Incorporating Large A Cations into Lead Iodide Perovskite Cages: Relaxed Goldschmidt Tolerance Factor and Impact on Exciton–Phonon Interaction

The stability and formation of a
perovskite structure is dictated
by the Goldschmidt tolerance factor as a general geometric guideline.
The tolerance factor has limited the choice of cations (A) in 3D lead
iodide perovskites (APbI3), an intriguing class of semiconductors
for high-performance photovoltaics and optoelectronics. Here, we show
the tolerance factor requirement is relaxed in 2D Ruddlesden–Popper
(RP) perovskites, enabling the incorporation of a variety of larger
cations beyond the methylammonium (MA), formamidinium, and cesium
ions in the lead iodide perovskite cages for the first time. This
is unequivocally confirmed with the single-crystal X-ray structure
of newly synthesized guanidinium (GA)-based (n-C6H13NH3)2(GA)­Pb2I7, which exhibits significantly enlarged and distorted
perovskite cage containing sterically constrained GA cation. Structural
comparison with (n-C6H13NH3)2(MA)­Pb2I7 reveals that
the structural stabilization originates from the mitigation of strain
accumulation and self-adjustable strain-balancing in 2D RP structures.
Furthermore, spectroscopic studies show a large A cation significantly
influences carrier dynamics and exciton–phonon interactions
through modulating the inorganic sublattice. These results enrich
the diverse families of perovskite materials, provide new insights
into the mechanistic role of A-site cations on their physical properties,
and have implications to solar device studies using engineered perovskite
thin films incorporating such large organic cations.