An electrochromic mirror electrode based on reversible uptake of hydrogen in nickel magnesium alloy films is reported. Thin, magnesium-rich Ni-Mg films prepared on glass substrates by cosputtering from Ni and Mg targets are mirror-like in appearance and have low visible transmittance. Upon exposure to hydrogen gas or on cathodic polarization in alkaline electrolyte, the films take up hydrogen and become transparent. When hydrogen is removed, the mirror properties are recovered. The transition is believed to result from reversible formation of Mg 2 NiH 4 and MgH 2 . A thin overlayer of palladium was found to enhance the kinetics of hydrogen insertion and extraction, and to protect the metal surface against oxidation.Devices capable of switching between mirror and transparent states may find applications in architectural and transportation energy conservation, lighting and displays, aerospace insolation control, and optical communications systems. Switchable mirrors based on rare earth hydrides were discovered by Huiberts et al., 1 who observed a reversible metal-to-insulator transition when a thin film (150 to 500 nm) of yttrium or lanthanum coated with a thin layer of palladium was exposed to hydrogen gas. The transition accompanies conversion of a metallic dihydride phase to a semiconducting trihydride. Rare earth-magnesium alloy films 2 were subsequently found to be superior to the pure lanthanides in maximum transparency and mirror-state reflectivity. Phase separation appears to occur when these alloys take up hydrogen, giving transparent MgH 2 and LnH 2-3 , both of which may participate in the switching mechanism. 3 Because the rare earths are highly vulnerable to oxidation, a Pd overlayer at least 5 nm thick is required for films exposed to air or to an alkaline electrolyte. Although the Pd catalyzes the uptake and removal of hydrogen, it limits the maximum transparency of the composite film to about 50%. 4 The influence of the Pd layer and of a Pd/AlO x composite cap layer on hydrogen uptake by rare earth-based films have been studied extensively by van Gogh et al. 5 and by van der Molen et al. In the case of nickel, a stoichiometric alloy phase, Mg 2 Ni, with the same Mg-Ni ratio as in the hydride, can be prepared from the elements. In Mg 2 Ni (Fig. 1a), there are Ni-Ni bonds (shown as hollow rods), two types of Ni-Mg bonds (solid rods) and three types of Mg-Mg bonds (not shown).10 The Ni-Ni bonds and Mg-Mg bonds are shorter in the alloy than in the pure elements. The alloy absorbs hydrogen without structural rearrangement up to a composition of Mg 2 NiH 0.3 .7 This phase has metallic properties similar to those of the pure alloy. Further introduction of hydrogen produces Mg 2 NiH 4 (Fig. 1b) Ni-Mg films were deposited by DC magnetron co-sputtering from 2 in diameter Ni and Mg (99.98%) targets onto glass substrates with and without transparent conductive coatings. The base pressure was 1.4 x 10 -7 Torr, process pressure 2 mTorr, Ni power 20 W, Mg power 22 W, target-tosubstrate distance 7.5 cm. Dep...

The exploration of the synthetic space of halide perovskites hinges on an enormous number of parameters requiring time-consuming experimentation to decouple and optimize. Here, the formation of the prototype material CH 3 NH 3 PbI 3 (MAPbI 3 ) is investigated at different time and length scales using multimodal in situ measurements to monitor the evolution of crystalline phases, morphology, and photoluminescence as a function of the lead precursors. Kinetically fast formation of crystalline precursor phases already during the spin-coat deposition is observed using lead iodide (PbI 2 ) or lead chloride (PbCl 2 ) routes. These precursor phases most likely template final MAPbI 3 film morphology. In particular, the emergence of the "needle-like" structure is shown to appear before film annealing. In situ photoluminescence measurements suggest nanoscale nucleation followed by rapid nuclei densification and growth. Using this multimodal in situ approach, different formation pathways can be identified either via precursor phases in the PbI 2 and PbCl 2 routes or direct perovskite formation from molecular building blocks as observed in the lead acetate (PbAc 2 ) route. Correlation of in situ results with photovoltaic device performance demonstrates the power of in situ multimodal techniques, paves the way to a fast screening of synthetic parameters, and ultimately leads to controlled synthetic procedures that yield high-efficiency devices.

