Measurement of the Optical Constants of Soda-Lime Glasses in Dependence of Iron Content and Modeling of Iron-Related Power Losses in Crystalline Si Solar Cell Modules

It is well known that the absorbance of soda-lime glass is very sensitive to the amount of iron contamination; therefore, it strongly affects the power output of mass-produced crystalline silicon solar cell modules. We use a combination of ellipsometry and transmission measurements to determine the optical constants, at wavelengths between 300 and 1690 nm, of soda-lime-silica glasses containing an iron content between 1 parts per thousand and 0.01 parts per thousand, measured with inductive coupled plasma optical emission spectroscopy. We derive two different semiempirical models for the extinction coefficient of soda-lime-silica glass as a function of its iron content: one model for iron alone and the other model for iron including other typical remaining coloring agents. Furthermore, we use ray tracing and spice simulations to predict the power losses in standard modules as a function of iron content in their cover glass sheet. Considering a module with 3.2-mm glass thickness, our results predict a decline in module output power due to iron content in the glass of 1.1% (3 W) for Fe2O3 = 0.1 parts per thousand and 9.8% (28W) for Fe2O3 = 1 parts per thousand.