REVIEW SUMMARY
◥SOLAR CELLS
Rethinking the A cation in halide perovskites
Jin-Wook Lee†, Shaun Tan†, Sang Il Seok, Yang Yang, Nam-Gyu Park*
BACKGROUND:Organic-inorganic lead halide
perovskites (OLHPs) represent a family of semi-
conducting materials that have attracted wide
interest for their optoelectronic applications.
By combining notable electronic and photo-
physical properties with low-temperature so-
lution processability using earth-abundant
materials, the field has experienced rapid
breakthroughs over the past decade. OLHP-
based optoelectronic devices have now been
established as an independent field of study.
In particular, solar cells that use OLHPs as the
photoactive layer have exceeded 25% power
conversion efficiency, which is competitive with
photovoltaics based on traditional inorganic
semiconductors such as silicon. However, un-
answered questions remain to be addressed
regarding the fundamental properties and our
understanding of OLHP materials.
ADVANCES:OLHPs have the general chemical
formula of ABX 3 , where A represents a mono-
valent cation, B represents the divalent lead
cation, and X is a halide anion. During its ini-
tial development, the A cation was limited to
mainly three candidates, namely methylam-
monium (MA+), formamidinium (FA+), and
cesium (Cs+), which were determined by size
and structural constraints to fit within the
spaces of the OLHP crystal lattice. Because the
A cation, by itself, does not directly contribute
to the OLHP band-edge construction, it was
traditionally believed to hardly affect the opto-
electronic properties of OLHPs.
In contrast to these presumptions, advance-
ments in recent years have unraveled the crit-
ical role of the A cation in determining the
optoelectronic and physicochemical proper-
ties of OLHPs. Major breakthroughs in the
field have been realized by using A cations to
fine-tune the properties of OLHPs, including
for (i) thermodynamic or kinetic stabiliza-
tion of desirable OLHP phases, especially the
metastable cubic formamidinium lead triiodide
polymorph; (ii) impeding the migration of
ions by electrostatic interactions and/or steric
effects; and (iii) surface and interface func-
tionalization and modification. These topics
are now at the forefront of OLHP research,
ultimately dictating the utility, function, per-
formance, and stability of OLHP-based opto-
electronic devices.In this review, we examine the important
developments enabled by the expanded role
of the A cation. To highlight its importance,
the vast majority of the record-breaking per-
formances for OLHP-based photovoltaics have
been enabled by breakthroughs related to the
versatility of the A cation, which we discuss.
Moreover, recently, bulky conjugated A cations
have even been demonstrated to participate in
constructing the OLHP band-edge structure,
further challenging a long-held traditional
notion regarding the role of the A cation.OUTLOOK:Future opportunities entail either
discovering new uses for existing A cation spe-
cies or identifying new A cations for existing
applications, or both simultaneously. When
looking toward improving the commercializa-
tion readiness of OLHPs, A cation applications
that involve addressing the module scale-up
challenges and long-term operational instability
issues must be further explored. This neces-
sitates a more comprehensive understanding
of the structure-property-performance-stabil-
ity correlations conferred by the A cation by
combining systematic experimental approaches
with first-principles theoretical studies.
▪RESEARCHSCIENCEscience.org 25 FEBRUARY 2022¥VOL 375 ISSUE 6583 835
The list of author affiliations is available in the full article online.
*Corresponding author. Email: [email protected] (S.I.S.);
[email protected] (Y.Y.); [email protected] (N.-G.P.)
These authors contributed equally to this work.
Cite this article as J.-W. Leeet al.,Science 375 , eabj1186
(2022). DOI: 10.1126/science.abj1186READ THE FULL ARTICLE AT
https://doi.org/10.1126/science.abj1186Modifying perovskites through A site cations.In recent years, important breakthroughs have been enabled by the expanded role and function of the A cation in tailoring the
physicochemical and optoelectronic properties of OLHPs. These advances ultimately dictate the utility, function, and performance of OLHP-based optoelectronic devices.