Formamidinium-lead-iodide (FAPbI3)-based perovskites with bandgap below 1.55 eV are of interest for photovoltaics in view of their close-to-ideal bandgap. Record-performance FAPbI3-based solar cells have relied on fabrication via the sequential-deposition method; however, these devices exhibit unstable output under illumination due to the difficulty of incorporating cesium cations (stabilizer) in sequentially deposited films. Here we devise a perovskite seeding method that efficiently incorporates cesium and beneficially modulates perovskite crystallization. First, perovskite seed crystals are embedded in the PbI2 film. The perovskite seeds serve as cesium sources and act as nuclei to facilitate crystallization during the formation of perovskite. Perovskite films with perovskite seeding growth exhibit a lowered trap density, and the resulting planar solar cells achieve stabilized efficiency of 21.5% with a high open-circuit voltage of 1.13 V and a fill factor that exceeds 80%. The Cs-containing FAPbI3-based devices show a striking improvement in operational stability and retain 60% of their initial efficiency after 140 h operation under one sun illumination.

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Copper-based materials are promising electrocatalysts for CO2 reduction. Prior studies show that the mixture of copper (I) and copper (0) at the catalyst surface enhances multi-carbon products from CO2 reduction; however, the stable presence of copper (I) remains the subject of debate. Here we report a copper on copper (I) composite that stabilizes copper (I) during CO2 reduction through the use of copper nitride as an underlying copper (I) species. We synthesize a copper-on-nitride catalyst that exhibits a Faradaic efficiency of 64 ± 2% for C2+ products. We achieve a 40-fold enhancement in the ratio of C2+ to the competing CH4 compared to the case of pure copper. We further show that the copper-on-nitride catalyst performs stable CO2 reduction over 30 h. Mechanistic studies suggest that the use of copper nitride contributes to reducing the CO dimerization energy barrier—a rate-limiting step in CO2 reduction to multi-carbon products.

Impressive progress in halide perovskite solar cells motivates further work to improve operating stability. It is known that ion-migration-driven decomposition represents a degradation pathway in perovskite solar cells and that it can occur within the perovskite material even in well-encapsulated devices. Here we find that quasi-two-dimensional (2.5D) perovskites suppress this ionmigration-induced degradation. Using TOF-SIMS, we confirm that iodide migration occurs in bulk perovskite photovoltaic devices operating at their maximum power point (MPP). We observe that iodine ions migrate across the spiro-OMeTAD layer to the spiro/ gold contact interface, oxidizing and deteriorating the gold at the interface. In contrast, we find that large ⟨n⟩ 2.5D perovskites exhibit a significantly reduced rate of ion migration compared to 3D devices and exhibit less than 1% relative PCE loss in over 80 h of continuous operation at MPP, whereas the PCE of 3D devices diminishes by more than 50% within the first 24 h.

Organic–inorganic hybrid perovskite solar cells (PSCs) have seen a rapid rise in power conversion efficiencies in recent years; however, they still suffer from interfacial recombination and charge extraction losses at interfaces between the perovskite absorber and the charge–transport layers. Here, in situ back‐contact passivation (BCP) that reduces interfacial and extraction losses between the perovskite absorber and the hole transport layer (HTL) is reported. A thin layer of nondoped semiconducting polymer at the perovskite/HTL interface is introduced and it is shown that the use of the semiconductor polymer permits—in contrast with previously studied insulator‐based passivants—the use of a relatively thick passivating layer. It is shown that a flat‐band alignment between the perovskite and polymer passivation layers achieves a high photovoltage and fill factor: the resultant BCP enables a photovoltage of 1.15 V and a fill factor of 83% in 1.53 eV bandgap PSCs, leading to an efficiency of 21.6% in planar solar cells.