We study the optical
response of CH3NH3PbI3 layers to
light solicitation under different environmental
gas and temperature conditions. The measurements were performed in
nonreactive (Ar or N2) and reactive (O2 or humid
air) gas in the range 40–80 °C crossing the tetragonal–cubic
transition (∼50 °C). With respect to truly inert Ar, the
use of N2 not only assures the reversibility of the optical
constants during thermal cycles but also improves the optical response
of the material. While in N2 and Ar atmospheres the optical
parameters of the material can be recovered at the end of the cycle,
in contrast, the presence of humidity in the air causes the absorption
coefficient to monotonically and inexorably decrease in the whole
visible range, especially after the lattice has moved to cubic. The
use of N2 thus represents an effective strategy to improve
the absorption under thermal operation conditions.
The advance of innovative photovoltaics based on hybrid perovskites is currently forced to face their stability and durability through the rationalization of the phenomena occurring into the lattice under conditions which mimic the material operation. In this framework, we study the structural modifications of MAPbI 3 layers by in situ structural and optical analyses upon recursive thermal cycles from 30 to 80 °C in different annealing environments. We reveal an acceleration of the material modification, above what expected, as the threshold of the tetragonal to cubic transition (∼50 °C) is surpassed. This produces discontinuities in the degradation rate, bandgap value, and dielectric behavior of the MAPbI 3 layer. The phenomenon is put in relationship with the order− disorder lattice modifications described by Car−Parrinello molecular dynamics calculations and reveals that the action of species from humid air becomes largely more effective above 50 °C for reasons related to the increased accessibility/ reactivity of the lattice which, in turn, impacts on defects generation.
We propose an up-scalable, reliable, contamination-free, rod-like TiO2 material grown by a new method based on sputtering deposition concepts which offers a multi-scale porosity, namely: an intra-rods nano-porosity (1–5 nm) arising from the Thornton’s conditions and an extra-rods meso-porosity (10–50 nm) originating from the spatial separation of the Titanium and Oxygen sources combined with a grazing Ti flux. The procedure is simple, since it does not require any template layer to trigger the nano-structuring, and versatile, since porosity and layer thickness can be easily tuned; it is empowered by the lack of contaminations/solvents and by the structural stability of the material (at least) up to 500 °C. Our material gains porosity, stability and infiltration capability superior if compared to conventionally sputtered TiO2 layers. Its competition level with chemically synthesized reference counterparts is doubly demonstrated: in Dye Sensitized Solar Cells, by the infiltration and chemisorption of N-719 dye (∼1 × 1020 molecules/cm3); and in Perovskite Solar Cells, by the capillary infiltration of solution processed CH3NH3PbI3 which allowed reaching efficiency of 11.7%. Based on the demonstrated attitude of the material to be functionalized, its surface activity could be differently tailored on other molecules or gas species or liquids to enlarge the range of application in different fields.
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