24^th World Nano Conference

Biography

Biography: Hikaru Kobayashi

Abstract

We have developed a method to fabricate ultralow reflectance (≤3%) Si using the surface structure chemical transfer (SSCT) method which simply involves contact of Pt catalyst with Si wafers immersed in H2O2+HF solutions. The SSCT treatment forms a nanocrystalline Si layer with the porosity decreasing with the depth and thus with the refractive index increasing with the depth. Although the graded refractive index can achieve ultralow reflectance, the nanocrystalline Si layer possesses an extremely large surface area, resulting in a high surface recombination rate. We have developed a surface passivation method called PSG method which includes spin-coating of phosphosilicate glass (PSG) and heat treatment at ~900ºC. The heat treatment melts PSG, resulting in its penetration into nanopores in the nanocrsytalline Si layer, formation of chemical bonds with Si nanocrystals, and thus, elimination of surface states. We have fabricated structure using the SSCT method and the PSG method. Despite the simple solar cell structure without antireflection coating, a high conversion efficiency of 19.8~20.4% has been achieved. The internal quantum efficiency (IQE) in the short wavelength region (≤400 nm) is nearly zero without passivation while it is greatly increased (e.g. 0.8 at 400 nm) by the PSG passivation method. Si nanocrystals in the nanocrystalline Si layer possess the same orientation, i.e. Si (100) orientation and thus, no grain boundaries are present in the nanocrsytalline Si layer. Therefore, recombination in the nanocrystalline Si layer is greatly suppressed. The nanocrystalline Si layer exhibits red photoluminescence, indicating that its band-gap energy increases due to the quantum confinement effect. The size of Si nanocrystals increases with the depth, resulting in the graded band-gap structure. Electron-hole pairs photo-generated in the nanocrsytalline Si layer are effectively separated by the graded band-gap structure, leading to the high IQE in the short wavelength region.