Spontaneous symmetry breaking in the optimization of subwavelength solar cell textures for light trapping

Light trapping in solar cells allows for increased efficiency and reduced materials cost. It is well known that a 4n² factor of enhancement in absorption can be achieved by randomly texturing the surface of the solar cell, where n is the refractive index of the material. However, this limit only holds when the thickness of the solar cell is much greater than the wavelength of light. In the subwavelength regime, the fundamental question remains unanswered: what surface texture realizes the optimal absorption enhancement? We turn to computational inverse electromagnetic design in order to find this optimal nanoscale texture for light trapping, and observe spontaneous symmetry breaking in the final design. We achieve a factor of 40 in enhancement at normal incidence and above 20 for angle-averaged incidence (averaged over an energy bandwidth of 1/8) for n= 3.5.