Optical Path Length Factor at Near-Bandgap Wavelengths in Si Solar Cells

摘要

By using the Rand and Basore (R&B) physical model for light trapping, a particular analytical expression for near-bandgap optical path length factor Zo is derived for the case where back surface reflectivity RBACK is equal to unity (RBACK = 1). This expression shows that according to the R&B physical model, at RBACK = 1, Zo has a finite value. At RBACK = 1, the R&B expression for Zo is shown to diverge. By using the original R&B derivation procedure, a new expression for Zo is obtained, which at RBACK = 1, reduces to the aforementioned particular expression for ideal back surface reflectance. The new expression is immune from the physical incongruity and tendency to overestimate Zo that affect the R&B expression. By using the new expression, a relationship between Zo and the Yablonovitch and Cody (Y&C) absorption-enhancement factor is derived for devices where parasitic absorption predominantly occurs at backside reflector. This relationship is shown to be in agreement with the literature. In the derivation, a connection between the approaches to light trapping of R&B and Y&C is exploited. This connection allows obtaining a new method to evaluate RBACK at near-bandgap wavelengths in most Si solar cells. The new method is checked on two real Si solar cells and found to provide plausible results. Compared to the method used by R&B, the method presented in this paper is shown to be more reliable