Visualizing light trapping within textured silicon solar cells

Abstract

Random pyramids are the most widely used texture in commercial monocrystalline silicon solar cells to trap weakly absorbed photons with near-bandgap energies. There has been steady improvement in efforts to model the light-trapping performance of random pyramids, including a shift from an assumed pyramid base angle of 54.7° (ideal-random pyramids) to smaller values that are consistent with measured average angles. However, simulations have not yet considered the effects of a distribution of base angles (real-random pyramids), which all real textured wafers have. In this contribution, we benchmark the light-trapping capability of real-random pyramids against ideal-random pyramids and Lambertian scatterers by performing ray tracing of an accurate three-dimensional topographical map of the surface of a textured silicon wafer measured using atomic force microscopy. The angular distribution function (ADF) of light rays within the wafer, calculated at each pass as rays bounce between the front and rear surfaces, reveals that real-random pyramids are superior to ideal-random pyramids in trapping light precisely because of the distribution in their base angle. In particular, the ADF inside a wafer with real-random pyramids evolves to be Lambertian within just two passes - by the time (non-absorbed) light re-arrives at the front surface. Furthermore, the total path-length enhancement of light reaches nearly 60 - exceeding that of a wafer with Lambertian surfaces - for narrow angles of incidence, though it falls short of the Lambertian reference for oblique angles.