Light trapping of crystalline Si solar cells by use of nanocrystalline Si layer plus pyramidal texture

Abstract

The surface structure chemical transfer (SSCT) method has been applied to fabrication of single crystalline Si solar cells with 170μm thickness. The SSCT method, which simply involves immersion of Si wafers in H2O2 plus HF solutions and contact of Pt catalyst with Si taking only ∼30s for 6 in. wafers, can decrease the reflectivity to less than 3% by the formation of a nanocrystalline Si layer. However, the reflectivity of the nanocrystalline Si layer/flat Si surface/rear Ag electrode structure in the wavelength region longer than 1000nm is high because of insufficient absorption of incident light. The reflectivity in the long wavelength region is greatly decreased by the formation of the nanocrystalline Si layer on pyramidal textured Si surfaces due to an increase in the optical path length. Deposition of phosphosilicate glass (PSG) on the nanocrystalline Si layer for formation of pn-junction does not change the ultralow reflectivity because the surface region of the nanocrystalline Si layer possesses a refractive index of 1.4 which is nearly the same as that of PSG of 1.4–1.5. The PSG layer is found to passivate the nanocrystalline Si layer, which is evident from an increase in the minority carrier lifetime from 12 to 44μs. Hydrogen treatment at 450°C further increases the minority carrier lifetime approximately to a doubled value. The solar cells with the <front Ag electrode/nanocrystalline Si layer/pyramidal Si substrate/boron-diffused back surface field/Ag rear electrode> structure show a high conversion efficiency of 18.5% in spite of the simple cell structure without antireflection coating. In this case, the short circuit photocurrent density is 40.1mA/cm2 under AM1.5 100mW/cm2 irradiation.