Hybrid halide perovskites represent one of the most promising solutions toward the fabrication of all solid nanostructured solar cells, with improved efficiency and long-term stability. This article aims at investigating the structural properties of iodide/chloride mixed-halide perovskites and correlating them with their photovoltaic performances. We found out that, independent of the components ratio in the precursor solution, Cl incorporation in an iodide-based structure, is possible only at relatively low concentration levels (below 3–4%). However, even if the material band gap remains substantially unchanged, the Cl doping dramatically improves the charge transport within the perovskite layer, explaining the outstanding performances of meso-superstructured solar cells based on this material.

Recovery of hydraulic conductivity after the induction of embolisms was studied in woody stems of laurel (Laurus nobilis). Previous experiments confirming the recovery of hydraulic conductivity when xylem pressure potential was less than ؊1 MPa were repeated, and new experiments were done to investigate the changes in solute composition in xylem vessels during refilling. Xylem sap collected by perfusion of excised stem segments showed elevated levels of several ions during refilling. Stem segments were frozen in liquid N 2 to view refilling vessels using cryoscanning electron microscopy. Vessels could be found in all three states of presumed refilling: (a) mostly water with a little air, (b) mostly air with a little water, or (c) water droplets extruding from vessel pits adjacent to living cells. Radiographic probe microanalysis of refilling vessels revealed nondetectable levels of dissolved solutes. Results are discussed in terms of proposed mechanisms of refilling in vessels while surrounding vessels were at a xylem pressure potential of less than ؊1 MPa. We have concluded that none of the existing paradigms explains the results.Over the past two decades, scientists have found substantial evidence that the vulnerability of xylem to cavitation is an important factor in the adaptation of plants to the environment (Tyree and Sperry, 1989;Cochard et al., 1992;Salleo and Lo Gullo, 1993). The cavitation (droughtinduced embolism) of xylem has been detected in stems (Cochard and Tyree, 1990), leaves (Kikuta et al., 1997), and roots (Mencuccini and Comstock, 1997) and has appeared to limit effectively the possible distribution areas of plant species (Cochard et al., 1992). For example, the vulnerability of Holm oak to xylem embolism caused by both drought and freeze stress (Lo Gullo and Salleo, 1993) provides a convincing explanation for the distribution versus elevation and latitude of this species (Pignatti, 1982) in the Mediterranean region.The threshold xylem pressure for cavitation is close to the typical midday xylem pressure of many species in the field (Kikuta et al., 1997). Such a narrow safety margin (Sperry, 1995) is intrinsically dangerous for plant survival under adverse environmental conditions but might be of some advantage in buffering leaf water status (Dixon et al., 1984;Salleo et al., 1997) and in inducing stomatal closure (Sperry, 1995).Debate still exists about the possible mechanisms involved in xylem refilling after cavitation events induced by drought (Tyree and Cochard, 1996) and freezing (Sperry, 1995) stress. The existing paradigm suggests that embolism removal must occur by gas dissolution in the surrounding water. Henry's law states that the solubility of a gas in water is proportional to the partial pressure of the gas species adjacent to the water. Since water in plants is saturated with air at atmospheric pressure, the paradigm requires that the embolism be above atmospheric pressure for the gases to dissolve. Some experiments on angiosperms and gymnosperms fit this paradigm Yang and Tyr...

Hybrid halide perovskites represent one of the most promising solutions toward the fabrication of all solid nanostructured solar cells with improved efficiency and long-term stability. This article aims at investigating the structural properties of the iodide/chloride mixed-halide perovskites and correlating them with the photovoltaic performances of the related sensitized solar cells. We found out that, independently on the components ratio in the precursor solution, Cl incorporation, in a I-based structure, is possible only at relatively low concentration levels (below 3-4%). However, even if the material band-gap remains substantially unchanged, incorporation of Cl as a dopant dramatically improves the charge transport within the perovskite layer, explaining the outstanding performances of meso-superstructured solar cells based on this material.

The influence of thermal treatments on the properties of mixed bromide-iodide organolead perovskites (MAPbI3−xBrx, MA=CH3NH3) is investigated in films prepared in air by single-step solution processes based on different precursor solutions. Initially, the bandgap energy (EG) dependence on composition is reconsidered on films obtained by mixtures of tri-halide solutions.An EG(x) relation is obtained that is expected to be independent of the film properties and can be used to assess perovskite composition. In these samples recombination centres are observed whose energy depth increases with x, likely involving the simultaneous presence of iodide and bromide, while the Urbach energy increases with the grain surface-to-volume ratio, which points out that the defects giving sub-bandgap absorption originate from grain boundaries. Tri-halide mixtures allow perovskite synthetic processes suitable for solar cell production, being fast and reproducible. A slight MABr excess in the solution made of MABr and PbI2 gives MAPbI2Br films free of PbI2 phases and with a high compositional stability, but non-radiative recombination channels can make the material not appropriate for high efficiency solar cells.Finally, the solution made of MAI and PbBr2 (3:1 molar ratio) is the less promising for solar cell production because its non-stoichiometric nature synthesis reproducibility an issue.

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