Wide-Band-Gap Perovskite Top Cells: Voltage Deficit and Halide Segregation

… more less Wide-band-gap perovskite top cells give less voltage than their gap promises. The voltage deficit is the gap, in eV, minus the Voc, in volts. Good 1.55 eV perovskite cells lose about 0.35 to 0.4 volts. Many 1.68 eV top cells lose more. There are three main causes. First, recombination at the contact layers. Recombination is when an electron and a hole meet and cancel, so their energy is lost. Second, the energy levels of the contact layers often line up worse with a wide-gap film. Third, halide segregation. Under light, iodide-rich patches form with a smaller gap. Charges fall into those patches, and the voltage drops toward the lower gap. A falling Voc during the first minutes in light, or a red shift in the film's light glow, hints at segregation. Several fixes are now common. Thin layers that heal the film's surface cut contact losses. Single-molecule-thick contact layers, called self-assembled monolayers, line up energies better. Mixed cations and a little chloride help keep the halides from moving. When you read a top-cell result, compute the deficit and see which fix the authors used. After this Concept you can work out a top cell's voltage deficit and name the likely cause from its Voc and glow data.