Suppressing nucleation over large areas: Although formamidinium-based lead iodide (PbI ₂ ) 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 PbI ₂ 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

One of the primary challenges in commercializing perovskite solar cells (PSCs) is achieving both high power conversion efficiency (PCE) and sufficient stability. We integrate wafer-scale continuous monolayer MoS ₂ buffers at the top and bottom of a perovskite layer through a transfer process. These films physically block ion migration of perovskite into carrier transport layers and chemically stabilize the formamidinium lead iodide phase through strong coordination interaction. Effective chemical passivation results from the formation of Pb-S bonds, and minority carriers are blocked through a type-I band alignment. Planar p-i-n PSCs (0.074 square centimeters) and modules (9.6 square centimeters) with MoS ₂ /perovskite/MoS ₂ configuration achieve PCEs up to 26.2% (certified steady-state PCE of 25.9%) and 22.8%, respectively. Moreover, the devices show excellent damp heat (85°C and 85% relative humidity) stability with <5% PCE loss after 1200 hours and notable high temperature (85°C) operational stability with <4% PCE loss after 1200 hours.