Complex phenomena are prevalent during the formation of materials, which affect their processing-structure-function relationships. Thin films of methylammonium lead iodide (CH3NH3PbI3, MAPI) are processed by spin coating, antisolvent drop, and annealing of colloidal precursors. The structure and properties of transient and stable phases formed during the process are reported, and the mechanistic insights of the underlying transitions are revealed by combining in situ data from grazing-incidence wide-angle X-ray scattering and photoluminescence spectroscopy. Here, we report the detailed insights on the embryonic stages of organic-inorganic perovskite formation. The physicochemical evolution during the conversion proceeds in four steps: i) An instant nucleation of polydisperse MAPI nanocrystals on antisolvent drop, ii) the instantaneous partial conversion of metastable nanocrystals into orthorhombic solvent-complex by cluster coalescence, iii) the thermal decomposition (dissolution) of the stable solvent-complex into plumboiodide fragments upon evaporation of solvent from the complex and iv) the formation (recrystallization) of cubic MAPI crystals in thin film.

Transparent thin films of copper(I) oxide prepared on conductive SnO2:F glass substrates by anodic oxidation of sputtered copper films or by direct electrodeposition of Cu2O transformed reversibly to opaque metallic copper films when reduced in alkaline electrolyte. In addition, the same Cu2O films transform reversibly to black copper(II) oxide when cycled at more anodic potentials. Copper oxide-to-copper switching covered a large dynamic range, from 85% and 10% photopic transmittance, with a coloration efficiency of about 32 cm2/C. Gradual deterioration of the switching range occurred over 20 to 100 cycles. This is tentatively ascribed to coarsening of the film and contact degradation caused by the 65% volume change on conversion of Cu to Cu2O. Switching between the two copper oxides (which have similar volumes) was more stable and more efficient (CE = 60 cm2/C), but covered a smaller transmittance range (60% to 44% T). Due to their large electrochemical storage capacity and tolerance for alkaline electrolytes, these cathodically coloring films may be useful as counter electrodes for anodically coloring electrode films such as nickel oxide or metal hydrides.

Thin, Pd-capped metallic films containing magnesium and first row transition metals (Mn, Fe, Co) switch reversibly from their initial reflecting state to visually transparent states when exposed to gaseous hydrogen or following cathodic polarization in an alkaline electrolyte. Reversion to the reflecting state is achieved by exposure to air or by anodic polarization. The films were prepared by co-sputtering from one magnesium target and one manganese, iron, or cobalt target. Both the dynamic optical switching range and the speed of the transition depend on the magnesium-transition metal ratio. Infrared spectra of films in the transparent, hydrided (deuterided) states support the presence of the intermetallic hydride phases Mg 3 MnH 7 , Mg 2 FeH 6 , and Mg 2 CoH 5 . * Following the discovery of the switchable mirror phenomenon in yttrium and lanthanum hydrides by Huiberts et al., 1 similar behavior was reported in rare earth-magnesium alloy films. 2 Recently, metaltransparent hydride switching was also found in nickel-magnesium thin films. 3 In all of these systems, a thin Pd overlayer (generally > 5 nm) is applied to catalyze absorption and desorption of hydrogen and to protect the readily oxidized rare earth and/or Mg from oxidation. Infrared internal reflectance spectroscopy was used to characterize the transparent Ni-Mg hydride films as a mixture of Mg 2 NiH 4 and MgH 2 . 3 Other ternary hydrides containing magnesium and a first row transition metal include Mg

Black-phase formamidinium lead iodide perovskite (FAPbI3), whilst the most promising species for efficient perovskite photovoltaics, is energetically unfavored at room temperature, and is thus always accompanied by undesirable yellow phases during crystallization 1,2,3,4 . The challenge to formulate the fast crystallization process of perovskite has limited the community in deriving unified guidelines for governing the formation of black-phase FAPbI3 5,6 . Here, through in-situ monitoring of the perovskite crystallization process, we report an oriented nucleation mechanism that acted as the key to avoid undesirable phases. This concept was applicable to improving the photovoltaic device performance under different film-processing scenarios. The small-area device demonstrated a power conversion efficiency of 25.4% (certified 25.0%), and the module (27.83 cm 2 ) achieved a champion aperture efficiency of 21.4% (certified). MainFormamidinium lead iodide perovskite (FAPbI3) features desirable bandgap and thermal resistance, and has thus emerged as the most promising candidate among the perovskite family for highly efficient photovoltaic devices 1,2,3,7,8 . However, the photoactive black-phase FAPbI3 is not energetically favorable at room temperature 4,9,10 .Polytype formation and other intermediate non-photoactive phases can readily occur, which undermines the photovoltaic performance. A few approaches have been developed to promote the formation of black-phase FAPbI3 at room temperature, such as adduct formation with PbI2 and solvent engineering using ionic liquids 11,12 .

Electroanalytical techniques indicate that voltage hysteresis in electrochemical conversion reactions has thermodynamic origins, which highlights the significant challenge to their prospects of application.

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