Optical Losses in Tandem Solar Cells: Reflection and Parasitic Absorption

… more less Optical losses in tandem solar cells are light that never reaches either absorber. There are two main kinds. The first is reflection. Each time light crosses from one layer to another with a different refractive index, some bounces back. A tandem has many such borders, so these add up. The second is parasitic absorption. That means a layer that is not an absorber still soaks up light and turns it into heat. Common culprits are the clear conducting oxides, the thin fullerene layer used to pull out electrons, and the layer that joins the two subcells. To find these losses, plot the EQE of each subcell and add them at each wavelength. EQE is the share of photons that become collected charge. The gap between that sum and 100 percent is light lost to reflection or parasitic absorption. A total reflection scan can split the two. Lost light lowers the current of whichever subcell it was meant for, and in a series stack that can make it the limiting one. Fixes include an anti-reflection coating on top, a textured surface, and thinner or more clear contact layers. Thinner contacts can raise resistance, so there is a trade-off with fill factor. After this Concept you can read summed EQE curves, estimate how much light a tandem loses, and name the layer to change first.