A Light-trapping Metric for Solar Cells With Application to Cadmium Telluride and Silicon

We examine an enhancement metric for characterizing light trapping in single-junction solar cells by comparing the measured quantum efficiency spectrum and the theoretical absorptance spectrum based on ergodic light scattering. This ideal enhancement is 4n², where n is the refractive index of the absorber layer. Using a uniform procedure, we have determined the effective enhancement from published results for many single-junction nanocrystalline, polycrystalline, and monocrystalline silicon cells, as well as for thin-film cadmium telluride (CdTe)-based cells. The largest effective enhancements were 33 for monocrystalline Si, 25 for nanocrystalline Si, and 10 for CdTe. The 4n² benchmarks are about 50 for Si and 36 for CdTe; for CdTe, 4n² light trapping adds about 1 mA/cm² to the photocurrent density of a 3-μm cell. We propose a procedure that separates the effects of parasitic absorption from incomplete scattering in determining the enhancement and show that the champion enhancement of 33 for silicon was mostly limited by parasitic absorption and not by inadequate scattering.