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.

泊松体结构的稳定性及其形成，受哥尔茨希德容差因子作为一般几何指导原则的支配。容差因子限制了三维铅碘化物泊松体（APbI3）中阳离子（A）的选择，而APbI3是一类令人着迷的半导体，适用于高性能光伏和光电子设备。在此，我们展示了在二维鲁德尔斯登-波珀（Ruddlesden–Popper，简称RP）泊松体中，对容差因子的要求得到了放宽，从而首次允许将多种较大的阳离子（超越甲基铵（MA）、甲酰胺铵和铯离子）纳入铅碘化物泊松体笼中。这一发现通过新合成的胍基（GA）型（n-C6H13NH3）2(GA)Pb2I7的单晶X射线结构得到了明确的证实，该结构展现出显著扩大并畸变的泊松体笼，其中包含空间受限的GA阳离子。与（n-C6H13NH3）2(MA)Pb2I7的结构比较揭示，结构稳定性的来源是缓解了二维RP结构中的应变积累和自调适的应变平衡。此外，光谱学研究显示，较大的A阳离子通过调节无机子晶格，对载流子动力学和激子-声子相互作用产生了显著影响。这些结果丰富了泊松体材料的多样家族，为A位阳离子在物理性质上的机制作用提供了新的见解，并对使用工程化泊松体薄膜的太阳能设备研究具有深远意义。