Perovskite solar cells (PSCs) have reached certified efficiencies of up to 23.7% but suffered from frailness and instability when exposed to ambient atmosphere. Zinc oxide (ZnO), when used as electron transport layer (ETL) on PSCs, gives rise to excellent electronic, optic, and photonic properties, yet the Lewis basic nature of ZnO surface leads to deprotonation of the perovskite layer, resulting in serious degradation of PSCs using ZnO as ETL. Here, we report a simple but effective strategy to convert ZnO surface into ZnS at the ZnO/perovskite interface by sulfidation. The sulfide on ZnO–ZnS surface binds strongly with Pb2+ and creates a novel pathway of electron transport to accelerate electron transfer and reduce interfacial charge recombination, yielding a champion efficiency of 20.7% with improved stability and no appreciable hysteresis. The model devices modified with sulfide maintained 88% of their initial performance for 1000 h under storage condition and 87% for 500 h under UV radiation. ZnS is demonstrated to act as both a cascade ETL and a passivating layer for enhancing the performance of PSCs.

Ni-based catalysts have been widely used in many important industrial heterogeneous processes such as hydrogenation and steam reforming owing to their sufficiently high activity yet significantly lower cost than that of alternative precious-metal-based catalysts. However, nickel catalysts are susceptible to deactivation. Understanding the adsorption and activation behavior of small molecules on the model catalyst surface is important to optimize the catalytic performance. Although many studies have been carried out in recent years, the initial oxidation process of nickel surface is still not fully understood, and the influence of the adsorption sequence of CO and O2 and their co-adsorption is controversial. In this study, the surface oxygen species on Ni(111) and the coadsorption of CO and O2 were explored using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). HREELS can provide useful information about the surface structure, surface-adsorbed species, adsorption sites, and interactions between surface oxygen species and CO on the surface. The results showed that there were two kinds of oxygen species after the oxidation of Ni(111), and the energy loss peaks at 54-58 meV were ascribed to surface chemisorbed oxygen species, and the peak at 69 meV to surface nickel oxide. The chemisorbed oxygen at low coverage displayed a LEED pattern of (2 × 2), revealing the formation of an ordered surface structure. As the amount of oxygen increased, the energy loss peak at 54 meV shifted to 58 meV. At an O2 partial pressure of 1 × 10 −8 Torr (1 Torr = 133.32 Pa), the AES ratio of O/Ni remained almost unchanged after dosing 48 L, which indicated that the surface nickel oxide was relatively stable. The surface chemisorbed oxygen species was less stable, which could change to surface nickel oxide after annealing in vacuum. CO adsorbed on Ni(111) at room temperature with tri-hollow and a-top sites. Upon annealing in vacuum, a-top CO weakened first and then disappeared completely at 520 K, whereas tri-hollow CO was much more stable. The pre-adsorption of CO could suppress O2 adsorption and oxidation of the Ni(111) surface. The presence of oxygen could then gradually remove and replace CO with O2. The surface oxygen species preferred the tri-hollow sites, resulting in more a-top adsorbed CO during the co-adsorption of CO and oxygen. The surface chemisorbed oxygen species were more active and could react with CO at room temperature; however, the surface nickel oxide was less active, and could only be reduced at a higher temperature and higher partial pressure of CO.

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