Superlattice solar cells on nanoimprinted photo-plasmon substrates

Thin film solar cells absorb only a small portion of the sunlight in one pass, which make light trapping a very important requirement for these solar cells. We report here the use of the two novel techniques (namely photonic and plasmonic) used in conjunction to achieve higher light trapping. When the thin film structure is deposited on a photo-plasmonic structure, which combines the light diffraction properties of a photonic structure with the reemission of light by a plasmonic structure, the current increases even more. We have obtained a 50% increase in current in a thin film of 0.9 micrometer by on a nanoimprinted photo-plasmon structure. In this study we also show the use of silicon alloys in a superlattice structure to further improve the photon absorption. The superlattice increases the quantum efficiency in the 600-700 nm range, and a properly designed photo-plasmon structure increases it in the 700-1000 nm range. A major advantage of the superlattice structure is that the crystallinity of the device remains constant throughout the thickness, without the need for changing hydrogen/silane dilution ratio. By using a-(Si, Ge) layers in the superlattice, one can shift the shoulder in optical absorption to longer wavelengths. Increasing the Ge content systematically increases the current in the cell.