Nanostructure Arrays Using Self-Assembly Templates for Broadband Omnidirectional Light Harvesting of Solar Cells

In this thesis, the periodic Si nanopillar arrays (NPAs) and Si nanowire arrays (NWAs) as an AR surfaces were fabricated by self-assembly method, and the optical properties of Si NWAs are discussed in detail. Finally, we apply the optical properties of nanostructures with different morphology on the tandem Si thin-film solar cell. The efficiency is enhanced 28%. First of all, periodic Si NPAs were fabricated by the colloidal lithography combined with catalytic etching. By varying the size of colloidal crystals using oxygen plasma etching, Si NPAs with desirable diameter and fill factor could be obtained. The Fresnel reflection can be eliminated effectively over broadband regions by NPAs; i.e., the wavelength-averaged specular reflectance is decreased to 0.70 % at the wavelengths of 200-1900 nm. The reflectance is reduced greatly for the incident angles up to 70° for both s- and p-polarized light. These excellent AR performances are attributed to light trapping effect and very low effective refractive indices, which can be modified by the fill factor of Si in the NPA layers. Second, large-area, periodic Si NWAs were fabricated by catalytic etching with anodic aluminum oxide as a patterning mask. The 100-nm-periodicity NWAs serve an AR function especially at the wavelengths of 200~400 nm, where the reflectance is decreased to be almost tenth of the value of the polished Si (from 62.9% to 7.9%). These NWAs show very low reflectance for broadband wavelengths and omnidirectional light incidence, attributed to the small periodicity and the stepped refractive index of NWA layer. The experimental results are confirmed by theoretical calculations. Raman scattering intensity was also found to be significantly increased with Si NWAs. The introduction of this industrial-scale self-assembly methodology for light harvesting greatly advances the development of Si-based optical devices. Finally, glass substrates with tunable nano- and microislands using self-assembly masks and reactive ion etching were successfully fabricated and applied for tandem Si thin-film solar cells. The morphology of islands greatly influence their optical properties: the microisland structure leads to high scattering, widening the distribution of optical waves within the solar cells, while the nanoisland structure enhances the transmittance and suppresses omnidirectional reflection. The two types of structures were further employed for device fabrications. The conversion efficiency is enhanced by 28%. The enhancement is attributed to the increased optical absorption through the improved transmittance and scattering across the layer interfaces. These studies should benefit the design and fabrication of future Si thin-film solar cells