6 research outputs found
Probing the Reactivity of ZnO with Perovskite Precursors
To achieve more stable
and efficient metal halide perovskite devices,
optimization of charge transport materials and their interfaces with
perovskites is crucial. ZnO on paper would make an ideal electron
transport layer in perovskite devices. This metal oxide has a large
bandgap, making it transparent to visible light; it can be easily
n-type doped, has a decent electron mobility, and is thought to be
chemically relatively inert. However, in combination with perovskites,
ZnO has turned out to be a source of instability, rapidly degrading
the performance of devices. In this work, we provide a comprehensive
experimental and computational study of the interaction between the
most common organic perovskite precursors and the surface of ZnO,
with the aim of understanding the observed instability. Using X-ray
photoelectron spectroscopy, we find a complete degradation of the
precursors in contact with ZnO and the formation of volatile species
as well as new surface bonds. Our computational work reveals that
different pristine and defected surface terminations of ZnO facilitate
the decomposition of the perovskite precursor molecules, mainly through
deprotonation, making the deposition of the latter on those surfaces
impossible without the use of passivation
Defects in Halide Perovskites: Does It Help to Switch from 3D to 2D?
Two-dimensional (2D) organic–inorganic hybrid
iodide perovskites
have been put forward in recent years as stable alternatives to their
three-dimensional (3D) counterparts. Using first-principles calculations,
we demonstrate that equilibrium concentrations of point defects in
the 2D perovskites PEA2PbI4, BA2PbI4, and PEA2SnI4 (PEA, phenethylammonium;
BA, butylammonium) are much lower than in comparable 3D perovskites.
Bonding disruptions by defects are more destructive in 2D than in
3D networks, making defect formation energetically more costly. The
stability of 2D Sn iodide perovskites can be further enhanced by alloying
with Pb. Should, however, point defects emerge in sizable concentrations
as a result of nonequilibrium growth conditions, for instance, then
those defects likely hamper the optoelectronic performance of the
2D perovskites, as they introduce deep traps. We suggest that trap
levels are responsible for the broad sub-bandgap emission in 2D perovskites
observed in experiments
Mixing I and Br in Inorganic Perovskites: Atomistic Insights from Reactive Molecular Dynamics Simulations
All-inorganic halide
perovskites have received a great deal of
attention as attractive alternatives to overcome the stability issues
of hybrid halide perovskites that are commonly associated with organic
cations. To find a compromise between the optoelectronic properties
of CsPbI3 and CsPbBr3, perovskites with CsPb(BrxI1–x)3 mixed compositions are commonly used. An additional benefit
is that without sacrificing the optoelectronic properties for applications
such as solar cells or light-emitting diodes, small amounts of Br
in CsPbI3 can prevent the inorganic perovskite from degrading
to a photo-inactive non-perovskite yellow phase. Despite indications
that strain in the perovskite lattice plays a role in the stabilization
of the material, a full understanding of such strain is lacking. Here,
we develop a reactive force field (ReaxFF) for perovskites starting
from our previous work for CsPbI3, and we extend this force
field to CsPbBr3 and mixed CsPb(BrxI1–x)3 compounds.
This force field is used in large-scale molecular dynamics simulations
to study perovskite phase transitions and the internal ion dynamics
associated with the phase transitions. We find that an increase of
the Br content lowers the temperature at which the perovskite reaches
a cubic structure. Specifically, by substituting Br for I, the smaller
ionic radius of Br induces a strain in the lattice that changes the
internal dynamics of the octahedra. Importantly, this effect propagates
through the perovskite lattice ranging up to distances of 2 nm, explaining
why small concentrations of Br in CsPb(BrxI1–x)3 (x ≤ 1/4) have a significant impact on the phase stability of
mixed halide perovskites
Mixing I and Br in Inorganic Perovskites: Atomistic Insights from Reactive Molecular Dynamics Simulations
All-inorganic halide
perovskites have received a great deal of
attention as attractive alternatives to overcome the stability issues
of hybrid halide perovskites that are commonly associated with organic
cations. To find a compromise between the optoelectronic properties
of CsPbI3 and CsPbBr3, perovskites with CsPb(BrxI1–x)3 mixed compositions are commonly used. An additional benefit
is that without sacrificing the optoelectronic properties for applications
such as solar cells or light-emitting diodes, small amounts of Br
in CsPbI3 can prevent the inorganic perovskite from degrading
to a photo-inactive non-perovskite yellow phase. Despite indications
that strain in the perovskite lattice plays a role in the stabilization
of the material, a full understanding of such strain is lacking. Here,
we develop a reactive force field (ReaxFF) for perovskites starting
from our previous work for CsPbI3, and we extend this force
field to CsPbBr3 and mixed CsPb(BrxI1–x)3 compounds.
This force field is used in large-scale molecular dynamics simulations
to study perovskite phase transitions and the internal ion dynamics
associated with the phase transitions. We find that an increase of
the Br content lowers the temperature at which the perovskite reaches
a cubic structure. Specifically, by substituting Br for I, the smaller
ionic radius of Br induces a strain in the lattice that changes the
internal dynamics of the octahedra. Importantly, this effect propagates
through the perovskite lattice ranging up to distances of 2 nm, explaining
why small concentrations of Br in CsPb(BrxI1–x)3 (x ≤ 1/4) have a significant impact on the phase stability of
mixed halide perovskites
Mixing I and Br in Inorganic Perovskites: Atomistic Insights from Reactive Molecular Dynamics Simulations
All-inorganic halide
perovskites have received a great deal of
attention as attractive alternatives to overcome the stability issues
of hybrid halide perovskites that are commonly associated with organic
cations. To find a compromise between the optoelectronic properties
of CsPbI3 and CsPbBr3, perovskites with CsPb(BrxI1–x)3 mixed compositions are commonly used. An additional benefit
is that without sacrificing the optoelectronic properties for applications
such as solar cells or light-emitting diodes, small amounts of Br
in CsPbI3 can prevent the inorganic perovskite from degrading
to a photo-inactive non-perovskite yellow phase. Despite indications
that strain in the perovskite lattice plays a role in the stabilization
of the material, a full understanding of such strain is lacking. Here,
we develop a reactive force field (ReaxFF) for perovskites starting
from our previous work for CsPbI3, and we extend this force
field to CsPbBr3 and mixed CsPb(BrxI1–x)3 compounds.
This force field is used in large-scale molecular dynamics simulations
to study perovskite phase transitions and the internal ion dynamics
associated with the phase transitions. We find that an increase of
the Br content lowers the temperature at which the perovskite reaches
a cubic structure. Specifically, by substituting Br for I, the smaller
ionic radius of Br induces a strain in the lattice that changes the
internal dynamics of the octahedra. Importantly, this effect propagates
through the perovskite lattice ranging up to distances of 2 nm, explaining
why small concentrations of Br in CsPb(BrxI1–x)3 (x ≤ 1/4) have a significant impact on the phase stability of
mixed halide perovskites
What Happens at Surfaces and Grain Boundaries of Halide Perovskites: Insights from Reactive Molecular Dynamics Simulations of CsPbI<sub>3</sub>
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 PbIx 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