Light-trapping Performance of Double-layer Conformal Grating Silicon Thin-film Solar Cells

High-efficiency solar cells based on nanostructures are a research hotspot in the photovoltaic field today. On the one hand， there is the research on new types of solar cells such as perovskite cells. On the other hand， silicon-based solar cells remain the most widely studied and applied type of cell. How to further improve the efficiency of silicon-based solar cells and reduce their cost still requires in-depth research. Silicon is an indirect bandgap semiconductor material， and its absorption of incident light in the near-infrared band is very weak. Only when the thickness of the absorption layer exceeds 200 µm can the cell achieve full absorption in a wide band （300~1 100 nm）. Reasonable light-trapping nanostructure design can enhance the light absorption performance of silicon thin-film solar cells. The morphology， size， material and spatial layout of these light-trapping nanostructures have a significant impact on the surface anti-reflection characteristics and light-trapping performance of silicon thin-film solar cells.In this paper， a double-layer conformal grating silicon thin-film solar cell structure was designed. The influence of different grating shapes on the light-trapping performance of silicon thin-film solar cells was systematically studied by using the concept of super-quadratic subwavelength gratings. By changing the degree of the grating’s profile function， the profile shape of the grating can be adjusted. Five representative grating profile curves （n=1， 2， 4， 10， ∞） were selected. As n increases， the grating profile shape can be gradually changed from triangular （n=1）， parabolic （n=2）， to rectangular （n→∞）. The silicon thin-film solar cells with the above five grating structures were optimized by using the finite-difference time-domain method. By comparing and analyzing the short-circuit current density， absorption spectrum， and light absorption enhancement spectrum of the five grating structures and the planar structure， the light-trapping effect law of the double-layer conformal grating silicon thin-film solar cell was explored. The internal light-trapping mechanism was analyzed using the electromagnetic field intensity distribution diagrams. In addition to qualitatively analyzing the light-trapping performance of each grating structure silicon thin-film solar cell， the concept of integral absorption was also introduced to quantitatively analyze their light absorption characteristics over the entire wavelength range. Moreover， the optimization calculation of the silicon thin-film solar cells with four types of bottom metal grating materials （Ag， Au， Cu， and Al） was carried out. A comparative analysis was conducted on the short-circuit current densities and absorption spectra of four types of bottom metal grating structures， and the light absorption enhancement effect of different bottom metal grating materials was explored. The mechanism was analyzed from the perspective of optical constants.The research results indicate that as n increases， the overall light-trapping performance of silicon thin-film solar cells decreases as the grating profile shape transforms from triangular （n = 1） to rectangular （n→∞）. Among them， the parabolic （n = 2） grating structure has superior light absorption performance in both the 300~750 nm and 750~1 100 nm bands. Because the n = 2 grating structure has excellent anti-reflection performance in the visible light band， and the LSPRs mode is excited on the surface of the bottom Ag grating in the near-infrared band， while optical waveguide mode coupling is achieved in the c-Si absorption layer. The triangular （n = 1） grating structure has better light-trapping performance in the 750~1 100 nm band， but its light absorption performance in the 300~750 nm band is poor. The results also show that the Jsc values of the silicon thin-film solar cells with the bottom metal gratings of Ag， Au， Cu and Al decrease in sequence， among which the bottom Ag grating structure has the best light-trapping performance and the bottom Al grating structure has the worst. The research results of this paper can provide theoretical guidance for the structural design of thin-film solar cells based on nano-grating structures.