The role of rear surface in thin silicon solar cells

The aim of this work is the study of the light-trapping ability of rear surface relief for thin silicon solar cells. With a simple model, the conditions needed to achieve efficient thin cells are discussed first. It is shown how the combined effect of back surface passivation and the confinement of light impacts the cell performance. If the illuminated face is not textured, light-trapping must be accomplished by the rear surface. A rear saw-tooth relief grating is proposed as a good back reflector producing also the tilt of reflected rays. Since technological limitations lead relief features to have sizes near the range of the optical wavelengths, the behavior of such structures is analyzed using a rigorous electromagnetic approach. The influence of the depth-to-period ratio of the grating in the internal reflectivity is analyzed. Finally, it is calculated that the back internal reflectivity is much higher when the exit medium is air than when it is aluminum.