Measuring Tandem Solar Cell Efficiency: Spectral Mismatch and the Limiting Subcell

… more less Measuring tandem solar cell efficiency is harder than measuring a single cell. A lab lamp, called a solar simulator, tries to copy the standard sunlight spectrum. No lamp is perfect. If it is a little too blue, the top cell makes extra current. If it is a little too red, the bottom cell does. In a series tandem, the subcell with less current sets the output. So a lamp error can change which subcell limits and make the efficiency look higher or lower than it truly is. This is called spectral mismatch. To fix it, labs first measure the EQE of each subcell. To do that, they shine a bias light that floods the other subcell, so only the cell under test limits the signal. From each EQE and the standard spectrum, they work out the current each subcell should make in real sunlight. Then they tune the lamp, often with several light sources, until both subcells make those currents. The J-V scan is then run slowly or held at max power, so the result is a stable value. This takes special gear, which is why trusted tandem records come from a few specialist test labs. After this Concept you can explain spectral mismatch and check whether a reported tandem efficiency used subcell EQE and a tuned lamp.