Light-trapping and back surface structures for polycrystalline silicon solar cells

Light-trapping in polycrystalline silicon solar cells is usually considered to be more difficult to implement than that in single crystal silicon sola r cells due to the random crystallographic orientations in various grains; Furthermore, if minority carrier diffusion length is on the order of or les s than solar cell thickness, which is the case of most cost-effective polyc rystalline silicon, the translation of optical gain, achieved from light-tr apping, into electrical gain will be rather limited, even with a perfect ba ck surface passivation. In this work, geometrical light-trapping structures are demonstrated using a simplified isotropic etching at polycrystalline s ilicon sur:faces. Combined with a back surface reflector (BSR), an enhanced absorption in the long wavelength region is measured with a low parasitic absorption. Different light-trapping structures are experimentally compared . To further examine the electrical gain from light-trapping, a three-termi nal solar cell structure is used. This structure allows three different bac k surface configurations to be realized in a single device: unpassivated, p assivated with a floating junction, and enhanced with a collecting junction . Results indicate that even with a relatively short minority-carrier diffu sion length the cur rent collection in the long wavelength region can be si gnificantly improved and the light-trapping effect is enhanced as well. Cop yright (C) 1999 John Wiley & Sons, Ltd.