The effect of rear surface passivation layer thickness on high efficiency solar cells with planar and scattering metal reflectors

Rear surface reflector of solar cell is designed to improve light collection capacity by allowing the low energy photons to go through multiple bounces inside the solar device before escaping. In this paper, we investigate the thickness effect of rear SiO₂ surface passivation layer on both optical and electrical properties of front-planar high efficiency PERT (Passivated Emitter and Rear Totally-Diffused) solar cells. Two kinds of metal reflectors are fabricated: the conventional planar reflectors by evaporated Al and the novel scattering reflectors by self assembled Ag nanoparticles. We find that the thickness dependence of rear SiO₂ layer (from 8 nm - 134 nm) on photocurrent shows an asymmetry for planar and scattering reflectors, moreover, the scattering reflectors perform much better than the planar reflectors under all tested SiO₂ thicknesses. A maximum current enhancement (calculated from wavelength 900 nm to 1200 nm) of 12.1% is presented for planar reflector with 134 nm SiO₂ film, and 18.4% for scattering reflector with the optimized 19 nm rear SiO₂ film. Additionally, by adding a detached metal mirror, the maximum current enhancement from scattering reflector jumps to 27.0%. Effective optical path length Z is calculated to study the light trapping (optical properties) under various SiO₂ thicknesses for both reflectors. Diffusion length L is calculated to track the electrical performance. It is shown that thicker SiO₂ is of benefit for both optical and electrical properties when planar Al reflector is used. However, for scattering reflectors, thinner SiO₂ is preferable for optical enhancement, but thicker SiO₂ is desirable for electrical gain. 19 nm SiO₂ is found to be the best choice for cells with scattering reflectors, considering both effects.