Effect of air mass on carrier losses in bifacial silicon heterojunction solar cells

We investigate the effect of incident spectra on current loss as a function of depth and voltage into high efficiency textured bifacial silicon heterojunction solar cells. We integrate thin-film ellipsometry measurements with a 3D optical model and a 2D electronic model and validate our model with measurements of external quantum efficiency and Suns-Voc. For front illumination at normal incidence, an increasing air mass of AM1.5 to 10 reduces current density loss due to parasitic absorption in ITO and a-Si:H from 8.1% to 4.0%, and increases recombination loss at maximum power from 4.2% to 4.7%, resulting in an overall increase in collected current (88.2% to 90.5%). Cell performance metrics are summarized as a function of air mass, with efficiency peaking at AM5.0 for front illuminated and rear illuminated cells with an albedo of unity. We further demonstrate the impact of spectra on bifacial efficiency by calculating rear-side performance with the spectral albedo of dry grass. Overall, current-collection and efficiency trends emphasize the importance of considering spectral effects in energy yield models. These results are of particular importance for cell structures with high bifaciality and significant spectral albedo contributions, locations with large proportions of diffuse light, and high air mass locations as in mid-to-high latitudes.