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.
Growth of Cu(In,Ga)Se2 (CIGS) absorbers under Cu‐poor conditions gives rise to incorporation of numerous defects into the bulk, whereas the same absorber grown under Cu‐rich conditions leads to a stoichiometric bulk with minimum defects. This suggests that CIGS absorbers grown under Cu‐rich conditions are more suitable for solar cell applications. However, the CIGS solar cell devices with record efficiencies have all been fabricated under Cu‐poor conditions, despite the expectations. Therefore, in the present work, both Cu‐poor and Cu‐rich CIGS cells are investigated, and the superior properties of the internal interfaces of the Cu‐poor CIGS cells, such as the p–n junction and grain boundaries, which always makes them the record‐efficiency devices, are shown. More precisely, by employing a correlative microscopy approach, the typical fingerprints for superior properties of internal interfaces necessary for maintaining a lower recombination activity in the cell is discovered. These are a Cu‐depleted and Cd‐enriched CIGS absorber surface, near the p–n junction, as well as a negative Cu factor (∆β) and high Na content (>1.5 at%) at the grain boundaries. Thus, this work provides key factors governing the device performance (efficiency), which can be considered in the design of next‐generation solar cells.
Studying the adsorbate interactions on a surface helps in understanding the growing surface morphologies and calculating the effective surface diffusion barriers. We study the interaction between Ga-Ga, N-N and Ga-N adatom pairs on the polar GaN(0001) surface using ab initio calculations based on density functional theory. The interaction energy between two adatoms on the surface does not seem to follow definite trends with increasing distance between the adatoms. The presence of a number of possible reconstructions on clean GaN(0001) and periodic effects due to the finite size complicate the analysis of the interactions. Various components of the total interaction energy are separated. We find that there is a large substrate lattice distortion caused due to Ga and N adatoms. The resulting elastic interaction is a major component of the interactions between the adatoms on the GaN(0001) surface. The dipolar interaction is much smaller in magnitude. We also evaluate the component of the interaction energy due to the substrate-mediated electronic interactions. The barriers for surface hopping of adatoms are significantly modified in the presence of other adatoms. We identify several possible surface hopping processes for Ga and N adatoms and calculate their barriers. In particular, we find that the N adatom has a lower barrier to move to an adjoining site on the other side of a neighboring Ga adatom. Kinetic Monte Carlo simulations are performed to see the effect of adatom interactions on the growing surface morphologies of GaN(0001). At the submonolayer growth stage, the fast diffusion of N adatoms located near Ga adatoms leads to more regular island features. In this way, we illustrate the role of adatom interactions in the initial surface nucleation and the morphologies of the growing GaN(0001) film.
The
adsorption of an adatom on the surface of a crystal is affected
by the surface coverage due to adatom–adatom and adatom–substrate
interactions. We study the dependence of adsorption energy of Ga and
N adatoms on wurtzite GaN(0001) as a function of decreasing surface
coverage from 0.25 to 0.04 monolayers through first-principles calculations.
The adsorption energies of Ga and N adatoms on the flat, clean GaN(0001)
substrate do not converge with decreasing coverage. Further, it appears
that the Ga and N adatoms significantly distort the substrate lattice
from the flat configuration. We found that these distortions increase
with increase in system size (or, equivalently, with reducing coverage).
This observation is counterintuitive since it is expected that lattice
distortions should decrease with decreasing surface coverage. We separate
the different contributions to the adsorption energy and identify
the part of adsorption energy that arises due to lattice distortions.
This contribution appears to be much larger than the contribution
due to dipolar interaction between adatoms and their periodic images.
This also allows us to identify some surface reconstructions of clean
GaN(0001) that have not been reported yet, which are lower in energy
than the flat clean substrate. The adsorption energies calculated
with respect to these reconstructed configurations converge with increasing
system size. This size-dependent or coverage-dependent study suggests
that the effect of lattice distortions should be carefully taken into
account when calculating adsorption energy for periodic systems from
first-principles methods.
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