All-Perovskite Tandem Solar Cells with Tin-Lead Bottom Cells

… more less All-perovskite tandem solar cells stack two perovskite layers instead of perovskite on silicon. The top cell has a wide band gap, about 1.8 eV, made rich in bromide. The bottom cell has a narrow band gap, about 1.25 eV, made by swapping about half the lead for tin. The two gaps split the sunlight much like a perovskite-silicon pair. The big appeal is that every layer can be made from ink or vapor at low heat. So the stack can be thin, light and even flexible, with no silicon wafer. There are four hard parts. First, tin oxidizes easily from Sn2+ to Sn4+ in air. That creates defects and leaks charge, so the bottom cell needs added reducing agents and care to keep air out. Second, the bottom absorber must be thick enough to catch the near-infrared light. Thick tin-lead films are harder to make clean. Third, the layer joining the two cells must survive the solvent of the next ink coated on it. Fourth, the bromide-rich top cell is prone to halide segregation under light, which lowers its voltage. Since the two cells are in series, current matching still applies. When you read such a result, check the stability data as well as the efficiency. After this Concept you can describe how an all-perovskite tandem is built and name the main reasons it is less mature than perovskite-silicon.