Minimizing Roughness Induced Optical Losses for a Four-Terminal CdTe/Si Tandem Solar Cell

Abstract

With high photovoltaic efficiencies, low production costs, and long-term stability of single junction cells, CdTe/Si four-terminal tandem solar cells are promising devices for surpassing single junction Si solar cell efficiency limits. High sub-band-gap transmission in the top junction is crucial to reach high efficiencies. Here, we study the impact of surface roughness on transmission using experiments and simulations, showing that the as-deposited texture both increases backscattering and parasitic absorption compared to flat surfaces due to electric field concentration in mesoscale surface features. Adding the ethylene vinyl acetate interlayer increases transmittance by reducing the index contrast at the back transparent conductive oxide. We show that these roughness-induced losses can be overcome by including high index optical coatings as additional interlayers, resulting in increased transmission through the CdTe cell and Si bottom cell efficiency that is comparable to a flat reference device.