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

Abstract

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 (APbI 3 ), 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 -C 6 H 13 NH 3 ) 2 (GA)Pb 2 I 7 , which exhibits significantly enlarged and distorted perovskite cage containing sterically constrained GA cation. Structural comparison with ( n -C 6 H 13 NH 3 ) 2 (MA)Pb 2 I 7 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.

Acetamidinium Cation to Confer Ion Immobilization and Structure Stabilization of Organometal Halide Perovskite Toward Long Life and High‐Efficiency p‐i‐n Planar Solar Cell via Air‐Processable Method

“…The undesirable ion migration will be retarded . Furthermore, the large size cation will increase the coordination with I − , thus stabilizing the crystal . Therefore, a novel cation, acetamidinium ion (Aa + cation), with a bigger ion radius, higher boiling point, and a larger amount of coordinated bonds with I − ions than that of conventional MA + , is induced into methyl ammonium lead triiodide perovskite (MAPbI 3 ) to stabilize crystal structure and retard the ion migration …”

Pressure‐Suppressed Carrier Trapping Leads to Enhanced Emission in Two‐Dimensional Perovskite (HA)2(GA)Pb2I7

“…At ambient conditions,asteep absorption edge is observed at 580 nm which is consistent with the previous report. [6] As summarized in Figure 1e,the band gap decreases initially up to 5.1 GPa with the color of the crystal changing from red to black, followed by an increase from 5.1 GPa to 10.3 GPa, and then gradually decreases with further pressurization. These trends of band gap narrowing followed by widening upon compression have been reported in other 2D lead halide perovskites,and are probably due to pressure-induced atomic distortion and amorphization.…”

“…This is probably due to its lower crystallographic symmetry (P1 and C2/c for GA and MA compounds,r espectively) and larger distortions of the Pb À I inorganic sublattice,i ndicating the stronger exciton-phonon coupling in (HA) 2 (GA)Pb 2 I 7 . [6] Interestingly,the PL intensity of (HA) 2 (GA)Pb 2 I 7 exhibits as harp increase during compression and reaches a1 2-fold enhancement at 1.59 GPa compared with that at ambient conditions (Figure 1a;S upporting Information, Figure S1). Then, the PL intensity exhibits ag radual decrease beyond 1.6 GPa and eventually disappears at 9.48 GPa, which is mainly due to the appearance of structural disorder.…”

“…[5] Because of the large GA cations, as trong exciton-phonon interaction in the heavily distorted inorganic sublattice results in the excitons to be localized, that is,t he formation of overmuch trapped excitons at ambient conditions.T hese trapped states would induce excessive nonradiative recombination by phonon-assisted energy transfer. [6,7,14] With increasing pressure,t he lattice contraction leads to phonon hardening which considerably reduces the exciton-phonon interaction and, thus,e nlarges the potential barrier for carrier trapping. [3a, 7, 15] Therefore,t he photogenerated carriers can hardly form the trapped states and the nonradiative recombination pathway is primarily blocked, resulting in an enhanced emission with more symmetric lineshape from the free excitons.…”

“…In 2D perovskite structures,many choices can be made for LA cations and the selection of A-site cations is also expanded. [3a,6] Guanidinium [C(NH 2 ) 3 ] + (GA + )h as been reported as al arger Ac ation that can be fitted into the RP perovskite cage, [6] which relaxes the Goldschmidt tolerance factor and increases the compositional tunability.T he incorporation of such over-sized GA + expands the perovskite cage and induces al arge octahedral distortion in the inorganic framework, which would lead to as trong exciton-phonon interaction that creates ah igh degree of charge carrier trapping in the 2D perovskites. [6,7] Importantly,t he significantly expanded cage of the GA perovskite,s etting it apart from other 2D perovskites,w ould enable us to reach the unexplored structural region which offers more tuning opportunities for fundamental study upon applying external stimuli such as temperature and pressure.…”