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

Abstract

Light-trapping in polycrystalline silicon solar cells is usually considered to be more difficult to implement than that in single crystal silicon solar cells due to the random crystallographic orientations in various grains. Furthermore, if minority carrier diffusion length is on the order of or less than solar cell thickness, which is the case of most cost-effective polycrystalline silicon, the translation of optical gain, achieved from light-trapping, into electrical gain will be rather limited, even with a perfect back surface passivation. In this work, geometrical light-trapping structures are demonstrated using a simplified isotropic etching at polycrystalline silicon surfaces. 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-terminal solar cell structure is used. This structure allows three different back surface configurations to be realized in a single device: unpassivated, passivated with a floating junction, and enhanced with a collecting junction. Results indicate that even with a relatively short minority-carrier diffusion length the current collection in the long wavelength region can be significantly improved and the light-trapping effect is enhanced as well. Copyright © 1999 John Wiley & Sons, Ltd.