Suppressing nucleation over large areas

Solar Cells Although formamidinium-based lead iodide (PbI2) perovskites have a favorable bandgap and good thermal stability, the difficulty in controlling nucleation makes it difficult to grow high-quality, large-area films compared with methylammonium counterparts. Bu et al. show that adding N -methyl-2-pyrrolidone to the perovskite precursors forms an adduct with PbI2 that promotes the formation of the desired black α-phase at room temperature. The addition of potassium hexafluorophosphate eliminated hysteresis by passivating interfacial defects and promoted long-term thermal stability at 85°C in unencapsulated devices. Large-area modules (17 square centimeters) achieved power conversion efficiencies of 20.4%. Science , abh1035, this issue p. [1327][1] [1]: /lookup/doi/10.1126/science.abh1035