Novel ultrathin LiF interlayers for efficient light harvesting in thin-film Si tandem solar cells

We have investigated the application of ultrathin lithium fluoride (LiF) interlayers for effective light harvesting in hydrogenated amorphous silicon (a-Si:H)/hydrogenated microcrystalline silicon (μc-Si:H) tandem solar cells. It is proved that the LiF interlayers are not suitable for intermediate reflectors of the tandem solar cells despite their low refractive index and low lateral conductivity. A poor vertical conductivity leads to the formation of a highly resistive tunnel junction. On the contrary, novel hydrogenated n-type silicon-oxide (n-SiOx:H)/LiF back reflectors are successfully employed in the tandem solar cells, reducing plasmonic absorption losses in nanotextured Al back contacts and providing effective refractive index grading. It is found that the ultrathin LiF interlayer mitigates nanotextures of the Al back contact. The spectral response of μc-Si:H bottom cells is markedly elevated in a near-infrared wavelength region. As a result, a conversion efficiency is improved by 8.1% compared to the reference cell with a conventional zinc oxide (ZnO) back reflector thanks to an increase in a short-circuit current by 5.5%. Consequently, the initial efficiency of 10.4% is attained.