In the last few years, perovskite solar cells (PSCs) became one of the most advanced technology in photovoltaics (PVs), reaching 25.5% certified power conversion efficiency (PCE) in single-junction cells. [1] This high-performance level, exceeding 80% of the thermodynamic bandgap limit of the perovskite, stems from the maximization of the solar spectrum absorption capability and reduction of the open-circuit voltage deficit through well-adjusted interfacial band level alignment and bulk/interface defects passivation to abate nonradiative recombination processes. [2] The reduction of interfacial energy losses due to energy misalignment is facilitated given the large richness of cationic and anionic substitution possibilities starting from the prototypical CH 3 NH 3 PbI 3 (MAPbI 3 ) composition, affording a very precise control of the optoelectronic characteristics. Through a wide exploration of composition, triple cation/double halide formulation Cs 0.05 (MA 0.17-FA 0.83 ) 0.95 Pb(Br 0.17 I 0.83 ) 3 (CsMAFA) rapidly emerged as one of the most efficient compositions for single-junction PSC [3,4] and also as a top cell in a monolithic perovskite/silicon tandem architecture reaching above 29% certified PCE with a slightly bromide richer composition. [5] However, the performance enhancement has progressed more rapidly than improving the stability, inhibiting the technology transfer to larger scale. Many efforts are now turned toward this objective through device encapsulation, [6,7] hydrophobic interfacial layers, [8] nanoscale 2D/3D structuration, [9] and defects passivation. [10][11][12][13] Topdown approaches that give further insights into the degradation pathways of the perovskite absorber and device stacks under operational conditions are highly desirable to propose rational ways to improve the stability at different scales from bulk and surface of the materials, interfaces, and finally on the entire device. Given the complexity in deciphering all possible contributions involved during the degradation, the first step ex situ investigations provide already relevant trends about material weaknesses. For instance, exposure of the conventional MAPbI 3 composition to a humid atmosphere showed rapid decomposition into PbI 2 and gas releases. [14] Depending on the relative humidity (RH)
Micro‐batteries are attractive miniaturized energy devices for new Internet of Things applications, but the lack of understanding of their degradation process during cycling hinders improving their performance. Here focused ion beam (FIB)‐lamella from LiMn1.5Ni0.5O4 (LMNO) thin‐film cathode is in situ cycled in a liquid electrolyte inside an electrochemical transmission electron microscope (TEM) holder to analyze structural and morphology changes upon (de)lithiation processes. A high‐quality electrical connection between the platinum (Pt) current collector of FIB‐lamella and the microchip's Pt working electrode is established, as confirmed by local two‐probe conductivity measurements. In situ cyclic voltammetry (CV) experiments show two redox activities at 4.41 and 4.58/4.54 V corresponding to the Ni2+/3+ and Ni3+/4+ couples, respectively. (S)TEM investigations of the cycled thin‐film reveal formation of voids and cracks, loss of contact with current collector, and presence of organic decomposition products. The 4D STEM ASTAR technique highlights the emergence of an amorphization process and a decrease in average grain size from 20 to 10 nm in the in situ cycled electrode. The present findings, obtained for the first time through the liquid electrochemical TEM study, provide several insights explaining the capacity fade of the LMNO thin‐film cathode typically observed upon cycling in a conventional liquid electrolyte.
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