leaky waveguide modes resonance, is presented. It is believed that the conclusions can provide a useful guide for the design of thin-film solar cells based on a dielectric subwavelength structure.