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Halide perovskites
make efficient solar cells but suffer from several
stability issues. The characterization of these degradation processes
is challenging because of the limited spatiotemporal resolution in
experiments and the absence of efficient computational methods to
study these reactive processes. Here, we present the first reactive
force field for molecular dynamics simulations of the phase instability
and the defect-induced degradation in CsPbI
3
. We find that
the phase transitions are driven by the anharmonic fluctuations of
the atoms in the perovskite lattice. At low temperatures, the Cs cations
tend to move away from their preferential positions, resulting in
worse contacts with the surrounding metal halide framework which initiates
the conversion to a nonperovskite phase. Moreover, our simulations
of defective structures reveal that, although both iodine vacancies
and interstitials are mobile in the perovskite lattice, the vacancies
have a detrimental effect on the stability, leading to the decomposition
of perovskites to PbI
2
.
Low‐dimensional perovskites have gained increasing attention recently, and engineering their material phases, structural patterning and interfacial properties is crucial for future perovskite‐based applications. Here a phase and heterostructure engineering on ultrathin perovskites, through the reversible cation exchange of hybrid perovskites and efficient surface functionalization of low‐dimensional materials, is demonstrated. Using PbI2 as precursor and template, perovskite nanosheets of varying thickness and hexagonal shape on diverse substrates is obtained. Multiple phases, such as PbI2, MAPbI3 and FAPbI3, can be flexibly designed and transformed as a single nanosheet. A perovskite nanosheet can be patterned using masks made of 2D materials, fabricating lateral heterostructures of perovskite and PbI2. Perovskite‐based vertical heterostructures show strong interfacial coupling with 2D materials. As a demonstration, monolayer MoS2/MAPbI3 stacks give a type‐II heterojunction. The ability to combine the optically efficient perovskites with versatile 2D materials creates possibilities for new designs and functionalities.
The commercialization of perovskite solar cells is hindered
by
the poor long-term stability of the metal halide perovskite (MHP)
light-absorbing layer. Solution processing, the common fabrication
method for MHPs, produces polycrystalline films with a wide variety
of defects, such as point defects, surfaces, and grain boundaries.
Although the optoelectronic effects of such defects have been widely
studied, the evaluation of their impact on the long-term stability
remains challenging. In particular, an understanding of the dynamics
of degradation reactions at the atomistic scale is lacking. In this
work, using reactive force field (ReaxFF) molecular dynamics simulations,
we investigate the effects of defects, in the forms of surfaces, surface
defects, and grain boundaries, on the stability of the inorganic halide
perovskite CsPbI3. Our simulations establish a stability
trend for a variety of surfaces, which correlates well with the occurrence
of these surfaces in experiments. We find that a perovskite surface
degrades by progressively changing the local geometry of PbI
x
octahedra from corner- to edge- to face-sharing.
Importantly, we find that Pb dangling bonds and the lack of steric
hindrance of I species are two crucial factors that induce degradation
reactions. Finally, we show that the stability of these surfaces can
be modulated by adjusting their atomistic details, by either creating
additional point defects or merging them to form grain boundaries.
While in general additional defects, particularly when clustered,
have a negative impact on the material stability, some grain boundaries
have a stabilizing effect, primarily because of the additional steric
hindrance.
Quasi-two-dimensional
(2D) Pb–Sn mixed perovskites show
great potential in applications of single and tandem photovoltaic
devices, but they suffer from low efficiencies due to the existence
of horizontal 2D phases. Here, we obtain a record high efficiency
of 18.06% based on 2D ⟨n⟩ = 5 Pb–Sn mixed perovskites (iso-BA2MA4(Pb
x
Sn1–x
)5I16, x =
0.7), by optimizing the crystal orientation through a regulation of
the Pb/Sn ratio. We find that Sn-rich precursors give rise to a mixture
of horizontal and vertical 2D phases. Interestingly, increasing the
Pb content can not only entirely suppress the unwanted horizontal
2D phase in the film but also enhance the growth of vertical 2D phases,
thus significantly improving the device performance and stability.
It is suggested that an increase of the Pb content in the Pb–Sn
mixed systems facilitates the incorporation of iso-butylammonium (iso-BA+) ligands in vertically
oriented perovskites because of the reduced lattice strain and increased
interaction between the organic ligands and inorganic framework. Our
work sheds light on the optimal conditions for fabricating stable
and efficient 2D Pb–Sn mixed perovskite solar cells.
The migration of defects plays an important role in the stability of halide perovskites. It is challenging to study defect migration with experiments or conventional computer simulations. The former lacks...
Ruthenium
(Ru) thin films are used as protective caps for the multilayer
mirrors in extreme ultraviolet lithography machines. When these mirrors
are exposed to atomic hydrogen (H), it can permeate through Ru, leading
to the formation of hydrogen-filled blisters on the mirrors. H has
been shown to exhibit low solubility in bulk Ru, but the nature of
H diffusion through Ru and its contribution to the mechanisms of blistering
remain unknown. This work makes use of reactive molecular dynamics
simulations to study the influence of imperfections in a Ru film on
the behavior of H. For the Ru/H system, a ReaxFF force field which
reproduces structures and energies obtained from quantum-mechanical
calculations was parametrized. Molecular dynamics simulations have
been performed with the newly developed force field to study the effect
of tilt and twist grain boundaries on the overall diffusion behavior
of H in Ru. Our simulations show that the tilt and twist grain boundaries
provide energetically favorable sites for hydrogen atoms and act as
sinks and highways for H. They therefore block H transport across
their planes and favor diffusion along their planes. This results
in the accumulation of hydrogen at the grain boundaries. The strong
effect of the grain boundaries on hydrogen diffusion suggests tailoring
the morphology of ruthenium thin films as a means to curb the rate
of hydrogen permeation.
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