A multi-institutional collaboration, co-led by scientists at the University of Cambridge and Okinawa Institute of Science and Technology Graduate University (OIST), has identified the source of efficiency-limiting defects in potential materials for next-generation solar cells and LEDs.
In the last decade, perovskites – a diverse range of materials with a specific crystal structure – have emerged as promising alternatives to silicon solar cells, as they are cheaper and greener to manufacture, while achieving a comparable level of efficiency.
However, perovskites
still
show
significant
performance losses and instabilities
, particularly in the specific materials that promise the highest ultimate efficiency
.
Most
research to date has focused on ways to remove
these losses
, but
their actual physical causes remain unknown
.
Now, i
n a
paper
published in
Nature
, researchers from Dr
Sam
Stranks’
s
group at Cambridge
’s
Department of Chemical Engineering and Biotechnology
and Cavendish Laboratory
,
and
Professor
Keshav Dani’s
Femtosecond Spectroscopy Unit
at
OIST
in Japan,
identify the source of the problem. Their discovery could
streamline
efforts to increase
the
efficiency
of perovskites
, bringing
them
closer to mass-market production.
Perovskite materials are much more tolerant of defects in their structure than silicon solar cells, and previous research carried out by
Stranks’
s
group found that to a certain extent,
some heterogeneity in their
composition
actually improves their
performance as solar c
ells and light-emitters
.
However, the current limitation of perovskite materials is the presence of a '
deep trap' caused by a defect, or minor blemish, in the material
.
These are areas in the material where
energised charge
carriers can get stuck and recombine, losing their energy to heat, rather than converting
it
into useful electricity or light. This recombination process can have a significant impact on the efficiency
and stability
of solar panels and LEDs.
Until now, very little was known about the cause of these traps
, in part because they appear to behave differently to traps in traditional solar cell materials
.
In 2015
,
Stranks
and colleagues
published
a paper in
Science
l
ooking
at the luminescence of
perovskites
, which
reveals
how good they are
at absorbing or emitting light
.
“
W
e found that
the material
was very heterogeneous
;
y
ou had quite
large regions that were bright and
luminescent and other regions that were really dark
,”
said
Stranks
.
“
These dark regions correspond to power losses in solar cells or LEDs.
But
what was causing th
e power loss
was always a mystery
,
especially because
perovskites
are otherwise so defect
-
tolerant
.
”
Due to limitations of standard imaging techniques, the group
couldn’t
tell if the darker areas were caused by one, large
trap site
, or many smaller traps, making it difficult to establish why they were forming
only
in certain regions
.
In 2017,
Dani’s group at OIST
made
a
movie
of how electrons
behave
in
semiconductors
after absorbing light. “
You can learn a lot from being able to see how charges move in a material or device after shining li
ght
.
For example, you c
an
see where they might be getting trapped,”
said Dani
.
“
However,
these
charges
are hard to visualise as they
move very fast
– on the timescale of a millionth of a billionth of a second;
and over very short distances
– on the length
scale of a
billionth of a
met
r
e
.
”
On hearing of
Dani’s work
,
Stranks
reached out to see if they could
work together to
address
the problem
visuali
s
ing
the dark regions in
perovskites
.
The team at OIST used a technique called
photoemission electron microscopy
(PEEM)
for the first time on perovskites
,
where they probed the material with ultraviolet light and built up an image based on how the
emitted
electrons scattered
.
When they looked at the material, t
hey
found
that the
dark regions contained
traps
,
around
10-100 nanometers in length,
which
were clusters of smaller atomic-sized trap sites. These trap clusters were spread unevenly throughout the perovskite material, explaining
the heterogeneous luminescence seen in
Stranks’s earlier research
.
When the researchers overlaid images of the trap sites onto images that showed the crystal grains of the perovskite material, they found that the trap clusters only formed at specific places, at the boundaries between certain grains.
To
understand why this
only
occurred at certain grain boundaries
, the group
s
worked
together
with Professor Paul Midgley’s team from
Cambridge’s
Department of M
a
terials Science and Metallurgy
using
a technique called
scanning electron
diffraction
to
create detailed images of the perovskite crystal structure
.
The project
team made use
of
the
electron
microscopy setup at the
e
PSIC
facility
at the Diamond Light Source Synchrotron
,
which has
specialised
equipment for
imaging
beam-sensitive
materials
, like perovskites
.
“Because
these materials are
very
beam
-
sensitive,
typical techniques that you would use
to probe local crystal structure on these length scales
will
quite quickly
change
the
material as you're looking at it
,
which can make interpreting the data very difficult,
” said
Tiarnan
Doherty
, a PhD student in
Stranks
’
s
group and
co-l
ead author of the study
.
“
Instead, we were able to use
very low exposure doses and
therefore
prevent
damage.
“
From the
work at OIST
, w
e knew where the
trap
clusters
w
ere
located
,
and at
ePSIC
,
we
scanned
around
those
same area
s
to see
the local structure.
W
e were
then
able to quickly pinpoint unexpected variations in the crystal
structure
around the
trap
clusters
.
”
The
group discovered that the trap clusters
only formed
at junctions where an area of the material with slightly distorted structure met an area with pristine structure.
“In perovskites
,
we have regular mosaic grains of material and m
ost of t
he grains are nice and pristine – the structure we would expect,”
said
Stranks
.
“
But e
very now
and again
,
you get a
grain that's slightly distorted and the chemistry of that
grain
is inhomogeneous. W
hat was really interesting and
which initially confused us
was that
it's not
the distorted grain
that's the trap
but
whe
re
that
grain meets a pristine grain; it's at that junction that the
traps
cluster
.
”
With this understanding of the nature of the traps
,
the team
at OIST
also u
sed
the
custom-buil
t
PEEM
instrumentation
to
visualise the dynamics of the charge carrier trapping process happening in the perovskite material.
“
This was possible as o
ne of the unique features of our PEEM setup is
that it can
image
ultra
fast processes
–
as short as femtoseconds
,” said
Andrew Winchester, a PhD student
in
Dani
’s
Unit, and
co-
lead author of this study
. “
We
found
that the trapping process was dominated by
charge carriers
diffusing to the trap clusters.
”
The
se
discover
ies
represent
a breakthrough in the quest to bring perovskites to the solar energy market.
“
We
still
don't know exactly why
the traps are
clustering there
,
but
we no
w
know
that they do form there, a
nd
seemingly
only there
,”
said
Stranks
.
“
T
hat's exciting because it means
we
now
know what to target to bring
up
the performances
of
perovskite
s
. W
e need to ta
rget those inhomogeneous phases or
get rid of these junctions
in some way
.
”
“The fact that charge carriers must first diffuse to the traps could also suggest other strategies to improve these devices,” said Dani. “Maybe we
could alter or control the arrangement of the trap clusters, without necessarily changing their average number, such that charge carriers are less likely to reach these defect sites
.”
The
team
s
’
research focused on one particular perovskite structure
.
The scientists
will now be investigating whether the cause of these trapping clusters is universal across
other
perovskite materials.
“Most of the progress in device performance has been
trial and error
and so far
,
this has been quite an inefficient process
,”
said
Stranks
. “
To date
,
it really hasn't been driven by
knowing a specific cause and
systematically
targeting that.
This is one of the first breakthroughs
that
will help us to use the fundamental science to
engineer more efficient devices
.”
Reference:
Tiarnan A.S. Doherty et al. '
Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites
.' Nature (2020). DOI: 10.1038/s41586-020-2184-1