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Layered 2D organic–inorganic hybrid perovskite is appearing as a rising star in the photovoltaic field, thanks to its superior moisture resistance by the organic spacer cations. Unfortunately, these cations lead to high exciton binding energy in the 2D perovskites, which suffers from lower efficiency in the devices. It thus requires a clear criterion to select/design appropriate organic spacer cations to improve the device efficiency based on this class of materials. Here, 2,2,2‐trifluoroethylamine (F3EA+) is introduced to combine with butylammonium (BA+) cations as mixed spacers. While BA+ enables self‐assembly of 2D perovskite crystals by van der Waals interaction, the introduction of F3EA+ spacers with a high dipole moment suppress nonradiative recombination and promote separation of photogenerated electron–hole pairs by taking the advantage of electronegativity of fluorine. The resultant solar cells based on [(BA)1–x(F3EA)x]2(MA)3Pb4I13 exhibit substantially increased open circuit voltage and fill factor compared with that of (BA)2(MA)3Pb4I13. The champion [(BA)0.94(F3EA)0.06]2(MA)3Pb4I13 solar cell yields a power conversion efficiency of 12.51%, which is among the best performances so far. These findings suggest an effective strategy to design organic spacer cations in layered perovskite for solar cells and other optoelectronic applications. A high dipole moment cation as a large organic spacer reduces the dielectric confinement effect and hence promotes separation of photogenerated electron–hole pairs in layered 2D perovskite materials, which leads to more efficient and stable perovskite solar cells. [ABSTRACT FROM AUTHOR]

A microfluidic-enabled electrochemical device has been developed to investigate electrochemically active nanomaterials under reaction conditions using in situ scanning transmission soft X-ray microscopy (STXM). In situ STXM measurements were conducted on electrodeposited Cu catalysts under electrochemical CO 2 reduction (CO 2 R) conditions. The study provides detailed, quantitative results about the changes in the morphology and chemical structure of the catalytic nanoparticles as a function of applied potentials. The deposited Cu nanoparticles initially contain both Cu(0) and Cu(I). As an increasingly cathodic potential is applied, the Cu(I) species gradually convert to Cu(0) over the potential range of +0.4 to 0 V versus the reversible hydrogen electrode (V RHE ). During this process, Cu(I) particles of various sizes are converted to metallic Cu at different reaction rates and at slightly different potentials, indicating a degree of heterogeneity in the electrochemical response of discrete particles. At CO 2 R relevant potentials, only metallic Cu is observed, and the morphology of the particles is fairly stable within the spatial resolution limits of STXM (∼40 nm). We also report in situ STXM studies of a working electrode with relatively thick Cu-based electrodeposits. The spatially resolved chemical analysis identifies that Cu-oxide species can persist under CO 2 R conditions, but only when the catalytic nanoparticles are electronically isolated from the working electrode and therefore are catalytically irrelevant. In summary, in situ STXM is presented as a technique to gain advanced morphological and spatially resolved chemical structure insights into electrochemically active nanomaterials, which was used to provide improved understanding regarding Cu nanomaterial catalysts under CO 2 reduction conditions.