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Stable but not quite cubic The black, photoactive phase of formamidinium (FA) perovskites, which is usually stabilized by cation alloying to avoid the formation of inactive hexagonal phases, is assumed to be cubic. High-resolution microscopy studies by Doherty et al . using nanoscale probes revealed that these FA-rich phases are not cubic but rather undergo slight tilting (by two degrees) of the octahedra. Black phases can have localized regions of hexagonal phases that nucleate degradation. Surface-bound ethylenediaminetetraacetic acid stabilized the tilted phase of pure FA lead triiodide against environmental degradation. —PDS

Understanding the nanoscopic chemical and structural changes that drive instabilities in emerging energy materials is essential for mitigating device degradation. The power conversion efficiency of halide perovskite photovoltaic devices has reached 25.7% in single junction and 29.8% in tandem perovskite/silicon cells 1,2 , yet retaining such performance under continuous operation has remained elusive 3 . Here, we develop a multimodal microscopy toolkit to reveal that in leading formamidinium-rich perovskite absorbers, nanoscale phase impurities including hexagonal polytype and lead iodide inclusions are not only traps for photo-excited carriers which themselves reduce performance 4,5 , but via the same trapping process are sites at which photochemical

Solar cells and optoelectronics based on lead halide perovskites are generating considerable interest but face challenges with the use of toxic lead. In this study, we fabricate and characterize lead-free perovskites based on germanium and tin solid solutions, CH3NH3Sn(1–x)Ge x I3 (0 ≤ x ≤ 1). We show that these perovskite compounds possess band gaps from 1.3 to 2.0 eV, which are suitable for a range of optoelectronic applications, from single junction devices and top cells for tandems to light-emitting layers. Their thermodynamic stability and electronic properties are calculated for all compositions and agree well with our experimental measurements. Our findings demonstrate an attractive family of lead-free perovskite semiconductors with a favorable band-gap range for efficient single-junction solar cells.

Electrochemical energy storage devices that can harvest energy from the environment and store it are increasingly important to address both energy poverty in developing parts of the world, as well as powering off-grid autonomous devices. Currently, batteries or supercapacitors connected to solar cells are used for these applications, but these frequently suffer from voltage mismatches and inefficiencies in the device packaging. This paper presents an optically and electrochemically active electrode for photo-rechargeable zinc-ion capacitors using vanadium oxide nanofibers. These rely on photo-excited charge carrier separation to charge the capacitors without any external photovoltaic or electrical devices. We found that silver nanowires are better than carbon based conductive additives as they support photo-excited holes transport and provides light scattering centers that enhance visible light absorption. The proposed capacitors show a ~ 63% capacity increase under illumination, photo-recharge in 30 minutes and ~ 99% capacity retention over 4000 cycles.