Light Trapping Textures Designed by Electromagnetic Optimization for Subwavelength Thick Solar Cells

Light trapping in solar cells allows for increased current and voltage, as\nwell as reduced materials cost. It is known that in geometrical optics, a\nmaximum 4n^2 absorption enhancement factor can be achieved by randomly\ntexturing the surface of the solar cell, where n is the material refractive\nindex. This ray-optics absorption enhancement limit only holds when the\nthickness of the solar cell is much greater than the optical wavelength. In\nsub-wavelength thin films, the fundamental questions remain unanswered: (1)\nwhat is the sub-wavelength absorption enhancement limit and (2) what surface\ntexture realizes this optimal absorption enhancement? We turn to computational\nelectromagnetic optimization in order to design nanoscale textures for light\ntrapping in sub-wavelength thin films. For high-index thin films, in the weakly\nabsorbing limit, our optimized surface textures yield an angle- and\nfrequency-averaged enhancement factor ~39. They perform roughly 30% better than\nrandomly textured structures, but they fall short of the ray optics enhancement\nlimit of 4n^2 ~ 50.\n