A structure of crystalline silicon thin film solar cells are proposed, which can absorb broadband spectrum. The antireflection (AR) coating is triangular diffraction grating and the back reflection layer is composed of air column photonic crystal (PC) of triangular lattice on the background of crystalline silicon. The influences of the diffraction grating and PC on the enhancement of the optical absorption in the cell are simulated using the rigorous coupled wave analysis and plane wave theory method. The results show that for the incident angles of 0°～60°, the AR coating has low reflection within the wavelength range of 380～1100 nm and the back reflection layer has high reflection within the wavelength range of 800～1100 nm. For the c-Si thin film solar cell with an active layer thickness of 20 μm, under the TM polarization state of the incident wave and when the white light is at a vertical incidence, the light absorption rate is increased by 18% from the triangular diffraction grating and by 6% from the two-dimensional PC. For the crystalline silicon thin film solar cells with AR coating and the back reflection layer, when the incident angle is less than 60°, the average absorption rate is 87.6% in the wavelength range of 380～850 nm and 49% in the wavelength range of 850～1000 nm.