High - Effi ciency Amorphous Silicon Solar Cell on a Periodic Nanocone Back Refl ector

Thin-fi lm silicon solar cells, including hydrogenated amorphous silicon (a-Si:H) single-junction and amorphous/microcrystalline silicon (a-Si:H/ μ c-Si:H) tandem-junction solar cells, are promising candidates for the global terawatt-scale deployment of photovoltaics thanks to the abundance and non-toxicity of their raw materials, and mature fabrication processes. [ 1 , 2 ] Compared to their traditional wafer-based crystalline counterparts, thin-fi lm silicon technologies, with typical silicon absorber thicknesses of only a few hundred nanometers, require much less active material, which can be deposited on a variety of low-cost substrates, such as glass, stainless steel, and plastic. Thinner absorber layers not only contribute to cost reduction but are also benefi cial in terms of energy-payback time. However, as light absorption in silicon becomes small near the bandgap, advanced light-trapping schemes must be implemented to achieve high effi ciencies with such thin silicon layers. [ 3 , 4 ] According to Yablonovitch’s theory, [ 5 ] a maximum absorption enhancement of up to 4 n 2 may be reached, where n is the refractive index of the active absorber material. Experimentally, light trapping is realized by introducing scattering at interfaces between neighboring layers that have different refractive indices. In the substrate confi guration (or n-i-p confi guration, describing the deposition sequence of n-type, intrinsic, and p-type silicon layers onto the substrate), this is achieved by texturing of the back refl ector. Thermally roughened silver fi lms, [ 6–9 ] silver-covered randomly oriented SnO 2 and ZnO pyramids grown by chemical vapor deposition, [ 10–13 ] and silver-covered vertically aligned ZnO nanorods grown by hydrothermal synthesis [ 14 ] have already been successfully used to enhance light absorption in n-i-p solar cells. Naughton et al. [ 15 ] fabricated arrays of silver-covered 1.6 μ m-high tapered Si nanopillars and used those as a back refl ector for coaxial n-i-p nanopillar solar