Tandem photonic-crystal thin films surpassing lambertian light-trapping limit

The maximum absorption of solar radiation over the broadest range of frequencies and incident angles using the thinnest material possible has important applications for renewable-energy generation. Complete random texturing of the film surface to increase the path length of light rays, first proposed nearly thirty years ago, has thus far remained the most effective approach for photon absorption over the widest set of conditions. Recent nanostructured designs involving resonant wave effects of photons have explored the possibility of superior performance though as of yet no proposal satisfying the dual requirements of enhanced and robust absorption over a large fraction of the solar spectrum has been made. Here we describe a general strategy for the design of absorbing semiconductor thin films based on a tandem structure where two partially-disordered photonic-crystal slabs, stacked vertically on top of each other, have large absorption that surpasses by a wide margin the Lambertian light-trapping limit over a broad bandwidth and angular range for a film with the same thickness as the combined layers. This tandem structure, the photonic analogue of the multi-junction solar cell, has almost double the improvement in the light trapping relative to a single-lattice design with equivalent thickness and is five times more effective than an unpatterned slab.