Beneficial effect of reducing symmetry on the enhancement of optical absorption of nanohole arrays

Nanostructure plays an important role in improving the absorption efficiency of thin film solar cells. In this paper, we have studied the dependence of the absorption efficiency of nanohole (NH) array on the shape and rotation angle of the NH. Results show that, in a large NH size range, the ultimate efficiency of a tilted elliptical NH array is higher than that of any vertical circular NH array. The ultimate efficiency of the elliptical NH array with its NH tilted at the angle when the array yields the maximum efficiency is 9.7% higher than the maximum efficiency that any vertical circular NH array can offer. Rotating the elliptical NHs around the axes perpendicular to the bottom of the NHs for an appropriate angle, the ultimate efficiency of the array enhances further and is 16.2% higher than that of the optimal vertical circular NH array. Compared with vertical NH arrays, the tilted and rotated NH structures present greater mode density of state, which can couple with the incident light. The reduction of symmetry is the main reason that leads to more resonance modes and greater absorption efficiency in the structure.