A multiple light paths analysis of the internal quantum efficiency (IQE) of a silicon solar cell with back reflector using grating structure to improve the light trapping is presented and the contributions of diffusion length of base regions to IQEs are simulated. An optical model for the determination of generation profiles of the cell is adopted and for a refractive index n material up to 4n(2) light paths are considered and compared with no light trapping structure. It is found that the spatial widths of the cell, the increase of diffusion length, the diffraction angle distribution, number of light trapping paths and transmitted angle can significantly affect the IQE(b) for lower absorption wavelength (i.e., 1000-1100 nm). With 4n(Si)(2) light trapping paths, the simulation results show that the best IQEs (approximate to IQE(b)) with transmitted angle can reach up to 73% which is 14% more than that with normal incident, the best achievable IQE(b) with grating structure is 49% at cell thickness w(b) = 50 mu m, and the IQE(b) with diffraction angle theta(m) = 60 degrees is 9% and 39% larger than that with transmitted angle theta(1) = 0 degrees and without light trapping, respectively. For the case of normal transmitted angle, the IQE(b) with diffusion length 1000 mu m is about 81% and is 37% higher than that with diffusion length 25 mu m. The obtained results can provide essential information for designing a high-efficiency solar cell. (C) 2010 Elsevier Ltd. All rights reserved.