Light Management in Silicon Nanostructures for Photovoltaics

The main challenge with the use of silicon for photovoltaics is that silicon is not a strong absorber of sunlight in the near-infrared region. Conventional silicon photovoltaics are thus typically of thicknesses 200 to 300 _m to ensure the absorption of most sunlight. However, single crystalline silicon solar cells require expensive manufacturing methods. So current solar cell technology is not as competitive as that of traditional energy sources. To promote and increase the silicon solar cell capacity, costs need to drop below $1/W. Increasing absorption of light in the absorber layers is critical issue for achieving high efficiency silicon solar cells. Various light trapping methods have been developed, experimentally and computationally. Light trapping with sub-wavelength nanostructures involves coupling light into localized resonant modes and guided resonance modes in active region to increase absorption. Nanophotonic light trapping strategies have used structuring of the silicon itself or patterning of dielectric materials on front and back of silicon.\ud \ud In order to continue developing cheap and high efficiency silicon solar cells, we studied several anti-reflection and light trapping structures with lower cost and higher absorption. First of all, tapered nanocone structure was proposed and studied and then fabricated via Bosch process. Secondly, high refractive index nanosphere scheme was investigated as a light trapping strategy which does not create new surface or interface and is easy to fabricate. Then inverse woodpile and woodpile photonic crystal structures were studied and proven to have superior light trapping ability due to the ability to engineer the photonic density of states.\ud \ud Finally, ultrathin silicon was fabricated via wet etching method, and then high refractive index nanosphere layers were coated and the power conversion efficiency was increased by 26.5%. Furthermore, metal nanomesh was used as front contact to substitute traditional indium tin oxide, and the power conversion efficiency was increased by 53% due to high haze factor and lower sheet resistance of the metal nanomesh. More importantly, with the metal nanomesh as front contact, the ultrathin silicon solar cells showed superior flexibility.\u