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Key Points Provenance changes at the outlet of the Hetao Basin indicate the desiccation and re‐integration of the upper Yellow River over the last ∼40 ka Paleo‐lake shorelines and geochemical proxies confirm that the west Hetao Basin contained the terminal lake for the desiccated Yellow River Climate‐river feedbacks across glacial‐interglacial cycles have implications for constraining terrestrial‐marine source‐to‐sink processes 

One of the organic components in the perovskite photo-absorber, the methylammonium cation, has been suggested to be a roadblock to the long-term operation of organic–inorganic hybrid perovskite-based solar cells. In this work we systematically explore the crystallographic and optical properties of the compositional space of mixed cation and mixed halide lead perovskites, where formamidinium (FA + ) is gradually replaced by cesium (Cs + ), and iodide (I − ) is substituted by bromide (Br − ), i.e. , Cs y FA 1− y Pb(Br x I 1− x ) 3 . Higher tolerance factors lead to more cubic structures, whereas lower tolerance factors lead to more orthorhombic structures. We find that while some correlation exists between the tolerance factor and structure, the tolerance factor does not provide a holistic understanding of whether or not a perovskite structure will fully form. By screening 26 solar cells with different compositions, our results show that Cs 1/6 FA 5/6 PbI 3 delivers the highest efficiency and long-term stability among the I-rich compositions. This work sheds light on the fundamental structure–property relationships in the Cs y FA 1− y Pb(Br x I 1− x ) 3 compositional space, providing vital insight to the design of durable perovskite materials. Our approach provides a library of structural and optoelectronic information for this compositional space. 

The impact of the bulky‐cation‐modified interfaces on halide perovskite solar cell stability is underexplored. In this work, the thermal instability of the bulky‐cation interface layers used in the state‐of‐the‐art solar cells is demonstrated. X‐ray photoelectron spectroscopy and synchrotron‐based grazing‐incidence X‐ray scattering measurements reveal significant changes in the chemical composition and structure at the surface of these films that occur under thermal stress. The changes impact charge‐carrier dynamics and device operation, as shown in transient photoluminescence, excitation correlation spectroscopy, and solar cells. The type of cation used for surface treatment affects the extent of these changes, where long carbon chains provide more stable interfaces. These results highlight that prolonged annealing of the treated interfaces is critical to enable reliable reporting of performances and to drive the selection of different bulky cations.