Chemists have found a universal mechanism of degradation of perovskite materials

Chinese
chemists have figured out
degradation three
different perovskite materials
through
education intermediate
phase APbX2.5
(X
= I,
Br) with
ordered vacancies halide ions.
Slow
degradation
maybe
the carbon coating that is
diffusion barrier for halide ions.
Results
research published
in the journal Advanced
Materials.

Pperovskite
solar cells are cheap, easy to obtain and effective: a battery of pure perovskites already show
efficiency
25.2 per cent,
and
tandems silicon-perovskite — 29.1 per cent.
Therefore,
if
such solar cells today
entered the market, they would have made
serious competition for the current leaders

solar elements on the basis of the net
silicon.
However, until commercialization
perovskites
adequate in-
stability:
they
degraded
under the action of sunlight, with
heating and
also
the presence of water and oxygen. On
the chemical mechanism of this process
still very little is known:
the final product
always been a halide
lead
PbХ2,
but
here is the interim
stage
process to track
failed. In addition
it was unclear whether
General mechanism of degradation of all
perovskite materials or each
the connection degrades on its own.

Chinese
chemists
under the leadership of the pan
Gao (Peng
Gao) of
Peking University
managed
to identify the mechanism of degradation of perovskite
which
reproduced for several different
materials. They
used in
situ
method
electron diffraction in selected area
— that is, the irradiated material electronic
the beam of
transmission electron microscope
and
watched as under
the action of this beam
changes gradually
the crystal lattice of the material.
Objects
the study was a thin film of two
different
perovskite materials — MAPbI3
and
MAPbBr3

on
transparent
conducting substrate covered with oxide
titanium, that is lower
half
perovskite
solar
elements.

Turned out
degradation
perovskites
happens
through loss
the halide ions and the formation of
phase APbX2.5.
It
the intermediate reaction product, which
when further degradation of the turns
halogenid
lead
PbХ2.
Jobs
(free
positions in the lattice)
halides
in
APbX2.5
ordered, that is, form a
the superstructure
— violation
the crystal structure, repetitive
with some regularity and creating
thus a new structure with others
the period of rotation. This
the superstructure the authors observed for
both
materials, which is pretty remarkable,
after initially crystalline
the structure of these materials is different —
have MAPbI3
tetragonal,
and

MAPbBr3


cubic. Earlier
similar
mechanisms of degradation were
also
fixed
for
two
other materials with the structure
perovskite — CsPbBr3
and
LaCoO3,
so
you can talk about the existence of universal
mechanism of degradation of these compounds.

For
slowing down
of degradation the authors proposed to use
carbon coating thickness
6-10 nanometers, which
caused by the method of precision etching.
Quantum-chemical
calculations show that a layer
too
thin to affect the scattering
electrons, but it can
to serve
diffusion barrier for halide ions
preventing their loss.
This
the hypothesis is confirmed experimentally:
if
apply a carbon coating only
the one hand,
that stabilization does not occur, because
the halide ions can leave the grid
through another
border. And
now, if coating both sides
the stability of perovskite material
noticeable
increases: in particular covered
carbon on both sides of the sample MAPbI3
could
to tolerate a dose of
exposure ten times greater than
the same sample without coating.

Similar
effect
effects
coating on the migration of halide ions
was
demonstrated
last month another
a group of chemists.
The authors
work covered
two-dimensional perovskite heterostructure
a layer of a hard polymer with aromatic
fragments, which let
the adjustment of the crystal lattice on the
the border of the two perovskites and much
slowed migration of halide ions.

Natalia Samoylova

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