The
behavior of alkali atom point defects in polycrystalline CuInSe2 is studied. In this work, three grain boundary models, one
coherent twin boundary and two twin boundaries with dislocation cores,
are considered. Total energy calculations show that all alkali metals
tend to segregate at the grain boundaries. In addition, the segregation
of alkali atoms is more pronounced at the grain boundaries with the
dislocation cores. The diffusion of alkali metals along and near grain
boundaries is studied as well. The results show that the diffusion
of alkali atoms in the grain boundary models is faster than within
the bulk. In addition, the ion exchange between Na and Rb atoms at
the grain boundaries leads to the Rb enrichment at the grain boundaries
and the increase of the Na concentration in the bulk. While the effects
of Na and Rb point defects on the electronic structure of the grain
boundary with the anion-core dislocation are similar, Rb atoms passivate
the grain boundary with the cation-core dislocation more effectively
than Na. This can explain the further improvement of the solar cell
performance after the RbF-postdeposition treatment.
Hydrogen bonds impact many material properties due to
the bond
directionality and collective strength. For them to exist, the atoms
connected to hydrogen should possess high electronegativity, the distance
between the interacting atoms should be less than their van der Waals
radii, and there should be a charge transfer between them. In hybrid
perovskites, the impact of hydrogen bonds is ascribed to several phenomena.
Among them are the effect of passivation at the surface, structural
stability of the mixed cation compounds, structural instability of
FAPbI3, and ferroelectric behavior. In this paper, we applied
a deterministic method to prove the existence of hydrogen bonds using
Raman spectroscopy. This approach was successfully applied to water
and is based on the emergence of a Raman mode as a result of the intermolecular
charge transfer, which is not measured for the isolated molecule.
With the help of DFT calculations, we attributed the Raman modes detected
at room temperature (RT) for MAI and FAI and compared them with the
Raman spectra of MAPbX3 and FAPbX3. This comparison
has shown that there are no hydrogen bonds in hybrid lead halide perovskites
at RT.
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