The effects of grating structure on the performance of PTB7  :  PC₇₀BM organic solar cells (OSCs) are considered. First, an OSC with a flat antireflection coating (ARC) is studied, and the effects of grating structures inside the active area of this solar cell are investigated. It is conceivable to increase the short circuit current density (J_SC) of solar cells using dissimilar gratings. Nonetheless, there are essential limitations that constrain usage of this method inside the OSCs. Conducting optimization processes based on particle swarm optimization algorithm, it is found that the highest achievable J_SC for three-grating structure is only 3.2% higher than the single structure grating. Consequently, we have used the grating structure inside the ARC layer of the solar cell. It is observed that using the Gaussian grating ARC layer could increase the overall J_SC of the solar cell by 21.1%, which is omnidirectional, as well.