Tech Briefs Magazine - August 2024 - 18

Creating 2D All-Organic Perovskites
All-organic perovskites offer distinct advantages over their all-inorganic counterparts.
Hong Kong Polytechnic University, Hung Hom, Hong Kong
P
erovskites are among the most researched
topics in materials science.
Recently, a research team led by Professor
LOH Kian Ping, Chair Professor of
Materials Physics and Chemistry and
Global STEM Professor of the Department
of Applied Physics of The Hong
Kong Polytechnic University (PolyU),
Dr. Kathy LENG, Assistant Professor of
the same department, together with Dr.
Hwa Seob CHOI, Postdoctoral Research
Fellow and First Author of the research
paper, has solved an age-old challenge
to synthesize all-organic two-dimensional
perovskites, extending the field into
the exciting realm of 2D materials. This
breakthrough opens up a new field of
2D all-organic perovskites, which holds
promise for both fundamental science
and potential applications.
Perovskites are named after their
structural resemblance to the mineral
calcium titanate perovskite, and are well
known for their fascinating properties
that can be applied in wide-ranging
fields such as solar cells, lighting, and catalysis.
With a fundamental chemical formula
of ABX3, perovskites possess the
ability to be finely tuned by adjusting the
A and B cations as well as the X anion,
paving the way for the development of
high-performance materials.
While perovskite was first discovered as
an inorganic compound, Loh's team has
focused its attention on the emerging class
of all-organic perovskites. In this new family,
A, B, and X constituents are organic molecules
rather than individual atoms like
metals or oxygen. The design principles for
creating three-dimensional perovskites using
organic components have only recently
been established. Significantly, all-organic
perovskites offer distinct advantages over
their all-inorganic counterparts, as they are
solution-processible and flexible, enabling
cost-effective fabrication.
Moreover, by manipulating the chemical
composition of the crystal, valuable
electromagnetic properties such as dielectric
properties, which finds applications
in electronics and capacitors, can be
precisely engineered.
18
Loh (right) and Choi (left) discovered a new method to make 2D all-organic perovskite and named
it the " Choi-Loh-v phase " (CL-v). (Image: PolyU)
Traditionally, researchers face challenges
in the synthesis of all-organic 3D perovskites
due to the restricted selection of
organic molecules that can fit with the crystal
structure. Recognizing this limitation,
Loh and his team proposed an innovative
approach: synthesizing all-organic perovskites
in the form of 2D layers instead of
3D crystals. This strategy aimed to overcome
the constraints imposed by bulky
molecules and facilitate the incorporation
of a broader range of organic ions. The anticipated
outcome was the emergence of
novel and extraordinary properties in
these materials.
Validating their prediction, the team
developed a new general class of layered
organic perovskites. Following the convention
for naming perovskites, they
called it the " Choi-Loh-v phase " (CL-v)
after Choi and Loh. These perovskites
comprise molecularly thin layers held together
by forces that hold graphite layers
together, the so-called van der Waals
forces - hence the " v " in CL-v. Compared
with the previously studied hybrid
2D perovskites, the CL-v phase is stabilized
by the addition of another B cation
into the unit cell and has the general formula
A2B2X4.
Using solution-phase chemistry, the
research team prepared a CL-v material
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known as CMD-N-P2, in which the A, B,
and X sites are occupied by CMD (a
chlorinated cyclic organic molecule),
ammonium and PF6−ions, respectively.
The expected crystal structure was confirmed
by high-resolution electron microscopy
carried out at cryogenic temperature.
These molecularly thin 2D
organic perovskites are fundamentally
different from traditional 3D minerals,
they are single crystalline in two dimensions
and can be exfoliated as hexagonal
flakes just a few nanometers thick -
20,000 times thinner than a human hair.
The solution-processibility of 2D organic
perovskites presents exciting opportunities
for their application in 2D electronics.
The Poly U team conducted measurements
on the dielectric constants of the
CL-v phase, yielding values ranging from
4.8 to 5.5. These values surpass those of
commonly used materials such as silicon
dioxide and hexagonal boron nitride.
Loh's research not only establishes an
entirely new class of all-organic perovskites
but also demonstrates how they
can be solution-processed in conjunction
with advanced fabrication technique
to enhance the performance of
2D electronic devices.
For more information, contact PolyU
at rio.general@polyu.edu.hk.
Tech Briefs, August 2024

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